ALUMINUM CHLORHYDROXIDE SUSPENSIONS 157 The suspensions were then hand-stirred for 2 min in order to wet the solid components. This was followed by mechanical stirring for 2 min with a Lightnin Mixer* at about 1165 rpm. The suspensions were then allowed to sit for 2 min, after which viscosity measurements were made. RESULTS AND DISCUSSION The concentration of fumed silica in the concentrate system has the most dramatic effect on apparent viscosity (hereafter referred to as vis- cosity) of all variables evaluated. Table I illustrates the marked increase in viscosity produced by small increases in fumed silica content. How- ever, the suspension stability of the finished aerosol formulation is equally dependent on the concentration of fumed silica. This was shown when the concentrate suspensions from Table I which contained different con- centrations of fumed silica were diluted to the aerosol form. The sedi- mentation volumes increased with increasing concentrations of fumed silica. This is the only viscosity-influencing variable in the concentra- tion suspension which correspondingly influences the suspension stabil- ity of the finished aerosol. The threshold level for viscosity between a pourable liquid and a nonpourable cream is approximately 5000 cps. For manufacturing pur- *Mixing Equipment Co. Inc., 136 Mt. Blvd., Rochester, N.Y. 14603. Table I Effect of Concentration of Ingredients on Viscosity Ingredient Viscosity Concentration (%) (cps) Fumed silica Fumed silica Fumed silica Water Water Water Aluminum chlorhydroxide Aluminum chlorhydroxide Hexachlorophene Hexachlorophene Hexachlorophene Benzethonium chloride Benzethonium chloride Perfume Perfume Perfume 3.0 1250 3.5 1950 4.0 (Control) 2900 7.0 (Control) 2900 7.5 7850 8.0 8950 33.0 (Control) 2900 41.0 8500 0 2900 O. 4 (Control) 2900 1.0 2900 0 2900 0.5 10,000 0 500 3.5 (Control) 2900 5.0 3700

158 .JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS poses, the viscosity of the concentrate suspension will determine whether conventional liquid filling equipment can be used or if a more costly and possibly less accurate cream filling system must be employed. Lot-to-lot variations in fumed silica may produce suspensions of dif- ferent viscosity (Table II). These differences may be caused by varia- tions in the surface area of the fumed silica from lot to lot which could produce different thickening efficiencies. It might also be caused by different amounts of adsorbed moisture in the fumed silica. This would affect the degree of dispersion, under the same conditions of shear. Table II Effect of Lot Variations on Viscosity Viscosity Ingredient Concentration (%) Lot No. (cps) Isopropyl ester 60.0 A (Control) 2900 Isopropyl ester 60.0 B 3450 Isopropyl ester 60.0 C 5500 Fumed silica 4.0 A (Control) 2900 Fumed silica 4.0 B 3800 The viscosity of the concentrate system is also considerably affected by the final concentration of water. Water can be contributed to the system from the aluminum chlorhydroxide, the fumed silica (to a lesser extent), and under humid conditions from the atmosphere. Because of the hy- groscopic nature of the concentrate suspension and some of its com- ponents, moisture control is very difficult in manufacturing. An ex- ample in Table I illustrates how the viscosity of a suspension increases as water is added in 0.5% increments. The initial concentration of water was determined by a modified Karl Fischer technique. Lot-to-lot variations in isopropyl ester frequently produce large vari- ations in viscosity (Table II). Attempts to determine analytically the differences between the three lots presented were not successful. No contamination could be detected through gas chromatography. The concentration of aluminum chlorhydfbxide in suspension will influence the viscosity of the concentrate system as shown in Table I. Viscosity increases with increasing concentrations of aluminum chlorhy- droxide. In addition, the particle size distribution of the aluminum chlorhy- droxide will also influence viscosity. As the particle size distribution includes smaller particles, the viscosity increases. In Table III, 70% of