140 JOURNAL OF COSMETIC SCIENCE BEYOND RHEOLOGY MODIFICATION: HYDROPHILICALLY MODIFIED SILICONE ELASTOMERS PROVIDE NEW BENEFITS Michael S. Starch, Jean E. Fiori and Zuchen Lin Dow Corning Corporation, Midland, Michigan Introduction: The introduction of silicone elastomers to our industry has provided fonnulators with new tools for modifying the theology and esthetic properties of silicone-based formulations. These silicone elastomers are based on crosslinked dimethicone and are typically supplied in the form of swollen gels that contain various silicone oils. One such elastomer with the INCI name Dimethicone Crosspolymer (DCP) has been shown to be useful for thickening various types of formulations, particularly anhydrous formulations, where cyclomethicone is the vehicle( This silicone elastomer also provides novel sensory properties that are unlike anything attainable with silicone fluids. The addition ofhydrophilic polyethylene glycol (PEG) functional groups to DCP affects both the chemical and rheological properties of the elastomer. This new class of materials, hereinafter referred to as PEG- modified DCP (PEG-DCP), has very diflkrent properties from the unmodified elastomer. The hydrophilic PEG substituents change the wetting properties of the elastomer and dramatically improve compatibility with polar ingredients 2. We have shown that PEG-DCP can be used as an emulsifier for oil-continuous as well as multiple emulsions. Reduced Syneresis: The utility of DCP and other elastomers prepared from non-polar silicones for thickening anhydrous antiperspirants is illustrated in Figure 1. This data was generated using a simple soft solid formula composed of elastomer, antiperspirant salt, and cyclomethicone. The solid line with triangles shows that the viscosity of the formula increases rapidly as the concentration of DCP is increased. The amount of syneresis for these formulas, (dotted line) however, is relatively insensitive to the DCP concentration. When PEG-DCP is used to prepare the formula, the viscosity is much lower, but syneresis is dramatically reduced by increasing the elastomer concentration to 4% in the formula. These results may be due to an associative thickening effect, whereby the PEG chains are driven to associate in the non-polar formula 450,000 400,000 350,000 300,000 ,B • 250,000 200,000 150,000 100,000 50,000 environment. Under shear conditions, including those used to measure formula viscosity, these associations break down, leading to low viscosity readings. We have found that the viscosity of such formulas is highly dependent on the shear history of the sample and that stirring the sample before measuring viscosity dramatically reduces viscosity. 0 0 2.5 3.0 3.5 4.0 4.5 Elastomer Concentration (wt %) 45 Viscosity - with DCg' • •'-- Vlscos*ty - w Ith FE43- ECP i Syneresls w •th PEG-EX):' - -a- - Syneres,s w ith DCg' I 4O 35 30 o•' 25 .• 2o g 15• 10 rlgUlC I Compatibility with Polar Ingredients: Silicone elastomers such as DCP that have no polar functionality are generally incompatible with organic oils. All the exceptions we have found to this rule are low molecular weight hydrocarbon oils. When mixed with DCP that has been swollen in cyclomethicone, organic oils such as fatty esters appear to alter the gel structure of the elastomer, leading to a loss of thickening efficiency. At higher concentrations, fatty

2000 ANNUAL SCIENTIFIC MEETING 141 esters can precipitate DCP, presumably due to poor solvation ofelastomer. The polar functionality of the PEG-DCP elastomer increases the range of organic oils that are compatible. The data shown in Table 1 illustrates this phenomenon with tocopherol. In these experiments, DCP and PEG-DCP were pre-blended with cyclomethicone and the compatibility of these blends with tocopherol was evaluated. The tocopherol was slowly added to the elastomer blends until the mixtures became hazy, indicating incompatibility. The amount oftocopherol required to produce incompatibility was recorded as the "saturation point" for the particular elastomer blend. Two PEG-DCP elastomers with different levels of PEG substitution were tested. The data indicates that the PEG functionality is responsible for the improved compatibility with tocopherol. Table 1: Compatibility of Silicone Elastomer / Cyclomethicone Blends with Tocopherol Elastomer PEG Appearance and Saturation Type Substitution stability of mixtures Point (w/w) DCP None I hazy, separates over time 1% PEG-DCP Low clear, stable over time 20 % PEG-DCP High clear, stable over time 28 % Emulsification: PEG-substituted silicones such as dimethicone copolyols are known in the industry for their ability to form water-in-oil (W/O) emulsions, where the continuous phase is composed primarily of cyclomethicone. One application for these W/O emulsions is clear antiperspirant gels, where an aqueous phase containing the antiperspirant salts is emulsified into a continuous silicone phase. We compared the emulsification performance of a widely used dimethicone copolyol with that ofa PEG-DCP elastomer in a simple antiperspirant gel formulation. We found that both produced stable W/O emulsion gels, but the viscosity of the gel with PEG-DCP was significantly higher (13,000 cP for the PEG-DCP gel versus less than 6,000 cP for gel with Dimethicone copolyol), due to the thickening effect of the elastomer in the continuous phase. In addition to simple W/O emulsions, PEG-DCP can be used to prepare multiple emulsions, including water-in-oil-in-water (W/O/W) emulsions 3 and propylene glycol-in-oil-in-water emulsions 4. In these systems, we believe the thickening effect of the silicone elastomer in the silicone oil phase helps to stabilize the initial W/O (or PG/O) emulsions and allows them to be dispersed in water without inversion. This offers the potential to formulate products with ingredients that may not be stable in conventional emulsions. Placing them in the innermost phase of a multiple emulsion could protect such ingredients from degradation. Summary: The development of silicone elastomers with hydrophilic functionality in the form of PEG substituents provides benefits beyond the thickening that can be achieved with non-functional silicone elastomers. We have shown that PEG-DCP, in particular, is compatible with a wider range of personal care ingredients, compared to the unmodified elastomer (DCP). When blended with cyclomethicone, PEG-DCP forms a silicone phase that can solubilize organic oil and will emulsify water to form stable W/O emulsions. The PEG-DCP provides thickening as well as emulsification, and this combination of properties allows the preparation of multiple emulsions. References: I) Schulz, Jr., W.J. Zhang, S., U.S. Patent 5,654,362 (August 5, 1997) 2) Schulz, Jr., W.J. Zhang, S., U.S. Patent 5,811,487 (September 22, 1998) 3) Lin, Z., Schulz, Jr.• W.J. Zhang, S., U.S. Patent 5,948,855 (September 9, 1999) 4) Lin, Z. Schulz, Jr., W.J. Smith, J. M., U.S. Patent 6,080,394 (June 27, 2000)