- Email: [email protected]
Mechanical properties of triple composites of polycarbonate, single-walled carbon nanotubes and carbon fibres
ARTICLE IN PRESS
Physica E 40 (2008) 2434–2439 www.elsevier.com/locate/physe
Mechanical properties of triple composites of polycarbonate, single-walled carbon nanotubes and carbon fibres B. Hornbostela,, P. Po¨tschkeb, J. Kotza, S. Rotha a
Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Leibniz-Institut fu¨r Polymerforschung Dresden e.V., Hohe Strasse 6, D-01069 Dresden, Germany
b
Available online 15 September 2007
Abstract The present work was undertaken to study the mechanical properties of reinforced polycarbonate systems, where the filler material consists of single-walled carbon nanotubes (SWCNTs), of carbon fibres (CF) or of both. The SWCNTs were taken from different sources, laser ablation and arc discharge, and carefully characterized before their incorporation into the matrix system. The loadings of the reinforcement material were varied from 1 to 35.5 wt% in the thermoplastic polymer. All composites were produced by melt extrusion. Experimental results show that small amounts of carbon nanotubes randomly distributed in thermoplastic matrix systems do not inevitably enhance the mechanical stability. Higher mechanical improvements could be attained by adding CF to the composite system. A triple composite of polycarbonate, PC/SWCNTs/CF reveals synergy effects in mechanical and electrical aspects. The composites were investigated by stress–strain measurements, dynamical mechanical analysis and hardness probing. r 2007 Elsevier B.V. All rights reserved. PACS: 62.23.Pq; 62.25.g Keywords: Carbon nanotubes; Carbon fibres; Polycarbonate; Mechanical properties; E-modulus; Tensile Strength; Hardness; Triple composites
1. Introduction The inherent unique mechanical and electrical properties of carbon nanotubes (CNTs) [1–3] have been receiving considerable attention. Concerning the mechanical side according to the literature [4–7] CNTs possess a tensile strength of up to 300 GPa at a maximal stiffness of E=1.7 TPa. Combined with their high aspect ratio and low density (1.3–1.8 kg/dm3) CNTs are a promising filler material for polymeric matrices in order to realize a strong light-weight material for practical macroscopic applications. Unfortunately, the strength of the CNTs can often not be transferred to the composite or to the matrix system. The most likely reason is the weak adherence of the filler to the host. This is amplified by impurities (like amorphous carbon or graphite particles) and by too short tubes. The first obstacle, the impurities, can theoretically be removed by purification of the nanotube material. Another Corresponding author.
E-mail address: [email protected] (B. Hornbostel). 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.08.100
approach to get rid of both obstructions simultaneously is the adaption and tuning of the synthesis process. However, both means are not trivial, because firstly, purification also comes with some defect introduction into the nanotubes themselves and, secondly, the synthesis mechanisms are not yet completely understood, so that the required fine tuning of the synthesis process is not possible at present. Another potential approach to take advantage of the tubes’ strength for the whole material system is to functionalize them chemically. This functionalization should then fulfill more than one function: (1) add molecules to the shell of the tube which can (covalently) bind to the matrix molecules; (2) these molecules also support the untangling/unbundling of the nanotube agglomerates/bundles to achieve a good dispersion without any clews in the matrix system, especially during the melt-extrusion process. This is important because larger agglomerates of nanofiller in the matrix are starting points for material failure. Again, functionalization is not trivial. CNTs as a filler material in a polymeric matrix have outstanding advantages concerning the electrical properties.
ARTICLE IN PRESS B. Hornbostel et al. / Physica E 40 (2008) 2434–2439
Already low loadings in intrinsically insulating host systems initiate electrical percolation and conduction [8–11], much earlier than for example carbon black (CB), which is state of the art for many practical applications nowadays. Nanotubes are also effective for shielding off or attenuating electromagnetic waves [12–14]. The effectiveness in these fields is rather low for carbon fibre (CF). However, CFs are thrifty in enhancing mechanical stability. Therefore, we conducted experiments of triple composites, consisting of PC, arc discharge sigle-walled carbon nanotubes (ArcDSWCNTs) and CF. The results on the percolation/conduction behaviour of these composites can be found in Refs. [8,9], while the results on the electromagnetic attenuation are given in Ref. [12]. In this publication we focus on the mechanical characteristics of melt-extruded polycarbonate (triple) composites.
