WO2005085525A1

WO2005085525A1 - Method and apparatus for transferring coating and/or processing agent to a fibrous web

Info

Publication number: WO2005085525A1
Application number: PCT/FI2005/050025
Authority: WO
Inventors: Reijo PIETIKÄINEN
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2004-03-09
Filing date: 2005-02-11
Publication date: 2005-09-15
Anticipated expiration: 2006-09-09
Also published as: JP5165368B2; JP2007527795A; FI20045065L; DE112005000532T5; FI117106B; FI20045065A0

Abstract

The invention relates to a method and apparatus for transferring a coating and/or processing agent to a paper/board web (W) with a processing apparatus, comprising a bearing surface established by at least one treatment element (5, 2), the fibrous web being contacted therewith. The method comprises employing the bearing surface of at least one (2, 5) of the treatment elements of the processing apparatus as a transfer surface, which is supplied with an agent (13) to be transferred to the fibrous web (W) prior to contacting the transfer surface with the fibrous web. Said transfer surface is heated for preheating the agent to be transferred prior to passing it to a pressing contact with the fibrous web.

Description

Method and apparatus for transferring coating and/or processing agent to a fibrous web

The present invention relates to a method and apparatus for transferring a coating and/or processing agent to a paper/board web with a processing apparatus, which is provided with at least one contact zone established by a web treatment element, such as a belt or a roll or the like, as well as by the web itself, and in which the agent is transferred in a first process step to the treatment element, from which the agent transfers in a second process step onto a surface of the fibrous web by way of a contact within the treatment zone.

In several prior known and commonly employed coating and surface sizing methods, the agent to be transferred is applied directly to the surface of a paper or board web. In spray coating, for example, the coating is sprayed as a fine mist directly onto a web, while in curtain coating and jet coating, the coating slip is trickled as a thin "curtain" or film directly onto a web. Prior known are also coating methods, in which the coating agent is applied first to one surface, for example to the surface of an applicator roll or a transfer belt, the examples of which include blade coating (applicator roll) and film coating (applicator roll).

A common feature in the above methods is that the methods are so-called wet coating processes. The agent to be transferred is applied to a fibrous web as a moist suspension or slip and the coated fibrous web must be dried downstream of the application. On the other hand, in a so-called dry coating process, the agent to be applied to a fibrous web is in a dry powdered form. The application is performed by charging the supply material particles electrically and by maneuvering the material flow in the application process as well as the bonding to a transfer surface by means of an electric field. The applied material layer must be additionally bonded, which is effected by means of a hot, pressing attachment nip. However, no drying is needed for the coated fibrous web.

It is a general objective of the present invention to provide an improved and enhanced method for transferring an agent to a fibrous web. One essential objective is to simplify and facilitate a consistent dispensing for an agent to be transferred, which is most conveniently achieved according to the invention by dispensing the agent first at a high precision onto a flat and smooth bearing surface, such as a roll or a belt, from which it subsequently proceeds to a fibrous web.

It is a second essential objective to improve the transfer and bonding of an applied agent to the surface of a fibrous web by raising the temperature of an agent to be transferred, such that the agent becomes softer, more fluid, and penetrates better into the fibrous web upon application. Accordingly, it is one objective of the invention to provide a simple, yet effective technical solution for heating an agent to be transferred in connection with application. A third essential objective is to enhance the adherence of an agent to be transferred to a fibrous web by designing a bonding process, in which the agent to be transferred is in a press contact with the fibrous web, to have a sufficient duration for giving the heated and plasticized agent enough time to adequately flow and penetrate into the pores of the fibrous web.

