High temperature silicon carbide impregnated insulating fabrics

High temperature insulating articles are provided having improved performance characteristics. The articles comprise fabrics of closely woven refractory or heat resistant fibers having particles of silicon carbide dispersed at least partially through the fabric and bonded to the fibers with an emulsifiable polyethylene wax. Such articles exhibit significantly increased high temperature emittance characteristics and an improved retention of integrity and flexibility after prolonged exposure to high temperature.

TECHNICAL FIELD 
This invention relates to high temperature, flexible, insulating fabrics of 
closely woven heat resistant fibers. More particularly, this invention 
relates to such fabrics having particles of silicon carbide dispersed 
through the fabric and bonded to the fibers. The fabrics of this invention 
exhibit a significantly increased high temperature emittance and a 
retention of their desired flexibility and insulation characteristics on 
exposure to exceptionally high temperature conditions over extended 
periods of time. 
The exterior surfaces of aerospace vehicles are subjected to high 
temperatures at various times during a space mission. Extremely high 
temperatures are encountered when the vehicle reenters the earth's 
atmosphere at high speed on its return flight. To protect against the 
disastrous effects of such high temperatures, a portion or all of the 
exposed metallic surfaces of an aerospace vehicle are covered with an 
insulating material. In the case of the Space Shuttle, in order to prevent 
the skin temperature from exceeding 175.degree. C., approximately 30,000 
tiles were affixed to exterior metallic surfaces of the vehicle. Each tile 
was approximately 15 centimeters square and varied in thickness from about 
2.5 centimeters to about 10 centimeters. Each tile consisted of a mass of 
non-woven, finely drawn and exceptionally pure silica fibers bonded 
together by partial fusion to form a rigid porous structure. The exterior 
or exposed surface of each tile in a critical location, such as the 
underside of the Space Shuttle, was coated with a dark silica coating in 
order to impart high temperature emittance characteristics to the surface 
of the tile. The acceptable performance of a tile depends on both the 
composition of the tile and the high temperature emittance characteristics 
of the exposed surface. 
The metallic skin of the space shuttle undergoes lateral dimensional 
changes as it encounters or develops widely varying temperatures during 
the space flight. To allow for such dimensional changes, the tiles 
described above are spaced slightly apart from each other when affixed to 
the exterior metallic surface of the vehicle. Some of these resulting 
spaces or gaps between the tiles, if left unfilled, would result in 
exposing the small area of the metallic skin at the bottom of the gap 
directly to the high temperatures encountered during reentry. Even though 
the area of the metallic skin of the vehicle exposed by virtue of such 
gaps is relatively small, the exposed area must be protected by 
appropriate insulation. Furthermore such insulation must have a sufficient 
flexibility and cushioning to keep the gap filled and still permit some 
lateral movement between the tiles. 
It is an object of this invention to provide an improved insulating 
material having utility as a gap filler in the above mentioned application 
as well as in other more conventional industrial applications. Other 
objects will become apparent from the description of the invention. 
BACKGROUND ART 
Compounds of silicon, including silicon carbide, have been disclosed in the 
art as having some utility as fillers or pigments in coating compositions 
(see U.S. Pat. No. 3,271,109 and U.S. Pat. No. 3,236,673). In all such 
cases, the ultimate sought for compositions were multicomponent mixtures 
containing various fillers, pigments, extenders, film formers, driers, 
etc. to be used as surface coatings for such conventional purposes as 
decorative effect of surface protection in normal use. Silicon carbide has 
also been disclosed as an ingredient in ablative compositions (see U.S. 
Pat. No. 3,623,904). The use of various materials as suspension agents in 
conventional coating compositions is also disclosed in the art. Such 
suspension agents include hydrogenated castor oil, aluminum distearate and 
emulsifiable polyethylene waxes (see U.S. Pat. No. 3,123,488 and U.S. Pat. 
No. 3,184,323). 
DISCLOSURE OF INVENTION 
According to this invention, a novel high temperature insulating article is 
provided which comprises a flexible, high temperature, insulating fabric 
of closely woven heat resistant fibers having finely divided particles of 
silicon carbide dispersed through the fabric and bonded to the fibers with 
an emulsifiable polyethylene wax. Particularly preferred articles of this 
invention are those wherein the fabric is closely woven heat resistant 
fibers of silica or alumina borosilicate. 
