Barrier paper for use in a paper or paperboard based packaging laminate

Abstract

The present invention relates to a barrier paper for use in a paper or paperboard based packaging laminate, said barrier paper comprising: a paper substrate, wherein said paper substrate is a multiply kraft liner having at least a first outermost ply comprising 80-95 wt% of bleached hardwood kraft pulp and 5-20 wt% of mineral filler, based on the total dry weight of the first ply, and a second ply comprising 80-100 wt% of unbleached pulp selected from the group consisting of softwood kraft pulp, softwood or hardwood chemi-thermomechanical pulp (CTMP), softwood or hardwood high-temperature chemi-thermomechanical pulp (HT-CTMP), and combinations thereof, and less than 5 wt% of mineral filler, based on the total dry weight of the second ply; a precoat layer applied on the paper substrate in contact with the first outermost ply; and an inorganic thin film coating applied on the precoat layer.

Description

[0001] BARRIER PAPER FOR USE IN A PAPER OR PAPERBOARD BASED PACKAGING LAMINATE

[0002] Technical field

[0003] The present disclosure relates to a barrier paper for use in a paper or paperboard based packaging laminate. More specifically, the present disclosure relates to a barrier paper comprising a precoat layer and an inorganic thin film coating applied on the precoat layer.

[0004] 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. In 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.

[0005] 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.

[0006] 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) layer and innermost layers of polyolefin and aluminum. The aluminum foil 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.

[0007] In the prior art, attempts have been made to replace the aluminum foil with more environmentally friendly and / or easier to recycle solutions, but so far with no real success.

[0008] A solution presented in the prior art is to prepare a barrier layer by providing a high-density paper or compact paper substrate with a vacuum deposited inorganic barrier coating layer. The vacuum deposited barrier coating layer may for example comprise or consist of AIOx, AI2O3 or SiOx. The vacuum coated barrier layer is then laminated to a paper or paperboard base layer to provide the base layer with improved barrier properties.

[0009] A problem with vacuum deposition techniques is that the paper substrate to be subjected to the vacuum deposition must have high smoothness and provide good adhesion to the vacuum deposited coating. For these reasons, thin high-density or compact paper substrates are typically used, and the substrates are also typically pre-coated or primer coated prior to vacuum deposition to improve smoothness and / or adhesion.

[0010] However, such vacuum coated high-density or compact papers may be difficult to recycle due to the compact structure of the substrate. In addition, such high- density or compact paper substrates may also cause problems in the vacuum coating and possible pre-coating steps due to their low mechanical strength and low stiffness.

[0011] Thus, there remains a need for improved solutions to replace 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

[0012] It 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.

[0013] It is a further object of the present disclosure, to provide an alternative to vacuum coated high-density paper or compact paper substrates which solves or ameliorates at least some of the problems associated with these materials.

[0014] It is a further object of the present disclosure to provide a barrier layer for a paper or paperboard based packaging laminate, such as a liquid or food packaging board, which barrier layer facilitates re-pulping of the packaging laminate as compared to packaging laminates using conventional combinations of plastic films and aluminum foils or using vacuum coated high-density paper or compact paper substrates.

[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] Inorganic thin film coatings, typically having 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, formed for example by vacuum deposition processes, such as physical vapor deposition (PVD) or chemical vapor deposition (CVD), can when applied to a suitable paper substrate provide good oxygen and water vapor barrier properties, comparable to the barrier properties of thicker aluminum foils. As the thickness of such inorganic thin film 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. Paper substrates for inorganic thin film coatings often suffer from either having too high roughness, or from having too poor mechanical strength and stiffness, or from being difficult to recycle.

[0017] The present invention is based on the realization that these problems can be overcome by providing the paper substrate in the form of a multiply kraft liner having at least a first outermost ply comprising 80-95 wt% of bleached hardwood kraft pulp and 5-20 wt% of mineral filler, based on the total dry weight of the first ply, and a second ply comprising 80-100 wt% of unbleached pulp and less than 5 wt% of mineral filler, based on the total dry weight of the second ply. The multiply structure of the kraft liner is an excellent substrate for inorganic thin film coating, since the outermost ply with bleached hardwood kraft pulp and pigments provides a good smooth surface for pre-coating and inorganic thin film coating, while the second and optional third ply provides the required mechanical properties to ensure processability and stability during pre-coating prior to vacuum coating.

[0018] Thus, the use of the multiply kraft liner makes it possible to provide a barrier paper with good gas, aroma and moisture barrier performance combined with good mechanical strength to ensure processability and convertability. The bulkier second and optional third layer further controls the heat distribution in the metallization (using high temperatures) whereby the shrinkage tendency is decreased. The bulkier second and optional third layer also facilitates re-pulping of the packaging laminate.

