The present invention relates to a method for coating a substrate having a plurality of grooves, such as a monolithic substrate used in a catalytic converter. In particular, the present invention relates to the rapid and uniform coating and rapid drying of a monolithic substrate with a coating medium. During the coating process, moisture remains in the grooves of the monolithic substrate. The moisture must therefore be removed in order for the coating to be dry and firm. In accordance with the present invention, removal of moisture and other volatiles is accomplished by applying a vacuum in a manner and for a period of time sufficient to remove moisture and other volatiles while maintaining the desired shape of the coating profile in the tank of.
Background technique:
Catalytic converters for removing and/or converting harmful components in exhaust gases are well known. Catalytic converters for this purpose are available in a variety of structures, one in the form of a catalytically coated rigid frame monolithic substrate or a honeycomb-type element with many longitudinal grooves to provide catalytically high surface area coating body. Rigid monolithic substrates are fabricated from ceramics and other materials. These materials and their structures are described, for example, in US3331787 and US3565830, which are incorporated herein by reference.
Monolithic substrates, especially monolithic substrates having a number of grooves, are coated with a slurry of catalytic and/or absorbent material (hereinafter, collectively referred to as "coating medium"). The slurry contains water and possibly other liquid components, which evaporate upon heating. Various methods of coating monolithic substrates with coating media are known in the art. From a cost standpoint, these methods have the disadvantage of minimizing the amount of coating medium applied, especially when expensive catalytically active noble metals such as platinum, palladium or rhodium are deposited as part of the coating medium Time. Not only is there a difficulty in coating the monolithic substrate, but it is also difficult to provide a strong, reproducible coating pattern in the tank.
One method of coating a prefabricated monolithic substrate is to pump the coating medium into each tank and then dry the coated substrate. Drying during the coating process is a necessary step in order to remove evaporated components (eg, water vapor) and to quickly fix the coating medium in the bath. Systems using dry operations cannot provide uniform coating thickness as well as uniform coating profile, where the coating medium is uniformly deposited over the same length of each slot.
It has been suggested to use a vacuum to draw the coating medium up from the tank. For example, US Pat. No. 4,384,014 to Peter D. Young discloses applying a vacuum over the monolithic substrate to remove air from the tank while drawing the coating medium upwards from the tank. The vacuum is then removed and excess coating medium is removed, preferably by gravity drainage, ie the excess coating medium is moved down the lower end or end of the tank.
US Patent No. 4,191,126 to James R. Reed et al. discloses dipping a monolithic substrate in a coating medium and using subatmospheric pressure to remove excess coating medium from the surface of the carrier. The vacuum is applied to remove obstructions in the grooves so that the coating medium can coat the surface of each groove.
An improvement in these systems is disclosed in US Pat. No. 4,609,563 to Thomas Shimrock et al., incorporated herein by reference. The system includes a method of vacuum coating a ceramic substrate element with a coating medium, wherein a predetermined amount of the coating medium is applied to the ceramic monolith substrate. The monolithic substrate is placed at a predetermined depth in a container, preferably of a predetermined size, containing a defined amount of coating medium to be applied to the substrate. The coating medium is then pulled up with a vacuum applied at the end opposite the end of the substrate immersed in the coating medium bath. There is no need to draw or remove excess coating medium from the substrate, nor does it require any pre-vacuum steps to remove air.
In Shimrock et al., a container containing a defined amount of coating medium (a dip pan is also known) is designed not only to be shaped to freely accommodate the substrate, but also to conform the substrate strictly to the coating to be applied. requirements for covering substrates. As exemplified in US4609563, if the shape of the monolithic substrate is oval, then the shape of the dip pan is also oval slightly larger than the size of the substrate itself.
Although smooth coatings produced by the method of US4609563 exceeded those produced by other reference methods, it was still difficult to obtain a uniform coating profile where the coating medium covered the same length of each groove. Additionally, because the method of the US4609563 patent preferably has a precisely shaped and sized dipping pan for each type of monolithic substrate, there is an added expense in having to use a variety of dipping pans of different sizes and shapes. Furthermore, the size of each dip pan is preferably slightly larger than the size of the substrate. Therefore, great care must be taken when placing such fragile substrates into the dip pan so that the substrate does not accidentally bump into the dip pan and cause damage.
U.S. Patent Application Serial No. 08/668,385, filed June 21, 1996 (Attorney File No. 3983) discloses an improved vacuum infusion coating method by applying a coating to a partially immersed coating medium. The substrate is subjected to a strong vacuum for a sufficient period of time while the coating medium is evenly pulled up into the grooves of the monolithic substrate to obtain a uniform coating profile. Substrates coated in this way are homogeneous but difficult to dry. This is because certain components of the coating medium evaporate under the coating conditions and form a vapor layer in the bath. The presence of this vapor layer prevents drying of the coating medium and further delays the production of coated substrates. Facilitating the drying process by increasing the strength of the vacuum during the vacuum injection of the coating medium may allow the coating medium to be pulled further into the tank than desired and ultimately create a non-uniform coating profile.
Therefore, if it is possible to speed up the drying of the substrate while maintaining a uniform coating profile in the groove of the substrate, then this would be a monolithic substrate coating, in particular, a monolithic substrate for use in catalytic converters A significant advance in the field of material coating.
