SE2230444A1 - A method for manufacturing a coated paper or paperboard product - Google Patents

Abstract

The present invention relates to a method for manufacturing a coated paper or paperboard product, said method comprising: a) providing a paper or paperboard substrate having a grammage of at least 100 g/m2, a density below 900 kg/m3, and a moisture content of at least 3.5 wt%, as determined according to ISO 638, said paper or paperboard substrate having a first main surface and a second main surface, b) forming a first water vapor barrier layer on the first main surface of the paper or paperboard substrate, said first water vapor barrier layer comprising a thermoplastic polymer, c) forming a second water vapor barrier layer on the second main surface of the paper or paperboard substrate, said second water vapor barrier layer comprising a thermoplastic polymer, and d) forming a vacuum coating layer on said first water vapor barrier layer to obtain the coated paper or paperboard product.

Description

A METHOD FOR MANUFACTURING A COATED PAPER OR PAPERBOARD PRODUCT Technical field The present disclosure relates to coated paper or paperboard products comprising a vacuum coating layer and to methods for their manufacture.

Background Coating of paper and paperboard with plastics is often employed to combine the mechanical properties of the paper or paperboard with the barrier and sealing properties of a plastic film. Paper or paperboard provided with even a relatively small amount of a suitable plastic material can provide the properties needed to make the paper or paperboard suitable for many demanding applications, for example as liquid or food packaging board. ln liquid or food packaging board, polyolefin coatings are frequently used as liquid barrier layers, heat sealing layers and adhesives. However, the recycling of such polymer coated board is difficult since it is difficult to separate the polymers from the fibers.

Also, in many cases the water vapor barrier properties of the polymer coated paper or paperboard are still insufficient unless the coating layers are thick or combinations of different polymer coating layers are used. Therefore, in order to ensure high water vapor barrier properties, the polymer coated paper or paperboard is often combined with one or more layers of aluminum foil. However, the addition of polymer and aluminum foil add significant costs and the combination of polymer coating layers and aluminum foils makes recycling of the materials more difficult. Also, due to its high carbon footprint there is a wish to replace aluminum foils in paper and paperboard based packaging materials with renewably sourced materials.

Aseptic packaging for long shelf-life products such as milk and juices are usually made from liquid or food packaging board comprising a multilayer paperboard based substrate, an outermost heat-sealable polyolefin (e.g. polyethylene, PE) 2 layer and innermost layers of polyolefin and aluminum. The aluminum foi| layer, needed to provide water vapor and oxygen barrier properties, is usually incorporated between layers of polyethylene to provide the following structure: PE/paperboard/PE/ aluminum/PE. ln the prior art, attempts have been made to replace the aluminum foi| with more environmentally friendly and/or easier to recycle solutions, but so far with no real SUCCSSS.

One solution presented in the prior art is to apply a very thin organic or inorganic barrier coating layer using vacuum deposition techniques, such as physical vapor deposition (PVD) or chemical vapor deposition (CVD). The vacuum deposited barrier coating layer may for example comprise or consist of AlOx, AlzOs or SiOx. These very thin coating layers, typically having a thickness in the range of 20-600 nm, and more preferably in the range of 50-250 nm, can when applied to a suitable substrate provide good oxygen and water vapor barrier properties, comparable to the barrier properties of thicker aluminum foils. As the thickness of the vacuum deposited coatings is typically at least an order of magnitude lower than the thickness of conventional foils, the metal content of the products can be dramatically reduced.

However, vacuum deposition coating performed directly on a paper or paperboard substrate to be coated, so called direct vacuum coating, or direct metallization, has been found to be problematic, especially for heavier paper or paperboard grades. The coating process takes place under vacuum, which means that the paper or paperboard substrate needs to degassed. ln addition to adding costs, the degassing also means that the substrate is dried to a very low moisture content. This drying and the subsequent remoisturizing to ambient moisture levels changes the mechanical properties of the paper or paperboard substrates. The drying will not only increase the cracking tendency and post-convertability of the paper or paperboard substrate, but there is also significant risk of cracking of the thin and sensitive vacuum coating layer as the substrate is remoisturized. Some additives commonly used in paper or paperboard may also be volatile and can migrate and hence cause deposits in the vacuum coating machine.

A solution presented in the prior art is to first provide a thin substrate in the form of a plastic film, or a high-density or compact paper with a vacuum coating layer, and then laminating the vacuum coated substrate to a paper or paperboard base layer to provide the base layer with improved barrier properties. However, this lamination type solution adds cost and complexity to the manufacturing method.

