Pad Batch Dyeing

Pad Batch Dyeing is one of the widely used technique for semi-continuous

dyeing process. It is mainly used in the dyeing of cellulosic fibre like cotton

or viscose (knit and woven fabric) with reactive dyes. Pad batch dyeing is

a textile dyeing process that offers some unique advantages in the form of

versatility, simplicity, and flexibility and a substantial reduction in capital

investment for equipment. It is primarily a cold method that is the reason

why it is sometimes referred to as the cold pad batch dyeing.

Working of a Cold Pad Dyeing Process

The technique or process used in pad-batch dyeing starts with saturating first the prepared fabric with pre-mixed

dye liquor. Then it is passed through rollers. The rollers, or padders, effectively forces the dyestuff into the fabric.

In the process, excess dye solution is also removed. After removal of excess dye stuff the fabric is subsequently

"batched". This batching is done by either storing it in rolls or in boxes. It takes a minimum of 4-12 hours. The

batches are generally enclosed by plastic films. This prevents absorption of carbon dioxide and water

evaporation. Finally as the reaction is complete the fabrics are washed. This is done by becks, beams, or any

other washing devices.

Special Features of Pad Batch Dyeing Process

Significant cost and waste reduction as compared to other conventional dyeing processes.

Total elimination of the need for salt and other specialty chemicals. For example there is no need for

anti-migrants, leveling agents and fixatives that are necessary in conventional dyebaths.

Optimum utilisation of dyes that eliminates specialty chemicals, cuts down chemical costs and waste

loads in the effluent. All this results in a formidable reduction in wastewater treatment costs.

Excellent wet fastness properties.

Pad batch dyeing cuts energy and water consumption owing to low bath ratio (dye:water) required for

the process. This is because unlike other dyeing processes it does not function at high temperatures.

A uniform dye quality is achieved with even color absorbency and colour fastness.

As compared to rope dyeing, Pad batch dyeing produces much lower defect levels.

In pad batch dyeing, qualities like high shade reliability and repeatability are common. This is because

of high reactivity dyes with rapid fixation rate and stability.

Lastly Pad batch dyeing can also improve product quality. The fabric undergoing the cold pad batch

dyeing process is able to retain an uniformly coloured appearance. It shows added luster and gives a

gentle feel. The fabric gives a brighter look in shades.

REACTIVE DYEING – Cold Pad Batch  

     Factors influencing Cold Pad Batch Dyeing

Substrate preparation and pH

Making up of  Colour and  Chemicals

Alkalie Proportionator

Fabric guiding system

The Pad box Dwell time in the pad box

Facilities to assist better application and pick of the colour.

Speed / Feeding rate and concentrationPad box pick up

Loading of the padding mangles

Selvedge thickness

Laboratory to Bulk reproducibility

Batch Rotation / reaction time

Washing off and Soaping.

Machine Cleaning 

In Process Quality Control Check List

 Reacatives -Cold Silicate Pad Batch

Exhaust method of dyeing and the related issues were discussed in the previous two articles

and the other important methods of dyeing of fabrics in open width with Reactives are:

       Cold Pad batch

       Pad – dry - Alkalie pad batch           

       Pad – dry or wet on wet Alkalie pad steam        

        Pad – dry - bake    

Cold pad batch system offers the most economical and most convenient method of dyeing

Reactives. The energy and water consumptions are the lowest and salt addition is totally

made redundant thus rendering it more eco friendly. Dyestuffs with relatively lower affinity

and high reactivity make them most suited for cold pad batch techniques. Because of the high

reactivity, fixation of the component colours is fully ensured and consequently

reproducibility of shades is more assured. Primarily it was being applied to woven fabrics and

with specialized features in the pad box and the guiding systems this is extended to knit wares

also. With the least inputs in terms of capital outlay energy, water, manpower and Right First

Time (RFT) capabilities, this method is the most cost effective option for dyeing substrates

that are amenable to padding operation.

Cold Silicate Pad Batch: 

This is the most commonly adopted method by most of the Process houses. Since the

Reactive colours are sensitive to alkalie to a lesser or greater extent and the general approach

is to apply the colour at neutral pH.and after the distrubution of the colour on the substrate,

the pH is raised for fixation. In an exhaust-dyeing situation we are able to provide the

required time dimension and Salt to facilitate exhaustion. In a pad situation, the exhaustion

phenomenon is replaced by the positive add on of the colour on the substrate and the

elaborate conditions of exhaustion and fixation are eliminated by padding the alkalie along

with the colour. The colour with the alkalie is mechanically squeezed between the mangles of

a padder and applied evenly on the substrate.  Since the alkalie is added with the colour the

primary precaution is to ensure that the reactive dyestuff does not hydrolyze. High (with

respect to speed) reactive colours generally are not stable and would loose the colour yield

due to hydrolysis with water. Even more stable reactive colours have their limitations in that

the different clolours in the recipe will have relatively different rates of hydrolysis and thus

not likely to give reproductive results. This problem has been overcome by the development

of ingenious alkalie proportionator units that help in mixing the alkalie and the dye in the

required proportions just before feeding into the pad bath, thus avoiding the propensity to

hydrolyze. Due to the high degree of reactivity, the dye fixes on the substrate during the

reaction dwell time that is dictated by the alkalie (pH.) and the time taken (rate of reaction)

by the slowest of the component dyes to fully react with the substrate at that pH.  Vinyl

Sulphones that are relatively slow reactive class of Colours are generally preferred for this

method of dyeing

 The alkalie proportionator is an equipment that facilitates mixing of the colour and the

alkalie in the required quantities and proportions just before the liquor is delivered to the pad

box for application; otherwise they are stored as two different stock solutions. The dye and

alkalie are mixed at a convenient volume ratio (generally 4:1) and correspondingly they are

made up to that extent stronger so that in the final liquor, the dyes and chemicals are present

in the required recipe concentrations The principle is very convincing but the problems arise

in the execution.

Factors Influencing Cold Pad Barch Dyeing

Substrate preparation and pH 

Substrate preparation has to be thorough as there is no scope for diffusion and migration

aspects of Exhaust dyeing and whatever applied through padding mangle should penetrate

uniformly across and along the fabric as the fabric speeds through the mangle at speed, where

the contact time could be just a few seconds. The preparation process is generally alkaline

and particularly after mercerization the fabric is rendered alkaline requiring effective

neutralization. This is best achieved with mineral acids preferably HCl; however the residual

mineral acid that would damage the cellulose needs to be neutralized again with Soda Ash or

Bicarbonate rendering  the pH invariably 8.5 or more and therefore again neutralized with

Acetic Acid.

Secondly the neutralization is carried out in the wash compartments of the Mercerizer

washing range and invariably the caustic carry over from the recuperator which is a wet on

wet leaching process shall not be efficient compounded by uneven expression at the mangle.

Unlike add-on mangles like dye pad the washing unit mangles invariably are less maintained

resulting in variation in alkalie across the width and length of the fabric causing different

levels of left over caustic reaching the neutralization zone. With limited contact time in a

speeding fabric movement in the washing range the neutralization invariably remains

incomplete in core areas and even in the surface areas where the caustic carried over is not

uniform as pointed out earlier.

Such a fabric .shall measure different pH levels across the width and length of the fabric

causing hydrolysis/spontaneous fixation however small selectively on the surface of the

fabric resulting in delta E variations exceeding 0.5 in the pad batch/continuous dyeing

systems causing selvedge centre variations, shade variation across the width and length

apparently insignificant but when cut and stitched juxtaposition would show marked

deviation in shade.  This problem is best addressed by the following procedure. It is

imperative that the caustic leached/extracted out at the recuperator of the Mercerizer should

be most efficient and the residual caustic carried over is minimal. A mineral acid passage

followed by neutralization of the mineral acid with Soda Ash and again a passage through

Acetic Acid to remove residual Soda Ash alkalinity does not guarantee uniform and or core

neutralization.  

Doing away with Mineral acid and employment of specialty chemicals -Organic acids based

Poly Carboxylates, substituted poly Carboxylic acids, hydroxy Carboxylic acids (Gluconates,

Citrates) and Sugar-Acrylic acid copolymers are in practice. The hydroxy Carboxylic Acids

and Sugar-Acrylic acid copolymers have the additional advantage of ready biodegradability.

Such Specialty chemicals like  Invatex AC of  Ciba or Sirrix 2UDI of Clariant or  Neutracid

Organic (organic +Inorganic buffers) of CHT, - through a dozing system  governed by a pH

control counter current closed circuit neutralization zone that give a steady uniform pH that is

faintly acidic (pH 6 to6.5) at the end of the range. With a higher  dissociation constant (Ka)

than that of Acetic Acid the organic protonizer ionizes more and faster and therefore

neutralizes more efficiently and faster; at the same time it does not tender or damage

cellulose Secondly the residual organic acid/ salt forms a buffer in the subsequent dyeing

operations giving a stable and uniform pH to start with. Fabric can be controlled to a uniform

pH of 6 to 6.5 (by extraction method) confirming that the core portion is also neutralized.

Such processed fabric when dyed by Pad- Batch technique invariably gives delta E variations

less than 0.5 when checked any where on the fabric. The neutralization system can be

retrofitted in a Mercierizer washing range with very minimal modifications to include

dozing /control.

Residual peroxide after peroxide bleach needs to be removed by peroxide ‘killers’ preferably

by specialty chemicals of enzymatic sources

    Making up of  Colour and Chemicals

The colours have to be dissolved and made up with cold water (in tropical countries with ice

cooled water) . as per the directions of the manufacturer to obtain a temperature of the liquor

below 20 deg C. The required amount of Urea is to be added for breaking of the hydrogen

bonding and disaggregation of the colour for easy dissolution. Filtering and cooling with ice

are recommended while making up to volume. The made up colour should be tested for

complete dissolution by drop test on the filter paper.  The Silicate should be from reliable

sources without contamination of heavy metals and Na2O : SiO2 ratio of 1 : 2 .1.The

chemical should be suitably diluted to obtain a 40˚ Tw solution and the recommended

quantity of Caustic Soda for the different depths of shades should be added and filtered

through a strainer. This would form the stock solution.  

