Reactive Dyes on Wool, Silk and Polyamide Fibers

3 Reactive Dyes on Wool, Silk and Polyamide Fibers 

Reactive Dyes on Wool and Silk. Reactive dyes produce brilliant shades on wool with good fastness. They differ from reactive dyes for cellulose fibers because the reactivity of the chemoactive groups in wool is considerably higher than that of the hydroxyl groups in cellulose. To achieve level dyeing on wool, dyes with reduced reactivity must be used, and an auxiliary agent added. 

Among the reactive groups in wool the most important are amino, imino, and hydroxyl groups. Reactions occur in a weakly acidic medium (pH 3-5) and include nucleophilic substitution of leaving groups (usually Cl, F, and, rarely, sulfonate or ammonium groups) or addition reactions to polar aliphatic double bonds. 

In the acidic range, ionic bonds are formed between dye and fiber, and the dye is still capable of migration. At pH 5, covalent binding to the fiber predominates. 

In the dyeing processfor wool, a nonionic or weakly cationic leveling agent is added to the liquor, and the pH adjusted to 3-4 with formic or acetic acid. The process is started at 40°C, and after some time the pH is adjusted to 5-6 with sodium dihydrogenphosphate. Dyeing is conducted at boiling temperature for 1 h. To eliminate hydrolyzed dye, an aftertreatment is performed at 80°C with ammonia (pH 8.5-9.0). The last rinsing bath is weakly acidified. 

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