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U.S. Patent Dec. 6,2011 Sheet 2 614 US 8,071,040 B2
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LOW SHEAR MOUNTING MAT FOR POLLUTION CONTROL DEVICES
This application claims the benefit of the filing date, under 35 U.S.C. §119(e), of U.S. Provisional Application Ser. No. 61/245,110, filed on Sep. 23, 2009.
A device for the treatment of exhaust gases, such as a catalytic converter or a diesel particulate trap. The device includes a fragile structure mounted Within a housing by a mounting mat that is disposed in a gap betWeen the housing and the fragile structure.
Exhaust gas treatment devices are used on automobiles to reduce atmospheric pollution from engine emissions. Examples of Widely used exhaust gas treatment devices include catalytic converters, diesel particulate traps and other pollution control devices.
A catalytic converter for treating exhaust gases of an automotive engine includes a housing, a fragile catalyst support structure for holding the catalyst that is used to effect the oxidation of carbon monoxide and hydrocarbons and the reduction of oxides of nitrogen, and a mounting mat disposed betWeen the outer surface of the fragile catalyst support structure and the inner surface of the housing to resiliently hold the fragile catalyst support structure Within the housing.
A diesel particulate trap for controlling pollution generated by diesel engines generally includes a housing, a fragile particulate filter or trap for collecting particulate from the diesel engine emissions, and a mounting mat that is disposed betWeen the outer surface of the filter or trap and the inner surface of the housing to resiliently hold the fragile filter or trap structure Within the housing.
The fragile structure generally comprises a monolithic structure manufactured from a frangible material of metal or a brittle, ceramic material such as aluminum oxide, silicon dioxide, magnesium oxide, zirconia, cordierite, silicon carbide and the like. These materials provide a skeleton type of structure With a plurality of gas floW channels. These monolithic structures can be so fragile that even small shock loads or stresses are often suflicient to crack or crush them. In order to protect the fragile structure from thermal and mechanical shock and other stresses noted above, as Well as to provide thermal insulation and a gas seal, a mounting mat is positioned Within the gap betWeen the fragile structure and the housing.
The mounting mat materials employed should be capable of satisfying any of a number of design or physical requirements set forth by the fragile structure manufacturers or the exhaust gas treatment device manufacturers. For example, the mounting mat material should be capable of exerting an effective residual holding pressure on the fragile structure, even When the exhaust gas treatment device has undergone Wide temperature fluctuations, Which causes significant expansion and contraction of the metal housing in relation to the fragile structure, Which in turn causes significant compression and release cycles for the mounting mats over a period of time.
Ceramic and metallic substrates used in exhaust gas treatment devices are most often mounted Within a metal housing With an inorganic fiber based mounting mat. This mounting mat material may contain only inorganic fibers. HoWever, the mounting mat material may also contain other types of fibers, organic binders, inorganic fillers and/or intumescent materials.
The mounting mat must function across a Wide range of operating temperatures to effectively hold the substrate in position. Substrates are subjected to axial forces acting on the substrate due to vibrations. The mounting mat also compensates for the fact that the metal housing expands more or less
than the substrate itself. Various exhaust gas treatment devices operate throughout a temperature range of ambient conditions of about 20° C. to about 1200° C. Therefore, mounting mats must provide robust holding pressure performance across this Wide temperature range.
As loW temperature applications become more prevalent either from more efficient engine design or an increase in popularity of diesel poWered vehicles, the desire for loW-cost mounting mats that perform Well at both loW and high temperatures has increased.
In loW temperature applications, such as turbocharged direct injection (TDI) diesel poWered vehicles, the exhaust temperature is typically about 1500 C., and may never exceed 300° C. It has been observed in the field that exhaust gas treatment devices utilized in such vehicles, Which are assembled With typical intumescent mats, fail With an unexpectedly high frequency.
