FR2954175A1 - Structure for filtering exhaust gas containing soot/solid particles in e.g. diesel engine, has filtering elements connected by joint cement, where structure satisfies specific equation comprising parameters such as walls apparent porosity - Google Patents

Abstract

The structure has honeycomb shaped monolithic unitary filtering elements connected together by joint cement, where the structure satisfies specific equation comprising parameters. The elements include channels that are closed by closures at ends to define inlet and outlet chambers opened along a respective gas intake and exhaust faces such that gas traverses porous filtering walls. The parameters are resistance to wrenching of the closures, mean value of inner perimeters of the chambers, mean length of the closures and apparent porosity of the walls. The mean length of the closures ranges from 3 to 7 mm. The apparent porosity of the walls ranges from 35 to 65 percentage.

Description

ASSEMBLED FILTER STRUCTURE

The present invention relates to the field of filter structures obtained by the assembly of a plurality of monolithic unitary filter elements of the honeycomb type, used for the filtration of solid particles contained in the exhaust gases of a diesel engine or gasoline. The said structures are capable of additionally incorporating a catalytic component that makes it possible to eliminate the polluting gases of the NO type, carbon monoxide CO or unburned HC hydrocarbons. The invention also relates to a method for obtaining such a filtering structure.

Filtration structures for soot contained in the exhaust gas of an internal combustion engine are well known in the prior art. These structures most often have a honeycomb structure, one of the faces of the structure for the admission of the exhaust gases to be filtered and the other side the exhaust of the filtered exhaust gases. The structure comprises, between the intake and discharge faces, a set of adjacent ducts or channels of axes parallel to each other separated by porous filtration walls, which ducts are closed to one or the other of their ends for delimiting input chambers opening along the inlet face and outlet chambers opening along the discharge face. For a good seal, the peripheral part of the structure is surrounded by a coating cement. The channels are alternately closed in an order such that the exhaust gases, during the crossing of the honeycomb body, are forced to pass through the sidewalls of the inlet channels to join the outlet channels. In this way, the particles or soot are deposited and accumulate on the porous walls of the filter body. Most often, the filter bodies are porous ceramic material, for example cordierite, based on aluminum titanate or silicon carbide. Ceramic or porous refractory materials based on silicon carbide or SiC obtained by high-temperature sintering are increasingly used for the manufacture of filters because of their high chemical inertia and their high refractoriness enable them to withstand mechanical stresses. important and realize structures whose walls are highly porous (ie more than 35% open porosity) and developing high filtration surfaces, which is favorable to obtain a low pressure drop. By pressure loss is meant the gas pressure difference existing between the inlet and the outlet of the filter. In known manner, during its implementation, the particulate filter is subjected to a succession of filtration phases (accumulation of soot) and regeneration (removal of soot). During the filtration phases, the soot particles emitted by the engine are retained and are deposited inside the filter. During the regeneration phases, the soot particles are burned inside the filter, in order to restore its filtration properties. The porous structure is then subjected to intense thermal and mechanical stresses, which can cause micro-cracking likely over time to cause a severe loss of filtration capacity of the unit, or even its complete deactivation. This phenomenon is particularly observed on monolithic filters of large diameter. To solve these problems and increase the life of the filters, it has been proposed more recently more complex filtration structures, combining in a filter block several unitary monolithic filter elements in honeycomb. The elements are most often assembled together by bonding by means of a cement of a ceramic nature, called in the following description seal cement or cement joint. Examples of such filter structures are for example described in patent applications EP 816 065, EP 1 142 619, EP 1 455 923 or WO 2004/090294.

