WO2007147949A2

WO2007147949A2 - Method and apparatus for treating a fibre web

Info

Publication number: WO2007147949A2
Application number: PCT/FI2007/050380
Authority: WO
Inventors: Pauli Kytönen
Original assignee: Metso Paper, Inc.
Priority date: 2006-06-21
Filing date: 2007-06-20
Publication date: 2007-12-27
Also published as: AT505875B1; FIU20060275U0; FI7399U1; CN101473090B; DE112007001442B4; DE112007001442T5; WO2007147949A3; AT505875A1; CN101473090A

Abstract

In the method, the web (1) to be treated is directed to run from a press nip (8) and treatment mixture is pressed into the web at this nip of rolls (8). The treatment mixture is distributed prior to directing the web into the nip from the spray nozzles to at least one side of the web (1) in such an amount that in the nip the treatment mixture is still wet. The present invention relates to a method and arrangement for treating a fiber web, the arrangement comprising applying a sizing mixture, a coating or the like treatment mixture onto especially board webs. The invention can also be used for paper webs and other corresponding fiber webs. The arrangement comprises at least one press nip (8), means (1) for directing the web (1) into the press nip (8) and means for distributing the wet treatment mixture so that a pool is formed at least between the nip formed by the means (8) forming the nip and the web (1). It further comprises means (20) for measuring the height of the surface of the pool and means for adjusting the height of the surface for controlling the penetration of the treatment material.

Description

Method and apparatus for treating a fibre web

The present invention relates to a method and arrangement for treating a fiber web, the arrangement comprising distributing a sizing mixture, a coating or the like treatment mixture onto especially board webs. The invention can also be used for paper webs and other corresponding fiber webs.

The present application relates to sizing (surface gluing), pigmenting and coating as well as treating the web by means of calandering. In the following, the invention is described using sizing as an example, sizing being the most preferred embodiment of the invention. The invention can, however, be applied to the other above-mentioned applications as well.

Usually, by sizing is meant aqueous mixture of starch applied to the surface of the board or paper and then pressed into the web in, for example, the nip between the rolls. Sizing is also used for increasing the strength of the web. Sizing also improves the surface properties and printability of the paper. In addition to starch, the sizing mixture can contain a number of additives and, in some cases, also latex can be used as the bonding agent of the glue. The invention can in principle be applied to all glue mixtures and different fiber webs. For some board grades, such as liner for corrugated board and fluting, strength is one of the most important properties. Because sizing can be used for considerably improving, among others, bending strength and surface strength, it is desirable to use large amounts of sizing with these board grades. Currently, the largest application amount can be achieved by means of pool application apparatuses, but their highest possible run speed is too low for modern requirements. Thus, modern board machines use film transfer application for distributing sizing. In film transfer application the sizing is first applied onto the surface of the film transfer roll, wherefrom it is transferred to the web. Board machines often utilize a simultaneous two-sided application, whereby the web runs through the nip formed by two film transfer rolls. Both of the rolls are provided with a dedicated application bar. These apparatuses can achieve a relatively large application amount on one surface of the web, 30-35 g/m² as wet film. Use of coating mixtures differs from surface coating in that is preferably to have the solid particles of the coating mixture to stay on the surface of the web.

