EP2876205A1

EP2876205A1 - Method for calendering a fiber web and calender

Info

Publication number: EP2876205A1
Application number: EP13193866.4A
Authority: EP
Inventors: Reijo PIETIKÄINEN; Tapio Anttila; Mika Viljanmaa
Original assignee: Valmet Technologies Oy
Current assignee: Valmet Technologies Oy
Priority date: 2013-11-21
Filing date: 2013-11-21
Publication date: 2015-05-27
Also published as: CN204311292U

Abstract

The invention relates to a method for calendering a fiber web, in which method at least one surface of the fiber web (W) is calendered in at least one calendering nip formed between two rolls. In the method at least one surface of the fiber web is roughened by calendering by at least one calender roll (11 R; 14R) with rough surface forming calendering nip with another calender roll (12) to surface roughness greater than the surface roughness of the fiber web when entering the calendering nip. In the method at least one surface of the fiber web is roughened by moisturizing the fiber web (W) during calendering or directly after calender and in the last calendering nip by at least one calender roll (11 R; 14R) with rough surface having surface roughness of 0,4 - 1,6 Ra. The method is utilized in production of a specialty paper web. The invention also relates to a calender of a fiber web machine comprising at least one calendering nip formed between two calender rolls, at least one hard calender roll (11) or at least one deflection compensated roll (14) and at least one soft calender roll (12). The calender has at least one calender roll (11 R; 14R) with rough surface forming calendering nip with another calender roll (12) to roughen at least one surface of the fiber web to surface roughness greater than the surface roughness of the fiber web when entering the calendering nip. The at least one calender roll with roughened surface has surface roughness 0,4 - 1,6 Ra. The calender comprises at least one moisturizing device (17). The calender is utilized in production of a specialty paper web.

