CN118871640A

CN118871640A - Coating nozzle, coating device and method

Info

Publication number: CN118871640A
Application number: CN202380025382.0A
Authority: CN
Inventors: B·门德兹-加伦
Original assignee: Voith Patent GmbH
Current assignee: Voith Patent GmbH
Priority date: 2022-03-09
Filing date: 2023-01-30
Publication date: 2024-10-29
Also published as: DE102022105518B4; DE102022105518A1; WO2023169740A1; EP4490356A1

Abstract

A coating device for applying a liquid or pasty coating medium to a running surface, in particular a rotating transfer roller, in a machine for manufacturing or processing a fiber web, wherein the coating device comprises a coating nozzle for producing a spray curtain consisting of the coating medium, characterized in that the coating device has a coating space, which is formed and limited by - the running surface, - at least a part of the wall of the coating nozzle, - the spray curtain, and - the position of the narrowest gap between the running surface and the coating nozzle, and wherein a thin film consisting of the flowing coating medium is applied on the boundary provided by at least a part of the wall of the coating nozzle.

Description

Coating nozzle, coating device and method

The present invention relates to a coating nozzle for applying a liquid or pasty coating medium to a running surface, in particular a running surface in a machine for producing or processing a fibrous web, and to a corresponding coating device and a coating method.

A number of coating processes for applying a coating medium to a paper or board web are known. In particular, so-called film presses are often used as coating devices for starch. In this case, a film of the coating medium is applied to a transfer roll and then transferred to the fibrous web in a transfer nip.

The coating medium is usually fed excessively onto a transfer roll and then metered to the desired coating amount by means of a suitable doctor blade metering system. Such doctor blade metering systems are known, for example, from DE 10 2004 029 565 A1.

In the context of the expanded design of film presses, coating rolls with very hard roll sleeves (0 to 5p & j, or more than 60 ShD) are used in the case of film presses operating at higher nip loads (80 to 180 kN/m). Since the contact area between the rolls in the roll nip is very small, the pressure of the substrate in the nip is high and the residence time is short, so that the penetration and distribution of starch in the fibrous web structure is better, whereby a higher efficiency of starch (higher strength with the same coating amount) can be achieved.

The use of very hard roller sleeves results in wear of the metering elements used in doctor blade metering devices because these metering elements are in direct contact with the rotating roller surface. The hard roller sleeve likewise wears or leaves behind marks, for example marks of the contour of the scraper bar.

The coating quantity of the volumetric metering element cannot be changed only slightly by changing the pressing pressure on the metering element, since the surface of the coating roller cannot be deformed due to the higher hardness. The coating quantity of the volumetric metering element cannot be slightly corrected anymore.

Thus, in the past, non-contact metering systems have been developed for use in particular with harder rolls, in which the coating medium is no longer fed excessively, but rather is metered before feeding.

One possible embodiment of such a coating device is to feed the coating medium onto the transfer roller in the form of a free-falling curtain. This is described, for example, in document EP 3830336 A1. The challenge faced by such a coating apparatus is to maintain very thin, free-falling curtains stable even under the harsh conditions of the production equipment.

Alternatively, a coating device is known, for example from EP 3332955 B1, in which the coating medium is sprayed onto the coating roller by means of a series of spray nozzles. The contact metering element can also be dispensed with. However, a disadvantage of such a coating device is that the spray nozzles used are easily contaminated or clogged during operation. The spray nozzle of EP 3332955 B1 is therefore designed to be removable individually for frequent cleaning. Alternatively, DE 20 2015 009 603 U1 proposes to provide a double set of nozzles, in which half of the nozzles are in operation and the remaining nozzles are in maintenance position.

Furthermore, a large number of individual spray nozzles are required for distributing the coating medium over the width of the coating roll or the material web to be coated. In order to achieve a uniform coating, the spray cone is also designed to overlap to a large extent, whereby the number of coating sprays is further increased. This increases not only the investment costs but also the maintenance costs, since even if the individual nozzles are defective or blocked, a uniform coating cannot be ensured.

In addition, secondary atomization of the coating material occurs during spray coating, which secondary atomization is not transferred to the coating roller but remains in the spray chamber. This secondary atomization can quickly contaminate the entire spray chamber and its internals. To prevent this, the spray chamber must be provided with a suction device, as shown in document EP 3332955 B1. Typically about 5% of the coating medium is output by the suction device and directed back for reuse.

The object of the present invention is to propose an alternative coating nozzle, and a corresponding coating device and coating method, which in particular overcome the difficulties of the prior art.

The object of the invention is also to propose a non-contact coating system which is not prone to contamination.

The object of the invention is also to propose a coating system which ensures a stable feeding of the coating medium onto the transfer roller, in particular in comparison with curtain coating.

According to the invention, the object is achieved by the design according to the independent claim. Further advantageous embodiments of the invention can be found in the dependent claims.

In terms of coating nozzles, the problem is solved by a coating nozzle for applying a liquid or pasty coating medium onto a running surface, in particular a running surface in a machine for producing or processing a fibrous web.

According to the invention, the coating nozzle comprises a fluid head which is designed to produce a film or curtain of coating medium, and a blowing head which is designed to produce a linear jet of gaseous medium. Furthermore, an impact wire is provided, at which the linear beam of gaseous medium impacts a film or curtain of coating medium to form a spray curtain. The blowing head is arranged in such a way that the spray curtain is oriented in the direction of the running surface. The running surface may in particular be the surface of a rotating roll or the surface of a fibrous web.

