1Blends Dyeing

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Blends Dyeing

1998

Society of Dyers and Colourists

John Shore

Formerly of BTTG-Shirley and ICI (now BASF), Manchester, UK

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Copyright © 1998 Society of Dyers and Colourists. All rights reserved. No partof this publication may be reproduced, stored in a retrieval system or transmittedin any form or by any means without the prior permission of the copyrightowners.

Published by the Society of Dyers and Colourists, PO Box 244, Perkin House,82 Grattan Road, Bradford, West Yorkshire BD1 2JB, England, on behalf of theDyers’ Company Publications Trust.

This book was produced under the auspices of the Dyers’ CompanyPublications Trust. The Trust was instituted by the Worshipful Company ofDyers of the City of London in 1971 to encourage the publication of textbooksand other aids to learning in the science and technology of colour and colorationand related fields. The Society of Dyers and Colourists acts as trustee to thefund, its Textbooks Committee being the Trust’s technical subcommittee.

Typeset by the Society of Dyers and Colourists and printed by H Charlesworth& Co. Ltd, Huddersfield, UK.

ISBN 0 901956 74 0

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Contents

Preface ix

CHAPTER 1 Why blending is necessary 1

1.1 Blending from the dyer’s viewpoint 11.2 The composition of blend fabrics 21.3 The relative importance of individual blends 31.4 Reasons for the development of fibre blends 51.5 Colour effects achieved by blending 101.6 Sighting colours for identification purposes 191.7 References 20

CHAPTER 2 Classification of fibre types and their blends 21

2.1 Classification of fibre types in terms of dyeability 212.2 Colour distribution attainable on binary blends 222.3 References 25

CHAPTER 3 Dynamic competition between fibre typesin the dyeing of blends 26

3.1 Introduction 263.2 The distribution of acid dyes on nylon/wool blends 293.3 The distribution of acid dyes on nylon/polyurethane blends 353.4 The cross-staining of wool by disperse dyes 363.5 The cross-staining of wool by basic dyes 413.6 The transfer of disperse dyes during thermofixation of

polyester/cellulosic blends 443.7 References 45

CHAPTER 4 Minimising incompatibility between dyesfrom different classes 46

4.1 Interaction between disperse dyes and reactive dyes 464.2 Interaction between disperse or vat dyes and basic dyes 474.3 Interaction between anionic dyes and basic dyes 484.4 References 52

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CHAPTER 5 Principles of design and colouring ofdifferential-dyeing blends 53

5.1 Design of differential-dyeing variant synthetic-polymer yarns 53

5.2 Dyeing of acid-dyeable nylon variants 575.3 Dyeing of acid-dyeable/basic-dyeable nylon

variants 615.4 Design of differential-dyeing cellulosic fabrics 635.5 Design of differential-dyeing wool keratin

derivatives 715.6 References 76

CHAPTER 6 Nylon/wool and other AA blends 77

6.1 Dyeing of nylon/wool blends 776.2 Blends of wool with other acid-dyeable fibres 796.3 Blends of nylon with other acid-dyeable fibres 826.4 Dyeing methods and dye selection for AA blends 846.5 References 85

CHAPTER 7 Wool/acrylic and other AB blends 86

7.1 Dyeing of wool/acrylic blends 867.2 Dyeing of nylon/acrylic blends 907.3 Blends of acid-dyeable and basic-dyeable acrylic

variants 917.4 Blends of modacrylic and acrylic fibres 937.5 Blends of amide fibres with modacrylic or acid-dyeable

acrylic variants 947.6 Blends of basic-dyeable polyester with wool or nylon 967.7 Dyeing methods and dye selection for AB blends 997.8 References 99

CHAPTER 8 Wool/cellulosic and other AC blends 100

8.1 Dyeing of wool/cellulosic blends 1008.2 Exhaust dyeing of nylon/cellulosic blends 1088.3 Continuous dyeing of nylon/cellulosic blends 1138.4 Dyeing methods and dye selection for AC blends 1158.5 References 118

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CHAPTER 9 Cellulosic/acrylic and other CB blends 119

9.1 Exhaust dyeing of cellulosic/acrylic blends 1199.2 Continuous dyeing of cellulosic/acrylic blends 1229.3 Blends of cellulosic fibres with modacrylic or

acid-dyeable acrylic variants 1249.4 Blends of basic-dyeable polyester with cotton 1269.5 Dyeing methods and dye selection for CB blends 1269.6 References 128

CHAPTER 10 Cotton/viscose and other CC blends 129

10.1 Properties and performance of cellulosic fibres in their blends 129

10.2 Dyeing behaviour of cellulosic fibres in their blends 133

10.3 Dyeing methods and dye selection for CC blends 13610.4 References 137

CHAPTER 11 Polyester/wool and other DA blends 138

11.1 Dyeing of polyester/wool blends 13811.2 Blends of cellulose acetate or triacetate with

wool 14911.3 Dyeing of polyester/nylon blends 15211.4 Blends of cellulose acetate or triacetate with

nylon 15411.5 Blends of poly(vinyl chloride) fibres with wool or

nylon 15711.6 Dyeing methods and dye selection for DA blends 16011.7 References 160

CHAPTER 12 Polyester/acrylic and other DB blends 161

12.1 Dyeing of polyester/acrylic blends 16112.2 Blends of cellulose acetate or triacetate with acrylic

fibres 16312.3 Dyeing of normal/basic-dyeable polyester blends 16512.4 Dyeing methods and dye selection for DB blends 16812.5 References 168

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CHAPTER 13 Polyester/cellulosic and other DC blends 169

13.1 Exhaust dyeing of polyester/cellulosic blends 16913.2 Continuous dyeing of polyester/cellulosic blends 18713.3 Blends of cellulose acetate or triacetate with cellulosic

fibres 19713.4 Blends of poly(vinyl chloride) fibres with cellulosic

fibres 20113.5 Dyeing methods and dye selection for DC blends 20113.6 References 204

CHAPTER 14 Triacetate/polyester and other DD blends 206

14.1 Dyeing properties of disperse-dyeable fibre blends 20614.2 Dyeing methods and dye selection for DD blends 21014.3 References 211

CHAPTER 15 Dyeing properties of three-componentblends 212

15.1 Introduction 21215.2 Dyeing of AAA blends 21315.3 Dyeing of AAB blends 21515.4 Dyeing of AAC blends 21615.5 Dyeing of CBA blends 21715.6 Dyeing of DAA blends 21715.7 Dyeing of DAC blends 21815.8 Dyeing of DBA blends 21915.9 Dyeing of DBC blends 22015.10 Dyeing of DDA blends 22115.11 Dyeing of DDC blends 22215.12 Dyeing methods and dye selection for three-component

blends 22215.13 References 225

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Preface

This book is an addition to the series on coloration technology issued by theTextbooks Committee of the Society of Dyers and Colourists under the aegisof the Dyers’ Company Publications Trust Management Committee, whichadministers the fund generously provided by the Worshipful Company ofDyers.

Earlier books on dyeing technology in this series, namely The dyeing ofsynthetic-polymer and acetate fibres (1979), The dyeing of cellulosics fibres(1986) and Wool dyeing (1992), each contained a chapter on the dyeing ofthose fibre blends most relevant to their respective titles. Inevitably, thisapproach lacked balance, and material on specific blends was either partiallyduplicated or, more often, entirely overlooked. When replacements for the1979 and 1986 books were under consideration in the early 1990s, thedecision was taken to produce a separate volume dedicated to the dyeing offibre blends. This book is the result of that change of approach.

Very few books have been devoted solely to this subject. The best knownis undoubtedly the ‘classic’ Dyeing of fibre blends (1966), written by RoyCheetham of Courtaulds. Invaluable in its time, Cheetham’s book was amine of practical information and detailed recommendations for everyconceivable blend. The treatment in this present book is intended to provideonly general guidelines in this respect, since a dyer encountering anunfamiliar blend for the first time cannot avoid undertaking preliminarydevelopment work. An attempt is made in the first five chapters of this bookto express some general principles applicable to the theme. A classification ofblends according to the dyeing properties of their component fibres isintroduced in Chapter 2. These categories form the respective topics of theremaining ten chapters on dyeing methods.

The author is indebted to the referee of this book and to Jim Park forvaluable comments and suggestions for improvement of the text. Gratefulthanks are due to Paul Dinsdale (the editor of the Society), Gina Walker(copy editing and proof reading) and Sue Petherbridge (typesetting andlayout).

JOHN SHORE

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CHAPTER 1

Why blending is necessary

1.1 BLENDING FROM THE DYER’S VIEWPOINT

The term ‘blending’ is used by the yarn manufacturer to describe specifically thesequence of processes required to convert two or more kinds of staple fibres intoa single yarn composed of an intimate mixture of the component fibres. Thismay be necessary to obtain a uniform yarn from different varieties of the samefibrous polymer, as in the blending of wool qualities differing in origin, or in theblending of two colours of mass-pigmented man-made fibre to give a target hue.Any blend must have acceptable properties for the spinner. Important factorsinclude the relative diameters, staple lengths and extensibilities of the fibrespresent. A mismatch can create a blend that has lower strength than that of eitherof the component fibre types. Polyester has an advantage over nylon in blendswith cotton in that its initial modulus matches that of cotton more closely.

To the dyer, however, the significant type of staple-fibre blend is that in whichthe components are two different fibrous polymers, each with its owncharacteristic dyeing properties. The term ‘blend’ has therefore been used moreloosely by the dyer to refer to any combination of fibre types, whether they occuras different filaments or staple fibres in the same yarn, or as different yarnsassembled in the same fabric or garment. This is the sense in which ‘blend’ isused here, the essential difference between the components being that of dyeingcharacteristics.

Blended-staple yarns occupy a highly important position alongside the majortypes of homogeneous staple-fibre yarns in the textile industry. Blends ofsynthetic fibres, notably polyester, with cellulosics are produced in suchquantities, for shirtings, dresswear, outerwear, rainwear, workwear andhousehold textiles, that continuous dyeing methods for these blends are asimportant as for the parent cellulosic fabrics. Polyester/wool blends areparticularly useful in suitings, dresswear and outerwear, whilst wool yarns in

2 WHY BLENDING IS NECESSARY

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hand-knitting, hosiery, knitwear and carpets have yielded much ground to nylon/wool blends.

Mixed-ply yarns have been incorporated in woven fabrics for many years,often to introduce special effect threads, or in more substantial proportion toconfer stretch, bulk or resilience. The contrasting dyeability of the componentyarns may give attractive marl effects and prove useful in carpets, knitwear orhand-knitting yarns. In support hosiery and foundation garments elastomericwarp yarns are often covered with nylon filament yarn.

Fabrics woven from polyester staple-core/cotton wrap yarns in both warp andweft directions can be successfully desized, bleached, dyed to solid shades andgiven a durable press finish without difficulty using conventional procedureswith only slight modifications. The finished fabrics are soft but exceptionallystrong. They are especially useful where high strength, durability, moistureabsorbency and easy-care performance are important features [1]. Fabricsconstructed from these staple-core yarns and from intimate-blend yarns havebeen compared before and after durable press finishing. The superior propertiesof the treated staple-core fabrics are attributed to the consolidation of thestronger but more extensible polyester staple in the core of the yarns [2].

Ingenious methods of combining man-made fibrous polymers in the sameextruded filament or bundle of filaments have been developed from time to timebut have failed to generate much more than novelty interest. Filaments madefrom two different polymers fused together within the material are known asbicomponent filaments [3,4]. Multifilament yarns, formed by the interminglingof two types of filament by extrusion from a special spinneret, contain a randomdistribution of the individual components [5].

1.2 THE COMPOSITION OF BLEND FABRICS

Staple-fibre yarn blends are long-established in woven fabrics and there is anexceedingly wide variety of fabric constructions woven or knitted from two (ormore) types of homogeneous yarn. Materials of the latter kind have often beenreferred to as ‘union fabrics’, but to avoid confusion this term will be avoidedhere. The broader description ‘blend fabrics’ will be used where necessary todescribe all types of construction made from two or more fibrous polymers orvariants that differ in dyeing characteristics, including filament unions, blended-staple fabrics, pile fabrics and carpets.

Apparel and domestic textiles are important for such blend fabrics, which mayexhibit desirable two-way differences in physical properties and often providescope for attractive multicolour patterning. The availability of wholly syntheticblend fabrics, such as polyester/acrylic dresswear, polyester/nylon outerwear or

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THE COMPOSITION OF BLEND FABRICS

nylon/acrylic half-hose, as well as the differential-dyeing variants of theseindividual fibres, offers considerable scope for striking coloured effects.

Pile fabrics play an important part in the upholstery and furnishings market.They often consist of a nylon or cellulosic backing fabric with a resilient pilemade from wool or acrylic staple. Cotton pile in nylon support fabric is widelyused in lightweight towelling, leisurewear and children’s clothing. The carpetindustry is a long-established outlet for fibre blends. Apart from the notableshare of nylon/wool blended staple in the traditional woven field, the availabilityof differential-dyeing nylon has simplified the production of multicoloureddesigns in tufted carpeting, made by needling the appropriately identified pileyarns into a suitable backing.

1.3 THE RELATIVE IMPORTANCE OF INDIVIDUAL BLENDS

It is often difficult to obtain detailed information on the relative demand fordifferent types of fibre blends. Statistics of production or consumption of textilefibres are almost always classified in terms of the total amount of each fibre type,irrespective of whether that fibre is used alone in a garment or other textile, or asa component of blended material. The figures in Tables 1.1 and 1.2 are takenfrom part of a confidential market research survey for 1985, in which theinformation was gathered for each market according to whether the amounts offibres were used alone or in one of several major categories of fibre blends. Inorder to exclude from consideration those industrial uses of fibres (normally notblended) where coloration is not a possibility, the statistics were limited to those

Table 1.1 Textile fibres available for coloration worldwide

Amount ProportionFibres (kg × 106) (%)

Cotton 11640 39.0Polyester/cellulosic blends 4520 15.2Nylon (including polyurethane) 3090 10.4Polyester 2840 9.5Acrylic (including modacrylic) 2210 7.4Viscose (including modal, polynosic) 2030 6.8Wool (including other animal fibres) 1560 5.2All other blends 1220 4.1Linen (including other bast fibres) 370 1.2Cellulose acetate and triacetate 285 1.0Silk 65 0.2

Total 29830


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Table 1.2 Blends of fibres available for coloration worldwide

Amount ProportionBlends (kg × 106) (%)

Polyester/cotton 3350 58.4Polyester/viscose 1170 20.4Miscellaneous blendsa 545 9.5Polyester/wool 410 7.1Wool/acrylic 80 1.4Other synthetic blendsb 75 1.3Other cotton blendsc 65 1.1Nylon/wool 45 0.8

Total 5740

a Includes wool/polyurethane, wool/viscose, cellulose acetate/nylon, ...b Includes nylon/acrylic, polyester/nylon, polyester/acrylic, ...c Includes nylon/cotton, cotton/acrylic, cotton/viscose, ...

quantities of each fibre or blend that were available for coloration, i.e. to bedyed, printed or finished as white apparel or household textiles.

Several interesting facts emerge from these tables. About 20% of the totalfibres in Table 1.1 are constituents of blended materials and about 80% of thistotal, broken down in Table 1.2, is represented by the polyester/cellulosic sector.As a substrate type, polyester/cellulosic is more significant than any of the threemain all-synthetic types and is second only to cotton in importance (Table 1.1).All other cotton or viscose blends are very much less significant than eitherpolyester/cotton or polyester/viscose. Polyester/wool is also a more importantblend than either nylon/wool or wool/acrylic, but here the differences in demandare less dramatic. The numerous ‘synthetic blends’ and ‘miscellaneous blends’making up the remaining 10% of the total in Table 1.2 are individually of minorsignificance but collectively they have presented a wide variety of problems tothose devising satisfactory dyeing procedures for them.

The blending and processing of an above-average proportion (i.e. more than20%) of total fibres in the form of blended materials is characteristic of therelatively complex and sophisticated textile industries found in economicallydeveloped or developing countries. In the Asia Pacific region this figure exceeds40% in some instances (Thailand, Malaysia and Indonesia) and is above averagein several others (Australia, Burma, PR China, Hong Kong, Japan, Korea,Philippines, Singapore and Taiwan). South Africa, Canada, USA, Mexico andBrazil are other markets with an above-average proportion of blended fibres.Most European textile industries process 10–20% of total fibres as blends, withabove-average values in Germany, Spain, Portugal, Greece, Bulgaria, Rumania

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and Poland. The overall figure for blends in the UK textile industry is 17.5% oftotal fibres, but the proportions represented by polyester/wool (42% of UKblends) and nylon/wool (16%) are substantially above average, reflecting thecontinuing importance of suitings and carpets respectively in the UK industry.

1.4 REASONS FOR THE DEVELOPMENT OF FIBRE BLENDS

Several interrelated factors may contribute to the justification for replacing ahomogeneous textile material by a blend:(1) Economy: the dilution of an expensive fibre by blending with a cheaper

substitute.(2) Durability: the incorporation of a more durable component to extend the

useful life of a relatively fragile fibre.(3) Physical properties: a compromise to take advantage of desirable

performance characteristics contributed by both fibre components.(4) Colour: the development of novel garment or fabric designs incorporating

multicolour effects.(5) Appearance: the attainment of attractive appearance and tactile qualities

using combinations of yarns of different lustre, crimp or denier, which stilldiffer in appearance even when dyed uniformly to the same colour.

1.4.1 Balance of economy and physical properties

Cellulosic fibres, especially viscose staple, have been used for many years inblending with more expensive wool or synthetic fibres. In such blends thebalance of physical properties is at least as important as economicconsiderations. During the 1930s the cheaper fibres from regenerated cellulose,i.e. viscose and cellulose acetate, as well as regenerated protein fibres helped tocompensate for fluctuations in the price of wool by providing blend yarns atmore stable prices in periods of high demand for wool. When synthetic staplefibres became available for blending in the 1950s, prices were high and blendingwith natural or regenerated fibres was a valuable means of establishing outletsfor them using existing methods of processing. As the price levels of syntheticfibres fell with the tremendous growth in competition and volume of productionthat followed, the cost differentials between these blends and the componentfibres lost most of their significance. In recent years there has been somemovement from 80:20 wool/nylon to 50:50 wool/polypropylene yarns in carpetson price grounds [6,7].

Fibre blending can be regarded as a contribution to fabric engineering. Byusing fibres that differ in absorbency, fabrics with specific moisture regain valuescan be created. With fibres that differ in denier, desired stiffness and drape

THE RELATIVE IMPORTANCE OF INDIVIDUAL BLENDS


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qualities can be designed into the fabric. Blends of synthetic fibres with naturalfibres offer the most valuable possibilities for combining desirable physicalproperties, because the two components are so dissimilar. In blends of polyesteror acrylic fibres with cotton or viscose the synthetic component provides creaserecovery, dimensional stability, tensile strength, abrasion resistance and easy-careproperties, whilst the cellulosic fibre contributes moisture absorption, antistaticcharacteristics and reduced pilling. The antistatic effect is particularly significant:for example, only 10–20% of viscose (or a smaller proportion of metallicfilaments) is required to confer antistatic properties on an acrylic fibre.

Apparel fabrics, hosiery and carpet yarns combining the durability and elasticrecovery of nylon with the warmth, bulk and softness of wool or high-bulkacrylic staple are important examples of a desirable balance of properties. Men’ssocks in 100% nylon were heavily promoted in the 1960s for their stretch, easy-care properties and durability compared with traditional wool socks. However,these garments had not been designed to meet comfort needs [8] and were soonperceived to be hot and uncomfortable when worn in shoes. It was at this timethat coarse-filament nylon blends with wool or cotton began to appear. Thisdevelopment resulted in nylon/wool and nylon/cotton socks that were morecomfortable and had the added benefits of dimensional stability with stretchproperties, easy-care laundering, attractive appearance and excellent durability.Spun-dyed yarns and differential-dyeing variants were exploited to provideincreased colour and design potential.

Stretch fabrics for leisurewear are available in a wide range of qualities, oftenbased on a crimped nylon warp with a wool, acrylic or viscose staple weft. Thedevelopment of durable flame-retardant finishes for conventional syntheticfabrics and their blends has proved difficult and there has been considerableexploitation of the inherent flame resistance and thermal insulation properties ofpoly(vinyl chloride) fibres, or certain modacrylic copolymers with chlorosubstitution, in blends with wool for thermal underwear, nightwear garments,children’s clothing and knitwear.

Many characteristics of all-wool cloths can be simulated by blending long-staple polyester or acrylic fibres with wool. These blends generally do not possessequivalent suitability for milling because of the absence of any directional frictioneffect with the smooth synthetic fibres, although small amounts (up to about20%) of these fibres can accelerate wool shrinkage during milling. Such blendsexhibit the valuable features of excellent dimensional stability (often at leastequal to shrink-resist wool), abrasion resistance and durable pleat retention.

The beneficial effect on crease recovery of blending polyester with wool isillustrated in Table 1.3. The value for the intact synthetic fibre alone is

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Table 1.3 Crease recovery of blended polyester and wool [9]

Crease recovery (%)a after dyeing at 105°C

Fabric No carrier With carrier

100% Wool 55 5555:45 Polyester/wool 74 68100% Polyester 80 74

a Data obtained by the CSIRO multiple pleat test

approximately 1.5 times that for wool, and when blended at a typical 55:45 ratiothe crease recovery of the blend fabric is significantly higher (74) than the valueanticipated from this composition (68). When dyed with carrier, however,changes in the fine structure of the polyester involving chain folding result in amarked reduction in crease recovery, by about 6% in this instance [9].

REASONS FOR THE DEVELOPMENT OF FIBRE BLENDS

The improvement in dimensional stability that takes place when wool isblended with an ester fibre is demonstrated in Figure 1.1. This records themarked decrease in milling shrinkage observed in worsted blend fabrics as theproportion of cellulose triacetate staple to 48s wool increases. In this instance theshrinkage is halved (or the dimensional stability is doubled) when the proportionof triacetate reaches about 40%. The stabilising effect of the man-made fibrecomponent is more pronounced in the case of coarser wool qualities.

20

10

20

30

40

50

40 60 80Triacetate in blend/%

Wef

t shr

inka

ge/%

Figure 1.1 Milling shrinkage of cellulose triacetate/wool


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1.4.2 Development of microfibre variants

Microfibres feel pleasant against the skin, combining the easy-care properties ofa synthetic fibre with the silky appearance and comfort of a natural fibre. Therate of growth of the market for polyester microfibres (less than 1 dtex perfilament) and supermicrofibres (<0.3 dtex fil–1) will depend on their success inpenetrating the polyester/cellulosic blend apparel sectors in Europe, the USA andAsia Pacific regions. So far the uses of polyester microfibres have been mainlyconfined to unblended (mainly filament) materials, such as:(1) fashionable woven outerwear suede and velour fabrics with attractive

handle and drape [10];(2) woven sportswear and skiwear with improved transfer of moisture;(3) polar fleece garments, which provide excellent thermal insulation [11];(4) tightly woven rainwear fabrics affording effective protection with

breathability;(5) warp- and weft-knitted twin-layer microliner fabrics [12];(6) imitation silks with attractive lustre and drape.

In 1992, microfibre variants represented less than 1% of total demand for allforms of polyester but by 2000 AD they are expected to achieve a 10–25%share. In woven fabrics, filament blends of polyester microfibres and viscose aregaining popularity in dresswear and blouses. Polyester microfilaments andcotton are being introduced into knitted sportswear. So far, unfortunately,spinning problems and pilling behaviour have inhibited the potential uses ofintimate staple blends of polyester microfibres with cotton [12].

Supermicrofibres have a filament diameter less than one-tenth of that of fibresin conventional filament or staple yarns (2–3 dtex fil–1). In contrast withstandard polyester, microfilament yarns of 0.6 denier or less cannot be package-or beam-dyed because the high density of the wet material prevents adequateliquor circulation. Fabrics woven from these yarns are preferably dyed inwinches, jets or overflow machines to preserve their bulky characteristics.Approximately twice as much disperse dye is required for microfilaments of 0.3–1.0 dtex fil–1 compared with conventional polyester of 2–3 dtex fil–1 for the samevisual depth. Cost increases of 15–20% are anticipated because of the higher dyeconcentrations that have to be used to dye standard-depth shades [13]. Onefactor contributing to this difference is the smaller proportion of microcrystallinematerial in the microfilament structure. The absorbed dye forms largeraggregates in the amorphous regions and the tinctorial power is correspondinglyreduced [11].

The rate of dye absorption is inversely proportional to the square root of the

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filament diameter. Thus another important difference between microfibres andconventional polyester is a markedly increased rate of dyeing, which tends to giverise more often to levelling problems. These can arise from:(1) commencing dyeing at too high a temperature;(2) raising the temperature too rapidly above 90°C;(3) incompatibility in mixture recipes because of differences in rates of

exhaustion;(4) poor fabric agitation at slow running speeds [14].

Improvements can be made by selecting dye combinations with greatercompatibility, starting at a low temperature and heating the dyebath at a slowrate of rise. A hold period at 90–100°C may well be advantageous [13], althoughdiffusion into the fibre only proceeds rapidly at 130°C. Rapid jet or overflowmachines are recommended, loaded with relatively short lengths of fabric tofacilitate agitation.

The exploitation of polyester microfibres blended with viscose in wovens orwith cotton in knitgoods will depend to some extent on parallel advances indeveloping new disperse dyes to meet these more stringent demands in dyeingmicrofibres. Such dyes must combine outstanding build-up, excellent fastnessand minimum cross-staining of the cellulosic component. Application techniquesmust optimise right-first-time productivity without sacrificing the aestheticappeal of the microfibre-based fabric constructions [12].

It is essential to select dyes of high fastness to light for microfibre polyester, asthe fastness ratings are inferior to those on standard polyester [15]. Thestandards for fastness to washing and rubbing are also lower than on standardpolyester because of the higher concentration of dye required to give the targetshade. Clearing of loose dye from the microfibre surfaces is more difficult andreduction clearing is always necessary on these variants. Post-dyeing heattreatments and inappropriate finishing chemicals often enhance these problems.Durable press characteristics of microfibre fabrics are inferior to those wovenfrom standard yarns and crease-resist treatments must be applied carefully toachieve satisfactory results [13].

In 1983 the introduction of Tactel (ICI) nylon heralded a marked revival ofinterest in nylon apparel. The most notable features of Tactel were enhancedaesthetic and comfort properties. Cotton-look Tactel fabrics became highlypopular for skiwear, anoraks, beachwear and track suits, combining theestablished assets of nylon, i.e. strength, easy-care performance and abrasionresistance, with enhanced handle and attractive appearance. In 1989 TactelMicro (<1 dtex fil–1) was introduced to yield fabrics that may be modified during

REASONS FOR THE DEVELOPMENT OF FIBRE BLENDS


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finishing by sueding or coating to give novel effects. Blends of Tactel Micro withcotton or wool have broadened the variety of constructions being offered togarment designers and consumers. Lightweight, soft, comfortable apparel fabricswith enhanced easy-care performance result from these blends [8]. Micro-filament nylon fabrics tend to float in winches or atmospheric jets because theydo not readily absorb liquid, but this problem can be obviated by pressure dyeingat 105–110°C. In blends with elastomerics such as Lycra (DUP), Tactel Microproduces a revolutionary combination of handle, comfort, garment fit and shaperetention in stretch-knit constructions that are highly suitable for bodywear andaerobic sportswear.

1.5 COLOUR EFFECTS ACHIEVED BY BLENDING

There are four major types of coloured effect (Figure 1.2) achieved by dyeing ablend of two fibres:(1) Solid: both fibres are dyed as closely as possible to the same hue, depth and

brightness.(2) Reserve: only one fibre is dyed and the other is kept as white as possible.(3) Shadow: the two fibres are dyed to the same hue and brightness but the

depth on one fibre is only a fraction of that on the other.(4) Contrast: usually the intention is to achieve the maximum possible contrast

in hue at approximately the same depth on both fibres, but sometimes moresubtle contrasts are preferred. In either case, optimum brightness on bothcomponent fibres enhances the pleasing appearance of the contrast effect.

Reserve, shadow and contrast effects are mainly of interest for mixed-plyhand-knitting yarns, fabric woven or knitted from homogeneous yarns, as wellas garments or tufted carpets made from differential-dyeing variants. The factthat synthetic fibres tend to absorb dyes less readily than the natural fibres is anadvantage in achieving reserve or contrast effects, which are thus less prone tocontamination by cross-staining problems.

1.5.1 Solid effects

A solid effect (sometimes called a union-dye effect) is most frequently theobjective of dyeing a binary blend, since most of those blends developed forreasons of economy, durability or physical properties, especially the blended-staple yarns, are not intended for use in multicoloured designs. The attainment ofa solid effect is most difficult with those blends in which an ester fibre that canonly be dyed with disperse dyes is blended with another disperse-dyeable type,i.e. cellulose acetate or any synthetic fibre. If nylon or an acrylic fibre is present,

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A

Shadow

B

Solidor

contrast

Colourand

reserve

Differences between fibre components A and B

App

lied

dept

h

SolidContrastShadowReserve

Matching hue and depthMatching depth but contrasting huesMatching hue at a ratio of depthsContrast between colour and white

50

50

33

67

Figure 1.2 Diagrammatic representation of colour effects in blends

COLOUR EFFECTS ACHIEVED BY BLENDING

there is some scope for adjustment towards solidity by shading with acid dyes orbasic dyes respectively, but distribution of the disperse dyes between the fibrecomponents can be controlled to only a limited extent by adjustment of dyeingtemperature or (more objectionably) by addition of a carrier. Solid dyeings onblends of cellulose acetate with polyester or acrylic fibres are impracticablebecause the acetate fibre is damaged under the relatively severe conditionsrequired to achieve reasonable depths on the synthetic component.

Much more control of distribution is possible in blends of nylon with wool,polyurethane or cellulosic fibres, because anionic reserving or blocking agentscan be added to control the degree of uptake of the anionic dyes by nylon belowthe saturation limit. Solid effects are not difficult to achieve on other types ofbinary blend, the easiest situation being found with blends of acid-dyeablevariants of nylon or acrylic fibres with basic-dyeable nylon, polyester or acrylicvariants, where the relative freedom from cross-staining gives optimumreproducibility of effect and control of shading.

1.5.2 Reserve effects

Cross-staining constitutes a serious problem if a reserve effect is required.Staining is more likely to occur in blends of fibres with distinctly different dyeing


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properties. It is especially prevalent in blends of a natural fibre with a syntheticone that can only be dyed with disperse dyes. In these circumstances it isimpracticable to dye the latter component without some cross-staining of thenatural fibre.

It is difficult to obtain a satisfactory reserve of any acid-dyeable fibre and noneof the blends with wool will give a reserve effect on the wool component.Reserve effects are impracticable on either component of blends of wool, nylonor polyurethane with one another. Nylon does not give a good reserve in itsblends with cellulose acetate, triacetate, polyester or cellulosic fibres. Acrylicfibres cannot be reserved in the presence of the disperse-dyeable ester fibres. Inblends of ester fibres with one another, only the less dyeable component can bereserved. This principle also applies to blends of normal and deep-dye variants,where the latter cannot be reserved. Both fibres can be reserved satisfactorily inblends of acrylic or the ester fibres with cellulosics, and in synthetic blends ofacid-dyeable and basic-dyeable variants.

Fastness standards may be impaired by the cross-staining of one fibre by aclass of dyes intended for the other component. Several measures can beconsidered with a view to minimising the degree of cross-staining in blendswhere this is a potential problem:(1) selection of dyes with the lowest affinity for the fibre to be reserved, as well

as those with the highest affinity for the component that is to be dyed withthem;

(2) selection of dyeing conditions that favour maximum exhaustion by thecomponent fibre to be dyed and hence the minimum cross-staining of theadjacent fibre;

(3) addition to the dyebath of a colourless agent that is preferentially absorbedon the dyeing sites of the fibre to be reserved and is able to act as a resistagainst subsequent staining by dyes with sorption behaviour similar to thatof the agent;

(4) a clearing treatment with a detergent to desorb, or a reducing agent todestroy, the dye stain that has been taken up by the adjacent fibre duringdyeing. In two-bath sequences it is often advisable to interpose a clearingstep after the first dyeing stage in order to remove any stain from thecomponent to be dyed in the second stage.

1.5.3 Shadow effects

The shadow effect (cumbersome expressions like two-tone, tone-in-tone, or tone-on-tone have also been used) may be regarded as an intermediate stage betweensolid and reserve effects (see Figure 1.2). The most pleasing shadow effects are

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obtained when the paler depth is between about one-third and one-half the depthof the deeper component. If the two depths are closer together, the effectapproaches that of incomplete solidity, whereas if the paler component is tooweak it resembles a stained reserve.

It is obviously much simpler to obtain shadow effects using one class of dyes –as in blends of cellulosic fibres with one another, with disperse dyes on blends oftriacetate with polyester or acetate, or with acid dyes on blends of nylon withwool or polyurethane – than to attempt to achieve a similar effect on thoseblends requiring different classes of dyes on the component fibres. The mostattractive shadow dyeings are seen on the differential-dyeing variants, such aspale-dye/deep-dye nylon or normal/deep-dye polyester, where the appeal of thedepth difference is not impaired by distracting differences of surface texture orlustre of the two components.

1.5.4 Contrast effects

Contrast effects (also called cross-dye or two-colour effects) represent theprimary justification for the development of differential-dyeing yarns and havecontributed much to the design of patterned apparel fabrics and tufted carpetingover the years. Colour contrasts cannot be obtained on blends in which the twofibres resemble one another too closely in dyeing properties, as in blends ofcellulosic fibres with one another, blends of ester fibres with one another, orblends of wool, nylon or polyurethane with one another. The best contrast effectsare shown by fabrics containing acid-dyeable with basic-dyeable synthetic yarns(e.g. nylon/acrylic blends), since the freedom from significant cross-stainingunder optimum dyeing conditions permits the contrast of complementary pairsi.e. red–green, blue–orange or violet–yellow.

In those blends where considerable cross-staining is unavoidable, thesharpness of the contrast is obviously seriously muted. In many instances onlypartial contrast effects are possible by using a disperse dye on both componentsand an ionic dye on the more dyeable component, as on normal/deep-dye nylonor the blends of ester fibres with ionic-dyeable fibres. In these circumstances, thehue on the more dyeable fibre is partly determined by that on the other fibre andthe resulting contrasts are limited (Table 1.4).

It is obviously easier, when selecting dyes for contrast effects, to dye the deeperand/or duller colour on the fibre component that is most prone to cross-staining.This general approach, however, may have to be modified if the two fibres differgreatly in abrasion resistance. For example, the polyester component of apolyester/viscose shirting fabric dyed in contrasting hues should be dyed moreheavily, because differential abrasion at the collar and cuffs leaves a higher



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Table 1.4 Limitations of colour contrast on a disperse-dyeable/ionic-dyeable blend

Colours of individual dyes Colours on component fibres

Disperse dye Ionic dye Disperse-dyeable Ionic-dyeable

Yellow Red Yellow OrangeYellow Blue Yellow GreenRed Yellow Red OrangeRed Blue Red VioletBlue Red Blue VioletBlue Yellow Blue Green

proportion of polyester on those edges and the abraded areas become muchmore evident if the polyester is dyed to a paler hue.

A particularly striking example of a pronounced contrast effect is provided bythe use of Lurex metallic threads in decorative apparel fabrics. Even here thefamiliar problem of staining by disperse dyes often arises. The degree of stainingof silver and gold Lurex threads by typical disperse dyes in the dyeing ofpolyester/Lurex blends by carrier and high-temperature methods has beentabulated. The stability of Lurex threads to various chemical treatments, andapplication of a sodium dithionite clearing treatment to minimise disperse dyestaining, have been examined [16].

1.5.5 Colour matching problems and colour measurement of dyed blends

The degree of solidity that will prove acceptable to the customer varies accordingto the end-product. Whilst piece-dyed fabrics woven from blended yarns call forhigh standards of solidity, carpet yarns may prove less critical, since for certaindesigns and qualities of carpet a slightly ‘broken’ appearance better simulatesthat presented by a blend of different qualities of wool. Furthermore, on tuftedcarpets and pile fabrics the upper surface of the material is uneven in height andthe interplay of incident lighting with the effects of differential crushing ensurethat the uniformity of appearance presented by a woven fabric can never beapproached.

Nevertheless, spinning of a staple blend must be carefully controlled. Even acarpet yarn containing 20:80 nylon/wool may give an unlevel appearance in thepiece if there are clumps of nylon fibres that have not been thoroughly openedand mixed. The minimum level of each component in a staple blend should be atleast 5% to ensure uniformity of mixing. Collaboration between ICS atNewbury and IWS at Ilkley in the 1980s resulted in a programme to enable fibre

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blending to be carried out by computer. This could be used to predict theproportions of various fibre-dyed components to match a target blended sample[17].

A particularly critical substrate from the viewpoint of solidity of shade ispolyester/wool, which is complicated by serious staining of the wool by dispersedyes. Another limitation is the need to employ a carrier to achieve satisfactoryyield and penetration on the polyester component at a temperature that does notseriously damage the wool. The problems of achieving solidity of hue, brightnessand saturation between textured polyester filament yarns and 55:45 polyester/wool staple yarns in the same fabric have been examined in detail [18], byreference to colorimetric data obtained by dyeing in the presence of variousconcentrations of carriers based on o-phenylphenol, trichlorobenzene, ormixtures of them.

Perhaps the most complex and intractable challenges have been faced by dyersof wool shoddy over the years since synthetic fibres became important. The threesources of raw material for the dyers of reclaimed waste are:(1) collected rags or worn-out garments;(2) cuttings and fents left over from the making-up of garments or household

textiles;(3) fibre producer’s waste, which is often uncoloured and does not require

sorting.

When military uniforms were all-wool materials, outworn uniforms were aprime source of recovered wool. Trade in these used garments declined, however,when polyester blends were adopted and difficulties arose at the stripping and re-dyeing stage. Special dye selections were devised, particularly from thoseproducts not affected by the iron content of the stripped wool arising from thecrude equipment in use at the time. There was significant conversion to wool/viscose unions, often dyed wool way only in fabric form.

Category 1 causes most problems as the rags and garments must be sorted bycolour and fibre type. They often contain blended materials that only becomeclearly identifiable after dyeing. Disperse dye staining is much more troublesomewhen dyeing a 99:1 blend of wool shoddy and polyester than in conventional55:45 polyester/wool blends [19]. Carrier dyeing of these synthetic fibreimpurities is highly inefficient. It is not difficult to achieve acceptable solidity onall-wool shoddy, but reclaimed acrylic material usually contains several differentacrylic variants that may differ widely in dyeability.

A novel method has been devised to cover synthetic fibre impurities in deep-shade dyeings on fabrics made from reclaimed wool [20]. This is to pass the wool


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fabric, together with a textured polyester fabric already dyed to a matchingshade, through the nip of a transfer printing calendar. The synthetic fibreimpurities in the reclaimed fabric are dyed by vapour-phase transfer but the woolfibres remain virtually unstained because the disperse dyes have such highaffinity for polyester at the heat transfer temperature. Conventional transfercoloration from a disperse dye transfer paper is uneconomic and carries a greaterrisk of wool staining. The textured polyester ‘reservoir’ fabric can be usedrepeatedly in this way with only very slow loss of disperse dye.

Shade matching problems are often more difficult to deal with in union fabricsthan in fabrics made from intimately blended yarns [21]. Recipes based ondifluoropyrimidine reactive dyes were applied to both components of a viscosewarp/nylon weft fabric dyed by the one-bath two-stage process. Reflectance datafor the dyed fabric and colour difference values between the viscose warp andnylon weft yarns were measured [22]. It was shown that objective matching ispossible and that the reproducibility of the matching operation can be improvedby careful selection of the dyes used.

The development of computer colour matching programmes for the dyeing ofblends presents specific difficulties. Dyes that cause minimal cross-staining arepreferred for better reproducibility of matching. Preliminary calibration work onthe laboratory scale must be carried out at the effective liquor ratio thatcorresponds to bulk conditions for the blend. All fibre components must bepresent in the calibration dyeings in order to account for the competitive effectsbetween them.

Fibre fragments taken from dyed blends can be used to produce felt-like discsthat retain the colour properties of both constituent fibres. These discs areprepared using a new type of press that has been described and illustrated [23].The discs are easy to prepare and are thus preferred to yarn windings forcalibration purposes. Two methods utilising this technique were described forprediction of the initial recipe for the acrylic pile and cotton backing of a cotton/acrylic velour fabric.

Instrumental techniques have been applied to the colour effects obtained ondifferential-dyeing nylon. Loop-pile carpet and pile yarns dyed with CI Acid Blue277 on deep-dye and normal variants, and CI Basic Yellow 45 on a basic-dyeablevariant, were measured using a colour computer system, a spectrophotometer tomeasure small areas of the design and a goniophotometric colorimeter [24].

Methods of determining the levelness of wool dyeings and of union dyeings onwool/cotton textiles by digital image analysis have been developed [25].Levelness values were derived from the standard deviations of the grey scaleratings corresponding to individual histograms of the dyed fabrics. The levelness

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of union dyeings could be assessed by analysing the shapes of the histogramsobtained from the wool/cotton dyeings.

Correct formulation of the pad liquor is an essential element in the qualityassurance of continuous dyeing. Transmission measurement is a viablealternative to control dyeings prepared in the laboratory to check pad liquors inbulk. Practical experience has demonstrated [26] that this method can be usednot only for soluble systems such as reactive dyes but also dispersions of disperseor vat dyes, as well as mixtures of soluble dyes with disperse dyes in polyester/cellulosic dyeing. With extensive automation of the measuring process, highoperational reliability and continuous monitoring are feasible.

Potential causes of incorrectly set pad liquors include:(1) varying dye deliveries from the supplier;(2) moisture sorption of powder brands;(3) sedimentation of liquid brands;(4) incorrect choice of dye from storage;(5) incorrect weighing;(6) incorrect dissolution or dispersion;(7) inadequate stirring during mixing;(8) soiling during transfer to pad trough;(9) lack of temperature control in storage or padding.

Commercial mixture recipes containing (a) disperse dyes with vat, vat leuco esteror reactive dyes, for the dyeing of polyester/cellulosic blends, or (b) disperse dyeswith 1:2 metal-complex or acid dyes, for polyester/wool blends, were examinedin pad liquors made up in readiness for continuous dyeing. Photometricmeasurements using analogue, digital and turbidity photometers [27] were usedas alternatives to conventional control dyeings to save time and running costs.Calibration curves confirmed that the Beer–Lambert law was sufficientlyapplicable to these mixtures, yielding adequate reproducibility for on-line controlpurposes. These approaches virtually eliminate the production of faulty batchesattributable to incorrectly set pad liquors, as well as freeing laboratory dyeingeffort for other tasks.

1.5.6 Colour effects on three-component blends

With the exception of nylon/wool/viscose blends in carpet yarns, ternary blendsare seldom dyed in solid shades because of the matching difficulties involved.Three-depth shadow effects are given by acid dyes on pale-dye/normal/deep-dyeor normal/deep-dye/ultra-deep nylon (see Chapter 15), but a three-way contrastwith primary colours on all three components is most difficult because of the



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problems of cross-staining. Multicoloured designs on ternary blends can bebased on two-way shadow or contrast effects with white reserve of the thirdcomponent, or more often, a two-way shadow effect with a contrasting hue onthe third fibre. Ternary blends on which disperse, basic and acid dyes can be usedwill give a limited range of three-way contrasts, but the hues on the ionic-dyeablevariants are dependent on the hue of the disperse dye, which cannot be restrictedto only one of the component fibres (Table 1.5).

Table 1.5 Limitations of colour contrast on a typical three-component blend

Colours of individual dyes Colours on component fibres

Disperse Basic Acid Disperse- Basic- Acid-dye dye dye dyeable dyeable dyeable

Yellow Red Blue Yellow Orange GreenRed Yellow Blue Red Orange VioletBlue Yellow Red Blue Green Violet

1.5.7 Scintillant effects on staple blends

Colour contrast effects on intimate blends of two different fibre types can beaccentuated by deliberately modifying the distribution of the constituent fibreswithin the yarns. In conventionally blended staple yarns sufficient doublings aregiven to ensure that this distribution is regular and uniform in cross-section. Thisuniformity is an inherent property of a conventional blend and it results inreproducible spinning and weaving characteristics and reliable in-serviceperformance.

Not all natural fibres exist as single or ultimate fibres, however. Flax andother bast fibres occur naturally in bundles of ultimate fibres held together byinterstitial material, varying in size from about 10 to 40 ultimates. When theseare dry spun the bundles do not break down but remain as groups within thespun yarn. Thus the intimacy of blend found in a conventional polyester/cottonis not found in a typical polyester/linen yarn. The flax bundles are distributedthrough a matrix of the individual polyester staple fibres (Figure 1.3).

This grouping of fibres can be utilised to produce striking colour contrasteffects using the dye selectivity of the constituent fibres. The fibre groups can beaccentuated by incorporating short fibres into the blend to produce slubs, or by

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Conventionalfibre blend

Groupedfibre blend

CellulosicfibresPolyesterfibres

Figure 1.3 Diagrammatic cross-sections of conventional and grouped polyester/cellulosicblend yarns [28]


injecting clumps of fibres into the yarn during spinning [28]. Fabrics made fromcoarse dry spun yarns exhibit particularly attractive scintillant effects and can beproduced with pronounced slub content. It is necessary to have at least 30% ofeach fibre present to make a significant contribution to the differential colourcontrast. Yarns with a short slub character are of great interest because theseslubs create focal points of colour that can be utilised by the designer. The mosteffective results are achieved when the paler or brighter colour is applied to theslub component so that it is highlighted against the darker background.

1.6 SIGHTING COLOURS FOR IDENTIFICATION PURPOSES

Sighting colours are especially useful in knitting or weaving plants that handle awide variety of man-made fibres and blended staple yarns. Selected low-fastnessdyes are used to stain the surface of the fibres, but they must be readily andcompletely removable in scouring before dyeing and finishing. They may becomedifficult to extract if the ‘sighted’ yarn is steamed or dry heat set before scouring.

Specially designed varieties of sighting colours include [29]:(1) an ionic dye complexed with a surfactant of opposite charge;(2) a water-soluble vinyl polymer associated with a dye of opposite charge;(3) a water-soluble starch derivative covalently linked to a reactive dye via the

hydroxy groups;(4) a water-soluble dye in which the replaceable hydrogen atoms in the

structure (as in OH, NH or CONH groups) are substituted by longpolyoxyethylene chains;

(5) an analogous polyoxyethylene-substituted disperse dye structure withsufficient hydrophobic character to inhibit penetration into the intermicellarregions of highly swollen cellulosic fibres.


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1.7 REFERENCES 1. A P S Sawhney, R J Harper, K Q Robert and.G F Ruppenicker, Text. Res. J., 61 (1991) 393.

2. L B Kimmel, A P S Sawhney, G F Ruppenicker and R J Harper, Text. Chem. Colorist, 26 (Mar1994) 22.

3. P A Koch, Textilveredlung, 7 (1972) 570; Chemiefasern und Textilind., 29/81 (1979) 431.

4. S Shiomura, Textile Asia, 22 (Sep 1991) 140. 5. P Lennox-Kerr, Text. Horizons, 11 (Jan 1991) 33.

6. P Lennox-Kerr, Text. Horizons, 2 (Nov 1982) 18.

7. S Roberts, Dyer, 178 (June 1993) 10. 8. L Jacques, J.S.D.C., 109 (1993) 315.

9. I B Angliss and J D Leeder, J.S.D.C., 93 (1977) 387.

10. G Jerg and J Baumann, Text. Chem. Colorist, 22 (Dec 1990) 12.11. A Lallam, J Michalowska, L Schacher and P Viallier, J.S.D.C., 113 (1997) 107.

12. P W Leadbetter and S Dervan, J.S.D.C., 108 (1992) 369.

13. J C Dupeuble, Chemiefasern und Textilind., 40/92 (1990) 986.14. D Wiegner, Chemiefasern und Textilind., 41/93 (1991) 148.

15. C L Chong, Textile Asia, 25 (Mar 1994) 59.

16. V Walther, Chemiefasern und Textilind., 35/87 (1985) 321.17. J Park, Wool Record, (Aug 1987) 23.

18. G Römer, Teinture et Apprets, No 145 (Dec 1974) 203.

19. K Barras, Dyer, 153 (13 June 1975) 612.20. M E Fielding, Dyer, 157 (21 Jan 1977) 68.

21. W Pape, Melliand Textilber., 69 (1988) 737.

22. A Gantsheva and E Kantscher, Textilveredlung, 26 (1991) 116.23. P Medilek, Melliand Textilber., 75 (1994) 822.

24. K Konno, I Hirai and T Gunji, J. Text. Mach. Soc. Japan, 37 (Apr 1992) 93.

25. J M Cardamone, W C Damert and W N Marmer, AATCC International Conference and

Exhibition, (Oct 1994) 246; Text. Chem. Colorist, 27 (Oct 1995) 13.

26. H P Locher and H Firmann, Textilveredlung, 26 (1991) 393.

27. V Reith, Melliand Textilber., 72 (1991) 774.28. B Hill and G Gray, J.S.D.C., 108 (1992) 419.

29. K Marquardt, Chemiefasern, 24 (1974) 940.

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21

CHAPTER 2

Classification of fibre types and their blends

2.1 CLASSIFICATION OF FIBRE TYPES IN TERMS OF DYEABILITY

The examples mentioned in Chapter 1 have given an indication of the widevariety of fibre blends available and the complex ways in which they may beassembled and dyed as blended yarns or in certain types of fabric, garment orcarpet. Before considering the fundamental principles of blends dyeing and themethods devised to colour them, it is useful to classify fibres and their blends interms of their dyeing characteristics. This is more appropriate in this context thenthe usual division into natural, regenerated and synthetic fibres.

Although more than one class of dyes is important on cellulosic fibres, anddisperse dyes are often used in pale depths on all types of synthetic fibre, a simpleclassification based on the classes of dyes used to obtain fast dyeings in fulldepths can be made (Table 2.1). This can then be applied to classify binaryblends (Table 2.2) and ternary blends (Table 15.1) in a similar way [1,2]. Thedyeing characteristics of the component fibres in full depths are particularlyimportant because many of the problems associated with the dyeing of blends aremore serious under such conditions. These problems include:(1) the interference with solidity caused by differences in the saturation limits

on the component fibres;(2) the greater degree of staining of reserved fibres, often making it necessary to

employ a two-bath method;(3) more critical fastness requirements that must be achieved adequately on

both fibres;(4) more serious incompatibility of dyes and auxiliaries of opposite charge at

higher concentrations.

The variations from alphabetical order in the codes for blend categories listed inTable 2.2 are deliberate. When referring to blends of polyester with natural fibresit is customary to name the synthetic fibre first, i.e. polyester/cotton (DC) or even‘poly/cotton’ is heard far more than cotton/polyester, which is usually

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Table 2.1 Classification of fibres by dyeing properties

A Fibres (dyed with acid dyes in full depths)

Wool and other animal fibresSilkNylon homopolymerAcid-dyeable nylon variantsPolyurethane fibresAcid-dyeable polypropyleneAcid-dyeable acrylic and modacrylic fibres

B Fibres (dyed with basic dyes in full depths)

Basic-dyeable acrylic and modacrylic fibresBasic-dyeable nylonBasic-dyeable polyester

C Fibres (dyed with cellulosic dyes in full depths)

CottonViscoseLyocell, modal and polynosic fibresLinen and other bast fibres

D Fibres (dyed with disperse dyes in full depths)

Cellulose acetateCellulose triacetatePolyester homopolymerDeep-dye polyester variantsPoly(vinyl chloride) fibres

encountered if cotton-rich blends are being considered. Furthermore, when afibre type is referred to by an adjectival term, such as ‘acrylic’ or ‘cellulosic’, it ispreferable to name this component second, as in nylon/acrylic (AB) or wool/cellulosic (AC) blends. Where both adjectival terms occur in the same category,i.e. the cellulosic/acrylic (CB) category, this order is preferred for the individualblends, such as cotton/acrylic and viscose/acrylic, rather than their reversals.

2.2 COLOUR DISTRIBUTION ATTAINABLE ON BINARY BLENDS

General comments can be made regarding the dyeing characteristics of the majorclasses of binary blends (Table 2.3). The AA blends, based mainly on nylon andthe protein fibres, are particularly important in knitting and carpet yarns.Physical properties usually provide the main justification for developing theseblends, which are often blended-staple yarns. Solid dyeings are therefore mostimportant and not too difficult to obtain because preferential uptake by nylon inpale depths can be controlled using reserving agents. These blends are ideally

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Table 2.2 Classification of binary blends

AA blends

Wool/silkWool/mohairWool/cashmereWool/angoraNylon/woolNylon/silkWool/polyurethaneNylon/polyurethaneWool/acid-dyeable polypropyleneNylon/acid-dyeable polypropyleneNormal/deep-dye nylon

AB blends

Wool/acrylic fibreSilk/acrylic fibreNylon/acrylic fibrePolyurethane/acrylic fibreAcid-dyeable polypropylene/acrylic fibreWool/modacrylic fibreMohair/modacrylic fibreNylon/modacrylic fibreAcid-dyeable/basic-dyeable acrylic fibreModacrylic fibre/acrylic fibreDeep-dye/basic-dyeable nylonWool/basic-dyeable polyesterNylon/basic-dyeable polyester

AC blends

Wool/cottonSilk/cottonNylon/cottonPolyurethane/cottonAcid-dyeable polypropylene/cottonWool/viscoseSilk/viscoseNylon/viscoseWool/modal fibreNylon/modal fibreNylon/linen

CB blends

Cotton/acrylic fibreViscose/acrylic fibreModal fibre/acrylic fibrePolynosic fibre/acrylic fibreCotton/modacrylic fibreViscose/modacrylic fibre

CC blends

Cotton/viscoseCotton/modal fibreCotton/polynosic fibreCotton/linenLinen/viscoseLinen/modal fibre

DA blends

Cellulose acetate/woolCellulose acetate/silkCellulose acetate/nylonCellulose triacetate/woolCellulose triacetate/silkCellulose triacetate/nylonPolyester/woolPolyester/silkPolyester/nylonPolyester/acid-dyeable polypropylenePoly(vinyl chloride)/woolPoly(vinyl chloride)/nylon

DB blends

Cellulose acetate/acrylic fibreCellulose triacetate/acrylic fibrePolyester/acrylic fibreCellulose acetate/modacrylic fibreCellulose triacetate/modacrylic fibrePolyester/modacrylic fibreNormal/basic-dyeable polyester

DC blends

Cellulose acetate/cottonCellulose triacetate/cottonPolyester/cottonPoly(vinyl chloride)/cottonCellulose acetate/viscoseCellulose triacetate/viscosePolyester/viscosePoly(vinyl chloride)/viscoseCellulose triacetate/modal fibrePolyester/modal fibrePolyester/polynosic fibrePolyester/linen

DD blends

Cellulose acetate/triacetateCellulose acetate/polyesterCellulose triacetate/polyesterNormal/deep-dye polyester

COLOUR DISTRIBUTION ATTAINABLE ON BINARY BLENDS

24 CLASSIFICATION OF FIBRE TYPES AND THEIR BLENDS

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suited to shadow effects because one class of dyes can be used for bothcomponents. Contrast and reserve effects are generally impracticable.

Optimum reproducibility of bright complementary colour contrasts isachieved on AB blends, containing nylon or a protein fibre with an acrylic orbasic-dyeable copolymer. Good reserve of the basic-dyeable fibre, or solid effects,can be obtained if required. Two-bath dyeing methods are generally preferred,because of the need to inhibit precipitation between the classes of dyes ofopposite charge.

The anionic dyes applicable to both components of an AC blend are, in mostcases, fully compatible with one another. This facilitates the exploitation of one-bath dyeing methods and gives ample opportunity for controlled shade matchingin either solid effects or colour contrasts. There is less interest in shadow orreserve effects on these blends.

Colour contrasts and solid effects are readily obtainable on CB blends. Eitherthe cellulosic or the acrylic fibre may be reserved if desired, so this is a versatile

Table 2.3 Colour effects attainable on binary blends

Colour effectBlend type(example) Solid Reserve Shadow Contrast

AA Use of Neither Easily Not(nylon/wool) auxiliaries component controlled possible

AB Easily Acrylic Seldom Wide range(nylon/acrylic) controlled reserve required available

AC Easily Cellulosic Seldom Wide range(nylon/cellulosic) controlled reserve required available

CB Easily Either Seldom Wide range(cellulosic/acrylic) controlled component required available

CC Dyeing Neither Viscose Not(cotton/viscose) conditions component deeper possible

DA Dyeing Polyester Seldom Limited(polyester/wool) conditions reserve required range

DB Easily Polyester Acrylic Limited(polyester/acrylic) controlled reserve deeper range

DC Easily Either Seldom Wide range(polyester/cellulosic) controlled component required available

DD Dyeing Polyester Easily Not(triacetate/polyester) conditions reserve controlled possible

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type of blend from the viewpoint of design. Two-bath methods are preferable,however, because basic dyes are incompatible with all classes of dyes for thecellulosic component.

Dye selection and control of dyeing conditions provide reasonable scope forsolidity on staple blends of the CC category, but there may have to be somesacrifice of optimum fastness and colour gamut. The CC blends are ideally suitedto shadow effects, as in brocade designs for furnishing or curtaining fabrics. Aswith the AA blends, reserve effects and colour contrasts are not attainable.

Blends of an ester fibre with nylon or a protein fibre (DA blends) are mostlyblended-staple yarns and solidity is therefore often important. The acid-dyeablecomponent cannot be reserved because of disperse dye staining, but good reserveof the ester fibre is attainable. Colour contrasts are limited because of the dullingeffect of the disperse dye on the acid-dyeable fibre.

The attainment of contrast or reserve effects by differential dyeing is the mainjustification for DB blends, which contain an ester fibre with an acrylic or otherbasic-dyeable copolymer. The basic-dyeable component cannot be reservedsatisfactorily, but good reserve of the ester fibre is possible. Contrast effects arelimited by cross-staining of disperse dye on the basic-dyeable yarn.The DC blends represent the most important category and solid effects are aprimary objective. Disperse dye staining of the cellulosic fibre is much less seriousthan for the DA and DB types, so either component of a DC blend can bereserved. Colour contrasts are possible but not of much interest in practice.

Disperse dyes offer the only possibility for colouring DD blends and thiscompletely eliminates the contrast option. Shadow and reserve effects areparticularly appropriate because the ester fibres differ so much in dyeability.Cellulosic acetate absorbs dye much more readily than the triacetate at lowtemperatures, making it easy to reserve the latter component. The degree ofdistribution on triacetate/polyester can be controlled using dyeing temperatureand carrier additions, but solid effects are difficult to achieve.

2.3 REFERENCES1. J Shore in The dyeing of synthetic-polymer and acetate fibres, Ed. D M Nunn (Bradford: SDC,

1979) 419.2. J R Aspland, Textile dyeing and coloration (North Carolina: AATCC, 1997) 331.

COLOUR DISTRIBUTION ATTAINABLE ON BINARY BLENDS

26 DYNAMIC COMPETITION BETWEEN FIBRE TYPES IN THE DYEING OF BLENDS

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CHAPTER 3

Dynamic competition between fibre types inthe dyeing of blends

3.1 INTRODUCTION

Dyeing systems in which more than one fibre type is present have been almostcompletely neglected by those researching the mechanisms of dyeing processes.This is not surprising, because the complications that arise when attempting topredict quantitatively the uptake of a single dye by one fibre type areconsiderable. Theories of dyeing established on the basis of carefulmeasurements of one such dye at various depths under a variety of dyeingconditions can seldom be transferred intact to other members of the same rangeof dyes, especially if these are distinctly different in structure from the first dyechosen. Further difficulties arise with the binary or ternary combinations of dyesthat are used routinely in practice to achieve the wide gamut of colours withwhich design colourists and dyers must work. Two dyes undergoing absorptionby the fibre simultaneously rarely reproduce the dyeing rate curves that they givealone. There is almost invariably an interaction between them, often (but notalways) resulting in a slower rate of uptake and a lower equilibrium exhaustionfor each of the dyes in combination, compared with the corresponding valueswhen applied individually.

