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Characteristics:
The name vat was derived from the large wooden vessel from which vat dyes were first applied. The first synthetic vat dye was an indigo created in 1879. Vat dyes are not only resistant to light, acids and alkalies but are also resistant to the strong oxidizing bleaches.
Vat dyes are characterized by their insolubility in water, they are chemically reduced for reaction with a fibre, then reoxidized to produce colour.
A wide choice of colours with good to excellent colour fastness properties is available in the vat dyes.
They are the class of dyes with all-round fastness properties. They are costly than any class of dyes.
Vat dyes are marketed under various trade names such as Benzanthrene, Caledon, or Durindon etc.
Suitability to textiles:
- Vat dyes are extensively used for dyeing natural and manmade cellulosics because of their high fastness to light, washing, rubbing, perspiration and even to hypochlorite bleaches.
- Vat dyes are fastest dyes for cotton, linen and rayon. They also may be applied to wool, nylon, polyester, acrylics and modacrylics with the use of mordant.
- They are the class of dyes which are used in the production of coloured shirtings, sarees, window curtains and such fabrics that are frequently subjected to laundering. These dyes have much use in prints.
Method of application:
Dyeing is carried out in 5 stages.
- Aqueous dispersion
- vatting
- Absorption of the dye molecule by the fibre.
- Reoxidation of the dye molecule with in the fibre.
- Soaping off.
- Aqueous dispersion: The insoluble vat dye is dispersed in water.
- vatting: This step involves the chemical reduction of the vat dye to produce the soluble, reduced or leuco form of the dye. This is achieved by reducing agent(sodium hydrosulphite), sodium hydroxide and water. This is called vatting. The dye stuff is carefully pasted with a wetting agent like T.R.O. This paste is added to the dye bath and allowed to stand for 20-30min. caustic soda and hydrosulphite should be added in required quantities for vatting. Only in this form they are absorbedby the material to be dyed.
- Absorption of the dye molecule by the fibre: The vatted dye molecules are substantive to cellulosic materials when this is introduced to the dye liquor. An electrolyte is added to the dye liquor and the temperature may be increased depending on the specific vat dye to achieve adequate exhaustion. The application of the dye molecule to the fibre occurs at temperature specific to a particular vat dye and occurs in the range from 20oc to 60oc. the addition of the electrolyte alters the equilibrium of the dye liquor so as to increase the substantivity of the dye molecule for the fibre. During this stage of dye application the textile material must be kept immersed in the dye liquor to prevent premature oxidation of the leuco compound. The dyeing is usually carried out for ¾ th to 1 hour.
- Reoxidation of the dye molecule with in the fibre: After dyeing the material is squeezed and exposed to atmosphere for proper oxidation, which develops the colour. They are reconverted to their original insoluble form in a subsequent oxidation process and there by fixed on the fibre. When using blues, the dyed fabric is first rinsed in water and later exposed to atmosphere.
- Soaping-off : During the previous stage some vat dye particles may be deposited on the surface of the textile material. This has to be removed to prevent poor rub-fastness. When complete oxidation takes place the material is washed in water and boiled in a liquor containing some suitable detergent, which removes the surface dye.
Fastness properties:
- The light fastness of textiles coloured with vat dyes have excellent light fastness rating about 7.
- The excellent wash fastness of textiles coloured with vat dyes is about 4-5.
- Vat dyes have always been very expensive compared with other class of dyes.
- These dyes are invariably used when good fastness properties are required.
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