TECHNICAL ASPECTS OF AEROSOL PACKAGING 361 Menthyl salicylate Methyl-para-dimethyl amino benzoate Homo-menthyl salicylate 2-Hydroxy-4-methoxy benzophenone Homo-menthyl salicylate has been suggested because its cooling effect on the skin is less than that of menthyl salicylate. •6 The following materials have been found to be satisfactory as vehicles for the above sun-screening agents: Paraffin oil Iso-propyl myristate "Cetiol V" (supplied by Deutsche Hydrierwerke, GMBH, Dusseldorf) Alcohol These vehicles may be used singly or blended to suit individual require- ments. The incorporation of silicones as water repellents, particularly in alcoholic sun tan lotions, has been suggested, '-7,•8 and silicone fluid MS.555 (supplied by Midland Silicones Ltd.) has been found to be compatible with the common propellents. Five to ten per cent sun-screening agent is dissolved in the vehicle(s) and 40-50 parts by weight of the lotion itself is packed with 60-50 parts by weight of propellent mixture. The propellent mixture itself normally consists of either a 50: 50 mixture of dichlorodifluoromethane and trichloro- monofluoromethane or a 40:60 mixture of the former with dichlorotetra- fluoroethane. HAIR LACQUERS The literature mentions three basic materials for use in aerosol hair lacquers, viz., ethyl cellulose, shellac and polyvinyl pyrrolidone. Experiments carried out with the ethyl cellulose formulations suggested by Philips'-g have not come up to expectations. The formulations themselves are indeed suitable for aerosol containers, but the resultant film is inclined to make the hair dull and lifeless. Shellac hair lacquers are suitable for aerosol packing providing unbleached' shellac is used and due attention is paid to the moisture content of the formulation and the type of alcohol used. Reference to these two points has been made earlier in this paper. Very interesting indeed is Witjens' formulation, which has been referred to already. 7 Shellac (unbleached, but dewaxed) 5.0 g. Castor oil, B.P. 2.3 g. Propylene glycol 2.3 g. Carbowax 1-6 g.

362 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Absolute alcohol Methylene chloride Charge: Hair lacquer (as above) Dichlorodifiuoromethane Trichloromonofluoromethane 36.0 g. 54.0 g. 75 parts by weight 46 ...... 46 ...... The most recent newcomer to aerosol hair lacquers is undoubtedly polyvinyl pyrollidone, .now freely available in this country. 3ø The PVP used in hair lacquers has a K value of 30 and a molecular weight of 40,000. One of the drawbacks of using PVP is its deliquescence, and the addition of a small amount of shellac has been suggested in order to overcome this. 3• Experi- mental packing of this formulation has shown that the shellac does not enter into any combination with the PVP and the desired effect is not achieved. A satisfactory formulation having all the desirable qualifications has been achieved by plasticising the PVP with dimethyl phthalate and adding a small amount of silicone fluid MS.555 (supplied by Midland Silicones Ltd.), a uniform film being formed, Wool wax alcohols and isopropyl myristate are other suitable additives and many more are referred to in the literature. a• No reference to perfumes has been made in the above formulations. While dosages may well be within the limits published 33 much will obviously depend on the perfume used and on the perfumer using it. LABORATORY PRACTICE It is hoped that the examples cited above have made it abundantly clear that a great deal of research is necessary before any particular pack can be approved. The first step, on receiving samples in the laboratory, is to test for compatibility between the product and the type of propellent which is suggested by experience. We have found the use ot ordinary lemonade "pop-bottles," suitably protected, to be very excellent for this purpose. If the first test is satisfactory, metal strips are inserted into another similarly filled bottle and the effects of the pack on the strips over a period of 7-10 days at room temperature is noted. If no deleterious effects are noticed by that time a number of aerosol containers are packed with what one hopes to be the ultimate formulation, and the containers are stored both at room temperature and at 95 ø F. for at least eight months. During that period various containers are destructively examined at regular intervals and the performance of the valve itself is checked frequently. A period of eight months may sound a little arbitrary, but it is a fact that any product which was passed as satisfactory under our test conditions has not failed when put on the market. For some time, the favourite method employed in the United States consisted of storing the