VAT DYES

Vat dyes are one of the important class in the synthetic dyes produced Vat dyes are one of the important class in the synthetic dyes produced. These dyes are characterised by their insolubility in water. They are applied to the fabric in a reduced, soluble form, which has affinity for the substrate and after the reduced dye been absorbed, the fabric is taken out from the dye bath and left in the air or immersed in solution, of a mild oxidising agent to reproduce the dyeing shades Vat dyes are, in general, fast to washing to light, etc.

Until early times of the present century, the only vat dyes known were those related to indigo. But due to the constant research work in the field, a new class, anthraquinone dyes was found and is of much prominence these days.

Indigo was first a naturally cultivated dye which was principally grown in every part of the world. Indigo has become quite important in dyeing full shades of navy blue on wool, for uniform cloth of good fasteners. While dyeing, it is reduced by hydrosulphite, to make the alkali soluble leuco derivative or to make indigo white.

Anthraquinone vat dyes are mostly solid in the insoluble oxidised form, sometimes as dry solids but more often as quneous pastes. They are applied in the reduced form as 'Vats', the reduction 'being affected by the use' of sodium hydrosulphite under strongly alkaline conditions.

Vat dyes are so named because of their ability to form a soluble alkali salt or vat from the insoluble dye. This is usually accomplished by an alkaline reducing agent, and the affinity of the colourless leuco salt for textile fibers, especially cellulosic fibers, is characteristic of the group.

Vat dyes are insoluble in water, solubilised by treatment with caustic soda and reducing agent, usually hyposulphite, the resulting leuco compounds have affinity for textile fiber, on exposure to air leuco compound inpregnated fiber reoxidises to the insoluble parent dye. Vat dyes mainly belong to indigoid and anthra quinoid classes and are characterised by high fastness, specially anthraquinoids, most valuable for dyeing and printing cotton, wool and silk pH is kept below a point at which damage to protein fiber may occur.

NaOH+Hydros                Reduction          Formation of soluble Na Compound O = Vat = O  ------------Ã   (HO -Vat - OH ---------------Ã   NaO - Vat - ONa

A wide range of different techniques are used in colouring processes with vat dyes. All processes involve three steps: · Vatting · Oxidation · After treatment.

vatting The step in which the reduction of the dyestuff into its leuco-form takes place is called vatting. Vat dyes are generally more difficult to reduce than sulphur dyes. Various reducing agents are used. Sodium dithionite (hydrosulphite) is still the most widely employed although it has some limits. Sodium dithionite is consumed by reduction of the dye and also by reaction with atmospheric oxygen, therefore an excess of reducing agent has to be used and various techniques have been proposed to reduce these losses. In addition sodium dithionite cannot be used for high temperature or pad-steam dyeing processes because over-reduction can occur with sensitive dyes. In these application conditions and also for printing, sulphoxylic acid derivatives are normally preferred.

oxidation After absorption by the fibre, the dye in its soluble leuco form is converted to the original pigment by oxidation. This process is carried out in the course of wet treatment (washing) by addition of oxidants such as hydrogen peroxide, perborate to the liquor.

After-treatment The final step consists in after-treating the material in weakly alkaline liquor with a detergent at boiling temperature. This soap treatment is not only aimed at removing pigment particles, but also allows the crystallisation of amorphous dye particles, which gives the material the final shade and the fastness properties typical of vat dyes.

Vat dyeing conditions can vary widely in terms of temperature and the amount of salt and alkali required, depending on the nature of the dye applied. Vat dyes are therefore divided into the following groups, according to their affinity for the fibre and the amount of alkali required for dyeing

Vat dyes are the primary choice where the highest degree of fastness to industrial laundering, weathering and light are required. Based on temperature, amount of caustic soda, hydrosulphite and salt, used in dyeing, vat dyes can be classified into four main groups:

IK dyes (I = Indanthren, K = cold) have low affinity, they are dyed at 20 - 30 °C and require little alkali and salt to increase dye absorption IW dyes (W = warm) have higher affinity, they are dyed at 40 - 45 °C with more alkali and little or no salt IN dyes (N = normal) are highly substantive and applied at 60 °C and require much alkali, but no addition of salt.

Solubilized Vat Dyes It's inconvenient to have to reduce your vat dyes in order to dissolve them. However, it is impractical to sell the reduced form of the dye, because it will oxidize in the air, back to the insoluble form. The solution to this problem is for the manufacturer to convert the soluble leuco acid form of the dye to the leuco ester, such as by reacting the leuco acid with sulfuric acid.

The leuco form of this solubilized dye can be regenerated by removing the ester group chemically (with sodium nitrite in dilute sulfuric acid) or by the action of light. The drawbacks of solubilized leuco esters are their greater expense and their poorer uptake into the fiber, which results in paler shades.