FI115405B - Calendering method especially for pre-calendering and paper processing line - Google Patents

Description

1115405

Calendering method especially for pre-calendering and paper processing line

The invention relates to a calendering method, in particular for pre-calendering, in which a paper web is passed through a calendering nip formed between two surfaces, whereby heat and pressure is applied to the surface of the calendering web at a certain distance in the direction of web travel. This is known as the so-called. a long nip that has a dimension in the web direction for a certain distance.

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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 introduction of a web through one or more nips formed between two pin-15n, 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. Thinking about the subsequent coating of the paper, the purpose of calendering is to bind loose particles to the surface and close the surface to make the coating layer! 20 flat.

Calendering is known to provide the desired quality of paper produced and processed in the previous steps t, such as. why smoothness and shine. At the same time, a certain final density is obtained on the paper. On the other hand, the calendering can be carried out as a pretreatment in order to obtain the desired properties on the surface of the paper for the subsequent treatment, e.g. coating.

Normally, when calendering a paper, its surface smoothing and polishing occurs at the expense of the paper thickness. The calender uses a line load and / or nip pressure as the control variable, which determines the quality of the paper surface and also the final thickness. Especially with low grades of paper, it has been impossible to perform calendering in which the paper thickness is not substantially *: *: would decrease in pursuit of the desired surface quality.

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As mentioned above, the calendering is accomplished by passing the web through one or more nips formed between the rotating rolls.

The nip may also be formed between a rotating roll and a strap passed over a counter roll or a special shoe. There may be several nipples, so that it is possible to form a calender, so called. a multi-nip calender with superimposed rolls between which nipples are formed.

In calendering, the web is exposed to temperature and pressure. Heat is most commonly transferred to the surface of the web through contact with the hot contact surface that guides it through the nip, for example the hard metal surface of the calender roll. By using a soft coating on the surface of at least one roll, or by passing through this other roll or special shoe element, an elastic strap is provided on both sides of the nip by the force of the elements being pressed against the nip to extend longer so-called. a long nip in which the nip pressure caused by the mutual loading of the elements (a pair of calender rolls or a calender roll and a shoe) is distributed over a longer area in the direction of web travel. An example of a shoe calender is disclosed in International Application WO 99/28551.

The significance of temperature in calendering is further discussed in Finnish Patent 74066, corresponding to US Patent 4 606 26 (temperature gradient calendering), and US Patent 5,033,373. International: '·· Publication WO99 / 67462 deals mainly with the pre-calendering of cardboard: 25 tia in a long nip while holding the sweats. Temperature and humidity values are not specified;

One factor that can further influence the calendering result

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. ···. in addition to space and pressure, there is moisture in the web before calendering. For example, ways are known to moisten the web, for example, on the side where it comes into contact with the hot surface in the calender nip. Such a procedure is known e.g. European Patent 617165, *; · 'wherein the strap over the back roll can also hold the web: a certain distance against the surface of the hot roll jacket.

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The length of the nip can be influenced by commonly used line loads; · *! by selecting a soft coating on the calender roll or a strap passed through the calender nip.

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After calendering, the paper can be post-treated in another way, which is pre-calendering. A typical post-calendering finish is coating, whereby a coating paste is applied to the surface of the calendered paper 5 by any suitable method. The coating paste is an aqueous mixture containing a pigment affecting the optical properties of the paper and a binder for fixing the pigment. The pre-calendering methods aim to make the paper surface suitable for coating, mainly to smooth, and in particular to make the base paper compact. Current coating methods are capable of applying very even amounts of coating.

A problem with the coating of the pre-calendered paper is that the water used in the coating relaxes the paper during the changes of the pre-calendering. The fibers return to their original shape, and when compressed in fish-tongue, the compressed fibers become "tubular" again. As a result, the surface of the paper tends to roughen again during coating. The same problem occurs in all surface finishing treatments after pre-calendering, where the paper surface comes into contact with water, such as surface sizing. For finished I paper, the problem may still occur with printing processes that contact the paper with water, particularly in offset printing. In principle, it is possible to get the surface smooth by: * ·· increasing the line load, ie by increasing the nip pressure, but then · 25 losing the properties of the paper elsewhere, ie the bulk of the paper suffers.

