FI20207086A1 - Method for sizing a fiber web and a forming section for a fiber web - Google Patents

Abstract

The invention relates to a method for forming a fiber web, in which at least part of the forming is done in a single-wire forming part. At least part of sizing of the fiber web is provided by applying sizing agent in solids content 10 — 40% by a curtain type application device (C10; C20) on the fiber web in forming section of a fiber web production line. The invention also relates to a forming section for a fiber web comprising at least one single-wire forming part. The forming section comprises at least a curtain type application device (C10; C20) for applying sizing agent in solids content 10 — 40% on the fiber web in forming section of a fiber web production line.

Description

Method for sizing a fiber web and a forming section for a fiber web The invention relates generally to producing fiber webs. Particularly the invention relates to a method for sizing a fiber web according to the preamble of the inde- pendent method claim and to a forming section for a fiber web according to the preamble of the independent forming section claim. As known from the prior art in fiber web machines, especially in paper and board machines, the fiber web is produced and treated in an assembly formed by a number of apparatuses arranged consecutively in a process line. A typical production and treatment line comprises a forming section comprising a headbox and a forming unit and a press section as well as a subsequent drying section and a reel-up. The pro- duction and treatment line can further comprise other devices and sections for fin- ishing the fiber web, for example, a size press, a calender, a coating section. The production and treatment line also comprises typically at least one winder for form- ing customer rolls as well as a roll packaging apparatus. In this description and the — following claims by fiber webs are meant especially containerboard or cartonboard webs. The task of a forming unit is to remove water from fiber suspension fed by the head- box. When the web is manufactured of watery fiber stock, water in the stock is re- moved on the forming section through a forming wire or forming wires for starting the formation of the web. Fibers remain on the forming wire or between the forming wires moving together. Depending on the grade of the web being manufactured, different types of stocks are used. The volume for which water can be removed from different stocks for achieving a web of good quality is a function of many factors, such as e.g. a function of the desired basis weight of the web, the design speed of S 25 the machine, and the desired level of fines, fibers and fill materials in the finished N product. Many types of devices are known on the forming unit such as foil strips, O suction boxes, turning rolls, suction rolls, and rolls provided with an open surface, 0 which are used in many different arrangements and arrays when trying to optimize I the volume, time and location of water being removed when forming the web. The = 30 manufacturing a high-quality end-product of desired grade is a function of the vol- 8 ume of dewatering, the dewatering method, the duration of dewatering, and the lo- S cation of dewatering.

O N A commonly used method of making a multi-ply fiber web is based on the use of several separate web forming units in which the different layers of the fiber web are caused to be drained in a layer by layer fashion either onto one another or onto separate wires, in which case they are combined with one another after partial de- watering. Typically in multi-ply / multilayer fiber web production, i.e. when producing a fiber web having more than one layers, term “multi-ply” is used when the layers are formed separately in the forming section and term “multilayer” is used when a multilayer headbox is used for feeding suspension layers to the forming section even though these terms multi-ply / multilayer are used very often synonymously and thus the difference can be defined by the context only. In production of fiber webs, for example in production of paper or board webs, sizing is used to alter the properties of a fiber web by adding sizing agents, for example starch, native starch, modified starch, cationic starch, microfibrillated cellulose, nan- ofibrillated cellulose, highly refined cellulose, AKD, ASA,SAE, pigments (GCC, PCC kaolin, talc), barrier chemicals (latex) or other sizing agents. Sizing can be divided to internal sizing and surface sizing. In internal sizing, some- times called stock sizing, the sizing agent is added to pulp in the wet end of the fiber — web machine before forming. In surface sizing the sizing agent is added onto the surface of the fiber web typically at the dry end of the fiber web machine. Sizing is used in order to improve paper web properties, in particular water resistance, water absorption properties, strength (SCT, burst), internal strength, tensile strength and bending stiffness. In addition, runnability as well as dusting tendency can be affected favorably. Sizing of a fiber web can also be cationic pulp sizing but as a disad- vantage of this known method is low sizing response when large amounts of sizing agent is used, complicated chemistry of the short circulation system, poor retention and in large sizing agent dosages degradation of the formation. In pulp sizing by multilayer headbox of a forming section for middle layer additives are used for fixa- — tion of the sizing agent and thus compound achieved is firmer containing larger par- N ticles and fixation to the fiber web is increased. It is also known to apply spray sizing N agent, for example so called spray starch, typically granular (uncooked) starch <Q slurry, in between the layers of a multi-ply fiber web. Disadvantages of this known 2 method are low sizing agent amount and misting. Also sizing agent profile in cross- E 30 machine direction is uneven, which increases amount of sizing agent because © weakest point in the fiber web determines the amount of sizing agent. Spray nozzle O are clogging easily and then it is needed to change spray pipe, which decreases N production efficiency.