Investigations revealed that, while fabrics of the preferred fibers are per 
se a good insulating composition, they were not acceptable in the 
aerospace application described above. Under the conditions encountered on 
reentry, excessively high temperatures would be encountered in the fabric 
and the metallic skin being protected, and the fabrics would become 
embrittled requiring a costly replacement following each flight. Such 
fabrics having finely divided particles of silicon carbide dispersed 
through the fabric and bonded to the fibers with an emulsifiable 
polyethylene wax were found to have such an increased high temperature 
emittance char-acteristic that, under the extreme conditions of reentry, a 
much lower maximum temperature within the fabric was maintained and the 
metallic skin temperature never exceeded a safe maximum level. 
The novel fabrics of this invention can be conveniently prepared by at 
least partially impregnating the woven fabric with a mixture of silicon 
carbide particles and an emulsifiable polyethylene wax dispersed in a 
suitable liquid organic vehicle or carrier. After impregnation of the 
fabric with such a mixture, the carrier evaporates leaving the silicon 
carbide particles dispersed at least partially throughout the fabric, 
including the surface thereof, bonded to the fibers with the emulsifiable 
polyethylene wax.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An impregnating composition was prepared by vigorously stirring with an 
electric laboratory stirrer a mixture of butyl alcohol, 1,000 mesh 
powdered silicon carbide and an emulsifiable polyethylene wax having a 
molecular weight of about 2,000 and an acid number of 7-14. The 
impregnating composition so prepared had the following composition with 
all percentages being expressed as percent by weight of total composition: 
______________________________________ 
Ingredient Percent 
______________________________________ 
Butyl Alcohol 82% 
Silicon carbide 12% 
Emulsifiable polyethylene wax 
6% 
______________________________________ 
Samples of two types of fabrics were employed in this test example. The 
first fabric consisted of closely woven silica fibers, the individual 
fibers having a diameter of 1-3 microns. The second fabric consisted of 
closely woven alumina borosilicate fibers, the individual fibers having a 
diameter of 10-12 microns. Fabric thickness among the various samples 
varied from 0.2 mm up to about 0.8 mm. 
The above mentioned fabrics were then used to prepare tile gap fillers 
sized to the thickness and depth of the gap to be filled. The gap filler 
consisted of a 0.2 mm thick Inconel foil, enveloped by the insulating 
fabric. The Inconel foil is for the purpose of providing dimensional 
stability to the filler, avoiding any sag in the filler once installed. In 
the case of the larger gaps, up to approximately 1 centimeter or more in 
thickness, two Inconel foils were used and a quantity of high purity 
silica fiber insulation was also inserted into the envelope between the 
two foils and between the foils and the enveloping fabric. In addition to 
providing a gap filler of the required thickness, these additional silica 
fibers provided the desired flexible cushioning effect between the 
adjacent tiles. 
Gap fillers so prepared were then impregnated with the mixture above 
described. Impregnation can be carried out by spraying or brushing the 
mixture onto the gap filler or by dipping the gap filler into the mixture. 
The mixture is applied in a quantity sufficient to provide thorough 
penetration of the mixture throughout the thickness of the fabric. 
After complete evaporation of the butyl alcohol, the thus treated gap 
fillers having silicon carbide particles and polyethylene wax dispersed on 
the surface and throughout the fabric bonded to the fibers, along with 
untreated gap fillers as controls, were subjected to the action of a 
Plasma Arc Jet which generated a surface temperature on the gap filler of 
approximately 2,300.degree. F. (1,260.degree. C.). In this test, untreated 
gap fillers had a high temperature emittance of approximately 0.35 whereas 
the treated gap fillers had a high temperature emittance of approximately 
0.75. The operating temperature of the treated gap filler was 
approximately 1,900.degree. F. (1,030.degree. C.) while the operating 
temperature of the untreated gap filler was approximately 2,300.degree. F. 
(1,260.degree. C.). During the test the untreated gap filler became 
embrittled and would not have been reusable had such an exposure been 
encountered on reentry. On the other hand, the treated gap fillers 
retained their integrity and flexibility and would not have had to be 
replaced for a second space flight. 
