FILM TRANSFER COATING

Abstract: The invention concerns a method in film transfer coating in a film press, in which press the nip (N) of the film press is formed out of two faces placed against each other and moving in the same direction so that said moving faces are pressed against each other in the nip. A coating agent or surface size is applied as a film (F1, F2) onto at least one of the moving faces that form the film press nip (N) and that are subjected to pressing, in which connection the coating agent or surface size is transferred in the nip (N) between the moving faces onto the paper or board web (W) running through the nip. After the nip the web is separated either directly from the nip or so that it follows one or the other of the moving faces. The web is separated from the nip so that, at least at one side of the web, the curve radius of the opening geometry is larger than 0.75 metre. The invention also concerns a film press intended for carrying out the method.

Description: FILM TRANSFER COATING The invention concerns a method in film transfer coating in a film press, in which press the nip of the film press is formed out of two faces placed against each other and moving in the same direction so that said moving faces are pressed against each other in the nip, and in which press a coating agent or surface size is applied as a film onto at least one of the moving faces that form the film press nip and that are subjected to pressing, in which connection the coating agent or surface size is transferred in the nip between the moving faces onto the paper or board web running through the nip, and after the nip the web is separated either directly from the nip or so that it follows one or the other of the moving faces. The invention also concerns a film press intended for carrying out the method, which press comprises a film press nip formed out of two faces that are placed against each other and that move in the same direction so that said moving faces are pressed against each other, through which nip the paper or board web has been passed, and which film press is provided with coating means for application of a coating agent or surface size as a film onto at least one of the moving faces that form the film press nip and that are subjected to pressing, from which face the coating agent or surface size has been arranged to be transferred in the film press nip onto the paper or board web, in which connection, after the nip, the web has been fitted to be separated either directly from the nip or so that it follows one or the other of the moving faces. When a film transfer technique is employed, in film size presses, the application of a size or of a paste onto a paper is carried out so that, depending on whether the paper is to be coated from one side only or from both sides, coating agent films of the desired thickness are applied, by means of applicator devices, onto the face/faces of one roll or both rolls in the film size press, in which connection these coating agent films are transferred onto the paper web in a nip formed by the size press rolls as the paper web runs through said nip. As a rule, the film rolls in film size presses are provided with resilient coatings, and the application onto the roll face can be carried out by using some known blade coater, a bar coater provided with a smooth or grooved coating bar, or an equivalent nozzle equipment. At present, in paper machines and so also in the film size presses connected with them, ever higher speeds are aimed at, in which connection, in particular, the increase in this speed has brought a new problem, i.e. formation of coating-agent mist at the outlet side of the press nip especially when running with large quantities of coating agent. The formation of mist arises from splitting of the coating agent film in the film press when the web is separated from the film roll. Thus, in such a case, the coating agent film splits so that most of the film remains on the paper, whereas a smaller portion remains on the face of the film roll. Excessive formation of mist can, in itself, already be quite an important problem, which is noticed in inferior quality of the coating on coated paper. A problem also arises from the fact that even a smaller amount of mist is deposited on the constructions of the machine and drops onto the paper web and spoils the quality of the paper, or that the mist spreads into the air in the room. It is a second significant problem in film presses how to be able to control the line of separation of the web from the roll in the film press along which the web follows after the nip. In the prior-art solutions, the point of separation of the web from the roll does not remain the same, but the point of separation "wanders" along the roll face. In this connection, fluttering may have occurred in the web, which may again have resulted in web breaks and in their well-known detrimental consequences. Further, wandering of the web separation point in the machine direction and also in the cross direction causes a serious problem of mist formation. Since the problems described above have come out significantly with increased web speeds only, the prior art includes a very low number of solutions by whose means it might be possible to eliminate these problems or by whose means it might be at least possible to reduce these problems to a significant extent. In fact, there are some earlier solutions, and of these solutions it is possible to mention, for example, the FI Patent No. 90,266, in whose solution attempts have been made to prevent formation of mist by using a particular hood construction. In a corresponding way, in the FI Patent No. 93,243, a solution has been described in which formation of mist in itself is not prevented, but this mist is recovered by means of mechanical devices fitted at the outlet side of the nip, or attempts are at least made to recover the mist, in which case the mist should not have access to spread to other constructions. Thus, this earlier