Engineering a Thermally and Electrically Conductive, Biodegradable, Mechanically Sustainable Nanocomposite via 3D Printing

The electronic industry has considerable demand for materials with good electrical and thermal conductivity, and for the convenience of manufacturing, it would be beneficial for the materials to have better mechanical properties in comparison to the currently actively researched material graphene nanoplatelets(GNPs). In response to such need, we intend to engineer nanocomposites with a blend of PLA, PBAT, and GNPs H-5 (average diameter of 5 microns) through 3D printing in this research. From the properties of individual materials, PLA is the currently commercialized 3D printing filament material; PBAT, as a ductile polymer, compensates for the brittleness of GNPs; GNPs H-5 contributes to good electrical and thermal conductivity. For environmental concerns, both PLA and PBAT are biodegradable polymers. From our previous studies, it had been concluded that the optimal ratio of PLA to PBAT is 1:3, creating percolation pathway for electrical conductivity. Contact angle tests and work of adhesion calculation indicated that GNPs preferred the phase of PBAT as opposed to PLA. The addition of PLA, at low concentration, would take up space and confine GNPs in PBAT, therefore improving electrical and thermal conductivity. Nevertheless, more PLA has the risk of making PBAT phase discontinuous. Inspired by this, in our research, we altered the concentration of GNPs (including 8wt.%, 12wt.%, 16wt.%, and 20wt.%) to find the composites with optimal thermal conductivity, electrical conductivity, and mechanical properties. The ratio of PLA to PBAT is fixed as 1:3. The 3D printing nozzle shear force is expected to further orientate the H-5 platelets.<br/><br/>Four-point electrical conductivity tests were performed on the 3D printed electrical test samples. As expected, electrical conductivity increases as GNPs concentration increases. For horizontally infilled samples, 8wt.% GNPs and 12wt.% GNPs samples were generally not electrically conductive. The electrical conductivity increases from 0 to 1.6 S/m when 16wt.% of GNPs is added. In addition, the orientation of infill during the 3D printing process has an effect on electrical conductivity. Vertically infilled samples are less electrically conductive than vertically infilled samples, however, the overall trend is similar. At 16wt.% GNPs, the electrical conductivity of horizontally infilled samples is approximately the value of Germanium, a classical semiconductor element. The thermal conductivity of samples were measured by a thermal camera while the samples were heated for a fixed interval of time. Thermal conductivity experiences minor increase as GNPs concentration increases. In comparison to pure PLA, 20wt.% GNPs samples are 350% as thermally conductive. Their thermal conductivity values are approximately the same as that of water at 50 Celsius degrees. This result support that an increase in the concentration of GNPs incurs an uplift in thermal conductivity. In the Instron test, the strain to stress diagram revealed the brittleness of high GNPs filling load samples. As the concentration of GNPs increases, the breaking elongation decreases, toughness decreases, and Young’s Modulus increases. Such result suggests that at the microscopic level, GNPs block the entanglement between polymers in the composites, giving rise to the decrease in material ductility. We expect the ideal blend of PLA, PBAT, and GNPs H-5 to have the proportion between 12wt.% and 16wt.%. If the concentration of GNPs is above 16wt.%, the material will be too brittle according to the mechanical tests data. Otherwise, if the concentration of GNPs equals or is below 12wt.%, the electrical conductivity of the material will be comparatively low for practical use.