Thus, there remains a need for improved solutions for replacing the combination of plastic films and aluminum foils in paper and paperboard based packaging materials, while maintaining acceptable liquid, water vapor, and oxygen barrier properties. At the same time, there is a need to replace the combination of plastic films and aluminum foils with alternatives that facilitate repulping and recycling of the used packaging materials.

Description of the invention lt is an object of the present disclosure to provide an alternative to the combination of plastic films and aluminum foils commonly used as barrier layers for providing water vapor barrier properties in packaging materials, such as liquid or food packaging board. lt is a further object of the present disclosure, to provide a method which allows for vacuum deposition coating to be performed directly on the paper or paperboard substrate to be coated, rather than first applying the vacuum coating layer to a thin substrate. lt is a further object of the present disclosure to provide a barrier layer, which has an oxygen transmission rate (OTR), measured according to the standard ASTM F1927 at 50% relative humidity and 23 °C, of less than 10 cc/m2/24h. lt is a further object of the present disclosure to provide a barrier layer, which has a water vapor transmission rate (WVTR), measured according to the standard ASTM F1249 at 50% relative humidity and 23 °C, of less than 10 g/m2/24h. 4 lt is a further object of the present disclosure to provide a coated paper or paperboard product, useful for example as a liquid or food packaging board, which facilitates repulping and recycling of the packaging board as compared to conventional packaging Iaminates using combinations of plastic films and aluminum foils.

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.

Vacuum deposition coating performed directly on a paper or paperboard substrate to be coated, so called direct vacuum coating, or direct metallization, has been found to be problematic, especially for heavier paper or paperboard grades. The coating process takes place under vacuum, which means that the paper or paperboard substrate needs to degassed. ln addition to adding costs, the degassing also means that the substrate is dried to a very low moisture content. This drying and the subsequent remoisturizing to ambient moisture levels changes the mechanical properties of the paper or paperboard substrates. The drying will not only increase the cracking tendency and post-convertability of the paper or paperboard substrate, but there is also significant risk of cracking of the thin and sensitive vacuum coating layer as the substrate is remoisturized. Some additives commonly used in paper or paperboard may also be volatile and can migrate and hence cause deposits in the vacuum coating machine.

The present invention solves this problem by first sealing each of the main surfaces of the paper or paperboard substrate with a water vapor barrier layer, such that the moisture content in the paper or paperboard substrate is retained in the substrate as vacuum coating treatment is performed.

According to a first aspect illustrated herein, there is provided a method for manufacturing a coated paper or paperboard product, said method comprising: a) providing a paper or paperboard substrate having a grammage of at least 100 g/m2, a density below 900 kg/m3, and a moisture content of at least 3.5 wt%, as determined according to ISO 638, said paper or paperboard substrate having a first main surface and a second main surface, b) forming a first water vapor barrier layer on the first main surface of the paper or paperboard substrate, said first water vapor barrier layer comprising a thermoplastic polymer, c) forming a second water vapor barrier layer on the second main surface of the paper or paperboard substrate, said second water vapor barrier layer comprising a thermoplastic polymer, and d) forming a vacuum coating layer on said first water vapor barrier layer to obtain the coated paper or paperboard product.

The paper or paperboard substrate (also referred to herein as "the substrate") is a sheet or web of material mainly formed from pulp of wood or other fibrous substances. The paper or paperboard substrate has a first main surface and a second main surface.

The method of the present disclosure is particularly useful for heavier paper or paperboard grades having a moisture content of at least 3.5 wt%. Thus, the paper or paperboard substrate used in the method should have a grammage of at least 100 g/m2, a density below 900 kg/m3, and a moisture content of at least 3.5 wt%, as determined according to ISO 638. ln some embodiments, the paper or paperboard substrate has a grammage in the range of 115-600 g/m2, preferably in the range of 150-450 g/m2, based on dry weight. Unless otherwise stated, the grammage is determined according to the standard ISO 536. ln some embodiments, the paper or paperboard substrate has a density in the range of 350-900 kg/m3, preferably in the range of 450-850 kg/m3, based on dry weight. Unless otherwise stated, the density is determined according to the standard ISO 534.

The combination of grammage and density typically results in a paper or paperboard substrate having a relatively high bending resistance. ln some 6 embodiments, the paper or paperboard substrate has a geometric mean bending resistance as measured according to ISO 2493 (L&W, 15°) above 150 mN, preferably above 170 mN, and more preferably above 190 mN.

Paper generally refers to a material manufactured in sheets or rolls from the pulp of wood or other fibrous substances comprising cellulose fibers, used for e.g. writing, drawing, or printing on, or as packaging material. Paper can either be bleached or unbleached and produced in a variety of thicknesses, depending on the end-use requirements.

