A method for manufacturing a multi-ply paperboard, and a multi-ply paperboard

Abstract

The present disclosure relates to a method for manufacturing a multiply paperboard on a paperboard machine. The method comprises the steps of - forming a middle-ply web from a first furnish comprising at least 20 wt% of CTMP on a first forming wire, - dewatering the middle-ply web on the first forming wire in a first dewatering zone, - dewatering the middle-ply web (3) between the first forming wire (1) and a second forming wire (6) in a second dewatering zone (5), which second dewatering zone (5) comprises at least one suction unit (7) and dewatering blades (8), which dewatering blades (8) are in contact with the second forming wire (6), and at least two loadable dewatering blades (9a, 9b, 9c, 9d) in contact with the first forming wire (1), wherein the loading pressure applied by each of the loadable dewatering blade (9a, 9b, 9c, 9d) is in the range of between 0.1 – 25 kPa -kPa, - forming a top-ply web from a second furnish and a back-ply web from a third furnish, - couching the said webs to form a three-ply web wherein the middle ply web is arranged between the top- and back ply web, - pressing the three-ply web in a press section, - drying the pressed web in a drying section and - reeling the web. The paperboard manufactured according to the present invention exhibits an excellent formation and a high z-strength.

Description

[0001] A method for manufacturing a multi-ply paperboard, and a multi-ply paperboard

[0002] Technical field

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

[0004] 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.

[0005] 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 such as delamination behavior during folding and creasing of the paperboard.

[0006] 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. Optionally, one or several layers of bonding agents are added between the plies to improve ply bond strength

[0007] To provide strength, smoothness, visual appearance 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.

[0008] Mechanical pulp or semimechanical 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 and light absorbing properties. Mechanical or semi-mechanical pulp also offers higher raw material efficiency and yield. Softwood CTMP is frequently employed in the middle ply in high-quality boards, as it provides high bulk and comprises a low content of shives. Chemical pulp is typically used in conjunction with mechanical pulp in the middle ply to enhance strength.

[0009] Another important property of the paperboard is the smoothness and formation, which relates to the uniformity of the fibers and other additives of the paperboard. Good formation is essential for the paperboard to ensure good dimensional stability, visual appearance and convertability and to have a smooth and even surface suitable for e.g. printing or mineral or barrier coating. One of the challenges faced by the paperboard manufacture is to ensure good formation and strength properties, including high z-strength, internal bonding, and ply-bond strength of the paperboard. This challenge is particularly significant when using high contents of high yield pulp and aiming to increase production efficiency, especially through cost-efficient drying methods, which is specifically intended to improve production.

[0010] Description of the invention

[0011] It is an object of the present invention to provide a method for manufacturing a paperboard with a good formation and excellent strength properties.

[0012] Another object of the present invention is to provide a method for manufacturing a paperboard with good formation, high bulk and excellent strength properties, especially high z-strength.

[0013] The above-mentioned objects, 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.

[0014] 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: - forming a middle-ply web from a first furnish comprising at least 20 wt% of CTMP on a first forming wire,

[0015] - dewatering the middle-ply web on the first forming wire in a first dewatering zone,

[0016] - dewatering the middle-ply web between the first forming wire and a second forming wire in a second dewatering zone, which second dewatering zone comprises at least one suction unit and dewatering blades, which dewatering blades are in contact with the second forming wire, and at least two loadable dewatering blades in contact with the first forming wire, wherein the loading pressure applied by each of the loadable dewatering blade is in the range of between 0.1 - 25 kPa,

[0017] - forming a top-ply web from a second furnish and a back-ply web from a third furnish,

[0018] - couching the said webs to form a three-ply web wherein the middle ply web is arranged between the top- and back ply web,

[0019] - pressing the three-ply web in a press section,

[0020] - drying the pressed web in a drying section and

[0021] - reeling the web.

