EP2547417A1

EP2547417A1 - Filtering structure, including plugging material

Info

Publication number: EP2547417A1
Application number: EP11713003A
Authority: EP
Inventors: Matthias Schumann
Original assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Current assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Priority date: 2010-03-19
Filing date: 2011-03-15
Publication date: 2013-01-23
Also published as: FR2957529B1; FR2957529A1; US20130011304A1; WO2011114050A1; JP2013522020A; CN102811789A

Abstract

The invention relates to a honeycomb structure that filters charged gases and particles, said structure being characterized in that: a) the filtering walls of said honeycomb structure are made of a material having, after curing, a mean heat expansion coefficient that is measured between 25° C and 1100° C and is less than 2.5 x 10^-6 K^-1, and b) the material making up the plugs includes a filler formed of refractive grains, the melting temperature of which is greater than 1500° C and the median diameter of which is between 5 and 50 microns, said material also including a vitreous bond phase.

Description

FILTERING STRUCTURE COMPRISING

The invention relates to the field of optionally catalytic filtering structures, in particular used in an exhaust line of a diesel type internal combustion engine.

Catalytic filters for the treatment of gases and the removal of soot from a diesel engine are well known in the prior art. These structures all most often have a honeycomb structure, one of the faces of the structure allowing the admission of the exhaust gas to be treated and the other side the evacuation of the treated exhaust gas. 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 walls. The ducts are closed at one or the other of their ends to delimit inlet chambers opening on the inlet face and outlet chambers opening along the discharge face. 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.

In known manner, during use, the particulate filter is subjected to a succession of filtration (soot accumulation) and regeneration (soot elimination) phases. During the filtration phases, the soot particles emitted by the engine are retained and are deposited inside the filter. During the phases of regeneration, the soot particles are burned inside the filter, in order to restore its filtration properties.

Most often, the filters are porous ceramic material, for example cordierite or silicon carbide.

Silicon carbide filters made with these structures are for example described in patent applications EP 816 065, EP 1 142 619, EP 1 455 923 or WO 2004/090294 and WO 2004/065088, to which the skilled person For example, reference may be made for more details and details, both for the description of filters according to the present invention and for their method of production. These filters advantageously have a high chemical inertness with respect to soot and hot gases but a coefficient of thermal expansion a little high, which leads, for the production of large filters, the need to assemble several monolithic elements by a cement joint or grouting in a filter block, in order to reduce their thermomechanical stresses. Due to the high mechanical strength of the recrystallized Si materials, it is possible to produce filters with thin filter walls and high porosity, with a very satisfactory filtration efficiency.

Cordierite filters have also been used for a long time because of their low cost. Due to the very low coefficient of thermal expansion of this material, in the normal operating temperature range of a filter it is possible to produce monolithic filters of larger size.

A material of the Titanate type of aluminum may also have an average coefficient of thermal expansion low, that is to say typically less than 2.5 × 10 ^-6 K ^-1 as measured according to the standards in force between 25 ° C and 1000 ° C. This material is also characterized by higher refractoriness and corrosion resistance than cordierite. It thus makes it possible to produce monolithic filters of large size, provided however that the thermal stability of the aluminum titanate is controlled, in particular during the regeneration phases of the filter. By thermal stability is meant the capacity of an aluminum titanate material, at high temperature, not to decompose into two distinct phases of titanium dioxide T1O ₂ and oxide of titanium. aluminum AI ₂ O ₃ , under normal conditions of use of a particulate filter.

Monolithic filters are thus described in the patent application WO 2004/011124, which proposes structures based on aluminum titanate for 60 to 90% by weight, reinforced with mullite, present at a level of 10 to 40% by weight. According to the authors, the filter thus obtained has improved durability. According to another embodiment, the patent application EP 1741684 describes a filter having a low coefficient of expansion and whose main phase of aluminum titanate is stabilized firstly by the substitution of a fraction of the Al atoms by Mg atoms. in the lattice Al ₂ Ti05 in a solid solution and secondly by substitution of a fraction of the Al atoms at the surface of said solid solution by Si atoms, provided in the structure by an additional intergranular phase of the type of potassium aluminosilicate and sodium, especially feldspar.

These monolithic structures are typically extruded and, before their cooking, closed at one and the other of their ends, most often by a similar material even identical to that constituting the filter walls, to define inlet chambers and outlet chambers as described above.

It turns out that the method of sealing or plugging with the materials usually used on both sides of an extruded structure, especially of large size, leads however to cracking of the filters especially in the zone corresponding to their bearing surface. on the baking rack. By large size is meant in particular structures of diameter greater than 100mm or section greater than 75 cm ² . Without this being considered definitively understood, these cracks would be due to constraints related to the difference of withdrawals within the structure between the clogged channels, that is to say before cooking the filter, and those not clogged. By the term "withdrawal", it is understood, in the sense of the present description, the difference, according to a dimension of the filter in question, for example the length of said dimension before and after baking. This phenomenon of removal of the material based on alumina titanate is often persistent at low temperature, that is to say at a temperature below 400 ° C, and especially at room temperature.

