A method for manufacturing a multi-ply paperboard

Abstract

The present invention relates to a method for manufacturing a coated paperboard. The method comprising providing a multiply paperboard web, which comprises a middle ply comprising at least 40 wt% by dry weight of CTMP and coating the said web in at least two coating steps. The first coating step comprises applying a first coating composition having a solid content of at least 65 wt% by means of jet application, followed by levelling the applied coating with a blade operating in stiff blade mode. The second coating step comprises applying a second coating composition having a solid content of at least 65 wt% by means of jet application, followed by levelling the applied coating with a blade operating in bent blade mode.

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.

[0006] Multiply paperboard used in packages, boxes, containers or cups for liquid, dry or frozen food is usually pigment coated to provide good printing performance and excellent appearance.

[0007] Paperboard substrates can be coated with pigment coating layers using various methods, such as roller, spray, curtain, blade, slot, immersion, gravure, or rod coating. Blade coating is often preferred for its advantages, such as high speed, high smoothness, and enhanced barrier properties, as it fills the cavities or pores effectively. Although this particular levelling method leads to higher smoothness, the risk of insufficient coverage is higher especially when using a paperboard substrate with a high content of mechanical fibers, such as CTMP, in its middle ply. Two or three pigment coating layers are oftentimes needed to provide a good surface for printing. A paperboard substrate comprising a high content of mechanical fibers has a high porosity and roughness, whereby the applied coating composition may penetrate the substrate, affecting e.g. density and bulk profiles negatively. A high content of CTMP in the middle ply further affect the formation and quality of the top ply negatively, potentially leading to adverse effects on the pigment coating. A high porosity and roughness may further lead to drainage of the wet coating composition, which may cause dilatant induced agglomeration and runnability issues during the levelling phase. Further challenges connected with blade coating includes weight- and profile control, runnability (especially metering and levelling), coating defects such as streaks, stalagmites, poor coverage and blade bleeding.

[0008] There is a need for an improved method to produce paperboard for packaging liquid, dry, and / or frozen food, with good printing performance and without coating runnability problems.

[0009] Description of the invention

[0010] It is an object of the present disclosure to provide a method to manufacture a paperboard with excellent appearance and which provides good printing performance.

[0011] It is a further object of the present disclosure to provide a method to manufacture a paperboard, which method facilitates the application of thin pigment coating layers without the runnability problems connected with the prior art.

[0012] 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.

[0013] According to a first aspect illustrated herein, there is provided a method for manufacturing a coated paperboard comprising the steps of: - providing a multiply paperboard web comprising a top ply, a back ply and at least one middle ply arranged between the top ply and the back ply, wherein the outer surface of the top ply forms a first side and the outer surface of the back ply forms a second side, wherein the middle ply comprises at least 40 wt.% by dry weight of CTMP,

[0014] - applying a first coating composition having a solid content of at least 65 wt% on the first side by means of jet application followed by levelling the applied first coating composition by means of a first blade, wherein the first blade has a first surface that is partly in contact with the applied coating, a second surface opposite to the first surface and a first blade tip, connecting said first and second surfaces and being in contact with the applied coating, wherein the first blade is operating in a stiff blade mode, thereby forming a first coating layer,

[0015] - applying a second coating composition having a solid content of at least 65 wt% on the first side by means of jet application followed by levelling the applied second coating composition by means of a second blade, wherein the second blade has a first surface that is partly in contact with the applied coating and a second surface opposite to the first surface and a second blade tip connecting said first and second surfaces and being in contact with the applied coating, wherein the second blade is operating in a bent blade mode, thereby forming a second coating layer.

[0016] The second coating layer is preferably the outermost coating layer, also referred to as a top-coat. The outermost coating layer provides a surface for printing.

[0017] It has surprisingly been found that applying high solid content coatings in two steps, using jet applicator followed by levelling with a blade operating in a stiff mode in the first coating step and in a bent mode in the second coating step, on a paperboard substrate with a high bulk, results in good runnability, effective coating hold out and excellent coating quality. The effective coating hold out prevents the penetration of the coating into the fiber substrate, resulting in increased coating strength and ensures excellent internal bonding strength (Scott Bond). The paperboard web comprising such high amount of CTMP in the middle ply has a high bulk. The high bulk of the web enhances its z-directional compressibility when subjected to the blade, especially the stiff blade, resulting in unexpectedly good runnability and high surface roughness, despite lower coat weights. Due to its compressible nature, the web conforms more effectively during the levelling process, leading to a more uniform shearing across the coating layers. This results in surprisingly good runnability, even at low coating weights. The utilization of a stiff blade mode for the first coating application and a bent blade mode for the outermost coating application has been proven to yield exceptional coating quality and runnability, while preserving the bulk of the paperboard web. Furthermore, this innovative approach enables the application of low coat weights while ensuring excellent coverage and coating performance also at high machine speeds.

