Patent Application
20090035512
The present invention relates to a honeycomb structure, a process for producing the honeycomb structure, and a bonding material. More particularly, the present invention relates to a honeycomb structure which can be suitably used, for example, in a carrier for catalyst loading, installed in internal combustion engine, boiler, chemical reactor, fuel cell reformer, etc., or in a filter for capture of particulates in exhaust gas; a process for producing the honeycomb structure; and a bonding material. Further particularly, the present invention relates to, for example, a honeycomb structure of large size wherein a plurality of honeycomb segments are reliably bonded to each other; a process for producing the honeycomb structure; and a bonding material suitably used in the process.
Ceramic-made honeycomb structures are in use, for example, in a carrier for catalyst loading, installed in internal combustion engine, boiler, chemical reactor, fuel cell reformer, etc., or in a filter for capture of particulates in exhaust gas, particularly diesel particulates (hereinafter, the filter for capture of diesel particulates is referred to as DPF).
Such a ceramic honeycomb structure is constituted by bonding, via an adhesive layer, a plurality of porous honeycomb segments each having a large number of passages divided by partition walls and extending in the axial direction of honeycomb segment (see, for example, Patent Document 1). That is, the ceramic honeycomb structure is constituted by combining square pole shape, porous honeycomb segments in rows and bonding them to each other via adhesive layers. The bonding is conducted by placing an adhesive layer between the to-be-adhered sides of two porous honeycomb segments and then applying a vibration to the honeycomb segments while applying a pressure to them. That is, the bonding step is conducted as follows. First, a first porous honeycomb segment having an undercoat layer formed on one to-be-bonded surface is placed on the lowermost position of the cut portion of a supporting jig. Then, a second porous honeycomb segment having an undercoat layer formed on one to-be-bonded surface and further having an adhesive coated on the undercoat layer is placed in close contact with the first porous honeycomb segment so that the to-be-bonded surfaces of the two honeycomb segments face each other via the adhesive. In this state, the end faces of the two honeycomb segments are pressed by a pressing plate for positioning of the two honeycomb segments. Further, a pressing jig is abutted to the second honeycomb segment to press it in a vertical direction and also give a vibration in a direction in which the to-be-bonded surfaces shift against each other. Thereby, the first and second honeycomb segments can be bonded to each other.
Then, a third porous honeycomb segment having an undercoat layer formed on one to-be-bonded surface and further having an adhesive coated on the undercoat layer is placed in close contact with the first honeycomb segment so that the other to-be-bonded surface of the first honeycomb segment and the to-be-bonded surface of the third honeycomb segment face each other via the adhesive. In this state, as in the case of the second honeycomb segment, the third honeycomb segment can be bonded to the first honeycomb segment. Further, a fourth porous honeycomb segment having undercoat layers formed on two to-be-bonded surfaces and further having an adhesive coated on each of the undercoat layers is placed between the second and third honeycomb segments in close contact with them. In this state, as in the cases of the second and third honeycomb segments, the fourth honeycomb segment can be bonded to the second and third honeycomb segments.
In the conventional bonding method, however, there is the following problem. Since pressure and vibration are applied in each bonding of porous honeycomb segments and such a bonding operation is repeated, the lower position honeycomb segments laminated earlier undergo vibration and pressure up to the bonding of last honeycomb segment; such forces act as a peeling force for honeycomb segments bonded to each other; as a result, the adhesive layers used for bonding of the lower position honeycomb segments are peeled, resulting in a reduction in adhesivity in part of the honeycomb structure obtained.
In order to solve the above problem and provide a bonding method for production of ceramic honeycomb structure, which can bond all honeycomb segments uniformly at an intended adhesivity by allowing the adhesive layers present between each honeycomb segments to be kept in their state when the honeycomb segments have been laminated, irrespective of the order of lamination of each honeycomb segment, there was proposed a bonding method for production of a ceramic honeycomb structure which is constituted by bonding, via adhesive layers, a plurality of porous honeycomb segments each having a large number of passages divided by partition walls and extending in the axial direction of honeycomb segment, which bonding method comprises laminating a required number of porous honeycomb segments via an adhesive layer present between the to-be-adhered sides of each two porous honeycomb segments and then applying a pressure to the total porous honeycomb segments via the outermost honeycomb segment, to bond the whole porous honeycomb segments simultaneously (see, for example, Patent Document 2).
