Fast relaxing sustainable soft vitrimer with enhanced recyclability

Vitrimers have been introduced to circumvent the lack of recyclability of traditional thermosets with permanent cross-linked structures, while preserving the advantages of structural stability and mechanical properties. The success of this lies in the successful incorporation of a robust networked structure to achieve reversible extensibility and toughness while preserving processability akin to thermoplastics. In this study, we report the synthesis of vitrimers utilizing 100% renewable and plant-based building blocks that exhibit transesterification exchange reaction (TER). The vitrimer was synthesized by solvent-free, high-shear reactive mixing of a biomass-derived lignin fraction enriched with carboxyl functionality and an epoxidized polyisoprene from natural rubber. The oxirane functionality in rubber reacts catalytically (zinc acetylacetonate) with carboxyl moieties in lignin to form esters at 180 °C. The ester linkages in the networked matrix undergo topological rearrangement upon heating above 180 °C, thus enabling (re)processability similar to thermoplastics. The material exhibits fast stress relaxation (characteristic relaxation time of <10 seconds) above 200 °C, which indicates the material's potential for use in rapid manufacturing of components and their recycling. This approach provides a pathway for circular and value-added utilization of lignin and subsequent use as a matrix for reinforced composites.