HK1084906A1 - Exhaust gas treatment device and method for making the same - Google Patents

Description

Exhaust gas treatment device and method for manufacturing same

Technical Field

The present invention provides a device for treating exhaust gas, such as a catalytic converter or a diesel particulate trap, having a fragile structure mounted within a housing, supported by one of the mounting mats disposed between the housing and the fragile structure.

Background

Catalytic converter assemblies for treating automotive or diesel exhaust contain a fragile structure, such as a catalyst support structure, for holding a catalyst that effects oxidation of carbon monoxide and hydrocarbons and reduction of nitrogen oxides. The fragile structure is mounted within a metal housing, preferably made of a fragile material, such as a monolithic structure made of metal, or a fragile, refractory ceramic material such as alumina, silica, magnesia, zirconia, cordierite, silicon carbide, and the like. These materials provide a structural skeleton type with numerous tiny flow channels. However, as noted above, these structures can be and often are quite fragile. In fact, these monolithic structures can be so brittle that small impact loads or stresses are often sufficient to fracture or shatter them.

The fragile structure is contained within a metal housing with a space or gap between an outer surface of the fragile structure and an inner surface of the housing. In order to protect the fragile structure from thermal and mechanical shock and other stresses mentioned above, while providing thermal insulation and gas sealing, it is known to provide at least one ply or mounting layer or support material in the gap between the fragile structure and the housing. For example, the disclosures of assignee's U.S. patent nos.4863700, 4999168, 5032441, 5580532, 5666726 and 6231818, all of which are incorporated herein by reference, disclose catalytic converter devices having a mounting or support material disposed in a gap between a housing and a fragile structure contained within the device to protect the fragile structure and hold it in place within the housing.

Currently, the materials used in mounting mats for catalytic converters and other exhaust gas treatment devices may be selected from relatively inexpensive materials, such as amorphous glass fibers, e.g., S-glass, and more expensive materials, such as high alumina ceramic oxide fibers. Intumescent as well as non-intumescent materials have been used and continue to be used in mounting mats, depending on the application and conditions in which the mounting mat is used.

The type of monolithic structure to be used, as well as the application and conditions of use of the mounting mat, must be determined prior to the selection of the mounting mat material. For example, while high temperature resistant mounting mat materials have been used that can withstand a wide range of high temperature applications, such as typically used in catalytic converters, lower temperature resistant, resilient, flexible materials may be used or more suitable for use in high G-loading applications, which use heavier substrates, such as those used in diesel catalyst structures and diesel particle traps.

In any event, the mounting mat material used should be capable of meeting the physical requirements set forth by any of a variety of designs or by the fragile structure manufacturer or catalytic converter manufacturer. For example, one or more layers of prior art mounting mat materials should preferably exert an effective residual holding pressure on the fragile structure even when the catalytic converter has experienced a wide range of temperature fluctuations, which results in significant expansion and contraction of the metal housing relative to the fragile structure, also referred to as the catalyst support structure, and thus in significant cycles of compression and release of the mounting mat over a period of time. Best, prior art mounting mats for high temperature applications have been found to be adequate to maintain fragile structures in the most demanding applications where temperatures reach above 900 ℃ and thermal cycling to room temperature often occurs.

Other mounting mats that need not be used in high temperature environments must have sufficient resiliency and flexibility to effectively retain the fragile structure with sufficient force or strength, but not crush the fragile structure during continued thermal cycling. Under normal operating conditions of the catalytic converter, the mounting mat requires a minimum shear strength of at least 5kPa to prevent fragile structures from being dislodged or damaged. The shear strength of the mounting mat is defined as the holding pressure of the mat multiplied by the coefficient of friction at the mat/fragile structure interface. The friction coefficient of a general mat in a catalytic converter in a use state is about 0.45. Thus, a mounting mat in high temperature applications, i.e., applications in which the temperature in a catalytic converter may rise to about 900 ℃ or more, should have an effective residual minimum holding pressure after 1000 test cycles at a heating surface temperature of about 900 ℃ under at least about 10 kPa.

For other exhaust gas treatment devices, such as diesel particle traps or diesel catalyst structures, it is understood that when these devices do not reach temperatures in high temperature catalytic converters, the weight of the fragile structure and the loading technique used require that the mounting mat used have a different effective residual minimum holding pressure than mentioned above. In these applications, it is preferred that the mounting mat achieve a relatively high minimum shear strength of at least about 25kPa to prevent the fragile structure from being dislodged or damaged. In such high G-load applications with heavy substrates, the coefficient of friction of the mounting mat product remains around 0.45 in use. Thus, the mounting mat in this type of application should have an effective residual minimum holding pressure after 1000 test cycles at a temperature of at least about 50kPa to about 300 ℃.

To date, many mounting mats have attempted to overcome the thermal cycling problems associated with high temperature applications by using high alumina or mullite ceramic fibers. In one known embodiment, ceramic fibers are made using an aqueous solution or a colloidal dispersion commonly referred to as an "organosol" or "sol-gel". While ceramic fibers formed by sol-gel processes can provide the high elasticity required to mount monolithic structures, the high cost of such fibers has forced manufacturers to find other less expensive solutions. In addition, these ceramic fibers generally have an average fiber diameter of less than 5 microns, and in some cases less than 3.5 microns. Thus, these fibers are respirable, i.e., inhaled into the lungs.

In other examples, fibrous mounting materials may be used in combination with other materials, such as intumescent materials and cushioning layers, in order to provide sufficient strength for maneuverability, resiliency, or to obtain sufficient holding pressure.

As an alternative to using sol-gel derived ceramic fibers, attempts have been made to form refractory ceramic fibers using melt processing techniques. Refractory ceramic fibers, i.e., fibers containing from about 45% to 60% alumina and from about 40% to about 55% silica, have been found to be satisfactory to high temperature catalytic converter manufacturers in only the last approximately decade or so by providing a mounting mat with sufficient elastic values to meet the needs of the manufacturer. However, mounting mats comprising such refractory ceramic fibers are not only expensive, but also difficult to manufacture, especially as they have to be subjected to process treatments. Care must be taken to ensure that it is substantially free of slag. One such application of refractory ceramic fibers as a high temperature catalytic converter mounting mat and the desired process to provide an adequate product has been disclosed in U.S. patent No. 5250269.

