U.S. patent application number 10/716820 was filed with the patent office on 2005-05-19 for composition and method for making ceramic filters.
Invention is credited to Gadkaree, Kishor P., Mach, Joseph F..
Application Number | 20050103232 10/716820 |
Document ID | / |
Family ID | 34574459 |
Filed Date | 2005-05-19 |
United States Patent
Application |
20050103232 |
Kind Code |
A1 |
Gadkaree, Kishor P. ; et
al. |
May 19, 2005 |
Composition and method for making ceramic filters
Abstract
The channel-plugging of porous ceramic honeycombs to provide
wall flow filter bodies therefrom is carried out using water-based
cements comprising ceramic powders and soluble alkali metal
silicates; the cements form durable plugs that are resistant to
thermal and chemical damage upon drying and without firing.
Inventors: |
Gadkaree, Kishor P.; (Big
Flats, NY) ; Mach, Joseph F.; (Lindley, NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
34574459 |
Appl. No.: |
10/716820 |
Filed: |
November 19, 2003 |
Current U.S.
Class: |
106/600 ;
106/626; 106/632; 106/635 |
Current CPC
Class: |
C04B 2111/00793
20130101; C04B 28/26 20130101; C04B 38/0012 20130101; C04B 14/10
20130101; C04B 14/305 20130101; C04B 14/324 20130101; C04B 14/042
20130101; C04B 14/303 20130101; C04B 14/042 20130101; C04B 28/26
20130101; C04B 2111/12 20130101 |
Class at
Publication: |
106/600 ;
106/632; 106/626; 106/635 |
International
Class: |
C04B 012/04 |
Claims
We claim:
1. A cement composition for plugging a ceramic honeycomb consisting
of a plastically shapeable blend comprising a ceramic powder, a
water-soluble alkali metal silicate, and water.
2. A cement composition in accordance with claim 1 wherein the
blend is essentially free of ceramic reinforcing fibers.
3. A cement composition in accordance with claim 1 wherein the
alkali metal silicate is potassium silicate.
4. A cement in accordance with claim 1 wherein the ceramic powder
consists essentially one or more powders selected from the group
consisting of cordierite, silicon carbide, silicon nitride,
aluminum titanate, clay, talc, alumina, and mixtures thereof.
5. A cement in accordance with claim 1 wherein the blend consists
essentially of ceramic powder and an alkali metal silicate
solution.
6. A method for selectively plugging channels of a ceramic
honeycomb comprising: introducing into the channels a plastically
shapeable cement composition comprising a blend of a ceramic
powder, a water-soluble alkali metal silicate, and water; and
drying the cement composition to form solid plugs.
7. A method in accordance with claim 6 wherein the blend is
essentially free of ceramic reinforcing fibers.
8. A method in accordance with claim 6 wherein the alkali metal
silicate is potassium silicate.
9. A method in accordance with claim 6 wherein the ceramic powder
consists essentially one or more powders selected from the group
consisting of cordierite, silicon carbide, silicon nitride,
aluminum titanate, clay, talc, alumina, and mixtures thereof.
10. A method in accordance with claim 6 wherein the blend consists
essentially of ceramic powder and an alkali metal silicate
solution.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to ceramic honeycomb filters
of wall-flow design, and more particularly to materials and methods
for plugging or manifolding such filters as required to provide for
the effective wall flow filtration of particulate-laden gases
flowing therethrough.
[0002] Honeycomb structures formed from ceramic materials have a
number of important uses including use as catalyst supports for
controlling emissions from combustion engines and stationary
pollutions sources such as power plants. They are also used as
porous particulate filter bodies for the filtration of
particulate-laden exhaust gases emitted by combustion engines such
as diesel engines. In the latter case, the bodies are modified
through the sealing or plugging of the ends of selected cells of
the honeycombs to provide a manifolded "wall-flow" filter
structure. The terms "sealed" and "sealing" as used herein refer to
both porous and non porous means of closing selected open
transverse cross-sectional areas of cells.
[0003] In current practice, the fabrication of such filters
involves plugging or otherwise sealing one end of each of the
channels or cells traversing a porous ceramic honeycomb body, a
first subset of cells being sealed at a first or inlet end face of
the honeycomb and the remaining cells being sealed at a second or
opposing outlet end face thereof. A particulate-contaminated fluid
such as an exhaust gas is supplied under pressure to the inlet face
and enters the body via those cells which have an open end at the
inlet face (i.e., "inlet" cells). Because these cells are sealed at
the opposite end face ("outlet" face) of the body, the contaminated
fluid is forced through the thin, porous walls into adjoining cells
which are sealed at the inlet face and open at the outlet face
(i.e., "outlet" cells). The solid particulate contaminant in the
fluid which is too large to pass through the porous openings in the
walls is left behind and a cleansed fluid exits the filter body
through the outlet cells for use.
