286 / 2018-09-25 16:54:02

Polyethylene/Polyethylene Grafted Polystyrene Composites for High Voltage Insulation Applications: Thermal, Electrical and Mechanical Behavior

polyethylene grafted polystyrene, composites, HV insulation systems, thermal, breakdown down, mechanical

The semi-conductive or insulating nanoparticles have been widely doped into polymer in order to prepare high-performance insulation materials and replace cross linking polyethylene (XLPE) considering its non-recycled properties. However, achieving the uniform dispersion of nanoparticles in polymers is the critical obstacle because it is easy to agglomerate, which lead to difficulty in controlling the properties of nanocomposites. In this study, we synthesized polyethylene grafted polystyrene grafting copolymer (PE-g-PS) by pre-irradiation suspension grafting technology. Then the PE/PE-g-PS composites with different content of PS (calculated from PE-g-PS) was prepared through melt blending in an internal mixer as potentials candidates in HV insulation systems including HVDC cables. The thermal, electrical, and mechanical properties of PE/PE-g-PS composites have been evaluated and compared to those of PE. In terms of PE-g-PS, its grafting degree was 10% measured by Soxhlet extraction and evidence of the grafting reaction was determined by FTIR. From the SEM, we observe that all PE/PE-g-PS composites present an obvious sea-island structure, and the number of PS particle is increased and the diameter of PS particles increased as the PS content (calculated from PE-g-PS) increase from 5% to 20%. Especially, when PS content reaches 20%, an obvious gap at the phase interface is observed indicating that the interface interaction is weak, but it also can be found that most of PS particles reveal fractured surface on section and no interfacial debonding occurs. Thermal properties of composites were measured from DSC. A slight decrease in crystallinity of PE in PE/PE-g-PS composites was observed, which indicated that the present of PS phase made against crystallization of PE. The crystallization temperature of composites increases gradually with the increasing PS content attributed to the PS side chain acting as heterogeneous nucleation agents. Two-parameter Weibull distribution plots as well as scale and shape parameters values corresponding to the AC breakdown tests were obtained to evaluate the breakdown strength of the composites and pure PE. The electrical breakdown strength increased gradually with increase of PS content and reached a maximum of 113.5kV/mm at 20wt% PS content, which is significantly superior to 94.63 kV/mm of the PE. This phenomenon was attributed to that the energy dissipation property of benzene ring increased the breakdown voltage of PE/PE-g-PS composites, compared to that of PE. Mechanical properties of both pure PE and PE/PE-g-PS composites have been characterized in tensile mode. The ultimate tensile strength UTS and elongation at break EB were detailed analyzed. UTS values corresponding to PE was 18.2 MPa. In PE/PE-g-PS composites, the addition of PE-g-PS induced small enhancements at 5 wt% PS content. However, at higher PS content, UTS values drops markedly to about 11.2 MPa at 20 wt% PS content. The increased of stiffness with the addition of PS particles formed by grafting reaction is expected due to their high rigidity. However, at elevated PS content, the bad adhesion and weak interaction at the interface impede the load stress transfer from the matrix PE phase to PS particles leading to reduced strength. The EB values of composites is reduced except 5 wt% PS content, which is expected due to the rigid nature of PS phase and bad adhesion at the interface. In sum, the PE/PE-g-PS composites with 5% and 10% PS content is more suitable as candidates due to their enhanced electrical properties and little change in mechanical properties.
Polyethylene/polyethylene grafted polystyrene composites for high voltage insulation applications: thermal, electrical and mechanical behavior.