Stock material or miscellaneous articles – Structurally defined web or sheet – Honeycomb-like
Reexamination Certificate
2002-12-23
2004-10-12
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Honeycomb-like
C428S304400, C428S310500, C264S044000, C264S628000, C264S631000, C264S652000, C264S656000, C264S657000, C264S660000, C264S661000, C422S122000, C422S177000, C422S180000, C422S222000
Reexamination Certificate
active
06803086
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a porous honeycomb structure body, the use thereof and a method for manufacturing the same. More specifically, it relates to a porous honeycomb structure body having a reduced pressure loss, with maintaining isostatic strength, by control of distributions of porosity and a pore diameter of the structure body, the use thereof and to a method for manufacturing the structure body. The porous honeycomb structure body of the present invention can be preferably used particularly as a particulate filter for exhaust gas purification, and a carrier for catalyst.
2. Description of the Related Art
Recently, the influence of particulate matter and NOx discharged from an automobile engine, especially a diesel engine or the like on environment has been increasingly receiving attention. Therefore, various research and developmental works have been made on the use of porous honeycomb structure body as important means for collecting and removing such deleterious substances.
For example, a honeycomb structure body is under developing to collect and remove particulate materials in exhaust gas by making exhaust gas flow into individual through holes having openings at one of the end faces of the honeycomb structure body comprising a plurality of through holes which are partitioned by porous partition walls and whose openings on an end face where an exhaust gas flows in and an end face where an exhaust gas flows out are alternately sealed, and passing it forcibly the partition walls thereof. A developmental work is also in progress, as a new approach to improve purification ability of such substances, to provide a catalyst made of a honeycomb structure body being composed of a porous structure all of which partition walls have a high porosity to which catalysts for decomposing HC and NOx are loaded in a relatively larger quantity.
Upon use of such a porous honeycomb structure body, it is housed in a metal case or the like by means of a holder with a fixed pressing force so that the structure body would not be displaced from the metal case under continuous vibrations from an engine or the like. Therefore, the structure body is required to have isostatic strength that enables the structure body to endure the pressing force. Especially, an attempt has been made to make the porosity in a honeycomb structure body higher, in responding to the demands in the reduction in pressure loss for less fuel consumption and high output, or the demands of the increase in the loaded amount of the catalysts for improvement of purification ability. Accordingly, it is strongly desired to have a honeycomb structure body provided with a sufficient isostatic strength with satisfying the demands for making the porosity in the honeycomb structure body higher. Incidentally, in the case of a honeycomb structure body installed in the path for exhaust gas, the flow amount of the exhaust gas in the center portion of the honeycomb structure body along with the perpendicular direction of the path for exhaust gas. Thus, the easiness in the flowability of exhaust gas and the decomposed amount of the HC, NOx or the like in the center portion of the honeycomb structure body awfully give influence on overall pressure loss and the purification ability. As a consequence, the development of the structure bodies and catalyst bodies having a structure capable of copping with the differences in the distribution in flowing gases is earnestly desired.
There has been proposed as a prior art porous honeycomb structure body to attain the objectives mentioned above a porous honeycomb structure body having a prolonged durability time for collection of particulate materials with a reduced frequency of regeneration treatment by constituting the honeycomb structure body to have “a porosity of 45 to 60%, pores with a pore diameter of 100 &mgr;m or more in a volume that corresponds to 10% or less of a total volume of all pores, and a relationship between a total of specific surfaces (Mm
2
/g) of all pores opening on the surface of the structure body and penetrating the structure body inwardly and surface roughness (N&mgr;m) on the surface of the structure body of 1,000M+85N≧530” (See Japanese Patent No. 2726616).
Further, there is disclosed a porous ceramic honeycomb structure body having significantly prolonged collection time, with the same collection efficiency and the same pressure loss, by having “a porosity of 40 to 55% and a total volume of pores having a diameter of 2 &mgr;m or less being 0.015 cc/g or less” (See Japanese Patent No. 2578176).
In addition, a cordierite honeycomb structure having a high collection rate, a small pressure loss and a small coefficient of thermal expansion simultaneously by having “a thermal expansion coefficient between 25° C. and 800° C. of 0.3×10
−6
/° C. or less, a porosity of 55 to 80%, an average pore diameter of 25 to 40 &mgr;m, and smalls pores each having a diameter of 5 to 40 &mgr;m and large pores each having a diameter of 40 to 100 &mgr;m as pores on surfaces of partition walls, the number of the small pores being 5 to 40 times as large as the number of the large pores” is also disclosed (JP-A-9-77573).
However, with respect to any of these honeycomb structure bodies, it has been never given any consideration to simultaneous satisfaction of such a characteristic as the reduction of pressure loss and the attainment in increased isostatic strength which are mutually contradictory one by controlling the pore distribution.
SUMMARY OF THE INVENTION
The present invention has been completed in view of the above problems. An object of the present invention is to provide a porous honeycomb structure body capable of satisfying simultaneously the characteristics of pressure loss and isostatic strength which are mutually contradictory properties, suitable for use particularly in an exhaust gas purification device installed in combustion equipment and which is usable, for example, for a structure body for collecting and removing particulate substances contained in exhaust gas or a catalyst body for decomposing HC, NOx and the like to remove them therefrom, a method for manufacturing the porous honeycomb structure body as well.
The intensive studies have been made so as to solve the above problems. As a result, firstly, we have found the phenomenon that the firing shrinkage of a honeycomb molded article containing a cordierite-forming raw material as a major component and carbon as a pore-forming agent becomes extremely evident when the product temperature of the molded article is within a temperature of 1,000 to 1,200° C., but the firing shrinkage of the molded article hardly occurs when the temperature of the molded article is outside the above-mentioned temperature range of 1,000 to 1,200° C.
Then, we have made further studies and found that a honeycomb structure body having a large pore diameter and porosity in a center portion which greatly influences a reduction in pressure loss can be obtained by using carbon as a pore-forming agent and controlling a temperature increasing rate of a firing environment so as to prevent carbon existing in the center portion of a molded article from burning out until the center portion of the molded article exceeds the above-mentioned temperature range. The present invention has been completed based on the present finding.
That is, according to the present invention, there is provided a porous honeycomb structure body having partition walls which contain cordierite as a primary crystal phase, a porosity of 40 to 75% and an average pore diameter of 10 to 50 &mgr;m, wherein the porosity and the pore diameter in a center portion of the honeycomb structure body are larger than those in a peripheral portion of the honeycomb structure body. In the present specification, the phrase “center portion” refers to a midpoint of a central axis of a honeycomb structure body or molded article or a partition wall portion which is the closest to the midpoint, while the phrase “peripheral portio
Muroi Yumi
Noguchi Yasushi
Wada Yukihisa
Boss Wendy
NGK Insulators Ltd.
Zimmerman John J.
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