WO2005031064A1

WO2005031064A1 - Method for controlling structural and functional characteristics of a fibrous web in a processing device for a fibrous web

Info

Publication number: WO2005031064A1
Application number: PCT/FI2004/050131
Authority: WO
Inventors: Maria Lepola; Henri Vaittinen; Reijo PIETIKÄINEN; Jari Ilomäki
Original assignee: Metso Paper, Inc.
Priority date: 2003-09-29
Filing date: 2004-09-13
Publication date: 2005-04-07
Also published as: DE112004001826T5

Abstract

The invention relates to a method for controlling structural and functional characteristics of a fibrous web in a processing device (1) for a fibrous web (W) which comprises a metal belt (2) arranged to rotate around a guide means (3), outside which belt is arranged at least one counter-element (5) forming a contact surface with the belt, in such a way that between the belt and counter-element is formed a web processing zone (N) through which the web to be processed is led. The processing device (1) optionally comprises at least one additional load roll (4) inside the metal belt loop. In the method, the form of the z-direction moisture distribution of the fibrous web is controlled by regulating the temperature of the surfaces in contact with the fibrous web (W) to control curling. The invention further comprises a method for controlling the roughening of a coated or uncoated fibrous web in the processing device and for reducing secondary roughening at subsequent process stages.

Description

Method for controlling structural and functional characteristics of a fibrous web in a processing device for a fibrous web

The present invention relates to a method applied in the manufacture of paper and board for controlling structural and functional characteristics of a fibrous web in a processing device for a fibrous web which device comprises a metal belt arranged to rotate around a guide means, outside which belt is arranged at least one counter- element forming a contact surface with the belt in such a way that between the belt and counter-element is formed a web processing zone through which the web to be processed is led, and in which processing device is optionally at least one additional load roll inside the metal belt loop.

One problem in the manufacture of paper and board and in the end product is the curling tendency of the fibrous web. Curling is most often caused by the unequal sidedness, or asymmetry, of the stress state in the direction of the plane of the fibrous web in the direction of thickness. The unequal sidedness of the stress state may be due to, for example, structural unequal sidedness of the web (such as fibre orientation, density, material distributions or fibre properties) combined with changes in the moisture level. A significant mechanism causing unequal sidedness is also the unequal sidedness of drying or in general the asymmetry of moisture flows in the dryer and finishing sections of a paper and board machine. To put it simply, it may be thought that the paper will curl to the side from which water is last removed, that is, the side dried last. The side dried first tends to shrink as it dries, but this side goes into a state of tensile stress while the structure is forced to remain plane-like due to the tension of the web. When under tensile stress, the creeping taking place in the fibre structure releases web-internal stresses and the structure stretches and loosens permanently. When also the other side finally dries, it also shrinks and curls the paper to the said side. /1/-/2/.

It should be noted that, depending on the nature of the drying process, drying may start from different sides of the web. When drying with low drying intensity (e.g. with a low temperature in the drying cylinder), the heat is conducted from the cylinder to the web, and further to the inner parts of the web. The temperature of the web rises relatively steadily, the temperature gradient is low and drying starts on the open wire side (cooler side). The drying front proceeds through the web, drying the cylinder-side surface last. With high intensity (high cylinder temperature, a Condebelt dryer and a metal belt calender), the situation is the opposite. Efficient heat transfer and the high temperature of the contact surface cause water to vaporize immediately when close to contact. Drying starts on the hot surface side, a strong temperature gradient is formed on the web, and moisture passes through the structure towards the cooler surface. Finally, the cooler side dries last.

In paper and board manufacture, another problem must be considered to be the so- called overdrying requirement. For example, in the dryer section of a SC paper production line, paper is first overdried to moisture content as low as about 2-4%. This web is, however, re-moistened with on-line moistening devices to moisture content of approximately 10% before calendering. There are two reasons for this procedure. Firstly, by means of overdrying, the fibre structure of the paper becomes keratinised, in other words the pores in the fibre wall close permanently, which stabilises the structure with a view to subsequent moisture changes. Another reason is the correction of CD- and MD-direction moisture profile errors. Correcting the profile by adjusting drying is inaccurate and difficult, whereas the re-moisturisation of an overdried web can be more accurately controlled. From the point of view of energy saving, overdrying and re-moistening are, however, extremely expensive.