An entirely different dimension is introduced when two fibres are present inthe same dyebath. A single dye may be distributed between them according to acomplex relationship that is determined by the differences in dyeabilitycharacteristics between the two substrates, the dyeing conditions and the applieddepth of the dye. An unequal distribution that may be found in an early stage ofthe process, arising from differing rates of uptake by the competing substrates,may later undergo a levelling effect as dye is desorbed from the initially moreheavily dyed fibre and is taken up by the other fibre type for which the dye hasinherently higher affinity.

When two different substrates are present in the same dyebath, initial kinetic

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conditions and later the energetic characteristics of the system will dictate whichof the substrates a given dye will tend to favour. More dye will ultimately beabsorbed by the substrate to which it is more substantive. This favouredsubstrate will require less energy for the sorption process because the dye–fibreforces of interaction are stronger. When a combination of two or three dyes isapplied to two different substrates, however, it does not always follow that thedyes present will all be absorbed preferentially by the same fibre component tothe same extent.

It is certainly possible in practice to choose a set of dyeing conditions in whicha trichromatic combination of dyes with similar dyeing properties will yield amatching hue on both substrates at equal depth (a solid effect) or at markedlydifferent depths (a shadow effect). To arrive at such ‘ideal’ combinations of dyesand dyeing conditions, however, implies considerable preliminary laboratorywork, especially when other important factors, such as fastness demands andnon-metameric matching, must be taken into account [1]. This task can befacilitated by deriving characteristic thermodynamic parameters of the dyeingsystem from the sorption behaviour of the individual dyes on the separatehomogeneous substrates. It is claimed that these parameters allow computationof the sorption behaviour of the dyes in combination on more than one substratesimultaneously [2].

The substantivity ratio (Df /Ds) for each dye on each substrate depends ontemperature, pH, liquor ratio, concentrations of dye and electrolyte, andparticularly in this case on the other dyes and substrates involved in thecompetitive sorption process. According to the Gouy-Chapman theory, thesubstantivity ratio is related to the electrolyte concentration or ionic strength andthe total amount of ionic charge imparted to the substrate by the absorption ofdye ions. This factor encapsulates the mutual restraining effects of the dyes onone another. The three characteristic constants in the Gouy-Chapman equationare as follows:A0 is a function of the standard affinity and thus the substantivity ratio under

the relevant conditions.A1 is proportional to the charge density and is a measure of the overall charge

on the substrate.A2 is related to the specific surface of the substrate accessible for dye sorption.

The simplest dyeing system that may be considered as representative of thedyeing of a blend is that in which one dye is distributed between two differentfibres that have broadly similar dyeing properties, so that the same dye will givean economic colour yield on both. Even if solidity of colour between them is the

INTRODUCTION


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objective, there is no guarantee that the dyeing conditions to achieve this at thetarget depth will ensure satisfactory penetration and fastness on both substrates.It is often easier to adopt this approach when a shadow effect is desired, becauseany inherent difference in dyeability between the substrates can be exploited inadjusting dye uptake levels towards the target difference in depth. The seriouslimitations of this simple dyeing system become clear when reserve and colourcontrast effects are considered. By definition it is usually extraordinarily difficultto suppress the dyeability of the less dyeable component to zero in order toreserve it as white or to dye it in another colour, which means introducing a dyefrom another dyeing class.

Another deceptively simple approach to the dyeing of a binary blend of fibresis to dye each in turn with an appropriate class of dyes in two completelyseparate dyeing processes, using the optimum conditions of application in eachcase just as if they were entirely separate substrates. At first sight, this appears anideal way in which to dye for solidity, reserve, shadow or contrast effects at will.In practice, however, technical limitations do arise here too. The wet fastnesscharacteristics of the dyeing achieved in the first process must be such that nosignificant desorption from it occurs during the second one. In other words, thefastness to cross-dyeing must be excellent. Thus the order in which the twoprocesses are carried out is important and the dyeing that requires the higherdyeing temperature is normally applied first.

The two-bath process is also less than ideal unless both classes of dyes are freefrom cross-staining problems, i.e. a class selected to dye one of the fibre typesshould not cause significant staining of the other fibre present. In a majority ofinstances, however, cross-staining of one or both fibre types must be taken intoaccount and a clearing process introduced after one or both of the dyeingprocesses. Thus the so-called ‘two-bath’ process may require several baths forcompletion. It is easy to see why this technically straightforward possibility turnsout to be the least attractive from an economic viewpoint.

Between these fairly obvious extremes of the single-class and the two-bathmethods there are two other compromises that offer greater flexibility than theformer and economic savings over the latter. These are the simultaneous one-bath and the one-bath two-stage methods. For clarification the four possibilitiesare summarised in Table 3.1. These will be referred to frequently in later chaptersand it is convenient to use the four hyphenated abbreviations listed in the firstcolumn to distinguish between them. In general, the progressive increase in costin moving down this list is compensated by a wider choice of suitable dyes andgreater freedom from the practical problems discussed in terms of typicalexamples in the remainder of this chapter. Another general rule is that the greater

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Table 3.1 Summary of general dyeing methods for binary blends

Method Dyebaths Dye classes Stages

Single-class One One OneOne-bath One Two simultaneously OneTwo-stage One Two in sequence TwoTwo-bath Two Two Two

INTRODUCTION

Scheme 3.1

[fibre]H3N COO– [fibre]H3N COOH+ H++ +

[fibre]H3N COO– [fibre]H2N COO–+ –OH + H2O+

Scheme 3.2

the depth of shade required, the more likely is it necessary to move to a methodlower down this list.

3.2 THE DISTRIBUTION OF ACID DYES ON NYLON/WOOL BLENDS

Nylon/wool blends are often dyed with a single class of anionic dyes, which maybe levelling acid, milling acid or metal-complex types. Ensuring solidity of shadeis normally the main requirement and this calls for careful selection ofcombinations of dyes with similar rates of dyeing and build-up characteristics.Partitioning of the dyes between the two fibres can be influenced by manyfactors, including dye structure, applied depth, dyebath pH, blend ratio and thequality of the component fibres.

Wool and nylon contain both basic and acidic groups, amongst which by farthe most important are amino and carboxyl groups respectively. Just like theparent amino acids from which all proteins are derived, both of these polymericamides show zwitterionic characteristics at pH values close to the isoelectricpoint, i.e. the pH at which the fibre contains equal numbers of protonated basicand ionised acidic groups. As the pH decreases below this point, the carboxylateanions are progressively neutralised by the adsorption of protons and the fibreacquires a net positive charge (Scheme 3.1).

Conversely, as the pH rises above the isoelectric point, the fibre becomesnegatively charged as a result of deprotonation of the amino groups byadsorption of hydroxide ions or other simple anions (Scheme 3.2).


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2010 40 60 80 100

20

40

60

80

100

Dyeing time/min

Exh

aust

ion/

%

Figure 3.1 Rate of dyeing of CI Acid Red 1 [3]

Figure 3.2 Rate of dyeing of CI Acid Red 41 [3] (for key see Figure 3.1)

The rate of dyeing of anionic dyes on nylon is much more rapid than on wool,particularly at 60–80°C and low applied depths, so that pale dyeings on nylon/wool show a marked preferential dyeing of the nylon. Even at 1% applied depth,for example, the uptake by nylon is much more rapid than by wool and thedifference in rate is greater for dyes with a lower degree of sulphonation. Figures3.1 and 3.2 are rate-of-dyeing curves for disulphonated and tetrasulphonateddyes (Figure 3.3) respectively on these two fibres, at 93°C and an initial pH of4.2 in aqueous acetic acid [3]. As dyeing proceeds, the pH rises towards 5 as aresult of sorption of acetic acid, particularly by wool.

Nylon

Wool

20 40 60 80 100

20

40

60

80

100

Dyeing time/min

Exh

aust

ion/

%

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Figure 3.3 Disulphonated and tetrasulphonated acid dyes

THE DISTRIBUTION OF ACID DYES ON NYLON/WOOL BLENDS

NH

N

O

NaO3S

SO3Na

HNCOCH3

NN

H

NaO3S

SO3Na

O SO3Na

NaO3S

CI Acid Red 1 CI Acid Red 41

The origin of the markedly different rates of dyeing lies in the differences inhydrophobic character between the two dyes and between the two fibres. Thenylon polymer is much more hydrophobic than wool and so it attracts the morehydrophobic of the two dyes preferentially. Thus the disulphonated dye showsthe most rapid rate of absorption on nylon, with a time of half dyeing (time for50% of the equilibrium exhaustion) of only about 2 minutes. Thetetrasulphonated dye is relatively hydrophilic and is therefore absorbed moreslowly by either fibre, showing the slowest rate of dyeing on wool, with a time ofhalf dyeing of approximately 3 hours.

Although nylon absorbs acid dyes more readily than does wool, partitionbetween the two fibres is not constant at all depths since the saturationconcentration on nylon is much lower than that on wool. The saturation limit onwool is not approached at the applied depth necessary to saturate the amine endgroup content of the nylon. In pale depths both the initial uptake and theultimate exhaustion are higher on nylon. In full depths, on the other hand, theinitial strike still occurs on nylon but eventually the wool becomes more heavilydyed because it has a much higher saturation uptake. At some intermediate depthdepending on dyeing conditions, there is a point at which both fibres are dyed tothe same depth even though the nylon reaches this equilibrium position morequickly.

This critical depth is specific for the dye and is much higher for amonosulphonated dye than for a disulphonated analogue, since a disulphonateddye requires about twice as many amine end groups for a given tinctorial yield(Figure 3.4). Blocking effects may occur if monosulphonates and disulphonatesare applied together, resulting in a heavier depth on the wool that is closer in hueto the disulphonated dye. Above the critical depth the nylon becomesprogressively more difficult to dye with levelling acid dyes and the distributionon the nylon/wool blend increasingly favours the wool.

The blocking of disulphonates by analogous monosulphonated dyes may be


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Figure 3.4 Electrostatic and hydrophobic bonding of acid dyes on nylon

exploited by applying mixtures of selected mono- and disulphonated pairs ofsimilar hue, varying the proportions according to applied depth in order tominimise the disparity in depth on the two fibre components. The preferredmonosulphonated dyes are mainly monoazo yellows and reds with anthra-quinone blues. The disulphonates have lower saturation values on nylon and sothey can only be used for pale and medium depths. They are virtually all yellowto red monoazo and disazo types, or violet to green anthraquinone derivatives.

Given the widely varying qualities of the two fibre types, as well as possiblevariations in the blend proportions, it is not surprising that specific dyeingconditions for solidity cannot be laid down, only guideline starting points forinitial experimentation [4]. Anionic agents capable of controlling the uptake ofanionic dyes by the nylon component below the critical depth are of two generaltypes:(1) levelling or blocking agents that are preferentially absorbed by nylon and

act as a partial reserve for levelling acid dyes;(2) retarding agents of higher relative molecular mass capable of controlling the

distribution of dyes of higher wet fastness on nylon/wool blends.

NH3

(CH2)6

NH

CO

–O3S N

CH2

CH2

CH2

CH2

CO

NH

NH3

(CH2)6

NH

CO

–O3S N

SO3–

H

N

O

NH3

CH2

CH2

CH2

(CH2)3

NH

CO

CI Acid Red 88

H

N

O

CI Acid Red 13

CO

NH

+

+

+

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Figure 3.5 Levelling or blocking agents for nylon/wool dyeing (R = long-chain alkyl)


Typical examples of the moderately substantive agents used to control the uptakeof levelling acid dyes are shown in Figure 3.5. They compete with these dyes forthe basic groups and facilitate the approach to an equilibrium distribution thatdoes not alter even on prolonged boiling, unless further additions of agent aremade to shift the equilibrium more in favour of the wool. Such products improvethe coverage of any dye-affinity variation in the nylon but have insufficientaffinity to exert a blocking effect with premetallised or milling acid dyes. A novelagent developed recently, however, permits either equalisation or a total reserveto be obtained even with metal-complex or milling dyes [5].

Retarding agents that control the initial uptake of these dyes by the nyloncomponent have higher affinity than the typical levelling agents defined in Figure3.5 Many of these are also used as syntan aftertreating agents for dyed nylon.These belong to a distinct group of condensates of formaldehyde with certainsulphonated phenols, thiophenols or naphthylaminesulphonic acids [6]. Therapid sorption of syntans by nylon is mainly attributed to electrostatic bondingbetween negatively charged sulpho groups in the syntan and the protonatedamino groups in the fibre. Hydrogen bonding between uncharged polar groupsand hydrophobic interaction between nonpolar moieties in the syntan and thenylon also contribute to the mechanism [4].

Maximum retarding effect is found when the syntan molecules are retainedclose to the fibre surface, since any treatment leading to diffusion of the syntaninto the fibre interior tends to lower its effectiveness. When used with levellingacid dyes, this type of agent does not prevent migration from wool to nylon onprolonged boiling. With metal-complex and milling acid dyes, however, theinitial distribution is preserved on boiling because these dyes show only limited

R SO3Na

R

SO3Na

R OSO3Na CH3(CH2)5 CH CH2CH

Sodium alkylbenzene sulphonate

CH(CH2)7COONa

OSO3Na

Sodium alkylnaphthalene sulphonate

Sodium alkanol sulphate Disodium sulphoricinoleate


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migration. It is not normally necessary to employ levelling or retarding agents onthe rare occasions when chrome dyes are preferred, since these give quite goodsolidity on nylon/wool blends at the heavy depths where they may offer anadvantage. Some of the syntans darken on exposure to light and this significantlylowers the light fastness of the dyeings.

Considerable attention has been given in recent years to devising suitableprocedures for dyeing wool at lower temperatures and near the isoelectric point(pH 4–5) in order to avoid the damage that inevitably occurs when dyeing at theboil. The low-temperature dyeing of wool offers:(1) optimum handle and durability;(2) improved carding, spinning and weaving performance;(3) brighter shades because of the lower degree of yellowing of the wool;(4) shorter dyeing cycles, higher productivity and lower process costs;(5) a better-quality end-product.

Problems associated with low-temperature dyeing include:(1) slower diffusion into the interior of the wool fibres, resulting in inadequate

wet fastness;(2) inadequate exhaustion and poor reproducibility.

The epicuticle of wool is the main barrier to penetration and this is mainlyresponsible for the dyeing problems. These problems can be overcome bydamaging the fibre scales in a chlorination process before dyeing, but this lowerswool quality too. The best results are achieved using monosulphonated 1:2metal-complex and milling acid dyes.

Shade partition between wool and nylon in blend dyeing is dependent ondyeing temperature. As already discussed, the tendency at the boil is for thenylon to absorb dye more quickly than wool and to dye more deeply below thecritical depth. This is easily corrected by adding an anionic agent to retard uptakeby the nylon and achieve solidity of shade over a range of depths. At lowertemperatures (e.g. 80°C) the wool is generally dyed more heavily than the nyloneven without an anionic agent present. As dyeing proceeds there is somemigration from wool to nylon until the final partition is reached, but this is adifferent equilibrium at the lower temperature than in conventional dyeing at theboil. In general, partition favours the wool and solidity is difficult to attain. Thisdifficulty can be overcome, however, using a specific equalising agent in place ofthe conventional anionic agent normally employed when dyeing at the boil [7].When dyes of high substantivity are applied below the boil, surface dyeing of thewool occurs to give dyeings of inadequate wet fastness.

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Premetallised and milling acid dyes (relative molecular mass 700–1000)demand more energy input for good fibre penetration. There is a close relation-ship between dye substantivity, exhaustion and diffusion into the fibre and this ismainly determined by the relative molecular mass and degree of sulphonation ofthe dye. Addition of an equalising agent shifts the partition in favour of the nylonat the lower temperatures, allowing solidity to be achieved at any temperature inthe 80–100°C range, depending on the amount of auxiliary required at a givendepth of shade. The optimum dyeing temperature is recipe-specific and atemperature selector system has been developed for use with the specificequalising agent. In pale depths (below ca. 0.6% total dye) the conventionalsyntan-type retarder may be used at a relatively low concentration [8].

3.3 THE DISTRIBUTION OF ACID DYES ON NYLON/POLYURETHANEBLENDS

The polyurethane elastomeric fibres vary in their ability to absorb anionic dyes,depending on their content of basic groups, but in general the equilibriumdistribution on a nylon/polyurethane blend tends to favour the nyloncomponent. The higher rate of dyeing of the elastomeric fibre at lowertemperatures, however, results in preferential dyeing of that component in theearly stage. There are two main methods of controlling this distribution, both ofthem depending on the use of dyeing auxiliaries.

Anionic agents, such as sodium alkanesulphonates, sulphated castor oil,disodium dinaphthylmethanedisulphonate or selected syntans, can be added topromote dye uptake by the polyurethane at equilibrium by becoming absorbedby the nylon and restraining subsequent migration of dye from the polyurethaneto the nylon. This method is more effective when dyeing with 1:2 metal-complexor milling acid dyes of the monosulphonate type with good levelling properties.The amount of agent required decreases progressively as the applied depth of thedyeing increases.

Cationic agents (Figure 3.6) will form labile ionic complexes with typicalanionic dyes. This type of complex is absorbed more slowly and tends to favourthe polyurethane component more than does the parent dye. Acid dyes of thedisulphonate type with only moderate levelling properties can be readilycontrolled on nylon/polyurethane materials by this method. The concentration ofcationic agent required increases with applied depth and the degree ofsulphonation of the dyes. A nonionic dispersing agent of the alkanol poly-oxyethylene class is necessary to solubilise the dye–agent complex (Figure 3.7).


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Figure 3.6 Cationic complexing agents for nylon/polyurethane dyeing

Figure 3.7 Schematic representation of solubilised dye–agent complex (R = long-chain alkyl)

CH3

N CH3R

CH3

O3S N

SO3–

CH3

N RCH3

CH3

R (OCH2CH 2)x OH

R (OCH2CH2)x OHCI Acid Red 13

as bis-alkyltrimethylammonium complex

H

N

O

Alkanol polyoxyethylene

+

+

3.4 THE CROSS-STAINING OF WOOL BY DISPERSE DYES

Wool keratin is the most sensitive of textile fibres towards staining by dyes of alltypes because it is a natural protein containing many different functional groups.The staining of wool by disperse dyes is a serious problem in dyeing blends withany of the ester fibres. Cellulose acetate suffers a loss in lustre if it is treated at theboil, as is normally necessary to dye the wool component during the second stageof a two-stage or two-bath sequence. Migration from the acetate to the woolincreases with dyeing time and temperature of the wool-dyeing stage. Migrationfrom cellulose triacetate to wool is slower under these conditions but treatmentat 105°C or at the boil with carrier, in order to attain full depth and penetrationon the triacetate, usually results in serious staining of the wool.

Polyester/wool presents the most difficult problem and studies have revealednumerous factors that may affect the degree of staining observed in practice [9].The stain on the wool is dull in hue and exhibits poor fastness to light and wettreatments. Staining implies a loss of colour yield on the polyester and makesshade matching more difficult because individual dyes differ in their propensityto staining. Bulky, low-twist wool becomes stained more easily then fine, high-twist yarns. Wool quality is another factor influencing the degree of staining.

NH33C16

CH3

N CH3H33C16

CH3

CH3

N CH2H35C17

CH3

Cetyltrimethylammoniumbromide

Cetylpyridiniumchloride

Stearyldimethylbenzylammoniumchloride

+++

Cl– Cl–Br –

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THE CROSS-STAINING OF WOOL BY DISPERSE DYES

Dye structure (molecular size, number, type and distribution of polar groups)has less influence on the initial level of staining than dyebath conditions and thequality of the dye dispersion. The mechanism of wool staining involves:(1) hydrogen bonding between proton donor groups (e.g. amino, amido,

hydroxy) in the disperse dye molecules and in wool keratin;(2) interaction by means of secondary (dipolar and van der Waals) forces

between the dye molecules and wool;(3) physical sorption of aggregated particles of disperse dyes on the scaly

surface of the wool fibre.

The preferred disperse dyes for these blends have low intrinsic saturation valueson wool, low tendency to aggregate at the dyeing temperature, rapid rates ofdiffusion into polyester and high equilibrium exhaustion. Mechanical retentionin yarn crevices may play a part in the initial deposition, since particle size andstability of the dye dispersion are important. Staining tends to decrease with theconcentration and anionic charge of the stabilising agents present in the dyedispersion, but the magnitude of the effect is specific to the types of dye andagent.

In a recent investigation of the kinetics of polyester/wool dyeing and the woolstaining problem, polyester and wool fabrics in the weight ratio 55:45 were usedto allow the disperse dye uptake by both components to be determinedindependently [10]. Dyeings with CI Disperse Blue 185 (Figure 3.8) and CI AcidRed 211 were carried out at the isoelectric point (pH 4.5) and wool damage wasassessed by measuring tensile properties and alkali solubility. Both disperse andacid dyes are absorbed by wool far more quickly and easily than the dispersedyes are taken up by the polyester at relatively low temperatures, because therates of diffusion in wool are so much more rapid. After only 30 minutes at110°C, therefore, the amount of disperse dye that has stained the wool is morethan twice that absorbed by the polyester (Figure 3.9).

NH2

NHCHHO

O2N O

O

CH2CH3

CH3

Figure 3.8 CI Disperse Blue 185


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Figure 3.9 Rate of uptake of disperse dye by polyester and wool [10]

As dyeing proceeds, desorption from the wool occurs at a rate determined bythe slow diffusion of the disperse dye into the polyester. The distributioncoefficient for a disperse dye between polyester and wool is controlled by thedifference in affinity values of the dye for the two substrates. After about an hourat 110°C, an equilibrium partition is approached but the degree of staining of thewool is not much less than after the first 15 minutes. As the disperse dye diffusesslowly into the polyester, the wool stain slowly desorbs at a rate that keeps thelow concentration of disperse dye in the dyebath roughly constant.

After a dyeing time of 30 minutes, wool staining is considerable at all dyeingtemperatures up to about 115°C (Figure 3.10). However, above around 120°Cthe diffusion of the disperse dye into the interior of the polyester fibre issufficiently rapid to give much lower staining of the wool. The pH dependence ofwool staining is not critical under the mildly acidic conditions preferred fordyeing both components of this blend (Figure 3.11). At pH values below theisoelectric point, wool staining is at a minimum. As the pH increases towards thealkaline side, so does the degree of wool staining.

When tested in the presence of the premetallised CI Acid Red 211, the stainingof wool by the disperse dye was markedly greater [11]. This was attributed topossible hindering of disperse dye desorption from the wool by the presence ofthe metal-complex dye. Since anionic dyes of this kind interact strongly withwool keratin they desorb again very slowly and incompletely. Sequestering agentssuch as ethylenediaminetetra-acetic acid (EDTA) and citric acid are useful to

Wool

Polyester

15 30 60 90 120

1

2

3

4

Dyeing time/min at 110 oC

Dye

upt

ake/

%

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Figure 3.10 Temperature dependence of disperse dye uptake [10] (for key see Figure 3.9)


2 3 5 74 6 82

3

4

Dyebath pH for 60 min at 110oC

Dye

upt

ake/

%

Figure 3.11 pH dependence of disperse dye uptake [10] (for key see Figure 3.9)

minimise wool staining, particularly the former. It is claimed that such agentsinteract with certain disperse dyes (e.g. the 1-amino-4-s-butylamino system in CIDisperse Blue 185) to hinder diffusion into wool (Figure 3.12). These agents have

30 60 90 120

1

2

3

4

Dyeing temperature/oC for 30 min

Dye

upt

ake/

%


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Figure 3.12 Interaction between CI Disperse Blue 185 and citric acid

NH3O2N

HO NH2CH

O

O

CH2

CH

CH2

OH

–OOC

–OOC

+

CH2CH3

+

CH3

low affinity for wool and formation of the dye–agent complex favours retentionof the disperse dye in the dyebath, allowing the polyester to absorb more dye as itis released from the complex at higher temperatures and improving the ultimateexhaustion.

When dyeing wool-rich blends, the problems of wool staining and theattainment of solidity at equilibrium are greatly aggravated. Uptake of dispersedyes by the polyester in full depths can be 25% less on a 20:80 polyester/woolblend than on a 50:50 blend [12]. If a 50:50 blend is dyed at a liquor ratio of10:1, then the individual components are each actually at 20:1 with respect tothe dyebath. When the blend ratio is 20:80, however, the polyester is being dyedat 50:1, making exhaustion far more difficult. To compound this, the liquor ratiofor the wool is only 12.5:1 and absorption of the disperse dyes by wool isfavoured preferentially. Less dye remains in the bath for dyeing the polyesterdirectly and this component becomes even more dependent on disperse dyetransfer from the wool [12].

Wool progressively loses its strength at elevated dyeing temperatures,especially about 110°C, although at this temperature the damage is not severe fordyeing times of 60 minutes or less. In the isoelectric region (pH 4–5) the keratinstructure is reinforced by electrostatic linkages between protonated aminogroups and carboxylate-containing amino acid residues. Hydrolysis of peptideand disulphide bonds is also at a minimum under these conditions. The loss instrength increases rapidly at pH values above 5 and temperatures above 110°C.The sensitivity of wool to degradation makes the use of a carrier essential to dyefull depths on the polyester at a relatively low temperature. Carrier additionlowers the degree of staining at a given temperature and also tends to lower thetemperature of maximum staining.

Carriers are able to minimise wool staining by accelerating the rate of uptakeof disperse dyes by polyester. It is believed that the sorption of carrier moleculesweakens the attractive forces between polyester segments, making them more

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mobile at lower temperatures than when the carrier is absent, thus changing theinternal structure of the fibre and lowering the glass-transition temperature.Hence carriers accelerate dye diffusion within polyester and so indirectly increasethe rate of transfer from wool to polyester. The chemical type of carrier exertsonly a marginal influence, the degree of staining tending to increase slightly in theseries: aryl esters < chlorobenzenes < aryl ethers < phenylphenols. Dyebathconditions are more important. Unfortunately, carriers are harmful to the healthof operatives and to the environment.

The wool staining problem is an important criterion in the decision whether todye polyester/wool sequentially in the one-bath mode or by the two-bath methodwith an intermediate clear to remove the disperse dye stain from the wool. In thetwo-stage sequence, the wool is cleared after dyeing by scouring with a nonionicdetergent at pH 4–5 and 50–70°C. This causes no significant damage to the woolbut is only moderately effective and this process is only suitable for pale andmedium depths because of fastness limitations. Nevertheless, it offers shorterprocessing times and higher productivity at lower cost. During the wool dyeingstage, disperse dye can migrate from the surface of the dyed polyester and causeback-staining of the wool [13]. There is no obvious relationship between thedegree of back-staining and the heat fastness class of the disperse dyes.

Full depths are usually dyed by a two-bath method. In the absence of theanionic dyes the blend can be given an intermediate reduction clear afterapplying the disperse dyes to the polyester. This is more effective than nonionicscouring, especially for azo disperse dyes, but the wool suffers some loss instrength and elasticity. When dyeing wool-rich blends, the two-bath sequencegives better shade partition than the one-bath method, especially when thepolyester component is dyed first [12]. Two-bath dyeing allows a wider choice ofdisperse dyes, since the wool staining is less important provided the intermediateclearing is adequate and the wool is dyed at a lower temperature than thepolyester. Certain anthraquinone-based disperse dyes, however, are reduced butnot destroyed by the reducing conditions. The degradation products may stilldiscolour the wool, especially after reoxidising back to the quinone form duringthe wool dyeing stage. In heavy depths the two-bath process gives improvedfastness to rubbing and perspiration [14]. These advantages are particularlysignificant for the dyeing of deep navy and black shades on polyester/wool [15].

3.5 THE CROSS-STAINING OF WOOL BY BASIC DYES

Although ultimately less serious than staining by disperse dyes, the uptake ofbasic dyes by wool in the initial stage of the one-bath dyeing process for wool/acrylic blends with milling acid and basic dyes can be troublesome. Above the


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Figure 3.13 Structures of typical milling acid and basic dyes

second-order transition temperature, however, dyeing of the acrylic componentcan take place. The thermodynamic affinity of a basic dye for the acrylic fibre ismuch higher than for wool. By the time that the dyebath reaches the boilingtemperature, most of the basic dye initially taken up by the wool has beentransferred to the acrylic component. This transfer proceeds during treatment ofthe fibre blend at the boil even after exhaustion of the basic dye from the dyebathis virtually complete. The degree of initial staining of the wool by the basic dyevaries with the type of acrylic fibre present in the blend. This variation becomesparticularly important when the applied depth approaches saturation of thedyeing sites in the acrylic fibre.

Cationic dyeing auxiliaries have been widely used in wool dyeing, ranging inionic character from the weakly basic alkylamine polyoxyethylene types to themuch more strongly basic fatty alkyl quaternary ammonium salts. Theseproducts are used as levelling agents and it is not surprising that the cationicretarders necessary when dyeing acrylic fibres with basic dyes behave in a similarmanner when added to the wool/acrylic dyeing system. The pronouncedretarding effect of a typical cationic retarder on the rate of dyeing of wool with amilling acid dye of the disulphonated anthraquinone type (Figure 3.13) isillustrated in Figure 3.14. The time of half dyeing is increased from about 22 to33 minutes by addition of the agent [16].

Dyeing rate curves for this milling acid dye and a typical monoazo basic dye(Figure 3.13) on a 50:50 wool/Orlon (DUP) blend in the presence (Figure 3.15)and absence (Figure 3.16) of the cationic retarder demonstrate that the anionic

O

O HN

HN

O2N N

Cl

N NCH2CH3

CH2CH2 N

CH3

CH3

CH3

X–

CI Acid Green 27

CI Basic Red 18

NaO3S CH2CH2CH2CH3

+

CH2CH2CH2CH3NaO3S

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THE CROSS-STAINING OF WOOL BY BASIC DYES

dye is highly sensitive to the agent of opposite charge. Under these conditions theagent increases the time of half dyeing from about 17 to 32 minutes for the aciddye but only about 26 to 32 minutes for the basic dye.

These results indicate that in the initial stage of a wool/acrylic dyeing thecationic retarder is either absorbed by the wool or forms with the acid dye alabile complex that has lower affinity for wool than the parent dye. Either effectwill significantly decrease the rate of dyeing of the acid dye. It is also evident that

CI Acid Green 27

CI Basic Red 18

10 20 25 35 45 50

20

40

60

80

100

Dyeing time/min

66 84 92 100

30

100 100

40

100 100Temperature/oC

Exh

aust

ion/

%

Figure 3.15 Rates of dyeing of wool/Orlon with 1% retarder [16]

1% Dye, no retarder

1% Dye, 1% retarder

10 20 30 40 50 60

20

40

60

80

100

Dyeing time/min

60 70 80 90 100 100Temperature/oC

Exh

aust

ion/

%

Figure 3.14 Rates of dyeing of CI Acid Green 27 on wool [16]


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5 10 15 30 35

20

40

60

80

100

Dyeing time/min

58 66 75

20

84 100

25

92 100Temperature/oC

Exh

aust

ion/

%

ca. 20% exhaustion of the basic dye occurs at temperatures below 80°C (i.e.below the glass-transition temperature of the acrylic fibre) and this dye must beabsorbed by the wool. This effect is irrespective of whether the cationic retarderis present or not, implying that complex formation between the acid dye and theretarder is the most probable explanation of the mechanism [16].

3.6 THE TRANSFER OF DISPERSE DYES DURING THERMOFIXATIONOF POLYESTER/CELLULOSIC BLENDS

The pad–thermofix dyeing of polyester/cellulosic fabrics is one of the few systemsof dyeing of blends that has been subjected to theoretical study. During the earlystages of padding and drying, much of the dye applied becomes deposited on therelatively more absorbent cellulosic component. Several possible mechanisms oftransfer of the disperse dyes from cellulose to polyester during thethermofixation stage have been proposed, but it is now widely accepted that thetransfer proceeds through the vapour phase [17]. The extent to which thistransfer takes place depends on the time and temperature of thermofixation.

As heating of the polyester/cellulosic fabric is continued, the total amount ofdisperse dye available for colouring the polyester decreases as a result ofvolatilisation into the atmosphere inside the heating chamber and deposition onthe inner surfaces of the latter. Some disperse dyes of low relative molecular mass(Mr) and relatively high volatility may suffer oxidative decomposition,

Figure 3.16 Rates of dyeing of wool/Orlon with no retarder [16] (for key see Figure 3.15)

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THE TRANSFER OF DISPERSE DYES DURING THERMOFIXATION

particularly if they contain sensitive substituents such as primary amino groups.It follows that a critical combination of fixation time and temperature givesoptimum yield of a specific dye, when the supply of dye from the reservoirprovided by the cellulosic component is sufficiently depleted for these progressivelosses to begin to favour desorption rather than adsorption of vapour at thepolyester surface.

Low-energy disperse dyes (approx. Mr <300) suffer relatively serious lossesunder the conditions required for optimum transfer and fixation on the polyester.Maximum transfer for these dyes is found at 200–210°C. For most dyes ofintermediate energy (approx. Mr 300–400) a temperature of 210–220°C isneeded. Good fixation of high-energy dyes (approx. Mr >400) often requires atemperature of 220–230°C, but optimum transfer is limited by the onset ofthermal degradation and yellowing of the cellulosic fibres, or even somesoftening of the polyester.

3.7 REFERENCES 1. J Park and J Shore, Rev. Prog. Coloration, 12 (1982) 1.

2. O Annen, H Gerber and B Seuthe, J.S.D.C., 108 (1992) 215.

3. B C Burdett, C C Cook and J C Guthrie, J.S.D.C., 93 (1977) 55. 4. T M Baldwinson in Colorants and auxiliaries, Vol. 2 Ed. J Shore (Bradford: SDC, 1990) 568.

5. Anon, Dyer. 177 (Apr 1992) 31.

6. C C Cook, Rev. Prog. Coloration, 12 (1982) 73. 7. A F Doran, Dyer, 176 (Aug 1991) 49.

8. A F Doran, J.S.D.C., 109 (1993) 15.

9. R E Lacey, V S Salvin and W A Schoeneberg, Am. Dyestuff Rep., 50 (1951) 978.10. J Wang and H Asnes, J.S.D.C., 107 (1991) 274.

11. J Wang and H Asnes, J.S.D.C., 107 (1991) 314.

12. A F Doran, unpublished work.13. K Türschmann and K H Röstermundt, Z. Ges. Textilind., 71 (1969) 326.

14. K H Röstermundt, Deutscher Färber Kalender, 80 (1976) 247.

15. W T Sherrill, Text. Chem. Colorist, 10 (1978) 210.16. D R Lemin, J.S.D.C., 91 (1975) 168.

17. C J Bent, T D Flynn and H H Sumner, J.S.D.C., 85 (1969) 606.

46 MINIMISING INCOMPATIBILITY BETWEEN DYES FROM DIFFERENT CLASSES

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46

CHAPTER 4

Minimising incompatibility between dyes fromdifferent classes

4.1 INTERACTION BETWEEN DISPERSE DYES AND REACTIVE DYES

The risk of interaction between these two dye classes is a significant factorlimiting the selection of suitable dyes for the one-bath pad–thermofix dyeing ofpolyester/cellulosic blends. These problems may arise as a result of covalentreaction between reactive dyes and certain disperse dyes, interaction betweenreactive dyes and dispersing agents, or instability of the dye dispersion systemunder the alkaline conditions of padding. Interaction leads to losses of yield onboth fibre components and may result in gelling or settling of the pad liquor.

High reactivity dyes are unsuitable because of chemical reaction in manycases. For example, the monochlorotriazine dye formed by reaction (Scheme 4.1)between CI Reactive Red 11 and the phenolic group in CI Disperse Yellow 1 wasisolated from a pad liquor containing 8 g l–1 sodium bicarbonate and identifiedby chromatographic analysis [1]. Further evidence for chemical reaction betweenmonochlorotriazine reactive dyes and primary amino or phenolic groups indisperse dyes has been presented [2]. The reaction products formed tend to beunstable and are readily decomposed by alkaline hydrolysis.

Since these problems of chemical reaction in the alkaline pad liquors necessaryfor one-bath application are closely connected with structural features of theindividual dyes, most of them can be avoided. Where possible, disperse dyes thatdo not contain nucleophilic primary amino or phenolic groups should beselected. Low-reactivity ranges of reactive dyes should be chosen in preference tohigh-reactivity types, so the selection of compatible disperse dyes that can beused becomes much wider.

Control of the pH of the pad liquor is a most important technique to minimiseproblems of chemical reaction between dyes from the two classes. Thus themixture of disperse and reactive dyes can be padded from a neutral bathcontaining migration inhibitor and sodium m-nitrobenzenesulphonate to prevent

47

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Scheme 4.1

reductive degradation of certain azo reactive dyes. After drying andthermofixation to ensure diffusion of the disperse dyes into the polyester fibres,the fabric is padded in an alkaline brine bath to minimise desorption of thereactive dyes, steamed to achieve fixation of these dyes on the cellulosiccomponent, rinsed cold and soaped at the boil. This two-stage method has fewrestrictions attributable to dye interaction and the choice of suitable dyes is muchgreater than in the simple pad–dry–thermofix application of both classes.

4.2 INTERACTION BETWEEN DISPERSE OR VAT DYES ANDBASIC DYES

In the one-bath dyeing of a typical DB blend (see Chapter 2), i.e. a blend of oneof the ester fibres with an acrylic fibre, it is most important to minimise the risksof incompatibility between a cationic species, such as one of the basic dyes or acationic retarder used with them for dyeing the acrylic fibre, and an anionicmoiety that has an important function in dyeing the ester fibre, such as a dispersedye stabiliser or a carrier emulsifying agent (Figure 4.1).

There are three aspects to minimising the risks of incompatibility in the one-bath dyeing of DB blends of the ester/acrylic type. An anionic retarder can beused to control uptake of the basic dyes by the acrylic fibre, but this tends to be

INTERACTION BETWEEN DISPERSE DYES AND REACTIVE DYES

N

NH

NaO3S

SO3Na

N

NN

Cl

Cl

HO

O2N

NO2NH

NH

NaO3S

SO3Na

N

NN

Cl

O

O2N

NO2NH

+

NaHCO3

O

NN

H

CI Disperse Yellow 1

ON

H

CI Reactive Red 11

SO3Na

SO3Na

+ NaCl + CO2 + H2O


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Figure 4.1 Possible interactions in dyeing a DB blend (R = long-chain alkyl)

less effective than a cationic retarder and more prone to restrain the ultimateexhaustion of the basic dyes. It is possible to select a carrier formulationcontaining a nonionic emulsifier instead of a conventional anionic carrier type.Anionic dispersing agents are best avoided but it is not normally possible toexclude the anionic stabilising agents that are already present in disperse dyeformulations as marketed.

More severe limitations apply to the simultaneous application of vat and basicdyes to cellulosic/acrylic CB blends. The dispersing agents present in vat dyeformulations are incompatible with the basic dyes and cationic retardersnormally used for dyeing acrylic fibres. A more serious problem, however, is thatalmost all basic dyes show chemical instability in even moderately alkalineconditions. The highly alkaline reducing conditions of a vat dyebath are evenmore extreme. One-bath methods generally are thus excluded and only a two-bath sequence can be considered.

4.3 INTERACTION BETWEEN ANIONIC DYES AND BASIC DYES

The range of bright colour contrasts is much wider on AB blends than on allother types of binary blend because the fibres carry opposite charges and ionicdyes are much more selective than disperse dyes. The opposite charges carried bythe dyes, however, lead to incompatibility in one-bath dyeing. There is a strong

O2N

Cl

N N NCH2CH3

CH2CH2 N

CH3

CH3

CH3–O3SO(CH2CH2O)x R

NH33C16–O3S

CH2

SO3–

CI Basic Red 18

N C16H33

Alkanol polyoxyethylene sulphate(carrier emulsifier)

+ +

Cetylpyridinium(retarder)

Cetylpyridinium(retarder)

Dinaphthylmethanedisulphonate(dispersant)

+

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Figure 4.2 Interaction between typical basic and acid dyes

tendency for water-soluble anionic dyes and basic dyes to interact (Figure 4.2). Itis highly likely that precipitation of the complex formed would occur even whenapplied in pale depths. Even where this does not occur the interaction seriouslyinterferes with reproducibility and causes increased cross-staining.

N

H3CCH3

CH3

HC CH N

H3C

O

O

HN

NHCH3

–O3S CH3

CI Basic Yellow 21

CI Acid Blue 27

+

INTERACTION BETWEEN ANIONIC DYES AND BASIC DYES

Addition of ca. 1% of an alkanol polyoxyethylene surfactant is sufficient toinhibit co-precipitation of basic dyes and milling acid dyes when used atconcentrations up to 1/1 standard depth in a one-bath exhaust dyeing process forwool/acrylic blends, for example [3]. With many combinations of basic dyes andthe relatively hydrophilic levelling acid dyes, it is possible to go to much heavierdepths with no problems. In practice, however, it is advisable, for dyeings above1/1 standard depth, to check that a specific combination of dyes will be suitablefor one-bath application.

Direct and reactive dyes present similar problems when applied with basicdyes in one-bath methods for the dyeing of cellulosic/acrylic CB blends. A furtherproblem in the case of reactive dyes is that an alkaline fixation stage must begiven to fix them on the cellulosic fibre. Almost all basic dyes are chemicallyunstable under alkaline conditions.

The best and most reliable method of dealing with these problems ofinteraction is to dye each fibre component separately. However, it has beenpossible to develop viable one-bath methods of applying acid/basic combinationsto AB blends and direct/basic combinations to CB blends. These involve theaddition of anti-precipitants to overcome the problems of dye interaction. Manyproprietary products are marketed specifically as anti-precipitants for use in


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Table 4.1 Chemical types of anti-precipitant [4,5]

OCH2CH2 unitsType Composition per molecule

Nonionic Fatty alcohol or alkylphenolpolyoxyethylenes ≥20

Nonionic Block copolymers of ethylene oxideand propylene oxide ≥40

Weakly Coco, oleyl, soya or tallow fattycationic amine polyoxyethylenes ≥20

Weakly Fatty alcohol or alkylphenolanionic polyoxyethylene sulphates ≥3

Weakly Fatty alcohol or alkylphenolanionic polyoxyethylene phosphates 4–12

Weakly Alkali metal, amine or alkanolamineanionic salts of fatty alcohol or alkylphenol

polyoxyethylene acetates ≥5

Amphoteric Alkyl chloride or dialkyl sulphatesalts of quaternary ammoniumpolyoxyethylene sulphates orphosphates ≥15

blend dyeing (Table 4.1) but some levelling agents used with acid dyes on woolor nylon, e.g. fatty amine polyoxyethylenes, will also function effectively in thisrole, either alone or with the addition of a nonionic dispersing agent (Figure 4.3).Nonionic agents are by far the most common anti-precipitants and theirsolubilising power by disaggregation is well known.

Numerous nonionic and weakly cationic surfactants with at least 20oxyethylene units per molecule are available, as well as block copolymers ofethylene oxide and propylene oxide. Suitable hydrophobic groups include fattyalcohols, alkylphenols, alkylamines and alkanolamines. Weakly anionic poly-oxyethylene sulphates and phosphates with polyoxyethylene chains approxi-mately 10 units in length are also widely available [4]. Mixed formulations oftenfunction better than single products and many proprietary products areempirically balanced mixtures [5].

In dye liquors containing substantial concentrations of both anionic and basicdyes, purely nonionic agents may be inadequate to effect complete solubilisation

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Figure 4.3 Schematic representation of solubilised acid dye–agent complex

Figure 4.4 Schematic representation of solubilised basic dye–agent complex

of the complex. If an anionic surfactant is added, this will tend to disrupt thedye–dye complex to form a basic dye–agent complex that responds more readilyto addition of the nonionic stabiliser (Figure 4.4). If strongly basic dyes arepresent, however, the complexes formed by this method may be too stable,leading to restraining of the basic dyes. Azo and anthraquinone derivatives witha localised charge tend to show this behaviour. Surface deposition may result inpoor fastness to rubbing and a tendency for the basic dye–agent complex to stainthe acid-dyeable fibre.

R = long-chain alkyl

Mixtures of another type that may be useful for minimising the cross-stainingof fibres by complexes of basic and acid dyes contain a weakly cationicalkylamine polyoxyethylene with a weakly anionic alkanol polyoxyethylenesulphate or phosphate. In many systems of interacting anionic and cationic

INTERACTION BETWEEN ANIONIC DYES AND BASIC DYES

O

O

NHCH3

HN

CH3–O3S

R NH

(CH2CH2O)x

(CH2CH2O)x H

H

R (OCH2CH2)x OH

+

CI Acid Blue 27

Alkanol polyoxyethylene

Alkylamine polyoxyethylene

R = long-chain alkyl

N

H3CCH3

CH3

HC CH N

H3C

R (OCH2CH2)x OH

CI Basic Yellow 21

–O3SO(CH2CH2O)x

Alkanol polyoxyethylene o

+

R

Alkanol polyoxyethylene sulphate


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surfactants an excess of either one may have a solubilising effect bydisaggregating the complex formed between them. The hybrid surfactantsgenerally characterised as weakly cationic or weakly anionic are particularlyuseful in this respect, since the oxyethylene chains inhibit ionisation and exert asolubilising and stabilising action on the complex entities present. Care should betaken to rinse off all traces of anti-precipitants after dyeing to avoid potentialproblems later. For example, residual nonionic surfactants can interfere with thesyntan aftertreatment of dyed nylon and its blends.

4.4 REFERENCES1. B Taylor and J Shore, unpublished work.

2. M Duscheva, L Jankov and K Dimov, Melliand Textilber., 56 (1975) 147.

3. D R Lemin, J.S.D.C., 91 (1975) 168.4. H D Pratt, Am. Dyestuff Rep., 68 (Sep 1979) 39.

5. T M Baldwinson in Colorants and auxiliaries, Vol. 2, Ed. J Shore (Bradford: SDC, 1990) 568.

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53

CHAPTER 5

Principles of design and colouring ofdifferential-dyeing blends

5.1 DESIGN OF DIFFERENTIAL-DYEING VARIANT SYNTHETIC-POLYMER YARNS

The development of novel fabric constructions by combining variants of thesame man-made fibre differing in lustre, denier or cross-sectional shape presentsno serious problems for the dyer. The introduction in the 1970s of acid-dyeableand basic-dyeable variants of the conventional synthetic fibres, with excellentresistance to staining by dyes of opposite charge to those for which they showaffinity, greatly enhanced the variety of multicoloured effects that could beachieved [1]. After initial popularity, the use of differential-dyeing yarns declinedsomewhat. However, in recent years there has been a revival of interest followingthe development of sophisticated tufting machine controllers with the ability toproduce a wide range of patterned effects, e.g. the shifting needle bar techniquecontrolled by computer, and this trend is continuing [2].

The advantages of differential dyeing of fabrics, compared with the long-established alternative of knitting or weaving with coloured yarns, include theavoidance of stockholding of dyed yarn, easier adaptation to minor changes infashion and quicker delivery of finished goods. These advantages provedparticularly relevant in tufted carpets, yielding an attractive range of bold orsubdued colour combinations in versatile designs. As indicated later, however,there are restrictions of colour gamut in certain styles and careful attention tostringent conditions of processing is necessary to ensure reproducible effects.

The use of differential-dyeing nylon yarns has made an impact on the stylingof automotive upholstery in multicolour patterns, ranging from tonals to boldsaturated colours. Colour with a white reserve is obtained with nylon andpolyester and shadow effects with the acid-dyeable nylon variants. Basic-dyeablenylon or polyester cannot be included, however, because basic dyes do not meetthe unusually high light fastness standards specified by the car manufacturers [3].

54 PRINCIPLES OF DESIGN AND COLOURING OF DIFFERENTIAL-DYEING BLENDS

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Four types of nylon yarn with dyeing properties different from those of theparent homopolymer were introduced in the 1970s for weaving, warp-knittingand weft-knitting, as well as carpets. Tufted carpeting, however, was the sectorwhere this development was fully exploited. The variant yarns introduced thenwere as follows:(1) Ultra-deep: acid-dyeable variant with a higher concentration (>80 milli-

equivalents per kg polymer) of basic groups than the deep-dye variant(2) Deep-dye: acid-dyeable variant with a higher concentration (60–70 milli-

equivalents per kg) of basic groups than the normal fibre(3) Normal fibre: poly(hexamethylene adipamide) containing 35–45 milli-

equivalents per kg basic groups(4) Pale-dye: acid-dyeable variant with a lower concentration (15–20 milli-

equivalents per kg) of basic groups than the normal fibre(5) Basic-dyeable: variant containing acidic groups to provide affinity for basic

dyes.

The amine end group content of nylon 6.6 can be increased by adding excessdiamine to the polymer salt or by lowering the Mr of the polymer by limiting theamount of water liberated during the melt polycondensation reaction betweenhexamethylenediamine and adipic acid [4]. The deep-dye and ultra-deepvariants, however, were obtained by the inclusion of a small proportion of aprimary aliphatic diamine containing a tertiary amino group, e.g. N-(2--aminoethyl)- or N,N-bis(2-aminoethyl)piperazine, in place of some of thehexamethylenediamine used in manufacture of the normal nylon 6.6 polymer(Figure 5.1).

CONH(CH2)6NHCO(CH2)4CONHCH2CH2 NCH2CH2

CH2CH2

N CH2CH2NHCO

Major component Minor component

Figure 5.1 Segment of deep-dye nylon 6.6 variant

This change in structure provided new basic sites to increase the equilibriumuptake of anionic dyes at a given pH. The pale-dye variant (sometimes called‘low-dyeing’ or ‘light-dyeing’) was made by reacting a proportion of the amineend groups in the normal polymer with a suitable blocking reagent such asγ-butyrolactone (Scheme 5.1).

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Scheme 5.1

Figure 5.2 Segment of basic-dyeable nylon 6.6 variant

Figure 5.3 Typical acid-dye-resist agent

NHCO(CH2)4CONH(CH2)6NHCO

SO3H

CONH(CH2)6NHCOMajor component

Minor component

NN

NNH

Cl

Cl

SO3H

DESIGN OF DIFFERENTIAL-DYEING VARIANT SYNTHETIC-POLYMER YARNS

Basic-dyeable (or acid-dye-resist) nylon 6.6 was produced by replacing someof the adipic acid used in the polymerisation by a suitable tribasic acid, such as 5-sulphoisophthalic acid (Figure 5.2). All these structural changes decrease thedegree of crystallinity of the nylon, so that the rates of diffusion of dyes into (andout of) the variant fibres are more rapid than in the homopolymer.

Similar multicolour effects can be derived from a single variant yarn, preferablya deep-dye type, using readily available reactive chemicals. Acid-dye-resist agents,e.g. condensates of the dihydroxyarylsulphone-formaldehyde-aminoarylsulphonatetype, offer the simplest approach for temporarily neutralising the basic groups ofan acid-dyeable variant yarn. Permanent acid-dye-resist effects can be obtainedby reaction of amino end groups with an active halogen derivative, such as4-(dichlorotriazinylamino)benzenesulphonic acid (Figure 5.3).

Deep-dye modifications of poly(ethylene terephthalate) were developed byvarying the proportions of ethylene glycol and dimethyl terephthalate andincluding other diols, such as propylene glycol [5], n-butylene glycol [6] ordiethylene glycol [7], or an aliphatic dicarboxylic acid, e.g. adipic acid(Figure 5.4), to lower the glass-transition temperature by increasing the extent of

CONH(CH2)6NH2 CONH(CH2)6NHCO(CH2)3OH

Blocked N-terminal groupN-terminal group

γ-Butyrolactone

+O

O


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Figure 5.4 Segments of deep-dye polyester variants

Figure 5.5 Segment of basic-dyeable polyester variant

COOOOCCH2CH2COO CH2CH2CH2CH2 OOC

COOOOCCH2CH2COO CH2CH2OCH2CH2 OOC

COOOOC CH2CH2 OOC CH2CH2CH2CH2 COO

Major component Minor component

amorphous regions in the polymer. Some of these copolymer fibres were designedfor dyeing in medium or full depths at the boil without addition of a carrier.Unfortunately, there are problems associated with excessive soiling of carrier-freepolyester variants under domestic washing conditions [8].

Basic-dyeable polyester copolymers, in which some of the terephthalic acidunits are replaced by a tribasic acid, e.g. 5-sulphoisophthalic acid (Figure 5.5),have proved useful in differential-dyeing blends, but anionic-dyeable polyesterfibres failed to progress beyond the development scale. The basic-dyeablevariants have a more accessible structure than normal polyester, lower strengthbut a reduced tendency to pilling in blends with wool. They are readily dyeablewith disperse dyes below 120°C in the absence of a carrier. The use of basic dyes,however, results in less staining of wool compared with disperse dyes.

Conventional acrylic fibres are readily dyed with basic dyes at the boil becausethey are copolymers of acrylonitrile with up to 15% of an inert comonomer, suchas an acrylate or methacrylate ester, to make the structure more amorphous andlower the glass-transition temperature. The anionic sites include end groupsarising from residues of the polymerisation catalyst (e.g. a persulphate or benzoylperoxide) as well as carboxylate groups introduced in the form of an acidic

COOOOC

SO3H

COO CH2CH2

Major component

OOC

Minor component

CH2CH2 OOC

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CH2CH

CN

CH2CH CH2CH

CONH2

NCH2CH

CN

CH2CH

n

CH2CH

COOCH3

COOHn

Majorcomponent

Minorcomponents

Majorcomponent

Minorcomponents

Acid-dyeable Basic-dyeable

Figure 5.6 Segments of acrylic-fibre copolymers

comonomer. Acid-dyeable acrylic fibres are also copolymers containing basiccomonomer units, such as vinyl-pyridine, acrylamide or methacrylamide(Figure 5.6).

DESIGN OF DIFFERENTIAL-DYEING VARIANT SYNTHETIC-POLYMER YARNS

Conventional acrylic fibres can also be rendered dyeable with acid dyes bytreatment with hydroxylamine sulphate (HONH3)2SO4 (a) alone at pH 4 and120°C, (b) in the presence of sodium tripolyphosphate, glycerol and sodiumalginate at pH 6, or (c) in the presence of benzyl alcohol as a plasticiser at 80°Cand pH 5. Substantivity for acid dyes is conferred by the introduction of basicsidechains into the polymer structure [9].

5.2 DYEING OF ACID-DYEABLE NYLON VARIANTS

It has been shown that many of the practical features of the differential dyeing ofacid-dyeable nylon variant yarns are consistent with a simple theoretical model.This is based on the assumption that the electrical phenomena determining thesorption at equilibrium by the individual substrates in competition with oneanother conform to a Donnan-type membrane equilibrium [10]. Attempts havebeen made to combine this model with colour match prediction calculations forfinite dyebath conditions with a view to improving performance in practicalsituations. The dyer, however, does not normally have access to sufficient priordetails about substrate variability that would be necessary to adopt aquantitative approach of this kind.

Blends of normal or pale-dye nylon with the deep-dye variant yarn are ideallysuitable for shadow effects. The distribution of an acid dye between thecomponents of these blends depends considerably on the dyeing conditions andthe molecular structure of the dye, especially the degree of sulphonation. At agiven pH in the neutral region, dyebath exhaustion of a series of dyes differingonly in degree of sulphonation decreases as the number of sulpho groups in themolecule increases. These differences in substantivity become more marked as


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Table 5.1 Variation of contrast with pH and dye affinity [11]

Neutral Contrast ContrastCI Acid affinity type change (%)

Blue 25 Very high Low 4Blue 41 High Medium 11Blue 175 Moderate High 20

the content of basic groups in the fibre increases, so that the deep-dye variant ismore sensitive than the normal fibre to differences in degree of sulphonationwithin the series of dyes. Consequently, more highly sulphonated dyes give betterdifferentiation between the variant yarns at a given pH than the less-sulphonateddyes with higher neutral-dyeing affinity.

The reason for this sensitivity is the variation with pH of the partition ofindividual dyes between the fibre variants. The relative sensitivity to pH of aciddyes of increasing polarity (Figure 5.7), and therefore of increasing contrasteffect, is shown in Figure 5.8. A carpet made from 50:50 pale-dye/deep-dyenylon was dyed at various pH values and the degree of contrast between the twofibre variants expressed as a ratio:

As the pH decreases so the contrast ratio approaches unity (i.e. soliditybetween the variants). The more polar the dyes used to obtain high contrast atpH 6.5 – the usual pH for differential-dyeing – the more marked the variations incontrast ratio if the dyebath pH is not closely controlled [11]. If the pH variesfrom batch to batch it will cause the partition to vary accordingly. Thisfluctuation will become apparent either as a strength difference between thedyeings of the two fibre variants or, less acceptably, a hue difference in a mixturerecipe. The dyebath pH preferred for a given blend of acid-dyeable nylonvariants is determined mainly by the most dyeable component. It is necessary tocontrol the pH carefully using a suitable buffer, preferably a mixture of sodiumdihydrogen phosphate and disodium hydrogen phosphate, to ensure optimumreproducibility of shadow and contrast effects.

If the desired degree of contrast can be achieved with dyes of high neutralaffinity, then these should be selected in the interest of good reproducibility. Table5.1 shows the percentage change in contrast that a pH variation of 6.5 ±0.3would produce for the three dyes compared in Figure 5.7 [11]. As indicated in

(5.1) Contrast ratio =

Dye uptake by deep -dye nylonDye uptake by pale -dye nylon

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Figure 5.7 Acid dyes of increasing polarity (25 < 41 < 175)

O

O

NH2

HN

SO3Na

NCH3

O CH3

O

O

HN

HN

H2C

SO3Na

H2C

SO3NaO

O

NH2

HN

SO3Na

CI Acid Blue 25

CI Acid Blue 175

CI Acid Blue 41

DYEING OF ACID-DYEABLE NYLON VARIANTS

CI Acid Blue 175

CI Acid Blue 41

CI Acid Blue 25

5 6 71

2

3

4

5

pH

Con

tras

t rat

io

Figure 5.8 Effect of pH on contrast ratio [11]

Figure 5.8, the higher the dyebath pH the greater the differentiation between thevariant yarns and the better the reservation of the less dyeable component.However, other factors must be taken into account in deciding the optimum


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dyebath pH. Total exhaustion begins to decrease at higher pH values and thedegree of differentiation attainable is limited by the need to achieve economicexhaustion on the more dyeable variant. If subdued shadow effects are required,dyes of lower sulphonation are preferred and are applied at a carefully controlledpH in the 5–6 region. For sharper differentiation, more highly sulphonated dyecan be used at pH 6–7.

There are three approaches to selection of dyes for binary blends of acid-dyeable nylon. They differ only in the dyes used and the colour effect obtained,since the same dyeing conditions can be employed in all cases:(1) Shadow effects using acid dyes giving differentiation between the variant

yarns. Selected direct dyes can also be used but these are more sensitive toany physical irregularities in the variant yarns.

(2) A limited degree of colour contrast using appropriate combinations ofmonosulphonated and disulphonated acid dyes. It is advisable to evaluatewhether the mixtures selected have satisfactory fastness to light on the deep-dye yarn.

(3) A wider but still limited degree of contrast using highly sulphonated aciddyes with selected disperse dyes that give the same depth on bothcomponents of the blend. The hue on the deep-dye nylon is dependent onthat of the disperse dye (section 1.5.4) and the wet fastness of these dyeingsis generally lower than that of contrast dyeings produced by method (2).

The acid dyes showing only moderate differentiation are mainly mono-sulphonated monoazo or anthraquinone dyes, whereas those dyes showingsharper differentiation are almost all disulphonated monoazo, disazo oranthraquinone types. The direct dyes recommended for method (1) are almost alldisazo disulphonates, although a few disazo tetrasulphonates can be used wheresharper differentiation is required. The disperse dyes preferred for solidity onthese blends in method (3) are mainly low-energy monoazo or anthraquinonetypes.

