Stock material or miscellaneous articles – Structurally defined web or sheet – Honeycomb-like
Reexamination Certificate
2002-12-30
2004-07-20
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Honeycomb-like
C428S116000, C428S034400, C428S188000, C428S220000, C428S689000, C428S698000, C428S702000, C422S180000, C422S222000, C055S523000, C502S407000, C502S414000, C264S630000, C264S631000
Reexamination Certificate
active
06764743
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a porous honeycomb structure and a process for production thereof. More particularly, the present invention relates to a honeycomb structure balanced in purification ability, end face durability and canning property, and suitably used, for example, in a carrier for a catalyst for purification of automobile exhaust gas, as well as to a process for production thereof.
BACKGROUND ART
Porous honeycomb structures are in wide use in a catalyst carrier (e.g. a carrier for catalyst for purification of automobile exhaust gas, a filter (e.g. a filter for diesel engine exhaust gas), etc. The regulation for exhaust gas is becoming stricter year by year in consideration of environmental problems; in order to respond to such a trend, catalysts for exhaust gas purification, when used for purification of the exhaust gas emitted from an automobile engine or the like, are required to have an improved purification ability. Meanwhile, a lower fuel consumption and a higher output are aimed strongly from the standpoint of engine development; in order to respond to such a situation, catalysts for exhaust gas purification are also required to have a lower pressure loss. Hence, in order to satisfy these requirements, there is a strong movement that by allowing a honeycomb structure to have a very small wall thickness; the gas permeability is enhanced to reduce the pressure loss and also, weight of the catalyst for exhaust gas purification is reduced to reduce the heat capacity, to improve the purification ability during warm-up.
Meanwhile, various foreign matter may come into the exhaust gas emitted from an engine, whereby a honeycomb structure may be subjected to scraping-off (erosion); therefore, improvement in erosion resistance is also required.
The above improvement in purification ability (e.g. lower heat capacity and lower pressure loss) and the above improvement in erosion resistance are generally contradictory to each other. For example, when a honeycomb structure is allowed to have an increased porosity and a smaller wall thickness in order to achieve a lower heat capacity, the honeycomb structure comes to have a lower strength and a lower erosion resistance.
In such applications, the honeycomb structure is used by being held by a metal-made can or the like and, in such use, the honeycomb structure need be held at certain strength so that there occurs no slippage between the honeycomb structure and the can. An improvement in canning property of the honeycomb structure, such as strength capable of withstanding the above strength or an ability to prevent slippage even at a low holding strength, is required.
For improvement of erosion resistance, there are described, in, for example, JP-A-2000-51710, a honeycomb structure in which cell walls are made thick only in the vicinities of the honeycomb structure end face, and a honeycomb structure in which the end face is made dense by coating the end face of a base honeycomb structure with a vitreous component such as glaze, water glass or the like, followed by firing. In the literature, there is a description that the porosity of the end face is 5 to 35%; however, there is disclosed only a honeycomb structure in which the porosity of other portion is 36%. Such a honeycomb structure is unable to satisfy all of low heat capacity, erosion resistance and canning property. Further, the literature makes no mention of a relation between porosity, wall thickness and surface roughness.
For improvement of canning property, there is disclosed, in JP-A-2-207846, a honeycomb structure in which the surface of outer wall is allowed to have unevenness by a method of applying a vibration in extrusion of the honeycomb structure. However, this method has a problem in that the wall is cut off in extrusion when the wall is thin; therefore, the method is difficult to apply to a honeycomb structure having a thin wall. Also, in JP-A-2-86847 is proposed a honeycomb structure in which the surface of outer wall is allowed to have an uneven part by applying sandblasting or spraying a ceramic material to the surface and the uneven surface is made more rough than another surface; and a process for production of such a honeycomb structure. This process, however, needs an additional step and is not economical. In these literatures as well, no mention is made on a relation between porosity, wall thickness and surface roughness, and no investigation is made on erosion resistance or heat capacity reduction.
DISCLOSURE OF THE INVENTION
The present invention has been made in view of the above-mentioned problems and aims at providing a honeycomb structure which satisfies requirements of improved purification ability, erosion resistance and improved canning property at a good balance and which is suitably used particularly in a carrier for catalyst for purification of automobile exhaust gas, etc.
The present invention further aims at a process for producing such a honeycomb structure, particularly a honeycomb structure of low porosity and good canning property.
In order to achieve the above aims, the present inventors made a study including various tests. As a result, the present inventors found out that while, in general, a high porosity and a thin cell wall are required in order to achieve a low heat capacity and the opposite is required in order to obtain improved erosion resistance, the influence of porosity on erosion resistance is greater when the wall is thinner. The present inventors also found out that in achieving a certain heat capacity, good erosion resistance is obtained by making thinner the wall and lower the porosity. The present inventors also found out that when the porosity is controlled at less than 30%, the canning property is reduced but this is caused by the smoothening of the surface of outer wall. The present inventors also found out that the canning property can be improved by allowing the average surface roughness (Ra
s
) to fall in 0.5 &mgr;m≦Ra
s
≦10 &mgr;m and/or allowing the average height (RzDIN
s
) on the surface of outer wall from a local maximum point to a next local minimum point to fall in 5 &mgr;m≦RzDIN
s
≦50 &mgr;m, and further that the above Ra
s
and RzDIN
s
can be achieved by allowing the pores having pore diameters of 1 &mgr;m or more to be 90% by volume or more of the total pore volume. The first aspect of the present invention is based on the above findings.
The first aspect of the present invention provides a honeycomb structure constituted by
porous cell walls which form a cell group consisting of a plurality of cells adjacent to each other, and
a honeycomb outer wall that surrounds and holds the outermost peripheral cells located at a circumference of the cell group,
characterized in that a porosity is 5% to less than 30%, a basic wall thickness (T
c
) of the cell walls is 0.030 mm≦T
c
<0.076 mm, an average surface roughness (Ra
s
) of the outer wall is 0.5 &mgr;m≦Ra
s
≦10 &mgr;m, and/or an average height (RzDIN
s
) on the surface of the outer wall from a local maximum point to a next local minimum point is 5 &mgr;m≦RzDIN
s
≦50 &mgr;m.
In the first aspect of the present invention, a preferred example is a honeycomb structure having the following characteristics. That is, in the first aspect of the present invention, it is preferred for improved canning property that Ra
s
is 0.5 &mgr;m≦Ra
s
≦10 &mgr;m and RzDIN
s
is 5 &mgr;m≦RzDIN
s
≦50 &mgr;m and, preferably, Ra
s
is 1.0 &mgr;m≦Ra
s
≦10 &mgr;m and RzDIN
s
is 10 &mgr;m≦RzDIN
s
≦50 &mgr;m, and an open frontal area (P) of honeycomb structure, of 83% or more is preferred for lower heat capacity, lower pressure loss and improved purification ability. Further, an outer wall thickness of 0.076 mm or more is preferred for improved canning property. Also, it is preferred that the average surface pore diameter of outer wall is larger than an average inside pore diameter of outer wall, in order to make Ra
s
and RzDIN
s
large. It is also preferred for higher loading power for catalyst that the
Kato Yasushi
Noguchi Yasushi
Ogawa Masato
Boss Wendy
Jones Deborah
NGK Insulators Ltd.
Oliff & Berridg,e PLC
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