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of stry accepted Abstract Therefore, polypropylene samples with different com- Composites: Part A 36 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. Keywords: A. Polymermatrix 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 mm 2 /s from 0.2 mm 2 /s for unfilled polypropylene 2,3. Such filled polymers with higher thermal conductivities than unfilled ones become more and more an important area of study because of the wide range of applications, e.g. in electronic packaging 46. 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. Cooling behaviour of particle injection moulding Bernd Weidenfeller a, * , Michael a Institute of Polymer Science and Plastics Processing, Technical University b GeoForschungsZentrum Potsdam, Section 4.1 Experimental Geochemi Received 25 June 2004; filled polypropylene during process Hofer b , Frank R. Schilling b Clausthal, Agricolastrasse 6, D-38678 Clausthal-Zellerfeld, Germany and Mineral Physics, Telegrafenberg, D-14473 Potsdam, Germany 4 July 2004 (2005) 345351 talc and SrFe 12 O 19 ) were prepared by extrusion and injection moulding using various volume fractions (050%). Magne- tite and barite are generally used to increase the weight of Koch-Str. 42, D-38678 Clausthal-Zellerfeld, Germany. Tel.: C49-5323- 723708; fax: C49-5323-723148. E-mail address: bernd.weidenfellertu-clausthal.de (B. Weidenfeller). * Corresponding author. Present address: Institute of Metallurgy, Robert- mercially available fillers (Fe 3 O 4 , BaSO 4 , Cu, glass fibres, 3. Experimental Talc, Mg 3 Si 4 O 10 OH 2 Strontium ferrite, SrFe 12 O 19 Copper, Cu Glass fibres l 11 :1.76G0.00,l 33 : 10.69G1.35, a:2.97,a:3.00G0.10, a:6.10G0.90 l 11 :401 l:1.21.5 13 14 15 2.0 1.5 15 11 Platelet Irregular Irregular Fibre 2.78 5.11 8.94 2.58 are tes: Part A 36 (2005) 345351 2. Theoretical considerations The Fourier law of heat transport in one dimension is given by vT vt Z a v 2 T vx 2 (1) with temperature T, time t, position x and thermal diffusivity a. In an homogeneous body, thermal diffusivity a and thermal conductivity l are interrelated by specific density r polypropylene, e.g. for bottle closures (cosmetics industry, cf. Ref. 10), strontium ferrite is used in polymer bonded magnets, glass fibres are used for the reinforcement of materials, and talc is an anti-blocking agent. However, copper was chosen as additional filler because of its high thermal conductivity compared to the other materials. The thermal properties of these injection moulded samples and the injection moulding behaviour were investigated and correlated to the amount and the kind of filler material. Table 1 Selected properties of filler materials Magnetite, Fe 3 O 4 Barite, BaSO 4 Thermal conduc- tivity (W/(m K) a:4.61G0.42, a:5.10,l 11 :9.7 l 11 :2.07G0.02,l 33 : 2.92G0.07, a:1.72G0.04 Reference 13 13 Mean particle diam- eter (mm) 9 1.5 Particle shape Irregular Irregular Density (g/cm 3 ) 5.1 4.48 a denotes measurements on monomineralic aggregates. Directions of anisotropy and l 33 are parallel to the crystallographic axes a, b and c, respectively. B. Weidenfeller et al. / Composi346 and specific heat capacity c p according to l Z c p ra (2) Assuming an injection moulding process with an isothermal filling stage for a polymer with a temperature T P and a constant temperature of the mould T M as well as a temperature independent thermal diffusivity a, an analytical solution of Eq. (1) results in 9 T ZT M C 4 p T P KT M ! X N nZ0 1 2nC1 exp K a2nC1 2 p 2 t s 2 C26C27 sin 2nC1px s C18C19 (3) In Eq. (3), s denotes the wall thickness of the injection moulded part and T the temperature of the moulding after 3.1. Materials Test materials were supplied by Minelco B.V. (The Netherlands). Minelco B.V. prepared in cooperation with RTP s.a.r.l (France) several polypropylene (PP) compounds with various fillers (Fe 3 O 4 , BaSO 4 , Cu, glass fibres, talc and SrFe 12 O 19 ) in an extrusion process similar to that described in Ref. 2. The filler materials are commonly used materials in industrial products. The filler particles do not have a time t after injection. Neglecting higher order terms, Eq. (3) can be reduced for the position xZs/2 to T ZT M C 4 p T P KT M expK ap 2 t s 2 C18C19C26C27 (4) Eq. (4) gives a relation between cooling rate and thermal diffusivity in an injection moulding process, where high thermal diffusivities result in a higher cooling rate and shorter process cycles. specified by the thermal conductivity tensor (l 11 , l 22 , l 33 ), where l 11 , l 22 surface coating which can affect thermal properties. Some selected properties of the filler materials are listed in Table 1. Fig. 1. Photograph of the used mould for the injection moulding experiments. The mould consists of a standard tensile test