Acid dye refers to the dyestuff containing a sulfonic acid group, a carboxyl, a hydroxy or other soluble group in the molecular structure and used in acid or neutral dye bath. According to the chemical structure it can be divided into azo type, triphenylmethane type, anthraquinone type, anthracene oxygen type and nitroso type pyrazolone type. Acid dyestuff is widely used, mainly as direct dyestuff for protein fibers (such as wool, silk) and polyamide fibers, protein. And the dyeing needs to be conducted in an acidic solution. It’s mainly used for nylon, wool, leather, silk and others. Direct dyes or basic dyes are applied in paper dyeing far more often than acid dyestuff.
According to the molecular structure and dyeing conditions of the acid dyestuff, it can be divided into strong acid dyes, weak acid dyes, acid mordant dyes, acid-premetallized dyes and neutral dyes. Acid dyestuff is mainly used for wool, silk, leather and nylon and other fibers with its vivid luster and wild chromatogram.
Strong acid dyes whose molecules are relatively small, have bright color and good levelness but a stronger affinity for the wool fibers, and need to be used in strong acidic dye bath (pH≤4). The main chemical structures of Strong acid dyes are azo, anthraquinone, triarylmethane and oxygen anthracene type. For example, acid Bright Yellow 2G (C. I. Acid Yellow 17), acid anthraquinone blue (C. I. Acid Blue 25), acid Lake Blue A (C. I. Acid Blue 7) and acid Rose Red B (C. I. Acid Red 52), etc. are all strong acid dyes.
Weak acid dyes have more complex molecules than strong acid dyes, and relatively less water-soluble groups. They have weaker affinity for wool fiber, and can be used in weakly acidic or nearly neutral dye bath. In addition to the ionic bonds between the anions of dyes and the cations of wool molecules, the two non-polar molecules combine by means of van der Waals attraction. So weak acid dyes have a poor levelness but an excellent wash fastness, therefore they are also known as milling dyes. The main chemical structures of weak acid dyes are the types of azo and anthraquinone. For example, weak acid black BR (C. I. Acid Black 24), weakly acidic Brilliant Blue RAW (C. I. Acid Blue 80) are weak acid dyes.
After treated by metal mordant (Chromium salts), acid mordant dyes generate the dye-metal complex on the fiber, which can improve their resistance to sunlight exposure and wetness. But such treatment will leave the dyed products in a darker hue, make their shade and color-matching less manageable and complicate the dyeing procedures.
Acidic metal complex dyes are a class acid dyes that can generate inner complex with some transition metals (such as copper, network, cobalt, etc.). In contrast with acid mordant dyes, acidic metal complex dyes generate the dye-metal complex with complexing agent first, and then dye the fiber, which is simpler than the dyeing procedure of acid mordant dyes.