Enhancing Strength and Toughness of Tannin-Based Adhesives via Topology-Controlled Hyperbranched Polyethers

Condensed tannins are promising biobased resource for wood adhesives due to their phenolic structures and sustainability. However, their insufficient strength and toughness hinder their practical applications in the wood-based panel industry. Herein, we introduced hyperbranched polyethers (HBPEs) as toughening modifiers with tunable topological architectures to regulate the cross-linked network of tannin-based adhesives. HBPEs with varied degree branching (DB) and molecular weights were synthesized by adjusting the initiator-to-monomer ratio, followed by amino-terminal functionalization to enhance the reactivity. As the monomer ratio, both DB and molecular weight of HBPEs increased simultaneously. The adhesive formulated with an optimal HBPE topology (DB = 0.53, 3% loading) exhibited outstanding bonding performance, achieving dry and wet shear strengths of 1.57 and 0.87 MPa, respectively, along with a high-fracture energy dissipation of 0.90 J under dry conditions and 0.62 J under wet conditions. Fourier transform infrared spectroscopy and solid-state 13C nuclear magnetic resonance confirmed covalent cross-linking between HBPEs and tannins via Stiff-based reactions and Michael addition reactions. Scanning electron microscopy and digital image correlation revealed that the flexible hyperbranched structure enhanced interfacial interlocking, regulated stress transfer, and mitigated stress concentration, thereby promoting the ductile fracture behavior. This work demonstrates that hyperbranched topology engineering is an effective strategy to simultaneously strengthen and toughen sustainable tannin adhesives.