JP2010001184A - Method for manufacturing exhaust gas filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress cracking caused by thermal stress and collapse caused by deterioration of strength by controlling combustion of a pore forming material and an organic binder and heat generation resulting from it in a process for firing a porous aluminum titanate sintered body which composes an exhaust gas filter. <P>SOLUTION: In a method for manufacturing an exhaust gas filter composed of a ceramic honeycomb structure having a large number of cells partitioned by porous partition walls by shaping a body obtained by adding a pore forming material and an auxiliary agent into a ceramic raw material powder and kneading into a honeycomb and firing the obtained honeycomb-shaped body, the ceramic honeycomb structure is composed of aluminum titanate as a substrate and a thermoplastic resin having a pyrolysis starting temperature of ≤400°C is used as the pore forming material. In the firing process, aluminum titanate is sintered by keeping a low oxygen atmosphere in which an oxygen concentration is 2% or below from the start of temperature elevation up to a predetermined oxygen introducing temperature of ≤1,100°C and introducing oxygen so that the oxygen concentration becomes 2% or above at the oxygen introducing temperature or above. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method of manufacturing a honeycomb-structured exhaust gas filter made of an aluminum titanate sintered body, which is used as an exhaust gas filter of an internal combustion engine.

  Treatment of fine particles discharged from an internal combustion engine, particularly a diesel engine, has become a problem, and an exhaust gas filter is installed in the exhaust passage to collect the exhaust fine particles. The exhaust gas filter is made of a ceramic honeycomb structure having a large number of cells in the flow path direction, and exhaust gas is circulated through a porous ceramic wall partitioning the cells so that exhaust particulates can be collected.

Conventionally, cordierite (2MgO · 2Al ₂ O ₃ · 5SiO ₂ ) having low thermal expansion and thermal shock resistance has been widely used as a ceramic material for exhaust gas filters (for example, Patent Document 1). In recent years, aluminum titanate (Al ₂ O ₃ · TiO ₂ ) has attracted attention as another material having an equivalent low thermal expansion coefficient and excellent thermal shock resistance (for example, Patent Documents 2 and 3). etc). Since aluminum titanate has a higher melting point than cordierite, it is expected to improve the heat resistance of the exhaust gas filter that is heated to high temperature by the combustion heat of exhaust particulates during regeneration.

Special table 2001-524451 gazette JP-A-8-215522 Japanese Patent No. 3185960 JP-A-8-323123

  The process for producing an exhaust gas filter generally includes a step of mixing and kneading ceramic material powder as a filter base material with a pore former and an organic binder, a step of extrusion molding to form a molded body of a predetermined shape, And a step of firing the molded body (for example, Patent Document 4). The pore former is used to make the honeycomb structure porous, and a method of adding a carbonaceous material such as carbon and firing in the air is generally used, and exhaust gas can be distributed to the cell walls of the honeycomb structure. Form pores.

  As a pore-forming material for aluminum titanate, Patent Document 2 uses carbon powder such as activated carbon, coke, and graphite, synthetic resin powder such as polyethylene, polystyrene, and polyolefin, and organic powder such as starch and wood powder. It is described in Patent Document 3 that a carbon powder that can easily adjust the particle diameter is suitably used to adjust the pore diameter as the particle diameter / aspect ratio.

  By the way, the exhaust gas filter usually has a desired porosity by adjusting the raw material particle size and the amount of pore-forming material added. However, when the amount of the pore former is increased in order to increase the porosity, the strength of the honeycomb structure is liable to be reduced, and there is a problem that a large amount of heat is generated during combustion. The organic binder added for molding also has the same problem, and many defects that cause cracks in the honeycomb structure have occurred due to the thermal stress generated by these heat generations.

  On the other hand, in Patent Document 1, as a method for controlling the firing of the cordierite honeycomb structure, a carbonaceous material such as an organic binder in an oxygen atmosphere lower than the atmosphere while introducing a low oxygen gas, for example, nitrogen gas, into a firing furnace. Release of the material is disclosed. Patent Document 4 discloses that in the production of an exhaust gas filter, the pore-forming agent added to the ceramic substrate is an ethylene-based resin powder that softens at 120 ° C. or less, thereby improving fluidity and changing the amount of the cell without changing the addition amount. It is described that the pores on the wall surface are enlarged.