In Condebelt drying, the fibrous web is dried under constant pressure while pressed between two flexible and compact steel belt loops. Between the metal belts is arranged a temperature difference in such a way that the upper belt is heated with steam and the lower is cooled with water. In connection with the lower cooled wire is a permeable wire in contact with the web. Heat is transferred from the hot belt to the paper web, vaporizing the water in it. The slightly superheated water vapour passes from the web, through the permeable wire, to the 'cold' side of the system, from where the vapour condenses onto the surface of the cooled belt and leaves the system as water. The heat released from condensation is transferred to the cooling water of the belt. It is important for the functioning of the process that there is as little air as possible between the two steel belts. This is why the air in the web is replaced by steam sucked through the wires. It is characteristic of the process that throughout the drying stage, a significant temperature difference acts between the surfaces of the web, and that moisture discharges one-sidedly in the direction of the cold surface.

A version of Condebelt drying is also known where a fibrous web is dried by conveying it between a roll and a metal belt (pat. appln. 810507, pat. appln.

843958). The principle is the same, but the second metal belt loop is replaced by a large-diameter cylinder or roll.

The aim of the present invention is to provide a method for controlling structural and functional characteristics of a fibrous web in a processing device for a fibrous web which comprises a metal belt arranged to rotate around a guide means, outside which belt is arranged at least one counter-element forming a contact surface with the belt in such a way that between the belt and the counter-element is formed a web processing zone through which the belt to be processed is led, and in which processing device is optionally at least one additional load roll inside the metal belt loop.

To achieve this aim, the method according to the invention is characterised in that in the method, the form of the z-direction moisture distribution of the fibrous web is controlled by regulating the temperature of the surfaces in contact with the fibrous web to control the curling of the web in a controlled manner.

By controlling the moisture distribution in the direction of thickness in connection with drying and finishing, for example calendering, the structural and functional characteristics of the web, such as density distribution in the z-direction, curling tendency and unequal sidedness of the surfaces (e.g. smoothness) can be adjusted. By means of the method, also the moisture profile can be adjusted in the CD- and MD-directions.

A further object of the invention is a method for controlling the roughening of a paper web in a processing device for a fibrous web and for reducing or preventing secondary roughening at subsequent process stages. A special embodiment of the present invention is newsprint, but the invention is also applicable to other paper or board grades.

In the series of books Papermaking Science and Technology, in the section on Papermaking Part 3, Finishing, edited by Jokio, M., published by Fapet Oy, Jyvaskyla 1999, 361 pages, p. 53-68, are described different paper grades and their uses. The descriptions below are examples of the values of fibrous webs currently used and of typical calendering conditions with reference to the said source publication /3/.

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

Newsprint usually contains 75-100% mechanical pulp, 0-25% chemical pulp and a maximum of 8% filler. Paper pulp may contain mechanical fibre, or as much as 100% recycled fibre. The filler content of recycled fibre may be higher than that of paper made from virgin fibre (up to 20%).

As general values for newsprint may be regarded the following: grammage 40-48.8 g/m², ash content 0-20%, PPS-slO roughness (ISO 8791-4) 3.0-4.5 μm, Bendtsen roughness (SCAN-P21:67) 100-200 ml/min, density 600-750 kg/m³, brightness 57- 63%, and opacity 90-96%.

Newsprint is calendered in a paper machine, in an on-line calender. Conventionally, this has been done with a 4-6-roll hard-nip calender. Newsprint is normally run at a speed of 1100 m/min - 1700 m/min. Line loads are 80-100 kN/m and water temperatures of the thermo roll 80 - 120°C.

