Critical solutions in_the_dyeing_of_cotton_textile_materials-libre(1)

Abstract: Over the decades there have been several papers on the coloration of

cotton-based textiles. The number of articles dealing with the processing of cotton,

including preparation, dyeing, and finishing, may be in the thousands. An

investigation of the possible causes of problems occurring in the coloration of

textiles revealed that a comprehensive review of case studies and scientific

analysis would be a welcome addition to the already rich pool of knowledge in

this area.

Key words: Cotton, troubleshooting, pretreatment, dyeing, dyes, colorants.

1. INTRODUCTIONCotton is the backbone of the world’s textile trade [1]. It has many qualities [2] and

countless end uses [3], which make it one of the most abundantly used textile fibres

in the world [4]. It is a seed hair of plant of genus Gossypium [5], the purest form of

cellulose found in nature. However, cotton is one of the most problematic fibres as far

as its general wet processing or dyeing is concerned. Quite frequently, the problems

in dyed cotton materials are not due to the actual dyeing process but due to some

latent defects introduced from previous production and processing stages. Often, the

root-cause(s) of a problem in the dyed material can be traced as far back as to the

cotton field. This monograph will address problems in the dyeing of cotton textile

materials in various forms. An overview of various textile operations for cotton will

be given in the beginning. Then, various key stages and factors involved in the

production of dyed cotton textile materials will be described in detail and problems

originating at each stage will be summarised.

1.1 Overview of Textile Operations for Cotton

The textile industry is comprised of a diverse, fragmented group of establishments

that receive and prepare fibres, transform fibres into yarn, convert the yarn into fabric

or related products, and dye and finish these materials at various stages of production.

Figure 1 shows some of the general steps involved in manufacturing cotton textiles.

Textiles generally go through three to four stages of production that may include

yarn formation, fabric formation, wet processing and textile fabrication [6]. Textile

fibres are converted into yarn by grouping and twisting operations used to bind them

together [7]. Although most textile fibres are processed using spinning operations,

the processes leading to spinning vary depending on whether the fibres are natural or

manmade. Figure 2 shows the different steps used in cotton yarn formation. Some of

CRITICAL SOLUTIONS IN THE DYEINGOF COTTON TEXTILE MATERIALS

R. Shamey and T. Husseindoi:10.1533/tepr.2005.0001

© The Textile Institute


2 Textile Progress doi:10.1533/tepr.2005.0001

Fig. 1 General steps in manufacturing cotton textile goods.

Yarn

Formation

Fabric

Formation

Wet

Processing

Fabrication

Warping

Sizing

Weaving

Printing

Finished Goods Sewing

Cutting

Finishing

Dyeing

Preparation

Knitting

Spinning

Fibre Preparation

Raw Cotton

Fig. 2 General steps in yarn and fabric formation.

Raw Cotton

Cleaning

Blending

Carding

Combing

Drawing

Drafting

Spinning

Yarn

Knitting

(Weft or Warp)

Warping

Sizing

Weaving

Fabric

doi:10.1533/tepr.2005.0001 Critical Solutions in the Dyeing of Cotton 3


these steps may be optional, depending on the type of yarn and spinning equipment

used.

The major methods for fabric manufacture are weaving and knitting, although

recently nonwoven constructions have become more popular. Before weaving, warp

yarns are first wound on large spools, or cones, which are placed on a rack called a

creel. From the creel, warp yarns are wound on a beam wherefrom they are passed

through a process known as sizing or slashing. The size solution forms a coating that

protects the yarns against snagging or abrasion during weaving. Fabrics are formed

from weaving by interlacing one set of yarns with another set oriented crosswise. In

the weaving operation, the lengthwise yarns that form the basic structure of the fabric

are called the warp and the crosswise yarns are called the filling, also referred to as

the weft [8, 9]. Knitted fabrics may be constructed by using hooked needles to

interlock one or more sets of yarns through a set of loops. The loops may be either

loosely or closely constructed, depending on the purpose of the fabric. Knitting is

performed using either weft or warp knitting processes [10].

Woven and knitted fabrics cannot usually be processed into apparel and other

finished goods until the fabrics have passed through several water-intensive wet

processing stages. Wet processing enhances the appearance, durability and serviceability

of fabrics by converting undyed and unfinished goods, known as grey or greige

goods, into finished consumers’ goods. Various stages of wet processing, shown in

Fig. 3, involve treating greige goods with chemical baths and often additional washing,

rinsing and drying steps [11]. Some of these stages may be optional, depending on

the style of fabric being manufactured or whether the material being wet-processed

is a yarn, or a knitted or woven fabric.

Some of the key steps in the treatment of cotton material include singeing, desizing,

scouring, bleaching, mercerizing, as well as dyeing and finishing.

Fig. 3 General steps in wet processing.

Finished

Fabric

Mechanical

Finishing

Chemical

Finishing

PrintingDyeing

Mercerising

Bleaching

Scouring

Desizing

Singeing



Singeing is a dry process that removes fibres protruding from yarns or fabrics.

Desizing is a wet process that removes the sizing material applied to the warp yarns

before weaving. Scouring is a cleaning process that removes impurities from fibres,

yarns or cloth through washing, usually with alkaline solutions. Bleaching is a chemical

process that decolourizes coloured impurities that are not removed by scouring and

prepares the cloth for further finishing processes such as dyeing or printing.

Mercerization is a chemical process to increase dyeability, lustre and appearance.

Dyeing operations are used at various stages of production to add colour to textiles

and increase product value. Dyeing can be performed using batch or continuous

processes. Common methods of batch or exhaust dyeing include package, beam,

beck, winch, jet and jig processing. Continuous dyeing processes typically consist of

dye application, dye fixation with chemicals or heat, and washing. Dyeing processes

may take place at any of several stages of the manufacturing process (fibres, yarn,

piece-dyeing). Stock dyeing is used to dye fibres; yarn dyeing is used to dye yarn;

and piece/fabric dyeing is done after the yarn has been constructed into fabric. Printing

is a localized or patternised coloration of the fabrics. Fabrics are printed with colour

and patterns using a variety of techniques and machine types. Finishing encompasses

chemical or mechanical treatments performed on fibre, yarn or fabric to improve

appearance, texture, or performance.

2. PROBLEMS ORIGINATING FROM COTTON FIBRE

2.1 Problems Caused by Immature and/or Dead Cotton

Although it a common practice to use the terms ‘dead’ and ‘immature’ interchangeably,

it is useful to use these terms to indicate two different levels of maturity in cotton

fibres. The normal mature cotton fibre is bean-shaped in cross-section and has a thick

cell-wall. The other extreme, dead cotton, has virtually no cell-wall thickness. The

intermediate range between mature and dead is classified as immature. The immature

(sometimes called thin-walled) fibre does have some secondary wall thickening. The

thinner wall of the immature fibre lacks the rigidity of mature cotton. This increased

flexibility of immature or dead fibres makes them prone to be mechanically knotted

into a clump during ginning, lint cleaning and carding. These neps or clusters of

fibres may resist dye and appear as white specks in the dyed material [12–16].

The distinction between dead and immature fibres is very important. Both dye

lighter than fully mature fibres but only immature fibres respond to mercerization or

any other swelling treatment. In contrast, dead fibres lack the ability to accept some

dye even if pre-treated with a swelling agent.

The white or light-coloured specks caused by immature/dead fibres may be of one

of the following three types. The first type of the defect occurs when a surface knot

of entangled immature fibres is flattened during processing and takes on a glazed,

shiny appearance. The knot then becomes a small, reflective mirror on the surface of

the dyed material. Its greater reflectance makes the knot appear lighter at some

viewing angles than the surrounding area although it has actually been dyed to the

same depth. The second type occurs when the fabric is poorly penetrated during

dyeing. Since the clumps of immature fibres are often loosely attached to the material,

they can be moved or knocked loose during subsequent processes. If the clump, or



the yarn behind it, is not properly penetrated during dyeing, a light spot will be seen

when the clump changes its position. The third type is the classic case of the clump

of immature or dead fibres not dyeing to the same depth as the surrounding material.

The coverage of immature cotton depends upon the following factors:

Fibre preparation: There are several stages in the fibre preparation where an

attempt can be made to decrease the amount of neps of the immature and/or dead

fibres that are usually clumped together [17]. It is important to try to remove these

clumps prior to the carding process. Once past the main cylinder of the card, the

clumped fibres go into the subsequently formed yarn and the fabric.

Preparation sequence: The preparation sequence has little, if any, impact on the

coverage of immature cotton. Only pre-treatments that swell the cell wall, giving

it greater thickness, are effective in improving the dyeability of immature cotton.

Swelling pre-treatment: Treatment with swelling agents at optimum concentration

(e.g. caustic soda with a 14% or greater concentration) is effective in swelling the

secondary wall of immature cotton, and improving its dyeing affinity. On the other

hand, dead cotton lacks the necessary cell-wall thickness to be effectively treated

by any type of swelling pre-treatment system.

Dye selection: Dyes vary widely in their ability to effectively eliminate the white

or off-shade specks. It is recommended that dye suppliers be consulted for data on

the immature cotton coverage capabilities of specific dyes. Since caustic pre-

treatment is ineffective in eliminating white or off-shade specks caused by dead

cotton, dye selection is the best alternative in this case. Although the exact mechanisms

are unknown, one theory is that dyes that cover dead cotton are those which do not

penetrate into the cellulose of the fibre (the core) but are deposited mainly in the

outside layer. This gives the dead fibre a ‘coloured’ skin.

After-treatments: Swelling treatments such as mercerization or ammonia treatment

may be effective after dyeing, as well as before, if the problem is the presence of

reflective surfaces and not a genuine difference in dye uptake by the immature

cotton. However, such a procedure is justified only in extreme cases, as there is an

inevitable change of shade even when the fabric is dyed with dyes that are resistant

to strong alkalis.

2.2 Problems Caused by Dyeability Variation in Cotton

The results of research [18] confirm the dyeability variations in cotton obtained from

different sources. It has been suggested that the substrate should be obtained from a

single source, wherever possible, in order to keep the dyeability variations to a

minimum. Since some dyestuffs are more sensitive to dyeability variations than

others; those dyes should be selected for dyeing which are less sensitive to dyeability

variation.

2.3 Problems Caused by Contaminants in Cotton

While cotton fibre may be as much as 96 % cellulose, there are other components

present which must be removed in preparation for a successful dyeing. Table 1 gives

a summary of naturally occurring impurities in cotton [19].

The level of contamination in cotton is affected by: geology of cultivation area;

soil constitution; weather conditions during the maturing period; cultivation techniques;



chemicals, pesticides and fertilizers; as well as harvesting techniques [20]. For the

dyer, the elements that pose the greatest threat are alkaline earth and heavy metal

contaminants such as calcium, magnesium, manganese, and iron. Depending on its

origin, raw cotton can exhibit widely different contents of alkaline earth and heavy

metal ions. Table 2 gives an example of the metal content of cotton having different

origins [21].

Table 1 Typical Composition of Raw Cotton

Component Proportion (%)

Cellulose 88.0–96.0Pectins 0.7–1.2Wax 0.4–1.0Proteins 1.1–1.9Ash 0.7–1.6Other organic compounds 0.5–1.0

Table 2 Metal Content of Cotton of Different Origins

Origin of CottonMetal Content (mg/kg)

Ca Mg Fe Cu Mn

Brazil Assai Piranha 3147 1156 680 6 30Brazil Sao Paulo 845 555 46 6 11Peru 700 440 13 < 1 < 1USA Texas 810 365 75 < 1 < 1USA California 600 540 40 < 1 < 1Egypt Makko 640 452 11 < 1 < 1

Levels of fats, oils and waxes present in cotton can be reduced to acceptable limits by

the action of alkali and surface-active products. In extreme cases, the use of solvent

and surface active mixtures may be necessary [22]. Pectins and the related substances

can be rendered soluble by the action of alkali, usually caustic soda, which also acts

as a swelling agent. Amino acids are also rendered soluble in the presence of alkali

by producing the corresponding sodium salts. Metals, however, cannot be adequately

removed by conventional alkaline processes since, in an alkaline medium, sequestering

agents cannot quantitatively separate the minerals of a complex structure containing

heavy metals. Moreover, in the alkaline pH region, cellulose swells rapidly and

strongly, thus impairing the transport of crystalline minerals from the core to the

periphery of the fibre. Demineralisation with organic or inorganic acid is more effective

as compared to the alkaline treatment process. However, regardless of the efficacy of

an acid treatment, the use of organic or inorganic acids for the demineralisation of

cellulosic fibres involves a number of disadvantages such as corrosion of machine

parts, difficulties in handling, and risk of fibre damage with strong inorganic acids,

while organic acids give lower demineralisation and are more volatile.

Speciality products based upon strongly acidic sequestering agents or a mixture of

sequestering agents with organic buffer systems are recently being used for

demineralisation of cotton. These products offer numerous advantages over conventional



acids such as hydrochloric acid or sulphuric acid. Some of the advantages are given

as follows:

• No corrosion

• No steam volatility

• No unpleasant odour

• Prevention of dissolved metal ions from re-precipitating

• Synergy with surfactants, improving the washing effect, dispersion power and

soil suspension capacity

• Lower ash content

• Improved degree of whiteness

• No fibre damage

However, with such an intensive demineralisation treatment, care must be taken that

magnesium ions are added in subsequent peroxide bleaches, in order to avoid fibre

damage in the bleach owing to insufficient stabilisation of hydrogen peroxide [23].

2.4 Effect of Cotton Colour Grade on the Colour Yield of Dyed Goods

The difference in the colour yield of cotton of different original colour grades, when

dyed after scouring and bleaching, is so small as to be explicable by experimental

variation [24].

A summary of dyeing problems originating from cotton fibre is given in Appendix

A.

3. PROBLEMS ORIGINATING IN YARN FORMATIONAs much as 25 percent of the faults responsible for downgrading cotton finished

garments may be attributed to yarn [25]. The key yarn parameters are as follows:

• Yarn count

• Twist per inch

• Twist direction

• Strength

• Type (open-end or ring-spun, combed or carded)

• Elongation at break

• Moisture content

• Hairiness/pilling characteristics

• Uniformity/variation

• Impurities/foreign matter

• Composition

• Single or ply

• Colour/shade

• Dyeability

• ‘Classimat’ majors [26]

Some common types of faults present in yarn are as follows:

• Neps

• Long thick places

• Short thick places



• Thin places

• Weak places

• Count variation

• Hairiness

• Dyeability variations [27–30]

The main causes of the dyeability variations in yarn are:

• Immature fibres

• Dead fibres

• Vegetable matter or other foreign matter

• Wrong twist

• Bad splice

• Neps

• Count variations

4. PROBLEMS ORIGINATING IN YARN WINDING FOR

PACKAGE DYEINGThe success of package dyeing, in terms of both levelness and yarn quality, is greatly

influenced by the degree of care taken in the preparation of the yarn packages [31].

It is often said that ‘Well wound is half dyed’ [32]. The standard of winding affects

the quality of dyed yarn to a great extent. A well wound package not only increases

the chances of level dyeing but it also minimises the risk of many other dyeing

problems [33].

The most important winding parameters are as follows:

• Winding system or type of winding

• Winding angle or package traverse

• The dye tube

• Winding ratio, i.e. the ratio of the inside tube diameter to the outside package

diameter [34, 35]

• Package density [36–38]

• Package type or concentricity

There are three types of winding in common use: wild or random winding; precision

winding; and digital step winding. A comparison of the three different types is given

in Table 3. The winding angle or package traverse depends upon the type of winding

Table 3 Comparison of Different Winding Systems

Wild Random Winding Precision Cross Winding Digital or Step Winders

Stable package Fragile package—must be Stable packagehandled with care

Constant winding density Density varies from Uniform homogeneous densityinside to out

Areas of ribboning are No ribboning No ribboningpossibleLiquor flow characteristics Good liquor flow Good liquor flow characteristicsare not optimum characteristics



system used. The winding angle remains the same in random winding. In precision

winding there is a decreasing winding angle, and in digital step winding each layer

has a slightly different angle from the previous one.

An important consideration in any package dyeing operation is the type of carrier

on which the yarn package is wound. A wide range of designs and materials has been

used as support media (dye tubes) for packages. Rockets, cones, springs, plastic

tubes and non-woven fabric centres have all found favour in certain regards. Each

system has its advantages and disadvantages. Ultimately, the decision lies with the

individual users based on the particular requirements of their businesses and the

circumstances in use [39].

The use of large diameter tubes is said to offer improved quality at no reduction

in productivity. Since the larger tube can hold an equivalent amount of yarn with less

yarn thickness, lower flow and reduced pressure create less yarn disturbance and

deliver a high quality product [40, 41].

Winding density is one of the most important package characteristics that affect

the quality of the dyed package [42–46]. Package density highly influences the flow

of dye liquor through the package and the exchange between dye liquor and the yarn.

As a result, density significantly affects the depth of shade and levelness of dyed

yarn. Uniform package density is essential to producing a perfect dyeing. Fluctuations

in winding density of ± 3% are regarded as very low, whereas differences of ± 5% to

8% are considered to be within the normal range [47]. If the package is too soft,

channelling of the dye liquor will result and ballooning may occur. Soft packages

also tend to have excessive yarn shifts when the dye liquor is forced through the

package, making subsequent operations, such as back-winding, more difficult because

the yarn tangles. If the package is too hard or dense, liquor circulation will be

restricted through the package and cause un-dyed spots where yarns cross over one

another. Higher winding densities within the area adjacent to the dyeing tube may

inhibit uniform dyeing conditions in all sectors of the yarn bobbin [48]. The higher

the compactness of the package, the lower is the liquor throughput [49]. The ideal

package is of uniform density throughout. It should be of sufficiently open construction

to permit dye liquor to flow freely, yet dense enough to prevent channelling of the

liquor through more accessible places.

In addition to levelness, package density also affects the shade depth. The inner

zone density influences the shade depth the most, and the outer zone the least.

Increasing the inner zone density decreases shade depth in all areas of the package.

Increasing the middle zone density increases shade depth in both the inner and the

middle zone, but decreases the outer zone shade depth. Increasing the outer zone

density increases the outer zone shade depth and decreases the inner zone shade

depth. Package density affects the inner zone shade depth the most and the outer zone

shade depth the least. To ensure the shade levelness among packages, the same

density profile should be used for all the packages. The influence of density profiles

on the levelness and the shade depth is eventually due to their effect on liquor flow

between and through the yarns. This indicates that the control of the dye liquor flow

is the most important factor in the success of package dyeing. The factors affecting

the density of the package, when surface winding, are different from those that

govern it in precision winding. The yarn supply and its position, speed of winding,



winding tension, and the pressure of the package on the winding drum all play an

important role in the build-up of the package, and various devices are available for

adjusting their effects in order to increase the possibility of producing packages that

are regular and even in density [50].

The shape of the package also has some influence on the pattern of the liquor flow.

Cheese-shaped packages of regular construction are shown to be ideally suited to

uniform liquor flow. Cones have certain disadvantages as compared to cylindrical

cheeses [51]. Parallel-sided packages are preferred on technical grounds, particularly

with regard to levelness [52]. In the case of cones, it has been found that at the centre

of the package the density is greater and more irregular than in the outer layers. In

contrast, the distribution of pressure in cheeses is more uniform. As the liquor flows

through the cones, an impact pressure builds up in the interior of the package, causing

the ends of the cones to bulge. The result is that the liquor cannot penetrate these

areas properly. Moreover, residual dyestuff is deposited in the area around the spacers,

as is sand and other suspended matter.

According to the maximum flow rate that can be achieved during the dyeing

process, there are three types of yarn package properties [53]: dyeable at low flow

rate, dyeable at medium flow rate and dyeable at high flow rate. Each type of package

has a particular flow-rate limit, above which it is not possible to work without

causing deformation, water channels and consequently all the associated defects.

Other factors that contribute to proper winding are as follows:

• Supply package quality

• Yarn delivery

• Tensioning device

• Winding speed

• Soft edges

• Package build

• Package holder pressure control

• Number of packages per spindle

A summary of problems caused by poor package winding is given in Appendix B.

