Note: Descriptions are shown in the official language in which they were submitted.
;iZ~5~2
Cl~RAMIC ~()AM
BACKGROUND OF THE INVENrrION
The invention relates to a ceramic filter having an open cell
foam structure ~or filtering molten metal, wherein the ceramic
is substantially phosphate-free and the individual grains or
crystals of ceramic are sintered together without binder being
present between the crystals, and the Eilters are manufactured
such -that: an open cell organic polymeric Eoam is impregnated
with an aqueous thixotropic slurry of ceramic materials; khe
surplus sLurry removed; the coated polymeric foam dried and
heated, as a result of which the organic polymeric foam decom-
poses and volatilizes; the resultant ceramic foam ~ired at an
elevated temperature and thus sintered, a ceralnic with open
cell foam structure.
It is known in the state of the art to employ porous ceramic
~oam materials as a filter for molten metals, especially
aluminum, as described for example in U.S. Patents 3'893'917,
` ~ 3'947'363~ 3'962'081, 4'024'056, 4'024'212, 4'075'303,
4'265'659, 4'342'644 and 4'343'704. These filters are produced
primarily from phosphate-bonded refractory materials and vari-
ous other additionsj firing at temperatures around 2000F to
~ature che bond. Such a process is described in U.S. Patent
3'962'081. Refractory materials of that kind are suitable for
use in the aluminum industry and easily withstand most alumi-
num alloys, which are usually cast a-t about 1300F. They are,
however, not suitable for many other potential applications
because oE low strength and poor chemical clurability. It i5
therefore an object of the present invention to develop a
material which exhibi~s the desired properties of the known
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ceramic material, especially the high poroslty, low pressure
drop, large yeometrical surface area and tortuous flow path,
but which overcomes khe above disadvantages of inade~uate
mechanical strength and refractory properties and poor chemi-
S cal durabili.ty. In addition it would be de~ireable -to ~evelop
a special material which could be rela~.lvely ~J~nply produced
and ls suitable for a number o applications, especially or
use in high temperature applications and for use with ferrous
metals and especially in filtration applications.
.
SUMMARY OF THE INVENTION
The product according to the invention should be characterized
in particular by way of high strength and special chemical
properties i.e. compared with the ceramic foam materials known
. to date superior mechanical, refractory and chemical proper-
ties.
:The filters according to the invention are characterized in
that the ceramic is prepared from a slurry containing at least
8 wt.-% phosphate blnder and that the ceramic open cell~struc-
ture comprizes a substantially phosphate-free ceramic material
: 20 having individual grains of ceramic material which are sinter-
ed together to minimize the porosity between the grains, or
that the ceramic is prepared from a slurry containin~ 3 to 15
wt.-~ of a gelled aluminum hydroxide b:inder, and the ceramic
foam is sintered at elevated temperatures SQ that the result-
ant ceramic has an open cell foam structure held toge~her by
! solid state, sintered ceramic bonding.
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A preferred version is such that the filter can be of a ceram~
ic prepared from a slurry containing at least 3 wt.-% phos-
phate binder and the resul-tant ceramic with an open cell foam
structure is of a substantially phosphate free ceramic materi-
S al, the individual grains of t~hich are sintered together tominimize the porosity between the grains.
The ilter according to the invention is such that it is sub-
stantially phosphate~free and preferably contains less than 2
wt.-~ phosphate - expressed as P2Os - and the individual ce-
ramic grains are preferably sintered together in order that
the porosLty b~tween the grains is as small as possible and so
that microporosity is produced. This microporosity should be
less than about 5 ~.
The ceramics from which the filters can be prepared are pref-
era~ly produced using a slurry containing 8-30 wt.-%, prefera-
bly 10-25 wt.-~, of phosphate binder.
A prefered binder is aluminum orthophosphate. aOwever, other
binders can also be employed, such as for example phosphoric
acid, aluminum phosphate, akali metal phosphates such as sodi-
um hexametaphosphate etc. The slurry can contain at least
8 wt.-~, preferably more than 10 wt.-% of phosphate binder
with reference to the weight of solid ceramic material. The
details of amounts are based on aluminum orthophosphate; if
other phosphate binders are employed, their amounts must be
calculated on the basis of aluminum orthophosphate.
