Tailored lignin-based polyurethane coatings with a high biomass content

New polyurethane coatings with a high biomass content (over 85 wt.%) and tailored properties are presented in this work. These materials are based on the copolymerization of a lignin-derived isocyanate (VA-NCO) with industrially available lignins obtained through different pulping processes, namely organosolv, kraft and soda methods. Firstly, VA-NCO was synthesized from vanillic acid, a lignin-derived product, and characterized through different spectroscopy methods. The optimal VA-NCO/lignins mass ratios for the different lignins was determined assessing the FTIR spectra and the sol-content of the thermally-crosslinked coatings until no free VA-NCO nor lignin was present in the crosslinked product. Based on the total -OH content of lignins determined by 31P-NMR (ranged from 4.04 to 5.58 mmol OH/g), differences on the reactivity of lignins were determined and attributed to the structural differences among them (i.e. molecular weight and repeating monolignol units). A thorough characterization of the PU coatings remarked the improved thermal stability, the tunability of the glass transition temperature and an increasing hydrophobicity when increasing the lignin content on the coatings. Moreover, the PU materials were found to exhibit a high adhesion on different materials, including wood, steel, aluminum and, especially glass. Hence, the results of this work demonstrate the applicability of lignin on the obtention high-lignin-content PU coatings with tuneable properties from untreated lignin and lignin-derived isocyanates.