WO2002081054A1 - Porous filter body and method - Google Patents

Abstract

Sintered non-oxide ceramic filter bodies of high strength and permeability, having pores in a size range particularly suitable for gas (e.g. diesel engine exhaust) filtration, are produced by firing filter preforms comprising selected mixtures of coarse and fine non-oxide particles to sinter the particles into porous ceramic filter bodies having a plurality of parallel inlet (10) and outlet channels (11). The high structural integrity of the preforms results from the extrusion of viscous plastic pastes of controlled viscosity at extrusion pressures of at least 30 bar and extrusion velocities of 60 mm to provide extrudates free of cracking and other defects.

Description

-1- SP01-079

POROUS FILTER BODY AND METHOD

This application claims the benefit of U.S. Provisional Application No. 60/282,516, filed 4/9/01 , entitled "Porous Filter Body and Method, by Lars Johannesen.

Background of the Invention The present invention relates to a method for producing a particle-based ceramic porous filter body which may be used for filtering particles from fluids. Filter bodies formed of porous silicon carbide (SiC), in particular filter bodies in the form of honeycomb filter bodies, are described in European Patent No. 0 336 883. The SiC filter bodies disclosed in that patent are of high thermal conductivity as well as high porosity. Thus they are particularly useful for the filtration of diesel engine exhaust since they can be regenerated without the localised heating that can damage conventional ceramic filters. European

Patent No. 0 692 995, which specifically relates to a method for closing the ends of the channels of such filter bodies, provides a further description of processes for the production of these filters.

Some effects of SiC particle size on the properties of these filters is noted in published Japanese patent application JP 03215374 A. That patent teaches the use silicon carbide powders of particular particle sizes to provide honeycomb filters of improved strength.

Notwithstanding prior work such on sintered filters, there remains a commercial need for filters offering improved durability and filtration performance that can be economically produced in high volumes through improved extrusion processes. It is a principal object of the present invention to provide improved production methods for such filters that ensure the efficient production of filter products of high quality. A further object is to develop production methods that enable significant improvements in the design of the fiiter body itself. Summary of the Invention In one aspect, the invention provides an improvement in the method for producing a ceramic porous filter body comprising extruding a viscous plastic paste containing non-oxide ceramic particles and an organic binder to form a "green" body, solidifying the "green" body by drying, and subjecting the dried solidified body to sintering under conditions sufficient to result in a porous structure. In accordance with the invention, the viscous plastic paste has rheological properties of viscosity and plasticity such that the extrusion results in a extrudate substantially without flaws or cracks at an extrusion pressure of at least 30 bar and an extrusion velocity of at least 60 mm per minute.

The rheological properties required to realize these results include an apparent viscosity for the plastic paste within the range of 5*10³ - 1*10⁵ Pa . s, as measured by a capillary rheometer at a temperature of 35°C within a shear rate range of 0.4 - 3.0 s^"1. Preferably, the paste will have a yield stress such that, when maintained at a temperature of 35°C, it will commence to flow when exposed to a pressure difference in the range of 100 - 400 kPa between the opposite ends of a rectilinear passage having an axial length of 80 mm and an inner diameter of 26 mm defined by a smooth, circularly cylindrical inner passage wall. Flow should also commence at a pressure difference of 400 - 800 kPa when the paste is maintained at a temperature of 25°C.

The preferred paste viscosities are within the range 8*10³-6*10⁴ Pa s, more preferably 1» 0⁴-3«10⁴, and still more preferably 1.5*10⁴-2.5*10⁴ Pa s. The yield stress is preferably such that the paste starts flowing when exposed to a pressure difference in the range of 100-300 kPa, preferably 150-250 kPa and more preferably 200-225 kPa between the opposite ends of the aforementioned passage at a paste temperature of 35°C.

The rheological properties of these pastes can generally be described as plastic. As hereinafter more fully described, the nature of the plasticity is shown in more detail with reference to Fig. 5 of the drawings, showing the relationship between extrusion velocity and extrusion pressure. Reference will also be made to Figs. 6, 7, and 8 of the drawings showing the temperature dependency of yield point through a capillary tube with an inlet pressure drop and without an inlet pressure drop, as well as the temperature dependency of the viscosity. As will be understood from the following description, one of the critical features of the present invention is to adjust the properties of the viscous plastic mass with respect to viscosity and plasticity in such a manner that the extrusion, especially when the extrudate is highly complex such as is the case for a honeycomb structure, will result in an extrudate substantially without flaws or cracks under production conditions. These conditions are defined, as a lower limit, by an extrusion pressure of at least 30 bar and an extrusion velocity of at least 60 mm per minute, the preferred production conditions, however, being characterised by a higher extrusion pressure, such as an extrusion pressure of at least 40 bar, or an extrusion pressure of at least 50 bar or even at least 60 bar, such as up to about 70 bar. The velocity with which the extrusion proceeds is preferably at least 100 mm per minute, more preferably at least 120 mm per minute and still more preferably at least 150 mm per minute.

The foregoing minimum conditions are established, however, while recognising that there is not necessarily a well-defined relationship between extrusion pressure and extrusion velocity. Thus it may be desired to utilise a higher extrusion pressure either to obtain a higher extrusion rate or to obtain a higher shape stability. The conditions for obtaining filter bodies substantially without flaws or cracks at these production parameters are the proper rheological properties, such as viscosity and plasticity properties, of the paste extruded, that is, parameters which confer to the paste a sufficient tensile strength, ultimate strain and fracture toughness to result in a smooth high quality filter body.

