U.S. patent application number 11/357949 was filed with the patent office on 2006-09-21 for honeycomb structure and manufacturing method thereof.
This patent application is currently assigned to NGK Insulators, Ltd.. Invention is credited to Yoshiro Ono.
Application Number | 20060210822 11/357949 |
Document ID | / |
Family ID | 36406071 |
Filed Date | 2006-09-21 |
United States Patent
Application |
20060210822 |
Kind Code |
A1 |
Ono; Yoshiro |
September 21, 2006 |
Honeycomb structure and manufacturing method thereof
Abstract
A honeycomb structure is made of cordierite and has a plurality
of cells separated from each other by porous partition walls and
functioning as fluid passages. The honeycomb structure has a
thermal expansion coefficient of 0.25.times.10.sup.-6/.degree. C.
or less, an average pore diameter is 3 to 8 .mu.m, and a porosity
of 25% or more. The honeycomb structure is made of cordierite
excellent in thermal shock resistance even with large pore
diameters and high porosity.
Inventors: |
Ono; Yoshiro; (Nagoya-city,
JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Assignee: |
NGK Insulators, Ltd.
Nagoya-city
JP
|
Family ID: |
36406071 |
Appl. No.: |
11/357949 |
Filed: |
February 22, 2006 |
Current U.S.
Class: |
428/593 ;
428/116 |
Current CPC
Class: |
F01N 2370/00 20130101;
Y02T 10/20 20130101; Y02T 10/12 20130101; B01J 35/04 20130101; B01J
35/002 20130101; C04B 2111/2084 20130101; Y10T 428/1234 20150115;
Y10T 428/24149 20150115; C04B 2111/00129 20130101; F01N 3/0222
20130101; C04B 38/0009 20130101; F01N 2330/06 20130101; C04B
2111/346 20130101; F01N 2330/30 20130101; C04B 2111/00793 20130101;
C04B 2111/0081 20130101; B01D 39/2093 20130101; C04B 38/0009
20130101; C04B 35/195 20130101; C04B 38/0054 20130101; C04B 38/0074
20130101 |
Class at
Publication: |
428/593 ;
428/116 |
International
Class: |
B32B 3/12 20060101
B32B003/12; B21D 39/00 20060101 B21D039/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 17, 2005 |
JP |
2005-077033 |
Claims
1. A honeycomb structure made of cordierite and having a plurality
of cells separated from each other by porous partition walls and
functioning as fluid passages, wherein the honeycomb structure has
a thermal expansion coefficient of 0.25.times.10.sup.-6/.degree. C.
or less, an average pore diameter is 3 to 8 .mu.m, and a porosity
of 25% or more.
2. A honeycomb structure according to claim 1, wherein a thermal
shock resistance temperature measured in a thermal shock resistance
test is 800.degree. C. or more.
3. A honeycomb structure according to claim 1, wherein a thermal
expansion coefficient is 0.2.times.10.sup.-6/.degree. C. or
less.
4. A method for manufacturing a honeycomb structure, comprising the
steps of: forming clay containing cordierite-forming raw material
satisfying the following conditions 1 and 2 in a honeycomb shape to
obtain a formed body, and drying and firing the formed body to
obtain the honeycomb structure having a plurality of cells
separated from each other by porous partition walls and functioning
as fluid passages and a thermal expansion coefficient of
0.25.times.10.sup.-6/.degree. C. or less, an average pore diameter
is 3 to 8 .mu.m, and a porosity of 25% or more: Condition 1: An
alumina source having a BET specific surface area of 10 m.sup.2/g
or more is contained in an amount of 13 to 28% by mass. Condition
2: Boehmite (Al.sub.2O.sub.3.H.sub.2O) having an average particle
size of 1 .mu.m or less is contained in at least a part of the
alumina source at a ratio of 0.45 to 15.8% by mass with respect to
the cordierite-forming raw material.
5. A method for manufacturing a honeycomb structure according to
claim 4, wherein the boehmite (Al.sub.2O.sub.3.H.sub.2O) has a BET
specific surface area of 80 m.sup.2/g or more.
6. A method for manufacturing a honeycomb structure according to
claim 4, wherein the cordierite-forming raw material further
contains 8% by mass or less of silica.
7. A method for manufacturing a honeycomb structure according to
claim 4, wherein the alumina source further contains alumina and/or
aluminum hydroxide.
8. A method for manufacturing a honeycomb structure according to
claim 4, wherein the alumina source has a BET specific surface area
of 20 m.sup.2/g or more.
Description
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
[0001] The present invention relates to a honeycomb structure and a
manufacturing method thereof. More specifically, the present
invention relates to a honeycomb structure made of cordierite
having excellent thermal shock resistance even with large pore
diameters and high porosity and to a method for manufacturing such
a honeycomb structure, the method being capable of manufacturing
such a honeycomb structure with high precision in size.
[0002] In recent years is high-lightened an influence of harmful
substances such as particulate and NO.sub.x contained in exhaust
gas discharged from automobile engines such as a diesel engine on
the environment. As an important means for removing such harmful
substances, a use of a honeycomb structure is variously
examined.
[0003] FIG. 1 is a perspective view schematically showing an
example of a honeycomb structure. As shown in FIG. 1, the honeycomb
structure 1 has a plurality of through-holes (cells 3) separated
from each other by porous partition walls 2 and functioning as
fluid passages. The honeycomb structure has a structure of plugging
in mutually different sites at both end faces where the cells are
open, and development of use of the honeycomb structure has been
proceeding as a filter (honeycomb filter) for trapping and removing
particulate matters in exhaust gas by allowing exhaust gas to flow
into each of the cells 3 opening at one end face to compulsorily
pass the exhaust gas through the porous partition walls 2. Also,
development of use of the honeycomb structure has been proceeding
as a catalyst body carrying a catalyst which decomposes HC and
NO.sub.x on the partition walls 2 to purify exhaust gas. As a
material constituting such a honeycomb structure, there has
suitably been used a cordierite having a low thermal expansion
coefficient and high thermal shock resistance.
