Boron Nitride in Thermoplastics

摘要

Boron Nitride (BN) is increasingly being used as a filler in thermoplastics tornenhance several material properties. Due to the high inherent thermal conductivity andrnseveral other desirable properties, BN powders are ideal fillers to make thermallyrnconductive thermoplastic composites. These BN-filled thermoplastics are used to makernthermally conductive housings and heat sinks as part of the thermal management solutionrnto dissipate heat generated in electronics, sensor and most recently, Light Emitting Diodern(LED) based lighting devices.rnBoron nitride is a synthetic ceramic material that is isoelectronic with carbon. Therntwo most common forms are hexagonal boron nitride (hBN), a soft form with arnhexagonal crystal structure comparable to graphite; and cubic boron nitride (cBN), a hardrnform analogous to diamond. Due to its inherent crystal structure, hexagonal BN isrnanisotropic, i.e. a number of physical properties depend on the direction within therncrystal. An important consequence of this anisotropy is that the final BN-thermoplasticrncomposites also exhibit anisotropic properties due to the alignment of BN crystals withinrnthe composite. To overcome the problem of anisotropy, Momentive PerformancernMaterials (“Momentive”) offers a unique line of “agglomerate” BN powders wherein arnnumber of platy BN crystals are agglomerated to make a larger particle. The orientationrnof BN crystals platelets within the agglomerated particle is effectively random, and as arnresult, the agglomerate particle exhibits more isotropic properties.rnWhile these agglomerate BN powder grades offer the possibility of isotropic BNthermoplasticrncomposite materials, maintaining their integrity during the various processrnsteps is a challenge. Due to their fragile nature, BN agglomerates frequently break downrninto their component platelets leading to anisotropic composites as explained earlier.rnThe relevance of BN-plastic composites as a material choice for LED lamprnhousings is discussed. The thermal conductivity of single crystal platelet and agglomeraternboron nitrides in thermoplastic resins is examined, along with the effect of various BNrnpowder properties such as purity, morphology and crystal size on thermal conductivity ofrnthe composites is explored. The anisotropic properties will be characterized usingrnthrough-plane and in-plane thermal conductivity measurements using the laser flashrntechnique. The effects of screw configuration during extrusion, and molding conditionsrnon the thermal conductivity are discussed. Other physical strength properties, such asrntensile strength, strain at break and dielectric strength are also examined and trade-offsrnfrom an application perspective discussed.