Page 384 - Basic Principles of Textile Coloration

PRE-PIGMENTATION DYEING METHODS 373

will reduce. Quinone vat dyes are generally difficult to reduce and easy to oxidise.
For sulphur dyes (Section 17.11) the reverse is true. A typical vat dye has a redox
potential around –0.80 V and oxidising agents with redox potentials more
positive than about –0.50 V will in principle give complete oxidation of the leuco
dye to the quinone. The rate of oxidation must also be acceptable. Oxidation of a
leuco vat dye can be achieved by exposing the goods to air, after an initial rinse to
remove adhering liquor, and therefore appears quite simple. Air oxidation,
however, is of little use for compact goods such as yarn packages, and is quite slow,
often requiring 30–40 min for complete oxidation. It may also result in side
reactions and off-shade dyeings. Chemical oxidants are therefore generally
preferred because they give more rapid and uniform oxidation throughout the
material. Sodium dichromate, hydrogen peroxide, or sodium perborate or
percarbonate are used at up to 40 °C. It is important that the initial oxidation is
carried out under alkaline conditions to avoid forming the vat acid, which is much
less soluble and hard to oxidise. Over-oxidation of some dyes, such as
Indanthrone, leads to changes in hue of the dyeing (Section 17.5.3).

17.6.2 Soaping
Soaping removes pigment deposited on the fibre surface that would wash or rub off
in use. The deposit is formed by oxidation of the leuco dye in the solution
adhering to the fibres after dyeing. Thorough rinsing after dyeing, using a weak
solution of alkaline hydros, helps to minimise this problem. Soaping involves
thoroughly washing the goods in a detergent solution at or near the boiling point.
It improves the light, washing and rubbing fastness of the dyeing. Some vat dyes
give a slight shade change on soaping so a standardised procedure is necessary for
exact colour reproducibility.

The soaping process probably involves re-crystallisation of the dispersed vat dye
particles obtained from the initial oxidation of the leuco dye. This is not a
spontaneous process and requires heat and water. The growth of the pigment
crystals inside the cotton fibre is limited by the pore size. It is this crystallisation
process that is responsible for the slight change in hue and the improvements in
the fastness properties.

17.7 PRE-PIGMENTATION DYEING METHODS
In many cases, leuco vat dyes have too high a substantivity for continuous dyeing
and preferential absorption by the cotton fibres would result in exaggerated tailing.