Preparation of novel cosmetic white pigments by mechanochemical treatment of zinc oxide with phosphoric acid

Abstract

ABSTRACT Zinc oxide has a photocatalytic activity, but in cosmetics it is used as a white pigment. When used in cosmetics, this photocatalytic activity causes some of the skin’s sebum to be broken down by the ultraviolet rays contained in sunlight. In this study, we attempted to mill zinc oxide with phosphoric acid to react the powder surface to zinc phosphate. Since zinc phosphate has no photocatalytic activity, the particles whose surface is replaced by zinc phosphate are expected to have little photocatalytic activity. The chemical composition, powder properties, photocatalytic activity, hue (visible light reflectance, L*a*b* value), and smoothness of the obtained powder materials were examined. A portion of the zinc oxide was reacted to zinc phosphate by milling with phosphoric acid. In some samples, the reaction to zinc phosphate was observed to be too advanced, resulting in larger particles. The photocatalytic activity of zinc oxide was successfully eliminated through milling. Although the samples in this study had sufficient whiteness, the whiteness of some of the samples decreased due to contamination by milling. The smoothness of the pigment powders produced in this study was improved by milling, especially by prolonged treatment and the use of low concentrations of phosphoric acid.