Thermal Conductivity and Mechanical Properties of Polymer Composites with Hexagonal Boron Nitride—A Comparison of Three Processing Methods: Injection Moulding, Powder Bed Fusion and Casting

Abstract

Materials providing heat dissipation and electrical insulation are required for many electronic and medical devices. Polymer composites with hexagonal boron nitride (hBN) may fulfil such requirements. The focus of this study is to compare composites with hBN fabricated by injection moulding (IM), powder bed fusion (PBF) and casting. The specimens were characterised by measuring thermal conductivity, tensile properties, hardness and hBN particle orientation. A thermoplastic polyurethane (TPU) was selected as the matrix for IM and PBF, and an epoxy was the matrix for casting. The maximum filler weight fractions were 65%, 55% and 40% for IM, casting and PBF, respectively. The highest thermal conductivity (2.1 W/m∙K) was measured for an IM specimen with 65 wt% hBN. However, cast specimens had the highest thermal conductivity for a given hBN fraction. The orientation of hBN platelets in the specimens was characterised by X-ray diffraction and compared with numerical simulations. The measured thermal conductivities were discussed by comparing them with four models from the literature (the effective medium approximation model, the Ordóñez-Miranda model, the Sun model, and the Lewis-Nielsen model). These models predicted quite different thermal conductivities vs. filler fraction. Adding hBN increased the hardness and tensile modulus, and the tensile strength at high hBN fractions. The strength had a minimum as the function of filler fraction, while the strain at break decreased. These trends can be explained by two mechanisms which occur when adding hBN: reinforcement and embrittlement.

Thermally Conductive Polymer Composites with Hexagonal Boron Nitride for Medical Device Thermal Management

“…This study is a continuation of our previous research on the encapsulation of interventional medical devices [33] and thermally conductive polymer composites [35]. This paper adds results for composites with different hBN powder types, and how the hBN platelets' orientation in the injection moulding process can be simulated.…”

“…In our previous work [35] we showed that injection moulding induces a preferred orientation of the platelets, with the platelet normal along the thickness direction of the specimens. The orientation varied through the thickness of the moulded specimens, and it increased with increasing hBN loading.…”

“…This is due to the preferred in-plane orientation of the hBN platelets in the former case, as shown by the XRD data in ref. [35]. Casting only resulted in a low preferred orientation, and powder bed fusion resulted in an almost random orientation.…”

“…The thermal conductivity of 2 mm thick specimens was indirectly determined by the non-contact, transient laser flash analysis (LFA) method. Details about the measurements are given in [35].…”

“…In future studies, the local platelet orientation induced in the injection moulding process can be simulated (as in Figure 5). From this, the local anisotropic thermal conductivity can be calculated with an appropriate model (some discussed in our previous study [35]). Finally, a thermal simulation can be performed, using the local anisotropic thermal conductivity as material input.…”