Efficient construction of boron nitride network in epoxy composites combining reaction-induced phase separation and three-roll milling

Abstract

Self-construct of filler network was developed to enhance the thermal conductivity of epoxy based composites. The selective distribution of hexagonal boron nitride (hBN) particles in epoxy/PES blends and enhanced dispersion of hBN were successfully obtained by reaction-induced phase separation (RIPS) and three-roll milling (TRM), respectively. The agglomeration of hBN particles in epoxy composite was significantly decreased by three-roll milling, and this was also supported by micromechanics theory, resulting in the construction of the highly efficient filler network. Compared with conventional epoxy/hBN composites and epoxy/PES/hBN composites prepared by mechanical stirring, the epoxy/PES/hBN composites fabricated by three-roll milling showed a great enhancement in thermal conduction and impact strength. The thermal conductivities of the epoxy/PES/hBN composites fabricated by three-roll milling reached 0.52 W/mK at a loading of 10 wt% hBN, which is almost 2.6 times that of the neat epoxy, 40.5% and 26.8% higher than that of epoxy/hBN composites and epoxy/PES/hBN composites without three-roll milling, respectively. Furthermore, the micron-size hBN has a greater advantage for thermal transfer in comparison with boron nitride nanosheets (BNNS). This study provides a readily scalable production method which allows for exfoliation of BN fillers as well as in situ construction of filler networks in thermosetting resin with an environmental friendly route.

Introduction

The upgrade and update of electronic components and large electrical equipment have put higher demands on the reliability of thermally conductive and insulating materials [[1], [2], [3]]. Stability and lifetime of energy systems such as solar power and light-emitting diodes (LEDs) have been significantly affected by thermal and insulating properties of materials [4,5]. Polymers have received widespread attention because of the wonderful processability and excellent dielectric property (such as epoxy resins), but most polymers exhibit lower thermal transfer capabilities (about 0.1–0.5 W/mK) [6,7]. Most ceramic fillers have been added to achieve high thermal conductivity and electrical insulation in polymer composites, such as aluminum nitride [8], hexagonal boron nitride [[9], [10], [11], [12]], silicon carbide [13], etc., which have been used for realizing high thermal conductivity enhancement in composites.

Addition of a large amount of fillers to the polymer matrix can greatly enhance the thermal conductive performance of composites. However, high viscosity and deteriorated mechanical property limit the production and processing of composites [14]. The construction of a continuous thermally conductive pathway to improve the utilization of fillers has become an essential issue in the preparation of high thermal conductivity composites [15,16]. One method is to build a three-dimensional interconnect filler network to obtain high thermal conductive composites at a low filler loading [17]. However, complicated processes of the method have limited applications in large-scale fabrication. Another approach is to construct segregated architecture, fillers with high thermal conductivity are located at the interfaces of polymer granules, forming a dense thermal conductive network by hot pressing [18]. The application in the commercial field is still limited in spite of the simple compression molding process, and it only fits for thermoplastic polymer composites. The selective distribution of fillers could also form continuous thermal conductive paths in two immiscible polymer blends [19]. The strategy has received more attention in thermoplastic/thermoplastic systems, which also been applied in electrically conductive composites [20]. In the thermoplastic/thermosetting resin system, thermal conductive network can be designed by reaction-induced phase separation (RIPS), and phase structure of final blends is mainly related to the content of thermoplastic matrix [21]. A suitable phase structure of polymer blends could be formed to function as a carrier for fillers to construct a continuous filler network structure, which could achieve thermal conductive paths at low filler loading. However, the efficiency of the formation of filler network still need to be improved.

Hexagonal boron nitride (hBN) has a wide band gap (5–6eV), which makes it have high electrical resistivity and low dielectric constant [22]. It also has a high thermal conductivity [23]. The lattice structure of a hBN monolayer is similar to graphene, and the properties of composites may be affected owing to the stacking and agglomeration of hBN particles [24]. A small amount of boron nitride flakes can be obtained by sonication in water or organic solvent [25], but the low yield of hBN flakes and high cost of organic solvents have limited the application in mass production. In addition, the functionalization of hBN reduces the interaction between the layers, which can obtain a certain amount of few-layer hBN [26]. With the aid of molten hydroxide, the insertion of cations (Na⁺ or K⁺) and anions (OH⁻) increases the self-curling energy of the hBN sheet, and it can also obtain the boron nitride nanosheet [27]. Another approach is exfoliation with a shear force, such as ball milling [28], or using a vortex fluidic device (VFD), the shear action from gravity and centrifugal force could exfoliate hBN particles [29]. However, the complicated processes and low yields have limited the mass production in commercial applications.

In this work, the hBN particles were used as thermally conductive filler and in situ exfoliated by three-roll milling, and thus reduce the adverse effects of stacking and agglomeration of hBN particles in the polymer blend. In addition, polyethersulphone (PES) was introduced into epoxy system, leading to construct an efficient filler interconnect network through reaction-induced phase separation. With different hBN content, the influence of three-roll milling on various properties of the composites has been discussed. Compared with the samples prepared by mechanical stirring, three-roll milling has improved the dispersion of hBN particles in the blend and enhanced the thermal conductive property and impact performance of the filled composites. For comparison, boron nitride nanosheet (BNNS) was also used in this system. Compared with the micro-size hexagonal boron nitride (hBN), BNNS has no obvious advantages for the enhancement of thermal conductivity.