[0022] It has been found that the use of loadable blades in the formation of a middle ply web comprising CTMP leads to improved formation while maintaining high z- strength, internal bonding, and ply-bond strength of the paperboard. The specific use of the loadable blades in accordance with the present invention efficiently improves formation and at the same time prevents crushing and backflow of water that may disrupt settled sheet formation. Additionally, the retention of fines and potential strength additives (such as starch and / or MFC) in the web is significantly enhanced. The retention of fines and strength additives can be particularly challenging when forming webs that contain a high amount of CTMP. The present invention addresses this issue.

[0023] If not specifically denoted otherwise, given % as used herein are weight% (wt%), and are calculated on the basis of a dry weight of 100 weight% of the respective object, such as a layer, a ply, web, a furnish or a composition. In embodiments, the loading pressure applied by each of the loadable dewatering blade is in the range of between 10 - 20 kPa. In embodiments, the loading pressure applied by at least one of the loadable dewatering blades is in the range of between 10-25 kPa, preferably between 15-25 kPa, or 15-20 kPa, or 18-20 kPa.

[0024] In embodiments, the second dewatering zone comprises at least two, preferably three or four suction units. Most preferably, the suction unit / s are multi-chamber suction unit / s. The dewatering blades (also referred to as foils) that are in contact with the second forming wire are preferably stationary dewatering blades in operation.

[0025] The second dewatering zone may comprise two to eight loadable dewatering blades, such as four or six loadable dewatering blades, that are in contact with the first forming section during operation. In embodiments, the second dewatering zone comprises, in the running direction of the middle-ply web, a first, an optional second, a third and an optional fourth loadable dewatering blade that are in contact with the first forming wire. In this embodiment, the loading pressure applied by each of the first and the optional second loadable dewatering blade is preferably between 0.1-12 kPa, preferably between 0.1-10 kPa , and the loading pressure applied by the third and the optional fourth loadable dewatering blade is preferably between 15-25 kPa, preferably in the range of 15-20 kPa. In some embodiments, only the first and the third dewatering may be in use and in contact with the first wire during operation. Alternatively, the first, second, third and fourth dewatering blades may all be in use and in contact with the first wire during operation. It has been found that these loading pressures enhances the formation of the board, while efficiently maintaining the z-strength, internal strength, and ply bond strength, including Scott Bond. The loading pressure applied by each of the loading blade may increase in the running direction of the web, e.g. so that the loading pressure of the second blade is higher than the first blade, the loading pressure of the third blade is higher than the loading pressure of the second blade and the loading pressure of the fourth blade is higher than the loading pressure of the third blade. The first furnish comprises at least 20 wt% chemi-thermal mechanical pulp (CTMP). The CTMP may be bleached or unbleached and may be derived from softwood or hardwood (e.g. birch or eucalyptus), or a mixture thereof. The remaining pulp in the first furnish may be kraft pulp or broke comprising kraft pulp and / or CTMP. In embodiments, the first furnish comprises 20 - 80 wt% CTMP, preferably 30 - 80 wt% CTMP or 40 - 80 wt% CTMP. In embodiments, the first furnish comprises at least 20 wt% broke, wherein the broke or part of the broke can be refined to a Schopper Riegler value of above 30 as determined by standard ISO 5267-1. In preferred embodiments, part of the broke, such as 1-20 wt% of the broke, can be refined to a SR value of above 70, preferably above 75: as determined by standard ISO 5267-1. The CTMP can be dried or never-dried pulp or mixtures thereof, preferably never-dried pulp.

[0026] The first furnish may further comprise a strength additive selected from the group consisting of cationic starch, anionic polymers, nanocrystalline cellulose, microfibrillated cellulose (MFC), polyvinylamine, chitosan, primary and secondary amines, polyethylene amines and modified polyacrylamides and combinations thereof. The anionic polymers may e.g. be anionic water soluble polymers such as carboxymethyl cellulose having a DS between 0.1 and 0.9. The first furnish may further comprise a formation additive, such as polysaccharide gum, e.g. CMC or guar gum.