Paperboard generally refers to strong, thick paper or cardboard comprising cellulose fibers used for example as flat substrates, trays, boxes and/or other types of packaging. Paperboard can either be bleached or unbleached and produced in a variety of thicknesses, depending on the end-use requirements. ln some embodiments, the paper or paperboard substrate is a multiply paperboard. The multiply paper or paperboard substrate is preferably comprised of two or more, more preferably three or more, cellulose-based plies. Each of the cellulose-based plies can have a certain composition of pulp fibers, such as bleached and/or unbleached Kraft pulp, sulfite pulp, dissolving pulp, thermomechanical pulp (TMP), chemi-thermomechanical pulp (CTMP), high- temperature CTMP (HT-CTMP), broke, and/or mixtures thereof. The different plies can have different grammages and/or thicknesses and may contain different amounts of additives, such as internal sizing agents.

As an example, the paper or paperboard substrate can be built up of one top-ply consisting of bleached or unbleached Kraft pulp, a mid-ply consisting of a mixture of bleached or unbleached Kraft pulp and CTMP, and a bottom-ply consisting of bleached or unbleached Kraft pulp, wherein the mid-ply has a higher thickness and/or lower density than both the top and bottom plies, respectively. ln some embodiments, the mid-ply has density below 750 kg/m3, preferably below 700 kg/m3, and more preferably below 675 kg/m3, and the top and bottom plies have a density above 750 kg/m3, preferably above 900 kg/m3. ln some embodiments the paper or paperboard substrate comprises a foam formed paperboard. ln some 7 embodiments at least one ply of a multiply paperboard substrate, preferably a mid- ply, is foam formed.

The paper or paperboard substrate may also be surface sized or impregnated. ln some embodiments, the paper or paperboard substrate is surface sized or impregnated on one or both sides with a surface sizing composition, preferably comprising a starch derivative, a cellulose derivative, or polyvinyl alcohol (PVOH) or a combination of thereof. The starch derivative may for example be a slightly modified, such as oxidised or cationised, starch. The cellulose derivative may for example be a sodium carboxymethyl cellulose with a degree of substitution higher than 0.4 such as in the range of 0.5-1 .5. The PVOH may be fully or partly hydrolysed. The surface sizing or impregnation may facilitate the release of the water vapor barrier layers from the paper or paperboard substrate during repulping. ln some embodiments, the grammage of the surface sizing composition is 0.2-10 g/m2, preferably 0.4-8 g/m2, and more preferably 0.8-5 g/m2 per side, based on dry weight.

The paper or paperboard substrate provided in step a) has a moisture content of at least 3.5 wt%, as determined according to ISO 638. This moisture content is typically suitable for subjecting the paper or paperboard substrate to further conversion to paper or paperboard products, e.g. by folding or forming operations. ln some embodiments, the paper or paperboard substrate has a moisture content in the range 3.5-9 wt%, preferably in the range of 4-8 wt%, as determined according to ISO 638.

The method of the present disclosure is particularly useful for paper or paperboard substrates comprising a hydrophobic internal sizing agent since some hydrophobic internal sizing agents may be volatile and can migrate and hence cause deposits in the vacuum coating machine. ln some embodiments, the paper or paperboard substrate comprises a hydrophobic internal sizing agent, preferably selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), a rosin sizing agent, a fatty acid, a fatty acid derivative, and combinations thereof. ln some embodiments, the paper or paperboard substrate comprises 0.1- 10 kg/tn of the hydrophobic internal sizing agent.

The paper or paperboard substrate itself, before the water vapor barrier layers are applied, will typically have a high permeability for water vapor. ln some embodiments, the paper or paperboard substrate provided in step a) has a water vapor transmission rate (WVTR), measured according to the standard ASTM F1249 at 50% relative humidity and 23 °C, of at least 100 g/m2/24h, preferably at least 150 g/m2/24h, and more preferably at least 200 g/m2/24h.

A coated paper or paperboard product is a material formed mainly from paper or paperboard, which has been coated on one or both main surfaces with at least one coating layers designed to improve the performance and/or appearance of the coated paper or paperboard product.

The coated paper or paperboard product typically has a first outermost surface intended to serve as the outside surface, or print side, and a second outermost surface intended to serve as the inside surface of a liquid or food packaging container. The side of the coated paper or paperboard product comprising the vacuum coating layer is typically intended to face the inside of a liquid or food packaging container.