Controlling the thickness of the paper is an essential part of the newsprint calender. Conventionally, the transverse thickness profile has been adjusted by means of hot/cold air jets, induction coils, and/or zone-adjustable calender rolls. Recent individually zone-adjustable rolls are able to adjust the transverse thickness profile without external devices. Since it has become increasingly easier to form the structure of paper (more de- inked pulp (DIP)) and improved smoothness is obtained on the former and press, the aim has been to reduce the number of nips in the calenders and thus also the line load.

Typical running conditions on a soft calender for newsprint using DIP base are 20- 80 kN/m at two soft nips and a temperature of 80-100°C. In some cases, even just one soft nip suffices, depending on the two-sidedness of the paper (which is dependent on the realisation of the press section of the paper machine).

TMP-based newsprint requires two soft calender nips and rather severe calendering conditions. Line loads vary typically within the range 250-350 kN/m and the temperature up to 160°C. TMP-based pulp compositions also require steaming to enhance the calendering effect. Steaming is used extremely efficiently in new paper machines provided with one-sided drying to control curling. For rough pulp compositions, precalendering has been considered also for the dryer section (breaker stack).

Wood-containing coated papers, such as MFC (machine finished coated), FCO (film coated offset), LWC (light weight coated), MWC (medium weight coated) and HWC (heavy weight coated) are often precalendered before coating and finally calendered after coating.

Coated papers containing mechanical pulp normally contain 45-75% mechanical pulp and 25-55% chemical pulp. Fillers are normally not used, with the exception of pigments obtained from coated waste paper. This gives a filler amount of about 5- 10% for the base paper. A typical grammage is 40-80 g/m².

Coated magazine paper (LWC = light weight coated) contains 40-60% mechanical pulp, 25-40% bleached softwood pulp, and 20-35% fillers and coaters. As general values for LWC paper may be regarded the following: grammage 40-70 g/m², Hunter gloss 50-65%, PPS S10 roughness 0.8-1.5 μm (offset) and 0.6-1.0 μm (roto), density 1100-1250 kg/m³, brightness 70-75%, and opacity 89-94%. The aim in precalendering is to reduce roughness and porosity to the required level before coating. Conventionally, LWC precalendering has been done by means of a two-roll machine calender comprising one water-heated roll and one deflection- compensated roll. Line loads typically vary between 10-40 kN/m and the water temperature is generally 80-100°C.

Controlling the thickness of the paper is an essential part of precalendering. Conventionally, the transverse thickness profile has been adjusted by means of hot/cold air jets, induction coils, and/or zone-adjustable calender rolls. Recent individually zone-adjustable rolls are able to adjust the transverse thickness profile without external devices.

As general values for MWC paper (medium weight coated) can be regarded the following: grammage 70-90 g/m², Hunter gloss 65-75%, PPS S10 roughness 0.6-1.0 μm, density 1150-1250 kg/m³, brightness 70-75%, and opacity 89-94%.

Conventionally, LWC and MWC paper are finally calendered with 10-12-roll supercalenders. Two or three off-line supercalenders are used per paper machine. The running speeds of the calenders vary between 600-800 m/min. Line loads are typically 300-350 kN/m and the water temperature of the thermo roll is 80-120°C.

As general values for MFC paper (machine finished coated) can be regarded the following: grammage 50-70 g/m², Hunter gloss 25-70%, PPS S10 roughness 2.2-2.8 μm, density 900-950 kg/m³, brightness 70-75%, and opacity 91-95%.

As general values for FCO paper (film coated offset) can be regarded the following: basis weight 40-70 g/m², Hunter gloss 45-55%, PPS S10 roughness 1.5-2.0 μm, density 1000-1050 kg/m³, brightness 70-75%, and opacity 91-95%. FCO is finally calendered either with 12-roll supercalenders or two-nip on-line soft calenders. Soft calendering requires rather severe calendering conditions, roll temperatures up to 200°C and line loads up to 350 kN/m. MFC paper is finally calendered on a two-nip on-line soft calender at relatively gentle calendering conditions due to low gloss targets. Roll temperatures are typically 70-90°C and line loads 70-120 kN/m. HWC (heavy weight coated) has a grammage of 100-135 g/m² and it can be coated even more than twice.

