Thermal, electrical and rheological behavior of high-density polyethylene/graphite composites

Thermally conductive high-density polyethylene (HDPE) composites with high loadings of flake graphite (FG) and expandable graphite (EG) were prepared by melt-blending process. Thermal conductivity of the composites was measured by hot disk method, and a very high value of 13.41 W/mK was obtained for HDPE/EG composite with 60 wt% EG, which is 28.3 times and 60.4 % higher than the values obtained for the HDPE and an HDPE/FG system with 60 wt% FG, respectively. Two-point method was applied to evaluate the electrical conductivity of the composites with various graphite loadings, and thermal percolation was observed in the vicinity of the electrical percolation threshold concentration. Dynamic rheology analysis was employed to define the geometrical changes caused by the interconnection of the inorganic fillers. The results showed that the in situ exfoliation of EG during the blending process could form more effective conductive pathways than those of FG-filled composites at the same graphite loadings. Such kind of pathways was responsible for the substantial improvement of the overall thermal and electrical conductivity of HDPE/EG composites. The results of dynamic rheology also confirmed that the thermally conductive networks were primarily formed at approximately 20 and 40 wt% of graphite concentrations for HDPE/EG and HDPE/FG composites, respectively.