Method for manufacturing a multiply paperboard and multiply paperboard

Abstract

The present disclosure provides a method of manufacturing a coated paperboard for inkjet printing. The method comprises providing a multi-ply paperboard substrate with a top-ply, back ply, and middle ply containing at least 40 wt% CTMP. A top-coating composition is applied to the first side, comprising binder and pigment at a 10:100 to 18:100 dry weight ratio. The pigment includes calcium carbonate and clay. The binder comprises 5-35 wt% styrene acrylate co-polymer and 65-95 wt% styrene acrylate vinyl acetate co- polymer. Binder and pigment constitute at least 90 wt% of the top-coating composition dry weight. The applied top-coating composition is dried to form a top-coating layer. The disclosure also provides a coated paperboard produced by this method.

Description

[0001] METHOD FOR MANUFACTURING A MULTIPLY PAPERBOARD AND MULTIPLY PAPERBOARD

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to coated paperboard suitable for inkjet printing, and more particularly to a method of manufacturing a coated paperboard for inkjet printing and a coated paperboard for inkjet printing.

[0004] BACKGROUND

[0005] Paperboard packaging is widely used across various industries due to its versatility, recyclability, and cost-effectiveness. In the printing industry, there is an increasing demand for high-quality inkjet printable paperboard that can provide excellent print quality also at high-speed printing. Inkjet printing technology has gained popularity in recent years due to its flexibility, print-on-demand, ability to produce high-quality graphics, and suitability for short print runs.

[0006] Conventional paperboard coatings for digital printing applications and especially for high speed inkjet printing often face challenges in achieving both good print quality and efficient drying of printed ink. Water-based or low viscosity solvent-based inks require coatings that can quickly absorb carrier liquids. These coatings are typically formulated with porous pigments, high surface area pigments, or resins such as crosslinked gel polymers. However, such coatings often result in low print gloss or prolonged drying times, making them unsuitable for high-speed coating and paperboard production processes. Additionally, such coatings are typically not suitable for on-line production as it will also negatively impact broke handling and reuse of the broke or mill or sheet plant rejects.

[0007] SUMMARY OF THE INVENTION

[0008] The inventors of the present invention have found that high-brightness, multiple-coated paperboards that incorporate substantial amounts of mechanical, high-yield pulpconsistently encounter difficulties in ensuring good print quality, efficient drying, and excellent ink durability. These challenges are compounded by the need to maintain smooth paperboard on-line coater operation and compatibility with fast production speeds. The use of high-yield pulp, such as chemithermomechanical pulp (CTMP), can provide benefits in terms of bulk and stiffness, but it also introduces complexities in achieving a smooth and uniform coating surface.

[0009] Additionally, the coating formulations intended for inkjet printing must strike a balance between ink absorption and surface properties. Coatings that are too absorbent can lead to excessive ink spreading and loss of image sharpness, while coatings that are not absorbent enough can result in poor ink adhesion and longer drying times. This balance becomes particularly challenging when dealing with high-speed production processes and the diverse range of inkjet inks available in the market.

[0010] It has been appreciated that a method and product are needed that overcome one or more of these problems. There is further a need for a coated paperboard that is suitable for high-speed inkjet printing and that may be recycled easily.

[0011] In a first aspect, a method of manufacturing a coated paperboard for inkjet printing is provided. The method comprises: providing a multi-ply paperboard substrate comprising at least a top-ply, a back ply and a middle ply arranged between the top-ply and the back ply, which middle ply comprises at least 40 wt% CTMP, based on the total dry weight of the middle 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; applying a top-coating composition on the first side, wherein the top-coating composition comprises binder and pigment at a bindecpigment dry weight ratio between 10:100 to 18:100, wherein the pigment comprises calcium carbonate and clay and wherein the binder comprises a styrene acrylate (SA) co-polymer in an amount in the range of 5-35 wt% and styrene acrylate vinyl acetate copolymer in an amount of 65-95 wt% as calculated on the total dry weight of the binder, and wherein the binder and pigment amount to at least 90 wt%, preferably to at least 95 wt%, of the dry weight of the top-coating composition; and drying the applied top-coating composition, thereby forming a top-coating layer.This method provides a coated paperboard with high smoothness and good inkjet printability, without the need for excessive calendering. The specific binder blend in combination with calcium carbonate and clay ensures optimal ink density, uniformity, and ink absorption when printed using water-based inkjet printing techniques. Moreover, the top-coating composition is particularly suitable for high-speed coating of paperboard comprising a high amount of bulky fibers, such as CTMP. The applied coating is easy to repulp and reuse in e.g. the middle ply, making it particularly suitable for on-line or off-line coating in an integrated paperboard mill. In addition, the coated paperboard exhibits high strength properties such as high z-strength and high internal bonding (Scott Bond). In a second aspect, a coated paperboard for inkjet printing is provided. The coated paperboard comprises: a multi-ply paperboard substrate comprising at least a top-ply, a back ply and a middle ply arranged between the top-ply and the back ply, which middle ply comprises at least 40 wt% CTMP based on the total dry weight of the middle 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; a top-coating layer applied on the first side, wherein the top-coating layer comprises binder and pigment at a bindecpigment dry weight ratio between 10:100 to 18:100, wherein the pigment comprises calcium carbonate and clay and wherein the binder comprises a styrene acrylate (SA) co-polymer in an amount in the range of 5-35 wt% and styrene acrylate vinyl acetate co-polymer in an amount of 65-95 wt% as calculated on the total dry weight of the binder, and wherein the binder and pigment amount to at least 90 wt% of the dry weight of the top-coating layer.