In low temperature catalytic converter applications, such as Turbocharged Direct Injection (TDI) diesel-powered vehicles, the exhaust gas temperature is typically around 150 ℃ and does not exceed 300 ℃. Different types of mounting mats may be used in these and other slightly elevated temperature applications. For many catalytic converter applications, intumescent mats, i.e. mounting mats made of an intumescent material such as graphite or vermiculite, have been used. It has recently been found that mounting mats made of intumescent materials may fail in these low temperature applications.

One possible reason for such failure is that the exhaust gas temperature may be too low to adequately expand typical intumescent vermiculite particles. Thus, such pads do not provide sufficient pressure against the fragile structure so as to tend to fail. Another possible cause is degradation of the organic binder system used in the intumescent mat and loss of holding pressure.

Accordingly, non-intumescent mat systems have been developed and are currently in common use in the industry. These materials are suitable for a wider temperature range than the prior art intumescent mats.

Non-intumescent mat systems are substantially free of intumescent materials such as graphite or vermiculite and, therefore, are substantially non-intumescent. The term "substantially non-intumescent" means that the intumescent mat does not readily expand in the thermal application for which the mat is intended. Of course, some degree of expansion of the pad will also occur based on its coefficient of thermal expansion, but the amount of such expansion is not significant and is minimized as compared to the expansion of a pad using an effective amount of the intumescent material. Heretofore, these non-intumescent mats have included high temperature resistant inorganic fibers, and optionally a binder. By high temperature resistant is meant that the fiber can be used at temperatures up to about 1260 ℃. Heretofore, non-intumescent mats have been known to generally comprise one or more types of fibers selected from alumina/silica (a product available from UnifraxCorporation, Niagara Falls, New York under the trade name FIBERFRAX), alumina/silica/magnesia (e.g., S2 glass by Owens Corning, Toledo, Ohio), depending on the application, the temperature range in which the mat is applied, and the type of monolith used.

Currently, the fibers used in prior art non-intumescent mounting mats for high temperature applications are typically high in alumina content. For example, refractory ceramic fibers consist essentially of alumina and silica, and typically contain about 45% to about 60% by weight alumina and about 40% to about 55% by weight silica. While other alumina/silica ceramic fibers, such as alumina or mullite ceramic fibers obtained by sol-gel processes, typically comprise greater than 50% alumina. S2-glass fibers generally include about 64% to about 66% silica, about 24% to about 25% alumina, and about 9% to about 10% magnesia. In general, it is believed that the more alumina that is used in the fiber, the higher the temperature at which the fiber can be used. For this reason, it has therefore been proposed to use fibers consisting essentially of alumina.

To avoid the high cost associated with the use of sol gel-derived, alumina-containing ceramic fibers, some mounting mat manufacturers have reverted to expensive pre-processing steps, such as roll bonding of the materials prior to mounting the mounting mat. Such roll bonding techniques, however, cannot be used in all catalytic converter applications. Other non-intumescent mounting mats are typically very thick and lack the required structural integrity and may even require treatment in a bag to prevent the mounting mat from breaking. These mounting mats are also difficult to cut to a size suitable for installation and must be further compressed until sufficient material is fitted in the gap between the catalyst support structure and the housing to support the installation.

The use of other types of materials has also been attempted in the manufacture of non-intumescent mounting mats for catalytic converters and other exhaust gas treatment devices for high temperature applications. For example, U.S. patent No.5380580 discloses a flexible, non-woven mounting mat comprising slag-free ceramic oxide fibers containing aluminosilicate fibers containing about 60% to about 85% by weight alumina and about 40% to about 15% by weight silica; a crystalline quartz fiber; or a combination of both. The alumina content of the aluminosilicate fiber is higher than that of the refractory ceramic fiber, but the aluminosilicate fiber is prepared by the sol-gel technology. On the other hand, crystalline quartz fibers are made of substantially pure silica (i.e., 99.9% silica). These fibers are made by a melt-drawing process using raw materials extracted from crystalline quartz, without leaching in any way. Such fibers are available under the trade name astroqurtz from j.p. stevens, Slater, New York or under the trade name QUARTZEL from Saint Gobain, Louisville, Kentucky. However, the cost of these quartz fibers has commercially hindered their use in mounting mats.

Similarly, U.S. patent No.5290522 discloses a nonwoven mounting mat for a catalytic converter that may comprise magnesia/alumina/silica fibers as are well known in the art and available from Owens Corning, Toledo, Ohio as S2-GLASS and ASTROQUARTZ quartz fibers as discussed in the above-referenced patents. In this patent it is clearly noted in comparative example 1 that a mounting mat containing commercially available leached glass fibers containing silica cannot pass the thermal shock test conducted by the patentee to determine a mounting mat suitable for use in a high temperature catalytic converter.

Mounting mats containing silica fibers in combination with intumescent materials were tested for use in catalytic converters, for example as described in german patent publication No. 19858025.

The preparation of leached glass fibers having a high silica content is described in detail in U.S. patent No.2624658, the disclosure of which is incorporated herein by reference in its entirety. Another method for producing leached glass fibers having a relatively high silica content is disclosed in european patent application publication No. 0973697. Although U.S. and european patent application publications disclose the preparation of leached glass fibers for high temperature resistant products made from synthetic fibers, there is no mention of any suitable fibers, or fibers that can be used as a mounting mat for an exhaust gas treatment device, such as a catalytic converter.

Disclosure of Invention

Broadly, melt-drawn, leached glass fibers having a relatively high silica content are used to form non-intumescent mounting mats for catalytic converters and other exhaust gas treatment devices. In certain embodiments, it has been found that heat treating the silica-containing leached glass fibers or mounting mats comprising the same prior to placement in a catalytic converter further enhances the holding pressure performance of the mounting mat.