[0004] Most common in the case of honeycombs with square channel
cross-sections is a masking design producing a checkerboard pattern
of plugs in each end of the honeycombs, such that each inlet cell
is surrounded on four sides by outlet cells, and vice versa.
Examples of such filter bodies and other wall-flow filter designs
that have been developed for these uses, are disclosed, for
example, in European Patent No. 0 043 694.
[0005] For the mass production of plugged wall flow filters it is
highly desirable to be able to seal selected cell ends as rapidly
and as inexpensively as possible. The hand-plugging if individual
cells is long and tedious and is not suited for the commercial
production of such filters. Accordingly numerous approaches to
simplify and speed the plugging operation have been developed. One
such approach, disclosed in U.S. Pat. No. 5,021,204, involves the
use of a rigid solid masking plate having a number of openings or
bores extending therethrough for directing suitable plugging
materials into the ends of the channels to be plugged.
[0006] Many alternative masking approaches employing flexible
polymeric masks or masking strips of impermeable and/or re-useable
materials have also been proposed. U.S. Pat. No. 4,411,856 provides
some examples. Further, a wide variety of compositions for the
plugging materials to be used in these processes is known,
including the foamed cement plugging materials disclosed in U.S.
Pat. No. 4,297,140.
[0007] A common disadvantage of most of the plugging processes and
materials developed to date is that they cannot successfully be
applied to ceramic honeycombs that have not been fired or otherwise
processed to develop high wall strength. Thus many of the known
plugging processes and materials are suitable only for application
to pre-fired honeycombs, and further require a second firing
treatment following the introduction of the plugging materials to
cure and bond the plugs firmly into the honeycomb structure. These
extra firing steps add significant cost to the filter manufacturing
process.
[0008] Another disadvantage of many of the known plugging
formulations is that they require the application of significant
extraneous heat or other energy in order to achieve sufficient
adherence and strength to withstand further handling in the course
of manufacture. So-called "cold set" plugging cements that would
form a durable mechanical seal without additional processing have
not been widely developed or used. Further, some cold set cements
that have been proposed for use employ ceramic fiber additives for
the purpose of plug reinforcement, and such additives are generally
to be avoided for environmental reasons.
[0009] Finally, any sealing system to be used for the manifolding
of ceramic honeycomb filter bodies must exhibit chemical and
physical stability sufficient to withstand extended use under the
conditions encountered by the filter bodies in use. These
conditions include high temperatures and chemical reactive
environments.
SUMMARY OF THE INVENTION
[0010] The present invention provides novel ceramic sealing or
cementing compositions and methods for using them that have
particular utility for the plugging of porous ceramic honeycomb
filter bodies. The cement compositions of the invention offer good
forming and wetting characteristics, yet will solidify upon drying
to form plugs that are intimately sealed to the channel walls of
porous ceramic honeycombs of the kinds presently favored for the
manufacture of such filter bodies. Post-firing of the plugs to
solidify and seal them to the honeycomb structure is not
required.
[0011] The cement compositions of the invention are plastically
shapeable blends of ceramic powder, water water-soluble alkali
metal silicates, and water. The ceramic powders are selected for
compatibility with the ceramic honeycombs to be plugged. That is,
they are selected to provide chemical and physical characteristics,
such as resistance to moisture at high temperatures and thermal
expansion characteristics, that are similar to those of the
honeycombs to be plugged.
[0012] The alkali metal silicate and water components of the
cementing compositions act together to form a dual purpose vehicle
and binder system, plasticizing the cements when wet and
rigidifying and stabilizing the cements after drying. Commercially
available alkali metal silicate solutions provide a convenient way
to efficiently compound and use cement batches containing these
constituents.
[0013] The dried cements are sufficiently strong and durable that
neither fiber reinforcement materials nor other additional
components need to be included in the compositions to enhance plug
properties. However, additions of other water-soluble or insoluble
constituents to these cementing compositions can be made where
needed to modify the physical properties or chemistry of the plugs
for specific applications.
[0014] The cementing compositions of the invention enable a
simplified plugging procedure for the manufacture of ceramic wall
flow filters. The cements can simply be distributed into selected
channels of the porous ceramic honeycombs to be processed, and
thereafter easily dried to form the final plugs. The plasticized
cements are generally compatible with known masking and
channel-filling processes and equipment that have been employed for
the plugging of ceramic honeycombs with conventional plugging
compounds.