According to another alternative, it has also been proposed a method for plugging or sealing the channels of an already sintered structure. The advantage of such a process can be to save a plugging operation, especially in the case of disposal of the filter after cooking, due to the presence of defects related to cooking or previous process steps but only revealed when cooking. In addition, according to another advantage of such a method, the cooking of ceramic structures of honeycomb type seems much more homogeneous when its channels are not clogged. The departure of gases from debinding could thus be facilitated, thus reducing the risks of cracking related to debinding. Such a process would ultimately make it possible, from a mixture of precursor materials initially more loaded with porogenic agent, finally more porous structures, and thus reduce the pressure loss associated with the filter operating in a line of exhaust, or even more easily integrate an additional catalytic function of exhaust gas depollution in said filter by depositing a coating based on active metals.

However, the methods described in the field involving the capping step after sintering or firing of the structure are also unsatisfactory, as has been observed by the Applicant. In particular, cracks still appear between the plugs and the walls of the closed channels during the additional cooking of said plugs. This problem could be this time attributed to a difference in dilatometric behavior between the material constituting the plug and that of the walls. The solutions proposed to date, consist in adapting the corking mixture to that of the wall material, in particular in the sense of a standardization of the dilatometric behavior of the materials. Thus, in the applications US2006 / 0272306 and WO2009 / 073092, it is described as fundamental principle to obtain a dilatometric curve to the firing of the closure material close to that of the already sintered material and constituting the walls of the structure. The structures developed according to these principles show a satisfactory adhesion of the plugs to the walls after the first heat treatment for corking plugs, for example at 1000 ° C. under air. It has however been found by the Applicant that the adhesion of the plugs to the walls of the filtering structure degrades sharply as the soot combustion cycles in the filter operation. In particular, cracks have appeared with the use of such plugging materials, for example after 10 thermal cycles between 500 and 1100 ° C., on a filter placed in an exhaust line of a diesel engine. It has thus been observed on the vast majority of filters studied a crack between the plug and the wall as will be illustrated later in the present description. This phenomenon can lead to insufficient sealing and therefore to a filter which has a filtration efficiency too low in operation. If plugs are detached from the walls of the structure during operation in the exhaust line, the filter can become ineffective and even have to be changed.

The present invention is designed specifically to filters of which the filter walls are made of a material having a low coefficient of thermal expansion means, that is to say less than 2.5x10 ^-6 K ^{^-1,} such as measured between 25 and 1100 ° C, and at least a portion of the channels is closed after sintering or cooking the honeycomb. The object of the present invention is thus to provide a honeycomb filtering structure that makes it possible to respond to all of the problems previously described, and in particular to have improved stability of the plugs and their cohesion with the walls, particularly in during successive cycles of regeneration of the filter during its implementation in an automobile exhaust line.

More particularly, the research carried out by the applicant has revealed that contrary to the indications and principles provided in the documents already published, in particular in the applications US2006 / 0272306 and WO2009 / 073092, to obtain a structure as previously described, it was not appropriate to adapt the mixing mixture to that of the wall material, in the sense of a standardization of the thermal expansion coefficients of the materials, but on the contrary a large difference between said coefficients, under certain conditions, could allow to obtain an adhesion durable, under the classic conditions of use of a particulate filter.

The research carried out has notably demonstrated that other parameters could be taken into account in order to obtain a filtering structure obtained by plugging after cooking, satisfying the problems previously exposed.

In its most general form, the present invention thus relates to a filtering structure of particles-loaded gases, of the honeycomb type, comprising a set of longitudinal adjacent channels of mutually parallel axes separated by porous filtering walls. said channels being alternately plugged at one or other end of the structure so as to define inlet channels and outlet channels for the gas to be filtered, and to force said gas to pass through the walls porous separating the inlet and outlet channels, said structure being characterized in that:

a) the filtering walls of said honeycomb structure consist of a material having after firing an average coefficient of thermal expansion, measured between 25 and 1100 ° C, lower than 2.5.10 ^"6 K ^{" 1} , and

b) the material constituting the plugs comprises:

a charge formed of refractory grains whose melting temperature is greater than 1300 ° C., or even greater than 1500 ° C. and whose median diameter is between 5 and 50 microns,

a vitreous binder phase whose composition corresponds to the following formulation, in weight% of the oxides: Si0 ₂ : between 50 and 95%,

RO: between 0.1 and 15%, where RO represents an alkaline earth oxide or the sum of the alkaline earth oxides in the glassy phase,

R ₂ '0: between 0.1 and 10%, R ₂ ' 0 representing an oxide of an alkali or the sum of the alkali oxides in the glassy phase,

AI ₂ O ₃ : less than 20%,

B ₂ O ₃ : less than 10%,

MgO: less than 5%,

c) the average coefficient of thermal expansion (TDC) of said material constituting the plugs, measured between 25 and 1100 ° C, without stress is at least equal to 4, 8.10 ^{~ 6} . K ^-1 , preferably at least equal to 5, 0.10 ^{~ 6} . K ^-1 . Preferably, the CTE coefficient is further lower than 10.10 ^-6 .K ^_1.

