WO2008040850A1

WO2008040850A1 - Method and arrangement in connection with curtain coating a fibrous web

Info

Publication number: WO2008040850A1
Application number: PCT/FI2007/050534
Authority: WO
Inventors: Topi Tynkkynen; Timo Nurmiainen
Original assignee: Metso Paper, Inc.
Priority date: 2006-10-03
Filing date: 2007-10-02
Publication date: 2008-04-10
Also published as: FI118925B; FI20065632A0

Abstract

The invention relates to a method in connection with curtain coating a fibrous web (W), said method comprising applying in a processing line (1) to at least one side of the fibrous web (W) at least one layer of coating agent by a curtain coating method. The method comprises adjusting a position of the fibrous web (W) in a cross-direction (CD) of the processing line (1) by means of guide elements (4) arranged for the fibrous web (W) for controlling a relative position between the fibrous web (W) and a coating curtain (22) from a curtain coater (20) in the cross- direction (CD) of the processing line (1). The invention relates also to an arrangement for implementing the method.

Description

Method and arrangement in connection with curtain coating a fibrous web

The invention relates to a method in connection with curtain coating a fibrous web. The invention relates also to an arrangement for implementing the method.

A common practice in curtain coating is so-called dry edge coating, wherein the coating applied to a fibrous web is excluded from the very edge zones of the web. The edge zones of a fibrous web not to be coated are typically in the order of 2-3 cm. The placement of a coating agent onto this edge zone, and most particularly even completely outside the web, must be absolutely avoided. The placement of a coating outside the intended coating area leads to an unnecessary increase in the amount of broke and also to a loss of raw materials and, especially, expensive coating agents. Admittedly, coating recovery equipment can be provided for the portion of coating placed outside the intended coating area or drifting even outside the web. Especially in multiply curtain coating, however, this coating material cannot be reused again in a coating process but, at best, it can only be recycled for some other appropriate application.

If a coating drifts beyond the edge of a fibrous web, there is another hazard of the coating agent working its way at the same time onto the reverse side of a web, causing both sealing of the reel and soiling of the entire roll assembly in a coating machine. This leads to a shutdown of the entire machine and to a tedious cleaning operation. Likewise, the rolls of a calender frequently in service on the same production line run a risk of becoming soiled in case the coating applied by curtain coating ends up in an edge zone or even completely outside the web and at the same time also on the reverse side of the web.

Another problem with the calender is that, if the coated area of a web, in other words the coating, drifts inside the confines of a calender's edge chamfers, the coated web strips lying within the confines of the chamfers shall have a thickness which clearly exceeds that of the actually calendered web. This may cause damage, especially to heated rolls. The roll develops spots from which heat does not conduct effectively enough. The roll heats up locally, i.e. develops a so-called "heat band". At the same time, the roll expands and the unevenly distributed and overgrown local stress may break the entire roll. Likewise, in the event that a web presently in the application process shifts in a cross-direction from the curtain, the web may develop pleat formations and cause a break in the coating process and a shutdown of the machine.

Admittedly, it is basically possible to work on the lateral dimensions of a coating curtain also by means of edge guides that can be employed in connection with a coating curtain. However, the main duty of an edge guide is to support and stabilize the curtain, not being practical in terms of active and high-speed adjustment of the width of a coating curtain.

It is an object of the invention to address the above problems, as well as to enable the flawless operation of a curtain coating process. A method of the invention is characterized by what is presented in the characterizing part of claim 1. The characterizing features for an arrangement of the invention are in turn presented in the characterizing section of claim 8. Preferred embodiments for the invention are disclosed in the dependent claims.

Thus, according to the invention, there is provided a lateral position adjustment for a fibrous web - hereinafter a web - such as to enable maintaining a relative position between the web and a curtain coater as desired and as constant as possible for preventing a relative movement therebetween from hampering the actual application process and for preventing the applied area from developing undesired changes with respect to the web. In the case of curtain coating, and especially dry edge coating, it has been found highly beneficial that the web be set to travel in a position as constant and central as possible with respect to the coater and hence, particularly, with respect to a coating curtain delivered thereby.

