APPLICATION DEVICE AND METHOD FOR COATING A FIBROUS MATERIAL WEB

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation, under 35 U.S.C. § 120, of copending International Patent Application PCT/EP2024/053786, filed Feb. 15, 2024, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German Patent Application DE 10 2023 106 575.0, filed Mar. 16, 2023; the prior applications are herewith incorporated by reference in their entirety.

FIELD AND BACKGROUND OF THE INVENTION

The invention relates to an application device for coating a fibrous material web, in particular a paper, packaging or cardboard web. The device has an application nozzle for applying a coating medium, in particular a starch solution, to a moving surface. The invention also relates to a method in which the device is used for coating a fiber web.

During the production or processing of fibrous material webs, or fiber webs for short, in particular of paper, packaging or cardboard webs, it is customary to provide these webs with one or more coatings.

These coatings may, for example, involve the application of pigment-containing coating colors or barrier media, for example on the basis of polyvinyl alcohol or micro/nano-fibrillated cellulose.

A further frequent use is the application of a starch solution to the fibrous web. As a result, firstly, the surface of the web can be influenced and, secondly, the strength properties of the fibrous web can also be improved by the application of starch.

Various application devices are known from the prior art.

The starch solution can be applied, for example, by means of a film press. In this case, an excess amount of coating medium is fed to the transfer roll, and then metered to the desired application amount by means of suitable doctor blade metering systems. A variation in the application amount is achieved by a greater or lesser amount of coating medium being doctored from the excess amount. For changing the application amount, a change in the doctor rod or doctor blade is generally necessary, for which purpose the machine has to be shut down.

Doctor (blade) metering systems of this type are described, for example, in German published patent application DE 10 2004 029 565 A1 and its counterpart U.S. Pat. No. 8,418,645 B2. In order to enable the starch to penetrate the fibrous web better, use has recently been made ever more frequently of transfer rolls with very hard surfaces. Such metering systems have proven disadvantageous since the hard roll surfaces cause the doctor blades to become very rapidly worn.

Alternatively, coaters are known, for example from European Patent No. EP 3 332 955 B1 and its counterpart U.S. Pat. No. 10,130,965 B2, in which the coating medium is sprayed onto the application rolls via a row of spray nozzles. In contrast with doctor blade metering systems, these systems manage without metering systems being in contact. Since the application amount is metered by adapting the application amount of the spray nozzles, the machine does not necessarily have to be shut down for this purpose.

However, the doctor blade metering systems and the spray nozzles have the disadvantage that the application amount can be varied only within relatively small limits.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide an application device and method which overcome a variety of disadvantages of the heretofore-known devices and methods of this general type and which provides for a device and a method that overcome the weaknesses of the prior art. It is a particular object of the invention to propose an application device in which the application amount can be varied within a wide range without the machine having to be shut down for this purpose. It is yet a further object of the invention to propose an application device which is both low in wear and low in maintenance.

With the above and other objects in view there is provided, in accordance with the invention, an application device for coating a fibrous material web, such as a paper web, a web of packaging material, or board material. The novel application device comprises:

• • an application nozzle for applying a coating medium to a moving surface, said application nozzle being formed with an outlet gap which extends in a width direction of said application nozzle and which is configured to produce a film of coating medium or a curtain of coating medium;
  • a device for varying an application amount of coating medium by varying said application nozzle; and
  • a control unit configured to determine a variation in the application amount from at least one characteristic value provided for a strength of the fibrous material web.

In other words, the objects of the invention are achieved with an application device for coating a fibrous material web, or fiber web, in particular a paper, packaging or cardboard web, wherein the application device comprises an application nozzle for applying a coating medium to a moving surface, and wherein the application nozzle has an outlet gap which extends in the width direction of the application nozzle and which is designed to produce a film of coating medium or a curtain of coating medium. Furthermore, the application device comprises means for varying the application amount. Finally, it is provided that the application device has a control unit which is designed to determine a variation in the application amount from at least one characteristic value provided for the strength of the fibrous web.

