WO2009040471A2

WO2009040471A2 - Method and arrangement for coating a web of fibre material with at least two layers of coating

Info

Publication number: WO2009040471A2
Application number: PCT/FI2008/050530
Authority: WO
Inventors: Jouni Haavisto; Simo Rahkonen
Original assignee: Metso Paper, Inc.
Priority date: 2007-09-27
Filing date: 2008-09-24
Publication date: 2009-04-02
Also published as: FI121391B; FI20075673A0; DE112008002205T5; FI20075673A; AT507747A2; AT507747B1; WO2009040471A3

Abstract

A method of and an arrangement for coating a web of paper, board or the like mainly comprising plant fibbers with at least two layers of treatment materials. In the method the web (3) is directed into the first coating step (1) and one layer of coating material is applied onto the first surface of the first web (3), subsequent to which the web is directed into the second coating step (10) and at least a second layer of coating material is applied onto the first surface of the web. A blade coater (1) is used in the first step and a contactless coater (10, 12) is used in the second step and the second coating layer is applied over an at least partly first coating layer.

Description

Method and arrangement for coating a web of fibre material with at least two layers of coating

The present invention relates to a method of coating a web of paper, board or the like mainly comprising plant fibbers with at least two layers of treatment materials.

The invention also relates to an arrangement for carrying out the method.

The most common method of producing a multi-layer coating is to make one coating at a time and to dry the product before applying the next coating layer. Usually, at least the coating layer itself is dry, the humidity of the substrate paper can increase from one layer to another. There are two principal reasons for drying the coating layers between applications. First, if the web is directed with contacting methods, such as rolls, the wet coating would stick to the surface of the rolls, causing contamination of the rolls and degrading the applied coating layer. On the other hand, the coating layer applied first must be able to withstand the stresses caused by the application of the next layer in order to be able to apply the layer at all. When using contacting applying methods, applying the subsequent coating layer on the previous moist or wet layer can be difficult and at least the total thickness of the produced coating will be low.

Various methods for applying a multi-layer coating without intermediate drying (wet-on-wet) have been developed. Usually these employ a film transfer coater for applying the first layer and the applying method of the next layer varies according to the desired properties of the final product. The problem of these known methods is achieving a sufficient smoothness without losing a sufficient coverage. For a good final smoothness a coarse board or paper product must either be strongly calandered or the irregularity volume of the substrate must be filled by blade coating.

Using contactless coating only it is difficult to achieve a smooth end product even by calandering, if the substrate paper is coarse. Blade coating has also been used as the first coating method of twin—layer coating and the applied coating layer has been dried in a normal way prior to applying the next layer .

The object of the invention is to provide a method of and an arrangement for coating fibrous material by means of which products having at least two coating layers can be produced while combining a good coverage of the coating layers with better smoothness than previously.

The aim of one embodiment of the invention is also to provide a coating method by means of which a two-layer coating can be carried out in a small space, such as in the place of a previous single-layer coating.

Further, the aim of one embodiment of the invention is to provide a coating arrangement in which multi-layer coating can be carried out using a smaller number of machine components than previously.

The invention is based on coating the fibrous web at least once by means of a blade coater and subsequent to this at least by means of contactless coater.

According to one preferred embodiment of the invention the contactless coating method is curtain coating. According to one preferred embodiment of the invention the contactless coating method is spray coating.

According to one preferred embodiment of the invention the contactless coating method is dry coating.

Further, according to one embodiment, paper with a coating layer applied to it by means of blade coating method is dried at the most until the common humidity percentage of the substrate paper and the coating layer is at least 6-8, more preferably at least 8-10, even more preferably at least 10 or more and most preferably the paper with its coating layer are not actively dried at all but the web is directly and without drying directed into a second coating step. If the paper is not actively dried, its humidity percentage can also be in some of the above-mentioned intervals .

Further, according to one embodiment, board with a coating layer applied to it by means of blade coating method is dried at the most until the common humidity percentage of the substrate board and the coating layer is at least 9-10, more preferably at least 10-13, even more preferably at least 13 or more and most preferably the board with its coating layer is not actively dried at all but it is directly and without drying directed into a second coating step.

