Direct dyes are water-soluble anion dyes. Most dye molecules contain sulfonic acid groups, and some have carboxyl groups. Van der Waals forces and hydrogen bonds bind the dye molecules and cellulose molecules.
Direct dyes are mainly used for dyeing cellulose fibers, but also for dyeing silk, paper and leather. During dyeing, the dye is directly attached to the dyeing fiber in the dye solution and adsorbed on the fiber through van der Waals force and hydrogen bond.
Acid dyes are a kind of water-soluble anionic dyes. The dye molecule contains acid groups such as sulfonic acid group and carboxyl group, which usually exist in the form of sodium salt. In the acid dyeing bath, it can bond with the amino group in the protein fiber molecule in an ionic bond, so it is called acid dye.
It is often used for dyeing silk, wool and polyamide fibers as well as leather. The acid dye dyes the fiber by its own affinity and combines with the fiber by an ionic bond. Acid mordant dyes have similar dyeing conditions to acid dyes, but they need some metal salts to form chelates on the fibers to obtain good washing resistance. Acid containing medium dyes, some of the acid dyes have chelated metal ions in the molecules, small tendency to hydrolysis, good dyeing fastness.