A method of manufacturing a heat-sealable paperboard for packaging of liquid, dry and / or frozen food; a heat-sealable paperboard and a package from said paperboard

Abstract

The present disclosure relates to a method of manufacturing a heat-sealable barrier-coated paperboard that can be used for packaging of various types of food, such as liquid, dry or frozen food. The method involves applying three layers of dispersion coating on a paperboard substrate using a blade coater. The middle layer contains pigments and a low amount of latex, preferably less than 40 wt%, while the top layer has a coat weight of between 5 – 12 gsm and a high amount of latex, preferably of between 50 – 95 wt%. The barrier-coated paperboard exhibits at least one of a Water Transfer Vapor Transfer Rate (WVTR) as measured using ASTM F1249-20 of less than 40 g / m2 / 24h, a water absorption between 0.2 – 15 g / m2 as measured using COBB 3600 in accordance with SCAN-P 12:64, and a KIT value of above 8 as measured using TAPPI 559. The method allows the application of the top layer with blade coating at higher coat weights without compromising the quality and performance of the coating.

Description

[0001] A METHOD OF MANUFACTURING A HEAT-SEALABLE PAPERBOARD FOR PACKAGING OF LIQUID, DRY AND / OR FROZEN FOOD; A HEAT-SEALABLE PAPERBOARD AND A PACKAGE FROM SAID PAPERBOARD

[0002] Field of the invention

[0003] 5 The present invention relates to a method of manufacturing a heat-sealable paperboard for packaging of liquid, dry and / or frozen food.

[0004] Background

[0005] Fiber based material used in packages or cups for liquids or frozen food is usually provided with barrier coatings both on the inside, which faces the packed item, and on the outside which provides the print-side of the package. The barrier coating applied on the inside makes the material resistant against e.g. liquids, grease, moisture, oxygen and / or aroma and enables it to withstand the influence of the packed item on the packing material. The barrier coating should also be heat-

[0006] 15 sealable. Barrier coating of paperboard has traditionally been dominated by lamination or extrusion coating with low-cost, readily available polyolefins, such as polyethylene (PE) or polypropylene (PP). Today, bio-based grades are also employed, including other bio-based polymers such as polylactic acid (PLA). Liquid packaging board is typically provided with a laminated polymer coating on both sides

[0007] 20 and often includes an additional aluminum foil or layer on the inside.

[0008] Extrusion coated packaging materials, including several layers of polyolefins and an aluminum foil may be difficult to re-pulp and recycle due to challenges in achieving high material efficiency and production efficient separation of the plastic coating from the paperboard. This has increased the interest in dispersion barrier coated paperboard, wherein an aqueous dispersion of fine polymer particles is applied on the paperboard surface. The coatings can be based on e.g., styrene-acrylate (SA) or styrene-butadiene (SB) latex or polyolefin dispersions. Several paperboard materials with dispersion coatings have been disclosed in relation to repulpable materials that cardboard which comprises a cardboard layer and a coating layer on at least one side, wherein a barrier layer is arranged between the coating layer and the cardboard containing binder and plate-like mineral pigment. The coating layer comprises lightrefracting pigment or mixture of pigments and a binder, which may be a synthetic latex of polymers of e.g. butadiene-styrene type copolymers. US 9771688 discloses a coated paperboard comprising a base coat and a top coat, each comprising binder and pigment. The teaching refers to styrene-acrylate copolymer or styrene-butadiene copolymer as binders. EP 2358942 discloses a recyclable coated paperboard coated with a first and second coating layer formed from an aqueous polymer dispersion, where each layer consists of about 70 to about 90 wt% of polymer emulsion and 10 to about 30 wt% pigments. W02020152635 discloses a heat-sealable paperboard comprising a paperboard substrate with a first dispersion coating layer comprising latex and pigments and a second dispersion coating layer comprising a polyolefin.

[0009] Some demanding end uses, such as the packaging of liquid- or frozen food, require high barrier properties that cannot be achieved by conventional one or two-layer dispersion barrier coatings. Applying three or more layers of dispersion barrier coatings can not only enhance the barrier performance and improve the coating uniformity and efficiency, but also expand the property-quality space.

[0010] Paperboard substrates can be coated with dispersion 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, low energy, high smoothness, and enhanced barrier properties, as it fills the pores effectively.

