reactive dyes

8/6/2019 Reactive Dyes

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Dyestuff Chemistry

Reactive Dyes



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Introduction to Reactive Dyes

Although fiber-reactive dyes have been a

goal for quite some time, the breakthroughcame fairly late, in 1954.

Prior to then, attempts to react the dye and

fibers involved harsh conditions that often

resulted in degradation of the textile.



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The best dyes, by far, to use for cotton and

other cellulose fibers are the fiber reactivedyes.

They are much brighter

longer-lasting, and easier-to-use



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Fiber reactive dye is the most permanent of

all dye types.Unlike other dyes, it actually forms a covalent

bond with the cellulose or protein molecule.



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Once the bond is formed, what you have is

one molecule, as the dye molecule hasbecome an actual part of the cellulose fiber

molecule



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The first fiber-reactive dyes were designed

for cellulose fibers, and they are still usedmostly in this way.

There are also commercially available fiber-

reactive dyes for protein and polyamide

fibers.



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In theory, fiber-reactive dyes have been

developed for other fibers, but these are notyet practical commercially.



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The first fiber-reactive dyes contained the

1,3-5-triazinyl group, and were shown byRattee and Stephen to react with cellulose in

mild alkali solution.

No significant fiber degradation occurred. ICI

launched a range of dyes based on thischemistry, called the Procion dyes.



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This new range was superior in every way to

vat and direct dyes, having excellent washfastness and a wide range of brilliant colors.

Procion dyes could also be applied in

batches, or continuously.



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Chemistry of Reactive dyes

Chromogen Fiber Reactivegroup

water solubilizing

group

Bridging group



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Reaction with Dye

This reaction takes place in presence of a base

Dye-Cl + H-O-cellulose base Dye-OH +salt

The bond formed is covalent which provides

excellent wash fastness and is stronger then the

hydrogen bonds of a direct dye on cellulose.



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Reaction with water

In reaction with water the reactive group ishydrolyzed and the dye looses its ability to

react with hydroxyl group of cotton or amino

groups of poly amide

Dye-Cl +H-OH base dye-OH + salt



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The chromogen is as mentioned before (Azo,

carbonyl or phthalocyanine class). The water solubilizing group (ionic groups,

often sulphonate salts), which has the

expected effect of improving the solubility,

since reactive dyes must be in solution for application to fibers.



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The bridging group links the chromogen and

the fiber-reactive group. Frequently the bridging group is an amino,

-NH-, group.

This is usually for convenience rather than

for any specific purpose.



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The fiber-reactive group is the only part of

the molecule able to react with the fiber.



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A cellulose polymer has hydroxyl functional

groups, and it is these that the reactive dyesutilize as nucleophiles.

Under alkali conditions, the cellulose-OH

groups are encouraged to deprotonate to

give cellulose-O- groups.



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These can then attack electron-poor regions

of the fiber-reactive group, and perform either aromatic nucleophilic substitution to

aromatics or nucleophilic addition to alkenes



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Reactive dyes can react both with water and

fiber but the main reaction is with fiber for 2reasons

Rate of chemical reaction with cellulose is

much faster than the reaction with water



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the probability of a diffused dye molecule

reacting with an OH group on cellulose ismuch larger than the cellulose group of water

In dyeing of cellulose with reactive most of

the dye reacts with fiber but some dye do

react with water.



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Unfixed dye composes of hydrolyzed dye

and unreacted dye The amount of unfixed dye can be 20-30% of

the total amount of dye used



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Properties of reactive dyes

Advantages

Available in a complete range of colors Very good wash fastness

Good to very good light fastness

High flexibility in the choice of method of application



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Easy to obtain level dyeing

Readily soluble in water



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Disadvantages

Cost of using reactive dyes is high Loss of substantial dye during application

Excessive time required for the dyeing process

Sensitive to oxidation



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Auxiliaries used in dyeing with

reactive

In the dyeing of cellulose with reactive dyes,

alkali is necessary it acts as a catalyst in the reaction between

the dyestuff and the fiber.



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The important point is not the type or amount

of alkali but rather the pH of the dye bath,which must be closely supervised.

Adding base increases the exhaustion of the

dye on the fiber



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It increases the rate of chemical reaction and

faster the rate of reaction, the faster the rateof exhaustion

Ionic nature of base further enhances the

exhaustion



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The most suitable pH for dyeing varies with

the temperature being approx. 11.5 for common warm dyeing

(dyeing at approx. 60 degrees )

10-11 for hot dyeing (80 )

12.5 for cold dyeing (40 degrees)



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Auxiliaries used in Reactive Dyeing

contd

Inorganic salts have two main functions in

exhaustion dyeing with reactive dyestuffs: Improving the affinity of the dyestuff

Acceleration of the dyestuff's association and

lowering of its solubility.



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Improving the affinity and thus the exhaustion

of dyestuff during primary exhaustion servesto raise the exhaustion rate of reactive

dyestuffs. Because reactive dyestuffs have a

lower affinity than direct dyestuffs, more

inorganic salt is required when using reactivedyestuffs in order to accelerate absorption



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The following graph displays the amount of

Glauber's salt used, the exhaustion rate and

the fixing rate for a conventional dye and a

high-fixation dye (Sumifix Supra E-XF).



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Because inorganic salts accelerate the

association of water-soluble dyestuffs and

lower the solubility of dyestuffs, an excess of

inorganic salt may in fact impede the

absorption of some dyestuffs, and so care is

necessary when one is used.



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The effectiveness of the inorganic salt is not

decided by the ratio of its quantity to the

quantity of fabric (o.w.f.) but rather by its

concentration in the dye bath (g/l)

so it is by lowering its concentration that the

effect of an inorganic salt can be reduced.



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Suitable fiber substrate

Cellulosic fibers

Protein fibers Polyamide fibers



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Methods of application

Exhaustion method

It consists of three stages Exhaust Dyeing

Fixation

After scouring



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Cold Pad-batch method

Semi- continuous method Fabric is padded with a solution of dyes and

the base and then wound onto a roller. which

are covered in plastic sheets

Left for 2hours to overnight depending on therate of fixation



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Continuous dyeing with reactive dyes

There are 2 methods currently in use byindustry

Two-Bath Method

the dyes and base are applied separately



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One-bath method

In this the dyes and the base are appliedtogether onto the fabric from the same

padding bath



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Chemical striping of reactive dyes

Once reactive dyes have been applied and

the unfixed, hydrolyzed dye has beenremoved.

there is no longer a possibility of

leveling or stripping by simple physicalmeans.



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If a reactive dyeing is unsatisfactory,

economics will probably dictate whatsteps must be taken to deal with the problem.



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For heavy shades, a reduction of about

20-40% in depth of shade can sometimesbe achieved by hydrolysis of the dye-fiber

bond with alkali in the presence of salt at

95C (200F)



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Depending on the individual dyes, complete

chemical stripping (color destruction)may be carried out in one of the

following four ways



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reduction only;

Reduction followed by oxidation;Oxidation only

oxidation followed by reduction.



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all traces of chemicals should be removed

prior to any re-dyeing attempts on a sampleof the stripped goods.



General fastness properties

light fastness- good to very good on most

fibers

Washing- good, sensitive to chlorine bleach

Dry cleaning- good

Perspiration- good

Crocking- good

reactive dyes

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