In the following description a number of measures for obtaining these properties of the paste will be discussed, as will the advantages of those properties for achieving further refinements in the design of filter bodies in order to adapt them to the increasingly critical requirements of fields of use such as for the removal of diesel soot particles from diesel engine exhaust. It will of course be understood that while the present specification and claims relate to a number of measures for obtaining the desirable properties of the paste, the invention is not limited to these specific measures, but rather relates to any process as defined above in which the properties ensuring the high quality at high production rates have been or are conferred to the paste, the attainment of these properties being assessable by relatively simple tests that can be performed by the person skilled in the art.

Preferably, the extruded paste will comprise both coarse and fine non- oxide ceramic particles as well as an organic binder. The content of the fine particles has a considerable influence on the viscosity and extrudability of the paste, with the extrudability of the paste improving with increasing content of the fine particles. These components and other possible components of the paste will be discussed in greater detail in the following, in particular in connection with the discussion of production of filter bodies where the coarse non-oxide ceramic particles are SiC particles.

The amount of fine particles in the extrusion batch can also have an important influence on the properties of the final porous filter body. It is normally preferred that the fine particles are not present in such an amount that they will tend, in the sintering process, to result in any substantial blocking of the porosities of the filter. Thus, the amount of the fine particles is preferably an amount in the range below an amount where substantial porosity reduction of the final filter body will occur in the sintering process. Porosity reduction can result from the settling of material originating from the consumed fine particles on the coarse particles outside the bridges or menisci present in the pore structure of the sintered body.

On the other hand, it is not desirable to use so small amounts of the fine particles that the final filter body will be weak. Therefore, the amount of the fine particles is preferably an amount in the range above an amount where a considerable further strength improvement due to bridge or menisci formation between neighbouring coarse particles can take place as a result of further addition of the fine particles. It can be determined experimentally the range necessary to produce neither substantial porosity reduction, nor further strength improvement of the final filter body. Thus the preferred amount of fine particles is in the range below the amount where substantial porosity reduction of the final filter body will occur in the sintering process and above the amount where a considerable further strength improvement due to bridge or menisci formation between neighbouring coarse particles can take place as a result of further addition of the fine particles.

The attainment of the necessary strength during the sintering of the filter bodies requires that there be a temperature "window" in the sintering range for the bodies wherein fine particle sublimation and/or diffusional transport can occur without substantial sublimation of the larger particle fraction of the batch. Whereas a window of 25°C is useful for this purpose, it is generally preferred to have a larger window, this object being achieved by selecting a fine particle fraction of a size such that the particles will be consumed wholly or partially by sublimation and/or diffusional transport at a temperature which is at least 100°C, more preferably at least 200°C, below the temperature at which the coarse particles will sublimate.

As mentioned above, a very important type of a filter is a honeycomb wall flow filter, and another aspect of the invention relates to a method for producing a ceramic porous honeycomb wall flow filter body. While the honeycomb wall flow filter body may be made with any desired wall thickness by the method according to the invention, including "traditional" thicknesses in the range of 0.8mm to 1.25mm, an important advantage of the rheological aspects of the invention is that the effective high quality production of honeycomb filters having relatively small wall thicknesses is enabled, whereby valuable new filter design possibilities are provided.

In accordance with the invention the cell wall thickness of a thin-walled honeycomb wall flow filter body may be in the range of 0.3-0.75mm, the particle size of the coarse particles being adapted to the wall thickness of the honeycomb so that the ratio between the average particle size (mean diameter) of the coarse particles and the wall thickness is at the most 1 :5, normally in the range of 1 :5-1 :30, such as in the range of 1 :6-1 :20, normally in the range of 1 :7-1 :20, often preferably in the range of 1 :7-1 :15, such as in the range of 1 :8- 1 :10 or 1 :8-1 :9.

As will be understood from the above explanation, the weight ratio between the coarse and the fine particles importantly affects the strength and porosity and the relationship between strength and porosity of the filter body. This relationship becomes particularly important at small wall thicknesses. For the development of adequate filter body strength, porosity and permeability during the firing of a dried body preform to sinter the non-oxide particles together in to the porous non-oxide ceramic, the ratio between the fine particles and the coarse particles may be anywhere between in the range between the "normal" 1 :5 or even 1 :4 on the higher side and 1 :30 on the lower side, but will, according to the present invention, preferably be in the range where the amount of the fine particles is relatively smaller, such as in the range of 1 :6- 1 :30, e.g., 1 :7-1 :30, more often, however, in the range of 1 :6-1 :15, such as in the range of 1 :8-1 :15 or often preferably in the range of 1 :9-1 :12, such as about 1 :10. As examples of the relatively small wall thickness may be mentioned a wall thickness is in the range of 0.4-0.7mm, more typically in the range of 0.5- 0.7mm and most preferably in the range of 0.5-0.6mm. In the case of these small wall thicknesses, it is preferred that the coarse particles are relatively small, e.g., that they have an average particle size, according to the FEPA gradation, in the range of 40-70 μm, such as in the range of 55-65 μm. A suitable particle size, as defined by FEPA Mesh size, is the size of FEPA Mesh F220-240 particles. The acronym FEPA stands for Federation of the European Producers of Abrasives. Whenever the present specification and claims refer to an average size "according to the FEPA gradation", this is intended to indicate the mean diameter of the particle fraction in question, in accordance with the tables of estimated mean diameters for grit sizes for aluminium oxide and silicon carbide grains published by FEPA. High sintered strength, porosity and permeability in accordance with the invention is achieved when the weight ratio between the fine and the coarse particles to be sintered to form the porous ceramic is in the preferred range of 1 :6-1 :15. The sintered material of the porous filter body in that case may have a strength, measured as 4-point bending strength, exceeding 20 mPa and a permeability (air) exceeding 1 -10^"12m². In a preferred embodiment the weight ratio between the fine and the coarse particles is in the range of 1 :9-1 :12, the strength of the filter material exceeds 25 mPa, and the permeability (air) exceeds 1.5.10^"l2m². The coarse SiC material may be FEPA mesh F230, and the fine SiC material may have an average particle size in the range of 0.3-1.5 μm, preferably in the range of 0.5-1.0 μm.