[0011] Although this particular levelling mode leads to higher smoothness, the risk of insufficient coverage is higher especially for first coating when using base with high content of mechanical fibers or if the formation or permeability of base paper is varying and / or high. However, applying more than two dispersion layers on paperboard by blade coating poses some challenges including coat weight and profile control, runnability (especially metering and levelling), and coating defects such as streaks, stalagmites or blade bleeding. These challenges make it difficult to apply higher coat weights or to run for example high aspects pigments especially in the last blade coating station. Therefore, there is a need for an improved method to produce a heat-sealable, recyclable paperboard for packaging liquid, dry, and / or frozen food, with high barrier properties.

[0012] Description of the invention

[0013] It is an object of the present disclosure to provide a method to manufacture a recyclable, heat-sealable paperboard with barriers against at least one of liquid, grease and / or moisture.

[0014] It is a further object of the present disclosure to provide a method to manufacture a recyclable, heat-sealable paperboard which method facilitates the application of at least three dispersion barrier coating layers at controlled coat weights, without the problems related to coating layer defects and runnability issues connected with the prior art methods.

[0015] The above mentioned objects, as well as other objects as will be realized by the skilled person in the light of the present disclosure, are achieved by the various aspects of the present disclosure.

[0016] According to a first aspect illustrated herein, there is provided a method of manufacturing a heat-sealable barrier-coated paperboard for packaging of liquid, dry and / or frozen food, the method comprising the steps of: providing a paperboard substrate comprising a first side and a second side,

[0017] - applying a first dispersion coating composition on the first side of the paperboard substrate by means of blade coating, forming a first dispersion coating layer,

[0018] - applying a second dispersion coating composition comprising latex and pigments on the first dispersion coating layer by means of blade coating, forming a second dispersion coating layer, wherein said second dispersion coating composition comprises less than 40 wt% latex as calculated on the total dry weight of said composition,

[0019] - applying a third dispersion coating composition comprising latex and pigments on the second dispersion coating layer by means of blade coating, forming a third dispersion coating layer, wherein said third dispersion coating composition comprises latex in an amount of between 50 - 95 wt%, wherein the third dispersion coating layer has a coat weight of between 5 - 12 gsm, preferably between 6 - 12 gsm, thereby forming a barrier-coated paperboard and wherein the barrier-coated paperboard exhibits at least one of a Water Transfer Vapor Transfer Rate (WVTR) of less than 40 g / m2 / 24h, preferably of less than 30 g / m2 / 24h, as measured using ASTM F1249-20, a water absorption between 0.2 - 15, preferably between 1 - 10 g / m2, most preferably between 1 - 5 g / m2, as measured using COBB 3600 in accordance with SCAN-P 12:64, and a KIT value of above 8, preferably above 10 and most preferably above 12 as measured using TAPPI 559.

[0020] The combination of a relatively low amount of latex in the second dispersion coating layer and a higher amount of latex in the third dispersion coating layer enables the application of a third dispersion coating layer using blade coating at higher coat weights without problems with coater runnability. The low amount of latex in the second coating layer gives rise to a second layer with a porosity and / or improved wet dispersion coating affinity that is optimized for the application of the third layer comprising a high amount of latex at quite high coat weights using blade coating. The method of the invention gives rise to fewer defects in the coating layers including fewer streaks, stalagmites or blade bleeding. In addition, good adhesion between the second and third dispersion coating layer is ensured. The barrier-coated paperboard produced by the method exhibits high barrier properties and is heat-sealable. The method also improves the CD profile for the second and third layers, which enhances the barrier performance. Moreover, the barrier-coated paperboard has good properties for creasing and folding, as well as low light transmission especially between 400 and 800 nm wavelengths, which is preferably below 5%, more preferably below 4%, and most preferably below 3%.

[0021] Blade coating as used herein refers to a method to form a coating layer on a substrate or on a previously applied coating layer by applying a coating composition in an excess on the substrate or on the previously applied coating layer, using for example roll applicator or jet applicator, and thereafter levelling the wet coating layer by scrape away or removing the excess coating by use of a flexible or stiff blade with hard or soft blade tip.

[0022] Dispersion coating composition as used herein refers to a dispersion of fine polymer particles or molecules and optionally pigments in a liquid medium. The first, second and / or third dispersion coating compositions are preferably aqueous dispersion coating compositions, i.e. the polymer particles or molecules and optionally pigments are uniformly dispersed or distributed in an aqueous medium. Preferably, the dispersions are made stable using one or several rheology modifiers and dispersing agents.

