FI94883B - Method and arrangement for double-sided coating of a thin printing paper web containing mechanical pulp - Google Patents

Abstract

Two-sided coating of lightweight printing paper (5) is effected in two successive stations (1,2) before respective drying units (3,4). In each station (12) is a soft roll (22) which coats one side of the paper (5) in a nip (N1) with a backing roll (24). The soft roll receives coating material from an applicator metering roll (20). The backing roll (24) is surfaced with metal, ceramic, polymer, polyurethane or rubber or any resilient material also on the surface of the coating roll (22).

Description

94883

Method and apparatus for double-sided coating of a thin printing paper web containing mechanical pulp

The invention relates to a method according to the preamble of claim 1 for double-sided coating of a thin printing paper web containing mechanical pulp, for example a paper web containing recycled fiber.

The invention also relates to an apparatus for double-sided coating a thin printing paper web containing mechanical pulp 10.

Thin printing paper webs containing mechanical pulp are traditionally coated with successive coating stations-15a, which are coated with short residence coaters.

Two consecutive coating stations are used because the thinness and wood content of the web do not allow simultaneous reciprocal coating. In a simultaneous coating operation, the web would become too wet and thus the runnability would suffer.

20 In addition, in the case of a simultaneous coating operation, it is difficult to measure the amount of coating.

Despite its many advantages, short residence coaters have several disadvantages. The air in the paste easily causes 25 stains. Due to the low application pressure and the short application distance, the absorption of water into the base paper and the consequent swelling of the fiber still takes place after the scraper blade.

This can impair the smoothness of the coated surface. In general, blade coating is critical for runnability with thin webs and especially with small amounts of coating.

Surprising problems have been encountered with web materials containing recycled fibers with blade coaters: the coating blades easily cause 35 streaks in this web material during coating.

Webs containing recycled fibers have a darker color and thus the opacity of the coating is a critical factor. Due to its operating principle, the blade coating gives an even 2 94883 surface and not an even layer of coating material. Because the web material is not uniform, the coverage of the blade coating is poor and the coated web material is mottled.

The object of the present invention is to obviate the drawbacks of the technique described above and to provide a completely new type of method and apparatus for double-sided coating a thin printing paper web containing mechanical pulp, for example a paper web containing recycled fiber.

10

The invention is based on the fact that the coating is carried out by a two-step film gluing pressing technique.

More specifically, the method 15 according to the invention is characterized by what is set forth in the characterizing part of claim 1.

The apparatus according to the invention, in turn, is characterized by what is set forth in the characterizing part of claim 4.

The invention provides considerable advantages.

The solution according to the invention avoids excessive wetting of the web, which achieves good runnability. With small amounts of coatings, a good coating result is achieved. A special advantage is achieved with web materials containing recycled fibers, whereby the streaks caused by the blade coating can be avoided. Since the film gluing pressing technique 30 forms a uniformly thick coating layer on the web, the coverage of the coating is good. For the same reason, better surface strength is obtained than blade coating. In addition, the measurement of the amount of coating is easy with permeable meters.

In the following, the invention will be examined in more detail by means of exemplary embodiments according to the accompanying figures.

Figure 1 shows a side view of a part of a paper machine, 3 94883, in which a coating device according to the invention is used.

Figure 2 shows a schematic view of an alternative coating device according to the invention.

5

Figure 3 shows in more detail a nozzle device suitable for a coater according to the invention.

In the context of the present invention, the film gluing technique refers to a technique in which a pressure is applied to a web in a nip formed by a film gluing roll and a counter roll, the circumferential speeds of both the film gluing roll and the counter roll being approximately equal to the web speed.

In the context of the invention, thin web material means web materials which weigh less than 65 g / m 2.

According to Figure 1, the apparatus according to the invention comprises a first film gluing press station 1 and a second film-20 gluing press station 2, between which a first drying unit 3 is placed. After the drying unit 3, a second coating unit 2 is located in the web path 5 The coating unit 1 consists of 25 film gluing rolls 22, a coating beam 20 and a film roll 22 counter roll 24. The coating beam 22 applies just the right amount of coating to the film gluing roll 22. than a film sizing roll so that the angle of detachment of the web 5 following the counter roll 22 from the nip N1 is as large as possible. Alternatively, a better runnability can be chosen, whereby the peel angle of the web 5 is made as small as possible, i.e. the web is guided in the direction of the film gluing roll 22.

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The first drying unit 3 consists of infra units 6 and 4 94883 drying cylinders 7.

The coating unit 2 is basically similar to the unit 1. Due to the way the web 5 is exported, station 2 forms a mirror-5 image of station 1. Station 2 is similarly followed by a drying unit 4 comprising an infrared dryer 8 and subsequent drying cylinders (not shown).

According to Figure 2, the coating beam 20 can be replaced by a so-called 10 with a gate roll coater, in which the coating material is transferred from the dosing pond 34 above the nip N2 between the transfer roll 30 and the dispensing roll 32 to the circumference of the dispensing roll 32 via the nip N2 and further to the surface of the film sizing roll 36 in the nip N3. From the surface of the film sizing roll 36, the coating material is transferred to the web 38 at the nip N4 between the film sizing roll 36 and the counter roll 39. According to the invention, the diameter of the counter roll 39 may be smaller than the diameter of the film roll 36.

The typical diameter of the film sizing roll 36 is 1000 mm and the diameter of the counter roll 39 is 800 mm, respectively.

Typically, in the solutions according to both previous examples, a film of coating material about 10 to 20 μιη thick is formed on the surface of the film sizing roll, which mainly (typically n.

