WO2003064766A2

WO2003064766A2 - A coating method involving the use of an electrical field

Info

Publication number: WO2003064766A2
Application number: PCT/FI2003/000079
Authority: WO
Inventors: Keijo KÄPYNEN; Päivi MIETTINEN; Kari Nurmi; Anna-Liisa Tammi; Jan-Erik Teirfolk
Original assignee: Raisio Chemicals Ltd
Priority date: 2002-01-31
Filing date: 2003-01-31
Publication date: 2003-08-07
Also published as: FI20020182A; FI115408B; WO2003064766A3; AU2003201981A1; FI20020182A0

Abstract

A method for coating a paper or paper board web at least on one side of the web by a coating color containing pigment particles and by applying a spray technique. The method comprises at least the following successive steps: (a) the web is electrically charged by conducting the web through an electrostatic field that is created between an electrode and a counterlectrode ; (b) the web that was electrically charged in step (a) is conducted to the coating station, and (c) the web conducted to the coating station is coated at least on one side by said coating color containing pigment particles and by applying the spray technique.

Description

IMPRONEMENT IN COATING OF PAPER OR PAPER BOARD WEB

The present invention relates to a method for coating paper or paper board web, particularly by using spray technique, to the use of said method and to the coating color employed in said method, as defined in the preambles of the independent claims set forth below.

Earlier, it is known to coat printing papers for instance by using the blade coating technique. However, blade coating requires a certain strength of the base paper. Therefore it has been possible to use blade coating mainly only when the content of the strength-giving chemical pulp fiber in the fiber material is high in the base paper. Blade coating has been applied for example to LWC (light weight coated) qualities, where the chemical pulp content in the fiber material is typically at least 40%, and to fine paper qualities, where the chemical pulp content of the fiber material is 100% or nearly 100%.

In general it can be maintained that the higher the chemical pulp fiber content in the fiber material of the base paper, the costlier the base paper. It would be advantageous to be able to coat also such wood-contained paper qualities where the content of chemical pulp fiber is low. Apparently this does not succeed by means of blade coating.

It has been found out that a coating applying film transfer (FS) technique does not require the same strength properties of the base paper as the blade coating. Consequently, by means of the film transfer technique, it is possible to coat both supercalendered (SC) and LWC paper qualities with a chemical pulp content below 40%. Even newsprint can be coated by the FS technique.

The level of quality achieved by FS coating is not, however, as high as in blade coating. One reason for this can be assumed to be the splitting of the film after a roll nip, i.e. an uneven distribution of the film between the paper and the roll surface. The surface created in coating is generally not as smooth as a surface created by blade coating. Thus the FS technique is not suitable for coating paper for gravure printing.

The lately developed coating based on spray technique has been expected to enable a high-quality coating of papers and paper boards containing wood, i.e. containing only a small amount of chemical pulp fibers. In the developed coating method based on spray technique, the coating color is conducted at a high pressure through several spray nozzles, arranged adjacently in a row, transversally across the web, towards the web to be coated. The coating station may include several rows of spray nozzles, located in succession in the proceeding direction of the web, in order to achieve a good coverage. By means of a high pressure, even over 100 bar, the coating color is attempted to be distributed in very small droplets that are directed as jets sprays to the web to be coated.

Part of the small color droplets remain hovering as mist in the air, in the area of the paper to be coated and of the spray nozzles. Part of the color droplets is not attached to the web but bounce back to the air. A large part of the mist originates from said ricocheted droplets. This mist forms various problems. From the coating machine, the mist is spread to the environment, unless the coating station is covered and the mist is removed by suction or by some other means. Apart from smudging the surfaces where it hits, the color that spreads around in the air as mist results in undesirable losses. Finally the color that is in the form of mist may end up in the waste water, where it is an extra burden.

