Method for manufacturing paperboard comprising a bulk ply

Abstract

A method for manufacturing paperboard in a paper-making machine, the method comprising the step of:forming a bulk ply by applying a pulp suspension comprising at least 50 wt% (based on the total dry weight of the pulp suspension) of high-bulk softwood pulp and 0.01 -10 wt% (based on the total dry weight of the pulp suspension) of a strength enhancement agent selected from the group consisting of highly refined cellulose, microfibrillated cellulose (MFC), a cationic strength additive, an anionic strength additive, or a combination thereof, from a headbox onto a moving wire to form a wet web, at least partially dewatering the wet web on the wire to obtain an at least partially dewatered web, and drying the at least partially dewatered web to obtain the bulk ply;wherein the wire is provided with a wire shaker which shakes the wire at frequency of 100-600 strokes per minute and a stroke length of 5-35 mm.

Description

The present invention relates to a method for manufacturing paperboard, particularly aimed at improving the quality of paperboard produced at high speeds in machines that use a high content of long and bulky softwood fibers, such as softwood chemi-thermomechanical pulp (SW CTMP), or softwood high temperature chemi-thermomechanical pulp (SW HT-CTMP).BackgroundIn paperboard manufacturing, particularly when producing paperboard with a high content of long and bulky softwood fibers, such as softwood chemithermomechanical pulp (SW CTMP), or softwood high temperature chemithermomechanical pulp (SW HT-CTMP), maintaining good formation and minimizing fiber flocculation becomes increasingly challenging.At high production speeds and fast drainage rates, the risk of uneven formation increases, leading to defects such as mottling and quality variations in coated or printed products. Furthermore, fibers often tend to align in the Machine Direction (MD), which negatively affects the mechanical properties of the paperboard, particularly the Cross-Direction / Machine Direction (CD / MD) strength ratio, and the dimensional stability of the final product.Several existing solutions, including headbox dilution systems, aim to improve fiber dispersion and reduce grammage variations. However, these methods fail to fully prevent re-flocculation of fibers on the wire, particularly at the critical early stages of web formation. As the pulp suspension is often highly dilute when applied to the wire, it allows fibers to move relatively freely, leading to fiber flocculation and poor retention of fines and chemicals such as strength additives.Although chemical additives, such as formation aids and strength additives, can mitigate some of these issues, they do not address the underlying problems of reflocculation and formation of micro-flocs.Mechanical solutions, such as wire shakers or breast roll shakers, have been introduced to enhance formation by applying a lateral shake to the wire, thereby inducing shear forces that help disperse fibers more evenly across the web. While effective at low speeds and lower grammages, wire shaking techniques have limitations when used in high-speed paperboard machines, as they can reduce the retention efficiency of fines and strength chemicals, leading to lower overall paperboard quality.From a technical and economical point of view, it would be preferable to find an improved method which allows for manufacture of paperboard with a high content of long and bulky softwood fibers, such as softwood chemi-thermomechanical pulp (SW CTMP), or softwood high temperature chemi-thermomechanical pulp (SW HT-CTMP), in a paper-making machine while maintaining good formation and minimizing fiber flocculation.Description of the inventionIt is an object of the present disclosure to provide a method for manufacturing paperboard in a paper-making machine using a pulp suspension comprising at least 50 wt% (based on the total dry weight of the pulp suspension) of high-bulk softwood pulp, which alleviates at least some of the above-mentioned problems associated with prior art methods.It is a further object of the present disclosure to provide a method for manufacturing paperboard in a paper-making machine using a pulp suspension comprising at least 50 wt% (based on the total dry weight of the pulp suspension) of high-bulk softwood pulp while maintaining good formation and minimizing fiber flocculation.The above-mentioned objects, as well as other objects as will be realized by the skilled person in the light of the present disclosure, are achieved by the various aspects of the present disclosure.The present invention provides an improved method for paperboard production that incorporates a wire shaking mechanism, such as a breast roll shaker, optimizing both formation and retention performance, even in high-speed paperboard machines with high content of bulky fibers. The invention addresses the shortcomings of existing methods by shaking the wire at specific frequencies and stroke lengths, effectively reducing fiber flocculation while maintaining retention efficiency, resulting in high-quality paperboard with enhanced mechanical properties.It was unexpectedly found that using a strength enhancement agent selected from the group consisting of highly refined cellulose (HRC), microfibrillated cellulose (MFC), a cationic strength additive, an anionic strength additive, or a combination thereof, in combination with the "coarse" high-bulk softwood pulp furnish, leads to less fines loss and improved strength enhancement agent retention when a wire shaking mechanism shakes the wire at specific frequencies and stroke lengths. Although not based on any specific theory, it is believed that the strength enhancement agent can interact more effectively during the forming process, preventing it from being lost in the drained water.According to a first aspect illustrated herein, there is provided a method for manufacturing a paperboard in a paper-making machine, the method comprising the step of:forming a bulk ply by applying a pulp suspension comprising at least 50 wt% (based on the total dry weight of the pulp suspension) of high-bulk softwood pulp and 0.01 -10 wt% (based on the total dry weight of the pulp suspension) of a strength enhancement agent selected from the group consisting of highly refined cellulose, microfibrillated cellulose (MFC), a cationic strength additive, an anionic strength additive, or a combination thereof, from a headbox onto a moving wire to form a wet web, at least partially dewatering the wet web on the wire to obtain an at least partially dewatered web, and drying the at least partially dewatered web to obtain the bulk ply;wherein the wire is provided with a wire shaker which shakes the wire at frequency of 100-600 strokes per minute and a stroke length of 5-35 mm.Paperboard generally refers to strong, thick paper or cardboard comprising cellulose fibers used for example as flat substrates, trays, boxes and / or other types of packaging. Paperboard can either be bleached or unbleached, coated or uncoated, and produced in a variety of thicknesses, depending on the end-use requirements.The inventive method is performed in a paper-making machine, more preferably in a Fourdrinier paper machine, optionally equipped with a hybrid former. A papermaking machine (or paper machine) is an industrial machine which is used in the pulp and paper industry to create paper or paperboard in large quantities at high speed. Modern paper-making machines are typically based on the principles of the Fourdrinier machine, which uses a moving woven mesh, a so-called "wire”, to create a continuous web by draining water from a fibrous material held in a pulp suspension and producing a continuously moving wet web of fiber. This wet web is then typically dried in the machine to produce a strong paper or paperboard web.The forming and dewatering steps of the inventive method are performed at the forming section of the paper machine, commonly called the wet end. The wet web is formed on the wire in the forming section of the paper machine.The pulp suspension is applied to the wire using a headbox. The function of the headbox is to dose and distribute the pulp suspension uniformly across the width of the wire. In the headbox, the pulp suspension pumped in a pipe is converted to a uniform flow with the same flow direction and essentially the same flow rate across the width of the wire.The headbox typically consists of a manifold distributor, flow stabilization elements and slice. The manifold distributor is a tapered header which converts the pipe flow into a rectangular flow through the slice opening with same velocity, quantity and thickness across the width of the wire.The wire is preferably an endless wire. The wire used in the inventive method preferably has relatively high porosity in order to allow fast dewatering of the pulp suspension.The wet web is partially dewatered on the wire. Dewatering of the web on the wire may be performed using methods and equipment known in the art, examples include but are not limited to table roll and foils (for example hydrofoils or vacufoils), suction boxes, friction less dewatering and ultra-sound assisted dewatering. Dewatering means that the dry solids content of the wet web is reduced compared to the dry solids content of the pulp suspension, but the dewatered web may still comprise a significant amount of water.The forming and dewatering steps of the inventive method may also be performed in a hybrid former. Hybrid formers combine the principles of both Fourdrinier and twin-wire forming technologies to enhance dewatering efficiency and sheet formation quality. These systems typically involve additional forming elements, such as an upper wire or belt, that apply pressure and assist in removing water from the wet web. Hybrid formers are particularly advantageous for improving