Abstract:
A holding material for a catalytic converter is interposed in a gap between a catalyst carrier and a metal casing receiving the catalyst carrier. The holding material includes a mat including alumina fiber and mullite fiber, wherein the alumina fiber and the mullite fiber are unitarily collected to form the holding material having a predetermined thickness.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a holding material for a catalytic converter, for example, for purging exhaust gas emitted from an automobile, and a method for producing the holding material for the catalytic converter.  
           [0003]    2. Description of the Related Art  
           [0004]    As known commonly, a catalytic converter for purging exhaust gas is mounted in a vehicle such as an automobile in order to remove detrimental components such as carbon monoxide, hydrocarbon, and nitrogen oxides from exhaust gas emitted from an engine of the vehicle. Generally, as shown in FIG. 2 which is a sectional view, the catalytic converter has a catalyst carrier  1  shaped like a cylinder, a metal casing  2  for receiving the catalyst carrier  1 , and a holding material  3  interposed in a gap between the catalyst carrier  1  and the casing  2  while mounted on the catalyst carrier  1 .  
           [0005]    Generally, the catalyst carrier  1  has a cylindrical honey-comb molded material, for example, made of cordierite, and a precious metal catalyst carried by the molded material. It is therefore necessary that the holding material  3  interposed in a gap between the catalyst carrier  1  and the casing  2  has a function for holding the catalyst carrier  1  safely to prevent the catalyst carrier  1  from being damaged by collision with the casing  2  due to vibration or the like during the running of the automobile, and a function for sealing the catalyst carrier  1  to prevent non-purged exhaust gas from leaking out through the gap between the catalyst carrier  1  and the casing  2 . Therefore, the holding material heretofore mainly used is a holding material formed by collection of alumina fiber, mullite fiber or other ceramic fiber into a mat shape having a predetermined thickness or a holding material molded into a cylindrical shape from the mat-like holding material.  
           [0006]    Recently, with the movement toward more rigid regulation of exhaust gas, there is a tendency for exhaust gas to be heated at a high temperature in order to increase efficiency of removing detrimental components from the exhaust gas. The ceramic fiber mat however has a problem that the ceramic fiber mat cannot be used at a temperature of not lower than 600° C. This is because when the ceramic fiber mat is heated while compressed, the ceramic fiber mat gets into a so-called heat set state in which the thickness of the ceramic fiber mat cannot be restored to its original thickness even if the ceramic fiber mat is released from the pressure. In addition, when an inorganic expanding material such as vermiculite is added to the ceramic fiber mat to form a so-called thermal expansion mat, heat resistance can be improved but the temperature is still limited to 700-800° C.  
           [0007]    The alumina fiber mat has an advantage in that the alumina fiber mat has heat resistance to endure a high temperature of about 1,000° C. and is low in decreasing rate of elastic recovery with the elapsed time. On the other hand, the alumina fiber mat has a disadvantage in that surface pressure is slightly reduced in comparison with other fiber mats in equal density. Accordingly, the cost increases because it is necessary to increase the density in order to increase the surface pressure.  
           [0008]    The mullite fiber mat has an advantage in that high surface pressure can be obtained in comparison with other inorganic fiber mats in equal density. On the other hand, the mullite fiber mat however has a disadvantage in that the mullite fiber mat deteriorates with the elapsed time because the decreasing rate of elastic recovery is high when heat stress is repeatedly applied on the mullite fiber mat.  
           [0009]    As described above, a holding material which has high surface pressure so that the catalyst carrier  1  can be held well, which has high sealing performance and which little deteriorates with the elapsed time has not been obtained yet in the related art. The present situation is that no measure can be taken in the related art against rising of the temperature in use of the catalytic converter.  
         SUMMARY OF THE INVENTION  
         [0010]    Accordingly, an object of the present invention is to provide a catalytic converter holding material in which more excellent catalyst carrier holding performance and exhaust gas sealing performance can be kept for a long term even at a high temperature.  
           [0011]    The present inventors have made eager examination to achieve the foregoing object. As a result, it has been found that the foregoing object can be achieved when a holding material for a catalytic converter, which is interposed in a gap between a catalyst carrier and a metal casing receiving the catalyst carrier, comprising:  
           [0012]    a mat including alumina fiber and mullite fiber,  
           [0013]    wherein the alumina fiber and the mullite fiber are unitarily collected to form the holding material.  
