COATER AND COATING METHOD

Abstract: The present invention relates to a coater and a coating method for application of coating mix to a web (1) of paper or board adapted to run over a backing roll (2), the coater comprising means for applying a layer of coating mix to the surface of the web (1), and a doctoring member (3) suitable for leveling the coating mix applied to the surface of the web (1) and for metering the amount of applied coating mix. The means for applying coating mix to the surface of the web (1) include at least one linear array of nozzles which is arranged to operate upstream from the doctoring member (3) in the travel direction of the web (1) and conprises at least two nozzles (6) that in a disposition close to the web (1) are adapted to direct dense jets (8) of coating mix to the surface of the web (1).

Description: Coater and coating method The present invention relates to a coater according to the preamble of claim 1 and a coating method according to the preamble of claim 8. In a conventional short-dwell -time applicator, the coating mix is applied to the surface of a moving web of paper from an applicator chamber which is filled with the coating mix and is located upstream from the doctor blade on the web travel. The rear end of the applicator chamber is formed by the doctor blade mounted on a blade support beam, while the front end is formed by a so-called meter- ing strip. Supportedly running on a backing roll, the paper web to be coated forms the third side of the applicator chamber. The coating mix adheres to the surface of the moving web when the web travels over the application zone that begins at the metering strip and ends at the doctor blade. In the cross-machine direction, the width of the application zone is defined by the end seals of the applicator chamber. The coating mix is introduced into the applicator chamber via an infeed slit adapted to the bottom of the chamber. Between the metering strip and the moving web of paper, there remains a gap via which a major portion of the coating mix pumped into the applicator chamber passes as a return flow out from the applicator chamber in a reverse direction to the web travel and is then collected to the coating mix circulation. Typically, the amount of the return flow is about 95 % of the overall volumetric flow of the coating mix pumped into the applicator chamber. The purpose of the reverse-direction flow is to curtail the entry of the boundary air layer traveling on the web surface into the application zone, assure even distribution of the coating mix flow and a sufficiently high coat replacement rate in the applicator chamber and keep the outflow gap between the metering strip and the web free from obstructions. The amount of return flow can be adjusted by way of controlling the distance of the metering strip from the paper web. Also the construction of the applicator chamber may be varied in order to modify the flow conditions within the chamber. The runnability of a shor -dwell coater is superior in regard to other coater types, thus making it highly preferred particularly in on-machine application. Its good runnability is based on a low application pressure and a short dwell time and zone length, the latter terms referring to the time from the instant of coating mix applica- tion to the instant of doctoring and, respectively, to the web travel from the point of application to the point of doctoring. The coating mix layer applied to the web surface contains solids slurried in water. After application, the coat applied to the web surface develops a superficial layer having a higher solids content. The relative thickness of this high-solids layer increases when the water of the applied coating mix is absorbed after application into the fiber of the underlying sheet of the base web. With a higher solids content of the applied coat and the resultant reduced flowability of the same, the force required for doctoring the applied coat increases. Respectively, the higher the solids content of the applied coat on the web, the greater will be the thickness of the coat remaining on the web surface after doctoring. If doctoring is performed by means of, e.g., a zero-angle doctor blade immediately after application, the solids content of the coating mix layer on the web surface is initially low, which means that the coat thickness on the web remains small after doctoring. Hence, it is possible to affect the thickness of the coat layer on the web surface as it leaves the doctoring step by way of controlling the dwell time in the coater. Due to the short dwell time and travel of the web in a short-dwell -time applicator, the coating mix and the water of the slurπed mix do not have much time to soak deep into the fiber of the base web and the mterfiber crevices thereof, whereby the degradation of web strength properties due to the wetting of the base web remains minimal. In a short-dwell coater, the doctoring force required for leveling the applied coating mix layer is smaller than what is needed in most other application methods, because the solids content of the coat applied to the web is still low during the doctoring step. An additional benefit of a short-dwell-time coater is that it is readily adaptable for application of thin coatings. On the other hand, the short dwell time causes postswell- mg of base web fiber that occurs only after the doctoring of the applied coat, whereby the surface pro- file of a short-dwell -time coated paper is coarser