348 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS TABLE II FLUORINATED HYDROCARBON PROPELLENTS AVAILABLE IN THE UNITED KINGDOM CHEMICAL NAME TRADE NAMES Gauge Pressure Boiling Pt. at 70 ø F. Trichloromonofluoromethane "Algofrene" 1 2-64 in. Hg. be- 74.7 ø F. "Arcton"9 low one atmo- "Isceon" 131 sphere Dichlorodifluoromethane "Algofrene" 2 70.12 p.s.i. -- 22 ø F. "Arcton" 6 "Freon" 12 "Isceon" 122 Dichloromonofluoromethane "Arcton" 7 8.38 p.s.i. 48 ø F Monochlorodifluoromethane "Arcton" 4 122.5 p.s.i. -- 41.4 ø F. Trichlorotrifluoroethane "Arcton" 63 -- 117.9 ø "Freon" 113 Dichlorotetrafluoroethane "Arcton" 33 12.87 p.s.i. 38.4 ø F. "Freon" 114 "Isceon" 224 The propellents dealt with so far are non-inflammable, odourless and non- toxic. A concise account of their properties is to be found in the March, 1956, issue of this Society's Journal. 6 A mixture of equal parts of dichlorodifluoromethane and trichloromono- fluoromethane has a pressure at 70 ø F. of 35 p.s.i.g. and is used to a very TABLE III Reg. Trade Names Manufacturers Suppliers .... "Algofrene" Montecatini SA., Milan, Italy J. Weil & Son, Ltd., London, E.C.2 "Arcton" Imperial Chemical Industries, Manufacturers Ltd., London, S.W. 1 -- "Freon" E.I. Du Pont de Nemours & Co. Brown & Forth, Ltd., Inc., Wilmington, U.S.A. London, N.W. 1 -- "Isceon" Imperial Smelting Corporation, Manufacturers Ltd., London, W. 1 large extent. A mixture of dichlorodifluoromethane and dichlorotetra- fluoroethane in the proportion 40: 60 has a somewhat similar pressure and with a greater stability is used in aqueous packs. The internal pressure of aerosols at room temperature is normally 35-40 p.s.i.g.

TECHNICAL ASPECTS OF AEROSOL PACKAGING 349 Butane, an inflammable but cheap propellent, is often used in Continental, i.e., European, cosmetic formulations, although I would advise against the use of butane in such packs on account of safety considerations. I have seen some experimental Belgian packs employing the even more inflammable dimethyl ether as propellent. Methylene chloride has been used successfully in hair lacquer formulations by Witjens, 7 replacing part of the normal propellent and part of the alcohol. TYPES OF PACK Aerosol packs can be divided into four main classes: Two-phase systems, Three-phase systems, Foams and Powders. TWO-PHASE SYSTEMS In these systems the liquid propellent is completely miscible with the product and there is only one liquid phase in addition to the propellent vapour, which occupies the headspace in the container. When the valve is operated, the pressure in the headspace forces the propellent product mixture up the syphon tube and out of the valve (Fig. 1). As soon as a drop of this mixture comes into contact with the atmosphere the liquid propellent changes into vapour and scatters the product into innumerable particles. The degree of scattering and the resulting particle size are determined principally by the relative amounts of product and propellent, and this brings us to the division of two-phase systems into the two sub-groups-- space sprays and surface sprays. In space sprays the amount of product in the pack is small compared with the amount of propellent and normally does not amount to more than 20 per cent by weight. Space sprays are true aerosols with particle sizes in the region of 50 microns, and examples of such packs are air fresheners, perfumes, bactericides, deodorants and insecticides. In surface sprays the amount of product approximately equals the amount of propellent. All the same, the amount of product may be as low as 30 per cent or as high as 75 per cent. The size oi the sprayed particles is in the region of 100-200 microns. Examples of cosmetic surface sprays are colognes, hair lacquers, sun screening oils, insect repellents and brilliantine. Valve design also plays a big part in determining the characteristics of space and surface sprays. The use of two-phase systems is not restricted to metal containers, but applies also to glass aerosols. When using the latter, however, the internal pressure is normally reduced to the so-called "ultra-low" figure of approxi- mately 15 p.s.i.g. THREE-PHASE SYSTEMS In three-phase systems the liquid propellent is immiscible with the product and there are two distinct liquid phases in addition to the propellent