fiber distribution, increasing dewatering rates, and achieving superior sheet uniformity. The method of the present disclosure, integrating bulky fibers with specific strength enhancement agents in conjunction with the application of shaking at a specific frequency and / or stroke length has been found useful to ensure an optimized drainage process. Specifically, too fast drainage can result in the premature immobilization of the wet fiber web, which inhibits adequate dewatering on the top side. This may be especially important in configurations such as hybrid formers or other systems employing two-sided dewatering, where effective water removal from the top side is desired for achieving optimal material properties.In some embodiments, the web leaving the wire is subsequently subjected to further dewatering of the web in a press section of the paper-making machine and drying of the web in a drying section of the paper-making machine.The optional further dewatering typically comprises pressing the web to squeeze out as much water as possible. The further dewatering may for example include passing the formed web between large rolls loaded under high pressure to squeeze out water. The removed water is received by a fabric or felt. The dewatering can be one-sided or two-sided. In some embodiments, the press section comprises at least one shoe press nip. In some embodiments, the press section comprises at least two shoe press nips.The drying may for example include drying the web by passing the web around a series of heated drying cylinders. Drying may typically reduce the water content in the web down to a level of about 2-18 wt%, preferably to about 3-13 wt%.In the method of the present disclosure, the wire is provided with a wire shaker which shakes the wire at frequency of 100-600 strokes per minute and a stroke length of 5-35 mm.The wire shaker operates by imparting a shaking or vibrating motion to the wire, also sometimes referred to as forming fabric, and to the wet web thereon during the initial stage of paper formation, when the fibers within the pulp suspension or wet web on the wire are in a dispersed state. In some embodiments, the wire shaker shakes the wire and the wet web thereon, when the consistency of the wet web is below 14 wt%, preferably below 12 wt%, and more preferably below 10 wt%. In some embodiments, the wire shaker shakes the wire and the wet web thereon, when the consistency of the wet web is even lower, such as below 8 wt% or below 6 wt%.The wire shaker generates an oscillating or vibrating motion primarily in the crossdirection (CD) of the machine, which is defined as the direction perpendicular to the machine direction (MD) in which the paper web travels. The oscillation may take various forms, including sinusoidal, reciprocating, or random, depending upon the specific configuration of the wire shaker and the intended effect on the fiber distribution.The shaking or vibrating motion serves the purpose of redistributing the fibers within the slurry in a more uniform manner, thereby reducing the occurrence of fiber clumps or "floes" and ensuring a consistent distribution of fibers across the width of the sheet. In addition, the shaking motion helps to ensure even drainage of water from the slurry, contributing to the formation of a uniform fiber mat on the wire.The wire shaker is typically equipped with a mechanical assembly, such as an eccentric shaft or cam mechanism, configured to generate the oscillating motion. In some embodiments, the shaking motion may be produced by a motorized mechanism. The shaking motion may be transmitted to the wire through one or more shaker arms or a similar linkage system, which directly engages with the frame supporting the wire and imparts the desired motion.In some embodiments, the wire shaker may take the form of a mechanical shaker incorporating cams and gears for generating the oscillating motion. In other embodiments, the wire shaker may utilize electronic or vibratory systems, including but not limited to vibratory motors or electronic control mechanisms, to achieve more precise control over the frequency and amplitude of the shaking motion.Examples of commercial wire shakers useful in the inventive method include, but are not limited to, the DuoShake 800 DG (Voith GmbH), the Breast Roll Shaker 120 (Valmet), and the Turboshaker (Bellmer). The breast roll is preferably, but not limited to, made of or comprising composite materials, such as a carbon fiber polymer composite, for example CarboForm from Voith.The wire shaker, as described, contributes to the improved formation of the paper web by promoting uniform fiber distribution, consistent web thickness, and enhanced or more homogenous water drainage. These improvements result in higher quality of the finished paper product, reduced incidence of sheet breakage, and improved printability and smoothness of the final sheet.The frequency and stroke length, or amplitude, of the shaking motion may be adjusted according to the type of paper being produced. The stroke length of the oscillation is controlled such that it remains within a range sufficient to redistribute fibers effectively without significantly disrupting the overall flow of the slurry on the wire.The present inventors have found that shaking the wire at a specific frequency and stroke length range is especially advantageous for pulp suspensions comprising at least 50 wt% (based on the total dry weight of the pulp suspension) of high-bulk softwood pulp. More specifically, the wire shaker should shake the wire at frequency of 100-600 strokes per minute and a stroke length of 5-35 mm.In some embodiments, the wire shaker shakes the wire at a frequency of 125-550 strokes per minute, preferably at a frequency of 150-500 strokes per minute.In some embodiments, the stroke length of the wire shaker is 8-32 mm, and more preferably 12-30 mm.In some embodiments, the wire is provided with a wire shaker which shakes the wire at frequency of 125-550 strokes per minute and a stroke length of 8-32 mm. In some embodiments, the wire is provided with a wire shaker which shakes the wire at frequency of 150-500 strokes per minute and a stroke length of 12-30 mm.In some embodiments, the wire shaker shakes the wire in the cross-direction (CD) of the paper-making machine, i.e. in a direction transverse to the direction of web travel.At high machine speeds, the water must be drained off faster, which may lead to poor retention, negatively affecting paperboard quality. The present invention allows for high machine speeds while still achieving the benefits of the wire shaker and good retention, particularly of the strength enhancement agent. In some embodiments, the machine speed is at least 450 m / min, preferably at least 550 m / min, and more preferably at least 650 m / min. In some embodiments, the machine speed even higher, such as at least 700 m / min, at least 750 m / min, or at least 800 m / min. In some embodiments the machine speed can be up to 1200 m / min, or up to 1300 m / min, or up to 1400 m / min.The use of a wire shaker described herein has been found particularly useful for paper-making machines with high machine widths. In wide machines, ensuring even distribution of fibers across the full width of the forming fabric becomes more difficult. Without effective shaking, fibers may tend to form clumps or accumulate unevenly, especially at the edges or in certain areas of the web. The wire shaker’s oscillating motion in the cross-direction (perpendicular to the machine's running direction) helps redistribute fibers uniformly across the entire width, preventing the formation of "floes" and ensuring a consistent fiber mat. On wider machines, effective water drainage is more challenging because water must drain evenly across the entire surface of the wire. A wire shaker enhances the drainage process by vibrating the wire, which helps release water more evenly across the wide surface. In some embodiments, the machine width is at least 5 meters, preferably at least 6 meters, such as in the range of 6-11 meters.Wide machines also often operate at high speeds, and the challenges of uniform fiber distribution and drainage are magnified under these conditions. A wire shaker enables the machine to handle both high speed and large width without sacrificing quality. By ensuring even distribution and drainage, the shaker helps maintain good formation and retention even at high speeds, where disturbances could otherwise lead to uneven fiber deposition.In some embodiments, the headbox consistency of the pulp suspension, i.e. the consistency of the pulp suspension when it is applied to the wire, is 1.0 wt% or less, preferably in the range of 0.1-1.0 wt%, and more preferably in the range of 0.4-1.0 or in the range of 0.45-0.9 wt%. In some embodiments the pulp suspension is diluted to the headbox consistency of 1.0 wt% or less in the headbox.In some embodiments, headbox SR value of the pulp suspension, i.e. the SR value of the pulp suspension when it is applied to the wire, is in the range of 14-35, preferably in the range of 16-33, and more preferably in the range of 18-30, as determined by standard ISO 5267-1.The pulp suspension of the method disclosed herein comprises at least 50 wt% (based on the total dry weight of the pulp suspension) of high-bulk softwood (SW) pulp. The high-bulk softwood pulp may preferably be obtained from spruce, pine, or a combination thereof. In some embodiments the high-bulk softwood pulp is obtained from >70% spruce and 0-30% pine. The high-bulk softwood pulp contributes to the structural characteristics of the resultant paper product, including but not limited to increased bulk, improved stiffness, and enhanced porosity. The specified proportion of high-bulk softwood pulp ensures that these beneficial properties are present in the final product. The high-bulk softwood pulp can be bleached or unbleached.In