Auxetic epoxy foams produced by solid state foaming

Auxetic epoxy resin foams were produced by solid-state foaming thanks to the use of properly shaped precursors. In fact, a re-entrant hexagonal shape of the precursors is preserved during foaming and results in a foam with a complex structure: a thin macro-structure with the re-entrant geometry filled with foam. The auxetic behavior was observed by using tensile tests at different temperatures (room temperature, 80?, and 100?). Indentation tests were also carried out to evaluate the gradient properties across the lines of the thin re-entrant macro-structure. In order to show that the auxetic behavior depended on the internal macro-structure, tests were also performed on foam panels obtained by cylindrical tablets and, therefore, with a standard-hexagonal macro-structure. In conclusion, the auxetic behavior was observed only for the foam panels with re-entrant hexagonal structure at 80?. In this case, a negative Poisson's ratio is immediately achieved at small strains and tends to a zero plateau value for longitudinal strains up to 1%.