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Search Completed | Title | INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (19) World Intellectual Property
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Text | INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (19) World Intellectual Property | 002
Process for producing hydrogels
The invention relates to a process for producing hydrogels based on a soluble salt of an acidic or amphoteric oxygen-containing molecular anion. Additionally disclosed is the use of the hydrogels for production of aerogels.
Aerogels are high-porosity solids in which up to 99.98% of the volume consists of pores. Aerogels can be produced on the basis of various materials, silica aerogels being the most well-known. However, they can also be formed from other acidic or amphoteric oxygen-containing molecular anions, for example titanates or aluminates. Aerogels can be obtained in this case especially via a sol-gel process to form a hydrogel, and subsequent drying. The internal structure of aerogels consists of a three- dimensional structure of primary particles which fuse to one another in a disordered manner during the sol-gel synthesis. The cavities present between the particles form the pores.
It is known that hydrogels, especially silica hydrogels, which can be produced by acidifying waterglass, can be dried under supercritical conditions to form microporous, three-dimensionally crosslinked products. Such a product obtained by supercritical drying, in the case of gels, is called aerogel. The supercritical drying completely or substantially eliminates the interfacial tension of the fluid present in the microporous, three-dimensionally crosslinked gel. The aim here is to substantially avoid shrinkage of the microporous, three-dimensionally crosslinked gel in the course of drying, since characteristic properties of the microporous, three-dimensionally crosslinked gels are entirely or partly lost in the course of shrinkage. Unlike the case of conventional drying with no particular provisions, in which the gels suffer a great contraction in volume and form xerogels, drying close to the critical point thus results only in a small contraction in volume (less than 15% by volume).
The prior art for production of aerogels by means of supercritical drying is described, for example, in detail in Reviews in Chemical Engineering, Volume 5, No. 1-4, p. 157-
198 (1988), in which the pioneering studies by Kistler are also mentioned.
WO-A-95 06 617 relates to hydrophobic silica aerogels which are obtainable by reacting a waterglass solution with an acid at a pH of 7.5 to 11, substantially removing ionic constituents from the hydrogel formed by washing with water or dilute aqueous solutions of inorganic bases while maintaining the pH of the hydrogel within the range from 7.5 to 11, displacing the aqueous phase present in the hydrogel by means of an alcohol and then supercritically drying the resulting alcogel.
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