# Priming Ceramic Materials for Better Waterproofing 

**Precoating with silica facilitates the application of a waterproofing
final coat.**

***Ames Research Center, Moffett Field, California***

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](http://www.nasatech.com/)** *under the [**Materials
category.**](http://www.nasatech.com/TSP/mat.html)*

*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; (650) 604-5104. Refer to ARC-14029.*

*Tech Briefs article published April 2000.*