Facile and cost-effective strategy for fabrication of polyamide 6 wrapped multi-walled carbon nanotube via anionic melt polymerization of ε-caprolactam
Graphical abstract
Introduction
The integration of smaller and lighter electronic devices can lead to malfunctions in the electronic devices because of static electricity and electromagnetic interference, putting the user at great risk [1], [2]. According to these trends, various studies have been performed to develop electrostatic discharge (ESD) or electromagnetic interference shielding effectiveness (EMI SE) materials [3], [4], [5]. ESD or EMI SE properties are known to have a significant relationship to the electrical conductivity of the material, and most polymers are difficult to apply to the ESD and EMI SE products because of the low electrical conductivity of the polymers [6], [7], [8], [9], [10], [11]. Polymer composites filled with carbon nanotubes (CNTs) are attracting attention because they can realize not only excellent electrical conductivity originated from the CNTs but also outstanding processability and lightweight characteristic due to the polymer matrix [12], [13], [14], [15], [16], [17], [18].
In composites containing CNT, the size, shape and dispersion of the filler have been identified as significant factors affecting the electrical conductivity of the composite [19], [20], [21], [22]. In particular, according to percolation theory considering the electron tunneling effect, the dispersion of the CNTs is recognized as a dominant physical factor in determining the electrical conductivity of the composite [23], [24], [25]. However, because the aggregation of CNTs by the van der Waals force hinders their uniform dispersion in the composite, the electrical conductivity of CNT filled composites is difficult to reach the theoretical results based on the percolation theory [21], [22], [23], [26], [27], [28].
To improve the dispersion of CNTs in the composite, methods of maximizing the mechanical mixing between the filler and the matrix, and of inhibiting the van der Waals force by covalent, non-covalent functionalization or polymer wrapping of CNTs have been reported [29], [30], [31], [32], [33], [34], [35]. Methods for maximizing the mechanical mixing are known to be difficult to induce uniform CNT dispersion, at least in the melt process, and the covalent, non-covalent functionalization or polymer wrapping methods are expensive and practically difficult to apply to mass production [17], [34], [35], [36], [37], [38]. Recently, based on in-situ polymerization using a low melt viscosity of cyclic butylene terephthalate (CBT), a type of in-situ polymerizable oligomer resin, the enhanced dispersion and electrical conductivities of composites incorporating nanofillers such as multi-walled CNTs (MWNTs) and graphene nanoplatelets (GNPs) have been reported [25], [39], [40], [41], [42], [43]. However, the mechanism of the proposed process has not been clearly identified, and it is difficult to apply it to ESD and EMI SE applications due to the high cost of the CBT resin (∼13 EUR).
ESD and EMI SE products with highly conductive polymer composites containing MWNTs are produced by a cost-effective combination of processes including both the extrusion process of MWNTs and polymer matrix and the injection molding process of the polymer composites. Ramasubramaniam et al. [9] reported that the composites can be used for ESD and EMI SE applications when the electrical conductivity is >10−5 S/m and 10 S/m, respectively. Therefore, there is an urgent need to develop a nanocomposite manufacturing process capable of enhancing the dispersion of MWNTs and reducing the incorporation amount of the filler without drastically increasing the processing cost. In this study, to induce a uniform dispersion of MWNTs, a nanocomposite manufacturing process combining both physical particle mixing using high-speed rotation and anionic in-situ polymerization of CL was proposed. Compared to the typical processing, enhanced dispersion of the MWNTs and electrical conductivity of the composites were achieved using the suggested processing, and electrical conductivities of 10−5 S/m and 10 S/m can be obtained by filling with only 1 and 3 wt% of MWNTs, respectively.
Section snippets
Materials
Acrylonitrile-butadiene-styrene (ABS, HR181, Kumho Petroleum Chemical Co., Seoul, Korea), which is widely used as a resin for ESD and EMI SE products requiring impact properties, was used as the matrix for the composites. The used matrix resin exhibited mechanical properties applicable to ESD and EMI SE products, with the molecular weight range of 50,000–200,000 g/mol. MWNTs (Jenotube 8, JEIO Co., Incheon, Korea) were used as a filler to improve the electrical conductivity of the composites.
Results and discussion
To confirm that the proposed process improved the electrical conductivity of the composites, the electrical conductivity of the composites was measured and is shown in Fig. 2. The composites fabricated using the proposed process exhibited overall enhanced electrical conductivity compared to the composites prepared by the typical process, and the percolation threshold was observed at a lower filler content. In particular, the electrical conductivities of the compounds filled with 1 and 3 wt%
Application
To apply the proposed process to the mass production of ESD and EMI SE products, optimization of the MWNT content of the master batch manufacturing process is required prior to melt mixing with the ABS resin. The higher the content of the master batch is, the less time it takes to fabricate, which improves productivity; however, MWNTs cannot be PA6-wrapped beyond a specific MWNT content in the master batch, which may reduce the dispersion effect of the fillers. Electrical conductivities and
Conclusion
To fabricate a nanocomposite based on a melt process that is able to induce a uniform dispersion of MWNTs and minimize the incorporation amount of expensive MWNTs, a nanocomposite manufacturing process that combined both physical particle mixing using high-speed rotation and anionic polymerization of CL was proposed. After the MWNTs were uniformly distributed by particle mixing, they were impregnated with the CL monomer due to the low melt viscosity, and then the MWNTs were wrapped with the PA6
Acknowledgements
NMR data were acquired on 400 MHz Solid state NMR spectrometer (AVANCE III HD, Bruker, Germany) at KBSI Western Seoul center (Seoul, Korea). This research was financially supported by Korea Institute of Science and Technology (KIST) Institutional Program, the Technological innovation R&D program of SMBA [S2394169], Basic Science Research Program (2017R1C1B5077037) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, the Industrial Technology Innovation
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