Method for coating a fibrous web

Abstract

A method for applying at least one application medium to a running fibrous web using an application aggregate, which method comprises the following steps: - providing a fibrous web having a temperature below 55°C before said web enters into the application aggregate; - applying the application medium to at least one side of the fibrous web; - drying the fibrous web by a first drying process in the form of air drying and a subsequent second drying process in the form of contact drying, wherein more than 50%, preferably between 80% and 90%, of the amount of water removed by the drying process is removed by air drying.

Description

[0001] The invention relates to a method for coating a fibrous web, in particular for generating a barrier effect according to the preamble of claim 1.

[0002] Paper and cardboard are very versatile materials, but they possess no or only very weak barrier properties, for example, against oxygen, moisture, or grease and oil attack. However, many packaging applications require a material with such barrier properties. Therefore, in order to use paper or cardboard—or more generally, fibrous webs—for such applications, they must be appropriately finished.

[0003] The coating of paper or cardboard to achieve barrier properties is known, for example, from EP 2 740 685 B2. Documents DE 10 2010 030081 A1, DE 10 2005 027831 A1 and DE 10 2010 029617 A1 also describe coating processes.

[0004] One suitable finishing method involves applying natural or synthetic polymers to the fiber web. Microfibrillated cellulose (MFC), in particular, is a natural polymer that can create very attractive barrier properties, for example against water vapor and grease, and is currently the subject of much research. For example, see publication CN110241658A.

[0005] One disadvantage of MFC, however, is that it is usually in the form of a highly viscous gel and cannot be processed efficiently in this form. For application to a fiber web, this substance must be highly diluted to achieve a flawless film application. This water must then be removed from the web by drying after application. It is important to ensure that the necessary, intensive drying process does not damage the applied barrier layer.

[0006] It is therefore an object of the invention to propose a method for coating a fibrous web with which even very highly diluted coating medium can be reliably applied.

[0007] A further object of the invention is to propose a method for the reliable production of an intact barrier screed on a fibrous web.

[0008] These problems are completely solved by a method according to independent claim 1. Further advantageous features of the embodiment according to the invention are found in the dependent claims.

[0009] The problem is solved by a method for applying at least one application medium to a moving fibrous web using an application unit, which comprises the following steps: a. Providing a fiber web with a temperature below 55°C before entering the coating unit. b. Applying the coating medium to at least one side of the fiber web. c. Drying the fiber web by first drying in the form of air drying and then a second drying in the form of contact drying. wherein more than 50%, preferably between 80% and 90% of the amount of water removed by drying is removed by air drying

[0010] It can be advantageous if, in order to provide a fiber web with a temperature below 55°C, the fiber web is cooled from a higher temperature, in particular from a temperature between 60°C and 90°C.

[0011] This upstream cooling can be achieved, for example, by guiding the fiber web over one or more cooled cylinders. Alternatively or additionally, cooling can be accomplished by applying cold air.

[0012] In another advantageous embodiment, cooling can be achieved via so-called "aquacooling." In this process, a suitable amount of water is applied to the fiber web, for example, by spraying it on. The web is then exposed to air.

[0013] In aquacooling, cooling is achieved primarily through water evaporation. The introduction of air serves primarily to remove the moisture that forms above the track, thus promoting the evaporation of further water and consequently, the further cooling of the track. Therefore, it is advantageous to apply air to the side(s) of the track to which water has been applied.

[0014] In advantageous embodiments, the coating medium can comprise natural or synthetic polymers. Examples include microfibrillated cellulose (MFC) or starch. Furthermore, the coating medium will typically contain a large amount of water. In advantageous applications, the coating medium will be adjusted such that the solids content is at least 10 wt%, and in particular between 20 wt% and 40 wt%.

[0015] Furthermore, it can be advantageous if the temperature of the application medium is less than 65°C.

[0016] In preferred embodiments, the process may further include step d. winding up the fibrous web, wherein the temperature of the material web during winding is 30°C or less.

[0017] To ensure such a temperature of the fiber web, it can be advantageous to arrange a cooling unit for downstream cooling – for example, in the form of a cooling cylinder – between the drying and the winding process.

