Deep Eutectic Solvent-Extracted Lignin for Flexible Polyurethane Foam Preparation

Lactic acid- and choline chloride-based deep eutectic solvents (DES) are applied for extracting lignin from pine wood. The ensuing DES-extracted lignin (DES lignin, DESL) has a purity higher than 90% and is rich in hydroxyl groups (about 5 mmol/g) without detectable sulfur. In-depth analyses show that DESL possesses good thermostability, low glass transition temperature (Tg), good polymer mobility, reduced cross-linking density, and rigidity. DESL was further subjected to an oxypropylation reaction to synthesize lignin-derived polyols (Lig-PO). Lig-PO carries functionalities that have both aromatic skeletons of isocyanate and hydroxyl groups of polyols, which are simultaneously leveraged in producing flexible polyurethane (PU) foam. The Lig-PO samples respectively derived from DESL, commercial alkali lignin (AL), and diluted acid corn stover lignin (DACSL) are subsequently used for flexible PU foam preparation with partial substitution (20 wt %) of petroleum-derived polyols. Compared to AL and DACSL, DESL-based PU (DESL-PU) foams have been the only ones capable of achieving highly homogeneous morphologies, well-oriented geometry, and superior thermal stability. DESL-PU foams have comparable compressive stress response with standard industrial flexible PU foam. The unique properties grant DESL to be an ideal biobased resource for PU foam production. This work demonstrates that DESL has a great potential for generating flexible biobased PU foams.