DE102022105500A1 - Commissioned work and procedures - Google Patents

Abstract

Applicator and method for coating a running fibrous web with a liquid or pasty application medium, comprising a metering device for applying the application medium to an application surface at a metering point and a treatment nip for transferring the application medium from the application surface to the fibrous web, the applicator comprises a conditioning device which is used for this purpose is set up to condition the application medium on the application surface between the dosing point and the treatment nip, characterized in that the conditioning device comprises a heating device and a revolving permeable belt, the heating device preferably being arranged below the application area, while the revolving permeable belt is above the application area is arranged, and wherein a contact zone is provided in the area of the heating device, in which the circumferential permeable band is in contact or can be brought into contact with the applied application medium.

Description

The invention relates to an applicator for coating a running fibrous web with a liquid or pasty application medium according to the preamble of claim 1 and to a coating process using such an applicator.

Depending on the desired application, fibrous webs, especially paper and cardboard webs, are coated with a variety of different substances. This includes, for example, an application of starch to increase strength, or an application of polymer or pigment to improve or modify the surface properties, or an application of polymer to create a barrier. The application of high-performance barrier coatings is becoming increasingly important, especially in the area of packaging papers and special papers, in order to give the fibrous web a certain level of gas, water and vapor tightness. This is very demanding.

The order can be done directly or indirectly.

In the case of a direct order, for example in the EP 0 791 687 is described, the application medium is applied directly to the web from a suitable metering device and then optionally pressed into the web in a treatment nip.

An alternative is the indirect order. This can be done, as in the paper published some time ago GB 108,340 A an application medium is applied to a roller surface and from there is transferred to the paper web in a treatment nip. The DE 10 2006 057 870 also describes indirect order systems. In particular, the metering of coating paint onto an endless belt and the transfer from this belt to the paper web are described there.

However, recent developments, particularly in the application of barrier coatings, require the application of application media that have a very low solids content and therefore a high proportion of water. The known applicators prove to be disadvantageous because they transfer almost all of the water to the paper web, which leads to a considerable drying requirement for the web. In addition, the very wet paper web can also tear easily, which leads to production losses due to web breaks or by reducing the production speed to avoid these breaks.

The suggestion is therefore to improve the application of such barrier coatings DE 10 2019 133 942 A1 an indirect application in which the coating medium is applied to an application surface and undergoes conditioning, in particular drying, before being transferred to the fibrous web. This means that the application medium can be applied with the low solids content necessary for processing, but is transferred to the paper web with a higher solids content, as a result of which less moisture is transferred to the fibrous web.

However, the applicant's experiments have shown that when using this process, the conditioning, in particular the drying, can in some cases lead to defects in the barrier coating in the form of wrinkles, cracks, blistering and the like. Such defects can be harmful if they destroy the integrity of the barrier film and thus the barrier effect.

It is therefore the object of the present invention to overcome the problems of the prior art. In particular, it is the object of the invention to propose an applicator and a method for coating with which the application of barrier media is also possible in a stable and reliable manner.

The object is achieved according to the invention by an embodiment according to the independent claim. Further advantageous embodiments of the present invention can be found in the subclaims.

With regard to the applicator, the task is solved by an applicator for coating a running fibrous web with a liquid or pasty application medium, comprising a metering device for applying the application medium to an application surface at a metering point and a treatment nip for transferring the application medium from the application surface to the fibrous web Applicator comprises a conditioning device which is designed to condition the application medium on the application surface between the dosing point and the treatment nip.

According to the invention it is provided that the conditioning device comprises a heating device and a revolving permeable belt, wherein the heating device is advantageously arranged below the application surface, while the revolving permeable belt is arranged above the application surface, and a contact zone is provided in the area of the heating device in which the surrounding permeable band with the given The correct application medium is in contact or can be brought into contact.

The inventors have recognized that - especially with thin films, for example thin films made of MFC and / or NFC or layers of highly ground pulp - convection drying or radiation drying can lead to defects in the coating, for example in the form of wrinkles, cracks, Bubble formation (“blistering”) and the like. Such defects are particularly damaging when applying barrier layers, as they destroy the integrity of the barrier film and thus the barrier effect.

