The present invention relates to a method for coating a fibrous web, such as papers and boards, by dry coating technique, the coating formed on the surface of a fibrous web being bonded by the application of heat and pressure.

In dry coating, a coating mix made up by pigment and bonding agents is transferred onto a paper web by using an electrically charged field. More specifically, the particles of a transfer material are charged with an electric charge, whereby the migration of the particles can be controlled by means of the charged field. I n prior known solutions, the charged field is generated by two or more electrodes at charging potentials different from each other, mounted on the opposite sides of a fibrous web. It is the principle that the powdery particles of a supplied material be charged by one electrode assembly to a certain charge level and the other electrode assembly, which is at a different charging potential, be used for generating a charged field for controlling the migration of material particles to be applied. It is particularly prior known that one electrode assembly is located in a supplied material flow or in its vicinity, for example in an application nozzle, and the other electrode assembly is placed on the opposite side of a fibrous web. Thus, the charged field, generated between the electrodes and working through the fibrous web, delivers the material particles onto the surface of the fibrous web. Finally, the coating layer applied onto the surface of the fibrous web is bonded by heating the coating and the fibrous web in a special bonding nip.

It is prior known to employ a pneumatically dispersing application nozzle in the application of a supplied material, for example a coating powder. The coating powder can be admixed in a flow of compressed air just upstream of the nozzle. I n view of charging the flow of air and particles emerging from the nozzle, the nozzle itself can be an electrode or the material flow nozzle can have an electrode in its vicinity.

It is an objective of the present invention to provide an improved method for coating a fibrous web by dry coating with the use of an electrically charged field and pressure contact. I n order to achieve objectives according to the invention, a method of the invention is characterized in that the method comprises forming at least one surface of a fibrous web with a coating of at least two component layers by applying a dry coating agent essentially in a single process step onto said at least one surface of the fibrous web, and/or onto a bearing surface coming to contact therewith in a subsequent process step, by means of application devices making use of an electrically charged field and capable of applying a desired number of component coating layers, and that the coating has its bonding process effected by means of a bonding mechanism constituted by a separate bonding nip or the nip or treatment zone of a calender, said calender being selected from the group of: a metal belt calender, some other belt calender, a soft, machine, shoe, super or multi-nip calender. Preferred embodiments of the invention are set forth in the dependent claims 2-4.

The bonding mechanism for use in a method of the invention may comprise e.g. a separate bonding nip or the nip or treatment zone of a conventional calender, such as a metal belt calender, some other belt calender, a soft, machine, shoe, super or multi-nip calender.

The inventive method enables the implementation of a multilayer coating process by dry coating technique. The benefits of a multilayer coating process are known as such from other coating methods, such as e.g. a multilayer curtain coating method. The benefits include, among others, the capability of selecting a layered sandwich structure in a manner optimal in view of the overall structure, whereby e.g. the surface can be opted for a coating agent beneficial in terms of printability or adhesive properties and the inner layers can be opted for a coating agent optimal in terms of bonding/reinforcement or density.

Rg. 1 shows schematically one dry coating arrangement for applying a method of the invention,

fig. 2 shows schematically a second dry coating arrangement for applying a method of the invention, figs. 3-4 show a dry coating arrangement for applying a method of the invention, in association with a metal belt calender,

fig. 5 shows a dry coating arrangement for applying a method of the invention, in association with a roll nip,

figs. 6-7 show a dry coating arrangement for applying a method of the invention, in association with a metal belt calender,

figs. 8-9 show a dry coating arrangement for applying a method of the invention, in association with a multi-nip calender.

I n reference to fig. 1 , there is shown a dry coating solution 110, including two coating units 100a and 100b disposed successively along a section between an unwinder 101 and a bonding nip N. Each coating unit comprises a high voltage source 108, negative electrodes 106 and an earth electrode 105, as well as coating agent feeding elements, including a pneumatic compressor 104, a fluid bed 109, and an application nozzle 103 for the application of coating material particles 111 onto the surface of a fibrous web W by means of an electrically charged field 107. Once the application of coating material layers is completed, the fibrous web is advanced through the heated bonding nip N for bonding all layers by the application of heat and compression pressure. The bonding process is followed by passing the fibrous web to a winder 102. A number of coating units can be mounted one after the other upstream of the bonding nip.

I n the embodiment of fig. 2, a dry coating apparatus 120 comprises likewise two coating units 100a and 100b, the first one 100a being disposed in a section between an unwinder 101 and a first bonding nip N-| and the second unit 100b being disposed in a section between the first bonding nip N-| and a second bonding nip N2 for bonding a first component layer lightly before the application of a following component layer. This solution enables the bonding and compacting of each component layer as desired, making it possible to provide the coating layers with a sort of gradient structure, for example by compacting a bottom layer lightly (little heat and pressure) and a top liner more strongly (plenty of heat, controlled pressure). There may be more component layers and the bonding nip can be provided downstream of each application device or downstream of a group consisting of two or more application devices.

Instead of the separate bonding nip N, N-| , N2, it is also possible to employ a calender, e.g. a metal belt calender, some other belt calender, a soft, machine, shoe, super or multi-nip calender, whereby the bonding can be effected by passing a coated fibrous web through one or more nips or through a treatment zone.

A metal belt calender 130 as shown in fig. 3 has three fibrous web treatment zones N3, N4, N5 arranged against the surface of a single counter-roll 5 on opposite sides of the counter-roll by means of three separate metal belt loops 2a, 2b, 2a. The metal belts 2a-2c are adapted to circle around guide rolls 3, said guide rolls being preferably adjustable for regulating the tightness and running of a metal belt. A dry coating unit 100a is disposed in a section between the first fibrous web treatment zone N3 and the second fibrous web treatment zone N4, and a second dry coating unit 100b is disposed in a section between the second fibrous web treatment zone N4 and the third fibrous web treatment zone N5. The first treatment zone N3 functions preferably as a pre-calender upstream of the first coating unit and the third treatment zone N5 functions preferably as a bonding/finishing calender. Optionally, the pre-calendering N3 can be accompanied by a pre-coating process 100.

