CA1042287A

CA1042287A - High temperature resistant material

Info

Publication number: CA1042287A
Application number: CA211,184A
Authority: CA
Inventors: Richard F. Shannon
Original assignee: Owens Corning Fiberglas Corp
Current assignee: Owens Corning
Priority date: 1974-03-14
Filing date: 1974-10-10
Publication date: 1978-11-14

Abstract

ABSTRACT OF THE DISCLOSURE
Open-celled silica foams coated with metal oxides or materials that can be converted to metal oxides are disclosed.
These coated foams are high temperature resistant materials and are employed as structural or insulation material.

Description

~42Z87 This invention relates to high temperature resistant open-celled silica foams. More par~icularly, this invention relates to open-celled silica foams coated with metal oxides or materials that can be converted to metal oxides.
High temperature resistant vitreous materials are known in the art. These vitreous materials are in the form of fibers, flakes, sheets or the like and have a maximum dimension in at least one direction not greater than 0.1 inch. At least one surface of the vitreous material is coated with a refractory metal oxide or a material that can be converted to a refractory metal oxide. These high temperature resistant materials are further described in United States Patent Numbers 2,901,379 and 3,232,782.
I now have discovered open-celled silica foams coated with at least one metal oxide or a material that can be ;`
converted to a metal oxide. The high temperature resistant open-celled silica oams of this invention have a coating of the metal oxide on all surfaces and cell walls of the foam. Unlike the silica fibers, fla~es and sheets of U. S. Patent Numbers `, 20 2,901,379 and 3,232,782, which have a maximum dimension in at least one direction not greater than 0.1 inch, the open-cell ~;
silica foams of this invention have no dimensional limitations.
The coated silica foams of this invention are highly temperature ~ resistant both before and after exposure to high temperatures.
rl The foams need not be exposed to high temperature prior to use ' and need not be replaced once exposed to high temperature in service. Not only are the foams of this invention resistant to high temperatures, but they also can be employed as structural materials.
The open-celled silica foam generally is coated with ;` , ~
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l~Z;~87 metal salts which may or may not be converted to the oxide form. The salt coating could be converted to a hydroxide form and remain there indefinitely until converted to the oxide form. In fire rated applications, the coating would remain in the hydroxide form until the silica structure was exposed to fire. In other applications such as refractory block appli-cations, the hydroxide form could be converted to the oxide form prior to use.
According to the present invention an article of manufacture comprises a body of silica foam having an adsorbent open-cell structure. Additionally, a coating of at least one metal oxide or a material that can be converted to at metal oxide is disposed on substantially all surfaces and cell walls of the body.
Silica foams can be formed directly from a pure silica melt or from sodium silicate. Sodium silicate denotes a crystal represented by the formula: Na2O . (SiO2)n. XH2O
wherein n i8 any number from 1 to 5 and x is the number 1 or larger.
; 20 Foamed products from sodium silicate are prepared by forming a mixture of a surface tension depressant and an ' aqueous solution of sodium silicate, subjecting the mixture to mixing with a gas at above atmospheric pressure until a wet ,~
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foam is formed, blending and reacting with the set foam an in-solubilizing agent in an amount sufficient to make a foamed product resistant to being solubilized by water. The foam then is rigidized before leaching and, if desired, cured, The cur-ing of the foam can be carried out in an autoclave.
Sodium then is leached from the sodium silicate foam to produce the open-celled silica foams employed in this inven- , i tion. The foams can be leached using hot water at an acid pH. ,-The water is heated to a temperature o~ at least 120F. with a

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r-104L22!37 p~ of about 6 being preferred. The leached foam then is dried.
Generally, the final product has a density, when dry, ranging up to 20 pounds per cubic foot, an average cell size ranging up to 300 microns in diameter, an average cell wall thickness ranging up to 16 microns and at least 50 cells per cubic millimeter. These foam products are used as structural or insulation materials. -The metal oxides employed in this invention are oxides ; of metals most capable of resisting high temperatures. Specific-metals include iron, tunasten, titanium, magnesium, aluminum, zirconium, chromium, nickel, cobalt, manganese, tin, bariùm, cerium and other rare earth metals. Preferred metals are ~ . .
,` chromium, zirconium, titanium, aluminum or iron. If desired, :~ .
mixtures of two or more metals can be employed such as spinel -~
forming metals where one of the metals is aluminum. ;~
Preferably, a salt solution of one of these metals is ;
applied to the open-cell silica foam after leaching. These sa~ts are converted to the corresponding oxide by heat or the corresponding hydroxide by gelling.
Salts such as aluminum sulfate, magnesium chloride, :;
zirconium chloride, chromium chloride or the like are applied to the silica foams. While salts such as metal sulfates and metal chlorides are preferred, salts of weak acids such as oxalates, formates, lactates, acetates or the like also can be employed.
Preferably, these salts should be water soluble. Examples of such salts are alumin~m acetate, aluminum lactate, beryillium oxalate, chromium acetate, chromium oxalate, magnesium acetate, magnesium formate, magnesium acetate, titanium oxalate, zirconium acetate and the like.
Apart ~rom physical and economical considerations,

