Developing Eco-friendly High-Strength Soy Adhesives with Improved Ductility through Multiphase Core–Shell Hyperbranched Polysiloxane

Development of nonformaldehyde biobased adhesives composed of renewable and low-cost biomass protein with high strength and ductility is desired to meet green and sustainable requirements for wood-based composites. Herein, a hyperbranched polysiloxane-terminated cardanol side group was incorporated for the first time with tannic acid in this effort, forming a fully biobased core–shell hybrid (TCA-HBSi) with high compatibility and reactivity. Subsequently, the synthesized TCA-HBSi was employed as a novel cross-linking agent in the preparation of modified soy protein (SP) resin. Because of the combination of rigid rings and flexible aliphatic chains, microphase-separated structures have been constructed in the interface between TCA-HBSi and brittle protein network. The structures not only promoted the dispersion of TCA-HBSi but also served as a multiple cross-linker to improve the interfacial interactions between TCA-HBSi and SP matrix, and hence facilitated the redistribution of mechanical stresses during loading. With such a synergistic multiphase structure, the TCA-HBSi-modified SP composite resins exhibited a more than 2-fold simultaneous increase in strength and toughness compared to the neat sample. Consequently, this strong but tough protein endowed the resins with substantially enhanced adhesive strength and water resistance when applied to plywood manufacturing. This work reveals that a modifier with “hard core, flexible shell” structures dispersed in biobased adhesive can more effectively improve the adhesive properties.