Kinetics of staining and bleaching

Kinetics of staining and stain removal have been studied with food dyes and natural colorants. Kinetics of staining indicate that the staining agent is adsorbed on the fiber surface and diffuses into the interior of the fibers. Similarities exist between staining and dyeing mechanisms of cotton, polyester and nylon fibers. Stain removal by nonoxidative detergency involves diffusion of the staining agent from the interior of fibers and desorption from the fiber surface. The kinetics of stain removal by bleaching with peroxyacids or activated soidum perborate depend on the affinity of the stain to fibers. When desorption of stain is rapid, bleaching of the stain in solution is the rate-determining step and stain removal obeys pseudo-first-order kinetics. When the stain is washfast, the concentration of the stain in fibers decreases with the square root of time, indicating a diffusion process. The peroxygen bleaching agent is sorbed in the subsurface of fibers or diffuses into the interior of fibers. If bleaching occurs in the outer perimeter of fibers, the rate-determining step may be the diffusion of stain molecules from the interior of fibers to the location of the bleaching agent. On the other hand, if bleaching occurs within fibers, the rate-determining step may be diffusion of the bleaching agent to the stain located within the fibers.