Synergistic effect of BN and MWCNT hybrid fillers on thermal conductivity and thermal stability of ultra-high-molecular-weight polyethylene composites with a segregated structure

The thermal conductivity of ultra-high-molecular-weight polyethylene (UHMWPE)/boron nitride particle (BNp), UHMWPE/BN sheet (BNs), and UHMWPE/(BN+MWCNT) hybrid filler composites with segregated structures was investigated using a powder mixture and hot-pressing process. The morphology of the fillers and composites was observed by optical microscopy, atomic force microscopy, and scanning electron microscopy, respectively. The results showed that the torispherical BNp filler contained particles of various dimensions, ranging from 200 to 500 nm, while the saucer-shaped BNs filler with irregular prominence on the edge exhibited uniformity of size, with widths of 100–150 nm and height of 3–5 nm. The networks of thermally conductive fillers and the interfacial thermal resistance at the filler boundaries played a major role in the thermal conductivity of the segregated composites, as revealed in an almost linear enhancement of conductivity with increasing filler content. In comparison to the 2D saucer-shaped BNs fillers, the varied size of the 0D BNp was more conducive to the formation of effective filler stacks, as the gaps between larger BNp fillers facilitated access by the smaller BNp fillers. The thermal conductivity of the UHMWPE composite with the addition of 50 wt% BNp increased from 0.4591 to 1.385 W/m·K, approximately 16.2 % higher than that of the UHMWPE/BNs composite (1.192 W/m·K). The synergistic effect of the BN+MWCNT hybrid fillers helped to improve the thermal conductivity of the UHMWPE composites. Compared with the 0D BNp filler, the 2D BNs was more readily entangled with 1D MWCNT and formed compact and overlapping thermally conductive networks. As such, the thermal conductivity of the UHMWPE/(BNs+MWCNT) hybrid filler composite (1.641 W/m·K for 50 wt% filler content) was superior to that of the UHMWPE/(BNp+MWCNT) composite (1.533 W/m·K). Additionally, the crystallization behavior and thermal stability of UHMWPE was almost unchanged in the presence of BN and BN+MWCNT hybrid fillers.