[0012] This coated paperboard structure provides excellent inkjet printability, dimensional stability, and convertibility, making it suitable for high-speed printing processes and various packaging applications.

[0013] In a third aspect, a method of manufacturing a printed paperboard is provided. The method comprises printing the coated paperboard according to the second aspect by means of inkjet printing on the coated first side. This method allows for high-quality inkjet printing on the coated paperboard, resulting in excellent print quality, ink density, and durability.The method may further comprise a step of applying a polymer layer or a varnish at least onto the printed first side. The application of a polymer layer or a varnish onto the printed surface provides additional protection and enhances the appearance of the printed paperboard, making it suitable for various packaging applications.

[0014] DETAILED DESCRIPTION

[0015] The present disclosure relates to a coated paperboard suitable for inkjet printing and a method of manufacturing such a coated paperboard. The coated paperboard comprises a multi-ply paperboard substrate with a top-coating layer applied on one side. The method of manufacturing involves applying and drying a top-coating composition on the paperboard substrate to form the top-coating layer.

[0016] The coated paperboard is designed to provide good print quality and excellent convertability while maintaining efficient production processes. The top-coating composition includes a specific combination of binder and pigment that allows for highspeed coating and paperboard production, particularly at speed of above 600 m / min, or above 800 m / min or even above 1000 m / min. The coating may be applied in-line or online. The method further effectively enables the repulp and reuse of broke, which may be added e.g. to the middle ply in amounts ranging from 5 wt% to 45 wt%, preferably in amounts ranging from 20 wt% to 45 wt% based on the total dry weight of the middle ply. The coated paperboard produced by the method may further be recycled easily.

[0017] The reject received from the repulping of the coated paperboard may be less than 12 wt%, preferably less than 10 wt%, preferably less than 7 wt% or less than 5 wt% based on the dry weight of the paperboard and determined according to PTS test method PTS-RH 021:2012 -cat 2.

[0018] The multi-ply paperboard substrate includes a middle ply containing a substantial amount of chemithermomechanical pulp (CTMP), which contributes to the bulk, rigidity and stiffness of the paperboard. The top-coating layer comprises a carefully selectedblend of binders and pigments that provide optimal ink absorption and print quality for inkjet printing applications.

[0019] The method of manufacturing the coated paperboard may include additional steps such as applying pre-coating and middle-coating layers before the top-coating layer. These additional layers can further enhance the printability and surface properties of the final product.

[0020] The resulting coated paperboard exhibits desirable properties for inkjet printing, including appropriate water or liquid absorption, surface roughness, and gloss. The paperboard also demonstrates good mechanical properties such as bending resistance, internal bond strength, and z-strength, making it suitable for various packaging applications. Although the paperboard produced by the method is particularly suitable for inkjet printing, it may also be used for dry or liquid toner digital printing technology, flexography, rotogravure or offset including hybrid printing such as combination of inkjet and flexography. The paperboard is further most suitable for sheet-fed or web-fed high speed printing, and high-speed inkjet print with aqueous based inks, primers or over-print varnishes, as such printing methods are more sensitive to dimensional stability and to the fact that ink must dry quickly. When the coated paperboard undergoes intense drying, required in certain printing methods, the moisture distribution across and through the paperboard may be negatively impacted, resulting in issues with flatness or curl. The paperboard of the invention, which comprise the top-coating layer and an optional precoating layer, can help solve these problems by controlling moisture loss during the drying process.

[0021] The multi-ply paperboard substrate comprises at least a top-ply, a back ply, and a middle ply arranged between the top-ply and the back ply. The middle ply comprises at least 40 wt% chemithermomechanical pulp (CTMP) based on the total dry weight of the middle ply. The CTMP may be HT-CTMP and may be derived from hardwood or softwood or combinations thereof. In embodiments, the middle ply comprises at least 50 wt% CTMP, or at least 70 wt% CTMP or at least 80 wt % CTMP. The remaining pulp in the middle ply may be broke and / or chemical pulp. This high CTMP content in the middle ply contributesto the bulk and stiffness of the paperboard substrate as well as the excellent runnability in the printing presses.

