A method, a paperboard product and use of a foam coater and a subsequent high- consistency metering size press

FIELD [0001] The present invention relates to manufacturing of paper and paperboard, and more particularly to methods for applying binders during said manufacturing.

BACKGROUND

[0002] In paper and paperboard manufacturing, surface sizing materials, usually starch solutions, are added to the surface of a mostly dried web, before the final drying. The purpose of the sizing is to improve surface strength, internal bond strength, stiffness and absorption properties, to bind fillers and fibres to the surface, and to reduce dusting.

[0003] Surface sizing is applied by means of a size press, consisting of two rollers. The web passes through the nip of the rollers and the solution comprising the sizing agent is applied to the surface of the web. [0004] In paperboard manufacturing, internal bond strength is an important parameter. In the known sizing methods, use of surface starch is limited to lower solids methods and lower molecular weight and branching in order to keep the viscosity low enough, so that penetration into the ply-zone and a sufficiently high internal bond strength can be achieved in the paperboard product. [0005] Surface sizing thus introduces more water to the web, which leads to an increased need for drying after the size press and increased energy costs of the process. For example, in paper manufacturing, rewetting due to surface sizing may reduce the sheet dryness by tens of percentage units, which as a result may limit the capacity of the machine if drying energy is a bottle neck of the process. [0006] Also, in the known methods, a rather large amount of a sizing solution needs to be applied to give an effective concentration of the sizing agent in the ply-zone.

[0007] Further, in the known methods, such as spray coating, wet-end application of binders is limited to low viscosity (such as less than 50 cP) solutions of particulate materials. [0008] These disadvantages can be at least partly overcome by means of the present invention.

SUMMARY OF THE INVENTION

[0009] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0010] According to a first aspect of the present invention, there is provided a method comprising: providing a web comprising fibrous material; applying a first binder composition in the form of a foamed composition onto the web; and subsequently applying a second binder composition onto the web; wherein the second binder composition has a higher solids content than the first binder composition.

[0011] Various embodiments of the first aspect may comprise at least one feature from the following bulleted list:

• Said providing of a web comprises forming a fibrous slush on a wire to obtain a wet web.

• Between said applying of the first binder and said applying of the second binder, the solids content of the web is increased, for example by removing water by suction, by pressing and/or by heating.

The first binder composition is applied onto a web having a solids content in the range 4 to 45 wt-%, such as 8 to 15 wt-%, calculated from the total weight of the web.

• Said first binder composition has a solids content of less than 20 wt-%, for example 0.1 to 15 wt-%, such as 0.5 to 10 wt-%.

• The first binder composition comprises a binder that is at least partly in a particulate form. · The first binder composition comprises nano structured cellulose or microstructured cellulose, such as cellulose fibrils, preferably having an average fibre diameter smaller than 1 pm. • The first binder composition comprises a binder that is at least partly in the form of an aqueous dispersion.

• At least a part of the first binder composition is retained in a surface part of the wet web.

• At least a part of the first binder composition penetrates into the wet web, preferably to a depth of at least 10%, preferably at least 20%, such as at least 30% of the total thickness of the wet web.

• Said first binder composition comprises starch, preferably at least a part of said starch being in a gelatinized form, and optionally a dry strength resin.

• Said first binder composition comprises polyvinylamine.

• Said first binder composition comprises starch.

• The first binder composition comprises less than 0.5 g/m ² of fillers.

• Said applying of the first binder composition is carried out as a foamed composition, preferably by means of a foam applicator or a foam coater.

• Said applying of the first binder composition is carried out in a wire section of a paperboard manufacturing process.

• Said applying of the first binder composition comprises applying the first binder composition onto one side of the wet web and providing suction from an opposite side of the wet web to make the binder composition penetrate into the wet web.

• After applying a first binder composition on a surface of a web, said web is combined with other webs, one or more of which have been optionally also treated with a first binder composition, to form a multi-ply structure.

• Subsequently water is removed from the multi-ply structure, for example by providing suction from either side of the multi-ply structure.

• Said applying of the first binder composition is carried out as one or more separately applied doses.

• The method is for manufacturing of a multi-ply paperboard product. • Said applying of the first binder composition is carried out separately onto one or more wet webs, which are then combined with each other and/or with further wet webs to form a multi-ply board.

• Any application of a binder in a foamed form is carried out before said combining of the webs.

• Said second binder composition has a different composition from that of the first binder composition, for example by comprising a different binder or binders.

• Said second binder composition has a different composition from that of the first binder composition, for example by comprising a different solids content.

• Said second binder composition is applied in a non-foamed form.

• Said second binder composition has a higher solids content than the first binder composition.

• Said second binder composition has a solids content of at least 5 wt-%, such as at least 20 wt-%, for example at least 30 wt-%.

• Said second binder composition comprises a surface size.

• Said second binder composition comprises a cationic starch.

• The second binder composition comprises less than 0.5 g/m ² of fillers.

• The method is for manufacturing of a multi-ply paperboard product.

• The second binder composition is applied on a surface of a dried multi-ply board which comprises one or more plies treated with the first binder composition.

• Said applying of the second binder composition is carried out by means of a size press suitable for high-consistency surface sizing, such as a high-consistency metering size press.

• Said method is carried out in the course of a paper or paperboard manufacturing process. • Said applying of the first binder composition and said applying of the second binder composition are carried out in a first step and in a subsequent or later second step, respectively, of the paper or paperboard manufacturing process.

• Said paper or paperboard manufacturing process does not include any step in which starch is sprayed in a non-foamed form on the web.

[0012] According to a second aspect of the present invention, there is provided a paperboard product obtained by the method according to the first aspect.

[0013] Various embodiments of the second aspect may comprise at least one feature from the following bulleted list: · The tensile strength index of the paperboard product is at least 43 kNm/kg in machine direction and at least 22 kNm/kg in cross direction.

