Bio-based tannin-furanic-silk adhesives: applications in plywood and chemical cross-linking mechanisms

Wood polyphenolic extracts, commonly called tannins, are excellent candidates for the production of bioplastics due to their abundance in nature, their commercial availability, and their reactivity. In particular, they were tested as wood adhesives with several hardeners, but their low moisture resistance and their rigidity reduced their technological interest. In the present study, we combined regenerated silk (RS) with tannin-furanic formulations to improve their properties. Three-layer plywood glued with these several fully renewable tannin-silk-furanic adhesives were tested for their mechanical properties: the modulus of elasticity, the modulus of rupture, and both dry and wet shear strength were enhanced when 20 wt % of RS was added. Initially, the cross section of the prepared samples was investigated by scanning electron microscopy, indicating a good dispersion of RS within the tannin-furanic matrix. Afterward, thermomechanical analysis of the adhesive highlighted that RS slows down the polymerization rate, decreasing the cross-linking kinetics of polyfurfuryl alcohol. Chemical investigations through ATR-FTIR and 13C-NMR show the formation of covalent bonds between RS and the furanic matrix. In summary, the combination of bioresources from the vegetal and animal kingdom allows the manufacturing of fully bio-based adhesives with enhanced mechanical properties and water resistance. This represents an important breakthrough in the exploitation of polyphenols, opening perspectives for their application in material science.