Microscopic evaluation of polymeric film properties of anhydrous sunscreen compositions and their relation to absorption and water resistance.

The aim of this study was to investigate the mechanism by which a VA/butyl maleate/isobornyl acrylate copolymer increases the SPF and water resistance of sunscreen formulations. Anhydrous sunscreen formulations with and without polymer were applied on polymethyl methacrylate (PMMA) plates and absorbance spectra were generated. Before immersion, the areas under the curve for the control and test samples were 98.49 and 117.09, respectively, and were 94.63 and 118.22, after immersion. Static and after-immersion, in vivo SPF values confirmed a boost in SPF and an increase in water resistance for the formulation containing the polymer (VA/butyl maleate/isobornyl acrylate copolymer). Digital imaging of sunscreen films combined with image analysis and contact angle measurements suggest that the polymer conformation changes upon exposure to water. The polymer forms a protective barrier over the sunscreen film upon exposure to water, which explains the enhancement in water resistance. The polymeric film formed has a different refractive index than the sunscreen film. The change in refractive indices causes diffraction of incident light, thus increasing its pathlength, leading to an increase in SPF.