Method of making diesel particulate filters

Porous ceramic articles are made by a method that allows the formed green body to be dried in a dielectric oven without arcing or shorting occurring while maintaining favorable physical properties. The method includes adding water insoluble cellulose and graphite to the ceramic-forming precursors as a burnout material. The method is particularly useful in forming porous cordierite articles that are extruded to form a honeycomb structure conventionally used as a particulate filter for the exhaust fluids of diesel engines. Such articles have a matrix of thin walls forming a multiplicity of open-ended cells extending from one end to another end of the honeycomb. The thin walls have a substantially smaller coefficient of thermal expansion in the direction parallel to the axes of the open-ended cells than in the direction transverse to the thin walls.

FIELD OF INVENTION 
The present invention relates to a method of making a porous ceramic 
article suitable for use as a diesel particulate filter and capable of 
being substantially dielectrically dried. 
BACKGROUND OF THE INVENTION 
It is well known that solid particulates and larger particles may be 
filtered from fluids (i.e., gases and/or liquids) by passing the 
particulate contaminated fluids through porous, walled honeycomb 
structures. U.S. Pat. No. 4,329,162 describes and claims honeycomb filters 
for removing carbonaceous solid particulates from diesel engine exhausts 
and other filtering applications. A typical diesel particulate filter 
("DPF") has a multiplicity of interconnected thin porous walls which 
define at least one inlet surface and one outlet surface on the filter and 
a multiplicity of hollow passages, or cells, extending through the filter 
from an inlet surface or an outlet surface or both. Inlet cells are open 
at least one inlet surface to admit contaminated fluid into the filter. 
The inlet cells are closed where they adjoin any outlet surface of the 
filter. Outlet cells are formed open at an outlet surface to discharge 
fluid which has passed through the filter. The outlet cells are similarly 
closed where they adjoin any inlet surface. The interconnected thin walls 
are provided with an internal interconnected open porosity which allows 
the fluid to pass from the inlet to the outlet cells while restraining a 
desired portion of the solid particulates in the fluid. 
The particulates are trapped in or collected on the surfaces of the thin 
walls defining the inlet cells. As the mass of collected particulates 
increases, back pressure across the filter increases and/or the flow rate 
of fluid through the filter decreases until an undesirable level of back 
pressure and/or flow rate is reached and the filter either is regenerated 
by removal of the trapped particulates or discarded. DPFs are typically 
installed in a housing which, like a muffler or catalytic converter, is 
inserted into the exhaust system of a diesel engine equipped vehicle. 
To produce the required porosity in a ceramic substrate to be used as a 
particulate filter, a "burnout" material is commonly added to and mixed 
with ceramic precursors prior to firing. This pore-forming material is 
burned out when the ceramic precursors are fired to produce the hardened 
ceramic body. The most common burnout material used in ceramic articles is 
graphite because it produces pores of optimal size and good overall 
porosity without swelling which can cause cracking or weakening of the 
ceramic article. 
Although ceramic ware prepared with up to 30 weight percent graphite 
exhibit acceptable physical properties, graphite burnout material is not 
without disadvantages. The most severe problem is the inability completely 
to dry graphite-containing ceramic ware dielectrically. A dielectric dryer 
utilizes a pair of opposing plates or electrodes to create a high 
frequency electrical field between the plates or electrodes. This 
"dielectric" field couples with the water in the ware, resulting in 
absorption of energy by the water. This energy absorption results in 
heating and evaporation of the water in the ware. Dielectric drying is the 
preferred method of drying ceramic ware because of the speed and 
uniformity with which the ceramic articles are dried. In addition, 
dielectric drying decreases cracking of the article during drying and 
increases the dimensional accuracy of the finished ware. 
