Method of firing ceramic honeycomb structural bodies

A method of firing ceramic honeycomb structural bodies, in which cordierite raw materials including at least raw kaolin is extruded to obtain a honeycomb structural formed body and the thus obtained honeycomb structural formed body is fired. The method includes the step of firing a honeycomb structural body having a rib thickness of not more than 4.6 mil under such a condition that a temperature ascending rate during 400-600.degree. C. is maintained under 70.degree. C./hr. As a preferred embodiment, a temperature ascending rate during 400-600.degree. C. is defined as under 70.degree. C./hr in the case of manufacturing a honeycomb structural body having a rib thickness of 4.6-4.0 mil, or, a temperature ascending rate during 400-600.degree. C. is defined as under 40.degree. C./hr in the case of manufacturing a honeycomb structural body having a rib thickness of 3.9-2.0 mil.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of firing ceramic honeycomb 
structural bodies, in which cordierite raw materials including at least 
raw kaolin is extruded to obtain a honeycomb structural formed body and 
the thus obtained honeycomb structural body is fired. 
2. Description of Related Art 
Generally, as a method of firing ceramic honeycomb structural bodies, in 
which cordierite raw materials including talc, kaolin, alumina and so on 
is extruded to obtain a honeycomb structural formed body and the thus 
obtained honeycomb structural body is fired, various firing methods have 
been known. For example, in Japanese Patent Laid-Open Publication No. 
1-249665, in order to prevent a crack generation due to an exothermic 
reaction in a decomposition temperature range of forming agents, it has 
been known the technique that a temperature ascending rate till the 
forming agent decomposition temperature is decreased as compared with that 
of over the forming agent decomposition temperature. That is to say, in 
this technique, the firing is performed in such a manner that a 
temperature ascending rate till 200.degree. C. is set to 80-90.degree. 
C./hr and a temperature ascending rate of over that temperature is set to 
100-120.degree. C./hr. 
Moreover, in Japanese Patent Laid-Open Publication No. 2-255576, in order 
to prevent a deformation of the ceramic honeycomb structural body, a 
temperature ascending rate is maintained under 60.degree. C. in a 
temperature range (1100-1180.degree. C.) in which the honeycomb structural 
body is shrunk by heat. Further, in Japanese Patent Laid-Open Publication 
No. 5-85856, in order to optimize the properties such as a water 
absorption rate and a thermal expansion coefficient rate, a temperature 
ascending rate is maintained (1) under 60.degree. C./hr in a temperature 
range (1100-1200.degree. C.) in which the honeycomb structural body is 
shrunk by heat, (2) over 80.degree. C./hr in a solid phase reaction 
temperature range (1200-1300.degree. C.), and (3) under 60.degree. C./hr 
in a liquid phase reaction temperature range (1300-keep temperature). 
On the other hand, as a carrier constructed by the honeycomb structural 
body used for purifying an exhaust gas from automobiles, it is necessary 
to increase a cell density so as to improve a purifying performance. In 
the honeycomb structural body, if the cell density is increased, a 
pressure loss is necessarily increased. Therefore, if the cell density is 
increased, an engine power is decreased. In order to reduce the pressure 
loss, it is necessary to make a rib thickness thinner. Generally, 
honeycomb structural bodies having a rib thickness of 6.0-6.6 mil, which 
are so-called as 6 mil body, are mainly used. However, recently, honeycomb 
structural bodies having a rib thickness of under 4.6 mil, which are 
so-called as thin wall body, are used increasingly. 
