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                                                                                                                    Application of Vat Dyes                         
Vat Yellow-molecular structure.

Dye Anion Solubility: The solubility of the sodium leuco derivatives of vat dyes varies considerably with structure, with some higher than 80 gpl and lower than 15 gpl. The C.I. Vat Blues 4, 6 and 14, based on indanthrones, have a tendency both to lose solubility on standing and , along with several others, to form insoluble salts of Ca, Mg or Fe in hard water. As mentioned earlier a sequestering agent can solve this later problem. At high temperatures, these same indanthrones are also subject to over-reduction, and C.I. Vat Blue 6 can lose its chloro groups. Additions of glucose or sodium nitrite may help eliminate over reduction.

Leuco Vat Dyeing: The earliest and the most fundamental vat dyeing process is the leuco-process in which the practical dyeing does not begin until the vat pigment is dissolved in caustic soda and hydrosulfite solution. Here we have a classical case of anions in solution, their sorption and diffusion in to the nonionic cellulosic fiber.

This type of dyeing process is applicable to cellulosic fibers and raw stock through sliver, yarn packages and yarn skeins, to knit or woven fabrics where liquor to goods ratios of 10:1 to 20:1 would be in the normal range. Its primary use is for heavy shades, which exhaust less rapidly and for which the possibility of unevenness is not so critical. The variety of machinery possible for dyeing fiber in all these forms makes any suggestion of a single optimal procedure absurd. But the variables that affect the levelness or uniformity on the one hand and the dyeing efficiency or exhaustion on the other are well understood. Careful consideration of these variables can be used to effect process optimization in any particular case.

• Temperature: The temperatures at the start of the dyeing is determined by the classification of the dye used (see table -1). Vatting is normally carried out prior to dyeing for about 10 minutes in the long liquor. During the dyeing the temperature can be gradually raised to about  80 C or even higher if the dyes are not sensitive to over reduction and a more stable alternative to hydro used as the reducing agent. Temperatures of 100 C (212 F) and a little higher may also be used, where leveling in lighter shades or penetration in tighter construction may be a problem. Towards the end of the dyeing cycle the temperature might be lowered to 40 to 50 C to increase the percentage exhaustion.

• Time:  The time at the maximum dyeing temperature can vary from about 45 minutes tat 60C to about 20 minutes at 100C, but will depend on the amount of circulation in the equipment.

• Salts:  Additional sodium chloride or sodium sulfate may be added to dye bath towards the end of the dyeing in order to improve exhaustion.

• Reducing system: The amounts of caustic soda and hydrosulfite used will depend not only on the concentration of the dye present but also on the temperature, whether the vessel is open to the air or enclosed, and on the area of exposed surface. The amount should be sufficient to maintain the dye in alkaline reduction condition throughout the dyeing. Any addition require care.

• Auxiliary Chemicals: Two kinds of agents which may be used in leuco-vat dyeing are leveling and stripping agents. They are essentially similar in their action, for both interact with the leuco-vat anions in solution to form complexes. These complexes lower the effective concentration of dye anions in the bath and hence can lower the rate of dyeing and increase the rate of stripping. The difference between the leveling and stripping agents in one of degree. The stability of leuco dye anion/agent complex is greater for stripping than leveling.                       

          Protective colloids, sequesterants or chelating agents may help facilitate dyeing of greige goods ( see the previous sub headings on preparation and water quality.

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