It has been found, however, that if the formed ceramic substrates 
containing a high level of graphite are dried dielectrically beyond some 
point (and before drying is complete), arcing or shorting takes place 
between the electrodes of the dryer and the ceramic ware. Arcing in the 
dielectric dryer can cause many problems including burning of the ware, 
cracking, or damage to the dryer. Also, because the ware cannot be fully 
dried dielectrically, drying must generally be completed in a conventional 
hot air oven. Due to the size of the ware typically used as particulate 
filters and non-uniformity of drying, considerable cracking of the ceramic 
can occur during hot air drying. Lastly, the use of graphite to develop 
porosity in the ceramic article results in a large exothermic reaction 
when the graphite is burned out. The reaction causes the inside of the 
ware to get much hotter than the outside during firing. These severe 
thermal gradients are another cause of cracking. 
Therefore, although the use of graphite as a burnout material has resulted 
in ceramic wares exhibiting good physical properties, there continues to 
be a need to improve the method of producing dimensionally accurate, 
durable porous ceramic substrates. 
SUMMARY OF THE INVENTION 
The present invention relates to an improved method of making porous 
ceramic articles suitable for use as, for example, filters to remove 
suspended particulates from the exhaust gas of diesel engines. It has been 
discovered, in accordance with the present invention, that reducing the 
level of graphite burnout material will prevent arcing during dielectric 
drying of the ceramic article. Unexpectedly, the substitution of graphite 
with an amount of water insoluble cellulose will allow the ceramic article 
to be dried dielectrically without arcing or shorting and, therefore, 
without the need for an additional "hot air" drying step. 
The present method of making a porous ceramic article comprises the steps 
of blending ceramic-forming precursors, graphite, and water insoluble 
cellulose with a vehicle and other desired forming aids to form a plastic 
mixture, forming the plastic mixture into a green body, dielectrically 
drying the green body, and firing the dried green body to form the desired 
porous ceramic article. 
The method of the present invention can be used to make a porous cordierite 
article. Graphite and water insoluble cellulose, and ceramic-forming 
precursors in amounts suitable to form an analytical batch composition by 
weight on an oxide basis of 9-20% MgO, 30-50% Al.sub.2 O.sub.3, and 
41-56.5% SiO.sub.2 are blended with a vehicle and other desired forming 
aids to form a plastic mixture. The plastic mixture is anisostatically 
formed into a green honeycomb and the green honeycomb is dielectrically 
dried. The dried green honeycomb is then fired under conditions effective 
to form a ceramic body consisting essentially of cordierite. 
The method of the present invention is particularly useful to make diesel 
particulate filters. Graphite, water insoluble cellulose, and 
ceramic-forming precursors in amounts effective to form a cordierite 
article are blended with a vehicle and forming aids to form a plastic 
mixture. The plastic mixture is anisostatically extruded to form a green 
honeycomb which is dielectrically dried and fired to form a cordierite 
article. The fired cordierite honeycomb is plugged and refired to form a 
diesel particulate filter. 
The substitution of a portion of the graphite burnout material with water 
insoluble cellulose allows the ceramic article to be completely dried 
dielectrically without arcing or shorting, while maintaining the desirable 
overall porosity and average pore size. Porous ceramic articles 
manufactured by the present process also exhibit a low incidence of 
cracking during drying. In addition, the use of a combination of cellulose 
and graphite also reduces the level of heat generated during any part of 
the firing because the cellulose burns out at a lower temperature than 
graphite. Therefore, any thermal gradients present will be spread out over 
a longer portion of the firing schedule and will reduce the thermal 
stresses accordingly. This, in turn, reduces the incidence of cracking 
during firing.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the present method, a new method of making a porous 
ceramic article is disclosed. The porous ceramic article formed by the 
method of the present invention is especially useful as a diesel 
particulate filter. The preferred embodiments disclosed below are, 
therefore, directed to that application of the present invention. 
