FI115408B - Method for Coating Paper or Cardboard, Use of the Method and Coating Paste - Google Patents

Description

115408

METHOD FOR COATING PAPER OR PAPERBOARD, USE AND METHOD OF COATING

FÖRFARANDE FÖR BESTRYKNINGSSMET AV EN PAPPERS- ELLER KARTONG-BANA, UTNYTTJANDE AV FÖRFARANDET OCH BESTRYKNINGSSMET

The present invention is directed to a method for coating a paper or board web as defined in the preamble of the independent claims below, in particular using a spray technique, to a process and to a coating paste for use in the process.

Previously, it is known to coat printing papers e.g. using blade coating technology. However, the blade coating requires a certain strength from the base paper. Therefore, blade coating has mainly been used only when the high-strength pulp fiber is high in the base paper. For example, blade coating has been used for LWC (Light Weight Coated) grades, where the pulp content is typically at least 40% of the fiber material, and for fine paper grades where the pulp content is 100% or nearly 100%.

. Generally, the greater the proportion of pulp fiber in the base paper; ,: the more expensive the base paper than the fibrous material is. It would be advantageous to be able to coat: also woody paper grades with a low proportion of pulp fiber. With blade coating this does not appear to be successful.

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: ,,,: It has been found that coating using film transfer technology (FS technology) does not *: '·. · Require similar strength properties to the base paper as the blade coating. Thus, the FS-:, ,, technology can be used to coat SC paper grades and LWC grades having a pulp content of less than 40%. FS technology can even coat ": newsprint.

2 115408 However, FS coating does not achieve the same quality as blade coating. One of the reasons for this may be the cracking of the film after the roll nip, i.e. the uneven distribution of the film between the paper and the surface of the roll. The surface formed during the coating is generally not as smooth as the surface produced by the blade coating. Thus, the FS-5 technique does not coat paper for intaglio printing.

Recently developed spray-based coating is expected to provide high quality coating of wood-based, low-fiber paper and board.

In the developed spray-based coating, the coating paste 10 is led under high pressure from a plurality of jets arranged side-by-side across the web towards the web to be coated. There may be several rows of jet nozzles at the coating station in succession in the direction of web travel to provide good coverage. The high pressure of the coating paste, up to more than 100 bar, is aimed at distributing the small paste into small droplets which are applied as jets to the film to be coated.

Some of the tiny droplets of paste remain suspended in the air between the paper to be coated and the spray nozzles. This mist creates a variety of problems. Fog spreading; ': From the coating machine to the environment unless the coating station is encased and fog removed' \ by suction or otherwise. The mist disappearing into the air creates unwanted dust. · · *. 20 disappeared, in addition to fogging the surfaces it hits. Finally, mist: \: pasta may end up in the waste water it loads.

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Achieving good quality has also proven to be a problem. From the nozzles: ·. | high pressure discharge paste spreads on poorly absorbent paper or board. To achieve the required coverage, the spray nozzles are placed; ''. · 25 to spray the pasta so that the edges of the pasta jets overlap slightly.

:.,.: However, this will easily result in a thicker coating layer:: · ·: where old jets overlap the jets. The higher the dry solids content of the coating paste, the more sensitive the old * 3 115408 is. This may be because the edges of the showers tend to dry more than the other showers. There has been some improvement in quality by reducing the dry matter content of the pigment paste, ie by adding water to the paste. For some grades, a clear deterioration in the spread of the paste 5 has been observed with a solids content of paste> 58%. Therefore, the dry solids content of the paste has been kept at <58%. However, in coating it would be advantageous to be able to avoid introducing unnecessary water to the web, for example, because the water must be dried after the coating, however. Also, the water-diluted coating paste or the water therein does not want to penetrate the web where it may adversely affect the properties of the web.

In addition, the spray technique has been found to require a relatively flat base paper, since in spray coating the entire surface of the base paper is coated with a uniformly thick paste layer, which consequently follows any irregularities in the base paper. To date, the most common pigment used in spray coating has been calcium carbonate, which is present in the form of small round particles in the paste. In spray coating, such paste does not smooth out unevenness in the base paper. In blade coating, the paste layer ':': on the contrary, smooths the surface irregularities of the base paper.