    Alkalie Proportionator 

The proportionator’s capability to pump the required quantity linearly and not at intervals has

a direct influence on the uniformity of the shade. Some of the simpler designs operate on the

level control principle and therefore the liquor flow would be as per the signals from the level

controller. The limits for this operation cannot be set very precisely such that there is no

perceptible delay in the intermittent flow of the liquor. Therefore there is bound to be certain

disruption in the continuity of the liquor flow. Where the substantiality of the dyestuff is

higher, there will be preferential absorption of the relevant colour from the bath resulting in

that much depletion in concentration of that colour. In ternary matchings where contribution

of all the colors is important for the shade like in a Grey or Khaki, variation in shade cannot

be ruled out.

Where the pad box volume is high this problem would be more pronounced. Colours with

moderate substantivity and high reactivity in terms of fixation and higher stability to

hydrolysis (in time dimension) would be more ideal.for cold silicate pad batch method of

dyeing.  In the context of exact colour matching, the reproducibility is difficult unless care is

taken to eliminate the variables. Modern Pad batch systems provide the answers to the

problems with Proportioning units that synchronize with the speed of the machine and the

liquor off take - to continuously mix dye and alkalie in the required proportion just before

feeding the pad box in a linear fashion.   

    Fabric guiding System 

The fabric guiding system should be able to feed the fabric in a fully open form without

selvedge curls, creases or distortion. In the case of warp knit ware the guiders should have the

capability to uncurl the selvedges. At the batching stage there should be expander system to

ensure the batch is wound without creases.  

    Pad box  

The pad box volume needs to be as low as possible. A narrow ‘U’ tube like pad box with a

dummy / spacer in the center would help achieve sufficient long dwell time and also reduce

the pad box volume.  While this arrangement helps immediate application (consumption) of

the alkalie mixed colour on the substrate, it may not as much help in the penetration of the

colour in to fabric substrate particularly heavy fabrics. Therefore it would be helpful to have

built in lay on rollers in the pad box to facilitate penetration by additional squeeze passages.

Where sensitive /thin/delicate fabrics are involved the lay on rollers may be by-passed or can

be replaced by dummies to reduce the liquor volume. .

The dwell time once established for a given pad box features, the same speed should be

maintained for the relevant fabric sort. Therefore it would be prudent to establish two or three

speed categories for different sort groups and maintain these conditions every time. As the

speed also would influence the liquor pick up and the preferential pick up of the dye, it plays

an important part in the shade reproduction. This aspect also emphasizes the importance of

the fabric preparation that needs to be absolutely perfect with respect to absorbency, evenness

of the whiteness and dryness as discussed under fabric preparation.

It is also necessary to maintain the padding liquor tmperature at below 20 deg. C for which

jacketed pad box with cold  (ice cold water in the tropical  countries where the temperatues

can go vdry high) to avoid  destabilisation of the bath by hydrolyzation of component colours

to different degrees depending on their Reactivity and consequent tailing with variation in

saturation and hue, particularly for colours with lower stability at higher temperatures..

    Loading of the Mangles 

The success of a pad batch system is in the capability of the pad box to uniformly apply the

colour on the fabric substrate. Different loading systems with ingenious designs to avoid

deflection of the padding mangles under load have been discussed at length in earlier issue

under padding. The choice of such systems will depend on fabric types in terms of their

construction, weight and width. Where there are frequent changes in the fabric

characteristics, care need to be exercised to provide for suitable facilities to accommodate

changes. Continuous running of narrow width fabric followed by a run on wide width fabric

could cause problem of center selvedge variation even with the modern mangles. Alternate

running of both wide and narrow width fabrics in frequent intervals would reduce this

problem. Also periodical buffing of the bowls would be advantageous.

    Thick Selvedge  

Fabrics with selvedges thicker than the body have always posed problems during winding

into a big batch after padding.  The batch tends to develop a ridge at the selvedges as it builds

up and beyond certain size it becomes unmanageable where the fabric starts rolling over at

the selvedge giving crimps or short creases oblique to the selvedge. In a stenter batching

operation this problem is over come by selvedge shifting device and this may not be

successful in a pad operation. When such observations are made the best solution would be to

limit the batch to that size where the problem is not there. Some of the process houses resort

to insertion of flat paper at intervals at both the selvedges as the batch builds up and even out

the ridge. This works to a point. The best and permanent solution to this problem is to ensure

that the selvedge construction is taken care of at the weaving stage

    Laboratory - Bulk Reproducibility 

The padding mangle expression plays an important role as the colour picked up is directly

related to the expression. The absorbency of the substrate besides the additional features like

lay on rollers would also influence the pick up as already discussed. Therefore,

standardization of these parameters to meet certain norms should be established, monitored

and controlled every time.  

While matching the shade in Laboratory, the bulk application parameters should be borne in

mind and the parameters for the lab Pad should be modified suitably such that the shade

produced in the lab pad is reproducible in the bulk. As the dyestuff is already mixed with

alkalie, the mobility of the dyestuff to migrate would be limited and the fixation phenomenon

would restrict such mobility, unlike in an exhaust-dyeing situation. 

The pressure applied on the padding mangle, the dwell time (function of the speed of the

machine and the length of fabric immersed in the pad liquor - i.e. starting from entry in to the

liquor level in the pad box to the nip) should be manipulated and established to obtain the

shade that would reproduce in bulk with the same recipe. This can be established carrying out

a few trials. This exercise would be easier where the configuration of Bulk and Laboratory

pad boxes are similar. 

Where the lab pad box does not provide the features available in the bulk, say as in the case

of an ordinary pad box without lay on rollers and the bulk padder having advanced features,

even under identical expressions, the laboratory matching would tend to give higher colour

yield than the bulk for the same recipe. The bulk would require increase in recipe

concentrations, particularly in heavier fabrics. In other words the estimates of cost based on

Lab recipe would be adverse. 

In this instance, the explanation for this phenomenon is that the colour picked up does not

penetrate in to the fabric substrate as much as it does in the bulk model due to better facilities

and hence in the lab match ring dyeing type of application results giving an apparent colour

yield on the surface. In bulk, relatively more inner substrate cross section also gets dyed and

therefore requires that much extra colour for obtaining the same shade. If the laboratory could

simulate the same level of ‘efficiency’ of the dye penetration as in bulk the laboratory recipe

would be reproducible every time. 

Process houses tend to believe that when expressions of the bulk and laboratory pad boxes

are made identical there is nothing further that can be done. They accept this as unavoidable

and provide a factor for conversion to bulk, which does not work every time as different

dyestuffs would penetrate to different extents and a single conversion factor would not be

valid. It would therefore be necessary to match any shade in Laboratory to the same

efficiency of penetration/diffusion across the cross section of the substrate in order to get

reproducible results as for as the padding mangle operation is concerned. In a real situation

quite a number of trials had to be taken to simulate bulk-dyeing results at the stage of

laboratory matching. Once such conditions and parameters are set, the Laboratory pad would

behave in the same fashion as the bulk and therefore each of the dyestuffs would tend to

behave similarly at the laboratory and bulk padding stages.Establishing laboratory padding

conditions and parameters that would correspond to bulk would solve most of the problems

related to Laboratory to bulk reproducibility.

The padded fabric may be checked for shade by drawing a sample and exposing the same

over a water bath in a micro oven that facilitates an accelerated fixation.

    Batch Rotation / Reaction time

The dwell time for reaction (fixation) to complete would vary with the alkalie concentration

and the class of reactive colours used. There are shock develop (short time of 4 to 6 Hours)

and the long cycle times of 12 to 18hours systems -whichever the process, the dwell time

period should not be compromised. Care should be taken to ensure the batch is protected

from water drops or acid fumes during the period of fixation. Polythene covers that tightly

enclose the batch would serve the purpose. It is also necessary to rotate the batch during the

period of fixation, lest the alkaline liquor should collect at the lower portions of the batch due

to gravity that could result in intermittent variation in shade along the length of the fabric

Though these are elementary precautions, the operators tend to ignore, particularly when

there is a breakdown of the rotating motor. It would be prudent to have alternate banks for

rotating the batches to take care of such breakdowns. 

    Washing off and Soaping

Soaping is an important operation where the washing and soaping sequence has to be

followed meticulously to ensure complete removal of the silicate and the hydrolyzed colour.

Silicate on fabric needs to be washed off in the first two compartments with warm over

flowing water and then followed by soaping at near boil (need to establish temperature charts

for different dyestuff combinations) with good anionic soaping agents with small additions of

polyphosphate in the wash baths.

While soaping, where Vinyl Sulphone based dyestuffs are involved, it is necessary to have

luke warm / preferably cold water over flow in the initial soaping baths to remove unfixed

colour followed by acidification to bring the pH to 5- 6 or neutral before raising the

temperature to boil to avoid the possible dye fibre bond cleavage, whereas this precaution is

not required in the case of Chloro Triazine based dyes.

Once the silicate is eased out soaping operation is rendered more efficient. Compartment nos.

3, 4 and 5 are with soaping chemicals. Compartment 6- washing off; 7 and 8 wash/rinse at

lower temperatures Nos.3 to 6.can be counter current An eight-compartment soaper with an

average of 20 meters capacity in each compartment should serve the purpose for a good

soaping for fabric weights up to 200 grms of plain weave at a speed of 50 meters/min. The

soaping compartments need to be totally enclosed to maintain temperature parameters.  The

guide rollers should be absolutely true and smooth on their ball bearings with tension

adjustments to ensure crease free passage of  the fabric through the soaping range.

 As the fabric weight increases either the speed need to be reduced or number of

compartments should increase. Wash boxes with advanced designs provide accessories to

facilitate good agitation and therefore are efficdient over a range of speeds. Also certain

levels of over flow in the soaping zone (compartments Nos. 3 to 5) may be necessary

particularly for heavy shades. The last two compartments need to be lukewarm or cold.