Uncoated magazine paper (SC = supercalendered) usually contains 50-70% mechanical pulp, 10-25% bleached softwood pulp, and 15-30% fillers. Typical values for calendered SC paper (including e.g. SC-C, SC-B, and SC-A/A+) include grammage 40-60 g/m², ash content 0-35%, Hunter gloss <20-50%, PPS S10 roughness 1.0-2.5 μm, density 700-1250 kg/m³, brightness 62-70%, and opacity 90-95%.

Woodfree printing papers made of chemical pulp, or fine papers, include uncoated - and coated - chemical-pulp based printing papers, in which the proportion of mechanical pulp is less than 10%.

Uncoated chemical-pulp based printing papers (WFU) contain 55-80% bleached birchwood pulp, 0-30% bleached softwood pulp, and 10-30% fillers. With WFU, the values vary considerably: grammage 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 to a great extent in accordance with the requirements and intended use. The following are typical values for once and twice coated chemical-pulp based printing paper: grammage of once coated 90 g/m², Hunter gloss 65-80%, PPS S10 roughness 0.75-2.2 μm, brightness 80-88%, and opacity 91-94%, and grammage of twice coated 130 g/m², Hunter gloss 70-80%, PPS S10 roughness 0.65-0.95 μm, brightness 83-90%, and opacity 95-97%.

Board manufacture uses chemical pulp, mechanical pulp and/or recycled pulp. Boards may be coated or uncoated. Boards can be divided, for example, into the following main groups according to their intended use.

Corrugated board, comprising a liner and fluting. Boxboards for making boxes and cases. Boxboards include liquid packaging boards (FBB = folding boxboard, LPB = liquid packaging board, WLC = white-lined chipboard, SBS = solid bleached sulphite, SUS = solid unbleached sulphite).

Graphic boards for making, for example, cards, files, folders, cases, covers, etc.

A large amount of water evaporates from paper in calendering when a high temperature and/or long dwell time are used. Substantial and rapid evaporation of water in connection with calendering roughens the paper, which is usually considered an adverse phenomenon.

The inventors of the present invention have found that the secondary roughening (post-roughening) tendency of the surface of paper seems to diminish after a long load/heat treatment zone at subsequent process stages (e.g. in printing and coating). The prevention of secondary roughening may also be due to a release of drying tensions when using a high moisture content, high temperature and long time period at the calendering stage.

Accordingly, the aim of the present invention is to provide a method by means of which the roughening of a coated or uncoated fibrous web can be regulated in a controlled manner at a certain stage of processing of the fibrous web, and by which method secondary roughening at subsequent process stages is prevented and the structure of the fibrous web is rendered more stable. To achieve this aim, the method according to the invention is characterised in that in the method, the entry moisture of the fibrous web led to the processing device is relatively high and at the same time is applied a higher processing temperature and the dwell time in the processing device is adjusted so that the desired post-processing moisture and surface stabilisation are obtained, that in the method is processed a paper web, the entry moisture of which on being led to the processing device is within the range from about 6 to about 50%, while the processing temperature about 100 to 400°C, and due to drying after processing, the final moisture is within the range from about 1 to 12%, the dwell time of the paper web in the processing device being about 5- 200 ms, and that in the method, a metal belt calender type processing device or shoe calender is used as the processing device. The method according to the invention is preferably implemented, for example, by using a metal belt calender which comprises a metal belt arranged to rotate around a guide means, outside which belt is arranged at least one counter-element forming a contact surface with the belt in such a way that between the belt and the counter- element is formed a belt processing zone through which the belt to be processed is led. Different implementations of the metal belt calender are described in greater detail, for example, in the international application WO 03/064761 Al. The metal belt calender may be located in the dryer section or at a point after it.