Essentially, the method of the present invention includes the steps of 
blending graphite and water insoluble cellulose with ceramic-forming 
precursors and an effective amount of vehicle and forming aids to form a 
plastic mixture. The plastic mixture is formed into a green body and 
dielectrically dried. The dried green body is then fired to form a porous 
ceramic article. 
The selection of the ceramic-forming precursors which will comprise the 
ceramic batch will depend on the desired fired composition. Any 
combination of ceramic-forming precursors that may be fired to form, for 
example, alumina, mullite, cordierite, zircon, silicon nitride, silicon 
carbide, spinel, zirconia, or forsterite, may be used as the 
ceramic-forming precursors of the present invention. 
Cordierite ceramic materials are generally preferred for diesel particulate 
filters due to their durability under the extreme temperatures and other 
physical or chemical conditions present in diesel engines. Exemplary 
cordierite ceramic materials and methods of making cordierite-containing 
honeycomb structures useful in the method of the present invention are 
disclosed in U.S. Pat. Nos. 3,885,977 and 4,001,028, the disclosures of 
which are hereby incorporated by reference. A particularly preferred 
method of making cordierite-containing ceramic articles is disclosed in my 
copending U.S. patent application Ser. No. 07/816,228 entitled "Modified 
Cordierite Precursors", filed Jan. 3, 1992. 
Typically, to form a cordierite ceramic article, talc, silica, alumina, 
aluminum hydroxides, and magnesia-yielding chemicals are used with the 
proportions of clay, talc, silica, aluminum hydroxides, and alumina chosen 
to produce cordierites analytically consisting essentially of 41-56.5% 
SiO.sub.2, 30-50% Al.sub.2 O.sub.3, and 9-20% MgO. The total weight of 
MgO, Al.sub.2 O.sub.3, and SiO.sub.2 is preferably at least 95% of the 
entire weight of the ceramic article. 
The crystals of cordierite, in accordance with the present invention, 
become preferentially oriented during the firing process. It is believed 
that this is accomplished by the use of raw materials in the form of flat, 
planar particles (i.e., platelets) rather than large isodimensional 
particles. Suitable platey materials are found in various talcs and 
delaminated clays, such as delaminated kaolin. 
The term platelet refers to the shape and geometry of a particle of 
material. The particle has two long dimensions and one short 
dimension--i.e. the length and width of the platelet are much larger than 
its thickness. The length and width need not be equal, but they must be 
much greater than the thickness of the platelet. 
The ceramic-forming precursors must be blended with an amount of water 
insoluble cellulose and graphite. As stated above, the cellulose and 
graphite serve as burnout material to achieve the requisite porosity in 
the ceramic article manufactured in accordance with the present invention. 
Any combination of graphite and water insoluble cellulose that provides 
the requisite physical properties and allows the formed green body to be 
dried dielectrically without arcing or shorting may be used. 
Generally, as the amount of water insoluble cellulose is increased, the 
amount of graphite should be decreased to maintain the desired overall 
porosity and optimal pore size. For the purposes of this invention, 
overall porosity is defined as the porosity within the walls of the 
substrate and does not include the open channels. Cellulose will generally 
produce about twice the porosity of a like amount of graphite. It is also 
desirable to keep the graphite level as high as possible without causing 
arcing or shorting during drying to prevent cracking due to the tendency 
of cellulose to swell. Preferably, about 10 to about 20 parts by weight 
graphite and about 4 to about 12 parts by weight cellulose should be added 
to 100 parts by weight of ceramic-forming precursors. The addition of 
about 15 to about 20 parts by weight graphite coupled with about 8 to 
about 10 parts by weight cellulose to 100 parts by weight ceramic-forming 
precursors is especially useful. 
