SOME ASPECTS OF THE SAFETY OF AEROSOL CONTAINERS 375 the maximum likely temperature, and any inherent hazard of the residual concentrate if the pack were inadequately pressurized. Mixed propellants It has been shown n that up to 25% by volume of propellant 12 may be replaced by propane or butane before a flammable mixture results this constitutes a saving in both cost and weight to the formulator. Reed developed a medium pressure blend of propellants 12 and 11 and isobutane (45:45:10) which is said to be non-flammable 2 and, which was available in the U.S.A. in 1957 la. A blend using larger amounts of the somewhat less volatile nbutane (proportions 20:50:30) was examined by us in 1954, and recommended to be labelled as for "butane". •Iore recently, we have noted that the presence in three products of 50% of a methylene chloride/trichlor- ethylene mixture failed to suppress the flammability due to 12% propane and 30% aromatic solvents. It was found that this mixture frequently caused failure of "0" seam containers stored at 55ø the maker subsequently submitted a less highly pressurized, non-flammable propellant system for his product. Chloroalkanes have also been used as pressure diluent and complementary solvent in 4-phase systems. We have exanfined a number of household product dispensers, fitted with vapour phase valves, in which roughly equal amounts of butane, water and methylene chloride are employed. The spray of these formulations is not flammable, whilst in the closed beaker test, the vigour with which the vapour will burn depends largely upon the relative proportions of the three components and also whether the dispensers are shaken before being discharged. Current research in the United States •2 has developed a high pressure blend containing dimethyl ether (15% by weight of liquid) and propellant 12. Having examined various proportions of these two gases in an apparatus similar to that employed by the U.S. Bureau of Mines, Scott et a112 claim that a vapour mixture of composition 30:70 by volume (i.e. 15'6•o by weight of liquid) is just below the lower explosive limit for all concentrations of this mixture in air. It is further stated that fractionation during slow leakage, giving rise to flammable "tail fractions" enriched in dimethyl ether (bp. --24ø), can be avoided by replacement of propel!ant 12 by a 90:10 mixture of 12 and 11. For comparison, the vapour pressures of propel!ant 12 and dimethyl ether at 21 ø are 5.7 and 5.1 arm respectively. It is not expected that this prope!lant blend will be used in U.K. production in the near future. Scott has also described a blend containing vinyl chloride 14 and investigated 15 high, medium and low pressure blends using the hydrocarbons propane, iso- and nbutane respectively. It is questionable whether any (commerci- ally) significant mixture of propane and propel!ant 12 is likely to be non- flammable. The maximum tolerable amount of isobutane for a non-

376 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS flammable medium pressure blend was found to be 14% (by weight of liquids) fractionation lowers this value to 12%. Reed •a, it has already been noted, recommended 10% isobutane when the diluents are a 1:1 mixture of propellants 12 and 11. For a low pressure blend, Scott •5 examined nbutane/ propellant 114 mixtures and concluded that the butane content must not exceed 8-5% by weight, which, allowing for fractionation reduces to under 7% in commercial practice, the resultant pressure would be too low. A blend used to pressurize perfumes contains nbutane and propellants 12 and 114 in the proportions 11:9:80. PROTECTIVE CAP All specifications require that the valve actuator button shall be adequately protected from accidental discharge by a cap or cover. Sweden cite an alternative threaded knob whilst for pyrethrin insecticides, the CPO 5 specify a cap of polyethylene conforming with Ministry of Defence Specifi- cation DEF 101. A cap may be considered unsatisfactory because it is too easily deformed or dislodged or if the clearance inside the cap, above the valve button, is too small. Laboratory examination of the cap reveals the dimensions and apparent construction, and a subjective estimate is made of the ease of horizontal dislodgement under both a steady applied force and short, abrupt blows. The internal clearance above the button is measured and then a series of increasing loads is applied vertically to the cap, up to a maximum of 200 lb or until the valve is discharged. Similar tests are made on containers that have cooled following storage at 55 ø . The equivalent pressure required to operate the valve is calculated from the area of the cap to which the critical load is applied. The clearance will depend upon the type of valve, length and design of button and height of the crimped shoulder of the cup above the domed end of the case. A wide variety of protective caps are available. Sturdy cylindrical tinplate caps, of 33 mm external diameter, fit snugly over the cup of the valve insert and normally effectively resist moderate horizontal and vertical pressure. These caps are usually about 19 mm tall, with a slightly domed end adding another 1 mm to the height occasionally, poor internal clearance above the valve button renders the cap unsatisfactory. A wider (50 ram) cap, fabricated from 0.4 mm tinplate, which fits over the full shoulder of the dispenser, has been found to be too flexible and the valve button is inadequately protected on objection, it was stated that this cap is only used for the "home market". A special applicator, with modified cap has been fitted to the dispensers of wood-worm preparations a miniature tinplate cap (diameter about 24 ram) fits over the button but inside the valve cup, whilst through a slot in the cap protrudes a polyethylene delivery tube connected by a brass adapter to the valve and terminating in a brass jet