  However, regarding the aluminum titanate honeycomb structure, it cannot be said that the behavior of the pore former and the organic binder in the firing process is sufficiently known, and there is a risk of quality deterioration due to generation of cracks and collapse due to large-scale heat generation. Patent Literature 3 proposes a method in which a molded body is fired in a non-oxidizing atmosphere and then heat-treated in an oxidizing atmosphere at a firing temperature or lower to burn a pore former such as carbon. In addition, there is a concern that carbon may remain and combustion may be insufficient, or conversely, combustion may occur at once, and control of combustion is not easy. As a countermeasure against cracking, for example, a method of reducing the thermal stress by slowing the firing rate is known, but it is necessary to control the temperature rise, and it takes time for firing, which is a factor that increases costs. It was.

  Further, in recent years, there is a demand for increasing the size of the exhaust gas filter, and the cell walls of the honeycomb structure tend to be thinned for the purpose of reducing the heat capacity. Accordingly, it is not easy to obtain a high-quality aluminum titanate sintered body with high productivity by burning and removing the pore former and the organic binder while maintaining the substrate strength.

  The present invention has been made on the basis of the above circumstances, and in the process of firing the porous aluminum titanate sintered body constituting the exhaust gas filter, the combustion of the pore former and the organic binder and the accompanying heat generation are controlled. An object of the present invention is to provide a method for producing a high-quality exhaust gas filter with high productivity without suppressing cracking due to thermal stress or collapse due to strength reduction and without increasing the time required for firing.

The invention of claim 1 for solving the above-mentioned problem is
A clay obtained by adding a pore former and an auxiliary agent to a ceramic raw material powder and kneading is formed into a honeycomb shape, and the obtained honeycomb formed body is fired to have a large number of cells partitioned by porous partition walls. A method for producing an exhaust gas filter comprising a ceramic honeycomb structure,
The ceramic honeycomb structure is based on aluminum titanate,
While using a thermoplastic resin having a pyrolysis start temperature of 400 ° C. or lower for the pore former,
In the step of raising the temperature of the honeycomb formed body to the firing temperature, the oxygen concentration is maintained in a low oxygen atmosphere of 2% or less from the start of temperature rise to a predetermined oxygen introduction temperature of 1100 ° C. or lower. Oxygen is introduced so that the oxygen concentration is greater than 2% to sinter aluminum titanate.

  A thermoplastic resin having a thermal decomposition start temperature of 400 ° C. or lower used as a pore former is greatly different in reaction during firing in an atmosphere. In the air atmosphere, the endothermic reaction due to the decomposition of the resin and the exothermic reaction due to the combustion of the decomposition gas are complex reactions, and when this reaction occurs in the wall of the honeycomb molded body, thermal stress is generated and leads to cracking. Guessed. On the other hand, when the temperature rise is started in the low oxygen atmosphere of the present invention, only an endothermic reaction due to the decomposition of the resin occurs, so that no thermal stress is generated due to heat generation, and firing cracks can be prevented.

  In addition, since oxygen is required at 1100 ° C. or higher where the reaction of aluminum titanate starts, the introduction of oxygen is started at a predetermined oxygen introduction temperature of 1100 ° C. or lower to sinter aluminum titanate. Temperature control is easy and the time required for firing can be shortened. Therefore, an exhaust gas filter made of a high-quality porous aluminum titanate sintered body can be manufactured with high productivity.

  In the invention of claim 2, the auxiliary agent includes an organic binder, the oxygen introduction temperature is set to a predetermined temperature which is not higher than 1100 ° C. and higher than the thermal decomposition completion temperature of the pore former and auxiliary agent. The

  The organic binder for ordinary honeycomb forming has a temperature at which the thermal decomposition is completed at 400 ° C. or higher in the firing step, and at least at a time of 400 ° C. or lower at which the thermoplastic resin as the pore former starts thermal decomposition. A part exists in the molded body. Therefore, by setting a low oxygen atmosphere up to the thermal decomposition completion temperature of the pore former and the auxiliary agent, while suppressing heat generation, the shape retention function of the organic binder suppresses strength reduction due to thermal decomposition, and cracks and Collapse can be prevented.

  In the manufacturing method of Claim 3, oxygen is introduce | transduced so that oxygen concentration may become 5% or more above the said oxygen introduction temperature.

  Preferably, by setting the oxygen concentration in the atmosphere after introducing oxygen to 5% or more, oxygen necessary for firing can be sufficiently supplied to obtain a high-quality aluminum titanate sintered body.

