Mechanical, electrical, and adhesive synergies in melt-processed hybrid bio-based TPU nanocomposites

Hybrid nanocomposites (NCs) based on a bio-based thermoplastic polyurethane (TPU) with multi-walled carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) as nanofillers, were obtained using a simple melt-mixing method. The effects of a) the GNP:CNT ratio, b) the total nanofiller content, and c) the aspect ratio of the CNTs on both the nanostructure and the thermal, electrical, mechanical, and adhesive properties of the NCs were studied in depth. Synergies were observed in the mechanical and electrical properties of the hybrid NCs when compared to the corresponding binary TPU/GNP and TPU/CNT NCs, regardless of either the GNP:CNT ratio or the aspect ratio of the CNTs. This was attributed to the enhanced dispersion of the GNPs in the presence of CNTs, caused by the intercalation of the two-dimensional graphene nanoplatelets among the one-dimensional carbon nanotubes. Consequently, the resulting conductive network was more efficient, and the reinforcing efficiency of the single nanofillers was improved. The findings of our study show that electrically conductive NCs with improved mechanical properties were achieved when part of the CNTs in the formulation was replaced by cheaper GNPs. Furthermore, a synergy was also observed in the adhesive properties of the hybrid NCs through their significantly higher lap shear strength than that of the pure TPU or binary reference NCs. In other words, by replacing part of the CNTs with GNPs, we were able to obtain hybrid TPU NCs which were cheaper, more effective, and higher performing than binary TPU/CNT and TPU/GNP NCs, pointing to their potential use as electrically conductive hot-melt adhesives.