Sustainable High-Performance Structural Adhesives With Intrinsic Thermo- and Piezoresistive Sensing Capabilities

Abstract

Epoxy adhesives are key enablers of lightweight, high-performance structures, providing efficient bonding of dissimilar materials for demanding engineering applications. Meanwhile, sustainable bio-based formulations with performances comparable to petroleum-derived systems are emerging as environmentally friendly alternatives. By incorporating carbon nanotubes (CNTs), these adhesives become multifunctional materials capable of simultaneously carrying loads and monitoring their structural state. Herein, a commercial Araldite epoxy and a bio-based resin, together with their composites containing 1, 3, and 5 wt% CNTs, were processed using a three-roll mill and systematically characterized. The bioresin exhibited higher tensile strength but lower strain-at-break than Araldite, while CNT incorporation caused only a slight reduction in the mechanical performance of both matrices. Although lap shear strength decreased after functionalization, all formulations remained suitable for semi-structural bonding applications. Thermoresistive sensitivity was maximized near the electrical percolation threshold, reaching ST ≈ −0.29 and −0.23% °C⁻¹ between 30°C and 100°C for Araldite and the bioresin, respectively. Load sensing was successfully demonstrated under shear loading in single-lap joints up to 50 N, with piezoresistive sensitivities approaching 300 Ω/N for Araldite containing 3 wt% CNTs and 25 Ω/N for the bioresin with 5 wt% CNTs. These findings demonstrate that both petroleum-based and bio-based epoxy adhesives can be transformed into sustainable multifunctional bonding materials with integrated thermo- and piezoresistive sensing capabilities for next-generation structural applications.