TECHNICAL ASPECTS OF AEROSOL PACKAGING 351 experienced when spraying directly on to the skin. This can be achieved by choosing propellents with low boiling points which will evaporate before the spray reaches the skin surface, and there will then be no cooling effect greater than when the particular product is applied in the conventional manner. Mina's suggestions for the use of high boiling propellents 9 are not at all practical, because trichlorotrifluoroethane with a boiling point of 117.9 ø F. does obviously not exert any significant pressure at normal room temperature. The disadvantage of three-phase systems is the poor, or even lack of, atomisation of the resultant spray. This is clearly demonstrated in the phase diagrams for low-pressure propellent-alcohol systems prepared by Reed. •o Sometimes special valve actuators are used to overcome this defect. Fig. 2. [U.S.P. 2,705,661. Three-phase system incorporating vaporisation promoters. 3--Liquid propellent layer. 6---Product layer. 7--Bubbles of propellent vapour. 8--Propellent vapour. 10--Porous chips.

352 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS FOAM PACKS Aqueous products such as hand cream, shampoo, shaving cream and tooth paste are examples of aerosol foams. A special foam head must be fitted to the valve (Fig. 3) and the presence of the syphon tube is optional. If the latter is omitted then the container is inverted during use. In foam packs the propellent does not usually amount to more than 10 per cent, andit is emulsified with the product. When the valve is operated the emulsion is forced out of the nozzle, whereupon the particles of liquid propellent change into gas within the emulsion, whipping the whole into a thick foam of innumerable particles. By varying the amount and type of propellent one• can obtain practically any type of foam quality desired. POWDER PACKS Aerosol powders consist of dispersions of the powdered product in the propellent, and in the not too distant future it may be possible to market REAM SPOUT MOUNTING DOMED GROMMET STEM ASKET ,I SPRING. • : 11 i• VALVE • ,, HOUSING ,. •[ /l :, SYPHON "" (optional,) ','" ,,,, I? iii I //, je •1 I BODY ',' ' ,? END Fig. 3. "Beer-can" type dispenser showing closed Precision valve with cream spout for foam dispensing.