Application nozzle, application unit, and method

Abstract

The invention relates to an application nozzle, to an application unit, and to a method for applying a liquid or pasty coating medium onto a running surface, in particular onto a running surface in a machine for producing or processing a fibrous web, wherein the application nozzle comprises a fluid head which is designed to produce a film from a coating medium or a curtain from a coating medium, and the application also additionally comprises a blowing head which is designed to generate a linear jet of gaseous medium. An impact line is additionally provided on which the linear jet of gaseous medium impacts the film or curtain made of the coating medium, thereby forming a spray curtain. The blowing head is arranged such that the spray curtain is oriented in the direction of the running surface.

Description

[0001] Application nozzle, application unit and process

[0002] The invention relates to an application nozzle for applying a liquid or pasty coating medium to a moving surface, in particular to a moving surface in a machine for producing or processing a fibrous web, as well as a corresponding application unit and application method.

[0003] A variety of coating processes are known for applying coating media to paper or board webs. Film presses are often used, particularly for starch application. A film of the coating medium is applied to a transfer roller and then transferred to the fiber web in a transfer nip.

[0004] Often, the coating medium is applied in excess to the transfer roller and then metered to the desired application quantity using suitable doctor blade metering systems. Such doctor blade metering systems are described, for example, in document DE102004029565 A1.

[0005] As part of the further development of the film press, the use of application rolls with very hard roll covers (0 to 5 P&J, or over 60 ShD) was introduced when operating the film press with higher nip loads (80 to 180 kN / m).

[0006] The high pressure and the short residence time of the substrate in the nip due to the very small contact zone between the rollers in the roller nip leads to a better penetration and distribution of the starch into the structure of the fibrous web, so that a higher efficiency of the starch (higher strengths with the same application quantity) can be achieved.

[0007] The use of very hard roller covers in doctor blade metering systems leads to increased wear on the metering elements used, as these metering elements are in direct contact with the rotating roller surface. The hard roller covers also experience wear or can develop marks, for example, on the profile of the doctor rods. The option of slightly varying the application quantity of the volumetric metering element by changing the contact pressure on the metering element is also no longer possible, as the surface of the application roller cannot be deformed due to its increased hardness. Thus, a slight adjustment of the application quantity of the volumetric metering element is no longer possible.

[0008] In the past, contactless dosing systems were therefore developed, particularly for use with hard rollers, in which the coating medium is no longer added in excess, but is dosed before it is applied.

[0009] One possible design for such applicators involves applying the coating medium to the transfer roller in the form of a freely falling curtain. This is described, for example, in document EP3830336 A1. One challenge with these applicators is keeping the very thin, freely falling curtain stable, even under the challenging conditions of a production facility.

[0010] Alternatively, application units are known from EP3332955 B1, for example, in which the coating medium is sprayed onto the application rollers via a series of spray nozzles. Here, too, contact dosing elements can be dispensed with. A disadvantage of such application units, however, is that the spray nozzles used easily become dirty or clogged during operation. For this reason, the spray nozzles of EP3332955 B1 are designed to be individually removable for frequent cleaning. Alternatively, DE 20 2015 009 603 U1 proposes providing a double set of nozzles, half of which are in operation and the others in the maintenance position.

[0011] In addition, distributing the coating medium across the width of the application roller or the material web to be coated requires a large number of individual spray nozzles. To achieve a uniform coating, the spray cones are also designed with significant overlap, further increasing the number of spray nozzles. This not only increases investment costs but also increases maintenance effort, since even a single defective or clogged nozzle can no longer guarantee a uniform coating.

[0012] Furthermore, during spray application, a secondary mist of coating material is created that is not transferred to the application roller and remains in the spray chamber. This secondary mist would quickly contaminate the entire spray chamber and its internal components. To prevent this, the spray chamber must be equipped with an extraction system, as described in EP3332955 B1. Typically, approximately 5% of the coating medium is removed via the extraction system and recycled for reuse.

[0013] It is an object of the present invention to propose an alternative application nozzle, as well as the associated application device and method, which in particular overcomes the difficulties of the prior art.

[0014] It is a further object of the present invention to propose a contactless application system which has a low tendency to become dirty.

[0015] It is also an object of the invention to propose an application system which ensures a stable application of the coating medium to the transfer roller, particularly in comparison to curtain application.

[0016] The object is achieved according to the invention by embodiments according to the independent claims. Further advantageous embodiments of the present invention can be found in the subclaims.

[0017] With regard to the application nozzle, the object is achieved by an application nozzle for applying a liquid or pasty coating medium to a moving surface, in particular to a moving surface in a machine for producing or processing a fibrous web.

[0018] According to the invention, the application nozzle comprises a fluid head designed to generate a film or curtain of coating medium, and the dispensing nozzle further comprises a blowing head designed to generate a linear jet of gaseous medium. Furthermore, an impact line is provided at which the linear jet of gaseous medium impacts the film or curtain of coating medium, forming a spray curtain. The blowing head is arranged such that the spray curtain is directed toward the moving surface.

[0019] The running surface can in particular be the surface of a rotating roller or the surface of the fibrous web.

[0020] The present invention maintains in a very creative way the advantages of curtain coating, namely the uniform, machine-wide application of the coating medium, but avoids the disadvantages of the delicate curtain.

[0021] In addition, with the proposed application nozzle, a spray application can also be used for large machine widths of 10 m and more without a massive increase in the number of spray nozzles and thus the risk of contamination.

[0022] For the purposes of this application, a linear jet of gaseous medium is understood to mean a jet of gaseous medium, in particular an air jet, that has a certain thickness and a certain extension (in the width direction of the moving surface), whereby this extension is many times (e.g., 10 times, 100 times, or more) greater than the thickness. Such a linear jet can be generated, for example, using a slot nozzle in which the length of the slot is many times greater than the thickness.

