Thermoplastic Insulation for High Voltage Cables
Licentiate thesis, 2020
High voltage direct current (HVDC) cables that seamlessly integrate renewables have an important contribution to the modern world’s efforts towards a more sustainable future. Extruded HVDC cables that efficiently and reliably transport electricity require robust insulation materials with good thermomechanical and dielectric properties. Peroxide-crosslinked polyethylene (XLPE) has been the conventional insulation material for extruded HVDC cables. However, peroxide crosslinking releases by-products that need to be removed, necessitating an expensive and time-consuming degassing procedure. Furthermore, XLPE is a thermoset material, which cannot be recycled by re-extrusion. Thermoplastic materials are therefore sought after as more sustainable material alternatives for HVDC cable insulation. Blends that contain isotactic polypropylene (iPP) are of great interest due to the thermomechanical reinforcement that its high melting crystals can offer. However, iPP alone is too brittle. Blends comprising iPP and softer components like polyethylene could give properties desired in cable insulation, but the incompatibility between iPP and LDPE must be considered. This thesis presents reactive compounding as a strategy to form PP–PE-type copolymers in-situ in a recyclable ternary blend comprising an ethylene-glycidyl methacrylate copolymer, a maleic anhydride-grafted polypropylene and low density polyethylene (LDPE). The material demonstrated excellent thermomechanical and DC dielectric properties, reflecting this novel strategy as a promising one for the design of recyclable insulation materials for future HVDC cables.