CA1296873C

CA1296873C - Refractory fiber products and method of making such products

Info

Publication number: CA1296873C
Application number: CA000538988A
Authority: CA
Inventors: Peter A. Wolter; Arthur N. Esposito
Original assignee: General Signal Corp
Current assignee: SPX Corp
Priority date: 1986-06-24
Filing date: 1987-06-05
Publication date: 1992-03-10
Anticipated expiration: 2009-03-10
Also published as: EP0251150B1; US4935178A; BR8703177A; DE3774944D1; JPS638275A; MX166806B; US5260011A; EP0251150A3; EP0251150A2

Abstract

ABSTRACT OF THE DISCLOSURE

The method of molding a ceramic product by forming a mixture of ceramic fibers with lengths less than one-fourth inch, water and a deflouculent, the mixture having a consistency which will not flow, placing the mixture in a water-absorbing mold, vibrating the mold to cause the mixture to flow into contact with the mold and eliminate voids in the mixture, allowing a portion of the water of the mixture to be absorbed by the mold leaving a hardened remnant in the mold, removing the hardened remnant from the mold, and heating the hardened remnant to sinter the fibers therein to each other.
In one example, granular alumina is mixed with the mass of fibers, water and the deflouculent to form the mixture. The products produced by the process have densities between 30 pounds per cubic foot and 150 pounds per cubic foot.

Description

~29~3'73 - REFRACTORY FIBER PRODUCTS AND METHOD
OF MAXING SVCH PRODUCTS

The present invention relates generally to molded ceramic products and to the methods for making such products. More specifically, the present invention relates to fiber ceramic products and to methods of molding such products.

INTRODUCTION
; .
Prior to the present invention,. ~ceramic products have been molded of various granular ceramia materials utilizing a plaster mold. In this commercially u~ed process, granular particles are mixed with as little water as possible to produce a mixture with a pudding consistency, and this mix~ure is placed within a recess in a plaster mold. The mixture does not contain sufficient water to permit the mixture to flow against the walls of the mold, and the mold is vibrated to cause the mixture to flow, there~y removing air pockets and causing the mixture to engage the walls of the recess in ~he. mold.
Moisture from the mixture is absorbed ~n the plaster of the plaster mold, and after a period of tlme, the mixture hardens to provide a green product, that - i8, a product which has sufEicient strength to hold its shape after removal from the mold. The green product is then heated to sin~er the ceramic granul~es ~to each other to provide a durablej solid ceramic product :

.;
a~ ~
. ' .
, ~6~73 Products produced in this manner are highly stable and may be utilized at elevated temperatures~ However, the material of the product is dense, making the products heavy. Hence, it is desirable to reduce the density o the ceramic product produced in the manner indicated above in oxder to produce a liyhter product without sacrificing strength.
It is also known that light weight silica product~ ma~ be produced with sllica fibers. As disclosed in United States Patent 3,500,444, issued to W.K. Hesse, et al. on March 10, 1970, entitled, ELECTRICAL HEATING UNIT WITH AN
INSULATING REFRACTORY SUPPORT, the ceramic fibers are filter molded from a dilute water suspension. In an example, Hesse sets forth a suspension which is 99~ water and 1% solids~ the solids including a binder, as well as the fibers. The resulting product is a product of low density, Hesse indicating a range of about 4 to about 30 pounds per cubic foot. In the process of Hesse, a large portion of the water of the suspension is removed roln~the mat by draining the water through a screen, the mat collecting on the screen. In practice, additional water is removed by applying pressure to the mat, and also vacuum is applied below the screen to facilitate the removal of water.
U.S. Patents 4,719,336 issued Januaxy 12, 1988 and 4,855,576 issued August 8, 1989 and which correspond to Canadian Application 536,411 filed May 5, 1987, of Duane L. Sterwald, assigned to the assignee of the present applicant, disclose vibrating the mold to facilitate removal of water from the mat.

~ ~ \
7~

To the ~nowledge of the present inventors, there is no pxocess for commercial production known prior to the present invention for produc~ng molded ceramic fiber products with densities greater than those of vacuum formed ceramic fiber products and les~ than molded gr~nular silica products, that is product~ having densi~1es between approximately 30 ~ounds per cubic foot and 150 pounds per cubic foot, without the use of excess binders and/or further fabrication. It is an object of the present invention to provide a process for molding ceramic fiber products having densi~ies in thi~ range.
Also, to the knowledge o the'present inventors, fiber ceramic products have not been made heretofore by casting using a plaster cast. Prior to the present invention, the requisite~
randomness of the ceramic fibers was obtained by suspending the fibers in a body ~f water. Products pxoduced by casting methods generally are, less,costly to produce and can be shaped in the casting process to ~liminate machining. It is an ob~ect of ,the present invention to provide a process for casting fiber ceramic products utilizing a water'absorbing~cast.
The low density composition of ceramic ibe,r products produced by 'known methods requires a binder to hold the fibers together during the processing of the product. In the absence of a binder, the mat produced by the filter molding process contains fibers which are in abutment with each other, but without adequate green stength to maintain ~he shape of the mat between the stepsi of removing the mat from the mold and sintering the mat. To overcome the lac~ of green strength, a binder is incorporated in the dilute suspension o ibers and .

