Method and assembly for coating a moving web

Abstract: The present invention relates to a method for coating a paper or paperboard web by at least two coat layers. The goal is to achieve a two-layer coating method capable of providing a smooth coat with high opacifying power. The invention is based on initially applying onto the web to be coated a first coat layer, whose surface is smoothed by means of a smoothing element, after which onto the partially or entirely dried, moist first coat layer of the web is applied a second coat layer using a transfer roll coater.

Description: The present invention relates to a method according to the preamble of claim 1 for coating a moving paperboard or paper web with at least two coat layers. The invention also concerns an assembly suitable for implementing the method. To improve the printability of paper and cardboard, one or more coat layers can be applied on these webs. The greater the number of coat layers the better the quality of the coated sheet with the additional possibility of using coating furnishes of different compositions. However, such use of two different coat layers causes a significant increase of production costs, because the coat application is conventionally made in several steps and the applied coat is dried prior to the application of the next layer. The coat can be applied either directly on the web surface using, e.g., a blade or rod coater, or alternatively, a transfer roll coater in which a coat film premetered on a transfer roll is applied in a roll nip on the web surface. Blade and rod coaters are characterized in that the coater seals up the surface texture voids of the base web and smooths the surface, whereby the thickness of the coat layer varies according to the volume of the surface texture voids in the base web. The final result is a smooth coat having an uneven brightness distribution combined with difficult control of smooth absorption of the coat. Transfer roll coating provides a coat with an essentially improved constant thickness, whereby easy control of coat absorption is attained though sufficient smoothness properties are a problem particularly with thicker grades of paper and paperboard. The transfer coat also achieves a relatively even coverage whereby the coated web brightness of a base web with, e.g., a low initial brightness, can be improved essentially using this coating method. Furthermore, since the transfer roll coating method dispenses with a doctor blade running in contact with the base web to be coated, the method offers excellent runnability with respect to abruptions in coating process. Two different approaches have been applied to utilize the advantageous properties of both coating methods. Transfer roll coating is used widely in precoat application, whereby the method can improve the coating result already through the increased total coat weight in which already a single doctored coat layer can assure sufficient coat smoothness. Conventionally, the precoat is dried prior to the subsequent final coat application, which typically is applied using a doctor blade coater. Also known in the art from US-A-2,937,955 is doctor blade application directly onto a semidry transfer precoat. A precondition to the use of this method is that the precoat has already settled or set so much that it can take the final coat application by means of the doctor blade coater. Setting of the coat can be speeded by, e.g., partial drying of the precoat in situations where the coat has not become sufficiently dewatered through absorption under pressure in the coating nip and due to moisture migration from the coat during the web travel over the free distance between the precoater and the final coater units. The web travel delay and coat setting time occurring between the transfer precoater and the doctor blade final coater can be extended by, e.g., increasing the web travel distance between the coater units thus achieving sufficient setting and dryness of the precoat through the extended moisture absorption time for the subsequent doctor blade coat application step. US -A-5,340,611 discloses a method in which the amount of coat applied in the transfer roll coating step is kept so small that the doctor blade of the knife applicator device cannot scrape the precoat away. However, such a method fails to achieve the maximum precoat weight possible by means of a transfer roll coater, and the first precoat layer remains very thin. According to the method, the smoothing element of second applicator device can be a levelling rod instead of a doctor blade. In pilot-scale test runs of this paperboard coating method, not even increasing the delay between the coating steps could make the precoat set at 10 g/m 2 coat weight sufficiently to avoid partial scraping-away of the already applied precoat in the subsequent doctor blade applicator. As the web speed is fixed by the speed at the coat transfer nip while the subsequent blade applicator causes a braking action on the web, a problem was also caused by the tendency of the web to produce a bag or slack in front of the smoothing blade of the applicator device resulting in web breaks. Hence, the method was not found practicable. The frequency of web breaks is further increased by the fact that the blade application step, which causes appreciably higher stress on the web than the preceding transfer roll coating step, is performed on a web of high moisture, whereby the web strength is obviously much lower than that of a dry web. Another approach to the utilization of the special characteristics of transfer and blade application steps is disclosed in FI patent application 941,803 by the applicant of the present invention, wherein the coater unit has a design permitting the use, according to the specifications of the final product and/or runnability criteria, of either a transfer or blade