WO2004007091A1

WO2004007091A1 - Coating head

Info

Publication number: WO2004007091A1
Application number: PCT/EP2003/007050
Authority: WO
Inventors: Erich Kranz
Original assignee: Mitsubishi Hitec Paper Flensburg Gmbh
Priority date: 2002-07-10
Filing date: 2003-07-02
Publication date: 2004-01-22
Also published as: DE50302079D1; DE10231276A1; AU2003281149A1; EP1521643B1; EP1521643A1; DE10231276B4; ATE314154T1

Abstract

The invention relates to a coating head (3) comprising a first half (1) and an adjacent second half (2), between which a gap (4) having a length L and a width B that is oriented perpendicular to the length L is embodied. Said gap (4) is connected to a color chamber (5) which is configured within at least one of the two halves (1, 2) of the coating head. Said color chamber (5) receives a coating material which is to be applied to the medium (12) that is to be coated via the gap (4) and a nozzle (6) forming the outer end of the gap. The color chamber (5) is provided with one or several upward-tapering recesses (7) which extend into vents (8). The invention also relates to the use of the inventive coating head and a coating method using the inventive coating head.

Description

coating head

The invention relates to a coating head for applying a coating composition to a moving medium, in particular a coating head for the curtain coating method for forming a liquid curtain from coating composition which is to be applied to a continuously moving, web-shaped medium, the use of the coating head according to the invention and a Coating process using the coating head according to the invention.

Coating heads of this type are known, for example, from EP 0 812 237 B1 or, in one embodiment as a horizontally operating extrusion coating head, from EP 0 673 684 B1 and essentially have a first and an adjacent second coating head half, between which one Gap is formed, the gap being connected to an ink chamber for accommodating the coating composition, and the coating composition being dispensed to the medium to be coated through the gap and a nozzle representing the outside end of the gap.

Since after the application of the coating material by means of an extrusion or curtain coating head, the applied coating is not further homogenized or otherwise treated, it is of great importance that the coating is carried out flawlessly. For example, the two protective rights EP 0 812 237 B1 and EP 0 673 684 B1 listed above attempt to solve the problem of achieving a uniform coating thickness without fluctuations, particularly in the case of thin coating jobs. In addition, however, it is of the utmost importance that the coating composition is free of trapped air in the form of air bubbles or foam, since each of such air bubbles leads to defects in the coating.

Numerous proposals have been made to form a coating mass without bubbles before it is applied to a medium. DE 40 32 833 A1 proposes a complete casting application device for highly viscous products with a casting head, storage container, return channel and pump, with the proviso that the flow of the product has to take place in a laminar manner without the formation of bubbles. This document thus formulates less an invention than the knowledge that turbulent movements of the product in the feed to the coating head cause blistering. Thanks to the use of pumps, necessary ink mass filters and long, partially curved lines, the idealized conditions of only laminar flows in the coating material feed to the application head can never be fully realized. For this reason, WO 84/02859 A 1 proposes a degassing container into which the coating composition is pumped from a storage container. The overflow mass containing foam and air bubbles is returned to the storage container from an overflow of this degassing container. The coating material discharged from the degassing container in the lower area is fed to a coating head and should then no longer have any foam or air bubbles.

In practice, all of the solutions that have been proposed up to now have not been convincing, since they either only allow too little inflow amounts to the coating heads per unit of time or have an unacceptable price-performance ratio, or they result in coating with a coating composition that is not completely degassed.

It is an object of this invention to present an aggregate with which a coating composition can be applied to a moving medium without foam or air bubbles. In particular, the new unit should be simple in construction, so that it is both inexpensive to buy and later to maintain. From the outset, it is also part of the catalog of requirements that the unit to be proposed can be used for the technology of applying coating compound by means of a liquid curtain.

After intensive considerations, it became apparent to the inventor that a convincing solution to the problem can only be found in a coating head that contains means for degassing the coating composition itself. Only in this way can it be excluded that previously degassed coating compound is not reintroduced into the coating compound by foam or air inclusions, which are caused by turbulent flows in pipelines laid between a degassing unit and a coating head, and foamed again.

This problem is solved with a coating head with a first half and an adjoining second half, between which a gap is formed with a gap length L and a gap width B oriented at right angles to the gap length L, the gap with at least one of the two coating head halves trained ink chamber is connected and the ink chamber is provided for accommodating a coating material through the gap and the outside of the gap end Representing nozzle is to be dispensed to the medium to be coated, characterized in that the ink chamber is provided with one or more bulges tapering upward, which open into ventilation openings.

If the paint chamber of the coating head according to the invention is preferably filled with coating material from a storage container via an intermediate degassing unit, residual foam from coating material with enclosed air bubbles collects in the bulges tapering upwards due to the lower specific mass of the coating material-air mixture contaminated with air bubbles. This mixture emerges from the ventilation openings and is ideally fed back to the storage container, for example via a vacuum vent. A variant is preferred in which a negative pressure is applied to the ventilation openings. The degassing performance of the coating head according to the invention can be increased considerably by means of such a preferably controllable suction device.