publication does not provide a solution for the mist formation itself either. Further, in the FI Patent No. 101,092, an improvement has been sought for the solutions in accordance with the patents mentioned above, and, thus, in the method described in this patent, action has been taken against the formation of coating agent mist itself so that, in the solution in accordance with the patent, in the coating process, attempts have been made to prevent formation of coating agent mist at the outlet side of the film press nip so that jets of steam are passed to the web side to be coated. With respect to the prior art, reference is made further to the US Patent 5, 496, 406, in which, in the film press, the web is supported after the nip by means of air support beams, which have been fitted so that a beam placed closer to the nip supports the web from the side of the roll which the web follows. The object of the present invention is to provide a novel, improved solution for the control of the line of separation of the web as well as for the problems related to the formation of mist of coating agent or surface size and to spreading of the mist. In view of achieving this objective, the method in accordance with the invention is mainly characterized in that the web is separated from the nip so that, at least at one side of the web, the curve radius of the opening geometry is larger than 0.75 metre. On the other hand, the film press in accordance with the invention is mainly charac- terized in that the curve radius of the opening geometry after the film press nip is, at least at one side of the web, larger than 0.75 metre. The most important advantages of the present invention, as compared with the prior- art solutions, are therein that, in the invention, attempts are made to keep the separation of the web from the film press nip as highly controlled as possible by means of a selection of the opening geometry after the film press nip in accordance with the invention. It is in particular owing to this opening geometry that, in the present invention, in itself, attempts are not made to prevent spreading of the mist alone, but attempts are made to prevent formation of mist completely. When no mist is formed at all, consequently mist does not cause problems either. By means of the opening geometry in accordance with the present invention after the film press, the geometry of the splitting of the coating can be brought under control, the separation of the web can be stabilized, and the speed of splitting of the film can be kept at a low level. In this way, formation of mist can be substantially eliminated, and the runnability can be made good. The further advantages and characteristic features of the invention will come out from the following detailed description of the invention. In the following, the invention will be described by way of example with reference to the figures in the accompanying drawing. Figure 1 is a fully schematic side view of a film press intended for two-sided coating or surface sizing of a web in which the method in accordance with the present invention is applied. Figure 2 is an illustration corresponding to Fig. 1 of a film press intended for onesided coating or surface sizing of a web in which the method in accordance with the present invention is applied. Figure 3 is an illustration corresponding to Figs. 1 and 2 of a belt embodiment of a film press in accordance with the present invention intended for one-sided coating or surface sizing of a web. In Fig. 1, the film press is denoted generally with the reference numeral 10. The film press 10 comprises a film press nip N, which is formed between a first and a second film press roll 1,2. The paper or board web W is passed into said nip N, in Fig. 1, as guided by a guide roll 5. The film press 10 in Fig. 1 coats or surface- sizes the web W from both sides, and, thus, each film press roll 1,2 is provided with applicator means 3,4, by whose means the coating agent or surface size is applied as a film F j ,F 2 onto the face l',2' of each roll. The films F^,V 2 are transferred in the film press nip N onto the web W, and the coated or surface-sized web W' is passed out of the nip N, in Fig. 1, so that it follows the face 2' of the second film press roll 2. The radii of the film press rolls 1,2 are denoted with the references R 1 and R 2 . Fig. 2 illustrates a film press, which is denoted generally with the reference denotation 10A. In a way similar to Fig. 1, the film press 10A comprises a film press nip N, which is formed between a first and a second film press roll 1A,2A. In Fig. 2, the paper or board web W is passed into said nip N as guided by the guide roll 5. The film press 10A shown in Fig. 2 differs from the solution shown in Fig. 1 in the respect that the film press 10A of Fig. 2 coats or surface-sizes the web W from one side, and, thus, just one of the rolls in the pair of rolls in the film press 10A, in Fig. 2 the first film press roll 1A, is provided with applicator means 3A for application of a coating agent or surface size as a film F 1 onto the face 1A' of the roll concerned. In Fig. 2A, the second roll 2A in the film press 10A operates just as a backup roll, by whose means the desired linear load is produced in the nip N. In the film press N, the film F j is transferred onto the web W, and the web W', which has been coated or surface-sized from one side, is passed out of the nip N, in Fig. 2, so that it follows the face 2A' of the second film press roll 2A. Also in Fig. 2, the radii of the film press rolls 1A,2A are denoted with the reference denotations Ri and R 2 . Fig. 3 shows a film press, which is denoted with the reference denotation 10B. The film press 10B comprises a film press N, which is, unlike Figs. 1 and 2, formed between a first and a second film press roll 1B,2B so that, in the solution shown in this figure, a belt IB' has been passed around the first film press roll IB, which belt runs through the nip N and which has been formed as an endless belt