[0018] The method of manufacturing a coated paperboard is preferably performed on a paperboard machine. Preferably, the coating steps are performed on-line, on the same machine, with the step of providing the multiply paperboard web. The web is preferably supported by a wire during the coating steps. The speed of the web in the coating thereof is preferably in the range of 600-1200 m / min, preferably 650- 1200 m / min, or 800-1200 m / min.

[0019] 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 layer, a ply, a furnish or a composition.

[0020] The multiply paperboard web is running in a direction D. Operating the first blade in a stiff blade mode includes that a tangent running from the first blade tip to a point 5 mm away from the first blade tip on the first surface forms an angle a with the running direction D of the moving web, which angle a is at least 30°, preferably in the range of from, 30° to 45°. The first blade has preferably a surface that is substantially straight, or that is only slightly curved. The first blade may be made from e.g. steel, or hardened steel. The blade tip can be steel, such as hardened steel, chromium carbide or tungsten carbide or a composite comprising ceramic and / or plastic coating. Operating the second blade in a bent blade mode includes that a tangent running from the second blade tip to a point 5 mm away from the second blade tip on the first surface forms an angle a’ with the running direction D of the moving web, which angle a’ is in the range of from 0° to 15°, preferably in the range of from 0° to 5°. The second blade is preferably a bent blade which preferably exhibits a convex curvature, with the center of the curve protruding towards the coating. The second blade is thus preferably bent more than the first blade. The second blade may be made from e.g. steel, such as hardened steel. The blade tip can be steel, such as hardened steel, chromium carbide or tungsten carbide or a composite comprising ceramic and / or plastic coating. The angles of the blades can be regulated by regulating the force applied on the blades.

[0021] In the context of this application, jet application refers to the process where the coating composition is fed through a pressurized nozzle, forming a jet that is applied on the web. In embodiments, the nozzle comprises a slot orifice defined by a lower and an upper lip, wherein the distance between the lower and upper lip (i.e. the slot orifice) is in the range of 0.2-1.4 mm. The nozzle is preferably applied with a pressure in the range of 0.1-2.0 bar. In embodiments, the jet tangent, i.e. the tangent running from the outlet of the nozzle and the point where the jet is applied on the web, forms a jet angle p with the running direction of the web D, which jet angle is within the range of from 30° to 43°. The conditions of the jet applicator, including e.g. jet or beam angle, slot orifice size, and applied pressure, can be adjusted to control the coating amounts and potential backflow.

[0022] In embodiments, the jet application of the first coating composition comprises applying the first coating composition on the web from a first nozzle having a first nozzle outlet, and the jet application of the second coating composition comprises applying the second coating composition on the web from a second nozzle having a second nozzle outlet, wherein the distance between the first nozzle outlet and the web is less than 14 mm, preferably less than 12 mm, and the distance between the second nozzle outlet and the web is less than 14 mm, less than 12 mm. The invention allows for a short distance between the nozzle and the web, even at high machine speeds. This facilitates a more uniform application and reduces the negative impact of air disturbances on the coating jet. In embodiments, the first coating composition is applied directly onto the paperboard web, forming a first coating layer, and the second coating composition is applied on the first coating layer.

[0023] In other embodiments, the method further comprises applying a third coating composition having a solid content of at least 65 wt% on the first coating layer by means of jet application followed by levelling the applied coating by means of a third blade, thereby forming a third coating layer. In these embodiments, the third coating layer is formed between the first coating layer and the second coating layer, and the second coating composition is applied on the third coating layer. The third blade may be operated in a stiff blade mode or in a bent blade mode.

[0024] In embodiments, the blade pressure of the first and the optional third blade is within the range of 0.2-1.8 bar, preferably within the range of 0.4-1 .3 bar.

[0025] It has been shown that high blade loads can be used when using a paperboard web as defined in the present disclosure. The high compressibility of the paperboard helps to mitigate the risks of streaks and shearing at the blade tip, even under high pressures. The thickness of the first, second and optional third blade is preferably in the range of 0.3-0.6 mm.

[0026] The first and the second coating layer has preferably each a grammage in the range of 4-12 g / m2, preferably in the range of 5-10 g / m2, while the optional third coating layer has preferably a grammage in the range of 4-11 g / m2, preferably in the range of 6-9 g / m2.

[0027] Preferably, the first and second coating composition each comprises pigment in an amount in the range of 80-90 wt%, preferably in the range of 86-90 wt%, and binder in an amount of 10-20 wt%, preferably 10-14 wt%, as calculated on the total dry weight of respective composition.