N In US patent publication 7,892,613 is disclosed a method of making a multi-ply pa- per or paperboard while reducing weight basis of a top ply, the method comprising the steps of: applying a mixture comprising starch and a filler to a surface of a base ply to form an intermediate layer, the base ply includes a preselected color; and applying the top ply to a surface of the intermediate layer wherein the intermediate layer is configured to obscure the color of the base ply when viewed through the top ply.

In connection with this known method a sizer comprising a special applicator that is positioned over the Fourdrinier machine is used and a liquid dispersion of additive is forced out of a narrow slot in the applicator and falls as a full-width curtain onto the wet stock.

An object of the invention is to create a method for sizing a fiber web and a forming section for a fiber web, in which the disadvantages and problems of prior art are eliminated or at least minimized.

A particular object of the invention is to provide cost-effectively a method for sizing a fiber web and a forming section for a fiber web, by which the problems relating to sizing agent distribution, outcome of sizing, especially relating to strength property results, retention of fiber webs, runnability and/or chemistry of the short circulation of the fiber web are solved.

In order to achieve the abovementioned objects, the method for sizing a fiber web according to the invention is mainly characterized by the features of the character- izing clause of the independent method claim and the and a forming section for a fiber web according to the invention is mainly characterized by the features of the characterizing clause of the independent forming section claim.

Advantageous em- bodiments and features are disclosed in the dependent claims.

According to the invention in the method for forming a fiber web, in which at least part of the forming is done in a single-wire forming part in which fiber web is on top o of a wire, in which at least part of sizing of the fiber web is provided by applying O 25 sizing agent by a curtain type application device on the fiber web in forming section O of a fiber web production line, at least part of sizing of the fiber web is provided by LO applying sizing agent in solids content 10 — 40% by the curtain type application de- _ vice. = © According to an advantageous feature of the invention the fiber web is a multi-ply O 30 fiber web, in which at least part of the forming is done in a twin-wire forming part S formed between a wire (10) for the bottom layer of the multi-ply fiber web and a wire N (20; 30) for the top layer of the multi-ply fiber web, in which the layers for the multi- ply fiber web are joined and treated layers combined in the twin-wire part, in which at least part of sizing of the multi-ply fiber web is provided by applying sizing agent by a curtain type application device (C10; C20; C30; C40) in between at least two layers of the multi-ply fiber web in forming section of a multi-ply fiber web production line.

According to an advantageous feature of the invention the sizing agent is applied by a slot-type or slide-type curtain application device.

According to an advantageous feature of the invention the sizing agent viscosity is between 40 — 2200 mPa s, advantageously 150 — 1200 mPa s, sizing agent flow rate is between 0,5 — 5 cm3/s/cm, advantageously 1 — 3 cm3/s/cm and / or the sizing agent surface tension is 20 — 70 mN/m, advantageously 30 — 50 mN/m.

— According to an advantageous feature of the invention suction and/or underpressure is created at the location of the curtain type application device a suction or an un- derpressure device on the opposite side of the multi-ply fiber web layer and its sup- port wire in relation to the curtain application device. Thus, absorption of the sizing agent into the fiber web layer is improved.

Advantageously the suction device is located on the opposite side of the multi-ply fiber web layer in relation to the curtain application device just before the application location of the sizing agent, whereby the suction effect eliminates or at least mini- mizes the disadvantages of the boundary air flow on the side of the multi-ply fiber web layer, where the application is provided.

According to an advantageous feature of the invention boundary air flow is turned off the running direction of the multi-ply fiber web before curtain application of the sizing agent by an air guide.

