Page 353 - Basic Principles of Textile Coloration

342 REACTIVE DYES

bleeding from the goods (Scheme 16.3). Dyeings with vinyl sulphone dyes have
maximum dye–fibre bond stability at around pH 4.5, whereas the corresponding
value for dyes based on halogenated nitrogen heterocycles is 6–7. The latter type
have dye–fibre bonds that are more sensitive to acid-catalysed hydrolysis.

Dye O Cell + H2O Dye OH + Cell OH
Scheme 16.3

The dyeing temperature and the nature and concentration of the alkali required
are determined by the reactivity of the dye, its degree of sulphonation and its
substantivity. For tightly twisted yarns and compact woven fabrics, the migration
phase of dyeing with low reactivity dyes can be carried out at higher temperatures
up to 120 °C to promote initial migration and penetration into the textile. This is
well above the later fixation temperature in the presence of alkali (80 °C). Azo
copper complex dyes will not withstand these conditions and tend to loose the
copper ion. For a higher fixation temperature, reaction with the fibre occurs at
lower pH, using a weaker alkali or a lower concentration of the usual alkali. As is
the case for salt addition, a deeper shade, or a higher liquor ratio, will require more
alkali. It is not usual to exceed a dyebath pH of 11, even with the less reactive dyes
since this invariably leads to lower colour yields because of dye hydrolysis.

The effects of increasing the dyeing pH during the fixation phase are complex
but usually involve an increase in the rates of reaction of the dye with the fibre
and with hydroxide ion. For polysulphonated dyes, one effect of dyeing at pH
above 11 is the decrease in substantivity of the dye for the increasingly anionic
dissociated cellulose. The greater negative charge of more cellulosate ions repels
the dye anions. In fact, some dyes actually desorb from the fibre into the dyebath
when the alkali is added at the start of the fixation stage giving a sudden decrease
in the degree of exhaustion at that point. In addition, with polysulphonated dyes,
the substantivity decreases as dye fixation proceeds because the cotton contains
more and more bound anionic dye molecules, which also repel the unfixed dye.
These effects can be counteracted by an increased concentration of salt in the
dyebath.

Dye hydrolysis is more pronounced and exhaustion is less at high liquor ratios.
Therefore, in winch dyeing, reactive dyes of higher substantivity are preferred. In
recent years, there has been a considerable shift to dyeing with reactive dyes on
machines with low liquor ratios. This gives more efficient dyeing, and reduces the