Preparation of novel cosmetic white pigment by pyrophosphate treatment of zinc oxide

Zinc oxide, which has photocatalytic activity, is used as a white pigment in cosmetics. With cosmetics containing zinc oxide, some sebum on the skin is broken down by the UV rays of the sun. In this study, in order to suppress the photocatalytic activity of zinc oxide, zinc oxide was added to an aqueous solution of sodium pyrophosphate and shaken, so that novel white pigment coated with zinc phosphate having no photocatalytic activity was prepared for use in cosmetics. The chemical composition, powder properties, photocatalytic activity, hue (visible light reflectance and L⁎ a⁎ b⁎ values), and smoothness of the resulting powder material were studied. Part of the zinc oxide reacted to zinc phosphate by this pyrophosphate treatment. The photocatalytic activity of zinc oxide was suppressed by this zinc phosphate coating. The resulting sample had sufficiently high reflectance in the visible light region and a high L⁎ value in the L⁎ a⁎ b⁎ color space. The treated samples had lower static friction resistance than the original zinc oxide. Judging from the results of photocatalytic activity, L⁎ value, and smoothness, the optimum conditions were the molar ratio P/Zn of 1/4 for 6 h of shaking the mixture. (author)