CA1082894A - Method of preparation of ceramic foam - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Ceramic foams possessing controlled permeability and uniformity are prepared by a combined rolling operation of impregnating an open-celled organic polymer foam material possessing a predetermined permeability and resilience with a fluid aqueous slurry of a thixotropic ceramic composition and immediate passage through at least two controlled roll gaps, wherein the rolls are preset to effect a temporary compression ranging from about 50 to 90% for the first pass and 70 to 90% for the second pass.
The resulting material is thereby uniformly impregnated with the required amount of slurry, and is then dried and heated to remove the organic foam component. The foams prepared in accordance with the present invention are useful as filters for molten metals, especially aluminum and its alloys.

Description

108Z894 ,r BACKGROUND OF THE INVENTION

The present invention relates generally to ceramic foams whlch are highly advantageous ror the flltratlon Or molten metals such as molten alumlnum, and partldularly to efflclent and economlcal methods for thelr preparatlon.
Porous ceramlc foams obtalned by treatlng lmpregnated open-celled organlc sponges or foams have been dlsclosed ln the prior art, as represented by U.S. Patent No.
3~090,094 to Schwartzwalder et al., U.S. Patent No.
3,097,930 to Holland, and U.S. Patent 3,111,396 to Ball. -~
Furthermore, the u~e of such ceramlc foams as fllter~
for molten metal, and partlcularly for the flltratlon of molten alumlnum and copper, ha6 been dlsclosed ln U.S.
Patent 3,893,917 lssued July 8, 1975 for "Molten Metal Fllter" and ln U.S. Patent 3,947,363 lssued March 10, 1976, by Mlchael J. Pryor et al.
As noted above, the prlor art suggests the preparatlon of ceramlc foam materlals. Thu~, U.S. Patent No. 3,111,396 to Ball sugge~ts that an organlc polymer foam after lmmerslon lnto a suspenslon of refractory materlal may subsequently be compressed by passage through rolls to effect the removal of excess refractory.
Thls technlque, whlch 18 comparable to a wlde varlety of conventlonal expulslon technlques used ln the art surfers from an lnherent disadYantage ln that the ~lurry 19 not uniformly dlstrlbuted through the bod~ of .- i - ,.
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108~894 ~.
the article. Thus, the outer area of the article tends to be more thinly coated with slurry than that near the center line. Such defects are particularly evident at l:he extremes of the permeability range found suitable for ~, use in the preparation of filters for molten metal, thus, bodies of high permeability tend to have undesirably weak surfaces and edges, while bodies of relatively low perme-ability tend to exhibit undesirable center line blockage.
Such defects render the resulting foams unsuitable for use in the filtration of molten metal.
The above difficulties were overcome by the process disclosed in our U.S. Patent 4,024,212, issued May 17, 1977, referred to above. This required an initial step of impregnation of the polymer foam with the slurry of ceramic composition, wherein the foam was subjected to repeated compression and recovery while completely immersed in the vigorously stirred or vibrated slurry. The impreg-nated foam was then removed from the impregnating tank and treated to remove excess slurry by passing it through two sets of rolls to effect twice-repeated compression and recovery, the roll gaps being preset to accomplish first a compression of 50-90o/o and then a compression of 70-90%.
However, such process was disadvantageous in requiring an excessive amount of handling of the polymeric foam slabs, which could deleteriously affect the distribution of the contained slurry and also the strength properties of the completed ceramic foam body. Also, the procedural steps could not readily be automated or be adapted for use in a continuous or semi-continuous mass production line.

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108289~ ~
Accordlngly, it is a prlncipal ob~ect of the present lnvention to provlde an efficlent and economical method for the preparatlon of ceramlc foam articles havlng predetermined pelrmeabllity propertles, wherein the operations of impreg-natlon of polymerlc foam wlth a slurry of ceramlc compositlon arld the removal therefrom of excess ~lurry are combined.
It 18 a further obJect of the present lnventlon to pro-vlde an efrlcient and economlcal method as aforesald which ylelds products possesslng lmproved strength propertles.
It 18 yet a further ob~ect of the present lnventlon to provide a readlly automated method for the production of ceramlc foam artlcles as aforesald whlch exhlblt structural unl~ormlty and freedom from defects such as center llne blockage and outer surrace weakness.
It 18 a stlll further ob~ect of the present lnventlon to provlde a method as arore~ald, whlch lends ltself to rapld commerclal-scale productlon technlques through the provlslon of a unltary operatlon whereln the step~ of slurry lmpregnatlon and of exce~s 31urry removal are effected ln rapld successlon.
Other ob~ects and advantages wlll be apparent from a careful revlew of the ensulng description.