Thus, a method according to the invention enables minimizing a positional fluctuation of the outer edges of a coating with respect to the edges of a web and thereby reducing a risk of the coating drifting into the edge zones and further completely outside the web. At the same time, this offers a possibility of reducing the uncoated edge zone and reducing a loss of material resulting from an uncoated web. A method of the invention enables reducing the size of an uncoated edge zone by as much as to the order of 5 mm or less. In practice, the adjustment is preferably performed by adjusting the position of a web with respect to the center axis of an entire production line, for example a coating line or an actual coating machine, most preferably by aligning the web with respect to the latter. The adjustment of a web position is feasible in practice by means of prior art equipment as well as methods for guiding a web, such as by means of one or more guide rolls or the like or a combination thereof.

The invention will now be described more closely with reference to the drawing which depicts two preferred embodiments of the invention and in which:

Rg. 1 shows in a highly diagrammatic view, obliquely from above, an arrangement of the invention in connection with a slide-fed curtain coater,

Rg. 2 shows in a schematic sectional view, described in an earlier Finnish patent No. 117176, the area at one edge of a nozzle beam.

In reference to fig. 1, there is diagrammatically shown a section of a processing line 1 for a fibrous web W, i.e. e.g. for a web of paper or board, e.g. a section of a paper or board machine, a coating line or some other machine intended for web processing, which can be either an off-line or an on-line machine. Therein, the web W is adapted to advance in the direction of an arrow V to pass under a curtain coater 20, by means of which a coating curtain 22 formed on a coating platform, in other words in this case on an applicator beam's runoff slide 28, is applied to the web surface. In addition to a slide-fed curtain coater shown in fig. 1, the present invention is naturally apt for use also in connection with a slot-fed curtain coater. The application is followed by running the web further to drying or other finishing equipment 32.

Especially in dry edge coating, it has been found important during the course of curtain coating to avoid a displacement of the coating curtain 22 and the web W relative to each other in a cross-direction CD of the web. This embodiment relates specifically to dry edge coating, wherein narrow strips 12 without coating are left on either side of a coating 14 along an edge zone of the web. In the prior art solution, the width of this edge zone 12 is typically in the order of at least 2-3 cm. For the sake of illustration, a width 10a, 10b of the edge zones 12 is clearly exaggerated in fig. 1. The arrangement according to the invention is naturally also apt for a so- called over-the-edge coating process, wherein the coating curtain extends beyond the edges of a web.

Possible changes occurring in the width of a web during the course of a run, as well as lateral movements of a web with respect to an adjacent coater, are nevertheless capable of causing displacement of the coating curtain 22 and the web W relative to each other in the cross-machine direction CD. This results in a hazard of the coating being applied on edge zones intended to remain uncoated and hence too close to either one or both edges of the web. In the worst case, some coating finds its way outside the web and at the same time easily also onto the reverse side of the web. As described, these situations lead to the foregoing problems and to a shutdown of the entire line.

According to the invention, in view of guiding the web W in a desired manner with respect to the coating curtain 22, there are now guide elements 4 provided in association with the coater 20. An adjustment carried out with the guide elements 4 is used to make sure that changes occurring either in the size or position of a web or possibly in the coating curtain 22 are unable to cause a delivery of the coating into the web's edge zone 12 or generally an excessive fluctuation in a lateral position of the web's coated portion.

In this context, the expression providing elements in association with the coater 20 is mainly used in reference to the fact that the guide elements are located, at least for the most part thereof, within the boundaries of the coater 20. In other words, such elements are located in the section downstream of a processing device 31 establishing a contact with the web and, on the other hand, in the section upstream of a next processing device located downstream of the coater. Consequently, an adjustment of the invention, at least for an essential part thereof, is performed within the section between a fibrous web attachment point 31 preceding the curtain coater 20 and an attachment point 32 following the curtain coater. For assisted guidance, it is of course possible to use elements located also in other parts of the processing line 1 for influencing the position of a web either directly or else indirectly by influencing other properties of the web, such as having a desired effect on the web's tendency to align itself.