The production of such filtering structures formed by the assembly of monolithic elements generally requires machining before the application of the external coating cement in order to give the filter a good seal and to give it a final external shape allowing its perfect integration into the exhaust line. This machining step, generally performed using abrasive wheels, is however difficult to implement without creating defects in the honeycomb structure, especially if the walls have a high porosity, typically close to or greater than 40%. or even 45, 50%, 55% or even 60%. The defects that may occur are, for example, the tearing off of the plugs or, more often, the breaking of the channel walls, or even the tearing of several successive peripheral channels in a radial direction. Such defects obviously impair the intrinsic properties of the filter by inducing in particular a significant decrease in its filtration area and significantly increasing the pressure loss associated with it in the exhaust line (in the state loaded with soot or in residues as in the unloaded state). In the end, if the step of resizing the filter is performed in poor conditions, the essential properties of the filter that are its filtration surface and its pressure drop, or even its robustness in extreme cases, can be significantly reduced, even after the application of the coating cement. For example, it is considered that only filters having locally less than 3 successive channels broken in a radial direction can be repaired by the application of the coating cement. The others are currently automatically rejected for the reasons previously described. Thus if the machining step is poorly controlled, it can thus lead to a drop in productivity and / or efficiency of the entire construction line of the filter. The object of the present invention is therefore to provide an improved filter to meet the problems described above, formed by assembling monolithic filter elements of the honeycomb type whose filtering walls are made of porous ceramic material, in particular of the silicon carbide type recrystallized, of porosity typically between 35 and 65%, said structure being substantially free from machining defects and thus having an optimum filtration efficiency. In particular, the research carried out by the applicant showed that different parameters had to be taken into account to obtain such a result. In addition, it has been found, and it is the object of the present invention, that these different parameters could be linked by a correlation relation allowing to determine in a simple and direct way, for example from the porosity target values. open porous walls, the other conditions that had to meet the filter to have sufficient resistance to the conditions of machining. In its most general form, the present invention thus provides a usable structure for the filtration of a gas charged with soot particles, said structure comprising a plurality of honeycomb filter elements connected together by a cement seal, said one or more elements comprising a set of adjacent channels of axes parallel to each other separated by porous walls, which channels are closed off by plugs at one or the other of their ends to delimit the entrance chambers opening along an inlet face of the gases and outlet chambers opening along a gas evacuation face so that the gas passes through the porous walls and a peripheral portion constituted by a coating cement protecting the or said elements. The structure according to the invention is characterized in that the following relation (1) is satisfied: (1): ## EQU1 ## wherein: R is the pull-out resistance of a plug obstructing a channel, expressed in MPa (megapascals), - P is the average value of the internal perimeters of the input channels and the output channels and is expressed in mm 30 (millimeters ), - L is the average length of the plugs, expressed in mm, - PO is the open porosity of the filter walls, expressed as a percentage.

For the purposes of the present invention, the term "pull-out resistance" of a stopper obstructing a channel is the pressure necessary to take off the plug from its channel, or alternatively to cause the rupture of the channel at the level of the adhesion zone. with said plug. By tearing off a plug, one thus means either the detachment thereof from the wall portions with which it is connected, or the rupture of said portions under the pressure exerted.

Conveniently, the pull-out resistance of a plug is achieved by subjecting the filter to a compression test of at least one plug and preferably a plurality of plugs distributed over the entry and exit surface. the filter, at room temperature, by applying a load on it. The load is applied in the longitudinal direction of the filter by means of a rod or a needle or a punch introduced into the channel blocked by the opposite end. The stem, whose end surface is smaller than that of the plug with which it is in contact but at least equal to 60% of the surface of the stopper, presses the stopper from the inside of the channel so as to expel or tear off the plug on the outside of the filter.

The speed of movement of the punch is for example between 3 and 5 mm / min. The resistance corresponds to the maximum force allowed by the plug brought to the end surface of the rod. The surface and shape of the stem are obviously a function of the size and shape of the cap. Typically the surface of the rod is for example greater than or equal to 0.7 mm 2. In the structures according to the invention, the open porosity of the filtering walls is preferably between 35% and 65% and the median pore diameter is most often between 5 and 25 microns. The porosity of the filter walls of the filter and the median pore diameter is measured by mercury porosimetry.