5. PROBLEMS ORIGINATING IN FABRIC FORMATIONWoven fabrics are produced by interlacing a group of warp and weft threads. Defects

in woven fabrics can be broadly grouped as yarn defects and process defects. Process

defects originate from the processes involved. Based on the processes, the defects in

the woven fabrics may be attributable to spinning, winding, warping, sizing, drawing-

in, pirn winding, loom-setting and handling [54]. The identification [55], definitions

[56], and images of defects [57] in woven fabrics and methods for their numerical

designation [58] are given in the respective references. Major problems that become

more apparent after dyeing but may be attributable to weaving include:

• Variation in the warp density of the cloth (wrong draw, missing end, double end)

• Selvedges thicker than the centre of the fabric

• Variation in size application on warp yarns

• Variation in drying of warp yarn after sizing

• Variation in warp tension during weaving



• Variation in weft density (missing pick, double pick)

• Variation in warp or weft yarns with respect to twist, twist direction, count,

hairiness, colour, tensile properties, fibre composition and/or spinning batch

• Fly or foreign matter or fibre woven into the fabric

Knitting is a process of making cloth with a single yarn or set of yarns moving in only

one direction, instead of two sets of yarns crossing each other, as in weaving. There

are two basic categories of knitting: Warp knitting and weft knitting. Warp knitting

works with multiple yarns running vertically and parallel to each other. The fabric is

constructed by manipulating these warp yarns simultaneously into loops which are

interconnected, e.g. Tricot, Raschel, Milanese, etc. Weft knitting works with one yarn

at a time running in a horizontal direction. The fabric is constructed by manipulating

the needle to form loops in horizontal courses built on top of each other, e.g. Circular,

Flat, Hosiery, etc. The largest proportion of knitted fabrics used today is weft knits

[10]. The faults in knitted fabrics can be categorized into those caused by yarn, those

in the course or length direction and those due to, or apparently due to dyeing [59,

60]. Major problems that become more apparent after dyeing but may be attributable

to knitting include [61–65]:

• Variation in course length (a ‘course’ is a row of loops across the width of a

knitted fabric)

• Variation in yarn with respect to count, twist, twist direction, hairiness, colour,

tensile properties, fibre composition, lubrication and/or spinning batch

• Variation in wale density (a ‘wale’ is a column of loops along the length of a

knitted fabric; ‘wale density’ is the number of loops per unit length measured

along a course)

• Vertical lines of distorted loops, of tuck stitches, or of cut stitches

• Fly or foreign matter knitted into the fabric

6. PROBLEMS CAUSED BY POOR WATER QUALITYThe use of water in textile dyeing and finishing is ubiquitous, and the role of water

in such processes is manifold [66]. Although it is difficult to state definitive water

demand for various processes, the raw material used in the greatest quantity in

virtually every stage of textile wet processing is water [67]. The quality of textiles

produced by any manufacturing operation which employs wet processes, such as

preparation, dyeing and finishing, is profoundly affected by the water quality [68].

Various textile processes are influenced in different ways by the presence of impurities

in the water supply and there are several major water use categories to be considered

including water for processing, potable purposes, utilities, and laboratory use. Each

requires different water-quality parameters. Process water (for preparation, dyeing,

and finishing) is to be mainly used for making concentrated bulk chemical stock

solutions, substrate treatment solutions, and washing. Potable water is for drinking

and food preparation. Utility use includes non-contact uses such as boiler use, equipment

cleaning etc.

Water from almost all supply sources contains impurities to some extent. The type

and amount of impurities depend upon the type of water source. The most common

impurities that may be present in water are as follows:



• Calcium and magnesium (hardness)

• Heavy metals, such as iron, copper, and manganese

• Aluminium

• Chlorine

• Miscellaneous anions (sulphide, fluoride, etc.)

• Sediments, clay, suspended matter

• Acidity, alkalinity, and buffers

• Oil and grease

• Dissolved solids

Contaminants from the water source are not the only ones found in textile water

supplies. There are major internal contributions, too. Common sources of internal

contamination are as follows:

• Clear well (used for water storage)

• Greige goods or other substrate

• Plumbing, valves, etc.

• Machinery

• Prior processes in the case of water reuse

There are many quick qualitative tests for detection of trace quantities of ions and

elements in water. There are also quantitative tests for determining the exact

concentration of cations such as calcium, magnesium, iron, copper, and manganese

in water. A description of quick spot tests for commonly occurring contaminants is

given by Smith and Rucker [68]. Analytical methods for water testing are given by

Thompson [69].

Water contaminants, especially metals, can have a substantial effect on many

textile wet processes. The effects are not always adverse but even when a process is

enhanced by water impurities, it is not desirable to have variance in processes and

product quality due to water quality changes. Such variations in the quality of water

make process and machinery optimisation and control difficult [70].

6.1 Problems in the Textile Laboratory

It is a common practice in some mills to use potable water for the laboratory supply

while using non-potable water for production processing. Since potable water is

usually chlorinated, it can alter the shade of dyeings and contributes to poor lab-to-

bulk reproducibility. Moreover, most work in analytical laboratories is done with

distilled and/or deionized water. However, many situations arising in textile wet

processing laboratories will require the use of process water in order to correlate well

with production. The laboratory technician must be able to realize when to use

process water and when to use distilled or deionized water.

6.2 Problems in Preparation Processes

Metallic ions in water can have a dramatic effect by either enhancing or inhibiting the

action of many preparation processes. All of the wet preparation processes are affected

in some way by metallic ion contaminants in water.

In enzymatic desizing, the metallic ions may cause inactivation of the enzymes,

resulting in poor size removal.



In scouring processes, calcium and magnesium ions (water hardness) cause the

most problems. These ions will precipitate soaps, forming a sticky insoluble substance

which deposits on the substrate. Such deposits impair the fabric handle, cause resist

in dyeing, attract soil to the material and cause inconsistent absorbency in subsequent

processes. Although most synthetic detergents used in scouring today do not precipitate

in the presence of calcium and magnesium ions, the fatty acid hydrolysis products

formed by the saponification of natural waxes, fats, and oils in the fibres will precipitate.

The formation of complexes with alkaline and alkaline earth salts drastically reduces

the solubility and the rate of dissolution of surfactants, thus impairing the wash

removal ability of the surfactants [71]. It is, therefore, imperative to use soft water in

the scouring process.

Bleaching with hydrogen peroxide is greatly affected, even by trace quantities of

metal ions in the water. The transition metal ions such as iron, copper, manganese,

zinc, nickel, cobalt and chromium catalyze decomposition of hydrogen peroxide

[72]. The decomposition is so rapid that it frequently occurs before any significant

bleaching can occur. In addition, the decomposition products attack cotton fibres

leading to their degradation. Bleaching baths containing these ions will therefore

lead to reduction in whiteness and high loss in fibre strength, as well as an increase

in fluidity. The alkaline earth metal (magnesium), on the other hand, produces beneficial

effects when present in peroxide bleaching solutions. These ions increase the stability

of hydrogen peroxide under alkaline bleaching conditions, and as a result increased

whiteness and less fibre degradation is obtained. Electrolytes of other metals may

have a harmful effect [73].

6.3 Problems in Dyeing Processes

The most commonly observed dyeing problems caused by poor water quality include

inconsistent shade, blotchy dyeing, filtering, spots, resists, poor washing off, and

poor fastness [74]. Inconsistent shade can be caused by chlorine contamination of the

process water or iron, copper and other metals. The action of copper on the dyestuff

can be prevented by a suitable complexing agent but not the action of iron. For iron,

purification of water prior to dyeing is recommended. Chelating agents are frequently

used in an attempt to eliminate the undesirable effect of these metals in process water,

but in many cases, the chelate itself may cause unpredictable effects such as shade

changes. The best strategy is to remove the metal from water before using it in

processing.

The presence of calcium and magnesium ions in the process water can cause

inconsistent and uneven washing-off of unfixed dyes, leading to blotches, and/or

inconsistent shade. Hexametaphosphates are effective sequestering agents for removing

these ions and are generally safe in the sense that they do not cause other undesirable

effects such as shade variations.

Blotchy dyeing can result from acidity or alkalinity in the water, depending upon

the application class of dyes. Even when the pH is neutral, water (and substrate) may

contain substantial alkalinity. This can have effects on exhaustion, levelling and

fixation of dyes. Similar types of defects can result from the residual chemicals,

especially alum (aluminium) in water.

Filtering in package dyeing, resists and spots can result from sediments, alum or



other residual flocking agents left over from water treatment, from organic contaminants,

from metal hydroxides (copper and iron), or from fatty acid/hardness metal complexes.

Generally, the stiffness of textile material dried after rinsing is greater, the higher the

solids content of the rinsing water [75].

In order to avoid the problems outlined above, water for textile processing has to

meet fairly stringent demands [76, 77]. The main requirements are as follows:

• Freedom from suspended solids and from substances that can give staining in

processing

• No great excess of acid or alkali

• Freedom from substances affecting the textile processes, such as iron, manganese,

Calcium or magnesium salts, and heavy metals

• Non-corrosiveness to tanks and pipelines, and

• Freedom from substances that give rise to foaming or unpleasant odour

Table 4 gives a summary of the requirements that the processing water has to meet

[32].

Table 4 Dyehouse Water Standard

Characteristic Permissible Limit

Colour ColourlessSmell OdourlesspH value Neutral pH 7–8Water hardness < 5 °dH (6.25°eH; 8.95°fH; 5.2 USA)Dissolved solids < 1 mg/lSolid deposits < 50 mg/lOrganic substances < 20 mg/l (KMnO4 consumption)Inorganic salts < 500 mg/lIron (Fe) < 0.1 mg/lManganese (Mn) < 0.02 mg/lCopper (Cu) < 0.005 mg/lNitrate ( NO 3

1– ) < 50 mg/lNitrite ( NO 2

1– ) < 5 mg/l

Table 5 gives the limits of impurities acceptable in water for steam boilers.

Table 5 Steam Boiler Feed Water Standard

Characteristic Acceptable Limit

Appearance Clear, without residuesResidual hardness < 0.05 °dHOxygen < 0.02 mg/lTemporary CO2 0 mg/lPermanent CO2 < 25 mg/lIron < 0.05 mg/lCopper < 0.01 mg/lpH (at 25 °C) > 9Conductivity (at 25 °C) < 2500 µS/cmPhosphate (PO4) 4–5 mg/lBoiler feed water temperature > 90 °C



Various measures and treatments may be employed in order to remove impurities

from water and to avoid problems in textile processing [76, 78], such as follows:

• Sedimentation and filtration treatments

• Softening treatments [such as cold lime-soda-softening or Zeolite softening]

• Reverse osmosis [79]

• The use of sequestering agents [80–83]

A summary of problems caused by poor water quality is given in Appendix C.

7. PROBLEMS IN SINGEINGTextiles are singed in order to improve their surface appearance and wearing properties

[84]. The burning-off of protruding fibre-ends which are not firmly bound in the

yarn, results in a clean surface which allows the structure of the fabric to be clearly

seen. Unsinged fabrics soil more easily than singed fabrics. The risk of cloudy dyeings

(a defect consisting of random, faintly defined uneven dyeing) with singed piece-

dyed articles in dark shades is considerably reduced, as randomly protruding fibres

cause a diffused reflection of light. Although cotton textile materials can be singed in

yarn [85], and knitted [86–88] as well as woven forms [84], singeing of woven

fabrics is much more common as compared to other forms. Two main methods of

singeing are direct flame singeing and indirect flame singeing [89].

There are singeing faults that are optically demonstrable and are quite easily

remedied during the actual working process. On the other hand there are singeing

faults that are not visible until after dyeing and that can no longer be repaired once

they have occurred.

A summary of problems in the singeing of woven fabrics is given in Appendix D.

8. PROBLEMS IN DESIZINGSizing has been considered as an ‘invention of the devil’ by some dyers and finishers

because it is the main source of many processing problems [90, 91]. Warp yarns are

coated with sizing agents prior to weaving in order to reduce their frictional properties,

decrease yarn breakages on the loom and improve weaving productivity by increasing

weft insertion speeds. The sizing agents are macromolecular, film-forming and fibre

bonding substances, which can be divided into two main types [92]: natural sizing

agents which include native and degraded starch and starch derivatives, cellulose

derivatives and protein sizes; and synthetic sizes which include polyvinyl alcohols,

polyacrylates and styrene–maleic acid copolymers. Starch-based sizing agents are

most commonly used for cotton yarns because of being economical and capable of

giving satisfactory weaving performance. Other products are also used, either alone

or in combination with starch sizes, when the higher cost can be off-set by improved

weaving efficiency. Some auxiliaries are also used in sizing for various functions and

include softening agents, lubricating agents, wetting agents, moistening agents, size

degrading agents, and fungicides. The desizing procedure depends on the type of

size. It is therefore necessary to know what type of size is on the fabric before

desizing. This can easily be determined by appropriate spot tests [93].

The sizing material present on warp yarns can act as a resist towards dyes and

chemicals in textile wet processing. It must therefore be removed before any subsequent



wet processing of the fabric. The factors on which the efficiency of size removal

depends are as follows:

• Viscosity of the size in solution

• Ease of dissolution of the size film on the yarn

• Amount of size applied

• Nature and the amount of the plasticizers

• Fabric construction

• Method of desizing

• Method of washing-off

Different methods of desizing are [94, 95]:

• Enzymatic desizing

• Oxidative desizing

• Acid steeping

• Rot steeping (use of bacteria)

• Desizing with hot caustic soda treatment

• Hot washing with detergents

The most commonly used methods for cotton are enzymatic desizing [96–98] and

oxidative desizing [99–101]. Acid steeping is a risky process and may result in the

degradation of cotton cellulose while rot steeping, hot caustic soda treatment and hot

washing with detergents are less efficient for the removal of the starch sizes.

Enzymatic desizing consists of three main steps: application of the enzyme, digestion

of the starch and removal of the digestion products. The common components of an

enzymatic desizing bath are as follows:

• Amylase enzyme

• pH stabiliser

• Chelating agent

• Salt

• Surfactant

• Optical brightener

The enzymes are only active within a specific range of pH, which must be maintained

by a suitable pH stabiliser. Chelating agents used to sequester calcium or combine

heavy metals may be injurious to the enzymes and must be tested before use. Certain

salts may be used to enhance the temperature stability of enzymes. Surfactants may

be used to improve the wettability of the fabric and improve the size removal. Generally,

non-ionic surfactants are suitable but it is always recommended to test the compatibility

of surfactants before use. Some brighteners may also be incorporated in the desizing

bath which may be carried through the end of the pre-treatment, resulting in improved

brightness but again, their compatibility must be ascertained before use. Enzymatic

desizing offers the following advantages [102]:

• No damage to the fibre

• No usage of aggressive chemicals

• Wide variety of application processes

• High biodegradability



Some disadvantages of enzymatic desizing include lower additional cleaning effect

towards other impurities, no effect on certain starches (e.g. tapioca starch) and possible

loss of effectiveness through enzyme poisons.

Oxidative desizing [103] can be effected by hydrogen peroxide [104, 105], chlorites,

hypochlorites, bromites, perborates or persulphates. Two important oxidative desizing

processes are [106]: the cold pad-batch process based on hydrogen peroxide with or

without the addition of persulphate; and the oxidative pad-steam alkaline cracking

process with hydrogen peroxide or persulphate. The advantages offered by oxidative

desizing are supplementary cleaning effect, effectiveness for tapioca starches and no

loss in effectiveness due to enzyme poisons. Some disadvantages include the possibility

of fibre attack, use of aggressive chemicals and less variety of application methods.

After desizing, the fabric should be systematically analyzed to determine the

uniformity and thoroughness of the treatment. It is first weighed to determine the

percent size removed. The results are compared with a sample known to have been

desized well in the lab. If the size is not adequately removed then either the treatment

or washing have not been thorough. Iodine spot tests are then conducted on the fabric

[107–109]. The fabric is not spotted randomly but from side-centre-side at different

points along the length of the fabric. The results of this evaluation give some idea of

the causes of any inadequate treatment.

Some of the most common problems in enzymatic desizing and their possible

causes are given in Appendix E.

9. PROBLEMS IN SCOURINGVarious aspects of cotton fabric preparation have been presented by Rosch [110–118]

and Sebb [119–124]. An important, if not the most important, operation in the pre-

treatment of cotton is the scouring or alkaline boil-off process. The purpose of alkaline

boil-off and the ensuing washing stage is to perform extensive fibre-cleaning by

ensuring a high degree of extraction of pectins, lignins, waxes and grease, proteins,

alkaline earth metals (Ca and Mg), heavy metals (iron, manganese and copper), low

molecular weight cellulose fragments, dirt and dust; and softening of husks. The

result is an increased responsiveness of cotton to subsequent processing [125]. The

process removes water insoluble materials such as oils, fats, and waxes from the

textile material. These impurities coat fibres and inhibit rapid wetting, absorbency

and absorption of dyes and chemical solutions. Oils and fats are removed by

saponification with hot sodium hydroxide solution. The process breaks the compounds

down into water-soluble glycerols and soaps. Unsaponifiable material such as waxes

and dirt are removed by emulsification. This requires the use of surfactants to disperse

the water-insoluble material into fine droplets or particles in the aqueous medium.

Both of these processes (saponification and emulsification) take place in a typical

scouring process. In addition, the scouring process softens and swells the motes to

facilitate their destruction during bleaching. Depending on the amount of impurities and

the reaction and wash conditions, the loss in weight of the raw cotton material due to

boil-off can reach up to seven percent or even higher in case of high-impurity cotton.

The important parameters of the scouring process are as follows:

• Concentration of caustic soda

• Type and concentration of auxiliaries



• Treatment temperature

• Reaction time

The higher the caustic soda concentration, the shorter can be the dwell time. In other

words, the shorter the dwell time, the higher the concentration required. The caustic

soda concentration normally employed neither affects the ash content nor the average

degree of polymerisation [DP] of cotton. Too high a concentration (e.g. > 8% o.w.f)

may result in a reduction in DP as well as yellowing of the cotton fibre. The higher

the concentration, the greater will be the fat removal. Due to the high degree of fat

removal, the absorbency will also increase but there may be harshness in the handle

of the material.

Two important auxiliaries used in scouring are chelating agents and surfactants.

Other auxiliaries that may sometimes be employed include antifoaming and anti-

creasing agents. Chelating agents are used to eliminate water hardness and heavy

metals, such as iron and copper which can affect the scouring process. These agents

bind polyvalent cations such as calcium and magnesium in water and in fibres, thus

preventing the precipitation of soaps. If polyvalent ions are present, insoluble soaps

may form, settle on the fabric and produce resist spots. There are four major types of

sequestering agents to choose from: inorganic polyphosphates, aminocarboxylic acids,

organophosphonic acids, and hydroxycarboxylic acids. The inorganic polyphosphates

such as sodium tripolyphosphate and sodium hexametaphosphate are probably the

best overall in that in addition to sequestering most metals they also aid in cleansing

the fibres. They may, however, hydrolyze at high temperature and loose their

effectiveness.

The aminocarboxylic acid types such as ethylenediaminetetraacetic acid (EDTA)

are very good in that they sequester most metal ions and are very stable under

alkaline conditions. They are the most used types. The organophosphonic acid types

such as ethylenediaminetetra (methylene phosphonic acid) are also very effective but

comparatively expensive. Oxalates and hydroxycarboxylic acids (citrates, etc.) are

excellent for sequestering iron but not effective for calcium and magnesium.

In order to quickly and effectively bring the chemicals to the textile material, i.e.

to improve their wettability and to ensure that the fibrous impurities will be removed

as far as possible, it is necessary to add surfactants with good wetting and washing/

emulsifying properties. A surfactant of optimal versatility to be used for preparation,

and in particular for the scouring and bleaching processes, ought to meet the following

requirements:

• It should have an excellent wetting ability within a wide temperature range

• It should permit a good washing effect and have a high emulsifying power for

natural fats, waxes and oils

• It should be resistant to oxidants and reducing agents

• It should be resistant to water-hardening substances

• It should be highly stable to alkalinity

• It should be biodegradable and non-toxic

Care should be taken in selecting the surfactants because of the inverse effect of

temperature on the solubility of non-ionic surfactants. If the process temperature is

above the cloud point of the surfactant, the surfactant may be ineffective and may



actually be deposited on the substrate. The surfactant used should have a cloud point

temperature just above the operating temperature, to be most effective. The cloud

point of non-ionic surfactants decreases in the presence of alkalis and electrolytes

and the degree to which it is lowered increases with concentration. The cloud point

should therefore be checked under application conditions to ensure that the surfactant

is effective under those conditions. The adverse effect of temperature on non-ionic

surfactants can be reduced by the addition of an anionic surfactant. Crypto-non-ionic

surfactants do not exhibit a cloud point. These are non-ionic surfactants that are

capped with an ionic group and they exhibit the excellent emulsifying properties of

non-ionics along with the good solubility properties of anionics.

