FEATURE “Automotive” Effect of particle size Particle size (ALF1 or ALF2) does not have any noticeable effect, as seen in Figures 2 and 3. The differences occurred when studying the uniformity of the two studied properties. The electrical and thermal conductivities and filler content were measured on 15 to 20 samples distributed in each injectionmoulded part (see Figure 1). Strong non-uniformity of both properties was found for thick fibres (ALF1); this was mainly due to the non-uniform distribution of the filler particles (Figure 4). Reducing the particle size (ALF2 fibres) significantly improved the uniformity of the filler distribution (Figure 5) and the dispersion of the measured properties around their mean values in the whole part was reduced by a factor of 2 [3]. g…/… Filled polymers with high thermal conductivity (Figure 1) were moulded using a semi-industrial injection machine. Small 15 x 15 x 2 or 4 mm samples were cut from the flat injection-moulded plates to analyse the thermal and electrical conductivities and the filler contents. Thermal conductivity was measured using a hot guarded plate apparatus designed for small samples analysis. Electrical conductivity was measured after deposition of a 100 nm silver electrode on both large faces of the samples. Main results The best results were obtained with aluminium fibres. Effect of filler content As shown in Figure 2, electrical conductivity increases sharply for a filler content around 30% by volume, which corresponds to the percolation threshold of the compound. Higher filler contents do not affect electrical conductivity. Thermal conductivity shows a different behaviour (Figure 2): it always increases as the filler content increases, the sharp increase being obtained at higher volumetric filler contents (44%). At this metal concentration, the composite exhibits a thermal conductivity 10 times higher than the pure polymer. Fig. 4: Distribution of ALF1 fibre content. Fig. 2: Electrical conductivity of PBT filled with aluminium fibres. Fig. 5: Distribution of ALF2 fibre content. Effect of particle orientation Optical observations showed that the particles were preferentially oriented along the main flow direction (x-axis). This explained the anisotropy of the injection-moulded parts, the in-plane thermal conductivity being 2.5 (y axis) to 4.5 (x axis) higher than the transversal one. Fig. 3: Thermal conductivity of PBT filled with aluminium fibres. JEC Composites Magazine / No34 July-August 2007