Method and apparatus for two-side coating of a thin printing paper web

Abstract: The invention relates to a method and apparatus for two-side coating of a light base weight printing paper web containing mechanical pulp or recycled fiber. According to the method the first side of a web (5) is coated in a first coating station (1), the first-side coat is dried at least partially in a dryer unit (3), the second side of the web (5) is coated subsequent to the drying of the first side in a second coating station (2), and the second-side coat is dried at least partially in a second dryer unit (4). According to the invention both the first-side coat and the second-side coat are formed by applying a required amount of coating mix onto the perimeter of a soft film-coating roll (22) and subsequently transferring said coat film to the web (5) in a nip (N₁) formed between a backing roll (24) and said film-coating roll (22).

Description: The present invention relates to a method according to the preamble of claim 1 for two-side coating of a thin printing paper web containing mechanical pulp, e.g., a paper web made of recycled fiber. The invention further relates to an apparatus according to the preamble of claim 4 suited to two-side coating of a thin printing paper web made of mechanical pulp. Thin printing paper webs containing mechanical pulp are conventionally coated on subsequent coating stations equipped with short-dwell coaters as the coater units. A combination of two subsequent coater stations is necessary as low base paper weight and high content of groundwood make single-run coating on both sides impossible. Two-side coating with conventional methods would excessively wet the web and thus impair its runnability. Furthermore, the measurement of coat weight in a two-side coating operation is difficult. Despite their multiple benefits, short-dwell coaters also have several drawbacks. Air entrapped in the coat paste easily causes mottling. Owing to the small linear application pressure and short application distance, wetting of the base web and subsequent fiber swelling occurs even after the web has passed the doctor blade. This may impair the coat smoothness. As a rule, doctor blade coating methods become critical with thin webs and particularly with light coat weights. Paper grades containing a high proportion of mechanical pulp and a high percentage of coat fillers such as, e.g., SC paper (wood containing high filler content super-calendered printing paper) make doctor blade coating impossible owing to the fragility of the base paper web. Web defects leading to brittleness result in low production yield and inferior runnability. Base paper grades containing recycled fiber have posed unexpected problems in doctor blade coaters: The coat is easily marked during coating by streaks caused by defective doctor blades. Recycled-fiber containing base paper grades have a darker color, thus making the opacifying power of the coat mix more critical. Owing to its operating principle, doctor blade coating tends to give a smooth coat, not a coat of uniform coat weight. As the base paper is not necessarily smooth, the opacifying power obtained in doctor blade coating is insufficient, resulting in mottling of the coated web. The Kimberly Clark-Mead coating process as described in Casey, "Pulp and Paper...", Wiley-Interscience Publication, New York 1983, page 2139, provides a separation of the functions of metering, distributing, and application of coating mixture. The metering function, which is the primary mechanism for adjusting coat weight, is carried out by a roll nip, which serves as the outlet from a coating pond. The distributing nip, which is located between the distributing and the applicator rolls, serves to reduce the coating film thickness and to distribute the film more evenly on the surface of the applicator roll, from where the coating film is transferred onto the web. From DE-U-91 00 291 it is known that light weight papers, which are easily broken, advantageously are two side coated in two separate coaters each followed by a dryer unit, which alternatively is an infrared dryer or a drum dryer. Both the first and the second coater comprise a soft applicator or film coating roll and a fountain coater with a doctoring bar or blade suitable for directly metering a coating mix film onto the perimeter of said film coating roll. But, the runnability of such a construction in high speeds is poor and there is no accurate control over the amount of the coating mix. It is an object of the present invention to overcome the disadvantages of the above-described prior-art technology and to achieve a method and apparatus for two-side coating of a thin printing paper web containing mechanical pulp such as, e.g., a paper web grade made of recycled fiber. The invention is based on performing the coating operation using a two-step compressive film-lamination technique. More specifically, the method according to the invention is principally characterized by what is stated in claim 1. Furthermore, the apparatus according to the invention is characterized by what is stated in claim 4. The invention provides significant benefits. The technique according to the invention halves the web wetting relative to single-run two-side coating. Thus, the invention provides good runnability. Further, good coat quality is attained at light coat weights. Particular benefit is achieved with base paper grades containing recycled fibers as the formation of streaks associated with doctor blade coating can be avoided. Since the compressive film-lamination technique forms a coat of uniform weight on the web, the coat has a high opacifying power. For the same reason, a higher burst index relative to doctor blade coating is attained. Moreover, the measurement of coat weight by means of rupturing testers is easy. The method according to the invention imposes minimal