The graphite may either be natural or synthetic. The average particle size 
and distribution of the graphite particles used in the method of the 
present invention may vary depending on, for example, the desired pore 
size after burnout. In general, the larger the particle size, the larger 
the pores. It is undesirable to achieve the requisite level of porosity 
using large pores (i.e., a ware having a mean pore diameter greater than 
30 .mu.m) because large pores decrease the strength of the final ceramic 
product. For a diesel particulate filter, it is preferred to have an 
overall porosity in the range of about 45 to about 55 percent and a mean 
pore diameter of about 8 to about 30 .mu.m. Especially preferred is a mean 
pore diameter of about 10 to about 30 .mu.m. Preferably the particle size 
of the graphite used to achieve the requisite porosity is in the range of 
about 5 to about 30 .mu.m, especially preferred graphite particles have a 
particle size of about 11 to 26 .mu.m. In addition, the mean pore diameter 
used to achieve the desired overall porosity may be adjusted by using a 
coarser (i.e., larger average particle size) talc. The use of a coarser 
talc to adjust the mean pore diameter of the ware will not adversely 
affect the fired product. 
The water insoluble cellulose used in the present method may come from a 
wide variety of sources. For example, wood fiber or pulp, vegetable 
fibers, cotton fibers, or synthetic cellulose are all suitable sources of 
cellulose. As discussed supra relative to graphite, the particle size of 
the cellulose added to the ceramic precursors may vary depending on, for 
example, the desired pore size after burnout. Preferably, the particle 
size of the cellulose is generally equivalent to the particle size of the 
graphite. An especially preferred cellulose for use in manufacturing 
diesel particulate filters is ALPHA-CEL.TM. C-100 cellulose, sold by 
International Filler Corp., North Tonawanda, N.Y. ALPHA-CEL.TM. C-100 
cellulose has the following particle size, as determined by screen 
analysis: 97-100 weight percent through 100 mesh, 55-60 weight percent 
through 200 mesh (United States Std. Sieve). 
The mixture of ceramic-precursors, water insoluble cellulose, and graphite 
is blended with vehicle and extrusion aids to achieve sufficient plastic 
flow to orient the platelets properly. Any suitable vehicle known in the 
art may be used in the method of the present invention. Water is a 
preferred vehicle. Extrusion aids, such as methylcellulose and sodium 
stearate, are also added in sufficient quantities to give the mixture 
formability and green strength prior to firing. Water, which also aids 
plastic formability, should be utilized at a level of 15-36 parts by 
weight to 100 parts by weight of dry material. 
Once a blend of raw materials in a plastically formable state is prepared, 
it can be subjected to a plastic flow or extrusion step which orients clay 
and talc platelets in the green body. In forming structures with thin web 
and thin ribbon material, the desired orientation of clay and talc 
platelets is in the plane of the webs. Other forming methods such as 
rolling and pressing of sheets, which may be assembled into honeycomb 
structures, can similarly be produced with a favorable orientation. 
In conventional isostatic forming methods, clay and talc particles of the 
batch tend to be left in the same orientation imparted during mixing and 
preforming preparation. By contrast, the present anisostatic method does 
not apply equal forces to all parts of the body being formed, and, 
therefore, the clay and talc platelets are caused to slip and rotate in 
the plastic batch while trying to reach a planar orientation. In pressing 
or extruding a ribbon of material, for example, the orientation results in 
an ideal configuration of the c-axis of the clay. The resulting cordierite 
crystals are oriented, after firing, to have the low expansion c-axes 
lying preferentially in the plane of the ribbon and the high expansion 
a-axes oriented transverse to that plane and parallel to the thin 
dimension. 
When forming an open celled, thin walled honeycomb structure, in accordance 
with the present invention, cordierite is oriented to have a low expansion 
along the axes of the cells and a high expansion across the thin wall (but 
not across the entire body normal to the cell axes). The effect of the 
high expansion direction is minimal, because the internal spaces in 
honeycomb allow expansion of the thin walls into the cells. A typical 
honeycomb structure useful with the present invention has a wall thickness 
of between about 0.076 and about 1.27 millimeters with cell densities of 
between about 1.4 cells/square centimeter to about 144 cells/square 
centimeter. The thickness of the thin walls is not critical for achieving 
proper orientation, but thinner walls enable more complete and more 
consistent planar orientation. A particularly preferred honeycomb 
structure for use as a diesel particulate filter is disclosed in U.S. Pat. 