           [0014]    For example, it has been found that the foregoing object can be achieved when a holding material constituted by a laminate of an alumina fiber mat and a mullite fiber mat is used in the condition that the alumina fiber mat is disposed on a side to come into contact with a catalyst carrier so that characteristics of the two mats can be combined with each other synergically.  
           [0015]    That is, in order to achieve the foregoing object, the invention provides a holding material for a catalytic converter having a catalyst carrier shaped like a cylinder, a casing for receiving the catalyst carrier, and a holding material interposed in a gap between the catalyst carrier and the casing while mounted on the catalyst carrier, the holding material being constituted by a laminate of an alumina fiber mat and a mullite fiber mat, the alumina fiber mat being constituted by alumina fiber collected into a predetermined thickness, the mullite fiber mat being constituted by mullite fiber collected into a predetermined thickness, the alumina fiber mat being disposed on a side where the alumina fiber mat comes into contact with the catalyst carrier.  
           [0016]    In the holding material according to the invention, because the aluminum fiber mat excellent in heat resistance is disposed on a high-temperature side to come into contact with the catalyst carrier, the deterioration of the mullite fiber mat with the elapsed time can be suppressed so that high surface pressure due to the mullite fiber mat can be kept for a long term.  
           [0017]    Further, it has been found that the foregoing object can be achieved when a holding material is molded out of a mixture of alumina fiber and mullite fiber into a mat shape or a shape according to the contour shape of a catalyst carrier so that characteristics of the two kinds of fiber are combined with each other synergically.  
           [0018]    That is, in order to achieve the foregoing object, the invention provides a catalytic converter holding material used in a catalytic converter having a catalyst carrier, a casing for receiving the catalyst carrier, and a holding material interposed in a gap between the catalyst carrier and the casing while mounted on the catalyst carrier, the holding material being constituted by a mixture of alumina fiber and mullite fiber collected into a mat shape having a predetermined thickness.  
           [0019]    The invention also provides a method of producing a holding material for a catalytic converter having a catalyst carrier, a casing for receiving the catalyst carrier, and a holding material interposed in a gap between the catalyst carrier and the casing while mounted on the catalyst carrier, the method including the steps of: forming a molding having a flat shape or a shape corresponding to a contour shape of the catalyst carrier by sucking and dehydrating an aqueous slurry containing alumina fiber and mullite fiber; and drying the molding.  
           [0020]    In the holding material according to the invention, a mixture of alumina fiber excellent in heat resistance and mullite fiber capable of being expected to have high surface pressure is used so that surface pressure sufficient to hold the catalyst carrier can be kept for a long term.  
           [0021]    The above-mentioned “mat” and “mixture of fiber collected into a mat shape” according to the present invention may be used in a shape of cylinder. Further, the word “mat” according to the present invention may be used to refer to a holding material after shaped like a cylinder. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a sectional view schematically showing a catalytic converter provided with a holding material according to the invention.  
         [0023]    [0023]FIG. 2 is a sectional view schematically showing a catalytic converter provided with a holding material in the related art and according to the invention.  
         [0024]    [0024]FIG. 3 is a graph showing measured results of the relation between density and surface pressure in the case is where the holding material is produced while the mixture ratio of alumina fiber to mullite fiber is changed. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    A holding material according to the present invention will be described below in detail.  
         [0026]    First Embodiment  
         [0027]    [0027]FIG. 1 is a sectional view, like FIG. 2, schematically showing a catalytic converter provided with a holding material according to a first embodiment of the invention. Like the related-art catalytic converter, the catalytic converter as shown in FIG. 1 has a catalyst carrier  1  shaped like a cylinder, a metal casing  2  for receiving the catalyst carrier  1 , and a holding material  3  interposed between a gap between the catalyst carrier  1  and the casing  2  while mounted on the catalyst carrier  1 . The holding material  3  according to the invention is constituted by a laminate of an alumina fiber mat  3 A and a mullite fiber mat  3 B.  
         [0028]    The laminate of the alumina fiber mat  3 A and the mullite fiber mat  3 B can be shaped like a flat sheet. To hold the catalyst carrier  1 , the laminate needs to be wound on the catalyst carrier  1  so that the alumina fiber mat  3 A comes into contact with the catalyst carrier  1 . The laminate may be shaped like a cylinder so that the alumina fiber mat  3 A is located on the inner side while the mullite fiber mat  3 B is located on the outer side. The holding material  3  shaped like a cylinder can be directly mounted on the catalyst carrier  1  to facilitate production of the catalytic converter.  
