Tuning Eco-Foaming of Kraft Lignin through Esterification: Effects on Chemical Structures and Thermal Stability

This study explores how anhydride esterification of kraft lignin (KL) influences an “eco-foaming” process that transforms 100% KL into rigid foams. KL was reacted with maleic (MA), succinic (SA), or phthalic (PA) anhydride in acetone. FTIR analysis confirmed hydroxyl consumption (diminished 3496/3372/3280 cm–1 bands) and formation of ester bonds at approximately 1712/1267/1130/1030 cm–1. The degrees of esterification (DE) ranged around 8.2% for MA, 7.9% for SA, and 6.3% for PA. Thermal analysis showed slightly higher glass-transition temperatures (Tg) and greater stability above 300 °C. When heated at 240 °C, the MA-KL foams exhibited smaller pores (∼23 μm), higher density (∼0.32 g·cm–3), and more closed-cell structures, which led to increased compressive strength and reduced thermal conductivity (0.043 ± 0.002 W·m–1·K–1) compared to unmodified KL foams (∼84 μm, 0.26 g·cm–3, 0.076 ± 0.005 W·m–1·K–1). TGA revealed improved thermal degradation resistance between 400 and 600 °C, likely due to denser, more closed-cell architectures and potential heat-induced cross-linking. Overall, esterification provides a controllable means to modify pore structure and foam performance, yielding lignin-based foams with enhanced mechanical strength, thermal insulation, and stability, suitable for sustainable building applications.