A honeycomb filter having these preferred parameters, and a cell pitch in the range 2-2.3 mm, and a wall thickness in the range of 0,5-0,7 mm will, when used for removing soot particles from diesel exhaust, have a very high soot loading capacity. That is, the increase of the pressure loss over the filter as soot accumulation builds up will be small because the filter area is high, while, at the same time, the thermal mass of the filter is substantially the same as in more conventional filters, e.g. SiC honeycomb filters with wall thickness of 0.8 mm or higher.

With respect to the fine particles, the average size is normally in the range of 0.3-3 μm, preferably in the range of 0.3-2 μm, such as in the range of 0.5-2 μm. Another way of expressing the size of the fine particles is by reference to their maximum size, where a size of 3μm is normally the maximum suitable or permissible size, the more preferred maximum size of the fine particles being 2 μm or even 1μm, as the smaller particles have a much higher tendency to be consumed in the sintering, such as will be explained in the following. Thus, a preferred average size of the fine particles is in the range of 0.5-0.8μm.

Description of the Drawings The invention may be further understood by reference to the appended drawings, wherein: Fig. 1 is scanning electron microscope photo of a typical micro-structure of the porous filter wall according to the invention,

Fig. 2 illustrates the flow principle of a wall flow filter, Fig. 3 is a graph showing the pore size of the porous structure of the filter as a function of the particle size of the coarse particles, Figs. 4a-4c show, schematically, the experimental capillary rheometer used in the measurements of the rheological data discussed herein, Fig. 5 is a graph showing pressure loss as a function of flow velocity when extruding paste for forming a filter body,

Figs. 6 and 7 are graphs showing temperature dependency of the yield point of the paste, Fig. 8 is a graph showing temperature dependency of viscosity of the paste, and

Figs. 9 and 10 are graphs showing strength and permeability of the filter body produced as a function of fine contents thereof.

Detailed Description

The flow principles involved in the operation of a wall flow filter are well known and schematically illustrated in Fig. 2 of the drawings. The honeycomb wall flow filter is of a very compact filter design enabling a large filter area within a restricted volume. The filter defines a plurality of parallel inlet and outlet channels or passages 10 and 11 , respectively, opening alternately in opposite ends of the filter body, whereby the openings of the passages in each end defines a chessboard-like pattern. When a particle loaded gas (black arrows) enters the inlet channels it has to pass through the filter walls 12 which trap the particles; the then-purified gas (white arrows) exits via the outlet channels. The composition of the non-oxide ceramic material to be utilized to manufacture the wall flow filters of the invention will be selected depending upon the particular filtration environment in which the filter is intended to operate. Where different sizes of non-oxide ceramic materials are to be used, the non-oxide ceramic material for the coarse particles is a ceramic material which is not an oxide and is suitably selected from the group consisting of SiC,

Si₃N , SiONC, mullite and aluminium titanate. Particularly useful materials for the coarse particles are non-oxide silicon ceramic particles, that is, non-oxide materials containing silicon such as SiC, Si₃N₄ and SiONC, among which SiC, Si₃N are preferred, and the presently most preferred material is SiC, particularly the commercially readily available alpha SiC.

The fine particles are preferably particles of the same material as the coarse particles, but also useful are mixtures wherein the coarse particles are SiC or Si₃N , and the fine particles are particles of mullite or aluminium titanate, or of mixtures thereof with SiC and/or Si₃N . Particles of the latter can reduce the necessary sintering temperature.

A preferred family of ceramic pastes based on SiC particulates includes the following main elements. These compositions can be varied to some extent depending on the choice of the coarse SiC fraction, and the actual geometry of the final product required:

Coarse SiC Fine SiC Organic binder

Lubricant Ethanol Water

Optionally 1 ,2-ethanediol (ethylene glycol) Optionally polyhydric alcohol

Optional sintering aids Pastes of these types are suitably produced by first mixing the coarse particles and the fine particles with the binder added in dry form, then adding the lubricant in the form of a solution or suspension in a liquid, and mixing, then adding any optional recycled material in the form of a mixture with aqueous phase, then adding water and optionally a polymeric alcohol and optionally a polyhydric alcohol, and mixing, and finally adding further aqueous phase up to the final content of aqueous phase, and mixing.

The extrusion is preferably performed in such a manner that the extrudate leaves the extruder at a velocity which is substantially identical all over the cross section of the extrudate, as this has been found to result in superior quality of the extrudate.

Particularly preferred are aqueous pastes containing a lower alcohol such as ethanol. The lower alcohol has a number of functions. Thus, the lower alcohol may function to secure that the gelation temperature of the paste is increased to a temperature above the extrusion temperature. The lower alcohol may also have an influence on the viscosity of the aqueous phase containing the organic binder, and in this case, it is preferred that the concentration of the lower alcohol is a concentration at which a curve representing viscosity of the aqueous phase of the paste as a function of the concentration of the lower alcohol shows a substantial plateau so that minor changes in the concentration of the lower alcohol will have only a small influence on the viscosity of the aqueous phase.

The following discussion sets forth more detailed parameters for preferred paste components and variations of components that may be resorted to for the manufacture of porous filter bodies from silicon carbide particles.

Coarse SiC powder:

F240-F120 grades (FEPA gradation) silicon carbide powders can be used, with the resulting pore sizes being in the range of 6-45 microns. The pore size of the sintered body is determined primarily by the selection of starting powder. The expected pore size can be described by the following simple linear relationship:

Pore size «0.59 * particle size - 18.9

Fig. 3 of the drawings is a graph based on measurements of the average pore size of filter samples produced with different sized coarse starting powders. The volume median measured by mercury intrusion is used. This relationship has been found to be valid for particle size from approx. 45 microns up to 250 microns.