[0027] In embodiments, the first furnish comprises CTMP and 0.5 - 7 wt%, preferably 2-7 wt% of microfibrillated cellulose (MFC) or highly refined cellulose fibers having a Schopper-Reigler (SR) value of 75 or higher, as determined by standard ISO 5267-1 , as calculated on the total dry weight of the first furnish The MFC or the highly refined pulp may be made from bleached kraft pulp from hardwood or softwood, or a mixture thereof. In embodiments, the MFC exhibits a water retention value (WRV) of more than 140%, preferably more than 160%, most preferably more than 170%, as measured using ISO 23714:2014

[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. The MFC is preferably softwood MFC. The method of the invention provides particular benefits when used to form middle-ply webs comprising CTMP and MFC or highly refined cellulose. When the web comprises MFC or highly refined cellulose, water retention within the web is more efficiently, allowing for higher loading blade pressures without risking web damage. This contributes to enhanced formation. The inclusion of MFC or highly refined cellulose may permit the loading pressure exerted by at least one of the loadable dewatering blades to be within the range of 15-25 kPa. This results in further improved formation. .

[0029] In embodiments, the CTMP comprises a fine fraction that is able to pass through a 200 Mesch screen in an amount of less than 20 wt%, preferably less than 15 wt% or less than 10 wt%, as calculated on the total weight of the CTMP. The amount of fine fraction is measured in accordance with SCAN-CM 66:05. The method of the invention enables the use of a CTMP pulp with such low amount of fine fraction while still achieving high z-direction strength, internal strength, and ply-bond strength. This is possible due to the excellent retention of the remaining fines, which also enables the use of washed pulp as described below

[0030] In embodiments, the CTMP is high temperature CTMP (HT-CTMP). 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 10%, more preferably less than 8% and most preferably less than 4% 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 ml and most preferably at least 650 ml as measured according to standard ISO 5367-2. Preferably, the HT-CTMP has a shaper 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 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.

[0031] In embodiments, the first furnish comprises in an amount of less than 15 kg / ton, preferably less than 12 kg / ton starch, such as in an amount in the range of 5 - 12 kg / ton or 5 - 10 kg / ton. The starch is preferably cationic starch or amphoteric starch. The method of the invention enables the use of less wet-end starch and still achieve high strength properties in the formed paperboard, since starch, as well as other strength additives, are effectively retained in the web. It is believed that the starch retention is above 60%, such as above 70% and most preferably above 80%. The starch retention can be determined e.g. by analyzing the starch content in white water taken from the first and / or second dewatering zone.

[0032] The formation of the middle-ply web is preferably performed at a speed of 4 - 12 tons / hours / m, preferably 4 - 10 tons / hours / m.

[0033] In embodiments, the middle ply web is formed by applying the first furnish on the first forming wire from a first headbox, wherein the consistency of the first furnish in the first headbox is 0.25 wt%-1 .1 wt%, preferably 0.25 - 0.85wt%, and more preferably 0.5 - 0.85wt%.

[0034] The method of the invention enables achieving good formation even when using slightly higher consistency levels. This is made possible by the mechanical retention of the MFC and fines, while maintaining energy efficiency.

[0035] Before entering the second dewatering zone, the middle ply web is dewatered on the first forming wire in the first dewatering zone. In the first dewatering zone, the web is dewatered on a single forming wire (i.e. on the first forming wire), resulting in one-sided dewatering. In embodiments, the middle -ply is dewatered in the first dewatering zone by passing the web on the first forming wire over a number of foils with adjustable angle and height. In addition to, or alternatively to, treatment with foils, the web may be subjected to dewatering by the application of vacuum in the first dewatering zone, preferably using vacuum boxes. The vacuum pressure applied on the web in the first dewatering zone is preferably within the range of 1- 10 kPa, and more preferably within the range of 1-7 kPa, or in the range of 1-5 kPa. In some embodiments, the vacuum pressure applied to the web in the first dewatering zone is at most 10 kPa, preferably at most 7 kPa, or at most 5 kPa. In this way the dewatering before the web is entered into the second dewatering zone is optimized for the subsequent treatment with loadable blades.