S According to the invention the forming section for a fiber web comprises at least one & single-wire forming part in which fiber web is on top of a wire, which forming section re 25 comprises at least a curtain type application device for applying sizing agent on the 0 fiber web in forming section of a fiber web production line, the forming section com- I prises at least the curtain type application device for applying sizing agent in solids > content 10 — 40%. > = According to the advantageous feature of the invention the forming section for a S 30 — multi-ply fiber web comprises at least one twin-wire forming part formed between a N wire for the bottom layer of the multi-ply fiber web and a wire for the top layer of the multi-ply fiber web, in which twin-wire part in the layers for the multi-ply fiber web are joined and treated layers combined, wherein the forming section comprises at least one curtain type application device for applying sizing agent in solids content — 40% by in between at least two layers of the multi-ply fiber web in forming section of a multi-ply fiber web production line. According to an advantageous feature of the invention the curtain type application 5 device is a slot-type or slide-type application device. According to an advantageous feature of the invention the forming section of the fiber web production line comprises a suction or an underpressure device on the opposite side of the fiber web and its support wire in relation to the curtain applica- tion device. Thus, absorption of the sizing agent into the fiber web layer is improved. 10 According to an advantageous feature of the invention the forming section of the fiber web production line comprises air guide, preferably an air curtain and/or a re- versing blade before the curtain type application device for turning boundary air flow off the running direction of the fiber web before curtain application of the sizing agent. According to an advantageous feature of the invention the sizing agent comprises cationic starch. According to an advantageous feature of the invention the sizing agent is applied at least in between layers that are middle layers of the fiber web. According to an advantageous feature of the invention the sizing agent is applied at least in between the top layer and the layer next to it. According to an advantageous feature of the invention the sizing agent applied pen- o etrates deep into the structure of the fiber web and advantageously the conveyance O and the penetration of the sizing agent in solids content 10 — 40% is guided to de- O sired direction and depth by a water-removal means and by controlling amount and LO 25 viscosity of the sizing agent, by conveying the sizing agent to both layers on each side of the boundary of the web layers to be joined the sizing in the boundary zone = between the layers joined is efficient.

O O According to an advantageous feature of the invention the fiber web is cartonboard, Q for example folding box board (FBB), white lined chipboard (WLC), solid bleached

O N 30 board (SBB), solid bleached sulphate (SBS) or liquid packaging board (LPB). In addition, the fiber web is advantageously containerboard, for example fluting,

kraftliner (KL), testliner (TL) or white top liner (WTL). Typically, these grades basis weights are between 100 to 400 g/m. The fiber web can have single or multi-ply structure and can be a coated or an un- coated fiber web. By the forming section according to the invention many advantages are achieved: Sizing agent high solids content decreases web dewatering need in forming and press section. High solids content ensures that reguired ply-bond and internal strength is achieved with smaller amount of sizing agent. Thus, grammage of the fiber web can be decreased as the critical strength properties are achieved by better effect of the sizing agents. Further, good retention is achieved due beneficial appli- cation to the inner structure and cost-savings are achieved as smaller amounts of sizing agent and thus the sizing agent remains within the fiber web and does not flush away with following water removal steps. This also makes possible to use higher amount of sizing agent and by this way it is possible to reduce amount of fibers. By this way production costs are reduced. High viscosity further improves sizing agent retention to the web. This saves amount of sizing agent and therefore saves production costs. In multi-ply fiber webs sizing agent between the plies cre- ates |-beam structure, which improves remarkable web stiffness. Additionally, good runnability properties of the fiber web are achieved as misting and other spreading as well as adherence to rolls and other components of the sizing agent is minimized. Also, chemistry of short circulation is easier as handling of the sizing agent can be isolated from the pulp circulation. Another benefit is that this removes need for dry end sizing and by that way fiber web machine drying capacity increases remarkably. The curtain type application device can be any kind of applicator which creates a S 25 — curtain-like sizing film, for example a slot-type or a slide-type curtain application de- N vice. Curtain can be single or multilayer, so it can contain single layer or several re layers with different sizing agents and properties. The slot-type curtain application 0 device is advantageous because dwell time is shortest from the nozzle to the fiber = web. a O 30 In the following the invention is explained in detail with reference to the accompa- = nying drawing to which the invention is not to be narrowly limited.

QA N In figure 1 is shown schematically an advantageous example of a forming section according to the invention.

