Bonding of ceramic fibers

A method of vacuum forming of aqueous, fibrous slurries and products produced thereby are disclosed. The method entails forming an aqueous slurry of ceramic fiber, cationic starch and silica sol. The silica has, based on total weight of the sol, about 50% silica having a particle size range of from about 7 nm to about 200 nm and a specific surface area of about 100 m.sup.2 /gm to about 10 m.sup.2 /gm. The slurry is passed through a porous screen under a vacuum pressure deposit the solids content onto the screen to produce high strength products.

FIELD OF THE INVENTION 
This invention relates to methods of vacuum forming of ceramic fibrous 
slurries into shaped products. 
BACKGROUND OF THE INVENTION 
U.S. Pat. No. 3,224,927 shows the use of cationic starch to precipitate 
silica binders onto refractory fibers for forming refractory papers and 
mats. Although the teachings of his patent are useful for manufacture of 
shaped ceramic fiber products, the amount of silica binder which can be 
flocked onto the ceramic fibers is limited by the flocking capacity of the 
cationic starch; namely, to about 1.5 units silica per unit of starch. In 
addition, the amount of starch which can be used cannot exceed about 8%. 
Otherwise, forming times are high and shapes stick to molds. Binder 
content and formulations therefore are restricted to levels that produce 
only moderately strong pieces, i.e., 80-120 PSI modulus of rupture. A need 
therefore exists for improved methods of vacuum forming shaped ceramic 
fiber products. 
SUMMARY OF THE INVENTION 
The invention relates to an aqueous ceramic slurry comprising ceramic 
fibers, cationic starch, and colloidal silica, a method of vacuum forming 
the slurry, and ceramic products formed by that method. The slurry 
typically has a solids content of about 0.5% to about 3% based on total 
weight of the slurry, about 0.5% to about 2% ceramic fiber based on the 
total weight of the slurry, about 0.01% to about 0.7% silica based on the 
total weight of the slurry, about 0.005% to about 0.2% cationic starch 
based on the total weight of the slurry, remainder water. The silica sol 
has, based on the weight of the sol, about 50% silica having a particle 
size range of from about 7 nm to about 200 nm and a specific surface area 
of about 100 m.sup.2 /gm to about 10 m.sup.2 /gm, remainder water. 
The method of vacuum forming the slurry entails passing the slurry through 
a porous screen under a vacuum pressure deposit the solids content of the 
slurry onto the screen to produce a shaped product. The ceramic products 
produce typically include ceramic fiber in an amount of about 62% to about 
96% by weight based on total weight of the ceramic product, about 2% to 
about 30% silica by weight based on total weight of the product, and about 
1% to about 8% of cationic starch by weight based on the total weight of 
the product. 
Having summarized the invention, the invention will now be described in 
detail by reference to the following detailed description and non-limiting 
examples. 
DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the invention, an aqueous slurry having ceramic fiber, 
silica sol having a large particle size and broad particle size 
distribution, and starch is vacuum formed to provide shaped products. The 
aqueous slurry of ceramic fiber, starch and silica sol has a solids 
content of about 0.5% to about 3% by weight based on the total weight of 
the slurry, preferably about 0.7% to about 1% solids by weight based on 
total weight of the slurry, about 0.5% to about 2% ceramic fiber by weight 
based on total weight of the slurry, preferably about 0.7% ceramic fiber 
by weight based on total weight of the slurry, about 0.01% to about 0.7% 
silica by weight based on total weight of the slurry, preferably about 
0.02% to about 0.21% silica by weight based on the total weight of the 
slurry, about 0.005% to about 0.2% cationic starch by weight based on 
total weight of the slurry, preferably about 0.005% to about 0.07% 
cationic starch by weight based on total weight of the slurry, remainder 
water. 
Optionally, a filler material such as ceramic fillers and organic fillers, 
preferably ceramic fillers, may be included in the aqueous slurry of 
ceramic fiber, silica sol, and starch to provide a modified slurry that 
also may be vacuum formed. The filler may be included in an amount of up 
to about 1% by weight based on the total weight of the modified slurry. 
