BARRIER PAPERS

Abstract

 A barrier paper, preferably for use in tropical conditions, comprising, among other layers, a primer layer (3) comprising a polymer or copolymer on a first base paper overcoat layer (2), a second base paper overcoat layer (4) comprising a second polymer on the primer layer, wherein the second polymer, characterized in that the primer layer (3) comprises, or consists of, a polymer or copolymer exhibiting a water absorption of less than 5%, and wherein the second base paper overcoat layer (4) comprises, or consists of, a second polymer exhibiting a water absorption that is higher than the water absorption exhibited by the polymer or copolymer of the primer layer (3).

Description

TECHNICAL FIELD

[0001] The present invention relates to barrier papers that are able to provide an excellent barrier against water vapor under tropical conditions and/or oxygen under ambient conditions.

PRIOR ART

[0002] In order to protect products against the effect of substances that can damage and/or spoil such products, products can be packaged in a packaging.

[0003] Thus, the packaging needs to be appropriately selected, depending on the packaged product, as well as depending on the substances that are to be kept from damaging the product.

[0004] When packaging goods that can spoil, such as foodstuffs, gases such as oxygen and water vapor should be kept from the foodstuffs, since on one hand, oxygen can lead to oxidation of fats (rancidity) and on the other hand, humidity can lead to the growth of microorganisms. The problem is exacerbated when the ambient temperature is high, since higher temperature accelerates not only oxidation and microorganism growth, but also their ingression into the package, which is why mostly the recommendation in packaged foodstuffs is to store them in a cool and dry place.

[0005] However, this is not always possible when foodstuffs are transported, exhibited or stored in tropical regions, i.e. in regions where high temperatures and humidity prevail in most seasons.

[0006] Historically, the packaging solutions that were available were in the form of polymer films that can be mass-produced, are robust as well as cost effective, and that can be heat-sealed using the proper equipment. For extra barrier against oxygen and moisture, such polymer films can be metallized with aluminum, other metals or metal oxides.

[0007] However, the polymer films have the problem of being sourced from non-renewable fossil raw materials, and of being virtually non-biodegradable, both of which are undesirable as a global effort to reduce the reliance on non-sustainable raw materials unfolds. Furthermore, complex constructs of polymer films are also not recyclable in existing waste processing infrastructure.

[0008] Paper packaging has been a traditional packaging solution for many decades, and lately, papers exhibiting good barrier properties against vapor and oxygen were developed as a replacement for metallized polymer films.

[0009] WO2022107007A1 provides a barrier film for a paper or paperboard-based packaging material, where the barrier film comprises a vacuum coating layer such as a metal layer and a protective coating layer comprising a cross-linked water-soluble polymer disposed on the vacuum coating layer. The vacuum layer, which in most cases is an aluminum layer, improves the barrier against oxygen and/or humidity. However, the oxygen and humidity barrier of the barrier papers that are reported are not on par at tropical conditions, unless the protective layer is formed.

[0010] WO2021260043A1 discloses a process for improving the barrier properties of a barrier layer comprised in a pulp-based substrate layer having an inorganic barrier layer, where a layer of an aqueous barrier composition that is applied to the pulp-based substrate layer to form a pulp-based substrate layer is dried such that the moisture content of the pulp-based substrate layer having a barrier layer is below 2.5 % by weight, based on the weight of the pulp-based substrate layer having a barrier layer.

[0011] However, when using paper based-packaging solutions in tropical conditions which include a thin metal layer, the barrier properties of the paper-based packaging solution degrade with time, thereby cutting short the shelf-life of the packaged goods.

[0012] It is believed that the degradation of the barrier properties of the paper-based packaging solutions is due to the ingress of moisture into the material of the barrier layers constituting the paper-based packaging solution, and in particular into the layers of vinyl alcohol-based barrier materials on which the metal layers are deposited. On one hand, the absorption of moisture leads to considerable volume change in these materials, and on the other hand, the absorption of moisture increases the permeability of the material to moisture, thereby exacerbating the volume change.

[0013] Thus, it is desirable to provide a packaging solution that is based at least in part on renewable raw materials, and which exhibits excellent barrier properties against oxygen at ambient and water vapour at tropical conditions, while at the same time exhibiting a longer shelf-life in tropical conditions.

SUMMARY OF THE INVENTION

[0014] The present invention provides a barrier paper which exhibits excellent barrier properties against water vapor at tropical conditions, even after prolonged exposure to tropical conditions, and when incorporated into a packaging, thereby extends the shelf-life of the packaged goods.

[0015] In particular, the barrier papers of the present invention exhibit a water vapor transmission rate (WVTR) of 1 g/m²*day or less at tropical conditions (38°C/90%RH), and/or an oxygen transmission rate (OTR) of 1 cm³/m²*day or less at ambient conditions (23°C/50RH). Moreover, the barrier papers of the present invention exhibits a water vapor transmission rate (WVTR) of 1 g/m²*day or less at tropical conditions (38°C/90%RH) after 1 000 hours and even after 1 500 hours at tropical conditions (38°C/90%RH). In one embodiment, the barrier papers of the present invention exhibits a water vapor transmission rate (WVTR) of 0.5 g/m²*day or less at tropical conditions (38°C/90%RH) after 1 000 hours and even after 1 500 hours at tropical conditions (38°C/90%RH).

[0016] It is a first object of the present invention to provide a barrier paper, preferably for use in tropical conditions, comprising, in this order:

a paper-based substrate layer, comprising, in this order,

a base paper layer,

a first base paper overcoat layer on the base paper layer, comprising a first polymer and optionally a pigment, wherein the first polymer is a copolymer of styrene,

a primer layer comprising a polymer or copolymer on the first base paper overcoat layer,

a second base paper overcoat layer comprising a second polymer on the primer layer,

wherein the second polymer is a polymer or copolymer of vinyl alcohol,

an inorganic layer, optionally applied via vapour deposition,

optionally, a protective layer comprising a third polymer on the inorganic layer, and/or

optionally, a print receiving layer comprising a pigment on the side of the base paper layer opposite the side of the first base paper overcoat layer,

characterized in that the primer layer comprises, or consists of, a polymer or copolymer exhibiting a water absorption after 24 hours at 38°C and 90% relative humidity of less than 5%, and wherein the second base paper overcoat layer comprises, or consists of, a second polymer exhibiting a water absorption after 24 hours at 38°C and 90% relative humidity that is higher than the water absorption at equilibrium exhibited by the polymer or copolymer of the primer layer.

