DE202022105761U1 - application device - Google Patents

Abstract

Application device for applying a liquid or pasty application medium, in particular an aqueous starch solution, to a moving surface (10), comprising a curtain application unit,
wherein the curtain applicator comprises an application nozzle (1) for dispensing the application medium and a support surface (4),
wherein the application nozzle (1) is designed as a slot nozzle (1), in which the application medium can be guided inside the nozzle along a straight channel gap (2) to a nozzle outlet (3),
characterized in that
the rectilinear channel gap (2) is inclined relative to the vertical (V), while the support surface (4) is wholly or partially either vertical or at a small angle relative to the vertical.

Description

The invention relates to an application device for applying a liquid or pasty application medium, in particular an aqueous starch solution, to a moving surface according to the preamble of claim 1.

The curtain application process (“Curtain Coating”) is used in the production or processing of material webs, in particular paper or cardboard webs, for a large number of applications. In addition to the classic application of pigmented coating colors, the curtain application process is also used for application and a wide variety of barrier media. The utility model document DE202019106814 U1 describes, for example, the use of a curtain coater to apply aqueous starch solutions.

The advantages of the curtain application process include non-contact application, which does not require dosing or leveling elements that touch the material web or the transfer rollers. In addition, the curtain application process allows the application of a uniformly thick, closed layer of the respective application medium to the web of material. Such an even and closed coverage of the surface to be coated is generally advantageous, but is of great importance when applying barrier media such as PVA or starch solutions.

In order to achieve such a uniform and closed application, it is essential that the curtain of application medium, which falls onto the surface essentially under the influence of gravity, can be reliably and stably ensured over the entire width of the material web.

Various difficulties arise here.

On the one hand, the surface tension of the application medium acts on the falling curtain in addition to gravity. As gravity acts to stretch the curtain (gravity stretch), surface tension tends to pull the curtain together, thus reducing its surface area. Especially with the ever-increasing working widths of the coating systems of up to 10m or more, it is becoming difficult to achieve a curtain that is uniform over the entire working width and also has a sufficient drop height.

In addition, the curtain hits a surface - a web of material in the case of direct coating or a transfer element such as a transfer roller in the case of indirect application - that is moving at very high speed. Speeds of 1500 m/min to 2000 m/min are the norm these days. Therefore, when the curtain makes contact with the moving surface, there is additional contact stretching due to the difference between the falling speed of the curtain and the (faster) speed of the surface.

In order to achieve a high quality application result, it is usually desirable to distribute the total stretch of the curtain as evenly as possible between the gravity stretch and the contact stretch. For this purpose, it is necessary, particularly when the surface runs at high speeds, for the curtain to fall at the highest possible speed shortly before it comes into contact with the surface. This requires a correspondingly high fall height. However, there are physical limits to the drop height of the curtain. With an increase in the drop height, the influence of the surface tension of the medium increases, as does the influence of small air movements in the vicinity of the curtain, so that from a certain point a stable curtain can no longer be reliably guaranteed. The thinner the curtain, i.e. the smaller the flow rate, the greater the influence of these effects.

To solve this problem, Scripture proposes DE 100 57 734 A1 proposes to provide a guide element in the fall path of the curtain, so that the curtain is guided by the guide element over a part of the fall height over its entire width. By stabilizing the curtain by means of the guide element, the total drop height of the curtain (=free curtain length+supported curtain length) and thus the achievable stretching of the shearing force can be increased.

However, in dealing with this solution, the inventors encountered a number of difficulties. In many existing systems, it is not readily possible to increase the drop height due to the limited installation space. In addition, it has surprisingly turned out that in some systems in which installation was possible, a satisfactory coating result could not be achieved.

It is therefore the object of the invention to overcome the problems of the prior art.

It is also an object of the invention to propose an application device that is still simple even when the installation space is difficult realizable, and is also easy to retrofit in existing systems.

It is also an object of the invention to propose a method by which a satisfactory coating result can be achieved even with low flow rates.

The objects are completely solved by an application device for applying a liquid or pasty application medium according to the characterizing part of claim 1, and by a method according to the characterizing part of claim 5.

Further advantageous embodiments of the present invention can be found in the dependent claims.