For heavier weight fabrics neutralization of core alkalie to satisfy the extraction method,

addition of specialty chemicals like Invatex AC by itself or mixed with Acetic Acid (to save

costs) may be helpful instead of only Acetic Acid in the seventh compartment with a dozing

system like what is mentioned under neutralization in the Mercerizer. Compartments 7 and 8

can be counter current.  

It is pertinent to caution that Bi-carbonate hardness in water is generally neglected. The water

may apparently show neutral pH in cold but the fabric rinsed with this water in the last

compartment will show alkalinity after drying. Where the water is bought from different

sources and if this aspect is not taken into consideration one can get different results despite

other stringent controls. Check for Bicarbonate hardness and include removal of bicarbonate

hardness sequence in the process water treatment. Otherwise provision to neutralize

bicarbonate if any is to be built in the dozing and control system in the neutralization

compartment

    Cleaning of Pad Box /Soaper  

Cleaning of the Pad box, feed lines stock tanks and the pump is an important function after

every shade change. Likewise the cloth guiders and both the fixed and rotating tension bars

are to be meticulously cleaned. It is a general principle to plan discretely.the sequence of

change of shades from light to dark or from dark to light depending on the day’s programme

for dyeing   Sensitive shades like yellows and blues and pastel shades that show up

contamination of colour glaringly need to be handled separately after thorough

cleaning.Similarly at the washing off and soaping stages also the precautions need to be taken

to empty and clean the different compartment bath after shade runs that could cause

contamination in the next shade.Washing of the Padding mangle and unloading and lifting

/separating the bowls after the run before stopping for the day is mandatory as the silicate is

difficult to wash off once dried up and the Mangle will be rendered unfit for carrying out

padding of colour.  

Cold Silicate Pad Batch Quality Control Check List

    Fabric ready for Dyeing

Parameters Method of

Checking

Frequency Norms/Limits Action

White/Yellownes

Index  

Whiteness  Yellowness

AATCC Test

Method110 of

1979  

  

 

Every lot 

Every lot

Left    Cen     Right  

   

--- 80 (not < 75) --

       0.07 to 0.08

Reject,re-

Process or

segregate

before  use

for dyeing

Absorbency Spot test Every lot < 2 Secs.

pH. Indicator Every  lot Left   Cen    Right   

----------  6 –6.5

-----------     

Drying Feel at

different

places

Every lot Dry / room

temperature

Defects Check lot card

remarks

Every lot  No compromise

 

*Caustic Soda addition to ˚40 Tw Silicate in the following table .

  Concentration Caustic (38˚ Be /72˚ Tw

/ 32.5% w/w or 450g p

l)

Light  <30gpl 6.5   g p l

Medium 30 to 50 g p l 11.5 g p l

Dark 50 to 60 g p l 16.5 g p l

Very dark >70 g p l 21.5 g p l

 

The Process house can decide on the alkalie additions based on their Silicate quality and

working results.

Dye Make up

ParametersMethod of

CheckingFrequency

Norms/Limits

Recipe Against Lab/Std. Every Recipe  

Make up tank

cleaningVisual

Before every

make up

 

Sequence of colour

/chemical addition

Against std.

sequence displayed 

 No tolerance

 Silicate strength  ˚ Tw.

Every time

silcate is made

up.

 No tolerance

Addition of Caustic As per table*Every shade

group.

 No tolerance

Make up of colour to

volume

Metering device or

by dip rod 

 No tolerance 

DissolutionSpot test on filter

paperEvery make up

Clear circular

spreading; No

sediments.

 

     Fixation / Batch Rotation 

Parameters Method of Checking

Dwell Time Tag the batch Indicate time of start and end

Covering of the Batch Polythene cover fully enclosed and secured

Precautions Keep away from steam, water spray or acid fumes

Rotation Ensure uninterrupted rotation

 

 

Dye Pad  

Parameters Method of checking Frequency Norms/Limits Action

Cleanliness Visual /Manual Start of every

shade

   

Feeding line Visual / Manual Start of every

padding

   

Pad box

liquor level

Visual At the start of

every

padding

   

Mangle

Pressure

Read on dial Start of

Padding

Asper setting L   

M    R

 

Speed Speedometer As per table    

Batching Visual Continuously

Even batch

without creases/

other defects*

Look for

ridges and

take action

The cloth

guiders

Should be fully

functional

     

 

If fluff or thread attaches to the bowl repeat spots may occur, the bowl should be  cleared of

the contamination 

Washing and Soaping 

Compartment 1 2 3 4 5 6 7 8

Temperature Display norm temp for each compartment. Check

and record – every 15 min. The dial thermometers

should be functional.

Additions of

soaping

chemicals

Display starting, feeding quantities and frequency of

addition for each bath, Monitor additions.

Alternatively dozing can be arranged.

 The efficiency of soaping may be checked - take a window sample at the delivery end nip

and sandwich between bleached poplin (without optical brightener); contact dry on a hot

steam cylinder and check any staining of the white. There should be no staining.

Dyeing of Reactives by Exhaust Method

DYEING OF REACTIVE DYES BY EXHAUST METHOD

REACTIVE DYES

EXHAUSTION PHASE

Primary Exhaustion Phase

AdsorptionDiffusionSubstantivity

REACTIVE DYES AND DIRECT COTTON DYES

Direct Cotton Dye Reactive Dyes Role of ElectrolytePartition /Distribution Coefficient and Degree of ExhaustionLiquor RatioTemperatureInfluence of pH.Influence of Substantivity

Migration phase

Secondary Exhaustion

Hydrolysis of Reactive dyes

Typical Examples

REACTIVE DYES

Choice of Reactive class of Dyes has become indispensable for application of colours on the cellulosics to provide bright range of shades with reasonably good fastness features. No other class of colours can boast of the versatile range of shades with unmatched brilliance, yet economically viable and cost effective that this class of dyes can offer. Even as Reactive dyes are most popular for dyeing solid shades it is equally sought after for various resist and discharge printing styles, thanks to its suitability to be resisted or discharged readily and effectively

The reaction mechanism is apparently simple in that on just altering the pH after exhaustion, formation of covalent bonds between the reactive group of the dye and the OH of cellulose proceeds. For the same reason of ready reactivity

with Cell OH groups, it reacts with Water also to get hydrolyzed in which state the dye behaves no better than a direct cotton dye. The management of the various factors/variables that govern the transport of dye uniformly from an aqueous bath to the cellulose substrate and its preferential reactivity to the fibre than to water is far more complex and critical to perform to obtain a satisfactory dyeing. As the shades invariably are tertiary matchings, the behaviour of individual dyes with different exhaustion and reactivity characteristics, all the more compounds the complexity of the problems of differential shade build up, variations, uneven dyeings, reproducibility, fastness etc multifold.

Though there are other methods of dyeing ‘Reactives’ like pad batch, pad –dry-cure or pad-dry-steam etc exhaust dyeing is practiced widely because of its flexibility to process fabrics in rope form and in the case of yarn and other packages, exhaust dyeing is the only alternative as on date. Tubular knit-ware, by its very physical form is more amenable to exhaust dyeing in ‘rope‘s form; however, advanced machineries obtainable in recent years claim satisfactory open width dyeing by Pad Batch technique.

The exhaust method of dyeing would include the following phases

1. Primary exhaustion phase /Migration 2. Secondary exhaustion phase, 3. Fixation (Reaction) phase -Secondary exhaustion and Fixation can run concurrently/over lapping. 4. Washing off phase.

Top

EXHAUSTION PHASE

Primary Exhaustion Phase

Exhaustion of dye from the dye bath to the cellulose during Primary Exhaustion phase is governed by the following three physical processes and the phenomenon of substantivity

AdsorptionDiffusion, Absorption/ Exhaustion/Migration

Adsorption

It would be relevant to briefly look at cellulose structure with respect to its Hydrogen bonding behaviour at the surface layers and in the interiors of the cellulose micro fibrils The interior layers contain both forms - 1Alpha and 1 Beta of Cellulose molecular chains that are packed compactly and there are intra molecular Hydrogen bonding parallel to the 1.4 Beta Glucoside link (OH of #2 to  #6 of  the succeeding glucose unit and  #3 OH with the ring O of the preceding Glucose Unit)  that stabilize the cellulose chain.

The other four hydroxyl groups are fully free for Hydrogen bonding. At the surface layers of cellulose even the O-3 (OH) and 2-6 Hydrogen bondings are reported to be absent and therefore all the six Hydroxyl groups in the Cellobiose repeat units at the surface are free to attract Hydrogen bonding with the water molecules.

Adsorption in an exhaust dyeing process is fundamentally the inter-phase phenomenon of a dye (solute) in its solution in water coming in to surface contact with the substrate and forming a surface layer/ coating. That is the starting phase for the rest of the diffusion and absorption phenomenon.  In the case of Cellulose exposed to a dye solution in water at slightly acidic pH there is no ionization of cellulose. However, with abundance of ‘free’ OH groups available at the surface

(six numbers in each of the repeat Cellobiose unit), water molecules are drawn in clusters around the cellulose molecules to form hydrogen bonds causing an overall charge separation. Resultant surface thus carries a negative charge known as the zeta potential

This surface negative charge would repel the advances of the negatively charged ionized dyestuff anions. The zeta potential is partially overcome due to the presence of large amount of dye anions, some of which are forced across the electron cloud through increase in energy (raise in temperature) or through mechanical agitation to come within the effective distance for the inter molecular forces like Wander Vaal’s forces/secondary valence forces to facilitate the dye anion to get adsorbed on the surface of cellulose. Presence of electrolyte also helps in providing the positive charge that can effectively neutralize the zeta potential and improve the adsorption. (Discussed under ‘Role of Electrolyte’)

Diffusion phenomenon takes over followed by the absorption and migration of dyestuff across the cellulose membrane. Diffusion is influenced by the concentration gradient across the interface of cellulose surface and dye bath, the surface area of the cotton substrate in contact with the dye bath, temperature and time and the physical characteristics of the substrate. This is termed as the primary exhaustion phase. The term exhaustion would include the collective phenomenon of adsorption, absorption diffusion and migration in that order.