Higher scouring temperatures will reduce treatment times and vice versa. At high

temperature, however, there will be complete removal of fats and waxes, which will

promote harsh handle of the material. Moreover, the cloud point of the surfactant also

has to be taken into account while applying high temperature.

In the case of pad-steam scouring, a typical process consists of the following

steps: Saturating the fabric with a solution of sodium hydroxide, surfactant and

sequestering agent; steaming; and thorough washing. After scouring, the material is

checked for thoroughness and uniformity of scouring as well as other scouring faults.

Appendix F gives most common problems in scouring, their possible causes, and

countermeasures.

10. PROBLEMS IN BLEACHINGCotton, like all natural fibres, has some natural colouring matter, which confers a

yellowish brown colour to the fibre. The purpose of bleaching is to remove this

colouring material and to confer a white appearance to the fibre. In addition to an

increase in whiteness, bleaching results in an increase in absorbency, levelness of

pre-treatment, and complete removal of seed husks and trash [126]. In the case of the

production of full white finished materials, the degree of whiteness is the main

requirement of bleaching. The amount of residual soil is also taken into consideration

because of the possibility of later yellowing of the material. In the case of pre-

treatment for dyeing, the degree of whiteness is not as important as, for example, the

cleanliness of the material, especially the metal content. Similar demands refer to the

production of medical articles. In this case, too, the metal content as well as the ash

content are important factors [127].

If whiteness is of primary importance, it requires a relatively large amount of

bleaching agent as well as a high operating temperature and a long dwell time.

Accurate regulation of the bleaching bath is a further obligatory requirement. Where

the destruction of trash, removal of seed husks and an increase in absorbency is a

prime necessity (e.g. for dyed goods), a high degree of alkalinity is all important. It

is, however, not the alkali alone that is responsible for these effects. The levelness of

pre-treatment can only be guaranteed if cotton of the same or equal origin is processed

in each bath. If this is not the case, suitable pre-treatment will have to be undertaken

to obtain, as closely as possible, the required uniformity. A pre-treatment with acid

and/or a chelating agent will even out (better still eliminate) varying quantities of

catalytic metallic compounds.

Although there are different bleaching agents that can be used for bleaching cotton,



hydrogen peroxide is, by far, the most commonly used bleaching agent today [128].

It is used to bleach at least 90% of all cotton and cotton blends, because of its

advantages over other bleaching agents. The nature of the cotton colour, its mechanism

of removal with hydrogen peroxide [129] and the basic rules for formulation of

bleaching liquors have been presented in detail elsewhere [120]. The mere formulation

of the correct initial bath concentration is not sufficient to ensure a controlled bleaching

process. Of equal importance are regular checks of the bath composition during the

operation. Such checks do not only contribute to an economic bleaching operation

but also allow an early tracing of the defects and failures of the system [122]. The

important parameters for bleaching with hydrogen peroxide are as follows:

• Concentration of hydrogen peroxide

• Concentration of alkali

• pH

• Temperature

• Time

• Nature and quality of the goods

• Water hardness and other impurities

• Types and concentration of auxiliaries

• Desired bleaching effect

• Available equipment, and stabilizer system employed [130, 131]

Most of these factors are inter-related, and all have a direct bearing on the production

rate, the cost and the bleaching quality. Though they operate collectively, it is better

to review them individually for the sake of clarity.

There are two concentrations to be considered: that based on the weight of the

goods and that based on the weight of the solution. All other factors being equal, the

concentration on the weight of the goods determines the final degree of whiteness. In

order to get adequate bleach there must be enough peroxide present from the start. On

the other hand, the peroxide concentration based on the weight of the solution will

determine the bleaching rate — the greater the solution concentration, the faster the

bleaching. No peroxide bleaching system ever uses up its entire peroxide charge for

active bleaching, as some is always ‘lost’ during normal process.

The alkalinity in the system is primarily responsible for producing the desired

scour properties and maintaining a reasonably constant pH at the desired level throughout

the bleaching cycle. The quantity of the alkali to be added depends above all on the

character of the goods, the finish required and the kind and quality of the other

ingredients in the liquor. The alkalinity is defined as the ‘amount’ of alkali in the

system and should be distinguished from the pH, which is a measure of the hydrogen

ion concentration in the solution. The pH value in peroxide bleaching is of extreme

importance because it influences bleaching effectiveness, fibre degradation and peroxide

stability in bleaching cotton fibres, as shown in Table 6.

With increasing pH, whiteness index increases to a maximum at a pH of 11.0 and

then decreases. Fibre degradation is at minimum at a pH of 9.0 but that which occurs

at a pH of 10.0 is well within acceptable values. Above a pH of 11.0, fibre degradation

is unacceptably severe. A pH range of 10.2–10.7 is considered optimum for bleaching

cotton with hydrogen peroxide. Lower pH values can lead to decreasing solubility of



sodium silicate stabiliser (see below) as well as lower whiteness due to less activation

of the peroxide [132].

By increasing the temperature, the degree of whiteness as well as its uniformity

increases. However, at too high a temperature, there is a possibility of a decrease in

the degree of polymerisation of the cotton. Moreover, due to good fat removal at high

temperatures such as 110 °C, the handle of the material can become harsh and the

sewability of woven cotton fabrics may also decrease. Time, temperature and

concentration of peroxide are all inter-related factors. At lower temperatures, longer

times and higher concentrations are required. As the temperature of bleaching increases,

shorter times and lower peroxide concentrations can be employed.

The amount of peroxide decomposed is greatly reduced with increasing weight of

cotton fibre in the bleach liquor. The raw fibre almost completely suppresses

decomposition, while the scoured fibre is somewhat less effective. The demineralised

fibre is the least effective stabiliser [133]. While impurities such as magnesium and

calcium may have a good stabilising effect when present in appropriate amounts,

other impurities such as iron, copper and manganese can have very harmful effect,

resulting in catalytic decomposition of hydrogen peroxide leading to fibre damage [134].

A good stabilising system is indispensable in bleaching cotton with hydrogen

peroxide. While sodium silicate is one of the most commonly used stabilisers, its use

may result in a harsh handle of the fabric as well as resist spots leading to spotty

dyeing. The best alternatives to sodium silicate are organic stabilisers or a combination

of silicate and organic stabilisers.

In addition to the most important ingredients of the bleaching recipe, namely

hydrogen peroxide, caustic soda and the stabilizer, auxiliaries are used sometimes to

aid the bleaching process. These may include surfactants and chelating agents. The

type and concentration of these auxiliaries also plays an important role in the bleach

effect obtained. The desired bleaching effect does not need necessarily be optimal

white. For goods-to-be-dyed, the main concern will normally be achieving good and

uniform absorbency.

The available equipment plays a role in determining which process criteria must

be taken into account such as: cold, hot or HT bleaching; dry-wet or wet-on-wet

impregnation; discontinuous or continuous processing; process control.

The most common problems in bleaching cotton with hydrogen peroxide are as

follows:

• Inadequate mote removal

• Low degree of whiteness

Table 6 Effect of pH on Bleaching Effectiveness, Fibre Degradation, and Peroxide Stability inBleaching Cotton Fibres

Initial pH Final pH Whiteness CUEN % PeroxideIndex Fluidity Remaining

8.0 4.4 66.8 5.48 72.59.0 8.7 67.3 1.44 71.6

10.1 9.9 71.3 2.44 63.311.0 11.7 72.2 7.29 7.012.0 12.4 69.5 17.8 2.0



• Uneven whiteness (or bleaching)

• Pinholes, tears, broken yarns, catalytic damage, loss in strength [135, 136]

• Resist marks

• Formation of oxycellulose

A summary of the possible causes of these problems and their countermeasures is

given in Appendix G.

It is not always possible to find the cause of these problems without detailed

analyses [72]. The most useful tests that can be carried out to check the effectiveness

of the bleaching process are for whiteness, absorbency and tensile strength. Checks

and measures are required also to assure level dyeing properties. After bleaching, for

example, the pH of the goods should be adjusted in the last rinse. Control of residual

moisture content (e.g. 7% for cotton) is part of the standard pre-treatment, which

should be uniform throughout the material [126].

11. PROBLEMS IN MERCERIZATIONMercerization is the treatment of cotton with a strong sodium hydroxide solution.

This process improves many properties of cotton fibres and may actually reduce or

eliminate some dyeing problems. Some of the properties of cotton fibres that are

improved by this process include [137, 138]:

• Increase in dye affinity

• Increase in chemical reactivity

• Increase in dimensional stability

• Increase in tensile strength

• Increase in lustre

• Increase in fabric smoothness

• Improvement in the handle

• Improvement in the appearance

There are many possible variations in the mercerization process. A review of technical

research and commercial developments in mercerisation has been given by Greenwood

[139]. Mercerization of cotton can be carried out on raw fibre [140], yarn, and knitted

[141–147] or woven fabric, and at any stage during preparation. Fabric may be mercerised

in greige form, after desizing, after scouring or after bleaching. The choice depends

upon the type of goods, the particular plant set-up, and the requirements of the final

mercerized fabric. Fabrics can be mercerized without tension to effect mainly an increase

in strength and dye affinity, or under tension to effect mainly an increase in the lustre [148].

The treatment may be wet-on-dry, wet-on-wet or add-on [149–151] at cold or hot tem-

peratures [152]. A comparison of cold and hot mercerization is given in Table 7 [153].

The most common of the various mercerization processes is that of treating the

fabric in the cold after bleaching with or without tension. The conventional method

of mercerization generally consists of the following steps:

• Padding the fabric through a strong sodium hydroxide solution

• Allowing time for the alkali to penetrate and swell the cotton fibres

• Framing to provide the tension required for lustre development

• Thorough rinsing to remove the alkali



The important mercerization parameters are as follows:

• Moisture content in the substrate for mercerization

• Concentration of caustic soda

• Penetration of caustic soda

• Temperature of caustic soda

• Wet pick-up

• Time of contact of the fabric with caustic soda

• Post-framing/tension on the material

• Washing/neutralization

If the fabric to be mercerized has a high moisture content, there may be a dilution of

the caustic soda concentration and the reaction between caustic and water generates

heat which may increase the bath temperature. The optimum concentration of sodium

hydroxide concentration is between 25 and 30% (48–54°Tw). Lower concentrations

will result in a lower degree of mercerization and less lustre. Higher concentrations

have no beneficial effect. A good wetting agent is necessary to improve penetration

of the caustic soda. The wetting agent must be stable and effective at the high alkaline

concentrations used [154], so only those wetting agents designed specifically for

mercerization should be used. The temperature of the bath can affect the degree of

mercerization. Swelling of the cotton and thus mercerization decreases with increasing

temperature [155]. The optimum temperature is 70–100 °F [21–38 °C]. Lower

temperatures do not affect the process adversely if the sodium hydroxide concentration

is in the proper range. At lower concentrations, the degree of mercerization increases

as the temperature decreases. Lower degrees of mercerization are obtained at

temperatures above l00 °F.

Wet pick-up in padding can affect mercerization in several ways. Less swelling

may occur at low wet pick-up, leading to incomplete mercerisation. The caustic

solution also plasticises the fabric so that it is easily stretched. At low wet pick-up

values, less plasticisation occurs and the fabric may tear during stretching on the

frame. Wet pick-up should be about 100%. The optimum time after padding is at least

30 seconds, to allow for the caustic to swell the cotton fibres before tension is applied

on the frame. Shorter times will result in incomplete mercerization.

As cotton fibres are swollen by the alkali, the fabric shrinks [156]. To obtain lustre

Table 7 Comparison of Conventional (Cold) and Hot Mercerization

Conventional Mercerization (10–20 °C) Hot Mercerization (70 °C)

Strong fibre swelling Less fibre swellingSlower swelling Rapid swellingSlower ‘relaxation’ Rapid ‘relaxation’Incomplete ‘relaxation’ Good ‘relaxation’Higher residual shrinkage Lower residual shrinkageSurface swelling Complete swellingUnevenness EvennessHarder hand Softer handNaOH diffusion inhibited Uninhibited NaOH diffusionLess lustre Optimised lustre



and shrinkage control, the fabric must be stretched on a frame. It should be stretched

in the width direction to its greige width or slightly more. No stretching in the length

direction is required unless extreme lustre is desired. If lengthways stretching is

needed, the frame speed should not exceed the padder speed by more than five

percent.

Removal of caustic soda from the fabric is very crucial for the development of

lustre and shrinkage control. The caustic soda solution concentration in the fabric

(not the rinse solution) should be reduced to less than 5% with the fabric still on the

frame. If not, low lustre and shrinkage of the fabric will occur. If the fabric shrinks

as it comes off the frame, the caustic concentration in the fabric has not been reduced

sufficiently. After the fabric comes off the frame, the remaining caustic should be

thoroughly rinsed out. It is difficult to remove residual amounts of caustic soda from

the fabric by rinsing alone, so they are usually neutralized with a dilute acid solution.

Care must be taken in using acetic acid for neutralization as some of the sodium

acetate formed may remain in the fabric and alter the pH in the subsequent wet

processes.

After mercerization, an analysis is carried out to determine the degree of

mercerization, which is specified by the Barium Number [157–160]. The Barium

Number obtained should be at least 130 and preferably 150. Low numbers result

from incomplete swelling of cotton fibres. A quick test for determination of the

degree of mercerization is to dye samples of the mercerized fabric along with a

sample known to be properly mercerized, using a direct dye such as C.I. Direct Blue

80. Any differences in the depth of the dyeings are indicative of different degrees of

mercerization. A red or blue dye should be used, since it is easier to observe differences

in depths of these colours visually. There is no standard test for analysis of the lustre

of mercerized fabric. It must be judged visually.

A summary of common problems in mercerization is given in Appendix H.

12. PROBLEMS IN DYEING WITH REACTIVE DYESReactive dyes are one of the most commonly used application class of dyes for cotton

materials, Two important aspects of reactive dyeing, namely dye variables and system

variables, are discussed in this section, along with important characteristics of

reactive dyeing such as exhaustion, migration and levelling, fixation and colour

yield, and washing-off and fastness. A significant portion of this section also deals

with the problem of the reproducibility and difficulties in obtaining right-first-time

dyeing.

12.1 Dye Variables in Reactive Dyeing

The major dye variables that affect reactive dyeing are dye chemistry, substantivity,

reactivity, diffusion coefficient and solubility. Each of these will be briefly discussed

below.

Dye chemistry: Reactive dyes have a wide variety in terms of their chemical structure

[161]. The two most important components of a reactive dye are the chromophore

and the reactive group.

The characteristics governed by the chromophore are colour gamut, light fastness,



chlorine/bleach fastness, solubility, affinity, and diffusion [162]. The chromophores

of most of the reactive dyes are azo, anthraquinone, or phthalocyanine [163]. Azo

dyes are dischargeable. Disazo dyes have the disadvantage of being much more

sensitive to reduction and many of them are difficult to wash-off. Anthraquinone

dyes exhibit relatively low substantivity and are easier to wash-off. Most of them

possess excellent fastness to light and to crease-resistant finishes, but they are not

dischargeable. Phthalocyanine dyes diffuse slowly and are difficult to wash-off [164].

Metal complex dyes containing copper possess rather dull hues, but show a high

degree of fastness to light and to crease-resistant finishes. Their substantivity is fairly

high; 1:2 complexes diffuse relatively slowly, so a longer time is needed to wash-out

unfixed dye completely.

The dye characteristics governed by the reactive group are reactivity, dye–fibre

bond stability, efficiency of reaction with the fibre, and affinity. Dyeing conditions,

especially the alkali requirement and temperature as well as the use of salt also

depend on the type of reactive group [165]. Dyes based on s-triazine do not have

good wet fastness properties in acidic media and, due to their high substantivity, have

poor wash-off properties. Similarly, dyes having a vinyl sulphone reactive system

have poor alkaline fastness. The chemical bond between the vinyl sulphone and the

cellulosic fibre is very stable to acid hydrolysis. The substantivity of hydrolysed by-

products of vinyl sulphone is low, so washing off is easy. Monochlorotriazines have

good fastness to light, perspiration and chlorine. The turquoise reactive dye shows an

optimum dyeing temperature that is generally about 20 °C higher than that of other

dyes with the same reactive group [166]. The fluorotriazine groups form linkages

with cellulose that are stable to alkaline media. Reactive dyes of dichloroquinoxaline,

monochlorotriazine and monofluorotriazine types show a tendency for lower resistance

to peroxide washing and dye–fibre bond stability [167]. A lower sensitivity to changes

in dyeing conditions (particularly temperature) is the most important characteristic

feature of the monochlorotriazine-vinyl sulphone heterobifunctional dyes. Dyeing

properties of some important reactive groups have been discussed in detail by various

authors [168–173].

Substantivity: Substantivity is more dependent on the chromophore as compared to

the reactive system. A higher dye substantivity may result in a lower dye solubility

[174], a higher primary exhaustion [175], a higher reaction rate for a given reactivity

[176], a higher efficiency of fixation [177], a lower diffusion coefficient, less sensitivity

of dye to the variation in processing conditions such as temperature and pH [178],

less diffusion, migration and levelness [179, 180], a higher risk of unlevel dyeing,

and more difficult removal of unfixed dye. Substantivity is the best measure of the

ability of a dye to cover dead or immature fibres. Covering power is best when the

substantivity is either high or very low [181]. An increase in the dye substantivity

may be effected by lower concentration of the dye, higher concentration of electrolyte

[182], lower temperature, higher pH (up to 11) and lower liquor to goods ratio [183].

Reactivity: A high dye reactivity entails a lower dyeing time and a lower efficiency

of fixation. (To improve the efficiency of fixation by reducing dye reactivity requires

a longer dyeing time and is, therefore, less effective than an increase in substantivity.)



Also there is a wider range of temperature and pH over which the dye can be applied.

Reactivity of a dye can be modified by altering the pH or temperature, or both. By a

suitable adjustment of pH and temperature, two dyes of intrinsically different reactivity

may be made to react at a similar rate.

Diffusion coefficient: Dyes with higher diffusion-coefficients usually result in better

levelling and more rapid dyeing. Diffusion is hindered by the dye that has reacted

with the fibre and the absorption of active dye is restrained by the presence of

hydrolysed dye. Different types of dyes have different diffusion characteristics. For

example, the order of decreasing diffusion is: unmetallised dyes, 1:1 metal-complex

dyes, 1:2 metal complex dyes; phthalocyanine dyes. An increase in the diffusion is

affected by increasing temperature, decreasing electrolyte concentration, adding urea

in the bath [184] and using dyes of low substantivity.

Solubility: Dyes of better solubility can diffuse easily and rapidly into the fibres,

resulting in better migration and levelling. An increase in dye solubility may be

effected by increasing the temperature, adding urea and decreasing the use of electrolytes.

12.2 System Variables in Reactive Dyeing

Temperature: A higher temperature in dyeing with reactive dyes results in a higher

rate of dyeing [185], lower colour yield [186], better dye penetration, rapid diffusion,

better levelling, easier shading, a higher risk of dye hydrolysis, and lower substantivity.

Raising the temperature appears to result in an opening-up of the cellulose structure,

increasing the accessibility of cellulose hydroxyls, enhancing the mobility as well as

the reactivity of dye molecules and overcoming the activation energy barrier of the

dyeing process, thereby increasing the level of molecular activity of the dye–fibre

system as well as dye–fibre interaction [187]. A comparison of hot and cold reactive

dyes has been given in [188, 189] along with some technical advantages of hot

reactive dyes over cold reactive dyes.

pH: The initial pH of the dyebath will be lower at the end of the dyeing by one half

to a whole unit, indicating that some alkali has been used up during dyeing. The

cellulosic fibre is responsible for some of this reduction, while a smaller part is used

by the dyestuff as it hydrolyses [190]. In discussing the effect of pH, account must be

taken of the internal pH of the fibre as well as the external pH of the solution. The

internal pH is always lower than the external pH of the solution. As the electrolyte

content of the bath is increased, the internal pH tends to equal the external pH. Since

the decomposition reaction is entirely in the external solution, the higher external pH

favours decomposition of the dye rather than reaction with the fibre. pH influences

primarily the concentration of the cellusate sites on the fibre. It also influences the

hydroxyl ion concentration in the bath and in the fibre. Raising the pH value by 1 unit

corresponds to a temperature rise of 20 °C. The dyeing rate is best improved by

raising the dyeing temperature once a pH of 11–12 is reached. Further increase in pH

will reduce the reaction rate as well as the efficiency of fixation. Different types of

alkalis, such as caustic soda, soda ash, sodium silicate or a combination of these



alkalis, are used in order to attain the required dyeing pH. The choice of alkali

usually depends upon the dye used, the dyeing method as well as other economic and

technical factors.