         [0029]    Mats suitably selected from known mats heretofore used in a holding material can be used as the alumina fiber mat  3 A and the mullite fiber mat  3 B.  
         [0030]    The mat preferably used as the alumina fiber mat  3 A is a mat, for example, made of alumina fiber containing 90% by weight or more of Al 2 O 3  (and SiO 2  as a residual component) and having low crystallinity in terms of X-ray crystallography. Generally, the fiber size of the alumina fiber is preferably selected to be in a range of 3 μm to 7 μm, and the fiber length of the alumina fiber is preferably selected to be in a range of from 0.05 mm to 5 mm.  
         [0031]    The mat preferably used as the mullite fiber mat  3 B is a mat made of mullite fiber having a mullite composition having an Al 2 O 3 /SiO 2  weight ratio of about 72/28 to about 80/20 and having low crystallinity in terms of X-ray crystallography. Generally, the fiber size of the mullite fiber is preferably selected to be in a range of 3 μm to 7 μm, and the fiber length of the mullite fiber is preferably selected to be in a range of from 0.05 mm to 5 mm.  
         [0032]    A method for obtaining the laminate of the alumina fiber mat  3 A and the mullite fiber mat  3 B is not limited but, for example, the laminate can be obtained by the following method. That is, a slurry containing alunima fiber and an organic binder and a slurry containing mullite fiber and an organic binder are prepared respectively in advance. First, the alumina fiber-containing slurry is molded by suction dehydration and dried to obtain an alumina fiber mat  3 A. Then, the mullite fiber-containing slurry is applied on one surface of the alumina fiber mat  3 A, molded by suction dehydration and dried to obtain a laminate of the alumina fiber mat  3 A and a mullite fiber mat  3 B. The thus obtained laminate is compressed into a predetermined thickness. Thus, the holding material according to the invention is accomplished.  
         [0033]    The holding material can be shaped like a cylinder as follows. First, a cylindrical mesh member (e.g., a cylindrical metal net) is used so that the cylindrical mesh member is immersed in the alumina fiber-containing slurry. The mesh member taken out is directly subjected to dehydration molding. The molded product is dried to produce a cylindrical alumina fiber mat  3 A. Then, the mullite fiber-containing slurry is applied on an outer surface of the cylindrical alumina fiber mat  3 A, molded by suction dehydration and dried. Finally, when the cylinder mesh member is removed, a cylindrical laminate of the alumina fiber mat  3 A located on the inner side and a mullite fiber mat  3 B located on the outer side is obtained. The thus obtained laminate is compressed into a predetermined thickness. Thus, the cylindrical holding material is obtained.  
         [0034]    In the invention, the density of the alumina fiber mat  3 A, the density of the mullite fiber mat  3 B, the thickness ratio between the alumina fiber mat  3 A and the mullite fiber mat  3 B and the total thickness of the holding material  3  can be determined suitably in accordance with the size, service temperature, etc. of the catalytic converter to which the holding material  3  is applied. Generally, the density of the alumina fiber mat  3 A and the density of the mullite fiber mat  3 B are both selected to be in a range of from 200 g/m 2  to 500 g/m 2 . In the invention, this density range can be used. To have heat resistance due to the alumina fiber mat  3 A and high surface pressure due to the mutllite fiber mat  3 B in good balance, it is preferable that the thickness of the alumina fiber mat  3 A is selected to be in a range of from 10% to 50% of the total thickness of the holding material  3  and the thickness of the mullite fiber mat  3 B is selected to be in a range of from 50% to 90% of the total thickness of the holding material  3 , when the density of the alumina fiber mat  3 A and the density of the mullite fiber mat  3 B are both in the aforementioned density range.  
       EXAMPLE OF FIRST EMBODIMENT  
       [0035]    The invention will be described below more specifically in connection with the following Example and Comparative Examples but the invention is not limited thereto at all.  
       Example 1  
       [0036]    Into water, 97 parts by weight of alumina fiber of low crystallinity containing 96% by weight of Al 2 O 3  (and SiO 2  as a residual component) and having a fiber size of about 4 μm and a fiber length of about 3 mm, and 5 parts by weight of an organic binder (acrylic emulsion) were dispersed to prepare an alumina fiber-containing aqueous slurry. On the other hand, 97 parts by weight of mullite fiber of low crystallinity containing 80% by weight of Al 2 O 3  (and SiO 2  as a residual component) and having a fiber size of about 4 μm and a fiber length of about 3 mm, and 3 parts by weight of an organic binder (acrylic emulsion) were dispersed into water to prepare a mullite fiber-containing aqueous slurry.  