Also the achieving of a good quality has turned out to be a problem. The color that is discharged at a high pressure from the nozzles is poorly distributed on the surface of an absorptive paper or paper board. In order to achieve a sufficient coverage, the spray nozzles are arranged to spray the color so that the edges of the color jets are somewhat overlapping. In that case, however, there are easily created streaks formed by the thicker coating layer at spots where the sprays are overlapping. Said streaks are created all the more easily, the higher the dry matter content in the coating color is. This may be due to the fact that the marginal areas of the spray jets tend to be dried more than the other jet areas. The quality has been somewhat improved by reducing the dry matter content of the pigment color, i.e. by adding water into the color. As regards the distribution of the color, there has been observed a clear weakening when the dry matter content in the color is 58% or more. Therefore it has been attempted to maintain the dry matter content below 58%. However, in coating it should be advantageous to avoid bringing excessive water to the web, for instance because the water must in any case be dried off the web after coating. Likewise it is not desirable that the water-diluted coating color or the water contained in the color should penetrate into the web, where it can have harmful effects in the properties of the web.

In addition, it has been found out that the spray technique requires a base paper that is relatively smooth, because in coating that uses spray technique, the whole surface of the base paper is coated with an evenly thick color layer, which thus conforms to the possible irregularities of the base paper. So far the pigment employed in coating with spray technique has been mainly calcium carbonate, which in the color is present as small round particles. In spray coating, this kind of color does not smooth the irregularities of the base paper. In blade coating, the color layer is made even with the blade, and this smoothes the irregularities of the base paper surface.

The publications US 3,833,467 and US 6,248,407 specify coating methods where the coating particles are electrically charged, and the paper web is earthed. This prevents the clod formation of the coating particles. In the publication EP 258 016 there is described a method for coating material, in which method both the coating material and the material to be coated are electrically charged, and their electrical charges are opposite. In that case the purpose is to achieve sufficiently small coating particles.

The publication DE 199 50 009 specifies a method for remoistening paper or cloth after printing. In this method, the paper or cloth is electrically charged, and water is sprayed on the material by using the spray technique.

The object of the present invention is to achieve and improvement in the above described problems in coating by means of the spray technique.

Thus the object of the invention is for instance to realize a coating method based on the spray technique by which a good quality in the coated surface can be achieved.

The object is for instance to realize a high-quality coating method whereby the pigment color is evenly distributed in the base paper, without creating streaks.

The object is also to enable the coating of wood-contained paper qualities or paper qualities containing less chemical pulp fiber than before by applying the spray technique.

The object is further to enable the utilization of various shapes of pigment particles that smooth the base paper in coating that applies the spray technique.

Still the object is to realize a method for coating a paper web, said method being more economical and safer to realize than the methods known in the prior art. An object of the invention also is to create a method whereby the spraying pressure of the coating color in spraying, when using the spray technique, is lower than in the methods of the prior art. A particular object of the invention is to realize a method where the amount of mist created of color droplets is minimized, and where the color droplets are attached as well as possible to the web, and are not ricocheted back to the air.

In order to achieve the above described objects, the present invention is characterized by what is defined in the characterizing parts of the independent claims set forth below.

Thus a typical method according to the invention for coating a paper or paper board web by a coating color containing pigment particles by applying the spray technique includes at least the following successive steps:

(a) the web is electrically charged by conducting the web through an electrostatic field created between an electrode and a counterelectrode,

(b) the web that is electrically charged in step (a) is conducted to the coating station, and

(c) the electrically charged web that is conducted to the coating station is coated, at least on one side, by said coating color containing pigment particles and by applying said spray technique.

Now it has been surprisingly found out that by charging the web electrically, the coating color can be evenly distributed on nearly all kinds of base papers or base paper boards, also those with absorptive qualities, by using the spray technique.

Surprisingly, coating color can be discharged through spray nozzles at a pressure lower than in the prior art, typically at a pressure of 5 - 130 bar, most typically at a pressure of 10 - 30 bar. Several advantages can thus be achieved. A lower pressure decreases the amount of created mist. Owing to the lower pressure, the nozzles have better durability than when a higher pressure is used. In that case the color can also contain plate-like particles, which use the nozzles have not endured earlier. For this reason, savings can also be achieved in the investment and operation expenses of the equipment.

In coating with spray technique, earlier mainly calcium carbonate based pigment, such as GCC, i.e. ground or PCC, i.e. precipitated carbonate, has been used. Surprisingly, on an electrically charged base paper or base paper board, also coating colors containing plate-like pigments can be distributed by the spray technique. Earlier it was assumed that only round pigment particles could be used in coating applying the spray technique. Now it has been found out that when using plate-like pigment particles in coating with the spray technique, it is possible to achieve a quality that is even remarkably better than earlier.

