METHOD OF PRODUCING POROUS CERAMIC HONEYCOMB STRUCTURAL BODIES

Abstract: A method of producing ceramic honeycomb structural bodies by firing extruded green bodies of ceramic raw material mixed with a pore forming agent is disclosed. As the pore forming agent, use is made of flammable carbon powder having a degree of loss in weight of 50% or more after a heat treatment at 650°C for one hour in air. By using such a flammable carbon powder as the pore forming agent, a retention time at about 1000°C for burning the pore forming agent in the firing process is shortened and as a result a firing time is shortened without deterioration of quality characteristics of resulting porous ceramic honeycomb structural bodies.

Description: The present invention relates to a method of producing porous ceramic honeycomb structural bodies especially for use as diesel particulate filters for collecting particulates contained in exhaust gas from a diesel engine, e.g. for automobiles. Hitherto, in a method of producing porous ceramic honeycomb structural bodies used as a diesel particulate filter, powder of ceramic raw material having a predetermined chemical composition is mixed with a pore forming agent and then kneaded to make an extrudable batch body. The batch body is extruded to shape green bodies and then dried and fired to obtain porous ceramic honeycomb structural bodies. For example, Japanese Patent Application Publication No. 60-31800 discloses a method of producing porous ceramic honeycomb structural bodies having a high dimensional accuracy by using graphite powder as an extruding lubricant and further discloses pore forming effects provided by the graphite powder. Also, Japanese Patent Application Publication No. 63-27303 (U.S. Patent No. 4279849) discloses a method of producing cordierite ceramic honeycomb structural bodies by using starch powder and pore forming effects provided by the starch powder. Moreover, it has been proposed to use coke or carbon powder as the pore forming agent, but the burning characteristics of such a coke and carbon powder in the firing process and the characteristics of ceramic honeycomb structural bodies produced by using the coke an carbon power have not been clarified. However, it was found that when the graphite powder is used as the pore forming agent, it is necessary to keep the ceramic honeycomb structural bodies at about 1000°C for a long time in the firing process to completely burn the graphite powder because of a property of non-inflammability of the graphite powder and as a result a long firing time is required. Moreover, under a rapid heating condition in the firing process, there sometimes occur unburned residues of the pore forming agent in the central portion of the fired honeycomb structural bodies and melt defects caused by violent burning of the unburned residues of the graphite powder at a high firing temperature. Such disadvantages are particularly noted in producing of large diesel particulate filters. Moreover, when the coke powder which is a kind of the carbon powder is used as the pore forming agent, the coefficient of thermal expansion of the produced ceramic honeycomb structural bodies becomes high owing to the great amount of impurities included in the coke powder and further when the coke powder is used together with the cordierite raw material, the orientation of particles of the cordierite raw material becomes wrong, since the coke powder has a rounded configuration, and as a result the coefficient of thermal expansion is high. When the starch powder such as wheat flour is used as the pore forming agent, if the amount of starch powder is large, firing breakage occurs and the control of firing condition is difficult and further the kneaded bodies are violently rotted if a condition of the kneaded bodies is not proper. It is an object of the present invention to eliminate the above mentioned drawbacks and to provide an improved method of producing porous ceramic honeycomb structural bodies for shortening the firing time and also obtaining a high yield. According to the present invention, there is provided a method of producing porous ceramic honeycomb structural bodies as set out in claim 1. By using the flammable carbon power defined in claim 1 as the pore forming agent according to the present invention, the retention time at about 1000°C for burning the pore forming agent can be shortened. As a result, the firing time can be shortened without rapid heating. Accordingly, there can be prevented the defect of unburned residue of the pore forming agent caused by the rapid heating and the melt defect caused by violent burning of the unburned agent at a high temperature. It is preferable that the amount of addition of the flammable carbon powder to the ceramic raw material is 15∼30 parts by weight, because if the amount of addition of the flammable carbon powder is less than 15 parts by weight, then the pressure loss of the filter may be large to decrease the collecting time, resulting in inferior characteristics of the diesel particulate filter and if the amount of addition of the flammable carbon powder is more than 30 parts by weight, then the strength of the filter may decrease and the firing time may greatly increase so that the unburned residues of the carbon powder often occur in the central portions of honeycomb structural bodies under the rapid heating condition, resulting in defective honeycomb structural bodies. It is further preferable that the grain size of the carbon powder is 20∼100µm in average particle diameter, because if the grain size is smaller than 20 µm, the diameter of the pores may be small and the ratio of fine pores is increased, resulting in a high pressure loss and short particulate collecting time and if the grain size is larger than 100 µm, the diameter of the pores may be large and as a result the particulate collecting efficiency of the porous ceramic honeycomb structural body drops, degrading the function of the filter. The carbon powder preferably has a chemical composition containing CaO and KNaO in amounts of not more than 0.1 wt%, respectively, since when the impurities contained in the carbon powder increase, the coefficient of thermal expansion of the honeycomb structural bodies becomes higher. Furthermore, it is preferable that the carbon powder has a flat configuration having an aspect ratio not less than 2, since when the carbon powder is particularly used together with the cordierite raw material, the particles of the raw material are not sufficiently orientated by using a carbon powder having an aspect ratio lower than 2, resulting in a high coefficient of thermal expansion of the honeycomb structural bodies. For a better understanding of features of the invention reference is made to the attached drawing, wherein: Fig. 1 is a diagram illustrating a firing condition used in an example of the present invention. In the following, an example of the present invention will be explained. This example is merely given in illustration of the invention and should not be interpreted into limit the scope of the invention. Example To 100 parts by weight of cordierite powder raw material having a chemical composition consisting of, as main components, SiO₂ 42-56 wt%, Aℓ₂O₃ 30∼45 wt% and MgO 12-16 wt% were added a pore forming agent having properties shown in Table 1 in an amount of addition shown in Table 2, 4 parts by weight of methyl cellulose and water as plasticizing medium. The resulting mixed raw material is kneaded to prepare extrudable batch bodies. Each batch body was extruded in a conventional manner to produce a cylindrical honeycomb structural body having an outer diameter of 230 mm, a height of 305 mm and a partition wall thickness of 430 µm with the number of cells being 16/cm². The porous ceramic honeycomb structural bodies obtained from each batch were dried and fired in various firing conditions shown in Fig. 1, in which O₂ concentration in air was controlled to be about 10% during the retention time since burning of the pore forming agent is affected by the O₂ concentration. The characteristics of the thus fired honeycomb structural bodies such as occurrence of cracks in fired bodies and occurrence of melt defects in the central portion of the fired bodies (defects caused by burning of the pore forming agent) were observed and the coefficient of thermal expansion at a temperature in 16 a range of 40°∼800°C was estimated. The results of the estimation is shown in Table 2. The carbon powders of Table 1 are crystalline carbon powder. From the results shown in Table 1, Table 2 and Fig. 1, it is appreciated that in the heat treatment wherein the honeycomb structural bodies were kept at 650°C for one hour in air according to the method of the present invention, by using the flammable carbon powder having a degree of loss in weight of 50% or more, the retention time at 1000°C for burning the pore forming agent can be shortened to thereby greatly shorten the firing time and as a result superior quality characteristics of the honeycomb structural bodies are obtained. On the other hand, when the starch powder of prior art was used as the pore forming agent, the burning was sufficient, but cracks were occurred and the quality characteristics was deteriorated. Also, when the graphite powder was used as the pore forming agent, the burning was wrong and a long retention time at 1000°C was required, moreover in a case of short retention time, the unburned residue of graphite powder was violently burned at a high temperature and the temperature is raised and as a result considerable melt faults were generated in the central portions of the honeycomb structural bodies. Furthermore, when the coke was used as the pore forming agent, the burning was sufficient, but because of much impurities, the orientation of the cordierite raw material was deteriorated and as a result the coefficient of thermal expansion was greatly increased. Although the invention has been described in the above example with respect to use of the cordierite raw material as the ceramic raw material, but other material such as mullite, aluminum titanate and the like may be used to attain the similar firing time shortening effect according to the present invention. It is seen from the above description that according to the method of producing the porous honeycomb structural bodies of the present invention, by using carbon powder having the prescribed properties, the firing time can be shortened without the unburned residue of the pore forming agent and the melt fault caused by violent burning of the pore forming agent and as a result superior porous honeycomb structural bodies for diesel particulate filters can be obtained.