[0023] The film or curtain of coating medium and / or the linear jet of gaseous medium can advantageously extend over at least 50%, preferably more than 80%, in particular the entire width of the running surface.

[0024] The idea behind the invention is to use a fluid head to create a film or curtain of coating medium of the desired width. This film / curtain can be guided toward the die head freely or with support under the influence of gravity. Advantageously, this fluid head can be designed as a "slot die" or "slide die," similar to the known curtain dies.

[0025] At the impact line, the film curtain is impinged upon by a jet of gaseous medium—usually air. These colliding media create a spray curtain. If the velocity of the gaseous medium is selected to be significantly higher than the velocity of the film curtain, the direction of the resulting spray curtain will essentially correspond to the direction of the jet of gaseous medium. Thus, by appropriately adjusting the blow head, the spray curtain can be directed toward the moving surface—for example, toward the surface of a rotating roller.

[0026] The spray curtain created in this way will not have the shape of a thin, freely falling curtain. Rather, the spray curtain, viewed from the side, will have the shape of a spray wedge and a certain opening angle 6. When referring to the direction of the spray curtain, this always refers to the central direction of this spray wedge.

[0027] The point of impact of the spray curtain should then be defined as the point on the running surface where the center of the spray wedge touches the running surface.

[0028] The terms "moving surface" and "moving surface" are used synonymously in this application. In most applications, such a moving surface can be the surface of a rotating roller or the surface of the fibrous web.

[0029] The coating medium may advantageously be a starch solution.

[0030] In the simplest case, the gaseous medium can be air.

[0031] A very significant advantage of the present invention is the fact that creating the spray curtain eliminates the need for a multitude of individual spray nozzles, which can become clogged with the coating medium. The coating medium can be dispensed, for example, from the low-maintenance wide-slot nozzles proven effective in curtain application. Furthermore, a uniform coating can be achieved without the overlap of the spray cones of neighboring spray nozzles otherwise necessary in spray application.

[0032] The blowing head for generating the jet of gaseous medium can also be very simple. In the simplest case, the blowing head can consist of a wide-slot nozzle that extends across the entire desired width and can be connected, for example, to a simple air blower or a compressed air supply.

[0033] If desired, the blow head can also be designed in a more complex manner and, for example, comprise a series of individual air nozzles arranged next to one another.

[0034] Since the die head hardly comes into direct contact with the coating medium and the air jet carries the medium away from the die head, the risk of contamination is very low.

[0035] By combining the fluid head and the blower head, this application nozzle is essentially a dual-component nozzle. This gives the application nozzle another highly advantageous feature: The dosage of the coating medium can be adjusted independently of the air jet speed. The speed of the spray curtain is achieved by changing the air flow rate that the air blower delivers through the blower head. This allows the application rate and spray speed to be adjusted independently of each other.

[0036] This means that, in contrast to curtain application, even small amounts of coating medium can still be applied stably.

[0037] In advantageous embodiments, it can be provided that the application nozzle comprises a support surface which is arranged such that the coating medium is discharged from the fluid head onto the support surface and is guided as a fluid film along the support surface.

[0038] Furthermore, the lower end of the support surface can form a separation edge, and the impact line can be located at this separation edge or at a distance of less than 5 mm from this separation edge. This separation edge can be provided with a structure, in particular with a wave structure or a sawtooth structure.

[0039] The supported film principle utilizes the adhesive force of the liquid on a flat or curved support surface to prevent shrinkage of the pre-metered liquid film on this support surface. Because the adhesive force of the material used (preferably stainless steel or rust-proof metals) for the support surface (blade, nozzle wall, or similar) is higher than the shrinkage force of surface tension, the coating medium adheres to the support surface with minimal shrinkage and flows downward under the influence of gravity, especially when this support surface is positioned vertically or at an angle of up to 45 degrees.

[0040] A particularly significant advantage of the supported film principle is that the flow rate or application rate of the film applied to this supported wall can be reduced to as little as 2.5 l / min / m or increased to over 100 l / min / m without causing film defects such as holes, thread formation, or so-called film bursts on this supporting surface. This allows a uniformly pre-metered film to be provided at the tear-off edge of the supporting surface for the subsequent spraying process.

[0041] At the lower edge of the support surface, the pre-metered film leaves the support surface. At this point, the supporting, shrinkage-preventing effect of the wall is lost, and surface tension begins to take effect again, causing stringing, bursting, and shrinkage in the film. This effect is more pronounced the lower the film thickness or the flow rate of the pre-metered liquid film on the support surface.

[0042] To avoid or minimize this surface tension effect at the lower edge of the support surface, another feature can be advantageous, eliminating the otherwise necessary use of surfactants or other surface tension-active agents. Liquid threads or droplets sometimes form at the lower edge of the support surface of the liquid film (usually when the film flow rate is reduced to below 8 l / min / m), which severely impair the uniformity of the film in the cross-machine direction (CD direction).

[0043] If this offered film is converted into a spray curtain by the linear jet of gaseous medium, holes, discontinuities or irregularities in the CD profile of the application quantity of the generated spray curtain can arise due to the surface tension effects described above.

[0044] To avoid this problem, it may be provided that instead of a straight separation edge of the supported liquid film, a structured, e.g. wavy or sawtooth-shaped separation edge is used at the lower edge of the support surface, thereby extending the effective length of the separation edge.

