HYDROPHILICALLY MODIFIED SILICONE ELASTOMERS 201 140000 .-. 120000 ---- 100000 ß • 80000 o • 60000 • 40000 • 20000 PEG/PPG-18/18 PEG/PPG-DCP PEG-DCP Dimethicone Figure 6. The effect of emulsifier type on the viscosity of w/o antiperspirant gels. of the silicone elastomer in the silicone phase stabilizes the initial w/s (or pg/s) emulsions and helps protect them against inversion when they are dispersed in water. Multiple emulsions offer 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 and perhaps modulate their release when the product is applied. Figure 7 shows a multiple emulsion that was made by dispersing a primary w/s emulsion into a second water phase (details about the preparation of this sample can be found in the Experimental section). COMPATIBILITY WITH POLAR INGREDIENTS We have established that hydrophilically modified silicone elastomers function as w/s Figure 7. Multiple emulsion photomicrograph (400x magnification).

202 JOURNAL OF COSMETIC SCIENCE emulsifiers, but there are other benefits of this modification. One additional benefit is improved compatibility with organic oils compared to DCP. The unmodified silicone elastomer is very sensitive to the presence of polar oils, even oils that are soluble in cyclopentasiloxane. Such oils tend to collapse elastomer gels that are swollen in cyclo- pentasiloxane, leading to a loss of thickening. Common organic oils such as fatty esters will have this effect even at relatively low addition levels. At higher concentrations, fatty esters can precipitate DCP, presumably due to poor solvation of the elastomer. The polar functionality of the PEG-DCP elastomer increases the range of organic oils that are compatible. To illustrate this phenomenon, the PEG-DCP samples from the designed experiment were blended with tocopherol. Tocopherol was chosen partly because it is completely miscible in cyclopentasiloxane and, therefore, any haziness observed in the samples could be attributed to interaction with the elastomer and not the solvent. The tocopherol was slowly added to the elastomer blends until the mixtures became hazy, indicating incompatibility. The concentration of tocopherol required to produce incompatibility was recorded as the "saturation point" for the particular elastomer blend. When tocopherol is blended with DCP in cyclopentasiloxane, the mixture is hazy even at addition levels of about 1%. The results for the PEG-DCP samples are shown in Figures 8 and 9. When long-chain PEG is used to make the PEG-DCP, the saturation point increases with higher levels of PEG substitution and decreases as the crosslink density increases (Figure 8). Figure 9 shows similar trends for PEG-DCP made with short-chain PEG, but overall the saturation points are lower. Taken together, these data clearly point to the role of PEG substitution in increasing compatibility with polar organic oils. This makes sense because more PEG substitution increases the overall polarity of the elastomer. Prieto and O'Lenick (5) have reported a similar relationship between PEG level and solubility in polar media for the dimethicone copolyols. The effect of crosslink density is also clear, but the explanation of why is not so easy. Perhaps the tighter self-association of more crosslinked elastomer makes it more sensitive to the solvent environment. USING PEG-DCP TO REDUCE SYNERESIS One of the applications that was first demonstrated for DCP was its use as a thickener 3O 25 20 Saturation Point 15 (wt %) lO 5 Low Crosslink 0 Medium Density High Process High Medium Parameter Low PEG Substitution Level Figure 8. The effect of PEG substitution level and crosslink density process parameter on the tocopherol saturation point of PEG-DCP made with long-chain PEG.