FIELD OF THE INVENTION:
This invention rotates to a method for selective plugging of channels in ceramic
honeycomb structure.
This invention further relates to a method of preparing solid plugs from green
ceramic material, for selective sealing of open ends of channels In ceramic
honeycomb structure.
BACKGROUND OF THE INVENTION:
Ceramic Honeycomb wall-flow filters are used for solid particulate filtration from
gases in which the particulates are mixed or suspended. The ceramic wait-flow
fitters are fabricated from a cellular or honeycomb structure in which mutually
adjoining cells or hollow passages extend through the structure between a pair of
its opposing end faces where the open, transverse cross-sections of the cells are
exposed. Fabrication of wall-flow fitters involves plugging selectively the end
faces of the ceils In checkered board pattern such that the cells open at one end
face are plugged at other end face. The walls of the cells in such a honeycomb
structure are thin and porous with high permeability, which allows the fluid to flow
through while preventing solid particles from flowing.
The ceramic wall-flow fitters used for exhaust emission nitration in diesel engines
are subjected to variable flow rates in which the pressure applied to the plugs are
variable and sometimes It is higher at higher emission rates. Due to continuous
Impingement of gases on the plugs In ceramic fitters over a period of time, the
erosion of (he plugs are fast and can get punctured if the strength of the plug
sintered along with ceramic honeycomb is not adequate.

DPF it fabricated using various materials such as cordierite, sillcon carbide etc.
The process or manufacturing Honeycomb involves extrusion molding, followed
by drying and then alternately plugging the end faces, followed by firing. Different
methods are available for plugging the end faces of Honeycomb.
U.S. Patent No. 4,329,162 also discloses ceramic honeycomb filters in which
selective seals are provided to the Inlet and outlet channels by injecting a
sinterabie material or other sealing cement into the appropriate ends of the
channels to form a "checkerboard" array of open/closed channels at each end
face.
Thermal expansion mismatch is one of the major disadvantages of using ceramic
cement for sealing of honeycomb cells and it increases the stress levels at the
end face of the honeycomb cells at higher temperatures, which may lead to
cracking of filters in use. Also the honeycombs sealed with ceramic cement can
only be used at low temperatures compared to that fabricated completely from
cordiertte.
U.S. Patent No. 4,410,591 describe alternate methods of fabricating a multiple
flow path body such as a stationary heat exchanger. A honeycomb structure is
provided having its cells arranged in columns across is open end faces, an open
end face of a honeycomb structure is dipped into a flowable resist material and
the resist material removed from selected columns by cutting it away together
with the common walls of the adjoining cells in the selected column or,
alternatively, the wads between the adjoining cells of the selected columns are
cut away at the open end face of the structure before dipping the end face into

the flowable resist material and the resist material is blown from the selected
columns using compressed air directed down the selected columns where the
adjoining cell walls had been removed. The end face was thereafter dipped into
slurry of cement to form a sealed channel across each of the selected columns.
The remaining flowable resist material was subsequently removed by heating. As
the cross-sectional density of cells in the honeycomb structure is increased, for
example to improve the efficiency of a filter body, the tolerances needed for the
removal of adjoining cell walls required by the Noll, et at method requires the
partial destruction of adjoining cells are sealed in a checkered or other possible
alternating cell patterns at the end faces.
European patent No. 42,301 discloses a method in which a ceramic honeycomb
is selectively sealed by bonding a sealing member to each end face of the
honeycomb. The sealing member has same outer shape as that of the
honeycomb and has network of alternate open channels separated by sealing
portions in checkered pattern or any other desired pattern. The open channels in
the seating member are located corresponding to the cells of the honeycomb
which are to be kept open. The extruded sealing members and green
honeycomb are separately dried before the sealing members are fitted to the
honeycomb body itself, preferably by means or any organic adhesive, a glass
material, or a ceramic material, followed by standard firing of the composite so
formed. The ceramic honeycomb fitters produced by above method can not be
sealed properly as the cells of the sealing member will have different drying
shrinkage than that in honeycomb resulting into mismatch of the cell openings in
honeycomb to the seating member. Otherwise, it requires exceptionally close
absolute dimensional matching of both the structures, which can only be met with
very high dimensional tolerances. It is impractical to fabricate the structures with
such high tolerances.

