What is Vat Dye? Theory of Vat Dye?

Vat Dyes:

The name vat was derived from the large wooden vessel from which vat dyes were first applied. Vat
dyes provide textile materials with the best colour fastness of all the dyes in common use. The fibres
most readily coloured with vat dyes are the natural and man-made cellulosic fibres. Vat dyes are more expensive and difficult to apply than other classes for cellulose such as directs, sulphurs, and reactive.
Indigo is a special case in the vat dye class. Indigo is attractive for its pleasing blue colour and for the
unique fading characteristics of garment dyed with it. Vat dyes are characterized by the presence of a
keto group. Vat dyes in keto form are water insoluble pigments.

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.

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.