Electrolyte: The addition of electrolyte results in an increase in the rate and extent of

exhaustion, increase in dye aggregation and a decrease in diffusion. The electrolyte

efficiency increases in the order: KCl < Na2SO4 < NaCl [191]. There may be impurities

present in the salt to be used, such as calcium sulphate, magnesium sulphate, iron,

copper and alkalinity, that can be a source of many dyeing problems [192].

Liquor ratio: At lower liquor ratios, there is a higher exhaustion [193] and higher

colour strength. An increase in colour strength may be attributed to greater availability

of dye active species in the vicinity of the cellulose macromolecules, at lower liquor

ratio.

Surfactants and other auxiliaries: It is possible to enhance dye uptake on cellulosic

fibres with the aid of suitable surfactants. Amongst all the systems, the highest dye

uptake is obtained with anionic surfactants [194]. Non-ionic surfactants may result in

a decrease in dye exhaustion and colour yield, and a change in shade. Some non-ionic

surfactants may slow down the dye hydrolysis [195]. Triethanolamine (TEA) is known

to enhance colour strength by enhancing the swellability and accessibility of the

cellulose structure. It may also modify the state of the dye, thereby enhancing its

reactivity and increasing the extent of covalent dye fixation.

12.3 Important Characteristics of Reactive Dyeings

The best guide to the dyeing performance of a reactive dye can be obtained from two

sources of information: the SERF profile and migration properties under application

conditions. The SERF profile is constructed by the determination of substantivity

factor, exhaustion factor, fixation percentage and rate of fixation. The performance of

a reactive dye can also be defined by the Reactive Dye Compatibility Matrix (RCM)

[196, 197]. The critical measures of performance are the substantivity equilibrium

(S), the migration index (MI), the level dyeing factor (LDF) and an index of the

reactivity of the dye (T50). Evaluation of these four measures of performance provides

a measure of the compatibility of the dye to provide right-first-time production. Right

first-time production is maximised if these fundamental measures of performance

within the RCM are set at:

Substantivity 70–80%

Migration index >90

LDF >70%

T50 a minimum of 10 minutes

In the following, some important characteristics of reactive dyeings, namely exhaustion,

migration, levelness, fixation and colour yield, washing-off, dye-fibre bond stability,

and fastness properties will be discussed.

Exhaustion: There are two types of exhaustion that relate to the application of reactive

dyes: primary exhaustion and secondary exhaustion. Primary exhaustion occurs before



the addition of the alkali, while secondary exhaustion takes place after the addition

of the alkali. Both the rate of exhaustion and the extent or degree of exhaustion are

important. The rate of exhaustion can be increased by selecting dyes of high substantivity,

increasing the temperature and increasing the electrolyte concentration. The degree

of exhaustion can be increased by selecting dyes of high substantivity, lowering the

temperature and increasing the electrolyte concentration.

Migration: The intrinsic properties of a reactive dye that affect migration are

substantivity, molecular structure, physical chemistry and stereochemistry. The higher

the dye substantivity, the lower is the migration. The external factors that affect

migration are: concentration of the dye, temperature, time, liquor ratio, liquor circulation

and the form of the textile material.

Levelness: Levelness of dyeing may be inhibited by high substantivity, lower dye

migration [198], too much salt in the dyebath [199], too high rate of exhaustion, too

high concentration of alkali [200], a rapid shift of dyebath pH, too high rate of

fixation, too high rate of rise of temperature [201] and poor liquor agitation. Levelling

is difficult to obtain in light shades and easier to obtain in dark shades. Addition of

salt in portions is recommended for light shades while for deep shades, salt can be

added all at one step.

Levelness can be achieved in two ways [202]: either by controlling the rate of

absorption so that a controlled absorption is obtained, or by using the migration

properties of the dyes to compensate for the unlevelness that has occurred during the

early stages of the process. Controlled absorption can be obtained by salt dosing,

alkali dosing, and/or controlling the rate of heating. During the primary exhaustion,

the dye is free to migrate. During the secondary exhaustion stage, dye migration is

poor. For pale dyeing shades (less than 1 % o.w.f.) the degree of primary exhaustion

is over 80% and the degree of secondary exhaustion is very small. Therefore control

of the primary exhaustion stage is very important if level dyeing is to be obtained.

The rate of primary exhaustion is dependent on the amount of electrolyte used.

Dosing or split addition of salt is recommended to obtain level dyeing. For medium

shades, both primary and secondary exhaustion steps are important for obtaining

level dyeing. Both controlled salt and alkali addition are important in this case. In the

case of deep shades, the all-in salt addition may be possible, but during the secondary

exhaustion, alkali dosing is important [203]. Dyes with high substantivity, low secondary

exhaustion, and low MI (Migration Index) values require controlled addition of

electrolyte after the addition of the dye. In contrast, dyes with low substantivity, high

secondary exhaustion, and medium to high migration index values require precise

control of liquor ratio, concentration of electrolyte, and addition profile of the fixation

alkali [204]. Table 8 gives a comparison of two different approaches to achieve level

dyeing.

Fixation and colour yield: The fixation and the colour yield depend upon the following

factors [205]:

• Fibre cross-section

• Porosity of the substrate



• Dye structure with respect to substantivity ratio, dye diffusion, reactivity, etc.

• Degree of fibre preparation

• Liquor ratio

• Concentration of salt and alkali

• Use of reaction catalyst

• Use of dye–fibre cross-linking agents

• Introduction of other chemical groups in the fibre

• Use of film-forming agents

• Chemical modification of cellulose

• After treatments

There are various ways to increase fixation and colour yield which include:

• Use of fixation accelerators

• Use of shorter liquor ratio

• Dyeing at low temperature (with decreasing temperature the substantivity for

fibre increases, causing increased exhaustion)

• Modification of chromophore and reactive group

• Use of dyes with high substantivity and high reactivity

• Treating cellulosic fibres with swelling agents

• Modification in appearance techniques

• Changing the morphology of fibre by chemical modification.

A uniform rise in rate of fixation can be obtained by: controlling the temperature of

the dyeing process suitably (possible for hot dyeing dyes only); adding alkali in

stages (it is virtually impossible, however, to prevent a sharp rise in fixation rate

whenever alkali is added); starting with a weaker alkali such as soda ash, and following

this with a stronger alkali, but only after a higher degree of fixation has been achieved;

progressive metering of alkali (such as the Remazol automet process); and adding salt

in stages (suitable for high substantivity dyes).

Washing-off of reactive dyes: The removal of unfixed dye takes place in three phases

[206]: dilution of dye and chemicals in solution and on the surface of the cellulose;

diffusion of the deeply-penetrated, unfixed, hydrolysed dye to the fibre surface; and

dilution and removal of the diffused-out dye. Goods are rinsed cold twice to remove

electrolyte, then rinsed hot to desorb some hydrolysed dye from the fibre prior to a

‘soaping process’ at or near the boil. A subsequent cold rinse completes the task of

Table 8 Ways to Obtain Level Dyeing

Control of Levelling Based on Migration Control of Levelling Based on ControlledAbsorption

A relatively low level of control may be A very good level of control is necessary tosufficient to get level dyeing get level dyeingPoor reproducibility Better reproducibilityPoor colour yield Better colour yieldDye additions or corrections may have Less need of additions and correctionsto be made



removing un-reacted and hydrolysed dye [207]. The factors which affect the washing

off of hydrolysed reactive dyes from the dyed material are as follows [208–212]:

• Dye substantivity

• Diffusion behaviour

• Reactive group

• Liquor ratio

• Washing temperature

• Electrolyte concentration

• pH

• Presence of calcium and magnesium ions in the ‘boiling soap’/hardness of water

• Liquor carry-over of the substrate

• Amount of unfixed dye

• Washing time

• Number of washing cycles/washing baths [213]

• Washing auxiliary employed

• Mechanical action

• Filling and draining

• Heating and cooling rates

Dye–fibre bond stability: Dye–fibre bond stability primarily depends upon the reactive

system. Dyes that react by a nuceophilic displacement mechanism show good stability

to alkali and, to different degrees, less stability to acid. Dyes that react by nucleophilic

addition give dye–fibre bonds with good stability to acid, but are less stable to alkali.

One of the most stable dye–fibre bonds is achieved with pyrimidinyl-based systems.

The triazine–cellulose bond is generally resistant to oxidative breakdown in the

presence of perborate, whereas this is a serious defect of some of the pyrimidine-

based systems. Dye–fibre bonds formed by monochlorotriazine dyes are less fast to

alkali (particularly at high temperature) than those formed between dichlorotriazinyl

dyes and cellulose. Vinyl sulphone dyes possess the same deficiency, but their higher

reactivity enables the problem to be avoided by the use of milder fixation conditions.

In case of pyrimidine dyes, the dye–fibre bond is more stable than in either of the

above two cases [214].

Fastness of reactive dyes: The factors that affect the fastness of reactive dyes are: the

chromophoric group, the stability of the dye–fibre bond and the completeness of the

removal of the unfixed dye. To maximise wet fastness, particularly in deep shades, it

is advisable to apply cationic after-treatments.

A summary of problems in dyeing with reactive dyes is given in Appendix I.

13. PROBLEMS IN DYEING WITH DIRECT DYESDirect dyes represent an extensive range of colorants that are easy to apply and also

are very economical [215–217]. There are three common ways to classify direct

dyes, namely, according to their chemical structure [218], according to their dyeing

properties, and according to their fastness properties. Of these three possible ways of

classifying direct dyes, the first is of least importance to the dyer, although of

considerable importance to those interested in dye chemistry [219]. According to the



Society of Dyers and Colourists’ classification, which is essentially based upon the

compatibility of different groups of direct dyes with one another under certain conditions

of batch dyeing, there are three classes of direct dyes: A, B and C. Class A consists

of self-levelling direct dyes. Dyes in this group have good levelling characteristics

and are capable of dyeing uniformly even when the electrolyte is added at the beginning

of the dyeing operation. They may require relatively large amounts of salt to exhaust

well. Class B consists of salt-controllable dyes. These dyes have relatively poor

levelling or migration characteristics. They can be batch dyed uniformly by controlled

addition of electrolyte, usually after the dyebath has reached the dyeing temperature.

Class C consists of salt- and temperature-controllable dyes. These dyes show relatively

poor levelling or migration and their substantivity increases rapidly with increasing

temperature. Their rate of dyeing is controlled by controlling the rate of rise of

temperature, as well as controlling the salt addition.

Important dyebath variables that influence the dyeing behaviour of direct dyes

include temperature, time of dyeing, liquor ratio, dye solubility, and presence of

electrolyte [220] and other auxiliaries.

Direct dyes can be applied by batch dyeing methods (on jigs, jet or package

dyeing machines), by semi-continuous methods (such as pad-batch or pad-roll) and

by continuous methods (such as pad-steam). Many direct dyes are suitable for application

by combined scouring and dyeing. In this process the usual practice is to employ soda

ash and non-ionic detergent. However, dyes containing amide groups are avoided

because of the risk of alkaline hydrolysis.

Direct dyes vary widely in their fastness properties, and staining effects on various

fibres. Most direct dyes, however, have limited wet fastness in medium to full shades

unless they are after-treated. The fastness of selected direct dyes can be improved in

several ways [221–224], such as the following:

• Treatment with cationic fixing agents

• Treatment with formaldehyde

• Treatment with copper salts such as copper sulphate

• Treatment with cationic agents and copper sulphate in combination

• Diazotisation and development

• Treatment with crosslinking agents or resins

An important consideration in dyeing with direct dyes is the ability of the dyes to

cover the immature cotton fibre neps, which has been explained, in most cases, in

terms of both the molecular weight and hydrogen bond formation capacity of the dye

molecules [225–227]. Given a similar capacity to form hydrogen bonds, dyes having

lower molecular weight show proportionately better nep coverage than those having

higher molecular weight. Table 9 gives Colour Index number of dyes with better

coverage of immature fibres [228].

A summary of common problems in the dyeing of cotton with direct dyes is given

in Appendix I.

14. PROBLEMS IN DYEING WITH SULPHUR DYESDespite their environmental concerns, which are constantly being addressed [229–

234], sulphur dyes occupy an important place for dyeing of inexpensive black, blue,



brown and green shades in medium to heavy depths on cellulosic fibres [235, 236].

The history, development and application of sulphur dyes have been widely reviewed

by various authors [237–248]. Sulphur dyes have been classified into four main

groups [249]: CI Sulphur dyes; CI Leuco Sulphur dyes; CI Solublised Sulphur dyes;

and CI Condensed Sulphur dyes. CI Sulphur dyes are water-insoluble, containing

sulphur both as an integral part of the chromophore and in attached polysulphide

chains. They are normally applied in the alkaline reduced (leuco) form from a sodium

sulphide solution and subsequently oxidised to the insoluble form on the fibre. Sulphur

dyes differ from the vat dyes in being easier to reduce but more difficult to re-oxidise,

different oxidants producing variations in hue and fastness properties. A leuco sulphur

dye has the same CI constitution number as the parent sulphur dye but exists as the

soluble leuco form of the parent dye together with a reducing agent in sufficient

quantity to make it suitable for application either directly or with only a small addition

of extra reducing agent. A solublised sulphur dye has a different constitution number

because it is a chemical derivative of the parent dye, non-substantive to cellulose but

converted to the substantive form during dyeing. Condensed sulphur dyes, although

containing sulphur, bear little resemblance to traditional sulphur dyes in their constitution

and method of manufacture. Sulphur dyes are available in various commercial forms

such as powders, pre-reduced powders, grains, dispersed powders, dispersed pastes,

liquids, and water soluble-brands.

The various steps in the application of sulphur dyes depend very much on their

type and commercial form. The main steps in the application of water-insoluble

sulphur dyes are as follows:

• Reduction, whereby the water-insoluble dye is converted into water-soluble form

• Application, whereby the solubilised dye is applied onto the substrate by a suitable

exhaust or continuous method

• Rinsing, whereby all loose colour is removed before the oxidation stage

• Oxidation, whereby the dye absorbed by the substrate is oxidised back into

water-insoluble form, and

• Soaping, which results in an increase in brightness as well as improved fastness

of the final shade

Various application methods for sulphur dyes, along with suggested recipes, have

been discussed in [243, 245, 246, 249, 250].

Table 9 Colour Index Number of Dyes with Better Coverageof Immature Fibres (Numbers in Brackets HaveLower Overall Coverage than Others)

Colour Colour Index Number

Yellow 7, 11, 27Orange (1, 15, 37, 102)Red 32 (20, 24, 76)Violet 9, 22, 66Blue 8, 26, 27, 98Green (1, 26)Brown 25, 29Black 3, 22, 39



The auxiliaries used in sulphur dyeing are: reducing agents, antioxidants, sequestering

agents, wetting agents, oxidising agents and fixation additives. The two most important

reducing agents for sulphur dyes are sodium sulphide [Na2S] and sodium hydrosulphide

[NaHS]. Caustic soda/sodium dithionite are conventional chemicals for vat dye reduction

but this system is difficult to control in the application of sulphur dyes and tends to

give inconsistent results except with certain sulphur vat dyes. A sodium carbonate/

sodium dithionite mixture is too weakly alkaline for the water-insoluble type sulphur

dyes and requires careful control if over-reduction and consequent low colour yield

are to be avoided. Glucose in the presence of alkali, usually caustic soda or a caustic

soda/soda ash mixture, has been used as another possible sulphur dye reducing agent,

but it is a weak reducing agent as compared to sodium sulphide or sodium hydrosulphide.

Other reducing agents such as thioglycol, hydroxyacetone and thiourea dioxide, have

had limited success. Sodium polysulphide and sodium borohydride can be used as

antioxidants to inhibit premature oxidation, promote better dyebath stability and

lessen the risk of bronzing, poor rubbing fastness and dark selvedges. Sequestering

agents are used where water quality is poor or variable, to avoid poor rubbing fastness

or unlevelness in the presence of multivalent ions in the dye liquor or in the substrate.

Wetting agents may be used to improve the wettability of the substrate. Although the

majority of sulphur dyes are unaffected by most wetting agents, some non-ionic

wetting agents may inhibit the dye uptake in exhaust dyeing or precipitate the dye as

a tarry leuco product.

Traditionally, the most preferred oxidising system has been sodium dichromate/

acetic acid because of its ability to rapidly and completely oxidise all reduced sulphur

dyes, resulting in good colour yield and fastness properties. Nevertheless, it has been

criticised increasingly on environmental grounds, and for its effects on handle and

sewability, especially with sulphur blacks. The addition of 1 g/l copper sulphate to

batchwise oxidation baths of sodium dichromate/acetic acid improves the light fastness

but may result in dulling of the shades, as well as harsher handle. It is not recommended

with sulphur blacks, where the presence of copper promotes acid tendering. Other

oxidising agents that have been tried as alternatives to sodium dichromate/acetic,

with various degrees of success, include [251, 252]: potassium iodate/acetic acid;

sodium bromate; hydrogen peroxide and peroxy compounds; and sodium chlorite.

Fixation additives, such as alkylating agents based on epichlorohydrin, give dyeings

of markedly improved washing fastness but often at the risk of some decrease in light

fastness. Moreover, in the event of the dyeing needing subsequent correction, alkylated

sulphur dyeings are difficult to strip and attempted removal will often entail destruction

of the dye chromogen.

Two special problems in dyeing with sulphur dyes are acid tendering and bronziness.

In severe conditions of heat and humidity, some sulphur dyeings, notably black, can

generate a small amount of sulphuric acid within the cellulosic fibres, leading to

tendering. AATCC Test Method 26-1994 (Ageing of sulphur dyed textiles) can be

used to determine whether the sulphur dyed textile material will deteriorate under

normal storage conditions [253]. Bronziness and other problems in sulphur dyeing

and their possible causes are summarised in Appendix J.



15. PROBLEMS IN DYEING WITH VAT DYESVat dyes remain the primary choice where the highest fastness to industrial laundering,

weathering and light are required [254]. Several primers [255–257] and reviews have

been published on progress in their development [258–265], and their application by

batch [266–270] as well as by continuous processes [271, 272]. This section gives

briefly some fundamentals of vat dyeing and reviews various problems in the dyeing

of cotton with vat dyes in an endeavour to consolidate the previous work done in this

regard [273–276].

Vat dyes are insoluble pigments, available in different forms [277]. Based on the

temperature and the amount of caustic soda, hydrosulphite and salt used in dyeing,

vat dyes can be classified into four main groups [278]: IN dyes require high temperature

and a large amount of caustic soda and sodium hydrosulphite; IW dyes require medium

temperature and a medium amount of caustic soda and sodium hydrosulphite with

salt added; IK dyes require low temperature and a small amount of caustic soda and

sodium hydrosulphite with salt added; and IN Special dyes require more caustic soda

and higher temperature than IN dyes. Generally, vat dyes have a very rapid strike, a

good degree of exhaustion and a very low rate of diffusion within the fibre. Vat dyes

of different chemical structure may differ in the solubility of their sodium leuco-vat,

stability towards over-reduction, stability towards over-oxidation, substantivity and

rate of diffusion. Commercial competitive dyes have fairly equal particle sizes. Large

particle sizes give dispersions of poor stability. For some vat dyes, colour yield

decreases with increasing particle size. The effect is generally dye-specific [279].