SUMMARY OF THE INVENTION
In accordance wlth the present lnventlon, ceramlc foam artlcles of controlled permeablllty and structural uniformlty are prepared by a process comprlsing provldlng an open-celled organic polymer foam materlal po~ses~lng a prede~ermined permeabillty and resllience, and ln a unltary operatlon ln whlch the steps are erfected ln rapld successlon, lmpregnatlng 1082~94 said polymer foam material with an aqueous slurry of a thixotropic ceramic composition which is maintained fluid to facilitate impregnation, and expelling excess slurry from said foam material by conducting at least two passes of said material through preset rolls to effect a temporary compression of about 50 to 90% for the first pass and 70 to 90/0 for the second pass. The resulting uniformly impregnated foam structure is then dried to volatilize moisture and heated to remove the organic foam component.
The resulting ceramic foam article is ready for use or may be further heated to sinter the ceramic material.
According to a broad aspect of the present invention there is provided a method of preparing a ceramic foam possessing controll~d permeability and structural uniformity. The method comprises the steps of providing a hydrophobic, reticulated organic polymer foam having a predetermined permeability and resilience. An aqueous slurry of thixotropic ceramic composition having a viscosity of 1 x 103 to 80 x 103cps is prepared. At least three rolls are mounted to furnish at least two gaps in close proximity between pairs of the rolls. The first gap is preset at a spacing of 50 to 90% of the thickness of the polymer foam and the second gap is spaced at 70 to 90% of the thick-ness. The polymer foam is advanced through the roll gaps while flowing the slurry over at least one surface of the polymer foam and into the first roll gap, thereby uniformly impregnating the polymer foam with the slurry.
According to a further broad aspect there is provided a step of drying and heating the impregnated form to remove the organic component therefrom.

10828~ -BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a preferred embodiment of the invention in schematic form, representing the production in a rolling operation of a uniformly impregnated polymeric foam sheet or slab by the application thereto of an aqueous suspension of ceramic composition just prior to passage through a triple roll assembly, by which the impregnated sheet is, in rapid succession, compressed, allowed to recover, and again compressed before recovery.
Figure ~ shows in schematic form a modification wherein the roll assembly shown in Figure 1 is replaced by two double roll assemblies for providing the compression and recovery stages.
DETAILED DESCRIPTION
In accordance with the present invention, the foregoing objects and advantages are readily obtained.
Ceramic foams possessing controlled permeability and structural uniformity are prepared by a process comprising providing an open-celled organic polymer foam material possessing a predetermined permeability and resilience, impregnating said polymer material with a fluid aqueous slurry of a thixotropic ceramic composition flowed thereon just prior to passage through a pair of roll gaps, preset to effect temporary successive compressions, amount-ing to about 50 to 90% of the thickness of said material for the first pass, and 70 to 90% of said thickness for the second pass. Following the combined impregnation and compression steps, the resulting uniformly impregnated foam material is heated to remove moisture and then the organic foam component. The resulting article is then ready for use, or may, if desired, be further heated to sinter the ceramic material.

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108Z8~4 The preparation of the ceramic foam articles of the present invention is dependent upon the observance of certain critical properties and parameters, as described in our U.S. Patent 3,962,081. Thus, the provision of the open-celled organic polymer foam material must be made with a view to control of the permeability and resilience of the final ceramic article, which may advantageously possess an air permeability in the range of 400 to 8,000 x 10 7 cm2 for use as filters for molten aluminum. The determination of air permeability is derived from the text Micromeretics by J. M. Dallavalle, published by Pitman, 1948, at page 263.
In accordance with the present invention, the air permeability of the resulting ceramic articles has been found to depend upon the permeability of the organic polymer foam employed in its preparation. For example, ceramic foams having - ` - 5a -.,~

