Epoxy foams were produced by means of solid-state foaming and their shape memory properties were evaluated together with other physical properties. Solid-state foaming consists of pressing thermosetting resin powders to produce solid tablets, heating the tablets at high temperature to generate both the formation of pores inside the resin and the resin polymerization. A nanoclay was added to the resin powder before pressing it up to a maximum content of 5 wt%. Unfilled and composite foams were characterized by density measurements and thermal analyses. Subsequently, foam samples underwent up to two thermo-mechanical cycles: each cycle consisted of the storage of a compressed shape and the subsequent thermal recovery. Compression tests were used to measure the effect of the thermo-mechanical cycles on the foam's mechanical performances and compressive toughness was extracted from the tests. It was observed that all the foams exhibited good shape memory properties also after cycling: nanoclay filler allows the foams to completely recover the initial shape and to increase the compressive and the specific compressive toughness.

A new foaming process has been developed for epoxy resins. Uncured epoxy tablets are fabricated by pressing commercial powders in a steel mold at room temperature and used as foam precursors. The tablets foam when heated in a muffle at high temperature. No blowing agent was added because the foaming mechanism depends on the uncured resin boiling point. The foaming temperature is set to be high enough to rapidly produce the resin boiling but not excessive to avoid the thermal degradation. During boiling, the epoxy resin polymerizes and the bubbles freeze in the final structure. Epoxy foams are obtained by heating the compacted tablets in cylindrical copper molds, having internal diameter equal to the tablet diameter. Several process parameters have been changed in the experiment to understand their correlation with the foaming efficiency. However, the foaming ratio (expressed in terms of the ratio between the final and the initial tablet height) is found to be mainly dependent on the initial tablet density. In optimal conditions, the foaming ratio can rise up to 6. Thermal tests have been performed to evaluate the epoxy powder behavior during the cure, whereas mechanical compression tests were used to evaluate the final performances of the foams.

No abstract

A solid-state process was used to produce epoxy foams with different contents of nano-clay (up to 10%wt). The foam properties were studied by means of numerous tests: X-ray analysis, dynamic mechanical analysis, mechanical tests (compression, flexure, indentation, stress relaxation). The straightening effect of the nano-clay filler was investigated and related to the loading conditions and the intrinsic properties of the resin matrix. Small and large foam samples were produced by changing the number of solid precursors during foaming. In the end, shape recovery tests showed that composite foams exhibit remarkably shape memory properties, at least at low filler contents.