Electrically conductive nanocomposites as heating elements for thermoplastics Joule welding

The potential of electrically conductive polycarbonate (PC) nanocomposites filled with carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) as heating elements to Joule weld PC substrates was investigated. Both GNPs/PC and CNTs/PC nanocomposites behaved as electrically percolated systems, with the CNTs rendering higher conductivities at considerably lower concentrations. Maximum values of 10.57 S/m and 0.028 S/m were achieved for the nanocomposite filled with 10 wt% loading of CNTs and 15 wt% loading of GNPs, respectively. The incorporation of 5 and 10 wt% loadings of CNTs into the polymer seemed to favour the development of denser conductive networks, capable of achieving the temperature required for an effective Joule welding of PC substrates under an applied voltage, whereas the addition of lower loadings of CNTs (i.e., ≤5 wt%) or GNPs at any loading ≤10 wt% rendered less conductive networks, not capable of achieving a high enough temperature for a successful Joule welding. The samples welded with the nanocomposite filled with 10 wt% CNTs showed higher lap shear strength (LSS) values (13.1–14.1 MPa) than those welded with the nanocomposite containing 5 wt% CNTs (12.3–12.9 MPa) due to the higher filler loading providing a higher conductivity and a more homogeneous Joule heating all throughout the heating element under the applied voltage. Likewise, increasing welding times, pressures and clamping distances led to progressively higher LSS of the Joule welded joints up to optimal values, with optimal times of 120 and 150 s being found for the nanocomposites containing 10 and 5 wt% loading of CNTs, respectively, and optimal pressure and clamping distance values of 1 MPa and 1.2 mm, respectively, being observed for both of them. This work demonstrates the potential of electrically conductive CNTs based polymer nanocomposites as heating elements for thermoplastics Joule welding, and highlights their formulation, applied voltage and welding parameters, including time, pressure and clamping distance, as key factors that can be strategically tuned to control the welding process and optimize the joints’ mechanical performance.