There are a variety of micro- and nanomaterials based on cellulose. Nanocellulose can be differentiated into cellulose nanocrystals (CNC), nanocrystalline cellulose (NCC), cellulose nanofibers (CNF or NFC) and bacterial nanocellulose (BNC).

Microfibrillated cellulose (MFC) - also known as “reticulated” cellulose, “superfine” cellulose or “cellulose nanofibrils”, microcrystalline cellulose (MCC), cellulose nanowhiskers (CNW).
All of these materials can be used as part of application media, especially barrier media. Application media with several components are often used here.
For example, MFC materials for use as an application medium may comprise: (a) nanofibrils and/or (b) fibrillar fines and/or (c) fiber fragments and/or (d) fibers, advantageously the nanostructures forming the main component.

In application works according to the present invention, an additional permeable tape is therefore used, which can be brought into contact with the application medium on the application surface from above in at least one contact zone. A heating device is advantageously provided below the application surface in the area of the contact zone. The conditioning, in particular the drying, of the application medium takes place here in the sandwich between the application surface and the surrounding permeable belt. The surrounding permeable band significantly improves the dimensional stability of the application medium or barrier film and reduces the tensions that arise during drying without the above-mentioned defects such as blisters, cracks, etc. occurring.
The surrounding belt is designed to be permeable so that the water vapor created during drying can be released into the environment and no bubbles or similar problems can arise from this vapor.

Arranging the heating device below the application surface is advantageous because the heating device can simply be designed as a contact heater, for example in the form of a heated cylinder or a heated plate. In this way, the application medium is dried from bottom to top, which is advantageous in order to avoid defects in the coating.
In general, however, it is also possible to arrange the at least one heating device above the application surface. When using convection or radiation heaters (e.g. IR emitters), these heating devices can be arranged both below and above the application surface.
It can also be provided that heating devices are provided both below and above the application surface.

The heating device used should in particular be able to generate a temperature of at least 60°C, preferably a temperature in the range between 70°C and 150°C, particularly preferably a temperature in the range between 80°C and 100°C.

The contact zone should extend at least over the entire length of the heating device (in the machine direction) in order to prevent the formation of film defects during the acute heating or drying phase.
However, it will usually be advantageous if the contact zone begins in front of the heating device and/or extends beyond the heating device. The length of the contact zone 300 can be more than 50cm, for example 100cm, 150cm or more.

In advantageous embodiments, the application surface can be provided by the surface of an application belt.
In particular, the surface of the application belt (or the entire application belt) can be provided by a metal belt or a closed polymer belt.
When using a closed polymer belt, the application surface will often be made of a rubber, a polyurethane or a silicone. The polymer layer of the endless belt can, for example, have a hardness between 50 and 97 Shore A. In particular, it can advantageously be provided that the application belt is guided through the treatment nip together with the fibrous web.
Depending on the application, the application surface can be smooth, roughened or structured. A matrix surface, such as fibers or mineral pigments embedded in a polymer, can also be advantageous.

In preferred embodiments, load means can be provided to bring the circulating permeable belt into contact with the applied application medium under pressure. The application of a certain pressure supports the effect of the surrounding permeable band in terms of dimensional stability and stresses.
In advantageous embodiments, loads between 0.5 kN/m and 4 kN/m, in particular between 1.5 kN/m and 2.5 kN/m, can be used.

The circumferential permeable band can be realized in various ways.
For example, it can be provided that the circumferential permeable band consists of or includes a fabric. Using a fabric has the great advantage that it is naturally permeable. It has been shown that, for example, the structures of known forming fabrics are suitable for use as a rotating permeable belt.

The MD and/or CD threads of such a fabric can each have diameters in the range from 0.1mm to 0.5mm. To avoid damage to the film of application medium, it has proven to be advantageous if the thread diameters are smaller than 0.4mm, in particular 0.3mm or smaller.

