Effect of crystalline morphology on electric and thermal properties of β-polypropylene for HVDC cable insulation

It is widely believed that mechanical and electrical properties of isotactic polypropylene (iPP) can be improved by introducing $\beta$-crystals. But current researches are focused on the influence of $\beta$-crystal content and spherulite size on the properties of iPP. In this paper, the effect of $\beta$-iPP crystal morphology on electrical and thermal properties of iPP used in HVDC cable insulation was studied. A kind of rare earth compounds $\beta$-nucleating agent WBG, which structure is simply expressed as $\left[\mathrm{La}^{3+} \mathrm{Ca}^{2+} \mathrm{P}_{\mathrm{m}} \mathrm{X}_{\mathrm{n}} \mathrm{Q}_{\mathrm{k}}\right]$ was used to generate rich $\beta$-crystalline structure in the compression-molded bars which were fabricated upon different molten temperatures. iPP samples with same crystallinity and $\beta$-crystal percentage but different crystal morphology were obtained. Three types of $\beta$-crystalline morphology, namely, $\beta$-spherulite, $\beta$-transcrystalline entity, and “flower”-like agglomerate of $\beta$-crystallites, were sequentially obtained with increasing molten temperatures (Tf). The morphology, crystallization behavior, DC breakdown strength and thermal property were characterized by polarized optical microscopy (POM), X-ray diffraction (XRD), thermogravimetric analysis (TG). Compared with the sample with $\beta$-spherulite, the DC breakdown strength and 10% weight loss temperature for the sample with “flower”-like agglomerate of $\beta$-crystallites increased by about 8% and 10.9 °C, respectively. The reason for the results may be related to the more fully developed microstructure in “flower”-like agglomerate. It can be concluded that, it is feasible to improve electric and thermal properties of iPP for HVDC cable insulation by introducing special $\beta$-iPP crystal morphology with fully developed microstructure by the method of controlling molten temperatures during the fabricated process.