In order to accomplish the above objectives, a method for transferring a coating and/or processing agent to a paper/board web with a processing apparatus, comprising a bearing surface established by at least one treatment element, the web to be treated being passed to a touching contact therewith, at least one of the treatment elements of said processing apparatus having its bearing surface used as a transfer surface, which is supplied with an agent to be transferred to the fibrous web prior to contacting the transfer surface with the fibrous web, is characterized in that said transfer surface has been heated for preheating the agent to be transferred prior to passing it to a pressing contact with the fibrous web. An apparatus of the invention for transferring a coating and/or processing agent to a paper/board web, said apparatus comprising at least one bearing surface established by treatment elements, the fibrous web being contacted therewith in a nip or treatment zone, at least one of the treatment elements of said apparatus having its bearing surface adapted to function as a transfer surface, which is supplied with an agent to be transferred to the fibrous web prior to contacting the transfer surface with the fibrous web, is in turn characterized in that said transfer surface is adapted to be heated for preheating the agent to be transferred prior to passing it to a pressing contact with the fibrous web.

In a method of the invention, the agent to be transferred is first applied to a moving transfer surface, which is passed to a transfer zone for a contact with a fibrous web, in which the applied agent transfers from the transfer surface onto a surface of the fibrous web. Thus, the transfer zone is constituted by a contact area, which is common to the transfer surface and the fibrous web and which is supplied with an agent to be transferred carried by the transfer surface and from which the fibrous web discharges carrying the applied agent on its surface. The transfer surface may particularly comprise a roll, a belt or wire or some other continuous surface, which is in contact with a fibrous web. In addition, the transfer zone may or may not have a roll, a wire or belt simultaneously on both sides of a fibrous web in contact with the fibrous web, which is the case in a press zone or nip established by a pair of rolls, a set comprising a roll and a belt loop or two opposing belt loops.

Thus, in a method of the invention, the surface establishing a contact with a fibrous web in the transfer zone is used as a transfer surface, to which the agent to be transferred is applied in such a way that, within the transfer zone at the latest, it is forced to a contact with the fibrous web and makes a transfer to the latter. The transfer surface is heated, such that the agent to be applied thereto and to be transferred to the fibrous web heats up even prior to winding up in a contact with the fibrous web and, thus, the agent to be transferred has already been heated during a transfer to the fibrous web surface taking place in the transfer zone. By virtue of preheating, the agent to be transferred flows and penetrates more readily into the pores of a fibrous web and has a stronger adherence to the fibrous web. The ultimate bonding to the fibrous web surface and the compaction of the transferred material layer shall take place in the transfer nip in response to heat and compression pressure.

A solution of the invention is especially beneficial in reference to dry coating as the applied dry coating heats up even before a bonding nip, whereby the only function left for the actual bonding nip is principally the compaction of applied agent and whereby the bonding nip can be dimensionally more modest, especially from the standpoint of heat transfer.

A solution of the invention is also suitable for the application of other than dry coating agents. The agent to be transferred may consist of a material either in liquid or solid state or a mixture of agents in these states, such as slurry or suspension. Furthermore, the agent to be transferred can be a material consisting of tiny solid particles, such as a powder or flour admixed in a liquid or gas phase. In particular, the agent to be transferred can be a coating material used for coating a fibrous web or a fibrous web processing agent, such as a sizing agent or a solution- type chemical used for improving the properties of a fibrous web, water used for moistening a web or a mixture of at least two of the above-mentioned agents.

The invention will now be described in more detail with reference to the accompanying drawings, in which:

fig. 1 shows schematically one solution of the invention embodied in connection with a metal-belt processing apparatus,

fig. 2 shows schematically a second solution of the invention also embodied in connection with a metal-belt processing apparatus,

fig. 3 shows schematically a third solution of the invention in connection with a roll nip,

fig. 4 shows schematically a fourth solution of the invention applied in connection with a roll,

fig. 5 shows schematically a fifth solution of the invention, and

fig. 6 shows schematically a sixth solution of the invention.