The inventive coated paper or paperboard product can provide both excellent oxygen barrier properties, water vapor barrier properties, and liquid barrier properties. ln the method of the present disclosure, the paper or paperboard substrate is first provided with a first water vapor barrier layer on the first main surface of the paper or paperboard substrate, and a second water vapor barrier layer on the second main surface of the paper or paperboard substrate, before a vacuum coating layer is formed on said first water vapor barrier layer to obtain the coated paper or paperboard product. ln other words, step d) of the inventive method is performed after both of the steps b) and c) have been performed. 9 Sealing each of the main surfaces of the paper or paperboard substrate with the water vapor barrier layers, helps to retain the moisture content in the paper or paperboard substrate as vacuum coating treatment is performed.

The first water vapor barrier layer is formed on the first main surface of the paper or paperboard substrate. The first water vapor barrier layer comprises a thermoplastic polymer. ln some embodiments, the thermoplastic polymer is a polyolefin or a mixture of polyolefins, more preferably a polyethylene, a polypropylene, or a mixture or copolymer thereof. ln some embodiments, the thermoplastic polymer is a polyethylene. ln preferred embodiments, the thermoplastic polymer is low density polyethylene (LDPE) or linear low density polyethylene (LLDPE). ln some embodiments, the first water vapor barrier layer is formed by dispersion coating with a dispersion comprising a thermoplastic polymer. Dispersion coating is a liquid film coating process wherein a dispersion comprising the thermoplastic polymer is spread out to a thin, uniform layer on the substrate and thereafter dried. The liquid phase of the dispersion is preferably water or an aqueous solution, but organic solvents or mixtures of water or aqueous solutions and organic solvents may also be used. The thermoplastic polymer is present in the dispersion in the form of polymer particles, such as a latex. The dispersion can be applied by contact or non-contact coating methods. Examples of useful coating methods include, but are not limited to rod coating, curtain coating, film press coating, cast coating, transfer coating, size press coating, flexographic coating, gate roll coating, twin roll HSM coating, blade coating, such as short dwell time blade coating, jet applicator coating, spray coating, gravure coating or reverse gravure coating. ln some embodiments, the first water vapor barrier layer is formed by extrusion coating of a thermoplastic polymer, or by dispersion coating with a dispersion comprising a thermoplastic polymer. ln some embodiments, the first water vapor barrier layer is formed by extrusion coating of a thermoplastic polymer, preferably a polyolefin or a mixture of polyolefins, more preferably a polyethylene, a polypropylene, or a mixture or copolymer thereof. ln preferred embodiments, the thermoplastic polymer is low density polyethylene (LDPE) or linear low density polyethylene (LLDPE). ln some embodiments, the thermoplastic polymer has a melting point in the range of 90-120 °C, preferably in the range of 100-110 °C. ln some embodiments, the first water vapor barrier layer has a grammage in the range of 6-25 g/m2, preferably in the range 8-20 g/m2.

Forming the first water vapor barrier layer on the first main surface of the paper or paperboard substrate allows for a relatively low melting point of the thermoplastic polymer and low grammage and of the layer. Specifically, the melting point of the thermoplastic polymer and grammage and of the layer may be lower as compared to the melting point of the thermoplastic polymer and grammage of a plastic film for a lamination type solution, wherein the plastic film is first provided with a vacuum coating layer, and then laminated to a paper or paperboard base layer. The low melting point of the thermoplastic polymer and low grammage and of the first water vapor barrier layer facilitates repulping and recycling of the paper or paperboard product.

To minimize the risk of pinholes, the water vapor barrier layer may preferably be applied in at least two different coating steps with drying or solidification of the coated layer between the coating steps.