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Diffusion

Diffusion process is explained by the relationship (Ficks Law of Diffusion in its simplest form.)  F =      -D (C1-C2) / L                 And  D = Do e -E/RT         WhereF = Mass flow of dye                gms/cm2 secD = Diffusion coefficient of the             dye m2/secD0 = Diffusion Coefficient at Infinite TemperatureC1 = Concentration of dye in the dye bath           g/cm3C2 = Concentration of dye on surface of the fiber            g/cm3

L = Thickness of the layer          cme, E, R = Constants (E activation Energy; e  exponential;  R  Universal Gas Constant)T = Temperature Kelvin

Applying the above relationship the following dynamics may be inferred during the diffusion / exhaustion stages of the dye to the cotton substrate.

F is the dyestuff sorbed across Unit area of the fiber surface in unit time (Rate)

Greater the surface area of the fiber in contact with the dye bath greater is the dyestuff sorbed.

 (C1-C2) concentration gradient during the process of diffusion. 

The concentration gradient at the initial stages would be higher and therefore the rate of dyestuff transport to the fibre phase will be correspondingly higher tending towards zero at equilibrium. 

D Diffusion coefficient Higher the Diffusion coefficient, lesser the time taken to reach the equilibrium. Time taken for dyeing 50% of the equilibrium depth of shade is an index of the speed 

Temperature Increase in Temperature increases Diffusion coefficient.

Since surface area is a factor, the characteristics of the fiber and construction would influence the diffusion. Nature of cotton from different sources would have different shape, cross section, micronaire, fineness, impurities, etc and different packing densities of the cellulose molecular chains thus altering the surface area characteristics.  The corollary is that thinner the fibre/count and lower the density factor greater is the surface area available and better would be the diffusion.

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Substantivity

The term substantivity is primarily a measure of the amount of the molecular dye chromophore that can penetrate/diffuse into the interstices of cellulose micro fibrils assisted by physical forces from an aqueous dye bath. This is influenced by the salt concentration in the dye bath, the liquor ratio, the temperature and the fibre surface area characteristics, besides the chemistry of the dye chromophore. Substantivity ratio is the unit concentration of dye on the fibre to the unit concentration of dye in the bath at the equilibrium state (both expressed in the same units)

The process of primary exhaustion proceeds to its limiting values dictated by the substantivity beyond which it ceases. In the absence of salt, the dye uptake by substantivity phenomenon as stated above is around 20 to 40% of the starting bath concentration or lower, a figure far too low to have any significant economically feasible colour yield. Therefore, as a general rule, without salt additions, substantvity by primary exhaustion of Reactive dye to cellulose cannot be improved or maximized, at the present status of Colouration technology.

[Efforts are on for reduced salt /salt-less systems based on changes in the chemistry of the dyes to exhibit reduced anionic behaviour, fibre substrate modification/sensitization to display cationic behavior to induce exhaustion with less/no salt, while retaining the reactive system for the ultimate fixation. Such developments are still in the R&D Labs and not presently available for bulk]

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REACTIVE DYES AND DIRECT COTTON DYES

Reactive and Direct Cotton dyes sport similar dye chromophoric structures but for the Reactive groups present in the Reactive dyes as opposed to Direct cotton dyes. The Reactive dyes are smaller sized more akin to Acid class of dyes (not necessarily as a general rule) with Reactive groups.

Direct Cotton Dye

Direct Cotton Dyes molecules are engineered to include some or all of the important features listed below 1. More number of hydrogen bonding groups, groups that would facilitate inter molecular attraction / diminish repelling forces and groups that can chelate with hydroxyl groups of the Cellulose 2. Molecules of sufficiently large enough size and shape that on aggregation could get trapped in the interstices of the Cellulose molecular chains thus difficult to be removed/washed off.. 3. Optimized number of solubilizing groups (invariably ‘-SO3Na’), just enough for the dye to go in to aqueous solution. Dyeing is invariably carried out at boil, to provide the heat energy to facilitate diffusion and migration. Higher temperatures can also cause de-aggregation and consequent de-sorption Since the dyes have good substantivity due to affinity caused by physical forces like Hydrogen bonding, metal chelation etc. there is less propensity to desorb and higher temperatures facilitates migration within the substrate forming the same physical

bonding at new sites (High substantivity always causes an initial ‘strike’ – aggregation of colour in most favourable loosely packed sites and migration to other sites to increase uniformity in dyeing is facilitated only by imparting energy.) Fastness characteristics are just adequate even for the most satisfactory dyes of its class due to bonding only by physical forces that are relatively week to the more powerful covalent bonds.

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Reactive Dyes

Reactive Dyes are capable of forming chemical covalent bonds with the Hydroxyl groups of cellulose fibre and therefore, better anchored to the substrate and not depend on the relatively weak physical forces to give better levels of fastness. All of the features that are desirable for a reasonably ‘fast to wash’ Direct Cotton dyes are not essential for Reactive class of dyes (because of the more strong covalent bond), though cannot be totally discarded as undesirable. Some of them could be counter productive. For example, Reactive Dyes with features listed under I and 2 of the Direct cotton dyes would exhibit problems of low migration and or difficulty to wash off the hydrolyzed dye. Certain quantity of Hydrolyzed dye is inevitable after the fixation stage and non removal of such unfixed dye would entail bleeding/staining of white during washing. Migration is facilitated by increase in temperature; but higher temperatures induce hydrolysis of Reactive dye during the fixation phase and therefore it would be necessary to bring down the temperature to the most favourble temperature for the reaction between dye stuff and substrate before alkalie addition can be made. There fore, in the case of Reactive dyes the following aspects are most important 1. Degree of Exhaustion of the dye bath on to the fibre (both primary and secondary) that is directly related to the substantivity should be maximized /optimized (assisted more by salt addition than by the physical forces). 2. The migration of the dye within the substrate during the primary exhaustion phase should be maximized. 3. Efficiency of reaction of the exhausted dye to the fibre should be maximized during fixation phase. 4. The kinetics of reactivity has the final influence on the success of dyeing, irrespective of high levels of success achieved in the exhaustion stages, though

exhaustion is an important (primary and or secondary) pre-requisite... 5. The above four aspects need to be performed within a reasonable span of time. 6. The corollary here is that the extent of hydrolysis of the dye during exhaustion and fixation stages needs to be minimized.

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Role of Electrolyte

Addition of electrolyte induces exhaustion both its rate and extent. Where the substantivity is lower the prime driving mechanism for diffusion /exhaustion of dye into the fibre is the concentration gradient across fibre/liquor interface and presence of common ion- i.e. electrolyte (Salt). The electrolyte, say, Sodium Chloride dissociates in water into Na+ and Cl - and Na+ has higher propensity to travel to the fibre /water interface and neutralize the negative charge thus facilitating the free transport of dye anion to be adsorbed onto the surface of the fibre and the subsequent diffusion/ absorption (exhaustion) to take place. Secondly, the dissociated NaCl ions are more associated with water than with the large molecular dye Chromophore with a few SO3Na or other solubilizing groups and thus occupy the limited available sites in the water effectively displacing the dye Chromophore.The distribution coefficient of dye therefore shifts towards fibre. It is not the quantity of the salt but its concentration that influences the degree of exhaustion. The degree of exhaustion increases with increasing concentrations of Salt to a limiting concentration. Higher concentrations of Salt result in aggregation of the dye in the dye bath itself and hence ‘it is salted out’ much in the same manner as in the manufacture of the dyestuff and less and less monomolecular dyes are available for reaching the fibre phase The optimal quantity of Salt in terms of concentration depends on the chemistry of the dye, its molecular size, its solubilizing groups, quality of water and the fibre substrate etc. Secondly, dyes displaying higher substantivity in the absence of salt would need lesser salt concentrations.

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Partition /Distribution Coefficient and Degree of Exhaustion

At a given liquor ratio and bath concentration of dyestuff and salt, the exhaustion of the dye proceeds from the liquor phase to the solid phase (cellulose) until it reaches an equilibrium. This state would be different for different solutes (dyestuffs) and the factors that contribute to this variability are their molecular size, ionic character, extent of hydrogen bonding groups, inter molecular forces,  temperature etc.  Such equilibrium, where the number of molecules absorbed is equal to the number of molecules desorbed at the cellulose/dye liquor interface, can safely be assumed to have been reached in a time span of infinity, i.e. at the end of Exhaustion phase or Partition of the dye from the liquor phase to the solid phase at a notional infinite time  It is desirable that the exhaustion proceeds at a satisfactory rate to achieve close to equilibrium exhaustion within a manageable /practicable time span a condition that is influenced by diffusion coefficient. Higher the diffusion coefficient faster the exhaustion as discussed earlier under diffusion...

The Partition/Distribution coefficient of a solute between two phases is calculated as the ratio of the concentration of the solute in one phase to the concentration of the solute in the other phase under equilibrium conditions

Interestingly, at the equilibrium state of exhaustion where the concentrations of dye on fibre and in the final bath tend to become steady and constant, it is an established fact that as the dye bath concentration is increased, the concentration in fiber phase at equilibrium though increases, does not do so linearly but progressively diminishes giving relatively lower distribution coefficient values. 

Degree of exhaustion is the ratio of the total amount of dye present in the cellulose at the end of exhaustion to the amount of dye present in the original bath before the start of the exhaustion process.