[0004] When a honeycomb structure is used as a filter, it is
naturally required that it has high trapping efficiency. After a
certain period of use, the honeycomb structure is generally
subjected to a regeneration treatment where accumulated soot is
combusted at high temperature to be burned down. When the
regeneration treatment is conducted with high frequency,
deterioration of the honeycomb structure is promoted. Therefore,
there is a requirement of reducing the number of regeneration
treatments with making trapping time longer.
[0005] Meanwhile, when a honeycomb structure is used as a catalyst
carrier, there has been required in recent years an increase in an
amount of catalyst to be loaded on the honeycomb structure in order
to further improve exhaust gas purification performance. In order
to increase the amount of catalyst to be loaded, it is desirable to
rise porosity of the honeycomb structure. The honeycomb structure
having larger pore diameters is more desirable. In any of the cases
that a honeycomb structure is used as a filter and that a honeycomb
structure is used as a catalyst body, the honeycomb structure is
disposed near the engine and exposes to thermal shock continuously.
Therefore, the honeycomb structure is required to have sufficient
thermal shock resistance.
[0006] A honeycomb structure made of cordierite can be manufactured
by preparing clay by the use of a cordierite-forming raw material
containing an alumina source, forming the clay obtained into a
honeycomb shape to obtain a honeycomb formed body, and drying and
firing the honeycomb formed body (see, e.g., JP-A-2003-40687).
There is also disclosed a honeycomb product made of cordierite
using boehmite having a BET specific surface area of 20 m.sup.2/g
as an alumina source for the purpose of enhancing thermal shock
resistance by reducing a thermal expansion coefficient (see, e.g.,
JP-A-2001-524452).
[0007] However, a honeycomb product manufactured in a method
disclosed in the Patent Document 2 has a small average pore
diameter of about 1.5 .mu.m. Therefore, since such a honeycomb
product has high strength, it is disadvantageous to catalyst
loading, while thermal shock resistance is improved, and a problem
of difficulty in loading a sufficient amount of catalyst
arises.
SUMMARY OF THE INVENTION
[0008] The present invention has been made in view of such a
problem prior art has and aims to provide a honeycomb structure
made of cordierite having excellent thermal shock resistance even
with large pore diameters and high porosity and to a method for
manufacturing a honeycomb structure made of cordierite having
excellent thermal shock resistance even with large pore diameters
and high porosity, the method being capable of manufacturing such a
honeycomb structure with high precision in size.
[0009] As a result of the present inventors' keen investigation to
achieve the above aim, the y found out that the above aim can be
achieved by allowing the cordierite-forming raw material as a raw
material for the manufacture to contain an alumina source having an
appropriate BET specific surface area at a predetermined ratio and
allowing the alumina source to contain fine boehmite at a
predetermined ratio, thereby enabling the thermal expansion
coefficient of the resultant honeycomb structure to be lowered and
enabling an appropriate distribution of pores to be imparted to the
honeycomb structures, which led to the accomplishment of the
present invention.
[0010] That is, according to the present invention, there is
provided the following honeycomb structure and manufacturing method
thereof.
[0011] [1] A honeycomb structure made of cordierite and having a
plurality of cells separated from each other by porous partition
walls and functioning as fluid passages,
[0012] wherein the honeycomb structure has a thermal expansion
coefficient of 0.25.times.10.sup.-6/.degree. C. or less, an average
pore diameter is 3 to 8 .mu.m, and a porosity of 25% or more.
[0013] [2] A honeycomb structure according to the above [1],
wherein a thermal shock resistance temperature measured in a
thermal shock resistance test is 800.degree. C. or more.
[0014] [3] A honeycomb structure according to the above [1] or [2],
wherein a thermal expansion coefficient is
0.2.times.10.sup.-6/.degree. C. or less.
[0015] [4] A method for manufacturing a honeycomb structure,
comprising the steps of:
[0016] forming clay containing cordierite-forming raw material
satisfying the following conditions 1 and 2 in a honeycomb shape to
obtain a formed body, and
[0017] drying and firing the formed body to obtain the honeycomb
structure having a plurality of cells separated from each other by
porous partition walls and functioning as fluid passages and a
thermal expansion coefficient of 0.25.times.10.sup.16/.degree. C.
or less, an average pore diameter is 3 to 8 .mu.m, and a porosity
of 25% or more:
Condition 1: An alumina source having a BET specific surface area
of 10 m.sup.2/g or more is contained in an amount of 13 to 28% by
mass.
Condition 2: Boehmite (Al.sub.2O.sub.3.H.sub.2O) having an average
particle size of 1 .mu.m or less is contained in at least a part of
the alumina source at a ratio of 0.45 to 15.8% by mass with respect
to the cordierite-forming raw material.
[0018] [5] A method for manufacturing a honeycomb structure
according to the above [4], wherein the boehmite
(Al.sub.2O.sub.3.H.sub.2O) has a BET specific surface area of 80
m.sup.2/g or more.
[0019] [6] A method for manufacturing a honeycomb structure
according to the above [4] or [5], wherein the cordierite-forming
raw material further contains 8% by mass or less of silica.
[0020] [7] A method for manufacturing a honeycomb structure
according to the above [4] or [6], wherein the alumina source
further contains alumina and/or aluminum hydroxide.
[0021] [8] A method for manufacturing a honeycomb structure
according to the above [4] or [7], wherein the alumina source has a
BET specific surface area of 20 m.sup.2/g or more.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a perpendicular view schematically showing an
example of a honeycomb structure.
