WO2005095711A1

WO2005095711A1 - A method for making a coated fibrous web

Info

Publication number: WO2005095711A1
Application number: PCT/FI2005/050099
Authority: WO
Inventors: Jouni Haavisto; Timo Nurmiainen; Seppo Luomi; Timo Kiiha; Jukka Koskinen; Stefan Kuni; Rauno Rantanen
Original assignee: Metso Paper, Inc.
Priority date: 2004-03-30
Filing date: 2005-03-22
Publication date: 2005-10-13
Also published as: JP2007530811A; FI20045110A; DE112005000680T5; FI20045110A0

Abstract

The invention relates to a method for making a coated fibrous web, said method comprising manufacturing and coating a bottom web in at least one coating process. The method comprises coating the bottom web in said at least one coating process with curtain coating and the coated fibrous web has its surface subjected to a finishing treatment in at least one additional process by using an apparatus which follows as closely as possible the coated web's topography.

Description

A method for making a coated fibrous web

The present invention relates to a method for making a coated fibrous web. More specifically, it relates to making a coated fibrous web apt for various printing applications. One particular object of application is additionally a method for making a white top liner, as well as the white top liner produced by the method.

Printing on the surface of paper or board requires a certain surface finish for enabling a consistent transfer of printing ink. The surface finish is based on the properties of a bottom web and a coating. One important factor relating to surface finish is the employed coating method, the most relevant being blade coating in its various forms. In blade coating, the coating material is applied first by a desired application method to the surface of a fibrous web, followed by adjusting the amount of coating by means of a doctor blade or doctor bar. One problem in blade coating is that it requires a high feed rate of coating material and thereby also a large capacity circulation in the machine. In practice, this represents both high investment costs and high operating costs. Likewise, replacing a grade of fibrous web to be coated with another is time consuming and thereby undermines efficiency. The large amount of coating material in use also means that a large portion of the coating material applied to the surface of a fibrous web must be removed for a desired coating thickness. A large amount of coating material to be removed necessitates a high line pressure for a blade used in doctoring. This is in turn likely to cause breaks in the fibrous web and thereby degrades runnability of the machine.

Another general coating method used currently for coating a fibrous web comprises a film transfer coater. The film transfer coater applies a mechanical stress to a web and is thus likely to cause breaks, thus degrading runnability of the machine. Other coating methods used currently for coating a fibrous web include e.g. size press, spray, curtain and air brush coating.

The use of curtain coating in paper/board web coating is becoming more popular as the force applied to the web thereby is much less than what is used in blade/bar or film transfer coating, thus causing fewer interruptions as a result of breaks in a presently coated web and thereby improving runnability. However, curtain coating is not equal for example to blade coating in terms of obtained smoothness, but it provides a coverage which is better than that achieved by blade coating. The surface finish produced by curtain coating is not as such adequate for printing purposes as printing ink is transferred from a printing press onto the surface of a fibrous web. Curtain coating has mainly been used in the manufacture of certain special grades of paper, such as thermosensitive grades and photographic grades.

Curtain coaters can be divided in slot-fed or slide-fed coaters. In a slide-fed curtain coater, a coating is fed by means of a nozzle unit onto an inclined slide, along which the coating flows towards an edge of the slide, the curtain being established as the coating comes off the edge of the slide. In slot-fed application bars, the coating is pumped through a distribution chamber into a narrow vertical slot, the curtain being established on its lip and flowing onto a web. Depending on the composition of a coating, the curtain coating applied in a single action may contain one or more layers.

The width of a resulting curtain is defined by edge guides, which are located, as suggested by their name, at the edges of a feeding slot/feeding lip.

It is one objective of the present invention to provide a new method for making a coated fibrous web, said method comprising manufacturing and coating a bottom web in at least one coating process and said method allowing for the manufacture of fibrous webs apt for printing. In order to achieve this objective, a method of the invention is characterized in that the method comprises treating the bottom web in the drying section of a manufacturing process with a Yankee cylinder or with a dryer based on the use of a metal belt and/or pretreating the bottom web by precalendering and/or precoating prior to coating the bottom web in said at least one coating process; that the bottom web is coated in said at least one coating process by curtain coating; and that the coated fibrous web has its surface subjected to finishing in at least one additional process with an apparatus selected from the group consisting of: a soft calender, a belt calender, a metal belt calender, a supercalender, a multi-roll calender with polymer rolls, a shoe calender, and a brush polisher. The finishing is performed on a dried coating. The prior art white top liner consists of an unbleached bottom layer, which can be made of virgin stock or recycled pulp, and of a top layer of bleached chemical pulp. The top layer has typically a basis weight of 30-60 g/m², which provides a sufficient coverage for a dark bottom layer. In order to upgrade the printing and other qualities of a surface, the basis weight must be within the range of 70-80 g/m². The process of making a separate layer requires a separate headbox in the machine and a separate pulp circulation and pulp making process for the pulp to be fed therein. In addition, increasing the thickness of body stock in the wet end has a direct impact on the production capacity of a machine by impeding drainage and thereby reducing output.

