EP1095187B1 - Method and device for improving the coating surface of strips of paper - Google Patents

Abstract

The invention relates to a method and device for improving the coating surface of a coating mass deposited on or applied to a paper web (8, 23) by a film press. An anionic aqueous coating mass or an anionic paper web are provided with an opposite electric charge by an electric charger, whereby the applied coating is attracted to the oppositely charged paper (8, 23) in a nip exit area of the film press. The electric charger is, for example, a corona generator (13, 17) or a direct current voltage source (9) that is connected to the paper (8, 23) and coating slip by electrodes (5, 5a, 7, 7a).

Description

When making paper, the paper webs are marked with a watery coating slip, also called coating color, spread. The coating colors consist of one or a mixture of different ones inorganic pigments and are made with one or a mixture of organic binders. It is known, these coating colors with so-called film presses on the Transfer paper webs. The coating color is on or applied to a rubber roller on both sides and then transferred from this roller or these rollers to the paper web. At the end of the contact with the coating slip Paper web cleavage occurs in the coating slip film. With this separation of a more or less viscous coating slip in itself, known rheological problems occur on the so-called flow lines or the orange peel effect to lead.

Because such film presses at speeds of 800 to 2,000 m / minute, occur when tearing apart the Spread from the wetted roller onto the contacted paper web small coating mass particles as free fog in the Room. You have to accept this fog more or less and checked him in practice by using a black box in the vicinity of the paper web running for drying, so to the nip output. An attempt was therefore made to determine the adhesive forces between the coating color and the roller, as well as the To influence cohesive forces within the coating color so that the fog at the nip exit and the so-called orange peel pattern is improved. However, these efforts resulted in unsatisfactory results in practice.

DE 32 35 368 A1 describes a method for painting Paper and cardboard known in which an aqueous paper coating slip is coated on a raw paper web. For improvement the penetration and migration of the aqueous paper coating slip it is intended that the paper first be corona discharged undergo. This is after the corona discharge device the base paper with a blade coater with a spread even layer of the aqueous coating slip. To coat the base paper, brush, air brush, Roller or knife coaters can be used. As for the coating methods specified there, only web speeds up to 800 m / min can be provided only relatively slow paper making processes performed become.

FR-A-1 447 984 describes a method for improving the Adhesion of a polyolefin film on a paper web known to to improve adhesion, the paper web with a high-frequency corona discharge is treated. Such Corona discharge device is not suitable for the orange peel effect and the spray when coating base paper to improve with an aqueous coating slip in film presses.

From US-A-4,576,831 is also a method for treatment a paper web with a high-frequency corona discharge known with which the adhesion of the coating slip improved shall be. When spreading using a film press there is no need to improve the adhesion, since this is already done by the pressure effect of the film press rollers.

US Pat. No. 3,498,865 describes the application of a polyfin layer known on a raw paper web, in which in a contact zone the melted polyfin in a limited electrical field is applied. Such a device is not suitable an aqueous coating slip with high throughput speeds apply evenly to a raw paper web.

The invention is therefore based on the object, the coating surface to improve the paper webs produced and to minimize the spray.

This object is achieved by the in claim 1 and claim 5 specified invention solved. Further training and advantageous embodiments are in the subclaims specified.

The invention has the advantage that with this method aqueous coating slip a more closed surface of the Dashes and therefore improved printability of the paper is achievable. Furthermore, one achieves with the inventive Process a printability that was previously only possible with thicker ones Stroke orders were possible, so that printed products can be produced with a lower mass per unit area are. Because these watery applied to film presses Coating compounds allow thinner strokes to be made with them Printed matter advantageously also more environmentally friendly dispose.

Furthermore, by avoiding the spray mist less Spread and less cleaning interruptions on the Coating equipment, such as film presses, is required, as a result the costs of the papers coated in this way can be reduced.

Fig. 1:

eine schematische Darstellung einer einseitig auftragenden Filmpresse mit einer elektrischen Ladungsvorrichtung;

Fig. 2:

eine schematische Darstellung einer zweiseitig aufragenden Filmpresse mit einer elektrischen Ladungsvorrichtung, und

Fig. 3:

eine schematische Darstellung einer zweiseitig auftragenden Filmpresse mit einem Koronagenerator für jede Seite als Ladungsvorrichtung.

The invention is explained in more detail using an exemplary embodiment which is illustrated in the drawing. Show it:

Fig. 1:

a schematic representation of a single-sided film press with an electric charging device;

Fig. 2:

a schematic representation of a double-sided upstanding film press with an electrical charging device, and

Fig. 3:

is a schematic representation of a double-sided film press with a corona generator for each side as a loading device.

1 of the drawing shows a schematic film press, through which the paper web 8 runs and on one side with an aqueous coating slip is provided, the counter roller 1 positive by an electric charging device ionized and therefore attractive for the anionic coating slip becomes.