When the thin wall honeycomb structural body having a rib thickness of 
under 4.6 mil is to be fired according to the known firing methods 
mentioned above, a crack is liable to be generated during the firing step 
as compared with the known honeycomb structural body having relatively 
thick wall thickness. Particularly, in order to obtain the honeycomb 
structural body having a thin wall thickness, a raw kaolin is used 
recently as a part of kaolin so as to improve a flowability of raw 
materials when they are passed through the die. In this case, since a 
crystallization water is removed from the raw kaolin at a temperature of 
400-600.degree. C. and this crystallization water removing reaction is a 
heat absorbing reaction, a temperature difference is generated in the 
honeycomb structural body during this heat absorbing reaction, so that a 
crack is liable to be generated in the honeycomb structural body. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to eliminate the drawbacks 
mentioned above and to provide a method of firing ceramic honeycomb 
structural bodies, in which a crack is not generated during a firing step 
in a honeycomb structural body having a thin wall thickness even if a raw 
kaolin is used as cordierite raw materials. 
According to the invention, a method of firing ceramic honeycomb structural 
bodies, in which cordierite raw materials including at least raw kaolin is 
extruded to obtain a honeycomb structural formed body and the thus 
obtained honeycomb structural formed body is fired, comprises the step of: 
firing a honeycomb structural body having a rib thickness of not more than 
4.6 mil under such a condition that a temperature ascending rate during 
400-600.degree. C. is maintained under 70.degree. C./hr. 
In the construction mentioned above, a temperature ascending rate is 
defined as under 70.degree. C./hr in the temperature range of 
400-600.degree. C. in which a crystallization water is removed from raw 
kaolin. Therefore, even if the ceramic honeycomb structural body having a 
rib thickness of under 4.6 mil is fired by using raw kaolin as cordierite 
raw materials, a crack generation does not occur. 
Moreover, in the present invention, it is preferred that a temperature 
ascending rate is defined as under 60.degree. C./hr in the temperature 
range of 400-600.degree. C. when the ceramic honeycomb structural body 
having a rib thickness of 4.6-4.0 mil is to be fired. This is because a 
crack generation can be effectively eliminated. In this preferred 
embodiment, if a temperature ascending rate during 400-500.degree. C. is 
defined as under 60.degree. C./hr and a temperature ascending rate during 
500-600.degree. C./hr is defined as a larger temperature ascending rate 
than that during 400-500.degree. C., a sufficient water removing reaction 
in raw kaolin can be performed since a temperature ascending rate during 
500-600.degree. C. can be improved. Further, in this preferred embodiment, 
if an additional amount of raw kaolin is defined as over 30% with respect 
to the total kaolin amount in the cordierite raw materials, a sufficient 
form maintaining performance can be obtained in the formed body and thus a 
deformation failure generation rate of the formed body can be decreased 
preferably. 
Furthermore, in the present invention, it is preferred that a temperature 
ascending rate is defined as under 40.degree. C./hr in the temperature 
range of 400-600.degree. C. when the ceramic honeycomb structural body 
having a rib thickness of 3.9-2.0 mil is to be fired. This is because a 
crack generation can be effectively eliminated. In this preferred 
embodiment, if a temperature ascending rate during 400-500.degree. C. is 
defined as under 30.degree. C./hr and a temperature ascending rate during 
500-600.degree. C./hr is defined as a larger temperature ascending rate 
than that during 400-500.degree. C., a sufficient water removing reaction 
in raw kaolin can be performed since a temperature ascending rate during 
500-600.degree. C. can be improved. Further, in this preferred embodiment, 
if an additional amount of raw kaolin is defined as over 40% with respect 
to the total kaolin amount in the cordierite raw materials, a sufficient 
formability can be obtained in the formed body and thus a deformation 
failure generation rate of the formed body can be decreased preferably. 
In this embodiment, if a temperature ascending rate is smaller than an 
upper limit thereof, a crack generation does not occur. Therefore, a 
particular lower limit of the temperature ascending rate is not defined. 
