## 3 Results
### Composite Foams Obtained through Almost Free Expansion Process  
#### 3.1.1 Morphology and Mechanical Properties  
The macroporous morphology of the composite foams at varying zeolite content is reported in Figure 2. All specimens evidenced a micro-structure without macroscopic defects indicating a suitable foaming process. A quite homogeneous cell geometry and distribution can be also highlighted. Although, some differences among foams, at varying zeolite content can be observed. In particular, at low zeolite content (foam Z20), the foam morphology is characterized by well-interconnected bubbles, suggesting the presence of micro channels among the bubbles. In fact, these foams are characterized by an open/close cell structure. The bubble walls are structurally compact, thick and defect-free. This suggests that mechanically unstable regions in the foams were not recognized. During the foaming process, the expansion of the siloxane matrix takes place because of hydrogen gas evolution induced by dehydrogenative coupling reaction. This leads to an expansion of the structure with progressively larger macropores. This effect is more evident as higher is the zeolite content in the composite foam. At increasing filler content, the viscosity of slurry significantly increases, thus hindering the hydrogen induced bubbling growth. As a consequence, foams with high zeolite content (foam Z60) showed numerous small bubbles homogeneously distributed. Some small and local macropores were occasionally evidenced. In order to better relate composite foam morphology and its mechanical performances, Table 2 summarizes apparent density (calculated as weight to volume ratio) and elastic properties for all composite zeolite foams. As reference, the not filled foam was also added. This information, can provide a direct practical information on the use and handling conditions of the material. Due to the zeolite filler addition, a gradual increase of elastic modulus and compression stress can be observed. In particular, the Foam Z60 showed an elastic modulus of 1.552 MPa, about two orders of magnitude higher than the unfilled one (Foam Z0, 0.031 MPa). As a consequence of the soft behavior of the microporous matrix, all composite foams exhibited a quite low compression stress. Although, as observed for the elastic modulus, a progressive increase at increasing the zeolite amount, was observed. However, although an increase in compressive stress takes place, the modulus increase limits the foam compressibility, representing a warning to pay attention during practical use and handling of the material. This behavior can be ascribed to the foam microstructure. At increasing amount of adsorbent filler in the foam, apparent density of the foam increases due to a densification effect of the composite material induced by the filler addition. The foaming ratio is gradually reduced as evidenced by comparing the density of Foam Z60 that is three times higher than unfilled one (foam Z0). In particular, based on calculated apparent density, it is possible to identify the foam void content (Vc), defined as  
\[\text{Vc}=\ 1-\text{ Apparent Density}/\text{Bulk Density} \tag{3}\]  
where the bulk density can be calculated through the rule of the mixture of the composite constituents. The void content decreases at increasing filler content. In particular for Z0 and Z60 batches a Vc of 72.2% and 47.8% was obtained. This confirms the limited foaming for high zeolite content composite specimens. The presence of the filler partially hinders the bubble growth during the foaming stage, mainly limiting the coalescence phenomena of the bubbles. In fact, the bubble dimension in the foams with higher amount of zeolite filler are characterized by quite lower average diameter size. The standard deviation is also reduced as confirmation of the less dispersed size distribution. However,