228 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS non-aerated form it is necessary to employ a propellant which, to all intent and purposes, is insoluble in the product. The only liquefied propellant which appears to fulfil this requirement is octafluorocyclobutane (Freon C318) •, which is, however, not yet approved by the U.S. Food and Drug Administration for use in edible products. Nitrogen has been found to be suitable •,5,6,7 for dispensing toothpaste in non-aerated form and, although it is generally referred to as an "insoluble" propellant, one must •ot lose sight of the fact that even the "insoluble" propellants are often soluble to a small degree. For example, the solubility coefficient of nitrogen in water, at a pressure of one atmosphere and at a temperature of 25 ø C. is 0.0143 •5 and in cottonseed oil, at 24-25 ø C., it is 0.061 •5 The marketing of toothpaste and similar products, pressurized in this manner, did not become a practical proposition until a valve capable of permitting the flow of viscous products became commercially available •. Quality Factors A number of factors must be determined for each product in order to ensure the ultimate success of the marketed package. Amongst these are viscosity, ullage and initial pressure. Viscosity is undoubtedly first and foremost, but is at the same time closely linked with the other two mentioned factors. In spite of any thoughts to the contrary, the viscosity of the product must be decreased when changing from the tube to the pressurized dispenser. In general terms, the viscosity of the product should be such that it is too thin to be squeezed from a tube and too thick to be poured from a bottle. With a toothpaste, for instance, it is naturally necessary to ensure that the paste does not run off the brush. The viscosity of the product has a direct bearing on the amount of residue left in the dispenser. If the viscosity is too high, then too much paste may adhere to the walls of the dispenser. The flow from the walls to the lower end of the dip tube will also be sluggish and should the dispenser have been stored on its side or upside down prior to use, then propellant rather than product might be expelled. This phenomenon is often referred to as cavitation. It has been suggested •6 that the interior of the dispenser might be coated with a lubricant in order to avoid cavitation. Should this be resorted to, then it is necessary to ensure that the lubricant is unaffected by any of the ingredients of the product. The suggestion of using flanged dip tubes to avoid cavitation does not appear to have been successful, as experience has shown this phenomenon to be rather a function of the concentrate formulation itself. The drop in pressure as the dispenser is emptied, as already referred to, will effect the amount of product residue. Ullage and initial gas pressure must therefore be very carefully correlated. The pressure required to expel

COMPRESSED GASES AS PROPELLANTS FOR COSMETIC PRODUCTS 229 the product residues when the dispenser is nearly empty can be determined experimentally and is also related to the product viscosity. The initial pressure is usually dictated by the strength of the dispenser, and by employ- ing Boyle's Law it is possible to calculate the ullage necessary. It may, however, be found that the ullage is of such magnitude that it could be preferable, particularly if the product viscosity cannot be adjusted, to incur somewhat larger product residues in order to decrease the ullage to reasonable proportions. So far as one can judge, it would appear to be United States practice to pack toothpastes into 6 oz. dispensers with an ullage of 30-33« per cent and an initial pressure, at 70 ø F, of 95-105 p.s.i.g. The pressure characteristics of nitrogen and similar gases are such that for every 10 ø F increase in temperature the pressure increases by only 2 p.s.i.g. It is therefore possible to utilize the conventional low-pressure dispensers, though at the moment only seamless containers* are employed. By reference to the gas laws it will be realized that only a very small quantity of gas is present in a 6 oz., or even a 20 oz., dispenser. Together with the low cost of propellants, such as nitrogen, this is obviously of great economic importance, but at the same time it creates almost the greatest danger. Accidental discharge of the propellant, or loss of the propellant due to mechanical faults, will lead to partial and possibly total inactivation of the pack. In order to avoid the former, the viscosity of the product must be right and it is necessary to ensure that the directions for use are followed by the consumer. Choice of the right container and valve together with proper quality control in the course of the filling operation can usually overcome loss of the propellant due to mechanical faults. Nitrogen and argon do not induce increased corrosion in the presence of water, as is the case with certain chlorofluorohydrocarbons. The chilling effect on the skin is also avoided. Oxygen, on the other hand, is almost certain to be present even if only in traces, and may well have an adverse effect on the perfume or flavour. Care must also be taken to ensure that the product viscosity does not alter with time and that there is no significant increase in the solubility of the propellant in the product on prolonged storage, which could affect the emission characteristics. These are but a few of the factors which will indicate that all pressurized packs, whatever the nature of the propellant, need careful pre-marketing storage tests. [Received: 14th October 111581 *Seamless Container •: One-piece body formed by pressing or drawing.