In the present description, when speaking of the measurement of thermal expansion coefficients, unless otherwise indicated, it is conventionally measured without stress (or load) on the analyzed material.

By RO is meant an alkaline earth oxide R preferably selected from the group consisting of Ca, Sr or Ba, or the sum in weight percentage of the oxides CaO, SrO or BaO, in the preceding formulation, if said phase vitreous contains more than one alkaline earth.

By R ₂ '0 is meant an oxide of an alkali R' preferably selected from the group consisting of Na, K, or the sum in weight percentage of the oxides Na ₂ <0 or K ₂ O, in the previous formulation if said glassy phase comprises more than one alkali.

The coefficient of thermal expansion of the material constituting the walls is measured under air according to the dilatometry techniques well known to those skilled in the art, such as for example reported in standard NFB40308. Thermal expansion, expressed as a percentage, corresponds to an elongation (if the variation is positive) or to a shrinkage (if the variation is negative) of the material under the effect of the increase of the temperature. The rate of increase is generally between 1 and 10 ° C / minute, preferably of the order of 5 ° C / minute. The measurement is made typically with dilatometers well known to those skilled in the art such as those of the Adamel or Setaram type including in particular an enclosure for the rise in temperature, a pusher in contact with a test piece of the test material provided with a displacement sensor for recording the dimensional variations of the sample. When no stress is applied, only a slight force is exerted on the pusher to maintain contact with the specimen, the pressure on the specimen being much less than 0.05 MPa. The test piece can be machined if necessary so as to obtain satisfactory flatness and parallelism of the contact face and the opposite face. Ideally, these faces should not show visible defects and the difference between any two measurements of length taken at the sliding foot between the contact face and the opposite face should be less than 0.2 mm typically for an average length of between 10 at 50mm. Preferably the test piece is of square section, its diagonal being typically between 0.1 and 0.5 times its length. Preferably the pusher is made of dense alumina so as to avoid any reaction with the material to be tested and the section of the end of the pusher in contact with the test piece is at least as large as that of the test piece in order to ensure contact with the entire face of the test piece on the side of the pusher.

According to another possible aspect of the present invention, the average coefficient of thermal expansion of said material constituting the plugs, measured between 25 and 1100 ° C and this time under a load of 0.1 MPa (MegaPascal), is preferably at least equal to 4.5.10 ^{~ 6} . K ^-1 , preferably at least equal to 5, 0.10 ^{~ 6} . K ^-1 .

It has been found in the context of the present invention that a pressure of 0.1 MPa appeared representative of the counter-pressure exerted by the walls of the structure on the plug material when the filter is subjected to a temperature rise. , especially during the regeneration phases experienced by a filter operating in an exhaust line. The measurement of the coefficient of thermal expansion under such a load makes it possible, according to the invention, to select more finely the materials able to enter into the constitution of the materials according to the invention. Such a coefficient of thermal expansion of the sealing material, under a load of 0.1 MPa, is measured under air, for example on a specimen of the sealing material after firing under the same conditions as previously described, the pressure exerted by the pusher on the specimen, ie the pressure calculated with respect to the contact face of the specimen being this time of 0.1 MPa. The coefficient of thermal expansion under the load is determined in the same manner as previously described for the coefficient of thermal expansion in the absence of stress. The reference state of the measurement is the starting state of the specimen under the load before warming up. In order to obtain the best accuracy, the dimensional variations under load are preferably measured on a structural sample in the direction of the largest dimension of the specimen.

According to the invention, a material is preferably chosen to constitute the plugs whose shrinkage, measured between 25 and 1100 ° C. and under a load of 0.1 MPa, is less than 2.5%, preferably less than 2, 0%. The shrinkage under load of the plugging material can be easily determined by simple analysis of the dilatometric curve obtained by measuring the coefficient of thermal expansion under the previous load of the test piece and by direct reading of the shrinkage value after heating at 1100 ° C and return to ambient. According to the present invention, the shrinkage of the material conventionally represents the difference according to a dimension of the test piece of the ceramic material, preferably the largest, measured before and after the heat treatment, relative to the initial dimension of said specimen .

The constituents of the compositions of the vitreous binding phase according to the criteria of the present invention can in particular vary in the following proportions, as a percentage by weight of the oxides:

S1O ₂ : between 65 and 95%, preferably between 70 and 90%,

CaO: between 0.5 and 15%,

Na ₂ 0: between 0.05 and 10%,

CaO + Na ₂ <0: between 3 and 25%, for example between 10 and 20%,

AI ₂ O ₃ : less than 15%, preferably less than 10%, B ₂ O ₃ : less than 10%, preferably less than 5%,

MgO: less than 5%.

In particular, according to a first embodiment of the invention, the vitreous binder phase can comprise, as a percentage by weight of the oxides:

Si0 ₂ : between 70 and 85%, preferably between 75 and 80%,

B ₂ O ₃ : between 1 and 10%, preferably between 1 and 5%,

CaO: between 5 and 15%,

AI2O3: between 4 and 10%,

SrO + BaO: less than 1%.

In the previous formulation, good adhesion results have been obtained especially when R ' ₂ 0, in the sense of previously described, is less than 3%, or even less than 1.5% or even less than 1%.

