On particle size distributions of aerosol sprays Table 1II. Valve/actuator systems * 647 Hardware ]- Valve housing Vapour Actuator combination orifice phase tap type (inches) (inches) 1 0.025 -- 1303 (Four-channel) 2 0.025 -- 015 Standard 3 0.080 -- 1303 (Four-channel) 4 0.080 -- 015 Standard 5 0.025 0.013 1303 (Four-channel) 6 0.025 0.013 015 Standard * Stem orifice 0.020 inches in all systems. ]' All valve/actuator systems ex. Metal Box Company Limited, UK. • The 1303 actuator has a four-channel swirl chamber for mechanical break-up of aerosol droplets. RESULTS The number/size information obtained for each spray was analysed as follows: the particle size information was computed on a mass basis and plotted on a logarithmic probability scale. From these plots the mass median diameter (the diameter above and below which lies 50 •o of the mass of the particles)was read off. The mass-median diameter was obtained in order to represent the coarseness of the spray. Typical cumulative size distribution curves are given in Fig. 4 (hardware combinations: 0.020 inch stem, 0.025 inch housing, 0.013 inch vapour phase tap and 1303 actuator). Particle D•ameter 300. (microns) 100. 50- lO- Product/ 5- a Propellent I Distance a.35/65 28cm b. 15/85 28 cm c C, 15/85 48cm 0.01 0"05 O.•i • 1•3 5'0 9•3•5 9'9 9•9 9999 Cumulative Weight Percentage Figure 4. The particle size distributions of aerosol systems studied. The condition of straight line plots as a criterion for log-normality was not applied because the data were limited by the extremes of the size ranges used and it had been assumed that all particles in a channel were equal in size to the mid point of that channel. Such assumptions can be further complicated by the unequal widths of the channels on some size ranges. An improved method of data analysis would be to describe the infor- mation obtained by the distribution function, from which parameters can be derived that can characterise the data. Raabe (20) has described the use of the log-normal function in particle size analysis, together with a maximum likelihood method for fitting and testing the fit of a log-normal function to grouped particle size data.

648 R. W. Pengilly and J. A. Keiner The size data for the 15/85 product/propellant aerosol are listed in Tables IV and V. Four repeat measurements of each valve/actuator combination gave a maximum varia- tion of d- 5 % in the computed mass-median diameters, and the values quoted are the arithmetic means of these results. Table IV. Mass-median diameters of 15/85 product propellant system at 48 cm from actuator (for fuller details see Tables 1 and 2) Housing Vapour phase tap Actuator Mass-median (inches) (inches) diameter (gm) 0-025 m Four-channel 9 0.025 -- Standard 10 0.080 -- Four-channel 11 0.080 -- Standard 10 0.025 0.013 Four-channel 8 0.025 0.013 Standard 10 0.080 0.013 Standard 10 0.080 0.013 Four-channel 9 The effect of evaporation of the volatile components of the spray can have a large influence on the particle size distribution as can be seen from a comparison of mass- median diameters derived from data at 28 and 48 cm from the actuator, where the diameters can change by a factor of three over a distance of 20 cm. These differences in mass-median diameter are most likely due to evaporation of the ethanol solvent in the formation, which has evaporated subsequently to the explosive vaporisation of the pro- pellant. For example, a 40 p,m diameter drop of the complete resin/solvent/propellant composition in Table Ilb will have evaporated to a drop of approximately 20 •tm diameter on loss of propellant and a drop of 9 •tm diameter on subsequent loss of solvent. This calculation assumes no condensation, disruption or agglomeration of the particles in flight and is based purely on the proportions of the constituents present in the formulation. The effect of a vapour phase tap is, in addition to reducing the discharge rate of the system (21), to reduce the particle size of the spray. The effect of the vapour phase tap is most marked at the higher product/propellant ratio of 35/65 (Table VI). For finer sprays (15/85 product/propellant aerosol), the vapour phase tap effect is small at 28 cm (Table V), and is negligible and difficult to detect if measurements are made at increasing distances from the actuator (e.g. 48 cm, Table IV). Table V. Mass-median diameters of 15/85 product propellant system at 28 cm from actuator (for fuller details see Tables 1 and 2) Housing Vapour phase tap Actuator Mass-median (inches) (inches) diameter (lam) 0.025 -- Four-channel 33 0-025 -- Standard 38 0.080 -- Four-channel 33 0.080 -- Standard 35 0.025 0.013 Four-channel 28 0.025 0.013 Standard 32 0.080 0.013 Four-channel 30 0.080 0.013 Standard 30