[0028] The pigment of the first and second coating composition may be chosen from the group of calcium carbonate, e.g. ground calcium carbonate (GCC) or precipitated calcium carbonate (PCC), or clay, e.g. kaolin clay, or combinations thereof. Preferably, the pigment of the first and second coating composition has a broad particle size distribution preferably having BET surface area less than 30 m2 / g and more preferably 8-25 m2 / g. Preferably, the pigment has a bimodal particle size distribution

[0029] In embodiments, the pigment in the second coating composition comprises 65-85 wt% calcium carbonate and 15-45 wt% clay, as calculated on the total solid content of the pigment. In embodiments, the pigment is clay, which clay substantially free from montmorillonite or swelling clays (<5 wt%). Such substances may cause reduction in solid content and might create high viscosity when complexing with thickeners

[0030] In embodiments, the pigment in the first coating composition comprises 80-100 wt% calcium carbonate and 0-20 wt% clay, as calculated on the total solid content of the pigment.

[0031] The binder is preferably a latex. The latex may have a mean particle size below 180 nm, preferably within the range of 60-160 nm, and glass transition temperature below 35 degrees Celsius (°C), preferably in the range of from -10°C to 30°C. The latex may further exhibit a size distribution that is monomodal or bimodal.

[0032] The term “latex” as used herein refers to an emulsion of polymers in an aqueous medium. Preferably, the polymers of the latex are synthetic polymers, preferably chosen from the group of styrene-butadiene latex, styrene-acrylate latex, acrylate latex, vinyl- acetate latex, polyvinyl-acetate latex, acrylate latex, vinyl acetateacrylate latex, styrene-butadiene-acrylonitrile latex, styrene-acrylate-acrylonitrile latex, styrene-butadiene-acrylate-acrylonitrile latex, styrene-maleic anhydride latex, styrene-acrylate-maleic anhydride latex, or mixture of the latexes.

[0033] The latex is preferably a styrene-butadiene (SB) latex or a styrene-acrylate (SA) latex, acrylate latex, vinyl acetate latex, poly-vinyl acetate latex or vinyl acetateacrylate latex, or mixture of these latexes. The binder may further comprise 0-5 wt% co-binders selected from starch, preferably modified starch, such as cross-linked starch or grafted starch, carboxymethyl cellulose (CMC), microfibrillated cellulose (MFC), polyvinyl alcohol (PVOH), or polyolefin.

[0034] In embodiments, the third coating composition comprises pigment in an amount of 80-90 % by dry weight, and binder in an amount of 10-20 wt%, as calculated on the total dry weight of said third coating composition.

[0035] The binder and pigment in the third coating composition may be selected from the same lists of binders and pigments as the binder and pigment in the first and second coating composition.

[0036] The first, second, and the optional third coating compositions may each include additives in an amount ranging from 0.1 to 1.5 wt%, based on the total solid content of the composition. These additives may include rheology modifiers or water retention agents. In some embodiments, each of the coating compositions may contain synthetic water retention and rheology modifiers (WRRM) in an amount of less than 1 wt%, preferably below 0.8 wt%, and more preferably within the range of 0.05-0.4 wt%, based on the total solid content of the composition. WRRM can also be known as associative or non-associative thickeners, or alkali swellable thickeners, and are typically based on acrylate or polyacrylate polymers derived from acrylic acid, methacrylic acid, or ethyl acrylate monomers. Synthetic polymers used as WRRM are highly effective in providing thickening properties. A higher concentration of WRRM is usually required when the web exhibits high roughness, as this leads to an increased risk of dewatering the coating dispersion. In the current case, it was unexpectedly discovered that the combination of high solids with the specific blade settings results in good runnability. Additionally, since WRRM are sensitive to metal ions, such as calcium ions, it may be beneficial to use these in conjunction with low molecular weight dispersants, such as sodium polyacrylate salts with a molecular weight of less than 10,000 g / mol, and preferably between 1,000 and 7,500 g / mol. The first, second, and optional third coating compositions may have a temperature in a range of 25-45 °C. Temperature control is particularly crucial when the coating is applied on-line and when the web is hot. This is especially important in embodiments where the amount of synthetic water retention and rheology modifiers (WRRM) in the coating composition is low, as precise temperature regulation is essential to ensure good runnability. WRRM are typically sensitive to temperature fluctuations, making consistent temperature management imperative for maintaining the effectiveness of these modifiers in the coating process.

[0037] In embodiments, each of the first, second and optional third coating composition has a water retention value below 130 g / m2, preferably in the range of 40-110 g / m2, and most preferably in the range of 50-90 g / m2, as measured using to TAPPI standard T701 pm-01.

[0038] The water retention value (WRV) defines the Abo Akademi Water retention value and is measured using TAPPI standard T701 pm-01. The WRV may be controlled by regulating the solid content and the choice of pigment in the coating composition. High solid content and pigment with a broad size distribution provides a low WRV. A low WRV ensures minimal migration of coating components and reduced penetration of the liquid phase into the board. Combined with a high solids content, this leads to rapid immobilization of the coating on the web, resulting in good coverage.