The invention retains the advantages of curtain coating, i.e. uniform coating of the coating medium over the machine width, in a very inventive manner, while avoiding the disadvantages of sensitive curtains. Furthermore, spray coating for large machine widths of 10m or more can also be used by the proposed coating nozzle without the number of coating nozzles being increased considerably and thereby avoiding the risk of contamination.

In the context of the present application, a linear jet of gaseous medium is understood to mean a jet of gaseous medium, in particular an air jet, having a certain thickness and a certain extent (in the width direction of the running surface), wherein the extent is greater than the thickness by a factor of (for example 10 times, 100 times or more). Such a linear beam can be produced, for example, by means of a slot nozzle in which the length of the slot is a multiple of the thickness.

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

The idea of the invention is to create a film or curtain of the desired width of the coating medium by means of a fluid head. The film/curtain can be guided freely or supportingly under the influence of gravity in the direction of the blowing head. Such a fluid head can advantageously be designed as a "Slot Die" or "Slide Die" in the type of a known curtain nozzle.

At the impact line, a linear jet of gaseous medium, usually air, is applied to the film and/or curtain. The curtain is formed by these mutually impinging media. If the velocity of the gaseous medium is chosen to be significantly higher than the velocity of the film/curtain, the direction of the generated spray curtain essentially corresponds to the direction of the beam of gaseous medium. The shower head can thus be arranged in a suitable manner in order to orient the shower curtain in the direction of the running surface, i.e. for example the surface of a rotating roller.

The spray curtain thus produced does not have the shape of a thin curtain that falls freely. Instead, the spray screen has a spray wedge shape as seen from the side and has a certain opening angle delta. In this category, the direction of the spray curtain always means the center direction of the spray wedge shape.

On the running surface, the position where the center of the spray wedge contacts the running surface is called the impact position of the spray curtain.

Within the scope of the present application, the terms "running surface" and "moving surface" are used synonymously. In most applications, such a running surface may be the surface of a rotating roll or the surface of a fibrous web. The coating medium may advantageously be a starch solution.

In the simplest case, the gaseous medium may be air.

A great advantage of the invention is that it is not necessary to use a large number of separate coating nozzles for creating the curtain, which may be clogged with coating medium. The coating medium can be delivered, for example, from low-maintenance wide-gap nozzles, which have been proven in curtain coating.

Furthermore, uniform coating can be achieved without the overlap of the spray cones of adjacent coating nozzles that would otherwise be necessary in spray coating.

The blow head for generating a jet of gaseous medium can also be of very simple construction. In the simplest case, the blow head can be formed by a wide-gap nozzle which extends over the entire desired width and can be connected, for example, to a simple fan or compressed air supply. If desired, the blow head can also be of more complex design, for example comprising a row of individual air nozzles arranged next to one another.

The risk of contamination is very low since the blow head is hardly in direct contact with the coating medium and the medium is blown off the blow head by the air jet.

By means of the combination of a fluid head and a blow head, the coating nozzle is in principle a bi-material nozzle. Thereby giving the coating nozzle other very advantageous properties. The metering of the amount of spray medium can be regulated independently of the speed of the air jet. The speed of the spray curtain is achieved by varying the amount of air delivered by the blower from the blowhead. Thereby enabling the coating amount and the ejection speed to be adjusted independently of each other.

Unlike curtain coating, this enables stable coating even with a small amount of coating medium.

In an advantageous embodiment, it can be provided that the coating nozzle comprises a support surface which is arranged in such a way that the coating medium is fed from the fluid head onto the support surface and is guided as a fluid film along the support surface.

Furthermore, it can be provided that the lower end of the support surface forms a separating edge and that an impact line is arranged at the separating edge or at a distance of less than 5mm from the separating edge.

The separating edge may be provided with a structure, in particular a wave structure or a saw tooth structure.

By the principle of a supported film, the adhesion of the liquid on a flat or curved support surface is utilized to avoid shrinkage of the film of liquid on the support surface, which is to be expected. Since the adhesion of the material used for the support surface (blade, nozzle wall or similar material), preferably stainless steel or stainless metal, is higher than the shrinkage force of the surface tension, the coating medium adheres to the support surface with minimal shrinkage and flows downwards by gravity, especially when the support surface is arranged vertically or at an inclination angle of not more than 45 degrees.

A particularly great advantage of the principle of supported films is that the flow or coating weight of the film coated on the walls of the support can be reduced to 2.5l/min/m or increased to more than 100l/min/m without causing film defects such as holes, lines or so-called film breaks in the support surface. This makes it possible to provide a uniformly pre-metered film for the subsequent shower at the separating edge of the support surface.

The pre-metered film leaves the support surface at the lower edge of the support surface. At this point, the wall loses its supporting and shrink-proof effect and the surface tension resumes its action, so that lines, cracks and shrinkage may occur in the film. The lower the thickness or flow of the film of liquid pre-metered on the support surface, the more pronounced this effect.

In order to avoid or minimize the effect of surface tension at the lower edge of the support surface, it may also be advantageous to avoid the use of surfactants or other surface tension-active adjuvants that are otherwise necessary.

Liquid filaments or droplets are sometimes formed at the lower edge of the support surface of the film of liquid (typically when the film flow drops below 8 l/min/m), which severely affects the uniformity of the film provided along the cross-machine direction (CD direction).

If the film provided is converted into a spray curtain by a linear beam of gaseous medium, holes, discontinuities or irregularities may be produced in the CD profile of the coating quantity of the spray curtain produced due to the above-mentioned surface tension effect.