[0019] According to a first aspect illustrated herein, there is provided a barrier paper for use in a paper or paperboard based packaging laminate, said barrier paper comprising: a paper substrate, wherein said paper substrate is a multiply kraft liner having at least a first outermost ply comprising 80-95 wt% of bleached hardwood kraft pulp and 5-20 wt% of mineral filler, based on the total dry weight of the first ply, and a second ply comprising 80-100 wt% of unbleached pulp selected from the group consisting of softwood kraft pulp, softwood or hardwood chemi-thermomechanical pulp (CTMP), softwood or hardwood high-temperature chemi-thermomechanical pulp (HT-CTMP), and combinations thereof, and less than 5 wt% of mineral filler, based on the total dry weight of the second ply; a precoat layer applied on the paper substrate in contact with the first outermost ply; and an inorganic thin film coating applied on the precoat layer.

[0020] The paper substrate is a multiply kraft liner having at least a first outermost ply and a second ply, wherein the first outermost ply and the second ply are formed from different pulp types. The first outermost ply comprises 80-95 wt% of bleached kraft pulp and 5-20 wt% of mineral filler, based on the total dry weight of the first ply, and the second ply comprises 80-100 wt% of unbleached pulp selected from the group consisting of softwood kraft pulp, softwood or hardwood chemi- thermomechanical pulp (CTMP), softwood or hardwood high-temperature chemi- thermomechanical pulp (HT-CTMP), and combinations thereof, and less than 5 wt% of mineral filler, based on the total dry weight of the second ply.

[0021] The first outermost ply comprising bleached kraft pulp ply with a relatively high amount of mineral filler offers a smooth surface excellent for applying barrier coatings. The second ply, and optional additional plies, provide the required mechanical properties to ensure processability of the barrier paper. The second ply, and optional additional plies, also importantly help to control the heat distribution during application of the inorganic thin film coating, which often involves subjecting the paper substrate to high temperatures. This control of the heat distribution decreases the shrinkage tendency of the paper substrate. Thus, the use of the multiply kraft liner makes it possible to provide a barrier paper with good gas, aroma and moisture barrier performance combined with good mechanical strength to ensure processability and convertability.

[0022] The multiply kraft liner can be manufactured in a paper or paperboard machine adapted for manufacturing of multiply kraft liner. Paper or paperboard machines for making multiply kraft liner are well known in the art. Typically, the machine layout comprises a stock handling section, a wet end section, a pressing and drying section and optionally a calendaring and / or coating section. In the wet end section, the plies may be formed individually, using different headboxes and laminated in a wet state, or formed together in a multiply headbox. If formed individually, the plies are typically laminated before the press and drying section of the paper machine.

[0023] In some embodiments, an interfacial layer comprising an additive, for example a strength enhancement agent or fines or reject obtained in a size fractionation or cleaning process, is formed at the interface between at least two adjacent plies of the multiply kraft liner. In some embodiments, the interfacial layer is applied using a separate water layer between the first and second ply using a multiply headbox. Applying additives in a water layer using a multiply headbox is well known to the skilled person.

[0024] In some embodiments, the interfacial layer comprises a strength enhancement agent. The strength enhancement agent may for example comprise a polysaccharide or a combination of polysaccharides, such as starch, cellulose or derivatives thereof, hemicellulose, or natural gums, such as guar gum or alginate. The polysaccharide or combination of polysaccharides may also comprise highly refined cellulose, microfibrillated cellulose (MFC), or nanocrystalline cellulose (NCC). The strength enhancement agent is preferably selected form the group consisting of a starch based strength enhancement agent, a cellulose based strength enhancement agent, and mixtures thereof. In some embodiments, the strength enhancement agent is a cellulose based strength enhancement agent. The cellulose based strength enhancement agent preferably comprises, or consists of, a fine cellulosic material such as highly refined cellulose or microfibrillated cellulose.

[0025] In some embodiments, the paper substrate has a grammage in the range of 60- 120 g / m2, preferably in the range of 70-110 g / m2, and more preferably in the range of 80-100 g / m2. A grammage in these ranges is important as it is thin enough to allow for the barrier paper to be conveniently used in a paper or paperboard based packaging laminate, yet thick enough to not be easily overdried. The grammage of the paper substrate is higher than the grammage of high-density papers usually employed in the prior art vacuum coated barrier layers resulting in considerable bending stiffness improvement.

[0026] In some embodiments, the bleached hardwood kraft pulp in the first outermost ply has a mean fiber length in the range of 0.6-1.2 mm, preferably in the range of 0.6- 1 mm, as determined according to ISO 16065-2.

[0027] In some embodiments, the bleached hardwood kraft pulp in the first outermost ply has a Schopper Riegler (SR) number in the range of 25-50, as determined according to ISO 5267-1.

