TECHNICAL ASPECTS OF AEROSOL PACKAGING 353 talcum and'-0•her dusting powders in this form. The chief problems en- countered in the formulation of these packs are the design of a satisfactory valve and control of the particle size. In order to avoid clogging of valves it has been suggested that large and/or needle-shaped particles be avoided. The material must be completely insoluble in the propellent in order to avoid agglomerative sedimentation and clumping in the container and crystalliza- tion in the valve passages, which would cause almost certain pack failure. n The following represents a typical formulation. Talc (50-70 microns) .... 10 per cent by weight High boiling additive (to reduce loss of powder during spraying and increase • adherence to skin) .... 0.5 per cent by weight Propellent ........ 89.5 per cent by weight CONTAINERS The following materials can and, in fact, are used for the packing of aerosol products: aluminium, tinplate, glass, nylon and other plastic materials. Aluminium Aluminium aerosol containers are commonly used for cosmetic products when external appearance and not cost are the determining factors. The sizes of aluminium aerosol containers range from 1 oz. to 20 oz. nominal capacity. The smaller sizes are eminently suitable for handbag-size sun-tan lotions and insect repellents. Aluminium containers must be internally lacquered when the following alcohols are present in the product: ethanol, n-propanol, n-butanol. Products containing iso-propanol can be packed in internally plain aluminium even with trichloromonofluoromethane as a constituent of the propellent mixture provided the total moisture content within the container does not exceed 0.05 per cent. Some of the common constituents of shaving cream and shampoo can also cause corrosion of internally plain aluminium. In the United States less than 1 per cent of the aerosol products utilising metal containers are packed in aluminium. Tinplate Most materials can be packed satisfactorily in internally plain tinplate aerosol containers which, as mentioned previously, are based on the beer can (Fig. 3 above). Tinplate aerosols are available with nominal capacities of only 6 oz. and 12 oz. When using internally plain tinplate dispensers, products containing ethanol, such as hair lacquers, have a shelf-life of 12 to 15 months at 77 ø F., provided the moisture content of the alcoholic con,-

354 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS centrate is kept to a minimum and does not exceed 1.5 per cent. Tinplate cans can also be internally lacquered. Glass Glass aerosols made their d•but in the U.S.A. within the last three years, and there is very little doubt that they play a very important role in the packing of cosmetic aerosols, particularly with corrosive packs such as anti- perspirants and with perfumed packs where even the slightest traces of metal may be fatal. These containers may be protected with metallic, plastic or composite outers. Sometimes they are unfortunately sold without any outer protection, when they must be considered as very dangerous packs indeed and their use is already proscribed in some of the North American States. I am fully convinced of the dangerous aspects of these packs, and there is no exaggeration in Wells's recent description • of the shattering of uncoated glass aerosols, as I have myself witnessed. The bottles contained 50 per cent by weight of ethanol and 50 per cent by weight of dichlorotetrafluoro- ethane with an internal pressure of 0.9 ats. excess. On shattering, pieces of glass were hurled as far as 40 feet. It is often argued that filled beer bottles have internal pressures which exceed 40 p.s.i.g. and yet such bottles are not considered as dangerous. Comparison between these two packs is not really fair, for in a beer bottle we are dealing with a dissolved gas, only a limited amount of which is available at the time of breakage of the bottle, whereas in an aerosol all the liquid propellent changes almost instantaneously to a gas which, after all, is the very principle of aerosols. The tremendous increase in volume which will occur on breakage (1 g. mol. liquid = 22,400 c.c. gas) will therefore be realised. An American glass manufacturer TM has stated that fragments from a shattered glass aerosol were propelled in a horizontal plane only, seldom travelling upward more than approximately 18 inches from the point of impact. Even such qualifications do not lessen the dangers of these packs, because there is no guarantee that breakage will occur on the ground. Supposing such breakage occurs on top of the dressing- table ? That these dangers are fully appreciated is shown by the fact that the biggest contract packer in this country is refusing to pack uncoated glass aerosols that a leading U.K. glass bottle manufacturer will not supply uncoated glass aerosol containers and that my company will not supply valves if they are to be fitted to uncoated glass aerosols. MEANS OF PROTECTING GLASS AEROSOLS As has already been stated above, the protection of glass aerosols can be achieved in three ways. 1. Metallic outers. Aluminium extrusions are normally used for this purpose and the packs look very attractive. In the U.S.A. this type of pack