Factors to control the alignment of surface treated titanium dioxide powders to maximize performance of sunscreens

OBJECTIVE: Titanium dioxide powders are contained in a large class of color cosmetics and sunscreen formulas. When they are used, a formation of uniform functional powder layer on skin is an important factor to show their functionality, such as aesthetic and UV protection. Attempts were made to extract the factors that affect on the UV shielding ability of deposited powder layer. METHODS: Seven kinds of surface treatments were conducted on nano-sized titanium dioxide powder to modify the surface characteristics. Dispersion samples were prepared by mixing these powders with liquids, such as mixed solutions of cyclopentasiloxane, isododecane, coconuts alkane and dimethicone using a disperser and a bead mill. The dispersions were applied using an applicator on cellulose triacetate film, polycarbonate film and polymethyl pentene film. Laser microscope observation and micro-gloss glossmeter analysis were carried out to assess the flatness of the deposited powder layers, and the UV shielding ability was evaluated using SPF analyzer. RESULTS: Factors whose influences on the structure and UV shielding ability of the deposited powder layer being analyzed were pseudo-HLB of the powders, liquids for preparing the dispersions, and material of the substrates. Higher UV shielding ability was attained when powders having pseudo-HLB at around 6 was employed independent from the kinds of liquids and substrates. Flatness of the deposited layer was found to enhance UV shielding ability of UV-B region, while that of UV-A region was scarcely influenced by the flatness. Employing lower surface tension liquids for preparing the dispersions and materials exhibiting lower polar component of surface free energy as substrates tended to enhance the UV shielding ability. CONCLUSION: Surface treatments conducted on the powders in this study were found to change UV shielding ability, especially UV-B shielding ability, and the relation between pseudo-HLB and UV shielding ability were scarcely influenced by the kinds of liquids. Both surface tension of liquids and polar component of surface free energy of substrates affected the UV shielding ability. It was suggested that pseudo-HLB calculated based on chemical structure becomes useful information to choose optimum surface treatment to make uniform powder alignment independent from the surrounding environment.