         [0037]    First, the alumina fiber-containing aqueous slurry was molded by suction dehydration and dried to produce an alumina fiber mat having grammage (areal density) of 400 g/m 2 . Then, the mullite fiber-containing aqueous slurry was applied on one surface of the alumina fiber mat, molded by suction dehydration and dried to thereby laminate a mullite fiber mat having grammage (areal density) of 600 g/m 2  on the alumina fiber mat. The thus obtained laminate of the alumina fiber mat and the mullite fiber mat was compressed to obtain a sheet-like holding material having a thickness of 7.5 mm and grammage (areal density) of 1,000 g/m 2 .  
         [0038]    Incidentally, the thickness ratio of the alumina fiber mat to the mullite fiber mat in the obtained sheet-like holding material was 33% (alumina fiber mat) to 67% (mullite fiber mat). The sheet-like holding material was compressed up to a thickness of 4 mm at a speed of 1 mm/min by a universal testing machine. Surface pressure in this condition was 90 kPa. Room temperature elastic recovery of the sheet-like holding material as a whole was 75%.  
       Comparative Example 1  
       [0039]    The alumina fiber-containing aqueous slurry used in Example 1 was used so that the slurry was molded by suction dehydration, dried and compressed to produce a sheet-like holding material having a thickness of 7.5 mm. The sheet-like holding material was compressed up to a thickness of 4 mm at a speed of 1 mm/min by the universal testing machine. Surface pressure in this condition was 70 kPa. Room temperature elastic recovery of the sheet-like holding material as a whole was 70%.  
       Comparative Example 2  
       [0040]    The mullite fiber-containing aqueous slurry used in Example 1 was used so that the slurry was molded by suction dehydration, dried and compressed to produce a sheet-like holding material having a thickness of 7.5 mm. The sheet-like holding material was compressed up to a thickness of 4 mm at a speed of 1 mm/min by the universal testing machine. Surface pressure in this condition was 100 kPa. Ordinary temperature elastic recovery of the sheet-like holding material as a whole was 80%.  
         [0041]    (Heat Resistance and Sealability Evaluation Test)  
         [0042]    The sheet-like holding material obtained in Example 1 was wound on a cordierite catalyst carrier of a cylindrical honey-comb structure having an outer diameter of 100 mm and a length of 100 mm, so that the alumina fiber mat came into contact with the catalyst carrier. Then, the resulting product was mounted in a stainless steel casing having an inner diameter of 108 mm and a length of 120 mm to produce a catalytic converter. For the sake of comparison, the holding materials obtained in Comparative Examples 1 and 2 were used to produce catalytic converters in the same manner as described above.  
         [0043]    Each of the produced catalytic converters was connected to an exhaust pipe of a gasoline engine. Exhaust gas was passed through the catalytic converter continuously for 24 hours. Gas emitted from the catalytic converter during the passage of the exhaust gas was analyzed to examine whether exhaust gas leaked from the holding material or not. After the passage of the exhaust gas was completed, the sheet-like holding material was taken out of each of the catalytic converters. Then, the elastic recovery of the sheet-like holding material was measured, so that the decreasing rate of elastic recovery from the initial elastic recovery was calculated. The holding material was further compressed up to a thickness of 4 mm at a speed of 1 mm/min by the universal testing machine so that surface pressure in this condition was measured. Leakage of exhaust gas from the holding material, the decreasing rate of elastic recovery and surface pressure were as shown in Table 1.  
                                                 TABLE 1                                       Comparative   Comparative           Example 1   Example 1   Example 2                                    Leakage of   Absent   Present   Present       Exhaust Gas       Room Temperature   75   70   80       Elastic Recovery (%)       Room Temperature   60   55   48       Elastic Recovery (%)       after Heating       Decreasing Rate (%)   80   79   60       of Elastic Recovery       Surface Pressure   90   70   100       (kPa) at Ordinary       State       Surface Pressure   80   63   50       (kPa) after Test                  
 
         [0044]    It was confirmed from Table 1 that the holding material constituted by a laminate of an alumina fiber mat and a mullite fiber mat in Example 1 was lower in the decreasing rate of elastic recovery than the holding material constituted by only the alumina fiber mat (Comparative Example 1) or only the mullite fiber mat (Comparative Example 2), that is, the holding material obtained in Example 1 was excellent in sealing performance.  
