Emperor Acid dyes are highly water soluble, and have better light fastness than basic dyes.The textile acid dyes are effective for protein fibers such as silk, wool, nylon and modified acrylics. They contain sulphonic acid groups, which are usually present as sodium sulphonate salts. These increase solubility in water, and give the dye molecules a negative charge. In an acidic solution, the -NH2 functionalities of the fibres are protonated to give a positive charge: -NH3+. This charge interacts with the negative dye charge, allowing the formation of ionic interactions. As well as this, Van-der-Waals bonds, dipolar bonds and hydrogen bonds are formed between dye and fibre. As a group, acid dyes can be divided into two sub-groups: acid-leveling or acid-milling.
Equalizing/leveling acid dyes: They have the highest level dyeing properties and are quite combinable in trichromatic shades. Their relatively small molecules allow high migration before fixation. However, they have low wet fastness and are normally not suited for apparel fabric.
Milling acid dyes: They have medium to high wet fastness, and some (in pale shades) have poor light fastness. These dyes are generally not combinable.
Metal complex acid dyes: More recent chemistry combined transition metals with dye precursors to produce metal complex acid dyes with the highest light fastness and wet fastness. These dyes are also very economical. They, however, produce duller shades.