While not intending to be limited by theory, one reason for these failures may be that the exhaust temperature is too loW to quickly burn off the organic binders, Which may at least partially liquefy Within the temperature range of ambient temperature to about 350° C. By “at least partially liquefy”, it is meant that the organic binders become softer, characterized by a reduction in viscosity, such that the organic binders may be at least partially floWable. As the organic binder begins to liquefy, the fibers Within the mounting mat may begin to slide past one another causing compaction of the mounting mat, Which results in negative expansion and a loss of shear strength and holding force of the mounting mat. From room temperature to about 200° C. the loss in holding force is gradual. HoWever, the loss in holding force is rapid from about 200° C. to about 250° C. When subsequently used in the loW temperature applications, the mats may fail to provide sufficient pressure against the fragile structure, and the exhaust gas treatment devices in Which the mounting mats are used may fail.
At temperatures above 350° C., the intumescent particles Which are typically present in the mounting mats expand and increase the holding force of the mat against the fragile structure. HoWever, in applications such as those described above in Which the mounting mats never experience temperatures above 350° C., the intumescent material is not exposed to a temperature sufficient to cause it to expand, and the mounting mats Will not benefit from the increased holding force provided by the expansion.
Previous attempts have been made at improving the loW temperature performance of mounting mat materials for exhaust gas treatment devices. One such attempt involves including expanding particles in the mounting mat Which expand (that is, increase in volume) throughout the temperature range Where the organic binder has a negative impact. Unfortunately, such expanding particles continue to expand at temperatures Well above the temperatures at Which the organic binders exhibit their negative impact on mat performance, and therefore provide undesirable expansion at higher temperatures.
What is needed in the industry is a flexible mounting mat for exhaust gas treatment devices Which can be easily installed and Which can function across a Wide range of inlet gas temperatures Without a significant loss in mat thickness and corresponding shear strength and holding pressure performance.
FIG. 1 shoWs a fragmentary vieW of an illustrative exhaust gas treatment device including the subject mounting mat.
FIG. 2 is a simplified schematic diagram of the apparatus used to test the subject mounting mat in comparison to prior art mounting mats.
FIG. 3 is a graph comparing the percent shear strength loss of the subject mounting mat and a prior art mounting mat as a function of hot face temperature (° C.).
FIG. 4 is a graph comparing the percent shear strength loss of the subject mounting mat and a prior art mounting mat as a function of hot face temperature (° C.).
A mounting mat for exhaust gas treatment device applications is provided. The mounting mat includes at least one ply or sheet that is comprised of heat resistant inorganic fibers, organic binder, and a colloidal inorganic oxide. According to certain embodiments, the mounting mat may optionally include an intumescent material. It has been unexpectedly found that the inclusion of a colloidal inorganic oxide, such as colloidal alumina, colloidal silica, or colloidal zirconia, in the mounting mat reduces the shear strain the mat experienced at temperatures of 350° C. and below. The mounting mat provides improved holding performance across a wide temperature range at relatively low cost.
A device for treating exhaust gases is also provided. The device includes an outer metallic housing, at least one fragile structure that is mounted within the housing by a mounting mat that is disposed between the inner surface of the housing and the outer surface of the fragile structure. The term “fragile structure” is intended to mean and include structures such as metal or ceramic monoliths or the like which may be fragile or frangible in nature, and would benefit from a mounting mat such as is described herein.
Catalytic converter catalyst structures generally include one or more porous tubular or honeycomb-like structures mounted by a thermally resistant material within a housing. Each structure may include from about 200 to about 900 or more channels or cells per square inch, depending upon the type of exhaust gas treatment device. A diesel particulate trap differs from a catalytic converter structure in that each channel or cell within the particulate trap is closed at one end. Particulate is collected from exhaust gases in the porous structure until regenerated by a high temperature burnout process. Non-automotive applications for the mounting mat may include catalytic converters for chemical industry emission (exhaust) stacks.
One illustrative form of a device for treating exhaust gases is designated by the numeral 10 in FIG. 1. It should be understood that the mounting mat is not intended to be limited to use in the device shown in FIG. 1, and so the shape is shown only as an illustrative embodiment. In fact, the mounting mat could be used to mount or support any fragile structure suitable for treating exhaust gases, such as a diesel catalyst structure, a diesel particulate trap, or the like.