The degassed coating compound is fed to a gap which ends in a nozzle as the outside end of the gap and releases the coating compound onto the medium to be coated. If the coating head is used to form a liquid curtain, the gap width B corresponds to the desired curtain thickness, while the gap length L corresponds to the curtain width. In many cases, this is chosen to be wider than the medium to be coated, so that part of the liquid curtain falls past the medium into a collecting tray, from where the coating material is returned to the storage container.

It has proven to be advantageous to precede the nozzle with a compensation chamber in which the pressure and flow conditions of the coating composition are standardized. The compensation chamber is designed as an expansion of the gap in its width over the entire gap length L in a limited area between the nozzle and the ink chamber. To supply the coating material, the compensation chamber is connected to the paint chamber, for example via individual downpipes or connecting pipes or preferably via an intermediate gap between the compensation chamber and the paint chamber.

The coating material is preferably supplied by means of a lateral inlet opening in the paint chamber, which is connected to the reservoir as far as possible via an intermediate degassing unit. Ideally, the ink chamber has also via an outlet opening, which is embedded in the opposite side of the ink chamber and is connected to a bypass coating material discharge. Excess coating material is fed back into the storage container via the bypass. Numerous tests have shown that a ratio of coating material supplied to coating material returned by bypass in a range from 1: 0.2 to 1: 2 can be regarded as particularly advantageous.

The present invention also includes the use of the coating head according to the invention for the application, in particular, of heat-sensitive coating compositions. Air bubble inclusions in heat-sensitive coatings lead to defective coatings and thus to the loss of the imaging function. Since the manufacturers of heat-sensitive recording materials are obliged to give a guarantee that the recording material is fully functional over its entire surface, incomplete coating can cause complaints to a large extent. Against this background it becomes clear why the structurally simple possibility of a particularly effective and error-free, integrated degassing coating head according to the present invention is particularly suitable for heat-sensitive coating compositions. Coating compositions of this type contain dye precursors of active substance and preferably organic color acceptors which react with them to form a color under the influence of heat. In addition to these essential components, other common additives in the heat-sensitive coating compositions include resins, waxes and stabilizers.

The coating head according to the invention can be operated both with other coating systems and in a multiple arrangement, so that wet-on-wet coatings are made possible. It is also conceivable to change the design of the proposed coating head in such a way that it is suitable for simultaneous multiple coating. In this case, several ink chambers with tapering bulges opening into ventilation openings, possibly several compensation chambers and several gaps, which are arranged one behind the other in the same orientation, are used within a multiple coating head.

To coat a web-shaped medium, the coating composition is preferably taken from a storage container via an intermediate degassing unit of the dyeing chamber of the coating head according to the invention supplied. If there are air pockets in the coating material, foam collects in the bulges of the ink chamber that taper upwards. This foam is removed by means of a preferably controllable suction device. For example, by means of vacuum deaerators, the coating mass fraction can be pulled out of the foam and returned to the storage container.

The coating composition not drawn off as foam from the ink chamber is fed to the nozzle of the coating head according to the invention. In a preferred process variant, under the influence of gravity, the coating composition forms a liquid curtain below the nozzle, which falls along a forming - dynamic - wetting line onto the web-shaped medium which is continuously moving relative to the coating head.

As already stated, it is preferred if the coating composition is heat-sensitive, for which purpose it contains dye precursors and color acceptors which react with them to form a color under the influence of heat. A paper web is preferred as the medium.

Another preferred method variant is the formation of a protective layer on the previously applied heat-sensitive coating. Both heat-sensitive and the protective layer covering them can be applied with the coating head according to the invention. This can be done with a time delay or simultaneously with a multi-coating head.

With the help of the figures, the invention is further clarified in the context of two construction examples:

Figure 1 shows a first variant of the coating head (3) according to the invention with a first half (1) and a second half (2), which is constructed mirror-inverted to the first half (1). In the upper area of the coating head (3), the two coating head halves (1, 2) adjoin one another precisely along a straight contact line K. Below the contact line K, a tubular ink chamber (5) is formed, which is oriented parallel to the contact line K and half of which is formed in the two coating head halves (1, 2). The ink chamber (5) is expanded at the top with conical bulges (7). The bulges (7) taper upwards and open into ventilation openings (8), each half along the contact line K. are formed in the two coating head halves (1, 2). The ink chamber (5) is slotted down the entire length of the tube and connected via the slit to a gap (4), half of which is formed in each of the two coating head halves (1, 2). The gap (4) has a gap length L with the same orientation as the contact line K and a gap width B perpendicular to the gap length L. The gap width B is thus determined by the distance between the two coating head halves (1, 2) in their lower region in accordance with their respective shapes. The gap (4) is bounded at the top by the slot in the ink chamber (5) and at the bottom by a nozzle (6), which represents the gap end at the lower end of the coating head (3). The nozzle (6) runs parallel to the contact line K.

The ink chamber (5), like the gap (4), is sealed laterally by closure means, not shown in FIG. 1. An opening (9, 10) is let into each of these closing means in the lateral extension of the ink chamber (5), one opening (9) for supplying the coating material and the other opening (10) for removing the coating material.

