Foaming of flat glass cullet using Si3N4 and MnO2 powders

The introduction of a compound capable of releasing oxygen, such as MnO2, greatly improves the foaming ability of Si3N4 used as foaming agent in soda-lime glass powder, leading to expansion at a relatively low temperature (800–850 °C) and short processing time (7–30 min). The effect is based on the supply of oxygen, in addition to that in the furnace atmosphere. At the highest level of porosity, however, the strength of foams is negatively affected by a coarse microstructure, determined by cell coalescence. The reduction of firing temperature or, above all, the reduction of the processing time, was found to limit the coalescence and significantly improve the strength of the foams.