The main stages in the dyeing of cotton with vat dyes are as follows:

• Conversion of insoluble vat pigment into soluble sodium leuco-vat anions

[reduction]

• Diffusion of sodium leuco-vat anions into cellulosic fibres

• Removal of excess alkali and reducing agents by washing off

• Oxidation of the soluble dye into insoluble pigmentary form within the cellulosic

fibres

• Soaping, during which the isolated molecules of vat pigments are re-orientated

and associate into a different, more crystalline form

Important requirements of vat dye reducing agents are a level of reducing power

(reduction potential) sufficient to reduce all commercial vat dyes to their water-

soluble form quickly and economically, and conversion of the vat dyes into products

from which the original pigment can be restored (no over-reduction). Various reducing

systems for vat dyes have been proposed and used [280–282]. The most common

type of reducing agent used for dyeing with vat dyes is sodium hydrosulphite, commonly

known as hydros but more correctly known as sodium dithionite, which has the

chemical formula Na2S2O4. Although a part of the hydros is used up in the reduction

of vat dyes, a large part of it may be destroyed by its reaction with oxygen in the air

(oxidation), particularly at higher temperatures. The rate of reduction of vat dyes

depends upon various factors, such as the particle size of the dye, the temperature,

time and pH during reduction and access of the reducing agent. The stability of

alkaline solutions of reducing agents may decrease with increased temperature, greater

exposure to air, greater agitation and lower concentration of the reducing agent. Vat



dyes of the indanthrene type may produce duller or greener shades at dyeing temperatures

higher than 60 °C, due to over-reduction. Over-reduction can be prevented by the use

of sodium nitrite if the reducing agent is hydrosulphite. In the case of thiourea oxide,

over-reduction cannot be prevented by nitrite.

The factors influencing the rate of dyeing with vat dyes include the type of substrate,

temperature, liquor ratio and concentration of dye and electrolyte. Mercerized cotton

gives a higher rate of dyeing compared with un-mercerized cotton, which in turn

gives a higher rate than grey material. At low temperature, the rate of exhaustion is

low, which might promote levelness but the rate of diffusion is also low. At high

temperature, the rate of exhaustion is high, which might decrease levelness but the

rate of diffusion is high. Maximum exhaustion, penetration and levelness can be

obtained by starting the dyeing at low temperatures in the leuco stage and slowly

raising the temperature. Some dyes may not be stable to very high temperatures, so

the stability of dyes to temperature must be taken into account. The reducing efficiency

of sodium hydrosulphite in caustic soda solutions at high temperatures decreases

rapidly in the presence of air. The higher the liquor ratio, the slower is the rate of

dyeing. Most of the dyes exhaust more rapidly at low concentrations, increasing the

risk of unlevel dyeing in light shades. Some have the same rate of dyeing irrespective

of the concentration. The higher the concentration of electrolyte, the higher is the rate

of dyeing.

The purpose of rinsing before oxidation is to remove any loose dye, excess of

reducing agent and alkali to lower the pH and establish conditions favourable for

oxidation. The higher the temperature and/or pH of the rinsing bath, the lower is the

colour strength. Very high pH and temperature during rinsing may also result in

dulling of the shade. The ideal is to do rinsing thoroughly at low temperature at a

rinsing bath pH value of 7.

The purpose of oxidation is to convert the water-soluble leuco form of the vat dye,

back into the insoluble pigment form. Important variables for the oxidizing step are

the type and concentration of oxidising agent, the type of pH regulator and pH during

oxidation, and temperature during oxidation. The oxidizing agent must provide a

level of oxidation potential sufficient to oxidize the reduced vat dye into insoluble

pigment, with no over-oxidation, i.e. beyond the oxidation state of the original

pigmentary form of the dye. Some criteria for selecting oxidising agents and a

comparison of different oxidising agents are given by Tigler [283]. Poor control of

pH during oxidation may result in uneven oxidation and a lower temperature may

result in slower oxidation. A pH below 7.5 should be avoided to prevent the possible

formation of acid leuco forms of vat dyes. The optimum pH for oxidation is 7.5–8.5.

The acid leuco form of vat dye is difficult to oxidize, has little affinity for fibre and

is easily rinsed out. The higher the temperature, the faster is the oxidation, the

optimum temperature being 120–140 °F.

The purpose of soaping after oxidation is to remove any dye that is not diffused

into the fibre and to stabilise the final shade. This results in improved fastness properties

and resistance to any shade change caused by a resin or other finish, or to consumer

use. Important soaping parameters are time, temperature and type and concentration

of soaping auxiliaries. Even when no detergent is used, the dyeings exhibit good

colour strength and good fastness properties. Washing with water alone tends to give



a slightly higher colour yield. It is best to carry out soaping without any detergent at

boiling temperature [284]. After soaping, the fabric is rinsed and dried.

Both exhaust and continuous dyeing methods are used to apply vat dyes. Exhaust

dyeing processes are mainly used for dyeing of loose stock, yarn and knitted fabrics

[285]. Woven fabrics can also be dyed by the exhaust method but for large batch

sizes, the continuous method is mostly uses.

Pad dyeing methods [286, 287] are usually a preference in the case of woven

fabrics, particularly if these are in large batches. The commonly used pad dyeing

methods are pad-jig, pad-steam and pad-Thermosol [288]. The most popular method

for dyeing woven fabrics in a continuous manner is the pad-dry-pad-steam method

[289–292], consisting of the following key steps:

• Impregnating the fabric in a bath containing vat dye, dispersing agent, anti-

migrant and a non-foaming wetting agent

• Squeezing the impregnated fabric to a given pick-up level

• Drying the fabric to achieve a uniform distribution of the vat pigment throughout

the fabric

• Impregnating the fabric with a solution of caustic soda and sodium hydrosulphite,

with the optional use of salt

• Expressing the impregnated fabric to a given pick-up level

• Steaming the fabric to bring about reduction of the dye to the soluble leuco form

and to promote diffusion of the dye into the cellulosic fibres

• Rinsing, oxidation, soaping, rinsing and drying the fabric

Intermediate drying is one of the most important steps in the pad-dry-pad-steam

process where the most common problem, ‘migration’, can take place [293, 294].

Important factors on which migration depends are: dye constitution; dye formulation;

pick-up; additives in the dye padder; residues of wetting agents and lubricants on the

fabric; fabric structure; and drying conditions. After drying, the fabric is padded with

an alkaline solution of sodium hydrosulphite, after which the fabric undergoes steaming.

Almost 40 % of vat dyeing problems are related to improper steaming conditions

[295]. Ideal steaming conditions are controlled temperature and moisture [296], freedom

from air [297], and sufficient dwell time. After steaming, the fabric undergoes rinsing,

oxidation and soaping.

The most important control steps in vat dyeing are reduction, absorption and

oxidation. The reduction and oxidation can best be controlled by metered addition of

chemicals [298]. The advantages of metered addition of hydrosulphite [299, 300] are

as follows:

• Better levelling by slower vatting

• No need of levelling agent

• Protection from over-reduction

• Control of initial rate of dyeing (strike)

• Possibility of warm pre-pigmentation to give optimum fabric/liquor movement

• Good reproducibility

• Reduction of sulphite/sulphate effluent pollution

• Automatic monitoring of vat state and redox potential by means of measuring

and regulating technology



Controlled dosage of hydrogen peroxide in the oxidation tank, together with the

measurement and control of pH can result in obtaining sufficient peroxide for the

oxidation of the dye as well as achieving an optimised dyeing procedure due to the

control of speed of oxidation [301].

A summary of problems in dyeing with vat dyes is given in Appendix K.

16. PROBLEMS IN DYEING WITH AZOIC DYES

Azoic dyes are the least commonly used dyes for dyeing cotton materials due to

disadvantages such as their complicated and time-consuming application procedure

[302], the limitations of hue selection and difficulties in calculating recipes [303].

Azoic dyes, unlike other dyes, are formed directly within the fibres by reacting

suitable diazo and coupling components inside the fibre [304]. The most commonly

used method for dyeing cotton materials with azoic dyes consists of treatment with

naphthol, intermediate treatment, development and after-treatment [305, 306].

Naphthols are insoluble in water, but their sodium salts (naphtholates) are water-

soluble or can be prepared as stable dispersions. When cotton is immersed in the

naphtholate solution, exhaustion takes place according to the substantivity of the

naphtholate ion. Low substantivity naphthols are mainly used for application by the

continuous method [307]. Naphthols of moderate substantivity can be applied by

both continuous or batch methods, adjusting the application conditions appropriately.

Naphthols of high substantivity are particularly suitable for batchwise methods.

The main components of a naphtholate solution are: surfactants, caustic soda,

formaldehyde and common salt or Glauber’s salt. The caustic soda converts the

insoluble naphthol into a water-soluble naphtholate. The presence of formaldehyde,

together with excess of caustic soda, provides good protection against the formation

of free naphthol, which may develop on exposure to atmospheric carbon dioxide or

in acid steam. The addition of electrolyte increases the exhaustion of naphthols and

is recommended for long-liquor applications but not in continuous methods where

high substantivity is undesirable. The exhaustion of naphthol decreases with increase

of temperature, so batchwise application is normally carried out at 20–30 °C, and

sometimes at 50 °C when improved wetting and penetration is necessary.

The treated substrate contains absorbed naphtholate as well as loosely-retained

naphtholate present only on the surface. It is imperative to remove this loosely-

retained naphtholate in order to achieve maximum fastness. The amount of loosely-

retained naphtholate can be reduced by hydroextraction, suction, squeezing, wringing

and/or rinsing, depending upon the form of the material. In continuous dyeing it is

advantageous to express the water on a pad mangle and then pass the fabric into a

hot-flue dryer. The naphtholated substrate is sensitive to light and, if the treated

substrate is exposed much to light before development, it may result in unlevelness

and change in the hue on subsequent development.

After naphtholation and intermediate treatment, the material is passed into a

development bath containing a dilute solution of a diazonium salt. This is produced

either by diazotization of a fast colour base, or by dissolving a fast colour salt. The

majority of fast colour salts result in developing baths of correct pH but, if required,

acetic acid is used to adjust the pH of the developing bath. The applied concentration of

a fast colour base or salt is related to the applied depth of naphthol and the liquor ratio.



After-treatment includes rinsing, acidification after development, and alkaline soaping

followed by rinsing. The after-treatment eliminates loose dyestuff and the residues of

the development component. Moreover, the final colour shade and optimised fastness

properties are achieved [308, 309].

A summary of problems in dyeing with azoic dyes is given in Appendix L.

17. POOR REPRODUCIBILITY IN DYEING OF COTTON

Poor shade reproducibility is one of the main causes of loss in productivity and

profits. It frequently occurs when a shade does not repeat properly and requires

corrective action such as dye or chemical additions, extra run time, boiling down,

stripping, re-dyeing and/or over-dyeing. Corrective/repair procedures require extra

time and processing, increasing the risk of physical damage. Practices such as stripping

or additions, increase also the risk of uneven dyeing and bath instability. All this not

only adds to the cost of production but also causes loss in productivity and profits.

Table 10 gives an idea of the relative increase in cost and loss in productivity and

profits for corrections over a right-first-time dyeing [197]. Just a small corrective

addition may entail up to a 10% increase in cost and a 20% decrease in productivity,

and slash the profits to about a half as compared to the right-first-time dyeing.

Table 10 The Cost of Non-conformance

Process Cost Productivity Profit

Blind dyeing (RFT) 100 100 100Small addition 110 80 48Large addition 135 64 –45Strip and redye 206 48 –375

There are three key areas where a good degree of reproducibility is a pre-requisite to

avoiding extra costs of dyeing, loss in productivity and, thereby, loss in profits:

(i) A recipe should give the same shade every time it is repeated in the lab (within

lab reproducibility or lab-to-lab reproducibility)

(ii) A recipe developed in the lab should give the same shade in the bulk dyeings as

it gives in the laboratory dyeings (lab-to-bulk reproducibility)

(iii) A recipe should give the same shade, all the time it is repeated in the bulk

dyeings

In order to attain the desired degree of reproducibility, there are quite a large number

of factors that must be taken into account, thoroughly observed and carefully controlled.

It might be convenient to describe these factors with reference to ‘materials and

inputs’, ‘machinery and equipment’, ‘process conditions’, and ‘methods and practices’.

Table 11 gives a summary of these factors [310–321].

Table 12 gives factors affecting dye selection and evaluation [311].

Table 13 shows the factors for reproducibility that can be monitored by lab checks

and those that can be controlled by standard operating procedures (S.O.P’s).

Some routinely performed tests for the evaluation of dyes are: moisture content;

strength as measured by reflectance values of dyeings or transmission of dye solutions



Table 11 Factors Affecting Reproducibility and Right-First-Time Dyeing

Substrate– Quality/characteristics of

cotton– Quality/characteristics of

yarn– Pre-treatment– Absorbency– pH– Residual alkalinity– Residual peroxide– Whiteness/colorimetric

coordinates– Dyeability– Validity with respect to

database– Moisture content– Conditioning– Weight

Dyestuff– Selection of dyes– Standardisation of dyes– Source of the dye sample– Moisture content of dyes– Strength of dyes– Weight of dyes– Adulteration of dyes/

impurities in dyes– Sensitivity of dyes to

changes in processconditions

– Compatibility of dyes– Reactivity of dyes– Distance of the dye colour

from the target colour– Number of dyes in the recipe– Distance of the colour to be

matched and the colour ofthe dye used in the recipe

– Metameric index of therecipe

Auxiliaries– Types of auxiliaries– Strength of auxiliaries– Impurities in auxiliaries– Amount/weight of

auxiliaries

Water– Impurities in water supply– Volume of dyebath

Steam– Impurities in steam supply

Process Conditions– Liquor to goods ratio– Fill water temperature– Fixation temperature– Rate of rise of temperature/

temperature gradient– Concentration of dye,

electrolyte, alkali and otherauxiliaries

– Addition profile of dye– Addition profile of

electrolyte/salt dosing– Conductivity– Addition profile of alkali/

alkali dosing– Fixation pH– Addition profile of

auxiliaries– Time (total time; before and

after the addition ofelectrolyte; before and afterthe addition of alkali; beforeand after the addition offixative or any otherauxiliary)

– Load size– Liquor level– Machine flow and liquor

reversal sequence– Method/conditions of

washing-off– Method/conditions of drying

Machine and Equipment– Leaking valves: steam, drain– Circulating pump or heat-

exchanger performance atoperating temperature

– Location and integrity oftemperature sensor

– Location and evenness ofsteam injection for heating

Accuracy and calibration of:– Pressure indicators and

controller– Flow indicators and

controller– Level gauge– Temperature indicator and

controller– Weighing balances– Spectrophotometer: inter

and intra instrumentcalibration

– Glassware such as pipettes,beakers, etc.

Methods and Practices– Frequent change of suppliers– Spurious supply of dyes– Improper storage of dyes– Improper labelling of dyes– Accuracy of weighing– Improper location of

balance, where there isturbulence

– Loss of the dye in the panof the weighing balance

– Spillage of solid dye priorto dissolution or after

– Cross-contamination ofvessels/materials

– Age of the dye solutions– Selection of wrong method

for dye strength evaluation– Blowing-out pipettes– Improper colour preparation– Calculation errors– Accuracy of dye recipe

formulation– Dispensing methods for

dyes and chemicals– Auxiliaries taken on the

weight of the fabric– Improper substitution of

Glauber’s salt with commonsalt

– Dye application method– Manner of drying the sample

for colour assessment– Conditioning of the sample

before colour assessment– Target shade too small or

soiled– Target shade for textiles in

paper/plastic– Dots/fluff in the area

scanned– Colour judgment– Type of colorimeter and

formula used– Database preparation for

computer colour matching– Make-up and geometry of

specimen–homogeneity,geometry and thickness

– Post dyeing operations– Poor housekeeping– Lack of training/

understanding, negligence,wrong attitude, wrongpractice



[322]; paper or thin layer chromatography; build-up test; pH sensitivity test; reactive

dye fixation; thermo migration; strike-migration test; SDC migration test; temperature

strike test; dusting [323]; solubility and solution stability [324]; electrolyte stability

of reactive dyes [325]; cold water solubility [326]; coverage properties; and fastness.

A good quality-control scheme for dyes usually consists of [311]: OSHA Form 20;

Table 12 Factors Affecting Dye Selection and Evaluation

Standardisation• Homogeneity• Absorption in solution• Analysis and identification

Storage stability• Variation in moisture content• Storage conditions

Solubility and physical form• Aqueous solubility• Crystal modification• Particle size• Commercial form

Health and safety• Dustiness• Trace metals• Eye and skin irritation• Acute toxicity• Long-term hazards• Biodegradation• Sludge adsorption• Fish toxicity

Cost-effectiveness• Shade area• Colour value• Build-up reproducibility

Dye application properties• Levelling and migration• Substantivity and diffusion• Reactivity and fixation• Sensitivity to temperature• pH and redox potential• Compatibility• Cross-staining• Transfer and vapour pressure• Efficiency of wash-off

In-service requirements• Coverage• Penetration• Fastness• Tendering of substrate• Influence of finishes

Table 13 Factors that can be Monitored by Lab Check and Those Controlled by Standard OperatingProcedures

Factor Monitor by Lab Check Control by SOP

Water quality XSubstrate dyeability XSubstrate preparation XSubstrate XDye selection XDye combination XDye moisture content XDye standardization XDye and chemical weighing Xand dispensingDyebath additives XLiquor ratio XpH XMachine flow and reversal XTime/temperature profile XColour assessment method X



manufacturer’s technical data; physical standards; satisfactory laboratory evaluation;

retention of samples for future references; and proper documentation.

Table 14 presents the permissible limits of variation of some of the factors for

reproducibility [314]. Some other limits of accuracy are given in Table 15 [314]

Table 15 Limits of Accuracy for Right-First-Time Dyeing

Factor ∆E (JPC 79)

Matching tolerance 0.3–0.5Cotton variability in dyeing 2.0Variability in water supply 3.0Instability of dye solution 3.0–5.0Computer prediction <1.0Variation in weighing of 5% 2.5Repeat knitting or card-wrapping 0.15for assessmentBatch levelness 0.2Lab dyeing reproducibility 0.8 (trained technician with class A glassware)

0.2 (untrained operative with automation)Spectrophotometer reproducibility 0.05–0.2

Table 14 Variation Permissible to Achieve Reproducibilityto Within ∆E 1 Unit

Factor Variation

Moisture content of dye 3.5%Moisture content of substrate 0.5%Weighing of substrate 0.5%Weighing of dyes and chemicals <0.5%Dye standardization <2.5%pH of the dyebath 0.35 units

18. DYEING PROCESSES FOR COTTONThere are three main types of processes for the dyeing of cotton textile materials:

batch, continuous and semi-continuous. Batch dyeing is sometimes called exhaust

dyeing because the dye is gradually transferred from a relatively large volume dyebath

to the material being dyed over a relatively long period of time. The dye is said to

exhaust from the dyebath to the substrate. The choice of a dyeing process depends

upon many factors including type of material (fibre, yarn, fabric, fabric construction,

garment), size of dyeing lots and quality requirements in the dyed material [327].

Modern dyeing machinery for the dyeing of cotton materials is based on three

principles: (1) circulation of the dye solution through the fibre, (2) circulation of the

fibre through the dye solution and (3) padding the dye solution onto the fibre. The

machinery for the exhaust processes is based upon the principles 1 and 2. Package

and beam dyeing machines are based upon the first principle while beck, jigger and

jet dyeing machines utilize the second principle. Pad-steam, pad-Thermosol, and

pad-exhaust machines such as pad-jig are based on the third principle [328]. This

section briefly gives the most common processes for the dyeing of cotton textile



materials, which are: package dyeing (for yarn), jet dyeing (for knitwear), and jigger

and pad dyeing for woven materials.

The term package dyeing usually refers to the dyeing of yarn that has been wound

on perforated cores so that dye liquor can be forced through the package, which may

be a tube, cheese or cone type. The yarn packages are placed on perforated spindles

on a frame which fits into a pressure vessel where dyeing takes place. The dye

formulation is pumped through the perforations in the spindles and package cores

into the yarn. The flow of liquor can be either from inside-to-outside of the package

or outside-to-inside. Periodic reversal of the flow results in better levelness of the

dyeing. A heat exchanger using high pressure steam as the heat source heats the dye

liquor in a package dye machine. An earlier review of developments in package

dyeing has been given by Fleming and Gaunt [329]. Types of package dyeing machines

[330, 331] and later refinements in package dyeing have been reviewed by Turner

[332] and some recent progress has been given by Tsui [333]. The most important

dyeing parameters in a package dyeing machine are as follows [334]:

• Liquor differential pressure (in-out and out-in)

• Liquor flow rate

• Liquor volume and liquor ratio

• Liquor temperature

• Circulation pump speed

• Static pressure

• Dyestuff and chemical preparation conditions

• Injection times and sequence of dyes and chemicals

• Liquor preparation and transfer times and sequences from/to reverse tank

• Liquor heating and cooling gradient

• Dyeing cycle times and sequences

The parameters given above are inter-related and must all be controlled carefully for

optimum dyeing to be achieved. Package dyeing, in spite of being simple and controllable

[335], is very prone to unlevelness. Success in attaining a good degree of levelness

is very much a direct result of package density and other theoretical considerations

[336].

