Title of Invention

METHOD OF PLUGGING CHANNELS IN A CERAMIC HONEYCOMB

Abstract

The present invention relates to a ceramic honeycomb wall-flow filter (2) is prepared by plugging channels (4) in a ceramic honeycomb by a method comprising the following. First, a mixture comprised of a dispersing liquid and ceramic powder is formed. Next, the mixture is inserted on one end of the channel (4) in an unplugged ceramic honeycomb such that the mixture flows to the other end where the mixture collects and forms a plugged ceramic honeycomb. Then, the plugged ceramic honeycomb is heated to a temperature sufficient to sinter the plugged ceramic honeycomb to form a porous sintered plugged ceramic honeycomb (2).

Full Text

METHOD OF MAKING WALL-FLOW MONOLITH FILTER This application claims the benefit of U.S. Provisional Application Serial No. 60/2:53,809, filed April 23, 2001. The invention relates to ceramic wall-flow tillers and methods o: making then. In particular, the invention relates to particulate traps-, such as diesel particulate traps. As air quality standards become more stringent, considerable efforts have fo.u ;*d on minimizing the paniculate matter emitted in diesel engine exhaust. A potential solution is a particulate trap inserted in the exhaust, system of a diesel engine. A honeycomb ceramic wUI-flow through filter, such as described in U.S. Pater:;: No. 4,276,0V i, has become the preferred type of particulate trap. These honeycomb filter; are made by extruding a paste comprised of water, binder and ceramic powders (for example, clay, mullite, silica, silicon carbide and alumina) that form, for example, cordierite upon firing. Clay* or water soluble binders are generally used to make the paste sufficiently p!a:;ti: to fo:in useable honeycombs. After the paste is extruded, the honeycomb is dried, debimlered and sir tered to form a honeycomb. The honeycomb is sintered, typically, to give sufficient strength co the :hin channel walls to survive insertion of a ceramic paste to plug trie chaincls. as described next. Finally, to make thl wai!-f'ow paniculate trap or filter, one half of the openings of one end of the sintered honeycomb are plugged with a paste comprised of a suitable powder, dispersion medium and binder. Then, on the other end, the channels not already plugged arc plugged with the past;. Subsequently, the plugged honeycomb is sintert i again to ft:mi the wall-flow paniculate trap. Unfortunately, this method suffers from a number of problems. For example, •he liquid in the puxe may be c-rawn into the porous walls of the fired honeycomb preferentiall) causing non-uniform drying shrinkage of the plug and ultimately cracks in the plug. A second problem is the necessity for multiple expensive steps (for example, at least :wo hi:-;h temperature firings) to manufacture the particulate trap. These multiple steps are typicaliy needed because the walls of a green ceramic honeycomb are thin and fragile they :end to deform and/or break when inserting the paste, which is particularly true when using a l;.-ge scale prccess. Another problem is the limited compositions that may be used for the plutj, due to the expansion of the fired honeycomb during the sintering shrinkage of the plui;. Accordingly, it would be desirable to provide a method for making wall-flow traps, for exam: !e. that avoids one or rrore of the problems cf the prior an, such as one of tl-o.'e described above. The invention is a method of plugging channels in a ceramic honeycomb :omp"ising: (a) forming a mixture comprised of a dispersing liquid and ceramic powder, (b) inserting the mixture at one end of the channel in an unplugged ceramic icncycomfc, such that the mixture flows to the other end v/here the mixture collects and OITTIS a plugged ceramic honeycomb and (c) heating the plugged ceramic honeycomb to a temperature sufficient to sinter the lugijed ceramic honeycomb to form a porous sintered plugged ceramic honeycomb. Thtr method is particularly useful, but not limited to, plugging of channels in T;iercomb wall-tlow filters. Surprising'v, these methods may be used to not only plug ihz u.n:ie!s, but also simultaneously provide a discriminating Lycr, for example, on the wails :iht outlet channels of a ceramic honeycomb wall-flow filter. The method may also be ed 3 provide cihcr useful materials on or in the walls of the filter (for