WO2009077643A1

WO2009077643A1 - Metal belt calender

Info

Publication number: WO2009077643A1
Application number: PCT/FI2008/050566
Authority: WO
Inventors: Stig Renvall
Original assignee: Metso Paper, Inc.
Priority date: 2007-12-17
Filing date: 2008-10-09
Publication date: 2009-06-25
Also published as: FI20075919A0; FI20075919L; AT508202B1; AT508202A3; DE112008003333T5; AT508202A2

Abstract

The invention relates to a metal belt calender (1), comprising a metal belt (3) adapted to extend around guiding rolls (2). Outside the belt is disposed at least one counter-roll (8) in a way to establish between the belt (3) and the counter-roll (8) a fibrous web treatment zone (PN), the fibrous web to be treated being passed there through. The calender comprises a first nip roll (5a), disposed outside the belt (3), establishing a soft nip with the counter- roll (8) and set in a location upstream of the treatment zone (PN) in the running direction of a web (W). The nip roll (5a) applies a press action to one side of the fibrous web. Outside the belt (3) is further disposed a second nip roll (5b), set in a location downstream of the treatment zone (PN) in the running direction of the web (W) to establish a soft nip with the belt circle guiding roll (2), said nip roll applying a press action to the other, opposite side of the fibrous web (W). In an intermediate location between said soft- nip establishing first (5a) and second (5b) nip rolls is disposed a third hard- surface nip roll (4), which establishes a hard nip with the counter-roll (8) or with the belt guiding roll (2).

Description

Metal belt calender

The present invention relates to a metal belt calender, comprising a metal belt adapted to extend around guiding rolls, outside said belt being disposed at least one counter-roll in a way to establish between the belt and the counter-roll a fibrous web treatment zone, the fibrous web to be treated being passed therethrough.

Calendering is a process, in which the surface properties and the thickness profile of various grades of paper and board are worked on to match the requirements of a printing method and further processing, such as e.g. a coating process, by compressing the paper between two or more compression elements, whereby the paper, due to mechanical working and heat, undergoes a deformation both in horizontal and vertical directions. Coated grades are typically pre-calendered prior to a coating process and post-calendered after the coating process.

Calendering is the final operation in a paper making process, which can be used for making a major difference in the properties of paper.

Regarding coated paper grades, calendering can be often broken down into two operations, pre-calendering prior to and final calendering after the coating process.

Paper and board grades are available in a wide variety and can be divided according to basis weight into two categories: papers with a single ply and a basis weight of 25-300 g/m² and boards made in multi-ply technique and with a basis weight of 100-600 g/m². As noted, the borderline between paper and board is flexible as board grades with lightest basis weights are lighter than the heaviest paper grades. Generally, paper is used for printing and board for packaging. The subsequently presented descriptions are examples of currently applied values for fibrous webs, and there may be considerable fluctuations from the disclosed values. The descriptions are mainly based on the source publication Papermaking Science and Technology, section Papermaking Part 3, Finishing, edited by Jokio, M., published by Fapet Oy, Jyvaskyla 1999, 361 pages.

Mechanical-pulp based, i.e. wood-containing printing papers include newsprint, uncoated magazine and coated magazine paper.

Newsprint is composed either completely of mechanical pulp or may contain some bleached softwood pulp (0-15%) and/or recycled fiber to replace some of the mechanical pulp. General values for newsprint can probably be regarded as follows: basis weight 40-48.8 g/m², ash content (SCAN-P 5:63) 0-20%, PPS SlO roughness (SCAN-P 76-95) 3.0-4.5 μm, Bendtsen roughness (SCAN-P21:67) 100-200 ml/min, density 600-750 kg/m³, brightness (ISO 2470:1999) 57-63% and opacity (ISO 2470:1998) 90-96%.

Uncoated magazine paper (SC = supercalendered) usually contains mechanical pulp to 50-70%, bleached softwood pulp to 10-25%, and fillers to 15-30%. Typical values for calendered SC paper (containing e.g. SC-C, SC-B and SC-A/A+) include basis weight 40-60 g/m², ash content (SCAN-P 5:63) 0-35%, Hunter gloss (ISO/DIS 8254/1) < 20-50%, PPS SlO roughness (SCAN-P 76:95) 1.0-2.5 μm, density 700-1250 kg/m³, brightness (ISO 2470:1999) 62-70% ja opacity (ISO 2470:1998) 90-95%.

