Thermal conductivity of polypropylene matrix composites filled with graphite and carbon black
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https://www.eduzhai.net International Journal of Composite M aterials 2013, 3(5): 136-140 DOI: 10.5923/j.cmaterials.20130305.04 Thermal Conductivity of Polypropylene Based Composite Materials Filled with Graphite and Carbon Black Muhammad Jahidul Hoque1,*, Suvra Chakraborty1, Monon Mahbub1, Md. Abdul Gafur2 1Department of M echanical Engineering, Bangladesh University of Engineering & Technology, Dhaka, 1000, Bangladesh 2Department of M aterials & M etallurgical Engineering, Bangladesh University of Engineering & Technology, Dhaka, 1000, Bangladesh Abstract The thermal conductivity of systems based on polypropylene, filled g raphite and carbon black have been studied. Graphite and carbon black having different particle shapes were used as fillers. The PP-Gr & PP-CB co mposites were prepared using extrusion and the hot compression mold technique. The composite preparation conditions allowed the formation of a random d istribution of filler particles in the polymer matrix. LEES method was followed for thermal conductivity measurement . Different weight percentage analysis for both type of filler in a single matrix were allowed. Analysis by varying total mass of the composite were also performed. The concentration dependence of the thermal conductivity shows no significant ju mp for the percolat ion threshold. Experimental results showed promising trend as described by Lichtenecker proposal and shows almost well-disposed accordance with theoretical values. Keywords Po ly mer Co mposites, Co mpression Molding, Perco lation Threshold, Packing Factor, Thermal Conductivity 1. Introduction Co mposite materials are made by co mbin ing reinforcement (fiber) with matrix (resin), and this combination of the fiber and matrix provide characteristics superior to either of the materials alone. The term co mposite material is used to describe the macroscopic combination of two or more materials. The fundamental goal in the production and application of co mposite materials is to achieve a desired performance fro m the co mposite that is not obtainable fro m the separate constituents or fro m other single phase materials. Nowadays, the use of composite materials in d ifferent fields of engineering (microelectronics, aeronautics and space, transport, etc.) is continually increasing. The interest for these materials arises fro m the fact that it is possible to develop new materials with properties adapted to specific applications. Moreover, in some cases, composite materials allo w the physical propert ies of each component used in the manufacturing process to be combined. Po ly mer co mp os it e are n o w an imp o rtant class o f engineering materials. The propert ies o f co mpos ites are largely influenced by the properties of their constituents and t h e d ist rib u t io n and in t eract io ns amo n g th em. Th e constituents usually interact in a synergic way, prov iding * Corresponding author: firstname.lastname@example.org (Muhammad Jahidul Hoque) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved properties that are not accounted for by a simp le volu me fraction of the components. Along with the volume fraction and the distribution of discrete units in the continuous phase, the interfac ial area plays an important role in determining the extent of interaction between the reinforcement and the matrix and in th is way the final properties of the co mposite. Versatile co mbination of the matrix and filler can be possible to improve and develop different properties of the composite materials. Such as polymer composites filled with meta l are of interest for many fields of engineering. Carbon based (graphite, carbon black) co mposites have exceptional mechanical properties wh ich are unequaled by other materials. The material is strong, stiff, and lightweight, corrosion resistive. Graphite co mposite is the material of choice for applications where lightweight & superior performance is paramount, such as components for spacecrafts, fighter aircrafts, and racecars. Today the composite marketplace is widespread. As reported recently by the SPI Co mposites Institute, the largest market is still in transportation (31%), but construction (19.7%), marine(12.4%), electrical/electronic equip ment (9.9%), consumer(5.8%), and appliance/business equipment are also large markets. The aircraft/airspace market represents only 0.8% which is surprising with regard to its importance in the orig ins of composite. The purpose of this study is to measure the thermal conductivity of polymer co mposite using a newer combination of polypropylene - graphite, polypropylenecarbon black. And to provide general informat ion and specifications on graphite composite materials and carbon black co mposite materials for designing lightweight, thermal International Journal of Composite M aterials 2013, 3(5): 136-140 137 conductive or resistive, high performance products