According to a preferred embodiment of the invention, the porous walls are essentially made of silicon carbide. By essentially silicon carbide, it is understood that the porous walls comprise more than 75% by weight of SiC and in particular more than 90%, or even more than 95% or even more than 98% by weight of SiC. Typically, the average length of the plugs is between 1 and 7 mm, preferably between 2 and 6 mm. The average length of the plugs is measured using calipers or a suitable gauge.

The average value of the internal perimeters of the input channels and the output channels is usually between 1 and 10 mm, for example between 3 and 7 mm. The average internal perimeter of the channels is determined by any known measurement technique or geometrically, from the shape of these channels. Advantageously, the volume or the surface of the input channels is greater than that of the output channels. For example, in the filtration structures according to the invention, the porous walls have corrugations so as to be concave with respect to the center of the inlet channels and convex in their middle relative to the center of the outlet channels. The experiments carried out by the applicant, some of which are reported later in the present description, have shown that the adaptation of the preceding parameters, by application of the relation (1) given previously, according to a first advantage, made it possible to preserve effective the external appearance of the assembled filters after their machining and therefore their expected intrinsic properties, that is to say, significantly reduced the removal of plugs or collapse walls previously observed. According to another advantage, the application of the present invention makes it possible to obtain filters having optimum performance, in terms of filtration efficiency, while retaining the intrinsic properties of the filter, in the sense previously described.

In particular, according to one of the essential aspects of the invention, it becomes possible, as a function of the target values of the porosity of the filtering walls and the geometry of the channels of the structure, by application of the preceding formula (1), to select in a simple way the formulations and the compositions of the plugs and their lengths, to obtain filters resistant to machining, that is to say having a minimum of defects in the sense previously described, while maintaining efficiency filtration quite satisfactory, especially when the pores of the walls are the highest, that is to say when they have a very low mechanical strength. The term "powder" conventionally means within the meaning of the present invention a set of grains or particles characterized by a grain size distribution or diameter generally centered and distributed around a mean or median diameter. By the terms "grain" or "particle" is meant a solid product individualized in a powder or a mixture of powders. Conventionally, the term "median size" or "median diameter", or "d50", is a mixture of particles or a set of grains, the size dividing the particles of this mixture or the grains of this set in first and second populations equal in number, these first and second populations having only particles or grains having a size greater than, or less than, the median size. Similarly, the percentile d10 of a powder corresponds to a grain size for which 10% by volume of the grains of the powder have a size less than or equal to d10 (and consequently for which 90% of the grains, by volume, have a size strictly greater than d10). D90 of a powder is the size of grains for which 90% by volume of the grains of the powder have a size less than or equal to d90 (and consequently for which 10% of the grains, by volume, have a strictly greater size at d9o).

The material constituting the walls preferably has an open porosity of between 35% and 65%, and even more preferably between 40% and 60%. Especially in the particle filter application, too low porosity leads to a too high pressure drop. Too high a porosity, however, leads to a level of mechanical resistance that is too low. The median diameter d 50 of the pores (for which 50% by volume of the pores have a smaller or equal diameter) constituting the porosity of the material is preferably between 5 and 25 microns, especially between 10 and 20 microns. In general, in the targeted applications, it is generally accepted that a too small pore diameter leads to excessive pressure loss, while too large median pore diameter leads to poor filtration efficiency. In general, the section of a monolithic element constituting the assembled structure is square, the width of the element being between 30 mm and 50 mm. Advantageously, the thickness of the walls is between 200 and 500 μm. The number of channels in the filter elements is preferably between 7.75 and 62 per cm 2, said channels having a section of about 0.5 to 9 mm 2, preferably between 0.8 and 5 mm 2. According to a possible embodiment, the volume of at least a portion of the input channels is different, in particular greater than that of at least a portion of the output channels. Advantageously, the filtering structure according to the invention is thus of the "asymmetrical" type, that is to say that the volume or the surface of the input channels is different and preferably greater than that of the output channels.

