White cotton clothes generally become yellow on exposure to direct sunlight due to ____________
Answers
Answer:
It depends on the substance, but most paints and dyes get their white color from titanium dioxide, which is known to yellow from exposure to ultraviolet light. Unfortunately, I do not know the mechanism of this action.
Nitrogen oxides in the atmosphere, and UV light are known to degrade the optical brighteners in fabrics. In fabrics, such as cotton, white is achieved by various bleaching processes, as well as the addition of optical brighteners, which fluoresce upon exposure to UV light. If the bleaching process isn't done correctly, that can lead to premature yellowing.
You also find optical brighteners in paper, which also uses bleaching as it's method of obtaining a white color. Residual acids from the paper-making process will eventually cause yellowing. Paper quality varies from "quick and dirty" newsprint, all the way to acid-free "archival" papers.
Again, I do not know the mechanism of these yellowing processes, but I'm sure there's a chemist lurking around here who could explain them.