  In the manufacturing method of Claim 4, oxygen is introduce | transduced so that oxygen concentration may become 20% or less above the said oxygen introduction temperature.

  Preferably, by setting the oxygen concentration in the atmosphere after introducing oxygen to 20% or less, the oxygen concentration necessary for firing is sufficiently supplied without increasing the oxygen concentration more than necessary, and high-quality aluminum titanate firing is performed. A ligation can be obtained.

  In the manufacturing method of Claim 5, the said oxygen introduction temperature is set to the predetermined temperature of 750 degreeC or more and 1100 degrees C or less.

  Preferably, if the oxygen introduction temperature is not lower than 750 ° C. and not higher than 1100 ° C., the temperature is sufficiently higher than the temperature at which the thermal decomposition of the auxiliary agent such as the thermoplastic resin or organic binder as the pore former is completed, and aluminum titanate Since the introduction of oxygen is started below the temperature at which the above reaction starts, it is possible to obtain a high-quality aluminum titanate sintered body by controlling the thermal decomposition and firing well.

  In the manufacturing method of Claim 6, the thermoplastic resin whose thermal decomposition start temperature is 300 degrees C or less is used for the said pore former.

  Preferably, if the thermal decomposition start temperature is 300 ° C. or lower, the temperature is sufficiently lower than the thermal decomposition completion temperature of the ordinary organic binder for forming a honeycomb. The shape can be maintained by the action of the organic binder present in the body, and the effect of preventing collapse is enhanced.

  In the manufacturing method according to claim 7, the thermoplastic resin having a thermal decomposition start temperature of 300 ° C. or lower is at least one selected from a methacrylic resin, an acrylic resin, a polyethylene resin, a PET resin, and a polystyrene resin. .

  Specifically, the above effects can be easily obtained by using these thermoplastic resins as pore formers.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIGS. 1B and 1C are schematic diagrams of an exhaust gas filter 1 to which the present invention is applied. For example, a diesel particulate filter that is installed in an exhaust passage of a diesel engine and collects exhaust particulates (particulates). (DPF) can be used. In the figure, the exhaust gas filter 1 is composed of a porous ceramic honeycomb structure 2 and is formed by partitioning a cylindrical wall with a porous partition wall 21 as shown in FIG. It has a large number of cells 22. As shown in FIG.1 (c), many cells 22 are provided in parallel with the flow direction (direction of the arrow of a figure) of waste gas, and an inside becomes a waste gas flow path, and each cell 22 is one end side (inlet side or Only one of the outlet side) is open, and the sealing material 11 is packed on the multi-end side. At this time, a large number of cells 22 are alternately plugged on both end faces in the axial direction of the ceramic honeycomb structure 2 to become exhaust gas inlets or outlets, and are adjacent to each other via a porous partition wall 21 having a large number of pores. A filter structure in which exhaust gas flows between the cells 22 is obtained.

The ceramic honeycomb structure 2 constituting the exhaust gas filter 1 is made of a porous sintered body mainly containing aluminum titanate. Aluminum titanate (Al ₂ O ₃ · TiO ₂ ) used as a base material has a low thermal expansion coefficient (3 × 10 ⁻⁶ / ° C. or less), has a thermal shock resistance, and is suitably used as an exhaust gas filter having a large temperature change. The Moreover, since the melting point (1850 ° C.) of aluminum titanate is higher than the melting point (1450 ° C.) of cordierite, there is little risk of damage or melting due to combustion heat during regeneration, and the durability of the exhaust gas filter 1 is improved. Let The same aluminum titanate as that of the base material is usually used for the plugging material 11 to be packed on the end face of the ceramic honeycomb structure 2, but aluminum magnesium titanate or the like can also be used.

  The porosity of the porous aluminum titanate sintered body to be the exhaust gas filter 1 is usually 20% to 70%, preferably about 30% to 60%. The larger the porosity, the more effective the pressure loss is reduced. However, the collection efficiency and the partition wall strength tend to decrease, and the both are set so that desired characteristics can be obtained. This porosity and pore diameter can be adjusted by the particle size of the ceramic raw material powder, the particle size of the pore former, and the amount added, which will be described later.

