CONTACTLESS LEVELING OF A WASHCOAT SUSPENSION

Abstract

The invention is directed to a process and an apparatus for coating substrates of motor vehicle exhaust gas catalysts. The invention particularly describes an improvement in such coating processes in which a suspension (washcoat) containing the catalytic material is applied to such a substrate (monolithic substrate) from above (metered charge process).

Claims

1. Process for coating a substrate monolith of the flow-through or wall-flow type with a washcoat suspension, characterized in that the washcoat suspension is applied from above to one end of an upright substrate monolith in a first step, a shear force in the form of pressure transmitted by gas is allowed in a subsequent step to act on the surface of the washcoat suspension in order to level it in a contactless manner, and the washcoat suspension is subsequently sucked into and/or pressed into the substrate monolith.

2. Process according to claim 1, characterized in that the shear force is generated by a method selected from the group consisting of a continuous gas stream, a pulsating gas stream, or a mixture thereof.

3. Process according to claim 1, characterized in that the pressure acting on the washcoat suspension by gas is reduced continuously or stepwise during the duration of the action.

4. Process according to claim 1, characterized in that the pressure acting as a result of gas or sound is greater than the flow limit of the washcoat suspension.

5. Process according to claim 1, characterized in that the pressure is generated by a pulsating gas/air stream from a sound- or pressure-wave generator.

6. Process according to claim 5, characterized in that the pressure is generated by ultrasound at a frequency of about 18,000 to about 90,000 Hz.

7. Process according to claim 5, characterized in that the pressure is generated by ultrasound for a period of about 0.1 to about 60 seconds.

8. Process according to claim 1, characterized in that the pressure is generated by a gas/air stream from an air nozzle.

9. Process according to claim 1, characterized in that the washcoat suspension has a viscosity of 0.01 to 100 Pa*s.

10. Process according to claim 1, characterized in that the washcoat suspension on the substrate monolith has a thickness of 0.5-15 cm.

11. Apparatus for coating a substrate monolith of the flow-through or wall-flow type with a washcoat suspension according to claim 1, comprising: a unit for locking the substrate monolith in the upright position; a unit for applying an amount of washcoat suspension from above to the upright substrate monolith; a unit for generating a shear force in the form of pressure which is transmitted to the surface of the washcoat suspension by air or gas.

12. Apparatus according to claim 11, characterized in that the apparatus has a unit which forms a collar around the substrate monolith in such a way that no washcoat suspension can run down the substrate monolith on the outside during the action of the pressure.

Description

[0056] FIG. 1:

[0057] The drawings in FIG. 1a-d represent various embodiments of a centrally rotatable gas/air nozzle. In this case, the length of the gas/air nozzle (flat nozzle, knife nozzle) is adapted to the diameter of the substrate. The type of gas/air stream can be influenced by the type and shape of the outlet opening. A laminar flow may be created by a continuous slot having a width of 0.5 to 5 mm, while a plurality of small outlet holes instead of the continuous slot generates a turbulent flow. A rotatably mounted flat nozzle can be rotated in a simple manner by a gas/air stream exiting orthogonally to the axis of rotation through a separate opening mounted at the end of the nozzle. Alternatively, other electrical or mechanical drives may also be used to rotate the nozzle.

[0058] FIG. 2:

[0059] FIG. 2 shows schematic diagrams of a gas/air nozzle, which is guided over the substrate in the x direction for the smoothing operation. This flat nozzle can also have a continuous slot or a multiplicity of individual air outlet holes.

[0060] FIG. 3:

[0061] The drawing shows a gas/air brush which corresponds in its size and shape (round, oval, angular) to the geometry of the substrate used. The discharge of the gas/air stream takes place through openings in the bottom plate, the size and number of which are selected in such a way that the resulting air stream can exert sufficient pressure on the washcoat surface to overcome the flow limit.

EXAMPLE 1

[0062] 50 g of a structurally viscous washcoat having a viscosity of 370 mPa*s at a shear rate of 150/s and a solid content of 42% are placed in a tray. Due to the flow limit of the washcoat, an irregular surface is formed during pouring. To smooth the surface, an ultrasonic generator with a frequency of 20 kHz and a power of 4 kW is used. Sound generation and transmission to the surrounding air takes place via a circular sonotrode with an end face of 3.8 cm.sup.2. After the ultrasonic generator is switched on, the distance between the sonotrode and the surface of the washcoat is continuously reduced until, at a distance of 1 to 2 cm, the sound pressure of the air waves generated by the ultrasonic oscillation effects a smoothing of the surface. The sound pressure required for smoothing can also be adjusted by controlling the ultrasonic power itself.

EXAMPLE 2

[0063] The washcoat of Example 1 is placed in a tray. Due to the pronounced flow limit of the washcoat used (approx. 100 Pa), an irregular surface is formed during filling into the tray. A flat nozzle having a slot width of 1 mm and a slot length of 6 cm is mounted above the surface of the washcoat. The knife nozzle is connected to a compressed air supply and operated at an air pressure of 6 bar. By lowering the nozzle to the surface of the ceramic suspension, at a distance of 10 cm from the surface, the flow limit of the washcoat is overcome and the liquid surface is leveled by the generated pressure of the air stream. The entire surface can be smoothed by a linear movement of the nozzle. By suitable selection of the parameters for the slot width, nozzle distance from the surface of the washcoat and air pressure, the person skilled in the art can select the performance of the generated laminar air stream such that the pressure applied to the liquid surface can be adjusted in a targeted manner to the flow limit of the washcoat used in each case.