With regard to the device, the object is achieved by an application device for applying a liquid or pasty application medium, in particular an aqueous starch solution, to a moving surface, comprising a curtain application unit, the curtain application unit comprising an application nozzle for dispensing the application medium and a support surface for guiding the curtain, and wherein the application nozzle is designed as a slot nozzle, in which the application medium can be guided to a nozzle outlet along a straight channel gap inside the nozzle. According to the invention, it is provided that the rectilinear channel gap is inclined relative to the vertical, while the supporting surface runs completely or partially vertically. If the support surface is not vertical for its entire length, at least the lower portion should be vertical prior to detaching the free curtain.

Alternatively, the supporting surface can also run at a small angle to the vertical instead of being completely vertical. This angle should be less than 45°, in particular less than 20°, preferably less than 10°, particularly preferably 5° and less. If it is said in the context of this application that the support surface should run vertically—unless otherwise explicitly described—this always includes the possibility that the support surface has such a small angle with respect to the vertical.

The support surface usually extends over the entire width of the curtain. With modern application devices, this can be up to 10 m or more.

In the case of curtain applicators, the use of such slot nozzles, in which the medium can be guided to the nozzle outlet via a straight channel gap, is widespread. Such nozzles are, for example, in the DE202019106814 shown. So far, however, these nozzles have been used in such a way that the duct gap runs in a vertical direction and is thus more or less an extension of the curtain. The nozzle has a certain overall height, which is due to the transverse distributor for the medium located above the channel gap, the necessary material thickness of the nozzle body and possible insulation, and cannot easily be reduced. As it is desired to increase the curtain length, the distance from the nozzle outlet to the surface must be increased. Depending on the structural conditions of the system, this may not be possible at all, since the application nozzle then comes into conflict with fixed components such as cross beams, etc. Or the desired curtain height can only be achieved at certain positions within the system due to the overall height of the nozzle, which significantly limits the freedom of design.

The inventors have recognized that, surprisingly, it is not at all necessary for the production of the curtain for the application medium to be guided vertically in the channel gap. It is perfectly sufficient if the application medium is transferred from the nozzle outlet to the support surface, and the support surface then runs completely or partially vertically. After passing through the support surface, the application medium then falls under the influence of gravity as a free curtain onto the surface.

Thus, in a device according to the invention, the channel gap or the entire application nozzle is inclined with respect to the vertical. This allows the curtain applicator to be more easily positioned so that the curtain strikes the point on the moving surface where desired or necessary. Structural restrictions due to support elements etc. can in many cases be circumvented by appropriately tilting the nozzle. As a result, this solution is also very well suited for retrofitting in existing systems.

It is possible that the nozzle or the channel is inclined in the running direction of the surface or against the running direction of the surface, which further increases the flexibility in use.

In order to ensure reliable outflow of the application medium from the nozzle outlet, it is often expedient if the angle between the straight channel gap and the vertical is a maximum of 75°. Angles between 5° and 45° are often selected.

Various materials can be used for the support surface. It can be made of metal, eg stainless steel, copper, brass or aluminium. Alternatively, the support surface can also be made of Be made of plastic, such as a polyurethane. Material combinations are also conceivable, such as a metal blade with a plastic coating, etc.

It can advantageously be provided that the supporting surface is pointed at its lower end and the angle of this point is between 15° and 45°, in particular between 20° and 30°. This makes it easier to detach the curtain from the tip of the support blade.

It can advantageously be provided that the length of the support surface is between 1 mm and 80 mm, in particular between 5 mm and 50 mm.

With regard to the method, the object is achieved by a method for applying a liquid or pasty application medium, in particular an aqueous starch solution, to a moving surface, the surface being the upper side of a material web, in particular made of paper or cardboard, in the case of direct application, and the surface in the case of indirect application a transfer element, preferably a transfer roller, which then transfers the application medium to the surface of the material web.

The application is carried out by means of a curtain application unit, which includes an application nozzle for dispensing the application medium and a support surface of the curtain, with the application medium being guided on the support surface after leaving the application nozzle and then falling onto the surface as a free curtain under the influence of gravity . The application nozzle is designed as a slot nozzle, in which the application medium is guided inside the nozzle along a straight channel gap to a nozzle outlet. According to the invention, the rectilinear channel gap is inclined with respect to the vertical, while the support surface is wholly or partially vertical.

A preferred application is the use of an application medium in the form of an aqueous starch solution. For these application media in particular, it can be advantageous that they do not contain any surfactants.

