Thermosetting polyurethanes from chemically-unmodified lignin: synthesis, characterization and prospective applications

One of the most common approaches to prepare lignin-based polyurethanes (PUs) is to chemically derivatize lignin to modify its structure and to allow its hydroxyl functions to be more readily available for reaction with isocyanates. However such an approach may reduce the competitive advantage of lignin over conventional systems based on fossil-derived polyols, as chemical modifications of lignin may increase both the cost and the environmental impact of the resulting PUs. Therefore, alternative strategies based on the direct use of lignin in co-reaction with suitable polyisocyanates without any preliminary chemical modification are to be preferred. To this end, new thermoset PU coatings from chemically-unmodified lignin are presented in this work. Such novel high-lignin-content thermoset PUs were prepared by recovering a sol fraction from a raw kraft lignin sample by solvent extraction and subsequently directly crosslinking such soluble lignin fraction with a toluene diisocyanate (TDI)-based polyisocyanate at different NCO/OH ratios. A complete characterization of the PU systems highlighted their improved thermal stability, better film forming ability and higher hydrophobic character compared with the uncrosslinked lignin precursor. Additionally, force-distance curve measurements performed by means of an atomic force microscope were employed to determine the elastic modulus of the PU materials. Finally, the lignin-based PU materials were found to exhibit high adhesion on different substrates, including glass, wood and metals. The results of our study provide a clear demonstration of a straightforward and widely applicable strategy to develop advanced high lignin content PU systems and highlight the potential of lignin-based thermoset PUs as high performance bio-derived coatings and adhesives.