Generally, an exhaust gas treatment device includes a housing: a fragile structure resiliently mounted within the housing; and a non-intumescent mounting mat disposed in the gap between the housing and the fragile structure, wherein the mounting mat comprises leached glass fibers formed by melting comprising at least 67 weight percent silica and applying one of the following minimum holding pressures for holding the fragile structure within the housing (i) at least about 10kPa, the gap bulk density being from about 0.3 to about 0.5g/cm after 1000 test cycles at a hot face temperature of about 900 ℃³A gap expansion of about 5%, or (ii) a gap bulk density of from about 0.3 to about 0.5g/cm after 1000 test cycles at a heating surface temperature of about 300 ℃ of at least about 50kPa³The gap expansion rate was about 2%.

Also provided is a method of making an exhaust gas treatment device comprising providing a mounting mat comprising silica-containing glass fibers formed by melting, wherein the melt-formed glass fibersGlass fibers are made by treating melt-drawn glass fibers, whereby the treated glass fibers have a silica content greater than the silica content of the glass fibers prior to treatment, and whereby the treated glass fibers contain at least 67 weight percent silica; wrapping the mounting mat around a fragile structure suitable for treating exhaust gases; and disposing the fragile structure and the mounting mat into the housing such that the mounting mat resiliently retains the fragile structure within the housing, wherein the mounting mat applies one of (i) at least about 10kPa, a gap bulk density of from about 0.3 to about 0.5g/cm after 1000 test cycles at a hot face temperature of about 900 ℃ to a minimum holding pressure for retaining the fragile structure within the housing³A gap expansion of about 5%, or (ii) a gap bulk density of from about 0.3 to about 0.5g/cm after 1000 test cycles at a heating surface temperature of about 300 ℃ of at least about 50kPa³The gap expansion rate was about 2%.

Drawings

FIG. 1 is a partial perspective view of a catalytic converter incorporating a mounting mat according to the present invention.

Detailed Description

A device for treating exhaust gas having a fragile structure mounted within a housing and supported by one of the mounting mats disposed between the housing and the fragile structure. It will be appreciated that the invention is not intended to be limited to use in the catalytic converter shown in the drawings, the shape of which is shown merely as an example to illustrate the invention. In fact, the mounting mat may be used to mount or support any fragile structure suitable for treating exhaust gases. Such as diesel catalyst structures, diesel particle traps, etc. The catalyst structure generally comprises one or more porous tubular or honeycomb structures, mounted in a housing by a heat resistant material. Each structure may include from about 200 to about 900 or more flutes or pockets per square inch of area, depending on the type of exhaust treatment device. The diesel particle trap differs from the catalyst structure in that each recess or pocket in the particle trap is closed at one end or the other. The particles are collected from the exhaust gas in the porous structure until regeneration by a high temperature combustion process. Non-automotive applications of the mounting mat of the present invention may include catalytic converters for chemical emission (exhaust) stacks. The term "fragile structure" refers to a structure comprising, for example, a metal or ceramic monolith or the like that is fragile or brittle in nature and can benefit from a mounting mat as described herein.

One representative form of an apparatus for treating exhaust gas is the catalytic converter shown and generally indicated by the numeral 10. The catalytic converter 10 may include a generally tubular housing 12, the housing 12 being made of two pieces of metal, such as high temperature resistant steel, held together by a flange 16. Alternatively, the housing may comprise a preformed cylinder into which the catalyst support structure surrounded by the mounting mat may be inserted. The housing 12 includes an inlet 14 at one end and an outlet (not shown) at the opposite end. The inlet 14 and the outlet have suitable structures at their outer ends, whereby they can be fastened to a conduit in the exhaust system of an internal combustion engine. The device 10 comprises a fragile catalyst support structure, such as a fragile ceramic monolith 18, which is supported and restrained within the housing 12 by a mounting mat 20 to be described below. The monolith 18 comprises a plurality of gas permeable channels extending axially from an inlet end surface at one end thereof to an outlet end surface at the opposite end thereof. The monolith 18 may be made of any suitable refractory metal or ceramic material in any manner and configuration known in the art. The cross-sectional configuration of the single piece is generally oval or circular, but other shapes are possible.

There is a distance or gap between the monolith and its housing that varies depending on the type and design of the device used, such as a catalytic converter, diesel catalyst structure, or diesel particle trap. A mounting pad 20 is filled in the gap to provide resilient support for the ceramic monolith 18. The resilient mounting mat 20 provides thermal insulation to the external environment and mechanical support to the catalyst support structure, protecting the fragile structure from mechanical impact.

In some embodiments, the mounting mat 20 comprises one or more melt-formed, high temperature resistant, amorphous leached glass fiber non-intumescent layers having a relatively high silica content, and optionally contains a binder, or other fibers suitable as a binder. The term "silica-rich" means that the amount of silica in the fiber is greater than the amount of any other component. Indeed, as discussed below, it will be appreciated that the silica content of these fibers after leaching is preferably higher than any other silica-containing glass fiber, including S-glass fibers, except for crystalline silica derived fibers or pure silica fibers.

The mounting mat is typically a unitary, substantially non-intumescent composite sheet or a thin layer of fusion-formed leached glass fibers containing silica and optionally minor amounts of alumina and other non-silica materials. The term "melt-formed" means that the fibers are made using melt processing techniques, rather than from a gel sol or other chemical dispersion techniques. The term "monolithic" means that the mounting mat has a self-supporting structure after manufacture and densification without the need for reinforcement or the addition of fiber, plastic or paper layers (including structures roll bonded to the mat) and without the need for disintegration to allow handling and manipulation. The term "substantially non-distending" is used as an illustration of the foregoing. Thus, it will be appreciated that in one embodiment, the mounting mat is free of intumescent materials, sol gel derived glass silica fibers and/or reinforcing layers.