The modified slurry having ceramic fiber, silica sol, starch, and ceramic 
filler has about 0.5% to about 3% solids by weight based on the total 
weight of the modified slurry, preferably about 0.07% to about 1.7% by 
weight of solids by weight based on the total weight of the modified 
slurry. Ceramic fibers are present in the modified slurry an amount of 
about 0.5% to about 2% by weight based on total weight of the modified 
slurry, preferably about 0.7% by weight based on the total weight of the 
modified slurry, silica is present in an amount of about 0.01% to about 
0.7% by weight based on total weight of the modified slurry, preferably 
about 0.02% to about 0.21% based on the total weight of the modified 
slurry, cationic starch is present in an amount of about 0.005% to about 
0.2% by weight based on total weight of the modified slurry, preferably 
about 0.01% to about 0.07% by weight based on the total weight of the 
modified slurry remainder water. Preferably the filler is a ceramic filler 
present in an amount of up to about 1.0% by weight based on the total 
weight of the modified slurry. The preferred silica sols employed in the 
aqueous slurries which are vacuum formed into dried ceramic products in 
accordance with the invention are aqueous, colloidal dispersions of 
discrete amorphous silicon dioxide particles in slightly alkaline water 
that includes, based on the total weight of the sol, about 50% silica, 
remainder water. These sols are available from Wesbond Corporation, 
Wilmington, Del. under the name Megasol.TM.. The sols may be used at a pH 
of about 8.0 to about 10.0, preferably at a pH of about 9.0 to about 9.5. 
The sols may be used in particle size ranges of about 7 nm to about 200 
nm, preferably in particle size ranges of about 8 nm to about 190 nm, most 
preferably at a particle size range of about 10 nm to about 180 nm. The 
sols may be used with specific surface areas varying from about 100 
m.sup.2 /gm to about 10 m.sup.2 /gm, preferably 80 m.sup.2 /gm to about 20 
m.sup.2 /gm, most preferably about 60 m.sup.2 /gm to about 27 m.sup.2 /gm. 
The sols may be used at titratable Na.sub.2 O contents of about 0.02% to 
about 0.35%, preferably about 0.1% to about 0.25%, most preferably about 
0.20% to about 0.22%. 
Silica sols such as Megasol.TM. which may be employed in the invention have 
larger particle size ranges and lower specific surface areas than prior 
art colloidal silica sols. These characteristics advantageously enables 
the use of much lower amounts of cationic starch to floc silica onto 
ceramic fibers, and to floc much larger amounts of silica onto the ceramic 
fibers. This enables manufacture of dried ceramic products such as ceramic 
fiberboard which have much lower organic content and higher strengths, and 
to produce products which sinter more slowly so that less shrinkage is 
experienced at elevated use temperatures. 
The cationic starches which may be employed in the aqueous slurries which 
are vacuum formed in accordance with the invention preferably are 
pregelationized cationic corn starches that have been treated with a 
cationic amine, cooked and flaked. These cationic starches are available 
under the tradename WESTAR+ from Wesbond Corporation, Wilmington, Del. 
These cationic starches have a cationic charge of about 0.18% N.sub.2 to 
about 0.22% N.sub.2, and a pH of about 4 to 8. Higher cationic charge 
starches (0.30% N) such as WESTAR+3 corn starch from Wesbond Corp. also 
may be employed. Other starches which can be used in the compositions and 
process disclosed herein include, but are not limited to SOLVATOSE Potato 
Starch, EMPRESOL Potato Starch, and STA-LOK potato starch. SOLVATOSE 
Potato Starch, available from American Key Products, Inc. Kearney, N.J., 
is a pregelationized cationic potato starch that has been treated with a 
cationic amine, cooked and flaked. This starch has a cationic charge, as 
measured by Nitrogen content, of about 0.30% N.sub.2. 
EMPRESOL Potato Starch, available from American Key Products, Inc. Kearney, 
N.J., also is a pregelationized cationic potato starch that has been 
treated with a cationic amine, cooked and flaked. This starch has a 
cationic charge, as measured by Nitrogen content, of about 0.30% N.sub.2. 
STA-LOK potato starch, available from Staley industrial Products, Decatur, 
Ill., is a pregelationized cationic potato starch that has been treated 
with a cationic amine, cooked and flaked. The starch has a cationic 
charge, as measured by Nitrogen content, of about 0.30% N.sub.2. 
