Method and assembly for coating a paper web

Abstract: The invention relates to a method and assembly for coating a moving paper or paperboard web (1), in which method coating mix is applied to the surface of the web (1) by means of a rotating applicator roll (11) with simultaneously transferring fresh coating mix onto the surface of said applicator roll. The coating mix applied to the web (1) is levelled with the help of smoothing means (4, 5, 6) acting against a backing roll (8). According to the invention, the web (1) to be coated is pressed against the applicator roll (11) with the help of an endless band (2), which is adapted to pass over at least said applicator roll (11), said backing roll (8) and said adjustment roll (10) placed in front of said applicator roll (11) in the machine direction of the web so that the band (2) with the web (1) backed by said band are together deflected to pass over said applicator roll (11) along their path between said adjustment roll (10) and said backing roll (8).

Description: The present invention relates to a method according to the preamble of claim 1 for coating a paper or paperboard web, in which method the coating mix is applied to the web being coated by means of an applicator roll. The invention also concerns an assembly suited for implementing said method. To improve the printability of paper, the base paper made in a paper machine may be coated with a variety of coating formulations. The applied coat must be smooth and have a constant coat weight after it is applied to a moving web in a slurried form and excess water is removed from the applied coat in dryers. The application of the coat may be divided into two steps comprising the application of the coat and subsequent smoothing of the coat combined with metering of the applied coat. A method used in the art is coat application to the paper web by means of a roll applicator, in which the web is passed in intimate contact over a rotating applicator roll. While the applicator roll is usually rotated along with the web at a speed lower than the web speed, counter-directional rotation and other types of differential speed arrangements are also known in the art. The coating mix is transferred onto the applicator roll surface by means of a metering device apparatus, or alternatively, the applicator roll is adapted to rotate in a coating mix pan. From the applicator roll surface, the coating mix is transferred to the surface of the moving web, and subsequent to application, the coat is further smoothed by doctoring away the excess coat with a doctor blade, levelling rod or air knife. The web to be coated can be pressed against the applicator roll by a variety of arrangements. In early coaters, a guide roll was placed to both sides of the applicator roll, and the web was kept against the applicator roll with help of the guide rolls. In such an assembly, the maximum usable applicator nip pressure between the web and the applicator roll is determined by the web tensile strength, which means that the maximum application pressure remains rather low. Such an arrangement is suited for low web speeds only, because the air layer travelling along with the web easily becomes entrained at high web speeds in the nip between the web and the applicator roll, thus causing surface defects in the coated web. Hence, this method is today used for coating webs other than paper in locations operated at sufficiently low web speeds. The web speed of such a low-speed coater may be increased marginally by injecting air to the backing side of the web, thus pressing the web against the applicator roll. In paper manufacture, roll application occurs against a backing roll. In such coaters, the web is passed via a gap or nip formed between the applicator roll and the backing roll, whereby a higher nip pressure can be used. By virtue of the elevated nip pressure, the coating mix can be efficiently forced into the base paper in the roll applicator, which in conjunction with the swelling of the fibers and increase in the bulk volume facilitates well-controlled application of heavy coats. The applied coat weight is initially often many-fold in comparison with the final coat weight. Therefore, the deep penetration of the coat and large volume of doctored coating mix requires a great doctoring force to be controlled in the smoothing of the coat. Such a great doctoring force in turn imposes a heavy stress on the moist web and causes web breaks. It has been noted, that the major portion of web breaks occur at blade coaters, which means that an improvement in the reliable function of coaters would have a major impact on the production efficiency of paper manufacture and total availability performance of the paper manufacturing machinery. US-A-5,368,893 describes a method and apparatus of above mentioned type. The apparatus comprises, in direction of web travel, an adjustable guide roll in front of the applicator roll capable of adjusting the lenght of the applicator zone. The applicator roll also comprises doctoring means for smoothing the layer of coating mix on the appalicator roll before the coating mix is transferred on the web. The coating layer is smoothed by an air knife. Since the doctoring power of an air knife is low, the coating mix is premetered on the applicator roll in order to limit the amount of applied coating close to the final desired coating weight to facilitate doctoring. The application distance, i.e. the angle over which the