Preparation and mechanical properties of thermosetting epoxy foams based on epoxy/ 2-ethyl-4-methylimidazol system with different curing agent contents

Epoxy/2-ethyl-4-methylimidazol system with different curing agent content was completely cured for foaming, and the effect of a systematic variation in 2-ethyl-4-methylimidazol content on the crosslinking density of cured epoxy resins was investigated. It was found that the crosslinking density of completed cured epoxy reduced as the 2-ethyl-4-methylimidazol content increased in certain range of contents (10–50 mol%). Then the precursors were foamed by a batch foaming process with supercritical carbon dioxide. The cellular morphologies of foamed epoxy resins were analyzed by scanning electron microscopy. The results revealed that the reduced crosslinking density would improve the foamability of cured epoxy resin. The microcellular epoxy foams could be obtained by maintaining a moderate crosslinking density, which can be controlled by varying 2-ethyl-4-methylimidazol content. For the completely cured epoxy with different curing agent content, when the crosslinking density of epoxy resin was 232.40 mol m^–3 (the 2-ethyl-4-methylimidazol content was 35 mol%) or lower, microcellular structure was obtained by adjusting the foaming conditions. The effects of foaming on the mechanical properties were also discussed. The results indicated that microcellular epoxy foams had higher impact strength but lower tensile strength and tensile modulus, validating that the introduction of microcellular structure in epoxy matrix was conducive to the improvement of the ductility of epoxy foams.