Feedstock-Dependent Properties of Degradable Thermosets Derived from Kraft Lignins and Epoxidized Linseed Oil

Abstract Brittleness and poor network control limit lignin-based thermosets. Highly bio-based thermosets (∼90% bio-based content) were prepared from oxidized spruce and eucalyptus kraft lignins and epoxidized linseed oil (ELO) via acid−epoxide curing, and poly(ethylene glycol) (PEG) was incorporated to improve toughness, yielding homogeneous networks with high gel content (>88%). Both thermosets showed comparable thermal stability (tan δ peak 93−109 °C; degradation onset above 250 °C), while lignin origin caused subtle differences in crosslinking efficiency, wettability, and thermomechanical behavior. WAXS revealed sandwich (89−91%) and T-shaped π−π stacking in both systems. Under alkaline conditions, rapid ester hydrolysis dissolved the network within 48 h, whereas minor mass loss occurred under near-neutral conditions. Spectroscopy indicated cleavage of the ester crosslinks, with the aromatic lignin framework largely intact, and decreased T-shaped stacking in spruce-derived thermosets. These results show how lignin origin and network design can tune material properties and end-of-life behavior.