[0022] The top-ply comprises at least 50 wt%, and in some examples at least 70 wt%, bleached hardwood kraft pulp, based on the total dry weight of the top-ply. This composition of the top-ply provides a smooth surface for coating and printing. The paperboard substrate may have a grammage range of 180-350 gsm, as measured according to ISO 536. In some examples, the grammage range may be 200-280 gsm. The bulk of the paperboard substrate may be in the range of 1.4-1.8 cm3 / g, preferably in the range of 1.4-1.8 cm3 / g. The surface properties of the paperboard substrate may be characterized by a Bendtsen roughness between 150-450 ml / min, as measured according to ISO 8791-2:2013. This roughness range provides a suitable base for subsequent coating applications. The substrate may further exhibit a water absorption of less than 45 g / m2, according to the COBB (30s) test method using ISO 535:2023

[0023] In some examples, the paperboard substrate may be surface sized on one or both sides. The surface sizing may be performed using starch and can be done by the use of a size press or a film press. The surface sizing composition may comprise at least 50 wt% starch, preferably at least 75 wt% starch, and 1-30 wt% of humectants. The humectants help in avoiding cracking during converting. The humectants may be selected from the group consisting of sorbitol, starch dextrins, polyethylene glycol and metal salts, or combinations thereof. The starch may be anionic, nonionic or weakly cationic starch or a combination of such starches. The surface size may be applied in an amount in the range of 0.4-4 gsm per side, or preferably 0.5-3 gsm per side. Surface sizing can improve the surface properties and printability of the paperboard substrate. Specifically, the surface sizing enables excellent dimensional stability and facilitates the converting of the coated paperboard. It also helps prevent excessive penetration of subsequently applied coatings, thereby improving the runnability during the coating process.

[0024] The combination of the multi-ply structure, the high CTMP content in the middle ply, and the composition of the top-ply results in a paperboard substrate with desirable bulk, stiffness, and surface properties suitable for subsequent coating and printing processes.The top-coating composition comprises binder and pigment at a bindecpigment dry weight ratio between 10:100 to 18:100. In some examples, the bindecpigment dry weight ratio may be between 10:100 to 16:100. The binder and pigment amount to at least 90 wt% of the dry weight of the top-coating composition.

[0025] The binder in the top-coating composition comprises a styrene acrylate (SA) co-polymer and a styrene acrylate vinyl acetate co-polymer. The styrene acrylate (SA) co-polymer is present in an amount in the range of 5-35 wt% and the styrene acrylate vinyl acetate copolymer is present in an amount of 65-95 wt%, as calculated on the total dry weight of the binder. In embodiments, the styrene acrylate (SA) co-polymer is present in an amount in the range of 10-30 wt%, while the styrene acrylate vinyl acetate co-polymer is present in an amount of 70-90 wt%, as calculated on the total dry weight of the binder.

[0026] In some examples, the binder in the top-coating composition may consist of the styrene acrylate (SA) co-polymer and the styrene acrylate vinyl acetate co-polymer without any additional binders. In other examples, the binder may include a co-binder in addition to the styrene acrylate (SA) co-polymer and the styrene acrylate vinyl acetate co-polymer. When present, the co-binder may be included in an amount of 1-15 wt%, or in some cases 1-10 wt%, as calculated on the total dry weight of the binder.

[0027] The co-binder, when included, may be selected from the group consisting of carboxy methyl cellulose (CMC), polyvinyl alcohol (PVOH), starch, styrene butadiene copolymer, polyvinyl acetate copolymer, styrene butadiene acrylonitrile co-polymer, styrene acrylate acrylonitrile co-polymer, styrene maleic anhydride co-polymer, styrene acrylate-maleic anhydride co-polymer, or mixtures thereof.

[0028] The styrene acrylate vinyl acetate co-polymer in the binder of the top-coating composition may have a glass transition temperature (Tg) of below 30°C, as measured using ASTM D7426-08 (2013). In some examples, the Tg may be below 25°C, or below 20°C. The low Tg of the styrene acrylate vinyl acetate co-polymer may contribute to the formation of a flexible and durable coating layer, while also ensuring adequate absorption of the carrier medium of the ink during the printing process.The binder is in the form of a latex in the coating composition. The latex particles preferably have a size (volume average particle diameter) in the range of 0.1-0.18 pm, more preferably in the range of 0.11-0.16 pm, as measured by dynamic light scattering, according to ISO 22412:2017. This particle size range contributes to optimal coating properties and printability. Preferably, the two binder components, i.e. the SA copolymer and the styrene acrylate vinyl acetate have different average particle sizes, resulting in a bimodal particle size distribution. This has shown to improve the rheological properties, especially at high solid contents.

[0029] The pigment in the top-coating composition comprises calcium carbonate and clay. In some examples, the pigment may comprise calcium carbonate in an amount in the range of 60-95 wt% and clay in an amount of 5-40 wt%, based on the dry weight of thepigment. The clay may preferably be kaolin clay. This composition of pigments provides an optimal balance between surface smoothness, gloss and ink absorption, contributing to high-quality inkjet printing results.