~9~87~

water. On sintering, however, the binder becomes unnecPssary, and further, under some conditions, tends to deteriorate the fibers. A fiber ceramic product is here described which does not include a binder.
The present in~entors have found that a water absorbing mold may be utilized to produce molded fiber ceramic products, provided: (1) the amount of water utilized in the mixture inserted into the water absorbing mold is sufficiently sma~1 that the mold can absorb the water, (2) the fibers are relatively short and of un1form cross-section so that the fibers will mix with the small amount of water present in a uniform manner, and (3) that a dispersing agent or deflocculent be utilized in the mixture with the water and the fibers to assure good contact between the water and the fibers. In addition, it is necessary in many cases to provide some means for retarding the rate at which water is absorbed in the water absorbent mold.
The present procedure makes use of a substantially uniform mixture of short ceramic fibers, a small quantity of water and a deflocculent. The mixture is worked into the recess of a water absorbing mold and caused to flow into contact with the walls of the mold by means of vibration. ~hereafter the mold absorbs water from the mixture and reduces the quantity of water sufficiently to allo~ the friction between abutting fibers to provide sufficient green strength to permit removal of the molded product from the mold. The product is removed from the mold, dried, and thereafter placed in an oven and sintered.
Products produced by this process ara unique in ~hat they may be cast with densities between 30 pounds per cubic foot and .~ .

150 pounds per cubic foot, and the invention includes products made by the process described above and the products themselves.
~ ore particularly in accordance with the invention there is provided the method of making a ceramic :~ 5 product comprising the steps of mixing a mass o ceramic material limited to the class consisting of ceramic fibers and granulated alumina and including ceramic fibers with a mass of water and a : deflocculen~ to ~orm a mix~ure, the fibers in the mixture havin~ average lengths less than one-fourth inch and 5ub9tantially uni~orm cross sections, the mixing continuing until the ceramic fibers are substantially uniformly distxibuted throughout the mixture and oriented sub.~tantially ; randomly with respect to each othe~, the ratio of water to ceramic material in the mixture being less than required to ~` provide a mixture consistency which will flow, thereafter placing the mixture into the recess o a mold constituted of liquid absorbent material J therea~ter subjecting the mold to vibration to cause the mixture to flow into contact with the mold and to eliminate voids i~ the mixture, thereafter, maintaining the mold and mixture at res~ ~ox a period of time ~: sufficient to allow a lar~e portion o~ the water from the ; mixture to become a~sorbed in the mold ~o permit ~he mixture ;; remnant remaining in the recess to harden, thereafter removing 2~5 ~ the miYture remnant:from the mold, and thereafter hea~ing the mixture remnant ~o sinter abutting fibers ~o each other.

~::

`~`~ : :

~ - 5 _ , 1 ~, ~l29~ 3 Embodiments of the invention will now be described with reference to the accompanying drawings wherein;
Figure 1 is a front elevational view of the molding equipment utilized in carrying out the processes embodying the present invention and in making the products of the processes and Figure 2 is a sectional view of the mold taken along the line 2-2 of Figure 1.
DETAILED__SCRIPTION OF PREFERRED EMBpDIMENTS OF THE INVENTION
Apparatus for caxrying out the present in~ention is illustrated in the figures. A mold of the type used for slip casting or thixotropic casting of ceramic products is shown at 10. Such molds are constructed of water absorbing materials, such as plaster of paris, and the mixture of ceramic material and wator placed in ~uch a mold containa sufficie tly small ;

5a -~ .

...