A method for manufacturing the exhaust gas filter 1 of the present invention will be described. As the base material of the ceramic honeycomb structure 2, aluminum titanate powder prepared by a generally known method can be used. The aluminum titanate powder is obtained by mixing TiO ₂ and Al ₂ O ₃ as raw materials in advance so as to have a predetermined molar ratio and firing, so that a homogeneous material can be obtained and the particle size can be easily adjusted. . The particle size of the aluminum titanate powder is not particularly limited, but can be appropriately selected within a range of 1 to 50 μm, for example. In the present invention, the aluminum titanate powder is mixed with a pore former and an auxiliary agent, kneaded to form a clay, a molding process using the clay to form a honeycomb-shaped body, and honeycomb molding The exhaust gas filter 1 made of a porous aluminum titanate sintered body is produced by a firing step of firing the body.

  In the mixing step, a pore former for making the ceramic honeycomb structure 2 porous, and an auxiliary agent including an organic binder and a lubricant are added to the aluminum titanate powder, which is a ceramic raw material powder. Organic binders and lubricants are auxiliary agents added for the purpose of improving shape retention and fluidity during molding. As organic binders, generally, methylcellulose binders such as methylcellulose and carboxymethylcellulose, polyvinyl alcohol, etc. And used together with a lubricant such as grease. In addition, a conventionally known molding aid can be added. These auxiliaries contribute to shape retention at the initial stage of firing, but are burned and removed by the subsequent temperature increase. For example, methylcellulose starts thermal decomposition at around 300 ° C., and when it exceeds 400 ° C., the function as an organic binder is greatly reduced.

  The pore former is one of the characteristics of the present invention, and a thermoplastic resin having a thermal decomposition start temperature of 400 ° C. or lower, preferably 300 ° C. or lower is used. The pore former starts thermal decomposition in the temperature rising process in the firing step, forms voids that become pores, and makes the ceramic honeycomb structure 2 porous. On the other hand, since the overall strength is reduced by the generation of the voids, it is difficult to maintain the shape. The present invention has been found that if the thermal decomposition starting temperature of the pore former is 400 ° C. or lower, the thermal decomposition is started and the holding power can be maintained before the organic binder is burned out in the firing step. . Preferably, thermal decomposition should be started while more organic binder is present. If a pore former that starts thermal decomposition at a lower temperature of 300 ° C. or lower is used, shape retention is improved.

  Preferably, the function of the organic binder is effectively used when the thermoplastic resin has a weight change due to thermal decomposition of the pore former of 50% or more, preferably 90% by 400 ° C. Can do. Specific examples include thermoplastic resins such as methacrylic resins, acrylic resins, polyethylene resins, polyethylene terephthalate resins, polystyrene resins, and the like. And

  Table 1 shows examples of thermoplastic resins having such characteristics such as polymethyl methacrylate (PMMA) resin, acrylic resin, polyethylene (PE) resin, polyethylene terephthalate (PET) resin, and polystyrene resin, which are methacrylic resins. The thermal decomposition temperature range is shown.

  In addition, the phenol resin written together in Table 1 is a thermosetting resin, and the thermal decomposition temperature range exists in the high temperature side compared with the pore former of this invention. In addition, since it does not decompose in the pyrolysis atmosphere (low oxygen atmosphere) of the present invention described later, it remains in the ceramic honeycomb structure 2 and generates heat due to combustion after the introduction of oxygen, causing cracks. Since carbon, which is a conventional pore former, has a high decomposition temperature and exhibits the same behavior, it is not used in the present invention.

  The addition amount of the pore former is usually set in the range of 5 to 50% with respect to the weight of the ceramic raw material powder. If the added amount is less than 5%, the porosity is reduced and the function of the exhaust gas filter 1 is deteriorated. If it is more than 50%, the porosity is increased and it is difficult to maintain the shape during firing. Preferably, the amount of pore-forming material added is in the range of 10 to 30%, and can be appropriately selected so that desired porosity and shape retention can be obtained. The particle size of the pore former determines the size of the voids that become pores, and is usually set appropriately in the range of 10 to 40 μm. The smaller the particle size, the smaller the pore size.

  In the mixing step, the above-described pore former, organic binder, lubricant and other auxiliary agents are added to and mixed with the aluminum titanate powder, which is the ceramic raw material powder, and kneaded using a known kneader. By doing so, it becomes a clay for molding. Next, in the forming step, the obtained kneaded material is extruded into a honeycomb shape using a known extruder to obtain a honeycomb formed body having a predetermined shape. Then, it heat-drys using a well-known microwave dryer, a hot air dryer, etc.

