]. Soc. Cosmet. Chem., 22, 153-160 (March 4, 1971) Physical Behavior of Aluminum Chlorhydroxide Suspensions for Aerosol Formulations J. M. CROTTY, B.S., R. L. RAYMOND, B.S., and B. SIEGAL, Ph.D. • Presented May 26-27, 1970, New York City Synopsis--In light of current theory, this paper describes several factors governing the char- acter of SUSPENSIONS containing ALUMINUM CHLORHYDROXIDE to be dispensed in AEROSOL form for antiperspirant use. In particular, systems employing fumed SILICA as the suspending agent are treated in detail. The effect of moisture content, polarity, and par- ticle size of the substituents upon viscosity and homogcneity are discussed. INTRODUCTION The formulation of nonaqueous, nonalcoholic suspensions of alu- minum chlorhydroxide for aerosol antiperspirants has presented a unique problem to the formulation chemist. This is due to the difference in physical behavior between the suspension as a concentrate and the sus- pension as a finished aerosol. The two systems differ in dilution, degree of particle sedimentation, and viscosity. In the finished aerosol, or dilute system, the life of suspension before soft settling of the particles, the degree of particle agglomeration or cak- ing, and the ease of particle redispersion are critical factors dependent on the concentration of the suspending agent. In the concentrate system, the viscosity of the suspension is the critical factor also dependent on the concentration of the suspending agent. * Gillette Co. Toiletries Div., Gillette Park, South Boston, Mass. 02106. 153

154 .JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS In order to achieve the desirable characteristics in the dilute form, it may be necessary to increase the viscosity of the concentrate beyond the level which will facilitate the manufacturing of the product. It is the purpose of this paper to discuss soIne physical parameters which affect the viscosity of aluminum chlorhydroxide suspensions as concentrates and their effect on the finished aerosol. SUSPENSIONS: GENERAL CONSIDERATIONS A suspension is a system in which insoluble solid particles of a mate- rial are dispersed throughout a liquid vehicle. The particles in suspen- sion settle under the force of gravity. This effect is qualitatively ex- pressed by Stokes' law which relates the rate of sedimentation to the dif- ference in densities between the particle and medium, times the square of the particle radius, divided by the viscosity of the vehicle. Kr• (d• - d,•) Rate = Stokes' law assumes that the particles are spheres of the same size, that there is no interaction between the particle and the medium, and that electrokinetic factors are not involved. This is rarely the case, especially in more concentrated suspensions. The limitations and extensions of Stokes' law have been reviewed elsewhere (1). Generally, there are two types of suspensions in practice: 1. Colloidally stable cr defiocculated suspensions. In this case, elec- trokinetic factors have been adjusted to allow the forces of repulsion be- tween particles to outweigh the van der Waals forces of energy associating particles. This will prevent the aggregation of the particles. Colloidally stable suspensions have been reviewed by Cartwright (2). 2. Controlled fiocculation suspensions. These are usually achieved by the addition of surface active ingredients or suspending agents which cause the particles to settle as loose riocs (open structures) which are re- dispersed easily. The system of immediate interest is a controlled fiocculation suspen- sion achieved by the addition of fumed or pyrogenic silica as the sus- pending agent. The mechanism of fumed silica as a suspending agent is as follows: Each particle has a very large surface area with hydroxyl groups attached to the silicon atoms on the particle surface. When the material is dis- persed in a suspension, its particles link together at the hydroxyl sites through hydrogen bonding. The effect produced is a three-dimensional