Table 1 presents typical values for mechanical-pulp containing coated papers. (MFC = machine finished coated, FCO = film coated offset, LWC = light weight coated, MWC = medium weight coated, HWC = heavy weight coated) Table 1

Coated magazine paper (LWC = light weight coated) contains mechanical pulp to 30-60%, bleached softwood pulp to 25-40%, and fillers and coaters to 20-35%.

HWC can be coated even more than twice.

Chemical-pulp produced, woodfree printing papers or fine papers include uncoated - and coated - chemical-pulp based printing papers, in which the portion of mechanical pulp is less than 10%.

Uncoated chemical-pulp based printing papers (WFU) contain bleached birchwood pulp to 55-80%, bleached softwood pulp to 0-30%, and fillers to 10-30%. The values with WFU vary widely: basis weight 50-90 g/m² (up to 240 g/m²), Bendtsen roughness 250-400 ml/min, brightness 86-92%, and opacity 83-98%.

In coated chemical-pulp based printing papers (WFC), the amounts of coating vary widely according to requirements and intended application. The following are typical values for once- or twice coated chemical-pulp based printing paper: once-coated basis weight 90 g/m², Hunter gloss 65-80%, PPS slO roughness 0.75-2.2 μm, brightness 80-88%, and opacity 91-94%, and twice-coated basis weight 130 g/m², Hunter gloss 70-80%, PPS slO roughness 0.65-0.95 μm, brightness 83-90%, and opacity 95-97%.

Release papers have a basis weight within the range of 25-150 g/m².

Other papers include, among others, sackkraft papers, tissues, and wallpaper bases.

Board grades make up a reasonably heterogeneous group, comprising high basis weight grades, the basis weight of which can be as high as 600 g/m², and low basis weight grades, the basis weight of which is about 120 g/m² , the grades possibly ranging from one based on primary fiber to one based by up to 100% on recycled pulp and from an uncoated grade to those with multiple coats. The board grades will be subsequently divided into coated and uncoated ones, because the coating has a major effect on the calendering process. The coated grades involve the use of both pre- calendering upstream of a coating device and a final calender downstream of the coating device. The uncoated grades are only subjected to final calendering. These two groups encompass a multitude of various board grades as follows:

Coated board:

-based on primary fiber (FBB=folding boxboard, SBS=solid bleached board, LPB=liquid packaging board, coated white top liner, carrier board) based on recycled fiber (WLC=white lined chipboard, coated recycled board).

Uncoated board: -based on primary fiber (kraft liner, white top liner, liquid packaging board) -based on recycled fiber (test liner)

Calenders are broken down into machine calenders, soft calenders, and multi-roll calenders. The machine calender has typically 1-2 nips and both rolls of the nip are hard rolls. The soft calender has generally 1-4 nips and at least one of the rolls making up a nip is coated with a soft coating. The multi-roll calender has usually 5-11 nips. The array of rolls in a multi-roll calender comprises both heated rolls and soft surface rolls.

Special calenders include, among others, a wet stack calender, a breaker stack, and long-nip calenders.

The wet stack calender is roughly identical to the multi-roll calender, but the calendering process is totally different. The wet stack calender makes efficient use of a moisture gradient, the moisture of a web arriving at the calender being only about 1-2%. The wet stack calender has water boxes for forming a water film on the web surface upstream of the nip, said film being pressed to the web surface in the nip. Accordingly, the web only becomes moist on the surface, thus calendering more at the surface than in the overdried interior. The wet stack calender is used as a pre-calender prior to a coating process for many board grades, such as e.g. solid bleached board (SBS).

The intermediate calender is a machine calender, which is located in the dryer section of a paper machine. The long-nip calenders include a shoe calender, which has a soft shell around the shoe roll and which has a nip length typically of 50-400 mm, as well as belt calenders. The traditional belt calender consists of a soft calender's thermo roll, belt circle, and a counter-roll inside the belt circle which can be a hard or soft roll. The belt runs around by way of the counter- roll and guide/tension rolls. A special application of the belt calender is a metal belt calender, in which the calendering belt comprises a metal belt which extends around guide rolls and, together with a counter-element, which is typically a roll, establishes a long nip zone whose length can be even more than 5000 mm. Inside the belt circle can be further provided a press element, e.g. a deflection-compensated roll, which enables a higher compression pressure nip point to be provided in the middle of the long nip. This type of metal belt calender has been described e.g. in the Applicant's earlier international application publication WO03064761A1.

