Improving impact strength and fracture toughness of epoxy resin through oligoester—A byproduct derived from the unsaturated polyester resin manufacturing process

Abstract Epoxy resin, a pivotal polymer with extensive applications, is hampered by inherent inflexibility and brittleness, limiting its potential in scenarios demanding resilience against external forces. This study addresses this limitation by introducing an innovative modification through the integration of epoxy‐oligoester (EOS), derived as a byproduct of unsaturated polyester resin synthesis. EOS, with an epoxy group content of 7.5% and an acid value of 0.25 mg KOH/g, functions as an effective internal toughening agent for epoxy resin. At 15 wt%, EOS significantly augments impact strength (67.7%) and fracture toughness (92.1%), achieved by reducing network density from 0.792 × 10 −3 to 0.678 × 10 −3 mol cm −3 , resulting in lower storage modulus ( E ′) and glass transition temperature. Additionally, the addition of 15 wt% EOS retards delamination and amplifies delamination energy. Mode I critical strain energy release rate (GIC) increases by 51.2%, from 608.1 to 911.5 J/m 2 , and Mode II critical strain energy release rate (GIP) rises by 51.7%, from 664.0 to 1007.6 J/m 2 . These findings underscore the promise of EOS‐modified epoxy in industries necessitating materials with elevated fracture toughness and superior resistance to delamination, such as transportation, maritime, and automotive sectors.