Claims: A method of producing a porous ceramic honeycomb structural body comprising the steps of mixing powder of ceramic raw material with a pore forming agent, kneading the mixture, extruding the kneaded mixture into a shaped green body and then drying and firing the green body, characterized in that said pore forming agent is a flammable carbon powder having a degree of loss in weight of 50% or more after a heat treatment at 650°C for one hour in air. A method as claimed in claim 1, wherein the amount of the flammable carbon powder added to the ceramic raw material is in the range 15 to 30 parts by weight per 100 parts of ceramic raw material. A method as claimed in claim 1 or claim 2, wherein the grain size of the carbon powder is in the range 20 to 100 µm in average particle diameter. A method as claimed in any one of claims 1 to 3, wherein the carbon powder has a chemical composition containing CaO and KNaO in amounts of not more than 0.1 wt%, respectively. A method as claimed in any one of claims 1 to 4, wherein the carbon powder has a flat configuration having an aspect ratio not less than 2. A diesel engine exhaust filter containing as a filter body a honeycomb structural body made by the method of any one of claims 1 to 5.

Inventors: HAMAGUCHI, KUNIKAZU, HAMANAKA, TOSHIYUKI, HARADA, TAKASHI, MIYAHARA, KAZUHIRO
Applicants: NGK INSULATORS, LTD.
Application year: 1989
Main classification: C04B38
IPC codes: C04B38, F02B3, B01D39, B01D46, B01J37, F01N3