Understanding sustainable polyurethane foams: A comparative analysis of foams from lignin and lignin hydrothermal liquefaction in alkaline vs. acidic conditions

With its natural abundance and low commercial utilisation, lignin offers a promising alternative to petroleum-based polyols in developing sustainable polyurethane foams (PUFs). This study explores the potential of hydrothermal liquefaction (HTL) modification technology for synthesising bio-based polyols, comparing it to the direct incorporation of lignin. The effects of HTL catalyst selection on PUF properties were systematically investigated using alkaline catalysts (K 2 CO 3 , NaOH, KOH) and acidic catalysts (formic acid, acetic acid). HTL of lignin generated bio-polyols enriched with phenolic monomers and a high hydroxyl content. Alkaline catalysts promoted methoxy group cleavage in the light oil fraction, with NaOH showing a concentration-dependent effect. In contrast, acidic catalyst concentration (e.g., acetic acid) exhibited minimal influence on methoxy group retention. In addition, PUFs were synthesised using bio-polyols with incremental substitutions of lignin and HTL-derived oils from NaOH and acetic acid. Remarkably, HTL oil-based foams maintained a cellular structure even at 100 wt% substitution, a feat unachievable with direct lignin incorporation. These bio-based foams exhibited enhanced urethane bond formation, resulting in significantly improved load-bearing properties (23.5–2006.1 kPa) compared to petroleum-based foams (26.7 kPa). Furthermore, PUFs synthesised under acidic conditions demonstrated greater fracture resistance, attributed to improved physical cross-linking due to methoxy group retention. This study underscores the critical role of HTL catalysts in tailoring PUF properties and provides valuable insights for developing sustainable, high-performance polyurethane materials. • Higher methoxy group cleavage in alkali-catalysis compared to acid-catalysis in HTL. • Higher methoxy group cleavage observed with increasing alkali concentration. • HTL oils permit 100 % substitution, which is not possible through unmodified lignin. • HTL oils from acetic acid resulted in PUFs with better compression properties.