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Abstract The effects of thermal properties of various fillers (magnetite, barite, copper, talc, glass fibres and strontium ferrite) in various proportions on the cooling behaviour of polypropylene matrix composites are investigated in an injection moulding process. A thermocouple in the cavity of the mould records the temperatures at the surface of the composite during injection moulding. From the slope of the cooling curves the thermal diffusivities of the composites are estimated and compared with thermal diffusivities at room temperature and elevated temperatures measured with a transient technique. The cooling curves show different merging sections affected by the after pressure, the diffusivity of the composite and the diffusivity of polypropylene matrix. The cooling behaviour depends on the anisotropic thermal diffusivity of the used composite, which is caused by the alignment of filler material due to the injection moulding process and the interconnectivity of the filler particles. The thermal diffusivity shows the highest value for 30 vol% talc filled polypropylene, whereas the shortest cooling time was found for 35 vol% copper filled polypropylene. The knowledge of the systematic variation of thermal transport properties of composites due to different filler material and filler proportions allows to optimize the mould process and to customize the heat flow properties. Furthermore, the strongly anisotropic thermal transport properties of talc filled polypropylene allow the design of composites with a predefined maximum heat flow capability to transport heat in a preferred direction.69808
Keywords: A. Polymer–matrix composites (PMCs); B. Thermal properties; E. Injection moulding; Particulate filler
1. Introduction
Commonly used plastics, such as polypropylene and polyamide, have a low thermal conductivity. However, new applications, mainly in automotive industries, e.g. for sensors or actuators, require new materials with an enhanced or high thermal conductivity [1]. By the addition of suitable fillers to plastics, the thermal behaviour of polymers can be changed systematically up to significant higher thermal diffusivity of O1.2 mm2/s from 0.2 mm2/s for unfilled polypropylene [2,3]. Such filled polymers with higher thermal conductivities than unfilled ones become
* Corresponding author. Present address: Institute of Metallurgy, Robert- Koch-Str. 42, D-38678 Clausthal-Zellerfeld, Germany. Tel.: C49-5323- 723708; fax: C49-5323-723148.
E-mail address: bernd.weidenfeller@tu-clausthal.de (B. Weidenfeller).
more and more an important area of study because of the wide range of applications, e.g. in electronic packaging [4–6]. The higher thermal conductivity can be achieved by the use of a suitable filler such as aluminium [1], carbon fibres and graphite [7], aluminium nitrides [6,8] or magnetite particles [2]. Also, the cooling behaviour in the mould of the injection moulding machine is influenced by the thermal properties of the polymer-filler composite. However, published values of thermal conductivities of the same filler materials in different polymer matrices vary drastically and a comparison of different materials is difficult or at least impossible [2].
Therefore, polypropylene samples with different com- mercially available fillers (Fe3O4, BaSO4, Cu, glass fibres, talc and SrFe12O19) were prepared by extrusion and injection moulding using various volume fractions (0–50%). Magne- tite and barite are generally used to increase the weight of
346 B. Weidenfeller et al. / Composites: Part A 36 (2005) 345–351
Table 1
Selected properties of filler materials
Magnetite, Fe3O4 Barite, BaSO4 Talc, Mg3[Si4O10][OH]2