[0015] Firing of the plugged honeycombs after drying of the cement
is possible, and may be preferred or even required where the
cements are used to plug green (dried but unfired) honeycombs. One
advantage of these cement compositions is that their handling,
forming, and drying characteristics, as well as their chemical and
physical behavior upon firing, are such as to make them a preferred
plugging material for the processing of unfired honeycombs. Neither
the strength nor the integrity and sealing characteristics of plugs
formed from these cement compositions appear to be adversely
affected by low to moderate temperature firing treatments.
DETAILED DESCRIPTION
[0016] A variety of different ceramic materials have been used for
the fabrication of porous ceramic honeycombs. Variations in
material composition as well as variations in channel wall
porosity, thickness and pore size are used by filter designers to
control filtration efficiency and pressure drop across the filters.
Examples of materials which have been used for such filters include
silicon carbide, silicon nitride, aluminum titanate, cordierite
(magnesium aluminosilicate), and various other porous silicate and
aluminosilicate ceramic compositions.
[0017] Regardless of the particular honeycomb material selected, it
is essential that arrays of channels in the honeycombs be durably
and effectively plugged to ensure that the filters perform their
primary function of wall flow filtration. The plugs may be porous
or non-porous, but they should not be a source of leaks permitting
particulate material to by-pass the filters, and they should be
sufficiently strong and chemically durable to resist displacement
under the high temperatures, stresses and vibrations encountered in
the course of engine exhaust system operation.
[0018] As previously noted, it is conventional practice to fire
ceramic honeycombs to be used for filter fabrication to high
temperatures prior to plugging, so that the honeycombs are strong
enough to withstand the plugging process. The fired filters are
then plugged with a cement, typically of a clay composition
chemically similar to the composition of the fired honeycombs, but
then must be re-fired to cure and bond the plugs into the
channels.
[0019] Depending upon the particular compositions of the honeycombs
and plugging cements selected for use, longer or shorter plugs and
higher or lower firing temperatures must be used to develop the
required plug durability. Where the bond strength between the
selected plugging cement and the honeycomb walls is relatively low,
deeper (longer) plugs must be used to increase the plug-honeycomb
bonding area and reach adequate plug retention strength. Longer
plugs are of course undesirable to the extent that they decrease
the available wall flow area and thereby increase filter pressure
drop. Longer and/or higher temperature cement sealing heat
treatments are of course undesirable for the reasons already
mentioned.
[0020] The cement formulations of the invention are particularly
advantageous because they can in many cases be compositionally
adjusted to match the compositions and properties of many of the
ceramics already found useful for the manufacture of porous
honeycomb filter substrates. Further, the cements offer good
setting characteristics at room temperatures, and form bonds with
many of these ceramics that are sufficiently strong even at modest
plug lengths to be put into use without a second firing treatment
to increase plug bond strength. And finally, plugs formed from
these compositions exhibit good plug integrity even in the absence
of strengthening additions of fibrous reinforcement materials. In
particular, they resist the high temperature strength deterioration
that is known to occur with some conventional cold setting cement
compositions either through second firings or as the result of
thermal spikes that can be experienced by these filters in the
course of use.
[0021] Among the water-soluble silicate binder components useful in
accordance with the invention are any of the commercially available
water-soluble alkali metal silicates, these conveniently being
available as powdered or flaked solids, or dissolved with water in
the form of alkali metal silicate solutions of various
concentrations and viscosities. Examples of such solutions include
standardized solutions of water-soluble sodium and potassium
silicates, including solutions such as the Kasil.RTM. series of
potassium silicate solutions and the A.RTM., N.RTM. and O.RTM.
series of sodium silicate solutions commercially available from the
PQ Corporation, Valley Forge, Pa., USA. These products are sold in
various solution viscosities and also various ratios of
SiO.sub.2/alkali oxide, the Kasil.RTM. series of products, for
example, being available in SiO.sub.2/K.sub.2O ratios ranging from
1.6 to 2.5. Accordingly the plasticity of the plugging cement
compositions can readily be adjusted by adjusting the relative
proportions of alkali silicate and ceramic components to be
included in the cement formulation as preferred to achieve good
plugging characteristics in the final ceramic powder/silicate/water
blend.
