Abstract:
Method and arrangement for coating a web in a film press nip defined by a pair of rotating rolls and through which the web runs and in which a coating agent is applied as a film onto a face of at least one of the rolls and transferred in the nip from the face of the roll(s) onto a respective side of the web to thereby coat the respective side of the web. Steam jets are applied to each side of the web that is being coated to prevent formation of coating-agent mist at a web-outlet side of the nip. The steam jets can be directed from a location after the nip to the web-outlet side of the nip and/or substantially to a point of separation of the web from contact with the roll(s).

Description:
FIELD OF THE INVENTION 
     The invention concerns a method in film transfer coating, in which the coating agent is applied as a film onto the face of at least one of the rolls that form a film press nip, from which roll face the coating agent is transferred in the nip between the rolls onto the paper or board web running through the nip. 
     Further, the invention concerns an equipment intended for carrying out the method in film transfer coating in connection with a film transfer press, which film transfer press comprises a film press nip formed by a pair of rolls, through which nip the paper or board web is passed, and which film transfer press is provided with coating devices for spreading a coating agent as a film onto the face of at least one of the rolls that form the film press nip, from which roll face the coating agent is arranged to be transferred in the nip between the rolls onto the paper or board web. 
     BACKGROUND OF THE INVENTION 
     By means of the film transfer technique, in film size presses the application of a size or paste onto the paper is carried out so that, depending on whether one side or both sides of the paper are supposed to be coated, coating agent films of the desired thickness are spread onto the face or faces of one or both of the rolls in the film size press by means of applicator devices, in which connection these coating agent films are transferred onto the paper web in the nip formed by the size press rolls as the paper web runs through said nip. As a rule, the film rolls in the film size presses are provided with resilient coatings, and the application onto the roll face can be carried out by using some prior-art blade coater, a bar coater provided with a smooth or grooved coating bar, or an equivalent nozzle equipment. At present, in paper machines and, thus, also in the related film size presses, ever higher speeds are aimed at, in which connection in particular this increased speed in size film presses has provided a new problem, i.e., in particular when running takes place with large coating quantities, formation of mist of the coating agent occurs at the outlet side of the press nip. The formation of mist in itself would not be a major problem, but a problem arises when this mist gathers on the constructions of the machine and drips from there onto the paper web and spoils the paper quality, or when the mist spreads into the air in the room. Since the problem has been encountered with higher significance only with increased web speeds, in the prior art at least substantially significant solutions have not been suggested for elimination of this problem. 
     Indeed, there are some prior-art solutions, and of them can be mentioned, 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. Similarly, in the FI Patent No. 93,243 a solution is suggested in which the formation of mist in itself is not prevented, but this mist is recovered, or at least attempts are made to recover the mist, by means of mechanical devices fitted at the outlet side of the nip, so that the mist should not have access to other constructions. Thus, this earlier publication does not provide a solution for the formation of mist itself either. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a method by whose means, in the film size press, formation of mist of coating agent and problems arising from same are avoided. In view of achieving this object, the method in accordance with the invention is mainly characterized in that, in the coating process, formation of coating-agent mist is prevented at the outlet side of the film press nip so that steam jets are applied at least to the side of the web that is being coated. 
     The equipment in accordance with the invention is mainly characterized in that, in connection with the film transfer press, steam supply devices are provided so as to apply steam jets at least to the side of the web that is being coated in order to prevent formation of coating-agent mist at the outlet side of the film press nip in connection with the coating process. 
     It is the most important advantage of the present invention over the prior-art solutions that, in the invention, in itself, attempts are not made just to prevent spreading of mist, but instead, attempts are made to prevent formation of mist completely. When no mist is formed at all, it does, consequently, not constitute problems either. The formation of mist is prevented in the present invention by means of spraying of steam, which provides the additional advantage that, by means of said steam jets, the web is supported and, thus, fluttering of the web is prevented, and it has been noticed that said fluttering in itself causes formation of mist. Further advantages and characteristic features of the invention will come out from the following detailed description of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, the invention will be described by way of example with reference to the figures in the accompanying drawing. 
