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Abstract
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Improving the through-plane thermal conductivity of high-density polyethylene (HDPE) pipe is essential to enhance geothermal energy utilization for ground-source heat pump (GSHP) systems. However, studies on improving the thermal conductivity in the radial direction of pipes are relatively few. Forcing anisotropic fillers into the highly oriented structure is an effective method to enhance the thermal conductivity of polymer composites. In this work, the through-plane thermal conductivity of HDPE pipe was greatly improved by regulating the orientation direction of graphite (Gt). Results indicated combining multistage stretching extrusion (MSE) and hot-pressingcutting process (ST) successfully converted the orientation of Gt from parallel to perpendicular to the extrusion direction, as a result, the through-plane thermal conductivity of HDPE/Gt-MSE+ST with 20 wt% Gt reached 3.75 W/(m·K), while the in-plane thermal conductivity was 2.5 W/(m·K). Meanwhile, HDPE/Gt-MSE+ST exhibited a yield strength comparable to that of HDPE and a greatly improved creep resistance, which was beneficial for the long-term resistance to inner fluid pressure. Furthermore, the conductivity resistance and breakdown strength of HDPE/Gt-MSE+ST composite meet the requirement for HDPE pipes used for the GSHP system. This paper offers a new method for the preparation of HDPE pipes with high thermal conductivity used for ground-source heat pump systems.
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