example, a catalyst tiudeation agen:) of a channel, while simultaneously forming the plugs. FIG. I is a front view showing one embodiment of a ceramic honevcomb tvr *iade according to the present inven.ion. FIG 2 is a side view of FIG. I with a part thereof cut away. FIG 3 is an enlarged schematic view of adjacent channels in the filter of FIG. FIG 4 is an enlarged cross-sectional view of a plugged portion of a channel i in-situ formed discriminating layer of the filter of FIG. I. Throughout the different views of the drawings, numeral 2 is a ceramic hom:yconiD body, numeral 4 is an inlet channel, numeral 5 is an outlet channel, numeral 6 a partition wall between adjacent channels, numeral 8 is an inlet plug, numeral 9 is an outlet plus; numeral 10 h an inlet end. numeral 11 is an outlet end and numeral 12 is a discriminaing hyer. U' lpluggeii eeraxic honeycomb: An unplugged honeycomb body is a ceramic honeycomb that has at least one channel that is not plugged. As an illustration, the honeycomb may have one half of the channels plugged on one end and no plugs on the other end (that is, half of the channels are unpegged). The unplugged ceramic honeycomb may be green, calcined or sintered, but is prcfeired to be either green or calcined. A green unplugged ceramic honeycomb is comprised of ceramic powders and an organic binder. Organic binder includes those, as dei.ciibed ir. Chaprcr 11 of Introduction to the Principles of Ceramic Processing. J. Reed, John Wiley and Scr.5, NY, N\\ 1988. A calcined unplugged ceramic honeycomb body is a gn;er unplugged ceramic honeycomb that has been heated to a calcining temperature su:;Ti( c:nt to remove :he organic binder and dehydrate any clay that may be present. A sircicd unplugged ceramic honeycomb is a green or calcined unplugged ceramic ho* e":omb :hat has been heated tfc a sintering temperature sufficient to fuse (sinter) the cei;iniic constituents into a monolithic cerimic. Plugged cen.mic horeycomb: A ;:ius;ged ceramic honeycomb is any of the aforementioned unplugged ceniric hontsyconbs tha: have at least one channel plugged with a mixture that forms a sin:;r,;c ceramic plug upon heading to a sintering temperature, thus forming a sintered pluj'g-rd ceramic honeycomb. Sintered plugged ceramic honeycomb: A plugged ceramic honeycomb that is heated to a sintering temperature sufficient to fuse (sinter) the ceramic constituents of the plug and, if necessary, the ceramic hom-vcomb into i monolithic ceramic. *> Referring to FIG. 1 through FIG. 4, which depicts one preferred embodiment o( the cer&inic honeycomb filter comprising a ceramic honeycomb body 2 that has a plur ility of parallel inlet and outlet channels 4 and 5, respectively, extending there through (that ii, from inlet end 10 to outlet end 1 t)f defined by porous partition walls 6, inlet channel plug? 3 and outlet channel plugs 9. The outlet channels 9 also have disposed on the surface of th: partition walls 6 a discriminating layer 12. When a gas or liquid 14 containing matter to be filtered is directed into inlet channels 4, the gas or liquid 14 passes through the palii on walls 6 and discriminating layer 12 and exits the outlet channels 5. Thus, the partition walls 6 and discriminating layer 12 filter out the matter from the gas or liquid 14. The ceramic honeycomb body 2 may be any useful ceramic that has sufficient poro: ty and strength to perform as a wall-flow filter. Evxurr.p es of useful ceramics include silicon carbide, silicon nitride, rnuiiite, CDrdienle, beta spodunenc, phosphate ceramics (for example, zirconium phosphate) or combinations thereof. Preferably, the ceramic is mullite, siJ: :cn carbide or ccrdierite. More preferably, the ceramic is mullite or silicon carbide. The muiiiie is preferably one that is tormed in the presence of a fluorine gas, such as those des:r:b«d in U.S. Patents 4,910,172; 4,911,902; 4,948,766; 5,098,455; 5,173,349; 5.1!