[0022] The ceramic component of the plugging formulation will be
selected so that it is compatible with the composition and
properties of the ceramic honeycombs to be plugged, as well
effective to secure the properties targeted for the cured plugs. In
the case of cordierite (magnesium aluminosilicate) ceramic
honeycombs, for example, the ceramic component may suitably
comprise powdered cordierite material, or it may include clay,
talc, and/or alumina powders or powder mixtures that can convert to
cordierite or cordierite-compatible ceramic phases upon firing. The
latter mixtures may be preferred for the plugging of green
(unfired) honeycombs, as conversion of the plugging material to
cordierite or cordierite-compatible phases such as alumina or
spinel can conveniently occur concurrently with the development of
cordierite ceramic phases in the plugged honeycomb during the
firing process. Similarly, for the plugging of silicon carbide,
silicon nitride, or aluminum titanate honeycombs, powders of the
silicon carbide, silicon nitride or aluminum titanate ceramic
materials used to form the honeycombs can comprise the ceramic
component of the plugging cement.
[0023] As the applications for the plugged wall-flow filters are
generally high-temperature applications, thermally stable
honeycombs and plugging cements are typically required. For that
reason, potassium silicate solutions are normally preferable to
sodium silicates because potassium ions exhibit lower mobility than
sodium ions at equivalent temperatures. However either alkali
formulation offers a cementing composition that will dry
efficiently to form strong well-bonded plugs without supplemental
firing.
[0024] Alkali ion selection is less important in cases where the
option to extract alkali from the plugs is exercised. Thus, one
process modification that is useful in carrying out plugging in
accordance with the invention is to subject the plugged honeycomb
to an ion-exchange treatment to extract alkali ions from the plugs.
This step can be accomplished by immersion of the plugged honeycomb
into a suitable ion-exchange medium, such as an aqueous ammonia
solution, to extract alkali from the plugs and leave only a
silica-bound ceramic plugging material after drying. Again, this
processing can be carried out at ambient temperatures and minimal
cost, as the need to perform a firing treatment to set the plugs is
still avoided.
[0025] An alternative to the use of an ion-exchange treatment to
reduce plug alkali content is the option of including reactive
salts of multivalent cations, such as calcium, aluminum, manganese,
magnesium, iron, zinc, nickel, chromium, copper or the like in the
cementing formulation. These salts can react with the binder or
ceramic components of the cements to form additional chemically
resistant compounds. Such reactions can occur and such compounds
can form either in the course of a post-plugging firing treatment
or in the course of subsequent use of the filter.
[0026] Other additives to these cementing formulations can include
conventional cementing materials such as the calcium aluminates
which can react with a variety of ceramic powders upon heating to
form strong, chemically resistant cementing phases. In all cases,
however, it is important to include proportions of the base ceramic
powder components that are sufficient to insure that the dried plug
remains thermally and chemically compatible with the ceramic
material forming the honeycomb. The inclusion of additives in
proportions that can introduce thermal stresses or objectionably
increase the reactivity of the plugging material is to be avoided,
such damaging proportions however being readily identifiable
through routine experimentation.
[0027] The invention may be further understood by reference to the
following representative examples, which are intended to be
illustrative rather than limiting.
EXAMPLE 1
Cordierite-Potassium Silicate Cement
[0028] A cordierite plugging cement suitable for the manifolding of
porous cordierite honeycomb bodies is first prepared. To formulate
such a cement, a quantity of powdered cordierite ceramic resulting
from the reaction-sintering of a clay-talc-alumina mixture is
classified to provide a powder feed of 10 .mu.m average particle
size. A quantity of this cordierite powder is then added to a
container of Kasil.RTM. 1 potassium silicate solution, a 29.1%
(weight) aqueous solution of a potassium silicate having a 2.5:1
SiO.sub.2:K.sub.2O weight ratio that is commercially available from
the PQ Corporation, Valley Forge, Pa., USA. The ceramic powder is
thoroughly mixed with that silicate to form a viscous paste cement
comprising 60% by weight of cordierite powder and the remainder
potassium silicate solution by weight.
[0029] The paste thus provided is next charged through a mask into
alternate channels of a cordierite ceramic honeycomb having a cell
density of 100 cells/in.sup.2 and a channel wall thickness of about
17 mils, the mask producing a checkerboard pattern of alternate
channels plugged by the cement across, the face of the honeycomb.
The paste thus applied is then allowed to dry under ambient
conditions, resulting in strong, hard cement plugs free of cracking
and showing no sign of separation from, or damage to, the channel
walls of the honeycomb.
[0030] For the purpose of evaluating the stability of the plugs
thus provided, the entire plugged honeycomb is heated to a
temperature of 1000.degree. C. in air, cooled to ambient
temperature, and re-examined. The strength and hardness of the
cement plugs are unaffected by this heat treatment, nor is any
separation, cracking or other damage to the plugs or surrounding
honeycomb structure observed.
