Influence of pH, concentration of sodium lactate as an additive and ultrasonic treatment on synthesis of zinc phosphate white pigments

Abstract

Zinc oxide that has the photocatalytic activity is used as a white pigment for cosmetics. A certain degree of sebum on the skin is decomposed by the ultraviolet radiation in sunlight. In this work, zinc phosphates were prepared from zinc nitrate and phosphoric acid at pH 5 and 7 with and without the addition of sodium lactate and ultrasonic treatment as a novel white pigment for use in cosmetics. The chemical composition, powder properties, photocatalytic activity, colour phase, moisture retention and smoothness of the zinc phosphates were studied. The obtained materials had a Zn/P ratio of about 1.5, which corresponds to zinc orthophosphate Zn3 (PO4 )2 . Samples prepared with ultrasonic treatment indicated the high ratios of large particles in scanning electron microscopy images and particle-size distributions. The photocatalytic activity of these zinc phosphate particles was too less to protect the sebum on the skin. The materials obtained and their thermal products at 100°C showed a high reflectance within the range of visible light. The slipping resistance and roughness of the powder were enough low for use in cosmetics.

Effect of mechanical treatment on the powder properties of zinc phosphate white pigments

“…Sodium lactate is effective for improving the moisture retention. In the previous work [15], the addition of sodium lactate also improves the moisture retention of zinc phosphate. This improvement is caused by the inherent water retention characteristic of sodium lactate and the easy incorporation of sodium lactate into zinc phosphate materials.…”

Influence of pH-control in phosphoric acid treatment of zinc oxide

“…Samples prepared at pH 1.65 had different particle sizes from the others. From the ICP results, this sample had a larger amount of zinc phosphate than the other samples (Table I), which affected the particle size distribution because zinc phosphate consisted of particles larger than 1 mm [12,13]. Other samples had particle sizes smaller than 1 mm.…”

“…Samples treated with phosphoric acid had larger particles than pristine zinc oxide. These samples had a possibility to form the aggregate in a drying process, because zinc phosphate and oxide are easy to form [12,13]. For cosmetic applications, inorganic pigments in solution are in common use without drying.…”

“…Samples prepared at pH 4 and 5 indicated weaker photocatalytic activity than those prepared at pH 6 and 7. The photocatalytic activity of zinc oxide was suppressed by the formation of zinc phosphate, which has no photocatalytic activity [12,13]. From the XRD results, samples prepared at pH 1.65 easily reacted with phosphoric acid; however, the large amount of zinc phosphate resulted in the cracking of the zinc phosphate shell, causing the photocatalytic activity of samples prepared at pH 1.65 to be stronger than those prepared at pH 4 and 5.…”

“…In earlier studies [12,13], we prepared zinc phosphate pigments with no catalytic activity. However, these zinc phosphates had particle sizes larger than 1 mm.…”