The production line for fibrous web may include several part processes, for example, so that the paper or board machine is followed by a calender, a web sizing unit, a calender, a coating machine, a calender and an final calender. These part processes may be realised as on-line or off-line solutions. Either all or only some of the part processes may be included. The calender may be any known calender. The processing device used for implementing the above methods is preferably a metal belt calender, which may be, for example, one of the above-mentioned calenders performing a part process or it may be located in a paper or board machine, for example, in the dryer section.

The invention is described in greater detail in the following, with reference to the appended drawings, in which:

Figure 1 shows z-direction moisture distribution of paper obtained from hot pressing tests performed with a static press test device in a situation where the pressing surfaces are impermeable,

Figure 2 shows a corresponding test series of hot pressing tests where one of the pressing surfaces is porous,

Figure 3 shows diagrammatically a metal belt calender according to the prior art, which is suitable for use in implementing the method according to the invention, Figure 4 shows the test results for roughening caused by the evaporation of water in newsprint,

Figure 5 shows the parts circled in Figure 4 separately as a bar diagram, and

Figure 6 shows the results of the roughening test for newsprint calendered with different calenders.

Figure 1 shows z-direction moisture distribution of paper obtained from hot pressing tests performed with a static press test device. One of the pressing surfaces of the test device was heated to a temperature of 110°C, while the other was maintained at a temperature of 30°C. The paper being pressed was divided into five different layers of equal strength (in the test there were 5 sheets in a pile), in such a way that the 1st layer was against the hotter contact surface and the 5th layer was against the colder surface. The tests were performed by applying pressing times of varying length (1, 2, 5 and 10 seconds). After each pressing pulse, the different layers in the sheet pile were rapidly separated, moved into plastic bags to prevent evaporation and weighed to determine moisture content. In the tests performed, the sheet piles had an initial moisture content of about 40%. The results of the test are shown in Figure 1, the vertical axis of the graph representing moisture content and the horizontal axis showing groups of bars corresponding to each pressing time, so that layers 1..5 correspond to the bars from left to right in groups. The results show that after a certain pressing time, the moisture distribution changes to strongly one-sided one, the hot surface side undergoing drying and the moisture moving in the direction of the cool surface. It should be noted that the pressing surfaces are impermeable, which means that the overall moisture of the sample will not fall, only the moisture distribution changes. It should furthermore be noted that the pressing time in the static test arrangement shown in the graph is considerably long compared with a real situation because it includes the closing and opening delays of the plates of the test device. However, the test shows that by regulating the temperature of the pressing surfaces and the pressing time, the z-direction movement of moisture and controlled moisture distribution described above are achieved. Figure 2 shows the results of a test series corresponding to Figure 1 in so-called Condebelt drying, that is, in a situation where a permeable wire is arranged on the cool surface side, in the pores of which wire water can condense. When studying the results, it is noted that drying starts on the hot surface side, from where the moisture moves through the structure towards the cool surface side. It is particularly noteworthy that the overall moisture of the web falls because the water condenses in the wire space on the cool side.

One particular object of application for the invention is metal belt calendering which is carried out, for example, to form the structure of the web or to compress the surface or to dry the web. By the arrangements according to the invention, adjustment of z-direction moisture distribution can be carried out in the said process and to it can be combined nip pressure control, the said measures aiming at controlled one-sided forming of the structure of the web, such as control of the one- sidedness of density distribution or surface roughness or of the curling tendency.

Another application of the invention is processing with a Condebelt-type device which is used, for example, for web drying and/or surface finishing. By controlling the extent of the moisture distribution of the web by the above means, the one-sidedness of the web structure and the curling tendency can be affected.