The silicon carbide used in this invention should be in finely divided 
state to facilitate a complete and uniform dispersion of the particles 
throughout the fabric. This is best accomplished using particles of 
silicon carbide fine enough to pass through a 1,000 mesh screen. 
The polyethylene waxes employed in this invention are emulsifiable 
polyethylene waxes having free carboxyl groups, a molecular weight of from 
about 1,500 to 6,000 and an acid number from about 2 to about 50. 
Preferably such waxes have a molecular weight of from about 2,000 to about 
2,500 and an acid number from about 7 to about 50. Particularly preferred 
is a wax having a molecular weight of about 2,000 and an acid number of 
about 7 to about 14. To facilitate handling, the polyethylene wax can be 
employed in the form of a premix with an alkyl alcohol, such as butyl 
alcohol, the alcohol being a softening agent for the wax, promoting rapid 
dispersion. In this particular application these waxes uniquely function 
as binding agents and sizing agents for the fabrics, binding the silicon 
particles to the fibers and enhancing the retention of flexibility in the 
fabric throughout exposure to high temperatures. 
While specific reference has been made to fabrics of closely woven silica 
or alumina borosilicate fibers, this invention is not thereby so limited. 
This invention contemplates the use of fabrics of other refractory fibers, 
their use being dependent upon the particular insulating conditions 
encountered. 
The degree of impregnation of the fabric can be varied substantially, 
depending upon the results desired. For example, in the case of gap 
fillers described herein, while the totality of the fabric envelope of the 
gap filler can be impregnated in accordance with this invention, 
satisfactory performance of such gap fillers was realized with partial 
impregnation, viz., when the gap filler was impregnated from the surface 
exposed to a depth of about 1/3 of the total vertical depth of the gap 
filler. Generally impregnation and surface coating of the fabric should be 
such as to produce an increase in the dry weight (silicon carbide and 
polyethylene wax combined) of the fabric in the amount of from about 0.02 
to about 0.1 gram per square inch (about 0.003 to about 0.02 gram per 
square centimeter) of fabric. Increasingly extreme temperature conditions 
will of necessity require a higher degree of impregnation. 
The ratio of the quantity of silicon carbide particles to the quantity of 
the particular polyethylene wax with which the fabric is impregnated is 
subject to substantial variation. The quantity of silicon carbide 
particles dispersed through the fabric is governed by the incremental 
increase in high temperature emittance desired. The quantity of 
polyethylene wax dispersed throughout the fabric is that quantity 
necessary to adhere the silicon carbide particles to the individual fibers 
while enhancing the retention of integrity and flexibility in the fabric 
throughout the exposure to high temperatures. Being amorphous in 
character, only minor amounts of the polyethylene wax need be employed. A 
weight ratio of silicon carbide particles to polyethylene wax ranging from 
about 1:1 to 10:1 are applicable. 
Impregnation of fabrics with silicon carbide particles and polyethylene 
waxes to produce the novel articles of this invention can be accomplished 
in any convenient manner. The technique described herein is particularly 
simple and effective. The content of such impregnating liquid mixtures can 
be substantially varied. Such mixtures containing from about 5 to about 
25% by weight of silicon carbide and from about 1 to about 10% by weight 
of the emulsifiable polyethylene wax are particularly useful. Many liquid 
organic materials can serve as the vehicle or carrier. The liquid vehicle 
or carrier should be such as to facilitate the preparation of a uniform 
and rather stable dispersion of the silicon carbide and polyethylene wax, 
facilitate penetration throughout the fabric or to the extent desired and 
have a volatility such that it promotes rapid drying of the treated 
fabric. The lower alkyl alcohols are particularly useful in this regard. 
Such alcohols would include methyl, ethyl, propyl, butyl, etc. alcohols. 
While the invention disclosed herein has been described in connection with 
its aerospace applications, the invention is not limited thereto. 
Furthermore, the particularly described total construction of the gap 
filler is not intended to be construed as a limitation on this invention. 
The fabric forming the envelope of the gap filler can be impregnated prior 
to fabrication of the gap filler with equally advantageous results. 
Fabrics of closely woven refractory fibers have wide industrial 
applications as insulating materials. Such fabrics for industrial 
applications having silicon carbide particles dispersed through the fabric 
and bonded to the individual fibers with an emulsifiable polyethylene wax 
in accordance with this invention will exhibit much improved performance 
characteristics.