Such a device is suitable in particular for the application of starch. An application of starch is namely an option for influencing the strength of a fibrous web. In the case of a web with strength values which are too low, a higher strength can be achieved by the application of starch, or of similar suitable coating media. However, in order to keep costs for the coating medium low, it is advantageous no longer to apply coating medium as necessary. The present application device, which comprises both means for varying the application amount and a control unit which determines an optimized application amount on the basis of one or more strength characteristic values, is therefore highly advantageous for enabling a cost-effective operation of the plant.

In combination with an appropriate actuator mechanism, the application device can also be very simply integrated in a closed control circuit.

As is known, the strength of a fibrous web can be described in various ways. Examples include the tear resistance and the tear propagation resistance in the machine direction and in the transverse direction, the bursting strength or the delamination strength, to name just a few. Conventionally, only an individual number or a small number of strength values rather than all of these strength values are relevant for a certain product. For said strength value or values relevant for the produced fibrous web, a characteristic value/characteristic values can be provided in the control unit.

The characteristic value or characteristic values can be determined directly by direct measurement or indirectly.

Since nowadays many strength measurements are still destructive measurements which can be carried out only in a laboratory, but not in the machine during operation, the characteristic values for the strength are frequently determined indirectly.

This can be undertaken, for example, by a variable being measured, of which it is known, e.g. from tests, that this variable correlates with the desired strength characteristic value.

Alternatively, more complex dependencies may also be used. The characteristic value of the strength can thus be modeled from a multiplicity of measurement variables. This may be, for example, a physical or else purely statistical model. Such model-based determinations of characteristic values have hitherto also been called “virtual sensors.”

Application nozzles with an outlet gap are already known per se. For example, German utility model DE 20 2020 107 431 U1 describes a curtain nozzle with a uniform nozzle gap. Commonly assigned German patent application DE 10 2022 105 518.3 discloses an application nozzle, in which a wide slot nozzle dispenses a curtain or film which is then atomized by means of a blowerg head.

These nozzles are advantageous in comparison to the doctor blade metering systems since the metering manages without contacting doctor blade elements and thus without wearing parts. In addition, an application using an excess is avoided, as a result of which a complicated recycling and processing of the excess application medium are rendered superfluous.

Compared to the spray nozzles, such application nozzles with an outlet gap have the advantage not only of being more favorable than a large number of spray nozzles, but also of having significantly less tendency to become soiled.

The means for varying the application amount can comprise, for example, a conveying system (e.g. a pump), the delivery rate of which is variable. This can be realized, for example, in the form of a speed-controlled pump. This embodiment is advantageous since it can generally be very rapidly and simply retrofitted even in existing application nozzles.

However, even these application nozzles continue to have the disadvantage that the application amount can generally be varied only within relatively small limits, which is entirely sufficient in some applications which have a relatively low variation range of the strength values, whereas in other applications a greater variation of the application amount may be necessary.

The inventors have now discovered that this disadvantage can be overcome, in a further embodiment of the invention, when a move is made away from a previous basic nozzle construction principle. Hitherto, the aim, e.g. in the construction of a curtain nozzle, was to keep the outlet gap as constant as possible. For example, the above-noted German utility model DE 20 2020 107 431 U1 makes a considerable effort to maintain the constant nature of the gap. This is because curtain coaters have previously been used virtually exclusively in the context of coating with pigment-containing coating colors. This accuracy is highly important here for a uniform covering of the web and for uniform surface properties associated therewith.

However, such nozzles will increasingly also be used in future for the application of starch. High accuracy of the gap is less significant here than the possibility of being able to vary the amount of applied starch even within a very wide range. The present invention therefore proposes designing the gap width of the outlet gap to be changeable.

