An idea that emerged 60 years ago has stalled attempts to convert underivatized co-product lignins into functional thermoplastics. Lignin macromolecules seem to be hydrodynamically compact, and so it was assumed that they must be cross-linked and therefore difficult to deform. If so, ductility in thermoplastics with a high proportion of hard lignin-structural segments could only be achieved with linkages through soft segments possessing a low glass-transition temperature. Recently, a paradigm shift has materialized from the creation of thermoplastics based entirely on methylated ball-milled softwood lignins that surpass polystyrene in tensile behavior. Moreover, plastics containing unmethylated ball-milled softwood lignin alone exhibit tensile properties similar to those of polyethylene. Even materials composed solely of (polyanionic) unmethylated softwood ligninsulfonates are just 1.6-fold weaker in tensile strength than the corresponding ball-milled lignin-based plastics. The constituent species in these new materials are associated complexes that intermesh through their peripheral components to establish continuity between neighboring macromolecular domains. It has long been recognized that individual softwood kraft lignin components interact noncovalently in a well-defined way to create complexes that associate further with one another to form very high molecular weight entities. From this point of view, industrial softwood kraft lignins should be promising candidates as starting materials for producing functional lignin-based plastics. Indeed, unmethylated Indulin AT alone is capable of forming polymeric materials that are comparable in tensile strength to lignin–butadiene polyurethane thermoplastics.

Prospects for high levels of industrial Kraft lignin (Indulin AT) in functional polymeric materials

255th National Meeting (2018)

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