The distribution of acid dyes between acid-dyeable variant yarns is affectedsignificantly by anionic levelling agents, such as alkylarylsulphonates andsulphated oils (Figure 3.5). These agents are preferentially absorbed by the deep-dye variant, so that the degree of differentiation is markedly reduced. Weaklycationic levelling agents for nylon, however, such as long-chain alkylaminepolyoxyethylenes, provide control of the rate of dyeing of acid-dyeable nylonblends without suppressing the differentiation, as they operate by complexingwith the anionic dyes. For similar reasons, a nonionic dispersing agent should beused in preference to anionic alternatives when using disperse and acid dyes for

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contrast effects by method (3). Acid-dyeable nylon blends may be aftertreatedwith a syntan to improve wet fastness, but if disperse dyes are present thetreatment temperature should not exceed 70°C or desorption may occur and thedegree of improvement in fastness is only marginal.

In the case of differential-dyeing tufted carpets or mats made with a jutebacking, it is particularly important to avoid too low a dyebath pH because thelignin impurities in the jute tend to become transferred to the nylon pile,especially the deeper-dyeing variants. This yellow-brown staining tends to dullthe brighter hues and to lower the light fastness because it discolours further onexposure to light. Staining at pH 6–8 is usually only slight and confined to thedeep-dye or ultra-deep yarns, which are often dyed to a full depth. Much of thelignin and identification sighting colours on the variant yarns can be removed byalkaline scouring at 80°C before dyeing. When dyeing unusually bright or palecolours it may be necessary to scour-bleach with alkaline dithionite at 70°C,followed by sodium perborate and a nonionic detergent at the same temperature.

5.3 DYEING OF ACID-DYEABLE/BASIC-DYEABLE NYLON VARIANTS

The diffusion kinetics of CI Basic Blues 3 and 9 (Figure 5.9) on a basic-dyeablenylon 6 variant containing 5-sulphoisophthalic acid N-terminal residues (Figure5.10) demonstrated that diffusion was much more rapid than on the parenthomopolymer [12]. Measurements of zeta potentials by the flow-potentialmethod showed that the modified fibres have an unusually high negative chargeat the surface, which decisively promotes sorption of the dye cations. The uptakeof dye can be regarded as an ion-exchange reaction. The agreement of dyesaturation data with the 5-sulphoisophthalic acid content confirmed that theseoxazine and thiazine dyes of the delocalised-charge type become boundstoichiometrically to the terminal sulphonated residues in the modified fibre [13].

The scope for bright colours on normal/basic-dyeable blends is narrower thanon deep-dye/basic-dyeable blends, because basic dyes stain normal nylon morethan the deep-dye variant. It is also more difficult to achieve heavier depths onnormal nylon without cross-staining of acid dyes on the basic-dyeablecomponent. For these reasons it is usual to dye normal/basic-dyeable com-binations in pale shades below one-third standard depth on both components.Anionic scouring agents should be avoided because they may complex with thebasic dyes and increase the basic dye staining of the acid-dyeable nylon variants.

After scouring with a nonionic detergent at 70°C, the basic and acid dyes areadded separately after an alkanol polyoxyethylene anti-precipitant, sodiumthiosulphate and a sequestering agent. A cationic retarder can be used to slow

DYEING OF ACID-DYEABLE NYLON VARIANTS


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NHCOCH2CH2CH2CH2CH2NHCO

SO3H

COOH

Figure 5.10 Sulpho-containing terminal group in basic-dyeable nylon 6

Figure 5.9 Basic blue dyes of the delocalised-charge type

down the initial strike of basic dyes on the basic-dyeable component at ambienttemperature, permitting improved levelling without impairing the ultimateexhaustion [14]. Dyeing proceeds readily at 85°C and pH 6. A phosphate bufferis essential to ensure stability of pH for reproducible results on the normal/basic-dyeable blend. When dyeing the deep-dye/basic-dyeable combination, however,pH control with ammonium acetate is satisfactory and more economical. Aciddye staining of the basic-dyeable nylon is cleared by boiling in dilute acetic acidat pH 4–5.

The acid dyes recommended for reserve of the basic-dyeable component of adifferential-dyeing nylon AB blend are mainly mono- or disulphonated monoazoor anthraquinone types. Levelling acid dyes of this type give better reserve ofbasic-dyeable nylon than do milling acid dyes or especially 1:2 metal-complextypes. Monoazo and anthraquinone basic dyes of the localised-charge type arepreferred for reserve of the acid-dyeable component of these blends. The lightfastness and wet fastness of basic dyes on basic-dyeable nylon are markedlyinferior to those of corresponding dyeings on acrylic fibres, and syntans are onlymoderately effective in improving wet fastness on this type of nylon. Manyanionic dyes on the acid-dyeable nylon variants show much better wet fastness,so it is preferable to dye the basic-dyeable fibre only to no more than a medium

N

O N(CH2CH3)2 X–(CH3CH2)2N

CI Basic Blue 3

+

N

S N(CH3)2 X–(CH3)2N

CI Basic Blue 9

+

63

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Scheme 5.2

depth and to select the acid-dyeable component for the heavier depth in thedesign.

5.4 DESIGN OF DIFFERENTIAL-DYEING CELLULOSIC FABRICS

5.4.1 Aminisation of cellulosic fibres

The concept of pretreating cellulosic material with a cationic compound in orderto confer affinity for acid dyes has been explored since long before reactive dyeswere discovered. In those days the approach was of little practical value becauseof poor fastness to washing of the cationic agents themselves and the dyeings thatcould be obtained with them. In the last twenty years or so, however, there hasbeen much activity in this area by applying the principles of reactive dyeing andreactant finishing to the application of these pretreating agents [15–19]. Much ofthis research has been mainly intended to improve the utilisation of reactive dyeson cotton or viscose, however, rather than the development of colour effects onbinary cellulosic blends.

Deep-dye viscose can be produced by incorporating additives containingamino groups into the spinning dope before extrusion. Thus aminoethylcellulose,prepared by condensation of 2-chloroethylamine with cellulose (Scheme 5.2)confers enhanced dyeability with direct or acid dyes under mildly acidic dyeingconditions. These primary amino dyeing sites enable reactive dyes to formcovalent bonds without requiring an alkaline fixation step after exhaustion.

DYEING OF ACID-DYEABLE/BASIC-DYEABLE NYLON VARIANTS

Yarns of deep-dye viscose can be knitted or woven into designs with normalviscose yarn. These are suitable for obtaining either of the following:(a) shadow effects with direct or alkali-fixed reactive dyes;(b) normal viscose reserve using acid dyes or neutral-fixed reactive dyes.

[cellulose] OH [cellulose] OCH2CH2NH2

[cellulose] OCH2CH2NH2 HO3S [dye] [cellulose] OCH2CH2NH3–O3S [dye]

[cellulose] OCH2CH2NH2 Cl [dye]

+

[cellulose] OCH2CH2NH

ClCH2CH2NH2

+

[dye]+

+


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Scheme 5.3

Scheme 5.4

By incorporating an acrylic yarn into the fabric, three-way shadow-contrast orreserve-contrast designs can be produced using basic dyes with these twoapproaches [20].

Diethylaminoethylated cotton or linen can be prepared in a similar way byreacting the substrate with the tertiary amine 2-chloroethyldiethylamine in thepresence of alkali at 95°C. The diethylaminoethyl substituents act as built-incatalysts, capable of initiating fixation of reactive dyes even in the absence ofalkali. A more effective approach to the aminisation of cotton fabric involvedpadding with caustic soda solution, followed by immersion in an acetonesolution of epichlorohydrin and triethanolamine. The etherifying agent isbelieved to be the reactive tertiary amine formed by the initial condensationbetween the starting materials (Scheme 5.3).

H2CO

CHCH2Cl CHCH2 N+

CH3

CH3

CH3+ N(CH3)3

Cl–

H2CO

5.4.2 Monofunctional quaternary ammonium reactants

Many of the early attempts to incorporate quaternary nitrogen sites in cellulosedepended on the use of epoxy reactants. The first product of this type on themarket [21] was Glytac A (Protex). This is readily available from epichloro-hydrin and trimethylamine (Scheme 5.4) [22].

Reaction with cellulose proceeds via the glycidyl group at alkaline pH.Reactive dyes can be fixed to the modified fibres at neutral pH without salt,conditions that are environmentally attractive. This behaviour has beenattributed to predomination of the zwitterionic form of the sidechain afteralkaline fixation (Scheme 5.5). Thus the anionic dye is absorbed on thequaternary site and, if reactive, it is attacked by the nucleophilic ionised alcoholicgroup nearby [23]. A reagent exhibiting some features in common with Glytac is1,1-dimethyl-3-hydroxyazetidinium chloride (DMAC). It reacts with cellulose bya similar ring-opening etherification. The results from the two quaternised

3 H2CO

CHCH2Cl (H2CO

CHCH2OCH2CH2)3N+ (HOCH2CH2)3N

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Scheme 5.6

cellulose ethers are similar, but there is marginally better reactive dye fixation tothe DMAC grouping [17].

Certain problems are characteristic of epoxy quaternising agents [15].Discoloration of the substrate can occur during the alkaline cure necessary forepoxy fixation. Loss of trimethylamine by thermal instability destroys theeffectiveness and simultaneously gives rise to objectionable odour in the treatedfabric. This difficulty can be only partly overcome by incorporating more bulkysubstituents at higher cost. Probably the most important limitation of theepoxypropyl agents is their very low substantivity. Not only are they unsuitablefor exhaust application but poor dye penetration arises from agent migration atthe drying stage of the pad–dry–bake application process.

The N-methylolation reaction that has been so important in traditionalchemical finishing can be exploited as a first step to the aminisation of celluloseusing N-methylolacrylamide. This acrylamidomethylated cellulose reacts readilywith ammonia or alkylamines to yield cellulose derivatives containing amino,imino or quaternary groups (Scheme 5.6). Dyeing tests on these derivativesshowed generally good colour yields and high fixation, but reactive dyes on thoseaminised with di- or trimethylamine gave poor fixation. The dye–fibre linkage islabile owing to the strongly electron-withdrawing quaternary group [23].

OCH2CHCH2 N+

CH3

CH3

CH3[cellulose]

OH

OH–Cl–

Scheme 5.5

OCH2CHCH2 N+

CH3

CH3

CH3 Cl–[cellulose]

O–

+ + H2O

DESIGN OF DIFFERENTIAL-DYEING CELLULOSIC FABRICS

Groups of the haloheterocyclic type found in traditional reactive dyes, such asaminochlorotriazine or difluoropyrimidine, have been exploited in aminisingagents that also contain mono- or bis-quaternary ammonium groups to boost theuptake of anionic dyes by the aminised substrate. Such agents react more readilywith cellulose and show higher thermal stability than the ring-opening types such

cell OH HOCH2NHCOCH CH2 OCH2NHCOCH CH2cell+

OCH2NHCOCH CH2cell H R OCH2NHCOCH2CH2 Rcell+

R = NH2, NHCH3, NHCH2CH2OH, N(CH3)2 or N(CH3)3Cl–+


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Figure 5.11 Cationic and anionic aminochlorotriazine agents

as Glytac or DMAC. Both classes of monofunctional reactive system, however,share the disadvantage of low substantivity for cellulose and must be applied bypadding. More complex multifunctional structures were designed for exhaustapplication [15] and these gave impressive colour yields. There are practicaldrawbacks to all these agents, however, including lower light fastness [17], huechanges and poor penetration into the fibre [15].

In a differential-dyeing evaluation of aminochlorotriazine agents, colourlesscationic and anionic products of this type (Figure 5.11) were applied to cottonyarns [24]. Yarn pretreated with the quaternary ammonium agents showedenhanced uptake of CI Acid Red 13 or CI Direct Blue 10. The degree ofdifferential uptake was dependent on the amount of agent applied, so fabricsexhibiting a range of shadow effects were obtained. Pretreatment with thesulphonaphthylamine agent gave cotton dyeable with CI Basic Yellow 15.Combinations of anionic- and cationic-modified yarns yielded contrast effectswith minimal cross-staining. The pretreated cotton samples were sufficientlystable to be converted to viscose or cellulose triacetate without loss of themodified dyeability, providing potential routes to differential-dyeing viscose ortriacetate variants.

This approach to differential-dyeing is flawed because the costly pretreatmentwith a sophisticated reactive agent is not justifiable. The fastness of the coloureffects to washing with detergent will be inadequate because the dyes are linkedto the ionic dyeing sites mainly by electrostatic bonds. In the case of basic dyes onthe sulphonaphthylamine sites, low fastness to light is also anticipated.

An alternative approach to the aminisation of cellulose involves esterificationusing chloropropionyl chloride. This chloropropionate ester condenses readily

NHCH2CH2 N

CH2CH3

CH2CH3

R

NN

N

Cl

HN

NH

NN

N

Cl

HN

X–

SO3Na

+

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Scheme 5.7

Scheme 5.8

with ammonia or alkylamines at 50°C to yield aminised cellulose derivatives(Scheme 5.7). If the aminisation treatment is carried out at the boil, however,chloropropionic acid is eliminated and the cellulose is regenerated. The esterbond remains intact during reactive dyeing but it is hydrolysed during alkalinesoaping at the boil [25].

[cellulose] OH NaO P

ONa

O

ONa

O P

ONa

O

ONa

[cellulose]+ + NaOH


Sodium or ammonium phosphates can be used to produce chambray or otherspecial colour effects on cotton fabrics. Yarn treated with the phosphorylatingagent (Scheme 5.8) is dried, cured and then woven as the weft with an untreatedwarp. The fabric is then piece dyed with reactive, vat or direct dyes. Only thewarps absorb dye because the phosphorylated cellulose resists these conventionaldyes for cotton. It can be cross-dyed, however, using 1:1 metal-complex dyes atpH 4. Interesting colour contrast effects are obtained by treating selected areas ofa cotton pile fabric with the phosphorylating agent before selective dyeing [26].These techniques offer the advantage of a cheap and effective modifying reactantbut the coordinate bonds linking the metal-complex dyes to the phosphatedyeing sites will show only limited fastness to acidic perspiration tests.

5.4.3 Polymeric cationic reactants

Many cationic polymers have been applied to cellulosic fibres with a view toenhancing uptake of anionic dyes [18]. It is considerably more difficult in theseinstances to interpret the precise mechanisms involved, apart from the obviousparticipation of electrostatic attraction between dye anions and basic groups inthe polymer. Recent studies have included the application to cotton [17] of thepolyamide-epichlorohydrin resin Hercosett 125 (Hercules), originally marketedas a shrink-resist treatment for wool. Improved dyeability with reactive dyes and

[cellulose] OH [cellulose] OCOCH2CH2Cl

[cellulose] OCOCH2CH2Cl H R [cellulose] OCOCH2CH2

+ ClCOCH2CH2Cl

R+

R = NH2, NHCH3, N(CH3)2 or N(CH3)3Cl–

50°C

+


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Scheme 5.10

NCH2

CH2

CHOH+

Cl–

+ N CH2CHCH2NHCH2CH2NH2

OH

H2NCH2CH2NH2

N CH2CHCH2NHCH2CH2NHCH2CHCH2

OH OH

N

Scheme 5.9

good wet fastness were obtained, but dullness of hue and impaired light fastnesswere disadvantages.

Incorporation of thiourea into the Hercosett polymer during application wasintended to overcome deficiencies when using low-reactivity dyes on Hercosettalone. Thiourea reacts with azetidinium groups in the resin to formisothiouronium groups. These are more strongly nucleophilic and improve thefixation of low-reactivity dyes. Some of the isothiouronium groups decomposeduring dyeing to yield thiol groups, which form further sites for dye fixation andmay also react with remaining azetidinium groups to form thioether crosslinks inthe resin (Scheme 5.9).

NCH2

CH2

CHOH SNH2

NH2

N SNH2

NH2

CH2CHCH2

OH

N SNH2

NH2

CH2CHCH2

OH

N ONH2

NH2

CH2CHCH2SH

OH

NCH2

CH2

CHOH

+

Cl–

N

+

HSCH2CHCH2

+

OH

Cl–

+

N

OH–

CH2CHCH2

+

OH

+S

Cl–

CH2CHCH2N+

OH

Cl–

Differences in colour yield were still observed between dyes of high and lowreactivity and therefore ethylenediamine was evaluated as an additive to theHercosett resin [27]. This reacts readily with azetidinium groups to formprimary, secondary and tertiary amino sites for fixation of reactive dyes (Scheme5.10). Enhanced substantivity, good brightness and excellent fastness to washingwere achieved but the light fastness was still impaired in most cases.

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Scheme 5.11

Cotton has little inherent substantivity for disperse dyes, but a water-solublebenzoylating agent, sodium benzoylthioglycolate, can be applied by a pad–dry–thermofix process in the presence of alkali (Scheme 5.11) to yield a benzoylatedcellulose for which disperse dyes show enhanced substantivity. Dyeings of highcolour strength and good fastness to ISO 3 washing are obtained [28]. Dispersedyeings on benzoylated cellulose exhibit much better fastness to washing thancorresponding dyeings on cotton esterified using long-chain acylating agents[29]. The ease of application of sodium benzoylthioglycolate is a great advantageover earlier processes based on the Schotten–Baumann reaction of cellulose withbenzoyl chloride. This process is of interest for the single-class dyeing of blendsof ester fibres with cellulosic fibres using disperse dyes to colour bothcomponents.


Polymer treatments devised to increase the versatility of cotton as a dyeablesubstrate [30] included:(1) treatment with a disperse-dyeable polymer followed by disperse dyeing;(2) addition of basic or acid dyes to a crosslinking formulation so that the dyes

are retained by interaction with the finish;(3) application of reactive or vat dyes with an anionic-dyeable polymer.

Bicoloured contrast effects can be produced by selective coating of the two sidesof the fabric with different combinations of dye and finish. Disperse and basicdyes exhibit poor light fastness on cotton although the brightness of basic dyesmay appear superficially attractive. Reactive dyes give the best balance ofproperties, since vat dyes sometimes show unlevelness under these conditions.

A two-stage dyeing sequence has been proposed for the contrast dyeing ofcotton terry towelling using reactive dyes [31]. The first step involves continuousdyeing of the fabric using a polyelectrolyte that promotes coloration of the tips ofthe terry towelling loops. Adverse drying conditions on hot cans (‘fry drying’)favour dye migration to the tips of the loops. The dried fabric is then paddedwith a highly alkaline concentrated salt solution to minimise dye desorption and

C

O

S CH2COONa HO [cellulose]+NaOH

Sodium benzoylthioglycolate C O [cellulose]

O

+ HSCH2COONa

Sodium thioglycolate


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steamed to complete alkaline fixation of the dyes. Unfixed dye is removed byrinsing. The second stage consists of uniformly dyeing on a winch or atmosphericjet in a contrasting colour also formulated with reactive dyes in conventionalexhaust dyeing.

5.4.4 Introduction of activated sites for nucleophilic dyes

In this approach the reactive function is incorporated into the substrate and areactive dyeing is carried out using a ‘non-reactive’ dye containing nucleophilicgroups. A model dye of this kind was prepared by reacting theaminochlorotriazine dye CI Reactive Red 58 with ethylene diamine to form a 2-aminoethylaminotriazine derivative (Scheme 5.12). Acrylamidomethylatedcellulose prepared by condensation of N-methylolacrylamide with cotton(Scheme 5.6) reacts readily with nucleophilic dyes of this kind, which show nohydrolysis during dyeing.

Scheme 5.12

High fixation is achieved either by exhaustion at pH 10.5 in salt solution orby pad–batch at the same pH for 24 hours [23]. These aminoalkyl dyes showzwitterionic character below pH 8 and this lowers the nucleophilicity of theprimary amino group. Nucleophilic dyes containing thiol groups would be morereactive than aminoalkyl analogues but there would be problems of toxicity,odour and a tendency to oxidise to disulphide. Aminoaryl or hydroxyalkylanalogues would be less reactive than the aminoalkyl derivatives.

The novel reactant 2,4-dichloro-6-(2′-pyridinoethylamino)-s-triazine(DCPEAT, Figure 5.12) was evaluated [32] as a means of activating cellulose bypad–batch application at pH 8.5 for 24 hours. After a cold water wash, themodified substrate was dyed with aminoalkyl dyes. High fixation was achievedby exhaust dyeing at pH 9 without salt. Nucleophilic dyes can also be fixed oncotton or nylon that has been pretreated [33] with the trifunctional reactant 2-chloro-4,6-bis(4′-sulphatoethylsulphonylanilino)-s-triazine (CSESAT, Figure5.12).

N

N N

HN Cl

NHAr

[dye]–O3S

N

N N

HN NHCH2CH2NH2

NHAr

+

[dye]

H2NCH2CH2NH2

–O3S

OH–

+ HCl

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Figure 5.12 Bifunctional and trifunctional reactants for cellulose

Developments in this area seem to be leading to daunting complexity ratherthan elegant simplicity. Certain aspects of this concept inspire unease rather thanconfidence:(1) The risk of premature hydrolysis leading to impaired fixation has been

transferred from the dye to the substrate.(2) The pretreatment with a colourless reactant necessary to activate the

substrate must be exceptionally uniform if dye fixation is to be consistent.(3) If penetration of the reactant is poor, ring dyeing will follow and the fastness

properties will be adversely affected.(4) Only the aminoalkyl dyes will react with the activated substrate and other

classes of dyes for unmodified cellulose may be inapplicable.(5) Cost implications give concern because both dye and substrate must be

specially modified before the desired reaction can occur.

5.5 DESIGN OF DIFFERENTIAL-DYEING WOOL KERATINDERIVATIVES

Wool is routinely chlorinated with a dilute acid solution of hypochlorite or withgaseous chlorine to give shrink-resist effects, whereby absorption of acid dyes isappreciably increased. The production of shadow effects on blends ofchlorinated and untreated wool depends not only on a selection ofmultisulphonated dyes but also on dyeing conditions. Dyeing at lowtemperatures will favour absorption by chlorinated wool, particularly in the caseof aggregated milling acid dyes that are less readily absorbed by the unmodifiedfibre except at higher temperatures. Certain chrome dyes will also producedistinctive colours on blends of chlorinated and unchlorinated wool. Levelling


N CH2CH2NH

N N

N Cl

Cl

HN

N N

N NH

Cl

CH2CH2SO2NaO3SO

DCPEAT

CSESAT

SO2CH2CH2 OSO3Na

+


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Figure 5.13 CI Mordant Black 9 chrome complex

acid dyes are unsuitable because of the rapid migration of these dyes even attemperatures below the boil.

The differential dyeing of wool can be achieved using several other techniques[34]:(1) localised pretreatment with acid-dye-resist agents of the dichlorotriazinyl-

aminoarylsulphonate type (Figure 5.3) before dyeing the untreated areaswith acid dyes;

(2) pretreatment with cationic agents such as Glytac A (section 5.4.2) to conferdeep-dye characteristics;

(3) overdyeing of fabrics containing wool yarns predyed with e.g. thechromium complex of CI Mordant Black 9 (Figure 5.13) to give colour/black effects;

(4) application of mordant dyes to prechromed and unchromed wool yarns, butthis is not of much value owing to poor shade reproducibility.

N N

SO3Na

OH

O

O

NN

NaO3S

OH

O

O

CrIII

Provided that the nucleophilic sites of reaction within the accessible regions ofwool keratin can be blocked, there will be no remaining opportunity for reactivedyes to become fixed. By choosing reactive dyes of high reactivity but relativelylow substantivity it should be possible to achieve a high degree of resist effect.The principal nucleophilic groups are primary amino (lysine), secondary imino(histidine) and free thiol groups (cysteine). A reactive dichlorotriazinyl-aminoarylsulphonate (Figure 5.3) effectively blocks these nucleophilic sites.

Acid-dye-resist agents of this kind that introduce extra anionic sidechaingroups into wool keratin will confer enhanced basic-dyeable character.Conversely, a reactive agent with a cationic centre, such as Glytac A (section

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Scheme 5.13

5.4.2), will act as a basic-dye-resist and confer increased acid-dyeable behaviour.Sandospace R (S) at weight gains of 8% or more resisted all the reactive, mor-dant and acid dyes tested to 70% resist or higher, except for a monosulphonatedacid dye. Uptake of the reactive and acid dyes was increased by prior reaction ofthe wool with Glytac A, a moderately effective resist treatment with respect tobasic dyes (40–80% resist) but hydrophobic dye–fibre forces appeared topredominate over cationic repulsion effects [35].

There is no universally effective reactive resist that will reserve all classes ofdyes normally applied to wool. Dyes of high Mr become attached to wool bynonpolar van der Waals forces between the hydrophobic dye anions andhydrophobic sidechains in the wool fibre, their strength being proportional to thearea of contact. High wet fastness is often achieved by increasing the Mr withsubstituents that do not form part of the chromogenic system, such as dodecyl orarylsulphonyl groups. As wool dyes become more hydrophobic they show agreater tendency to aggregate in the dyebath. Aggregation within voids in thekeratin structure is probably partly responsible for the poor migration, high wetfastness and excellent light fastness of these dyes.

The benzoylation of wool using benzoic anhydride forms benzoylaminoresidues on lysine sidechains and N-terminal end groups, as well as benzoateester groups on the hydroxy-containing sidechains of serine and threonine(Scheme 5.13). This destroys most of the sites for reactive dye fixation and alsoacts as an effective resist for acid dyes with two or more sulphonate groups.

OO

O

[wool] NH2

[wool] OH

OCO

NHCO[wool]

[wool]

+ + H2O

DESIGN OF DIFFERENTIAL-DYEING WOOL KERATIN DERIVATIVES

If a water-soluble bifunctional arylating agent (Scheme 5.14) is applied byexhaustion at pH 6.5 together with conventional disperse dyes, absorption bywool takes place with the agent held initially by electrostatic attraction andhydrophobic interaction as if it were a colourless milling acid dye. Fixationoccurs during subsequent treatment for an hour at the boil, the activevinylsulphone group being released by β-elimination and reaction taking place


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with nucleophilic groups in the fibre. Some reaction also occurs via the lessreactive chlorotriazine group. Hydrophobic groupings attached to the wool inthis way confer enhanced substantivity for disperse dyes. This approach is ofinterest for the single-class dyeing of blends of ester fibres with wool usingdisperse dyes to colour both components [36].

Sulphamic acid reacts with the amino groups of lysine sidechains and the N-terminal end groups in wool to form ammonium sulphamate groups, and withthe hydroxy groups of serine and threonine to form ammonium sulphate groups(Scheme 5.15). Wool that has been treated with sulphamic acid and urea in apad–dry–bake process shows increased uptake of basic dyes. Milling acid dyesthat depend mainly on hydrophobic dye–fibre interaction are poorly resisted bysulphamylated wool, but levelling acid dyes absorbed mainly by electrostatic

Scheme 5.15

[wool] NH2

[wool] OH

[wool] NHSO3– NH4

++

[wool] OSO3– NH4

+

HO3SNH2

HO3SNH2+

Scheme 5.14

N

NN

SO2CH2CH2NH

NH

HN NH

[wool]

[wool]

+ NaHSO4HN N

NN

NH

Cl

SO2CH CH2

CH2 H2N [wool]

N

NN

SO2CH

Cl

HN NH

H2N [wool]

+

+

N

NN

SO2CH2CH2OSO3Na

Cl

HN NH

75

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DESIGN OF DIFFERENTIAL-DYEING WOOL KERATIN DERIVATIVES

attraction are strongly resisted. Reactive dyes, almost all of which are highlysulphonated with no hydrophobic alkyl substituents, are also effectively resisted.

Sulphamylated wool shows much higher exhaustion of typical basic dyes thanacrylic fibres. These contain only about 50–150 milli-equivalents per kg ofanionic groups, whereas wool treated with sulphamic acid contains up to ca.1000 milli-equivalents per kg of acidic dyeing sites [37]. Sulphamylation of woolhas a marked effect on the light fastness of basic dyes. The fastness ratings on themodified wool are much higher than on untreated wool (Table 5.2). This bringsthe ratings of the modified wool close to values obtained for the same dyes onacrylic fibres. This effect is attributed to the beneficial influence of the stronglyanionic groups introduced by the sulphamylation reactions. Basic dyes onuntreated wool are absorbed because of the presence of the carboxyl groups inaspartic and glutamic acid sidechains and the C-terminal end groups.Unfortunately, the fastness to washing of basic dyes on untreated andsulphamylated wool is extremely poor. The treatment with sulphamic acid makesthe wool more hydrophilic. Marginal improvement of the wet fastness is possibleby aftertreatment with tannic acid but this causes dulling of the dyeings.

The treatment of merino wool with ethanolamine in aqueous isopropanol toconfer enhanced dyeability and a shrink-resist effect was investigated using afactorial design experiment with three variables [38]. These effects ofethanolamine have been attributed to alkaline hydrolysis of the cystinedisulphide groups to form dehydroalanine residues that can react with the amineto form β-aminoalanine groups, which are able to interact with anionic dyes. Avinylsulphone reactive dye was used to assess the improvement in dyeability. Theoptimum conditions found were 1 mol l–1 ethanolamine in 50% aqueousisopropanol at 55°C, which gave the best dyeability and shrink resistance withonly 3% weight loss and acceptable whiteness.

Table 5.2 Exhaustion and light fastness of typical basic dyes [37]

Exhaustion (%) Light fastness

2% o.w.f. of Acrylic Sulphamylated Acrylic Untreated SulphamylatedCI Basic fibre wool fibre wool wool

Yellow 11 71 99 6 4 6Red 51 30 97 6 3–4 6Blue 3 33 98 4–5 1 4


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5.6 REFERENCES 1. C Baur, Teinture et Apprets, 144 (Oct 1974) 163.

2. J Park, J.S.D.C., 109 (1993) 133. 3. A Anton and J C Browne, Text. Chem. Colorist, 16 (Sep 1984) 135.

4. A Anton, Text. Chem. Colorist, 13 (Feb 1981) 46.

5. R Gutmann, T Barth and H Herlinger, Chemiefasern und Textilind., 40/92 (1990) 104. 6. T Sato, Chemiefasern und Textilind., 40/92 (1990) 35.

7. M D Teli and N M Prasad, Am. Dyestuff Rep., 80 (June 1991) 18.

8. S M Doughty, Rev. Prog. Coloration, 16 (1986) 25. 9. M M Marie, Am. Dyestuff Rep., 82 (Sep 1993) 86.

10. R McGregor, AATCC Int. Tech. Conf., (Oct 1976) 124.

11. T L Dawson and B P Roberts, J.S.D.C., 95 (1979) 47.12. H Muller and V Rossbach, Text. Research J., 47 (1977) 44.

13. H Muller, Text. Research J., 47 (1977) 71.

14. M A Herlant, Am. Dyestuff Rep., 81 (June 1992) 15.15. G E Evans, J Shore and C V Stead, J.S.D.C., 100 (1984) 304.

16. R L Stone and R J Harper, AATCC International Conference and Exhibition, (Oct 1986) 214.

17. D M Lewis and X P Lei, Text. Chem. Colorist, 21 (Oct 1989) 23.18. J Shore, Rev. Prog. Coloration, 21 (1991) 23.

19. J Shore, Indian J. Fibre Text. Res., (Mar 1996) 1.

20. R Aitken, J.S.D.C., 99 (1983) 150.21. M Rupin, G Veaute and J Balland, Textilveredlung, 5 (1970) 829.

22. T S Wu and K M Chen, J.S.D.C., 108 (1992) 388.

23. D M Lewis and X P Lei, J.S.D.C., 107 (1991) 102.24. J A Clipson and G A F Roberts, J.S.D.C., 105 (1989) 158.

25. X P Lei and D M Lewis, Dyes and Pigments, 16 (1991) 273.

26. E J Blanchard, J T Lofton, J S Bruno and G A Gautreaux, Text. Chem. Colorist,11 (Apr 1979) 76.

27. X P Lei and D M Lewis, J.S.D.C., 106 (1990) 352.

28. D M Lewis and P J Broadbent, J.S.D.C., 113 (1997) 159.29. E Einsele, H Sadeli and H Herlinger, Melliand Textilber., 63 (1981) 967; 64 (1982) 61.

30. R J Harper, E J Blanchard, J T Lofton, J S Bruno and G A Gautreaux, Text. Chem. Colorist,

6 (Sep 1974) 201.31. J N Etters, Am. Dyestuff Rep., 83 (Sep 1993) 70.

32. D M Lewis and X P Lei, AATCC International Conference and Exhibition, (Oct 1992) 259.

33. D M Lewis and Y C Ho, AATCC International Conference and Exhibition, (Oct 1994) 419.34. A C Welham, Am. Dyestuff Rep., 81 (Oct 1992) 15.

35. V A Bell, D M Lewis and M T Pailthorpe, J.S.D.C., 100 (1984) 223.

36. D M Lewis, J.S.D.C., 113 (1997) 193.37. B A Cameron and M T Pailthorpe, J.S.D.C., 103 (1987) 257.

38. L Coderch, A M Manich, J L Narra and P Erra, J.S.D.C., 107 (1991) 19.

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77

CHAPTER 6

Nylon/wool and other AA blends

6.1 DYEING OF NYLON/WOOL BLENDS

Blends in which both fibres are dyeable with anionic dyes in full depths (AAblends) are usually developed because of a desirable balance of physicalcharacteristics. The attainment of solid colour effects is therefore usuallynecessary. As soon as nylon was readily available for commercial outlets after theSecond World War, it was recognised that the exceptional strength, durabilityand abrasion resistance made it the ideal fibre for blending with wool. Blendedtwill fabrics gave the same performance as the all-wool counterpart but at halfthe weight. Hand-knitting yarns and half-hose blended from 20:80 to 40:60nylon/wool give a valuable combination of softness and strength. Stretch fabricswoven from a crimped nylon warp and a wool weft are established in wintersportswear, particularly skiwear, and leisure clothing.

Woven carpets and high-quality tufted carpets are most important outlets forblended-staple nylon/wool yarns, usually containing about 20% of nylon to con-fer improved durability and abrasion resistance whilst retaining the absorbency,softness and antistatic qualities of the natural fibre. An interesting anomaly ofthe 1990s market, however, is a demand for carpets made from blended nylon/wool but dyed and woven to simulate sisal, a natural cellulosic fibre viewed as afashionable floor covering by ecology-conscious consumers [1].

The quality of the wool and its base colour will influence the visual effectobtained between the two fibre types when dyed. Even when the depth of colouris the same, the yellowness of the wool can result in the shade appearing dulleron that component. Blends containing nylon 6 show higher saturation uptakeand critical depth than nylon 6.6 blends and require more retarding or levellingagent. Supplies of the same nylon type from different sources, or even differentmerges from the same source, may also vary in dyeing properties [2].

Good solidity is generally easier to achieve on staple-fibre blends, such ashand-knitting yarns or carpets, because the blend often contains only 10–20% ofnylon and the distribution can be controlled using an appropriate auxiliary.

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Solidity is a more serious problem when dyeing nylon/wool furnishings orsportswear woven with a crimped nylon warp and a wool weft. Apart from thequality and proportions of wool and nylon present, the distribution of dyebetween them depends on several factors already discussed (section 3.2),including the constitution and degree of sulphonation of the dyes, applied depth,agent concentration, pH and temperature of dyeing [3].

Like the disulphonated levelling acid dyes, the 1:1 metal-complex dyes havelow substantivity for nylon and the critical depth on nylon/wool is relatively lowcompared with monosulphonated acid dyes and 1:2 metal-complexes. Dyes ofthe 1:1 metal-complex type and the levelling acid disulphonates are dyed at pH2–3 with formic or sulphuric acid and salt at the boil. Monosulphonated aciddyes and premetallised 1:2 types dye nylon more readily, so they are applied tonylon/wool at pH 5–6 with ammonium acetate and acetic acid. A near-neutralpH is necessary to control levelness in pale depths, but more acetic acid can beused for full depths.

Medium or heavy depths on nylon/wool carpet blends are usually dyed with1:2 premetallised dyes in order to ensure good fastness to light and shampooing.The monosulphonated type offers the best economic compromise of goodpartition with moderate usage of retarding agent. Partition is, however,dependent on control of pH, being improved in favour of the wool withdecreasing pH and increasing temperature. The control of both of these factorscan be ensured using an automatic dosing system [4]. Coverage of physicalvariations in the nylon is limited but an alkanol polyoxyethylene levelling agentgives some improvement. Heavy, dull colours can be dyed by a two-stage methodin which the nylon is first dyed preferentially with 1:2 metal-complexes at pH 6and the wool is then filled in with selected 1:1 metal-complex types.

It is difficult to attain satisfactory solidity on nylon/wool with chrome dyes,which tend to favour wool. Some solid hues can be based on mixtures of selectedchrome and 1:2 premetallised types. The chrome dyes giving acceptable solidityon nylon/wool are mainly monosulphonated naphthylazo derivatives of Mr

350–450. They are used only for economical dull orange, red, brown and blackdyeings of high wet fastness. At such depths no anionic retarder is required.Chroming of the dye is more difficult on nylon/wool than on nylon alonebecause the chromium tends to be absorbed preferentially by the wool. Chelationwith the dye can be improved by adding a mild reducing agent, such as sodiumthiosulphate.

Novel black 1:2 metal-complex azo dyes containing chelated iron(III) insteadof chromium(III) atoms have been synthesised. Results from in vitro hazardtesting reveal that these iron complexes are satisfactory with regard to

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genotoxicity [5]. The fastness properties of these new dyes on both nylon andwool are comparable with those of their chromium analogues. It is anticipatedthat they will be strong candidates for nylon/wool dyeing in carpets orfurnishings requiring high light fastness [6].

Disperse dyes are rarely used on nylon/wool because their light fastness andwet fastness ratings are low, and the heavy stain on wool has poor fastness tolight, rubbing and perspiration. If heavy depths are dyed with disulphonatedlevelling acid dyes above the critical depth, disperse dyes can be applied with analkanol polyoxyethylene dispersant from the same bath to fill in the nyloncomponent. The disperse dyes chosen for this shading purpose are mainly low-energy monoazo or anthraquinone types. Owing to the cross-staining of woolthat inevitably occurs, the fastness of the dyeing may be impaired.

If dye partition between nylon and wool is unequal, particularly if there is anoff-tone shade variation, this can lead to a stripy appearance in plain woven ortufted carpet constructions. Further problems can arise after wear, for if there is amarked tonal difference between the fibre components in a loop-pileconstruction, the carpet begins to show pronounced local colour changes wherethe wool fibres become abraded away in worn areas [4].

Unmetallised acid dyes fade more slowly in wool than in nylon and the fadingmechanism appears to be different, probably because oxidative degradation innylon is not inhibited, whereas the reducing environment of the wool fibre has aretarding effect. The differences are also consistent with the formation of dyeaggregates in wool, which has a much higher amine end group content [7].

6.2 BLENDS OF WOOL WITH OTHER ACID-DYEABLE FIBRES

Owing to its characteristic lustre and excellent physical and chemical properties,silk has remained the traditional dress fabric for kimonos and saris in Asia. It isbecoming increasingly important in other forms of dresswear, either alone orblended with wool, cotton, nylon or polyester. Natural silk is blended in equalproportions with wool to make high-class apparel, contributing lustre andstrength. Intimate blends are usually dyed in solid shades, but it is not difficult toachieve shadow effects because of the higher initial rate of dyeing of the silkcomponent, especially at low pH. Silk yarns are also used as effect threads in finewool dress fabrics. Effect threads are preferably yarn dyed before weaving andthe wool is then cross-dyed, making allowance for staining of the silk at thisstage.

Attempts have been made to dye the wool and reserve the silk, althoughcareful dye selection is necessary and considerable skill is required to formulate awide range of shades in this way. Any cross-staining cannot be completely

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cleared without affecting the shade on the wool. Strongly acidic dyebathsincrease the depth on the silk component. Wool is favoured at higher pH valuesand at pH 5–6 the silk is almost completely reserved. Monosulphonated millingacid dyes of Mr 450–550 give the best reserve under these conditions.

If solidity is required on wool/silk blends it is necessary to dye at a low pHbelow the boil. Under these conditions, however, levelling acid and 1:1 metal-complex dyes do not give adequate fastness on the wool. It is possible to applysome selected milling acid dyes but level dyeing is difficult. Common salt and amildly cationic retarder are necessary to control the rate of dyeing. Thedistribution of direct, acid and metal-complex dyes between wool and silk hasbeen examined by dyeing simulated blends [8].

Blends of natural silk and virgin wool can be dyed at ambient temperaturewith high-reactivity dyes for the silk and then the wool is filled in with millingacid or 1:2 metal-complex dyes at pH 5–6 and 90°C, to minimise cross-stainingof the reactive-dyed silk. The fastness to light, water and alkaline perspiration ofreactive dyes from the aminofluorotriazine and α-bromoacrylamide classes issatisfactory and superior to that of copper-complex direct or 1:2 chromium-complex acid dyes on silk [9].

Besides wool, the animal fibres of interest include mohair and cashmere fromspecies of goat, alpaca and vicuna from camel species, and angora fur fromrabbits. These are relatively scarce and costly but may be blended with wool toincrease lustre and give a distinctive appearance. Blends of wool with silk,mohair, cashmere or alpaca are largely subject to the dictates of fashion. Hand-knitting yarns are luxury items and processing costs tend to be low relative toretail prices, so that more attention can be paid to high quality rather than prod-uctivity and materials cost. Typical blends for such yarns include wool/angora,wool/cashmere, wool/mohair and nylon/mohair. Some cashmere is diluted withfine wools for economic reasons but may still carry a cashmere label.

The outstanding properties of angora and cashmere in knitwear apparel arewell known. These fibres will not withstand prolonged boiling, so reproduciblecolour matching and first-class levelness are essential [10]. Angora is onlyprocessed in blends with wool, sometimes with the addition of a smallproportion of nylon to improve the durability. For economic and technicalreasons 1:2 metal-complex and milling acid dyes are preferred. Chrome dyes and1:1 metal-complex types are seldom used because strongly acidic dyebaths maydamage the angora. A simple test using CI Acid Red 18 and CI Basic Blue 9(Figure 6.1) can be used to assess the degree of oxidation damage to the angora.The bluer the staining with this mixture, the greater the degree of damage [11].In pale dyeings, any nylon present dyes more intensely, whereas in full depths it is

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the wool that gives the deeper shade. Solidity can be achieved using a retardingagent, the addition needed being dependent on the applied depth and blendproportion, particularly the amount and quality of any nylon present. Theamount of retarder required varies inversely with dye concentration.

Major outlets for wool/mohair blends are worsted outerwear and suitings.Such fabrics may be made from intimate blends for both warp and weft, but theyoften consist of a mohair warp with a botany wool weft. The enhanced lustreand good wear properties make these blends suitable for lightweight suitings anddresswear. Wool/mohair fabrics may be piece dyed with 1:2 premetallised dyes,or more economically with levelling acid dyes. If the blend is to be used insuitings, it is customary to dye the wool and mohair separately in sliver or topform for subsequent blending.

The rate of dyeing and equilibrium exhaustion on mohair fibres are higherthan for wool fibres of similar diameter. Visual and instrumental assessment ofdepth of shade, however, shows little difference between the two fibre types dyedseparately with the same dyebath concentration of an acid dye. Thesemeasurements support the view that the pronounced surface lustre associatedwith mohair is responsible for its apparently slightly lower content of absorbeddye when compared with other less-lustrous wools dyed from the same bath[12]. Mohair and wool show a similar tendency to yellow as a result of aqueousoxidation or thermal treatments. Urea-bisulphite solubility data indicate thatmohair suffers less modification under mild conditions but this position isgradually reversed with increasing severity of treatment [13]. Thus the loss inweight during aqueous treatment is ultimately greater for mohair than for wool.

The dyeing of wool/polyurethane blends has much in common with thedyeing of blends of nylon with either of these fibres. Anionic dyes are used in asimilar way with anionic agents (sections 3.2 and 3.3) to control the tendency ofmilling and 1:2 premetallised dyes to favour the polyurethane component.Chrome and metal-complex dyes generally give better wet fastness on wool/polyurethane than milling acid dyes, but 1:1 metal-complex and levelling acid

Figure 6.1 Components of stain test for damage in angora fibres

BLENDS OF WOOL WITH OTHER ACID-DYEABLE FIBRES

NaO3S N

SO3Na

NaO3S N

S N(CH3)2 X–(CH3)2N

CI Acid Red 18

H

N

O

CI Basic Blue 9

+


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dyes are unsuitable because of degradation of the polyurethane under thestrongly acidic dyeing conditions required.

Blends of acid-dyeable polypropylene staple with wool are of interest fortufted carpets, upholstery and certain apparel outlets, such as men’s socks. Bothfibres are dyeable with anionic dyes and the blend gives a full handle at lowfabric weight per unit area, only moderate formation of electrostatic surfacecharge and a high fibre–fibre frictional coefficient. This leads to softness ofhandle and a high resistance to pilling. Blends of 50:50 to 20:80 wool/acid-dyeable polypropylene are also of growing importance in Axminster carpets [1].

It is difficult to control the distribution of anionic dyes between these fibresowing to their high substantivity but slow rate of diffusion in the syntheticcomponent. At temperatures below about 75°C the polypropylene is only ring-dyed. Only at 80°C and above does diffusion into the polymer matrix becomeappreciable and dyeing of the acid-dyeable fibrillar regions begin.

The attainment of satisfactory solidity entails commencing dyeing at 75–80°Cand pH 3 to 5, according to applied depth. The initial distribution favours thepolypropylene surface but as the temperature approaches the boil the woolbecomes heavily dyed. Penetration of the acid-dyeable regions of the syntheticfibre proceeds steadily at the boil and some improvement in solidity is possible atthis stage with those dyes showing good migration properties. A weakly cationiclevelling agent of the alkylamine polyoxyethylene type is recommended withmetal-complex dyes and the level dyeing of milling acid and chrome dyes can bepromoted by addition of a nonionic agent of the alkanol polyoxyethylene class.

6.3 BLENDS OF NYLON WITH OTHER ACID-DYEABLE FIBRES

Elastomeric polyurethane yarns are mostly of interest to provide stretchproperties for characteristic nylon outlets in the knitting industry. Warp-knitswimwear and sportswear, and Raschel-knit foundation garments, underwear,surgical hose and half-hose tops, are important outlets for nylon/polyurethaneblends. Although the elastomeric fibre is often covered by the nylon in therelaxed fabric, penetration of the close construction during dyeing is essentialbecause the elastomeric fibre is revealed on stretching the fabric. Careful controlof processing tensions, as well as the pH, time and temperature of treatment, isnecessary to preserve the optimum strength and elastic properties ofpolyurethane fibres. Nylon/polyurethane fabrics are normally scoured at60–70°C with tetrasodium pyrophosphate and an anionic detergent.

Disperse dyes diffuse into polyurethane fibres even more readily than intonylon or cellulose acetate, but the wet fastness properties of the dyeings arecorrespondingly low. Selected dyes give solidity on nylon/polyurethane, but for

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acceptable wet fastness these dyes can only be used for pale depths on hosiery orwarp-knit stretch garments. The preferred dyes have good migration propertiesand are mainly monoazo yellows with anthraquinone reds and blues.

Anionic dyes are widely used for solid effects on nylon/polyurethane. Thefactors influencing partition between these components have been outlined insection 3.3. Shadow effects are of no interest because the pale-dyed polyurethaneis revealed only on stretching the fabric. Reserve or contrast effects areimpracticable. Acid dyes are most useful for bright full depths and moderatedepths of all hues. Levelling acid dyes are absorbed preferentially by thepolyurethane at 40–60°C, but migrate in favour of the nylon at highertemperatures. Some milling acid dyes favour the polyurethane considerably andthese dyes do not migrate readily to nylon.

Metal-complex dyes generally give better wet fastness on polyurethane thanmost acid dyes but 1:1 metal-complexes are unsuitable because the stronglyacidic dyeing conditions required would impair the physical properties of theelastomeric fibre. Duller and heavier depths are usually dyed with 1:2 metal-complex dyes, but the economy offered by chrome dyes is still preferred in someinstances. Chrome blacks give the best solidity and fastness. Poor migration is aproblem with 1:2 metal-complex dyes and these dyes are generally more sensitiveto dye-affinity variations in the nylon filament yarns, which often form the outersurface of a nylon/polyurethane fabric. Basic complexing agents such asethylenediaminetetra-acetic acid or hydroxylamine derivatives are used asprotecting agents to minimise the acidic degradation of polyurethane whendyeing nylon/polyurethane blends at low pH [14].

Acid dyes readily giving solid effects on nylon/polyurethane are mainly yellowto red monoazo monosulphonates and violet to blue anthraquinonemonosulphonates with generally good levelling properties but only moderate wetfastness. The wet fastness of acid dyes on nylon/polyurethane can be improvedwith syntan aftertreatment. Alternatively, better wet fastness but only moderatemigration and coverage properties can be achieved using selected yellow to redmonoazo disulphonates, red to blue disazo disulphonates and blue to greendisulphonated anthraquinone dyes. The preferred 1:2 metal-complex dyes aremonoazo types with no more than one solubilising group in general. Most of thechrome dyes suitable for this blend are monoazo structures. There are moremonosulphonates than disulphonates, but some have one or two additionalcarboxyl groups, one of which usually participates in complexing with thechromium atom.

Many of the problems encountered in one-bath dyeing methods for wool/silkblends (section 6.2) are even more critical on nylon/silk. Experimental disperse

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NR

RN N SO2CH2CH2OSO3H R CH3 or CH2CH2OH

Figure 6.2 Experimental reactive disperse dyes for nylon/silk blends

Table 6.1 Dye selections for AA blends

Colour Dyeing DyeBlend effect method selection

Wool/silk Solid Two-stage Reactive dyes on silk, then milling acidand 1:2 metal-complex dyeson wool at 90°C

Silk Single-class Monosulphonated milling acid dyesreserve at pH 5–6

Wool/mohair Solid Single-class Levelling acid or 1:2 metal-complex dyes

Wool/angora Solid Single-class Milling acid and 1:2 metal-complex dyeswith anionic retarder

Nylon/wool Solid Single-class Monosulphonated 1:2 metal-complexand acid dyes with anionic retarder

Wool/polyurethane Solid Single-class Milling acid and 1:2 metal-complex dyeswith retarder

Nylon/polyurethane Solid Single-class Chrome, 1:2 metal-complex or millingacid dyes with retarder

Wool/acid-dyeable Solid Single-class Chrome, 1:2 metal-complex orpolypropylene milling acid dyes at pH 3.5

reactive dyes of the sulphatoethylsulphone type (Figure 6.2) have been evaluatedby applying them to nylon and silk fabrics simultaneously [15]. Maximumuptake occurred at pH 8 on silk and at pH 6–8 on nylon. Optimum soliditybetween the two fibres was found at pH 6. The ratio of fixation to exhaustionwas very high. Exhaustion and fixation increased slowly with dyeing time andtemperature, even after 4 hours at 90°C, and the rates of dyeing were similar onthe two fibres. Regrettably, no fastness values were recorded for these dyeings.

6.4 DYEING METHODS AND DYE SELECTION FOR AA BLENDS

The preferred dyeing method for all binary blends of acid-dyeable fibres is theapplication (Table 6.1) of a single class of anionic dyes, usually premetallised 1:2

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or milling acid types, with an anionic retarder to control uptake by thecomponent that is dyed preferentially. This component is wool in its blends withsilk or angora, but the other component in wool blends with mohair, nylon,polyurethane or acid-dyeable polypropylene. In nylon/polyurethane blends it isthe polyurethane that is preferentially dyed in the early stage. It is possible toreserve silk in its blends with wool or nylon but in general there is virtually nointerest in shadow, reserve or contrast effects on AA blends.

6.5 REFERENCES 1. S Roberts, Dyer, 178 (Jun 1993) 10.

2. J Park, Carpet Manufacturer Int., (Autumn 1987) 21. 3. D Schwer, H Ritter and K Zesiger, Textilveredlung, 16 (1981) 479.

4. T L Dawson, Rev. Prog. Coloration, 15 (1985) 29.

5. H S Freeman, J Sokolowska-Gajda, A Reife, Z D Claxton and V S Houk, AATCC International

Conference and Exhibition, (Oct 1993) 254.

6. J Sokolowska-Gajda, H S Freeman and A Reife, AATCC International Conference and

Exhibition, (Oct 1994) 388. 7. H L Needles and I Weatherall, Text. Chem. Colorist, 24 (Dec 1992) 7.

8. J H Qian and Z T Song, Proc. 7th Int. Wool Text. Res. Conf., Tokyo (1985) 249.

9. R Rohrer, Textilveredlung, 20 (1985) 85.10. J A Galek, Dyer, 163 (22 Feb 1980) 133.

11. F Sakli, M van Parys, R Dubois and J Knott, Melliand Textilber., 69 (1988) 191.

12. M B Roberts and E Gee, SAWTRI Bull., 11 (Sep l977) 32.13. M B Roberts, SAWTRI Tech. Report, 351 (1977).

14. D Schwer, Textilveredlung, 23 (1988) 296.

15. M Dohmyou, Y Shimizu and M Kimura, J.S.D.C., 106 (1990) 395.

DYEING METHODS AND DYE SELECTION FOR AA BLENDS

86 WOOL/ACRYLIC AND OTHER AB BLENDS

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86

CHAPTER 7

Wool/acrylic and other AB blends

7.1 DYEING OF WOOL/ACRYLIC BLENDS

The dimensional stability, strength and abrasion resistance of wool/acrylic blends(usually 50:50 to 20:80) are superior to those of all-wool fabrics. They are ofparticular interest for garments in which thermal insulation is important, as inwoven or knitted sweaters, skirts and outerwear. Blends containing the coarserquality wools are used in blankets and floor coverings. Acrylic fibres dyed in gelform or as loose stock before blending with wool are stable to conventional woolprocesses, such as milling, decatising and blowing. Acrylic fibres may also beblended with mohair, angora or silk to lower costs and improve physicalperformance.

Wool/acrylic blends may be carbonised to remove vegetable debris from thewool and it is sometimes possible to carbonise after dyeing. Basic dyes showinglittle or no change of hue when subjected to carbonising with sulphuric acid,followed by prompt neutralisation, are mainly yellow, orange and red methinedyes with selected azo reds and anthraquinone blues. Certain other basic dyeschange colour substantially on carbonising, however. Although a subsequenttreatment with ammonia at 40°C will fully restore the original hue there isinvariably a loss in depth.

Worsted-spun wool/acrylic yarns are scoured at pH 5 and 60°C with anonionic detergent. Woollen-spun yarns are scoured at 30°C with soda ash andan anionic detergent. Blend yarns containing high-bulk acrylic fibres should befully relaxed before dyeing by autoclave treatment in saturated steam or byimmersion in boiling water in the dyeing vessel. Blend fabrics are prepared fordyeing by conventional techniques for all-wool materials but precautions shouldbe taken to allow for the thermoplastic properties of the acrylic fibres.Relaxation to remove inherent tensions is important, particularly if the fabric isto be subsequently steamed or pleated.

Blends in the AB category are of particular interest for reserve or colour

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contrast effects because the fibre components are dyeable with ionic dyes ofappropriate charge. Good solidity is also important, however, and can beobtained without difficulty when required. Although either component can bereserved, it is often more convenient to reserve the acrylic fibre using anionicdyes. The range of bright colour contrasts is much wider on AB blends than onall other types of binary blend because the fibres carry opposite charges and ionicdyes are much more selective than disperse dyes. The opposite charges carried bythe dyes, however, can lead to incompatibility in one-bath dyeing (section 4.3).

Disperse dyes are of little interest for the acrylic component of wool/acrylicblends because of inadequate fastness and severe staining of the wool. It isnecessary that the dyed material has adequate fastness to pleating, pressing andironing, particularly in the case of knitwear and jersey fabrics. Combinations ofdisperse and acid dyes can only be used in pale depths. Because of the relativelyslow rate of absorption of disperse dyes by acrylic fibres, these often give rise toconsiderable staining of wool (section 3.4). There is markedly less cross-stainingby basic dyes because these have much higher affinity for acrylic fibres.Dispersion stability is much more important in package or beam dyeing, as is thelimited efficiency of clearing treatments.

Wool/acrylic blends can be readily dyed to reserve the acrylic fibre. It is lessconvenient to reserve the wool because of the cross-staining by basic dyes(section 3.5). Solidity of shade is often required in dyeing these blends fordresswear or knitwear and is invariably specified for carpet yarns. Solidity isreadily obtained by applying combinations of anionic and basic dyes with ananti-precipitant, either at pH 2–3 for 1:1 metal-complex types or at pH 6–7 formilling acid dyes or 1:2 metal-complexes, with or without sulpho groups.

Cationic retarders are not required, except for pastel shades, as the anionicdyes exert a marked retarding effect. Mildly cationic agents of the alkylaminepolyoxyethylene type form water-soluble complexes with 1:1 metal-complexdyes under strongly acidic conditions. The alternative method of improvingcompatibility by complexing the basic dyes with an anionic retarder gives bettercontrol of the rate of dyeing of the acrylic component (section 4.3).

For pale depths the dyebath is set at 50°C and pH 4–5 with acetic acid, salt,an anionic retarder and an alkanol polyoxyethylene anti-precipitant. The anionicand basic dyes are added separately and then the temperature is raised and heldat 90°C to slow down the rate of uptake of the basic dyes. Finally, the woolcomponent is dyed to shade with the anionic dyes at the boil. Bettercompatibility is found in intermediate depths by commencing at 50°C and pH 6–7 with salt, nonionic anti-precipitant and neutral-dyeing anionic dyes. After

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raising slowly to 80°C to promote level dyeing of the wool, acetic acid (to givepH 4–5), an anionic retarder and the basic dyes are added and dyeing of theacrylic component completed at the boil.

In those instances where adding the basic dyes at 80°C gives levellingproblems, particularly when 1:1 metal-complex dyes are being used on woolwith an alkylamine polyoxyethylene complexing agent, it may be preferable toset the initial dyebath at 60°C and pH 2–3 (sulphuric acid) with the basic dyesand complexing agent, and to raise the temperature to 80°C before adding thepremetallised dyes. Under these conditions the rates of dyeing of the two classesof dyes are more closely synchronised in the final stage at the boil. Thezwitterionic character of the 1:1 metal-complexes at low pH confers greatercompatibility with basic dyes than the more anionic neutral-dyeing dyes.

At one time, wool/acrylic blends were mainly dyed by two-bath methods inmedium or heavy depths to avoid cross-staining of wool or dye precipitationproblems. Two-stage processes are now usually employed with the basic dyesapplied first at the boil, followed by cooling to 60°C, adding the anionic dyesand completing the wool dyeing at the boil. Heavy depths may still be obtainedby a two-bath sequence with the basic dyes and a cationic retarder at the boil andpH 5 in the first stage. After an intermediate scour with nonionic detergent at80–90°C, or with acidified formaldehyde-sulphoxylate at 70–75°C if necessaryto clear the basic dye stain, the wool component may be dyed to shade at the boiland pH 6–7 with 1:2 metal-complex or milling acid dyes in the presence of analkylamine polyoxyethylene levelling agent.

The degree of staining of conventional acrylic fibres by reactive, premetallisedor milling acid dyes is very slight and does not present a problem in the dyeing ofwool/acrylic blends. Because of this, when dyeing union fabrics where light anddark contrasting colours are present in the design, e.g. black/yellow, it is usual todye the wool to the darker colour where possible [1]. On the other hand, sincewool contains amino acid residues with carboxyl-containing sidechains, as wellas C-terminal end groups that are ionised under the mildly acidic dyeingconditions usually used for these blends, sites are available for the sorption ofbasic dyes (Figure 7.1). Thus there will always be some sorption by wool and theextent to which this occurs is of considerable practical interest. At the boil,decomposition products of the wool are produced, particularly at pH 7 or above,that have a reducing action on certain basic dyes, thus necessitating careful dyeselection [2].