Another preferred version is such that the filters can ~e made
of a ceramic prepared from a slurry containing 3 to 15 wt.-%
of a gelled aluminum hydroxide binder, and the ceramic foam is
sintered at elevated temperatures so that the ceramic results
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as an open cell foam structure in which the cera~ic material
exhibits a solid state sintered ceramic bond. Such a filter is
substantially free of binder i.e. free of glassy phases which
lower the strength of the product. The structure also exhibits
a form of mechanical strengthening which can oe called dry
sintering (German = trockene Sinterung).
The ceramic of the filter was, preferably, prepared from a
slurry containing 3-15 wt.-~ gelled Boehmite.
Alumina, or aluminum oxide, is used by way of preference as
the ceramic material for the filter.
The quantities o~ binder employed always refer to the total
weight of dry material of the ceramic composition i.e. without
~aking into account the water added ln the preparation oE the
slurry.
The process Eor manufacturing the filter is such that a reti-
cula-ted organic polymer foam with open cell structure is im-
pregnated with an aqueous slurry having thixotropic properties
and containing the ceramic materials and binder, the excess
slurry removed from the then coated polymer foam which is sub-
sequently heated so that the organic polymer fvam decomposesand volatili2es, the resultant ceramic Eoam is fired at an
elevated temperature thus producing a ceramic foam with a re-
ticulated structure of ceramic material, the filter as a whole
having an open cell structure with a plurality of intercon-
nected voids surrounded by a network of ceramic material.
!
In accordance with the process of the present invention ahydrophobic, reticulated organic polymer foam, preferably
polyurethane foam, is employed.
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The slurry contains at least 3 wt.-~ phosphate binder and sin-
tering is performed such that a ceramic with an open cell
structure of substantially phosphate-free material is obtain-
ed, in which the individual grains of ceramic material are
sintered together to minimize the porosity between the grains,
or the slurry contains 3-15 wt.-% of a gelled aluminum hydro-
xide binder, and the ceramic foam is sintered in such a manner
at elevated temperatures that a ceramic with open cell foam
structure of ceramic material with solid state sintered cera-
mic bonding is obtained.
The slurry can contain at least 8 wt.-~, as a rule 8 to 30
wt.-~, usefulLy at least 10 wt.-~ and pre~erably 10 to 25
wt.-% phosphate binder.
The slurry can also contain 3 to 15 -wt.-% of a gelled aluminum
hydroxide binder, and the ceramic foam can be sintered at ele
vated temperatures so that a ceramic with an open cell foam
structure of ceramic material with solid state, sintered cera-
mic bonding is obtained.
According to a process of the presen~ invention a hydrophobic
reticulated organic polymer foam, preferably polyurethane foam
can be: impregnated with an aqueous slurry having thixotropic
properties and containing ceramic materials and at least
8 wt.-~, preferably more than 10 wt.-~ of phosphate ~inder;
dried and heated to remove the organic polymer foam; fired at
an ele~7ated -temperature to ~701atilize the phosphate, as
result of which a substantially phosphate-free material is ob-
tained; following which the refractory material is sintered to
produce a material in which the individual ceramic grains are
so densely fused together that the porosity between the grains
is kept as small as possible.
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According to the invention the ceramic with open cell foam
structure is produced starting from an open cell, preferably
hydrophobic, flexible organic foam. Suitable substances are
polymer foa~s such as, for example, polyurethane foams and
cellulose derivative ~oams.
~ny combustable and deformable organic foam which is resili-
ent, i.e. after deforming can return to its original shape~ is
normally suitable for that purpose. The foam can feature i-
100, preferably S0-75 pores per inch. The Eo~l~ must burn off
or volatilize at a temperature below the Eiring temperature of
the ceramic material which is employed.
The aqueous ceramic slurry employed is thixotropic and should
exhibit good fluidity and be comprized of an aqueous suspen-
sion of the ceramic intended for use in the material, the
fraction of said ceramic not being critical and amounting to
10 to 50 wt.-~. Typical ceramic materials which may be em-
ployed contain alumina, bauxite, zirconium dioxide, zirconia,
mullite and silicon carbide or mixtures thereof. ~he grain
size should correspond to 75 mesh and Einer, preferably 270 to
400 mesh and finer. Alumina is in fact the preferred refracto-
ry material, however any refractory material can be used which
i5 suitable for use ~ith a phosphate binder and has a sinter-
ing temperature that is higher than the temperature required
to drive off the phosphorus.
