Catalytic converter assemblies for treating exhaust gases of automotive and diesel engines contain a fragile structure, such as a catalyst support structure, for holding the catalyst, used to effect the oxidation of carbon monoxide and hydrocarbons and the reduction of oxides of nitrogen, the fragile structure being mounted within a metal housing. The fragile structure is preferably made of a frangible material, such as a monolithic structure formed of metal or a brittle, fireproof 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 tiny flow channels. However, as noted hereinabove, these structures can be, and oftentimes are, very fragile. In fact, these monolithic structures can be so fragile that small shock loads or stresses are often sufficient to crack or crush them.
The fragile structure is contained within a metal housing, with a space or gap between the external surface of the fragile structure and the internal surface of the housing. In order to protect the fragile structure from thermal and mechanical shock and other stresses noted above, as well as to provide thermal insulation, it is known to position at least one ply or layer of mounting or support material within the gap between the fragile structure and the housing. For example, assignee's U.S. Pat. Nos. 4,863,700, 4,999,168, 5,032,441, and 5,580,532, the disclosure of each of which is incorporated herein by reference, disclose catalytic converter devices having a mounting or support material disposed within the gap between the housing and the fragile structure contained in the devices to protect the fragile structure and otherwise hold it in place within the housing.
In low temperature catalytic converter applications, such as turbocharged direct injection (TDI) diesel powered vehicles, the exhaust temperature is typically about 150.degree. C., and may never exceed 300.degree. C. It has been observed in the field that catalytic converters, that are assembled with typical intumescent mats, fail with an unexpectedly high frequency.
One reason for these failures is that the exhaust temperature is too low to expand the intumescent, typically vermiculite, particles. This has even been found in converters that have been pre-heated to about 500.degree. C. to pre-expand the intumescent particles. When subsequently used in the low temperature TDI application, the mats fail to provide sufficient pressure against the fragile structure and thus fail. It should be noted that converters used in gasoline engines overcome this initial loss in holding force as the converter continues to heat up to the final operating temperature, which may be as high as 900.degree. C. At temperatures above 350.degree. C., the intumescent particles expand and increase the holding force of the mat against the fragile structure.
A second reason for these failures is that organic binder systems used in the intumescent mat products degrade and cause a loss in the holding force. From room temperature to about 200.degree. C. the loss in holding force is gradual; however, the loss in holding force is rapid from about 200.degree. C. to about 250.degree. C., as shown in FIG. 3.
FIG. 2 shows the performance of prior art intumescent mats in a 1000 cycle test at 300.degree. C. with a gap between the fragile structure and the shell of about 4.0 to about 4.1 mm. All mats were preheated at 500.degree. C. for one hour to pre-expand the intumescent material (vermiculite). In the 1000-cycle test, the mat must maintain a pressure of greater than 15 psi at all times to provide adequate holding force on the fragile structure. FIG. 2 shows a loss in holding force with the eventual failure after about 500 cycles. The data presented in this graph correlates well with the failures observed with converters mounted with conventional intumescent mounting mats used in TDI diesel applications operating at less than 300.degree. C. The test procedure and specific results of the tests of prior intumescent mats are set forth in detail below.
Non-intumescent mat systems are known in the art. Fibers such as SAFFIL.RTM. (from ICI, United Kingdom) and MAFTEC.RTM. (from Mitsubishi Chemicals, Japan) may be used to mount fragile structures for use over a wide temperature range. These fiber only products contain no intumescent material, such as vermiculite, to increase the holding force as the converter is heated. These mats are composed of polycrystalline fibers with a high Young's Modulus (greater than 20-40.times.10.sup.6 psi) which function as ceramic springs to provide the required holding force against the fragile structure. These products provide adequate function in turbocharged direct injection (TDI) diesel converters.
Historically, these products have been dry layed without the addition of organic binder; as a result, the thickness of these products is typically greater than 18 mm making them difficult to install in converters, as described in the patents referenced above. Further, the cost of these products has been prohibitively high as compared to intumescent mats. Recently, a new generation of these products have been provided with improved handling and installation by vacuum packing, or by the addition of organic binders and sometimes additional stitching or needling to achieve a thinner and more flexible mat. A thickness of less than 10 mm can be achieved by these means. However, testing of the new generation mats in the 150.degree.-300.degree. C. temperature range has shown lower holding force than for the first generation mats.
The first such product of this new generation is described in U.S. Pat. No. 5,580,532, which claims a flexible polycrystalline ceramic fiber mat for use in mounting catalytic converters, particularly useful in the operating temperature range of 750.degree. C. to 1200.degree. C. Flexibility is achieved by impregnating a mat with various organic binders. All of the binders referenced in this patent, however, yield a mat with lower performance in the 150.degree.-300.degree. C. operating temperature range of a TDI diesel converter. However, satisfactory performance may still be achieved due to the high Young's modulus of the fibers used in these mats.
European Patent Application EP803643 discloses a mat product made with mineral fibers over a very wide composition range (0-99 wt. % Al.sub.2 O.sub.3, 1-99.8 wt. %SiO.sub.2) bonded with a binder to produce a thin, flexible mat for mounting fragile structures. The fibers are further defined as preferably having compositions in the range of 95 wt. % Al.sub.2 O.sub.3, or 75 wt. % Al.sub.2 O.sub.3 --25 wt. % SiO.sub.2. The application states that only fibers with a high elastic modulus will provide sufficient holding force to support the fragile structure as the converter heats and cools during use. Fibers used in prior art intumescent mat products are stated not to be suitable. The application describes the use of conventional organic binders, such as acrylic latex, for applications where the temperature is high enough to burn-out the binder, such as above 500.degree. C. For low temperature applications, such as with diesel engines in the 220.degree.-300.degree. C. range, the application states that conventional organic binders thermally degrade and become hard. Upon thermal cycling of the converter, the hardened mat is no longer capable of maintaining adequate holding force on the fragile structure and failure results. The application states that alternative binders which do not harden, such as a silicone binder, may successfully be used in this temperature range.
In U.S. Pat. No. 4,929,429 and 5,028,397, the comparative examples show that even when melt formed ceramic fibers have been treated to reduce the shot content to as low as 5%, these fibers still lack the requisite resiliency to adequately hold the fragile structure in the converter shell, as is described in U.S. Pat. No. 5,250,269. The U.S. Pat. No. 5,250,269 describes how adequate resiliency can be achieved by first heat treating melt formed ceramic fibers, such as CERAFIBER.RTM. (Thermal Ceramics, Augusta, Ga.). Comparative examples of mats made with melt formed ceramic fibers without treatment failed in both laboratory testing and converter hot shake testing.
What is needed in the industry is a mat that can function at an average mat temperature range from ambient temperature up to at least about 350.degree. C. and can be installed in exhaust gas treatment devices such as TDI diesel catalytic converters and the like without a loss in holding force.
It is an object of the present invention to provide a mat that can function throughout an average mat temperature range from ambient temperature up to at least about 350.degree. C. while maintaining a holding force of at least about 15 psi in exhaust gas treatment devices such as TDI diesel catalytic converters and the like.
It is another object of the present invention to provide a mat that is sufficiently thin and sufficiently flexible to be easily handled and installed in exhaust gas treatment devices such as TDI diesel catalytic converters and the like.