[0018] Since a single coating is usually insufficient to achieve a barrier effect, it can be advantageously provided that a further application medium is applied to the moving fiber web by means of a further application unit, and that this further application unit is arranged in the direction of travel of the fiber web before or after the application unit.

[0019] In this context, it can be particularly advantageous if the fiber web undergoes further drying after the subsequent application unit, whereby the fiber web is heated to more than 60°C, in particular to a temperature between 65°C and 90°C.

[0020] To ensure that the fiber web is at most 55°C when it enters the application unit, despite this high web temperature, the described upstream cooling is very helpful.

[0021] Suitable application units for the application medium and / or the further application medium can be units for film application, curtain application or spray application.

[0022] Further advantageous embodiments of the invention are explained using an exemplary embodiment with reference to the drawing. The features mentioned can be advantageously implemented not only in the combination shown, but also individually combined with one another. Figure 1 schematically shows a method according to one aspect of the invention.

[0023] Figure 1This document describes a method for applying two layers of coating medium to a paper or cardboard web. The fiber web first receives a primer coat by applying another coating medium in a separate coating unit 100. This primer coat can be, for example, a starch coating, which can be applied to one or both sides. Alternatively, a pigment coating can be applied in the separate coating unit 100. The primer coat can be applied using a known coating method. In particular, the separate coating unit 100 can be a film coating unit, a curtain coating unit, or a spray coating unit. In other embodiments of the method, the primer coat itself can consist of several layers. These additional primer layers can be applied to the fiber web before the separate coating unit 100.

[0024] Since applying the additional coating medium also introduces a large amount of moisture onto the web, a further drying step (200) follows the application unit. This further drying step (200) can be any suitable drying method, such as air, infrared, or contact drying, or combinations thereof.

[0025] Regardless of the type of further drying, the fiber web is heated considerably. Typically, after further drying, the fiber web has a temperature of more than 60°C, often between 65°C and 90°C.

[0026] To provide a fiber web at a temperature of 55°C or less, the process of Figure 1A pre-cooling system 5 is provided. This can be achieved, for example, by guiding the fiber web over one or more cooled cylinders. Alternatively or additionally, cooling can be carried out by applying cold air. The use of so-called aquacooling can also be advantageous. In this process, the web is cooled to such an extent that it has a temperature of only 55°C or less before entering the application unit 10. Depending on the application, the temperature of the fiber web can also be between 40°C and 50°C.

[0027] The cooled web is coated with the coating medium in the subsequent coating unit 10. The coating medium can be the same as the subsequent coating medium or a different one. Preferably, the coating medium is a barrier medium and comprises natural or synthetic polymers, in particular microfibrillated cellulose (MFC). With such a barrier coating, it is particularly important that the coating is continuous and free of craters or holes. The methods according to the various aspects of the present invention are very advantageous for this purpose.

[0028] It should be noted that when applying a coating to a hot surface, a large amount of heat energy is suddenly introduced into the coating medium. The coating medium, or the water it contains, then immediately begins to evaporate, which can lead to craters or holes in the coating. Therefore, precise temperature control of the surface before application is crucial, especially when applying a barrier coating.

[0029] It will often be necessary to apply more than 6 g / m², and in particular 8 g / m² or more, of solids in the coating unit. Coating is generally carried out on one side only, and film coating units, curtain coating units, or spray coating units are very suitable. It can be advantageous if the temperature of the coating medium is not too far above the temperature of the fiber web, in particular if the temperature of the coating medium is a maximum of 20°C, preferably a maximum of 10°C, above the temperature of the fiber web. Generally, the temperature of the coating medium should be 65°C or below. Cooling of the coating medium may be necessary for this purpose.

[0030] In advantageous applications, the coating medium is adjusted so that the solids content is at least 10 wt%, particularly between 20 wt% and 40 wt%. Nevertheless, a large amount of moisture is still applied to the fiber web with the coating medium, which must subsequently be removed by drying 20. The drying process 20 comprises several stages. First, air drying 22 takes place, in which air, particularly warm air, is applied to the fiber web, especially to the coated side of the fiber web. This is followed by contact drying 24. Here, the fiber web can, for example, be passed over one or more heated cylinders. The temperature of the drying cylinders can be kept relatively low. In advantageous embodiments, the surface temperature of these drying cylinders is 80°C or less.The two parts 22, 24 of the drying process are coordinated in such a way that at least 50%, preferably between 80% and 90%, of the total amount of water removed by the drying process is removed by the air drying process 22.