In figure 2 is shown schematically another advantageous example of a forming sec- tion according to the invention. In figure 3 is shown schematically a curtain application device and an air guide — steam box arrangement according to an advantageous example. In figure 4 is shown schematically yet another advantageous example of a forming section according to the invention. During the course of the following description like numbers and signs will be used to identify like elements according to the different views which illustrate the invention and its advantageous examples. In the figures some repetitive reference signs have — been omitted for clarity reasons. In figure 1 is shown an example of a forming section for a fiber web, in this example for a two-layer multi-ply fiber web. The forming section comprises a headbox M10; M20 for each layer, from which the stock suspension is fed to the forming unit for each layer beginning as one wire part comprising a wire 10 for the bottom layer of — the multi-ply fiber web and a wire 20 for the top layer of the multi-ply fiber web, each wire comprising rolls 12, 22 for guiding, tensioning and/or driving the wire 10; 20 as an endless loop. The stock suspension M10; M20 is first fed onto the wire 10; 20 and thereafter the stock on the wire is guided past inside the loop of the wire 10; 20 located water removal means 11; 21; 15; 25. Inside of the loop of wire 10; 20 at least one suction means 15; 25 are located. During the run of the stock on the one wire part the water removal is substantially horizontal. A curtain application device C10; C20 is located at the run of the stock layer the one wire part for applying sizing agent in solids content between 10 — 40% by a curtain type application device onto the o stock layer and to be in between at the layers of the multi-ply fiber web in forming O 25 — section. Sizing agent comprises advantageously modified starch, which surface ten- O sion is adjusted to level 30 — 50 mN/m by other chemicals. The curtain application LO device C10; C20 is advantageously located such that the suction means 15; 25 are > located on the opposite side of the multi-ply fiber web inside of the loop of the wires = 10; 20.

O O 30 Advantageously, only one application device C10; C20, which device C10 is most S advantageously applicating the sizing agent onto the fiber web layer on the lower S wire 10. The application device C20 for the sizing agent can also be only applicating onto the fiber web on the upper wire 20. It is also possible to use two application devices C10; C20 as shown in the example of the figure 1. Also, more than one application devices C10; C20 can be used consecutively for one fiber web layer. The stock forming the top layer of the multi-ply fiber web guided on the wire 20 is after the one-wire part guided downwards towards the first wire 10 and the runs of the wire 10 for the bottom layer and the wire 20 for the top layer are united by a combining roll 23 to form a twin wire part and the webs for the bottom layer and the top layer are guided into a gap formed between the wires 10; 20 forming the twin- wire part of the forming unit. Web layers have been joined together by a combining roll 23 at the beginning of the twin-wire forming part. Combining roll 23 is situated in — the top layer wire loop 20. Thus, the sizing agent applied in solids content between 10 — 40% by the curtain application devices C10; C20 onto each layer of the web is guided in between the layers. Sizing agent comprises advantageously modified starch, which surface tension is adjusted to level 30 — 50 mN/m by other chemicals. Advantageously, after applying the sizing agent in form of curtain by the curtain ap- plication device C10 its conveyance and penetration into the web layer is assisted such that the sizing agent is forwarded deeper into the web layer on which the sizing agent is applied by guiding the sizing agent to desired direction. In the example of figure 1 a suction means 17 are provided inside the lower wire loop 10 and after the application of the sizing agent by the curtain application device C10 but before the web layers are joined by the combining roll 23 at the beginning of the twin-wire forming part in order to assist the conveyance and penetration of the sizing agent into the web layer. Additionally advantageously, after applying the sizing agent by the curtain applica- tion device C20 and after the web layers have been joined by the combining roll 23 conveyance of the sizing agent is further enhanced by guiding the direction of con- N veyance of the sizing agent from the surface over the joining boundary towards the N web layer on the other side of the boundary of the joining web layers. In the example <Q of figure 1 a suction means 18 are provided inside the lower wire loop 10 and after 2 the application of the sizing agent in form of curtain by the curtain application device E 30 C20 and after the web layers have been joined by the combining roll 23 at the be- © ginning of the twin-wire forming part in order to enhance the conveyance of the siz- O ing agent over the boundary between the web layers joined.

O O After the joining of the layers the multi-ply fiber web is guided on the wire 10 sup- porting the bottom side of the multi-ply fiber web as a one wire part during which run support foils 16 located inside the wire 10 loop. The support foils 16 do not deviate the run of the wire 10 only remove water from the bottom surface of the wire 10 and support the run of the wire 10 as the multi-ply fiber web is guided via suction roll 13 towards a pick-up roll 41 for transferring the multi-ply fiber web to a first press fabric 40 of a press section. The press section also comprises a second press fabric 50 with a roll 51. The press fabrics 40, 50 comprise rolls 42, 52 for guiding, tensioning and/or driving the fabrics 40; 50 as an endless loop. In the press section the multi- ply fiber web is guided between the first press fabric 40 and the second press fabric 50 to a press nip formed between a first press roll 45 and a second press roll 50. After the press section the fiber web is guided to a drying section (not shown).