European folding board is typically manufactured by using a Yankee cylinder, which provides smoothness on the top surface of a sheet. The web must be delivered into and out of a Yankee cylinder at a certain web surface moisture for optimal final smoothness and runnability. This implies that a fixed diameter of the dryer imposes a limit on the range of speed at which the web can be run around a Yankee cylinder. The range of basis weight, which can be efficiently run on the machine, is quite narrow, and modern high machine speeds would require unrealistically large diameters for Yankee cylinders.

Hence, alternative pre-calendering methods make objects of major interest. Hard-nip calenders are used in new coated board machines for smoothing the web prior to coating without compromising efficiency and with a maximal output capacity. However, a sustainable smoothness cannot be achieved to the extent of what is accomplished by wet stack calenders or a Yankee cylinder. A higher roll temperature provides better smoothness with lower nip loads, and bulk as well as stiffness can be spared. A high roll temperature softens fibers of the actual surface, giving considerable smoothness to the basic sheet. During a coating process, however, the water originating from coating penetrates deeper into the sheet, resulting in the swelling of fibers and renewed roughening of the basic sheet's surface.

A smoothening effect continuing throughout the coating process is achieved, provided that the web is surface-moistened prior to the treatment in a hot nip. A steam generator can be used for this purpose, but that functions by condensing the vapor for a film of water on the surface of a low-temperature web and calls for cooling of the web. A more effective tool is to employ an air-atomized micro-drop moistening device. Positioning the unit is critical as the dwell time has an effect on the water penetration depth in the web. A more permanent smoothening result is obtained by having a long dwell time and a large amount of water.

A hard nip, as established either between two rollers or by using a metal belt, provides a good large-scale smoothness, whereas a soft nip provides a good small-scale smoothness. The softest nip is obtained by means of an OptiDwell shoe calender, in which the reduction of bulk, as well as stiffness, is highly marginal. What can be anticipated in addition to a small-scale smoothness is a highly uniform absorption of the coating agent, as well as absorption during the printing period, and the result is a printed product without gloss mottles. A viable option to the shoe calender is soft calendering, which is implemented by using roll coatings of a relatively soft material.

The combination of a moistening device, hot soft-nip and hard-nip technologies provides a large-scale smoothness which competes with wet stack and Yankee cylinders while resulting in a better small-scale smoothness with 4-7% higher bulk and density. A metal belt calender has been found to provide paper and board with desired properties which have been earlier obtained by said calender technologies, with the exception of a small-scale smoothness. Paper and board webs, which are difficult to calender, such as board webs made from North American softwood species, do not create a sufficient small-scale smoothness in a metal belt calender.

It is an objective of the present invention to provide a remedy to this shortfall by developing an improved metal belt calender, which enables both a large-scale and a small-scale smoothness better than before to be attained on either side of a web. In order to accomplish this objective, a metal belt calender according to the invention is characterized in that the calender comprises a first nip roll, disposed outside the belt, establishing a soft nip with the counter-roll and set in a location upstream of the treatment zone in the running direction of a web, said nip roll applying a compression effect on one side of the fibrous web; a second nip roll, disposed likewise outside the belt, establishing a soft nip with the belt circle guiding roll and set in a location downstream of the treatment zone in the running direction of the web, said nip roll applying a compression effect on the other, opposite side of the fibrous web; and a third hard-surface nip roll, which is disposed in an intermediate location between said soft-nip establishing first and second nip rolls, and which establishes a hard nip with the counter-roll or with the belt guiding roll.