According to another possible embodiment, the composition of the vitreous binder phase corresponds to the following formulation, in weight% of the oxides:

Si0 ₂ : between 80 and 90%

Na ₂ 0: between 3 and 10%

CaO: between 1 and 10%, preferably between 2 and 6% MgO: between 0.1 and 5%, preferably between 0.5 and 3% B ₂ O ₃ : less than 5%, preferably less than 2%

AI ₂ O ₃ : less than 2%, preferably less than 1% SrO + BaO: less than 1%

K ₂ O: less than 1%

According to a third possible embodiment, the composition of the vitreous binder phase corresponds to the following formulation, in weight% of the oxides:

Si0 ₂ : between 80 and 90%

Na ₂ <0: between 1 and 10%, preferably between 2 and 6% K ₂ 0: between 1 and 10%, preferably between 1 and 5% CaO: between 1 and 10%, preferably between 2 and 6%

SrO + BaO: between 3 and 10%, preferably between 5 and

10%

B ₂ O ₃ : less than 5%, preferably less than 2% Al ₂ O ₃ : less than 3%, preferably less than 2%. In the filtering structure according to the invention, the refractory grains consist of at least one material chosen from silicon carbide, alumina, zirconia, silica, titanium oxide, magnesia and aluminum titanate. , mullite, cordierite, aluminum titanate, preferably aluminum titanate or cordierite.

It is obvious that the capping material according to the invention can respond to all possible combinations between the different domains and initial values. and / or preferred constituents previously described and between the various possible combinations of the constituent elements of the capping material (composition of the grains and the glassy phase). In order not to unnecessarily burden the present description, all the possible combinations of said constituents are not described in the present description but they must however be considered as envisaged by the applicant in the context of the present description (in particular of two, three combinations or more) .

In addition, the material constituting the plugs of the first end and the material constituting the plugs of the second end may have a different chemical composition.

The present invention also relates to a catalytic filter obtained from a structure as previously described and by deposition, preferably by impregnation, of at least one supported or preferably unsupported active catalytic phase, typically comprising at least one precious metal such as Pt and / or Rh and / or Pd and optionally an oxide such as CeC> 2, ZrC> 2, Ce0 ₂ ^~ Zr0 ₂ for the treatment of polluting gases of CO or HC and / or NOx type and / or the combustion of soot. Such a filter finds particular application as a catalytic support in an exhaust line of a diesel or gasoline engine or as a particulate filter in a diesel engine exhaust line.

The present invention relates to an exhaust line, comprising a filtering structure as previously described.

In the present description, the following definitions are given:

At least a part of the channels is closed after sintering or cooking of the honeycomb. not all channels are closed after sintering. Thus the inlet channels can be closed before sintering the structure while the outlet channels are closed after sintering of the structure.

The term "material constituting the plugs" means that at least one plug of the filtering structure is constituted by this material.

By the term "based on", it is understood that said walls comprise at least 50% by weight and preferably at least 70% by weight, or even at least 90 or even 98% by weight of said material.

For the purposes of the present description, the term median diameter, or ds o, of a mixture of particles or of a set of grains, means the size dividing the particles of this mixture or the grains of this mixture into first and second equal populations. in volume, these first and second populations comprising only particles or grains having a size greater than or less than the median diameter respectively.

By the term "powder" is conventionally meant within the meaning of the present invention a set of grains or particles characterized by a size distribution or grain 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.

The present invention also relates to a method of manufacturing a structure as previously described, comprising the following main steps:

a) preparation of a composition based on the material constituting the structure and shaped, in particular by extruding through a die of said material, a honeycomb structure,

b) optionally drying said structure in air according to a technique chosen from hot air drying, drying by microwave drying, drying by lyophilization at a temperature below 130 ° C or a combination of said techniques,

c) firing said structure, optionally comprising an initial debinding step,

d) preparing a composition for obtaining a capping material as previously described and filling with said composition, channels of said fired structure,

e) heat treatment for baking plugs placed on the ends of the baked structure.

A conventional method of manufacturing a honeycomb structure according to the present invention is given below without it being considered as limiting a particular procedure.

In general, the material constituting the walls of the structures obtained according to the invention preferably has an open porosity of between 20% and 65%, and preferably between 35% 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 dso, by volume, of the pores constituting the porosity of the material is preferably between 5 and 30 microns, preferably between 8 and 25 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. Advantageously, the thickness of the walls is between 0.2 to 1.0 mm, preferably 0.2 to 0.5 mm. The number of channels in the filter elements is preferably between 7.75 and 62 per cm ² , said channels typically having a cross section of about 0.5 to 9 mm ² .

For example, said structure according to the invention can also be obtained from an initial mixture of grains based on aluminum titanate and / or cordierite. Advantageously, according to this mode, the aluminum titanate or cordierite-based powder has a median diameter of less than 60 microns.

Preferably, the walls of the structure are made of a porous ceramic material based on aluminum titanate. Said porous walls may also incorporate other phases or elements in minor proportions, that is to say generally any addition known to stabilize the main phase of the aluminum titanate type.