[0028] In some embodiments, the bleached hardwood kraft pulp in the first outermost ply is a bleached eucalyptus kraft pulp. Using pulp formed from hardwood fibers, such as eucalyptus fibers, in the first outermost ply improves the formation and further contributes to the smooth surface suitable for subsequent application of the precoat layer and the inorganic thin film coating. In some embodiments, the KAPPA number of the bleached eucalyptus kraft pulp is below 60, preferably below 50 and more preferably below 40, as determined according to ISO 302:2015. In some embodiments, the ISO brightness of the bleached eucalyptus kraft pulp is above 80, preferably above 85, and more preferably above 87 as determined according to ISO 2470. Such bleached eucalyptus kraft pulp exhibits less problems with taste and odor, and improved formation. It is believed that lower KAPPA and high ISO brightness in the above specified ranges gives not only a better visual appearance but also better formation and a synergistic effect on the barrier properties of the finished product together with the precoat layer and the inorganic thin film coating. Normally, the KAPPA and ISO brightness of this ply would not be considered relevant since this the surface of the ply will be coated and hence not visible.

[0029] In some embodiments, the mineral filler is selected from the group consisting of clay (for example calcined clay or high platy clay), talc, CaCOs (for example ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), or in-situ produced calcium carbonate), TiO2, AI2O3, aluminium trihydrate, SiO2, kaolin, bentonite, and phyllosilicates, or a combination thereof. In preferred embodiments, the mineral filler comprises calcium carbonate (CaCOs), for example ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), or in-situ produced calcium carbonate. The mineral filler preferably comprises high aspect ratio filler particles, e.g. flaky particles. The average particle size of the mineral filler may be in the range of 0.5-6 pm, such as in the range of 0.6-5 pm. The amount of particles having a particle size below 2 pm may for example be determined by sedigraph measurements. In some embodiments, the amount of particles having a particle size below 2 pm is at least 30 wt%, and preferably in the range of 40-99 wt% based on the total weight of the mineral filler.

[0030] In some embodiments, the second ply comprises less than 3 wt% of mineral filler, based on the total dry weight of the second ply.

[0031] In some embodiments, the unbleached pulp in the second ply is an unbleached softwood kraft pulp. Using pulp formed from softwood fibers in the second ply further contributes to the strength properties which improves the processability of the barrier paper.

[0032] In some embodiments, the unbleached pulp in the second ply is an unbleached CTMP or HT-CTMP.

[0033] In some embodiments, the multiply kraft liner has a third ply comprising 80-100 wt% of kraft pulp, based on the total dry weight of the third ply.

[0034] In some embodiments, the third ply is an outermost ply. In this embodiment, the barrier paper comprises a first outermost ply, formed by the first ply and provided with a pre-coat layer and an inorganic thin film coating, a middle ply formed by the second ply and a second outermost ply formed by the third ply.

[0035] In some embodiments, the kraft pulp of the third ply is an unbleached kraft pulp. Using pulp formed from softwood fibers in the third ply further contributes to the strength properties which improves the processability of the barrier paper. In some embodiments, the kraft pulp of the third ply is a hardwood and / or a softwood kraft pulp. In preferred embodiments, the kraft pulp of the third ply is a softwood kraft pulp.

[0036] In some embodiments, the third ply comprises less than 5 wt%, preferably less than 3 wt%, of mineral filler, based on the total dry weight of the third ply.

[0037] In some embodiments, the paper substrate has a density below 900 kg / m3, preferably below 800 kg / m3, and more preferably below 700 kg / m3. In some embodiments, the density of the paper substrate is above 450 kg / m3, preferably above 500 kg / m3. The density of the paper substrate may preferably be in the range of 450 - 900 kg / m3, preferably in the range of 500 - 800 kg / m3or 500 - 900 kg / m3.

[0038] The paper substrate will typically have a bulk above 1.20 g / cm3, preferably above 1.25 g / cm3, and more preferably above 1 .30 g / cm3or above 1 .35 g / cm3or above 1.40 cm3 / g, such as in the range of such as 1.20-1.80 g / cm3or 1.30-1.60 g / cm3. A high bulk, such as above 1.20 g / cm3, provides rigidity and runnability for inorganic thin film coating and for further converting in packaging lines.

[0039] The paper substrate will typically have will typically have a specific formation of less than 1.5 g0 5 / m, preferably less than 1.3 g0 5 / m, and more preferably less than 1 g°5 / m as determined according to SCAN-P 92:09.

[0040] In some embodiments, the paper substrate has one or more of: a bursting strength above 400 kPa as determined according to ISO 2758, a tear strength above 600 mN in both the machine direction (MD) and the cross direction (CD), as determined according to ISO 1974, and a tensile stiffness above 900 kN / m in the machine direction (MD) as determined according to ISO 1924-3. These properties are advantageous in the coating and converting processing of the barrier paper.

