Tannin-like Lignin for Sustainable Synthesis of Water-Resistant Adhesives

Lignin, one of the most abundant yet underutilized biopolymers, offers strong potential for sustainable adhesive development but is limited by poor reactivity and water resistance. Here, we report a scalable, green two-step valorization strategy to transform kraft lignin into tannin-like adhesives with underwater bonding capabilities. Microwave-assisted depolymerization in isopropanol efficiently cleaved β-O-4 linkages, yielding low-molecular-weight oligomers with improved reactivity. Subsequent phenolic functionalization using resorcinol in an acidic deep eutectic solvent selectively formed Cα-CAr linkages, increasing the phenolic hydroxyl content by 56.6%, as confirmed by fourier transform infrared spectroscopy (FTIR) and 2D-HSQC NMR. The functionalized lignins were cross-linked with epoxidized soybean oil and malic acid to prepare fully biobased adhesives. The resulting materials exhibited high thermal stability (onset at 325 °C), excellent dry-state lap shear strength (3.3 MPa on aluminum), and remarkable water resistance, retaining 87% of bonding strength after 24 h immersion. Furthermore, dynamic ester bonds imparted intrinsic self-healing properties, enabling recovery of 71 and 64% of tensile strength under dry and wet conditions, respectively, after five damage-healing cycles. This work provides a sustainable platform for lignin valorization into high-performance adhesives, contributing to green chemistry and engineering principles and enabling applications in underwater repair, structural bonding, and eco-friendly packaging.