Page 352 - Basic Principles of Textile Coloration

BATCH DYEING OF COTTON WITH REACTIVE DYES 341

Reactive dyes are very versatile and allow a variety of different approaches for
controlling the rate of dye absorption, and the degree of dye migration and dye
fixation. Salt additions, temperature variations and alkali additions are used alone
or in combination to control the dyeing process. Reactive dyes have recently been
classified as [1]:
(1) alkali-controllable dyes, which have relatively high reactivity and only

moderate substantivity. They are applied at relatively low temperatures and
level dyeing requires careful control of the addition of alkali to initiate the
fixation stage. Examples include DCT, DFCP and VS reactive dyes;
(2) salt-controllable dyes. These are dyes of relatively low reactivity towards
cotton under alkaline conditions and therefore the dyeing temperature will be
as high as 80 °C. They have appreciable substantivity and level dyeing
requires careful addition of salt to promote exhaustion. Examples in this class
include TCP, MCT as well as MFT reactive dyes;
(3) temperature-controllable dyes, which undergo fixation at high temperatures
even under neutral conditions. The NT dyes are in this class.

The objective of dyeing with reactive dyes is to obtain the maximum degree of
reaction between the dye and fibre, with a minimum of dye lost through hydrolysis
of the reactive group, and under conditions where the colour of the dyed material
is uniform. Dyes with different reactive groups and molecular structures require
different amounts of added salt to obtain economical exhaustion. Some reactive
dyes, including most of the oldest, have relatively simple molecular structures and
quite low substantivity for cellulosic fibres. They need high concentrations of salt,
up to 100 g l–1, particularly when dyeing deep shades or using long (high) liquor
ratios. At such high salt concentrations, there is always a risk of dye precipitation,
particularly when dyeing at low temperatures. Although Glauber’s salt is more
expensive than sodium chloride, it is preferred for dyes prone to aggregation at low
dyeing temperatures, such as the turquoise copper phthalocyanine reactive dyes.
The advantage of dyes with low substantivity is that they diffuse easily in the fibres
and are easy to wash out of the material after dyeing. The higher the substantivity
of the reactive dye for the cellulose, the higher the bath exhaustion and the
greater the chance of reaction with the fibre, but the greater the difficulty of
removing unfixed dye during the final washing.

For dyeings with vinyl sulphone dyes, it is advisable to ensure that the residual
alkali has been removed or neutralised prior to soaping since hydroxide ion can
catalyse hydrolysis of the ether type dye–fibre bond and result in additional colour