A jigger or jig consists of a trough for the dye or chemical liquor. Fabric from a

roll on one side of the machine is run through the liquor in the trough and wound on

a roll on the opposite side of the jig. When the second roll is full, the drive is reversed,

and the fabric is transferred through the liquor back to the first roll. Live steam

injected into the bottom of the trough through a perforated pipe across the width of

the jig heats the liquor. Some modern jigs also have heat exchangers for indirect

heating. Covering the top of the jig minimises the heat loss to the atmosphere, keeps

the temperature uniform on all parts of the fabric and minimises exposure of the

liquor and the cloth to air. Minimising exposure to air is important when using

sulphur or vat dyes since these dyes can be oxidized by atmospheric oxygen. Jigs

exert considerable lengthwise tension on the fabric and are more suitable for the

dyeing of woven than knitted fabrics. Since the fabric is handled in open-width, a jig

is very suitable for fabrics which crease when dyed in rope form. Some typical

problems that may be encountered in conventional jig machines are as follows:



• Temperature control from side-to-side and end-to-end of the roll

• Tension control from end-to-end

• Constant speed control from end-to-end

• Prevention of creases

• Prevention of air

Although these problems have been overcome by many manufacturers [337–345],

expert monitoring is required to obtain quality dyeing in jig dyeing.

Continuous dyeing is usually defined as a dyeing method where a relatively

concentrated dye solution is applied evenly across the entire width of the fabric

passing through it in a continuous manner. The application of colorant solution is

usually accomplished by padding but also may be done by other means. Padding is

followed by subsequent fixation of the dye by chemical or thermal means. Continuous

dyeing is predominantly used for woven fabrics. However, machinery is also available

for both open width and tubular knits. When processing knits, the fabric must be

subjected to low and uniform tension for maintaining the desired aesthetics. Padding

techniques must be altered to properly handle tubular knit goods because edge lines

can occur if good dye penetration is not obtained or if the hardness of the pad rolls

is not correct.

In the pad batch method, the fabric ready-for-dyeing is impregnated with dye

liquor, excess liquor is squeezed out on a mangle, the fabric is batched onto rolls or

held in boxes for 2–12 hours, and then covered with plastic film to prevent adsorption

of carbon dioxide from air or evaporation of water. Subsequently, the fabric is washed

off in any of the conventional ways, depending upon the available equipment.

Typical problems encountered in pad dyeing are lengthwise shade variation (also

called tailing or ending) [346–349] and widthways shade variation (also known as

listing or side-centre-side shade variation) [350–353]. The dyeing problems occurring

in a continuous dyeing range may be attributable to the dye padder, pre-drying, the

Thermosol unit, the chemical padder, the steamer, and the wash boxes [354, 355].

19. SUMMARYThis monograph describes various key stages for the manufacture of dyed cotton

materials and reviews possible problems introduced at each stage. Dyed cotton materials

are not produced in a straightforward one-step process but there are many processes

involved, each with a number of variations and each variation with a number of

variables. In addition, textile dyeing is characterized by a large number of variables,

each with a distinct degree of effect on the final outcome of the process. The assortment

of so many variables, as well as the inter-relation among these factors, makes right-

first-time dyeing quite demanding, and troubleshooting faulty dyeings even more

exacting and arduous.

By experience, a trouble-shooter can reduce the number of probable causes of

problems, but confirmation of the exact cause can be difficult. A best estimate,

possible through a process of elimination, requires answers to a series of questions

and/or actual laboratory tests. Although some of the defects can be analyzed by the

practical dyer, in many cases, they can be analyzed only by a special textile laboratory,

set-up for this purpose, with qualified personnel and special equipment. A satisfactory

diagnosis entails: a well-equipped testing laboratory, extensive experience in testing,



and expert knowledge about several textile processing stages, the interaction between

the process variables, and the structural features that determine the properties of the

material; as well as a knack of problem-solving.

Defects in dyed cotton materials can be attributed to innumerable causes ranging

from poor quality of fibre, faulty spinning, inappropriate yarn package formation,

improper weaving or knitting, impurities in water, poor standardisation of dyes and

chemicals, lack of control of the variables in the processes involved, machine

malfunctions to human errors. This monograph addresses the most common problems

in the dyeing of cotton textile materials in various forms. An overview of various

textile operations for cotton is given and various key stages and factors involved in

the production of dyed cotton textile materials are examined in detail and problems

originating at each stage are summarised. As quality requirements are becoming

more stringent in textiles, human expertise in such a specialized area as dyeing is

becoming more limited and expensive. We are aware that sufficient weight was not

given to all aspects of production and coloration, and additional detailed examination

of techniques would be required to understand the underlying cause of specific problems.

However, we hope that readers will find this monograph a useful source of information

for the troubleshooting of common problems in the dyeing of cotton-based textile

substrates.

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USEFUL TERMS AND DEFINITIONS

Affinity: The quantitative expression of substantivity. It is the difference between the

chemical potential of the dye in its standard state in the fibre and the corresponding

potential in the dyebath. Note: Affinity is usually expressed in units of calories

(joules) per mole. Use of this term in a qualitative sense, synonymous with substantivity,

is deprecated.

Batchwise processing: Processing of material as lots or batches in which the whole

of each batch is subjected to one stage of the process at a time. It is the opposite of

continuous processing.

Beam dyeing: Dyeing of textile material wound onto a hollow perforated roller

(beam) through the perforations of which dye-liquor is circulated.

Beck/winch: An open vessel, formerly made of wood or iron, nowadays of stainless

steel, for the wet processing of textile materials.

Bleaching: The procedure of improving the whiteness of textile material, with or

without the removal of natural colouring matter and/or extraneous substances, by a

bleaching agent.

Bleaching agent: A chemical reagent capable of destroying partly or completely the

natural colouring matter of textile fibres, yarns, or fabrics, and leaving them white or

considerably lighter in colour. Examples are oxidizing and reducing agents. Amongst

the former, hydrogen peroxide is widely used.

Chromophore: The part of a molecular structure of an organic dye or pigment

responsible for colour.

Colour yield: The depth of colour obtained when a standard weight of colorant is

applied to a substrate under specified conditions.

Desizing: The removal of size from fabrics.

Diffusion: Movement of the dye molecules from the surface of the fibre to the

interior of the fibre.

Exhaustion: The proportion of dye or other substrate taken up by a substrate at any

stage of a process to the amount originally available.

Fastness: The property of resistance to an agency named (e.g. washing, light, rubbing,

crocking, gas-fumes). Note: On the standard scale, five grades are usually recognized,

from 5, signifying unaffected, to 1, grossly changed. For lightness, eight grades are

used, 8 representing the highest degree of fastness.

Fixation: Immobilization of the dye molecules inside the fibre. Note: Different methods

include ‘insolubilization’ (e.g. for vat and sulfur dyes in cotton; polymeric binders

with pigments), ‘chemical bonds’ (e.g. hydrogen bonding for direct dyes in cotton),

‘ionic bonding’ (e.g. acid dyes in wool and nylon, and basic dyes in acrylic), covalent

bonding (e.g. reactive dyes in cotton) and solubility in the fibre (e.g. disperse dyes in

polyester, nylon and acetate).



Grey (greige): Woven or knitted fabrics as they leave the loom or knitting machine,

i.e. before any bleaching, dyeing or finishing treatment has been given to them. Some

of these fabrics, however, may contain dyed or finished yarns. Note: In some countries,

particularly in the Northern American continent, the term greige is used. For woven

goods, the term loomstate is frequently used as an alternative. In the linen and lace

trades, the term brown goods is used.

Jet-dyeing machine: (a) A machine for dyeing fabric in rope form in which the fabric

is carried through a narrow throat by dye-liquor circulated at a high velocity.

(b) A machine for dyeing garments in which the garments are circulated by jets of

liquid rather than by mechanical means.

Jig/jigger: A machine in which fabric in open width is transferred repeatedly back

and forth from one roller to another and passes each time through a dyebath or other

liquid of relatively small volume. Jigs are frequently used for dyeing, scouring,

bleaching and finishing.

Levelness: Uniformity of dye or chemical distribution across the substrate.

Lustre (luster, US): The display of different intensities of light, reflected both specularly

and diffusely from different parts of a surface exposed to the same incident light.

High lustre is associated with gross differences of this kind, and empirical measurements

of lustre depend on the ratio of the intensities of reflected light for specified angles

of incidence and viewing. Note: This definition makes these differences in intensity

of light the keypoint, since these form the chief subjective impression on the observer

of lustre. Both specular and diffuse light must be present together, for, if diffuse light

only is present, the surface is matt, not lustrous, whereas, if specular light only is

present, the surface is mirror-like, and again not lustrous. The phrase ‘exposed to the

same incident light’ has been included to rule out shadow effects, which have no part

in lustre proper. The general term ‘surface’ is intended to apply to fibres, yarns, and

fabrics, and indeed to other surfaces, e.g. that of a pearl (though there the differently

reflecting parts are very close together). In the second sentence of the definition,

lustre is regarded as a positive function of the differences, the appropriate adjective

of intensification being ‘high’.

Mercerization: The treatment of cellulosic textiles in yarns or fabric form with a

concentrated solution of caustic alkali whereby the fibres are swollen, the strength

and dye affinity of the materials are increased, and the handle is modified. The

process takes its name from its discoverer, John Mercer (1884).

Mote: There are two broad categories:

(a) Fuzzy motes. The largest of this type of mote consists of whole aborted or

immature seeds covered with fuzz fibers and sometimes also with very short lint

fibres, the development of which has ceased at a very early stage. Small fuzzy motes

originate as either undeveloped or fully grown seeds, which are broken in ginning

and disintegrate still further in the opening, cleaning and carding processes.

(b) Bearded motes. Pieces of seed coat with fairly long lint fibres attached.

Note 1: Both classes of mote become entangled with the lint cotton and, when they

are present in quantity, their complete elimination is impossible except by combing.



Note 2: Fuzzy and bearded motes carrying only a small piece of barely visible seed-

coat are frequently termed seed-coat neps.

Package dyeing: A method of dyeing in which the liquor is circulated radially through

a wound package. Note: Wound packages include slubbing in top form and cheeses

or cones of yarn.

Piece-dyeing: Dyeing in fabric form.

Pilling: Small accumulations of fibres on the surface of a fabric. Pills can develop

during wear, are held to the fabric by an entanglement with the surface fibers of the

material, and are usually composed of the same fibres as those from which the fabric

is made.

Reactive dye: A dye that, under suitable conditions, is capable of reacting chemically

with a substrate to form a covalent dye–substrate linkage.

Resist: (a) A substance applied to a substrate to prevent the uptake or fixation of a dye

in a subsequent operation. Note: The substance functions by forming a mechanical

barrier, by reacting chemically with the dye or substrate, or by altering conditions

(e.g. pH value) locally so that development cannot occur. Imperfect preparation of

the substrate may cause a resist as a fault. (b) In printing plate or roller making, a

coating of, for example, light-hardened gelatin which protects from the action of the

etching solution those areas of the plate or roller which are not required to be etched.

Scouring: The treatment of textile materials in aqueous or other solutions in order to

remove natural fats, waxes, proteins and other constituents, as well as dirt, oil and

other impurities.

Sequestering agents: A chemical capable of reacting with metallic ions so that they

become part of a complex anion. The principle is used to extract calcium ions from

hard water, iron (II) and copper ions from peroxide bleach liquors and various metallic

ions from dyebaths, by forming a water-soluble complex in which the metal is held

in a non-ionizable form.

Shade: (a) A common term loosely employed to broadly describe a particular colour

or depth, e.g., pale shade, 2% shade, mode shade and fashion shade. (b) To bring

about relatively small modifications in the colour of a substrate in dyeing by adding

further small amount of dye, especially with the object of matching more accurately

with a given pattern.

Singeing: To remove, by burning against a hot plate, in a flame, or by infra-red

radiation, unwanted surface hairs or filaments. The operation is usually performed as

a preliminary to bleaching and finishing.

Stripping: Destroying or removing the dye or finish from a fibre.

Size: A gelatinous film-forming substance, in solution or dispersion, applied normally

to warps but sometimes to wefts, generally before weaving.

Note 1: The main types are carbohydrates and their derivatives, gelatin, and animal

glues, although other substances, such as linseed oil, poly (acrylic acid), and poly

(vinyl alcohol) are also used.



Note 2: The objects of sizing prior to weaving are to protect the yarns from abrasion

in healds and reed and against each other; to strengthen them; and by the addition of

oils and fats, to lubricate them.

Note 3: A size may be applied to carpets (e.g. starch) and occasionally to wool fabrics

(e.g. animal glue).

Sizing: A process in which warp yarns are sized during transfer from warper’s beams

to loom beams. Two or more size boxes may be used in parallel and/or in tandem if

the warp sheet is too dense for effective sizing in one box, or if it contains yarns with

different fugitive tints. Slasher sizing is also known as slashing.

Slashing: See sizing

Substantivity: The attraction between a substrate and a dye or other substrate under

the precise conditions of test whereby the latter is selectively extracted from the

application medium by the substrate.

Surfactant/Surface Active Agent: An agent, soluble or dispersible in a liquid, which

reduces the surface tension of the liquid.

CHEMICAL SYMBOLS OF SOME OF THE REAGENTS

Chlorine Dioxide (ClO2)

Hypochlorous Acid (HClO)

Sodium Hypochlorite (NaOCl)

Hydrogen Peroxide (H2O2)

Potassium Citrate Monohydrate (C6H5O7K3.H2O)

Potassium Oxalate (K2[C2O4])

Sodium Acetate (CH3COONa.3H2O)

Sodium Bromite (NaBrO2)

Sodium Chlorate (NaClO3)

Sodium Chlorite (NaClO2)

Sodium Citrate Dihydrate (C6H5O7Na3.2H2O)

Sodium Nitrate (NaNO3)

Sodium Perborate (NaBO3.H2O)

Sodium Persulphate (Na2S2O8)

Sodium Phosphate (NaH2PO42H2O)



Problem

Acid tendering

Bronziness

Dye spots/resist spots/stains/blotches/filtering/dark colouredareas

APPENDIX A: Summary and Solutions to Problems Originating from

Cotton Fibre

Possible Cause

Inadequate washing/neutralisation of the dyed fabric

1. Insufficient quantity ofsodium sulphide2. Degraded quality of sodiumsulphide3. Too high concentration ofsalt4. Too high concentration ofalkali5. Too long exposure of dyedgoods to air before being after-treated6. Too short liquor ratio7. Too high temperature duringdyeing8. Water hardness

9. Presence of calcium ormagnesium in cotton10. Failure to remove excessliquor before dyeing11. Premature oxidation ofreduced dye

12. Excessively heavy shade

1. Poor pre-treatment2. Residual contaminants in thesubstrate3. Water contamination

4. Contaminants in salt, alkali,etc.5. Soiling of material by air-borneparticles of powdered dyes6. Incompatibility of dyebathassistants7. Poor stability of dyedispersion

Countermeasure

1. Thorough washing/neutralisationof the fabric after dyeing – Rinsingwell before oxidation and soapingafter oxidation2. Use of alkaline bath in the finalrinse3. Use of sodium acetate and sodaash for neutralisation4. Storing the fabric at lowtemperature and humidity5. Resin finishing

1. Optimum quantity of sodiumsulphide2. Good quality control of sodiumsulphide3. Optimum concentration of salt

4. Optimum concentration of alkali

5. No long exposure of dyed goodsto air before being after-treated

6. Optimum liquor ratio7. Optimum temperature duringdyeing8. Use of soft water or appropriatesequestrants9. Demineralisation of cotton oruse of appropriate sequestrants10. Removal of excess liquorbefore dyeing11a. Excess quantity of sodiumsulphide11b. Exclusion of air from insidethe machine12. Use of dyes with hightinctorial strength

1. Good pre-treatment2. Good pre-treatment and washingafter pre-treatment3. Water purification or use ofsequestrants4. Good quality control ofcommodity chemicals5. Good housekeeping and use ofliquid dyes if appropriate6. Use of compatible dyebathassistants7a. Good dispersion stability7b. Optimum control oftemperature, pH, etc.



Poor washing fastness

Poor washing off

Poor rubbing fastness

Shade change/inconsistent shade

8. Too high rate of dye strikeon the substrate9. Short liquor ratio10. More dense/compact yarnareas appearing dark due to lessscattering of light11. Improper rinsing/washing-off after dyeing

1. Inadequate rinsing of thefabric before oxidation2. Premature or over-oxidationof the dye3. Poor washing-off after-treatment

1. Dye substantivity, too high

2. Dyes of low diffusioncoefficient3. Short liquor ratio of thewashing bath4. Low temperature of thewashing liquor5. High electrolyteconcentration in the washingbath6. Water hardness

7. Inadequate washing time8. Inadequate number of washcycles/wash baths9. Low mechanical action10. Misuse of dyebath assistants

1. Dyes of poor rubbingfastness2. Catalytic damage of thematerial3. Presence of polyvalent ions

4. Too high moisture in the testmaterial5. Improper use of finishingagents6. Un-Mercerized cotton7. Incomplete diffusion andfixation8. Incomplete washing-off

1. Residual peroxide, alkalinityor other contaminants in thesubstrate2. Water hardness

8. Optimum process control

9. Optimum liquor ratio10. Good quality control of thesubstrate

11. Optimum rinsing/washing-offafter dyeing

1. Thorough rinsing of the fabricbefore oxidation2. Optimum oxidation

3. Thorough washing-off (see belowthe cause of poor washing-off)

1. Use of dyes of optimumsubstantivity2. Use of dyes of optimumdiffusion coefficient3. Optimum liquor ratio of thewashing bath4. Optimum temperature of thewashing liquor5a. Use of low salt dyes5b. Optimum rinsing cycle/sequence6. Use of soft water and/orsequestrants7. Optimum washing time8. Optimum number of washcycles/wash baths9. Optimum mechanical action10. Compatible dyebath assistants

1. Use of dyes of good rubbingfastness2. No catalytic damage of thematerial3. Use of appropriate complexingagents4. Proper conditioning of the testmaterial5. Proper use of finishing agents

6. Use of Mercerized cotton7. Use of optimum dyeingconditions8. Optimum washing-off

1. Substrate free fromcontamination before dyeing

2. Use of soft water or appropriatesequestrants

Problem Possible Cause Countermeasure



3. Contamination of dyebathwith calcium, magnesium,chlorine or heavy metals4. Improper use of dyebathassistants, e.g. surfactants,sequestrants, fixatives,lubricating agents

1. Low dye diffusion2. High dye affinity3. Dyes of poor migration4. Improper reduction of the dye5. Too much salt6. Rate of dyeing too high7. Rapid rate of rise oftemperature8. Low liquor flow rate9. Low mechanical action10. Poor rinsing beforeoxidation11. Improper oxidation of thereduced dye12. Uneven washing/soapingafter-treatments13. Presence of iron/copper

14. Improper use of surfactants

1. Poor pre-treatment2. Dye substantivity too high

3. Dyes of poor diffusioncoefficient

1. Poor pre-treatment2. Residual peroxide in thesubstrate3. Incomplete or over-reductionof the dye4. Too low dye substantivity

5. Too low concentration ofelectrolyte6. Too low dye concentration7. Too high liquor ratio8. Too short dyeing time9. Incomplete or over-oxidationof the reduced dye10. Improper use of surfactants11. Loss of dye or chemicalsduring weighing/dispensing12. Water hardness

Poor levelling/unevendyeing

Low diffusion/penetration

Poor colour yield

3. Use of appropriate sequestrants

4. Use of compatible dyebathassistants

1. Use of dyes of good diffusion2. Use of dyes of optimum affinity3. Use of dyes of good migration4. Optimum reduction of the dye5. Optimum concentration of salt6. Optimum rate of dyeing7. Optimum rate of rise oftemperature8. Optimum liquor flow rate9. Optimum mechanical action10. Thorough rinsing beforeoxidation11. Optimum oxidation of thereduced dye12. Thorough and uniformwashing/soaping after-treatments13. Use of appropriate complexingagents14. Use of compatible surfactants

1. Good pre-treatment2. Use of dyes of optimumsubstantivity3. Use of dyes of good diffusioncoefficient4. Use of optimum dyeingconditions

1. Good pre-treatment2. No residual peroxide in thesubstrate3. Optimum reduction of the dye

4. Use of dyes of optimumsubstantivity5. Optimum concentration ofelectrolyte6. Optimum dye concentration7. Optimum liquor ratio8. Optimum dyeing time9. Optimum oxidation of thereduced dye10. Optimum use of surfactants11. Careful weighing/dispensing ofdyes and chemicals12. Use of soft water orappropriate sequestrants




1. Too long dyeing time (due tocorrective/repair procedures)2. Too high mechanical orhydraulic action3. Too high temperature(increases the sensitivity of thematerial to physical damage)

1. Low dyeing temperature2. Low concentration ofelectrolyte

1. Too high mechanical action2. Improper surfactant3. Too high concentration ofsurfactant4. Introduction of air into thedyeing machine

1. Too high dye concentration2. Too high electrolyte content3. Too low temperature4. Poor stability of dyedispersion

The presence of broken fibreson the surface at the damagedportions leads to strongerscattering of light causing themto appear lighter

1. Incomplete reduction of dyes

1. A surface knot of entangledimmature fibres is flattenedduring processing and takes ona glazed, shiny appearance.

2. Clumps of immature fibresloosely attached to the yarnsthat are poorly penetrated bythe dye, move or knock loose toreveal the white or lightly dyedarea3. Dead cotton of poordyeability4. Immature cotton of poordyeability

Heavy metals

Physical damage

Slow rate ofexhaustion

Foaming

Dye aggregation

Light coloured areas/spots on the yarn

Dull shades

White or light-coloured specks inotherwise deep dyedmaterial

Incomplete removal ofsize in enzymaticdesizing

1. Right First Time Dyeing

2. Optimum mechanical orhydraulic action3. Optimum temperature

1. Optimum dyeing temperature2. Optimum concentration ofelectrolyte

1. Optimum mechanical action2. Use of low-foaming surfactant3. Optimum concentration ofsurfactant4. Prevention of air entry into thedyeing machine

1. Optimum dye concentration2. Optimum electrolyte content3. Optimum temperature4. Good stability of dye dispersion(by ensuring optimum temperature,pH, etc.)

