Effect of β-crystals on the mechanical and electrica properties of β-nucleated isotactic polypropylene

Polypropylene has great potential as a thermoplastic insulation material for high voltage direct current (HVDC) cable. However, its mechanical properties restrict its application, particularly at low temperature. In this paper, isotactic polypropylene (iPP) with the β-nucleating agent WBG was prepared by the melt blending method. The crystallinity, β-crystal proportion and crystalline morphology were analyzed by wide-angle X-ray diffraction, differential scanning calorimetry, scanning electron microscopy and polarized optical microscopy. The results showed that the crystallinity and β-crystal proportion increased as the WBG concentration increased. Three types of β-crystal morphology, namely β-spherulite, flower-like agglomerate and β-quasi-spherulite, were sequentially obtained with increasing WBG concentration. The mechanical properties, space charge distribution, thermally stimulated discharge current and DC electrical breakdown strength were investigated. Compared with pure iPP, the Izod impact strength and DC breakdown strength of iPP/0.1%WBG were improved by 238 and 16%, respectively, and the fracture energy and brittleness temperature of iPP/0.3%WBG were improved by 420 and 80%, respectively. In addition, the space charge accumulation was suppressed in the range of 0.1–0.5 wt% WBG. Chain coupling between the β-crystals and the amorphous phase, particularly in the flower-like agglomerates, was considered to play an important role in improving the mechanical and electrical performance.