Vitrification-Controlled Curing of Epoxidized Linseed Oil Resins: Kinetics, TTT Analysis, and Thermomechanical Performance with Anhydride and Amine Curing Agents

Epoxidized linseed oil (ELO) is a promising biobased alternative to petrochemical epoxy precursors, with a curing behavior strongly influenced by the chemistry of the curing agent and the resulting network architecture. In this study, ELO was synthesized via the Shi epoxidation approach and subsequently cured with either methyltetrahydrophthalic anhydride (ELO-AD) or a fatty amine agent (ELO-AM). Although epoxy–amine reactions exhibit lower activation energies (19–26 kJ·mol–1) than the epoxy–anhydride system (40–45 kJ·mol–1), the overall curing efficiency is governed by vitrification and diffusion constraints rather than intrinsic chemical reactivity. The anhydride-cured system forms a dense network with higher glass transition temperature (Tg = 46–94 °C), higher cross-link density (υ = 1.19 × 103 mol·m–3), and gel content (97.95 ± 0.16%) together with rapid viscosity growth (kη up to 0.83 min–1 at 140 °C), resulting in shorter characteristic times in time–temperature–transformation (TTT) diagrams. The amine-cured system forms a more flexible, lower Tg (−10 to 19 °C) network with lower υ (4.77 × 102 mol·m–3), and gel content (79.87 ± 4.45%), consistent with a less interconnected netword and greater diffusion limitations at advanced conversion. These structural and kinetic differences translate directly into macroscopic performance. ELO-AD exhibits superior Young’s modulus and strength (2.0 GPa and ≈ 46 MPa, respectively), whereas ELO-AM shows enhanced ductility (strain at break ≈ 26%) and higher thermal stability (DTG peak ≈ 485 °C). Overall, this study establishes quantitative structure–process–property relationships for ELO-based epoxy systems, with practical guidelines for selecting curing agents and processing windows for sustainable thermoset resins.