Fine SiC powder:

FCP 10-FCP 15 SiC powder materials, commercially available from Norton, are normally preferred. FCP (Fine Ceramic Powder) particle sizes correspond to the number of square meters of surface area per gram of the powder product sold. Table 1 below shows equivalent particle diameters for various FCP products wherein the average particle size of each sample material as well as the particle sizes corresponding to the 10% and 90% (weight) points on the particle size distribution curve (normally distributed) are reported.

Table 1 - SiC Particle Size Distributions

10% (μm) 50% (μm) 90% (μm)

FCP-15 0,18 0,5 1 ,0

FCP-13 0,31 0,8 1 ,6

FCP-10 0,30 0,9 2,1

FCP-07 0,40 2,0 6,5

As mentioned above, the fine SiC powder is added primarily to sinter the coarse particles together. During sintering, the grain growth of the coarse particles takes place by consumption of the fines. The fines are volatilised upon heating and re-deposited at the grain contacts between the coarse particles, leading to the formation of grain boundaries and a strong, integral filter product.

Filter pore size is also influenced by the quantity of fine powder introduced. Up to a limit of approximately 15% addition of fines, no reduction of pore size is seen as all the fines are consumed during sintering/recrystallization to build up the grain boundaries. The amount of fines and the particle size of the fines are thus chosen to give a desired compromise between permeability and strength of the sintered body under the specific sintering conditions, as well as to secure the paste plasticity that is required for the extrusion production of a well-structured porous product.

Organic binder:

The binder is added with the objective of obtaining the desired rheological properties of the paste, by adding plasticity to the mixture of ceramic particles, and to add strength to the 'green' products after drying. The amount of the organic binder should be adjusted to the actual liquid phase content in the ceramic paste. If the binder content is not high enough, the viscosity of the paste may not be high enough to prevent a loss of liquid phase during extrusion, which is unacceptable. On the other hand, the content of the organic binder should not be so high that it results in a too high viscosity.

The organic binder may suitably be a cellulose or a cellulose derivative, such as a cellulose ether. The presently preferred cellulose ether is methyl hydroxyethyl cellulose ether, which may, for example, be present in an amount of 3-8, preferably 4.5-6 and more preferred 5.2-5.6 per cent by weight of the total amount of paste. Other cellulose binders may of course be used.

An example of a preferred organic binder is Tylose MH300-P2 which is the trade name for an organic polymer, a cellulose derivative, methyl hydroxyethyl-cellulose ether. The Tylose binder is a water soluble polymer that becomes 'saturated' when all hydroxyl groups are hydrolysed. The number 300 refers to the viscosity in a 2 wt% aqueous solution. Other cellulose derivatives with various other substituents may be alternatively or additionally used as the organic binder, provided their properties are found to be suitable for the purpose.

Lower alcohol:

In addition its function to control the gelation of the binder and to lower the surface tension of the liquid phase, a lower alcohol such as ethanol will result in an increased wetting of the ceramic particles. In addition, the easy vaporisation of ethanol is desirable as the green bodies are getting stable after a very short initial drying phase which already takes place immediately after extrusion.

Also, a water-alcohol liquid phase is advantageous for use with the preferred binders as the addition of alcohol shifts the binder gelation temperature higher. The presently preferred alcohol is ethanol. Without the addition of ethanol, undesirable gelation of the binder in the ceramic paste during extrusion has been observed.

As a secondary effect, the viscosity of the binder system increases with increasing ethanol content in the liquid phase, until it reaches a plateau at approximately 20-50% by weight of ethanol. With a higher ethanol content, the viscosity decreases rapidly. In practice, therefore, the ethanol content is therefore chosen at approximately 20-35 % by weight of the liquid phase. Lubricant:

The paste normally contains a lubricant, such as stearic acid or a wax, the lubricant preferably being in a finely divided form such as micro-particles of stearic acid or a wax emulsion. Stearic acid is particularly effective to decrease friction during extrusion, although wax and other organic materials with similar properties have been found to be usable. Pristerene 4900 flakes from Unichema International constitutes a useful stearic acid source.

Polyhydric alcohol (optional): Ethylene glycol, 1 ,2-ethanediol, or another suitable polyhydric alcohol may be added as an optional plasticiser for the batch.

PVA (optional):

A preferred component of the paste from the standpoint of paste uniformity is polyvinyl alcohol. This material is added as a solution in water to act as a secondary organic binder. It has been found that such a secondary organic binder, of a chemical principle different from the first binder, preferably a polymeric alcohol, can contribute advantageously to level out any minor differences in behaviour of the paste from batch to batch and thus to obtaining a uniform and highly reproducible production. Examples of suitable PVA products are BDH 30573 from BDH Laboratory Supplies, Poole, BD15 1TD, England, and Rdh 63018 from Riedel-de Haen AG, 30926 Seelze, Germany. The Rdh 63018 product has a viscosity of 4-6 mPa*s as a 4% by weight solution in water at 20°C.

Sintering aids (optional):

Sintering aids known from the literature to be suitable for improving the properties of a sintered body can be added to the material in the form of Al- or B-containing compounds. For both of the above mentioned types of sintering additives the addition should be kept at a level lower than the solid solution level of approx. 1 % atom, to make sure that no secondary phase can be formed and concentrated in the grain boundaries. The addition of sintering additives also has an influence on other relevant properties of SiC, the most pronounced influence being the effect on thermal conductivity. Undoped SiC has a thermal conductivity of approx. 75 W/m*k, which, according to the literature (Ceramic Bulletin, 67, No. 12, 1988, pp 1961-1963), is decreased to approx. 60 W/m*k when doped with Al. On the other hand SiC doped with B or

Be has a thermal conductivity of 170 W/m*k or 260 W/m*k, respectively.