sample and a test bar for the measurement of thermal diffusivity. time curves the same injection moulding parameters for all composite materials were chosen. The used injection machine. The position of the thermocouple for temperature measurements is Part A 36 (2005) 345351 347 3.2. Thermal diffusivity measurements The thermal diffusivity of the polymers is measured by a transient method 12, closely related to laser-flash experi- ments 11. The used transient technique is especially optimized for measurements of polyphase aggregates. A temperature signal is transferred to the upper side of the sample and registered by a thermocouple. The transferred temperature signal starts a thermal equilibration process in the specimen, which is recorded by a thermocouple as the difference between samples rear surface and a constant temperature in a furnace and which is used for the evaluation of thermal diffusivity. A least squares algorithm is used to determine the thermal diffusivity, while varying systematically the thermal diffusivity value in an especially Fig. 2. Mold with cavity for preparing test samples in an injection moulding marked by an arrow. B. Weidenfeller et al. / Composites: designed finite-difference scheme. A detailed description of the apparatus is given by Schilling 12. The accuracy of the measurements of the polyphase aggregates is 3%. For thermal diffusivity measurements, small cylinders of 10 mm diameter and 56 mm height were cut out of the injection-moulded rods (cf. Fig. 1). 3.3. Injection moulding With an injection moulding machine (Allrounder 320C 600-250, Arburg, Germany) standard samples for measuring tensile properties together with a rod for thermal measure- ments of 10 mm diameter and 130 mm length were prepared in one mould (cf. Fig. 1). In the cavity of the tensile test bar a chromel alumel (Type K) thermocouple was applied. During injection moulding experiments the temperature was recorded every 0.5 s by a digital multimeter and stored in a personal computer. The position of the thermocouple at the sample surface and its position in the cavity of the ejector are shown in Figs. 1 and 2, respectively. The thermocouple submerges approximately 0.2 mm into moulding parameters are listed in Table 2. The resultant characteristic times of the injection moulding cycle are tabled in Table 3. 4. Results and discussion In Fig. 3, the cooling behaviour of polypropylene without and with various fractions of magnetite filler are presented. the cavity. Therefore, a good thermal contact between polymer and thermocouple even after shrinkage 10 of the moulding is ensured. For a better comparison of the recorded temperature Table 3 Characteristic times in one injection moulding cycle starting with the injection of the polymer into the cavity at time t i ZK8.5 s until the ejection of the mould at t f Z68 s Injection time (s) K8.52 Dwell time (s) 29 Cooling time (s) 954 Open/close time ejection time (s) 5468 Total cycle time (s) 76.5 These times define the time axis (abscissa) of Figs. 3 and 6. Table 2 Injection moulding parameters during preparation of sample rods for measurements of thermal diffusivity by transient technique Mass (polymer) temperature (8C) 200 Mould temperature (8C) 20 Cycle time (s) 76.5 Injection time (s) 10.5 Dosing time (s) 12.4 Holding pressure time (s) 7.0 Injection pressure (Pa) 6!10 7 ylene composites with various filler fractions of Fe 3 O 4 . The symbols are measured tes: Part A 36 (2005) 345351 At a time t 0 Z0 s the temperature measured by the thermocouple reaches a maximum value around 200 8C. With increasing time the observed temperature decreases. After tZ54 s the mould opens and the cooling behaviour recorded with the thermocouple changes because it is no longer in contact with the injection moulded material. Due to the large diameter of the rod, the time (54 s) until the mould is opened and the injection moulded parts are ejected is chosen relatively high to ensure that the parts are surely solidified. It can be seen in Fig. 3 that the slope of the curve changes significantly after tz9 s, which corresponds to the time Fig. 3. Comparison of cooling curves of unfilled polypropylene with polyprop values; the lines are regression lines (cf. text). B. Weidenfeller et al. / Composi348 where the after pressure is removed. Additionally, Fig. 3 points out that the composite in the cavity cools down faster with increasing magnetite fraction. To reach a temperature of TZ60 8Ca temperature far below the solidification of the samplethe polypropylene needs in the described exper- iment a time of tZ50.5 s, whereas cooling time of polypropylene with 50 vol% Fe 3 O 4 is reduced to tZ30.9 s (cf. Table 4). The reduced cooling time is in good agreement with the increased thermal diffusivity of magnetite filled