[0036] The inventors have found that by controlling the solid content of the web when entering the second dewatering zone, it is possible to apply higher loading pressures from the dewatering blades without compromising the integrity of the web. A relatively low solid content (i.e. high water content) of the web may be maintained by incorporating of MFC or highly refined pulp into the web and / or by regulating the vacuum pressure applied in the first dewatering zone - specifically keeping it at a lower level.

[0037] In embodiments, the middle ply web is formed by applying the first furnish on the first forming wire from a first headbox, wherein the water retention value (WVR) of the first furnish in the headbox is at least 90%, or at least 100%, or at least 110% preferably in the range of 100-180%, or 100-160%, or 100-140%, or 110-160%, as measured using ISO 23714:2014. Such high WVR of the first furnish further facilitates the dewatering with high loading pressures from the dewatering blades without damaging the web.

[0038] In embodiments, the top-ply web is formed by applying the second furnish on a forming wire from a second headbox and the back-ply web is formed by applying the third furnish on a forming wire from a third headbox, wherein the consistency of the second furnish in the second headbox and the third furnish in the third headbox is 0.10 - 0.40 wt%, preferably 0.2 - 0.25 wt%. The second furnish and the third furnish preferably comprises somewhat longer fibers, whereby a lower consistency is needed to ensure good formation.

[0039] The top-ply web and the back-ply web may be dewatered by use of adjustable foils in the same manner as the middle ply web.

[0040] The solid content of each ply at the couching of the plies to form the three-ply web is preferably in the range of 8 - 12 wt%. Solid contents within these ranges improves ply bonding and optimizes the following pressing steps. In embodiments, the dryness difference between plies to be couched is within the range of 1 - 4 wt%.

[0041] Starch may be applied between the plies before couching, preferably by means of spraying or curtain coating. The starch may be applied in an amount in the range of 0.1 - 5 gsm, preferably 0.1 - 2 gsm, calculated on the dry weight of starch. The starch may be applied in the form of a foam. The starch may further comprise additives, e.g. MFC. Starch can be added in particulate or granular form, partially dissolved, swelled or fully cooked (i.e. fully dissolved) or as a mixture thereof. The starch content in the applied suspension can range from 2-30 wt%, depending on the application technique.

[0042] In embodiments, the solid content of the web after the press section and before the drying section is at least 43 wt%, preferably at least 45 wt%, preferably in the range of 47 - 57 wt%.

[0043] It has been found that by using CTMP in the middle ply in combination with the described forming and specific use of loading blades, the web can be pressed to higher solid contents in the press section. In this way, shrinkage in the following drying steps can be reduced whereby the produced paperboard exhibits a higher smoothness, and higher bending stiffness. In addition, a lower pressure in the inline calendering is needed to ensure the required smoothness, whereby the strength properties and the bulk of the paperboard web can be maintained. In embodiments, the three-ply web is subjected to in-line calendaring between the drying section and the reeling, preferably in a soft nip calendar with a pressure range of 10 - 30 kN / m or a hard nip calendar with a pressure range of 20 - 60 kN / m, preferably 20 - 50 kN / m.

[0044] The method of the invention enables the desired smoothness of the paperboard to be achieved with lower pressure during in-line calendaring, thereby maintaining the bulk without excessive densification. In embodiments wherein a hard nip calendar is used, the web is subjected to the in-line calendar before potential coating of the web. Steam is preferably added to the web prior to the hard nip. In this way, lower calendar loads may be applied whereby a higher bulk can be achieved. In embodiments wherein a soft nip calendar is used, the web is subjected to the calendaring after potential coating. The temperature of the calendar roll in the calendaring is preferably in the range of 100 - 200 °C.

[0045] In embodiments, the second furnish, forming the top-ply web, exhibits a Schopper Riegler (SR) value of at least 27, preferably in the range of 28 - 35, as determined by standard ISO 5267-1 .

