Bio-Based Tannin-Furanic-Silk Adhesives: Applications in Plywood and Chemical Cross-linking Mechanisms

Wood polyphenolic extracts, commonly called tannins,are excellentcandidates for the production of bioplastics due to their abundancein nature, their commercial availability, and their reactivity. Inparticular, they were tested as wood adhesives with several hardeners,but their low moisture resistance and their rigidity reduced theirtechnological interest. In the present study, we combined regeneratedsilk (RS) with tannin-furanic formulations to improve their properties.Three-layer plywood glued with these several fully renewable tannin-silk-furanicadhesives were tested for their mechanical properties: the modulusof elasticity, the modulus of rupture, and both dry and wet shearstrength were enhanced when 20 wt % of RS was added. Initially, thecross section of the prepared samples was investigated by scanningelectron microscopy, indicating a good dispersion of RS within thetannin-furanic matrix. Afterward, thermomechanical analysis of theadhesive highlighted that RS slows down the polymerization rate, decreasingthe cross-linking kinetics of polyfurfuryl alcohol. Chemical investigationsthrough ATR-FTIR and C-13-NMR show the formation of covalentbonds between RS and the furanic matrix. In summary, the combinationof bioresources from the vegetal and animal kingdom allows the manufacturingof fully bio-based adhesives with enhanced mechanical properties andwater resistance. This represents an important breakthrough in theexploitation of polyphenols, opening perspectives for their applicationin material science.