U.S. Patent No. 4,411,856 proposes the use of a mask completely covering one
end face of the monolith, this mask being provided with passages and with peg-
like extensions on the side facing the monolith. The pegs are placed into the
ducts that are not to be sealed whereas the composition is introduced through
the passages in the mask info the end regions of the ducts to be seated. An
attempt is made to take the aforementioned deviation of the monolith from a
desired geometry Into account by making the mask utilized of an elastic material.
This method may be applicable within limits. There is, though, the problem that
the spacing of the above-mentioned pegs to the passages Is fixed so that the
compensation of (he deviations of the monolith from the desired geometry,
obtained by the elasticity of the mask, remains restricted to very low values.
US Patent No. 5,766,393 discloses a method where expansive material is used
to temporarily seal the alternate holes of Honeycomb in checkerboard pattern. A
ceramic material paste is used to plug the remaining notes. The green
honeycomb thus fabricated is sintered to produce diesel particulate fitter. The
expensive materials used for sealing are burnt during the process of flring and
forms opening in checkerboard pattern.
In another US patent 6,627,123, a powdered material is charged Into selected
cells of honeycomb and then ft is compacted by vibration, centrifuge, manual
compaction or some other method. After compaction of the powdered material
into the cells of the honeycomb laser energy is applied to heat it sufficiently to
become flowable which is further solidifies after cooling to form plugs.

The fitters produced by such method does not ensure uniform plug length and it
will result to differentia! back pressure when the fitter is used for gas filtration.
Due to this, a mechanical stress win be generated during application and It may
result in detoriation of mechanical strength of fitter.
Also, during the process of plugging of green honeycomb with a plastic material,
usually the material is either filled into the cells by applying high pressure or a
material with high flowability is used to improve the workability. In case of highly
plastic material, the pressure applied on the honeycomb end face is high which
may damage or deform the cells in the process while in case of highly fiowable
material the dispersing medium diffuses into the cell wall which is adequate to
soften the cell watts.
Another problem with above method is the high shrinkage of plugging material
compared to the honeycomb, which is being plugged. Due to this the green
honeycombs are subjected to high stress during drying. This may lead to crack
generation in the filters after firing.
During the process of plugging honeycombs by above method, when laser
energy is applied the heating of powdered material is only done while adjoining
cells, which are open, are not subjected to any heating. As a result of this,
temperature differential is attained at different locations on the end surface of the
honeycomb. Also the center portion of the honeycomb Is at low temperature than
the end faces. Due to this a high level of stress is generating which can cause
cracking of honeycomb.

Apart from this, a close control on heating of powdered material Is also needed,
as the plugging material, which is being heated, can conduct enough heat to
deform the cell walls.
In another US Patent No. 5,062,911 the method of plugging green honeycomb
structure is disclosed where dried green honeycomb is wetted, preferably with
water, to re-plastictze the ceramic material at the face and a stilt-wet end cap
body is then firmly fitted onto each face with sufficient force to Integrate the
plasticized ceramic material of each into a substantially continuous phase. Any
excess material from the end cap that extends beyond the sides of the
honeycomb Itself as a result of this pressing operation can be trimmed off so that
the edges of the end caps are flush with the sides of the honeycomb. The end
caps are then selectively perforated by a hand-held punch or tool having a pin
that is sized and shaped appropriately to pierce the end cap in a manner
congruent with the size and shape of the channel openings to be exposed.
In the methods explained for plugging of honeycombs, a flowable material is
used. The flowable material is charged into the honeycomb cells by capillary
action in certain cases while sometimes a plastic paste is pushed into the cells by
applying pressure. Charging of material with high flowabllty into the honeycomb
cells Is mostly preferred as It gives higher plug length compared to that achieved
by using plastic mass. The desirable consistency for material with high flowabllity
for this purpose is achieved only at higher water content and the quantity of
material charged into the cells of honeycomb Is much higher than the material
quantity of cell walls with which it makes contact. Due to this, localized softening
and deformation of cells in the green honeycombs is seen as the binders in
honeycomb readily absorb water from charged material.

Another drawback of these methods include the generation of high stress
locations on the end face of the honeycomb cells due to high drying shrinkage of
materials used for plugging. This can lead to crack generation at end face of the
cells in honeycomb fitters.
Non-uniformity in plug length is also commonly seen in the fitters produced by
above methods, which further leads to lower mechanical durability when used for
gas filtration application at higher temperatures.
OBJECTS OF THE INVENTION:
It is therefore, an object of this invention to propose a method of preparing solid
plugs for selective sealing of channels in ceramic honeycomb structure which
helps to avoid softening and deformation of cells during the method of plugging
dried green ceramic honeycombs.
It is further object of this invention to propose a method of preparing solid plugs
for selective sealing of channels In ceramic honeycomb structure which leads to
uniform length of the plugs across the section of plugs between two end faces.
Another object of this invention to propose a method of preparing solid plugs for
selective sealing of channels in ceramic honeycomb structure to reduce stress
level in the honeycomb cells occurring due to higher drying shrinkage.
Yet another object o( this Invent ton to propose a method of preparing solid plugs
for selective sealing of channels in ceramic honeycomb structure which have
better filtration efficiency and lower pressure drop.