A method of obtaining a structure according to the invention is now described. As already described said structure comprises a plurality of honeycomb filter elements, interconnected by a joint cement and a peripheral portion constituted by a coating cement protecting said elements. A synthesis method, in accordance with that described in the application EP 1142619A1 to which reference will be made for the details of the implementation, typically comprises the following steps. a) preparation of a mixture of powders, for example based on silicon carbide, and shaped, in particular by extrusion, the honeycomb filtering elements, b) preferably drying and removal of the organic material, in particular by an intermediate heat treatment and / or by use of microwaves, c) plugging and sealing of the ducts of the filtering elements, d) cooking the body under a non-oxidizing atmosphere at a sintering temperature of between 1500 ° C. and 2400 ° C., preferably greater than 1900 ° C, or even greater than 2000 ° C, to obtain filter elements whose filtering walls, for example recrystallized SiC, have a mean porosity of preferably between 35 and 65%, e) assembly of the filter elements by means of a cement preferably followed by a drying heat treatment; f) machining of the assembly to obtain a typically round or oval outer shape filter; g) deposition of a coating external sealing by means of a cement followed preferably by drying, or even baking between 200 and 1100 ° C.

Typically, during step a) of forming the monoliths, binding agents and optionally plasticizers may be added to the powder mixture, for example different initial SiC particle size fractions. These binding agents or plasticizers are for example chosen from the range of polysaccharides and cellulose derivatives, PVA, PEG, or even derivatives of lignones or chemical setting agents such as phosphoric acid or sodium silicate, provided that these are compatible with the cooking process. In a known manner, in order to obtain porosity levels of the walls of the structure that are compatible with use as a particulate filter, that is to say between typically 35 and 65%, it is generally necessary to introduce more in the mixture of organic porogens. These organic porogens are vaporized at higher or lower temperatures during cooking. Pore forming agents such as polyethylene, polystyrene, starch or graphite are described in applications JP 08-281036 or EP 1 541 538. The rheology of the plastic mixture thus obtained is easily controlled by water additions in quantity. appropriate. The shaping of the porous product is preferably carried out so as to produce pieces of various shapes according to any known technique, in particular by extrusion. The size of the granules and fractions of SiC particles constituting the initial mixture of particles is adapted according to the techniques in force to the thickness of the part to be carried out so as to ensure the necessary properties of porosity, mechanical strength and appearance for the desired application. Clogging of the monolithic elements during step c) is preferably carried out before the cooking of these elements (step d) according to techniques well known in the prior art. Preferably the plugs are made of a material with a coefficient of expansion close to that constituting the filtering walls, preferably made of an SiC-based material such as the filtering walls. The corking paste is generally obtained in conventional manner by mixing an oily or aqueous system with one or more SiC powders of different median diameter, for example three powders of SiC of different median diameter. The SiC powders used in the corking paste preferably have a composition greater than 98% by weight of SiC, preferably in the form of a. The capping paste may comprise one or more powders of particles having a median diameter d 50 of between 5 and 100 μm, preferably between 10 and 50 μm, and a particle powder having a median diameter less than 5 μm, for example close to 1 μm. An inorganic sintering agent of the alumina or mullite type having a median diameter d50 of between 5 and 100 μm, for example close to 10 μm, may optionally be used to increase the density of the plugs formed. Organic materials can be incorporated into the sealing paste to modify the final properties and in particular the porosity and the properties of adhesion to the porous walls. Such materials include porogens, solvents, dispersants and anti-foaming agents. The organic porogenic agents are for example chosen from polyethylene, polystyrene, starch, graphite or possibly tire crumb from pneumatic waste recycling in order to reduce the economic cost and the environmental impact. The amount of these porogens can be between 0 and 20% by weight, preferably between 0 and 10% by weight to obtain plugs having a porosity of between 30 and 70%. The dispersing agents are, for example, chosen from polyoxyalkylene amine derivatives. The anti-foaming agents are, for example, chosen from alkyl phenol ether phosphate derivatives. The oily system consists in particular of the addition of a solvent derived from benzoate. The choice of agents and their respective amount is determined so as to obtain preferably a paste whose viscosity for a shear rate of 0.6 s-1 is between 400 and 600 Pa.s, typically measured by the aid of a Haake viscometer equipped with a mobile cross. Optional removal of the solvents according to step b) can be achieved by heat treatment or alternatively by the use of microwaves for a time sufficient to bring the water content not chemically bound to less than 1% by weight. . Of course, other known equivalent means can be envisaged without departing from the scope of the present invention.