Degree of Exhaustion in terms of distribution coefficient and liquor ratio is given by the relationship

Where   E          Degree of Exhaustion K          Partition coefficientL          Material Liquor Ratio

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Liquor Ratio

Recipe of x% owf (on weight of fabric) in terms of absolute quantity would be present in the starting dye bath but its concentration in the dye bath would vary depending on the liquor ratio

The recipe equivalent dye % on the fabric after the completion of dyeing would not be x% but would tend towards x% - depending on the efficiency of dyeing/the substantivity /reactivity of the dye. In an ionic kind of reactions like Acid dyes on wool the degree of exhaustion would proceed to almost to .100% subject to the dye present in the dye bath does not exceed the saturation capacity of the reacting sites present in the substrate.- the limiting degree of exhaustion in this case.

In a model scenario where the liquor ratio is changed to a higher one: Amount of dyestuff expressed owf, when present in the higher liquor ratio would register proportionately a lower concentration of the dye in the starting bath and consequently lower concentration gradient at the fibre liquor interface resulting in lesser rate of diffusion of the dye from liquor phase to fiber phase

Numerical Example

Cas Recip Substrat Amount of Liquo Liquor Dye

e e owfe Weight 

dye on fibre*

rRatio

Volume (Wt)

bath Concn.

I 1% 100 Kgs     

1.0 Kgs1: 5 500L 2 gpl

2 1% 100 Kgs   1.0

Kgs1:10

1000L  

1 gpl

*Arrow indicates’ tending towards’

Only 50% of the dye molecules are available at the interface for adsorption and diffusion in case 2.and therefore the rate of diffusion will be lowered and it would take relatively far longer time  to reach the equilibrium state.In case 1 starting from 1:10 going to 1:5, the increased concentration of dye in the bath would increase the rate of diffusion (increased concentration gradient) and take shorter time for exhaustion.

The relationship E= K/ (K+L) as discussed under Distribution coefficient (K); any increase in L would diminish the E –the degree of exhaustion. Such a situation would entail higher starting concentration of the dye and or increase in concentration of Salt to ‘occupy the available sites in water’ (as explained earlier under salt concentration) in a larger volume of water to displace the dye anion to shift the distribution coefficient to the fiber phase. But increased  salt addition cannot always fully compensate for the adverse exhaustion behaviour but only to a point  (as discussed under Role of Electrolyte) Therefore, not only increase in concentration of the dye, but also that of salt will be necessary (barring certain marginal cases) - quantitative aspects governed by the  substatnivity characteristics of the dyestuff.Such a situation would be more pronounced in the case of low/poor substantive dyes compared to the dyes with better substantivity. There are ready reckoners for recipe correction available for changes in liquor ratios from the dyestuff manufacturers but they are only for guidance. As individual dyes would behave differently, an intelligent understanding and application of the given information only can give meaningful results.

The corollary is that a change in liquor ratio would affect the least in dyes with high substantivity and most in those with poor substantivity

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Temperature

Temperature of the bath is another factor influencing exhaustion As explained earlier presence of salt increases the substantivity facilitating aggregation of the dye in the fiber phase Increase in temperature in the case of high substantive dyes as in the case of direct cotton dyes help in the migration of the dye within the substrate but in the case of dyes that are less substantive increase in temperatures could be counterproductive Temperature up to 50 deg C contributes to de-aggregation of the molecules of dye, both in fibre and water phases; but relatively less in fiber phase and more in the water phase. Therefore the net effect is that there are more de-aggregated monomolecular dye free to move towards the fiber phase than that is desorbed from the fibre and therefore the exhaustion proceeds. There is a maxima in the exhaustion curves of dyes of low substantivity at temperature around 40 to 50 deg C. beyond which increase in temperatures results in decreasing degrees of exhaustion explained by the higher degree of de-aggregation of the dye in the fiber phase and lesser physical forces to resist desorption, unlike in the case of substantive direct cotton dyes; annulling the influence of salt..

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Influence of pH.

The pH is relevant to the Reactivity aspect and not considered as a factor in the exhaustion process. Also, the dye bath pH during the exhaustion phase is maintained at 5.5 to 6. As long as the pH of the bath is slightly acidic, no reaction can take place and therefore primary exhaustion and bringing the temperature close to the reaction temperatures can be carried out conveniently.

Influence of Substantivity

High substantivity facilitates exhaustion process; also requiring less concentration of salt for exhaustion but for the same reason migration of the dye would be restricted resulting in unlevel dyeing. However dyes with medium+ substantivity engineered to provide the balance in the molecular structure to promote migration and good reactivity

that matches the exhaustion curve (primary and secondary) would give the best results both in terms of dye yield and washing efficiency. Poor substantive dyes that are also not sensitive to electrolyte additions are poor builders and therefore will give poor yields.

High substantivity .dyes with low reactivity (Fixation) falling below the exhaustion levels would result in high levels of unfixed and hydrolyzed dye to be washed off and the dye and its hydrolyzed version also being highly substantive, the washing efforts also will be high requiring more water, energy and mechanical efforts

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Migration phase

Since fiber surface area is a factor in diffusion process, the exhaustion would proceed to locations where relatively more surface area is presented like in the amorphous areas and less densely packed crystalline areas in that order in the cellulose and therefore the dye concentration within the cellulose substrate would not be uniform/even. Such a situation would result in uneven build up of the dye both in hue and intensity. In a trichromatic mixture the situation could be worse. The process of Migration of the exhausted dye depends on the molecular size of the dye its spatial profile (Steric) and the solubilizing groups present. The other external factors would relate to temperature, machinery used and the package profiles and densities (in case of package dyeings).  Raising the temperature would provide the required thermal energy; but cannot be increased arbitrarily due to limitations discussed under ‘Temperature’. Both exhaustion and migrations can be maximized /improved by better mechanical agitations that would facilitate intimate surface area contact of the cellulose with dye liquor and by improved flow designs that facilitate better liquor exchange at the fiber liquor inter-phase.

Migration phase should precede the fixation phase as once the reactive dye forms a covalent bond with Cell O- it is anchored strongly and cannot be shifted.

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Secondary Exhaustion

The observations and inferences in the above deliberations related to primary exhaustion in a Reactive exhaust dyeing process are incomplete without the final fixation. When Alkali is added, the cellulose ionizes to form Cell-O- and H+ (Cell O– Na+) and starts forming covalent bonds with the reactive functional groups of the dye Chromophore. When more and more of dye anions are covalently bond, the distribution coefficient shifts to fiber phase effecting further exhaustion due to deficiency of dye anions in the cellulose phase and dye bath concentration starts depleting further. The degree of alkalinity in terms of pH plays a major role in shifting the fixation of dye to its hydrolysis reacting with water. Any exhaustion during this stage if it is hydrolyzed dye it would be far more undesirable In a reactive dye system therefore, primary exhaustion alone does not govern the efficiency of dyeing. The degree of secondary exhaustion also would influence the efficiency. During the secondary exhaustion when alkalie is added, there is a second reaction that also sets in motion in parallel ( i.e. the hydrolysis of the Reactive dye with water) in competition to the fixation of the dye that is the primary aim. The dye anion is equally facilitated to react with OH of water to form the hydrolyzed dye in which state the dye is as good as a direct dye with all its ‘undesirable’ characteristics. It is the reactive group in the dye, pH and temperature that influence the hydrolysis of dye in preference to reacting with cellulose. It becomes critical that the hydrolysis is curbed to maximize efficiency. The relationship between temperature and reactivity is that higher temperatures require lower alkalinity; to optimize on hydrolysis. They can be broadly grouped under ‘High’ ‘Medium’ and ‘Low’ categories requiring 40º C. 60 º C and 80º.C respectively - levels of pH 12.5 for High (cold dyeing), 11.5 for Medium (Warm) and 10 - 11.0 for Low (Hot Dyeing) for the reaction to proceed more favorably towards the substrate. The term more reactive is used in the sense that it requires lesser levels of alkalinity and lower temperatures (and not the reaction itself. Given the right temperatures, alkalinity and time the reaction proceeds to completion in all cases.)

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Hydrolysis of Reactive dyes

The most critical part of the Reactive dyeing is the actual fixation where the covalent bond takes place between the Cellulose O -   and the Reactive group of the Dye Chromophore.

1. Cynuryl chloride based dyes

2. Vinyl Sulphones

Dye-SO2 –CH2-CH2-OSO3H    (MINUS)      H2SO4       Dye SO2-CH=CH2

The electron attracting Sulphone group causes electron deficiency on the terminal carbon atom enabling neucleophylic attack to take place. . (Addition reaction)

Dye-SO2-CH=CH2 + O-R1-         Dye-SO2-CH -- CH2-OR1                                                H (+)         Dye-SO2-CH 2- CH2-OR1

Where [-O-R1] is [-O Cellulose] or [-OH] of water, etc.The liberated acid in both the two reactions is continuously neutralized by alkalie for the forward reaction to proceed during the fixation process..Efficiency of Reactive dyeing (Rate of Fixation /Rate of Hydrolysis) for a given exhaust dyeing process has been expressed in mathematical terms making use of  the competing First order /pseudo first order rate constants of the reaction of the dye with the cellulose and the dye hydrolysis with water , the equilibrium concentration of the dye on fabric and  concentration of dye in  the aqueous phase (For details please refer Chapter 4 of ‘The Dyeing of Cellulosic Fibres’ by Maurice R Fox and Harry H Sumner Edited by Clifford Preston 1986  - SDC Publication}

It has also been emphasized that the expression is too ideal and relates to certain assumptions and conditions that are

not practically achievable in the real situation. However, the broad principles are applicable and the direction of the reactions proceeds towards the ideal. To whatever extent the variables can be controlled and maintained, the results achieved could be optimized and also reproduced maintaining the same conditions and controls every time.