DETAILED DESCRIPTION OF THE INVENTION
[0023] A honeycomb structure of the present invention takes effect
of excellent thermal shock resistance even with large pore
diameters and high porosity. In addition, according to a method for
manufacturing a honeycomb structure of the present invention, a
honeycomb structure made of cordierite having excellent thermal
shock resistance even with large pore diameters and high porosity
can be manufactured with high precision in size.
[0024] The best mode for carrying the present invention will
hereinbelow be described. However, the present invention is by no
means limited to the following embodiments, and it should be
understood that even if an appropriate change, improvement, or the
like, may be given to the following embodiments within the range of
not deviating from the gist of the present invention, they are
within the range of the present invention on the basis of those
skilled in the art.
[0025] First, an embodiment of a honeycomb structure of the present
invention will be described. A honeycomb structure of the present
embodiment is made of cordierite and has a plurality of cells
separated from each other by porous partition walls and functioning
as fluid passages with a thermal expansion coefficient of
0.25.times.10.sup.-6/.degree. C. or less, an average pore diameter
of 3 to 8 .mu.m, and a porosity of 25% or more. Description on
details are as follows:
[0026] A honeycomb structure of the present invention has a thermal
expansion coefficient is 25.times.10.sup.-6/.degree. C. or less.
This is within the range of having excellent thermal shock
resistance in the case that all honeycomb structures and shapes are
taken into consideration. When the thermal expansion coefficient is
above 25.times.10.sup.-6/.degree. C., sufficient thermal shock
resistance cannot be obtained in the case of a structure having
high porosity and large capacity. From the viewpoint of imparting
superior thermal shock resistance, it is preferable that a
honeycomb structure of the present embodiment has a thermal
expansion coefficient of 0.2.times.10.sup.-6/.degree. C. or
less.
[0027] A honeycomb structure of the present embodiment has an
average pore diameter of 3 to 8 .mu.m. When the average pore
diameter is below 3 .mu.m, it is difficult to obtain sufficient
trapping efficiency in the case of using the honeycomb structure as
a filter. In the case of using the honeycomb structure as a
catalyst carrier, it is difficult to load a sufficient amount of
catalyst on the honeycomb structure. On the other hand, when an
average pore diameter is above 8 .mu.m, it is difficult to secure
sufficient physical strength. Incidentally, from the viewpoint of
securing physical strength with securing trapping efficiency and
enabling a sufficient amount of catalyst to be loaded, it is
preferable that a honeycomb structure of the present embodiment has
an average pore diameter of 4 to 6 .mu.m.
[0028] In addition, a honeycomb structure of the present embodiment
has a porosity of 25% or more. When the porosity is below 25%, it
is difficult to obtain sufficient trapping efficiency in the case
of using the honeycomb structure as a filter. In the case of using
the honeycomb structure as a catalyst carrier, it is difficult to
load a sufficient amount of catalyst on the honeycomb structure.
Incidentally, from the viewpoint of securing physical strength with
securing trapping efficiency and enabling a sufficient amount of
catalyst to be loaded, it is preferable that a honeycomb structure
of the present embodiment has a porosity of 30% or more. Though
there is no particular limitation on the upper limit of the
porosity of a honeycomb structure of the present embodiment, the
porosity is preferably 70% or less in consideration of practical
producibility, maintaining necessary physical strength, and the
like.
[0029] As described above, a honeycomb structure of the present
embodiment has a small thermal expansion coefficient and excellent
thermal shock resistance. Specifically, a honeycomb structure of
the present embodiment has a thermal shock resistance temperature
measured by a thermal shock resistance test of preferably
800.degree. C. or more, further preferably 850.degree. C. or more.
Though there is no particular limitation on the upper limit of the
thermal shock resistance temperature measured by a thermal shock
resistance test of the honeycomb structure of the present
embodiment, it is 950.degree. C. with regard to the one having
required porosity and a practical shape and size from the view
point.
[0030] Here, a method for carrying out the "thermal shock
resistance test" in the present specification is described. In the
thermal shock resistance test, a honeycomb structure is put in an
electric furnace heated up to a predetermined temperature in
advance to be heated for a sufficient period of time, followed by
being air-cooled. A thermal shock due to quick cooling may cause
cracks on a side face, on an end face, or inside the structure. In
the case that no crack is caused upon cooling down to room
temperature, the structure is considered that it has cleared the
heating temperature. Cracks are detected by eye observation, strike
sound, and the like. The honeycomb structure which has cleared the
heating temperature is heated until a crack is caused by 50.degree.
C. The highest temperature with no crack caused is defined as
"thermal shock resistance temperature (.degree. C.).
[0031] In the present specification, "average pore diameter" means
a value measured by a method of mercury penetration and can be
measured by a mercury penetration type porosimeter. "Porosity"
means a value (Po) calculated by the following formula (1) from the
whole pore volume (V) measured by the method of mercury penetration
of the honeycomb structure (porous body) and absolute specific
gravity (d.sub.t) (d.sub.t=2.52 g/cm.sup.2 in the case of
cordierite). Po={V/(V+1/d.sub.t)}.times.100 (1)
[0032] An example of the whole structure of a honeycomb structure
of the present embodiment is shown in Table 1. However, the whole
shape and the cell shape of a honeycomb structure are not limited
to this. Examples of the whole shape include a quadrangular prism
and a triangular prism besides a cylinder as shown in FIG. 1.
Examples of the cell shape (shape of cells 3 in a cross section
perpendicular to the fluid flow direction) include a hexagon, a
triangle, and a circle besides a square as shown in FIG. 1.
