Incendivity of discharges from electrostatically charged plastics

Electrostatic charge densities of 1·1-2·3 nc cm^-2 can be generated over areas of polyethylene (polythene) sheet by rubbing. The nature and magnitude of electric discharges from the polyethylene sheet to an earthed electrode are a function of the relative humidity of the atmosphere and of the size of the electrode. The discharges change from corona to spark type if the radius of a hemispherical electrode is increased from 1 to 10 mm; with the latter-sized electrode the sparks can contain up to 0·23 μC of charge. Such sparks, which have an equivalent electrical energy of 0·67-0·92 mJ, can ignite flammable mixtures of coal gas, methane, acetone, methanol, toluene, cyclohexane and dioxane with air. For a fixed electrode system the incendivity of a spark is directly related to the electric charge in it.