Thermally conductive phase-change materials for energy storage based on low-density polyethylene, soft Fischer-Tropsch wax and graphite

Phase change materials based on graphite-filled wax/polyethylene blends could find application as thermal energy storage materials. Such compounds, comprising wax to polyethylene in a 3:2 proportion, were prepared by twin screw compounding. Two types of graphite were used in an attempt to improve the thermal conductivity of the compounds. Expanded graphite enhanced the thermal conductivity by more than 200% at a loading of 10 wt.%, compared to a ca. 60% improvement with natural graphite flakes at the same loading. The TGA results showed that all the compounds underwent a two-step degradation. In all cases the mass % ratios of the two degradation steps were roughly 3:2 for wax:LDPE, which confirms that the wax evaporated completely before the degradation of LDPE started. The DSC results suggest that the heat energy storing capacity of the wax is not influenced by the other components as long as heating is restricted to temperatures just above the melting point of the wax. It is also apparent that the presence of both forms of graphite enhanced the rate of heat transfer to the PCMs. The DMA results show that the presence of wax had a softening effect, while the presence of graphite opposed this softening effect by reinforcing the PCM composites. (C) 2011 Elsevier B.V. All rights reserved.