web is turned over the applicator roll can be adjusted for different paper or board grades by the adjustable guide roll. Even though this apparatus is better suited for coating methods using air knife doctoring than earlier apparatuses, it has still several drawbacks of apparatuses described above. As mentioned, the application force and web tension has to be kept very low to avoid web breaks. This may cause on high speeds entrainment of air between the web and the applicator roll even though the guide roll is arranged close to the applicator roll. The method does not allow any adjustment of the application force and is only suitable for thick and strong paper or cardboard grades that can whitstand high tension even when wet. If the web tension is lowered, the risk for uneven coating rises rapidly. Also, even for thickest cardboard grades the application pressure remains low compared to pressures available when backing roll is used. Therefore no penetration of the coating mix on the web can be achieved. Further, because of the freely travelling wet web, only air knife doctoring can be used since the wet web cannot withstand stresses imposed to the web by blade or rod doctors. Thereby the use of this method and apparatus is limited to the low speeds applicable to air knife doctoring. One further drawback caused by the low or even nonexistent application pressure is that the coating mix layer forms a hydrofoil between the web and the application roll. This foil lifts the web off the roll whereby the contact distance can not be accurately adjusted as the contact point and the leaving point of the web and the coating layer may travel on the surface of the roll. GB-A-2 272 850 describes an apparatus for coating a moving web wherein the coating mix is spread on the web against first backing roll and smoothed against second backing roll. The application apparatus used herein is usually a blade measured applicator unit or corresponding applicator forming a linear mechanical force against the web and the first backing roll. The backing rolls may be substituted with a backing belt in order to save space. This document does not relate to problems typical for roll application methods. GB-A-1 337 429 describes a coating method wherein a moving web is pressed against a surface of an applicator roll by a band. The band is guided by guide rolls and one of the rolls located downstream after the applicator roll is adjustable. The apparatus enables adjustment of the zone of contact over the applicator roll and the application pressure. The applied coating layer is smoothed and levelled by doctoring and the doctor is positioned after the location where the web leaves the band. Therefore no support is provided between the band and the doctor. As stated above, the strength of the wet web is extremely low at this point and web breaks may occur. At concurrent high web speeds, roll applicators are hampered by splashing of the coating mix, which is caused by the splitting of the coat film as the web exits the nip between the applicator roll and the backing roll. Here, a part of the coat film remains adhered to the web, while the other part sticks to the surface of the applicator roll, whereby the coat film will become extended between the web and the applicator roll forming droplets that are centrifugally thrown out from the nip. The larger the tangential exit angle of the web leaving the applicator roll, the more splashing will occur, because this condition is related to a more violent splitting of the coat film. Hence, concurrent coaters tend to use large-diameter rolls to reduce the tangential exit angle of the web leaving the applicator roll. Simultaneously, the larger diameter of the applicator roll extends the application zone length and improves the application result. However, fabrication of such large-diameter applicator rolls to exact tolerances combined with good dynamic balancing is expensive, and with the trend toward larger-diameter applicator rolls, the design and location of the coater in a factory building becomes ever more difficult simply due to the larger size of the coater machinery. As roll applicators, however, can achieve heavy coats with good quality of the coat, reduction or elimination of the above-described shortcomings is most challenging. It is an object of the present invention to provide a roll applicator apparatus with a wider control range of application zone parameters and essentially improved runnability. The goal of the invention is achieved by supporting the web during coating by means of a backing band against the applicator roll and at the doctoring step. More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the applicator assembly according to the invention is characterized by what is stated in the characterizing part of claim 9. The invention provides significant benefits. The invention facilitates significantly wider adjustment of coating parameters. The distance between the point of application and the point of doctoring can be made adjustable, whereby the amount of water penetrating into the web, and thus, the degree of drying of the coat prior to doctoring can be controlled. By increasing this distance, more water can penetrate into the web, thus increasing the solids content of the applied coat. Result-ingly, a thicker coat can be left on the web during doctoring stage. The application nip can be made longer than in a backing roll assembly, because the backing band may be arranged