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J. Cosmet. Sci., 63, 213–221 (May/June 2012) 213 Microscopic evaluation of polymeric fi lm properties of anhydrous sunscreen compositions and their relation to absorption and water resistance DONALD PRETTYPAUL and HANI FARES, Ashland Specialty Ingredients, 1361 Alps Road, Wayne, NJ 07470. Accepted for publication November 15, 2011. Synopsis The aim of this study was to investigate the mechanism by which a VA/butyl maleate/isobornyl acrylate co- polymer 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 confi rmed 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 fi lms 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 fi lm upon exposure to water, which explains the enhancement in water resistance. The poly- meric fi lm formed has a different refractive index than the sunscreen fi lm. The change in refractive indices causes diffraction of incident light, thus increasing its pathlength, leading to an increase in SPF. INTRODUCTION Polymers have been used in sunscreen products for many years. The primary purpose of adding polymers to sunscreens is to impart water resistance to the formulation. The use of polymers evolved through the years and their use is not limited to increasing water resistance, but has been extended to boosting SPF. Over the years, scientists have studied several facets of sunscreen formulations and pub- lished papers depicting mechanistic information. The solvent effect on spectrum shift was studied by Agrapidis-Paloympis et al. (1) and explained thoroughly. The photostabil- ity of sunscreens has received a lot of attention from scientists (2–4). Broad spectrum coverage and UVA damage to the skin has also been covered (5), as well as in vitro meth- odologies for SPF measurements (6). The mechanism by which polymers boost SPF and increase water resistance has not received much attention. Sayre (7) and Jones (8) de- scribed the effect of fi lm thickness and the effect of polymeric beads on increasing the pathlength of radiation, respectively, as a means of boosting SPF.