Direct fabrication of high-strength epoxy adhesive from black liquor via amide-ester H-bond network

Most industrial adhesives are derived from petroleum-based feedstocks, raising concerns about sustainability and environmental impact. Although bio-based alternatives have been explored, they often suffer from limited mechanical strength and poor water resistance. In this study, a high-performance bio-based epoxy adhesive was developed using raw black liquor as a direct source of kraft lignin. The synthesis involved epoxidation followed by cross-linking polymerization with diethylenetriamine and maleic anhydride. After thermal curing at 180 °C for 2 h, the adhesive exhibited an exceptional lap shear strength of 32 MPa on aluminum substrates, outperforming all previously reported lignin-based adhesives and even most commercial adhesives. This enhanced adhesion is attributed to structural amido-ester covalent bonds in the adhesive, as well as the multiple three-dimensional hydrogen bond networks formed between the hydroxyl/amino groups on the amido-ester chains and the substrate interface. The adhesive also demonstrated a level of water resistance. The experimental results were strongly supported by DFT and MD simulations, which confirmed the role of interfacial hydrogen bonding in adhesion. This work presents a scalable approach for valorizing lignin-rich industrial waste streams into advanced adhesive systems with competitive performance.