Absorption in the Exhaustion Process

In the dissolved form, the vatted dye is present as the sodium leuco compound, either as a single molecule or as a complex of a few dye molecules. In the exhaustion process, the higher the affinity, the more complete is the absorption by the fiber. The degree of absorption of dyes depends also on dye concentration, liquor ratio, temperature, and the electrolyte present in the dyebath. It is generally between 80 and 90 %. 

After absorption by the fiber, the leuco dye is converted to the original pigment by oxidation and, in this way, fixed to the fiber. Oxidation can be achieved with atmospheric oxygen or by the addition of oxidants, such as hydrogen peroxide, perborate, or 3-nitrobenzene sulfonic acid, to the liquor. 

After the dyeing process, the material is usually rinsed with water to remove unfixed dye and decrease the alkalinity of the liquor. Depending on the oxidizing agent, oxidation is carried out at pH 9-12 and 50-60°C. 

The leuco dye present in wastewater is also converted to water-insoluble pigment, which can easily be separated mechanically or adsorbed on clarification sludge. Thus, colorization of the wastewater by vat dyes is negligible. 

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