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This paper focuses on novel insulation polypropylene/poly(ethylene-co-octene) (PP/POE) nanocomposites for High Voltage Direct Current (HVDC) cable application. The composites contain silica modified by a solvent-free method using silanes differing in polarity and functional moieties. Thermogravimetric Analysis and Fourier Transform Infrared Spectroscopy showed that the solvent-free method is an effective way to modify silica by silanes. Silica/PP/POE nanocomposites were prepared in a mini twin-screw compounder, and the effect of silica on crystallization, dispersibility and dielectric properties of the samples was investigated. Differential Scanning Calorimetry results showed that the unmodified and modified silicas acted as nucleation agents and increased the onset of the crystallization temperature of the polymeric matrix. Scanning Electron Microscopy images showed that the silica is mostly located in the PP phase matrix. For the PP/POE nanocomposites filled with unpolar silica, a higher trap density (measured by Thermally Stimulated Depolarization Current, TSDC) was found; this might be caused by the larger interfacial area due to a better dispersion of the unpolar silica in the polymeric matrix. Polar silicas introduce deeper traps than the unpolar ones, which is most likely due to the hetero-atom introduction. Nitrogen atoms were found to have the strongest effect on the charge trapping properties. According to these results, amine-modified silica is a promising candidate for PP/POE nanocomposites for HVDC cable applications.

Abstract: As a part of serial work about the toughening effect of elastomer and nucleating agent on polypropylene (PP), this work is focused on the microstructure changes of PP matrix in PP/elastomer blends with β-phase nucleating agent (β-NA) during the uniaxial tensile deformation process. The microstructure changes have been investigated through differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXD) measurements. The results show that there is a transformation of β-PP to α-PP, which is dependent on the local strain of the tensile-deformed specimen during the deformation process. The bigger the local strain, the high the degree of the β→α transformation is. At the later stage, β-PP is completely changed into α-PP. The presence of elastomer, especially at high load, prevents such transformation, possibly leading to more β-PP participating in the later deformation process of the fracture, which most likely results in the great improvement of fracture toughness of PP/elastomer/β-NA. Further results show that the β→α transformation occurs mainly in the necking region of the specimen during the deformation process. [Copyright &y& Elsevier]