A method for manufacturing a multi-ply paperboard

Abstract

The invention pertains to a method for producing multiply paperboard on a paper machine. The process involves forming a paperboard web with a top ply, back ply, and at least one middle ply, where the middle ply consists of at least 40 wt% Chemi-Thermo Mechanical Pulp (CTMP). The back ply is made of at least 80 wt% unbleached kraft pulp with a kappa number of at least 40. The paperboard web undergoes dewatering, pressing, drying, and cooling. It is then calendered in a hard nip with a load ranging from 10-50 kN / m to achieve high smoothness while maintaining the bulk of the paperboard. This method, which incorporates CTMP in the middle ply and high kappa number unbleached kraft pulp in the back ply, benefits from reduced calendering pressures due to pre-cooling, optimizing the paperboard's properties.

Description

[0001] A METHOD FOR MANUFACTURING A MULTI-PLY PAPERBOARD

[0002] Technical field

[0003] The present disclosure relates to a method for manufacturing a multiply paperboard.

[0004] Paperboard is a widely used material for various packaging applications due to its strength, durability, and printability. Typically, paperboard consists of one to five layers or plies comprising cellulose fibers. Paperboard intended for packaging of food or liquid often consists of multiple plies, which enables a higher resistance to bending compared to single-ply paperboard. Multi-ply paperboard generally includes top and back plies, along with one or more middle plies. Optionally, bonding agents may be added between the plies to improve the strength of the bond between them.

[0005] In high-quality paperboards, the middle ply often comprises hardwood and / or softwood chemi-thermo-mechanical pulp (CTMP), which provides a high bulk. The middle ply may further comprise broke and chemical pulp. The outer plies, namely the top ply and back ply, are typically made from bleached and / or unbleached kraft pulp. For liquid packaging applications, the top ply is usually made from bleached pulp, while the back ply is made from unbleached pulp.

[0006] The production of paperboard involves several stages, including the formation of a web from a mixture of fibers, pressing and drying the web to remove water and moisture, and subsequent calendering to improve surface smoothness and thickness uniformity. However, one significant problem encountered during the drying process is the occurrence of shrinkage of the board. Shrinkage is usually more pronounced at the edges of the web, leading to an uneven surface- and density profile.

[0007] To mitigate the quality effects caused by shrinkage, one common solution is to apply high nip loads during the subsequent calendering process. By exerting significant pressure on the paperboard web, the calendering process can alleviate some of the issues caused by shrinkage. However, this approach has its limitations. Applying high nip loads in the calendering process leads to a reduction in the bulk of the paperboard, which decreases the bending stiffness and rigidity of the board. A reduction in bulk and loss in stiffness further increases the need for a higher amount of cellulose fibers in the manufacturing process and / or a higher grammage, which has a detrimental effect on fiber efficiency and sustainability efforts. Therefore, there is a need for an improved method of producing paperboard that addresses the shrinkage problem without sacrificing bulk and other desirable characteristics.

[0008] Description of the invention

[0009] It is an object of the present invention to provide a method for manufacturing a paperboard with an even surface- and density profile while maintaining a high bulk.

[0010] Another object of the present invention is to provide a method for manufacturing a paperboard, which addresses the shrinkage problem caused in the drying of the paperboard web, without decreasing the bulk.

[0011] The above-mentioned object, as well as other objects as will be realized by the skilled person in light of the present disclosure, are achieved by the various aspects of the present disclosure.

[0012] According to a first aspect illustrated herein, there is provided a method for manufacturing a multiply paperboard on a paperboard machine comprising the steps of:

[0013] - forming a multiply paperboard web comprising a top ply, a back ply and at least one middle ply arranged between said top and back ply, wherein the middle ply is made from a middle ply furnish comprising at least 40 wt% (based on the total dry weight of the middle ply furnish) of chemi-thermo-mechanical pulp (CTMP), and wherein the top ply is made from a top ply furnish and the back ply is made from a back ply furnish, wherein the back ply furnish comprises at least 80 wt% (based on the total dry weight of the back ply furnish) of unbleached kraft pulp having a kappa number according to ISO 302:2015 of at least 40,

[0014] - dewatering, pressing and drying the thereby formed multiply paperboard web, forming a dried multiply paperboard web,

[0015] - cooling the dried multiply paperboard web,

[0016] - calendering the cooled web in a hard nip with a nip load of 10 - 50kN / m, preferably 10 - 40 kN / m, more preferably of 10 - 30 kN / m and

[0017] - reeling the web.

