A method for manufacturing a multi-ply paperboard

Abstract

The invention discloses a method for manufacturing a multi-ply paperboard comprising the steps of: - forming a web comprising at least a top ply, a back ply and a middle ply, wherein the middle ply is formed from a first furnish comprising at least 50 wt% of a CTMP or HT-CTMP, or a mixture thereof, and 0.1–15 wt% of a first fiber-based bulking agent based on the total dry weight of said first furnish, wherein the first fiber-based bulking agent has a Schopper Riegler (SR) value in the range of 70-100 as measured according to ISO 5267-1:1999, a tensile index less than 70 Nm / g as measured according to ISO 1924-3:2011 and an apparent bulk density less than 900 kg / m3 as measured according to ISO 534:2011, - subjecting the thereof formed web to pressing in at least one shoe press, and - drying the web.

Description

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

[0002] Technical field

[0003] The present disclosure relates to a method for manufacturing a multi-ply paperboard.

[0004] Background

[0005] Paperboard intended for conversion into packages using fast-running automatic machines must possess the requisite strength to withstand the strain and stress associated with converting processes. Additionally, it must exhibit high bending resistance, not only to facilitate smooth converting operations but also to ensure optimal package performance. For products with extended shelf life, the package must also possess excellent barrier properties against light, oxygen, and moisture.

[0006] The bulk of paperboard (inverse of density) is a significant property contributing to its thickness. Increased thickness enhances the bending stiffness of the board and enables the papermaker to reduce the amount of fibers used, resulting in cost savings. However, higher bulk often leads to a decrease in internal strength. One of the challenges faced by papermakers is to increase the bulk of the paperboard while maintaining its strength and to ensure for the convertability (folding and creasing) of the paperboard.

[0007] Typically, paperboard consists of 1-5 plies (layers). Paperboard intended for conversion usually comprises multiple plies, exhibiting higher bending resistance index compared to single-ply paperboard. Multi-ply paperboard generally consists of top and back plies, along with one or more middle plies. The middle plies provide bulk to the paperboard. Optionally, one or several layers of bonding agents are added between the plies to improve ply bond strength

[0008] To provide strength and excellent printing properties, chemical pulp is commonly used in the top and back plies of the board. The middle ply may contain both mechanical pulp and / or chemical pulp. Mechanical pulp or semi-mechanical pulp, such as bleached or unbleached CTMP (chemi-thermomechanical pulp), is oftentimes preferred due to its lower cost compared to chemical pulp and its bulk inducing properties. Mechanical or semi-mechanical pulp also offers higher raw material efficiency and yield. Softwood CTMP is frequently employed in the middle ply or bulk layer in high-quality boards, as it provides high bulk and comprises a low content of shives. Chemical pulp is typically used in conjunction with the mechanical pulp in the middle ply to enhance strength.

[0009] In recent years, high-temperature chemi-thermomechanical pulp (HT-CTMP) has been preferred in certain applications when forming the bulk layer. HT-CTMP refers to further development of the CTMP process, wherein the material has been subjected to higher pre-heating temperature before the refining step in the manufacturing process, resulting in a pulp with a higher opacity and / or higher content of longer fibers, which further improves the bulk and the stiffness of a paperboard made thereof. In addition, less refining energy is needed. On the other hand, a higher content of longer fibers or bulky fibers may affect formation and the flocculation behavior of the fibers negatively. Moreover, high yield pulp based on for example CTMP and HT-CTMP will require dry strength bonding agents, especially when targeting higher bulk.

[0010] Dry bond strength can be improved with natural polysaccharides such as starch as well as synthetic polymers. Recently, nanocellulose or fine microfibrillated cellulose has been used to improve strength properties such as tensile strength, burst strength and compression strength of the paperboard.

[0011] Unfortunately, many of the available strength enhancing additives increases the drainage resistance and density of the paperboard, which in turn may impact e.g. stiffness and mechanical behavior. Moreover, the use of a high content of water soluble polysaccharides and / or nanocellulose increases the risk of low (self-) retention and may further consume a higher content of retention and drainage aids. It is a challenge to improve the retention of strength chemicals when using a high content of nanocellulose.

[0012] The patent publication WO15087293 discloses a paperboard comprising HT- CTMP and strength additives, such as MFC and starch. The addition of MFC to paperboard may however increase the density of the paperboard, whereby the bulk is reduced. To control sheet density and especially the density profile (z- direction) and to increase or maintain high bulk, it is common to optimize dewatering and wet pressing by using extended nips, such as shoe presses, in the pressing section in combination with low nip loads. However, the utilization of shoe presses for preserving bulk in MFC-containing paperboard poses challenges due to the difficulty in dewatering the board effectively, resulting in the requirement of high nip loads. This might lead to uneven (z-)distribution of strength additives, which may result in delamination, reduced strength and problems with dimensional stability.