than that rendered by a great number of other types of coaters . Particularly at high web speeds and in the application of thick coats, the use of the short-dwell-time application technique is limited by streakiness of the coated web surface and inferior cross-machine profile quality of the applied coat. While the reasons leading to streakiness have not been resolved in full depth, one probable factor acting herein has been traced to the lateral and longitudinal turbulences of the coating mix in the applicator chamber, uneven wetting line profile of the web being coated, entry of boundary layer air with the web into the applicator chamber and variations in the amount of coating mix adhering to the base web. Herein, the term wetting line is used for making reference to the cross- machine line along which the base web meets the coating mix being applied. A remedy to the streakiness of the coat profile hampering short-dwell-time applicators has been sought, among other attempts, from the use of shorter applicator chamber and replacement of the stiff metering strip of the applicator chamber with a perforated, flexible dam blade designed to reduce turbulences in the applicator chamber. While these arrangements have been able to move the onset of streaki- ness to web speeds higher than those used in the prior art, it has been found that a distinctly streaky coat surface quality will again appear at practical web speeds as low as 2000 m/s. Other attempts to solve the streakiness problem of short- dwell-time application has been sought by placing close to the doctor blade a jet applicator beam, wherefrom the coating mix is applied to the web surface being coated in a noncontacting manner by ejecting the coat from a narrow-siit jet nozzle extending over the width of the web. This arrangement provides the characteristic bene- fits of the jet-application technique, such as a smooth coating layer of the web surface, combined with the short dwell time of short-dwell-time applicators. As modern papermaking machines are from 8 to 10 m wide, the narrow slit of the jet nozzle beam assembly is difficult to manufacture with a precision extending over the entire cross-machine width of the web to be coated. Moreover, the jet nozzle beam assembly requires complicated and expensive accessories. Jet application is also hampered by the frequent plugging of the narrow slit of the jet nozzle and the air pocket that develops under the doctor blade, both of these factors tending to degrade the quality of the applied coat. In some cases it may be complicated to arrange the jet nozzle beam sufficiently close to the doctor blade holder beam. Due to the above- described reasons, it is difficult to manufacture a flawlessly functional short-dwell jet applicator. It is an object of the present invention to provide a novel type of coater and coating method capable of over- coming the above-described problems. The goal of the invention is achieved by virtue of applying the coating mix to the web surface with the help of fan nozzles that have a fanned and vertically flattened spraying pattern and are placed in front of the doctor blade upstream on the travel of the web. In the embodiment according to the invention, the operating pressure of nozzles is kept relatively low and, resultingly, the spraying pattern of the nozzles is homogeneous, whereby the flow of coat leaving the nozzles is not atomized in contrast to what takes place in conventional spraying nozzles, for instance. There are disposed a plurality of nozzles over the cross-machine width of the web and the nozzles are mounted on a feed tube which is aligned to extend over the width of the web to be coated and is adapted to supply the coating mix to the nozzles. Also in the machine direction, there may a number of nozzle arrays operating in succession. More specifically, the coater according to the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the coating method according to the invention is characterized by what is stated in the characterizing part of claim 8. The invention offers significant benefits, A coater according to the invention has no closed applicator chamber as is conventional in a short-dwell-time applicator, thus allowing the coater to provide a smooth coat even at the highest web speeds exceeding 2000 m/min. Further, the coater according to the invention has an extremely simple construction, which makes it essentially less costly than the conventional jet applicator de- scribed above. The nozzle operating pressure of the coater according to the invention is low and thus there is no need for a supply of atomizing air, whereby also the nozzle structure design can be substantially simpler than that of conventional spraying nozzles, for instance. With the help of the feed tube, the volumetric supply rates and pressures of the coating mix are distributed in a relatively uniform manner between the individual nozzles, whereby also the profile of the coat applied to the web surface becomes uniform. By virtue of using a feed tube for nozzle support, the nozzles can be brought closer to the doctor blade than what would be possible for a narrow-slit jet applicator beam, thus permitting the present embodiment to use a short dwell travel if so required. In the coater embodiment according to the invention, the nozzle orifice has a larger diameter than the gap width of a narrow-slit jet nozzle, whereby the plugging tendency of the nozzles is reduced in regard to the performance of a