As mentioned above, the glass fibers are preferably treated to increase the silica content of the fibers. That is, when the fibers are first melt processed, such as by melt drawing the fibers and producing fibers, these glass fibers generally include many non-silicon oxides and other components. That is, they may have the characteristics of, for example, fiberglass. They were not originally formed from pure silica fibers such as fibers derived from crystalline quartz as disclosed in U.S. patent nos. 5290522 or 5380580. Rather, these "impure" glass fibers must be treated to remove non-silicon oxides, such as alumina, sodium oxide, boron oxide and any other water or acid soluble components present, thereby producing fibers with a higher silica content than that of the glass fibers prior to treatment. The silica content of the resulting leached glass fibers depends on the amount of non-silica oxides and other components initially present and the extent to which these materials are extracted from the fibers.

Leaching is a preferred method of treating glass fibers to increase the silica content of the fibers. The glass fibers may be leached in any manner and using any technique known in the art. Typically, leaching can be performed by placing the melt-formed glass fibers in an acid solution or other solution suitable for extracting non-silicon oxides and other components from the fibers. As previously mentioned, a more detailed description of various known leaching techniques is described in U.S. patent application No2624658 and European patent application publication No.0973697, but such techniques are not limited thereto.

The silica purity of these glass fibers after leaching is much higher than before leaching. Typically, leached glass fibers have a silica content of at least 67 weight percent. This is higher than the silica content of S-glass. More preferably, the leached glass fibers comprise at least 90 weight percent, and even more preferably, from about 90 weight percent to less than 99 weight percent silica. It can be understood that: these fibers have a higher silica content than any other known silica-containing glass fibers, including S-glass fibers, except for quartz fibers or pure silica fibers containing greater than 99.9% silica.

In some embodiments, the glass fibers comprise from about 93 weight percent to about 95 weight percent silica with the balance being non-silicon oxides, such as alumina, sodium oxide, and other alkali or alkaline earth metal oxides. The alumina content is preferably about 4 to 6 weight percent, while other ceramic oxides and components, including sodium oxide, typically comprise less than about 1 weight percent of leached glass fibers. In some embodiments, it is preferred that the leached fibers comprise less than 1 weight percent of alkali or alkaline earth metals. It will be appreciated that not all of the non-silicon oxides need to be removed from the leached glass fibers. However, leached glass fibers require that the silica content exceed the alumina content, and more preferably, by weight, by at least about 67%. The fibers are also substantially free of slag.

Importantly, these leached glass fibers are relatively inexpensive compared to fibers obtained from ceramic fibers such as high alumina fibers and particularly the crystalline quartz described above. The average fiber diameter of the leached glass fibers is preferably greater than at least about 3.5 microns, and more preferably greater than about 5 microns. On average, glass fibers typically have a diameter of about 9 microns. An average fiber diameter of from about 5 to 14 microns is preferred. Thus, the leached glass fibers of the present invention are not respirable. The leached glass fibers can be provided in any form commonly used in the manufacture of mounting mats. In some embodiments, the fibers are staple cut fibers. Prior to leaching, it will be appreciated that the fibers may be prepared by any method known in the art, but are generally formed using known melt processing techniques, such as by melt spinning or melt drawing processes, in a manner that provides a cost effective fiber manufacturing process. In some embodiments, the glass fibers are melt drawn.

Examples of leached glass fibers having relatively high silica content and suitable for use in making mounting mats for catalytic converters or other known gas treatment devices include the leached glass fiber commercial product available under the trade name BELCOTEX from belchem fiber Materials GmbH, germany, and the leached glass fiber commercial product available under the registered trade name REFRASIL from Hitco Carbon Composites, inc. BELCOTEX fibers are a standard type staple fiber pre-yarn (staple fiber pre-yarn). These fibers have an average fineness of about 550tex and are typically made from alumina-modified silicic acid. BELCOTEX fibers generally contain about 94.5% silica, about 4.5% alumina, less than 0.5% sodium oxide, and less than 0.5% other components. They have an average fiber diameter of about 9 microns and a melting point in the range of 1500 ℃ to 1550 ℃. These fibers are heat resistant up to 1100 ℃, generally slag free and binder free.

REFRASIL fibres, like BELCOTEX fibres, are leached glass fibres with a high content of amorphous silica, used to provide thermal insulation for applications in the range 1000 ℃ to 1100 ℃. The fibers have a diameter of between about 6 microns and about 13 microns and a melting point of about 1700 ℃. The fibers, after leaching, generally have a silica content of about 95% by weight. The alumina is present in an amount of about 4% by weight and the other components are present in an amount of 1% or less.

Leached glass fibers useful as mounting mats for exhaust gas treatment devices, such as catalytic converters, are fibers made by melting with a relatively high silica content. It is believed that there is essentially no catalytic converter mounting mat made from leached glass fibers in commerce. Leached glass fibers may have been tested either to fail to maintain a sufficient and effective minimum holding pressure during thermal cycling for use as a mounting mat in an exhaust gas treatment device or to be used with a wide variety of other materials, such as intumescent materials, that help provide sufficient and effective residual holding pressure for the mounting mat.

Leached glass fibers that are formed into a mat shape have been found to generally provide less strength to the mat. That is, leaching of non-silicon materials, including alumina, from the glass fibers is expected to reduce the retention of the mat until the mat has a lower shear strength. While it may be the case that the mounting mat comprises leached glass fibers containing silica to initially provide a sufficient minimum holding pressure within the housing of the catalytic converter for holding the fragile structure, mechanical or thermal cycling of the mat will rapidly destroy its ability to maintain the minimum holding pressure. Accordingly, leached glass fibers having higher silica content have been increasingly used to prepare catalytic converter mounting mats. Such a fact is confirmed in us patent 5290522.

However, we have found that by further processing, the holding pressure performance of the leached glass fibers prior to forming the mounting mat or mounting mats made from these fibers after forming, even after cycling, is sufficiently improved to be suitable for use in an exhaust gas treatment device. Without these additional treatments, however, mounting mats containing such leached glass fibers do not maintain a minimum holding pressure sufficient to hold the fragile structure within the housing of the exhaust gas treatment device after being subjected to thermal cycling.