Film transfer coating apparatuses do, however, have their limitations that reduce their effectiveness and make them less economical. In the nip of film transfer rolls a good penetration of glue into the fiber structure of the web is achieved, but the length of the nip is relatively short, whereby the actual absorption into the web only takes place after the nip. As the drying of the glue must in practice be begun immediately after the nip in order to make the web dry to touch and for directing it, the total absorption time is short. Even though film transfer presses or coating apparatuses can be used for distributing a large amount of treatment material, with large amounts of coating material a pool can be formed between the surface of the web and the surface of the film transfer roll. A high web speed increases the risk of forming of pools, but a pool can be formed even at low speeds. The forming of this so-called mini pool limits the maximum application amount, because the pool causes splashes, oscillation of the web and because of its strong turbulence it limits even passage of air through the nip. In a film transfer nip the web is strongly wetted when using large amounts of glue, causing driveability problems. As the web is wetted, it becomes weaker, whereby it is easier torn. The moisture and drying also change the dimensions of the web and the changes of dimensions must be taken into account by adjusting the web tension and possibly with spreading rolls. The application chambers of the film transfer coating apparatuses are very carefully designed for ensuring even distribution of glue. However, material is constantly removed from the surface of the fiber web, and the material can be transferred to the application chamber either directly on the surface of the film transfer roll or along with the machine circulation. This causes disturbances especially when it ends up under the scraper bar and can cause even clogging in the application chamber. Further, the scraper bars and their holder cradles wear fast, causing short service intervals. This causes high service costs and production cuts due to service interruptions. These problems are highlighted in machines using recycled fiber, because recycled fiber will always include abrasive foreign matter, as most important of which can be mentioned the very abrasive pigment coating particles included with the fibers. As recycled fiber is a very important raw material in the production of board, these problems are especially emphasized in machines used for production packing board. During application, air and impurities can easily be entrained into the machine circulation and they must subsequently be removed by means of efficient deaerators and screen apparatuses. These apparatuses are expensive and the necessary machine circulation is long and its volume will be large.

The problems and limitations occurring during film transfer application are differently emphasized in the production of different products. It is obvious that with strong board grades the danger of web breakages is not as significant as with thinner papers possibly made of recycled fibers. On the other hand, in the production of print paper or board the wear and soiling of the apparatuses is reduced.

In the coating or sizing of paper or board the amount of treatment material transferred onto the web by the rolls of the film transfer coating apparatus can be measured by exposing the application roll to infra-red radiation subsequent to scraping, by means of which the amount of water retained in the glue or coating material layer on the roll is determined on the basis of the attenuation of the radiation. The amount of treatment material transferred to the web can be calculated on the basis of the determined amount of water, solids content of the treatment material and the transfer coefficient between the web and the application roll. With this method it is difficult to determine the generation and size of the mini pool. The amount of coating material retained by the web can also be estimated on the basis of the amount of glue scraped from the rolls and returned to the machine tank. Even with this method it is difficult to control the pool before the nip, even though in principle it can also be determined with some precision, as the difference between the amounts of glue fed into the nip and returned into the machine tank.

The aim of the invention is to provide an arrangement by means of which a widely variable amount of treatment material can be distributed onto the surface of a moving fiber web with a controllable penetration and to avoid the limitations occurring in conventional film transfer or pool presses, such a spilling of treatment material and web breakages. The invention is based on distributing the treatment material mixture onto at least one surface of the web, subsequent to which the web is directed into a nip between two rolls located at a distance from the distribution point, into which nip a mini pool is formed by introducing treatment material, the height of the surface of the pool being measured and the height of the pool being adjusted such that a correct, desired penetration of the treatment material is achieved and web breakages are avoided.

The arrangement and method according to the invention are characterized by what is disclosed in the characterizing portions of independent patent claims.

Considerable advantages are achieved by means of the invention.

