Dyeing with vat dyes:

The application of vat dyes to cellulosic materials occurs in five stages.

Aqueous Dispersion:

The insoluble vat dye is dispersed in water.

Vatting:

This step involves the chemical reduction of the vat dye to produce the soluble, reduced or leuco form of the dye. This is achieved by Sodium Hydrosulphite, Sodium Hydroxide and water. The sodium hydrosulphite chemically reduces the vat dye in the alkaline conditions created by the presence of sodium hydroxide.

Absorption of dye molecules by the fibre:

The vatted dye molecules are substantive to the cellulosic material when this is introduced into the dye liquor. To achieve adequate exhaustion, an electrolyte is added to the dye liquor and the temperature may be increased depending on the specific vat dye. The application of the dye molecule to the fibre occurs at temperatures specific to a particular vat dye and occurs in a range from 200 C to 600 C. The addition of the electrolyte alters the equilibrium of the dye liquor so as to increase the substantivity of the dye molecules for the fibre. During this stage of dye application the textile material must be kept immersed in the dye liquor to prevent premature oxidation of the leuco compound.

Figure: Water soluble leuco form of indigo

Re-oxidation of dye molecules within the fibre:

Once within the polymer system of the fibre the leuco form of the vat dye has to be oxidized and converted to its original colour and the insoluble form of the dye. Oxidation of the leuco compound can be achieved by atmospheric oxygen although this is somewhat slow. In practice, a mild oxidizing reagent such as sodium perborate is used to convert the soluble leuco compound into the original insoluble vat dye.

Soaping-off vat dyes:

During the previous stage some insoluble vat dye may be deposited on the surface of the textile material. This has to be removed to prevent poor rub-fastness as well as a possible change of shade due to the subsequent removal of this surface deposit. Soaping-off, which is the boiling of the dyed material in a liquor containing some suitable detergent, removes this surface dye. The term soaping-off was derived from the fact that before the development of detergents, soap was used to remove the surface dye.