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J. Soc. Cosmet. Chem. 28 641-650 (1977) ¸ 1977 Society of the Cosmetic Chemists of Great Britain The influence of some formulation variables and valve/actuator designs on the particle size distributions of aerosol sprays R. W. PENGILLY and J. A. KEINER Unilever Research Isleworth Laboratory, 455 London Road, Isleworth, Middlesex, TW7 5AB Received 23 February 1977 Synopsis Knowledge of the size and distribution of particles produced from cosmetic aerosol products is iml• ortant not only from the viewpoint of product optimisation but also from considerations of potential inhalation characteristics. A discussion is given of methods suitable for determination of the respirable fraction of the spray or a complete particle size distribution. Results are presented for the particle size distributions of some aerosol formulations with differing compositions and levels of propellant. The effects of certain actuator designs and valve specifications are also presented. These results are discussed in terms of different particle formation mechanisms due to differences in formulation, mechanical action of the valve and actuator and subsequent changes in size of the particles. It is concluded that the particle size distribution of a cosmetic aerosol product only has meaning when the formulation, dispensing, and sampling details are also specified. INTRODUCTION The production of particulate material from a pressurised pack is a complex sequence of events, much of which is little or poorly understood. Knowledge of the size and dis- tribution of particles produced from cosmetic aerosol products is important for product optimisation and consideration of potential inhalation characteristics. Much of the information in the literature on aerosol spray particle size appears at first sight to be contradictory. For example, in the case of hairspray products, Brunner et al. (1) found that 50 • by weight of the particles were 30 I•m or more, whereas Ripe et al. (2) found that, of the hairsprays they examined, 49 % of the particles had a diameter of less than 15 I•m. These differences may be due to many factors, including variations in sampling and sizing techniques (as suggested by Cambridge (3)), and widely differing formulation/valve-actuator systems. These variables are known to exert a large influence on spray particle size. The object of this paper is to consider some of the methods that have been applied to determining the particle size distributions of aerosol sprays, and to examine the influence of some formulation and hardware design variables on the particle size of some model aerosol systems, with a view to presenting a more unified picture of some of the important variables. 641