Effects of Structural Changes on Mechanical Properties of XLPE With Different Cross-Linking Modes During Thermal-Oxidative Aging: A Theoretical Analysis

The thermal oxidative aging of cross-linked polyethylene (XLPE) during cable operation is a primary factor leading to changes in its mechanical properties. Notably, these properties exhibit an initial increase followed by a decrease as aging progresses. In this study, the molecular models of silane-XLPE (Si-XLPE) and peroxide-XLPE (p-XLPE) were constructed to analyze their thermal-oxidative aging processes using ReaxFF simulations. The research focused on understanding how structural changes in XLPE during aging impact its mechanical properties from the perspective of intermolecular forces. The simulation results reveal that Si-XLPE initially degrades from the silane cross-linking bonds, eventually forming small hydrocarbon molecules. In contrast, p-XLPE experiences the formation of C = O and C = C bonds during its structural degradation. As the degree of aging increases, both the density and cohesive energy density of XLPE initially rise and then decline, while the free volume fraction exhibits a decrease followed by an increase. This indicates that the early stages of aging enhance the intermolecular forces within XLPE, which explains the observed pattern in mechanical property changes. Furthermore, the study finds that Si-XLPE consistently exhibits stronger intermolecular forces than p-XLPE across all stages of aging, contributing to its superior mechanical properties. These simulation results effectively explain the mechanical behavior observed in the experimental studies of XLPE. This article is expected to provide some new perspectives and ideas on constructing the structure-activity relationship between the microstructure and macroscopic properties of XLPE.