350 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Fig. 1. Spray producing dispenser showing Continental valve open. vapour phase. The heavy liquid propellent forms a pool at the bottom of the container, and the product layer, usually aqueous, forms the middle layer intermediate between the liquid and gaseous propellent. In this type of pack the syphon tube does not reach to the bottom of the container, but is immersed only in the product. When the valve is operated, the propellent vapour in the headspace forces the product up the syphon tube and out of the valve a small amount of liquid propellent then vaporises to fill the increased headspace. Porous chips and similar promoters of vaporisation may be added to the pack. 8 (Fig. 2.) Three-phase systems present the only possibility of pressure packing aqueous products which are meant to be sprayed rather than to be ejected as foams. The use of three-phase systems is also advocated when it is desired to avoid contact between sensitive membranes and the propellent, as might be the case with burn dressings. As no propellent is sprayed, no cooling effect is experienced. In this connec- tion it might not be amiss to mention means of overcoming the cooling effect

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.