Cross-Linking Agent\nBased on the Dynamic Bond and\nResultant Epoxy Adhesive with a High Mechanical Property

Due to the high cross-linking density of cured epoxy\nresins resulting\nin the inherent brittleness of adhesives, epoxy adhesives are less\nresistant to crack initiation and growth and often fail in a brittle\nmanner. Thus, promoting maximum elongation and ductility without sacrificing\nmechanical strength remains challenging. In this study, according\nto molecular structure design, through using amide bonds to combine\nflexible aliphatic amines and furan structures by a simple transesterification\nmethod, a polyamide curing agent, <i>N</i>,<i>N</i>-bis(6-aminohexyl)furan-2,5-dicarboxamide oligomer (FDAH), was obtained.\nThis curing agent was used to prepare a series of high-performance\nepoxy adhesives based on noncovalent interactions. Noncovalent bond\ninteractions could provide additional aid in energy dissipation and\ndynamic behavior, while flexible aliphatic chains ensure enough flexibility\nin the network. On the one hand, the adhesion strength of the adhesive\nwas improved by introducing noncovalent interactions into and between\nmolecular chains. On the other hand, the structure of furan-amide\nand the characteristics of flexible aliphatic amines were combined\nto adjust the cross-linking density of structural adhesives, thereby\nimproving their mechanical properties. By adjusting the content of\nthe modified curing agent in the epoxy network, the molecular structure,\ncuring behavior, and properties of the cured epoxy adhesive were systematically\ncharacterized, and it was found that the prepared adhesive had high\ntensile strength (62.4 MPa), large elongation at break (14%), excellent\nadhesion performance, suitable sizing, and curing window. The strengthening\nmechanism of reversible dynamic bonds on adhesives was also preliminarily\nelucidated. Overall, this work provides more possibilities for tuning\nthe physical properties of cross-linked networks, and using similar\nstrategies to enhance rigid polymers can achieve a good combination\nof high strength and scalability.