* • · · ·: ··: * The degree of fiber relaxation and re-hardening respectively can be controlled to some extent, e.g., by the dry matter, content, and coating method of the coating paste, but this limits the coating-30 process and in any case re-hardening. In order to prevent roughening from affecting the surface properties of the coated paper, it is necessary to increase the amount of coating, which is not a desirable solution; The problem has occurred, for example, with mechanical pulp; mainly containing base papers which cannot achieve good coverage in the coating process with small amounts of coating.

. Other paper grades may also have the same problems. On the other hand, it is desirable to keep the amount of coating ·· * in certain, e.g. paper quality defined '·' * · * target values.

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The object of the invention is to eliminate the aforementioned drawbacks and to improve the surface quality in pre-calendering, whereby the amount of coating, coating composition and coating process can be more freely selected in the coating process following calendering. To accomplish this purpose, the method according to the invention is essentially characterized in what is set forth in the characterizing part of the appended claim 1.

The web is introduced into the calendering nip at a suitably high inlet humidity and passed through this calendering nip at a relatively low calendering pressure using a sufficiently long residence time obtained by the long calendering nip with the hot surface of the nip and a sufficiently high temperature at this surface. When a moist web is obtained at 15 calendering nipples under pressure for a sufficient period of time to contact a hot surface, the web surface temperature can be raised above the glass transition temperature of the fibers, Tg, which is moisture dependent. In this way, the surface of the paper is brought into a state where the plastic deformation of the fibers is permanent. In this case, it is sufficient that deformation is achieved on the fibers of the surface layer of the paper which have been problematic by the roughening caused by relaxation, and that the fibers in the middle of the paper in the z-direction can be left unaffected. Fibers in the middle of the paper can also, especially: '·· thin paper grades, also experience deformation and sweat;.! i 25 goes at this point, but due to the good surface quality, calendering after coating can be done using less pressure than usual, which saves the overall sweat. For final calendering of coated papers, i I ·; ··. in this case, for example, instead of a multi-nip calender, use a 2-nip soft calender 30 at a high surface temperature.

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It should be noted that the surface temperature of the hot roller must be above the glass transition temperature in order to bring the surface fibers to the glass transition temperature. With a sufficiently long residence time in the calender nip, the surface temperature of the surface in contact with the web 35 is between 200 ° C and 300 ° C, preferably above 250 ° C, to obtain sufficient effect at higher machine speeds (shorter residence times). For example, the temperature may be * · 250-300 ° C. For practical reasons, it is not advisable to keep the surface temperature of the roll at more than 300 ° C, but in principle it is possible to use temperatures higher than 300 ° C in the invention. If two successive calendering nips are used to treat different sides, the other preferably has a higher temperature than the first, since the lower humidity 5 in the second nip requires a higher temperature to achieve the same deformation.

The long nip provides low pressure and a long residence time, which, on the one hand, saves sweat and, on the other hand, raises the fiber temperature to high. Calendering input moisture and calendering temperature are variables by which one or both variables can be adjusted to suit the surface quality of the calendering result. For example, it is possible to keep one variable constant and change another to obtain the desired smoothness and density on the surface. High humidity is also advantageous in the sense that after calendering, the final humidity is not too low, even if high temperatures are used.

In addition to the pre-calendering of coated papers, the method can also be used for calendering that completes the final surface quality of the product, i.e., the final calendering of uncoated papers.

·. Calendering is performed as described above on paper which is i. ·. 25, prior to calendering, left or moistened with a higher moisture content than 5%, whereby its moisture content is more than 5%. In addition, it is possible to dry the web after calendering. In this case, the inlet moisture of the web is preferably about 10-20%, and the roughness of the paper obtained in calendering is lower than the target roughness

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After calendering at such high humidity, it is dried to the final moisture, i.e. after drying, which has already naturally occurred during calendering, it is further dried, the paper surface roughening to the desired level. Calendering the paper with the combined effect of high temperature, high humidity, and long residence time results in the largest stresses being released during 35 drying operations and the fibers' original shape is partially restored. This achieves the target roughness, and the surface roughened by the drying process is no longer further roughened when exposed to water in the subsequent process, for example in the coating process or printing process. Such paper is less sensitive than paper calendered at a lower moisture content than those mentioned after drying. The inlet moisture may also be selected such that, at the temperature used and the dwell time 5, and possibly due to the number of nipples, the paper already reaches the target moisture prior to coating during calendering without further drying.

