CNTs | Processing
Figure 1: Electrical resistivity versus SWCNT concentration in LLDPE with MFI values of 20 (black curve) and 5 (red curve) g/10 min at 190°C and 2.16kg
Figure 2: Volume resistivity as a function of polyethylene MFI. Samples prepared with polyethylene wax (blue curve) and without wax (black curve)
loading SWCNTs into polymers with low fluidity, higher shear forces are needed. However, another possible way to disperse SWCNTs in low-fluidity polymers is to increase the polymer fluidity. This can be achieved by including additives, in this case polyethylene-based wax. Figure 2 shows the electrical volume resistivity of a number of PE grades with MFI values in the range 0.1 to 65 g/10 min. Dilution of the 2wt% SWCNT masterbatch in high-fluidity PE resulted in the construction of conductive networks and, as a consequence, conductive polyethylene was achieved at a lower level – at 0.1 wt% of SWCNTs. Dilution in low-fluidity PE does not result in electrical conductivity at this SWCNT loading level. However, the addition of 2wt% of polyethylene-based wax to the low fluidity PE increases the polymer fluidity, increasing MFI and resulting in shear sufficient for SWCNT dispersion and the construction of a conductive network.
compression moulded using a Dr Collin P200PV press at 170°C for 15 min. A PE/SWCNT masterbatch containing 2wt% SWCNT
was produced using a two-step procedure. A powder of SWCNTs was mixed with LLDPE (MFI 20 g/10 min at 190°C and 2.16kg) then extruded at 250°C and 300 rpm and granulated at the end of the extrusion line. For the dilution step, granular masterbatch and polyethylene was mixed together and extruded at 250°C and 300 rpm.
Conductivity results Figure 1 shows the volume electrical resistivity of the samples prepared by the dilution of 2wt% masterbatch to SWCNT concentrations of 0.01, 0.05, 0.10 and 0.20wt%. In this LLDPE MFI 20 resin the percolation threshold can be seen to be very low, at an SWCNT loading level of between 0.01 and 0.05wt%. However, for the same masterbatch dilution and processing condi- tions but using an LLDPE MFI 5 resin the percolation threshold is reached at a much higher SWCNT loading level, close to 0.2wt%. The difference in percolation behaviour for the low-MFI
polyethylene can be explained by the shear forces, which are insufficient for dispersing the SWCNTs and construct- ing the percolation network. The data shows that, for
58 COMPOUNDING WORLD | September 2016
Conclusions Many of the PE resins developed for industrial applica- tions are low MFI grades. Previous studies have shown that, to reach required conductivity levels in low-fluidity polymers through the addition of SWCNTs, the extrusion conditions had to be modified to use high screw rotation speeds with significantly higher temperatures. Such compounding conditions cannot always be applied for polymer modifications due to possible polymer degradation processes. This research shows that the use of an additive that increases polymer fluidity could help achieve the required electrical conductivity without significant modification of the compounding conditions. This research demonstrates that melt mixing with
standard twin-screw extrusion equipment can be used to produce conductive PE at ultralow loading levels of SWCNTs in comparison with other conductive fillers and over a wide range of MFIs. It also shows that the use of fluidity increasing additives such as polyethylene wax can enable creation of a conductive network without significant modification of compounding conditions.
About the authors: This article is based on a technical paper titled “The benefits of SWCNT for producing conductive plastic composites” by Evgeniy Ilin, Alexandr Bezrodny and Mikhail Predtechenskiy at OCSiAL in Luxembourg. The full version of the paper, including references, can be downloaded here. Luxembourg headquartered OCSiAL operates what it claims is the world’s largest industrial plant for production of single wall carbon nanotubes with a capacity of up to 10 tonnes/year. Its TUBALL products contain more than 75% of SWCNTs. ❙
www.ocsial.com
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