[0013] There thus remains a need for a method for manufacturing a paperboard with high bulk and yet excellent strength properties, which method further enables high retention and an even distribution of strength chemicals.

[0014] Description of the invention

[0015] It is an object of the present disclosure to provide a method for manufacturing a paperboard with a high bulk while maintaining excellent strength properties.

[0016] Another object of the present disclosure is to provide a method for manufacturing paperboard that is energy-efficient and yields high-quality paperboard with exceptional strength properties.

[0017] 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.

[0018] According to a first aspect illustrated herein, there is provided a method for manufacturing a multi-ply paperboard comprising the steps of:

[0019] - forming a web comprising at least a top ply, a back ply, and a middle ply arranged between the top ply and the back ply, wherein the middle ply is formed from a first furnish comprising at least 50 wt% of a CTMP or a HT- CTMP, or a mixture thereof, and 0.1 - 15 wt% of a first fiber-based bulking agent based on the total dry weight of said first furnish, wherein said first fiberbased bulking agent has a Schopper Riegler (SR) value in the range of 70 - 100 as measured according to standard ISO 5267-1:1999, a tensile index less than 70 Nm / g as measured according to standard ISO 1924-3:2011 and an apparent bulk density less than 900 kg / m3as measured according to standard ISO 534:2011,

[0020] - subjecting the thereof formed web comprising at least the top ply, the back ply and the middle ply to pressing in at least one shoe press, and

[0021] - drying the web.

[0022] It has been found that utilizing the aforementioned furnish mixture, which includes HT-CTMP or CTMP, or a mixture thereof, along with the first fiber-based bulking agent having an SR value within the specified range, a tensile index below the specified value and an apparent bulk density less than the specified value enables efficient permeability and / or dewatering using a shoe press and results in a paperboard with enhanced bulk with low densification and controlled shrinkage behavior. Moreover, the method according to the present disclosure has shown to improve the retention of strength additives and result in a more even distribution of the strength additives in the z-direction of the middle ply. It further enables a higher content of CTMP or HT-CTMP, or a mixture thereof, in the middle ply of the paperboard, although CTMP or HT-CTMP, or a mixture thereof, comprises bulky and / or long fibers, without affecting the strength and dimensional stability negatively.

[0023] The term “high-temperature chemi-thermomechanical pulp (HT-CTMP)” as used herein refers to pulp that has been pre-heated to a temperature of at least 140 °C, preferably of at least 150 °C, or even more preferably of at least 160 °C prior to the refining step. The HT-CTMP may be produced from hardwood or softwood or a combination thereof. The HT-CTMP can be bleached or unbleached or delignified pulp. Preferably, the fibers of the HT-CTMP have a length-weighted mean fiber length of at least 0.7 mm and more preferably of at least 0.8 mm, such as between 0.7 - 2.5 mm or between 0.8 - 2.5 mm, as measured according to standard ISO 16065-2 and / or a mass fraction of fines of less than 15%, more preferably less than 10% and most preferably less than 8% as measured according to standard ISO 10376:2011. The freeness (CSF) of the HT-CTMP pulp is preferably at least 550 ml, more preferably at least 600 and most preferably at least 650 ml as measured according to standard ISO 5367-2. The HT-CTMP pulp may be made from hardwood or softwood. The HT-CTMP may further be made from a mixture of hardwood and softwood, such as birch-HT-CTMP and spruce HT-CTMP, preferably with a ratio birch:spruce of 100:0 to 50:50, such as 90:10, 80:20, 70:30 and / or 60:40. Preferably, the HT-CTMP has a shape factor of at least 80%, more preferably at least 85% and most preferably at least 88%. The shape factor defines the straightness of the fibers and is defined as the maximum extension length of the fiber (projected length) divided by the true length of the fiber (along the fiber contour). The shape factor can be calculated according to the formula S = 10Oxl / L, wherein I = the projected length and L = the true length. In the manufacturing of the HT-CTMP, the fibers have preferably been subjected to gentle refining, preferably by high-consistency (HC) refining, and optionally to treatment in at least one latency chest. The fibers may further have been subjected to at least one washing step. In this way, the shape factor, the permeability and the bulk of the pulp are improved.

