Polyurethanes (PUs) are made through the reaction of polyols with isocyanates to form urethane linkages. Currently, PUs are primarily synthesized from petroleum-based raw materials and are used for various applications such as foams, adhesives, coatings, and elastomers. In recent years, there has been considerable interest in synthesizing PU from sustainable raw materials such as vegetable oils, tannins, and lignin. Lignin, as the second most abundant natural polymer, has great potential to replace petroleum-based polyol in PU formulations. However, lignins obtained from different biomass sources (hardwood, softwood, and annual crops) and extraction processes (kraft, soda, organosolv and enzymatic hydrolysis) have widely varying properties, which could affect their reactivities toward isocyanates, thus the final product's performances. Therefore, it is imperative to measure lignin properties and its reactivity with isocyanate to identify the most important characteristics of lignin that can affect its reactivity/ suitability for polyurethane applications.In this study, the properties of twenty different lignins isolated from different biomass sources and isolation methods were evaluated, such as ash content, elemental analysis, hydroxyl content using Phosphorus Nuclear Magnetic Resonance spectroscopy (31P NMR), and molar mass through Gel Permeation Chromatography (GPC).The reactivity of these lignins toward isocyanate was also measured using titration, Fourier transform infrared (FTIR) spectroscopy, and 31P NMR. In addition, the activation energy and cure kinetics of the reaction of oven-dried lignins with isocyanate were analyzed using differential scanning calorimetry (DSC).℗ Among the various tested lignins, softwood lignins had an overall higher reactivity toward phenyl isocyanate, and annual crop lignins had the least reactivity. According to the statistical℗ analysis and modeling correlation between lignin properties and their reactivity, aliphatic hydroxy, guaiacyl (GOH), and sodium content had the highest impact on reactivity results. Also, on average, softwood lignins contained a higher amount of aliphatic OH, GOH, and sodium (Na), thus justifying their higher reactivity with isocyanate than hardwood and annual crops lignins. Additionally, the direct effect of sodium on the reactivity was also examined by comparing the reactivity of washed and unwashed lignin samples. After washing, the sodium content of lignin samples was significantly lower, thus their reactivities were significantly reduced. Sodium greatly affects the reactivity of lignin because it acts as a catalyst in increasing the reaction of lignin with isocyanate, while also causing the trimerization/dimerization reaction of isocyanates. The reaction of isocyanate compounds with each other catalyzed with the presence of strong metal ions such as sodium, creates a false positive error, when the reactivity is measured through titration and FTIR techniques that are focused on the amount of unreacted isocyanates at the end of reaction periods. Therefore, we recommend the 31P NMR method as the most reliable approach for directly measuring the reactivity of functional groups in lignins with isocyanate, especially for lignin samples that have high level of impurities (such as sodium content (> 0.1%)). Since 31P NMR technique can quantitively determine the amount of different functional groups of lignin before and after the reaction, the results will not be affected by the amount of isocyanate that reacted with each other and it is focused on what percent of hydroxyl groups of lignin are reacted with isocyanates. According to the 31P NMR results, the reactivity of different functional groups of lignins with phenyl isocyanate is as follows: aliphatic >> guaiacyl (GOH)>= p-hydroxyphenyl (HOH) = syringyl (SOH) >> carboxylic acid (COOH). The results of this novel study can help both researchers from academia and industry to select the most suitable lignins for polyurethane applications, and thus create more sustainable products.
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