Acid dyes are mainly used for dyeing protein fibers and nylon. Almost equivalent amino and carboxyl groups H2N-W-COOH are contained in wool, silk and polyamide fibers. When the fiber enters the aqueous solution, the amino and carboxyl groups dissociate in water. Form zwitterion + H3N—W—COO-. The pH value of wool's isoelectric point is 4.2 ~ 4.8. When the pH of the solution is greater than the isoelectric point, the surface of the fiber is negatively charged; when the pH value of the solution is small and the isoelectric point of the fiber, the surface of the fiber is positively charged.
Acid dyes are ionized into D-SO3- and Na + in the dyeing solution. In the strong acid dye bath, the fiber surface is positively charged, and the dye ion is negatively charged. The fiber and dye are combined by ionic bonds; in the weak acid bath and in In the sex bath, the surface of the fiber is negatively charged, and the dye ion is also negatively charged. At this time, the fiber and the dye are combined by van der Waals force and hydrogen bonding.