The Benefits of SWCNT for Producing Conductive Plastic Composites

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1 The Benefits of SWCNT for Producing Conductive Plastic Composites Evgeniy Ilin 1, Alexandr Bezrodny 1 and Mikhail Predtechenskiy 1,2 1 OCSiAl Ltd,1 Rue de la Poudrerie, L-3364, Leudelange, Grand-Duché de Luxembourg 2 Kutateladze Institute of Thermophysics, 1 Lavrenteva Avenue, Novosibirsk, Russia. Abstract New electrically conductive SWCNT-polyethylene composites with ultralow percolation thresholds have been developed. The melt mixing of different grades of polyethylene with SWCNTs was performed as a function of melt flow index (MFI) of polyethylene, achieving electrical resistivity in the range ohm*cm at wt% of SWCNTs in compressionmoulded samples. Such low percolation thresholds show that TUBALL SWCNTs can achieve perfect dispersability using a melt mixing approach, which makes these SWCNTs very promising for conductive polymers applications. Introduction Since the discovery of carbon nanotubes (CNTs) by Iijima in 1991 [1], they have attracted great interest due to the wide range of potential industrial applications. As a conductive and reinforcing additive for polymers, CNTs are a more promising material than the other known and commercially available carbon-based fillers such as carbon black, carbon fibre and graphite due to their high l to d aspect ratio. This high aspect ratio of CNTs leads to a reduction in the percolation threshold in comparison with other carbon-based fillers. For example, the loading level of carbon black for achieving percolation starts from 5 10 wt% [2 4] while the required loading level of CNTs is much less. In comparison with multi wall carbon nanotubes (MWCNTs), which start to percolate at a loading level of around 1 wt%, single wall carbon nanotubes (SWCNTs) can achieve percolation at a loading level of just wt% The ultralow percolation thresholds of SWCNTs open up new possibilities for the design of conductive polymer materials. For example, it is possible to colour plastics filled with SWCNTs while still retaining the antistatic properties of the final composite. Properties such as these make SWCNTs the most promising agent for designing various conductive plastics and for improving the physico-mechanical properties of these composites. SWCNTs are now being used for creating

2 new polymer composites for applications such as electrical engineering, high-voltage engineering, electromagnetic interference shielding, antistatic materials, and electrostatic paintings and coatings. However, the incorporation of SWCNTs into the polymer matrix is still a challenge. It is well known that SWCNTs occur as bundles [5,6]. The dispersion of SWCNTs and the dispersion quality in the polymer matrix are the key parameters of the composite. For thermoplastic materials, there are three main approaches for incorporating SWCNTs into the polymer matrix: in situ polymerisation [7 10], solvent-based methods [7,11 14] and melt mixing [7,11,15 19]. The first two approaches allow the preparation of a high-quality SWCNT dispersion in the polymer. However, these two methods are quite difficult to use at the industrial scale due to their complexity. In contrast, the melt mixing technique, which is widely used in the polymer industry and is a readily scalable and solvent-free method, seems to be an ideal approach for SWCNT incorporation into thermoplastic matrixes. In this paper, we demonstrate SWCNT melt mixing conditions with various polyethylene grades. Polyethylene is the most widely used plastic in industry. The extrusion conditions of SWCNTs with polyethylene are discussed as a function of polyethylene melt flow index (MFI). All the composites demonstrate antistatic properties in the range ohm*cm with ultralow SWCNT loading levels of wt%. Materials and equipment The polyethylene grades with MFI=20 and 5 g/10 min at 190 C and 2.16 kg were used for the masterbatch preparation and masterbatch dilution steps. The polyethylene granules were used as received. According to our preliminary studies, polyethylene wax allows the dispersion quality to be improved, especially in the case of polyethylene grades with low MFI. TUBALL SWCNTs produced by OCSiAl were used as received for masterbatch preparation. For the preparation of the SWCNT masterbatch in polyethylene and the masterbatch dilution to final composites, a Haake Polylab OS PTW twin screw co-rotating extruder with a screw diameter of 16 mm and L/D of 40 was used. All the samples were compression moulded using a Dr. Collin GmbH P200PV press at 170 C for 15 min. Experimental part The masterbatch containing 2 wt% of SWCNTs was produced via a two-step procedure using linear low-density polyethylene (LLDPE) with MFI 20 g/10 min at 190 C and 2.16 kg. A powder of pristine SWCNTs was mixed with the polyethylene granules. This mixture was

