Factors of the Breakdown Performance Discrepancy Between Polypropylene Cable Insulation and Material

The recyclable polypropylene (PP) cable insulation has attracted increasing attention for its environmental benefits and excellent performance. However, compared with the corresponding PP material, the breakdown strength of PP cable insulation after extrusion is often reduced. To investigate the factors responsible for this "material-to-cable"discrepancy, this study compares the AC breakdown properties of pressed samples (PPs), ring-cut insulation films (PPc), and heat-treated PPc (H-PPc). Phase morphology and crystalline structure were characterized using SEM and XRD, respectively. The results show that PPc exhibits an overall reduction in AC breakdown strength with pronounced radial variation across the insulation thickness, while heat treatment can achieve partial recovery. Structural characterizations indicate that extrusion leads to enlarged rubber-phase size with a more heterogeneous distribution, reduced crystallinity, and altered crystal-phase fractions. Heat treatment promotes crystal rearrangement and refinement, which mitigates interfacial weak regions and thereby improves the breakdown strength. The breakdown strength and crystallinity of the XLPE reference sample remained almost unchanged, indicating that the PP system is more sensitive to processing conditions. This study provides a basis for enhancing the insulation process and designing post-treatment for PP cables.