Below the nozzle (6) in the extension of the gap (4), a liquid curtain is shown, which runs vertically downwards along the nozzle (6) and in the lower region through the wetting line (13) as the cutting line of the liquid curtain with the medium to be coated (12 ) is limited. The wetting line (13) runs parallel to the contact line K. The direction of movement of the medium (12) runs at right angles to the wetting line (13) and parallel to the gap width B.

FIG. 2 shows a further variant of the coating head (3 ^' ) according to the invention. In addition to the first variant according to FIG. 1, an additional tubular compensation chamber (11) is formed below the ink chamber (5), which is oriented parallel to the contact line K and half of which is formed in the two coating head halves (1, 2). Again the ink chamber (5) is slotted down over the entire length of the tube. Now, however, the ink chamber (5) is connected via this slot to an intermediate gap (14), half of which is formed in each of the two coating head halves (1, 2). At the bottom, the intermediate gap (14) opens into an upper opening slot of the compensation chamber (11), which extends over the entire length of the tube. This has a second slot at the bottom over the entire length of the tube and is connected via this second slot to a slot (4) which, as already described for FIG. 1, opens into the nozzle (6). Not shown in FIG. 2 are closure means which, in addition to the ink chamber (5) and the gap (4), now also laterally seal the compensation chamber (11) and the intermediate gap (14).

FIG. 3 shows a simplified installation situation of the coating head (3, 3 ^' ) according to the invention without possible pumps and / or further degassing units. A storage container (17) contains a coating material which is supplied from the storage container (17) via the coating material supply (15) to the coating head (3, 3 ^' ) and further via the inlet opening (9) (not shown) to the paint chamber (5). An excessively supplied coating material from the ink chamber (5) of the bypass coating material discharge, in short: the bypass (16), which in turn is connected to the storage container (17), is supplied via an outlet opening (10) opposite the inlet opening (9) ,

FIG. 4 shows an overall coating curtain (18) formed by a multiple coating head. Such a multiple coating head comprises a plurality, in the case on which FIG. 4 is based, three individual coating heads. For this purpose, the nozzles of the three individual coating heads, each dimensioned by their gap width B, which determines the thickness of the individual coating curtains (19), and by their gap length L, which determines the width of the individual coating curtains (19), are combined so that the total coating curtain (18) has the same width the individual coating curtain widths and with a thickness corresponding to the sum of all individual coating curtain thicknesses.

1 first half of the coating head second half of the coating head

3 ^' : coating head

gap

5 color chamber

6 nozzle

7 bulges

8 vents

9 inlet opening

1 0: outlet opening

1 1: Compensation chamber

1 2: Medium to be coated

1 3: wetting line

1 4: intermediate gap

1 5: Coating material supply

1 S: Bypass (coating mass discharge)

1 7: Storage container

1 8: Total coating curtain

1 9: Single coating curtains

B gap width

K contact line

L gap length

Claims

claims

1. coating head (3, 3 ^' ) for applying a coating composition to a moving medium (12)

^■ with a first half (1) and an adjoining second half (2),

^■ between which a gap (4) is formed with a gap length L and oriented at right angles to the gap length L gap width B,

^■ the gap (4) being connected to an ink chamber (5) formed at least in one of the two coating head halves (1, 2)

^■ and the ink chamber (5) is provided for accommodating a coating material which is to be dispensed through the gap and a nozzle (6) representing the outside of the gap to the medium (12) to be coated, characterized in that

^■ the ink chamber (5) is provided with one or more bulges (7) tapering upwards, which open into ventilation openings (8).

2. Coating head according to claim 1, characterized in that the gap (4) at least in one of the two coating head halves (1, 2) is partially expanded to an equalizing chamber (11) which is between the nozzle (6) and the ink chamber (5) is positioned, the ink chamber (5) and the compensation chamber (11) being connected to one another.

3. Coating head according to one of claims 1 or 2, characterized in that the ink chamber (5) has an inlet opening (9) which is connected to a coating material supply, and an outlet opening (10) opposite the inlet opening (9) which has a bypass - Coating mass discharge is connected.

4. Coating head according to one of claims 1 to 3, characterized in that the ventilation openings (8) are connected to a suction device.

5. Multiple coating head, comprising a plurality of individual coating heads each according to one of claims 1 to 4, wherein the nozzle of the multiple coating Layering head is formed at least from the nozzles of the individual coating heads arranged one behind the other with the same orientation.

6. Use of a coating head according to one of claims 1 to 4 for applying a coating composition to a moving medium (12), wherein the coating composition contains dye precursors and color acceptors reacting with them under the influence of heat to form a color.

7. A coating method comprising forming a liquid curtain from a coating composition by means of a coating head according to one of claims 1 to 4, wherein the liquid curtain along the wetting line (13) onto a web-shaped medium moving continuously below the coating head (3, 3 ^' ) (12) hits.

8. Coating method according to claim 7, characterized in that the coating composition contains dye precursors and color acceptors reacting with them under the influence of heat.

9. Coating method according to one of the two claims 7 or 8, characterized in that the medium (12) is a paper web.

10. Coating method according to one of claims 8 and 9, characterized in that a protective layer is subsequently applied to the applied heat-sensitive coating composition.