Lignin as a Key Component in Lignin-Containing Cellulose Nanofibrils for Enhancing the Performance of Polymeric Diphenylmethane Diisocyanate Wood Adhesives

Lignin-containing cellulose nanofibrils (LCNFs), an emerging family of nature-based nanomaterials, have been successfully applied in many commercial polymer systems to enhance their properties and functional performance. In particular, LCNFs were used in polymeric diphenylmethane diisocyanate (pMDI) wood adhesives as a functional compound to significantly reinforce bonding properties. In order to elucidate the mechanisms for the interactions between lignin in LCNFs and pMDI wood adhesive, 13C–1H HSQC 2D-NMR experiments were carried out on dissolved ball-milled LCNFs to reveal the molecular–structural characteristics of lignin and polysaccharides in LCNFs and identify hydroxyl groups contained in these biomolecules that can be involved in reactions with the pMDI model compound. Moreover, a lignin-free CNF was prepared by chlorite treatment as a control group to illustrate the role of the lignin component in affecting the resulting adhesives. It was found that lignin was the integral component in LCNFs that is responsible for the increased reactivity as shown by curing kinetics, the improved thermal stability as shown by TGA, and the higher compatibility with nonpolar reagents as shown by lap-shear tests. The fundamental knowledge gained from this study allowed us to better understand the function of the lignin component as a distinctive feature of LCNFs, and insights into the role of lignin in improving pMDI wood adhesive performance would be highly beneficial for applying LCNFs as a novel sustainable nano-bio-reinforcing agent for a wider range of polymeric systems.