Soybean protein-based adhesives are limited in their application due to their poor wet bonding strength and poor water resistance. Herein, we prepared a novel, environmentally friendly soybean protein-based adhesive by adding tannin-based resin (TR) to improve the performance of water resistance and wet bonding strength. The active sites of TR reacted with the soybean protein and its functional groups and formed strong cross-linked network structures, which improved the cross-link density of the adhesives and then improved the water resistance. The residual rate increased to 81.06% when 20 wt%TR was added, and the water resistance bonding strength reached 1.07 MPa, which fully met the Chinese national requirements for plywood (Class II, ≥0.7 MPa). SEM observations were performed on the fracture surfaces of all modified SPI adhesives after curing. The modified adhesive has a denser and smooth cross-section. Based on the TG and DTG plots, the thermal stability performance of the TR-modified SPI adhesive was improved when TR was added. The total weight loss of the adhesive decreased from 65.13% to 58.87%. This study provides a method for preparing low-cost and high-performance, environmentally friendly adhesives.

Wood adhesive was prepared using Broussonetia papyrifera waste leaf protein as the raw material. The performance of the B. papyrifera leaf protein adhesive compared to soy protein was investigated using X-ray diffraction, Fourier transform infrared spectroscopy, and differential scanning calorimetry. The results indicated that both B. papyrifera leaf protein and soy protein were spherical proteins that could easily form three-dimensional crosslinked network structures and were of potential for protein adhesive preparation. The B. papyrifera leaf and soy protein-based adhesives had similar curing behaviors, but the crosslinking reaction of B. papyrifera leaf protein-based adhesive seems to be more complex than that of the soy protein-based adhesive. The B. papyrifera leaf protein-based adhesive had a lower increasing trend of particle size and crystallinity than the soy-based protein adhesive, and its water resistance and bonding strength were also weaker. The plywood with BP leaf protein adhesive had dry and wet shear strengths of 0.93 MPa and 0.59 MPa, respectively. These results are promising for future industrial production using Broussonetia papyrifera waste leaf protein as a new protein wood adhesive in the wood industry.

Wood adhesive is very important to the quality of wood-based panels in the forest product industry. Researchers are currently focused on developing green and environmentally friendly adhesives using biomass materials. Soybean meal (SM), a cheap and high-protein byproduct from soybean processing, is considered a green biomaterial resource for adhesive production. In this study, SM was modified using urea and sodium hydroxide as denaturants, and two cross-linking agents were prepared to incorporate into the soybean meal adhesive for bonding reactions. Sodium hydroxide added to soybean protein caused structural damage to the protein structure, which allowed the peptide chains to unfold extensively, forming a low-molecular-weight peptide mixture with a good size distribution and exposing numerous active functional groups for cross-linking with the tannin–phenol–formaldehyde resin (TR) and wood. Adding the epoxy resin CA and TR enabled the cross-linking agents to react with the active functional groups on proteins, forming a cross-linked network structure. As a result, the adhesive’s strength, residual rate, and solid content were improved.