[0023] 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 acetate-acrylate 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 these latexes. 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 average particle size of the latex emulsion is preferably 50 -500 nm such as 80-250 nm. The size distribution could be both mono or bimodal as well as multimodal. Preferably, the size distribution is at least bimodal since this improves the runnability o the coating machine.

[0024] The latex has preferably a glass transition temperature (Tg) of between -20 to +60 °C, preferably between -15 to +45 °C. The latex may further comprise thickeners and / or defoamers.

[0025] The paperboard substrate is preferably a multi-layer paperboard, comprising a top ply and a back ply and one or several middle plies. The middle ply may provide bulk and / or rigidity. A multi-layer paperboard may be made using several headboxes or using a multilayer headbox. Preferably, the paperboard substrate, also denoted baseboard, is a three-ply paperboard comprising a top ply, a middle ply and a back ply. The paperboard substrate may have a basis weight in the range of 120 - 400 gsm, such as of about 150 gsm, preferably of about 200 gsm, or of about 300 gsm. A multilayer paperboard is particularly suitable for liquid and / or food packaging. The one or more middle plies can contain bleached or unbleached sulphate / kraft pulp and bleached or unbleached CTMP, which advantageously provide additional bulk. The CTMP may be unbleached or bleached HT-CTMP. The pulp can be hardwood and / or softwood, and / or dried or never-dried, and / or fresh or recycled. The middle ply may comprise fibres originating from chemi-thermomechanical pulp (CTMP) or thermomechanical pulp (TMP). Preferably, the middle ply comprises sulphate pulp, broke and CTMP. The top and back ply preferably comprise bleached and / or unbleached sulphate pulp. Top and back may preferably not comprise softwood CTMP fibres. The paperboard substrate has preferably a density of less than 950 kg / m3, more preferably of less than 900 or less than 850 kg / m3. The paperboard substrate may have a surface roughness of at least 135 ml / min, preferably in the range of 150 - 160 ml / m, as measured by the Bendtsen method according to ISO 8791-2:2013.

[0026] Unless otherwise stated in the standard methods referred to herein, all measurements and testing are conducted at a temperature of 23 °C with a relative humidity of 50% (RH).

[0027] If not specifically denoted otherwise, given % are weight%, and are calculated on the basis of a dry weight of 100 weight% of the respective object, such as of a layer or of a composition.

[0028] The coat weight or grammage refers to the weight expressed as grams per square meter, gsm or g / m2. As used herein, gsm and g / m2may be used interchangeable.

[0029] In embodiments, the surface roughness of the formed second dispersion coating layer defines ratio Ra / Rz within the range of 0.30 - 0.35, preferably 0.32, as measured using standard ISO 21920-1 . It has been found that the forming of a second dispersion coating layer with a surface roughness within this range improves the blade coating of the third dispersion coating composition significantly. The surface roughness of the second dispersion coating layer may be controlled by e.g., the latex-pigment ratio, the average particle size of pigments, the glass-transition temperature of the latex, the drying temperature and moisture content prior to the third coating and / or the amount of co-binders or rheology modifiers. It can further be controlled by adjusting the blade angle and blade pressure in the blade coating. The surface roughness of the formed second dispersion coating layer within the said range defines the surface roughness of the second dispersion coating layer before the application of the third coating layer.

[0030] Preferably, the second coating composition comprises latex in an amount of less than 30 wt%, preferably in an amount of 15 - 30 wt%. The second dispersion coating composition may further comprise the pigments in an amount in a range of 60 - 95 wt%, preferably in a range of 70 - 85 wt%, based on the dry solid content of the second dispersion coating composition.

[0031] The latex of the second coating composition is preferably chosen from the group of polyvinyl acetate latex and styrene acrylate latex. These latexes provide a quite high porosity. The latex may further be a mixture of these latexes. In embodiments the latex comprises 100 - 50 wt% styrene acrylate latex and 0 - 50 wt% polyvinyl acetate latex, as calculated on the dry weight of the latex.

[0032] In embodiments, the pigments in the second dispersion coating composition have shape factor as of less than 10, preferably of less than 8. In this context, the term "shape factor" refers to the average value (calculated on a weight average basis) of the ratio between the mean particle diameter and particle thickness. This measurement is conducted utilizing the electrical conductivity method and the as apparatus outlined in U.S. Pat. No. 5,128,606.

