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Vat Yellow-molecular structure.

Batch Dyeing of Vat Dyes:

Batch dyeing characteristics:  No single classification of vat dyes by dyeing characteristics has been as useful  or as generally accepted as has the classification of direct dyes in to groups A, B and C.

One method of classification of vat dyes in to four principle sub-groups: IK, IW, IN and IN Special, which still leaves a few dyes out, such as C.I. Vat Black 9. This classification is based on the different substantivities of the leuco vat anions and the corresponding differences in dyeing temperatures and the salt, caustic soda and hydro concentrations necessary to give the best over all dyeing results. The importance of these traditional groupings is restricted to batch dyeing with a leuco vat anions. substantivities of the leuco vat anions and the corresponding differences in dyeing temperatures and the salt, caustic soda and hydro concentrations necessary to give the best over all dyeing results. The importance of these traditional groupings is restricted to batch dyeing with a leuco vat anions.

The 'I' stands for 'Indanthrene' . "K" stand for the German word, Kalt, meaning cold. Dyes (reduced leuco vat anions) in this group are dyed at room temperature with a relative high salt concentration ( common salt or anhydrous sodium sulphate) and relatively low sodium hydroxide (caustic soda) concentration, all of which tend to promote higher substantivity and exhaustion of equilibrium.Indanthrene' . "K" stand for the German word, Kalt, meaning cold. Dyes (reduced leuco vat anions) in this group are dyed at room temperature with a relative high salt concentration ( common salt or anhydrous sodium sulphate) and relatively low sodium hydroxide (caustic soda) concentration, all of which tend to promote higher substantivity and exhaustion of equilibrium.

"W" stands  for German/English word warm. Dyes in this group are more substantive and can be dyed at 40 to 50 C (100 to 120 F) with less salt and slightly more alkali.

"N" stands for the German/English word normal. Such dyes are even more substantive, require more alkali but no salt, and can be dyed at 60 C (140 F). The IN special dyes require even more alkali.

Table-1 shows the conditions under which 2% o.w.g. shades can be dyed at 10:1 liquor to goods ratio for IK, IN and IW dyes. The concentrations of chemicals increase with increasing dye concentration and liquor to goods ratio. For more details a reputable dye supplier should be consulted since different dye organizations use different methods of categorization. Also read carefully the units in which the concentrations are expressed. Conveniently, few of the vat dyes in use today fall outside the IW and IN categories. When it is necessary to dye mixtures of IW and IN dyes, the IN conditions are to be selected because the higher temperature and lower salt are least likely to cause unevenness due to too rapid dyeing.

Conditions for dyeing leuco vat anions at 10:1 liquor to goods ratio.

Batch Dyeing Processes: There are two distinct types of vat dyeing processes which will be discussed here. They may be characterized as leuco and pre-pigmentation methods, and there are also dyeing methods which are hybrids. But before we proceed to look at the different processes in turn, it would be desirable to reiterate a caution, given earlier, regarding preparation and water quality. 

Preparation and water quality:   When dyeing cotton with water soluble anions under conditions of high substantivity, as is the case with leuco-vat anions, it is imperative to have the goods as free as possible from any impurities which might be distributed non-uniformly and which are hydrophobic (e.g. oil, fats and waxes), or which can interact with the dye anions to give hydrophobic (insoluble) products; e.g., the calcium and magnesium cations present in the process water and in the cotton itself, from the irrigation water. Anything which might stand in the way of uniformly wetting the goods, with dye solution could cause un-level dyeing. The better the preparation, the more uniform the dyeing. Careful preparation may not put money in the bank, but it could stop second quality goods from eating away assets. The use of sequestering agents or protective colloids which complex with calcium and magnesium ions is recommended. Oxidative bleaching pretreatments along with the use of strong alkalis, dispersing agents and sequestering agents often precede dyeing; but it is possible with vat dyes to take advantage of the highly alkaline dye baths and both prepare and dye greige goods at the same time.

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