Page 396 - Basic Principles of Textile Coloration

BATCH DYEING PROCEDURES WITH SULPHUR DYES 385

this and some sodium nitrite to minimise stainless corrosion. Sodium bromate
oxidation is carried out in dilute acetic acid solution at about pH 4.

Colour bronzing and crocking problems arise once a critical depth of shade has
been exceeded. Deep shades must therefore be well soaped to remove residual
surface colour. This improves shade stability, decreases bronziness and improves
the colour brightness and washing fastness. Soaping is less critical in pale and
medium depths. Unlike the quinone vat dyes, however, crystallisation of sulphur
dye particles does not occur in this process. Excessively bronzy black dyeings can
be treated with alkaline sodium sulphide solution to remove the surface pigment
responsible for the bronzy appearance. Bronzing can be minimised by ensuring that
the dye in the dyebath is always fully reduced, that the fabric is not over-exposed
to air during dyeing, and that it is well rinsed immediately after dyeing. Blacks may
be soaped with olive oil and soap.

Some dyeings are treated with copper sulphate, or with this and sodium
dichromate, to improve the light fastness, and in some cases the wet fastness.
Fabrics for use inside rubber articles should not be copper treated. Some yellow-
brown dyes are treated with copper sulphate to improve light fastness but the
effect is lost on washing. The use of chromium salts is now declining because of
their adverse environmental impact.

Because of their dull colours, dyeings with sulphur dyes are often topped with
the much brighter basic dyes. The sulphur dye pigment acts as a mordant for the
cationic dyes. Dyeings with sulphur dyes may also be shaded with some sulphide
stable direct dyes, but this tends to lower the washing fastness.

Alkylation of the leuco thiols under alkaline conditions with epichlorohydrin
derivatives (3-chloropropene oxide) before oxidation renders them insoluble. Oxi-
dation is then no longer required. Some dyes, however, may benefit from an oxi-
dative treatment so as to develop the full colour. The alkylating agents may have
high molecular weight or be polyfunctional so that the alkylated product has very
good washing fastness in the presence of the peroxy compounds in domestic
detergents.

17.12.4 Dyeing with soluble sulphur dyes

Large amounts of such dyes are sold in liquid form. They are of two types – water-
soluble reduced leuco dyes, and solubilised sulphur dyes – the former being far
more important. The water soluble leuco dyes are completely in solution and
contain far less insoluble matter than a sulphur dye powder. This is beneficial in