Influence of ultrasonic-assisted supercritical carbon dioxide foaming process on microcellular thermosetting epoxy foams: Morphology and thermal-mechanical properties

Abstract

The preparation of microcellular thermosetting epoxy foam with good cell morphology and uniform cell distribution is still a big challenge because of its highly cross-linking chemical networks, Herein, introducing ultrasound into the traditional supercritical carbon dioxide foaming process was reported to improve the foamability and thermal-mechanical properties of thermosetting epoxy resin. The influence of ultrasonic conditions on the cell morphology was investigated firstly, which indicated that higher ultrasonic power and controllable duty cycle resulted in outstanding foamability, with the average cell size decreasing to about 200 nm and cell density increasing to the level of 1014. The foaming mechanism with outstanding foamability of thermosetting epoxy is strongly related to the ultrasonic cavitation effect, which is positive to more uniform cell nucleating sites and then more small cells. Furthermore, the thermos-mechanical properties of foamed epoxy were investigated. The glass transition temperature and the storage modulus of the fabricated epoxy foams determined presents increasing tendency with the introduced higher ultrasonic power and duty cycle, which partially related to the high absorption of carbon dioxide by the mechanical effect of ultrasound and the enhanced thermal effect of ultrasonic field. Smaller cell size and higher cell density are also positive to the higher dynamic mechanical properties. This work provides a new strategy with the introduced ultrasound into the supercritical microcellular foaming for optimum the related cell morphology and also thermo-mechanical properties of polymeric foams.