A piece of flexible organic foam is impregnated with aqueous
ceramic slurry so that the fiber-like webs are coated there-
with and the voids are filled therewith. NormallY, the foam is
simply i~mersed in the slurry for a short time which suffices
to ensure complete impregnation of the foamv if necessary
pressed together then allowed to expand again.
3L2~232
The impregnated foam is then compressed to expel sufficient
slurry to leave the fiber-like web portion of the organic foam
uniformly coated with slurry and with a plurality of blocked
pores throughout the body -to increase flow path tortuosity in
the final product. One may, for example in a continuous proc-
ess, pass the impregnated foam through one or more pre-set
pairs of rolls to effect the desired expulsion of slurry from
the foam and achieve the desired degree of impregnation. Nat-
urally, this expulsion may be done manually by simply squeez-
ing the flexible Eoam ma~erial to the desired extent. At thisstage the Eoam is still Elexib~e and may i~ desired be formed
into configurations suitable or specific ~lltration tasks
i.e. into curved plates, hollow cylinders etc. It is then nec-
essary to hold the formed foam in position by conventional
means until the organic substrate is decomposed, or preferably
until the ceramic is sintered. ~he impregnated foam is then
dried by any suitable means, such as air drying, accelerated
drying at a temperature from 210 to 1300F for from 15 minu-tes
to o hours, or by microwave drying. The air ~rying may be
achieved in from 8 to 24 hours. After drying, -the thus treated
eoam is burnt off and heated further until the ceramic coating
has sintered.
This firing should take place at a temperature of at least
3020F, preferably at least 3050F, and preferably at that
temperature for from 15 minutes to 10 hours in order to decom-
pose and drive off the web of flexible foam then drive off the
phosphate fraction and finally sinter the ceramic.
The resultant product is a porous, fused ceramic foam material
which is subskantially ~ree of phosphate i.e. has less than
2 wt.-~ of phosphate expressed as P2O5, and is characterized
- by superior mechanical, thermal and chemical properties than
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g
ceramic foam ma-terials heretofore known. The ceramic foam is
characterized by having an open cell structure with a plurali-
-ty of interconnected voids surrounded by a web o~ ceramic. In
that ceramic web the individual grains are tigh~ly coalesced
i.e. sintered together to minimize -the porosity between ~he
grains and to obtain a microporosity which should be less than
about 5 ~. Usefully the ceramic in form of an open cell struc-
~ure features a plurality of pore blockages, uniformly distri-
buted throughout the structure to increase the tortuosity of
the ~low path.
The aforesaid process according to the invention resulks in
the entire structure shrinking by up to 15 ~ per unit Length,
resulting in ceramic bodie~ which starting from polymer foams
with 5-lO0 pores per linear inch, exhibit 4,3 to 115 pores per
linear inch, reducing the macro or gross porosity of the en-
tire Eilter. In the case of a filter produced according to the
process of the invention the macroporosity is about as
without that said processing this value is 90 ~.
Further, the process according to the present invention can be
performed in such a manner that essentially a gelled aluminum
hydroxide binder is employed instead of the phosphate binder~
,~
The process according to the invention is performed essential-
ly as described above; the changes arising within the scope of
the invention are clear from the following descriptions
In accordance with the present invention the ceramic with open
cell foam structure is prepared from an open cell, preferably
hydrophobic flexible foam material having a plurality of in-
terconnected voids surrounded by a web of said flexiole foam
material. Typical material which may be used includes the
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polymeric foams such as polyure-thane foams and the cellulosic
foams. Generally, any combustible organic plastic foam may be
used which has resilience and ability to recover its original
shape. The foam must burn out or volatilize at below the fir-
ing temperature of the ceramic material which is employed.
The aqueous cerarnic slurry which is employed, should exhibit apseudoplasticity, should be thixotropic have a relatively
C~,mD~s~ ~
~; ' high degree of fluidity and be 0~4~ of an aqueous suspen-
~; sion of the ceramic intended for use in the material. Typical
ceramic materials which may be employed include preferably
alumina and bauxite and also others such as zirconium oxide,
zircona sand, chromium oxide, cordierite, mullite etc. in each
case with a grain size of 75 l~esh and less, and preferably
with a grain si2e of 270-~00 mesh and finer.