         [0045]    Second Embodiment  
         [0046]    [0046]FIG. 2 is a sectional view schematically showing a catalytic converter provided with a holding material according to a second embodiment of the invention. The configuration of the catalytic converter is the same as that of a related-art catalytic converter except that the holding material  3  according to the invention is constituted by a mixture of alumina fiber and mullite fiber.  
         [0047]    The holding material  3  can be shaped like a flat mat. When the holding material  3  is molded into a shape according to the contour shape (e.g., cylindrical shape in FIG. 2) of a catalyst carrier  1 , the holding material  3  can be directly mounted on the catalyst carrier  1  to facilitate production of the catalytic converter.  
         [0048]    Materials suitably selected from known materials heretofore used in a holding material can be used as the alumina fiber and the mullite fiber. The material preferably used as the alumina fiber is fiber containing 90% by weight or more of Al 2 O 3  (and SiO 2  as a residual component), having low crystallinity in terms of X-ray crystallography and having a mean fiber size of 3 μm to 7 μm and a wet volume of 400 cc/5 g to 1,000 cc/5 g. On the other hand, the material preferably used as the mullite fiber is a mullite composition having an Al 2 O 3 /SiO 2  weight ratio of about 72/28 to about 80/20, having low crystallinity in terms of X-ray crystallography and having a mean fiber size of 3 μm to 7 μm and a wet volume of 400 cc/5 g to 1,000 cc/5 g.  
         [0049]    The wet volume is calculated by a method having the following steps:  
         [0050]    (1) weighing 5 g of a dried fiber material by a weigher with accuracy of two or more decimal places;  
         [0051]    (2) putting the weighed fiber material into a glass beaker having a weight of 500 g;  
         [0052]    (3) putting about 400 cc of distilled water at a temperature of 20-25° C. into the glass beaker prepared in the step (2) and dispersing the fiber material into the distilled water (by an ultrasonic cleaner if necessary) while stirring carefully by a stirrer so that the fiber material is not cut;  
         [0053]    (4) transferring the content of the beaker prepared in the step (3) into a 1,000 ml graduated measuring cylinder and adding distilled water into the graduated measuring cylinder up to the scale of 1,000 cc;  
         [0054]    (5) ten-times repeating a process of stirring the content of the graduated measuring cylinder prepared in the step (4) by turning the graduated measuring cylinder upside down while blocking an opening of the graduated measuring cylinder with the palm of a hand or the like carefully to prevent water from leaking out;  
         [0055]    (6) measuring the sedimentation volume of fiber by eye observation after placing the graduated measuring cylinder quietly under room temperature for 30 minutes after the stop of the stirring; and  
         [0056]    (7) applying the aforementioned procedure to three samples and taking an average of the measured values as a measured value.  
         [0057]    The mixture ratio of alumina fiber to mullite fiber is preferably selected to be in a range of from 10:90 to 90:10 in order to have heat resistance and surface pressure in good balance. If the mixture ratio is out of the range, the object of the invention cannot be achieved because characteristic of one of the two kinds of fiber is superior to that of the other.  
         [0058]    A part of the alumina fiber or a part of the mullite fiber may be replaced by at least one member selected from the group consisting of silica-alumina fiber, silica fiber, glass fiber, rock wool and biodegradable fiber. By this replacement, unique characteristic of the additive fiber can be added to the holding material  3  or reduction in cost can be attained. A known or available material may be used as each of these kinds of additive fiber. The amount of the additive fiber can be selected suitably in accordance with the kind of the additive fiber if heat resistance and surface pressure to be able to be attained according to the invention are not lowered.  
         [0059]    The holding material  3  according to the invention can be obtained as follows. For example, a slurry containing alumina fiber and mullite fiber and further containing the aforementioned additive fiber and an organic binder is molded by suction dehydration. The molded product is dried and compressed into a predetermined thickness. The holding material  3  can be shaped like a cylinder by the following method. A cylindrical mesh member (e.g., a cylindrical metal net) is used so that the slurry is molded by suction dehydration and dried. Then, the holding material  3  shaped like a cylinder is obtained when the cylinder mesh member is removed.  
         [0060]    In the invention, the density and thickness of the holding material  3  at the time of mounting of the holding material  3  can be determined suitably in accordance with the size, service temperature, etc. of the catalytic converter to which the holding material  3  is applied. Generally, the density is selected to be in a range of from 0.2 g/m 2  to 0.6 g/m 2 . In the invention, this density range can be used.  
