PREPRINTS OF THE 1998 ANNUAL SCIENTIFIC MEETING 83 NOVEL SILICONE THICKENING TECHNOLOGY: A REVIEW OF DEVELOPMENTAL APPROACH AND PRACTICAL APPLICATIONS Michael S. Starch and Carol A. Hoag Dow Corning Corporation, Midland, Michigan 48686 Introduction The widespread use of low molecular weight silicones such as cyclomethicone in a variety of personal care products has led to the need for efficient and cost-effective means to thicken these silicones. The ability to thicken cyclomethicone has obvious benefits in formulations such as anhydrous antiperspirants where cyclomethicone is the primary vehicle. In these systems, control over formula viscosity is important for proper dispensing and reducing stability problems such as syneresis. In other types of formulas, the aesthetics of the formula can be altered by the use of thickened silicones. Several methods of thickening cyclomethicone are already known and practiced in the industry. These include the use of silicone gum (high molecular weight dimethicones or dimethiconols) blends or inorganic materials such as fumed silica. The silicone gum blends are relatively inefficient and the concentrations required to achieve significant thickening can produce a greasy feel. Also, silicone gums do not produce a yield value when blended with cyclomethicone, so they are not effective in preventing settling. On the other hand, fumed silica is an efficient thickening agent for cyclomethicone and will produce a yield value su?.,.cicn! to i.mprove fo..,•..u!a stabi!i•, but *.he ½s*•e!ics of si!ico-•cyc!o..'n. et•icone mixtures are not always suitable for the intended application. A Better Solution: Silicone Elastomers (Three-Dimensional Silicone Polymers) A new class of materials has recently been developed to address the problem of thickening cyclomethicone. These are three-dimensional, or loosely crosslinked, silicone polymers which provide efficient thickening and improved aesthetics relative to other thickening additives. The presence of a small amount of crosslinking between silicone polymer chains produces a qualitative change in the properties in the material. These are no longer simple fluids, but are properly categorized as elastomers which have an extended three-dimensional structure. When cyclomethicone is blended with these materials, a swollen network is formed which can accommodate very large amounts of cyclomethicone without showing any sign of syneresis. Given the intimate nature of this blend, we expected that some of the cyclomethicone would be strongly bound. To confirm this, we followed the weight loss at 40 ø C of samples of elastomer blend and cyclomethicone. Figure I shows the results, which were somewhat surprising. The weight loss curves indicate the volatility of the cyclomethicone was essentially unaffected by the elastomer. Routes to Silicone Elastomer / Fluid Blends Several methods for preparing silicone ela,stomer/fiuid blends have been explored in our laboratories. One method involves the preparation of a cured silicone elastomer that is blended with silicone fluid to produce the swollen network. The process of blending cured elastomer with silicone fluid requires very high shear mixing and the viscosity of the blend tends to increase over time due to residual curing activity of the elastomer. This approach is also expensive because it requires two separate manufacturing steps. Another approach, the "in-situ method", involves curing the elastomer in the presence ofcyclomethicone. With careful control of processing conditions, this method is capable of producing consistent blends with stable viscosities. Given the advantages of the in-situ method, that was the route that we pursued. However, competitive patents prevented us from using conventional silicone elastomer chemistry, so an elastomer based on an organic crosslinker was developed. This work led to a material which has been assigned the INCI name of cyclomethicone and dimethicone crosspolymer. Rheological Properties As stated earlier, this new class of silicon•.thickeners are elastomers, and therefore demonstrate elastomeric behavior when examined using dynamic theological measurement techniques. To illustrate this, Figure 1 shows the results from a frequency sweep experiment. As the frequency increases, we see a steady

84 JOURNAL OF COSMETIC SCIENCE increase in G', which is the elastic storage modulus. Under other conditions, the elastomer blend behaves like a high solids dispersion where the viscosity depends on the shear rate. Figure 2 shows the a results of a stress sweep experiment where the sample was subjected to increasing stress which caused a steady increase in shear rate with the attendant decrease in viscosity. Applications The most common type of products which utilize cyclomethicone as the vehicle are anhydrous antiperspirants. These products generally require a thickener to achieve adequate stability, so this was the first application we explored. To compare the thickening performance of a silicone elastomer to other additives, several antiperspirant creams were prepared using equal concentrations of three thickening agents. The rheological properties of these simple formulas containing only antiperspirant salt (25%), thickener (4%), and cyclomethicone (71%) were studied using the stress sweep method. The results, shown in Figure 3, indicate that the silicone elastomer and the fumed silica provide comparable thickening effects, although the prototype with silica had a higher viscosity under all conditions. In contrast, the silicone gum (dimethiconol) provides a much lower viscosity, especially at low shear rates. Conclusion A new class of silicone materials has been developed which offers an effective way to modify the theology ofcyclomethicone and other low viscosity silicone fluids. Preliminary work has shown that they are effective thickeners in two types of formulations. Other benefits and applications for these materials will no doubt be found as they gain wider acceptance in the industry. Figure 1: Weight Lo• 84 40 C Figure 2: Dynamic Frequeficy Sweep fo•SHtcono E•tom• I , , Figure 3: St•ss Sweep foc Silicone Eli&tm.er Blend Figure 4: Stress Swe4tm foc 111nm Antlpempl•lnt Crlmm• • ,,