Mainly Acid dye is being used to dye textile at low pH and to dye specifically WOOL, SILK, and NYLON and not Cotton Fabrics. Few Acid Dyes are used as food colorants and one of the uses in the medical field for organelles as well.
These dyes belong to an acidic group of families such as SO3H & COOH and when the ionic bond is established between protonated – NH2 group of fiber and acid group of dye can be applied on Wool, Silk, and Nylon.
Classes of Acid Dyes:
Acidic Dyes are mostly divided into various classes according to their fastness requirements & Dyeing Behavior which actually covers their wet fastness, migration ability, and dying pH.
- Leveling acid dyes:
- – Relatively low molecular weights
- – Migrate more readily before fixation
- – Exhibit low wet fastness
- – Not normally suited for use as apparel fabric
- – They require an acidic dye bath, often using sulfuric acid and sodium sulfate mixtures (pH2-4), together with leveling agents such as ethoxylated fatty amines.
- Milling dyes:
- – High molecule weight
- – Migrates slowly
- – Exhibit wet fastness
- – Useful for dyeing wool materials
- – Milling acid dyes are sometimes called ‘Neutral acid dyes’ as they do not require an acidic dye bath. They are commonly applied using Acetic acid (pH4-7).
- Metal complex acid dyes:
- – High molecular weights
- – Low mobility
- – High wet fastness
- – Commonly used on nylon and synthetic polyamide fibers.
- – Metal complex acid dyes are economical. However, they produce relatively dull shades.