Valorisation of waste to yield recyclable composites of elemental sulfur and lignin†
Lignin is the second-most abundant biopolymer in nature and remains a severely underutilized waste product of agriculture and paper production. Sulfur is the most underutilized byproduct of petroleum and natural gas processing industries. On their own, both sulfur and lignin exhibit very poor mechanical properties. In the current work, a strategy for preparing more durable composites of sulfur and lignin, LSx, is described. Composites LSx were prepared by reaction of allyl lignin with elemental sulfur, whereby some of the sulfur forms polysulfide crosslinks with lignin to yield a three-dimensional network. Even relatively small quantities (<5 wt%) of the polysulfide-crosslinked lignin network provides up to a 3.4-fold increase in mechanical reinforcement over sulfur alone, as measured by the storage moduli and flexural strength determined from dynamic mechanical analysis (temperature dependence and stress–strain analysis). Notably, LSx composites could be repeatedly remelted and recast after pulverization without loss of mechanical strength. These initial studies suggest potential practical applications of lignin and sulfur waste streams in the ongoing quest towards more sustainable, recyclable structural materials.