Polyorganosiloxanes for Coating Porous Ceramic Insulation

Ames Research Center, Moffett Field, California

Liquid and paste polyorganosiloxane formulations (which become high-temperature- and oxidation-resistant organosilicone-based ceramics upon curing) have been invented for use in (1) protecting porous, lightweight ceramic thermal insulation materials against aeroconvective thermal degradation and (2) repairing and bonding such materials. These formulations were originally intended especially for application to the fibrous refractory composite insulation (FRCI) tiles that protect parts of the space shuttles during re-entry into the terrestrial atmosphere; they may also be suitable for application to similar insulating tiles in laboratory and industrial furnaces.

A formulation of this type starts out as a mixture of (1) one or more liquid di- and tri-functional organosilanes; (2) one or more suitable fillers to enhance chemical, mechanical, and/or thermal properties of the uncured mixture and/or the organosilicone end product; and (3) water. The functionality of the organosilanes resides in alkoxy groups attached to the silicon atoms. Before application to a ceramic thermal-insulation surface that one seeks to protect or bond, the mixture is allowed to cure partially at room temperature by the hydrolysis and partial condensation of the organosilanes with the water, yielding liquid polyorganosiloxanes (incompletely polymerized organosilicones) with unreacted silanol groups. Also formed in the condensation reaction are alcohols, which become dissolved in the remaining water. The resulting mixture can be applied to the surface of the ceramic insulation by spraying, brushing, rolling, flowing, or other conventional technique.

After application, the mixture is allowed to cure at room temperature to become a soft solid coat. The mixture continues to cure at room temperature, eventually becoming a hard polyorganosiloxane coat. The continued curing occurs by condensation of the unreacted silanol groups. If desired, curing can be accelerated by heating. When exposed to still higher temperature (especially in an extreme oxidative and aeroconvective environment like that experienced by a spacecraft during re-entry), the coat becomes an oxidation-resistant and thermally stable protective ceramic on the underlying ceramic insulation.

A low-viscosity, watery formulation of this type, with or without one or more fillers, would ordinarily be applied to a porous ceramic tile and allowed to soak into a surface layer of pores so that, upon curing, it could form a protective surface layer within and on the ceramic substrate to prevent the entry of hot gases. A viscous liquid formulation containing larger amounts of fillers could be applied to the surface of the ceramic tile to form a hard, impermeable layer on the external surface of the ceramic. A highly viscous formulation in the form of a paste could be suitable as an adhesive and/or filler for fabrication or repair. For example, the paste could be used to fill small holes caused by chipping or to cement plugs in place to fill larger holes (see figure).

This work was done by Daniel B. Leiser, 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. 5,985,433). 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-14077.

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