The invention will be described hereinbelow by way of example in reference to its various configurations. Fig. 1 illustrates a mechanical solution of the invention, comprising an endless metal belt loop 2 as well as a counter-element 5 functioning as its outside bearing surface, a fibrous web W being passed through a contact therebetween. The belt 2 and the roll 5 establish therebetween an extended treatment zone N, the compression pressure effect of which, applied to the fibrous web, can be regulated by a tension of the belt 2 as well as by the load of a possible additional support roll 4 inside the belt loop.

In the embodiment of fig. 1, an agent 13 to be transferred is dispensed onto at least one bearing surface, either onto the belt 2 by means of an application device 11 or onto the surface of the roll 5 by means of an application device 11'. Application can also be performed to both surfaces at the same time.

In the embodiment of fig. 1, at least one of the bearing surfaces, either the metal belt 2 or the roll 5, is adapted to be heated. The metal belt can be heated with a separate heating device 6, such as for example an induction heater, an infrared radiator or a gas burner. The heating may be based also on resistive heating. The heating may also be effected by means of heated support rolls. The roll 5 can be adapted to be heated, for example by means of conventional heating procedures for a thermal roll. When necessary, the bearing surfaces are provided in vicinity thereof with a doctoring or other such cleaning system 7, 7' for preparing the bearing surface for the next cycle in an endless run.

The application devices 11, 11' shown in the embodiment of fig. 1 can be similar to those used in a dry coating process, whereby the agent flowing out of the application device is charged with an electrode assembly (not shown) set in the vicinity of the flow and whereby the metal belt 2 and/or the roll 5 is/are further provided with an earth electrode (not shown). Hence, the particles of agent to be transferred flowing out of the application device will be charged with a potential difference sufficient with respect to the transfer surface 2, 5 for having them work their way to the transfer surface. While coming to contact with the heated transfer surface, such particles heat up prior to making contact with the fibrous web W. Particles 13a are preferably adapted to establish a contact with the fibrous web in a sufficiently long treatment zone between the belt and the roll 5, such that in the treatment zone the fibrous web will be subjected to a heating and compression effect, by virtue of which the agent to be transferred bonds permanently to the fibrous web surface.

In a particularly beneficial case, the agent to be transferred comprises a dry, pulverous material used in pigment or dry barrier coating, primarily a mixture of pigment and bonding agents or polymer. Instead of a particulate dry coating agent, it is also possible to apply a material in the form of a solution, fine spray, slurry, paste or some other wet blend. In addition to a coating pigment and binder mixture, the material can be e.g. a sizing agent, a solution-type chemical, water or the like.

In addition to an electric-field applying method as described above in reference to fig. 1, the dispensing of an agent to be transferred to a transfer surface can also be effected by other known principles, such as for example by brushing, spraying, trickling, or by bonding through indirect contact, and especially by methods applying these principles, such as spray coating, blade coating, jet coating, roll application, etc.

The transfer surface is doctored or cleaned preferably downstream of a transfer nip by using an actuator 7, 7' before another dose of agent to be transferred is dispensed thereon. An anti-adhesive chemical can be applied to the transfer surface during the course of a cycle.

Fig. 2 illustrates an embodiment, which is in principle similar to that shown in fig. 1, but the bearing surface comprises a second belt loop 2' instead of the roll 5. Other main principles are the same as those in fig. 1.

Fig. 3 shows an embodiment of the invention, in which two rolls 5, 5' establish therebetween a nip or treatment zone for passing a fibrous web W therethrough. The treatment zone operation can be controlled by regulating nip pressure and temperature of the rolls. In this embodiment, the supply agent is dispensed by means of application devices 11, 11' to the outer surfaces of the rolls 5, 5' working as bearing surfaces. The application can be done to just either one of the surfaces 5, 5' or to both at the same time. At least one of the bearing surfaces is heated with the result that the applied agent heats up before coming to contact with the fibrous web W in the transfer zone. Heating of the rolls can be effected by conventional arrangements. The rolls are cleaned with the devices 7, 7' before another dose of agent is dispensed thereon.

