312 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS + + + + + + + +++++ + Figure 1. Aerosol can charging. potpourri of electrical terms such as voltage, charge, capacitance, and energy. However, when more clearly understood, it is seen that energy is the chief factor that determines the ability of a spark discharge to ignite a flammable gas mixture. The amount of spark energy required for ignition reportedly depends upon the gas composition, electrode geometry, and other conditions (1), but for a butane/air mixture the minimum ignition energy has been indicated to be near 0.2 millijoules (2). While 0.2 millijoules is a credible value, further experimentation is warranted to confirm its relevance in the case of spark discharges from can surfaces in clouds of commercial aerosol products. II. DISCUSSION A. ELECTRICAL CAPACITOR RELATIONSHIPS An aerosol can may be considered as one electrode of an electrical capacitor, the other being earth ground or nearby grounded metal. Thus, a brief review of electrical capacitor relationships is helpful to understanding of the factors affecting the energy of an electrostatic charge. A capacitor is constructed by juxtaposing two conductors to permit attractive interaction by virtue of equal and opposite charges on their surfaces. The common parallel plate type of capacitor is illustrated in Figure 2. If a battery is connected across the capacitor as shown, electrons are withdrawn from the left hand plate and forced upon the right hand plate, thus, producing excess charges, + Q and -Q, on the inner surfaces of the plates. Charge, then, is defined as

ELECTROSTATIC CHARGE ON AEROSOL CANS 313 + + + ,[,_ Figure 2. Electrical capacitor relationships. the quantity of electrification, expressed in units of coulombs, resulting from a surplus or deficit of electrons. The coulombic charge may be either measured directly with a special electrometer (coulombmeter) or calculated by: Q (coulombs) = C (farads) x V (volts). Likewise, V, the voltage (electric pressure) and C, the capacitance (charge storage capability) can be measured with suitable instruments or calculated as: V_ Q c Q Normally, the energy of a charge is not directly measureable, but is calculated by Q2 E(joules) = 1/2 QV = 1/2 CV 2- 2C Where: Q = charge in coulombs, V = voltage in volts, C = capacitance in farads. Thus, to calculate the charge energy, two of the three parameters--charge, capaci- tance, and voltage--must be known. For further illustration, the charging of a capacitor by a voltage is roughly comparable to the filling of a pressure vessel by a source of gas at a given pressure. The capacitance determines the amount of electrical charge just as the volume determines the amount of gas that the filled vessel will contain. The capacitance of a parallel plate capacitor is directly proportional to the surface area of the plates and inversely proportional to the distance separating them. Likewise, the smaller an aerosol can, the smaller its capacitance and the closer to grounded metal, the greater the capacitance. It should be noted that the coulombic charge, once developed, is relatively constant, being independent of changes in the can environment if corona and conductive losses are prevented. Can capacitance, on the other hand, is condition-dependent and variable, and with it vary also the voltage and energy. The effect of can capacitance on the charge energy may be illustrated by visualizing two cans which differ significantly in size but which bear the same amount of coulombic charge. The charge on the smaller can of lower capacitance is necessarily more dense, and consequently possesses a higher potential energy, and will release