Basic dyes of the localised-charge monoazo and anthraquinone types arewidely used, but cyanine and oxazine types are also important. The staining ofwool by basic dyes is particularly troublesome in the early stage of dyeing at

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DYEING OF WOOL/ACRYLIC BLENDS

NH

CH

CO

CH2COO–CONH CH COO–

R

Aspartic acid residue C-terminal residue

Figure 7.1 Sites in wool for sorption of basic dyes

temperatures below 80°C. As the boil is approached, transfer of basic dyes fromwool to the acrylic component proceeds, so that the wool exerts an effectiveretarding influence on the acrylic dyeing process (section 3.5). This migrationand hence the final distribution of the basic dyes depends on many factors,including the presence of surfactants (anti-precipitants and levelling agents),electrolyte, time, temperature and pH. Thus the degree of wool staining isdetermined by dyebath conditions and is minimised by dyeing for at least onehour at the boil and pH 5. Nonionic auxiliaries promote transfer to the acrylicfibre but cationic products tend to increase staining of the wool.

Certain basic dyes of small molecular size exhibit exceptionally goodmigration properties and are reasonably compatible with anionic dyes, allowingminimal use of nonionic anti-precipitant [3]. These dyes have proved especiallysuitable for dyeing wool/acrylic blends. IWS fastness requirements must be metfor machine-washable performance of shrink-resist garments made from theseblends [4]. Dye selection for both components is important. Basic dyes should beused on the acrylic fibre and reactive, premetallised or chrome dyes for the wool.Mordant dyes are usually chosen for black and navy blue on economic grounds,a pH of 3.5 being used for chroming.

The use of basic dyes and α-bromoacrylamide or vinylsulphone reactive dyesis a popular one-bath method, as a wide range of bright hues can be obtainedwith optimum fastness [4,5] and simultaneous yarn bulking and dyeing ispossible. Pale dyeings can be produced with mixtures of reactive or selected 1:1metal-complex dyes and basic dyes at pH 4–5 together with special levellingagents that also function as precipitation inhibitors. In medium and full depths,ammonia treatment after dyeing is recommended to complete the fixation ofreactive dyes on the wool and give dyeings of good fastness to perspiration [6].

Problems of interaction with basic dyes are encountered in the one-bathdyeing of these blends in the following order of increasing difficulty: chrome <reactive < 1:1 metal-complex < levelling acid < milling acid < sulphonated 1:2metal-complex < unsulphonated 1:2 metal-complex. Hence chrome and reactivedyes give the best compatibility with basic dyes and can be used for wool in the


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isoelectric region at pH 4–5. The high degree of exhaustion of chrome dyes leadsto virtually complete avoidance of precipitation. With chrome dyes the oxidativechromium compounds present in the aftertreatment bath help to counteract anytendency for the basic dyes to be decomposed by any reductive products formedin wool at the boil.

7.2 DYEING OF NYLON/ACRYLIC BLENDS

Nylon/acrylic blends are used mainly for half-hose, knitted sweaters, sportswearand swimwear, blankets, furnishing fabrics and floor coverings. These have awool-like handle and appearance, but better tensile strength, abrasion resistanceand durability. Nylon contributes more to the strength, extensibility, wrinklerecovery and resistance to wear, whilst the acrylic fibre confers softness, bulk andwarmth. Blends of acrylic fibres with nylon are more durable but less bulky thanwool/acrylic blends.

Nylon/acrylic blends (typically 20:80) have been popular in carpets for manyyears, particularly for Axminster designs [7]. The physical characteristics ofDralon U325(BAY) acrylic fibre in blended carpet yarns have been described.This modified fibre has high substantivity for basic dyes and virtually completeexhaustion of the dyebath can be achieved in a short time at 80–85°C [8]. It issuitable not only for yarn dyeing but also for piece dyeing on a carpet winch orjet machine. Blends containing 20–30% nylon for carpets can be dyed onwinches or atmospheric jets with basic and acid dyes in the presence of a cationicretarder, a migration assistant and a nonionic anti-precipitant.

Thus velour carpets tufted from 30:70 nylon/Dralon yarn can be dyed by firstapplying acid dyes to the nylon as the temperature is raised to 70°C, then addingthe basic dyes and heating to 80–85°C to dye the Dralon U325 component.Although these carpets have good resistance to pile deformation during dyeing,slow indirect cooling to 60°C is recommended at the end of the dyeing cycle [8].

Nylon/acrylic apparel fabrics are normally scoured with dilute ammonia andan alkylphenol polyoxyethylene detergent at 50–60°C and then dyed by a one-bath method. Thus selected levelling acid dyes may be complexed with a weaklycationic alkylamine polyoxyethylene and added to a bath containing Glauber’ssalt and acetic acid at pH 4–5 and 40°C before the basic dyes. After the acid dyeshave become absorbed by the nylon at 80–85°C, the dyebath is heated to the boiland the acrylic component dyed to shade with the basic dyes. Any cross-stainingof nylon by the basic dyes may be cleared using acidified formaldehyde-sulphoxylate at pH 4–5 and 70°C.

An alternative one-bath method is based on a similar temperature/time cyclebut the initial dyebath is set at pH 2–3 (sulphuric acid) with the basic dyes, an

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alkanol polyoxyethylene sulphate complexing agent and an alkanol polyoxy-ethylene anti-precipitant. When the temperature reaches 80°C, selected mono- ordisulphonated monoazo 1:1 chromium-complexes are added and heatingcontinued slowly to the boil.

Full depths are usually dyed by a more prolonged two-stage process. Theacrylic fibre is first dyed with the basic dyes at the boil and pH 4–5. The dyebathis then cooled slowly to 70°C and adjusted to pH 2–3 with sulphuric acid. The1:1 metal-complex dyes and an alkylamine polyoxyethylene complexing agentare added and the nylon component dyed to shade at the boil.

The opposite sequence is preferred for full depths dyed with 1:2 metal-complex dyes. The nylon is dyed first at the boil with the selected premetalliseddyes in dilute ammonia and the alkanol polyoxyethylene sulphate complexingsystem. After cooling the dyebath slowly to 80°C, the acrylic fibre is dyedconventionally with basic dyes at the boil and pH 4–5. If a two-bath method ispreferred for optimum fastness and freedom from any risk of co-precipitation,the basic dyes are applied first at the boil and pH 4–5. After an intermediaterinse, a fresh dyebath is set at pH 6–7 and the nylon is dyed at 80–85°C withneutral-dyeing 1:2 metal-complex or milling acid dyes.

Polyurethane/acrylic blends for sweaters and leisurewear are usually madefrom core-spun yarns and complete solidity is not essential. Basic dye stains showlow fastness on the polyurethane component, however, so that it is oftennecessary to use a two-bath method. Selected basic dyes are applied to the acrylicfibre at the boil from a near-neutral bath and an intermediate clear is given usingsodium dithionite, soda ash and a nonionic detergent. The polyurethane is thenfilled in with selected neutral-dyeing 1:2 metal-complex or milling acid dyes.Some chrome dyes are also suitable on the polyurethane if heavy, dull dyeings arerequired.

7.3 BLENDS OF ACID-DYEABLE AND BASIC-DYEABLE ACRYLICVARIANTS

Conventional acrylic fibres made from acrylonitrile and up to 15% of an inertcomonomer (section 5.1) are readily dyeable with basic dyes at the boil. Acid-dyeable acrylic variants contain basic comonomer units that provide sites forsorption of anionic dyes. Blends of acid-dyeable and basic-dyeable acrylic fibresare used in typical wool outlets, such as jersey dresswear, sweaters, hand-knittingyarns, blankets and pile fabrics.

Basic-dyeable and acid-dyeable acrylic fibres both absorb basic dyes underneutral conditions, but the uptake of basic dyes by the acid-dyeable variantdecreases markedly as the pH falls from 7 to 2. The exhaustion of these dyes on

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basic-dyeable acrylic fibres is almost independent of pH over this range. Basicand acid dyes, therefore, can be applied selectively to the two types of fibre at pH2 because most basic dyes are stable and reserve the acid-dyeable variantreasonably well under these conditions. Chlorite bleaching of blends containingacid-dyeable acrylic fibres results in increased uptake of basic dyes. It is usual toprescour with an alkanol polyoxyethylene at 60–70°C before dyeing.

Blends of acid-dyeable and basic-dyeable acrylic fibres offer interestingpossibilities for solid, reserve or colour contrast effects. The basic-dyeablevariants can only be dyed with disperse or basic dyes, whereas acid-dyeable typescan be dyed with either of these classes as well as direct, levelling acid,premetallised and chrome dyes. The inherent wet fastness of anionic dyes onacid-dyeable acrylic fibres is far superior to that of the same dyes on wool ornylon, or of direct dyes on cellulosic fibres [9].

The 1:2 metal-complex dyes are not used extensively but selected monoazo1:1 metal-complex types (Mr 450–550) are widely used for deep shades of goodfastness to light and wet treatments. These and the levelling acid dyes for brightershades are applied with sulphuric acid at pH 2–3 to obtain full yield, penetrationand fastness. Chrome dyes of the monoazo monosulphonate type (Mr 350–450)can also be used for dark shades of good fastness to light, washing and pleating.Chrome dyes sensitive to low pH should be avoided, as a strongly acidic dyebathis essential for optimum yield.

Disperse dyes with good dyeing properties and adequate pleating fastness onacid-dyeable acrylic fibres are the intermediate-energy nitro, monoazo andanthraquinone types (Mr 300–400). Basic dyes exhibit similar fastness propertieson both types of acrylic fibre but give higher yields and dye more rapidly on thebasic-dyeable fibre, requiring a cationic retarder to control the rate ofabsorption. Shadow effects are achieved at neutral pH but at pH 2 all of thebasic dyes are absorbed by the basic-dyeable variant, leaving the acid-dyeablefibre reserved. Basic dyes stable to strongly acidic conditions and giving effectivereservation of the acid-dyeable variant are almost all delocalised-charge types,including yellow, orange and red methines or monoazothiazole derivatives andoxazine blues.

Since basic-dyeable acrylic fibres have no affinity for acid dyes it is possible toreserve them in blends with acid-dyeable variants. The wet fastness of acid dyeson these fibres is superior to that on wool but light fastness is sometimes inferior.Levelling acid dyes that offer optimum light fastness and good reservation of thebasic-dyeable component are mainly mono- or disulphonated monoazo oranthraquinone derivatives.

The recommended method of application is at the boil and pH 2–3 with

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sulphuric acid and Glauber’s salt to obtain adequate penetration and fastness ofthese dyes. Careful control of temperature rise is necessary because the rate ofstrike is rapid and the migration properties on these fibres are relatively poor. Analkylamine polyoxyethylene levelling agent helps to decrease the initial rate ofdyeing. The dyebath is cooled back to 80°C before shading additions are made.

Reserve of the acid-dyeable component is obtained with basic dyes and asuitable anionic retarder at pH 2. Dyes of the methine, cyanine, monoazo,oxazine and anthraquinone types are usually satisfactory. Colour contrasts canbe produced on these blends with suitable combinations of basic and levellingacid dyes by a one-bath method, although the most economical effects are thosewith a deep shade on the acid-dyeable type and a paler depth on the basic-dyeable component.

Complexing between the basic dyes and a combination of anionic andnonionic anti-precipitants, or between the acid dyes and a weakly cationicalkylamine polyoxyethylene (section 4.3), must be adopted in order to minimisethe risk of co-precipitation and each of these measures exerts a retardinginfluence on the corresponding class of dyes. Specific retarding agents for thebasic dyes should be avoided if possible when dyeing solid or contrast effects onthis type of blend. Cationic retarders are preferentially absorbed by the basic-dyeable fibre and this may impair the development of crimp in high-bulk yarns.Absorption of an anionic retarder by the acid-dyeable variant may causerestraining of the acid dyes in heavier depths.

The levelling acid dyes are applied at 80°C and pH 2 with Glauber’s salt andan alkanol polyoxyethylene anti-precipitant. The basic dyes are then added andboth components dyed to shade at the boil. Scouring at 70°C with an alkanolpolyoxyethylene detergent clears any stain of basic dyes from the acid-dyeablevariant. Full depths are dyed by a two-stage method. The acid-dyeablecomponent is first dyed at the boil and pH 2–3 with the levelling acid dyes andsalt. After cooling to 60–70°C, the basic dyes, nonionic anti-precipitant andmore salt are added and the basic-dyeable fibre is dyed to shade at the boil.

7.4 BLENDS OF MODACRYLIC AND ACRYLIC FIBRES

Modacrylic fibres, e.g. Dynel (Union Carbide), that contain less than 85%acrylonitrile with other inert or basic comonomers, form blends with acrylicfibres that are mainly used in traditional wool outlets. Blends of Dynel with anacrylic fibre in pile fabrics and floor coverings are less flammable than the acrylicfibre alone. There is a legal requirement to have at least 15% modacrylic fibre inan acrylic carpet. A blend of 50:50 modacrylic/acrylic fibres may be dyed by a

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one-bath method at the boil with disperse dyes for the modacrylic componentand basic dyes for the acrylic fibre. A dependent range of colour contrasts withlight fastness 5 can be obtained in this way, although the fastness ratings of thebasic dyes on acrylic are approximately 1 to 1.5 units higher than the dispersedyes on the modacrylic fibre.

Dynel is delustred during dyeing at temperatures above 80°C but the lustrecan be restored by subsequent treatment at a higher temperature, i.e. by drying at120–130°C or by hydrosetting. For example, after dyeing at 80°C the lustre isrestored at 105°C. After dyeing at the boil a treatment at 120°C is required, andif dyed under pressure at 105°C the lustre returns at 130°C. Increased quantitiesof salt in the dyebath may also be used to maintain lustre in circumstances wherethe adoption of higher temperatures in drying or hydrosetting is not acceptable.

Blends of modacrylic fibres with conventional acrylic fibres may be dyed insolid or shadow effects with basic dyes applied to both components by a two-stage method. The acrylic component is dyed preferentially by temperaturecontrol in the absence of a retarder, raising slowly to the boil to avoidunlevelness. When the target depth has been reached on the acrylic fibre, thedyebath is cooled to 80°C, a butyl benzoate carrier is added and the modacrylicfibre dyed to shade at the lower temperature. Blends of modacrylic fibres withacid-dyeable acrylic variants provide more scope for reserve and contrast effects.Selected disperse dyes will give satisfactory solidity in pale or medium depths butfastness ratings are barely adequate. Reserve effects or bright colour contrasts inmoderate or full depths are obtained by methods similar to those alreadyoutlined for acid-dyeable/basic-dyeable acrylic blends (section 7.3).

7.5 BLENDS OF AMIDE FIBRES WITH MODACRYLIC OR ACID-DYEABLE ACRYLIC VARIANTS

Further AB blends of minor importance are those containing an amide fibre(wool, silk, mohair or nylon) with a modacrylic or acid-dyeable acrylic variant.Dynel (Union Carbide) is a modacrylic fibre made from acrylonitrile and vinylchloride. The controlled shrinkage properties of the fibre can be turned topractical use in the manufacture of bulked fabrics and of backing yarns in knittedpile fabrics. Dynel has been blended with amide fibres and used in pile fabricsintended for apparel or furnishings.

Blends containing 75:25 Dynel/mohair are scoured and dyed at 80°C, carebeing taken to avoid fabric distortion. Basic dyes are used on the modacrylicfibre and then milling acid dyes on the mohair. Butyl benzoate is recommendedto give adequate colour yields on Dynel at 80°C. Pale and many medium depthscan be achieved by a two-stage sequence but for deep shades it is advisable to

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operate a two-bath method with an intermediate clear of the basic dyes from themohair. The basic dyes must be selected for minimum staining of mohair.Clearing is carried out at 60°C with a mildly acidic solution of zincformaldehyde-sulphoxylate (Figure 7.2).

Figure 7.2 Zinc formaldehyde-sulphoxylate

BLENDS OF AMIDE FIBRES WITH ACRYLIC VARIANTS

Possible dyed effects on Dynel/wool include solidity of shade, shadow orcontrast effects, normally obtained by one-bath methods. Selected basic dyes areapplied to the modacrylic fibre and 1:2 metal-complex or milling acid dyes to thewool. A butyl benzoate carrier for Dynel, a levelling agent and an anti-precipitant are also required. The fabric is first treated with these agents at 40°Cand the dissolved anionic dyes added. The temperature is raised to 70°C, the pHlowered to 4–5 (acetic acid) and the basic dyes added gradually before raising thetemperature slowly to the boil.

Verel (Eastman) is a modacrylic fibre of high flame resistance and a softhandle. It has been blended with wool for use in pile fabrics and floor coverings,in 30:70 blends with wool in knitwear and in 50:50 blends for half-hose. Thedyes used include disperse, basic and 1:2 metal-complex dyes. The premetalliseddyes give satisfactory fastness to light and washing for apparel or furnishingfabrics. Verel is preferably dyed at 80–90°C, or at 70–80°C in the presence of anorganophosphate carrier. If it is dyed at or near the boil, however, loss of lustreand deformation of the fibre take place, resulting in fabric creasing. Verel andwool can be dyed simultaneously with selected 1:2 metal-complex dyes. Woolcan also be dyed and Verel reserved using 1:1 metal-complex types applied at alow pH.

Fabrics containing 50:50 Dynel/nylon may be dyed in colour contrasts or withreservation of the modacrylic component. Those acid dyes for nylon with leastaffinity for Dynel, mainly disulphonated premetallised or milling acid dyes of Mr

700–800, are preferred, a levelling agent being recommended with the 1:2 metal-complex types. Suitable basic dyes for Dynel showing good reserve of nylon aredelocalised-charge structures of the azomethine, azothiazole and azotriazoleclasses.

The nylon component of blends with acid-dyeable acrylic fibres may be dyedwith selected acid dyes that show good neutral-dyeing affinity to reserve theacrylic variant. Nylon can be almost completely reserved by applying basic dyesin the presence of 3% urea after dissolving with the aid of methanol or anonionic surfactant. The dyebath pH changes from slightly acidic to slightly

O S O CH2OHHO– Zn2+


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COO CH2CH2 OOC COO

SO3H

CH2CH2 OOC

COO CH2CH2 OH HO

SO3H

CH2CH2 OOC

HOOC COOH

+ +

Scheme 7.1

alkaline during dyeing. Solid or contrast effects are achieved by a one-bathmethod with selected milling acid and basic dyes in the presence of a nonionicanti-precipitant.

In general, acid-dyeable acrylic fibres will readily absorb the dyes usuallyapplied to wool but the basic groups in the acrylic variant are less strongly basicthan those in wool and adequate dyebath exhaustion requires appreciably higherconcentrations of acid. Under normal wool-dyeing conditions a heavier depth isattained on the wool, giving a shadow effect. Solidity of shade can be achieved,however, by adding a nonionic agent that controls the rate of dyeing of the woolas well as promoting levelness. The most critical factor is the acid concentrationneeded to achieve solidity in the presence of the retarder. Suitable dyes includeselected milling acid or chrome dyes, as well as certain premetallised types.

Wool may be dyed and the acid-dyeable acrylic reserved by applying millingacid or 1:2 metal-complex dyes with good neutral-dyeing affinity in the presenceof a weakly cationic levelling agent. The best reserve effect is obtained at90–95°C. Basic dyes can be applied to the acrylic component at low pH if alimited range of colour contrasts is desired.

7.6 BLENDS OF BASIC-DYEABLE POLYESTER WITH WOOL OR NYLON

Although more costly than normal polyester, the basic-dyeable variant (section5.1) can be used in blends with nylon or wool to achieve a wider and moreattractive gamut of coloured effects, since the problems of cross-staining of woolor nylon by disperse dyes are absent. One-bath methods using basic and aciddyes are available for bright colour contrasts or reserve styles. Solid or shadoweffects can also be obtained without difficulty, but are less often required.Disperse dyes show lower light fastness on the basic-dyeable variant than on

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Table 7.1 Dye selections for AB blends

Blend Colour effect Dyeing method Dye selection

Wool/acrylic Acrylic Single-class Reactive, metal-complex or millingreserve acid dyes

Solid or One-bath Premetallised or milling acid dyescontrast (pale depths) and basic dyes with anti-precipitant

Two-stage Basic dyes with retarder, then 1:2(full depths) metal-complex or milling acid dyes

One-bath Reactive dyes and migrating basic(machine- dyes with levelling/stabilisingwashable) agents

Nylon/acrylic Solid or One-bath Levelling acid or 1:1 metal-complexcontrast (pale depths) dyes and basic dyes with

anti-precipitant

Two-stage Basic dyes at pH 4–5, then pre-(full depths) metallised dyes at appropriate pH

Polyurethane/ Solid Two-bath Basic dyes, reduction clear, then 1:2acrylic metal-complex or milling acid dyes

Wool/modacrylic Solid or One-bath Premetallised or milling acid dyescontrast and basic dyes with carrier and

anti-precipitant

Mohair/ Solid or Two-stage Basic dyes with carrier, then millingmodacrylic contrast acid dyes at pH 6–7

BLENDS OF BASIC-DYEABLE POLYESTER WITH WOOL OR NYLON

normal polyester and basic dyes on basic-dyeable polyester are significantly lessfast to light than on acrylic fibres.

A blend of basic-dyeable polyester and nylon can be dyed with excellentcontrast by the following one-bath method. After scouring at 60–70°C withsoda ash and a nonionic detergent, the dyebath is prepared at 50°C and pH 5with an alkanol sulphate as a retarder for the basic dyes, an alkanolpolyoxyethylene anti-precipitant, Glauber’s salt to inhibit hydrolysis of the basic-dyeable variant (Scheme 7.1), and finally the acid dyes and the basic dyesseparately. The temperature is raised slowly from 80°C to the boil and the twocomponents dyed simultaneously. Similar methods have been devised for blendsof basic-dyeable polyester with wool [10]. A reserve effect on either fibre can beobtained using the above procedure by simply omitting the appropriate class ofdyes from the recipe. When using basic dyes only, the reserve of the wool ornylon can be improved by a final treatment with sodium dithionite, ammoniaand a nonionic detergent at 50°C.


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Nylon/modacrylic Modacrylic Single-class Disulphonated 1:2 metal-complexreserve or milling acid dyes

Contrast One-bath Premetallised or milling acid dyesand basic dyes with anti-precipitant

Acid-dyeable/ Acid-dyeable Single-class Delocalised-charge basic dyes withbasic dyeable reserve anionic retarder at pH 2acrylic

Basic-dyeable Single-class Monoazo 1:1 metal-complex orreserve levelling acid dyes at pH 2–3

Solid or One-bath Levelling acid dyes and basic dyescontrast (pale depths) with anti-precipitant at pH 2–3

Two-stage Levelling acid dyes, then(full depths) basic dyes

Modacrylic/acrylic Solid Single-class Basic dyes at the boil, then at 80°Cwith carrier

Solid or One-bath Disperse dyes and basic dyescontrast

Wool/acid-dyeable Acrylic Single-class Premetallised or milling acid dyesacrylic reserve at 90°C

Solid or Single-class Selected chrome, metal-complex orshadow milling dyes

Limited Two-stage Basic dyes at low pH, thencontrast premetallised or milling acid dyes

at pH 7

Nylon/acid-dyeable Acrylic Single-class Premetallised or milling acid dyesacrylic reserve at pH 6–7

Nylon Single-class Basic dyes with ureareserve

Solid or One-bath Milling acid dyes and basic dyes withcontrast anti-precipitant

Wool or nylon/ Nylon Single-class Basic dyes with cationic retarderbasic-dyeable reservepolyester

Polyester Single-class Premetallised or milling acid dyesreserve at pH 6–7

Contrast One-bath Acid dyes and basic dyes withanti-precipitant

Table 7.1 Continued


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DYEING METHODS AND DYE SELECTION FOR AB BLENDS

7.7 DYEING METHODS AND DYE SELECTION FOR AB BLENDS

This range of versatile blends offers valuable opportunities for bright colourcontrast, shadow or reserve effects with relative freedom from cross-stainingproblems. One-bath methods, normally using basic dyes and acid dyes with anonionic anti-precipitant, are available in most cases to give solid or contrasteffects, although two-stage procedures may be preferred for full depths on thevarious blends with acrylic variants, the latter component being dyed first (Table7.1). Excellent white reserve effects are attainable on all the synthetic fibrecomponents in these blends. Reactive dyes are recommended to meet machine-washable standards on wool/acrylic blends.

7.8 REFERENCES 1. D R Lemin. J.S.D.C., 91 (1975) 168.

2. W Haertl, Textil Praxis, 44 (1989) 285; Melliand Textilber., 70 (1989) 354; Textilveredlung, 24(1989) 214.

3. R Parham, Am. Dyestuff Rep., 71 (Sep 1982) 42.

4. H Flensberg and A Laepple, Textilveredlung, 26 (1991) 342.

5. R Hüls, Textilveredlung, 10 (1975) 399. 6. W G Prinzel, Textilveredlung, 18 (1983) 230.


8. R Block and J Honsel, Chemiefasern und Textilind., 34/86 (1984) 345. 9. K Nagawa, Japan Textile News, No. 251 (Oct 1975) 74.

10. J Park and S Davis, J.S.D.C., 89 (1973) 37.

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CHAPTER 8

Wool/cellulosic and other AC blends

8.1 DYEING OF WOOL/CELLULOSIC BLENDS

The wool/cotton blend is superior in durability to all-wool fabrics but there is aloss in other desirable characteristics, such as handle, drape, pleat retention andcrease recovery. There is a resurgence of interest in blending these two naturalfibres throughout the developed world, where such blends in garmentstraditionally made from cotton are seen as conferring desirability and exclusivityin high-quality dresswear and shirting fabrics [1]. Developments in shrink-resisttreatment of the wool component have greatly improved the washability of suchmaterials, which offer value, comfort, versatility and styling.

Typical examples of traditional union fabrics with a cotton warp and awoollen weft include blazer cloths, gabardine rainwear, shirtings and pyjamas.The traditional ‘linsey-woolsey’ fabric, closely woven for household or appareluses, was made from a linen warp to give strength and a wool weft to provide theaesthetic qualities of the construction. Blended worsted yarns containingapproximately equal proportions of wool and cotton have been long-establishedin knitwear, dresswear, underwear, children’s clothing, lightweight shirtings,pyjama cloths and blankets. For washable wear such blends have been stabilisedby a gaseous chlorination treatment. The original Viyella shirt fabric was a wool/cotton blend.

The 20:80 wool/cotton yarn is the best for achieving washable apparelwithout the use of chlorinated wool. Draw blending and intimate blending yielddifferent fabric properties but both give satisfactory dyeing and finishingperformance in woven or knitted constructions. Finishing techniques andformulations have been optimised to provide fully washable performance witheasy-care and low shrinkage properties [2].

Attractive pile fabrics are made with a cotton backing cloth and a worsted-spun pile yarn of either mohair or wool. Mohair gives higher lustre and bettercrush resistance than wool. Fabrics of this kind have been used to cover toys andas outerwear. Grey cotton is often chosen as a backing cloth unless a white is

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specified, when the fabric is peroxide bleached to minimise any risk of damage tothe pile. After scouring on the winch, mohair pile fabrics are dyed in open width.

Higher tensile strength and lower shrinkage on washing are positive attributescontributed by viscose to blends with wool. Traditional outlets for wool/viscoseblends include lightweight suiting, outerwear, dresswear, knitwear, blankets andfloor coverings. These are usually made from intimate blends of the two fibresbut outerwear, dresswear and knitwear may include two-fold or fancy yarns fornovelty effects. There has never been more than limited interest in blends of silkwith viscose, usually encountered in dresswear if at all. Economy-priced blazercloths have been made with a viscose warp and a wool/viscose blended weft. Pilefabrics for low-cost apparel are sometimes made with a wool pile and a viscosebacking cloth.

Ramie has distinctive features that do not appear to be fully recognised. Thetensile strength is high and an outstanding feature is the high wet strength [3].Attractive lustre and good abrasion resistance make the fibre applicable innumerous outlets. Blends of 70:30 wool/ramie have been used for dress fabrics.The satisfactory dimensional stability of these blends makes them suitable asshirtings, although the texture is somewhat heavy and firm. Such blends maygive rise to ‘prickle-itch’ problems, however, when worn next to the skin. Ramiehas dyeing properties similar to those of cotton and these blends are normallypiece dyed by a one-bath method using direct and milling acid dyes.

Traditionally, solid shades on blended wool/cotton apparel fabrics and wool/viscose woven carpeting were dyed with prepared mixtures of direct and aciddyes called ‘union dyes’, selected to give matching colours on the two fibres withgood fastness to light. Some dyers preferred to formulate their own recipes,however, containing direct dyes selected to dye both components together withacid dyes to adjust the shade on the wool. Self-levelling or temperature-controllable direct dyes with a low degree of sulphonation are substantive toboth substrates, whereas multisulphonated salt-controllable types often tend toreserve the wool [4]. Dyeing under slightly acidic conditions with ammoniumsulphate at 90°C gives optimum partition of the direct dyes. The wet fastnessachieved in this way is largely determined by that of the direct dyes on thecellulosic component but some improvement is possible by aftertreatment with acationic dye-fixing agent [5].

Mohair/cotton pile fabrics are dyed in open width with direct and milling aciddyes from the same bath, or by a two-stage process involving application oflevelling acid dyes to the mohair pile from a sulphuric acid bath, followed bydyeing of the cotton backing with suitable direct dyes. The latter method ensures

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a superior handle and appearance of the pile but some acidic hydrolysis of thecotton may occur in the first stage.

Blends containing chlorinated or stripped wool, or wool damaged in wetprocessing, are likely to offer difficulty in union dyeing. Blends of chlorinated ordamaged wool with cotton are more difficult to dye level than those containingvirgin wool because of more rapid absorption of direct dyes by the wool. It isnecessary to add an anionic retarding agent to slow down this absorption. Wetfastness on chlorinated or damaged wool is also inferior to that on the intactsubstrate. Pale or medium depths in bright shades on cotton and chlorinatedwool can be achieved with high-reactivity dyes applied at low temperatures,however, together with an anionic retarder.

Solidity of shade is normally required on intimate blends of wool and viscosefor dresswear, knitting yarns and carpets, but in knitwear and dresswear the twofibre types are sometimes dyed in contrasting colours. One-bath dyeing withcombinations of direct and acid dyes is generally used, although appropriatereactive dyes can be applied to either fibre type and direct or acid dyes used to fillin the other component. Optimum fastness is given by reactive dyes on theviscose component, followed by milling acid or 1:2 metal-complex dyes on thewool. Vat, sulphur and azoic dyes are not considered for these blends because ofthe strongly alkaline dyeing conditions necessary, which would damage bothfibre types.

Direct dyes should be selected with good build-up on viscose to minimisecross-staining of the wool. The preferred dyes are mostly disazo tetra-sulphonates, particularly those of the symmetrical diarylurea type. Preferred dyesfor the wool are disulphonated milling acid dyes of Mr 500–800 andunsulphonated 1:2 metal-complex monoazo types of Mr 850–950. Levelling acidor 1:1 metal-complex dyes should be avoided because the sulphuric acid requiredfor adequate exhaustion would damage the cellulosic fibre.

The absorption of direct dyes by wool can be reduced using anionic retardingagents of the syntan type. Dyeing for long periods at the boil or under acidicconditions will result in increased absorption of direct dyes by the woolcomponent. Chemically damaged wool also absorbs direct dyes more quickly.Alkaline conditions favour the absorption of acid dyes by viscose and also tendto damage the wool fibre. The products of wool hydrolysis may cause reductiveattack of certain sensitive azo direct and acid dyes in the dyebath. All theseundesirable complications can be minimised using ammonium sulphate tomaintain an acidic pH.

Advantages of dyeing wool/cellulosic blends under mildly acidic conditionsinclude those listed on the following page.

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(1) The method is applicable to a wide range of blend proportions. (2) Better penetration of yarn or fabric is achieved by the freedom to dye at the

boil without fibre damage. (3) Higher exhaustion of the dyebath results in better shade reproducibility. (4) Independent shading of the two fibres is facilitated. (5) Contrast effects can be produced by selective dyeing of the two fibres

simultaneously with direct and acid dyes. (6) Mildly acidic dyebaths permit selective dyeing of the wool fibre and reserve

of the cellulosic component without significant hydrolytic damage of thecellulose.

(7) Dyeing wool under acidic conditions results in improved handle and lessrisk of damage. Pile fabrics dyed in this way are more resilient and lustrous.

(8) Satisfactory results may be obtained on blends of carbonised wool withcellulosic fibres that are difficult to dye under neutral conditions.

(9) Sensitive azo direct or acid dyes are less likely to be chemically attacked.(10) Rubbing fastness is improved compared with dyeing under neutral

conditions.

A two-stage process for the exhaust dyeing of wool/viscose blends with directand acid dyes has been examined by a factorial design method. The influence ofseven independent parameters (dyeing time, concentrations of auxiliaries, pHand temperature of the two stages) on colour yield and fastness was evaluated.The optimum conditions were found to be acid dyeing for one hour at pH 4.5and 100°C with urea but no reserving agent (to inhibit direct dye staining ofwool), followed by direct dyeing at pH 7 and 95°C. Significant improvements infastness to perspiration and wet rubbing were achieved [6].

Exhaust dyeing techniques are well established using two classes of fibre-specific reactive dyes on wool/cotton blends. Pad–batch is also a viable option,though not yet fully adopted in the production environment [7]. Bifunctionalaminochlorotriazine-sulphatoethylsulphone dyes (Figure 8.1) exhibit a highdegree of fixation on wool under acidic conditions and are particularly suitablefor wool/cellulosic blends. Two-stage and two-bath dyeing methods have beendevised [8] to give high colour yields with good levelling and excellent fastness ongarments, hosiery, knitgoods, yarn and loose stock. The relationships betweenexhaustion, fixation and dyebath pH have been illustrated and fastness ratingson wool recorded [9].

Using a special selection of difluoropyrimidine reactive dyes (Figure 8.2),wool/cellulosic blends can be dyed with good solidity by a two-stage procedure.The cellulosic component is dyed first at low temperature and high pH, followed


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Figure 8.2 Typical difluoropyrimidine cellulose-reactive dye

Figure 8.1 Typical bifunctional cellulose-reactive dye

SO3Na

NSO2CH2CH2OSO3Na

N

H O

NaO3S

N

NN

HN NH

SO3Na

Cl

SO3Na

NH3CON

HNH

N

N

Cl

O

NaO3SF

F

by adjustment to an acidic pH and an increase in temperature to dye the wool.The pH changes are controlled by a programmable multiproduct injection devicecapable of providing continuous monitoring of pH [10]. The main advantages ofthis method are a considerable reduction in dyeing time and improved woolquality on blended goods where high wet fastness is demanded.

Vinylsulphone dyes of the cellulose-reactive type (Figure 8.3) can be applied towool/cotton blend fabrics by the one-bath pad–batch method using sodiumsilicate and caustic soda. The wool should be prechlorinated to give improvedsubstantivity for these vinylsulphone dyes. After padding at ambient temperatureand batching overnight, the alkali is washed out by cold rinsing and the dyeing issoaped and rinsed under neutral or mildly acidic conditions. Advantages of thistechnique include low energy consumption, effective penetration of thick fabrics,maintenance of wool quality, high wet fastness, good reproducibility andsatisfactory yield compared with exhaust dyeing methods [11].

When dyeing wool/cellulosic blends with reactive dyes in one-bath methods,blue dyes derived from bromamine acid (1-amino-2-bromoanthraquinone-4-sulphonic acid) can not be selected in most instances because they tend to reactpreferentially with wool. Nevertheless, azo reactive blues can be absorbed andfixed about equally on both components if applied under appropriate conditions.The lower uptake by cellulose of bromamine acid derivatives is probably

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Figure 8.4 CI Reactive Blue 2

Figure 8.3 Typical vinylsulphone cellulose-reactive dye


attributable to their non-planarity. It has been shown that the triazine ring of CIReactive Blue 2 (Figure 8.4), and of other dyes containing the triazinylamino-anilinoanthraquinone grouping, is twisted by almost 90 degrees from the planeof the anthraquinone nucleus. All three NH links in such structures causetwisting of the adjacent ring system on both sides [12].

A serious drawback of most reactive dyeing techniques for wool/cellulosicblends is the adverse effect of alkaline fixation treatment on the quality of thedyed wool fibres. The influence of various concentrations of sodium carbonateon degradation of the wool fibres in a wool/cotton blend was estimated in termsof urea-bisulphite solubility. It was demonstrated that an acceptable two-stageexhaust method entails dyeing the wool first from a mildly acidic dyebath andthen dyeing the cotton with salt and alkali at a pH of no more than 10, and atemperature of not more than 50°C for a dyeing time of not more than 1 hour[13].

In a later study of this problem, samples of merino slubbing were treated (a)under exhaust dyeing conditions with 2–12 g l–1 sodium carbonate and salt at40°C, or (b) as recommended for pad–batch dyeing in carbonate/hydroxidemixtures at pH 10–13 and ambient temperature. The wool samples wereanalysed in detail to assess the degree of damage [14]. Batching treatments forvarious times (20–50 hours) at ambient temperature in the pH range 10.5–12.5

OCH3

N

O2SN

H

SO3Na

O

CH2CH2OSO3Na

SO3Na

NH2O

O HN

NN

NCl

NHSO3Na

HN

SO3Na


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were carried out on a wool/cotton fabric. Amino acid analysis after enzymatichydrolysis of the wool component revealed the detailed effects of the alkalinedegradation. Only in the case of cystine was decomposition extensive. Aminority of the aspartic and glutamic acid residues showed deaminationreactions and certain other amino acid units had undergone racemisation [15].

Dyeing of the wool portion in a wool/cotton blend presents few difficultiesand high wet fastness on machine-washable goods can be attained using wool-reactive dyes, chrome dyes or, in certain cases, 1:2 metal-complex or milling aciddyes with a suitable aftertreatment. As already noted, however, it is the cottonportion that causes problems. Until the 1980s, all the dye classes that couldprovide satisfactory fastness on cotton required strongly alkaline conditions.These caused significant damage to the wool and consequently impairedacceptability and performance of the finished garment in use.

Those vat dyes that are capable of reduction with relatively lowconcentrations of caustic soda can be used but even with these dyes decreasedabrasion resistance is observed. Certain azoic combinations can also be appliedbut fastness to rubbing is limited and the processing sequence is complicated.High-reactivity cellulose-reactive dyes are often adopted but these generallyrequire a two-bath or at least a two-stage process, as already discussed. With anappropriate cationic pretreatment for the cotton, however, such as pad–dry–cureapplication of dimethyloldihydroxyethyleneurea and choline chloride, wool/cotton blends can be dyed by a one-bath method using selected reactive dyesdesigned for wool [16,17].

The introduction of Indosol SF(S) reactant-fixable dyes (Figure 8.5) in the1980s provided the opportunity to use fast dyes for wool (wool-reactive,premetallised or milling acid dyes) with them in a one-bath process. The dyebathis set at pH 6 with a syntan to minimise cross-staining of the wool by the Indosoldyes. The reactant-fixable dyes and 1:2 metal-complex or milling acid dyes areapplied simultaneously at the boil in the presence of Glauber’s salt. Therecommended aftertreating agent is applied from a fresh bath of Glauber’s saltand soda ash solution at 40°C. This enhances the wet fastness of the anionic dyeson wool as well as the reactant-fixable dyes on cotton [1].

For certain deep shades chrome dyes are preferred for the wool but a two-bathtechnique is necessary in these cases, the wool being dyed and chromed first atthe boil and pH 4. The cotton is then dyed at the boil from a fresh bathcontaining the reactant-fixable dyes, electrolyte, an alkylamine polyoxy-ethylenelevelling agent and a weakly anionic blocking agent to minimise surface stainingof the dyed wool by the dyes. If the undyed wool is shrink-resist treated by anoxidative process such as chlorination, the affinity of the wool for all anionic

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Figure 8.5 Typical reactant-fixable direct dye


NaO3S

O CuII

OH2

OH2NO2S

NN

SO3Na

OCuII

H2O

OSO2NH2

N N

N

H

dyes is increased substantially. In these circumstances, the reactant-fixable dyescan be used for both fibres, with the degree of uptake by wool being controlledby means of a blocking agent of the syntan type.

Reactive dyes represent an obvious choice for dyeing wool/linen blends inview of their brightness and high wet fastness. Application of reactive dyes to thelinen component can give rise to bleeding of acid dyes from wool, especiallywhen exhaustion and fixation temperatures higher than 40°C have to be used.The alkaline conditions required to fix the reactive dyes on linen may causedamage of the wool fibre as well as dye desorption. Bleeding of the dyed woolcan be minimised if a reserving agent of the syntan type is added during thealkaline fixation step of the linen dyeing stage [18]. The dyeing of blends ofshrink-resist wool with linen by a one-bath process often leads to differentialuptake. Deeper dyeing of the wool component can be inhibited using reactivedichlorotriazinyl-substituted anionic auxiliaries in carefully controlled amounts[19].

Further problems include tendering of the linen by acid and hydrolyticdegradation of the wool keratin by alkali. Several reactive dyes were evaluatedon wool/linen yarns and fabrics at pH 4.5 (ammonium sulphate and acetic acid),followed by fixation to wool in ammonia solution at pH 8, rinsing andneutralisation with acetic acid. The dyeings were tested for levelness and fastnessto rubbing and perspiration. Damage of the wool was assessed by alkalisolubility, urea–bisulphite solubility, cysteine content and wet strength [20]. Thesubstantivity of the linen can be enhanced using a suitable cationic pretreatingagent. This yields a solid effect with wool-reactive dyes but lowers the lightfastness.

Blends of silk and cellulosic fibres can be dyed with vinylsulphone reactivedyes (Figure 8.3) using the normal pad–batch conditions devised for all-cellulosicfabrics. The fastness of these dyes on silk is generally good, the light fastnessbeing comparable with that established on cellulosic fibres [21]. Decompositionof silk can occur during dyeing under extreme pH conditions and this mainly


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Scheme 8.1

entails rupture of peptide bonds to form new chain-terminal groups, withprogressive lowering of Mr (Scheme 8.1).

To investigate this behaviour, degummed silk was treated in blank dyebaths atvarious pH values at the boil and tested for strength, extensibility, N-terminalamino group content, viscosity and protein loss. The results confirmed that silk isrelatively stable under dyeing conditions between pH 4 and pH 9. This moderatestability, even in the mildly alkaline region, indicates that the dyeing of silk/cotton blends with high-reactivity dyes is much less damaging to the proteinstructure than the dyeing of wool/cotton blends in the same way [22].

8.2 EXHAUST DYEING OF NYLON/CELLULOSIC BLENDS

Serious problems were encountered with the early nylon/cotton blends in the1950s. Blends containing less than 50% nylon were actually weaker than all-cotton yarns. Owing to the lower modulus of the nylon, the load on the yarn as itwas extended was increasingly borne by the cotton fibres in the blend. Thisproblem was solved by developing nylon with a stress–strain curve closer to thatof cotton [23].

Nylon/cellulosic staple blends containing 10–30% nylon with cotton orviscose are used in lightweight suiting and dresswear, leisure shirts and half-hose.Many of these blends, as well as workwear fabrics with a 25:75 nylon/cottonwarp and a cotton weft, or 20:80 nylon/viscose carpet yarns, contain relativelyminor proportions of nylon and acceptable solid effects are not difficult toachieve. Similar considerations apply to pile fabrics, such as upholstery with anylon pile in a woven cotton backing, or cotton-pile terry towelling with a weft-knit nylon backing for beachwear, children’s clothing or leisure shirts, whereslight two-sided differentiation may present no problem in made-up garments orcovers.

Tactel (ICI) nylon/cotton blends have been strongly promoted in sportswear.Good solidity of hue and depth is more critical in 50:50 blends and in unionfabrics, such as nylon warp stretch fabrics containing cotton or nylon/cottonwefts for swimwear and narrow fabrics, crimped nylon warp/viscose weftsportswear or swimwear, nylon/viscose filament dresswear, or cotton warp/nylonweft constructions for uniforms, rainwear and workwear.

CONH CH

R1

CONH CH

R2

CONH

CONH CH

R1

COOH H2N CH CONH

R2

CONHCONH+

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EXHAUST DYEING OF NYLON/CELLULOSIC BLENDS

Warp-knitted velvet fabrics with a viscose pile and a nylon backing are usefulas furnishing fabrics, outerwear, trimmings and lining fabrics, often on costgrounds. Dyeing and printing are usually completed before raising. Overprintingof these dyed fabrics with metallic pigments is popular [24]. Plush velour andvelvet fabrics with a nylon or nylon/wool pile and a cotton backing areencountered in the upholstery sector. Occasionally the pile is made from noveltyyarns in which the filaments vary in denier and crystallinity along their length, sothat an attractive shadow effect is obtained within each filament.

When jet bleaching nylon/cotton blends with hydrogen peroxide at the boil,the amount of peroxide should be decreased according to the proportion ofnylon present and complex organic bases are added as protective agents tominimise oxidative damage of the nylon, i.e. deamination of N-terminal endgroups. An organic stabiliser for the peroxide, e.g. an aminocarboxylate,aminophosphonate or hydroxycarboxylate, should be present as this has asequestering action on any Fe(III) or Cu(II) ions present, which may causecatalytic degradation of the cellulose. Peroxide scavengers containingthiosulphite may be used to ensure that there is no residual peroxide in the goodsat the end of the bleaching operation [25].

There are various possibilities regarding the choice of dye classes for solideffects on nylon/cellulosic blends. Apart from fastness considerations, the choiceof dye system is much influenced by blend construction. Single-class methods aremainly used where the nylon is a minor component, i.e. where only the cellulosicfibre plays a significant part in the surface appearance of the blend fabric, thenylon occupying the interior or the reverse side of the construction.

Reserve, shadow and limited contrast effects are practicable on nylon/cellulosic blends, but seldom encountered in practice. Shadow effects aresometimes required in certain woven upholstery designs, for example. The nylonmay be reserved by applying selected direct dyes to the cellulosic fibre at 80–90°C, with the usual salt addition and a syntan to protect the nylon from cross-staining. Many direct dyes are suitable, but the most important are thesalt-controllable disazo or trisazo types with two to four anionic groups permolecule. A smaller range of yellow to red disazo dyes with two solubilisinggroups and a disulphonated phthalocyanine blue have been recommended forsolid effects in pale or medium depths with good levelling characteristics. Solidityis favoured by applying these dyes at pH 5–6 with sodium dihydrogen phosphateas buffer and with limited salt addition at the boil.

A wider gamut in good solidity is attainable if disperse and direct dyes areapplied by a one-bath method at pH 8 and 70°C. An alkanol polyoxyethylene isrecommended as dispersing and levelling agent, together with a syntan to


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minimise uptake of direct dyes by the nylon. Multisulphonated direct dyes of thesalt-controllable type are used and the preferred disperse dyes are low-energytypes with good levelling properties on nylon. The wet fastness properties ofnylon/cellulosic blends dyed in this way are severely limited and the method isrestricted to low-quality fabrics.

Many suitable neutral-dyeing 1:2 metal-complex and milling acid dyes areavailable to give good reserve of the cellulosic fibre when applied to the nyloncomponent by conventional methods in the presence of ammonium acetate.Disulphonated disazo and anthraquinone dyes with excellent wet fastness, butonly moderate levelling properties, can be used widely. Coverage of dye-affinityvariations in the nylon is much less of a problem than on filament nylon fabrics,especially when nylon/cellulosic staple-blends are to be dyed. Most of thepremetallised dyes used are monosulphonated monoazo 1:2 chromiumcomplexes.

If premetallised or milling acid dyes of this kind are applied with the salt-controllable direct dyes already described above, solid-effect dyeings of goodfastness to light and moderately good wet fastness can be obtained economicallyon nylon/cellulosic blends. This method is especially important where both fibresmake a major contribution to the appearance of the material. It is often useful toinclude a syntan to inhibit staining of the nylon by the direct dyes.

Dyeing commences with the acid dyes, a weakly cationic alkylaminepolyoxyethylene retarder if necessary and the minimum amount of syntan,depending on the applied depth and the direct dyes selected. If a cationic levellingagent is found necessary, sufficient of an alkanol polyoxyethylene anti-precipitant should be added to solubilise the dye–retarder complex. The dyebathis buffered to an optimum pH between 5–6 for full depths of milling acid dyesand 7–8 for pale depths of premetallised dyes. The direct dyes are added at about60°C and the temperature raised to the boil, adding salt to promote exhaustionof the direct dyes by the cellulosic fibre.

Bright hues with excellent fastness can be achieved on nylon/cellulosic blendsusing reactive dyes. Unfortunately, reactive dyes are highly sensitive to the type ofnylon present and to dye-affinity variations in filament nylon. Many reactivedyes contain several sulphonic acid groups per molecule and pronouncedblocking on the nylon component is observed when attempting to apply amultisulphonated dye in the presence of a less-sulphonated dye of higher affinity.The problems of incompatibility arising from this phenomenon are particularlydifficult when dyeing the blend because differences in distribution of individualdyes between the component fibres are accentuated.

In spite of these difficulties, methods employing one class of reactive dyes to

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colour both fibres have been established, especially for those fabrics composedmainly of cotton in which the nylon is hidden when the fabric is made into agarment. Weft-knitted terry towelling with a strong nylon filament base and anabsorbent cotton pile has proved popular for children’s wear, beachwear andleisure shirts. Bright hues with very good fastness to washing and light arerequired. This is usually achieved with reactive dyes and metal-complex ormilling acid dyes, but a wide variety of reactive dyes can be used satisfactorily onboth fibres if the proportion of nylon is not too high.

In a typical three-stage method from a single bath, the reactive dyes are firstexhausted on to the nylon from weakly acidic solution in the absence of salt.Electrolyte is then added to promote further uptake by the cellulosic fibre and afinal alkaline fixation treatment is given. Some control over the distributionbetween nylon and cellulose is possible by selection of dyebath pH, temperatureand electrolyte concentration. Nylon is favoured at low applied depths but thedistribution shifts in favour of the cellulosic fibre as the saturation level of nylonis approached.

Reactive dyes with good neutral-dyeing properties in the presence of salt canbe applied by a simpler two-stage sequence of neutral exhaustion and alkalinefixation, as for 100% cellulosic materials. Most metal-complex reactive dyes, aswell as multisulphonated unmetallised types, require pH values of 4 or lower forreasonable uniformity of distribution. This gives some risk of degradation of thecellulosic fibre, especially if it is viscose, and may lead to inefficient utilisation ofdye on the cellulosic component by acid hydrolysis of the reactive group.

Many 1:2 metal-complex and several milling acid dyes are fast to soda boiling.This means that they can be applied with reactive dyes in the two-stage method,provided no serious interaction occurs. Reactive dyes are applied in the presenceof alkali, together with non-reactive metal-complex dyes for the nylon. Free acidis added to give pH 7 and the temperature raised to the boil to fill in the nylonportion. There is only slight staining of the cellulosic fibre under these conditions.It is important to keep the pH slightly alkaline during washing and rinsing toavoid possible reaction of nylon with residual reactive dyes.

When selecting reactive and milling acid dyes for this blend, it is more usual toadopt a two-bath method that gives a wider choice of suitable dyes. In this case,an important criterion is that the bond between the reactive dye and cellulosemust be stable to acid hydrolysis during dyeing of the nylon component in fulldepths. Anionic dyes with high neutral-dyeing affinity are therefore preferred andvinylsulphone reactive dyes (Figure 8.3) are generally unsuitable.

The facility to attain high wet fastness standards on nylon/cellulosic blends bya one-bath technique at mildly acidic pH is a substantial advantage over the two-


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Figure 8.6 Typical bis(aminonicotinotriazine) reactive dye

bath or two-stage procedures based on reactive dyes [1]. Reactant-fixable directdyes (Figure 8.5) and 1:2 metal-complex or milling acid dyes are appliedsimultaneously. The bath is set at pH 6 with Glauber’s salt and a syntan tominimise staining of the nylon by the reactant-fixable dyes. The 1:2 metal-complex or milling acid dyes are added with a weakly cationic retarder and anonionic anti-precipitant if necessary. The reactant-fixable dyes are then addedand dyeing of both fibres is completed at the boil.

Kayacelon React (KYK) bis(aminonicotinotriazine) dyes (Figure 8.6) are high-temperature neutral-fixing reactive dyes especially suitable for the one-bathdyeing of nylon/cellulosic blends. By choosing acid dyes that are relativelyinsensitive to salt it is possible to dye solid shades on nylon/cotton in one bath.The reproducibility of the method depends on buffering the dyebath carefully topH 6–7 with a phosphate buffer [26].

Knitwear made from polyurethane/cotton blends, in which the elastomericfibre may range from 5% to 20%, has been widely popular in recent years forstretch garments, such as skiwear, sportswear, underwear and leisure clothing. Ifthe content of polyurethane is higher than about 8%, the slitted tubular-knitgoods show a strong tendency to roll at the edges, leading to a ‘cigarette’ formthat is most difficult to penetrate during dyeing. This problem has been solved bypresetting on a stenter at 190°C after slitting and then either carefully ‘bagstitching’ to restore the tubular form, or preferably dyeing in an overflowmachine capable of achieving satisfactory penetration and levelness [27].

Blends of polyurethane and cellulosic fibres are dyed by methods similar tothose for nylon/cellulosic blends. Monosulphonated monoazo or disulphonatedanthraquinone acid dyes, or 1:2 metal-complex monoazo types, are preferred for

N

N

N

N

HN NH N

NN

N

HN NH

NNNN

H H

NaO3S SO3Na

O O

COO––OOC

SO3Na NaO3S

SO3Na NaO3S

+ +

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the polyurethane component and practically all direct dyes for cellulose will givelow staining of polyurethane fibres at pH 8–8.5. Similar considerations apply tothe dyeing of blends of acid-dyeable polypropylene with cellulosic fibres. Selectedpremetallised or milling acid dyes are applied to the synthetic component at theboil and pH 3–4 in the presence of an anionic retarder. Selected direct dyes arethen applied from the same bath after adjusting to 70°C and pH 7–8. A syntan isincluded to minimise staining of the polypropylene by the direct dyes.

8.3 CONTINUOUS DYEING OF NYLON/CELLULOSIC BLENDS

The carpet industry is the most important field in which AC blends have beenused in quantities sufficient to justify the development of continuous dyeingprocesses. Another area for the adoption of continuous methods has been thedyeing of heavy-duty nylon/cotton woven fabrics, which are closely related to themuch more extensively used polyester/cotton blends for this outlet. Similarprinciples of application are relevant for the continuous dyeing of plush andvelvet upholstery fabrics made in nylon pile/cotton backing constructions.Continuous dyeing systems have also been developed for nylon warp/cotton weftsportswear materials that have proved exceptionally popular in recent years [28].

Warps in these sportswear fabrics are usually treated with acrylate sizes sothat desizing with enzymes is unnecessary. Pretreatment comprises singeing, coldbleaching, alkaline boiling out, washing-off and drying. After dyeing, thesegoods are often given a hydrophobic finish usually based on a glyoxal-fluoro-carbon resin or a silicone polymer. This can contribute to improvements in fast-ness of anionic dyes to perspiration and water, as well as giving the desired finish.

Modified viscose carpets containing only 10–20% nylon can be dyedcontinuously with direct dyes that give an acceptably solid effect on the twofibres. The preferred dyes are mostly disazo types with two solubilising groupsand good levelling properties, but for mode shades the best results are obtainedwith salt-controllable dyes. Migration of the disazo disulphonates during dryingand steaming may lead to colour variations in mixture dyeings. Temperature-controllable direct dyes tend to give poor penetration and inferior fastness torubbing. A wetting and levelling agent of the nonylphenol polyoxyethylene typeis added to the pad liquor, with urea if necessary to improve solubility of thedirect dyes. Solidity at full depths requires careful control of pH and anionicsurfactants should be avoided as these may interfere with uptake of the directdyes by nylon.

An alternative process, recommended for plush or velvet upholstery with anylon pile and cotton backing, is based on the same selection of acid or metal-complex dyes used on all-nylon carpets, together with the salt-controllable


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multisulphonated direct dyes that give low staining of nylon. Suitable foamingagents are added to improve levelness and penetration. These avoid undesirablemigration and frosting of the nylon pile. Steaming for 5–15 minutes is necessaryaccording to depth applied. Continuous methods that include direct dyes sufferfrom problems of substantive tailing and differential affinity for the two fibres, aswell as limited brightness and inferior fastness to light and wet treatments inmany instances.

Reactive dyes for the cellulosic component offer advantages in most of theseaspects and they are suitable for one-bath application methods in general.Satisfactory fastness in pale depths is given by padding with reactive and dispersedyes, urea and an anionic migration inhibitor, followed by thermofixation at180–200°C. Low-energy monoazo and anthraquinone disperse dyes arerecommended. Slight discoloration and stiffening of the fabric may occur if ahigh proportion of nylon is present. Batching for two hours after padding mayimprove fixation of the reactive dyes when viscose is the cellulosic component.

A modification of this process can be used to dye full depths with reactive dyesand selected metal-complex or acid dyes. Less urea is required and afterthermofixation under alkaline conditions the dyes on nylon are developed fullyby an acid shock treatment in dilute formic acid solution at the boil. Analternative one-bath sequence for reactive dyes and unsulphonated 1:2 metal-complex dyes is:(1) neutral pad–dry–thermofix treatment for fixation of the metal-complex

dyes on nylon;(2) padding with caustic soda in near-saturated brine;(3) batching to fix the reactive dyes on the cellulosic fibre.

The relative merits of wet steaming without intermediate drying, a pad–steamprocess with intermediate drying and a pad–batch–steam sequence for thefixation of metal-complex and acid dyes on the nylon component have beenevaluated in terms of the resulting wet fastness [28]. Selected reactive or reactant-fixable dyes can be used for the cellulosic fibre. Improvements in colour fastnessare possible using a syntan to fix the anionic dyes on the nylon and a cationicaftertreatment for the reactant-fixable dyes.

Dull dyeings of high fastness to light and wet treatments may be required onnylon/cotton fabrics for workwear or uniforms. Vat and metal-complex dyes areoften used in these circumstances, although selected sulphur and milling aciddyes provide more economical recipes where fastness standards permit. Stabilityof the premetallised and milling acid dyes to reduction and oxidation is an

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CONTINUOUS DYEING OF NYLON/CELLULOSIC BLENDS

Table 8.1 Dye selections for exhaust dyeing of AC blends


Wool/cotton Wool One-bath Multisulphonatedreserve salt-controllable direct dyes

Solid One-bath Acid dyes andlow-sulphonated direct dyes

Wool/cotton Solid Single-class Monofunctional or bifunctional(machine- cellulose-reactive dyeswashable)

One-bath Premetallised or milling acid dyesand reactant-fixable dyes

Two-stage Wool-reactive and thencellulose-reactive dyes

Two-bath Chrome dyes, thenreactant-fixable dyes

Chlorinated Solid Single-class High-reactivity dyes withwool/cotton anionic retarder

One-bath Acid dyes and direct dyeswith anionic retarder

Mohair/ Solid One-bath Milling acid dyes and direct dyescotton

Two-stage Levelling acid dyes at pH 2–3,then direct dyes

Wool/viscose Viscose Single-class Disulphonated milling acid dyes orreserve unsulphonated 1:2 metal-complex

dyes

essential criterion of selection for application with vat or sulphur dyes. Sulphurdyes can be used for the cotton by a simplified method in which the metal-complex dyes are applied alone by pad–dry–thermofix and the reduced sulphurdyes included in a subsequent chemical pad–steam stage. A costly two-bathapplication of 1:2 metal-complex dyes by pad–steam on nylon, followed by a vatpigment pad–dry–chemical pad–steam process for the cotton, may be necessaryto achieve maximum fastness.