some embodiments, the pulp suspension comprises at least 60 wt% (based on the total dry weight of the pulp suspension), and more preferably at least 70 wt% (based on the total dry weight of the pulp suspension), of the high-bulk softwood pulp. The higher proportions of high-bulk softwood pulp further enhance the bulk and stiffness of the paper product, making it particularly suitable for applications requiring high resilience and thickness. By increasing the proportion of high-bulk softwood pulp beyond 50 wt%, improvements in formation, structural integrity, and overall strength of the paper product can be realized, while also maintaining costeffectiveness and efficient processability. The higher content, specifically at 70 wt%, is particularly advantageous in the production of paperboard grades where superior bulk and rigidity are required.In some embodiments, the high-bulk softwood pulp is a high-yield softwood pulp. The term "high-yield" refers to the fact that a large portion of the wood (usually over 85%) is converted into pulp during the production process. High-yield pulps are typically characterized by their high content of lignin and hemicellulose. The use of high-yield softwood pulp provides multiple advantages, including increased bulk and lower cost per unit mass of pulp, as well as reduced consumption of raw wood material. Additionally, the higher yield enhances the fiber retention properties of the resulting paper product, contributing to improved overall sheet formation, thickness, and strength characteristics, while also maintaining environmental sustainability.In some embodiments, the high-bulk softwood pulp is softwood (SW) chemithermomechanical pulp (SW CTMP) or softwood high-temperature chemithermomechanical pulp (SW HT-CTMP), or a combination thereof. SW CTMP and SW HT-CTMP are mechanical pulps that combine chemical pre-treatment with thermomechanical refining to produce fibers that have high bulk and good bonding strength. These pulps are particularly advantageous in applications where high bulk is required without compromising paper strength. SW HT-CTMP, which has undergone high-temperature treatment, further enhances the fiber properties by improving bulk and reducing fines content. The use of SW CTMP, SW HT-CTMP, or a combination thereof, in the disclosed method, provides the ability to customize the characteristics of the paperboard according to specific performance requirements.Softwood chemi-thermomechanical pulp (SW CTMP) is a high-bulk mechanical pulp produced by subjecting softwood fibers to a chemical pre-treatment followed by thermomechanical refining. In the SW CTMP process, softwood chips are first impregnated with a chemical solution, typically consisting of sodium sulfite or similar compounds, which partially softens the lignin in the wood fibers. Following this chemical pre-treatment, the softened wood chips undergo mechanical refining at elevated temperatures, which mechanically separates the fibers while maintaining a high degree of fiber integrity. The resulting pulp retains much of the lignin content, providing high bulk and stiffness properties, while the chemical pretreatment enhances fiber bonding and strength. SW CTMP is a high-yield pulp, typically prepared with a yield of at least 85%, such as at least 88%, at least 90%, or at least 91%.Softwood high-temperature chemi-thermomechanical pulp (SW HT-CTMP) is a variant of SW CTMP wherein the refining process is conducted at higher temperatures, typically in the range of 150-200 °C. The higher temperature during refining allows for more effective defiberization and fibrillation, resulting in improved bulk and reduced fines generation. The increased temperature also further softens the lignin in the softwood fibers, enhancing the flexibility and strength of the fibers without compromising the bulk properties. The higher temperature also allows for the SW HT-CTMP to be made with lower chemical dosing or faster chemical impregnation than SW CTMP. SW HT-CTMP is particularly advantageous for producing paper products that demand both high bulk and improved strength characteristics.In some embodiments, the high-bulk softwood pulp has a Canadian standard Freeness (CSF) value in the range of 300-750, 350-750, 350-600 ml. The CSF value is a measure of the drainage rate of a pulp suspension and is indicative of the ability of the fibers to retain water. A higher CSF value corresponds to coarser fibers with greater bulk and faster drainage rates, which are desirable in certain paper or paperboard grades where rapid water removal and increased sheet bulk are required. The disclosed freeness range allows the high-bulk softwood pulp to provide a balance between drainage efficiency and fiber bonding strength. By selecting the high-bulk softwood pulp with a specific freeness value within these ranges, the method ensures that the pulp retains sufficient water for fiber bonding during sheet formation while enabling efficient water removal, contributing to both process efficiency and the final product's structural properties.In some embodiments, the high-bulk softwood pulp, or at least a part thereof, is never dried pulp.In some embodiments, the high-bulk softwood pulp has a bulk value of at least 2.9 cm3 / g, preferably at least 3.0 cm3 / g, and more preferably at least 3.2 cm3 / g, such as in the range of 3.5-5.5 cm3 / g, as determined in accordance with ISO 534:2005 (50 kPa), for a pulp having a Canadian Standard Freeness of approximately 550 ml, based on the sheet preparation method specified in ISO 5269-1:2005.In some embodiments, the high-bulk softwood pulp has a fiber length Lc(l) of above 1.2 mm, preferably above 1.3 mm, and more preferably above 1.4 mm as determined according to ISO 16065. In embodiments, the high-bulk softwood pulp has a fiber length Lc(l) in the range of 1.2 - 3.5 mm, or 1.3 - 3.3 mm or 1 .4 - 3.3 mm. The fiber length Lc(l) can for example be measured with a Valmet FS5 fiber analyzer.In some embodiments, the high-bulk softwood pulp has a fines mass fraction of less than 10 %, more preferably less than 8 %, and most preferably less than 4 %, as determined in accordance with ISO 10376:2011 .In some embodiments, the high-bulk softwood pulp contains extractives in an amount less than 0.4%, and more preferably less than 0.35%, as measured by the SCAN-CM 49:03 standard (acetone extraction).In some embodiments, the high-bulk softwood pulp has a shives content (Sommerville 0.15 mm) of less than 0.4%, as determined in accordance with TAPPI T 275-18.In some embodiments, the high-bulk softwood pulp exhibits an ISO Brightness of greater than 50 %, preferably greater than 52 %, as measured in accordance with ISO 2470-1:2009.In some embodiments, the high-bulk softwood pulp has a tensile index of at least 18 Nm / g, and more preferably at least 20 Nm / g, as determined in accordance with ISO 1924-3:2011 , for a pulp having a Canadian Standard Freeness of approximately 550 ml, with sheets prepared according to the method outlined in ISO 5269-1:2005.In some embodiments, the pulp suspension further comprises 1-40 wt% (based on the total dry weight of the pulp suspension) of a hardwood chemithermomechanical pulp (FIW CTMP) or hardwood high-temperature chemithermomechanical pulp (HW HT-CTMP), or a combination thereof. The inclusion of HW CTMP or HW HT-CTMP enhances the smoothness and formation properties of the bulk ply or multi-ply paperboard. Hardwood fibers are shorter and finer than softwood fibers, providing improved sheet formation and surface quality, which is beneficial for applications requiring fine printing or coating.In some embodiments, the pulp suspension further comprises 1-20 wt% (based on the total dry weight of the pulp suspension) of softwood kraft pulp having a kappa number according to ISO 302:205 of at least 40, preferably at least 60, or at least 80. The addition of softwood kraft pulp, especially with a higher kappa number, enhances the mechanical strength and durability of the paperboard. Softwood kraft pulp helps to provide tear resistance and tensile strength to the bulk ply. The use of kraft pulp with a kappa number of at least 40, preferably higher, ensures that the pulp suspension retains its bulk while providing significant strength improvements.In some embodiments, the pulp suspension further comprises 1-40 wt% (based on the total dry weight of the pulp suspension) of broke pulp. In some embodiments, the pulp suspension comprises 1-35 wt%, such as in the range of 2-30 wt%, (based on the total dry weight of the pulp suspension) of broke pulp. Broke pulp refers to recycled or off-spec paper material from the papermaking process, which is incorporated into the pulp suspension to improve resource efficiency and reduce material waste. The broke may for example be disintegrated, screened, and refined to an SR value in the range of 18-40, more preferably 20-35, and most preferably 22-32. All Schopper Riegler (SR) values mentioned in the present disclosure are determined according to standard ISO 5267-1 . The inclusion of up to 40 wt% broke pulp offers economic and environmental benefits by reducing raw material consumption and promoting the sustainability of the papermaking process.In addition to the high-bulk softwood pulp, the pulp suspension disclosed herein further comprises 0.01 to 10 wt% (based on the total dry weight of the pulp suspension) of a strength enhancement agent. In some embodiments, the pulp suspension comprises 0.1 wt% to 10 wt%, or 0.5 wt% to 10 wt%, or 1 wt% to 10 wt%, (based on the total dry weight of the pulp suspension) of the strength enhancement agent. In some embodiments, the pulp suspension comprises 0.5 wt% to 10 wt% (based on the total dry weight of the pulp suspension) of the strength enhancement agent.The strength enhancement agent is selected from the group consisting of highly refined cellulose (HRC), microfibrillated