Enhanced DC Breakdown Strength of XLPE via Denser Cross-Linking Network Formation with Liquid Polybutadiene

Due to the increased voltage in high-voltage direct current (HVDC) cables, the insulation layer thickens, posing challenges in removing cross-linking byproducts, which influences the electrical properties of cross-linked polyethylene (XLPE). This research aimed to enhance DC breakdown properties while reducing the cross-linker dosage and maintaining the cross-linking degree. Liquid polybutadiene (LPB) is incorporated into the base resin, namely, low-density polyethylene (LDPE), and cross-linked at lower cross-linker level. The results show that after the addition of 1.5 phr LPB, the DC breakdown strength of XLPE increased by at least 15.13%. The heat elongation rate decreased from 138.61% to 35.00%, indicating denser cross-linking network formation. Infrared spectrum analysis indicated the creation of new cross-links by additional vinyl groups from LPB and the suppression of ineffective cross-linking reactions. Positron annihilation lifetime spectroscopy revealed a decrease in the average radius of free-volume holes from 0.322 to 0.319 nm at 30 °C, when the cross-linking density changed from 6284 to 5157 g/mol. Based on the free-volume breakdown theory, the elevated breakdown strength can be attributed to the smaller free-volume holes. This research presents a novel strategy for developing improved XLPE and offers a solution to compromised electrical properties affected by cross-linking byproducts.