[0018] The inventors to the present invention have found that the use of CTMP in the middle ply and unbleached kraft pulp with a high kappa number in the back ply in a multiply paperboard and applying a cooling step between the drying and calendering, allow for reduced pressures in the subsequent hard nip calendering process, while still achieving an even surface profile, especially an improved CD roughness profile. In this way, the bulk of the paperboard is further preserved. By incorporating a high kappa pulp in the back ply, which high kappa pulp has a significant amount of lignin, the shrinkage of the web during the drying process can be effectively countered. This, in turn, enables the use of reduced pressures in calendering. In addition, applying a cooling step before calendering further enhances the calendering effect. The speed of the web in the calendering is preferably within the range of 550 - 1200 m / min, or 550 - 1100 m / min, preferably in the range of 800 - 1000 m / min. Preferably, the hard nip is the only nip in the calendering of the web.

[0019] A hard nip in the context of the application refers to a nip formed between two hard rolls, preferably steel rolls. In embodiments, the two hard rolls are steel rolls without any rubber coating. The hard rolls may have a hardness of at least 500 HV20.

[0020] If not specifically denoted otherwise, given % as used herein are weight%, and are calculated on the basis of a dry weight of 100 weight% of the respective object, such as a furnish, a layer, or a ply. The kappa number is measured according to ISO 302:2015, without a temperature correction. In embodiments, the cooling of the web is performed so that the temperature of at least 50% of its thickness is in the range of 10 - 70 °C, preferably in the range of 10 - 50°C. The cooling of the web to these temperatures further enhances the calendering step. It has been shown that cooling of a web having an outer ply comprising a high amount of unbleached kraft pulp with a high kappa boost the calendering even further without destroying the bulk. The coarse fibers in the back ply enables more efficient cooling of the middle ply, whereby the bulk is preserved in the calendering.

[0021] The cooling may be performed by using a cooling device including cold air and / or chilling rolls. This can be performed by applying a cold air flow towards the web, preferably towards both sides of the web. The air flow may preferably have temperature within the range of 10 - 50 °C, preferably 10 - 40 °C, and a speed of 20 - 100 m / s, preferably 40 - 80 m / s. Such an air flow further removes potential particles from the surface of the web, whereby the following calendering step is further enhanced. Preferably, the cooling is achieved solely by directing a flow of cold air toward the web. In embodiments, the air flow is directed to both sides of the web by means of nozzles. The method of the invention allows for a simple cooling process, without the need for additional cooling steps.

[0022] Alternatively, the cooling may be performed by spraying water preferably at a temperature of 10 - 70 °C, more preferably of 15 - 65 °C, to the surface of the top- and back ply. Such applied water evaporates on the surface, which contributes to an efficient cooling of the web.

[0023] The method may further comprise a moisturizing step, wherein the top side of the web is moisturized prior to the calendering step. The moisturizing can be performed by adding water or steam to the surface of the top side, preferably in an amount of 0.1-10 gsm and more pref. 0.1-5 gsm. This step should take place in the vicinity of the hard nip calender, preferably such that the delay time between moisturizing and the hard nip calender is less than 1 s such as 0.01-0.8 s. In this way, the moisture is concentrated to the surface of the board and hence act primarily as softener or lubricant for the outer ply The moisturizing step may be performed after the cooling. In embodiments, the method comprises a step of moisturizing the web by applying steam to the surface of the top ply, which step is performed after the cooling step and before the calendering step.

[0024] The application of steam onto the cool surface leads to the condensation of water, thereby enhancing the calendering step. To further enhance the effect of steam application, the web can be subjected to electrical treatment, such as corona treatment, before the stream treatment or by applying charged droplets on an earthed web.

