Thermally bonded fibrous product

A thermally bonded fibrous product is composed of a sintered blend of aluminosilicate fibers, silica powder and boron nitride powder.

BACKGROUND OF THE INVENTION 
This invention relates to thermally bonded aluminosilica products and, more 
particularly, to thermally bonded aluminosilicate fibrous products of a 
novel and advantageous composition suited for use as diesel soot filters, 
kiln furniture, combustor liners, burner tubes and other high temperature 
applications. 
Ceramic fibers are often used to provide thermal insulation from very high 
tempertures. For many applications, it is convenient and often necessary 
to form the fibers into rigid shapes such as boards or specialty 
configurations. Two approaches are used to accomplish bonding of ceramic 
fibers to obtain these shapes. Chemical bonding is the most common. A heat 
or air setting binder, usually in the liquid form, is added to the fiber 
and through drying, the application of low to moderate heat, or chemical 
reaction, the binder becomes rigid and literally glues the fibers into a 
rigid structure. Such binders include colloidal oxide suspensions, 
silicate solutions and thermoplastic suspensions. The strength of these 
systems are relatively low, typically having 20 to 50 psi flexural 
strength properties. More binder can be added to increase the strength. 
However, the additional binder adversely affects thermal insulating 
properties. 
A second family of rigid ceramic shapes has been developed that utilizes 
more expensive firing schedules to sinter or fuse the fibers into a rigid 
structure. Flexural strengths are 10 to 20 times that of chemical bonding 
at the same density with no sacrifice in insulating properties. 
Difficulties in matching thermal expansion and preventing destructive 
crystal growth in the fibers have been experienced. Some success has been 
reportedly achieved through the use of a blend of silica fibers and 
aluminoborosilicate fibers. This technology still depends on the use of 
very expensive, high purity fibers and teaches against the use of lower 
grade fibers or lower grade particulates. 
SUMMARY OF THE INVENTION 
In accordance with the invention, a thermally bonded fibrous product is 
provided from compositions comprising a sintered blend of aluminosilicate 
fibers, silica powder, rather than silica fiber, and boron nitride powder. 
The thermally bonded fibrous products of the invention advantageously 
provide inexpensive constructions of high thermal insulating value with 
improved strength at relatively low densities. 
In the product of the invention, the components of the product, on a weight 
basis, consist essentially of 75% aluminosilicate fiber, 20% silica powder 
(-325 mesh) and 5% boron nitride powder (-325 mesh) sintered by firing at 
the temperature of at least 2350.degree. F. (1288.degree. C.). The use of 
silica powder as a bond former, rather than silica fibers, is unique in 
that conventional thermal bonding theory teaches the use of silica fiber 
as a bonding agent.