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'' ' '' ', , .''~' 1&~42~87 there is no upper limit on the thickness or weight of metal coating that can be employed in this invention. Generally, the coating ranges from 0.5 to 100 weight percent based on the weight of silica foam. P~eferably, this percent ranges from 1 to 50 and more preferably from 3 to 20. Since all surfaces and cell walls of the silica foam are covered by the coating, the thickness of the coating will depend upon the amount of coatlng employed. ;~
The metal salts are converted, by heat, to metal oxidesO It!is our belief that the metal oxides react at elevated temperature with the surface of the foam to form a metal silicate surface area. The entire silica foam is not converted to a metal silicate, but only the oxide in contact with the silica and exposed to elevated temperatures. In these exposed areas, the resulting product may be a metal silicate i sandwiched between a metal oxide and silica foam. It is not ~-~
I necessary to subject the coated foam to elevated temperatures i in order to produce the metal silicate. The foam can be i fabricated into a finished article and the final conversion of the metal oxide to a metal silicate coating need never occur if the particular product is never subjected to a sufficiently ; high service temperature for the conversion. The metal oxide coating is capable of such conversion at any time.
It is desirable to gel the coating to retain the metal in place on the sample. Otherwise, the salt has a tendency to mi~rate to the surfaces of the sample. This can be accomplished in situ on the silica foam by immersing the foam first in a solution of the metal salt and then, while still wet pulling a hydroxide solution such as an ammonium hydroxide solution through the silica foam. In the alternative, gelation of the "

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coating can be carried out by passing ammonia gas through the -sample. The gelling converts the metal salt to a metal hydroxide. The metal hydroxide coated silica foams then are dried and can be heated as discussed in the previous paragraph.
The coating of the surface of each cell wall of the silica foam with an opacifying metal salt, which then is converted to a metal oxide, improves the thermal conductivity (K) of 6 pcf silica foam over 0.60 K at 500F. mean temperature to 0.50 K at 0.5 at 500F. mean temperature. The metal silicate need not be formed to get the opacifying characteristics in the foam. Thermal conductivity (K) is measured in BTU/inch/hr./

~1 sq. ft./F. '~' Specific composites and processes embodying the principles of this invention are set forth in the ollowing examples:
EX~MPLE I
An a~ueous sodium silicate solution of about 40 percent solids and having a ratio of silicon dioxide to sodium ~ ;~
oxide of a~out 3.25 to 1, by weight, was placed in a pressurized, 20 stirred container. Distilled tall oil acid was placed in the `
container. Air then was fed into the container at a pressure ` slightly in excess of 20 psi. Thereafter, the contents of the container were thoroughly mixed at this pressure. The resulting wet foam was blended thoroughly with sodium fluorosilicate (75 percent solids slurry in water).
; The resulting material was placed in molds having the following sizes in inches: 6 x 6 x 2; 18 x 12 x 2; and 36 x 18 x

4~ Samples of each size were oven cured at a temperature of about 200F. at atmospheric pressure.
The samples were leached to remove sodium by washing ;
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with hot water having a pH of about 6, leaving a pure open-cell silica foam.
EXAMPLE II
The process of Example I was repeated except that the samples were cured in an autoclave under steam. Steam was introduced into the autoclave over a period of 30 minutes, held at a pressure of 250 psi for 3~ hours and reduced to atmospheric pressure over an additional time period of 30 minutes.
EXAMPLE III (Control) ....
A sample of silica foam prepared according to the ;~
procedures of Example II was exposed to the flame of a propane torch for periods of time ranging from 30 seconds to 5 minutes.
The cells of the foam melted and flowed into a molten state.
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Surface erosion also occurred where the flame touched the surface.