In order to avoid this problem, it can be provided that a structured, for example wavy or zigzag-shaped separating edge is used at the lower edge of the support surface instead of a straight separating edge which supports the liquid film, whereby the effective length of the separating edge is prolonged.

By means of the wavy or zigzag-shaped or other suitably structured geometry, the wetting angle of the application medium with respect to the separating edge can be optimally utilized, so that the surface tension of the application medium itself is used for better wetting, optimal contact and even distribution of liquid on the edge.

In many cases, chemical solutions using expensive surfactants can be avoided or bypassed by such mechanical solutions (serrated edges or fine wavy edges).

In a particularly preferred embodiment, it can be provided that the support surface is provided in whole or in part by a wall of the blow head.

This is also advantageous because the distance from the flow head to the blow head and thus the distance over which the film or curtain of coating medium runs cannot be increased any more. In practice, this distance is generally less than 50cm, in particular less than 30cm. It is often difficult to mount a separate component as a support surface.

This problem can be solved very simply by designing the wall or a part of the wall of the blow head such that it is suitable as a support surface for the film of coating medium. This part of the wall of the blow head can in particular be made of metal, such as stainless steel, and can also be designed smooth, i.e. without screws, accessories or other structural elements (such as embossed numbers or letters).

It is generally advantageous that the support surface is smooth and does not allow the coating medium to build up on the support surface. The support surface may thus be treated to improve smoothness and/or reduce the tendency to adhere. Examples of suitable treatments are listed below:

● Polytetrafluoroethylene, i.e., coated with a fluoroplastic such as PTFE (polytetrafluoroethylene) to prevent or minimize adhesion.

● For example, plastic coating with polyethylene

● Polishing

● Electrolytic polishing or plasma polishing

The treatments listed here are particularly advantageous if the support surface is composed entirely or partly of metal, such as stainless steel.

In a further preferred embodiment, it can be provided that the length of the screen, and thus the distance of the impact line from the running surface in the direction of the screen, is at most 80mm, in particular less than 20mm, particularly preferably less than 10mm.

In spray coating known from the prior art, the distance of the nozzles from the surface, and thus the length of the spray curtain, is about 100mm, to ensure that the result of the superimposed spray pattern of the individual nozzles is sufficiently good and uniform.

According to the prior art, a spray screen having a length of less than 50mm is more difficult to adjust, whereas a spray screen having a length of less than 20mm cannot be adjusted at all, because otherwise uncovered areas, strong CD profile irregularities and holes in the spray pattern would result. The coating nozzle proposed here makes it possible to adjust very simply the particularly short lengths of the spray curtain below 50 mm. In the test, even a spray curtain of 3mm in length works well.

The shorter the length of the spray screen, the more uniform the spray pattern and the more uniform the CD profile of the film coated on the rotating roll or on the web supported on the rotating roll by means of the spraying device.

In addition, secondary atomization by using a shorter spray curtain reduces or does not occur. One of the problems of conventional spray coating can thereby be avoided or minimized in a particularly simple manner. Thus, the coating nozzle or coating device according to aspects of the present invention is typically designed without a separate suction device for secondary atomization, which significantly reduces the acquisition and running costs.

Due to the shorter length of the spray screen, the spray screen is able to create a higher dynamic head on the impact line or impact zone on the moving surface, which is beneficial in suppressing and limiting the negative effects of the air boundary layer.

Due to the shorter length of the spray screen, the drop velocity of the spray drops is significantly less reduced than for longer flight distances. The drop velocity of the spray curtain decreases approximately as the square of the flight length of the spray curtain. Therefore, the dynamic head generated at the surface is significantly greater when the length of the curtain is shorter, so that the air boundary layer can be better and more effectively controlled or avoided.

In order to avoid or at least significantly reduce secondary atomization, other advantageous possibilities are provided within the scope of the invention, which may be used alone or in combination.

For example, it can be provided that the direction of the spray screen has a component opposite to the direction of movement of the running surface.

The spray curtain, which is arranged opposite the direction of movement of the surface, for example the roller surface, prevents or minimizes the generation of secondary atomization, since the transfer rate of the coating medium on the surface is thereby improved. Another advantage of this arrangement is that the spray curtain itself thereby reduces the air boundary layer of the moving surface. The spray curtain acts here like a non-contact active air boundary layer limiter ("AirCut").

It is furthermore advantageous to avoid secondary atomization that the spray curtain is impacted onto the running surface at a gentle impact angle α.

The impact angle α here means the angle enclosed by the direction of the spray curtain at the impact point and the tangent to the running surface. The impingement angle α is in the range of 0 ° to 90 °.

In this case, a relatively gentle angle of attack α of at most 60 °, in particular between 10 ° and 50 °, has proven to be advantageous.

In an advantageous embodiment, the blowing head can be designed to be movable, in particular rotatable, so that the direction of the spray screen and thus also the impact angle α can be adapted in a simple manner.

Furthermore, the movable design of the blowing head and/or the fluid head also facilitates their movement from the working position into the maintenance position.

The feature of the spray coating in a direction opposite to the direction of movement of the surface, together with the as gentle average angle of attack α as possible in the range of 10 degrees to a maximum of 60 degrees, contributes to the uniformity of the film of coating medium applied on the moving surface. The reversal of the direction of the spray curtain after contact of the spray curtain with the substrate thus formed homogenizes the film and leads to a better adhesion of the film on the running surface.

The feature of spray coating in a direction opposite to the direction of movement of the moving surface with an average impingement angle as gentle as possible in the range of 10 to maximally 60 degrees and a short distance or short length of the spray curtain, for example <10mm, is very advantageous for reducing the influence of the air boundary layer transported by the running surface. The curtain is blown against the direction of the air boundary layer. Whereby the dynamic head due to the accelerated spray curtain slows down, deflects the air boundary layer and minimizes its impact on the spray delivery rate.