[0017] Without wishing to be held to a particular theory, it is believed that when the primer layer comprises, or consists of, a polymer or copolymer exhibiting a water absorption after 24 hours at tropical conditions (38°C, 90%RH) of less than 5%, and even when the second base paper overcoat layer comprises, or consists of, a second polymer exhibiting a water absorption at equilibrium that is higher than the water absorption exhibited by the polymer or copolymer of the primer layer, the uptake of water in the primer layer is lowered. This, in turn, reduces the available water that may then ingress the second layer and reduces the volume expansion of both primer and second base paper overcoat layer.

[0018] It is a second object of the present invention to provide a food packaging material comprising the barrier paper according to the first object, where the food packaging material is configured such that the barrier paper is in direct contact with a food when in use.

[0019] It is a third object of the present invention to provide a use of a barrier paper according to the first object, in a manufacturing process of a food packaging material according to the second object, or in a food packaging material according to the second object of the invention.

[0020] It is a fourth object of the present invention to provide a process to produce a barrier paper, preferably a barrier paper according to the first object of the present invention, the barrier paper comprising

a paper-based substrate layer, comprising, in this order,

a base paper layer,

a first base paper overcoat layer comprising a first polymer on the base paper layer, wherein the first polymer is a copolymer of styrene, and optionally a pigment,

a primer layer comprising an polymer or copolymer on the first base paper overcoat layer,

a second base paper overcoat layer comprising a second polymer on the primer layer,

wherein the second polymer is a polymer or copolymer of vinyl alcohol an inorganic layer,

optionally, a protective layer comprising a third polymer and/or optionally, a print receiving layer comprising a pigment on the side of the base paper layer opposite the side of the first base paper overcoat layer,

wherein the primer layer comprising an olefin polymer or olefin copolymer exhibits a water absorption at equilibrium of less than 5%, and wherein the olefin polymer or copolymer of the primer layer is preferably different from the first and second polymer
wherein the process may include the steps of, in the drying section of a paper or paperboard machine equipped with a drying device:

drying a dewatered paper base layer precursor, using the drying device, to form the paper base layer; or

drying a dewatered base paper layer precursor coated with a first base paper overcoat layer coating composition, using the drying device, to form the base paper layer and the first base paper overcoat layer,

wherein the drying device is configured to prevent drying shrinkage, and wherein preferably the drying device configured to prevent drying shrinkage is a Yankee dryer.

[0021] Alternatively, instead of drying a dewatered base paper layer precursor, the process may include the step of drying a dewatered base paper layer precursor coated with a coating composition for the first base paper overcoat layer comprising a polymer, using the drying device, to form a dried base paper layer coated with the first base paper overcoat layer comprising a polymer.

[0022] Further embodiments of the invention are laid down in the dependent claims.

FIGURES

[0023] 

Fig. 1. shows the evolution of the weight of silica gel packed with a barrier paper according to the present invention, as per Example Paper 2, when placed at 38°C and 90% relative humidity.
The amount of water absorbed in the silica gel increases with time, as water vapor continuously permeates the barrier paper. As can be seen, the weight increase follows a linear path over a period of about 1500 hours and the estimated water vapor transmission rate, which is annotated in the graph, remains constant over about 1500 hours at about 0.35 g/m²/d.

Fig. 2 shows the evolution of the weight of silica gel packed with a barrier paper, as per Example Paper 1, when placed at 38°C and 90% relative humidity. The amount of water absorbed in the silica gel increases with time, as water vapor continuously permeates the barrier paper. As can be seen, the weight increase follows a non-linear path over a period of about 800 hours, and the estimated water vapor transmission rate, which is annotated in the graph, increases over 1500 hours from about 1.65 g/m²/d in the first 400 hours to about 4.3 g/m²/d after about 1000 to 1500 hours.

Fig. 3 shows a barrier paper according to the present invention comprising a base paper layer (1), a first base paper overcoat layer (2) on the base paper layer, comprising a first polymer and a pigment, wherein the first polymer is a copolymer of styrene, a primer layer (3) comprising a polymer or copolymer on the first base paper overcoat layer, a second base paper overcoat layer (4) comprising a second polymer, on the primer layer, wherein the second polymer is a polymer or copolymer of vinyl alcohol, an inorganic layer (5), a protective layer (6) comprising a third polymer on the inorganic layer, and a print receiving layer (7) comprising a pigment on the side of the base paper layer opposite the side of the first base paper overcoat layer (2).

DESCRIPTION OF PREFERRED EMBODIMENTS

[0024] It is a first object of the present invention to provide a barrier paper, preferably for use in tropical conditions, comprising, in this order:

a paper-based substrate layer, comprising, in this order,

a base paper layer,

a first base paper overcoat layer on the base paper layer, comprising a first polymer and optionally a pigment, wherein the first polymer is a copolymer of styrene,

a primer layer comprising a polymer or copolymer on the first base paper overcoat layer,

a second base paper overcoat layer comprising a second polymer on the primer layer, wherein the second polymer is a polymer or copolymer of vinyl alcohol,

an inorganic layer applied via vapour deposition,

optionally, a protective layer comprising a third polymer on the inorganic layer, and/or

optionally, a print receiving layer comprising a pigment on the side of the base paper layer opposite the side of the first base paper overcoat layer,

characterized in that the primer layer comprises, or consists of, a polymer or copolymer exhibiting a water absorption after 24 hours at 38°C and 90% relative humidity of less than 5%, and wherein the second base paper overcoat layer comprises, or consists of, a second polymer exhibiting a water absorption after 24 hours at 38°C and 90% relative humidity that is higher than the water absorption at equilibrium exhibited by the polymer or copolymer of the primer layer.

[0025] The water absorption of a material, such as for example a polymer, in tropical conditions, e.g. after 24 hours in at 38°C and 90% RH, is determined as described below.