[0045] By means of a wavy or sawtooth-shaped or otherwise suitably structured geometry, the wetting angle of the coating medium relative to the tear-off edge can be optimally utilized so that the surface tension of the coating medium itself ensures better wetting, optimal contact and homogeneous distribution of the liquid over this edge.

[0046] This mechanical solution (sawtooth edge or finely waved edge) can in many cases avoid or circumvent the otherwise necessary chemical solution using expensive surfactants.

[0047] In particularly preferred embodiments, it can be provided that the support surface is provided entirely or partially by the wall of the blow head.

[0048] This is also advantageous because the distance from the fluid head to the die head—and thus the distance along which the film or curtain of coating medium runs—cannot be made arbitrarily large. In practice, this distance is often less than 50 cm, especially less than 30 cm. Installing a separate component as a support surface is often difficult in these cases.

[0049] This problem can be easily circumvented by designing the wall of the die head, or a portion thereof, in such a way that it serves as a support surface for a film of application medium. In particular, this portion of the die head wall can be made of metal, e.g., stainless steel, and can also be smooth, meaning it does not have any screws, attachments, or other structural elements (such as embossed numbers or text).

[0050] In general, it is advantageous if the support surface is smooth and does not allow any coating medium to accumulate on it. Therefore, the support surface can be treated to increase smoothness and / or reduce adhesion. Some examples of suitable treatments are listed below:

[0051] • Teflonization, i.e. coating with fluorine-containing plastics such as PTFE (polytetrafluoroethylene) to prevent or minimize adhesion

[0052] • Plastic coating e.g. with a polyethylene

[0053] • Grind

[0054] • Electropolishing or plasma polishing

[0055] The treatments listed here are particularly advantageous if the support surface is made entirely or partially of a metal, for example stainless steel.

[0056] In further preferred embodiments, it can be provided that the length of the spray curtain, and thus the distance of the impact line from the running surface along the direction of the spray curtain, is a maximum of 80 mm, in particular less than 20 mm, particularly preferably less than 10 mm.

[0057] In the spray application known from the prior art, a distance of the nozzle from the surface, and thus a spray curtain length of approximately 100 mm, is used to ensure that the result of the superimposed spray patterns of the individual nozzles is sufficiently good and homogeneous. Spray curtains with lengths of less than 50 mm are difficult to adjust according to the state of the art, and lengths under 20 mm are impossible to adjust at all, as this would result in uncovered areas, severe CD profile irregularities, and holes in the spray pattern.

[0058] The application nozzle proposed here allows for very short spray curtain lengths of less than 50 mm to be easily adjusted. In tests, even spray curtains as short as 3 mm worked well.

[0059] The shorter the length of the spray curtain, the more uniform the application pattern and the CD profile of the liquid film applied by this spray device onto a rotating roller or onto a web supported on it.

[0060] Furthermore, the use of shorter spray curtains results in less or no secondary mist. Thus, one of the problems of conventional spray application can be easily avoided or minimized. Application nozzles or applicators according to aspects of the present invention can therefore often be designed without a dedicated extraction system for the secondary mist, which significantly reduces both acquisition and operating costs.

[0061] Due to the shorter length of the spray curtain, it develops a higher dynamic pressure on the impact line or on the impact area on the moving surface, which is advantageous in terms of deceleration and limiting the negative effects of the air boundary layer.

[0062] Due to the shorter length of the spray curtain, the droplet velocity decreases significantly less than with a longer flight distance. The droplet velocity of the spray curtain decreases approximately by the square of the spray curtain's flight length. Thus, with a shorter spray curtain length, a significantly greater dynamic pressure of the spray curtain can be generated on the surface, allowing the air boundary layer to be better and more efficiently controlled or avoided.

[0063] To avoid, or at least significantly reduce, secondary mist, further advantageous options are available within the scope of the present invention, each of which can be used individually or in combination. For example, the direction of the spray curtain can be provided with a component that is directed opposite to the direction of movement of the moving surface.

[0064] The spray curtain, positioned opposite to the direction of movement of the surface, such as the roller surface, prevents or minimizes the formation of secondary mist by improving the transfer rate of the coating medium to the surface. A further advantage of this arrangement is the fact that the spray curtain itself reduces the air boundary layer of the moving surface. The spray curtain acts like a contactless, active air boundary layer limiter ("AirCut").

[0065] Furthermore, to avoid secondary mist, it is also advantageous if the spray curtain hits the moving surface at a flat angle of impact a.

[0066] The angle of impact α is the angle formed by the direction of the spray curtain at the point of impact with the tangent of the moving surface. The angle of impact α ranges between 0° and 90°.

[0067] A relatively flat angle of incidence a of maximum 60°, especially angles of incidence a between 10° and 50°, has proven to be very advantageous.

[0068] In advantageous embodiments, the blow head can be designed to be movable, in particular rotatable, in such a way that the direction of the spray curtain, and thus also the angle of impact a, can be easily adjusted.

[0069] In addition, a movable design of the blow head and / or the fluid head can also be advantageous in order to move them from a working position to a maintenance position.

[0070] The characteristic of spray application in the opposite direction to the direction of surface movement, combined with a flattest possible average angle of impact (α) in the range of 10 to a maximum of 60 degrees, promotes the uniformity of the coating film applied to the moving surface. The resulting reversal of the direction of the spray curtain after contact with the substrate results in film homogenization and improved film adhesion to the moving surface.

[0071] The feature of spray application in the opposite direction to the direction of movement of the moving surface, at the flattest possible average angle of impact in the range of 10 to max. 60 degrees, along with a small distance or a short spray curtain length (e.g., < 10 mm), is very advantageous for reducing the influence of the air boundary layer transported by the moving surface. The spray curtain blows against the direction of the air boundary layer. As a result, the air boundary layer is decelerated and redirected due to the dynamic pressure generated by the accelerated spray curtain, minimizing its effect on the spray transfer rate.