The elements for guiding a web may comprise the use of prior art equipment, such as e.g. traditional web guides, for example one or more guide rolls. The example of fig. 1 comprises a guide roll 4 disposed upstream of a coater 20 in the running direction of a web. It is also conceivable that guide elements for guiding a web be disposed on both sides of the coater 20, such as a device 4b indicated by a dashed line, or that web guide elements be disposed in a direct contact or integrally with the coater, such as a device 4c indicated by a dashed line.

What is essential is that the elements 4 for guiding a web be disposed in association with the coater 20 at least in such a way that said elements will be located downstream of a so-called web supporting point or attachment point preceding the coater 20 and, on the other hand, upstream of a web supporting or attachment point following the coater. Here, the term supporting and attachment points are used in reference to a contact point of the web or a point at which a free displacement of the web in a crosswise direction without outside assistance is essentially impeded. Such a supporting point can be established e.g. by nips or other web supporting or pressing devices in a momentary firm contact with the web. In fig. 1, these devices or points are indicated by reference numerals 31 and 32. However, it should be specifically emphasized that the above-mentioned supporting or attachment point does not refer e.g. to such a roll or the like guide element, which supports or guides a web and thereby establishes a support point for the web and the main purpose of which is thus to support a web or its running, and which do not particularly impede a possible displacement or transfer of the web in a cross-machine direction. In addition, the present invention is applicable in connection with curtain coating performed both for a supported and an unsupported web. Hence, neither in the course of a supported web application method do guide rolls or the like intended for supporting a web necessarily refer to the above-mentioned web attachment points.

In addition to guide rolls mentioned by way of example, it is conceivable to employ a wide range of various methods and devices for guiding a web in a manner according to the invention. Other possible guide elements for guiding a web include for example a leading roll, especially an unwinder in a conical configuration provided with a crosswise manipulation or swiveling feature, an application roll and feasible changes in its friction properties or providing a vacuum in association therewith. It is generally plausible to mention various elements and methods of the prior art for establishing a crosswise draw in the paper web. It is conceivable to use an inequal draw for the ends of a wire loop in contact with paper, an asymmetric tension profile for a web, changing the orientation profile of paper with a headbox, asymmetric drying or wetting (with a calender, a press) etc. Thus, the invention is by no means limited to the use of just one certain web guiding device or method. In a method of the invention, it is also possible to employ both active and passive devices or elements for guiding a web and the operation may proceed both manually and, as particularly preferred, in an automated fashion. Preferably, at least some of the devices or elements are nevertheless active for providing an adjustment result as speedy and precise as possible. In particular, this is achieved with measuring instruments like those indicated by reference numeral 8 for confirming both individual and established relative positions of a web and a coating and for determining necessary adjustment procedures.

By controlling a crosswise CD position of the web W and, if necessary, also its width in a desired manner, the edge zone 12 remaining on either side of the web can be kept more constant and in a desired size. Although the objective is indeed to explicitly coincide in a desired manner the relative positions of the web W and the coating curtain 22 of a curtain coater in the cross-direction CD of the processing line 1, the adjustment of a web position can be implemented in practice with respect to other checkpoints. An objective is, for example, to place the web in a symmetrical alignment relative to the center axis of a coater and/or a coating curtain and/or, as particularly preferred, relative to a center axis 26 of the entire machine or in this case the processing line 1. Performing a web adjustment in a manner centered explicitly relative to the center axis 26 provides the additional benefit in that the edge guards of a curtain coater can also be likewise adjusted relative to the center axis of a machine and thereby the entire adjustment arrangement can be simplified.