No. 4,329,162 to Pitcher, Jr. 
Besides honeycomb structures, other shapes can be extruded or otherwise 
formed, and the anisotropy of the expansion will be controlled by the 
orientation imparted to the clay platelets during forming. 
The formed green body is dielectrically dried. Dielectric drying can be 
done in a dielectric oven having electrodes on either side of or above and 
below the green body. In accordance with the present invention, the green 
body may be dried completely in a dielectric oven, dispensing with a need 
for an additional drying step in a hot air oven, for example. For the 
purposes of this invention, a completely dried green body can still have 
some moisture content. To be completely dried, the green body should be 
dried to remove all the water that surrounds the particles during the 
extrusion and most of the interstitial water between the particles after 
the particles come in contact. Interstitial water is the water in the 
remaining voids between the tightly packed particles. There may be a small 
amount of water adsorbed on the particle surfaces, however. As is known in 
the art, this small amount of water does not adversely affect the physical 
properties of the resulting ceramic ware. 
The firing range for the formed cordierite body should be 
1340.degree.-1440.degree. C. with a soak time sufficient to yield a 
substantially complete reaction to the cordierite phase. Soak times of 
6-12 hours may be used. 
It is also possible to form cordierite bodies without clay or talc from a 
blend of 12-16 wt % magnesium oxide, 35-41 wt % aluminum oxide, and 43-53 
wt % silica, as taught by U.S. patent application Ser. No. 07/654,528 
entitled "Fabrication of Cordierite Bodies", now U.S. Pat. No. 5,114,644. 
Once forming aids are added to this blend to form a green body, the body 
can be dried and fired to form a cordierite-containing article. Such 
firing is carried out by heating to a temperature of 
1000.degree.-1200.degree. C. and increasing that temperature at a rate of 
100.degree. C./hour to a level of 1350.degree. to 1450.degree. C. 
The fired ceramic honeycomb can be plugged and refired to enhance the 
article's filtering properties. In a preferred embodiment, alternate cells 
of a honeycomb article such as described in U.S. Pat. No. 4,329,162 to 
Pitcher are plugged adjacent to each endface in a checkered-style pattern 
such that those cells plugged at the inlet end face are open at the outlet 
end face and vice versa. Plugs can be formed by injecting a plastically 
formable ceramic cement into the desired cell ends with an air-operated 
sealant gun. The amount of plugging cement injected into the cell ends can 
be controlled by measuring the time that operative air pressure is applied 
to the sealant gun. The depth or length a plug extends into the cell can 
vary widely. Useful lengths are in the range of about 5 to 15 mm, 
preferably about 9.5 to 13 mm. 
The plugging cement used in the present method can be any known foaming or 
nonfoaming ceramic cement. Preferably, the ceramic cement should be 
durable in the face of high heat as well as the chemical and physical 
conditions typically encountered in modern exhaust systems. Preferably, a 
foaming cement is used to counteract the drying and firing shrinkage which 
commonly occurs when using nonfoaming cements. Preferred ceramic cements 
are disclosed in U.S. Pat. No. 4,329,162 to Pitcher and U.S. Pat. No. 
4,297,140 to Paisley. 
After injecting the ceramic cement, the previously fired ceramic article is 
refired. This firing is carried out by heating to a temperature of about 
1350.degree. to 1440.degree. C. within 60 hours. 
EXAMPLES 
Preparation of all experimental Samples 1-63 followed one basic procedure. 