[0046] The smoothness of the paperboard can further be improved by forming the top ply from a furnish having a comparatively high refining level. The use of a highly refined pulp in the top ply may, with conventional technologies, lead to a “too dense” top ply, leading to delamination problems in the following drying step. The method of the invention enables the use of a comparatively highly refined top ply furnish, without causing these problems.

[0047] Preferably, second furnish, forming the top ply, comprises bleached or unbleached kraft pulp from softwood or hardwood or from a mixture of softwood and hardwood. In embodiments, the second furnish comprises at least 50 wt%, preferably at least 70 wt% or at least 100 wt% of bleached or unbleached kraft pulp from hardwood.

[0048] The third furnish, forming the back ply web, may comprise bleached or unbleached kraft pulp from softwood or hardwood or from a mixture of softwood and hardwood. In a preferred embodiment, the back ply is made of unbleached kraft pulp.

[0049] In embodiments, the first furnish, forming the middle-ply, comprises 50-100 wt% CTMP or HT-CTMP from birch

[0050] In embodiments, the pressing of the three-ply web in the press section comprises subjecting the web to pressing in at least a first and a second shoe press nip.

[0051] Preferably, the pressing of the web in the press section comprises subjecting the web to pressing in a first, a second and a third shoe press nip, wherein the nip pressure in the first shoe press nip is at most 800kN / m, preferably in the range of 500 - 800 kN / m, the nip pressure in the second shoe press nip is in the range of 700 - 1500 kN / m, preferably in the range of 800 - 1200 kN / m and the nip pressure in the third shoe press nip is in the range of 600 - 1500 kN / m, preferably in the range of 700 - 1200 kN / m.

[0052] The middle-ply is made from a furnish comprising CTMP, which contains fines, and strength chemicals. If high nip loads are used in the first shoe press, it may result in drainage resistance, which can cause uneven z-distribution of the materials in the web. To address this issue, it has been demonstrated that the combination of three shoe presses in series is effective. In this method, the first nip has a comparatively low line load, while the second and third nips have high nip loads. This approach results in a multi-ply board with unfirm formation and high bulk.

[0053] The web speed in the press section is preferably between 450 - 1200 m / min, more preferably around 800 - 1000 m / min.

[0054] In embodiments, at least one shoe press is double felted. One, two or all three of the shoe presses may be double felted. In preferred embodiments, the first and the second shoe presses (in the running direction of the web) are double felted, while the last one is one-felted. In this way the smoothness of the ply facing the unfelted shoe roll is enhanced. In addition, the contact with the drying cylinders in the following drying step is enhanced, leading to improved drying effectiveness. At least one of the shoe presses may include a grooved sleeve, preferably an interrupted grooved sleeve. The temperature of the web in the press section is preferably within the range of 42 - 55 °C, more preferably in the range of 48 - 52 °C. Steam may be applied on the paperboard web prior to or in the beginning of the press section. In the embodiment, wherein three shoe presses are used in series, the sleeve used in the first and second press, in the running direction of the web, may preferably comprise interrupted grooves, while the sleeve used in the third shoe press may comprise interrupted or continuous grooves. The void volume of sleeves may be in the range of from 400 - 600 cm3 / m2. A sleeve in the context of the application refers to a covering made of a resilient and durable material, which fits over the pressing shoe and applies pressure to the paperboard web as it passes through the press nip.

[0055] Preferably, there are no or few / short open draws between the press section and the drying section. This approach enhances internal strength and reduces the need for speed difference.