Ceramic fibers which can be employed in the slurries which are vacuum 
formed in accordance with the invention include, but are not limited to 
aluminosilicate fibers such as "Fiberfrax" Regular fibers, "Fiberfrax" 
6000 fibers from Unifrax Corporation, Niagara Falls, N.Y., "Fiberfrax" 
Spun fibers from Unifrax Corporation, and "Kaowool" Ceramic Fibers from 
Thermal Ceramics, Augusta, Ga. Preferably, the ceramic fibers are any of 
"Fiberfrax" 6000 fibers, "Fiberfrax" Spun Fibers, and "Fiberfrax" Regular 
fibers. These ceramic fibers may be used in dimensions of about 2-3 
microns diameter and about four inches length. 
"Fiberfrax" Regular fibers have about 47-53% alumina, 48-53% silica, about 
0.1% Fe.sub.2 O.sub.3, about 0.1% TiO.sub.2, about 0.1-1.3% Na.sub.2 O, 
and about 0.5% trace impurities. "Fiberfrax" 6000 fibers and "Fiberfrax" 
Spun Fibers, are, according to Fiberfrax Co., made from Kaolin. 
"Fiberfrax" 6000 and "Fiberfrax" Spun fibers typically have 45-51% 
alumina, 46-52% silica, about 0.8-1.1% Fe.sub.2 O.sub.3, about 1.0-1.8% 
TiO.sub.2, about 0.1-0.2% Na.sub.2 O, and about 1.0% trace impurities. 
Other ceramic fibers which may be employed include but are not limited to 
alumina fiber, silica fibers such as those sold under the tradename 
"Maxsil" by McAllister Mils, Independence, Va., glass fibers such as 
"Insulfrax" from Unifrax Corporation, Niagara Falls, N.Y., mineral wools, 
and other fibers designed to operate at high temperatures; i.e. above 
1400.degree. F., also may be used as ceramic fibers in the invention. 
Optionally, organic fibers may be included with the ceramic fibers. 
Examples of organic fibers which may be employed include but are not 
limited to cellulose fibers, aramid fibers, and polyethylene fibers. 
In accordance with the invention, an aqueous ceramic fiber-water mixture is 
formed by adding ceramic fibers, optionally with organic fibers such as 
those above, to water. Optional fillers such as ceramic and organic 
fillers may be included. Examples of ceramic fillers include but are not 
limited to oxides such as alumina, alumino-silicates such as Mullite, and 
clays such as Kyanite. Examples of organic fillers include but are not 
limited to cellulose and polyethylene. The fillers may be employed in 
fiber, pulp or powder form. 
The fiber-water mixture, optionally including filler, then is subjected to 
moderate agitation by a propeller mixer to disperse the fibers and to 
ensure that uniform flocs can be formed. Thereafter, cationic starch is 
added with moderate agitation for about 5-10 minutes to hydrate the 
starch. The resulting fiber-starch-water composition has a pH of about 
4-8, a total solids content of about 0.5% to about 3% by weight based on 
the total weight of the fiber-starch-water composition, preferably about 
0.7% to about 0.8% total solids content based on the total weight of the 
fiber-starch-water composition, about 0.5% to about 2.7% by weight ceramic 
fiber based on the total weight of the fiber-starch-water composition, 
preferably 0.7% by weight ceramic fiber based on the total weight of the 
fiber-starch-water composition, about 0.005% to about 0.3% starch by 
weight based on the total weight of the fiber-starch-water composition, 
preferably about 0.01% to about 0.07% by weight starch based on the total 
weight of the fiber-starch-water composition, remainder water. 
After producing the above described fiber-starch-water composition, 
sufficient Megasol.TM. silica sol is added to achieve about 4-30% by 
weight silica based on the weight of the fiber in the fiber-starch-water 
composition. The Megasol.TM. is added to the fiber-starch-water 
composition during moderate mixing to floc the fibers into 3-dimensional 
flocs. The amount of Megasol.TM. silica sol added is controlled to achieve 
a ratio of silica to starch of about 1:1 to about 5:1, preferably about 
2:1 to about 4:1, most preferably about 2:1 to about 3:1. 
The resulting aqueous ceramic fiber-starch-silica slurry has three 
dimensional flocs and can be vacuum formed onto a screen mold to yield a 
shaped preform. Vacuum pressures of about 20 inches Hg to about 29 inches 
Hg typically are employed during vacuum forming. Vacuum forming of the 
slurries may be performed to produce products of any desired thickness and 
shape. Typically, the aqueous slurries are vacuum formed to provide 
preforms of about 1 to about 4 inches thick. 
