Technical feature | thermally conductive
Carbon-based fi llers can be effective options for enhancing thermal conductivity in polymer applications that do not require electrical insulation. Imerys Graphite & Carbon’s Daniele Bonacchi provides some selection and application advice
The carbon approach to thermal conductivity
Metal replacement by polymer compounds is increasing demand for thermally conductive plastic materials in applications that require heat dissipation or heat transfer. The use of polymers offers substantial advantages compared to standard metal-based technologies in terms of weight reduction, corrosion resistance, design fl exibility and manufacturing cost reduction [1]. Since polymers are inherently thermally insulating
(<0.5 W/m.K), conductive fi llers must be added in order to provide the required thermal conductivity. Filled
polymers will never reach the thermal conductivity
values of pure metals (>200 W/m.K). However, in many technical applications of thermally conductive plastic compounds such a high level of thermal conductivity is not required and the achievable thermal conductivity range of 0.5 to 20 W/m.K is often suffi cient to fulfi l the requirements. It should be noted, though, that the thermal conductivity of a plastic compound might be an anisotropic property. The causes of such anisotropy are many: polymer crystallinity; anisotropic thermal conduc-
tivity of the additive; and additive orientation are usually considered the most important [2-3]. Carbon-based fi llers are promising
thermally conductive additives and there are various allotropes of carbon available, including graphite, carbon nanotube (CNT), carbon black and graphene. However, when these additives are added to a polymer above a certain threshold in weight percent they also give rise to electrical conductivity, so they cannot be used as such in applica- tions that require electrical insulation. Where high electrical resistivity is needed, ceramic powders such as boron nitride, aluminum oxide and nitride can be used, although the cost associated with high performance additives such as born nitride is typically high [4] while the other options are known to be abrasive to compounding machines and injection moulding systems [5]. Beside electrical conductivity, there are other
numerous properties that must be taken into account, such as compound viscosity or mechanical properties. When choosing a carbon additive, a material form and additive dosing level appropriate for the fi nal applica- tion has to be selected. LED heat sinks are the most demanding applications
in terms of thermal conductivity; they are usually made by injection moulding of engineering polymers contain- ing a thermally conductive additive, usually at high loadings. In LED heat sinks both in-plane and through plane thermal conductivity are important, as well as the specifi c heat sink design, and it is very diffi cult to predict the fi nal cooling power. Nevertheless, an
www.compoundingworld.com
Main image: Graphite can provide enhanced thermal
conductivity for polymer
applications
where electrical insulation is not required
Left: An SEM image of graphite structure
February 2016 | COMPOUNDING WORLD 57
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