Principles of Application of Vat Dyes

Generally, the application of vat dyes to textile materials involves four distinct steps.

a)Vatting in which the insoluble commercial dye is reduced and solidities (vatted) by using Sodium hydrosulphite (hydrose) and sodium hydroxide (NaoH).

b) 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.

c) Oxidation, in which the soluble form of the dye absorbed by the fibre, 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).

d) After treatment, 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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