Dyeing of Reactives by Exhaust Method

8/2/2019 Dyeing of Reactives by Exhaust Method

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Dyeing of Reactives by Exhaust Method

DYEING OF REACTIVE DYES BY EXHAUST METHOD

REACTIVE DYES

EXHAUSTION PHASE

Primary Exhaustion Phase

Adsorption

Diffusion

Substantivity

REACTIVE DYES AND DIRECT COTTON DYES

Direct Cotton Dye

Reactive Dyes

Role of Electrolyte

Partition /Distribution Coefficient and Degree of Exhaustion

Liquor Ratio

Temperature

Influence 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 mostpopular for dyeing solid shades it is equally sought after for various resist

and discharge printing styles, thanks to its suitability to be resisted ordischarged readily and effectively

The reaction mechanism is apparently simple in that on just altering the pHafter exhaustion, formation of covalent bonds between the reactive group of
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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. Themanagement 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 toperform to obtain a satisfactory dyeing. As the shades invariably are tertiarymatchings, 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 inropes 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.

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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 andthe phenomenon of substantivity

AdsorptionDiffusion,

Absorption/ Exhaustion/Migration

Adsorption

It would be relevant to briefly look at cellulose structure with respect to itsHydrogen 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
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(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 arereported 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 thestarting 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 OHgroups 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 thezeta potential

This surface negative charge would repel the advances of the negatively

charged ionized dyestuff anions. The zeta potential is partially overcome dueto 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 Vaals forces/secondary valence forces to

facilitate the dye anion to get adsorbed on the surface of cellulose. Presence


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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 theconcentration 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 istermed 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

Where

F = Mass flow of dye gms/cm2 secD = Diffusion coefficient of the dye m2/sec

D0 = Diffusion Coefficient at Infinite Temperature

C1 = Concentration of dye in the dye bath g/cm3

C2 = Concentration of dye on surface of the fiber g/cm3L = Thickness of the layer cm

e, E, R = Constants (E activation Energy; e exponential; R Universal GasConstant)T = Temperature Kelvin

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

F is the dyestuff sorbed across

Unit area of the fiber surface inunit time (Rate)

Greater the surface area of the

fiber in contact with the dye bathgreater is the dyestuff sorbed.

(C1-C2) concentration gradient

during the process of diffusion.

The concentration gradient at the

initial stages would be higherand therefore the rate of dyestuff

transport to the fibre phase will

be correspondingly highertending towards zero at

equilibrium.


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D Diffusion coefficient Higher the Diffusion coefficient,

lesser the time taken to reach the

equilibrium. Time taken fordyeing 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 constructionwould 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 thesurface 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, thetemperature and the fibre surface area characteristics, besides the chemistry

of the dye chromophore. Substantivity ratio is the unit concentration of dyeon the fibre to the unit concentration of dye in the bath at the equilibriumstate (both expressed in the same units)


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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% ofthe 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 tocellulose cannot be improved or maximized, at the present status ofColouration 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 ofdyes (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


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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 thusdifficult 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 energyto 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 highertemperatures 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 byimparting 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 theHydroxyl 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 totallydiscarded as undesirable. Some of them could be counter productive. For

example, Reactive Dyes with features listed under I and 2 of the Directcotton 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 wouldentail 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 tobring 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 mostimportant 1. Degree of Exhaustion of the dye bath on to the fibre (bothprimary 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 theexhausted dye to the fibre should be maximized during fixation phase. 4. The

kinetics of reactivity has the final influence on the success of dyeing,


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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 oftime. 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 /exhaustionof 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 thesurface of the fibre and the subsequent diffusion/ absorption (exhaustion) to

take place. Secondly, the dissociated NaCl ions are more associated withwater 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 thesalt 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 areavailable for reaching the fibre phase The optimal quantity of Salt in terms of

concentration depends on the chemistry of the dye, its molecular size, itssolubilizing 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 fordifferent 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 numberof 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 isdesirable 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 iscalculated 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 solinearly 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 isgiven by the relationship


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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

CaseRecipe

owf

Substrate

Weight

Amount ofdye on

fibre*

Liquor

Ratio

LiquorVolume

(Wt)

Dyebath

Concn.

