Principles of Vat Dyeing

Vat dyes are water-insoluble, organic pigments that are used to dye cotton and other cellulose libers. The principle of vat dyeing is based on chemical reduction of these dyes to the leuco compounds, which are soluble in aqueous alkali and exhibit fiber affinity, followed by reoxidation within the fiber to the water-insoluble starting dye. 

1) Indigoid vat dyes are derivatives of indigo and thioindigo (see Section 3.5). Leucoindigo compounds have a comparatively low affinity for fibers. Thus, these dyes are used mainly in textile printing rather than dyeing (see Section 4.4.6). 

2) The leuco compounds of anthraquinoid dyes have a high affinity for the cellulose fiber and give dyeings that meet the highest fastness requirements in use and processing. 

Commercial anthraquinoid vat dyes are dye preparations that consist of a vat-table colored pigment and a dispersing agent. Such vattablepigments are polycyclic quinoid compounds that contain two or more carbonyl groups in a closed system of conjugated double bonds (Section 3.4). 

The dyeing behavior of vat dyes, i.e., their absorptive behavior and leveling capacity, is determined basically by the substantivity and diffusion of the leuco compounds. The chemical stability and insolubility in water of the dye pigment that results from reoxidation in the fiber account for the generally unsurpassed light-, wet-, and weatherfastness of vat dyeings. 

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