Indigo dyes, the vegetable dyes known to ancient Indians, was dyed on textile materials in wooden vats (tubs). Dyes similar to indigo in their application are called ‘vat dyes’. Actually indigo is the first number of vat dyes.
Bond formation with fibers:
They are insoluble in water so they cannot be directly applied on cottons or other fibers. They have to be temporarily converted into a water soluble form, having sufficient affinity for fibers. During the dyeing process, it is this soluble form of the dye that is applied on cotton, followed by re conversion of the soluble form into the original insoluble form. As a result, the insoluble dye is trapped in the fiber substance and cannot come out during soaping or any other wet treatments. So the conversion of water insoluble dyes to water soluble dyes and then re conversion of water soluble dyes to water insoluble dyes are carried out in two steps-
01. Solubilizing step
(A) Reduction of the dye into the weakly acidic leuco form
(B) Salt formation by neutralizing these weakly acidic leuco vat dyes by NaOH to give a water soluble product. This is called solubilizing step.
Reduction followed by solubilizing is called ‘vatting of the dye’. On reduction, indigo gives indigo white, a colorless and water insoluble product, this dissolves in a solution of NaOH.
02. Oxidation step.