2. Experimental section SWCNT material was produced in home-made synthesis reactors as described in Refs. [8,15,16]. The collected SWCNT material was carefully homogenized mechanically. Afterwards the content of SWCNTs was estimated by means of our quality control protocol [8,17,18]. For the ArcD-SWCNTs a purity of approximately 40%, was determined, which is quite reasonable for ArcD-material. The mean length of these tubes was found to be in the range of 4–6 mm. For the laser ablation tubes (LASWCNTs) we found 60% purity at lengths of up to 18 mm. The SWCNT-material was used without any previous purification steps, i.e. acid treatment. It should be emphasized that the investigations were performed with large SWCNT batches of at least 25 g for the laser ablated material and 200 g for the ArcD-SWCNTs.
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The polycarbonate (PC) used for this study was PC Iupilon E2000 (Mitsubishi Engineering Plastics) and Makrolon 2805 (Bayer Materialscience). Two-phase composites, consisting of PC and SWCNTs, were produced by the melt-extrusion method under direct incorporation of the SWCNTs as described in Ref. [8]. Here, a co-rotating Micro Compounder (DACA Instruments, Goleta, CA, USA) was used. The extruded strands were taken to be post-processed to discs with thicknesses of 0.3 mmoto45 mm in a hot press. From these discs specimens for the mechanical characterization were punched out. The PC triple composites (and the references) with ArcD-SWCNTs and carbon micro-fibres (CF, Tenax-U 201, fibre length 3 mm, density 1.8 kg/dm3) were also prepared by the melt-extrusion method, however, utilizing a Rheomex Haake PTW 16/25D [9]. Following an injection molding process (machine Babyplast 6/10) 1.4 mm thick specimens were fabricated from the extruded triple composites (and the references). All specimens complied laterally to the draw bars of type 1BB of the standard DIN EN ISO 527-2 for stress–strain tests in pulling machines. The stress–stain tests were effected with a Zwick/Roell Z2.5/TN1S at a constant velocity of 1 mm/min for the modulus of elasticity E and at 10 mm/min for the tensile strength s. Three-point dynamical mechanical analyses (DMA) on the triple composites were effected. Here, a Netzsch DMA was utilized. In this analysis the type 1BB specimen were put into the measuring device; however, the characteristically protruding end parts were cut off. The testing frequency was set to 1 Hz while the heating/cooling rate was 2 K/min. DMA occurred in N2 atmosphere in the temperature range 30–180 1C. Additional testing on the triple composites was performed with a universal hardness testing machine (Fischerscope H100VP XY). Simultaneously, the universal
Fig. 1. E-modulus (a) and yield strength (b) of two PC/SWCNT composites over the loading of filler material. Both graphs show in contradiction to the expectations a decrease of the mechanical strength with increasing nanomaterial content or just a slight increase, respectively. The intrinsically high E-modulus (1.7 TPa) or tensile strength (300 GPa), respectively, of SWCNTs cannot be transferred to the composite system.