>4 154; 5,198,007; 5,252,272 and 5,340,516. The grains of the mullite preferably have an average aspect ratio of at least 2, more preferably at least 5, and most preferably at least 10. Generally, the porosity of the ceramic honeycomb body 2 is from 30 percent to 80 percent porous. Preferably, the ceramic honeycomb body 2 is 40 percent to 70 percent porou:;. The plugs 8 and 9 may be any porosity sufficient to act effectively as a plug. Generally, the plug;:.; S and 9 may be any ceramic composition including essentially the same cerami: composition as the ceramic honeycomb body 2. Essentially the same composition means tne plugs 8 and/or 9 have essentially the same chemistry and microstructure as the In one preferred embodiment of the ceramic wall-flow filter, the plug or plugs 8 and 9 have me same composition as the ceramic honeycomb body 2. In this embodiment, there may or may not be a discriminating layer 12. Preferably, there is a discriminating lay or 12 that has essentially the same chemistry as the ceramic honeycomb boey. In another preferred embodiment, the sintered ceramic honeycomb body 2 has an inlet plug $ at the outlet end 11 that has a different composition than an outlet plug 9 at the inlet end 10. Different composition means that, after sintering, the compositions have a reidily discernable microstructural difference (for example, porosity, crystalline structure or gra n size) or chemical difference by typically employed techniques for characterizing ceramics. Essentially this means that the differences are not readily discernable by the aforementioned techniques. Preferably, cne half of all of the channels of the honeycomb body 2 (that is, the in let channels 4) are plugged at one end and the remaining channels not plujgg* i on the one end are plugged at the other end (that is, outlet channels 5 are plugged at the inl;t end). Ev;n more preferably, the outlet plugs 9 have essentially the same chemistry but different micro* tucture than iniet plugs 8. It is, further, preferred that this embodiment ilso luve a d scrim rating layer 12. Most preferably, the out'e1: plugs 9 have essentially the iam<: composition as the discriminating layer and iniet plugs have essentially same: ceramic honeycomb body> The discriminating layer 12 may be any material useful for making a filter, so Ions as the average pore size of the discriminating layer 12 is substantially Jess than the aveng--; pore ;;ize of toe ceramic honeycomb body. Suitable materials include those described for the ceramic honeycomb body. "Substantially less" generally means that the discriminating layer average pore size is at most three quarter the average pore size of the ceramic honeycomb body. Preferably, the average pore size cf the discriminating layer 12 is at most one h llf ard more preferably, at most one quarter the average pore size of the ceramic honeycomb body 2. In addition, the ceramic honeycomb wall-flow filter 1 may have a catalyst on c;" v. ithin at least one partition wall 6 or discriminating layer 12. The catalyst may be any cati yst suitable cci catalyze, for example, the combustion of soot particles or oxidation of CO (carbon monoxide) or NOx (nitrogen oxides). Exemplary catalysts include the tclliwimas. A first exemplary catalyst is directly a bound-metal catalyst, such as noble iTist.iU, base metal:; and combinations thereof. Examples of noble metal catalysts include pi itinurn, rhodium, palladium, ruthenium, rhenium, silver and alloys thereof. Examples of base metal catalysts include copper, chromium, iron, cobalt, nickel, zinc, manganese, vanadium, titanium, scandium and combinations thereof. The metal catalyst, preferably, is in the; form of a metal, but may be present as an inorganic compound, such as an oxide, nitride anc carbide, or as a defect structure within the ceramic grains of the porous catalyst support. The metal may be applied by any suitable technique, such as those known in the arc. :;or example, the metal catalyst may be applied by chemical vapor deposition. A second exemplary catalyst is one that is incorporated into the lattice stricture of the ceramic grains of the aforementioned catalyst honeycomb. For example, an elcm;nt ma/ be Cs. Zr, La, Mg: Ca, a metal clement describee in the previous paragraph or cornbinatiens thereof. These elements may be incorporated n any suitable manner, such &> those Known in the an. A thrd exempl ary catalyst is a combination of ceramic panicles having metal deposited thereon. These are typically referred to as wash-ccats. Generally, wash-coats consist of micrometer-sized ceramic panicles, such as zeolite, iluminosilicate, silica, ceria, zircoua, barium oxide, barium carbonate and alumina panicles that have metal deposited thereon. The meial may be any previously described for directly deposited metal. A panicuiarly