~96~3~3 quantitie~ of water that the absorption of the water from the mixture in the walls o the mold permit~ the ceramic material in the mold to harden.
The mola 10 contains a recess 12 ~haped to form ~he de~ired product, and in the illu~tratlon of Figures 1 and 2, the rece~s 12 is ~haped to form a cylindr~cal tube. The re~ss 12 has a cylindrical outer wall 14 and cylindrical inner wall 16~ In addition, the recess has a circular bottom wall 18.
The mold 10 rest~ upon a vibration table 20. which is provided with a frame 2Z supporting a ta~le top 24 through a plurality of compliant spacers 26 disposed at intervals about the perimeter of the table 24. A vibrator 28 is mounted on the unders.ide of the table top 24, and operatlon of the vibrator 28 causes the table.top 24 to vibrate with re~pect to the frame 22.
To cast a product with ceramic fibers, a mixture of ceramic fiber and water is prepared. In order to: produce a physically strong ceramic body, it is nece~sary for the fibers ~o be randomly distrlbuted throughout thb body, and accordlngly, the fibers must be randomly di~tribut~d in the mlxture. I the ~ibers are too long, they will tend to ball, or otherwise become aligned, and acco~dingly, the f~bers must be relatively ~hort. Further,. if ~ibers are relatively long, the fiber~ will pack le3s den~ely i~ the mold. ~The inventors have found that the length of the ceramic fibers ~hould not exc~ed 1/4 inch on ~verage.
Ceramic fiber~ axe produced under high temperaturei and generally are provided with ball~ or shot at the ends thereof.

The presence o balls at the ends of ~he ibers adversely - 6 ~

8~73 affects ~he distribution of the fiber~ in the mold, and fibers should be processed to remove the balls from the fibers and the balls dlscarded pr~or to mixlng ~he fibers with water. The cross section of the fiber~ should be uniform with respect to each other and with respect to the length of the fibers in order to promote high density products.
The fiber~ contain hlgh refractory compositions, such as ~ilica, alumina, zirconia~ alumina-silica composltions, includin~ those alumina-~lica compositions containlng titania and/or zixconia, and synthetically produced inorganlc fibers which-exhibit resistance to deterioration ln temperatures up to the order of 2000 to 4000F. Such fibers are more fully ~: , . .. .. .
described in an article enti~led "Critical Evaluatlon of the Inorganic Fi~ers" in Product Engineering~ August 3, 1964~ pages , 96 through 100. Suitable fibers are available commercially and B include FIBERM~X and FIBERFRAX available from the Carborundum Company, ZIRCAR zirconia flbers available from Zircar Products, Inc., SAFFIL or XAOWOOL ceramic fibers available from ~abcocX
and Wilcox Company. Such fibers have diameter ranging from approximately 2 to approxim~tely 6 mi~ron~ and are rel~tively uniform throughout the length~ of the fibers. The shot or ball content should not exceed 25% of the ~o~al by weight.
;The fibers are mixed with w~ter u~lng a low intensity mixer to produce a uniform distribution of fibers within the mlxture. There must be suf~iclen~ water present to fill all voids between ~iber~, bu~ as little water as possible should be used in the mixture, since the water must be ab~orbed in the ::: mol d in order to pexmit the cexamic product to harden. Because ~ Tr ~ rk 7 ~Z~ 73 of the fact that the den~ity of fiber ceramic product~ is significantly lower than that of granular ceramic products, considerably more water is utilized in the mixture containing f~bers as a percentage of the t~tal. The amount of water necessary in the mixture should be just sufficient t~ permit ths mixture to flow under vibration, hut not flow in the a~sence of vibration, as in conven~ional thixotropic casting p`rocesse~.
; It is necessary to provide a dlsper~ing agent in the mixture to make certain that contact is made with all portions of the ceramic fiber. Dispexsing agent~ are -available commercially, and DAR~A~ #7 di~tr~uted by R. T. Vanderbilt Company, Inc. has proven to be satisfactory.
The mold must contain a sufficient mass to absorb the water from the mixture inserted in the recess of the mold. One suitable material for the mold is plaster o paris which i5 a dehydrated gypsum. The water content oE the mixture i5 approximately equal to that ` of the fibers by weight.
Thixotropic casting of granular ceramic~ i8 achieved using up ; to approximately 1~% water and 85% granulDr material, and hence . .
the present process requires the mold to absorb more than ~ix tlmes the amount of water used by the prior art thixotropic casting process.
The fact that such a relatively large ~uantity of water must be transferred from the mixture 1n the reces~ of the mold to the mold ex~ends the time for hardening of the green product, and also increases the likelihood of nonuniform transfer of water to the mold, thus leaving wet regions in the r c~,cl e, 1~ k - 8 -, green product which may not ha~re adequate green strength.
It is desirable to xetard the flow ~f moistuxe from themlxture in the recess of the mold in order to obtain a more uniform distribution of the mixture in the mold and the moisture in the product. For this purpose, a water retardant i8 mixed with the mixture of fiber~ and water. Alginate~ have proven to be sati~factory for this purpose. Sodium alginate and ammonium alginate have pro~en to be particularly ~uitable, and have the effect of providing a skin on the wall of the plaster mold to produc:e a smoo$h product. Sodium or ammonium alginate ls mixed with the water and fiber prior to placlng the mixture in the mold. The alginates may be omitted from the mixture, and the mold coated with polyvinyl alcohol as an alternative.
The pxocess for producing a sintered ceramic product starts with the prepaxation of the liquid component of the ... . .