Which sintering additives should be added will depend on the intended use of the filter body and the particular requirements dictated by that use. One important purpose for utilising such additives with SiC filter bodies is to obtain a high TSP (Thermal Shock Parameter) when producing honeycomb filters having a small wall thickness.

Preparation of the ceramic paste:

To obtain best homogeneity and plasticity in the ceramic paste upon mixing, the ceramic paste should be mixed according to some general and to some specific guidelines. Also the incorporation of recycled (used) material in the process should to follow certain guidelines.

To obtain the desired properties of the ceramic paste, a specific order of combination of the components is preferred. A desirable first step is mixing coarse and fine SiC powders with the methyl hydroxylethyl cellulose binder

(Tylose), the latter being added as powder or fine granules, without addition of any liquid phase. In this way the binder is evenly distributed in the ceramic material.

The next step may be a dissolution of the stearic acid lubricant in heated ethanol followed by addition of the alcohol-stearic acid solution to the dry components with mixing to ensure even distribution of the precipitated micro grained stearic acid in the ceramic material. As the solution is cooled by contact and mixing with the ceramic material, the stearic acid precipitates as micro grains already distributed evenly in the ceramic material. Where wax is to be employed as a lubricant, a wax-containing emulsion of wax in water can be added together with the batch water. Licomer PE 02 wax emulsion or Micro

Wax, both from Hoechst, are suitable for this purpose. The last batch addition may be water, with or without the inclusion in the water of the optional PVA and ethylene glycol binder constituents. Mixing of the ceramic paste to secure even liquid distribution and dissolution of the organic binder in the liquid phase then follows.

It is also possible to include recycled batch materials of similar composition to newly prepared batches. This is best accomplished by replacing any missing liquid phase from the recycled material, through the addition of appropriate amounts of ethanol and water to pulverized or sectioned recycled ceramic bodies, prior to combination with the other batch constituents.

The mixing of the ceramic paste is suitably performed in a planetary mixer, such as a commercial R series Eirich mixer adapted for mixing of ceramic pastes. During the mixing, the mixture undergoes a phase transition from a non-uniform blend to a uniform, coherent mass.

Examples of specific paste mixtures that may be prepared with the materials and in accordance with the procedures hereinabove set forth are as follows:

Example I

Component Weight, q

Coarse SiC particles

(FEPA Mesh 180, mean particle size 69 μm) 13500

Fine SiC particles, FCP 15C 2700

Tylose MH 300P2 1125

PVA Rdh 63018, 7% solution 270

Stearic acid 270

1 ,2-ethanediol (ethylene glycol) 90

Ethanol 1025

Water 2150

Sub-total of intial blend 21130 Recycled material 3500 Extra ethanol 20 Extra water 40

Total Paste Components 24690 Example II

Coarse SiC particles

(FEPA Mesh F180, mean particle size 69 μm) 15000

Fine SiC particles, FCP 15C 3000

Tylose MH 300P2 1275

PVA Rdh 63018, 7% solution 300

Stearic acid 350

1 ,2-ethanediol (ethylene glycol) 100

Ethanol 1300

Water 2360

Total Paste Components 23685

Determining when a batch viscosity and plasticity effective to provide extruded honeycomb filter bodies of high structural quality have been achieved is best accomplished through capillary extrusions. Figs. 4a-4c of the drawings show a longitudinal cross-section of a capillary design for an experimental capillary rheometer useful for generating the necessary data.

Fig. 4a shows an extruder outlet comprising a base part 13, which may be connected to the extruder (not shown) and an outlet tube 14a. The outlet tube 14a defines an inner part 15a defining a cylindrical passage with a large diameter and an outer part 16a defining a cylindrical passage with a smaller diameter. The outer part 16a may be replaced by outer parts 16b and 16c having a shorter length, but the same diameter as illustrated in Figs. 4b and 4c. A pressure transducer 17 for measuring the pressure within the base part 13 is arranged in a transverse bore or pocket.

Rheological determinations were performed on pastes provided in accordance with the invention using a labscale extruder fitted with capillary tubes such as illustrated in Figs. 4a-4c. to provide a capillary rheometer system. The extruder used was a Handle lab scale extruder fitted with an extruder outlet incorporating one of five different capillaries : 013mmxL60mm,

013mmxL120mm, 026mmxL60mm, 026mmxL150mm and 026mmxL240mm. For all capillaries an upstream reduction from 080mm (part 15a) was provided as shown in Figs. 4a-4c. By using capillaries of the same diameter with identical inlet section, the pressure drop with origin in the inlet section can be eliminated, and the theory of viscous flow in a capillary can be applied to calculate the viscosity of different shear rates. Besides the inlet pressure drop can be calculated, arising from the converging flow into the capillary entrance. In this converging flow elements of the extrusion compound are subjected to a stretching type of deformation. This high entrance pressure drop offers a method of estimating the elastic properties of the compound.

In the above mentioned capillaries the aspect ratio is in the same range as that of the actual extrusion dies, thereby simulating the actual process during the determination of the rheological properties of the paste. As the importance of the end-effects are known not to be negligible with the extrusion dies being used for the production of porous filter bodies, it would also be quite inappropriate to base the evaluation of the compound on steady shearing viscosity only. Since these compounds have non-Newtonian fluid characteristics, it was decided to perform as many measurements as possible at different shear rates to evaluate the viscosity change with shear rate. The compounds were expected to exhibit plastic behavior including a well defined yield stress and, after a short nonlinear region, a linear relationship between shear stress and shear rate. Capillary measurements were therefore conducted at different shear rates in the entire temperature range from approximately 20 C to approximately 40 C.