loop by means of turning rolls 6,7,8. The paper or board web W is passed into said nip N, in Fig. 3, as guided by the guide roll 5. The film press nip 10B shown in Fig. 3 coats or surface-sizes the web W from one side, and for this purpose the film press 10B belt IB' is provided with an applicator device 3B, by whose means the coating agent or surface size is applied as a film F j onto the belt IB'. Similarly to Fig. 2, the second roll 2B in the film press 10B operates exclusively as a backup roll, by whose means the desired linear load can be produced in the nip N. The film F j is transferred in the film press N from the belt IB' onto the web W, and the web W', which has been coated or surface-sized from one side, is passed out of the nip N, in Fig. 3, so that it follows the belt IB', from which it is separated after the nip N. Figs. 1 to 3 are just exemplifying illustrations of how the coating and/or surface sizing can be carried out in a film press and how the geometry of a film press can be controlled, the invention being, however, not confined to these exemplifying embodiments alone. Of the starting points that resulted in the present invention, it can be stated briefly that, as was already described above, in a film press, the formation of mist after the nip is increased when the running speed becomes higher. It has been noticed in test runs that, if the web W' is taken out of the nip N by means of a belt in the tangen- tial direction, the formation of mist is of a substantially lower extent than without a belt. A situation of a belt can be approached when the diameters of the rolls that form the nip N in the film press are made larger. To a great extent, this interdependence complies with the formula of normal acceleration: v = = \ V2 ** (1/R, + 1/R 2 ), wherein v = normal acceleration V = running speed R 1 and R 2 = radii of the rolls. In the solution in accordance with the present invention, a pair of rolls is employed which exceeds the value 1.33 of the prior-art size presses with the largest rolls 1.5 m + 1.5 m, computed from the formula 1/R 1 + 1/R 2 . It is advantageous to increase the diameter of the hard-faced backup roll, because this roll is replaced very rarely. This can be achieved by using a backup roll whose diameter is larger than 1.5 metres. It is, however, possible that at least one of the rolls that form the nip has a diameter larger than 1.5. When a belt is employed, it is preferable to arrange the taking out of the web in the direction of the tangent of the film roll, either directly out of the nip or so that the web follows the film roll. In the case of two belts, i.e. in a case in which there is a belt at both sides of the nip, the taking out of the web would take place in compliance with the requirements of size of said roll diameters or so that the belts are separated from the film on one of the belts at an invariable angle. Before the web is separated from one of the film belts, the coating should be preferably dried on the belt in order to increase the filter layer formed on the coating, which is known to reduce the formation of mist. Thus, in the method in accordance with the present invention, the nip N in the film press is formed out of two faces (roll face / belt) placed one against the other and moving in the same direction, which faces are loaded by pressing them against each other in the nip. The coating agent or surface size is applied as a film F l5 F 2 onto at least one of the moving faces that form the film press nip N and that are subjected to pressure, in which connection the coating agent or surface size is transferred in the nip N between the moving faces onto the paper or board web W running through the nip. After the nip, the web is separated either directly from the nip or so that it follows one of the moving faces. According to the invention, the web is separated from the nip so that the curve radius of the opening geometry is larger than 0.75 m at least at one side of the web. Thus, in the case of a roll nip, the diameter of at least one of the rolls is larger than 1.5 metres. In one-sided coating or surface-sizing of the web, after the nip the web is made to follow the moving face subjected to pressure and placed at the web side opposite in relation to the web face to be coated or surface-sized before the web is passed out of the film press. At least one (cf. Fig. 3) or both of the moving faces subjected to pressure and forming the nip can be formed out of a belt that runs through the nip and that has been formed as an endless loop. On the other hand, one or both of the moving faces that form the nip and that are subjected to pressure can consist of a roll face. In one-sided coating or surface-sizing of a web, the moving face subjected to pressure and placed at the web side opposite in relation to the web face to be coated or surface-sized can be hard, in particular a metal face, or, on the other hand, it can consist of a soft coating, in particular of a polymer coating. Further, in one-sided coating or surface-sizing of a web, the moving face subjected to pressure and placed at the web side opposite in relation to the web face to be coated or surface-sized can be doctored with water or moistened with water mist or steam in order to facilitate the separation of the web. Further, said face can be provided with a coating that repels the coating agent of surface size. When the film press nip is made of two moving belts subjected to pressure, after the nip, before the web is separated from the belt, the coating can be dried in order to increase the filter layer formed by the coating. Above, the invention has been described by way of example with reference to the preferred embodiments illustrated in the figures in the accompanying drawing. The invention is, however, not confined to the exemplifying embodiments illustrated in the figures alone, but different embodiments of the invention can show variation within the scope of the inventive idea defined in the accompanying patent claims.