More specifically, the arrangement and method according to the invention are characterized by what is disclosed in the characterizing parts of the independent patent claims.

The preferred embodiments of the invention are more specifically defined in the dependent claims.

Considerable advantages are achieved by means of the invention.

Relatively good smoothness is required from at least print papers but also from other coated surfaces to be further treated in order to achieve good further treatability . In printing too uneven a surface can easily lead to uneven printing results. The problem of contour-following coating methods , such as curtain and spray coating is the too high roughness level in the final product subsequent to coating caused by the too high roughness of the substrate paper. Pre-calandering can, of course, be used for smoothening the substrate paper and it does even out the peaks of substrate paper, but the depressions of the substrate paper are, correspondingly, not treated. The best means for smoothening the substrate paper and thus producing a smooth final product is precoating with a blade effectively filling the depressions in the substrate paper. Because contactless coating, such as spray or curtain coating, forms a uniformly thick layer following the contour of the underlying surface, the smooth surface smoothened by means of a doctor blade of a blade coater is copied also as the surface of the coating layer produced by means of a curtain coater and thus good smoothness is achieved.

The idea of the invention is to combine a blade precoating and a surface-following contactless surface coating, either dry coating, spray coating or, most preferably, curtain coating into a combined, compact process . This is achieved by reducing or eliminating the normal drying after blade coating prior to the contactless coating. As the contactless coating does not stress the paper web, such a process can also be carried out with good driveability in practice. As there is no contact with the web, the risk of web breakages is low and the applying of the layer following the blade coating does not scrape or otherwise displace the previously applied, possibly very dry layer. The proposed layouts allow a considerable saving of space. This allows constructing a shorter machine hall, more preferable frame structures for the coating station and a smaller number of web guide rolls in comparison to, for example, two separate coating stations. In total, these add up to considerable cost savings when building a machine capable of double coating the web. The compact layout allows efficient use of the surrounding space of the coating stations in both length and height directions . This also allows a small reduction of total drying.

The humidity values presented above for paper and board are absolute limit values at their lower threshold. With the at least one drier located downstream of the blade coating it is usually possible to achieve a maximum of 85% dry solids content for the coating layer which can be considered the limit for the so-called contact dryness of the coating. If the moisture level is at the lower limit or less than it (6-8 for paper, 9-10 for board), the coated surface is already contact dry and the web can be guided by means of e.g. application roll.

So, if the first coating layer is made with a blade coater, achieving a value below the lower threshold is usually only possible by using two driers. The idea of the invention is to keep the amount of drying downstream of the blade coating so small that the surface is wetter than contact dry. This will also allow reduction of the drying power after the first coating step. In order to do this, the coating of the second step must be contactless so that coating of the second step does not damage the still wet first coating layer. For the quality of the coated surface the blade coating of the first layer and the contactless coating provide the advantage that blade coating forms an even surface and it fills the pores of the substrate paper or board. As the coating layer accurately follows the contours of the lower surface when using contactless coating methods it will also be smooth. Simultaneously a good coverage of the coating, typical of contactless coating methods, is achieved.

In the following, the invention is disclosed in more detail by means of reference to the appended drawings .

Figure 1 is a schematic side view of an embodiment of the invention.

Figure 2 is a schematic side view of a second embodiment of the invention.

