Super-Strong and Water-Resistant Silk Fibroin/Gelatin Biomass-Based Wood Adhesives via β-Sheet Structure and Interfacial Schiff Base Reaction

Aiming at the low strength and poor water resistance of traditional biomass adhesives and the environmental risks of petroleum-based adhesives, this study developed a formaldehyde-free silk fibroin/gelatin (SF/GEL) biomass wood adhesive based on the synergistic effect of β-sheet structure and interfacial Schiff base reaction. Aldehyde groups were introduced on wood surfaces via “NaOH pretreatment-NaIO4 oxidation” modification. The β-sheet structure of silk fibroin enhanced the cohesion of the adhesive layer, while gelatin enhanced wettability. Under hot-pressing, aldehyde groups reacted with amino groups to form Schiff base covalent bonds, which combined with mechanical interlocking to realize interface strengthening. Under the optimal process (SF/GEL = 1:1, hot-pressing at 140 °C for 6 min), the dry strength of eucalyptus plywood reached 3.75 ± 0.25 MPa, and remained 2.95 ± 0.37 MPa after cyclic aging (soaking the specimens in boiling water for 4 h, followed by drying for 20 h, repeating this cycle twice; then testing the bonding strength after soaking the specimens in cold water for 1 h), exceeding the bonding strength requirement of 0.7 MPa specified in the GB/T 17657-2022 national standard. The dry strength of poplar plywood was 2.75 ± 0.30 MPa, and remained 1.98 ± 0.28 MPa after cyclic aging, also exceeding the 0.7 MPa bonding strength requirement of the GB/T 17657-2022 national standard. With renewable raw materials and environmentally friendly process, this adhesive provides a green and high-performance solution for the wood processing industry.