Incidentally, a honeycomb structure of the present embodiment can
be used as a catalyst carrier by loading a catalyst on a porous
partition wall surface or in pores. Alternatively, a honeycomb
structure of the present embodiment can be used as a filter by
alternately plugging one opening and the other opening of a
plurality of cells.
[0033] When a honeycomb structure of the present embodiment is used
as a catalyst carrier, it is preferable that the honeycomb
structure has a cell density of 6 to 1500 cells/inch.sup.2 (0.9 to
233 cells/cm.sup.2) and a partition wall thickness of 50 to 2000
.mu.m (about 2 to 79 mil). Also, the honeycomb structure has a
length (whole length of a honeycomb structure) of usually 60 to 300
mm, preferably 100 to 250 mm, in the fluid flow direction.
[0034] Next, an embodiment of a method for manufacturing a
honeycomb structure will be described. A method for manufacturing a
honeycomb structure of the present embodiment includes the steps
of: forming clay containing cordierite-forming raw material
satisfying the following conditions 1 and 2 in a honeycomb shape to
obtain a formed body, and drying and firing the formed body to
obtain the honeycomb structure having a plurality of cells
separated from each other by porous partition walls and functioning
as fluid passages and a thermal expansion coefficient of
0.25.times.10.sup.-6/.degree. C. or less, an average pore diameter
is 3 to 8 .mu.m, and a porosity of 25% or more:
Condition 1: An alumina source having a BET specific surface area
of 10 m.sup.2/g or more is contained in an amount of 13 to 28% by
mass.
Condition 2: Boehmite (Al.sub.2O.sub.3H.sub.2O) having an average
particle size of 1 .mu.m or less is contained in at least a part of
the alumina source at a ratio of 0.45 to 15.8% by mass with respect
to the cordierite-forming raw material.
[0035] A cordierite-forming raw material used in a method for
manufacturing a honeycomb structure of the present embodiment
contains 13 to 28% by mass of an alumina source having a BET
specific surface area of 10 m.sup.2/g or more (Condition 1). By
allowing the cordierite-forming raw material to contain a fine
alumina source having a predetermined BET specific surface area or
more at a predetermined ratio or more, a honeycomb structure having
a small thermal expansion coefficient and excellent thermal shock
resistance. When the alumina source has a BET specific surface area
of below 10 m.sup.2/g, it is impossible to obtain a honeycomb
structure having a sufficiently small thermal expansion coefficient
and excellent thermal shock resistance. When the ratio of the
alumina source having a BET specific surface area of 10 m.sup.2/g
or more is below 13% by mass, it is impossible to obtain a
honeycomb structure having a sufficiently small thermal expansion
coefficient and excellent thermal shock resistance. On the other
hand, when the ratio is above 28% by mass, shrinkage in size upon
firing is large, and the honeycomb structure tends to become dense.
Therefore, it has a tendency of having lowered porosity,
deterioration in size precision, and increase in shape
inferiority.
[0036] The alumina source contained in the cordierite-forming raw
material has a BET specific surface area of preferably 20 m.sup.2/g
or more, further preferably 40 m.sup.2/g or more. In addition, the
ratio of the alumina source contained in the cordierite-forming raw
material is preferably 8.8 to 27.4% by mass, and further preferably
15.0 to 22.0% by mass.
[0037] In the cordierite-forming material to be used in a method
for manufacturing a honeycomb structure of the present embodiment,
boehmite (Al.sub.2O.sub.3.H.sub.2O) having an average particle size
of 1 .mu.m or less is contained in at least a part of the alumina
source at a ratio of 0.45 to 15.8% by mass with respect to the
cordierite-forming raw material (Condition 2). When fine boehmite
having an average particle size of 1 .mu.m or less is used as at
least a part of the alumina source at a predetermined ratio, the
thermal expansion coefficient is lowered because a
cordierite-forming reaction is accelerated, and a honeycomb
structure suitable as a catalyst carrier can be produced. When the
boehmite has an average particle size of above 1 .mu.m, the thermal
expansion coefficient of the honeycomb structure obtained cannot be
lowered. When the ratio of the boehmite having an average particle
size of below 1 .mu.m to the cordierite-forming raw material is
lower than 0.45% by mass, thermal shock resistance of the honeycomb
structure obtained cannot be enhanced sufficiently from the same
viewpoint. On the other hand, when the ratio is above 15.8% by
mass, a large amount of shrinkage is caused upon drying and firing,
and it is difficult to produce a honeycomb structure having an
aimed structure with high precision in size.
[0038] The boehmite contained in an alumina source may be either
boehmite or an imitative boehmite with an average particle size of
preferably 1 .mu.m or less, more preferably 0.5 .mu.m or less. The
ratio of the boehmite contained in the alumina source is preferably
1.0 to 14.0% by mass, further preferably 4.0 to 12.0% by mass with
respect to the cordierite-forming raw material. Incidentally, when
the ratio of the boehmite contained is increased, it has the
advantage of lowering the thermal expansion coefficient of the
honeycomb structure obtained, and further, firing temperature can
preferably be lowered.
[0039] In the present specification, "average particle size" means
a value of 50% particle size measured by a laser
diffraction/scattering type particle size measuring device (e.g.,
Commercial name: "LA-910" produced by Horiba, Ltd.) where a light
diffraction method is applied as the measurement principle.
Incidentally, the measurement is performed in the condition that
the raw material is completely dispersed in water.
[0040] In a method for manufacturing a honeycomb structure of the
present embodiment, the boehmite contained in at least a part of
the alumina source has a BET specific surface area of preferably 80
m.sup.2/g or more, further preferably 100 80 M.sup.2/g or more, and
particularly preferably 150 m.sup.2/g or more. When a BET specific
surface area of the boehmite exceeds the above value, a honeycomb
structure having excellent thermal shock resistance can be
manufactured.