Description

In general present invention relates to a method for calendering a fiber web and a calender. More especially the present invention relates to a method for calendering a fiber web according to the preamble part of claim 1 and to a calender according to preamble part of claim 7.
Calendering is generally carried out in order to improve the properties; surface properties, like smoothness and gloss, and caliber of a fiber web such as a paper or board web. Calendering has different functions depending on the location of the calender in the fiber web production line and on the fiber web grade to be produced. For example when producing coated fiber web grades a precalender is used before coating and after coating the fiber web is calendered in final calender.
Calender can be an incorporated part of the fiber web production line or the coating machine i.e. an online-calender. In case a calender forms an independent unit, it is called offline-calender. Into online-calenders the fiber web is guided from the drying section of the fiber web machine or of the coater. Into offline-calenders the fiber web is guided from a reel-up.
In calendering the web is passed into a nip, i.e. calendering nip, formed between rolls that are pressed against each other, in which nip the web becomes deformed as by the action of temperature, moisture and nip pressure. It is also known that surface roughness of calender rolls have an effect to smoothness of the finished fiber web. Also moisturizing of the fiber web, especially changes of moisture content, have an effect to smoothness of the finished fiber web. Re-drying does not recover the lost smoothness i.e. the change is irreversible.
Calendering has an effect to various properties of the fiber web, for example smoothness, density, transparency, opacity etc. Thus in some cases some required property values lead to difficulties in achieving other required values of properties of the fiber web. For example smoothness of the fiber web may rise to too high level in order to achieve in calendering at needed loads and temperatures the required density, transparency and opacity levels.
Various different types of calenders are used in calendering of fiber webs: in hard nip calenders (machine calenders) there are typically 1 - 2 nips and the web is calendered in a nip between two hard rolls, in soft calenders there are typically 1 - 4 nips and at least one of the nip rolls is a soft surface polymer roll. In hard nip calenders and in soft calenders one of nip rolls is a heatable thermo roll. Multiroll calenders have usually 5 - 11 nips i.e. 6 - 12 calender rolls, of which 2 - 5 are thermo rolls and 4 - 7 soft surfaced, polymer coated rolls. Super calenders have usually also several nips and in a typical super calender, the hard rolls are of metal, usually steel and/or cast iron, and the soft rolls have been paper-filled or fabric-filled.
In WO2005052253 is disclosed a method in calendering a paper web or corresponding fiber web, in which the paper web is calendered in a multiroll calender, wherein the paper web is calendered in a stack of rolls formed by at least three rolls, wherein calendering nips are formed by thermo rolls and by a calendering rolls with a soft surface. The paper web is calendered in at least one calendering nip formed by a thermo roll with a matt surface and by a calendering roll with a soft surface in order to produce a paper web with a matt surface.
In WO2007077296 is disclosed a method and a device of calendering a fiber web, such as paper, board or a corresponding web, so that the fiber web is passed through a nip zone formed by two hard surfaces so that the fiber web is calendered in at least one hard nip zone, in which nip zone at least one surface pressing the web is arranged as a hard matte-surface rough surface manufactured to have an Ra roughness of 0.2 to 1.0 µm, preferably an Ra roughness of 0.4 to 0.6 µm, in order to calender the fibrous web to be mattelike.
Paper and board are available in a wide variety of types and can be divided according to basis weight in two grades: papers with a single ply and a basis weight of 25 - 300 g/m² and boards manufactured in multi-ply technology and having a basis weight of 150 - 600 m/m². It should be noted that the borderline between paper and board is flexible since board grades with lightest basis weights are lighter than the heaviest paper grades. Generally speaking, paper is used for printing and board for packaging.
The subsequent descriptions are examples of values presently applied 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, edited by Rautiainen, P., and published by Paper Engineers' Association, Helsinki 2009, 404 pages.
Mechanical-pulp based, i.e. wood-containing printing papers include newsprint, uncoated magazine and coated magazine paper.
Today's newsprint furnishes mostly contain between 80 and 100 % deinked pulp (DIP). The rest of the furnish is mechanical pulp (typically TMP). However, there is also newsprint made of 100 % mechanical fiber furnishes. DIP based newsprint may contain up to 20 % filler. The filler content of a virgin-fiber based newsprint furnish is about 8 %.
General values for CSWO newsprint can be regarded as follows: basis weight 40-48.8 g/m², PPS s10 roughness (SCAN-P 76-95) 4.0 - 4.5 µm, Bendtsen roughness (SCAN-P21:67) 150 ml/min, density 600 - 750 kg/m³, brightness (ISO 2470:1999) 58 - 59 %, and opacity (ISO 2470:1998) 92 - 95 %.
Uncoated magazine paper (SC-supercalendered) grades usually contain 50 % - 75 % mechanical pulp, 5 % - 25 % chemical pulp, and 10 % - 35 % filler. The paper may also contain DIP. 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 s10 roughness (SCAN-P 76:95) 1.0 - 2.5 µm, density 700 - 1250 kg/m³, brightness (ISO 2470:1999) 62 - 75 %, and opacity (ISO 2470:1998) 90 - 95 %.
Coated mechanical papers include for example MFC (machine finished coated), LWC (lightweight coated), MWC (medium weight coated), and HWC (heavy weight coated) grades. Coated mechanical papers usually contain 45 % - 75 % mechanical or recycled fiber and 25 % - 55 % chemical pulp. Semi-chemical pulps are typical in LWC paper grades made in the Far East. The filler content is about 5 % - 10 %. The grammage is typically in the range 40 - 80 g/m².
General values for LWC paper can be regarded as follows: basis weight 40-70 g/m², Hunter gloss 50 - 65 %, PPS S10 roughness 1.0 - 1.5 µm (offset) and 0.6 - 1.0 µm (roto), density 1100 - 1250 kg/m³, brightness 70 - 75 %, and opacity 89 - 94 %.
General values for MFC paper (machine finished coated) can be regarded as follows: basis weight 48 - 70 g/m², Hunter gloss 25 - 40 %, PPS S10 roughness 2.2 - 2.8 µm, density 900 - 950 kg/m³, brightness 70 - 75 %, and opacity 91 - 95 %.
General values for MWC paper (medium weight coated) can be regarded as follows: basis weight 70 - 90 g/ m², Hunter gloss 65 - 70 %, PPS S10 roughness 0.6 - 1.0 µm, density 1150 - 1250 kg/m³, brightness 70 - 75 %, and opacity 89 - 94 %.
Wood-free paper is divided into two segments: uncoated and coated. Conventionally, furnish of wood-free papers consists of bleached chemical pulp, with less than 10 % mechanical pulp.
Typical values are for uncoated WFU Copy paper: grammage 70 - 80 g/m², Bendtsen roughness 150 - 250 ml/min and bulk > 1.3 cm³/g; for uncoated offset paper: grammage 60 - 240 g/m², Bendtsen roughness 100 - 200 ml/min and bulk 1.2 - 1.3 cm³/g; and for color copy paper: grammage 100 g/m², Bendtsen roughness < 50 ml/min and bulk 1.1 cm³/g.
