j. Cosmet. Sci., 54, 193-205 (March/April 2003) Beyond rheology modification: Hydrophilically modified silicone elastomers provide new benefits MICHAEL S. STARCH, JEAN E. FIORI, and ZUCHEN LIN, Dow Corning Corporation, P.O. Box 994, Mid/and, MI 48686-0994. Accepted for publication July 24, 2002. Presented at the Annual Scientific Meeting of the Society of Cosmetic Chemist•3 New York, December 7-8, 2000. Synopsis The development of silicone elastomers with hydrophilic functionality in the form of PEG substituents provides benefits beyond the rheology modification (thickening) that can be achieved with silicone elas- tomers that lack functional substituents. We have shown that PEG-modified dimethicone crosspolymer (PEG-DCP) is an effective emulsifier for water-in-silicone (w/s) emulsions where the continuous phase is cyclopentasiloxane. The utility of PEG-DCP for producing simple antiperspirant gels and also multiple emulsions (w/s/w) was demonstrated. Additional benefits for PEG-DCP such as improved compatibility with polar organic oils and reduced syneresis in anhydrous antiperspirants were also shown to be associated with the addition of hydrophilic groups to the silicone elastomer. The performance and physical properties of a series of PEG-DCP samples from a two-level factoffal design were evaluated. The variables studied were the level of PEG substitution, the length of the PEG chains, and crosslink density. All of the PEG-DCP samples were synthesized and tested in the form of dispersions in cyclopentasiloxane. Variations in the composition of PEG-DCP such as the level of PEG substitution and the length of the PEG chain were shown to have a significant effect on the viscosity of the PEG-DCP and its emulsification performance in a simple w/s emulsion. Variation of a process parameter that affects the crosslink density of the elastomer also changed the emulsification properties of the PEG-DCP. INTRODUCTION The introduction of silicone elastomers to the personal care industry has provided formulators with new tools for modifying the rheology and aesthetic properties of silicone-based formulations. These silicone elastomers are based on crosslinked dimethi- cone and are typically supplied in the form of swollen gels that contain various silicone oils. Cyclopentasiloxane in particular is the silicone fluid most often used as the swelling solvent, although other silicone fluids such as dimethicone are also used. One such elastomer with the INCI name dimethicone crosspolymer (DCP) has been shown to be useful for thickening various types of formulations, particularly anhydrous antiperspi- rant formulations, where cyclopentasiloxane is the vehicle (1). This silicone elastomer 193

194 JOURNAL OF COSMETIC SCIENCE also provides novel sensory properties that are unlike anything attainable with silicone fluids. The unique properties of the DCP/cyclopentasiloxane blend have been attributed to the extended structure of the elastomer that has the ability to absorb large amounts of silicone fluid. When the elastomer is synthesized in cyclopentasiloxane, the elastomer and cyclopentasiloxane form what is essentially one large gel in the reaction vessel. This gel is then sheared to produce gel particles that thicken silicone oils in much the same way that carbomer gel particles thicken aqueous formulations. These soft gel particles are what provide the unique aesthetic properties in many formulations. The addition of hydrophilic polyethylene glycol (PEG) functional groups to DCP affects both the chemical and rheological properties of the silicone elastomer. This new class of materials, hereinafter referred to as PEG-modified DCP (PEG-DCP), encompasses a wide variety of different materials that range from liquids to elastomeric solids. The PEG-DCPs have very different properties from the unmodified silicone elastomer. The hydrophilic PEG substituents change the wetting properties of the elastomer and dra- matically improve compatibility with polar ingredients (2). The PEG-DCPs are a new class of polymeric emulsifiers that are useful for preparing water-in-silicone (w/s) emul- sions where the continuous phase consists primarily of low-viscosity silicone oils such as cyclopentasiloxane. We have also shown that PEG-DCP can be used to prepare multiple emulsions. SILICONE ELASTOMER CHEMISTRY All of the silicone elastomers that are made for use as ingredients in personal care formulations are based on the hydrosilylation reaction. This reaction is used to form the crosslinks between the silicone polymer chains and also to attach functional substituents such as PEG. The hydrosilylation reaction involves the addition of a vinyl group (CH2--CH-R) to a silicon hydride (Sill), using a platinum catalyst. This reaction pro- ceeds very rapidly and produces a chemically stable linkage between the silicone and the organic group. Since the silicon hydride in effect adds across the vinyl group, there are no by-products. To make a silicone precursor polymer for the hydrosilylation reaction, the silicon hydride groups are introduced randomly along the silicone chain, typically by copolymerization of methylhydrogen siloxane and dimethyl siloxane units. These pre- cursor polymers can then be crosslinked with an o•,to-diene or a vinyl-terminated silicone polymer (e.g. vinyldimethicone). The PEG substituents are incorporated in the form of allyloxy-terminated PEGs that are added to the precursor polymer using the same hydrosilylation reaction. To prepare PEG-DCP, the silicone precursor polymer is reacted with allyloxy- terminated PEG and crosslinker (o•,to-diene) in a solvent. For our work, the solvent was cyclopentasiloxane and the samples we evaluated were dispersions of PEG-DCP in this solvent. After completion of the hydrosilylation reaction, the elastomer dispersions were subjected to high shear mixing. When the viscosity of the elastomer dispersion is low (e.g., when the crosslink density is low), the high shear mixing has little effect, but when the hydrosilylation produces a thick elastomer dispersion (gel), the shearing step con- verts the material to a paste composed of small gel particles. All of the work we are reporting here was done with PEG-DCP made from the same