[0025] The moisture content of the web after the cooling and the optional moisturizing step is preferably within the range of 4 - 18 wt%, preferably 6 - 16 wt% and most preferably 8 - 14 wt%.

[0026] In embodiments, the hard nip is formed between a heated calender roll or thermo roll and a backing roll, wherein the surface temperature of the heated calender roll is above 100 °C, preferably in the range of 150 - 300 °C. The heated calender roll may be heated using steam or thermal oil. The heated calender roll may be applied with a temperature gradient system, wherein the calender roll is divided into multiple zones, equipped with its own heating system. This allows for precise temperature control creating a temperature gradient across the width of the roll. The backing roll is typically not actively heated, but its temperature may be increased due to contact with the heated the web.

[0027] In embodiments, the surface temperature of the backing roll is above 80 °C, preferably in the range of 100 - 230 °C. Preferably, the heated calender roll is in contact with the top ply of the web, while the backing roll is in contact with the back ply of the web. The elevated temperatures of the backing roll cause the lignin in the high kappa pulp of the back ply to undergo plasticization and cross-linking, resulting in a significant improvement in strength, especially tensile strength, and smoothness of the back ply. The diameter of the heated roll is preferably at least 700 mm, more preferably within the range of 750 - 1500 mm, e.g. 800 - 1200 mm. The diameter of the backing roll is preferably within the range of 500 - 1200 mm. The heated calender roll and the backing roll are preferably hard rolls.

[0028] In embodiments, the backing roll comprises deflection compensation. Such a roll is designed to address the issue of roll deflection, which can negatively impact the uniformity and quality of the paperboard web. Deflection compensation can be achieved through the use of various techniques, such as hydraulic or pneumatic loading system, including e.g. the use of a hydraulically deflection-compensated swimming roll as the backing roll. Such a deflection compensation is especially relevant for very wide machines, such as machines with a width of more than 7 m, or more than 7.5 m, or more than 8 m.

[0029] The forming, dewatering, pressing and drying of the web prior to the cooling and calendering steps can be made using conventional techniques.

[0030] The forming of the multiply paperboard web may be made by forming a middle ply web, a top-ply web and a back-ply web on separate wires using different headboxes. These webs are then dewatered on their respective wires and subsequently combined or “couched” to form a multiply web. In an alternative embodiment, at least two of the webs may be made on a wire using a multiply headbox. Preferably, the pressing of the multi-ply web comprises subjecting the web to pressing in at least one shoe press nip, such as in a first and a second shoe press nip or in a first, second and third shoe press nip. While the first and second shoe press nips are preferably double-felted, the third shoe press nip is preferably one-felted nip. In one embodiment, the web is subjected to pressing in a first and a second shoe press nip followed by a smoothing press. The use of high kappa unbleached pulp in the back ply allows for higher nip loads in the pressing thereof. The nip load in each of the shoe press nips may be within the range of 500 - 1500 kN / m, such as in the range of 800 - 1050 kN / m. In the drying step, the wet web is preferably passed through a series of heated drying cylinders. Preferably, the dried multiply paperboard web has a moisture content of 2 - 18 wt%, such as in the range of 5 - 10 wt% or 10 - 18 wt%.

[0031] In embodiments, the middle ply furnish comprises at least 60 wt%, preferably between 60 - 90 wt% (based on the total dry weight of the middle ply furnish) of CTMP. The CTMP in the middle ply furnish may be derived from softwood or hardwood fibers, or a mixture thereof and may be bleached or unbleached. In embodiments, the CTMP is high-temperature CTMP (HT-CTMP). In addition to CTMP, the middle ply furnish may comprise kraft pulp and broke. The CTMP or HT-CTMP can be dried or never-dried pulps or mixtures thereof.