By varying the gap width, a very large range of application amounts can be elegantly covered. If, for example, the gap width is increased from 0.5 mm to 1 mm, a doubling of the application amount can therefore be achieved. If expanded to 2 mm, there is even a fourfold increase. Such large variations are advantageous for the operator of the plants since, as a result, for example, a broad range of types of paper having very different requirements and application amount can be produced in the same plant. And this can be achieved—given a corresponding design of the application device—without the plant being shut down.

Alternatively or additionally, the variation in the gap width can also be used for adapting the internal pressure of the application nozzle.

The nozzle internal pressure is an important parameter in a curtain nozzle or even in the described nozzles with a blowing head. As a rule, the coating material is namely not intended to be pressed out of the outlet gap but is intended to leave the gap freely under the action of gravity. The nozzle internal pressure should therefore not become too high. However, depending on the rheology of the coating medium, with the same delivery rate and same gap widths different nozzle internal pressures may arise. With an application nozzle according to one aspect of the invention, the gap width can now be adapted in such a way that the nozzle internal pressure lies within the desired range.

It is helpful in this respect if a suitable sensor system is provided for determining this nozzle internal pressure. This measurement may take place directly in the interior of the application nozzle. However, since such an internal sensor system can potentially become more easily soiled, it is generally more expedient to carry out the determination of the pressure in an inlet of the coating medium.

The gap width can be varied in different ways or with different actuating means.

For example, a fixed gap having a large gap width can be provided, which is covered to a greater or lesser extent by means of a displaceable cover. Alternatively, it can be provided that at least one wall of the outlet gap is formed from a movable wall element. The outlet gap can then be made narrower or wider per se by said wall element.

The cover or the wall element can be moved, for example, via one or more actuating screws, threaded spindles or similar actuators. Since the width of the application nozzle may be 1 to 10 m or more, it will frequently be expedient, for changing the gap width, to provide a plurality of actuators which are distributed over the width of the application nozzle. The actuators may be moved manually or also by motor.

In order to achieve a uniform gap width over the width of the application nozzle, it is often advantageous if the actuators are distributed uniformly over the width of the application nozzle such that the distances between adjacent actuators over the width of the application nozzle are identical or differ only by less than 10%.

Other actuating means for varying the gap width are also conceivable.

Advantageously, the application device is configured in such a way that, by means of the means for varying the application amount, e.g. by changing the gap width, the application amount can be varied between a minimum amount and a maximum amount, the maximum amount being at least 30%, in particular at least 50%, preferably at least 100% above the minimum amount.

In order to be able to vary the gap width of the outlet gap without having to stop the plant, it is advantageous in particular if the gap width can be changed by motor and/or hydraulically and/or pneumatically.

The application nozzle conventionally extends over the entire width of the fibrous web to be coated or over the entire width of the moving surface.

In modern plants, the width of the fibrous web, and therefore the width of the application nozzle, can be up to 10 meters or more. It will therefore frequently be expedient, in order to change the gap width, to provide a plurality of actuators which are distributed over the width of the application nozzle.

Furthermore, the application device can have a control unit which is designed to automatically change the gap width. This control unit can be integrated, for example, in the central machine and plant control and in particular also in the same control unit with which the variation of the application amount is determined. A high degree of automation is therefore possible and, for example, the gap width can be adjusted on the basis of measured or predefined values from the machine control.

Embodiments can be provided in which the application nozzle is designed as a curtain nozzle, in which the coating medium is deposited in the form of a freely falling curtain under the influence of gravity from the outlet gap onto the moving surface. This then constitutes an improvement to the curtain nozzle described in DE 20 2020 107 431 U1.

In particularly preferred embodiments, it can be provided that the application nozzle furthermore comprises a blowing head which is designed to produce a linear jet of gaseous medium, and wherein, furthermore, an impact line is provided at which the linear jet of gaseous medium impacts against the film or curtain of coating medium so as to form a spray curtain, and wherein the blowing head is arranged in such a way that the spray curtain is directed toward the moving surface. The gaseous medium can be in particular air.

This then constitutes an improvement to the application device described in our prior German patent application DE 10 2022 105 518.3.

