FORMULATING AEROSOIJS TO OBTAIN SPRAY PATTERNS 295 firms the existence of such condi- tions. At - 10øF., accidentally broken bottles containing propellent 1l, 12 (75 per cent 11-25 per cent 12) were projecting glass fragments further than those containing pro- pellent 114. During the drop test at 100øF., however, breakage of bottles with propellent 114 was accompanied by a greater detona- tion and a farther reaching frag- mentation of glass than in the cases of mixed propellents 11, 12 (75 per cent 11-25 per cent 12). It should LIqIJID ?I•lPtI•?IIRS 'F Figure 6.- Flashing of hydrocarbon propel- lents. be stressed again that when an aerosol bottle breaks, not the whole amount of liquid propellent vaporizes instantaneously, but only a part of it. The amount of instantaneous flashing propellent is higher, the higher its specific heat, the lower its latent heat of vaporization, the lower its boiling point and the higher the initial temperature of the aerosol contents. 4. The e•ect of solubility on flashing of aerosols In a two-phase aerosol system (liquid and vapor phase) where the liquid propellent is completely miscible with the concentrate, the latter usually retards the flashing. This can be proven by the experiment which shows that a 100 per cent pure propellent will deliver a finer spray pattern than compounded with other higher boiling ingredients. Also, by increasing the ratio of propellent to concentrate, a finer spray will be obtained. In a three-phase aerosol system, (two liquid phases and one vapor phase), where the liquid propellent and concentrate form separate layers, the concentrate does not influence significantly the flashing of the propellent. The propellent behaves here very much as if it would exist by itself, and its amount of flashing is greater than if it were mixed with the concentrate. This fact explains why at drop testing, bottles with three-phase formulation broke more forcefully than those with two-phase formulations, containing the same amount and type of propellent. 5. The e•ect of (a) viscosity, (b) density and (c) surface tension (a) The average particle size is always increasing with an increase in viscosity of a product. Too high viscosity may cause a streamy jet instead of a spray or may even clog the valve. It is difficult to predict the viscosity of solutions consisting of two or more constituents even with known viscosities. There exists a critical viscosity limit still permitting the delivery of an acceptable spray.

296 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (b) The average drop size will change with variations in liquid density. Here it is important to differentiate between density of concentrates and propellents. When the density of the concentrate decreases, the drops become larger however, when the density of the propellent decreases, the drops become smaller. This is due to the fact that the volumetric flow through valve orifice is constant. Thus, the particles of concentrate are surrounded with a larger volume of propellent when the density of the propellent is low, and therefore, the atomization is facilitated. For this reason,when formulating aerosols with Genetron 101 instead ofpropellent 11 and 12, a lower percentage by weight of Genetron 101 is required for ob- taining the same spray pattern. (c) The average drop size becomes larger when the surface tension increases. Surface tension is related to the difference between liquid density and vapor density. Because surface tension is also a measure of energy to produce a new surface, it has a definite influence on spray forma- tion. Propellents and solvents have usually low surface tension. Low surface tension aids dispersion, because less energy is required to separate the particles. Certain additives may be used in formulation to decrease their surface tension and to obtain a better spray pattern. This method is widely used in aerosol emulsions. Fo^M FORM^TtON The above properties of aerosol sprays have also an effect upon foam formation. In aerosol foam products, like shaving creams and shampoos, the propellent is present in the container as a separate layer on the top or the bottom, and is also emulsified in the concentrate. Only a small part of propellent-concentrate mixture is soluble in each other. The foam quality depends upon the type and amount of concentrate and propellent. Generally with a given concentrate, high boiling propellents produce a wet, slowly growing foam, while low boiling propellents deliver a stiff, dry and elastic foam. Higher amounts of propellent in relation to the same con- centrate will result in stiffer and drier foams. Too wet foams are impractical in use, too dry and elastic foams are lacking in wetting and spreading properties and are obviously poor in quality. It is absolutely necessary to balance properly the aerosol foam formulation in order to obtain the best possible result. The knowledge of interaction of physical and chemical properties makes the development of new products easier for the cosmetic chemist. Thus, should he choose to replace the halogenated propellents in a foam aerosol by pure hydrocarbon propellents (such as butane, isobutane, propane or their mixtures) the theoretical evaluation of the necessary adjustments in the formulation will save him a lot of time. First of all, mixture of hydrocarbon propellents delivering nearly the same pressure as the halo-