• The tensile stiffness index of the paperboard product is at least 5.5 kN/m in machine direction and at least 2.4 kN/m in cross direction.

The Scott bond of the paperboard product is at least 130 J/rrf.

• The bending stiffness index of the paperboard product is at least 0.34 Nm ⁷ /kg ³ in cross direction and at least 0.68 Nm7/kg ³ in machine direction as measured by Taber 15° ISO 2493 and Grammage ISO 536.

The paperboard product is folding boxboard.

The paperboard product is liner board. [0014] According to a third aspect of the present invention, there is provided use of a foam coater and a subsequent high-consistency metering size press for applying binder compositions to a web in a first step and in a subsequent second step, respectively, in a manufacturing process of paperboard.

[0015] Advantages of the invention

[0016] The present invention makes it possible to improve both inter-ply and intra- ply strength in multilayer paperboard products. [0017] The present invention enables more efficient optimization of binder properties.

[0018] The present invention may facilitate control of penetration depth of sizing agents. [0019] The present invention may increase the range of achievable surface binder penetration for product quality optimization.

[0020] The present invention makes it possible to reduce drying costs during paper and paperboard manufacturing.

[0021] In some embodiments, binder addition to the interface between adjacent plies in the wet-end of a paperboard manufacturing process increases internal bond strength and resistance to delamination during printing and converting of the paperboard product.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIGURE 1A shows a micrograph of a two-ply structure onto which a binder composition comprising starch was applied independently onto one ply, before combining the plies, in the wet end of a paperboard manufacturing process by means of a foam applicator.

[0023] FIGURE IB shows a micrograph of a control sample. In this case the foam composition was otherwise the same but did not contain any starch or other binders.

[0024] FIGURE 2A shows a micrograph of a three-ply structure onto which a binder composition comprising 15 wt-% starch was applied by means of a high consistency metering size press.

[0025] FIGURE 2B shows a micrograph of a three-ply structure onto which a binder composition comprising 25 wt-% starch was applied by means of a high consistency metering size press. [0026] FIGURE 2C shows a micrograph of a three-ply structure onto which a binder composition comprising 30 wt-% starch was applied by means of a high consistency metering size press. [0027] FIGURES 3A to 3C show micrographs of three-ply structures and illustrate the use of foam-applied binders of lower solids content between top and middle plies in combination with the application of a binder in higher solids concentration to the surface of the combined multi-ply structure.

EMBODIMENTS

[0028] DEFINITIONS

[0029] Unless otherwise stated herein or clear from the context, any percentages referred to herein are expressed as percent by weight based on a total weight of the respective composition. [0030] As used herein, the term “average particle size” refers to the number average particle size based on a largest linear dimension of the particles (also referred to as “diameter”) as determined using a technique known to those skilled in the art, such as Scanning Electron Microscopy, Transmission Electron Microscopy, and/or a Light Scattering technique. [0031] In the present context, the term “paperboard” is to be understood to designate a fibrous web which can be used as such as a board, typically having a grammage in the range indicated below, or which can be used as a part of a board or a converted board. The board or converted board can be uncoated or coated.

[0032] The present invention may be used in the manufacturing of paper or paperboard grades from 1-ply to multiply structures, thus comprising one or more plies, preferably 1 to 3 plies.

[0033] “Grammage” indicates how many grams one square meter weighs. The grammage of paperboard is typically from 90 to 600 g/m ² .

[0034] “Bulk” expresses the specific volume of a material. Bulk is the inverse of density.

[0035] In the present context, the term “filler” typically refers to any conventional filler or combination of fillers, such as precipitated calcium carbonate, kaolin, talc, ground calcium carbonate, kaolin clay, calcined clay, synthetic silicate, titanium dioxide, plastic pigment or any other mineral or organic filler or pigment. [0036] An improved method for applying a binder composition to a web in a manufacturing process of paper or paperboard has been discovered. In the method, advantageously, binder compositions are applied in at least two stages. A first binder composition is preferably applied to a wet web by means of a foam process in a wire section or like of a paper or paperboard manufacturing process. A second binder composition is preferably a high-consistency binder applied during a later stage, to a dried web, in the paper or paperboard manufacturing process.

[0037] In the present invention, the splitting of binder application to be performed in two separate steps and at two separate units results in unexpected advantages. The characteristics of each binder may be better optimized in view of the desired functionalities in the end product and in view of limitations imposed by and opportunities provided by the unit or process stage in which the binder application takes place.

[0038] In some embodiments, the amount of applied binder or surface sizing agent, such as starch, may be reduced in comparison to conventional methods. Additionally, the qualities of the binder that provide strength and stiffness may be maximized. For example, the added binder amount may be decreased without compromising quality. Also the need to dry the web after the sizing step may be reduced.

[0039] In some embodiments, it may be possible to increase the strength of paperboard by binder application in two stages. The first stage may strengthen the web more deeply. The second stage, being the high consistency surface sizing, may introduce binder more on the surface. Together these two stages may make it possible to use less water which has to be dried.

[0040] Some embodiments may alleviate the problem of lowered production due to the bottleneck of drying requirements. [0041] Some embodiments may enable increasing the running speed and production volume.

[0042] In some embodiments, application of functional agents such as binders in a foamed form in the wet-end of the paper or paperboard manufacturing process also enables incorporation of materials that improve resistance to delamination in the final product. [0043] We have observed that a combination of particulate and dispersed binders may be advantageous for the purpose of improving delamination resistance and strength, stiffness and other functional properties, such as sizing and wet strength.

[0044] A typical process of producing paperboard with a board machine line is described in the following. The process may be sub-divided into the steps of pulping, stock preparation, forming (wire section), wet pressing, drying, surface sizing, drying, pre calendering, coating, drying, calendering, and reeling.