In any of the above two Patent Documents, however, the bonding material is pressed and spread by application of load; therefore, the bonding material needs to have fluidity. Hence, there was an inconvenience that the bonding material shrinks between the bonding and the appearance of strength at bonded portion (heating is necessary in any of the above two Patent Documents) and there occurs the non-uniformity of bonding width or the shifting of bonding portion. The non-uniformity of bonding width or the shifting of bonding portion invites stress concentration during the actual use of the honeycomb structure obtained, causing a problem of generation of inconveniences such as crack formation and the like.
The present invention has been made in view of the above-mentioned problems of prior art. The present invention aims at providing a honeycomb structure wherein a plurality of honeycomb segments are reliably bonded to each other without causing inconveniences such as cracking, peeling and the like at the bonding portions of honeycomb segments; a process for producing a honeycomb structure having the above-mentioned properties; and a bonding material which can bond to-be-bonded bodies to each other without causing inconveniences such as cracking, peeling and the like at the bonding portions of to-be-bonded bodies.
According to the present invention, there are provided a honeycomb structure, a process for producing the honeycomb structure, and a bonding material, all described below.
[1] A ceramic-made honeycomb structure produced by bonding, with a bonding material, a plurality of honeycomb segments each comprising a cell structure having a plurality of cells divided by porous partition walls and functioning as a fluid passage, and a porous outer wall provided at the periphery of the cell structure, to each other at the outer walls of honeycomb segments to obtain an integral body, wherein the bonding of honeycomb segments is made with a bonding material containing 0.1 mass % or more of a heat-gelling organic binder having a thickening-initiating temperature of 75° C. or lower.
[2] A ceramic-made honeycomb structure produced by bonding, with a bonding material, a plurality of honeycomb segments each comprising a cell structure having a plurality of cells divided by porous partition walls and functioning as a fluid passage, and a porous outer wall provided at the periphery of the cell structure, to each other at the outer walls of honeycomb segments to obtain an integral body, wherein the bonding of honeycomb segments is made with a bonding material containing a thermosetting resin.
[3] A process for producing a ceramic-made honeycomb structure, which comprises a bonding step of bonding, with a bonding material, a plurality of honeycomb segments each comprising a cell structure having a plurality of cells divided by porous partition walls and functioning as a fluid passage, and a porous outer wall provided at the periphery of the cell structure, to each other at the outer walls of honeycomb segments to obtain an integral body, in which bonding step, a bonding material containing 0.1 mass % or more of a heat-gelling organic binder having a thickening-initiating temperature of 75° C. or lower is coated on each outer wall, the coated bonding material is dried at a temperature of 100° C. or higher to form a bonding layer, and the outer walls are bonded to each other via the bonding layer.
[4] A process for producing a ceramic-made honeycomb structure, which comprises a bonding step of bonding, with a bonding material, a plurality of honeycomb segments each comprising a cell structure having a plurality of cells divided by porous partition walls and functioning as a fluid passage, and a porous outer wall provided at the periphery of the cell structure, to each other at the outer walls of honeycomb segments to obtain an integral body, in which bonding step, a bonding material containing 0.1 mass % or more of a heat-gelling organic binder having a thickening-initiating temperature of 75° C. or lower is coated on each outer wall, the coated bonding material is dried by applying a microwave, to form a bonding layer, and the outer walls are bonded to each other via the bonding layer.
[5] A bonding material for honeycomb structure, containing 0.1 mass % or more of a heat-gelling organic binder having a thickening-initiating temperature of 75° C. or lower.
[6] A bonding material for honeycomb structure containing plastic resin inorganic particles and colloidal oxide other than an organic binder described above, containing a thermosetting resin and inorganic particles and/or a colloidal oxide.
[7] A bonding method for bonding of honeycomb segments, which comprises heating using a bonding material set forth in [5] or [6].
In the present invention, by adding a heat-gelling organic binder having a thickening-initiating temperature of 75° C. or lower or a thermosetting resin, the resulting bonding material can be dried or can express a strength, more quickly. Thereby, there can be suppressed the dispersion of bonding width or the shifting of bonding position, which have occurred with bonding materials having fluidity before drying, making it possible to obtain a honeycomb structure wherein a plurality of honeycomb segments are reliably bonded to each other without causing bonding inconveniences such as cracking, peeling and the like at the bonding portions.
