Page 383 - Basic Principles of Textile Coloration

372 VAT DYES

This is responsible for the rather poor chlorine fastness of indanthrone. The reaction
is reversible and treatment of an over-oxidised dyeing with dilute alkaline hydros
solution followed by air oxidation regenerates blue Indanthrone.

Partial formation of the green azine derivative can occur during air oxidation if
the fabric still contains a relatively high concentration of NaOH solution. Rinsing
out the excess alkali from the dyeing is therefore necessary before significant
oxidation by oxygen has occurred. Some of the chlorinated indanthrone vat dyes
are prone to elimination or substitution of a chlorine atom unless vatting is carried
out with care. Although the indanthrone type dyes represent a rather special case,
it is obvious that vatting of the insoluble pigment to produce a solution of the
leuco derivative for dyeing is not always a simple operation. It is important to
follow the supplier’s recommendations for all stages of the dyeing process.

17.5.4 Aftertreatment of the dyeing
Once the actual dyeing process is complete, the goods go through a series of
operations:
(1) rinsing to remove adhering exhausted dye liquor and chemicals. A reductive

rinse with dilute alkaline hydros solution may be needed for heavy shades;
(2) oxidising the leuco dye to the pigment form using hydrogen peroxide and

acetic acid;
(3) soaping at or near the boil. For soaping in a package machine, the first step

may be dispersion at 60 °C to remove loose surface colour, followed by
soaping at or near the boil. This is important to avoid filtration of eliminated
dye particles by the package;
(4) neutralising the dyeing with acetic acid solution and final rinsing.

These processes are an essential part of the overall procedure and must be
correctly carried out if the dyeing is to have the optimum fastness properties. The
oxidation and soaping stages are discussed in the following section.

17.6 OXIDATION AND SOAPING AFTER DYEING

17.6.1 Oxidation
An oxidising agent has a much more positive redox potential than the compound
that it can oxidise or, considered from the opposite perspective, the compound
being oxidised has a more negative redox potential than the oxidising agent that it