After cooling, cell damage was visible to the eye, i~e., holes - , :.
were left in the surfaces of the foam which had been exposed to !
the flame of the propane torch.
EXAMPLE IV
A sample of silica foam prepared according to the , 20 procedures of Example II was dipped in an aqueous solution of chromic sulfate (10 percent solids in water). While still wet, a 5 percent by weight solution of ammonium hydroxide was pulled through the sample to gel the metal salt to a metal hydroxide. The sample was oven dried and the coating of chromic hydroxide was 17.0 weight percent based on the weight of the silica foam sample. The coated sample was exposed to the flame of a propane torch for periods of time ranging from 30 seconds to five minutes. No melting of cells of the foam was observed. `
After cooling, cell integrity was not damaged and no differences between the fired and unfired foam could be observed.

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EXA~LE V
The procedures of Example IV were repeated except that ferric chloride was employed instead of chromic sulfate. The coating of ferric hydroxide on the silica foam sample was 15.2 weight percent based on the weight of the silica foam sample.
No melting of cells of the foam was observed. After cooling, cell integrit~ was not damaged and no differences between the fired and unfired foam could be observed.
EXAMPLE VI
The procedures of Example IV-were repeated except that aluminum chloride was employed instead of chromic sulfate. The coating of aluminum hydroxide on the silica foam sample was 35.2 weight percent based on the weight of the silica foam sample.
No melting of cells of the foam was observed. Again, after cooling, cel~ integrity was not damaged and no differences between the fired and un~ired foam could be observed.
A comparison of the result of Example III with the results of Examples IV to VI reveals the advantage of employing the metal coatings of this invention. The coated open cell silica foams of this invention provide excellent resistance to high temperature as all of the surfaces of each cell wall are coated with the metal coati.ngs of this invention. These coated silica foams can be readily used as structural material. For example these coated or impregnated silica foams can be used in high temperature block insulation having a thickness of 2 inches or in acoustical ceiling panels which are 2' x 4' x 5/8"
or 4' x 4' x 5/8". These materials retain their structural properties even at elevated temperatures.
Although this invention has been described in ` 30 considerable detail, it must be understood that such detail is . ~ ., i .

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~42287 for the purpose of illustration only and that many variations and modifications can be made by one skilled in the art without departing from the scope and spirit thereof.

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Claims

The embodiments of the invention in which an ex-clusive property or privilege is claimed are defined as follows:

1. An article of manufacture comprising:
a body of silica foam having an adsorbent open-cell structure; and a coating of at least one metal oxide or a material that can be converted to a metal oxide disposed on substantially all surfaces and cell walls of said body.

2. An article of manufacture according to claim 1 wherein the coating ranges from 0.5 to 100 weight percent based on the weight of the body of silica foam.

3. An article of manufacture according to claim 1 or claim 2 wherein the coating ranges from 1 to 50 weight percent based on the weight of the body of silica foam.

4. An article of manufacture according to claim 1 or claim 2 wherein the coating ranges from 3 to 20 weight percent based on the weight of the body of silica foam.

5. An article of manufacture according to claim 1 wherein the coating is initially applied to the body of silica foam as salt solution which is then converted to a metal oxide or a metal hydroxide.

6. An article of manufacture according to claim 5 wherein the salt solution is gelled with a hydroxide solution.

7. An article of manufacture according to claim 6 wherein the hydroxide solution is an ammonium hydroxide solution.

8. An article of manufacture according to claims 1, 2 or 5 wherein the metal oxides are oxides of iron, tungsten, titanium, magnesium, aluminum, zirconium, chromium, nickel, cobalt, manganese, tin, barium, cerium and other rare earth metals.

9. An article or manufacture according to claims 1, 2 or 5 wherein the metal oxides are oxides of chromium, zirconium, titanium, aluminum or iron.

10. An article of manufacture according to claims 5, 6 or 7 wherein the metal salts are metal sulfates, metal chlorides or metal salts of weak acids.

11. An article of manufacture according to claims 5, 6 or 7 wherein the metal salts are metal sulfates or metal chlorides.

12. An article of manufacture according to claims 5, 6 or 7 wherein the metal salts are chromic sulfate, ferric chloride, or aluminum chloride.

13. An article of manufacture according to claim 7 wherein the body of open-cell silica foam has a density, when dry, ranging up to 20 pounds per cubic foot, an average cell size ranging up to 300 microns in diameter, an average cell wall thickness ranging up to 16 microns with at least 50 cells per cubic millimeter and wherein the body has a minimum dimension in all directions of at least 2 inches.

14. An article of manufacture according to claim 1 wherein a portion of the coated silica foam is exposed to elevated temperatures such that the exposed portion of the coated silica foam is a metal silicate sandwiched between the coating and the silica foam.

15. An article of manufacture according to claim 13 having a minimum dimension in all directions of at least 5/8 inch.