mixture, namely mixing together water, the flow retardant in a range o 0.5 to 4 grams per 1000 cublc centimeters of water, : . . . .
and the dispersing agent in a ~ange of 0.5 to 4 cubic centimeters per 100 cubic centimeters of water.~ Therea~ter, a mass of flbers of proper length and uniformity i8 placed in a mlxing bowl and a quantity of the llquid component is added to the fi~ers and mixed wlth the fiber~ ln the mixing bowl. If desired, th~ solid component may include refràctory ceram~c powders, such as silica or alumina, a~ well as ceramic fibers, thereby increaslng the density of the product. Additional .. ;

liquid component is added until the mixture of fibers and li~uld component ha~ a pudd:ln~ consi~torlc~, that i8l a .
~ _ 9 _ , 37~

con~l~tency which wlll not ~low except u~der mechanical . '` agitation. ' , ~ , .,,,~:; . . ~hereafter the m~xture of fibers,and liquid component i~
'. . transferred ~y hand .into the reces~, of:i~the' mold, and ~he vibrator is placed in operation,to make .the mixture of li~uid component and fibers sufficiently flowable to obtain good contact with the walls o~ the recess in the mold, W~en ~he recess is filled, and all air pockets have been removed and good contact i~ achieved with the walls of the mold, ~he vibrator is turned. off. The mixture in the recess of the mold ' is then allowed to harden. After a perlod of time, the mixture ,. will become sufi.ciently hard that actuation of the vibrator will not cause the mixture to become fluid. At this time the : product has ~ufficient green strength to be removed from the mold and permitted to air.dxy. The product may also be dried ` in a low temperature oven if de~ired~ , Thereafter, the green product is placed in an oven and . the temperature rai~ed ~uficient to cause sintering of the ibers.' The i~ers ~ecome attached to each other'at the points ' of contact ln the product, thus formlng a ~rong rig~d body.
` The followlng are speciflc example~ o products produced according to the present invention. 290 gram~ of ceramic fibers consistlng of 95~ AL203 and 5~ SI02 obtained from A Babcock ~ Wilcox Compuny under the name SAFFIL were chopped to le~s than 1/4 inch ~n average length. The liquid component of the slurry W~8 m~de up by mlxing 2 gram~ of nmmonium alglnate obtained from TIC ~ums Inc. with 1000 cubic centimetexs of water and mixing ~her~with 5 cubic centimeters of a Trc~ c h ~rk 10 .
, .

polyel~ctrolyte dispersing agent in an aqueous solution known . rl S
as DARVAN No . 7 obtained from R. T. Vanderbilt Company, Inc.
The fiber component was placed in a low intensity mixer and 320 cubic centimeters of the liquid component was added to the fiber component and mixed for 13 minutes to f~rm a pudding consistency. The mix was then hand fed into a tile mold for producing a tile 1/2 inch thick by 2 inches high by 9 inches long made with pot~ery plaster obtained from Georgia Pacific Co. under the tradename K-60. After one hour the green product was removed from the mold and permitted to air dry for a period of about 24 hours. The green product was then fired in a furna~e at 1560C. for 3 hours. The finished product had a density of 63 pounds per cubic foot.
An example of a refractory fiber ceramic product which includes refractory powders is as follows. The liquid component o~ the mixture was irst prepared by mixing 2 grams of ammonium alginate per 1000 cubic centimeters of water, and A
. .
dispersant such as DARVAN No. 7 in the amount of 2 cubic centimeters per 100 cubic centimeters of water. Fibers obtained from Carborundum Company under the trademark FIBERFRAX
and chopped ko an average length o~ approxima~ely 1/8 inch and processed so that the shot content was less than 25% were mixed with a refractory powder in the form of Alcoa Chemical Division's T~64 tabular alumina (-325 mesh~ in equal proportions by weight. 400 grams of the dry mixture of tabular alumin~ nnd f ib~rs w~ tht~n pl~cetl in ~ mixing bowl, and tl-c liquid component was added to the dry component and admixed in the mixing bowl to pxoduce a pudding consistency. The mixture ~ T~ ~l c ~