[0024] The term “fiber-based bulking agent” is herein intended to mean an agent which is fiber-based and has bulking properties. The fiber-based bulking agent is fibrillated cellulose and is produced by fibrillating cellulose containing pulp, such as virgin pulp. The fiber-based bulking agent may be produced from CTMP or HT-CTMP, preferably HT-CTMP.

[0025] The first furnish comprises at least 50 wt% of CTMP or HT-CTMP, or a mixture thereof, based on the total dry weight of the first furnish. Preferably, the first furnish comprises CTMP or HT-CTMP, or a mixture thereof, in the range of 50 - 99.1 wt%, more preferably in the range of 60 - 90 wt%, or in the range of 70 - 85 wt%, and the first fiber-based bulking agent in the range of 0.1 - 15 wt%, more preferably in the range of 0.1 - 10 wt%, or in the range of 0.1 - 5 wt%, based on the total dry weight of the first furnish. The remaining pulp in the first furnish may be chemical pulp, such as Kraft pulp, preferably bleached or unbleached Kraft pulp from hardwood, and may at least partly originate from broke.

[0026] Paperboard generally refers to strong, thick paper or cardboard comprising cellulose fibers used for boxes and other types of packaging. Typically, uncoated paperboard has a grammage of between 150 - 400 gsm, preferably 180 - 400 gsm, or 200 - 350 gsm and a density of between 500 - 900 kg / m3, preferably 600 - 850 kg / m3. The process of the present disclosure is particularly advantageous for the production of Folding Boxboard (FBB) used for e.g. food packaging applications such as cereal boxes, frozen food packaging and confectionery boxes. Other suitable end-uses include Clay Coated News Back (CCNB), which is a recycled paperboard coated with a layer of clay on the top side and often has a grey or brown backside and is used for e.g. food packaging boxes, takeout containers and retail packaging and greaseproof paperboard, which has a special treatment that provides resistance to grease and oil used for packaging of fatty and greasy food items like bakery products, fast food and fried snacks. Further suitable end-uses include Food Service Board (FSB), Coated Unbleached Kraft paperboard (CUK), White-Top Kraft Liner (WTKL), coated White-Top Kraft Liner (cWTKL), Cup Board, or Liquid Packaging Board (LPB).

[0027] In embodiments, the first furnish further comprises a strength additive selected from the group consisting of cationic starch, anionic polymers, Kraft pulp-based microfibri Hated cellulose (MFC), polyvinylamine, chitosan, primary and secondary amines, polyethylene amines and modified polyacrylamides and combinations thereof. The first furnish may comprise such additional strength additives in an amount of 1 - 5 wt% based on the total dry weight of the first furnish. The anionic polymer may be e.g. carboxymethyl cellulose (CMC) and / or anionic polyacrylamide (A-PAM). In embodiments, the first furnish comprises less than 3 wt%, preferably less than 2 wt% or less than 1 .5 wt% of Kraft pulp-based MFC, such as in the range of 0.1 - 3 wt%, or 0.1 - 2 wt% or 0.1 - 1.5 wt% of Kraft pulpbased MFC, based on the total dry weight of the first furnish.

[0028] 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. 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.

[0029] The term “Kraft pulp-based microfibri Hated cellulose (MFC)” as used herein refers to MFC made from Kraft pulp or made from a pulp mixture (i.e. , a mixture comprising two or more different types of pulp) comprising at least 50 wt%, preferably at least 60 wt%, Kraft pulp. Preferably the Kraft pulp-based MFC is made from never-dried Kraft pulp having a lignin content less than 5 wt%. The Kraft pulp may be bleached or unbleached. Preferably, the Kraft pulp may be made from softwood or hardwood. The Kraft pulp-based MFC may have an SR value in the range of 85 - 100, preferably in the range of 90 - 100, or in the range of 90 - 98, as measured by standard ISO 5267-1 :1999.

[0030] In embodiments, the first furnish comprises an anionic strength additive and a cationic strength additive. The anionic additive is preferably an anionic polysaccharide selected from nanocrystalline cellulose, Kraft pulp-based MFC, CMC or sodium carboxymethyl cellulose, anionic starch, alginate, and hemicellulose, whereas the cationic additive is preferably a cationic polysaccharide, such as cationic starch.