  FIG. 1A shows an example of the atmosphere and temperature control in the firing step of the present invention. The firing step is a characteristic part of the present invention. First, the dried honeycomb formed body is placed in a firing furnace having a predetermined low oxygen atmosphere (first atmosphere), and the temperature rise is started. Auxiliaries such as pore formers, organic binders and lubricants in the molded body are thermally decomposed. This low oxygen atmosphere is maintained at a pre-set oxygen introduction temperature of 1100 ° C. or lower, and when a predetermined oxygen introduction temperature is reached, oxygen is introduced to form a predetermined atmosphere (second atmosphere), and a predetermined firing temperature. The aluminum titanate is sintered by raising the temperature to and holding.

  The first atmosphere in the firing step is a low oxygen atmosphere having an oxygen concentration of 2% or less by volume, and as an atmosphere gas other than oxygen, for example, nitrogen gas can be used. In an atmosphere having a low oxygen concentration of 2% or less, oxidation of the pore former of the present invention made of the above-described thermoplastic resin is suppressed, and only thermal decomposition that is an endothermic reaction proceeds. Preferably, a low oxygen atmosphere of 1.5% or less is preferable, and since there is no thermal stress accompanying heat generation up to the oxygen introduction temperature, firing cracks do not occur.

  In a low oxygen atmosphere, organic binders behave similarly. For this reason, in the present invention, the oxygen introduction temperature is set to 1100 ° C. or lower at which aluminum titanate starts the reaction, preferably a predetermined temperature that is equal to or higher than the thermal decomposition completion temperature of the pore former and the organic binder. Until the thermal decomposition of the binder is completed, heat generation is suppressed as a low oxygen atmosphere (first atmosphere). Specifically, as shown in Table 1, the thermal decomposition completion temperature of the pore former is about 400 ° C. to 500 ° C., and methyl cellulose generally used as an organic binder is about 450 ° C. to 500 ° C. Since the thermal decomposition is almost completed, the oxygen introduction temperature is preferably set to a temperature higher than 450 ° C., usually 500 ° C. to 1100 ° C. At this time, if the oxygen introduction temperature is low, the organic matter (residual carbon) remaining after thermal decomposition may burn at a time and cracks may occur. Preferably, the oxygen introduction temperature is set to a predetermined temperature of 750 ° C. to 1100 ° C. Then, the above effect can be easily obtained.

  The second atmosphere of the firing step is an oxygen introduction atmosphere having an oxygen concentration of greater than 2% by volume, preferably 5% or more, and contains oxygen such as air in the firing furnace so that a predetermined oxygen concentration is obtained. Introduce gas. If the oxygen concentration is greater than 2%, oxygen necessary for the combustion of residual carbon and the reaction of aluminum titanate from 1100 ° C. can be supplied, and if the oxygen concentration is preferably 5% or more, sufficient oxygen can be supplied. Supplying can promote the sintering of aluminum titanate. At this time, an atmospheric atmosphere (oxygen concentration of 21%) can be used. However, if the oxygen concentration is high, there is a risk that a large amount of heat is generated due to the combustion of residual carbon. Preferably, the oxygen concentration is lower than that of the atmospheric atmosphere. It is good to adjust to.

  The temperature raising rate is not particularly limited, and in the first atmosphere in which thermal decomposition occurs, it is preferable to raise the temperature at a relatively slow rate of, for example, about 10 to 20 ° C./hour and gradually advance the thermal decomposition. However, if the thermal decomposition completion temperature is exceeded, the rate of temperature increase can be shortened by increasing the rate of temperature increase. Even in the second atmosphere after the oxygen introduction temperature, a large amount of heat generation due to combustion of the pore former or auxiliary agent does not occur as in the prior art, so the temperature rise rate should be increased and the temperature raised quickly to the firing temperature. Can do. The firing temperature is usually in the range of 1200 ° C. to 1500 ° C., and is retained for a predetermined time to obtain a porous aluminum titanate sintered body that becomes the exhaust gas filter 1 of the present invention.

