| PATENT ASSIGNEE'S COUNTRY | USA |
| UPDATE | 07.00 |
| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 11.07.00 |
| PATENT TITLE |
Low CTE cordierite bodies with narrow pore size distribution and method of making same |
| PATENT ABSTRACT |
Cordierite body of CTE at 25-800.degree. C. of .ltoreq.4.times.10.sup.-7 C, at least 85% of porosity having pore diameter of 0.5-5.0.mu.; or >4-6.times.10.sup.-7 C.sup.-1, porosity at least 30 vol %, at least 85% of porosity has pore diameter 0.5-5.0.mu.. Raw materials talc, Al.sub.2 O.sub.3 source, and kaolin, calcined kaolin, and/or silica, and optionally spinel, particle diameter of talc .ltoreq.3.0.mu., of Al.sub.2 O.sub.3 source <2.0.mu., kaolin is <35 wt % of raw materials when particle diameter is <2.0.mu., are blended with vehicle and aids into plastic mixture. Green body is formed, dried, fired at 1370.degree. C.-1435.degree. C. When particle diameter of talc is <2.0.mu., and Al.sub.2 O.sub.3 source is <20 wt % of raw materials, and dispersible high surface area Al.sub.2 O.sub.3 source having particle diameter of <0.3.mu., is <5.0 wt % of raw materials, and particle diameter of kaolin is <2.0.mu., heating rate from 1150.degree. C.-1275.degree. C. is >200.degree. C./hr. When particle diameter of talc is .gtoreq.2.0.mu., and Al.sub.2 O.sub.3 source is <20 wt % of raw materials, and dispersible high surface area Al.sub.2 O.sub.3 source having particle diameter of <0.3.mu., is <5.0 wt % of raw materials, and particle diameter of kaolin is <2.0.mu., heating rate from 1150.degree. C.-1275.degree. C. is >50.degree. C./hr and <600.degree. C./hr. When Al.sub.2 O.sub.3 source is less than 20 wt % of raw materials, and dispersible Al.sub.2 O.sub.3 source having particle diameter <0.3.mu.is .gtoreq.5.0 wt % of raw materials, and particle diameter of kaolin is <2.0.mu., heating rate from 1150.degree. C.-1275.degree. C. is >50.degree. C./hr. When particle diameter of kaolin is >2.0.mu., heating rate from 1150.degree. C.-1275.degree. C. is <600.degree. C./hr and >30.degree. C./hr. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | 07.07.99 |
| PATENT REFERENCES CITED | This data is not available for free |
| PATENT PARENT CASE TEXT | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. A method of producing a cordierite body, the method comprising: a) selecting cordicrite-forming raw materials comprising talc, an Al.sub.2 O.sub.3 -forming source, and one or more of the components of kaolin, calcined kaolin, and silica, with optional additions of spinel, wherein the mean particle diameter of the talc is not greater than about 3.0 micrometers, and the mean particle diameter of the Al.sub.2 O.sub.3 -forming source is not greater than about 2.0 micrometers, and the amount of kaolin, if present, is less than about 35% by weight of the raw materials when the mean particle diameter of the kaolin is less than about 2.0 micrometers; b) intimately blending the raw materials with an effective amount of vehicle and forming aids to impart plastic formability and green strength to the raw materials and form a plastic mixture therefrom; c) forming said raw materials into a green body; d) drying the green body; and e) firing said green body at temperature of about 1370.degree. C. to 1435.degree. C., wherein when the mean particle diameter of the talc is less than about 2.0 micrometers, and the amount of the Al.sub.2 O.sub.3 -forming source is less than about 20% by weight of the raw materials, and dispersible high surface area Al.sub.2 O.sub.3 -forming source having a mean particle diameter of less than about 0.3 micrometers, if present, constitutes less than about 5.0% by weight of the raw materials, and the mean particle diameter of the kaolin is less than about 2.0 micrometers, the heating rate between 1150.degree. C. and 1275.degree. C. is greater than about 200.degree. C./hr, wherein when the mean particle diameter of the talc is not less than about 2.0 micrometers, and the amount of Al.sub.2 O.sub.3 -forming source is less than about 20% by weight of the raw materials, and dispersible high surface area Al.sub.2 O.sub.3 -forming source having a mean particle diameter of less than about 0.3 micrometers, if present, constitutes less than about 5.0% by weight of the raw materials, and the mean particle diameter of the kaolin is less than about 2.0 micrometers, the heating rate between 1150.degree. C. and 1275.degree. C. is greater than about 50.degree. C./hr and less than about 600.degree. C./hr, wherein when the amount of Al.sub.2 O.sub.3 -forming source is less than about 20% by weight of the raw materials, and dispersible Al.sub.2 O.sub.3 -forming source having a mean particle diameter of less than