[0041] In a method for manufacturing a honeycomb structure of the
present embodiment, it is preferable that silica is contained in
the cordierite-forming raw material at a ratio of 8% by mass or
less. When silica is contained in the cordierite-forming raw
material at a predetermined ratio, lowering of the porosity by
boehmite can be complemented by a pore-increasing effect of silica,
and a honeycomb structure having a desired porosity and pore
diameter can be obtained. In addition, shrinkage upon firing can be
reduced, and a honeycomb structure having an aimed structure can be
manufactured with high size precision. Therefore, by using silica,
an amount of boehmite can be increased, and a honeycomb structure
having low thermal expansion can be obtained. Incidentally, the
lower limit of the ratio of silica contained in the
cordierite-forming raw material is not particularly limited, it is
preferably 5% by mass or more in order to exhibit an effect of
containing silica.
[0042] In a method for manufacturing a honeycomb structure of the
present embodiment, it is preferable that alumina and/or aluminum
hydroxide is further contained in the alumina source because a
honeycomb structure having higher porosity and further suitable as
a catalyst carrier can be manufactured. Incidentally, the ratio of
alumina contained in the case that alumina is contained in the
alumina source is preferably 20% by mass or less, further
preferably 5 to 18% by mass, with respect to the cordierite-forming
raw material. The ratio of aluminum hydroxide in the case that
aluminum hydroxide is contained in the alumina source is preferably
28% by mass or less, further preferably 4 to 23% by mass, with
respect to the cordierite-forming raw material. However, among
alumina, for example, activated alumina obtained by calcining
aluminum hydroxide and having a high specific surface area is not
preferable because it increases the thermal expansion coefficient
of the resultant honeycomb structure.
[0043] Next, a method for manufacturing a honeycomb structure of
the present embodiment is described in more detail by each step.
First, an alumina source raw material, a silica source raw
material, and a magnesia source raw material, which serve as an
alumina source, a silica source, and a magnesia source in a
cordierite composition, are added to obtain a cordierite-forming
raw material. To the cordierite-forming raw material is added a
dispersion medium such as water, and they are mixed and kneaded to
give clay. The cordierite-forming raw material means a compound
containing a silica source, an alumina source, and a magnesia
source and convertible into cordierite by firing. Usually, it is
the one in which these particles are mixed so that it has the
theoretical composition (2MgO.2Al.sub.2O.sub.3.5SiO.sub.2) of
cordierite after firing.
[0044] In the present specification, "alumina source" means
alumina, aluminum hydroxide, active alumina, or boehmite. Though
particles of kaolin (Al.sub.2O.sub.3.2SiO.sub.2.2H.sub.2O) and
mullite (3Al.sub.2O.sub.3.2SiO.sub.2) can generally be used as an
alumina source because they play roles of an alumina source and a
silica source, they are not included in the "alumina source" of the
present specification.
[0045] As a silica source, particles of silica, a silica-containing
composite oxide, a substance converted into silica by firing, or
the like, can be used. Specifically, particles of silica
(SiO.sub.2) including quartz, kaolin
(Al.sub.2O.sub.3.2SiO.sub.2.2H.sub.2O), talc
(3MgO.4SiO.sub.2.H.sub.2O), mullite (3Al.sub.2O.sub.3.2SiO.sub.2),
or the like, can be used. Though there is no particular limitation
on the average particle size of the silica source particles, there
are suitably used silica source particles having an average
particle size of about 5 to 50 .mu.m in the case of quartz, about 2
to 10 .mu.m in the case of kaolin, about 5 to 40 .mu.m in the case
of talc, and about 2 to 20 .mu.m in the case of mullite.
[0046] As a magnesia source, particles of magnesia, a
magnesia-containing composite oxide, a substance converted into
magnesia by firing, or the like, can be used. Specifically,
particles of talc, magnesite (MgCO.sub.3), or the like, can be
used, and talc particles are preferable among them. Though there is
no particular limitation on the average particle size of the
magnesia source particles, there are suitably used magnesia source
particles having an average particle size of about 5 to 40 .mu.m
(preferably 10 to 30 .mu.m) in the case of talc, and about 4 to 8
.mu.m in the case of magnesia particles.
[0047] Though examples of a dispersion medium added to the
cordierite-forming raw material include water and a mixed solvent
of water and an organic solvent such as alcohol, water can be used
particularly suitably. When a cordierite-forming raw material and a
dispersion medium are mixed together and kneaded, additives such as
a pore-forming material, an organic binder, and a dispersant may
further be added thereto.
[0048] Examples of the pore-forming material include carbon such as
graphite, flour, starch, a phenol resin, poly(methyl methacrylate),
polyethylene, and poly(ethylene terephthalate). Among them, a micro
capsule made of an organic resin such as an acrylic resin can
particularly suitably be used.
[0049] There may suitably be used; as an organic binder,
hydroxypropylmethyl cellulose, methyl cellulose, hydroxyethyl
cellulose, carboxylmethyl cellulose, poly(vinyl alcohol), or the
like. There may suitably be used, as a dispersant, a substance
having a surface active effect, for example, ethylene glycol,
dextrin, fatty acid soap, and polyalcohol.
[0050] Incidentally, mixing and kneading of a cordierite-forming
raw material and a dispersion medium may be conducted by a known
mixing/kneading method. However, for the mixing, there is used a
mixer capable of rotating a stirring blade at a high speed of 500
rpm or more (preferably 1000 rpm or more) and excellent in stirring
force and dispersing force, and the mixture is conducted with
applying shearing force. By such a mixing method, a cohesive mass
contained in each raw material particles and causing an internal
defect of a honeycomb structure obtained can be pulverized and
disappeared.
[0051] The mixing can be conducted with a conventionally known
mixer, for example, a sigma kneader, a ribbon mixer, or the like.