conventional jet applicator nozzle. As the mutual displacement of the nozzles, as well as the distance of the linear nozzle array from the doctor blade holder beam, is easy to change, the dwell travel of the web in the coater is readily adjustable. Further, the replacement and cleaning of nozzles is easy, since the entire lightweight feed tube with its nozzles is detachable from the coater station. Also the cleaning of nozzles can be automated if so desired. The embodiment according to the invention does not need any straightness compensation for the feed tube inasmuch the line of application from a cross-machine array need not be as straight as is required in a conventional jet applicator, for instance. In the following, the invention will be examined in greater detail by making reference to exemplifying em- bodiments illustrated in the appended drawings in which FIG. 1 shows schematically a sectional view of an embodiment of coater according to the invention; and FIG. 2 shows a linear nozzle array of a coater according to the invention aligned in the cross-machine direction over the web. Referring to the drawing, a paper web 1 to be coated, which is supported by a backing roll 2, is shown running in the direction indicated by arrow 7. A doctor blade 3 is mounted on a blade holder beam 4. Nozzles 6 for coating mix application are mounted in a parallel disposition on a coating mix feed tube 5 that is aligned to extend over the width of the web 1. The jets 8 exiting the nozzles 6 has a fan-shaped and vertically flattened pattern, thus making the pattern of the jet 8 leaving the nozzle 6 to widen most substantially in the width direction of the web 1. The jet 8 emitted from the nozzles 6 is dense, because it is not atomized with the help of a separate atomizing-air supply as is the case in conventional spraying nozzles, for instance. At the point of incidence on the web 1, the pattern of the jet 8 substantially thin in the travel direction of the web 1 as compared with the broadness of the jet in the width direction of the web 1. There are at least two pieces of the nozzles 6 mounted in a linear array on the feed tube 5 over the width of the web 1. The term linear nozzle array refers to a line of at least two nozzles 6 in a parallel disposition in the width direction of the web 1, whereby the nozzles need not necessarily be located at identical distances from the doctoring member 3. The number of nozzles 6 needed in the applicator is dependent on such factors as the fan angle of the jet 8 emitted by the nozzles 6 and the distance of the nozzles 6 from the web 1. The mutual disposition of the nozzles 6 of a linear array is advantageously about 20-200 mm. The feed pressure of the nozzles 6 is relatively low, advantageously less than 10 bar, typically about 0.3- 6 bar. The fan angle of the jet 8 leaving the nozzle 6 in the width direction of the web 1 is typically about 15- 150°. The required fan angle is determined, among other things, by the feed pressure of the nozzle 6. In the coater according to the invention, the dwell travel defined as the distance from the point of incidence on the web 1 of the jet emitted by the nozzle 6 to the tip of the doctor blade 3 is advantageously about 20-800 mm. The coating mix jet is directed in the travel direction of the web 1 so that the angle between the tangent of the backing roll 2 and the fanned jet at the point of incidence of the jet on the web 1 is advantageously about 20- 90°. The distance of the coating mix feed tube 5, with the nozzles 6 mounted thereon, from the surface of the web 1 to be coated can be varied. Additionally, the angle of incidence on the web 1 of the jets 8 emitted by the nozzles 6 can be adjusted by rotating the feed tube 5. Coating mix is pumped along the feed tube 5 to the nozzles 6, wherefrom the coating mix is further applied to the surface of the web 1 moving supported on the backing roll 2. After application, the excess coating mix is removed from the blade-side surface of the web 1 and the surface of the applied coat is leveled by means of a doctoring member such as a doctor blade 3 mounted on a blade holder beam 4. The excess coating mix doctored away from the surface of the web 1 can be returned back to the coating mix circulation. The thickness of the applied coat remaining after doctoring can be affected by varying the dwell time. If the dwell time is made short, the water entrained in the coating mix layer applied to the surface of the web 1 has rather no time to soak into the fiber of the base web, thus making the thickness of the applied coat thin, particularly if a zero-angle doctor blade is used for leveling. If the dwell time is extend- ed, a proportionally larger portion of the water contained in the applied coating mix has time to soak into the base web fiber, whereby also the coating layer remaining particularly after zero-angle doctoring is respectively thicker. In addition to those described above, the invention may have alternative embodiments. When necessary, it is possible to apply a thicker coat to the web 1 by way of arranging the nozzles 6 into a number of successively operating linear arrays, whereby each one of the linear arrays can be mounted on a separate feed tube 5 extending in the width direction over the web 1. In lieu of a doctor blade, the doctoring member 3 can be a doctoring bar or air-knife doctor. Furthermore, the coating mix can be applied if so desired to an unsupported web or to a web running on a support belt or wire.