However, if a temperature ascending rate becomes lower, a total firing 
time becomes longer correspondingly. Therefore, from a viewpoint of 
production efficiency, it is preferred to perform a firing at a 
temperature ascending rate near the upper limit thereof. Moreover, in the 
present invention, a unit "mil" means a thickness and 1 mil corresponds to 
25.4 .mu.m.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Hereinafter, a method of firing ceramic honeycomb structural bodies 
according to the invention will be explained. At first, fine talc, kaolin, 
alumina and the other cordierite generation raw materials are mixed to 
obtain a mixture, a composition of which lies in a range of SiO: 42-56 wt 
%, Al.sub.2 O.sub.3 : 30-45 wt %, and MgO: 12-16 wt % showing a cordierite 
theoretical chemical composition of 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2. 
Then, forming agents and/or poring agents are added to the mixture, and 
the thus obtained mixture is extruded to obtain a formed body. Then, the 
formed body is dried up to obtain a ceramic honeycomb formed body. 
In this case, it is preferred to set an additional amount of raw kaolin 
over 30% with respect to a total kaolin amount in cordierite raw materials 
when a honeycomb structural body having a rib thickness of 4.6-4.0 mil is 
to be formed and also it is preferred to set an additional amount of raw 
kaolin over 40% with respect to a total kaolin amount in cordierite raw 
materials when a honeycomb structural body having a rib thickness of 
3.9-2.0 mil is to be formed. These reasons are as follows. That is to say, 
it is necessary to narrow a slit width of die used for extruding so as to 
make a rib thickness thin. If the slit width is narrow, a pressure loss 
becomes larger and thus a forming rate is decreased. In order to increase 
the forming rate, one idea, such that a water component in a ceramic batch 
is increased, is known. However, in such a case, since the ceramic 
honeycomb formed body extruded through slits of the die has not a 
sufficient form maintaining performance, the ceramic honeycomb formed body 
is liable to be deformed by its own weight. In order to prevent such a 
deformation, we find that it is effective to increase an additional amount 
of raw kaolin with respect to a total kaolin amount in cordierite raw 
materials. Therefore, in the case that an additional amount of raw kaolin 
is increase as mentioned above, a sufficient flowability can be obtained 
even if a water component in the batch is not so increased. 
Moreover, if all of or a part of alumina serving as a supply source of 
alumina component in cordierite raw materials is substituted by aluminum 
hydroxide, a thermal expansion coefficient of the honeycomb structural 
body can be decreased, and thus this is a preferred embodiment. 
Further, as a fine talc to be used, it is preferred to use a fine talc 
including particularly low alkali component. Moreover, in order to make 
particle sizes of talc and kaolin fine, it is preferred to use a calcined 
talc which is effective for preventing a crack generation of the honeycomb 
structural body due to a shrinkage during drying and firing steps. In this 
case, it is preferred to use a calcined talc having the same fine particle 
size as that of raw materials. As the forming agents, (1) organic binders 
such as methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, 
starchy paste, flour, glycerin, (2) surface-active agents, and (3) wax are 
used selectively for its purpose. Moreover, as the poring agents, 
graphite, starch and sawdust are used selectively for its purpose. 
After that, when the thus obtained ceramic honeycomb structural body is 
fired, it is preferred to control a temperature ascending rate as follows. 
In one preferred embodiment, a temperature ascending rate during 
400-600.degree. C. is defined as under 70.degree. C./hr, when the 
honeycomb structural body having a rib thickness of 4.6-4.0 mil is to be 
fired. In the another preferred embodiment, a temperature ascending rate 
during 400-600.degree. C. is defined as under 40.degree. C./hr, when the 
honeycomb structural body having a rib thickness of 3.9-2.0 mil is to be 
fired. In the temperature regions other than the above mentioned 
temperature range, the same temperature ascending rate as that of the 
known firing method can be used. 
Moreover, in the further another preferred embodiment, a temperature 
ascending rate during 400-500.degree. C. is defined as under 60.degree. 
C./hr and a temperature ascending rate during 500-600.degree. C. is 
defined as a larger temperature ascending rate than that during 
400-500.degree. C. Further, in another preferred embodiment, a temperature 
ascending rate during 400-500.degree. C. is defined as under 30.degree. 