Materials for the surrounding permeable belt include the usual plastics such as PET, PA6, PA6.10, PET+PBT, PA6.12+PA6.6. PBT, PEN-PET mixtures, PET-PU mixtures etc. suitable.

Alternatively or additionally, it can be provided that the circumferential permeable band consists of or comprises a membrane or a perforated film. The film can consist, for example, of PET and/or a polyurethane (PU) or a laminate, for example of PET and PA.
Films or membranes have the advantage that they are very smooth per se and therefore there is little risk that the structure of the permeable tape will damage the film of application medium. The desired permeability can be achieved through a performance, for example using laser perforation or classic needle perforation.

It has proven to be advantageous if the circumferential permeable band has a permeability to air of 250 cfm - 800 cfm, in particular 450 cmf - 600 cfm.

The permeability or air permeability is defined as the air throughput per unit area at a certain differential pressure and is usually given in the paper industry as cfm/ft2 (“cubic feet per minute/square feet) at 0.5 inch water column. ASTM D737-04 is the standard test method for air permeability, particularly of textile fabrics.

Depending on the application, it can also be advantageous if the conditioning device also comprises a further heating device and a further revolving permeable belt, the further heating device preferably being arranged below the application surface, while the further revolving permeable belt is arranged above the application surface, and wherein in In the area of the further heating device, at least one further contact zone is provided, in which the further circumferential permeable band is in contact or can be brought into contact with the applied application medium.
Everything that has been said so far about the circulating permeable band, the heating device and the contact zone also applies, mutatis mutandis, to the further circulating permeable band, the further heating device and the further contact zone.
By providing multiple heating devices and permeable belts, the drying process of the coating medium can be controlled even better.
For example, a very fine circumferential permeable belt can be used for the initially very liquid application medium, while after the initial drying a further circumferential permeable belt can be used as a coarser fabric or a belt with greater or smaller permeability.
A different type of heating can also be used for the further heating device, for example radiation drying, than in the first heating device, which can be designed, for example, as contact heating.

The use of a third, fourth, etc. heating device or a third, fourth, etc. circulating permeable belt is also possible.

With regard to the method, the object is achieved by a method for coating a running fibrous web with a liquid or pasty application medium, at least one applicator according to one aspect of the invention being used for the coating.

It is particularly advantageous if a barrier medium is used as the application medium, in particular an aqueous suspension with microfibrillated cellulose (MFC) and/or nanofibrillated cellulose (NFC). With these barrier coatings, the avoidance of defects in the coating such as blistering is particularly crucial.

In preferred embodiments of the method it can be provided that the application medium has a dry content of at least 1% by weight, in particular between 1.5% by weight and 5% by weight, at the beginning of the contact zone.
Barrier media in particular often have to be conveyed in a very high dilution, i.e. with a very low dry content, and also applied to the application surface.
In order to avoid an excessive reduction in the strength of the fibrous web due to moistening, the moisture is reduced by conditioning, in particular drying, before the treatment nip. It has been shown that with a dry content of 3% by weight, in particular between 3% by weight and 5% by weight, even demanding coating media of MFC or NFC can still be conveyed and applied sufficiently well, and sufficient moisture can still be removed via the conditioning without this conditioning must be excessively large.

In frequent versions, between 4 g/m ² and 40 g/m ² , in particular between 8 g/m ² and 25 g/m ² dry matter of application medium are applied to the application surface. With smaller application quantities, a closed coating can be difficult to achieve, especially with barrier coatings. With larger application quantities, a lot of water is applied at the low dry contents used, which is difficult to remove.

In preferred embodiments, it can be provided that the application medium is applied to the application surface at the metering point in a smaller width than the fibrous web to be coated.
By applying pressure to the application medium through the rotating permeable belt, not only the formation of defects can be reduced. The still very liquid application medium can also compress - at least at the beginning of the contact zone. This allows the medium to spread, which leads to a widening of the applied layer of application medium.

For the same reason, it is advantageous if the circulating, permeable band is also wider, in particular 10% or 20% wider, than the application medium applied to the application surface at the dosing point.

The invention is explained below with reference to figures. The invention is explained below with reference to figures. The invention is not limited to the embodiments shown here.