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B. Hornbostel et al. / Physica E 40 (2008) 2434–2439
hardness (HU), the intruding hardness (HIH) and the intruding modulus (EIH) were determined. The die of the imprints was of pyramid-shaped geometry (Vickers). The final force on the die was 490 mN. 3. Results and discussion Fig. 1 shows the results of stress–strain measurements on two-phase composites with PC and SWCNTs. A strong increase of the strength in comparison to the pure PC is not observed. This is especially true for the less pure (40%) and shorter ArcD-SWCNTs, where the values drop with the nanofiller content. For the longer and purer (60%) LA-SWCNTs a small improvement in the yield strength
can be observed. The weak behaviour can mainly be attributed to the low adhesion of the components to each other. In order to take advantage of the impressive electrical properties of SWCNTs, i.e. conductivity [8,9] or attenuation of electromagnetic waves [12] without losing the mechanical properties of the pure PC, experiments with smaller ratios of CF were initiated. Initially, the total loading of filler in the host material was kept at 12.5 wt%. The stress–strain characteristics of these triple composites with PC/ArcD-SWCNTs/CF are shown in Fig. 2. The data indicates that CFs raise the modulus of elasticity slightly. In the case of the replacement of CF ratios by ArcD-SWCNTs the E-modulus can be kept in the region of the E-modulus of the pure PC. At PC:ArcD-SWCNTs:CF ¼ 87.5:7.5:5
Fig. 2. E-modulus (a) and yield szS or rupture strength szR (b) of three-phase composites of PC/ArcD-SWCNTs/CF. The bottom abscissa depicts the loading of ArcD-SWCNT material while the top abscissa shows the content of CF. The total loading is kept at 12.5 wt% always.
Fig. 3. Dynamical mechanical analysis (DMA) of triple composites with PC/ArcD-SWCNTs/CF. A pronounced rise in the dynamical E-modulus can be observed for the composites with a constant loading of 12.5 wt% (a). Even higher is the ascent of the triple composites with a constant ArcD-SWCNT loading of 2.5 wt% and larger, variable CF content.
ARTICLE IN PRESS B. Hornbostel et al. / Physica E 40 (2008) 2434–2439
[wt%] the E-modulus drops slightly under the value of the pure matrix and the spread grows. Therefore, in this case the formulation should not exceed 7.5 wt% ArcD-SWCNTs to avoid a strong descent in the E-modulus. The positive effect of the CF in the composites’ E0 -moduli can also be found in DMA measurements, Fig. 3(a). The dynamical E0 -moduli increase distinctly. The drop in the tensile strengths cannot be prevented by putting in small ratios of CF as seen in Fig. 2(b). Therefore experiments with higher loadings of CF and just low loadings of SWCNT material were recommenced. Now the loading of SWCNT material was kept constant at 2.5 wt%, which is well above the electrical percolation threshold [8,9]. The DMA measurements of these triple composites show a stronger rise in E0 as expected. Ascending mechanical values are also observed in the stress–strain
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measurements for the E-modulus (Fig. 4(a)) and for the tensile strengths (Fig. 4(b)). Further evaluations were performed by the utilization of an instrumented indentation test for hardness and materials according to ISO 14577. The advantage for using such a universal hardness probing machine is the simultaneous determination of several material properties, like universal hardness HU, intruding hardness HIT and intruding E-modulus EIT. Here, the universal hardness HU corresponds to the Martens hardness because a pyramidshaped probe was used. Fig. 5(a) shows that HU and HIT can be lifted to higher values with nanotubes. At PC:SWCNTs:CF ¼ 87.5:7.5:5 the values peak with HU150 N/mm2. In comparison HU of the reference PC: CF ¼ 87.5:12.5 is only 140 N/mm2. In both cases the total loadings were kept constant at 12.5 wt%. At higher
Fig. 4. Stress–strain measurements of the E-modulus (a) and the tensile strengths (b) of triple composites with high loadings of CF and constantly 2.5 wt% ArcD-SWCNTs. Over the references at 0 wt% carbon fibres a distinctive rise is present. The yield strength szS becomes congruent with the rupture strength szR, because the brittleness increases by SWCNTs.
Fig. 5. Results of the measurements of the hardnesses HU and HIT (a), and the moduli of elasticity EIT and E (b) for the composites with a total filler material loading of 12.5 wt% (except reference with 100% PC). The modulus of elasticity E of the stress–strain measurements is faced EIT in Fig. 5(b): the behaviour of E corresponds EIT.