1 is a honeycomb structure; 2 is a partition wall; 3 is a cell; 5 is a cell structure; 7 is an outer wall; 8 is a bonding layer; 12 is a honeycomb segment; 36 is a bonded body; 42 is a diamond tool; 43 is a coating material; 44 is a leveling plate; 45 is a shaft; 46 is a handle; 47 is an abutting jig; 48 is a periphery-coating machine; and 50 is an outer wall.
The mode for carrying out the present invention is described below. However, the present invention is in no way restricted to the following mode and it should be construed that, as long as there is no deviation from the scope of the present invention, design change, modification, etc. can be added appropriately based on the ordinary knowledge possessed by those skilled in the art.
Organic binder refers generally to an organic bonding material, and bonding material refers to a material which is used for bonding or fixing solids of same kind or different kinds to form a part, a product, etc. In the case of ceramic production, the organic binder means organic compounds which are added to enable molding of ceramic raw material powder and impart the strength necessary for maintenance of the shape of the molding obtained. Therefore, as representative organic binders, there can be mentioned, for example, natural starch, gelatin, agar, cellulose derivatives such as semi-synthesized alkyl cellulose (e.g. methyl cellulose) and carboxymethyl cellulose, and synthetic, water-soluble polymers such as polyvinyl alcohol, polyacrylic acid type polymer, polyacrylamide and polyethylene oxide, etc.
Gelling refers generally to a phenomenon in which a sol is changes to a gel. It occurs when a sol is cooled, or when a solvent, a salt or the like is added to a sol, or when a mechanical impact is applied to a sol. For example, when an aqueous gelatin solution is cooled, there occurs a transition from sol to gel; and the gel returns to a sol when heated. As such an example, agarose (a main component of agar) is also known.
In the present invention, there is used, as a constituent component, an organic binder which shows a so-called heat-gelling phenomenon in which, contrary to the above gelling, an aqueous solution thereof becomes a gel when heated and, when cooled, the gel returns to the original aqueous solution.
Heat-gelling refers generally to a phenomenon in which an aqueous alkyl cellulose or hydroxyalkyl cellulose solution becomes a gel when heated and, when cooled, the gel returns to the original aqueous solution. Such an aqueous solution showing heat-gelling, which is reversible phenomenon, which solution has different viscosity restore temperatures when heating and cooling, that is, a hysteresis property. As substances showing heat-gelling, there can be generally mentioned hydroxyethyl methyl cellulose, hydroxyethyl ethyl cellulose, hydroxypropyl methyl cellulose, hydroxypropyl ethyl cellulose, hydroxybutyl methyl cellulose, etc. In the present invention as well, water-soluble cellulose is used preferably.
In the present invention, as the organic binder, there is used only an organic binder which gels upon heating and whose gelling temperature is 75° C. or lower. As preferred organic binders, there can be mentioned, for example, water-soluble cellulose derivatives. As the cellulose derivatives, there can be mentioned, for example, methyl cellulose, hydroxypropyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose and hydroxybutyl cellulose. Of these, those having a gelling temperature of 75° C. or lower are used in the present invention.
Thickening agent refers generally to a material which is added to increase the viscosity of a fluid. Cellulose derivatives such as methyl cellulose, carboxymethyl cellulose and the like are a thickening agent dispersible in water.
Therefore, in the first embodiment, there is used a bonding material containing, of the above-mentioned preferred organic binders, an organic binder which shows a heat-gelling phenomenon and has a thickening-initiating temperature of 75° C. or lower in the heat-gelling phenomenon. The thickening action of the organic binder suppresses the fluidity of the bonding material.
For example, an aqueous solution of methyl cellulose becomes a gel when heated and, when cooled, becomes a sol. It is considered that the crosslinking zone brought about by gelling is formed by a hydrophobic interaction. In-depth studies have been made and various products are being marketed by various companies. With respect to, for example, methyl cellulose, products of different properties are being marketed by Sin-Etsu Chemical Co., Ltd. under the trade name of “Metolose”.
