Rate of Exhaustion Compatibility of Dyes

8/12/2019 Rate of Exhaustion Compatibility of Dyes

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Terminology

By Hammad MohsinSpr 2009

Exhaustion The degree of dyebath exhaustion as a

function of time describes the rate andextent of the dyeing process.

DEFINE ? the mass of dye taken up by the material

divided by the total initial mass of dye inthe bath, but for a bath of constant volume


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C0 are the concentrations of dye in thedyebath initially and

Cs at some time dur ing the process,

Exhaustion rate as f(time)


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The degree of exhaustion of a dye atequilibrium is higher the greater the

substantivity of the dye for the fibre beingdyed.

Often, a very substantive dye will g ive a highinitial rate of absorption, or strike.

Substantivity is the attraction between dyeand fibre whereby the dye is selectivelyabsorbed by the fibre and the bath becomesless concentrated.

Both the initial strike and the rate ofexhaustion vary with the dyeing conditions.

Cautious about? Initial Strike Overall rate of dyeing Depth of color in given time Equilibrium conditions

Dyers prefer a high degree of exhaustionin a relatively short dyeing time


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variations in dye exhaustion (a) reaction with the fibre (b) temperature (c) chemical additions, for dyeing cotton with a reactive dye of low

substantivity and reactivity

Liquor Ratio gives the weight of the bath solution

relative to the weight of the dry materialbeing dyed.

Since the density of the solution is usually

very close to 1.00 kg l1, the liquor-to-goods ratio is usually given in l kg1


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An increase in the liquor ratio causes adecrease in the degree of exhaustion,under the given conditions,

decrease in the depth of colour of thedyeing.

If dyeing is described by a simple equilibrium constant K inthe form of the ratio of the concentration of dye in the fibre(C f g dye kg1 fibre) and the concentration in the bath ( C s gdye kg1 solution), K will be given by:

where E is the fractional exhaustion and L is the liquor ratio.


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Because K is a constant, under givenconditions, an increase in the liquor ratio Lis accompanied by a correspondingdecrease in the equilibrium exhaustion E .

Rate of dyeing and Strike Dyeing rates has more practical significance

than equilibrium conditions

Why? To continue at equilibrium isuneconomical

Slow dyeing involves long dyeing times withincreased risk of fibre damage and dyedecomposition, particularly at higher dyeingtemperatures. It is too costly.

On the other hand, very rapid dyeing will usuallyresult in the colour being unlevel.


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Dyeing consists of three steps:(1) transfer of dye from the bulk of the solution to

the fibre surface;(2) adsorption of dye onto the fibre surface;(3) diffusion of adsorbed dye into the fibre interior.

The adsorption equilibrium is usually rapid, and

the overall rate of dyeing depends on the rate ofdiffusion of the dye into the fibres.

This implies that the rate at which dyeliquor arrives at the surface of thefibres does not control the dyeing rate


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Dyeings on filaments that are finer alsoappear paler. These two effects haveconsiderable impact on the dyeing of thenewer synthetic microfibres.

STRIKE: The initial rate of dyeing (theinitial slope of exhaustion versus time)

High Strike Rapid strike by a dye often results in initial

unlevelness and must be avoided forthose dyes that

PROBLEM?? cannot subsequently

migrate from heavily to lightly dyed areasof the fabric.


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Strike DependencyThe strike depends on the dyeing temperature, the dyeing pH, and the addition of chemicals.

Even for dyes of moderate and low str ike, theobjective of uniform dyeing of the fibre mass israrely achieved during the initial stages of theoperation.

This is because of irregularities in the materialsconstruction, in the fibre packing

and in the distr ibution of residual impurit ies, as wellas differences in temperature

and flow rate of the solution in contact with thefibres.

Strike Test After a few minutes of dyeing a sample of

fabric, a second undyed sample is added

the two samples are then dyed together

for several minutes, removed and rinsed.


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Results

For dyes of high strike, the first sample ismuch deeper in colour than the second

Dyeing of the two samples may becontinued to establish how long it takes forthem to have the same final colour.

Other version Alternatively, a series of samples may be dyed in

succession for a constant short period in the same bath.

As each dyed sample is removed, it is replaced by anundyed one, and so on.

After the test, the coloured samples are placed side by side in the order they were dyed. Rapid dyeing dyes give

a short series of heavily dyed samples, the later samples being much paler. For

slow dyeing dyes, the colour is less deep but the depth is much more uniform over the

entire range of samples. This type of test may be carried out with a constant or

variable bath temperature.


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Effect of Temperature and Dyeing

rate The rate of dyeing increases with anincrease in temperature but the finalexhaustion may increase or decreasedepending upon the particular dyeingsystem.

Figure illustrates that increasing thetemperature increases the initial rate ofdyeing.

This is the usual effect of temperature onthe rate of a process.

Dyebath exhaustion as a function of timefor slowly (b) and rapidly (a) dyeing

dyes at two temperatures, where T1 > T2


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Le Chateliers principle predicts this effect oftemperature for an exothermic dyeing process.

This principle states that a system at equilibrium

reacts to an applied constraint in a manner thatminimises its effect.

Dyeing is usually an exothermic process (heatreleasing); the interaction between the dye and fibremolecules is stronger than between the dye andwater molecules in solution.

The dyeing equilibr ium reacts to an increase intemperature by absorbing more heat energy.

It therefore shifts in the endothermic di rection (heatabsorbing), dye desorbing from the fibres so that thefinal exhaustion is less at the higher temperature.

Dye compatibility In combination colors, dye mixtures are

used. These should all be dyes with about the

same rate of dyeing They should have very similar rates of

exhaustion when used in mixtures underthe given dyeing conditions.

dye absorb at the same rate= nochange in hue = always on requiredshade


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Mixtures of dyes that build up on shade,with no change in hue, are said to becompatible

Testing dye compatibility Dyeing of dye mixtures in big number but

at different times. a sample of the dyed fabric is removed

from a given container. replaced with an identical piece of undyed

material. Each dyeing is then continued for thesame total time.


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The series of samples removed from thedyebath, arranged in order of the samplingtime, will have decreasing colour depths.

The series of replacement samples, inorder of their introduction time, will haveincreasing colour depths.

RESULT: For compatible dyes, all thedyed samples will have the same hue, with

colour depths in relation to the actualamount of time spent in the dyebath.

Migration and leveling Migration of the dye demonstrates that the dye

can be desorbed from more heavily dyed fibresand re-absorbed on more lightly dyed ones.

While migration is important for level dyeing, ithas two major drawbacks.

1- the dyes ability to desorb from dyed fibresduring migration usually means that thedyeings have lower fastness to washing.


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Dyes of very high washing fastness areessentially non-migrating dyes for which leveldyeing depends upon very careful control of therate of dye uptake by the material.

2- with migrating dyes is that good migrationmay result in lower exhaustion, againbecause of their ability to desorb from thefibres.

Migration Test Migration tests characterise a dyes levelling

behaviour. Undyed fabric is treated with an identical dyed

sample in a blank dyebath, with all the requireddyeing chemicals but no added dye, under thesame conditions as in dyeing

After a given time, the degree of transfer ofcolour from the original dyed to the undyedsample is evaluated.


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a dye capable of migration easily transfersto the undyed fabric and after a short timeboth samples will have the same amountof dye and the same colour depth.

There will also usually be some dye in thesolution.

Exactly the opposite is found for non-migrating dyes. Little dye transfers fromthe dyed to the undyed fabric.

Rate of Exhaustion Compatibility of Dyes

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