After producing the vacuum formed shapes, the preforms are removed from the 
mold and dried. Typically, drying is performed at about 250.degree. F. for 
about 3-4 hours to yield a dried product. Other drying conditions may be 
used depending on the composition and thickness of the preform. 
Thereafter, the dried product optionally may be fired at elevated 
temperatures such as about 1800.degree. F. for about one hour. Other 
firing temperatures and conditions may be used depending on the 
composition and thickness of the dried product. 
The dried products produced by the process described herein typically 
include ceramic fiber in an amount of about 62% to about 96% by weight 
based on total weight of the dried product, preferably about 72% to about 
94% by weight ceramic fiber based on the total weight of the dried 
product, about 2% to about 30% by weight silica based on total weight of 
the product, preferably about 4% to about 21% by weight silica based on 
total weight of the product, and about 1% to about 8% by weight cationic 
starch based on total weight of the product, preferably about 2% to about 
7% by weight cationic starch based on the total weight of the product. The 
dried products produced by the process described herein typically have a 
modulus of rupture ("MOR") of about 100 PSI to about 500 PSI, a density of 
about 14 lb/ft.sup.3 to about 25 lb/ft.sup.3, and a Shore hardness of 
about 60 to about 80. 
The use in the invention of the silica sol compositions disclosed herein 
having the wide range of silica particle sizes and low surface areas 
advantageously enables an increase of silica binder content of about 200% 
to about 300% compared to prior art sols to achieve products which have 
dried and fired strengths more than twice that which can be obtained with 
prior art silica sols having smaller particles and narrower particle size 
ranges. The dried products produced by the process described herein have 
increased strength which translate into more durable products. 
The dried products optionally may be fired at elevated temperatures such as 
at about 1800.degree. F. for about one hour. Firing of the dried products 
yields fired ceramic articles which have ceramic fiber in an amount of 
about 67% to about 98% by weight based on the total weight of the fired 
article, preferably about 77% to about 96% by weight ceramic fiber based 
on the total weight of the fired article, and about 2% to about 33% by 
weight silica based on the total weight of the fired article, preferably 
about 4% to about 23% silica by weight based on the total weight of the 
fired article. The fired articles typically have a high modulus of rupture 
("MOR") of about 60 PSI to about 200 PSI, and a fired linear shrinkage of 
about 1% to about 1.2%. The fired articles produced by the process 
described herein have increased strength which translates into a more 
durable finished product.

EXAMPLES 
The following non-limiting examples further illustrate this invention. All 
parts and percentages are expressed in terms of parts by weight based on 
fiber weight unless otherwise noted. Modulus of rupture data is obtained 
by breaking test bars which measure 3 inches wide by 3.5 inches long by 
0.3-0.5 inches thick as cut from vacuum formed products. Using a 2 inch 
span, the test bars are center loaded to failure in flexure. Modulus of 
rupture values are calculated using the formula: 
EQU R=(3WI)/(2bd.sup.2) 
Where: 
R=modulus of rupture in lbs/in.sup.2 
W=load in pounds at which the specimen failed 
I=distance (span) in inches between the center-lines of the lower bearing 
edges 
b=width of specimen in inches 
d=depth of specimen in inches 
Example 1 
A dilute slurry is prepared containing 80 grams of "Fiberfrax" 
aluminosilicate bulk fiber in 25 pounds (3 gallons) of water. To this 
slurry, 4 grams of Westar+ Cationic Corn Flaked Starch (5% by weight of 
fiber) is added dry and mixed for 5 minutes to allow starch to hydrate. 
Next, 24 grams of Megasol.TM. (50% solids) is added to floc the starch and 
fibers together in a three dimensional floc, which is then vacuum formed 
through a 6.5 inch.times.6.5 inch.times.1 inch screen mold. The shape is 
removed from the mold and dried at 250.degree. F. until thoroughly dry (3 
to 4 hours). Strength, density, and shrinkage properties of this composite 
product are given below. 
Weight Ratio of fiber: silica: starch=100:15:5 
Silica: Starch=3:1 
Density, Dried=15.0 lbs/ft.sup.3 
Modulus of rupture (MOR), Dried=214 PSI 
Modulus of rupture (Fired 1 hour at 1800.degree. F.)=90 PSI 
Fired linear shrinkage=1.0% 
Examples 2-6 
In Examples 2-6, Example 1 is repeated using silica to starch ratios from 
1:1 to 4:1 for both Megasol and a commonly used sol, Ludox HS 40, 
available from DuPont Corp. Ludox HS40 has the following properties: 
__________________________________________________________________________ 
Silica solids, by weight 
40% 
Surface Area, sq. meters/gm. 
