Vat Dyes | Indigo Dyes | Application of Vat Dyes

Vat dye has been around since the early days of mankind and the most prominent of it is the indigo dye which was used by the Egyptians in 200 BC. So in this article, I’ve tried to give a glimpse about the process of vat dyeing along with the dyeing process for indigo dyes which gives us such beautiful blue shades in denim fabrics.

What are vat dyes?

Vat dyes are a class of dyes which are classified by the method of their application. In ancient times, dyeing was often carried out in wooden vat or bucket, thus assigning the name ‘vat dye’. These are insoluble in water and non-ionic in nature mainly due to the presence of C=O chromophore in its structure.

The dyeing process generally involves three basic steps:

• Preparation of the vat, i.e. reduction of dye to its leuco form followed by its conversion to sodium salt.
• Dyeing
• Oxidation to parent dye

Basic principle of dyeing with vat dye

 Vat dye has no initial affinity for cellulosic fibre and thus need to be chemically changed (reduced) to make it soluble and absorbed into the fibre. Once inside the fibre the dye oxidise back to insoluble form and gets trapped by establishing linkage with the fibre through H-bonds and van der waals forces which make it less likely for the dye to wash out providing superior fastness properties. In the process the carbonyl groups are the chromophores which are changed to >C=OH groups on reduction and then to soluble sodium derivatives (>C=ONa) in presence of alkali like sodium hydrosulphite. From there, oxidation can be carried out through open air, hydrogen peroxide, sodium perborate or hypochloride to restore original dye structure, trapping the dye molecules inside the fibre to develop the required shade.

Alos Read: Fabric Dyeing Process - An introduction for beginners

2. Are they the same as indigo dyes?

Indigo dyes come under Indigoid which is a vat dye and thus is the most utilised type of vat dye. It is an organic compound with a distinctive blue colour which becomes the base for various blue shades found in denim fabrics.

Structure of indigo dye

They are typically applied to yarn in rope form through consecutive dip to build up the shade. After each dip it becomes visible to see the indigo colour developing. When the yarn is out of the first dip box it starts off as a yellow colour which is in reduced or soluble form. Then it is raised in the air by rollers called ‘skying’, which results in oxidation into a light shade of blue. With each dip the shade darkens until the desired depth is achieved. Indigo dye has an excellent light fastness property but as the colour is only applied on the surface of the yarn it can also be washed down to different levels producing a range of beautiful blues so popular in denim.

In what type of garments Vat dyes are normally used?

Vat dyes are widely used for colouring cellulosic fibre, especially cotton fibre along with viscose rayon, leather and other fibres.

Indigo dye is the most used vat dye as it becomes the base for dyeing denim fabrics. Apart from that it can also be used in blend like polyester/cotton blend e.g. t-shirt in the second bath for dyeing the cotton fibre. But because of the high alkali concentration required for the dye bath, pure vat cannot be used on animal fibres (wool, natural silk, and various hairs).

Ref: Fundamentals and Practices in Coloration of Textiles, J.N. Chakraborty.