cellulose (MFC), a cationic strength additive, an anionic strength additive, or a combination thereof. The strength enhancement agent is preferably plant based. The inclusion of the strength enhancement agent in the pulp suspension is designed to improve the mechanical properties of the final paper product, including tensile strength, compression strength, burst strength, and internal bonding strength, without significantly compromising the bulk and formation properties conferred by the high-bulk softwood pulp.The concentration of the strength enhancement agent within the range of 0.01 to 10 wt% is specifically chosen to achieve an optimal balance between enhancing the mechanical performance of the paper and maintaining efficient processability. Lower concentrations, such as 0.01 wt%, provide a measurable improvement in paper strength, particularly in grades requiring minimal strength enhancement, while higher concentrations, up to 10 wt%, offer significant reinforcement of fiber bonding and sheet integrity in more demanding applications.In some embodiments, the strength enhancement agent comprises highly refined cellulose. Highly refined cellulose (HRC), through mechanical or chemical processing, has a fine, fibrillated structure that allows it to enhance fiber bonding within the paper web. It improves the tensile and tear strength of the sheet by increasing the surface area available for hydrogen bonding between fibers, while also maintaining the bulk and porosity benefits of the high-bulk softwood pulp. Highly refined cellulose typically has an SR value in the range of 50-84 as determined by standard ISO 5267-1. This strength enhancement agent is particularly beneficial in paper products where strength is a priority but excessive densification should be avoided. The highly refined cellulose can be produced from wood cellulose fibers, both from hardwood and softwood fibers or a combination thereof. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It is preferably made from wood pulp including pulp from virgin fiber, e.g. mechanical, chemical and / or thermomechanical pulps. It can also be made from broke or recycled paper.In some embodiments, the pulp suspension comprises 0.1 wt% to 10 wt%, preferably 0.5 wt% to 10 wt% (based on the total dry weight of the pulp suspension) of highly refined cellulose. In some embodiments, the highly refined cellulose has an SR value in the range of 50-84, preferably in the range of 55-84, or in the range of 60-84, as determined by standard ISO 5267-1.In some embodiments, the strength enhancement agent comprises microfibrillated cellulose (MFC). MFC consists of cellulose fibers that have been mechanically or chemically treated to produce a network of nanoscale fibrils, which significantly enhance fiber bonding and the mechanical properties of the paper. The addition of MFC at the disclosed levels improves the strength of the paper by promoting fiber entanglement and increasing the fiber-to-fiber bonding surface area, resulting in a paper product with high tensile, burst, and tear strength. MFC shall in the context of the patent application mean a cellulose particle, fiber or fibril having a width or diameter of from 20 nm to 1000 nm. Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment steps is usually required in order to make MFC manufacturing both energy efficient and sustainable. The cellulose fibers of the pulp used when producing MFC may thus be native or pre-treated enzymatically or chemically, for example to reduce the quantity of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, wherein the cellulose molecules contain functional groups other (or more) than found in the original cellulose. Such groups include, among others, carboxymethyl (CM), aldehyde and / or carboxyl groups (cellulose obtained by N-oxyl mediated oxidation, for example "TEMPO"), or quaternary ammonium (cationic cellulose). After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC. MFC can be produced from wood cellulose fibers, both from hardwood and softwood fibers. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It is preferably made from wood pulp including pulp from virgin fiber, e.g. mechanical, chemical and / or thermomechanical pulps. It can also be made from broke or recycled paper or packaging board. In some embodiments, the pulp suspension comprises 0.1 wt% to 10 wt%, preferably 0.5 wt% to 10 wt% (based on the total dry weight of the pulp suspension) of microfibrillated cellulose (MFC). In some embodiments, the microfibrillated cellulose (MFC) has an SR value in the range of 85-100, preferably in the range of 90-100, or in the range of 90-98, as determined by standard ISO 5267-1 .In some embodiments, the strength enhancement agent comprises a cationic strength additive. Cationic strength additives, which are positively charged, are effective at improving paper strength by enhancing the electrostatic attraction between fibers, particularly when combined with anionic fibers or fillers. These additives form strong ionic bonds within the fiber network, which increases the internal bonding strength of the paper sheet. Cationic strength additives are especially useful in paper grades that require high dry and / or wet strength or resistance to mechanical stress, as they improve fiber retention and bonding even in the presence of high moisture content. In some embodiments, the pulp suspension comprises 0.01-5 wt%, preferably 0.1-3 wt%, and more preferably 0.1-2 wt% (based on the total dry weight of the pulp suspension) of a cationic strength additive, preferably a cationic polymer, more preferably a cationic polysaccharide, such as a cationic starch.In other embodiments, the strength enhancement agent comprises an anionic strength additive. Anionic strength additives, which are negatively charged, interact with cationic components within the pulp to promote strong fiber bonding. These additives are particularly advantageous in neutral or alkaline papermaking processes, where they improve the tensile and tear strength of the paper. Anionic strength additives further contribute to the overall strength and consistency of the final paper product, and also improve fiber formation on the wire. In some embodiments, the pulp suspension comprises 0.01-5 wt%, preferably 0.1-3 wt%, and more preferably 0.1-2 wt% (based on the total dry weight of the pulp suspension) of an anionic strength additive, preferably selected from the group consisting of anionic nanocrystalline cellulose, anionic microfibrillated cellulose (MFC), anionic carboxymethyl cellulose (CMC), anionic starch, alginate, hemicellulose, and combinations thereof. Anionic MFC may for example be oxidized or carboxylated MFC. The anionic CMC, for example sodium carboxymethyl cellulose, may typically have a molecular weight above 50000 g / mol, such as above 100 000 g / mol, and a degree of substitution above 0.2, such as in the range of 0.3-1.5.In some embodiments, the strength enhancement agent comprises a combination of two or more strength enhancement agents selected from highly refined cellulose, microfibrillated cellulose, cationic strength additives, and anionic strength additives. The combination of different strength enhancement agents allows for synergistic improvements in paper strength while maintaining the desirable bulk, porosity, and formation characteristics imparted by the high-bulk softwood pulp. For example, the use of both cationic and anionic additives can optimize fiber retention and bonding, while the addition of highly refined cellulose or MFC enhances tensile and tear strength without significantly reducing bulk. In some embodiments, the strength enhancement agent comprises a combination of a cationic strength additive and an anionic strength additive. In some embodiments, the strength enhancement agent comprises a combination of a cationic polymer, more preferably a cationic polysaccharide, such as a cationic starch and an anionic strength additive selected from the group consisting of anionic nanocrystalline cellulose, anionic microfibrillated cellulose (MFC), anionic carboxymethyl cellulose (CMC), anionic starch, alginate, hemicellulose, and combinations thereof. The cationic starch may for example be a high molecular weight starch made from potato or a crosslinked or branched starch comprising a high content of amylopectin. The starch may also be an amphoteric starch.The addition of 0.01 to 10 wt% of a strength enhancement agent ensures that the pulp suspension retains high performance across a variety of paper grades and applications. The disclosed range allows for flexibility in adjusting the amount of strength enhancement agent based on the desired properties of the final product. Lower concentrations may for example be suitable for applications where bulk and softness are prioritized, while higher concentrations may for example be applicable for high-strength paper grades such as packaging, specialty papers, or highperformance board.In some embodiments, the temperature of the pulp suspension when it leaves the headbox is in the range of 35-85 °C, preferably in the range of 40-65 °C. These temperature ranges improve the retention of strength enhancement agents and promotes faster drainage, which is advantageous for high-speed, high-width paper machines. It ensures optimal processing conditions for high-bulk softwood pulp, particularly SW CTMP and SW HT-CTMP, and facilitates dewatering and drying..The inventive method is especially useful when combined with two-sided dewatering of the wet web. For example using a twin-wire former or a hybrid former.In some embodiments, the wet web is subjected to two-sided dewatering on the wire. Two-sided dewatering process involves the simultaneous removal of water from both the top and bottom surfaces of the pulp suspension as it forms into a paper web on the wire or forming section of the paper machine. In this process, dewatering