Journey to Polymeric Materials Composed Exclusively of Simple Lignin Derivatives

Between 1955 and 1960, theories about lignin configuration were vacillating between random-coil and cross-linked "microgel" representations for macromolecular lignin chains. Light scattering was important in these early studies, but it was difficult to deal adequately with lignin fluorescence at the 546 nm incident wavelength being used. Cross-linking then prevailed, largely because of the hydrodynamic compactness of high molecular weight lignin species. The conceptual ramifications of this paradigm led to 40 wt % incorporation limits (or less) for lignins in cohesive polymeric materials. In due course, however, further evidence for a random-coil description of individual lignin components materialized; it became less obvious why simple lignin derivatives could not, on their own, form promising polymeric materials. The first plastics composed solely of a (native) ball-milled softwood lignin are similar to polyethylene in tensile behavior. Blending with just 5 wt % tetrabromobisphenol A (a flame retardant) results in a material that surpasses polystyrene decisively. Prior methylation of the ball-milled lignin produces markedly better results, with and without small quantities of blend components. Even quite challenging lignin derivatives like the (polyanionic) sulfonates perform auspiciously in formulations with particular aliphatic polyesters. The macromolecular species in polymeric materials with very high lignin-derivative contents are associated lignin complexes rather than individual lignin macromolecules. The interactions of blend components with these complexes are instrumental in determining the mechanical properties of contemporary lignin-based plastics.