~082894 permeabillties ln the range of from about 800 to about

2,200 x 10-7 cm2 have been prepared from polyurethane foam materlals havlng air permeabilitie~ ranging from 4,500 to 5,400 x 10-7 cm2. Further, the selection of raw foam permeabllity ln the range o~ i2% racllltates the preparation of a ceramlc foam havlng a permeabllity predetermined to wlthln a range of ~5%.
In addltlon to the control of permeability, the foams of the present lnventlon must possess structural uniformity and a particular range of cell size. Structural unlformlty has been ~ound to relate to the resllience of the organlc polymer foam precursor. Particularly, resilience may be determlned wlth reference to certain standards set forth ln the ASTM-D-1564-71 which refers to the propertles of compresslon set and reillience as measured by ball rebound.
Compresslon set, determined by the compression load deflec-tlon test, measures the extent to whlch the foam returns to lts orl~lnal slze or thickne~s after compresslon to a stated reduction such as, for example, 50%. Foams found sultable in accordance wlth the invention exhiblt a compres-slon set of less than 30~ at 50% compresslon, and thu~
return to at least 70% of their orlginal thlckne~q after compresslon ls released. Re~ilience, determlned by the ball rebound test, measures the resl3tance that the materlal exhlbits to compression by the helght of rebound of a steel ball dropped from a stated dlstance onto a foam ~ample. The percentage of return of the ball to the original height ls noted, and foams sultable in the present lnvention have been found to be those possesslng ball rebounds of greater than 25%.

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The above propertles, usually measured in tests run under dry condltlons, must be ~ubstantially retalned in an aqueous envlronment a~, for example, durlng lmpregnation wlth and unlform dlstrlbutlon of the aqueous ceramlc slurry in accordance wlth the present lnventlon. Accordlngly, it has been round that hydrophoblc foams perform better and are pre~erred to hydrophlllc foams, as the latter su~fer conslderable 1088 Or reslllence ln aqueous environments.
Thls loss Or reslllence 18 evldent ln the occ~rence of the aforenoted defect of center llne blockage.
Wlth the above-noted crlterla ln mlnd, organlc polymer foam materlals whlch may be employed in the present inventlon lnclude a wide varlety o~ hlghly reslllent, reticulated hydro-phoblc materlals such as the polyester and polyether polyurethanes, Ruch as "hlgh resillence" or "cold cure"
urethane materlals whlch utlllze polymerlc lsocyanates in thelr formulatlon; polyvlnyl foams such as polyvlnyl chlorlde, polyvlnyl acetate, and polyvlnyl copolymers;
polyurethanes coated wlth polyethylene or polyslloxane polymers and copolymer~; and foams prepared from sultable reslns such as celluloslc derivative~. The foams must burn out or volatllize at below the firlng temperature o~
the ceramlc materlal wlth which they are impre~n~ted.
As noted earller, the dlmenslons Or the foam should corre-spond roughly to the dlmenslon~ of the desired ceramlc artlcle. Thus, for example, a foam materlal havlng a thlckne~s ranglng from about lf 2" to 4" is employed when the resultlng c~ramlc foam 1R to functlon as molten metal fllter.

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In addition to the propertles of permeabillty and unlformlty, the above noted polymerlc materlals must pos~ess a pore slze wlthin deflned llmlts ln order to render them el~rectlve in the preparatlon Or molten metal fllters. Pore or cell slze has been found to be important to the ~tructural unlformlty of the ceramlc ~oam and should preferably lle wlthln the range of 5 ppl (pores per linear inch) and 50 ppl .
The control of the above noted varlables contrlbutes to the structural unlformlty and permeability of the resultlng filter and directly arfects metal ~low rate and efrectlvene~s through the tortuoslty of the flow path.
Though these ractors are slgnlficant, additlonal factors wlll be dlscussed herelnbelow which comblne to provlde rurther control of the final ceramlc foam artlcle.
The organlc foam selected with reference to the above dlscusslon 18 then lmpregnated wlth a slurry of a thlxotroplc ceramlc materlal. The property of thlxotropy is lmportant to the present lnventlon as it afrects the unlformlty of structure and strength of the ~lnal ceramic foam article.
Thixotropic materlals are those which display a high resl~tance to flow under low rates of shear and correspond-ingly, a low reslstance to ~low under relatively high rates o~ shear. As thls relates to the method of the present lnventlon, the ceramic ~lurry must have sufficlent fluidlty durln~ condltlons of flow to enter and fill ~he void~ of the organic foam material rapidly and thereby coat the surrounding polymer web9 and at the same tlme be capable of quickly regalnlng sufficient viscosl~y under stag~ant conditlons to resist running out or draining from the foam CON-69~