8.4 DYEING METHODS AND DYE SELECTION FOR AC BLENDS

Solid effects are mainly of interest on these blends. They can be readily obtained(Table 8.1) by single-class (reactive dyes) or one-bath methods on blends of wool,


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Table 8.1 Continued


Wool/viscose Solid or One-bath Milling acid dyes and direct dyescontrast

Wool-reactive dyes and direct dyes

Milling acid dyes andcellulose-reactive dyes

Wool/linen Solid Two-stage Milling acid dyes, thencellulose-reactive dyes

Wool/ramie Solid One-bath Milling acid dyes and direct dyes

Nylon/ Nylon Single-class Salt-controllable direct dyescellulosic reserve with syntan

Cellulosic Single-class Monosulphonated 1:2reserve metal-complex or disulphonatedmilling acid dyes

Solid Single-class Disulphonated disazo and(pale depths) phthalocyanine direct dyes

Nylon/cellulosic Solid Single-class Reactive dyes at pH 4–5,then salt and alkali

One-bath Low-energy disperse dyesand salt-controllabledirect dyes

Premetallised or milling aciddyes and salt-controllabledirect dyes

Premetallised or milling aciddyes and reactant-fixable dyes

Selected acid dyes andbis(aminonicotinotriazine)neutral-fixing dyes

Two-stage Reactive dyes with alkali, thenselected premetallised and millingacid dyes

Polyurethane/ Polyurethane Single-class Direct dyes at pH 8 with syntancotton reserve

Solid One-bath Premetallised or milling acid dyesand direct dyes at pH 8

Acid-dyeable Solid Two-stage Premetallised or milling acid dyespolypropylene/ at pH 4, then direct dyes at pH 8cotton

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DYEING METHODS AND DYE SELECTION FOR AC BLENDS

Table 8.2 Dye selections for continuous dyeing of nylon/cellulosic blends


Nylon/ Solid Pad–dry–steam Salt-controllable direct dyescellulosic

Premetallised or milling acid dyesand salt-controllable direct dyes

Premetallised or milling acid dyesand reactant-fixable dyes

Pad–dry–thermofix Low-energy disperse dyes andreactive dyes

Pad–dry–thermofix– Selected premetallised or millingacid shock acid dyes and reactive dyes

Pad–dry–thermofix– Unsulphonated 1:2 metal-complexpad–batch dyes and reactive dyes

Pad–dry–thermofix– Selected premetallised dyes, thenchemical pad–steam pre-reduced sulphur dyes

Pad–dry–steam, Selected 1:2 metal-complex dyes,pad–dry–chemical then vat dyespad–steam

chlorinated wool, mohair, nylon or polyurethane with cotton, or blends of woolor nylon with viscose. Two-stage procedures are necessary for wool/linen or acid-dyeable polypropylene/cotton blends, the non-cellulosic fibre being dyed first.The higher wet fastness standards provided by reactive dyes with premetallisedor milling acid dyes on nylon/cellulosic blends or machine-washable wool/cottongoods also require two-stage or two-bath processes. On wool/cotton the wool isdyed first but on nylon/cotton it is preferable to first dye the cotton.

Continuous dyeing methods for solid effects on nylon/cellulosic blends rangefrom low-cost pad–steam dyeing with direct dyes or the pad–thermofix disperse/reactive process, to much more elaborate and costly two-bath sequences for highall-round fastness performance. Premetallised and milling acid dyes by pad–steam are mostly used to colour the nylon but the class preferred on the cellulosicfibre may be direct, reactant-fixable, reactive, sulphur or vat dyes (Table 8.2).


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8.5 REFERENCES 1. J A Hook and A C Welham, J.S.D.C., 104 (1988) 329.

2. R L Stone, R H Wang and G P Morton, Text. Chem. Colorist, 18 (Aug 1986) 11. 3. T P Nevell in Cellulosics dyeing, Ed. J Shore (Bradford: SDC, 1995) 6.


5. T M Baldwinson in Colorants and auxiliaries, Vol. 2, Ed. J Shore (Bradford: SDC, 1990) 444. 6. M M Muratova, T Y Rosinskaya, L I Belenkii and L L Vidrevich, Tekstil. Prom., 12 (Dec 1980)

48.

7. N E Houser, AATCC Nat. Tech. Conf., (Oct 1985) 116. 8. K Imada, M Sasakura and T Yoshida, Text. Chem. Colorist, 22 (Nov 1990) 18.

9. A N Lee, Dyer, 178 (Apr 1993) 30.

10. D Hildebrand, Proc. 7th Int. Wool Text. Res. Conf., (Tokyo), Vol. V (1985) 239.11. H Putze and G Dillmann, Textilveredlung, 15 (1980) 457.

12. M Matsui, U Meyer and H Zollinger, J.S.D.C., 104 (1988) 425.

13. H Zahn, I Steeken and U Altenhofen, Chemiefasern und Textilind., 31/83 (1981) 684.14. I Steenken, I Souren, U Altenhofen and H Zahn, Textil Praxis, 39 (1984) 1146.

15. U Altenhofen, I Souren and H Zahn, Textilveredlung, 20 (1985) 144.

16. J M Cardamone, AATCC International Conference and Exhibition, (Oct 1994) 7.17. J M Cardamone, W N Marmer, E J Blanchard, A H Lambert and J Bulan-Brady, Text. Chem.

Colorist, 28 (Nov 1996) 19.

18. I Steenken, I Funken and G Blankenburg, Textilveredlung, 21 (1986) 128.19. J Haarer, H Thomas and H Höcker, Melliand Textilber., 76 (1995) 1003.

20. G Kratz, A Funder, H Thomas and H Höcker, Melliand Textilber., 70 (1989) 128.

21. H Putze, Textil Praxis, 39 (1984) 1051.22. B Vogt, U Altenhofen and H Zahn, Textilveredlung, 20 (1985) 90.

23. R M Hoffman and R W Peterson, J. Text. Inst., 49 (1958) 418.

24. G Wünsch, Textilveredlung, 24 (1989) 57.25. D R Wallwork, Textile Technology Internat., (1990) 229.

26. A N Lee, Dyer, 179 (Apr 1994) 29.

27. Brazzoli SpA, Dyer, 179 (Aug 1994) 31.28. O Annen, F Somm and R Buser, Textilveredlung, 22 (1987) 19.

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119

CHAPTER 9

Cellulosic/acrylic and other CB blends

9.1 EXHAUST DYEING OF CELLULOSIC/ACRYLIC BLENDS

Cellulosic/acrylic blends are the most important of the acrylic fibre blends. Thecellulosic fibre contributes economy, moisture regain and antistatic properties. Inapparel outlets, the acrylic component is included for heat insulation, creaserecovery and abrasion resistance. Important characteristics of cellulosic/acrylicfibre blends in upholstery, pile fabrics and tufted carpets are appearanceretention, resilience and wear resistance.

Cotton/acrylic blends are widely used in the rapidly expanding sportswearand leisurewear sectors. Fine-spun 20:80 to 50:50 cotton/acrylic yarns are usedin lightweight woven suiting, dresswear and sportswear, or knitted underwear,leisurewear and swimwear. Blends of viscose and acrylic fibres are used in skirts,dresswear, linings and hosiery yarns. It may be desirable to insert durable pleatsin dresswear garments with the aid of a reactant resin finish. Modacrylic fibres inblends with cotton or viscose (20:80 to 50:50) have been exploited forunderwear, hosiery, leisure clothing and dresswear, particularly pleated skirts.

High-bulk acrylic and modal fibre long-staple yarns are suitable for dresswearand suitings with a wool-like appearance. Fleece fabrics are made from woollen-spun acrylic fibre pile in a cotton backing. Fancy high-bulk acrylic jerseyknitgoods containing linen or viscose slubs are of interest for furnishing fabricsas well as apparel. Viscose/acrylic staple yarns have proved effective as high-twistpile for non-crush carpets, as they are significantly cheaper than all-wool orwool/viscose carpets.

Package dyeing of cotton/acrylic yarns is important. Careful prescouring ofthe cotton is essential using trisodium phosphate at 60–70°C and an alkanolpolyoxyethylene detergent. This is necessary to remove residual lubricants and,in the case of unrelaxed yarns containing high-bulk acrylic, to avoid fixation ofcontaminants during relaxation by steaming. Good whiteness is achieved on thecellulosic component by bleaching with silicate-stabilised hydrogen peroxide at70°C and pH 9–10. Careful control is necessary to avoid yellowing of the acrylicfibre.

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The selection of dyes for cellulosic/acrylic blends is complicated by the factthat disperse dyes show poor build-up and limited fastness on the acryliccomponent, so that basic dyes must be used in most instances. Problems ofincompatibility between these dyes and the various classes of anionic dyesrequired to meet appropriate fastness demands on the cellulosic fibre (sections4.2 and 4.3) are therefore characteristic of such blends, in contrast to those ofcellulosics with other synthetic fibres. Cross-staining is less of a problem,however, and a good reserve can be obtained on either component, althoughsolid effects are most often required.

The choice of best dyeing method is determined by the colour effect andgamut required, as well as the wet fastness specification. Cellulosic/acrylic blendscan be readily dyed to reserve the acrylic fibre by pretreating with a syntan at 60–70°C and then dyeing with salt-controllable direct dyes at 70°C and pH 7–8. It isless convenient to reserve the cellulosic component because staining by basic dyesmay be troublesome, particularly with regenerated cellulosic fibres or withcotton that has suffered oxidative damage during scouring or bleaching. Thecarboxyl groups formed as a result of alkaline oxidation of cellulose provideanionic sites for the sorption of basic dyes. Basic dyes are applied to the acryliccomponent with a cationic retarder at the boil. The rates of exhaustion of basicdyes of various types on acrylic fibres have been analysed in self shades andcombination recipes using high-pressure liquid chromatography [1]. Thepreferred dyes for reserving the cellulosic component are mainly of the localised-charge monoazo or anthraquinone types. Any basic dye staining of the cellulosicfibre can be removed using sodium dithionite and a nonionic detergent at 60°C.

Solidity or contrast effects can be easily obtained by one-bath methods. Themost economical approach for pale depths is one-bath application of disperseand direct dyes. Wet fastness is seldom acceptable at intermediate depths, but thelevelling properties of disperse dyes are much superior to those of basic dyes onacrylic fibres. The disperse dyes recommended for pale depths on cellulosic/acrylic blends are mainly low- and intermediate-energy dyes of Mr 230–350,particularly nitro, monoazo and anthraquinone types. The preferred direct dyesare almost all salt-controllable disazo, polyazo or stilbene types with three orfour anionic groups.

The blend is dyed at the boil and pH 4–5 in the presence of an anionicdispersing agent. Disperse dyes are partly absorbed by the cellulosic componentin the early stage of dyeing, but transfer in favour of the acrylic fibre proceeds asthe dyeing temperature approaches the boil. The acrylic component is dyed first,then the dyebath temperature is lowered to 80°C, Glauber’s salt added and the

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EXHAUST DYEING OF CELLULOSIC/ACRYLIC BLENDS

cellulosic fibre filled in with the direct dyes. It is not difficult to remove theresidual stain from the cellulosic fibre by subsequent scouring.

Cellulosic/acrylic blends can be dyed in pale or medium depths by a one-bathprocess with direct and basic dyes, but there is a risk of co-precipitation of thesedyes. Careful selection of basic dyes and the addition of a nonionic anti-precipitant are necessary. Problems of incompatibility can be virtually overcomeby the selection of basic dyes of small molecular size and the relativelyhydrophilic multisulphonated direct dyes with a low tendency to aggregate in theabsence of electrolyte [2].

The cellulosic fibre is dyed first at 80°C and pH 5–6 (acetate–acetic buffer)with the direct dyes, Glauber’s salt and an alkanol polyoxyethylene anti-precipitant. The basic dyes and an anionic retarder are then added and thetemperature raised to the boil to approach target depth on the acryliccomponent. If necessary, more Glauber’s salt may be added to improveexhaustion of the direct dyes. The acidic dyebath is important to avoidaggregation and possible precipitation of the dyes. The direct dyes must showsatisfactory exhaustion under these mildly acidic conditions that are preferred forapplying basic dyes. The direct dyes tend to retard the uptake of the basic dyes bythe acrylic component and may give restraining of final exhaustion in heavydepths. An advantage of dyeing under mildly acidic conditions is to avoid therisk of reductive decomposition of some direct dyes under alkaline conditions atelevated temperatures.

Medium or heavy depths may be dyed by a more lengthy procedure thatminimises any risk of incompatibility. In the two-stage sequence, the basic dyesare applied with an alkanol polyoxyethylene anti-precipitant at pH 5–6, raisingthe temperature slowly from 80°C to the boil. When dyeing of the acrylic fibre iscomplete, the temperature is lowered again to 80°C, the direct dyes andGlauber’s salt are added and dyeing of the cellulosic component is completed atthis temperature. A two-bath method is similar, except that the basic dyes areapplied at pH 4–5 with a cationic retarder and the direct dyeing is commenced at40°C and pH 7, after an intermediate clear with a nonionic detergent.

Better fastness of the cellulosic fibre is given by reactive, reactant-fixable or vatdyes. Basic dyes and reactive dyes are preferred for bright hues of high wetfastness. Basic dyes would be restrained by the presence of reactive dyes and theytend to be unstable to the conditions of alkaline fixation, so that two-bathmethods are normally necessary. Full depths are usually achieved by dyeing theacrylic fibre first with basic dyes. The temperature of the reactive dyeing stagemust not exceed the glass-transition temperature of the dyed acrylic fibre in order


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to avoid possible desorption of basic dyes [3]. Data have been given on thestability to cross-dyeing of basic dyes tested over the pH range 2 to 9 [2]. There isa risk of change in hue of basic dyeings during the alkaline fixation and soapingof reactive dyeings on the cellulosic component. The vinylsulphone reactive dyesare stable to cross-dyeing with basic dyes at pH 5. Advantage may be taken ofthis in a two-stage sequence, in which the alkaline fixation bath for the reactivedyeing is subsequently adjusted to pH 5 with acetic acid in order to dye theacrylic fibre with the basic dyes.

A two-stage process with reactant-fixable copper-complex direct dyes andbasic dyes is suitable to achieve high wet fastness in pale and medium depths [4].The reactant-fixable dyes are applied first to the cellulosic component at pH 5–6(acetate–acetic buffer) and 70°C in the presence of electrolyte. The basic dyes arethen added with an alkylamine polyoxyethylene as anti-precipitant and mildlycationic retarder, and the acrylic fibre is dyed at the boil. An appropriate cationicfixing agent is used to aftertreat the dyeings. Advantages of this process include ashort dyeing time, good level dyeing behaviour and high standards ofreproducibility and fastness performance [5].

When dyeing yarn in package form it is usual to select vat dyes for thecellulosic fibre. Problems of incompatibility are more evident when vat dyes areused with basic dyes. A two-bath sequence is necessary because the anionicdispersing agents in the vat dye formulations would restrain uptake of the basicdyes and promote staining of the cellulose, at the same time causing instability ofthe vat dye dispersion. If the cellulosic fibre were dyed first, the vat dye would actas a mordant and the basic dyes would stain the strongly anionic fibre surface.This stain would not be removed completely by soaping [2]. Thus it is preferableto dye the acrylic component first, because the basic dyes are fast to cross-dyeing,and the reducing conditions of vat dyeing at 50–60°C help to remove the basicdye stain from the cellulosic fibre. Some vat dyes stain the acrylic fibresignificantly and allowance for this must be made in shade matching. Thepreferred vat dyes for minimum staining of the acrylic fibre are mainly thepolycyclic quinones and their halogenated derivatives, including thedibromopyranthrones, indanthrones, dichloro- and dibromoiso-violanthrones,and alkoxyviolanthrones [6].

9.2 CONTINUOUS DYEING OF CELLULOSIC/ACRYLIC BLENDS

These are the only acrylic blends produced in sufficient quantity to justifycontinuous dyeing. The principles of dye selection in relation to compatibilityand fastness requirements are essentially similar to the analogous batchwisemethods, but the application techniques have much in common with the

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continuous dyeing of polyester/cellulosic blends. Blends of acrylic fibres witheither cotton or viscose can be processed satisfactorily on continuous ranges aftersingeing, desizing and scouring in open width.

For blends containing a high proportion of acrylic fibre with cotton, it is oftenpossible to obtain an acceptably solid appearance and moderate fastness in paledepths by dyeing the acrylic fibre only using disperse dyes applied by pad–dry–thermofix. Higher fastness in pale or medium depths is attainable on theseacrylic-rich blends using basic dyes by pad–steam, leaving the cellulosic fibreundyed. The fabric is padded at 70°C with the basic dyes, the liquor beingadjusted to pH 5 with citric acid. The fabric is then steamed at 100–103°C, but itmay be necessary to give a prolonged steaming with certain basic dyes. If reserveof the cellulosic fibre is desired, this may require a clearing treatment in ananionic detergent solution at 90°C, or in a reduction clearing bath at 70°C in thecase of deeper shades.

For pale depths of outstanding fastness, selected vat dyes can be applied togive reasonable solidity on both fibres. These are applied by pigment paddingfrom dispersion and drying, followed by chemical padding. Salt (100 g l–1) isadded to the alkaline dithionite solution to minimise transfer from the acrylicfibre to the cellulose during subsequent steaming. Halogenated vat dyes of thepyranthrone, indanthrone and isoviolanthrone types, as well as alkoxyderivatives of violanthrone, are especially suitable [6].

Solid dyeings on cotton/acrylic blends can also be obtained using liquid brandsof sulphur dyes to colour both fibres. Advantages of this approach overalternative two-stage systems are the lower costs of dyes and chemicals, a shorterand simpler dyeing cycle, satisfactory fastness and improved coverage ofimmature cotton [7]. On the other hand, of course, sulphur dyes are much morerestricted to a gamut of relatively dull hues.

Basic and direct dyes may be applied together in the pad–steam process, usingan alkanol polyoxyethylene sulphate to complex with the basic dyes and analkanol polyoxyethylene to disperse the complex in the pad liquor (section 4.3),but there are serious limitations. The depth on the acrylic fibre is limited by thestability of the dye–agent complex and the restraining influence of the anionicagent and the direct dyes. A fixation accelerator is also required to promote yieldand penetration of the basic dyes into the acrylic component during steaming.These agents are usually water-insoluble aryl ethers containing halogeno orcyano substituents [8]. A product of this type is emulsified in the pad liquortogether with an anti-precipitant system. Selection of the direct dyes is restrictedby their limited solubility in the pad liquor at pH 5, slow diffusion into thecellulosic fibre during steaming and only moderate wet fastness.

CONTINUOUS DYEING OF CELLULOSIC/ACRYLIC BLENDS


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Conventional basic dyes are not compatible with vat dye dispersions butselected basic dyes have been marketed in complex form as liquids stabilised withanionic dispersing agents. These are compatible with liquid brands of vat dyesfor continuous dyeing. Before dyeing with these complexes, the anionic groups inthe acrylic fibre must be converted to the ammonium salt form by pretreatmentwith an ammonium salt at 80°C. If these basic dye complexes are applied tountreated acrylic fibres with anionic groups in their usual sodium salt form, thecomplexes do not dissociate and give relatively low yields with poor fastness.

The pretreated cellulosic/acrylic fabric is padded with the complexed basicdyes together with selected vat dyes at 50°C and pH 8–9 (phosphate buffer).During thermofixation at 200°C the complexes are transferred from thecellulosic to the acrylic fibre and also dissociate to give the parent basic dyes.These blends show high wet fastness after the residual basic dye stain has beencleared from the cellulosic fibre during conventional chemical pad–steamfixation of the vat dyes. Optimum fastness in full depths on untreated cellulosic/acrylic blends can be achieved by pad–steam dyeing with conventional basicdyes, followed by the usual pigment pad–dry–chemical pad–steam sequence forvat dyeing of the cellulosic fibre. This relatively costly process gives a wide rangeof shades of high fastness to light and wet treatments. The application of vat dyesis instrumental in removing any basic dye that may have stained the cellulosicfibre in the first stage.

Basic and reactive dyes are generally incompatible at only moderateconcentrations in the pad liquor, especially the localised-charge basic dyeswith the most highly sulphonated members of the reactive class.Nevertheless, these two classes yield exceptionally bright hues for deepcontrast effects of excellent fastness, applied by a two-bath sequence ofconventional pad–steam processes. If a resin finish is required for thecellulosic component, the fabric should be treated under conditions thatminimise discoloration of the acrylic fibres, particularly with regard tocuring temperature. The amount of resin applied should be determined bythe cellulosic content of the blend. Fabrics containing more than 50% ofacrylic fibre do not normally require a resin finish.

9.3 BLENDS OF CELLULOSIC FIBRES WITH MODACRYLIC OR ACID-DYEABLE ACRYLIC VARIANTS

Dynel (Union Carbide) modacrylic fibre is used in 20:80 to 50:50 blends withcotton or viscose for half-hose, underwear and nightwear. The modacryliccomponent contributes dimensional stability, improved handle and good

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launderability. Woven goods include pile fabrics with a cotton warp and a 70:30cotton/Dynel weft. Dynel is resistant to mercerising and peroxide bleaching ofthe cotton. It is usual to dye solid shades on these blends but a wide range ofcolour contrasts is possible because the dyes preferred for Dynel do not normallystain the cellulosic fibre.

Shades of moderate fastness (light 5–6 and mild washing tests) are obtainedwith disperse and direct dyes by a one-bath method. Higher wet fastness requiresvat, sulphur or reactive dyes for the cotton and basic dyes for Dynel but a two-stage process is unavoidable. The general approach is to dye the Dynel at the boiland pH 5 with basic dyes and then to fill in the cotton under normal conditionsfor the cellulosic dyes chosen.

Verel modacrylic fibre is blended with cotton for sportswear and underwear,typically as 25:75 blends. Flannel-type fabrics can be made from 75:25 Verel/viscose yarns. No resin finish is necessary for this modacrylic-rich blend. Solidshades can be obtained on Verel/cellulosic blends using disperse or 1:2 metal-complex dyes on Verel and direct dyes on the cotton or viscose. Either fibre canbe reserved and contrast or shadow effects are also possible if desired.

A full range of dyed effects can be produced on blends of acid-dyeable acrylicvariants with cellulosic fibres. Several techniques are available to dye the acrylicand reserve the cellulosic component:(1) The acrylic fibre is dyed with basic dyes by the urea method. The dyes

should be dissolved in hot water and either methanol or a nonionicsurfactant, rather than acetic acid. Dyeing takes place in the presence of 3%urea and the dyebath pH gradually rises from slightly acidic to slightlyalkaline during the course of dyeing. The dyed material is rinsed well andgiven a mild scour with sodium hypochlorite at 40°C to clear the cellulosicfibres.

(2) Selected chrome dyes may be applied from an acidic dyebath. It is essentialto scour with a nonionic detergent and tetrasodium pyrophosphate afterchroming is complete. These dyes have satisfactory fastness to perspirationand steam pleating.

(3) The acid-dyeable acrylic fibre is dyed with selected levelling acid dyes fromphosphoric acid solution, followed by a neutralising scouring treatment asfor chrome dyeings. The light fastness of these dyes may be lower whendyed with phosphoric acid. Preferred dyes include selected monoazo oranthraquinone disulphonates of Mr 400–600. The fastness of levelling aciddyes is generally lower than that of basic or chrome dyes, so this approach isusually confined to bright shades on lower-quality goods.

BLENDS OF CELLULOSIC FIBRES WITH ACRYLIC VARIANTS


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It is less convenient to dye the cellulosic fibre and reserve the acid-dyeable acrylicvariant because of the affinity of the latter for most classes of dyes. Selected salt-controllable direct dyes of the multisulphonated type can be applied after thematerial has been pretreated at 60–70°C with a weakly cationic surfactant. Thebest reserve of the acid-dyeable acrylic fibre is achieved by dyeing at pH 8 and70°C. The wet fastness is improved by finishing with a crease-resist resin. If solidshades are required a two-bath method is preferred, first dyeing the acrylicvariant with basic or chrome dyes and then filling in the cellulosic fibre withdirect dyes at pH 8.

9.4 BLENDS OF BASIC-DYEABLE POLYESTER WITH COTTON

Knitted fabrics made from cotton and basic-dyeable polyester fibres can bedyed in contrasting colours using direct and basic dyes applied together witha nonionic anti-precipitant by a one-bath method at 120°C in a jet oroverflow machine. Aftertreatment with a cationic fixing agent or a reactantresin is advisable to attain satisfactory wet fastness on the cotton. Improvedfastness without resin treatment is possible using reactive and basic dyes. Atwo-bath method is necessary because of the problems of interaction [9]. Thebasic dyes may be applied first, followed by aminochlorotriazine dyes underconventional conditions at 80°C. An inverse process is preferred forvinylsulphone dyes and basic dyes, because these reactive dyeings are stableto the mildly acidic conditions necessary for dyeing the basic-dyeablepolyester component.

9.5 DYEING METHODS AND DYE SELECTION FOR CB BLENDS

Reserve of the basic-dyeable component in these blends can be readily obtainedusing salt-controllable direct dyes, but it is less convenient to reserve thecellulosic fibre because of potential staining by basic dyes. Solid or contrasteffects are attainable in various ways (Table 9.1), according to the fastnessperformance required and the processing costs that can be tolerated. One-bathmethods with direct dyes and disperse or basic dyes offer the simplest approachin pale or medium depths. Various two-stage or two-bath sequences are availableusing reactive, reactant-fixable, sulphur or vat dyes before or after basic dyeingof the non-cellulosic component.

Continuous dyeing methods for solid effects on cellulosic/acrylic blends rangefrom low-cost coloration of the acrylic fibre only, using disperse or basic dyesespecially on acrylic-rich materials, to elaborate two-bath sequences entailingdouble steaming operations. The more costly processes necessary for high all-

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Table 9.1 Dye selections for exhaust dyeing of CB blends


Cellulosic/ Acrylic Single-class Syntan pretreatment, thenacrylic reserve salt-controllable direct dyes

Cellulosic Single-class Localised-charge basic dyesreserve

Solid or One-bath Direct dyes and disperse dyes atcontrast (pale depths) pH 4–5

One-bath Multisulphonated direct dyes andmigrating basic dyes withanti-precipitant

Two-stage Basic dyes at the boil, then directdyes at 80°C

Reactant-fixable dyes at 70°C, thenbasic dyes at the boil

Vinylsulphone reactive dyes, thenbasic dyes at pH 5

Two-bath Basic dyes at the boil, thenaminochlorotriazine dyes at 80°C

Basic dyes at the boil, then vat dyesat 50°C

Cellulosic/ Solid One-bath Direct dyes and disperse dyesmodacrylic

Two-stage or Basic dyes at the boil, then vat,two-bath sulphur or reactive dyes

Cellulosic/acid- Acrylic Single-class Multisulphonated salt-controllabledyeable acrylic reserve direct dyes at pH 8 and 70°C

Cellulosic Single-class Basic dyes with 3% ureareserve

Chrome dyes at pH 4–5

Levelling acid dyes from phosphoricacid solution

Solid Two-bath Basic dyes or chrome dyes, thensalt-controllable direct dyes at pH 8

Cotton/basic- Solid or One-bath Direct dyes and basic dyes withdyeable contrast anti-precipitant at 120°Cpolyester

Two-bath Basic dyes, then aminochlorotriazinereactive dyes at 80°C

Vinylsulphone reactive dyes, thenbasic dyes at pH 5

DYEING METHODS AND DYE SELECTION FOR CB BLENDS


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Table 9.2 Dye selections for continuous dyeing of cellulosic/acrylic blends


Cellulosic/ Solid Pad–dry–thermofix Disperse dyesacrylic (pale depths)

Pad–dry–steam Basic dyes with citric acid

Direct dyes and basic dyeswith anti-precipitant andfixation accelerator

Pad–dry–chemical Halogenated vat dyes orpad–steam sulphur dyes

Pad–dry–thermofix– Ammonium salt pretreatment,chemical pad–steam then vat dye liquids and

stabilised basic dyes at pH 8–9

Pad–dry–steam, Conventional basic dyes,pad–dry–chemical then vat dyespad–steam

Pad–dry–steam, Conventional basic dyes,pad–dry–steam then reactive dyes

round fastness require reactive or vat dyes for the cellulosic component and basicdyes for the acrylic fibre (Table 9.2).

9.6 REFERENCES 1. M White, F Schlaeppi, N E Houser and J T Larkins, AATCC Nat. Tech. Conf. (Oct 1983) 280. 2. A Laeppli and R Jenny, Textilveredlung, 23 (1988) 248.

3. W Haertl, Textil Praxis, 44 (1989) 285; Melliand Textilber., 70 (1989) 354; Textilveredlung, 24(1989) 214.

4. J A Hook and A C Welham, J.S.D.C., 104 (1988) 329.

5. Anon, Chemiefasern und Textilind., 34/86 (1984) 752.

6. F R Latham in Cellulosic dyeing, Ed. J Shore (Bradford: SDC 1995) 246. 7. H M Tobin, Am. Dyestuff Rep., 70 (Sep 1981) 32.

8. H Fischer, Textilveredlung, 13 (1978) 449.

9. M A Herlant, Text. Chem. Colorist, 17 (June 1985) 117; Am. Dyestuff Rep., 74 (Sep 1985) 55,(Oct 1985) 37.

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129

CHAPTER 10

Cotton/viscose and other CC blends

10.1 PROPERTIES AND PERFORMANCE OF CELLULOSIC FIBRES INTHEIR BLENDS

Bast fibres consist of bundles of thick-walled cells held together by non-cellulosicmaterial. The ultimate fibres in flax and ramie are about 30 µm in diameter. Flaxfibres are about 30 mm in length but those of ramie are unusually long (ca. 150mm). In the bast bundles the ends of these individual fibres overlap. Flax andramie are separated into their ultimate fibres before spinning into fine yarns.

Separation of flax fibre bundles from the harvested stems is time-consuming.During prolonged ‘retting’ or soaking in water, the effect of bacterial action onthe intercellular material loosens the fibres sufficiently for mechanical separationby ‘scutching’ and ‘hackling’. The mechanical decortication of ramie can beachieved without preliminary retting.

Bast fibres contain far less cellulose than does cotton (Table 10.1). Theintercellular material includes pectins, hemicelluloses and lignins. Much of this isremoved when flax or ramie is scoured. Inadequately scoured goods are difficultto dye level because the bast fibre and the non-cellulosic impurities differ indyeability.

Table 10.1 Composition of typical natural cellulosic fibres [1]

Proportion of dry weight (%)

Raw Decorticated GreyConstituent flax ramie cotton

Cellulose 80.1 83.3 94.0Intercellular material 10.5 7.5 2.5Wax 2.6 0.2 0.6Ash 1.5 2.1 1.2Residual material 5.3 6.9 1.7

130 COTTON/VISCOSE AND OTHER CC BLENDS

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Traditional viscose filaments have been produced by extruding a solution ofsodium cellulose xanthate dissolved in aqueous sodium hydroxide through aspinneret into a coagulating bath containing sodium sulphate and sulphuric acid.The filaments are stretched mechanically during regeneration and theirproperties are determined by the concentration of the viscose solution, the rate ofcoagulation and the rate of stretching. The coagulation process is controlled bytemperature and additives such as zinc sulphate or glucose. Regular viscosediffers from cotton in being non-fibrillar, having no central lumen and having amuch lower degree of polymerisation (DP). Although consisting wholly ofcellulose, the skin and core of viscose filaments differ somewhat insupramolecular structure. The relative proportions of skin and core varyaccording to the conditions of coagulation.

The presence of viscose in blends with cotton improves the appearance byimparting more lustre and firmness of handle. The regenerated fibre providesadditional absorbency, which is useful in towelling constructions. Apparel usesfor cotton/viscose blends include poplin shirts, blouses, dresswear, knitwear,leisure garments, T-shirts, underwear and children’s clothing. These blends offercomfort appeal with good wear and laundering properties. The optimum blendcomposition for wear resistance is approximately 70:30 cotton/viscose.

Terry towelling may be made from 50:50 or 65:35 cotton/viscose blends forgreater absorbency than all-cotton cloths. An important traditional use forcotton/viscose unions is in brocade material for curtains and furnishings in whichthe viscose appears on the surface in the form of floral designs. Theseconstructions often contain both filament and staple yarns. Pile fabrics aresometimes made with a viscose pile in a cotton backing fabric. These are aneconomical alternative to wool pile/cotton backing fabrics.

Hollow viscose fibres, such as Viloft (Courtaulds), have been produced in anattempt to simulate the natural lumen of cotton. Sodium carbonate isincorporated in the spinning dope. When this is extruded into the acidiccoagulating bath the carbon dioxide formed inside the filament creates acontinuous hollow central channel. Careful control of the conditions of thecarbonate decomposition reaction is necessary to obtain a reproducible product[2]. The hollow structure of the fibre imparts high torsional rigidity leading to anattractive handle with higher bulk and fabric cover than regular viscose at thesame fabric density. Hollow viscose fibres have a lower density (1.15 g cm–3) andhigher water imbibition (130%) than regular viscose (1.52 g cm–3 and 90%),giving good insulation, extra absorbency and comfort.

These characteristics make an important contribution to the appeal of cotton/Viloft and polyester/Viloft blends. These have been exploited successfully in

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PROPERTIES AND PERFORMANCE OF CELLULOSIC FIBRES IN THEIR BLENDS

knitted underwear, sportswear, leisure garments, dresswear and towelling. Inroller towels, for example, a polyester/Viloft warp yarn offers substantiallygreater strength than traditional cotton yet contributes even higher absorbency tothe construction [2].

Compared with cotton, regular viscose suffers from the disadvantage of muchlower breaking strength, particularly when wet. This is seldom a problem inapparel but it renders viscose unsuitable for more critical end uses. The term‘modal’ was introduced to describe all regenerated cellulosic fibres havingtenacities in the conditioned state and wet moduli at 5% extension above certaindefined values [1]. Modal fibres are made with an above-normal concentrationof zinc sulphate in the coagulating bath and ‘modifiers’ (such as dimethylamineor cyclohexylamine) in the spinning dope. These fibres have a higher initial wetmodulus than regular viscose but lower than that of the polynosic fibres.

Polynosic fibres have been defined as having low wet extension even underalkaline conditions, high knot strength and a higher DP than regular viscose. Thedistinctive features of their method of production include:(1) a xanthate solution of viscosity higher than that used in the manufacture of

regular viscose, achieved using an aged pulp of intrinsically higher DP ratherthan a more concentrated solution;

(2) a coagulating bath of low salt concentration with no modifiers or otheradditives;

(3) a lower temperature of extrusion than regular viscose.

Under these conditions, high stretch (up to 300%) can be achieved. Polynosicfibres are highly oriented and have stress–strain curves closely similar to those ofcotton, rather than to other regenerated cellulosic fibres. They appear to befibrillar in structure and are largely unaffected by dilute solutions (up to 8%) ofsodium hydroxide, which will dissolve as much as 25% of regular viscose.Polynosic fibres show improved laundering performance and give good yarnstrength in blends with cotton but they are generally less useful for blending withpolyester fibres [2].

Modal and polynosic fibres are finding increasing application in blends withcotton for knitgoods. Vincel (Courtaulds) in a polynosic fibre that is used aloneor in blends with cotton, viscose or polyester staple fibres. A popular blend forapparel fabrics is 50:50 cotton/Vincel. Blends of cotton with modal fibres areparticularly important in woven dresswear and lightweight suitings. These modal-rich blends (80:20 to 55:45) are usually designed to exploit the lustre, drape andsoftness of the regenerated fibre, whereas in cotton-rich mixtures the strength,washability and durability of the natural fibre make important contributions.


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CH2

NCH2

O

CH3

OCH2

CH2

Figure 10.1 Solvent for manufacture of lyocell regenerated cellulosic fibres

The degree of swelling of modal and polynosic fibres alone and in 50:50blends with cotton was compared with an all-cotton control in solutions ofsodium hydroxide at concentrations up to 250 g l–1 and temperatures in therange 20–80°C. At ambient temperature the regenerated fibres were muchmore swollen than cotton and the 50:50 blends showed the expectedintermediate degree of swelling. As the temperature of treatment increasedtowards 80°C, all fibre types and blends showed enhanced swelling, so thatthe behaviour of the individual fibres and their blends became more closelysimilar. Under mercerising conditions of high alkali concentration and lowtemperature for a short time, the modal fibres and cotton control behavedsimilarly [3].

The adverse effects of conventional viscose manufacturing plants on theenvironment has been recognised for many years. Only recently, however, hasCourtaulds plc established commercial production of a regenerated cellulosicfibre using a non-aqueous solvent method. Tencel (Courtaulds) is a ‘lyocell’ fibreobtained by continuous dissolution of wood pulp in mesomorphic N-methylmorpholine-N-oxide (Figure 10.1) and extrusion into a dilute aqueoussolution of the amine oxide to precipitate the regenerated fibre [4]. The dilutedsolvent is then purified and reused at the continuous dissolving stage, so that theprocess is environmentally innocuous.

Tencel has a bright lustre and a circular cross-section. The tenacity (wetand dry) is markedly higher than that of cotton or any other type ofregenerated cellulosic fibre (Table 10.2). The wet tenacity is only about 15%lower than the dry value and is markedly higher than that of cotton. Theexceptionally high wet modulus results in very low shrinkage, about 2% inwarp and weft [5]. Tencel is fibrillar in structure and resembles cotton evenmore closely than modal fibres in its behaviour under stress and capacity forabsorbing liquid water. Because of the close similarity between the stress–strain curves of Tencel and cotton, it can contribute to the strength of theblended yarn even at low blend levels. An interesting feature of Tencel is that

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PROPERTIES AND PERFORMANCE OF CELLULOSIC FIBRES IN THEIR BLENDS

the conversion of fibre strength to yarn strength is considerably higher thanfor other cellulosic fibre types, because of the high cohesion between theclosely packed fibres of circular cross-section in the yarn. Tencel improvesthe performance of blends with cotton by enhancing strength, lustre, yarnregularity, spinning and wear performance.

10.2 DYEING BEHAVIOUR OF CELLULOSIC FIBRES IN THEIR BLENDS

Linen and its blends with cotton have been used traditionally in fine wovenapparel and household textiles, notably tablecloths, napkins, curtains andfurnishings. Ramie/cotton blends (60:40 to 50:50) are of interest for woven orknitted leisure clothing. Linen fabrics and blends of linen or ramie with cottonwill withstand an alkaline scour at the boil, followed if necessary by a combinedperoxide/chlorite bleach. Dyeing in rope form with direct dyes is followed byconventional resin finishing. Treatment with liquid ammonia at 20 m min–1 canbe carried out before or after dyeing to enhance the performance of the finalfinish. It is essential to carefully neutralise any retained ammonia by treatment inrope form with acetic acid solution. Effective finishing is also important toachieve optimum wet fastness of the direct dyes [6].

The traditional growing of flax has been resumed in Saxony since 1993.Fabrics woven from open-end yarns spun from 40% short-staple linen and 60%modal fibres are being produced. Various preparation sequences have beenevaluated, including enzymatic desizing in a jet machine and cold pad–batchbleaching, or continuous pad–steam scour-bleach treatment with alkalineperoxide. The preferred dyeing process is exhaust dyeing with reactive dyes,which offer excellent reproducibility, levelness, penetration and fastnessperformance [7].

Direct dyes usually dye viscose and mercerised cotton preferentially because agreater surface area is available for sorption on these substrates than on

Table 10.2 Typical physical properties of cellulosic fibre types [4]

Fibre tenacity (cN tex–1) Moisture WaterElongation regain imbibition

Fibre type Wet Dry (%) (%) (%)

Viscose 10–15 22–26 20–25 13 90Modal 19–21 34–36 13–15 12.5 75Lyocell 34–38 40–42 13–15 11.5 65Cotton 26–30 20–24 7–9 8 50


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unmercerised cotton. The order of increasing substantivity on various cellulosicfibres does not differ significantly from dye to dye, except in the case ofphthalocyanine blues. These show higher substantivity for cotton than forviscose or modal fibres. This is because their affinity for regenerated cellulose islower in spite of its greater accessibility [8].The higher dyeability of mercerisedcotton is attributable to the lower surface charge on this substrate. Thisdifference is less marked at higher electrolyte concentrations but under theseconditions direct dyes show slower migration and inferior levelling.

Solidity of shade with direct dyes on cotton/viscose blends varies considerablywith dye structure but it can be controlled by adjusting salt concentration anddyeing temperature. The substantivity of direct dyes for cellulose isapproximately inversely related to their degree of sulphonation. Direct dyesgiving good solidity on cotton/viscose blends tend to be mainly disazotetrasulphonates, including some copper-complex types. Pale shades present littledifficulty when dyed in the absence of salt. Solidity in deeper shades is achievedmore readily with little or no salt present at the boil.

Where salt must be used for medium and full depths in order to attaineconomical exhaustion, sometimes solidity can be ensured only by dyeing at atemperature as low at 60°C. The optimum conditions vary from one dye toanother and result in reduced penetration and lower wet fastness. Blends ofmercerised cotton and viscose, however, will often give good solidity in fulldepths by dyeing at the boil with only low concentrations of salt (0–5 g l–1)because of the higher dyeability of mercerised cotton. The development ofviscose microfibres (section 1.4.2) has enabled colour yields and reflectancevalues to be obtained with direct and reactive dyes that are close to those oncotton. As a result, the attainment of solid effects on fabrics containing viscosemicrofibres and cotton is now easier than on conventional cotton/viscose blends[9].

Solidity of shade is normally aimed at in the dyeing of cotton/viscose blendsand it is not feasible to attempt reservation of either fibre. Vat or sulphur dyes areoften used because it is generally more difficult to achieve solidity with direct orreactive dyes. Brocades and other furnishing fabrics woven into designs formedby raised viscose wefts on a cotton warp ground are not as critical as intimateblends where lack of solidity gives an objectionable skittery appearance. As withdirect dyes, the degree of solidity attainable with vat dyes depends on dyeselection and conditions of application. Preferred vat dyes include acridones,carbazoles, indanthrone blues and violanthrone blues and greens. If conventionaldyeing conditions are applied, regular viscose will invariably be dyed moreheavily than the cotton. A decrease in dyeing temperature favours the cotton, so

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Table 10.3 Differences in dyeability between various cellulosicfibres [10]

Equilibrium exhaustion (%) of CI Direct

Fibre Yellow 106 Red 80 Blue 218

Prima 80 99 74Avril 74 92 66Viscose 66 90 66Viloft 66 88 66Cotton 59 84 50

DYEING BEHAVIOUR OF CELLULOSIC FIBRES IN THEIR BLENDS

cold-dyeing vat dyes are more suitable in general. Pigment padding methods tendto give better solidity than batchwise dyeing on the jig at 30–50°C.

The proportions of crystalline material in regenerated cellulosic fibres areabout 40% in regular viscose, 50% in modal fibres and 65% in polynosics,compared with 70% in cotton. As crystallinity increases the water imbibitionand dyeability decrease accordingly. Thus direct dye uptake under a given set ofconditions generally increases in the order: cotton < polynosics < modal fibres <regular viscose.

The uptake of selected direct dyes by the hollow viscose fibre Viloft(Courtaulds) and two crimped modal fibres Avril (Avtex Fibers) and Prima (ITTRayonier) has been compared with cotton and regular viscose as controls (Table10.3). CI Direct Red 80 is a tetrazo hexasulphonate of unusually high affinity,whereas Blue 218 (a copper-complex disazo tetrasulphonate) and Yellow 106have only moderate affinity for cellulose. The percentage exhaustion atequilibrium was consistently lower on cotton than on any of the regeneratedcellulosic fibres, as expected. The dyeability of Viloft was closely similar to thatof regular viscose. Prima was consistently more dyeable than Avril, which was inturn more dyeable than regular or hollow viscose.

Methods of dyeing modal fibres and cotton/modal blends with direct, reactiveor vat dyes have been reviewed [11]. The selection of reactive dyes for theproduction of shadow effects on 50:50 cotton/modal blends was outlined.Exhaust dyeing systems for loose stock, yarn, knitted and woven fabrics weredescribed. Several padding methods on fabrics made from these blends weredetailed, including pad–batch or pad–thermofix with reactive dyes, pad–jig orpad–steam with vat dyes, and pad–develop with vat leuco esters.


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The dye affinity of polynosic fibres such as Vincel (Courtaulds) can beenhanced by pretreatment with sodium hydroxide solution. The concentrationshould not exceed 60 g l–1 or there may be a deterioration in physical properties.Most dyes are absorbed more slowly and migrate less readily on Vincel than onregular viscose. Blends of Vincel with either cotton or viscose can be dyed in solidshades with carefully selected direct, reactive, vat or sulphur dyes, many dyesbeing most suitable at specific depths of shade. The fastness properties of most ofthese dyes are the same on Vincel and regular viscose. It is not practicable to dyeVincel and reserve the other cellulosic fibre, although attractive shadow effectscan be produced with selected dyes and controlled dyeing conditions.

A minority of direct dyes, especially from the self-levelling and temperature-controllable classes, will give solid shades in pale depths on cotton/Vincel.Selected high-reactivity dyes can also be used, either by pad–batch or exhaustapplication. Certain bright reactive dyes appear slightly duller on Vincel than oncotton or viscose. Vat dyes are usually selected from those applicable at 50°C orhigher temperatures and are preferably applied by pigment padding to improvepenetration and solidity of shade. Selected sulphur dyes can be used in mediumand full depths.

Any of the dye classes used for cotton can be applied to Tencel (Courtaulds)lyocell fibre. Consistency of Tencel in terms of dyeability is routinely monitoredusing dyes known to be sensitive to potential variations in this importantproperty [4]. In exhaust dyeing the colour yield on Tencel is similar to that onviscose and greater than that on cotton. Thus care is required when dyeingcotton/Tencel blends because of preferential uptake by Tencel. This makesshadow effects much easier to achieve than solidity in exhaust dyeing. Viscose/Tencel blends, on the other hand, readily give solid effects.

The yield of reactive dyes on Tencel is exceptional by all dyeing methods andespecially by printing. Thus alkaline treatment analogous to causticisation ofviscose or mercerisation of cotton is not necessary for Tencel. Blends of cottonand Tencel can be mercerised, however. By careful modification of conventionalsemi-continuous dyeing methods, solid dyeings with reactive dyes can beachieved on 50:50 cotton/Tencel, using either pad–batch application or a sodiummetasilicate development technique.

10.3 DYEING METHODS AND DYE SELECTION FOR CC BLENDS

Blends of cellulosic fibres with one other are ideally suitable for shadow effects,especially those in which unmercerised cotton is the paler component and aregenerated cellulosic fibre is the deeper one. A wide selection of dyes and dyeing

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Table 10.4 Dye selections for CC blends

Blend Colour effect Dye selection

Cotton/linen Solid or shadow Direct dyes or vat dyesCotton/ramie

Mercerised Solid Direct dyes at low salt concentrationcotton/viscose at the boil

Unmercerised Solid Disazo tetrasulphonated direct dyescotton/viscose at 60°C

Cotton/viscose Solid or shadow Selected vat dyes at 20–30°C

Cotton/viscose Solid or shadow Direct or reactive dyesmicrofibres

Cotton/modal Solid or shadow Direct, reactive or vat dyes

Cotton/polynosic Solid Selected self-levelling andCotton/lyocell temperature-controllable direct dyes

Selected vat dyes at 50°C

Selected high-reactivity dyes by pad–batch

Viscose/polynosic Solid or shadow Direct, reactive or vat dyesViscose/lyocell

DYEING METHODS AND DYE SELECTION FOR CC BLENDS

conditions yields satisfactory solidity on blends of mercerised cotton or regularviscose with one another or with other regenerated cellulosics. Dye selection anddyeing techniques are more restricted when dyeing solid effects on unmercerisedcotton blends (Table 10.4).

10.4 REFERENCES 1. T P Nevell in Cellulosics dyeing, Ed. J Shore (Bradford: SDC, 1995) 6.

2. R Aitken, J.S.D.C., 99 (1983) 150. 3. D Bechter, H Herlinger and E Pelz, Textil Praxis, 41 (1986) 59.

4. J M Taylor and P Mears, J.S.D.C., 107 (1991) 64; Chem. Brit. 30 (1994) 628.

5. I D Holme, Dyer, 178 (Oct 1993) 13. 6. G Kratz and A Funder, Melliand Textilber., 68 (1987) 775.

7. H Hellwich, Melliand Textilber., 78 (1997) 346.

8. O Annen, H Gerber and B Seuthe, Melliand Textilber., 72 (1991) 1015; J.S.D.C., 108 (1992) 215. 9. D Hildebrand and F Stöhr, Melliand Textilber., 73 (1992) 281.

10. V Davis and R R King, J.S.D.C., 100 (1984) 342.

11. W Schaumann, Textilveredlung, 22 (1987) 15.

138 POLYESTER/WOOL AND OTHER DA BLENDS

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138

CHAPTER 11

Polyester/wool and other DA blends

11.1 DYEING OF POLYESTER/WOOL BLENDS

11.1.1 Properties and preparation of polyester/wool blends

The achievement of a desirable combination of physical properties is usually themain justification for utilising a DA blend. Strength, abrasion resistance, creaserecovery and durable pleating characteristics are contributed by the polyesterfibre component. Virtually all goods made from polyester/wool blends areintended for outerwear, typically suitings, dresses and skirts. Modified polyesterfibres with improved resistance to pilling have been blended with wool in knittedjersey dresswear. This can eliminate the need to singe polyester/wool fabrics, atreatment not usually available in wool processing. Singeing may also introducedyeability differences. Unlike all-wool jersey, polyester/wool fabrics can often bedyed on the beam and these blends show better dimensional stability on washing.

Such blends were developed in the early 1950s and have been established inwoven suitings ever since. The important 55:45 polyester/wool blend arose fromthe realisation that this is the minimum polyester content that allows durablepleating of the blend fabric. Reducing the wool content lowers the aestheticappeal but decreasing the polyester proportion makes it no longer possible toretain pleated effects after washing. The most important blend in the USA is an80:20 fabric, composed of a textured filament polyester warp and a 55:45polyester/wool blended staple weft. In Western Europe another luxurious fabricis a 20:80 blend, containing a 55:45 blended staple warp and a pure wool weft.Smaller market niches exist, e.g. a 40:60 polyester/wool blend for luxuryautomotive fabrics. This specific outlet puts high demands on both fabric anddye performance. To meet these demands the two fibres are usually dyedseparately as loose stock or tops and subsequently blended [1].

In recent years the ‘re-discovery’ of natural fibres has given increased emphasisto the aesthetic appeal of wool-rich blends, rather than the optimum balance ofcomfort, wear and easy-care properties that is provided by the popular 55:45

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blend. In these blends the content of polyester may be as low as 20–30% and theproduction of solid shades using conventional one-bath dyeing methods can bequite difficult. The polyester component is often dyed much weaker in depththan expected [2].

Polyester fibres will withstand the normal processes used to prepare woolfabrics, such as carbonising and milling. The stentering temperature must not betoo high when carbonising polyester/wool fabrics, however, because appreciabledamage and yellowing of the polyester component may occur under suchconditions. Neutralised fabrics should have a slightly acidic pH to avoid possibledamage to the wool [3]. It is usual to give a crabbing treatment to polyester/woolfabrics to minimise creasing during winch or jet scouring and subsequent dyeing.Jet dyeing has a mild milling action on these goods and yields a softer handlecompared with the somewhat crisper feel characteristic of beam-dyed fabrics.

Careful preparation of the fabric prior to beam dyeing is most important.Preshrinking is necessary to prevent any moiré effect (water marking) that mayarise from differential shrinkage on the beam [4]. Presetting at 170–190°Cprotects against rope creasing or possible shrinkage in beam dyeing. Highersetting temperatures cause yellowing of the wool. Heat setting improves thehandle, resilience, crease resistance, dimensional stability, shrink resistance andpilling performance of the goods. It does, however, reduce the dyeability of thepolyester component after setting. This may aggravate wool staining.

Scouring with an anionic detergent and soda ash eliminates the risk of residualnonionic detergent being carried out into the dyebath and adversely affecting thedispersion stability of disperse dyes. Polyester/wool knitted fabrics may bescoured in the jet machine with ammonia at 40°C before dyeing. Bright and/orpastel shades may require a preliminary mild bleaching treatment. The wool maybe given either an oxidative or a reductive bleach, whereas the polyester onlyrequires treatment with a fluorescent brightening agent.

11.1.2 Stages at which dyeing may be carried out

The dyer of polyester/wool has three options:(1) Dyeing each fibre type separately as loose stock or tops before blending.(2) Dyeing as an intimately blended yarn.(3) Dyeing as fabric in rope form or open width.

Dyeing separately prior to blending allows the choice of dyes of maximumfastness, with level dyeing performance being of secondary importance. Thepolyester can be dyed at 130°C with high-energy disperse dyes of maximum

DYEING OF POLYESTER/WOOL BLENDS


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sublimation fastness. Wool as loose stock or tops can be dyed with chromereactive, 1:2 metal-complex or milling acid dyes.

Dyeing in yarn or piece form allows greater flexibility with shorter lead timesin production and lower stockholding. The most important factor in polyester/wool yarn dyeing is shrinkage of the yarn, for which 3–5% is consideredacceptable [5]. In contrast to polyester/wool fabrics, yarns blended from thesetwo fibres are not heat set before dyeing. Since disperse dyes show higher affinityfor unset polyester, high- or intermediate-energy dyes can be applied to the yarnat 105°C to give higher fastness to sublimation and less staining of the wool.Dyeing at these later stages (especially as fabric) demands better level dyeingbehaviour from the dyes. This introduces constraints on the level of fastnessattainable from the disperse dyes, which must be low- or intermediate-energytypes in piece dyeing.

Beam dyeing is particularly suited to flat woven constructions and thosefabrics where felting could be a problem, i.e. wool-rich blends. Since the fabric isheld stationary and the liquor percolates through it, there is no mechanical actionon the cloth to induce felting shrinkage. The beam is less effective on structuredfabrics where the jet or overflow machine helps to retain fabric handle and bulk.A specific problem associated with dyeing polyester/wool yarn or piece inenclosed machinery at temperatures above the boil is control of dyebath pH.Chemical changes brought about in wool by heat can cause the pH to rise. It ifrises above pH 7 the disperse dye dispersion can become unstable, as well ascausing further damage to the wool. Acetate–acetic acid buffer systems are oftenused for their economy and relative freedom from effluent problems.

11.1.3 Selection of dyes and carriers

The dyeing of polyester/wool is almost always directed towards solidity ratherthan differential effects, because unfortunately the most troublesome of all cross-staining problems is the staining of wool by disperse dyes (section 3.4). Thisblend is most frequently dyed using disperse dyes for the polyester and millingacid or 1:2 metal-complex dyes for the wool. Matched formulations containingboth disperse and anionic dyes are commercially available. Since neutral toslightly acidic conditions are required for both dye classes there are no conflictingpH requirements. The critical parameter is that of temperature, since the toptemperature (130°C) that would allow all disperse dyes to be used would alsocause unacceptable degradation of the wool. A compromise temperature withinthe range 95–120°C is adopted and disperse dyes of low- or intermediate-energyclasses are selected to perform well at the chosen temperature.

The temperature at which the dyeing machine operates is the most important

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factor affecting the length of the dyeing process, the choice and colour yield ofthe disperse dyes and the amount of carrier required. The two important factorsgoverning disperse dye selection are:(1) the temperature at which the polyester/wool is to be dyed;(2) the degree to which the disperse dye stains the wool.

The higher the dyebath temperature, the wider the choice of dyes, particularly formedium or heavy depths of shade. Disperse dyes without carrier have virtuallyno substantivity for polyester below 85°C but above this point the dyeing ratedoubles for each 5°C rise in temperature. The slow rate of diffusion of dyes intothe polyester is an indirect cause of the preferential staining of wool below theboil. Even at low temperatures wool is rapidly penetrated by disperse dyes,especially if dyed in the presence of a carrier. Some low-energy azo disperse dyesare taken up by wool extremely rapidly.

High-energy dyes do not build up satisfactorily on polyester at the boileven in the presence of a carrier, compared with low-energy dyes with betterlevelling properties but lower fastness to heat. Only pale depths can be dyedwith intermediate-energy dyes at the boil. Medium depths with these dyesrequire a dyeing temperature of 105°C at least. Good carrier-dyeingproperties and low staining or ease of clearing from wool are given byselected intermediate-energy dyes of the nitro, monoazo and especiallyanthraquinone types. Many anthraquinone dyes are absorbed quickly ontothe polyester surface. Those blue and navy dyes that build up satisfactorily atthe boil tend to be anthraquinone-based and relatively expensive tomanufacture.

The inherently more cost-effective azo types will not build up to full depths atthe boil, showing inferior levelling and more staining of the wool. To gain thebenefit of this cost-effectiveness, it is necessary to dye at 105°C or above. Fullnavy or black shades are best dyed with intermediate-energy dyes at 110–120°Cusing a wool protective agent. If maximum colour yield and fastness areessential, high-energy dyes should be applied to the polyester at 130°C beforeblending with predyed wool. Even if the depth on the polyester is slightly heavier,the blend may still give an appearance of solidity because of the higher lustre ofthe synthetic fibre.

Several factors must be considered in selecting a suitable carrier for polyester/wool dyeing. These include the types of dyeing equipment available, the degreeof staining of the wool and the relation between dye yield and appliedconcentration of carrier. Carriers exert a plasticising influence on the polyesterstructure and cause the glass-transition temperature to be lowered, although the



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enhanced swelling that accompanies carrier treatment is not necessarilyassociated with acceleration of diffusion of the dyes into the fibre. An importantaspect of carrier dyeing is the concentration–efficiency profile, i.e. the range ofcarrier concentrations within which maximum acceleration of dyeing takesplace. This occurs when the voids in the polyester fibre become saturated withcarrier [1]. Further additions of carrier above this saturation limit merely result ina corresponding increase in carrier concentration in the dyebath. This isaccompanied by a restraining action on the disperse dyes present. This isdetrimental to colour yield and hence wastes both carrier and dye.

It is known that o-phenylphenol tends to accentuate wool staining more thanmethylnaphthalene or phenolic esters like methyl cresotinate. These are in turninferior in this respect to diphenyl, trichlorobenzene and the inert esters like butylbenzoate. Diphenyl, o-phenylphenol and trichlorobenzene tend to cause morewool damage and yellowing on exposure to light than do ester carriers.Nevertheless o-phenylphenol has been much used traditionally for polyester/wool dyeing on ground of cost-effectiveness. Odour has been a problem withmethylnaphthalene or methyl salicylate and trichlorobenzene has been longregarded as a serious toxic hazard. Diphenyl has tended to give localised carrierspotting and the esters are inferior in cost-effectiveness.

There is relatively little cross-staining of the polyester fibre by anionic dyes forwool. Levelling acid dyes give negligible staining and the reserve of polyester by1:1 metal-complex or milling acid dyes is good to very good, with 1:2 metal-complexes and chrome dyes moderate to good in general. Generally speaking,unsulphonated 1:2 metal-complex dyes stain polyester more than theirsulphonated analogues and premetallised azo dyes more so than unmetallisedazo acid dyes. Anionic dyes begin to dye the wool at 40–50°C and dyebathexhaustion is virtually complete after about 30 minutes at the boil. Dye selectionfor wool is almost independent of dyeing temperature.

Wool dye selection can thus be made on grounds of wet fastness, since thelevelling and coverage is adequate under the conditions required to obtain asatisfactory dyeing of the polyester fibre. Neutral dyeing of polyester/wool blendsis not recommended for three reasons:(1) Damage of the wool is much less in the isoelectric region (pH 4.5–5) compared

with pH 6–8 conditions.(2) The release of wool breakdown products causes the dyebath pH to rise,

resulting in instability of the disperse dye dispersion.(3) Certain azo disperse dyes susceptible to reduction are adversely affected by

high temperature dyeing, as a result of the progressive hydrolysis of cystineresidues in wool.

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In the dyeing of these blends an optimum pH 5–5.5 should be maintained whenusing sensitive azo disperse dyes and, if necessary, a reduction inhibitor such asformaldehyde should be present in the dyebath [6].

Neutral-dyeing 1:2 metal-complex and milling acid dyes are preferred forpolyester/wool because 1:1 metal-complex and levelling acid dyes requirestrongly acidic dyebaths where disperse dye staining is severe. Chrome dyes aregiven an oxidative aftertreatment that can damage the wool and change the hueof the dyed material [7]. A one-bath method for polyester/wool yarn usingdisperse and chrome dyes allows full reduction clearing to eliminate loosedisperse dye without causing a significant shade change or loss of depth of thewool dyeing [8].

