EP1841917B1

EP1841917B1 - Supporting arrangement in the curtain coating of a paper/board web

Info

Publication number: EP1841917B1
Application number: EP05813209A
Authority: EP
Inventors: Heikki Vatanen; Jukka Koskinen; Timo Nurmiainen
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2005-01-27
Filing date: 2005-12-02
Publication date: 2013-02-13
Anticipated expiration: 2025-12-02
Also published as: FI20055040A0; FI117175B; JP4677456B2; WO2006079679A1; EP1841917A1; EP1841917A4; JP2008527186A

Description

The present invention relates to a web supporting arrangement in the curtain coating of a paper/board web, wherein a web to be coated is conveyed through a curtain coating station, at which a coating curtain flowing down from an application device proceeds onto the web surface to form a single- or multilayer coating thereon.
Curtain coaters can be categorized in slot-fed and slide-fed coaters. In the slide-fed curtain coater, a coating is fed by means of a nozzle assembly onto an inclined plane, the coating flowing down towards an edge of the plane constituting a feeding lip, the curtain forming as the coating falls off the feeding lip. In slot-fed application beams, a coating is pumped by way of a distribution chamber into a narrow vertical slot, the curtain forming at its lip and falling onto a web. A coating can be applied in one or more layers. The resulting curtain is maintained at a desired width by means of an edge guide which, as suggested by its name, is located along the edge of a feeding slot/feeding lip.
A multitude of various grades of paper and board are in existence and can be sorted by basis weight in two classes: papers of a single ply and with a basis weight of 25-300 g/m<2> and boards made in multiply technique and with a basis weight of 150-600 g/m<2>. As noted, there is a floating line between paper and board, since boards of the lightest basis weight are lighter than the heaviest papers. Generally, paper is used for printing and board for packaging. Papers and boards can be coated or uncoated.
The following descriptions are examples of presently used values for fibrous webs to be coated and substantial deviations from the given values may occur. The descriptions are mainly based on the source publication Papermaking Science and Technology, .
Coated magazine paper (LWC = light weight coated) contains mechanical pulp at 40-60%, bleached softwood pulp at 25-40% and fillers and coatings at 20-35%. General values for LWC-paper can considered as follows: basis weight 40-70 g/m², Hunter gloss 50-65%, PPS S10 roughness 0.8-1.5 µm (offset) and 0.6-1.0 µm (roto), density 1100-1250 kg/m³, brightness 70-75% and opacity 89-94%.
General values for MFC-paper (machine finished coated) can be considered as follows: basis weight 50-70 g/m², Hunter gloss 25-70%, PPS S10 roughness 2.2-2.8 µm, density 900-950 kg/m³, brightness 70-75% and opacity 91-95%.
General values for FCO-paper (film coated offset) can be considered as follows: basis weight 40-70 g/m², Hunter gloss 45-55%, PPS S10 roughness 1.5-2.0 µm, density 1000-1050 kg/m³, brightness 70-75% and opacity 91-95%.
General values for MWC-paper (medium weight coated) can be considered as follows: basis weight 70-90 g/m², Hunter gloss 65-75%, PPS S10 roughness 0.6-1.0 µm, density 1150-1250 kg/m³, brightness 70-75% and opacity 89-94%.
HWC (heavy weight coated) has a basis weight of 100-135 g/m² and can be coated even more than twice.
In coated pulp-based woodfree printing papers or fine papers (WFC), the amounts of coating vary to a great extent according to demands and intended use. The following are typical values for once and twice coated pulp-based printing paper: once coated basis weight 90 g/m², Hunter gloss 65-80%, PPS s10 roughness 0.75-2.2 µm, brightness 80-88% and opacity 91-94%, and for twice coated, basis weight 130 g/m<2>, Hunter gloss 70-80%, PPS SIO roughness 0.65-0.95 [mu]m, brightness 83-90% and opacity 95-97%.
Boards make up quite a heterogeneous group, including high-basis weight grades with a basis weight as high as 500 g/m<2> and low-basis weight grades with a basis weight of about 120 g/m<2>, the grades ranging from those based on primary fiber to those based by 100% on recycled fiber and from uncoated ones to those with multiple coating. Coated boards include the following:

primary-fber based (folding board (FBB = folding boxboard), bleached pulp board (SBS = solid bleached board), liquid packaging board (LPB = liquid packaging board), coated white top liner, carrier board)
recycled-fiber based (white lined chipboard (WLC = white lined chipboard), coated recycled board).