[0039] The first, second and optional third coating composition each has a solid content of at least 65 wt%, preferably of at least 66 wt% or at least 67 wt%, such as in the range of 65-75 wt%, or 66-75 wt% or 68-75 wt%. The upper limit for the solid content depends on the viscosity of the composition.

[0040] The Brookfield viscosity of the first, second and optionally third coating composition is preferably in the range of 500-2000 mPas, preferably in the range of 600-1500 mPas, as determined according to standard SCAN P 50:84 at 23 °C, 100 rpm. The method may further comprises applying a fourth coating composition on the second side of the paperboard web. The fourth coating composition may have a solid content of at least 65 wt% or at least 68 wt% and be applied by means of jet application followed by levelling the applied coating by means of a fourth blade. The fourth blade may operate in stiff or bent blade mode. The fourth coating composition may be a pigment coating composition comprising pigment and binder. In preferred embodiments, the fourth coating composition comprises pigment in an amount of 80-90 wt%, preferably 86-92 wt%, by dry weight and binder in an amount of 10-20 wt%, preferably 8-14 wt%, by dry weight.

[0041] The method of the invention may further comprise steps of drying the applied coating composition after each coating step, preferably to a moisture content in the range of 4-14 wt%. The drying is preferably performed using non-contact drying methods such as hot air, infrared (IR) dryers, impingement dryers or steam heated dryers. The IR driers are preferably electric IR driers, which ensure significant reduction in CO2 footprint. The drying process preferably involves a combination of an electric IR dryer and an air turn dryer. The air turn dryers collect energy from the IR dryer and, optionally, from the heated web. This method enhances the efficiency of the drying process and ensures robust strength of the coating.

[0042] In embodiments, the multiply paperboard web is provided by the step of

[0043] - forming a first multiply paperboard web,

[0044] - pressing and drying said first paperboard web,

[0045] - subjecting the dried first paperboard web to calendering in a precalendering step.

[0046] Preferably, the pre-calendering step comprises calendering the web in a hard-nip with a nip load of 10-50 kN / m, preferably 10-40 kN / m. The method of the invention enables the use of a paperboard with a high amount of CTMP in the middle ply, which paperboard has not been subjected to any calendering, or subjected to calendering using low nip loads and yet achieve a smooth surface for printing. The said pre-calendering step is preferably the only calendering step prior to the coating. The said hard-nip is preferably the only nip in the pre-calendering step. 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.

[0047] The method may further comprise the step of calendering the coated paperboard web. The calendering of the coated paperboard web may be performed in a soft or hard nip, preferably in a soft nip. Preferably, the step of calendering the coated paperboard web comprises subjecting the web to calendering in at least one calendering nip, wherein the highest nip load in the calendering is below 70 kN / m, preferably below 50 kN / m, most preferably below 30 kN / m, such as in the range of 10-70 kN / m, or 10-50 kN / m or 10-30 kN / m. The calender temperature can range from 30 to 250 °C.

[0048] It was unexpectedly discovered that despite the high roughness and substantial content of CTMP in the middle ply, the smoothness achieved after coating was remarkably good. Consequently, the calendering pressure can be significantly reduced or even omitted entirely, thereby preserving the higher bulk and enhancing the rigidity of the paperboard.

[0049] In embodiments, the middle ply comprises at least 50 wt%, preferably between 50-90 wt%, or 60-90 wt%, or 75-90 wt% of CTMP, as calculated on the total dry weight of the middle ply. The CTMP in the middle ply may be derived from softwood (e.g. spruce, fir or pine, or mixes thereof) or hardwood fibers (e.g birch, eucalyptus, beech, aspen acacia, maple or poplar), or a mixture thereof and may be bleached or unbleached. In embodiments, the CTMP is high-temperature CTMP (HT-CTMP). In embodiments, the CTMP is derived from a mixture of at least two different softwood species, such as at 60-99.9 wt% of spruce, and 0.1- 40 wt% of pine. In other embodiments, the CTMP is derived from a mixture of softwood and hardwood, e.g. 60-99 wt% of softwood and 1-40 wt% of hardwood. In one preferred example, the CTMP is HT-CTMP whereof 80 wt% is derived from spruce and 20 wt% is derived from birch. In other embodiments, the CTMP is derived from a mixture of at least two different hardwood species. The CTMP generated from softwood has preferably a bulk value of at least 2.5 cm3 / g, preferably at least 2.9, and more preferably at least 3.2 cm3 / g, such as in the range of 3.5-5.5 cm3 / g, as determined in accordance with ISO 534:2005 (50 kPa), for a pulp having a Canadian Standard Freeness of approximately 550 ml, based on the sheet preparation method specified in ISO 5269-1:2005

[0050] In embodiments, the CTMP generated from softwood contains extractives in an amount less than 0.4 wt%, and more preferably less than 0.35 wt%, as measured by the SCAN-CM 49:03 standard (acetone extraction), and / or has a shives content (Sommerville 0.15 mm) of less than 0.4 wt%, as determined in accordance with TAPPI T 275-18. The CTMP generated from softwood may exhibit an ISO Brightness of greater than 50 %, preferably greater than 52 %, as measured in accordance with ISO 2470-1:2009.

