Study on Thermal Conductivity of BNNs/Mg(OH)2/LDPE Composites Based on Melt Blending Method

Enhancing the thermal conductivity of insulation materials can reduce the operating temperature and thus increase the transmission capacity of power cables. In this work, low density polyethylene (LDPE) with the fillers of magnesium hydroxide (Mg(OH)2) was selected as the base insulation materials for fire retardant. Boron nitride nanosheets (BNNs) were selected as the fillers to increase the thermal conductivity. To imitate the melt extrusion in cable manufacturing process, composites with different filler ratios were prepared by HAAKE Polylab OS and then molded at high pressure. The thermal conductivities of the composites were measured by LFA447 nanoflash and the microstructures of composites were observed by the scanning electron microscope (SEM). The filler orientation was characterized by diffraction of X-rays (XRD) to represent the anisotropic index of the composites. The results indicated that BNNs were inclined to be oriented along the extrusion plane because of the axial pressure in molding process. As a result, the composites showed anisotropic thermal conductivity. For example, in the composites with 20wt% BNNs, the thermal conductivity in through-plane direction reached 3.97W/(m·K), which was 1240% of that of LDPE. Furthermore, with the addition of Mg(OH)2, the thermal conductivity of the composite was further improved, and the thinner flake samples showed stronger anisotropy in thermal property. It was shown that the multi-packing structure molded by the simple melt-blending process was a feasible method to fabricate high efficiency thermally conductive composites.