A typical coating color when coating a mechanical printing paper, such as newsprint and LWC, where the content of mechanical pulp is great, now contains a pigment mixture having

70 - 90%, typically about 80% carbonate and

10 - 30%, typically about 20% plate-like pigment.

A typical coating color in the coating of supercalendered rotogravure paper respectively contains a pigment mixture having

30 - 50%, typically about 40% carbonate and

50 - 70%, typically about 60% plate-like pigment.

It seems that plate-like pigment particles also smooth irregularities of the base paper. It can be assumed that plate-like pigment particles, when hitting the electrically charged surface, are arranged on the surface on their face in the surface direction, thus forming a smoothing layer over the surface irregularities. Typically plate-like pigment particles are kaolin or talcum particles or a mixture of these.

When using the spray technique, the thickness of the coating can be easily adjusted by adjusting the amount of the color to be sprayed.

When applying the spray technique according to the invention, the web can be electrically charged in step (a) by conducting the web through a corona discharge device or another respective electrostatic field formed between an electrode and a counterelectrode. Advantageously the employed counterelectrode can be a surface made of some electroconductive material that in step (a) guides the passage of the web, said surface being for instance a hitch roll, for example a chromium-plated roll. Thus said guiding surface can be earthed in order to create a suitable charge in the web. The electrostatic field is advantageously directed to proceed, at least for the major part, at right angles to the web.

The web is electrically charged in step (a) by an electrode, by which there is typically generated a voltage of 60 kN or under 60 kN, most typically 30 kN or under 30 kN. In that case the electric current is typically 10 mA/m or under 10 mA/m, most typically 5 mA/m or under 5 mA/m.

When applying the spray technique according to the invention, the web is advantageously conducted, in a vertical position, from step (a) to step (c). Advantageously the web is conducted in a vertical position also through the coating station in step (c). Typically the hitch roll, in connection with which the web is electrically charged, is arranged on a higher level than the coating station proper, so that the web proceeds in an essentially vertical position downwardly from the hitch roll towards the coating station, and in a vertical position at least through the main part of the coating station. When so desired, the passage of the web can naturally be arranged in another direction.

The arrangement according to the invention can advantageously be used in coating stations where the web is coated simultaneously on both sides. When desired, the web can also be coated on one side only.

In coating applying the spray technique, there are used several spray nozzles arranged in a row across the web, typically so that the distance between the nozzles is 40 - 130 mm. When desired, several rows of nozzles can also be arranged on top of each other in the coating station. In that case the nozzles that are arranged in succession in the proceeding direction of the web are advantageously positioned in a somewhat overlapping manner. Advantageously the nozzles are earthed.

When applying the coating method based on the spray technique according to the invention, the dry matter content of the coating color can be higher than what has been possible in the prior art when applying the coating method. Consequently, the dry matter content of the coating color can be 58% or more, even 60% or more, but generally 70% or less. Thus the feeding of excessive water on the web is avoided, and savings are achieved in the drying expenses.

A positive electric charge of the base paper helps in distributing a color with a high dry matter content evenly over the whole web.

In addition, an electric charge in the web essentially decreases the creation of a mist cloud in the coating station. In ordinary spray coating, the very small droplets that tend to be separated from the color spray are in the solution according to the invention conducted, with the color jet, towards the web to be coated. Thus it can be assumed that even the edges of the color jet are not dried in a similar way as in earlier spray coating. Consequently also the streaks caused by drier color are avoided in coating.

The coating method based on the spray technique according to the invention is suited to be used in the coating of various different wood-containing paper qualities. The method can be used in coating for instance newsprint or LWC paper by pigment color. According to the invention, in manufacturing the paper to be coated, the content of chemical pulp fiber in the pulp can generally be reduced. Likewise, the method according to the invention is suited to be used in coating paper containing mainly chemical pulp fiber, for example paper containing 80 - 100% chemical pulp fiber, by pigment color.

The invention is described in more detail below with reference the appended drawings, where

Figure 1 schematically and by way of example illustrates a coating device applying the spray technique, where the web is electrically charged by the method according to the invention, and

Figure 2 illustrates a cross-section of the device of figure 1, along the line AA.