[0033] The pigments in the second dispersion coating composition are preferably selected from the group of sodium carbonate, calcium carbonate and phyllosilicates such as clay and combinations thereof. The calcium carbonate may be ground calcium carbonate (GCC) or precipitated calcium carbonate (PCC). In preferred embodiments, the pigments in the second dispersion coating composition are sodium carbonate, most preferably sodium bicarbonate. The partly soluble character of sodium carbonate optimizes the porosity of the second dispersion coating layer. In other preferred embodiments, the pigments are GCC having a mean particle size distribution D95, D90, D60 which means that 95 wt% or 90 wt% or 60 wt% has a particle of less than 2 urn. The size distribution can hence be of different shapes or further engineered by making mixtures of said pigments or by adjusting the steepness or slope of the particle size distribution curve. Preferably, the pigment suspension is substantially free from pigments larger than 5 urn or more pref, of those larger than 10 urn, since these will further leads to runnability problems when applied particularly in the top coating.

[0034] In embodiments, the latex in the second dispersion coating composition has a glass transition temperature (Tg) of at least 25 °C, preferably of at least 30 °C. The glass transition temperature is determined by differential scanning calorimetry method ASTM D7426 - 08(2013). Such a hard latex optimizes the surface roughness of the second dispersion coating layer and thereby facilitates the blade coating of the third dispersion coating layer. In embodiments, the latex is polyvinyl acetate latex or styrene acrylate latex or a mixture thereof, wherein the glass transition temperature (Tg) of the latex Is at least 25 °C, preferably of at least 30 °C, such as between 25 to 60 °C or 30 to 60 °C.

[0035] In embodiments, the method further comprises a step of drying the second dispersion coating layer before the application of the third dispersion coating composition, wherein the step of drying is performed so that the paperboard substrate reaches a temperature of at least 100 °C, preferably of a temperature in the range of 100 - 150 °C. When the second layer of coating is exposed to an excess drying, the porosity of the layer is further optimized, whereby the adhesion of the third dispersion coating layer is enhanced. The drying may be performed by the use of IR and / or hot air. The method may further comprise a step of applying steam, moisture or atomized water to the second dispersion coating layer after the step of drying the second dispersion coating layer and before the application of the third dispersion coating layer. Steam may be applied so that the moisture content of the paperboard including the first and second dispersion barrier coating layers reaches a moisture content in the range of 3 - 15 wt% before the application of the third dispersion coating composition. Preferably, steam, moisture or atomized water is applied in an amount of 0.2-12 gsm.

[0036] In an alternative embodiment, the method comprises a step of drying the second dispersion coating layer before the application of the third dispersion coating composition, wherein the step of drying is performed so that the paperboard substrate reaches a moisture content in the range of 3 - 15 wt% before the application of the third dispersion coating composition. Thus, in this embodiment, no steam is applied.

[0037] The application of the third dispersion coating on the second dispersion barrier coating layer while the paperboard is moist (3 - 15 wt%) enhances the adhesion of the third dispersion coating layer further.

[0038] The second dispersion coating composition is preferably applied in an amount to form a coat weight of the second dispersion coating layer in a range of 4 - 12 gsm, preferably 5 - 10 gsm, and most preferably 6 - 10 gsm.

[0039] Preferably, the third dispersion coating composition comprises latex in an amount of 50 - 95, more preferably in an amount of 70 - 95 wt% and the pigments in a range of 5 - 50 wt%, more preferably in an amount of 5 - 30 wt%. The pigments in the third dispersion coating composition may be selected from the group consisting of calcium carbonate and clay and / or combinations thereof.

[0040] In embodiments, the Brookfield viscosity of the third dispersion coating composition is in the range of 150 - 2500 mPas, most preferably in the range of 200 - 1500 mPas. The Brookfield viscosity is determined at 23 °C, using a Brookfield viscosimeter at a rotational speed of 100 rpm according to SCAN-P 50:84. The invention is particularly advantageous in connection with the application of coating compositions with viscosities within this range, since it is particularly difficult to achieve the desired coat weight when applying dispersion barrier coatings with such low viscosities. Moreover, the coating compositions have shear thinning behavior which makes the interaction between the second and third coating layer more critical.