The second water vapor barrier layer is formed on the second main surface of the paper or paperboard substrate. The second water vapor barrier layer comprises a thermoplastic polymer. The second water vapor barrier layer may be of the same type as the first water vapor barrier layer, or different. ln some embodiments, the second water vapor barrier layer is formed by extrusion coating of a thermoplastic polymer, or by dispersion coating with a dispersion comprising a thermoplastic polymer. ln some embodiments, the thermoplastic polymer is a polyolefin or a mixture of polyolefins, more preferably a polyethylene, a polypropylene, or a mixture or copolymer thereof. ln preferred embodiments, the 11 thermoplastic polymer is low density polyethylene (LDPE) or linear low density polyethylene (LLDPE). ln some embodiments, the second water vapor barrier layer is formed by dispersion coating with a dispersion comprising a thermoplastic polymer. Dispersion coating is a liquid film coating process wherein a dispersion comprising the thermoplastic polymer is spread out to a thin, uniform layer on the substrate and thereafter dried. The liquid phase of the dispersion is preferably water or an aqueous solution, but organic solvents or mixtures of water or aqueous solutions and organic solvents may also be used. The thermoplastic polymer is present in the dispersion in the form of polymer particles, such as a latex. The dispersion can be applied by contact or non-contact coating methods. Examples of useful coating methods include, but are not limited to rod coating, curtain coating, film press coating, cast coating, transfer coating, size press coating, flexographic coating, gate roll coating, twin roll HSM coating, blade coating, such as short dwell time blade coating, jet applicator coating, spray coating, gravure coating or reverse gravure coating. ln some embodiments, the second water vapor barrier layer is formed by extrusion coating of a thermoplastic polymer, or by dispersion coating with a dispersion comprising a thermoplastic polymer. ln some embodiments, the second water vapor barrier layer is formed by extrusion coating of a thermoplastic polymer, preferably a polyolefin or a mixture of polyolefins, more preferably a polyethylene, a polypropylene, or a mixture or copolymer thereof. ln preferred embodiments, the thermoplastic polymer is low density polyethylene (LDPE) or linear low density polyethylene (LLDPE). ln some embodiments, the thermoplastic polymer has a melting point in the range of 90-120 °C, preferably in the range of 100-110 °C. ln some embodiments, the second water vapor barrier layer has a grammage in the range of 2-35 g/m2, preferably in the range of 2-25 g/m2, and more preferably in the range 4-20 g/m2. To minimize the risk of pinholes, the water vapor barrier 12 layer may preferably be applied in at least two different coating steps with drying or solidification of the coated layer between the coating steps.

The layers applied to the paper or paperboard substrate in steps b) and c) are water vapor barrier layers. The term water vapor barrier layer as used herein refers to a layer which prevents or significantly restricts passage of water vapor across the coated surface. More specifically, the first and second water vapor barrier layers each reduce the water vapor transmission rate (WVTR) of the paper or paperboard substrate coated with the water vapor barrier layer, measured according to the standard ASTM F1249 at 50% relative humidity and 23 °C, to less than 75 g/m2/24h, preferably less than 50 g/m2/24h, and more preferably less than 30 g/m2/24h.

The reduction of the WVTR caused by the first and second water vapor barrier layers significantly reduces the drying required for, or caused by, the subsequent vacuum coating treatment.

The first and second water vapor barrier layers also act as liquid water barrier layers. ln some embodiments, the first and second water vapor barrier layers each have a Cobbeoo value measured according to the standard ISO 535, of less than 50 g/m2.

When the first and the second water vapor barrier layer have been formed on the paper or paperboard substrate, the vacuum coating layer is formed on said first water vapor barrier layer to obtain the coated paper or paperboard product.

Vacuum coating refers to a family of processes used to deposit layers of metals, metal oxides and other inorganic and organic compositions, typically atom-by- atom or molecule-by-molecule, on a solid surface. Multiple layers of the same or different materials can be combined. The process can be further specified based on the vapor source; physical vapor deposition (PVD) uses a liquid or solid source and chemical vapor deposition (CVD) uses a chemical vapor. 13 Vacuum coating typically results in very thin coatings. ln some embodiments, the vacuum coating layer has a thickness in the range of 10-600 nm, preferably in the range of 10-250 nm, and more preferably in the range of 50-250 nm. This may be compared to conventional aluminum foils used in packaging laminates, which foils typically have thickness in the range of about 3-12 um. ln some embodiments, the vacuum coating layer is formed on the first vapor barrier layer by physical vapor deposition (PVD) or chemical vapor deposition (CVD). The terms PVD and CVD also include atomic layer deposition (ALD) and deposition of diamond like carbon (DLC) by PVD or CVD.

The vacuum coating layer may be inorganic or organic. ln some embodiments, the vacuum coating layer is an inorganic vacuum coating layer, such as a metal, metal oxide, or ceramic vacuum coating layer. ln some embodiments, the vacuum coating layer comprises a metal or metal oxide selected from the group consisting of aluminum, magnesium, silicon, copper, aluminum oxides, magnesium oxides, silicon oxides, and combinations thereof, preferably aluminum or an aluminum oxide. vacuum coating layer is an inorganic vacuum coated layer, such as a metal, metal oxide, or ceramic vacuum coating layer.

One preferred type of vacuum coating, often used for its barrier properties, in particular water vapor barrier properties, is an aluminum metal physical vapor deposition (PVD) coating. Such a coating, substantially consisting of aluminum metal, may typically have a thickness of from 50 to 250 nm, although a thickness even lower than 50 nm may also be useful, and even preferred in some embodiments. The thickness of the vacuum coating layer corresponds to less than 1 % of the aluminum metal material typically present in an aluminum foil of conventional thickness for packaging, i.e. 6.3 um. Thus, in some embodiments, the vacuum coating layer comprises aluminum. 14 The thickness of the vacuum coating layer may also be characterized by the optical density of the layer. ln some embodiments the vacuum coating layer has an optical density above 1.8, preferably above 2.0, above 2.5, above 2.7, or above 3.0.