[0030] In some examples, the pigment may comprise nano clay in an amount of below 5 wt%, preferably of below 3 wt% calculated on the total dry weight of the pigment. The use of a low amount of nano clay, or even no nano clay, may improve the runnability of the coating process and allow for high drying speeds in the subsequent drying step. In the context of this disclosure, nano clay refers to clay that has at least one dimension in the range of 1-100 nanometers. The pigment may further comprise silica in an amount of below 1 wt%. In embodiments. In embodiments, the pigment may comprise no silica. The pigment may comprise a total amount of nano clay and silica in an amount of less than 5wt% or less than 1 wt%. In embodiments, the pigment may comprise no silica or nanoclay.

[0031] The pigment in the top-coating composition may have a surface area of below 30 m2 / g as measured using ISO 9277:2022. In some examples, the pigment in the top-coating composition may have a surface area in the range of below 25 m2 / g, or below 20 m2 / g, such as in the range of 5-25 m2 / g. The use of pigments with such low surface area incombination with the specific binder blend may provide a surface suitable for inkjet printing.

[0032] The top-coating composition may be applied to the first side of the paperboard substrate using various coating techniques. These techniques may include roller, spray, curtain, blade, slot die, gravure or reverse gravure, film press or rod coating. In some examples, a preferred coating technique may involve the use of a jet applicator followed by blade levelling, preferably using bent blade mode.

[0033] The top-coating composition may be applied in an amount to form a top-coating layer with a coat weight in the range of 5 - 10 gsm, preferably 5-9 gsm. This coat weight range may provide an optimal balance between coating performance and material usage. The speed of the substrate during the coating step may be in the range of 600 - 1200 m / min. In some examples, the speed may be in the range of 800 - 1100 m / min. These high coating speeds may allow for efficient production processes.

[0034] After application, the top-coating composition may be dried to form the top-coating layer. The drying step may be performed using conventional drying techniques such as infrared drying, hot air drying, or a combination thereof. The drying process is preferably carried out to achieve a moisture content of the paperboard of between 3 and 12 wt% and to a surface temperature of the coated side in the range of 70°-120°C, such as 75-115°C, or 70-95°C, such as 70-90°C. Drying to such moisture content in combination with the binder Tg and chemistry of the latex, provide suitable ink carrier acceptance. The surface temperature may further be controlled by applying a cooling step after the last coating and drying step. It has been found that drying to too low moisture contents may result in too high latex film formation or coalescence which affects the print quality and ink drying negatively.

[0035] The top-coating composition may include additives conventionally used in pigment coating compositions. These additives may be present in an amount ranging from 0.1-10 wt%, or 0.1-5 wt%, based on the total dry weight of the composition. Examples of suchadditives may include rheology modifiers or water retention agents. These additives may help control the flow properties and drying characteristics of the coating composition. The combination of the specific binder blend, pigment composition, and coating parameters may result in a top-coating layer with desirable properties for inkjet printing applications. The coating process and subsequent drying step may be optimized to achieve the desired coat weight, surface properties, and production efficiency.

[0036] In addition to the top-coating layer, the method of manufacturing the coated paperboard may include applying one or more additional coating layers to enhance the properties of the final product. These additional layers may include a pre-coating layer and a middlecoating layer.

[0037] A pre-coating composition may be applied on the first side of the paperboard substrate to form a pre-coating layer and is preferably applied directly to the paperboard substrate, which may be surface sized. This pre-coating step may be performed before applying the top-coating composition. The pre-coating composition may comprise binder and pigment at a bindecpigment weight ratio of between 10:100 to 18:100. The binder and pigment may amount to at least 90 wt% of the pre-coating composition. The pre-coating composition may have a solid content of at least 63 wt%. In some examples, the solid content may be in the range of 63 - 77 wt%. In other examples, the solid content may be in the range of 65-75 wt%. The pre-coating composition may be applied in an amount to form a pre-coating layer with a coat weight in the range of 6-10 gsm, which provides optimal coverage while maintaining cost-effectiveness.

[0038] The pigment in the pre-coating composition may be chosen from the group of clay, such as kaolin clay, calcium carbonate, or mixtures thereof. In embodiments, the pigment in the pre-coating is calcium carbonate. The binder of the pre-coating composition may be selected from the group consisting of styrene-acrylate (SA) co-polymer, styrenebutadiene (SB) co-polymer, styrene acrylate vinyl acetate co-polymer, or mixtures thereof.A middle-coating composition may be applied on the first side of the paperboard substrate to form a middle-coating layer. This middle-coating step may be performed after applying the pre-coating composition and before applying the top-coating layer. The middle-coating composition may comprise binder and pigment at a bindecpigment weight ratio of between 10:100 to 18:100. The binder and pigment may amount to at least 90 wt% of the middle-coating composition.

