Properties of Iron Powder Filled High Density Polyethylene

PROPERTIES OF IRON POWDER FILLED HIGH DENSITY POLYETHYLENE Mihai Rusu *, Nicoleta Sofian Daniela Rusu 2, Eugen Neagu 3 and Rodica Neagu 3 1 "Gh Asachi" Technical University, Faculty of Industrial Chemistry, Department of Macromolecules, Bd. D.Mangeron, No.71, 6600 Iasi, Romania 2 "Gr. Τ. Popa" University of Medicine and Pharmacy, Faculty of Biomedical Engineering, Biomaterials Department, Str. Universitatii, No. 16, 6600 Iasi, Romania "Gh. Asachi" Technical University, Faculty of Materials Science, Department of Physics, Bd. D.Mangeron, No. 68, 6600 Iasi, Romania ABSTRACT HDPE/iron powder composites have been characterized from the point of view of mechanical, thermal and electrical properties. Results for iron powder contents varying between 0 and 24 % by volume are presented. Generally, the composites show poorer mechanical properties as compared with the unfilled polymer. The density and hardness of HDPE/iron composites are higher than those of HDPE. The incorporation of iron powder in HDPE increases the thermal diffusivity and conductivity and decreases the specific heat and the electrical resistivity. The experimental data were interpreted using appropriate theoretical models. Key Words: Polymer composites; High­density polyethylene; Iron powder; Metal­filled polymers. Corresponding author. Fax: +4032271311; E­mail: mrusu@ch.tuiasi.ro 469 Brought to you by | HEC Bibliotheque Maryriam ET J. Authenticated Download Date | 6/8/15 4:18 PM Vol. 21, No. 5, 2001 Iron Powder Filled High Density Polyethylene INTRODUCTION Within the group of composite materials, those containing powder fillers /1,2/ occupy a special place and, among them, special mention should be made of the mixtures consisting of thermoplastic polymers and metal fillers /3­9/. Composites consisting of highly conductive filler powder dispersed in a flexible, insulating polymer matrix are commonly used in electronic applications for die attach, solderless connectors, thermistors and pressure­ sensing elements. Other uses of such composites include electromagnetic shielding and antistatic devices as well as chemical sensors 111. The composites made by incorporation of powdery metal fillers into thermoplastic polymers combine the advantageous properties of metals and plastics, offering cost effectiveness and rapid fabrication rate with a wide range of design flexibility, light weight, non­corrosiveness etc 111. In this paper, we report a study of the mechanical, thermal and electrical properties of high­density polyethylene (HDPE)/iron powder composites. EXPERIMENTAL Materials The following materials have been used in the study: • high­density polyethylene (HDPE), RIGIDEX type, A 52 BB/088 (British Resin Products Ltd., England), with a melt flow index of 0.88g/10 min.; • iron powder with particle size in the range of 50­100 μ πι (S.C. Sinterom S.A.Cluj­Napoca, Romania), with a density of 7.8 g/cm 3 , and a thermal conductivity of 80.2 W/m­K. Composites preparation The composite materials have been prepared by compounding HDPE with different amounts of metallic powder on a laboratory­rolling machine with rollers heated at 155°C. The mixing time was about 10 min after the formation of the HDPE sheet on the anterior cylinder of the rolling machine and addition of the iron powder. The mixtures have been transformed through pressing (preheating for 10 min, pressure for 5 min at 160°C and 15 MPa, cooling through pressure) in 1 mm and 4 mm thick plates respectively, from 470 Brought to you by | HEC Bibliotheque Maryriam ET J. Authenticated Download Date | 6/8/15 4:18 PM Μ. Rusu et al. which Journal of Polymer Engineering samples have been cut for the determination of the physical, mechanical, thermal and electrical characteristics. Thus, composite materials with iron powder concentrations ranging between 0 and 24 % by volume have been prepared. Measurements Tensile measurements were carried out using a TIRA TEST 2200 (Germany) (5 cm initial gap between clamps, and the mobile clamp speed of 5 cm/min.), according to ASTM­ D638. Flexural modulus was measured with an Instron Universal Testing Machine (Type 1121), with a three points charge applicator, according to ASTM­D790. The Izod notched impact tests were performed on a FIE (Type IT­0.42) machine, according to ASTM­D256, at room temperature (samples were previously notched with a "V" shape). Thermal conductivity, thermal diffusivity and specific heat were measured using the transient method, based on the heat transfer in a semi­ infinite solid theory /10/. The volume resistivity was measured at room temperature, between parallel copper plate electrodes, using a Keithley 602 electrometer. In this paper most of the composite properties are expressed as relative values (ratio between composite properties and those of unfilled polymer). RESULTS AND DISCUSSION 1. Mechanical properties Tensile characteristics have been determined from the stress­strain curves (Figure 1). From these curves one may easily notice the increase in composites brittleness with the increase in iron powder content /11,12/. The tensile strength experimental data were analyzed by comparison with some predictive models, in order to understand the formation of weak structures in these composites /13,14/. The most common expressions of tensile strength dependence on the composition of two­phase composites are based on the first power law [Eq. (1)] and the two­thirds power law [Eq. (2)]: — = (1­Φ) (1) 471 Brought to you by | HEC Bibliotheque Maryriam ET J. Authenticated Download Date | 6/8/15 4:18 PM Iron Powder Filled High Density Polyethylene Vol. 21, No. 5, 2001 0 2 4 6 8 10 trnmigaSIF 12 St r a i n (mm) Fig. I :yxwvutsrponmlihgfedcbaTSPHFED Stress­strain curves for HDPE/iron composites with different concentrations of iron powder: 0% vol.(l); 4 % vol.(2); 8% vol.(3); 12% vol.(4); 16% voi.(5); 20% vol.(6) and 24% vol.(7). = (1­Φ2/3) ^ σ (2) Ρ where σ ς and σ ρ denote the tensile strength of the composite and matrix respectively, and Φ is the volume fraction of filler. The power laws originate from the relation between the area fraction and inclusions volume fraction /15/. For a completely random distribution of the dispersed phase, the first power law relationship, and for the case of spherical inclusions, the two­thirds power law with appropriate weighting factor, can be derived on a simple mathematical consideration. The following models were used: ^ ( Ι ­ Φ ) * ! σ 472 (3) Ρ Brought to you by | HEC Bibliotheque Maryriam ET J. Authenticated Download Date | 6/8/15 4:18 PM Journal of Polymer Engineering Μ. Rusu et αϊ. σ σ 2,3 )K2 (4) (1­Κ3Φ2/3) (5) = (1­Φ Ρ = Ρ = exp (­ΑΓ 4 Φ) ( 6) These equations describe the non­adhesion type structure. In the first power law [Eq. (3)] the parameter K | accounts for the weakness in the structure introduced through discontinuity in stress transfer or formation of stress concentration points at the filler­polymer interface, in analogy to parameter K 2 from Nielsen's model [Eq. (4)] /15/. When K| (or K 2 ) are equal to 1, it means that there is no stress concentration effect. In the meantime, the lower the value of Κ ι (or K 2 ), the greater the stress concentration effect. The weighting factor K 3 from Eq. (5) describes the adhesion quality between polymer matrix and inclusions and depends on the details of the model. For example, K 3 = l . l represents dense hexagonal packing in the plane of highest density; K 3 =l .21 describes the extreme case of poor adhesion, with spherical inclusions for the minimum cross section between spherical beads, and K 3 =l stands for strain consideration. In general, the lower the value of K 3 , below 1.21, the better the adhesion is /13/. In analogy to some non­polymeric materials, such as metals and ceramics, and according to the porosity model [Eq. (6)] inclusions may be considered as pores or voids in polymer blends and composites. As a result of non­ adhesion at the interface, the pores are assumed to play no significant role on the mechanical properties of the composites. The K 4 parameter describes stress concentration; the higher the value of K4, the higher stress concentration /13/. Figure 2 shows the relative tensile yield strength as a function of iron volume fraction. The experimental data and the predicted curves corresponding to the theoretical models presented above are plotted on the same diagram, in order to determine the model parameters and for a better interpretation of the results. One may observe that the experimental curve approaches the diagrams corresponding to Eqs. (2), (3), (4), (5) and (6), for which the fitting parameters are K,=0.90, K 2 =0.98, K 3 =1.21 and respectively, Brought to you by | HEC Bibliotheque Maryriam473 ET J. Authenticated Download Date | 6/8/15 4:18 PM Iron Powder Filled High Density Vol. 21. No. 5. 