[0026] The method for determining the water absorption was based on the method of ISO62:2008 (Determination of water absorption) except that instead of a square, tubular or rod-shaped specimen, a disc-shaped specimen having an area of 0.0018 m² and of a thickness of 0.6 mm was used. Also, in order to ensure a truly dry sample, the specimen was dried at 120°C, instead of drying it at 50°C, before the specimen was exposed to tropical conditions for 24 hours, i.e. 38°C and 90 RH.

[0027] For each coating composition that was evaluated, three test specimens were tested. An empty Lenz DURAN^® Medium Form Weighing Bottle with NS 50/12 Cap was dried in an oven for at least 2 hours at 120 °C. After drying, the bottles were closed in the oven and subsequently transferred to a desiccator to allow them to cool down, until room temperature was reached. The dry cup mass (M₀) was determined to a precision of 1 mg, using the analytical scale. A film having an area of 0.0018 m² and of a thickness of 0.6 mm of fresh coating composition was added to the bottle -The fresh coating sample was then dried, in the cup with the lid removed, in an oven for 2 hours +/- 5 min at 120°C.

[0028] After drying, the dry sample was in placed the desiccator to allow the sample to cool down to room temperature.

[0029] The dry mass of the bottle containing the dried sample was determined on an analytical scale to a precision of 1 mg (M₂). Next, the sample was conditioned for 24 hours in a climate cabinet at tropical conditions (38°C, 90% RH). After conditioning, the mass of the bottle containing the sample was determined on an analytical balance to a precision of 1 mg within 1 min following the removal from the climate cabinet (M₃).

[0030] Thus, in the context of the present invention, the water absorption, by weight, expressed as a percentage was calculated following the formula below:

[0031] Thus, in general, the formula for the calculation of the water absorption, by weight,of a given material, such as a polymer, is:

where the wet weight is the weight of the material to be tested after 2 hours at 120°C and 24 hours at 38°C and 90% relative humidity, the dry weight is the weight of the material to be tested after 2 hours at 120°C.

[0032] The barrier paper according to the first object of the present invention exhibits an excellent barrier property against at least water vapor, and in addition also exhibits excellent oxygen barrier, especially at tropical conditions and over a prolonged time. In some embodiments, the barrier paper may additionally exhibit a barrier property against other substances, such as for example, oil, fat, grease, air, liquid water and so on. It is understood that the present invention does not necessarily preclude the addition of further layers configured to confer additional barrier properties against substances other than oxygen and/or oxygen.

[0033] In one preferred embodiment of the first object of the present invention, the inorganic layer (5) may be of metallic aluminum (Al⁰), for example, when the inorganic layer (5) is a layer of aluminum applied by physical or chemical vapor deposition. In another preferred embodiment of the barrier paper according to the first object, the inorganic layer (5) consists of a metal, such as aluminum, or a metal oxide, such as aluminum oxide, or silicon, or silicon dioxide. More preferably, the inorganic layer (5) consists of a metal layer, wherein the metal is aluminum. In another preferred embodiment of the barrier paper according to the first object, the inorganic layer (5) comprises, preferably consists of, combinations of said metal and said non-metal, such as for example a combination of aluminum (Al⁰) and aluminium oxide.

[0034] In one preferred embodiment of the first object of the present invention the barrier paper exhibits a water vapor transmission rate (WVTR) of less than 1 g/m²*day or less when measured at 38°C and 90% relative humidity according to ISO 2528.

[0035] In one preferred embodiment of the barrier paper according to the first object of the present invention, the barrier paper exhibits an oxygen transmission rate (OTR) of 1 cm³/m²*day or less when measured at 23°C and 50% relative humidity according to ISO 2528.

[0036] In one preferred embodiment of the first object of the present invention the barrier paper exhibits a water vapor transmission rate (WVTR), of less than 1 g/m²*day or less, when measured at 38°C and 90% relative humidity according to ISO 2528, and an oxygen transmission rate (OTR) of 1 cm³/m²*day or less, when measured at 23°C and 50% relative humidity according to ISO 2528.

[0037] The barrier paper according to the first object of the present invention comprises, a paper-based substrate layer comprising, in this order, a base paper layer(1), a first base paper overcoat layer (2), a primer layer (3), a second base paper overcoat layer (4); an inorganic layer (5), and optionally, a protective layer (6), which are formed on top of each other, i.e. the first base paper overcoat layer (2) is formed on the base paper layer (1), the primer layer is formed on the first base paper overcoat layer, the second base paper overcoat layer is formed on the primer layer, the inorganic layer is formed on the second base paper overcoat layer, and the protective layer is formed on the inorganic layer. It is understood that the above layers may be directly in contact with each other, or they may not be, for example, when other layers are included in between them.

[0038] The order of the base paper layer (1), first base paper overcoat layer (2), primer layer (3), second base paper overcoat layer (4), inorganic layer (5), and optionally, the protective layer (6) is chosen such that on one side, the second base paper overcoat layer (4) is protected from water ingress via the primer layer (3) and the inorganic layer (5) is protected from damage by the protective layer (6), when present. Thus, when the protective layer (6) is present, the inorganic layer (5) is sandwiched between the second base paper overcoat layer (4) and the protective layer (6). Thus, in the barrier paper according to the first object of the present invention, the inorganic layer (5) is not exposed to, but shielded from, damage like scuffs or scratches, while at the same time the second base paper overcoat layer (4) is shielded from water ingress by the primer layer (3) and to some degree by the first base paper overcoat layer (2).

[0039] Within the barrier paper according to the first object of the present invention, the paper-based layer (1) having the first base paper overcoat layer (2), primer layer (3) and second base paper overcoat layer (4) on it, is configured, at least on one side, for receiving the inorganic layer (5), which is deposited on the side of the substrate layer configured to receive the inorganic layer (5), by various processes that yield a thin film deposition such as for example atomic layer deposition (ALD), chemical vapor deposition (CVD) or physical vapor deposition (PVD).

[0040] The base paper layer (1) may be a paper or paper-board layer, which may be formed from a mixture of paper pulp and mineral particles such as fillers, pigments or sizing materials.