In figure 2 is shown an example of a forming section for a multi-ply fiber web, in this example for a three-layer fiber web. The forming section comprises a headbox M10; M20; M30 for each layer, from which the stock suspension is fed to the forming unit for each layer beginning as one wire part comprising a wire 10 for the bottom layer of the multi-ply fiber web and a wire 20 for the middle layer of the multi-ply fiber web and a wire 30 the top layer of the multi-ply fiber web, each wire comprising rolls 12, 22;32 for guiding and driving the wire 10; 20; 32 as an endless loop. The stock suspension is first fed onto the wire 10; 20; 30 and thereafter the removal means 11; 15; 21; 25; 31; 35, which can be for example forming shoes and/or suction de- vices, each loop comprising at least one suction device 15; 25; 35. The run of the wire 10; 20; 30 during this water removal on the one wire part is substantially hori- zontal. A curtain application device C10; C20; C30; C40 is located at the run of the stock layer the one wire part for applying sizing agent in solids content between 10 — 40% by a curtain type application device onto the stock layer and to be in between at the layers of the multi-ply fiber web in forming section. Sizing agent comprises advantageously modified starch, which surface tension is adjusted to level 30 — 50 mN/m by other chemicals. The curtain application device C10; C20; C30; C40 is o advantageously located such that the suction means 15; 25; 35 are located on the O opposite side of the multi-ply fiber web inside of the loop of the wires 10, 20, 30. S Advantageously, only one application device C10; C20; C30; C40 for the sizing 2 agent is used, which device C10 is most advantageously applicating the sizing agent E 30 onto the fiber web layer on the lower wire 10. The application device C20 for the © sizing agent can also be only applicating onto the fiber web on the wire 20 or the O application device C30 for the sizing agent can also be only applicating onto the S fiber web on the wire 30. Additionally, yet another application device C40 can be N used to apply the sizing agent in solids content between 10 — 40% on to the web layer on the lower wire after joining the web layer on the lower wire 10 and the web layer on the upper wire 20. Sizing agent comprises advantageously modified starch,

which surface tension is adjusted to level 30 — 50 mN/m by other chemicals.

It is also possible to use two application devices C10; C20; C30; C40 or three application devices C10; C20; C30; C40 or all four application devices C10; C20; C30; C40 as shown in the example of the figure 2. Also, more than one application devices C10; C20; C30; C40 can be used consecutively for one fiber web layer.

The stock forming the middle layer of the multi-ply fiber web guided on the wire 20 is after the one-wire part guided downwards towards the first wire 10 and the runs of the wire 10 for the bottom layer and the wire 20 for the middle layer are united by a combining roll 23 to form a twin wire part and the webs for the bottom layer and the middle layer are guided into a gap formed between the wires 10; 20 forming the twin-wire part of the forming unit.

Web layers have been joined together by a com- bining roll 23 at the beginning of the twin-wire forming part.

Combining roll 23 is situated in the middle layer wire loop 20. Thus, the sizing agent applied in solids content between 10 — 40% by the curtain application devices C10; C20 onto the bottom and middle layer of the web is guided in between the layers.

Sizing agent comprises advantageously modified starch, which surface tension is adjusted to level 30 — 50 mN/m by other chemicals.

Advantageously, after applying the sizing agent in solids content between 10 — 40% by the curtain application device C10 its conveyance and penetration into the web layer is assisted such that the sizing agent is forwarded deeper into the web layer on which the sizing agent is applied by guiding the sizing agent to desired direction.

In the example of figure 2 a suction means 17 are provided inside the lower wire loop 10 and after the application of the sizing agent by the curtain application device C10 but before the web layers are joined by the combining roll 23 at the beginning of the twin-wire forming part in order to assist the conveyance and penetration of S the sizing agent into the web layer. re Additionally advantageously, after applying the sizing agent in solids content be- 0 tween 10 — 40% by the curtain application device C20 and after the web layers I have been joined by the combining roll 23 conveyance of the sizing agent is further = 30 enhanced by guiding the direction of conveyance of the sizing agent from the sur- 8 face over the joining boundary towards the web layer on the other side of the bound- S ary of the joining web layers.

In the example of figure 2 a suction means 18 are S provided inside the lower wire loop 10 and after the application of the sizing agent in solids content between 10 — 40% by the curtain application device C20 and after the web layers have been joined by the combining roll 23 at the beginning of the twin-wire forming part in order to enhance the conveyance of the sizing agent over the boundary between the web layers joined.

