Method of producing porous ceramic filter, using cordierite composition including talc and silica powders

A method of producing a porous ceramic filter, using a cordierite precursor or starting composition including a talc powder component and a silica powder component, such that particles of the talc powder component and the silica powder component whose size is not less than 150 .mu.m constitute not more than 3% by weight of the starting composition, while particles of the talc and silica powder components whose size is not more than 45 .mu.m constitute not more than 25% by weight of the starting composition. A green body for the porous ceramic honeycomb filter formed of this starting composition is fired to react the starting composition to form cordierite and produce the desired porous ceramic honeycomb filter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of producing a porous ceramic 
filter formed of a cordierite composition, and more particularly to a 
method of producing such ceramic filter suitable for removing soot or 
other particulate matter contained in exhaust gases emitted by a diesel 
engine. 
2. Discussion of the Prior Art 
Recently, various types of porous honeycomb filters using a cordierite 
honeycomb structure with porous partition walls have been proposed as a 
porous ceramic filter capable of functioning to filter fluids such as 
gases. For example, porous honeycomb filters are used as a so-called 
"diesel particulate filter" for removing particulate matter from exhaust 
gases emitted by a diesel engine. These diesel particulate filters are 
roughly classified into a high-trapping-efficiency type, and a 
low-trapping-efficiency type. These two types are selectively employed 
depending upon the specific requirement. 
As a porous ceramic honeycomb filter having improved filtering capability, 
laid-open Publication No. 61-129015 of unexamined Japanese Patent 
Application proposes an exhaust emission purifying filter in which the 
size of the pores formed adjacent to the surfaces of the partition walls 
of the honeycomb structure is specifically controlled. Described more 
particularly, those pores consist of relatively small pores whose 
diameters fall within a range of 5-40.mu.m, and relatively large pores 
whose diameters fall within a range of 40-100.mu.m. This exhaust emission 
purifying filter is prepared from a ceramic composition with which a 
suitable foaming or blowing agent is mixed. A desired green honeycomb 
structure formed of this ceramic composition is fired at an elevated 
temperature, whereby pores are formed in the partition walls of the fired 
honeycomb structure, due to heating of the ceramic composition in the 
presence of the foaming agent mixed therein. 
Laid-open Publication No. 61-54750 of examined Japanese Patent Application 
discloses porous honeycomb filters in a wide range of trapping capacity 
from a high--trapping-efficiency type to a low-trapping-efficiency type. 
The porous honeycomb filters disclosed therein have controlled open 
porosities (ratio of a volume of open pores to that of non-open pores) and 
controlled average sizes of the pores. Further, laid-open Publication No. 
58-70814 of unexamined Japanese Patent Application teaches that the 
pressure loss of a porous ceramic honeycomb filter can be lowered by 
forming the partition walls of the honeycomb structure with large pores 
having 100 .mu.m or larger sizes, for example. 
Generally, the following three characteristics are important in determining 
the overall filtering function or capability of a porous ceramic honeycomb 
filter. These characteristics arc: a) trapping efficiency (ratio of the 
particulate matter removed from a subject fluid, to those not removed); b) 
pressure loss (amount of pressure drop of the subject fluid flowing 
through the filter); and c) nominal operation time (time duration from the 
commencement of use of the filter to the time at which the pressure loss 
increases to an upper limit). In this respect, it is significant to note 
that the trapping efficiency is proportional to the pressure loss. Namely, 
an increase in the trapping efficiency results in an undesirable increase 
in the pressure loss, and a consequent decrease in the operation time. If 
the filter is adapted for a comparatively reduced amount of pressure loss, 
the operation time can be prolonged, but the trapping efficiency is 
unfavorably lowered. 
The most important characteristic of the porous ceramic honeycomb filter is 
the trapping time, i.e., the time duration for which the filter can 
operate with the pressure loss held below the permissible upper limit. For 
the reason indicated above, however, it has been considered difficult to 
increase the trapping time while maintaining a sufficiently high trapping 
efficiency. In this respect, it is noted that an increase in the nominal 
operation time of a porous ceramic honeycomb filter means a decrease in 
the required volume of the filter for a specific application, and the 
decrease in the required volume contributes to an improvement in the 
thermal shock or stress resistance of the filter. Therefore, it is 
desirable to increase the operation time (life expectancy) of the filter, 
particularly where the contaminated or clogged filter can be reclaimed by 
burning out the contaminants or particulate matters, as in the case of the 
diesel particulate filter used for a diesel engine. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method of 
producing an improved porous ceramic filter which has a prolonged nominal 
operation time and a reduced pressure loss, while maintaining a 
sufficiently high trapping efficiency. 
The above object may be achieved according to the principle of the present 
invention, which provides a method of producing a porous ceramic honeycomb 
filter comprising the steps of (a) preparing a starting composition 
including a talc powder component and a silica powder component, such that 
particles of the talc powder component and the silica powder component 
whose size is not less than 150 .mu.m constitute not more than 3% by 
weight of the starting composition, while particles of the talc and silica 
powder components whose size is not more than 45.mu.m constitute not more 
than 25% by weight of the starting composition; (b) forming a green body 
for the porous ceramic filter, by using the prepared starting composition; 
and (c) firing the green body to react the starting composition and 
produce the porous ceramic honeycomb filter. 