about 0.3 micrometers constitutes not less than about 5.0% by weight of the raw materials, and the mean particle diameter of the kaolin is less than about 2.0 micrometers, the heating rate between 1150.degree. C. and 1275.degree. C. is greater than about 50.degree. C./hr, and wherein when the mean particle diameter of the kaolin is greater than about 2.0 micrometers, the heating rate between 1150.degree. C. and 1275.degree. C. is less than about 600.degree. C./hr and greater than about 30.degree. C./hr, to produce a body having a composition consisting essentially of in weight percent about 49% to 53% SiO.sub.2, 33% to 38% Al.sub.2 O.sub.3, and 12% to 16% MgO and having one of the following sets of properties: a mean coefficient of thermal expansion at 25-800.degree. C. of .ltoreq.4.times.10.sup.-7 .degree. C..sup.-1, and a porosity wherein not less than about 85% of the total porosity has a pore diameter between about 0.5 micrometers and 5.0 micrometers, or a mean coefficient of thermal expansion at 25-800.degree. C. of >4.times.10.sup.-7 .degree. C..sup.-1, but .ltoreq.6.times.10.sup.-7 .degree. C..sup.-1, a total porosity of not less than about 30% by volume, wherein not less than about 85% of the total porosity has a pore diameter of between about 0.5 micrometers and 5.0 micrometers. 2. A method of claim 1 wherein the amount of Al.sub.2 O.sub.3 -forming source is at least about 20% by weight of the raw materials. 3. A method of claim 1 wherein kaolin is present as a raw material and has a mean particle diameter of no greater than about 2.0 micrometers, and wherein the amount of Al.sub.2 O.sub.3 -forming source is no greater than about 20% by weight of the raw materials. 4. A method of claim 3 wherein the amount of dispersible high surface area Al.sub.2 O.sub.3 -forming source is not less than about 5.0% by weight of the raw materials. 5. A method of claim 4 wherein the dispersible high surface area Al.sub.2 O.sub.3 -forming source is selected from the group consisting of boehmite, pseudoboehmite, and combinations thereof. 6. A cordierite body having a mean coefficient of thermal expansion at 25-800.degree. C. of .ltoreq.4.times.10.sup.-7 C.sup.-1 and a porosity wherein not less than about 85% of the total porosity has a pore diameter between about 0.5 micrometers and 5.0 micrometers. 7. A cordierite body having a mean coefficient of thermal expansion of >4.times.10.sup.-7 C.sup.-1 but <6.times.10.sup.-7 C.sup.-1, a total porosity not less than about 30% by volume, wherein not less than about 85% of the total porosity has a pore diameter between about 0.5 micrometers and 5.0 micrometers. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
This invention relates to cordierite bodies having a unique combination of low coefficient of thermal expansion (CTE) and narrow pore size distribution. This is accomplished by use of selected combinations of raw materials and firing schedules. More particularly, the bodies are honeycomb structures that find use as substrates for catalytic reactions, and for filtration applications. BACKGROUND OF THE INVENTION Cordierite bodies having honeycomb structures are especially suited for but not limited to use as substrates for catalysts for converting automotive exhaust, for example, or as diesel particulate filters or as regenerator cores. Use of cordierite is favorable in these applications because of its good thermal shock resistance. The thermal shock resistance is inversely proportional to the coefficient of thermal expansion (CTE). That is, honeycombs with low thermal expansion have good thermal shock resistance and can survive the wide temperature fluctuations that are encountered in the application. In some cases, such as for thin-walled honeycomb substrates, it is desirable to decrease the total porosity so as to increase strength. However, this reduction in porosity results in a decrease in percent loading of the washcoats, which contain the catalysts, so that in some cases it becomes necessary to coat the substrate several times in order to build up a washcoat layer of the desired thickness. This multiple coating process adds cost to the final product. It is desirable for cordierite bodies to have a narrow pore size distribution of pores having a diameter of less than about 10 micrometers. The advantage of a narrow pore size distribution is that it enhances washcoat pick-up so that the desired thickness of the washcoat layer can be achieved with a single coating step without requiring a multiple coating process. Up to now, a body having both low CTE and narrow pore size distribution in the less than 10 micrometer range has not been achieved. For the above reasons, it would be highly desirable and an advancement in the art to have cordierite bodies with both of these properties. The present invention provides such bodies and a method of making them. SUMMARY OF THE INVENTION In accordance with one aspect of the invention, there is provided a cordierite body of CTE at 25-800.degree. C. of .ltoreq.4.times.10.sup.