once lmpregnatlon 18 complete. It has been ~ound ln aLccordance wlth the present invention that certaln ceramic materlals prepared ln comblnatlon wlth partlcular alr elettlng agents and temporary blnders display the deslred thlxotroplc character to successfully conduct lmpregnatlon and the ~ubsequent unl~orm dlstrlbutlon throughout the polymerlc foam.
As the ceramic ~lurry which is employed hereln may vary accordlng to the end use of the foam, a wlde varlety o~ ceramic materials o~ varylng refractoriness may be employed. Partlcularly, such materlals as alumlna, chromla, zlrconia, magnesla, tltanla, silica and mixtures thereof may be present. Such material~ are noted for their relatlvely hlgh rerractorlness or ablllty to serve in hlgh temperature sltuatlons. However, other materlals of lesser refractorlness such as mulllte, calclned clay and varlous glasses Or hlgh softenlng temperature may be employed hereln either alone or in comblnatlon wlth each other and wlth more r~rractory materlals to prepare the resultlng roam article. Insorar as the utlllty Or the resultlng artlcle as a molten metal ~llter ls concerned~ the only requirement placed on a selectlon of the partlcular ceramlc materlals ls that they provlde the artlcle wlth sufflclent reslstance to the chemlcal attack of the molten alloys over the exposure tlmes lnvolved ln filtratlon. A partlcular composition which has been quccessfully employed hereln comprise~ a mlxture o~ alumina and chromla.
The above composltlon also includes a room temperature blnder or alr settlng agent whlch provides green strength to the ~lurry, particularly during the bake out and the CoN-69-M

optlonal slntering operatlons where the foam is sub~ected to thermal stress. A wlde varlety of alr settin~ or binder agents are known ln the art which would be sultable in thls regard. Thus, for example, the composltlon Or the pre~ent lnllentlon may employ such materials as colloldal alumlnum orthophosphate, alkall metal slllcates such as ~odlum and potasslum sillcate, ethyl silicate, alumlnum hydroxychlorlde, magneslum orthoborate, and the llke.
The blnder or air setting agent is generally employed ln a 50% aqueous ~olutlon whlch may be employed in the range Or from 5 to about 50% of the total slurry. Preferably, the blnder solutlon ls employed ln a range of from 25 to 35%
of the slurry.
In addltlon to the blnder noted above, certaln agents hereln referred to as rheologlcal agents are employed which serve to promote the deslred thlxotropic property Or the slurry. Several materials are known whlch may serve as rheologlcal agents, among them certaln organlc materlals such as carboxymethyl cellulose and hydroxyethyl cellulose, and certaln lnorganlc materlals such as bentonlte and kaolin.
Of the materlals avallable ln this regard, bentonite has been ~ound to be partlcularly preferred. Bentonlte is a naturally occurrlng clay composed primarlly Or aluminum and varlous ~lllcates, usually lncludlng quantities of magneslum and lron. In additlon to lts promotion Or the thlxotroplc propertles of the slurry, bentonlte performs a small settlng or blnding function, as certaln glassy phases are produced upon firing of the artlcle whlch yleld lncreased strength ln the final foam structure. In addition to bentonite, a small amount of kaolin may also be employed CoN-69-M
~08Z894 whlch provldes both binding and rheologlcal lmprovement to the final slurry in the same manner as bentonite.
~aolln is a clay composed primarily of alumlna and silica.
~Taturally, one could employ the chemlcal equlvalents of the aforenoted materlals to approxlmate their compositions.
The general range of addltion of the rheological agents of the present lnventlon 18 wlthin about 0.1 to about 12% by weight of the slurry. In a preferred embodiment, the rheo-logical agents are added in an amount ranging from about 0.5-2% by welght.
Though, as indicated above, the thixotropic ceramic material may be prepared ln a wide variety of formulatlons, a particular compositlon has been determined to be eminently sultable which comprises alumlna ln an amount ranglng from about 40-80~, and preferably from about 45-50%, chromla ln an amount ranging up to about 20~, and preferably from about 10-15%, kaolin ln an amount ranglng up to about 10%, and preferably from about 2-5%, bentonlte ln an amount ranglng from about 0.1-10%, and preferably from about 0.5-2%, colloldal alumlnum orthophosphate (50~ solutlon) ln an amount of from about 5-50%, and preferably from about 25-35%~
Addltlonal water may be added to the above formulatlon ln amounts ranglng up to about 20~, and preferably from about 5-10% for the purpose of ad~ustlng vlscosity, dlscussed ln detall hereinbelow. Though the foregolng formulatlon ls suggested ln lt~ preferred range~, lt ls to be under~tood that the invention 1~ not llmlted thereto, as other formulatlons may be prepared from the lngredients reclted earller.