Thus, in this context, the term center axis is used in reference to each machine- specifically determined median, extending in a main machine direction lengthwise relative to a cross-machine direction. Generally, the center axis settles in the middle of a treated web, i.e. is located in the middle of a running line set up for the web itself. It should be noted that in this respect the web adjustment need not be necessarily performed by using the very center axis of a machine as a reference point, but the adjustment can also be effected by means of some other reference point and also at the time when the web is in the process of having itself aligned relative to the center axis.

Like a positional adjustment of the web itself, it is also possible to effect a positional adjustment of the coater 20 and further the coating curtain 22 by adjusting it the same way relative to the center axis 26 of a machine, preferably by aligning it relative to the center axis 26 of a processing line. The adjustment is carried out for example in such a way that the center axes 26 of the device 20 and the line 1 are adjustable to extend in alignment with each other.

On the other hand, the relative alignment of a web and a coating curtain can also be effected for example by setting up either the right or the left edge of both the web and the coating curtain relative to each other, as desired. Thus, the adjustment in terms of the other edge can be effected for example by way of controlling the width of both a web and a curtain.

Assistance in the guidance of a web can be provided by instruments 8, which measure both a position of the web and, if necessary, also a width of the web or e.g. established relative positions of the web and the coating, and which are connected to the web guide elements 4. A disposition of the elements 4 for guiding a web between the web attachment points 31 and 32 enables at the same time a maximum adjustment clearance. Preferably, the above-mentioned web attachment points are disposed on either side of the coater 20, such that the distance therebetween is at least twice the width of a web. The larger the gap between the attachment points, the larger the adjustment clearance for a crosswise position of the web.

In addition to the fact that the invention enables checking or impeding the passage of a coating into the edge zone 12 intended to remain uncoated or even beyond the edge zone, the method according to the invention provides also a means of reducing a width of the edge zone 12. Thereby, it is possible both to reduce the amount of paper going to broke and also to increase the output of coated product. The method of the invention enables reducing the size 10a of edge zones by as much as to the order of 5 mm or even less 10b than that.

According to one embodiment of the invention, in association with the adjustment of a web position is further provided at least a partial adjustment for the width and position of a coating curtain, which can be performed while observing also the width and/or the position of a web. Such an adjustment has been described more precisely in the Applicant's earlier Finnish patent No. 117176. Disclosed therein is a device for adjusting the application width of a slide-fed curtain coater, i.e. also the width of a coating curtain. As shown schematically in fig. 2, the apparatus comprises a laterally movable adjustment device 40, which is disposed on at least one of the edge zones of a runoff slide 28 and which, in the embodiment shown in fig. 2, includes a cover element 41, a blade element 42, and a bottom element 50. The blade element 42 restricts a coating color 29 trickling along the runoff slide in terms of its lateral extent S. Thus, according to the invention, it is possible both to perform the adjustment of a lateral position of the web W relative to the curtain and further, as necessary, to adjust the actual coating curtain 22 in terms of its width and position. At the same time, the arrangement of fig. 2 enables maintaining a feeding profile of the beam unchanged irrespective of the application width.

The adjustment device is preferably adapted for a stepless displacement in view of determining a desired application width. Displacement can be automated and can be effected in such a way that the adjustment of width can be implemented, if necessary, even during the course of a run, in which case a guidance of the adjustment device may be based e.g. on a continuous tracking of the web's edge. Such an adjustment arrangement is now proposed to be combined with the adjustment arrangement presented in this application regarding the relative position of a web and a coating curtain.

As an additional benefit, the above-described adjustment solution for the width or position of a coating curtain allows at the same time a treatment as desired for the coating color emerging from a single nozzle slot. In addition, the stream of coating color exceeding the application width can be returned by way of pipe elements 46 back to the machine circulation. A guide element, consisting of the bottom element 50 and the cover element 41, is namely provided with chute members (not shown) coincident with each nozzle slot, which enable this coating color outside the limits of the application width to flow into the pipe elements 46 present in the bottom element 50. Thus, for example some of the layers of coating color may be recyclable back into the machine circulation and to be re-fed into the feeding chamber. This is a major advantage, especially in the case of multiply curtain coating in which the intermingling of various coating color layers would otherwise impede a recycling of the coating agent back to the supply for reuse.