• 1 zeigt ein Auftragswerk, wie es aus dem Stand der Technik bekannt ist.
• 2 zeigt ein Auftragswerk gemäß einem Aspekt der vorliegenden Erfindung
• 3a zeigt schematisch den Trocknungsprozess gemäß dem Stand der Technik
• 3b zeigt den Trocknungsprozess des Auftragsmediums in einem Auftragswerk gemäß Ausführungen der vorliegenden Erfindung.

The figures show in detail:

• 1 shows a commissioned work as is known from the prior art.
• 2 shows a commissioned work according to an aspect of the present invention
• 3a shows schematically the drying process according to the state of the art
• 3b shows the drying process of the application medium in an application work according to embodiments of the present invention.

1 shows an application work known from the prior art for coating a running fibrous web 4. An application medium is applied to an application surface 6 by means of a metering device 2. The application area 6 is in the in 1 shown embodiment provided by the surface of an endless application belt 6a.

In the 1 Dosing device 2 shown uses a squeegee, especially a roller squeegee for squeegeeing off application medium applied in excess.

Alternatively, for example, a spray application can also be provided. Dosing using a nozzle, as is known from so-called curtain coaters, is also possible. To do this, however, the path of the endless application belt 6a must be changed accordingly, which is shown in 1 from the dosing point 20 to the transfer roller runs vertically, or essentially vertically, while for dosing by means of a freely falling curtain, the application surface 6 must run at least largely horizontally.

The application medium is transported on the application surface 6 from the metering point 20 to the treatment nip 3, and is transferred to the fibrous web 4 in this treatment nip 3. When executed in 1 The fibrous web 4 and the endless application belt 6a pass together through the treatment nip 3. This treatment nip 3 is designed, for example, as a roller nip.

After passing through the treatment nip 3, the fibrous web 4 and the endless application belt 6a are separated. In order to avoid contamination or to ensure precise dosing by the dosing device 2, devices for cleaning the belt 6a or the application surface 6 can be provided between the outlet from the treatment nip 3 and the dosing point 20 - for example in the form of cleaning scrapers and / or cleaning nozzles. A certain distance is provided between the dosing point 20 and the treatment gnip 3, which can advantageously be more than 1m, in particular more than 2m. A conditioning device 1 is arranged along this route in order to condition the application medium on the application surface 6. In the 1 Conditioning device 1 shown is designed as a non-contact drying device 1. This can be, for example, a heating device 100 for IR irradiation. This heating device is arranged above the application surface 6.

2 In contrast, shows a commissioned work according to one aspect of the invention. It differs from the well-known commissioned work 1 only in the design of the conditioning device 1. The conditioning device 1 of the commissioned work 2 also includes a heating device 100. However, this heating device 100 is arranged below the application belt 6.

In addition, the conditioning device 1 according to 2 another revolving permeable band 200. The revolving permeable band 200 is arranged above the application surface 6. The rotating belt 200 is in contact with the applied application medium in the area of a contact zone 300 - if necessary using suitable load means - or can be brought into contact with the application medium there. The contact zone 300 is in the design of 2 longer than the length of the heating device 100. It can be provided that the contact zone 300 begins before the heating device 100 and / or ends after the heating device 100. The length of the contact zone 300 can be more than 50cm, for example 100cm, 150cm or more.

