WO2012163787A1

WO2012163787A1 - Method for producing a woodfree coated or lightweight coated paper web

Info

Publication number: WO2012163787A1
Application number: PCT/EP2012/059679
Authority: WO
Inventors: John Bergman
Original assignee: Voith Patent Gmbh
Priority date: 2011-05-30
Filing date: 2012-05-24
Publication date: 2012-12-06
Also published as: DE102011076713A1

Abstract

The invention relates to a method for producing a woodfree coated or lightweight coated paper web (1), a pigment-containing, liquid to pastelike application medium (4, 5) being applied to at least one side (1a) of the web (1), as it runs through a coating machine, by means of curtain coating. According to the invention, a wetting agent (8) is applied to the same side (1a) of the web before the single-coat or multi-coat of the pigment-containing application medium (4, 5) is applied, to raise the moisture content of the web surface (1a) from approximately 3% to up to 15%.

Description

Method for producing a wood-free coated or lightly coated paper web

The invention relates to a method for producing a wood-free coated or lightly coated paper web, wherein on at least one side of the running through a coater web a pigment-containing, liquid to pasty application medium is applied by curtain coating.

From DE 10 2007 029 328 A1 a method for producing a substantially wood-free paper web (WFC - WoodFree Coated) is known. In this method, the web is provided with a double line on at least one side of the web, with the last coating being made by a non-contact application method, such as curtain coating. One wants to avoid previously used doctor blade coating process, because previously by the action of the doctor element a relatively heavy stress on the web and web breaks were recorded.

Wood-free paper is so-called graphic paper.

WO 2005/059245 A1 and WO 2005/052252 A1 disclose processes for producing an LWC paper (Light Weight Coated Paper). As the name implies, such a paper has a coating of only light weight. Above all, it should be easy to print and suitable for gravure printing. Such papers are used for catalogs and magazines. At present, LWC paper is provided with a blade or film coating.

A curtain coating method is known from DE A1 100 12 344. From DE A1 10 2007 000 782 the application of at least two layers with a curtain applicator is known. As a curtain applicator here is a slot die (English: Slot Die) into consideration, from the substantially gravity following the application medium from above falls down on the web page to be coated in the form of a closed curtain and there is deposited as a contour line. From EP-A1 1 255 615 another embodiment of a curtain applicator is known, which is referred to in professional circles as a slide layer nozzle (English: Slide Die). Here the Auftragmediunn only falls on the substrate to be painted after it has evened on a sliding surface of the nozzle.

The curtain coating is contactless when using the two types of commissioned work. That is, the amount of application that is applied remains on the paper. There is no doctor blade doctoring (e.g., doctor blade or blade).

Very often it has been found that the coating layer applied with the curtain applicator is scarred or craters, i. uncovered bodies located between 20 μιτι and 150 μιτι are large. This is a quality defect because later on the web to be applied pressure then also quality defects, i. Has printing defects. The said craters are sometimes larger than those of printed grids. Craters larger than 50 μιτι are even visually recognizable and particularly disturbing.

It is therefore an object of the invention to provide an improved method for applying application medium to the surface of a moving paper web for producing a wood-free coated or lightly coated web, in which craters of greater than 25 μιτι not occur, or at least reduced.

The object of the invention is achieved by a method according to claim 1. Advantageous embodiments emerge from the dependent claims.

After initially a fiber suspension is prepared, then introduced the fiber suspension in a headbox and a fibrous web is produced in a former, followed by dewatering in a press and then drying the web. Subsequently, a coating of the paper web is carried out either in online mode (without previous rolling and again Unrolling the web) or in offline mode (ie with a separate from the web-making machine coating machine) for the purpose of finishing or to improve the printability of the web. As a coating method is preferably thought of the known curtain coating.

According to the invention, it is provided that a moistening agent is applied to the same side of the web before said one or more layers curtain coating or application of the pigment-containing coating medium, so that the moisture content of the web surface of about 3% is increased to up to 15%.

It has turned out, quite surprisingly, that no disturbing craters have formed on the painted web surface.

The applied coating layer is now very even and ideal for later printing. Apparently, the spraying of the humectant has caused a displacement of air from the existing surface pores of the paper. This achieves better wetting of the coating medium on the paper web surface. The surface of the coated paper web is characterized by a uniform surface and smoothness.

According to the humidifying agent is sprayed in an amount of about 3 I / h and under a pressure of about 3 bar, wherein a drop size of the humidifying agent to be sprayed of less than 20 μιτι is used.

The humectant is water or demineralized water. If desired, additives such as surfactants such as surfactants may be blended. The advantage of this measure is that the penetration of the humectant into the web can be forced, but also delayed. For example, a medium weight polymer could be used, which is the better crosslinking of the paper fibers with the coating medium is used.

It is advantageous with regard to the absorption capacity of the paper web for the moistening agent or its spray droplets if, during the application of the moistening agent, the web is drawn off on its opposite side. For this purpose, the web can be guided via a suction device, for example a suction roller.

