Succinic Acid as a Sustainable Curing Agent for High-Performance, Rapidly Reprocessable Epoxy Vitrimers

Abstract

This work explores the use of succinic acid (SA) for developing a series of high-performance vitrimers with fast network rearrangement starting from a diepoxy oligomer (DGEBA). Succinic, being a short-chain biobased acid, permitted the formation of tailored epoxy vitrimers with desired properties, while its stability and crystalline nature provided curing latency. By tailoring the molar ratios of the reactive end-groups, catalyst type [Zn(acac)₂ or Sn(oct)₂] and loading, cross-link densities, reactive moieties, and mechanical and viscoelastic properties were readily manipulated. The resulting vitrimers exhibited a densely cross-linked network (T_g ∼ 94–133 °C, E ∼ 2.3–3 GPa, σ_max ∼ 61–68 MPa) combined with a small statistical distance between hydroxyl-ester moieties, therefore fast reprocessing/relaxation (e.g., 700 s at 160 °C), while being thermally stable (T_2% ∼ 330 °C, T_d ∼ 400 °C). All prepared vitrimers were easily reprocessable, achieving a retention of E even after three reprocessing cycles and 75 to 90% recovery of tensile strength (σ_max). Excess epoxy allowed for homopolymerization reactions, yielding a number of nonexchangeable bonds and fewer hydroxyl moieties, which increased brittleness while suppressing creep and relaxation abilities. For the 1:0.75 vitrimer, which provided the best balance between high cross-link density and rapid relaxation, the effect of Sn(oct)₂ loading (0, 1, 2.5, and 5% mol) was further explored. The present study offers a fine-tuning of vitrimer properties without altering any of the formulations and/or manufacturing processes associated with typical thermosets’ production at an industrial level.