applicator as alternative methods yet employing the same coat dryer for both coating methods. Because this apparatus fails to utilize the benefits of two-layer coating, its principal benefit is to increase the flexibility of production, but not to improve the quality of the end product over comparable one-layer coating methods. WO-A-92/00419 discloses a blade coating method wherein the second coating layer is spread on a wet first coating layer either by a short-dwell applicator or a roll applicator. If short-dwell applicator is used as the second applicator, the method has same drawbacks as the method described in US-A-5,340,611. The roll applicator described in WO-A-92/00419 is a kiss roll applicator wherein the applicator roll is immersed partially into coating mix and lifts coating mix by its rotation on the surface of the web. Typical for this kind of applicators are a very large amount of applied liquid or coating mix, typically in range of 200 - 250g/m 2 . Since the amount of coating mix attached on the applicator roll and on the web can not be effectively limited or controlled, the excess coating mix has to be doctored away from the surface of the web. This is usually done with a doctor blade and in WO-A-92/00419 even two successive doctor blades are used. Since the amount of applied coating is very big, heavy doctoring forces must be used in order to obtain desired thickness of coating layer. As described above, doctoring of the second layer after the application and leveling of the first coating layer affects effectively the first coating layer also, whereby the total thickness of the final coating layer is not the sum of both applied and smoothed coating layers but considerably less. Further, the distance between application of the first and the second coating layer is very short, whereby the first coating layer has no time to set and is still in liquid state, whereby it is easily removable by subsequent doctoring. However, particularly thicker grades of paper and paperboard present a need for combining the benefits of both the transfer and the blade coating methods. Yet, due to lack of sufficient footprint and investment capital, intermediate drying of the precoat is not always possible. In such a case the only remaining alternative is the above-described type of wet-on-wet coating, in which the present methods fail to provide sufficiently heavy total coat thicknesses in which the high opacifying power of the transfer roll coating method and the good smoothness given by blade application could be utilized in an optimal manner. It is an object of the present invention to achieve a two-layer coating method capable of applying a smooth coat of high opacifying power. The goal of the invention is accomplished by initially applying a first coat layer onto the surface of the web to be coated which is levelled by means of a smoothing element and subsequently applying a second coat layer by means of a transfer roll applicator onto the web carrying the first, partially or entirely dried, moist coat layer. More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the apparatus according to the invention is characterized by what is stated in the characterizing part of claim 8. The invention offers significant benefits. The invention makes it possible to apply a very heavy coat on the web even without intermediate drying, whereby test runs have proved as heavy total coat weights as 20 g/m 2 possible so that a 10 g/m 2 coat was applied in a blade coater after which another 10 g/m 2 coat was applied in a subsequent transfer roll applicator. The coat will be sufficiently smooth sealing up properly the voids of the web surface texture simultaneously giving good brightness. The blade coating step can be carried out using a low-cost coat furnish capable of smoothing the web surface texture while the transfer roll application step is performed using a higher-cost fine-dispersed coat capable of imparting good printability and coverage. Since the transfer roll coating step does not damage the first coat layer, the smooth precoat makes it possible for the transfer roll to give a smoother final surface than is conventional in the art. The blade of the first coater keeps the web entering the transfer roll coating step tensioned thus maintaining the web between the coater units free of bags. The web runnability remains good as the transfer roll imposes only a minor load on the moist web. In the following the invention is described in greater detail with reference to the appended diagrams illustrating two embodiments of the assembly suitable for implementing the invention. The invention can be carried out using conventional coaters. Referring to Fig. 1, the embodiment shown therein has the blade coating step implemented as a short dwell time coater, while the transfer roll coater is an applicator suited for coating both sides of the web. The short dwell time coater 1 comprises a backing roll 2 and a short-dwell applicator having a blade holder beam 3 incorporating an application chamber 4, in which one of the chamber walls is formed by a doctor blade 5 adapted to the outgoing side of the web travel. The blade 5 is loaded by an adjustable loading assembly 6,which is also suitable for controlling the cross-machine profile of the applied coat. The coating mix is delivered into the applicator chamber 4, wherefrom it is applied onto the surface of the web 7. A portion of the coating mix reflows backward to the web travel and is recirculated. From the short-dwell coater the web is passe to the transfer roll coater where a second coat layer is applied to the web surface. Again referring to Fig. 1, the apparatus shown therein has two transfer rolls 8, 9 forming a nip through which the web 7 is passed. In the illustrated embodiment, the film of coating is metered onto the surfaces of the rolls 8, 9 by means