Another parameter affecting the formation of secondary atomize is the opening angle delta of the spray curtain.

A smaller opening angle results in a more compact, easier-to-handle spray screen which can in particular also be more easily arranged at a gentle angle of attack α with respect to the running surface.

A small opening angle δ can be achieved, for example, by a suitably designed nozzle of the blow head. In particular, it can be provided that the blow head has a lip, preferably an adjustable lip, at the outlet.

The opening angle may also be influenced by the viscosity of the coating medium or the air speed of the blow head. The low viscosity and higher air velocity result in a smaller opening angle of the wedge shape of the spray screen.

The following table shows exemplary advantageous embodiments:

Features (e.g. a character)	Example 1	Example 2
			Opening angle delta	25°	20°
Length of spray curtain	10mm	5mm
			Angle of impact alpha	50°	30°
Direction of spray curtain	Opposite to the direction of movement	Opposite to the direction of movement

In terms of a coating device, the technical problem is solved by a coating device for applying a liquid or pasty coating medium onto a running surface, in particular in a machine for producing or processing a fibrous web, wherein the coating device comprises at least one coating nozzle according to one aspect of the invention.

In a preferred embodiment, the running surface can be provided by the surface of a rotating transfer roller.

Alternatively, the running surface may also be provided by a running fibrous web. The fiber web can run freely during the application of the application medium or can run, for example, supported by rollers.

For double-sided coating, it may be advantageous for the coating device to have two transfer rolls, which together form a roll nip. It may be provided that the coating device may have two coating nozzles according to aspects of the invention, wherein a first coating nozzle applies the coating medium onto the first transfer roller and a second coating nozzle applies the coating medium onto the second transfer roller.

One or two transfer rolls with very hard roll sleeves (0 to 5p & j or more than 60 ShD) may be provided. Higher nip loads (80 to 180 kN/m) can be used when the coating apparatus is running.

One of the two transfer rollers may be embodied here as a controlled deflection roller ("Controlled Deflection Roll"). This is very important in pairs of hard transfer rolls to ensure a uniform profile across the width of the coating device, even if a plurality of different operating conditions should be achieved.

The one or more coating nozzles are preferably arranged such that the spray curtain impinges on the surface of the roll between 7 o 'clock and 12 o' clock, preferably between 8 o 'clock and 12 o' clock, in particular between 8 o 'clock and 10 o' clock.

The positions are to be understood here by reference to the clockwise rotating drum, respectively.

By selecting the proper position of the spray curtain, additional effects associated with minimizing the impact of the air boundary layer and avoiding secondary atomization can be achieved when setting the proper impingement angle of the spray curtain. These effects are particularly reflected in the region between the 7 o 'clock and 10 o' clock positions of the coating region, in particular between the 8 o 'clock and 10 o' clock positions. This region is therefore preferred. The area between 10 o 'clock and 12 o' clock is also a usable, although not optimal, location area for the spray screen. Depending on the design of the coating apparatus, a region between 12 and 15 points is sometimes also possible. However, the tendency to form here by secondary atomization is significantly more pronounced, so that this region is not preferred.

The impact zone is located in the region between 7:00 and 9:00, in particular between 8:00 and 9:00, on the transfer roll (or on the fibrous web supported on the rotating roll), which offers the advantage that the drops which occasionally form on the lips of the nozzle are guided out of the coating surface vertically by the action of gravity, so that no drop marks are produced on the coating film.

In terms of the method, the technical problem is solved by a method for applying a coating medium, in particular a starch solution, to a running surface, in particular a running surface in a machine for producing or processing a fibrous web, which method is carried out by means of a coating device designed according to one aspect of the invention.

The device and method proposed here are particularly suitable for producing and spraying fine to coarse sprays of coating media based on low-viscosity liquids or liquid mixtures.

Although the invention is particularly suitable for the coating of starch solutions, the invention is not limited to any coating medium.

The coating medium may be, for example:

● Water or water containing polar or nonpolar soluble additives (e.g., salts, colorants),

● With other liquids (binders, thickeners, extended rheology thickeners, alcohols or any miscible or immiscible ingredients),

● Aqueous solutions of starches from various natural starches (corn, wheat, potato, tapioca, barley, rice, etc.). Gelatinization and degradation of the starch required for the production of the starch solution may be carried out by different methods: purely chemical gelatinization, thermochemical gelatinization, purely thermal gelatinization, enzymatic gelatinization, combinations of different processes,

● Paint colorants with finely dispersed solid particles, such as CaCO ₃, kaolin, talc or other mineral pigments, etc., having a coarse, medium or fine aspect ratio, or artificial or polymeric pigments, or similar artificial or polymeric pigments,

● An aqueous dispersion of a fiber-based material, for example an aqueous dispersion of cellulose fibers of nanocellulose, microfibrillated cellulose, nanocellulose, staple fibers or long fibers,

● Any low viscosity liquid having a bohler fly (Brookfield) viscosity of less than 300mPas, preferably less than 200mPas, measured at 100rpm, 60 ℃ with spindle 4.

The optimal viscosity of the coating medium is below 200mPas, in particular in the range between 0.1 and 160mPas for the Bowler's fly viscosity (100 rpm/55 ℃, spindle 4).

In particular for the use of starch solutions in the method described here, the solids content of the coating medium is preferably in the range between 1% and 35%.