[0031] Most preferably, the cationic starch is a high Mw (i.e., average molecular weight is higher than 2 000 000 Dalton and preferably above 3 000 000 Dalton and most preferably above 4 000 000 Dalton) 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 method of the present disclosure allows for a low amount of starch dosing, such as 0.1 - 3 wt%, or preferably 0.1 - 2 wt%, based on the total dry weight of the first furnish, since the retention of starch is improved. The first furnish may further comprise internal sizing agents, preferably chosen from the group of alkyl ketene dimer (AKD), alkyl succinic anhydride (ASA), styrene acrylate or styrene maleic anhydride, waxes, and rosin resin, or a combination thereof, preferably added to the first furnish in an amount of in the range of 0.02 - 1.5 wt%, or preferably in an amount in the range of 0.1 - 0.5 wt%, based on the total dry weight of the first furnish. When internal sizing agents are used in combination with the first fiber-based bulking agent as an additive, the obtained product shows reduced drying shrinkage due to higher press solids after press section and due to lower drainage resistance of the first fiber-based bulking agent.

[0032] In embodiments, the first furnish is formed by adding the first fiber-based bulking agent and at least part of the internal sizing agents as a pre-mixture to a furnish comprising CTMP, HT-CTMP or a mixture thereof. When at least a part of the surface sizing agents is added as a pre-mixture with the first fiber-based bulking agent the retention of the surface sizing agents is further improved.

[0033] In embodiments, the first furnish is formed by adding a pre-mixture of the first fiber-based bulking agent and a strength additive, such as an anionic polymer, such as CMC or A-PAM, having a degree of substitution of less than 0.7, preferably of less than 0.6, and most preferably of between 0.05 - 0.5, to a furnish comprising CTMP, HT-CTMP or a mixture thereof. Such a quality of anionic polymer stabilizes the first fiber-based bulking agent but has less solubility in water whereby the retention onto the first fiber-based bulking agent is enhanced. Moreover, this has also a positive effect on the formation of the produced paperboard.

[0034] In embodiments, the first furnish is formed by adding a pre-mixture of the first fiber-based bulking agent and an anionic polymer, such as CMC or A-PAM, having an ash content of higher than 1 wt%, preferably higher than 5 wt%, most preferably higher than 10 wt%, such as in the range of 1 - 60 wt%, or 5 - 60 wt% or 10 - 40 wt%, to a furnish comprising CTMP, HT-CTMP or a mixture thereof. In this way, the affinity of the anionic polymer to the fiber is improved. The term “ash” as used herein refers to inorganic material, for example added electrolytes and residuals from the anionic polymer manufacturing process.

[0035] In embodiments, the first furnish is formed by adding a pre-mixture of the first fiber-based bulking agent and a polysaccharide, preferably an anionic, non-ionic or amphoteric polysaccharide, to a furnish comprising CTMP, HT-CTMP or a mixture thereof.

[0036] The first fiber-based bulking agent, having an SR value of 70-100 as measured according to standard ISO 5267-1 :1999, a tensile index less than 70 Nm / g as measured according to standard ISO 1924-3:2011 and an apparent bulk density less than 900 kg / m3as measured according to standard ISO 534:2011 , as mentioned above may be produced by fibrillating (virgin and preferably never- dried) CTMP or HT-CTMP, preferably HT-CTMP. The CTMP or HT-CTMP may be bleached or unbleached. The CTMP or HT-CTMP may be produced from hardwood or softwood pulp or a combination thereof. Preferably, the CTMP or HT- CTMP is produced from softwood pulp, most preferably never dried softwood Kraft pulp. The hardwood may be from e.g., birch, eucalyptus, aspen or maple leaf, or a mixture thereof. The softwood may be from e.g., spruce or pine, or a mixture thereof. In embodiments, the first fiber-based bulking agent is produced by fibrillating CTMP or HT-CTMP, wherein the CTMP or HT-CTMP is produced from hardwood. In other embodiments, the first fiber-based bulking agent is produced by fibrillating CTMP or HT-CTMP, wherein the CTMP or HT-CTMP is produced from a mixture of hardwood and softwood. The fibrillation can be accomplished in one or several steps. In embodiments, the fiber-based bulking agent may be fractionized prior to or after a fibrillation step. This helps to adjust the aspect ratio and hence the bulk enhancing effect. In other embodiments, the first fiber-based bulking agent is subjected to refining to exhibit the SR value of 70 - 100. In these embodiments, the refining step is optimized to achieve the desired properties and the first fiber-based bulking agent is not fractionated.

[0037] In embodiments, the first fiber-based bulking agent is produced by subjecting CTMP or HT-CTMP to a first refining step, preferably at low consistency (1-5 wt%), whereafter the refined pulp is subjected to a fibrillation step using, for example, a second refiner or high shear fibrillation equipment. In some embodiments, the accept from the first refining step is subjected to a second refining / fibrillation step at low consistency refining performed in series to produce the first fiber-based bulking agent having the specified SR value, tensile index and apparent bulk density. In some other embodiments, there is at least a third low consistency refining step between the first refining step and the second fibrillation step. In these embodiments, fibrillation is fine tuning the fiber suspension to the specified tensile index and apparent bulk density, whereas the first and second refining are used to gradually refine and, optionally, cut the fibers in order to adjust the aspect ratio of the material.

