Green and High-Value Polyurethane Foams: Lignin-Microcrystalline Cellulose Biocomposites From Industrial Byproducts

The growing demand for sustainable polymeric materials has driven efforts to replace petroleum-based polyols in rigid polyurethane foams (RPFs) with renewable alternatives. Lignin, owing to its abundant hydroxyl groups and aromatic structure, is considered a promising candidate; however, its intrinsic rigidity and structural heterogeneity often limit its incorporation and compromise foam performance. In this study, industrial Kraft lignin (KL) was liquefied using 1,4-butanediol and employed as a bio-based polyol for RPF fabrication. Process optimization demonstrated that catalyst, surfactant, and water contents played critical roles in regulating foam density and compressive strength. To enhance mechanical performance, microcrystalline cellulose (MCC) derived from tobacco stems was introduced as a reinforcing phase. At an MCC loading of 5 wt%, the compressive strength at 10% strain increased by 34%, from 171.6 ± 7.5 to 229.7 ± 12.6 kPa, with enhancement up to 50% strain, while the apparent density increased slightly from 85.4 ± 2.0 to 93.5 ± 2.6 kg·m −3 . MCC incorporation also reduced the water contact angle from 110.3° to 100.0°. Morphological analysis revealed that increased system viscosity during foaming led to gas entrapment and larger, more irregular cellular structures. Overall, this work demonstrates an effective strategy for valorizing industrial byproducts into high-performance, sustainable RPFs.