Pressure-triggered cross-linking enables soybean meal adhesives with extended pot life and enhanced wet bonding performance

Soybean meal (SM) adhesives are regarded as promising alternatives to petroleum-based adhesives for promoting the sustainable development of the wood industry. However, the conflict between the high cross-linking efficiency of cross-linkers and the pot life of adhesives severely limits the industrial application of SM adhesives. Herein, a pressure-triggered microcapsule cross-linking strategy was proposed. Pressure-responsive PAPI microcapsules with reactive cores were prepared through interfacial polymerization between chain extenders and polymeric methylene diphenyl diisocyanate (PAPI), and subsequently incorporated into SM adhesives to construct a pressure-triggered cross-linking system. The mechanism underlying stable encapsulation and pressure-triggered release was clarified by analyzing the interfacial chain-extension reaction, shell characteristics, core retention, and pressure-induced morphological changes of the microcapsules. The microcapsules prepared using polyethylene glycol 200 as the chain extender and an emulsification speed of 1400 rpm exhibited suitable shell stability and effective pressure-triggered release behavior. Moreover, tannic acid (TA) was introduced to enhance the mold resistance and further improve the bonding performance of the adhesives through TA-induced non-covalent interactions. The developed adhesive still maintained a wet shear strength of 0.74 MPa after 24 h of storage. Meanwhile, the pot life of the adhesive was prolonged by 12 times compared with that of conventional cross-linking SM adhesives. This work provides a new strategy for developing sustainable protein-based adhesives with both high bonding strength and long pot life, reducing resource waste and improving production continuity.