The invention will now be described in more detail with reference to the accompanying silicon drawings, in which Figure 1 schematically illustrates the calender nip used in the method of the invention, Figure 2 illustrates the use of the invention in pre-calendering when only one side is calendered; Fig. 4 illustrates the use of the invention in pre-calendering according to another embodiment, Fig. 5 illustrates the use of the invention in final calendering when the fishery is performed on one side only, and. Figure 6 shows double-sided final calendering.

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Figure 1 shows schematically N · · · 30 used in the invention. The nipple is the so-called. a long nip formed by loading one another against a hard surface, a rotatable roll 1 and a soft, elastic continuous material 2a, which may be a soft roll cover of another rotary calender roll, a strap over a fixed shoe element or a strap over a rotating counter roll. The nip is then formed by the combined effect of the load and the elasticity of the material. The length of the nip is at least 50 mm, preferably at least 70 mm. Hard roll surface and material

A calenderable paper web W has been introduced between Ia 2a so that it passes: * ·. · Under pressure for a certain distance in the nip N. The roll surface 1 of the roll 1 and · · 7 · 115405 ial rial 2a move substantially at the web speed W. The pressure in the nip is determined by the mutual loading of the hard surface roller 1 and the counter element (soft roller roller body, counter roll or shoe element) supporting / supporting the elastic material 2a, which may also be expressed as line pressure (load force / paper web width). In a known manner, the pressure varies in the longitudinal direction of the calender nip so that it initially increases gradually to a maximum value approximately in the center of the nip and then gradually decreases to its original pressure. By adjusting the shoe, the pressure-10 profile and the nip length can be changed.

The hard surface roll 1 is a so-called roller. a thermal roll which may be heated in any known manner so that its surface temperature rises so high that it undergoes a plastic deformation of the surface of the paper web to be calendered at a certain contact time under nip pressure with a residence time of nip length. With a long nip and high humidity, it is possible to influence the fiber temperature to rise significantly above the glass transition temperatures of the polymers contained in the fibers. For this purpose, the surface temperature T of the roll 1 by then 20 into the calender nip is preferably at least 200 ° C, and more preferably at least 250 ° C.

The retention time of the web at nip N is the length of the nip divided by the web speed of the web. The web of this time is under road contact under pressure; '' 25 to a surface of a roll at a temperature from which heat is transferred to the web.

At current high machine speeds, it is important to provide a nipple long enough to maintain the web under sufficient pressure and temperature for a sufficient period of time. Shoe Element Geometry (Shape and Position): ..]: You can also influence the pressure distribution at the long nip.

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Figure 2 illustrates the use of the invention in pre-calendering. Paper. ··. is calendered in the long nip N between the hard roll 1 and the soft surface calendering element 2. The soft surface calendering element 2 in the figure is a rotating fish -: i .: 35 blade roll with a soft roll coating, but can also be formed by when. ···. this is a so-called. shoe calender, or it may be a combination of a rotating counter roll and an endless belt passed over and through the nip. The 115005 W input moisture of the web 8 to nip N is more than 5%. In addition to high ductility, the web does not dry out excessively, and its outlet moisture content is achieved, e.g., to at least 3.0%.

5 After the web W has passed through the calender nip N, its surface has reached a state where it is permanently plasticized. The web W is then subjected to a process of applying an aqueous agent to its surface, in particular to a coating process carried out in a coating unit S, wherein at least one surface of the web which has been pre-calendered against a hot surface and wherein the fibers have a permanent as a coating by uniformly applying a thin layer of coating material to the surface of the roll 4 and transferring it in a nip N 'between the rolls 3,4 to the surface of the web. Other coating methods also come into play. The coating may also be a surface sizing wherein a water-based surface sizing is applied to the paper surface. The paper is then dried in a known manner and rolled in a reel. Other coating methods, such as blade coating, may also be used.

Figure 3 shows a pre-calendering process in which the web is precalendered before coating by performing the same treatment on both sides.

! In the first pre-calendering unit C1, the web W is calendered in the long nip N between the hard roll 1 and the soft-surface calendering element 2. In the second pre-calendering unit C2, the web 25 has a soft hot surface, now the order of the elements has been changed. The second nip preferably employs a higher temperature than the first. Similarly, a relatively high calendering input moisture (e.g., about 20%) can be used for both sides of the treatment because the web dries more. The inlet humidity of the other nip can be about 10%. After double-sided treatment, the web is coated on both sides. as film transfer coating in coating unit S by rolls 3 and 4.