ARTICLE IN PRESS B. Hornbostel et al. / Physica E 40 (2008) 2434–2439
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Fig. 6. HU, HIT and EIT of composites with a ArcD-SWCNT content of 2.5 wt% and high CF loadings (except references).
nanotube and lower CF loadings the mechanical values drop again, as already observed in the stress–strain measurements before. The modulus of elasticity E of the stress–strain measurements is faced EIT in Fig. 5(b): the behaviour of E corresponds to that of EIT. Fig. 6 shows HU, HIT and EIT of the triple composites with 2.5 wt% ArcD-SWCNTs and high CF content, plus the references. Again, an increase of the mechanical values can be observed for the triple composites. The introduction of SWCNTs supports the increase of the hardnesses, Fig. 6(a). The value of EIT (Fig. 6(b)) from PC:ArcD-SWCNTs: CF ¼ 64.5:0:35.5 is slightly below that of the comparable composite PC:ArcD-SWCNTs:CF ¼ 65:2.5:32.5. However in consideration of the error bars and the slightly lower total loading of filler material, the differences are compensated, so that EIT corresponds to the E measurements from Fig. 4(a) and to the DMA from Fig. 3(b). 4. Conclusions Reinforcement effects of triple composites consisting of PC, ArcD-SWCNTs and CF have been studied. In detail the tensile strengths s, E-moduli, dynamical E0 -moduli, universal hardnesses (HU), intruding hardnesses (HIH) and intruding moduli (EIH) were determined. While for pure PC/SWCNT composites an ascent of the mechanical properties is not inevitably attainable, simple triple composites with CF fulfill the expectations of increased strength. It is for this reason that the electrical benefits of CNTs can be exploited for electrical conduction [8,9] and for electromagnetic shielding [12] in insulating dielectric matrices without sacrificing, or with even increasing, the mechanical strength of the material system. Acknowledgements We are grateful to Steffi Mu¨ller (University of Stuttgart, IKT) and Didier Garray (CRIF/WTCM, Seraing (Lie`ge))
for discussions and support. Furthermore we thank Stephanie Gelse and Franziska Koch (MPI fu¨r Festko¨rperforschung, Stuttgart) for their assistance. The work was financially supported within the projects CANAPE, CARDECOM, SPANG and NANOSPARK of the sixth framework programme of the European Union.
References [1] S. Subramoney, Adv. Mater. 10 (1998) 1157. [2] J.P. Salvetat, G.A.D. Briggs, J.M. Bonard, R.R. Bacsa, A.J. Kulik, T. Stockli, N.A. Burnham, L. Forro, Phys. Rev. Lett. 82 (1999) 944. [3] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998. [4] M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, Nature 38 (1996) 678. [5] E.W. Wong, P.E. Sheehan, C.M. Lieber, Science 277 (1997) 1971. [6] B.G. Demcyk, Y.M. Wang, J. Cumings, M. Hetman, W. Hen, A. Zettl, et al., Mater. Sci. Eng. A 334 (2002) 173. [7] D. Qian, G.J. Wagner, W.K. Liu, M.F. Yu, R.S. Ruoff, Appl. Mech. Rev. 55 (2002) 495. [8] B. Hornbostel, P. Po¨tschke, J. Kotz, S. Roth, Phys. Status Solidi B 243 (2006) 3445. [9] B. Hornbostel, P. Po¨tschke, D. Kornfeld, J. Kotz, S. Roth, Electrical percolation and conductivity in polycaronate/singled-walled carbon nanotube composites, J. Nanostruct. Polym. Nanocompos., 2007, accepted for publication. [10] M.S.P. Shaffer, A.H. Windle, Adv. Mater. 11 (1999) 937. [11] J. Sandler, M.S.P. Shaffer, T. Prasse, W. Bauhofer, K. Schulte, A.H. Windle, Polymer 40 (1999) 5967. [12] B. Hornbostel, U. Leute, P. Po¨tschke, J. Kotz, D. Kornfeld, P.-W. Chiu, S. Roth, Attenuation of electromagnetic waves by carbon nanotube composites, Physica E, 2007, submitted for publication. [13] Y. Wang, X. Jing, Polym. Adv. Technol. (2005) 16344. [14] N. Li, Y. Huang, F. Du, X. He, X. Lin, H. Gao, et al., Nano Lett. 6 (2006) 1141. [15] B. Hornbostel, M. Haluska, J. Cech, U. Dettlaff, S. Roth, Math. Phys. Chem. 222 (2005) 1. [16] B. Hornbostel, M. Haluska, M.D. Lima, A. Nish, J. Kotz, D. Kornfeld, S. Roth, Nanotubes from laser ablation: study of selected synthesis parameters, 2007, in preparation. [17] U. Dettlaff-Weglikowska, J. Wang, J. Liang, B. Hornbostel, J. Cech, S. Roth, AIP Conf. Proc. 786 (2005) 129.
