Why Are They Called Vat Dyes?
The name "vat dye" comes from the traditional process of preparing and applying these dyes in a vat. Vat dyes are generally insoluble in water in their normal oxidised form. To apply them to cotton, they are chemically reduced in an alkaline bath to produce a soluble leuco form. After the dye enters the fibre, oxidation converts it back to its insoluble coloured form.
What Happens During Vat Dyeing?
The process is based on two important chemical stages: reduction and oxidation. Reduction changes the dye into a form that can be applied to the fibre. Oxidation then restores the original coloured form, which remains trapped within the fibre structure.
Main Steps of Vat Dyeing
- Prepare the fabric and dye bath.
- Reduce the vat dye under the required alkaline conditions.
- Apply the reduced dye to the textile material.
- Allow sufficient penetration and circulation.
- Oxidise the dye to develop its original colour.
- Wash and carry out the required after-treatment.
Vat Dyes and Indigo
Indigo is a well-known vat dye used for denim. Its characteristic behaviour during repeated dipping and oxidation helps create the familiar denim appearance. The dye may remain more concentrated near the yarn surface under typical denim dyeing conditions, contributing to the development of a worn or faded look during later processing.
Why Are Vat Dyes Important?
Vat dyes are valued for their colour fastness properties on cellulosic fibres when properly applied. However, the dyeing process requires careful control of reduction, alkalinity, temperature and oxidation.
FAQs
Why is the term vat dye used? It comes from the traditional vat-based preparation and application process.
Are vat dyes soluble in water? Their normal oxidised form is generally insoluble; reduction produces a soluble leuco form.
Is indigo a vat dye? Yes. Indigo is a well-known example of a vat dye.
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