[0041] Suitable paper pulp may be sourced from either virgin or recycled pulp sources. An example of a recycled pulp source is deinked pulp. Virgin pulp sources are plant materials such as wood, rice straw, bagasse, bamboo, sisal, and others. In a preferred embodiment, the paper pulp forming the base paper layer is chemical wood pulp such as Kraft or sulfite wood pulp, and is more preferably a bleached chemical wood pulp. Suitable bleached chemical wood pulps are, for example, northern bleached softwood kraft (NBSK), southern bleached softwood kraft (SBSK) or dissolving pulp. The mineral particles in the base paper layer are formed of minerals chosen from calcium carbonate, talcum, gypsum or titanium dioxide, aluminum phyllosilicate clays such as kaolin clay.

[0042] Within the barrier paper according to the first object of the present invention, in the paper-based substrate layer may be a calendared or super-calendared or the base paper layer (1) may be a calendared or super-calendared paper or paperboard. Calendaring and super-calendaring can densify the base paper substrate layer or base paper layer, and provide a smoother surface on the base paper substrate layer or base paper layer (1), which is advantageous when applying liquid coating compositions onto the base paper layer for the formation of the first base paper overcoat layer (2).

[0043] Within the barrier paper according to the first object of the present invention the first base paper overcoat layer (2) comprising a first polymer and optionally a pigment is preferably in direct contact with at least one side of the base paper layer (1). The first base paper overcoat layer (2) may have a coat weight of about 0.5 to 20 g/m², preferably in an amount of 2 to 8 g/m², more preferably in a an amount of 3 to 6 g/m². The first base paper overcoat layer (2) comprises a polymer or copolymer of styrene, which may be water-soluble or non water-soluble, and may be comprised in the first base paper overcoat layer (2) in an amount of 0.5 to 20 g/m², preferably in an amount of 2 to 8 g/m², more preferably in a an amount of 3 to 6 g/m².

[0044] A purpose of the first base paper overcoat layer (2) is to decrease the porosity of the surface of the base paper layer, i.e. to increase holdout for the following primer layer (3).

[0045] For example, the first base paper overcoat layer (2) may comprise a non water-soluble styrene-butadiene copolymer or styrene-acrylic copolymer, which may be comprised in the first base paper overcoat layer (2) in an amount of about 0.5 to about 20 g/m², preferably in an amount of 2 to 8 g/m², more preferably in a an amount of 3 to 6 g/m². In a preferred embodiment, the styrene-butadiene copolymer or styrene-acrylic copolymer is an emulsion polymer.

[0046] In general, the first base paper overcoat layer (2) is formed by applying a water-based coating composition comprising the polymer of the first base paper overcoat layer (2) on top of the base paper layer (1). Depending on the polymer used, the liquid coating composition may be a solution, a latex or a dispersion, or a mixture comprising two or more thereof.

[0047] In general, when a non water-soluble polymer such as styrene-butadiene is used as the polymer, the first base paper overcoat layer (2) is formed by applying an aqueous latex of the polymer on one side of the base paper layer (2) via a suitable process such as for example spray coating, curtain coating, roller coating or blade coating, and drying the aqueous latex to form the first base paper overcoat layer (2).

[0048] The first base paper overcoat layer (2) may further comprise a pigment, which may comprise a inorganic or organic pigment. Suitable inorganic pigments are mineral pigments such as clay minerals that comprise aluminium (Al), which aluminium may be in the form of an oxide of aluminum, such as for example alumina (Al₂O₃). Such a clay minerals include for example kaolin, which is a widely used clay in papermaking and paperboard making. Furthermore, the pigment of the first base paper overcoat layer (2) may alternatively or additionally comprise a pigment that is chosen from a calcium carbonate, such as ground calcium carbonate or precipitated calcium carbonate.

[0049] For example, the first base paper overcoat layer (2) may comprise styrene-butadiene copolymer or styrene-acrylic copolymer, or a mixture thereof as the polymer of the first base paper overcoat layer, which may be comprised in the first base paper overcoat layer in an amount of about 0.5 to about 20 g/m², preferably in an amount of 2 to 8 g/m², more preferably in an amount of 3 to 6 g/m², optionally together with a pigment such as for example a clay mineral that comprises aluminium (Al), such as kaolin.

[0050] The purpose of the primer layer (3) is, among others, to increase holdout for the following second base paper overcoat layer (4) while at the same providing a barrier against water ingress into the adjoining second base paper overcoat layer (4). It was found that in multiple state of the art papers, a double layer of an ethylene vinyl alcohol copolymer is used as the barrier layer underlying an inorganic layer, and that the barrier properties of such a construct can be further improved by replacing part of the double layer of ethylene vinyl alcohol copolymer with a primer layer (3) as described hereinbelow. In particular, the shelf life of the barrier paper in tropical conditions can be extended versus a barrier paper having double layer of ethylene vinyl alcohol copolymer.

[0051] For example, the primer layer (3) may comprise a polymer or copolymer of an olefin, such as ethylene or propylene or styrene or butadiene ; or may comprise an ethylene (meth)acrylic acid copolymer or may be a polycondensation polymer or copolymer, or may be a polymer or copolymer of styrene . In particular, the polymer or copolymer of the primer layer (3) may be a polymer or copolymer of an alpha-olefin, such as 1-hexene or 1-butene with ethylene or may be a polycondensation polymer or copolymer such as a polyamide or a polyester. Suitable polyamides may be aliphatic polyamides such as PA46, PA410, PA510, PA6, PA, 610, PA66, PA610, PA612, PA1010, PA11, PA12 or PA1212. Suitable polyesters may be polyethylene terephthalate (PET) or PETG. In one preferred embodiment of the first object of the present invention the primer layer (3) comprises, or consists of, a copolymer of styrene such as for example styrene-butadiene copolymer or styrene-acrylic copolymer. In a preferred embodiment, the styrene-butadiene copolymer or styrene-acrylic copolymer is an emulsion polymer. The primer layer (3) may further comprise a polymer of styrene. For example, a polymer of styrene is polystyrene (PS).