Aluminum oxide vacuum coating layers also known as AlOx coatings can provide similar barrier properties as aluminum metal coatings, but have the added advantage of thin AlOx coatings being transparent to visible light. ln some embodiments, the vacuum coating layer is an organic vacuum coating layer. ln some embodiments, the vacuum coating layer comprises carbon.

The organic vacuum coating layer may for example be a vacuum coated carbon layer, such as a diamond-like carbon (DLC) layer formed from carbon or organic compounds. ln some embodiments, the vacuum coating layer has a thickness in the range of 10-600 nm, preferably in the range of 10-250 nm, and more preferably in the range of 50-250 nm.

A vacuum coating layer is formed at least on the first water vapor barrier layer. ln some embodiments, a vacuum coating layer is formed on both of said first and second water vapor barrier layer. The vacuum coating layer formed on the second water vapor barrier layer may be identical to, or different from the vacuum coating layer formed on the first water vapor barrier layer.

The first and second water vapor barrier layers formed on the main surfaces of the paper or paperboard substrate prior to the vacuum coating treatment, act to retain the moisture content in the paper or paperboard substrate as the vacuum coating treatment is performed. Thus, the moisture content of the coated paper or paperboard substrate is substantially the same, or only slightly lower, preferably less than 3 wt% lower, more preferably less than 2 wt% lower, and most preferably less than 1 wt% lower, than the moisture content of the uncoated paper or paperboard substrate. ln some embodiments, the paper or paperboard substrate in the obtained coated paper or paperboard product has a moisture content in the range 3.5-9 wt%, preferably in the range of 4-8 wt%, as determined according to ISO 638.

The inventive coated paper or paperboard product can provide both excellent oxygen barrier properties, water vapor barrier properties, and liquid barrier properties. ln some embodiments, the obtained coated paper or paperboard product has an oxygen transmission rate (OTR), measured according to the standard ASTM D- 3985 at 50% relative humidity and 23 °C, of less than 10 cc/m2/24h, preferably less than 5 cc/m2/24h, and more preferably less than 2 cc/m2/24h. ln some embodiments, the obtained coated paper or paperboard product has a water vapor transmission rate (WVTR), measured according to the standard ASTM F1249 at 50% relative humidity and 23 °C, of less than 10 g/m2/24h, preferably less than 5 g/m2/24h, and more preferably less than 2 g/m2/24h. ln some embodiments, a protective coating layer is formed on the vacuum coating layer. The protective coating layer protects the thin and sensitive vacuum coating layer form being scratched or damaged, which could othen/vise lead to deterioration of the barrier properties afforded by the vacuum coating layer. The protective coating layer may also provide additional barrier properties, or other properties, to the coated paper or paperboard product. The protective coating layer is preferably also heat-sealable. ln some embodiments, the inventive method further comprises: e) forming a protective coating layer on said vacuum coating layer. ln some embodiments, the protective coating layer is a polyethylene layer. ln some embodiments, the grammage of protective coating layer is in the range of 8-50 g/m2, preferably in the range of 12-50 g/m2. 16 The coated paper or paperboard product obtained by the method according to the first aspect is useful for manufacturing paper or paperboard containers, particularly liquid or food packaging containers. Thus, according to a second aspect illustrated herein, there is provided a method for manufacturing a container, particularly a liquid or food packaging container, said method comprising: a) manufacturing a coated paper or paperboard product according to the method described herein with reference to the first aspect; and b) converting the coated paper or paperboard product into a container.

The coated paper or paperboard product typically has a first outermost surface intended to serve as the outside surface, or print side, and a second outermost surface intended to serve as the inside surface of a liquid or food packaging container. The side of the coated paper or paperboard product comprising the vacuum coating layer is typically intended to face the inside of a liquid or food packaging container. Thus, in some embodiments the vacuum coating layer faces the inside of the container.

The coated paper or paperboard products obtained by the inventive methods have a unique structure wherein a paper or paperboard substrate having a grammage of at least 100 g/m2, a density below 900 kg/m3, and a moisture content of at least 3.5 wt% is sandwiched between a first and a second water vapor barrier layer and a vacuum coating layer formed on at least the said first water vapor barrier layer, wherein a the first water vapor barrier layer is formed of polyethylene having a melting point in the range of 90-120 °C and has a grammage in the range of 8-20 g/m2.

