J. Soc. Cosmetic Chemdsts, 16, 359-363 (1965) A New Procedure for the Preparation of Polyethylene-Mineral Oil Gels PAUL THAU, B.S., and CHARLES FOX, B.A.* Synopsis--A new procedure is described for the preparation of polyethylene-mineral oil gels. This procedure utilizes high shear mixing equipment to maintain a fine dispersion of the polyethylene wax during a critical phasc of the cooling process. The consistencies of gels produced by this procedure are independent of cooling rates. In contrast to existing methods of preparation, this innovation offers advantages in terms of simplified processing and greater formulation flexibility. INTRODUCTION In recent years, polyethylene-mineral oil gels have been employed as vehicles in various dermatologicals (1). The advantages of this type of base over the conventional oleaginous bases are: (a) very little change in consistency over a wide range in temperature, (b) excellent stability at elevated temperatures, (c) improved spreadability (2, 3, 4), (d) release rates of various medicaments which exceeds that from other oleaginous bases (5), and (e) bland and inert characteristics. This type of vehicle is, of course, also useful for the preparation of various cosmetic formulations. However, the previously published procedure (6) for preparing these polyethylene-mineral oil gels has limitations which severely reduce its application to the manufacture of cosmetic preparations. For example, a temperature of 130øC is re- quired to dissolve the high molecular weight (approximately 20,000) polyethylene in mineral oil, and a three hour holding period is said to be required to effect solution. The solution which is then allowed to cool to a temperature just above its cloud point (95øC-100øC) must then * Warner-Lambert Research Institute, Morris Plains, N.J. 07950. 359

3(50 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS be "shock cooled" to a temperature just above its gel point. This rapid cooling is accomplished by spreading a thin film of the hot solution on a constant temperature, water cooled, metal surface and removing the film from the surface when it reaches $0øC. This type of processing requires highly specialized equipment, and the high processing tempera- ture precludes the addition of volatile and heat sensitive materials. The new simplified process for preparing polyethylene-mineral oil gels, which utilizes commonly available high shear mixing equipment in conjunction with lower molecular weight polyethylene waxes, is readily adaptable to the preparation of cosmetic and pharmaceutical formula- tions. The use of lower molecular weight (1500-2000) polyethylene waxes makes it possible to reduce the dissolution temperature from 130øC to ,•)0øC-95øC and the holding time from three hours to one hour. However, a higher concentration of wax is required to obtain desirable consistencies. The Eppenbach Homo-Mixer ©*, utilizing a high speed turbine-stator mechanism with a fixed clearance, has been found to give satisfactory high-shear mixing. RESULTS To prepare a basic gel using this procedure, 8-15% of polyethylene wax (M.W. 1500-.2000) is dissolved in mineral oil at 90øC-95øC, using a propeller type mixer with moderate agitation. After complete solu- tion is achieved, the propeller stirrer is replaced with an Eppenbach Homo-Mixer. The speed of the Homo-Mixer is adjusted to produce moderate turbulence in the solution. This mixing operation is con- tinued until the mixture has air-cooled to 65øC (approximately 10øC below the cloud point). The Homo-Mixer is then replaced by a con- ventional loop or paddle mixer to minimize air entrapment in the formulation. Cooling with moderate agitation is continued until the temperature drops to about 45øC, and the preparation is then poured into tubes or jars. The mixture forms a gel after packaging as further cooling occurs, and it reaches maximum consistency within 24 hours. When perfumed formulations are prepared, the fragrance is conveniently incorporated after the mixture has cooled to 50 øC. Polyethylene-mineral oil gels prepared by this high shear process have distinct advantages over identical compositions prepared com- pletely with conventional loop, paddle, or propeller mixers. Processing with the Homo-Mixer within the described temperature range enhances * Gifford-Wood Co., 420 Lexington Ave., New York, N.Y. 10017.