The most important advantage of the invention is the widely variable application amount. Especially with large application amounts the arrangement according to the invention is preferable to the earlier methods. Due to the long retention time even a large amount of treatment material will have time to set onto the surface of the web and inside web and it will not be pressed out in the nip of the rolls, hi principle, the arrangement allows the use of as large an amount of glue or coating material as necessary within the requirements currently known products. If necessary, the large application amount also makes it possible to produce new kinds of products. The amount of treatment material and the profile can be adjusted by means of the feed volumes of the nozzles or by changing the nozzle size. Retention time is adjusted simply by changing the distance between the application unit and the press rolls. The final adjustment of the penetration of the glue is made by changing the nip pressure of the nip of press rolls. Another factor having an effect on the penetration is the surface height of the formed mini pool. According to the invention the height of the pool surface is measured and the application amount increased, if necessary, or the distance between the nip and application is reduced. With this adjustment increasing the pool height too much in relation to the web speed is to be avoided, as this will cause the above-mentioned problems of spillage and web breakages. Most preferably the application is simultaneously performed on both sides of the web, whereby only one treatment station and the driers required by it are needed. However, different amounts of treatment material mixture can be distributed on different sides of the web or even different treatment material mixtures can be used. The arrangement also makes double application directly on wet surface possible. This is made by forming a roll nip as in a usual film transfer unit so that the press rolls have usual application bars, whereby a second layer of glue is transferred to the web in the nip. The treatment mix can be distributed hot onto the surface of the web, whereby it is usually less viscous and dries faster on the surface of the web. Evaporation of water is also enhanced by the possibility of directing the web to the application apparatus directly from the drier when still hot.

A calander nip can also be used as the roll nip, for example so that one of the rolls is a soft roll and the other roll is a heatable roll. Correspondingly, one of the rolls or even both rolls can be replaced by belts or the nip can be formed from a shoe calander. Due to the heatable roll or belt and the long, preferably adjustable retention time the drying after pressing can be eliminated, if the drying effect transferred to the web at the nip is sufficient. Thus, by means of the invention, application, nip pressing of application, calendering and drying can be replaced by only application and a hot calander nip. Especially expensive fluid bed or infrared driers can then be omitted and the acquisition price of the system can be reduced and it can be arranged in a small space, which is preferable especially when renewing the production line. With starch-containing glues and different coating mixtures adhesion of glue to the hot roller or belt surfaces can prevent use of this arrangement. In connection with sizing or coating choice of material is critical when using heating in order to avoid adhesion.

In addition to the above, the weaknesses of gluing by film transfer press are avoided, the weaknesses being inadequate penetration of glue into the web, i.e. limitations in the strength of the product, directing a large amount of glue through the film transfer nip, i.e. avoiding formation of a mini pool in the roll nip and fast wear of application bars and bar cradles especially in machines using recycled fiber. As there are no application chambers, clogging of chambers by material loosening from the web is avoided. Material loosened from the web cannot get into the machine circulation, so there's no need to screen the glue mixture for removing impurities. Usually a return circulation is not even necessary, because all glue mixture to be applied can be directed onto the web. In case an excess of glue must be collected from the rolls or spray chambers, it can be easily led back into the feed tank. In this case the amounts are small and the required screening apparatuses are inexpensive. In addition to these, a disadvantage of pool presses is the speed limitation caused by the spilling of the pool, which limits their application possibilities with the currently required web speeds.

In the following, the invention is examined by means of examples and by reference to the appended drawings.

Figure 1 is a schematic illustration of one embodiment of the invention seen from the side.

Figure 2 is a schematic illustration of another embodiment of the invention seen from the side.

Figure 3 is a detail of figure 2.

In figure 1, the gluing station according to the invention is shown arranged into the production line. In the direction of movement of the web 1 the first component is the drying cylinder group 2, from which the web 1 is directed straight to the application apparatus 3. Subsequent to the application apparatus 3 are the press rolls 4 and an air drier 5, from which the web is directed to further processing. Between the drying cylinder group 2 and the application apparatus 3 there are measurement instruments 6 for monitoring the web 1.