[0051] In embodiments, the CTMP generated from hardwood has a bulk value of at least 2.5 cm3 / g, preferably at least 2.9 cm3 / g, and more preferably at least 3.2 cm3 / g, such as in the range of 3.2-7.0 cm3 / g, as determined in accordance with ISO 534:2005 (50 kPa), for a pulp having a Canadian Standard Freeness of approximately 550 ml, based on the sheet preparation method specified in ISO 5269-1 :2005.

[0052] In embodiments, the CTMP generated from hardwood contains extractives in an amount less than 0.4 wt%, and more preferably less than 0.35 wt%, as measured by the SCAN-CM 49:03 standard (acetone extraction) and / or has a shives content (Sommerville 0.15 mm) of less than 0.4%, as determined in accordance with TAPPI T 275-18.

[0053] The hardwood CTMP may further exhibit an ISO Brightness of greater than 50 %, preferably greater than 52 %, as measured in accordance with ISO 2470-1 :2009.

[0054] In some embodiments, the CTMP is specifically bleached CTMP or HT CTMP having an ISO Brightness of greater than 78 %, preferably greater than 80 %, and most preferably greater than 82% as measured in accordance with ISO 2470- 1:2009. A preferred a grade is bleached hardwood HT CTMP. In addition to CTMP, the middle ply may comprise kraft pulp and / or broke. In embodiments, the middle ply comprises 20-50 wt% of broke, as calculated on the total dry weight of said middle ply.

[0055] In embodiments, the middle ply comprises 50-100 wt% by dry weight of CTMP and 0-50 wt% by dry weight of broke, preferably 50-80 wt% CTMP and 20-50 wt% broke. Broke refers to defective or unusable paperboard that is generated in the manufacturing process of the multiply paperboard and may be coated or uncoated. As the broke used in the disclosed process comprise a high content of CTMP, broke can be used in a high amount without affecting the bulk of the middle ply negatively. In embodiments, the broke is disintegrated, screened and optionally refined to adjust the bulk-tensile strength behavior. In embodiments the bulk of the broke used is preferably 1.7 cm3 / g, more preferably at least 2.2 cm3 / g, as determined in accordance with ISO 534:2005 (50 kPa), for a pulp having a Canadian Standard Freeness of approximately 550 ml, based on the sheet preparation method specified in ISO 5269-1 :2005

[0056] The middle ply may further comprise 0-40 wt%, preferably 0-30 wt% or 0-20 wt% by dry weight of kraft pulp and / or recycled pulp, as calculated on the total dry weight of the middle ply.

[0057] The middle ply may further comprises 0.01-15 wt%, or 1-10 wt% by dry weight of a strength enhancement agent preferably selected from the group consisting of highly refined cellulose, microfibrillated cellulose (MFC), a cationic strength additive, an anionic strength additive.

[0058] The middle ply may have a density of less than 550 kg / m3, preferably less than 500 kg / m3as measured according to standard ISO 534:2011. In some embodiments, the middle ply may have a density in the range of 200-500 kg / m3 or in the range of 200-400 kg / m3, as measured according to standard ISO 534:2011. 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 536:2019. 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 536:2019.

[0059] In embodiments, the middle ply has a low specific formation value. In some embodiments, the middle ply has a specific formation value of higher than 0.5 gA0.5 / m, preferably higher than 0.6 gA0.5 / m, and more preferably higher than 0.7 gA0.5 / m, such as between 0.7 and 1.8 gA0.5 / m as measured according to SCAN-P 92:09. A higher specific formation value indicates a less uniform distribution of fibers across the paperboard, leading to worse surface properties, which also affects the quality of the top ply.

[0060] The paperboard web used in the method of the invention further comprises a top ply and a back ply, wherein the outer surface of the top ply forms a first side and the outer surface of the back ply forms a second side. The first side is subjected to coating in accordance with the invention disclosed herein and preferably forms the print side in a packaging produced from the coated paperboard. In embodiments, the top ply and the back ply each has a grammage in the range of 15 gsm to 120 gsm, and more preferably in the range of 20 gsm to 90 gsm, as measured according to standard ISO 536:2019. This grammage range ensures that the outer plies provide sufficient surface quality and strength without compromising the middle ply’s contribution to overall thickness and rigidity of the multi-ply paperboard.