ARTICLE IN PRESS B. Hornbostel et al. / Physica E 40 (2008) 2434–2439 [18] For the sample characterisation and quality control of CNTs an extensive protocol was compiled by the consortium of the EU-project SPANG. It is hoped that by applying this protocol reproducible results will be obtained at different laboratories. To facilitate cooperation with other consortia and quite generally with colleagues
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worldwide, the protocol is also spread outside the EU project SPANG. The protocol is regularly updated. We appreciate any interest for the protocol and any feedback from colleagues working in the field.
Physica E 40 (2008) 2434–2439 www.elsevier.com/locate/physe
Mechanical properties of triple composites of polycarbonate, single-walled carbon nanotubes and carbon fibres B. Hornbostela,, P. Po¨tschkeb, J. Kotza, S. Rotha a
Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Leibniz-Institut fu¨r Polymerforschung Dresden e.V., Hohe Strasse 6, D-01069 Dresden, Germany
b
Available online 15 September 2007
Abstract The present work was undertaken to study the mechanical properties of reinforced polycarbonate systems, where the filler material consists of single-walled carbon nanotubes (SWCNTs), of carbon fibres (CF) or of both. The SWCNTs were taken from different sources, laser ablation and arc discharge, and carefully characterized before their incorporation into the matrix system. The loadings of the reinforcement material were varied from 1 to 35.5 wt% in the thermoplastic polymer. All composites were produced by melt extrusion. Experimental results show that small amounts of carbon nanotubes randomly distributed in thermoplastic matrix systems do not inevitably enhance the mechanical stability. Higher mechanical improvements could be attained by adding CF to the composite system. A triple composite of polycarbonate, PC/SWCNTs/CF reveals synergy effects in mechanical and electrical aspects. The composites were investigated by stress–strain measurements, dynamical mechanical analysis and hardness probing. r 2007 Elsevier B.V. All rights reserved. PACS: 62.23.Pq; 62.25.g Keywords: Carbon nanotubes; Carbon fibres; Polycarbonate; Mechanical properties; E-modulus; Tensile Strength; Hardness; Triple composites
1. Introduction The inherent unique mechanical and electrical properties of carbon nanotubes (CNTs) [1–3] have been receiving considerable attention. Concerning the mechanical side according to the literature [4–7] CNTs possess a tensile strength of up to 300 GPa at a maximal stiffness of E=1.7 TPa. Combined with their high aspect ratio and low density (1.3–1.8 kg/dm3) CNTs are a promising filler material for polymeric matrices in order to realize a strong light-weight material for practical macroscopic applications. Unfortunately, the strength of the CNTs can often not be transferred to the composite or to the matrix system. The most likely reason is the weak adherence of the filler to the host. This is amplified by impurities (like amorphous carbon or graphite particles) and by too short tubes. The first obstacle, the impurities, can theoretically be removed by purification of the nanotube material. Another Corresponding author.
