Enhanced Dielectric Breakdown Property of Polypropylene Based on Mesoscopic Structure Modulation by Crystal Phase Transformation for High Voltage Power Cable Insulation

As an environmentally friendly polymer material, isotactic polypropylene (pp), possesses excellent dielectric properties while it can be recycled, so it is regarded as having promising application prospects in the field of high voltage power cable insulation. However, the increasing operating voltage rating also puts higher demands on its insulation reliability. In this study, an intrinsic modulation method for pp insulation based on mesoscopic structure modulation by crystal phase transformation is proposed, which not only effectively improved the dielectric properties but also enhanced the mechanical toughness synergistically. The transformation of the crystal phase from the alpha-crystal to beta-crystal within the PP/beta-NA samples was successfully achieved by the solution blending method. The crystallization efficiency is greatly promoted, and the crystal structure is further improved at the same time. As the proportion of beta-crystal gradually increases, the elongation at break could be raised to 451% at maximum. The difference in dielectric parameters between the crystal and amorphous regions is caused by the variation in the molecular chain density and arrangement, which is also the main reason for further triggering the high-intensity partial discharges and large-area electrical tree degradation in the amorphous region between the spherocrystal boundary at the mesoscopic scale. Compared with the premodification, the introduction of beta-crystal effectively alleviated the problem of electric field distortion. Among them, the modified PP-beta-0.2 sample had 16.8 kV/mm lower maximum electric field, 2258 fewer total partial discharges, 420 mu m(2) less electrical tree cumulative damage area, and 28% higher breakdown strength. Accordingly, it also has promising applications in the manufacturing of high-voltage power cable insulation.