[0052] The primer layer (3) comprises, or consists of, a polymer or copolymer exhibiting a water absorption after 24 hours at 38°C and 90% relative humidity, of less than 5%, preferably of less than 4% more preferably of less than 3% and most preferably of less than 2%.

[0053] The primer layer (3) may have a coat weight of about 0.5 to about 6 g/m², preferably in an amount of 1 to 4 g/m², more preferably in an amount of 2 to 3 g/m². In particular, the polymer or copolymer of the primer layer (3) may be comprised in the primer layer (3) in an amount of about 0.5 to about 6 g/m², preferably in an amount of 1 to 4 g/m², more preferably in an amount of 2 to 3 g/m².

[0054] In general, the primer layer (3) is formed by applying a water-based coating composition, which in general is liquid, comprising the polymer or copolymer of the primer layer on top of the first base paper overcoat layer. Depending on the polymer used, the liquid coating composition may be a solution, a latex or a dispersion, or a mixture comprising two or more thereof.

[0055] Within the barrier paper according to the first object of the present invention, the second base paper overcoat layer (4) comprising a polymer is preferably in direct contact with the primer layer (3). The second base paper overcoat layer (4) may have a coat weight of about 2 to 8 g/m², preferably in an amount of 3 to 6 g/m², more preferably in a an amount of 3 to 4 g/m². The second base paper overcoat layer (4) may comprise, or consist of, a p olymer or copolymer of vinyl alcohol, which may be water-soluble or non water-soluble, and which may be comprised in the second base paper overcoat layer (4) in an amount of 2 to 8 g/m², preferably in an amount of 3 to 6 g/m², more preferably in a an amount of 3 to 4 g/m².

[0056] A purpose of the second base paper overcoat layer (4) is to decrease the number of, and preferably eliminate any, pinholes of the surface of the primer layer (3) for an efficient subsequent application of the inorganic layer (5), free of defects. Thus, in a preferred embodiment of the present invention, the second base paper overcoat layer (4) is pinhole free, or has less than 5 pinholes per m².

[0057] It has been found that the smoothness of the second base paper overcoat layer (4) can be improved when the total amount of polymer comprised in the second base paper overcoat layer (4) is applied in a coating process that consists of two or three coating steps, each applying a fraction of the total amount polymer comprised in the second base paper overcoat layer (4) instead of applying the total amount polymer comprised in the second base paper overcoat (4) in a single coating step. For instance, half of the total amount of polymer may be applied in a first coating step and the remaining half of the total amount of polymer may be applied in a second coating step. Suitable coating techniques that may be used for the application of the coating composition that results in the second base paper overcoat layer (4) are for example spray coating, curtain coating, roller coating or blade coating. In a preferred embodiment, the coating composition is an aqueous coating composition such as for example a solution of the polymer or a latex of the polymer. It is understood that in each coating step, the coating composition is dried after being applied, i.e. when multiple coating steps are carried out, the coating composition is dried between coating steps and after the last coating step. Alternatively, it is possible to coat wet-on-wet when using curtain coating in conjunction with a dual layer slot die.

[0058] In one embodiment of the barrier paper according to the first object of the present invention, a suitable polymer comprised in the second base paper overcoat layer (4) may be a polymer or copolymer of vinyl alcohol, such as polyvinyl alcohol or ethylene vinyl alcohol (EVOH) copolymer.

[0059] For example, a suitable water-soluble polymer may be a water-soluble type of ethylene vinyl alcohol (EVOH) copolymer.

[0060] In a preferred embodiment, the second base paper overcoat layer (4) comprises ethylene vinyl alcohol copolymer (EVOH) as the sole polymer of the second base paper overcoat layer (4), which may be a water-soluble type of ethylene vinyl alcohol copolymer (EVOH). More preferably, the second base paper overcoat layer (4) comprises ethylene vinyl alcohol copolymer (EVOH) or polyvinyl alcohol, as the case may be, as the sole polymer in an amount of at least 90 weight percent, even more preferably in an amount of at least 95 weight percent, based on the total weight of the second base paper overcoat layer (4).

[0061] The water-soluble type of ethylene vinyl alcohol copolymer (EVOH) may preferably have an ethylene content of less than 20 mol%.

[0062] In a preferred embodiment, the second base paper overcoat layer (4) comprises ethylene vinyl alcohol copolymer (EVOH) having an ethylene content from about 20 to about 55 mol % or having an ethylene content of less than 20 mol%.

[0063] The second polymer comprised in the second base paper overcoat layer (4) exhibits a water absorption after 24 hours at 38°C and 90% relative humidity that is higher than the water absorption after 24 hours at 38°C and 90% relative humidity exhibited by the polymer or copolymer comprised in the primer layer. For instance, this means that the second polymer may exhibit a water absorption also below 5%, provided that the polymer or copolymer comprised in the primer layer (3) has an even lower water absorption at equilibrium. In a particular embodiment, the second polymer may exhibit a water absorption after 24 hours at 38°C and 90% relative humidity of more than 5%, or between 5% and 15%, preferably of more than 8%, or between 8% and 12%.

[0064] In a preferred embodiment, both the protective layer (6) and the primer layer (3) comprise the same polymer as sole polymer, or the same combination of polymers. In particular, both the protective layer (6) and the primer layer (3) may comprise a polymer or copolymer of an olefin, in particular a polymer or copolymer of an alpha-olefin such as polyethylene, or a copolymer of ethylene (meth)acrylate copolymer.

[0065] The barrier paper according to the first object of the present invention comprises an inorganic layer (5), that is applied via vapor deposition. The inorganic layer may be formed of a metal layer that comprises, or consists of, a metal or a metal oxide. Alternatively, the inorganic layer (5) may be formed of a non-metal layer that comprises, or consists of, a non-metal such as for example silicon or a non-metal oxide such as silicon dioxide.

