A mild recycling of flame-retarding bisphenol-A epoxy towards formaldehyde-free wood adhesive by introducing hydrazone/ester dual reversible bonds

Degradation of epoxy resins in industry usually requires harsh conditions due to the three-dimensional cross-linked networks, making the products difficult to degrade and recycle, which leads to unacceptable environmental problems. Therefore, it is urgent to prepare a recyclable epoxy resin that can be degraded under mild conditions. Apart from the issue of recycling, another major limitation of epoxy resin applications is its high flammability. Herein, we presented a method of preparing the bisphenol-A epoxy resins with mild degradation and recycling performance by introducing vanillin-based flame-retarding epoxy monomers and ester reversible bonds. The prepared epoxy resins exhibited a high tensile strength of 83 MPa and intrinsic flame retardancy, reaching the vertical burning (UL-94) V-0 level. Attributing to ester exchange reactions, the resins could be completely depolymerized into the hydroxyl groups terminated oligomers by alcoholysis in ethylene glycol (EG). More importantly, the oligomers could be re-cured and up-recycled to be utilized as formaldehyde-free and multiple cycle-use plywood adhesive, and the adhesive strength could reach 4.5 MPa, meeting the ISO 1096–2014 standard. Interestingly, the solvent could be well recovered without causing secondary pollution. This work would provide an approach for the preparation of intrinsically flame-retardant and recyclable bisphenol-A epoxy resins.