Development of a strong and conductive soy protein adhesive by building a hybrid structure based on multifunctional wood composite materials

In this study, a strong and multifunctional soy protein adhesive was designed and developed. It was then combined with wood units to prepare a clean and electromagnetic shielding wood-based panel via simple hot press. Specifically, bio-based raw material was prepared by grafting pyrrole (PY) and dopamine hydrochloride (DOPA) in sequence on to soy protein isolate (SPI), and then combined with AgNO3 and a self-synthesized bio cross-linker epoxidized quercetin (EQC), to develop a strong and conductive soy protein-based adhesive with mildew-proof and flame-retardant properties. A covalent cross-linked structure and noncovalent bond (hydrogen bonds and ionic bonds) were formed among SPI, EQC, DOPA, and Ag+. As well, a hybrid structure was formed via in situ polymerization of Ag NPs, resulting in an adhesive with improved water resistance. Thus, the dry and wet shear strengths of plywood bonded with resultant adhesive increased by 78% and 52.4%, respectively, relative to those of the SPI adhesive. The modified adhesive exhibited good mildew resistance (shelf life ＞ 15 d) due to the natural antibacterial properties of quercetin, and the dynamic equilibrium of Ag NPs and Ag+ in the presence of DOPA. Meanwhile, the LOI of the modified adhesive was 41.3%, which is higher than that of SPI, improving flame retardancy of the resultant plywood. Notably, the obtained adhesive exhibited conductivity owing to the presence of polypyrrole (up to 7.09 S/cm), which can be used to develop reflection-based particle board with electromagnetic shielding ability. This study provides a high-potential and feasible strategy for developing multifunctional biomass composites to replace metal and polymer materials, which is an important and practical approach to promote sustainable development of materials and reduce carbon emissions.