C./hr and a temperature ascending rate during 500-600.degree. C. is 
defined as a larger temperature ascending rate than that during 
400-500.degree. C. 
In the preferred embodiment of the thin wall honeycomb structural body 
which is a manufacturing object of the invention, a temperature ascending 
rate for the honeycomb structural body having a rib thickness of 4.6-4.0 
mil is different from that for the honeycomb structural body having a rib 
thickness of 3.9-2.0 mil. This is because an amount of raw kaolin required 
for manufacturing the thin wall honeycomb structural bodies having 
respective rib thickness is different. That is to say, it is necessary to 
increase an amount of raw kaolin if a rib thickness becomes thinner. 
Then, actual experiments will be explained. Hereinafter, in an experiment 
1, a temperature ascending rate during 400-600.degree. C. is varied to 
investigate a crack generation rate. Moreover, in an experiment 2, as a 
preferred embodiment, a relation between formability and additional amount 
of raw kaolin is investigated. 
Experiment 1 
According to the method mentioned above, as shown in the following Table 1 
and Table 2, ceramic raw materials such as kaolin including a 
predetermined amount of raw kaolin, talc, and alumina raw materials were 
prepared and mixed to obtain a mixture having a cordierite chemical 
composition. Then, methyl cellulose was added in the thus obtained mixture 
as a forming agent to obtain a plasticized mixture. Then, the thus 
obtained plasticized mixture was extruded and dried up to prepare a 
ceramic honeycomb formed body. All the prepared ceramic honeycomb formed 
bodies had the same dimension such as an elliptical shape having long 
diameter: 180 mm.times.short diameter: 120 mm.times.length: 100 mm. Then, 
the thus prepared ceramic honeycomb formed bodies were fired according to 
a predetermined heat curve in a firing furnace. Then, a percentage of the 
ceramic honeycomb structural bodies in which a crack was generated after 
the firing was calculated, and an estimation was performed on the basis of 
the thus calculated result. The firing was performed in such a manner that 
a temperature ascending rate in a temperature range of 400-600.degree. C. 
was varied as shown in the Following Table 1 and Table 2. The results were 
also shown in Table 1 and Table 2. In Table 1 and Table 2, the estimation 
was performed as follows; a symbol ".largecircle.": usable and a symbol 
"x": unusable. 
TABLE 1 
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Rib thickness after firing (mil) 
4.6 4.0 
Percentage of raw kaolin amount 30 35 
with respect to total kaolin 
amount (%) 
Temperature ascending rate 
80 70 60 60 70 60 50 40 
during 400-500.degree. C. (.degree. C./hr) 
Temperature ascending rate 80 70 60 70 70 60 50 50 
during 500-600.degree. C. (.degree. C./hr) 
Firing crack generation rate (%) 4.0 0.5 0.0 0.2 5.0 0.4 0.0 0.2 
Estimation 
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TABLE 2 
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Rib thickness after firing 
3.9 3.5 3.0 2.0 
(mil) 
Percentage of raw kaolin 40 40 40 45 
amount with respect to 
total kaolin amount (%) 
Temperature ascending 
50 
40 
30 
30 
50 
40 
30 
30 
40 
30 
20 
20 
35 
25 
15 
15 
rate during 400-500.degree. C. 
(.degree. C./hr) 
Temperature ascending 50 40 30 40 50 40 30 40 40 30 20 30 35 25 15 
25 
rate during 500-600.degree. C. 
(.degree. C./hr) 
Firing crack generation 3.0 0.3 0.0 0.1 6.0 0.7 0.0 0.1 5.0 0.6 0.0 
0.1 5.0 0.6 0.2 0.2 
rate (%) 
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From the results shown in Table 1 and Table 2, it is understood that a thin 
wall ceramic honeycomb structural body having a rib thickness under 4.6 
mil, in which no crack is generated, can be obtained by defining a 
temperature ascending rate in a temperature range of 400-600.degree. C. as 
under 70.degree. C./hr in which a crystallization water is removed from 
raw kaolin, even if a firing is performed by using raw kaolin as 
cordierite raw materials. 
