320 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS contacting with a probe from a power supply. The ungrounded lead from a Keithley electrometer was then contacted with the can to measure the charge (coulombs). The capacitances of the suspended cans were then calculated from C -- Q/V. Average capacitance values from several determinations were 4.0, 4.5, and 5.5 pf for 6-, 8-, and 16-oz cans, respectively. Further calculation, by Q = •, shows that for a can capacitance of 4 pf the potentially incendive 0.2-millijoule energy level would be reached at a charge of 4 x 10 -8 coulombs. However, it is again cautioned that the relevance of this energy value to that needed to ignite a vapor of formulated aerosol product has not been confirmed and additional work in this regard is recommended. D. CHARGE DEVELOPMENT WITH COMMERCIAL PRODUCTS Table I gives data from static electrification studies in the Phillips Petroleum Company laboratories using the direct method of charge measurement in can-puncture tests. The results show that the amount of charge which developed depended both upon the presence and the type of a powder component in the product. Thus, the charge that developed was insignificant when the can contained propellant only. When 11% aluminum chlorohydrate was included with the propellant, the static charge was larger but with energy still well below 0.2 millijoules. However, when 11% talc was incorporated, a charge of high energy, 49 millijoules, developed. This charge would have reached a calculated voltage of 157,000 V on the 4 pf capacitance of the can if it had not been prevented by the relatively high capacitance input of the coulombmeter. The data also show the charge magnitude that various commercial products have been observed to generate on the cans after puncture. Additionally the data show that static quenching agents offer hope for reducing the charge. lll. CONCLUSION In conclusion, the energy of an electrostatic charge largely determines its potential incendivity. Calculation of the energy requires measurement of two out of three electrical parameters--charge, voltage, and capacitance. Charge and minimum can capacitance are the preferred parameters for measurement in order to calculate the Table I Charge Measurement on Punctured Aerosol Cans of Dry Powder-Containing Products Charge Voltage Energy Product (coulom bs) (volts) (milli joules) A46 © Propellant (no powder) A46 + 11% Talc A46 + 11% Aluminum Chlorohydrate Antiperspirant A Antiperspirant A + Antistat (0.6%) Antiperspirant B Antiperspirant B + Antistat (0.5%) Cleaner A Cleaner B Christmas Decorating Snow Antifungal Powder Spray 1.2 x 10 -9 300 .0002 6.3 x 10 -7 157,500 4900 1.9 x 10 -8 4,750 .05 6.6 x 10 -7 163,125 53.4 9.1 x 10 -8 22,750 1.0 1.8 x 10 -7 45,000 4.2 4.6 x 10 -8 11,670 0.27 2.7 x 10 -6 675,000 912 1.6 x 10 -7 40,000 3.2 4.4 x 10 -7 110,000 26.6 2.9 x 10 -7 72,500 10.5

ELECTROSTATIC CHARGE ON AEROSOL CANS 321 greatest energy that a charge on an aerosol can will attain by the equation E = Q2/2C. Of the methods under consideration, the direct method of charge measurement with a Keithley or other high-quality coulombmeter seems preferable for simplicity, preci- sion, and inter-laboratory accuracy. The nominal minimum capacitance of aerosol cans was found to be about 4-5.5 pf comparative experiments by others are encouraged. Using a can capacitance value of 4 pf, the potentially incendive 0.2 millijoule-energy level is reached at a charge of 4 x 10 -8 coulombs. Charge energies greatly exceeding 0.2 are frequently observed to develop on punctured cans of commercial aerosol products in laboratory experiments. Incorporation of optimum amounts of specific antistatic agents offers promise for reducing the charge to non-incendive levels. IV. ACKNOWLEDGMENT The assistance of E. Ann Claytot with experimental procedures discussed herein is gratefully acknowledged. V. REFERENCES (1) Benard Lewis and Guenther Von Elbe, Combustion, Flames, and Explosions of Gases, Academic Press Inc., New York, (1961) pp. 325-346. (2) J. M. C. Roberts and John F. Hughes, IEEE Transactions on Industry Applications, Vol. 1A-15, No. 1, 104-108, 0anuary/February 1979). (3) R. E. Reusser, M. T. O'Shaughnessy and R. P. Williams, "Measuring In-Plant Electrostatic Charge," AerosolAge, 24 (3), 17-23 (1979). (4) F. G. Eichel, "Electrostatics," Chemical Engineering, 153-167 (March 13, 1967). (5) J. R. Greaves and B. Makin, "A Grid Collection Method for the Measurement of Electrostatic Charge in Aerosol Systems," Aerosol Age, 25 (2), 18 (February, 1980).