Sulfur dye
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Sulfur dyes are the biggest volume dyes manufactured for cotton. They are cheap, generally have good wash-fastness and are easy to apply. The dyes are absorbed by cotton from a bath containing sodium sulfide and are insolubilised within the fibre by oxidation. During this process the dyes form complex larger molecules which is the basis of their good wash-fastness. These dyes produce robust dyeings that have all round fastness except to chlorine. Due to the high polluting nature of the let out dye-baths, slowly sulfur dyes are being phased out. Sulfur dyes are primarily used for dark colors such as blacks, browns, and dark blues. The deep indigo blues of denim blue jeans are a product of sulfur dyes.
Recent advances in dyeing technologies have allowed the substitution of toxic sulfide reducing agents. Glucose is now used and both low sulfide and zero sulfide products are available.
Even new developments are done in the field of reducing the dye by means of electro chemical reducing. But this work is just on research stage and is expected to come to industry very soon which will eradacate the problems of polluting sulfide.
Sulfur dyes are water insoluble. They have to be treated with a reducing agent and an alkali at temperature of around 80 degree celsius where the dye breaks in small particle which then becomes water soluble and hence it can be absorbed on the fabric. It is allowed to exhaust i.e. allowed to go on to the fabric by means of heating and adding common salt which facilitates the dye to go to the fabric surface. After the adsorption of the dye on fibre surface it is removed from the dye solution and then taken for oxidation. Now in oxidation step the dye which had broken in smaller paritcles joins and again forms the parent dye which is insoluble in water by means of oxygen. The oxidation can be done in air as well if it needrd to be done faster than oxidising agents like Hydrogen peroxide etc can be used. Now as the dye has become water in-soluble in fibre so it wont bleed in the water when washed and will hence not stain other clothes. But it had poor fastness to rubbing. i.e. The dye from the fibre comes out gradually if the dyed fabric is rubbed against any thing, even on its surafce. Also the fastenss to chlorite bleach is poor because chlorite breaks the colour imparting group in the dye and hence the coloured part becomes colourless. Sulfur dye are very cheap dye. Out of all the sulfur dyes 50% production is done of the sulfur black colour as it is very important from point of industry because all black colour dyed fabric need very high concetration of black colour to dye the fabric and give it the jet black colour appearace. But as sulfur dyes are cheap it is more used where very heavy black colour is to be produced.
Sulfur dyes does not have a pure red colour in its shade bank. A pink or scarlet colour is available.
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