The analysis of the mechanical properties in compression of the starch based foams produced show how incorporating natural reinforcements to the starch-based polymer matrix it is possible to increase the strength of the pure foams, especially in the case of adding grape waste. From the evaluation of the cellular structure grape particles were the only natural reinforcement which did not considerably change the cellular structure at least in terms of cell size. On the contrary, barley straw fibers and cardoon waste produced a decrease of the cell size. Nevertheless, all the foams produced presented in general highly inhomogeneous cellular structures, with and without fibers, which results in in a wide distribution of cell sizes (NSD values higher than 0.5).
The energy absorbed per unit volume of the foam reinforced with barley straw fibers is higher than those of the other foams. This could be due to the fact that the toughness of the starch polymer matrix was increased by the addition of these fibers.
From the comparison of the mechanical properties of the solid and those of the correspondent foams it is appreciated how the compressive modulus of the foams produced is higher than the compressive modulus of the solid. This is due to the loss of water during the foaming process. The initial solid precursor is a rubbery polymer because of the high amount of water that plasticize the polymer matrix while the final foam is composed of rigid glassy cell walls and struts. A modellization based on the cubic cell model proposed by Gibson and Ashby (1997) allowed estimating the compressive modulus of the cell walls and struts by assuming that the exponent n was equal to 2. The estimated compressive modulus of the cell walls and struts of the foams is approximately 50 times higher than those of the used-solid precursors.