Page 380 - Basic Principles of Textile Coloration

DYEING COTTON WITH LEUCO VAT DYES 369

recommended correspond to the IN, IW and IK methods described in Section
17.4.2.

The rate of diffusion of a typical leuco vat dye into cotton is about half that of a
direct dye at the same temperature. The high rate of dyeing of leuco vat dyes is
therefore a consequence of their high substantivity for cellulose. Since the
standard affinities of leuco vat dyes are no higher than those of direct dyes, the
high substantivity and rate of dyeing are caused more by the high electrolyte
content of the dyebath than by high inherent affinity.

The initial strike can be retarded using polyethylene oxide complexing agents.
The initially formed dye–auxiliary complex presumably slowly breaks down to
gradually liberate free leuco dye, thus controlling the initial rate of adsorption.
Both non-ionic and cationic retardants are used but these decrease the
exhaustion. They can also be used for levelling of uneven dyeings. Such dyeing
auxiliaries should be added to the dyebath after vatting. The dye manufacturer’s
recommendations should be consulted since these agents are often dye-specific.
Similar chemicals can be used for stripping faulty dyeings. This is not easy because
of the high substantivity of the leuco vat dyes and the stripping auxiliaries used
form much more stable complexes with the leuco dyes than those obtained from
levelling agents.

17.5 DYEING COTTON WITH LEUCO VAT DYES

17.5.1 Preparation
As always, good preparation of the goods is essential to avoid a non-uniform
distribution of residual impurities that might hinder level dyeing or that might
interact with the dyes. Water quality is important since the calcium and
magnesium salts of some leuco dyes may precipitate if hard water is used. The use
of sequestrants or protective colloids is often recommended.

17.5.2 Dyeing conditions
Quinone vat dyes are used on all forms of cellulosic materials. The vat is usually
pre-prepared and added to the dyebath containing the goods, possibly in
successive portions. Vat dyes of high substantivity are often used for dyeing deeper
shades where the effect of poor levelling is less visible. They usually follow a
Freundlich adsorption isotherm (Section 11.1.1) so that using more dye gives