Premetallised and milling acid dyes for wool tend to exhaust too rapidly at pH4.5–5, especially the unsulphonated types and pale or medium depths ofmonosulphonated 1:2 metal-complex dyes, resulting in unlevel dyeings. Dispersedye staining is also more severe. For these reasons the dyebath is normallybuffered at pH 5.0–5.5, just above the ‘ideal’ isoelectric zone, necessitatingcareful control of treatment time. Most milling acid dyes show satisfactoryexhaustion and levelling after 10 minutes at 110°C in this region of pH.

Thus premetallised dyes, supplemented by selected milling acid dyes in brightshades, are widely used. The choice between unsulphonated, monosulphonatedor disulphonated 1:2 metal-complex types is related to dyebath conditions,coverage and levelling requirements, as well as the target level of wet fastness insubsequent tests. Unsulphonated premetallised dyes applied from a weakly acidicbath show better levelling properties than sulphonated types but are moreexpensive and lower in wet fastness, particularly in respect of staining of adjacentnylon, a common lining material in polyester/wool garments.

Monosulphonated 1:2 metal-complex dyes have good coverage and levellingproperties at the high temperatures necessary for polyester/wool dyeing as well asgood wet fastness and better cost-effectiveness than unsulphonated analogues.They are usually applied within the range of pH 5–6, according to depth ofshade. Disulphonated 1:2 premetallised dyes show more limited coverage,although this is still adequate at high temperature. The wet fastness is the best ofthe three types and these dyes are usually the most cost-effective. Disulphonateddyes are typically used for full depths and are best applied at pH 5.

The choice between the one-bath and two-bath methods with disperse andneutral-dyeing acid dyes depends on applied depth and target fastnessrequirements. The one-bath method is more economical and gives satisfactoryfastness properties in pale or medium depths. The two-bath sequence gives morereproducible solidity or brighter contrast effects as well as optimum fastness in



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full depths. In the one-bath process the disperse and acid dyes are appliedtogether at pH 5–6 with a dinaphthylmethanedisulphonate dispersing agent andthe selected type and concentration of carrier. The disperse dye stain is clearedfrom the wool with a nonionic detergent at 60–70°C. Low-energy disperse dyesof relatively low affinity for polyester tend to cause more staining in the one-bathprocess than high-energy dyes, particularly at longer liquor ratios.

In the two-bath method the polyester component is dyed conventionally withthe disperse dyes alone under the conditions specified above. Surface depositionon the polyester and the stain on the wool are removed by reduction clearing at45–50°C with sodium dithionite, ammonia and nonionic detergent. The wool isthen cross-dyed at the boil with 1:2 metal-complex or milling acid dyes from anammonium acetate–acetic acid bath containing an alkylamine polyoxyethylenelevelling agent. The disperse dyes selected for the two-bath method should showminimum transfer from polyester to wool under these conditions. Dyes of highfastness to sublimation do not transfer so readily as those of moderate fastness.

Quite recently, a series of new benzothienylazo disperse dyes (Figure 11.1) hasbeen evaluated [9]. These are capable of dyeing both components of a polyester/wool blend to yield satisfactory exhaustion and fastness (Table 11.1). Dyeingwas carried out at pH 4.5 and the boil in the presence of methyl salicylate ascarrier. The wool was invariably dyed more deeply than the polyester,presumably because all the coupling components selected were phenolic typeswith some anionic character. Substituted aniline-type couplers are almost alwaysused in the synthesis of conventional monoazo disperse dyes. In view of thelimited range of relatively dull hues obtained, this approach is not of muchpractical value so far unless dyes of brighter hue, especially blues, can bediscovered to augment them.

Figure 11.1 Benzothienylazo dyes for single-class polyester/wool dyeing

11.1.4 Wool damage and wool protective agents

The dyeing of polyester/wool blends presents difficulties of severe damage ofwool at the top temperature normally used to dye polyester with high-energydyes. The range of disperse dyes performing adequately in carrier dyeing at105°C is limited. Some carriers are toxic (e.g. trichlorobenzene) and all carriersare harmful in the working environment. There are considerable advantages to

SN

X

N R

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Table 11.1 Hue and fastness properties of benzothienylazo dyes [9]

Light fastness Wash fastnessSubstituent Coupler Hue on(X) (R) polyester/wool P P/W W P P/W W

CN Resorcinol Yellowish orange 7 6 6 4 5 5CN Naphthalene-2,3-diol Reddish brown 7 7 6 4 5 5CN Naphthalene-2,7-diol Reddish orange 6 7 7 5 4 5CN 8-Hydroxyquinoline Reddish violet 7 6 7 4 5 5CN 2-Naphthol Reddish orange 7 7 7 4 4 5COOEt Resorcinol Reddish brown 6 7 6 4 5 5COOEt Naphthalene-2,3-diol Reddish violet 6 6 7 4 4 5COOEt Naphthalene-2,7-diol Reddish violet 6 7 6 5 5 5COOEt 8-Hydroxyquinoline Reddish brown 7 7 6 4 5 5COOEt 4-Chloro-1-naphthol Reddish violet 7 7 6 4 5 4

P = polyesterW = woolP/W = polyester/wool

be gained by dyeing at 110°C or above, in terms of colour yield, levelling,fastness and shorter processing cycles. Against this must be balanced the physicaland chemical degradation of the wool keratin when treated under theseconditions. Aqueous hydrolysis results in breakage of electrostatic linkagesbetween oppositely charged sidechains. Dyeing wool at the boil and pH 2–4results mainly in hydrolysis of peptide bonds (depolymerisation). At pH 5 andabove the main effect is hydrolysis of the disulphide bonds in cystine units(breaking of crosslinks). Damage is considerable at 120°C and any pH.

These chemical changes result in lower strength and abrasion resistance,accompanied by increased elongation, alkali solubility and yellowing. The extentof damage to the wool depends on dyebath temperature, pH and treatment time.The degree of yellowing of the wool is greater at 120°C than at 110°C but theeffect of yellowing is less critical when dyeing in dark shades. For these dyeingsthe higher temperature favours higher exhaustion of intermediate-energydisperse dyes. Such problems are especially acute with ultra-lightweightdresswear made from polyester microfibres blended with exceptionally finewool.

In order to minimise wool damage, it is necessary to add a reagent that willchemically modify the molecular structure of the wool keratin, stabilising certainreactive sidechains and forming new crosslinks that replace those lost duringhydrolysis. For many years, formaldehyde was the only reagent to confereffective protection in this way (Scheme 11.1). The crosslinking action of



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Scheme 11.1

formaldehyde causes embrittlement of wool, slightly reducing the elongation atbreak and markedly lowering the urea-bisulphite solubility. When dyeing in thepresence of 5% o.w.f. formaldehyde (30% solution), the treatment time shouldnot exceed 60–90 minutes at 110°C or 30–40 minutes at 120°C.

Several further precautions are necessary if formaldehyde is added:(1) Careful dye selection is important because certain dyes undergo a colour

change in the presence of formaldehyde at 110–120°C.(2) Treatment should be within the isoelectric region for wool (pH 4.5–5) and

the maximum permitted dyeing time should not be exceeded.

[wool] CH2S SCH2 [wool] [wool] CH2SH HSCH2 [wool]

[wool] CH2SCH2SCH2 [wool]

CYS CYS link

[wool] (CH2)4 NH2

[wool] (CH2)4 NHCH2OH [wool] (CH2)4NHCH2NH(CH2)4 [wool]

LYS LYS link

CH2[wool] OH CH2[wool] OH

CH2OH

CH2OH

N

CH2[wool]

H CH2[wool] OH

CH2OH

CH2NH(CH2)4 [wool]

NHOCH2

CH2[wool]

CH2OH

+

Cysteine (CYS) HCHO

Lysine (LYS)

NHOCH2

CH2[wool]

CH2NH(CH2)4

HCHO

Methylollysine

H2O

2HCHO

[wool]

LYS

Tyrosine (TYR) Dimethyloltyrosine

TRY LYS link

LYS

Tryptophan (TRY)

2HCHO

LYS

Dimethyloltryptophan

TYR LYS link

Cystine

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Scheme 11.2

(3) Ammonium salts for pH control should be replaced by an acetate–aceticacid or mixed phosphate buffer, because ammonia reacts with formal-dehyde to form hexamethylene tetramine.

(4) Formaldehyde vapour is environmentally hazardous and this increasinglyrestricts the circumstances in which it can be used. This trend favoursthe replacement of formaldehyde by precursor compounds of theN-methylolamide class. Disadvantages of this process include residualodour, variations in fabric strength and harshness of handle, especially withbeam-dyed fabrics [10].

The leading formaldehyde precursor used for this purpose isdimethylolethyleneurea (DMEU). This functions as a wool protective agent byslow decomposition to release formaldehyde into the dyebath. There is littlerelease of the active species +CH2OH at pH 4–6 and 70°C but at110–120°C DMEU is almost completely dissociated into ethyleneurea andformaldehyde (Scheme 11.2).

Scheme 11.3

The addition of dimethyloldihydroxyethyleneurea (DMDHEU) when dyeingpolyester/wool permits treatment for up to one hour at 120°C, resulting inimproved colour yields on the polyester component without detrimental effect onthe physical properties and structure of the wool. By a similar mechanism(Scheme 11.3), DMDHEU acts as a stabiliser for the wool in pressure dyeing andaccelerates the rate of dyeing of the polyester fibre without adversely affecting theequilibrium exhaustion of the acid dyes [11].

Reactive dyes also exert a protective effect on wool keratin. They are believedto minimise damage by reacting with the cysteine residues formed by cystine

NNC

O

HOH2C CH2OH NHHNC

O

2H+

+ 2 +CH2OH

DMEU EU

NNC

O

HOH2C CH2OH

HO OH

NHHNC

O

DHEU

HO OH

2H+

DMDHEU

+ 2 +CH2OH



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hydrolysis (Scheme 11.1). This concept has been exploited to develop colourlessprotective agents that imitate the behaviour of reactive dyes [12]. These moresophisticated products are likely to be less cost-effective than formaldehydeprecursors but they do avoid the hazards of exposure to formaldehyde vapour.

In a recent investigation of this problem, botany wool fabrics were treatedwith solutions of potential protective agents at 120°C or 130°C and pH 4. Wooldamage was assessed in terms of tear strength and wet bursting strength. Theaddition of either potassium bromate (KBrO3), to counteract the reductivehydrolysis of cystine disulphide crosslinks, or sodium hydrogen maleate(HOOC–CH=CH–COONa), to react with the thiol groups of the cysteineformed, improved the strength retention. Both products caused stiffening of thefabric but the handle of the bromate-treated wool was superior to that treatedwith sodium hydrogen maleate [13]. Dimethylolethyleneurea and selectedvinylsulphone and α−bromoacrylamide reactive dyes gave some protection butthe wool damage was still substantial at 130°C.

11.1.5 Future prospects for polyester/wool blends

The availability of deep-dye and basic-dyeable polyester yarns (section 5.1) haswidened the range of possibilities of fabric design in piece-dyed polyester/wool.Deep-dye polyester staple fibres blended with wool can be dyed without the costor pollution problems associated with carriers and pressure-dyeing equipment isnot essential. The melting point and initial modulus of deep-dye polyester,however, are lower than the normal fibre. Trevira 350 (HOE) is a low-pill staplepolyester of high dyeability but lower tensile strength and resistance to abrasionthan the standard homopolymer. Pretreatment of Trevira 350/wool blends entailsan emulsified solvent scour to remove oil stains, followed by drying and heatsetting. The dyebath is set at pH 5–6 with a levelling agent and a crease lubricant.A sequestering agent is not normally required. The wool component is oftendyed with 1:2 metal-complex dyes. Dyeing is generally carried out on anoverflow machine at 105°C. Under these conditions the colour yield on Trevira350 is 5–10% higher than on standard polyester [14].

Carriers have been widely recognised to be injurious to the health ofoperatives working with them. Carrier vapours are hazardous and pollute theenvironment where they are used. Residual traces present in the dyed fabric maybe released on subsequent heat treatment. These products are being increasinglyrestricted from use in sophisticated markets. Polyester/wool fabrics may still bedyed with carriers in those developing countries where environmental laws areless rigorous than in highly developed economies. This is typical of a generaltrend in which harmful products or processes that have been abandoned in

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developed countries for environmental reasons may still be undertaken in low-wage or less-regulated industries.

Several possible options exist to minimise the future use of carriers bypolyester/wool dyers. These include:(1) Improved carriers with no significant environmental problems may be

developed, such as ethylenediaminetetramethylphosphonic acid (EDTMP).The surprising effectiveness of this compound (Figure 11.2) in enhancing theuptake of disperse dyes by polyester fibres [15] has not yet been demonstratedon a commercial scale.

(2) Improved wool protective agents may be developed for use in dyebaths at120°C. The existing products intended for this purpose are not impressivelyeffective (section 11.1.4).

(3) Improved deep-dye polyester fibres may be developed for dyeing in blendswith wool at 105°C or lower temperatures. The existing variants of this typeare rather costly and show other disadvantages (section 5.1).

(4) Producer-coloured or stock-dyed polyester may be blended with scouredwool that may be cross-dyed later in yarn or piece form. This approach isless versatile than those listed above.

(5) Producer-coloured or stock-dyed polyester may be blended with woolalready predyed as loose stock or tops. This is even less versatile but thereare no restrictions on dye selection and this approach yields a product ofmaximum fastness.

Figure 11.2 Potential carrier to enhance disperse dye uptake

O

PHO H2C

HO

O

PHO H2C

HO

N CH2CH2

O

P OHCH2

OH

O

P OHCH2

OH

N

EDTMP

11.2 BLENDS OF CELLULOSE ACETATE OR TRIACETATE WITH WOOL

Worsted-spun blends of cellulose acetate/wool are cheap and attractive for hand-knitting yarns. The full handle, elastic recovery and resilience of knitted wool arenot much affected by inclusion of 30–40% cellulose acetate fibre. The acetatecomponent of the blend contributes improvements in crease recovery anddimensional stability during washing. Blends of 70:30 wool/acetate becameimportant for a time in woven carpet constructions. Although more expensive



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than viscose, cellulose acetate offered better resilience and coverage comparedwith wool/viscose blends [16]. Apparel end uses such as blouses, skirts andsuitings are made from blends of wool with cellulose acetate or triacetate.

Triacetate/wool is used for dresswear and leisure clothing, but this blend is lessimportant than acetate/wool or polyester/wool. Triacetate/wool fabrics havegood crease recovery and dimensional stability (Figure 1.1), but wool is degradedby the ‘S finish’ usually given to 100% triacetate fabrics. This is a surfacesaponification in 3% sodium hydroxide solution at 80–90°C, often given as anantistatic and antisoiling treatment. It is difficult to set triacetate/wool fabricswithout damaging the wool. Durable pleating can be introduced if the triacetatecontent exceeds 60%, however, and triacetate/wool is more resistant thanacetate/wool to the boiling conditions necessary to dye the wool satisfactorily.

Cellulose acetate/wool fabrics will not withstand carbonising. The fabrics areeasily damaged and scouring or milling processes must be carried out withminimum mechanical friction under mild conditions of alkalinity and tem-perature. A suitable sequence is:(1) crabbing at pH 5–6 and 80–85°C with an alkylphenol polyoxyethylene;(2) scouring at 40–50°C with ammonia and a nonionic detergent.

The staining of wool by disperse dyes increases with decreasing pH and becomesparticularly serious if the saturation limit of the cellulose acetate is exceeded. Theacid conditions at the boil required to apply 1:1 metal-complex or levelling aciddyes to wool would damage cellulose acetate and cause more severe disperse dyestaining of the wool. Pale and medium depths are dyed at 85–90°C or below toavoid loss of lustre by the acetate fibres. Full depths must be dyed at the boil toachieve optimum fastness of the anionic dyes on the wool, but the decreasedlustre of the acetate component is less obvious in full depths.

Disperse dyes are absorbed mainly by the acetate component below the boil,but migration in favour of the wool proceeds when the boil is reached. Onprolonged boiling some migration back to the cellulose acetate may occur if thesaturation limit has not been exceeded, but this is usually accompanied bydamage to the acetate fibre. Disperse dyes for cellulose acetate giving minimumstaining of wool are mainly low-energy (Mr 220–300) monoazo, nitro orquinoline yellows and oranges, together with intermediate-energy (Mr 300–380)monoazo reds and anthraquinone violets, blues and greens.

Knitting yarns normally require a one-bath method with disperse and aciddyes to give either solidity of shade or contrast effects, or with acid dyes alone togive reservation of the acetate. Colour contrast effects are often a feature of two-fold acetate/wool yarns and these can usually be achieved by dyeing the fibres

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simultaneously. The dyes for wool can be milling acid or 1:2 metal-complextypes, according to brightness and wet fastness required. Neutral-dyeing millingacid dyes of Mr 550–850, mainly disulphonates of disazo, anthraquinone,xanthene or triarylmethane chromogens are used for acetate reserve effects, orfor shadow and contrast effects in conjunction with disperse dyes.

The disperse and milling acid dyes are applied at pH 5–6 from an ammoniumacetate–acetic acid bath with an alkanol polyoxyethylene dispersing agent.Neutral-dyeing 1:2 metal-complex dyes are applied in a similar way at pH 6–7(ammonium acetate) with a weakly cationic alkylamine polyoxyethylenelevelling agent. The surface staining of the wool can be cleared by scouring witha nonionic detergent at 40–50°C. If necessary, sodium dithionite may be addedfor more effective clearing, especially if azo disperse dyes are present. Brightercontrast effects and better fastness in full depths are achieved by dyeing thecellulose acetate first, giving an intermediate clear and then dyeing the wool atpH 6–7 in a fresh bath. The disperse dyes on the acetate must be selected towithstand these cross-dyeing conditions without migrating to the wool.

The tendency of cellulose acetate to delustre in aqueous treatments attemperatures above 85°C has always been a problem for the dyers of blends inwhich the other fibre gives optimum dyeability under these conditions. This hasbeen overcome by the introduction in 1987 of Xtol (Courtaulds) fibre. This canbe dyed at the boil without delustring. The physical properties of Xtol areidentical with those of conventional cellulose acetate fibres. Thus it retains thesoft handle, rich lustre and comfort properties of traditional acetate apparel andconventional scouring and finishing processes can still be used.

The stability at temperatures up to the boil allows a much wider selection ofdisperse dyes to be used on Xtol. The restriction of conventional acetate to 85°Cmeant that only disperse dyes that exhausted well at that temperature, i.e. thelow-energy types with only moderate to poor wet fastness, could be used. Manyof the higher-energy dyes developed for polyester dyeing can be used on Xtol andthese dyeings show excellent fastness to washing at 50°C [17].

Intimate blends of wool and cellulose triacetate are usually spun on theworsted system and then piece-dyed for solidity, contrast or reserve effects.The cross-staining of wool with disperse dyes is more pronounced in theseblends than in wool/acetate blends, because the dyeing rate on triacetate isso much slower. Carriers will accelerate this rate but their use is deprecatedon environmental grounds. Before dyeing, triacetate/wool fabrics arescoured at 50–60°C with ammonia and an anionic detergent. Triacetate ismore resistant than the secondary acetate to wool dyeing conditions, but toensure preferential dyeing of triacetate by the disperse dyes it is essential to

BLENDS OF CELLULOSE ACETATE OR TRIACETATE WITH WOOL


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dye this blend at 105°C, or at the boil with a carrier. These conditions causemore severe disperse dye staining than in cellulose acetate/wool blends at85–90°C. Cross-staining of the wool can be minimised by including anonionic dispersant in the dyebath. Recommended disperse dyes of theintermediate-energy class (Mr 300–400) are mainly nitrodiphenylamineyellows, monoazo reds and anthraquinone blues.

In a one-bath method with selected disperse and milling acid dyes in pale ormedium depths, both classes of dyes are applied together at the boil and pH 6–7(ammonium acetate), with an anionic dispersing agent and butyl benzoate ordiethyl phthalate as carrier. A two-bath procedure entails dyeing the triacetatefirst at 105°C and the wool later in a fresh bath at the boil. The latter sequence ispreferred for full depths where 1:2 metal-complex dyes may be required andwool staining is a particularly serious problem.

Migration in favour of the triacetate increases with temperature, dyeing time,pH and concentration of carrier, so that relatively severe dyeing conditions arepreferred for optimum yield. The disperse dye stain is cleared from the woolusing an anionic detergent at 60–70°C. It may be necessary to add sodiumdithionite and ammonia but the physical properties of the wool may suffer. Thetwo-bath method for full depths on triacetate/wool is easier than on acetate/woolbecause there is less tendency for disperse dyes to transfer from triacetate duringcross-dyeing of the wool at the boil.

11.3 DYEING OF POLYESTER/NYLON BLENDS

Polyester/nylon blends exhibit exceptional strength and durability in robustouterwear and protective clothing. Half-hose is an important outlet for polyester/nylon as staple yarns or filament unions. The two fibres may be intimatelyblended in warp or weft yarns, or may be woven or knitted as separate yarns inthe form of designs that are usually coloured in distinctive hues. It is customaryto dye the intimate blends in solid shades.

Polyester became an important carpet fibre in the USA around 1970 becauseof the popularity at that time of heavyweight shag-pile constructions. Theattractive appearance of polyester yarns was retained longer than that of nylonbecause of their superior heat set retention. However, when polyester was usedlater in lighter-weight semi-shag and saxony constructions it was found to haveinadequate abrasion performance and resistance to pile deformation. Theabrasion resistance of nylon is about three times that of polyester [18]. Blends of50:50 polyester/nylon were used for lighter-weight carpets in the USA but an80:20 polyester/nylon blend was introduced in the UK for the so-called ‘splush’

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carpeting, mainly for bedrooms and bathrooms, and this has remained popularfor many years [16].

Most of the problems encountered with polyester/nylon carpet blends arisewith stock-dyed yarns at the heat-setting stage. During autoclave or continuoussteam setting, some low-energy disperse dyes migrate into the spinning lubricanton the fibre surface and give a yarn with inferior fastness to rubbing. High-energy disperse dyes, if dyed for too short a time at top temperature, yield ringdyeings that give rise to unacceptable colour changes during setting as a result offurther dye diffusion within the fibre.

The affinity of disperse dyes for polyester is higher than for nylon, but the rateof diffusion in nylon is much more rapid than in polyester. Most disperse dyes,therefore, dye the nylon component of a polyester/nylon blend more heavily inthe absence of a carrier at the boil, but the polyester is favoured at highertemperatures or when at dyeing at the boil with a carrier. Carriers of the arylester or trichlorobenzene types have been preferred to o-phenylphenol ordiphenyl because they give a more satisfactory partition and relatively lowresidual odour. The use of carriers, however, is seldom acceptable nowadaysbecause of their adverse environmental impact.

The principle of producing solid shades of moderate fastness on polyester/nylon using disperse dyes alone is determined essentially by dye selection anddyeing temperature. The most rapidly diffusing dyes are likely to give the bestresults. Nylon absorbs almost all of the disperse dye present at 60°C but as thetemperature is raised the rate of transfer to the polyester can be controlled by therate of temperature rise. In high-temperature dyeing at 120°C or under carrier-dyeing conditions at the boil most of the nitro and aminoketone (yellow),monoazo (yellow to red) and anthraquinone (red to blue) disperse dyes colourpolyester more readily, but some disazo orange and aminonaphthoquinone bluedyes still favour the nylon.

Shadow effects can be produced without difficulty but the best reserve iswith polyester, using acid dyes on the nylon. It is difficult to dye polyesterwith complete reserve of the nylon. Many acid dyes give an excellent reserveof polyester when applied at pH 4 and 90–95°C with the usual levellingagents for nylon dyeing. Neutral-dyeing 1:2 metal-complex dyes can beapplied in a similar way at pH 7–8. Multisulphonated reactive dyes atpH 3–4 give an outstanding reserve of the polyester, although both of theseclasses of dyes are rather more sensitive to dye-affinity variations in thenylon. The whiteness of the polyester can be enhanced by applying adisperse-type fluorescent brightening agent at the boil after dyeing andaftertreatment of the nylon.

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Patterned contrast effects are produced by cross-dyeing half-hose knitted fromnylon and polyester yarns. In the two-stage method, the darker colour is dyed onthe nylon with milling acid or 1:2 metal-complex dyes and the lighter colour isthen dyed on the polyester. Disperse dyes selected for their relatively low affinityfor nylon are mainly intermediate-energy monoazo or anthraquinone types of Mr

300–380. The nylon is dyed with the anionic dyes at 70°C from an ammoniumacetate–acetic acid bath, then the disperse dyes are added, the temperature raisedand dyeing continued at 120°C to achieve the target depth on the polyester.

Where a deep colour is required on the polyester, it is necessary to use a two-bath process. More attractive independent contrasts are possible in this way bydyeing the polyester first at 120°C. The disperse dye stain is desorbed from thenylon in an intermediate clear with a nonionic detergent or destructively strippedwith an alkaline dithionite treatment at 70°C. The anionic dyes are applied froma fresh bath at the same temperature, pH control being necessary to facilitateexhaustion. Even so, the contrast is difficult to control owing to some transfer ofdisperse dyes from the polyester during the second stage of the process [19]. Asyntan aftertreatment is given finally to improve wet fastness on the nylon.

11.4 BLENDS OF CELLULOSE ACETATE OR TRIACETATEWITH NYLON

High tensile strength and abrasion resistance are important contributions madeby nylon to these blends. In woven or knitted dresswear, linings andundergarments, the cellulose ester fibre usually predominates and blends rangingfrom 85:15 to 65:35 are typical. Cellulose acetate/nylon has been used in stapleblends for hand-knitting yarns or as nylon warp/acetate weft woven filamentdresswear. Special effects are attainable using ply yarns, e.g. a crimped nylonyarn twisted with a bright cellulose acetate filament yarn. Triacetate/nylonfabrics often exhibit better elastic properties and abrasion resistance. Theseblends are suitable for hosiery, knitwear and sportswear. Warp-knitted velvetfabrics with a cellulose acetate or triacetate pile and a nylon backing are useful asfurnishing fabrics, outerwear, trimmings and lining fabrics, particularly on costgrounds. Overprinting of dyed fabrics with metallic pigments is popular. Dyeingand printing are completed prior to raising [20].

Cellulose acetate/nylon blends cannot be preset in steam or hot air because ofthe sensitivity of the acetate fibre to heat, but the dyeing process exerts ahydrosetting action to stabilise the material. Intimate blends of these fibres forknitting yarns are dyed to a wide range of shades, mainly in pale depths. Selecteddisperse dyes can be applied at 80°C with a nonionic dispersing agent to give

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shadow effects. Owing to low wash fastness this method is only suitable for paleor medium depths [19].

Whilst it is theoretically possible to use disperse dyes to colour bothcomponents of a 50:50 blend, in practice it is found that most of the dye isabsorbed by the acetate component, especially at neutral or slightly alkaline pH.Acceptable solidity on 80:20 blends is given by simple monoazo or1,4-disubstituted anthraquinone dyes. The light fastness of many disperse dyesvaries appreciably between nylon and cellulose acetate.

Triacetate/nylon fabrics are usually prescoured at 70°C with a nonionicdetergent, stenter set at 210°C and then given an S finish. Solid dyeings can beachieved using disperse dyes at the boil with an anionic dispersing agent and, ifnecessary, addition of a butyl benzoate or diethyl phthalate carrier. Some dyesshow better solidity in the presence of a carrier, whereas others with lowersubstantivity for nylon give a more solid effect without carrier. The concentrationof carrier can be adjusted to shift the balance of partition towards either fibrewhen carrier dyeing at the boil.

High-temperature dyeing at 120°C offers greater latitude in dye selection,improved exhaustion and penetration of both fibres [21]. The best solidity isgiven by certain low-energy monoazo and anthraquinone dyes. Several orangeand brown monoazo and disazo dyes of higher fastness tend to favour the nyloncomponent too much. If problems of solidity arise due to preferential dyeing ofthe nylon at high temperature, this can be improved by pretreating the blend at50–60°C with a carrier suitable for triacetate. The use of a carrier should beregarded as a last resort, however, because of the adverse environmental impactof these products.

Solidity is not easy to achieve on cellulose acetate/nylon or triacetate/nylonbecause most disperse dyes show a marked bathochromic shift on nyloncompared with their respective hues on the cellulose acetate or triacetatecomponent: yellows appear redder, reds bluer, and blues greener on nylon. Thiseffect makes staple blends look skittery (lacking solidity) if a substantialproportion of nylon is present. The difference in hue may be attributable to thepreferential absorption of the more bathochromic ionised species (Scheme 11.4)of such dyes by the amine end groups in nylon. Both fibre components arecapable of hydrophobic interaction and hydrogen bonding with either chargedor uncharged forms of these dyes (Figure 11.3). The colour difference is of littleinterest for contrast effects on filament unions because the hue on the nylon isusually duller than that on the acetate or triacetate fibre.

A good reserve of cellulose acetate or triacetate is obtained using 1:2 metal-

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O2N NH

NO2

OH O2N NH

NO2

O–

O

O NH2

OH

Mid-yellow

OH–

H+

O

O NH2

O–

Reddish yellow

CI Disperse Yellow 1

H+

OH–

Red

CI Disperse Red 15

Violet

Scheme 11.4

Figure 11.3 Dye–fibre bonding of CI Disperse Yellow 1 on acetate and nylon

complex or milling acid dyes on the nylon component at 80°C (acetate/nylon) or120°C (triacetate/nylon) and pH 5–6 with ammonium acetate–acetic acid.Dependent colour contrasts with the nylon heavier in depth are given by dispersedyes selected for minimum staining of nylon and neutral-dyeing acid dyes to fillin the nylon component. When anionic dyes are used in these contrast dyeings,careful pH control is necessary for reproducible results. The preferred millingacid dyes are mainly disazo disulphonates (yellow to red) and monosulphonated(violet to blue) and disulphonated (blue to green) anthraquinone dyes withrelatively high wet fastness performance.

O2N NH

NO2

OH

HCHC CH

O

O2N NH

NO2

O–

O2N NH

NO2

OH

NH3

(CH2)6

NH

CO

(CH2)4

C

+

O

Reddish yellow

NH

Mid-yellow

O

Nylon fibre

COCH3H2C

Acetate fibre

Mid-yellow

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The two classes of dyes are compatible but a two-stage sequence may givebetter control of solidity than does a one-bath process. A carrier may benecessary to assist dyeing of the triacetate. Moderate or full depths of disperseand anionic dyes are afterscoured at 50–60°C with a nonionic detergent, or ifnecessary with sodium dithionite as a reduction clearing agent. Thoroughscouring is essential where good wet fastness is important, because the dispersedyes have only low fastness on the nylon component.

11.5 BLENDS OF POLY(VINYL CHLORIDE) FIBRES WITH WOOLOR NYLON

Much of the poly(vinyl chloride) or PVC fibre used for textile applications isfound in DA blends with wool or nylon. PVC fibre is an economical componentof staple blends and exhibits the unusual properties of inherent flame resistanceand a high degree of shrinkage when heated above 60°C. Although the shrinkagephenomenon limits the conditions of processing to some extent, it does enableunique effects to be produced in speciality fabrics. It is essential to ensure that thecomponent fibres are thoroughly blended, as the PVC fibre may contract at theboil to 50% of its original length. During processing of the blend the PVC fibrestend to become concentrated in the interior of the yarn, so that dyeing of thePVC component may be unnecessary if it is present only in a small proportion.Solid pale or medium depths, shadow effects and PVC reserve are all possible inblends with wool or nylon.

Staple PVC fibres blended with wool at the 10–20% level contribute strengthand bulk to the yarn for hand-knitting or as a weft in blanket manufacture. Iflatent-shrinkage properties are required without resorting to milling, 20–25% ofthe PVC fibre can be incorporated and the fabric shrunk by scouring or stenterdrying at 80–90°C. Owing to the heavyweight characteristics obtained, theseblends are of more interest for outerwear or uniforms rather than dressgoods orlight suitings.

Latent-shrinkage fibres are usually dyed as staple or combed tops at 50–60°Cwith a trichlorobenzene carrier before blending with dyed wool. The preshrunkPVC fibres can be dyed at 90–95°C as a blend with undyed wool. Selected low-energy monoazo and anthraquinone disperse dyes give minimum cross-stainingof wool when applied with acid dyes in a one-bath method. Full depths may bedyed by a two-bath sequence:(1) PVC fibre dyed with disperse dyes at 90–95°C;(2) wool stain cleared using nonionic detergent and ammonia at 50–60°C;(3) wool dyed at the boil with premetallised or chrome dyes selected for

minimum staining of PVC fibre.

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Knitted interlock made from a 70:30 blend of nylon and PVC fibre for washableapparel is relaxed in boiling water, desized, dyed and stenter dried. Such fabricsare processed in open width throughout under minimum tension to allowoptimum shrinkage, giving a felt-like material that can be raised to give a suededappearance and handle. The PVC fibre migrates inwards to such an extent thatoften only the nylon needs to be dyed.

Selected disperse dyes will give a similar depth on nylon and PVC fibre, butlight fastness on the PVC component is often a problem. Low-energy nitro,monoazo and 1,4-disubstituted anthraquinone dyes give the best solidity ingeneral. Most of the other disperse dyes favour nylon or give different tones onthe two fibres, nylon again showing the more bathochromic hue. These blendsare dyed at the boil with a nonionic dispersing agent and a trichlorobenzenecarrier if necessary to promote uptake by the PVC fibre. Acid dyes can be used tofill in the nylon. Full depths should be reduction cleared after dyeing usingsodium dithionite, soda ash and a nonionic dispersing agent.

Table 11.2 Dye selections for dyeing of DA blends


Polyester/wool Polyester Single-class 1:1 metal-complex or milling acid dyes atreserve appropriate pH

Solid One-bath Intermediate-energy disperse dyes andmonosulphonated 1:2 metal-complexdyes at 105°C with carrier

High-energy disperse dyes anddisulphonated 1:2 metal-complex dyes at120°C with wool protective agent

Two-bath Selected disperse dyes at 120°C, then1:2 metal-complex or milling acid dyesat the boil

Acetate/wool Acetate Single-class Disulphonated milling acid dyesreserve

Solid or One-bath Selected disperse dyes and neutral-dyeingcontrast acid dyes at pH 6

Two-bath Selected disperse dyes at 80°C, then 1:2(full depths) metal-complex or milling acid dyes

Triacetate/ Solid One-bath Intermediate-energy disperse dyes andwool milling acid dyes at the boil with ester

carrier

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Table 11.2 Continued


Triacetate/ Solid Two-bath Intermediate-energy disperse dyes atwool (full depths) 105°C, then 1:2 metal-complex dyes at

the boil

Polyester/ Polyester Single-class Acid dyes at pH 4 and 90°Cnylon reserve

Solid or Single-class Low-energy disperse dyes at 110–120°Cshadow

Solid or Two-stage Darker hue with neutral-dyeing acid dyescontrast at 70°C, then intermediate-energy

disperse dyes at 120°C

Two-bath Intermediate-energy disperse dyes at(full depths) 120°C, then anionic dyes at 70°C with

syntan aftertreatment

Acetate/nylon Acetate Single-class Premetallised or milling acid dyes at 80°Creserve

Solid or Single-class Low-energy disperse dyes at 80°Cshadow (pale depths)

Solid or Two-stage Selected disperse dyes, then milling acidcontrast dyes at 80°C

Triacetate/ Triacetate Single-class Premetallised or milling acid dyes at 120°Cnylon reserve

Solid Single-class Low-energy disperse dyes at 120°C(pale depths)

Two-stage Selected disperse dyes, then milling aciddyes at 120°C

PVC/wool PVC Single-class Premetallised acid dyes at the boilreserve

Solid One-bath Low-energy disperse dyes and acid dyes(pale depths) at 90–95°C

Two-bath Disperse dyes at 90–95°C, then(full depths) premetallised or chrome dyes at the boil

PVC/nylon PVC Single-class Milling acid dyes at the boilreserve

Solid Single-class Low-energy disperse dyes at the boil(pale depths)

One-bath Selected disperse dyes and milling aciddyes at the boil

BLENDS OF POLY(VINYL CHLORIDE) FIBRES WITH WOOL OR NYLON


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11.6 DYEING METHODS AND DYE SELECTION FOR DA BLENDS

In these blends a good reserve can invariably be achieved on the ester fibre orPVC fibre using acid dyes at the appropriate pH and temperature for the acid-dyeable component. Intermediate-energy disperse dyes and neutral-dyeing aciddyes are compatible in one-bath processes for solid shades on blends of the esterfibres with wool, although full depths require a two-bath sequence with anintermediate clear to remove the disperse dye staining from the wool. Soliditywith low-energy disperse dyes in pale depths is possible on the other blends in theDA category (Table 11.2) but more elaborate two-stage or two-bath proceduresare necessary for medium or full depths, the ester fibre or PVC fibre being dyedfirst.

11.7 REFERENCES 1. S M Doughty, Rev. Prog. Coloration, 16 (1986) 25. 2. A F Doran, unpublished work.

3. M Brenner and H Zahn, Melliand Textilber., 64 (1983) 845.

4. H H Konrad and K Türschmann, Textile Praxis, 33 (1978) 932. 5. K H Röstermundt, Deutscher Färber Kalender, 80 (1976) 247.

6. P Rube and D Wegerle, Melliand Textilber., 57 (1976) 496.

7. T Balchin, Am. Dyestuff Rep., 72 (Sep 1983) 27. 8. M Drewniak, Am. Dyestuff Rep., 68 (Jun 1979) 45.

9. T H Afifi and A Z Sayed, J.S.D.C., 113 (1997) 256.

10. G Römer, Textilveredlung, 14 (1979) 332.11. E D Katcher and M P Neznakomova, Textil Praxis, 37 (1982) 637.

12. D M Lewis, Rev. Prog. Coloration, 19 (1989) 49.

13. C M Carr, J.S.D.C., 108 (1992) 531.14. K H Röstermundt, Textil Praxis, 47 (1992) 649.

15. Y Riad, S M Hamza, H M El-Nahas and A A El-Bardan, J.S.D.C., 106 (1990) 25.

16. T L Dawson, Rev. Prog. Coloration, 15 (1985) 29.17. J M Taylor and P Mears, J.S.D.C., 107 (1991) 64.

18. T L Dawson and B P Roberts, J.S.D.C., 93 (1977) 83.

19. H W Partridge, Rev. Prog. Coloration, 6 (1975) 56.20. G Wünsch, Textilveredlung, 24 (1989) 57.

21. P L Moriarty, Text. Chem. Colorist, 14 (Aug 1982) 148.

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161

CHAPTER 12

Polyester/acrylic and other DB blends

12.1 DYEING OF POLYESTER/ACRYLIC BLENDS

Cotton-spun blends of polyester and acrylic fibres (often 85:15 to 70:30), orpolyester fabrics containing acrylic effect yarns, are important in woven orknitted upholstery and furnishings, outerwear, easy-care suiting, dressgoods,sportswear and leisure clothing. As the major component, polyester confersstrength, easy-care properties, dimensional stability and high whiteness. Acrylicfibre as the minor component contributes a soft, natural handle, high bulk andcover, improved comfort in wear and versatility in coloration. Polyester/acrylicfibre blends are usually piece-dyed and can be readily processed in garment form,owing to their excellent shape retention, crease recovery and abrasion resistance.Blending of polyester with modacrylic fibres is usually intended to takeadvantage of the inherent flame resistance of the modacrylic component. Therelatively low abrasion resistance of the latter is greatly compensated by thepresence of polyester in the blend [1].

Polyester/acrylic fabrics are scoured at pH 4 with a nonionic detergent andheat set at 185–190°C before dyeing. Pale or medium depths of selected dispersedyes can be dyed on the polyester with satisfactory reserve of the acrylic fibreusing a methylnaphthalene carrier at the boil. Yellow to orange aminoketone ordisazo dyes give the best reserve, but selected low-energy red to blueanthraquinone dyes are also acceptable under these conditions. An excellentreserve of the polyester component is given at the boil and pH 5 with selectedbasic dyes and a cationic retarder, e.g. cetyltrimethylammonium bromide.

The same method can be employed for solid, shadow or contrast effects byadding an alkanol polyoxyethylene as anti-precipitant, appropriate disperse dyesand a carrier formulated with a nonionic emulsifying system. Anionic dispersingagents are best avoided because they may either complex with the cationicretarder and increase staining of the acrylic fibre by the disperse dyes, or complexwith the basic dyes and cause unacceptable restraining. Anionic emulsifying

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systems for the carrier would also show these defects and may exhibit loweremulsion stability in the presence of the cationic dyes or retarder.

Maximum brightness in contrast effects may be limited because of staining ofthe acrylic fibre by the disperse dyes. The adverse effect of disperse dye stainingon the light fastness of basic dyes on the acrylic component has been quantified[2]. The disperse dye stain should be cleared by treatment at 60°C with sodiumdithionite, ammonia and a nonionic detergent.

An alternative two-stage approach is to use an anionic retarder for the basicdyes. In the first stage the surface of the polyester is ring-dyed with disperse dyesat pH 7–8 (sodium acetate) and 80°C in a compatible system containing ananionic retarder, a conventional anionic emulsion of the carrier and a nonionicanti-precipitant. The pH is then adjusted to 5 with acetic acid, the basic dyesadded and the dyeing temperature increased to the boil to dye the acrylic fibreand promote diffusion of the disperse dyes into the interior of the polyester. Afinal scour at 60°C with a nonionic detergent, ammonia and dithionite completesthe procedure.

Kayacryl ED (KYK) basic dyes contain as counter-ion an arylsulphonate thatacts as an anionic retarder and migrating agent. This facilitates compatibilitywith conventional anionic formulations of disperse dyes and auxiliaries. No anti-precipitant is required and the fastness properties are equal to those ofconventional basic dyes. Superior levelling is shown without the customary useof a cationic retarder. There is less staining of equipment and savings of time,labour, water, energy and auxiliaries [3]. The Kayacryl ED dyes migrate muchmore readily than conventional basic dyes and show good stability over a widerange of pH.

These one-bath or two-bath dyeing methods on winch, jet or package dyeingmachines offer reproducible contrast effects at low cost [4]. In general, it ispreferable for the acrylic fibre to be dyed more deeply if reserve, shadow orcontrast effects at full depths are required. Solid or contrast dyeings in full depthsshould be dyed by a two-bath method for optimum fastness and control ofcolour matching. The polyester component is dyed first because a basic dyeingon the acrylic fibre may show slight bleeding later in the disperse dyebath,especially if treated at high temperature. For optimum solidity, the polyestershould be dyed slightly heavier than the target shade to allow for slight transferto the acrylic fibre in the second stage. Anionic dispersing agents (and an anioniccarrier emulsion if necessary) can be used in the first bath and a cationic retarderselected for the basic dyes without restriction. An intermediate scour with anonionic detergent at 80°C is necessary to clear the disperse dye stain and toremove any residual carrier present.

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BLENDS OF CELLULOSE ACETATE OR TRIACETATE WITH ACRYLIC FIBRES

12.2 BLENDS OF CELLULOSE ACETATE OR TRIACETATE WITHACRYLIC FIBRES

Staple 50:50 or 60:40 blends of cellulose acetate or triacetate with acrylic fibresare used in jersey dresswear, high-bulk sweaters, hand-knitting yarns, sportswear,tropical suitings and woven furnishing fabrics. The cellulose ester fibrecontributes a smooth, silky handle, lustrous appearance and good dimensionalstability, giving an attractive combination of properties with the fuller handle,easy-care performance and heat insulation contributed by the acrylic component.

Reserve and contrast effects are of relatively low interest on cellulose acetate/acrylic fabrics because they are usually made from blended-staple yarns andcross-staining is a problem. Solid effects in pale depths can be obtained withdisperse dyes applied at 75–80°C and shaded if necessary with basic dyes for theacrylic component, although this fibre is only ring-dyed under these conditions.Higher temperatures are best avoided because the lustre of the acetate fibrewould be impaired.

Full depths are achieved by a compromise two-bath method. The acrylicfibre is dyed for the minimum time at the boil and pH 4–5 (acetic acid) withthe basic dyes and a cationic retarder. The dyebath is then cooled to 80°Cand run to drain. The disperse dyes are then applied at 70–80°C and pH 6from a fresh bath containing a nonionic dispersing agent to achieve thetarget depth on the acetate fibre. This method does not give optimumfastness in heavy depths on the acrylic fibre but the degree of damage to thecellulose acetate is just about tolerable.

Much improved control of the dyeing of cellulose acetate/acrylic blends hasbecome possible with the introduction by Courtaulds in 1987 of Xtol celluloseacetate fibre, which can be dyed at the boil without delustring. Fancy mixed-plyyarns containing Xtol and Courtelle (Courtaulds) can be dyed in a wide range ofsolid, shadow or contrast effects using selected disperse and basic dyes in a one-bath method. Reserve effects are problematical, particularly alongside deepshades, but acrylic reserve is probably the best choice.

Although acrylic fibres are readily dyeable with low-energy disperse dyes, thehigh-energy dyes recommended for Xtol give little or no cross-staining of theacrylic fibre in 50:50 blends with Xtol. Basic dyes for the acrylic fibre need morecare in selection because they tend to stain most fibres including Xtol [5]. It ispossible to achieve minimal cross-staining of Xtol by careful selection, however.The highest possible dyeing temperature should be used and the dyeing timeshould be sufficient to allow migration of the basic dyes from Xtol to Courtelle.Reduction clearing after dyeing may assist in reducing the ultimate staining of theacrylic fibre.


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Triacetate/acrylic blends can be dyed to solid shades, shadow (with the acrylicas the heavier depth), contrast and triacetate reserve. It is not possible to dye thetriacetate with reserve of the acrylic fibre. A good reserve of the ester fibre in atriacetate/acrylic fabric is obtained by dyeing with basic dyes selected forminimum staining of triacetate using a nonionic dispersing agent. Any stain onthe triacetate is cleared with slightly acidified hypochlorite at ambienttemperature, followed by a sodium bisulphite rinse.

Solid and contrast effects are achieved by a one-bath method using selecteddisperse and basic dyes. At temperatures below the boil the basic dyes tend tostain the triacetate, but migration to the acrylic fibre proceeds at the boil. Dyes inwhich the charge is delocalised (yellow to red methines and cyanines, oxazineblues and triarylmethane greens) migrate more readily than dyes with a localisedcharge (azo and anthraquinone derivatives).

When dyeing colour contrasts, the acrylic component should preferably bedyed to a deeper colour than the triacetate in order to minimise undesirablecross-staining of the acrylic fibre by the disperse dyes. Satisfactory one-bathdyeing is achieved at the boil and pH 5–7 (phosphate buffer) with sodiumN-methyloleylaminoethanesulphonate as a weakly anionic retarding agent(Figure 12.1). The basic dyes and disperse dyes are added separately in thatorder, followed by an aryl ester carrier in an anionic emulsion. Full depths shouldbe scoured at 60°C with a nonionic detergent to remove the residual basic dyestain from the triacetate.

H3CN

H(CH2)7COCH2CH2SO3

– Na+CH3(CH2)7CH C

Figure 12.1 Anionic retarder for basic dyes

Solid or contrast effects can be produced on blends of cellulose acetate ortriacetate with acid-dyeable acrylic variants by two-stage methods using acid orchrome dyes for the acrylic fibre first, then neutralising and applying selecteddisperse dyes to the ester fibre at 70°C (acetate) or at the boil with an aryl estercarrier (triacetate). Dyes requiring only moderately acidic conditions should beselected for the acrylic variant. The acrylic fibre can be reserved by applyingdisperse dyes that have good affinity for the ester fibre at the appropriate dyeingtemperature. The ester fibre is reserved by applying levelling acid dyes at a

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moderately acidic pH, preferably using phosphoric acid. Chrome dyes may beused for higher fastness but metal-complex types tend to stain the ester fibres.

12.3 DYEING OF NORMAL/BASIC-DYEABLE POLYESTER BLENDS

Polyester copolymer variant yarns of the basic-dyeable type (section 5.1) are usedin combination with the homopolymer for reserve, shadow and contrast effects.The basic-dyeable copolymer is more accessible than the homopolymer and inmost instances shows higher yields and rates of dyeing with disperse dyes,although some anthraquinone derivatives do not behave in this way. Basic-dyeable polyester exhibits lower tensile strength than the homopolymer andmuch lower abrasion resistance. It is more readily degraded by acid or alkali thanthe homopolymer, so that it should be dyed at pH 5–6 and a temperature nohigher than 120°C. Glauber’s salt addition minimises degradation within theselimits. Caustic soda should be avoided and reduction clearing with ammonia anddithionite should not be carried out above 60°C.

Careful scouring with a nonionic detergent and soda ash at 60–70°C toremove spinning oils is important. Presetting at 165°C is also recommended.Satisfactory reserve of the homopolymer is achieved at the boil and pH 5 usingbasic dyes, Glauber’s salt and a nonionic carrier emulsion. Aryl ester carrierspromote optimum reserve of the normal polyester when using basic dyes only.The preferred basic dyes for this blend are mainly methine, cyanine, monoazo oroxazine types. Practically all of them except certain orange to red monoazo dyesreserve normal polyester well. The residual stain is cleared from thehomopolymer at 50°C using sodium dithionite, ammonia, a nonionic detergentand Glauber’s salt to protect the basic-dyeable polyester from hydrolysis.Pyrazoline fluorescent brightening agents cannot be used in reserve effects on thehomopolymer because they tend to promote damage of the basic-dyeablevariant. Compounds based on benzoxazolyl- and benzofuranyl-benzimidazolederivatives (Figure 12.2) are preferred because they are resistant to chloritebleaching [6].

Shadow effects with disperse dyes on these blends show the sharpestdifferentiation using slow-diffusing high-energy dyes from a long liquor at about80°C, if necessary with a low concentration of an ester carrier, but the normalpolyester is badly ring-dyed under these conditions. The best approach to solidityis to use rapid-diffusing low-energy dyes at the boil with a diphenyl carrier, or at120°C with an ester carrier to assist levelling. Some subtle contrasts are possiblewith selected low- and high-energy disperse dyes in mixtures, but the scope forselection is extremely limited.

BLENDS OF CELLULOSE ACETATE OR TRIACETATE WITH ACRYLIC FIBRES


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Figure 12.2 Structural types of fluorescent brightening agents

The most widely used disperse dyes for contrast effects with basic dyes aremainly quinoline and anthraquinone derivatives. Cationic retarders may be usedespecially in pale depths, but they tend to be less effective than on acrylic fibresbecause the rate of diffusion of the dye is already much slower on basic-dyeablepolyester and restraining may be excessive in full depths. Weakly cationicretarders of the alkylamine polyoxyethylene type promote levelling in the hightemperature method at 120°C. Some useful azo and anthraquinone disperse dyesshow light fastness at least one blue-scale rating lower on basic-dyeable polyestercompared with the homopolymer. All basic dyes show lower light fastness onbasic-dyeable polyester than on acrylic fibres and this limits their selection forpile fabrics, upholstery and carpets. In most cases, the monoazo types with alocalised charge tend to show better fastness to light than do dyes with adelocalised charge on the molecule.

Anionic scouring or dispersing agents and carriers formulated with anionicemulsifiers should be avoided. Diphenyl and aryl ester carriers in nonionicemulsifying systems are preferred for their compatibility with basic dyes, ease ofremoval from the fibre and minimal influence on light fastness. The disperse andbasic dyes are added separately to a solution of an alkanol polyoxyethylene anti-precipitant before the nonionic carrier emulsion. The ester carriers accelerate therate of uptake of basic as well as disperse dyes and they give better migration

R1 NN

R2

O

N

N

N

R2R1CH3

CH3

O N

N

R2R1CH3

CH3

Substituted 1,3-diphenylpyrazolines

Substituted benzoxazolyl-benzimidazoles

Substituted benzofuranyl-benzimidazoles

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Table 12.1 Dye selections for DB blends


Polyester/ Acrylic Single-class Low-energy disperse dyes withacrylic reserve methylnaphthalene carrier

Polyester Single-class Basic dyes with cationic retarderreserve

Solid or One-bath Disperse dyes and basic dyes withcontrast nonionic carrier system and

anti-precipitant

Two-stage Disperse dyes at 80°C, then basicdyes at the boil with anionic retarderand carrier

Two-bath Disperse dyes at 120°C, then basic(full depths) dyes with cationic retarder

Acetate/ Solid Single-class Disperse dyes at 80°Cacrylic (pale depths)

Two-bath Basic dyes briefly at the boil with(full depths) cationic retarder, then disperse

dyes at 80°C

Xtol/ Solid or One-bath High-energy disperse dyes andCourtelle contrast selected basic dyes at the boil(Courtaulds) with anti-precipitant

Triacetate/ Triacetate Single-class Basic dyes with nonionic dispersingacrylic reserve agent

Solid or One-bath Disperse dyes and delocalised-contrast charge basic dyes at the boil with

anionic retarder and carrier

Acetate/ Acetate Single-class Chrome or levelling acid dyesacid-dyeable reserve at pH 4acrylic

Solid or Two-stage Chrome or levelling acid dyes, thencontrast disperse dyes at 80°C

Triacetate/ Triacetate Single-class Chrome or levelling acid dyesacid-dyeable reserve at pH 4acrylic

Solid or Two-stage Chrome or levelling acid dyes, thencontrast disperse dyes at the boil with aryl

ester carrier

Normal/ Homopolymer Single-class Delocalised-charge basic dyes andbasic-dyeable reserve nonionic aryl ester carrier emulsionpolyester at the boil

Shadow Single-class High-energy disperse dyes at 80°C

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Normal/ Solid Single-class Low-energy disperse dyes at 120°Cbasic-dyeable with aryl ester carrierpolyester

Contrast One-bath Disperse dyes and basic dyes at120°C with nonionic aryl estercarrier and anti-precipitant

under high temperature conditions. Diphenyl gives the most economical yields atthe boil and tends to favour the disperse dyes, so that this type of carrier is oftenpreferred for contrast effects in full depths. Where possible, however, it ispreferable to dye at high temperature and to use carriers sparingly, if at all. Fulldepths are cleared with dithionite and ammonia as already described.

12.4 DYEING METHODS AND DYE SELECTION FOR DB BLENDS

The disperse-dyeable component of DB blends containing polyester or cellulosetriacetate can be readily reserved. Solid effects with disperse dyes only can beachieved in pale depths on acetate/acrylic and normal/basic-dyeable polyesterblends, but better control of shade matching is possible using basic and dispersedyes. One-bath methods for solid or contrast effects are available for polyester ortriacetate with acrylic fibres, as well as normal/basic-dyeable polyester. Two-stage or two-bath sequences become necessary for full depths on the blendscontaining conventional or acid-dyeable acrylic variants (Table 12.1).

12.5 REFERENCES1. I R Hardin, Man-made fibres: their origin and development, Eds. R B Seymour and R S Porter

(London: Elsevier, 1993).

2. J Jeths, Chemiefasern und Textilind., 25/77 (1975) 356.3. R Parkham, Am. Dyestuff Rep., 82 (Sep 1993) 79.

4. F T Wallenberger, Text. Research J., 50 (1980) 289.

5. J M Taylor and P Mears, J.S.D.C., 107 (1991) 64.6. T Martini, Chemiefasern und Textilind., 38/90 (1988) 827.

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169

CHAPTER 13

Polyester/cellulosic and other DC blends

13.1 EXHAUST DYEING OF POLYESTER/CELLULOSIC BLENDS

13.1.1 Properties and preparation of polyester/cellulosic blends

Blends of ester fibres with cotton or viscose are produced in greater quantity thanthe corresponding blends with wool (section 1.3). Factors contributing to thissituation have been the relative ease of processing, effective clearing andversatility of application, leading to a wide range of dyed and finished effects.Without question the exploitation of polyester/cellulosic blends represents themost successful compromise between the contrasting physical properties ofsynthetic and natural fibres. Polyester/cellulosic yarns are used in sewing threadsand slub effects for apparel. Woven polyester/cellulosic fabrics are important inshirting, sheeting, dressgoods, outerwear and workwear. Woven staple 67:33polyester/cotton and 50:50 polyester/viscose blends in numerous constructionsform the well-established basis of this field and many of these fabrics areproduced in sufficient quantities to justify continuous dyeing.

Polyester/cellulosic knitgoods include fleece knits, interlocks and jerseys,sportswear, T-shirts and dresswear. Knitted fabrics are less appropriate forcontinuous dyeing owing to their lower dimensional stability but thedevelopment of atmospheric jet machines has made it possible to dye thesefabrics satisfactorily. Presetting can often be avoided if there is no risk of creasingin the jet dyeing machine and the brief dyeing cycle, short liquor ratio, highdegree of turbulence and vigorous washing conditions make this techniquehighly efficient.

As the most important fibre blend, ranging in characteristics from lightweightpoplin shirting to heavy drill workwear fabric, polyester/cotton is sufficientlyfamiliar to require little description. Polyester/linen blends are noteworthy as aluxury alternative to polyester/cotton in high-quality fashion goods, tablewareand bedlinen. Care must be taken not to mar the characteristic nature of the linentexture.

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The primary shortcoming of woven polyester/cotton blends in the late 1950swas their inability to create fabrics that would retain creases once they had beenmade into garments. The breakthrough was the development of the durable pressfinish that allowed deferred curing of the garment. This was constructed and thecreases set in before complete reaction of the finish with the cotton component ofthe blend. This procedure resulted in a garment that would retain its creasesthrough a prolonged series of wash–wear cycles. Initially, the process was appliedto all-cotton fabrics in the early 1960s but severe problems occurred because ofthe adverse effect of the durable press finish on strength and abrasion resistance.When it was later adopted for polyester/cotton blends, however, the durabilityand crease resistance of the polyester made an impressive contribution to thefinish. Improvements were also made to the finish formulations, to blending ofthe fibres in the yarn and to the garment pressing methods.

Polyester/viscose is an essential blend for apparel, replacing polyester to agreat extent over the last decade. The superior comfort of this blend over thesynthetic fibre alone is without question and it is this, together with thecapability to accept chemical finishes that produce fabric qualitiesunrecognisable from the starting material, that has led to the outstandingpopularity of this blend in apparel [1]. Viscose fibres can be chemically crimpedin manufacture by selecting regeneration conditions after extrusion that givefilaments with an asymmetric cross-section, such as less acid in the spinning bathand a higher bath temperature. The asymmetry causes the filaments to formcontinuous helical curves that impart the crimp characteristics. Sarille(Courtaulds) is a crimped viscose fibre that has been particularly successful withpolyester, including 65:35 Sarille-rich blends for dressgoods [2].

Blends of polyester with regular or crimped viscose, modal or polynosic fibresare important in such outlets as lightweight tropical suiting, fashionwear,raincoats, leisure clothing and sportswear. Blends with viscose, and especiallymodal or polynosic fibres, are more suitable than cotton blends for knitgoods inview of their higher lustre and softness. Polyester/polynosic fibre blends haveparticularly good dimensional stability for use in tubular-knitted fabrics andgarments [3]. Polyester/Vincel blends with polyester as the main component areof interest for rainwear, and for lightweight lawn constructions in summerdresswear and blouses.

A valuable advantage for polyester in blends with cotton is its outstandingresistance to the rather severe preparation necessary for raw cotton beforedyeing. Thorough preparation of polyester/cellulosic woven fabrics is theessential prelude to successful dyeing, especially if a continuous method is to beused. Any subsequent unlevelness or stains are almost always related to

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inadequate preparation. About 70% of typical dyeing faults are attributable topoor penetration [4].

The nature of the size used on warp yarns of woven goods should be identifiedso that it can be effectively removed. Rapid identification of sizes is now muchmore sophisticated than it was ten years ago [1]. Weavers often change sizeformulations without prior notification and this can lead to serious dyeingproblems. Size removal outside the dyeing machine is greatly preferred tominimise machine occupancy and avoid build-up of residual contaminants.Neutral pH washing is the best for water-soluble sizes that respond well tovigorous treatment, but complete removal may require enzyme desizing beforealkaline scouring. Hot- or cold-active enzymes are available for batchwise orcontinuous desizing.

