Soft and hard epoxy resins used as insulating sealants in nuclear power plants were aged simultaneously by heat and gamma rays at 80, 90, and 100°C, and the changes in their various properties induced as a result of their degradation were measured. The thermal, mechanical, dielectric, and spectroscopic properties hardly change in the hard epoxy resin, whereas they change drastically in the soft epoxy resin, indicating that the latter resin becomes hard and then brittle. This big difference in mechanical property between the two resins is attributable to a significantly low glass transition temperature of −30°C in the soft resin compared to that of 120°C in the hard resin. Although the hard epoxy resin with the high glass transition temperature exhibits thermal and mechanical properties insensitive to the aging conditions, its complex permittivity at temperatures higher than 140 °C shows an evident increase in accord with the progress of degradation. This phenomenon can be an indicator of the degradation of this resin. The effects of degradation on the absorption spectra in a THz frequency range are also examined.
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