-7 C.sup.-1, with at least 85% of its total porosity having a mean pore diameter of 0.5-5.0 micrometers. In accordance with another aspect of the invention, there is provided a cordierite body of CTE at 25-800.degree. C. of >4-6.times.10.sup.-7 C.sup.-1, a total porosity of at least 30 vol %, at least 85% of its total porosity having a pore diameter 0.5-5.0.mu.. In accordance with another aspect of the invention, there is provided a method of making the above-described cordierite bodies that involves intimately blending the raw materials of talc, an Al.sub.2 O.sub.3 -forming source, and one or more of kaolin, calcined kaolin, and silica, and optionally spinel with vehicle and forming aids into a plastic mixture. The mean particle diameter of the talc is .ltoreq.3.0 micrometers, that of the Al.sub.2 O.sub.3 -forming source is .ltoreq.2.0 micrometers. The kaolin, if present, is <35 wt % of raw materials when particle diameter is <2.0 micrometers. A green body is formed, which is dried and fired at 1370.degree. C.-1435.degree. C. When the mean particle diameter of talc is <2.0 micrometers, and the Al.sub.2 O.sub.3 -forming source is <20 wt % of raw materials, and dispersible high surface area Al.sub.2 O.sub.3 -forming source having a particle diameter of <0.3 micrometers, is <5.0 wt % of the raw materials, and the mean particle diameter of the kaolin is <2.0 micrometers, the heating rate from 1150.degree. C.-1275.degree. C. is >200.degree. C./hr. When the mean particle diameter of the talc is .gtoreq.2.0 micrometers, and the Al.sub.2 O.sub.3 -forming source is <20 wt % of the raw materials, and dispersible high surface area Al.sub.2 O.sub.3 -forming source having a mean particle diameter of <0.3 micrometers is <5.0 wt % of the raw materials, and the mean particle diameter of the kaolin is <2.0 micrometers, the heating rate from 1150.degree. C.-1275.degree. C. is >50.degree. C./hr and <600.degree. C./hr. When the Al.sub.2 O.sub.3 -forming source is les than 20 wt % of the raw materials, and dispersible Al.sub.2 O.sub.3 -forming source having particle diameter of <0.3 micrometers is .gtoreq.5.0 wt % of the raw materials, and the mean particle diameter of the kaolin is <2.0 micrometers, the heating rate from 1150.degree. C.-1275.degree. C. is >50.degree. C./hr. When the mean particle diameter of the kaolin is >2.0 micrometers, the heating rate from 1150.degree. C.-1275.degree. C. is <600.degree. C./hr and >30.degree. C./hr. DETAILED DESCRIPTION OF THE INVENTION This invention relates to cordierite bodies having a unique combination of low thermal expansion or CTE measured at 25-800.degree. C., and narrow pore size distribution. The bodies are made by a process that involves selection of specific combinations of raw materials and firing conditions. According to this invention, CTEs are the mean expansions from 25-800.degree. C. measured by dilatometry. In honeycombs it is the mean expansion along the direction parallel to the length of the open channels. Unless otherwise specified particle size is expressed as mean particle diameter. Particle size is measured by a sedimentation technique. Porosity is total porosity measured by mercury porosimetry and is expressed as volume percent. The Raw Materials The success of the present invention in obtaining low CTE and very narrow pore size distributions between 0.5 and 5.0 micrometers is based upon the use of fine talc in combination with certain raw materials and firing schedules to maintain low CTE. The use of fine talc enables the attainment of very high fractions of porosity between 0.5 and 5.0 micrometers. However, finer talcs have a tendency to result in a body with a higher CTE due to reduced microcracking. To restore CTEs to desirable low values, the other raw materials must be selectively chosen and, for some raw material combinations, certain restrictions must be placed upon the heating rates during firing. To achieve the cordierite composition of about 49% to 53% SiO.sub.2, about 12% to 16% MgO, and about 33% to 38% Al.sub.2 O.sub.3 of low thermal expansion and narrow pore size distribution, the raw materials that are utilized are talc, an Al.sub.2 O.sub.3 -forming source, one or more of the components of kaolin, calcined kaolin, and silica. Optionally, spinel can be a raw material. The talc must have a mean particle diameter of no greater than 3.0 micrometers. By