Curtain coating is suitable for making basically all the above-mentioned coatable grades. Curtain coating does not provide a smoothness equal to what is achieved by blade coating, but it is better than blade coating in terms of coverage.
The document EP 1 676 957 A2 discloses a device for applying a liquid to pasty medium to one side of a moving paper, board or other fibrous web during its production and/or finishing. The device has a curtain applicator which discharges the application medium as a free-fall, self-contained curtain which moves substantially under the force of gravity directly onto one web side or coating side of the moving fibrous web. The device, moreover, comprises a collecting device being provided for collecting application medium in the event of a break in the fibrous web and/or for discharged side edges of the curtain during the operation phase of the applicator.
From document WO 2004/027151 A1 a method for changing the running conditions of a curtain coater is known. In the method, the impact angle between a plane passing congruently with the direction of travel of a track comprised of a paper/board web and a plane passing congruently with the direction of travel of a coating curtain hitting the impact point of the paper/board web is changed. The paper/board web is supported by underneath support means from below at a point following the impact point. To change the impact angle, the direction of travel of the track is changed within the area surrounding the impact point by adjusting the location of the underneath support means.
One problem in curtain coating is to provide a support for the web within a coating curtain impact zone for providing the web with a coating layer which is as consistent as possible in terms of quality. In prior art solutions, the web support over the course of a coating process has been typically implemented in two different techniques: the support is provided in line with a coating curtain impact point or the web is provided with a support by means of supporting elements placed on either side of the impact point. In practice, when coating is effected in line with a support, the employed supporting element is obviously a roll. A wide and/or high-speed web calls for the use of a large-diameter roll, which is a bulky solution. In addition, a large rotating roll brings along lots of air to the application zone and the angle adjustment of an impact point is not possible in practice. When coating is applied to an unsupported web over the area between supporting rolls placed on either side of the impact point, the air supplied by the rolls is not usually a major problem, but in certain situations the web vibration may become a problem. Furthermore, this solution is likewise bulky.
It is an objective of the invention to provide a solution capable of essentially eliminating the foregoing problems of the prior art. In order to accomplish this objective, a web supporting arrangement of the invention is provided that comprises supporting elements placed on a side of the web opposite to the surface of the web which is to be coated, including a relatively small diameter backing roll which is arranged downstream of an impact point and a bearing element which is equipped with a sliding surface and located upstream of the roll. The bearing element is adjustable for supporting the web between a first position upstream of the impact point, whereby the coating is effected on the web being unsupported by the bearing element, and a second position in line with the impact point, whereby the coating is effected on the web being supported by the bearing element.
The inventive solution enables performing the coating either on a supported web or on an unsupported web, as desired. Further benefits of the solution include a compact size and elimination of vibrations resulting from the use of large rolls, as well as less powerful air flows in the vicinity of a coating curtain. In the event of web breaks, the web is not able to stuff itself between the sliding surface and a boundary air remover, which can easily happen when using a roll.
The backing roll position is preferably adapted to be adjustable in such a way that the impact angle of a curtain relative to the web is variable by shifting the roll's position. The sliding surface has its tip preferably adapted to be adjustable to various angular positions, whereby, when coating is effected on a supported web, the impact angle can be varied by modifying the tip's angular position and, if necessary, also by repositioning the roll.
The invention will now be described with reference to the accompanying drawings, in which:

Fig. 1: shows schematically one embodiment of the invention with the coating effected on a supported web.

