Liquid waterproofing agents based on polyorganosiloxanes have been invented for use in treating porous, lightweight, fibrous ceramic thermal-insulation materials in both tile (rigid) and blanket (flexible) forms. Whereas silane-based waterproofing materials developed previously for this purpose are toxic and volatile and must be applied in tedious procedures (involving repeated injection at multiple locations by use of syringes), the present formulations are nontoxic and nonvolatile and can be applied by ordinary coating procedures.
Waterproofing of lightweight, fibrous ceramic thermal-insulation materials is needed for the following reasons: These materials are often hygroscopic. Because of its porosity and hygroscopicity, such a material can absorb as much as five times its own weight in water. In addition to adding unacceptably to the weight of the insulation, absorbed water could give rise to freeze/thaw damage or to damage from explosive vaporization upon sudden exposure to very high temperature.
A waterproofing agent of the present type is formulated as an aqueous solution of di- and tri-alkoxyfunctional organosilanes, the molecules of which contain hydrocarbyl groups of between 1 and 10 carbon atoms. The functionality resides in alkoxide groups attached to the silicon atoms. Before application to a ceramic thermal-insulation material, the solution is allowed to cure partially at room temperature by the hydrolysis and partial condensation of the organosilanes with the water, yielding low-molecular-weight polyorganosiloxanes with unreacted silanol groups. Also formed in the condensation reaction are small amounts of alcohols, which become dissolved in the water. If desired, alcohol can be added to the solution to facilitate drying; however, it has been found that the addition of water alone yields satisfactory results. Thus, unlike previously developed waterproofing materials, which generally contain such toxic, volatile solvents as toluene, xylene, naphtha, and/or lacquer thinner, this waterproofing agent is relatively nontoxic and nonvolatile.
The resulting solution can be applied to the ceramic insulation that one seeks to render waterproof by any suitable conventional coating technique — for example, spraying, brushing, rolling, or flowing. The solution penetrates the ceramic substrate to some depth by capillary action in the pores. Thus, what is formed is not a continuous coat on the exterior surface that would seal against penetration of all liquids and gases; instead, the interior surfaces of the pores become coated with a thin layer of waterproofing material that adds little to the overall weight of the ceramic. The waterproofing treatment is completed by mild heating (typically to no more than about 100 °C) to drive the condensation reaction to completion and to dry the coated ceramic substrate. Heating can be effected by use of a heat gun, heat lamp or any other convenient means.
This work was done by Daniel B. Leiser, Domenick E. Cagliostro, Ming-ta S. Hsu, and Timothy S. Chen of Ames Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.
This invention has been patented by NASA (U.S. Patent No. 5,766,322). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel,
Ames Research Center,
Refer to ARC-14068.