[0031] This drying process is highly advantageous for achieving a smooth, flawless coating. Investigations by the applicant have shown that excessively intense drying – i.e., with an excessively high mass transfer rate [kg / m²< *h] – such as infrared (IR) drying and / or excessively high web temperatures, leads to craters or holes in the coating, thereby completely or partially destroying the barrier effect.

[0032] By pre-cooling - or in other versions, e.g. in offline systems: by providing a suitably cool web, the application medium is not applied to a hot web, which, as mentioned, can lead to craters or holes.

[0033] Air drying 22 is a relatively mild drying process compared to IR drying, resulting in evaporation with a low mass transfer rate. Since the air is blown onto the coating from above, the fiber web does not heat up significantly.

[0034] Only after a large portion of the moisture (> 50 wt%) has been removed by air drying 22 has the applied coating hardened sufficiently to allow further drying by contact drying 24 without compromising the integrity of the coating. In contact drying 24, drying typically occurs by heating the fiber web from the uncoated side. If contact drying 24 were to begin too early, before the coating has hardened sufficiently, the coating would be damaged by the drying screen or the cylinder surface.

[0035] Despite the hardening of the coating achieved by air drying 22, it is advantageous to make contact drying 24 comparatively gentle. Therefore, the surface temperature of these drying cylinders should preferably be 80°C or less.

[0036] The fully dried fiber web is wound into a so-called tambour in a 40-degree reel for further processing. To minimize material transport within the coating, and thus losses in barrier effectiveness, the temperature of the fiber web during winding should not exceed 30°C. To ensure this, the [product / service] features... Figure 1The illustrated embodiment includes a downstream cooling stage 30 between the drying stage 20 and the winding stage 40 to cool the fiber web to a temperature of 30°C or less. The same units are suitable for implementing the downstream cooling stage 30 as described for the upstream cooling stage 5. While such downstream cooling 30 is very advantageous, it may be omitted depending on the subsequent processing of the fiber web. Reference symbol list

[0037] 5 Upstream cooling 10 Application unit 20 Drying 22 Air drying 24 Contact drying 30 Downstream cooling 40 Winding 100 Additional application unit 200 Additional drying

Claims

1. A method of applying at least one application medium to a moving fibre web by means of an application unit, comprising the following steps a. providing a fibrous web at a temperature of below 55°C before entering the application unit b. applying the application medium to at least one side of the fibrous web c. drying the fibrous web by a first drying process in the form of air drying and a subsequent second drying process in the form of contact drying wherein more than 50%, preferably between 80% and 90%, of the amount of water removed by the drying is removed by the air drying.

2. Process according to claim 1, characterised in that, in order to provide a fibrous web with a temperature of below 55°C, the fibrous web is cooled from a higher temperature, in particular from a temperature between 60°C and 90°C.

3. Method according to claim 2, characterised in that, in order to cool the fibrous web, moisture is first applied to the fibrous web and then the fibrous web is exposed to air.

4. Method according to one of the preceding claims, characterised in that the application medium comprises natural or synthetic polymers, in particular microfibrillated cellulose (MFC).

5. Method according to claim 4, characterised in that the application medium comprises microfibrillated cellulose (MFC).

6. Method according to one of the preceding claims, characterised in that the temperature of the application medium is less than 20° , in particular 10° or less above the temperature of the fibrous web at the inlet to the application unit.

7. Method according to one of the preceding claims, characterised in that the method further comprises the step of d. Rolling up the fibrous web, wherein the temperature of the material web during rolling up is 30°C or less.

8. Method according to one of the preceding claims, characterised in that a further application medium is applied to the running fibrous web with the aid of a further application unit, and this further application unit is arranged upstream of the application unit in the running direction of the fibrous web.

9. Method according to claim 8, characterised in that the fibrous web undergoes further drying after the further application unit, as a result of which the fibrous web is heated to more than 60°C, in particular to a temperature between 65°C and 90°C.

10. Method according to one of the preceding claims, characterised in that the application medium and / or the further application medium is applied to the fibrous web by means of film application, curtain application or spray application.

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