[0041] In some embodiments, the paper substrate has a wet tensile strength in the machine direction above 0.2 kN / m, preferably above 0.3 kN / m, and more preferably in the range of 0.3 - 1 kN / m, as determined according to ISO 3781 :2011. The wet strength is important for the application of the precoat layer and the inorganic thin film coating. However, the paper substrate is still recyclable due to the high bulk.

[0042] The paper substrate itself, without the precoat layer and the inorganic thin film coating, will typically have a high permeability for gases, such as oxygen, air and carbon dioxide. In some embodiments, the paper substrate has a Gurley Hill value below 5000 s / 100ml, preferably below 2000 s / 100ml, and more preferably below 1000 s / 100ml, as measured according to standard ISO 5636-5.

[0043] The paper substrate itself, without the precoat layer and the inorganic thin film coating, will typically have a high permeability for water vapor. In some embodiments, the paper substrate has a water vapor transmission rate (WVTR), measured according to the standard ASTM F1249-20 at 50% relative humidity and 23 °C, of above 200 g / m2 / 24h, and more typically above 1000 g / m2 / 24h.

[0044] The paper substrate itself, without the precoat layer and the inorganic thin film coating, will typically have low or no resistance to oil and grease penetration.

[0045] The paper substrate may also be surface sized. In some embodiments, the paper substrate is surface sized on one or both sides with a surface sizing composition, preferably comprising starch or a starch derivative, or a combination of starch or a starch derivative and microfibrillated cellulose.

[0046] On the optionally surface sized paper substrate a precoat layer is applied. The precoat layer renders the surface of the paper substrate smoother and less porous before the inorganic thin film coating is applied. The precoat layer may also improve the adhesion of the inorganic thin film coating. Preferably, the precoat layer may also improve the gas, water vapor, and / or liquid barrier properties of the coated substrate.

[0047] The precoat layer will also provide a barrier against migration of low molecular weight substances from the paper substrate. This may be especially useful in the method according to the present disclosure, since some components in the paper may be volatile or prone to migration and hence cause deposits in the vacuum coating machine. Without being bound to any scientific theory, it is believed a polymeric precoat layer can provide not only good adhesion to the inorganic thin film coating, but also good barrier for migration of volatile components.

[0048] The precoat layer may be applied by any suitable method known in the art. The precoat layer may for example be applied as a solution or dispersion in an aqueous or organic solvent carrier using liquid coating methods known in the art, in melt form using extrusion coating, or in the form of a solid film by lamination.

[0049] The precoat layer is preferably formed by means of a liquid film coating process, i.e. in the form of a solution or dispersion which, on application, is spread out to a thin, uniform layer on the substrate and thereafter dried. The liquid phase of the solution or 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 one or more polymers may be present in the solution or dispersion in dissolved form or in the form of polymer particles, such as a latex. The precoat layer 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, impregnation, 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.

[0050] To minimize the risk of pinholes in the precoat layer, the precoat layer may preferably be applied in at least two different coating steps with drying of the coated film between the steps.

[0051] In some embodiments, at least one precoat layer is applied in the form of a foam. Foam coating is advantageous as it allows for film forming at higher solids content and lower water content compared to a non-foamed coating. The lower water content of a foam coating also reduces the problems with rewetting of the paper substrate. The foam may be formed using a polymeric or non-polymeric foaming agent. Examples of polymeric foaming agents include PVOH, hydrophobically modified starch, and hydrophobically modified ethyl hydroxyethyl cellulose.

[0052] Typically, the precoat layer will comprise one or more polymers. The precoat layer may be comprised entirely of the one or more polymers, or it may also further comprise other additives for facilitating the coating process or improving the properties of the precoat layer.

[0053] In some embodiments, the precoat layer comprises at least 50 wt% of a polymer or mixture of polymers based on dry weight.

[0054] In some embodiments, the precoat layer comprises a latex coating, preferably an SA latex coating or an SB latex coating.

[0055] In some embodiments, the precoat layer comprises a polymer selected from the group consisting of a polyvinyl alcohol (PVOH), an ethylene vinyl alcohol (EVOH), a polyurethane, a polysaccharide, and derivatives thereof and combinations thereof, preferably PVOH.

[0056] The polysaccharide may be a natural polysaccharide or a chemically modified polysaccharide, for example a chemically modified cellulose, such as a carboxymethyl cellulose (CMC).

