A reusable silica surface insulation material is provided by bonding amorphous silica fibers with colloidal silica at an elevated temperature. The surface insulation is ordinarily manufactured in the form of blocks (i.e., tiles)

Goldstein, H. E.

Leiser, D. B.

Smith, M.

English

NASA Technical Reports Server

United States Patent [I91 
Fletcher et al. 
[ 1 1  I 3,952,083 
[451 Apr. 20, 1976 
[541 SILICA REUSABLE SURFACE INSULATION 
[ 761 Inventors: 
[22] Filed: 
James C. Fletcher, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of Howard E. Goldstein, 
Saratoga, Calif.; Marnell Smith; 
Daniel Leiser, both of San Jose, 
Calif. 
Dec. 26, 1973 
[21] Appl. No.: 427,775 
[52] U.S. C1. .................................... 264163; 264160; 
[51]  Int. CL2 ...................... F27D 7/04; C04B 35/14 
[58] Field of Search .......................... 264160, 63, 66; 
264166 
259181 A 
(561 References Cited 
UNITED STATES PATENTS 
2,847,314 8/1958 Fisher ................................... 264163 
3,549,473 1211970 Le Blanc et al. ..................... 264163 
3,102,279 911 972 Ardary et al .......................... 264160 
OTHER PUBLICATIONS 
“An Analysis of Solids-Solids Blenders,” Patterson P. 
Kelly Advertising circular, Nov. 1, 1967. 
Primary Examiner-Robert F. White 
Assistant Examiner-John Parrish 
Attorney, Agent, or Firm-Darrell G. Brekke; Armand 
G .  Morin, Sr.; John R. Manning 
ABSTRACT [571 
A reusable silica surface insulation (RSI) material is 
provided by bonding amorphous silica fibers with col- 
loidal silica at an elevated temperature. The surface 
insulation is ordinarily manufactured in the form of 
blocks (Le., tiles). 
6 Claims, 1 Drawing Figure 
FIBER, COLLOIDAL SILICA , ADDITIVES, 
DEIONIZED WATER DEIONIZED WATER 
BINDER MIXING WASHING (5 \ 
13’ 
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U.S. Patent ~ p r i l 2 0 ,  1976 
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3,952,083 
3,952,083 
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surface impurities by washing alternately in hydrochlo- 
SILICA REUSABLE SURFACE INSULATION ric acid (pH=3) and deionized water. This treatment 
The invention described herein was made in the per- generally has a small effect on bulk chemical composi- 
formance of work under a NASA contract and is sub- tion. The small quantities of impurities removed have a 
ject to the provisions of Section 305 of the National 5 significant effect on devitrification and shrinkage with 
Aeronautics and Space Act of 1958, Public Law some fiber lots, however. Average fiber diameter 
85-568 (72 Stat. 435; 42 U.S.C. 2457). diamater of the Microquartz 108 fiber is 1 to 2.5 mi- 
crons. Larger fibers up to 6 microns can also be used. 
Fiber length should be less than ?4 inch. or stratifica- BACKGROUND OF THE INVENTION 
1. Field of the Invention 
Insulating tiles which are reusable and suitable for 
2. Description of the Prior Art 
In the past, reusable surface insulation materials have 
been formed by bonding ceramic fibers with an inor- 
ganic binder. Among the fibers used have been silica, 
mullite, alumino silicate and zirconia. A large variety of 
binders has been used, including organometallic pre- 
cursors, colloidal oxides and finely divided glasses. The 
RSI materials are furnace cured to obtain the desired 
properties. 
The prior art composition suffer from a number of 
disadvantages. The currently available silica reusable 
surface insulation qaterials are made by a very com- 
plex process that requires from two to four weeks in 
Pro The material of the present invention can 
be three days. Fythermore, the mechanical 
properties and/or thermal stability of the prior art ma- 
terials are inferior to those of the present invention. 
The materials of the present invention are superior to 
the prior art materials in that they are less expensive, 
require less time to fabricate and have superior me- 
chanical and/or thermal properties. 
