Page 357 - Basic Principles of Textile Coloration

346 REACTIVE DYES

compounded if high salt concentrations are also present. Newer ranges of reactive
dyes, particularly those with more than one reactive group that give higher
fixation, have attempted to address the problem of colour in the effluent with
some success. Several dye manufacturers now offer reactive dyes requiring smaller
amounts of salt for exhaust dyeing. The use of less salt demands a higher degree of
dye substantivity but this impedes efficient washing-off after dyeing. This is
counteracted by using dyes that give good fixation.

It is possible to eliminate colour bleeding and staining of adjacent material
when an article is first washed during use by aftertreating the dyeing with a
cationic fixative. This type of product reacts with any residual unfixed anionic dye,
forming an organic salt of greatly increased molecular size and of lower water
solubility and diffusion rate. Such cationic fixatives lack permanence on repeated
washing but this is not a problem since the unfixed dye will have gradually been
removed by that point. They may, however, reduce the light fastness of the dyeing
and are therefore more suitable for treatment of deep shades. Such an
aftertreatment is not a remedy for inefficient washing-off of unfixed dye. If the
amount of unfixed dye remaining in the goods is significant there is a risk of the
precipitated dye–auxiliary complex rubbing off, particularly on wet abrasion.

Dyeings with a few red DCT reactive dyes on cotton are prone to increased
colour bleeding because of hydrolysis of the dye–fibre bond. Unreacted chlorine
atoms in the dye’s reactive group may hydrolyse under warm humid storage
conditions, liberating HCl. This catalyses the hydrolysis of the dye–fibre bond.
The colour bleeding can be counteracted by adding a polyamine to the final rinse
water that reacts with any residual reactive chlorine atoms in the bound dye.

As for direct dyes, some reactive dyes may be reduced by the cellulose when
dyeing at high temperatures in presence of alkali. This can lead to a significant
decrease in colour strength when dyeing viscose with reactive dyes, for example.
Addition of the mild oxidant m-nitrobenzene sulphonate usually prevents this.

There is always a risk of anionic reactive dyes being precipitated by calcium,
magnesium or heavy metal ions in the water supply, or of the formation of
insoluble hydroxides of these metals under the alkaline dyeing conditions. To
avoid these problems, a limited addition of a polyphosphate sequestering agent
may be required. EDTA should be avoided with azo copper complex dyes since
demetallisation can occur with a dramatic change in hue. The copper in the stable
copper phthalocyanine dyes is, however, unaffected by EDTA.

Finally, stripping reactive dyes is by no means easy. Hydrolysis of the dye–fibre
bond using hot acetic acid solution, followed by good washing, may give partial