Claims: Claims 1. A method in film transfer coating in a film press, in which press the nip (N) of the film press is formed out of two faces placed against each other and moving in the same direction so that said moving faces are pressed against each other in the nip, and in which press a coating agent or surface size is applied as a film (F^ F 2 ) onto at least one of the moving faces that form the film press nip (N) and that are subjected to pressing, in which connection the coating agent or surface size is transferred in the nip (N) between the moving faces onto the paper or board web (W) running through the nip, and after the nip the web is separated either directly from the nip or so that it follows one or the other of the moving faces, characterized in that the web is separated from the nip so that, at least at one side of the web, the curve radius of the opening geometry is larger than 0.75 metre. 2. A method as claimed in claim 1, characterized in that, in one-sided coating or surface-sizing of the web, after the nip, the web is made to follow the moving face subjected to pressure and placed at the web side opposite in relation to the web face to be coated or surface-sized before the web is passed out of the film press. 3. A method as claimed in claim 1 or 2, characterized in that at least one or both of the moving faces that form the nip and that are subjected to pressure is/are composed of a belt running through the nip and formed as an endless loop. 4. A method as claimed in claim 1 or 2, characterized in that at least one or both of the moving faces that form the nip and that are subjected to pressure is/are composed of a roll face. 5. A method as claimed in any of the preceding claims, characterized in that, in one-sided coating or surface-sizing of the web, the moving face subjected to pressure and placed at the web side opposite in relation to the web face to be coated or surface-sized is hard, in particular a metal face. 6. A method as claimed in any of the claims 1 to 4, characterized in that, in onesided coating or surface-sizing of the web, the moving face subjected to pressure and placed at the web side opposite in relation to the web face to be coated or surface- sized is composed of a soft coating, in particular of a polymer coating. 7. A method as claimed in any of the preceding claims, characterized in that, in one-sided coating or surface-sizing of the web, the moving face subjected to pressure and placed at the web side opposite in relation to the web face to be coated or surface-sized is doctored with water or moistened with water mist or steam in order to facilitate the separation of the web. 8. A method as claimed in any of the preceding claims, characterized in that, in one-sided coating or surface-sizing of the web, the moving face subjected to pressure and placed at the web side opposite in relation to the web face to be coated or surface-sized is composed of a coating that repels the coating agent or surface size. 9. A method as claimed in any of the preceding claims, in which the nip in the film press is composed of two moving belts subjected to pressure, characterized in that, after the nip, before the web is separated from the belt, the coating is dried so as to increase the filter layer formed by the coating. 10. A film press intended for carrying out the method as claimed in any of the preceding claims, which press comprises a film press nip (N) formed out of two faces that are placed against each other and that move in the same direction so that said moving faces are pressed against each other, through which nip the paper or board web (W) has been passed, and which film press is provided with coating means for application of a coating agent or surface size as a film (Fi , F 2 ) onto at least one of the moving faces that form the film press nip (N) and that are subjected to pressing, from which face the coating agent or surface size has been arranged to be transferred in the film press nip onto the paper or board web (W), in which connection, after the nip, the web has been fitted to be separated either directly from the nip or so that it follows one or the other of the moving faces, characterized in that the curve radius of the opening geometry after the film press nip is, at least at one side of the web, larger than 0.75 metre. 11. A film press as claimed in claim 10, characterized in that, in a film press that coats or surface-sizes the web from one side, after the nip, the web has been made to follow the moving face subject to pressure and placed at the web side opposite in relation to the web face to be coated or surface-sized before the web is passed out of the film press. 12. A film press as claimed in claim 10 or 11, characterized in that at least one or both of the moving faces subjected to pressure and forming the nip comprise(s) a belt that runs through the nip and that has been formed as an endless loop. 13. A film press as claimed in claim 10 or 11, characterized in that at least one or both of the moving faces subjected to pressure and forming the nip comprise(s) a face of a film press roll. 14. A film press as claimed in any of the claims 10 to 13, characterized in that, in a film press that coats or surface-sizes the web from one side, the moving face subjected to pressure and placed at the web side opposite in relation to the web face to be coated or surface-sized is hard, in particular a metal face. 15. A film press as claimed in any of the claims 10 to 13, characterized in that, in a film press that coats or surface-sizes the web from one side, the moving face sub- jected to pressure and placed at the web side opposite in relation to the web face to be coated or surface-sized comprises a soft coating, in particular a polymer coating. 16. A film press as claimed in any of the claims 10 to 15, characterized in that, in a film press that coats or surface-sizes the web from one side, the moving face subjected to pressure and placed at the web side opposite in relation to the web face to be coated or surface-sized is provided with means for doctoring of said face with water or for moistening of the face with water mist or steam in order to facilitate the separation of the web. 17. A film press as claimed in any of the claims 10 to 16, characterized in that, in a film press that coats or surface-sizes the web from one side, the moving face subjected to pressure and placed at the web side opposite in relation to the web face to be coated or surface-sized is provided with a coating that repels the coating agent or the surface size. 18. A film press as claimed in any of the claims 10 to 17, in which the nip in the film press has been formed out of two moving belts subjected to pressure, characterized in that the film press is provided with means for drying the coating after the nip before the web is separated from the belt in order to increase the filter layer formed by the coating.

Inventors: RANTANEN RAUNO, RANTANEN, RAUNO
Applicants: RANTANEN RAUNO, VALMET CORP, RANTANEN, RAUNO, VALMET CORPORATION
Application year: 1999
Main classification: D21H23
IPC codes: B05C1, B05C9, D21H23