Vat dye, a water-insoluble dye class that is widely used on cellulose fibers like cotton, jute, flax etc. Chemical constitution of vat dyes affects their dyeing properties and fastness behavior. Dyeing with vat dyes kind of similar with the “Sulphur dyeing”. Vat dyes possess overall the best fastness properties. Though they are insoluble in water, antraquinones are mostly soluble in hot DMSO.
These dyes are non-ionic and develop anion on solubilization and possess excellent affinity for cellulosics and retained by the fiber with hydrogen bond and van der walls force on oxidation. Though their molecular size is too small than the pores of cellulose, after diffusion and oxidation they crystallizes to form big aggregates during soaping and thus shows superior fastness properties.
Chemical constitution of vat dyes
In the case of Indigo, vatting involves reduction of a pair of conjugated carbonyl groups. For most vat dyes, these carbonyl groups are present in an anthraquinone or polycyclic quinone. The chemical structure of a vat dye influences the water solubility, substantivity and rate of diffusion into the cotton of the leuco compound used in dyeing.
Types of Vat dyes
- On the basis of chemical structure, vat dyes are derived into major two classes:
- Indigoids: Containing the chromophore
Indigoids are subdivided into two classes:
2. Anthraquinoid: These are mainly two types:
a) Sulphurised antraquinone
b) Antraquinone ester
- On the basis of application method:
The German Interessen Gemeinschaft für Farbenindustrie (IG) developed one popular classification for their Indanthrene range of vat dyes based on leuco compound substantivity and the required dyeing conditions.
Another types of Vat Dye
- The IN (indanthrene normal) group: These dyes require the use of concentrated NaOH and high vatting (60 °C) and dyeing temperatures (60 °C). No salt is added to the dyebath because of the high substantivity of the leuco dyes for cotton.
- The IW (indanthrene warm) group: These dyes require only moderate amounts of NaOH and lower vatting (50 °C) and dyeing temperatures (50 °C). The leuco forms of these dyes have moderate substantivity for cotton and some addition of salt is needed during dyeing to aid exhaustion
- The IK group: These dyes only need a low concentration of NaOH with low vatting (40 °C) and dyeing temperatures (20 °C). These dyes have low substantivity for cotton and need considerable salt for good dye bath exhaustion. Some have amide groups that would be hydrolysed under the vatting and dyeing conditions used for IN and IW dyes.
To Sum Up!
These dyes produce excellent color on cellulose with a wide shade range. Not only cellulosic fibers but also protein fibers can be dyed with vat dyes. Jeans or denims that add a new era to our fashion industry, also dyed with vat dyes.