So far, surfactants have often been added to the application media in order to reduce the surface tension and thus increase the stability of the curtain. With the guided curtain wetting the support surface, the curtain film adheres to this support surface, so that the shrinkage of the guided curtain by surface tension cannot act. This means that the usually very expensive tenside can be dispensed with and the stability of the curtain can be mechanically increased.

Since the transverse contraction of the curtain is significantly reduced due to the adhesion of the curtain film to the support surface, very simple edge guide elements can also be used to keep the format width of the curtain constant.

In advantageous variants of the method it can be provided that the dynamic surface tension of the application medium is between 30 mN/m and 70 mN/m, in particular between 40 mN/m and 60 mN/m. With these very high surface tensions, the curtain is usually so unstable when applied without support that great heights can no longer be achieved with freely falling curtains. However, application media with such high surface tensions can still be used in methods according to aspects of this invention.

The dynamic surface tension is usually determined using a bubble tensiometer.

Furthermore, it can be advantageous that the surface tension of the surface is higher, in particular at least 10% higher, than the dynamic surface tension of the application medium.

Measuring methods for the surface tensions are known to those skilled in the art. The surface tension of the surface can be measured by measuring the contact angle. The dynamic surface tension can be carried out, for example, using the bubble pressure method.

Furthermore, it can be provided that the amount of application medium is 35 liters/min/m or less, preferably 20 liters/min/m, 10 liters/min/m or 8 liters/min/m or less, particularly preferably 6 ml/min /m or less. Even with such low flow rates, a stable curtain can still be created.

It is very advantageous if the free curtain strikes the moving surface at an angle of between 20° and 160°, in particular between 30° and 150°. The inventors have recognized that if the surface rises too sharply at the point of impact of the curtain, the effect of the back flow can increase, ie a migration of parts of the application medium counter to the direction of movement of the surface. Furthermore, the inventors have recognized that if the surface drops too sharply at the point of impact of the curtain, there is a risk that parts of the air boundary layer entrained by the surface will get between the application medium and the surface and thereby damage the integrity of the coating. This finding also explains why in some systems in which an installation kon conventional devices was possible, sometimes no satisfactory coating result could be achieved

This is where the present invention proves to be particularly advantageous. For example, in indirect coating, if the curtain falls on the surface of a rotating roll, the angle of impact of the curtain (falling vertically) depends solely on the positioning of the curtain applicator assembly relative to the roll. The present invention gives the user a high degree of freedom in the selection of positioning, since far less consideration is given to the spatial conditions of the system during positioning, since the inclination of the nozzle allows great freedom in positioning.

In further embodiments it can be provided that the application medium has a “low shear” viscosity in the range from 5 mPas to 2000 mPas.

The “low shear” viscosity or “low shear viscocity” is measured according to Brookfield at 100 rpm/SP2 and a temperature between 20°C and 60°C. SP2 designates the "spindle 2")

In further versions it can be provided that the application medium has a “high shear” viscosity in the range from 2 mPas to 80 mPas

The "high shear" viscosity or "high shear viscocity" is measured according to Haake at a shear rate of 500,000 to over 1,000,000 s-1.

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

• 1 zeigt schematisch einen Vorhang mit Defekten (Platzer)
• 1a zeigt einen Vorhang, der aus einer Auftragsvorrichtung gemäß einem Aspekt der Erfindung stammt
• 2 zeigt schematisch eine Auftragsvorrichtung gemäß einem weiteren Aspekt der vorliegenden Erfindung
• 3 zeigt schematisch eine Auftragsvorrichtung gemäß einem weiteren Aspekt der vorliegenden Erfindung

The figures show in detail:

• 1 shows a curtain with defects (bursts)
• 1a Figure 12 shows a curtain resulting from an applicator according to one aspect of the invention
• 2 shows schematically an application device according to a further aspect of the present invention
• 3 shows schematically an application device according to a further aspect of the present invention

1 shows an example of a freely falling curtain 5, such as is emitted from conventional curtain application nozzles. The application medium is released here over a width B as a curtain 5, which falls down under the influence of gravity. As a result, the application medium is accelerated and stretched (gravity stretching) in the direction of gravitational acceleration. At the same time, the surface tension acts on the application medium. On the one hand, this leads to the curtain 5 constricting at the lateral edges. In the course of the free fall, the curtain becomes thinner and thinner due to the stretching, so that after a length L the surface tension becomes strong enough to tear the curtain 5 open. There are so-called curtain bursts 6. An order using this curtain is only possible up to a fall height L. L is therefore also referred to as the “usable length” or “usable height” of the curtain.