After the joining of the layers the multi-ply fiber web is guided on the wire 10 sup- porting the bottom side of the multi-ply fiber web as a one wire part during which run support foils 14 located inside the wire 10 loop.

The support foils 14 do not deviate the run of the wire 10 only remove water from the bottom surface of the wire 10 and support the run of the wire 10 as the multi-ply fiber web is guided towards next twin wire part.

The stock forming the top layer of the multi-ply fiber web guided on the wire 30 is after the one-wire part guided downwards towards the first wire 10 and the runs of the wire 10 for the bottom and middle layers and the wire 30 for the top layer are united by a combining roll 33 to form a twin wire part and the webs for the bottom and middle layers and the top layer are guided into a gap formed between the wires 10; 30 forming the twin-wire part of the forming unit.

Web layers has been joined together by a combining roll 33 at the beginning of the twin-wire forming part.

Combining roll 33 is situated in the top layer wire loop 30. Thus, the sizing agent applied in form of curtain by the curtain application device C30 onto the top layer of the web is guided in between the top layer and the combined bottom-middle layer and the sizing agent applied in solids content between 10 — 40% by the curtain application device C40 is guided in between the top layer ad the combined bottom- middle layer.

Advantageously, after applying the sizing agent in solids content between 10 — 40% by the curtain application devices C30; C40 and after the web layers have been joined by the combining roll 33 conveyance of the sizing agent is further enhanced by guiding the direction of conveyance of the sizing agent from the surface in to the web layer on the lower wire 10 and over the joining boundary towards the web layer N on the other side of the boundary of the joining web layers.

In the example of figure N 2 a suction means 18 are provided inside the lower wire loop 10 and after the appli- <Q cation of the sizing agent in solids content between 10 — 40% by the curtain appli- 2 cation devices C30; C40 and after the web layers have been joined by the combin- E 30 ing roll 33 at the beginning of the twin-wire forming part in order to enhance the © conveyance of the sizing agent over the boundary between the web layers joined.

S After the joining of the layers the multi-ply fiber web is guided on the wire 10 sup- S porting the bottom side of the multi-ply fiber web as a one wire part during which run support foils 16 located inside the wire 10 loop.

The support foils 16 do not deviate the run of the wire 10 only remove water from the bottom surface of the wire 10 and support the run of the wire 10 as the multi-ply fiber web is guided via suction roll 13 towards a pick-up roll 41 for transferring the multi-ply fiber web to a first press fabric 40 of a press section. The press section also comprises a second press fabric 50 with a roll 51. The press fabrics 40, 50 comprise rolls 42, 52 for guiding, tensioning and/or driving the fabrics 40; 50 as an endless loop. In the press section the multi- ply fiber web is guided between the first press fabric 40 and the second press fabric 50 to a press nip formed between a first press roll 45 and a second press roll 50. After the press section the fiber web is guided to a drying section (not shown). Correspondingly the application of the sizing agent in solids content between 10 — 40% can be provided for multi-ply fiber webs with more layers than in the examples of figures 1 and 2. In figure 3 is shown a curtain application device C10; C20; C30; C40 and an air guide 61 — steam box 62 arrangement 60. The curtain application device C10; C20; C30; C40 is advantageously located such that the suction means 15; 25; 35 are located on the opposite side of the fiber web W and the wire 10; 20; 30. A suction or an underpressure device can be just before, under or just after curtain type ap- plication device. In the example as the air guide a reversing blade or air foil 61 is used before the curtain application device C10; C20; C30; C40 for turning boundary air flow A off the running direction of the fiber web W before curtain application of the sizing agent. An air curtain can also be used as the air guide. The air guide 61 distance to the curtain application device C10; C20; C30; C40 can be adjusted and air guide 61 can be also after the curtain application device C10; C20; C30; C40 (not shown). A steam-box 62 is provided just after the air guide 61. The steam-box 62 can be also after the curtain type application device (not shown). If the suction / underpressure device 15; 25; 35 is just before the curtain type application device, it > 25 reduces the amount of boundary air.