1. Good quality control of thesubstrate2. Optimum singeing

1. Optimum reduction of dye

1a. Swelling treatment(Mercerization or ammoniatreatment) before or after dyeing1b. Selection of dyes with goodcoverage properties2a. Increase in dye penetration2b. Swelling treatment(Mercerization or ammoniatreatment) before dyeing

3. Selection of dyes with bettercoverage properties4a. Swelling treatment(Mercerization or ammoniatreatment) before dyeing4b. Selection of dyes with bettercoverage properties

1. Use of sequestering agents2. Demineralisation













1. Obtaining the substrate from asingle source wherever possible2. Use of dyes with minimumsensitivity to dyeability variation

Good fibre preparation andcleaning (during spinning)

1. Careful handling and storage2. Good fibre preparation, cleaningand pre-treatment

Alkaline earth metals

Alkaline earth metals and/orheavy metals







Heavy metals

Substrate obtained fromdifferent sources

Seed capsules, leaves, branches,etc. and /or Neps

1. Foreign fibres2. Residues from insecticide,growth regulators, defoliants,etc.3. Dirt, dust from storage

Less removal of oilsand fats due tobreaking of emulsions(Improper scouring)

Less stability ofperoxide bath due toblockage of stabilisers(Improper bleaching)

Harsh handle of thesubstrate due todeposition ofinsoluble alkalineearth salts

Lowering ofwhiteness due toformation of insolubleproducts with opticalbrighteners

Low solubility of dyes

Dye stains due toformation of insolubledye products

Change in the tone ofdyeing

Low washing fastnessof reactive dyes dueto hindrance in theremoval of hydrolyseddye

Decrease in fibrestrength (Bleachingdamage)

Dyeability variation

General impairment

Resist spots




APPENDIX B: Summary and Solutions to Problems in Yarn Winding

Problem

Channelling

Unlevel dyeing

Swelled or puffy packageshoulders

White or light yarn streaks inotherwise deep dyed yarn

Leakage or poor liquor flowthrough the edges of the dyepackage

Package deformation

Pressure or lustre marks oninner yarn layers

Different shades in the inner,middle and outer layers of apackage

Possible Cause

1. Uneven package density2. Too soft package winding

Uneven package density

1. Too soft package winding2. Foaming in the bath

1. Too soft package winding2. Foaming in the bath

1. Differences in the packagedensities2. Unsatisfactory spacersealing

1. Differences in packagedensity from package topackage, and within apackage2. Improperly woundpackages3. All the perforations of thedye tube not covered withyarn4. Pressed density of thecompressed tubes not same5. Damaged tubes6. Faulty spacers7. Defective locking caps8. Shrinkage and deformationof plastic tubes subjected tohigh temperature9. Too high liquor flow

Too high a winding orpressing density, or highresidual shrinkage

Non-uniform winding and/orpressing density

Countermeasure

1. Uniform package density2. Optimum package winding

Uniform package density

1. Optimum package winding2. Use of anti-foaming agent

1. Optimum package winding2. Use of anti-foaming agent

1. Uniform package densities

2. Optimum spacer sealing

1. Uniform package densityfrom package to package, andwithin a package

2. Optimum package winding

3. All the perforations of thedye tube covered with yarn

4. Similar pressed density ofthe compressed tubes5. No damaged tubes6. No faulty spacers7. No defective locking caps8. Use of good quality dyetubes

9. Optimum liquor flow

Optimum winding andpressing density and noresidual shrinkage

Uniform winding andpressing density

APPENDIX C: Summary and Solutions to Problems Caused by Poor

Water Quality

Problem

Poor removal of starchsizes

Possible Cause

1. Water hardness

2. Heavy metals

Countermeasure

1. Appropriate water treatmentprior to use in processing2. Use of suitable sequestrantsduring processing



Harsh handle of thesubstrate

Inconsistent absorbencyafter scouring

Tendency of the substrateto attract soil

Decreased solubility andrate of dissolution ofsurfactants

Decreased wash-removalability of surfactants

Catalytic decompositionof hydrogen peroxideleading to fibredegradation, loss in fibrestrength, increase influidity and reduction inwhiteness

Inconsistent shade

Inconsistent shade andblotches due toinconsistent and unevenwashing off

1. Water hardness

2. Alkaline and alkaline earthmetals3. High solid content in therinsing water

1. Water hardness

2. Alkaline and alkaline earthmetals

1. Water hardness

2. Alkaline and alkaline earthmetals3. Greasy contaminants

1. Water hardness


1. Water hardness


Transition metal ions (iron,copper, manganese, zinc,nickel, cobalt and chromium)Alkaline earth metals (otherthan magnesium)

ChlorineIron, copper or other metalsChelates such as EDTA,DTPA, NTA, and HEDTACalcium and magnesium(hardness) in the processwaterAcidity or alkalinity in waterFluorescent brighteners inwater

Sediments, alum or otherresidual flocking agents leftover from water treatment,from organic contaminants,from metal hydroxides(copper and iron), or fromfatty acid/hardness metalcomplexes












Appropriate water treatment priorto use in processing







High solid content in therinsing water

High solid content in therinsing waterWater hardnessHeavy metalsSulphates, sulphites,sulphides, or chloridesSilica

Calcium ions

Calcium ions

Calcium ions

Surfactants in water

Dissolved carbon dioxide,dissolved oxygen

Filtering in packagedyeing

Dye resists, stains and/orspots

Loss of colour depth

Difficulty in the removalof hydrolysed dye

Decrease in the wetfastness

Foaming

Corrosion or rusting ofmachine parts


APPENDIX D: Summary and Solutions to Problems in Singeing

Problem

Incomplete singeing

Uneven singeing(widthways)

Possible Cause

1. Too low flame intensity2. Too fast fabric speed3. Too far distance betweenthe fabric and the burner4. Inappropriate singeingposition (not severe enough)5. Too much moisture in thefabric incoming for singeing

1. Non-uniform moisturecontent across the fabricwidth2. Non-uniform flameintensity across the fabricwidth3. Uneven distance betweenthe burner and the fabric

Countermeasure

1. Optimum flame intensity2. Optimum fabric speed3. Optimum distance between thefabric and the burner4. Optimum singeing position

5. No excess moisture in thefabric incoming for singeing

1. Uniform moisture contentacross the fabric width

2. Uniform flame intensity acrossthe fabric width

3. Uniform distance between thefabric and the burner



Uneven singeing(lengthways)

Thermal damage orReduction in tearstrength

1. Non-uniform moisturecontent along the fabriclength2. Non-uniform flameintensity along the fabriclength3. Change in fabric speedduring singeing4. Change in the distancebetween the fabric and theburner along the length

1. Too high flame intensity2. Too slow fabric speed3. Too close distance betweenthe fabric and the burner4. Inappropriate singeingposition (too severe)

1. Uniform moisture contentalong the fabric length

2. Uniform flame intensityalong the fabric length

3. Uniform fabric speedduring singeing4. Uniform distance betweenthe fabric and the burneralong the length

1. Optimum flame intensity2. Optimum fabric speed3. Optimum distance betweenthe fabric and the burner4. Optimum singeing position

APPENDIX E: Summary and Solutions to Problems in Desizing

Problem

Incomplete desizing

Uneven desizing(widthways)

Uneven desizing(lengthways)

Cause

1. Inappropriate desizing bathpH2. Inappropriate desizing-bathtemperature3. Insufficient fabric pick-up

4. Insufficient digestion time5. Poor enzyme activity6. Deactivation of enzyme dueto presence of metals or othercontaminants7. Ineffective wetting agent

8. Incompatible wetting agent

1. Uneven pad pressure(across the width)2. Non-uniform padtemperature3. Non-uniform chemicalconcentration in the bath

1. Uneven pick-up(along the length)2. Preferential drying of outerlayers of the batch

3. Temperature variationduring digestion

Countermeasure

1. Optimum pH

2. Optimum temperature

3a. Optimum squeeze pressure3b. Use of wetting agent4. Optimum digestion time5. Use of good enzymes6a. Use of soft water6b. Use of appropriatesequestering agents7. Use of good and effectivewetting agent8. Use of compatible wettingagent

1. Uniform squeeze pressure

2. Uniform bath temperature

3. Uniform chemicalconcentration

1. Uniform pick-up along thefabric length2a. Covering the batch withpolythene or other suitable sheet2b. Keep the batch rolling3a. Covering the batch withpolythene or other suitable sheet3b. Keep the batch rolling




Uneven desizing(random)

1. Poor wetting agent

2. Inappropriate bathtemperature3. Foaming in the bath4. Improper use of defoamer5. Uneven liquor distributionduring padding6. Non-uniform washing afterdesizing

1. Use of effective andcompatible wetting agent2. Optimum bath temperature

3. Use of appropriate defoamers4. Use of appropriate defoamers5. Uniform liquor distributionduring padding6. Thorough and uniformwashing after desizing

APPENDIX F: Summary and Solutions to Problems in Scouring

Problem

Inadequate scouring orInadequate absorbency orHigh residual impurities(batch scouring of yarnor fabric)

Inadequate scouring orInadequate absorbency orHigh residual impurities(Pad-steam scouring offabric)

Uneven scouring (randomunevenness whenscouring in fabric form)

Uneven scouring (randomunevenness whenscouring yarn in packageform)

Possible Cause

1. Too low concentration ofscouring chemicals2. Incompatible or ineffectivesurfactant/wetting agent3. Too low scouringtemperature4. Inadequate scouring time5. Inadequate washing afterscouring

1. Too low concentration ofscouring chemicals2. Incompatible or ineffectivesurfactant/wetting agent3. Too low steamingtemperature4. Inadequate steaming time5. Inadequate washing afterscouring

1. Poor stability of surfactant/wetting agent (cloud pointbelow applicationtemperature)2. Water hardness orineffective chelating agents3. Non-uniform and/orineffective washing afterscouring4. Improper use of defoamer

(all above causes for randomunevenness when scouring infabric form, and)1. Uneven package density2. Yarn variations

Countermeasure

1. Optimum concentration ofscouring chemicals2. Compatible and effectivesurfactant/wetting agent3. Optimum scouring temperature

4. Optimum scouring time5. Optimum washing afterscouring

1. Optimum concentration ofscouring chemicals2. Compatible and effectivesurfactant/wetting agent3. Optimum steamingtemperature4. Optimum steaming time5. Optimum washing afterscouring

1. Suitable selection and properuse of surfactant/wetting agent

2. Use of soft water or effectivechelating agents3. Uniform and thorough washingafter scouring

4. Suitable selection and properuse of defoamer

1. Uniform package density2. No yarn variations (Goodquality control of incoming yarn)

Problem Cause Countermeasure



1. Uniform pad pressure2. Uniform bath temperature

3. Uniform chemicalconcentration in the bath

1. Uniform concentration ofscouring chemicals with time

2. Uniform moisture content inthe incoming fabric along thelength

Optimum concentration ofalkali during scouring

1. Use of soft water orappropriate chelating agents2a. Careful selection ofscouring auxiliaries2b. Thorough washing afterscouring

1. Optimum alkaliconcentration2. Optimum dwell time

1a. Exclusion of air1b. Use of mild reducing agent

2a. Water purification2b. Use of appropriatecomplexing agent2c. Demineralisation (if ironpresent in the textile material)

1. Uneven pad pressure2. Non-uniform temperatureacross the bath3. Non-uniform chemicalconcentration across the bath

1. Variation in the concentrationof scouring chemicals withtime2. Variation in the moisturecontent of the incoming fabricalong the length

Complete loss of natural oils/fats due to too high alkaliconcentration

1. Deposits of insoluble salts ofsurfactants2. Redeposition of impurities

1. Too high alkali concentration

2. Too long dwell time

1. Presence of air in themachine, leading to theformation of oxycellulose2. Contamination of iron

Uneven scouring(widthways unevennessin pad-steam scouring)

Uneven scouring(lengthways unevennessin pad-steam scouring)

Harsh handle

Resist marks

Yellowing of the goods

Tendering or damage orloss in strength


APPENDIX G: Summary and Solutions to Problems in Bleaching

Problem

Low degree of whiteness(Bleaching yarn or fabricin batch form)

Low degree of whiteness(Bleaching fabric by pad-

Possible Cause

1. Inadequate concentration ofhydrogen peroxide2. Inadequate alkaliconcentration3. Too low bleaching pH4. Too short bleaching time5. Too low bleachingtemperature6. Residual sodium acetate afterneutralization

1. Inadequate concentration ofhydrogen peroxide

Countermeasure

1. Optimum concentration ofhydrogen peroxide2. Optimum alkaliconcentration3. Optimum bleaching pH4. Optimum bleaching time5. Optimum bleachingtemperature6. Thorough rinsing afterneutralization

1. Optimum concentration ofhydrogen peroxide



steam process)

Uneven whiteness(random)

Uneven whiteness(lengthways) (bleachingof fabrics by pad steamprocess)

Uneven whiteness(widthways) (bleachingof fabrics by pad steamprocess)

Harsh handle

Fibre degradation ORReduction in fibrestrength

2. Inadequate alkaliconcentration3. Inadequate pick-up

4. Too low bleaching pH5. Too short steaming time6. Too low steamingtemperature7. Residual sodium acetate afterneutralization

1. Use of inappropriatesurfactants2. Water hardness

3. Irregular chemical feeding4. Condensation or water marks5. Foaming in the bath6. Inappropriate use ofdefoamer7. Ineffective and/or non-uniform washing afterbleaching

1. Non-uniform pick-up withtime2. Variation in chemicalconcentration with time3. Variation in steamingconditions with time4. Variation in the fabric speed

1. Uneven pad pressure (acrossthe fabric width)2. Non-uniform bathtemperature3. Non-uniform chemicalconcentration

1. Silicate deposits

2. Too high concentration ofalkali3. Too high bleaching/steamingtemperature

1. Metal contaminants

2. Optimum alkaliconcentration3a. Optimum pick-up3b. Use of good wetting agents4. Optimum bleaching pH5. Optimum steaming time6. Optimum steamingtemperature7. Thorough rinsing afterneutralization

1. Appropriate/compatiblesurfactants2. Soft water or use ofsequestering agents3. Optimum chemical feeding4. Optimum steaming conditions5. Appropriate use of defoamer6. Appropriate use of defoamer

7. Thorough and uniformwashing after bleaching

1. Uniform pick-up with time

2. Uniform chemicalconcentration with time3. Uniform steaming conditionswith time4. Uniform fabric speed

1. Uniform pad pressure (acrossthe fabric width)2. Uniform bath temperature

3. Uniform chemicalconcentration

1a. Use of organic stabilisers1b. Optimum control of pH(low pH reduces silicatesolubility)1c. Thorough washing afterbleaching2. Optimum concentration ofalkali3. Optimum bleaching/steamingtemperature

1a. Demineralisation to removemetals from the fibre1b. Treatment of water toremove metal contaminants1c. Use of appropriatecomplexing agents




2. Use of appropriatestabiliser(s)3. Optimum bleaching pH4. Optimum condition of time,temperature, and concentrationof peroxide

1a. Demineralisation to removemetals from the fibre1b. Treatment of water toremove metal contaminants1c. Use of appropriatecomplexing agents/ stabiliser(s)

1. Optimum bleaching pH/alkalinity2. Adequate softening of motesduring scouring

1a. Use of alternative stabiliser(s)1b. Appropriate ratio ofNa2O:SiO2

1c. Optimum pH duringbleaching and washing afterbleaching2a. Proper stabilisation of thebleaching liquor2b. Optimum bleachingconditions2c. Use of appropriatecomplexing agents for metalcontaminants

Optimum alkalinity in thebleach liquor

Optimum alkalinity in thebleach liquor

Optimum bleaching temperatureand alkalinity

2. Unstabilised hydrogenperoxide3. Too high bleaching pH4. Extreme condition of time,temperature, and concentrationof peroxide

1. Localised fibre degradationusually due to heavy metalpresence

1. Too low bleaching pH/alkalinity2. Inadequate softening ofmotes during scouring

1. Silicate deposits

2. Oxycellulose formation

Too high alkalinity in thebleach liquor

Too high alkalinity in thebleach liquor

Too high bleaching temperatureand/or alkalinity

Pinholes, broken yarns,tears

Inadequate mote removal

Resist spots

Loss in voluminouscharacter of the material

Decrease in the elasticityof the material

Low sewability of thematerial


APPENDIX H: Summary and Solutions to Problems in Mercerization

Problem

Incompletemercerization

Possible Cause

1. Low concentration of sodiumhydroxide2. Inappropriate wetting agent3. Inappropriate temperature ofthe incoming fabric or thepadder4. Low pick-up5. Insufficient contact time

Countermeasure

1. Optimum concentration ofsodium hydroxide2. Appropriate wetting agent3. Appropriate temperature ofthe incoming fabric or thepadder4. Optimum pick-up5. Optimum contact time



Low increase in lustre

Uneven mercerization(width-wise)

Uneven mercerization(length-wise)

Uneven mercerization(random)

Tearing of the fabric

Poor shrinkagecontrol

1. Low concentration of sodiumhydroxide2. Inappropriate temperature ofthe incoming fabric or the padder3. Low pick-up4. Insufficient contact time5. Insufficient fabric stretchingwhile on the frame6. Too much caustic on the fabricas it comes off the frame

1. Uneven pad temperature2. Non-uniform bath temperature3. Non-uniform alkaliconcentration in the bath4. Non-uniform moisture in thefabric across the width

1. Dilution of the bath with time

2. Increase in bath temperaturewith time3. Length-wise variation in themoisture content of the fabric4. Variation in the pad pressureduring the process5. Variation in pick-up along thefabric length

1. Ineffective and/or incompatiblewetting agent

1. Low concentration of sodiumhydroxide2. Low pick-up

1. Insufficient fabric stretchingwhile on the frame2. Too much caustic on the fabricas it comes off the frame

1. Optimum concentration ofsodium hydroxide2. Appropriate temperature of theincoming fabric or the padder3. Optimum pick-up4. Optimum contact time5. Optimum fabric stretchingwhile on the frame6. Removal of excess causticfrom the fabric before it comesoff the frame

1. Even pad temperature2. Uniform bath temperature3. Uniform alkali concentrationin the bath4. Uniform moisture in the fabricacross the width

1. Uniform moisture content inthe fabric2. No variation in bathtemperature with time3. No length-wise variation in themoisture content of the fabric4. No variation in the padpressure during the process5. No variation in pick-up alongthe fabric length

