Coating processes – With pretreatment of the base – Heating or drying pretreatment
Reexamination Certificate
1999-01-12
2001-10-30
Beck, Shrive P. (Department: 1762)
Coating processes
With pretreatment of the base
Heating or drying pretreatment
C427S324000, C427S326000, C427S361000, C427S377000, C427S421100, C427S424000
Reexamination Certificate
active
06309704
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention.
The present invention relates to a device for direct or indirect application of a liquid or viscous coating medium onto a moving material web, specifically a paper or cardboard web.
2. Description of the Related Art.
Devices for direct or indirect coating of a moving material web are generally known in the current state of the art. Reference can be made to the VOITH SULZER article “Advances in Coating Equipment” by Dr. M. Kustermann. This article may be acquired from the assignee under order number P3008.
The process of coating medium application by use of an applicator unit onto the moving material web is generally influenced by a series of disturbing influences. Even when the coating medium is supplied to the material web, irregular air flows across the operating width of the material web that is to be coated may negatively influence the coating result. Particularly at high operating speeds, these air currents originate from the boundary air layer that is carried along by the material web on its surface. It also has a negative effect on the coating result if the absorption capacity of the material web that is to be coated varies in its longitudinal or flow direction, or in its cross direction. This specifically addresses the absorption capacity of the material web, which is at least one factor responsible for how quickly the coating medium dewaters into the material web. It is also detrimental if, based on the given physical characteristics such as surface tension and viscose elasticity, undesirable structures occur in the applied medium. In indirect application methods, there are additional irregularities in the surface structure of the coated material web, resulting for example, from the so-called film splitting effect. In direct application methods—for example when utilizing an open jet nozzle coater with a downstream doctoring device, a “Spray Coating” coater, a “Curtain Coating” coater or similar problems may develop. These problems include effects such as doctoring streaks, grooving when utilizing profiled metering rods, ink splashes or film splitting when utilizing smooth metering rods, and metering rod flooding (this term is generally used for excessive build up of coating medium at the end of a metering rod), which may negatively influence the coating result.
SUMMARY OF THE INVENTION
The present invention further develops an applicator unit of the generic concept so that the negative influence of the previously explained effects upon the achieved coating result are at least reduced, if not entirely eliminated.
An applicator unit is provided which, in the direct application method, applies the coating medium in the form of a coating layer directly onto the material web at an application point. In the indirect application method, the coating medium is first applied onto an applicator element, for example an applicator roll, which then, at an application point, transfers the coating medium to the material web in the form of a coating layer. A drying device for drying of the coating layer is provided downstream from the point of application, viewed in the web flow direction.
A device for moistening and/or warming of the material web and/or the coating medium or the application layer is provided prior to the dryer, when viewed in the direction of material web flow. An advantage of the method according to the invention is to be found in that the coating medium dries slower, thereby reducing its viscose elasticity.
The result is that the coating medium remains free-flowing, so that the differences in applied coating thickness and irregularities in the surface structure of the coating layer can be uniformly distributed. Collectively, an independent leveling of the coating layer results. Moistening and/or warming may be effected by use of vapor, preferably water vapor which is caused by evaporation of a liquid in the area of the applicator unit and/or outside the applicator unit and is then supplied to the applicator unit. Additionally, or alternatively, moistening may also occur through spraying of a liquid, preferably water. In addition to a certain level of dilution of the coating medium, the warming of the coating medium that is preferably achieved with vapor also results in a viscosity reduction, which ensures better flow of the coating medium.
The utilization of water, be it in the form of water vapor or in the form of a water spray mist, offers the advantage that the applied water can again be removed in the downstream dryer and, therefore, the structure of the coating layer is not impaired.
The moistening and/or warming device as provided under the invention may be located at one or more of the locations discussed below:
For example, the moistening and/or warming device may be located immediately prior to the point of application, when viewed in direction of web flow. If it releases the vapor or the spray mist in the direction toward the point of application, the material web and the coating medium could be moistened simultaneously. Moistening of the material web reduces variations in its absorption and provides for uniform dewatering of the coating medium into the material web. Moistening and/or warming of the coating medium reduces its surface tension and reduces its viscosity, homogenizing the wetting of the material web with the coating medium and increasing the uniformity of the layer that is to be applied, even prior to the transfer to the material web.
Additionally, the influence of boundary air layer which, particularly at high material web speeds is carried along by the web and impairs the coating result, is reduced.
The moistening and/or warming device which is located before the applicator unit may, for example, include a vapor supply line which, in the area of the applicator unit, is equipped with at least one vapor outlet opening on the side facing the application area. Particularly with a view toward reducing the influence of the boundary air layer that is transported along by the material web, the vapor outlet opening forming side sections of the vapor supply line or of the side sections following the vapor outlet opening, viewed in vapor flow direction, are preferably tapered in the direction of vapor flow. An excessive amount of vapor nay be blown into the application nip through this vapor outlet nozzle, so that at least a portion of the introduced vapor will again leave the application area along the material web surface, although in opposite direction to the web flow direction. However, it is also possible to introduce only a sufficient amount of vapor which can be absorbed by the material web and/or the coating medium and which, therefore, can be carried along.
The moistening and/or warming device may also be located following the point of application, viewed in direction of material web flow. For example, in direct application methods, it may be located between the point of application and a device for leveling and/or metering the coating layer which would be installed downstream from the point of application, viewed in direction of web flow. However, it is also possible that the moistening and/or warming device is provided following a device. for leveling and/or metering the coating layer, which would be installed downstream from the point of application, viewed in direction of web flow.
When the moistening and/or warming device is installed after the point of application when viewed in direction of material web flow, it is preferable that the moistening and/or warming device directs the vapor or the spray mist substantially orthogonally to the material web surface, since this permits effective moistening of the coating layer surface. In addition, the kinetic energy of the vapor can basically be used for averaging.
For example, the moistening and/or warming device may include a vapor supply line which is equipped with at least one vapor outlet opening in the side section facing the material web.
In order to ensure sufficient moistening of the coating layer, it is further suggested that a v
Hess Harald
Kurtz Rudiger
Mendez-Gallon Benjamin
Beck Shrive P.
Calcagni Jennifer
Taylor & Aust P.C.
Voith Sulzer Papiertechnik Patent GmbH
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