160 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS are obtainable in either plastic coated style or uncoated style. Uncoated glass aerosol bottles have been given silicone treatments to improve their abrasion resistance. The Chemical Specialties Manufacturers Association has set up a glass aerosol committee for the purpose of setting up standards for testing glass aerosol products. Standard methods will be set up for (1) pressure determination, (2) drop testing and (3) pre-testing. The recom- mended maximum pressure for the plastic coated glass aerosol bottle is 25 lb./sq.in. gauge at 70øF. and for the uncoated aerosol bottle, 15 lb./sq.in. gauge at 70øF. There are currently available two plastic containers for aerosol products. One of these containers is made of Nylon and is available in 2 oz. and 4 oz. sizes. Although this container is suitable for many nonaqueous products, tests indicate that it is permeable to water. Another plastic container currently available in a 11/2 oz. size is made of reelamine-formaldehyde. Although it appears this container is impermeable to water, it has not as yet undergone adequate use-testing. Plastic as well as glass containers offer esthetic advantages as a package and are quite useful where corrosion of metal containers is a problem. It is perhaps the present aerosol valve which has made the growth of the aerosol industry possible. Only as a result of the development of such plastics as Nylon, polyethylene and Teflon has it been possible to design and economically manufacture an aerosol valve which can meet today's rigid specifications. Any engineer presented with this problem as recently as 10 years ago would have deemed it impossible. Yet today there are several spray and foam valve manufacturers. Valves are made with vari- ous metals, plastics and elastomers. The component parts of a valve con- sist of a izup, a housing, a dip tube, one or more gaskets, a spring and a button. In one instance a diaphragm is used in place of a spring. The dip tube is nearly always made of polyethylene. The cups are usually tin plated steel. Other component parts are made of Nylon, Teflon, brass, nickel plated steel and nickel plated copper. Springs are usually made of steel and stainless steel, and gaskets are made of synthetic and natural rubber. The buttons in most instances are made of polyethylene, although they may be made of other plastic materials. Many sizes of internal ori- fices as well as button orifices are available. They range from about 0.013 to 0.040 inches, although special orifice openings under and above these values have been made. There are two ways to check the valve for use with a particular product. One is to make up several samples with the product in question and then check the spray delivery rate after aging at room and elevated temperatures. The other method is to immerse the various component parts of the valve in a pressure tube with the propellent and the formulation and then check for corrosion of the metal parts and swelling or shrinkage of the elastomeric and plastic parts. This latter

FORMULATING FOR PRESSURE 161 method is desirable since it can be determined which of the component parts of the valve are unsuitable. In some instances it has been found that cor- rosion has taken place only at the spring, in which case it is necessary to make a change only in this part. Important developments other than valves which have made the aerosol industry possible are improvements in electroplating of steel plate, improve- ments in can manufacture and coating of cans. It is needless to say, of course, that without the chlorofiuoroethanes and methanes there could be no aerosol products as we know them today. The research and develop- ment that went into all of these things probably ran into many millions of dollars, yet I am sure we would all agree in looking at the results that these expenditures were wise. With any aerosol formulation that is to be packaged in a metal container, tinplate or aluminum, there is always the problem of corrosion to consider. The problem of corrosion, as previously mentioned, is aggravated by the use of a propellent which will hydrolyze in the presence of water. Of the nonhydrogen containing propellents, Freon 11 and Genetron 11 are the most susceptible to hydrolysis. The hydrogen containing propellents are more susceptible to alkaline hydrolysis than the completely fiuorinated propellents (5). The nonfiuorinated compounds such as methyl chloride and methylene chloride undergo hydrolysis most readily. Their use in the presence of water, therefore, will cause hydrochloric acid formation. Corrosion of the container or any of the metal component parts of the valve such as the spring or cup is important because of the possibility of the following: 1. Change or loss of perfume odor. 2. Clogging of the valve by corrosion particles. 3. Change in color of product. 4. Perforation of the container. One of the first indications of corrosion in hair lacquer products has been the development of a sour metallic odor in the product. Many times this odor change could be detected before corrosion would be visible to the naked eye. Valve problems have often been caused by rust formation or precipita- tion in the container. With the very small orifice sizes used, it does not require the formation of much sedimentation before clogging occurs. The blocking by corrosion products of the internal orifices makes the container completely useless. Whereas it is possible to reclaim an aerosol container in which the button orifice is plugged by the expedient use of a straight pin, such a simple procedure is not possible where the internal orifice is closed. The development or change of color of an aerosol product because of cor- rosion is not desirable because of possible discoloration of hair or skin in