with graphite and carbon black composites. In our study we prepared sample through several steps such as particle t reatment, extruding(init ial mixing of polymer & filler), hot pressing (final sample), samp le preparation. A co mmon poly mer materials polypropylene is used as matrix for both of the filler graphite and carbon black. Using standard procedure of preparing sample the thermal conductivity of the matrix is measured with the help of LEES apparatus. It was intended to find out the trend of the thermal conductivity of polymer co mposite for different percentage content of graphite and carbon black as filler in certain weight of poly mer composite and also by varying the total weight content of single poly mer co mposite. Co mparison of the experimental result with theoretical model result and analyze any degree of divergence of these result and to find out the reason of any discrepancy if any was also studied. 2. Thermal Conductivity of the Composite Several investigations of polymer co mposites with dispersed fillers show the absence of percolation behavior of the thermal conductivity k with increasing dispersed filler concentration[3-6]. The realistic filler geo metric characteristics connected with the packing factor F value have not been taken into account. This is due to the fact that the thermal conductivities of the d ispersed filler kfand of the polymer matrix kp are co mparable to each other, their ratio not being more than 103, whereas the filler electrical conductivity ϭf is 1010–1020 times larger than the polymer conductivity ϭp. The model p redicts the percolation threshold appearance only if the ratio of fille r conductivity to polymer conductivity is larger than 105. In fact, the percolation theory is applied only to systems having conductive sites (or bonds) in a non-conductive mediu m. Lichtenecker proposed such a dependence, that the same function describes both the conductivity and resistance: k= f( kp; kf ;φ); 1/ k = f (1/kp; 1/ kf ;φ) . The following function fulfills the above conditions: k= kp (1 -φ )kfφ (1) Taking the logarith m, we get: log k=(1-φ) logkp+ logkf φ or logk=logkp+(logkf-logkp)φ (2) where φ is the content of the filler in the poly mer co mposite where φ = mfρp/( mfρp +mpρf) mf = Mass of the filler mp = Mass of the polymer ρp = Density of the filler ρf = Density of the poly mer Topological peculiarities of the systems filled with dispersed filler has not taken into account in Eqs. (1)–(2). When such systems are filled, the thermal conductivity of the composites increases fro m kp to the largest kF value within the 0 < φ < F concentration interval. An equation describing the concentration dependence of thermal conductivity has been proposed for that case in , logk=logkp+(logkF–logkp)(φ/F)N (3) Ev idently, if F=1 and N=1, then Eq.(3) is identical to Eq. (2) and the behavior of the system follows Lichtenecker’s dependence. The value of F=1 means that the filler phase possesses such properties that it fills up all co mposite volume at φ=F (limit content), i.e. the filler content changes fro m 0 to 1 volu me portion and kF=kf . This case is realized for systems having continuous second phase, for example, alloys or solutions. If the second phase is a dispersed filler, then the F value is taken into account by φ/F, which also changes fro m 0 to 1.This evaluation of F can help further to determine the value of N and trend of the findings. 3. Test for Thermal Conductivity According to ASTM Method C 201 the thermal conductivity can be measured. An alternative method for the test of therma l conductivity is LEES and Chorlton’s method. The thermal conductivity of a poorly conductive poly mer composite having thin layer is measured by this method. Difficulty in maintaining the face at uniform temperature overcame by placing a good conductor copper, of exact ly the same diameter as the experimental slab on each side of poor conductor and in measuring that temperature. If T1= Temperature of top copper plate at steady state. T2= Temperature of bottom copper plate at steady state. A= Cross sectional area of poly mer co mposite s amp le. K= Thermal conductivity of polymer co mposite s amp le. d= Thickness of the slab polymer co mposite sample. then the quantity of heat conducted per second through composite sample is Q=kA(T1-T2)/d (4) At the steady state this heat Q is rad iated per second from bottom copper plate. If m and s be the mass and specific heat of bottom copper plate and dT/dt be its cooling rate at temperature T2, then the heat loss (radiated per second from bottom copper plate ) is Q= ms (d T/d t) (5) dT/dt is determined by performing a subsidiary experiment. Fro m equation (4) and (5) the thermal conductivity of poly mer co mposite is k= [ ms (d T/d t)d ]/[A (T1 -T2 )] (6) 4. Experimental 4.1. Materials 138 M uhammad Jahidul Hoque et al.: Thermal Conductivity of Polypropylene Based Composite M aterials Filled with Graphite and Carbon Black 4.1.1. Poly mer The polymer used in this study is polypropylene (PP) and it is isotactic(Melting point 176°C ) type PP. Density: 850kg/m3. Thermal conductivity 0.1~0.22 W/ mK. In isotactic PP all methyl groups positioned at the same side with respect to the backbone of the polymer chain without any branching . results are compared for different co mbination. Dependence of the thermal conductivity on filler volu me content for the composites of carbon black studied are shown in Figure1. & Figure2. 