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Typical Examples

Reviewing the critical variables that govern the dyeing Efficiency in a Reactive dyeing process, the following few examples will highlight the pros and cons of the factors discussed. 1. Low Primary exhaustion (P) and high Reactivity (R) (incidentally higher secondary exhaustion) Where P is low and Fixation is high

Initial exhaustion phase will not be critical as less amount of dye is transported. On addition of alkalie the reaction starts and the secondary exhaustion proceeds as more and more of the dye takes part in the reaction. During this phase the

competing reaction - hydrolyzation of the un-exhausted dye close to the substrate phase and in the dye bath also starts and it would become critical to control minimize this aspect of the reaction. The direction and rate of reaction towards covalent bonding with substrate have to be controlled by careful manipulation of pH and temperature. That would require precision instruments /plc controls. Secondly since the exhaustion is low and better part of the dye exhaustion takes place in the secondary phase, migration would be affected and the dyeing would be non uniform.

Where P, Substantivity and R are high

Primary exhaustion would be high and whatever exhausted would be fixed. In this case it would be critical during exhaustion phase as the substantivity is high and migration could be a problem.

Higher temperatures need to be resorted to for migration and that would not be in favour with Dyes of the low reaction temperatures in view of its high reactivity. Such a situation would warrant graduated salt additions to avoid initial strike – linear or step wise in order to facilitate phased migration. It would require cooling if higher temperatures were to be adopted. Because of the high reactivity pH control to maintain low and constant alkaline pH through out the reaction/fixation phase would be critical. Depending on the hot or cold class of colours the temperature maintenance will be critical.   In the above example where the substantivity before salt addition is relatively lower but enhanced by salt addition, migration would be better facilitated. It could be possible to standardize on an isothermal dyeing sequence starting with salt bath

The desirable features of the dyestuff would be to posess reasonbly good substantivity and migration capability, a good exhaustion percntage including the seondary exhustion  that are achievabe within a paracticable time dimension and  reactivity that matches the degree of exhaustion so that all the exhausated dye is fixed.

This would mean ideally that the curves S and F should super impose at the concluding stages of the dyeing process. Such a dyeing would require least effort for soaping,  However such an ideal system

is not practicable but efforts should be to move towards the ideal system Dyes with similar substantivity that are moderate and having good primary exhaustion (assisted by salt addition) and migration potentials and also a relatively lower secondary exhaustion with reactivity reaching close to equilibrium exhaustion would be the most suitable choice where auto dozing and sophisticated control systems are not available.

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Evaluation of Substantivity

A very useful and simple practical method to assess

substantivity of the Reactive dyestuffs in the lab based on chromatographic principles is given in the article “Effects of Dye Substantivity in the Dyeing of Cotton with Reactive Dyes” a prize winning article By Canadian Association of Textile Colourists and Chemists in TCC Nov 91).

The individual process house labs can conveniently assess substantivity of the dyes and group them for using in their recipe mixtures. The dyestuff manufacturers themselves recommend colours that have similar substantivity features; however it would be safe to assess in ones own lab unless supplied by propriety manufacturers.

Evaluation of Migration Index

Ref.material Practical method to evaluate migration Index  “Reactive Dye Selection and Process Development for Exhaust Dyeing of Cellulose”BY M.J. Bradbury, P. S. Collishaw and S. Moorhouse, ZENECA Colours, Blackley, England. August1995, Vol. 27, No. 8

Dyeing of Reactives - Further Options

 Pad colour – dry - Alkalie pad batch 

 Pad Colour - Dry - Chemical Pad - Steam    

 Pad Bake System

 The Problems and Precautions  

Pad colour – dry - Alkalie pad batch 

Selected Mono-chloro Triazine based dyes are mostly suited for this method of dyeing. Di-

coloro Triazine may also be used but as this class of dyestuffs can be dyed more easily at

lesser costs by other methods may not be an attractive proposition. Mono - Chloro Triazines

require higher temperatures for dyeing and therefore the cost of additional drying would not

be a constraint. Also, when the fabric is hot flue dried without alkalie in the system the colour

is evenly applied and the subsequent alkalie padding would ensure even fixation of the dye.

Due to padding of dried fabric at both the stages of dyeing and alkalie application better

control of the add on of the respective dyes and chemicals is possible and hence

reproducibility is far more assured.  

The fabric after dye pad and drying need to be air cooled by passage through skier rollers and

batched up. Covering with polythene sheets during the wait for subsequent alkalie pad would

help protect exposure to steam or fumes in a dye house atmosphere. Alternatively the system

can be made continuous. Choice of alkalie and its concentration in the pad would depend on

the class of colour and percent shade on weight of fabric and salt concentration of around 30

to 50 g p l should be sufficient – this needs to be established. Addition of 5 to 10% of colour

to the starting pad volume, depending on the shade and experience would help avoid initial

bleeding into the bath and consequent tailing in the first few pieces.  

The problem aspect of this method of dyeing is the drying part. Unless the hot flue features

are capable of providing the right conditions and controls, it could end up in face to face

variations, traveling creases, tailing etc. Anti-migration agents, which do not react with the

Reactives, need to be used. Small quantity of Sodium alginate itself can increase the viscosity

during the drying process and prevent migration when mixed with  the colour-padding bath.   

The alkalie-padded material in batch form should be covered with polythene and stored under

rotation, as discussed under Silicate pad batch for 2 to 20 hours depending on the reactivity

and the class of colours used. It is important to group the colours based on their colour yields

while matching the shades.                                           

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Pad Colour - Dry - Chemical Pad - Steam    

Pad colour - dry – chemical pad - steam instead of batch is much more effective and gives

better reproducibility of shades except for the higher energy costs. But the advantages gained

make it a very good option for dyeing of Reactives. The system can be made continuous and

the shades can be continuously monitored and controlled. The exhaustion and the related

influencing factors are totally absent and therefore control is that much more easy and

effective. Steaming is with saturated steam at around 102˚ C discussed under steamer  and

steaming time preferably 40 sec to 60 sec. Longer dwell time may be necessary for certain

colour/ alkalie combinations /substrates. Though the air free atmosphere inside the chamber

required for Vat colour development by steaming is not a  necessity in the case of Reactives,

in order that the steam is saturated and under an incremental positive pressure it is necessary

to ensure  full loading of steam 

 Padding the colour followed by alkalie padding (Wet on Wet) and steaming is an alternative

method that can be successful only if the control systems are absolutely fool proof. The

positive add on of the alkali should be consistent and the alkalie concentration /pH of the bath

should be maintained same throughout the run. With the modern padding mangles and

positive add on nips like Flex nips the confidence level to obtain RFT and consistency in

reproducibility is high.

Pad Bake System:

 In the pad bake systems, hither to only lighter shades were possible and with the advent of

the concept of having residual moisture in the baking zone, fixation levels have increased and

more dark shades and consistency are possible. High reactive cold dyeing dyes are best suited

to this process. The desirability of a transport vehicle to enable the dye and the alkalie to

migrate / penetrate in to the substrate is achieved by having a 25% RH (moisture level) in the

baking chamber. The second requirement of the reaction between the cellulose and the

Reactive dye is better facilitated by the optimum temperature and moisture present in the

system for the efficient fixation. The conventional addition of Urea is eliminated and the

necessity of salt as in the case of exhaust method is not applicable, thus providing an

environmentally friendly dyeing process.  

Specially selected cold dyeing Procions of MX type are primarily chosen for applications

developed by the Zeneca /BASF// Monforts in their Econtrol Maxi Dyeing system –ie.

padding the colour with Sodium Bi-Carbonate and Primasol NF (Wetting agent) drying

through their Thermex Hot Flue. (without any intermediate infrared drying) in an atmosphere

of 25% RH (moisture) inside the hot flue. The temperature in the hot flue is 120˚ C and the

time of treatment is 2 min under the established conditions. The fixation claimed is far better

than the pad batch or pad steam methods. The hot flue is provided with steam injection and

moisture monitor and control units that take care of the climate inside the hot flue.

The dyestuff with Bi-Carbonate is stable for over 6 hours and hence can be managed by

careful planning to avoid storage of the prepared dye in excess of 6 hours. In the E control

Flexi Option 1 system Alternative dyestuffs of bi-functional type like Procion CX or Basilen

F-M/F dyes with lower reactivity can also be employed by having stronger alkalies like

Caustic soda and or Soda Ash, but due to lower pad bath stability, alkalie proportionator will

be required and the rest of the drying under ‘Econtrol conditions would apply to give

satisfactory fixation.  . 

In the Econtrol Flexi Option 2 system dyestuffs other than Procion MX type, may be dyed

without a Pad/Alkalie dozer using milder alkalie systems like Soda Ash only in the dye pad

bath to ensure a working bath stability followed by a Econtrol hot flue (with 25% RH)

treatment and a further extra treatment of baking at 160˚ C for 1 min or steaming at 120˚ C

for 1 min. This option avoids the use of Urea and also Alkali/dye dozer; yet provide a

satisfactory dyeing system.

 The E control RTN Process for one bath dyeing of Polyester cellulosic blends employing

Procion CX / MX dyes and Dispersol C-VS and XF/SF dyes and selected Dispersol C and

Palanil dyes has also been  developed by the same group and is considered a successful

process. The single bath padding and drying under Econtrol system is further thermo-fixed at

210˚ C. Please refer BASF / The E Control ® Process Technical Information TVT 309 April

1997 for full details   

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The Problems and Pprecautions  

The problems that may arise shall be related to the following: 

·       Fabric preparation

·       Choice of colours

·       Choice of the available options

·       Pad bath capacity and Dye / alkalie dozing

·       Migration

·       Conditions in the hot flue.

 Moisture in the hot flue is an important factor for the colour yield and therefore the steam

injection and humidity control system should be in order to have the exacting requirements

without compromise. Rest of the aspects have been deliberated and the quality checks

desirable have all been enumerated in detail earlier

 While soaping, where Vinyl Sulphone based dyestuffs are involved, it is necessary to have

luke warm / preferably cold water over flow in the initial soaping baths to remove unfixed

colour followed by acidification to bring the pH to 5- 6 or neutral before raising the

temperature to boil to avoid the possible dye fibre bond cleavage, whereas this precaution is

not required in the case of Chloro Triazine based dyes.  