[0038] In embodiments in which the first fiber-based bulking agent is produced by fibrillating HT-CTMP, the pulp or chips have preferably been subjected to preheating at a temperature above 160 °C, more preferably above 170 °C, such as above 175 °C or 180 °C, prior to the refining. A higher treatment temperature provides a higher softening of lignin and hence more efficient pre-refining prior to fibrillation, i.e. higher SR value between pre-refining and fibrillation. One benefit of this treatment step is that the bulk enhancing effect is greater.

[0039] In embodiments in which the first fiber-based bulking agent is produced by fibrillating HT-CTMP, the utilized HT-CTMP is preferably a never-dried HT-CTMP having an SR value of 9 - 25, such as 15 - 25, as measured according to standard ISO 5267-1 :1999, a KAPPA value higher than 80 as measured according to standard ISO 302:2015 and a hemicellulose content above 15 wt% as measured according to SCAN-CM 71 :09.

[0040] Preferably, the first fiber-based bulking agent comprises at least 50 wt%, or at least 80 wt% or at least 90 wt% or at least 95 wt% of fibrillated CTMP or HT-CTMP fibers as calculated on the total dry weight of the first fiber-based bulking agent. As mentioned above, the SR value of the first fiber-based bulking agent is between 70 - 100 as measured according to standard ISO 5267-1 :1999. In some embodiments, the SR value of the first fiber-based bulking agent is between 70 - 90 or 75 - 90 or 80 - 90 or 85-90, as measured according to standard ISO 5267- 1 :1999. In some embodiments, the SR value of the first fiber-based bulking agent is between 70 - 95 or 75 - 95 or 80 - 95 or 85 - 95, as measured according to standard ISO 5267-1 :1999. In some embodiments, the SR value of the first fiberbased bulking agent is between 75 - 100 or 80 - 100 or 85 - 100 or 90 - 100, as measured according to standard ISO 5267-1 :1999.

[0041] As mentioned above, the first fiber-based bulking agent has a tensile index less than 70 Nm / g as measured according to standard ISO 1924-3:2011. Preferably, the first fiber-based bulking agent has a tensile index less than 60 Nm / g as measured according to standard ISO 1924-3:2011. The tensile index is measured for 60 gsm sheets, comprising 100 wt% fiber based bulking agent, prepared by following the standard ISO 5269-1 :2005, but improving the retention efficiency by applying a filter paper, such as a filter paper, preferably a wet-strengthened filter paper, having a retention level of 8-12 pm and a filtration speed of 20 s / 10 ml (e.g., Ahlstrdm Munktell 1289), on the wire.

[0042] As mentioned above, the first fiber-based bulking agent has an apparent bulk density less than 900 kg / m3as measured according to standard ISO 534:2011 . Preferably, the first fiber-based bulking agent has an apparent bulk density of 500 - 900 kg / m3, most preferably 550 - 850 kg / m3, as measured according to standard ISO 534:2011 for sheets as described above.

[0043] In embodiments, the first fiber-based bulking agent has a water retention value (WRV) of less than 200%, preferably less than 180%, most preferably less than 170%, as measured according to standard ISO 23714:2014.

[0044] In embodiments, the first fiber-based bulking agent has a mean fibril area of fibers having a length of at least 0.2 mm (ISO 16065-2:2014) of at least 10%, preferably of at least 15, and more preferably at least 20%, such as 20-60%. The term “mean fibril area” as used herein refers to the length weighted mean fibril are and can be determined using a Fiber Tester Plus device (ABB L&W).

[0045] The paperboard may comprise further bulk providing middle plies, such as a first and a second middle ply or a first, a second and a third middle ply. Such additional middle plies may be formed from the first furnish. In embodiments, the top ply is formed from a second furnish comprising 80 - 100 wt% of hardwood Kraft pulp, preferably bleached hardwood Kraft pulp, based on the total dry weight of said second furnish. Such a composition of the top ply improves the formation, surface smoothness and the print quality of the paperboard. In embodiments, the second furnish comprises 80 - 99.9 wt% of hardwood Kraft pulp and 0.1 - 15 wt%, preferably 0.1 - 10 wt% and most preferably 0.1 - 5 wt% of a second fiber-based bulking agent, based on the total dry weight of said second furnish. The remaining pulp of the second furnish may be e.g. softwood Kraft pulp.

