Soybean meal-based adhesives are attractive potential environmentally friendly replacements for formaldehyde-based adhesives. However, the low strength and poor water resistance of soybean meal-based adhesives limit their practical application. This study was conducted to develop a natural fiber-reinforced soybean meal-based adhesive with enhanced water resistance and bonding strength. Pulp fiber (PF), poplar wood fiber (WF), and bagasse fiber (BF) were added as fillers into the soybean meal-based adhesive to enhance its performance via hydrogen bonding between the PF and the soybean meal system. The enhanced adhesive exhibited a strong crosslinking structure characterized by multi-interfacial interactions wherein PF served as a bridging ligament and released residual stress into the crosslinking network. The crosslinked structure and improved interfacial interactions were confirmed by Fourier transform infrared (FTIR) spectrophotometry, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) measurements. Plywood bonded with 4 wt % PF-containing soybean meal-based adhesive exhibited a wet shear strength (1.14 MPa) exceeding that of plywood bonded with the control group by 75.4% due to the stable crosslinking network having efficiently transformed stress and prevented the permeation of water molecules.

It remains a great challenge to fabricate bio-based soy protein isolate (SPI) composite film with both favorable water resistance and excellent mechanical performance. In this study, waterborne epoxy emulsions (WEU), which are low-cost epoxy crosslinkers, together with mussel-inspired dopamine-decorated silk fiber (PSF), were used to synergistically improve the water resistance and mechanical properties of SPI-based film. A stable crosslinking network was generated in SPI-based films via multiple physical and chemical combinations of WEU, PSF, and soy protein matrixes, and was confirmed by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray diffraction (XRD), and solid state ¹³C nuclear magnetic resonance (¹³C NMR). As expected, remarkable improvement in both water resistance and Young’s modulus (up to 370%) was simultaneously achieved in SPI-based film. The fabricated SPI-based film also exhibited favorable thermostability. This study could provide a simple and environmentally friendly approach to fabricate high-performance SPI-based film composites in food packaging, food preservation, and additive carrier fields.