Electrically Conductive Composite Materials from Carbon Nanotube Decorated Polymer Powder Particles

Abstract

The aim of this work was to investigate the production of electrically conductive composites with a segregated MWCNT network through the use of MWCNT decorated polymer powder particles. The focus was on open-porous samples with appropriate characteristics for microfiltration membranes, produced through pressureless sintering. Compression moulding was also performed to produce non-porous composites and additionally investigate the influence of the processing methods on the formation and quality of the resulting MWCNT network. The decorated powder was received from the project partner FutureCarbon GmbH and processed at the Institute of Polymer Research of Helmholtz-Zentrum Geesthacht. Decorated powders with MWCNT concentrations of 0.2 wt% to 3.0 wt% were investigated for polyetherimide (PEI) with the addition of results for a first-phase, larger test powder at 5.0 wt%. Ultra-high molecular weight polyethylene (UHMWPE) with MWCNT concentrations of 0.2 wt% up to 5.0 wt% was processed and analysed. For polypropylene (PP), only the concentration of 5.0 wt% was used for two different particle size contingents. The polymers varied in material properties and powder particle sizes and shapes. Investigations on the received decorated powders showed that the results of the decoration process appear to strongly depend on the powder properties. The effectiveness of the decoration process is characterised by the surface-coverage of the powder particles with MWCNT and the amount, size and localisation of MWCNT agglomerates. Results from scanning electron microscope (SEM) investigations showed that the different powders were decorated with varying effectiveness, depending on their specific characteristics. The hard amorphous polyetherimide was effectively decorated for all concentrations, while the smaller, less dense and softer UHMWPE showed an unexpected, strong dependency of the effectiveness of the powder particle decoration on the MWCNT concentration. The also soft polypropylene was effectively decorated with MWCNT, due to its larger powder particle diameters. MWCNT agglomerates were numerous for polyetherimide, but they were relatively small compared to the powder particles, located in cracks and grooves and attached to the powder particles. The same was observed for polypropylene, with the MWCNT being attached to the powder particles predominantly at irregularities of the surface structure. The surface of the UHMWPE particles also showed prominent grooves and gaps, but the MWCNT agglomerates did not form predominantly at the powder particles, but rather away from them as unbound agglomerates, comparable to or much larger than the powder particles in size, depending on the MWCNT concentration. A competitive process between surface-coverage by individual MWCNT on the powder particles and the formation of agglomerates away from the particle surfaces was assumed from the observation of the decorated powders through SEM investigations. The effectiveness of the decoration process and therefore powder quality proved to be a strong influence on the electrical and mechanical properties of the processed composite material. The electrical conductivity of PEI/MWCNT showed a predictable dependency on the MWCNT concentration with significantly increasing values with increasing concentration. For PP, it was found that the larger powder particles provided the highest electrical conductivity value, due to them possessing less surface area per weight compared to the smaller particles. This explains the strong deviation in electrical conductivity values between the MWCNT concentrations for UHMWPE. While 1.0 wt%, 3.0 wt% and 5.0 wt% showed a significant coverage of the powder particles surfaces in scanning electron micrographs, 0.2 wt%, 0.5 wt% and 1.5 wt% to 2.5 wt% showed particles with fully non-covered or only partially covered parts of their surface. These powders therefore show significantly lower conductivity values compared to the powders with 1.0 wt%, 3.0 wt% and 5.0 wt% MWCNT. The sintered membranes for PEI and UHMWPE showed water-permeable, open-porous structures. Analysis of the pore sizes through capillary flow porometry showed that the mean flow pore diameter for pristine PEI and for PEI/MWCNT up to a MWCNT concentration of 1.5 wt% was above (> 10 µm) the values expected for microfiltration membranes, while 2.0 wt% to 3.0 wt% were in an appropriate diameter range (< 10 µm). Because of their smaller particle diameter, UHMWPE and UHMWPE/MWCNT showed the smallest mean pore flow diameters. For PEI and UHMWPE, differences in the electrical conductivity of sintered samples compared to compression moulded ones were found. For UHMWPE, the compression moulded samples were generally more conductive. In comparison, the sintered samples for PEI were generally more conductive than the compression moulded ones with one exception. From the comparison of the sintered samples to the compression moulded ones and between the different used powder particles, it was found that a high surface concentration and homogeneous surface coverage, together with a high melt viscosity, is desired for high conductivity values for compression moulded samples. For open porous membranes, smaller powder particles with high surface coverage but moderate melt viscosity are preferable. Additionally, an amorphous polymer is preferred to avoid influences of the crystallisation of the polymer matrix on the MWCNT network. It was shown that the sintering and compression moulding of MWCNT-decorated polymer powder particles can produce composites with very high electrical conductivity values. The electrical conductivity results from a segregated MWCNT network distributed through the polymer matrix.