Al.sub.2 O.sub.3 -forming source is meant Al.sub.2 O.sub.3 itself or other material having low water solubility which when fired converts to Al.sub.2 O.sub.3. Some typical Al.sub.2 O.sub.3 -forming components include alumina, Al(OH).sub.3 (also known as aluminum trihydrate or the mineral gibbsite), or aluminum oxide hydroxide (also known as aluminum monohydrate or the mineral boehmite or pseudo-boehmite. Dispersible high surface area Al.sub.2 O.sub.3 -forming component or source can be provided as the powder or as a sol. By dispersible is meant that the agglomerates of very fine particles can be broken up and dispersed into the constituent particles having a mean particle diameter of less than about 0.3 micrometers. By high surface area is meant a surface area greater than about 10 m.sup.2 /g and preferably greater than about 4 m.sup.2 /g. Such powders can include boehmite, pscudobochmitc, gamma-phase alumina, delta-phase alumina, or other so-called transition aluminas. The Al.sub.2 O.sub.3 -forming source must have a mean particle diameter of no greater than 2.0 micrometers, and preferably a specific surface area greater than about 5 m.sup.2 /g. It is preferred that the amount of Al.sub.2 O.sub.3 -forming source be at least about 20% by weight of the raw materials to allow the broadest range in heating rates and still obtain low CTE bodies. The mean particle diameter of the kaolin, if present, can range between about 0.2 and 10 micrometers. However, if the mean particle size is less than about 2 micrometers, the amount of such kaolin used must be less than about 35 wt. % of the total raw material charge. The balance of the Al.sub.2 O.sub.3 required to form cordierite is supplied by calcined kaolin or an Al.sub.2 O.sub.3 -forming source, and the balance of the SiO.sub.2 being supplied by calcined kaolin or silica powder. It is preferred that the amount of Al.sub.2 O.sub.3 -forming source provided as a dispersible high surface area Al.sub.2 O.sub.3 -forming component be not less than about 5% by weight of the raw material charge. The raw materials are blended with vehicle and forming aids that impart plastic formability and green strength to the raw materials when they are shaped into a body. When the forming is done by extrusion, the extrusion aids are most typically cellulose ether organic binder, and a lubricant such as sodium ammonium or diglycol stearate, although the invention is not limited to these. The organic binder contributes to the plasticity of the mixture for shaping into a body. The plasticizing organic binder according to the present invention refers to cellulose ether binders. Some typical organic binders according to the present invention are methylcellulose, ethylhydroxy ethylcellulose, hydroxybutyl methylcellulose, hydroxymethylcellulose, hydroxypropyl methylcellulose, hydroxyethyl methylcellulose, hydroxybutylcellulose, hydroxycthylcellulose, hydroxypropylcellulose, sodium carboxy methylcellulose, and mixtures thereof. Methylcellulose and/or methylcellulose derivatives are especially suited as organic binders in the practice of the present invention with methylcellulose, hydroxypropyl methylcellulose, or combinations of these being preferred. Preferred components of cellulose ethers are Methocel A4M, F4M, F240, and K75M from Dow Chemical Co. Methocel A4M is a methylcellulose, while Methocel F4M, F240, and K75M are hydroxypropyl methylcellulose. The organic binder content is typically is about 3% to 6%, based on the raw material. The vehicle can be inorganic, i.e. consisting largely of water, which is typically but not exclusively about 28% to 46%; or it can be organic. The use of water is preferred, although evaporable organic liquids such as lower alkanols can be wholly or partly substituted as desired. The weight percents of the organic binder, vehicle and other additives are calculated as superadditions with respect to the raw materials. The mixture is then formed into a green body. The preferred forming method is by extrusion through a die. Extrusion can be done by using a hydraulic ram extrusion press, or a two stage de-airing single auger extruder, or a twin screw mixer with a die assembly attached to the discharge end. In the latter, the proper screw elements are chosen according to material and other process conditions in order to build up sufficient pressure to force the batch material through the die. The bodies according to the present invention can have any convenient size and shape. However, the process is especially suited to production of cellular monolith bodies such as honeycombs. Cellular bodies find use in a number of applications such as catalyst carriers, filters such as diesel particulate