to conform to the contour of the applicator roll and the application zone length in the machine direction can be controlled by adjusting the position of the movable guide roll. By virtue of the longer application zone in the nip, the application result and the quality of the applied coat are improved. The tangential exit angle between the band-backed outgoing web and the applicator roll can be made smaller than that of a backing-roll supported web, whereby a smoother splitting of the coat film is obtained resulting in less splashing. The backing band supports the web so that the dynamic forces imposed thereon by web speed and tensile stress variations will be inflicted on the backing band, rather than on the web, resulting an essential reduction in the magnitude of web-breaking forces. Furthermore, the use of a band to back-up the web is capable of flattening pouches caused-by swelling of the web, since both the band and the web are passed in a concave manner over the applicator roll and in a convex manner over the doctor blade. In the following the invention will be examined in greater detail by making reference to the appended drawing illustrating a preferred embodiment of the invention. Referring to the diagram, the coater apparatus shown therein utilizes a convention arrangement for applying a coating mix onto a web 1. The smoothing device is a doctor blade unit comprising a support beam 5 with a doctor blade assembly 4 attached thereto, a coating mix pan 7 and rotating arms 6, which are connected at their one ends with a pivotal joint 14 to the frame 3 of the coater and at their other ends to the ends of the support beam 5. In the vicinity of the doctor blade unit 4 - 7 is adapted a large-diameter backing roll 8 for backing the web 1 at the point of doctoring. The web 1 is passed via the nip between the backing roll 8 and the doctor blade. As the different versions of doctor blade assemblies and their function are known to those versed in the art, a detailed description of these parts of the coater may be omitted herein. The applicator assembly of the coater comprises an applicator roll 11, a nozzle applicator device 12 and coating mix pan 13. Supported all its way by an endless band 2, the web 1 to be coated is adapted to pass over the surface of the applicator roll 11 and through the nip between the backing roll 8 and the doctor blade unit 4 - 7. The band 2 enters the applicator roll 11 over an adjustment roll 10 and therefrom travels to the backing roll 8 passing over the surface of the roll. From the backing roll 8, the band is guided via three guide rolls 9 back to the adjustment roll 10. The paper or paperboard web 1 to be coated meets the band at the adjustment roll 10. The positions of the adjustment roll 10, the applicator roll 11 and the backing roll are adapted so as to make the band 2 pass over a curved segment of the surface of the applicator roll 11. In this fashion, the application nip can be made longer, which improves the transfer of the coating mix onto the web 1. The application zone length can be varied by moving the adjustment roll 10 in the direction of arrow b relative to the applicator roll 11. While a vertical movement of adjustment is used in the embodiment shown in the diagram, other movement directions of the adjustment roll 10 may be necessary if the travel of the band 2 and the location of the rolls are altered. By moving the adjustment roll 10 downward, the application zone length will be increased, and respectively, an upward movement of the roll shortens the application nip length. Obviously, a compensation facility for the band travel length change due to a change in the position of the adjustment roll 10 must be arranged. Such a compensation is easiest to implement by means of a separate tensioning roll, whose movement may be adapted to occur automatically with the movement of the adjustment roll 10, whereby the band tension can be kept constant during the adjustment of the application zone length. The tensioning roll may be one of the guide rolls 9, or alternatively, the adjustment roll 8 may be arranged to move along a nonlinear path so curved that a constant band tension is maintained. In practice, the band tension adjustment is required because it offers a method of controlling the application pressure. As the pressure p in the application nip is proportional to the quotient of the tension k of the band 2 divided by the radius r of the applicator roll 11, the application nip pressure can be controlled by either adjusting the band tension or altering the radius of the applicator roll 11. Since the latter alternative of changing the applicator roll radius is cumbersome, nip pressure adjustment by means of a separate tensioning roll is more advantageous. While it is also feasible to design the path of the adjustment roll 10 such that the roll acts as a tensioning roll only, this arrangement fails to offer the adjustment facility of the application nip zone length. The actual web tension control in coating takes place by adjusting the web tension with the help of differential speed control over the coater station. In the most preferred embodiment of the invention, the distance between the applicator roll 11 and the coater 4 - 7 is made adjustable in the direction marked by arrow a in the drawing. Referring to the drawing, it is obvious that as a change of the position of the applicator roll 11 also affects the application zone length, it is advantageous in this