[0031] To further evaluate the chemical stability of cement plugs
provided from this composition, a set of plugged honeycomb samples
fired to 1000.degree. C. and a second set of samples incorporating
dried but unfired plugs are subjected to acid leaching in aqueous
0.01N nitric acid solutions for 24 hours. Neither the dried plugs
nor the dried and fired plugs are adversely affected by the acid
treatment.
EXAMPLE 2
Comparative Strength Testing
[0032] The thermal stability of the plugging cement of Example 1 is
evaluated by comparing the strengths of new and heat-treated cement
samples formulated as in that Example with samples made from a
conventional plugging cement. The tests are carried out on MOR
(modulus of rupture) strength testing bars of the two cements, by
stressing the bars to breakage under 4-point bending stress.
[0033] The conventional plugging cement employed for testing has a
base composition consisting, in weight percent, of about 66%
cordierite powder, 26.8% Fiberfrax.RTM. QF-180 ceramic fiber
reinforcement, 6.4% Ludox.RTM. colloidal silica binder, and 0.8% of
a methyl cellulose temporary binder. A water vehicle is blended
into this mixture in a proportion of about 26.8 parts water for
each 100 parts cement mix by weight. All MOR tests are carried out
at an ambient temperature of 25.degree. C.
[0034] Representative results for such tests are reported in Table
A below.
1TABLE A Thermal Durability Tests Heat Treatment Cement Composition
Temperature M.O.R.-kPa (Psi) Standard (Ludox) cement none 2158 kPa
(313 .+-. 40 psi) " 500.degree. C. 462 kPa (67 .+-. 20 psi) "
1000.degree. C. 2137 kPa (310 .+-. 10 psi) Example 1 cement none
12514 kPa (1815 .+-. 200 psi) " 500.degree. C. 11321 kPa (1642 .+-.
95 psi) " 1000.degree. C. 12066 kPa (1750 .+-. 100 psi)
[0035] The significantly higher strengths of the Example 1 cements,
both before and after heat treatment, are evident from the data in
Table A. Particularly important is the capability of the Example 1
cements to avoid the large reductions in strength observed for the
conventional cements at intermediate heating temperatures. The
several-fold increase in cement strength observed to result from
the use of the Example 1 composition will not only increase the
durability of the plugs but will also permit the use of shorter
plugs having a reduced impact on the wall flow filtration area of
the filters.
EXAMPLE 3
Cordierite-Sodium Silicate Cement
[0036] The cement compounding, plugging, drying, and firing
procedures of Example 1 are repeated, except that the Kasil.RTM.
potassium silicate solution of Example 1 is replaced by a sodium
silicate solution. The sodium silicate solution used is N.RTM.
sodium silicate solution commercially available from the PQ
Corporation, Valley Forge, Pa., a 37.5% (weight) aqueous solution
of a sodium silicate having a 3.22:1 SiO.sub.2:Na.sub.2O weight
ratio. The resulting plugged honeycombs provided as in this Example
3 are inspected and tested following the procedures described in
Example 1, and all of the inspection and testing results are
substantially the same.
EXAMPLE 4
Cordierite/Calcium Aluminate--Potassium Silicate Cement
[0037] A mixture consisting, in weight percent, of 30% of
Kasil.RTM. 1 potassium silicate solution, 35% of cordierite powder
as described in Example 1, and 35% of a calcium aluminate powder
(average particle size 10 microns, commercially available from
Lafarge Calcium Aluminates, Chesapeake, Va. 23324) is prepared and
thoroughly mixed to provide a cordierite/calcium aluminate-based
cementing composition. The cement paste thus provided is similar in
handling characteristics to the cement of Example 1, and is
similarly charged through a plugging mask into alternating channels
of a cordierite ceramic honeycomb of the same geometry and
composition as the honeycomb of Example 1.
[0038] The silicate/cordierite/calcium aluminate plugs thus
provided are dried as described in Example 1, and again produce
strong, hard ceramic plugs that are well-bonded and sealed to the
channel walls of the cordierite honeycombs. Neither acid leaching
in 0.01N aqueous nitric acid nor firing of the plugged honeycombs
to 1000.degree. C. to thermally age the plugged structures as
described in Example 1 produces any significant deterioration in
plug or honeycomb integrity.
[0039] While the invention hereinabove set forth has been described
with reference to certain specific and detailed embodiments and
examples thereof, it will be apparent that those embodiments and
examples have been presented for purposes of illustration only, and
not for the purpose of circumscribing the invention as it may be
practiced within the scope of the appended claims.
* * * * *