preferred wash-coat catalyst coating is one comprised of alumina particles having a noble m^a! thereon. It is understood that the wash-coat may be comprised of more than one meal oxide, such as alumina, having oxides of at least one of zirconium, barium, lanthanum, magnesium and cerium. A fourth exemplary catalyst is a perovskite-type catalyst comprising a metal oxide composition, such as those described by Golden in U.S. Patent No. 5,939,354. A fifth exemplary catalyst is one that is formed by and deposited on the catalyst support by calcining at a temperature of from 300°C to 3000°C, a composition that comprises (a) an aqueous salt solution containing at least one metal salt and (b) an amfhiphinc ethylene oxide containing copolymer, wherein the copolymer has an average molecular weight of greater than 400, an ethylene oxide content of 5 to 90 percent and an KLIi of between -1.5 and 15, as described by Gruenbaucr, el: al., PCT Patent Application No. V'0:'9/l 8-809. J"r. addition, the catalyst may also be one as described by U.S. Patent No. 5,>3QJs433 and FCT Patent Application No. WO99/03627. Li performing the method of plugging channels in a ceramic honeycomb, a mixture comprised of a dispersing liquid and a ceramic powder is formed. The mixture is then inserted into ai: least one channel of an unplugged ceramic honeycomb, such that the mixt: re flows to the other end where the mixture collects and forms a plugged ceramic honeycomb. The plugged ceramic honeycomb is then heated l:o a temperature sufficient to sinte:" the plugs and fuse them to the ceramic honeycomb and, if necessary, sinter the ccr ar.ic honeycomb toe- (that is, sinter the ceramic honeycomb if it is a green or calcined cerarric honeycomb). In other words, the plugged ceramic honeycomb is heated sufficiently to :b: m a porous smiered plugged ceramic honeycomb. The mixture nust be fluid enough to be inserted into one end of a channel of the gusen ceramic honeycomb and subsequently flow through the channel and collect at the othsr end of the channel, for example, from the mere exertion of gravity. Thus, the mixture may tisposit, for example, a discriminating layer on die partition walls and form the inlet or out et plug, after removing a sufficient amount of dispersing liquid to give the collected mi.Htvre enough integrity to form the plug Generally, the viscosity of the mixture is at most 10(30 >:entipoise (cp), more preferably the mixture has a viscosity of at most 200 cp, even more preferably at most 100 cp and most preferably at most 20 cp. The mixture may be formed by any suitable method, such as those known in the an Suitable methods include those described in Chapter 17 of Introduction to the Prircip'es of Ceramic Processing, J. Reed. John Wiley and Sons, NY, 1988. The dispersing liquid may be, for example, water, any organic liquid, such as an .slohol, aliphatic, glycol, ketone, ether, aldehyde, ester, aromatic, alkene, alkyne, carboxylic acid, carboxylic acid chloride, amide, amine, nttrile, nitro, sulfide, sulfoxide, stilfone, oiganometalJic or mixtures thereof. Preferably, the dispersing liquid is water, an aliphatic, dkene or alcohol. More preferably, the liquid is an alcohol. Preferably, the alcohol is a methanol, propanoic ethanol or combinations thereof. Most preferably, the abohol is propanol. The :eramic powder may be any ceramic powder useful to form the plugs, suc\ as ceramic powders that form ceramics, such as silicon carbide, silicon nitride, muHitc cordierite, beta The mixture may contain other useful components, such as those known in the art of making ceramic suspensions. Examples of other useful components include dtspcr.ants, deflo:cuiams, flocculants, piasticizers, defoamer:>, lubricants and preservatives, su:h ix those desci:ed in Chapters 10-12 of Introduction to the Principles of Ceramic Pr-rossing J. Reel John Wiley and Sons, NY, 1988. A preferred binder in :he mixture is one; t!"