Novel and high-performance tannin-polyamine adhesive: New insight into phenol-amine chemistry

Condensed tannin-polyethyleneimine (CT-PEI) adhesives have shown high wood bonding performance in earlier studies. However, high cost caused by complicated synthesis of PEI, as well as use of toxic monomers lead to challenges which limits the application in wood industry. In this study, the melamine-based highly branched polyamine (MBHBP) synthesized by a facile, green, and low-cost strategy was proved to be an ideal candidate for replacing PEI in fabricating polyamine-tannin adhesives. The plywood bonding performance tests showed that the shear strength of three-ply plywood remained to be 1.76 MPa after soaking the specimens in boiling water for 3 h, superior to most of the previously reported tannin-based adhesives. When soaking time was extended to 72 h, the strength remained above 1.20 MPa, indicating excellent water resistance of the CT-MBHBP adhesive. For the first time, the previously proposed mechanism for polyamine-catechol crosslinking reactions was verified by using model reaction combined with ESI-MS analysis on the reaction products. The results confirmed that both Schiff base formation and Michael addition contribute to the formation of crosslinking networks. The phenol-amine chemistry was further understood.