[0057] In some embodiments, the precoat layer comprises at least 50 wt% of a water- soluble polymer or mixture of water-soluble polymers based on dry weight. The water-soluble polymer of the precoat layer is soluble in cold water or soluble in hot water, e.g. at a temperature below 100 °C or even above 100 °C, for a given period of time. The water-soluble polymer in addition to acting as an adhesive for the inorganic thin film coating, also facilitates separation of the inorganic thin film coating and optional additional plastic layers applied on top of the precoat layer or inorganic thin film coating during repulping. In some embodiments, the water- soluble polymer is selected from the group consisting of a polyvinyl alcohol (PVOH), an ethylene vinyl alcohol (EVOH), a chemically modified cellulose, a starch, an alginate, and a hemicellulose. In some embodiments, the water-soluble polymer is selected from the group consisting of a polyvinyl alcohol (PVOH), an ethylene vinyl alcohol (EVOH), a carboxymethyl cellulose (CMC), a starch, an alginate, and a hemicellulose, preferably a PVOH.

[0058] In some embodiments, the precoat layer comprises at least 50 wt% of a PVOH, preferably at least 70 wt% of a PVOH, based on the total dry weight of the precoat layer.

[0059] In some embodiments, the PVOH has a degree of hydrolysis in the range of 80-99 mol%, preferably in the range of 85-99 mol%.

[0060] In some embodiments, the PVOH has a pH value in the range of 3-11 , preferably in the range of 4-10, and more preferably in the range of 5-9. PVOH with pH values outside these ranges may dissolve mineral filler, such as CaCOs, present in the first outermost ply and cause problems with an uneven PVOH layer or poor adhesion.

[0061] In some embodiments, the PVOH has an ash content of less than 4 wt%, preferably less than 3 wt%, and more preferably less than 2.5 wt%. In some embodiments, the PVOH is a washed PVOH.

[0062] In some embodiments, the precoat layer further comprises a crosslinking agent capable of crosslinking the water-soluble polymer. The crosslinking agent may advantageously be applied together with the water-soluble polymer, and then activated, e.g. by heat or radiation, when the precoat layer is in contact with the inorganic thin film coating. Crosslinking improves the water vapor barrier properties of the precoat layer. Suitable crosslinking agents include, but are not limited to polyfunctional organic acids or aldehydes, such as citric acid, glyoxal, and glutaraldehyde. In some embodiments, the crosslinking agent is an organic acid, and more preferably citric acid. The concentration of the crosslinking agent may for example be 1-20 wt%, preferably 1-15 wt%, based on the dry weight of the precoat layer. In some embodiments, the precoat layer comprises PVOH and citric acid. Crosslinking of the PVOH with citric acid improves the water vapor barrier properties of the precoat layer. Additionally, the crosslinking of the PVOH with citric acid in contact with the inorganic thin film coating has been found to further improve adhesion of the inorganic thin film coating and the overall water vapor barrier properties of the vacuum coated paper.

[0063] In some embodiments, the precoat layer further comprises up to 50 wt% of a nanocellulose, such as nanocrystalline cellulose, based on the dry weight of the precoat layer.

[0064] In some embodiments, the grammage of the precoat layer is in the range of 1-20 g / m2, preferably in the range of 2-15 g / m2, more preferably in the range of 3-12 g / m2, based on dry weight. Without being bound to any theory, it is believed that a grammage according to these ranges may provide not only good adhesion to metal layer, but also good barrier for migration of volatile components from the paper substrate during the vacuum coating process.

[0065] In some embodiments the barrier paper may further comprise a coating layer similar or identical to the precoat layer but applied of the opposite side of the paper substrate. Such a coating layer may provide enhanced dimensional stability to the barrier paper and prevent curl.

[0066] The bursting strength of the paper substrate is preferably improved, or at least retained with the precoat layer. Thus, in some embodiments the paper substrate with the precoat layer has a bursting strength above 400 kPa as determined according to ISO 2758.

[0067] On the precoat layer an inorganic thin film coating is applied to obtain the barrier paper.

[0068] In some embodiments, the inorganic thin film coating layer is a vacuum coated inorganic thin film coating layer. 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.

[0069] Vacuum coating typically results in very thin coatings. In some embodiments, the inorganic thin film coating 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 pm.

[0070] In some embodiments, the inorganic thin film coating layer is applied to the precoat layer by physical vapor deposition (PVD) or chemical vapor deposition (CVD).

[0071] In some embodiments, the inorganic thin film coating layer comprises a metal, a metal oxide, or a ceramic.

[0072] In some embodiments, the inorganic thin film 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 an aluminum oxide.

[0073] 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 inorganic thin film coating 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 pm. Thus, in some embodiments, the inorganic thin film coating comprises aluminum. The thickness of the inorganic thin film coating may also be characterized by the optical density of the layer. In some embodiments the inorganic thin film coating has an optical density above 1.8, preferably above 2.0, above 2.5, above 2.7, or above 3.0.