&t is .nluble in the dispersing liquid, but not soluble in water. \* The mixture may also contain binders. Examples of binders include c.i.'ulose ethers, such as those described in Chapter 11 of Introduction to the Principles of Cetamic Processing, I. Reed, John Wiley and Sons, NY, NY, 1988. Preferably, the binder is a msthylcellulcss or ethylceilulose, such as those available from The Dow Chemical Company under the trademarks METHOCEL and ETHOCEL. Preferably, the binder dissolves in Jie d spsrsing liquid, but not water, such as ETHOCEL. After forming the mixture, it is inserted into a channel of an unplugged ceramic honeycomb to form plugs (that is, form a plugged ceramic honeycomb). Insertion into a channel may be accomplished by any suitable method, such as those known in the art. For example, the mixture may be poured, squirted or injected into the channel. The- dispersing liquid may be removed by any suitable method, such as by drying in air, drying by application of heat or vacuum, or by removing it by blocking the char;iel ends on one end of the ceramic honeycomb body with a porous medium that removes ths dispersing liquid by capillary action. An example of such a porous medium is plasu;r of Paris, such a In another preferred embodiment, the unplugged ceramic honeycomb has at least one plug in a channel on one end and at least one channel that has no plugs (that is, opwi channel). Subsequently, the mixture is inserted into the open channel. The mixture is insured in tic ope a channel on the same end as the plug in the plugged channel, such that the mixture flows down the open channel and forms a plug in the open channel at the other end of :he honeycomb. After insertion of the mixture (that is, formation of the plugged ceramic honeycomb), the pegged ceramic honeycomb is heated :o a sintering temperature sufficient to sinter the plugs and, if necessary, sinter the ceramic honeycomb to form the plugged sin:;r ;c ceramic hcreycomb. In general, the plugged sintered ceramic honeycomb is from 30 pe:cent tc 80 percent porous and, preferably, from 40 percent to 70 percent porous. Th: sintering temperature is dependent on the ceramic to be formed but, in genen J, is at least 500°C. Preferably, the sintering temperature is at least 1000°C and more prersnbly, ti least 1100°C to preferably, at most 2200°C, more preferably, at most 1750°C and m:«t preferably, at most 1400°C. The heating to the sintering temperature may be carried out in any suitable mannc" or heating apparatus and under any suitable atmosphere or combinations of atmospheres, such as those known in the art for making the particular sintered ceramic desire;!. In performing the method, an unplugged ceramic honeycomb that has been calcined may be used. Generally, the calcining temperature is insufficient to substantially sinter the green ceramic honeycomb, but is sufficient to remove any organic binders and dehydrate any clay, such thai: the dehydrated clay substantially fails to be rehydrated when contacted with water. The calcining temperature may be any temperature suitable to substantially renove any organic binders or dehydrate any clay present in the honeycomb. Generally, if then: is a c ay present, the temperature should be sufficient to cause the clay to substantially fail to rehydrate "Substantially fail to rehydrate" is gencrall> when 90 percent by weight of t ur clay fail to rehydrate when placed in water for 24 hours. Preferably, the calcining ternp:rature is g;:sa: enough that 99 percent and more preferably, all of the dehydrated clay fails lo rehydrate when placed in water. Generally, the calcining temperature is from 400°C to at most 1000°C. More preferably, :he calcining temperature is at least 500, more preferably at least 600, most preferably at lea*:" 650 to preferably at most 950, more preferably at most 900, and most preferably at most 8:50°C The; calcining atmosphere may be any atmosphere suitable for dehydrating the clay or toth. Examples include air, vacuum, inert atmospheres (for example, noble gases), nitrogen or combinations Vhereof. The method and apparatus for heating to the caleir.ing temperature may be any suitable apparatus, such as those known in the art. Thr plugged calcined ceramic honeycomb is then sintered in the same manner as previously described to form the sintered plugged ceramic honeycomb. Finally, an unplugged sintered ceramic honeycomb may also be used- The unplugged sintered ceramic honeycomb nay be mace by any suitable method. For example, the sintering may be the same as described for sintering the plugs. After the plugs are inserted, the plugs are sintered as described previously to form the porous sintered plugged cerarrc honeycomb. Example 1 A 37,2"cells per cm" green honeycomb was made by extruding and drying a p.:st> mixture