It is an object of the present invention to provide an improvement on the above problems in spray coating.

The purpose of the invention is, for example, to provide a coating method based on a spray technique which can achieve good quality with respect to the coated / surface.

: * ·. j 25 The purpose is, for example, to provide a high quality coating method whereby: the pigment paste is evenly applied to the base paper without any old ones.

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It is also intended to enable the coating of wood-containing or less pulp-containing paper grades using spray technology.

A further object is to enable the use of various base paper-leveling pigment particles in a spray-applied coating.

For the purposes of the foregoing purposes, the present invention is known as defined in the characterizing parts of the following independent claims.

A typical process of the invention for coating a paper or board web 10 with a coating paste containing pigment particles using a spray technique thus comprises at least the following successive steps: (a) electrically charging the web through an electrostatic field formed between the electrode and counter electrode, (b) the charged web is fed to 15 coating stations and (c) an electrically charged web fed to the coating station is coated with at least one side of said coating pigment containing pigment particles using said spray technique.

:. '.: It has now surprisingly been found that by electronically charging the web, the spray paste can be applied evenly to almost any kind of base paper or paperboard, including absorbent.

* · '...: The coating paste can unexpectedly be unloaded from the spray nozzles: /. j at a lower pressure, typically 5-130 bar, most typically 10-30: ... · bar. In this way, several advantages can be achieved. Lower pressure '"* · 25 reduces the amount of mist produced. Due to lower pressure, the» t · 115408 nozzles are more durable than at higher pressures.

The paste may then also contain plate-like particles which the nozzles have not previously been able to use. Due to the above, savings can also be made in the investment and operating costs of the equipment.

In the past, spray-based coatings have previously used mainly calcium carbonate based pigments such as GCC or PCC carbonate. Unexpectedly, spray pastes with sheet-like pigments can also be applied to the electrically charged base paper or board. In the past, it has been imagined that only circular pigment particles 10 can be used in spray coating applications. It has now been found that the use of plate-like pigment particles in spray-applied coating can achieve even much better quality than before.

A typical coating paste for high mechanical pulp coating of mechanical printing paper, such as newsprint and LWC, then contains a pigment mixture containing from 70% to 90%, typically about 80% carbonate and; '; 10-30%, typically about 20%, of the lamellar pigment.

. ·, Typical coating paste for gravure paper coating »·> 20, respectively, of pigment mixture with * * *

Ti. " 30-50%, typically about 40% carbonate and: 50-70%, typically about 60% lamellar pigment.

'. The plate-like pigment particles also appear to smooth out. · · ·. irregularities in the base paper. It is contemplated that the plate-like pigment particles 25, when struck on the electrically charged surface, will settle on the surface of the flattened surface, thereby forming a surface smoothing layer over the surface irregularities. The plate-like pigment particles are typically kaolin or talc particles or any mixture thereof.

5 With spray technology, the thickness of the coating can be easily adjusted by adjusting the amount of paste to be sprayed.

In applying the spray technology of the invention, the web can be charged electrically in step (a) by passing the web through a corona discharge device or other electrostatic field formed between the corresponding electrode and the counter electrode. Preferably, the yoke electrode 10 may be a surface made of electrically flaking material and guiding the web passage in step (a), such as a turning roll, for example a chromium-plated roll. This guiding surface can then be grounded to provide a suitable charge to the web. The electrostatic field is preferably directed to travel at least substantially perpendicular to the web.

The web is electrically charged in step (a) with an electrode typically providing a voltage of <60 kV, most typically <30 kV. The current is then: • typically <10 mA / m, most typically <5 mA / m.

; ; When applying the spray technique according to the invention, the web is preferably guided: vertically from step (a) to step (c). The web is preferably guided in an upright position 20 also through the coating station in step (c). Typically, the pivoting roll, in which the web is charged electrically, is arranged at a level higher than the actual coating position so that the web runs vertically downward from the pivoting roll towards the coating station and vertically at least »! through the bulk of the coating station. If desired, the course of the web can of course be tracked 1 also in the other direction.

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The solution of the invention can advantageously be used in coating stations where the web is simultaneously coated on both sides. If desired, only one side of the web may be coated.

