Oil Thickening with Organoclay Enhances the Ultraviolet Absorption Ability of Sunscreen on a Skin-mimicking Substrate

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Abstract
The performance of sunscreen products depends on their ultraviolet (UV) absorption ability through the film formed on the skin surface upon their application. Therefore, it is important that a uniform film is formed on the uneven skin surface for effective sunscreen performance. Because most UV filters are oil soluble, we hypothesized in this study that increasing the viscosity of the oil phase of a sunscreen product can improve the performance of the sunscreen. We first examined the association between the concentration of the oil thickener and the UV absorption ability of the sunscreen product using a skin-mimicking substrate (SMS). Among all thickeners examined (petrolatum, dextrin palmitate, silica silylate, and organoclay), organoclay and silica silylate significantly increased the UV absorbance of sunscreen on the SMS in a concentration-dependent manner. Thereafter, we examined film uniformity to elucidate the mechanism underlying the observed increase in UV absorption. The uniformity of film thickness on the SMS increased with increasing organoclay content, based on decreased standard deviations of film thickness. Our results showed that increasing the viscosity of the oil phase with organoclay resulted in the formation of a uniform film by preventing the sunscreen from flowing into the grooves when applied on the SMS, thereby increasing UV absorbance by more than two-fold that of sunscreen without organoclay. Thus, the use of thickeners, such as organoclay, increases the viscosity of the oil phase at a low shear rate after the high shear of application. This is an effective strategy for improving the overall quality and performance of sunscreen products.

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