[0032] In embodiments, the middle ply furnish comprises 50 - 100 wt% (based on the total dry weight of the middle ply furnish) of CTMP and 0 - 50 wt% by dry weight of broke, preferably 50 - 80 wt% CTMP and 20 - 50 wt% broke. The middle ply furnish may further comprise 0 - 20 wt% (based on the total dry weight of the middle ply furnish) of kraft pulp. Broke refers to defective or unusable paperboard, or edges or trim waste, that is generated in the manufacturing process of the multiply paperboard.

[0033] In embodiments, the middle ply furnish comprises 20 - 50 wt% (based on the total dry weight of the middle ply furnish) of broke, which broke comprises 1 - 30 wt% by dry weight of unbleached kraft pulp having a kappa number according to ISO 302:2015 of at least 40, preferably of at least 80. The broke generated during the claimed process comprises 1 - 30 wt% by dry weight of high kappa kraft pulp from the back ply. Utilizing this broke in the middle ply is advantageous because it does not significantly contribute to increased density, unlike broke comprising a higher amount of lower kappa kraft pulp. The broke may be coated broke, thus comprising a small amount of pigments and latex. Th middle ply furnish may comprise 0.1 - 5 wt% pigments and latex from coated broke (based on the total dry weight of the middle ply furnish). The latex is preferably styrene acrylate latex or styrene butadiene latex or mixtures thereof. The cooling of the web in accordance with the invention is especially advantageous when forming the middle ply from a furnish comprising small amounts of pigments and latex, as the web is then less prone to collapse during the following calendering step.

[0034] In embodiments, the middle ply furnish further comprises 0.01 - 10 wt% (based on the total dry weight of the middle ply furnish) of a strength enhancement agent selected from the group consisting of highly refined cellulose, microfibri Hated cellulose (MFC), a cationic strength additive, an anionic strength additive.

[0035] The cationic strength enhancement agent is preferably cationic starch. The anionic polymer strength additive is preferably anionic polysaccharide such as anionic starch or anionic-carboxymethyl cellulose (CMC). The highly refined cellulose has preferably a Schopper-Reigler (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. Microfibrillated cellulose (MFC) shall in the context of this patent application mean a cellulose particle, fiber or fibril having a width or diameter of from 20 nm to 1000 nm. 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. The anionic CMC has preferably a degree of substitution of higher than 0.3 and a mean molecular weight of above 30 000 g / mol, preferably above 50 000 g / mol.

[0036] In embodiments, the middle ply furnish comprises 0.5 - 5 wt%, preferably 1 - 3 wt% of starch, 0.3 - 5 wt%, preferably 0.5 - 2 wt% of MFC and 0.02 - 0.2 wt% of anionic strength enhancement agent, preferably anionic CMC, all wt% calculated on the total dry weight of the middle ply furnish.

[0037] In embodiments, the back ply furnish comprises at least 75 wt%, preferably between 90 - 100 wt% (based on the total dry weight of the back ply furnish) of unbleached kraft pulp having a kappa number according to ISO 302:2015 of at least 40. In one embodiment, the back ply comprises 100 wt% by dry weight of unbleached kraft pulp with the said kappa number.

[0038] The unbleached kraft pulp in the back ply may be generated from softwood or hardwood fibers. In embodiments, the unbleached kraft pulp are made from 70 - 100 wt% softwood fibers and 30 - 0 wt% hardwood fibers, as calculated on the total dry weight of fibers in the pulp.

[0039] Preferably, the unbleached kraft pulp in the back ply furnish has a kappa number according to ISO 302:2015 of at least 60, preferably at least 80, more preferably at least 90, such as between 60 - 110, or 80 - 110, or 80 - 90, or 90 - 110. The unbleached kraft pulp in the back ply furnish may further have a Schopper Riegler (SR) value in the range of 18 - 35, such as in the range of 20 - 32, as determined by standard ISO 5267-1 . MFC may be added prior to the refining of the high kappa pulp. In this way, the refining is facilitated and the energy efficiency improved.

[0040] In embodiments, the unbleached kraft pulp in the back ply furnish has a shives content of less than 1 wt%, preferably less than 0.4 wt%, as calculated on the dry weight of said unbleached kraft pulp and as measured using TAPPI T275 standard, (Sommerville of 0.15 mm).

