Lignin is the most abundant natural aromatic polymer and contains both aliphatic and aromatic hydroxyl groups with excellent potential to replace polyol in formulations of polyurethane resins. However, inaccessibility of hydroxyl functional groups embedded in the lignin structure is a major limiting factor to broaden its application as polyol. Thus, lignin modification is necessary to increase the number of hydroxyl groups, or make them more accessible to react with isocyanate. In this study, ionic liquids (ILs) as a green recyclable solvent was used to increase the hydroxyl content of Organosolv lignin samples isolated from both hardwoods and softwoods. Modified lignin with bromide based ionic liquid has been analyzed by size exclusion chromatography (GPC), quantitative phosphorus nuclear magnetic resonance spectroscopy (31P NMR), quantitative 2D (heteronuclear single quantum coherence) HSQC NMR and XPS (X-ray photoelectron Spectroscopy). Increase in both aliphatic and aromatic hydroxyl functional group, decrease in molecular weight, particle size and B-O-4 linkages in lignin samples was observed after the treatment. Lignin’s reactivity towards isocyanates was improved after ionic liquid treatment compared to untreated samples. We have also further investigated this reaction by studying chemical structures of lignin model compounds (vanillin, coniferyl alcohol, guaiacylglycerl-beta-guiacyl ether) before and after ionic liquid treatment, using advanced analytical techniques such as: proton, carbon and phosphorus NMR, FTIR (Fourier transform infrared spectroscopy), LC-MS (liquid chromatography-mass spectroscopy). The results showed the breakage of B-O-4 linkage in the lignin model compounds. Lignin modification with ionic liquid can be an efficient and environmental friendly technique to increase lignin reactivity as a biobased polyol in polyurethane resin synthesis applications.

Sustainable pathways to increase lignin reactivity for resin applications

2019 Fall National Meeting

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