The application apparatus 3 comprises press rolls 8 supported by bearings on the frame 7, the rolls forming a nip through which the treated web 1 passes. The rolls 8 and the frame 7 can be parts of a usual film transfer coating apparatus, so in this aspect the application apparatus corresponds to a film transfer coating apparatus from which the application apparatuses of rolls 8 are removed. The web 1 is directed to run directly downwards to the rolls 8 via web guide rolls 9. The area between guide rolls 9 and the press rolls 8 is closed on both sides of the web by means of casing 10. Thus the web 1 runs in a space enclosed by guide rolls 9, press rolls 8 and the casing 10. The edges of the web 1 and any openings can be sealed by means of rubber or sheet metal sealing battens. The necessary amount of spray nozzles 11 are provided in the casing 10 for distributing atomized glue onto the surface of the web. Most preferably pressure- atomizing nozzles can be used for achieving an even coverage without the drying effect of atomizing gas. With glue, the pressure over the nozzles can be fairly small, usually less than 1 Mpa, usually about 1-10 bar. However, the water content of the treatment material mixture is so high that for example atomizing air can hardly detrimentally dry the mixture inside the casing with its high moisture. If necessary, water vapour can be used as atomizing gas, whereby no drying occurs and the treatment material mixture can simultaneously be heated. In this embodiment there is no need for a treatment material recovery system, because the condensed treatment material mixture in the formed chamber and on the walls of casing or the spray in the casing drops or falls to press rolls 8 and is thus transferred to the web 1. The walls of the casing 10 can be heated, whereby the spray adhered on the wall does not dry. In case the flow of treatment material to the press rolls 8 causes faults in the web 1, excess treatment material can be collected away by means of, for example, troughs in the lower portion of the casing 10 and it can be directed back to the machine circulation or the pulper.

A number of spray nozzle rows can be used, if necessary. Thus, when one row of spray nozzles is out of order for the duration of, for example, washing, the machine can still be run by using other nozzles. The apparatus according to the invention can most preferably be used for two-sided coating, but naturally only one side can be coated, if desired. During web breakages and other production interruptions the nozzle is turned away from to the casing, towards the washing trough for example 180° or 90°, whereby the nozzles can be checked, washed or changed. The walls of the casing can easily be opened for washing. In the above-mentioned embodiment the retention time can be adjusted by changing the position of the nozzle rows in casing 10. Thus, however, the casing becomes large and the adjustment travel usually short.

In a solution according to figures 2 and 3 the whole nozzle casing 10 is moved in relation to the press nip 8. In this embodiment the web is directed from the guide roll 9 directly into the nip 8 of press rolls so that both sides of the web engage the press rolls 8 simultaneously. Thus the retention time and distance between application and the pressing is made equal on both sides of the web 1. The nozzle casing 10 is arranged on a guide track 12 and it can be moved along the guide track for changing the distance between the application point and the nip between press roll 8. The length of this guide track 12 can be quite large. In practice, only available space and the length of free travel the web can controllably by directed, limit the length of the guide track 12 and the longest retention time.

According to the invention, the retention time is adjusted for forming the desired amount of treatment material on the web subsequent to the nip. The adjustment can be made as an one-time change of amount of treatment material or other factor of production or quality, such as web speed, material of the bottom web or other such aspect. Alternatively the adjustment can be continuous on the basis of measurement. The adjustment can be based on the measurement of the surface height of the mini pool, the measurement of the treatment material subsequent to the nip of the web, measurement of the profile or the quality of the surface or, for example, adjustment on the basis of the web speed.

An essential feature is the measurement and adjustment of the height of the surface of the mini pool for achieving correct penetration. It is obvious that the measurement can not be carried out by means of touching elements, so the measurement must be made by means of a non-touching sensor 20 (figure 2), such as laser, an inductive sensor or an infra-red sensor. The amount of the treatment material to be applied can primarily be adjusted by means of the measurement. The application amount is controlled by controlling the feed amount of the application apparatus, which is taken controlled by the control of the coating apparatus according to the information input into the control system or a computer. Because penetration is also essentially influenced by the retention time between application and pressing, i.e. the setting time of the treatment material onto the surface of the web, it can be advantageous to combine the adjustment of retention time with the measurement of the surface height of the mini pool. The mini pool is always formed in the nip, regardless of the design of the nip, when there is a trough-like space before the nip. Thus measurement of the height of the mini pool is suitable for controlling both roll nips and belt nips. Increasing the surface level of the mini pool increases the penetration and the amount of the treatment material transferred to the web and reducing the surface height correspondingly reduces these. The highest possible surface height, dependent on, for example, the properties of the treatment material and web speed, can be determined for each nip type, above which formation of eddies starts in the pool, together with splashing, causing dirtying and a risk of web breakage. Measurement of surface height can be used for making sure that this height is not exceeded. The limit height can be determined either by experience or by calculations and the limit value can be made more accurate by test runs and with information collected during the run time of the apparatus.