In coated pulp-based printing papers (WFC), the amounts of coating vary widely in accordance with requirements and intended application. The following are typical values for once- and twice-coated, pulp-based printing paper: once-coated basis weight 90 g/m², Hunter gloss 65 - 80 %, PPS s10 roughness 0.75 - 1.1 µm, brightness 80 - 88 %, and opacity 91 - 94 %, and twice-coated basis weight 130 g/m², Hunter gloss 70 - 80 %, PPS S10 roughness 0.65 - 0.95 µm, brightness 83 - 90 %, and opacity 95 - 97 %.
Containerboard includes both linerboard and corrugating medium. Liners are divided according to their furnish base into kraft-liner, recycled liner and white top liner. Liners are typically 1- to 3-ply boards with grammages varying in the range 100 - 300 g/m².
Linerboards are generally uncoated, but the production of coated white-top liner is increasing to meet higher demands for printability.
The main carton board grades are folding boxboard (FBB), white-lined chipboard
(WLC), solid bleached board (SBS) and liquid packaging board (LPB). In general, these grades are typically used for different kinds of packaging of consumer goods. Carton board grades vary from one- up to five-ply boards (150 - 400 g/m²). The top side is usually coated with from one to three layers (20 - 40 g/m²), the back side has less coating or no coating at all. There is a wide range of different quality data for the same board grade. FBB has the highest bulk thanks to the mechanical or chemi-mechanical pulp used in the middle layer of the base board. The middle layer of WLC consists mainly of recycled fiber, whereas SBS is made from chemical pulp, exclusively.
FBB's bulk typically is between 1.1 - 1.9 cm³/g whereas WLC is on range 1.1 - 1.6 cm³/g and SBS 0.95 - 1.3 cm³/g. The PPS-s10-smoothess is respectively for FBB between 0.8 - 2.1 µm, for WLC 1.3 - 4.5 µm and for SBS 0.7 - 2.1 µm.
Release paper is used in label base paper in various end-use applications, such as food packaging and office labels. The most common release paper in Europe is supercalendered glassine paper coated with silicone to provide good release properties.
Specialty papers are used for various purposes and comprise for example industrial papers, label papers, poster papers, décor papers and release papers.
Typical values for supercalendered release papers are basis weight 60 - 95 g/m², caliper 55 - 79 µm, IGT 12-15 cm, Cobb Unger for dense side 0.9 - 1.6 g/m² and for open side 1.2 - 2.5 g/m².
Coated label paper is used as face paper for release, but also for coated backing paper and flexible packages. Coated label paper has a grammage of 60 - 120 g/m² and is typically sized or pre-coated with a sizer and single-blade coated on one side. Some typical paper properties for coated and calendered label paper are basis weight 50 - 100 g/m², Hunter gloss 70 - 85 %, PPS s10 roughness 0.6 - 1.0 µm, Bekk smoothness 1500 - 2000 s and caliper 45 - 90 µm.
In particular the present invention relates to calendering of specialty papers, in particular of release papers. Typically specialty papers, in particular release papers, are calendered in a super calender, in which roughness of the calender rolls is about 0,2 Ra. Moisture contents of the paper web after calendering is 8 - 10 % and generally after-drying, for example by air-borne dryers and drying cylinders, is used in order to reach the desired final moisture content. Roughness of the finished specialty paper web, in particular release paper web, is an important factor for example in view of the later siliconizing process of the web. Roughness of the finished specialty paper, in particular release paper, is also important in view of oil absorption properties. In the present there is a need to increase portion of cheaper short fiber pulp in production of release paper webs but this leads to too high smoothness values at the required level of density, opacity and transparency of the web. Thus portion of long fiber pulp has been high, about 40 - 50 %, in pulp used for production of release paper webs.
An object of the present invention is to create a method for calendering a fiber web and a calender, by which a fiber web with desired roughness values of the web surface is produced.
Another object of the present invention is providing a method and a calender to recover desired roughness of the fiber web, which has been calendered to too high smoothness values.
A particular object of the present invention is to provide a method and a calender for producing specialty paper web, in particular release paper web, by which finished calendered web with lower smoothness (Bekk 1500 s), required transparency (45 - 50 %) and density (1100 - 1250 kg/m³) is produced.
In order to achieve the above mentioned objects and those that will be disclosed later, the method for calendering a fiber web according to the invention is mainly characterized by the features of the characterizing part of claim 1. The calender according to the invention is in turn mainly characterized by the features of the characterizing part of claim 7.
According to the invention in the method at least one surface of the fiber web is roughened by at least one calender roll with rough surface to surface roughness greater than the surface roughness of the fiber web when entering the calendering nip and by moisturizing the fiber web during calendering or directly after calender and at least one surface of the fiber web is roughened in the last calendering nip of the calender by at least one calender roll with rough surface having surface roughness of 0,4 - 1,6 Ra. The invention is utilized in production of specialty paper web, in particular release paper web.
According to an advantageous feature the at least one calender roll has a rough surface with surface roughness 0,4 - 1,6 Ra.
According to an advantageous feature all hard calender rolls of the calender have rough surface having same surface roughness of 0,4 - 1,6 Ra.
According to an advantageous feature the roughness value of the surface of the hard calender rolls increases in the travel direction of the fiber web in the calender.
According to an advantageous aspect of the invention the fiber web is moisturized between travel to the calendering nip formed by the at least one calender roll with rough surface from the preceding calendering nip.
According to another advantageous feature of the invention the fiber web is dried after the calendering and moisturizing.
According to another advantageous feature of the invention the temperature of the fiber web is controlled during calendering by controlling temperature of at least one calender roll in order to reach desired roughness value of the at least one surface of the fiber web to be roughened.
According to another advantageous feature of the invention the roughness of the at least one surface of the fiber web is controlled during calendering by controlling nip pressure of at least one calender nip in order to reach desired roughness value of the at least one surface of the fiber web to be roughened.
According to an advantageous feature of the invention the fiber web is moisturized in the calender during the run of the fiber web between two successive calendering nips.
In the following the invention is described with reference to the accompanying drawing in which