It is also expedient if the moistening agent is applied spatially separated from the subsequent curtain coating.

This can be done by a climate chamber or cabin is arranged to the subsequent curtain applicator or just a barrier between applicator and sprayer is installed. This avoids negative influences on curtain stability.

In this context, it is also expedient if the moistening agent is applied even before removal of the air boundary layer flowing with the web takes place. The subsequently delivered single or multi-layer curtain which is sensitive to air currents can thereby fall down on the web with even greater stability.

As already stated, the curtain coating can also be multi-layered. For this purpose, it is provided that two so-called single-curtain nozzles arranged one behind the other are used for dispensing two individual curtains or a unit forming double curtain nozzle or a multi-curtain nozzle. Of course, only the delivery of a single curtain is possible.

The curtain coating for the application of the individual layer or of several layers or layers is carried out with known curtain slot dies or slot dies and / or known sliding layer nozzles or slide dies. These application units apply the coating colors very evenly and without excess to the paper or cardboard surface. Here are the respective Application media in separate single-layer or multi-layer curtains, substantially in accordance with gravity, delivered to the pre-moistened paper. Since the curtain application is 1: 1, that is applied only in the amount that is to remain on the web surface, you save expensive application medium.

In the case of multi-ply application, lower values should be selected for the coat weight, for the viscosity and for the solids content of the lowermost applied layer of the application medium than for the overlying layer or layers. For example, the lowermost layer has a solids content of about 30 or 40% and top layer 60 or 70%.

So that the previously applied moisture is maintained, it is also advantageous if for the lowest layer of the application medium, a coating weight between 1 and 5g / m ² , preferably 2 to 4g / m ² , a viscosity BR100 between 20 and 500mPas, preferably BR100 between 30 and 300mPas is chosen.

Preferably, the uppermost layer (top coat layer) and one possible intermediate layer becomes wet-in-wet, i. applied without application of an intermediate drying on the already applied to the paper surface bottom layer (base coat layer). The top layer anchors itself particularly well with the base layer or the intermediate layer as a result of the wet-on-wet application. In addition, costs for the unneeded drying facilities are saved.

To produce a wood-free coated or a light-weight coated paper web, a porosity measured according to Bendtsen has proved to be particularly suitable, which is equal to or greater than 200 ml / min.

Following the treatment of the first side of the web, the second side of the web can also be treated analogously to the first side of the web.

With the help of the method according to the invention, both wood-free paper (WFC) - so-called graphic paper, as well as LWC catalog or Produce magazine paper in high quality.

The invention will be explained in more detail with reference to an embodiment. It shows a schematic representation of the figure by way of example a curtain applicator with which the inventive method is performed.

In the single figure, a paper web 1 can be seen. This is prepared by first preparing a fiber suspension. Subsequently, the fiber suspension is introduced into a headbox and a paper web is produced in a former. Subsequently, the wet paper web is subjected to dewatering in a press, followed by drying of the web. This manufacturing process is generally designated by M (for Manufacturing) in the figure.

The paper web to be coated has a porosity measured according to Bendtsen of about 200 ml / min.

The pulp or paper web runs in the example shown in the direction L over a support surface. 2

Above the web 1 and spaced from it, a curtain applicator 3 is arranged, which is designed in the example as a so-called twin so double slot nozzle.

However, if desired, it would also be possible to arrange two applicators in the direction of travel L of the web 1 in succession. Likewise, as not shown here, these could also be designed into a structural unit as a so-called multi die for dispensing even further coating layers. The applicator 3 is for dispensing of a coating medium 4 and 5 or pigment-containing coating color in the form of two single-layer curtains 4 a and 5 a determined that form a total of a double-layered curtain in the example. These curtains 4 a, 5 a fall substantially downwards due to gravity to the web surface 1 a. The curtains and the double curtain then generates on the surface 1 a of the paper web 1, a contour layer 6. For the bottom layer of the application medium 4 as Base layer 6a of the curtain 4a, a line weight of about 3 g / m ² , a viscosity BR100 of about 200mPas and a solids content of about 35 is selected.

For the second application medium 5 or curtain 5a, the same coating color as in the case of curtain 4a or a different composition can be used again. The curtain 5 a generates on the surface of the paper web 1 the upper layer of the contour layer or forms a cover layer 6 b.

Since unwanted air boundary layers are carried along with the high-speed paper or board web 1 (over 1200 m / min), an air boundary layer removal or fighting device 7 is present in front of the coater 3. In addition, collecting devices, such as trays or gutters, are available for application medium collected when the curtain is started and stopped. These are not shown for clarity reasons.

