Preparation and characterization of thermoplastic polyurethanes using partially acetylated kraft lignin

Thermoplastic polyurethanes were prepared using 90 % acetylated softwood kraft lignin, polyethylene glycol and 4,4-methylene diphenyl diisocyanate. Due to the glass transitions of the compatible soft and hard segment mixtures, the polyurethanes exhibited the first glass transitions at −40 to 10 °C, and the transition temperatures increased with increasing hard segment content. Due to the glass transitions of the microphase separated hard domains, the second glass transitions occurred at 150 °C. The viscous responses during the second transitions decreased as the separated hard domains-induced chemical and physical crosslinks increased. The Young’s modulus and tensile strength increased with increasing hard segment content, whereas the breaking strain decreased. The phase morphology changed from an isolated hard domain structure to an interconnected one as the physical crosslinks increased, which caused drastic changes in the increasing or decreasing tendency of the tensile strength or breaking strain. Because of the phase morphology, the polyurethanes exhibited viscoplasticity or viscoelasticity.