2001 Polyethylene 1.0 eq.(1) •eq.(3), IK·, =0.901 eq.(2) eq.(4),lK 2 =0.98} eq.(5),(K 3 =1.21) eq.(6UK4=3) ο 0) α 0.4 0 Fig. 2: 5 10 15 20 Iron powder ( %, vol. ) 25 Relative tensile yield strength (­Δ ­) of HDPE/iron composites as a function of filler content and comparison with theoretical models described by the eqs. (l)­(6). K 4 =3. T h e s e values indicate a poor adhesion between the t w o phases and a small stress concentration effect. An explanation of the relative tensile yield strength decrease when increasing the iron powder content consists in the incorporation of large defects (the iron particles) that act as stress concentration points. Furthermore, it is known that H D P E is a carbohydrate polymer, with a relatively low level of intermolecular interaction forces; its mechanical strength is considerably due to the semi­crystalline structure. Incorporation of any inclusions in this polymer, and especially of fillers, induces modification of the degree of crystallinity and, implicitly, of its mechanical characteristics 121. T w o mathematical models have been considered for calculating tensile elongation of the polymeric composites with p o w d e r fillers [Eqs. (7) and (8)] /10.13/: = (1­φ1/3) (7) ερ 474 Brought to you by | HEC Bibliotheque Maryriam ET J. Authenticated Download Date | 6/8/15 4:18 PM Journal of Polymer Engineering Μ. RUSH et AL. ) = (1­Κ5Φ2/1yxwvutsrqponmlihgfedcbaTRPKHFEDA (8) ε Ρ where: e c ­ tensile elongation of the composite; ε ρ ­ tensile elongation of the polymer; Φ — volume fraction of filler; K 5 ­ constant depending on the dimension of disperse particles and on the treatments applied to them (K 5 =1.21). The diagrams plotted according to these models and the experimental curve on relative tensile yield elongation are presented in Figure 3. Analysis of these diagrams shows that for low (up to 8% vol.) and high (above 16% vol.) iron powder contents the tensile yield elongation decreases slowly, but the decrease becomes more significant for a concentration range between 8 and 16% vol., which corresponds to the formation of particles agglomerates. Fig. 3: Relative tensile yield elongation (­Δ ­) of HDPE/iron composites as a function of filler content and comparison with theoretical models described by the eqs. (7) and (8). Brought to you by | HEC Bibliotheque Maryriam475 ET J. Authenticated Download Date | 6/8/15 4:18 PM Iron Powder Filled High Density Polyethylene Vol. 21, No. 5, 2001 For low contents of metal powder in the mixtures the experimental diagram plotting the evolution of relative tensile yield elongation is placed above the diagram obtained for Eq. (8). The increase of the filler concentration above 16% vol. produces a decrease of the experimental data below the predicted values. The breaking of a material under impact is caused by the rapid crack propagation through the bulk. The crack propagation rate is inversely proportional to the material impact strength. A polymeric material with good impact strength absorbs most of the impact energy, while crack propagation is very slow. The formation of microfissures perpendicular on the propagating direction of the main fissure is one of the mechanisms that explain the slowing down of the rate of fissure propagation. The capacity of forming microfissures during the impact, the rate of their formation and the amount of absorbed energy depend on the polymer nature, on its morphology and molecular weight etc /14Λ In the case of HDPE/iron composite materials, both Charpy notched impact strength and Izod notched impact strength decrease considerably with the introduction of the first amounts of iron powder in the polymer (Figure 4). Such a sudden decrease of impact strength is recorded up to a 4 % vol. iron powder content. This behavior may be explained by the rapid propagation of the fissures induced as a result of the impact along the particle­polymer interface, due to the weak adhesion between the two phases. The flexural modulus of the HDPE/iron composite materials decreases with the increase of the iron powder content in the mixtures (Figure 5). The decrease is more significant in the concentration domain assuring an individually dispersion of the metal particles (up to 4 % vol.). For filler contents between 4 and 12% vol. the flexural modulus is less influenced by the increase of iron particle concentration, but