Novel Thermoplastic Material Concepts for High Voltage Cable Insulation - Engineering Immiscible Blends for a Sustainable Future

Doctoral thesis, 2021

To cope with our growing demand for energy in a sustainable way, efficient long-distance power transmission via high voltage direct current (HVDC) cables is crucial – these cables facilitate the integration of renewable energy into our power networks. For reliable and efficient power transmission, underground and undersea cables require robust insulation materials that possess a high level of mechanical integrity, a low direct-current (DC) electrical conductivity and a high thermal conductivity at the elevated temperatures experienced during cable operation. There is growing interest in thermoplastic materials that fulfill these requirements since thermoplastics offer the possibility for mechanical recycling by melt-reprocessing, and allow for more energy efficient cable production.

In this thesis, it is shown that thermoplastic blends of low-density polyethylene (LDPE) and isotactic polypropylene (iPP) can be engineered towards HVDC cable insulation applications despite the immiscibility between LDPE and iPP. Reactive compounding was explored as a strategy for compatibilising iPP and LDPE, resulting in a material concept that exhibited good thermomechanical properties while maintaining low DC electrical conductivity and thermoplasticity. Blends comprising iPP, LDPE and a styrenic copolymer were also investigated. This led to another thermoplastic material concept where the blend composition could be tuned to simultaneously attain appropriate mechanical stiffness, DC electrical conductivity and thermal conductivity. Further, the addition of aluminium oxide nanoparticles was found to reduce the already low DC electrical conductivity of such blends. The novel material concepts described in this thesis may facilitate the design of thermoplastic insulation materials for HVDC cables of the future.