JOURNAL OF COSMETIC SCIENCE 96 high ratios of silicone acrylate (hard) to SPE (soft), shown here 10:0 (all silicone acrylate), color fi lms are brittle and can be chipped off through abrasions. Contact transfer test was conducted to understand the extent of pigment transfer from color cosmetic fi lms to white substrates upon contact (3). Shown in Figure 6 are four water-in-oil liquid foundation prototypes with the same amount of “Hard” silicone acry- late and the same “Hard” to “Soft” ratio (3:1), varying only silicone emulsifi ers (all fl uids). Emulsifi ers A and B both showed incompatibility with silicone acrylate and decent mis- cibility with artifi cial sebum oil (data not shown). Emulsifi ers C and D both showed good compatibility with the silicone acrylate however, Emulsifi er D showed the highest de- gree of immiscibility with artifi cial sebum oil. For the test, liquid foundations were coated and dried on nylon fi lter papers. In some cases, cosmetic fi lms dried on fi lter papers were impregnated with artifi cial sebum prior to the test. Figure 6B shows a digital cam- era image taken after contact transfer tests with both cosmetic fi lms and pigment traces transferred. As shown in the top row, without sebum, there is no observable pigment transfer for all four foundation fi lms. However, shown in the bottom row of Figure 6B, with a sebum pretreatment, four formulations showed different degrees of pigment trans- fer, which were documented by colorimeter measurement (see Figure 6C). Emulsifi er D, having both good compatibility with the silicone acrylate and the highest degree of im- miscibility with artifi cial sebum oil, led to formulation with best sebum resistance. DISCUSSION While there are different technical approaches toward long-lasting performance, today’s prevailing technologies in color cosmetics are based on using various polymeric “fi lm Figure 6. Contact transfer test. (A) Compositions of water-in-silicone liquid foundations, varying only silicone emulsifi ers. (B) A digital camera image taken after contact transfer tests, having both cosmetic fi lms (on fi lter paper) and pigment traces transferred to white substrates. (C) White substrates’ color increase after contact transfer test (with sebum impregnation, the bottom row in B).

LONG-WEAR SEBUM-RESISTANT COSMETICS 97 formers.” Virtually, every color cosmetic currently on the market with “long lasting” claims contains at least one organic or silicone “fi lm former,” such as PVP-type copoly- mers, acrylate-type copolymers, polyethylene, silicone MQ resins, silicone resin waxes, and silicone acrylates (1–2). Despite all the developments, to date, no general formulation guidelines have been clearly articulated to achieve the benefi ts of long wear. Regarding silicone-based technologies, over the past decades, researchers from Revlon, Procter & Gamble, L’Oreal, Estee Lauder, Shiseido, and others adopted a variety of silicone materi- als for long-wear color cosmetics, including silicone gums, silicone waxes, silicone resins, silicone polyamide, and silicone pressure-sensitive adhesives (5–9). However, other than a vague concept of “MQ + plasticizer,” fundamental understandings and formulation knowledge remain in the minds of very few skilled cosmetic chemists (1–3). Our “Soft” + “Hard” formulation strategy focuses on studying interactions among non- volatile components, especially polymeric nonvolatiles, in a given formulation chassis. Our defi nition of “Hard” components includes a number of materials such as MQ resins, acrylates, resin waxes, silica silylates, and different tackifi ers. “Soft” components include the other ingredients typically found in formulations, e.g., liquid emulsifi ers, rheology modifi ers, and shine-enhancing fl uids. Understanding the intrinsic material properties of “Soft” + “Hard” nonvolatile blends may enable us to better interpret/predict relevant cosmetic fi lms’ long-wear properties. For instance, for prototypes in Figures 5A and 6A, upon application on skin and evapora- tion of volatile fl uids, we hypothesized that the combination of SPE and silicone acrylate are the main components of a polymer matrix that is responsible for adhering pigments onto the skin. The particular silicone acrylate studied is a hard and relatively brittle ma- terial in its neat form. Our results suggest that the silicone acrylate is able to form com- patible blends with several SPEs, with good miscibility at polymer chain segment level. Rheology and DSC studies show that a “soft” SPE (compatible with silicone acrylate) is able to lower the blends’ glass transition temperature, making it an effective plasticizer of silicone acrylate. Conversely, the silicone acrylate is able to raise the glass transition temperature, making silicone acrylate an effective “tackifi er” for compatible SPE. In a sense, consistent to our fi lm properties and fl exibility studies, the SPE may effectively mobilize the “hard” acrylate segments, increasing fl exibility to the fi lm. Likewise, the “hard” acrylate segment can effectively “toughen” the “soft” SPE, introducing cohesion strength to the fi lm. At right ratios, these combined polymeric nonvolatiles may form an optimized matrix to “glue” pigment particles on the skin. Insights around the intrinsic properties of a given pigment “glue” blend can be gained through its viscoelastic profi le. Consistently with the understanding that sebum negatively impacts long-wear perfor- mance of color cosmetics, our studies confi rmed that in vitro testings with artifi cial sebum oil pretreatment lead to more color transfer than without sebum pretreatment. Perspira- tion and sebum may alter a blend of nonvolatiles’ pigment “gluing” effi cacy. If sebum is miscible with nonvolatile components, it effectively reduces the “Hard” to “Soft” ratio, further plasticizes the blend of nonvolatiles, and results in a “softer” and more “fl uidic” matrix on skin. This leads to weakened pigment binding to skin, easier removal upon rub-off, and more color transfer upon contact. On the other hand, if each component of a nonvolatile blend can be carefully selected with reduced sebum miscibility, the nonvola- tile matrix formed for pigment binding may be less prone to sebum’s plasticization, thus may reduce the detrimental effects of sebum. Based on all our learnings, we developed a systematic formulation strategy for longer wear and sebum resistance benefi ts.