With this process it is not necessary to use a phosphate or
another inorganic binder.
~he aqueous sLurry contains a gelled aluminum hydroxide.
In accordance with the present invention aluminum hydroxides,
preferably boehmite, but also the other aluminum-hydroxide-
monohydrates and aluminum-hydroxide-trihydrates, is used as a
temporary binder and rheological agent. First a colloidal dis-
persion is made with an acid that substantially volatilizes
under firing temperature, preferably nitric acid but also hy-
drochloric, sulfuric or others. This is followed by the prepa-
ration of a thixotropic slurry with the desired ceramic mate-
rial, preferably alumina.
Generally, one uses from 3-l5 % alumina by weight based on dry
materials, and a hydrate:acid ratio of from 2:1 to 5:1, pref-
~.%~229~
erably about 3:t~ all based on 70 ~ acid. In the subsequentpreparation of the thixotropic slurry one can use small
amounts or additives i~ desired, for example, rheological
agents, supplemental binders, dispersants and the like. The
water content is not especially critical, for example 10-50 5
water based on total weigh-t can readily be used. The water
co~ponent simply obtains reasonable fluidity to impregnate the
~oarn and coat the fibers. An advantage of this procedure is
that no residual materials, such as phosphate that are poten-
tially reactive with molten metal, are left in the resultantproduct.
Thus, in accordance with the processing of the present inven-
tion one Eirst gels the aluminum hydroxide with the acid and
adds the ceramic components and water to Eorm the slurry. If
desired, one could admix all components together. The slurry
is then used to prepare the ceramic with the open cell foam
structure.
The flexible Eoam material is impregnated with the aqueous
ceramic slurry so that the fiber-like webs are coated there-
with and the voids filled therewith. Normally, it is preferredsimply to immerse the foam in the slurry Eor a short period of
time sufficient to ensure complete impregnation of the Eoam.
;: ~
The impregnated foam is then co~npressed to expel a portion of
the slurry while leaving the fiber-like web portion coated
therewith and a plurality of pore blockages throughout ~he
body to increase flow path tortuosity i.e. homogeneously dis-
tributed -throughout the ceramic body rather than grouped to-
gether. In a continuous operation one may pass -the impreynated
foam through a pre-set roller to effect the de~ired expulsion
of slurry from the foam and leave the desired arnount impreg-
1;2~i22~
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nated therein. Naturally, this may be done manually by simply
squeezing the flexible foam ~naterial to the desired extent~ At
this stage, the foam is still flexible and may if desired be
formed into configurations suitable for specific ~iltration
tasXs i.e. into curved plates, hollow cylindersr etc. It is
necessary to hold the formed foam in position by conventional
means until the organic substrate is decomposed, or preferably
until the ceramic is sintered. The impregnated foam is then
dried by any suitable means, such as air drying, accelerated
drying at a temperature of from 100 to 700~C for from 15 min-
utes to 6 hours, or by microwave drying. Air drying may be
achieved in from 8 to 24 hours. After drying, the material is
heated at an elevated temperature to sinter the ceramic coat-
ing on the Eiber-like webs leavlng the plurali-ty of ~locked
pores as described above.
In accordance with the present invention, the drying procedure
first yields crystallites of alumina, initially gamma alumina
eventually transformed to alpha alumina. This provides suffi-
cient green strength for handling and firing.
The actual firing conditions depend on the ceramic. Generally,
temperatures in excess of 2000F and preferably in excess of
2500~ for at least 15 minutes and generally at least I hour
at temperature and generally less than 10 hours in order to
volatilize the web of flexible foam and sinter the ceramic to
;~ 25 form the solid state sintered ceramic bond.
The resultant product is a porous, solid state ceramic foam
material which is substantially free of organic components,
and is characterized by mechanical, thermal and chemical prop-
erties superior to those of ceramic foam material heretofore
known. The ceramic foam is characterized by having an open
22~32
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cell structure with a plurality of interconnected voids sur-
rounded by a web of ceramic which, when dry and sintered is
absolutely free of poten-tially harmful binders or glassy or
clay phases. The ceramic with the open cell foam structure is
a solid state, sintered product especially useful for hi~h
temperature applications such as filtration of iron, ferrous
alloys and steel.