It has surprisingly been found that heat treating the fibers provides significantly higher holding pressure performance, particularly for high temperature applications, although higher holding pressures can be achieved in lower temperature applications. In a particular embodiment, the leached glass fibers (or mounting mat comprising the leached glass fibers) may be heat treated at a temperature in a range of at least about 900 ℃ or greater, preferably from about 900 ℃ to about 1100 ℃, so that the mounting mat using the fibers can apply the lowest desired holding pressure in an exhaust gas treatment device even after 1000 cycles of expansion and contraction. Heretofore, by heat treating these fibers, one would not have expected the mounting mat to maintain an effective minimum holding pressure after thermal cycling.

It has been found that heat treating fibers having higher silica content also improves many of the physical properties of the fibers. For example, the creep strength of the fiber is increased, as well as the surface area of the fiber. Without being bound by any theory, it is believed that heat treating the leached glass fibers pre-shrinks the fibers. It is also known that low shrinkage is beneficial for good creep strength. It is also believed that heat treating the fibers also removes excess aqueous components from the fibers, thereby providing increased surface area. Thus, the use of leached glass silica fibers, particularly treated leached glass silica fibers, can be used to make mounting mats having all of the excellent physical properties of fiberglass, but are suitable for higher temperature applications, having a melting point in excess of that of fiberglass.

The heat treatment of the leached glass fibers can be performed before the mounting mat is made or after the mounting mat is made. When heat treated after the mounting mat is made, the mounting mat is heat treated at a temperature of at least 900 ℃ for an effective period of time to meet or exceed the required effective minimum holding pressure for holding the fragile structure within the applied housing. Similarly, when heat treated prior to making the mounting mat, the leached glass fibers may preferably be heat treated at a temperature of at least 900 ℃ for an effective period of time such that a minimum holding pressure for holding the fragile structure within the housing is met when making the mounting mat. The specific amount of time for heat treatment can vary widely depending on the thickness of the mounting mat, the uniformity of heating, the type of heat source used, the temperature rise time, and the temperature of the heat source, among other conditions. All of these variables are readily understood by those skilled in the art, and thus the effective time for performing the heat treatment at temperatures of 900 ℃ or above can be readily determined without undue experimentation.

In general, it has been recognized that heat treatment can be carried out for periods of time ranging from 15 minutes or less, where a relatively small, thin pad and excellent uniform heat source is used, to periods of time exceeding 1 hour, where a larger, thicker pad is used (excluding temperature rise and fall times). In some embodiments, the mounting mat or leached glass fibers are heat treated at a temperature between about 900 ℃ to 1100 ℃ for more than 1 hour. It will further be appreciated that the leached glass fibers and/or mounting mats made therefrom may alternatively be heat treated at a relatively low heating temperature, such as 300 ℃. However, the length of time necessary to obtain a satisfactory mounting mat with the required effective holding pressure is not commercially viable if the heat treatment is carried out for more than 24 hours. It is within the scope of the present invention to heat treat at any time and temperature regime below the time and/or temperature that causes devitrification of the fiber to achieve the same beneficial effects as described above. Typically, the fibers or mat may be heat treated at or above the intended use temperature. It is noted that heat treatment at lower temperatures may affect the utility of the mounting mat in applications requiring thermal cycling above the heat treatment temperature.

In order to maintain a minimum holding pressure to hold the fragile structure within the housing, the leached glass fibers used in the mounting mat may be treated using other methods, such as ion exchange or diffusion to enhance the creep strength of the fibers. However, it is to be understood that virtually any method by which the leached glass fibers or the mounting mat can be treated so as to maintain a minimum holding pressure for the mounting mat to hold the fragile structure within the housing after thermal cycling can be used.

The mounting mat preferably uses leached glass fibers containing silica up to 100% by weight. However, in other embodiments, the mounting mat may optionally comprise other known fibers, such as alumina/silica fibers, or other ceramic or glass fibers suitable for making the mounting mat for a particular temperature application. Thus, alumina/silica fibers, such as refractory ceramic fibers, may be selectively used for high temperature or wide temperature range applications. Other ceramic or glass fibers, such as S-glass, may be used with leached glass silica fibers in similar or lower temperature applications. However, in such instances, the mounting mat preferably comprises at least 50% by weight silica-containing leached glass fibers. In other words, the majority of the fibers used to make the mounting mat are leached glass fibers containing silica, and in a more preferred embodiment, at least 80% by weight of the fibers are leached glass fibers containing silica.

In some alternative embodiments, fibers such as S2-glass may be added to the mounting mat in amounts of from greater than 0 to about 50% by weight, based on 100% by weight of the entire mounting mat. It is expected that these glass fibers will be used primarily for low temperature applications due to their melting point temperature, among other reasons.

In other alternative embodiments, the mounting mat may include refractory ceramic fibers in addition to leached glass fibers. When refractory ceramic fibers, i.e., alumina/silica fibers or the like, are used, they are present in an amount of from greater than 0 to less than about 50 percent by weight, based on 100 percent by weight of the entire mounting mat.

As previously mentioned, the mounting mat may or may not contain a binder. When a binder is used, the components are mixed to form a mixture or slurry. The slurry of fibers and binder is then formed into the structure of the mat and the binder is removed, thereby providing a mounting mat comprising substantially only the heat treated fibers (and optionally other fibers). Typically, the binder that is removed is used to initially bind the fibers together. The binder used is generally an organic binder. By "removed" it is meant that the binder will eventually burn off of the mounting mat, leaving only the leached glass fibers (and other ceramic or glass fibers if such fibers are used) as the mounting mat for supporting the fragile structure.

Suitable binders include water soluble binders and water insoluble binders, but it is preferred that the binder used be a reactive, thermosetting latex which, after curing, is a flexible material that can be removed from the mounting mat by burning as described above. Examples of suitable binders or resins include, but are not limited to, acrylic water-based latexes, styrene-butadiene, vinyl pyridine, acrylonitrile, vinyl chloride, polyurethane, and the like. Other resins include low temperature, flexible thermosetting resins such as unsaturated polyesters, epoxies, and polyvinyl esters. Preferably, about 5% to about 10% latex is used, with about 8% being most preferred. The solvent for the binder may comprise water, or a suitable organic solvent, such as acetone, for the binder used. The concentration of binder in the solvent (if used) can be determined by conventional methods based on the desired binder loading and the workability (viscosity, solids content, etc.) of the binder system.