Both the outlet gap and the linear jet of gaseous medium conventionally extend over the entire width of the moving surface or of the fibrous web.

The application device according to aspects of the invention can be used for coating fibrous webs on one side or both sides.

For coating on both sides, it can be provided, for example, that the application device has a first application nozzle for application to a first moving surface and a second application nozzle for application to a second moving surface, the first moving surface and the second moving surface each being designed as a surface of a transfer roll, and the two transfer rolls forming a transfer nip in which the coating medium can be transferred to the fibrous web.

The same or different coating media can be applied on both sides.

One or both transfer rolls can be designed as hard rolls. It is intended to be understood by this that the surface of the first transfer roll and/or the surface of the second transfer roll has/have a hardness of over 55 ShoreD, in particular over 90 ShoreD.

An application device for the application of starch, which application device comprises two hard rolls, is referred to as a “hard-nip-sizer.” A particularly good penetration of the starch into the fibrous web can therefore be achieved.

With the above and other objects in view there is also provided, in accordance with the invention, a method for coating a fibrous web, in particular a paper, packaging or cardboard web, in which a coating medium is applied to a moving surface by means of an application device, the application device being designed according to one aspect of the invention.

In other words, the novel method comprises:

• • providing an application device as described above;
  • determining a value for a strength of the fibrous material web, and coating the fibrous material web with a coating medium by the application device, and thereby varying the application amount depending on the characteristic value.

In preferred embodiments, the coating medium can be a starch solution. Alternatively, a multiplicity of barrier media can also be used as the coating medium. These can be in particular also fiber-containing media, such as MFC (micro-fibrillated cellulose) or NFC (nano-fibrillated cellulose).

Advantageously, it can be provided that the coating takes place indirectly, with the coating medium first of all being applied to a moving surface in the form of a transfer roll, and being transferred from there in a transfer nip to the fibrous web. Such an indirect application can also be carried out, for example, by means of a “hard-nip-sizer”.

In preferred embodiments, it can be provided that a characteristic value for the strength of the fibrous web is determined, and the application amount varies depending on said characteristic value. The application amount may be varied by an operator after assessing the characteristic value. However, it is particularly advantageous if said change takes place automatically by means of the control unit. For this purpose, default values or default curves can be stored in the control unit, and the control unit can determine an optimum delivery capacity or an optimum gap width from the characteristic value and the default values. This gap width can then be adjusted automatically via motorized or hydraulic and/or pneumatic actuators. An operator intervention can therefore even be completely dispensed with.

The delivery capacity can also be varied in automated fashion and without operator intervention, e.g. via a rotational speed of the pump.

The characteristic value can be determined directly by direct measurement or indirectly.

Since nowadays many strength measurements are still destructive measurements which can be carried out only in a laboratory, but not in the machine during operation, the characteristic values for the strength are frequently determined indirectly.

This can be undertaken, for example, by a variable being measured, of which it is known, e.g. from tests, that this variable correlates with the desired strength characteristic value.

Alternatively, more complex dependencies may also be used. The characteristic value of the strength can thus be modeled from a multiplicity of measurement variables. This may be, for example, a physical or else purely statistical model. Such model-based determinations of characteristic values have hitherto also been called ‘virtual sensors’.

A highly advantageous use of the present invention is improving the changing of a type in a papermaking machine. It can thus namely be provided that, in the event of a change from a first type of fibrous web to a second type, the application amount is automatically changed from a first default value to a second default value by changing the gap width. This simplifies the changing of the type for the operator. In addition, it also provides the operator with more flexibility in production planning since it is now possible to change even from a type having a high requirement for, e.g. starch, directly to a type having a low starch requirement without needing to shut down the machine. This is in particular also advantageous since, during a change of type, the provision of a strength characteristic value is frequently not possible at all or possible only with a high degree of inaccuracy.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in an application device and method, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A and 1B show an application nozzle according to one aspect of the invention;

