Synergistic effect of graphite fillers on electrical and mechanical properties of Co-continuous PVDF/PP composites

This study explores the development of conductive co-continuous polyvinylidene fluoride/polypropylene/graphite (PVDF/PP/GR) composites with varying PVDF/PP ratios (60:40 and 70:30) to evaluate the effects of single and hybrid GR fillers on composites’ electrical conductivity, morphology, thermal stability, and mechanical properties. The composites were prepared with GR fillers ranging from 20 to 60 wt.% and particle sizes between 5.9 and 564 µm. Morphological analysis using scanning electron microscopy (SEM), supported by selective solvent extraction of the PVDF phase, confirmed a well-developed co-continuous structure at a PVDF/PP ratio of 70:30. Key performance metrics, including through-plane electrical resistivity (inverse of conductivity), thermal degradation, and mechanical strength, were systematically analyzed. The results showed that the addition of 60 wt.% GR reduced melting temperature while improved overall crystallinity and the degradation temperature from 440°C to 480°C, with a corresponding increase in residual ash content from 15 to 70 wt.%. Among the single-filler composites, medium-sized GR particles (17.8 µm) at 60 wt.% in the PVDF/PP (70:30) system exhibited the lowest electrical resistivity (3.41 Ω·cm). Additionally, the incorporation of 60 wt.% hybrid GR fillers, mixture of medium (17.8 µm) and large (561 µm) particles in an 80:20 ratio, further reduced resistivity to 1.17 Ω·cm, showing the synergistic effect of hybrid GR fillers of different sizes. Small GR particles act as bridges between the larger ones, leading to a more electrically conductive GR network. Mechanical testing revealed a flexural modulus ranging from 0.9 to 9.9 GPa and a flexural strength ranging from 7.0 to 26.3 MPa. The compressive elastic modulus varied between 0.1 and 0.2 GPa, while the compressive strength ranged from 7.8 to 22.9 MPa. These results highlight the suitability of PVDF/PP/GR composites as electrically conductive materials, particularly for applications in proton exchange membrane fuel cell (PEMFC) bipolar plates, as their main properties are in the range aimed by the U.S. Department of Energy (DoE).