E-mail address: [email protected] (B. Hornbostel). 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.08.100
approach to get rid of both obstructions simultaneously is the adaption and tuning of the synthesis process. However, both means are not trivial, because firstly, purification also comes with some defect introduction into the nanotubes themselves and, secondly, the synthesis mechanisms are not yet completely understood, so that the required fine tuning of the synthesis process is not possible at present. Another potential approach to take advantage of the tubes’ strength for the whole material system is to functionalize them chemically. This functionalization should then fulfill more than one function: (1) add molecules to the shell of the tube which can (covalently) bind to the matrix molecules; (2) these molecules also support the untangling/unbundling of the nanotube agglomerates/bundles to achieve a good dispersion without any clews in the matrix system, especially during the melt-extrusion process. This is important because larger agglomerates of nanofiller in the matrix are starting points for material failure. Again, functionalization is not trivial. CNTs as a filler material in a polymeric matrix have outstanding advantages concerning the electrical properties.
ARTICLE IN PRESS B. Hornbostel et al. / Physica E 40 (2008) 2434–2439
Already low loadings in intrinsically insulating host systems initiate electrical percolation and conduction [8–11], much earlier than for example carbon black (CB), which is state of the art for many practical applications nowadays. Nanotubes are also effective for shielding off or attenuating electromagnetic waves [12–14]. The effectiveness in these fields is rather low for carbon fibre (CF). However, CFs are thrifty in enhancing mechanical stability. Therefore, we conducted experiments of triple composites, consisting of PC, arc discharge sigle-walled carbon nanotubes (ArcDSWCNTs) and CF. The results on the percolation/conduction behaviour of these composites can be found in Refs. [8,9], while the results on the electromagnetic attenuation are given in Ref. [12]. In this publication we focus on the mechanical characteristics of melt-extruded polycarbonate (triple) composites.
2. Experimental section SWCNT material was produced in home-made synthesis reactors as described in Refs. [8,15,16]. The collected SWCNT material was carefully homogenized mechanically. Afterwards the content of SWCNTs was estimated by means of our quality control protocol [8,17,18]. For the ArcD-SWCNTs a purity of approximately 40%, was determined, which is quite reasonable for ArcD-material. The mean length of these tubes was found to be in the range of 4–6 mm. For the laser ablation tubes (LASWCNTs) we found 60% purity at lengths of up to 18 mm. The SWCNT-material was used without any previous purification steps, i.e. acid treatment. It should be emphasized that the investigations were performed with large SWCNT batches of at least 25 g for the laser ablated material and 200 g for the ArcD-SWCNTs.
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The polycarbonate (PC) used for this study was PC Iupilon E2000 (Mitsubishi Engineering Plastics) and Makrolon 2805 (Bayer Materialscience). Two-phase composites, consisting of PC and SWCNTs, were produced by the melt-extrusion method under direct incorporation of the SWCNTs as described in Ref. [8]. Here, a co-rotating Micro Compounder (DACA Instruments, Goleta, CA, USA) was used. The extruded strands were taken to be post-processed to discs with thicknesses of 0.3 mmoto45 mm in a hot press. From these discs specimens for the mechanical characterization were punched out. The PC triple composites (and the references) with ArcD-SWCNTs and carbon micro-fibres (CF, Tenax-U 201, fibre length 3 mm, density 1.8 kg/dm3) were also prepared by the melt-extrusion method, however, utilizing a Rheomex Haake PTW 16/25D [9]. Following an injection molding process (machine Babyplast 6/10) 1.4 mm thick specimens were fabricated from the extruded triple composites (and the references). All specimens complied laterally to the draw bars of type 1BB of the standard DIN EN ISO 527-2 for stress–strain tests in pulling machines. The stress–stain tests were effected with a Zwick/Roell Z2.5/TN1S at a constant velocity of 1 mm/min for the modulus of elasticity E and at 10 mm/min for the tensile strength s. Three-point dynamical mechanical analyses (DMA) on the triple composites were effected. Here, a Netzsch DMA was utilized. In this analysis the type 1BB specimen were put into the measuring device; however, the characteristically protruding end parts were cut off. The testing frequency was set to 1 Hz while the heating/cooling rate was 2 K/min. DMA occurred in N2 atmosphere in the temperature range 30–180 1C. Additional testing on the triple composites was performed with a universal hardness testing machine (Fischerscope H100VP XY). Simultaneously, the universal
Fig. 1. E-modulus (a) and yield strength (b) of two PC/SWCNT composites over the loading of filler material. Both graphs show in contradiction to the expectations a decrease of the mechanical strength with increasing nanomaterial content or just a slight increase, respectively. The intrinsically high E-modulus (1.7 TPa) or tensile strength (300 GPa), respectively, of SWCNTs cannot be transferred to the composite system.