230 
Particle Size, nanometers 
12 avg. 
Na2O, weight % 0.41 
pH 9.7 
Dried MOR Fired MOR** 
Example 
Wt. Ratio* 
Silica:Starch 
Megasol .TM. 
Ludox HS 
Megasol 
Ludox HS 
__________________________________________________________________________ 
2 100:5:5 
1:1 160 117 57 50 
3 100:10:5 
2:1 195 88 79 48 
4 100:15:5 
3:1 214 70 90 37 
5 100:20:5 
4:1 222 68 98 66 
6 100:7.5:2.5 
3:1 117 57 66 27 
__________________________________________________________________________ 
*fiber:silica:starch ratio 
**1 hour at 1800.degree. F. 
Example 7 
A dilute slurry is prepared by adding 80 grams of "Fiberfrax 6000" 
aluminosilicate bulk fiber to 25 pounds (3 gallons) of water. To this 
slurry, 4 grams of WESTAR+ Cationic Corn Flaked Starch (5% by weight of 
fiber) is added dry and mixed for 5 minutes to allow the starch to 
hydrate. Next, 24 grams of Megasol.TM. (50% solids) is added to floc the 
starch and fibers together into three dimensional flocs. The flocced 
material is then vacuum formed through a 6.5 inch.times.6.5 inch.times.1 
inch screen mold to yield a shaped preform. The preform is removed from 
the screen mold and dried at 250.degree. F. until thoroughly dry (3 to 4 
hours). Strength. density, and shrinkage properties of this composite 
product are given below. 
Weight Ratio (fiber:silica:starch)=100:15:5 
Silica:Starch=3:1 
Density,Dried=15.2 lbs./ft.sup.3 
Modulus of rupture, Dried=217 PSI 
Modulus of rupture (Fired 1 hour at 1800.degree. F.)=119 PSI 
Fired linear shrinkage =1.2% 
Example 8 
A dilute slurry is prepared that contains 80 grams of "Fiberfrax" Regular 
aluminosilicate fiber in 25 pounds (3 gallons) of water. To this slurry, 4 
grams of Westar+ Cationic Corn Flaked Starch (5% by weight of fiber) is 
added dry and mixed for 5 minutes to allow the starch to hydrate. Next, 24 
grams of Megasol.TM. (50% solids) is added to floc the starch and fibers 
together in a three dimensional floc, which is then vacuum formed through 
a 6.5 inch.times.6.5 inch.times.1 inch screen mold. The shape is removed 
form the mold and dried at 250.degree. F. until thoroughly dry (3 to 4 
hours). Strength, density, and shrinkage properties of this composite 
product are given below. 
Weight Ratio (fiber:silica:starch)=100:15:5 
Silica Starch=3:1 
Density Dried=16.8 lbs./ft.sup.3 
Modulus of rupture, Dried=250 PSI 
Modulus of rupture (Fired 1 hour at 1800.degree. F.)=131 PSI 
Fired linear shrinkage =1.2% 
Example 9 
A dilute slurry is prepared that contains 80 grams of "Fiberfrax" Regular 
aluminosilicate fiber in 25 pounds (3 gallons) of water. To this slurry, 8 
grams of Westar+3 Cationic Corn Flaked Starch (10% by weight of fiber) is 
added dry and mixed for 5 minutes to allow the starch to hydrate. Next, 48 
grams of Megasol.TM. (50% solids) is added to floc the starch and fibers 
together into three dimensional flocs, which is then vacuum formed through 
a 6.5 inch.times.6.5 inch.times.1 inch screen mold. The shape is removed 
form the mold and dried at 250.degree. F. until thoroughly dry (3 to 4 
hours). Strength, density, and shrinkage properties of this composite 
product are given below. 
Weight ratio (fiber:silica:starch)=100:30:10 
Silica starch=3:1 
Density dried=24.3 lbs./ft.sup.3 
Modulus of rupture, dried=502 psi 
Modulus of rupture (fired 1 hour at 1800.degree. F.)=200 psi 
Fired linear shrinkage =1.2%