Recyclable and Flexible Polyester Thermosets Derived from Microwave-Processed Lignin

Commercial Kraft lignin was successfully valorized into recyclable and flexible polyester thermosets. Kraft lignin was first processed into more well-defined lignin fractions via liquid-solid extraction in green solvents (methanol and ethanol) or microwave-processed in ethanol at different temperatures (80 and 160 degrees C). The lignin fractions were then cross-linked in one pot with PEG and citric acid into polyester thermosets. In comparison to the thermoset synthesized from the original Kraft lignin, the thermosets synthesized from all the ethanol-extracted fractions were more homogenous and flexible as reflected by a sharper alpha-transition and over 100% increase in elongation at break. Furthermore, recycling of two of the prepared thermosets was evaluated (i.e., the thermoset prepared from the original lignin and the one prepared from microwave-processed lignin). Both materials could be rapidly recycled back into liquid resins under microwave hydrolysis and recured into thermosets under the original curing conditions. However, good quality films and comparable mechanical properties were only obtained after recuring the recycled resin originally synthesized from the microwave-processed lignin. Microwave processing compared to liquid-solid extraction paved the way for higher yields of narrow dispersity lignin fractions and matching final properties, providing an attractive approach to recyclable flexible lignin thermosets.