Page 392 - Basic Principles of Textile Coloration

SULPHUR DYES 381

with or dissolved in hot water and the solution used directly for dyeing, or after
addition of more sodium sulphide. These dyes eliminate the difficulties of vatting,
because reduction of sulphur dyes is not rapid.

Solubilised sulphur dyes are thiosulphate derivatives of the leuco compounds
and are available in powder or liquid form. They are prepared by reacting
disulphides with sodium sulphite to give the thiosulphate ester of the thiol. These
are called Bunte salts (Scheme 17.5). These dyes are applied in the presence of
alkali and reducing agent (Na2S) to generate the sulphur dye that then undergoes
further reduction to the leuco dye. Like the sulphate esters of leuco vat dyes, the
thiosulphate esters have little substantivity for cotton. They are also used in paper
dyeing where they are precipitated by the fillers added to the pulp.

Dye S S Dye + Na2SO3 Dye S SO3Na + Dye SNa

2Dye S SO3Na + Na2S Dye S S S Dye + 2Na2SO3

Dye S S S Dye + Na2SO3 Dye S S Dye + Na2S2O3

Scheme 17.5

The sulphurised vat dyes, such as CI Vat Blue 43 and Sulphur Red 10, bridge
the vat and sulphur dye classes. They are manufactured by the same kinds of
procedures as for sulphur dyes but are applied like vat dyes using hydros for
vatting.

17.11.3 Characteristics of sulphur and leuco sulphur dyes on cotton
Sulphur dyes have the dullest range of colours of all dye classes but are relatively
inexpensive. They are used to dye medium to deep, dull shades on cellulosic
materials. There are several excellent blacks giving dyeings with good wet fastness
properties. In fact, when black, and deep brown, blue and dull olive green shades
are needed, with good washing and satisfactory light fastness at reasonable cost,
sulphur dyes are irreplaceable. There are few green sulphur dyes and no true reds.
There are, however, an abundance of blacks, blues, yellows and browns. On a
world basis, sulphur dyes constitute one of the major dye classes.

Sulphur dyes are used in cotton dyeing for woven goods using jig dyeing machines
and also in continuous dyeing. They are commonly used for the continuous dyeing