The basic experimental batch was about 1000 grams in weight for the oxide 
and mineral portions of the batch. Because the batches were not normalized 
back to 100 parts in every instance, this number is not always exact 
depending on the type of raw material used. The batch was weighed into a 
large, wide mouth NALGENE.RTM. bottle, sold by Nalgene Co., a subsidiary 
of Sybron Corp., Rochester, N.Y., followed by the addition of binder, 
methylcellulose, and the extrusion aid. The bottle was then placed in a 
TURBULA.RTM. mixer, sold by Glen Mills, Inc., Maywood, N.J., to dry blend 
for approximately 10 minutes. 
After the batch ingredients were mixed, they were transferred into a mix 
muller pan. The muller had a mixer wheel and scrapers to keep the wet 
batch from sticking on the pan or wheel. The mixer was started and water 
was added slowly while mixing. After all the water was added, mixing was 
continued to plasticize the batch. Because of the size of the muller, the 
batch generally formed small granules rather than large plastic masses. 
After plasticizing, the batch was extruded. 
Extrusion of Samples 1-50 was carried out on a small, ram-type extruder. 
The batch was first passed through a noodle dye using vacuum deairing to 
remove air from the batch material. This was done twice by placing the 
material back into the extruder barrel. After two passes, the die was 
changed to a 16 cells per square centimeter, 0.432 millimeter wall 
thickness die. Again, the barrel was deaired and 3.12 centimeter diameter 
pieces were made. The extruded green bodies were wrapped in foil to 
prevent rapid drying of the surface and then placed in a hot-air drying 
oven to dry over a two day period. The samples were cut to approximately 
7.5 centimeter lengths and placed into alumina setter boxes. The length of 
each sample was measured before and after firing and the firing shrinkage 
was calculated. The firing was carried out in a gas fired, Bickeey-type 
3000 kiln. The schedule used was a standard automotive 64 hour firing 
schedule, which is shown below: 
25.degree.-200.degree. C. in 2.0 hours; 
200.degree.-325.degree. C. in 5.0 hours; 
325.degree.-450.degree. C. in 2.5 hours; 
450.degree.-600.degree. C. in 6.5 hours; 
600.degree.-900.degree. C. in 3.0 hours; 
900.degree.-1100.degree. C. in 4.0 hours; 
1100.degree.-1130.degree. C. in 2.0 hours; 
1130.degree.-1160.degree. C. in 2.0 hours; 
1160.degree.-1265.degree. C. in 4.0 hours; 
1265.degree.-1320.degree. C. in 3.0 hours; 
1320.degree.-1390.degree. C. in 6.0 hours; 
Hold @ 1390.degree. C. for 8.0 hours; 
1390.degree.-650.degree. C. in 8.0 hours; 
650.degree.-100.degree. C. in 5.0 hours. 
Extrusion of Samples 51-54 was carried out on a large ram extruder. Web 
articles having a 14.15 centimeter diameter, 16 cells/square centimeter, 
and a 0.43 millimeter wall thickness were produced. The large extrusions 
were not wrapped in foil. These large, extruded green bodies were cut to 
the desired length and placed on contoured, nonconducting (i.e., wooden) 
setters and placed into a laboratory dielectric oven with electrodes above 
and below the ware. The ware remained in the oven for six to ten minutes 
to completely dry the ware. The ware was rotated 90.degree. part way 
through the drying cycle to achieve even heating and drying. 
The fired samples were tested for overall porosity, pore size, and density. 
Also, micrographs and visual observation of the web surfaces and fractured 
edges was used to evaluate the pore structure. Standard mercury intrusion 
under high pressure was used to calculate porosity, pore size, and 
density. As the pressure is increased, the mercury is forced into finer 
and finer pores allowing a plot of pore size versus porosity to be drawn. 
Various base compositions were used in the examples. Table I lists the 
ceramic-precursors of the various base compositions used in the examples 
in terms of parts by weight. The base compositions are shown in Table I 
without the graphite or cellulose additions. Approximate average particle 
sizes ("APS") are listed in parentheses. All average particle sizes are in 
microns. 