elements, such as vacuum boxes, foils, suction rolls or suction boxes, shoes or sleeves, may be positioned on one or both sides of the forming fabric to extract water from the pulp suspension in a controlled manner. This differs from traditional one-sided dewatering, where water is typically removed only through the bottom side of the wire. Two-sided dewatering is advantageous for the pulp suspension of the present disclosure, as it promotes uniform fiber distribution, more even distribution and retention of strength additives, improved bulk and formation consistency, and increased drainage efficiency.Two-sided dewatering ensures even water removal from both sides, leading to more uniform fiber distribution and sheet formation. By evenly dewatering, strength enhancement agents like microfibrillated cellulose and cationic / anionic strength additives are better distributed and retained in the fiber matrix, improving the paper’s tensile strength and bonding. The balanced water removal prevents fiber compaction, preserving the high bulk properties of the softwood pulp and maintaining the desired sheet thickness and structure.In some embodiments, the wet web is formed by hybrid forming. Hybrid formers combine the principles of both one-sided and two-sided dewatering technologies to enhance dewatering efficiency and sheet formation quality. Thus, in some embodiments, the wet web is first subjected to one-sides dewatering and then subjected to two-sided dewatering.An advantage of using a wire shaker for bulky fibers in accordance with the present disclosure, is that the waterline can be adjusted and better controlled. Two-sided dewatering is preferably applied before the waterline in order to enable better formation and distribution of the components in the thickness direction of the web. In some embodiments, two-sided dewatering is preferably applied when the consistency of the wet web is below 10 wt%, preferably below 8 wt%, and more preferably below 6 wt%.In some embodiments, the bulk ply has a density of less than 500 kg / m3, preferably less than 400 kg / m3 as measured according to standard ISO 534. In some embodiments, the bulk ply may have a density in the range of 200-500 kg / m3 or in the range of 200-400 kg / m3, as measured according to standard ISO 534In some embodiments, the bulk ply has a grammage in the range of 50-350 gsm, preferably in the range of 50-250 gsm, as measured according to standard ISO 536. In various embodiments, the bulk ply may have a grammage in the range of 50-300 gsm, or in the range of 50-250 gsm, or in the range of 60-175 gsm, as measured according to standard ISO 536.The method of the present disclosure leads to a bulk ply having a low specific formation value. In some embodiments, the bulk ply has a specific formation value of less than 0.8 gΛ0.5 / m, preferably less than 0.7 gΛ0.5 / m, and more preferably less than 0.6 gΛ0.5 / m, as measured according to SCAN-P 92:09. A lower specific formation value indicates a more uniform distribution of fibers across the paperboard, leading to improved formation, smoother surfaces, and enhanced strength.The pulp suspension discussed herein can be used for forming a bulk ply of a multi-ply paperboard, where the bulk ply contributes to the overall thickness, rigidity, and mechanical strength of the paperboard structure. The high-bulk softwood pulp, comprising at least 50 wt% of the pulp suspension, along with the 0.01-10 wt% of strength enhancement agents, enables the formation of a robust and voluminous bulk ply that provides several key advantages for the final paperboard product.The bulk ply as disclosed herein can either be used as a standalone paperboard or as a bulk ply in a multi-ply paperboard. The high-bulk softwood pulp and strength enhancement agents allow for the formation of a robust bulk ply that imparts thickness, rigidity, and strength to the final product.In some embodiments, the bulk ply is formed together with one or more additional plies to form a multi-ply paperboard. This configuration enhances the overall strength, stiffness, and functionality of the paperboard by combining layers with distinct properties. The bulk ply provides the necessary thickness and structural support, while the additional plies contribute to for example surface quality and printability.The bulk ply formed from the pulp suspension disclosed herein can be formed together with one or more additional plies to form a multi-ply paperboard using various methods, including but not limited to couching.In some embodiments, the bulk ply is formed together with one or more additional plies by a couching process. Couching involves bringing the bulk ply into contact with the one or more additional plies while they are still in a wet state. The plies are pressed together under controlled pressure, causing the fibers at the interfaces of the plies to interlock and bond, creating a multi-ply structure. This method allows for efficient lamination of the plies without the need for adhesives, resulting in a cohesive, high-strength paperboard.In some embodiments, the bulk ply is formed together with a top ply and a back ply to form a middle ply in a multi-ply paperboard. The middle ply, made from the high-bulk pulp suspension, offers the necessary bulk and rigidity, while the top and back plies typically enhance the surface properties and provide additional strength. This configuration is particularly useful for high-performance packaging and printing applications where structural integrity and surface smoothness are required.In some embodiments, the different plies are formed on different wires and then couched together to create a multi-ply paperboard. In some embodiments, the back ply is formed on a first wire, the bulk ply is formed on a second wire, and the top ply is formed on a third wire, and the plies then couched together to create a multi-ply paperboard. In other embodiments, two of the plies, for example the back ply and the bulk ply, or the bulk ply and the front ply, may be formed on the same wire and then couched together with the third ply to create a multi-ply paperboard. In some embodiments, the bulk ply is formed as the middle layer, and the top and back plies are couched together with the bulk ply to create a multi-ply paperboard. During the couching process the plies are pressed together, improving the bonding between the layers and forming a solid, uniform paperboard structure. This multiply construction ensures that the bulk ply contributes to the overall thickness and rigidity, while the top and back plies provide the necessary surface characteristics and additional strength.In some embodiments, the bulk ply and additional plies may be couched together either sequentially or simultaneously. In a sequential couching process, the bulk ply is formed first, followed by the application and bonding of the top and back plies in separate steps. In a simultaneous couching process, the top, bulk, and back plies are formed and bonded together in a single operation. In an alternative couching process, two of the plies are formed first, followed by the application and bonding of the third ply in a separate step.In some embodiments, the method for manufacturing paperboard in a papermaking machine comprises the steps of:a) forming a back ply by applying a pulp suspension comprising at least 50 wt% (based on the total dry weight of the pulp suspension) of bleached or unbleached kraft pulp from a headbox onto a moving wire to form a wet web, at least partially dewatering the wet web on the wire to obtain an at least partially dewatered web; b) forming a bulk ply by applying a pulp suspension comprising at least 50 wt% (based on the total dry weight of the pulp suspension) of high-bulk softwood pulp and 0.01 -10 wt% (based on the total dry weight of the pulp suspension) of a strength enhancement agent selected from the group consisting of highly refined cellulose, microfibrillated cellulose (MFC), a cationic strength additive, an anionic strength additive, or a combination thereof, from a headbox onto a moving wire to form a wet web, and at least partially dewatering the wet web on the wire to obtain an at least partially dewatered web;c) forming a top ply by applying a pulp suspension comprising at least 50 wt% (based on the total dry weight of the pulp suspension) of bleached or unbleached kraft pulp from a headbox onto a moving wire to form a wet web, at least partially dewatering the wet web on the wire to obtain an at least partially dewatered web; d) couching the at least partially dewatered webs together to form a multi-ply web; ande) drying the multi-ply web to obtain the multi-ply paperboard;wherein the wire for the bulk ply is provided with a wire shaker which shakes the wire at frequency of 100-600 strokes per minute and a stroke length of 5-35 mm.In some embodiments, the top ply comprises at least 50 wt% (based on total dry weight) of bleached or unbleached kraft pulp, preferably bleached kraft pulp. The kraft pulp in the top ply provides additional strength to the paperboard. The use of bleached kraft pulp in the top ply enhances the paperboard’s formation and printability, providing a smooth, bright surface suitable for high-quality printing and finishing. The top ply may add to the aesthetic and functional qualities of the multiply paperboard. In alternative embodiments, the top ply comprises more than 65% bleached hardwood kraft pulp, such as birch or eucalyptus pulp, refined to a Schopper-Riegler (SR) value above 23, for example refined to a Schopper-Riegler (SR) value in the range of 25 to 40.In some embodiments, the back ply comprises at least 50 wt% (based on total dry weight) of bleached or unbleached kraft pulp, preferably unbleached kraft pulp. The kraft pulp in the back ply provides additional strength and dimensional stability to the paperboard. In some embodiments, the back ply comprises