[0041] A low shives content enhances the surface quality of the paperboard, resulting in a smoother appearance, and contributes to an improved formation.

[0042] Preferably, the unbleached kraft pulp in the back ply furnish has fiber content generated from pine of at least 50 wt%, preferably at leat 70% or at least 80%, such as in an amount of 70 - 100 wt%, as calculated on the total dry weight of fibers in the pulp. The unbleached kraft pulp may further have a fiber content generated from spruce in at least 10 wt%, such as in the range of 10 - 30 wt% as calculated on the total dry weight of fibers in the pulp. In embodiments, the unbleached kraft pulp in the back ply furnish has a fiber content generated from pine in an amount of 70 - 90 wt% and a fiber content generated from spruce in an amount of 10 - 30 wt%, as calculated on the total dry weight of fibers in the pulp.

[0043] The unbleached kraft pulp in the back ply furnish has preferably an extractives content according to SCAN-CM 49 of less than 0.5 wt%.

[0044] The inclusion of a high kappa number pulp in the paperboard-making process can increase the yield, but it may also result in decreased overall strength properties of the paperboard, including lower tensile strength, tear strength, and stiffness. However, incorporating a high kappa pulp at least partly from spruce and / or ensuring a low content of extractives can enhance tear resistance and overall strength, effectively mitigating the potential strength loss associated with the high kappa number.

[0045] The unbleached kraft pulp may be fractionated to ensure the preferred shives content. The extractive content may be controlled by subjecting the pulp to washing and screening.

[0046] Preferably, the fibers of the unbleached kraft pulp in the back ply furnish have a fiber length Lc(l) of above 1.5 mm, or preferably above 1.6 mm, or preferably above 1.9 mm, more preferably above 2.0 mm in accordance with ISO 16065. The fiber length Lc(l) is preferably defining the fiber length of the pulp in the headbox. Preferably, the pulp comprises at least 70 wt% never-dried pulp. In embodiments, the pulp is preferably never-dried pulp. The water retention value of the unbleached kraft pulp is preferably within the range of 1.2 - 2.1 g / g, more preferably 1.3 to 2.0 or 1.4 - 1.9 g / g, as measured using TAPPI Method T441. These properties will further improve the tear strength. The air resistance (Gurley Hill) of the unbleached pulp is preferably lower than 45 s / 100 ml, preferably lower than 30 s / 100 ml, as measured on ISO-sheets (ISO 5269-1). The use of a pulp with such low air resistance optimizes the dewatering in the press section, e.g. using a shoe press.

[0047] The unbleached kraft pulp used in the back ply furnish may further preferably exhibit one or more of the following properties:

[0048] - An ISO brightness of 15 - 40%, preferably 17 - 35, more preferably 19 - 25%, as measured using ISO 2470-1.

[0049] - FS5 curl of at least 5%, preferably at least 6%, as measured on ISO- sheets (ISO 5269-1) made from the pulp.

[0050] - Scott Bond of at least 500 J / m2, or at least 600 J / m2, preferably at least 675 J / m2, more preferably at least 700 J / m2as measured using TAPPI T 569 and measured on ISO sheets (ISO 5369-1) of 150 gsm made from the pulp. In embodiments, the back ply furnish comprises 0.01 - 10 wt% (based on the total dry weight of the back ply furnish) 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.

[0051] The cationic strength enhancement agent is preferably cationic starch. The anionic polymer strength additive is preferably anionic carboxymethyl cellulose (CMC), which preferably has a degree of substitution of higher than 0.3 and a mean molecular weight of above 30 000 g / mol, preferably above 50 000 g / mol. The highly refined cellulose has preferably a Schopper-Reigler (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 embodiments, the back ply comprises cationic starch, CMC and MFC or highly refined cellulose.