in figure 1 is shown schematically one example of a calender according to the invention,
in figure 2 is shown schematically another example of a calender according to the invention and
in figure 3 is shown schematically yet another example of a calender according to the invention.

During the course of this description like numbers and signs will be used to identify like elements according to the different examples which illustrate the invention.
Figure 1 schematically show an example of multiroll calender 10 with a stack of calender rolls 11, 12, 14, in which a fiber web W is calendered in the calendering nips between the calender rolls. By reference numerals 12 a calender roll that is a resilient-surfaced calender roll, i.e. a soft calender roll 12, is indicated and by reference numeral 11 a calender roll that is a hard calender roll, for example a thermo roll, is indicated. The uppermost and lowermost rolls 14 of the calender roll stack are a deflection compensated rolls. Soft calender rolls 12 are soft-surfaced, for example polymer-surfaced deflection, rolls and smooth-surfaced rolls 12 are metal rolls, or thermo rolls either heatable and/or coolable thermo rolls. Fly rolls guiding the fiber W web in the calender 10 are indicated by reference numeral 15 and spreader rolls by reference numeral 16. Moisturizing devices 17 are located in pockets formed by the calender rolls in the stack, fly roll 15 and fiber web W. In this calender 10 only one side of the fiber web is calendered and the calender 10 comprises two rolls with roughened surface for roughening the surface of the fiber web to be calendered, last hard calender roll 11 R of the stack and lowermost deflection compensated roll 14.
Figure 2 schematically show an example of multiroll calender 10 with a stack of calender rolls 11, 12, in which a fiber web W is calendered in the calendering nips between the calender rolls. By reference numerals 12 a calender roll that is a resilient-surfaced calender roll, i.e. a soft calender roll 12, is indicated and by reference numeral 11 a calender roll that is a hard calender roll, for example a thermo roll, is indicated. Soft calender rolls 12 are soft-surfaced, for example polymer-surfaced deflection, rolls and smooth-surfaced rolls 12 are metal rolls, or thermo rolls either heatable and/or coolable thermo rolls. Fly rolls guiding the fiber W web in the calender 10 are indicated by reference numeral 15. In this calender 10 both sides of the fiber web is calendered and a reversing nip is located in the middle of the calender 10 between two soft calender rolls 12. The calender 10 comprises two rolls with rough surface for roughening the surface of the fiber web to be calendered, last hard calender roll 11 R of the stack above the reversing nip and the last hard calender roll 11 R at lowermost location in the stack of rolls of the calender 10.
Figure 3 schematically show an example of a soft calender 10 with four calender rolls 11, 12, in which a fiber web W is calendered in the calendering nips between the calender rolls 12, 11; 11; 12. By reference numerals 12 a calender roll that is a resilient-surfaced calender roll, i.e. a soft calender roll 12, is indicated and by reference numeral 11 a calender roll that is a hard calender roll, for example a thermo roll, is indicated. Soft calender rolls 12 are soft-surfaced, for example polymer-surfaced deflection, rolls and smooth-surfaced rolls 12 are metal rolls, or thermo rolls either heatable and/or coolable thermo rolls. In this calender 10 both sides of the fiber web W is calendered and the calender 10 comprises two rolls with rough surface for roughening the surface of the fiber web W to be calendered, hard calender rolls11R of both calendering nips of the calender 10.
In the calenders 10 of the examples of the figs. 1 - 3 at least one surface of the fiber web W is roughened by at least one calender roll 11 R, 14R to surface roughness greater than the surface roughness of the fiber web W when entering the calendering nip formed by the at least one calender roll 11 R, 14R and another calender roll 12. In the example of figure 1 the fiber web W surface is also roughened by moisturizing the fiber web by the moisturizing devices 17 during calendering during the run of the fiber web W between two successive calendering nips. The at least one calender roll 11 R, 14R has a rough surface with surface roughness greater than 0,4 - 1,6 Ra.
According to one advantageous example the invention is utilized in production of release paper web. In this example the fiber web is guided to the calender, in which at least one calender roll has a rough surface, in moisture content of 13 - 15%. In the first stage the fiber web is calendered with high load such that desired density and transparency is achieved. After the first calendering stage the moisture content of the fiber web is 8 - 10 %. The fiber web is moisturized such that surface moisture content of the web is about 15 % by non-contacting moisturizing method, for example by spray-moisturizing, microjet-moisturizing. The application amount of water is for example 3 - 8 g/m². After moisturizing the fiber web is dried by air borne dryers or by drying cylinders to desired final moisture content. The side of the fiber web that will be roughened by the above treatment is the side that later will be siliconized.
According to one example the fiber web is moisturized at the stack of the calender rolls, advantageously on the lower stack of calender rolls by applying moisture onto the fiber web by pond-moisturizing method, for example by wet stack or by mini-pond in at least one calendering nip. The fiber web is moisturized such that the outer surface of the fiber web has moisture content that causes the fibers to indurate such that later when the fiber web dries the surface of the fiber web will roughen.