[0056] The CTMP used in the middle-ply is preferably washed in at least three different steps in order to reduce metal content, and residuals from pre-impregnation and optional bleaching. In embodiments, the CTMP in the first furnish comprises an Mn content of less than 2 mg / kg. Preferably, the CTMP comprises less than 0.8, preferably less than 0.25 wt% extractives based on extraction in acetone according to standard SCAN CM-49:03. The CTMP may further exhibit at least one of a chemical oxygen demand (COD) according to DIN 38409-H41 / SFS 5504:1988 of less than 7 mg / kg and / or a biochemical oxygen demand (BOD) according to DIN EN 1899-1 :1998-05 of less than

[0057] In embodiments, the CTMP may be washed in a washing method including the steps of:

[0058] - subjecting the CTMP to a first washing step at a consistency of 5 - 10 wt%, preferably by use of a twin roll press, and press dewatered to a consistency of at least 20 wt%, preferably 22 - 35 wt %, resulting in a first CTMP suspension

[0059] - optionally subjecting the first CTMP suspension to a high consistency bleaching

[0060] - diluting the first CTMP suspension to a dry solid content of less than 10 wt%,

[0061] - subjecting the diluted first CTMP suspension to a second washing step at a consistency of 5 - 10 wt%, preferably by use of a twin roll press.

[0062] Preferably, the said steps are repeated at least two times. After the said washing steps, the formed CTMP may be subjected to dilution, LC-refining and screening. Thereafter, the CTMP may be thickened and subjected to a final press dewatering step at a consistency of 5 - 10 wt% to a consistency of at least 22 wt%.

[0063] Washing CTMP in this way improves the organoleptic properties of the resulting paperboard. Washing the CTMP can however also result in reduced strength properties. The method of the invention enables the use of washed CTMP while still achieving excellent formation and high strength properties.

[0064] Additionally, the method may involve surface sizing the web on at least one side, preferably before calendaring using a film press. The sizing agent may comprise starch and / or MFC, in an amount in the range of 1 to 5 gsm, preferably 1 to 3 gsm. The method may further comprise a step of surface sizing the web on at least on one side.

[0065] In embodiments, the method further comprises the steps of applying at least two coating layers, preferably three coating layers, on a first side of the three-ply web, and optionally one coating layer on a second side of the web, which coating steps are applied integrated on the paperboard machine.

[0066] The coating layers applied on the first side may be pigment coating layers, providing a surface excellent for printing, or a surface excellent for applying further barrier coating layers. Such pigment coating layers may comprise binder and pigments at a binderpigment ratio of from 8:100 to 25:100, preferably 10:100 to 20:100 such as 15:100. The pigments may be e.g. Calcium carbonate or clay and the binders may comprise latex, such as SA or SB latex.

[0067] Alternatively, or additionally, the coating layers or at least one of the coating layers applied on the first side may provide a barrier against at least one of oxygen, grease and / or water vapor. Such barrier coating layers may provide a barrier against condensation. In these embodiments, the coating layers may comprise binders and optionally pigments at a binderpigment ratio of 20:100 to 200:100, or preferably 25:100 to 50:100 or 35: 100 to 50:100 or, alternatively 100:100 to 150:100. The binder could e.g. be starch and / or latex.

[0068] The optional coating layer applied on the second side could be either a pigment coating layer or a barrier coating layer as defined above.

[0069] The coating layers may preferably be applied by means of blade coating and / or curtain coating.

[0070] The method of the invention enables the application of dispersion coatings on-line in the paperboard machine and still obtain excellent film forming.

[0071] According to a second aspect illustrated herein, there is provided a paperboard manufactured according to the method of the first aspect.

[0072] The paperboard may exhibit at least one of a bulk in the range of 1.3 - 2.1 cm3 / g according to ISO 534:2005 and a surface roughness (PPS-10) of less than 2 pm as measure using standard method ISO 8791-4, preferably less than 1.5 pm, or even less than 1 pm. 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 has preferably a grammage (ISO 536) of between 150 - 400 gsm, preferably 180 - 400 gsm, or 200 - 350 gsm and / or a density of between 500 - 900 kg / m3, preferably 600 - 850 kg / m3. The paperboard may further exhibit at least one of the following:

[0073] - a Scott Bond of at least 150 J / m2, preferably at least 175 J / m2(Tappi T569 pm-00), such as in the range of 150 - 350 J / m2 - a bending resistance index GM 15° (ISO 2493) of at least 15 Nm6 / kg3, preferably at least 16 Nm6 / kg3, such as in the range of 15 - 20 Nm6 / kg3(e.g. a paperboard with a grammage of 220 gsm may have a bending resistance 15 GM° of around 200 mN, a paperboard with a grammage of 235 gsm may have a bending resistance 15 GM° of around 230 mN and a paperboard with a grammage of 350 may have a bending resistance 15° GM of around 645 mN),

[0074] - z-strength of at least 200 kPa, preferably at least 250 kPa, as measured according to SCAN- P 80.