TABLE I 
______________________________________ 
A B C D E 
______________________________________ 
TALC 95-28 (APS 5-9) 
40.21 -- -- -- -- 
TALC 95-27 (APS 5-9) 
-- 40.78 40.78 
40.78 
40.78 
Calcined Clay 21.17 26.48 26.48 
20.00 
15.00 
(APS 1.5) 
HYDRITE MP (raw clay) 
25.15 15.37 -- -- -- 
(APS 7) 
KAOPAQUE-20 (raw clay) 
-- -- 15.37 
15.37 
15.37 
(APS 1.5-2.0) 
Al.sub.2 O.sub.3 (APS 4.0-4.5) 
13.47 15.34 -- -- -- 
Al(OH).sub.3 (APS 3.5-4.0) 
-- -- 23.42 
27.97 
31.48 
SiO.sub.2 (APS 4.0-4.5) 
-- 2.00 2.00 
5.51 
8.20 
______________________________________ 
TALC 95-28 is a talc having a broad particle size distribution (i.e., a 
larger number of coarse particles) and TALC 95-27 is a talc with a 
narrower particle size distribution. Both TALC 95-27 and TALC 95-28 are 
available from Pfizer, Inc., New York, N.Y. HYDRITE MP and KAOPAQUE 20 are 
available from Georgia Kaolin Company, Elizabeth, N.J. 
4.00 parts by weight water soluble methylcellulose and 0.50 parts by weight 
percent sodium stearate were added to 100 parts by weight of the 
ceramic-forming precursors as a plasticizer/extrusion aid. 
The results of these tests are listed in Table II below. 
TABLE II 
______________________________________ 
Graphite/ 
Cellulose Porosity 
Pore Size 
Sample # 
Base Comp. Weight % (%) (.mu.m) 
______________________________________ 
1 A 30/0 49.8 8.9 
2 A 20/2 49.1 8.8 
3 A 20/5 51.6 10.7 
4 A 30/0 49.0 9.8 
5 A 15/2 46.3 7.39 
6 A 15/5 47.2 7.28 
7 A 10/2 44.6 6.25 
8 A 10/5 44.9 6.66 
9 A 30/0 52.1 10.30 
10 A 20/2 -- -- 
11 A 20/4 47.7 7.02 
12 A 20/6 50.2 8.64 
13 A 15/4 45.6 7.31 
14 A 15/6 49.0 8.22 
15 C 30/0 46.4 6.55 
16 C 30/0 51.8 7.21 
17 D 30/0 53.4 6.73 
18 D 30/0 48.9 7.31 
19 E 5/0 44.2 3.56 
20 E 10/0 46.5 4.12 
21 E 15/0 47.0 4.91 
22 E 20/0 49.2 6.11 
23 E 0/2 44.0 3.84 
24 E 0/4 45.8 4.43 
25 E 0/6 47.0 5.26 
26 E 0/10 49.4 6.97 
27 B 20/0 48.6 6.76 
28 B 25/0 52.1 10.6 
29 B 30/0 50.9 7.72 
30 E 0/0 42.1 2.55 
31 E 5/2 46.5 3.76 
32 E 5/4 46.9 4.46 
33 E 5/6 48.2 5.67 
34 E 5/8 49.1 6.67 
35 E 10/2 48.4 4.96 
36 E 10/4 47.4 5.55 
37 E 10/6 52.3 7.01 
38 E 10/8 52.4 7.72 
39 E 15/2 49.3 5.36 
40 E 15/4 51.2 6.24 
41 E 15/6 51.3 7.59 
42 E 15/8 52.3 7.36 
43 E 20/2 51.8 6.74 
44 E 20/4 54.3 7.51 
45 E 20/6 53.2 7.49 
46 E 20/8 55.6 8.10 
47 E 10/6 49.8 6.27 
48 E 10/8 51.8 6.78 
49 E 15/4 51.1 5.74 
50 E 15/6 54.3 6.62 
51 E 0/8 -- -- 
52 E 10/6 47.9 6.02 
53 E 20/2 50.5 6.24 
54 E 20/8 54.6 7.50 
______________________________________ 
Table II illustrates that green bodies prepared with a combination of 
graphite and water insoluble cellulose as a burnout material may be 
prepared and exhibit the requisite overall porosity and mean pore 
diameter. No cracking occurred in the samples containing both graphite and 
cellulose. However, Sample 51, made with only 8 weight percent cellulose 
and no graphite, cracked badly. Samples 51 through 54 were all large 
extrusions containing up to 20 weight Percent graphite which were 
completely dried dielectrically without arcing or shorting occurring. 
