Silanization is performed in a single treatment at moderate temperature and pressure.
A relatively simple silanization process has been developed for waterproofing or rewaterproofing aerogels, xerogels, and aerogel/tile composites, and other, similar low-density, highly microporous materials. Such materials are potentially attractive for a variety of applications — especially for thermal-insulation panels that are required to be thin and lightweight. Unfortunately, such materials are also hydrophilic and tend to collapse after adsorbing water from the air. Hence, an effective means of waterproofing is necessary to enable practical exploitation of aerogels and the like.
Older processes for waterproofing aerogels are time-consuming, labor-intensive, and expensive, relative to the present process. Each of the older processes includes a number of different chemical-treatment steps, and some include the use of toxic halogenated surface-modifying compounds, pressures as high as hundreds of atmospheres, and/or temperatures as high as 1,000 °C.
In contrast, the present silanization process involves a single step, pressure and temperature near ambient values, and no use of toxic halogenated compounds. In this process, an aerogel object that has been dried and otherwise fully formed is exposed to the vapor(s) of one or more silicon- containing organic compound(s) at effective concentration(s) in a closed chamber at controlled temperature and pressure. Suitable treatment compounds include silazanes, alkoxysilanes, and silanes, which react with polar active sites in the aerogel. The temperature need not be particularly high, as long as it is sufficient to sustain vaporization of the treatment compound(s) at the chamber pressure, which typically lies in the range between 76 mm of Hg [0.1 atmosphere (≈10 kPa)] and 1.5 m of Hg [≈2 atmospheres (≈0.2 MPa)].
In one of several experiments, some samples of carbon aerogels, xerogels, and a carbon-aerogel/carbon-xerogel composite were placed in a plastic bag along with an open vial of methyldiethoxysilane (MDES) and the bag was then sealed. Other samples of the same materials were left untreated. After 24 hours in the bag, the treated samples were shown to have been waterproofed in that they exhibited low degrees of water pickup, whereas the untreated samples showed much higher degrees of water pickup (see table).
This work was done by Ming-Ta S. Hsu, Timothy S. Chen, Susan White, and Daniel J. Rasky of Ames Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Ames Research Center, (650) 604-5104. Refer to ARC-14254.