[0074] Aluminum oxide inorganic thin film coatings also known as AIOx coatings can provide similar barrier properties as aluminum metal coatings, but have the added advantage of thin AIOx coatings being transparent to visible light.

[0075] In some embodiments, the inorganic thin film 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.

[0076] The coating of the paper substrate with the precoat layer and inorganic thin film coating significantly improves the oxygen and water vapor barrier properties of the coated paper as compared to the uncoated paper substrate.

[0077] In some embodiments, the barrier paper has an oxygen transmission rate (OTR), measured according to the standard ASTM F1927-20 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 1 cc / m2 / 24h.

[0078] In some embodiments, the barrier paper has a water vapor transmission rate (WVTR), measured according to the standard ASTM F1249-20 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 1 g / m2 / 24h.

[0079] In addition to providing good oxygen and water vapor barrier properties, the inventive vacuum coated paper may also form a good barrier for other gases, as well as aromas and odors.

[0080] In some embodiments, the barrier paper has a mean light transmission at 400 -

[0081] 800 nm of below 5 %, preferably below 3 %, and more preferably below 2 %. The combination of unbleached fibers in the second ply, and optional third ply, a white first outermost ply comprising bleached fibers and filler, and an inorganic thin film coating provides excellent opacity and UV and light barrier properties to the barrier paper.

[0082] In addition to improving the processability and providing a surface suitable for application of the inorganic thin film coating, the use of a multiply kraft liner as the paper substrate also improves the recyclability as compared to similar structures using high-density paper as the paper substrate. The high-density papers often prepared from highly refined pulp, are typically difficult to repulp (i.e. to disintegrate and resuspend in water). The pulp of the multiply kraft liner on the other hand, and particularly the unbleached pulp of the second and optional additional plies, selected from the group consisting of softwood kraft pulp, softwood or hardwood chemi-thermomechanical pulp (CTMP), softwood or hardwood high-temperature chemi-thermomechanical pulp (HT-CTMP), and combinations thereof, exhibits much faster disintegration and lower drainage resistance, and is much easier to repulp and recycle as compared to the high-density paper products. The disintegration, cleaning and washing, fractionation, and removal of impurities becomes easier, which leads to low recycling reject rates. Also, the amount of chemicals required for the recycling can be reduced.

[0083] In some embodiments, the barrier paper has a total reject rate according to PTS- RH 021 / 97 test method for Category II products of less than 20 %, preferably less than 10 %, more preferably less than 5 %.

[0084] In some embodiments, the fiber fraction obtained from the PTS-RH 021 / 97 test method for Category II products has a Schopper Riegler (SR) number of less than 30, preferably less than 25, and more preferably less than 20, as determined according to ISO 5267-1.

[0085] The filler content of the barrier paper, the paper substrate, or of plies of the paper substrate, can be determined by determining the ash content in accordance with ISO 1762:2019 (at 525 °C). According to a second aspect illustrated herein, there is provided a paper or paperboard based packaging laminate comprising: a paper or paperboard base layer, and a barrier paper layer comprising a barrier paper according to first aspect. wherein the barrier paper layer is laminated to the paper or paperboard base layer, by an adhesive tie layer.

[0086] In some embodiments of the paper or paperboard based packaging laminate, the barrier paper layer is laminated to the paper or paperboard base layer with the inorganic thin film coating facing the paper or paperboard base layer. In alternative embodiments, the barrier paper layer is laminated to the paper or paperboard base layer with the inorganic thin film coating oriented to face outward, away from the paper or paperboard base layer.

[0087] In some embodiments of the paper or paperboard based packaging laminate, the adhesive tie layer comprises a water based adhesive, e.g. a water based adhesive comprising ethylene vinyl acetate co-polymer, polyvinyl alcohol (PVOH) and / or acrylics.

[0088] In some embodiments of the paper or paperboard based packaging laminate, the adhesive tie layer comprises a polyolefin based adhesive. The polyethylene is preferably polyethylene or a polyethylene derivative. In some embodiments of the paper or paperboard based packaging laminate, the adhesive tie layer comprises an ethylene vinyl alcohol (EVOH) based adhesive. Since the multiply kraft liner is easier to repulp during recycling than for example high-density papers used in the prior art, it is possible to use a polyolefin or EVOH based adhesive in the laminate without causing problems in the recycling thereof. The adhesive tie layer may be applied by dispersion coating or by extrusion coating. Paper generally refers to a material manufactured in thin sheets from the pulp of wood or other fibrous substances comprising cellulose fibers, used for writing, drawing, or printing on, or as packaging material.