Accordingly, it is one particular object of the present invention to provide a new method for making a white top liner, enabling an increase of production capacity and a substantial cost saving. In order to achieve this objective, a method of the invention is characterized in that the method comprises forming at least a substantial portion of the white top layer from a single- or multilayer coating material applied on top of the bottom layer by a curtain coating method. A white top liner of the invention is in turn characterized in that at least a substantial portion of the white top layer consists of a single- or multilayer coating material applied on top of the bottom layer by a curtain coating method.

By replacing, according to the invention, a layer of bleached chemical pulp with a layer of coating color for a top layer of white top liner, the process can be shifted to the dry end, whereby the velocity potential of a board machine increases in the wet end provides an increase in capacity. The price of coating color is only about half or less than that of the price of bleached chemical pulp, whereby the inventive solution offers considerable cost savings as well. In addition, this obviates the need of setting up a separate headbox for forming a top layer, along with a pulp circulation associated therewith.

Curtain coating allows for the application of high quantities of coating in a single coating position, especially in a multilayer curtain coating position. Curtain coating is also capable of providing an excellent coverage. The composition of coating created in curtain coating follows very precisely the topographic structure of a product being coated. It has been discovered in the invention that, by performing on a curtain- coated fibrous web a finishing treatment with an apparatus capable of following as closely as possible the topographic structure of the coated web, the result will be a surface gloss and smoothness qualified for printing. In addition, the finishing treatment can be used for reducing the size of pores present in the coating and for homogenizing the distribution thereof, i.e. to have pore sizes approaching each other. It is further desirable, prior to curtain coating, to prepare a bottom web to be coated for good smoothness in terms of its topography in order to provide good printing qualities for the surface after coating and finishing.

The finishing apparatus comprises preferably a calender, which may have one or more nips or other surface treating elements and which may have one or more heated bearing surface in the above-mentioned calendering nip, zone or element. Said heated bearing surface may comprise e.g. a roll or a belt. The calender can be selected for example from the following group consisting of: a soft calender, a belt calender, a metal belt calender, a supercalender, a multi-roll calender with polymer rolls, and a shoe calender. The calendering conditions are preferably as follows: temperature is within the range of about 50 to about 400°C and nip pressure within the range of about 2 to about 600 kN/m.

Using a brush polisher as the finishing apparatus makes an effective counter-action against mottling caused by repolishing in easily polished coatings. The brush polisher is provided with brush rolls delivering an elastic pressure on the surface of a presently treated fibrous web and additionally there is a comparatively high relative speed between the brush rolls and the fibrous web. This combination of pressure and speed provides high-level smoothness and polish rates without a loss of bulk.

The method may further involve the use of moistening and/or steam-treating a fibrous web to be coated before or during the pretreatment process, and a coated fibrous web can be moistened and/or steam-treated on its uncoated side before or during the finishing process. The qualities of a bottom web can be influenced for example by optimizing a stock composition and/or providing the bottom web with a laminar structure by using a laminating headbox. The stock composition is preferably obtained by using mechanical pulp, chemical pulp and/or more than about 25% deinked recycled pulp (DIP). Regarding the surface of a bottom web, the objective is a topography as flat as possible and an appropriate pore structure, which is accomplished, for example, by supplementing the bottom web's stock composition with a fine filler such that its portion at least in the top layer exceeds about 10%. With a laminating headbox this is achieved by supplementing the top-layer forming material with the required amount of a filler-concentration increasing constituent or some other suitable finely divided material. In the event of using a headbox which only produces a single layer, the proportion of fine filler in the entire stock composition can be raised to a rate which exceeds about 10%.

In addition to or instead of modifications to stock composition, the qualities of a bottom web surface can also be upgraded by subjecting the bottom web to appropriate pretreatment processes, such as e.g. precalendering and/or precoating. The employed precalender can be e.g. a hard-nip calender, a soft calender, a shoe calender, a belt calender, a metal belt calender, a WetStack calender, a supercalender or a multi-roll calender, e.g. a multi-roll calender equipped with polymer rolls. In precalendering, the process conditions can be selected e.g. as follows: temperature within the range of about 50-200°C and nip pressure within the range of about 10-400 kl\l/m. This results in fibers plasticizing during precalendering and the structure of a fibrous web is made as stable as possible while preserving the bulk of a fibrous web.

Precoating can be performed by using e.g. blade coating, film coating, spray coating or curtain coating. It is preferred that curtain coating be effected with a bottom web having a smoother surface than those used in blade coating or other currently employed coating methods.

