Briefly Description of Dyeing with Sulphur Dyes ?

Dyeing with Sulphur Dyes:

Initially the goods are wet out in the bath. Since the dyeing liquor contains appreciable amounts of sulphide, copper fittings must be avoided. If wetting or penetrating agents are used these should be of the anionic type since non-ionic surfactants form stable, non-substantive complexes with the leucothiols. An anionic product such as phosphated 2-ethylhexanol is suitable. The bath may then be set at 40°C with some sodium polysulphide. Polysulphides in the leucodyebath prevent premature oxidation of the dye and reduce the tendency to bronziness of deep dyeing of blues, navies and blacks. An addition of a sequestrant such as EDTA avoids precipitation of the leucothiolate by calcium and magnesium ions. The leuco dye is then added slowly and, since the leuco dyes only have low to moderate substantivity for cellulose, some salt may be added initially, or in portions during dyeing, to promote exhaustion. After dyeing the goods are rinsed, the leuco dye oxidised and the dyeing is soaped as for a conventional vat dye.

Sulphur dyes usually have acceptable substantivity, particularly in the presence of salts, so that stripping in a fresh reducing bath is not easy. Dyeing is often conducted at the boil but this decreases the degree of exhaustion. Sulphur dyes require less salt than reactive dyes and usually have reasonable exhaustion. Low sulphideleuco dyes require more salt and no polysulphide. They do not give good exhaustion in heavy shades and the use of a low liquor ratio is recommended. For popular shades such as black, it has long been common practice to use a standing bath. This is a dye bath that is re-used for subsequent dyeings after addition of more reduced dye. Any free sulphur that tends to accumulate is dissolved by addition of sodium sulphite to give thiosulphate. This prevents it sticking to the goods. The actual dyeing temperature can vary. At higher temperatures around the boil, the bath exhaustion is less but penetration of the leuco dye into the fibres is better than at lower temperatures.