Taking reactive dyes as an example, in theory, due to the covalent bond between dyes and fibers, washing with water cannot easily cause dye desorption, fading and bleeding. Therefore, the color fastness of reactive dyes to soaping depends on Quantity of unfixed dyes (hydrolyzed dyes and small amounts of unreacted dyes). If the hydrolyzed dye cannot be completely removed by soaping, the subsequent washing will continue to fade. Soaping fastness is also related to the bond-forming stability of bond dyes, and bond-breaking dyes will also be washed out with water.
Therefore, the most important factors affecting the color fastness to soaping are the dye structure and properties, followed by the dyeing and post-dyeing processes. Reactive dyes have high fixation rate, or slow hydrolysis rate, less hydrolyzed dyes, and less dyes to be removed by washing. Unfixed dyes and hydrolyzed dyes have low directness, good water solubility, not easy to stain, and easy to wash off. However, with high dye concentration and a large amount of residual dye, it is not easy to wash off.