3 extruded at 250 C and 300 rpm and granulated at the end of the extrusion line. For the dilution of the masterbatch, the granules of masterbatch and polyethylene were mixed together and extruded at 250 C and 300 rpm. Results and discussions Figure 1 shows the volume electrical resistivity of the samples prepared by the dilution of 2 wt% masterbatch to the SWCNT concentrations of 0.01, 0.05, 0.1 and 0.2 wt%. In the case of the masterbatch dilution in the same polyethylene (LLDPE MFI 20 g/10 min at 190 C and 2.16 kg), the percolation threshold seems to be very low, at an SWCNT loading level of between 0.01 and 0.05 wt% (the black curve in Figure 1). In the case of the masterbatch dilution using the same conditions in polyethylene with MFI 5 g/10 min (low-density polyethylene with MFI 5 g/10 min at 190 C and 2.16 kg), the percolation threshold is reached at a much higher SWCNT loading level close to 0.2 wt% (the red curve in Figure 1). Figure 1. Electrical resistivity versus SWCNT concentration in polyethylene grades with MFI 20 and 5 g/10 min at 190 C and 2.16 kg. The difference in percolation behaviour for low-mfi polyethylene could be explained by the shear forces not being high enough for constructing the percolation network. These data show that, for loading SWCNTs into polymers with low fluidity, higher shear forces are needed. However, another possible way to disperse SWCNTs in low-fluidity polymers is based on increasing the polymer fluidity, which can be done by including some additives. In this work, we

4 used polyethylene-based wax. Figure 2 shows the electrical volume resistivity of a number of polyethylene grades with MFI values in the range 0.1 to 65 g/10 min. Figure 2. Volume resistivity as a function of polyethylene MFI. Samples prepared using polyethylene wax (blue curve) and without the wax (black curve). Dilution of the 2 wt% SWCNT masterbatch in high-fluidity polyethylene grades leads to the construction of conductive networks and, as a consequence, conductive polyethylene is achieved at 0.1 wt% of SWCNTs, whereas dilution in low-fluidity polyethylene grades does not modify the electrical conductivity at this SWCNT loading level. However, the addition of 2 wt% of polyethylene-based wax tends to increase the polymer fluidity, resulting in an increase of MFI and in shear forces sufficient for SWCNT dispersion and the construction of a conductive network. Given the wide range of polyethylene grades with low MFI that have been developed for a number of different industrial applications and products, the use of an additive that increases polymer fluidity could help to modify plastics in term of conductivity without requiring significant modification of the compounding conditions. It was shown in previous studies that, to reach the required conductivity in low-fluidity polymers by the addition of SWCNTs, the extrusion conditions had to be modified to use high screw rotation speeds with significantly higher temperatures. Such compounding conditions can not be always applied for polymer modifications due to possible polymer degradation processes. The approach proposed in this work is simple and avoids polymer degradation during the compounding step.