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hardness (HU), the intruding hardness (HIH) and the intruding modulus (EIH) were determined. The die of the imprints was of pyramid-shaped geometry (Vickers). The final force on the die was 490 mN. 3. Results and discussion Fig. 1 shows the results of stress–strain measurements on two-phase composites with PC and SWCNTs. A strong increase of the strength in comparison to the pure PC is not observed. This is especially true for the less pure (40%) and shorter ArcD-SWCNTs, where the values drop with the nanofiller content. For the longer and purer (60%) LA-SWCNTs a small improvement in the yield strength
can be observed. The weak behaviour can mainly be attributed to the low adhesion of the components to each other. In order to take advantage of the impressive electrical properties of SWCNTs, i.e. conductivity [8,9] or attenuation of electromagnetic waves [12] without losing the mechanical properties of the pure PC, experiments with smaller ratios of CF were initiated. Initially, the total loading of filler in the host material was kept at 12.5 wt%. The stress–strain characteristics of these triple composites with PC/ArcD-SWCNTs/CF are shown in Fig. 2. The data indicates that CFs raise the modulus of elasticity slightly. In the case of the replacement of CF ratios by ArcD-SWCNTs the E-modulus can be kept in the region of the E-modulus of the pure PC. At PC:ArcD-SWCNTs:CF ¼ 87.5:7.5:5
Fig. 2. E-modulus (a) and yield szS or rupture strength szR (b) of three-phase composites of PC/ArcD-SWCNTs/CF. The bottom abscissa depicts the loading of ArcD-SWCNT material while the top abscissa shows the content of CF. The total loading is kept at 12.5 wt% always.
Fig. 3. Dynamical mechanical analysis (DMA) of triple composites with PC/ArcD-SWCNTs/CF. A pronounced rise in the dynamical E-modulus can be observed for the composites with a constant loading of 12.5 wt% (a). Even higher is the ascent of the triple composites with a constant ArcD-SWCNT loading of 2.5 wt% and larger, variable CF content.
ARTICLE IN PRESS B. Hornbostel et al. / Physica E 40 (2008) 2434–2439
[wt%] the E-modulus drops slightly under the value of the pure matrix and the spread grows. Therefore, in this case the formulation should not exceed 7.5 wt% ArcD-SWCNTs to avoid a strong descent in the E-modulus. The positive effect of the CF in the composites’ E0 -moduli can also be found in DMA measurements, Fig. 3(a). The dynamical E0 -moduli increase distinctly. The drop in the tensile strengths cannot be prevented by putting in small ratios of CF as seen in Fig. 2(b). Therefore experiments with higher loadings of CF and just low loadings of SWCNT material were recommenced. Now the loading of SWCNT material was kept constant at 2.5 wt%, which is well above the electrical percolation threshold [8,9]. The DMA measurements of these triple composites show a stronger rise in E0 as expected. Ascending mechanical values are also observed in the stress–strain
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measurements for the E-modulus (Fig. 4(a)) and for the tensile strengths (Fig. 4(b)). Further evaluations were performed by the utilization of an instrumented indentation test for hardness and materials according to ISO 14577. The advantage for using such a universal hardness probing machine is the simultaneous determination of several material properties, like universal hardness HU, intruding hardness HIT and intruding E-modulus EIT. Here, the universal hardness HU corresponds to the Martens hardness because a pyramidshaped probe was used. Fig. 5(a) shows that HU and HIT can be lifted to higher values with nanotubes. At PC:SWCNTs:CF ¼ 87.5:7.5:5 the values peak with HU150 N/mm2. In comparison HU of the reference PC: CF ¼ 87.5:12.5 is only 140 N/mm2. In both cases the total loadings were kept constant at 12.5 wt%. At higher
Fig. 4. Stress–strain measurements of the E-modulus (a) and the tensile strengths (b) of triple composites with high loadings of CF and constantly 2.5 wt% ArcD-SWCNTs. Over the references at 0 wt% carbon fibres a distinctive rise is present. The yield strength szS becomes congruent with the rupture strength szR, because the brittleness increases by SWCNTs.
