Classification And Properties of Vat Dyes (Part 1)

Definition of Vat Dye:

Vat dyes are water insoluble pigments. They are called dyes because chemical reduction in alkaline solution converts the pigment into a water soluble leuco form with substantively for cotton.

The overall process therefore involves three key steps:

I.Reduction of pigment to the soluble leuco compound ,a process called vatting.

II.Absorption of the leuco compound by the cotton during dyeing.

III.Oxidation of the absorbed leuco compound in the cotton, reforming the insoluble pigment inside the fibers.

Classification of Vat Dye:

For quinine vat dye there is no single classification according to dyeing properties as is the case for the direct dyes.The German Interessen Gemeinschaftfur Farbenindustrie (IG) developed on popular classification for their indanthrene range of vat dye based on leuco compound substantivity and the required dyeing condition.There were three main types:

I.The IN group of dyes require the use of concentrated NaOH and high vatting (60C) and dyeing temperatures (60C).No salt is added to the due bath because of the high substantivity of the leuco dyes for cotton.

II.The IW group of dyes require only moderate amount of NaOH and lower vatting and dyeing temperature (50C).The leuco form of this dyes have moderate substantivity for cotton and some addition of salt is needed during dyeing to aid exhaustion.

III.The IK group of dye only need a low concentration of NaOH with low vatting and dyeing temperature(20C).These dye have low substantivity for cotton and need considerable salt for good dye bath exhaustion.

Chemical Classification Of Vat Dyes :

The vat dyes are to be divided into several groups, depending upon their chemical nature and origin, as follows:

(a) Indigo, including both natural and Synthetic
(b) Thio-indigo dyes, containing sulphur
(c) Indigo derivatives, such as the brom-indigos; usually not derived directly from indigo itself, but built up synthetically.
(d) Anthraquinone derivatives, including the various Indanthrene, Marione, Algol dyes, some Helindone, and others.

(e) Carbazol derivatives, of which Hydron Blue is the chief representative