[0075] - an specific formation of less than 0.7 sqrt(g / m2), preferably less than 0.6 sqrt(g / m2), or less than 0.5 sqrt(g / m2), preferably in the range of 0.2 - 0.7 sqrt(g / m2), or 0.3 - 0.6 sqrt(g / m2), as measured on the paperboard as un-coated using SCAN-p 92:09, or combinations thereof. 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), Food Service Board (FSB), White-Top Kraft Liner (WTKL), coated White-Top Kraft Liner (cWTKL), Cup Board, and Liquid Packaging Board (LPB).

[0076] The invention is defined by the claims, which detail the distinctive features of the method and the paperboard.

[0077] A preferred embodiment of the invention is presented in the following description, with reference to the accompanying figure.

[0078] Fig.1 is a schematic figure of a forming section of a paperboard machine that may be utilized in association with the invention.

[0079] In the embodiment of the invention schematically shown in fig. 1 , the first furnish comprising at least 20 wt% CTMP is applied on a first forming wire (1) from a first headbox (2), forming a middle-ply web (3). The middle-ply web (3) is dewatered on the first forming wire (1) in a first dewatering zone (4). In the dewatering of the middle-ply web (3) in the first dewatering zone (4), the middle-ply web, on the first forming wire (1) is preferably passed over a set of adjustable foils (10). Thereafter, the middle-ply web is dewatered in a second dewatering zone (5) between the first forming wire (1) and a second forming wire (6). The middle-ply web (3) is thus subjected to two-sided dewatering in the second dewatering zone (5). The second dewatering zone (5) includes at least one suction unit (7) and dewatering blades (8) in contact with the second forming wire (6) and loadable dewatering blades (9) in contact with the first forming wire (1). The dewatering blades (8) that are in contact with the second forming wire (6) are preferably stationary during operation. In the method of the invention, the loading pressure applied by the loadable dewatering blades is in the range of 0.1-25 kPa. Preferably, the loading pressure applied by the first and the second dewatering blades is in the range of 0.1-12 kPa, while the loading pressure applied by the third and second dewatering blades is in the range of 15-25 kPa.

[0080] The suction unit / s(7) is preferably arranged on the same side of the web as the stationary dewatering blades (8). Preferably, the stationary and the loadable dewatering blades (8, 9) are arranged in series such that at least one of the loadable dewatering blade (9a, 9b, 9c, 9d) is arranged to be in contact with the first forming wire (1) between two subsequent stationary dewatering blades (8) in the running direction (11) of the web.

[0081] Following the forming section, the middle-ply may proceed to the pressing, drying and reeling section as previously disclosed, however not illustrated in Figure 1.

[0082] 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 middle-ply web (3) from a first furnish comprising at least 20 wt% of CTMP on a first forming wire (1),- dewatering the middle-ply web (3) on the first forming wire (1) in a first dewatering zone (4),- dewatering the middle-ply web (3) between the first forming wire (1) and a second forming wire (6) in a second dewatering zone (5), which second dewatering zone (5) comprises at least one suction unit (7) and dewatering blades (8), which dewatering blades (8) are in contact with the second forming wire (6), and at least two loadable dewatering blades (9a, 9b, 9c, 9d) in contact with the first forming wire (1), wherein the loading pressure applied by each of the loadable dewatering blade (9a, 9b, 9c, 9d) is in the range of between 0.1 - 25 kPa,- forming a top-ply web from a second furnish and a back-ply web from a third furnish,- couching the said webs to form a three-ply web wherein the middle ply web is arranged between the top- and back ply web,- pressing the three-ply web in a press section,- drying the pressed web in a drying section and- reeling the web.