Fig. 4 depicts an embodiment, in which the agent 13 to be applied comprises e.g. a dry coating mixture of a pigment and bonding agent in a particulate form in a dry pulverous state. In the exemplary embodiment of fig. 4, the orifice of an application device 12 is provided with electrodes 21 for charging particles 13 upon their exit from the application device. A roll 15 is connected to an earth electrode 10 and constitutes at the same time a transfer surface, which is the first destination for charged particles 13a flowing out of the application device 12. The roll 15, which functions as a transfer surface, is then passed, along with particles present on its surface, to a contact with the fibrous web W for delivering the particles onto a surface of the fibrous web. The contact takes place preferably in a roll nip N (zone II) established with a counter-roll 16 or slightly upstream thereof (zone I). The roll 15 is heated, whereby the supplied particles 13 heat up even before the contact I or II, in which the final bonding occurs in response to compression and heat applied to the web in the roll nip N.

Fig. 5 illustrates a very simple embodiment of the invention, which only comprises a heated roll 5 and a fibrous W in contact with its outer surface. An application device 11 is used to dispense an agent to be transferred to the roll surface functioning as a bearing and transfer surface, at which the agent to be transferred heats up and from which the agent proceeds to the fibrous web in a transfer zone (not shown). A contact pressure existing in the transfer zone is only developed by tensile forces F of the fibrous web, resulting in a poor contact and heat transfer. Therefore, it is an objective to maximize a covering length and the method is best suited for slow running speeds and thick grades (boards). However, since it is applied first to a heated roll surface, the agent heats up even before a transfer zone and there is no need for a particularly high-grade heat transfer contact. It is nevertheless beneficial in this embodiment, for example in the case of dry coating, to employ agents having a glass transition temperature Tg which is low, e.g. in the order of 20- 100°C.

Fig. 6 shows an embodiment, in which the agent to be applied is transferred through the use of an electric field to a fibrous web W with an application device 12. The application device 12 has its orifice provided with electrodes 21 for charging particles 13 upon their exit from the application device. A roll 15 is connected to an earth electrode 10 and constitutes at the same time a transfer surface, which is the first destination for charged particles 13a flowing out of the application device 12. The roll 15, which functions as a transfer surface, is then passed, along with particles present on its surface, to a contact with the fibrous web W for delivering the particles onto a surface of the fibrous web. The bearing surface, in this case a thermal roll 15, is heated and hence the applied agent heats up before coming to contact with the fibrous web W in the transfer zone. A pressure existing in the contact is also in this embodiment developed solely by a web tension. Because the contact pressure is very low, thus resulting in a modest heat transfer, it is beneficial in this embodiment to employ a transfer agent, for example a dry coating, the treatment of which can be performed at a low temperature. It is particularly beneficial to employ a dry coating agent having a glass transition temperature Tg which is low, in the order of 20-100°C.

The invention has been described above by way of example, in reference to figs. 1- 6. With regard to the belt loop, reference is made to a metal belt solution in which a belt of steel is indeed particularly useful. However, the use of other metal belts and even non-metallic belts and wires is possible. For example, it is conceivable, in appropriate cases, to make use of dryer wires or other fabric-reinforced polymer wires.

The invention is particularly suitable for coating, surface sizing, as well as moistening fibrous webs of paper and board applied in connection with a production line, both as an on-line and off-line solution. The invention finds application also in the production of other web-like products, such as coated fiberboards.

Claims

1. A method for transferring a coating and/or processing agent to a paper/board web (W) with a processing apparatus, comprising a bearing surface established by at least one treatment element (5, 2, 15), the web (W) to be treated being passed to a touching contact therewith, at least one (2, 5, 15) of the treatment elements of said processing apparatus having its bearing surface used as a transfer surface, which is supplied with an agent (13) to be transferred to the web (W) prior to contacting the transfer surface with the web, characterized in that said transfer surface is heated for preheating the agent to be transferred prior to passing it to a pressing contact with the web.