According to a third aspect illustrated herein, there is provided a coated paper or paperboard product comprising: a paper or paperboard substrate having a grammage of at least 100 g/m2, a density below 900 kg/m3, and a moisture content of at least 3.5 wt%, as determined according to ISO 638, said paper or paperboard substrate having a first main surface and a second main surface; 17 a first water vapor barrier layer formed on the first main surface of the paper or paperboard substrate, said first water vapor barrier layer comprising a thermoplastic polymer, a second water vapor barrier layer formed on the second main surface of the paper or paperboard substrate, said second water vapor barrier layer comprising a thermoplastic polymer, and a vacuum coating layer formed on at least the said first water vapor barrier layer, wherein the first water vapor barrier layer is formed of polyethylene having a melting point in the range of 90-120 °C and has a grammage in the range of 8-20 g/m2. ln some embodiments, the thermoplastic polymer is a polyolefin or a mixture of polyolefins, more preferably a polyethylene, a polypropylene, or a mixture or copolymer thereof. ln some embodiments, the thermoplastic polymer is a polyethylene. ln preferred embodiments, the thermoplastic polymer is low density polyethylene (LDPE) or linear low density polyethylene (LLDPE).

The coated paper or paperboard product, the paper or paperboard substrate, the first water vapor barrier layer, the second water vapor barrier layer, and the vacuum coating layer may be further defined as described above with reference to the first aspect.

The coated paper or paperboard product can provide an alternative to conventional materials using aluminum foil layers, which can more readily be repulped and recycled. ln some embodiments, the coated paper or paperboard product has a reject rate according to PTS RH 021/97 of less than 30 %, preferably less than 20 %, more preferably less than 10%.

Generally, while the products, polymers, materials, layers and processes are described in terms of "comprising" various components or steps, the products, 18 polymers, materials, layers and processes can also "consist essentially of' or "consist of' the various components and steps.

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 can be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. ln addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims (31)

1. A method for manufacturing a coated paper or paperboard product, said method comprising: a) providing a paper or paperboard substrate having a grammage of at least 100 g/m2, a density below 900 kg/m3, and a moisture content of at least 3.5 wt%, as determined according to ISO 638, said paper or paperboard substrate having a first main surface and a second main surface, b) forming a first water vapor barrier layer on the first main surface of the paper or paperboard substrate, said first water vapor barrier layer comprising a thermoplastic polymer, c) forming a second water vapor barrier layer on the second main surface of the paper or paperboard substrate, said second water vapor barrier layer comprising a thermoplastic polymer, and d) forming a vacuum coating layer on said first water vapor barrier layer to obtain the coated paper or paperboard product.

2. The method according to claim 1, wherein the paper or paperboard substrate has a grammage in the range of 115-600 g/m2, preferably in the range of 150-450 g/m2, based on dry weight.

3. The method according to any one of the preceding claims, wherein the paper or paperboard substrate has a density in the range of 350-900 kg/m3, preferably in the range of 450-850 kg/m3, based on dry weight.

4. The method according to any one of the preceding claims, wherein the paper or paperboard substrate has a geometric mean bending resistance as measured according to ISO 2493 (L&W, 15°) above 150 mN, preferably above 170 mN, and more preferably above 190 mN.

5. The method according to any one of the preceding claims, wherein the paper or paperboard substrate is a multiply paperboard.

6. The method according to any one of the preceding claims, wherein the paper or paperboard substrate comprises a foam formed paperboard.

7. The method according to any one of the preceding claims, wherein the paper or paperboard substrate has a moisture content in the range 3.5-9 wt%, preferably in the range of 4-8 wt%, as determined according to ISO

8. The method according to any one of the preceding claims, wherein the paper or paperboard substrate comprises a hydrophobic internal sizing agent, preferably selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), a rosin sizing agent, a fatty acid, a fatty acid derivative, and combinations thereof.

9. The method according to any one of the preceding claims, wherein the paper or paperboard substrate has a water vapor transmission rate (WVTR), measured according to the standard ASTM F1249 at 50% relative humidity and 23 °C, of at least 100 g/m2/24h, preferably at least 150 g/m2/24h, and more preferably at least 200 g/m2/24h.

10. The method according to any one of the preceding claims, wherein the first water vapor barrier layer is formed by extrusion coating of a thermoplastic polymer, or by dispersion coating with a dispersion comprising a thermoplastic polymer.

11. The method according to any one of the preceding claims, wherein the first water vapor barrier layer is formed by extrusion coating of a thermoplastic polymer, preferably a polyolefin or a mixture of polyolefins, more preferably a polyethylene, a polypropylene, or a mixture thereof.