[0072] Another parameter that influences the formation of secondary mist is the opening angle 6 of the spray curtain.

[0073] A small opening angle allows for a more compact and easier to handle spray curtain, which can also be arranged more easily at a flat angle of incidence a to the running surface.

[0074] A small opening angle 6 can be achieved, for example, by a suitable design of the nozzle of the blowing head. In particular, the blowing head can be provided with a lip, preferably an adjustable lip, at the outlet.

[0075] The opening angle can also be influenced by the viscosity of the coating medium or the air velocity of the nozzle. Lower viscosity and higher air velocities result in a smaller opening angle of the spray curtain wedge.

[0076] The following table shows some examples of advantageous designs:

[0077] With regard to the application unit, the object is achieved by an application unit for applying a liquid or pasty coating medium to a moving surface, in particular to a moving surface in a machine for producing or processing a fibrous web, wherein the application unit comprises at least one application nozzle according to one aspect of the invention.

[0078] In preferred embodiments, the running surface may be provided by the surface of a rotating transfer roller.

[0079] Alternatively, the running surface can also be provided by a moving fibrous web. This fibrous web can run either freely or supported, for example, by a roller, during application of the coating medium.

[0080] For double-sided application, it may be advantageous if the applicator has two transfer rollers that together form a roller nip. The applicator can then have two application nozzles according to one aspect of the invention, with the first application nozzle applying the coating medium to the first transfer roller, and the second application nozzle applying the coating medium to the second transfer roller.

[0081] One or two transfer rolls with very hard roll covers (0 to 5 P&J, or over 60 ShD) can be provided. Higher nip loads (80 to 180 kN / m) can be used during application.

[0082] One of the two transfer rollers can be designed as a controlled deflection roll. This is particularly important for pairs of hard transfer rollers to ensure a uniform profile across the entire width of the application unit, even when a variety of different operating modes are to be realized. Preferably, one or more of the application nozzles are arranged so that the spray curtain impinges on the surface of the roller between the 7 o'clock position and the 12 o'clock position, preferably between the 8 o'clock position and the 12 o'clock position, in particular between the 8 o'clock and the 10 o'clock positions.

[0083] The position is to be understood with regard to a roller rotating clockwise.

[0084] By selecting the appropriate position of the spray curtain, additional effects can be achieved by setting the appropriate impact angle of the spray curtain in connection with minimizing the influence of the air boundary layer and avoiding secondary mist. These effects are particularly evident in the area between the 7 and 10 o'clock position, especially between the 8 and 10 o'clock position of the application zone. This zone is therefore preferred. The area between 10 o'clock and 12 o'clock is also a usable, albeit less optimal, positioning area for the spray curtain. The area between 12 and 15 o'clock is sometimes also possible, depending on the design of the application unit. However, since the tendency for secondary mist to form is noticeably more pronounced there, this area is less preferred.

[0085] The position of the impact zone in the area between the 7:00 and 9:00 o'clock position, in particular between the 8:00 and 9:00 o'clock position on the transfer roller (or on a fiber web supported on a rotating roller) offers the advantage that any drops forming on the lip of the nozzle can be removed vertically and outside the application area by the effect of gravity in such a way that no drop marks can be generated on the application film.

[0086] With regard to the method, the object is achieved by a method for applying a coating medium, in particular a starch solution, to a moving surface, in particular to a moving surface in a machine for producing or processing a fibrous web, the application being carried out by means of an application unit according to one aspect of the invention.

[0087] The devices and methods proposed here are particularly well suited for generating and spraying fine to coarse spray made from coating media based on low-viscosity liquids or liquid mixtures.

[0088] Although the present invention is particularly well suited for the application of starch solution, it is not limited to any coating medium.

[0089] Coating media can be, for example:

[0090] • Pure water or water with polar or non-polar soluble additives (e.g. salts, dyes,)

[0091] • Water mixed with other liquids (with binder, with thickener, with extensional rheology thickener, with alcohol, or any kind of miscible or immiscible components)

[0092] • Aqueous starch solutions made from all types of natural starch (corn, wheat, potato, tapioca, barley, rice, etc.). The gelatinization and degradation of the starch to produce the starch solutions can be achieved using various methods: pure chemical gelatinization, thermo-chemical gelatinization, pure thermal gelatinization, enzymatic gelatinization, and combinations of different processes.

[0093] • Coating paints with finely dispersed, solid micro-particles, such as mineral coating pigments such as CaCO3, kaolins, talc or other mineral pigments, etc... with coarse, medium or fine aspect ratio or synthetic or polymer pigments, or similar synthetic or polymer pigments Aqueous dispersions of fiber-based substances such as nano-fibrillated cellulose, micro-fibrillated cellulose, nano-cellulose, cellulose fiber with short or long fiber

[0094] • Any low viscosity liquids with Brookfield viscosity < 300 mPas, preferably < 200 mPas, measured at 100 rpm, 60°C, spindle 4.

[0095] The optimal viscosity level of the coating medium is below 200 mPas, especially in the range between 0.1 and 160 mPas for the Brookfield viscosity (100 rpm / 55°C, spindle 4)

[0096] The solids content of the coating medium, especially when using starch solutions for a process described here, is advantageously in the range of 1 to 35%.

[0097] The specific gravity of the coating medium can be in the range of 0.8 g / cm 3 up to 1.3 g / cm 3 lay.

[0098] A process according to one of the aspects of the invention can be carried out at a processing temperature for the coating medium of 50°C to 90°C. Temperatures up to 100°C are also possible.