Next, specific examples for confirming the effects of the present invention will be shown.
Among the thermoplastic resins exemplified in Table 1 as the pore former of the present invention, the thermal decomposition characteristics of polymethyl methacrylate (PMMA) resin were examined. FIG. 2 (a) shows the endothermic heat generation and weight change when the temperature is increased in a conventional air atmosphere, and FIG. 2 (b) shows the endothermic heat generation and weight change when the temperature is increased in the low oxygen atmosphere of the present invention. Shown in The measurement was performed using a known differential thermal analyzer, and the thermal decomposition characteristics (TG-DTA) evaluated under the following conditions are shown in a schematic diagram with the temperature as the horizontal axis.
Evaluation items: TG (differential thermal balance) and DTA (differential thermal analysis)
Atmosphere: Air atmosphere, low oxygen atmosphere (oxygen concentration: 1%)
Evaluation temperature range: 0 ° C to 500 ° C

  As is clear from comparison between FIGS. 2A and 2B, the reaction of the PMMA resin used as the pore former of the present invention varies greatly depending on the atmosphere. In the conventional air atmosphere shown in FIG. 2A, the endothermic reaction due to the decomposition of the resin and the exothermic reaction due to the combustion of the decomposition gas are complicated reactions. When this reaction occurs in the wall of the ceramic honeycomb structure 2, thermal stress is generated and leads to cracking. On the other hand, in the low oxygen atmosphere of the present invention shown in FIG. 2 (b), only the endothermic reaction by the decomposition of the resin proceeds, and the thermal decomposition is completed at 400 ° C. Therefore, even if this reaction proceeds in the wall of the ceramic honeycomb structure 2, cracks due to thermal stress do not occur.

  3 (a) and 3 (b) show the results of examining the thermal decomposition characteristics (TG-DTA) of methyl cellulose, which is an organic binder, using the same measurement method. FIG. 3 (a) shows the endothermic heat generation and weight change when the temperature is raised to 0 to 500 ° C. in a conventional air atmosphere, up to 0 to 500 ° C. in the low oxygen atmosphere of the present invention (oxygen concentration: 1%). FIG. 3B shows the heat absorption and heat change and the weight change when the temperature is raised.

  As is apparent from a comparison of FIGS. 2 and 3, methyl cellulose, which is an organic binder, also exhibits the same behavior as the PMMA resin, which is the pore-forming material of the present invention. It can be seen that the endothermic reaction due to the decomposition of the resin proceeds to suppress the exothermic reaction. Further, since the thermal decomposition of the PMMA resin is completed while methylcellulose is present, the shape can be maintained even if voids are generated by the thermal decomposition, and the collapse due to the strength reduction can be suppressed.

  4 and 5 are the same measurement methods, and among the thermoplastic resins shown in Table 1 as examples of the pore former of the present invention, polyethylene (PE) resin, polyethylene terephthalate (PET) resin, acrylic resin It is the result of investigating the thermal decomposition characteristic (TG-DTA) of. For each, the endothermic heat generation and weight change when the temperature is raised to 0 to 500 ° C. in the low oxygen atmosphere (oxygen concentration: 1%) of the present invention are shown in FIGS. Further, FIGS. 5A to 5C show the endothermic heat generation and weight change when the temperature is raised to 0 to 500 ° C. in a conventional air atmosphere.

  4 and 5, when other thermoplastic resin of Table 1 is used as the pore former of the present invention, it shows the same behavior as PMMA resin, and the low oxygen atmosphere of the present invention. Thus, it can be seen that the endothermic reaction due to the decomposition of the resin proceeds and the exothermic reaction can be suppressed. In addition, the thermoplastic pyrolysis that becomes the pore-forming material is almost completed while methylcellulose is present, so even if voids are generated by pyrolysis, the shape can be maintained and the collapse due to the decrease in strength is suppressed. be able to.

  Next, as Example 1, a ceramic honeycomb structure 2 to be an exhaust gas filter 1 of the present invention was prototyped by the manufacturing method described above. First, with respect to 100 parts by weight of the aluminum titanate powder as the ceramic raw material powder, 20% by weight of the PMMA resin as the pore former of the present invention, 12% by weight of methyl cellulose as the organic binder, and a commercially available lubricant (Unilube: 2.8 wt% of Nippon Oil & Fats Co., Ltd., trade name) was added, and water was added and mixed. This mixture is kneaded using a known kneader to form a molding clay, and the obtained clay is extruded into a honeycomb shape shown in FIG. 1 (b) using a known extruder. A honeycomb formed body having a predetermined shape was obtained. Then, it heat-dried using the well-known microwave dryer.

