Polycarbonate nanocomposites for electromagnetic shielding

Appropriate toughening strategies yield polycarbonate/carbon nanotube composites with suitable conductivity and mechanical properties for the manufacture of electromagnetic-shielding device housings.The combination of polymers with conductive nanofillers such as carbon nanotubes (CNTs)—cylindrical carbon molecules with nanoscale diameter—has generated considerable interest in…

Appropriate toughening strategies yield polycarbonate/carbon nanotube composites with suitable conductivity and mechanical properties for the manufacture of electromagnetic-shielding device housings.The combination of polymers with conductive nanofillers such as carbon nanotubes (CNTs)—cylindrical carbon molecules with nanoscale diameter—has generated considerable interest in industrial research. Such composites would find many practical applications in the manufacture of electronic devices and sensors, as electromagnetic interference (EMI) shielding materials to replace metal housings (which are more unwieldy, expensive, and corrosion-prone).1On an industrial scale, melt mixing is the method most widely employed to produce polymer/CNT composites. However, it does not usually achieve complete dispersion of nanotubes and their agglomerates inside the polymer matrix, and so fails to optimize mechanical performance. Moreover, obtaining a composite with sufficient electrical conductivity for EMI shielding generally requires adding large amounts of CNTs, which likewise tends to impair the mechanical properties of the matrix.2 Thus, novel strategies for processing polymer/CNT composites are needed, capable of yielding the desired electrical properties without causing loss of mechanical performance.3To address this need, in our study4 we investigated polycarbonate (PC) composites containing different amounts of multiwalled carbon nanotubes (MWNTs)—a type of CNT composed of multiple concentric cylinders—prepared using a twin-screw extruder. Our goal was to achieve a good balance of electrical conductivity and mechanical properties, and to this end we tested the effects of several toughening strategies on these composites. First of all, we tried adding small amounts of a copolymer, cyclic polybutylene terephthalate (CBT), as a processing aid to improve the rheological and mechanical properties of the PC/MWNT composites. We also compared how different molding techniques (injection and compression molding) and subsequent thermal treatment (annealing) affected the performance of the materials. In this way, we were able to assess how different processing methods can influence the electrical and mechanical properties, shielding effectiveness, and morphology of PC/MWNT composites.We prepared the nanocomposites by mixing PC pellets with appropriate amounts of a commercial masterbatch containing 15% by weight (wt%) MWNTs, to obtain two final nanocomposites incorporating 2 and 5wt% filler loadings (2MWNT and 5MWNT) in a PC matrix. We also prepared batches of these composites into which 5wt% CBT (relative to the weight of neat PC) was incorporated during the extrusion process. In this way we obtained nanocomposite pellets with two different loadings of conductive filler, prepared both with and without the CBT copolymer. To test the industrial process performance of these different nanocomposites, we processed a portion of the pellet samples by compression molding followed by rapid cooling, and processed another portion by injection molding. We also annealed some of the injection-molded specimens at 150°Cfor a period of 5h.Figure 1.Shear stress and melt viscosity of polycarbonate (PC) composites incorporating 5% by weight (wt%) multiwalled nanotubes (MWNTs), containing different amounts of a cyclic polybutylene terephthalate (CBT) copolymer, as measured in a miniextruder at 290°C(Minilab Haake Rheomex CTW 5, Thermo Scientific).Our results show that addition of 5wt% CBT indeed improved the processability of the PC/5MWNT composites (see Figure 1) by lowering the melt viscosity. This means that the CBT acts as an external lubricant, capable of diminishing the frictional forces between the melt and the machine during extrusion, which is of relevance for prospective industrial applications. We also found that the dynamic storage modulus (see Figure 2), dynamic loss modulus, and dynamic viscosity (these last two not shown) all increased with increasing loadings of the conductive MWNT filler. This significant change in rheological behavior denotes a transition from liquid-like to solid-like behavior,5 which we associate with the formation of an interconnected structure that also favors electrical properties.Figure 2.Storage modulus (G  ′ ) versus frequency (ω) of neat PC and of PC nanocomposites with 2 and 5wt% filler loadings (2MWNT and 5MWNT) with and without addition of 5wt% CBT (5CBT), measured in an ARES rheometer (TA Instruments, parallel plate 25mm, gap 2mm, 260°C). We measured the electrical conductivity of the different composites processed in different ways (injection and compression molding, and