1a FIG. 12, on the other hand, shows a curtain 5 which, after leaving the nozzle outlet 3, is guided on a supporting surface 4 before it falls further onto the surface 10 as a free curtain 5 under the influence of the shearing force. In execution in 1a it can be seen that the support surface 4 can also be made wider than the width B of the curtain. However, it should be at least as wide as the curtain 5 to be supported. The free length Lf is supplemented here by the supported length Lg, which increases the usable length L of the curtain 5 and thus the achievable gravity stretch accordingly. However, the overall height of the curtain applicator unit is increased by the supported length Lg. This length Lg can be 80 mm, for example, which can lead to problems with the existing installation space, especially when retrofitting existing systems. This problem can be avoided by providing a channel gap 2 which is inclined with respect to the vertical V.

the 2 shows a possible application device according to an aspect of the present invention. An application nozzle 1 in the form of a slot nozzle 1 is shown here. Inside the application nozzle 1, the application medium is guided in a reservoir 7, which makes the application medium available over the entire width B, via a straight channel 2 to the nozzle outlet 3. After the nozzle outlet 3 , the application medium—for example an aqueous starch solution—is transferred to the support surface 4 . The support surface 4 is arranged vertically so that the application medium falls or flows down along the support surface 4 as a supported curtain 5 . In this embodiment, the support surface 4 is pointed at its lower end. The angle of this point can be between 15° and 45°, in particular between 20° and 30°. At this point, the application medium detaches itself from the support surface and falls as a free curtain 5 onto the moving surface 10, which in 1 is realized in the form of a transfer roller 11.

The channel 2 is inclined by the angle α with respect to the vertical. The angle α is advantageously less than 75° since this has a positive effect on the flow behavior of the application medium in the channel 2 . An angle α between 5° and 45° can often be selected.

The curtain 5 strikes the surface 10, 11 at an angle β. Here the angle β is less than 90°. Too small an impact angle β makes the curtain application more susceptible to the air boundary layer, which is transported through the moving surface 10,11 into the gusset between the curtain 5 and the surface 10,11. The angle β should advantageously not be less than 20°. When applied to a roller 11, the angle β depends on the position of the point of impact.

The device in 3 differs from the in 2 only in that the moving surface 10 is a material web 12, for example a paper web 12 or cardboard web 12 here. It should also be noted that the direction of movement of the material web 12 is from left to right, while the direction of rotation of the roller 11 is in 2 runs from right to left. Since the web of material 12 is guided obliquely upwards, the curtain strikes at an angle β that is greater than 90°. If the angle β is too large, the risk of so-called back flow can increase, ie a migration of parts of the application medium against the direction of movement of the surface 10. The angle β should therefore advantageously be selected to be less than 160°. A change in this angle β can be implemented with direct application by changing the path of the web (steeper or flatter) at the point of impact.

Reference List

1

application nozzle

2

straight channel gap

3

nozzle outlet

4

support surface

5

Curtain

6

curtain bursting

7

lateral distribution

10

moving surface

11

transfer roller

12

web of material

B

width of the curtain

V

vertical

L

length of the curtain

LG

supported length

lf

free length

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited 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.

Patent Literature Cited

• DE 202019106814 U1 [0002]
• DE 10057734 A1 [0009]
• EN 202019106814 [0019]

Claims (4)

Application device for applying a liquid or pasty application medium, in particular an aqueous starch solution, to a moving surface (10), comprising a curtain application unit, the curtain application unit having an application nozzle (1) for dispensing the application medium, and a support surface (4), the application nozzle ( 1) is designed as a slot nozzle (1), in which the application medium can be guided inside the nozzle along a straight channel gap (2) to a nozzle outlet (3), characterized in that the straight channel gap (2) is inclined relative to the vertical (V). , while the support surface (4) is either wholly or partially vertical or runs at a small angle to the vertical. application device claim 1 characterized in that the angle (α) between the straight channel gap (2) and the vertical (V) is at most 75°, in particular between 5° and 45°. Application device according to one of Claims 1 or 2 , characterized in that the supporting surface (4) is pointed at its lower end and the angle of this point is between 15° and 45°, in particular between 20° and 30°. Application device according to one of the preceding claims, characterized in that the length of the supporting surface (4) is between 1 mm and 80 mm, in particular between 5 mm and 50 mm.

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