       EXAMPLE OF SECOND EMBODIMENT  
       [0061]    The invention will be described below more specifically in connection with the following Examples and Comparative Examples but the invention is not limited thereto at all.  
         [0062]    Alumina fiber of low crystallinity containing 96% by weight of Al 2 O 3  (and SiO 2  as a residual component) and having a mean fiber size of about 3 μm and a wet volume of 800 cc/5 g was mixed with mullite fiber of low crystallinity containing 80% by weight of Al 2 O 3  (and SiO 2  as a residual component) and having a mean fiber size of about 4 μm and a wet volume of 800 cc/5 g while the mixture ratio (weight ratio) of alumina fiber to mullite fiber was changed variously to (1) 100% of alumina fiber, (2) 75% of alumina fiber and 25% of mullite fiber, (3) 50% of alumina fiber and 50% of mullite fiber, (4) 25% of alumina fiber and 75% of mullite fiber, and (5) 100% of mullite fiber. With respect to 100 parts by weight of each of the thus obtained mixtures, 8 parts, by weight of an organic binder (acrylic emulsion) were dispersed into water in order to prepare an aqueous slurry. The aqueous slurry was molded by suction dehydration, dried and shaped like a mat. The mat was compressed to obtain surface specific gravity (grammage) of 1200 g/m 2 .  
         [0063]    Each of the thus obtained molded products was heated at 700° C. for an hour so that the organic binder was removed. Then, the molded product was cut into sample pieces of 25 mm square. Each of the sample pieces was compressed at a cross head speed of 1 mm/min by a universal testing machine (manufactured by Shimadzu Corporation) so that load per density (surface pressure) in this condition was measured. FIG. 3 is a graph showing results of the measurement. When comparison is made in equal density, it is obvious that surface pressure decreases as the mixture percentage of alumina fiber increases.  
         [0064]    (Heat Resistance and Sealability Evaluation Test)  
         [0065]    The holding materials constituted by a mixture of alumina fiber (25%) and mullite fiber (75%) was wound on a cordierite catalyst carrier of a cylindrical honey-comb structure having an outer diameter of 100 mm and a length of 100 mm and then mounted in a stainless steel casing having an inner diameter of 108 mm and a length of 120 mm to produce a catalytic converter. For the sake of comparison, the holding material constituted by only alumina fiber and the holding material constituted by only mullite fiber were used to produce catalytic converters in the same manner as described above.  
         [0066]    Each of the produced catalytic converters was connected to an exhaust pipe of a gasoline engine. Exhaust gas was passed through the catalytic converter continuously for 24 hours. Gas emitted from the catalytic converter during the passage of the exhaust gas was analyzed to examine whether exhaust gas leaked from the holding material or not. After the passage of the exhaust gas was completed, the sheet-like holding material was taken out of each of the catalytic converters. The elastic recovery of the sheet-like holding material was measured, so that the decreasing rate of elastic recovery from the initial elastic recovery was calculated. The holding material after the completion of the passage of the exhaust gas was further compressed up to a thickness of 4 mm by the universal testing machine so that surface pressure in this condition was measured. Leakage of exhaust gas from the holding material, the decreasing rate of elastic recovery and surface pressure after use were as shown in Table 2.  
                                                     TABLE 2                                       Decreasing                   Rate of   Surface           Leakage of   Elastic   Pressure           Exhaust Gas   Recovery   after Use                                        Alumina Fiber +   None   80%   82 kPa           Mullite Fiber           Alumina Fiber   None   80%   63 kPa           alone           Mullite Fiber   None   60%   50 kPa           alone                      
 
         [0067]    It was confirmed from Table 2 that the holding material constituted by a mixture of alumina fiber and mullite fiber was lower in the decreasing rate of elastic recovery than the holding material constituted by only alumina fiber or only mullite fiber, that is, the holding material constituted by a mixture of alumina fiber and mullite fiber was excellent in sealing performance.  
         [0068]    As described above, in the holding material according to the invention, both excellent catalyst carrier holding performance and excellent exhaust gas sealing performance can be kept even under a high temperature, so that improvement in heat resistance of the catalytic converter and elongation of the life of the catalytic converter can be attained. In addition, the filling density of the holding material can be reduced to produce high surface pressure, so that the holding material is excellent in cost performance.  
         [0069]    Further, in the holding material according to the invention, both excellent catalyst carrier holding performance and excellent exhaust gas sealing performance can be kept even under a high temperature, so that improvement in heat resistance of the catalytic converter and elongation of the life of the catalytic converter can be attained.