Effect of High Relative Humidity on the Thermal Aging of Composite Epoxy Insulation Materials

In tropical environments, the aging of electrical insulation materials is susceptible to the high humidity of the environment. This work investigates the effect of high relative humidity (RH) on thermal aging of Diglycidyl Ether of Bisphenol A (DGEBA)/Anhydride/Wollastonite composite epoxy insulation material. The aging test at 95°C without humidity control finds that this material exhibited exceptional resistance to thermal aging. On the other hand, high relative humidity significantly accelerated thermal degradation of material via the adsorbed water molecules that attack the ester bonds of epoxy resin matrix, resulting in hydrolysis of the ester bonds. The mechanical strength of material decreased by about 43.4% after hygrothermal aging at 85% RH after 2018 hours under the same temperature. By curve fitting based on the relationship between material mechanical strength and aging times at 95°C and 85% RH, life expectancy at 95°C and 85% RH was 2135 hours. Meanwhile, the Fourier transform infrared (FTIR) absorption peak of ester carbonyl bond (C=O) on the material surface exhibited a correlation with the degradation in the mechanical strength. At the end-of-lifetime point (when the mechanical strength decreased to 50%), there was a 78.6% decrease in the ester bond absorption peak due to hydrolysis and thermal aging. And the glass transition temperature of material decreased from 116 to 105°C, the heat capacity of material increased by 10%, the decomposition temperature of material decreased from 412 to 404°C. As the mechanical test is destructive, this work proposes an easier and non-destructive method to identify the extent of aging by investigating the infrared ester C=O absorption peak of DGEBA/Anhydride/Wollastonite composite epoxy material using FTIR spectrometer.