Dielectric, thermal and mechanical properties of polypropylene/ultra‒high molecular weight polyethylene nanocomposites for power cables

Abstract

Cross‒linked polyethylene (XLPE), which is the current state of the art insulation system for HV cable systems, is difficult to recycle due to its crosslinked, thus thermoset, nature. In efforts of achieving more sustainable cable systems over the complete lifecycle of the asset, alternative insulation systems have been investigated. One major limitation for XLPE in terms of the thermal stability, thus maximum operating temperature, is the fact that the base polymer for XLPE systems, branched low‒density PE, has an inherently low melting point. As a consequence, even after cross‒linking, the resulting compound undergoes a softening transition around the temperature the base polymer would melt but is held together by the cross‒links. A replacement for PE should have an inherently higher melting point, which, if high enough, could mean that a cross‒linker would not necessarily be required. This would make the material easier to recycle by design.Further, the material needs to be cheap in bulk, easy to process and thus compatible with extrusion processes common in the HV cable sector, have a low loss and dielectric constant, and have adequate thermal conductivity. Polypropylene (PP) fits many of these requirements. However, PP without any additives is too brittle to be used in HV cables, and the thermal conductivity of PP is significantly lower than that of PE‒based compounds, which means that the increased thermal headroom by virtue of a higher melting point is rather restricted.Present work shows the effects of surface‒modified magnesium oxide (MgO) nanofillers on the AC breakdown strength, thermal conductivity, and mechanical properties of PP, as well as composites with ultra‒high molecular weight polyethylene (UHMWPE). If UHMWPE is mixed in with PP, the resulting polymers show separate crystallisation peaks, signifying insufficient miscibility. When MgO nanoparticles are mixed in, the composites move from separate crystallisation to co‒crystallisation peaks, which suggests that the addition of the nanofiller improves the miscibility by acting as a compatibilizer. The UHMWPE improves thermal conductivity from 0.21 W/m∙K to 0.31 W/m∙K due to its long molecular chains. Mechanical elasticity of PP/UHMWPE blends is enhanced due to the weak bonding strength at the interfaces resulting from the incompatibility between PP and UHMWPE. The addition of MgO improves bonding between the otherwise separate PP and UHMWPE phases, further improving the mechanical and dielectric breakdown strength of these blends. While the AC breakdown strength drops for PP/UHMWPE blends, the addition of nanoscale MgO not only recovers but slightly improves breakdown strength values.

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Identifiers

ORCID for Thomas Andritsch: orcid.org/0000-0002-3462-022X

ORCID for Phichet Ketsamee: orcid.org/0000-0003-1733-6806

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