For capillary measurements of these types the apparent viscosity is given, by definition: η_a =τ_w/γw wherein τ_w is the wall shear stress and γ_w is the wall shear rate. The wall shear stress in a capillary tube at steady shearing is given as: τ_w= ΔP/4 * dc/L wherein ΔP is the pressure drop across the length L, d_c is the capillary diameter, and L is the capillary length. The wall shear rate for a Newtonian fluid (often called the apparent wall shear rate) is given as: γ_w = 8V/dc as the velocity profile is parabolic. In the case of non-Newtonian fluids the velocity profile deviates from parabolic , which means that the wall shear rate will be different from that given above. The correct wall shear rate may be found by using the Rabinowitch correction: γ_w = 8V/d_c * (3n^, +1)/4n' wherein n' = d(lnτ_w)/d(ln(8V/d_c)), i.e., n' is the slope of the graph of lnτ_w versus ln(8V/do).

Measuring results on a typical SiC powder-containing plastic paste mixture such as Example II above, prepared as above described and tested by means of the described capillary rheometer using capillary tubes such as illustrated in Figs. 4a-4c, are shown in Table 2 below. Included in Table 2 are corresponding_. data sets of temperature, extrusion velocity, and pressure, for a number of runs recorded at different temperatures in the range from 25 °C up to 40°C with increments of approximately 5 °C. At each temperature the yield point was measured by assuming that the shear rate reached zero 60 seconds after the auger of the extruder was turned off.

Table 2. Rheological data for Ceramic Paste.

Capillary,026 x L160 Capillary, 026 x L240

T (°C) P(Bar) P, 60 sec V T (°C) P(Bar) P, 60 sec V (Bar) (cm/min) (Bar) (cm/min)

25.1 30.0 8.9 39.0 26.1 48.0 15.2 28.5

25.2 29.2 8.5 36.0 26.3 44.0 14.4 24.0

25.1 25.2 8.3 15.0 26.4 39.8 14.3 15.0

30.0 , 25.6 6.9 37.5 30.1 42.6 10.3 39.0

30.6 24.7 6.9 28.5 30.1 39.3 10.4 28.5

30.0 22.3 7.0 16.5 30.0 34.8 10.1 16.5

35.1 23.1 6.2 33.0 35.0 37.4 8.2 42.0

35.0 22.9 6.2 26.5 35.0 35.5 8.4 30.0

35.3 20.1 6.3 15.0 35.0 31.7 8.5 16.5

39.9 22.9 5.5 43.5 39.9 35.7 7.5 45.0

40.3 21.8 6.1 33.0 40.0 33.4 7.5 33.0

39.7 21.0 6.1 21.0 39.6 30.1 7.6 18.0 Fig. 5 of the drawings shows the rheological properties of the paste of Example II assessed by measuring the pressure loss through a capillary tube having a length of 80 mm and a diameter of 26 mm as a function of flow velocity. The graph is obtained by measuring how the extrusion pressure through the capillary tube depends on extrusion velocity at four different temperatures in a range covering the actual operating temperature, when extruding the plastic compound. As the graphs are based on interpolated data, some actual measurement points are plotted, which confirms that the interpolations are accurate. The graphs shown in Figs. 6, 7, and 8 are also based on data taken from the Example II paste. At all temperatures this paste has a typical plastic profile with a yield point considerable lower than the intercept with the y-axis defined by the close to rectilinear profile of the paste at extrusion velocities higher than 0.0015 m/s. Fig. 6 illustrates the temperature dependency of the yield point of this paste when extruding through a capillary having a diameter of 26 mm and a length of 240 mm including the entrance pressure drop. As Fig. 6 illustrates, the yield point of the paste decreases continuously when the temperature increases, the decrease being most pronounced in the range from approximately 26 °C to approximately 30°C. Over that range the pressure is reduced by approximately 33% corresponding to a decrease of 1.25 Bar/°C.

The yield point does not have an inflexion point nor reach a 'plateau like' temperature range.

By comparing Fig. 6 and Fig. 7 of the drawing characterising this paste a qualitative impression of the elastic and plastic properties of the Example II compound can be made. In the lower temperature region the dominant pressure drop is the pressure drop arising from viscous flow in the capillary, while the dominant pressure drop in the higher temperature range is the inlet pressure drop, which is related to elastic properties. Fig. 7 illustrates the temperature dependency of the yield point of this paste by extrusion through a capillary having an inner diameter of 26 mm and a length of 80 mm. The graph shows that the yield point decreases continuously when the temperature increases, most pronounced in the range from approximately 26 °C to approximately 30°C, where the pressure is reduced with approximately 46% corresponding to a decrease of 0.7 Bar/°C. Again the yield point does not have an inflexion point nor reach a 'plateau like' temperature range.

By using the above equations, the viscosity of the paste has been calculated in the temperature range from approximately 25-40°C. The temperature dependency of the viscosity is shown in Figure 8, with the viscosity of the paste is based on the experimental results described in Table 2.

Fig. 8 illustrates the temperature dependency of viscosity, measured with a capillary rheometer principle. The graph shows that the viscosity decreases continuously when the temperature increases. This effect is most pronounced in the range from approximately 26 °C to approximately 30°C, where the viscosity is reduced with approximately 33% corresponding to a decrease of 3.5510³ Pa s/°C. Like the yield point, the viscosity of this paste does not have an inflexion point nor reach a 'plateau like' temperature range. A further example of a plastic paste that. is particularly well suited for the production of a thin-walled monolith for a honeycomb wall flow filter body is set forth as Example III below:

Example III Component Weight, g

Coarse SiC particles

(FEPA Mesh 230, mean particle size 53 μm) 12500

Fine SiC particles, FCP 15C 2500

Tylose MH 300P2 1060 PVA Rdh 63018, 7% solution 250

Stearic acid 290

1 ,2-ethanediol (ethylene glycol) 125

Ethanol 855

Water 2080 Total Paste Components 19660

A paste having the composition of Example III can be extruded to provide thin-walled honeycomb bodies useful for forming wall flow filters by conventional means. Typically, the pastes are first pre-extruded to provide slugs of material, these desirably being aged for several days for conditioning and equilibration purposes. The aging period should be sufficiently long to achieve full reaction of the organic binder, thereby obtaining the required viscosity and plasticity necessary for the re-extrusion of the ceramic paste/intermediate cylindrical bodies into the desired cylindrical honeycomb bodies.