Moreover, from the results shown in Table 1, it is understood that it is 
preferred to define a temperature ascending rate in a temperature range of 
400-600.degree. C. as under 70.degree. C./hr in the case of manufacturing 
the honeycomb structural body having a rib thickness of 4.6-4.0 mil. 
Further, it is understood that a temperature ascending rate during 
500-600.degree. C. can be increased while a crack generation rate is 
maintained at a low level, if a temperature ascending rate in a 
temperature range of 400-500.degree. C. is defined as under 60.degree. 
C./hr, and thus a production efficiency can be improved. We think this is 
because a sufficient water removing reaction can be performed by 
decreasing a temperature ascending rate during 400-500.degree. C. 
Further, from the results shown in Table 2, it is understood that it is 
necessary to define a temperature ascending rate in a temperature range of 
400-600.degree. C. as under 40.degree. C./hr in the case of manufacturing 
the honeycomb structural body having a rib thickness of 3.9-2.0 mil. 
Moreover, as is the same as the results shown in Table 1, it is understood 
that a temperature ascending rate during 500-600.degree. C. can be 
increased while a crack generation rate is maintained at a low level, if a 
temperature ascending rate in a temperature range of 400-500.degree. C. is 
defined as under 30.degree. C./hr, and thus a production efficiency can be 
improved. 
Experiment 2 
As is the same as the experiment 1, ceramic honeycomb formed bodies each 
having a predetermined amount of raw kaolin in which a rib thickness was 
varied respectively as shown in the following Table 3 and Table 4. Then, a 
percentage of the ceramic honeycomb formed bodies in which a deformation 
failure occurred during the forming step, and then an estimation was 
performed on the basis of the results. In Table 3 and Table 4, the 
estimation was performed as follows; a symbol ".largecircle.": usable and 
a symbol "x": unusable. 
TABLE 3 
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Rib thickness after firing (mil) 
4.6 4.0 
Percentage of raw kaolin 
20 30 40 50 25 35 45 55 
amount with respect to total 
kaolin amount in raw material 
powders (%) 
Deformation failure generation 15 0.5 0.3 0.2 17 0.6 0.4 0.1 
rate during forming (%) 
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TABLE 4 
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Rib thickness afier firing 
3.9 3.5 3.0 2.0 
(mil) 
Percentage of raw kaolin 
30 
40 
50 
60 
30 
40 
50 
60 
35 
45 
55 
65 
40 
50 
60 
70 
amount with respect to 
total kaolin amount in raw 
material powders (%) 
Deformation failure 4 0.4 0.3 0.1 23 0.7 0.5 0.2 18 0.4 0.3 0.1 
8.7 0.7 0.4 0.3 
generation rate during 
forming (%) 
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From the results shown in Table 3, it is understood that it is preferred to 
define a percentage of raw kaolin amount with respect to a total kaolin 
amount in raw material powders as over 30% in the case of manufacturing 
the honeycomb structural body having a rib thickness of 4.6-4.0 mil. 
Moreover, from the results shown in Table 4, it is understood that it is 
preferred to define a percentage of raw kaolin amount with respect to a 
total kaolin amount in raw material powders as over 40% in the case of 
manufacturing the honeycomb structural body having a rib thickness of 
3.9-2.0 mil. 
As is clearly understood from the above explanations, according to the 
invention, it is possible to obtain a ceramic honeycomb structural body, 
in which no crack is generated during the firing step even if raw kaolin 
is used as cordierite raw materials, when a thin wall ceramic honeycomb 
structural body including at least raw kaolin as cordierite raw materials.