[0045] Mechanical pulping is used to manufacture fibrous products such as printing and writing papers, paperboard, newsprint or tissue. Mechanical pulp provides high bulk and good opacity. Mechanical pulp can be combined with chemical pulps to produce a mixture of properties and characteristics. Chemical pulp is pulp which is prepared from cooked wood chips. Hybrid pulping methods, e.g. chemithermomechanical pulping (CTMP), use a combination of chemical and thermal treatment. Chemithermomechanical pulp is a preferred raw material in the process and products of the present invention, in particular less refined CTMP. In multilayered products, one or more layers may be of this CTMP, while other layers may be produced from other types of pulp.

[0046] The pulp raw material is processed to form a stock, a fibrous slush. Stock preparation systems modify the raw materials in such a way that the stock supplied to the board machine line suits the requirements of the board machine line. The quality of the stock supplied to the board machine line influences the quality of the paperboard. Various additives may be added to the stock.

[0047] The fibrous slush is conducted from one or several headbox(es) onto moving wire section(s). On the wire section the fibrous slush is drained typically with foils, foilboxes, vacuum boxes or vacuum rolls. As a result, a board web is obtained. After the wire section the dry content of the web is typically between 18 and 30 %, calculated from the total weight of the web.

[0048] From the wire section the web is conducted to the press section, where dewatering and consolidation is carried out in roll nips. The dry content is increased up to 40 to 50% after the press section.

[0049] After the press section, the web is directed to the drying section. In the drying section, typically hot cylinders are used for evaporating water from the web. [0050] After the drying section the board machine has a surface sizing unit, which is followed by after drying, coating and calendaring units, whereby the final surface of the paperboard is made.

[0051] The invention concerns the application of sizing agents or binders in the course of a paper or paperboard manufacturing process. By means of the invention it is possible to dispense with a conventional size press, such as a pond size press, which are adapted for application of low-consistency binders.

[0052] In a preferred embodiment, the present method comprises the following steps: providing a web comprising fibrous material; applying a first binder composition in the form of a foamed composition onto the web; and subsequently applying a second binder composition onto the web; wherein the second binder composition has a high solids content, preferably a higher solids content than the first binder composition, preferably such that the amount of water to be removed in the afterdrier is reduced. Advantageously, the first binder composition is applied in a forming section, and the second binder composition is applied in a surface sizing section of a paperboard machine.

[0053] In the present invention, the starting material for preparing said web comprising fibrous material may comprise any suitable lignocellulosic fibrous material, such as wood pulp, for example mechanical and/or chemical pulp. Various additives may be added to the pulp, such as binders, hydrophobic sizing agents, retention aids or combinations thereof.

[0054] In some embodiments, a first binder composition in a foamed form may be applied on a web in such a manufacturing stage in which the web has a solids content of not more than 45 wt-% and not less than 4 wt-%, more preferably in the range 6 and 30 wt- % and most effectively in the range 8 to 15 wt-%. [0055] The web-forming technology may be any suitable technology and may comprise any web-forming method in which a wet web is part of the process. By “wet web” it is referred to a web the solids content of which is not more than 50 wt-%.

[0056] In one embodiment, said forming is carried out by means of a Fourdrinier, a gap former or a hybrid former. [0057] In one embodiment, said forming is carried out by means of an air-laid or other non- woven forming technology.

[0058] Preferably, the first binder composition is applied in a forming stage, onto a wet web. The wet web may be located on a moving wire, and the first binder composition is applied from above the wire onto the exposed surface of the wet web.

[0059] In some embodiments, the end product is a multi-ply paperboard product. In its manufacturing process, the webs that are to generate the multiple plies of the end product are formed independently. At least one of the multiple webs may be treated with the first binder composition. Preferably, application of the first binder composition is carried out before combining the webs to form a multi-ply board. Preferably one ply where joints are made by combining webs has been treated with the first binder. After combining the treated web with other ply or plies the board is directed to pressing and pre-drying sections.

[0060] In one embodiment, the first binder composition comprises a binder that is at least partly in a particulate form. The binder composition may comprise particles having an average particle size of greater than 20 pm. An advantage of including a particulate binder is that the particles may be retained on the surface of the wet web onto which the binder composition is applied. As a result, the treated wet web is well adapted for being combined with other webs and to form a multi-ply board. The binder particles on the surfaces of the webs increase bonding between the webs and thus inter-ply bonding in the multi-ply product is improved.

[0061] Advantageously, the first binder composition is applied on at least one layer of a multilayer product before the layers are combined with each other, whereby inter-ply strength may be improved.

[0062] In one embodiment, at least a part of the first binder composition penetrates into the wet web, preferably to a depth of at least 10%, preferably at least 20%, such as at least 30% of the total thickness of the wet web.

[0063] In one embodiment, a substantial part, such as most of the first binder composition is retained in a surface part of the wet web, such as within the part nearest to the surface of the wet web forming at most 15%, for example at most 10%, such as at most 5% of the total thickness of the wet web. Said surface will become the ply interface after combining multiple webs with each other.

[0064] In one embodiment, the concentration profile of the first binder shows a gradient away from the surface such that the largest concentration is at the surface and the concentration decreases when moving away from the surface and deeper into the web.

[0065] By means of the present method, it may be possible to better control the concentration profile of the binder or binders within the ply or plies.

[0066] Advantageously, the first binder may eventually cross the ply interface and become distributed to a second ply as there is preferably a gradient of the first binder moving in the direction of the second ply. The binder may thus continue movement after combining the plies, and forces in pressing and drying drive liquid contained within the web toward the surfaces in both directions, carrying and depositing some of the binder with the movement.