The removal of binders or debinding is preferably carried out in air and at a temperature preferably below 700 ° C, so as to ensure sufficient mechanical strength before sintering and avoid uncontrolled oxidation of SiC. The cooking of the unitary honeycomb filter elements is carried out at high temperature depending on the nature of the material used for the preparation of the filtering walls. The cooking temperature is generally between 1500 and 2400 ° C. For SiC-based walls, the firing is carried out at a temperature greater than 1800 ° C., preferably greater than 2000 ° C., more preferably greater than 2100 ° C. but less than 2400 ° C. Preferably, said cooking is conducted under a non-oxidizing atmosphere, for example Argon. The average thickness of the jointing material between the elements used during step e) is preferably between 0.5 and 4 mm, in particular at least 1 mm. For small thicknesses, the mechanical strength of the filter is low and the flatness dispersion of the monolithic elements can then generate local thermomechanical stresses and reduce stress relaxation by the grouting material. If the thickness is too high, the pressure drop of the filter becomes too strong, especially since the monolithic elements are numerous, that is to say that the number of joints in the filter section perpendicular to the axis the filter is high. The grouting material is understood here as a moldable composition formed by a particulate and / or fibrous mix, dry or wet, capable of setting in mass able to have a sufficient mechanical strength at ambient temperature or after drying and / or heat treatment of which the temperature will not exceed the softening or subsidence temperature which defines the refractoriness of the material (s) constituting the monolithic elements. Mouldable means a composition capable of plastic deformation necessary for the display on the joint face of the monolithic elements and having a sufficient adhesion with respect to these elements so as to make them integral or to allow the manipulation of the filter assembled immediately after the grouting operation, or if necessary after heat or chemical treatment or other treatment such as ultraviolet irradiation. The grouting material of the honeycomb filtering unit elements preferably comprises particles and / or fibers of ceramic or refractory material, chosen from non-oxides, such as SiC, aluminum nitride and / or silicon, aluminum oxynitride, or among oxides, especially comprising Al 2 O 3, SiO 2, Cr 2 O 3, MgO, ZrO 2, or any of their mixtures. Preferably the composition comprises at least 20% SiC. The grouting material is preferably a ceramic and / or refractory cement. Preferably, the monolithic filter elements are based on SiC and are assembled by a jointing material whose thermal conductivity is greater than or equal to 0.1 W / m.K for any temperature between 20 and 800 ° C. A high thermal conductivity of the grouting material advantageously makes it possible to homogenize the heat transfers in the filter while a low thermal conductivity, especially less than 0.1 W / mK (measurement typically performed at a temperature of 600 ° C.) contributes to increase thermal gradients and thermomechanical stresses in the seal and within the filter. The monolithic unitary honeycomb filter elements are preferably assembled by partial bonding, in the sense that the space between the monolithic elements may not be completely filled by the grouting material, so as to relax the thermomechanical stresses on the surface. filter, as described for example in applications EP 1 726 800 or FR 2 833 857. Jointing material configurations as described in applications WO 2005/084782 or WO2004 / 090294, which involve areas of adhesion little or no between the grouting material and the filter element and areas of strong adhesion between the grouting material and the honeycomb filter element are also conceivable. The assembled filter preferably has a coating cement integral with the assembled filter, affixed during step g), in particular of the same mineral composition as the grouting material in order to increase the mechanical strength. The depollution device may further comprise a catalytic coating for the treatment of CO or HC and / or NOx type polluting gases.