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In addit~on to lts thlxotroplc propertles, the ceramlc slurry of the pre~ent invention must possess a care~ully controlled vlscoslty durlng the tlme of lmpregnatlon, ln order to enable the achlevement Or a reproducibly unirorm ceramlc artlcle. The desired range of vlscoslty has been found to be from l x 103 to 80 x 103 cp~ (centlpolse), and prePerably wlthin the range of lO x 103 to 40 x 103 cps.
Vlscoslty ls regulated durlng the formulatlon of the slurry and must be wlthln the above ranges at the time the organic polymer foam ls lmprengated. As noted above, a convenlent way of regulatlng and thereby controlling viscoslty ls through the varlatlon ln exce~s water content withln the speclried ranges. For the purposes of the present lnventlon, vlsco~lty is measured at 25C with a #6 splndle, Brookfleld RVT Vlscometer at 20 rpm arter 15 minutes rotatlon, the slurry havin~ prevlously been mlxed ln an 80-quart Hobart Mlxer at 60 rpm for 30 minutes.
Rererrlng to Flgure l Or the drawlngs, slabs l of retlculated polyurethane foam havlng pore sizes between 5 and 50 ppl, preferably between 25 and 35 ppl, and deslred slze, ~uch as any convenlent length, a width of 4 to 36 inches, and thlckness of 1/4 to 4 lnches, for example, 17 inches wide and 2 inches thick, are passed successlvely through a V2 inch gap between rolls 2 and 3 and then through the closely adJacent gap of 3/16 to 3/8 inch between rolls 2 and 4. The rolls have a grit surface, may be of convenient size, such as 3 inches in -diameter and sc¢ewhat longer than the slab width, and are mounted in a houslng at each end, supported ln base 5, th~e ~haft ends belng geared together to rotate at the same speed, for example at 12.5 rpm. The leading edge of ~he slab may be lmmersed - 12 _ CoN-69-M
108~8~4 in the slurry before enterlng the flr~t roll gap, and during the passage through the roll stand, slurry 6 18 caused to flow from contalner 7 over the surface of roll 2 and slab 1 at a rate such as to malntaln a pool or supply B of slurry at the gap entrance. On leaving the first gap, slab 1 qulckly tends to recover its orlginal thickness because Or lts reslllence, and then 1~ lmmediately compressed while traverslng the second gap, rollowlng whlch lt recovers lts orl~lnal thlckness, and is dlscharged to conveyor ~.
The serles of alternate compressions and recoverles of the slab serve to maintaln the slurry in fluid condition and thus facllltate the uniform distribution of slurry in the slab. Subsequently, the retained slurry is ln a qulescent state and lts vlscoslty under~oes a rapid rise because of its thixotropic propertles, belng held firmly wlthln the slab durlng handllng and whlle belng drled. The foregolng ~equence may be repeated once or twice to lnsure the desired unlrormity in the dlstrlbutlon of slurry retalned ln the ~lab, preferably after turnlng the slab 180 end over end.
Alternatlvely, the lmpregnated slab emerglng from the roll stand may be passed through one or more similarly arranged roll stands. A drlp tank 10 1~ provlded below the roll stand to collect exces~ ~lurry and drlppings from the slab :J
edges and the rolls, which may be returned steadlly or intermittently to the maln slurry reservoir, not shown, where it i~ malntained fluld and ready for use by adequate stirrlng or vlbratlon.
Among variations in the apparatus as deccribed above which may be advantageous at times, rolls 2 ancl 3 may be - ¦

3~ mounted side ~y side wlth their shaft centers in the same CoN-69-M

substantlally horizontal plane, so that a pool of slurry 1~ maintained ln contact wlth both main surfaces of the slab at the gap entrance, wlth roll 4 retalned in positlon be!low the outer two rolls and approxlmately half way b~tween them, so that the slab ls not unduly dlstorted durlng its passage between the gaps.
The embodiment shown ln Flgure 2 provides a pair of two-hlgh roll stands which may be used to replace the triple roll stand ln the apparatus assembly shown ln Flgure 1. The rirst and second gaps are provided, respec-tively, between roll~ _ and 12 of the first ~tand and between rolls 13 and 14 of the second stand, functionlng ln the same manner as descrlbed above with re3pect to the embodlment of Flgure 1.
The unlformly lmpregnated slabs are tran~ferred from conveyor 9 and may then be drled and, i~ deslredJ flred to provlde a fused ceramlc foam article. The drylng and heatlng sequence 18 employed for the prlmary purpose of removlng water and then the organic polymer foam from the artlcle. Generally, conventlonal drylng technlque3 mPy be employed, but it ~hould be kept in mlnd that a sultable heatlng rate for the removal of the foam ~hould take into account the heat provlded by the oxldatlon of the foam ltself. The effect oP thls phenomenon ls partlcularly notlceable ln the heating of large masses Or the foam where a s~gnlrlcant volume of the heatlng chamber may be occupled by the artlcles. In ~uch caseR, it may be necessary to malntain the material at a temperature ranglng from 400-700F to avoid excesslve heat up resultlng from chemlc~l reaction which may cause the ceramic ~ilaments to rupture -- l Ll -. . .