at least 50 wt% (based on total dry weight) of bleached or unbleached softwood kraft pulp. In some embodiments, the bleached or unbleached softwood kraft pulp has an SR value in the range of 18-32, preferably in the range of 20-28, as determined by standard ISO 5267-1. In some embodiments, the back ply comprises at least 50 wt% (based on total dry weight) of bleached or unbleached hardwood kraft pulp. The bleached or unbleached hardwood kraft pulp may for example be obtained from birch or eucalyptus. In some embodiments, the bleached or unbleached hardwood kraft pulp has an SR value in the range of 18-37, preferably in the range of 20-30, such as in the range of 20-25 or in the range of 25-35, as determined by standard ISO 5267-1.When combined with top and back plies made from kraft pulp, the multi-ply paperboard exhibits improved surface quality, printability, strength (tensile, tear, compression), and structural integrity, suitable for a wide range of applications.The multi-ply paperboard may for example be used for Folding Box Board (FBB), Food Service Board (FSB), Solid Unbleached board (SUB), Coated Unbleached Kraft paperboard (CUK), White-Top Kraft Liner (WTKL), coated White-Top Kraft Liner (cWTKL), Cup Board, or Liquid Packaging Board (LPB).In some embodiments, the multi-ply paperboard is a white-top paperboard. In white-top paperboard the top ply is made from bleached kraft pulp, and optionally pigment coated, to provide a white, printable surface, while the bulk ply and back ply offer the necessary bulk and structural support. This combination is advantageous for applications that require both visual appeal and strength.In some embodiments, the top ply and the back ply each have a grammage in the range of 15 gsm to 120 gsm, and more preferably in the range of 20 gsm to 90 gsm, as measured according to standard ISO 536. This grammage range ensures that the outer plies provide sufficient surface quality and strength without compromising the bulk ply’s contribution to overall thickness and rigidity of the multi-ply paperboard.In some embodiments, the ratio of the grammage of top ply, and the bulk ply and the ratio of the grammage of back ply and the bulk ply, are both at least 1 :2, preferably in the range of 1:2 to 1 :8, and more preferably in the range of 1 :3 to 1 :5. These grammage ratios ensure that the bulk ply provides the majority of the thickness and rigidity while the top and back plies contribute to surface characteristics and additional structural support. In some embodiments, the grammage of the back ply is up to 40 % lower than the grammage of the top ply.In some embodiments, the multi-ply structure can comprise one or more additional plies added between the two outer plies (top ply and back ply) to further enhance the properties of the paperboard. These additional layers may comprise various types of pulp or other fibrous materials, depending on the desired characteristics of the final product. For example, a reinforcing layer may be incorporated to increase tensile strength, or a barrier layer may be included to improve moisture resistance. The inclusion of additional layers allows for customization of the paperboard structure, enabling optimization for specific applications such as packaging, printing, or specialty paperboard products.The use of a wire shaker as described herein is particularly useful for bulk plies. However, a wire shaker may also be used for any additional ply of a multi-ply structure as described herein, and particularly for any additional bulk ply of a multiply structure as described herein. In some embodiments, a wire shaker is used for the bulk ply and the back ply, but not for the top ply. Frequency and stroke length may be adapted according to the composition of each ply.In some embodiments, a wire shaker is used for the back ply. In some embodiments, the wire for the back ply is provided with a wire shaker which shakes the wire at frequency of 150-600, such as in the range of 200-550 strokes per minute or 250-500 strokes per minute. In some embodiments, the wire for the back ply is provided with a wire shaker which shakes the wire at a stroke length of 5-25 mm. When the back ply comprises at least 50 wt% (based on total dry weight) of bleached or unbleached softwood kraft pulp a longer stroke length of 6-25 mm, such as 8-25 mm or 10-25 mm may preferably be used. When the back ply comprises at least 50 wt% (based on total dry weight) of bleached or unbleached hardwood kraft pulp a shorter stroke length of 5-20 mm, such as 6-18 mm or 6-14 mm may preferably be used.In some embodiments, the headbox consistency for the back ply is 0.8 wt% or less, preferably in the range of 0.1-0.8 wt% or in the range of 0.3-0.8 wt%. In some embodiments the pulp suspension for the back ply is diluted to the headbox consistency of 0.8 wt% or less in the headbox.In some embodiments, the headbox consistency for the top ply is 0.6 wt% or less, preferably in the range of 0.2-0.6 wt%, and more preferably in the range of 0.3-0.5 wt%. In some embodiments the pulp suspension for the top ply is diluted to the headbox consistency of 0.6 wt% or less in the headbox.In some embodiments, spray starch may be applied between the plies to enhance the bonding strength and cohesion of the multi-ply paperboard. The starch may preferably be applied in an amount of 0.1 to 3 gsm. The starch acts as a natural adhesive, improving fiber-to-fiber bonding between the layers. Spray starch contributes to the overall internal strength of the paperboard by promoting stronger ply adhesion, thereby increasing the resistance of the board to delamination and improving its structural integrity.In some embodiments, the multi-ply paperboard has a grammage in the range of 120 gsm to 600 gsm, as measured according to standard ISO 536. In various embodiments, the multi-ply paperboard may have a grammage in the range of 120 to 140 gsm, 150 to 175 gsm, 180 to 250 gsm, 250 to 375 gsm, or 380 to 500 gsm, as measured according to standard ISO 536.In some embodiments, the multi-ply paperboard has a density of less than 600 kg / m3, preferably less than 500 kg / m3 as measured according to standard ISO 534.In some embodiments, the multi-ply paperboard has a tensile index MD / CD ratio in the range of 1 .5-2.8. preferably in the range of 1.7-2.4, as determined according to ISO 1924-3:2005.In some embodiments, the multi-ply paperboard has a Scott Bond strength of at least 100 J / m2, more preferably at least 150 J / m2.In some embodiments, the multi-ply paperboard has a specific formation value of less than 0.8 gΛ0.5 / m, preferably less than 0.7 gΛ0.5 / m, and more preferably less than 0.6 gΛ0.5 / m, as measured according to SCAN-P 92:09 for the top ply. A lower specific formation value indicates a more uniform distribution of fibers across the paperboard, leading to improved formation, smoother surfaces, and enhanced strength.The specific formation of the bulk ply or paperboard can be measured online using specialized formation sensors or imaging systems integrated into the papermaking process. These systems typically employ high-resolution cameras or laser-based scanning devices to continuously monitor the fiber distribution and structure of the paperboard as it forms on the wire. The formation sensor captures real-time data related to the uniformity of the fiber mat, evaluating the fiber density, flocculation, and overall consistency of the sheet. The specific formation reflects the distribution of fibers across the sheet, with lower values indicating better (more uniform) formation.The formation sensor is typically positioned along the forming section of the paper machine, close to the wire, where the pulp suspension begins to form the bulk ply. The sensor can analyze formation at various stages, from the initial wet web to the partially dried state, allowing for continuous assessment of fiber uniformity throughout the process.Online measurement of specific formation can be used to adjust the settings of the wire shaker in real time, optimizing the paperboard formation process. Based on the data collected from the formation sensor, the parameters of the wire shaker, such as shaking frequency and stroke length, can be automatically adjusted to improve, for example, the formation quality.As an example, if the online measurement indicates poor formation, such as areas of excessive fiber flocculation, the system can increase the shaking frequency to enhance fiber dispersion and reduce clumping. As another example, if the formation sensor detects non-uniform fiber distribution in the sheet, the stroke length of the wire shaker can be increased to apply greater oscillatory motion, promoting more even fiber deposition across the width of the forming fabric. The shaker can also be using stored data for analyzing and predicting, e.g., quality of formation and hence propose or adjust settings based on stroke length or stroke length and frequency.These real-time adjustments, guided by the online specific formation data, allow the papermaking machine to dynamically respond to variations in fiber distribution and optimize the uniformity of the bulk ply. Benefits include more homogenous dewatering, improved sheet uniformity, reduced defects, optimized wire shaker performance, and enhanced product qualityThus, in some embodiments, the method of the present disclosure further comprises online measurement of specific formation of the bulk ply or paperboard, and real-time adjustment of the frequency and / or stroke length of the wire shaker based on the online measurement.Generally, while the products, polymers, materials, layers and processes are described in terms of "comprising” various components or steps, the products, polymers, materials, layers and processes can also “consist essentially of or “consist of the various components and steps.While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