[0099] The surface tension of the coating medium, especially the starch solution, can range from 30 mN / m to 70 mN / m (50°C). The range of usable surface tensions can be expanded, among other things, by using a structured tear-off edge.

[0100] Using methods according to aspects of the present invention, the application rate of coating medium per surface can be up to 100 l / m / min, in particular between 2.5 l / m / min and 50 l / m / min. Dosing takes place via the application nozzle without the use of contacting dosing elements, which also enables the use of hard transfer rollers. The linear jet of gaseous medium at the outlet of the blow head can have a speed of more than 5 m / s, in particular more than 10 m / s or more than 12 m / s. One advantage of the invention is that the speed of the jet of gaseous medium - and thus the speed of the spray curtain - is largely decoupled from the application rate of coating medium.

[0101] The described discharge nozzles or applicators have proven very advantageous, among other things, because they can completely or largely prevent the occurrence of secondary mist. However, it can happen that the remaining secondary mist is still strong enough to lead to permanent contamination of the application nozzles.

[0102] In principle, it is also possible to equip the application nozzles or applicators according to aspects of the invention with suction devices, as is known from the prior art. However, for cost reasons, it is advantageous if such a suction device can be dispensed with.

[0103] To this end, the inventors have developed a solution that is not only very advantageous in combination with commissioned works according to aspects of the invention, but also represents a further inventive idea independently of this.

[0104] What is proposed is an applicator for applying a liquid or pasty coating medium to a moving surface, in particular to a moving surface in a machine for producing or processing a fibrous web, wherein the applicator can advantageously be an applicator according to one aspect of the invention.

[0105] The application unit according to the further inventive idea comprises an application nozzle for generating a spray curtain.

[0106] The application unit further comprises an application space which is formed and limited by the running surface • at least a part of the wall of the application nozzle

[0107] • the spray curtain

[0108] • the location of the narrowest gap between the running surface and the application nozzle.

[0109] If a secondary mist is created inside the application area, the application medium can in principle accumulate on the boundary surfaces, but does not cause permanent contamination there.

[0110] If the medium deposits on the moving surface, this is not critical, as it is already covered with coating medium—or, in the case of reversed movement, is immediately covered with coating medium by the spray curtain—and the deposition of the secondary mist does not disrupt the deposited film. The additional coating medium is simply transported further along with the moving surface.

[0111] Deposits on the spray curtain are also uncritical, since the movement of the spray curtain carries the coating medium of the secondary mist towards the moving surface and deposits it on it.

[0112] No deposits can form in the open gap between the moving surface and the application nozzle. Escaping secondary mist through this gap can be prevented, as described later, by adjusting the geometry and, if necessary, additional sealing measures.

[0113] To prevent deposits on the remaining wall, the further inventive concept provides for a film of flowing coating medium to be applied to the boundary provided by at least part of the wall of the application nozzle. The wall is thus continuously flushed with coating medium ("flushing medium") during operation of the application unit. Any secondary mist that deposits on this wall is simply removed with the flushing medium. Since the secondary mist consists of the same coating medium as the flushing medium, the medium remains pure and can be used for coating.

[0114] In this way, extraction of the secondary mist can either be avoided entirely or accomplished by a very simple and energy-efficient device.

[0115] An applicator with an applicator according to an aspect of the further inventive idea can in principle be used for all spray applicators known from the prior art.

[0116] The rinsing medium can be collected without much effort and fed to the spray nozzles.

[0117] Due to the structure of the application nozzles according to aspects of the present invention, such an application space for such application nozzles can be realized particularly easily.

[0118] An advantageous embodiment is, for example, an application unit according to one aspect of the invention, wherein the application unit has an application space which is formed and delimited by:

[0119] • the running surface

[0120] • the wall of the blow head

[0121] • the spray curtain

[0122] • the location of the narrowest gap between the moving surface and the die head, and wherein the coating medium is applied from the fluid head to the wall of the die head in such a way that the entire wall of the die head inside the application space is covered with flowing coating medium.

[0123] Since, in advantageous embodiments of the invention, the application medium is applied from the fluid head onto a support surface, with this support surface being provided by the wall of the blow head, the flushing of this boundary of the application space can be achieved in a completely natural manner. The coating medium used as the flushing medium, together with any collected secondary mist, is then conveyed as a spray curtain onto the moving surface by the jet of gaseous medium upon reaching the separation edge. A separate discharge of the flushing medium is not necessary here.

[0124] The part of the wall of the blow head that serves as a support surface can also be treated to prevent adhesion, for example by:

[0125] • Teflonization, i.e. coating with fluorine-containing plastics such as PTFE (polytetrafluoroethylene) to prevent or minimize adhesion

[0126] • Plastic coating e.g. with a polyethylene

[0127] • Grind

[0128] • Electropolishing or plasma polishing

[0129] To prevent secondary mist from escaping from the front sides of the application unit, the application chamber can be closed off at both front sides by a sealing surface.

[0130] While secondary mist deposits can, in principle, occur on these sealing surfaces, this does not disrupt ongoing operations, and the sealing surfaces can be easily cleaned by operating personnel during a brief machine downtime. A separate edge flush or a double-walled wall flushed with cold water can clean the external sealing surfaces of deposits through condensation of steam, but such a device is generally not necessary.

[0131] The layer thickness of the flushing medium on the wall of the application nozzle or the blowing head can be, for example, between 5 μm and 1000 μm, especially between 10 μm and 500 μm. To prevent large amounts of secondary mist from escaping through the gap between the moving surface—usually a transfer roller—and the application nozzle, it is advantageous if the narrowest gap between the moving surface and the application nozzle—for example, the blowing head—is between 2 mm and 20 mm wide.