[0067] In a preferred embodiment, the first binder is applied as a foam layer on one surface of a wet web. By using vacuum and/or pressure the foam and water content in the web can be moved, whereby the binder becomes re-distributed from the surface. An advantage of this process is that by controlling the method and direction of water removal, a control of the final distribution of the binder can be managed.

[0068] As a further advantage or an alternative advantage, the present method may use less water (via using higher solids content) requiring less drying energy in the second binder application step.

[0069] Advantageously, a first binder composition is applied on one or more of such surfaces of individual webs that are to face a surface of another ply in a multi-ply structure. In one embodiment, a first binder composition is applied on each of such surfaces of individual webs that are to face a surface of another ply in a multi-ply structure. Each surface to be treated may be treated with its respective first binder composition and amount. In this way it is possible to optimize the inter-ply bond in each interface between adjacent plies in a multi-ply structure.

[0070] The first binder composition may be any suitable binder composition as long as it can be applied in a wet end of a paper machine or a paperboard machine. [0071] In one embodiment, the solids content of the first binder composition, which is to be turned to a foamed form, is in the range 0.1 to 20 wt-%, such as 1 to 10 wt-%, for example 1 to 5 wt-% from the total weight of the composition.

[0072] In one embodiment, the solids content of the first binder composition, which is to be turned to a foamed form, is less than 5 wt-%.

[0073] An advantage of applying the first binder in a foamed form is that a wider range of binders and additives and solids contents can be more easily used in the composition to be foamed.

[0074] In one embodiment, the viscosity of the first binder composition before being foamed is in the range 1 to 1500 cP, for example 50 to 1500 cP.

[0075] The first binder composition may comprise natural starches (dissolved and/or undissolved/uncooked particles), modified starches (dissolved and/or undissolved/uncooked particles), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVOH) (dissolved and undissolved form), polyacrylamides (PAM), polyvinylamines (PVAm), polyimides, latex binders (such as SBR and acrylics combinations), synthetic starches and combinations thereof.

[0076] In addition to binders, the first binder composition may also comprise functional additives, such as internal sizing agents, wet strength resins, barrier materials and combinations thereof. [0077] In a preferred embodiment, the first binder composition comprises at least one binder that is capable of improving inter-ply binding strength.

[0078] In one embodiment, said first binder composition comprises one or more of the following: uncooked native starch, cooked native starch, uncooked cationic starch, cooked cationic starch, and a combination of cooked cationic starch and uncooked cationic starch.

[0079] In one embodiment, said first binder composition comprises a mixture of a dry strength resin, such as cationic polyvinylamine, and a natural binder, such as uncooked or cooked cationic starch. [0080] In one embodiment, said first binder composition comprises a polyvinylamine based polymer.

[0081] In one embodiment, said first binder composition comprises cationic starch.

[0082] In one embodiment, said first binder composition comprises a polyacrylamide based polymer.

[0083] In one embodiment, said first binder composition comprises a mixture of polyvinylamine and starch.

[0084] In one embodiment, said first binder composition comprises a latex binder based on styrene-acrylic copolymers. [0085] In one embodiment, the first binder composition comprises cellulose fibrils.

Cellulose fibrils may also be known as or referred to as nanocellulose, nano or microfibrillated cellulose. Cellulose fibrils have a narrow diameter < 1 pm, more typically 0.02 to 0.04 pm diameters, however, cellulose fibrils may be non-homogenous and may contain some material up to 5 pm in diameter. Cellulose fibril length also varies over a wide range from 0.1 to > 1000 pm, more typically the fibril length is 0.5 to 200 pm. Cellulose fibrils can be derived from a variety of sources, such as from wood pulp or from bacteria. Wood based raw materials are preferred.

[0086] Advantages of using cellulose fibrils in a first binder composition include high binding potential for intra and inter ply strength, improved stiffness, improved tensile strength and compression resistance. When applied in the first binder composition, reduction in drainage rate is avoided and there is less alignment in the machine running direction which improves the cross machine properties more so than if they were added to the furnish before forming the web.

[0087] In one embodiment, said first binder composition comprises starch, and preferably at least a part of said starch is in a gelatinized form. Said first binder composition preferably forms a particle- formed binder.

[0088] In a preferred embodiment, said first binder composition comprises a particle based binder and a suspended and/or dissolved polymeric binder in weight/weight ratios ranging from 4:1 to 1:4, preferably 2:1 to 1:2. The particle based binder may be selected from the group of uncooked starches, cellulose fibrils, undissolved PVOH and combinations thereof, preferably uncooked natural or modified starch.

[0089] The dissolved and/or suspended polymeric binder may be selected from the group of both natural and synthetically derived binders, for example comprising natural starches (dissolved and/or undissolved/uncooked particles), modified starches (dissolved and/or undissolved/uncooked particles), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVOH) (dissolved and undissolved form), polyacrylamides (PAM), polyvinylamines (PVAm), polyimides, latex binders (such as SBR and acrylics combinations), synthetic starches and combinations thereof, preferably a cationic polymer based on polyvinylamine.

[0090] The combination of a particle based binder and a suspended and/or dissolved polymeric binder provides the advantages of the particle based binder, which due to it’s size remains at or very near the ply zone providing inter-ply strength, and the advantages of the suspended and/or dissolved polymeric binder, which moves with the foam and liquid phase into the adjacent plies providing intra-ply strength to each ply around the ply zone.

[0091] In some embodiments, there are substantially no fillers included in the first binder composition.

[0092] In an embodiment, the amount of fillers (i.e. basis weight) in the first binder composition is less than 5 g/m ² , for example less than 3 g/m ² , for example less than 1 g/m ² , such as less than 0.5 g/m ² .