Instead of a binder, the mounting mat may include other fibers in addition to leached glass fibers to hold the mounting mat together. These fibers may be used in amounts of from greater than 0 to about 20% by weight, based on 100% by weight of the total composition, to aid in the bonding together of the leached glass fibers.

Mounting mats comprising fibrous leached glass silica can be prepared using any of the known techniques commonly used to prepare mounting mats. For example, leached glass fibers can be mixed with a binder or other fibers useful as a binder to form a mixture or slurry using a papermaking process. Any mixing means may be used, but preferably, when a binder is used, the consistency of the fiber-containing ingredients is about 0.25% to 5% or solids content (0.25-5 parts solids and 99.75-95 parts water). The slurry is then diluted with water to facilitate its formation, which eventually flocculates under the action of flocculants and retention aid chemicals. The flocculated mixture or slurry may then be placed on a paper machine to form a fibrous layer containing paper. Alternatively, such a layer may be formed by vacuum casting of a slurry. In each case, drying is generally carried out in a drying oven. For a more detailed description of standard papermaking techniques, reference is made to U.S. patent No.3458329, the disclosure of which is incorporated herein by reference. It will be appreciated that when a binder is used, the leached glass fibers are subjected to a heat treatment, and the step of heat treating the fibers should be performed prior to adding the binder or binding fibers to the leached glass fibers.

In other embodiments, the leached glass fibers may be processed into a mat using conventional methods, such as dry air layering. The mat has very little structural integrity at this stage and is too thick relative to the mounting mats of conventional catalytic converters and diesel particle traps. The resulting mat may then be dry needled as is known in the art to densify the mat and increase its strength. The heat treatment of the fibers may be performed before the mat is formed or after the mat is subjected to a needling process.

When dry air layering techniques are used, the mat may alternatively be treated by adding a binder to the mat by impregnation to form a discontinuous fiber composite. In this technique, the binder is added after mat formation, rather than the prepreg that forms the mat as in the conventional papermaking techniques described hereinabove. This method of making the mat helps to maintain fiber length by reducing breakage. However, it is understood that the heat treatment may be performed before any binder is added.

The method of impregnating the mat with the binder includes completely immersing the mat in the liquid binder system or alternatively spraying the mat. In a continuous process, a fibrous mat that can be transported in rolls is unwound and moved, for example on a conveyor belt or a scrim, past a spray nozzle for applying a binder to the mat. Alternatively, the pad may pass through the spray nozzle under the force of gravity. The mat/binder prepreg is then passed between press rolls which remove excess liquid to densify the prepreg to its desired thickness. The densified prepreg is then passed through an oven to remove any remaining solvent and, if necessary, to partially cure the binder to form a composite. The drying and curing temperatures depend, inter alia, on the binder and, if used, the solvent used. The composite may then be cut or rolled for storage or transport.

The mounting mat may also be manufactured in a batch process by dipping a portion of the mat into a liquid binder, removing the prepreg and applying pressure to remove excess liquid, followed by drying to form a composite and storing or cutting to the desired size.

It is noted that the density of mounting mats made from these leached glass fibers may be too low to be readily used in catalytic converter applications. Therefore, further densification should preferably be performed by any means known in the art to provide higher strength. One way of densifying is to needle the fibers to entangle and entangle them. Additionally or alternatively, a hydraulic winding method may be used. Other alternatives are rolling with a press roll to press the fibers into a mat form. Any of these methods of densification of the mat, or a combination of these methods, can be readily employed to obtain the correct and desired form of the mounting mat.

Regardless of the method used above, the composite can be cut, such as by molding, to form mounting mats of precise shape and dimensions with reproducible tolerances. Densified by needling or the like, the mounting mat 20 exhibits suitable handling properties, that is, it is easy to handle, not as brittle as many other fibrous mats or mats that can be broken in the hand. Which can be easily and flexibly mounted or wrapped around the catalyst support structure 18 or similar fragile structure without cracking and then disposed within the catalytic converter housing 12. Generally, the fragile structure surrounded by the mounting mat may be inserted into the housing or the housing may be built around the fragile structure surrounded by the mounting mat.

Further, it has been surprisingly found that the mounting mats of the present invention are capable of maintaining a minimum holding pressure of at least 50kPa, and a gap bulk density of the mat of from about 0.3 to about 0.5g/cm after 1000 mechanical cycles in a standard 1000 week gap expansion test (gap expansion test) at a hot face temperature of about 300 ℃³The gap expansion rate is about 2%. It will be appreciated that this test is particularly suitable for mounting mats used to hold heavier substrates in low temperature high G load applications. Exhaust gas treatment devices for such applications include diesel catalyst structures and diesel particulate traps. For high temperature applications, such as typical catalytic converters, it has been found that the mounting mat is capable of maintaining a minimum holding pressure of at least 10kPa with a gap bulk density of from about 0.3 to about 0.5g/cm after 1000 mechanical cycles in a standard 1000 cycle gap expansion test at a heating face temperature of about 900 deg.C³The gap expansion rate is about 5%.

The term "cycling" means that the gap between the monolith (i.e., fragile structure) and the housing is opened and closed at a specific distance and at a predetermined rate. To simulate practical conditions, the expansion of the gap between the casing and a brittle structure of a given diameter can be determined by calculating the coefficient of thermal expansion of a conventional casing at temperatures of, for example, around 900 ℃. The final mat basis weight was then selected to meet the test criteria and provided a minimum holding pressure (Pmin) of greater than about 10kPa after 1000 cycles. The objective is to provide adequate support at the lowest cost, and therefore the minimum basis weight is selected to meet greater than about 10kPa required. While some prior art non-intumescent mounting mats also have the ability to maintain a "high" minimum pressure after 1000 cycles at a heated face temperature of at least about 900 ℃, those mats contain very expensive ceramic fibers made from a gel sol having an alumina content of at least 30% or more, or fibers derived from crystalline quartz, or both. The prior art non-intumescent mats do not contain leached glass fibers having more than about 67% silica.