In spray coating applications, a plurality of spray nozzles are arranged in a row 5 across the web, typically with a nozzle spacing of 40 to 130 mm.

Multiple nozzle rows can be provided on top of the coating station if desired. Thus, the nozzles arranged one after the other in the direction of web travel are preferably arranged slightly overlapping. The nozzles are preferably grounded.

When applying the spray technique based on the spray technique 10 of the invention, the dry matter content of the coating paste may be higher than what has previously been possible with the spray technology. The solids content of the coating paste may be> 58%, even more than 60%, however, generally <70%. This avoids the introduction of excess water onto the web and saves on drying costs.

The positive electrical charge of the base paper promotes the distribution of high dry solids paste evenly throughout the web.

In addition, the electrical charge of the web prevents the formation of a fog cloud at the coating station. In a conventional spray coating, • · \; the aspirating tiny tiny droplets in the solution of the invention are directed with the spray jet towards the web to be coated. Thus, it can be assumed that '; * ·] Even the edges of 20 spray jets do not dry in the same way as in previous spray coating. This also avoids the use of a drier paste in the coating.

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The spray technique of the invention is suitable for use in coating a variety of wood-containing paper grades.

. * .. '25 The method is suitable for use e.g. • coating of newsprint or LWC paper with pigment paste. In the manufacture of paper to be coated according to the invention, the proportion of pulp fiber in the pulp can generally be reduced.

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The process of the invention is also suitable for use with a pigment paste for coating a pulp-containing paper, such as 80-100% pulp-containing paper.

The invention will now be further described with reference to the accompanying drawings, in which:

Figure 1 shows schematically and by way of example a coating apparatus employing a spray technique in which the web is electrically charged by the process of the invention; and

Figure 2 is a cross-sectional view of the device of Figure 1 along line AA.

10 shows a web coating device 10 comprising a device 12 for electrically charging the web and a coating station 14 for coating the electrified web 10 '.

The device 12 for electrifying the web comprises a charge electrode 16 and a grounded pivot roll 18 serving as a counter-electrode guiding the web. The web 10 is imaged; '15 to pass through an electric field 20 formed between the electrode 16 and the turning roll 18, whereby the web is electrically charged. The electric field is perpendicular to the web.

· · The electrically charged web 10 'is transported vertically to the coating station 14 below the roller roll 18' *.

In the coating station shown in Figure 1, there are two overlapping nozzle rows 22, 24 and 22 ', 24' of spray nozzles 26 on both sides 20 of web 10 '.

; However, there may be only one row of nozzles when applying the solution of the invention. Correspondingly, the nozzle row (s) may be arranged on only one side of the web.

,; The nozzles are grounded. Coating spray jets 28 are sprayed from the nozzles towards the web 10 '. The coating paste jets 28 distribute the paste evenly onto the web 10 '.

2: It can be seen from Figure 2 that each nozzle row 22, 24 and 22 ', 24' has a plurality of nozzles 26 spaced apart such that the discharge jets substantially cover the entire width thereof. In different overlapping rows, the showers are slightly overlapped to ensure coverage. Perhaps the most important idea of the present invention is to electrically charge a paper or board web over a rotating roll prior to the coating station, and then to spray spray preferably on both sides of the web simultaneously. In this way, a very smooth coated surface can be obtained, which also enables the production of gravure papers where the smoothness of the paper is an important factor affecting printability. Electronisation of the web also substantially eliminates the formation of pasta mist.

The spray-based coating according to the invention provides many advantages over the coating based on the prior spray technique, such as: and in such a way that the runnability of the web remains good; and - the solution according to the invention also avoids the formation of a paste mist, i.e. * · • · * / avoids dirty surfaces of the paste, used to remove the paste mist * # • · ''. from equipment and the passage of paste into waste water. The coating station does not need to be enclosed or provided to prevent spreading of the · ·, · · \ 20 paste mist: ', · a powerful suction system to remove the mist. Also, no equipment is needed in the fog

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»T · •, t: for condensation or absorption in water. The non-fogging of the paste thus also reduces the formation of waste water and thus allows the coating station to be built in a smaller space than before.

* * · T '·, · 25 Spray coating according to the invention is primarily * »* * * ,, / intended for coating paper grades containing wood fiber,»: ·! where the benefits are potentially greatest. However, it has been found that this coating process can advantageously also be used for fine paper coating.