Cotton is usually the minor component in a polyester/cotton blend so thatalkaline scouring, peroxide bleaching and mercerising usually provide adequatepreparation. Most polyester/cellulosic fabrics may be winch scoured in anionicdetergent and soda ash at 70–80°C. Cold pad–batch peroxide bleaching offers alow-cost alternative to batchwise processes to increase the production rate andquality of polyester/cotton knitgoods [5].

Cold mercerising after bleaching improves the absorbency, lustre anddimensional stability of polyester/cotton and improves the colour yield of vat orreactive dyeings. It is not recommended for viscose or modal fibre blends,however. The dyeability of the cellulosic component in these blends can beenhanced by cold causticising followed by thorough rinsing.

Crease recovery, dimensional stability and pilling resistance are all improvedby stenter setting of polyester/cellulosic fabrics. This is normally carried out at180–200°C on a stenter either before or after dyeing. The higher temperature isparticularly relevant for closely woven poplins or similar constructions destinedfor continuous dyeing. Heat setting before dyeing reduces the tendency to creaseif subsequently dyed in rope form. Dyeability variations arising from differencesin thermal history of the polyester may also be less evident. Sensitivity to thesedifferences is greater in batchwise dyeing. If heat setting is carried out beforepreparation, care must be taken to ensure that there are no oil stains or othercontaminants that might become set into the material and prove more difficult toremove later [6]. Heat setting after dyeing removes minor creases introduced inrope dyeing and also stabilises the fabric structure at its finished width.

Singeing of both sides of the fabric is essential for polyester/cellulosic stapleblends, but this treatment should be carried out after batchwise dyeing. Themicroscopic beads of fused polymer formed on the tips of the projectingpolyester fibres take up dye more readily than the intact fibres in the body of the

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fabric. Because of their amorphous character these produce an unacceptableskittery or speckled appearance, especially after batchwise dyeing. Cropping tosheer off the protruding fibre ends is an alternative that avoids this problem.

13.1.2 Disperse dyeing of the polyester component

Polyester/cellulosic yarns are mostly dyed as cones or cheeses, or on beams,under high-temperature conditions. Fabrics are generally dyed on a beam, in a jetor overflow machine, or on a continuous range. In circulating-liquor machinesthe initial dye uptake should be uniform throughout the load to ensure that thefinal dyeing is level. A rapid rate of dyeing and a slow rate of circulation of thedye liquor may both contribute to unlevel dyeing problems. To ensure uniformuptake the increment of exhaustion for each cycle of liquor through the machineshould be not greater than about 2% exhaustion per cycle. The drains of suchmachines should be designed so that it is safe to release the exhausted liquorwhile still under pressure but this technique is effectively limited to circulating-liquor machines.

Beam dyeing is unsuitable for polyester/cellulosic fabrics of very lowpermeability, constructions with a sculptured surface and knitgoods that areoften difficult to wind evenly with suitable tension on to a beam. Jet dyeingmachines can cause crushing or creasing of fabrics with a sensitive surface,such as velvet or corduroy. After jet dyeing it is desirable to cool the dyebathslowly to about 80°C in order to avoid the risk of creasing of the dyedmaterial. Certain lightweight cloths may also give difficulty in the jetmachine owing to the excessive length of an economical machine load. Thecompletely relaxed and tensionless dyeing conditions in a jet or overflowmachine allow relaxation, bulking and shrinkage that is physically preventedin the beam machine. Thus the softer, bulkier handle and subdued lustreprovided by jet dyeing may be preferred for polyester/viscose knitgoods, forexample, whereas the firmer, flatter handle and stronger reflections frombeam-dyed goods are usually favoured for polyester/linen and manypolyester/cotton fabrics [6].

Many of the comments on wool staining by disperse dyes (section 3.4) areapplicable to the staining of cellulosic fibres, but these are less sensitive than woolto the relatively severe conditions required to clear the stain. Staining isaggravated by low pH and poor dispersion stability, as well as marginally bynonionic agents and carrier chemicals. The disperse dye stain has low fastness tolight and wet treatments. On polyester/cellulosics the degree of staining is lessthan on wool, prolonged boiling favours migration to polyester without severedamage to the cellulose and the stain can be reduction cleared safely.

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Cotton staining in the dyebath at 130°C with those dyes having lowsubstantivity for cotton is usually negligible. After the hold period at toptemperature, residual dye remains in solution at 130°C and if the dyebath isdrained by blowing out under pressure, the deposition of loose dye on thefibre surfaces is minimised. On the other hand, if the dyebath is cooledslowly the dissolved dye will re-precipitate and undesirable deposition willoccur.

For the highest fastness the unfixed disperse dyes on the polyester surface andthe stain on the cellulosic component should be removed by a thorough soapingwith detergent or by a suitable reduction clearing. In many cases the clearingtreatment can be incorporated in the subsequent dyeing of the cellulosic fibre, asin the reduction, reoxidation and soaping of vat or sulphur dyes. Where reactivedyes are used and the highest fastness standard is required it may be necessary togive a reduction clearing treatment separately before the reactive dyes areapplied. Disperse dyes tend to stain the lignin component of linen, so thatreduction clearing of the stain is more important than with cotton [6]. Vat orreactive dyes are normally chosen for linen and it is dyed in open width to avoidcreasing. Clearing can be combined with the reduction stage of vat dyeing on thisblend.

Disperse dyes of all relevant chemical types have been used in the batchwisedyeing of polyester/cellulosic blends but the trend has been in favour of the high-energy dyes, especially where durable press finishes are to be applied. Not onlyhave the levels of sublimation fastness in chemical finishing and end-userequirements become more severe, but there have been improvements in thedesign and operation of jet and overflow machines to process these blends. High-energy dyes show optimum yield and levelling properties under these conditions.Low-energy dyes are now only of marginal interest for excellent levelling in paledepths. The affinity of disperse dyes for cellulose is a factor of practicalsignificance in selecting suitable dyes for either batchwise or continuousmethods. In continuous dyeing the selection has to take into account the rapidityof clearing of the stained cellulosic component and the sensitivity of the stainingto variables arising during thermofixation [7].

Light fastness on polyester is usually adequate and after dyeing the dyes areprotected from chemical attack at moderate temperatures by the hydrophobicnature and relative impermeability of the fibre. Nevertheless, some dyes aresensitive to alkaline conditions or to the presence of heavy metal ions at relativelylow concentrations. Thus disperse dyes are normally applied under slightly acidicconditions (pH 5) and a sequestering agent is normally used with those dyesknown to be sensitive to trace metals.



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The rate of exhaustion of a disperse dye by polyester is controlled by therate at which the temperature is raised. At some temperature between 80°Cand 120°C the dyeing rate for that dye reaches a maximum. The temperaturerange over which the dyeing rate is at this maximum is known as the ‘criticaldyeing temperature’ (CDT). Slow-diffusing high-energy dyes have a highCDT, whereas more rapidly-diffusing dyes have a lower CDT. Specific valuesof CDT depend on the rate of temperature rise, dye concentration, liquorflow rate, liquor ratio and the substrate to be dyed. Rapid-dyeing proceduresdepend on adding the disperse dyes at a temperature just below the CDT andthen raising the temperature slowly in the vicinity of the CDT to ensure thatthe exhaustion rate that just permits level dyeing is not exceeded. Thetemperature is then raised from just above the CDT to the top dyeingtemperature at the maximum rate.

In polyester/cellulosic dyeing the need for levelling agents with disperse dyes isless critical than when dyeing polyester alone. In the early stage of a batchwisedyeing, or in the first padding of a continuous process, the cellulosic fibresabsorb a substantial proportion of the disperse dyes applied. These subsequentlymigrate to the polyester as the top dyeing temperature or the thermofixation stepis reached. The cellulosic component is thus, in a sense, acting as a retarding orlevelling agent for the disperse dyes [8].

Nevertheless, many dyers still prefer to add a levelling agent, especially forpale depths. The efficiency of ethoxylated nonionic surfactants used as levellingagents during accelerated heating in the early stage of exhaust dyeing of thepolyester component was studied with a trichromatic combination of low-energydisperse dyes. Criteria for the selection of nonionic levelling agents include cost-effectiveness, ease of handling, effect on dye yield, minimal foaming and ease ofremoval from the substrate by rinsing [9].

Oligo-soaps are polyethylene glycol fatty acid esters of high Mr and generalformula: RCOO(CH2CH2O)nCH2CH2OH. These agents provide excellent dyedispersing and solubilising characteristics. By forming a complex molecularmatrix with the polyester surface during dyeing, these compounds give excellentdye levelling under a wide variety of conditions [10]. Their low-foamingproperties eliminate the need for mixtures of nonionic levelling agents, dispersingagents and defoamers in jet dyeing systems.

Fastness to sublimation of disperse dyes is not a criterion for selectingthose that yield optimum wet fastness performance in finished polyester/cellulosic fabrics. Those found suitable in this respect must show minimumtendency to thermomigration or desorption in aqueous media after heattreatment at temperatures above 140°C. They should also have minimum

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Figure 13.1 Azo disperse dye structures capable of solubilisation by alkali (Ar = aryl)

substantivity for adjacent fibres, notably nylon, in wet fastness tests.Selection of dyes that show minimum staining of the cellulosic fibres is alsoadvisable.

Unsatisfactory wash fastness of disperse dyes on the polyester fibre,particularly with regard to staining of adjacent nylon, may arise fromaftertreatments such as soaping at the boil. These are normally necessary whendyeing the cellulosic fibre with vat, sulphur, reactive or azoic dyes. Even if areduction clear is given after the boiling soap treatment, unfixed disperse dyemay still be present on the polyester surface. This problem can be minimised byselecting disperse dyes that do not migrate readily to the fibre surface when thefabric is soaped at the boil. An alternative approach is to use reactant-fixabledirect dyes on the cellulosic component that do not require a boiling soaptreatment [11].

Many disperse dyes show lower light fastness on polyester microfibres (section1.4.2) compared with standard polyester, so this becomes a further factor in dyeselection [12]. The relatively higher amounts of disperse dyes needed onmicrofibres have a significant influence on build-up. Small differences in build-upbetween dyes on conventional polyester are exaggerated on polyestermicrofibres. The high applied concentrations needed to achieve full depths onmicrofibres play a critical part in the resultant limited wash fastness after post-stentering. Traditionally acceptable dyes for conventional recipes can showunacceptable results when applied to microfibres.

Major dyemakers have put considerable research effort into developing anew generation of disperse dyes designed to optimise wash fastness andminimise the cross-staining of the cellulosic fibre [13]. If the Dispersol XF(BASF) dyes, for example, are compared with conventional monoazo andanthraquinone disperse dyes, the differences in wash fastness and cross-staining performance are magnified when tested on polyester microfibres.Diester-containing azo disperse dyes and certain azothiophene blues (Figure13.1) that are capable of being rendered soluble by a mild alkaline

HN

N NNAr1

CH2CH2COOCH3

CH2CH2COOCH3 SO2N N

NO2

N Ar2

COCH3



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aftertreatment offer considerable benefits when dyeing polyester microfibre/cellulosic blends. These include:(1) minimal cross-staining of the cellulosic fibre;(2) minimal processing times because the alkaline fixation stage for reactive

dyes clears the disperse dye stain;(3) avoidance of a reduction clear with dithionite;(4) achieving good wash fastness standards after post-stentering.

13.1.3 Direct dyeing of the cellulosic component

Several possible batchwise dyeing methods are available for polyester/cellulosicblends, based on the use of disperse dyes and the various classes of dyes for thecellulosic fibre. The choice between these possibilities depends mainly onrequirements of hue, depth and fastness properties. Direct dyes are popular dueto their economy, compatibility, robustness and adequate fastness in pale depths.Their major weakness is their poor wet fastness in depths above about half-standard depth [1]. The wet fastness of direct dyeings can be enhancedsubstantially, however, when finishing with a durable press reactant togetherwith a cationic fixing agent [14].

Most direct dyes give a good reserve of polyester. An economical one-bathbleaching and direct dyeing system for achieving an excellent polyester reserveeffect entails treating the scoured polyester/cotton fabric with sodium carbonate,hydrogen peroxide, silicate stabiliser and a hexametaphosphate sequestrant atpH 10 and the boil to bleach the cotton in the presence of selected peroxide-stable direct dyes [15]. After cooling to 85°C, salt is added and the temperatureraised to 95°C to achieve full exhaustion of the direct dyes.

Solidity on blended staple yarns is usually the colour effect required. Disperseand direct dyes can be applied in a cheap and simple one-bath process, but therelatively low fastness levels are inadequate for many polyester/cellulosic outlets.Disperse/direct combinations are thus mostly used at the cheaper end of themarket. Because of the limited wet fastness of direct dyes, staining of thecellulosic fibre by disperse dyes is less important than with other combinations.The direct dyebath serves as a soaping bath to give a mild clearing of the dispersedyeing.

A practical advantage that direct dyeing offers over reactive dyeing of theseblends is the markedly lower concentration of electrolyte necessary. The saltconcentration (10–15 g l–1) is rarely sufficient to adversely affect the dispersionstability of the disperse dyes, although instability occasionally arises in short-liquor (package or beam) equipment. Similarly, neither the dispersing agents norany levelling agent required would normally interfere with the direct dyeing

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process. On the other hand, many direct dyes are adversely affected at the hightemperature (120–130°C) and acidic pH (4.5–5.5) usually preferred for dispersedyeing. It is necessary to ensure that the direct dyes are sufficiently soluble andchemically stable under these conditions. The influence of pH and additions ofelectrolyte, carrier, levelling and sequestering agents on the stability and dyeingbehaviour of direct dyes selected for the one-bath method has been tabulated[16]. The vulnerability of most direct dyes to alkaline reducing conditionsprecludes conventional reduction clearing of the dyed polyester component [8].

General recommendations have been given for the package dyeing ofpolyester/cotton yarns with direct and disperse dyes by the one-bath method,either at high temperature [17] or using 1–2% of a mixture of diphenyl andtrichlorobenzene as carrier [18]. Disperse/direct recipes are important for blendsof polyester with viscose or other regenerated cellulosic variants for suitingmaterials, where fast-to-light direct dyes can be used without the need for highwash fastness. Aftertreatment and resin finishing of disperse/direct dyeings cangive materials of acceptable if still limited wet fastness.

In the one-bath method with disperse and direct dyes, the polyester fibre isdyed at pH 6 with a disodium dinaphthylmethanedisulphonate dispersing agentat 130°C on the beam or in a jet machine. The dyebath is then cooled to 90°C,salt is added and dyeing continued until the cellulosic fibre reaches the targetdepth. Aftertreatment with a cationic fixing agent is given where appropriate toimprove the fastness properties of the direct dyes. These are selected for stabilityin the high-temperature dyebath and are mainly self-levelling or salt-controllabledisazo multisulphonated dyes. A wider choice of direct dyes can be used in thetwo-bath method with intermediate reduction clearing. This alternative givesmoderately good fastness in pale or medium depths, provided the fabric is givena durable resin finish.

13.1.4 Reactant-fixable direct dyes for the cellulosic component

Reactant-fixable dyes are those copper-complex direct dyes (Figure 8.5) that aresuitable for aftertreatment with certain Indosol (Clariant) cationic fixing agents,yielding exceptionally good fastness to washing. Not all direct dyes respond inacceptable ways to these treatments. In general, bright unmetallised direct dyesshow poor light fastness after treatments with Indosol agents. Selectedunmetallised reactive dyes have been used to provide bright hues that supplementthe limited shade gamut of the reactant-fixable range [11].

Substantial savings in processing time, labour costs, chemicals, water andeffluent treatment are claimed for reactant-fixable dyes, compared with vat orreactive alternatives. These savings are even more significant when dyeing



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cellulosic blends than in the dyeing of all-cotton or all-viscose materials [19]. Thereasons for this arise from the fact that the dyeing of these blends can betechnically complex and time-consuming. In most cases the Indosol systemallows for one-bath dyeing, leading to specific savings over other conventionalcellulosic dyeing methods. Reduction clearing or soaping at the boil can beavoided, leading to further reductions in time, energy and water consumption.The soaping and rinsing of reactive dyeings are much more time-consuming thancationic aftertreatment [1].

The cationic nature of Indosol E-50 (Clariant) is multifunctional and onemolecule of fixing agent can interact electrostatically with several sulphonategroups in the same or different Indosol dye molecules. Furthermore, chelatinggroups in the fixing agent are able to form coordination bonds with the copperatoms in the dye molecules. These two features result in the formation of a muchmore stable complex between dye and agent than when simple cationic fixingagents are used to fix conventional direct dyes by electrostatic bonding only. Thefastness level achieved corresponds to that of domestic laundering at 50°C.Indosol E-50 is particularly suitable for treatment of knitted underwear,sportswear and hosiery. In the case of Indosol EF (Clariant), the electrostatic andcoordination bonds formed as described above are further reinforced becausethis agent is capable of reacting with a hydroxy group in cellulose to form acovalent ether bond, similar to that produced in a typical reactive dyeing. Thisadditional fixation leads to high levels of colour fastness at all applied depths,e.g. laundering at 60°C. Although Indosol EF is applied by exhaustion at 40°Crather than the 60°C used for Indosol E-50, a further alkaline treatment withcaustic soda, or alternatively a high-temperature cure at 150°C, is necessary toinitiate the reaction of Indosol EF with cellulose.

Most woven cellulosic blend fabrics require at least a moderate degree ofcrease recovery to exhibit satisfactory easy-care properties. Indosol CR(Clariant) completely replaces the cellulose reactant resins that confer easy-care properties by crosslinking. It can only be applied by a pad–dry–bakesequence. A cationic fixing agent of the Indosol E-50 type is reacted with aconventional reactant of the dimethyloldihydroxyethyleneurea type to giveIndosol CR. Formation of the dye–Indosol E-50 complex occurs initially, buton curing the reactant crosslinks the cellulose segments and forms a three-dimensional matrix in which the dye–agent complex is held permanently.Thus exceptionally high wash fastness is achieved, allowing full depths toyield satisfactory fastness to washing tests at the boil. Microfibre blends ofpolyester and viscose have been dyed successfully with disperse and reactant-fixable dyes.

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The use of selected disperse dyes for the polyester allows for rapid rates oftemperature rise and short duration at 130°C, minimising possible decom-position of the copper-complex reactant-fixable dyes. This process has clearadvantages:(1) One-bath operation simplifies control and improves productivity [20].(2) In-to-out times are reduced to between 3.5 hours (Indosol CR) and 4.5

hours (Indosol E-50/EF).(3) Only 5–15 g l–1 Glauber’s salt is required, compared with 20–100 g l–1

electrolyte in reactive dyeing.(4) The levelling power of the reactant-fixable dyes is excellent at 130°C.(5) Shading additions are easy to make before Indosol aftertreatment.

The most obvious drawback of this approach is the restricted gamut inbright shades. A copper-specific sequestering agent Plexophor SFI (Clariant)must be used to protect certain disperse blue dyes that are sensitive to copperions when dyeing in fully-flooded jet machines. Ethylenediaminetetra-aceticacid is too powerful, causing demetallisation of the reactant-fixable dyes,whereas phosphates have little ability to sequester copper [21]. Shadingadditions to the aftertreated dyeing can be difficult because of the highlycationic surface charge. Addition of anionic dyes results in rapid strike andhence unlevelness. An anionic surfactant must be added before any shadingwith the reactant-fixable dyes.

Two popular routes to black dyeings on polyester/viscose blends are based oneither CI Direct Black 22 (Figure 13.2) or the reactant-fixable system. Black 22after resin treatment shows exceptional fastness on viscose and is a classic dye ofthe ortho, para primary amino or hydroxy type where the molecules of dye cancondense with free formaldehyde or the N-methylol groups of the reactant. Aproblem with Black 22 is the necessity for alkali to maintain solubility in dyeingand this affects the stability of the disperse dyes adversely. A two-stage procedureis necessary, dyeing the polyester at pH 5 and 130°C and then cooling andadjusting to an alkaline pH to dye the viscose. The use of reactant-fixable dyesavoids the need for this pH swing [1].

In a more recent development, Clariant have introduced the Optisal/Optifixsystem, supported by a computer program. This consists of a range of bright,metal-free direct dyes with high exhaustion values and low salt content. Thesefeatures result in minimal contamination of waste waters. The stability of thesedyes at 130°C allows them to be used in a one-bath method with disperse dyeson polyester/cellulosic blends. Fixation of the Optisal dyes to cellulose occursduring an alkaline aftertreatment with a cationic reactant, Optifix F Liquid



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Figure 13.2 CI Direct Black 22

(Clariant), which does not release formaldehyde. These dyeings will meetdomestic laundering requirements up to 50°C [22].

13.1.5 Reactive dyeing of the cellulosic component

Reactive dyeing is by far the most important technique adopted for exhaustdyeing of the cellulosic portion of these blends. Reactive dyes can be used for afull gamut of hues at virtually all depths. When dyeing a polyester-rich blend,such as 70:30 for example, with reactive dyes at a typical liquor ratio of 15:1, theeffective liquor:cellulose ratio is 15:0.3 or 50:1. Hence it is essential to select forbatchwise dyeing reactive dyes that give high fixation at a long liquor ratio. Theuse of reactive dyes of low substantivity for the exhaust dyeing of such blends isinefficient, expensive and difficult to reproduce. Reactive dyes are almostcompletely free from cross-staining of the polyester. The only exceptions are thephthalocyanine-based blues and greens that may give slight staining in someinstances.

It is with this class of dyes on these particular blends, however, that the totaldyeing time can be the most prolonged. The original processing systems for theseblends usually consisted of:(1) conventional high-temperature application of the disperse dyes;(2) reduction clearing to remove any disperse dye staining from the cellulosic

fibre;(3) conventional reactive dyeing in a fresh bath at the pH, temperature and

electrolyte concentration appropriate for the reactive system selected;(4) rinsing and soaping at the boil as usual.

Total load-to-unload times could be as long as 12 hours and even then additionalshading corrections might be required. Detailed cost comparisons have shownthat the two-bath method with disperse and reactive dyes costs in labour, energy,

N

SO3Na

N

NH2

H2N

O

NN

HN

N

HSO3Na

NaO3S

ON N

NH2

H2N

NaO3S

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dyes and chemicals about three times as much as the one-bath alternative withdisperse and direct dyes [23]. Traditionally, this two-bath sequence withintermediate clearing was considered essential for optimum fastness of soliddyeings or maximum brightness of contrast effects in full depths. There wasvirtually no risk of interaction between the disperse and reactive dyes. Theselection of disperse dyes was unrestricted and common salt could be used freelyin the reactive dyeing bath.

Preparation of blends of microfibre polyester with viscose for sportswear byscouring, drying and heat setting should be followed by a two-bath dyeingsequence. The polyester is dyed first on the jet at 130°C. This is followed by areduction clear, and then the viscose is dyed with vinylsulphone dyes [12].Options have been presented for rationalising dyeing sequences on polyester/cotton compared with the traditional bleach, polyester dyeing, reduction clearand reactive dyeing. Novel concepts for relocating the bleaching step were con-sidered and the savings attainable from rationalisation demonstrated [24].

During the 1980s, reactive dye manufacturers optimised exhaust dyeingtechniques for these blends with a view to reducing overall dyeing times andlimiting the number of soaping and rinsing steps. Developments were mainlyaimed at reducing the time and energy requirements without affecting thefastness of the dyed goods. When dyeing in combination with conventionalalkali-fixing reactive dyes, some compromise in processing is required. Ideally,alkali-sensitive disperse dyes should be fully absorbed by the polyester beforealkali is added to the dyebath. Likewise, the disperse dyes should not beintroduced until the reactive dyes are fully fixed and the dyebath neutralised,otherwise lower or erratic yields may result [6]. The particular process chosenalso depends on the type and utilisation of the available machinery.Opportunities for automation, short load-to-unload times, low energyrequirements and savings of water and chemicals are important [25].

Most of these developments followed either of two distinct approaches:(1) Two-stage processes in which the polyester was dyed first and then alkali

added at a later stage to induce fixation of the reactive dyes, usually fromthe low-reactivity ranges.

(2) The ‘reverse’ two-stage or two-bath processes, in which the cellulosic fibrewas dyed first and then after an intermediate rinse the polyester was high-temperature dyed. This sequence was particularly suitable for vinylsulphonedyes.

Although dyeing times can be reduced by up to 4 hours using these techniques,there still remains the serious disadvantage of the high concentrations (50–100



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g␣ l–1) of electrolytes required. This interferes with dispersion stability bysuppressing the essential ionising mechanism of the anionic polyelectrolytedispersing agents through a common-ion effect, thus decreasing the charge onthe dispersed particles and promoting agglomeration. The interference isvariable, frequently affecting one manufacturer’s formulation of a particular dyemore than another’s, or even varying between successive batches of the sameformulation. Dispersion stability depends on many parameters such as batchquality of the dispersing agents and the physical form of the disperse dye, notmerely particle size distribution but also crystal form. This problem may bealleviated by the judicious use of additional dispersing agents, of which theethoxylated phosphate type is particularly effective [8].

Another problem in disperse/reactive systems is the difficulty of reconciling thedifferent pH requirements of the two classes of dyes. Conventional reactive dyesneed alkaline conditions (pH 10–12) for fixation. Although a few disperse dyeswill tolerate such alkalinity during dyeing, many containing vulnerableheterocyclic rings or hydrolysable ester groups (Figure 13.1) must be dyed belowpH 6. Nevertheless, practically all disperse dyeings will withstand quite severealkalinity once the dyes have thoroughly penetrated the polyester fibre.

In the conventional two-stage approach, the initial disperse dyeing at hightemperature ensures uniform wetting out of the cellulosic fibres. Disperse dyestaining is normally removed during subsequent alkaline fixation of the low-reactivity dyes at 80–100°C, but disperse dyes with ester groups that can behydrolysed at pH 11 and 80°C are preferred. Selection of the disperse dyes islimited, however, to those with good dispersion stability in the presence ofelectrolyte. For this reason, the use of Glauber’s salt rather than common salt isessential. The reactive groups in the low-reactivity dyes are sufficiently stable towithstand the conditions of high-temperature dyeing of the polyester at pH 5–6(phosphate buffer). High-reactivity dyes are generally unsuitable because of therisk of interaction with the disperse dyes (section 4.1).

A mild oxidising agent such as sodium m-nitrobenzenesulphonate is added toinhibit reduction of azo reactive dyes at 130°C, especially when dyeing polyester/viscose blends. This agent is less effective for azo disperse dyes or under acidicconditions. The reducing action of viscose can be attributed to:(1) high aldehyde end group content arising from oxidative damage in

bleaching;(2) residual sulphur in the form of sodium sulphide or carbon disulphide from

xanthate regeneration in manufacture. Sulphide levels as low as 10 mg l–1 inenclosed systems can attack some azo direct or reactive dyes [1].

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In a more versatile alternative technique, the disperse and reactive dyes areapplied together at high temperature and pH 4.5–6. Salt, if added at thestart, would promote exhaustion of the reactive dyes but could then exert itsmaximum deleterious effect on the dispersion stability. After exhausting thedisperse dyes at high temperature, the dyebath is cooled to 80°C. This is thebest point at which to add the salt, since the residual concentration ofdisperse dyes is too low for them to be adversely affected. After the low-reactivity dyes have been exhausted by the cellulosic fibre, alkali (usuallysodium carbonate) is added to bring about fixation. Good batch-to-batchreproducibility of hue is related to the relative sensitivity of such systems toprocess variables as demonstrated in the BASF process for the rapid dyeingof these blends, which has been applied successfully to microfibre polyester/viscose [25]. As with direct dyeings, reduction clearing of the disperse dyeingmust be avoided because of the sensitivity of azo reactive dyes to reduction.Some clearing takes place, however, during the alkaline fixation and soapingto desorb unfixed and hydrolysed reactive dyes.

In the Sumitomo RPD-Supra two-stage method, disperse dyes are applied tothe polyester in the first stage and Sumifix Supra (NSK) bifunctional reactivedyes (Figure 8.1) are applied to the cellulosic fibre in the second stage. Theseaminochlorotriazine-sulphatoethylsulphone dyes exhibit high substantivity andreactivity, resulting in excellent fixation. They level well and show low sensitivityto variations in time and temperature, giving highly reproducible dyeings [26].The levelling of reactive dyes is closely related to substantivity, migration andrate of fixation. In the case of Sumifix Supra structures, chromogens have beenselected from a wide range of chemical classes and optimised in substantivity andreactivity to ensure a high degree of levelness. The reaction rate can be controlledmainly by dyebath pH and temperature. Owing to the difference in reactivitybetween the two functional groups, optimum dyeing conditions cover a relativelywide range of conditions [27,28].

With the high-reactivity ranges of reactive dyes, it is necessary to completefixation on the cellulosic fibre before applying the disperse dyes to the polyester.After dyeing with the high-reactivity dyes at the appropriate pH, temperatureand salt concentration in the presence of sodium m-nitrobenzenesulphonate,followed by the relevant alkaline fixation conditions, the pH is adjusted to 6–6.5with acetic acid. The disperse dyes and disodium dinaphthylmethane-disulphonate as dispersing agent are added and the polyester is dyed to the targetdepth at 130°C.

High-temperature dyeing is a highly efficient alternative to soaping at the boil



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as a means of desorbing the unfixed reactive dyes at a rapid rate of diffusion inthe absence of electrolyte. Shade reproducibility depends on the stability of thedye–fibre bonds in the reactive-dyed cellulosic fibre to the high-temperaturepolyester dyeing stage. Any final shading of the cellulosic fibre requires a freshbath to be set after clearing of the disperse dyeing.

This two-stage method is particularly appropriate for vinylsulphone reactivedyes because the dye-fibre bond formed by the nucleophilic addition mechanism[29] is more resistant to acid hydrolysis than that formed by most other types ofreactive dyes. Thus vinylsulphone dyeings are not impaired by the mildly acidicdyebath preferred for applying disperse dyes to the polyester fibre [30]. Soapingafter application of the vinylsulphone dyes is unnecessary because any unfixeddye is readily removed in the final wash-off after the disperse dyeing stage.

This reverse approach is useful for piece dyeing on a jet machine, but is lesssuitable for beam dyeing or package dyeing of yarn than the sequence in whichthe polyester is dyed first, owing to greater aggregation of the unsorbed dispersedyes in the high concentration of electrolyte present in the dyebath. Advantagesof the Then Airflow aerodynamic jet dyeing machine for this process are said tobe optimum fabric quality, freedom from creasing, no foaming, a high degree ofreproducibility and ease of cleaning of the machine after use [31].

Exhaust dyeing of the cellulosic fibre with Levafix E (DyStar) dichloro-quinoxaline and Levafix E-A (DyStar) difluoropyrimidine dyes proceeds withsalt at 40–60°C, followed by fixation at pH 9.5–11.5. Dyeing of the polyesterwith disperse dyes is completed at pH 4–6 and 130°C. The principles andpractice of automatic metering of the chemicals have been detailed with specialreference to Levametering (DyStar) systems [32]. This ensures optimum fixationconditions for both components of the blend, giving excellent reproducibility,level dyeing and high colour yields. Full automation of the dyeing processeliminates the dead time that occurs with manual control. The time savingsenhance productivity and lower the costs of labour and energy.

Kayacelon React (KYK) reactive dyes contain two aminonicotinotriazinereactive groups per molecule (Figure 8.6). They readily react with cellulose,eliminating the nicotino group to form a covalent bond that is identical with thatgiven by analogous dyes of the well-established bis(aminochlorotriazine) type.The substitution reaction occurs with nonionised hydroxy groups in celluloseunder neutral dyeing conditions at high temperature, although if necessary thereaction can be induced at lower temperatures under mildly alkaline conditions[27,33]. A one-bath process for dyeing polyester/cotton blends with disperse dyesand Kayacelon React dyes entails application at pH 7 and 130°C with salt and anonionic levelling agent. After dyeing, the fabric is soaped at the boil and rinsed

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as in conventional reactive dyeing methods. The use of a fixing agent is notrecommended for pale depths, but a product of the long-chain alkylammoniumtype is used for medium-depth dyeings and a polyamine auxiliary used for fulldepths.

13.1.6 Water-insoluble colorants for polyester/cellulosic blends

Insoluble vat and sulphur dyes often show moderate to severe cross-staining ofthe polyester component in these blends. In the case of vat dyes, this cansometimes be exploited in using them to colour both fibres simultaneously. Someof the benzamidoanthraquinone vat dyes of fairly low Mr dye polyester morereadily than the cellulosic fibre at 130°C and can be used for solid effects on thetwo fibres in pale depths. Certain olive green, khaki, brown and black dyes ofrelatively complex structure give duller and more bathochromic hues on thecellulosic fibre compared with the polyester.

Many polycyclic vat dyes give a good reserve of polyester fibres when appliedby the usual alkaline leuco methods. These are also useful when dyeing solid orcontrast effects with disperse dyes for the polyester. They include the halogenatedderivatives of anthanthrone or pyranthrone, the indanthrone and violanthroneblues and greens, as well as most of the acridone and carbazole types. The colourgamut is restricted in the bright orange, red and violet sectors, however.

Two-stage dyeing with disperse and vat dyes is a valuable method of achievingexcellent fastness to light and washing at all depths. The disperse and vat dyesmay be applied at pH 5–6 with an anionic dispersing agent. After dyeing thepolyester at 130°C, the temperature is quickly lowered to 85°C and the vat dyesare reduced with alkaline dithionite. The cellulosic fibre is dyed to shade at anappropriate temperature within the range20–60°C, followed by reoxidation at 50°C and soaping at the boil. Staining ofthe polyester fibre by the vat dyes may complicate shade matching. The vatdyeing conditions in the second stage act as a reduction clearing bath for thedisperse dyeing.

Selection of the vat dyes is restricted by problems of instability in high-temperature dyeing. This difficulty can be overcome by introducing the vatpigment dispersion after cooling to 85°C, but before adding the reducing system.Owing to the rapid consumption of sodium dithionite, vat dyes are applied moreeasily in machines completely filled with liquor, i.e. beams and fully-flooded jetmachines, rather than in only partly flooded jets or overflow machines. Onlyselected vat dyes are sufficiently stable in the reduced form for addition at 85°C,including flavanthrone yellow, the indanthrone and violanthrone blues andgreens, as well as several of the carbazole types. In the case of the indanthrone



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blues, carbazole browns and olive greens, sodium nitrite should be added toprevent over-reduction.

Well known drawbacks of vat dyeing processes include the difficulty ofmaintaining satisfactory reducing conditions and the need to avoid over- orunder-reduction. These problems can lead to unlevelness, poor reproducibility ofshade and, in some cases, poor fastness to rubbing. Sodium dithionite isespecially unattractive from the environmental viewpoint. Ecologicallyinnocuous methods for reducing vat dyes currently under consideration include:(1) electrochemical reduction using a mediator;(2) organic reducing agents such as hydroxyacetone;(3) iron pentacarbonyl or other Fe(II) complexes.

Iron complexes with ligands derived from triethanolamine or gluconic acid havebeen investigated with selected Indanthren (BASF) dyes applied by exhaustdyeing. Iron salt requirements, various molar ratios of Fe(II) salt to gluconic acid,the influence of caustic soda concentration and the presence of Fe(III) ions,levelling behaviour and over-reduction were investigated [34].

Two-bath methods of applying disperse dyes followed by vat or sulphur dyesusually present no serious limitations. The reducing conditions in the seconddyebath are often sufficient to clear the surface deposition of disperse dyeswithout an intermediate clear. Alkaline dithionite should be used to solubilise thesulphur dyes because sodium sulphide may damage the polyester fibre.

The low cost of sulphur dyes makes disperse/sulphur dye recipes useful fordull, heavy depths, but there are obvious limitations of shade. They are usuallysatisfactory when the wash fastness requirements are less stringent than thetypical standards for disperse/vat dyeings. With certain heavy shades it may benecessary to introduce an intermediate clearing step. Sulphur black dyeing is arather laborious procedure but it does produce dyeings of excellent bloom andwash fastness. Either pre-reduced or solubilised sulphur dyes may be used. Thepre-reduced approach is more economical but it does cause more staining of thedyeing vessel.

Pigment coloration of bleached polyester/cotton garments, e.g. knitted leisureshirts, can be carried out after treating them with a suitable cationic binder.Exhaust coloration with the dispersion of organic pigments and a compatibleanionic dispersing agent is followed by stonewashing with a surfactant, dry heatfixation of the binder and finally a heat setting treatment to stabilise the fabric.Colour yield, levelness of coloration and fastness to rubbing are all potentialproblems [35]. Attention must be paid to the time and temperature ofpretreatment, binder concentration, pH and the effects of stonewashing on

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fastness to rubbing, yield and levelness. Single-class application to fibre blends isa major advantage claimed for this process.

13.2 CONTINUOUS DYEING OF POLYESTER/CELLULOSIC BLENDS

13.2.1 Disperse dyeing of the polyester component

The continuous dyeing of woven polyester/cellulosic blend fabrics is a mostimportant sector of the textile dyeing industry. Statistics for the USA have shownthat more than half of the polyester fibre processed is destined for dyed polyester/cellulosic fabrics, more than half of which are dyed con-tinuously by pad–thermofix processes. Continuous processing of long runs to a given colourensures consistently high yields and reproducible uniformity at a much moreeconomical price per metre than batchwise methods. The length of run to eachcolour normally exceeds 5000 m, so a typical order in several colours will takeseveral hours to process through each stage. The trend, however, is for shorterruns. Dyeing and heat setting of woven fabrics can be achieved simultaneouslywithout the risk of rope creasing. Crease formation, however, can still arise incontinuous processing and it is responsible for more seconds quality than anyother fault.

Thorough preparation is especially important before the continuousdyeing of polyester/cotton. Each stage can be carried out continuously, i.e.singeing, enzyme desizing, alkaline peroxide pad–steam scour-bleach,mercerising and heat setting before dyeing. Typical fabrics prepared in thisway include shirting, light suiting, rainwear, workwear, military and civilianuniforms. This preparation sequence merely removes the surface impuritiesfrom the polyester fibres, which are not significantly penetrated by thealkaline peroxide liquor applied to the cotton. Mercerising enhances thecolour yield, lustre and dimensional stability of the fabric, as well as itsappearance by improving the coverage of immature cotton neps. Thecrystalline structure of the cotton cellulose is modified [36], swelling thefibres and eliminating their convolutions, but the polyester is not materiallyaffected. Heat setting is obligatory to minimise subsequent dimensionalchanges at the high temper-atures reached in thermosol dyeing [37].

Polyester/cellulosic fabrics should be padded evenly with the dyedispersion and a nonionic wetting agent at pH 5–6. A migration inhibitor ofthe anionic polyelectrolyte type, e.g. sodium alginate, polyacrylamide,poly(vinyl alcohol) or carboxymethylcellulose, is normally included and thisgels during drying to ultimately form a solid film in which much of the dye is



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entrapped. Migration inhibitors of the polyacrylamide type do not interferewith dye transfer in thermofixation as much as does alginate [6]. Liquidbrands of disperse dyes are preferred to solid forms, as they are less prone tomigrate during drying and they give less staining of the cellulosic fibres.Higher colour yields are attainable and the pad liquors are easier to preparein the large quantities normally required for continuous dyeing. On the otherhand, liquid dyes can settle on long storage and thus require thoroughagitation before weighing. When stored in partly used containers they mayvary in strength owing to loss of water or deposition of solids at the air/liquid interface.

Prior to thermofixation the disperse dyes are present in the interstices ofthe fabric in a highly aggregated state and are embedded, together with thedispersants and wetting agent, in this solidified matrix of polyelectrolyte.During drying and thermofixation the smaller aggregates release disperse dyemolecules that migrate to the fibre surfaces. Further disaggregation proceedsand there is a build-up of dye near these surfaces [38]. Diffusion into theinterior of the polyester fibres can only take place when the disperse dyes arein the monomolecular form and the temperature has reached that of thethermofixation chamber.

A vertical infrared predrying zone minimises the tendency towardsstreakiness or two-sided effects if insufficient migration inhibitor has beenused. This treatment should reduce the moisture content of the fabric from50–60% after padding to 25–30%, so that drying can be completed in a hot-flue machine at 100–120°C or on modulated drying cylinders (scaled from80°C to 140°C) without significant migration [39]. Problems encounteredduring padding and intermediate drying include the induction period whilethe fabric is being heated up and staining of the cellulosic fibres by thedisperse dyes, which is greater than in batchwise dyeing. These factors areboth dependent on the type of drying unit, the fabric construction and thecomposition of the pad liquor. The use of optimised dispersing systemsfacilitates the transfer of disperse dyes from the surface of the cellulosicfibres to the polyester during intermediate drying to give the highestattainable yield on the polyester [40].

Migration during intermediate drying can cause many faults that aredifficult to rectify. It is particularly troublesome in the continuous dyeing ofknitted fabrics containing polyester/cellulosic yarns. Migration may occur asa result of high liquor retention after padding, a slow initial rate of drying ora high residual moisture content after the predrying step [41]. Faults arisingfrom dye migration include:

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(1) Ending: the drying rate and visual depth vary along the length of a run.(2) Streakiness: variable rates of drying over the fabric surface.(3) Frosting: dye migration from the interior to the surface of the fabric.(4) Two-sidedness: one surface of the fabric is hotter than the other during

drying, giving a visible difference in depth with the hotter side deeper.(5) Dark selvedges: these regions hotter than the fabric centre.(6) Light selvedges: these regions cooler than the fabric centre.(7) Poor penetration: disturbance of the weave reveals undyed regions at the

crossovers and inside the threads.

With the exception of creasing, shading across the width of the fabric is thebiggest single cause of customer rejects [39]. Several tests are available to assessmigration, namely the ‘sandwich’, ‘watch glass’, ‘glass plate’, ‘hot air’ and ‘fold’tests [42].

Parameters that influence migration are dye class and constitution, physicalform of the dyes, dyebath additives and preparation of the substrate. In general,disperse dyes in liquid form perform better in this respect than those formulatedas grains or powders. Quality control problems are more likely when handlingliquid brands, however, because of the risk of sedimentation on storage.Differences in migration behaviour between ‘equivalent’ disperse dyes with thesame CI generic name may arise because different formulating agents are present.

Even when the fabric is evenly and fully penetrated during padding, the finaldyeing may exhibit poor penetration because of migration or inadequatethermofixation. During drying, water evaporates from the fabric surface and isreplaced by capillary flow of dye liquor from the interior. The dyed material willshow inferior fastness to washing and wear points of the garments, such ascollars, cuffs and elbows, will tend to develop into lightly coloured folds orpatches [6].

In a study of the behaviour of disperse dyes padded on to a 70:30 polyester/cotton blend, the dye applied was found to be distributed with only 30% of thetotal amount on the polyester and the remainder on the cotton. Thus theconcentration of disperse dye before thermofixation was more than five timesgreater on the cotton than on the polyester. During thermofixation most of thedye was found to be transferred from cotton to polyester by volatilisation intothe vapour phase [43]. Factors contributing to staining of the cotton include thechemical nature and concentration of migration inhibitor present, the dyebathpH and the chemical structure of the dyes used [44]. At unusually hightemperatures, or if the treatment time is unduly prolonged, dye is lost byvolatilisation and contamination of the interior surfaces of the thermofixation

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unit. Subsequent dyeings may be spoiled by volatilisation and redeposition of thiscondensed dye on the new material.

Optimum fixation conditions depend on the design and efficiency of thethermofixation equipment as well as the depth and sublimation fastness of thedyeing, but the treatment temperature is usually within the range 205–220°C.Although high-energy disperse dyes require somewhat critical conditions foroptimum transfer and fixation, they are the most widely used dyes for pad–thermofix methods, especially in full depths. The optimum temperature ofthermofixation is that at which the maximum amount of dye is transferred to thepolyester without significant loss by volatilisation or contamination. Treatmentat this temperature ensures optimum colour yield and fastness, highreproducibility, low sensitivity to temperature variations and colour stabilityduring subsequent durable finishing. Too low a temperature of thermofixation ortime of treatment may prevent complete transfer of dye from cotton to polyester,resulting in excessive staining of the cotton and poor yield on the polyester.Listing arising from differential temperature distribution may be found afterthermosol treatment. Contact with the base-plates on a thermosol pin stentermay cause off-shade selvedge marking [39].

The effective time of thermofixation at a given temperature is not the same asthe nominal duration of the treatment. Air is a poor heat-transfer medium and itmay take 45–60 seconds before the fabric temperature is within a degree or twoof the air temperature in a conventional hot-air thermofixation unit. The timetaken also depends critically on the fabric weight and construction. The heavierthe fabric, the longer it takes to attain equilibrium temperature. It is importantthat the material has been dried uniformly before entering the thermofixationunit, otherwise the initial heating stage will be used to evaporate this residualmoisture rather than raising the cloth temperature to its maximum. This willresult in uneven and inadequate dye fixation [6].

Reduction clearing after the thermosol treatment is desirable to attainoptimum fastness on the polyester and to clear any disperse dye stain from thecellulosic fibres. Reduction treatment must be avoided, however, if the cotton hasalready been dyed. When vat dyes are applied the reducing conditions necessaryare often sufficient to clear the unfixed disperse dyes effectively, so that a separateclearing treatment is unnecessary. Those disperse dyes that contain alkali-hydrolysable groupings, or certain heterocyclic diazo components such as thenitrothiophene blues (Figure 13.1) can be cleared by alkaline washing rather thanreduction clearing. Alkaline washing is substantially cheaper than reductionclearing. It is applicable after dyeing with reactive dyes and it may be combinedwith the washing-off of unfixed reactive dyes.

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Many dyed polyester/cellulosic fabrics are subjected to fairly severe heat treat-ments during subsequent durable finishing and garment manufacture. Theseprocesses may induce further migration of low- or intermediate-energy dispersedyes from the polyester to the cellulosic fibres or the resin film, resulting in pos-sible colour changes or inferior fastness to rubbing and wet treatments. Theselection of high-energy disperse dyes helps to minimise these problems.

The severity of the thermomigration effect depends on several factors:(1) constitution and applied depth of the dyes used;(2) heat treatment history of the polyester fibres;(3) degree of penetration of the polyester after thermofixation;(4) degree of staining of the cellulosic fibres;(5) temperature and duration of the heat treatment that causes the

thermomigration;(6) presence of other contaminants on the surface of the fibre components.

Important counter measures [45,46] that can be taken include:(1) careful dye selection to avoid dyes prone to thermomigration;(2) adequate thermofixation conditions to ensure optimum penetration of the

polyester fibres;(3) thorough post-clearing to minimise residual staining of the cellulosic fibres;(4) lowering the curing temperature in subsequent resin finishing;(5) minimal application of softeners and antistatic agents at the finishing stage.

13.2.2 Water-soluble dyes for the cellulosic component

The economically attractive one-bath batchwise process with disperse and directdyes cannot be adapted readily to continuous dyeing. Direct dyes exhibit onlylimited solubility and are highly aggregated at padding concentrations. Theirhigh substantivity leads to shade matching problems because of rapid depletionfrom the pad liquor (tailing). Steaming times would have to be prolongedbecause direct dyes diffuse only slowly into the cellulosic fibres. The possibility ofpad–thermofix application of the disperse dyes, reduction clearing and thenbatchwise dyeing with direct dyes is of no interest because of high processing costto achieve only moderate fastness.

Reactive and vat dyes are the main alternatives for dyeing the cellulosiccomponent. Reactive dyes give an excellent reserve of the polyester. Reserve ofthe cellulosic component is much less desirable, especially in polyester/cottonworkwear fabrics or polyester/viscose blends in general, because the cellulosicfibre is preferentially abraded during wear. One-bath or two-bath processes



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based on disperse and reactive dyes offer exceptional brightness and very goodfastness up to medium depths, especially for polyester/viscose dresswear orshirting.

The simplest method of reserving the polyester fibre is to pad with high-reactivity dyes, sodium bicarbonate and salt to minimise migration, followed bydrying at 90°C. Urea is usually added to improve solubility during padding andto enhance the colour yield. For maximum fixation in full depths or underadverse drying conditions, a brief steaming treatment may be given beforesoaping at the boil. In an alternative process, low-reactivity dyes are applied withsoda ash and urea, dried and reacted by thermofixation at160–200°C. Improved results are obtained from either method on polyester/viscose blends if the padded fabric is batched and stored for 1–2 hours beforedrying. Better storage stability of the pad liquor is ensured if the fabric is paddedwith the dyes in neutral solution and dried, then padded again in caustic sodaand salt solution, steamed, rinsed cold and soaped at the boil.

Selected disperse and reactive dyes can be fixed simultaneously by the simpleand economical one-bath pad–dry–thermofix process. Dye selection is criticalbecause of the risk of interaction between disperse and reactive dyes underalkaline conditions (section 4.1). Suitable disperse and high-reactivity dyes arepadded with urea, sodium bicarbonate and migration inhibitor. Sodiumm-nitrobenzenesulphonate is added to prevent reduction of certain azo reactivedyes, particularly on polyester/viscose fabrics. After drying, the fabric isthermofixed at 200–220°C and soaped at the boil.

Urea concentrations greater than about 50 g l–1 induce excessive staining ofthe cellulosic fibres under these conditions, resulting in unsatisfactory fastnessand inferior yield on the polyester. Another disadvantage of urea is that itdecomposes above 135°C (Scheme 13.1) and the ammonia and cyanic acidformed are objectionable. Although more costly, dicyandiamide (cyano-guanidine) and dicyanoguanidine (Figure 13.3) are preferable to urea, becausethey are thermally stable and do not give rise to toxic by-products [47].

A more versatile selection of dyes is possible in two-stage methods using pad–dry–thermofix application of disperse dyes to the polyester, followed by pad–steam, alkali–pad or alkali–shock treatment to fix the reactive dyes on thecellulosic fibre. Less productive, but giving the widest freedom in terms of dyeselection is the semi-continuous two-bath sequence of conventional pad–dry–thermofix to apply the disperse dyes and then cold pad–batch dyeing with thereactive dyes. High-reactivity dyes offer higher productivity than low-reactivitytypes because of the short batching times required [48].

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Scheme 13.1

HN CNH

NH2

CNHN C

NH CN

Dicyandiamide

NH CN

Dicyanoguanidine

Figure 13.3 Alternatives to urea in thermosol dyeing

H2N C

NH2

O HN C

NH2

OH N C OH

Urea

+ NH3

Cyanic acid

The advantages of semi-continuous dyeing methods for these blends include:(1) suitability for lengths to a colour that are insufficient to justify fully

continuous processing;(2) optimum use of readily available equipment;(3) higher productivity and better uniformity at the padding stage than by

batchwise dyeing;(4) continuity of colour over long runs compared with production of several

discrete batchwise dyeings;(5) suitability for contrast effects with versatile control of shade matching.

Useful alternatives to the above semi-continuous process are the pad–batch–beam and pad–batch–jet sequences, which are more productive with high-reactivity dyes and especially suitable for vinylsulphone dyes. The blend fabric ispadded with the reactive dyes and the usual amounts of alkali and salt and thenbatched for 2–24 hours according to dye reactivity. It is then transferred to ahigh-temperature beam or jet machine, rinsed and soaped at the boil. Afteraddition of the disperse dyes, dispersing agent and acetic acid to pH 6, thepolyester component is dyed at 130°C in the usual way. These processes offer theadvantages of optimum yields of the reactive dyes with minimum occupation ofthe available pressure-dyeing equipment.

This approach is suitable for tubular-knit polyester/cellulosic fabrics by dyeingthe cellulosic component first using a padding unit (e.g. BeauTech, Calator orJawatex) designed specifically for pad–batch processing of knitted fabrics. Thepolyester component is then jet-dyed with disperse dyes. Significant savings overtraditional exhaust processes for knitgoods are claimed [49]. These semi-continuous techniques are particularly versatile for contrast effects since one run



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on the pad–batch equipment can be divided into a series of batches for beam orjet dyeing to different hues in the second stage.

13.2.3 Water-insoluble dyes for polyester/cellulosic blends

Selected members of the vat dye range will dye both components of a polyester/cellulosic blend in most shades up to medium depth. At full depths the cellulosicfibre is dyed more deeply and a skittery effect is evident. The target shade shouldbe matched with a single dye or binary mixture if possible. Trichromaticcombinations of dyes that differ widely in hue should be avoided. The bestsolidity is given by selected indigoid and thioindigoid dyes but these have limitedfastness. Good solidity and better fastness are provided in the yellow to orangesector by polycyclic quinone derivatives.

Suitable dyes are padded at 30–40°C with a wetting agent of thesulphoricinoleate type and a migration inhibitor. After drying andthermofixation at 200–220°C to promote diffusion of the vat dyes into thepolyester, the fabric is cooled in air and the dyes on the cellulosic componentreduced by padding in alkaline dithionite. Because of the previously hightemperature of the thermofixation stage, special attention must be given toensure that the fabric is adequately cooled before chemical padding and that thepad liquor temperature is kept below 30°C. After steaming, the dyeing isreoxidised and soaped at the boil as usual.

The same process sequence is used to apply mixtures of selected disperse andvat dyes at the initial padding stage. Matched mixtures of selected disperse andvat dyes have been commercially available for many years. The critical factor isthe stability of the disperse dyes to reducing systems, once they have been fixedon the polyester. The chemical pad–steam stage is effective in clearing thedisperse dye stain from the cellulosic fibres as well as reducing the vat dyespresent. Only in unusually heavy depths is it necessary to apply the vat dyes froma separate bath. The most severe requirements of fastness to rubbing, washing,chlorine and dry heat in all depths for uniforms or workwear are met withselected high-energy disperse dyes and vat dyes [50].

The disperse dyes are selected for stability under the reducing conditions of thepad–steam stage and the vat dyes should give only limited staining of thepolyester under thermofixation conditions. The higher the treatment temperatureis above 190°C, the greater is the degree of staining. Minimal staining is shownby the indanthrone blues and many of the acridone and carbazole types.Satisfactory results can be achieved with dyes giving an intermediate degree of

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staining, if disperse dyes are used to adjust the hue on the polyester. Vat dyes ofthis kind include violanthrone and isoviolanthrone derivatives, as well as most ofthe benzamidoanthraquinones, oxazoles and thiazoles [51].

Vat leuco ester dyes will give a solid effect in pale depths on the twocomponents of a polyester/cellulosic blend. These dyes are padded at 60–80°Cwith soda ash, sodium nitrite and a wetting agent. The dyeing on the cellulosicfibre is developed on a jig or a continuous washing range by immersion of thefabric in dilute sulphuric acid at 20–40°C, neutralisation with soda ash at 40°Cand soaping at the boil. In this process the vat leuco esters dye the cellulose andinitially stain the surface of the polyester.

Final rinsing, drying and heat setting at 200–210°C achieves optimum fastnessand yield of the vat dyes on the polyester. The fabric must be completely freefrom alkali at the thermofixation stage to avoid discoloration of the cellulosicfibre. The oxidised forms of these dyes diffuse into the polyester duringthermofixation, which is an essential part of the dyeing process as it develops andstabilises the colour. A hydrosetting treatment on the beam at 130°C may begiven to achieve penetration of the polyester if no thermofixation equipment isavailable.

In applications requiring high fastness in pale depths this method of dyeingboth fibre components simultaneously is elegant and easy to use. The onlydrawbacks are that there is only a limited possibility of adjusting the proportionsof dye between the two fibres and the products are costly. In pale depths both ofthese disadvantages are of minor significance.

The solubilised vat leuco esters can also be applied in combination withdisperse dyes. The blend fabric is padded at pH 5–6 with, in addition to the usualmigration inhibitor, a mixture of anionic and nonionic auxiliaries. Drying andthermofixation to dye the polyester with the disperse dyes initiates preliminarydecomposition of the vat leuco esters. These are then fully fixed by padding inthe usual nitrite and sulphuric acid, then developing on a jig or washing range[8].

Most disperse dyes in dispersion are unable to withstand the reducing systemsrequired for sulphur dyeing, but once they have been absorbed within thepolyester fibre they are quite stable. Disperse dyes are applied together withdispersed or solubilised sulphur dyes by padding at pH 4.5–5.5 and drying,followed by thermofixation at 190–210°C. Padding with alkaline dithionite,steaming, reoxidation to fix the sulphur dyes and soaping at the boil completesthe process. Sodium sulphide or hydrosulphide reduction is avoided because ithas an adverse effect on the polyester fibres.



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An alternative process involves padding with disperse dyes, solubilised sulphurdyes and the reducing agent thiourea dioxide at pH 5–6, followed by drying andthermo-fixation to fix both classes of dyes. In this case, however, the thioureadioxide restricts the choice of disperse dyes since some azo dyes are attacked by it[8].

Dyeing problems of various kinds may be associated with chemical pad–steamdevelopment of vat or sulphur dyes [52]. Inadequate yield may result fromincorrect formulation or subsequent decomposition of the chemical pad liquor.Listing at the selvedges, foaming, shade variation or condensation spotting canall arise in the steaming stage necessary to fix the dyes on the cellulosiccomponent. To obtain full development of shade, it is absolutely essential thatthere should be no air in the steamer. Ineffective washing to remove chemicalresidues, insufficient oxidising chemicals to reoxidise the leuco forms of the dyesand deposition of unfixed dye on the surface of the fibres may lead to fastnessproblems after final rinsing. Reoxidation of indanthrone blues above pH 9 mayresult in over-oxidation, leading to greener and duller shades.

Azoic combinations still offer possibilities for colouring the cellulosic componentin certain bright shade areas. In red shades, for example, this approach is moreeconomical and gives higher fastness to chlorine than reactive dyeing. Continuinginterest in these products is attributable to the availability of stabilised liquid brandsfor ease of handling, low costs of machinery and labour, high productivity andflexibility of operation. Computer programs are available on disc for IBM-compatible systems. These supply complete recipes including precise applicationprocedures that ensure simple and reliable application in the dyehouse [53].

In a low-cost one-bath process, the fabric is padded at 40°C with a stabilisedazoic diazo component (Fast Salt), an azoic coupler (Naphtol) and a matchingmixture of disperse dyes for the polyester, together with an acid donor (sodiummonochloroacetate) and urea or dicyandiamide to optimise coupling [54]. Afterdrying at 130°C the fabric is thermofixed at 210°C, rinsed and soaped at theboil.

A versatile semi-continuous sequence entails conventional pad–dry–thermofixdyeing of the polyester with disperse dyes, followed by padding at 50°C with anazoic Fast Salt and a Naphtol dissolved in an alkaline solution containingmethylated spirit (formaldehyde must be excluded). After impregnation thefabric is batched for 1–2 hours, developed in acetic acid solution and soaped atthe boil to remove unreacted azoic components [55].

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13.3 BLENDS OF CELLULOSE ACETATE OR TRIACETATE WITHCELLULOSIC FIBRES

When cellulose acetate yarns were introduced in the 1920s, they createdconsiderable interest because of their potential for cross-dyeing effects withcellulosic yarns. In blends with viscose, the acetate fibres confer aconsiderable improvement in crease recovery. Cellulose acetate is used incotton-spun blends for a range of apparel end uses, mainly dresswear.Cellulose acetate/viscose woven fabrics made from intimate staple blendsand conventional viscose filament constructions with acetate filament effectthreads were formerly popular for crepe dresswear, gabardines, tropicalsuitings, leisure shirts, underwear and children’s wear. Two-fold yarns forcolour contrast effects, and viscose filament warps with acetate or acetate/viscose staple wefts, may also be encountered. Acetate/viscose fabrics are stillsignificant for tailored ladieswear and ‘shot silk’ linings. Acetate/cottonbrocades have been used for many years in floral patterned curtains andupholstery.

When cellulose triacetate was first introduced in the 1950s, it was recognisedas a cheaper alternative to polyester with strength and wearing propertiesintermediate between those of secondary cellulose acetate and polyester. It isquicker-drying and more stable to boiling dyebaths than the secondary acetate. Ithas better pill resistance than polyester, but is inferior to polyester in abrasionresistance and dimensional stability.