The advantages described above are illustrated by the nonlimiting examples which follow, illustrating certain embodiments of the invention. The following examples allow a comparison with the products obtained according to the prior methods.

Examples

In a first step, SiC-based honeycombs are usually synthesized.

More precisely, 70% by weight of a SiC powder whose grains have a median diameter d 50 of 10 microns is mixed with 30% by weight of a second SiC powder whose grains have a median diameter d 50 of 0. , 5 micron, in a first mode comparable to the powder mixture described in EP 1 142 619. For the purposes of the present description, the median diameter d5o denotes the diameter of the particles such that respectively 50% of the total population of said particles has a size smaller than this diameter.

Four mixtures were made with different levels of porogen of the polyethylene type in respective proportions equal to 0% (Examples 1 to 5); 5% (Examples 6 to 11); 12% (Examples 12 to 15) and 15% (Examples 16 to 19) of the total weight of the grains of SiC so as to obtain filtration unit elements of different porosity, after baking. For each mixture, a methylcellulose-type conditioning additive in a proportion equal to 10% by weight of the total weight of the SiC grains is also added.

The quantity of water required is then added and kneaded to obtain a homogeneous paste whose plasticity allows the extrusion through a die of a honeycomb structure so as to produce monolithic blocks characterized by a wave layout of the internal channels, such as those described in connection with Figure 3 of the application WO 05/016491. Three types of geometries were extruded in a cross section, the wall undulation being characterized by an asymmetry rate as defined in WO 05/016491. The asymmetry rate is equal to 7% for the geometries 1 and 2 as reported in Table 1 and 11% for the geometry 3 as reported in Table 1. The dimensional characteristics of these structures after extrusion are given in FIG. Table 1: Geometry of the channels and 1 2 3 of the monolith Examples 6 to 10 11 to 19 5 191 cpsi * 281 cpsi * 169 cpsi * Density of channels either or 29, 6 43, 5 26, 2 channels / cm2 channels / cm2 channels / cm2 Asymmetry rate 7% 7% 11% of the walls Thickness average 350} gym 320} gym 340} gym walls Length of the monolith 15,24 cm 15,24 cm 15,24 cm (unitary element in nest bee) Width of the monolith 3.6 cm 3.6 cm 3.6 cm (unitary honeycomb element) * channels per square inch Table 1 The green monoliths obtained by microwave are then dried for a time sufficient to bring the water content not chemically bound to less than 1% by mass.

The channels of each face of the monoliths are alternately plugged according to well-known techniques, for example described in application WO2004 / 065088. Various capping formulations have been used and reported in Table 2 below. In Table 1: - the SiC 1 powder is characterized by the following particle size distribution: d10 = 10 μm, d50 = 35 μm and d90 = 80 μm, - the SiC 2 powder is characterized by the following particle size distribution: d10 = 4 μm, d50 = 13 μm and d90 = 28 μm, the SiC 3 powder is characterized by the following particle size distribution: d10 = 0.5 μm, d50 = 1.5 μm and d90 = 6 μm.

The capping mixture is composed of a dispersing agent derived from polyoxyalkylene amine, an antifoaming agent derived from alkyl phenol ether phosphate and a solvent derived from benzoate (the three constituting the oily mixture). The blowing agent is of the polyethylene type. The cooking aid is of the fine alumina type as described above. The respective amount of these agents is determined so as to obtain a paste whose viscosity for a shear rate of 0.6 s-1 is 500 Pa.s, measured using a Haake viscometer equipped with a mobile cross. The monoliths are then baked under Argon with a rise in temperature of 20 ° C / hour until reaching a maximum temperature of 2100 ° C which is maintained for 6 hours.

The monoliths are then assembled by sixteen elements according to conventional techniques by bonding using a cement of the following chemical composition: 72% by weight of SiC, 15% by weight of Al2O3r 11% by weight of SiO2, the remainder being constituted by impurities, mainly Fe2O3 and alkali and alkaline earth metal oxides. The average thickness of the joint between two adjacent blocks is of the order of 2 mm. The assembly is then machined in order to form assembled filters of cylindrical shape of about 14.4 cm in diameter.