In operation, the catalytic converter experiences significant temperature variations. Due to the difference in the coefficients of thermal expansion, the housing may expand more than the support structure 18, so that the gap between these elements increases slightly. In a typical case, the gap may expand and contract by about 0.25 to about 50mm during thermal cycling of the converter. The thickness and mounting density of the mounting mat are selected to maintain a minimum holding pressure of at least about 10kPa under all conditions to prevent the fragile structure from vibrating loose. The mounting pressure exerted by the mounting mat 20 under these conditions can accommodate the thermal characteristics of the assembly without compromising the physical integrity of the constituent components.

For mounting mats used for low temperature applications, the test was performed at about 300 ℃. However, the test was performed in the same manner as the high temperature test described above. However, in view of the differences in loading applications and the fact that heavier catalyst structures are often used, it is necessary to have a higher minimum holding pressure. Thus, as noted above, the mat must provide a holding pressure of at least 50kPa to the fragile structure after 1000 test cycles with a heating surface temperature of about 300 ℃.

Having described the general aspects of the invention, a more particular description is now provided by way of example. It is to be understood that these examples are for illustration purposes only and are not to be considered limiting in any respect unless otherwise noted. These examples are intended only to illustrate embodiments of the present invention.

To illustrate embodiments of the present invention, BELCOTEX leached glass fibers were heat treated at a temperature between about 900 ℃ and 1100 ℃ for 2 hours. The heat treated fibers were then blended together with about 6.5% by weight of a fiber suitable for thermally bonding leached glass silica fibers. The combined fibers are then wet formed into the shape of a mat. The mat was dried in a furnace at a temperature of about 60 c. The dried mat was then hot pressed to a density of 148kg/m³Left and right. The pressed mat is formed into a suitable final form for use as a mounting mat for catalytic converter applications.

In another embodiment, the invention is illustrated by first placing leached glass silica fibers wet in a mat. The mat was then needle punched to a density of 148kg/m³Left and right. The needle punched pad is then heat treated at a temperature of between about 900 ℃ and 1100 ℃ for 2 hours. After heat treatment, the mat is formed into its final product form for use as a mounting mat for catalytic converter applications.

Other mounting mats comprising leached glass silica fibers obtained from other manufacturers were also made using essentially one of the above methods, and the mat or fibers were heat treated at a temperature of at least 900 ℃. For comparison, sample mats containing leached glass fibers with higher silica content were prepared, but the fibers or mats were not heat treated. These pads are considered to be in an "as is" state.

In at least one example, a "pre-shrunk" leached glass silica fiber mat was tested. This fiber mat is commercially available for different uses, but has never been tested or suggested to be suitable for use as a mounting mat for an exhaust gas treatment device. The term "pre-shrinking" means that the leached glass silica fibers have been further treated to reduce the amount of shrinkage of the mat. In many applications, it is desirable that the fibrous mat retain its shape and not shrink in high temperature applications.

Each pad was subjected to a high temperature (900 deg.C) 1000 cycle gap expansion test. The test conditions included a set of constant parameters for comparison of the sample pads. These test parameters included three different heating surface temperatures of 300 deg.C, 600 deg.C and 900 deg.C, and a gap bulk density of 0.3g/cm³And the gap expansion rate is about 5%. Given these parameters and the basis weight of known mounting mats, a gap of 2.9 to 5.45mm was used in this test to achieve 0.3g/cm³Gap bulk density of (a). For the low temperature (300 ℃)1000 cycle test, a gap of 4.13mm and an expansion rate of about 2% were used.

It will be appreciated that one of ordinary skill in the art will be able to perform this 1000-week test using these set-up parameters without undue experimentation. That is, the above-described set parameters will enable one of ordinary skill in the art to make similar comparisons of the effective holding pressure of the pad, regardless of the pad characteristics or the size of the gap. For the particular mats tested here, gap sizes of 2.9 to 5.45mm were considered appropriate based on known characteristics of the mat such as basis weight and other set parameters such as gap bulk density. It is also understood that: the gap bulk density of the mounting mat will vary depending on the application. For some applications, it may be possible to use a lower gap bulk density, e.g., 0.3g/cm³An acceptable minimum holding pressure cannot be obtained, but at higher interstitial bulk densities, e.g. 0.5g/cm³An acceptable minimum holding pressure can be achieved. Thus, the test can be performed using any gap bulk density suitable for the application, generally falling within about 0.3 to 0.5g/cm³Within the range of (1).

The results of these tests are shown in table I below.

TABLE 1

1000 cycle gap expansion test results (unit: kPa)

Example numbering/testing	1 original state	2 Heat treatment	3 original state	4 Pre-shrinking	5 Heat treatment	6 original state	7 Heat treatment
								Pmin1000@0.3GBD&300℃	7.93	124	14.73	63.16	98.43	12.4	122.06
Pmin 1000@0.3GBD&900℃	2.00	98.79	4.07	39.71	29.44	2.41	41.36

P fixed gap @0.3GBD& 300℃	24.82	264.04	21.71	114.44	110.10	34.88	89.14
								P fixed gap @0.3GBD& 600℃	6.98	259.01	10.86	122.09	114.72	11.72	81.42
P fixed gap @0.3GBD& 900℃	7.75	253.56	8.14	126.37	115.54	7.72	97.69

Observing the results of the tests, it can be seen that all the treated leached glass fiber mats started in the initial tests at 300 ℃, 600 ℃ and 900 ℃ with a holding pressure (P) that greatly exceeded the minimum holding pressure required for the application of high temperature (i.e. Pmin ═ 10kPa) and low temperature (i.e. Pmin ═ 50kPa)_{Fixed clearance}Results). However, the "as-received" leached glass silica fiber mat did not maintain sufficient holding pressure after the initial cycle, let alone after 1000 cycles. By comparison, the treated, leached glass fiber mat was used to maintain an effective holding pressure or load (Pmin) that exceeded the minimum required value throughout the 1000 th cycle in the above test. For high temperature applications, it can be seen that the minimum effective holding pressure (Pmin) of the treated fibers after 1000 cycles at 900 ℃ is around 29kPa, well above the minimum of 10 kPa.