In addition, the kneading can be conducted with a conventionally
known kneader, for example, a sigma kneader, a Banbury mixer, a
screw type extrusion kneader, or the like. Particularly, it is
preferable to use a kneader (so-called vacuum kneading machine or
biaxial continuous kneading extruder) provided with a vacuum
apparatus (e.g., vacuum pump) in that clay having no defect and
good formability can be obtained.
[0052] By forming the obtained clay by a forming method such as an
extrusion forming method, a honeycomb formed body having a
plurality of cells separated from each other by partition walls can
be obtained. A preferable extrusion forming method is a method
using a die having a desired cell shape, partition wall thickness,
and cell density.
[0053] Next, the obtained honeycomb formed body is dried to obtain
a honeycomb dried body. There is no particular limitation on a
drying method, and there may be employed a conventionally known
method such as hot air drying, microwave drying, drying under
reduced pressure, vacuum drying, or freeze drying. Among them, a
drying method in combination of hot air drying, microwave drying,
and dielectric drying is preferable in that the whole formed body
can be dried quickly and uniformly.
[0054] Next, the honeycomb dried body is fired to obtain a
honeycomb structure. Since the firing conditions (temperature and
time) depend on kind of each of the raw material particles
constituting the honeycomb formed body, they may appropriately be
determined according to these kinds. For example, it is preferable
to fire at 1410 to 1440.degree. C. for three to ten hours. When the
firing conditions (temperature and time) are below the above
ranges, cordierite crystallization of aggregate raw material
particles tends to be insufficient. On the other hand, when the
firing conditions exceed the above ranges, generated cordierite
tends to melt.
[0055] It is preferable to remove organic substances (pore-forming
material, organic binder, dispersant, etc.) in the honeycomb dried
body by combustion (calcination) before firing or in a temperature
rise process in firing because removal of the organic substances
can further be accelerated. The combustion temperature of the
organic binder is about 200.degree. C., and the combustion
temperature of the pore-forming temperature is about 300 to
1000.degree. C. Therefore, the calcination temperature may be about
200 to 1000.degree. C. Though the calcination time is not
particularly limited, it is generally about 10 to 100 hours.
EXAMPLE
[0056] The present invention is hereinbelow described specifically
on the basis of Examples. However, the present invention is not
limited to these Examples. Incidentally, methods of various
property values are shown below.
[Thermal Shock Resistance Test]
[0057] A honeycomb structure is put in an electric furnace heated
in advance at a predetermined temperature to be heated for
sufficient period of time till the whole honeycomb structure is at
a predetermined temperature, followed by being air-cooled. A
thermal shock due to quick cooling may cause cracks on a side face,
on an end face, or inside the structure. In the case that no crack
is caused upon cooling down to room temperature, the structure is
considered that it has cleared the heating temperature. Cracks are
detected by eye observation, strike sound, and the like. The
honeycomb structure which has cleared the heating temperature is
heated until a crack is caused by 50.degree. C. The highest
temperature with no crack caused is defined as "thermal shock
resistance temperature (.degree. C.).
[Percentage of Water Absorption]
[0058] After the honeycomb structure was boiled at 100.degree. C.
to allow it to absorb water sufficiently, humid air at 55.degree.
C. (humidity of 96% or more) was sent into the structure for three
minutes, and surplus moisture adhering to a partition wall surface
is removed. The amount of moisture (ratio to the mass of the
honeycomb structure) absorbed in the honeycomb structure (taken by
pored in the partition walls) is calculated from the following
formula (2) as "percentage of water absorption". Percentage of
water absorption (%)=([mass of honeycomb structure after absorbing
water]-[mass of honeycomb structure before absorbing water])/[mass
of honeycomb structure before absorbing water] (2) [Drying/Firing
Shrinkage Ratio]
[0059] The drying shrinkage ratio and the firing shrinkage ratio
are calculated according to the following formulae (3) and (4).
Incidentally, "size" in the following formulae (3) and (4) means
"diameter" of the honeycomb structure. Drying shrinkage ratio
(%)=([size upon forming]-[size after drying])/[size upon forming]
(3) Firing shrinkage ratio (%)=([size after drying]-[size after
firing])/[size after drying] (4)
Example 1
[0060] As shown in Table 1, 21% by mass of talc (average particle
size: 8 .mu.m, BET specific surface area: 11 m.sup.2/g), 19% by
mass of talc (average particle size: 11 .mu.m, BET specific surface
area: 5 m.sup.2/g), 12% by mass of kaolin (average particle size: 9
.mu.m, BET specific surface area: 7 m.sup.2/g), 32% by mass of
calcined kaolin (average particle size: 3 .mu.m, BET specific
surface area: 10 m.sup.2/g), and 16% by mass of boehmite (average
particle size: 0.5 .mu.m, BET specific surface area: 20 m.sup.2/g)
were mixed together to prepare a cordierite-forming raw material.
To 100 parts by mass of the prepared cordierite-forming raw
material were added 7 parts by mass of an organic binder (methyl
cellulose, hydroxypropylmethyl cellulose), 1.5 parts by mass of a
surfactant (sodium stearate), and 31 parts by mass of water, and
they were put in a mixer for mixing for three minutes to obtain a
wet mixture.
[0061] The obtained wet mixture was put in a screw type extrusion
kneader and kneaded to give clay in a cylindrical shape. This clay
was put in an extrusion forming machine for extrusion forming to
give a honeycomb formed body. The obtained honeycomb formed body
was subjected to dielectric drying and hot air drying, and then cut
at both ends so as to have predetermined dimensions. Thus, a
honeycomb dried body was obtained. The dried body obtained was
fired at 1420 to 1440.degree. C. for five hours to produce a
cylindrical honeycomb structure (Example 1). The honeycomb
structure had dimensions of 106 mm in diameter and 100 mm in whole
length (length of passages) with a partition wall thickness of 63
.mu.m and a cell density of 900 cells/in.sup.2 (This cell structure
has low thermal shock resistance temperature measured in the case
of carrying out the thermal shock resistance test). Various
property values of the honeycomb structure are shown in Table
3.