[0052] In embodiments, the top ply furnish comprises at least 80 wt% (based on the total dry weight of the top ply furnish) of bleached kraft pulp. The use of bleached kraft pulp in the top ply enhances the smoothness of the top ply, which preferably forms the print side in a thereof formed packaging. The unbleached kraft pulp in the back ply furnish and the kraft pulp in the top ply furnish may be derived from hardwood or softwood fibers, or a mixture thereof. In preferred embodiments, the unbleached kraft pulp in the back ply furnish is derived from softwood fibers, and the kraft pulp in the top ply furnish is derived from hardwood fibers.

[0053] The method may further comprise a step of surface sizing the web on at least on one side, preferably on the top ply side. The amount if surface sizing is preferably in the range of 0.5 - 4 g / m2per side. The surface size may comprise at least 50 wt% of starch such as cationic, anionic or non-ionic starch and may be applied by use of a sizing film press. The surface size may further comprise additives like e.g. pigments, optical brightening agents, softeners, cross-linkers or co-binders such as synthetic polymers or latexes. The surface sizing step can be performed prior to the cooling step, or after the calendering step. In embodiments, the surface sizing is applied before the cooling, the optional moisturizing step and the calendering. If the surface sizing is applied before a moisturizing step, the moisture added is hindered from penetrating into the middle ply, whereby the calendering is more efficient.

[0054] The grammage of the multi-ply paperboard may be within the range of 125 - 500 g / m2, preferably within the range of 200 - 450 g / m2. In embodiments, the grammage is within the range of 230 - 450 g / m2. The grammage may be measured in accordance with ISO 536:2019.

[0055] In some embodiments, the middle 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 middle 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. In embodiments, the back ply and the top ply may each have a grammage in the range of 20 - 60 gsm, preferably in the range of 25 - 55 gsm.

[0056] According to a second aspect illustrated herein, there is provided a paperboard manufactured according to the method of the first aspect. The paperboard comprises a top ply, a back ply and at least one middle ply arranged between the top ply and the back ply.

[0057] The Bendtsen roughness according to ISO 8791-2 of the outer surface of the top ply is preferably below 620 ml / min, more preferably within the range of 200 - 600 ml / min, or within the range of 200 - 550 ml / min.

[0058] The Bendtsen roughness according to ISO 8791-2 of the outer surface of the back ply is preferably below 1050 ml / min, more preferably within the range of 400 - 1000 ml / min, or within the range of 400 - 850 ml / min.

[0059] Preferably, the CD roughness of the back ply surface has a variation of less than 25%, and more preferably less than 20%, or less than 15%, as calculated on the mean roughness value determined for the center of the web. The brightness of the outer surface of the top ply is preferably at least 70%, more preferably at least 80 %, as measured using ISO 2470 / 1:2016. The top ply side has preferably a Cobb 30s (ISO 535:2014) value of below 45 g / m2, preferably within the range of 20 - 40 g / m2The brightness of the outer surface of the back ply is preferably less than 50%, such as in the range of 18 - 49, or 20 - 49 %, or 30 - 49 %, as measured using ISO 2470 / 1 :2016. The COBB 30 s of the back ply side is preferably within the range of 25 - 55 g / m2.

[0060] The paperboard may exhibit a bulk of at least 1.0 cm3 / g, preferably at least 1.5 cm3 / g, or at least 1.8 cm3 / g, as measured using ISO 534:2011. In embodiments, the paperboard exhibits a bulk in the range of 1.3 - 2.1 cm3 / g, or in the range of 1.5 - 2.1 cm3 / g or in the range of 1.8 - 2.1 cm3 / g. The paperboard may further exhibit a taint value of less than 0.5 according to the Robinson chocolate test as measured using standard EN 1230-2-2009, the multicomparison test. The paperboard may further have an internal bond strength (Scott Bond) of at least 100 J / m2, preferably of at least 150 J / m2, as measured using TAPPI T 569. The paperboard may further exhibit a bending resistance MD / CD ratio within the range of 1.8 and 2.5 and more pref, between 2.0 and 2.4 according to standard ISO 2493 (L&W 15 degrees)

[0061] Paperboard manufactured according to the method described in the first aspect is useful for various carton board applications, including but not limited to Folding Box Board (FBB), Clay-Coated Unbleached Kraft (CUK), Food Service Board (FSB), White-Top Kraft Liner (WTKL), coated White-Top Kraft Liner (cWTKL), Cup Board, and Liquid Packaging Board (LPB).