FIG. 2 shows an application device according to a further aspect of the invention; and

FIG. 3 is a highly diagrammatic view of a device for the simultaneous coating of a fiber web on both sides.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A and 1B show an application nozzle 1 which is designed as a curtain nozzle 1. The coating medium, for example a starch solution, is guided in a distribution channel 10 and distributed over the width of the application nozzle 1. The width of the application nozzle 1 conventionally extends over the entire width of the moving surface 4 or of the fibrous web to be coated. The coating medium exits from the application nozzle 1 through an outlet gap 2 and then falls downward in the form of a curtain 14 under the influence of gravity onto a moving surface.

In order to vary the gap width W, an actuating means 3, of the application nozzle 1 of FIGS. 1A and 1B comprise a wall of the outlet gap 2 formed from a movable wall element 7. The outlet gap 2 can then be made narrower or wider by the wall element 7. FIG. 1A shows a position with a large gap width W. In order to vary the application amount, this gap width W, as shown in FIG. 1B, can be reduced. For this purpose, the actuating means 3 of the application nozzle 1 have suitable actuators 9. An actuating screw 9 is shown by way of example. Since the width of the application nozzle 1 can be up to 10 m or more, it is frequently expedient, in order to change the gap width W, to provide a plurality of actuating screws 9 or other actuators which are distributed over the width of the application nozzle 1. The actuating screws 9 can be moved manually or else by motor.

For automation of the adjustment, a motorized or else a hydraulic and/or pneumatic drive of the actuators is advantageous. The latter can be connected to a control unit which can automatically change the gap width W depending on default values without the plant having to be shut down for this purpose.

The application device in FIG. 2 shows an application nozzle 1 which applies the coating medium to a moving surface 4 in the form of a transfer roll 4. The coating medium can then be transferred in a transfer nip from the transfer roll 4 to the fibrous web.

The application nozzle 1 comprises a nozzle as illustrated in FIGS. 1A and 1B. In contrast thereto, the coating medium in FIG. 2 does not fall downward in the form of a curtain 14, but rather is deposited as a film onto a support surface 8 and guided by said support surface 8. Furthermore, the application nozzle 1 further comprises a blower head 5 which is designed to produce a linear jet of gaseous medium. The gaseous medium can be air. The linear jet of gaseous medium impacts at an impact line onto the film of coating medium so as to form a spray curtain 6. The blower head 5 is arranged in such a way that the spray curtain 6 is directed toward the moving surface 4.

The application amount is also changed here by varying the gap width W of the outlet gap 2. This takes place, as described in FIGS. 1A and 1B, via suitable actuating means 3. The blower head 5 or the linear jet of gaseous medium does not contribute to varying the application amount. By this means, inter alia, the atomization of the spray mist 6 or the impact angle of the spray mist 6 can be adapted.

FIG. 3 illustrates a so-called “hard-nip-sizer” with two rolls 4 with a moving surface 4a and a moving surface 4b, respectively. The two rolls 4 are disposed to form a hard nip 16 through which the web 17 travels. In this case, the coating solution is applied indirectly, in that it is first deposited on the moving surface 4a, 4b by way of a (free-falling) curtain 14 of by way of a spray curtain 6—as illustrated in FIG. 2, before it is transferred onto the web 17 in the nip 16. Both of the application devices 1a and 1b have application nozzles with variable profiles of the gap widths. That is, the application of the coating medium onto the moving surfaces 4a, 4b, may be effected by the first embodiment illustrated in FIGS. 1A and 1B, or by the second embodiment illustrated in FIG. 2.

The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention:

• • 1 application nozzle
  • 2 outlet gap
  • 3 actuating means
  • 4 moving surface
  • 5 blower head
  • 6 spray curtain
  • 7 wall element
  • 8 support surface
  • 9 actuator, actuating screw
  • 10 distribution channel
  • 14 (free-falling) curtain of coating medium
  • 16 (hard) nip
  • 17 fibrous web
  • W gap width