Novel Reusable Wood Adhesive Comprising Disulfide Bond–Modified Polyurethane

Abstract

The repeated bonding properties of wood adhesives directly influence the service life of wood. The covalent cross-linked networks of conventional thermosetting wood adhesives endow them with excellent performance but suppress the mobility of molecular chains, hindering the repeated bonding after adhesive failure via molecular chain reorganization. Here, we develop a thermoplastic hyperbranched polyurethane wood adhesive, which contains a prepolymer composed of cashew phenol, polyol, and isocyanate groups as the branching chains, a hyperbranched polyester as the core, and lipoic acid with disulfide (S–S) bonds as the terminal groups. When this adhesive breaks or cracks under external force and the broken surfaces are joined together, the molecular chains on both sides of the adhesive converge toward the breakage points driven by the highly branched structure of the hyperbranched polymer, promoting contact between the S–S bonds on both sides. Under heating conditions, dynamic reversible exchange reactions occur, stitching the cracks, and enabling the adhesive to bond repeatedly. Benefiting from this advantage, HPU-LA (100%) can retain more than 85% of its original bonding strength even after undergoing five repeated bonding cycles at 80 °C. This study addresses the issue of resource waste caused by the nonreusability of conventional adhesives as well as has notable research implications for extending the service life of wood.