The nozzle casing 10 comprises nozzle bars 14 on both sides of the web 1. The teachings of the above example apply to the operation, location and serviceability of the nozzle 11. The design of the nozzle bar 10 must, however, be different, because the rolls

8, 10 do not seal the inside of the casing 10 against the environment. The web 1 arrives to the casing 10 along a narrow inlet slit 15. The air entrained with the web 1 forms an overpressure into the narrow slit and prevents the treatment material spray from escaping the casing therethrough. However, only an inconsiderable amount of air is allowed to enter the casing and the operation of the nozzles is not disturbed. Here, the nozzles 11 are directed in the same direction with the web 1, whereby the spray of the nozzle 16 is carried along the air possibly still entrained by the web 1 along with it and it is directed onto the web 1. In the trailing direction of the web 1 there is a narrow outlet slit 19 as well. The slit is surrounded on both sides by heatable chambers 17. The chambers function to prevent the treatment material from drying on the trailing side of the web 1. A heatable plate or, for example, a heatable surface can be used in lieu of the chambers. Excess treatment material spray is removed by means of outlet channels 18 arranged on the side of the chambers. If necessary, suction can be used for spray removal or a small overpressure can be formed inside the casing 10 by means of the air transported along with the web. Air can also be blown into the outlet slit at the trailing end of the web by means of an air knife-like apparatus. The separate treatment material distribution apparatus according to this embodiment can be arranged to work in any position of the web, so that the routing of the track 1 is relatively free as long as there is a sufficiently long straight portion before the press rolls.

The mixture distributed within the invention onto the web 1 must be wet as it arrives into the press nip. By wet is here meant that the nip pressure can be used for especially influencing the penetration of glue into the web, i.e. the glue mixture can be pressed inside the fiber layer of the web. Otherwise the nip would be useless. The distance between the distribution of the glue mixture and the press nip as well as the nip pressure can be used for adjusting the penetration into the web 1 to be as desired. The glue mixture can, however, be distributed onto the web even at a fixed distance from the nip, but in this case an essential part of adjustability is lost. The glue mixture is usually hot when produced, about 85°C, and the glue can be heated for keeping it in this temperature either by means of heat exchangers prior to feeding it to the nozzles or electrically even in the nozzles themselves or by means of a heating medium circulating in the nozzle bar, for example. Heating the glue lowers its viscosity, improves atomizing in the nozzle and enhances the evaporation of water on the surface of the web. As the web comes from the roll of the last drier roll group, it is usually fairly hot, so water is quite efficiently evaporated from the hot glue mixture hitting the surface of the hot web.

Instead of film transfer rolls, a calander nip, for example, can be used in the method, the nip having a roll with a soft surface for the second roll and a heatable hard roll as the second roll. Both rolls can also be soft or hard and one or more of the rolls can be heatable. If the rolls are different from each other, two roll nips might be necessary. In this case, however, the effect of the second roll nip to the penetration of the treatment material is small, because the surface of the web can be too dry after the first roll nip. Instead of the roll nip, a belt or shoe calander comprising one or two subsequent nips can be used. If a metal belt is used in the calander, the belt can easily be heated or cooled for changing the surface properties of the web.