[0046] The second fiber-based bulking agent utilized in the second furnish may be further defined and produced as set out above for the first fiber-based bulking agent utilized in the first furnish. Thus, the second fiber-based bulking agent of the second furnish has a Schopper Riegler (SR) value in the range of 70 - 100 as measured according to standard ISO 5267-1 :1999, a tensile index less than 70 Nm / g as measured according to standard ISO 1924-3:2011 and an apparent bulk density less than 900 kg / m3as measured according to standard ISO 534:2011 .

[0047] In embodiments, the back ply is formed from a third furnish comprising 80 - 100 wt% of bleached or unbleached Kraft pulp from softwood or hardwood based on the total dry weight of the third furnish. Optionally, the third furnish may comprise 0.1 - 15 wt%, preferably 0.1 - 10 wt%, most preferably 0.1 - 5 wt%, of a third fiber-based bulking agent, based on the total dry weight of the third furnish. The third fiber-based bulking agent may be further defined and produced as set out above for the first fiber-based bulking agent utilized in the first furnish. Thus, the third fiber-based bulking agent of the central layer composition has a Schopper Riegler (SR) value in the range of 70-100 as measured according to standard ISO 5267-1 :1999, a tensile index less than 70 Nm / g as measured according to standard ISO 1924-3:2011 and an apparent bulk density less than 900 kg / m3as measured according to standard ISO 534:2011 .

[0048] As mentioned above, the method of the first aspect comprises a step of forming a web comprising at least a top ply, a back ply and a middle ply arranged between the top ply and the back ply, i.e., a wet web comprising at least three plies is formed (i. e. , a wet multi-ply web is formed). The method of the first aspect comprises also a step of subjecting the thereof formed web to pressing in at least one shoe press, i.e., the formed wet web is subjected to wet pressing in at least one shoe press. Furthermore, the method of the first aspect comprises a step of drying the web, i.e., drying the web to a dried web.

[0049] The top ply, middle ply and back ply may be formed by use of a multi-layer headbox. The method may further comprise forming a central layer in-between the top ply and the middle ply or in-between the middle ply and the back ply, using the multi-layer headbox. The multi-layer headbox preferably includes separate guide devices, such as guiding tubes, for guiding the furnishes and an aqueous composition forming the central layer to a nozzle, from which the furnishes and the central layer composition emerge via a gap onto a forming wire.

[0050] In alternative embodiments, the top ply, middle ply and back ply may be formed as separate webs on separate wires, wherein the method includes the step of couching said webs to form the multi-ply web. In the embodiment wherein the paperboard comprises more than one middle ply, the middle plies may be formed by use of a multilayer headbox.

[0051] The multi-ply web may also be formed by using wet-in-wet-forming, where the same long wire is used for forming first one ply with one head box and then forming consecutive plies on the same wire on top of previously formed plies. This method is preferably complemented with top wires for upwards dewatering after each of the consecutive plies that are formed on the previously formed plies.

[0052] The use of multi-layer headbox can also be used in both wet- in-wet-forming and with separately formed layers.

[0053] The web comprising the top-, middle- and back ply may be dewatered on a dewatering wire by use of a sleeve roll prior to the step of pressing the web. The sleeve roll may be curved to change the wrap angle of the dewatering wire on the sleeve roll, thereby increasing the dewatering capacity. It has been found that the first furnish mixture of the present disclosure facilitates the use of such a sleeve roll in the dewatering without causing problems with two-sidedness. Two- sidedness of paperboard may induce problems with curl, formation, uneven distribution of fines, delamination at the converting of the paperboard etc.

[0054] The multilayer web is preferably subjected to pressing in at least a first and a second shoe press. The present disclosure enables higher line loads in the shoe presses compared to using polymeric bulking agents, while preserving the bulk. In embodiments, the line load of the first and the second shoe press is between 200

[0055] - 1000 kN / m, preferably in the range of 200 - 800 kN / m. The line load of the first shoe press may e.g. be between 200 - 800 kN / m and the line load of the second shoe press may be between 600 - 1500 kN / m. In embodiments, the web is subjected to pressing in a first, a second and a third shoe press nip. In these embodiments, the line load of the first shoe press may be e.g., in the range of 500

[0056] - 800 kN / m and the line load of the second shoe press may be e.g., in the range of 700 kN / m - 1500 kN / m, preferably in the range of 800 - 1200 kN / m, and the line load of the third shoe press nip may be e.g., in the range of 600 - 1500 kN / m, preferably in the range of 700 - 1200 kN / m. The use of the claimed furnish mixture in the middle ply reduces the risk of migration of the strength additives at the pressing of the paperboard. Migration of strength additives may cause undesired two-sidedness. The web speed in the press section is preferably between 450 - 1200 m / min, more preferably between 800 - 1000 m / min.

