Boron Nitride Powders Improving Thermal Conductivity in Polymers

Boron nitride (BN) is increasingly being used as a filler in thermoplastics, primarily to increase the thermal conductivity of the resins. Filler-reinforced plastics are being considered to replace traditionally metal parts in a number of markets, with applications such as under-hood automotive parts, sensors and housings for motors, LEDs, and other electronic devices.

Boron nitride is a synthetic ceramic material that is isoelectronic with carbon. Like carbon, boron nitride exists in multiple allotropic forms. The two most common forms are hexagonal boron nitride (hBN), a soft form with a hexagonal crystal structure comparable to graphite; and cubic boron nitride (cBN), a hard form with a crystal structure analogous to diamond. Hexagonal boron nitride can be converted to the cubic form using a high-temperature, high-pressure process.

Hexagonal boron nitride crystals are made of planar sheets of covalently bonded boron and nitrogen atoms that make the a-b plane of the crystal. Van der Waal’s forces hold multiple layers of such BN planes together in the c direction. An important consequence of this crystal structure is that the crystals have anisotropic properties, i.e. the properties in the crystal’s a-b plane are different from the through-plane properties. For example, the in-plane thermal conductivity has been estimated to be > 300 W/mK, while the through-plane conductivity is only about 3 W/mK.

When BN powders are used as fillers in resins, the BN-resin composite materials also demonstrate anisotropic properties, largely determined by the orientation of the platy BN crystals in the final part. To overcome this problem of anisotropy, boron nitride powders have been developed which are agglomerates of single crystal. In such agglomerate grades, platy BN crystals are held together to form a larger particle to randomize their orientation. Such BN powder grades, broadly called agglomerate grades, demonstrate more isotropic properties than do single crystal BN grades.

One of the biggest challenges of using agglomerate BN grades is to preserve the structure through all the processing steps. BN agglomerates are relatively weak and are likely to break down into their component platelet crystals if sheared aggressively during processing. In the case of thermoplastics, the BN agglomerates can break down during the extrusion step and/or the molding step. Both the screw configuration during extrusion and the flow configuration during molding determine the extent of shear of the BN agglomerates. These processing steps should be monitored and controlled to preserve the agglomerates’ beneficial structure.

The thermal conductivity of single crystal platelet and agglomerate boron nitrides in a thermoplastic resin is examined in this paper to consider and explain the effect of particle morphology. The anisotropic properties will be characterized using through-plane and in-plane thermal conductivity measurements on BN-plastic composite parts using laser flash measurements. The effect of screw configuration during extrusion and molding conditions on the thermal conductivity and other physical strength properties, such as tensile strength strain at break, will also be examined and the trade-offs will be discussed. 

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