Fig. 5. Results of the measurements of the hardnesses HU and HIT (a), and the moduli of elasticity EIT and E (b) for the composites with a total filler material loading of 12.5 wt% (except reference with 100% PC). The modulus of elasticity E of the stress–strain measurements is faced EIT in Fig. 5(b): the behaviour of E corresponds EIT.
ARTICLE IN PRESS B. Hornbostel et al. / Physica E 40 (2008) 2434–2439
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Fig. 6. HU, HIT and EIT of composites with a ArcD-SWCNT content of 2.5 wt% and high CF loadings (except references).
nanotube and lower CF loadings the mechanical values drop again, as already observed in the stress–strain measurements before. The modulus of elasticity E of the stress–strain measurements is faced EIT in Fig. 5(b): the behaviour of E corresponds to that of EIT. Fig. 6 shows HU, HIT and EIT of the triple composites with 2.5 wt% ArcD-SWCNTs and high CF content, plus the references. Again, an increase of the mechanical values can be observed for the triple composites. The introduction of SWCNTs supports the increase of the hardnesses, Fig. 6(a). The value of EIT (Fig. 6(b)) from PC:ArcD-SWCNTs: CF ¼ 64.5:0:35.5 is slightly below that of the comparable composite PC:ArcD-SWCNTs:CF ¼ 65:2.5:32.5. However in consideration of the error bars and the slightly lower total loading of filler material, the differences are compensated, so that EIT corresponds to the E measurements from Fig. 4(a) and to the DMA from Fig. 3(b). 4. Conclusions Reinforcement effects of triple composites consisting of PC, ArcD-SWCNTs and CF have been studied. In detail the tensile strengths s, E-moduli, dynamical E0 -moduli, universal hardnesses (HU), intruding hardnesses (HIH) and intruding moduli (EIH) were determined. While for pure PC/SWCNT composites an ascent of the mechanical properties is not inevitably attainable, simple triple composites with CF fulfill the expectations of increased strength. It is for this reason that the electrical benefits of CNTs can be exploited for electrical conduction [8,9] and for electromagnetic shielding [12] in insulating dielectric matrices without sacrificing, or with even increasing, the mechanical strength of the material system. Acknowledgements We are grateful to Steffi Mu¨ller (University of Stuttgart, IKT) and Didier Garray (CRIF/WTCM, Seraing (Lie`ge))
for discussions and support. Furthermore we thank Stephanie Gelse and Franziska Koch (MPI fu¨r Festko¨rperforschung, Stuttgart) for their assistance. The work was financially supported within the projects CANAPE, CARDECOM, SPANG and NANOSPARK of the sixth framework programme of the European Union.
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ARTICLE IN PRESS B. Hornbostel et al. / Physica E 40 (2008) 2434–2439 [18] For the sample characterisation and quality control of CNTs an extensive protocol was compiled by the consortium of the EU-project SPANG. It is hoped that by applying this protocol reproducible results will be obtained at different laboratories. To facilitate cooperation with other consortia and quite generally with colleagues
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worldwide, the protocol is also spread outside the EU project SPANG. The protocol is regularly updated. We appreciate any interest for the protocol and any feedback from colleagues working in the field.