Fig. 1 illustrates one embodiment of the invention, in which a coating curtain 3 flowing down from a nozzle assembly's 1 feeding lip 2 is adapted to strike on the surface of a web W at a location in which the web lies on top of a supporting surface. Reference numeral 4 represents an arrangement for removing boundary air carried along with the web W.
The inventive web supporting arrangement comprises a relatively small-diameter roll 6, which is adapted to support the web W downstream of an impact point 12. The roll 6 is adapted to be set at various positions, thus allowing, for example, the modification of an impact angle between the web W and the coating curtain 3. The supporting arrangement additionally comprises an adjustable bearing element designated by reference numeral 5, which is mounted on a beam member 11 extending in a cross-web direction. The bearing element 5 is provided with sliding means 7, 8 and is pivotable by means of the beam 11 for displacing the sliding means 7, 8 in the direction indicated by an arrow 13. The sliding means 7, 8 are preferably designed for an arcuate exterior as regards the sliding surfaces thereof in contact with the underside of the web. When viewed in the web proceeding direction, the sliding means may also consist of a single-piece block or more than two pieces. A tip portion 8 is preferably adapted to pivot as indicated by an arrow 12 for altering the proceeding direction of the web W in view of modifying an impact angle between the web and the coating curtain as desired. At this time, if necessary, the roll 6 can also be shifted as called for by the required adjustment of an impact angle. Between the web-contacting sliding surface of the sliding means 7, 8 and the underside of the web W is preferably provided a fluid lubrication, for example by means of air or another gas, for reducing friction and/or impeding web dusting. Alternatively or in addition to this fluid lubrication, the sliding surface is preferably designed for low friction and/or wear resistance by using a friction reducing and/or wear resistance enhancing coating or surface treatment or by using a wear resistant material, such as e.g. a ceramic material or stainless steel, for making a sliding-surface containing sliding element or a section thereof. The sliding element may consist of a base member made e.g. in stainless steel, having integrated therewith a sliding-surface containing element of a wear resistant material. The sliding means have the tip portion 8 thereof adapted to pivot about a fulcrum 9, e.g. by means of a spring-loaded adjustment element 10 shown in the illustrated embodiment. This spring-loaded adjustment element allows the tip portion 8 to flex as necessary, e.g. in response to shocks caused by possible web breaks.
The distance of the sliding means 7, 8 relative to the boundary air removing equipment 4 is preferably adapted to be adjustable, such that a gap between the coatable web and the boundary air removing equipment can be adjusted to a desired size.

Claims

A web supporting arrangement in the curtain coating of a paper/board web, wherein the arrangement comprises supporting elements placed on a side of the web opposite to the surface of the web which is to be coated, including a relatively small diameter backing roll (6) being arranged downstream of an impact point (12) and a bearing element (5), being equipped with sliding means (7, 8) and located upstream of the roll (6), characterized that the bearing element (5) is adjustable for supporting the web (W) between a first position upstream of the impact point (12), whereby the coating is effected on the web being unsupported by the bearing element (5) and a second position in line with the impact point (12), whereby the coating is effected on the web being supported by the bearing element (5).
The web supporting arrangement as set forth in claim 1, characterized in that the sliding means (7, 8) have a sliding surface which is low in friction and/or wear resistant.
The web supporting arrangement as set forth in claim 2, characterized in that the sliding surface is designed for low friction and/or wear resistance by means of a coating or surface treatment capable of reducing friction and/or enhancing wear resistance.
The web supporting arrangement as set forth in claim 2, characterized in that the sliding-surface containing sliding element (7, 8) or a portion thereof is made in a wear resistant material.
The web supporting arrangement as set forth in claim 4, characterized in that the material comprises a ceramic material.
The web supporting arrangement as set forth in any of claims 1 - 5, characterized in that between the sliding surface of the sliding means (7, 8) and the underside of the web (W) is provided a fluid lubrication for reducing friction and/or impeding web dusting.
The web supporting arrangement as set forth in claim 6, characterized in that the fluid comprises air or another gas.
The web supporting arrangement as set forth in any of the preceding claims, characterized in that the sliding means are provided with a flexible tip portion (8).
The web supporting arrangement as set forth in any of the preceding claims, characterized in that the sliding means (7, 8) have the sliding surface thereof adjustable for controlling a gap between the web and boundary air removing equipment located above the web upstream of an impact point.
The web supporting arrangement as set forth in any of the preceding claims, characterized in that the backing roll (6) has its position adjustable for varying the impact angle of the curtain.