Sulfur dyes are synthetic organic dyes that are formed by the thionation or sulphurisation of organic intermediates containing nitro or amino groups. Sulfur dyes have sulfur bonds in their molecules. Sulfur dyes are highly colored water-insoluble dyes. Some sulfur dyes are partially soluble in water. They have no direct affinity for cellulose fibers. To make them substantive, they are converted to soluble forms by treatment with a reducing agent. Sulfur dyes have good lightfastness. Due to the strong alkaline conditions, they are not suitable for wool. Sulfur dyes are cheap and easy to manufacture. Sulfur dyes have moderate to good heat resistance and chemical resistance. Due to their poor fastness to chlorine, they are not suitable for products that are bleached with hypochlorite.
Sulfur dyes are mainly used for the dyeing of textile cellulosic materials, or blends of cellulosic fibers with synthetic fibers such as acrylic fibers, polyamide and polyester. They can also be used for a limited number of silk and paper applications. Soluble sulfur dyes are used for the dyeing of certain types of leather. Low water solubility of sulfur dye is the basis for good wash fastness of these dyed fabrics. Since the dye is usually insoluble in water, it does not seep when washed in water and does not stain other clothes. Most colors are available, especially deep shades, rich blacks, and navy, but not bright red and orange.
Sulfur dyeing mechanism
Sulfur dyes are highly colored, water-insoluble compounds that must be converted into water-soluble forms before being applied to textile materials. The conversion is performed by treatment with a reducing agent such as aqueous Na2S solution. They are absorbed on the fiber surface and then reconverted back to the original water-insoluble dye form by oxidation. In addition to cellulose fibers, sulfur dyes can also be used to dye staple fibers and yarn.
Figure 1. Mechanism of dyeing with sulfur dye