The 3a and 3b show schematically the advantages of the present invention in comparison to the prior art. Both figures show a section of a conditioning device 1. An application surface 6 is shown, onto which an application medium is applied. In 3a is the situation in a commissioned work 1 to see. The heating device 100 is mounted above the application surface 6. This means that the application medium dries from top to bottom. After some time, a somewhat solidified film 8 will form in the upper area of the application medium, while in the lower area the application medium will continue to be in the form of a liquid bevel 7.
If the drying continues, the water vapor that forms in the liquid phase 7 can only escape by passing through the closed film 8 that has already been formed and thus breaking the film 8 open again. In this way, errors such as blistering can occur.
3b On the other hand, shows the situation in a commissioned work like in 2 shown. The heating device 100 is attached below the application surface 6. In this case, the application medium is dried from below. The solidified film 8 forms on the application surface 6, the still liquid bevel 7 lies on top. Water vapor created during further drying can escape further upwards.
In addition, it is in 3b a permeable belt 200 is provided which is in contact with the coating medium over the heater 100. The circumferential permeable belt 200 significantly improves the dimensional stability of the application medium or the barrier film and reduces the tensions that arise during drying without causing the defects mentioned. The circumferential permeable belt significantly improves the dimensional stability of the application medium or the The barrier film is significantly improved and the tensions that arise during drying are reduced without causing defects such as wrinkles, cracks, etc.
Since the band 200 is also permeable, the resulting water vapor can also escape through the permeable band 200 without it reaching the bottom 3a described effects such as blistering occur.

Reference symbol list

1

Conditioning device

3

Treatment nip

6

Application area

7

liquid bevel

8th

solidified film

20

Dosing point

100

Heating device

200

circumferential permeable band

300

Contact zone

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0791687 [0004]
• GB 108340 A [0005]
• DE 102006057870 [0005]
• DE 102019133942 A1 [0007]

Claims (13)

Applicator for coating a running fibrous web (4) with a liquid or pasty application medium, comprising a metering device (2) for applying the application medium to an application surface (6) at a metering point (20) and a treatment nip (3) for transferring the application medium from the Application surface (6) on the fibrous web (4), the application unit comprises at least one conditioning device (1) which is designed to condition the application medium on the application surface (6) between the dosing point (20) and the treatment nip (3), thereby characterized in that the conditioning device (1) comprises a heating device (100) and a rotating permeable belt (200), the heating device (100) preferably being arranged below the application surface (6), while the rotating permeable belt (200) above the application surface (6) is arranged, and wherein in the area of the heating device (100) at least one contact zone (300) is provided, in which the circumferential permeable band (200) is in contact or can be brought into contact with the applied application medium. Commissioned work Claim 1 , characterized in that the application surface (6) is provided by the surface of an application belt (6a). Commissioned work Claim 2 , characterized in that the surface of the application belt (6a) is provided by a metal belt or a closed polymer belt. Applicator according to one of the preceding claims, characterized in that load means are provided in order to bring the rotating permeable belt (200) into contact with the applied application medium under pressure. Applicable work according to one of the preceding claims, characterized in that the circumferential permeable band (200) consists of or comprises a fabric. Commissioned work based on one of the Claims 1 until 4 , characterized in that the circumferential permeable band (200) consists of or comprises a membrane or a perforated film. Applicable work according to one of the preceding claims, characterized in that the circumferential permeable band (200) has a permeability to air of 250 cfm - 800 cfm, in particular 450 cmf - 600 cfm. Applicator according to one of the preceding claims, characterized in that the conditioning device (1) also comprises a further heating device (100) and a further circulating permeable belt (200), the further heating device (100) preferably being arranged below the application surface (6). , while the further revolving permeable band (200) is arranged above the application surface (6), and in which in the area of the further heating device (100) at least one further contact zone (300) is provided, in which the further revolving permeable band (200) is in contact or can be brought into contact with the applied application medium. Method for coating a running fibrous web (4) with a liquid or pasty application medium, characterized in that at least one applicator according to one of the preceding claims is used for the coating. Procedure according to Claim 9 , characterized in that a barrier medium is used as the application medium, in particular an aqueous suspension with microfibrillated cellulose (MFC) and/or nanofibrillated cellulose (NFC). Procedure according to one of the Claims 9 or 10 , characterized in that the application medium at the beginning of the contact zone (300) has a dry content of at least 1% by weight, in particular between 1.5% by weight and 5% by weight. Procedure according to one of the Claims 9 until 11 , characterized in that between 4 g/m ² and 40 g/m ² , in particular between 8 g/m ² and 25 g/m ² dry matter of application medium are applied to the application surface (6). Procedure according to one of the Claims 9 until 12 , characterized in that the application medium is applied to the application surface (6) at the metering point (20) in a smaller width than the fibrous web (4) to be coated.

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