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Characteristics:
These dyes are so called because they contain sulphur atom in their molecules.
Sulphur dyes were first made in 1879.
These dyes are fast to washing, light and perspiration but excessive chlorine bleaching will strip the colour.
Sulphur dyes in general are insoluble in water.
Sulphur dyes produce dull colours such as navy blue, brown and black.
They are used for black more than any other class of dyes.
If textile materials dyed with sulphur dyes are stored for a greater length of time, the fabrics become tender.
Sulphur dyes are relatively low in cost; and when applied correctly they perform well.
Sulphur dyed fabrics have good colour fastness to washing but are sensitive to chlorine bleaches.
Suitability to textiles:
The fibres most readily colored with sulphur dyes are natural and manmade cellulosic fibres particularly cottons and Rayon’s. Traditionally they were used for dark shades such as browns, blacks and navy blues but some of the newer sulphur dyes are available in bright colours.
Applicable to stock, yarn and piece goods, used for heavy woven and knitted cotton as well as linen and jute.
They are excellent for Khaki and for the heavy piece goods used in work clothes.
Method of application:
Sulphur dyes are insoluble in water and must be made soluble with the aid of caustic soda, and sodium sulfide, sulphur dyeing is done at high temperature and with large quantity of salt, which helps to drive the color into the fabric. The role of sodium sulphide or sodium hydrosulphite is to reduce the sulphur dye to water soluble form or leucoform. In this reduced or leuco form, sulphur dyes are substantive to Cellulosic fibres. For adequate exhaustion, an electrolyte such as NaCl can be added to the dye liquor.
To obtain adequate penetration and a satisfactory rate of dyeing, heating of dye liquor is needed. This increases the energy of the constituents of the dye liquor, increasing the rate of dyeing and ensures adequate penetration of the fibre polymer system.
Once the dye is within the fibre polymer, the reduced sulphur dye is converted to its original insoluble form by oxidation.
After rinsing, the fabric is oxidized to the desired shade by exposure to the air or chemically by the use of acetic acid or sodium per borate.
The oxidation process must be carefully controlled became penetration of dye is retarded by premature oxidation.
Excess chemicals and dye must be completely removed through washing.
Sulphur dyes penetrate more thoroughly than any other dye because of the high temperature and the alkalinity of the dye bath.
Fastness properties:
- The light fastness of the sulphur dyed textile materials is fair rating about 4. This fair light fastness may be improved by an after treatment with Meta salts to increase the light fastness of about 5.
- Wash fastness rating of sulphur dyed textile materials is fair rating about 3-4.
- Sulphur dyes have a colour range which is mainly limited to black, brown, blue and olive.
- Sulphur dyed textile materials may show a metallic or bronze sheen which is referred to as bronzing. This effect is undesirable as it detracts from the aesthetic appeal of the dyed textile material, as well as giving rise to poor rub-fastness.
The relatively low cost of sulphur dyes has meant their continued use particularly for dark colours such as navy and black
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