Abstract
Water-tree resistances of styrene block copolymer/polypropylene (SEBS/PP) composites are investigated by characterizing crystallization structures in correlation with the dynamic mechanical properties to elucidate the micro-structure mechanism of improving insulation performances, in which the accelerated aging experiments of water trees are performed with water-knife electrodes. The water-tree morphology in spherulites, melt-crystallization characteristics and lamella structures of the composite materials are observed and analyzed by polarizing microscopy (PLM), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), respectively. Dynamic relaxation and stress-strain characteristics are specifically studied by means of a dynamic thermomechanical analyzer (DMA) and electronic tension machine, respectively. No water-tree aging occurs in both the highly crystalline PP and the noncrystalline SEBS elastomer, while the water trees arising in SEBS/PP composites still has a significantly lower size than that in low-density polyethylene (LDPE). Compared with LDPE, the PP matrix of the SEBS/PP composite represent a higher crystallinity with a larger crystallization size in consistence with its higher mechanical strength and lower dynamic relaxation loss. SEBS molecules agglomerate as a “island” phase, and PP molecules crystallize into thin and short lamellae in composites, leading to the blurred spherulite boundary and the appreciable slips between lamellae under external force. The high crystallinity of the PP matrix and the strong resistance to slips between lamellae in the SEBS/PP composite essentially account for the remarkable inhibition on water-tree growth.
Highlights
Green initiative and recyclable thermoplastic materials are promised in advance to be applied for the extruded plastic insulated power cable that can acquire excellent electrical and mechanical properties and good processing performances [1,2], in which polypropylene (PP) represents multiple high performances simultaneously in mechanical strength, heat resistance and electrical insulation [3,4].Since 2002, when the world’s first 0.6 kV and 22 kV PP cables were produced by blending polyethylene (PE) and antioxidants into a syndiotactic PP (SPP) matrix for cable main insulation, it has been suggested that the alternative current (AC) breakdown strength and dielectric loss of PP material are well-qualified for manufacturing insulated cables in future
Even though no observable water trees arise in the pure materials of PP and styrene-ethylene/butylene-styrene triblock copolymer (SEBS) after the water tree-aging experiments, the SEBS/PP composites show a substantial morphology of the water trees essentially existing in amorphous regions between spherulites, implying that SEBS fillers have introduced void structures into the PP matrix for accommodating water accumulations
The dispersion morphologies of SEBS in the PP matrix further reveal that polystyrene fragments in SEBS molecules lead to a preferential structure of the amorphous morphology that is incompatible with PP crystallization
Summary
Green initiative and recyclable thermoplastic materials are promised in advance to be applied for the extruded plastic insulated power cable that can acquire excellent electrical and mechanical properties and good processing performances [1,2], in which polypropylene (PP) represents multiple high performances simultaneously in mechanical strength, heat resistance and electrical insulation [3,4]. In the inevitable humid environment, water will gradually be absorbed into the insulation layer of the power cable and form a water tree under the AC electric field to cause insulation degradation and dielectric failure, which generally occurs in medium-voltage power cables [5,6,7,8] It has been substantially demonstrated from sufficient experiments that the inception and growth of water trees do not rely on external factors, such as electric field strength and frequency, aging time and ambient temperature but, are affected by internal factors such as crystallization morphology, type and the content of additives in the insulating materials [9,10,11,12]. The crystallization characteristics and mechanical properties of the SEBS/PP composites and LDPE are investigated to elucidate the intrinsic mechanism of the water-tree formations correlated with crystallization morphology
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