Acid dyes are highly soluble in water and have good light fastness compared to simple basic dyes. The acid dyes are most effective for the protein fibers like silk, wool, modified acrylics, and Nylon. They have sulphonic acid groups that are normally present as the sodium sulphonate salts. It increases the solubility in water and gives the dye molecules a negative charge. So as a group, acid dyes are divided into two subgroups – acid leveling or acid milling.

Division of acid dye as per their dyeing characteristics!

Acid dyes are mostly classified as per their dyeing behavior, particularly in relation to the dyeing pH, their ability to migrate during the process of dying, and their washing fastness. The molecular weight and the sulfonation degree of the dye molecule identify these characteristics of dyeing. The classification is based on the behavior of dyeing like below –

• Leveling dye
• Fast acid dye
• Milling acid dyes
• Super milling acid dyes

Milling is the procedure in which the woolen material is treated in a weak alkaline solution with the considerable action of a mechanic for promoting the felting. Dyes of good fastness to miling is important to prevent color bleeding during the procedure.

What are the properties of acid dyes?

Since these are sold as the sodium salt, they form such a large anion in the aqueous medium.

Acid dye properties are discussed below –

• These dyes are anionic in nature.
• These dyes are perfect for silk, wool, modified acrylics, and polyamide.
• These are applicable from the strong acidic to neutral pH bath.
• These dyes have no affinity for cotton cellulose, hence it is not suitable for cellulosic.
• These dyes combine with fiber by the hydrogen bonds, Waals forces, or via ionic linkages.

Which color are acid dyes available?

There are various colors available of acid dyes –

• Acid yellow dye
• Acid orange dyes
• Acid violet dyes
• Acid red dyes
• Acid blue dyes
• Acid green dyes
• Acid brown dyes
• Acid black dyes

Applications of acid dyes – where it is used?

It is useful in various sectors –

• It is useful in textiles, fertilizers and leather.
• Acid dyes are also used in detergents and metal anodization.
• It acts like the coolants for engines.
• It is used in wool for resulting in the ionic or salt link between the dye molecule and fiber polymer.
• It is also useful in silk with various colors. Silk exerts affinity for acid dyes at low temperature and dyeing is commenced at 40 degree celsius.