[0039] The middle-coating composition may have a solid content of at least 65 wt%, at the application step. In some examples, the solid content may be in the range of 65-80 wt%. The middle-coating composition may be applied in an amount to form a middle-coating layer with a coat weight in the range of 6-10 gsm.

[0040] The binder and pigment of the middle-coating composition may be selected from the same groups of binders and pigments as those used in the pre-coating composition. Each of the pre-coating and middle-coating compositions may be applied using conventional coating techniques, such as roller, spray, curtain, blade, slot die, gravure, or rod coating. A preferred technique may include the use of a jet applicator followed by blade levelling.

[0041] The application of these additional coating layers may provide a smoother surface for the subsequent top-coating layer, potentially improving the final print quality and surface properties of the coated paperboard. The surface roughness of the coated paperboard, comprising the optional pre-coating layer and the optional middle-coating layer, before the application of the top-coating layer, and measured using the Parker Print Surf (PPS) method at 10 kgf / cm2(PPS-10), may be less than 4 |im. The high solid content of both the pre-coating and middle-coating compositions may allow for efficient drying and may help prevent issues such as fiber roughening or swelling during the coating process. In embodiments, the total coat weight of the pre-, middle- and top-coating layer is at most 15 gsm, preferably at most 12 gsm. In these embodiments, there are preferably no additional coating layers comprising pigments on the first side.The coated paperboard produced by the method described herein exhibits several desirable characteristics that make it suitable for inkjet printing and various converting and packaging applications. These characteristics include specific ranges for water absorption, surface roughness, gloss, bending resistance, internal bond strength, and z-strength.

[0042] The surface roughness of the coated paperboard, as measured on the coated top-side (including the top-coating layer) using the Parker Print Surf (PPS) method at 10 kgf / cm2(PPS-10), may be in the range of 0.6-1.8 |im as determined according to ISO 8791-4:2021. In some examples, the surface roughness may be in the range of 0.7-1.4 |im. This range of surface roughness provides a suitable balance between ink receptivity and print quality. It also provided very good laser code readability and good offset printing quality.

[0043] The coated paperboard and the top coating formulation are also suitable for other digital printing processes, such as dry and liquid toner technology. Furthermore, as demonstrated in the example, the coating interacts well with varnishes, such as UV varnish, which resulted in a high gloss finish.

[0044] The Cobb-Unger (30s) value of the coated paperboard, as measured on the coated topside, may be less than 10 g / m2, preferably less than 8 g / m2, according to SCAN-P 37:77. This allows for appropriate ink absorption during the printing process while maintaining the structural integrity of the paperboard.

[0045] The coated paperboard may exhibit a z-strength of at least 200 kPa, as measured according to ISO 15754: 2009. In some examples, the z-strength may be at least 210 kPa. The z-strength is a measure of the internal bond strength of the paperboard in the z-direction (thickness direction) and is indicative of the paperboard's resistance to delamination. A high z-strength further makes the paperboard suitable for converting, including sheeting, creasing, folding and printing.

[0046] The gloss of the coated paperboard, as measured on the coated top-side at a 75° angle using ISO 8254-1:2009, may be above 40%. In some examples, the gloss may be in the range of 40-60%. This level of gloss contributes to the visual appeal of the printed productwhile maintaining good ink receptivity. In addition, high gloss is important for laser code readers, e.g. ID or 2D codes.

[0047] The internal bond strength of the coated paperboard, as measured by the Scott Bond test according to TAPPI T 569, may be at least 120 j / m2. This internal bond strength ensures good structural integrity and resistance to delamination during converting processes. The bending resistance of the coated paperboard, measured at a 15° angle in the machine direction (MD), may be in the range of 185-800 mN as determined according to ISO 2493-1:2010. This range of bending resistance provides sufficient stiffness for various packaging applications while allowing for appropriate flexibility.

[0048] These characteristics of the coated paperboard may be achieved through the combination of the multi-ply substrate composition, the specific coating formulations, and the coating process parameters described herein. The balance of these properties contributes to the overall performance of the coated paperboard in inkjet printing applications and subsequent converting processes.

[0049] The coated paperboard produced by the method described herein may be used for manufacturing a printed paperboard by means of inkjet printing on the coated first side. The inkjet printing process may involve applying water-based or solvent-based inks to the coated surface of the paperboard using inkjet printing technology. It may also involve UV inks or solvent free inks. The specific combination of binders and pigments in the topcoating layer may provide optimal ink absorption and print quality for inkjet printing applications.

[0050] In some examples, the inkjet printing process may be performed using sheet-fed or web-fed high-speed printing equipment, or direct printing on boxes or packages The coated paperboard may be particularly suitable for these high-speed printing methods due to its dimensional stability and quick ink drying properties. The method of the invention further reduces the need for the application of a primer prior to the printing.