     FIG. 1 is a fully schematic side view of a film press in which the method in accordance with the present invention is utilized. 
     FIG. 2 is a schematic illustration of an enlarged detail from the area A in FIG.  1 . 
     FIGS. 3A and 3B are schematic illustrations of further embodiments of the invention, applied to single-sided coating of the web. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, the film press is denoted generally with the reference numeral  10 . The film press  10  comprises a film press frame  11  mounted on a foundation C, on which frame  1  the film press rolls  12 , 16  are mounted in a conventional way. The bearing housing  13  of the first film press roll, i.e. the lower roll, is mounted directly on the frame  11  of the film press and attached to the frame rigidly. On the other hand, the bearing housing  17  of the second film press roll, i.e. the upper roll, is mounted on a loading arm  19 , which is linked pivotally on the film press frame  11  by means of a pivot shaft placed in the cross direction of the machine. Between the loading arm  19  and the film press frame  11 , loading cylinders  22  are fitted, by whose means the loading arm  19  is loaded in order to produce a nip pressure of the desired extent in the nip N formed by the lower roll  12  and the upper roll  16 . Either one of the film press rolls  12 , 16 , preferably the upper roll  16 , or, alternatively, both of the film press rolls  12 , 16  can be variable-crown rolls in order that the nip N pressure could be brought to the desired level in the cross direction of the machine. In the solution shown in FIG. 1, each film press rolls  12 , 16  is provided with coating means  15 , 21 , by whose means the size films or equivalent coating agent films are spread and smoothed onto the faces  14 , 18  of said rolls. Of course, it is fully obvious that, if the web W supposed to be coated from one side only, the film press roll placed at the corresponding side of the web W is provided with a coating device, in which case the film press roll at the opposite side is not coated. 
     In the solution shown in FIG. 1, when the web W runs through the nip N, the coating agent films are transferred from the roll  12 , 16  faces  14 , 18  onto the web W′. The web W is passed into the film press nip N over a guide roll  23 , which is mounted on a stationary frame  24  mounted on the foundation C. After the film press nip N, the web W′ is passed over a spreader roll  25  to further processing. The construction and the operation of the coating devices  15 , 21  are not described in detail in this connection, because they can be conventional, known from the prior art, and, thus, they do not constitute a part of the present invention. Nor will the means of support of the spreader roll  25  be described in more detail in this connection. In particular in pigment coating, after the film size press, as a rule, airborne web dryers are employed before a drying cylinder. 
     The method in accordance with the present invention is based on supply of steam onto the web W, and most advantageously the steam is supplied as steam jets to the outlet side of the nip N. This is seen in FIG. 1 in the area A and in more detail in FIG.  2 . Steam can also be supplied onto the web W before the nip N, as is shown in the area B in FIG.  1 . This will be reverted to briefly later. 
     As was already described above in relation to FIG. 1, in film press coating, the coating agent is applied as films F 1 ,F 2  onto the faces of the film press rolls  12 , 16  that form the nip N, and the web W is passed through said film press nip N, in which connection, in the nip, the coating agent films F 1 ,F 2  are substantially transferred onto the web W′, as is illustrated in FIG.  2 . Further, as was described above, in particular new and high-speed machines involve the problem that, in particular when running takes place at high speeds and with large coating quantities, at the outlet side of the film press nip N formation of coating-agent mist occurs. Thus, in the present invention, this problem has been solved so that, at least at the outlet side of the nip N, at least on the side of the web W′ that is supposed to be coated, steam supply means  30 , 31  are provided, from which steam jets are passed after the nip N to the point of separation between the web W′ and the roll  12 , 16 . With this procedure, it has been possible to exclude formation of mist completely, even though it is not known exactly what this elimination of mist is based on. 
     Concerning this, a number of different alternative theories can be presented, whose correctness has not been proved with certainty. According to one theory, the steam “condenses” the mist onto the web W′. A theory has also been presented according to which, in the steam phase, the film splitting filaments are shorter because of the difference in surface energy between air and steam. Further, it has been suggested that the steam jet stabilizes the web separation line. Further, it has been suggested that the steam raises the temperature of the roll face or forms an aqueous film on the roll face, in which connection the splitting of the film is facilitated. 