, 25 90%) adheres to the web in the nip formed by the film sizing roll and the counter roll.

The absorption of the coating paste and the water in it is proportional to the pressure level of the nip and the time of application of the pressure, i.e. the width of 30 nips. The width of the nip, on the other hand, unambiguously depends on the diameters and hardness of the rollers and the pressure level mainly on the line load of the nip and in addition on the driving speed. Thus, the high penetration of the coating is caused by the high application pressure acting on the relatively wide nip.

The maximum pressure is typically 1000 kPa overpressure and the nip width is more than 15 mm. Good results according to the invention are obtained above pressure values of 500 kPa and nip widths of 10 nm. The desired nip widths, on the other hand, are achieved with hard rollers with diameters larger than 600 mm. In particular, the diameter of the film sizing roll should be greater than 600 mm and the diameter of the counter roll greater than 600 mm. In order to achieve a minimum pressure of 500 kPa, the line load must be at least 20 kN / m with normal paste. In the traditional brush roll application, the application pressure is only 50 to 100 kPa and, correspondingly, under the conventional scraper blade 10, a pressure of 1000 kPa acts only at a distance of less than 1 mm.

The pressures in a traditional short residence coater are even lower. The solution according to the invention is suitable for typical web speeds of 400 to 1500 m / min. The line load of the nip is typically between 20 and 50, preferably about 35 kN / m. Polyurethane, rubber or other flexible material is used as the coating on both the counter roll 7 and the film sizing roll 4. The P&J readings of the rolls are typically about 0 to 40. In the solutions according to the invention, the film sizing roll is always the so-called a soft roll having a surface material of, for example, polyurethane.

As shown in Figure 3, the nozzle device of the film sizing roll 40 comprises a low angle coating blade 42 attached to the frame structure 41. Between the frame structure 41 and the blade 25 42 there is a load hose 43 by which the pressure and position of the blade 42 can be adjusted. The paste 46 is in the coating space formed by the front wall 44 and the blade 41, from where it is transferred in a controlled manner to the surface of the roll 40. The front wall 44 is attached to the frame of the apparatus by support structures 45. This nozzle device is known in the art and the solution is described in more detail e.g. in Finnish patent publication 81734. The blade 42 can alternatively be replaced by a rod scraper.

In one preferred embodiment of the invention, the counter roll has a metal surface. The metal surface is preferably, for example, chromium. A roll coating of ceramic material can also be used. A counter-roll with a metal or ceramic surface tentatively calend the coating. These rolls with 6,94883 steel cores can also be cooled, causing water to condense on the surface of the roll, which in turn makes it easier to keep the roll clean. The better heat resistance and abrasion resistance of a roll with a metal or ceramic surface also allow the use of steam and scrapers to clean the roll.

According to the invention, a groove rod dosing device or a spraying device can also be used as a nozzle device for applying the coating to the film roll, with which the coating is sprayed onto the surface of the roll.

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Claims (10)

A method for double-sided coating of a thin printing paper web containing mechanical pulp, for example a web containing recycled fiber, wherein - the first side of the web (5) is coated at a first coating station (1), 10. the first coating layer is at least partially dried by drying equipment (3); the web (5), the second side is coated after the first half-curing the second coating 15 of relay station (2), and - a second coating layer over at least part of the second drying apparatus (4), 20 characterized in that - the first and the second coating layer is required for applying the päällystemää-RA on the surface of the film roll (22) rotating at a circumferential speed approximately equal to the speed of the web (5), the coating being transferred to the web (5) in a nip (N 1) between the counter roll (24) and the film roll (22). Method according to Claim 1, characterized in that a metal-faced roll is used as the counter roll (24) for the preliminary calendering of the web (5). A method according to claim 1, wherein a web with a basis weight of up to 65 g is coated, characterized in that a roll having a diameter smaller than the diameter of the film roll (22) is used as a counter roll (24) to promote rapid release of the coating layer. 94883 8 4. An apparatus comprising mechanical pulp thin painopape-rirainan (5), for example, recycled fiber-containing web for simultaneous coating, the apparatus comprising a first 5 of the coating station (1) of the web (5) of the first half of coating, - a first drying device (3) of the web (5) drying for 10 - a second coating station (2) of the web (5) for coating the second side, and - a second drying apparatus (4) of the web (5) dry for 15 vausta, characterized in that - the first and second coating units 20 (1, 2 ) comprise - a film roll (22) on the surface of which a coating layer of the desired thickness can be formed and which roll can be rotated at a circumferential speed approximately corresponding to the web speed, - coating material application means (20) on which the coating layer can be formed on the film roll 22 , - a counter roll (24) forming a nip (N.) with the bogies (22) and the counter roll (24) is rotatable at a circumferential speed approximately equal to the web speed, whereby the web (5) can be guided through said nip (N 1). 9 94883 Apparatus according to Claim 4, characterized in that the counter roll (24) has a smaller diameter than the film roll (22). Apparatus according to claim 4 or 5, characterized in that the counter roll (24) has a metal surface for the preliminary calendering of the web (32). Apparatus according to claim 4 or 5, characterized in that the counter roll (24) has a ceramic surface for the preliminary calendering of the web (32). 7 94883 Apparatus according to one of the preceding claims, characterized in that the counter roll (24) is provided with cooling equipment. Apparatus according to one of the preceding claims, characterized in that the counter roll (24) is provided with a steam cleaning apparatus. 20 Apparatus according to one of the preceding claims, characterized in that the counter roll (24) is provided with a scraper cleaning device. • «Λ m f ♦ ♦ · ία 94883