[0093] In the present context, the term “foam” may refer to a gas suspended or dispersed in a liquid medium. The gas component may be any gas or gas mixture which is capable of creating foam with a liquid medium. The gas may be for example air, nitrogen, oxygen, carbon dioxide, inert gases or similar or any combination or mixture thereof. The liquid medium may be any liquid medium as long as it allows a foamed composition to be generated and applied to a web and is compatible with eventual additives or functional agents desired. The liquid medium may be for example water, ethanol, organic solvents or similar or mixtures thereof. Examples of suitable foam components are given for example in US 4571360.

[0094] Characteristics of the foam, such as density, bubble size and stability (half- life), may be as described in US 4099913 and US 4023526. [0095] In one embodiment, the density of the foam is in the range 80 to 600 g/L, preferably 100 to 450 g/L, for example 125 to 200 g/L.

[0096] In one embodiment, the half-life of the foam is in the range 60 to 7200 s.

For example, the half-life of the foam may be at least 300 s, preferably at least 600 s. [0097] In one embodiment, the bubble size of the foam, expressed as the average diameter of the bubbles at atmospheric pressure, is in the range 10 to 500 pm, preferably 20 to 200 pm, more preferably 70 to 150 pm.

[0098] In preferred embodiments, applying of the first binder composition is carried out in a forming or wire section of a paperboard manufacturing process on a wet web. [0099] In one embodiment, the first binder composition is applied as a foamed composition to a wet web. Preferably, the wet web has a solids content of not more than 45 wt-% and not less than 4 wt-%, more preferably in the range 6 and 30 wt-%, and most effectively in the range 8 to 15 wt-%.

[00100] In one embodiment, the first binder composition is applied as a foamed composition to the web after the wet-dry line and before the couching point, which is the point where multiple separate plies are combined to form a multiply board.

[00101] The foam can be applied to all plies or to only some of the plies, for example to at least one ply of the end product, and effective delamination resistance may still be achieved. A dewatering direction sequence may be used in order to define the ply or plies in which the components of the binder composition are to be located and have impact to the properties of said ply or plies.

[00102] In a preferred embodiment, the first binder composition is in the form of a foamed composition.

[00103] The general steps of generating and applying a foam may be as described in US 4435965.

[00104] In some embodiments, said applying of the first binder composition is carried out by means of a foam applicator or a foam coater.

[00105] Suction and/or pressure may be used to control the distribution and confinement of the first binder composition after or during the step of applying the foam. [00106] Direction application may be effective in controlling the distribution of the first binder composition. By the expression “direction application” it is referred to a method in which, as a first step, a vacuum or alternatively pressure is applied to the web containing the applied foam in order to move the foam deeper away from the web surface. As a second step, a vacuum or alternatively pressure is applied to the web in order to move the foam in the opposite direction. An advantage of this method is that it may be ensured that the foamed composition becomes distributed across the entire thickness of the web, such as on both sides or surfaces of the web, in order to improve inter-ply bonding on both sides of the treated web or ply upon subsequent combination of said web with other webs.

[00107] In some embodiments, said applying of the first binder composition comprises applying the first binder composition onto one side of the wet web and providing suction from an opposite side of the wet web to make the binder composition penetrate deeper into the wet web.

[00108] In some embodiments, after applying a first binder composition on a surface of a wet web, said web is combined with other wet webs to form a multi-ply structure, and subsequently water is removed from the obtained multi-ply structure by providing suction from either side of the multi-ply structure. One or more of the webs to be combined may have been treated with a respective first binder composition on one or both sides of the web.

[00109] In one embodiment, said applying of the first binder composition is carried out as one or more separately applied doses. In each dose the applied amount may be different.

[00110] In some embodiments, the method is for manufacturing of a multi-ply paperboard product, and said applying of the first binder composition is carried out separately onto one or more wet webs, which are then combined with each other and/or with further wet webs to form a multi-ply board.

[00111] Advantageously, the method of the present invention comprises application of two separate binder compositions, a first and a second binder composition, in separate steps. Preferably, the compositions and/or consistencies of the first and the second binder compositions differ from each other. [00112] A function of the first binder composition may be that it provides binding strength at the ply- zone, thus making it possible to use a higher solids content second binder composition in a later stage without reduced product quality, such as peel strength.

[00113] In a preferred embodiment, the first binder composition is applied as a foamed composition, and the second binder composition is applied as a non-foamed composition, such as a suspension or a dispersion.

[00114] Preferably, the first binder composition is applied on a wire section and the second binder composition is applied after the wire section.

[00115] In some embodiments, both the first binder composition and the second binder composition comprise the same binder compound or compounds, such as starch, which may be cooked or uncooked starch. For example, the first binder composition may comprise starch, water and a foaming agent, and the second binder composition may comprise starch and water and be free from any foaming agents.

[00116] Preferably, the second binder composition is applied onto a web onto which the first binder composition has already been applied and which has been dried or from which water has been removed to obtain a web with a higher solids content.

[00117] Said second binder solution preferably comprises a surface size, and the application of the surface size typically produces a surface size layer on or into the web.

[00118] The surface size typically comprises a synthetic water-soluble polymer or a natural polymer or a derivative thereof.

[00119] The surface sizes applicable in some embodiments of the present invention may be divided into several groups, whereby the main division is between cationic and anionic surface sizes. In addition to or as an alternative to these, some reactive sizes, such as alkyl ketene dimer (AKD), may be used as a surface size. [00120] Cationic surface sizes include cationic starches and starch derivatives as well as corresponding carbohydrate-based natural polymers. Of synthetic polymers, for example styrene/acrylate copolymers (SA), polyvinyl alcohols, polyurethanes and alkylated urethanes may be used. [00121] Anionic surface sizes include anionic starches and starch derivatives and corresponding carbohydrate-based natural polymers such as carboxymethylcellulose and its salts, alkyl celluloses such as methyl cellulose and ethyl cellulose. Synthetic polymers include: styrene / maleic acid copolymer (SMA), diisobutylene / maleic anhydride, styrene acrylate copolymers, acrylonitrile / acrylate copolymers, and polyurethanes and similar latex products containing the same chemical functionalities.

