Principles of Application of Vat Dyes

Vat dyes are insoluble and cannot be used directly on textile materials and requires vatting. Vat dye mainly used for cellulose fiber dyeing but in protein fiber dyeing pH should be controlled. Dyeing process is difficult from other dye. Among all the dyes, vat dye has the best fastness properties though rubbing fastness is not good.
Worker in vat dyeing
Fig: Worker in vat dyeing (Image courtesy: http://www.aquila-style.com)
Stages or Chemistry or Mechanism of dyeing with vat dyes:
Generally, the application of vat dyes to textile materials involves five distinct steps.

a) Aqueous dispersion: The insoluble vat dyes, in this stage dispersed in water.

b) Vatting: Vatting in which the insoluble commercial dye is reduced and solidities (vatted) by using Sodium hydrosulphite (hydrose) and Sodium hydroxide (NaOH). Sodium hydrosulphite work as a reducing agent. Again salt formation by neutralising their hydroxide to give a water soluble product. Sodium hydroxide is used as a solubilising agent.

c) Dye absorption: Dyeing, in which the soluble sodium salt of the leuco vat dye is absorbed by the textile material from an alkaline reducing medium in the presence of either a retarding agent or an exhausting agent depending on the rate of dyeing. In this stage, the textile material must be immersed in dye liquor to prevent oxidation of leuco compound.

d) Reoxidation of vat dye: Oxidation, in which the soluble form of the dye absorbed by the fiber, is reconverted into the original insoluble dye by atmospheric oxygen (Airing) or by the use of “chemical oxidation.” (i.e. involving the use of a chemical like sodium per borate or potassium dichromate or Hydrogen per oxide).

e) After treatment: During the previous stage some insoluble vat dye may be disposited on the surface of the textile material. Soaping – off, in which the dyed material is subjected to a treatment either boiling soap or other detergent solution in order to get the proper tone by way of aggregation of smaller dye particles into bigger ones and also to get the optimum fastness, especially rubbing fastness by removing the surface deposited dye particles.
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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.


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