JEC COMPOSITES MAGAZINE - Issue #34 - July/August 2007 - 41

Raw material Filled polymers with high thermal conductivity* Adding a conductive filler in a polymer is an effective way to improve the thermal conductivity of plastics. Extensive thermal and composition measurements on injection-moulded polybutylene terephthalate plates show that the best filler consists of short aluminium fibres, and that thermal conductivity can be increased by a factor of 2, 6 and 10 using 20, 42 and 44 % vol. aluminium, respectively. BERTRAND GARNIER, RESEARCH SCIENTIST, CNRS & FLORIN DANES, PROFESSOR ENGINEERING SCHOOL OF THE UNIVERSITY OF NANTES, THERMOKINETICS LABORATORY (UMR CNRS), NANTES, FRANCE filled polymers, but their high cost and lack of stability are major drawbacks. The challenge of thermally conductive polymers Thermally conductive filled polymers are heterogeneous materials containing at least two solid phases: a continuous insulating phase and a disperse conductive phase. The latter is introduced as a filler during the compound processing stage; therefore, the filler particles are distributed irregularly in the end product. Fig. 1: Sample locations in the injection-moulded part (L=100mm). Applications relate to heat transfer in the car industry (radiators, hoods, battery cases for electric vehicles ), electronics or electrical engineering (components, cases ). The use of injection-moulded filled polymers would make it possible to produce geometrically complex parts while saving weight compared to the metal version. In many applications, the suitable thermal conductivity in the transversal direction of the plate is 2 to 4 W.m-1.K-1 instead of the lower conductivity of unfilled technical polymers (from 0.15 to 0.3 W.m-1.K-1). The in-plane conductivity is less relevant for the end-user, although high values can help prevent hot points. The practical difficulty when producing thermally conductive filled polymers is to retain the technical characteristics of the bare polymer. As the presence of filler leads to poor mechanical properties (inducing brittleness, for example), great care must be taken to achieve not only good conductivity, but also stability, innocuousness, mechanical quality and, last but not least, low price. Intrinsically conductive polymers are an alternative to The ease of production of thermally conductive filled polymers depends on the type of polymeric material, decreasing in the following order: inks, adhesives/mastics, rubbers, thermosets, and thermoplastics. The first two classes are already technically mature, and rubbers are often made conductive by blending with carbon black; but thermosets are seldom made conductive by filling. Finally, the commercial availability of conductive thermoplastics, though repeatedly announced, has never become effective. Materials, processing and measuring methods Tests were performed using polybutylene terephthalate (PBT) and various types of fillers such as carbon black, ceramics, boron nitride, and aluminium. Two sets of aluminium fibres (ALF1 and ALF2) with the same average length (1.1 mm) and with different average diameters (respectively 90 μm and 160 μm) were considered. The polymer and filler were mixed using a twin screw extruder. Flat plates with dimensions 100 x 100 x 2 or 4 mm No34 July-August 2007 / JEC Composites Magazine

JEC COMPOSITES MAGAZINE - Issue #34 - July/August 2007

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