The means to avoid / control all the sources of problems are available and the will to make

use of these rests solely with the dyer. One must understand their limitations and choose the

methods; blaming the methods or the equipment without proper assessment of their

capabilities can hardly solve the problems.

Technical brochures from the dye manufacturers are only for guidance and one has to evolve

one’s process and methods based on these guidance provided. All technical brochures

conclude with a disclaimer that the information given is not guaranteed. There was the other

day, a leading manufacturer of dyes lamenting on the user ignoring the basics of the

minimum water quality or the preparation of the substrate and complaining about the non-

performance of the dye. This lament  from the dye manufacturer sure has substance and the

water quality and substrate preparation cannot be ignored. Avoidance of substandard quality

of dyestuffs/ chemicals is the responsibility of the user and so is his duty  to comply with the

quality norms of process water and substrate preparation to get the right results.

BLIND DYEING.    

Why Blind Dyeing

   Concept of Success in Dyeing Process in the 20th Century   

Key  Factors Affecting Dyeing Process               

   Quality of water                  

   Substrate preparation            

   Dyeability of substrate / Selection of Dyes         

   Weight of substrate / Weighing of Dyes and Chemicals

   Dispensing methods of Dyes and Chemicals                       

   Standardization of  Dyes            

   Moisture content of dyes            

   Moisture content of substrate at the time of weighing               

   Dye bath additives                 

   M:L ratio                      

   Time / Temperature /pH profiles.                  

   Machine Charaacteristics

Lab to Bulk Reproduction               

Role of Laboratory               

Management 

   Substrate - Yarn / Fabric:                       

   Dyes and Chemicals:              

   Automation:                   

   Computer Colour Matching:      

   Administration:                  

   Switchover to Blind Dyeing:              

Conclusion:

 

 

Why Blind Dyeing

 In the present context of global market and stiff competition with no protection like quota

system, unless the suppliers are able to provide quality at the right price and delivery at the

right time / just in time, they cannot be successful in the race for survival. Gone are the days

when it was a sellers market when one could get away with supply of substandard goods with

impunity at prices dictated by the suppliers. The buyers had no choice then; but now with

open market they are exposed to better quality and features at competitive prices that too at a

short notice.  They are now better informed and aware of their rights to get the best value for

their money. Meeting their requirements is the main thrust of the industry today.This scenario

emphasizes the need to reduce costs, improve quality and features and ensure timely delivery

besides reduce wastes and environmental pollution. In the area of dyeing, Right First Time (R

F T) is the only answer to achieve the needs emphasized.  The following table indicates the

cost of re processing.                                                      

  Relative Cost 

RFT   100

 + Sampling  105

 + One Shade Correction  135

 + One re-Dyeing  175

Higher the success rate of R F T greater is the possibility of reducing costs.  The QUALITY

also suffers by re processing, as the characteristics of the substrate are invariably affected.

Due to re- processing the cycle time is increased and thus there is a delay; there is a

production loss and also loss of capacity utilization. - A chain of undesirable consequences.  

Concept of Success in Dyeing Process in the 20th Century 

Dyeing processes down the ages have been closely guarded secrets handed down by word of

mouth and practice more by trial and error than understanding it scientifically. During the 60s

and 70s it was accepted that variations in dyeings are inevitable and hence skill is required to

correct. Greater this skill one possessed, greater he was sought after. Dyeing was considered

a fine art and dyers concentrated on their skill, unmindful of the consequences discussed

earlier. However a skilled dyer was able to minimize the stages of correction and therefore

contributed to relative reduction of losses. In the 80s and 90s, the concept of dyeing changed

and presently it is increasingly understood as a chemical reaction and therefore treated as a

Science. Like any other chemical reaction, it is governed by certain conditions and factors –

both physical and chemical that are predictable but not so easily controllable. The success in

dyeing requires a scientific approach and does not merely depend on skill. By monitoring,

manipulating and controlling the various factors and variables that govern the dyeing process,

it is possible to produce reproducible results consistently both in laboratory and in bulk.   It

was not possible in the 60s, as there had not been much advanced technology available for

controlling the variables. With the advent of plc, instrumentation, automatic dozing and

dispensing systems, Computer colour matching etc., it is possible to precisely monitor,

manipulate and control all variables / factors successfully. Skill required now is the

understanding of the physical and chemical reaction aspects that govern the processes that

convert the grey goods in to finished product. In other words any deficiency in the

intermediary processes in the sequential chain would reflect deficiency in the subsequent

processes in the chain and in the finished product. In any  conversion operation from raw

material to finished product  involving several intermediary processing units  each one of

them operate as  both ‘in-put’ and ‘out put’ entities-  i.e.  receiving from the immediate

predecessor as ‘in put’, the intermediate process unit does some work on it and delivers as

‘out put’ to the next unit for further processing –this chain continues until the finished

product is materialized.  In each of the input – out put transactions unless the quality of ‘out

put’ fulfills the requisite compliance of quality standards of the  receiving Unit, irrespective

of good work don on the material the product  would suffer quality compliance. The forgoing

discussions assume that the different substrates have undergone the requisite preparatory

processes to be fit for the dyeing different classes of dyestuffs

Key Factors Affecting Dyeing Process   

The majority of the factors and variables that govern a dyeing process are detailed below: 

Quality of Water 

The quality of water is an all important factor that can cause undue problems. Provided one

knows the tolerances that can be sustained in dyeing different classes of dyestuffs on different

substrates and techniques followed, certain liberties may be taken in the water

quality.However in a modern Process House processing variety of classes of colours on

different substrates employing different techniques it is prudent to have an optimally

acceptable water tolerance standards .Water quality prescribed by Messrs Thies and Japanese

sources  Appendix 1

 Substrate preparation  

Substrate preparation is an important phase in achieving quality dyeing and finishing.

Preparation required for different class of dyestuffs and dying processes may vary; may be

batch wise, semi continuous or continuous but the criteria for a substrate to be fit for dyeing a

particular class of colour and by a dyeing technique are to be met. Typical substrate

preparation for cotton dyeing (check list for quality parameters)   

 Dyeability of substrate / Selection of Dyes 

The dyestuff or the technique employed should be amenable for the dyeability of the

substrate. Using the wrong dyestuff or quantity of dyestuff beyond the saturation capacity of

the substrate cannot guarantee consistency even when other factors are conducive.  Colours

applied by techniques that are not suitable need to be avoided.

 ·        Choice of improper colours for exhaust dyeing in machines like winches, jiggers or

Soft flow/jet dyeing machines; ·        Resorting to high percentage of colours to obtain dark

shades. In a pad/ batch or pad /Continuous dyeing systems the pick up of colour in the pad

mangle is important. If the dye picked up differs during the operation 

 ·        either due to variations in the concentration of the feed dye liquor 

 ·        or differential substantivity amongst the component

individual colours picked up will vary and therefore correspondingly shade in the obtained

will also vary. Dyestuffs having varying substantivity in the recipe would be preferentially

picked up differentially from the pad box bath by the substrate by ‘exhaustion’/ ‘wicking’

phenomenon. If the shade is a single component matching it could be possible to feed the pad

box with higher concentration to compensate for the phenomenon mentioned; whereas for a

binary or tertiary match with widely differing substantivity this option is not possible.

Therefore selection of suitable colours with similar characteristics need not be over

emphasized  

 Weight of substrate / Weighing of Dyes and Chemicals  

This aspect is particularly important for Batch processes. Variation in the weight of substrate

for a given colour recipe and substrate weight obviously would case n the shade variation

from batch to batch. Similarly Weighment of dyes and chemicals for different batches has to

be accurate to give consistency 

 Dispensing methods of Dyes and Chemicals 

Dyes and chemicals are needed to be dispensed to the dyeing system. There should be

uniformity and consistency in handling and dispensing –sequence of additions, the mode of

additions like stock solutions or solid additions as the case may be should be consistently the

same  

 Standardization of Dyes 

Standardization is outside the control of the dyeing plant. The only method to control is to get

the supplies from propriety manufacturers of repute. Have good quality control laboratory to

check dyestuff against standards 

 Moisture content of dyes 

Moisture levels particularly in the case of Reactive Colours are critical and once dyestuff of

standard quality is obtained, ensure that it is immediately closed air tight every time it is

opened for dispensation. Secondly, sooner the colours are used better it is to reduce the

moisture related problems including hydrolysis for the dye 

 Moisture content of substrate at the time of weighing 

The moisture in the substrate is a relatively manageable phenomenon by good house keeping

principles and work practices.  The cotton substrate can absorb up to 8% at standard

conditions of 20deg C and 65%RH. It is necessary to dry the fabric to these conditions with

Moisture control Instruments. In the bygone days the material used to be  invariably bone

dried and in order to facilitate uniformity in conditioning  we used to pass this material

through a steaming zone followed by passing over a chilled cylinder (Cold water circulated)

before taken for dyeing.. 

Dye bath additives : Dye bath additives need to be consistently the same both in quality and

quantity. 

 M;L Ratio 

M: L is an important factor that can cause major havoc in consistency and reproducibility.

Refer article on Dyeing of Reactives by Exhaust Method 

 Time / Temperature /pH profiles.  : Refer   Dyeing of Reactives by Exhaust Method  

Machine characteristics 

It is essential to know the dyeing equipment’s characteristics and capability. Machineries are

made to perform for certain range of capabilities and that need to be respected. Selection of

machinery for a given dyeing operation is important. In case of a batch wise operation m: l

ratio is an important factor. Similarly different transport mechanism and feed system/ nozzle

size etc  like in the case of Soft flow and jet or winch  Machines; Heavy duty jiggers or

tensionless jiggers for delicate fabric operations  The designs are available for every need and

care need to be take to employ the most suited machinery for the  purpose.    Similar variables

apply to after treatment processes also. Blind Dyeing, which means, Right First Time dyeing

will not yield high success rate unless the above variables are controlled precisely

Laboratory to Bulk Reproduction 

The fundamental requirement in a laboratory is to ensure that the reproducibility of shades

within the laboratory is consistent. Technician to technician and for the same technician, from

one dyeing to another repeat dyeing, the Delta   E variations range as high as 3.5 to 4.5 units. 