of* alumina, clay, binders and water at Advanced Ceramics Incorporated (Atlanta, GA). The green honeycomb was cut to a length of 150 mm. Half the channels on one end of the green honeycomb were plugged with the same pasty mixture used to make the honeycomb so as to form a checkerboard pattern of plugs on this end (first plugged end). The 'oneycomb was placed in a clamp v/ith the first plugged end facing up (that is, the other or second end was facing down). Mullite powder (Baikalox MULCR, Baikowski International, Charlotte, NC), having an average particle size of 3 micrometers was mixed with 2 propanol and 3 percent by weight of ethyl cellulose (ETHOCEL, The Dow Chemical, Midland, MI) to form a slurry haWr£ 10 percent by weight mullite. The slurry was fluid and easily poured. The slurry was then poured into the unplugged channels of the first plugged end. The slurry flowed into £iid down the channels, coating the walls of the channels and collecting at the other end of ihc honejcomb. The slurry collected at the second end and formed a checkerboard pai::cru of plugs cue to capillary action in the channels not plugged in the first plugged end, to form a piugged green honeycomb filter. After drying, the plugged green honeycomb fiker was heated to I000°C to renov- the binders £nd lightly sinter the oxides. The lightly sintered honeycomb was then convened to acicular mullite using the process described by Moyer, et ah, U.S. Patent No. 5,1!>S 007. The resultant honeycomb wall-flow filter had a discriminating layer of fine needles of mullite where the mullite slurry contacted the walls of the honeycomb. The pluj;s on the first end had essentially the same mullite microstructure as the honeycomb, whereas the plugs on the second end had a. mullite microstructure resembling the discriminating layer. E x ample 2 A green honeycomb plugged on one end was prepared by the same method above. The green honeycomb plugged on one end was then heated to 1000°C to remove the binders and lightly sinter the oxides. The same mullite powder of Example I was mixed with a 4 weight percent solution of METHOCEL in water to form a slurry having 10 percent by weight mullite. The slurry was poured :lown the open channels of the first end in the same manner as Example 1 to font) plugs at the second end. After drying, the honeycomb was heated to 600°C to rcnove the METHOCEL binder from the plugs at the second end. After this, the plugged honeycomb was converted to acicular mullite using the process described by Moyer, et al., U.S. Patent No. 5, J 98,007. The resultant honeycomb wall-flow filter had essentially the samt: microstructure characteristics as the filter in Example I. 1. The invention is a. method of plugging channels in a ceramic hoieycomb comprising; (a) fDrminjj a mixture comprised of a dispersing liquid and ceramic powder, (b) inserting i:he mixture at one end of the channel in an unplugged ceramic hone^comh, such ihat the mixture flows to the other end where the mixture collects and for-ns a plu^iged ceramic honeycomb and (c) h sarin;! the plugged ceramic honeycomb to a temperature sufficient to sinter the plugged ceramic iioneycomb to form a porous sintered plugged ceramic honeycomb. 2. The method of Claim I wherein the dispersing liquid is water or an alcoh'M. 3. The method of Claim 2 wherein the dispersing liquid is methanol, propa-.ol, cthanol or mixtures thereof. 4. The method of Claim 3 wherein the dispersing liquid is propanol. 5. The metliod of Claim I wherein inserting of the mixture is performed by i iS'^rting the mi».r.ire at one end of the channel in the green ceramic honeycomb and allowing the -nixtur^ to flow to the other end, which is blocked, such that the mixture colhrci;; and forms .i plug. ^ 6. The method of Claim 5 wherein the other end is blocked by a porous bod;* capable of reprieving the dispersing liquid of the mixture. 7. The method of Claim 5 wherein the mixture, as it flows through the channel, deposits ceramic powder on a wall of the channel, such that upon heating of step (c) a di:;crinuiiating layer is formed on the wall of the channel. 8. The method of Claim 1 wherein the mixture contains a catalyst. 9. A method of plugging channels in a ceramic honeycomb substantially as herein described with reference to the accompanying drawings.

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