Valorization of lignin for advanced material applications: a review

Abstract

Lignin is the second most abundant natural biopolymer after cellulose, constituting between 18 wt% and 35 wt% of lignocellulosic biomass. Its renewability, vast availability, and sustainability make it a desirable alternative and/or complementary macromolecule to conventional polymers. However, due to its relatively short and branched molecular size, high polarity and moisture absorption, lack of melt processability, and inferior dispersibility in nonpolar polymer matrices, its extensive application in polymers is limited. For effective and sizable application of lignin in polymers and composite materials, functionalization and modifications are necessary. This review paper intends to discuss the valorization of lignin as feedstock to produce alternative chemicals for converting lignin moieties into value-added products which can be used in advanced applications. A comprehensive overview of various physicochemical modification and functionalization routes of lignin is critically reviewed. Furthermore, the engineering of lignin bulk structures into nanomaterials with various morphologies for advanced material applications is covered. Certain lignin structures exhibit antibacterial and antifungal properties, thus fueling the potential employment of lignin in bioactive materials. The most significant advances in the valorization of lignin including lignin-incorporating sunscreen formulations, shape-memory applications, carbon fibers, additive manufacturing, pickering emulsion stabilizers, self-healing elastomers, etc. are critically reviewed. Lastly, perspectives for future development and opportunities for the effective valorization of lignin for material applications are discussed.