Triacetate/cellulosic blends are mainly of interest for dresswear, suiting andskirts, many of the garments being designed with durable pleated effects. Thetriacetate component contributes easy-care properties, durable pleating andcrease recovery. Triacetate/viscose, more important than blends with cotton, isused for children’s clothing, leisure shirts, pleated dresswear and lightweightsuiting. Triacetate/polynosic blends have also proved interesting for woven andknitted ladieswear fabrics. A resin finish is required if the proportion of viscoseor polynosic fibre exceeds 30%.

Intact cellulose acetate shows only slight staining by dyes for cellulose, butacetate/cotton must be bleached carefully under mildly acidic conditions becausepartial saponification of the ester groups greatly increases subsequent staining ofthe acetate component. If it is necessary to bleach cellulose acetate/viscose fabricsfor reserve effects or pale bright dyeings, this can be carried out with silicate-stabilised hydrogen peroxide and an anionic detergent at 70°C. Normally it is



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only necessary to singe, open-width relax in boiling water and then scour withanionic detergent and ammonia at 60–70°C.

Reserve of viscose using selected disperse dyes for the acetate may be difficult.The effect depends on the origin of the viscose and the efficiency of desizing andperoxide bleaching, as well as the dyeing conditions. It is important to desizethoroughly, since any residual size in the warp yarns will accentuate staining bydisperse dyes and thus require a more severe clearing treatment after dyeing.Viscose staining also depends on dyeing time and temperature, liquor ratio andthe selection of anionic dispersing agents. The preferred disperse dyes that giveoptimum reserve of viscose are mainly low-energy dyes (Mr 230–300), includingdinitrodiphenylamine (yellow), nitroaniline monoazo (yellow to red) or 1,4-disubstituted anthraquinone (red to blue) types.

The stain on the viscose can usually be cleared by alkaline reduction atambient temperature. Sodium dithionite in trisodium orthophosphate solution issatisfactory with most azo disperse dyes. However, an oxidative clear withsodium hypochlorite at a mildly alkaline pH, followed by an antichlor treatmentwith sodium bisulphite, is often more effective for anthraquinone disperse dyes.Either type of clearing treatment must be carried out under ambient conditionsto avoid decomposition of the disperse dyes on the cellulose acetate. Where bothtypes of disperse dye chromogen are present, it may be necessary to give bothclearing treatments in sequence with the reduction clear first. The essentialmechanisms are azo fission and leuco-anthraquinonoid solubilisation in thereductive process and destruction of the anthraquinone chromogen by oxidativeattack [8].

If a neutral scouring and bleaching sequence is given to avoid saponificationof the cellulose acetate, this component can be reserved using selected salt-controllable direct dyes of the multisulphonated type with salt at 80°C. Solideffects on acetate/viscose are traditionally obtained by a one-bath method at 75–80°C and pH 6–7 with the disperse and direct dyes, salt and disodiumdinaphthylmethanedisulphonate as the dispersing agent. Care is required becausesome copper-complex direct dyes are able to modify the hue of certain dispersedyeings and the salt addition may lower the dispersion stability of some dispersedyes.

Acetate/viscose dresswear or suiting fabrics are often dyed in colour contrasts,or either fibre may be reserved. These blends have been particularly importantalso in ‘shot silk’ contrast effects on filament viscose warp/acetate weft liningfabrics, where the component yarns are dyed in complementary contrasts such asred–green or blue–gold. Optimum contrast in full depths requires a two-bath

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sequence in which the cellulose acetate is dyed first, an intermediate clear atambient temperature is given and then the cellulosic fibre is shaded to target withdirect dyes. Either dithionite reduction or hypochlorite oxidation may beemployed as necessary with these high-contrast effects in which the acetate fibreis usually dyed to the heavier depth. It is easier to obtain clear bright shades onthe cellulosic fibre in these instances by clearing the disperse dye stain beforeapplying the direct dyes.

Acetate/viscose blend fabrics for apparel are dyed on the winch or jigaccording to fabric construction. Winch dyeing is avoided if there is likely to beserious rope creasing. Dresswear or leisure shirting is usually plain woven from50:50 acetate/viscose staple blend yarns, or contains an acetate filament warpand staple viscose weft that can be dyed in contrasting colours. The dyed fabric isnormally resin-treated to achieve the required dimensional stability and wetfastness.

Acetate/cotton curtaining and furnishing fabrics are dyed with disperse anddirect dyes selected from those with adequate light fastness for these outlets. Ithas also been possible to produce solid effects on these blends using selected vatdyes applied by pigment padding in the presence of disodiumdinaphthylmethanedisulphonate as dispersing agent. After drying, the fabric isdeveloped at the boil in a solution of sodium sulphite and sodium formaldehyde-sulphoxylate, reoxidised and soaped. Woven fabrics prone to creasing aredeveloped on the jig and knitted constructions on the winch.

Traditionally, however, there has been little point in using dyes of high fastnesson the cellulosic component of an acetate/cellulosic blend because the wetfastness attainable on the acetate using low-energy disperse dyes was relativelypoor. Furthermore, the acetate fibre would be delustred by the boiling soapaftertreatment needed to develop optimum fastness with reactive dyes on thecellulosic component. The introduction of Xtol (Courtaulds) acetate fibre in1987 enabled all unfixed reactive dyes to be completely removed from thecellulosic fibre at the boil without delustring of the acetate fibre [56]. Togetherwith the higher wet fastness attainable with high-energy disperse dyes on Xtol,much improved performance can be achieved compared with what waspreviously possible on such blends.

Solid shades are most common on triacetate/cellulosic blends but contrasteffects and reservation of either fibre type are practicable. Most triacetate blendfabrics are dyed on the winch or jet and adequate preparation is essential. Fabricsfor reserve effects or pale dyeings are enzyme desized and peroxide bleached.Triacetate is more resistant to saponification under alkaline conditions than the

BLENDS OF CELLULOSE ACETATE OR TRIACETATE WITH CELLULOSIC FIBRES


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secondary acetate, and triacetate/cotton fabrics may be cold mercerised. Theyshould not be S finished (section 11.2) or boiled with alkali, however, as thisaggravates cross-staining of the triacetate by direct dyes.

Selected disperse dyes will give a satisfactory reserve of the cellulosic fibre.These can be applied at the boil and pH 5–6 with an ester carrier and disodiumdinaphthylmethanedisulphonate as dispersing agent. Triacetate absorbs dispersedyes more slowly than acetate and this increases the likelihood of cross-stainingof the cellulosic fibre. Adequate clearing should be attainable using an anionicdetergent at 70°C. Carrier addition can be avoided by jet dyeing at 120°C but itmay then be necessary to reduction clear the disperse dye stain using sodiumdithionite at 40°C.

The preferred disperse dyes for reserving the cellulosic fibre and showing goodstability to subsequent cross-dyeing with direct dyes and salt are mainlyintermediate-energy dyes (Mr 300–400) with adequate fastness to pleating,particularly monoazo and disazo (yellow to orange), chloronitroaniline monoazo(red) and tri- or tetra-substituted anthraquinone (red to blue) types. Undamagedcellulose triacetate can be reserved with selected multi-sulphonated self-levellingand salt-controllable direct dyes, particularly stilbene derivatives and disazotetrasulphonates, including copper-complex types and symmetrical derivatives ofdiarylurea middle components. It is usual to give a cationic aftertreatment toimprove wet fastness and avoid marking-off onto the undyed triacetatecomponent. Resin treatment is then necessary to enhance crease recovery,dimensional stability and colour fastness. Vat dyes are unsuitable for triacetatereserve effects because the triacetate fibre is partially saponified and cross-stainedduring vat dyeing.

Solid and contrast effects are readily obtained by a two-stage process based onthe method already described for cellulosic fibre reserve with disperse dyes.Direct and disperse dyes are added initially and after dyeing of the triacetate thedyebath is cooled to 60°C, salt is added and dyeing continued at 80–90°C totarget depth on the cellulosic fibre. Full depths show better fastness or brightercontrast effects from a two-bath sequence with an intermediate clear of thedisperse dye stain using alkaline dithionite at 40–50°C.

Better wet fastness is achieved on triacetate/cellulosic blends using disperseand vat dyes applied by a two-bath method. Vat dyes withstand the conditions ofdisperse dye application better than do direct dyes and therefore the cellulosicfibre is dyed first in this case. Selected cold-dyeing vat dyes are applied at pH 9and 45°C. Some staining of the triacetate occurs as a result of partialsaponification, but the stain has good fastness and the triacetate can be filled in

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with disperse dyes from a fresh bath. The preferred vat dyes [51] includebenzamidoanthraquinones, heterocyclic anthraquinone derivatives (acridones,oxazoles and thiazoles) and halogenated polycyclic quinones (anthanthrone,dibenzopyrenedione and indanthrone).

Bright dyeings with good wet fastness can be obtained from a three-stagesequence with disperse dyes applied by the conventional ester carrier process,then exhaustion of the reactive dyes on to the cellulosic fibre with salt and finallyan alkaline fixation stage, cold rinsing and soaping at the boil.

A limited range of specific shades can be achieved on triacetate/viscose blendfabrics using selected azoic diazo and coupling components. Solidity andlevelness were claimed to be acceptable and fastness to various agencies,including wet and dry rubbing, was satisfactory [57].

13.4 BLENDS OF POLY(VINYL CHLORIDE) FIBRES WITH CELLULOSICFIBRES

Jersey fabrics in this category are made by the controlled shrinkage, dyeing,stenter drying and raising of interlock knitted from staple blends of poly(vinylchloride) or PVC fibres with viscose or modal fibres. These constructions aresuitable for swimwear, sportswear, outerwear and upholstery fabrics. Wovencotton or modal fibre blends with PVC fibre are also of interest, including a70:30 cotton/PVC staple blend for corduroy constructions and a cotton warp/PVC filament weft fabric for car upholstery. Processing is usually in open widthto minimise creasing.

After desizing and scouring with caustic soda (cotton blends) or soda ash(viscose or modal blends) and an anionic detergent, these fabrics can be dyed bya one-bath method using disperse and direct dyes at 80–95°C with dispersingagent and salt. Better wet fastness is achieved by applying disperse and vat dyesin a two-bath sequence. The conventional vat dyebath in the second stage clearsthe disperse dye stain from the cellulosic fibre.

13.5 DYEING METHODS AND DYE SELECTION FOR DC BLENDS

Reserve or contrast effects are seldom in demand on polyester/cellulosic fabrics,but on blends of cellulosics with cellulose acetate or triacetate, especially onacetate/viscose fabrics, they do make a significant contribution. The simple one-bath process with disperse and direct dyes is important on cellulosic blends withthe cellulose ester fibres or PVC fibre, but is only used on low-quality goods inthe polyester/cellulosic sector. Two-stage techniques with disperse and reactive

BLENDS OF CELLULOSE ACETATE OR TRIACETATE WITH CELLULOSIC FIBRES


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Table 13.1 Dye selections for exhaust dyeing of DC blends


Polyester/ Polyester Single-class Direct dyes stable to peroxide bleach;cellulosic reserve selected polycyclic vat dyes

Solid Single-class Benzamidoanthraquinone vat dyes of(pale depths) relatively low Mr

One-bath Disperse dyes and disazomultisulphonated direct dyes at pH 6

Disperse dyes and reactant-fixabledyes with copper-specific sequesterant

Disperse dyes and nicotinotriazinereactive dyes at 130°C

Two-stage Disperse dyes at 130°C, thenlow-reactivity dyes at 80–95°C

High-reactivity dyes at lowtemperature, then disperse dyesat 130°C

Disperse dyes at 130°C, then selectedvat dyes at 20–60°C

Two-bath Disperse dyes at 130°C, then sulphur(full depths) dyes with dithionite

Acetate/ Acetate Single-class Salt-controllable multisulphonatedcellulosic reserve direct dyes at 80°C

Cellulosic Single-class Low-energy disperse dyes at 80°Creserve

Solid One-bath Low-energy disperse dyes andsalt-controllable direct dyes at pH 6–7and 80°C

Contrast Two-bath Disperse dyes at 80°C, reductive oroxidative clear at 20°C, then directdyes at 80°C

Xtol/ Solid or Two-stage High-energy disperse dyes at the boil,cellulosic contrast then reactive dyes at appropriate

temperature

Triacetate/ Triacetate Single-class Selected multisulphonated direct dyescellulosic reserve with cationic aftertreatment

Cellulosic Single-class Intermediate-energy disperse dyesreserve at 120°C

Solid or Two-stage Disperse dyes at 120°C, then directcontrast dyes at 90°C

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Triacetate/ Solid or Two-bath Selected vat dyes at 45°C, thencellulosic contrast intermediate-energy disperse dyes

at 120°C

PVC/ Solid One-bath Low-energy disperse dyes and directcellulosic dyes at 95°C

Two-bath Disperse dyes at 95°C, thencold-dyeing vat dyes

Table 13.2 Dye selections for continuous dyeing of polyester/cellulosic blends


Polyester/ Polyester Pad–dry High-reactivity dyes with sodiumcellulosic reserve bicarbonate and urea

Pad–dry– Low-reactivity dyes with sodiumthermofix bicarbonate and urea

Solid Pad–dry– Selected disperse dyes andthermofix high-reactivity dyes with sodium

bicarbonate and urea

Disperse dyes and stabilised azoiccomponents with acid donor andhydrotropic agent

Pad–dry–thermofix– Disperse dyes, then high-reactivitypad–batch dyes

Pad–batch–beam Vinylsulphone reactive dyes, thenor –jet disperse dyes at 130°C

Pad–jig develop– Vat leuco esters for both fibresdry–thermofix (pale depths)

Pad–dry–thermofix– Disperse dyes, then low-reactivitypad–steam dyes

Selected vat dyes for both fibres(pale or medium depths)

High-energy disperse dyes, thenselected vat dyes

High-energy disperse dyes, thendispersed or solubilised sulphurdyes

DYEING METHODS AND DYE SELECTION FOR DC BLENDS


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dyes represent the most important exhaust dyeing approach on polyester/cellulosic or Xtol (Courtaulds) acetate/cellulosic blends. Two-stage or two-bathprocesses with disperse and vat dyes offer high standards of all-round fastnessperformance on blends of polyester, cellulose triacetate or PVC fibre with thevarious natural or regenerated cellulosic fibres (Table 13.1).

Excellent polyester reserve effects are attainable on polyester/cellulosic fabricsusing highly productive pad–dry or pad–dry–thermofix application of reactivedyes. Solidity on the two fibre components up to medium depths is attainableusing vat dyes or their leuco esters alone, or a simple pad–dry–thermofix processwith selected disperse and reactive dyes. Higher all-round fastness and a widerselection of dyes are offered by various two-stage sequences involvingthermofixation of the disperse dyes on polyester and then chemical pad–steamfixation of vat, sulphur or reactive dyes. Compromise semi-continuous methodssuch as pad–dry–thermofix–pad–batch or pad–batch–beam (or –jet) have alsobeen found useful for disperse and reactive dyes (Table 13.2).

13.6 REFERENCES 1. J A Hook, J.S.D.C., 108 (1992) 367. 2. R Aitken, J.S.D.C., 99 (1983) 150.

3. G Henze, H Bille, W Thonig and G Schmidt, Textile Asia, 5 (Jul 1974) 44.

4. W Prager and M J Blom, Text. Chem. Colorist, 11 (Jan 1979) 11. 5. W S Hickman and R J Chisholm, Australian Text., 13 (Mar–Apr 1993) 24.

6. W J Marshall in The dyeing of cellulosic fibres, Ed. C Preston (Bradford: SDC, 1986) 320.

7. R T Norris, Textilveredlung, 12 (1977) 258. 8. T M Baldwinson in Colorants and auxiliaries, Vol. 2, Ed. J Shore (Bradford: SDC, 1990) 568.

9. F Schlaeppi, R D Wagner and J L McNeill, Text. Chem. Colorist, 14 (1982) 257.

10. K Miyata, AATCC International Conference and Exhibition, (Oct 1992) 121.11. H Tiefenbacher, Textil Praxis, 37 (1982) 812; Chemiefasern und Textilind., 35/87 (1985) 797.

12. K H Röstermundt, Textil Praxis, 46 (1991) 56.

13. P W Leadbetter and S Dervan, J.S.D.C., 108 (1992) 369.14. M A Herlant, Text. Chem. Colorist, 17 (Jun 1985) 117; Am. Dyestuff Rep., 74 (Sep 1985) 55;

(Oct 1985) 37.

15. N E Houser, J C Martin and M White, Am. Dyestuff Rep., 70 (Sep 1981) 19.16. N E Houser and M White, AATCC Nat. Tech. Conf., (Oct 1976) 105.

17. T D Fulmer, America’s Textiles Internat., 18 (Jan 1989) 54.

18. T A Waldrop, Am. Dyestuff Rep., 75 (Sep 1986) 22.19. D Monney, Dyer, (Apr 1994) 32.

20. U Kreig, Australian Text., 10 (Jan 1990) 38.

21. J A Hook and A C Welham, J.S.D.C., 104 (1988) 329.22. Anon, Dyer, (Jul 1995) 14.

23. Y Sato, Am. Dyestuff Rep., 72 (Sep 1983) 30.

24. R W Chalk and N E Houser, Text. Chem. Colorist, 20 (Nov 1988) 17.

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REFERENCES

25. I D Menzies and P W Leadbetter, AATCC Nat. Tech. Conf., (Oct 1985) 108.

26. S Abeta and K Imada, Am. Dyestuff Rep., 74 (May 1985) 2527. A N Lee, Dyer, (Apr 1994) 29.

28. S Abeta and K Imada, AATCC Nat. Tech. Conf., (Oct 1985) 112.

29. C V Stead in Colorants and auxiliaries, Vol. l, Ed. J Shore (Bradford: SDC, 1990) 324.30. G H Kenyon, Am. Dyestuff Rep., 68 (Mar 1979) 19.

31. H U von der Eltz and R Adrion, Textil Praxis, 45 (1990) 1023.

32. D Hildebrand, B Renziehausen and D Hellmann, Melliand Textilber., 69 (1988) 895.33. N Morimura and M Ojima, Am. Dyestuff Rep., 74 (Feb 1985) 28.

34. B Semet and G E Grüninger, Melliand Textilber., 76 (1995) 161.

35. C L Chong, S Q Li and K W Yeung, Am. Dyestuff Rep., 81 (May 1992) 17.36. T P Nevell in Cellulosics dyeing, Ed. J Shore (Bradford: SDC, 1995) 16.

37. J Pashley, J.S.D.C., 109 (1993) 379.

38. H U von der Eltz and J Müller, Internat. Text. Bull., No. 2 (1978) 1.39. H D Moorhouse, Rev. Prog. Coloration, 26 (1996) 20.

40. F Somm, Textilveredlung, 23 (1988) 257.

41. F Somm, C Oschatz and H Lehmann, Teintex, 42 (Mar 1977) 125.42. F Somm and R Buser, Textilveredlung, 19 (1984) 359.

43. C J Bent, T D Flynn and H H Sumner, J.S.D.C., 85 (1969) 606.

44. W Shimizu and J W Rucker, Am. Dyestuff Rep., 84 (Apr 1995) 32.45. P Richter, AATCC Nat. Tech. Conf., (Oct 1983) 255.

46. H U von der Eltz and R Kuhn, Melliand Textilber., 67 (1986) 336.

47. W Marschner and D Hildebrand, Chemiefasern und Textilind., 31/83 (1981) 153.48. R Buser, M Capponi and F Somm, Textilveredlung, 12 (1979) 106.

49. G Harding, Australasian Text., 6 (Jan–Feb 1986) 22.50. C Otte, Textilveredlung, 18 (1983) 269.

51. F R Latham in Cellulosics dyeing, Ed. J Shore (Bradford: SDC, 1995) 247.

52. L R Smith and O E Melton, Text. Chem. Colorist, 14 (May 1982) 113.53. P Frey, Textil Praxis, 48 (1993) 521.

54. H Kaiser, Chemiefasern und Textilind., 26/78 (1976) 316.

55. P Frey, Text. J. of Australia, 49 (Aug 1974) 36.56. J M Taylor and P Mears, J.S.D.C., 107 (1991) 64.

57. L A Kovkin and N A Tikhomirova, Intensif. tekhnol. otdelki i modif. tekstil. materialov,

(1984) 53.

206 TRIACETATE/POLYESTER AND OTHER DD BLENDS

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206

CHAPTER 14

Triacetate/polyester and other DD blends

14.1 DYEING PROPERTIES OF DISPERSE-DYEABLE FIBRE BLENDS

Cellulose acetate, triacetate and polyester differ greatly in dyeability at a giventemperature. The low-energy dye 1,4-diaminoanthraquinone (CI Disperse Violet1) yields a 50% exhaustion value at 40°C on acetate, 70°C on triacetate and over100°C on polyester. The dyeing rate is much more rapid on cellulose acetate, sothat it is virtually impossible to achieve solid effects on either acetate/triacetate oracetate/polyester blends. Shadow and reserve effects have been of some intereston mixed-ply yarns. Triacetate/polyester offers more scope for achieving solidityand this blend has proved moderately useful in dresswear fabrics woven fromfilament yarns. Shadow effects and a limited degree of reserve of polyester withappropriate selections of dye and carrier are also possible. Triacetate/polyester istherefore by far the most versatile of the DD blends.

It is usual to scour, stenter set and S finish as for 100% triacetate fabrics (section11.2). The presetting treatment markedly lowers the dyeability of the triacetatecomponent without greatly affecting the polyester, so that solid effects becomeeasier to obtain. During dyeing the triacetate is dyed preferentially in the earlystage, but migration from triacetate to polyester occurs later as the dyes diffuseslowly into the polyester. Eventually a solid effect is achieved, the time dependingon the recipe and depth of shade. It is easier to attain acceptable solidity at shortliquor ratios and in pale depths. At longer liquor ratios preferential absorptionby the triacetate component becomes more obvious and it is progressively moredifficult to transfer the dyes to the polyester [1].

Cellulose triacetate and polyester are therefore sufficiently close in dyeingproperties to give good solidity with selected dyes under controlled dyeingconditions. There are four possible methods of obtaining good solidity on triacetate/polyester blends by batchwise dyeing:(1) Monoazo, disazo or anthraquinone disperse dyes of intermediate or high

fastness to sublimation (Mr 300–450) are the most suitable for this purposein the conventional high-temperature process at 120°C.

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(2) A two-stage method, dyeing first at 130°C to achieve satisfactory yield andpenetration on the polyester fibre, followed by a period at the boil withoutcarrier to bring the triacetate fibre to the same depth.

(3) Selected dyes will give good solidity in the presence of an aryl ester or diphenylcarrier at the boil. Anthraquinone dyes of intermediate sublimation fastnessare particularly appropriate. Few azo dyes give adequate solidity under theseconditions.

(4) In an alternative carrier-dyeing method, selected dyes favouring polyester atthe boil are used together with others that favour triacetate. This results incomplicated recipes with up to six component dyes.

These blends are almost always dyed at high temperature nowadays. Theselection of intermediate- to high-energy dyes applied at 120°C is the preferredapproach. The two-stage method is time-consuming but provides better controlfrom the viewpoint of shade matching. Carrier-dyeing methods are much moreunattractive because of the adverse effect of these products on the workingenvironment. Combinations of dyes that are partially selective for dyeing therespective fibres do give more control of colour matching to achieve a solideffect, however.

In high-temperature dyeing small quantities of carrier are still sometimesadded to shift the balance in favour of the polyester. Ester-based carriers, such asmethyl salicylate or butyl benzoate, are particularly effective [2]. Phenoliccarriers lower the light fastness, especially on the triacetate component, and aredifficult to remove without appreciable stripping of dye from the triacetate. Inaddition to its toxicity, trichlorobenzene causes moderate swelling of thetriacetate fibres and is best avoided.

The dyes favouring polyester under carrier-dyeing conditions at the boil arethe high-energy azo and anthraquinone types. Low-energy monoazo and nitrodyes generally favour the triacetate fibres. If those dyes showing a preference fortriacetate are applied alone at 90°C with an ester carrier, shadow and polyesterreserve effects can be readily obtained.

Shadow effects represent the only possibility on cellulose acetate/triacetateblends for dresswear. Solidity, reserve and contrast effects are all excluded. Theacetate is dyed more deeply and there may be differences in hue between thecomponent fibres when applying combination recipes in mode shades. Dyeing at60°C favours the acetate fibre to the greatest extent and as the dyeingtemperature is increased the difference in depth decreases, although even at 95°Csolidity is not generally achieved and the acetate fibre is delustred above 85–

DYEING PROPERTIES OF DISPERSE-DYEABLE FIBRE BLENDS


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PAGE TITLE

90°C. The introduction in 1987 of Xtol (Courtaulds) acetate fibre [3] has offeredthe possibility of dyeing acetate/triacetate blends at the boil to achieve near-solidity without delustring but this possibility does not widen the colouringpotential of these blends very much. Simple low-energy yellow, orange and reddyes of the substituted p-nitrophenylazoaniline type give the best approach tosolidity. Ester carriers do promote uptake of dyes by the triacetate component,but these agents cause unacceptable swelling of the acetate fibres.

Solidity is virtually impossible to attain on polyester/acetate blends becausethe rate of dyeing is so much higher on the acetate. If a reserve of the polyester isrequired the acetate can be dyed at 60–70°C with a nonionic dispersing agentusing mainly monoazo yellow to red dyes of relatively high energy(Mr 330–450) containing N-hydroxyethyl or N-acetoxyethyl substituents in thecoupling amine, as well as violets and blues of the 1,4-disubstitutedanthraquinone type.

Shadow effects on this blend require higher dyeing temperatures and longertimes. Unfortunately, such severe conditions tend to degrade conventionalacetate fibres badly, leading to delustring, fibre swelling and loss in tensilestrength of the blend fabric. Much better shadow dyeings are obtained on blendsof Xtol (Courtaulds) acetate fibre with polyester, since high-energy dyes can beeffectively absorbed by both components without delustring of the Xtol fibres.

It is sometimes possible to use combinations of slow- and rapid-diffusingdisperse dyes in the presence of a carrier to increase the rate of dyeing on thepolyester. It is particularly important, however, to check the effect of thecarrier on the tensile strength of the acetate fibres. Rapid-diffusing dyes arelow-energy (Mr 230–270) monoazo and anthraquinone types. The slow-diffusing dyes are similar in structural types but higher in energy (Mr

270–350). Carrier-dyeing methods, however, are becoming increasinglyobjectionable for ecological reasons.

Shadow effects with low- and intermediate-energy disperse dyes are readilycontrolled on fabrics woven from normal and deep-dye polyester yarns (section5.1). Dyes with similar rates of exhaustion on these two fibre variants should beselected for mode shades [4]. The rate of temperature rise should be slowed in thecritical region and an alkanol polyoxyethylene retarder used to minimiseunlevelness. Good liquor circulation or vigorous agitation of the fabric arenecessary. Unlevel dyeings can be corrected by treatment with an ester carrierand a dispersing agent at pH 5–6 and the boil.

The light fastness of disperse dyes on deep-dye polyester variants designed fordyeing at the boil without carrier is inferior to that on normal polyester. Thisdeficiency is attributed to the presence of alkylene ether groups in the main chain

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tBu

OH

tBu

O

CH2

C CH2H2C

CH2

O

tButBu

OH

OO

tBu

OH

tBu

tBu

HO

tBu

NN

N

tAm

OH

tAm

AntioxidanttBu = tertiary butyl

Ultraviolet absorbertAm = tertiary amyl

Figure 14.1 Stabilising agents for disperse dyes on deep-dye polyester

of the copolymer (section 5.1). The addition of stabilisers to the dyebath hasbeen examined with a view to overcoming this drawback [5]. An antioxidantIrganox 1010 (Ciba) and an ultraviolet absorber Tinuvin 328 (Ciba) of thehindered phenol type were used (Figure 14.1).

DYEING PROPERTIES OF DISPERSE-DYEABLE FIBRE BLENDS

Figure 14.2 Disperse dyes evaluated with stabilising agents

O

O OH

NH2

O

O2N

CN

N N NCH2CH2OCOCH3

CH2CH2OCOCH3

O

O

N

Dyes of high light fastness

Red

O2N N N NCH3

CH3

CI Disperse Red 60

OrangeCH3

CH3

Dyes of low light fastness

CI Disperse Red 82


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Table 14.1 Disperse dye selections for DD blends


Triacetate/ Polyester One-stage Low-energy monoazo and nitropolyester reserve or dyes at 90°C with ester carrier

shadow

Solid One-stage Intermediate- or high-energydyes at 120°C

Intermediate-energyanthraquinone dyes with estercarrier at the boil

High-energy dyes for polyesterand low-energy dyes fortriacetate with ester carrier

Two-stage Polyester dyes at 130°C, thentriacetate dyes at the boil

Acetate/ Shadow One-stage Low-energy dyes at 60–80°Ctriacetate

Polyester/ Polyester One-stage High-energy monoazo andacetate reserve low-energy anthraquinone dyes

at 60–70°C

Polyester/Xtol Shadow One-stage High energy dyes at the boil(Courtaulds) without carrier

Normal/ Shadow One-stage Low- and intermediate-energydeep-dye dyes at 120°Cpolyester

Two disperse dyes of high light fastness and two of inferior fastness (Figure14.2) were applied at 130°C in the presence of these additives to polyester fabricsknitted from the two variant yarns. Improvements in xenon-arc light fastnessratings of 0.5 to 1 point were obtained with each additive. The ultravioletabsorber was marginally more effective than the antioxidant and in some cases asynergistic effect was observed using them in combination.

14.2 DYEING METHODS AND DYE SELECTION FOR DD BLENDS

Among these blends a reserve effect can only be achieved on polyester in itsblends with cellulose acetate or triacetate. There are several methods forattaining good solidity on triacetate/polyester blends (Table 14.1) but this is theonly DD blend on which this effect is practicable. The two methods that require

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DYEING METHODS AND DYE SELECTION FOR DD BLENDS

the use of a carrier to achieve this are of little or no interest nowadays. All DDblends will readily yield shadow effects, of course, this being the inevitable resultof the widely different rates of dyeing between these disperse-dyeable fibres.

14.3 REFERENCES1. V R Adomas, R R Zhyamaitaitene and V V Brazauskas, Issled. svoistv. syrya i pererab. ego v

shelkov. prom-sti., M (1984 ) 141.2. T M Baldwinson in Colorants and auxiliaries, Vol. 2, Ed. J Shore (Bradford: SDC, 1990) 568.

3. J M Taylor and P Mears, J.S.D.C., 107 (1991) 64.

4. J Hürter, Melliand Textilber., 63 (1982) 296.5. B Küster and H Herlinger, Textil Praxis, 40 (1985) 406.

212 DYEING PROPERTIES OF THREE-COMPONENT BLENDS

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CHAPTER 15

Dyeing properties of three-component blends

15.1 INTRODUCTION

Extension of the classification scheme in Chapter 2 to the case of three-component blends follows logically to give categories such as those listed inTable 15.1. Clearly, there are twenty possible combinations of three from fourfibre types that could be assembled but in practice not all of them are useful. Theten categories included in Table 15.1 are those in which actual three-componentblends have been encountered. It is often advantageous to approach each of theseblends as a one-off set of requirements. Nevertheless, the general principlesadopted in dealing with the binary blends offer the best basis for developing acoherent dyeing method for a three-way blend.

The selection of fibre blend components and their proportions is as critical forthree-component blends as it is for the more familiar binary blends, but theaddition of a third component offers no guarantee of producing a fabric withenhanced characteristics overall. Apparel fabrics for blazers, outerwear, flannelsand velours, traditionally manufactured with a cotton warp and a wool weft,may be made with a blended wool/viscose or nylon/viscose weft, for example, oneconomic grounds. Four-component blends are occasionally encountered, too,since dresswear or other items of apparel may be woven from dissimilar blendedstaple yarns in both warp and weft.

An interesting account has appeared of pushing back the frontiers of what isachievable in multicolour piece dyeing for the woven suitings sector by the GibbsBurge dyehouse in Australia [1]. Regular production was established of four-colour designs on multifibre fabrics, sometimes including six man-made fibres(acrylic, modacrylic, nylon, viscose, regular and modified polyester). Numerousfancy yarns (marls, mélanges, slub yarns, bouclés, donegals) were incorporatedin design styling. Initially these were included as stripe effects, but then graduallyintroduced into more complex constructions to extend the versatility ofappearance, colour and texture effects.

At the outset, limitations were anticipated in fastness to light, washing,perspiration and rubbing, cross-staining, differential abrasion, differential

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Table 15.1 Classification of ternary blends

shrinkage and cockling. These proved to be less critical than expected, providingcareful compromises in dye selection and dyeing conditions were reached. Ingeneral, most customers were prepared to accept wider tolerances in order toobtain novel effects with greater intrinsic appeal [1].

15.2 DYEING OF AAA BLENDS

Ternary AAA blends of acid-dyeable nylon variants, e.g. pale-dye/normal/deep-dye or normal/deep-dye/ultra-deep, are less versatile than the binary blends(section 5.2) because the range of coloured effects attainable is limited mainly to

AAA blendsNylon/wool/polyurethanePale-dye/normal/deep-dye nylonNormal/deep-dye/ultra-deep nylon

AAB BlendsNylon/wool/acrylic fibrePolyurethane/wool/basic-dyeable

polyesterNylon/modacrylic fibre/acrylic fibreNylon/acid-dyeable acrylic fibre/basic-

dyeable acrylic fibreModacrylic fibre/acid-dyeable acrylic

fibre/basic-dyeable acrylic fibreNormal/deep-dye/basic-dyeable nylon

AAC blendsNylon/wool/cottonNylon/wool/viscoseNylon/polyurethane/cottonNormal nylon/deep-dye nylon/cottonNormal nylon/deep-dye nylon/viscose

CBA blendsCotton/acrylic fibre/nylonCotton/basic-dyeable acrylic fibre/acid-

dyeable acrylic fibreCotton/modacrylic fibre/acrylic fibre

DAA blendsCellulose acetate/nylon/woolCellulose triacetate/nylon/woolPolyester/nylon/woolPolyester/polyurethane/wool

DAC blendsCellulose acetate/nylon/cottonCellulose triacetate/nylon/cottonPolyester/nylon/cottonPolyester/polyurethane/cottonCellulose acetate/wool/viscoseCellulose acetate/wool/linenCellulose acetate/nylon/viscoseCellulose triacetate/nylon/viscosePolyester/nylon/viscose

DBA blendsPolyester/acrylic fibre/woolNormal polyester/basic-dyeable

polyester/woolNormal polyester/basic-dyeable

polyester/nylonPoly(vinyl chloride)/acrylic fibre/woolPoly(vinyl chloride)/acrylic fibre/nylon

DBC blendsPolyester/acrylic fibre/cottonPolyester/acrylic fibre/viscoseNormal polyester/basic-dyeable

polyester/cottonNormal polyester/basic-dyeable

polyester/viscose

DDA blendsCellulose acetate/polyester/nylonCellulose acetate/poly(vinyl chloride)/wool

DDC blendsCellulose triacetate/polyester/cottonCellulose triacetate/polyester/viscose

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three-way shadow effects. The cost of manufacture is similar to that for AABnylon blends containing two acid-dyeable variants with a basic-dyeable yarn.These offer considerably wider possibilities for contrast and reserve effects.Control of pH is most important for reproducible results on ternary acid-dyeableblends. A relatively slight change in total exhaustion may significantly alter thedistribution between the variant yarns.

Acid dyes for these blends differ in selectivity according to structural type. Thedifferentiation within the range pH 4–7 tends to increase with degree ofsulphonation. Levelling acid dyes with three or four sulpho groups show selectivebehaviour even in pale depths, but monosulphonates give marked differencesonly in full depths applied at pH 7–8. Premetallised dyes and the morehydrophobic ‘supermilling’ acid dyes with high wet fastness properties showsuch poor differentiation that they are of little value for these blends.

Three-way shadow effects are given by a range of monoazo or mono-sulphonated anthraquinone dyes. The normal nylon component can be reserved(if necessary at a slightly higher pH) using a similar series of dyes that arepreferentially absorbed by the deep-dye and ultra-deep variants. These aremainly mono- or disulphonated anthraquinone dyes. Many disulphonates andtrisulphonates, especially monoazo and disazo types, will give a satisfactoryreserve of the normal and deep-dye yarns. These dyes are selectively absorbed bythe ultra-deep variant under neutral conditions where hydrophobic dye–fibrebonding predominates.

Four approaches to dye selection are available to exploit the colouringpossibilities with these groups of dyes:(1) A short but versatile range of hues for three-way shadow effects contains a

red and a green that allow reserve of the normal nylon at a higher pH.(2) The same range can be used in conjunction with more highly sulphonated

types that dye the ultra-deep variant selectively. Such selections yield ashadow effect on the normal and deep-dye yarns with a much deepercontrasting (but dependent) hue on the ultra-deep nylon.

(3) Selected dyes that reserve only normal nylon can be used with the morehighly sulphonated types to obtain a dependent contrast on the moredyeable variants with the normal nylon reserved.

(4) Alternatively, selected disperse dyes can also be added to give a three-waydependent colour contrast. The ultra-deep yarn is invariably dyed veryheavily to a dull hue in these effects. It absorbs all three dye types and thedisperse dyes often show some preference for the ultra-deep component.This limits the attractiveness of the coloured design and the wet fastness thatcan be achieved.

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Polyurethane fibres are sometimes used in nylon/wool fabrics to provideenhanced stretch properties. Fast dyeings on this ternary blend are best obtainedwith 1:2 metal-complex dyes. Coverage of physical variations in the nyloncomponent is less serious than for nylon/polyurethane blends (section 6.3). As inthe application of these dyes to nylon/wool (section 6.1), a syntan is added toretard uptake by the nylon component and an alkanol polyoxy-ethylene is usedto improve levelling.

15.3 DYEING OF AAB BLENDSCore-spun polyurethane yarns are sometimes used to confer better stretchcharacteristics in polyester/wool fabrics. The use of disperse dyes for thepolyester often results in low fastness of the dyed polyurethane to spottingwith chlorinated solvents. If basic-dyeable polyester is used in blends withwool and polyurethane, however, this problem no longer arises since ionicdyes can be applied to all three fibres. The dyebath is set with an alkanolpolyoxyethylene as anti-precipitant, an anionic retarder for the basic dyes,an aryl ester carrier formulated with a nonionic emulsifier, acetic acid andthe basic dyes. These dyes are exhausted on to the basic-dyeable polyester atthe boil. The dyebath is then cooled to 70°C and the two acid-dyeablecomponents dyed simultaneously using suitable neutral-dyeing anionic dyeswith an anionic agent to control the distribution between the wool andpolyurethane.

Knitted blends of basic-dyeable and acid-dyeable acrylic yarns are sometimesphysically strengthened by including nylon. The one-bath method with selectedbasic and acid dyes at pH 4–5 already described for nylon/acrylic blends (section7.2) will give attractive contrasts between nylon and the basic-dyeable acrylicfibre while reserving the acid-dyeable acrylic fibre. At lower pH, the acid dyeswill give a shadow effect on the two acid-dyeable components. Under near-neutral conditions, the basic dyes will give a similar effect on the two acrylicvariants.

Blends of normal, deep-dye and basic-dyeable nylon provide a wider rangeof colouring possibilities than either the ternary AAA combinations or thebinary AB blends. There are still limitations imposed by cross-staining,however. The dyeing method is essentially that already described for normal/basic-dyeable nylon, using a phosphate buffer at pH 6 (section 5.3). Thecoloured effects are varied by dye selection. Certain monosulphonated aciddyes will give a shadow effect on the acid-dyeable components and reservethe basic-dyeable yarn. If basic dyes of the localised-charge type are added, a

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shadow and contrast effect is obtained. A dependent contrast with reserve ofthe basic-dyeable nylon is given by selected mono- and disulphonated aciddyes in combination (section 5.2). If the disulphonated acid dyes and basicdyes are used together, the normal nylon can be reserved with anindependent contrast on the other two fibres, but there are limitations ofdepth. Finally, a restricted gamut of dependent three-colour contrasts ispossible using both types of acid dyes together with basic dyes.

15.4 DYEING OF AAC BLENDS

Traditionally, the most important AAC blend was the nylon/wool/viscosecombination for carpet yarns but this is now seldom encountered. Blends ofthe AAC type are sometimes found in decorative uniforms or blazer-clothdesigns, examples being a cotton or nylon/viscose warp with a nylon/woolweft or a nylon staple warp and a wool/viscose weft. All of these may bereadily dyed by a one-bath method under neutral conditions using selectedmetal-complex or acid dyes as discussed for nylon/wool (section 6.1),together with salt-controllable direct dyes mainly of the disazo or trisazotetrasulphonate type. Salt addition promotes exhaustion of the direct dyes bythe cellulosic component and a syntan is required to minimise uptake ofthese dyes by nylon.

Nylon/polyurethane/cotton blends containing 10–20% of the elastomericfibre are important in knitted underwear. Stretch corduroy fabrics containingthese three components can be dyed semi-continuously by first dyeing thecotton with reactive dyes by cold pad–batch, sulphur dyes by pad–steam, orvat leuco esters by a pad–develop method [2]. The two acid-dyeable fibresare then filled in by exhaust dyeing with 1:2 metal-complex or milling aciddyes in the presence of an appropriate auxiliary agent to control thedistribution between the nylon and polyurethane components (section 3.3).

Normal and deep-dye nylon yarns have been used in blends with cottonfor upholstery fabrics. These can be dyed with selected direct and acid dyesas already discussed for nylon/cellulosic blends (section 8.2). The preferredacid dyes are those mono- and disulphonated types that show good reserveof the cotton as well as satisfactory differentiation of the nylon variantyarns. Suitable direct dyes are mainly of the salt-controllable type with up tofour sulphonate groups. These give acceptable reserve of the nylon variantyarns when applied in the presence of a syntan. In this way a restrictedgamut of shadow with contrast effects is possible, but the wet fastnessproperties are limited.

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15.5 DYEING OF CBA BLENDS

Carpets and rugs are sometimes made with an acrylic/modacrylic pile, one of theobjectives being to use the modacrylic fibre to reduce the risk of flammability.Such a blend of 60:40 acid-dyeable acrylic/Dynel (Union Carbide) may be usedas the pile on a cotton backing. The dyeing procedure is to use a one-bathmethod, selecting basic dyes for the Dynel, 1:2 metal-complex dyes for the acid-dyeable acrylic variant and salt-controllable direct dyes for the cotton. The use ofan anti-precipitant system and careful dye selection are essential. A butylbenzoate carrier favours absorption of the basic dyes by Dynel and the dyebathshould be maintained at pH 6 with ammonium sulphate. Wet fastness of thecotton backing can be improved by a conventional treatment with a cationicfixing agent.

Carpets made from basic-dyeable and acid-dyeable acrylic fibres on a jute orcotton backing fabric, however, are more difficult to dye in deep contrastinghues. The need to dye at pH 2 to minimise basic dye staining of the acid-dyeablecomponent (section 7.3) degrades the cellulosic fibre backing. Staining of theacid-dyeable fibre by lignin impurities from the jute (section 5.2) may also occur.For these reasons it is preferable to employ more subtle muted contrasts on theseconstructions.

15.6 DYEING OF DAA BLENDS

So-called ‘booster’ blends of low-pill polyester staple with long-staple wool forjersey-knitted suitings, outerwear and casual wear contain 10% nylon staple toprovide a favourable balance of pilling resistance and increased durability insuitings [3]. These offer an interesting challenge to the attainment of acceptablesolidity. In a one-bath method for pale or medium depths, all three fibres aredyed simultaneously using disperse and 1:2 metal-complex or milling acid dyes,followed by soaping to remove disperse dye staining from the wool and from thesurface of the polyester.

The polyester, and to some extent the nylon, can be dyed first with dispersedyes and carrier at the boil or under pressure at 105°C in a two-bath method.After reduction clearing, the wool and nylon components are dyed in a freshbath with the selected anionic dyes. Attainment of optimum solidity on thisblend requires:(1) selection of disperse dyes that favour polyester rather than nylon (section

11.3);(2) selection of 1:2 metal-complex and milling acid dyes showing a suitable

partition between wool and nylon at the target depth (section 6.1);

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(3) the use of a syntan as a retarding agent on the nylon when dyeing in paledepths.

Nylon may be used to improve the tensile strength and abrasion resistance ofcellulose ester/wool blends, as in 33:33:33 cellulose acetate/wool/nylon or65:30:5 triacetate/wool/nylon. Two-bath dyeing methods are preferred for theseblends. Disperse dyes in the low- to intermediate-energy range (Mr

220–380) are selected for cellulose acetate to give minimum staining of wool at80°C (section 11.2). Preferred disperse dyes for triacetate are the intermediate-energy types (Mr 300–400), including nitrodiphenylamine yellows, monoazoreds and anthraquinone violets and blues. These are applied at the boil in thepresence of an ester carrier. In both instances the disperse dye stain is clearedfrom the wool using a nonionic detergent at 50°C. The nylon and woolcomponents are filled in with neutral-dyeing 1:2 metal-complex or milling aciddyes using appropriate auxiliaries (section 3.2). Chrome dyes are also suitable fordeep shades. Fabrics of this type that are triacetate-rich can be heat set at 200°Cafter dyeing to enhance dimensional stability and improve the wet fastness of thedisperse dyes.

Core-spun polyurethane yarns may be included in the weft direction in wovenpolyester/wool fabrics to give improved stretch properties in skiwear andsportswear. The polyurethane fibre readily absorbs most disperse dyes and thismay result in low fastness to solvent spotting. Disperse dyes for use on theseblends, therefore, are selected for low solubility in dry cleaning solvents. Two-stage or two-bath methods are necessary, applying these disperse dyes with acarrier at the boil followed by 1:2 metal-complex or milling acid dyes for thewool. These methods are analogous to the corresponding processes in theabsence of polyurethane (section 11.1). Carriers of the aryl ester or trichloro-benzene type are preferred, however, because o-phenylphenol may damage thepolyurethane fibre.

15.7 DYEING OF DAC BLENDS

Solidity is difficult to achieve on polyester/nylon/cotton blends. It is oftenadvisable to leave the cotton undyed or pastel dyed because of problems ofcolour matching or differential abrasion of the cotton (section 1.5.4). Thesynthetic components can then be dyed in solid or preferably contrasting hues bythe methods devised for polyester/nylon blends (section 11.3).

Such considerations do not apply to the dyeing of those polyester/cotton orpolyester/viscose outerwear fabrics containing 5–10% of core-spun polyurethane

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weft yarns to impart enhanced stretch to the construction. However, dispersedyes used on these blends should be selected for low solubility in chlorinatedorganic solvents because of their limited fastness on the polyurethanecomponent. Conventional exhaust methods of applying these dyes, together withdirect dyes or followed by reactive dyes for the cellulosic fibre, are analogous tothe corresponding processes in the absence of polyurethane (section 13.1).

Cellulose acetate/wool/viscose blends are normally dyed by a two-bathmethod. The acetate is dyed first at 80°C and the stain on the other twocomponents is reduction cleared with dithionite and ammonia. Direct andneutral-dyeing acid dyes are then applied from a fresh bath in the presence ofsalt. Shirting and pleated dresswear woven from 60:20:20 or 55:15:30 triacetate/nylon/viscose blends may be dyed by a one-bath method based on the selectionof disperse dyes for solidity on the triacetate and nylon compo-nents (section11.4). Direct dyes of the self-levelling and salt-controllable disazomultisulphonate types are used to fill in the viscose.

A problem of unlevel dyeing was encountered on a union fabric woven from alinen warp and a triacetate/nylon blended-staple weft. It was found that the mostcritical stage of processing was steaming, which could lead to irregular stripyfaults in the vat-dyed linen warps. A factorial design was used to establish theoptimum conditions for steam fixation in order to minimise the fault [4].

15.8 DYEING OF DBA BLENDS

Fabrics containing these three types of yarn are generally intended for three-colour contrast effects or two-colour contrast with the disperse-dyeable fibrereserved. Solidity is not often required, but polyester/wool/acrylic blended-stapleyarns for overcoats or suitings are dyed in solid shades by a two-bath method[5]. Bright contrasts in complementary hues can be obtained if only the twoionic-dyeable fibres are dyed. If disperse dyes are used as well they tend to dye (orstain) all three fibre types to some extent, giving a dependent hue (Table 1.5) onthe basic-dyeable fibre and a duller hue or clearing problems on the acid-dyeablecomponent, especially when this is wool.

Many disperse dyes will give three-way shadow effects on blends of nylonwith normal and basic-dyeable polyester, the distribution being controlled bydyeing temperature and addition of carrier (section 12.3), but the low wetfastness on the nylon is a limiting factor. Better fastness is given by two-wayshadow with reserve of the acid-dyeable component of this type of blendusing a carrier and selected disperse dyes to reserve nylon or wool, typicallylow-energy anthraquinone dyes (Mr 220–300). Better control of the nylon

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reserve and shadow or dependent contrast effects on the two polyestervariants is provided by the method already described using disperse andbasic dyes (section 12.3). Anthraquinone basic dyes are generally unsuitable,the preferred types being mainly methine, monoazo, triarylmethane,xanthene and oxazine.

All of these shadow or reserve effects based on disperse dyes are moresuccessful when the acid-dyeable fibre is nylon rather than wool, which is tooeasily stained by disperse dyes and difficult to clear satisfactorily (section 3.4).The method already described (section 7.6) for contrast effects on basic-dyeablepolyester and nylon or wool using basic and acid dyes with an alkanolpolyoxyethylene anti-precipitant can be used on these blends to reserve thenormal polyester fibre. Selected premetallised, milling acid or reactive dyes areused on the acid-dyeable fibre.

Dependent three-colour contrasts (Table 1.5) on blends of nylon or woolwith normal and basic-dyeable polyester are obtained by a two-stage methodat pH 5–6 (ammonium acetate–acetic acid). The acid-dyeable component isdyed with acid dyes at 75°C in the presence of an alkanol polyoxyethylenedispersing agent as anti-precipitant and Glauber’s salt to protect the basic-dyeable variant from hydrolysis. The basic dyes are then added, followed bythe disperse dyes and a diphenyl or aryl ester carrier formulated with anonionic emulsifying system. The target shades on the normal and basic-dyeable polyester are achieved at the boil. Possible incompatibility betweensome basic dyes and anionic dispersing agents in the disperse dyeformulations can be minimised by adding the disperse dyes along with theacid dyes at the lower temperature. However, this does increase the dispersedye staining of the acid-dyeable fibre.

15.9 DYEING OF DBC BLENDS

A troublesome characteristic of dyed contrast effects on polyester/cellulosicblends is for the cellulosic component to be partially lost by differential abrasionon exposed edges of the garment. Resin treatments, particularly the severe curingconditions necessary for durable press processes, accentuate the tendency for thisto occur. The fault is especially obvious when the polyester is less heavily dyed, sothat the abraded areas have a frosted appearance. An improvement in thisrespect is shown by blends of normal and basic-dyeable polyester with acellulosic fibre. If the polyester homopolymer is dyed to the palest depth, thebasic-dyeable copolymer to the heaviest depth and the cellulosic component to

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an intermediate level, preferential abrasion of the cellulosic fibre does not alterthe apparent balance of the design too drastically. Similar considerations apply toblends of polyester and acrylic fibres with cotton or viscose.

Three-colour designs of limited fastness on these DBC blends are obtainableby a simple one-bath method. Selected intermediate-energy disperse dyes andsalt-controllable direct dyes are applied at pH 4 and 70°C in the presence of analkanol polyoxyethylene anti-precipitant, as well as Glauber’s salt to protect thebasic-dyeable polyester and to promote exhaustion of the direct dyes on to thecellulosic fibre. The basic dyes and a suitable carrier formulated with a nonionicemulsifier are added and the basic-dyeable polyester is dyed to shade at the boil.

A brighter gamut of contrasts with much better wet fastness on the cellulosicfibre is offered by a two-stage method. Selected disperse and reactive dyes areadded with Glauber’s salt and an alkanol polyoxyethylene anti-precipitant. Thereactive dyes are exhausted on to the cellulosic fibre at 60°C and then fixed at anappropriate alkaline pH and temperature. The pH is adjusted to 5 with aceticacid, basic dyes and a nonionic carrier emulsion are added and the dyeing of thepolyester variant yarns is completed at the boil.

A three-fibre blend of interest in household textiles has a 65:35 polyester/cotton warp and an 80:20 acrylic/cotton weft. Multicoloured or reserve effectscan be produced but if all three fibre types are to be dyed to a solid shade a three-bath process, applying disperse, vat and basic dyes in that order, may benecessary. The cost of this sequence can only be justified to achieve exceptionalfastness demands. If the vat dyes can be replaced by direct dyes, a moreeconomical two-bath sequence of disperse and direct dyes followed by basic dyesis practicable at a lower level of fastness.

15.10 DYEING OF DDA BLENDS

The attainment of acceptable solidity and fastness is often difficult on theseblends and cross-staining makes multicoloured effects of little interest. Forexample, the components of a 25:50:25 cellulose acetate/polyester/nylondresswear fabric can be dyed acceptably solid up to medium depth with alimited selection of low-energy disperse dyes applied at 80–90°C with carrier.Adequate yield and fastness on the polyester, however, can only be attainedunder conditions that damage the cellulose acetate. Similarly, a blend of67:18:15 wool/PVC fibre/acetate formerly used in lightweight uniforms wasmost difficult to dye in piece form because of the physical and chemicalsensitivities of the component fibres. Both the wool and PVC fibres are prone

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to shrinkage, but not necessarily to comparable extents, and the celluloseacetate tends to become delustred at temperatures normally preferred fordyeing wool. Both wool and cellulose acetate may suffer hydrolytic damageif treated under alkaline conditions. Wool is badly stained by the dispersedyes that must be used to dye the other two components.

15.11 DYEING OF DDC BLENDS

Dresswear fabrics made from 65:15:20 or 30:40:30 cellulose triacetate/polyester/viscose blends can be dyed by a one-bath method. This depends onthe selection of disperse dyes for optimum solidity on triacetate/polyester(section 14.1) and direct dyes that give the best reserve of unsaponifiedtriacetate without suffering significant decomposition in the high-temperature dyeing process at 120°C (section 13.3). The preferred dispersedyes are monoazo or disazo intermediate-energy types (Mr 300–380) and themost suitable direct dyes are of the self-levelling and salt-controllable disazomultisulphonate types. An addition of sodium m-nitrobenzenesulphonateassists in minimising the risk of slight decomposition of the direct dyes athigh temperature.

15.12 DYEING METHODS AND DYE SELECTION FOR THREE-COMPONENT BLENDS

It is difficult to summarise the numerous and varied methods that have to beadopted for the exhaust dyeing of blends containing three different fibretypes (Table 15.2). Solid effects are usually what is required on those blends(AAA, AAB, AAC and DAA) that contain any two of the major acid-dyeablefibres (wool, nylon or polyurethane). The same limitation is true for blendsthat contain two ester fibres (DDA or DDC) and the DAC blends of an esterfibre and an amide fibre with a cellulosic fibre.

The full versatility of colouring possibilities, i.e. three-way shadow,shadow/reserve, contrast/reserve, shadow/contrast and three-way contrasteffects, is only offered by those blends that contain at least two dyeabilityvariants of the same synthetic fibre. These can be the differential-dyeingnylon variants in AAA, AAB or AAC blends, acid-dyeable/basic-dyeableacrylic fibres (AAB blends) or normal/basic-dyeable polyester (DBA or DBCblends).

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Table 15.2 Dye selections for three-component blends

Blend Composition Colour effect Dye selection

AAA Pale-dye/normal/ Three-way Monoazo or anthraquinone levellingdeep-dye nylon shadow acid dyes

Normal/deep-dye/ Reserve (n)/ Monosulphonated or disulphonatedultra-deep nylon shadow acid dyes

Shadow/ Disulphonated and multisulphonatedcontrast acid dyes

Reserve (n)/ Monosulphonated and multisulphonatedcontrast acid dyes

Three-way Monosulphonated and multisulphonatedcontrast acid dyes with disperse dyes

Nylon/wool/ Solid 1:2 metal-complex dyespolyurethane

AAB Polyurethane/wool/ Solid Basic dyes with anionic retarder, thenbasic-dyeable neutral-dyeing acid dyespolyester

Nylon/acid-dyeable Reserve (a-d)/ Acid dyes and basic dyes at pH 4–5acrylic/basic- contrastdyeable acrylic

Shadow/ Acid dyes and basic dyes at pH 2 (acidcontrast shadow) or pH 7 (basic shadow)

Normal/deep-dye/ Shadow/ Selected monosulphonated acid dyesbasic-dyeable reserve (b-d)nylon

Shadow/ Monosulphonated acid dyes withcontrast localised-charge basic dyes

Contrast/ Selected monosulphonated andreserve (b-d) disulphonated acid dyes

Reserve (n)/ Disulphonated acid dyes with localised-contrast charge basic dyes

AAC Nylon/wool/ Solid Selected 1:2 metal-complex or millingcellulosic acid dyes and salt-controllable direct

dyes with syntan

Nylon/polyurethane/ Solid Reactive dyes by pad–batch, then 1:2cotton metal-complex or milling acid dyes

Normal nylon/ Shadow/ Monosulphonated and disulphonateddeep-dye nylon/ contrast acid dyes and salt-controllable directcotton dyes with syntan

CBA Cotton/modacrylic/ Solid or Salt-controllable direct dyes, basic dyesacrylic contrast and 1:2 metal-complex dyes with aryl

ester carrier and anti-precipitant

DAA Polyester/nylon/ Solid Disperse dyes and 1:2 metal-complex orwool milling acid dyes with syntan

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DAA Acetate/nylon/wool Solid Low- to intermediate-energy dispersedyes at 80°C, then 1:2 metal-complex ormilling acid dyes with syntan

Triacetate/nylon/ Solid Intermediate-energy disperse dyes withwool carrier at the boil, then 1:2 metal-complex

or milling acid dyes with syntan

Polyester/wool/ Solid Disperse dyes with carrier, then 1:2polyurethane metal-complex or milling acid dyes

DAC Polyester/nylon/ Contrast/ Neutral-dyeing acid dyes at 70°C, thencotton reserve (c) intermediate-energy disperse dyes

at 120°C

Polyester/ Solid Disperse dyes and disazopolyurethane/ multisulphonated direct dyescellulosic

Acetate/wool/ Solid Low-energy disperse dyes at 80°C, thenviscose direct dyes and neutral-dyeing acid dyes

Triacetate/nylon/ Solid Low-energy disperse dyes and disazoviscose multisulphonated direct dyes

at 120°C

DBA Normal polyester/ Three-way Selected disperse dyes with carrierbasic-dyeable shadowpolyester/nylon

Shadow/ Low energy disperse dyes with carrierreserve (n)

Contrast/ Selected disperse dyes and basic dyesreserve (n) at 120°C

Reserve (p)/ Basic dyes and neutral-dyeing acid dyescontrast with anti-precipitant

Three-way Acid dyes at 75°C, then basic dyes andcontrast disperse dyes with carrier and

anti-precipitant

DBC Normal polyester/ Three-way Intermediate-energy disperse dyes,basic-dyeable contrast basic dyes and salt-controllable directpolyester/ dyes with carrier and anti-precipitantcellulosic

Disperse dyes and reactive dyes withanti-precipitant, then basic dyes withnonionic carrier at the boil

DDA Acetate/polyester/ Solid Selected low-energy disperse dyes atnylon (pale depths) 80°C with carrier

DDC Triacetate/ Solid Intermediate-energy disperse dyes andpolyester/viscose disazo multisulphonated direct dyes

at 120°C


Blend Composition Colour effect Dye selection

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REFERENCES

15.13 REFERENCES1. A Lester, Australian Text., 2 (May–June 1982) 18.

2. F Somm, Textilveredlung, 15 (1980) 7.3. J B Timmis, Dyer, 153 (4 Apr 1975) 363.

4. L N Nazarenko, R D Efremov and L V Danileika, Kiev tekhnol. I legkoi prom.-sti., Kiev

(1988) 4.5. M V Karve, Indian Text. J., 87 (1977) 123.

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