Over the years, there has been a growing need for sustainable alternatives to replace petroleum-based polyols in the formulation of flexible polyurethane (PU) foams. Lignin, being the second most abundant natural polymer after cellulose, possesses various hydroxyl functionalities, making it a good polyol replacement. However, the incorporation of lignin in polyurethane flexible foams has been hampered by lignin’s rigid structure, high hydroxyl value, poor solubility in co-polyols, and low reactivity towards isocyanate. Therefore, it is important to overcome these limitations to fully harness lignin’s potential in flexible polyurethane foam applications. This study presents various modification strategies to synthesize liquid lignin polyols suitable for flexible PU formulations. Lignin was first oxyalkylated using varying molar ratios of propylene carbonate (PC) (4, 5, and 10 equivalents (eq)), and the resulting liquid lignin polyols were directly incorporated into flexible PU foam formulations. While the inclusion of lignin polyols increased the biobased carbon content of the foams, excessive residual PC adversely affected key mechanical properties such as tensile strength, tear resistance, and compression force deflection (CFD). Fourier-transform infrared (FTIR) spectroscopy indicated that unreacted PC interfered with microphase separation, weakening intermolecular interactions and reducing the overall strength of the lignin-based foams. A lignin-to-PC molar ratio of 1:5 was identified as optimal, providing a balance between processability and thermomechanical performance. Next, various co-polyols were used to further minimize PC loading in the lignin oxyalkylation reaction from 5 to 2eq. The results indicated that high ethylene oxide (EO)-based polyols showed the best compatibility with lignin oxyalkylation reaction and promoted lignin’s phenolic hydroxyl group’s reactivity with PC, while propylene oxide (PO)-based and bio-based polyols (such as castor oil, soy polyols, and cardanol-based polyols) showed poor compatibility, resulting in a solid–liquid two-phase mixture during the reaction. Partial least squares (PLS) modeling (R2Y = 86%, Q2Y = 67%, using two components and cross-validation) revealed a strong positive correlation between the co-polyol’s EO content and compatibility, and a strong negative correlation between compatibility and the co-polyol’s average molecular weight. Following optimization of the oxyalkylation reaction, a flexible PU foam incorporating 20% lignin polyol in place of petroleum-based polyols demonstrated enhanced mechanical properties and met industry standards for automotive seating. Additionally, a foam formulation with 50% total polyol replacement, comprising 20% lignin polyol and 30% soy polyol, met all performance criteria except elongation at break, while showing improved CFD compared to commercial counterparts. Finally, high-performance novel lignin-based polycarbonate polyols were prepared via a two-step process involving oxyalkylation of lignin with propylene carbonate, followed by transesterification with dimethyl carbonate. The resulting polycarbonate lignin polyols exhibited hydroxyl values (111–179 mg KOH/g) and viscosities (11,660–25,950 mPa.s) suitable for flexible PU foam formulations. In-depth Nuclear Magnetic Resonance (NMR) analysis confirmed the grafting of long polyether chains onto lignin during the oxyalkylation step and the introduction of multiple carbonate linkages at the end of the transesterification step. Foams were formulated by replacing up to 40% of petroleum-based polyols with synthesized polycarbonate lignin polyols. Additionally, flexible PU foams were prepared by replacing 60% of conventional polyol with a combination of synthesized lignin polyol and soy polyol. Formulated foams demonstrated superior mechanical properties, including enhanced tensile strength, and load-bearing properties, compared to petroleum-based foams. Additionally, the foams exhibited improved thermal stability, shock absorption, and biodegradability.
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