Page 366 - Basic Principles of Textile Coloration

REACTIVE DYES FOR WOOL 355

both nucleophilic addition to the double bond and nucleophilic substitution of the
bromine atom (Figure 16.8). They can form a three-membered aziridine ring that
can react further with the protein resulting in a new crosslink. The actual dyes are
probably dibromopropionamides, which eliminate HBr on dissolving in hot water.
Methyltaurine-ethylsulphones and 2-sulphatoethylsulphones form the vinyl
sulphone reactive group relatively slowly at pH 5–6 (1 h at the boil). This allows
some levelling during dyeing before the vinyl sulphone dye reacts with the wool
and becomes immobilised. 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 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 non-
uniformity 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.

16.6.2 Batch dyeing of wool with reactive dyes

Reactive dyes are usually applied to wool at pH 5–6 using ammonium salts, and
acetic acid as required. At higher pH values, exhaustion is too low, and at lower
values rapid dye uptake gives unlevel dyeings. Slightly higher pH values are used
for dyeing paler shades (pH 5.5–6.0) and lower values (pH 5.0–5.5) for deep
shades. Reactive dyes often give quite good exhaustion at temperatures below the
boil but the dyeing temperature will eventually be raised to 100 °C to ensure that
reaction with the wool is as complete as possible. Some procedures recommend a
holding stage at an intermediate temperature of 65–70 °C for 15–20 min to allow
the dye to migrate before it reacts with the wool.

Because of their tendency to give unlevel, skittery dyeings, reactive dyes are
usually applied to wool in the presence of proprietary levelling agents. These are
often amphoteric, having both cationic and anionic groups in the molecule. In
contrast to most levelling agents, which decrease the dyeing rate, the auxiliary