Acid dyes which are anionic, water soluble and normally applied from acidic bath. Acid dyes are applied to the fibre under acid conditions. Acid dyes have very good shade range and standard fastness properties. Wet fastness of acid dyes can be improved after-treating the dyeing in some cases. Compatibility within combination is important as blocking effects and differential coverage of yarn irregularities may result if selection is not carefully done. Acid dyes fix to the fibre by hydrogen bonding, vander waals forces and ionic linkages. There are 3 kinds of acid dyes such as neutral acid dyes, weak acid dyes, strong acid dyes.
Use of Acid Dyes:
Acid dyes are normally used for dyeing natural protein like wool and silk. It's also used for dyeing synthetic polyamide like nylon and sometimes acrylic.
Dyeing Process of Wool Fibre:
Wool fibre polymer represents as:
H2N-W-COOH → Wool
That is to say wool polymer has amino group of one side and carboxylic group on the other side. In certain condition it became:
H2N - W - COOH → H3N+ - W - COO-
The attached hydrogen the carboxylic group is transferred or replaced by amino group in other. Thus when wool fibre is taken in an acid solution, a negative ionized carboxylic group of wool molecule takes H+ from HCI and produced D electrically neutralized carboxylic acid group.
HCI → H+ - CI-
NH3+ - W - COO- + HCI → CI NH3 - W - COOH
When wool is immersed in acid solution the concentration of acid reduces and the absorption of acid by wool fibre increases. This absorption continues until equilibrium absorption.