Claims: What is claimed is: 1. Coater for application of coating mix to a web (1) of paper or board adapted to run over a backing roll (2) , the coater comprising - means for applying a layer of coating mix to the surface of the web (1) , and - a doctoring member (3) suitable for leveling the coating mix applied to the surface of the web (1) and for metering the amount of applied coating mix, c h a r a c t e r i z e d in that said means for applying coating mix to the surface of the web (1) include at least one linear array of nozzles which is arranged to operate upstream from the doctoring member (3) in the travel direction of the web (1) and comprises at least two nozzles (6) that in a disposition close to the web (1) are adapted to direct dense jets (8) of coating mix to the surface of the web (1) . 2. Coater according to claim 1, c h a r a c t e r - i z e d in that the nozzles (6) disposed in the form of a linear array are mounted on a coating mix feed tube (5) . 3. Coater according to claim 1, c h a r a c t e r - i z e d in that the mutual distance between the nozzles (6) of the linear array is 20-200 mm. 4. Coater according to claim 1, c h a r a c t e r i z e d in that the dwell travel of the web as seen from the point of coating mix application from the linear nozzle array disposed closest to the doctoring member (3) is 20-800 mm. 5. Coater according to claim 1, c h a r a c t e r i z e d in that the fan-shaped jet (8) of coating mix applied from the nozzles (6) has an exit angle of 15- 150°. 6. Coater according to claim 1, c h a r a c t e r i z e d in that the jet (8) applied from the nozzles (6) is directed along the travel direction of the web so that the angle between the tangent of the backing roll (2) and the jet (8) at the incidence point of the jet on the web (1) is 20-90°. 7. Coater according to claim 1, c h a r a c t e r - i z e d in that feed pressure of the nozzles (6) is about 0.3-6 bar . 8. Method for coating a moving web (1) of paper or board, the method comprising the steps of - passing the web (1) to be coated supported by a backing roll (2) to a coater station, - applying coating mix to the surface of the web (1) , and - leveling with the help of a doctoring member (3) c h a r a c t e r i z e d in that the coating mix is applied to the surface of the web (1) from at least one linear array of nozzles arranged to operate upstream from the doctoring member (3) in the travel direction of the web (1) , said array comprising at least two nozzles (6) that in a disposition close to the web (1) are adapted to direct dense jets (8) of coating mix to the surface of the web (1) . 9. Method according to claim 8, c h a r a c t e r i z e d in that the thickness of the coat applied to the surface of the web (1) and remaining after doctoring is be affected by varying the dwell travel of the web in the coater.

Inventors: KIIHA TIMO, KOSKINEN JUKKA, KIIHA, TIMO, KOSKINEN, JUKKA
Applicants: KIIHA TIMO, KOSKINEN JUKKA, VALMET CORP, KIIHA, TIMO, KOSKINEN, JUKKA, VALMET CORPORATION
Application year: 2000
Main classification: B05B1
IPC codes: B05B1, B05C5, B05C11, D21H23, D21H25