LIQUID SULPHUR BLACK DYE

Sulfur dyes are most commonly used dyes manufactured for cotton in terms of volume. They are cheap, generally have good wash-fastness, and are easy to apply. Sulfur dyes are predominantly black, brown, and dark blue. Red sulfur dyes are unknown, although a pink or lighter scarlet color is available.

The most important member of the class is Sulfur Black 1. It is produced by the reaction of 2, 4-dinitrophenol and sodium sulfide in hot water. Like many sulfur dyes, details on the chemical reactions are poorly understood. It is accepted that the sulfide reduces the nitro groups to aniline derivatives, which are thought to form indophenol-containing intermediates that are further crosslinked by reaction with sulfur. The result are insoluble, high molecular weight species. Sulfur Black 1 is imperfectly understood, and the material is probably heterogeneous. It is speculated to be a polymer consisting of thianthrene and phenothiazine subunits.

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 are 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 effected 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.

The sulphur dyes have been in use as cotton dyes for over ninety years. As far as the application is concerned the sulphur dyes are akin to the vat dyes since they are water insoluble substances which first must be reduced to a soluble form by means of sodium sulphide, which is applied to cotton fibre and the dye being regenerated by oxidation (in air). This group of dyes is produced by heating relatively simple aromatic substance with sulphur or a sulphur compound where by aromatic rings are by disulfphide or disulphoxide bridges. These links are converted by the action of sodium sulphide into -SNa groups, smaller water soluble molecules being produced. On oxidation in air the molecules are linked together once more as sulphides.