Distinct Improvement in UV Light Protection Properties by TiO2 Dispersion Using a Thin Film Spinning Technique

Fine particles of titanium oxide (TiO2) or zinc oxide (ZnO) with a diameter in the nanometer region are often used for cosmetic materials because they potentially have excellent optical properties such as transparency or ultraviolet (UV) light protection. This study proposes a method to improve UV protection properties of those particles through the optimization of manufacturing processes. The method is based on the application of a thin film spinning high speed mixer for the dispersion of TiO2 fine particles. We investigated the effect of turbulent liquid flow produced by the mixer on the particle size distribution, storage stability and UV light protection properties. Using this technique, flocculation of fine particles was effectively disintegrated with increasing rotor speed to produce monodispersed uniform dispersion which showed sharp particle size distribution dependent upon the rotor speed of the mixer. In the case of a peripheral speed higher than 30 m/s, no precipitation was observed when the samples were left standing at room temperature more than a month. When we evaluated the UV light protection effects of TiO2 fine particles, it was found that UV transmittance measured by spectrophotometer, and sun protection factor (SPF) obtained by an SPF analyzer remarkably improved when the peripheral speed of the mixer was raised. Interestingly, there was a critical rotor speed at 30 m/s which showed significant increase in SPF. This threshold was typical of thin film spinning high speed mixers but not observed with a conventional homogenizing mixer. This remarkable improvement may be the result of the sharp particle size distribution produced by the turbulent liquid flow at a higher range of rotor speed. We report on the importance of manufacturing processes that contribute to the quality of functional products.