The electrical charge of the base paper can at least partially replace the carbonate pigment used in the prior art spray-based applications, the powdered or GCC or the precipitated PCC pigment, with a pigment containing lamellar or lamellar pigment particles such as kaolin or talc. The addition of kaolin and / or talc can improve the coverage and printability of the coating, particularly in offset printing. Smooth spray coated paper can also be used for gravure printing.

The invention is not intended to be limited to the above embodiments, but is intended to be widely used within the scope defined by the following claims.

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Claims (21)

115408 A method for coating a paper or cardboard web with a coating paste containing pigment particles at least on one side of the web using a spray technique 5, characterized in that the method comprises at least the following successive steps (a) electrically charging the web through an electrostatic field formed between the electrode and (b) in step (a), the electrically charged web is fed to a coating station and (c) the web fed to the coating station is coated on at least one side with said pigment particulate coating paste using said spray technique. A method according to claim 1, characterized in that the web is electrically charged in step (a) by conducting the web »! (; through an electrostatic;; field formed between the corona discharge device and the counter electrode. * t Method according to Claim 1 or 2, characterized in that a counter-electrode is used which is made of an electrically conductive material and guides the web in step (a). t t Method according to claim 1 or 2, characterized in that. the counter electrode is a swivel roll, typically a grounded swivel roll. '; The method according to claim 1 or 2, characterized in that the direction of the electrostatic field in step (a) is at least substantially perpendicular to the web. 115408 6. A method according to claim 1, characterized in that the web is guided vertically through the main part of step (b). A method according to claim 1, characterized in that the web is guided vertically through the main part of the coating station. A method according to claim 1, characterized in that the web is electrically charged in step (a) with an electrode providing a voltage of <60 kV, typically <30 kV, and / or <10 mA / m, typically <5 mA / m. A method according to claim 1, characterized in that the web 10 is coated on both sides of the web in a coating position. A method according to claim 1, characterized in that, in the coating station, the web is coated with a coating paste which is sprayed from the spray nozzles at a pressure of 5 to 130 bar, typically 10 to 30 bar. A process according to claim 1, characterized in that the dry matter content of the coating paste is> 58%, typically> 60% but <70%. A method according to claim 1, characterized in that the coating station *, '· has at least one row of spray * ... * nozzles arranged across the web, the nozzles being spaced from 40 to 100 mm apart. 1 A method according to claim 12, characterized in that two or more rows of spray nozzles are blocked below the web at the coating station. » The method according to claim 1, characterized in that the spray nozzles are grounded. 115408 Process for coating mechanical printing paper such as newsprint or LWC paper according to any one of claims 1 to 14, characterized in that the pigment paste contains a pigment mixture of 5 70-90%, typically about 80%, carbonate and 10-30%, typically about 20%, sheet-like pigment. A process for coating gravure paper according to any one of claims 1 to 14, characterized in that the pigment paste contains a pigment mixture of 10 to 30%, typically about 40% carbonate and 50 to 70%, typically about 60% sheet-like pigment. Process according to Claim 15 or 16, characterized in that the plate-like pigment particles consist mainly of kaolin or talc particles or a mixture thereof. The process according to any one of claims 1 to 17, as described in claims 1 to 17. * use of woody paper such as newsprint or LWC paper • i «i * *. for coating with pigment paste. I 'I • I '* 19. The method i t' according to any one of claims 1-17 above; 'Use for coating pigment paste mainly on pulp-containing paper, such as 80-100% pulp-containing paper. i ' Coating paste for use in coating mechanical printing paper such as newsprint or LWC using one of the methods of claims 1-17 mentioned above, characterized in that the coating paste contains a pigment mixture having a t 115408 of 70-90%, typically about 80%, carbonate and 10 to 30%, typically about 20%, of a sheet-like pigment, typically kaolin or talc, or a mixture thereof. A coating paste for use in coating a printing paper using any one of the above-mentioned methods according to claims 1-17, characterized in that the coating paste contains a pigment mixture of 30-50%, typically about 40% carbonate and 50-70%, typically about 60% lamellar. pigment material, typically kaolin or talc, or a mixture thereof. »« · 115408