  This honeycomb formed body was fired under the conditions shown in Table 2. The temperature raising process was controlled as shown in FIG. First, the honeycomb formed body was placed in a firing furnace having a low oxygen atmosphere with an oxygen concentration of 1.5%, and the temperature was raised to 750 ° C. which is an oxygen introduction temperature. At this time, from the start of the temperature increase to 500 ° C. at which the thermal decomposition of the pore former, the organic binder and the lubricant in the honeycomb molded body is almost completed, a constant temperature increase rate (15 ° C./h) is set. In the above, the temperature rising rate was increased. When the temperature reached 750 ° C., air as an oxygen-containing gas was introduced so that the oxygen concentration was 10%, and the temperature was raised to 1400 ° C., which is the firing temperature. While maintaining the oxygen concentration, the sintered body was held at the firing temperature for about 10 hours to obtain a porous aluminum titanate sintered body.

  FIG. 6 (a) is a schematic view showing the state of the ceramic honeycomb structure 2 made of the porous aluminum titanate sintered body obtained in this manner, having a good appearance, and being cracked or broken inside. Was not seen. FIG. 6B is a schematic diagram showing a state of the ceramic honeycomb structure 2 made of a porous aluminum titanate sintered body obtained by firing in the air using conventional carbon as a pore-forming material. It can be seen that there are a plurality of cracks extending to the outer surface and the inside, and appearance defects have occurred.

In the ceramic honeycomb structure 2 obtained by the present invention, the wall thickness of the partition wall 22 is, for example, 0.1 to 1.0 mm, preferably 0.2 to 0.5 mm, and the cell density is, for example, 25 to 500 cells / inch ² , preferably Is made of a porous aluminum titanate sintered body having a porosity of 200 to 400 cells / inch ² and a porosity of, for example, 20 to 70%, preferably 30 to 60%. The porous aluminum titanate sintered body has an average pore diameter of, for example, 5 to 30 μm, preferably 10 to 20 μm, and a thermal expansion coefficient of, for example, 3.0 × 10 ⁻⁶ / ° C. or less, preferably 1 × 10 ⁻⁶ / ° C. It is as follows.

  Further, in the same manner, as shown in Table 2, the conditions such as the oxygen introduction temperature, the oxygen concentration in the first atmosphere, or the oxygen concentration in the second atmosphere after the oxygen introduction were changed, and the honeycomb molded body was changed. Firing was performed to obtain a porous aluminum titanate sintered body. The conditions and the state of the obtained ceramic honeycomb structure 2 are shown in Table 2 together with the results of Example 1 as Examples 2 to 4 and Comparative Examples 1 to 5.

  As is apparent from Table 2, the oxygen introduction temperature is 750 ° C. or 1100 ° C., the oxygen concentration in the first atmosphere is 1.5%, and the oxygen concentration in the second atmosphere is greater than 2.0% and 10.0%. % In Examples 1-4, the ceramic honeycomb structure 2 in a good state as shown in FIG. 6A was obtained. On the other hand, even if the oxygen introduction temperature is 750 ° C. or 1100 ° C., in Comparative Examples 1 and 2 in which the oxygen concentration of the second atmosphere is 2.0% or 1.5%, necessary oxygen cannot be supplied. Aluminum titanate did not become a sintered body. Further, in Comparative Example 3 in which the oxygen introduction temperature is 400 ° C. and the oxygen concentration of the second atmosphere thereafter is 10.0%, as in Comparative Example 4 baked in the atmosphere from room temperature, firing cracks due to heat generation, Collapse was seen. Moreover, the oxygen introduction temperature was set to 1300 ° C., which is higher than the temperature at which the reaction of aluminum titanate starts, and then the comparative example 5 fired in the air did not become an aluminum titanate sintered body as a whole.

  From the above results, in order to control the thermal decomposition of the pore former and the organic binder, the oxygen introduction temperature is set to be higher than 400 ° C. and not higher than 1100 ° C., preferably 750 ° C. to 1100 ° C. In a low oxygen atmosphere of 2% or less, preferably 1.5% or less, and after introduction of oxygen, in an atmosphere having an oxygen concentration of greater than 2%, preferably greater than 2% and 10% or less. By firing, the exhaust gas filter 1 without cracking or collapsing can be obtained.

  Here, Table 3 shows the influence of the oxygen concentration after the introduction of oxygen on the ceramic honeycomb structure 2. As shown in Table 2, both Example 2 and Comparative Example 1 were fired under the same conditions except that the oxygen concentration in the second atmosphere after introducing oxygen was 5% and 1.5%, respectively. (Oxygen introduction temperature 750 ° C., oxygen concentration up to oxygen introduction temperature 1.5%). Regarding the ceramic honeycomb structure 2 fired under each condition, the results of the crystal structure analysis by the X-ray diffraction method and the results of the crystal structure observation by the electron microscope are shown in Table 3. It was divided into parts.

