Degradation of Mechanical and Dielectric Properties of Flame-Retardant Ethylene Propylene Rubber by Thermal Aging

Thermal degradation was performed at 175, 200, and 250 °C from 100 to 2000 h on flame-retardant ethylene propylene rubber (FR-EPR) with the same compositions as those used in the insulation of a safety-related cable in nuclear power plants. For these samples, indenter modulus (IM), elongation at break, tensile strength, and complex permittivity were measured to clarify mechanical and dielectric properties. Fourier transform infrared absorption and differential scanning calorimetric spectra were also measured to determine structural changes. As a result, it was found that FR-EPR becomes hard by oxidation and crosslinking, and that it becomes brittle as the degradation becomes more severe. In addition, such changes in mechanical properties by the aging occur much earlier than the changes in dielectric properties. Regarding this, IM is a good indicator to examine the degradation degree of FR-EPR.