[0132] With a larger gap, more secondary mist will escape from the application area. With a smaller gap, there is a risk that the moving surface will come into contact with the application nozzle – for example, due to slight vibrations in the system. With gap widths of over 2 mm, even thicker layers of rinsing medium (500 pm - 1 mm) can still be reliably applied.

[0133] To further seal the application chamber, a nozzle for generating an airflow can also be provided. The airflow is directed or can be directed from outside the application chamber to the narrowest gap. The airflow of this nozzle can be adjusted so that the secondary mist remains within the application chamber.

[0134] Finally, it can be advantageous if the application chamber is shaped so that the tangent of the application roller intersects the wall of the blow head inside the application chamber at the point of impact of the spray curtain. Thus, when the spray curtain impacts the transfer roller, tangentially projected droplets of coating medium land on the nozzle wall and can be picked up and transported further by the flowing, liquid film of the rinsing medium on the nozzle wall. This prevents these droplets from being projected through the gap and leaving the application chamber.

[0135] If the kinetic energy of the splashes or secondary spray is so high that reflection occurs after impact on this rinsed surface, these reflected droplets can be intercepted by the moving surface of the rotating applicator roller and transported further. Increasing the length of the spray curtain reduces the impact velocity of the spray on the moving surface, thereby minimizing or eliminating reflection of secondary spray upon impact.

[0136] The invention is explained below with reference to figures. The invention is not limited to these embodiments.

[0137] The figures show in detail:

[0138] Figure 1 shows an application nozzle according to one aspect of the invention

[0139] Figure 2 shows an application nozzle according to a further aspect of the invention

[0140] Figure 2a shows a section of an application nozzle according to a further aspect of the invention

[0141] Figure 3 shows schematically an application nozzle according to a further aspect of the invention as well as an application space according to a further inventive idea

[0142] Figure 4 shows a commissioned work according to one aspect of the invention

[0143] Figure 1 shows an application nozzle 1 as part of an application unit 10 for applying a liquid or pasty coating medium to a moving surface 4 according to one aspect of the present invention.

[0144] In the embodiment shown here, the moving surface 4 is formed by a moving fibrous web. In alternative embodiments, however, this can also be provided, for example, by the surface 4 of a transfer roller or similar.

[0145] In the embodiment shown in Figure 1, the application nozzle 1 comprises a fluid head 2, which is designed in the manner of a conventional wide-slot nozzle and discharges a freely falling curtain of coating medium 14. Furthermore, the application nozzle comprises a blowing head 3 designed to generate a linear jet of gaseous medium 6, typically an air jet 6. Both the curtain 14 and the air jet 6 extend homogeneously in the transverse direction across the entire width of the surface 4 to be coated.

[0146] As the curtain 14 falls downward due to gravity, the air jet 6 is directed toward the moving surface 4. The air jet 6 strikes the curtain 14 at an impact line 8. Due to the speed of the air jet 6, the coating medium of the curtain 14 is converted into a spray curtain 5. The direction R of the spray curtain 5 is largely identical to the direction of the air jet 6 and is directed toward the moving surface 4.

[0147] The spray curtain 5 also extends over the entire width of the surface 4 to be coated. The application to the running surface 4 is also uniform over the entire width, thus eliminating the time-consuming overlapping of a large number of individual nozzles in the width direction known from the prior art.

[0148] In contrast to the freely falling curtain of coating medium 14, the spray curtain 5 is not a thin curtain, but rather, viewed from the side, has approximately the shape of a spray wedge 5, which has a certain opening angle 6. The direction R of the spray curtain 5 is referred to as the central direction R of this spray wedge 5.

[0149] The point of impact 13 of the spray curtain 5 is the point on the running surface 4 at which the center of the spray wedge 5 touches the running surface 4.

[0150] The angle a between the direction of the spray mist at the point of impact 13 and the moving surface 4 is referred to as the angle of impact a. The angle of impact a is a right angle in Figure 1. Advantageously, the angle of impact a is chosen to be significantly smaller, in particular 60° or less.

[0151] Figure 2 shows an application nozzle 1 as part of an application unit 10 according to a further aspect of the invention. In the application nozzle 1 shown here, the coating medium is provided from the fluid head 2 not in the form of a curtain, but in the form of a supported film 15. A part of the wall of the blow head 3 serves as the support surface for the film. The blow head 3 or the corresponding wall is arranged such that the film 15 flows downwards along the wall under the influence of gravity. The supporting wall can be straight, as shown in Figure 2, or curved. Due to the principle of the supported film 15, the adhesive force of the coating medium on a flat or curved support surface is used to prevent shrinkage of the liquid film 15 pre-metered onto this support surface.The impact line 8, at which the jet of gaseous medium 6 from the blow head 3 strikes the film 15 of coating medium, is located in this embodiment at a lower end of the tear-off edge 8a forming the support surface or at a distance of less than 5 mm from this tear-off edge 8a.

[0152] In order to avoid any possible droplet formation at this separation edge 8a due to surface tension effects, it can be provided that instead of a straight separation edge 8a of the supported liquid film, a wavy or sawtooth-shaped separation edge 8a is used at the lower edge of the support surface, whereby the effective length of the separation edge is extended.

[0153] A wavy or sawtooth-shaped geometry allows for optimal use of the coating medium's wetting angle relative to the tear-off edge 8a, so that the surface tension of the coating medium itself ensures better wetting, optimal contact, and homogeneous distribution of the liquid across this edge 8a. This can, in many cases, avoid or circumvent the otherwise necessary chemical solution using expensive surfactants.

[0154] In Figure 2a, the details around the tear-off edge 8a are shown again in an enlarged form.