. * · ·. Other coating methods may also be used.

»· · 35 The effect on calendering can be enhanced by wetting the surface of the web. Figure 4 shows a pre-wetting unit 7 upstream of the calender, in which the web is moistened with a higher moisture content than usual. Thus, its moisture content by the web to the calender 9 115405 I nip is more than 5%. Known wetting devices that accurately dispense a certain amount of water onto the web surface may be used for wetting. If the web is moistened to a moisture content of about 10-20%, its roughening can be reduced in subsequent processing steps by allowing it to harden to the desired level after drying after calendering. Following the calender is a drying unit 5 where the drying on the web is carried out so that the paper surface roughens to the desired level. Although the figure shows one multicylinder drying group in which the web is brought into contact with hot surface drying cylinders, drying may also be performed using radiation heat, for example, using known infrared dryers or a combination of both. The number of drying cylinders may be other than shown in the figure, and some of the cylinders may be non-heated suction rolls. After the drying unit 5, the web is passed to a coating unit S, which may be any of the above-described, or the paper is printed in a printing process.

Pre-moistening is not required if the web is left to be suitably moist with the preceding drying section. The humidifying unit 7 can then be left out.

Further, it is possible that Figure 4 shows a reciprocal calender 20 as in Figure 3.

The paper web W dries in the calendar depending on the residence time and temperature. If the web dries to about 5 - 8% humidity, it can be run over ·. unit 3 with an air drying step. Alternatively .·. The web may be dried in the drying unit 5 to this moisture.

Figure 5 illustrates the use of the invention in final calendering, whereby the difference from the previous pictures is that the web is no longer processed; ··: after the calendering according to the invention, it is rolled into a reel. The surface quality of the paper is then advantageous when the paper is used, for example, for printing purposes, whereby water does not cause hardening during the printing process. The treatment may be the same as in figure 2, ie high inlet humidity, long nip and high temperature.

35 ·; · * Figure 6 shows a final calendering using a 2-nip calender, in which the hot caliper in the latter calender nip comes against a side of the web that has not been treated in the previous 10 1 15405 nips, i.e. the thermo rolls 1 come across the web. The arrangement is thus the same as in Figure 3.

Calendering and possible pre-wetting can be performed in the paper-5 production line as an on-line process, whereby the calendar is placed in a so-called "paper" machine. dry head. It is also possible to perform calendering and coating in-line as an off-line process on paper produced on a different machine and then rolled. It is also possible that the paper is rolled after calendering, as in Figs. 5 and 6, and this rolled paper, which thus has suitable surface properties due to calendering, is coated in a different coating unit as off-line treatment with respect to calendering.

The invention is particularly suitable for the pre-calendering of mechanical pulp-based papers, such as LWC base papers, but the invention is also suitable for the pre-calendering of other coated wood-containing and wood-free printing papers. While the advantages of the invention are clear, especially in the manufacture of relatively thin coated grades (LWC and ULWC), the kek-I 20 is not limited to the amount of coating and can also be used for thicker coated grades. The invention is also applicable to the final calendering of uncoated printing papers. The invention will now be further described by some examples which do not. 25 are not to be construed as limiting the invention.

40 g / m2 basis weight paper with 35% chemical pulp and 65% mechanical pulp and 5% filler content was subjected to pre-calendering tests with a hard ... 30 rolls and soft-surface polymer rolls in the form of relatively short nip forming rolls, and other pre-calendering tests with 70-100 mm long nip so-called. a shoe. The line speed was 1000 m / min. The results are • presented in the following table., 35 • a »a · 11 1 15405

Table. Results of pre-calendering tests on 40 g / m2 base paper in soft calendering and long nip calendering.