under thermal stress. The exact temperature will be dictated by the particular organic foam base used.
As lndlcated above, the ceramic foam may, if desired, be rurther heat treated or flred to fuse the ceramlc partlcles lnto a hlghly rerractory structure. As noted earller, thls practlce ls optlonal, as, for example, in the employment of the foam artlcles of the present invention as ~llters ror molten aluminum, lt has been found that the foam materlal need only be heat treated at a temperature of ~rom l,OOO to l,100F to remove the organic component.
The re~ulting article would be sultable as such for use wlth molten alumlnum alloy at temperatures as high as 1,400F. In ~uch use, the air setting or blnding agent would provlde the necessary strength to the article, and the rull slnterlng treatment would not be requlred.
Utlllzlng the method dlsclosed above, ceramlc foams may be prepared whlch range ln thlckness from 1/4" to 4"
and may be Or an area ranglng up to about several square feet. The foams would pos3ess, based on the raw foam employed, pore counts of from about 5 to 50 ppi, wlth permeabllltles ranglng from about 100 x 10 7 cm2 to 10,000 x 10-7 cm2 and bulk densltles of from 0.2 gm/cm3 to 1 gm/cm3.
In the lnstance ~here the foim artlcles of the present lnventlon are utlllzed as ~ilters for molten metal, alr permeabilltles may range from about 400 to 8,000 ~ 10-7 cm2 and pore counts may range ~rom about 5 to 45 ppl.
Naturally, as noted earller, both permeabllities and pore slzes may be varled to suit the particular mode of end use o~ the article. Thus, for example, a relatlvely ---108Z~394 fine filter may be prepared which would possess an air permeability of from 400 to 2,500 x 10 7 cm2 and a pore count of from 20 to 45 ppi. Such an article would be u~eful in the filtration of aluminum alloys of the 5,000 series. ~owever, if the input metal is particularly dirty, one should preliminarily filter the metal through a relatively coarse ceramic foam filter having a pore size of between 5 and 20 ppi, and an air permeability ranging from 2,500 to 8,000 x 10 7 cm2. This may be accomplished by providing a single filter with a gradation of properties or by using a series of filters of differing porosity.
In accordance with the present invention, the advantages and features of the above method will be more -readily understandable from a consideration of the following comparative examples, Example I being representative of the process of U.S. Patent No. 4,024,212 issued May 17, 1977 and Example II in accordance with the present invention.
EXAMPLE I
A polyester polyurethane foam material was provided having a thickness of 2", containing 30 pores per linear inch and having an air permeability of 4,697 x 10 7 cm . An aqueous ceramic slurry containing 47% alumina, 13% chromia, 3.5% kaolin, 1% bentonite, and 29% aluminum orthophosphate (50% aqueous solution) was mixed in an 80-quart Hobart mixer at 60 rpm for 1 hour. After 1/2 hour of mixing a sample was removed for viscosity testing. The sample showed that the slurry had a viscosity of 32.0 x 103 cps at 25C measured with a #6 spindle, Brookfield RVT Viscometer at 20 rpm after 15 minutes testing. A

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~lab Or the ~oam materlal was immersed in the ~lurry and repeatedly compressed and expanded with a plunger devlce l'or about 30 3econd~ whlle the bath of slurry was vlbrated alt 60 cycle3 per second in order to flll the volds wlth l31urry. The polymerlc roam slab thus lmpregnated was taken from the slurry and pas~ed through grlt-coated rolls ln a triple roll stand preset to provide 75% and 87.5%
reductlon~ ln thickness to expel the excess slurry. The roll~ were of 3 lnch dlameter and were drlven at a speed Or 12.5 rpm. The artlcle exhlblted substantlally complete sprlng-back after rolllng was completed.
The slab was then drled in an oven at 150F for 1 hour and at 200F for 2 hours, and was then heated from 200F to 500F at 100F/hr., and held at 500F for 8 hours, ln order to remove the polyurethane flbers wlthout collap~lng the ceramlc web. The resultlng materlal was then fired ln a kiln uslng a heating rate Or 170F/hr.
to l,800F, followed by a furnace cool.
The fired slab proved to be sound and the surface wa~
reslstant to spalllng. Its permeabllity was measured as 1554 x 10-7 cm2 and its bulk denslty as 0.39 gm/cm3, and the product displayed an average modulus of rupture of 18.3 psl.