The film press consists of two film press rollers, the the lower one represents the application roller 3 and the upper one the counter roller 1, which are both provided with a rubber covering. The rollers 1, 3 are driven in opposite directions and transported between its contact surfaces, the paper web 8 in a predetermined Direction. The application roller 3 of the film press is with a Pre-coating device 4, through which the coating slip is applied to the application roller 3. The coating slip becomes even, for example, with a known squeegee applied to the surface of the application roller 3 and taken in the direction of rotation as film 6 on the roller 3 and on the lower side 2 of the paper web 8 passing through it. Since the two rollers 1, 3 against each other under a predetermined pressure run, the aqueous coating slip is partially dewatered, so that a certain layer of the coating slip on the lower paper web side 2 is applied.

The pre-coating device 4, which optimally distributes the coating slip an electrode (cathode) 5 and is with the dispersed coating slip, which as electrokinetic System can be considered in electrical contact. The negative pole is a voltage source 9 with this Electrode 5 connected. On the counter roller 1 is a co-rotating Metal roller 7 arranged as a second electrode (anode) acts and connected to the positive pole of the DC voltage source 9 is. The rubber covering of the upper film press roller 1 is made made of an electrically conductive rubber mixture, so that the Paper 8 is charged with a positive electrical charge becomes. The electrodes 5, 7 and the voltage source 9, as well as an electrically conductive roller coverings Charging device electrical charging, a DC voltage source 9 is selected, which provides a voltage of approximately 150 to 2,000 V. With others Embodiments can also handle higher voltages 10,000 V can be used.

Fig. 2 of the drawing shows schematically a film press for coating a paper web on both sides. This is what it is about essentially a film press as shown in FIG Drawing described with various additional devices is provided in order to enable a double-sided coating. The same parts are therefore also with the provided with the same reference numerals as in Fig. 1 of the drawing.

2 of the drawing, two film press rolls 1, 3 are shown, which are both designed as application rollers and through their contact surfaces, the paper web 8 continuously passes. Through these two application rollers 1, 3 the Paper web 8 wetted on both sides with coating slip. This is on a pre-coating device 4, 4a is provided for each application roller 1, 3, by the help of a known, not shown Doctor device on the surface of the application rollers 3, 1 evenly spread the coating slip. This creates a film 6, 6a of coating slip on the roller surfaces, the partially on both sides 2, 2a of the passing Paper web 8 is applied.

For better application of the coating slip on both sides 2, 2a of the paper web 8 are the two pre-coating devices 4, 4a connected to a DC voltage source 9. Put it there the pre-coating devices 4, 4a electrodes (cathodes) represents that with the disperse coating slip that acts as an electrokinetic System can be viewed in electrical Are in contact.

Furthermore, two guide rollers 7a, 7b are provided whose contact surfaces the paper web 8 runs and the application rollers 1, 3 is supplied. These guide rollers exist made of electrically conductive material or are with an electrical conductive surface material coated. Stand there at least these conductive roller surfaces with the DC voltage source 9 and represent the opposite Electrode (anode). In this way, the continuous Paper web 8 with a positive electrical charge charged.

3 of the drawing, a film press is shown schematically, through which a two-sided order on the paper web 23 is reached. They are used as an electrical charging device two spray electrodes (corona generators) 13, 17 are provided, the opposite charge of the paper web 23 opposite of the coating slip. The paper web 23 is one Deflection roller 16 transported to a first guide roller 15. This guide roller 15 is connected to a compression spring 14 which the paper web 23 close to the right-hand corona generator 17 passes. This will make the right paper web surface provided with an electrical charge. This will make the paper contrary to the coating mass ionized by the upper Atomic position of the paper web 23 is oxidized. This corona oxidation leads to hydrophilization and thus to faster water absorption. With sufficient voltage can also be reactive on the cellulose fiber wall Groups are formed that improve the line character.

Above the right-hand corona generator 17 is a left-hand one Corona generator 13 with compression spring provided 19 guide roller 18 provided the oxidation of the left paper web surface causes.

Subsequently, the paper web 23 runs through two oppositely driven Film press rollers 20, 12. Horizontal next to these Film press rollers 20, 12 is still another metering roller 11, 21 arranged, which are also counter to the respective Film press roller 12, 20 is driven. The metering roller 11, 21 has a textured surface that uniform wetting of the film press roller 12, 20 with the Spreadable serves. In the upper nip 10, 22 of the respective Film press roller 12, 20 and the associated metering roller 11, 21 the coating slip is entered and over the surface of the Film press roller 11, 21 on the two paper side surfaces of the continuous paper web 23 applied. The Application of the coating slip corresponding to that described in FIG. 1 Method.

