Multifunctional bio-based structural adhesives: Integrating recycled carbon fibers for electrothermal healing

The permanent, irreversible architecture of conventional petroleum-based structural adhesives presents an environmental bottleneck at end-of-life. This study addresses this challenge by presenting a multifunctional, bio-based structural adhesive developed with circular economy principles in mind. The system combines a vanillin-derived epoxy matrix with mechanically recycled carbon fibers (RCF). The resin, diglycidyl ether of vanillyl alcohol (DGEVA), is cured with 4,4′-aminophenyl disulfide (4AFD), forming a catalyst-free covalent adaptable network (CAN) based on dynamic disulfide (S–S) bonds. The addition of 20 phr RCF leads to a marked improvement in mechanical properties: the bulk adhesive reaches an ultimate tensile strength of 51 MPa, while the shear strength of CMC/CMC joints increases from 33 MPa to 50 MPa. Concurrently, the formation of a percolated fiber network introduces electrical conductivity (20.5 S/m), enabling efficient Joule heating. Under an applied voltage of 3 V, the composite heats up to 170 °C, exceeding both the glass transition temperature (Tg ≈ 127 °C) and the activation threshold for bond exchange. This localized heating promotes rapid stress relaxation and network rearrangement, enabling in situ electrothermal healing, with crack volume reductions up to 58%. Furthermore, reprocessability is confirmed through multiple rebonding cycles in oven. Overall, these results outline a viable alternative to conventional petroleum-based thermoset adhesives, combining structural performance with recyclability and functional responsiveness