The invention is very well suited for renewing or replacing an existing pool glue press or a film transfer press. In this case, the technical components needed for forming a pool or, correspondingly, application chambers and coating material or glue circulations are removed from the apparatus. According to the invention these application methods are replaced by arranging a spray application bar before the roll nip. Such a renewal is relatively inexpensive and with it the both operation speed and the amount of used treatment material can be essentially increased. Due to the larger treatment material amounts the apparatus can be used for producing a larger range of products than previously.

The invention can also be used in connection with usual film transfer coating apparatuses.

Claims

We claim:

1. An arrangement for treating a paper or board web (1) or a corresponding fiber web, the arrangement being used for applying a treatment mixture onto the surface of the web (1), the arrangement comprising

- at least one press nip (8),

- means for directing the web (1) into the press nip (8), and

- means for distributing a wet treatment material mixture so that a pool is formed at least between the means (8) forming the nip and the web (1).

characterized by means (20) for measuring the height of the surface of the pool and by means for adjusting the height of the surface for controlling the penetration of the treatment material.

2. An arrangement according to claim 1, characterized in that the means for distributing the treatment mixture are spray nozzles (11) arranged at a distance from the press nip (8) for introducing treatment mixture onto at least one surface of the web (1), the distance being adjustable in the direction of travel of the web.

3. An arrangement according to claim 1 or 2, characterized in that spray nozzles (11) are arranged on both sides of the web (1).

4. An arrangement according to any of claims 1-3, characterized by means (12, 13) for changing the distance between the spray nozzles and the nip.

5. An arrangement according to any of the above-mentioned claims, characterized in that the means for forming the nip are two rolls (8) and the web (1) is routed to run through the nip between these two rolls (8), preferably film transfer rolls.

6. An arrangement according to any of the above-mentioned claims, characterized in that the nip is formed by at least one film transfer roll with means arranged thereon for introducing treatment material into the nip via the roll.

7. An arrangement according to any of claims 1-6, characterized in that at least one of the means forming the nip is a belt and the web (1) can be directed to run through the nip between the at least one roll and the belt.

8. An arrangement according to any of claims 4-7, characterized in that the surface of the at least one roll or belt is soft.

9. An arrangement according to any of claims 4-8, characterized in that the surface of the at least one roll or belt is hard.

10. An arrangement according to any of claims 4-9, characterized in that the at least one roll or belt is heatable.

11. An arrangement according to any of claims 4-10, characterized in that the at least one roll or belt is coolable.

12. A method for treating a paper or board web (1) or a corresponding fiber web, the arrangement being used for applying a treatment mixture onto the surface of the web (1), the arrangement comprising

- taking the web (1) into the press nip (8),

- treating the web (1) in at least one press nip (8),

- distributing a wet treatment material mixture so that a pool is formed at least between the means (8) forming the nip and the web (1).

characterized in that the height of the surface of the pool is measured and the height of the surface is adjusted for controlling the penetration of the treatment material.

13. A method according to claim 12, characterized in that the treatment mixture is distributed by spray nozzles (11) arranged at a distance from the press nip (8) for introducing treatment mixture onto at least one surface of the web (1), the distance being adjustable in the direction of travel of the web.

14. A method according to claim 12 or 13, characterized in that treatment material is distributed from the spray nozzles (11) to both sides of the web (1).

15. A method according to any of claims 12-14, characterized in that the distance between the spray nozzles and the nip is changed.

6. A method according any of claims 12-15, characterized in that the nip is formed by two rolls (8) and the web (1) is routed to run through the nip between these two rolls (8), preferably film transfer rolls.

17. A method according to any of claims 12-16, characterized in that the nip is formed by at least one film transfer roll with means arranged thereon for introducing treatment material into the nip via the roll.

18. A method according to any of claims 12-17, characterized in that at least one of the means forming the nip is a belt and the web (1) is directed to run through the nip between the at least one roll and the belt.

19. A method according to any of claims 15-18, characterized in that at least one roll or belt is heated.

20. A method according to any of claims 15-19, characterized in that at least one roll or belt is cooled.