Catalytic converter 10 may include a generally tubular housing 12 formed of two pieces of metal, for example, high temperature resistant steel, held together by flange 16. Alternatively, the housing may include a preformed canister into which a mounting mat-wrapped fragile structure is inserted. Housing 12 includes an inlet 14 at one end and an outlet (not shown) at its opposite end. The inlet 14 and outlet are suitably formed at their outer ends whereby they may be secured to conduits in the exhaust system of an internal combustion engine. Device 10 contains a fragile structure, such as a frangible ceramic monolith 18, which is supported and restrained within housing 12 by a mounting mat 20. Monolith 18 includes a plurality of gas pervious passages that extend axially from its inlet at one end to its outlet at its opposite end. Monolith 18 may be constructed of any suitable refractory metal or ceramic material in any known manner and configuration. Monoliths are typically oval or round in cross-sectional configuration, but other shapes are possible.
The monolith is spaced from inner surfaces of the housing by a distance or a gap, which will vary according to the type and design of the device utilized, for example, a catalytic converter, a diesel catalyst structure, or a diesel particulate trap. This gap is filled with a mounting mat 20 to provide resilient support to the ceramic monolith 18. The resilient mounting mat 20 provides both thermal insulation to the external environment and mechanical support to the fragile structure, thereby protecting the fragile structure from mechanical shock across a wide range of exhaust gas treatment device operating temperatures.
In general, the mounting mat includes high temperature resistant ceramic fibers comprising alumina and/or high temperature resistant biosoluble inorganic fibers, organic binder which at least partially liquefies at elevated temperature prior to binder burnout, colloidal inorganic oxide and optionally at least one type of intumescent material. The mounting mat 20 is capable of providing a holding pressure suflicient to resiliently hold the fragile catalyst support structure 18 within a housing 12 of an exhaust gas treatment device 10 throughout a wide temperature range.
The high temperature resistant inorganic fibers utilized in the subject mounting mat can withstand the mounting mat forming process, withstand the operating temperatures of the exhaust gas treatment devices, and provide the minimum holding pressure performance for holding fragile structure within the exhaust gas treatment device housing at the operating temperatures. Without limitation, suitable inorganic fibers that may be used to prepare the mounting mat and exhaust gas treatment device include high alumina polycrystalline fibers; mullite fibers; refractory ceramic fibers such as alumino-silicate fibers or kaolin fibers; alumina-zirconiasilica fibers; alumina-magnesia-silica fibers such as S-glass fibers or S2-glass fibers; E-glass fibers, biosoluble alkaline earth silicate fibers such as calcia-magnesia-silica fibers or magnesia-silica fibers, or combinations thereof.
According to certain embodiments, the high temperature resistant inorganic fibers that are used to prepare the mounting mat comprise ceramic fibers comprising alumina. Without limitation, suitable ceramic fibers include alumina fibers, mullite fibers, alumino-silicate fibers, alumina-zirconiasilica fibers, and similar fibers. High alumina polycrystalline fibers may comprise the fiberization product of about 72 to about 100 weight percent alumina and about 0 to about 28 weight percent silica. A suitable alumino-silicate ceramic fiber is commercially available from Unifrax I LLC (Niagara Falls, N.Y) under the registered trademark FIBERFRAX. The FIBERFRAX® ceramic fibers comprise the fiberization product of a melt comprising about 45 to about 75 weight percent alumina and about 25 to about 55 weight percent silica. The FIBERFRAX® fibers exhibit operating temperatures of up to about 1540° C. and a melting point up to about 1870° C. The FIBERFRAX® fibers can be easily formed into high temperature resistant sheets and papers.
In certain embodiments, alumino-silicate fiber may comprise from about 40 weight percent to about 60 weight percent Al2O3 and about 60 weight percent to about 40 weight percent SiO2. The alumina/silica/magnesia glass fiber typically comprises from about 64 weight percent to about 66 weight percent SiO2, from about 24 weight percent to about 25 weight percent Al2O3, and from about 9 weight percent to about 11 weight percent MgO.
The E-glass fiber typically comprises from about 52 weight percent to about 56 weight percent SiO2, from about 16 weight percent to about 25 weight percent CaO, from about 12 weight percent to about 16 weight percent Al2O3, from about 5 weight percent to about 10 weight percent BZO3, up to