Rigid polyurethane foams refined by the lignin oligomers from catalytic upstream biorefining process

Bio-rigid polyurethane foams (RPFs) were synthesized using the poplar sawdust lignin oligomers that were fractionized by catalytic upstream biorefining (CUB) as the substitute of unsustainable petrol-based polyols. On this basis, as the primary hydroxyl group providers for the RPFs, the lignin oligomers were isolated from the crude CUB oils that were obtained using the pulping liquors with different n-propanol/H2O ratios. Results demonstrated the CUB lignin oligomers were generally possessed low molecular weight, good miscibility, and high hydroxyl groups content, irrespective of the pulping liquor constituents, affording to highly cross-linked polyurethane networks. Moreover, the CUB pulping liquor's composition and the lignin oligomer substitution rate were proved as two decisive factors to the morphology and mechanical properties of the resulted RPFs. Compared to other groups, the RPF substituted with the lignin oligomers obtained from R7/3 (refer to n-propanol/H2O of 7/3 (v/v) in the pulping liquor) and a polyol substitution rate of 30% exhibited better mechanical properties and a highly assembled nanosized lignin complex. Additionally, the compressive strength and the apparent density of RPFs were highly dependent on the hydroxyl group content in CUB lignin oligomers, while the inner-molecular bonds such as the syringyl/guaiacyl ratios and β-O-4 bonds were also affected to the compressive strength of RPFs. The RPFs prepared by CUB lignin oligomer in the current work provides a foundation for future valorization of the side-streams from lignin-first biorefining processes.