embodiment to use the adjustment roll 10 for setting the application zone length, while the band tension is adjusted with a separate tensioning roll. In this fashion, the other control parameters can be kept constant during the adjustment of one control variable. The application process in this embodiment can be controlled in a flexible manner by altering the application zone length, application pressure and dwell time between the point of application and the point of doctoring. Besides, conventional adjustments of doctor blade profile, angle and loading offer additional control parameters as well as the comtrol of the speed of the applicator roll. The applicator assembly may thus be optimized to maximize the coat quality under given running conditions. The external factors affecting the control of the coater include the properties of the coating mix applied in combination with the base paper, web speed and desired coat weight. To achieve a constant coat profile, an extremely smooth surface and a constant thickness of the band are important in the selection of the band material. Hence, the band must have a smooth surface that may contain only small micropores. It must be noted that the web cannot be adhered to a nonporous band by means of, e.g., a vacuum applied to the rear side of the band. However, relatively strong adherence of the web to the band surface is provided by adhesion between the moist band and the web. Additionally, the adhesion is slightly increased due to the static charging of the web prior to coating as well as after coating due to increased adhesion of the wet web to the band surface. Moreover, the applicator roll and the levelling device impose an additional pressure keeping the web against the band. The band material may be selected from the group of pliable metals such as stainless steel, brass and aluminium and different polymers that can be processed into a smooth band of sufficient tensile strength. The function of the above-described assembly is as follows. The web 1 to be coated is passed to the coater from the previous paper-making stage and is guided to meet the band 2 at the adjustment roll 10. The roll 1 attaches by adhesion to the band 2 and passes on the band to the applicator roll 11. On the approach side of the web 1 onto the applicator roll 11 is adapted a nozzle applicator device 12 suited for metering coating mix onto the applicator roll 11. The applicator roll 11 rotates codirectionally with the machine direction of the web 1 at a reduced speed of approx. 15 - 25 % with respect to the speed of the web 1 and transfers the coating mix from its surface onto the web 1. An excess amount of coat is applied onto the applicator roll 11, and that part of the coating mix not transferred from the roll to the web 1 is taken back to the coating mix pan 13. The applicator roll 11 can be scraped clean from the coat, or alternatively, the coat may be allowed to circulate on the surface of the roll 11 back to the application zone. In the application zone, the weight of the thick coat layer transferred onto the web 1 is very high, typically approx. 200 - 250 g/m 2 . Because the maximum coat weight that can be applied in a controlled manner in a single coating step of roll application to the web surface is approx. 20 g/m 2 , the applied coat must be doctored heavily to achieve a desired coat thickness. Conventionally, the coat is smoothed with the help of a doctor blade that imposes a strong stress on the web. In the assembly according to the invention, the stress imposed on the web is further transmitted to the band, whereby the risk of web breaks is reduced essentially. The excess coat removed by the doctor blade from the surface of the web 1 is taken back to the coating mix pan 7 of the doctor blade unit. The entire doctor blade unit is supported by the above-described rotating arms 6, thus facilitating the rotation of the entire assembly away from the backing roll 8 for cleaning, blade change and other maintenance operations. From the doctor blade unit 4 - 7, the web 1 is passed supported by the band 2 onto the first guide roll 9, which serves to detach the web from the band 2 and pass the web further to the dryer. Besides that described above, the present invention may have alternative embodiments. Instead of a doctor blade, the smoothing of the coat can be made using a levelling rod or an air knife, but these devices may not necessarily provide sufficiently strong levelling force for a large amount of applied coat, thus limiting the use of these alternatives to special applications. Different arrangements of the guide roll set and other rolls are also possible, because the approach and exit directions of the web may be varied in different coater designs. In fact, the assembly according to the invention is easy to modify according to local needs. One feasible design has the band adapted to pass over the backing roll, the guide roll and the applicator roll only. However, in such an assembly the angle of the band over the backing roll, and respectively, the angle of the band over the adjustment roll become very large, which means that the band must be made from a sufficiently pliable material to permit sustained bending of the band over the rolls at high web speeds. In addition to the above-described alternatives, one feasible design has the applicator roll made movable toward the band and away therefrom, thus facilitating adjustment of the application zone length and band tension.