[0155] The jet 6, for example an air jet 6, strikes the film of coating medium 15 at the impact line 8, forming a spray curtain 5 of coating medium. This spray curtain 5 is wedge-shaped when viewed from the side.

[0156] A small opening angle 6 is often desired for this spray curtain 5, for example to reduce the formation of a secondary mist.

[0157] A small opening angle 6 allows a more compact and easier to handle spray curtain, which in particular can also be arranged more easily at a flat angle of incidence a to the running surface 4.

[0158] In order to adjust the opening angle 6, the embodiments according to Figure 2 and Figure 2a provide that the blow head 3 has a lip 9, preferably an adjustable lip 9, at the outlet.

[0159] The length L of the spray curtain 5 can be chosen to be very short according to aspects of the present invention, and in particular can be 30 mm, 20 mm or less. Figure 3 shows a further embodiment according to aspects of the invention and the further inventive concept. Similar to Figure 2, the coating medium is discharged from the fluid head 2 onto the wall 11 of the blow head 3, which serves as a supporting surface for the film of coating medium 15. The film 15 again runs downwards along the wall 11 of the blow head 3. Figure 3 shows that this wall 11 serving as a supporting surface can also be curved. The impact line 8 is again provided at the tear-off edge 8a at the lower end of the wall 11. The spray curtain 5 generated is directed towards the moving surface 4, which in Figure 3 is exemplified as a transfer roller, for example in a film applicator.

[0160] In the embodiment shown here, the direction (R) of the spray curtain (5) advantageously has a component that is directed opposite to the direction of movement of the moving surface (4). The impact angle α, at which the spray curtain 5 strikes the moving surface 4 at the impact point 13, is selected here as a shallow angle α below 60°, in particular between 10° and 50°.

[0161] The feature of spray application in the opposite direction to the direction of movement of surface 4, together with a flattest possible average angle of impact a in the range of 10 to a maximum of 60 degrees, promotes the uniformity of the film of coating medium applied to the moving surface 4. The resulting reversal of direction of the spray curtain 5 after contact with the moving surface 4 results in film homogenization and improved film adhesion to the moving surface 4.

[0162] The application unit 10 in Figure 3 comprises an application space 20. This application space 20 is formed and limited by

[0163] • the running surface 4

[0164] • the wall of the blow head 1

[0165] • the spray curtain 5

[0166] • the location of the narrowest gap 7 between the running surface 4 and the

[0167] • Blow head 3. The entire wall 11 of the blow head 3 inside the application chamber 20 is covered with flowing coating medium. Likewise, the moving surface 4 inside the application chamber 20 is covered with coating medium. The latter is very advantageous in terms of the further inventive concept, but not absolutely necessary.

[0168] The length L of the spray curtain 5 can again be chosen to be very small, for example 30mm, 20mm or less, in particular 10mm.

[0169] If a secondary mist should now arise inside the application chamber 20, it can be deposited on one of the boundaries of the application chamber 20 covered with flowing or moving coating medium, or leave it through the gap 7.

[0170] To prevent secondary mist from escaping from the gap, gap 7 should be selected to be relatively small. It should not exceed 20 mm. However, since both the wall 11 and the moving surface 4 are covered with a film of coating medium 15 during operation, gap 7 should not be selected too narrowly to avoid contact. Therefore, a gap height of 2 mm should normally not be exceeded.

[0171] It is advantageous if the blow head 3, as shown in Figure 3, is designed to be movable, in particular rotatable. This allows not only the impact angle a but also the height of the gap 7 to be regulated.

[0172] Additionally, further means for sealing the gap 7 can also be provided. For example, it is possible to provide a nozzle—not explicitly shown in Figure 3—for generating an air flow, wherein the air flow is directed or can be directed from outside the application chamber 20 onto the narrowest gap 7. The counterpressure thus generated can further prevent secondary mist from escaping from the application chamber 20. Contact of the secondary mist with the spray curtain 5 is also uncritical, since the coating medium of the secondary mist is then simply transported with the spray curtain 5 onto the moving surface 4.

[0173] If the medium is deposited on the moving surface 4, this is not critical, since this is already covered with coating medium - or in the case of the opposite direction of movement is immediately covered with coating medium by the spray curtain 5 - and the deposition of the secondary mist does not cause any disturbances in the deposited film.

[0174] A deposit of coating medium on the wall 11 of the blow head 3, which here represents the upper boundary of the application chamber 20, is also uncritical, since this wall 11 is continuously flushed by the film 15 of coating medium. The coating medium also serves as a flushing medium. Any deposits of the secondary mist are then carried away by the flushing medium of the same substance and subsequently reapplied to the moving surface 4 as spray mist 5.

[0175] The application space 20 in Figure 3 is realized - e.g. by the arrangement and orientation of the blow head 3 or the shape of the wall 11 - in such a way that the tangent 12 of the running surface 4 of the transfer roller at the point of impact 13 of the spray curtain 5 intersects the wall 11 of the blow head in the interior of the application space 20.

[0176] Figure 4 shows an applicator 10 according to one aspect of the invention. The applicator 10 comprises two transfer rollers, each of whose surfaces is applied with a coating medium. The coating medium is then transferred to a fibrous web in a transfer nip.

[0177] At least one application nozzle 1 is provided on each of the transfer rollers. At least one, in particular both, application nozzles 1 can be application nozzles according to one aspect of the invention. Alternatively or additionally, at least one, in particular both application nozzles 1 can have an application chamber 20 according to one aspect of the further inventive concept.

[0178] In Figure 4, the application nozzles 1 are arranged such that the point of impact 13 of the coating medium, in particular the point of impact of the spray curtain 5 in the case of an application nozzle according to one aspect of the invention, is located at the 9-11hr position of the transfer roller.