Ideally, in a sophisticated laboratory with modern equipments and standardized practices the  

E variation shall not be more than 0.5 units. 

The general practice followed by a number of dye houses for translating laboratory to bulk is

to start with a recipe  - 10% to 15% lower and add one or more colours after taking out a

sample in the bulk to correct the shade. This practice has always involved time consuming

and cumbersome re-working procedures, yet, never given very satisfactory results. The

dyeing time cycles doubled or tripled thus resulting in resulted in the escalation of costs. 

 Experience shows that many problems for successful conversion of laboratory. scale to bulk

is    caused by wrong recipe choice.  This reality emphasizes the need to have a thorough

understanding of the dyestuff supplier and their product characteristics, the choice of colours

and also good understanding with the customers as to the tolerance limits for shade variation

to ensure continuity in the reproduction of acceptable shades, batch after batch.

 It has been found that many of the factors and variables that govern the dyeing process are

different in laboratory and in bulk. Most common offenders are Water quality, Type of

Substrate, Source of Dyes, Dye application method M: L ratio, pH. of the bath, Time

/Temperature profile, Method of assessing colour, Weight basis for recipe calculation. The

following example will show the significance of these variables. The limits of accuracy that

must be imposed on the most important variables to obtain reproducibility to a within a Delta

E of 1 unit are shown below.                     

Factor  Variation 

 Moisture Content of dye 3.5% 

 Moisture Content of Substrate  0.5%

 Weighment of substrate  0.5%

 Weighment of Dyes and Chemicals  <0.5%

 Dye Standardization  <2.5%

 pH of Dye-bath  0.35 Units

 In other words farther the deviations greater the variations in reproducibility. * Delta E

difference within 0.5 is necessary for more stringent matching and shading up pieces for

supplies to uniforms'.therefore the  critical factors need to be further controlled to reduce

variations.

Even having standardized these variables, it is necessary to measure and or monitor any

known or suspected changes in the sources of raw materials or machinery/procedures. There

must be a close coordination between the laboratory and production staff. 

The Role of Laboratory 

For blind dyeing operation to be successful, the laboratory has to meet the following

requirements: 

1     High degree of accuracy in laboratory Matching. The laboratory needs to be equipped

with automatic dispensing systems that can vouch for accuracy manned by technicians with

high discipline and perfection in work ethics to consistently follow standard procedures.

There need to be constant dialogue with the production department for the feed back on bulk

reproduction to fine tune the techniques and systems.   Process details are to be standardized

for both laboratory and bulk 

2.   Compatible dyeing methods that can be reproduced / co-related in bulk.

3.      Choice of Dyes that are compatible, give level dyeings and are robust to process

conditions. By this, it is meant, that the dyes and chemicals should have flexibility and

tolerance to accept certain degree of variations in the processing parameters and yet give the

desired results, like salt tolerance, pH. Tolerance, Temperature range etc. However it is   

4.      Accuracy of weighing and measuring should be of the highest order both in laboratory

and in bulk. 

 5.      Modern laboratory techniques, computer colour matching, electronic balances and

pipettes- all assist in improving efficiency and accuracy in laboratory. 

6.      Substrate used for laboratory matching should be of the same source as in bulk;

undergone same preparatory treatment in bulk. 

7.      After initial matching trials, the final shade for approval should be carried out on a

machine and system that can simulate the exact conditions in the bulk. Laboratory dyeing

machines are now available with sophisticated automation and instrumentation facilities.

However certain experimentation may be required to scale up to bulk    

Management 

   Substrate - Yarn / Fabric: 

This is the most difficult area to control, as high degree of discipline is required to source the

Grey.  Cotton being a natural fibre, its characteristics shall vary according to its geographical

and morphological factors; also on the grade of cotton. Sourcing should be done where you

have a close coordination with the supplier as to the specifications of the substrate. Same kind

of mix should be used for entire production of a particular requirement. Compromise on this

aspect for any reasons could jeopardize the ultimate end result. The management should take

a holistic view and operate. Even in the case of synthetics like polyester, merge numbers,

batch numbers are given for this purpose and good house keeping should be mandatory. In a

composite Textile Mills, it could be possible, but, in the case of commission processing units,

the same cannot be said.  Here again, management should have a good rapport with the

supplier units and prescribe specifications to which the supplies are to be made. In the

absence of such a possibility, a sample of Grey should be processed along with a bulk in

advance and dyeing trials taken with the help of computer colour matching and the necessary

corrections made at the laboratory stage itself.   

   Dyes and Chemicals:  

This area again demands greater discipline. Under no circumstances, can the constituents of

the recipe be changed between the laboratory and the bulk. Even the brands used in lab

should not be altered. Different brands of the same generic CI numbers of colours may have

different characteristics, standardization, strengths etc., thus may cause variations in the

dyeing behavior. Common excuses given in the dye houses are that the matching was done a

few months earlier and when the order came there was no stock of the particular brand of

colour, sometimes the entire recipe colours are not available. . Product substitution is done

for other considerations like cost, non-availability of funds and consequent refusal of supplies

by the supplier, etc.Presently in Reactives, which are the most popular class of colours used

for cellulosics, trichromatic system of colours are available which comprise of essentially

three colours -i.e. A blue, a red, and an yellow which can produce all the shades in the colour

chart. These colours are specially engineered to have similar dyeing characteristics for

substantivity, exhaustion, reactivity and washing off properties. A Navy blue, a Black and a

Turquoise supplement these. Specially designed dyeing systems are available to ensure high

degree of reliability. Chemicals and auxiliaries should be tested for consistent purity and

strength. The physical parameters like powder form or liquid form, concentrations etc. should

be consistently maintained.   Water should be of consistent quality with only permissible

hardness - preferably not more than 5 ppm both for lab and bulk. 

   Automation: 

Modern technology, particularly microprocessors have brought high level of controls to the

dyeing process. The process cycles, dozing, MPI- multi product injection systems, Time /

temperature profiles are programmable and there is very little human intervention once the

system is set up, thus ensuring reliability and consistency. Use of fixed weights of substrate,

constant batch sizes and m: l ratios are to be meticulously practiced. Due to automation in

dispensing and dozing of dyes and chemicals at consistent concentrations, the pH profiles are

also possible to be maintained. 

   Computer Colour Matching: 

Computer colour matching system has brought in a new dimension to the speed of matching

operations. The accuracy and predictability also have improved considerably. With a

rationalized range of compatible dyes, the system is very effective. The database should be

updated for any changes in the substrate or method of dyeing or dye quality etc. The light

source against which samples are to be matched needs to be specified and extent of

metamerism needs to be sorted out to avoid controversies later.  

Administration: 

The production planning and control should monitor and maintain records of material

movement in the process house right from the Grey receipt. Information should be available

in respect of product traceability. . They should be in close coordination with the various

departments and supply information on source fabric / yarn, batch size, process

specifications, management information etc. Recipe control and traceability in relation to

specific batches are very desirable.  With the advent of computers, all these control

information details are possible to be maintained. This enables quick analysis of the problems

/ errors, if any, for taking corrective action on variables that have been responsible and to

update the system to make it more efficient and successful. 

   Switch over to Blind Dyeing:  

Switch over process is not an easy exercise. It requires planning and execution of various

issues discussed as above. Involvement of all concerned with implicit faith in achieving the

objective is needed. There is a possibility of existing staff resisting to change and this

attitudinal resistance should be anticipated. It is desirable to induct open-minded technicians

with science background and to train them straightaway in the blind dyeing techniques.

Before that, the infrastructural requirements are to be provided. The apprehension, that there

are far too many variables that are to be addressed could be discouraging, but; it should be

understood that the modern technology provides for taking care of any number of variables

effectively. These are days of precision landing of spacecrafts on far of planets, which calls

for far more variables to be comprehended. Secondly in a processing environment

consistency in following and maintaining repeat procedures – not necessarily the ideal -

strictly under control is bound to get consistency in the end results. 

Conclusion 

It is established that Right First Time- Blind dyeing is possible with the availability of

modern technology. It is also emphasized that success rate in this system contributes vitally

to the three important requirements for survival in the Global Market in the changed

environment of free competition i.e. 1) Quality with better features, 2) Competitive price and

3) Timely / Just in time delivery 

Appendix 1

Dyehouse water quality

  Parameters Extracted from Textilmaschinen   Thies  

For efficient operation of the plant, as well as optimum dyeing efficiency. Watersupplied for

both direct and indirect processes, e.g. cooling, should meet thefollowing

specification.Mechanical or applications problems arising from using water which does not

reachthis standard, are not covered by our warranties and guaranties. 

Permissible concentration

Colour colourless

Smell odourless

pH-value neutral pH 7-8

Water hardness < 5 °dH (6,25°eH; 8,95°fH; 5,2 USA)

Suspended solids < 1 mg/l

Filterable soldids < 50 mg/l

Organic substances < 20 mg/l (KMnO4- consumption)

Solid residues from evaporation < 500 mg/l

Iron (Fe) < 0,1 mg/l

Manganese (Mn) < 0,02 mg/l

Copper (Cu) < 0,005 mg/l

Nitrate (NO31-) < 50 mg/l

Nitrite (NO21-) < 5 mg/l   

Japanese Sources 

This table shows a typical water quality standard for dyeing.  

Parameter Standard value

Total hardness 0~30ppm

SiO2 15~20ppm

Bicarbonate 0

pH 6.5~7.4

Fe 0.05ppm＞

Mn 0.05ppm＞

Ca 3.0ppm＞

Mg 0.5~1.0ppm

Al 0.5~1.0ppm

Turbidity 3＞

Color 4＞

Asai; Shin-senshokukako-koza Vol 2. p199. Kyouritsu Publishing(1972)