Examples 2 to 21, Comparative Examples 1 to 17
[0062] Honeycomb structures (Examples 2 to 21, Comparative Examples
1 to 17) were manufactured in the same manner as in Example 1
described above except for varying average particle sizes (.mu.m),
BET specific surface areas (m.sup.2/g), and mix proportions (mass
%) of the various raw materials used as shown in Tables 1, 2, 4,
and 5. Various property values of the honeycomb structures
manufactured are shown in Tables 3 and 6. TABLE-US-00001 TABLE 1
Example Average BET specific 1 2 3 particle surface area Mix
proportion size (.mu.m) (m.sup.2/g) (mass %) Talc 8 11 21 12 12 11
5 19 28 28 Kaolin 9 7 12 17 17 Calcined kaolin 3 10 32 17 17 Silica
4 3.5 6 6 Alumina Alumina 4 1.5 14 14 source Boehmite 0.11 160 6
0.5 20 16 0.5 80 6 Alumina Average particle size (.mu.m) 0.5 2.9 3
source BET specific surface area (m.sup.2/g) 20 48.2 24.8 Cell
Partition wall thickness (.mu.m) 63 63 63 structure Cell density
(cell/inch.sup.2) 900 900 900
[0063] TABLE-US-00002 TABLE 2 BET Average specific particle surface
Example size area 4 5 6 7 8 9 10 11 12 (.mu.m) (m.sup.2/g) Mix
proportion (mass %) Talc 8 11 13 12 12 12 12 12 12 12 12 11 5 28 29
29 28 28 30 30 28 28 Kaolin 9 7 17 17 17 17 17 16 16 19 15 Calcined
kaolin 3 10 17 17 17 17 17 16 16 19 15 Silica 4 3.5 5 5 5 5 5 5 5 3
7 Alumina Alumina 4 1.5 18 16 14 13 11 9 7 13 16 source Aluminum 2
15 0 0 0 0 0 0 0 0 0 hydroxide Boehmite 0.11 160 2 4 6 8 10 12 14 6
7 Alumina Average particle size (.mu.m) 3.6 3.2 2.8 2.5 2.1 1.8 1.5
2.8 2.9 source BET specific surface area 17.7 33.3 48.6 63.3 77.7
91.6 105.2 48.9 48.3 (m.sup.2/g) Cell Partition wall thickness
(.mu.m) 63 63 63 63 63 63 63 63 63 structure Cell density
(cell/inch.sup.2) 900 900 900 900 900 900 900 900 900 BET Average
specific particle surface Example size area 13 14 15 16 17 18 19 20
21 (.mu.m) (m.sup.2/g) Mix proportion (mass %) Talc 8 11 11 12 12
21 21 21 18 18 18 11 5 27 27 26 19 19 19 21 21 21 Kaolin 9 7 15 15
16 21 21 21 16 16 16 Calcined kaolin 3 10 15 15 16 21 21 21 20 20
20 Silica 4 3.5 5 5 5 0 0 0 4 4 4 Alumina Alumina 4 1.5 23 0 0 8 8
7 9 8 8 source Aluminum 2 15 0 18 13 8 8 7 12 12 11 hydroxide
Boehmite 0.11 160 4 9 12 0.48 1.19 2.38 0.48 1.19 2.38 Alumina
Average particle size (.mu.m) 3.4 2.8 2.1 2.9 2.8 2.6 2.8 2.7 2.5
source BET specific surface area 25.3 50.6 77.9 12.5 18.8 29.3 12.9
17.9 26.4 (m.sup.2/g) Cell Partition wall thickness (.mu.m) 63 63
63 63 63 63 63 63 63 structure Cell density (cell/inch.sup.2) 900
900 900 900 900 900 900 900 900
[0064] TABLE-US-00003 TABLE 3 Thermal Thermal shock Water Firing
Drying + Firing expansion resistance absorption Average pore
shrinkage shrinkage coefficient temperature ratio Porosity diameter
ratio ratio (.times.10.sup.-6/.degree. C.) (.degree. C.) (mass %)
(%) (.mu.m) (%) (%) Example 1 0.22 800 14.3 27.2 3.72 8.4 13.1
Example 2 0.13 850 18.7 31.9 4.55 3.2 8.6 Example 3 0.25 800 21.2
35.5 4.43 3.1 7.8 Example 4 0.19 800 21.9 35.9 4.87 2.1 6.2 Example
5 0.16 800 19.5 32.8 4.6 2.7 7.2 Example 6 0.13 850 18.2 31 4.44
3.2 8.1 Example 7 0.1 850 16.1 30 4.22 3.8 9.1 Example 8 0.08 900
13.9 28 4 4.4 10.4 Example 9 0.06 900 12.4 26.5 3.76 5 11.2 Example
10 0.05 900 10.4 25.1 3.11 5.5 12.3 Example 11 0.15 850 17.3 30.6
4.3 3.8 8.5 Example 12 0.17 850 18.9 32.6 4.5 2.7 7.7 Example 13
0.16 800 18.3 34.3 3.95 6.2 11.3 Example 14 0.1 850 15.6 32 3.65
6.7 12.2 Example 15 0.05 900 11.8 28.3 3.22 7 13.9 Example 16 0.25
750 15.9 26 4.59 6.1 10.7 Example 17 0.21 800 15.3 25.3 4.54 6.1
10.8 Example 18 0.2 800 15.4 25.2 4.51 6.2 11.1 Example 19 0.25 700
21.3 34.3 4.6 4.5 8.9 Example 20 0.21 750 20.6 33.5 4.72 4.5 9
Example 21 0.19 800 19.8 33.3 4.62 4.7 9.5
[0065] TABLE-US-00004 TABLE 4 BET Average specific particle surface
Comparative Example size area 1 2 3 4 5 6 7 8 9 10 (.mu.m)
