HYDROPHILICALLY MODIFIED SILICONE ELASTOMERS 203 Saturation Point (wt %) 25' 2O 10 5' 0 .- High Medium Low Medium Crosslink Density High Process Parameter PEG Substitution Level Figure 9. The effect of PEG substitution level and crosslink density process parameter on the tocopherol saturation point of PEG-DCP made with short-chain PEG. for anhydrous antiperspirants. The utility of DCP for thickening a simple anhydrous roll-on antiperspirant based on cyclopentasiloxane is illustrated in Figure 10. This graph compares the thickening of DCP to that obtained with an organomodified clay (Quater- nium-18 Hectorite). Although it is an effective thickener, formulations based on DCP do exhibit syneresis unless the concentration of DCP is quite high (7% elastomer). Syneresis in this context refers to the separation of cyclopentasiloxane from the formula and is quite undesirable because the silicone fluid tends to leak and coat the outside of the package. Syneresis can appear over time, or when the formula is subjected to shear forces as when it is dispensed from the package. We used a centrifuge test as a measure of the potential for a sample to exhibit syneresis. A series of simple antiperspirant soft solid formulas was prepared using DCP (Dow Corning © 9040 Silicone Elastomer Blend) and PEG-DCP made with long-chain PEG, a high level of PEG substitution, and low crosslink density. Figure 11 shows the .... 500 •, 400 8 • 200 "100 o o m 0 2 3 4 5 Rheology Modifer Added (solids basis) -*- Quaternium-18 Hectorite --- DCP Figure 10. Comparison between DCP and organomodified clay as thickeners for an anhydrous roll-on antiperspirant.

204 JOURNAL OF COSMETIC SCIENCE 450,000 - 45 400,000 •. 40 350,000 -. 35 300,000 ' 30 • i 250,000 25 .• 200,000 20 • 150,000 15 • 100,000 10 50,000 • 5 o • , T Io 2.5 3.0 3.5 4.0 4.5 Viscosity - with DCP Viscosity - with PEG-DCP Syneresis with PEG-DCP Syneresis with DCP Elastomer Concentration (wt %) Figure 11. Viscosity and syneresis for anhydrous antiperspirants thickened with DCP versus PEG-DCP. viscosity and syneresis values for these formulas. Syneresis is reduced as the elastomer concentration increases for both DCP and PEG-DCP, but the effect is much more pronounced for the PEG-DCP. For DCP, the viscosity increase with each increment of elastomer level is such that the viscosity of the formula becomes unmanageable before syneresis is reduced to values close to zero. On the other hand, each increment of added PEG-DCP produced a large reduction in syneresis, while the viscosity was essentially unchanged. It should be noted that the viscosities shown for the formulas made with PEG-DCP are somewhat misleading because these systems are strongly shear thinning. As a result, the measured viscosity is quite variable, depending on the shear rate used. If the sample is subjected to shear--for example, by shaking the container vigorously-- the viscosity will be much lower than it was before shaking, but will increase over time. This increase in viscosity over time may be a consequence of chain entanglement be- tween loosely crosslinked elastomer chains, or an association between the PEG substitu- ents. Whatever the cause of the reversible shear thinning effect, we have found PEG- DCP quite useful for making stable low-viscosity antiperspirants such as roll-ons, and we believe this effect would be useful for aerosol antiperspirants as well. CONCLUSIONS We have shown that a new class of silicone elastomers, PEG-modified dimethicone crosspolymer (PEG-DCP), can provide benefits that are different from the thickening effects that can be achieved using silicone elastomers without functional substituents. Our work has demonstrated that PEG-DCP has utility as an emulsifier for preparing w/s emulsions and multiple (w/s/w) emulsions where the silicone phase is cyclopentasilox- ane. The addition of the hydrophilic PEG substituents to the silicone elastomer improves compatibility with polar organic oils, and this was illustrated with tocopherol. It was found that PEG-DCP is not as effective as the non-functionalized elastomer (DCP) for thickening anhydrous antiperspirants, but is more effective for reducing syneresis. In order to evaluate the effects of variations in composition and processing on the performance and physical properties of PEG-DCP, samples from a two-level factorial design were evaluated. The variables studied were the level of PEG substitution, the length of the PEG chains, and crosslink density. In general, PEG-DCP samples made