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There is much interest in the development of replacement materials for crosslinked polyethylene (XLPE) that are both recyclable (i.e. thermoplastic) and capable of high temperature operation. Thermally, polypropylene is the ideal choice, although its stiffness and low electrical breakdown strength make for a challenging materials design problem. We report here on the compositional optimization of a propylene homopolymer/propylene-ethylene copolymer blend in terms of its dynamic mechanical properties and thin film electrical breakdown strength. The extrusion of a trial minicable using the optimized blend is also discussed, which is shown to exhibit a significantly improved electrical performance, as gauged by its DC breakdown strength, than an XLPE-insulated reference.

The structure and chemistry of two electrical trees (designated Tree A and Tree B) grown in low density polyethylene have been studied by a combination of confocal Raman microprobe spectroscopy, optical microscopy and scanning electron microscopy. Despite being grown under similar conditions (A, 30 °C and 13.5 kV; B, 20 °C and 13.5 kV), these two trees exhibit very different structures. Tree A exhibits a branched structure while Tree B is more bush-like. In Tree A, the very tips of the structure are made up of hollow tubules, which exhibit just the Raman signature of polyethylene. On moving towards the high voltage needle electrode, fluorescent decomposition products are first detected which, subsequently, are replaced by disordered graphitic carbon. From the relative intensity of the graphitic sp2 G and D Raman bands, the constituent graphitic domains are estimated to be ∼4 nm in size, which leads to a local tree channel resistance per unit length of 1–10 Ω µm−1. These structures are therefore sufficiently conducting to prevent local electrical discharge activity. In Tree B, the observed fluorescence increases continuously from the growth tips to the needle. Here, the tree channels are not sufficiently conducting to prevent electrical discharge activity within the body of the tree. These results are discussed in terms of mechanisms of tree growth and, in particular, the chemical processes involved.

Crosslinked polyethylene (XLPE) has a successful history as a cable insulation material. Nevertheless, in recent years, as environmental awareness has grown, concerns about the ease with which it can be recycled have emerged. Although technologies have been developed for XLPE recycling, this report concentrates instead on the development of a thermoplastic alternative. Specifically, a 20 : 80 blend of high density and low density polyethylene (HDPE : LDPE) was selected and subjected to a non-isothermal crystallization procedure. It was found that, provided the cooling rate falls between 0.5 and 10 K min -1 , the blend exhibits superior breakdown strengths and high temperature mechanical stiffness compared to XLPE. A trial cable was then extruded from this blend using such a cooling rate. The breakdown behavior of the morphologically-designed cable was finally compared with that of LDPE and XLPE reference systems.