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Version: acceptedVersion × Subject: Electrical properties and parameters × Subject: Polymers ×

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    Influence of mixing conditions on carbon nanotube shortening and curling in polycarbonate composites
    (Melville, NY : AIP, 2017) Krause, Beate; Carval, J.; Pötschke, Petra
    Polycarbonate composites containing multiwalled carbon nanotubes (MWCNTs, 0.2-2.0 wt%) were melt mixed in small scale at different conditions of screw speed and mixing time to vary the specific mechanical energy (SME) input between 0.4 and 4.0 kWh/kg. Next to the electrical properties of compression molded plates and the MWCNT macrodispersion also the nanotube length and shape were analyzed. For this, the matrix of the composites with 0.75 wt% MWCNT loading was dissolved and the remaining nanotubes were investigated using TEM. It was found that with increasing SME input the number of remaining CNT agglomerates decreases. The MWCNT length decreased from initially about 1.4 micrometers towards 350 nanometers at a SME of 4 kWh/kg and the mean curling values were also reduced. The electrical percolation threshold increases with SME from about 0.4 wt% to 0.6 wt%.
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    Development of a polymer composite with high electrical conductivity and improved impact strength for the application as bipolar plate
    (Melville, NY : AIP, 2016) Hopmann, C.; Windeck, C.; Cohnen, A.; Onken, J.; Krause, Beate; Pötschke, Petra; Hickmann, T.
    Bipolar plates constitute the most important structural component in fuel cell stacks. Highly filled thermoplastic composites with high electrical conductivity obtain an increasing importance in the design of bipolar plates as alternative to conventional metallic systems. Thermoplastics (e.g. PP) have suitable properties such as a good processability, chemical resistance, light weight and low production costs. As thermoplastics have low electrical conductivities, conductive fillers have to be included in the matrix. A high content of such fillers (e.g. graphite) in excess of 80 wt.-% is necessary to achieve the desired electrical properties. However, materials with such high filler contents embrittle readily. The workability in injection and compression molding is difficult and the mechanical stability is insufficient in case of strain deformation. As consequence, material failure and an inacceptable amount of damaged goods can be observed during the processing. As no suitable thermoplastic system is available for better mechanical properties, the induction and dispersion of a rubber phase in the thermoplastic matrix can be used to increase the impact strength of the conductive composite. In this research work a ternary composite, based on PP as matrix, EPDM as impact modifier and synthetic graphite as conductive filler, was developed. The material was produced using a 26 mm co-rotating, intermeshing twin-screw extruder. The amounts of PP, EPDM and graphite were varied systematically and a process window was defined that enables improved impact strength and high electrical conductivity of the new material. The results indicate that impact strength can be enhanced by about 99 % with an EPDM content of 30 wt.-% in the PP matrix. The electrical conductivity decreases in a small range with increasing content of EPDM, but the conductivity is still excellent for producing bipolar plates.