As apparent from Table 3, in Example 2, the crystal growth of aluminum titanate was confirmed in both the outer peripheral portion and the central portion of the ceramic honeycomb structure 2. On the other hand, in Comparative Example 1, although the crystal of aluminum titanate was confirmed in the outer peripheral portion of the ceramic honeycomb structure 2, fine particles of titania (TiO ₂ ) and alumina (Al ₂ O ₃ ) were observed in the central portion. It can be seen that the reaction of aluminum titanate does not proceed to the inside when the oxygen concentration is low.

  As described above, according to the present invention, the thermal decomposition reaction during firing is controlled, the pore former is thermally decomposed in the presence of an organic binder to improve shape retention, and high quality is achieved. Productivity can be improved by shortening the firing time.

(A) is a figure for demonstrating the temperature rising at the time of baking, and control of atmosphere in the manufacturing method of this invention, (b), (c) is an exhaust gas filter manufactured by this invention method, respectively. It is the whole perspective view and whole schematic sectional drawing. (A) is a figure which shows the temperature characteristic in the air atmosphere of PMMA resin which is a pore making material, (b) is a figure which shows the temperature characteristic in a low oxygen atmosphere. (A) is a figure which shows the temperature characteristic in the air atmosphere of the methylcellulose which is an organic binder, (b) is a figure which shows the temperature characteristic in a low oxygen atmosphere. (A), (b), (c) is a figure which shows the temperature characteristic in the low oxygen atmosphere of PE resin, PET resin, and acrylic resin which are pore forming materials, respectively. (A), (b), (c) is a figure which shows the temperature characteristic in the air atmosphere of PE resin, PET resin, and acrylic resin which are pore forming materials, respectively. (A) is a schematic block diagram which shows the external appearance of the exhaust gas filter manufactured by the method of this invention, (b) is a schematic block diagram which shows the external appearance of the exhaust gas filter manufactured by the conventional method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Exhaust gas filter 11 Sealing material 2 Ceramic honeycomb structure 21 Porous partition 22 Cell

Claims (7)

A clay obtained by adding a pore former and an auxiliary agent to a ceramic raw material powder and kneading is formed into a honeycomb shape, and the obtained honeycomb formed body is fired to have a large number of cells partitioned by porous partition walls. A method for producing an exhaust gas filter comprising a ceramic honeycomb structure,
The ceramic honeycomb structure is based on aluminum titanate,
While using a thermoplastic resin having a pyrolysis start temperature of 400 ° C. or lower for the pore former,
In the step of raising the temperature of the honeycomb formed body to the firing temperature, the oxygen concentration is maintained in a low oxygen atmosphere of 2% or less from the start of temperature rise to a predetermined oxygen introduction temperature of 1100 ° C. or lower. A method for producing an exhaust gas filter, wherein oxygen is introduced so that the oxygen concentration is greater than 2% to sinter aluminum titanate. The auxiliary agent includes an organic binder,
The method for producing an exhaust gas filter according to claim 1, wherein the oxygen introduction temperature is set to a predetermined temperature that is 1100 ° C or lower and is higher than a thermal decomposition completion temperature of the pore former and the organic binder.   The method for producing an exhaust gas filter according to claim 1 or 2, wherein oxygen is introduced so that the oxygen concentration is 5% or more at the oxygen introduction temperature or higher.   The method for producing an exhaust gas filter according to any one of claims 1 to 3, wherein oxygen is introduced so that the oxygen concentration is 20% or less above the oxygen introduction temperature.   The method for producing an exhaust gas filter according to any one of claims 1 to 4, wherein the oxygen introduction temperature is set to a predetermined temperature of 750 ° C or higher and 1100 ° C or lower.   The method for producing an exhaust gas filter according to any one of claims 1 to 5, wherein a thermoplastic resin having a thermal decomposition start temperature of 300 ° C or lower is used for the pore former.   The exhaust gas filter according to claim 6, wherein the thermoplastic resin having a thermal decomposition start temperature of 300 ° C or lower is at least one selected from a methacrylic resin, an acrylic resin, a polyethylene resin, a PET resin, and a polystyrene resin. Production method.

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