In order to reduce or avoid an otherwise frequently occurring crater or scarring formation in the coating layer 6a, 6b, before the curtain application of the pigment-containing application medium, a wetting agent 8 is applied to the same side of the web with a spraying device 9, so that the moisture content of the web surface is approx 3% is increased up to 15%. For this purpose, a device marketed by the applicant "ModulePro-Nozzle" or so-called Danfoss spray nozzles, for example of the type "Nessie 180Z1940 0, 15" is used. The humectant 8 is sprayed in an amount of 3 l / h and under a pressure of 3 bar. This achieves drop sizes of aufzusprühenden humectant of less than 20 μιτι.

As humectant water is used in the example.

It can be seen in the figure that during the application of the moistening agent 8, the web 1 runs on its opposite side via a suction device 10. As a suction device here is connected to a vacuum generating device 1 1 suction roller 12 is provided. By the suction of the web 1 succeeds a faster and better transfer of the humectant 8 on the web. 1

So that the sensitive curtain is not disturbed in its stability, a Curtain applicators 3 and 4 enclosing climatic chamber 13 is present. The spray humidifier 9 is preferably outside the climate chamber, i. seen in the direction L of the paper web 1, arranged in front of the chamber 13. This ensures spatially separate dispensing of the moistening agent 8 from the subsequent coating by means of the curtains 4a, 5a. Both orders can not influence each other negatively.

Dashed lines in the figure indicate a possible further variant of the arrangement of the spray-moistening device 9.

The humidifying device 9 is seen in this variant in the running direction L of the paper web 1 while disposed within the climatic chamber 13, but even before the air boundary layer control device 7. That is, the humidifying agent 8 is applied before removing the flowing with the web 1 air boundary layer takes place , Again, this ensures that the curtain can fall undisturbed. Following the curtain coating, the web 1 is dried. By way of example, only a contactless deflection and drying unit, a so-called Airturn 14 is shown. Further treatment steps, such as, for example, coating of the other side of the web 1 b, renewed drying by means of contactless drying devices and cylinder driers, and subsequent smoothing of the web can also take place, but are not shown in the figure. LIST OF REFERENCE NUMBERS

1 paper web

1 a track surface

1 b other side of the track

2 support surface

3 curtain applicator

4 application medium

4a curtain

5 application medium

5a curtain

6 contour layer

6a base layer

6b topcoat

7 air boundary layer control device

8 humectants

9 spraying device

10 suction device

1 1 Negative pressure generation

12 suction roll

13 climate chamber

14 Airturn

L running direction

Claims

claims

1 . Method for producing a wood-free coated or lightly coated paper web (1), wherein on at least one side (1a) of the web (1) running through a coater, a pigment-containing, liquid to pasty application medium (4, 5) is applied by curtain coating,

characterized in that

before the single- or multi-layer application of the pigment-containing coating medium (4, 5), a wetting agent (8) on the same web page (1 a) is applied, so that the moisture content of the web surface (1 a) of about 3% up to 15 % is increased.

2. The method according to claim 1,

characterized in that

the moistening agent (8) is sprayed on at about 3 l / h and under a pressure of about 3 bar, wherein a droplet size of the moistening agent to be sprayed of less than 20 μιτι is used

3. The method according to claim 1 or 2,

characterized in that

as humectant (8) water or demineralized water is selected, if desired, additives, for example surface-active substances, such as surfactants are mixed.

4. The method according to claim 1 to 3,

characterized in that

during the application of the moistening agent (8), the web (1) on its opposite side (1 b) is sucked.

5. The method according to claim 1 to 4, characterized in that

the moistening agent (8) is applied even before the air boundary layer flowing with the web (1) is removed.

6. The method according to claim 1 to 5,

characterized in that

the moistening agent (8) is applied spatially separated from the subsequent curtain coating.

7. The method according to claim 6,

characterized in that

the curtain coating is preferably multi-layered and is carried out with the aid of two successively arranged so-called single-curtain nozzle, or a double-curtain nozzle or a multi-curtain nozzle.

8. The method according to claim 6 or 7,

characterized in that

for the coating of the paper web (1) a known Vorhangschlitzdüse or slot Die (3) and / or a known Gleitschichtdüse or Slide Die is used.

9. The method according to at least one of claims 6 to 8,

characterized in that

the coating weight, the viscosity and the solids content of the lowest applied layer or a base layer (6a) of the application medium (4) is chosen to be lower than in the case of the layer (6b) above it.

10. The method according to claim 9,

characterized in that

for the lowermost layer (6a) of the application medium, a coating weight between 1 and 5 g / m ² , preferably 2 to 4 g / m ² , a viscosity BR100 between 20 and 500 mPas, preferably BR100 between 30 and 300 mPas and a Solids content between 30 and 40% is selected.

1 . Method according to at least one of the preceding claims,

characterized in that

in the paper web to be spread (1) is selected one which is used a porosity measured according to Bendtsen, which is greater than 200ml / min.

2. The method according to claim 1 to 1 1,

characterized in that

after the treatment of the first web page (1 a), the second web page (1 b) is treated analogously to the first web page (1 a).