(m/.sup.2g) Mix proportion (mass %) Talc 8 11 21 12 20 22 21 21 21
21 12 12 11 5 19 27 18 19 19 19 19 19 29 29 Kaolin 9 7 21 16 12 22
12 12 12 12 16 16 Calcined kaolin 3 10 21 19 30 22 32 32 32 32 17
17 Silica 4 3.5 4 5.5 5.5 Alumina Alumina 4 1.5 9 9 15 15 source 1
10 15 Aluminum 2 15 9 13 hydroxide Boehmite 0.22 100 16 0.12 130 16
0.11 160 16 0.09 180 16 6 230 6 4 200 6 active 8 20 20 alumina
Alumina Average particle size (.mu.m) 3 2.8 8 1 0.1 0.1 0.1 0.2 406
4 source BET specific surface 8.3 9.5 20 10 180 160 130 100 67.5 59
area (m.sup.2/g) Cell Partition wall thickness (.mu.m) 63 63 63 63
63 63 63 63 63 63 structure Cell density (cell/inch.sup.2) 900 900
900 900 900 900 900 900 900 900
[0066] TABLE-US-00005 TABLE 5 BET Average specific particle surface
Comparative Example size area 11 12 13 14 15 16 17 (.mu.m)
(m/.sup.2g) Mix proportion (mass %) Talc 8 11 21 13 12 11 12 2 18
11 5 19 29 29 27 27 18 21 Kaolin 9 7 6 17 16 15 16 22 16 Calcined
Kaolin 3 10 16 17 16 15 16 22 2 Silica 4 3.5 1 5 5 5 5 4 Alumina
Alumina 4 1.5 19 6 27 8 9 9 source Aluminum 2 15 9 12 hydroxide
Boehmite 0.11 160 28 16 16 Alumina Average particle size (.mu.m)
0.1 4 1.1 4 1.4 3 2.8 source BET specific surface 160 1.5 118.4 1.5
106.7 9.3 10.3 area (m.sup.2/g) Cell Partition wall thickness
(.mu.m) 63 63 63 63 63 63 63 structure Cell density
(cell/inch.sup.2) 900 900 900 900 900 900 900
[0067] TABLE-US-00006 TABLE 6 Thermal Thermal shock Water Firing
Drying + Firing expansion resistance absorption Average pore
shrinkage shrinkage coefficient temperature ratio Porosity diameter
ratio ratio (.times.10.sup.-6/.degree. C.) (.degree. C.) (mass %)
(%) (.mu.m) (%) (%) Comp. Ex. 1 0.3 750 16.3 27.4 4.84 6.7 10.7
Comp. Ex. 2 0.3 700 21.3 35.1 4.55 4.4 8.6 Comp. Ex. 3 0.67 600
21.9 29 7.89 6.9 11.3 Comp. Ex. 4 0.58 600 14.3 29.1 4 7.3 10.8
Comp. Ex. 5 0.01 900 2 11.7 2.11 10.7 18.8 Comp. Ex. 6 0.01 900 2
11.6 1.99 11 18.2 Comp. Ex. 7 0.07 900 2.3 13.1 2.17 11.1 17.9
Comp. Ex. 8 0.11 900 4.3 17 2.44 10.5 17.1 Comp. Ex. 9 0.58 650
26.4 39 5.82 1.7 5.8 Comp. Ex. 10 0.45 650 24.9 37.7 5.33 1.8 6.5
Comp. Ex. 11 0.07 900 4.7 18.1 1.65 8.4 18.7 Comp. Ex. 12 0.45 650
24.5 37.7 5.03 1.6 4.8 Comp. Ex. 13 0.03 900 9.2 23.6 3.08 6.1 13.3
Comp. Ex. 14 0.28 700 20.2 36.2 3.99 5.8 10.5 Comp. Ex. 15 0.02 900
9.1 25.3 2.99 7.4 14.9 Comp. Ex. 16 0.28 750 16.2 27.3 4.48 6.1
10.3 Comp. Ex. 17 0.27 700 21.3 34.7 4.39 4.4 8.8
[0068] It is clear from the results shown in Tables 3 and 6 that a
honeycomb structure having a small thermal expansion coefficient
and excellent thermal shock resistance when a fine boehmite is used
as an alumina source. Further, it became clear that a honeycomb
structure can have further excellent thermal shock resistance when
boehmite having a high BET specific surface area is used as an
alumina source (see Examples 2 and 3). Incidentally, when an excess
amount of boehmite is used, deformation is prone to be caused
because of high drying/firing shrinkage ratio, and a honeycomb
structure obtained tends to have lowered porosity (see Comparative
Example 5 to 8, 11, 13, and 15). However, it became clear that, by
allowing a cordierite-forming raw material to contain silica, water
absorption ratio and shrinkage ratio can be corrected, and it is
possible to manufacture a honeycomb structure having a low thermal
expansion coefficient, excellent thermal shock resistance, high
porosity, and large average pore diameter.
[0069] A honeycomb structure of the present invention has excellent
thermal shock resistance even with large pore diameters and high
porosity. Therefore, it is suitable as a filter for removing
harmful substances such as particulate and NO.sub.x contained in
exhaust gas discharged from automobile engines such as a diesel
engine or as a catalyst carrier.
* * * * *