FI118388B - Procedure for calendering paper - Google Patents

Description

118388

Method for calendering paper

The present invention relates to a method for paper calendering, wherein a paper web at a given moisture is calendered by means of heat and pressure.

After drying the paper, the surface structure of the web is made desirable by mechanical treatment on the surface, calendering. There are many methods of fish sharpening, but all of them have in common the passing of the web 10 through one or more nips formed between two surfaces, normally rotating roll surfaces. The purpose of calendering is to improve the quality of the paper by squeezing it to a certain constant end thickness and, in particular, smoothing its surface. Moisture is known to improve the ductility of the fibers contained in the paper during calendering. As a result, calendering is performed at a specific moisture content, the "target calendering moisture". Increasing the calendering moisture also improves the strength properties of the calendered paper.

20 Such a method, in which the paper web is calendered at a given humidity by means of heat and pressure, is thus intended to provide paper with the desired surface properties and structure.

* ·· * »• ·: Calendar refers to multi-roll, soft, belt, metal belt or. · * ··. * 25 long nip calendars.

Papermaking methods in which paper is first dried by means of a drying section from which it is conveyed in a continuous web to the calender. During drying of the paper, the drying section can:.: *: 30 perform a wetting of the web to correct the moisture profile of the paper.

Calendered paper has been proposed to be produced firstly by calendering the paper making process, whereby:. calendering moisture, that is:. * ·· target calendering moisture.

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Finnish Patent 105839, corresponding to WO 00/03087, discloses a method in which a paper web is first dried with a drying section to a certain first moisture content, after which the web moisture is added to a second web treatment moisture content, whereupon the web is after which the web is rolled onto the paper rolls thus formed. The residence time from wetting to paper handling by calendering is 0.2 to 2.0 seconds, according to the patent.

According to a preferred embodiment of this patent 105839, papermaking is carried out in a continuous process by drying the formed web so that the desired moisture content of the web decreases to a desired first moisture content, preferably <6%, typically 2-6% depending on the case, followed by , to a treatment moisture content of the web which is preferably> 8%, most preferably 8-12%, i.e. higher than the first moisture content, after which the web is introduced substantially immediately into the calendering.

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With the method described above, e.g. the dimensional stability of the paper is improved when the fibers undergo a first drying step. This. ···! '' Over-drying '' is justified by the fact that the moisture expansion potential of the paper decreases as its lowest moisture content during papermaking decreases. This means more peacefully. ***; * behaving web, for example in printing.

Thus, it is a well-known fact that increasing the moisture content of the paper improves the calendering result. In general, the problem is that, despite calendering, 30, the web is still roughened again after calendering. Thus the smoothness of the surface obtained by calendering is met. partially offset.

The reason for roughening is due to changes in the structure of the fibers and fiber network during drying. As water exits the fibers, the fibers shrink, especially in the transverse direction, and their structure changes.

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The shrinkage of individual fibers through fiber bonds affects the shape of the entire street network and apparently also the roughness of the paper surface.

The object of the invention is to eliminate the above drawbacks and to provide a method in which the behavior of the fibers can be controlled. To accomplish this purpose, the method of the invention is essentially characterized in that the paper web is calendered with its surface fiber moisture at FCP (fiber collapse point) such that said paper fibers dry out to a moisture content lower than FCP equivalent during calendering.

Most of the fiber shrinkage occurs when a certain dry matter content is reached. This point is the so-called. fiber Collapse point, FCP, which has no significant effect on fiber morphology or chemical composition. FCP is located within a narrow range of individual fibers with a solids content of about 85-88%, i.e. a moisture content of 12-15%. This is a narrower humidity range than previously recommended for calendering.

The paper web is calendered in its fiber FCP so that the paper dries to a lower final moisture content than the fiber FCP. Significant shrinkage and roughening of surface fibers will no longer occur. ·· *! after calendering, since the shrinkage and drying of the fibers occurs during calendering. Calendering can be done with one or more I / 25 nipples using appropriate temperatures.

♦ · ··· • ♦ * · ·: It is possible to determine the FCP of the pulp fibers and use: ···: the moisture value as the minimum calorific input moisture content (target moisture). When the web comes out of the calender, its humidity has dropped below FCP. Thus, it is essential that the FCP of the paper raw material fibers is known, and the calendering is performed such that the moisture in these fibers. ·. *: Passes through the FCP during calendering.

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Each type of fiber raw material has its own FCP, which can be set to 35 exact numerical values. This is the dry matter content achieved during drying, where the cross-sectional area of the fiber and the width of the fiber fall sharply. For each fiber type, FCP can be determined e.g.

4, 118388, by the microscopic measurement method described by Nanko, H., Asano, S. and Ohsawa, J./'Shrinking Behavior of Pulp Fibers During Drying ', 1991 International Paper Physics Conference, p. 365-373. Some exemplary exact values are e.g. 5 following: bleached Japanese beech pulp 88%, bleached poplar pulp 85%.

Since the fibers on the surface of the web largely determine the properties of the web, and on the other hand, the shaping applied to the web during calendering should be surface-oriented, minimizing the structure of the product, it is essential that the surface fibers specifically meet said conditions for FCP. at least they are in FCP calendering. The fibers in the middle portion are preferably at a lower moisture content than FCP. Re-wetting after the drying section is preferably performed on one or both sides, preferably so that the wetting effect is limited to the fibers of the corresponding surface to a certain depth (in the z-direction). This can be accomplished by appropriate dosing of wetting water.

The invention will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 schematically illustrates the evolution of the moisture content of the web according to the method of the invention, and!: ··! Figure 2 schematically shows an apparatus using the method of the invention.