1. A method for manufacturing paperboard in a paper machine, the method comprising the step of:forming a bulk layer by applying a fiber suspension comprising at least 50 wt%, based on the total dry weight of the fiber suspension, of high-bulk softwood pulp, wherein the high-bulk softwood pulp is softwood chemithermomechanical pulp (SW CTMP) or softwood high-temperature chemithermomechanical pulp (SW HT-CTMP), or a combination thereof, and 0.01-10 wt%, based on the total dry weight of the fiber suspension, of a strength improving agent selected from the group consisting of highly refined cellulose with a Schopper Riegler value (SR) in the range of 50-84 determined according to standard ISO 5267-1, microfibrillated cellulose (MFC), a cationic strength additive, an anionic strength additive, or a combination thereof, from a headbox onto a moving wire to form a wet web, at least partially dewatering the wet web onto the wire to obtain an at least partially dewatered web, and drying the at least partially dewatered web to obtain the bulk layer;Characterized by the fact that the water wire is equipped with a wire shaker that shakes the wire with a frequency of 100-600 strokes per minute and a stroke length of 5-35 mm.

2. The method according to claim 1, wherein the wire shaker shakes the wire at a frequency of 125-550 strokes per minute, preferably at a frequency of 150-500 strokes per minute.

3. The method according to any one of the preceding claims, wherein the stroke length of the wire shaker is 8-32 mm, and more preferably 12-30 mm.

4. The method according to any one of the preceding claims, wherein the wire shaker shakes the wire in the cross-machine direction (CD) of the paper machine.

5. The method according to any one of the preceding claims, wherein the machine speed is at least 450 m / min, preferably at least 550 m / min and more preferably at least 650 m / min.

6. The method according to any one of the preceding claims, wherein the machine width is at least 5 meters, preferably at least 6 meters, such as in the range of 6-11 meters.

7. The method according to any one of the preceding claims, wherein the headbox concentration is 1.0 wt% or lower, preferably in the range of 0.1-1.0 wt%, and more preferably in the range of 0.4-1.0 or in the range of 0.45-0.9 wt%.

8. The method according to any one of the preceding claims, wherein the fiber suspension comprises at least 60% by weight, based on the total dry weight of the fiber suspension, and more preferably at least 70% by weight, based on the total dry weight of the fiber suspension, of the high-bulk softwood pulp.

