Improving toughness of epoxy resin by in‐situ formed secondary network during tertiary amine initiated curing

Abstract The mechanical properties of epoxy resin cured by tertiary amine initiated ring‐open reaction were investigated. Special curing procedures that first at 130°C for different times (facilitating chain transfer/reducing crosslinking density) and then 80°C/2 h–130°C/2 h were used. For cured specimens, the glass transition temperature showed a continuous decrease with increasing time at the first step of curing ( t 1st step ), and the swelling ratio also increased. These changes pointed to a monotonic decrease in crosslinking density. Mechanical properties were characterized by three‐point bending and tensile test. Both the deflection and strain at break showed a sudden increase when t 1st step reached 30 min, corresponding well to occurrence of gelation at 130°C. The fracture surface after tensile test was further examined. Rough surface with rupture of fibrillar structure was found when t 1st step reached 30 min. These observations all suggest an abrupt increase of toughness related to gelation ( t 1st step = 30 min). It is explained by formation of network at the first step of curing, which has lower crosslinking density and serves as a secondary toughening network.