[0089] Paperboard generally refers to strong, thick paper or cardboard comprising cellulose fibers used for boxes and other types of packaging. Paperboard can either be bleached or unbleached, coated or uncoated, and produced in a variety of thicknesses, depending on the end use requirements. Paperboard may be a single ply material, or a multiply material comprised of two or more plies. A common type of multiply paperboard is comprised of a lower density mid-ply (also sometimes referred to as “bulk ply”) sandwiched between two higher density outer plies. The lower density mid-ply may typically have a density below 750 kg / m3, preferably below 700, below 650, below 600, below 550, below 500, below 450, below 400 or below 350 kg / m3. The higher density outer plies typically have a density at least 100 kg / m3higher than the mid-ply, preferably at least 200 kg / m3higher than the mid-ply.

[0090] A paper or paperboard based packaging laminate is a packaging material formed mainly from paperboard. The paper or paperboard base layer can be made from pulp, including pulp from virgin fiber, e.g. mechanical, chemical and / or thermomechanical pulps. It can also be made from broke or recycled paper. In addition to the paper or paperboard, the paper or paperboard based packaging laminate may comprise additional layers or coatings designed to improve the performance and / or appearance of the packaging laminate.

[0091] The paper or paperboard based packaging laminate 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 packaging container. The side of the paper or paperboard base layer comprising the inventive barrier paper is preferably intended to serve as the inside surface of a packaging container.

[0092] In some embodiments, the paper or paperboard base layer has a grammage of at least 100 g / m2. In some embodiments, the paper or paperboard base layer has a grammage of at least 150 g / m2, 200 g / m2, 250 g / m2, 300 g / m2, 350 g / m2, or 400 g / m2. The grammage of the paper or paperboard base layer is preferably below 1000 g / m2, 800 g / m2, or 600 g / m2. Unless otherwise stated, the grammage is determined according to the standard ISO 536.

[0093] In some embodiments, the paper or paperboard base layer has a density below 700 kg / m3, preferably below 600 kg / m3. Unless otherwise stated, the density is determined according to the standard ISO 534.

[0094] The paper or paperboard base layer may be a single ply paperboard or a multiply paperboard. In some embodiments, the paper or paperboard base layer is a multiply paperboard. In some embodiments the paper or paperboard base layer is a multiply paperboard comprised of two or more plies. In some embodiments the paper or paperboard base layer is a multiply paperboard comprised of three or more plies. In some embodiments the paper or paperboard base layer is a multiply paperboard comprised of a lower density mid-ply sandwiched between two higher density outer plies.

[0095] The paper or paperboard based packaging laminate can provide an alternative to conventional materials using aluminum foil layers, which can more readily be repulped and recycled. In some embodiments, the paper or paperboard based packaging laminate has a reject rate according to PTS RH 021 / 97 of less than 30 %, preferably less than 20 %, more preferably less than 10%.

[0096] In some embodiments, the paper or paperboard based packaging laminate further comprises an outermost polymer coating layer on one or both sides of the paper or paperboard based packaging laminate. The outermost polymer layer(s) preferably provide liquid barrier properties and mechanical protection for the paper or paperboard based packaging laminate surface. The outermost polymer layer is preferably also heat-sealable.

[0097] The outermost polymer layers may comprise any of the thermoplastic polymers commonly used in protective and / or heat-sealable layers in paper or paperboard based packaging laminates in general or polymers used in liquid or food packaging board in particular. Examples include polyethylene (PE), polyethylene terephthalate (PET), polyethylene furanoate (PEF), polypropylene (PP), polyhydroxyalkanoates (PHA), polylactic acid (PLA), polybutylene succinate (PBS), polyglycolic acid (PGA), starch and cellulose. Polyethylenes, especially low density polyethylene (LDPE) and high density polyethylene (HDPE), are the most common and versatile polymers used in liquid or food packaging board. The polymer layer may also be latex based, such as styrene acrylic (SA) latex and / or styrene butadiene (SB) latex. The polymers used are preferably manufactured from renewable materials.

[0098] In some embodiments, the paper or paperboard based packaging laminate comprises a first outermost polymer layer, preferably a polyethylene layer, arranged on the paper or paperboard substrate.

[0099] In some embodiments, the paper or paperboard based packaging laminate further comprises a second outermost polymer layer, preferably a polyethylene layer, arranged on the vacuum coating layer.

[0100] The outermost polymer layers may of course interfere with repulpability but may still be required or desired in some applications. The additional polymer layers may for example be applied by extrusion coating, film lamination or dispersion coating.

[0101] Thermoplastic polymers, are useful since they can be conveniently processed by extrusion coating techniques to form very thin and homogenous films with good liquid barrier properties. In some embodiments, the additional polymer layer comprises polypropylene or polyethylene. In preferred embodiments, the outermost polymer layers comprise polyethylene, more preferably LDPE or HDPE.