filters, molten metal filters, regenerator cores, etc. Generally honeycomb cell densities range from 235 cells/cm.sup.2 (about 1500 cells/in.sup.2) to 15 cells/cm.sup.2 (about 100 cells/in.sup.2). Wall (web) thicknesses range typically from about 0.07 to about 0.6 mm (about 3 to about 25 mils). The external size and shape of the body is controlled by the application, e.g. in automotive applications by engine size and space available for mounting, etc. This invention is especially advantageous for honeycombs having very thin walls, e.g. .ltoreq.0.13 mm (5 mils). Thinner walled honeycombs can be made e.g. 0.025-0.1 mm (1-4 mils) for some of the inventive mixtures especially those that contain clay, alumina, and a talc all of which have a mean particle size of <3 micrometers in diameter. The green body is then dried according to conventional procedures for green cordicrite bodies such as e.g. oven or dielectric drying. The dried body is then fired at a temperature of about 1370.degree. C. to 1435.degree. C. Depending on the raw material combination, the firing conditions will vary. For example, when the mean particle diameter of the talc is less than about 2.0 micrometers, and the amount of the Al.sub.2 O.sub.3 -forming source is less than about 20% by weight of the raw materials, and dispersible high surface area Al.sub.2 O.sub.3 -forming source having a mean particle diameter of less than about 0.3 micrometers, if present, constitutes less than about 5.0% by weight of the raw materials, and the mean particle diameter of the kaolin is less than about 2.0 micrometers, the heating rate between 1150.degree. C. and 1275.degree. C. is greater than about 200.degree. C./hr to yield the microstructure required for microcracking and low CTE. When the mean particle diameter of the talc is not less than about 2.0 micrometers, and the amount of Al.sub.2 O.sub.3 -forming source is less than about 20% by weight of the raw materials, and dispersible high surface area Al.sub.2 O.sub.3 -forming source having a mean particle diameter of less than about 0.3 micrometers, if present, constitutes less than about 5.0% by weight of the raw materials, and the mean particle diameter of the kaolin is less than about 2.0 micrometers, the heating rate between 1150.degree. C. and 1275.degree. C. is greater than about 50.degree. C./hr and is less than about 600.degree. C./hr. When the amount of Al.sub.2 O.sub.3 -forming source is less than about 20% by weight of the raw materials, and dispersible high surface area Al.sub.2 O.sub.3 -forming source having a mean particle diameter of less than about 0.3 micrometers is present in an amount not less than about 5.0% by weight of the raw materials, and the mean particle diameter of the kaolin is less than about 2.0 micrometers, the heating rate between 1150.degree. C. and 1275.degree. C. is greater than about 50.degree. C./hr. When the mean particle diameter of the kaolin is greater than about 2.0 micrometers, the heating rate between 1150.degree. C. and 1275.degree. C. is less than about 600.degree. C./hr and greater than about 30.degree. C./hr. The fired body is then cooled to room temperature in as short a time as is practical. The cordierite bodies of the present invention are characterized by either (1) a mean coefficient of thermal expansion at 25-800.degree. C. of .ltoreq.4.times.10.sup.-7 C.sup.-1 ; or (2) a mean coefficient of thermal expansion of >4.times.10.sup.-7 C.sup.-1 but .ltoreq.6.times.10.sup.-7 C.sup.-1, and a total porosity not less than about 30% by volume. When the CTE is less than 4.times.10.sup.-7 C.sup.-1 the total porosity can have any value but is preferably greater than about 18%. In all cases, at least about 85% of the total porosity lies between about 0.5 micrometers and 5.0 micrometers. The porous microcracked bodies of the present invention find use as catalytic substrates because the pore size distribution is advantageous for picking up and holding a washcoat. The methods of the present invention are especially suitable for manufacture of thin-walled 0.152 mm (<0.006 inches) and ultra thin walled 0.102 mm (<0.004 inches) honeycomb bodies of high cell density. Additionally, the porosities and pore size distributions of bodies prepared by the inventive methods are much less sensitive to changes in heating rates during firing than for bodies prepared by conventional methods involving use of coarser talcs. It is believed that these properties should result in less variability in washcoat loading for substrates fired in various locations within a kiln. The narrow pore size distribution can also be useful in certain filtration applications. |
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