9. The method according to any one of the preceding claims, wherein the high-bulk softwood pulp is a high-yield softwood pulp.

10. The method according to any one of the preceding claims, wherein the high-bulk softwood pulp has a Canadian Standard Freeness value in the range of 300-750, 350-750, 350-600 ml.

11. The method according to any one of the preceding claims, wherein the fiber suspension further comprises 1-40% by weight, based on the total dry weight of the fiber suspension, of a leaf chemi-thermomechanical pulp (HW CTMP) or leaf high-temperature chemi-thermomechanical pulp (HW HT-CTMP), or a combination thereof.

12. The method according to any one of the preceding claims, wherein the fiber suspension further comprises 1-20% by weight, based on the total dry weight of the fiber suspension, of softwood pulp with a Kappa number according to ISO 302:205 of at least 40, preferably at least 60 or at least 80.

13. The method according to any one of the preceding claims, wherein the fiber suspension further comprises 1-40% by weight, based on the total dry weight of the fiber suspension, of recycled fiber pulp.

14. The method according to any one of the preceding claims, wherein the fiber suspension comprises 0.5% by weight to 10% by weight, based on the total dry weight of the fiber suspension, of the strength-improving agent.

15. The method according to any one of the preceding claims, wherein the fiber suspension comprises 0.1 wt% to 10 wt%, preferably 0.5 wt% to 10 wt%, based on the total dry weight of the fiber suspension, of highly refined cellulose.

16. The method according to any one of the preceding claims, wherein the fiber suspension comprises 0.1 wt% to 10 wt%, preferably 0.5 wt% to 10 wt%, based on the total dry weight of the fiber suspension, of microfibrillated cellulose (MFC).

17. The method according to any one of the preceding claims, wherein the microfibrillated cellulose (MFC) has an SR value in the range of 85-100 determined according to standard ISO 5267-1.

18. The method according to any one of the preceding claims, wherein the fiber suspension comprises 0.01-5% by weight, preferably 0.1-3% by weight and more preferably 0.1-2% by weight, based on the total dry weight of the fiber suspension, of a cationic strength additive, preferably a cationic polymer, more preferably a cationic polysaccharide, such as cationic starch.

19. The method according to any one of the preceding claims, wherein the fiber suspension comprises 0.01-5% by weight, preferably 0.1-3% by weight and more preferably 0.1-2% by weight, based on the total dry weight of the fiber suspension, of an anionic strength additive, preferably selected from the group consisting of anionic nanocrystalline cellulose, anionic microfibrillated cellulose (MFC), anionic carboxymethyl cellulose (CMC), anionic starch, alginate, hemicellulose and combinations thereof.

20. The method according to any one of the preceding claims, wherein the temperature of the fiber suspension when it leaves the headbox is in the range of 35-85°C, preferably in the range of 40-65°C.

21. The method according to any one of the preceding claims, wherein the wet web is subjected to double-sided dewatering on the wire.

22. The method according to any one of the preceding claims, wherein the bulk layer has a density of less than 500 kg / m3, preferably less than 400 kg / m3, measured according to standard ISO 534.

23. The method according to any one of the preceding claims, wherein the bulk layer has a basis weight in the range of 50-350 g / m2, preferably in the range of 50-250 g / m2, measured according to standard ISO 536.

24. The method according to any one of the preceding claims, wherein the bulk layer has a specific formation value lower than 0.8 gΛ0.5 / m, preferably lower than 0.7 gΛ0.5 / m and more preferably lower than 0.6 gΛ0.5 / m, measured according to SCAN-P 92:09.

25. The method according to any one of the preceding claims, wherein the bulk layer is formed together with one or more additional layers to form a multi-layer carton.

26. The method according to any one of the preceding claims, wherein the bulk layer is formed together with a surface layer and a bottom layer to form an intermediate layer in a multi-layer carton.

27. The method according to any one of the preceding claims, comprising the steps of:a) forming a bottom layer by applying a fiber suspension comprising at least 50% by weight, based on the total dry weight of the fiber suspension, of bleached or unbleached kraft pulp from a headbox onto a moving wire to form a wet web, at least partially dewatering the wet web onto the wire to obtain an at least partially dewatered web;b) forming a bulk layer by applying a fiber suspension comprising at least 50 wt.%, based on the total dry weight of the fiber suspension, of high-bulk softwood pulp, wherein the high-bulk softwood pulp is softwood chemi-thermomechanical pulp (SW CTMP) or softwood high-temperature chemi-thermomechanical pulp (SW HT-CTMP), or a combination thereof, and 0.01-10 wt.%, based on the total dry weight of the fiber suspension, of a strength improving agent selected from the group consisting of highly refined cellulose with a Schopper Riegler value (SR) in the range of 50-84 determined according to standard ISO 5267-1, microfibrillated cellulose (MFC), a cationic strength additive, an anionic strength additive, or a combination thereof, from a headbox onto a moving wire to form a wet web, and at least partially dewatering the wet web onto the wire to obtain at least partially drained track;c) forming a surface layer by applying a fiber suspension comprising at least 50% by weight, based on the total dry weight of the fiber suspension, of bleached or unbleached kraft pulp from a headbox onto a moving wire to form a wet web, at least partially dewatering the wet web onto the wire to obtain an at least partially dewatered web;d) combining the at least partially dewatered webs to form a multi-layer web; ande) drying the multilayer web to obtain the multilayer board;wherein the wire for the bulk layer is provided with a wire shaker that shakes the wire with a frequency of 100-600 strokes per minute and a stroke length of 5-35 mm.

28. The method of claim 27, wherein the headbox concentration of the fiber suspension to the surface layer is 0.6 wt% or lower, preferably in the range of 0.2-0.6 wt%, and more preferably in the range of 0.3-0.5 wt%.

29. The method according to any one of claims 27-28, wherein the headbox concentration of the fiber suspension to the bottom layer is 0.8 wt% or lower, preferably in the range of 0.1-0.8 wt%, and more preferably in the range of 0.3-0.8 wt%.

30. The method according to claim 26, wherein the surface layer comprises at least 50% by weight, based on total dry weight, of bleached or unbleached kraft pulp, preferably bleached kraft pulp.

31. The method according to any one of claims 26-30, wherein the bottom layer comprises at least 50% by weight, based on total dry weight, of bleached or unbleached kraft pulp, preferably unbleached kraft pulp.

32. The method according to any one of claims 26-31, wherein the multi-layer board is a white top board.

33. The method according to any one of claims 26-32, wherein the surface layer and the bottom layer each have a basis weight in the range of 15 g / m2 to 120 g / m2, measured according to standard ISO 536.

34. The method according to any one of claims 26-33, wherein the ratio between the surface density n of the surface layer and the bulk layer and the ratio betweenthe basis weight of the bottom layer and the bulk layer are both at least 1:2, preferably in the range of 1:2 to 1:8 and more preferably in the range of 1:3 to 1:5.

35. The method according to any one of claims 26-34, wherein the multi-layer cardboard has a basis weight in the range of 120 g / m2 to 600 g / m2, measured according to standard ISO 536.

36. The method according to any one of claims 26-35, wherein the multi-layer board has a specific formation value lower than 0.8 gΛ0.5 / m, preferably lower than 0.7 gΛ0.5 / m and more preferably lower than 0.6 gΛ0.5 / m, measured according to SCAN-P 92:09.