A sustainable vitrimer from depolymerized lignin: Synthesis and reworkable application

This study reports a sustainable vitrimer synthesized from epoxidized and depolymerized lignosulfonate (EpddLS) via a fully solvent-free, water-based process under ambient conditions. Lignosulfonate is demethylated under mild acidic conditions and further depolymerized and functionalized with epoxide groups to enhance its reactivity. The resulting EpddLS is blended with DGEBA and cured with methyl nadic anhydride (MNA) in the presence of zinc stearate to introduce dynamic covalent transesterification bonds. The vitrimer exhibits rapid stress relaxation (τ ≈ 2 s at 160 °C) with an activation energy (Ea) of 45-55 kJ/mol and a crosslinking density (νₑ ≈ 5.0-9.1 × 10³ mol m^-3), reflecting efficient topological rearrangement and a robust, densely crosslinked network. The optimal formulation (EpddLS-80 wt%) achieves a high glass transition temperature (∼100 °C), excellent mechanical integrity, and outstanding thermal stability. Dynamic bond exchange enables thermal reprocessing, reworkability, and self-healing while maintaining structural performance. Moreover, the aromatic and sulfur-rich nature of lignosulfonate imparts intrinsic, additive-free flame retardancy (UL-94 V-0) through condensed-phase char formation and melt-flow suppression. This work establishes a direct link between lignin chemistry, crosslink density, and dynamic covalent behavior, enabling circular, flame-resistant, and repairable bio-based thermosets for sustainable structural and encapsulation applications.