Why does the dielectric strength (volts/mil) drop as the thickness of the Epoxy or urethane is increased?
The dielectric strength, which is the measure of the ability of a cured epoxy or polyurethane to withstand voltage, is directly related to the amount of impurities contained within the specimen being tested. As a rule, the thicker the specimen under test, the lower the volts/mil results will be because the thicker sample will contain more impurities per unit volume. Epoxy and urethane products containing fillers will exhibit lower volts/mil values because additional impurities are contained in the fillers. Similarly, unfilled epoxies contain less impurities and will yield higher volts/mil figures. The typical values for epoxies range from 425 volts per mil (0.125 in. thickness) to 1800 volts per mil (0.001 in. thickness). It must be considered that the volume and the amount of impurities within do not follow a direct relationship. For example, doubling the volume of the epoxy will not double the amount of impurities contained within the specimen. Although the volts per mil will slightl