12. The method according to any one of the preceding claims, wherein the first water vapor barrier layer has a grammage in the range of 6-25 g/m2, preferably in the range 8-20 g/m

13. The method according to any one of the preceding claims, wherein the second water vapor barrier layer is formed by extrusion coating of a thermoplastic polymer, or by dispersion coating with a dispersion comprising a thermoplastic polymer.

14. The method according to any one of the preceding claims, wherein the second water vapor barrier layer has a grammage in the range of 2-35 g/m2, preferably in the range of 2-25 g/mz, and more preferably in the range 4-20 g/m

15. The method according to any one of the preceding claims, wherein the thermoplastic polymer has a melting point in the range of 90-120 °C, preferably in the range of 100-110 °C.

16. The method according to any one of the preceding claims, wherein the first and second water vapor barrier layers each reduce the water vapor transmission rate (WVTR) of the paper or paperboard substrate coated with the water vapor barrier layer, measured according to the standard ASTM F1249 at 50% relative humidity and 23 °C. to less than 75 g/m2/24h, preferably less than 50 g/m2/24h, and more preferably less than 30 g/m2/24h.

17. The method according to any one of the preceding claims, wherein the first and second water vapor barrier layers each have a Cobbeoo value measured according to the standard ISO 535, of less than 50 g/m

18. The method according to any one of the preceding claims, wherein the vacuum coating layer is formed on the first vapor barrier layer by physical vapor deposition (PVD) or chemical vapor deposition (CVD).

19. The method according to any one of the preceding claims, wherein the vacuum coating layer is an inorganic vacuum coating layer, such as a metal, metal oxide, or ceramic vacuum coating layer.

20. The method according to any one of the preceding claims, wherein the vacuum coating layer comprises a metal or metal oxide selected from the groupconsisting of aluminum, magnesium, silicon, copper, aluminum oxides, magnesium oxides, silicon oxides, and combinations thereof, preferably aluminum or an aluminum oxide.

21. The method according to any one of claims 1-20, wherein the vacuum coating layer is an organic vacuum coating layer.

22. The method according to claim 21, wherein the vacuum coating layer comprises carbon.

23. The method according to any one of the preceding claims, wherein the vacuum coating layer has a thickness in the range of 10-600 nm, preferably in the range of 10-250 nm, and more preferably in the range of 50-250 nm.

24. The method according to any one of the preceding claims, wherein the vacuum coating layer is formed on both of said first and second water vapor barrier layer.

25. The method according to any one of the preceding claims, wherein the paper or paperboard substrate in the obtained coated paper or paperboard product has a moisture content in the range 3.5-9 wt%, preferably in the range of 4-8 wt%, as determined according to ISO

26. The method according to any one of the preceding claims, wherein the obtained coated paper or paperboard product has an oxygen transmission rate (OTR), measured according to the standard ASTM D-3985 at 50% relative humidity and 23 °C, of less than 10 cc/m2/24h, preferably less than 5 cc/m2/24h, and more preferably less than 2 cc/m2/24h.

27. The method according to any one of the preceding claims, wherein the obtained coated paper or paperboard product has a water vapor transmission rate (WVTR), measured according to the standard ASTM F1249 at 50% relative humidity and 23 °C, of less than 10 g/m2/24h, preferably less than 5 g/m2/24h, and more preferably less than 2 g/m2/24h.

28. The method according to any one of the preceding claims, further comprising: e) forming a protective coating layer on said vacuum coating layer.

29. A method for manufacturing a container, particularly a liquid or food packaging container, said method comprising: a) manufacturing a coated paper or paperboard product according to any one of claims 1-28; and b) converting the coated paper or paperboard product into a container.

30. The method according to claim 29, wherein the vacuum coating layer faces the inside of the container.

31. A coated paper or paperboard product comprising: a paper or paperboard substrate having a grammage of at least 100 g/m2, a density below 900 kg/m3, and a moisture content of at least 3.5 wt%, as determined according to ISO 638, said paper or paperboard substrate having a first main surface and a second main surface; a first water vapor barrier layer formed on the first main surface of the paper or paperboard substrate, said first water vapor barrier layer comprising a thermoplastic polymer, a second water vapor barrier layer formed on the second main surface of the paper or paperboard substrate, said second water vapor barrier layer comprising a thermoplastic polymer, and a vacuum coating layer formed on at least the said first water vapor barrier layer, wherein the first water vapor barrier layer is formed of polyethylene having a melting point in the range of 90-120 °C and has a grammage in the range of 8-g/m 24