EXAMPLE II
Thls example employed ~labs of the same lot Or polyester polyurethane materlal and a 31urry, havlng vlsco~lty measured as 30.5 x 103 cps, of the same composltion as used ln Example I, which were treated in accordance with the combined impregnation-rolllng method of thls lnventlon, as shown in Figure 1, uslng a trlple roll stand of 3 inch diameter rolls rotated at 12.5 rpm.
Thle polymerlc foam slab was advanced lnto the flrst roll ~ap, preset to provlde a 75% reductlon ln thlckness, through a ~upply Or slurry flowing thereto at a rate such a~ to malntain a pool Or slurry, contacting the slab and roll surraces at the gap entrance. As the slab advanced beyond the gap, lt recovered ln thickness and then lmmedlately entered the second gap, preset to accompllsh an 87.5%
reductlon ln thickness before recovery to the orlglnal thicknes~. The emerged slab was then turned end over end through 1~0 and the above lmpregnation rolling process was repeated, uslng the above procedure, resulting ln a unlrormly lmpregnated slab whlch exhiblted complete recovery of the orlglnal thlckness and the retentlon of greater reslllence throughout the process than was evldent ln Example I. The slabs were then drled and heated as set forth ln Example I, the product belng gound and havlng L
a surface reslstant to spalllng. The permeablllty was measured as 1456 x 10-7 cm2, the bulk denslty as o.38 gm/cm3, and average modulus of rupture as 21.1 p91.
Thus, the method of thls example elimlnates procedural step~ essentlal ln the prevlous method and results ln a produc~ of improved strength propertles. Thl~ result ls probably accomplished because of the decreased dlstortlon errects, lnternal abrasion of polymeric fllaments and webs, and~or ~tructural deterioratlon whlch may otherwlse occur during the exce3slve handllng steps prevlously consldered necessary.

,~

CoN-69-M

Furthermore it 18 apparent that the procedural ~teps embodied ln Example I are not well adapted for automation, als would be de~lred for operation by mass productlon m~thods. In contrast, the component operatlng step~
embodled ln Example II may readily be automated, includlng the advancement of indlvldual polymerlc foam slabs, photoelectrlc control of the flow Or slurry to contact the roll and slab surrace Just prlor to the rolllng steps, and tran~fer of the lmpregnated slabs to the drylng and heatlng zone. It may be noted that the above applles llkewlse to the comblned lmpregnatlon and rolllng of contlnuous lengths of polymerlc foam, whlch may be sub-dlvlde to final form when convenlent, such as after the dryln~ and heatlng treatment. Particularly ror ~uch use, it may be advantageous to provlde that the lowermost roll or rolls be partly lmmersed ln the slurry contalned ln the dlp tank to lnsure the presence of excess slurry for contact wlth the lower surface of the polymerlc foam durlng the lmpregnation-rolllng operatlon.
In summary, the preferred embodlments ln accordance wlth thls lnventlon comprise the use of organlc polymerlc foam materlal as above descrlbed, characterlzed by havlng :
25 to 35 ppl (pores per lnch), permeablllty o~ 4000 to 8000 x 10-7 cm2, and thlckness of abou~ 2 to 4 lnches;
the use of aqueous suspenslons of finely-dlvlded refractory composltlon as specified above, havlng a vlscosity of 25 to 35 x 103 cp~; the productlon Or cer~mlc foam product havlng a permeablllty of 1,000 to 3,000 x 10 7 cm ; and ..
subJecting the polymerlc foam lmmedlately arter lts lnltlal contact with the slurry to multiple cycles of temporary compresslon and recovery by passage through ad~acent roll gaps, the first compresslon amountlng to 50% to 80% and subsequent compre~slons being the same as the flrst or greater, up to a maximum of about 90%.
Unless otherwlse speclfled all percentages expressed hereln are ln terms of percent by welght.
This invention may be embodled ln other ~orms or carrled out ln other ways wlthout departlng from the spirlt or es3ential characterlstlcs thereof. The present embodi-ment~ are therefore to be con~ldered as ln all respects lllustratlve and not restrlctlve, the scope of the lnventlon belng lndlcated by the appended clalms and all changes whlch come wlthln the meanlng and range of equlvalency are lntended to be embraced thereln.

