J. Cosmet. Sci., 69, 203–212 (May/June 2018) 203 Investigating Film Properties of Polymers Used in Anhydrous Sunscreen Formulations Using Scanning Electron Microscopy (SEM): Polymer/Polymer Interactions and Their Relation to Vapor Transmission WILLIAM THOMPSON, DIANE KENNEDY, TIMOTHY GILLECE, ROGER L. MCMULLEN, LARRY SENAK, and HANI FARES, Ashland Specialty Ingredients G.P. Bridgewater, NJ (W.T., D.K., T.G., R.L.M., L.S., H.F.) Accepted for publication May 2, 2018. Synopsis A mechanistic understanding of the role of polymers in waterproofi ng anhydrous sunscreen formulations has been hypothesized in the past, but has never been clearly established. In this article, we demonstrate the utility of fi eld emission scanning electron microscopy (SEM) to generate images of sunscreen fi lms 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 magnifi cations. Vapor transmission data were collected using an evaporimeter to understand the permeability of these fi lms in the presence and absence of fi lm formers. Examination of the SEM images reveals that after spraying the product onto a layer of corneocytes, the sunscreen fi lters formed a hydrophobic barrier over the skin, whereas added polymers formed fi lms over the sunscreen layer. The shape of the fi lm formed by various polymers and its porosity were infl uenced by chemistry and concentration. When more than one polymer was incorporated in the sunscreen formulation, the interactions between the polymers infl uenced the formation of the fi lm. 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 fi lm 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 fi lm 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 fi ngerprint 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. INTRODUCTION Over the past few years, aerosol sunscreen formulations have been gaining popularity among consumers, as compared to traditional creams and lotions. This is mainly Address all correspondence to Hani Fares at hfares@ashland.com.
JOURNAL OF COSMETIC SCIENCE 204 because of convenience of application, especially on children. The use of polymers in such formulations has become a standard practice as polymers impart water resistance and contribute to boosting the sun protection factor (SPF) of such preparations. In many instances, polymers affect sensorial properties of the formulations as well. During the past several years, most of the research conducted on the use of polymers in sunscreen formulations has been centered around developing new methodologies for testing in vitro SPF and water resistance (1–3). Recently, the effect of polymers on sensorial attributes was also investigated (4). The mechanism by which poly- mers affect waterproofi ng of sunscreens was described by Prettypaul and Fares several years ago (5). Although the authors described mechanistic information on the forma- tion of a polymeric fi lm, additional information on the fi lm properties needed to be investigated. In this study, we developed a direct method by which one can visualize polymer fi lms after they are sprayed onto stratum corneum sheets. The method allows us to study the interaction of various polymer combinations on fi lm surfaces and their ability to form a continuous fi lm on the surface. We used vapor transmission data for such fi lms to under- stand the breathability of polymer mixtures on the skin and how it is related to in vitro water resistance. MATERIALS AND METHODS In this study, we introduce a methodological approach for investigating sunscreen fi lm formation on the skin. Specifi cally, we probed the interactions of the primary fi lm former, VA/butyl maleate/isobornyl acrylate copolymer, with two commonly used fi lm formers in spray formulations, namely, acrylates/dimethicone and hydroxypropyl cellulose. All formulations used in this study are displayed in Table I. The chassis developed was quite simple and contains typical ultraviolet A (UVA) and ultraviolet B (UVB) sun- screens, functional polymers, and alcohol (the diluent). The formulations were then aero- solized as described in the following paragraph. MATERIALS Butyl methoxydibenzoylmethane (Escalol™ 517), benzophenone-3 (Escalol™ 567), ho- mosalate (Escalol™ HMS), ethylhexyl salicylate (Escalol™ 587), octocrylene (Escalol™ 597), isostearyl neopentanoate (Ceraphyl™ 375), VA/butyl maleate/isobornyl acrylate copolymer (Advantage™ Plus), and hydroxypropyl cellulose (Klucel™ G CS) were sup- plied by Ashland Specialty Ingredients G.P. (Covington, KY). Acrylates/dimethicone copolymer (KP 545L) was obtained from Shin-Etsu Chemicals (Tokyo, Japan). Alcohol, SD 40-B (200 proof), was provided by Pride Chemical Solutions (Holtsville, NY) and butylene glycol was purchased from Thermo Fisher Scientifi c (Waltham, MA). In addi- tion, tests were conducted with a commercial SPF 30 aerosol sunscreen formulation con- taining the following ingredients: avobenzone, octocrylene, oxybenzone, SD alcohol 40-B, isobutane, dimethicone, tocopherol, ascorbyl palmitate, VA/butyl maleate, isobornyl acrylate copolymer, and fragrance.
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