     As was stated above, it has not been possible to present a fully reliable theory concerning the prevention of mist formation, but, in any case, it has been established with certainty, with a great surprise, that passing of steam jets to the outlet side of the nip N to the point of separation between the web W′ and the roll  12 , 16  makes the mist disappear. 
     With reference to FIG. 1 above, the further possibility is provided that the web W can already be steam-treated before the nip N. In this way the temperatures of the films F 1 ,F 2  could be made higher, which in itself would reduce the mist formation. In FIG. 1 it is illustrated schematically that the steam supply means  32 , 33  placed before the nip N are placed quite far before the nip N. It might, however, be preferable that said devices are placed in direct vicinity of the nip N, in which case the steam jets could be directed either at the web W, at the coating agent films present on the faces  14 , 18  of the film press rolls, or directly into the nip N, i.e. into the gap between the roll face  18 / 14  and the web W. The primary solution is, however, that shown in more detail in FIG. 2, i.e. spraying of steam to the outlet side of the nip. 
     FIGS. 3A and 3B show further embodiments of the invention, which embodiments are, in the illustrations in the figures, applied to single-sided coating of the web. In the solutions as shown in these figures, a coating device  15  has been fitted in connection with one roll  12  only, of the rolls that form the film press nip N, which coating device applies a film of coating agent onto the face of said roll  12 . In said exemplifying embodiments, at the outlet side of the nip N, a collecting equipment  40   a , 40   b  is fitted, which is supposed to prevent spreading of steam or coating-agent mist that may possibly have been formed in the coating process into the air in the room. The embodiments shown in FIGS. 3A and 3B are highly similar to one another, so that, in the following, they will be examined together. The collecting equipment  40   a , 40   b  comprises a basin or trough provided with walls  41   a , 41   b , which basin or trough is, as is shown in the figures, preferably fitted in the gap between the web W and the roll  12  provided with a coating device  15 , at the outlet side of the nip N. The collecting equipment  40   a , 40   b  is fitted at a very short distance from said roll  12 , preferably so that the wall of the collecting equipment  40   a , 40   b  placed towards the roll  12  passes along the roll  12  face. The collecting equipment  40   a , 40   b  is connected with a suction duct  44   a , 44   b , through which a suction and at least a partial vacuum are applied to the collecting equipment  40   a , 40   b , so that, by the effect of said vacuum, the steam and the coating-agent mist are gathered in the collecting trough. Further, the collecting equipment  40   a , 40   b  is supposed to take advantage of the condensation effect, and therefore preferably at least one of the walls of the collecting equipment  40   a , 40   b  is provided with cooling. Further, the collecting equipment  40   a , 40   b  is provided with a top cover  42   a , 42   b , which is fitted in the vicinity of the web W. In the exemplifying embodiment shown in FIG. 3B, the top cover  42   b  is further provided with sides extending above the edges of the web W, and in partial opposed relationship to a roll  26  over which the web is guided. The top cover  42   a , 42   b  is arranged to be displaceable and adjustable in the lateral direction in compliance with the web width. Therefore, the collecting equipment  40   a , 40   b  is provided with guides  43   a , 43   b , along which the top cover  42   a , 42   b  can be displaced. 
     Since, in the coating process, the formation of the coating-agent mist takes place expressly at the outlet side of the nip N, it is fully possible to use a collecting equipment  40   a , 40   b  as shown in FIGS. 3A and 3B alone to collect and to dispose of the coating-agent mist that has been formed, without the supply of steam onto the web W that was described above. Even by means of the collecting equipment  40   a , 40   b  alone, it is possible to reduce the coating-agent mist at least substantially. Of course, it is possible and even advantageous to make use of the collecting equipment as shown in FIGS. 3A and 3B in connection with the supply of steam, in which case the formation of coating-agent mist can be excluded most efficiently. 
     Above, the invention has been described by way of example with reference to preferred exemplifying embodiments of the invention illustrated in the figures in the accompanying drawing. The invention is, however, not confined to the exemplifying embodiments shown 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.