[0057] In embodiments, the at least one shoe press forms a shoe press nip and the temperature of the web in the shoe press nip is in the range of 20 - 75 °C, preferably in the range of 35 - 75 °C and most preferably in the range of 40 - 75 °C. It has been found that the addition of the fiber-based bulking agent to the furnish in accordance with the present disclosure enhances the positive influence of temperature on water removal.

[0058] In embodiments, the method further comprises the step of reeling the web (after the step of drying the web) on a reel spool to form a paperboard roll, wherein the paperboard roll is loaded with two rider rolls during the reeling. In this way, the bulk is maintained, and issues related to two-sidedness is reduced. The paperboard manufactured by the method of the present disclosure preferably exhibits a Scott Bond of at least 180 J / m2, or preferably at least 200 J / m2or at least 220 J / m2as measured according to standard TAPPI 569 and / or a tensile stiffness index (GM) of at least 5 kNm / g, or preferably at least 6 kNm / g or at least 10 kNm / g, as measured according to standard ISO 1924-3:2005, and / or a thickness of between 250 and 800 pm, and / or a PTS recyclability of the base board defining a reject level of less than 10%, or less than 5% according to PTS RH 021 / 97 test method for Category II. Also, the paperboard manufactured by the method of the present disclosure has a low density and preferably a Z-strength above 200 kPa, more preferably above 350 kPa such as above 380 kPa, based on SCAN-P 80:98. Preferably, the tensile index (CD) for the paperboard is preferably above 20 Nm / g, more preferably above 22 Nm / g, most preferably above 25 Nm / g, according to standard ISO 1924-3:2011. In one embodiment, the paperboard manufactured by the method of the disclosure exhibits 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.

[0059] In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.

[0060] Example 1

[0061] In Example 1 a fiber-based bulking agent was produced. The raw materials used to produce the fiber-based bulking agent was bleached spruce CTMP which was low-consistency refined (4 wt%) to fines enriched material (>30 wt%) and then subjected to fibrillation by using high intensity refiner (Masuko supermass colloider).

[0062] As comparative examples, a softwood Kraft pulp, which was enzyme treated prior to fibrillation, was used (Reference 1) and a commercial grade MFC (Celish KY100S, Daicel FineChem Ltd.) was used (Reference 2). 60 g / m2sheets were prepared from the pure MFC and fiber-based bulking agent by dewatering through a filter paper based on the standard ISO 5269-1 :2005 and were then gently wet pressed and dried. The samples were then conditioned and tested. The apparent bulk density was determined according to the standard ISO 534:2011. The tensile index was determined according to the standard ISO 1924- 3:2011 and the grammage was determined according to the standard ISO 536:2019.

[0063] Drainage time (s) was determined based on sheet forming according to standard ISO 5269-1 :2005, in which the time to dewater the samples was determined ( / .e. until no water layer on fiber).

[0064] The term fines as used herein generally refers to particles significantly smaller in size than cellulose fibers. In some embodiments, the term cellulose fines as used herein refers to fine cellulosic or non-cellulosic particles, which are able to pass through a mesh 200 sieve (equivalent hole diameter 76 pm) of a conventional laboratory fractionation device (SCAN-CM 66:05). The fines content may for example be determined according to ISO 10376:2011 by sieving through a Mesh 200 sieve. (The Schopper Riegler of the samples were determined with ISO 5267- 1:1999).

[0065] The results in Table 1 shows that the fibrillated CTMP fines gives, i.e. bulking agent, gives very low drainage resistance and low tensile index properties. Also, the apparent bulk density is clearly lower than for the commercial MFC.

[0066] Table 1 Example 2

[0067] The same fiber based bulking agent as in Example 1 was further added to CTMP furnish (refined to 550-500 ml according to ISO 5267-2:2001) in an amount of 10 wt% (based on CTMP content, dry / dry) together with 1 .5 wt% cationic starch (based on dry content of CTMP fiber). The furnish was mixed and sheets were prepared based on ISO 5269-1 :2005. The obtained wet pressed and dried sheets were then analysed. The apparent bulk density of the sheet was 420 kg / m3(ISO 534:2011), a Z-strength of 400 kPa (SCAN-P 80:98) and tensile index of 27 Nm / g (ISO 1924-3:2011).

[0068] A corresponding sample (reference) without the fiber-based bulking agent and the starch had a significantly lower Z-strength (150 kPa) at a density of 350 kg / m3.