the increase of filler content above 12% induces its sharper decrease. A general analysis of the mechanical properties of HDPE/iron powder composites indicates that the introduction of the metallic filler into HDPE causes a decrease of the most of these properties /11,12/. However, the mechanical properties of HDPE/iron composites are still interesting for many applications, as one can see from Table 1. Both the density and the hardness of the HDPE/iron composites increase with the increase of the filler content in the mixture. Nevertheless, while the increase of density is directly proportional with the content of iron powder, 476 Brought to you by | HEC Bibliotheque Maryriam ET J. Authenticated Download Date | 6/8/15 4:18 PM Journal of Polymer Μ. Rusu et al. Engineering Iron p o w d e r (Vi.vol.) Fig. 4:zyxvutsrponmlihgfedcaVRPIHEDC Relative Izod notched impact strength (1) and relative Charpy notched impact strength (2) of H D P E / i r o n composites as a f u n c t i o n of filler content. Fig. 5: Variation of relative flexural modulus (­o­) of HDPE/iron composites as a function of filler content. 477 Brought to you by | HEC Bibliotheque Maryriam ET J. Authenticated Download Date | 6/8/15 4:18 PM Iron Powder Filled High Density Polyethylene Vol. 21, No. 5, 2001 Table 1 Mechanical, thermal and electrical properties of HDPE/iron composites VOLUME CONCENTRATION OF IRON P O W D E R (%) ^^QOMPOSITES NO 4 0 8 12 16 20 24 PROPERTIES 1 Tensile yield strength, N/mm 2 33.1 28.7 27.6 24.0 23.8 20.5 18.2 2 Tensile yield elongation, 10.3 9.6 9.5 6.1 9.1 6.0 5.1 % 3 4.09 1.03 0.93 0.91 0.87 0.82 0.81 215 73.8 71.9 69.2 56.9 51.8 35.9 Density, g/cm 3 0.96 1.31 Shore D hardness, 62.0 62.3 63.8 64.0 64.8 65.0 65.6 124 125 127 128 134 1.52 1.44 1.65 1.93 1.86 2.89 4.29 Charpy notched impact strength, N cm/cm 2 4 Izod notched impact strength, KJ/m 5 6 2 1.68 1.99 2.31 2.56 2.99 °Sh D 7 Vicat softening 140 142 temperature, °C 8 Thermal difusivity, m 2 /s •ΙΟ"7 •ΙΟ"7 •ΙΟ"7 •ΙΟ"7 •ΙΟ"7 •ΙΟ"7 •ΙΟ"7 9 Thermal conductivity, 0.50 0.51 0.61 0.66 0.68 0.79 1.33 3.62 2.54 2.14 1.59 1.51 1.34 1.04 •106 •106 •106 •106 •106 •106 8.59 2.42 1.52 1.01 6.31 5.59 6.11 14 4 3 W/m­K 10 Specific heat, J/kg­K •106 11 Electrical volume resistivity, Ω ­m •10 14 •10 14 •10 •10' •10 •103 •102 the diagram corresponding to hardness exhibits an S­shape (Figure 6). Analysis of this latter diagram demonstrates that the most important increase of composite hardness occurs in the range of filler contents that corresponds 478 Brought to you by | HEC Bibliotheque Maryriam ET J. Authenticated Download Date | 6/8/15 4:18 PM Μ. Rusu et al. Journal of Polymer Engineering Ifl U) υ ca c •o ί­ ο •C Ο ν c. ο n in ν > a €j a. Iron powder (% f voll tsrpomlihgeaTF Fig. 6: Variation of relative density (­o­) and relative Shore D hardness (­Δ ­) of HDPE/iron composites as a function of file content to the coexistence of both individually dispersed particles and particle agglomerations (5 ­ 20% vol.). 2.Thermal properties Vicat softening temperature increases slowly with the increase of iron powder content (Figure 7). The largest slope of the diagram appears once again in the filler concentration domain in which the metal particles are dispersed both individually and as agglomerates. A melting heat of 39.87 J/g characterizes the composite material containing 24% vol. iron powder, which is much lower than that of HDPE, i.e. 155.0 J/g (Figure 8). In the composite containing 24% vol. iron powder only the polyethylene ­ present as 28% by weight ­ melts. Keeping this in mind and calculating the theoretical melting heat, a value of 142.14 J/g ­ 479 Brought to you by | HEC Bibliotheque Maryriam ET J. Authenticated Download Date | 6/8/15 4:18 PM Iron Powder Filled High Density Polyethylene Vol. 21, No. 5. 