Lignin reinforcement in thermosets composites

With its highly branched structure, its functional groups, and its free ring positions in the aromatic monomers, lignin is a promising bio-based candidate for network formation in resin formulations for thermosets and thermoset composites. Of particular interest are phenol-formaldehyde, polyurethane, and epoxy systems which are reviewed in separate sections. Various kinds of lignin are considered, including common kraft lignin and lignosulfonates but also the more recent organosolv and enzymatic hydrolysis lignins. A few examples for the development of cross-linked polymers with lignin which do not fall in the three categories are also given. It becomes clear that with the right structure (molar mass, functional groups) and the right modification (e.g., purification, fractionation, chemical derivatization) lignin can play an important role in future thermoset systems.