Acid dyes are anionic, soluble in water, and are essentially applied from the acidic bath. These dyes possess acidic groups, such as SO3H and COOH.
Why It’s Called Acid Dyes?
Acid dyes are so-called because, in the first place, the dyeing process is carried out in an acidic aqueous solution containing mineral and organic acid, and secondly they were nearly all sodium salts of organic acids and the anion is the active colored component.
Properties of Acid Dyes
These dyes are anionic in nature.
These dyes are suitable for wool, silk, polyamide, and modified
These are applied from a strongly acidic to neutral pH bath.
These dyes have no affinity for cotton cellulose, hence not
suitable for cellulosic.
These dyes combine with the fiber by hydrogen bonds, Vander
Waals forces or through ionic linkages.
Chemical structure of acid dyes
Most of the acid dyes belong to the following three main structural molecules,
1. Anthraquinon type
2. Azo dye type
3. Triphenylmethane type.
Many blue dyes have this structure as their basic shape. The structure predominates in the Leveling class of acid dye.
Azo dyes type
The structure of azo dyes is based on azobenzene, PhN=N-Ph. Officially Azo dyes are a separate class of dyes primarily used on cellulose fibers such as cotton and rayon; but many acid dyes have a similar structure; most give different shades of red.
Acid dyes having structures related to triphenylmethane predominate in the milling class of dye. There are many yellow and green dyes commercially applied to fibers that are related to triphenylmethane.
Fastness Properties of Acid Dyes
The wet and light fastness properties of the acid dyes vary from poor to excellent, depending upon the molecular structure of the dyes. The fastness properties as per the category are as follows:
Neutral acid dyes: since these dyes have very good leveling and migration properties, and have a low affinity for the fiber, therefore the wet fastness properties of this class are generally poor.
Weak acid dyes or half milling dyes: These dyes have a medium to good affinity for the fiber and are generally applied in a weakly acidic bath, shows medium to good wet fastness properties.
Strong acid dyes or super milling dyes: These dyes have poor exhaustion properties, therefore applied under very strong acidic condition, exhibit good fastness properties.
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