The specific gravity of the coating medium may be in the range of 0.8g/cm 3to 1.3g/cm 3.

The method according to one aspect of the invention may be carried out at a processing temperature of 50 ℃ to 90 ℃ for the coating medium. Temperatures of not more than 100℃are also possible.

The surface tension of the coating medium, in particular of the starch solution, can be from 30mN/m to 70mN/m (50 ℃). The range of surface tension that can be used can be extended by using a structured separation edge.

The coating medium can be applied to each surface in an amount of up to 100l/m/min, in particular from 2.5l/m/min to 50l/m/min, by the method according to aspects of the invention. Metering is carried out by means of the coating nozzle without the use of a contact metering element, whereby hard transfer rolls can also be used.

The linear beam velocity at the outlet of the blowing head, which is formed by the gaseous medium, can be more than 5m/s, in particular more than 10m/s or more than 12 m/s. An advantage of the invention is that the speed of the jet of gaseous medium and thus the speed of the spray screen is largely decoupled from the application quantity of the application medium.

The described discharge nozzle or coating device has proved to be very advantageous, since the occurrence of secondary atomization can thereby be completely or largely avoided. However, it may also occur that the residual secondary mist is still sufficient to cause fouling of the coating nozzle.

In principle, it is also possible to provide the coating nozzle or the coating device according to the various aspects of the invention with a suction device known from the prior art. However, for reasons of cost, it is advantageous to dispense with such a suction device.

For this reason, the inventors developed a solution that is not only advantageous in connection with the coating device according to the various aspects of the invention, but also forms a further inventive concept independent thereof.

A coating device for applying a liquid or pasty coating medium to a running surface, in particular in a machine for producing or processing a fibrous web, is proposed, wherein the coating device can advantageously comprise a coating device according to one aspect of the invention.

The coating device here comprises a coating nozzle for producing a spray curtain.

The coating device also has a coating space passing through

● The running surface,

● At least a part of the wall of the coating nozzle,

● The spraying curtain,

● The position of the narrowest gap between the running surface and the coating nozzle

Forming and defining a boundary.

If a secondary atomization occurs in the coating space, no persistent dirt is produced there, although in principle the coating medium may accumulate at the boundary surfaces.

The deposition of the medium on the running surface is not critical, since the running surface is originally covered by the coating medium or is immediately covered by the curtain to coat the medium when the direction of movement is reversed, and the secondary atomized deposition does not interfere in the deposited film. Additional coating medium is carried along with the moving surface.

Deposition on the spray screen is also not critical, as movement of the spray screen carries and deposits the secondarily atomized coating medium toward and onto the moving surface.

Deposition is not possible at the open gap between the running surface and the coating nozzle. As will be described later, the escape of secondary mist through this gap can be avoided by adapting the geometry and, if necessary, taking additional sealing measures.

In order to avoid the formation of deposits on the remaining wall sections, it is provided according to a further inventive concept that a film of a flowing coating medium is applied at the boundary provided by at least a part of the wall section of the coating nozzle. Thus, during operation of the coating device, the wall is continuously rinsed by the coating medium ("rinsing medium"). The secondary mist deposited on the wall is carried away here by the flushing medium. Since the secondary mist consists of the same coating medium as the flushing medium, the medium remains clean and can be used for coating.

This makes it possible to completely avoid the suction of the secondary atomization or to achieve this by means of very simple and energy-saving means.

According to one aspect of the other inventive concept, the coating device with spray heads can in principle be used for all spray coating devices known from the prior art.

The flushing medium can be collected and fed into the coating nozzle at little effort.

Due to the construction of the coating nozzle according to aspects of the invention, the coating space for such a coating nozzle can be realized particularly easily.

An advantageous embodiment is, for example, a coating device according to one aspect of the invention, wherein the coating device has a coating space which passes through

● The running surface,

● The wall part of the blowing head,

● The spraying curtain,

● The position of the narrowest gap between the running surface and the blowhead

Forming and defining a boundary, and wherein a coating medium is applied from the fluid head onto the wall of the gas head such that the entire wall of the gas head is covered by the flowing coating medium within the application space.

In an advantageous embodiment of the invention, the coating medium is fed from the flow head onto a support surface, wherein the support surface is provided by the wall of the blow head, so that flushing of the boundary of the coating space can be achieved in a completely natural manner. The coating medium used as a flushing medium is transported together with the collected secondary mist onto the running surface as a spray curtain by a jet of gaseous medium when reaching the separating edge. No separate discharge of flushing medium is necessary here.

The portion of the wall of the blow head that is the support surface may also be treated to prevent adhesion, for example as follows:

● For example, plastic coating with polyethylene

● Polishing

● Electrolytic polishing or plasma polishing

In order to avoid secondary mist escaping from the end of the coating device, it can be provided that the coating space is closed at both end sides by sealing surfaces.

In principle, secondary mist deposits can occur on these sealing surfaces. However, this does not affect the ongoing operation and the sealing surfaces can be cleaned very simply by the operator during a brief stoppage. A separate edge flushing device or a wall with a double-walled cold water flushing can be used to remove deposits from the outer sealing surface by condensation of water vapor, however such a device is generally not required.

The layer thickness of the flushing medium on the wall of the coating nozzle or the blow head can be between 5p μm and 1000 μm, in particular between 10 μm and 500 μm.

In order to prevent a large amount of secondary mist from escaping through the gap between the running surface, typically the running surface of the transfer roll, and the coating nozzle, it is advantageous that the narrowest gap between the running surface and the coating nozzle, e.g. the blow head, is preferably 2mm to 20mm wide.

