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Characteristics:
Direct dyes are also called substantive colours because of their excellent substantivity for cellulosic textile materials.
These dyes are easily applied to the cellulosic fibres from simple solutions in water hence, these are called cotton dyes. Turmeric is the only natural direct dye stuff that has a direct affinity for cotton.
Bottiger was the person to discover Congo red (direct dye) in 1894.
Advantages of direct dyes are ease of application, low cost, wide range of available colours.
Suitability to textiles:
- The fabrics most readily coloured with direct dyes are the man-made and natural cellulosic fibres; that is, cotton, viscose fibres and cuprammonium rayon etc.
- Much used for printing on dischargeable dyed backgrounds.
Method of application:
The application of direct dyes to the cellulosic fabrics is a simple method. These dyes are applied to the cellulosic fibres from aqueous liquor to which electrolyte called sodium salt is added. The cellulosic fibre in the dye liquor has negatively charged surface attracting sodium cations which are positively charged. The presence of the chloride ions in the dye liquor assists the dye anion to leave the dye liquor and enter the fibre polymer system. The application of heat to the dye liquor increases the energy of the dye components of the dye liquor, swells the fibre and accelerates the rate at which dye occurs.
While dyeing light shades uneven dyeing may occur this can be avoided by reducing the salt percentage. Soap or Turkey red oil may be added to the dye bath to obtain greater evenness.20 percent of the common salt on the weight of the material is used generally to exhaust the dye bath. In case of heavy shades, more salt can be added to the dye liquor to enable better exhaustion. The fabric should be rinsed well immediately after it is removed from the dye bath in order to avoid deposition of salt on the material.
For dyeing light shades the material to liquor ratio is 1:20 while for dark shades it is 1:15.
Fastness properties:
- Fabrics which are dyed and printed with direct colours have a moderate light fastness rating about 3. A relative short exposure to direct sunlight is enough to initiate degradation of the dye molecule.
- Fabrics dyed with direct dyes have relatively poor wash fastness rating about 2-3. The loss of dye is seen as fading of the cellulosic textile materials.
- Direct dyes are relatively easy to apply to cellulosic textiles. However, although relatively easy to remove which gave rise to the saying “easy on & easy off”.
- The fastness to light and fastness to washing of direct dyed textile materials can be improved by the treatment with solutions of metallic salts.
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