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Characteristics:
These are sometimes referred to as ice dyes (or) developed colours. These are also called as azoic dyes because of the reason that their molecules contain an azo group.
These dyes are mostly used in the handloom industry for dyeing of cellulosic fibres.
These dye stuffs are characterized by their beauty, brightness of shades and their all round fastness properties.
Yellow, golden yellow, orange, brown, scarlet, red, maroon, blue, violet and black shades are produced regularly by the cotton dyers.
The first azoic colour was discovered by Thomas and Robert holiday who produced a bluish red shade.
Suitability to textiles:
- Azoic dyes are used to a great extent on manmade and natural cellulosic fibres, for special purposes it is used on nylon and acetate.
- These dyes are extensively used for printing since colours are dischargeable and work with other groups.
Method of application:
The method of applying these dyes is somewhat similar to that of developed dyes, as it involves diazotizing. The fabric is first immersed in napthol, which impregnates the fibres; it is then dipped in to the diazotized colour bath (Base).
Dyeing or printing with azoic dyes is a two stage process. The first stage of dyeing with azoic dyes is called naptholation which involves dissolving the napthol called naptholation which involves dissolving the napthol in water using sodium hydroxide. The fibre is immersed in the solution of napthol. To assist penetration room temperature is adequate for cotton but for viscose rayon a temperature of 80-850c is used. The material is squeezed for any excess solution remaining in the material.
The second stage is called diazotization which involves the preparation of the di azo compounds by converting this component to the soluble diazonium salt. For this sodium nitrite and hydrochloric acid is added to the dye liquor. To this dye liquor ice may be added, because of the reason that diazonium salts are stable at low temperatures.
Once the fabric is treated with napthol, It is passed through the liquor containing diazonuium salt. The reaction between napthol and diazonium salt is referred to as coupling. Diazotization is preferably carried out in vessels constructed of wood, plastic, earth ware or stainless steel. Iron, copper, zinc vessels should not be used. Care must be taken at each stage of dyeing that most dye formation occurs within the polymer system of the fibre.
Fastness properties:
- Fabrics dyed and printed with azoic dyes have very good to excellent light fastness ranging from 6-7. It requires very prolonged exposure to sunlight for azoic dyed fabrics to be adversely affected.
- Because of the insoluble nature if the azoic dyes, fabric dyed with azoic dyes have very good wash fastness property rating about 4-5. This means they are unaffected during laundering.
- Azoic dyes are characterized by their very bright red and orange colours.
- Textile materials dyed with azoic dyes suffer from poor rub-fastness. Which might be controlled through careful soaping –off at all stages of dyeing.
- At times azoic dyes may cause a matt or delustered effect. This effect, called blinding, is more common on viscose rayon than on cotton.
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