High temperature vulcanised silicone rubber (SR) has excellent hydrophobicity and thus is widely applied both in indoor and outdoor insulation. According to the analysis of different experimental ageing tests and field investigation of SR, temperature and humidity are considered to be the fundamental factors during degradation. Hygrothermal ageing was proposed in this study to simulate the chalking state of the SR, finally leading to the irreversible degradation far below the thermal decomposition temperature. The variation of morphology, hardness and polydimethylsiloxane (PDMS) content was quantified during the yearlong hygrothermal ageing test. It is implied that the gradual process of whitening and hardening are accompanied by the decline of the PDMS content. Finally, ageing delamination in SR and its degradation mechanism were proposed.

Aramid fiber (AF)/epoxy composites are often used as mechanical and insulating materials for ultra-high voltage gas-insulated switchgear (GIS) insulation pull rods. However, the product performance is believed to be restricted by the poor adhesion between AF and the resin matrix, affecting the operation reliability of GIS. In this work, a cascade process was applied to modify the AF surface to improve interfacial behavior. The AF was first treated with air dielectric barrier discharge at atmospheric pressure to obtain a rough surface and then reacted with toluene-2,4-diisocyanate (TDI) to introduce polar functional groups onto the surface. The plasma discharge power and treatment time were investigated as major parameters, and the aging effect was studied. The modified AF was characterized by scanning electron microscope, Fourier transform infrared spectrometer, x-ray photoelectron spectroscopy, atomic force microscope, and filament yarn tensile test. The interfacial behavior of the AF/epoxy composite was analyzed by interfacial shear strength (IFSS) test of micro-bond specimen and interlaminar shear strength (ILSS) test of Naval Ordnance Laboratory rings. Experimental results showed that polar functional groups were introduced onto the AF surface. The IFSS and ILSS of AF/epoxy composite were increased by 32.8% and 20.2% at most, respectively, and the tensile strength of the AF was mostly preserved. Moreover, with the increase in plasma discharge power, the interfacial strength of the modified AF/epoxy interface increased firstly and then decreased. The AF modified by TDI retained a relatively good modification effect after aging for some time. This study presents a cascade process for the AF surface modification with simplicity, effectiveness, and resource-saving, which is suitable for industrial applications.

This study successfully improved the heat resistance of epoxy resin by optimizing the ratio of bisphenol A-type epoxy resin matrix and methyl tetrahydrophthalic anhydride curing agent, and using modified imidazole accelerator instead of amine curing agent. Infrared spectroscopy (FTIR) and thermogravimetric (TG) analysis have confirmed that high heat-resistant epoxy resin has higher thermal stability, and its thermal decomposition initiation temperature is significantly increased, making it suitable for high-temperature operating environments. In addition, high heat-resistant epoxy resin exhibits significant optimization in dielectric properties, with reduced dielectric constant, dielectric loss, and conductivity at high temperatures, demonstrating dielectric properties more suitable for high-temperature operating environments. By using the Kramers Kronig relationship, the dielectric relaxation characteristics of high heat-resistant epoxy resin were further revealed.