1. Use of effective andcompatible wetting agent

1. Optimum concentration ofsodium hydroxide2. Optimum pick-up

1. Optimum fabric stretchingwhile on the frame2. Removal of excess causticfrom the fabric before it comesoff the frame


APPENDIX I: Summary and Solutions to Problems in Dyeing with

Direct and Reactive Dyes

Problem


Possible Cause


Countermeasure

1. Good pre-treatment2. Good pre-treatment andwashing after pre-treatment3. Water purification or use ofsequestrants



Poor washingfastness

Poor washing off

Poor rubbingfastness

4. Contaminants in salt, alkali,etc.5. Soiling of material by air-borne particles of powdered dyes6. Incompatibility of dyebathassistants7. Too high rate of dye strike onthe substrate (due to too highreactivity)8. Short liquor ratio9. More dense/compact yarnareas appearing dark due to lessscattering of light10. Improper rinsing/washing-offafter dyeing11. Dye aggregation

1. Inadequate washing-off ofhydrolysed dye (in case ofreactive dyes)2. Inadequate removal of looselyretained dye (for direct dyes)3. Inherent low fastnessproperties of the dyes


2. Dyes of low diffusioncoefficient3. Short liquor ratio of thewashing bath4. Low temperature of thewashing liquor5. High electrolyte concentrationin the washing bath

6. Water hardness

7. Inadequate washing time8. Inadequate number of washcycles/wash baths9. Low mechanical action10. Misuse of dyebath assistants11. Disazo, 1:2 Metal Complex,and Phthalocyanine dyes aredifficult to wash-off

1. Dyes of poor rubbing fastness

2. Catalytic damage of thematerial3. Presence of polyvalent ions

4. Good quality control ofcommodity chemicals5. Good housekeeping and use ofliquid dyes if appropriate6. Use of compatible dyebathassistants7. Optimum process control


10. Optimum rinsing/washing-offafter dyeing11. See causes of the problem‘Dye aggregation’ below

1 & 2. Optimum washing off (Seecauses of ‘Poor washing-off’below)

3. Use of cationic fixing agents orother fastness improving after-treatment

1. Use of dyes of optimumsubstantivity2. Use of dyes of optimumdiffusion coefficient3. Optimum liquor ratio of thewashing bath4. Optimum temperature of thewashing liquor5a. Use of low salt dyes5b. Optimum rinsing cycle/sequence6. Use of soft water and/orsequestrants7. Optimum washing time8. Optimum number of washcycles/wash baths9. Optimum mechanical action10. Compatible dyebath assistants11a. Use of alternative dyes ifpossible or11b. Use of more severe washing-off1. Use of dyes of good rubbingfastness2. No catalytic damage of thematerial3. Use of appropriate complexingagents




4. Proper conditioning of the testmaterial5. Proper use of finishing agents6. Use of mercerized cotton7. Use of optimum dyeingconditions8. Optimum washing-off

1. Substrate free fromcontamination before dyeing2. Use of soft water or appropriatesequestrants3. Use of appropriate sequestrants


1. Use of dyes of good solubility2. Use of dyes of good diffusion3. Use of dyes of optimum affinity4. Use of dyes of good migration5. Optimum concentration of salt6. Optimum rate of dyeing7. Optimum rate of rise oftemperature8. Optimum pH control9. Optimum liquor flow rate10. Optimum mechanical action11. Alkali added optimally12. Use of alternative dyes oroptimum dyeing conditions13. Use of appropriate complexingagents14. Use of compatible surfactants

1. Good pre-treatment2. Use of dyes of optimumsubstantivity3. Use of dyes of good solubility4. Use of dyes of good diffusioncoefficient5. Use of optimum dyeing conditions

1. Good pre-treatment2. No residual peroxide in thesubstrate3a. Careful storage of dyes3b. Use of freshly prepared dyesolution3c. Optimum dyeing conditions4. Use of dyes of optimumsubstantivity

4. Too high moisture in the testmaterial5. Improper use of finishing agents6. Un-mercerized cotton7. Incomplete diffusion andfixation8. Incomplete washing-off

1. Residual peroxide, alkalinity orother contaminants in the substrate2. Water hardness

3. Contamination of dyebath withcalcium, magnesium, chlorine orheavy metals4. Improper use of dyebathassistants, e.g. surfactants,sequestrants, fixatives, lubricatingagents

1. Poor dye solubility2. Low dye diffusion3. High dye affinity4. Dyes of poor migration5. Too much salt6. Rate of dyeing too high7. Rapid rate of rise oftemperature8. Rapid shift of dyebath pH9. Low liquor flow rate10. Low mechanical action11. Alkali added too soon12. Metal complex orPhthalocyanine dyes13. Presence of iron/copper


1. Poor pre-treatment2. Dye substantivity, too high

3. Poor dye solubility4. Dyes of poor diffusioncoefficient

1. Poor pre-treatment2. Residual peroxide in thesubstrate3. Dye hydrolysis (only in caseof reactive dyes)

4. Too low dye substantivity




Poor colour yield




5. Optimum concentration ofelectrolyte6. Optimum concentration of alkali7. Optimum dye concentration8. Optimum liquor ratio9. Optimum dyeing time10. Optimum use of surfactants11. Careful weighing/dispensing ofdyes and chemicals12. Use of soft water orappropriate sequestrants

1. Right first time dyeing


1. Good dye solubility2. Optimum dyeing temperature3. Optimum concentration ofelectrolyte


1. Optimum dye concentration2. Optimum electrolyte content3. Optimum temperature4. Good dye solubility


Use of alternative dyes

1. Use of dyes of optimumsubstantivity2. Optimum temperature3. Optimum concentration ofelectrolyte

5. Too low concentration ofelectrolyte6. Too low concentration of alkali7. Too low dye concentration8. Too high liquor ratio9. Too short dyeing time10. Improper use of surfactants11. Loss of dye or chemicalsduring weighing/dispensing12. Water hardness


1. Low dye solubility2. Low dyeing temperature3. Low concentration ofelectrolyte


1. Too high dye concentration2. Too high electrolyte content3. Too low temperature4. Lower dye solubility

The presence of broken fibres onthe surface at the damagedportions leads to strongerscattering of light causing themto appear lighter

Metal complex dyes containingcopper possess rather dull shades

1. High substantivity dyes

2. Low temperature3. High concentration ofelectrolyte

Physical damage


Foaming

Dye aggregation


Dull shades

Lower dye solubility




APPENDIX J: Summary and Solutions to Problems in Dyeing with

Sulphur Dyes

Problem

Acid tendering

Bronziness


Possible Cause

Inadequate washing/neutralisationof the dyed fabric

1. Insufficient quantity of sodiumsulphide2. Degraded quality of sodiumsulphide3. Too high concentration of salt4. Too high concentration ofalkali5. Too long exposure of dyedgoods to air before being after-treated6. Too short liquor ratio7. Too high temperature duringdyeing8. Water hardness

9. Presence of calcium ormagnesium in cotton10. Failure to remove excessliquor before dyeing11. Premature oxidation ofreduced dye

12. Excessively heavy shade


4. Contaminants in salt, alkali,etc.5. Soiling of material by air-borne particles of powdered dyes6. Incompatibility of dyebathassistants7. Poor stability of dye dispersion

Countermeasure

1. Thorough washing/neutralisation of the fabric afterdyeing – Rinsing well beforeoxidation, and soaping afteroxidation2. Use of alkaline bath in thefinal rinse3. Use of sodium acetate andsoda ash for neutralisation4. Storing the fabric at lowtemperature and humidity5. Resin finishing

1. Optimum quantity of sodiumsulphide2. Good quality control ofsodium sulphide3. Optimum concentration of salt4. Optimum concentration ofalkali5. No long exposure of dyedgoods to air before being after-treated6. Optimum liquor ratio7. Optimum temperature duringdyeing8. Use of soft water orappropriate sequestrants9. Demineralisation of cotton oruse of appropriate sequestrants10. Removal of excess liquorbefore dyeing11a. Excess quantity of sodiumsulphide11b. Exclusion of air from insidethe machine12. Use of dyes with hightinctorial strength

1. Good pre-treatment2. Good pre-treatment andwashing after pre-treatment3. Water purification or use ofsequestrants4. Good quality control ofcommodity chemicals5. Good housekeeping and use ofliquid dyes if appropriate6. Use of compatible dyebathassistants7a. Good dispersion stability7b. Optimum control oftemperature, pH, etc.



8. Optimum process control




3. Thorough washing-off (seebelow the cause of ‘Poorwashing-off’)



6. Use of mercerized cotton7. Use of optimum dyeingconditions8. Optimum washing-off

1. Substrate free fromcontamination before dyeing2. Use of soft water orappropriate sequestrants3. Use of appropriate sequestrants

8. Too high rate of dye strike onthe substrate9. Short liquor ratio10. More dense/compact yarnareas appearing dark due to lessscattering of light11. Improper rinsing/washing-offafter dyeing

1. Inadequate rinsing of thefabric before oxidation2. Premature or over-oxidation ofthe dye3. Poor washing-off after-treatment



6. Water hardness




4. Too high moisture in the testmaterial5. Improper use of finishingagents6. Un-mercerized cotton7. Incomplete diffusion andfixation8. Incomplete washing-off

1. Residual peroxide, alkalinity orother contaminants in the substrate2. Water hardness

3. Contamination of dyebath with


Poor washing off







1. Use of dyes of good diffusion2. Use of dyes of optimumaffinity3. Use of dyes of good migration4. Optimum reduction of the dye5. Optimum concentration of salt6. Optimum rate of dyeing7. Optimum rate of rise oftemperature8. Optimum liquor flow rate9. Optimum mechanical action10. Thorough rinsing beforeoxidation11. Optimum oxidation of thereduced dye12. Thorough and uniformwashing/soaping after-treatments13. Use of appropriatecomplexing agents14. Use of compatible surfactants

1. Good pre-treatment2. Use of dyes of optimumsubstantivity3. Use of dyes of good diffusioncoefficient4. Use of optimum dyeingconditions

1. Good pre-treatment2. No residual peroxide in thesubstrate3. Optimum reduction of the dye

4. Use of dyes of optimumsubstantivity5. Optimum concentration ofelectrolyte6. Optimum dye concentration7. Optimum liquor ratio8. Optimum dyeing time9. Optimum oxidation of thereduced dye10. Optimum use of surfactants11. Careful weighing/dispensingof dyes and chemicals12. Use of soft water orappropriate sequestrants

calcium, magnesium, chlorine orheavy metals4. Improper use of dyebathassistants, e.g. surfactants,sequestrants, fixatives, lubricatingagents

1. Low dye diffusion2. High dye affinity

3. Dyes of poor migration4. Improper reduction of the dye5. Too much salt6. Rate of dyeing too high7. Rapid rate of rise oftemperature8. Low liquor flow rate9. Low mechanical action10. Poor rinsing before oxidation

11. Improper oxidation of thereduced dye12. Uneven washing/soapingafter-treatments13. Presence of iron/copper


1. Poor pre-treatment2. Dye substantivity too high






Poor colour yield








1. Optimum dye concentration2. Optimum electrolyte content3. Optimum temperature4. Good stability of dyedispersion (by ensuring optimumtemperature, pH, etc.)


1. Optimum reduction of dye2a. Use of appropriatecomplexing agents2b. Demineralisation, if metalspresent in the substrate

1. Prevention of air inside themachine2. Optimum amount of sodiumsulphide and/or alkali




1. Too high dye concentration2. Too high electrolyte content3. Too low temperature4. Poor stability of dye dispersion


1. Incomplete reduction of dyes2. Presence of heavy metals

1. Presence of air inside themachine2. Insufficient amount of sodiumsulphide and/or alkali

Physical damage


Foaming

Dye aggregation


Dull shades

Premature oxidation


APPENDIX K: Summary and Solutions to Problems in Dyeing with

Vat Dyes

Problem


Possible Cause


Countermeasure

1. Good pre-treatment2. Good pre-treatment andwashing after pre-treatment3. Water purification or use ofsequestrants




Poor washing off


4. Contaminants in salt, alkali,etc.5. Soiling of material by air-borne particles of powdered dyes6. Incompatibility of dyebathassistants7. Poor stability of dye dispersion

8. Too high rate of dye strike onthe substrate9. Short liquor ratio10. More dense/compact yarnareas appearing dark due to lessscattering of light11. Improper rinsing/washing-offafter dyeing

1. Inadequate rinsing of thefabric before oxidation2. Premature or over-oxidation ofthe dye3. Poor washing-off after-treatment



6. Water hardness




4. Too high moisture in the testmaterial5. Improper use of finishingagents

4. Good quality control ofcommodity chemicals5. Good housekeeping and use ofliquid dyes if appropriate6. Use of compatible dyebathassistants7a. Good dispersion stability7b. Optimum control oftemperature, pH, etc.8. Optimum process control




3. Thorough washing-off (seebelow the cause of ‘Poorwashing-off’)


1. Use dyes of good rubbingfastness2. No catalytic damage of thematerial3. Use of appropriate complexingagents4. Proper conditioning of the testmaterial5. Proper use of finishing agents




6. Use of mercerized cotton7. Use of optimum dyeingconditions8. Optimum washing-off


2. Use of soft water orappropriate sequestrants3. Use of appropriatesequestrants


1. Use of dyes of good diffusion2. Use of dyes of optimumaffinity3. Use of dyes of goodmigration4. Optimum reduction of the dye5. Optimum concentration of salt6. Optimum rate of dyeing7. Optimum rate of rise oftemperature8. Optimum liquor flow rate9. Optimum mechanical action10. Thorough rinsing beforeoxidation11. Optimum oxidation of thereduced dye12. Thorough and uniformwashing/soaping after-treatments13. Use of appropriatecomplexing agents14. Use of compatible surfactants

1. Good pre-treatment2. Use of dyes of optimumsubstantivity3. Use of dyes of gooddiffusion coefficient4. Use of optimum dyeingconditions

1. Good pre-treatment2. No residual peroxide in thesubstrate3. Optimum reduction of thedye4. Use of dyes of optimumsubstantivity




Poor colour yield

6. Un-mercerized cotton7. Incomplete diffusion andfixation8. Incomplete washing-off


3. Contamination of dyebathwith calcium, magnesium,chlorine or heavy metals4. Improper use of dyebathassistants, e.g. surfactants,sequestrants, fixatives,lubricating agents

1. Low dye diffusion2. High dye affinity

3. Dyes of poor migration

4. Improper reduction of the dye5. Too much salt6. Rate of dyeing too high7. Rapid rate of rise oftemperature8. Low liquor flow rate9. Low mechanical action10. Poor rinsing beforeoxidation11. Improper oxidation of thereduced dye12. Uneven washing/soapingafter-treatments

13. Presence of iron/copper


1. Poor pre-treatment2. Dye substantivity, too high






5. Optimum concentration ofelectrolyte6. Optimum dye concentration7. Optimum liquor ratio8. Optimum dyeing time9. Optimum oxidation of thereduced dye10. Optimum use of surfactants11. Careful weighing/dispensingof dyes and chemicals12. Use of soft water orappropriate sequestrants





1. Optimum dye concentration2. Optimum electrolyte content3. Optimum temperature4. Good stability of dyedispersion (by ensuring optimumtemperature, pH, etc.)


1. Optimum reduction of dye2a. Use of appropriatecomplexing agents2b. Demineralisation, if metalspresent in the substrate

1. Prevention of air inside themachine2. Optimum amount of sodiumdithionite and/or alkali





1. Too high dye concentration2. Too high electrolyte content3. Too low temperature4. Poor stability of dye dispersion


1. Incomplete reduction of dyes2. Presence of heavy metals

1. Presence of air inside themachine2. Insufficient amount of sodiumdithionite and/or alkali

Physical damage


Foaming

Dye aggregation


Dull shades

Premature oxidation




APPENDIX L: Summary and Solutions to Problems in Dyeing with

Azoic Dyes

Problem



Poor washing off

Possible Cause


4. Contaminants in salt, alkali,etc.5. Soiling of material by air-borne particles of powdered dyes6. Incompatibility of dyebathassistants7. Too high rate of dye strike onthe substrate (due to too highreactivity)8. Short liquor ratio9. More dense/compact yarnareas appearing dark due to lessscattering of light10. Improper rinsing/washing-offafter dyeing11. Aggregation of naphthol

1. Inadequate removal of looselyretained naphtholate beforedevelopment2. Inadequate removal of looselyretained dye after development

1. Naphthol substantivity, toohigh2. Naphthol of low diffusioncoefficient3. Short liquor ratio of thewashing bath4. Low temperature of thewashing liquor5. High electrolyte concentrationin the washing bath6. Water hardness


Countermeasure

1. Good pre-treatment2. Good pre-treatment andwashing after pre-treatment3. Water purification or use ofsequestrants4. Good quality control ofcommodity chemicals5. Good housekeeping and use ofliquid dyes if appropriate6. Use of compatible dyebathassistants7. Optimum process control


10. Optimum rinsing/washing-offafter dyeing11a. Proper preparation ofsolution11b. Optimum electrolyteconcentration (as little aspossible)11c. Good agitation/mechanicalaction

1. Optimum removal of looselyretained naphtholate beforedevelopment2. Optimum removal of looselyretained dye after development

1. Use of naphthols of optimumsubstantivity2. Use of naphthols of optimumdiffusion coefficient3. Optimum liquor ratio of thewashing bath4. Optimum temperature of thewashing liquor5. Optimum rinsing cycle/sequence6. Use of soft water and/orsequestrants7. Optimum washing time8. Optimum number of washcycles/wash baths9. Optimum mechanical action10. Compatible dyebathassistants




6. Use of mercerized cotton7. Use of optimum dyeingconditions8. Optimum removal of looselyretained naphtholate beforedevelopment9. Optimum removal of looselyretained dye after development


2. Use of soft water orappropriate sequestrants3. Use of appropriatesequestrants


1. No/little exposure ofnaphtholated substrate to lightbefore development2. No use of mixtures of fastcolour salts and bases together3. Optimum concentration of salt4. Optimum rate of dyeing5. Optimum rate of rise oftemperature6. Optimum pH control7. Optimum liquor flow rate8. Optimum mechanical action9. Use of appropriate complexingagents10. Use of compatible surfactants

1. Good pre-treatment2. No residual peroxide in thesubstrate3. Use of naphthol of optimumsubstantivity4. Optimum concentration ofelectrolyte



4. Too high moisture in the testmaterial5. Improper use of finishingagents6. Un-mercerized cotton7. Incomplete diffusion andfixation8. Inadequate removal of looselyretained naphtholate beforedevelopment9. Inadequate removal of looselyretained dye after development


3. Contamination of dyebath withcalcium, magnesium, chlorine orheavy metals4. Improper use of dyebathassistants, e.g. surfactants,sequestrants, fixatives, lubricatingagents, etc.

1. Exposure of naphtholatedsubstrate to light beforedevelopment2. Use of mixtures of fast coloursalts and bases together3. Too much salt4. Rate of dyeing too high5. Rapid rate of rise oftemperature6. Rapid shift of dyebath pH7. Low liquor flow rate8. Low mechanical action9. Presence of iron/copper


1. Poor pre-treatment2. Residual peroxide in thesubstrate3. Use of low substantivitynaphthol4. Too low concentration ofelectrolyte




Poor colour yield




5. Too low concentration of alkali6. Too long liquor ratio7. Insufficient time for naphtholapplication and development8. Improper use of surfactants9. Loss of chemicals duringweighing/dispensing10. Water hardness

1. Too long dyeing time (due tocorrective/repair procedures)2. Too high mechanical orhydraulic action

1. Low naphthol solubility2. Low dyeing temperature3. Low concentration ofelectrolyte


Too high electrolyteconcentration


Presence of heavy metals

Physical damage


Foaming

Aggregation ofnaphthol


Dull shades

5. Optimum concentration of alkali6. Optimum liquor ratio7. Optimum application/development time8. Optimum use of surfactants9. Careful weighing/dispensing ofchemicals10. Use of soft water orappropriate sequestrants


2. Optimum mechanical orhydraulic action

1. Good naphthol solubility2. Optimum dyeing temperature3. Optimum concentration ofelectrolyte


Optimum electrolyteconcentration


1a. Use of appropriatecomplexing agents1b. Demineralisation, if metalspresent in the substrate

Problems Possible causes Countermeasures

Critical solutions in_the_dyeing_of_cotton_textile_materials-libre(1)

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Transcript of Critical solutions in_the_dyeing_of_cotton_textile_materials-libre(1)