[0066] In a preferred embodiment of the barrier paper according to the first object, the inorganic layer (5) has a thickness of less than 500 nm, preferably of from 5 to 250 nm, more preferably of from 110 to 250 nm. The thickness of the inorganic layer (5) is inferior to traditional metal layers used in paper-based barrier laminates, which include a metal foil having a thickness in excess of 5 microns or more. The inorganic layer (5) of the barrier paper according to the first object is not a foil, but corresponds rather to a thin film layer due to its inferior thickness. Accordingly, methods of thin film deposition such as for example sputtering, atomic layer deposition (ALD) or vacuum deposition methods such as physical or chemical vapor deposition (PVD/CVD), in particular e-beam evaporation PVD, may be used to form the inorganic layer (5) on the second base paper overcoat layer (4) of the barrier paper according to the first object.

[0067] In a preferred embodiment of the barrier paper according to the first object, the inorganic layer (5) consists of a metal, such as aluminum, or a metal oxide, such as aluminum oxide, or silicon, or silicon dioxide. More preferably, the inorganic layer (5) consists of a metal layer, wherein the metal is aluminum. In another preferred embodiment of the barrier paper according to the first object, the inorganic layer (5) comprises, preferably consists of, combinations of said metal and said non-metal, such as for example a combination of aluminium (Al) and aluminium oxide.

[0068] The barrier paper according to the first object of the present invention may optionally comprise a protective layer (6) comprising a third polymer, such as for example a polymer or copolymer of an olefin, in particular a polymer or copolymer of an alpha-olefin, such as polyethylene, or may comprise an ethylene (meth)acrylate copolymer as the third polymer. For example, a suitable polyolefin may be polyethylene or polypropylene. The protective layer (6) comprising a third polymer protects the inorganic layer against damage such as scuffing, scratching and such. In addition, the protective layer (6) may also provide further barrier functionality and heat-sealing functionality. Accordingly, the protective layer (6) comprises, or consists of, a heat-sealable polymer or copolymer of an olefin.

[0069] In a preferred embodiment of the barrier paper according to the first object, the protective layer (6) comprises, or consists of, an emulsion polymer or copolymer of an olefin, such as polyethylene, or comprises, or consists of, an ethylene (meth)acrylate copolymer. Further the protective layer (6) is preferably not a foam layer.

[0070] In a preferred embodiment of the barrier paper according to the first object, the polymer comprised in the protective layer (6) is a thermoplastic polymer or copolymer, preferably a non-cross-linked, thermoplastic polymer or copolymer, such as polymer or copolymer of an olefin, which may be heat-sealable. Preferably, the thermoplastic polyolefin is chosen from polyethylene, polypropylene, ethylene vinyl acetate or ethylene (meth)acrylic acid copolymer.

[0071] The protective layer (6) may be formed by applying an aqueous latex of the polymer on one side of the inorganic layer (5) via a suitable process such as for example spray coating, curtain coating, roller coating, slot die coating or blade coating, and drying the aqueous latex to form the protective layer (6).

[0072] In a preferred embodiment of the barrier paper according to the first object, the combined amount of primer layer (3) and second base paper overcoat layer (4) is of 4 to 8 g/m², preferably of 4 to 6 g/m² and more preferably of 5 to 6 g/m². In particular when the combined amount of primer layer (3) and second base paper overcoat layer (4) is below 10 g/m², the barrier paper according to the first object can be recycled more conveniently.

[0073] It is a second object of the present invention to provide a food packaging comprising the one or more sheets of barrier paper according to the first object of the invention, wherein the barrier paper comprises the print receiving layer (7) and said print receiving layer (7) is printed with an image, and wherein the barrier paper comprises the protective layer (6) and said protective layer (7) is heat-sealed, preferably to itself.

[0074] The barrier paper according to the first object of the invention may incorporate an image receiving layer (7), in particular when used in a food packaging. The image receiving layer (7) comprises a pigment and preferably comprises a clay pigment, more preferably a kaolin clay pigment, and/or comprises a calcium carbonate pigment, where the calcium carbonate is a ground calcium carbonate or a precipitated calcium carbonate, or mixtures thereof. The food packaging according to the second object of the present invention may comprise one or more sheets of barrier paper according to the first object of the present invention, wherein preferably the barrier paper comprises the print receiving layer (7), and said print receiving layer (7) is printed with an image, and/or wherein the barrier paper comprises the protective layer (6) and said protective layer (6) is heat-sealed, to a container and forms a lid or to itself to form a pouch.

[0075] The food packaging according to the second object of the present invention may have any form that is suitable for packaging foods. For example, the packaging material may be in the form of a food container that is preformed, such as a tray, a cup, a bottle, a box, a dish, a capsule, a sachet, a bag, a pouch, or a holding cone. Alternatively, the packaging material may be in the form of a sheet that can be a packaging sheet, a wrapping sheet, a separating sheet and so on. Alternatively, the barrier paper may be comprised in a food packaging material as a part of a food container, where the food packaging material is in the form of a cap or a lid of the food container, especially a peelable lid.

[0076] It is a third object of the present invention to provide a use of the barrier paper according to the first object, in a food packaging.

[0077] The barrier paper according to the first object can be used in any manufacturing process of a food packaging material. In particular, when manufacturing process comprises a step of applying a print receiving layer to the food packaging material, the barrier paper according to the first object comprises a print receiving layer (7) that is adjacent to the paper-based substrate layer, on the side opposite of the layers having a barrier functionality, e.g. of the inorganic layer (5).

[0078] It is a fourth object of the present invention to provides process to produce a barrier paper, preferably a barrier paper according to the first object of the present invention, the barrier paper comprising

a paper-based substrate layer, comprising, in this order,

a base paper layer (1),

a first base paper overcoat layer (2) comprising a first polymer on the base paper layer, wherein the first polymer is a copolymer of styrene,

a primer layer (3) comprising an polymer or copolymer on the first base paper overcoat layer,

a second base paper overcoat layer (4) comprising a second polymer on the primer layer, wherein the second polymer is a polymer or copolymer of vinyl alcohol an inorganic layer (5),

optionally, a protective layer (6) comprising a third polymer and/or optionally, a print receiving layer (7) comprising a pigment on the side of the base paper layer opposite the side of the first base paper overcoat layer (2)

wherein the primer layer (3) comprising an olefin polymer or olefin copolymer exhibits a water absorption after 24 hours at 38°C and 90% relative humidity of less than 5%, and wherein the olefin polymer or copolymer of the primer layer (3) is preferably different from the first and second polymer,
wherein the process may include the steps of, in the drying section of a paper or paperboard machine equipped with a drying device:

drying a dewatered paper base layer precursor, using the drying device, to form the paper base layer (1); or

drying a dewatered base paper layer precursor coated with a first base paper overcoat layer coating composition, using the drying device, to form the base paper layer (1) and the first base paper overcoat layer (2),

wherein the drying device is configured to prevent drying shrinkage, and wherein preferably the drying device configured to prevent drying shrinkage is a Yankee dryer.

