|This article does not cite any references or sources. (December 2009)|
Sulfur dyes are the most commonly used dyes manufactured for cotton in terms of volume. 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 or sodium hydrosulfite and are made insoluble within the fiber by oxidation. During this process these dyes form complex larger molecules which is the basis of their good wash-fastness.
These dyes have good all round fastness except to chlorine. Due to the highly polluting nature of the dye-bath effluent, 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.
Future developments in the field of reducing dye levels by means of electro-chemical processes are promising. This work is just in the research stage but is expected to come to industry very soon. This may eradicate the problems of polluting sulfides.
Sulfur dyes are water insoluble. They have to be treated with a reducing agent and an alkali at temperature of around 80 degrees Celsius where the dye breaks into small particles which then becomes water soluble and hence can be absorbed by the fabric.
Heating and adding a substance like common salt facilitates the absorption. After this the fabric is removed from the dye solution and then taken for oxidation. During the oxidation step the small particles of dye once more form the parent dye which is insoluble in water.
This oxidation can be done in air or by using oxidizing agents like hydrogen peroxide or sodium bromate in a mildly acidic solution. Now as the dye has become water insoluble in fiber so it will not bleed in water when washed and will not stain other clothes. However the dye may have poor fastness to rubbing, that is the dye from the fiber may come out gradually if the fabric is rubbed against. Also the fastness to hypochlorite bleach is poor because hypochlorite breaks the color imparting group in the dye and hence the colored part becomes colorless.
Sulfur dyes are very inexpensive and very important to the dyeing industry. Out of all the sulfur dyes perhaps 50% of production is of the sulfur black color as black is the most popular fabric color.
Sulfur dyes do not have any pure red color in its shade range. A pink or lighter Scarlet color is available.