The manufacturing method according to the invention most often comprises a step of kneading the initial mixture of powders into a homogeneous product in the form of a paste, a step of extruding a raw product shaped through a suitable die so as to obtain monoliths of the honeycomb type, a drying step of the monoliths obtained, possibly an assembly step and a cooking step carried out under air or in an oxidizing atmosphere at a temperature not exceeding 1800 ° C. preferably not exceeding 1650 ° C.

The capping step, carried out after the cooking of the honeycomb monoliths, can be carried out according to the process described for example in US Pat. No. 4,557,773 or EP 1,500,482, for example. Clogging mixtures are mixtures of particles, dry or moist, suitable for mass. The caking or hardening of these mixtures after Clogging of the channels of the structure may result from drying or, for example, curing of a resin. Finally, the heating makes it possible to accelerate the evaporation of the water or of the residual liquid after curing.

All the refractory powders conventionally used as filler in the capping material, which comprise a mixture of refractory grains whose median diameter is between 5 and 50 microns, can be used, taking into account, of course, the composition of the material constituting the walls. filter. The refractory powders may for example be powders based on silicon carbide and / or alumina and / or zirconia and / or silica and / or titanium oxide and / or magnesia or mixed powders, in particular aluminum titanate or mullite. Preferably, the refractory powders are molten products. The use of sintered products is also possible. Preferably, the refractory powders represent more than 50%, preferably more than 70% of the mass of the dry mineral material of the capping mixture.

In a preferred embodiment, the capping mixture comprises at least one aluminum titanate powder which is at least 50% preferably at least 80% by weight of the particulate mixture. Even more preferably, the aluminum titanate powder is the only refractory powder used in the capping mixture.

A vitreous binder phase around the grains previously described and constituting the filler of the clogging material can be obtained from the melting of the corresponding precursor oxides Si0 ₂ , RO, R ^ O, B ₂ O ₃ , etc. introduced in a mixture in the appropriate proportions with said grains. The whole is brought to a temperature sufficient to form an essentially vitreous phase coating the grains of the charge, thus forming the constituent material of the plugs. Alternatively, it is also possible to use a glass powder of the desired final composition, that is to say as described above, directly in admixture with the filler, the assembly then being heated to obtain final capping material. The glass powder then used is preferably of median diameter between 5 and 50 microns.

The corking mixture also preferably comprises a temporary binder and / or chemical to promote its implementation, in particular the rheology adapted according to the capping process used.

These binders can be chosen from the following nonlimiting list:

organic temporary binders, such as resins, especially thermosetting resins, that is to say formed of at least one polymer convertible by heat treatment (heat, radiation) or ^{physico¬chemical} (catalysis, hardener) into infusible material and insoluble. The thermosetting resins thus take their final form at the first hardening, the reversibility being impossible. Thermosetting resins include in particular phenolic resins, silicone-based or epoxy resins, other temporary binders such as derivatives of cellulose or lignin, such as carboxymethylcellulose, dextrin, polyvinyl alcohols, polyethylene glycols,

chemical setting agents such as phosphoric acid, polyphosphates of alkali metals or alumino-phosphates, or sodium silicate and its derivatives,

inorganic binders, such as silica gels or silica in colloidal form; binder based on silica gel and / or alumina and / or zirconium chemical setting agents, such as

phosphoric acid, aluminum monophosphate, etc.

Closures made by sealing the structure after firing may also include other organic additions such as lubricants or plasticizers.

The capping mixture may optionally comprise a pore-forming agent, for example chosen from cellulose derivatives, acrylic particles, graphite particles and their mixtures, incorporated into a particulate capping mixture in order to create porosity to relax the particles. constraints on the walls and / or possibly lighten the filter. However, the amount must not be too high, for example it must be less than 25% by weight relative to the mineral composition of the capping mixture in order to have a sufficient seal.

The invention relates to a honeycomb particle filter having a structure as previously described, suitable for filtering the exhaust gas of a motor vehicle. Such a filter may comprise a single monolithic element or be obtained by the association, by bonding with a joint cement, of a plurality of monolithic elements in honeycomb. Such a filter may optionally comprise an outer coating applied for example after firing the structure before or after plugging of the channels. It preferably comprises particles and / or fibers of ceramic or of refractory material, chosen from oxides, in particular comprising Al ₂ O ₃ , SiO ₂ , MgO, TiO ₂ , ZrO ₂ , Cr ₂ O ₃ or any one of their mixtures, or even among the non-oxides, such as SiC, aluminum and / or silicon nitride, aluminum oxynitride, etc.

The invention and its advantages will be better understood on reading the nonlimiting examples which follow. In the examples, all the percentages are given by weight, in particular oxides.

