Soy-Based Adhesives Functionalized with Pressure-Responsive Crosslinker Microcapsules for Enhanced Wet Adhesion

With excellent bonding performance, formaldehyde-based adhesives have been widely used for the production of woody materials. However, these adhesives could potentially cause harm to human health and the environment. Herein, we report an eco-friendly soy-based adhesive (SBA) by constructing a pressure-responsive crosslinker system with a core–shell structure. Isophorone diisocyanate (IPDI) was encapsulated in microcapsules as the core, and a shell mainly consisting of polyurethane was used to hinder the crosslinking reaction between the soy protein and IPDI, thus avoiding an increase in viscosity of the adhesive. As the crosslinker and curing agent for the SBA, the encapsulated IPDI in the microcapsules could be released under an external pressure, which induces in situ crosslinking of IPDI with wood and soy protein, promoting the curing reaction of the SBA and building chemical bridges between the SBA and wood, thereby decreasing curing temperature and improving water resistance of the SBA. The SBA modified with the pressure-responsive crosslinker system exhibited improved wet shear strength, moderate viscosity, low curing temperature, and very low cytotoxicity, showing great potential as an alternative to formaldehyde-based adhesives. Furthermore, the pressure-responsive microcapsule crosslinker system has been also successfully applied to functionalize many other biomass-derived adhesives (i.e., cottonseed protein, peanut meal, oxidized starch, and sodium alginate) to dramatically improve their water resistance and mechanical properties. This work provides a versatile strategy to develop formaldehyde-free, sustainable, and high-performance bio-based adhesives.