Preferably, the bonding material used in the present invention contains any of inorganic particles and a colloidal oxide.
As the inorganic particles, there can be mentioned powders of ceramics such as cordierite, silicon carbide, silicon nitride, alumina, mullite, zirconia, zirconium phosphate, aluminum titanate, titania and the like; and powders obtained by combining two or more kinds of the above powders. The bonding material can have an improved affinity by being appropriately selected in view of the matching with the material of honeycomb segment (kind of ceramic) on which the bonding material is to be coated. In particular, a cordierite powder or a silicon carbide powder can be used preferably.
As the colloidal oxide, there can be mentioned, for example, silica sol, alumina sol, colloidal silica and colloidal alumina. They may be used singly or in combination of two or more kinds.
In the present invention, as to the cell density of honeycomb segment (the number of cells per unit honeycomb sectional area intersecting the fluid passages of honeycomb segment at right angles), there is no particular restriction. However, too small a cell density results in an insufficient geometric surface area and a too large a cell density results in too large a pressure loss; therefore, none of them is preferred. The cell density is preferably 0.9 to 310 cells/cm2 (6 to 2,000 cells/in2). As to the shape of cell section (cell section intersecting the fluid passage at right angles), there is no particular restriction, either. The cell sectional shape may be any selected from polygons such as triangle, tetragon and hexagon, circle, oval, corrugation, etc.; however, it is preferably a triangle, a tetragon or a hexagon from the standpoint of production. As to the thickness of partition wall, there is no particular restriction, either. However, too small a partition wall thickness results in an insufficient strength of honeycomb segment and too large a thickness results in too large a pressure loss; therefore, none of them is preferred. The thickness of partition wall is preferably 50 to 2,000 μm.
As to the shape of honeycomb segment, there is no particular restriction, either, and any shape can be employed. For example, it is preferred that a square pole shape such as shown in
As to the shape of the section of honeycomb structure intersecting the fluid passages of honeycomb structure at right angles, there is no particular restriction, either, and any shape can be selected from round shape such as true circle, oval, ellipse, or polygon such as triangle, tetragon or pentagon, amorphous shape, etc. When the honeycomb structure of the first invention is used as a catalyst carrier incorporated into an internal combustion engine, a boiler, a chemical reactor, a fuel cell reformer, etc., a metal having a catalytic activity may be preferably loaded on the honeycomb structure. Representative metals having a catalytic activity include platinum (Pt), palladium (Pd), rhodium (Rd), etc. At least one kind of these is preferably loaded on the honeycomb structure.
Meanwhile, when the honeycomb structure of the present invention is used as a filter (e.g. DPF) for capture/removal of particulate substance (soot) present in exhaust gas, it is preferred that a given number of cells are plugged at one end face of the honeycomb structure, the remainder of cells are plugged at other end face, and neighboring cells, when seen at each end face, are plugged alternately (each end face looks checkerwise and each cell is plugged at either of the two ends). Owing to such plugging, for example, a soot-containing exhaust gas incoming from one end face of honeycomb structure passes through the partition walls and leaves from other end face of honeycomb structure. In passing of exhaust gas through partition walls, the porous partition walls function as a filter and can capture the soot. As the captured soot accumulates on the partition walls, an increase in pressure loss appears; as a result, engine undergoes a load and there appears a reduction in fuel consumption and operability; therefore, it becomes necessary to periodically burn and remove the soot using a heating means such as heater and regenerate the function of the filter. In order to promote the burning during the regeneration, the above-mentioned metal having a catalytic activity may be loaded on the honeycomb structure.
The honeycomb structure obtained by bonding is subjected to hot-air drying or the like, to vaporize the water contained in the bonding material and allow the honeycomb structure to express a strength necessary for shape maintenance. In this case, the temperature in the vicinity of the bonding material shows no increase and remains stagnant at 75 to 100° C. owing to the heat of vaporization of water. During the period, each bonded portion has strong areas and weak areas, which gives rise to the non-uniformity of shrinkage and the shifting of bonded portion, caused by vibration or self-weight. Hence, by adding a heat-gelling organic binder having a thickening-initiating temperature of 80° C. or lower, a strength is imparted to each bonded portion prior to the stagnation of temperature rise. The effect of heat-gelling is larger as the addition amount of organic binder is larger and, when the addition amount is smaller than 0.1 mass %, the effect does not spread to all the particles of bonding material and no sufficient effect is expected. The effect is expectable when bonding is made with a bonding material containing the organic binder preferably in an amount of 1 mass % or more, and the effect is particularly striking when the content of organic binder is 5 mass % or more. The effect is further striking when the organic binder has a thickening-initiating temperature of 75° C. or lower. As particularly preferred heat-gelling organic binders, there can be mentioned, for example, cellulose derivatives such as methyl cellulose, hydroxypropyl methyl cellulose and the like.