QA N In figure 4 is shown an example of a forming section for a multi-ply fiber web, in this re example for a two-layer fiber web. The forming section comprises a headbox M10; 0 M20 for each layer, from which the stock suspension is fed to the forming unit for I each layer beginning as one wire part comprising a wire 10 for the bottom layer of - 30 the multi-ply fiber web and a wire 20 for the top layer of the multi-ply fiber web, each 8 wire comprising rolls 12, 22 for guiding, tensioning and/or driving the wire 10; 20 as S an endless loop. The stock suspension M10; M20 is first fed onto the wire 10; 20 S and thereafter the stock on the wire is guided past inside the loop of the wire 10; 20 located water removal means 11; 21; 15; 25. Inside of the loop of wire 10; 20 at least one suction means 15; 25 are located. During the run of the stock on the one wire part the water removal is substantially horizontal. A curtain application device C10;

C20 is located at the run of the stock layer the one wire part for applying sizing agent in form of curtain by a curtain type application device onto the stock layer and to be in between at the layers of the multi-ply fiber web in forming section.

The curtain application device C10; C20 is advantageously located such that the suction means 15; 25 are located on the opposite side of the multi-ply fiber web inside of the loop of the wires 10; 20. Advantageously, only one application device C10; C20 for the curtain is used, which device C10 is most advantageously applicating the sizing agent onto the fiber web layer on the lower wire 10. The application device C20 for the sizing agent can also — be only applicating onto the fiber web on the upper wire 20. It is also possible to use two application devices C10; C20 as shown in the example of the figure 4. Also, more than one application devices C10; C20 can be used consecutively for one fiber web layer.

The stock forming the top layer of the multi-ply fiber web guided on the wire 20 is after the one-wire part guided downwards towards the first wire 10 and the runs of the wire 10 for the bottom layer and the wire 20 for the top layer are united by a combining roll 23 to form a twin wire part and the webs for the bottom layer and the top layer are guided into a gap formed between the wires 10; 20 forming the twin- wire part of the forming unit.

Web layers have been joined together by a combining roll 23 at the beginning of the twin-wire forming part.

Combining roll 23 is situated in the top layer wire loop 20. Thus, the sizing agent applied in solids content between 10 — 40% by the curtain application devices C10; C20 onto each layer of the web is guided in between the layers.

Sizing agent comprises advantageously modified starch, which surface tension is adjusted to level 30 — 50 mN/m by other chemicals.

S 25 Advantageously, after applying the sizing agent in solids content between 10 — 40% N by the curtain application device C10 its conveyance and penetration into the web 3 layer is assisted such that the sizing agent is forwarded deeper into the web layer Lo on which the sizing agent is applied in solids content between 10 — 40% by guiding I the curtain to desired direction.

In the example of figure 4 a suction means 17 are = 30 provided after the application of the sizing agent in solids content between 10 — 4096 8 by the curtain application device C10 but before the web layers are joined by the 5 combining roll 23 at the beginning of the twin-wire forming part in order to assist the S conveyance and penetration of the sizing agent into the web layer.

Additionally advantageously, after applying the sizing agent in solids content be- tween 10 — 40% by the curtain application device C20 and after the web layers have been joined by the combining roll 23 conveyance of the sizing agent is further enhanced by guiding the direction of conveyance of the sizing agent from the sur- face over the joining boundary towards the web layer on the other side of the bound- ary of the joining web layers. In the example of figure 4 a sleeve roll 27 is provided inside the lower wire loop 10 after the application of the sizing agent in solids content between 10 — 40% by the curtain application device C20 and after the web layers have been joined by the combining roll 23 at the beginning of the twin-wire forming part in order to enhance the conveyance of the sizing agent over the boundary be- tween the web layers joined and deeper towards the top surface of the multi-ply fiber — web. Alternatively, this can be provided by locating a suction means (now shown) inside the upper wire loop and thus causing the conveyance of the sizing agent towards the top surface of the multi-ply fiber web. A sleeve roll 27 is a roll that com- prises a stationary support shaft, an belt loop, which is led to circle around the sta- tionary support shaft, that the sleeve roll further comprises at least one curvilinear dewatering zone consisting of two partial curves such that the radius of curvature of a first partial curve is greater than the radius of curvature of a second partial curve following the first partial curve in the travel direction of belt loop. A sleeve roll con- figuration is disclosed for example in EP patent application 17164069 of the appli- cant.