[00122] Many of the above surface sizes are provided in the form of viscous solutions formed from the sodium or ammonium salts of the corresponding polycarboxylic acids.

[00123] The concentration of the surface size in the second binder solution is generally about 0.01 to 25% by weight, typically about 15% by weight.

[00124] The amount of surface size (i.e. basis weight) in the second binder composition is generally about 0.1 to 10 g/m ² , especially about 0.2 to 5 g/m ² , for example about 0.3 to 3 g/m ² .

[00125] The amount of surface size (i.e. basis weight) in the second binder composition may be at least 0.1 g/m ² , especially at least 0.3 g/m ² , for example at least 0.5 g/m ² .

[00126] In some embodiments, the second binder may be starch, which can be modified or unmodified starch, preferably starch derived from wheat, potato, rice, com or cassava. [00127] In one embodiment, the second binder composition comprises cooked starch and is preferably entirely free from uncooked starch.

[00128] In one embodiment, said second binder composition has a solids content of at least 5 wt-%, such as at least 10 wt-%, such as at least 20 wt-%, for example at least 30 wt-

%. [00129] In one embodiment, said second binder composition comprises or consists of a high-solids content binder composition, preferably having a solids content of at least 30 wt-%.

[00130] In some embodiments, there are substantially no fillers included in the second binder composition. [00131] In an embodiment, the amount of fillers (i.e. basis weight) in the second binder composition is less than 5 g/m ² , for example less than 3 g/m ² , for example less than 1 g/m ² , such as less than 0.5 g/m ² .

[00132] Preferably, the present method is for manufacturing of a multi-ply paperboard product such that the second binder composition is applied on at least one surface of a multi-ply board which comprises plies that at least one of which has been treated with the first binder composition.

[00133] Typically, the multi-ply board has been at least partly dried before applying the second binder composition. Such partial drying may be carried out by means of wet pressing and pre-drying stages.

[00134] Preferably, before or upon applying the second binder composition the dryness of the web is in the range 85 to 100%.

[00135] In some embodiments, said applying of the second binder composition is carried out by means of a size press suitable for high-consistency surface sizing, such as a high-consistency metering size press. Advantageously, a conventional size press, such as a pond press, is not used.

[00136] In some embodiments, said applying of the second binder composition is carried out by means of a metering size press, preferably a high-consistency metering size press. [00137] Preferably, after the application of the second binder composition the dryness of the web is maximized to reduce the energy needed to remove the water added with the surface sizing.

[00138] The present method may be carried out in the course of a paperboard manufacturing process. [00139] In one embodiment, except for the first and the second binder compositions no further binder compositions are applied to the web during said paperboard manufacturing process.

[00140] The present invention also concerns a multiply paperboard product obtained by the method according to the invention. [00141] In one embodiment, the tensile strength index of the paperboard product is at least 43 kNm/kg in machine direction and at least 22 kNm/kg in cross direction.

[00142] In one embodiment, the tensile stiffness index of the paperboard product is at least 5.5 kN/m in machine direction and at least 2.4 kN/m in cross direction. [00143] In one embodiment, the Scott bond of the paperboard product is at least 130

J/m ² .

[00144] In one embodiment, the bending stiffness index of the paperboard product is at least 0.34 Nm ⁷ /kg ³ in cross direction and at least 0.68 Nm ⁷ /kg ³ in machine direction as measured by Taber 15° ISO 2493 and Grammage ISO 536. [00145] In a preferred embodiment, the paperboard product is or comprises folding boxboard or liner board.

[00146] “Folding boxboard”, also referred to as FBB or by the DIN Standard 19303 codes of GC or UC, is a paperboard grade made up of multiple layers of chemical and mechanical pulp. Generally this grade is made up of mechanical pulp in between two layers of chemical pulp. The top layer is typically of bleached chemical pulp with an optional pigment coating.

[00147] “Liner board” typically refers to a strong stiff board that is used as the flat covering over a corrugating medium in corrugated fibreboard. Corrugated fibreboard is a material consisting of a fluted corrugated sheet (corrugated medium) and one or two flat liner boards.

[00148] In one embodiment, the grammage of the paperboard product is in the range 90 to 600 g/m ² , such as 90 to 450 g/m ² , for example 150 to 500 g/m ² .

[00149] In one embodiment, the paperboard product is coated or uncoated folding boxboard in which the pulp of the middle layer comprises or consists of mechanical pulp and the pulps of the top and the bottom layers comprise of consist of chemical pulp.

[00150] The mechanical pulp of the middle layer may be selected from the following group: ground wood (GW), pressure ground wood (PGW), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), bleached chemithermomechanical pulp (BCTMP), semichemical pulp, and combinations thereof, preferably BCTMP. [00151] The chemical pulp of the top and the bottom layers may be bleached chemical pulp, in particular bleached kraft pulp, and it may comprise softwood, such as spruce or pine or mixtures thereof, or hardwood, such as birch, poplar, aspen, alder, maple, eucalypt tropical hardwood, or mixtures thereof, or it may comprise or consists of a mixture of softwood and hardwood chemical pulp.

[00152] In one embodiment, a two-ply liner board is manufactured. Each of the two plies is formed independently. The print surface ply is formed by a Fourdrinier and the base ply is formed by a hybrid former. The two plies are couched together at the combining roll such that the top side of the surface ply meets the top side of the base ply. The foam can be applied to the top side of the base ply, preferably after the hybrid unit and before the combining roll. Alternatively, the foam can be applied to the top side of the surface ply, preferably after the wet/dry line and before the combining roll. Regardless of which ply is treated, the foam is applied before the combining point. Preferably at least one vacuum box is placed below the foam application point.