Calendars- Pre-calendering (soft / hard) carpet __sole_____

Nipple pressure (kN / m) ___ 60__60__150__150

Thermal roll surface temperature (° C) ___ 50__100__50__100

Density (kg / m3) __ 585__685__691__721__731

Humidity (%) __ 4.2__4J ____ 3J) __ 3.6 PPS roughness, ts / ws (m) __ 6.26 / 8.17 6.20 / 5.65 6.15 / 5.52 6.07 / 5.08 5, 90 / 5.12

Bendtsen-sileys, ts / ws (ml / min) __ 510/710 505/400 509/385 510/250 515/260

Bendtsen Air Permeability (ml / min) __ 250__2 £ 1__235__231__221

Cobb-Unger oil absorption, ts / ws (g / m2) 21.0 / 16.5 20.1 / 14.2 19.5 / 13.6 19.9 / 14.3 19.4 / 13.1

Calendars- Pre-Calendering (long nip) for carpet _-E2t2 _____

Nipple pressure (kN / m) ___ 200__400__400__400

Thermal roll surface temperature (° C) ___ 200__200__290__290

Density (kg / m3) __ 585__645__691__743__746

Humidity (%) __ 4j2__3JS__3__2J> __ x __10J__x__x__x__4.5 PPS Roughness, ts / ws (pm) __ 6.26 / 8.17 5.98 / 5.27 5.82 / 3.70 5.66 / 2.99 5.44 / 2.66: Bendtsen-sileys, ts / ws (ml / min) __ 510/710 505/230 440/198 425/191 408/198

Bendtsen air permeability (ml / min) __ 250__201__151__110__98

Icobb-Unger olyabsorption, ts / ws (g / m2) 21.0 / 16.5 19.4 / 13.5 17.9 / 11.6 16.1 / 8.2 15.1 / 6.5 • ..: 5 From the results, it can be noted that the sealing of the surface against the thermal roll of the paper is greatly improved when the humidity is raised by more than 10% and the temperature is 200 ... 300 ° C by long nip calendering. The oil absorption obtained was hot at 290 ° C on the bottom side of the roll · · *: (wire bond in the table) 6.5 g / m 2 with a calendering inlet moisture of 10.1%.

Claims (9)

A calendering process, in particular for preliminary calendering, wherein a paper web (W) is passed through a calender nip 5 (N) formed between two surfaces, heat being conducted in the nip to the surface of the web to be calendered and subjected to pressure on the web. a fixed length in the running direction of the web (W), characterized in that during the calendering a long, at least 70 mm long calendering nip is used, in which the paper is contacted with the surface of a heated calendering element, such as a roller (1), whose surface temperature is at least 200 ° C, e.g. between 200 ... 300 ° C, and the paper has an input moisture content of 10-20%, with the remaining deformation time of the web in the nip and with the cooperation of the temperature of the surface existing in contact with the web (W) in the nip, a residual deformation is achieved. the fibers on the surface of the paper. 15 Process according to claim 1, characterized in that the surface temperature is at least 250 ° C, preferably between 250 ... 300 ° C. Method according to claim 1 or 2, characterized in that after the calendering, at least the surface of the paper web (W) is coated to which the interaction of the temperature and the holding time is directed to achieve a residual deformation. Method according to any of the preceding claims, characterized in that the paper web (W) is dried after calendering. Method according to Claim 3 or 4, characterized in that the coating is carried out in the same line with the calendering. * · • »30 6. A method according to any of the preceding claims, characterized in that a corresponding treatment is performed for both sides of the bath. 7. Treatment line for paper, comprising a calender for treating the surface of the paper and a coating unit for coating the paper, characterized in that the 115405 - drying calender there are means arranged to increase the moisture content of the paper web to an input moisture content of 10 - 20%, or to leave an input moisture content of 10 - 20% in the paper web, - the calender comprises in the running direction of the paper web (W) a minimum nip area 70 (5 mm) long ( N) formed between the surface of a heated calender element, such as a roller (1), and the surface (2a) of a second calender element, - the surface of the heated calender element is arranged at a temperature of at least 200 ° C, and 10. in the web running direction (W) after the calender, there is a coating unit (S) with coating means (3, 4) which are provided at least on the side of the web (W) which has been against the surface of the heated calender element, such as a roller ( 1).: 15 Processing line according to claim 7, characterized in that the calender comprises two calender units (C1, C2) in which the surface of the calender element heated in the nip area, such as the surface of the roller (1), is on different sides of the web, and in the course direction of the web. after calender units (C1, C2), one or more coating units (S) are provided with means for performing both sides coating. Treatment line according to claim 7 or 8, characterized in that after the calender before the coating unit (S) there is a drying unit (5). '} 25 4 · * · • J • »· • I 1 •« · t • · • 1 * »♦ 1» • · · • · • · · ·