Wollastonite-diopside glass-ceramic foams from supercritical carbon dioxide-assisted extrusion of a silicone resin and inorganic fillers

Biphasic ceramics based on wollastonite (CaSiO3) and diopside (CaMgSi2O6) have been recently proposed as valid biomaterials. Silicone resins, filled with inorganic reactive powders, have a great potential in both synthesis of the desired phases and in the shaping in form of highly porous foams. In particular, glass-ceramic foams have been already obtained from a liquid silicone, filled with CaO and MgO precursors and with hydrated salts, such as sodium borate and phosphate. The foaming is due to the release of water vapour within the polymer (at 300-350 °C), before its conversion into amorphous silica, and reaction with Ca and Mg oxide, by heat treatment (above 1000 °C). In this paper we discuss a significant extension of the approach, referring to a well-known commercial solid silicone. In order to prevent the cross-linking of the silicone (that would compromise the foaming), the polymer and the fillers were mixed by means of conventional polymer extrusion assisted by supercritical carbon dioxide. Finely powdered extrudates could be easily foamed, at 350 °C, due to the release of water vapour and CO2. Owing to the uniform cellular structure (with a total porosity of about 80%) and the absence of cracks, the final products, after firing at 1100 °C, showed an excellent crushing strength (in the order of 10 MPa), far exceeding that of previously developed foams.