The method of the present invention which uses the starting composition 
prepared as described above permits the production of a porous ceramic 
honeycomb filter whose major crystal phase is cordierite and whose pores 
consist principally of pores having diameters of 10-50 .mu.m, with a small 
number of pores whose diameters are 100 .mu.m or more. This porous ceramic 
honeycomb filter is given a relatively long nominal operation time, 
without lowering the filtering efficiency. It is recognized that the pores 
whose diameters are not less than 100 .mu.m greatly influence the trapping 
efficiency and the pressure loss, namely, an increase in the number of the 
pores whose diameters are 100 .mu.m or more will considerably reduce the 
filtering efficiency. It is also noted that the pores whose diameters fall 
within a range of 10-50 .mu.m contribute to an increase in the operation 
time of the filter, and to a decrease in the pressure loss. More 
specifically, the pressure loss of a fluid flowing through the filter can 
be minimized to effectively remove the particulate matter from the fluid, 
by increasing the number of such relatively small pores of 10-50 .mu.m. In 
other words, the trapping function or capability of the porous ceramic 
honeycomb filter can be substantially improved, by increasing the number 
of such small pores of 10-50 .mu.m, while controlling the number of the 
pores whose sizes are smaller than 10 .mu.m or larger than 100 .mu.m, as 
described above. Thus, the porous ceramic honeycomb filter produced 
according to the present method has a prolonged operation time with a 
reduced pressure loss, while maintaining a high trapping efficiency (90% 
or more). The present method is particularly suitable for producing a 
porous ceramic honeycomb filter for emissions from a diesel engine, i.e., 
"diesel particulate filter". 
The porous ceramic honeycomb filter produced according to the method of the 
present invention generally has an open porosity of 45-60%, a pore volume 
of not more than 10% which consists of the pores whose diameters are 100 
.mu.m or more, and a pore volume of not less than 65% which consists of 
the pores whose diameters fall within a range of 10-50 .mu.m. Preferably, 
the pore volume provided by the pores whose diameters are 100 .mu.m or 
more is 5% or less, while the pore volume provided by the pores whose 
diameters are 10-50 .mu.m is 70% or more. If the open porosity of the 
filter is less than 45%, the operation time tends to be shortened with a 
relatively large pressure loss of the subject fluid, even if the pore size 
is controlled as described above. If the open porosity of the filter 
exceeds 60%, the mechanical strength of the filter is insufficient, and a 
pore-forming agent such as graphite must be used in a large amount, which 
unfavorably increases the required firing time and consequently the 
required production cycle time. 
According to the method of the present invention for producing a porous 
ceramic honeycomb filter, there is first prepared a starting composition 
which includes a talc powder component such as talc or calcined talc, and 
a silica powder component such as non-crystalline silica, and which 
further includes suitable additives such as kaoline, calcined kaoline, 
alumina and aluminum hydroxide. Generally, the starting composition 
comprises 42-56% by weight of SiO.sub.2, 30-45% by weight of Al.sub.2 
O.sub.3 and 12-16% by weight of MgO. 
This cordierite composition is prepared according to the principle of the 
invention, such that the particles of the talc powder and the silica 
powder whose size is 150 .mu.m or larger constitute 3% or less by weight 
of the entire starting composition, while the particles of the talc and 
silica powders whose size is 45 .mu.m or less constitute 25% or less by 
weight of the entire starting composition. The use of the thus prepared 
starting composition permits the produced porous ceramic honeycomb filter 
to have an effectively increased nominal operation time with a limited 
increase in the pressure loss, as well as a trapping efficiency as high as 
90% or more. The volume of the pores having 100 .mu.m or larger sizes can 
be further reduced while the volume of the pores having 10-50 .mu.m sizes 
can be increased, if the talc and silica particles whose size is 150 .mu.m 
or more constitute not more than 1% by weight of the entire starting 
composition and the particles whose size is 45 .mu.m or less constitute 
not more than 20% by weight of the entire starting composition. 
For adjusting the porosity and other properties of the filter to be 
produced, a pore-forming agent such as graphite is added to the starting 
composition prepared as described above. Further, a plasticizer and a 
binder which are conventionally used are added to the obtained mixture, to 
plasticize the mixture into a formable batch for extrusion. By using the 
thus prepared batch, a green honeycomb body for the desired porous ceramic 
honeycomb filter is formed by extrusion. The green body is dried, and the 
dried green body is fired at a temperature between 1380.degree. C. and 
1440.degree. C. In this way, the desired porous ceramic body is produced. 