[0051] After the inkjet printing process, a polymer layer or a varnish may be applied onto the printed first side of the paperboard. This polymer layer may serve various purposes, suchas providing additional protection to the printed surface, enhancing the appearance of the printed image, or improving the barrier properties of the paperboard.

[0052] In some examples, the polymer layer may be one or several polyolefin layers, with a total grammage preferably within the range of 7-30 gsm. The polyolefin layer may be applied to the printed first side using extrusion coating or lamination techniques.

[0053] The varnish may e.g. be a UV varnish or a dispersion varnish.

[0054] The application of a polymer layer to the printed surface may provide exceptional adhesion due to the specific properties of the coated paperboard. The combination of the multi-ply substrate, the coating layers, and the inkjet-printed surface may create a suitable foundation for strong adhesion of the polymer or varnish layer.

[0055] The polymer layer may be selected based on the specific requirements of the final product and may e.g. comprise different types of polyolefins such as polyethylene or polypropylene.

[0056] To compensate for a tendency to curl, a moisturizing solution or dispersion may be applied on the second side of the paperboard substrate. This moisturizing step may be performed after the application and drying of any of the coating layers on the first side. The moisturizing solution or dispersion may comprise water, starch, or a mixture of starch and pigments. The moisturizing solution or dispersion may be applied in an amount ranging from 1 to 9 grams per square meter (gsm). In some examples, the application amount may be in the range of 1 to 4 gsm. This range of application amounts may provide sufficient moisture to counteract the curl while avoiding excessive wetting of the paperboard.

[0057] The application of the moisturizing solution or dispersion may help balance the moisture content between the two sides of the paperboard, thereby reducing or eliminating curl. This curl compensation may contribute to improved flatness of the final product, which may be beneficial for subsequent printing and converting processes.The combination of the multi-ply substrate structure, the specific coating formulations, and the curl compensation technique may result in a coated paperboard with excellent dimensional stability and printability. The high chemithermomechanical pulp (CTMP) content in the middle ply may provide bulk and stiffness to the paperboard, while the carefully selected binder and pigment compositions in the coating layers may ensure optimal ink absorption and print quality for inkjet printing.

[0058] The application of multiple coating layers, including the optional pre-coating and middlecoating layers, may contribute to a smooth and uniform surface for inkjet printing. The high solid content of these coating compositions may allow for efficient drying and may help prevent issues such as fiber roughening or swelling during the coating process. This may result in a coated paperboard with good surface properties and excellent coater runnability.

[0059] The specific combination of binders in the top-coating layer, including the styrene acrylate (SA) co-polymer and the styrene acrylate vinyl acetate co-polymer, may provide a balance of properties that contribute to both print quality and convertibility. The low Tg of the styrene acrylate vinyl acetate co-polymer may enhance the flexibility of the coating, while the styrene acrylate (SA) co-polymer may contribute to the overall durability and print receptivity of the surface.

[0060] The carefully selected pigment composition, including calcium carbonate and clay, may provide optimal ink absorption and surface properties for inkjet printing. The low surface area of the pigments used in the top-coating composition may contribute to improved runnability during the coating process and may allow for high drying speeds.

[0061] The interaction of these various elements may result in a coated paperboard that exhibits desirable characteristics for inkjet printing applications, including appropriate water absorption, surface roughness, and gloss. The coated paperboard may also demonstrate good mechanical properties such as bending resistance, internal bond strength, and z-strength, making the paperboard suitable for various packaging applications and subsequent converting processes.EXPERIMENTAL

[0062] The effectiveness of the coated paperboard produced by the method described herein has been demonstrated through experimental results. An example of the invention and a comparative example were prepared and evaluated.

[0063] In the inventive example, a 240 gsm 3-ply base board was used, comprising a high content of CTMP in the middle ply (70 wt% HT CTMP and 30% Broke), while both top and back sides comprised bleached hardwood kraft pulp. The base board top side was coated with 3 pigment coating layers (pre-, middle, and top coating) with a total coat weight of less than 25 gsm. Notably, the total content of synthetic binder was less than 1.5 wt% in the whole product, which is significantly less than for normal inkjet papers.

[0064] The first coating (pre-coating layer) was applied using a jet applicator and blade coater to an amount of 8.1 gsm at a speed of 900 m / min and subsequently dried with IR and air dryers to a moisture content of 7.3 wt%. The coating comprised 100 pph calcium carbonate (Omya HydroCarb60 and Omya HydrocarbGO, Omya Int AG), 14 parts of styrene / acrylate latex, and 0.5 parts of rheology modifiers. The coating had a viscosity of 1100 mPas (Brookfield, 100 rpm as measured according to SCAN-P 50:84) and a solid content of 67.8 wt%.