DETAILED DESCRIPTION 
The invention comprises the construction or formation of thermally bonded 
fibrous products, comprising a sintered blend of primarily aluminosilicate 
fibers, silica powder and boron nitride powder, that are low in cost, and 
have high-strength, low-density and high thermal insulating value with 
improved machineability. Specifically, the products comprising the 
composition of this invention are particularly suited for use as diesel 
soot filters, kiln furniture, combustor liners and burner tubes. 
Suitable fibers for the practice of this invention consist essentially of 
approximately equal parts of alumina and silica such as KAOWOOL brand 
ceramic fiber marketed by the Babcock & Wilcox Company. The 
aluminosilicate fibers have average diameters of about 3 microns. 
The particulates employed in the formation of the products of the invention 
include -325 mesh silica powder and smaller amounts of -325 mesh boron 
nitride powder. Suitable silica powders include high purity quartz silica 
powder (-325 mesh), as is available from Atlantic Equipment Engineers, and 
high purity amorphous fumed silica (12 millimicron), marketed under the 
trademark AEROSIL 200 by Degussa. Boron nitride powder marketed as UCAR 
HCV by Union Carbide has been found to provide a suitable boron nitride. 
The following examples are illustrative and explanatory of portions of the 
invention and not restrictive thereof. 
A number of billets measuring 8.5 inches (25.4 cm) square and 1.5 inches 
(3.8 cm) thick were prepared from compositions of aluminosilicate fiber, 
silica powder and boron nitride powder was first added to deionized water. 
Starting materials included a first system having, by weight, 76.2% of 
aluminosilicate fiber, 19% silica powder and 4.8% boron nitride powder, 
and a second system having, by weight, 89.5% aluminosilicate fiber, 7.2% 
amphorous fumed silica powder and 3.3% boron nitride powder. The solution 
was mixed in a high shear Arde Barinco CJ-4 lab mixer for a sufficient 
time to disperse the solids within the solution. The aluminosilicate fiber 
was then added and the modified solution was further mixed for a 
sufficient time to disperse the fiber component in the solution. After 
dispersion, a small amount of Betz 1260 polymer was added to flocculate 
the solution. 
Billets were formed from the flocculated fiber/powder slurry in a 
casting/pressing tower. This tower had a top and bottom platen, each 
platen constructed such that a vacuum could be applied and liquid removed 
through the platen. After the slurry was added, it was allowed to 
free-drain. The resultant pad was then pressed by movement of the bottom 
platen to a desired thickness. During the processing operation, a vacuum 
was applied to the top and bottom platen. 
The pad was oven dried, at approximately 250.degree. F. (121.degree. C.) 
until thoroughly dry. The dried parts were then fired in an electric 
furnace, at a heat-up rate of about 400.degree. F./hr (205.degree. C./hr 
and held at the sintering temperature for 90 minutes. The aluminosilicate 
fiber-silica powder-boron nitride composition was then fired at 
2350.degree. F. (1287.degree. C.), the aluminosilicate-fumed silica-boron 
nitride was fired at 2500.degree. F. (1371.degree. C.). 
Room temperature modulus of rupture and density were determined in the 
as-fired condition, and after reheats to 1800.degree. F. (982.degree. C.), 
2100.degree. F. (1148.degree. C.) and 2400.degree. F. (1316.degree. C.). 
Table I reflects the values of these properties for sample billets formed 
from the first system composition while Table II reflects the values of 
these properties for sample billets formed of the second system 
composition. 
The billets displayed no shrinkage at reheat temperatues of 1800.degree. F. 
(982.degree. C.) and 2100.degree. F. (1149.degree. C.). At a reheat 
temperature of 2400.degree. F. (1316.degree. C.), sample billets of the 
first system composition experienced less than 7% volumetric shrinkage and 
sample billets of the second system composition experienced less than 2% 
volumetric shrinkage. 
Analysis has indicated that boron nitride is released upon firing, above 
the temperature of 1200.degree. F. (649.5.degree. C.) and fluxes the 
silica powder to form a fused bond at the fiber intersections, thereby 
producing an extremely strong shape. In addition, the boron nitride aids 
in the stabilization of the fused bond against crystallization. When 
blends in the desired portions, by weight, of approximately 75% 
aluminosilicate fiber, are formed, and fired, a very strong, lightweight 
insulation results. 
TABLE I 
______________________________________ 
-325 Mesh Silica Powder 
Reheat 
After 8 hr 
After 48 hr 
After 8 hr 
As @ 1800.degree. F. 
@ 2100.degree. F. 
@ 2400.degree. F. 
Fired (982.degree. C.) 
(1149.degree. C.) 
(1316.degree. C.) 
______________________________________ 
MOR, psi 860 805 820 1025 
(kg/cm.sup.-2) 
(60.4) (56.6) (57.6) (72.0) 
Density 31 31 29 31 
pcf (496.6) (496.6) (464.6) (496.6) 
(kg/m.sup.3) 
MOR/Density 
28 26 28 33 
(Psi/Pcf) 
Firing 
Temp. 
.degree.F. 2650 2650 2350 2350 
(.degree.C.) 
(1454) (1454) (1288) (1288) 
______________________________________ 
TABLE II 
______________________________________ 
Fumed Silica Powder 
Reheat 
After 48 hr. 
After 8 hr. 
As @ 2100.degree. F. 
@ 2400.degree. F. 
Fired (1149.degree. C.) 
(1316.degree. C.) 
______________________________________ 
MOR, psi 380 430 460 
(kg/cm.sup.2) 
(26.7) (30.2) (32.3) 
Density 25 27 27 
pcf (464.6) (432.5) (432.5) 
(kg/m.sup.3) 
MOR/Density 15 16 17 
(Psi/Pcf) 
Firing 
Temp. 
.degree.F. 2500 2500 2500 
(.degree.C.) (1371) (1371) (1371) 
______________________________________