2. A method as set forth in claim 1, characterized in that the transfer surface is contacted with the fibrous web in a nip or treatment zone (N).

3. A method as set forth in claim 1 or 2, characterized in that, in the process of application to the transfer surface, the agent to be transferred is in a wet pasty consistency in the form of slurry or suspension or solution or spray.

4. A method as set forth in claim 3, characterized in that the agent to be transferred to the fibrous web comprises water or a water soluble chemical.

5. A method as set forth in claim 3 or 4, characterized in that the application method has been chosen from the group of: blade coating, film coating, spray coating, curtain coating, cast coating.

6. A method as set forth in claim 1 or 2, characterized in that, in the process of application to the transfer surface, the agent to be transferred is in a dry pulverous or powdered form.

7. A method as set forth in claim 6, characterized in that the agent to be transferred to the fibrous web comprises a dry coating material.

8. A method as set forth in claim 6 or 7, characterized in that the agent to be transferred is in the form of powder or in some other particulate form, which is passed onto a transfer surface by using an electrically charged field, the transfer surface functioning as one electrode and the path or its vicinity of a flow carrying the agent to be transferred being provided with at least one second electrode (11), such that the flowing material particles are charged with a charging potential sufficient with respect to the electrode constituted by the transfer surface for having charged particles (13a) make their way to the transfer surface.

9. A method as set forth in any of claims 1-8, characterized in that the processing apparatus comprises a metal belt loop (2), adapted to travel around at least one support element (3), and at least one counter-element (5) outside the belt loop for establishing, together with the belt loop, an extended treatment zone for the fibrous web.

10. A method as set forth in any of claims 1-8, characterized in that the processing apparatus comprises a nip established by two opposing rolls.

11. A method as set forth in any of claims 1-8, characterized in that the processing apparatus comprises a contact area established by a contact between one roll and the fibrous web.

12. A method as set forth in any of claims 1-11, characterized in that an agent is transferred simultaneously to both surfaces of the fibrous web.

13. A method as set forth in any of claims 1-11, characterized in that an agent is only transferred to one surface of the fibrous web.

14. A method as set forth in any of the preceding claims, characterized in that the processing apparatus is used not only for bonding an agent to be transferred to the fibrous web but also for the calendering treatment of a fibrous web in a nip or treatment zone.

15. An apparatus for transferring a coating and/or processing agent to a paper/board web (W), said apparatus comprising at least one bearing surface established by treatment elements /5, 2), the web (W) being contacted therewith in a nip or treatment zone (N), at least one (2, 5; 2'; 5'; 15) of the treatment elements of said apparatus having its bearing surface adapted to function as a transfer surface, which is supplied with an agent (13) to be transferred to the web (W) prior to contacting the transfer surface with the web, characterized in that said transfer surface is adapted to be heated for preheating the agent to be transferred prior to passing it to a pressing contact with the web.

16. An apparatus as set forth in claim 15, characterized in that it comprises a metal belt loop (2) adapted to travel around at least one support element (3) and at least one counter-element (5; 2') outside the belt loop for establishing, together with the belt loop, an extended treatment zone for the fibrous web.

17. An apparatus as set forth in claim 15, characterized in that it comprises a nip established by two opposing rolls (5, 5'; 15, 16).

18. An apparatus as set forth in claim 15, characterized in that it comprises a roll (5; 15) for establishing a contact with the fibrous web (W).

19. An apparatus as set forth in any of claims 15-18, characterized in that the transfer surface (15) is adapted to function as an electrode, and that the apparatus further comprises at least one second electrode (21) disposed in the path of an agent to be transferred, whereby flowing material particles (13) are chargeable with a charging potential sufficient with respect to the electrode constituted by the transfer surface for having charged particles (13a) transferred to the transfer surface by means of an electrically charged field.