Hence, according to the invention, it is an objective of the pretreatment to provide a high large-scale smoothness (low Bendtsen roughness), the Bendtsen roughness number (SCAN-P21-.67) being preferably about 0 to about 200 ml/min, while the goal in currently employed coating methods is typically a Bendtsen roughness number of about 250 ml/min prior to the actual coating process. The pretreatment is used in an effort to provide the PPS slO roughness (SCAN-P 76:95) value of about 0 to about 2,0 μm for the surface of a bottom web, while the goal in currently employed coating methods is typically a value of about 3-3,5 μm in order to avoid problems in the spreading of a coating material during the actual coating process.

Precalendering and precoating can be performed in one and the same process or in a desired relative sequence, e.g. precalendering first and then precoating or vice versa.

The topographic flatness of a bottom web surface can also be influenced according to the invention by using in the drying section of a bottom web manufacturing process a Yankee cylinder or a dryer based on the use of a metal belt, e.g. a Condebelt dryer or a metal-belt calender type dryer so as to provide the surface with porosity and smoothness qualities as favourable as possible prior to eventual pretreatment processes of the surface (precalendering and/or precoating) or prior to coating the surface. Integrating the use of a Yankee cylinder or a metal belt dryer in the drying section with a pretreatment of the surface of a bottom web prior to the actual coating process makes it possible to achieve extremely high smoothness values for the bottom web surface (Bendtsen roughness about 0-100 ml/min and PPS s-10 roughness about 0-1 μm).

Claims

1. A method for making a coated fibrous web, said method comprising manufacturing and coating a bottom web in at least one coating process, characterized in that the method comprises treating the bottom web in the drying section of a manufacturing process with a Yankee cylinder or with a dryer based on the use of a metal belt and/or pretreating the bottom web by precalendering and/or precoating prior to coating the bottom web in said at least one coating process; that the bottom web is coated in said at least one coating process by curtain coating; and that the coated fibrous web has its surface subjected to finishing in at least one additional process with an apparatus selected from the group consisting of: a soft calender, a belt calender, a metal belt calender, a supercalender, a multi- roll calender with polymer rolls, a shoe calender, and a brush polisher.

2. A method as set forth in claim 1, characterized in that the finishing is performed within the temperature range of about 50 to about 400°C with a nip pressure within the range of about 2 to about 600 kl\l/m.

3. A method as set forth in claim 1 or 2, characterized in that the coated fibrous web has its uncoated side moistened and/or steam-treated prior to or during the finishing process.

4. A method as set forth in any of claims 1-3, characterized in that the method comprises using mechanical pulp and/or chemical pulp and/or deinked recycled pulp (DIP) for the bottom web's pulp composition in excess of about 25%.

5. A method as set forth in any of claims 1-4, characterized in that the bottom web has more than 10% of fine filler in its top layer for providing the surface with a topography as smooth as possible.

6. A method as set forth in claim 1, wherein the bottom web is pretreated, characterized in that the method comprises performing the pretreatment with a calender selected from the group consisting of: a hard-nip calender, a soft calender, a shoe calender, a belt calender, a metal belt calender, a WetStack calender, a supercalender or a multi-roll calender, e.g. a multi-roll calender provided with polymer rolls.

7. A method as set forth in claim 6, characterized in that the precalendering is performed within the temperature range of about 50-200°C with a nip pressure within the range of about 10-400 kN/m.

8. A method as set forth in claim 1, wherein the bottom web is pretreated, characterized in that the fibrous web is moistened and/or steam-treated prior to or during the pretreatment process.

9. A method as set forth in claim 1, wherein the bottom web is pretreated, characterized in that the pretreatment is performed by precoating and that the precoating is effected by using blade coating, film transfer coating, spray coating or curtain coating.

10. A method as set forth in claim 9, characterized in that the precoating provides the bottom web's surface with a Bendtsen roughness number (SCAN-P21:67) of about 0 to about 200 ml/min.

11. A method as set forth in claim 9 or 10, characterized in that the precoating provides the bottom web's surface with a PPS slO roughness number (SCAN-P 76:95) of about 0 to about 2,0 μm.

12. A method for making white top liner, which consists of a bottom layer and a white top layer, characterized in that the method comprises forming at least a substantial portion of the white top layer from a single layer or multilayer coating material applied on top of the bottom layer by a curtain coating method, and that the method comprises treating the bottom web in the drying section of a manufacturing process with a Yankee cylinder or with a dryer based on the use of a metal belt and/or pretreating the bottom web by precalendering and/or precoating prior to coating the bottom web by a curtain coating method.

13. A method as set forth in claim 12, characterized in that the white top layer is formed entirely from a coating material.

14. A white top liner, consisting of a bottom layer and a white top layer, characterized in that at least a substantial portion of the white top layer consists of a single- or multilayer coating material applied on top of the bottom layer by a curtain coating method.