A screw extruder adapted for the extrusion of porous SiC is then used to form the slugs into thin-walled honeycomb structures. The extruder is suitably an auger extruder designed for the extrusion of coarse grained materials in complex shapes. Rotational movement of the ceramic paste within the extruder should be suppressed, a result achieved by using a double auger in combination with noodle dies and mass knives. The auger and the linings in the extruder are suitably provided with heating or cooling means to control batch temperature and reduce thermal gradients therein. High temperatures can induce undesirable gelling , while excessive temperature gradients lead to non- uniform extrusions. Conventional honeycomb extrusion dies, desirably including conventional wear coatings to extend die life, can be used.

If desired, the extruder and batch slug may be preheated to an optimal extrusion temperature, typically in the 25-35°C. range, so that the extrusion can be initiated at pressures in the range of 5-10 bar. Thereafter continuous extrusion can take place effectively at extrusion rates producing 50-150mm of extruded product length per minute at extrusion pressures of 30-50 bar and extrusion temperatures of 25-35°C, with pastes having the rheological properties hereinabove described. Extruded honeycomb ceramic bodies produced as described may be dried and fired using known techniques. To control shrinkage and the resultant possibility of cracking, drying should be carried out in such a way as to maximize the uniformity of the drying rate across all sections of the extruded pieces. Firing the bodies to achieve non-oxide particle sintering can be carried out in graphite-insulated furnaces, most preferably in the firing range 2200°C to

2430°C depending of the particle size of the starting material. Protective nitrogen or argon atmospheres are used. As noted above, the porosity and permeability of the sintered ceramic products can be effectively controlled by controlling the particle size distribution of the non-oxide particles included in the batch. Fig. 9 of the drawings illustrates the relationship between the strength and permeability of a sintered silicon carbide product and the amount and particle size of a fine silicon carbide fraction included in a batch containing both coarse and fine silicon carbide powders. In this case, the effects illustrated are for the case of a batch comprising varying proportions of Norton FCP-15 fine SiC powder having an average particle size of 0.5 μm. As is evident from a study of Fig. 9, a fines content of 9.09 wt% can maximize the level of permeability provided while still retaining a sintered product strength as high as that obtained through the inclusion of 16.67 wt% of the same fine powder in the batch.

Similar data is plotted in Fig. 10 of the drawing for the case where a Norton FCP-10 fine ceramic powder having an average particle size of 0.9 μm is included as the fine powder fraction of the batch. That data indicates that a slightly higher or identical permeability and a substantially increased strength are observed at a batch content of 9.09 wt% fines, when a compared to a higher batch addition of 16.67 wt%.

Products produced in accordance with the invention have both a honeycomb structure of high structural quality and a pore structure well adapted for the filtration of fine particulates such as exhaust soot from diesel exhaust streams. Fig. 1 of the drawings is a scanning electron micrograph showing the pore micro-structure of a sintered wall portion of a typical silicon carbide honeycomb filter body provided in accordance with the invention. The porous filter wall comprises coarse particles bonded together in contact points, thereby defining the porosity of the filter. The coarse particles are fragments of larger crystallites, which upon crushing form irregular particles. The low aspect ratio of these particles, typically in the range of 1 :2, are only changed slightly during the sintering process.

Claims

CLAIMS:

1. A method for producing a ceramic porous filter body, said method comprising: extruding a viscous plastic paste containing non-oxide ceramic particles, and an organic binder to form a "green" body, solidifying the "green" body by drying, and subjecting the dried solidified body to sintering under conditions sufficient to result in a porous structure, the viscous plastic paste having such rheological properties with respect to viscosity and plasticity that the extrusion results in a extrudate substantially without flaws or cracks at an extrusion pressure of at least 30 bar and an extrusion velocity of at least 60 mm per minute, these rheological properties being: an apparent viscosity within the range of 5 0³- 1 10⁵ Pa s, as measured by a capillary rheometer at a temperature of 35°C and within a shear rate range of 0.4 - 3.0 s^~\ and such a yield stress that the paste starts flowing through a rectilinear passage, which is defined by a smooth, circularly cylindrical inner passage wall and which has an axial length of 80 mm and an inner diameter of 26 mm, when the paste is exposed to a pressure difference in the range of 100 - 400 kPa between the opposite ends of the passage and maintained at a temperature of 35°C as well as when the paste is exposed to a pressure difference of 400 - 800 kPa and maintained at a temperature of 25°C.

2. A method according to claim 1 , wherein the apparent viscosity is within the range 8 0³- 610⁴ Pa s.

3. A method according to claim 2, wherein the apparent viscosity is 1 0⁴ - 3 0⁴, preferably 1.5 10⁴- 2.5 10⁴ Pa s.

4. A method according to any of the claims 1-3, wherein the yield stress is such that the paste starts flowing when exposed to a pressure difference in the range of 100 - 300 kPa, preferably 150-250 kPa and more preferred 200 - 225 kPa between the opposite ends of the passage and maintained at a temperature of 35°C.

5. A method according to any of the claims 1-4, wherein the yield stress is such that the paste starts flowing when exposed to a pressure difference in the range of 500 - 750 kPa, preferably 550-700 kPa and more preferred 600 - 650 kPa between the opposite ends of the passage and maintained at a temperature of 25°C.

6. A method according to any of the preceding claims, wherein the viscous plastic paste comprises a bimodal particle system containing coarse non-oxide ceramic particles and fine particles of the same non-oxide ceramic and/or of a different ceramic material.

