Sulphur dyes have been classified into four main groups: CI Sulphur dyes; CI Leuco Sulphur dyes; CI Solublised Sulphur dyes; and CI Condensed Sulphur dyes.
CI Sulphur dyes are water-insoluble, containing sulphur both as an integral part of the chromophore and in attached polysulphide chains. They are normally applied in the alkaline reduced form from a sodium sulphide solution and subsequently oxidized to the insoluble form on the fibre.
Sulphur dyes differ from the vat dyes in being easier to reduce but more difficult to re-oxidise, different oxidants producing variations in hue and fastness properties. A leuco sulphur dye has the same CI constitution number as the parent sulphur dye but exists as the soluble leuco form of the parent dye together with a reducing agent in sufficient quantity to make it 8llitable for application either directly or with only a small addition of extra I8ducing agent.
A solublised sulphur dye has a different constitution number because it is a chemical derivative of the parent dye, non-substantive to cellulose but converted to the substantive form dyeing.
Condensed sulphur dyes, although containing sulphur, bear little resemblance to traditional sulphur dyes -their constitution and method of manufacture.
Sulphur dyes are available in various commercial forms such as powders, pre-reduced powders, grains, dispersed powders, dispersed pastest, liquids and water soluble-brands.
Application Method of Sulphur Dyes
Sulfur dyes are water-insoluble. In the presence of a reducing agent and at alkali pH's at elevated temperature of around 80 °C, the dye particles disintegrate, which then becomes water-soluble and hence can be absorbed by the fabric. Sodium sulfide or sodium hydrosulfide is suitable reducing agents. Common salt facilitates the absorption. After the fabric is removed from the dye solution, it is allowed to stand in air whereupon the dye is regenerated by oxidation. The regenerated parent dye is insoluble in water. Oxidation can also be affected in air or by hydrogen peroxide or sodium bromate in a mildly acidic solution.
The low water solubility is the basis of the good wash-fastness of these dyed fabrics. These dyes have good all round fastness except to chlorine bleaches. Because the dye is water-insoluble, it will not bleed when washed in water and will not stain other clothes. The dye, however, may have poor fastness to rubbing. The dyes are bleached by hypochlorite bleach.