[0069] Although the fiber-based bulking material has low strength properties, the strength when adding to the furnish is surprisingly high and the results show that there is a synergistic effect both from fast drainage and good interaction with cationic starch, which thus enables high internal Z-strength without significant densification of the sheet.

Claims

CLAIMS1. A method for manufacturing a multi-ply paperboard comprising the steps of:- forming a web comprising at least a top ply, a back ply, and a middle ply arranged between the top ply and the back ply, wherein the middle ply is formed from a first furnish comprising at least 50 wt% of a CTMP or HT-CTMP, or a mixture thereof, and 0.1 - 15 wt% of a first fiber-based bulking agent based on the total dry weight of said first furnish, wherein said first fiber-based bulking agent has a Schopper Riegler (SR) value in the range of 70 - 100 as measured according to standard ISO 5267- 1 :1999, a tensile index less than 70 Nm / g as measured according to standard ISO 1924-3:2011 and an apparent bulk density less than 900 kg / m3as measured according to standard ISO 534:2011 ,- subjecting the thereof formed web comprising at least the top ply, the back ply and the middle ply to pressing in at least one shoe press, and- drying the web.

2. A method according to claim 1 , wherein the first furnish further comprises a strength additive selected from the group consisting of cationic starch, anionic polymers, Kraft pulp-based microfi bril lated cellulose, polyvinylamine, chitosan, primary and secondary amines, polyethylene amines and modified polyacrylamides and combinations thereof.

3. A method according to any one of the preceding claims, wherein the HT- CTMP has a freeness (CSF) of at least 600 ml, preferably at least 650 ml, as measured according to standard ISO 5367-2:1999.

4. A method according to any one of the preceding claims, wherein the first furnish further comprises internal sizing agents, preferably chosen from the group of alkyl ketene dimer (AKD), alkyl succinic anhydride (ASA) and rosin resin, or a combination thereof.

5. A method according to any one of the preceding claims, wherein the first furnish is formed by adding a pre-mixture of the first fiber-based bulkingagent and a strength additive to a furnish comprising CTMP or HT-CTMP, or a mixture thereof.

6. A method according to any one of the preceding claims, wherein the first furnish is formed by adding a pre-mixture of the first fiber-based bulking agent and a polysaccharide to a furnish comprising CTMP or HT-CTMP, or a mixture thereof.

7. A method according to any one of the preceding claims, wherein the SR value of the first fiber-based bulking agent is between 75 - 95 as measured according to standard ISO 5267-1:1999.

8. A method according to any one of the preceding claims, wherein the SR value of the first fiber-based bulking agent is between 85 - 90 as measured according to standard ISO 5267-1:1999.

9. A method according to any one of the preceding claims, wherein the first fiber-based bulking agent has a tensile index less than 60 Nm / g as measured according to standard ISO 1924-3:2011.

10. A method according to any one of the preceding claims, wherein the first fiber-based bulking agent has an apparent bulk density of 500-900 kg / m3as measured according to standard ISO 534:2011.11 . A method according to any one of the preceding claims, wherein the first fiber-based bulking agent has a water retention value (WRV) less than 180% as measured according to standard ISO 23714:201412. A method according to any one of the preceding claims, wherein the first fiber-based bulking agent comprises at least 50 wt% of fibrillated CTMP or HT-CTMP fibers as calculated on the total dry weight of the first fiber-based bulking agent.

13. A method according to any one of the preceding claims, wherein the top ply is formed from a second furnish comprising 80 - 100 wt% of hardwoodKraft pulp, preferably bleached hardwood Kraft pulp, based on the total dry weight of said second furnish.

14. A method according to claim 13, wherein the second furnish further comprises 0.1 - 15 wt% of a second fiber-based bulking agent based on the total dry weight of said second furnish, wherein the second fiber-based bulking agent has a Schopper Riegler (SR) value in the range of 70 - 100 as measured according to standard ISO 5267-1 :1999, a tensile index less than 70 Nm / g as measured according to standard ISO 1924-3:2011 and an apparent bulk density less than 900 kg / m3as measured according to standard ISO 534:2011.

15. A method according to any one of the preceding claims, wherein the back ply is formed from a third furnish comprising 80 - 100 wt% of bleached or unbleached Kraft pulp from hardwood based on the total dry weight of said third furnish.

16. A method according to any one of the preceding claims, wherein the at least one shoe press forms a shoe press nip and wherein the temperature of the web in the shoe press nip is in the range of 20 - 75 °C, preferably in the range of 35 - 75 °C and most preferably in the range of 40 - 75 °C.

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