Improved dielectric properties of polypropylene insulation via crystal structure regulation using a rare earth nucleating agent

Abstract

The polypropylene (PP)-based composites have emerged as a promising alternative to conventional cross-linked polyethylene (XLPE) for high-voltage direct current (HVDC) cable insulation applications, with significant advantages such as excellent performance and green environmental protection. This study investigates the impact of rare earth β-nucleating agent WBG-II on the micro-structural and dielectric properties of PP composites, with additive concentrations systematically varied from 0.1% to 1.0% (by mass). Quantitative analysis revealed that nucleating agent incorporation induces pronounced micro-structural modifications in PP. Specifically, at 0.5 wt% additive loading, the β-crystal content and degree of crystallinity reached optimal values of 54.4% and 83.7%, respectively. Correspondingly, the breakdown field strength (E_b) exhibited a significant enhancement, achieving 267.17 kV/mm, a 50.06% increase compared to the untreated PP matrix. However, further increasing the nucleating agent content to 1.0 wt% resulted in a pronounced decline in both β-crystal fraction (38.27%) and overall crystallinity (80.7%), concomitant with deterioration in dielectric performance. Mechanistic analysis indicates that heterogeneous nucleation triggered by the nucleating agent enhances both crystallinity and β-crystal content, while also increasing the interfacial area between micro-crystals, thereby increasing the average trap density, increasing the probability of carrier capture, effectively hindering charge injection, and ultimately enhancing dielectric performance. Conversely, excessive nucleating agent concentration induces severe agglomeration, disrupting nucleation kinetics and thereby compromising both crystal quality and dielectric characteristics. This study establishes a theoretical framework for optimizing eco-friendly PP-based HVDC cable insulation through targeted crystal structure engineering.