4.1.2. The Fillers The fillers used are graphite and carbon black. Graphite(gr): Graphite powder with the average particle size ranging fro m 10 to 20 µm. Black in co lor, density: 2160kg / m3, thermal conductivity: 119~165 W/ mK, Carbon -graphite%=30-70. Carbon bl ack(CB ): CB is furnace black type, has density 1800~2100 Kg/ m3, therma l conductivity is 0.141for grade 6, 0.182fo r grade 6 spheron. 4.2. Sample Preparation For samp le of pure polypropylene, moisture removal of polymer is done by placing required amount of PP in a furnace at 50℃ for 3 hours. PP-GR, PP-CB mixtures were prepared by extrusion process, by gradually adding the appropriate amount of the filler in the poly mer matrix. The matrix-filler mixing was performed at a temperature slightly above the melt ing point of the poly mer. And the moderate speed of the rotor is maintained. For ho mogeneous mixing of the polymer and filler this mixing process is repeated for at least 3 times. Then noodles from the ext ruder turned into small tablet for co mpression molding. Co mpression molding is done at sufficient temperature and liver oil pressure and special water cooling arrangement were maintained with water jacket. Square samp le fro m compression mold ing is then grinded for making circular shape with required dimension and treated to prepare it for measuring therma l conductivity using LEES apparatus. Figure 1. Concentration dependence of the thermal conductivity of the composite: PP-CB(grade 6): square-theoretical conductivity, rombosexperimental conductivity 4.3. Characterization LEES apparatus was used for thermal conductivity measurement. It consist of two copper disc. Oil bath, oil supplying pump, o il heating arrangement and digital temperature measurement arrangement .Po ly mer co mposite was placed between two good conducting copper disc in the LEES apparatus. Hot silicon oil is supplied by centrifugal pump for heating the tested sample and then cooling rate is measured by using cooling temperature for a certain time limit . At steady state conduction heat from the sample equals the heat radiation and convection with the surrounding from copper disc. Then thermal conductivity of the poly mer composite is measured by equalizing Fourier’s law of conduction and Newton’s cooling law. Figure 2. Concentration dependence of the thermal conductivity of the composite: PP-CB(grade 6- spheron) square -theoretical conductivity, rombos- experimental conductivity 5. Results and Discussion All findings of theoretical(literature) and experimental Figure 3. Concentration dependence of the thermal conductivity of the composite:PP-Graphite with high carbon content International Journal of Composite M aterials 2013, 3(5): 136-140 139 Dependence of the thermal conductivity on filler volu me content for the composites of graphite studied are shown in Figure3. & Figure4. Co mparison of both the experimental and theoretical observation. Poly mer co mposite filled with high carbon content graphite shows less discrepancy of experimental & theoretical observation than low carbon content graphite filled composite. And in some filler volu me content theoretical proposal leads experimental conductivity for composite material having graphite with lo w carbon content. Figure 6. Comparison of thermal conductivity of PP-Graphite(lCC) & PP-CB (grade6). Square: PP-Gr, Rombos:PP-CB 6. Conclusions Figure 4. Concentration dependence of the thermal conductivity of the composite:PP-Graphite with low carbon content Low carbon content graphite filled poly mer co mposite shows steady trend after certain filler content, which is absent in PP-Graphite (HCC) co mposite. Spheron Carbon black has higher thermal conductivity in co mparison witn grade 6 carbon black, but much less than PP-Graphite compos ite as s een from both Figure5. & Figure6. for mos t of the filler volu me content The result obtained with the investigation of polymer composite filled with graphite and carbon black allow us to draw the following conclusion. In explaining the concentration dependence of thermal conductivity of polymer co mposite filled with carbon black of grade 6 exh ibit more convenient result by follo wing the proposal of theoretical mode l. Both type graphite filled co mposite shows increasing trend of therma l conductivity, where sa mple having graphite of high carbon percentage shows more increment. But after 19% filler content PP-Graphite(LCC) composite shows steady trend violating the absence of percolation behavior of the thermal conductivity with increasing dispersed filler concentration. 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