composites due to the high thermal diffusivity of the particles (cf. Table 1) which leads, regarding Eq. (4), to an increased cooling rate. The temperature time dependence in Fig. 3 does not follow a simple linear behaviour expected for temperaturetime curves by Eq. (4) in a logarithmic plot. Only for the unfilled polypropylene the measured values can be fitted with a single straight line between approximately 15 and 54 s. The slope of this line leads to a diffusivity of az0.21 mm 2 /s (cf. Eq. (4). The other measured cooling curves of the polypropylene-magnetite composites are fitted in each case with two straight lines, for the high temperature (a 1 ) and low temperature (a 2 ) region. The thermal diffusiv- ities estimated from the slopes of the regression lines are a 1 (15 s!t!40 s)z0.24 mm 2 /s and a 2 (41 s!t! 54 s)z0.19 mm 2 /s for PP with 15 vol% Fe 3 O 4 , a 1 (12 s! t!33 s)z0.29 mm 2 /s and a 2 (34 s!t!54 s)z0.19 mm 2 /s for PP with 30 vol% Fe 3 O 4 ,anda 1 (9 s!t! 22 s)z0.33 mm 2 /s and a 2 (28 s!t!54 s)z0.16 mm 2 /s for PP with 50 vol% Fe 3 O 4 (cf. Table 5). It is remarkable that the calculated thermal diffusivities a 1 of the higher temperature parts of the cooling curves are a little bit lower than the diffusivities measured with the transient technique, while the calculated thermal diffusivities a 2 of the lower temperature parts of the cooling curves meet the measured diffusivity values Table 4 Time t to cool down a polypropylene-filler composite from a mass (polymer) temperature of T M Z200 down to 60 8C Composite Filler fraction (vol%) t (from 200 to 60 8C) (s) PP 0 50.5 PPCFe 3 O 4 15 46.4 PPCFe 3 O 4 30 40.5 PPCFe 3 O 4 45 34.6 PPCFe 3 O 4 50 34.9 PPCBaSO 4 15 44.3 PPCBaSO 4 30 40.7 PPCBaSO 4 45 35.6 PPCCu 15 40.5 PPCCu 30 33.8 PPCCu 35 29.0 PPCglass fibres 15 46.0 PPCglass fibres 30 41.8 PPCglass fibres 35 40.8 PPCtalc 15 45.7 PPCtalc 30 42.5 PPCSrFe 12 O 19 30 40.9 The cooling is measured in situ within a cavity of the mould by a K-type thermocouple. of unfilled polypropylene quite well (cf. Table 5 and Fig. 4). Fig. 4 shows the measured thermal diffusivity data of the investigated samples at ambient conditions. It can be seen that the thermal diffusivity of the magnetite-polypropylene 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 Table 5 Thermal diffusivity estimated from the cooling behaviour of injection moulded polypropylene-filler composites using the slope of the regression lines (a 1 , a 2 ) (cf. Fig. 3) compared to thermal diffusivity values measured by the transient method (a) Composite Regression lines Transient method a 1 (mm 2 /s) a 2 (mm 2 /s) a (mm 2 /s) PP 0.21 (12055 8C) 0.19 (26 8C) PPC15 vol% Fe 3 O 4 0.24 (12067 8C) 0.19 (6751 8C) 0.27 (26 8C) PPC30 vol% Fe 3 O 4 0.29 (12068 8C) 0.19 (6845 8C) 0.35 (26 8C) PPC50 vol% Fe 3 O 4 0.33 (12577 8C) 0.16 (6745 8C) 0.48 (26 8C) The temperature values in parenthesis give the temperature region of the regression lines and the ambient temperature during the measurement with the transient technique. B. Weidenfeller et al. / Composites: Part A 36 (2005) 345351 349 composite is increased from aZ0.19 for unfilled poly- propylene up to aZ0.48 (PPC50 vol% Fe 3 O 4 ) with increasing magnetite loading. Therefore, the cooling time becomes shorter for higher magnetite filler fractions (Fig. 3). One reason for the change in the slope of the cooling curves shown in Fig. 3 is a change of the thermal diffusivity with temperature which is shown in Fig. 5 for magnetite and barite polypropylene composites with 45 vol% filler fraction. With increasing temperature thermal diffusivity decreases. Therefore, the values derived from mould experiments should be smaller than the measured values of the composites at room tempera- tures. Thermal diffusivity of the PP matrix is mainly caused by phonons and is related to the mean sound velocity v and mean free path length l of phonons according to a Z 1 3 vl (5) 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. solidification temperature T S no crystallites in the poly- propylene matrix are present, but below T S 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 samples 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. tes: Part A 36 (2005) 345351 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 Fig. 6. Comparison of the cooling behaviour of polypropylene matrix composites machine. B. Weidenfeller et al. / Composi350 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 from the thermal conductivity which is approximately 40 times higher than that of talc. This might be related to the poor interconnectivity of the particles in the composite, which was shown by Weidenfeller et al. 3. It was shown that the interconnectivity, which is a relative measure
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