After the joining of the layers the multi-ply fiber web is guided on the wire 10 sup- porting the bottom side of the multi-ply fiber web as a one wire part during which run support foils 16 located inside the wire 10 loop. The support foils 16 do not deviate the run of the wire 10 only remove water from the bottom surface of the wire 10 and support the run of the wire 10 as the multi-ply fiber web is guided via suction roll 13 — towards a pick-up roll 41 for transferring the multi-ply fiber web to a first press fabric o 40 of a press section. The press section also comprises a second press fabric 50 O with a roll 51. The press fabrics 40, 50 comprise rolls 42, 52 for guiding, tensioning O and/or driving the fabrics 40; 50 as an endless loop. In the press section the multi- LO ply fiber web is guided between the first press fabric 40 and the second press fabric > 30 50 to a press nip formed between a first press roll 45 and a second press roll 50. = After the press section the fiber web is guided to a drying section (not shown).

3 Above only some advantageous examples of the inventions have been described S to which examples the invention is not to be narrowly limited and many modifications N and alterations are possible within the invention.

Claims (10)

Claims

1. Method for forming a fiber web, in which at least part of the forming is done in a single-wire forming part in which fiber web is on top of a wire (10), in which at least part of sizing of the fiber web is provided by applying sizing agent by a curtain type application device (C10; C20; C30; C40) on the fiber web in forming section of a fiber web production line, characterized in that at least part of sizing of the fiber web is provided by applying sizing agent in solids content 10 — 40% by the curtain type application device (C10; C20; C30; C40).

2. Method according to claim 1, characterized in that the fiber web is a multi- ply fiber web, in which at least part of the forming is done in a twin-wire form- ing part formed between a wire (10) for the bottom layer of the multi-ply fiber web and a wire (20; 30) for the top layer of the multi-ply fiber web, in which the layers for the multi-ply fiber web are joined and treated layers combined in the twin-wire part, in which at least part of sizing of the multi-ply fiber web is provided by applying sizing agent by a curtain type application device (C10; C20; C30; C40) in between at least two layers of the multi-ply fiber web in forming section of a multi-ply fiber web production line.

3. Method according to claim 1 or 2, characterized in that the sizing agent is applied by a slot-type or slide-type curtain application device (C10; C20; C30; C40).

4. Method according to claim 1 or 2, characterized in that the sizing agent viscosity is between 40 — 2200 mPa s, advantageously 150 — 1200 mPa s, o sizing agent flow rate is between 0,5 — 5 cm3/s/cm, advantageously 1 — 3 O 25 cm3/s/cm and / or the sizing agent surface tension is 20 — 70 mN/m, advan- ro tageously 30 — 50 mN/m. 2 5. Method according to any of previous claims, characterized in that suction E and/or underpressure is created at the location of the curtain application de- © vice (C10; C20; C30; C40) by a suction or an underpressure device (15; 25; O 30 51) on the opposite side of the fiber web and its support wire (10; 20; 30) in N relation to the curtain application device.

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6. Method according to any of previous claims, characterized in that boundary air flow is turned off the running direction of the fiber web before curtain ap- plication of the sizing agent by an air guide (61).

7. Forming section for a fiber web comprising at least one single-wire forming part in which fiber web is on top of a wire (10), which forming section com- prises at least a curtain type application device (C10; C20; C30; C40) for applying sizing agent on the fiber web in forming section of a fiber web pro- duction line, characterized in that the forming section comprises at least the curtain type application device (C10; C20; C30; C40) for applying sizing agent in solids content 10 — 40%.

8. Forming section according to claim 7, characterized in that the forming sec- tion for a multi-ply fiber web comprising at least one twin-wire forming part formed between a wire (10) for the bottom layer of the multi-ply fiber web and a wire (20; 30) for the top layer of the multi-ply fiber web, in which twin-wire partin the layers for the multi-ply fiber web are joined and treated layers com- bined, which forming section comprise at least a curtain type application de- vice (C10; C20; C30; C40) for applying sizing agent in between at least two layers of the multi-ply fiber web in forming section of a multi-ply fiber produc- tion line.

9. Forming section according to claim 7 or 8, characterized in that the curtain type application device (C10; C20; C30; C40) is a slot-type or slide-type cur- tain application device (C10; C20; C30).

10.Forming section according to any of claims 7 - 9, characterized in that the forming section of the fiber web production line comprises a suction or an underpressure device (15; 25; 35) on the opposite side of the fiber web and its support wire in relation to the curtain type application device (C10; C20; C30; C40) and/or the forming section comprises air guide (61), preferably an S 25 air curtain and/or a reversing blade before the slot-type application device N (C10; C20; C30; C40) for turning boundary air flow off the running direction 3 of the fiber web before curtain application of the sizing agent..

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