annealing) with a LORESTA-GP resistivity meter. The results (data not shown) indicate that the PC composites with 5wt% MWNTs—irrespective of processing method—have sufficient electrical conductivity (>10−2S/cm) for EMI shielding applications.6 These conductivity values remain similar with addition of CBT, regardless of the process technique subsequently used. For nanocomposites containing 5wt% MWNTs that are compression-molded, a model based on classical electromagnetic theory7 predicts a shielding effectiveness of ~40dB (see Figure 3), which would be adequate for use in electronics housings. On the other hand, when these same composites are injection-molded the shielding effectiveness is lower, because injection molding causes the MWNTs to become highly oriented, thus interrupting tube-to-tube contacts and so reducing conductivity.Figure 3.Theoretical shielding effectiveness (SE) predictions, according to Colaneri,7for nanocomposite samples processed by different methods. Figure 4.Fracture toughness (KIQ) of the nanocomposite samples processed by different methods. Figure 5.Light microscope image of a PC/5MWNT/5CBT injection-molded specimen shows the morphology of carbon nanotube agglomerates. Flexural tests (2mm min−1, ISO 178) showed that adding the CBT copolymer had a positive effect on the nanocomposites. A combination of CBT addition and annealing optimized the flexural parameters, yielding improvements of up to 30% in the flexural modulus (data not shown). In fracture tests (ASTM D4045, SENB specimens), all the nanocomposites presented a brittle or semibrittle fracture curve. Fracture toughness was found to decrease with increasing MWNT loading, but this drawback can be partially counteracted by annealing after injection molding, or by using compression molding instead. Overall, compression molding seems to be the most suitable processing technique for composites containing CBT (see Figure 4). Microscopic fracture analysis confirmed the existence of different toughening mechanisms, such as immobilizing of polymer chains at the surface of the nanotubes, and ‘pull-out’ (a partial detaching of the nanotubes from the matrix, which absorbs a lot of energy and so enhances material toughness). In compression molding and with CBT addition, specimens were able to absorb more work of fracture. The processing method strongly influenced nanotube agglomeration (in terms of both the number of agglomerates and their shape). In compression-molded specimens, large agglomerates had a more flattened and curved appearance, which seems to be the most suitable morphology for improving the electrical conductivity of composites. For injection-molded samples containing CBT, the nanotube agglomerates were more oriented and exhibited a skin-core morphology, i.e., with the largest ones near the center and the smallest ones along the specimen edge (see Figure 5). Thus, CBT addition improves the processability of PC/MWNT composites in both extrusion and the ensuing injection molding, and can be combined with annealing strategies to enhance electrical conductivity.In summary, our findings show that it is possible to obtain PC/MWNT nanocomposites for EMI shielding applications while minimizing impairment of mechanical performance, using processing techniques that can easily be scaled up for industrial production. We plan to follow up this work by studying nanocomposites with different crystalline matrices to investigate how the degree of crystallinity affects the electrical and electromagnetic shielding properties of the material.AuthorsMaria José AbadUniversity of A CoruñaSantiago Garc # ia PardoUniversity of A CoruñaLaura ArboledaUniversity of A CoruñaAna AresUniversity of A CoruñaXoán Garc # iaUniversity of A CoruñaSonia DopicoUniversity of A CoruñaReferencesT. W. Chou, L. Gao, E. T. Thostenson, Z. Zhang and J. H. Byun, An assessment of the science and technology of carbon nanotube-based fibers and composites, Compos. Sci. Technol. 70, pp. 1, 2010. R. Socher, B. Krause, M. T. Müller, R. Boldt and P. Pötschke, The influence of matrix viscosity on MWCNT dispersion and electrical properties in different thermoplastic nanocomposites, Polymer 53, pp. 495, 2012. Y. H. Man, Z. C. Li and Z. J. Zhang, Interface-dependent mechanical properties in MWNT-filled polycarbonate, Mater. Trans. JIM 50, pp. 1355, 2009. S. G. Pardo, L. Arboleda, A. Ares, X. Garc # ia, S. Dopico and M. J. Abad, Toughening strategies of carbon nanotube/polycarbonate composites with electromagnetic interference shielding properties, Polym. Compos., 2013. M. J. Abad, A. Ares, R. Noguerol, X. García, C. Cerecedo, V. Valcárcel, J. M. Caamaño and F. Guitián, Rheology and thermal behavior of polyamide reinforced with alumina whiskers, Polym. Compos. 33, pp. 2207, 2012. J. Amarasekera, Conductive plastics for electrical and electronic applications, Reinf. Plast. 49, pp. 38, 2005. N. F. Colaneri and L.W. Shacklette, EMI shielding measurements of conductive polymer blends, IEEE T. Instrum. Meas. 41, pp. 291, 1992. DOI:  10.2417/spepro.005144

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