The Electric Current Conducted in Epoxy Resin and the Mechanism of its Dielectric Breakdown

Abstract

Epoxy resin has hitherto been extensively used as insulating material by virtue of its favorable properties both in its electric conducting and mechanical engineering functions and in its capacity for being cured at room temperature. In view of the fact, however, that the mechanism of its dielectric breakdown has not yet been made clear enough, experimentary researches have been conducted from various standpoints. Three types of epoxy resin were selected for specimens, bisphenoidal, novolac and cycloaliphatic, and measurement was made of their electric condition before their breakdown, and of the electric field in the breakdown, in order to ascertain the mechanism of dielectric breakdown of the material. Measurement was also made, on the other hand, of the γ-ray irradiated samples with respect to their electric current conducted in them, and to their dielectric breakdown, and study was made concerning how to determine the effective irradiation. In other words measurement was made of the electric current at the various degrees of voltage applied, and of the temperature. The temperature dependence in dielectric breakdown strength was also measured. The results obtained are summarized as follows.(1) The electric current conducted in the epoxy resin before its dielectric breakdown is the ionic current which is show by the formula exp (KE) in the high electric field.(2) There is slight variation in the property of the electric current e. g. its temperature dependence among the rest, dependent on the kind of the material, epoxy resin, and the curing agent, through which the current is conducted. Abrupt increase of current is generally observable at a point higher than transition temperature for glass. Decrease of current which is sometimes observable at temperature a little over 100°∼120°C is considered to be an hygroscopically affected incident. It is the commonest phenomenon with epoxy resin reinforced with curing agent of acid anhydrous type that it, being of smaller hygroscopicity than the same material with curing agent of amine type, rarely presents decrease in the electric current conducted in the material.(3) Irradiation promotes conductivity. This is considered to be due to the increase in density of charged particles caused by irradiation.(4) The voltage required for dielectric breakdown depends upon the thickness of the specimen, and upon the lapse of time required for the rise of voltage. The voltage decreases according to the rise of temperature. The mechanism of dielectric breakdown, though it is electronic when but a short time is required for the rise of voltage, is considered to be therefore impulsively thermal when a longer time than circ. 5×10-5sec is required for the rise of voltage.