HYDROPHILICALLY MODIFIED SILICONE ELASTOMERS 199 Viscosity (cP) 160000ß 140000,- 120000- 100000 • 80000 ß 60000--' 40000 Y 20000 ß Low Crosslink 0 " ' '•'"• Medium Density High Process High Medium Parameter Low PEG Substitution Level Figure 2. The effect of PEG substitution level and crosslink density process parameter on the viscosity of PEG-DCP made with short-chain PEG. 90 Y, 80' 70=, 60" Water Uptake 50 (wt %) 40:: 30 •: 20 lO 0.•', High Medium PEG Substitution Level Low Crosslink Medium Density High Process Parameter Low Figure 3. The effect of PEG substitution level and crosslink density process parameter on water uptake for PEG-DCP based on long-chain PEG. consists primarily of cyclomethicone (cyclotetrasiloxane and/or cyclopentasiloxane) is in antiperspirant gels. In these products, the water phase consists of an aqueous solution of antiperspirant salt, and often the refractive index of the water and silicone phase are matched to produce a transparent emulsion. For our investigation, we compared a commercial silicone emulsifier, PEG/PPG-18/18 dimethicone (Dow Corning © 5225C Formulation Aid), with two elastomeric emulsifiers. The first was similar to the PEG- DCP samples discussed previously. It was made with a high level of PEG substitution and low crosslink density. For the other elastomeric emulsifier, we used a copolymer of polyethylene glycol and polypropylene glycol (PEG/PPG) as the hydrophilic group in order to make it more similar to the dimethicone copolyol emulsifier. This second type of elastomeric emulsifier is referred to as PEG/PPG-DCP. Both elastomeric emulsifiers produced stable antiperspirant gels. Figure 6 shows the viscosity for the gels based on the elastomeric emulsifiers and the PEG/PPG-18/18

200 JOURNAL OF COSMETIC SCIENCE 80: 70 60-- Water Uptake 50 •: (wt %) 20-- 10 .--' High Medium PEG Substitution Level Crosslink Density I Process Parameter Low Figure 4. The effect of PEG substitution level and crosslink density process parameter on water uptake for PEG-DCP made with short-chain PEG. lOO 90 80 70 60 50 4 lO 6 8 Concentration of PEG-DCP (wt. %) --*- Low CDPP/Low PEG --=- High CDPP/High PEG ] Figure 5. The effect of dilution level on water uptake for two PEG-DCP samples made with short-chain PEG. dimethicone. The viscosity of the gel made with PEG/PPG-DCP was much lower than that of the gel made with PEG-DCP, and although it appeared to be stable over time, differences were noted when the gels were rubbed on a hard surface. This "rub-out" test is a measure of how stable the gel is when exposed to very high shear, such as when the formula is applied to the skin. An acceptable gel maintains its integrity when rubbed out, but when the emulsion breaks under high shear and releases the aqueous antiper- spirant, this is unacceptable because this can produce stickiness and a more noticeable white residue when the formula dries. The gels made with dimethicone copolyol and PEG-DCP passed the "rub-out" test, but the gel made with PEG/PPG-DCP failed. MULTIPLE EMULSIONS In addition to simple w/s emulsions, PEG-DCP can be used to prepare multiple emul- sions, including water-in-silicone-in-water (w/s/w) emulsions (3) and propylene glycol- in-silicone-in-water emulsions (4). In these systems, we believe that the thickening effect