Still another object of this invention to propose a method of preparing solid plugs
for selective sealing of channels in ceramic honeycomb structure, which provides
any desired length of plugs even with higher ceil density.
DESCRIPTION OF THE INVENTION:
The various aspects of the invention are better understood with reference to the
accompanying drawings, in which:
Fig 1 depicts schematically the step of forming plugs by using plug-forming
apparatus.
Fig 2 depicts schematically the stainless steel wires for cutting plugs into required
length.
Fig 3 depicts schematically a member used for removing plugs formed by using
apparatus shown in Fig 1.
Fig 4 depicts schematically the step of covering selective cells In a ceramic
honeycomb with a mask.
Fig 5 depicts schematically the step of applying ceramic slurry into cells of the
honeycomb.
Fig 6 depicts schematically the method of selectively applying ceramic slurry Into
the cells of the honeycomb.
Fig 7 depicts schematically the step of plugging honeycomb cells selectively with
solld ceramic plugs.
DETAILED DESCRIPTION OF THE INVENTION:
Thus according to this Invention is provided a method for selective sealing of
channels in ceramic honeycomb structure comprising, forming dried green plugs
of ceramic material, followed by preparing a ceramic slurry containing a binder,
selectively applying said slurry Into the cells of a honeycomb structure, and
sealing the same by inserting the plugs.

The step of forming the ceramic plugs comprises charging a plastic ceramic mix
of plugging material into the channels of fired honeycomb or plug forming
apparatus.
The ceramic material and slurry used are preferably of same composition as that
used for fabricating honeycombs. However, it can also be made out of materials
with thermal expansion nearer to that of honeycomb. The ceramic materials used
for plug, slurry and honeycomb can be of cordlerlte, silicon, carbide, silicon
nitride, mullite, alumina, zirconia, aluminium titanate etc. depending upon
application. For low thermal expansion application cordlerlte or silicon carbide is
predominantly used whiIe for high heat resistant application mullite, alumina
zirconia or aluminium titanate are used.
Further, the method explained can also be used for plugging fired honeycombs
where fired plugs can be inserted into the cells by using ceramic cement with
thermal expansion nearer to that of honeycomb. This method of plugging can
also be used for honeycombs having circular, rectangular or trigonal cells.
The shrinkage of the material plugged into the celts of honeycomb is significantly
less and hence generates very small stress on the cell walls, thereby reducing
the chances of crack generation in ceramic fitters.
Fig 1 depicts an exemplary apparatus for forming dry green ceramic plugs. The
apparatus can be made out of any corrosion resistant material, particularly from
stainless steel. The apparatus has channels 1 extending in longitudinal direction
and, which is open at end faces and with another opening along the surface 2 as
shown in Fig 1. The channels extend parallel to each other separated by partition
3.

The width of the partition are equal and can be varied for which a typical value
here is 2.1mm. The channels are having square cross section for which the width
is 2.1mm. Further, the apparatus has fine slits 4 of width 0.2mm and at equal
distance of typically 7 mm from each other. These silts have depth of 2.1mm and
the bottom 5 has circular hole of diameter 0.25 mm extending along the bottom in
transverse direction of the apparatus.
For making dried green ceramic plugs, ceramic raw materials with same
composition as used in honeycomb extrusion are mixed for 4 hrs in a cone
blender and then mixed with 20% water in a high shear mixer for 15 to 30
minutes. The mixed body prepared is kept for aging upto 12 hrs in a seated
container to avoid any moisture loss. Aging of body is done to get uniform
distribution of moisture within the body. The aged body is mixed again in high
shear mixer to get homogenous plastic body. The wet plastic ceramic mix is Wed
into the channels 1 of apparatus.
After filling up of wet plastic ceramic mix Into the channels 1 of apparatus In Fig
1, a stainless steel sheet of typically 1 mm thickness and having a sharp edge at
one end is used to wipe the excess material oozed out on the surface of the
apparatus.
Further, stainless steel wires of typically 0.4 mm with metal beads at the ends as
shown in Fig 2 are used to cut the plugs into required length. For cutting the
plugs fitted into the channels to required lengths the wire Is kept in stretched
condition and run manually across the slits 4 from the surface to the end of the
slit 5 in Fig 1.