Samples 1-14 (Base Composition A) indicate that the porosity of the 
graphite/cellulose containing samples (Samples 2,3, 5-8, 10-14) ranged 
from slightly below to slightly above the porosity exhibited by Base 
Composition A with only graphite added (Samples 1,4,9) Pore size tends to 
decrease with lower levels of graphite, indicating that it is preferable 
to maintain as high a level of graphite as possible while still preventing 
arcing or shorting during drying. Analyses of the data regarding Base 
Compositions B (Samples 27-29), C (15-16), D (Samples 17-18), and E 
(Samples 19-26, 30-54) indicates that similar results may be obtained with 
other base compositions. 
Samples 55-63 were prepared according to base composition E of Table I with 
the substitution of an equivalent amount of a coarser talc for the TALC 
95-27. Three talc sizes were obtained by screening a coarse talc at 120, 
140, and 170 mesh Unit Standard Sieves. The coarse talc used was 99-48 
TALC, available from Pfizer, Inc., New York, N.Y. The average particle 
size of the coarse talc screened at 120 mesh was about 63 .mu.m. The 
average particle size of the coarse talc screened at 140 mesh was about 58 
.mu.m. The average particle size of the coarse talc screened at 170 mesh 
was about 52 .mu.m. The screened coarse talc was blended in varying ratios 
with TALC 95-28. In each of Samples 55-63, 20 parts by weight graphite and 
10 parts by weight cellulose were added to the modified base composition. 
The results of the talc substitution on the graphite and cellulose 
containing batch are listed in Table III below. 
TABLE III 
______________________________________ 
Talc Talc Porosity 
Pore Size 
Sample Coarse/Fine 
Sieve (%) (.mu.m) 
______________________________________ 
55 25/75 120 53.7 13.2 
56 25/75 140 53.4 12.2 
57 25/75 170 54.3 12.3 
58 37.5/62.5 120 51.7 15.4 
59 37.5/62.5 140 54.0 14.3 
60 37.5/62.5 170 54.3 13.2 
61 50/50 120 53.8 15.6 
62 50/50 140 54.5 15.1 
63 50/50 170 53.5 14.0 
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As shown by Table III, the mean pore diameter of the ware may be adjusted 
by substituting a coarser talc for a finer talc. As the ratio of coarse 
talc to fine talc increased, the pore sizes got progressively larger. In 
addition, as the coarse talc fraction changed within each ratio from 120 
to 170 mesh (i.e., coarser to finer), the pore size decreased. The pore 
sizes in each of the samples is in a range suitable to use the ware as a 
diesel particulate filter. 
In summary, graphite may be substituted with water-insoluble cellulose to 
prevent arcing and/or shorting during dielectric drying of the ceramic 
article. The use of cellulose in combination with graphite will allow 
complete dielectric drying while not compromising the overall porosity and 
pore size requirements for dimensionally accurate ceramic articles desired 
for use as diesel particulate filters. 
Although the invention has been described in detail for the purpose of 
illustration, it is understood that such detail is solely for that 
purpose, and variations can be made therein by those skilled in the art 
without departing from the spirit and scope of the invention which is 
defined by the following claims.