[00153] Turning now to the drawings, exemplary embodiments of the invention are described.

[00154] FIGURES 1A and IB illustrate use of starch as a first binder and how the first binder may be located in and directed to the interface between two plies. The samples have been stained with an iodine solution which colours the starch binder to have a dark blue to violet colour. The darkest area is at the ply interface where the starch was applied. The purple colour (the dark area) extending in both directions shows the distribution of the starch in the resultant paperboard.

[00155] FIGURE 1A shows a micrograph of a multi-ply structure according to an embodiment of the present invention. A binder composition comprising starch (first binder) was applied independently onto the top surface of the bottom ply, before combining the plies. The starch was applied in the wet-end of a paperboard manufacturing process by means of a foam applicator.

[00156] FIGURE IB shows a micrograph of a control sample. In this case the foam composition was otherwise the same but did not contain starch or any other binder.

[00157] It can be observed that in FIGURE 1A the majority of the applied starch remains at the ply interface. A minor part of the starch has penetrated deeper into both layers. The amount of starch is indicated by the intensity of colour (darkness). It can be observed that a gradient extending into the bottom to a depth of about 50% of the bottom ply thickness and further a gradient of binder exits extending into the top-ply nearly to the surface. [00158] FIGURES 2A, 2B and 2C illustrate use of starch as a binder having a high solids content (second binder) and how such a binder can be retained in a surface part of a multi-ply structure (three-ply paperboard). The solids content of the binder composition was varied from 15 wt-% to 30 wt-%.

[00159] FIGURE 2 A shows as a cross-sectional view a structure onto which a binder composition comprising 15 wt-% starch was applied.

[00160] FIGURE 2B shows as a cross-sectional view a structure onto which a binder composition comprising 25 wt-% starch was applied.

[00161] FIGURE 2C shows as a cross-sectional view a structure onto which a binder composition comprising 30 wt-% starch was applied. [00162] An advantage of the high solids content of the second binder is that the binder will not penetrate deeply into the structure, whereby the amount of binder and concurrently the amount of water that becomes applied may be reduced. Thus less drying may be needed and the drying energy costs may be decreased.

[00163] By means of applying a lower solids content binder composition in a wire section it is possible to achieve a good ply bond between adjacent plies. Therefore, there is no need for a subsequent, higher solids content binder composition to penetrate to the ply interface. Instead, the higher solids content binder composition may be located and have an effect only in a surface part of the structure, for example it may reduce dusting and increase abrasion resistance of the surface. [00164] FIGURES 3A to 3C serve to illustrate the use of foam-applied binders of lower solids content (first binder) between top and middle plies to achieve good ply-bond in combination with the application of a binder in higher solids concentration (second binder) to the surface of the combined multi-ply board where the amount of the second binder applied to the top surface has been reduced by 33 to 36% to reduce the penetration and reduce the water to be removed by drying. Generation of the samples pictured in FIGURES 3 A to 3 C is described in Example 1 below.

[00165] FIGURE 3A shows a cross-sectional view of a triple coated three ply paper board which has no first binder added. It illustrates a reference example of a typical three- ply folding box board. The cross-section has been stained with an iodine solution which shows starch as dark blue to violet and the intensity of the colour is relative to the concentration of the starch. Other components do not stain to blue to violet. Note that the dark blue colour near the top surface extends through the top-ply to the interface with the middle ply.

[00166] FIGURE 3B shows a cross-section view of a triple coated three-ply paper board of the same fibre and surface coating components as FIGURE 3A except that sprayed on particle starch has been excluded, dry strength resin DS#2 has been added via foam application in the wet end to the surface of the middle ply just prior to combining with the top ply, and the amount of surface starch has been reduced by 33%. By comparing the illustration to FIGURE 3A it can be observed that the surface starch does not penetrate as far into the structure.

[00167] FIGURE 3C shows a cross-section view of a triple coated three-ply paper board of the same fibre and surface coating components as FIGURE 3B except that the first binder composition as applied via foam in the wet-end includes dry strength resin DS#2 and starch. In comparison with the illustration in FIGURE 3B it is clear that the starch extends from the fibre surface through the top-ply and into the top-side of the middle ply. Further by comparing with the illustration in FIGURE 3A is can be seen that in FIGURE 3A starch stops penetrating abruptly near the top of the mid-ply; however, in FIGURE 3C the starch can be seen to extend in a gradient (colour intensity gradually reducing) into the middle ply to roughly 30% of the thickness of the middle-ply.

[00168] Example 1: Wet-end surface addition of strength agents and reduced surface sizing

[00169] Pilot scale wet end chemical application was installed on a typical 3-ply paper board machine.

[00170] Three plies are formed separately and then joined in the wet-state prior to wet-pressing. Surface plies (plies 1 and 3, or top and bottom plies respectively) are made using typical Fourdrinier technology. The middle ply (ply 2) is made with a hybrid system of a typical Fourdrinier and top dewatering unit placed after initial dewatering. Plies are combined such that the middle-ply upper surface makes a joint with the upper surface of the top ply with both forming fabric supporting the combined plies. Vacuum is used to draw the combined webs toward the top-ply forming fabric. The mid-ply forming fabric is guided away from the lower surface of the mid-ply. A second joint is made by bringing the exposed lower side of the mid-ply into contact with the upper side of the bottom ply. The three combined plies and supported on both sides by forming fabrics from top and back plies. As with the first joint, vacuum is applied to draw the combined web toward the bottom ply and the top-ply forming fabric is guided away exposing the lower or “wire” side of the top ply. The 3-ply combined web is transferred out of the forming section and into the pressing section using felts to pick-up the web from the bottom forming fabric and then supporting the bottom side with a second felt. Wet web solids content was measured to be in the range 15.0 to 15.3% just before transfer to the press-section.

