VAT DYE

History:

the vat dye is found amongst the oldest natural coloring matters used for textiles. Indigo has been known in India since the earliest periods of which historical records exist. According to the writings of Julius Caesar the ancient Britons used Woad to stain their Bodies and faces and Tyrian purple was exported from Tyre to the Mediterranean countries nearly 4000 years ago. Tyrian purple is extracted from a shell fish and is therefore of animal origin but woad and indigo exist in plants, combined with glucose in the form of glucosides the vat dyes are all insoluble in water and cannot be used for dyeing without modification when treated with reducing agents they are converted into leuco compounds all of which are soluble in water in the presence of alkalis. These leuco compounds are substantive towards cellulose and reoxidize to the insoluble coloured pigment within the fibre when exposed to air. The leuco compounds are often colourless or of quite a different colour from the product of oxidation.
Vat dyes so named: the word vat means vessel. The dye takes their generic name from vatting , the vat dyes are naturally obtained colouring matter from the ancient time and kept into wooden vat and make solubilise in vat by the process of fermentation – so it is called vat dye.

Properties of vat dyes:

1. Vat dye is water insoluble and can’t be applied directly on textile material.
2. Mainly use fir cellulose fibre dyeing but in protein fibre dyeing PH should be controlled.
3. Rubbing fastness is not good.
4. Various shades is found.
5. Dyeing process is difficult.
6. Costly.
7. Washing fastness of vat dye is very good with rating 4-5.

Classification of vat dyes:

For quinone vat dyes, there is no single classification according to dyeing properties as is the case for the direct dyes. The German interessen Gemeinschaft fur farbenindustrie (IG) developed one popular classification for their indanthrene rangr of vat dyes based on leuco compound substantivity and the required dyeing conditions.

Three main types:

1. The 1N (indanthrene normal) group of dyeing temperatures (60C) and dyeing temperatures (60C). no salt is added to the dyebath because of the high substantivity of the leuco dyes for cotton;

2. The IW (indanthrene Warm) group of dyes requires the use of concentrated NaOH and lower vatting (50C) and dyeing temperatures (50C). the leuco forms of these dyes have moderate substantivity for cotton and some addition of salt is needed during dyeing to aid exhaustion;

3.The IK group of dyes only need a low concentration of NaOH with low vatting (40C) and dyeing temperatures (20C). these dyes have low substantivity for cotton and need considerable salt for good dyebath exhaustion. Some have amide groups that would be hydrolyswd under the vatting and dyeing consitions used for IN and IW dyes.

There are special processes for some black vat dyes that require an oxidative aftertreatment to develop the full black colour. Table 17.1 compares the characteristics of these three tupes of vat dye . the required concentrations of hydros, caustic soda and salt increase with increasing amounts of dye in the bath and with increasing liquor ratio.

There are various other classifications of vat dyeing methods . the SDC recommend tests to determine the best dyeing methods. In this , the colour strengths of dyeing produced under different dyeing conditions are compared with those of standarad dyeing using a grey scale . this test applies only toanthrawuinone dyes . thereare also SDC tests for determine the strike, migration and leveling characteristics of vat dyes . different companies have different classification systems for thir vat dyer. Because vatting and dyeing conditions vary from one another, the suppliers’ recommendations should be consulted.

17.5.3 problems with anthraquinone dyes

A number if chemical problems aruse with some quinone vat dyes . these include:
1. Multiple reduction steps for poluquinones such as iudanthrone;
2. Isomerusm of leuco compounds to oxanthrones;
3. Hydrolysis of amide groups;
4. Over-oxidation after dyeing;
5. Dehalogenation of some dyes,

To minimize these types of problems , the supplier’s recommendations for vatting and dyeing must be followed.
Indanthrone (Cl var blue 4) and some of its derivatives show a number of these problems indanthrone has two anthraquinoe residues in its molecule. The normal blue leuco compound used in dyeing is that corresponding to the reduction of one of the anthraquinone groups (5, in figure 17.5). If both anthraquinone groups are reduced. The final product (6) gives a brownish yellow solution. Has poor substantivity for cotton and is more difficult to oxidize. Such over-reduction produces duller blue dyeing of lower colour yield.