[0061] In some embodiments, the ratio of the grammage of the top ply, and the middle ply and the ratio of the grammage of back ply and the middle ply, are both at least 1 :2, preferably in the range of 1 :2 to 1 :8, and more preferably in the range of 1 :3 to 1 :5. These grammage ratios ensure that the middle ply provides the majority of the thickness and rigidity while the top and back plies contribute to surface characteristics and additional structural support. In some embodiments, the grammage of the back ply is up to 40 % lower than the grammage of the top ply.

[0062] In some embodiments, the multi-ply structure can comprise one or more additional plies added between the two outer plies (top ply and back ply) to further enhance the properties of the paperboard. These additional layers may comprise various types of pulp or other fibrous materials, depending on the desired characteristics of the final product. For example, a reinforcing layer may be incorporated to increase tensile strength, or a barrier layer may be included to improve moisture resistance. The inclusion of additional layers allows for customization of the paperboard structure, enabling optimization for specific applications such as packaging, printing, or specialty paperboard products.

[0063] Of particular importance is the top ply composition. In the top ply, the fiber is preferably at least 50 wt% kraft pulp such as bleached kraft pulp and more preferably bleached hardwood kraft pulp, which has been refined to a SR between 18 and 40 and more pref. 18 to 35 according to ISO 5267-1 :1999. In embodiments, the top ply comprises 70-100 wt% of kraft pulp, preferably bleached kraft pulp, as calculated on the total dry weight of the top ply.

[0064] The surface roughness (Bendtsen roughness) of the top side before coating is preferably at least 300 ml / min, preferably in the range of 300-500 ml / min, as measured by the Bendtsen method according to ISO 8791-2:2013. The method of the invention enables pigment coating of a paperboard with a comparatively rough surface and yet achieve a smooth surface printing.

[0065] The back ply may comprise bleached or unbleached kraft pulp from softwood and / or hardwood.

[0066] The multi-ply paperboard web used in the method of the invention has preferably a density of less than 850 kg / m3and more preferably less than 750 kg / m3and most preferably less than 650 kg / m3. The said web has preferably a bulk that is higher than 1.4 cm3 / g, as determined according to ISO 534:2005, and / or an ash content less than 5 wt% and more preferably less than 4 wt % and most preferably less than 3 wt%.

[0067] 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).

[0068] In embodiments, the second coating layer forms an outermost coating layer.

[0069] Preferably, the coated paperboard exhibits, as measured on the second coating layer, at least one of a surface roughness (Bendtsen roughness) of less than 20 ml / min, preferably less than 10 ml / min, as determined using the Bendtsen method according to ISO 8791-2:2013, and a surface roughness (PPS-10) of less than of less than 2 pm, preferably of less than 1.5 pm, most preferably of less than 1 pm, as determined using standard method ISO 8791-4:2021. embodiment

[0070] In the following, a preferred embodiment of the method of the invention is described.

[0071] Fig. 1 is a schematic drawing of a method according to an embodiment of the invention.

[0072] Fig 2 is a schematic drawing of the operation of the jet applicator according to an embodiment of the invention.

[0073] In accordance with Fig. 1 , a multiply paperboard web (1) is running in a direction (D). A first coating composition is applied on a first side (1a) of the web (1) by means of a first jet applicator (2). The applied coating is levelled by use of a first blade (3). The first blade (3) has a first surface (3a) that partly contacts the applied coating, a second surface (3b) opposed to said first surface (3a) and a first blade tip (4) connecting said first and second surface (3a, 3b). The first blade tip (4) is the edge of the blade (3) that is in contact with the applied coating. During operation, the outermost region of the blade tip (4) forms a first tip angle (a) with the of the moving web (1). The first tip angle (a) defines the angle between a tangent (5) on the first surface of the blade (3a) and the running direction (D) of the moving web (1). Said tangent (5) defines a tangent running from the first blade tip (4) to a points mm away from the tip (4) on the first blade’s first surface (3a). In accordance with the invention, the first blade (3) is operating in a stiff blade mode, wherein the first tip angle a is at least 30°, preferably within the range of from 30° to 45°. The applied and levelled coating composition forms a first coating layer on the first side of the web (1a). In the schematic Fig. 1 , the first blade is straight. However, the first blade may also be slightly curved.

[0074] A second coating composition is applied on the first side of the coated, preferably dried, web (1a) by means of a second jet applicator (6). The applied second coating is levelled by use of a second blade (7). The second blade (7) has a first surface (7a) that partly contacts the applied coating, a second surface (7b) opposed to said first surface (7a) and a second blade tip (8) connecting said first and second surface (7a, 7b). The second blade tip (8) is the edge of the blade (7) that is in contact with the applied coating. During operation, the outermost region of the blade tip (8) forms a second tip angle (o’) with the moving web (1). The second tip angle (o’) defines the angle between a tangent (9) on the second blade (7a) and the running direction (D) of the moving web (1). The tangent (9) on the second blade (7a) defines a tangent running from the second blade tip (8) to a point 5 mm away from the tip (8) on the second blade’s first surface (7a).