BRIEF DESCRIPTION OF THE DRAWINGS 
The sole FIGURE of the drawing is a pictorial dia- 
gram showing the steps necessary for preparing insulat- 
ing materials in accordance with the present invention. 
use at high temperatures. 
10 tion, during molding, and cracking of the resulting tile 
may occur. If the fiber is too long for efficient use, it is 
chopped before use. . 
The quantity of fiber, relative to the quantity of col- 
loidal silica can be varied through a rather broad range 
15 with satisfactory results. For a quantity of fiber of from 
200 to 400 gms, it has been found suitable to use from 
1,000 to 3,000 gms. of binder solution which consists of 
0.5 to 1.0% colloidal silica, 0.025% to 1% reagent 
grade starch and 1 to 5 ml. ammonia. 
The binder solution is dispersed in a high speed mixer 
and then blended with the drained fibers and water in a 
V-b!ender for 30 minutes. The pH is adjusted to 9.0 
with ammonia. An intensifier bar in the V-blender is 
used to break up fiber clumps. The use of the V- 
25 blender with an intensifier bar is unique to this inven- 
tion. It allows use of a relatively viscous slurry, and 
gives a better fiber dispersion than other mixing meth- 
ods. The resulting superior material is largely due to 
this mixing process. The slurry is poured into a mold for 
30 either single direction or multi-direction pressing and 
rapidly pressed at 10-20 psi. Final tile density is con- 
trolled by fiber content and molded dimensions and 
may vary from 7 to 30 Ib/ft3. Great care must be taken 
to avoid contamination of the raw materials during the 
35 process. All utensils and mixing vessels are polyethy- 
lene or stainless steel and only deionized water is used 
in any step in the process. The tile is then dried in an 
oven for 18 hours. The temperature is raised at the rate 
20 
- 
of 20F per hour to a maximum temperature of 300°F 
40 and held for the remainder of the cycle. The tile is 
taken directlv from the drving oven and daced in the 
DESCRIPTION OF THE PREFERRED 
EMBODIMENTS ' 
. Y  
aterial of the present invention is firing furnace and fired. A furnace must be used that 
fabricated utilizing high purity silica fibers, colloidal has no alkali or alkaline earth oxide impurities in the 
(fumed) silica as a binder and reagent grade starch as a vicinity of the tile being fired. A temperature rise rate 
temporary binder. Tiles or other desired shapes can be 45 of 300"F/hr. or less is required to avoid cracking and 
made having densities of from 7 to 30 pounds per cubic warping the 6 X 6 X 3 inch tiles. For larger tiles, slower 
foot. The silica fiber and silica binder are selected so heating may be required. The time-at-temperature and 
that they will be stable with respect to devitrification at the exact maximum firing temperature are dependent 
temperatures over 2000°F for long periods of time. The on the fiber used. Less stable fiber lots are fired for 
starch must be pure in order not to leave a residue 50 shorter time and at a lower temperature. Shrinkage of 
which would contaminate the finished product, but, as each fiber lot differs during processing and requires 
.will be later apparent, it is burned out during a process- different initial as-cast densities in order to obtain the 
ing step and does not form a part of the finished com- same final tile density. Maximum firing temperature 
position. The final composition preferably contains varies from 2200°F to 2400'F. The firing cycle allows 
from 95 to 98% of silica fibers with the balance being 55 sufficient time for sintering and stabilization relative to 
colloidal silica binder. shrinkage while not allowing excessive shrinkage or any 
Various raw materi an be utilized in practicing dcviation of the tile. The starch burns out at a tempera- 
the invention and one ble form of silica fiber is that ture of about 800"F, leaving a pure silica unbonded 
known as Microquartz 108 fiber manufacturcd by structure. At about 2300°F fiber-to-fiber sintering oc- 
ville, which is at least 98.5% pure silica. 60 curs at a fairly rapid rate. It appears that the final tile 
uitable starting material is the high purity properties are determined by fiber-to-fiber sintering at 
ena, Cali- least as much as they are by fiber-to-binder sintering. 
