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| Fig: Types of reactive dyes for wool |
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| Fig: Reactions of bromoacrylamido reactive dyes with wool | | | |
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BATCH DYEING OF WOOL WITH REACTIVE DYES
Wool fibre is a animal protein fibre. The chemical construction of the wool fibre is very much complex but contains a reactive functional group that used to react with the reactive group of reactive dyes. The somewhat late development of fibre-reactive dyes was partly caused by a lack of appreciation of the considerable reactivity of made of cellulose fibres (specially cotton fibre) or proteins fibres (specially wool fibre).
A number of dyes developed for wool with 2-sulphatoethylsulphone or chloroacetylamino groups were not immediately recognised as reactive dyes. In fact, the development of reactive dyes really started with the introduction of the Procion reactive dyes for cotton by ICI in 1955. Despite the many possible reactive groups in reactive dyes capable of covalent bond formation with nucleophilic groups in wool, only a limited number of types of reactive dye have been commercially successful. Figure-1 shows the major types of reactive groups. The dye chromophores are essentially those used for cotton fibre reactive dyeing. The most important reactive groups in wool fibre are all nucleophilic and are found mainly in the side-chains of amino acid residues.
They are, in order of decreasing reactivity, thiol (the –SH of cysteine), amino (–NH– and –NH2 of say histidine and lysine) and hydroxyl groups (–OH of serine or tyrosine). Difluorochloropyrimidines undergo aromatic nucleophilic substitution of one or both fluorine atoms, the fluorine between the two nitrogen atoms being the most reactive. Bromoacrylamido groups are stable in boiling water at pH 7 and react by both nucleophilic addition to the double bond and nucleophilic substitution of the bromine atom. They can form a three-membered aziridine ring that can react further with the protein fibre resulting in a new crosslink. The actual reactive dyes are probably dibromopropionamides, which eliminate HBr on dissolving in hot water. Methyltaurine-ethylsulphones and 2 sulphatoethylsulphones form the vinyl sulphone reactive group of reactive dyes relatively slowly at pH 5–6 (1 h at the boil). This allows some textile levelling during dyeing before the vinyl sulphone dyes reacts with the wool fibre and becomes immobilized.
This is useful in hank and winch dyeing where the liquor/goods interchange is less favourable. In fact, all commercial reactive dyes for wool have absorption rates that are greater than the rate of reaction with the wool fibre to allow some migration. Chloroacetylamino groups (–NHCOCH2Cl) react by an SN2 mechanism (Scheme 16.5). Wool reactive dyes are applied like acid dyes in weakly acidic solution. The degree of exhaustion and fixation are usually very high and clearing of unfixed dye from the goods may only be needed for deep shades. Reactive dyes for wool tend to be unlevel dyeing and are prone to give skittery dyeings. They are used more on loose fibre and slubbing than on piece goods, where they accentuate fibre nonuniformity and poor, uneven fabric preparation. A number of amphoteric or weakly cationic auxiliary products are available to assist level dyeing. Despite their good light fastness and very good washing fastness, they are still not widely used, partly because of their high cost. Red to maroon shades are very popular but there are no black reactive dyes available that can match the chrome blacks on wool fibre.
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