2. A method according to claim 1, wherein the second dewatering zone comprises, in the running direction (11) of the middle-ply web, a first, an optional second, a third and an optional fourth loadable dewatering blade (9a, 9b, 9c, 9d) that are in contact with the first forming wire (1), and wherein the loading pressure applied by each of the first and the optional second loadable dewatering blade (9a, 9b) is between 0.1-12 kPa, preferably between 0.1-10 kPa , and the loading pressure applied by the third and the optional fourth loadable dewatering blade (9c, 9d) is between 15-25 kPa, preferably between 15-20 kPa, or 18-20 kPa.

3. A method according to anyone of the preceding claims, wherein the first furnish comprises CTMP and 0.5 - 7 wt% of microfibrillated cellulose (MFC) or highly refined cellulose fibers having a Schopper-Reigler (SR) value of 75 or higher, as determined by standard ISO 5267-1.

4. A method according to anyone of the preceding claims, wherein the loading pressure applied by at least one of the loadable dewatering blades is in the range of 15-25 kPa.

5. A method according to anyone of the preceding claims, wherein the dewatering of the web (3) in first dewatering zone (4) comprises subjecting the web to vacuum at a vacuum pressure of at most 10 kPa, preferably at most 7 kPa, and most preferably at most 5 kPa.

6. A method according to anyone of the preceding claims, wherein the middle ply web is formed by applying the first furnish on the first forming wire from a first headbox, wherein the water retention value (WVR) of the first furnish in the headbox is at least 90%, or at least 100%, preferably in the range of 100-180%, or 100-160%, as measured using ISO 23714:20147. A method according to anyone of the preceding claims, wherein CTMP comprises a fine fraction that is able to pass through a 200 Mesh screen in an amount of less than 20 wt%, as calculated on the total dry weight of the CTMP.

8. A method according to anyone of the preceding claims, wherein the CTMP is high temperature CTMP (HT-CTMP).

9. A method according to anyone of the preceding claims, wherein the first furnish comprises starch in an amount of less than 15 kg / ton, preferably less than 12 kg / ton.

10. A method according to anyone of the preceding claims, wherein the middle- ply web is formed by applying the first furnish on the first forming wire (1) from a first headbox (2), wherein the consistency of the first furnish in thefirst headbox (2) is 0.25-1 .1 wt%, prefarebly 0.25 - 0.85 wt%, more preferably preferably 0.5 - 0.85 wt%.

11. A method according to anyone of the preceding claims, wherein the top-ply web is formed by applying the second furnish on a forming wire from a second headbox and the back-ply web is formed by applying the third furnish on a forming wire from a third headbox, and wherein the consistency of the second furnish in the second headbox and the third furnish in the third headbox is 0.10 - 0.40 wt%, preferably 0.2 - 0.25 wt%.

12. A method according to anyone of the preceding claims, wherein the solid content of the web after the press section and before the drying section is at least 43 wt%, preferably at least 45 wt%, preferably in the range of 47 - 57 wt%.

13. A method according to anyone of the preceding claims, wherein the three- ply web is subjected to in-line calendaring between the drying section and the reeling, preferably in a soft nip calendar with a pressure range of 10 - 30 kN / m or a hard nip calendar with a pressure range of 20 - 60 kN / m, preferably 20 - 50 kN / m.

14. A method according to anyone of the preceding claims, wherein the top ply is formed from a second furnish, which second furnish exhibits a Schopper Riegler (SR) value of at least 27, preferably in the range of 28 - 35, as determined by standard ISO 5267-1.

15. A method according to anyone of the preceding claims, wherein the CTMP comprises less than 0.8, preferably less than 0.25 wt% extractives.

16. A method according to anyone of the preceding claims, wherein the CTMP has a chemical oxygen demand (COD) according to DIN 38409-H41 / SFS 5504:1988 of less than 7 mg / kg.

17. A paperboard manufactured according to anyone of the claims 1 - 16.

Images