[0062] 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

CLAIMS1. A method for manufacturing a multiply paperboard on a paperboard machine comprising the steps of:- forming a multiply paperboard web comprising a top ply, a back ply and at least one middle ply arranged between said top and back ply, wherein the middle ply is made from a middle ply furnish comprising at least 40 wt%, based on the total dry weight of the middle ply furnish, of CTMP, and wherein the top ply is made from a top ply furnish and the back ply is made from a back ply furnish, wherein the back ply furnish comprises at least 80 wt% (based on the total dry weight of the back ply furnish) of unbleached kraft pulp having a kappa number according to ISO 302:2015 of at least 40,- dewatering, pressing and drying the thereby formed multiply paperboard web, forming a dried multiply paperboard web,- cooling the dried multiply paperboard web,- calendering the cooled web in a hard nip with a nip load of 10 - 50kN / m, preferably 10 - 40 kN / m, and- reeling the web.

2. A method according to claim 1, wherein the cooling is performed solely by directing a flow of cold air to the web, which air has a temperature in the range of 10 - 50 °C, preferably 10 - 40 °C, and a speed of 20 - 100 m / s, preferably 40 - 80 m / s.

3. A method according to anyone of the preceding claims, further comprising a step of moisturizing the web by applying steam to the surface of the top ply, which moisturizing step is performed after the cooling step and before the calendering step.

4. A method according to anyone of the preceding claims, wherein the hard nip is formed between a heated calender roll and a backing roll, wherein the surface temperature of the heated calender roll is above 100 °C, preferably in the range of 150 - 300 °C.

5. A method according to anyone of claim 4, wherein the surface temperature of the backing roll is above 80 °C, preferably in the range of 100 - 230 °C.

6. A method according to anyone of the preceding claims, wherein the middle ply furnish comprises at least 60 wt%, preferably between 60 - 90 wt%, based on the total dry weight of the middle ply furnish, of CTMP.

7. A method according to anyone of the preceding claims, wherein the middle ply furnish comprises 0.1-5 weight% pigments and latex from coated broke, based on the total dry weight of the middle ply furnish.

8. A method according to anyone of the preceding claims, wherein the back ply furnish comprises at least 75 wt%, preferably between 90 - 100 wt%, based on the total dry weight of the back ply furnish, of unbleached kraft pulp having a kappa number according to ISO 302:2015 of at least 40.

9. A method according to anyone of the preceding claims, wherein the unbleached kraft pulp in the back ply furnish has a kappa number according to ISO 302:2015 of at least 60, preferably at least 80, more preferably at least 90, such as between 60 - 110, or 80 - 110 or 90 - 110.

10. A method according to anyone of the preceding claims, wherein the unbleached kraft pulp in the back ply furnish has a fiber content generated from pine of at least 50 wt%, preferably in the range of 50 - 60 wt%, as calculated on the dry weight of the total fiber content in the back ply furnish.

11. A method according to anyone of the preceding claims, wherein the back ply furnish comprises 0.01 - 10 wt%, based on the total dry weight of the back ply furnish, 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.

12. A paperboard manufactured according to anyone of the claims 1 - 11 , which paperboard comprises a top ply, a back ply and at least one middle ply arranged between the top ply and the back ply.

13. A paperboard according to claim 12, wherein the Bendtsen roughness according to ISO 8791-2 of the outer surface of the top ply is below 620 ml / min, preferably within the range of 200 - 600 ml / min.

14. A paperboard according to anyone of the claims 12-13, wherein the Bendtsen roughness according to ISO 8791-2 of the outer surface of the back ply is below 1050 ml / min, preferably within the range of 400 - 1000 ml / min.

15. A paperboard according to anyone of the claims 12-14, wherein the paperboard exhibits a bulk of at least 1 cm3 / g, preferably at least 1.5 cm3 / g, as measured using ISO 534:2011.

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