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SULPHUR DYES 379

oxidation conditions for each dye vary so it is important to follow the supplier’s
instructions. The use of these dyes was never widespread and has declined in
recent times. They are not particularly competitive because of their high cost.

17.11 SULPHUR DYES

17.11.1 Chemical nature of sulphur dyes

Sulphur dyes are a type of vat dye used for dyeing cellulosic fibres. The insoluble
pigment is converted into the substantive leuco compound by reduction with
sodium sulphide and the leuco form is subsequently oxidised inside the fibre.
Sulphur dyes are manufactured by heating organic materials with sodium
polysulphide in aqueous or alcoholic solution, or by baking with sodium sulphide
and sulphur. A variety of organic compounds are used including amines, nitro
compounds, and phenols and their derivatives. These dyes almost always contain
loosely bound sulphur and liberate hydrogen sulphide when treated with acidic
solutions of reducing agents such as stannous chloride.

The chemistry of sulphur dyes is very complex and little is known of the
molecular structures of the dyestuffs produced. The Colour Index only gives the
structures of the starting materials that are used to produce these dyes and even
then the information can be quite misleading. Two products manufactured from
the same starting materials are unlikely to have similar properties even though
they may have the same CI number. The actual dyestuffs produced from the same
materials by different processes may have quite different dye contents, dyeing
characteristics and environmental impacts. Because of the large amounts of
sodium sulphide used in the manufacture and application of these dyes, much of
which will eventually be found in the effluent, they pose a significant
environmental problem. In this respect, sulphur dyestuffs produced in the
developed Western nations are much more environmentally friendly.

A variety of structural units have been proposed for various sulphur dyes. It is
generally accepted that these dyes are polymeric having sulphur containing
aromatic heterocyclic units such as thiazines and thiazoles linked by di- or
polysulphide bonds (Figure 17.8). On treatment of an aqueous dispersion of the
insoluble pigment with sodium sulphide, the sulphide links are reduced forming
individual heteroaromatic units with thiol groups (Scheme 17.4). These are
soluble in the alkaline solution in the form of thiolate ions that have low to
medium substantivity for cellulose. Over-reduction is possible if other groups are