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.

PREPARATION OF POLVETHYLENE-MINERAL OIL GELS 361 the gelling action of the polyethylene, as the data in Table I clearly show. This, in turn, results in a more elegant product with more desirable spreading characteristics. The consistency of gels prepared by conventional mixing procedures varies as a function of cooling rates. Softer gels result as the cooling rate decreases. This variable is sub- stantially eliminated when the high shear mixing process is employed, as shown in Table I. The gels prepared by the high shear process are much more stable at all storage temperatures than those prepared by conventional mixing and have a low temperature viscosity index. They do not liquefy at high summer temperatures nor harden excessively at low winter tem- peratures. The effect of temperature on cone penetration of a poly- ethylene-mineral oil gel and of Petrolatum U.S.P. is shown in Fig. 1. In addition, the polyethylene-mineral oil base can tolerate the addition of a high concentration of solids without making it unduly pasty. For TABLE I Consistency of Polyethylene Wax-Mineral Oil Gels as a Function of Processing Procedure Cooling Composition Rate? A B C D øC/min. Polyethylene wax M.W. 1500 • Polyethylene wax--M.W. 2000 • Polyethylene wax--M.W. 7000 •' Light white mineral oil c Heavy mineral oil d Penetration readingsZ (0.1 min. div., "Precision" penetrometer with brass COliC) High shear mixing process Conventional nfixing process 120 g. 120 g ....... ...... 120 g .... ......... 120 g. 880 g .... 880 g. 880 g. ... 880 g ....... ß . 215 212 210 370 270 190 195 191 300 • 0.35 ......... O. 50 ......... 3.00 370 620 34() • O. 35 ......... 0.50 ......... 3.00 • Allied Chemical Corporation, Solvay Process Div,, 40 Rector St., New York, N.Y. 10006. • Eastman Chcmical Products, Inc., Kingsport, Tenn. c 65/75 Saybolt Universal seconds at 100øF. d 300/360 Saybolt Universal seconds at 100øF. • Gel displays slight oil separation at time of measure•nent. / ASTM Designation: D217-60T. a Cooling rate controlled from 90 ø to 45øC. Preparation •vas then poured into jars and tubes.