Examples of realization:

a) production of a fused aluminum titanate powder:

In all the examples the percentages are given by weight. In a preliminary step, Aluminum Titanate was prepared from the following raw materials:

about 40% by weight of alumina with a purity ratio of Al ₂ O ₃ greater than 99.5% and a median diameter of ₅ o of 90 μιτι, sold under the reference AR75® by the company Pechiney,

about 50% by weight of titanium oxide in rutile form, comprising more than 95% of TiO 2,

about 1% of zirconia having a median diameter dso of about 120 μιτι, marketed by the company Europe Minerais,

^- about 5 ~ 6 weight of silica with a purity level in S1O ₂ greater than 99.5% and a median diameter dso of about 210 μιη, marketed by Sifraco,

^- About 4 ~ 6 weight of a magnesia powder with a MgO purity level greater than 98% and more than 80% of particles having a diameter of between 0.25 and 1 mm, marketed by the company Nedmag.

The mixture of the initial reactive oxides was melted in an electric arc furnace, under air, with an electric oxidizing step. The molten mixture was then cast into a CS mold so as to obtain rapid cooling. The product obtained is crushed and sieved to obtain powders of different size fractions. More specifically, the grinding and the sieving are carried out under conditions allowing the final obtaining of two particle size fractions:

a particle size fraction characterized by a median diameter dso substantially equal to 50 microns, referred to as the coarse fraction according to the present invention,

a particle size fraction characterized by a median diameter dso substantially equal to 30 microns, referred to as the intermediate fraction according to the present invention,

a particle size fraction characterized by a median diameter dso substantially equal to 1.5 microns and referred to as the fine fraction according to the present invention.

For the purpose of the present description, the median diameter dso denotes the diameter of the particles, measured by sedigraphy, below which 50% by volume of the population is found. Microprobe analysis shows that all the grains of the melt phase thus obtained have the following composition, as a percentage by weight of the oxides (Table 1):

Tab water 1 b) production of cooked monoliths

At first, we synthesized a series of dry raw monoliths as follows:

In a kneader, powders are mixed according to the following composition:

100% of a mixture of two aluminum titanate powders previously produced by electrofusion, approximately 75% of a first powder with a median diameter of 50 μm and 25% of a second powder with a median diameter of 1.5 μm.

We add, with respect to the total mass of the mixture:

- 4% by weight of an organic binder of the cellulose type,

15% by weight of a blowing agent,

5% of plasticizer derived from ethylene glycol,

- 2% lubricant (oil),

0.1% surfactant,

approximately 20% of water so as to obtain, according to the techniques of the art, a homogeneous paste after kneading whose plasticity allows the extrusion through a die of a honeycomb structure which after cooking presents the dimensional characteristics according to Table 2.

The green microwave monoliths are then dried for a time sufficient to bring the water content not chemically bound to less than 1 ~ 6 by mass.

The dry raw monoliths are then cooked, without the channels being blocked, under air progressively until reaching a temperature of 1450 ° C which is maintained for 4 hours.

Table 2 The porosity characteristics were measured by high-pressure mercury porosimetry analyzes carried out with a Micromeritics 9500 porosimeter. EXAMPLE 1

The baked monoliths are then capped at each of their ends, according to the classical chessboard configuration (every other channel), with a corking mixture corresponding to the following formulation (in parts by weight): 100 parts of a mixture of an aluminum titanate powder previously produced by electro-fusion, ground in such a way that its median grain diameter is equal to 30 μm,

31 parts of Elkem 971U silica,

25 parts of the sodoboric glass powder FX300 sold by the company Reidt, the median diameter of which is 22 μm and the chemical composition of which is given in Table 3,

1.5 parts of organic binder of the cellulose type,

0.6 parts of carboxylic acid dispersant,

- about 45 parts of water.

The monoliths whose channels are alternately plugged in a conventional chessboard pattern, are then subjected to a heat treatment to a final temperature of 1100 ° C, which is maintained for 1 hour.

The experimental protocol of Example la is identical to that of Example 1 but differs only in that the filler is a cordierite powder of substantially the same particle size.

Example 2 and 2a:

In contrast to Example 1, the fired monoliths are plugged on the side of the end or the bearing face on the cooking medium using a corking mixture corresponding to the following formulation (in parts by weight):

100 parts of a mixture of aluminum titanate powder previously carried out by electrofusion, with a median diameter of 30 μm,

31 parts of Elkem 971U silica,

25 parts of the soda-lime glass powder ST300 marketed by the company Reidt, the median diameter of which is 22 μm and the chemical composition of which is given in Table 3,

1.5 parts of organic binder of the cellulose type,

0.6 parts of carboxylic acid dispersant,

- about 45 parts of water.

Example 2a differs from Example 2 only in that the load is this time obtained from the cordierite powder of Example 1a.

Example 3 and 3a:

In contrast to Examples 1 and 2, the cooked monoliths are plugged on the side of the end or of the support surface on the cooking support using a corking mixture corresponding to the following formulation (in parts weight):

31 parts of Elkem 971U silica,

25 parts of the Barium-Strontium-sodium-potassium potassium powder sold by the company Reidt, of which the median diameter is 22 μm and the chemical composition of which is given in Table 3,

1.5 parts of organic binder of the cellulose type,

0.6 parts of carboxylic acid dispersant, about 45 parts of water.

