316 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS constitute a great monetary loss, but will also adversely affect public acceptance of this new type of packaging which the author sin- cerely feels has great market po- tentialities and consumer ad- vantages. Low-pressure aeros½l consists of a mixture of an active ingredient and a propellant in a sealed con- tainer. This container has a spe- cially designed valve to dispense the product. Part of the propellant remains in solutien as a liquid and part changes to a gas and fills the space above the liquid portion. The pressure exerted by this gaseous phase forces the solution out of the dip tube at the bottom of the can, up through the discharge valve. At the opening of the valve the propellant instantly changes into a gas thereby dispersing the con- centrate into small particles. When the proportion of propellant is high, we produce a very fine, rela- tively dry, spray as characterized by the space aerosols. We can produce a wet spray by using less propellant as characterized by the paints and plastic coatings. The foam type products are partial emulsions of the propellant and active ingredient. When the valve is opened, the liquid propellant in the emulsion changes to a gas to give the foam effect. The more propellant used, the drier the foam. In considering stability testing of aerosols it is well to know some- thing about the production problems in filling these products. It is even more important to take into consideration the production prob- lems involved in aerosol products as related to original formulations than with the usual cosmetic. In the spray type of aerosol, the filling operation takes place in two steps--one in which the concen- trate or active ingredient is meas- ured accurately into the container, and second in which the propellant is added to complete the contents. The amount of material used as the concentrate may vary from 1 to 65 per cent of the formulation. Since the propellants are liquids boiling far below room temperature, it is important that this concen- trate be pre-cooled to temperatures that will permit accurate addition of the propellants with a minimum loss. In order to accomplish this, extensive refrigeration equipment is required. Basically it consists of pumping the concentrate through a cooler into a refrigerated filling tank. A constant head of material is maintained in the filling tank by a series of automatic valves. The filling nozzle on the tank is acti- vated by a microswitch set off by the container as it passes the filling nozzle. The microswitch in turn controls an electronic interval timer which holds the nozzle valve open for sufficient time to allow the required amount of concentrate to flow into the can. With proper maintenance, an accuracy of plus or minus 2 per cent can be maintained. At this point in the operation it is necessary to make constant check weights to insure the accuracy of the concentrate fill. After passing

STABILITY TESTING OF LOW-PRESSURE AEROSOLS 317 the filling head the container with the cool concentrate passes under the propellant filling head, which operates in the same manner, ex- cept that two or more filling nozzles are uses to introduce the pre-cooled propellant. Again check weights are maintained to insure the accu- racy of the completed contents. Due to the very low temperatures encountered at the nozzles, moisture condensation presents a real prob- lem. It is important that moisture be eliminated from these cans since the propellants hydrolyze and form corrosive acids. It is therefore necessary to make ample provision for the removal of this accumu- lation or prevent its formation by the use of heated dry air or air- conditioned rooms surrounding the filling heads. At this point the can and contents are at a temperature usually below 0øF., and there is little evidence of boiling or gas loss. The containers then pass through to the capper and are seamed. After seaming, the cans are passed through a series of hot water baths. The temperature of these baths is adjusted so that the contents of the can is heated to a temperature of 130 ø F. In these baths, leakers, lights, overfilled cans, or defective cans can be observed and removed. The cans are. then dried, passed through an automatic labeler and cartoned. The above operations are basic for the standard type of spray aerosol. The water base or foam products such as shampoo and shaving cream are handled quite differently since the amount of propellant used is small in comparison to the spray type dispenser. In this case the product is introduced in liquid form at room temperature or above. The valve is attached, crimped into position, and the propellant intro- duced through the valve as a liquid using a special type pressure piston filler. Check weighings after both filling procedures are again main- tained to assure ,the accuracy of the fill and the cans are sent through the water bath for testing. The first step in producing a successful aerosol is to carefully check the physical compatibility of the concentrate with the pro- pellant. This is particularly im- portant with the spray type prod- ucts since a homogenous mixture is absolutely necessary for the proper functioning of the package. With the foam type products it is not absolutely necessary but a more desirable product is obtained if we have complete compatibility. A preliminary compatibility test is made at room temperature using Freon 11. This material boils at approximately 24øC. and can therefore be handled conveniently as a liquid at room temperatures. The concentrate is placed in a glass-stoppered graduated cylinder. Freon 11 is then'added in small increments, the stopper replaced, and the mixture shaken. In this way we can determine the complete solubility range of the material under investigation in the pro- pellant. In many instances we may have separation in two or more