Investigation on the Printability of Recycled Thermoplastic Polyurethane/Carbon Nanotube Nanocomposites

Although the production of polymer/carbon nanotube (CNT) nanocomposites has grown exponentially over the last years for a variety of applications, the availability of polymer/CNT filaments for use in commercial 3D printing systems is very limited, and, currently, little is known about the printability of recycled polymer/CNT nanocomposites. In this respect, the fused filament fabrication (FFF) of recycled thermoplastic polyurethane (TPU)/CNT nanocomposites was investigated with a special focus on the piezoresistive behavior. Mechanically recycled and virgin TPU/CNT nanocomposites with different CNT contents (0.5, 1, 3, and 5 wt% by weight) were subjected to filament extrusion and FFF, and the changes induced by mechanical recycling, CNT contents, and infill orientation were monitored by melt flow index, thermal, mechanical, electrical, and piezoresistive properties. It was found that the recycled TPU nanocomposites exhibit very good printability with mechanical and electrical properties that are generally comparable with those for the virgin nanocomposites, the decrease of the elongation at break at 5 wt% CNTs being the primary challenge for the mechanical recycling of TPU/CNT nanocomposites. The 3D-printed recycled TPU/CNT nanocomposites with 3 wt% and 5 wt% CNTs have very good strain sensing behavior with tunable sensitivity by varying the printing conditions. When targeting strains that suit the human motion (0–68%), the recycled TPU/CNT film sensors printed with 0-infill orientation show higher sensitivity (gauge factor up to 115 and 20 for 3 wt% and 5 wt%, respectively) compared with the virgin TPU/CNT film sensors (gauge factor up to 50 and 15 for 3 wt% and 5 wt%, respectively). The findings of this work provide guidance for assessing the potential of using recycled TPU/CNT nanocomposites for 3D printing strain sensors for a wide range of human motions.