Figure 1 shows a coating device of the web 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 an earthed hitch roll 18 that guides the web and serves as a counterelectrode. The web 10 is arranged to pass through the electric field 20 created between the electrode 16 and the hitch roll 18, so that the web is electrically charged. The electric field is directed at right angles to the web. The electrically charged web 10' is brought as a vertical input to the coating station 14 located underneath the hitch roll 18.

In the coating station illustrated in figure 1, on both sides of the web 10', there are arranged two superimposed rows 22, 24 and 22', 24' of spray nozzles 26. When applying the arrangement according to the invention, the nozzles can also be arranged in one row only. Respectively, the nozzle row or rows can be arranged on one side of the web only. The nozzles are earthed. Through the nozzles, coating color jets 28 are sprayed towards the web 10'. The coating color jets 28 distribute the color evenly on the web 10'. From figure 2 it is seen that each nozzle row 22, 24 and 22', 24' includes several nozzles 26, and the mutual distances of said nozzles are arranged to be such that the jets sprayed through the nozzles in principle cover the whole web along the whole width thereof. In the separate superimposed rows the jets are arranged in a somewhat overlapping manner in order to ensure a complete coverage. Maybe the most important idea of this invention is to charge the paper or board web electrically on top of the hitch roll preceding the coating station, and to perform the spray coating thereafter, advantageously on both sides of the web simultaneously. Thus there is realized an extremely smooth coated surface that makes it possible to manufacture also gravure printing papers, where the paper smoothness is an important factor that affects the printability. The electrical charging of the web also essentially reduces the creation of color mist.

Several different advantages are achieved by the coating method according to the invention, when compared with the earlier coating, based on the spray technique, such as:

- by means of the arrangement according to the invention, it is possible to distribute the coating color, even a color with a relatively high dry matter content, evenly onto a web, so that a high-quality coated web surface is obtained, and that the web runability remains good; and

- by means of the arrangement according to the invention, any harmful formation of color mist is essentially reduced. This reduces the occurrence of smudging of the surfaces. Now it is not absolutely necessary to provide separate devices for removing color mist. An uncontrolled proceeding of the color into waste waters is essentially reduced. It is not necessary to cover the coating station in order to prevent the color mist from spreading, and neither is it necessary to provide the coating station with a powerful suction system in order to remove the mist. Likewise, there are not necessarily needed means for condensating the mist or for absorbing it into water. Thus the more limited atomization of the color also reduces the creation of waste waters. Consequently the coating station can be built in a space that is smaller than in the prior art, when the number of auxiliary devices earlier needed because of the color mist is reduced or even eliminated.

The coating based on the spray technique according to the invention is primarily meant to be used in coating paper qualities containing wood-contained fiber, because there the advantages to be achieved may possibly be the greatest. However, it has been found out that this coating method can advantageously be used in the manufacturing of fine grade paper, too.

By using the electrical charging of the base paper, it is possible to at least partly replace the ground carbonate color GCC or precipitated carbonate color PCC that were used in the earlier applications based on the spray technique by plate-like pigment, such as kaolin or talcum. By means of a kaolin or talcum addition, the coverage and printability of the coating can be improved, particularly in offset printing. A smooth paper coated by the spray technique can also be used in gravure printing.

The invention is described in more detail in the following example.

There was carried out an experiment where a paper web was electrically charged according to the invention described above, before the spray coating unit, by means of the device described in figure 1. This experiment is below called the experiment according to the invention. On the other hand, there was also carried out an experiment where the paper web was not electrically charged, and this experiment is below called the reference experiment.

In spray coating, there was used a color where the employed pigment was ground carbonate. The dry matter content in the color was 55%. The base paper was of the LWC quality, 52 g/m , and the amount of coating was 9 g/m . The web speed was 500 m/min, and the distance of the web from the spray nozzle was 90 mm.

It was found out that in the experiment according to the invention, the atomization of the color into the environment was clearly more limited than in the reference experiment. When observed by a light microscope, the paper coated in the experiment according to the invention was more homogeneous and smooth than the paper coated in the reference experiment. Likewise it was found out the shape of the spray jet in the experiment according to the invention was sharper than in the reference experiment.

