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Fig. 4. Thermal diffusivity values of injection moulded polypropylene samples with different fillers and various filler proportions measured by a transient technique at room temperature (cf. text). Solid lines are plotted to guide eyes.
Above the solidification temperature of the PP matrix (around 110 8C, DSC measurements) the thermal diffusivity of the matrix is reduced due to the lowered bulk modulus K which results in a reduced phonon velocity (vz(K/r)0.5) and reduced mean free path length of phonons in a liquid (Einstein approximation). Furthermore, above solidification temperature TS no crystallites in the poly- propylene matrix are present, but below TS a crystallization in the polypropylene matrix appears, and the degree of crystallization as well as the bulk modulus of the composite is dependent on the amount of filler [16]. The presence or absence of crystallites affects the bulk modulus K and the phonon free path. Other reasons for the discrepancy between diffusivity values of the different experiments are the non-isobaric conditions in the injection moulding process and the non-isothermal conditions along the sample’s thickness.
The cooling behaviour of magnetite, barite, glass fibre, talc, hard ferrite and copper fillers in comparison with the unfilled polypropylene are plotted in Fig. 6. Only the cooling behaviour of the unfilled and the copper filled polypropylene show significant differences to the other composites.
Fig. 5. Temperature dependence of thermal diffusivity of magnetite and barite filled polypropylene with a filler content of 45 vol%. The symbols represent measured values, the lines are deduced by linear regression.
350 B. Weidenfeller et al. / Composites: Part A 36 (2005) 345–351
Fig. 6. Comparison of the cooling behaviour of polypropylene matrix composites filled with filler fraction of 30 vol% in the cavity of an injection moulding machine.
The copper filled composite cools down much faster than the other investigated composites. The temperature of the unfilled polypropylene is during the whole injection moulding process higher than the temperature of the other investigated materials. The cooling behaviour of the other composite materials does not show large differences. The temperatures of the magnetite loaded PP is a little bit lower than the temperatures of the barite filled PP at the same cooling time. The temperatures of the strontium ferrite polypropylene composite again are a little bit lower than those of the magnetite filled polymers.
While the measured thermal diffusivity of the talc filled polypropylene is much higher than the thermal diffusivity of the other investigated materials and even much higher than that of the copper filled polypropylene, the cooling behaviour of talc is smaller than that of the other investigated materials. Weidenfeller et al. [3] report in the talc filled composite an alignment of the talc particles oriented along their direction of highest thermal conduc- tivity in the direction of the flow, due to the moulding process. The measurements of thermal diffusivity are made parallel to this axis of highest thermal conductivity, whereas the temperature measurements in the injection moulding process reveal the diffusivity perpendicular to the flow direction. This implies that the talc filled PP samples have a strong anisotropy with a maximum in the flow direction and a minimum perpendicular to the flow. The anisotropy of the injection moulded specimens due to the geometry of the particles is shown in Ref. [3].
In spite of the high thermal conductivity of the copper (cf. Table 1) compared to the other used filler materials, the cooling behaviour is relative poor and the measured temperatures in the cavity are not as significant different from those of the other composites as could be expected