[00171] Consolidation of the 3-ply wet web was accomplished by pressing through two nips while supported by felts and a third press unsupported. Consolidated web solids content was estimated to be in the range 45 to 48% leaving the press-section.

[00172] The consolidated web is then dried on steam heated drums to a water content in the range 8.5 to 11.0 wt-%. Dry drums are divided into 5 sections and steam load was monitored to determine if chemical application had any effect on drying demand.

[00173] After the fifth drying section, surface size is applied to both top and back surfaces via a metering size press. For this test, #4 rod is used to meter the surface size and enough is applied to ensure the surface size penetrates to both ply interfaces to achieve good resistance to delamination. Test points were made reducing the applied surface size on the top side by 1/3. The reduction was achieved by changing the rod size to #2.

[00174] The sized web is then dried to target of 9 wt-% water content using IR pre heater and steam heated drums. Again steam pressure was monitored to assess impact on drying capacity of the reduced size-press addition.

[00175] Upon drying the sized web was calendered and then triple coated on the top surface, calendered again and wound on to a jumbo reel. Samples of the finished paperboard were taken from the outside wraps of the jumbo reel to test key performance properties. Further, small rolls were made after slitting and rewinding, subsequently cut into sheets and then printed via 4-colour offset printing. Printed samples were evaluated for delamination defects.

[00176] Implementation of a step of applying a foamed binder solution is described in the following.

[00177] Chemical application was accomplished by applying a foamed solution of binder(s) (first binder(s)) to the upper surface of the mid-ply. Application point was after the top dewatering and before forming the joint with the top-ply and located over the middle of a vacuum box which was followed by another vacuum box prior to the combining point. The mid-ply web solids content before chemical application was in the range 12.2 to 12.5 wt-% and 13.0 to 13.5 wt-% at the combining point. Pilot foam application system consisted of a chemical feed system, foam generation device and a narrow applicator.

[00178] Binders, foaming agent and water were prepared and stored in separate containers. Each container was connected to a metering pump. Different chemical dopes were achieved by control of pumping rates from each container.

[00179] Foaming was accomplished in a rotor stator foam mixer. Chemical dope from the metering pumps was fed into the foam mixer along with pressurized air to develop a foam of target 150 g/L density. Foam generation variables were held constant during the test allowing for some small adjustments to foaming agent concentration and rotor speed.

[00180] Pressurized foam flows from the foam generator to the applicator mounted above the moving mid-ply wet web. Foam is ejected from a fixed gap slot at a fixed distance above the web such that a 65 g/m ² uniform layer of foam is applied to the moving web. [00181] Reference material incorporated 0.5 g/m ² uncooked starch sprayed onto the mid-ply web via mist showers just before the combining point between with the top-ply (foam applicator approximately 0.5 m upstream of the mist shower).

[00182] Foam applicator applied materials (DS#1, DS#2 and DS#3) consisted of three different binders for dry strength used singly or in combination: DS#1 Dry strength agent is cationic polyvinyl amine based polymer DS#2 Dry strength agent is polyacrylamide based polymer

DS#3 Cationic starch of medium substitution

[00183] The foamed binders were applied at 0.3 to 3 wt-% solids content.

[00184] Surface sizing (second binder) was a cationic starch of low substitution applied at 12.6 wt-% solids content.

[00185] Paperboard: 290 gsm triple coated top surface

[00186] Quality evaluation: Each test point was sampled in two locations along the same cross-direction strip taken from the outside wraps of the jumbo roll. The two positions were selected such that one was within the area where the pilot applicator applied chemicals via foam. The location was chosen to be near the center of the application area to avoid any effects of uneven application or response near the edges. The second point was chosen from an area outside the chemical applied area. Peel test (average perpendicular force needed to separate plies) was used to indicate resistance to delamination. Finally, sheets from inside the coated area were printed via offset 4-colour process and visually examined for delamination defects.

Table 1.

* TP8 sheets from within the chemical application area and from outside the chemical application area were printed.

** Steam pressure requirements for the first 5 sections was constant within normal variation throughout the test.

*** ’’Out” refers to the area of the web outside the foam applied chemistry region and “In” refers to the area within the foam applied chemistry region

[00187] The results in Table 1 show that application of binders that would not be practical with conventional wet-end surface dosing methods (like mist showers) can be achieved using the present foam application methods and still deliver good resistance to print de lamination without spray starch and with 1/3 reduction in surface sizing binder. Further, drying requirements after surface sizing were reduced by 25% even though an additional 60 g/m ² of water was added with the foam in the wet-end and water load reduction at the size press was only 7 g/m ² . As a result, either energy savings could be achieved or an increase in machine capacity for drying limited production is made possible.

[00188] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[00189] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment.

[00190] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[00191] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. [00192] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[00193] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or “an”, i.e. a singular form, throughout this document does not exclude a plurality.

INDUSTRIAF APPFICABIFITY

[00194] The method according to the present invention is applicable at least in the manufacturing of folding box board and liner board.

CITATION FIST

Patent Literature US4571360 US4099913 US4435965 US4023526

Non Patent Literature

Eklund, R. W. and Hobbs, O. L, High Speed Application of Foamed Starch “Foamcote”, to a paper web, Proceedings TAPPI Coating Conference, pp. 83 - 89, 1986.

Skelton, L, Foam assisted dewatering - a new technology emerges. PTI March 1987, pp. 431 - 436.

Kinnunen-Raudaskoski, K. et.al, Novel thin functional coatings for paper by foam coating, Tappi Journal, Vol. 16, No. 4, April 2017, pp. 179 - 186.