I 1% 100 Kgs1.0 Kgs

1: 5 500L 2 gpl

2 1% 100 Kgs1.0 Kgs

1:10 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 andit 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 Ethe degree of exhaustion.


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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 todisplace 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 ofElectrolyte) Therefore, not only increase in concentration of the dye, but alsothat 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/poorsubstantive 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 applicationof the given information only can give meaningful results.

The corollary is that a change in liquor ratio would affect the least in dyeswith 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 caseof 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 lesssubstantive increase in temperatures could be counterproductive Temperatureup to 50 deg C contributes to de-aggregation of the molecules of dye, both infibre 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 isdesorbed 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 indecreasing 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 theinfluence of salt..


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Influence of pH.

The pH is relevant to the Reactivity aspect and not considered as a factor inthe 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 thetemperature close to the reaction temperatures can be carried outconveniently.

Influence of Substantivity

High substantivity facilitates exhaustion process; also requiring less

concentration of salt for exhaustion but for the same reason migration of thedye 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 interms 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 dyeto 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

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 inthe 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 unevenbuild 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. Theother 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; butcannot 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


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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 ONa+) and starts forming covalent bonds with the reactivefunctional groups of the dye Chromophore. When more and more of dye

anions are covalently bond, the distribution coefficient shifts to fiber phaseeffecting further exhaustion due to deficiency of dye anions in the cellulose

phase and dye bath concentration starts depleting further. The degree ofalkalinity 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 systemtherefore, 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 theReactive 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 toform 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 andtemperature 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 highertemperatures require lower alkalinity; to optimize on hydrolysis. They can be

broadly grouped under High Medium and Low categories requiring 40

C. 60 C and 80.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 andlower 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 thecovalent bond takes place between the Cellulose O - and the Reactive group

of the Dye Chromophore.

1. Cynuryl chloride based dyes

2. Vinyl Sulphones

Dye-SO2CH2-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-OR1H (+) 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 CliffordPreston 1986 - SDC Publication}

It has also been emphasized that the expression is too ideal and relates tocertain 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 optimizedand also reproduced maintaining the same conditions and controls every

time.


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Top

Typical Examples

Reviewing the critical variables that govern the dyeing Efficiency in a

Reactive dyeing process, the following few examples will highlight the prosand 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. Onaddition 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 wouldbecome 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 islow 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 highPrimary exhaustion would be high andwhatever exhausted would be fixed. In this

case it would be critical during exhaustion

phase as the substantivity is high andmigration could be a problem. Higher

temperatures need to be resorted to for

migration and that would not be in favourwith Dyes of the low reaction temperatures in

view of its high reactivity. Such a situation

would warrant graduated salt additions to

avoid initial strikelinear or step wise in order to facilitate phasedmigration. 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 becritical. 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


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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 timedimension 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 theconcluding stages of the dyeing process.

Such a dyeing would require least effortfor soaping, However such an ideal

system is not practicable but efforts

should be to move towards the ideal

system Dyes with similar substantivitythat are moderate and having good primary exhaustion (assisted by salt

addition) and migration potentials and also a relatively lower secondaryexhaustion with reactivity reaching close to equilibrium exhaustion would be

the most suitable choice where auto dozing and sophisticated control systems

are not available.

Top

Evaluation of Substantivity

A very useful and simple practical method to assess substantivity of theReactive dyestuffs in the lab based on chromatographic principles is given inthe 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 ofthe 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

CelluloseBY M.J. Bradbury, P. S. Collishaw and S. Moorhouse, ZENECA Colours,

Blackley, England. August1995, Vol. 27, No. 8


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Dyeing of Reactives by Exhaust Method

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