A configuration 140 as shown in fig. 4 comprises a metal belt 2 extending around guide elements 3 and a counter-roll 5 disposed outside the metal belt, establishing a fibrous web treatment zone Ng between the belt 2 and the roll 5. Coating units are represented by reference numerals 115a-115c and earth electrodes by reference numeral 105. This arrangement enables establishing a two-ply structure on one surface of a fibrous web W by transferring particles of coating material from the coating unit 115a onto the metal belt's 2 surface and from the coating unit 115b onto the fibrous web's W surface, followed by advancing the fibrous web into the treatment zone Ng, in which the coating material applied to the metal belt's surface transfers onto the fibrous web's surface on top of the applied coating material layer and each layer bonds to the fibrous web by the application of pressure and heat. I n order to provide a thermal effect, the roll 5 and/or the belt 2 are/is heated. I n the discussed embodiment, the other surface of a fibrous web is only formed with a single-ply coating by using the roll's 5 surface as a transfer surface, but it is also conceivable to implement the coating of this other side as a two-ply coating by providing a device similar to the application device 115b also on this other side of the fibrous web.

A configuration 150 as shown in fig. 5 comprises a press nip N7 established by two rolls 5a, 5b. The coating material is applied onto the surface of the roll 5a and the surface of a fibrous web, as well as onto the surface of the roll 5b, prior to advancing the fibrous web into the press nip N7 to establish, as in the embodiment of fig. 4, a two-ply coating on one side of the fibrous web and a single-ply coating on the other side.

A configuration 20 as shown in fig. 6 comprises two belt loops 2a, 2b and a counter- roll 5 therebetween for two treatment zones Ns and Ng. Coating units 100a and 100b are disposed in a section between the treatment zones Ns and Ng. A fibrous web W-| is first passed to the pre-calendering process Ns and then to a first coating process 100a, followed by advancing it over a reversing roll 7 to a second coating process 100b and thence to the bonding/finish calendering process Ng. Optionally, a pre-coating process 100 can be performed as early as in association with the pre- calendering process Ns-

A configuration 30 as shown in fig. 7 comprises a first metal belt loop 2a and a first counter-roll 5a to establish therebetween a first treatment zone N-J Q, as well as a second metal belt loop 2b and a second counter-roll 5b to establish therebetween a second treatment zone N-| 2- The counter-rolls 5a, 5b are positioned so as to mutually establish a heated bonding nip N-| -| for a coating material. This configuration enables a variety of treatments for a fibrous web, e.g. such that a fibrous web W-| is first passed for a pre-calendering process through the treatment zone N-J Q and then through a dry coating process 100a and over a reversing roll 7 further through a second dry coating process 100b, from which it is advanced into the bonding nip N-| 1 for bonding the coating material to the fibrous web. This operation can be followed by delivering a coated web W2 out of the apparatus for further processing, such as e.g. winding. Downstream of the bonding nip N-| -| , the web may optionally have a third coating process 100c, from which the web can be passed over a reversing roll 7a into the second treatment zone N-|2 for a bonding/finish calendering process. The apparatus may also be provided with a fourth coating process 10Od, which may function e.g. as a pre-coating step upstream of the pre-calendering process N-| Q-

I n the configuration of fig. 8, a first coating unit 110a is mounted in a section between two nips N21 and N22 of a multi-nip calender and a second coating unit 100b in a section between two other nips N23 and N24. The configuration of fig. 9 has a first coating unit 110a disposed in a section between two roll assemblies of a multi-nip calender and a second coating unit 100b in a section between two nips N35 and N35 of a second roll assembly. I n the embodiments of figs. 8 and 9, the number of coating units can be increased to provide a multiply treatment on at least one side of a fibrous web and at least a single-ply treatment on the other side.

I n the foregoing embodiments, the surface of a roll or a belt is preferably treated in a way to deny bonding of a coating material thereto. This can be accomplished e.g. by forming the surface with a Teflon coating or by treating it with an appropriate releasing chemical.

The configurations shown in the embodiments of figs. 3 and 6-9 enable implementing the pre-calendering process of a fibrous web upstream of the application of a dry coating material to establish a flat base, which makes it easier to apply an even layer of coating thereto. It is particularly preferred that pre- calendering, coating, and final calendering be effected with one apparatus for a short web conveying distance and for preserving the web heat and moisture better between various processes. I n view of pre-calendering, it is also possible, if necessary, to include in the various foregoing embodiments a separate pre-calender upstream of the apparatus containing a coating unit. The pre-calender can be selected from the number, including: a soft, machine, multi-nip, shoe or belt calender. The belt calender comprises preferably a metal belt calender. The above pre-calendering is also feasible in association with a traditional single-ply dry coating process to facilitate the establishment of a smooth coating. The above-described fibrous web coating procedures and/or possible pretreatment (pre-calendering) and/or final treatment (bonding/finish calendering) can be implemented on a fibrous web making line both as on-line and off-line solutions. More specifically, all process steps (manufacturing, pre-calendering, coating and possible finish calendering of a fibrous web) can be effected as an on-line solution (without intermediate winding operations). The finishing processes can also be effected off-line (after intermediate winding) with respect to a fibrous web making machine (e.g. a papermaking machine). Even in this case, the finishing processes (pre-calendering, coating, and finish calendering) can preferably be totally integrated relative to each other as on-line processes (no intermediate winding operations therebetween) or some of those can be effected as separated by intermediate winding operations.