An improved process for waterproofing ceramic materials — especially lightweight ceramic insulation materials in both rigid (tile) and flexible (blanket) form — has been invented. In this process, traditional waterproofing agents are used, but before the waterproofing agents are applied, the ceramic substrates are first coated with thin layers of silica, as explained below.
As used here, "waterproofing" of ceramics means rendering them water-repellent, not making them impervious. In situations in which ceramics are exposed to water, the need for waterproofing arises as follows: These materials are very porous (void volumes exceed 90 percent in some cases). They are also hygroscopic — typically capable of absorbing three to five times their own weight of water. Water can thus add undesirably to the weight of these materials. These materials can also be damaged by explosive vaporization of trapped water during rapid heating, and by the anomalous expansion and contraction of trapped water during freezing and thawing. Thus, what is needed is a means to reduce the intrusion of water to minimize these deleterious effects.
The traditional waterproofing agents preferred for this purpose are silanes, substituted silanes, silazanes, and mixtures of these. These agents can be applied easily to silica ceramics, but cannot be applied easily to other (e.g., alumina) ceramics. The present invention is based on the discovery that even a difficult-to-coat ceramic can easily be coated with a traditional silicon-containing waterproofing agent, provided that it is first coated with a thin layer of silica. "Coated" as used here does not mean covered on the exterior macroscopic surface; it means covered on all microscopic exterior and interior pore surfaces and interstices.
The silica coating is formed by first coating with a silica precursor and converting the precursor to silica either in situ during the coating process or subsequently by oxidative pyrolysis. Some of the traditional waterproofing agents mentioned above are also useful as silica precursors; other silica precursors include silicones and siloxanes. A silica precursor can be applied to a ceramic substrate by any of a number of techniques; for example, solution coating, vapor deposition, low-pressure chemical deposition, immersion, or injection. The silica coating is typically thin enough (thickness of the order of 1 —¼m or less) to be flexible, so that it can be used on either a rigid or a flexible ceramic substrate without risk of cracking and consequent exposure of the ceramic to moisture.
In an experiment to demonstrate the invention, a piece of ceramic insulation containing alumina batting was coated by contact with methyltrimethoxysilane vapor (the silica precursor), then further coated with dimethylethoxysilane waterproofing agent. The piece was then heated in air at 1,000 °C for two hours, destroying the waterproofing and pyrolyzing the silica precursor to silica. The waterproofing agent was then reapplied. The waterproofed piece was found to absorb less than 5 percent of its weight in water — an acceptably small amount of water in the application for which this particular piece of insulation was designed.
This work was done by Domenick A. Cagliostro and Ming-ta S. Hsu of Ames Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tspunder the Materials category.
This invention has been patented by NASA (U.S. Patent No. 5,814,397). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel,
Ames Research Center;
Refer to ARC-14029.