Page 397 - Basic Principles of Textile Coloration

386 VAT DYES

package dyeing. They often still require a small addition of sodium sulphide or
another reducing agent. These liquids contain the stabilised leuco dye, sodium
sulphoxyate-formaldehyde or sulphide and sodium carbonate. The solubilised
sulphur dyes are thiosulphate esters prepared from the leuco thiols with sodium
sulphite (Scheme 17.5). They usually have low substantivity for cellulose and are
useful for package, padding and pad–jig applications. Reduction is necessary before
or during dyeing and the usual aftertreatments are needed.

17.13 CONTINUOUS DYEING WITH SULPHUR DYES
Sulphur dyes are used for continuous dyeing of cotton goods using a pad–steam–
wash process, with three groups of wash boxes for rinsing, oxidising and soaping.
Padding may take place at up to 80 °C to reduce the substantivity of the leuco dye
for the cotton fibres. This reduces the problems of selective absorption and the
resulting initial colour tailing that it causes. As for other vat dyes, steaming is
carried out in air-free saturated steam. Then the dyeing is rinsed at 40–60 °C and
oxidised with sodium bromate plus metavanadate catalyst at pH 4 in the presence
of acetic acid. In the remaining wash boxes, the best possible soaping and rinsing is
done. For black dyes tending to produce sulphuric acid by oxidation on storage, a
final soda ash rinse may be added. In some instances a better appearance results
using a two-pad method. This involves padding with the sulphur dye suspension or
solution, followed by intermediate drying, padding with sodium sulphide solution,
steaming, and the usual aftertreatment sequence.

17.14 ENVIRONMENTAL CONCERNS
Waste sulphur dye baths, and the sulphide ion they contain, liberate hydrogen
sulphide on acidification. These solutions cannot be directly released into water
streams without treatment. Residual pad bath liquor from continuous dyeing is
usually mixed with the residual oxidising bath liquor to oxidise the excess sulphide.
Exhausted dyebaths can be oxidised with hydrogen peroxide. Oxidation of sulphide
ions generates the less noxious thiosulphate and sulphate ions. This particular
effluent problem can be partially solved by using glucose and caustic soda as the
reducing agent but the vatting temperature must be 90–95 °C. This particular
reducing system can be used with solubilised sulphur dyes, with leuco sulphur dyes as
the additional reductant, and even with sulphur dyes, replacing part of the sodium
sulphide.