Claims: A method of coating a moving paper or cardboard web (1), in which method the web (1) is passed to the coater, coating mix is applied to a surface of the web (1) by means of a rotating applicator roll (11) with simultaneously transferring fresh coating mix onto the surface of said applicator roll (11), the coating mix applied to the web (1) is levelled with a smoothing means (4, 5, 6) acting against a backing roll (8), and the web (1) to be coated is pressed against the applicator roll (11) with an endless band (2), characterized in that the endless band is adapted to pass over at least said applicator roll (11), said backing roll (8) and an adjustment roll (10) placed in front of said applicator roll (11) in the machine direction of the web so that the band (2) with the web (1) backed by said band are together deflected to pass over said applicator roll (11) along their path between said adjustment roll (10) and said backing roll (8). A method as defined in claim 1, characterized in that the mutual position of said adjustment roll (10) and said applicator roll (11) is adjusted so as to permit a change of the tangential approach angle of the band (2) with the web (1) backed by the band to the applicator roll (11), simultaneously changing the applicator zone length. A method as defined in claim 1 or 2, characterized in adjusting the mutual distance between said applicator roll (11) and said smoothing means (4, 5, 6). A method as defined in any of foregoing claims, characterized in that the tension of said band (2) is adjusted by a tensioning roll (9) acting a guide roll for said band (2). A method as defined in any of foregoing claims, characterized in that the tension of said band (2) is adjusted through the position adjustment of said applicator roll (11). A method as defined in claim 2, characterized in that the mutual position between said adjustment roll (10) and said applicator roll (11) is adjusted through the position adjustment of said adjustment roll (10) so that the tangential approach angle of the band (2) with the web (1) backed by the band to the applicator roll (11) is changed, simultaneously changing the applicator zone length. A method as defined in claim 2, characterized in that the mutual position between said adjustment roll (10) and said applicator roll (11) is adjusted through the position adjustment of said applicator roll (11) so that the tangential approach angle of the band (2) with the web (1) backed by the band to the applicator roll (11) is changed, simultaneously changing the applicator zone length. A method as defined in claim 3, characterized in that the mutual distance between said applicator roll (11) and said smoothing means (4, 5, 6) is adjusted through the position adjustment of said applicator roll (11). An assembly for coating a moving paper or cardboard web (1), said assembly comprising a rotatable applicator roll (11) for applying a coating mix to a surface of said web (1), an applicator device (12) for transferring fresh coating mix onto said applicator roll (11), a backing roll (8) placed after said applicator roll (11) in the machine direction of said web (1), smoothing means (4, 5, 6) adapted to operate backed by said backing roll (8) for levelling of the coating mix applied to the web (1), and an endless band (2) adapted to pass over at least said applicator roll (11) characterized in that an adjustment roll (10) is placed in front of said applicator roll (11), and the endless band (2) is adapted to pass over at least said applicator roll (11), said backing roll (8) and said adjustment roll (10) placed in front of said applicator roll (11) in the machine direction of the web so that the band (2) with the web (1) backed by said band are together deflected to pass over said applicator roll (11) along their path between said adjustment roll (10) and said backing roll (8). An assembly as defined in claim 9, characterized by means for adjusting the mutual position between said adjustment roll (10) and said applicator roll (11) so that the tangential approach angle of the band (2) with the web (1) backed by the band to the applicator roll (11) is changed, simultaneously changing the applicator zone length. An assembly as defined in claim 9 or 10, characterized by means for adjusting the mutual distance between said applicator roll (11) and said smoothing means (4, 5, 6). An assembly as defined in any of foregoing claims 9 - 11, characterized by a tensioning roll (9) adapted to guide said band (2) so as to permit tension control of said band (2). An assembly as defined in any of foregoing claims 9 - 12, characterized by means for altering the tension of said band (2) through the position adjustment of said applicator roll (11). An assembly as defined in claim 10, characterized by means for moving said adjustment roll (10) so that the tangential approach angle of the band (2) with the web (1) backed by the band to the applicator roll (11) is changed, simultaneously changing the applicator zone length. An assembly as defined in claim 10, characterized by means for moving said applicator roll (11) so that the tangential approach angle of the band (2) with the web (1) backed by the band to the applicator roll (11) is changed, simultaneously changing the applicator zone length. An assembly as defined in claim 11, characterized by means for moving said applicator roll (11) relative to said smoothing means (4, 5, 6). An assembly as defined in any of foregoing claims 9 - 16, c h a r a c t e r i z e d in that said smoothing means is a doctor blade unit (4, 5, 6).

Inventors: KINNUNEN JORMA, KINNUNEN, JORMA
Applicants: METSO PAPER INC, METSO PAPER, INC.
Application year: 1996
Main classification: D21H23
IPC codes: B05C1, D21H23, B05C3, B05D7, B05C11