The coated papers were measured as regards glare and smoothness. The results are shown in Table 1. Glare is given in percentages, and smoothness is PPS 10 smoothness in micrometers. Table 1

In the table it is seen that in the experiment according to the invention, there was obtained coated paper that had a smoother surface and a better glare than the paper obtained from the reference experiment.

The invention is not to be restricted to the above described embodiments only, but it can be applied widely within the scope of the appended claims.

Claims

1. A method for coating a paper or paper board web at least on one side of the web by a coating color containing pigment particles and by applying a spray technique, characterized in that the method comprises at least the following successive steps:

(a) the web is electrically charged by conducting the web through an electrostatic field that is created between an electrode and a counterelectrode,

(b) the web that was electrically charged in step (a) is conducted to the coating station, and

(c) the web conducted to the coating station is coated at least on one side by said coating color containing pigment particles by applying said spray technique.

2. 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 created between a corona discharge device and a counterelectrode.

3. A method according to claim 1 or 2, characterized in that the employed counterelectrode is a surface made of an electroconductive material that guides the passage of the web in step (a).

4. A method according to claim 1 or 2, characterized in that the employed counterelectrode is a hitch roll, typically an earthed hitch roll.

5. A method according to claim 1 or 2, characterized in that the direction of the electrostatic field in step (a) is at least for the major part perpendicular to the web.

6. A method according to claim 1, characterized in that the web is conducted in an essentially vertical position through the main part of step (b).

7. A method according to claim 1, characterized in that the web is conducted in an essentially vertical position through the main part of the coating station.

8. A method according to claim 1, characterized in that the web is electrically charged in step (a) by an electrode that generates a voltage that is below 60 kN, typically below 30 kV, and/or an electric current that is below 10 mA/m, typically below 5 mA/m.

9. A method according to claim 1, characterized in that the web is coated in the coating station on both sides of the web.

10. A method according to claim 1, characterized in that in the coating station, the web is coated by a coating color that is sprayed through spray nozzles at a pressure of 5 - 130 bar, typically at a pressure of 10 - 30 bar.

11. A method according to claim 1, characterized in that the dry matter content in the coating color is over 58%, typically over 60% but under 70 %.

12. A method according to claim 1, characterized in that in the coating station, there is over the web arranged at least one row of spray nozzles, wherein the mutual distance between the nozzles is 40 - 100 mm.

13. A method according to claim 12, characterized in that in the coating station, there are over the web arranged two or more superimposed rows of spray nozzles.

14. A method according to claim 1, characterized in that the spray nozzles are earthed.

15. A method according to any of the claims 1 - 14, for coating mechanical printing paper, such as newsprint or LWC paper, characterized in that the pigment color contains a pigment mixture comprising

70 - 90%, typically about 80%, carbonate and

10 - 30%, typically about 20%, plate-like pigment.

16. A method according to any of the claims 1 - 14 for coating gravure paper, characterized in that the pigment color contains a pigment mixture comprising

30 - 50%, typically about 40% carbonate and

50 - 70%, typically about 60% plate-like pigment.

17. A method according to claim 15 or 16, characterized in that the plate-like pigment particles mainly contain kaolin or talcum particles or a mixture thereof.

18. The use of a method according to any of the preceding claims 1 - 17 for coating wood-contained paper, such as newsprint or LWC paper, by pigment color.

19. The use of a method according to any of the preceding claims 1 - 17 for coating paper containing mainly chemical pulp fiber, such as a paper containing 80 - 100% chemical pulp fiber, by pigment color.

20. A coating color to be used for coating mechanical printing paper, such as newsprint or LWC paper, by using a method defined in any of the preceding claims 1 - 17, characterized in that the coating color contains a pigment mixture comprising

70 - 90%, typically about 80%, carbonate and

10 - 30%, typically about 20%, plate-like pigment material, typically kaolin or talcum or a mixture thereof.

21. A coating color for use for coating gravure printing paper by using a method defined in any of the preceding claims 1-17, characterized in that the coating color contains a pigment mixture comprising

30 - 50%, typically about 40% carbonate and

50 - 70%, typically about 60% plate-like pigment material, typically kaolin or talcum or a mixture thereof.

22. The use of paper or paper board coated by a method according to any of the preceding claims 1 - 17 in gravure printing.