Polymeric aerogels

Relative to base-catalyzed gels, those eatalyzed with acid have a larger number of small pores, and it is difficult to dry them without significant pore collapse. However, a two-step process in which the precursors are initially hydrolyzed with acid, followed by gelation under basic conditions, does lead to robust polymeric aerogels. In fact, this process yields the lowest density aerogels on record. 

Perrut M, Francais E (1999) Process and equipment for drying polymeric aerogel in the presence of a supercritical fluid. US patent 5 962-539. 

Polymeric aerogels were developed as binders for foundry sands in the last decade to solve certain problems, which are typical for classical organic binders. In the last few years, organic and inorganic aerogels were developed in granular form as additive to foundry sands to improve certain properties such as metal penetration, fineness, and collapsibility. 

Polymeric aerogel based on resorcinol and formaldehyde was first tried by Briick and Ratke [4], to see if they can be used to bind foundry sands. They used the fact that Resorcinol-Formaldehyde (RF) aerogel can be gelled and dried under ambient conditions. 

Polymeric aerogels based on RF were developed in the last years for application in foundry practice as a new nanostructured binder for foundry sands. Their properties allow, for instance, to replace conventional polymeric binders, since they solve certain problems of core removal, penetration, surface reactions, blow holes, and other casting defects especially in light metal casting. RF aerogels as polymeric binders are at the verge of being used by industry. 

MUow B, Ratke L (2010) BTX free decomposition of polymeric aerogel binders, hit J Foundry Res, submitted... 

Carbon aerogel Aerogel obtained by pyrolysis of an organic (polymeric) aerogel under protective atmosphere... 

The first phase in the process is the formation of the sol . A sol is a colloidal suspension of solid particles in a liquid. Colloids are solid particles with diameters of 1-100 nm. After a certain period, the colloidal particles and condensed silica species link to form a gel - an interconnected, rigid network with pores of submicrometer dimensions and polymeric chains whose average length is greater than one micrometer. After the sol-gel transition, the solvent phase is removed from the interconnected pore network. If removed by conventional drying such as evaporation, so-called xerogels are obtained, if removed via supercritical evacuation, the product is an aerogel . 

The results obtained clearly demonstrate that sulfate ions promote the consolidation of titania morphology in nanometer scales and the formation of a crystalline, anatase phase in aerogels dried using supercritical carbon dioxide. This trend is consistently demonstrated by adsorption experiments as well as SAXS and XRD studies. The presence of platinum promotes the formation of a fine polymeric structure of titania in nanometric scales. After calcination all samples exhibit a similar morphology, yet with a notable difference in texture parameters. 

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