Hierarchically structured glass monoliths based on polyurethane foams as template

Hierarchically porous glass foams were prepared via a combination of a replication technique and a phase separation of sodium borosilicate glasses. Open-pore polyurethane foams were impregnated with a slurry containing sodium borosilicate glass powder, binders, solvents and stabilizers. The composite was calcinated and sintered whereupon the organic polymer was decomposed and the monolith was compacted. A phase separation was initiated by an additional thermal treatment. The sodium-rich borate phase in the phase separated glass was removed with hydrochloric acid. Finally, the secondary silica species within the pores generated by the acid treatment were removed with sodium hydroxide solution. The monoliths were characterized by electron microscopy, nitrogen sorption, μ-CT and mercury porosimetry. Pore diameters – obtained from the template structure – were achieved in a range of 0.4–1.0 mm. The following phase separation and the coupled acid-alkaline leaching lead to an additional pore system within the glass framework.