The method with the above-described devices for application the coating slip on the paper web 8, 23 is based on following principle:

The aqueous coating slips provide an electrokinetic System, with each pigment having a zeta potential in it owns aqueous system. The coating slips are dispersed, because these pigments have electrokinetic repulsive forces feature.

The invention was therefore based on the knowledge of this electrokinetic Forces in splitting the coating slip film to use and thus improve the transfer to paper 8, 23. Since all common coating colors are anionic and should also be transferred to anionic paper, the electrokinetic repulsive forces had to be overcome for this to achieve a more homogeneous stroke application. Therefore, the method according to the invention is a transfer device, by which the ionogenicity is influenced in such a way that the electrokinetic repulsive forces between the paper 8, 23 and the pigments of the coating slip and thus the coating slip be picked up yourself. The coating slip can both cationic and the paper anionic or vice versa. The only thing that is decisive is the opposite Charge difference between paper 8, 23 and Coating material so that there is an electrokinetic attraction comes between the coating slip and the paper 8, 23. It would be also conceivable, immediately cationic coating slips with anionic Use paper as here too the electrokinetic Forces would affect the coating process favorably. This is currently not common, given the problems it presents could not solve so far. In any case, but would also the use of cationic coating slips Process for increasing the electrical charge difference serve and thus an improvement of the stroke order cause.

So it turned out to be completely surprising with the inventive method Effect shown that with such an opposite electrical charging of the coating slip and the paper 8, 23 creates a more homogeneous application without the orange peel effect and also the spray at the nip outlet of the film press roller 3 can be reduced. Because at the outlet of the paper web, the one-sided, mostly double-sided part of the on the film press roller pre-dosed coating color, finds the inevitable Cleavage takes place in the coating slip. There is one Part of the water has already struck the paper web Concentration, or vice versa, the solids content has already increased, but the mobility of the coating slip must still be preserved. Adhesion plays in this gap zone both on the paper surface and on the dispensing roller 1.3 plays a crucial role. In practice it has it has therefore been shown that each of these splitting processes torn out mutually repulsive because the same Pigments and binders carrying zeta potential, as a spray with anionic charge to the cathode and with cationic charge tend to the anode and thus reduce this mist. simultaneously is the coated paper web coming out of the gap more homogeneous, which improves printability after drying.

Through the electrical charging and transfer of the coating slip and of the paper 8, 23 was therefore able to apply a more homogeneous line and a minimization of the spray can be achieved. there it doesn't depend on the polarity of the charged elements like of paper 8, 23 and the coating slip, but only on an opposite charge difference that the electrokinetic Attraction can take effect. The invention can therefore also be implemented in other configurations, as far as an opposite charge can be achieved, through which the electrokinetic forces of attraction take effect can.

Claims (8)

1. A method of improving the coating surface of the aqueous coating material applied to a web of base paper (8, 23) by means of a paper coating machine which additionally contains a charger affecting the web of base paper, characterised in that the web of base paper (8, 23) passes through two film press rollers (1, 3; 12, 20) of a paper coating machine rotating in opposite directions, the aqueous coating material being transferred to the web of base paper (8, 23) by means of at least one film press roller (1, 3; 12, 20), the charger ionising the coating material and the paper in opposite ways.
2. The method of claim 1, characterised in that the web of paper (8) is ionised by means of an electrode (7, 7a, 7b) connected to one pole of a d.c. voltage source (9), the other pole of the d.c. voltage source (9) being connected to the coating material.
3. The method of claim 1, characterised in that the web of paper (23) is ionised by means of at least one scattering electrode (13, 17) (corona generator).
4. The method of one of the preceding claims, characterised in that the web of base paper (8, 23) is coated by the film press (1, 3; 12, 20) at a speed of 800 to 2 000 m/min.
5. The method of one of the preceding claims, characterised in that the aqueous coating material is partly dehydrated by the two film press rollers (1, 3; 12, 20) running in opposite directions, whereby a thin, predetermined, uniform layer of coating material is applied to the web of base paper (8, 23).
6. Apparatus for carrying out the method of one of preceding claims 1 to 5,
   characterised in that a film press containing at least two film press rollers (1, 3; 12, 20) is provided to coat the web of base paper (8, 23) with the aqueous coating material, and that an electric charger is provided to ionise the base paper (8, 23).
7. The apparatus of claim 6, characterised in that the charger comprises an anode (7, 7a, 7b) and a cathode (5, 5a) which are connected to a d.c. voltage source, one electrode (5, 7, 7a, 7b) being electrically connected respectively to the web of base paper (8) and to the coating material.
8. The apparatus of claim 6, characterised in that the charger contains at least one spray electrode (corona generator) (13, 17) which is arranged near the web of paper (23) and used to ionise at least one surface of the paper.

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