3D printing boron nitride nanosheets filled thermoplastic polyurethane composites with enhanced mechanical and thermal conductive properties

The rapid development of high-power integrated electronic devices for 5 G system has created an urgent requirement for thermally conductive and electrically insulating materials, which has attracted significant research interest, especially for the engineering materials with mechanical flexibility and anisotropic thermal conductivity. In this study, exfoliated hexagonal boron nitride nanosheets (BNNSs) were incorporated in thermoplastic polyurethane (TPU) with varying contents of 10, 20 and 30 wt% through the solution-melt compounding. Then samples for testing were fabricated from the composites by 3D printing, and their thermal conductivity and tensile properties were evaluated to explore the structure-activity relationship. Results showed that the effects of melt squeezing and shear flow occurred during in 3D printing process were dominant in enabling the orientation of BNNSs within the TPU matrix, which was found to be more dependent on the ratio (dn/h) of nozzle diameter dn to layer thickness h and less dependent on the printing speed v. Such in-plane orientation of BNNSs combined with the rastering structure of the deposited strands in the warps and wefts and the low density of inner voids were helpful to improve the mechanical and thermal conductive performance of printed BNNS/TPU composites, demonstrating a much better retention of tensile strength and greatly increased thermal conductivity than those of molded samples via hot compression. In addition, the performance of printed composites was also beneficial from the size effect of BNNSs, which could not be achieved without nano exfoliation. As a result, the tensile strength and elongation at failure of the BNNS/TPU composites with 30 wt% loading were 106.2% and 116% higher than those of printed BN/TPU composites, respectively. In addition, anisotropic thermal conductivity of printed BNNS/TPU in the printing direction and thickness direction increases from 1.51 to 1.80 Wm−1K−1 and from 1.26 to 1.30 Wm−1K−1, representing increases of 19.21% and 3.17%, respectively.