2001 Fig.yxwvutsrponmlihgfedcbaXVTQPMJHFEDA 7: Variation of relative Vicat softening temperature of HDPE/iron composites as a function of filler content. which is slightly lower than that corresponding to the unfilled HDPE ­ may be obtained. This slight decrease in the melting heat of the polyethylene contained in the composite may be explained through the decrease of its crystallinity degree. The above assertion is also supported by the result obtained from the estimation of the crystallinity degree (X), with the following relation /16/: X =­^­­100, ίioo where: [%] (9) Q „ ­ melting heat of polyethylene, J/g; Q J O O ­ melting heat of a 100% crystalline polyethylene, whose value is estimated at 270.03 J/g. All these calculations lead to the conclusion that the crystallinity degree of the H D P E from the composite is of 52.64%, versus 62.4% ­ for the unfilled polymer. The modification of the melting heat does not influence to any great extent the melting point of HDPE from the composite material (Figure 8). The literature provides several relations trying to approximate the thermal conductivity 480 of the composites containing fillers, such as: Maxwell's Brought to you by | HEC Bibliotheque Maryriam ET J. Authenticated Download Date | 6/8/15 4:18 PM Μ. Rusu et al. Fig. 8: Journal of Polymer Engineering DSC diagrams for HDPE (full lines) and HDPE/iron (24% vol.) composite (dashed lines). equation (10), Cheng and Vachon's equation (11), Lewis and Nielsen's equation (12), and Agari and Uno's equation (13)/17,18/: Kc _Km+2Kp+ Kp 2Φ(Κη, ­K p ) Kc K 1­Λ/3Φ/2 Kc 1 + ΑΒΦ K 1­βφψ ' P P (10) Km+2Kp­<t>(Km­Kp) log Kc = Φ • C 2 · lc (11) K„. / Κ pn — 1 1—Φ Β = —— ­ and Ψ = 1 + ­ — ^ ­ ^ ­ Φ Km/Kp+A Φ„2 (12) (13) Brought to you by | HEC Bibliotheque Maryriam481 ET J. Authenticated Download Date | 6/8/15 4:18 PM Iron Powder Filled High Density Polyethylene Vol. 21, No. 5, 2001 where: Kc,Kp and K m ­ represent the thermal conductivity values of the composite, polymer and, respectively, filler; Φ the volumetric fraction of the disperse filler; A constant depending on the shape and orientation of the disperse particles (for randomly distributed spherical particles, A = l , 5 while for randomly distributed aggregates of spherical particles, A=3); ­ the maximum packing fraction of disperse phase (for randomly C,, C 2 ­ experimentally determined constants of order unity. C, is a Φη, distributed spherical particles, Φ„,=0,637); measure of the effect of the dispersed particles on the secondary structure of the polymer, like crystallinity and the crystal size. C 2 measures the ease of the particles to form conductive chains. The more easily particles are gathered to form conductive chains, the more thermal conductivity of the particles contributes to change thermal conductivity of the composite and C 2 becomes closer to 1. Thermal diffusivity and thermal conductivity of the composite material increase, while specific heat decreases with the increase of filler content in mixtures (Figure 9, Table 1). For iron powder contents below 20% vol. the experimental diagram of thermal conductivity approaches the curves corresponding to Eqs. (10), (12) (with A=1.5) and Eq. (13) (with C,=0.99 and C 2 =0.30). When filler concentration increases above 20% vol., the experimental conductivity becomes higher than the values predicted by the theoretical models. It is known that percolation conductivity appears when the ratio between filler conductivity and polymer conductivity is above 10 s /19­21/. Because the ratio between iron thermal conductivity (80.2 W/m­K) and HDPE thermal conductivity (0.505 W/m­K) is only 158.81, the thermal conductivity of HDPE /iron composite does not have a percolation threshold. For a possible explanation of the manner in which the thermal conductivity varies when increasing the iron powder content, an attempt was made to correlate the values obtained for this characteristic with the structure of the composite, by means of optical microscopy (Figure 10). For filler content up to 5% vol., the studies of optical microscopy showed that the metal powder particles are dispersed individually in the polymeric 482 Brought to you by | HEC Bibliotheque Maryriam ET J. Authenticated Download Date | 6/8/15 4:18 PM Μ. Rusu et at. Journal of Polymer Engineering σ­ αι •C If 11 ν a, > > o 5 Φ <u er a Iron powder (%,vol.) Fig. 9: Relative thermal conductivity ( ­ · ­ ) , relative thermal diffusivity (­ο­) and relative specific heat (­Δ ­) for HDPE/iron composites as a function of filler content and comparison with theoretical models for thermal conductivity described by the eqs. (10), (11), (12) and (13). matrix (Figure 10 a). An increase of the filler content in the mixture initiates particles agglomeration, so that in the concentration interval ranging between 5 and 20% vol., the metal powder is found inserted in the polymeric matrix both as individual particles and as agglomerations of particles (Figure 10 b,c). When the content of iron powder exceeds 20% vol., almost all