Processing | CNTs
Researchers at OCSiAL have produced electrically conductive PE/SWCNT composites with ultralow percolation thresholds through optimised dispersion by flow modification. This article summarises the work
Study shows flow key to CNT dispersion
Carbon nanotubes (CNTs) have attracted considerable interest as a conductive additive for polymers due to their very high aspect ratio compared to commercially available carbon-based fillers such as carbon black, carbon fibre and graphite. This high aspect ratio results in a reduction in the percolation threshold for electrical conductitivity (the minimum level where a connected network is established). The loading level of carbon black to achieve percolation, for example, is in the range from 5–10 wt%. Multi wall carbon nanotubes (MWCNTs) 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 0.001 wt%. The ultralow percolation threshold of SWCNTs opens
up new possibilities for the design of conductive polymer materials. For example, it is possible to produce coloured plastics filled with SWCNTs while still retaining antistatic properties. This makes SWCNTs a promising option for designing conductive plastics and for improving the physico-mechanical properties of these composites. However, the incorporation of SWCNTs into the polymer matrix remains a challenge. It is well known that SWCNTs occur as bundles.
Dispersion of the SWCNTs and the quality of that dispersion in the polymer matrix are the key param- eters in achieving an optimum composite. For thermoplastic materials, there are three main
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approaches to incorporating SWCNTs into a polymer matrix: in situ polymerisation; solvent-based methods; and melt mixing. The first two approaches allow preparation of a high quality SWCNT dispersion but are difficult to use at an industrial scale. Melt mixing is widely used in the polymer industry and, as a readily scalable and solvent-free method, seems well suited to SWCNT incorporation into thermoplastic matrixes.
Mixing conditions OCSiAL’s researchers examined SWCNT melt mixing conditions with various polyethylene (PE) grades and produced a range of composites displaying antistatic properties in the range 104
–108
SWCNT loading levels of 0.05–0.1wt%. The extrusion conditions of SWCNT with PE were explored as a function of the polymer’s melt flow index (MFI). PE grades with MFI of 20 and 5 g/10 min at 190°C
and 2.16kg were used for the masterbatch preparation and masterbatch dilution steps. The PE granules were used as received and polyethylene wax employed as an additive to improve dispersion in the low MFI grades. The SWCNTs (TUBALL from OCSiAL) were also used as received. Both PE/SWCNT masterbatch and dilution to the final composite was performed in a Haake Polylab OS PTW twin screw co-rotating extruder with a screw diameter of 16mm and L/D of 40. Samples were then
September 2016 | COMPOUNDING WORLD 57
Main image: Use of flow
modifiers may allow SWCNTs
to be dispersed in polymers without major compounding process
modification, according to OCSiAL
researchers ohm*cm with ultralow
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