Thermally Conductive Hexagonal Boron Nitride-Polyethylene Nanocomposites: Effect of Processing Method and Filler Size, Exfoliation, and Alignment on Thermal Conductivity

Abstract

Applications of polymers in thermal management have recently gained significant interest. Rapid heat transfer demand in thermal management applications requires enhancing the low thermal conductivity of polymers. We developed polyethylene (PE)-hexagonal boron nitride (hBN) composites with hBN loading between 5 and 40 vol%. The impacts of the composite fabrication method, the PE type, and hBN size, exfoliation, and alignment on the composite morphology, thermal conductivity, thermal stability, and mechanical properties were thoroughly investigated. Our results indicate increasing the hBN loading increases κc and enhance the mechanical properties and the thermal stability of the composites. Moreover, dry ball milling of hBN and PE yields composites with higher κc than melt extrusion. The processing-induced hBN alignment significantly affects κc. Melt extrusion followed by compression molding produces composites with hBN aligned parallel to the sheet surface at the skin layers (~fixed 250 µm each) and randomly aligned at the core of the sheet. This processing-induced hBN alignment led to κc dependence on the sheet thickness as the core layer has higher conductivity than the skin layers. By controlling the alignment of hBN in LLDPE, κc was greatly enhanced, reaching up to ~600 increase in the κc of the unaligned hBN composites. Finally, the exfoliation of hBN proved effective in increasing κc and improving the composites’ mechanical properties and thermal stability but led to significant processing challenges due to the very high melt viscosity.