[0065] After drying, a second coating (middle-coating layer) was applied using a jet applicator and a blade coater, based on a 100% calcium carbonate recipe similar to the first coating. The coating was applied at 900 m / min with a coating amount of 7.0 gsm (dry coat weight after drying). After drying with IR and air dryers, the moisture content of the paperboard was about 7.3 wt%.

[0066] A third and final coating (top-coating layer) was then applied using jet applicator and blade coater, with the composition shown in Table 1.

[0067] A comparative example was also prepared with similar pre- and middle-coating layers but with a different top coating composition, also shown in Table 1. The GCC-1 is a high surface area calcium carbonate designed for faster absorption and e.g. inkjet inks, whereas the fine calcium carbonate used in the invention had a BET surface area of 15g / m2 which can be determined with for example ISO 9277:2022 and the fine clay had a specific surface area of 21 m2 / g according to ISO 9277:2022.

[0068] The top coating was dried with IR dryers and hot air to a moisture content of 7-8 wt%. Surface temperature after drying was between 75 and 90 degrees Celsius. Samples were not calendered.

[0069] Table 1 - top coating composition

[0070]

[0071] The properties of the coated board samples are shown in table 2.

[0072] The properties (PPS Smoothness, Brightness, Cobb Unger and gloss) were determined for the top-side (print side). Hexanal content was measured by using static headspace Gas Chromatrography with Flame Ionization Detection (GC / FID) using Hexanal in triacetin as calibration. The samples were heated to 90 °C for 40 minutes.Table 2 - Results for the coated paperboard comprising top coating

[0073]

[0074] The samples were then tested for inkjet printability, offset printability, laser coding and (uv) varnishing. Both the comparative paperboard and the coated paperboard according to the invention were printed using Videojet 8510 printer with black water based dye ink and printed at 300 dpi resolution for coding tests with speed of 50 m / min (print quality) and 25 m / min (drying time). The samples were also printed on sheet fed heat-set offset printer and varnished with UV-varnish using anilox roller.

[0075] Table 3 below shows the results from the printing trials.

[0076] Table s Results from printing trials

[0077] > <

[0078]

[0079] The results confirmed that the use of styrene acrylate vinyl acetate latex in the inventive example provided surprisingly good smoothness and good inkjet printability. Although thecomparative example comprised a modified calcium carbonate pigment and s / A latex, the comparative example did not provide sufficient smoothness or inkjet printability. The trial demonstrated that a bulky paperboard can be coated for inkjet printing with fairly low amount of coating and without using special resins or high surface area pigments such as silica. Also, it was shown that the coating could be applied at high speeds without causing problems with base board delamination. In fact, the inventive example showed improved internal strength properties along with high smoothness, which provided good inkjet printability, high UV varnish gloss, and good laser coding. These experimental results highlight the effectiveness of the coated paperboard produced by the method described herein. The combination of the multi-ply substrate with high CTMP content, the specific coating formulations, and the coating process parameters resulted in a product that exhibits excellent printability and convertibility while maintaining good structural integrity. The low hexanal content further enables a higher content of broke in the middle ply.

[0080] Features of any of the examples or embodiments outlined above may be combined to create additional examples or embodiments without losing the intended effect. It should be understood that the description of an embodiment or example provided above is by way of example only, and various modifications could be made by one skilled in the art. Furthermore, one skilled in the art will recognise that numerous further modifications and combinations of various aspects are possible. Accordingly, the described aspects are intended to encompass all such alterations, modifications, and variations that fall within the scope of the appended claims.

Claims

CLAIMS1. A method of manufacturing a coated paperboard for inkjet printing, the method comprising the steps of:providing a multi-ply paperboard substrate comprising at least at top-ply, a back ply and a middle ply arranged between the top-ply and the back ply, which middle ply comprises at least 40 wt% CTMP based on the total dry weight of the middle 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, andapplying a top-coating composition on the first side, wherein the top-coating composition comprises binder and pigment at a bindecpigment dry weight ratio between 10:100 to 18:100,wherein the pigment comprises calcium carbonate and clay and wherein the binder comprises a styrene acrylate (SA) co-polymer in an amount in the range of 5-35 wt% and styrene acrylate vinyl acetate copolymer in an amount of 65-95 wt% as calculated on the total dry weight of the binder, andwherein the binder and pigment amount to at least 90 wt% of the dry weight of the top-coating composition,drying the applied top-coating composition, thereby forming a top-coating layer.

2. A method according to claim 1, wherein the pigment comprises calcium carbonate in an amount in the range of 60-95 wt% and clay in an amount of 5-40 wt%, based on the dry weight of the pigment..

3. A method according to anyone of the preceding claims, wherein the pigment comprises nano clay in an amount of below 5 wt% calculated on the total dry weight of the pigment.

4. A method according to anyone of the preceding claims, wherein the pigment in the top-coating composition has a surface area of below 30 m2 / g as measured using ISO 9277:2022.