The pores are formed in the fired filter, due to firing reaction of the 
starting composition, based primarily on the framework constituted by the 
talc powder component such as talc or calcined talc and the silica powder 
component such as non-crystalline silica. In particular, the use of the 
silica powder component which is typically non-crystalline silica 
facilitates the adjustment of the pore size, since the silica powder 
remains stable at a higher temperature than the other materials, and is 
melted and diffused at 1350.degree. C. or higher. The silica powder 
permits the formation of the pores having a relatively constant size, 
which corresponds to the particle size of the starting powder. 
While the pores are formed based on the framework constituted by the 
pore-forming agent and other additives as well as the talc and silica 
powder components, the process in which the pores arc formed can be 
explained as follows. Initially, the pore-forming agent such as graphite 
disappears or is burnt out at a temperature in the neighborhood of 
1000.degree. C. Then, the reaction of the talc powder occurs, and a liquid 
phase of cordierite appears at a temperature of about 1280.degree. C. or 
higher, causing minute pores to be formed by the framework of the talc 
powder component. A subsequent reaction of this component will cause 
reduction of the framework. On the other hand, the silica powder component 
is melted and diffused at a further elevated temperature of about 
1350.degree. C. or higher, creating minute pores. Therefore, the size of 
the pores formed later by the framework of the silica powder controls the 
size of the pores formed in the filter produced, such that the 
substantially constant size of the pores in the filter corresponds to the 
particle size of the silica powder. For forming the pores whose sizes fall 
within a range of 10-50 .mu.m, it is particularly desirable to use the 
silica powder whose average particle size ranges from 30 .mu.m to 50 
.mu.m. 
In the case of a porous ceramic honeycomb filter like the "diesel 
particulate filter", it is important to reduce the coefficient of thermal 
expansion and increase the resistance to thermal stresses. Such a porous 
ceramic honeycomb filter is heat-treated to burn out the contaminants such 
as soot deposited on the porous partition walls, when the pressure loss of 
the subject fluid reaches a permissible upper limit during use. In this 
instance, there is a possibility of cracking of the filter due to 
different temperatures within the body of the filter during the firing to 
burn out the contaminants. In view of this possibility, it is required to 
reduce the thermal expansion coefficient and improve thermal stress 
resistance of the filter. Where the porous ceramic honeycomb filter is 
formed of a cordierite composition, the filter is generally required to 
have a relatively large pore size and a relatively high porosity, for 
assuring an adequate relationship among the filtering efficiency, pressure 
loss and operation time (life expectancy). In the case of a raw cordierite 
precursor material, however, the starting composition is required to have 
a relatively large particle size, which makes it difficult to reduce the 
thermal expansion coefficient of the filter produced. 
As indicated above, the use of a silica powder component (e.g., 
non-crystalline silica) whose average particle size is 30-50 .mu.m is 
effective to control the pore size distribution of the produced filter so 
as to increase the number of the 10-50 .mu.m pores. If the same object is 
to be attained by means of the talc powder component, the average particle 
size of the talc powder should be as large as 100 .mu.m or more. In this 
case, too, the filter suffers from an excessively high coefficient of 
thermal expansion. 
In view of the above, it is preferable to use the talc powder component 
whose average particle size is 80 .mu.m or less. Further, it is 
recommended to use the silica powder component whose average particle size 
ranges from 30 .mu.m to 50 .mu.m, so that the number of the 10-50 .mu.m 
pores is increased. In other words, the thermal expansion coefficient of 
the filter produced is considerably high, when the average particle size 
of the talc powder is 100 .mu.m or more. To avoid this, the talc powder 
composition is adjusted so that the average particle size is as small as 
possible, and the maximum average particle size of the talc powder is 
preferably 80 .mu.m, as indicated above. Further, the non-crystalline 
silica or other silica powder is adjusted to assure a relatively large 
number of the 10-50 .mu.m pores. If a porous filter having a honeycomb 
structure is formed from the cordierite composition adjusted as described 
above, the coefficient of thermal expansion of the filter is reduced to 
0.8 or smaller along the A axis, and 1.4 or smaller along the B axis. 
Further, the filter is capable of exhibiting a sufficiently high thermal 
shock resistance at a temperature of 850.degree. C. or higher, where the 
filter has an outside diameter of 118mm and a height of 150 mm. 
It will be generally understood from the above explanation that the thermal 
expansion of the ceramic filter prepared from a starting composition is 
largely affected by the particle size of the talc powder, while some 
filters such as the diesel particulate filter which have a large pore size 
inherently require the use of a talc powder whose particle size is 
relatively large, which results in increasing the thermal expansion 
coefficient of the filter produced. According to the present invention, 
however, the pores having a relatively large size are formed by using the 
silica powder (non-crystalline silica) having a particle size larger than 
that of the conventionally used silica powder, while the thermal expansion 
coefficient of the filter is lowered by using the talc powder having a 
comparatively small particle size. The use of the relatively coarse silica 
powder whose average particle size is 30-50 .mu.m permits the use of a 
relatively fine talc powder, thereby making it possible to suitably 
control the size of the minute pores formed in the filter produced.