Vat dye and its application in textile:
Definition:
Vat dye is a type of dye that has to be dissolved in a vessel called VAT by a solution of a reducing agent called sodium hydrosulphide or hydrogen and caustic soda (NaOH). https://bdfabric.com/stripping-of-acid-from-wool/
Vat dye and its application in textile-introduction:
Blue or indigo is an ancient color used in the Indian subcontinent. In Indigo, the color was also common in parts of Egypt and the Roman Empire. In ancient times, this indigo color obtained from nature was dissolved by fermentation and reduced by specially made wooden pots, especially organic matter. This wooden vessel was known as Vat. Hence the name VAT Dye. Other dyes that are dyed soluble like Indigo are called Vat Dye and this method is called Vatting.
Vat dye is insoluble in water so it cannot be used in textile products without vatting. When this dye is processed by a reducing agent it turns into a leuco compound, which is soluble in water in the presence of alkali. Vatting is the process by which vat dye is converted into a leuko compound. A leuko compound reacts with cellulose. When it is again exposed to oxygen in the air, it is re-oxidized in the fiber to form an insoluble color.
Vat dye and its application in textile. There are many types of coloring groups in VAT dye. However, one or more carbonyl (> C = O) groups are present in each dyestuff. Vat dyes are insoluble due to the presence of this carbonyl group.
When this dye is first processed by the reducing agent hydrogen (Na2S204), the carbonyl (> C = O) group is converted into leuko compound (≥ C – OH). A leuko compound reacts with sodium hydroxide (NaOH) to form the sodium salt of leuko compound, which is soluble in water. It is then dyed with an aqueous solution. When the product is left exposed to air after dyeing, it re-becomes an insoluble color compound in the presence of oxygen in the air.
Classification of vat dyes:
(A) VAT dye can be divided into two parts based on the use of alkali. E.g.
1. Strong alkali vat dye
2. Weak alkali vat dye
1. Strong alkali vat dye:
To get more color shades, high-density alkali (NaOH) is used in the dye bath.
2. Weak alkali vat dye:
A small amount of alkali (NaOH) is used for the dye-bath solution for the middle shade. Weak alkalis are used for good dyeing.
(B) VAT dye can be divided into two parts based on the chemical composition. E.g.
1. Indigoids
2. Anthraquinones
Indigoids can be further divided into two parts. E.g.
# Indigotin
# Thioindigotin
