The most common type of extruded power cable insulation is based on cross-linked polyethylene (XLPE), which cannot be recycled as a thermoplastic material. Hence, thermoplastic insulation materials currently receive considerable attention because they would allow recycling through re-melting. In particular blends of polyethylene (PE) and polypropylene (PP) would be a compelling alternative to XLPE, provided that the poor compatibility of the two polymers can be overcome. Here, we establish an alternative approach that exploits the in situ formation of a PE–PP-type copolymer through reactive compounding. Ternary blends of an ethylene-glycidyl methacrylate copolymer, a maleic anhydride-grafted polypropylene, and up to 70 wt % low-density polyethylene (LDPE) are compounded at 170 °C. Covalent bonds form through reaction between epoxy and carboxyl groups, leading to a PE–PP-type copolymer that shows good compatibility with LDPE. The in situ generated PE–PP copolymer arrests creep above the melting temperature of LDPE, mediated by a continuous network that is held together by PP crystallites. Recyclability is confirmed by reprocessing at 170 °C. Furthermore, the here investigated formulations feature a low direct-current electrical conductivity of ∼4 × 10–14 S m–1 at 70 °C and 30 kV mm–1, on a par with values measured for LDPE and XLPE. Evidently, in situ formation of a PE–PP-type copolymer through reactive compounding is a promising approach that may enable the design of thermoplastic insulation materials for power cables.

High‐voltage direct‐current power cables are vital for the efficient transport of electricity derived from renewable sources of energy. The most widely used material for high‐voltage power cable insulation – low‐density polyethylene (LDPE) – is usually crosslinked with peroxides, a process that releases unwanted by‐products. Hence, by‐product‐free crosslinking concepts that mitigate the associated increase in electrical conductivity are in high demand. Click chemistry‐type crosslinking of polyethylene copolymer mixtures that contain glycidyl methacrylate and acrylic acid co‐monomers is a promising alternative, provided that the curing reaction can be controlled. Here, we demonstrate that the rate of the curing reaction can be adjusted by tuning the number of epoxy and carboxyl groups. Both dilution of copolymer mixtures with neat LDPE and the selection of copolymers with a lower co‐monomer content have an equivalent effect on the curing speed. Ternary blends that contain 50 wt% of neat LDPE feature an extended extrusion window of up to 170 °C. Instead, at 200 °C rapid curing is possible, leading to thermosets with a low direct‐current electrical conductivity of about 10−16 S cm−1 at an electric field of 20 kV mm−1 and 70 °C. The conductivity of the blends explored here is comparable to or even lower than values measured for both ultraclean LDPE and a peroxide‐cured commercial crosslinked polyethylene grade. Hence, click chemistry curing represents a promising alternative to radical crosslinking with peroxides. © 2019 Society of Chemical Industry

To design reliable high voltage cables, clean materials with superior insulating properties capable of operating at high electric field levels at elevated temperatures are required. This study aims at the electrical characterization of a byproduct-free crosslinked copolymer blend, which is seen as a promising alternative to conventional peroxide crosslinked polyethylene currently used for high voltage direct current cable insulation. The characterization entails direct current (DC) conductivity, dielectric response and surface potential decay measurements at different temperatures and electric field levels. In order to quantify the insulating performance of the new material, the electrical properties of the copolymer blend are compared with those of two reference materials; i.e., low-density polyethylene (LDPE) and peroxide crosslinked polyethylene (XLPE). It is found that, for electric fields of 10–50 kV/mm and temperatures varying from 30 °C to 70 °C, the DC conductivity of the copolymer blend is in the range of 10−17–10−13 S/m, which is close to the conductivity of crosslinked polyethylene. Furthermore, the loss tangent of the copolymer blend is about three to four times lower than that of crosslinked polyethylene and its magnitude is on the level of 0.01 at 50 °C and 0.12 at 70 °C (measured at 0.1 mHz and 6.66 kV/mm). The apparent conductivity and trap density distributions deduced from surface potential decay measurements also confirmed that the new material has electrical properties at least as good as currently used insulation materials based on XLPE (not byproduct-free). Thus, the proposed byproduct-free crosslinked copolymer blend has a high potential as a prospective insulation medium for extruded high voltage DC cables.

The impact of the composition of blends comprising low-density polyethylene (LDPE), isotactic polypropylene (PP) and a styrenic copolymer additive on the thermomechanical properties as well as the direct-current (DC) electrical and thermal conductivity is investigated. The presence of 5 weight percent (wt%) of the styrenic copolymer strongly reduces the amount of PP that is needed to enhance the storage modulus above the melting temperature of LDPE from 40 to 24 wt%. At the same time, the copolymer improves the consistency of the thermomechanical properties of the resulting ternary blends. While both the DC electrical and thermal conductivity strongly decrease with PP content, the addition of the styrenic copolymer appears to have little influence on either property. Evidently, PP in combination with small amounts of a styrenic copolymer not only allows to reinforce LDPE at elevated temperatures but also functions as an electrical conductivity-reducing additive, which makes such thermoplastic ternary formulations possible candidates for the insulation of high-voltage power cables.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

The impact of a small amount of polystyrene-b-poly(ethylene-co-butylene)-bpolystyrene (SEBS) on the thermomechanical and electrical properties of blends comprising low-density polyethylene (LDPE) and isotactic polypropylene (PP) is investigated. SEBS is found to assemble at the PP:LDPE interface as well as within isolated PP domains. The addition of 10 wt% SEBS significantly increases the storage modulus between the melting temperatures of the two polyolefins, 110 and 160 C, and results in improved resistance to creep during both tensile deformation as well as compression. Furthermore, the ternary blends display a very low direct-current (DC) conductivity as low as 3.4 Â 10 À15 S m À1 at 70 C and 30 kV mm À1 , which is considerably lower than values measured for neat LDPE. The here presented type of ternary blend shows potential as an insulation material for high-voltage direct current power cables.