mechanical stress on the web. Two-side single-run coating requires a long path of unsupported pulling of the web prior to the web support roll to give the coat a possibility of drying prior to touching the support roll. In the technique according to the invention, the uncoated side can be supported by a roll immediately after the coating of the other side, thus achieving a significant reduction in coater unit size. In the following the invention is described in greater detail with the help of exemplifying embodiments illustrated in the annexed drawing in which Figure 1 is a side elevation view of a part of a paper machine incorporating a coater apparatus according to the invention. Figure 2 shows in greater detail a nozzle assembly suited to implement the coater apparatus according to the invention. In conjunction with the present invention, the term compressive film-lamination technique is used to refer to a technique in which the web during coating is subjected to a linear pressure in a nip formed between a film-coating roll and a backing roll so as maintain the peripheral speeds of both the film-coating roll and the backing roll approximately equal to the web speed. In conjunction with the present invention, the term light web material refers to web materials having a base weight of less than 65 g/m 2 . With reference to Fig. 1, the apparatus according to the invention comprises a first compressive film-lamination station 1 and a second compressive film-lamination station 2, having a first drying unit 3 placed between them. The web path is configured so that subsequent to the first drying unit 3 on the path of a web 5 is placed the second coating station 2, followed by a second drying unit 4. Basically, both drying units have a similar construction. The coating station 1 is comprised of a film-coating roll 22, a coater application means 20 and a backing roll 24 of the film-coating roll 22. The coater application means 20 serves for applying a desired amount of coating mix onto the film-coating roll 22, wherefrom the coating mix is subsequently transferred in the nip N 1 to the web 5. To make the coating mix adhere to the web with the greatest smoothness free from orange peel effect, the backing roll 24 has a smaller diameter than the film-coating roll, whereby the angle at which the web 5 conforming to the backing roll 24 exits the nip N 1 is maximized. In an alternative approach the web 5 is guided toward the film-coating roll 22, whereby also a good coat surface quality is obtained. The first drying unit 3 is comprised of infra-red dryers 6 and dryer drums 7. Basically, the coating station 2 has a similar construction to that of coating station 1. Owing to the routing of the web 5, the station 1 is mirrored from station 2. The station 2 is analogously followed by the second drying unit 4 comprising an infra-red dryer 8 with subsequent drying cylinders (not shown). With reference to Fig. 2, a nozzle assembly for a film-coating roll 40 is comprised of a coater blade 42 tilted to an acute angle and mounted to a frame structure 41. Between the frame structure 41 and the blade 42 is placed a loading hose 43 suited to controlling the linear pressure and position of the blade 42. The coating mix 46 is contained in a metering fountain formed between a front wall 44 and the blade 42, wherefrom the mix is transferred in a controlled manner onto the perimeter of the coating roll 40. The front wall 44 is mounted by means of support elements 45 onto the chassis of the apparatus. Such a nozzle assembly is known in the art. The blade 42 can alternatively be replaced by a doctoring bar. The film-coating roll 40 has a diameter of 1000 mm typical, and the backing roll 24 has a diameter of 800 mm, respectively. In a preferred embodiment of the invention the backing roll has a metal surface. The metal surface can be of chromium, for instance. Also ceramic or polymer covered backing rolls are usable. A metal or ceramic covered backing roll performs initial calendering of the coated web. Such coated rolls with a steel core can also be provided with chilling, whereby condensation of moisture onto the roll perimeter is attained, which in turn aids keeping the roll clean. Owing to the improved thermal and wear resistance of a metal or ceramic covered roll, also steam and scraper blades can be used for keeping the rolls clean. Characterizingly, in the above-described exemplifying embodiments the surface of the film-coating roll is adjusted to carry an approx. 7...15 µm thick film of the coating mix, a major portion (typically approx. 80 %) of which is adhered to the web in the nip between the film-coating roll and the backing roll. Thus, the web is coated with a coat of 7...15 g/m 2 dry weight. The adsorption of the coating mix paste and the water contained therein is related to the magnitude of the nip pressure and the duration of said pressure, that is, the width of the nip. On the other hand, the nip width is solely determined by the diameters of the rolls and their hardness, while the magnitude of the nip pressure is principally determined by the linear loading of the nip, and additionally, by the web speed. Accordingly, the good penetration of the coat into the web is achieved by means of a high linear application pressure imposed in a relatively wide nip. The maximum nip pressure typically is 1000 kPa gauge and the nip width is in excess of 15 mm. Good results according to the invention have been obtained by keeping the nip pressure above 500 kPa and the nip width greater than 10 mm. Such desirable nip widths can be attained by means of hard rolls with diameters in excess of 600 mm. Particularly the film-coating roll must have a diameter greater than 600 mm, whereby the backing roll must also have a diameter in excess of 600 mm. To achieve the minimum pressure limit of 500 kPa, the linear nip loading must be at least 20 kN/m for a typical coating mix paste. In conventional kiss roll coating, the application pressure is only approx. 