The aforesaid structure results in increasing desirable
physical properties, such as increased mechanical, thermal
and chemical properties. ~ny small amounts of organic
additives are driven of in the Eiring process. 'rhe small
amounts oE inor~anic additives that may be used iE desired
would not detract frorn properties, for example, less than 1-3
wt.-~ each of sintering agents, such as zinc oxide, grain
growth inhibitors, such as magnesium oxide or inorganic
rheological aids such as clays, montmorillonite, bentonite,
kaolin or organic rheological agents.
Various processes for preparing ceramic foams for filtering
molten metal are also described in U.S. Patents 3,962,0~1,
4,075,303 and 4,024,212.
The filters according to the invention, because of their
special mechanical, thermal and chemical properties, are
suitable for filtering molten metals such as iron, ferrous
alloys and steels.
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~XAMPLE 1
Process procedure using phosphate binder:
Various samples were prepared by impregnating a commercially
available, hydrophobic, reticulated polyurethane foam material
with a ceramic slurry conkaining as solids 98 ~ alumina and
2 % montmorilloni~e as a rheological aid. About 30 ~ aluminum
orthophosphate binder in a 50 ~ aqueous solution based on to-
tal weight was added, The samples were dried and fired at tem-
peratures of 2000~ 28~0F, 2980F, 3020F, 3050F and 3090F
for 5 hours. The samples were then subjected to chemical anal-
; ysis, compressive strength tests and chemical resistance
tests. The results are set forth hereinbelow.
The results clearly showed that samples fired at around 2000Fcontained substantially in excess of 2 wt.-% phosphate expres-
t5 sed as P20s, but samples fLred at 3020F and higher were sub-
stantially phosphate~free i.e. contained less than 2 wt.-%
phosphate expressed as P20s. In addition, in all cases the
samples fired at 3020P and above were characteri~ed oy having
individual grains of ceramic tightly fused together so that a
minimum in porosity between the grains was obtained; the re~
sultant microporosity amounded to less than 5 ~.
The following tables I and II show various properties obtained
after firing at the various temperatures; in each case the
thickness of the starting material was 2.01 inch (~.10 cm).
5Z~:9~
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TABLE I
Compressive strength, bul~ density and thickness of ceramic
~oams Eired at various temperatures.
Firing F2000 1 2870 ¦ 2980 ¦ 3020 ¦ 305~ j 3090
5 No. of samples I ¦
tested .8 ¦ 1 0 ¦ 9 ¦ 11 ¦ 1 0 ¦ 1 0
Compressive strength
kg/cm210.28 ¦11.01 1 9.70 ¦10.12 ¦14.30 ¦1~.52
psi*146 ¦ 157 ¦ 13~ ¦ 144 ¦ 203 ¦ 263
10 Apparent bulk density
gm/cm30-31 ¦ 0-36 1 0-33 1 0.37 1 0-37 1 0-39
lb/Et3**19.2 ¦ 22.3 ¦ 20.4 1 22.9 1 22.9 1 24.
Thickness
cm5-10 1 4-87 1 4-82 1 4-64 1 4-41 1 4-48
lS in***2-0~ 92 ¦ 1 90 ¦ 1-83 ¦ 1-74 j ~-77
* psi = pounds per square inch (pound per &.45 cm2)
** lb = pound; ft = foot - 30.5 cm
: *** in = inch = 2.54 cm
:
The strength data shown in table I, above, show a strong in-
crease at temperatures above 3050~F while acceptable strength
is obtained at 3020F. The data al~so sho~ that the apparent
density increases and the thicknes~s decreases indicating
shrinkage which in turn is related to the loss of poroslty in
the walls i.e. microporosity.
l~iZ292
~ 16 -
TABLE II
Chemical resistance of ceramic foams fired at various tempera-
tures.
Firing temperature ~eight Loss (~) in:
F C10%NaOH 10~HNO3 10~H2SO~ 10~HCL 10'~CH3COOH
2000 1093Samples disin-tegrated in the solutions
2870 1577 13.4 12.5 12.6 10.5 1.5
2980 1635 10.4 10.5 10.6 10.6 1.4
3020 1660 8.8 10.1 8.1 9.2 1.6
103050 1676 3.6 4.9 4~4 5.5 1.1
3090 1699 ~.7 2.0 2.~ 1.4 0.9
-
. .