[0041] In some embodiments, the first, second and / or third dispersion coating composition has an AAGWR water retention value (Abo Akademi Water retention value) below 250 g / m2, preferably in the range of 30-150 g / m2, as determined according to TAPPI standard T701 pm-01. Such a water retention ensures good runnability.

[0042] In addition to latex and pigments, the second and / or third dispersion coating composition may comprise additives conventionally included in dispersion coating compositions, such as thickeners, defoaming agents, dispersing agents, crosslinkers, rheology modifiers and / or lubricants. The second and / or third dispersion coating composition may further comprise co-binders such as e.g., starch and polyvinyl alcohol (PVOH). In embodiments, the second and / or third dispersion coating composition comprises co-binders in an amount of between 0.5 - 15 wt%, preferably of between 0.5 - 10 wt%. The second dispersion coating composition preferably comprises co-binders and rheology modifiers in a total amount of 0.1 - 10 wt%. The solid content of the second and / or third dispersion coating composition is preferably above 35 wt%, most preferably above 40 wt%.

[0043] The first dispersion coating composition may comprise binder and pigments and is preferably applied in an amount to form a first dispersion coating layer with a coat weight in the range of 2 - 12 gsm, preferably in the range of 4 - 10 gsm. The binder in the first dispersion coating composition may be selected from the group consisting of starch, carboxymethyl cellulose (CMC), microfibrillated cellulose (MFC), polyvinyl alcohol (PVOH), polyolefin, styrene acrylate copolymer or styrene butadiene copolymer. In a preferred embodiment, the binder in the first dispersion coating composition is a water-soluble binder, preferably selected from the group of polyvinyl alcohol, starch, carboxymethyl cellulose (CMC) and microfibrillated cellulose (MFC). The use of a water-soluble binder in the first dispersion coating composition facilitates the recycling of the barrier-coated paperboard and the separation of fibers from the dispersion coating. The first dispersion coating composition may comprise the binder in an amount in a range of 10 - 50 wt% and the pigments in a range of 90 - 50 wt%. In preferred embodiments, the first dispersion coating composition may comprise the binder in an amount in a range of 20 - 35 wt% and the pigments in a range of 80 - 65 wt%.

[0044] Microfibrillated cellulose (MFC) shall in the context of the patent application mean a cellulose particle, fiber or fibril having a width or diameter of from 20 nm to 1000 nm. Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment steps is usually required in order to make MFC manufacturing both energy efficient and sustainable. The cellulose fibers of the pulp used when producing MFC may thus be native or pre-treated enzymatically or chemically, for example to reduce the quantity of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, wherein the cellulose molecules contain functional groups other (or more) than found in the original cellulose. Such groups include, among others, carboxymethyl (CM), aldehyde and / or carboxyl groups (cellulose obtained by N-oxyl mediated oxidation, for example “TEMPO”), or quaternary ammonium (cationic cellulose). After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC.

[0045] In addition to binder and pigments, the first dispersion coating composition may also comprise additives conventionally included in dispersion coating compositions, such as thickeners, defoaming agents, dispersing agents, cross-linkers and lubricants.

[0046] In embodiments, the method further comprises the step of surface sizing the paperboard substrate before the step of applying the first dispersion coating composition. The surface sizing is preferably done by use of a metering size press. Preferably, the paperboard substrate is surface sized with anionic or cationic starch, or cationic polymer, microfibrillated cellulose (MFC) or combinations thereof. Preferably, the surface sizing is applied in an a coat weight of 0.5 - 5 gsm per side of the paperboard substrate. This further improves the barrier properties and facilitates the recycling of the product.

[0047] In embodiments, the first, second and / or third dispersion coating composition is applied in an excess and wherein the excess coating is scraped off and reused in any of the first, second and / or third dispersion coating composition.

[0048] The reject received from repulping the heat-sealable paperboard may be less than 12 wt% preferably less than 10 wt%, more preferably less than 7 wt% based on 100 % dry weight of the paperboard and determined according to PTS test method RH021 / 97 (Category II).

[0049] The method of the invention may further comprise a step of drying the barrier-coated paperboard and a step of chilling the dried paperboard, preferably to a temperature of 40 - 60 °C. The chilling in combination with the specific structure of the three coating layers increases the blocking resistance of the produced barrier-coated paperboard.

