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Figure 1: Influence of extruder screw speed on volume resistivity at 0.1 and 0.2wt% TUBALL loadings
good dispersion required to form the necessary SWCNT network in the polymer a critical shear stress must be achieved during compounding that overcomes the physical entanglement and cohesive forces between the carbon nanotubes. It was observed that a very low
concentration of carbon nanotubes does not modify the electrical conductivity of the polymer considerably. However, as the dosage increases and the percolation threshold is reached, the electrical conductivity increases over a very narrow concentration range. For the polymer system in this study this transition was observed in a concentration range significantly lower (0.1% - 0.2% SWCNT) than for standard conductive fillers (as shown in the chart on the previous page). These experiments were performed
over a screw speed range of between 250rpm and 750rpm. For this highly fluid MFI 25 polymer system it was clear that better results were obtained at higher screw speeds (Figure 1). However, it is known from previous studies that too high a shear level can lead to degradation of CNT integrity. As shear is influenced by both the processing parameters and polymer viscosity, some level of polymer system-specific optimization will be required to achieve good dispersion and distribution of SWCNTs without destroy- ing their characteristics. Twin screw extruders offer the capability to design specific optimized screw profiles. One experiment compared a distributive screw profile to a dispersive
Figure 2: Influence of process temperature on volume resistivity at 0.1 and 0.2wt% TUBALL loadings
profile during the masterbatching step. The higher shear achieved by the dispersive screw allowed a resistivity of 6.20E+03 ohm*cm to be reached for the compound after dilution at 0.2wt% SWC- NT loading while the material produced using the lower shear distributive screw was measured at 6,00E+04 ohm*cm. The study investigated two different
feeding options for TUBALL incorporation during the masterbatching step. The resistivity values were found to be lower when the SWCNTs were introduced with the polymer pellets through the main hopper (6.40E+03 ohm*cm at 0.2%SWC- NT) than when they were added sepa- rately through a side feeder (2.80E+05 ohm*cm at same dosage). It was also seen that increased
processing temperature during the masterbatching and dilution step resulted in a reduction of resistivity in the compound (Figure 2). Higher processing temperatures would be expected to reduce the applied shear during mixing but this effect is thought to be counter- balanced by reduced Van der Waals interaction between the entangled CNTs.
Conclusions This experimental work has demonstrated that melt mixing using standard twin screw compounding equipment allows preparation of conductive polypropylene compounds at very low dosages of TUBALL through a two-step process involving the preparation of a masterbatch. Selection of process conditions was
Table 1: Volume resistivity of PP masterbatches containing 2wt% TUBALL SWCNTs prepared under different process conditions
Screw speed Low Low High High
CNT/IL Feeding Side Feeder Main hopper Side Feeder Main hopper
Volume resistivity (Ohm*cm) 6.3E+01 1.0E+02 1.3E+01 3.7E+01
found to significantly affect the final performance of the compound. For the selected formulations, it was observed that compounding conditions generating higher levels of shear and using higher process temperatures provided the highest conductivities. It was concluded that increased shear helps the dispersion and distribution of the SWCNT, creating a conductive network at significantly lower dosage levels than standard conductive additives. As this study was carried out at lab scale, it is expected that optimization of formulations and process conditions for industrial scale production would allow a further reduction in the amount of SWCNT required to achieve a specific conductivity. While this preliminary investigation
was limited to polypropylene, OCSiAl has observed similar trends in polyethylene compounds and believes this study sets a baseline for development in a broader range of thermoplastics including engineering resins.
About OCSiAl OCSiAl is the first company to develop breakthrough technology for single-wall carbon nanotube production, enabling large-scale commercial use. In 2014, OCSiAl entered the nanomaterials market with the universal additive TUBALL, which contains >75% SWCNTs. OCSiAl is developing technologies based on SWCNT for a range of applications including lithium-ion batteries, elasto- mers, transparent conductive films, composite materials and others. It has regional offices in the USA, Luxembourg, Russia, Korea, China, Hong Kong and India.
For more information visit
www.ocsial.com or contact Dr. Christian Maus, OCSiAl Development and Support Leader,
christian.maus@
ocsial.com
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