[0102] In some embodiments, the outermost polymer layers are formed by extrusion coating of the polymer onto a surface of the paper or paperboard substrate or laminate. Extrusion coating is a process by which a molten plastic material is applied to a substrate to form a very thin, smooth and uniform layer. The coating can be formed by the extruded plastic itself, or the molten plastic can be used as an adhesive to laminate a solid plastic film onto the substrate. Common plastic resins used in extrusion coating include polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET).

[0103] The basis weight of each of the outermost polymer layers is preferably less than 50 g / m2. In order to achieve a continuous and substantially defect free film, a basis weight of the outermost polymer layer of at least 8 g / m2, preferably at least 12 g / m2is typically required. In some embodiments, the basis weight of the outermost polymer layer is in the range of 8-50 g / m2, preferably in the range of 12-50 g / m2.

[0104] Generally, while the products, polymers, materials, layers and processes are described in terms of “comprising” various components or steps, the products, polymers, materials, layers and processes can also “consist essentially of” or “consist of” the various components and steps.

[0105] 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. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

CLAIMS1 . A barrier paper for use in a paper or paperboard based packaging laminate, said barrier paper comprising: a paper substrate, wherein said paper substrate is a multiply kraft liner having at least a first outermost ply comprising 80-95 wt% of bleached hardwood kraft pulp and 5-20 wt% of mineral filler, based on the total dry weight of the first ply, and a second ply comprising 80-100 wt% of unbleached pulp selected from the group consisting of softwood kraft pulp, softwood or hardwood chemi-thermomechanical pulp (CTMP), softwood or hardwood high-temperature chemi-thermomechanical pulp (HT-CTMP), and combinations thereof, and less than 5 wt% of mineral filler, based on the total dry weight of the second ply; a precoat layer applied on the paper substrate in contact with the first outermost ply; and an inorganic thin film coating applied on the precoat layer.

2. The barrier paper according to claim 1 , wherein the paper substrate has a grammage in the range of 60-120 g / m2, preferably in the range of 70-110 g / m2, and more preferably in the range of 80-100 g / m2.

3. The barrier paper according to any one of the preceding claims, wherein the second ply comprises less than 3 wt% of mineral filler, based on the total dry weight of the second ply.

4. The barrier paper according to any one of the preceding claims, wherein the unbleached pulp in the second ply is an unbleached softwood kraft pulp.

5. The barrier paper according to any one of the claims 1 - 3, wherein the unbleached pulp in the second ply is an unbleached CTMP or HT-CTMP.

6. The barrier paper according to any one of the preceding claims, wherein the multiply kraft liner has a third ply comprising 80-100 wt% of kraft pulp, based on the total dry weight of the third ply.

7. The barrier paper according to claim 6, wherein the kraft pulp of the third ply is an unbleached kraft pulp.

8. The barrier paper according to any one of the claims 6-7, wherein the third ply comprises less than 5 wt%, preferably less than 3 wt%, of mineral filler, based on the total dry weight of the third ply.

9. The barrier paper according to any one of the preceding claims, wherein the precoat layer comprises a latex coating, preferably an SA latex coating or an SB latex coating.

10. The barrier paper according to any one of the claims 1 - 8, wherein the precoat layer comprises a polymer selected from the group consisting of a polyvinyl alcohol (PVOH), an ethylene vinyl alcohol (EVOH), a polyurethane, a polysaccharide, and derivatives thereof and combinations thereof, preferably PVOH.

11. The barrier paper according to any one of the preceding claims, wherein the inorganic thin film coating layer is a vacuum coated inorganic thin film coating layer.

12. The barrier paper according to any one of the preceding claims, wherein the inorganic thin film coating layer comprises a metal, a metal oxide, or a ceramic.

13. The barrier paper according to any one of the preceding claims, wherein the inorganic thin film 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.

14. A paper or paperboard based packaging laminate comprising:a paper or paperboard base layer, and a barrier paper layer comprising a barrier paper according to any one of claims 1- 30, wherein the barrier paper layer is laminated to the paper or paperboard base layer by an adhesive tie layer.

15. The paper or paperboard based packaging laminate according to claim 14, wherein the barrier paper layer is laminated to the paper or paperboard base layer with the inorganic thin film coating facing the paper or paperboard base layer.

16. The paper or paperboard based packaging laminate according to any one of claims 14-15, wherein the adhesive tie layer comprises a water based adhesive.

17. The paper or paperboard based packaging laminate according to any one of claims 14-15, wherein the adhesive tie layer comprises a polyolefin based adhesive.

18. The paper or paperboard based packaging laminate according to any one of claims 14-18, further comprising an outermost polymer coating layer on one or both sides of the paper or paperboard based packaging laminate.

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