5. A method according to anyone of the preceding claims, wherein the styrene acrylate vinyl acetate co-polymer has a glass transition temperature (Tg) of below 30, preferably below 25, or below 20 °C, as measured using ASTM D7426-08 (2013).

6. A method according to anyone of the preceding claims, wherein the top-coating composition is applied in an amount to form a top-coating layer with a coat weight in the range of 5 - 10 gsm.

7. A method according to anyone of the preceding claims, wherein the method further comprises a step of applying a pre-coating composition on the first side forming a pre-coating layer, which step is performed before the step of applying the top-coating composition, wherein the pre-coating composition comprises binder and pigment at a bindecpigment weight ratio of between 10:100 to 18:100, and wherein the binder and pigment amount to at least 90 wt% of the pre-coating composition.

8. A method according to claim 7, wherein the pre-coating composition has a solid content of at least 63 wt%, preferably in the range of 63 - 77 wt%, more preferably in the range of 65-75 wt%.

9. A method according to anyone of the claims 7-8, wherein the pre-coating composition is applied in an amount to form a pre-coating layer with a coat weight in the range of 6-10 gsm.

10. A method according to anyone of the claims 7-10, wherein the method further comprises a step of applying a middle-coating composition on the first sideforming a middle-coating layer, which step is performed after the step of applying the pre-coating composition and before the step of applying the top-coating layer, wherein the middle-coating composition comprises binder and pigment at a bindecpigment weight ratio of between 10:100 to 18:100, and wherein the binder and pigment amount to at least 90 wt% of the middle-coating composition.

11. A method according to claim 10, wherein the middle-coating composition has a solid content of at least 65 wt%, preferably in the range of 65-80 wt% and is applied in an amount to form a middle-coating layer with a coat weight in the range of 6- 10 gsm.

12. A method according to anyone of the preceding claims, wherein the top-ply comprises at least 50 wt%, preferably at least 70wt% bleached hardwood kraft pulp, based on the total dry weight of the top-ply.

13. A method according to anyone of the preceding claims, further comprising a step of applying a moisturizing solution or dispersion on the second side to compensate for curl.

14. A coated paperboard for inkjet printing, comprising:a multi-ply paperboard substrate comprising at least at top-ply, a back ply and a middle ply arranged between the top-ply and the back ply, which middle ply comprises at least 40 wt% CTMP based on the total dry weight of the middle 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,a top-coating layer applied on the first side,wherein the top-coating layer comprises binder and pigment at a bindecpigment dry weight ratio between 10:100 to 18:100, wherein the pigment comprises calcium carbonate and clay andwherein the binder comprises a styrene acrylate (SA) co-polymer in an amount in the range of 5-35 wt% and styrene acrylate vinyl acetate co-polymer in an amount of 65-95 wt% as calculated on the total dry weight of the binder, andwherein the binder and pigment amount to at least 90 wt% of the dry weight of the top-coating layer.

15. A coated paperboard according to claim 14, wherein the pigment in the topcoating layer has a surface area of below 30 m2 / g as measured using ISO 9277:2022.

16. A paperboard according to anyone of the claims 14-15, wherein the top-coating layer has a coat weight in the range of 5 - 10 gsm.

17. A paperboard according to anyone of the claims 14-16, wherein the paperboard further comprises a pre-coating layer applied on the first side between the top-ply and the top-coating layer, wherein the pre-coating layer comprises binder and pigment at a bindecpigment weight ratio of between 10:100 to 18:100, and wherein the binder and pigment amount to at least 90 wt% of the pre-coating layer.

18. A paperboard according to anyone of the claims 14-17, wherein the paperboard further comprises a middle-coating layer applied on the first side between the pre-coating layer and the top-coating layer, wherein the middle-coating layer comprises binder and pigment at a bindecpigment weight ratio of between 10:100 to 18:100, and wherein the binder and pigment amount to at least 90 wt% of the middle-coating layer.

19. A coated paperboard according to anyone of the claims 14-18, wherein the coated paperboard exhibits, as measured on the coated top-side, at least one of:a COBB-Unger value of less than 10 g / m2according SCAN-P 3 " TI,a surface roughness (PPS-10) in the range of 0.6-1.8 um, preferably 0.7-1.4, as measured using ISO 8791-4:2021,a gloss (75°) of above 40%, preferably in the range of 40-60%, as measured using ISO 8254-1:2009, or a combination thereof.

20. A coated paperboard according to anyone of the claims 14-19, wherein the coated paperboard exhibits at least one of:an internal bond strength (Scott Bond) of at least 120 j / m2, as measured according to TAPPI T 569a z-strength of at least 200 kPa, preferably at least 210 kPa, as measured according to ISO 15754: 2009.

21. A method of manufacturing a printed paperboard comprising printing the coated paperboard according to anyone of the claims 11-20 by means of inkjet printing on the coated first side.

22. A method according to claim 21, which method further comprises a step of applying a polymer layer or a varnish onto the printed first side.

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