[0050] The method may further comprise the step of applying moisture, e.g. water, to the second side of the paperboard in order to control or prevent curl.

[0051] The method may further comprise the step of applying a polymer layer on the second side of the paperboard substrate. The polymer layer applied on the second side is preferably an extruded or laminated polymer layer, preferably of a polyolefin such as polyethylene or polypropylene. In this embodiment, the paperboard may further be retortable or autoclavable. In other embodiments, the polymer layer on the second side is a dispersion coated polymer layer. The dispersion coated polymer layer may further include pigments.

[0052] According to a second aspect illustrated herein, there is provided a heat-sealable paperboard for the packaging of liquid, solid or frozen food, produced by the method according to the first aspect. The heat-sealable paperboard may also be used for the packaging of liquid or semi-liquid home care and personal care products. The barrier- coated paperboard may further be suitable for ultrasonic sealing.

[0053] According to a third aspect illustrated herein, there is provided a package for liquid and / or frozen food made from the heat-sealable paperboard according to the second aspect. The package is preferably made by use of heat-sealing. The third dispersion coating layer preferably faces the inner side of such a package, while the optional polymer layer applied on the second side of the paperboard faces the outside of the package. In preferred embodiments, the third dispersion coating layer forms the inner surface of the package, while the optional polymer layer applied on the second side forms the outer / printing surface of the package. In alternative embodiments, the third dispersion coating layer forms the outer / printing surface of the package, while the optional polymer layer applied on the second side forms the inner side.

[0054] A “printing surface” refers to the surface adapted to be printed. The “inner surface” refers to the surface adapted to be in contact with a content of the package

[0055] Further features of the present invention will become apparent from the example and figure, wherein:

[0056] Fig. 1 is a schematic drawing of a heat-sealable barrier-coated paperboard according to an embodiment of the invention.

[0057] The heat-sealable barrier-coated paperboard as shown in Figure 1 comprises a paperboard substrate (1), comprising three plies, a middle ply (1a), a top ply (1b), and a back ply (1c). The middle layer (1a) may comprise CTMP, sulphate pulp and broke. The top ply (1b) and the back ply (1c) may comprise sulphate pulp, most preferably bleached or unbleached sulphate pulp from hardwood. The paperboard substrate comprises a first side and a second side, wherein the top ply (1 b) is arranged on the second side and the back ply (1c) on the first side. The first side refers to the inside of a packaging such as a container made from the heat-sealable paperboard, while the second side refers to the print side of such a packaging.

[0058] On the first side of the heat-sealable paperboard, on the back ply (1c), the paperboard substrate (1) is applied with a first dispersion coating layer (2), forming a first inside layer. The first dispersion coating layer (2) comprises a binder and pigments and is applied by means of blade coating. A second dispersion coating layer (3) comprising a latex and pigments is applied on the first dispersion coating layer (2) by means of blade coating. The second coating layer comprises latex in an amount of less than 40wt%, preferably in the range of 15 - 30 wt%. The second dispersion coating layer (3) forms a second inside layer. A third dispersion coating layer (4) is applied on the second dispersion coating layer (3) by means of blade coating. The third dispersion coating layer (4) forms a third inside layer.

[0059] The third dispersion coating layer (4) is adapted to be in contact with the content of a container produced from the paperboard.

[0060] On the second side of the paperboard, the top ply (1b) is coated with a polymer coating layer (5).

[0061] While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not to be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

Claims1. A method of manufacturing a heat-sealable barrier-coated paperboard for packaging of liquid, dry and / or frozen food, the method comprising the steps of:- providing a paperboard substrate (1) comprising a first side and a second side,- applying a first dispersion coating composition on the first side of the paperboard substrate (1) by means of blade coating, forming a first dispersion coating layer (2),- applying a second dispersion coating composition comprising latex and pigments on the first dispersion coating layer (2) by means of blade coating, forming a second dispersion coating layer (3), wherein said second dispersion coating composition comprises less than 40 wt% latex,- applying a third dispersion coating composition comprising latex and pigments on the second dispersion coating layer (3) by means of blade coating, forming a third dispersion coating layer (4), wherein said third dispersion coating composition comprises latex in an amount of between 50 - 95 wt%, wherein the third dispersion coating layer has a coat weight of between 5 - 12 gsm, thereby forming a barrier-coated paperboard and wherein the barrier-coated paperboard exhibits at least one of a Water Transfer Vapor Transfer Rate (WVTR) of less than 40 g / m2 / 24h as measured using ASTM F 1249-20, a water absorption of between 0.2 - 15 g / m2as measured using COBB 3600 in accordance with SCAN-P 12:64, and a KIT value of above 8 as measured using TAPPI 559.