Shrinkage and sintering during firing are much more 
fiber is its rapid with binder than without, allowing shorter firing 
e fiber not 65 times and thus, increasing long-term stability of thc tile. 
have a total bulk than 0.3% In general, shrinkage during firing is 25-45% by vol- 
and the total alkali t should be ume. Warpage during firing can be a significant prob- 
below 0.06%. The fibers are usua4ly treated to remove lem because the tile has essentially no strength at the 
3,952,083 
3 
firing temperature. Temperature gradients in the tile 
are likely to cause warping. Warping can also occur 
due to sagging of an unsupported tile during firing. 
After firing, the tiles are machined to obtain exact 
dimensions. Usually about 0.5 cm of material is ma- 
chined off. 
Frequently, it is desirable to modify the optical prop- 
erties of the tile and this can be accomplished by add- 
ing an imprcgant to the slurry in concentrations of 0.1 
to 2% solids. The impregnant should be finally divided 
and preferably is smaller than 300 mesh. Various ox- 
ides such as chromium and cobalt oxide as well as 
silicon carbide can be used. 
The binder is a finely divided silica. The material sold 
under the trade name CAB-0-SIL M5 by the Cabot 
Corporation has been found suitable. CAB-0-SIL M5 
is a colloidal silica prepared by a vapor phase hydroly- 
sis of silicon tetrachloride. It has high chemical purity, 
low water content, an enormous external surface area 
and a high degree of particle separation. Because of 
these properties, it functions in extremely small quanti- 
ties as a binder in forming the insulation of the present 
invention. 
Any purified starch can be used in carrying out the 
invention and it is normally preferred to use chemically 
pure reagent grade starch. 
The steps in carrying out the present invention are 
shown in the drawing. Great care must be taken to 
avoid contamination of the raw materials during the 
process and all of the utensils and mixing vessels are 
polyethylene or stainless steel and only deionized water 
is used in any step of the process. The fibers are first 
washed in a drum 1 with hydrochloric acid for about 
one hour at a pH of 3 using pure nitrogen as the agitat- 
ing agent. This is done several times until the fiber is 
sufficiently clean. At the same time the binder solution, 
which consists of amorphous silica, starch and water, is 
prepared in a mixer 3 and the fibers and the binder are 
added together and blended thoroughly in a blender 5 
as a very viscous slurry. After blending, the materials 
are shaped in a mold 7 and then dried in oven 9. The 
dry tiles are then fired in kiln 11 for the required length 
of time and temperature to produce the finished tiles 
13. The tiles are then machined to a desired size and 
shape. 
The following non-limiting examples illustrate pre- 
ferred embodiments of the present invention. 
EXAMPLE 1 
In accordance with this example, tiles 5 X 5 X 1% 
inches having a density of 15 pounds per cubic foot 
were produced. About 260 grams of Microquartz 108 
fiber were placed in a polyethylene barrel with 8 gal- 
lons of deionized water and 10 to 20 ml. of hydrochlo- 
ric acid were added to bring the pH to about 3. Pure 
nitrogen was bubbled through the mixture to agitate 
the fiber and aid in the washing. Washing was contin- 
ued for 2 hours. The acidic water was drained off and 
fresh deionized water was added and the mixture again 
agitated with pure nitrogen for 10 minutes. This pro- 
cess was repeated until the rinse water had a pH of 
about 6, which ordinarily requires 3 rinses. 
In a separate vessel, the binder was prepared by com- 
bining 20 grams of reagent grade starch, 30 grams of 
CAB-0-SIL M5 and 2 ml NH,OH and 1,000 grams 
deionized water; the binder was mixed for 2 minutes in 
a suitable mixer suck as a Waring blender. The wet 
fiber and binder slurry were combined in a stainlcss 
4 
steel twin shell V-blender having 8 quart capacity. 