Example 3a differs from Example 3 only in that the load is this time obtained from the cordierite powder of Example 1a. Example 4 and 4a:

Unlike the previous examples, the baked monoliths are plugged on the side of the end or of the support surface on the baking support using a corking mixture with a corking mixture corresponding to the following formulation (in parts weight):

31 parts of Elkem 971U silica,

25 parts of the calcoalumino-boric glass powder HK300 sold by the company Reidt, whose median diameter is 22 μm and the chemical composition of which is given in Table 3,

1.5 parts of organic binder of the cellulose type,

0.6 parts of carboxylic acid dispersant,

- about 45 parts of water.

Monoliths whose channels are alternately plugged according to a traditional chessboard diagram, are then subjected to a heat treatment up to a final temperature of 1100 ° C, which is maintained for 1 hour.

Example 4a differs from Example 4 only in that the load is this time obtained from the cordierite powder of Example 1a.

The average coefficient of thermal expansion of the material constituting the walls of the monolith was measured, on a bar of said fired material, of dimension lcm> <2.5mm> <2.5mm, under air at a rate of temperature rise of 5 ° C / min until reaching a temperature of 1100 ° C, using a Setaram dilatometer. The determination of the coefficient of thermal expansion of the material constituting the walls is carried out from room temperature (25 ° C.) to 1100 ° C.

The coefficient of unloaded and underloaded thermal expansion of the sealing material was measured on a strip of dimensions 0.7 cm × 0.7 cm × 15 mm made with the sealing material, after the latter has undergone heat treatment beforehand. temperature of 1100 ° C, for 1 hour, so as to obtain a sintered sealing material representative of the plugs constituting the filters according to the preceding examples.

The measurement of the coefficient was made from the ambient temperature at 1100 ° C. according to a rate of rise in temperature of 5 ° C./min under air by means of a vertical dilatometer (of the Setaram type). Without load, the sensor represents a pressure of less than 0.05 MPa so as to ensure constant contact with the specimen during the test. When a load was applied, it corresponded to a stress of 0.1 MPa applied in a direction according to the largest dimension of the test piece.

The adhesion of the monolith plug was evaluated on the plugged structure after thermal treatment of plugs. A first evaluation was made first on the initial filtering structure, before any thermal treatment, by observation, under a scanning electron microscope, of the cap / wall interface of the monolith on a polished sample, in a longitudinal section. It is notably noted the presence (or absence) of microcracks or discontinuity of the structure at said interface. All the samples according to Examples 1 to 4 and 4a indicate satisfactory adhesion initially (that is to say before thermal cycling), corresponding to a perfect continuity of material at the plug / wall interface.

The durability of the adhesion of the plugs to the monolith was then evaluated by subjecting the filters tested several thermal cycles, representative of the most stringent conditions of use of a filter arranged in an exhaust line. Each cycle corresponds to heating between 500 ° C and 1100 ° C with a ramp of 5 ° C / min and a return to 500 ° C. The cycle is repeated 10 times.

As shown in Table 3, the filters according to Examples 1, 1a, 2a and 3a comprise plugs made of a material not in accordance with the present invention. In particular, it can be seen in the data reported in Table 3 that the coefficient of thermal expansion (CDT) of these materials, at ambient pressure and without stress, is less than 4.8><10 ^{~ 6} K ^-1 . Similarly, all these materials have, under a load of 0.1 MPa, a CDT value of less than 4.5><10^"6 K ^{" 1} .

After the durability test, a scanning electron microscope observation of the channel / plug interface of the filters according to the non-compliant examples 1, 1a, 2a, 3a, revealed the presence of cracks. between wall and cork on all samples tested.

As shown in Table 3, the filters according to Examples 2, 3, 4 and 4a comprise plugs made of a material according to the present invention. In particular, it can be seen in the data shown in Table 3 that the coefficient of thermal expansion (CDT) of these materials, at ambient pressure, is greater than 4.8><10 ^{~ 6} K ^-1 when the measurement is made without stress and greater than 4.5><10 ^{~ 6} K ^-1 under a load of 0.1 MPa.

After the durability test, a scanning electron microscopic observation of the channel / plug interface of the filters according to Examples 2, 3, 4 and 4a in accordance with the invention has demonstrated a continuity of material between the wall and the stopper on all the tested samples, that is to say an adhesion between the stoppers and the walls. In particular, only the monoliths according to the invention, for which the composition of the glass and the filler component of the plug material have been chosen to obtain a coefficient of average thermal expansion, at ambient pressure, greater than 4.8 × 10 ^-6. K ^-1 , have a quite remarkable adhesion, after successive annealing.

We also see that the glass formulation described in Examples 4 and 4a is particularly advantageous because it leads to a satisfactory adhesion after the durability test, regardless of the chemical nature of the load used (cordierite or aluminum titanate).

A formulation of plugs according to the compositions described in the prior documents, for example according to US2006 / 027306, characterized by low coefficients of thermal expansion of the material forming the plugs, particularly close to those of aluminum titanate constituting the walls of the structure, has also been subjected to the durability test described above. As shown in Table 3, an electron microscope observation revealed the presence of cracks between the walls and plugs on the samples tested.