The samples obtained were evaluated and characterized according to the following procedures:

A- Dimensional measurements of the plugs and perimeter of the channels: The length L of the plugs is measured with the help of a caliper, on 10 plugs randomly distributed on each face of the filter. The internal perimeter of the channels is calculated geometrically and verified by image analysis.

B- Measurement of the porosity of the material constituting the matrix. The open porosity of the material constituting the walls of the monoliths as well as the median diameter d 50 of the pores were determined according to the standard techniques of high-pressure porosimetry of mercury, with a porosimeter of the Micromeritics 9500 type. By median diameter of the pores d 50, means a value for which 50% by volume of the pores are less than said value.

C- Measurement of the resistance to tearing off the plugs. The resistance to tearing of a plug from the wall in a channel that it obstructs is carried out at ambient temperature, under a pressure applied thereto in a longitudinal direction of the filter according to the following experimental protocol: on 10 plugs randomly distributed in the filter a pressure (load) by displacement of a hardened steel rod pressed against it and whose diameter is smaller than the smallest width of a channel, said rod being moved according to a pressure speed of 4mm / min. The load is applied in the longitudinal direction of the filter through the rod, introduced into the channel blocked by the end opposite the plug. The rod, whose end surface is smaller than that of the plug with which it is in contact, presses the plug from the inside of the channel so as to expel or pull the plug towards the outside of the filter. An Instron press connected to the rod and equipped with a displacement sensor and a force sensor so as to measure the maximum force admissible by the plug, ie the force at break (in Newton) when the plug is detached from the walls of the channel that it obstructs or when the walls break. The breaking strength (in MPa) is then determined by dividing the value of the force at break measured by the surface of the rod, in mm2.

D-Filtration Efficiency Measurement The filtration efficiency of the filter device is determined by measuring the amount of smoke emitted at the outlet of the filter relative to the input quantity. To do this, a smoke meter is placed upstream and downstream of the particulate filter, the latter being placed on the engine exhaust line operated at full power at 4000 rpm for 30 minutes. The smoke meter is used to determine the amount of soot particles emitted through a measurement of blackening due to smoke. The filtration efficiency index must remain above 90% for the test to be considered satisfactory. As reported in Table 2, the following notes were assigned to each of the filters: +: filtration efficiency greater than 90%: filtration efficiency less than 90%. E- external appearance after machining As reported in Table 2, the following notes were assigned to each of the filters after their observation: +++: no detectable defects after machining, ++: slight defect after machining, at most one channel defective locally, +: wall tearing locally affecting two successive channels at most, at least three successive channels have a wall tear in a radial direction. The main characteristics and results obtained are summarized in Table 2 given below.

