Improving the photostability of bleached silk without reducing its whiteness

Bleached silk is rapidly yellowed by exposure to the ultraviolet radiation present in sunlight. The conventional application of a water-soluble hydroxyphenyl benzotriazole ultraviolet absorber (such as UVFast W) to bleached silk reduces its rate of photoyellowing but has a negative impact on the whiteness of the bleached silk, largely cancelling out the improvements in whiteness achieved during bleaching. Therefore, a series of bleaching processes and aftertreatments using UVFast W on silk were investigated, with the measurement of reflection spectrum, whiteness, yellowness, and strength before and after exposure to ultraviolet radiation. The results show that whiteness of silk treated by oxidation–reduction double bleaching can be increased, and the whiteness of double-bleached silk with 4% UVFast W aftertreatment was even slightly higher than that of the hydrogen-peroxide-bleached silk. After ultraviolet irradiation, double-bleached silk samples with 4% UVFast W treatment resulted in both the highest whiteness and optimum photostability by comparison with unbleached standard silk and hydrogen-peroxide-bleached silk samples with and without 4% UVFast W treatment, and its strength difference was superior to the samples, except for standard silk with 4% UVFast W treatment after exposure to UV radiation for 18 h.