_ 20 --

Claims (20)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of preparing a ceramic foam possessing controlled permeability and structural uniformity which comprises:
A. providing a hydrophobic, reticulated organic polymer foam possessing a predetermined permeability and resilience;
B. preparing an aqueous slurry of a thixotropic ceramic composition having a viscosity within the range of 1 x 103 to 80 x 103 cps;
C. providing at least three rolls mounted to furnish at least two gaps in close proximity between pairs of said rolls, the first gap being preset at a spacing of 50-90% of the thickness of said polymer foam and the second gap at 70-90% of said thickness;
D. Passing said polymer foam through said roll gaps while flowing said slurry over at least one roll and a surface of said polymer foam to establish a supply of said slurry at the entrance to said first gap, thereby uniformly impregnating said polymer foam with said slurry; and E. drying and heating said impregnated foam to remove the organic component therefrom.

2. The method of Claim 1 wherein said roll gaps are provided by a triple-roll stand.

3. The method of Claim 1 wherein said roll gaps are provided by a pair of two-roll stands.

4. The method of Claim l wherein the said first roll gap is preset at a spacing equal to 50% to 90% of the thickness of said polymer roam.

5. The method of Claim l wherein the said second gap is preset at a spacing equal to 70% to 90% of the thickness of said polymer roam.

6. The method of Claim l wherein the said polymer foam has a thickness of 0.25 inch to 4 inches.

7. The method of Claim l wherein the said polymer foam is selected from the group consisting of polyester polyurethanes, polyether polyurethanes, polyvinyl foam materials, and cellulosic derivatives.

8. The method of Claim l wherein the said polymeric roam has a resilience measured by the ball rebound test of greater than 25%, a compression set not greater than 30% at 50% compression, a permeability of 4000 to 8000 x 10-7 cm2 and a pore size of 5 to 50 ppi.

9. The method of Claim l wherein said aqueous slurry contains a ceramic composition comprising materials selected from the group consisting of alumina, chromia, zirconia, magnesia, titania, silica, mullite, calcined clay, and mixtures thereof.

10. The method Or Claim 9 wherein said composition further comprises 2.5 to 25% of an air setting agent and 0.1 to 12% of a rheological agent.

11. The method Or Claim 10 wherein said air setting agent is selected from the group consisting of colloidal aluminum orthophosphate, alkali metal silicates, ethyl silicate, aluminum hydroxychloride, magnesium orthoborate and mixtures thereof.

12. The method Or Claim 10 wherein said rheological agent is selected from the group consisting of organic cellulosic derivatives, bentonite, kaolif and mixtures thereof.

13. The method Or Claim 9 wherein said slurry comprises 40-80% alumina, up to 20% chromia, 2.5 to 25% aluminum orthophosphate, up to about 10% kaolin and about 0.1-10%
bentonite.

14. The method Or Claim 13 wherein said slurry comprises 45-50% alumina, 10-15% chromia, 12.5-17.5%
aluminum orthophosphate, 2-5% kaolin, and .5-2% bentonite.

15. The method of Claim l wherein the viscosity of said slurry 10 10 x 103 to 40 x 103 cps.

16. The method of Claim l wherein said drying is conducted at a temperature ranging from about 400-700°F.

17. The method of Claim 16 wherein said foam material is further heated to sinter said ceramic composition entrained therein.

18. The method of Claim 1 wherein the said polymer foam has a permeability of 4000 to 8000 x 10-7 cm2, the said aqueous slurry has a viscosity of 25 to 35 cps, the said first roll gap being preset at a spacing of 50 to 80%
of the thickness of the said polymer roam, and the said second gap being preset at a spacing at least the same as the first and up to 90% of the thickness of the said polymer foam.

19. In preparing a ceramic foam having controlled permeability and structural uniformity, the method comprising:
A. providing a hydrophobic, reticulated organic polymer foam having a predetermined permeability and resilience;
B. preparing an aqueous slurry of a thixotropic ceramic composition having a viscosity of 1 x 103 to 80 x 103 cps;

C. providing at least three rolls mounted to furnish at least two gaps in close proximity between pairs of said rolls, the first gap being preset at a spacing of 50 to 90% of the thickness of said polymer roam and the second gap at 70 to 90% of the said thickness; and D. advancing said polymer foam through said roll gaps while flowing said slurry over at least one surface of said polymer foam and into said first roll gap, thereby uniformly impregnating said polymer foam with said slurry.

20. The method of Claim 19 wherein said polymer foam is advanced a plurality of times through the said roll gaps.