The Vat colorants take their name from a wooden tub or vat (meaning ‘Vessel’) in which the process of reduction was first performed. A special dye bath is used wherein the colorant is applied on the fabric by reducing it to a soluble form. The reduction process is performed by adding a strong reducing agent.It is important to add some reducing agent such as hydro sulphite because according to the basic nature, vat colorants are insoluble in water and cannot be applied directly. Therefore, they must be solubilized before use. Before, we move further to explore the properties of these coloring agents let’s take a look on the process in which insoluble form of water is converted into soluble form.
To solubilize Vat dye, the blend of chemicals ‘sodium hydroxide’ (such as caustic soda, or lye) and ‘sodium dithionite’ (also known as sodium hydrosulfite, which is used as a reducing agent) is used.
This is the foremost step involved in dying, using vat agents and is called ‘batting’. The fabric is dipped into this leuco form. After the dye is absorbed by the fiber, the soluble form is converted into pigment through oxidation. For this, the fabric is taken out from the bath and oxidized.
As a final step, the textile is passed through a special treatment called “soaping”. In soap treatment, the dyed fabric is boiled in a mixture of weakly alkaline liquor and detergent. The goal of this treatment is to remove pigment particles in order to give the material, a final shade with color fastness.
These dyes can be classified based on their application conditions, which can be one of the following:
- IN (Bayer classification) or CI (Ciba classification) dyes (Hot)
- IW or CII dyes (Warm)
- IK or CIII dyes (Cold)
- IN special dyes
- Vat agents are used for coloring and printing materials like wool and cotton.
- They are insoluble in water and need to pass through a special process, called reduction, in order to convert dyestuff into its leuco-form.
- Diversity of color shades can be produced.
- All classes that come under Vat coloring agents have different light fastness properties. Higher rate of light fatness is 7 and lower rate is 5 (which is better than light fatness properties of other dyes) on the rasssssste of 8.