A series of the thermal conductive polyamide-6 (PA6) composites filled with boron nitride (BN) were prepared by two methods, including melting method (MM) and solution method (SM). The thermal conductivity, morphology, crystallization behavior, thermal stability, and rheological properties of PA6 composites were investigated. The results showed that the thermal conductivity of two methods increased with increase in the filler content, the thermal conductivity of PA6/BN composites containing 40 wt% BN prepared by melting method was up to 1.02W·m^-1·K^-1, while the thermal conductivity of PA6/BN composites prepared by solution method was up to 1.44 W·m^-1·K^-1 at the same filler content.

Keywords: polyamide-6, boron nitride, melting method, solution method, thermal conductivity

With the development of electronic packaging technology, the electronic devices tend to bear ultra-thin, lightweight and multifunctional properties. Large amounts of heat generated or accumulated in the electronic devices should be dissipated quickly in order to prolong the service life of electronic products. ^1-3 Generally, common polymers have relatively low thermal conductivity (typically only 0.2 W·m^-1·K^-1),⁴ which are not suitable for thermal materials application. The incorporation of highly thermal conductive fillers into polymers to develop high-performance thermal-conductive composites had been mostly desired so far.⁵ The composites based on crystalline polymers (e.g., polypropylene,⁶ polyamide-6,^7,8) provided enhanced thermal conductivity as compared to those based on amorphous polymers. And the thermal conductivities of composites were highly enhanced by using various thermal conductive fillers, such as, graphites, graphene, carbon nanotubes (CNTs), carbon fibers (CFs) and boron nitride (BN).⁹
Many articles have been published on improving the thermal conductivity of polymer composites.^10,11 Besides, the direction of different preparation methods were also extensively investigated. Various approaches had been taken for the formation of the polymer composites: in situ polymerization, solution blending, powder blending, roller mixing and melt mixing, according to the different states of the process.¹² The different preparation methods would affect the dispersion, distribution, orientation and length to diameter ratio of the filler in the matrix,¹³ and then influence the thermal conductivity of the composites. For example, Agari et al.¹⁴ prepared PE/graphite composites by four methods, and the thermal conductivity became higher in the following order: melt mixture < rollmilled mixture ≈ solution mixture < powder mixture. A majority of current researches have proved that the solution mixing effectively improved the dispersion of the filler in the polymer matrix, reduced aggregation which further improved the thermal conductivity.^15-17
As a widely used engineering plastic, PA6 offered good thermal stability, chemical resistance and excellent mechanical properties.¹⁸ And the conductive filler of hexagonal BN (h-BN) was a lamellar material with a graphite-like structure in which planar networks of BN hexagons were regularly stacked.^19,20 It has the highest thermal conductivity among ceramic materials and also an electronic insulator. In addition, bulky hexagonal BN can be exfoliated into mono- and/or fewlayers of BNNSs in various solvents thus obtained high aspect ratio,^21-23 Therefore, BN-filled polymer composites, especially the lamellar BN, not only inherit the insulating properties of the polymer matrix but also impart polymer composites thermal conductivity in comparison to conventional fillers, such as, Al₂O₃, ZnO, AlN, and SiC.^24-26
Liu et al.¹⁵ reported that the maximum value of thermal conductivity of carbon fiber/PA66 composites containing 40 wt% carbon fiber prepared by SM achieved 2.537W·m^-1·K^-1, which was superior to carbon fiber/PA66 composites prepared by EM at the same filler content. Hong et al.²⁷ demonstrated that aluminum nitride (AlN) and boron nitride (BN) were found to be the ideal heat dissipation materials for thermal packaging after he investigated the properties of AlN/BN epoxy composites.
The aim of this paper was to investigate the preparation and thermally conductive, morphology and crystallization properties of polyamide composites based on BN filler. Two methods were used to prepare the PA6 composites which including melting method and solution method. The results will be discussed and provided theoretical bases for the preparation of composites with excellent thermal conductivity.