The apparatus in Fig 1 filled with wet ceramic mix in its channels Is kept into an
oven at 1100C for 4 Hrs to dry and then removed for cooling it to ambient
temperature. The ceramic material in the apparatus shrinks after drying and can
be easily removed by pushing it from the end face of the channels using a
member shown in Fig 3 with square cross section of 0.25 mm width and 50 mm
length. By this method solid dried green plugs having square cross-section and
of 6 mm length an 0.25 mm width are prepared. The plugs 3 shown in Fig 5 are
formed by such method are having equal length measured across any section
between two-ends of the plug.
Dried green ceramic plugs can also be prepared by ruling up plastic mix Into a
fired ceramic honeycomb and then drying it into oven at 110°C for 4 Hrs. To get
suitable length and cross section of plug, fired honeycomb slices of required
length are cut from the same lot of honeycombs, which are dried and kept for
plugging. The cross section of plugs formed by this method exactly matches the
cells of the dried green honeycombs, which are to be plugged. This method is
adopted preferably if the deformation in cells of honeycomb is more.
A ceramic slurry used in the process of plugging honeycombs. It is preferably
prepared out of same material as that used for extrusion of honeycombs. For
preparing ceramic slurry 46% kaolin, 41% Talc and 13% Alumina by weight is
mixed and again blended with 5% methyl cellulose by weight for 2 hrs in a cone
blender. Further, 35% water is added to the mix of ceramic materials and organic
binder and then mixed for 30 minutes by using a stlrrer.
To apply a thin coating of ceramic slurry on the cell walls at the end face 1 of the

honeycomb In Fig 4, a mask 2 with openings 2a in checkered pattern as shown
in Fig 4 is used. The mask 2 can be made out of any material preferably with
stainless steel sheet. Openings made in the mask are of the same dimension as
that In honeycomb. The green honeycomb to be plugged with dried green plugs
is kept vertically on a flat surface and the mask is placed on the end face of the
honeycomb in such a way that it covers the cells of honeycomb in checkerboard
pattern.
A thin coat of ceramic slurry is applied on the surface of the cell walls at the end
face as depicted in Fig 5. An enlarged view of cell walls across the section taken
along the ling X-X in honeycomb coated with ceramic slurry is shown in Fig 7.
For applying ceramic slurry into the cells a fine brush is used. The brush is
dipped into ceramic slurry to adhere sufficient quantity of material into It and then
the material adhering to the brush is discharged by mopping it on the surface of
the ceils of honeycomb which are kept open for plugging. The ceramic slurry
when applied using a brush Is easlly absorbed into the cells due to gravitational
force as well as capillary action. So, the quantity of material discharged into cells
depends on the consistency of slurry made and dimension of the cells. For
plugging of green honeycombs the ceramic slurry applied is very small in quantity
In order to avoid any softening of cell walls. Also, It Is required to get a thin coat
of slurry on the surface of cell walls.
To optimize the quantity of slurry being discharged Into the cells, a pick and place
mechanism can also be used where material flow can be controlled. A small
quantity of slurry is picked and released into the cells selected for plugging as

shown in Fig 6. During this process air is blown from the other end of the channel
in which slurry is released to achieve a required length of slurry coating from the
end face of the cells, The air blown from the other end of the channels also
increases the evaporation of water present in slurry. Thus, a very small quantity
of water remains in the slurry coated on the honeycomb cell wails and which is
not adequate to diffuse into the cell walls thereby preventing the softening of ceil
walls. Plug also may be soaked with slurry by using brush or other means.
The plugs formed are now inserted into the ceils fully to match one of the end
surfaces of the plugs to the end face of the honeycomb.
The method of masking and then plugging of cells is done on the other end face
of the honeycomb as explained earlier in such a way that the cells which are
plugged on opposite side are open at this end.

WE CLAIM:
1. A method for selective sealing of channels in ceramic honeycomb
structure comprising, forming dried green plugs of ceramic material,
followed by preparing a ceramic slurry containing a binder, selectively
applying said slurry into the cells of a honeycomb structure, and sealing the
same by inserting the plugs.
2. The method wherein the ceramic material used for plugging has a thermal
expansion similar to that of the honeycomb.
3. The method as claimed In claims 1 and 2, wherein the material used for
plug, slurry and honeycomb is selected from cordiertite, silicon carbide,
silicon nitride, mullite, alumina and aluminium titanate and zirconia.
4. The method as claimed in claim 1, wherein the step of preparing ceramic
plugs comprises charging a plastic ceramic mix of plugging material into
the channels of a plug forming apparatus and drying it.
5. The method as claimed in claim 4, wherein said plug forming apparatus is
a fired honeycomb.
6. The method as claimed in claim 5, wherein said fired honeycomb is
produced from honeycombs to be plugged.

7. The process as claimed In claim 1, wherein said ceramic honeycomb has
10 to 600 cells per square Inch, preferably 100 to 400 cells per square
inch.
8. The method as claimed in claim 1, wherein the cross section of the cells in
the honeycomb structure can be square, circular, rectangular or the like.

The invention relates to a method for selective sealing of channels in ceramic honeycomb structure comprising, forming dried green plugs of ceramic material, followed by preparing a ceramic slurry containing a binder, selectively applying said slurry into the cells of a honeycomb structure, and sealing the same by
inserting the plugs.