The larger the width, the more secondary mist permeates out of the coating space. At smaller widths there is a risk that the running surface comes into contact with the coating nozzle, for example when the apparatus is slightly vibrated. When the gap width exceeds 2mm, it can be reliably achieved even if the layer thickness of the flushing medium is large (500 μm to 1 mm).

In order to further seal the application space, a nozzle for generating an air flow can also be provided, wherein the air flow is directed from outside the application space towards the narrowest gap or the air flow can be directed from outside the application space towards the narrowest gap. The air flow of the nozzle can be adjusted such that the secondary mist remains in the coating space.

Finally, it can also be advantageous if the coating space is shaped such that a tangent to the coating roller at the impingement point of the spray curtain intersects a wall of the blowing head located in the interior of the coating space. Whereby when the spray curtain impinges on the transfer roller, the tangentially thrown-off droplets of the coating medium land on the nozzle wall and can be taken up and transported further by the flowing film of liquid of the flushing medium on the nozzle wall. In particular, these droplets are prevented from being thrown through the gap and out of the application space.

If the kinetic energy of the splashed droplets or secondary jets is so high that reflections occur after striking the rinsed surface, these reflected droplets can be caught and transported further by the running surface of the rotating applicator roll.

Increasing the length of the spray screen reduces the impact velocity of the spray on the running surface, thereby minimizing or avoiding reflections of the secondary mist upon impact.

The invention is described below with reference to the drawings. The invention and other inventive concepts are not limited to these designs. In the drawings:

figure 1 shows a coating nozzle according to one aspect of the invention,

Figure 2 shows a coating nozzle according to another aspect of the invention,

Figure 2a shows a cut-away view of a coating nozzle according to another aspect of the invention,

Fig. 3 schematically shows a coating nozzle according to another aspect of the present invention and a coating space according to another inventive concept,

Fig. 4 illustrates a coating apparatus according to one aspect of the present invention.

Fig. 1 shows a coating nozzle 1, which is part of a coating device 10, according to one aspect of the invention, the coating device 10 being used for applying a liquid or pasty coating medium onto a running surface 4.

In the embodiment shown here, the moving surface 4 is formed by a running fibrous web. In alternative embodiments, however, the moving surface may also be provided, for example, by the surface 4 of a transfer roller or similar device.

In the embodiment shown in fig. 1, the coating nozzle 1 comprises a fluid head 2 which is designed in the form of a classical wide-gap nozzle and outputs a free-falling curtain of coating medium. The coating nozzle further comprises a blow head 3, which is designed to produce a linear jet of gaseous medium 6, usually an air jet 6. The curtain 14 and the air jet 6 extend uniformly in the transverse direction over the entire width of the surface 4 to be coated.

The curtain 14 falls downwards due to gravity and the air jet 6 is directed towards the running surface 4. The air beam 6 hits the curtain 14 at the impact line 8. The coating medium of the curtain 14 is converted into a spray curtain 5 by the speed of the air jet 6. The direction R of the spray screen 5 corresponds to a large extent to the direction of the air jet 6 and is oriented toward the running surface 4.

The spray curtain 5 also extends over the entire width of the surface 4 to be coated. In this case, the coating on the running surface 4 is also uniform across the width, so that a complex overlapping of a plurality of coating nozzles in the width direction, as known from the prior art, is dispensed with.

Unlike the freely falling curtain of coating medium 14, the spray curtain 5 is not a thin curtain, but rather, viewed from the side, takes the shape of a spray wedge 5, which has a certain opening angle δ. The central direction R of the spray wedge 5 is referred to as the direction R of the spray curtain 5.

The position on the running surface where the centre of the spray wedge 5 is in contact with the running surface 4 is called the impact position 13 of the spray curtain 5. The angle α between the direction of the spray at the impingement position 13 and the running surface 4 is called the impingement angle α. The impact angle α in fig. 1 is a right angle. The angle of attack α is advantageously chosen to be significantly smaller, in particular 60 ° or less.

Fig. 2 shows a coating nozzle according to another aspect of the invention as part of a coating device 10. In the coating nozzle 1 shown here, the coating medium is not provided in the form of a curtain from the fluid head 2, but in the form of a supported film 15. A part of the wall of the blow head 3 is used here as a support surface for the film. The blowing head 3 or the corresponding wall is arranged in such a way that the film 15 flows down the wall under the influence of gravity. The wall of the support may be straight as shown in fig. 2 or curved. By the principle of the supported film 15, the adhesion of the coating medium on a flat or curved support surface is used to avoid shrinkage of the film 15 of the liquid which is to be measured on the support surface. In this embodiment, the impact line 8 of the jet of gaseous medium 6, which impinges on the film 15 of coating medium, which is emitted from the blow head 3, is located at the lower end of the separating edge 8a forming the support surface or at a distance of less than 5mm from this separating edge 8 a.

In order to avoid the formation of drops at the separation edge 8a, which may be due to the influence of surface tension, it may be provided that instead of a straight separation edge 8a supporting the liquid film, a separation edge 8a, for example of wavy or zigzag shape, is used at the lower edge of the support surface, whereby the effective length of the separation edge is prolonged.

The wave-like or zigzag geometry makes it possible to optimally use the wetting angle of the coating medium with respect to the separating edge 8a, so that the surface tension of the coating medium itself is used for better wetting, optimal contact and even distribution of liquid on this edge 8 a. Thereby avoiding or bypassing chemical solutions that use expensive surfactants.