In the context of this application blade coating means coating methods in which the coating layer is smoothened to the surface of the fibrous web subsequent to applying. Such methods include short dwell time coating, combination of jet applying and a doctor blade, various roll applying methods combined with a subsequent doctoring and smoothening and, for example, spray or curtain application followed by doctoring. The term contactless coating refers to methods in which the coating material is applied as e.g. a curtain, spray or dust onto the surface of the fibrous web without mechanically supporting the flow of the applied material as it flows from the application apparatus to the web. In the solution of figure 1 the blade coating station 1 is arranged on the floor surface 2 of the machine hall, the web to be coated arriving from under the floor level . The web is directed through the machine along the desired route by means of guide rolls 4 attached to the frame 5. First the frame is directed to the blade coating station 1 in which the web circulates via a counter roll 6. In the direction of travel of the web the first is the jet application apparatus 7 and the web 3 travels between the counter roll 6 and the jet application apparatus 7. The first coating layer is applied to the web by means of the jet application apparatus. Next, the web travels forward on the surface of the counter roll 7 to the scraper blade that smoothens the surface of the coating. From the blade coating station the web is directed upwards again by means of directing with the guide rolls 4, supporting from the uncoated side. In this example there is a one-sided drier 9 subsequent to the blade coater for drying the first coating layer. The necessity of the drier 9 depends on the process conditions, such as the initial moisture of the substrate web, the coating mixture and the amount of the applied coating material. However, because in the next coating step the coating is applied in a contactless manner, the high water content of the coating material is not a problem, because even a fragile coating stays on the web during contactless coating. The increase of the moisture of the substrate web is neither a problem, because no large stresses are subjected to the web in the following steps, so even a wet web stays intact in the process without risk of web breakages. Thus, the use of the drier is largely determined by whether there is room for the drier 9 between the blade coater and the contactless coater in the machine line. If there is room, the drier 9 can be used for FI2008/050530

increasing the adjustment possibilities of the process. The power of the driers is limited to be smaller than the power needed for fully drying the first coating layer. Most preferably the drier is a one-sided air drier using blower air blown in a high temperature of over 400 ⁰C. Such a drier allows an efficient drying shock and dewatering in a small room.

From the drier 9 the web travels to a spray coater 10 and from there further to a conventional drier part 11. In the spray coater 10 is coated the same side of the web as with the blade coater 1. In this case there is a contactless drier apparatus at the beginning of the drier part 11 and the web is directed from the spray coater 10 in a contactless manner to the drier part 11. This allows two- sided coating of the web by means of a spray coater. However, as a one-sided blade coater 1 is used in this case, the spray coater will also be one-sided, because otherwise the coating would be asymmetric. Thus a two-sided coating can be accomplished by using two successive apparatuses according to figure 1. A drier part 11 is usually provided between the coating steps of the different sides. However, if the first coating step is performed by means of a two-sided blade coater with a subsequent two- sided spray coater, it is possible to achieve a relatively short and compact coating process in which both sides are treated with at least two coating layers . When using either a one-sided or two-sided blade coating the spray coater can be one- or two-sided depending on the produced paper grade. With a two-sided station, coating material or glue can be applied to the other side, i.e. back side, for controlling curving with some special papers, such as label papers.

The coating station or apparatus of figure 2 is otherwise similar to that in figure 1, but there is a curtain coater 12 instead of the spray coater 10. A curtain coater has some advantages due to which it is very preferable in connection with this invention. Being a contactless method, the driveability of the curtain coater is good as it does not exert stresses on the web. However, it can simultaneously achieve both quite low and high doses of coating material, whereby it provides great adjustability for the process. One quite significant point is that it is possible to produce a multi-layer coating at one coating run. Thereby the curtain coater has two or more successive application slots by means of which a multi-layer curtain or a number of sequenced application runs can be formed. Usually the number of application slots is one, two, three or four, but in principle the number can be larger as well. When this is combined with the smooth surface produced with a blade coater, great optimization possibilities are provided. Thus a curtain coater producing a multi-layer coating is a very preferable embodiment .

The method allows, for example, the use of an expensive coating material as a thin layer only in the uppermost layer while using less expensive materials in other layers . On the other hand, the multi-layer coating allows producing print papers customized to meet the needs of a customer by optimization of each layer. It is further possible to utilize the possibility of a curtain coater for small application amounts. This, further combined with the good smoothness made possible by the blade coater, makes it possible to inexpensively improve the printing properties of a paper to a new level. Thus it is possible to market new grades either replacing previous, more expensive grades, improving the printability of previous grades or creating totally new paper grades with their new possibilities of use. These advantages apply as well for board as other printable fibrous web materials .