of a rod applicator 12, 13 incorporating analogously with a short-dwell coater an application chamber 10 in which one of the chamber walls is formed by a levelling rod 11 adapted to the outgoing side of the web 7. The levelling rod 11 meters on the surface of the roll 9 a coating film which is transferred to the surface of the web in the nip between the rolls 8, 9. To the surface of the other roll 8 forming the nip may additionally by means of a rod applicator be metered coating mix, water, size or dispersion in order to prevent warping of the web or to subject the other side of the web to a treatment. Between the blade and the transfer roll coating steps is adapted a guide roll 14 for guiding the web to the transfer roll coating step. This roll 14 may be arranged movable, whereby the travel distance of the web 7 between the coating steps can be controlled by adjusting its position. Such an arrangement gives limited possibility of affecting moisture absorption from the coat layer applied by means of the blade coating step, whereby the first coat layer will become dryer prior to the application of the second coat layer even without resorting to intermediate drying. Referring to Fig. 2, another embodiment of the invention is shown therein having a single roll pair 15, 16 serving for both the blade coating and transfer roll coating steps. In this arrangement the web is first passed onto a roll acting as both the backing roll of the blade coating step and the backing roll of the transfer roll 15. The blade coating is performed in a conventional manner against the backing roll 16 after which the web 7 is passed to a guide roll 17 over which the web is passed to the nip between the backing roll 16 and the transfer roll 15 in which nip the second coat layer is applied to the surface of the web 7. The design of this apparatus is extremely compact permitting two-layer coating in a very small space. In addition to those described above, the present invention may have alternative embodiments. The blade coating step may be replaced by a levelling rod performing the application of the first coat layer. Furthermore, the transfer roll coating step may be arranged to coat the web two-sidedly, whereby one side of the web will be coated by a single coat layer and the other side by two layers. Two-layer coating can be applied alternatively to either the bottom side or the upper side of the web depending on the coater layout. Intermediate drying can be arranged between the blade and the transfer roll coating steps, whereby heavier coat weights may be applied and the smooth precoat applied by means of the doctor blade or rod can be better utilized. The blade coating step may be carried out using coaters of the short dwell time type described above, or alternatively, using transfer roll, spray nozzle or similar application method followed by levelling with a rod or blade.

Claims: A method of two-layer coating a paper or paperboard web, in which method onto the web is applied a first coat layer by means of a first coater, onto the web and on its first coated side is next applied a second coat layer by means of a second coater, and during the first coating step the surface of the coat is levelled after the application of the coat by means of a smoothing element, characterized in that during the second coating step coating mix film is premetered and levelled on a transfer roll and the coat film is then applied on the web by the transfer roll. A method as defined in claim 1, characterized in that the coat is applied during the second coating step over nondried first coat layer. A method as defined in claim 1 or 2, characterized in that the travel distance of the web from the first coating step to the second coating step is lengthened in order to promote the setting of the coat layer applied during the first coating step. A method as defined in claim 1 or 3, characterized in that the coat layer applied during the first coating step is dried prior to the application of the second coat layer. A method as defined in any foregoing claim, characterized in that to the web side opposite to the web side to be two-layer coated is subjected to treatment with a furnish. A method as defined in any foregoing claim, characterized in that the first coat layer is smoothed by means of a doctor blade. A method as defined in any of foregoing claims 1 - 5, characterized in that the first coat layer is smoothed by means of a levelling rod. An assembly for two-layer coating a paper or paperboard web, said assembly comprising a first coater (1) for applying a first coat layer onto the web, a second coater for applying a second coat layer onto the first coated surface of the web, and said first coater incorporates a smoothing element (5) for levelling the applied coat layer, characterized in that said second coater is a transfer roll coater comprising at least one transfer roll (8, 9, 15) and at least one means for applying a premetered and levelled film (11) of coating on the transfer roll. An assembly as defined in claim 8, characterized by a guide roll (14) serving to increase the travel distance of the web from the first coating step to the second coating step in order to promote the setting of the coat layer applied during the first coating step. An assembly as defined in claim 8 or 9, characterized by a dryer for drying the coat layer which is applied during the first coating step prior to the application of the second coat layer An assembly as defined in any of claims 8 - 10, characterized in that said first coater is a blade coater. An assembly as defined in any of claims 8 - 10, characterized in that said first coater is a rod coater.

Inventors: RANTANEN RAUNO, RANTANEN, RAUNO
Applicants: METSO PAPER INC, METSO PAPER, INC.
Application year: 1997
Main classification: D21H23
IPC codes: B05C1, B05C11, D21H23, B05C9, D21H25, B05D1