The metal belt calender solution is described in the following. The metal belt calender may be, for example, of the type shown in Figure 3, which comprises a calendering belt 2 made of metal which rotates around guide rolls 3, of which guide rolls a least a part are movable to enable adjusting of the belt 2 tension and/or the length of the processing zone as desired. The calendering belt 2 runs around a roll 5 arranged outside it, whereby the calendering area is formed between the belt 2 and the roll 5. The material web W to be calendered travels through the calendering area, whereby the desired pressure impulse and thermal effect are exerted on it as a function of time. Figure 3 shows, with a line of dots and dashes 9, the shape of the pressure impulse when a nip roll 4 acting as a pressing means which presses the belt against the roll 5 is arranged inside the calendering belt 2, thus forming a higher pressure nip area inside the calendering area. The dash line 8, on the other hand, shows the shape of the pressure impulse when the contact pressure affecting the calendering area is formed only by means of belt 2 tightness, while the nip roll 4 is out of pressing contact with the belt 2 (or when the nip roll 4 is not installed inside the belt at all 2). By contact pressure is referred to the sum of pressure impulses exerted on the web W in the calendering area between the belt 2 and the counter-element 5, which are due to belt 2 tension and/or pressing force exerted by possible belt-internal pressing means 4. Roll 5, like nip roll 4, may or may not be a deflection compensated roll, and it is selected from a group including: a flexible surface roll, such as a polymer-coated roll, a rubber-coated roll or elastomere- coated roll, a shoe roll, a thermo roll and a filled roll. In the metal belt calender shown in Figure 3, the nip roll 4 is a shoe roll. Figure 6 refers to the heating means, such as an induction heater, an infrared radiator, a gas burner or a capacitive heater. In a metal belt calender may be used elevated temperatures, for example, exceeding about 100°C to over about 200°C and even up to 400°C, depending on the application.

By regulating the contact temperature on the different sides of the paper, the temperature difference acting across the paper is used to control the direction of travel of the water and the intensity of the transfer in the structure of the paper. In a metal belt calender, both the belt and the thermo roll acting as counter-element can be heated to the same temperature. In the pressing zone the paper web is, therefore, against heated surfaces on both sides, whereby the web is heated starting on both surfaces symmetrically. In this situation, water begins to transfer and condense in the cooler middle parts of the web. By varying the effective period of the pressing zone and the temperature of the surfaces, the intensity of the symmetric heat and moisture distribution can be controlled. After the pressing zone, water leaves the web symmetrically and the bending moment causing the curling tendency will not occur.

When the web is heated one-sidedly, water will tend to move in the direction of the colder surface, whereby after the pressing zone, water leaves mainly through the surface on this side of the paper, and the curling tendency thus arises in this direction. By adjusting the effective period of the pressing zone, the temperature level of the surfaces and the temperature difference between the surfaces, the magnitude of the moisture gradient formed, and thus the curling tendency, can be affected.

In successive processes, the last process is the most decisive. The water collecting on the cold surface is doctored or removed by other means. The hot surface may be a thermo roll and the cold surface a belt (or vice versa). In addition to one-sided heating, the colder surface may be cooled (e.g. through cold water circulation or blasting) to increase the temperature difference between the surfaces. Through this one-sided heating, the moisture profile can be evened out in the CD- and MD- directions.

In connection with both of the cases described above, that is, symmetrical or onesided moisture and temperature gradients, the force and timing of the mechanical pressing can be adjusted, for example, by means of the additional load roll, to adjust the z-profile of the structure of the web in a controlled manner. In a preferred embodiment of the invention, one of the contact surfaces is heated hotter to guide the water to the colder side and once a strong moisture gradient has formed, a load pulse is directed at the web by means of the additional load roll 4 arranged inside the metal belt loop 2, whereby the web W is compacted more on its moister side, thus causing the z-direction density gradient to form on the web.

In a process according to a second aspect of the invention, the entry moisture is high and the calendering conditions such that the paper dries to the desired final moisture. During drying (= evaporation after the nip), the surface is roughened, which prevents roughening at subsequent process stages.

