Investigating Film Properties of Polymers Used in Anhydrous Sunscreen Formulations Using Scanning Electron Microscopy (SEM): Polymer/Polymer Interactions and Their Relation to Vapor Transmission.

A mechanistic understanding of the role of polymers in waterproofing anhydrous sunscreen formulations has been hypothesized in the past, but has never been clearly established. In this article, we demonstrate the utility of field emission scanning electron microscopy (SEM) to generate images of sunscreen films in the presence and absence of several polymers. VA/butyl maleate/isobornyl acrylate copolymer was studied alone and in combination with hydroxypropyl cellulose and acrylates/dimethicone copolymer. Anhydrous sunscreen formulations were sprayed onto stratum corneum substrates and left to dry. SEM micrographs of treated stratum corneum sections were then collected at various magnifications. Vapor transmission data were collected using an evaporimeter to understand the permeability of these films in the presence and absence of film formers. Examination of the SEM images reveals that after spraying the product onto a layer of corneocytes, the sunscreen filters formed a hydrophobic barrier over the skin, whereas added polymers formed films over the sunscreen layer. The shape of the film formed by various polymers and its porosity were influenced by chemistry and concentration. When more than one polymer was incorporated in the sunscreen formulation, the interactions between the polymers influenced the formation of the film. Cumulative evaporimeter data indicated that the sunscreen phase had the highest reduction in cumulative evaporation rate (39.3%/h) followed by the addition of a film former to the spray, which reached an additional reduction of 17.9%/h in the best case. This method was also used to examine the film properties of a commercial sun protection factor 30 sunscreen product containing VA/butyl maleate/isobornyl acrylate copolymer. SEM micrographs of the commercial product applied to skin showed the same fingerprint as prototype formulations containing VA/butyl maleate/isobornyl acrylate copolymer. Overall, this method can be used by sun care scientists in the development and optimization of anhydrous sunscreen sprays.