Reference signs used in the drawing

11: hard calender roll
11R: hard calender roll with rough surface
12: soft calender roll
14: deflection compensated roll
14R: deflection compensated roll with rough surface
15: fly roll
16: spreader roll
17: moisturizing device
W: fiber web

Claims

Method for calendering a fiber web, in which method at least one surface of the fiber web (W) is calendered in at least one calendering nip formed between two rolls, characterized in that in the method at least one surface of the fiber web is roughened by calendering by at least one calender roll (11 R; 14R) with rough surface forming calendering nip with another calender roll (12) to surface roughness greater than the surface roughness of the fiber web when entering the calendering nip, that in the method at least one surface of the fiber web is roughened by moisturizing the fiber web (W) during calendering or directly after calender and in the last calendering nip by at least one calender roll (11 R; 14R) with rough surface having surface roughness of 0,4 - 1,6 Ra and that the method is utilized in production of a paper web, in particular release paper web.
Method according to claim 1, characterized in that in the method the fiber web (W) is moisturized between travel to the calendering nip formed by the at least one calender roll (11 R, 14R) with rough surface from the preceding calendering nip.
Method according to claim 1 or 2, characterized in that in the method the fiber web is dried after the calendering and moisturizing.
Method according to claim 1, characterized in that in the method the temperature of the fiber web (W) is controlled during calendering by controlling temperature of at least one calender roll in order to reach desired roughness value of the at least one surface of the fiber web.
Method according to claim 1, characterized in that in the method the roughness of the at least one surface of the fiber web (W) is controlled during calendering by controlling nip pressure of at least one calender nip in order to reach desired roughness value of the at least one surface of the fiber web to be roughened.
Method according to claim 1, characterized in that in the method the fiber web is moisturized in the calender during the run of the fiber web between two successive calendering nips.
Calender of a fiber web machine comprising at least one calendering nip formed between two calender rolls, at least one hard calender roll (11) or at least one deflection compensated roll (14) and at least one soft calender roll (12), characterized in that the calender has at least one calender roll (11 R; 14R) with rough surface forming calendering nip with another calender roll (12) to roughen at least one surface of the fiber web to surface roughness greater than the surface roughness of the fiber web when entering the calendering nip, that at least one calender roll with roughened surface has surface roughness 0,4 - 1,6 Ra, that the calender comprises at least one moisturizing device (17) and that the calender is utilized in production a specialty paper web, in particular release paper web.
Calender according to claim 8, characterized in that the at least one calender roll having rough surface is a hard calender roll (11 R) or a deflection compensated roll (14R).
Calender according to claim 8, characterized in that all hard calender rolls of the calender (10) have a rough surface having same surface roughness of 0,4 - 1,6 Ra.
Calender according to claim 8, characterized in that the roughness value of the surface of hard calender rolls of the calender (10) increases in the travel direction of the fiber web (W) in the calender (10).