[0179] One or two transfer rolls with very hard roll covers (0 to 5 P&J, or over 60 ShD) can be provided. When operating the application unit 10, higher nip loads (80 to 180 kN / m) can be used.

[0180] One of the two transfer rollers can be designed as a controlled deflection roll. This is particularly important for pairs of hard transfer rollers to ensure a uniform profile across the entire width of the application unit 10, even when a variety of different operating modes are to be implemented.

[0181] List of reference symbols

[0182] 1 application nozzle

[0183] 2 fluid head

[0184] 3 Blow head

[0185] 4 running surface

[0186] 5 spray curtain

[0187] 6 Jet of gaseous medium, air jet

[0188] 7 gap

[0189] 8 Impact line

[0190] 8a tear-off edge

[0191] 9 Lippe

[0192] 10 commissioned works

[0193] 11 Wall with coating medium as rinsing medium

[0194] 12 Tangent

[0195] 13 Impact point of the spray curtain

[0196] 14 Curtain of coating medium

[0197] 15 Film of coating medium

[0198] 20 order room

[0199] R Direction of the spray curtain

[0200] L Length of spray curtain a Angle of incidence

[0201] 6 opening angles

Claims

Patent claims 1. Application nozzle (1) for applying a liquid or pasty coating medium to a moving surface (4), in particular to a moving surface (4) in a machine for producing or processing a fibrous web, characterized in that the application nozzle (1) comprises a fluid head (2) configured to produce a film of coating medium (15) or a curtain of coating medium (14) and the dispensing nozzle (1) further comprises a blow head (2) configured to produce a linear jet of gaseous medium (6), wherein an impact line (8) is further provided at which the linear jet of gaseous medium (6) impacts the film (15) or curtain (14) of coating medium, forming a spray curtain (5), and wherein the blow head (3) is arranged such that the spray curtain (5) is directed in the direction of the moving surface (4). 2 Application nozzle (1 ) according to claim 1 , characterized in that the application nozzle (1 ) comprises a support surface which is arranged such that the coating medium is discharged from the fluid head (2) onto the support surface and is guided as a fluid film (15) along the support surface, wherein the support surface is provided in particular wholly or partially by the wall of the blow head (3). 3 Application nozzle (1 ) according to one of the preceding claims, characterized in that the lower end of the support surface forms a tear-off edge (8a), and the impact line (8) is provided at this tear-off edge (8a) or at a distance of less than 5mm from this tear-off edge (8a). 4 Application nozzle (1 ) according to claim 3, characterized in that the tear-off edge (8a) is provided with a structure, in particular with a wave structure or a sawtooth structure. 5 Application nozzle (1 ) according to one of the preceding claims, characterized in that the length (L) of the spray curtain (5) from the impact line (8) to the running surface (4) is a maximum of 50mm, in particular less than 20mm, particularly preferably less than 10mm. 6 Application nozzle (1 ) according to one of the preceding claims, characterized in that the direction (R) of the spray curtain (5) has a component which is directed against the direction of movement of the moving surface (4). 7 Application nozzle (1 ) according to one of the preceding claims, characterized in that the opening angle 6 of the spray curtain (5) is less than 30°, in particular between 25° and 10°. 8 Application nozzle (1 ) according to one of the preceding claims, characterized in that the blowing head (3) has a lip (9) at the outlet, in particular an adjustable lip (9) which is suitable to influence the direction and / or the opening angle of the spray curtain.

9. Application unit (10) for applying a liquid or pasty coating medium to a moving surface (4), in particular to a moving surface (4) in a machine for producing or processing a fibrous web, wherein the moving surface (4) is provided in particular by the surface (10) of a rotating transfer roller, characterized in that the application unit (10) comprises at least one application nozzle (1) according to one of the preceding claims.

10. Contract work (10) according to claim 9, characterized in that the spray curtain (5) is located between the 7 o'clock position and the 12 o'clock position, in particular between the 8 o'clock and the 10 o'clock position, especially preferably between the 8 o'clock position and the 9 o'clock position, impacts the surface (4) of the transmission roller. 11 Contract work (10) according to one of claims 9 or 10, characterized in that the contract work (10) has a contract space (20) which is formed and bounded by • the running surface (4) • the wall of the blow head (11 ) • the spray curtain (5) • the point of the narrowest gap (7) between the running surface (4) and the blow head (3), and wherein the coating medium from the fluid head (2) is applied to the wall (11) of the blow head (3) such that the entire wall (11 ) of the blow head (3) inside the application chamber (20) is covered with coating medium. 12 Contract work (10) according to claim 11 , characterized in that the narrowest gap (7) between the running surface (4) and the blow head (3) is between 2mm and 20mm wide.

13. Application device (10) according to one of claims 11 or 12, characterized in that a nozzle is provided for generating an airflow, wherein the airflow is directed or can be directed from outside the application space (20) to the narrowest gap (7).

14. Contract work (10) according to one of claims 11 to 13, characterized in that the contract space (20) is shaped such that the tangent (12) the transfer roller (4) at the point of impact (13) of the spray curtain (5) cuts the wall of the blow head (3) inside the application chamber (20). A method for applying a coating medium, in particular a starch solution, to a moving surface (4), in particular to a moving surface (4) in a machine for producing or processing a fibrous web, characterized in that the application is carried out by means of an application unit (10) according to one of claims 9 to 14. A method according to claim 15, characterized in that the linear jet of gaseous medium (6) at the outlet of the blow head (3) has a velocity of more than 5 m / s, in particular more than 10 m / s or more than 12 m / s.

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