Recyclable Polyethylene Insulation via Reactive Compounding with a Maleic Anhydride-Grafted Polypropylene

Abstract

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 × 10-14S m-1at 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.