7. A method according to any of the preceding claims, wherein the weight ratio between the fine and the coarse particles is in the range between 1 :6 and 1 :30.

8. A method according to claim 7, wherein the ratio is in the range of 1 :9-1:12.

9. A method according to any of the preceding claims, wherein the coarse ceramic particles are particles of SiC, Si₃N , SiONC, mullite and/or aluminium titanate.

10. A method according to claim 9, herein the coarse non-oxide ceramic particles are non-oxide silicon ceramic particles are particles, in particular particles of SiC or Si₃N .

11. A method according to claim 10, wherein the coarse particles are particles of SiC.

12. A method according to claim 11 , wherein the SiC is alpha SiC.

13. A method according to any of the preceding claims, wherein the fine particles are particles of the same material as the coarse particles.

14. A method according to any of the claims 1-13, wherein the coarse particles are SiC or Si₃N , and the fine particles are particles of mullite or aluminium titanate, or of mixtures thereof with SiC and/or Si₃N₄.

15. A method according to any of the preceding claims, wherein the viscous plastic paste has such properties with respect to viscosity and plasticity that the extrusion results in a coherent extrudate substantially without flaws or cracks at an extrusion pressure of at least 30 bar.

16. A method according to claim 15, wherein the viscous plastic paste has such properties with respect to viscosity and plasticity that the extrusion results in a coherent extrudate substantially without flaws or cracks at an extrusion pressure of at least 40 bar.

17. A method according to claim 16, wherein the viscous plastic paste has such properties with respect to viscosity and plasticity that the extrusion results in a coherent extrudate substantially without flaws or cracks at an extrusion pressure of at least 50 bar.

18. A method according to any of the preceding claims, wherein the extrusion is performed in such a manner that the extrudate leaves the extruder at a velocity, which is substantially identical all over the cross section of the extrudate

19. A method according to claim 18, wherein the temperature difference, during the extrusion, between a paste part corresponding to a middle part of the cross- section of the extrudate and a paste part corresponding to a circumference part of the extrudate is at the most 4°C.

20. A method according to claim 19, wherein the temperature difference is at the most 1°C.

21. A method for producing a porous non-oxide ceramic honeycomb wall flow filter body having a cell wall thickness in the range of 0.3-1.25mm comprising the step of firing a dried honeycomb preform having cell walls containing a mixture of coarse and fine non-oxide particles to sinter the particles into a porous ceramic, wherein; the fine non-oxide particles have a maximum average particle size of 3μm; the coarse non-oxide particles have a minimum average particle size of 40μm; and the weight ratio of fine non-oxide particles to coarse non-oxide particles does not exceed 1 :4.

22. A method for producing a porous non-oxide ceramic honeycomb wall flow filter body having a cell wall thickness in the range of 0.3-0.75mm comprising the step of firing a dried honeycomb preform having cell walls containing a mixture of coarse and fine non-oxide particles to sinter the particles into a porous ceramic, wherein; the fine non-oxide particles have an average size in the range of 0.3- 3μm; the coarse non-oxide particles have an average size in the range of 40- 70μm; the weight ratio of fine non-oxide particles to coarse non-oxide particles is in the range of 1 :5 to 1 :30, and the ratio of the average coarse particle size to the cell wall thickness of the porous ceramic is in the range of 1 :5 to 1 :30.

23. A method in accordance with claim 22 wherein the cell wall thickness of the porous ceramic is in range of 0.4-0.7mm and wherein the ratio of the average coarse particle size to the cell wall thickness of the porous ceramic is in the range of 1 :6 to 1 :20.

24. A method in accordance with claim 21 wherein the fine non-oxide particles have an average size in the range of 0.5-1.Oμm.

25. A method in accordance with claim 21 wherein the weight ratio of fine non- oxide particles to coarse non-oxide particles is in the range of 1:6 to 1:15.

26. A method in accordance with claim 21 wherein the coarse non-oxide particles have an average size in the range of 55-65μm.

27. A sintered non-oxide ceramic honeycomb wall flow filter body having a mean pore size in the range of 6-45μm, a 4-point bending strength exceeding 20 mPa, a cell wall thickness in the range of 0.3-1.25mm, and an air permeability exceeding 1 -10^"12 m², said body being produced by firing a dried honeycomb preform containing a mixture of coarse and fine non-oxide particles to sinter the particles into a porous ceramic, wherein the fine non-oxide particles have a maximum average particle size of 3μm, the coarse non-oxide particles have a minimum average particle size of 40μm, and the weight ratio of fine non-oxide particles to coarse non-oxide particles does not exceed 1 :4.

28. A sintered non-oxide ceramic honeycomb wall flow filter body having a mean pore size in the range of 6-45μm, a 4-point bending strength exceeding

25 mPa, a cell wall thickness in the range of in the range of 0.3-0.75mm, and an air permeability exceeding 1.5.10^"l2m², said body being produced by firing a dried honeycomb preform containing a mixture of coarse and fine non-oxide particles to sinter the particles into a porous ceramic, wherein the fine non- oxide particles have the fine non-oxide particles have an average size in the range of 0.3-3μm, the coarse non-oxide particles have an average size in the range of 40-70μm, the weight ratio of fine non-oxide particles to coarse non- oxide particles is in the range of 1 :5 to 1 :30, and the ratio of the average coarse particle size to the cell wall thickness of the filter body is in the range of 1 :5 to 1 :30.

29. A sintered non-oxide ceramic honeycomb wall flow filter body in accordance with claim 27 wherein the non-oxide ceramic material is a material selected from the group consisting of SiC, Si3N4, SiONC, mullite and aluminium titanate.

30. A sintered non-oxide ceramic honeycomb wall flow filter body in accordance with claim 27 wherein the non-oxide ceramic material is alpha SiC.