Page 393 - Basic Principles of Textile Coloration

382 VAT DYES

of corduroy. They are now being used more widely in jet machines. Sulphur dyes are
also used for dyeing denim olive, brown and maroon, rather than the traditional
Indigo blue, as well as to ‘bottom’ or ‘top’ Indigo dyed cotton warps. To ‘bottom’ or
‘top’ means that a sulphur dye is applied either before or after the Indigo. The
fastness to wet processes and to crocking can be varied almost as required to satisfy
the demand for the faded look so popular for denim. This is achieved by allowing
premature oxidation of the leuco dye during dyeing, by using short dyeing times so
that there is inadequate time for dye penetration into the fibres, and by poor rinsing
and soaping after dyeing. The dyeings can then be subsequently treated to produce
the faded worn look by removing the surface colour.

Although cellulosic goods dyed with sulphur dyes usually have good washing
fastness, it can be further improved by resin finishing. The light fastness varies
from moderate to good in heavy shades. A major characteristic of sulphur dyes is
the poor fastness to chlorine, which distinguishes them from most quinone vat
dyes. Dyeings with sulphur dyes cannot be bleached with hypochlorite. In fact,
these dyes are readily distinguished from other cotton dyes by their dark, dull
colours and the bleaching that occurs when a dyeing is spotted with hypochlorite
and allowed to dry.

Cotton dyed with some sulphur blacks becomes tendered on storing under
warm humid conditions. This is a consequence of the formation of sulphuric acid
from oxidation of the sulphur dye in the fibres. It can be minimised by thorough
washing after dyeing before the oxidation of the leuco dye, by a final alkaline
rinsing with soda ash solution, and by resin finishing. Such tendering is avoided by
dichromate oxidation of the leuco dye.

17.12 BATCH DYEING PROCEDURES WITH SULPHUR DYES

17.12.1 Vatting sulphur dyes

The major reducing agent for sulphur dyes is sodium sulphide, possibly containing
some polysulphides. Sulphide ion is strongly hydrolysed in solution and gives a pH
almost as high as the same concentration of NaOH (Scheme 17.6). Since hydrogen
sulphide is a diprotic acid, the species present in an alkaline solution depends upon
the pH and may be sulphide or hydrosulphide. Polysulphide ions are all in equilib-
rium with themselves and sulphide and hydrosulphide ions. Formation of polysul-
phide ions decreases the pH of solutions of sulphide ion (Scheme 17.6). These
polymeric ions are not very sensitive to air. They therefore inhibit oxidation of
sulphide and hydrosulphide and help to prevent ‘bronzy’ dyeings with a metallic