Coating processes – With post-treatment of coating or coating material – Solid treating member or material contacts coating
Reexamination Certificate
1999-05-21
2001-04-17
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Solid treating member or material contacts coating
C427S358000, C427S361000, C427S395000, C427S411000, C427S420000, C427S424000
Reexamination Certificate
active
06217940
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for coating paperboard with at least two coating layers.
The invention also relates to an arrangement for implementing the method.
BACKGROUND OF THE INVENTION
Paperboard can be coated with one or more coating layers in order to improve its printability. The more coating layers are applied, the higher the attained paper quality, simultaneously enabling the use of coating agents having different properties. However, the use of two coating layers entails a significant increase in production costs because the coating is usually carried out in separate steps, and the applied coating layer has been dried prior to the application of the next layer.
The coating can be performed either directly onto the board surface by means of, e.g., a blade or a rod coater, or by means of a film transfer coater whereby film which has been apportioned onto a film roll is transferred onto the paper surface in a roll nip. It is typical of the blade and rod coating methods that the doctor blade fills the roughness volume of the coating underlayer and evens out the surface whereby the thickness of the coat varies in accordance with the roughness volume variations of the coating underlayer. A smooth coat results, having an uneven brightness coverage, and the uniform absorption properties of the coat are difficult to control.
In film transfer coating, a coat of an essentially more uniform thickness is obtained rendering it easy to control the absorption properties, but sufficient smoothness properties pose a problem, particularly in the case of thicker papers and boards. The coating also provides quite an even coverage, whereby, for example, the brightness of a coating underlayer of low brightness can be significantly improved by means of this method. Furthermore, as no doctor blade trailing along the coating underlayer is involved in film transfer coating, the method offers excellent runnability with respect to coating breaks.
When paperboard is manufactured furnished with two or several coating layers, the investment costs involved are significant, because the coated web must be dried prior to the application of the next layer. This results in great length of the coating line, wherefore it requires a lot of room in the factory hall. The drying also requires a significant amount of thermal energy. Thus, it would be of advantage to apply the second coating layer onto a wet or damp first layer without any intermediate drying step. This, however, leads to problems in that the application and evening out of the second coating layer scrapes most of the first wet coating layer off the board surface and a thin final coat results, whereby at least some of the advantages of the duplex coating are lost.
SUMMARY OF THE INVENTION
The present invention is directed to a method and apparatus by which a paperboard web can be provided with two coating layers without any intermediate drying.
The invention is based on applying a second coating layer onto the first coating layer by means of a nozzle applicator device. The first layer is advantageously applied by means of a film transfer coater.
In more detail, the method according to the invention is characterized by what is stated in the characterizing part of claim
1
.
The apparatus according to the invention, then, is characterized by what is stated in the characterizing part of claim
13
.
The invention offers considerable benefits.
The most important benefits attained by the invention relate to improving the quality of the coated paper board. The coating method according to the invention achieves better coverage resulting in good optical smoothness of the surface. The coated surface obtained is very smooth and glossy. The unexpectedly good optical properties attained especially by means of one embodiment of the invention are based on the fact that, during the second coating step, the coating particles are preoriented in the direction of the web surface, thus achieving good coverage and brightness properties. Combining this with a coating layer of uniform thickness obtained by means of a film transfer coater will achieve a surface of essentially more even color than those produced by conventional methods. When performed by a nozzle applicator device comprising a small-angle blade or by an application nozzle, the application of the second layer does not impose similar strain on the damp first layer as does, for example, short-dwell application, and thus, the first coating layer is not scraped off the web surface when the second layer is applied. Thus, the solution according to the invention achieves a greater total coat weight than the previous wet-on-wet methods. By means of a film transfer coater, the web underside can be wetted in a controlled manner, thus avoiding warping of the board caused by water absorption from the coat by the web. The wetting of the underside of the web can be done with the same apparatus as the actual coating. The duplex coating according to the invention provides considerable savings in space compared to ordinary duplex coating where the web is dried prior to the application of the next layer. The invention is especially beneficial in coating paper board with an on-line coating apparatus since the web coming from the board making machine is usually hot, whereby the dry matter content rises rapidly when the coating mix of the first coating layer comes in contact with the hot web.
In addition to a film transfer coater, other methods can be used to apply the first coating layer. It is a fact that a film transfer coater usually provides the best coating quality and good runnability, but a similar coating layer can be achieved by means of applicator roll and pool coaters, a curtain coater apparatus, and nozzle coating devices and spray coaters, or even spray application devices. It is also feasible to use a device for applying the first coating layer, which evens out the coat by means of a doctor device, for example a short-dwell applicator, but this will achieve a poorer first-layer coverage than the above-listed devices. Correspondingly, a smoother first-layer surface is attained, resulting in a smoother final surface.
In the following, the invention is described in more detail by means of the annexed drawings.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.
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Cameron Erma
Cohen & Pontani, Lieberman & Pavane
Valmet Corporation
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