In one embodiment, there is further proposed a concurrent crosswise adjustment for either a web or a coating curtain or both by removing a desired-size strip from one or both edge zones thereof. In terms of a web, this can be performed by cutting as known in the art, in terms of a curtain by means of adjustment elements as described in the above-mentioned additional embodiment.

Claims

1. A method in connection with curtain coating a fibrous web (W), said method comprising applying in a processing line (1) to at least one side of the fibrous web (W) at least one layer of coating agent by a curtain coating method, characterized in that a position of the fibrous web (W) in a cross-direction (CD) of the processing line (1) is adjusted by means of guide elements (4) arranged for the fibrous web (W) for controlling a relative position between the fibrous web (W) and a coating curtain (22) from a curtain coater (20) in the cross-direction (CD) of the processing line (1).

2. A method as set forth in claim 1, characterized in that said positional adjustment of the fibrous web (W) is performed in practice by adjusting a position of the fibrous web (W) in the cross-direction (CD) with respect to a center axis (26) of the processing line (1).

3. A method as set forth in claim 2, characterized in that the fibrous web (W) is aligned with respect to the center axis (26) of the processing line (1).

4. A method as set forth in any of claims 1-3, characterized in that a position of the coater (20) and/or the coating curtain (22) in the cross-direction (CD) is adjusted relative to the center axis (26) of the processing line (1) or aligned with respect to the center axis (26).

5. A method as set forth in any of claims 1-4, characterized in that the method comprises adjusting a position of the coating curtain (22) in the cross-direction (CD) of the processing line (1) by changing a width of the coating curtain (22) from one or both edges of the coating curtain (22) by means of adjustment elements (40) included in the coater (20).

6. A method as set forth in any of claims 1-5, characterized in that the method is applied to dry edge coating.

7. A method as set forth in any of claims 1-6, characterized in that the curtain coater (20) comprises the use of a slide-fed (28) curtain coater or a slot-fed curtain coater.

8. An arrangement in connection with curtain coating a fibrous web (W), said arrangement comprising a curtain coater (20) disposed in a fibrous-web processing line (1) for applying at least one layer of coating agent by a curtain coating method to at least one side of the fibrous web (W), characterized in that, for the fibrous web (W), there are provided fibrous-web (W) guide elements (40) for adjusting a position of the fibrous web (W) and thereby controlling a relative position between the fibrous web (W) and a coating curtain (22) from the curtain coater (20) in a cross-direction (CD) of the processing line (1).

9. An arrangement as set forth in claim 8, characterized in that, viewed in a running direction (V) of the fibrous web (W), said elements (4) are disposed upstream of the curtain coater (20), downstream thereof, in a direct contact therewith and/or integrally with the curtain coater (20) or in a combination of these.

10. An arrangement as set forth in claim 8, characterized in that said elements (4) are adapted to be positionally adjustable relative to the coater (20) in a main direction (MD) and/or a cross-direction (CD) of the processing line (1).

11. An arrangement as set forth in any of claims 8-10, characterized in that said guide elements (4) include one or more guide rolls or conical leading rolls or the like devices, by means of which an alignment of the fibrous web (W) is adjustable in the cross-direction (CD) of the processing line (1).

12. An arrangement as set forth in any of claims 8-11, characterized in that at least some of said guide elements (4) are actively functional and/or automated.

13. An arrangement as set forth in any of claims 8-12, characterized in that the coater (20) is provided with adjustment elements (40) for adjusting a position of the coating curtain (22) in the cross-direction (CD) by changing a width of the coating curtain (22) from one or both of its edges.