The process according to the second aspect of the invention was tested with newsprint. The test results are presented in the following, based on reference /4/. Newsprint (51 g/m²) was calendered in the test with a metal belt calender in conditions causing much water to evaporate during calendering. In this case, 3-8 g/m² of water evaporated depending on calendering conditions. Since the final moisture with newsprint is constant (typically 7-9%), the entry moisture must be increased when evaporation increases. In the tests, the aim was to maintain the final moisture constant (8.5%). This means that the entry moisture was varied in accordance with the process conditions. Below are shown the entry moistures used in the tests:

- 100°C, 40 ms -> about 12%

- 150°C, 40 ms -> about 14-15%

- 200°C, 40 ms -> about 16-20%

Figure 4 shows roughening due to extensive evaporation of water. In the Figure can be seen that simultaneous increasing of calendering temperature and entry moisture roughens the paper and its density is reducing. In Figure 4 are circled points where the conditions were identical (line load 30kN/m, processing time 40 ms, final moisture 8.5%) with different entry moisture and processing temperature values. Roughening is seen particularly clearly when the calendering temperature is raised from 150°C to 200°C (entry moisture increased simultaneously). Moderate roughening appears already when the temperature is raised from 100°C to 150°C. Figure 5 shows the points circled in Figure 4 as a separate bar diagram.

When assessing the results, it should be remembered that raising the calendering temperature and moisture normally reduces roughness through the plasticizing of the paper. Plasticizing was very probably the factor that decreased the roughness values in the test. Roughening due to the effect of the evaporation of water is, therefore, likely to be greater than shown in Figures 4 and 5, because the plasticizing effect acts in the other direction.

Figure 6 shows the results of the roughening test for papers calendered with different calenders (in the Figures, LN = long nip = metal belt calender). As seen in Figure 6, raising the temperature and calendering moisture reduces the roughening tendency as the calendering temperature increases. Compared with machine and soft calendering, which are presently the most common methods used in newsprint calendering, the difference is quite considerable. In shoe calendering, the relatively long nip also causes considerable evaporation of water, thus also giving a rather stable surface. In the process according to the invention, a substantial amount of water evaporates during calendering (e.g. more than about 1 g/m², preferably within the range from about 2 g/m² to about 8 g/m², or even more than 8 g/m²), whereupon the paper roughens and the surface stabilises. The calendering conditions are selected so as to make rapid evaporation possible.

- For example in metal belt calendering, the values may be as follows: temperature >120°C, preferably about 150-250°C, dwell time > 10 ms.

- For example in shoe calendering, the values may be as follows: temperature >120°C, preferably about 150-250°C, nip length more than 30 mm, preferably about 50-300 mm.

An alternative embodiment of the process is such that the evaporation of water after calendering is arranged by means of various drying devices if evaporation following calendering is insufficient. The drying device may be, for example, an infrared dryer, a suspending dryer, a drying cylinder or drying cylinder group.

The most preferred application of the invention is to paper grades containing a large quantity of mechanical pulp. Mechanical pulp has a higher roughening tendency than paper grades containing chemical pulp. The invention is particularly well suited for newsprint or paper grades similar to newsprint, but the scope of protection of the invention also covers its application to other paper grades, such as, for example, LWC precalendering or other coated grades containing mechanical pulp, whereby the aim is to prevent secondary roughening in coating. The invention may also be applied to grades containing chemical pulp (coated or uncoated).

Bibliography:

/l/ Papermaking Science and Technology, book series: Papermaking Part 2, Drying, p. 351-361, ed. Karlsson, M., publ. Fapet Oy, Jyvaskyla, Finland, 1999.

/2/ Papermaking Science and Technology, book series: Papermaking Part 16, Paper Physics, Kajanto, I. & Niskanen, K., "Dimensional stability", p. 223-259, (ed. Niskanen, K., publ. Fapet Oy, Jyvaskyla, Finland, 1998. /3/ Papermaking Science and Technology, book series: Papermaking Part 3, Finishing, ed. Jokio, M., publ. Fapet Oy, Jyvaskyla, Finland, 1999, 361 pages, p. 53- 68. /4/ Thesis for diploma by Maria Lepola: "Kalenterointiparametrien vaikutus pitkanippikalanterissa", Espoo, Finland, 8.9.2003.