Paper or board comprising more than two coating layers can also be produced by adding more coating stations to the line. For example, a dry coater could be located after the contactless coater, application by means of which after forming a new coating layer would also dry the web, thus decreasing the total drying power. The dry coating can also be performed before blade coating or some other coating method can be used before dry coating. It is essential for the invention that the process comprise one blade coating step for achieving smoothness and subsequent to this the next coating layer is applied onto the smooth surface by means of a contactless method for achieving a good coverage and, due to the contour-coating, the good smoothness of the surface is also copied onto the surface of this layer. According to the invention it is also possible to produce various asymmetric products and, if necessary, to treat only one side of the web. The method can also be combined with pre-calandering or final calandering.

On the basis of the above-mentioned examples it will be obvious that a number of solutions, differing from the above-mentioned embodiments, are possible within the scope of the invention. Thus, it is not the intention to limit the invention to the above-mentioned examples only, but the patent protection is to be considered within the full spectrum of the appended patent claims .

Claims

We claim:

1. A method of coating a web of paper, board or the like mainly comprising plant fibbers with at least two layers of treatment materials, in which method:

- the web (3) is directed into a first coating step (1) and one layer of coating material is applied onto the first surface of the web (3) , and

- the web (3) is directed into a second coating step (10) and at least a second layer of coating material is applied onto the first surface of the web, characterized in that

- a blade coater (1) is used in the first step,

- a contactless coater (10, 12) is used in the second step, and

- the second coating layer is applied over at least a partly wet first coating layer.

2. A method according to claim 1, characterized in that the contactless coating method is curtain coating.

3. A method according to claim 1 , characterized in that the contactless coating method is spray coating.

4. A method according to claim 1, characterized in that the contactless coating method is dry coating.

5. A method of producing paper according to any of claims 1-4, characterized in that the coating layer applied by means of blade coating method (1) and the substrate paper is dried at the most until the common humidity percentage of the substrate paper and the coating layer is at least 6- 8%, more preferably at least 8-10%, even more preferably at least 10% and most preferably the coating layer is not actively dried at all, but it is directed directly and without drying into the second coating step.

6. A method of producing board according to any of claims 1-4, characterized in that the coating layer applied by means of blade coating method (1) and the substrate board is dried at the most until the common humidity percentage of the substrate board and the coating layer is at least 9- 10%, more preferably at least 10-13%, even more preferably at least 13% and most preferably the coating layer is not actively dried at all, but it is directed directly and without drying into the second coating step.

7. A method according to any of claims 2, 5 or 6, characterized in that a number of coating layers is applied by means of a curtain coater (12) .

8. A method according to any of claims 1-7, characterized in that two-sided coaters are used, whereby both sides of the web are treated simultaneously.

9. A method according to any of claims 1-7, characterized in that one-sided coaters are used, whereby both sides of the web are treated successively.

10. A method according to any of claims 1-7, characterized in that the first side of the web (3) is coated at least once before the blade coating (1) .

11. An arrangement for coating a web of paper, board or the like mainly comprising plant fibbers with at least two layers of treatment materials, the arrangement comprising: - a first coating station (1) for applying one coating layer to the first surface of the web (3), and - a second coating station (10) for applying a another coating layer onto the first surface of the web, characterized in that

- the first coating station comprises a blade coater (D,

- the second coating station comprises a contactless coater (10, 12) , and

- the power of the driers (9) between the first and second coating station is limited so as to be smaller than the power needed for fully drying the first coating layer.

12. An arrangement according to claim 11, characterized in that the contactless coating method is a curtain coater (12) .

13. An arrangement according to claim 11 , characterized in that the contactless coating method is a spray coater (10) .

14. An arrangement according to claim 11, characterized in that the contactless coating method is a dry coater (12) .

15. An arrangement according to any of claims 11-14, characterized in that one-sided coaters are used.

16. An arrangement according to any of claims 11-14, characterized in that two-sided coaters are used.