The chemical resistance of the ceramic was determined oy im-
mersing the pre-weighed filter samples in the specified solu-
tions for 5 1/2 days. The samples were then removed from the
solutions, rinsed, dried and reweighed. The 2000F samples
disintegrated or softened to the point that they could not be
removed from the solutions intact, so no weight loss aata was
recorded. Weight losses for the other samples are shownO There
appears to be a pronounced change at 3020F and even more so
at 3050~F.
Thus, as can be seen ero~ the foreyoing, a ceramic roam mate-
rial having superior strength properties and chemical durabil-
ity is obtained.
~zs~z~
EXAMPLE 2
Process procedure using aluminum hydroxide blnder:
A thixotropic ceramic slurry was prepared by first preparing
a gel consisting of the following:
1853 grams of boehmite
476 milliliters of concentrated nitric acid
9060 milliliters of water.
Three portions of this gel were added to a dry powder blend of:
74 kilograms oE alumina
79 grams of magnesia
~n additional 300 milliliters o;E water was added to this mi~ture
and the entire batch was thoroughly mixed in a hi~h speed
intensive mixer.
The aforesaid thixotropic ceramic slurry was used to impregnate
nominal 20 pore per inch, open cell, flexible polyurethane foam
blocks to a green density of about 10% of theoretical so that
the fiber-like webs of the foam were coated therewith and the
voids filled therewith. Impregnation was accomplished by
immersing the foam in the slurry and using preset rolls to
compress the foam and expel a portion of the slurry while
leaving the fiber-like web portion coated therewith and with
- a plurality of blocked pores throughout the body to increase
flow path tortuosity.
The resultant impregnated foams were dried and heated to re-
move the organic component therefrom and fired to produce a
~, '
- 18 -
solid state, sintered ceramic bond by heating to approxiLnately
3000E f~r 1 hour. ~he resultant product is a porous, Eused
ceramic foam material substantially free of organic components
and characterized by excellent mechanical, thermal and chemi-
S cal pro2ertiesO The linear ~iring shrinkage was about 15 ~such that the resulting ~ired ceramic Eoam had a fired nominal
pore size of 23 pores per inch and a Eired density o~ about
15 ~ o~ theoretical.
EXAMPLE 3
; 10 Ceramic Eoams prepared in accordance with Example 1 and naving
fired di~ensions of 2" x 2" x 1" were used in an investment
casting operation to filter high quality stainless steel cast-
ings. The ceramic filters were placed directly in the mold
cavity and were under poured. The resulting castings were su~-
1S stantially free of inclusion related defects.
;
E~AMPLE 4
Ceramic foams prepared in accordance with Example 1 and having
fired dimensions of 4" x 4" x 1" were used to filter stainless
steel fan housings. In this case a top pour arrangement was
employed. ~igh quality castings were obtained. Weld repair,
normally required ~or such castings and which is generally due
to inclusions, was substantially eliminated.
~25i2;2~;~
, g
EXAMPLE 5
Ceramic foam prepared in accordance with Example 1 and having
flred dimensions of ~" x 4" x 1" were used to filter a high
tempera-kure nickel-aluminum-bronze alloy -test casting. A nigh
quality casting ~as obtained. This casting would normally re
~uixe many hour!3 o~ weld repair; however, ~he resul.tant c~st
ing o~ this example reduced the weld work substantially.
EX~MPLE 6
A thixotropic slurry was prepared as in Example 1 and was used
to impregnate a nominal 10 pore per inch, open ceLl, flexible
polyuret'nane foam block as in Example 1. The resultant impreg~
nated foam was microwave dried and t`nen ~ired to 3000F to
produce a solid state, sintered ceramic bond and a porous,
fused ceramic foam material substantially free of organlc com-
ponents characteri2ed oy excellent mechanical, thermal andchemical properties. The fired ceramic foam, measuring 4" x
~" x t" with a fired pore size of about 12 pores per inch, was
used to filter a low alloy steel casting. Inclusion related
defects were substantially eliminated.
~hus as can be seen from the foregoing, a ceramic foam mate-
rial having superior strength properties and chemical durabil-
ity is obtained.