[0079] A paper machine or a paperboard machine includes several sections, starting with a wet section, or wet end, where a web of wet paper precursor is formed by discharging a paper furnish onto a formation table, where the furnish is drained of some water mainly by suction. After the wet section, in the press section, the paper precursor is passed through dewatering devices which remove water via mechanical action, such as for example rolls that squeeze the paper precursor to dewater the paper precursor. At this point, the "dewatered" paper precursor, which is still moist, transits into the dryer section, where drying devices such as heated rolls, infrared heaters, and hot air heaters remove the remaining moisture from the dewatered paper precursor.

[0080] When dried in the dryer section, the dewatered paper precursor tends to shrink, in both machine direction as well as in cross direction, unless the drying shrinkage is prevented in some way.

[0081] In the process according to the fourth object of the present invention, in the drying section of the paper or paperboard machine equipped with a drying device, the dewatered paper base layer precursor is dried to form the paper base layer (1) using said drying device, which drying device is configured to prevent drying shrinkage.

[0082] In one embodiment of the process according to the fourth object of the present invention, the paper base layer (1) thus obtained after drying may be subsequently coated with a coating composition for the first base paper overcoat layer (2) comprising a polymer. Depending on the polymer used, the liquid coating composition may be a solution, a latex or a dispersion, or a mixture comprising two or more thereof. Accordingly, the process according to this embodiment of the fourth object of the present invention includes the step of coating the paper base layer (1) with a coating composition for the first base paper overcoat layer (2) comprising a polymer.

[0083] In another embodiment of the process according to the fourth object of the present invention, the paper base layer precursor may be coated with a liquid coating composition for the first base paper overcoat layer (2) comprising a polymer, i.e. prior to drying the paper base layer precursor. Depending on the polymer used, the liquid coating composition may be a solution, a latex or a dispersion, or a mixture comprising two or more thereof. Accordingly, the process according to this embodiment of the fourth object of the present invention includes the step of coating the paper base layer precursor with a liquid coating composition for the first base paper overcoat layer (2) comprising a polymer, in the wet section of the paper machine or the paperboard machine.

EXAMPLES

[0084] Three test barrier papers were produced, two according to the present invention and one as a comparative and are described below. All papers were manufactured on the basis of the same base paper layer and the same first base paper overcoat layer, which comprised a first polymer and a pigment, wherein the first polymer was a copolymer of styrene.

[0085] Paper 1 : a primer layer was formed on the first base paper overcoat layer in an amount of 3 g/m². The primer layer was made of ethylene vinyl alcohol copolymer (EVOH) having a water absorption after 24 hours at 38°C and 90% relative humidity of about 10.4%. A second base paper overcoat layer was obtained by coating the same composition as in the primer layer on the primer layer. The thus formed double EVOH layer had a weight of about 6 g/m². Subsequently, an inorganic layer of aluminium (Al) was formed by vapour deposition on the second base paper overcoat layer and finally, a protective layer of ethylene acrylic acid copolymer (EAA) was in an amount of about 4 g/m² was formed on the inorganic layer.

[0086] Paper 2 : a primer layer was formed on the first base paper overcoat layer in an amount of 2 g/m². The primer layer was made of styrene butadiene copolymer having a water absorption after 24 hours at 38°C and 90% relative humidity of about 1.9 %. A second base paper overcoat layer was obtained by coating the primer layer with a coating composition of EVOH having a water absorption after 24 hours at 38°C and 90% relative humidity of about 10.4 %. The thus formed EVOH second base paper overcoat layer had a weight of about 3 g/m². Subsequently, a inorganic layer of aluminium (Al) was formed by vapour deposition on the second base paper overcoat layer and finally, a protective layer of ethylene acrylic acid copolymer (EAA) was in an amount of about 4 g/m² was formed on the inorganic layer. Paper 2, with a total coat weight of 9 g/m² (2+3+4), showed lower WVTR and over a longer time period, when compared to Paper 1, even at a total coat weight that was 10% lower, see Table 1.

[0087] Paper 3 : a primer layer was formed on the first base paper overcoat layer in an amount of 2 g/m². The primer layer was made of styrene butadiene copolymer having a water absorption after 24 hours at 38°C and 90% relative humidity of about 1.9 %. A second base paper overcoat layer was obtained by coating the primer layer with a coating composition of EVOH having a water absorption after 24 hours at 38°C and 90% relative humidity of about 10.4 %. The thus formed EVOH second base paper overcoat layer had a weight of about 4 g/m². Subsequently, a inorganic layer of aluminium (Al) was formed by vapour deposition on the second base paper overcoat layer and finally, a protective layer of ethylene acrylic acid copolymer (EAA) was in an amount of about 4 g/m² was formed on the inorganic layer. Paper 3, with a total coat weight of 10 g/m² (2+4+4), showed lower WVTR and over a longer time period, when compared to Paper 1. When compared to Paper 2, the WVTR was comparable over the same period of time, thus emphasizing the role of the primer layer over an increase in the amount of EVOH, see Table 1.

Table 1:

	Paper 1	Paper 2	Paper 3
Combined coat weight primer and second base paper overcoat layer	6	5	6
WVTR1	1.6	0.3	0.3
WVTR2	4.3	0.3	0.3
OTR	0.01	0.3	n.m

1measured up to 50 hours. 2measured between 1000 and 1500 hours.

[0088] As can be seen in the above Table, Paper 2 and 3 barrier paper reach exceptionally low WVTR, at 38°C and 90 % RH, and maintain the exceptionally low WVTR over the course of more than 1000 hours.