The apparatus according to the invention expands an application range for the successful use of metal belt technology. In some cases, moistening may be needed for softening the fibrous web upstream of the apparatus for improving the smoothness result even further. The apparatus of the invention combines the attainment of a good small-scale smoothness offered by a soft nip with a large-scale smoothness obtainable by means of a hard nip on both sides of a web. It is prior known that a soft nip produces a good small-scale smoothness (PPS slO roughness (SCAN-P 76-95)), but does not a major effect on large-scale smoothness (Bendtsen roughness (SCAN-

P21:67)). A hard nip or a metal belt calender produces a reverse outcome, i.e. a good large-scale smoothness, but the small-scale smoothness falls short of what is obtained by a soft nip. The metal belt calender according to the invention lends itself particularly well to board making, which is currently practiced by using wet stack calenders or Yankee cylinders. The solution according to the invention enables improving stiffness by about 5% with respect to the prior art methods, while the other board properties remain the same or become better. The wet stack calender, for example, offers a good smoothness prior to coating and an adequate absorption resistance for the coating. However, the web undergoes a substantial compaction with a reduction of bulk and stiffness. In addition, web breaks are commonplace in wet stack calenders. The machine's production capacity suffers, because the web must be subjected to drying upstream of the set of rolls, and the added water must also be drained.

The invention will now be described with reference to the accompanying drawing, the sole figure of which depicts one embodiment for a metal belt calender of the invention in a diagrammatic view of principle.

As shown in fig. 1, the metal belt calender comprises a metal belt 3, which is adapted to extend around heated belt guiding rolls 2 and which establishes a long calendering zone PN with a counter-roll 8 disposed outside the belt circle. The counter-roll 8 is preferably a heated roll. A fibrous web W is adapted to travel through said long nip. On the web traveling path, upstream of the long nip PN, is provided a soft-surface nip roll 5a for treating the fibrous web on one side, and in a location downstream of the long nip PN is provided a second soft-surface nip roll 5b for treating the fibrous web on the opposite side. The nip roll 5b is set in register with the heated guide roll 2 inside the belt circle, establishing a soft nip therewith. The fibrous web W is guided by means of a deflection roll 7 away from the latter soft nip. The apparatus of fig. 1 further comprises a hard-surface nip roll 4 inside the belt circle 3 for establishing a hard nip at an intermediate location between said soft nips 5a, 8 and 5b, 2 in the calendering zone PN. A hard-nip establishing nip roll can also conceivably be placed outside the belt circle, in which case it can be positioned against e.g. the same guide roll 2 as the latter soft-nip establishing nip roll 5b, upstream of said nip roll 5b as regarded in the web running direction.

The solution according to the invention is capable of providing both a good large-scale and small-scale bilateral smoothness for a fibrous web. In the apparatus according to the invention, the metal belt 3 has its temperature preferably adapted to be substantially equal to that of its counter-roll 8, whereby the roll 8 and the belt 3 can be heated or cooled.

Claims

1. A metal belt calender (1), comprising a metal belt (3) adapted to extend around guiding rolls (2), outside said belt being disposed at least one counter-roll (8) in a way to establish between the belt (3) and the counter- roll (8) a fibrous web treatment zone (PN), the fibrous web to be treated being passed therethrough, characterized in that the calender comprises a first nip roll (5a), disposed outside the belt (3), establishing a soft nip with the counter-roll (8) and set in a location upstream of the treatment zone (PN) in the running direction of a web (W), said nip roll applying a compression effect on one side of the fibrous web; a second nip roll (5b), disposed likewise outside the belt (3), establishing a soft nip with the belt circle guiding roll (2) and set in a location downstream of the treatment zone (PN) in the running direction of the web (W), said nip roll applying a compression effect on the other, opposite side of the fibrous web (W); and a third hard-surface nip roll (4), which is disposed in an intermediate location between said soft-nip establishing first (5a) and second (5b) nip rolls, and which establishes a hard nip with the counter-roll (8) or with the belt guiding roll (2).

2. A metal belt calender as set forth in claim 1, characterized in that the hard-nip establishing nip roll (4) is disposed inside the belt circle (3), thus establishing a nip with the counter-roll (8).

3. A metal belt calender as set forth in claim 1, characterized in that the hard-nip establishing nip roll (4) is disposed outside the belt circle (3), thus establishing a nip with the belt guiding roll (2).

4. A metal belt calender as set forth in any of the preceding claims, characterized in that the soft-nip establishing roll (5a, 5b) is selected from a group, including: a polymer-coated roll, a rubber-coated roll, an elastomer- coated roll, and a shoe roll.