Mussel-Inspired Underwater Epoxy Adhesive Based on Dynamic Dithioacetal-Driven Stepwise Curing

Robust underwater adhesives hold high application value, yet are restricted by the challenges of room-temperature underwater curing and negative effect of interfacial water. Inspired by mussels that form firm underwater bonds via catechol-containing byssus proteins, herein, the six-membered cyclic dithioacetal epoxy curing agent (DM-1) is synthesized via the condensation reaction between bio-based protocatechualdehyde and dihydrolipoic acid for constructing advanced underwater epoxy adhesives by dynamic dithioacetal-driven stepwise curing. During curing, DM-1 first generates linear polydithioacetal through a dynamic dithioacetal-driven ring-opening reaction, followed by the reaction of the generated terminal thiol groups with epoxy groups. Subsequently, the side-chain carboxyl groups continue to react with epoxy groups, accompanied by the dissociation-recombination process of dithioacetal bonds to achieve cross-linking. Benefiting from the synergistic effect of the interfacial strength provided by hydrophobic polydithioacetal chains and catechol groups, and the cohesive strength derived from multiple underwater curing processes, the adhesive exhibits superior underwater adhesive bonding strength on stainless steel (11.2 MPa) to that achieved in air (3.7 MPa), environmental tolerance and recyclability, providing a feasible strategy for the development of high-performance underwater epoxy adhesives.