* CaO + MgO ** Aluminum Titanate Table 3

Claims

1. Filtering structure of particles-loaded gases, of the honeycomb type, comprising a set of adjacent longitudinal channels of mutually parallel axes separated by porous filtering walls, said channels being alternately plugged to one or the other other ends of the structure so as to define inlet channels and outlet channels for the gas to be filtered, and so as to force said gas to pass through the porous walls separating the inlet and outlet channels, said structure characterized in that:

b) the material constituting the plugs comprises:

a filler formed of refractory grains whose melting temperature is greater than 1500 ° C. and whose median diameter is between 5 and 50 microns,

a vitreous binder phase whose composition corresponds to the following formulation, as a weight percentage of the corresponding oxides:

Si0 ₂ : between 50 and 95%,

AI ₂ O ₃ : less than 20%,

B ₂ O ₃ : less than 10%,

MgO: less than 5%.

2. Filtering structure according to claim 1, wherein the average thermal expansion coefficient of said material constituting the plugs, measured between 25 and 1100 ° C and without stress, is at least equal to 4, 8.10 ^{~ 6} . K ^-1 , preferably at least equal to 5, 0.10 ^{~ 6} . K ^-1 .

3. Filtering structure according to one of claims 1 or 2, wherein the average coefficient of thermal expansion of said material constituting the caps, measured between 25 and 1100 ° C under a load of 0.1 MPa, is at least 4 , 5.10 ^{~ 6} . K ^-1 , preferably at least equal to 5, 0.10 ^{~ 6} . K ^-1 .

4. filtering structure according to one of the preceding claims, wherein the shrinkage of the material constituting the caps, measured between 25 and 1100 ° C under a load of 0.1 MPa, is less than 2.5%, preferably less than 2.0%.

5. Filtering structure according to one of the preceding claims, wherein the composition of the glassy binder phase corresponds to the following formulation, in weight% of the oxides:

S1O ₂ : between 65 and 95%, preferably between 70 and 90%,

CaO: between 0.5 and 15%,

Na ₂ 0: between 0.05 and 10%,

CaO + Na ₂ O: between 3 and 25%,

AI ₂ O ₃ : less than 15%, preferably less than 10%,

B ₂ O ₃ : less than 10%, preferably less than 5%,

MgO: less than 5%.

6. Filtering structure according to one of the preceding claims, wherein the composition of the glassy binder phase corresponds to the following formulation, in weight% of the oxides: S1O ₂ : between 70 and 85%, preferably between 75 and 80%, B ₂ O ₃ : between 1 and 10%, preferably between 1 and 5%,

CaO: between 5 and 15%,

AI2O3: between 4 and 10%,

SrO + BaO: less than 1%.

7. Filtering structure according to the preceding claim, wherein R ' ₂ O is less than 3%.

8. Filtering structure according to one of claims 1 to 4, wherein the composition of the glassy binder phase corresponds to the following formulation, in weight% of the oxides:

Si0 ₂ : between 80 and 90%

Na ₂ 0: between 3 and 10%

CaO: between 1 and 10%, preferably between 2 and 6%

MgO: between 0.1 and 5%, preferably between 0.5 and 3% B ₂ O ₃ : less than 5%, preferably less than 2%

AI ₂ O ₃ : less than 2%, preferably less than 1% SrO + BaO: less than 1%

K ₂ O: less than 1%

9. Filter structure according to one of claims 1 to 5, wherein the composition of the glassy binder phase corresponds to the following formulation, in weight% of the oxides:

Si0 ₂ : between 80 and 90%

a ₂ 0: between 1 and 10%, preferably between 2 and 6%

K ₂ 0: between 1 and 10%, preferably between 1 and 5%

CaO: between 1 and 10%, preferably between 2 and 6%

SrO + BaO: between 3 and 10%, preferably between 5 and 10% B ₂ O ₃ : less than 5%, preferably less than 2%

AI ₂ O ₃ : less than 3%, preferably less than 2%.

10. Filtering structure according to one of the preceding claims, wherein the refractory grains are constituted by at least one material selected from silicon carbide, alumina, zirconia, silica, titanium oxide, magnesia, aluminum titanate, mullite, cordierite, aluminum titanate, preferably aluminum titanate or cordierite.

11. Filter structure according to one of the preceding claims, wherein the porous walls are made of a material based on aluminum Titanate or Cordierite.

12. Filtering structure according to one of the preceding claims, wherein the material constituting the plugs of the first end and the material constituting the plugs of the second end have a different chemical composition.

13. Filtering structure according to one of the preceding claims further comprising a supported catalytic phase supported or preferably unsupported, typically comprising at least one precious metal such as Pt and / or Rh and / or Pd and optionally an oxide such as CeO2 , rO2, CeO2 ^~ Zr0 ₂ .

14. Exhaust line, comprising a filter structure according to one of the preceding claims.

15. Method of manufacturing a structure according to one of the preceding claims, comprising the following main steps: a) preparation of a composition based on the material constituting the structure and shaped, in particular by extrusion through a die of said material, of a honeycomb structure,

c) firing said structure, optionally comprising an initial debinding step,

d) preparing a composition for obtaining a sealing material according to one of claims 1 to 9 and filling with said composition, channels of said fired structure,

e) heat treatment for baking plugs placed on the ends of the baked structure.