PROPELLENT IN AEROSOL PRODUCTS 139 CROSS-SECTION OF TYPICAL AEROSOL CROSS-SECTION OF TYPICAL SPACE • OR PACKAGE FOR FOAM-TYPE PRODUCT. SURFACE-COATING AEROSOL PACKAGE. "'--',,.,•.__. • ( rely '-'"'•r,.,•) • 8 .d._...---• ½ / ealneT _•UTES 8Y • U • LI PR[S.SlNG TO •1 I I !• ONE SIDE • U,.I_ AEROSOL FOAM ,) Figure 1. what might be described as flash evaporation the instant it leaves the valve orifice and is exposed to atmospheric pressure. In the case of spray prod- ucts, the flash evaporation shatters the droplets of liquid leaving the valve into many, very small droplets. With the foam products, where the lique- fied propellent gas is emulsified in the aqueous phase, the flash evaporation of the propellent expands the water solution into a foam full of the pro- pellent gas. Bo•]• Po•s ^•t) V^vo]• P]•.sst•s The boiling points and vapor pressures are probably of greatest concern since they are the basic properties which make the aerosol product function properly. These are shown in Table 3. It is important to note that the boiling points and vapor pressures cover a very broad range. It is this wide range which makes possible the versatile character of these propel-

140 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS TABLE 3--BOILING POINTS AND VAPOR PRESSURES Propellent •--•--Boiling Point •, •--Vapor Pressure, psig.--- øF. øC. 70øF. 130øF. "Freon-11" 74.8 23.8 2.6* 24.3 "Freon-114" 38.4 3.6 12.9 58.8 "Freon-IT' -21.6 -29.8 70.1 180 "Freon-22" -41.4 -40.8 123 299 * Inches of mercury, vacuum. lents. Very few aerosol products are propelled by a single compound. The usual propellent is a solution of two compounds. We might consider "Freon-12" as the basic propellent which is diluted with "Freon-11" or "Freon-114" to reduce its pressure to the range permitted by the I.C.C. regulations. A chart showing the pressure ranges of these solutions is given in Fig. 2. "Freon-22" holds a somewhat unique position in that its RANGE OF PRESSURES OBTAINABLE AT 70 F. WITH VARIOUS "FREON" PROPELLENTS "FREON-11"--•"FREON-12" SOLUTIONS i • I I [ "FREON-114"--"FREONo12"SOLUTIONS [ "FREON-114"--"FREON-22" SOLUTIONS [ "FREON-12"--"FREON-22" SOLUTIONS "FREON.11 .... FREON-114" "FRE•)N.12 .... FREON-22" 90 100 110 120 VACUUM I[ PRESSURE- Pounds per squarg inch Inches of M©rcur¾ "FREON" COMPOUNDS AND THEIR GAUGE PRESSURES Figure 2. vapor pressure is exceptionally high for an aerosol propellent but because of its excellent solvent properties, it finds use in certain aerosol products which could not be formulated successfully otherwise. The dilution effect afforded by "Freon-11" and "Freon-114" perhaps is an oversimplification. Each compound has the properties required of a propellent to a limited de- gree and thus functions as more than a mere diluent. This is particularly true of"Freon-114" which in some cases can be used alone as a propellent. The point to remember is that "Freon-12" in most cases cannot be diluted