additives feature | Thermal conductivity in plane (x/y)
5 4 3 2 1 0
bulk (x/y/z)
be increased by a factor of up to 20 with sufficient cooling provided. This is only achievable using improved thermally
conductive fillers. The complex requirements for these fillers limit the selection to just a few candidates. This is another reason why the market development of thermally conductive compounds for electrically insulating applications has been slow in the past. First of all, the filler’s thermal conductivity needs
20 30 hBN filler level in wt. % Fig. 2: Thermal properTies oF pa66/hBN compouNds
new automotive drive systems, issued their new specifications for high-volume products. The second obstacle has been overcome by new
thermally conductive functional fillers developed for thermoplastic compounding described below in this article.
The third obstacle arose when r&d activities focused
on developing compounds with thermal conductivity as close as possible to the metals to be replaced. However, recent engineering studies have shown that thermal conductivities in the range of 2-5 w/m.K are sufficient for most applications [1, 2]
. This allows for lower filler
contents and improved mechanical performance of thermally-conductive compounds at reasonable cost. it is widely accepted that the replacement of metals
with plastics enables major reductions in system costs. nevertheless, purchasing-driven approaches have focused on cost/kg of fillers or compounds, rather than on system cost, and have impeded the success of thermally-conductive compounds in the past. This article will show how this last hurdle can been over- come by modern approaches.
Recent achievements A growing number of compounders are able to supply electrically insulating, but thermally conductive thermoplastic engineering compounds with realistic thermal conductivities from 2-5 w/m.K [6, 8]
45
to be at least a magnitude higher than the required thermal conductivity of the compound, i.e. > 50 w/ m.K. This rules out common low-cost mineral or glass fillers. Additional requirements for high electrical insulation rule out known graphite, carbon fibre and metal fillers. This limits the candidates principally to ceramic fillers, such as Ain, Si3 Al2
n4 o3
, Zno and hexagonal boron nitride (hBn). most early attempts were made with Al2
o3 , Sic, fillers,
mainly due to the material’s ready availability and assumed low cost. However these attempts caused problems in thermoplastic processing that limited market acceptance. Al2
o3 can cause unacceptable
levels of wear in compounding extruders or injection moulding systems, leading to prohibitive processing costs [3]
. Thermal conductivities in excess of 2 w/m.K can
only be reached at filler levels higher than 50 vol.% at which point wear problems become even more severe. At the same time, fracture toughness and elongation at break are reduced to levels outside of typical specifications [3]
. High abrasiveness is a common feature of all the
above-listed ceramic fillers, with the sole exception of hBn. Both hBn and hexagonal graphite show comparable lattice structure and therefore similar self-lubricating and anti-wear properties. This is not only a prerequisite for avoiding wear problems, but also helps to reduce shear forces in extrusion at high filler levels. in addition, hBn yields much higher thermal
conductivities in compounds than are predicted from theoretical models like lewis-nielsen [5]
. This is due
to its platelet structure, expressed as a high aspect ratio (diameter/thickness ratio). The lewis-nielsen model does not offer good
news to developers of thermally conductive com- pounds: even at Al2
o3 . Although far
lower than most metals, this nevertheless allows a dramatic improvement in thermal management compared with unfilled polymers such as polyamide 6 (0.25 w/m.K). in simple terms, the thermal transfer can
26 compounding world | February 2012 filler levels of 30 vol.%, the
predicted and measured thermal conductivity is still only 1 w/m.K. At the same filler level, a boron-nitride-filled
compound can reach as much as 5 w/m.K [Fig 2]. However, the lewis-nielsen model predicts thermal conductivity of only 2 w/m.K, even with 30 vol.% hBn
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Thermal conductivity in W/m.K
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