Fig. 2a shows again in an enlarged detail around the separation edge 8 a. The jet 6, for example an air jet 6, impinges here on a film 15 of the coating medium at an impingement line 8, as a result of which a spray curtain 5 of the coating medium is formed. The screen 5 is wedge-shaped in side view.

A smaller opening angle delta is generally desired for the spray curtain 5, for example, to reduce the formation of secondary atomizers.

A smaller opening angle δ results in a more compact, easier-to-handle spray screen which can in particular also be more easily arranged at a gentle impact angle α with respect to the running surface 4.

In order to adjust the opening angle δ, it is provided in the embodiment according to fig. 2 and 2a that the blow head 3 has a lip 9, preferably an adjustable lip 9, at the outlet.

According to various aspects of the invention, the length L of the spray screen 5 can be selected to be very short, especially 30mm, 20mm or less.

Fig. 3 illustrates another design in accordance with various aspects and other inventive concepts of the present invention. Similar to fig. 2, the coating medium is fed out from the flow head 2 onto the wall 11 of the blow head 3, which wall serves as a support surface for the film formed by the coating medium 15. The film 15 is still running downwards at the wall 11 of the blow head 3. Fig. 3 shows that the wall 11 as a support surface can also be designed in a curved manner. The impact line 8 is still provided at the separating edge 8a at the end of the lower part of the wall portion 11. The generated spray curtain 5 is oriented in the direction of the moving surface 4, in fig. 3 the moving surface 4 being designed for example as a transfer roll, for example in a film coating apparatus.

In the embodiment shown here, the direction (R) of the spray screen (5) advantageously has a component which points counter to the direction of movement of the running surface (4). The impact angle α of the spray curtain 5 against the running surface 4 at the impact location 13 is here selected to be a gentle angle α of 60 ° or less, in particular 10 ° to 50 °.

The feature of the spray coating in a direction opposite to the direction of movement of the surface 4, together with the as gentle average impact angle α as possible in the range from 10 degrees to a maximum of 60 degrees, contributes to the uniformity of the film of coating medium applied on the moving surface 4. The reversal of the direction of the spray curtain 5 after contact with the running surface 4 thus formed homogenizes the film and gives a better adhesion of the film on the running surface 4.

The coating device 10 in fig. 3 comprises a coating space 20. The coating space 20 is constituted and defined by

● Running surface 4

● Wall 11 of the blow head

● Spray curtain 5

● The position of the narrowest gap 7 between the running surface 4 and the blowing head 3

● Blow head 3

The entire wall 11 of the blow head 3 in the coating space 20 is covered with the flowing coating medium. The moving surface 4 is also covered by the coating medium in the coating space 20. The latter is very advantageous in the context of the further inventive concept, but is not necessarily essential.

The length L of the spray screen 5 can still be chosen very small, for example 30mm, 20mm or less, in particular 10mm.

If secondary atomization is formed in the interior of the coating space 20, the secondary mist may be deposited on the boundary of the coating space 20 covered by the flowing or moving coating medium or leave the coating space.

To avoid secondary mist escaping from the gap, the slit 7 should be chosen smaller. Here should not exceed 20mm. However, in operation, the wall 11 and the running surface 4 are covered by a film of the coating medium 15, so that the gap 7 cannot be selected too narrow and contact is avoided. Therefore, the gap height should generally not exceed 2mm.

In this case, it is advantageous if the blow head 3 is designed to be movable, in particular rotatable, as shown in fig. 3. Thereby not only the impact angle α but also the height of the gap 7 can be adjusted.

In addition, other means for sealing the gap 7 can be provided. For example, it is possible to provide a coating nozzle, not explicitly shown in fig. 3, for generating an air flow, wherein the air flow is directed from outside the coating space 20 towards the narrowest gap 7 or the air flow can be directed from outside the coating space 20 towards the narrowest gap 7. The back pressure that can be generated in this way can additionally prevent secondary mist from escaping from the application space 20.

The contact of the secondary mist with the spray screen 5 is also not problematic, since the secondarily atomized coating medium is transported with the spray screen 5 onto the moving surface 4.

The deposition of the medium on the running surface 4 is not problematic, since the running surface is originally covered by the coating medium or is immediately covered by the curtain to coat the medium when the direction of movement is reversed, and the deposition of the secondary mist does not interfere in the deposited film.

The deposition of the coating medium on the wall 11 of the blow head 3 is also not problematic, since this wall 11 is continuously flushed by the film 15 of coating medium, which forms the upper boundary of the coating space 20. The coating medium is used here at the same time as a rinsing medium. Possible deposits of the secondary spray are carried away by the flushing medium of the same material and then recoated onto the running surface 4 in the form of a spray 5.

The spray space 20 in fig. 3 is realized, for example, by the arrangement and orientation of the blowing 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 impact location 13 of the spray curtain 5 intersects the wall 11 of the blowing head located in the coating space 20.

Fig. 4 illustrates a coating apparatus 10 according to one aspect of the present invention. The coating device 10 includes two transfer rolls, on the surfaces of which coating media are coated, respectively. The coating medium is transferred onto the fibrous web randomly in the transfer nip.

At least one coating nozzle 1 is provided on each transfer roll. At least one, in particular two, coating nozzles 1 can be coating nozzles according to one aspect of the invention. Alternatively or additionally, at least one coating nozzle 1, in particular two coating nozzles 1, may have a coating space 20 according to an aspect of another inventive concept.

In fig. 4, the coating nozzle 1 is arranged such that the impingement position 13 of the coating medium, in particular of the spray curtain 5 in the case of a coating nozzle according to one aspect of the invention, is located at the 9 o' clock position of the transfer roll.