• · • · ·

Figure 1 shows the evolution of the moisture content of the web obtained by the method of the invention. It shows how the formed web is dried to desirably reduce the moisture content of the web to a certain first moisture content M1, after which the moisture of the web is added to the second processing moisture M2 of the web. , which is at least relative to the moisture of the surface fibers: · * ·: 35 higher or at least equal to the above FCP, after which the web * »is subjected substantially immediately to calendering where its surface properties are affected and during which the moisture of the web drops below the FCP of the fibers.

Figure 2 shows an apparatus employing the method of the invention. The figure shows the paper machine so-called. dry head. The figure shows a paper web W, defined by means of various cylinders and rollers and other web guiding means, which is originally formed by a forming machine part of a papermaking machine from an aqueous fiber suspension which has been dewatered in successive 10 stages . In the drying section D, where the paper is dried to a certain moisture content, the web is led to the calender C. The drying section usually uses a plurality of heated cylinders over which the web passes in contact. When the web leaves 15 of the drying section D, it is at the first moisture content M1. With the web opening W between the drying section D and the calender C, the paper web W is again moistened with the humidifying device 1 to another, higher moisture content M2, which is the inlet moisture of the calender C. This moisture is selected such that at least the surface to be calendered has a moisture content at least equal to the FCP of the fibers.

: * · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·. dries to less than FCP moisture in the fibers (calender output ·. **: humidity), i.e. moisture level M3. The moisture of the fibers as time passes, ··· 25 through FCP during calendering. The drying of the fibers occurs due to the contact of the web with the heated surface of the calender space. Thus, it is not a matter of arbitrarily or previously selected moisture · · '·· *' moisture content, but specifically of the fiber moisture content (and, respectively, 30 dry solids) of the FCP.

• φ · •.! Ant useam::: Figure 2 shows an on-line multi-nip calender with a plurality of • · ·. ·· / heated rolls with which the web comes into contact with the corresponding *. Typically for a multi nip calender, the same surface of the web • m 35 comes into contact with two or more heated surfaces V-j as it passes through the calender. Figure 2 shows how the web will be treated in this way on both sides. In addition, the pre-calender moistening 6 118388 with the moistening device 1 is mutual. Reference numeral 2 denotes a roller for adjusting the dwell time between wetting and calendering, for example, by moving the rolls relative to one another such that the distance traveled by the web from the wetting apparatus 1 to the calender 5 C is changed. Letter S denotes measuring devices (gauges with humidity sensors) that can automatically monitor humidifying devices 1 with the information provided by the web, so that the surface to be calendered will achieve the desired target humidity with respect to the FCP before going to the calender. The FCP of the fibers used as the raw material for the paper can be determined separately in the laboratory, and this assay result can be used to set said target moisture.

After calendering, the web W having the aforementioned third moisture content of 15 M3 is rolled on a reel R in the reel thus obtained.

customer rolls can be formed in the post-treatment process with a winder (not shown).

Although a multi-nip calender is described above, a ka-20 calender with only one nip, such as a metal belt calender or a soft calender, can also be used. The nip may be, for example, a so-called nip. long nip where one side of the paper web becomes longer contact heated: * ··; with a calender roll. As another aspect of the FCP moisture passing through the nip.

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The invention provides the following advantages: Since FCP is based on cellulose-based fibers used in papermaking, typically at a moisture content of 12: 30 -15%, i.e. relatively high humidity, is easier on the calendar and calendering can be accomplished with lighter calendars.

··· * - Increasing humidity also improves strength properties.

The transverse shrinkage profile of the web in the calendar due to FCP:. * ·: 35 is better prevented than with the drying section,: Λ: so the transverse shrinkage profile is 7 118388 smoother, which also results in a better dimensional stability profile of the paper.

If a pulp of two or more types of fiber having different FCPs is used as the raw material, the processing FCP will be selected according to the main component (assuming that the different types of fibers are relatively uniformly distributed in the paper in the z direction, i.e. . For this reason, it is preferable to use a pulp having at least 75% by weight, preferably at least 80% by weight, of the same fiber to the FCP. It is also possible to select different fiber types of the blend so that, although they are different raw materials, they have the same FCP.

By paper is meant here all fibrous web-like products to be calendered, regardless of the basis weight, characterized by a specific FCP of surface fibers.

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Claims (7)

118388 A method of paper calendering, wherein a paper web of a specific moisture, characterized by a specific FCP (fiber 5 collapse point) of surface fibers, is calendered by heat and pressure, characterized in that the paper web is calendered with its surface fiber moisture at that particular FCP. said fibers dry during calendering to a final humidity lower than the humidity equivalent to FCP. 10 A method according to claim 1, characterized in that the paper moisture of the fibers in the paper web for calendering is equal to their moisture content in FCP. 3. A method according to claim 1, characterized in that the paper web fibers have an inlet moisture content higher than their moisture content in FCP, and the paper web fibers reach a moisture equivalent to FCP during calendering and dry during calendering from FCP to final moisture. 20 A method according to any one of the preceding claims, known in the art. The fact that the paper dries during calendering to a final moisture of *: .. * · * in the range of 2-10%, especially 5-9%. • · • · • · · · * · *: 25 A method according to any one of the preceding claims, characterized in that the paper web is calendered in FCP for fiber moisture by multi-nip calendering by passing the paper web through a plurality of heated rolls and a nip formed by a soft surface roll. · * ... 30 The method according to any one of the preceding claims, known as * ··. the fact that the paper is rolled at the final humidity achieved by calendering * ". Method according to one of the preceding claims, characterized in that the FCP of the fibers used in the pulp is determined * *. * ··. separately prior to calendering, and the calendering conditions are adjusted by · · * FCP obtained from this assay. 118388