Lignin

Lignin is the second most abundant sustainable polymer, and the most abundant sustainable source of aromatic polymers. Technical lignin is produced in forms featuring varying properties, and has several advantages: good mechanical properties, CO₂ neutrality, thermal stability, chemical resistance, flame retardancy, and antimicrobial and antioxidant properties. Chemistry, types, properties, fabrication routes of technical grades, market data, and applications of lignin are illustrated. Lignin is contained in a few commercial filaments for additive manufacturing (AM), and also studied as a filler in polymers for AM, aiming at reducing the amount of polymer and hence the material cost, possibly enhancing and customizing the feedstock’s performance, and replacing non-sustainable fillers such as glass and carbon and, in that case, forming a fully sustainable composite material. In most technical literature reviewed, lignin did not generate a composite superior to its matrix in mechanical performance. Two groups of studies on lignin are here reported. In one group, lignin was blended with various polymers that are also offered as AM grades, such as acrylonitrile butadiene styrene (ABS), high density polyethylene, polypropylene, polyethylene terephthalate, polystyrene (PS), polyurethane, and the conclusions of those studies may be insightful for AM feedstocks. In the other group of technical papers lignin was specifically evaluated as AM feedstocks in combination with ABS, acrylates, PS, carbon fibers, and rubber, and the sustainable polymers polyhydroxybutanoate, and polylactic acid. Other technical aspects addressed for lignin-polymer composites by R&D are improvement of interfacial adhesion using chemical modification and compatibilizers, and enhancement of mechanical performance by adding plasticizers. Being lignin available as powder and fiber, it has been mostly leveraged for two families of AM processes, vat photopolymerization, and extrusion. Lignin is a versatile ingredient for AM and can drive the use of AM, f.e. enabling multifunctional parts, but its efficiency in strengthening and stiffening polymer matrices must be improved through a scientific understanding of the aspects involved.