50...100 kPa, while a conventional doctor blade coater can achieve a pressure of 1000 kPa over a nip width of less than 1 mm. In a conventional short-dwell coater the encountered levels of application pressure are yet lower. The technique according to the invention is suited to web speeds of 400...1500 m/min typical. The linear nip loading is typically in the range of 20...50 kN/m, advantageously approx. 35 kN/m. The coating material of both the backing roll 24 and the film-coating roll 22 is polyurethane, rubber or any suitable resilient material. The P&J numbers of the rolls are typically in the range of approx. 0...40. The film-coating roll employed in the embodiments according to the invention is invariably a so-called soft roll with a surface material of polyurethane, for instance. The table below gives exemplifying compositions of coating mixes suited for advantageous use according to the invention: Coating color component Coating mix composition 1 Coating mix composition 2 Calcium carbonate pigment 100 parts Kaolin pigment 100 parts Starch binder 10 parts 6 parts Synthetic binder 4 parts 6 parts Additives 2 parts 0.5 parts Solids content 55 % 58 % Viscosity (Brookfield 100) 500 cP 800 cP

Claims: A method for two-side coating of a light base weight printing paper web containing mechanical pulp or recycled fiber, in which method the first side of a web (5) is coated in a first coating station (1); the first side coat is dried at least partially in a dryer unit (3); the second side of the web (5) is coated subsequent to the drying of the first side in a second coating station (2); the second side coat is dried at least partially in a second dryer unit (4); both the first side coat and the second side coat are formed by applying a required amount of coating mix onto the perimeter of a soft film coating roll (22) rotated at a peripheral speed approximately equal to the speed of the web (5) and subsequently transferring said coat film to the web (5) in a nip (N 1 ) formed between a backing roll (24) and said film coating roll (22), characterized by combining the following steps: a nozzle assembly containing a metering fountain is used for transferring the coating mix in a controlled manner onto the perimeter of the film coating roll (22,40) ; the first and the second dryer units (3,4) are comprised of infrared dryers (6) and dryer drums (7), and the nip pressure in the nip (N 1 ) is kept above 500 kPa and the nip width is kept greater than 10 mm, and that a backing roll (24) is used which has a smaller diameter than the film coating roll. A method as defined in claim 1, characterized in that a web of 65 g/m 2 maximum is coated. A method as defined in claim 1 or 2, characterized in that the backing roll (24) used is a metal-covered roll for initial calendering of the web (5). An apparatus for two-side coating of a light base weight printing paper web (5) containing mechanical pulp or possibly recycled fiber, said apparatus comprising a first coating station (1) for coating the first side of a web (5); a first dryer unit (3) for drying said web; a second coating station (2) for coating the second side of said web (5); and a second dryer unit (4) for drying said web (5), whereby both the first and the second coating station (1,2) comprise a soft film coating roll (22) suitable for receiving onto its perimeter a coating mix film of desired thickness and said roll being rotatable at a peripheral speed approximately equal to the web speed; coating mix application means (20) suitable for metering a coating mix film onto the perimeter of said film coating roll (22), and a backing roll (24) capable of forming a nip (N 1 ) with the film coating roll (22) and said backing roll (24) being rotatable at a peripheral speed approximately equal to the web speed, whereby the web (5) can be routed through said nip (N 1 ), characterized by combining the following features: a nozzle assembly (42,44) containing a metering fountain formed between a front wall (44) and a blade (42) tilted to an acute angle, wherefrom the coating mix (46) is transferred in a controlled manner onto the perimeter of the coating roll (22,40); first and second dryer units (3,4) comprised of infrared dryers (6) and dryer drums (7); the film coating roll (22) and backing roll (24) having a diameter greater than 600 mm, whereby the backing roll (24) has a smaller diameter than the film coating roll and the apparatus being capable of giving a linear nip loading of at least 20 kN/m for a typical coating mix paste. An apparatus as defined in claim 4, characterized in that the backing roll (24) is metal-covered for initial calendering of the web (5). An apparatus as defined in claim 4, characterized in that the backing roll (24) is ceramic-covered for initial calendering of the web (5). An apparatus as defined in claim 4, characterized in that the backing roll (24) is polymer-covered for initial calendering of the web (5). An apparatus as defined in one of claims 4 to 7, characterized in that the backing roll (24) is provided with chilling means. An apparatus as defined in one of claims 4 to 8, characterized in that the backing roll (24) is provided with steam cleaning means. An apparatus as defined in one of claims 4 to 8, characterized in that the backing roll (24) is provided with scraper cleaning means. An apparatus according to claim 4, characterized in that the coating material of both the backing roll (24) and the film-coating roll (22) is polyurethane, rubber or any suitable resilient material.

Inventors: RANTANEN RAUNO, VIKMAN KAI, RANTANEN, RAUNO, VIKMAN, KAI
Applicants: METSAE SERLA OYJ, VALMET CORP, METSAE-SERLA OYJ, VALMET CORPORATION
Application year: 1993
Main classification: D21H23
IPC codes: D21H11, B05C1, D21H23, B05D1, B05C3, B05D7, B05C9, D21H19