additives feature | Thermal conductivity
50 45 40 35 30 25 20 15 10 5 0
elongation at break in % fracture toughness in kJ/m2 Elastic modulus in gPa
insulation and a low density of 2.2 g/cm³. An established application of the material is replacing talcum in cosmetic products. The electronics industry also uses boron nitride in gap fillers for thermal management of integrated circuits. other applications range from sintered parts to coatings in contact with liquid metals in the foundry industry and the metallization of plastic film. modern processing technologies with continuous
furnaces, as used at ESK ceramics, allow processing of high volumes with excellent product performance at reasonable cost. Based in Kempten, germany, ESK is a worldwide leader in hBn products and the major producer in Europe.
20 30 hBn filler level in wt. %
Fig 3: MEChaniCal ProPErTiES oF Pa66/hBn CoMPoundS wiTh diFFErEnT FillEr loadingS
filler. This can be explained by the theoretical model itself: the lewis-nielsen-model assumes filler particles have an aspect ratio of 1, i.e. spherical particles, which is true for most ceramic fillers. However platelets or fibres allow for multiple particle-to-particle paths, and therefore thermal conductivity is preferentially enhanced in the direction of fibre or platelet orientation. The Halpin-Tsai model [4]
takes these factors into
account and predicts a strong increase in thermal conductivity with aspect ratio [1]
. At 40 vol.% Bn filler
level and an aspect ratio of 30, thermal conductivity of 5 w/m.K is to be expected and measured in compounds if appropriately processed. ESK ceram- ics’ Boronid hexagonal boron nitride shows such high aspect ratios (Fig 1) and yields the expected results as can be seen in Fig 2. So far, hBn has had only very limited applications
in thermoplastic compounding but – thanks to its unique properties – we predict it is well on the way to becoming a standard filler in thermally conductive engineering compounds.
Fig 4: SEM of Boronid
TCP15-100 free-flowing agglomerates
28
Production and applications Hexagonal boron nitride is not found in nature; it therefore has to be synthesized from readily available boron- and nitride-rich pre-cursors, such as melamine and boric acid. The graphitic hexagonal structure requires thermal processing at high temperatures (up to 2,000°c) in order to form large crystalline platelets of high purity and excellent thermal conductivity. pure white in colour, hBn offers excellent electrical
compounding world | February 2012
www.compoundingworld.com 45
Processing hBN in twin-screw extruders The high aspect ratio and small particle size (typically 1-20 µm diameter) of hBn result in high specific surface areas in the range of 2-20 m²/g, low tap densities and consequently poor flow characteristics. in the first compounding applications, this caused
major problems in twin-screw extrusion. This was even more difficult at the high filler levels used in early developments and compromised processing cost and stability. ESK ceramics has therefore developed easy-to-
process, free flowing agglomerates with high tap density (> 0.4 g/cm³). They allow very high productiv- ity rates, even at increased filler levels in twin-screw extrusion. The agglomerates (see Fig 4) are easily dispersed by the shear forces in the thermoplastic melt. For concentrations up to 30-40 vol.%, it is preferable to use a gravimetric side feeder. not only has the improved processing of this new
filler type (Boronid Tcp15-100) proved a break- through, it is the first time a combination of com- pound thermal conductivity and reasonable mechan- ical performance has been achieved with hBn fillers.
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