A thermosetting resin can show an effect as an accelerator for drying and curing, too. The thermosetting resin refers generally to a compound (e.g. a natural or synthetic resin) which can show the insolubility or non-fusibility exhibited by liquid or plastic substance when catalyzed, heated or given an energy (e.g. light). The thermosetting resin of the present invention, when heated, gives rise to a crosslinking reaction between the molecules and is converted to a non-fusible, insoluble polymer having a three-dimensional network structure. There can be mentioned, for example, urea resin, melamine resin, phenolic resin, epoxy resin, unsaturated polyester resin and acrylic resin. Any of these resins has a chemically reactive functional group in the molecule and their properties when cured differ depending upon their chemical compositions. In the present invention, a thermosetting resin can be used as an accelerator for drying and curing, and a phenolic resin, an epoxy resin, etc. are particularly preferred.
In using a bonding material containing a heat-gelling organic binder or a thermosetting resin, it is effective, for arrival at a thickening-initiating temperature or a thermosetting temperature in a short time, to conduct drying by applying a micro wave, or to conduct drying by dielectric heating, or to conduct drying by heating a honeycomb structure not only from outside but also from inside. The temperature elevation time up to thickening-initiating temperature or thermosetting temperature can be shortened by using heated honeycomb segments. It is also effective to conduct bonding using a bonding material and immediately subjecting the resulting body to an atmosphere of 1000C.
In the present invention, “drying” of bonding material refers to that the liquid component in the bonding material is vaporized at a temperature at which the components in the bonding material causes no fusion or the like, that is, causes substantially no firing and, as a result, the bonding material is solidified (gelled). Thus, the honeycomb structure of the present embodiment is obtained by drying alone (no firing takes place) of bonding material, resultant formation of bonding layer, and subsequent bonding of honeycomb segments to each other at their outer walls; therefore, in the honeycomb structure, there hardly appear bonding defects such as cracking in bonding layer, peeling of bonding layer per se and the like, caused, for example, by differences in thermal expansion coefficient and shrinkage ratio between bonding layer and honeycomb segment.
Also, in the honeycomb structure of the present embodiment, the bonding layer is formed by drying alone (no firing takes place) of bonding material and, thereby, honeycomb segments are bonded to each other at the outer walls; therefore, the effect that bonding defects hardly appear, is striking particularly when the honeycomb structure is large.
At least part of the periphery of the honeycomb structure (bonded body) obtained by bonding of honeycomb segments may be removed as necessary. Specifically explaining, as shown in
When at least part of the periphery of the bonded body 36 has been removed, a coating material 43 is coated on the removed portions to form an outer wall 50 of a honeycomb structure 1 as shown in
In coating of the coating material, a periphery-coating machine 48 such as shown in
The bonding material used in the present invention forms a bonding layer by simply being dried without being fired and thereby can bond to-be-bonded bodies to each other; therefore, an effect that bonding defects hardly appear, appears strikingly particularly when each to-be-bonded body is large (the coating area by bonding material is large).
When the bonding material of the present invention is coated on the bonding surface of each porous, to-be-bonded body, the difference in shrinkage between different sites of the bonding layer formed arises hardly and there hardly arises inconveniences in bonding layer, such as cracking, peeling and the like. Therefore, the bonding material of the present invention is preferably used in mutual bonding of bonding surfaces (outer walls) of a plurality of porous honeycomb segments composed of ceramic. As the ceramic constituting each porous honeycomb segment, there can be mentioned, for example, cordierite, silicon carbide, silicon nitride, alumina, mullite, zirconia, zirconium phosphate, aluminum titanate, silicon carbide-silicon composite material, and titania. In using these bonding materials, it is effective, in order to allow the bonding material to arrive at a thickening-initiating temperature or a thermosetting temperature in a short time, to employ a method of applying a micro wave to conduct drying; a method of heating a honeycomb structure not only from outside but also from inside to promote drying; a method of using heated honeycomb segments to shorten the temperature elevation time up to thickening-initiating temperature or thermosetting temperature; and so forth. It is also effective to employ a method of subjecting a bonded body right after bonding to an atmosphere of 100° C.