Claims

1. A method for controlling structural and functional characteristics of a fibrous web in a processing device (1) for a fibrous web (W) which comprises a metal belt (2) arranged to rotate around a guide means (3), outside which belt is arranged at least one counter-element (5) forming a contact surface with the belt, in such a way that between the belt and counter-element is formed a web processing zone (N) through which the web to be processed is led, and in which processing device (1) is optionally at least one additional load roll (4) inside the metal belt loop, characterised in that in the method, the form of the z-direction moisture distribution of the fibrous web is controlled by regulating the temperature of the surfaces in contact with the fibrous web (W) to control the curling of the web in a controlled manner.

2. A method as claimed in claim 1, characterised in that in the method, both the belt (2) and the counter-element (5) are heated to essentially the same temperature to remove water approximately symmetrically to prevent curling of the web at subsequent process stages.

3. A method as claimed in claim 1, characterised in that in the method, one contact surface is heated hotter and the other colder and the temperature difference between the said surfaces is regulated to control the curling of the web in a controlled manner.

4. A method as claimed in claim 1, characterised in that in the method, one of the contact surfaces is heated hotter and the other colder to guide the water to the colder side, and that once a strong moisture gradient has formed, a load pulse is directed at the web (W) by means of at least one additional load roll (4) arranged inside the metal belt loop (2), whereby the web W becomes more compacted on its moister side, thus causing a z-direction density gradient to form on the web.

5. A method for controlling structural and functional characteristics of a fibrous web in a drying device (1) for a fibrous web (W) which comprises a metal belt loop forming an impermeable surface, outside of which metal belt is arranged a metal belt loop forming another impermeable surface or drying cylinder or roll, in such a way that between the said impermeable surfaces is formed a web drying zone, through which the belt to be dried is led together with the porous wire, characterised in that in the method, the form of the z-direction moisture distribution of the fibrous web is controlled by regulating the temperature of the said impermeable surfaces to control the curling of the web in a controlled manner.

6. A method for controlling the roughening of a coated or uncoated fibrous web in a processing device for a fibrous web and for reducing or preventing secondary roughening at subsequent process stages, characterised in that in the method, the entry moisture of the fibrous web led to the processing device is relatively high, and at the same time is applied a higher processing temperature and the dwell time in the processing device is adjusted so that the desired post-processing moisture for roughening the fibrous web and for stabilising the surface is reached, that in the method is processed a paper web, the entry moisture of which on being led to the processing device is within the range from about 6% to about 50%, while the processing temperature is within the range from about 100 to 400°C, and due to the drying after processing, the final moisture is within the range from about 1% to 12%, the dwell time of the paper web in the processing device being about 5-200 ms, and that a metal belt calender type processing device or shoe calender is used as the processing device in the method.

7. A method as claimed in claim 6, characterised in that in the method is calendered newsprint, in which method the entry moisture of the paper web when led to the calender is within the range from about 12% to about 20%, while the processing temperature is within the range from about 140°C to 200°C, and due to drying after calendering, the final moisture is within the range from about 7% to 9%.

8. A method as claimed in claim 7, characterised in that in the method, the dwell time of the paper web in the calender is about 40 ms or more.

9. A method as claimed in any of the claims 6 to 8, characterised in that the metal belt calender type processing device used for implementing the method is located in the dryer section or at a point following it.

10. A method as claimed in claim 9, characterised in that the metal belt calender is located in the central area of the dryer section, and that the entry moisture of the paper web led into the metal belt calender is about 20%, while the processing temperature is about 200°C and the exit moisture about 10%, in which method the final drying after calendering to the desired final moisture is carried out within the end area of the dryer section.