Deionized water was added to bring the total water to 
7,000 grams. The ingredients were mixed for 30 min- 
utes with the blender running on a 5-minute-on and 
5-minute-off cycle. The slurry was then transferred to a 
mold and pressed to a 6 X 6 X 2%inches tile at a pres- 
sure of 10-20 psi. The bottom and sides of the mold 
were perforated and covered with a 200 mesh stainless 
steel screen to allow excess water to flow out. The tiles 
I o  were then dried in a drying oven on an 18-hour cycle 
beginning at a temperature of 50°F for one hour which 
was increased at a rate of 20°F per hour to 220°F and 
held for four hours and the temperature then raised 20" 
per hour to 300°F and maintained at this temperature 
l 5  until the tiles were placed in a curing furnace. In the 
furnace the curing cycle was 300°F per hour to 2300°F. 
held 2% hours, lowered 300°F per hour to 1800°F when 
the furnace was turned off and allowed to cool to 
300°F. The total firing time was 24 hours. The tile was 
2o now machined from 5% X 5% X 2 inches in accordance 
with usual machining practice to the desired size. 
EXAMPLE 11 
In accordance with this example, a similar tile was 
25 made except that the density was 9 pounds per cubic 
foot. The starting materials were 300 grams of Mi- 
croquartz 108 washed in the same manner as Example 
I and the binder was 30 grams of reagent grade starch, 
3o 50 grams CAB-0-SIL M5 and 2 ml NH,OH. This mix- 
ture was added to 1,000 grams deionized water and 
mixed for 2 minutes. 
The materials were combined in twin shell V- 
Blender; deionized water was added to bring the total 
35 water to 8,000 grams; the wet fiber was mixed in the 
same manner as Example 1. 
pressed in a 7 X 7 X 2% inches 
same manner as in Example 1. 
300°F per hour to 2350"F, held for 4 M hours lowered 
40 300" per hour to 1800°F and the furnace was turned 
off. After cooling as in Example 1, the tile was 6% X 6% 
X 2% inches and was then machined to a 6 X 6 X 2 
inches final size. 
EXAMPLE 111 
A tile having a density of 12% pounds per cubic foot 
was prepared which contained silicon crabide 
opacifier. 
One thousand grams of Microquartz 108 fiber was 
50 washed in 15 gallons of deionized water and 40-50 ml 
HCI and rinsed as in Example 1. In a separate vessel 
100 grams starch, 175 grams CAB-0-SIL M5 and 25 
grams S i c  (300 mesh) and 4 ml NH,OH in 300 grams 
of deionized water were mixed in a blender. The mixing 
55 was continued for 4 minutes. The wet fiber and the 
binder-opacifier slurry were placed in a stainless steel 
"intensifier" twin-shell V-blender having a one-cubic- 
foot capacity. Deionized water was added to bring the 
total up to 25,000 grams. The ingredients were mixed 
60 for 45 minutes with the intensifier running on a 5- 
minute-on, 5-minute-off cycle. The slurry was then 
pressed in a 10% X 10% X 4 inches mold and dried as 
in Example 1. The tile'was cured by placing the mate- 
rial in furnace and increasing the temperature by 200°F 
65 per hour to 2350"F, holding for 2 hours, lowering 200" 
per hour to 1800°F and turning the furnace off. When 
the furnace had cooled to about 500"F, the tile was 
removed. It was then machined to 9 X 9 X 3 inches. 
45 
3,952,083 
5 6 
of 95 to 98% by weight silica fibers and 2 to 5% by 
weight colloidal silica binder, and EXAMPLE IV 
In this example, a tile having a density of 15 pounds 
per cubic foot was made to demonstrate reimpregna- 
tion Of the tile with binder. The process of Example 1 
was repeated except that 240 grams of microquartz 
fiber was used with a binder consisting of 90 grams 
CAB-O-SIL, 2 ml NH40H, 20 grams starch and 3,000 
grams Of deionized water. The curing cycle was 300°F 
per hour to 2250°F, held for 1 %  hours, lowered 300°F 10 2o0F/hr. to 3 0 0 0 ~  where the tile is dried for about 10 
per hour to 1800°F and the furnace turned off. The tile 
was removed when the furnace temperature was below 4. The method of clatm 1 wherein the silica fibers are 
500'F. 