The results of the significant 1000 cycle gap expansion test demonstrated for the non-intumescent mounting mat for exhaust gas treatment devices according to the present invention, as well as the relatively low cost associated with the manufacture of leached glass fibers comprising silica compared to gel sol ceramic fibers or fibers derived from crystalline quartz, are advantageous for the catalytic converter and diesel particle trap industries. The mounting mat can be die cut, is operable in low profile as a resilient support, provides ease of handling, and is in a flexible form so as to be able to provide complete wrapping of the catalyst support structure without cracking if desired. Alternatively, the mounting mat may be wrapped entirely around the entire circumference or circumference of at least a portion of the catalyst support structure. The mounting mat may also be partially wrapped and, if desired, include end seals as currently used in some conventional converter devices to prevent gas bypass flow.

The mounting mats described above are also beneficial for many applications such as conventional automotive catalytic converters, motorcycle and other small engine machinery, automotive pre-converters, and high temperature gaskets and gaskets, even future electric vehicle floor catalytic converter systems. In general, they may be used in any application where a pad or gasket is required at room temperature to apply a holding pressure, and more importantly at elevated temperatures of around 20 ℃ to at least around 1100 ℃, including providing the ability to maintain a holding pressure during thermal cycling.

The mounting mats described above may also be used in catalytic converters in the chemical industry that are disposed in exhaust gas or exhaust stack assemblies, including stack assemblies that contain fragile honeycomb structures that require protective mounting.

The present invention is not limited to the specific embodiments described above, but includes modifications, variations and equivalent embodiments defined by the following claims. The above embodiments are not necessarily of an alternative nature and different embodiments may be combined to provide the desired characteristics.

Claims (13)

1. An apparatus for treating exhaust gas, comprising:

a housing;

a fragile structure resiliently mounted within the housing; and

a non-intumescent mounting mat disposed in the gap between the housing and the fragile structure, wherein the mounting mat comprises heat treated leached glass fibers formed by melting comprising at least 67 weight percent silica, wherein the mounting mat applies one of the following minimum holding pressures for holding the fragile structure within the housing(i) At least 10kPa, and a mat gap bulk density of 0.3 to 0.5g/cm after 1000 test cycles at a heated face temperature of 900 DEG C³A gap expansion of 5%, or (ii) a gap bulk density of 0.3 to 0.5g/cm after 1000 test cycles at a heated face temperature of 300 ℃ of at least 50kPa³The gap expansion rate was 2%.

2. The apparatus of claim 1, wherein said housing has an inlet at one end and an outlet at the opposite end through which the exhaust gas flows; and wherein said fragile structure has an outer surface, an inlet end surface at one end in communication with said inlet of said housing, and an outlet end surface at an opposite end in communication with said outlet of said housing.

3. The apparatus of claim 1, wherein the leached glass fibers comprise at least 90 weight percent silica.

4. The apparatus of claim 1, wherein said leached glass fibers comprise 93 to 95 weight percent silica, and from 4 to 6 weight percent alumina.

5. The apparatus of claim 1, wherein said mounting mat comprises from 50 to 100 weight percent of said leached glass fibers.

6. The apparatus of claim 1, wherein the leached glass fibers are melt drawn.

7. The apparatus of claim 1, further characterized by at least one of the following features:

(i) wherein the mounting mat is substantially free of binder;

(ii) wherein the diameter of the leached glass fibers is greater than 3.5 microns; or

(iii) Wherein the leached glass fibers are substantially free of slag.

8. The apparatus of claim 1 wherein the mounting mat comprises from greater than 0 to 50 weight percent S2-glass fibers or refractory ceramic fibers, based on 100 weight percent of the entire mat.

9. The apparatus of claim 1 wherein (i) the mounting mat, (ii) or the leached glass silica fibers prior to forming the mounting mat, are heat treated at a temperature of at least 900 ℃ for an effective period of time to achieve an effective minimum holding pressure for holding the fragile structure within the housing.

10. The apparatus of claim 1, wherein the apparatus is a catalytic converter or a diesel particle trap.

11. A method of making the device of claim 1 for treating exhaust gas, comprising:

(a) providing a mounting mat containing silica-containing glass fibers formed by melting, wherein the melt-formed glass fibers are formed by:

(i) leaching the fusion-formed glass fiber, whereby the leached glass fiber has a silica content greater than the silica content of the glass fiber prior to treatment, and whereby the treated glass fiber contains at least 67 weight percent silica;

(ii) heat treating the leached glass fibers prior to forming the mounting mat or heat treating the leached glass fiber mounting mat prior to wrapping around the fragile structure;

(b) wrapping the mounting mat around a fragile structure suitable for treating exhaust gases; and

(c) disposing the fragile structure and the mounting pad into the housing such that the mounting pad resiliently retains the fragile structure within the housing, wherein the mounting pad applies the following for retaining the fragile structure within the housingAt least 10kPa, and a mat gap bulk density of 0.3 to 0.5g/cm after 1000 test cycles at a heated face temperature of 900 DEG C³A gap expansion of 5%, or (ii) a gap bulk density of 0.3 to 0.5g/cm after 1000 test cycles at a heated face temperature of 300 ℃ of at least 50kPa³The gap expansion rate was 2%.

12. The method of claim 11, wherein said step of leaching the melt-formed glass fibers comprises leaching the melt-formed glass fibers in an acid solution.

13. The method of claim 11, wherein the step of heat treating the fibers or mounting mat comprises heat treating at a temperature of at least 900 ℃ for an effective period of time to achieve an effective minimum holding pressure for holding the fragile structure within the housing.