CLEAR GEL ANTIPERSPIRANTS 227 mixture is mixed rapidly for a very short time (typically less than 15 seconds), and rapidly poured into containers for subsequent gellation and evaluation. The speed at which the sticks are prepared is important because the gel usually sets up in less than a minute. The addition of the hot glycol phase to the cooler but refluxing active phase causes a considerable evolution of potentially flammable vapor that must be handled with due care. The stability of the various solid gels is evaluated by placing a one-ounce clear glass jar of the solid gel in an oven set at 60 ø C. Two weeks in the 60 ø C oven without com- pletely liquefying indicates the formulation will probably exhibit adequate stability at the lower conventional temperature stability conditions. In our experience, one day at 60 ø C generally corresponds to two weeks at 45 ø C, so that two weeks at 60 ø C corre- sponds to over half a year at 45 ø C. The complete liquefication point is used as the stability evaluation criterion because it is a less subjective end point than the typical loss of hardness test for a solid gel. Generally a solid gel will maintain adequate gel hardness for at least two thirds of the time that it takes to liquefy completely at 60 ø C. The best signal that the solid gel is beginning to deteriorate is the presence of the characteristic almond odor of benzaldehyde that results from the decomposition of DBS. The odor is evident long before any other physical evidence of gel deterioration is noticeable. All of the materials employed were used as received from their supplier. Anhydrous materials were used because of the deleterious effect of water on the formulations. RESULTS AND DISCUSSION The liquefaction of the gel is caused by the decomposition of the DBS gellant. The mechanisms for the decomposition of acetals are the acid catalyzed reaction with water (hydrolysis) or with an alcohol (transacetalation). Hydrolysis is minimized by limiting the water content of the raw materials in the formulation. Table I shows a series of formulations comparing the stabilities obtained with formulations containing various ratios of solvents: butylene glycol to hexylene glycol and also ethanol to isopropanol. This experiment was conducted to determine if transacetalation can be minimized by choice of solvents. The stability of constant glycol formulas (1, 2, and 3 4, 5, and 6 7, 8, and 9 10, 11, and 12) marginally increases as the formula increases in isopropanol content and decreases in ethanol content. The stability of constant alcohol formulas (1, 4, 7, and 10 2, 5, 8, and 11 3, 6, 9, and 12) marginally decreases as the formula increases in hexylene glycol content and decreases in butylene glycol content. Controlling the choice of the stick solvent (ethanol, isopropanol, butylene glycol, hex- ylene glycol), while affecting stability somewhat, does not provide an appreciable in- crease in stability. Table II presents the data for five possible stability additives, magne- sium sulfate, cocamide MEA, zinc acetate, methenamine (hexamethylene tetramine), and acetamide MEA. It was postulated that the anhydrous magnesium sulfate would tie up the free water introduced from the raw materials. The others were chosen as possible buffering agents. Cocamide MEA and acetamide MEA are amides that act as Lewis bases and also contain a small amount of free MEA. Zinc acetate is an organic solvent-soluble inorganic base.

228 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table I Effect of Alcohol/Glycol on Gel Stability Formulation Constants % Dibenzylidene Sorbitol (1) 3.0 Steareth- 100 1.0 Aluminum Chlorohydrex (2) 10.0 Stearic Acid, Triple Pressed 0.5 Hydroxypropylcellulose 0.2 Cyclomethicone 5.0 Formulation Variables Formula Number 1 2 3 4 5 6 7 8 9 10 11 12 % % % % % % % % % % % % Butylene Glycol -- -- 10.0 10.0 10.0 20.0 20.0 20.0 30.0 30.0 30.0 HexyleneGlycol 30.0 30.0 30.0 20.0 20.0 20.0 10.0 10.0 10.0 -- -- -- Ethanol, Anhy. 50.3 33.5 16.8 50.3 33.5 16.8 50.3 33.5 16.8 50.3 33.5 16.8 Isopropanol -- 16.8 33.5 -- 16.8 33.5 -- 16.8 33.5 -- 16.8 33.5 Stability Data: Days Until Completely Liquid @ 60øC 2 3 3 2 3 4-6 3 4-6 4-6 3 4-6 4-6 @ 45øC 45 45 45 41 50 64 41 50 64 41 52 71 (1) Millithix 925 ©, Milliken Chemicals, Spartanburg, SC. (2) Rehydrol II ©, Reheis Chemical Company, Berkeley Heights, NJ. Methenamine is an amine. These additives were investigated utilizing the 16 formula- tions in Table II. Formula 7 from Table I is identical to formula 13 in Table II. The 45øC data for the two identical formulas evaluated at different times shows the variability inherent in this type of measurement. Since a statistical analysis of the data in Table II was anticipated, only one half of the 25 factorial design was necessary. However, a statistical analysis was not performed because of missing data points. A number of the formulations were so stable that an end point was not reached after 8 months at 60øC. The study was discontinued at this point because ethanol loss was becoming a concern at the high temperature. The inorganic salts, magnesium sulfate and zinc acetate, were evaluated at 0.3% be- cause of solubility constraints. The amides, cocamide MEA and acetamide MEA, be- cause of their lower buffering capacity were evaluated at 5 %. The amine, methenamine, because of its propensity to darken was evaluated at 0.1%. The comparison of the stability results for formula 14 to 15, 17, and 21 16 to 15, 19, and 23 18 to 17, 19, and 25 and 22 to 21, 23, and 25 shows that magnesium sulfate, at the levels studied, is the least effective stabilizing active of the five. The comparison of the stability results for formula 14 to 16, 18, and 22 15 to 16, 19, and 23 17 to 18, 19, and 25 and 21 to 22, 23, and 25 shows that at the levels studied, cocamide MEA is more effective than magnesium sulfate but less effective than the other three additives. Roehi disclosed cocamide MEA as a DBS stability aid. The comparison of the stability results for formula 15 to 14, 17, and 21 16 to 14, 18, and 22 19 to 17, 18, and 25 and 23 to 21, 22, and 25 shows that at the levels studied, zinc acetate is more effective than magnesium sulfate and cocamide MEA but less effective than methenamine and acetamide MEA.