The present invention is described in more depth by way of Examples. However, the present invention is not restricted to these Examples.
A silicon carbide powder and a silicon powder were mixed as raw materials for honeycomb segment, at a mass ratio of 80:20. Thereto were added, as pore formers, starch and a foamed resin. There were further added methyl cellulose, hydroxypropoxyl methyl cellulose, a surfactant and water to prepare plastic clay. The clay was subjected to extrusion and the extrudate was dried by a micro wave and hot air to obtain a honeycomb segment formed body having a partition wall thickness of 310 μm, a cell density of 46.5 cells/cm2 (300 cells/in2), a square sectional shape of 35 mm×35 mm, and a length of 152 mm. The cell ends of the honeycomb segment formed body were plugged so that the two end faces of the formed body looked checkerwise. That is, the plugging was conducted so that each cell is plugged at one end and each two neighboring cells are plugged at different ends. As the plugging agent, the same materials as the raw materials for honeycomb segment were used. After the plugging of cell ends, drying was conducted; then, degreasing was conducted in the atmosphere at about 400° C.; thereafter, firing was conducted in an Ar inert atmosphere at about 1,450° C. to obtain a honeycomb segment of porous structure in which SiC crystal particles were bonded by Si.
There were mixed 40 mass % of a silicon carbide powder as inorganic particles, 30 mass % of an aqueous solution containing 40 mass % of a silica gel, as an inorganic binder, 1 mass % of a puddle, and 29 mass % of an aluminosilicate fiber. Water was added to the mixture, followed by kneading for 30 minutes using a mixer, to obtain a bonding material A (a standard bonding material) containing no organic binder. In this bonding material was compounded, as a heat-gelling organic binder, methyl cellulose having a thickening-initiating temperature of 70° C. or 85° C., or, as a thermosetting resin, a phenolic resin, in the proportion shown in Table 1, to prepare bonding materials B to F.
Using each of the bonding materials shown in Table 1, 16 same honeycomb segments were bonded with a target bonding width of 1 mm; drying was conducted at 200° C. for 2 hours; then, dispersion of bonding width and shifting of bonding were observed. The dispersion of bonding width was measured at various bonding sites (24 sites). The shifting of bonding was confirmed by counting the number of sites where the shifting by two cells or more was observed. Further, the periphery of each bonded body obtained was ground so that the bonded body after grinding had a cylindrical shape; a coating material was coated on the ground surface; and a heat treatment was applied at 500° C. for 2 hours to obtain each honeycomb structure. Each honeycomb structure was fitted to the exhaust pipe of diesel engine; soot was accumulated inside the honeycomb structure in an amount of 8 g/L and then the honeycomb structure was regenerated; this operation was repeated 50 times. After the test, the presence or absence of cracking at the end face of honeycomb structure was examined. The test results are shown in Table 2.
The honeycomb structures of the present invention each obtained by bonding with a bonding material containing 0.2 to 6 mass % of a heat-gelling organic binder having a thickening-initiating temperature of 75° C. or lower, showed neither shifting of bonding nor after-test cracking. The honeycomb structure obtained by bonding with a bonding material containing a thermosetting resin, showed as well neither shifting of bonding nor after-test cracking.
With the honeycomb structure, process for production thereof and bonding material, according to the present invention, there can be suppressed the dispersion of bonding width and shifting of bonding, which have hitherto arisen in bonding materials having fluidity before drying; therefore, they can be suitably used, for example, in a carrier for catalyst loading, installed in internal combustion engine, boiler, chemical reactor, fuel cell reformer, etc., or in a filter for capture of particulates in exhaust gas.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-081796 | Mar 2006 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2007/056108 | Mar 2007 | US |
| Child | 12233225 | US |
