4.9. Vat dyes

Vat are made in the form of powders submicron with non-dusting finish. They are readily wetted on the contact with water and form a stable dispersion. The size of the pigment particles and the composition of the accompanying chemicals are selected so that the dye is easily dispersed, do not sediment, and are quickly reduced.

Vat dyes are essentially insoluble in water and incapable of dyeing fibers directly. However, reduction (Na₂S₂O₄) in alkaline (NaOH) liquor produces the water soluble alkali solt of the dye, which, in this leuco form, has an affinity for the textile fiber. Subsequent oxidation reforms the original insoluble dye.

Indigo-molecular structure Vat Yellow-molecular structure

The process of dyeing with vat dyes includes four steps:

- vatting, i.e. formation of the dye leuco compound КрСОNa;

- adsorption of the dye leuco compound by the fiber and diffusion into the fiber;

- oxidation of the leuco compound adsorbed by the fiber, i.e. conversion of the dye back into the insoluble form КрС = 0;

- aftertreatment in which the dyeing obtains the true and stable shade and optimum fastness properties.

Vatting can be done in two ways:

- in the concentrated, so-called stock vat;

- in a dye liquor.

By the end caustic soda action on the dye pig­ment, a sodium salt of the leuco compound is formed during vatting. This usually differs from the original oxidized form of the dye as far as color is concerned. Vat dyes exhaust quickly onto the fiber from aqueous solutions. Oxidation of the leucocompound takes place on the fiber either in a medium of air oxygen or is performed with the aid of oxidizing agents. The latter method is more frequent. In some cases the dyeing must be additionally acidified to remove alkali and by-products of the oxidation from the fibers. It is possible to oxidize and acidify in the same bath simultaneously.

The aim of after treatment is stabilizing the shade and achieving maximum fastness properties. Therefore, the dyed material is soaped at the boil, usually under alkaline conditions. The dye pigment is re-crystallized whereby crystal aggregates are formed which are compact and more resistant to the effects of light. Vat dyes can deliver dyeing with highest levels of washing and wet fastness properties. For staining of adjacent cotton goods and shade changes on washing, vat dyeing are rarely rated less than 4-5 on a scale of 5.

Dyeing vat dyes

For work carrying out there are following solutions: vat dye in the form of 10 % suspensions, dry hydrosulphite (Na₂S₂0₄), 35 % NaOH, and 10 % solution of salt (NaCl).

For dyeing take the sample of the bleached cotton fabric, viscose fabric and a polyester fiber.

Solution Components:

Dye. . . . . . . . . . . . . . . . . . . . . .3 % from weight of a fiber

NaOH (35 %). . . . . . . . . . . . . . 20—25 ml/l

Reducing agent Na₂S₂O₄. . . . .13—14 г/л

Sodium chloride NaCl. . . . . . .10 % from weight of a fiber

Water. . . . . . . . . . . . . . . . . . . . X

To translate vat dye in the soluble form (leuco compound), it is necessary to reduce it. For this purpose in a porcelain glass on 100 ml measure the calculated quantity of suspension of dye (preliminary to shake up) and add the calculated quantity NaOH and 36 ml hot (70°С) waters. Then into a glass enter dry Reducing agent Na₂S₂O₄, carefully mix and leave to stand 10-15 min at temperature 60°С (not to overheat!) to a complete recovery and dye dissolution. Thus color of reactionary weight varies, and the solution gets intensive coloring (to note change of color in a writing-book).

In prepared concentrated a dye solution add hot water to the calculated volume and the moistened and wrung out samples immerse. Continue dyeing at 50-60°С during 20 min, after that add the calculated quantity of NaCl to the solution and continue dyeing of 30 min more at the same temperature. During dyeing a yarn hank continuously overturns, thus it is necessary to watch that the yarn all time was under level the dye solution.

After the termination of dyeing the material is left by 20-30 mines for oxidation on air. Color of a fiber thus varies and gets the coloring corresponding to color of insoluble dye.