2. The method according to claim 1 , wherein the surface roughness of the formed second coating layer (3) defines a ratio Ra / Rz within the range of 0.30 - 0.35, preferably 0.32, as measured using standard ISO 21920-1.

3. The method according to any one of claims 1 - 2, wherein the second coating composition comprises latex in an amount of less than 30 wt%, preferably in an amount of 15 - 30 wt%.

4. The method according to anyone of the preceding claims, the second dispersion coating composition comprises the pigments in an amount in a range of of 70 - 85 wt%,5. The method according to anyone of the preceding claims, wherein the pigments in the second dispersion coating composition have shape factor of less than 10.

6. The method according to anyone of the claims 4 and 5, wherein the pigments in the second dispersion coating composition are selected from the group of sodium carbonate, calcium carbonate and clay and combinations thereof.

7. The method according to anyone of the preceding claims, wherein the latex in the second dispersion coating composition has a glass transition temperature (Tg) of at least 25 °C, preferably of at least 30 °C.

8. The method according to anyone of the preceding claims, further comprising a step of drying the second dispersion coating layer before the application of the third dispersion coating composition, and wherein the step of drying is performed so that the paperboard substrate (1) reaches a temperature of at least 100 °C, preferably of a temperature in the range of 100 - 150 °C.

9. The method according to claim 8, further comprising a step of applying steam to the second dispersion coating layer (3) after the step of drying the second dispersion coating layer and before the application of the third dispersion coating layer.

10. The method according to anyone of the preceding claims, wherein the second dispersion coating composition is applied in an amount to form a coat weight of the second dispersion coating layer in a range of 4 - 12 gsm, preferably 5 - 10 gsm, and most preferably 6 - 10 gsm.

11. The method according to anyone of the preceding claims, wherein the third dispersion coating composition comprises the pigments in a range of 5 - 50 wt%.

12. The method according to anyone of the preceding claims, wherein the first dispersion coating composition comprises a binder and optionally a pigment and is applied in an amount to form a first dispersion coating layer (2) with a coat weight in the range of 2 - 10 gsm, preferably in the range of 4 - 6 gsm.

13. The method according to anyone of the preceding claims, wherein the binder in the first dispersion coating composition is selected from the group consisting of starch, carboxymethyl cellulose (CMC), microfibrillated cellulose (MFC), polyvinyl alcohol (PVOH), polyolefin, styrene acrylate copolymer or styrene butadiene copolymer.

14. The method according to claim 13, wherein the binder in the first dispersion coating composition is a water-soluble binder, preferably selected from the group of polyvinyl alcohol, starch, carboxymethyl cellulose (CMC) and microfibrillated cellulose (MFC).

15. The method according to anyone of the claims 12 - 14, wherein the first dispersion coating composition comprises the binder in an amount in a range of 10 - 50 wt% and the pigment in a range of 90 - 50 wt%.

16. The method according to anyone of the preceding claims, further comprising the step of surface sizing the paperboard substrate (1) before the step of applying the first dispersion coating composition.

17. The method according to claim 16, wherein the paperboard substrate is surface sized with anionic or cationic starch, or cationic polymer or microfibri Hated cellulose (MFC) or combinations thereof.

18. The method according to anyone of the preceding claims wherein the first, second and / or third dispersion coating composition is applied in an excess and wherein the excess coating is scraped off and reused in any of the first, second and / or third dispersion coating composition.

19. The method according to anyone of the preceding claims, wherein the reject received from repulping the heat-sealable paperboard is less than 12 wt% preferably less than 10 wt%, more preferably less than 7 wt% based on 100 % dry weight of the paperboard and determined according to PTS test method RH021 / 97 (Category II).

20. The method according to anyone of the preceding claims, further comprising the step of applying a polymer layer (5) on the second side of the paperboard substrate (1).21 . A heat-sealable paperboard for the packaging of liquid, solid or frozen food, produced by the method according to anyone of the claims 1 - 20.

22. A package for liquid, dry and / or frozen food made from the heat-sealable paperboard according to claim 21.

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