One or two transfer rolls with very hard roll sleeves (0 to 5p & j or more than 60 ShD) can be provided herein. Higher nip loads (80 to 180 kN/m) may be used when the coating apparatus 10 is in operation.

One of the two transfer rolls can be designed here as a controlled deflection roll. This is very important in pairs of hard transfer rolls to ensure a uniform profile across the width of the coating device, even if a plurality of different operating conditions should be achieved.

List of reference numerals

1 Coating nozzle

2 Fluid head

3 Blowing head

4 Running surface

5 Spray curtain

6 Spray of gaseous medium, air spray

7 Gap

8 Impact wire

8A separation edge

9 Lip

10 Coating device

11 Having a wall of a coating medium as flushing medium

12 Tangent line

13 Impact position of spray curtain

14 Curtain of coating medium

15 Film of coating Medium

20 Coating spaces

R direction of spray curtain

Length of L spray curtain

Alpha angle of impact

Delta opening angle

Claims

1. Coating device (10) for applying a liquid or pasty coating medium to a running surface (4), in particular a rotating transfer roll (4), in a machine for producing or processing a fibrous web, wherein the coating device (10) comprises a coating nozzle (1) for producing a spray curtain (5) of coating medium, characterized in that the coating device (10) has a coating space (20) through which the coating space passes

● The running surface (4),

● At least a part of the wall of the coating nozzle (1),

● The spraying curtain (5),

● The position of the narrowest gap (7) between the running surface (4) and the coating nozzle (1)

The boundary is formed and defined such that,

And wherein a film made of a flowing coating medium (15) is coated on the boundary provided by at least a part of the wall of the coating nozzle (1).

2. Coating device (10) according to claim 1, characterized in that the coating nozzle (1) comprises a fluid head (2) designed for producing a film of coating medium (15), and the coating nozzle (1) further comprises a blowing head (2) designed for producing a linear beam of gaseous medium (6), wherein an impact line (8) is also provided, at which the linear beam of gaseous medium (6) impacts the film of coating medium (15) or a curtain (14) to form a spray curtain (5), and wherein the blowing head (3) is arranged such that the spray curtain (5) is oriented in the direction of the running surface (4).

3. The coating device (10) according to claim 2, wherein the coating space (20) is filled with a liquid

● The running surface (4),

● A wall part (11) of the blowing head,

● The spraying curtain (5),

● The position of the narrowest gap (7) between the running surface (4) and the blow head (3)

The boundary is formed and defined such that,

And wherein a coating medium is applied from the fluid head (2) onto the wall (11) of the gas blowing head (3) such that the entire wall (11) of the gas blowing head (3) is covered by the flowing coating medium in the application space (20).

4. The coating device (10) according to any 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 equal to a maximum of 50mm, in particular less than 20mm, particularly preferably less than 10mm.

5. The coating device (10) according to any of the preceding claims, characterized in that the narrowest gap (7) between the running surface (4) and the coating nozzle (1), in particular the blow head (3), is 2mm to 20mm wide.

6. Coating device (10) according to any one of the preceding claims, characterized in that the layer thickness of the coating medium on the wall (11) of the coating nozzle, in particular on the wall (11) of the blow head (3), is between 5 μm and 1000 μm, in particular between 10 μm and 500 μm.

7. Coating device (10) according to any of the preceding claims, characterized in that a nozzle is provided for generating an air flow, wherein the air flow is directed or orientable from outside the coating space towards the narrowest gap (7).

8. The coating device (10) according to any one of the preceding claims, characterized in that the coating space (20) is shaped such that a tangent (12) of the transfer roll (4) at the impingement position (13) of the spray curtain (5) intersects a wall portion of the blow head (3) located within the coating space (20).

9. The coating device (10) according to any one of the preceding claims, wherein the coating space (20) is closed at both end sides by sealing surfaces, respectively.

10. The coating device (10) according to any one of the preceding claims, wherein the direction (R) of the curtain (5) has a component directed opposite to the direction of movement of the running surface (4).

11. The coating device (10) according to any one of the preceding claims, wherein the curtain (5) impinges the running surface (4) with an impingement angle α of at most 60 °, in particular an angle of 10 ° to 50 °.

12. The coating device (10) according to any one of the preceding claims, characterized in that the running surface (4) is provided by the surface (10) of a rotating transfer roll, and the spray curtain (5) impinges on the surface (4) of the transfer roll especially between 7 o 'clock and 12 o' clock, preferably between 8 o 'clock and 10 o' clock, particularly preferably between 8 o 'clock and 9 o' clock.

13. Coating device (10) according to any one of claims 2 to 12, characterized in that the blowing head (3) is designed to be movable, in particular rotatable, in order to change the direction (R) of the spray curtain (5).

14. Method for applying a coating medium, in particular a starch solution, onto a running surface (4), in particular onto a transfer roll (4) in a machine for manufacturing or processing a fibrous web, characterized in that the application is effected by means of an application device (10) according to any of the preceding claims.

15. The method according to claim 14, characterized in that the coating medium has a bohler fly viscosity of between 0.1 and 300mPas, preferably less than 200mPas, in particular less than 160mPas, measured at 100rpm, 60 ℃, with a spindle 4.

16. The method according to claim 14 or 15, characterized in that the coating medium has a coating weight of less than 100l/m/min, in particular between 2.5l/m/min and 50 l/m/min.

17. The method according to any one of claims 14 to 16, characterized in that the coating medium has a solids content of between 1% and 35% and/or (in the case of 50 ℃) a surface tension of between 30mN/m and 70 mN/m.