Biomimetic epoxy adhesive capable of large-scale preparation: From structural underwater bonding to hydrothermal durability

Biomimetic adhesives inspired by mussels are promising bonding materials in both dry and wet conditions. However, few such materials are suitable for application as underwater construction adhesives, where much higher bonding performance (≥5 MPa) and scale production are urgently required. Herein, a biomimetic catechol-based Mannich base epoxy curing agent (CMB) was prepared by the one-pot condensation reaction of 4-tert-butylcatechol, paraformaldehyde, and diethylene triamine on a hectogram scale. Further, a fast-curing two-part epoxy adhesive was designed by combining the CMB with the diglycidyl ether of hydrogenated bisphenol A. Benefiting from the independent crosslink-point strategy and synergic conditions, the CMB epoxy adhesive could maintain its dry bonding strength (9.0 MPa), even in a 4 °C underwater environment, which is the highest value that has been achieved to the best of our knowledge. Moreover, with a moderate post-curing process, the dry bonding strength on sandblasted Al (20.4 MPa) matches that were obtained on anodized Al and siloxane-treated Al, on which the obtained bonding strength represents the upper limit of an adhesive because of their well-known optimal surfaces for bonding. To the best of our knowledge, no previously reported catechol-assisted adhesive could achieve the same strength as siloxane-treated surfaces. Accelerated ageing experiments showed that the catechol groups had good endurance under hydrothermal conditions. The developed CMB epoxy adhesive has great potential as a high-performance structural adhesive and the findings of this study contribute to the understanding of the important chemical properties necessary for the design of biomimetic adhesives.