Development of mainly plant protein-derived plywood bioadhesives via soy protein isolate fiber self-reinforced soybean meal composites

Self-reinforced composites (SRCs) exhibit excellent interface compatibilities and have similar constituents for easy recycling, thereby making them competitive with traditional polymeric composites in various industrial areas. However, there is still a lack of research regarding the biodegradable plant protein-based SRCs plywood adhesives with rigid strength. In this study, the “self-gluing” concept of the binding of the soy protein isolate fibers (SPFs) network skeleton with the soybean meal (SM) matrix was explored. To avoid the SPFs’ aggregation and to improve their dispersity, a batchwise feeding of SM was applied before and after the SPFs were added. The 1,2,3-propanetriol-diglycidyl-ether (PTGE) was the cross-linker, which readily formed dense physicochemical interactions with the multiple side groups (COOH/NH2/OH) in both the SPFs reinforcement phase and the SM matrix. A set of SPFs addition amounts (0.5%–2.0%) were employed in order to manufacture the self-reinforced SPFs/SM/PTGE plywood adhesives, and their bonded three-layer plywood was produced for testing. The wet shear strength of the plywood bonded with the SPFs/SM/PTGE adhesives fulfilled China’s interior furnishing requirements (≥0.7 MPa, GB/T 9846-2015). The SPFs/SM/PTGE adhesive bonded plywood reached its highest wet shear strength of 1.12 MPa (1.5 wt.% SPFs addition), which was a 70% increase compared to the unmodified SM/PTGE-bonded plywood sample. The morphology, the infrared spectra, the crystallinity observations, and the thermal behaviors of the SPFs/SM/PTGE adhesives showed fine SPFs-to-SM interfacial bonding and a dense crosslink network in the SPFs/SM/PTGE adhesive systems. In this study, the “self-reinforced” concept of biodegradable plant protein-based composites was evaluated and showed a great potential to apply into the plywood adhesive systems.