5 Conclusions This work shows that the well-known and widely used industrial method of melt mixing with standard twin-screw extrusion equipment allows the design of conductive polyethylene, over a wide MFI range of this polymer, at ultralow loading levels of SWCNTs in comparison with other conductive fillers. Conductive low-fluidity SWCNT-polyethylene composites could be developed without significant modification of compounding conditions thanks to additives such as polyethylenebased wax, which lead to an increase of polymer fluidity and consequently the easier construction of a conductive network. SWCNTs are thus a very promising conductive additive for constructing conductive polyethylene with a wide range of applications. References 1. S. Iijima. Helical microtubules of graphitic carbon. Nature. 1999, vol. 354, Y. Kanbur. Electrical and mechanical properties of polypropylene/carbon black composites. Journal of Reinforced Plastics and Composites. 2009, vol E. P. Mamunya, V. V. Davidenko and E. V. Lebedev. Effect of polymer-filler interfacial interactions on percolation conductivity of thermoplastics filled with carbon black. Composite Interfaces. 1997, vol. 4, L. Rejon, A. Rosas-Zavala, J. Porcayo-Calderon and V. M. Castano. Percolation phenomena in carbon black-filled polymer concrete. Polymer Engineering and Science. 2000, vol A. Kis, G. Csányi, J.-P. Salvetat, T.-N. Lee, E. Couteau, A. J. Kulik, W. Benoit, J. Brugger and L. Forró. Reinforcement of single-walled carbon nanotube bundles by intertube bridging. Nature Materials. 2004, J.-H. Du, J. Bai and H.-M. Cheng. The present status and key problems of carbon nanotube based polymer composites. Polymer Letters. 2007, vol. 1, S. Sathyanarayana and C. Hu bner. Thermoplastic nanocomposites with carbon nanotubes. Structural Nanocomposites. 2014, vol. 8, T. H. Kim, C. Doe, S. R. Kline and S. Choi. Water-redispersible isolated single-walled carbon nanotubes fabricated by in situ polymerization of micelles. Advanced Materials. 2007, vol. 9,

6 9. M. Lahelin, M. Annala, A. Nyka nen, J. Ruokolainen and J. Seppa la. In situ polymerized nanocomposites: polystyrene/cnt and poly(methyl methacrylate)/cnt composites. Composite Science and Technology. 2011, vol. 21, C. Park, Z. Ounaies and K. A. Watson, Dispersion of single wall carbon nanotubes by in situ polymerization under sonication. Chemical Physics Letters. 2002, vol. 364, P.-C. Ma, N. A. Siddiqui, G. Marom and J.-K. Kim. Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: a review. Composites: Part A. 2010, vol. 41, S. D. Bergin, V. Nicolosi and P. V. Streich. Towards solutions of single-walled carbon nanotubes in common solvents. Advanced Materials. 2008, vol. 20, C.-K. Chang and J.-Y. Hwang. Influence of solvent on the dispersion of single-walled carbon nanotubes in polymer matrix and the photovoltaic performance. Journal of Physical Chemistry. 2010, vol. 114, F. Du, J. E. Fischer and K. I. Winey. Coagulation method for preparing single-walled carbon nanotube/poly(methyl methacrylate) composites and their modulus, electrical conductivity, and thermal stability. Journal of Polymer Science: Part B: Polymer Physics vol. 41, P. Pötschke, A. R. Bhattacharyya, A. Janke, S. Pegel, A. Leonhardt, C. Täschner, M. Ritschel, S. Roth, B. Hornbostel and J Cech. Melt mixing as method to disperse carbon nanotubes into thermoplastic polymers. Fullerenes, Nanotubes and Carbon Nanostructures. 2005, vol. 13, B. Hornbostel, P. Pötschke, J. Kotz and S. Roth. Single-walled carbon nanotubes/polycarbonate composites: basic electrical and mechanical properties. Physica Status Solidi (B). 2006, vol. 243, B. Krause, P. Po tschke, E. Ilin, et al. Melt mixed SWCNT-polypropylene composites with very low electrical percolation. Polymer. 2016, 98, S. Haider, Y. Khan, W. A. Almasry and A. Haider. Thermoplastic nanocomposites and their processing techniques. In: Thermoplastic Composite Materials. 2012, F.-L. Jin and S.-J. Park. A review of the preparation and properties of carbon nanotubesreinforced polymer compositess. Carbon Letters. 2011, vol. 12,