Study on the preparation and properties of lignin-based flexible polyurethane foams with NaCl as a medium

Currently, most polyols used in polyurethane foam (PUF) synthesis are derived from petrochemicals. However, lignin as the most abundant aromatic biopolymer rich in hydroxyl groups, is a suitable filler for synthesizing polyurethane foam. Therefore, in this study, Kraft lignin (KL) was utilized as a partial substitute for flexible polyethylene glycol 400 (PEG400). After adding 15 % KL, NaCl was introduced as a medium for synthesizing LFPUF and Polydimethylsiloxane (PDMS) was used as a foam stabilizer. Lignin-based flexible polyurethane foam (LFPUF) with high elasticity was synthesized. The aromatic ring structure of KL acted as a reinforcement for the foam, while the flexible PEG400 provided excellent flexibility to LFPUF. Na⁺ interacts with oxygen atoms in the long chain of PEG to form a cyclic crown ether structure. This promotes proximity and ordering of the hydroxyl groups, thereby facilitating proton transfer and optimising the cell size of the foam. Ultimately, the foam synthesized with 3 % NaCl (LFPUF-3%NaCl) exhibited an average cell size and hole size of 484.1 and 183.6 μm, respectively, significantly smaller than those of the foam synthesized without NaCl (843.5 and 249.3 μm, respectively). The foam resilience and elastic recovery of LFPUF-3%NaCl were 39.56 and 99.03 %, respectively, which was higher than that of the foam synthesized without NaCl (20.36 % and 97.08 %, respectively). In addition, LFPUF-3%NaCl maintained a high elastic recovery of 97.61 % after 20 cycles of compression. The egg drop test demonstrated that the foam effectively provided protective cushioning for fragile items.