4. machining the tile to the desired dimensions. 
2. The process of claim 1 wherein said silica fibers 
5 are acid washed and neutralized prior to mixture with 
the said binder. 
3. The process of claim 1 wherein the tiles are dried 
at a heating rate of 20"F/hr. to 220"F, held for approxi- 
mately 4 hours, and then the temperature is raised at 
hours. 
less than % inch in length. 
5. The method of claim 1 wherein the amount of 
dispersed in the said slurry ranges from 
1.25% to 15% by weight based on the weight of silica 
fibers. 
6. A method of making a reusable silica surface insu- 
hours in a dilute Hcl solu- 
tion at pH3, said fibers comprising less than 0.06% by 
weight of alkali and alkaline earth impurities, 
b. washing the fibers in (a) in deionized water until the 
pH is at  least 6, 
c. preparing a binder by mixing colloidal silica, starch, 
and ammonical deionized water, 
d. blending the fibers of ( b )  with the binder in ( c )  to 
form a slurry, 
The tile was then machined to 5% x 5% x 2 inches 
and placed in a mixture of 3,000 grams deionized wa- 15 colloidal 
ter* 90 grams CAB-o-SIL and 
uum was applied (29 inches Hg) for 30 minutes. The 
tile was dried and cured as described in Example 1 .  The 
tile was then trimmed to 5 X 5 X 1% inches. The result 2o lation material comprising the steps of: 
was a tile with similar strength to the tile of Example 1 
but with a higher modulous of elasticity. 
Although certain specific quantities and operating 
conditions have been given, it will be obvious to those 
skilled in the art that many variations can be made in 
the examples shown without departing from the spirit 25 
of the invention. 
m1 NH40H and 
a. washing silica fibers for 
We claim: 
1. A method of making a reusable silica surface 
lation material comprising the steps of: 
a. forming a slurry of silica fibers and a binder compris- 30 e. mixing the blend ( d )  for 30 minutes in a twin-shell 
f. molding said slurry into the shape of a tile at a pres- 
Ibs. per 'quare inch, 
ing colloidal silica, starch and water by mixing said V-blender equipped with an intensifier bar, 
slurry for 30 minutes in a twin-shell V-blender 
equipped with an intensifier bar, said silica fiber com- 
line earth impurities, 35 ture of 300"F., and 
b. molding said slurry into a tile at a pressure of 10-20 
Ibs. per square inch, 
c. drying said tile on an 18 hour cycle up to a tempera- 
ture of 300"F., and 
d. firing said tile according to a curing cycle compris- 40 
ing: 
1. heating said tile in a furnace at the rate of 200°F. 
to 3 0 0 0 ~ .  per hour to a maximum temperature of 
2250°F. to 2350"F., 
2. holding said tile at said maximum temperature for 45 
1 % to 2% hours, 
3. lowering the temperature at the rate of 200°F. to 
300°F. per hour to a temperature of 1800"F., turn- 
ing off the furnace and allowing the tile to cool to 
30O0F.-50OoF. before removing the tile from the 50 
furnace, said firing step providing a tile consisting 
sure Of 
prising less than 0.06% by weight of alkali and alka- g. drying said tile On an hour cyc1e to a tempera- 
h. firing said tile in accordance with a curing cycle 
which comprises: 
1. heating said tile in a furnace at the rate of 200°F. 
to 300°F. per hour to a maximum temperature of 
2250°F. to 2350"F.9 
2. holding said tile at said maximum temperature for 
1% to 2% hours, 
3. lowering the temperature at the rate of 200°F. to 
300°F. per hour to a temperature of 1800"F., turn- 
ing Off the furnace and allowing the tile to COO1 to 
300"F.-500"F. before removing the tile from the 
furnace, said firing step providing a tile consisting 
of 95 to 98% by weight silica fibers and 2 to 5% by 
weight colloidal silica binder, and 
4. machining the tile to the desired dimensions. * * * * *  
5 5  
60 
65 


Silica reusable surface insulation

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