Vat dyes are a class of dyes that are classified by their method of application. Vat dyeing refers to the process of dyeing in a vat or bucket. Most dyes, such as fiber-reactive dyes, direct dyes, and acid dyes can be used in a vat dyeing. Cotton, wool, and other fibers can be dyed with vat dyes.
The original vat dye is indigo, once obtained from plants but now produced synthetically. Indigo was first recovered from the water-soluble glucoside in plants, and then the insoluble blue product was dissolved in wooden vat by a process later known as vatting. This is the origin of the name of vat dye.
Vat dyes are the most important dyes for dyeing on cotton and cellulose fibers. They have excellent fastness, including washing, light, chlorine and rubbing fastness. The main use of Vat Blue 1 is to dye cotton yarns, which is mainly used for the production of denim for jeans. A small amount of Vat Blue 1 is used to dye wool and silk. Vat Blue 1 is an indigo derivative that is also used as a colorant. About 20,000 tons of Vat Blue 1are produced annually, mainly for the production of jeans. It is also used as a food coloring agent.
Classification of Vat Dyes
Anthraquinone vat dyes: The anthraquinoneacridones are capable of producing hues from orange to blue and can be prepared, for example, by reaction of sulfonic acid with carboxylic acid which produces Vat Red 35.
Fused ring polycyclic vat dyes: A large number of vat dyes belong to the category of fused ring polycyclic vat dyes, which contains carbonyl groups. The color, constitution and theory of color of these structures have received attention.
Indigoid vat dyes: Vat dye chemistry would not be completed if without indigo, whose structure was elucidated in 1883. Indigo is applied as its water-soluble form, produced by the reduction with sodium dithionite, and it is still used extensively for the dyeing of denim.