[0075] In accordance with the invention, the second blade is operating in bent blade mode, wherein the second tip angle (o’) is less than 15°, such as in the range of from 0° to 15°, preferably less than 5°, most preferably in the range of from 1° to 5°

[0076] Fig. 2 illustrates the operation of a jet applicator according to one embodiment of the invention. Fig 2 shows the web (1), a jet applicator (2) comprising a nozzle (2a) and a jet (10) that is emitted from the nozzle (2a) onto the moving web (1). The jet (10) is emitted from the nozzle and forms an angle (p), called the jet angle, with the running direction of the web (D). The jet angle (P) defines the angle of the jet tangent and the running direction of the web (D). The jet tangent is the tangent running from the outlet of the nozzle (2a) and at the point where the jet is applied on the web (1). According to one embodiment of the invention, the jet angle ((P) ranges from 30° to 43°. The jet applicator (2) is positioned so that the distance between the outlet of the nozzle (2a) and the first side (1a) of the web is less than 14 mm, preferably less than 12 mm.

[0077] A 245 gsm three ply board substrate with a middle ply comprising 65 wt% HT- CTMP, 25 wt% broke and 5 wt% refined kraft pulp, as calculated on the total dry weight of said board substrate was provided. The board substrate was surfacesized with starch (2 gsm on each side), dried and then calendered with hard nip at 180 °C and at a nip pressure of 50 kN / m and at a web speed of 400 m / min. As shown in Table 1 , the board substrate, after surface-sizing and calendaring, had a grammage of about 250 gsm, a thickness of 430 urn and bulk of 1.71 cm3 / g.

[0078] A first pre-coating composition was applied on the top side of the board substate, which pre-coating composition comprised 100 pph Calcium carbonate, 14 pph of Styrene acrylate latex and 0.3 parts of synthetic rheology modifier. As used herein, “pph” refers to “parts per hundred pigment” and specifies the amount of each component relative to 100 parts per weight of the pigment component. The precoating composition had a solid content of 67 wt% and Brookfield viscosity value of 1028 mPas determined at 23 °C. The coating was applied on the surface-sized and calendered board substrate using a jet applicator operated with a jet angle of 37°, and tip angle of 40° with a blade extension of 16 mm and operated in stiff mode. The coat weight of the pre-coating layer was 8 g / m2and the coater was run at 700 m / min.

[0079] A middle coating layer was applied by applying a middle-coating composition on the pre-coating layer. The middle-coating composition had the same composition as the pre-coating composition and was applied using the same coater mode and settings as in pre-coating step. The applied coat weight was 7 gsm. A top coating layer was applied on the middle-coating layer. The top-coating composition comprised 75 pph calcium carbonate and 25 pph kaolin, 14 pph of styrene acrylate latex and 0.2 parts of synethic rheology modifier. The top-coating composition had a solid content of 67 wt% and Brookfield viscosity of 1100 mPas. The coating was applied with jet applicator with jet angle of 37°, whereas levelling was made in bent blade mode with a tip angle less than 5° and beam angle of 27°. The coater speed was 700 m / min and the applied coat weight was 7 g / m2. Target moisture content for final product was 7-8 wt% The results of the coating trial are summarized in Table 1. The results show that the coating quality and the runnability was excellent, although a rough board substrate comprising a high content of CTMP in the middle ply was used.

[0080] Table

Claims

Claims1. A method for manufacturing a coated paperboard comprising the steps of:- providing a multiply paperboard web (1) comprising a top ply, a back ply and at least one middle ply arranged between the top ply and the back ply, wherein the outer surface of the top ply forms a first side (1a), and the outer surface of the back ply forms a second side, wherein the middle ply comprises at least 40 wt.% by dry weight of CTMP,- applying a first coating composition having a solid content of at least 65 wt% on the first side (1a) by means of jet application followed by levelling the applied first coating composition by means of a first blade (3), wherein the first blade has a first surface (3a) that is partly in contact with the applied coating, a second surface (3b) opposite to the first surface (3a) and a first blade tip (4) connecting said first and second surfaces (3a, 3b) and being in contact with the applied coating, wherein the first blade (3) is operating in a stiff blade mode, thereby forming a first coating layer,- applying a second coating composition having a solid content of at least 65 wt% on the first side by means of jet application followed by levelling the applied second coating composition by means of a second blade (7), wherein the second blade has a first surface (7a) that is partly in contact with the applied coating and a second surface (7b) opposite to the first surface (7a) and a second blade tip (8) connecting said first and second surfaces (7a, 7b) and being in contact with the applied coating, wherein the second blade is operating in a bent blade mode, thereby forming a second coating layer.

