High modulus and strength polyurethane film synthesized from lignin-based polyol with various lignin contents and NCO/OH molar ratios

This study explored the synthesis of lignin-polyol (LP) by liquefaction of Kraft lignin in polyethylene glycol and glycerol. LP23 (23 wt% lignin) and LP41 (41 wt% lignin) were obtained with remarkable liquefaction yields of 97.43 % and 93.75 %, respectively. LP-based polyurethane film (LP-PUF) was then fabricated by reacting LP23 and LP41 with hexamethylene diisocyanate. Mechanical properties and hydrophobicity of LP-PUF were optimized by varying NCO/OH molar ratios. LP23-PUF1.9 with NCO/OH of 1.9 showed excellent tensile strength of 34.28 MPa and Young's modulus of 233.27 MPa, while LP41-PUF2.0 with NCO/OH of 2.0 exhibited good tensile strength of 26.03 MPa and Young's modulus of 260.66 MPa. LP23-PUF2.0 displayed high hydrophobicity as indicated by the water contact angle of 96.8° and low surface free energy of 15.68 mN/m. Glass transition temperature (Tg) of LP-PUF varied depending on lignin content and NCO/OH molar ratio. Specifically, the increase of KL content from 23 % to 42 % induced big rise of Tg by 8 °C. The NCO/OH induced change of Tg was relatively weak, as indicated by the small increase of −2.71 °C of LP41-PU1.4 to −1.81 °C of LP41-PUF2.0. The results of this work offer insights for lignin utilization as structural component of high modulus and high strength polyurethane film.