filler particles form agglomerates, being in direct contact with each other (Figure 10 d). Under such circumstances a 3D network­type structure made of metal particles is formed and the thermal conductivity of composite material presents a sharp increase. The propagating rate of the thermal flow through a non­metallic solid depends on the coupling intensity of the vibration movements of the atoms Brought to you by | HEC Bibliotheque Maryriam 483 ET J. Authenticated Download Date | 6/8/15 4:18 PM Iron Powder Filled High Density Polyethylene Vol. 21, No. 5, 2001 (c) (d) Fig. 10:ywvutsrponmlkjihgfedcbaPMIHED Microscopic photographs of HDPE filled with 4% (a), 12% (b), 16% (c) and 2 4 % (d) by volume iron particles. and groups of adjacent atoms. Intense couplings occur in the materials with covalent bonds, the thermal transmission showing a slight deficit in the case of highly ordered crystalline networks. In the case of metals, the process of thermal energy transmission from one point to another, by means of mobile electrons, conceals the contribution of the atomic network vibration movements to thermal conduction, the first one being a much more efficient process. In the case of HDPE/iron composite we can assume that when the filler content reaches 2 0 % vol., a change of thermal conduction mechanism occurs, from conduction through atomic network vibration movements, specific for polymers, to conduction by means of mobile electrons, specific for metals. 484 Brought to you by | HEC Bibliotheque Maryriam ET J. Authenticated Download Date | 6/8/15 4:18 PM Journal of Polymer Engineering Μ. Rusu et al. 3.ELECTRICAL PROPERTIES The electrical conductivity of HDPE is dramatically influenced by the addition of iron powder. The electrical volume resistivity of HDPE decreases by 12 orders of magnitude with the addition of iron powder (see Table 1), which corresponds to a proportional increase of electrical conductivity. Figure 11 shows the plot of log (electrical volume resistivity) against metal content. The sudden decline in the electrical resistivity of composite material is explained by the percolation theory 1221. The theory implies network or path arrays for conduction not necessarily by metal­to­metal contact. A quantum mechanical tunneling mechanism has been proposed, whereby the electrons can hop or tunnel through the insulating region between adjacent particles up to distances of several nanometres. The electric field strength between neighboring filler entities may be high enough to cause complete breakdown of the matrix or adsorbed surface layers. The so­called percolation threshold I 5 Fig. 11: I I I 10 15 20 Iron p o w d e r (%,vol.) I 25 — Variation of log(electrical volume resistivity) for HDPE/iron composites as a function of filler content. Brought to you by | HEC Bibliotheque Maryriam485 ET J. Authenticated Download Date | 6/8/15 4:18 PM Iron Powder Filled High Density Polyethylene Vol. 21, No. 5, 2001 represents the lowest concentration of conducting particles at which continuous conducting chains are formed /23/. The percolation threshold for the studied composite material corresponds to an iron powder content of 12% by volume, for which a sharp fall in electrical resistivity (of 11 orders of magnitude) has been observed. CONCLUSIONS The results reported here demonstrate that the addition of iron powder to HDPE induces a decrease of the mechanical properties, in comparison with those of the unfilled polymer. The density and hardness of HDPE/iron composites are higher than those for the unfilled polymer and increase with the filler content. The incorporation of iron powder in HDPE increases the thermal diffusivity and thermal conductivity and decreases the specific heat. The electrical resistivity of HDPE is greatly decreased by addition of iron powder, indicating a proportional increase of electrical conductivity. BIBLIOGRAPHY 1. A.A. Berlin, S.A. Volfson, S.S. Enikolopian and S.S. Negmatov, Principles of Polymer Composites, Akademie Verlag, Berlin, 1986. 2. V.A. Paharenko, V.G. 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Neagu, J. Thermoplastic Composite Materials, 14(1), 20 (2001). 22. G R. Ruschau, S. Yoshikawa and R.E. Newnham, J. Appl. Phys., 72(3), 953 (1992). 23. L. Yang and D.L. Schruben, Polym. Eng. Sei., 34(14), 1109 (1994). Brought to you by | HEC Bibliotheque Maryriam 487 ET J. Authenticated Download Date | 6/8/15 4:18 PM