2. A method according to claim 1 , wherein multiply paperboard web (1) is running in a direction D, and wherein a tangent running from the first blade tip (4) to a point 5 mm away from the first blade tip on the first blade’s first surface (3a) forms an angle a with the running direction D of the moving web (1), which angle a is at least 30°, preferably in the range of 30° to 45°, and wherein a tangent running from the second blade tip (8) to a point 5 mm away from the second blade tip on the second blade tip’s first surface(7a) forms an angle a’ with the running direction D of the moving web (1), which angle a’ is in the range of from 0° to 15°, preferably in the range of from 0° to 5°.

3. A method according to claim 1 , wherein the jet application of the first coating composition comprises applying the first coating composition on the web from a first nozzle (2) having a first nozzle outlet (2a), and the jet application of the second coating composition comprises applying the second coating composition on the web (1) from a second nozzle (6) having a second nozzle outlet, wherein the distance between the first nozzle outlet (2a) and the web (1) is less than 14 mm, preferably less than 12 mm, and the distance between the second nozzle outlet and the web is less than 14 mm, less than 12 mm.

4. A method according to anyone of the preceding claims, wherein the method further comprises applying a third coating composition having a solid content of at least 65 wt% on the first coating layer by means of jet application followed by levelling the applied coating by means of a third blade, thereby forming a third coating layer.

5. A method according to anyone of the preceding claims, wherein the blade pressure of the first and the optional third blade is within the range of 0.2- 1.8 bar, preferably within the range of 0.4-1.3 bar.

6. A method according to anyone of the preceding claims, wherein the first and the second coating layer each has a grammage in the range of 4-12 g / m2, preferably in the range of 5-10 g / m2.

7. A method according to anyone of the claims 4-5, wherein the third coating layer has a grammage in the range of 4-11 g / m2, preferably in the range of 6-9 g / m2.

8. A method according to anyone of the preceding claims, wherein the first and second coating composition each comprises pigment in an amount of80-90 wt%, preferably 86-90 wt%, by dry weight and binder in an amount of 10-20 wt%, preferably 10-14 wt%, by dry weight.

9. A method according to claim 8, wherein the pigment in the second coating composition comprises 65-85 wt% calcium carbonate and 15-45 wt% clay, as calculated on the total solid content of the pigment.

10. A method according to anyone of the claims 8-9, wherein the pigment in the first coating composition comprises 80-100 wt% calcium carbonate and 0-20 wt% clay, as calculated on the total solid content of the pigment.11 . A method according to anyone of the preceding claims wherein each of the first, second and optional third coating composition has a water retention value of below 130 g / m2, preferably in the range of 40-110 g / m2, as measured using to TAPPI standard T701 pm-01.

12. A method according to anyone of the preceding claims, wherein the multiply paperboard web is provided by the step of- forming a first multiply paperboard web,- pressing and drying said first paperboard web,- subjecting the dried first paperboard web to calendering in a precalendering step.

13. A method according to claim 12, wherein the pre-calendering step comprises calendering the web in a hard-nip with a nip load of 10-50 kN / m, preferably 10-40 kN / m.

14. A method according to anyone of the preceding claims, further comprising the step of calendering the coated paperboard web.

15. A method according to claim 14, wherein the step of calendering the coated paperboard web comprises subjecting the web to calendering in at least one calendering nip, wherein the highest nip load in the calendering is below 70 kN / m, preferably below 50 kN / m, most preferably below 30 kN / m.

16. A method according to anyone of the preceding claims, wherein the middle ply comprises at least 50 wt%, preferably between 50-90 wt% of CTMP, as calculated on the total dry weight of the said ply.

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

18. A method according to anyone of the preceding claims, wherein the middle ply comprises 20-50 wt% of broke, as calculated on the total dry weight of said middle ply.

19. A method according to anyone of the preceding claims, wherein the top ply comprises 70-100 wt% of kraft pulp, as calculated on the total dry weight of the top ply.

20. A method according to anyone of the preceding claims, wherein the surface roughness (Bendtsen roughness) of the top side before coating is at least 250 ml / min, preferably in the range of 250-500 ml / min, as measured by the Bendtsen method according to ISO 8791-2:2013.

21. A method according to anyone of the preceding claims, wherein the second coating layer forms an outermost coating layer, and wherein the coated paperboard exhibits, as measured on the second coating layer, at least one of a surface roughness (Bendtsen roughness) of less than 20 ml / min, preferably less than 10 ml / min, as measured by the Bendtsen method according to ISO 8791-2:2013, and a surface roughness (PPS-10) of less than of less than 2 pm, preferably of less than 1.5 pm, most preferably of less than 1 pm, as measured using standard method ISO 8791-4:2021.

Images