[0089] Water vapor transmission rate (WVTR) was measured at 38°C and 90% relative humidity according to ISO 2528.

[0090] Oxygen transmission rate (OTR) was measured at 23°C and 50% relative humidity according to ISO 2528.

[0091] As can be seen from Figure 1, the weight of silica gel packed with Paper 1 increases due to the absorption of water in the silica gel, in a non-linear manner for the Paper 1, whereas for Paper 2, as shown in Fig. 2, the weight increase follows a linear path over an extended period. This means that the rate of water vapor transmission and thereby, the increase of weight per time unit, increases in Paper 1, while Paper 2 maintains the initial water vapor transmission rate.

Claims

1. A barrier paper, preferably for use in tropical conditions, comprising, in this order:

a paper-based substrate layer, comprising, in this order,

a base paper layer (1),

a first base paper overcoat layer (2) on the base paper layer, comprising a first polymer and optionally a pigment, wherein the first polymer is a copolymer of styrene,

a primer layer (3) comprising a polymer or copolymer on the first base paper overcoat layer,

a second base paper overcoat layer (4) comprising a second polymer, on the primer layer, wherein the second polymer is a polymer or copolymer of vinyl alcohol,

an inorganic layer (5), optionally applied via vapour deposition, optionally, a protective layer (6) comprising a third polymer on the inorganic layer, and/or

optionally, a print receiving layer (7) comprising a pigment on the side of the base paper layer opposite the side of the first base paper overcoat layer,

characterized in that the primer layer comprises, or consists of, a polymer or copolymer exhibiting a water absorption of less than 5% after 24 hours at 38°C and 90% relative humidity, and wherein the second base paper overcoat layer comprises, or consists of, a second polymer exhibiting a water absorption after 24 hours at 38°C and 90% relative humidity that is higher than the water absorption after 24 hours at 38°C and 90% relative humidity exhibited by the polymer or copolymer of the primer layer.

2. The barrier paper according to claim 1, wherein the inorganic layer comprises, preferably consists of, a metal such as aluminium (Al), a metal oxide such as aluminium oxide or comprises, preferably consists of, a non-metal such a silicon or a non-metal oxide such as silicon oxide, or comprises, preferably consists of, combinations of said metal and said non-metal such as a combination of aluminium (Al) and aluminium oxide, and/or wherein the inorganic layer is applied via chemical vapour deposition or physical vapour deposition.

3. The barrier paper according to claim 1 or 2, wherein the polymer or copolymer of the primer layer is a polymer or copolymer of an olefin, a polymer or copolymer of styrene, or is a polycondensation polymer or copolymer, or is an ethylene (meth)acrylic acid copolymer.

4. The barrier paper according to any preceding claim, wherein the polymer or copolymer of the primer layer is a polymer or copolymer having a water absorption after 24 hours at 38°C and 90% relative humidity of less than 2%.

5. The barrier paper according to any preceding claim, wherein the first polymer comprised in the first base paper overcoat layer is comprised in an amount of 0.5 to 20 g/m², preferably in an amount of 2 to 8 g/m², more preferably in an amount of 3 to 6 g/m² and/or wherein the polymer comprised in the second base paper overcoat layer is comprised in an amount of 2 to 8 g/m², preferably in an amount of 3 to 6 g/m², more preferably in an amount of 3 to 4 g/m², and/or the polymer or copolymer of the primer layer may be comprised in the primer layer in an amount of about 0.5 to about 6 g/m², preferably in an amount of 1 to 4 g/m², more preferably in an amount of 2 to 3 g/m².

6. The barrier paper according to any preceding claim, wherein the combined amount of primer layer and second base paper overcoat layer is of 4 to 8 g/m², preferably of 5 to 6 g/m².

7. The barrier paper according to any preceding claim, wherein the first polymer or copolymer comprised in the first base paper overcoat layer is styrene-butadiene copolymer or styrene-acrylic copolymer.

8. The barrier paper according to any preceding claim, wherein the inorganic layer has a thickness of less than 500 nm, preferably of from 5 to 250 nm.

9. The barrier paper according to any preceding claim, wherein the second polymer comprised in the second base paper overcoat layer comprises a polymer or copolymer of vinyl alcohol, such as polyvinyl alcohol (PVOH) or ethylene vinyl alcohol (EVOH) copolymer, as the sole polymer of the second base paper overcoat layer or comprises a mixture of polymers or copolymers of vinyl alcohol, such as a mixture of polyvinyl alcohol (PVOH) and ethylene vinyl alcohol (EVOH) copolymer.

10. The barrier paper according to any preceding claim, wherein the pigment in the print receiving layer comprises a clay pigment, preferably a kaolin clay pigment, and/or the pigment in the print receiving layer comprises a calcium carbonate pigment coating, where the calcium carbonate is a ground calcium carbonate or a precipitated calcium carbonate, or mixtures thereof.

11. The barrier paper according to any preceding claim, wherein the amount of second base paper overcoat layer is obtained by applying a fraction 1/N of said amount in N passes on the first base paper overcoat, in particular wherein N is 2.

12. The barrier paper according to any preceding claim, wherein the protective layer is heat-sealable and wherein the third polymer is a thermoplastic polymer such as a polymer or copolymer of an olefin, and the protective layer is obtained by applying an aqueous dispersion comprising a polymer or copolymer of an olefin, preferably chosen from polyethylene, polypropylene, ethylene vinyl acetate (EVA) or by applying an aqueous dispersion comprising an ethylene acrylic acid copolymer (EAA) or ethylene methacrylic acid copolymer (EMAA).

13. The barrier paper according to any preceding claim, wherein the barrier paper has a WVTR of less than 1 g/m²*day at when measured according to ISO 2528 at 38°C and 90% relative humidity, after 1 000 hours at 38°C and 90% relative humidity.

14. A food packaging comprising one or more sheets of barrier paper according to any one of the preceding claims, wherein the barrier paper comprises the print receiving layer and said print receiving layer is preferably printed with an image, and wherein the barrier paper comprises the protective layer and said protective layer is heat-sealed, preferably to itself .

15. Use of a barrier paper according to any one of claims 1 to 13, in a food packaging according to claim 14.

Drawing