Ex1 Ex2 Ex3 Ex4 Ex5 Ex6 Ex7 Ex8 Ex9 Percentage by weight of SiC powder 1 0 0 52.9 58.7 52.9 0 52.57 52.9 69.8 percentage by weight of SiC 2 powder 57.2 58 , 5 17.6 0 17.6 58.5 17.5 17.6 0 percent by weight of the SiC powder 3 28.2 28.8 17.6 14.7 17.6 28.8 17.5 17, 6 17,4 percentage weight of cooking aid 1,71 0 0 0 0 0 0,35 0 0 percentage weight of blowing agent 0 0 0 9,2 0 0 0 0 0 percentage weight oily mixture 12,9 12.6 11.9 17.5 11.9 12.6 12.0 11.9 12.8 L = Length of caps L (in mm) 4 4.5 4.5 5 4.5 3.5 3 5 5 , 0 P = Average perimeter of inlet and outlet channels (in mm) 5.96 5.96 5.96 5.96 6.39 5.96 5.96 5.96 5.96 PO = Porosity of the filter wall (in%) 39 39 39 39 39 47 47 47 47 Median pore diameter (in μm) 10 10 10 10 10 14 14 14 14 R = resistance to crushing of the caps (in Mpa) 391 341 294 177, 6 281 480.9 220.5 168 180 K '= (R x D) / L 583 452 389 212 399 819 438 200 215 K' / (1000 x exp (-0.095xPO)) 24 18 16 9 16 71 38 17 19 Effectiveness of filtration + + + - + + + + + Machining aspect - + ++ ++ ++ - - + ++ Ex10 Exil Ex12 Ex13 Ex14 Ex15 Ex16 Ex17 Ex18 Ex19 weight percentage of SiC powder 1 58.7 52 , 9 52.9 67.1 58.7 63.4 63.4 61.0 58.7 69.8 percentage weight of SiC powder 2 0 17.6 17.6 0 0 0 0 0 0 0 percentage by weight SiC powder 3 14.7 17.6 17.6 16.8 14.7 15.9 15.9 15.3 14.7 17.4 percentage weight of cooking aid 0 0 0 0 0 0 0 0 0 0 percentage weight of blowing agent 9.2 0 0 3.2 9.2 6.5 6.5 8.0 9.2 0 percentage weight oily mixture 17.5 11.9 11.9 12, 9 17.5 14.3 14.3 15.7 17.5 12.8 L = Length of plugs L (in mm) 5.0 5 3 3 3.5 2.5 3.5 4 4 4 P = Average perimeter inlet and outlet channels (in mm) 5.96 4.83 4.83 4.83 4.83 4.83 4.83 4.83 4.83 4.83 OP = Porosity of the filter wall ( in%) 47 47 55 55 55 55 59 59 59 59 Median pore diameter (in pm) 14 14 18 18 18 18 20 20 20 20 20 R = resistance to crushing of caps (in Mpa) 38 215 100 63 30 45 38 32 24 70 K '= (R x P) / L 45 208 161 101 41 87 52 39 29 85 K '/ (1000 x exp (-0.095xPO)) 4 18 30 19 8 16 14 11 8 23 Filtration efficiency - + + + - + + + - + Appearance machining ++ + - + ++ + + ++ ++ - Table 2 It is seen in Table 2 that Examples 2, 3, 5, 8, 9, 11, 13 and 15 to 17, satisfying the relation (1) according to the invention, have the best compromise between their filtration efficiency and their appearance in the machining.

Claims (7)

REVENDICATIONS1. A filter structure for a gas charged with soot particles, said structure comprising a plurality of honeycomb filter elements interconnected by a joint cement, the at least one element comprising a set of adjacent channels of parallel axes between them separated by porous walls, which channels are closed by plugs at one or other of their ends to define inlet chambers opening along a gas intake face and outlet chambers opening on a gas evacuation face such that the gas passes through the porous walls and a peripheral portion constituted by a coating cement protecting the one or more said elements, said structure being characterized in that it satisfies the following relationship (1): 10 <- [RxP / [1000 x L x exp (-0.095xPO)] in which: -R is the tear resistance of a plug obstructing a channel and is expressed in MPa , - P is the va their average of the internal perimeters of the inlet channels and the outlet channels and is expressed in mm (millimeters), -L is the average length of the plugs, expressed in mm, - PO is the open porosity of the filter walls, expressed in percentage . 2. Structure according to claim 1, wherein the open porosity of the filter walls is between 35% and 65% and wherein the median pore diameter is between 5 and 25 microns. 3. Structure according to one of claims 1 or 2 wherein the porous walls are essentially silicon carbide. 4. Structure according to one of the preceding claims, wherein the average length of the plugs is between 1 and 7 mm. 5. Structure according to one of the preceding claims, wherein the average value of the internal perimeters of the input channels and the output channels is between 1 and 10 mm, preferably between 3 and 7 mm. 6. Structure according to one of the preceding claims, wherein the volume or the surface of the input channels is greater than that of the output channels. 7. Filtering structure according to one of the preceding claims, wherein the porous walls have corrugations so as to be concave with respect to the center of the inlet channels and convex in their middle relative to the center of the outlet channels.