Comprehensive characterization of sustainably synthesized lignin-based polyols towards thermoset materials

Abstract

Lignin, the most abundant aromatic biopolymer, has a high potential as an alternative to fossil resources in the chemical industry, especially within thermoset materials not only as a filler material but as reactant contributing to product properties. However, incorporating lignin is challenging because of the low and non-uniform reactivity of the functional groups and lack of characterization understanding. This study evaluated both ethylene carbonate and glycerol carbonate to obtain a polyol with solely aliphatic OH functionalities by modifying four different organosolv lignins from beech and poplar. The modification with ethylene carbonate yielded a polyol with aliphatic OH content in agreement with the theoretically calculated aliphatic OH content by conversion of native aliphatic and phenolic OH functionalities and COOH moieties. The modification with glycerol carbonate resulted in a more complex product with a lower aliphatic OH content than expected. This bias is caused by (1) non-reactivity of native alOH moieties with glycerol carbonate and (2) the formation of cyclic CO3 moieties on top of lignin. The hypothesis regarding loss of low molecular weight fragments in the work up was refuted. In addition to 31P NMR as a widely used analysis, 13C NMR was applied throughout the study and was successfully used in the aliphatic OH content determination in OL feedstocks as well as contributing to glycerol carbonate modified lignin with direct quantification of the 1,2-diol content and cyclic CO3 content. The uniform and improved reactivity towards thermoset development was demonstrated by an esterification with propionic acid as a proof-of-concept for e.g. polyester development.