~Z~ 3 was then hand fed into a pla~ter mold, and the mold was sub~ected to vibration until ;the mold was ~ull and air pockets were removed from the mold. The vibration was stopped and the mold was allowed to absorb enough water from the mixture so th~t further vibxation would not cause the mixture to become fluid.
Thereafter, the product was stripped from the -mold and the product was permitted to air dry for a period of 24 hours.
The air dried produc~ was then placed in a furnace at 2450 Fahrenheit for a period of 4 hours to slnter. The product had a density of 70 pounds per cubic foot.
In both e~amples, the vibrator operated at a frequency of S0 cycles. The agitat~on of the mlxture in the mold is only required to make the mlxture fluid enough to fill ~he mold and to xemove air pockets from the mold. The process is not sensitive to the frequency of the vibration and the vibration need only be sufficient to cause the mixture to become fluid.
Likewise, the mixer utilized to mix the li~uid component and the dry component of the mixture is not critical. Mixers manufactured by Hobart Manu~acturiny Company for kitchen use, such as the Kitchenaid Model K~45 or Hobart Model A-200 have proven to be satisfactory~
Products produced by the process de~crib~d above have considerably lower dens~ties th~n c~eramic product~ produced by the casting process using aggregate ceramlc materials, but higher den3ities th~n those produced by the vacuum forming method using aexamic ibers. Further, the density of the product produced under the present invention may be controlled "

~2~i8~

more readily than fiber ceramic products produced by the filtermethod, In add~tlon, the densit~ of products produced under the present invention extend over a range of densities lying generally between the density of filter formed fiber ceramic products and the density of ceramic products produced by the thixotropic casting process. Further, products produced according to the present invention can be expected to have longer life than the fiber ceramic products requiring a binder.
The present invention permits the production of lower density product~ than can be produced by the thixotropic casting method, thus requiring less energy for sintering and reducing the quantity of ceramic materials in the product from that required by the thixotropic casting process. Likewise, the present invention permits the use of a plaster mold casting process for fiber ceramic pxoducts which is significantly less costly than a vacuum filtering fiber ceramic process, both in the elimination o the need for a vacuum and in the energy requixed to dry the green product.
Those skllled in the art will devise many processes, products, and uses for the present invention over and above those here disclosed. It is therefore intended that the scope of the present inven~ion be not limited by ~he foregoing specification, but rather only by the appended claim~.

Claims

1. The method of making a ceramic product comprising the steps of mixing a mass of ceramic material limited to the class consisting of ceramic fibers and granulated alumina and including ceramic fibers with a mass of water and a deflocculent to form a mixture, the fibers in the mixture having average lengths less than one-fourth inch and substantially uniform cross sections. the mixing continuing until the ceramic fibers are substantially uniformly distributed throughout the mixture and oriented substantially randomly with: respect to each other, the ratio of water to ceramic material in the mixture being less than required to provide a mixture consistency which will flow, thereafter placing the mixture into the recess of a mold constituted of liquid absorbent material, whereafter subjecting the mold to vibration to cause the mixture to flow into contact with the mold and to eliminate voids in the mixture, thereafter, maintaining the mold and mixture at rest for a period of time sufficient to allow a large portion of the water from the mixture to become absorbed in the mold to permit the mixture remnant remaining in the recess to harden, thereafter removing the mixture remnant from the mold, and thereafter heating the mixture remnant to sinter abutting fibers to each other.

2. The method of making a ceramic product comprising the steps of claim 1 including the step of retarding the flow of water from the mixture into the mold.

3. The method of making a ceramic produce comprising the steps of claim 2 wherein the flow of water into the mold is retarded by admixing an alginate with the mass of fibers, deflocculent and water.

4. The method of making a ceramic product comprising the steps of claim 3 wherein the alginate is ammonium alginate.

5. The method of making a ceramic product comprising the steps of claim 2 wherein the flow of water into the mold is retarded by coating the walls of the mold with polyvinyl alcohol.

6. The method of making a ceramic product comprising the steps of claim 1 wherein the mass of the water in the mixture comprises a mass of the same order of magnitude as the mass of the ceramic content of the mixture placed into the mold.

7. The method of making a ceramic product comprising the steps of claim 1 wherein the mold is produced of plaster of paris.

8. The method of making a ceramic product comprising the steps of claim 1 wherein the fibers have cross sections between 2 and 6 microns.

9. The method of making a ceramic product comprising the steps of claim 1 in combination with the step of mixing granular ceramic particles with the ceramic fibers, water and deflocculent before placing the mixture in the mold.

10. The method of making a ceramic product comprising the steps of claim 9 wherein the granular ceramic particles comprise tabular alumina.