EP4172412A1

EP4172412A1 - Water-resistant fibrous material and method for manufacturing it

Info

Publication number: EP4172412A1
Application number: EP21830264.4A
Authority: EP
Inventors: Susanne HANSSON; Raija BÅDENLID; Kaj Backfolk; Isto Heiskanen
Original assignee: Stora Enso Oyj
Current assignee: Stora Enso Oyj
Priority date: 2020-06-24
Filing date: 2021-06-24
Publication date: 2023-05-03
Also published as: CA3186605A1; US20230243105A1; SE544175C2; SE2050753A1; EP4172412A4; KR20230026353A; CN115917083A; WO2021260606A1

Abstract

The present invention relates to a water-resistant fibrous material, such as flexible sheet, paper or paperboard, comprising a foam-formed fibrous ply, wherein said foam-formed fibrous ply has been subjected to grafting with a fatty acid halide to render the foam-formed fibrous ply water-resistant. The present invention further relates to a method for manufacturing the water-resistant fibrous material.

Description

WATER-RESISTANT FIBROUS MATERIAL Technical field The present disclosure relates to water-resistant fibrous material for use in wet or damp environments. More specifically, the present disclosure relates to water- resistant fibrous material comprising a foam-formed fibrous ply. Background Foam-forming technology enables production of recyclable and sustainable materials for several applications including lightweight packaging materials, construction materials and boards. Foam-forming enables utilization of longer fibers than in traditional papermaking forming processes, improved control of web structure, and manufacturing of paper and board having low density compared to conventionally formed paper and board. In foam-forming technology an aqueous foam is used instead of water as a carrier medium in the manufacturing of fiber-based products. The presence of air bubbles in the aqueous medium changes the properties of the medium fundamentally. The technology enables higher headbox consistencies and production of structures with excellent formation and high bulk. Other benefits include reduced water usage and energy savings. Resistance to water and other liquids is an important property in many paper, paperboard or containerboard applications. Some examples include packaging, such as boxes, bags, corrugated board and other containers; fresh and aseptic liquid packaging; boxes, bags, corrugated board, trays, or cups for hot, cold, dry, wet and frozen food and beverages; products for outdoor use such as boxes, bags, signs and posters; pots, trays and covers for plants; packages for construction materials, and construction material. Paper, paperboard or containerboard for use in wet or damp environments is usually treated with sizing agents to enhance certain qualities; and above all, to increase the resistance to penetration of water and other liquids into the paper or paperboard. There are two main types of sizing: internal sizing and surface sizing. For internal sizing, chemicals are added to the pulp at the wet end, for example alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) or rosin sizing agent. Common surface-sizing agents include, e.g., starch or acrylic co-polymers. Coating of paper, paperboard or containerboard with plastics is often employed to combine the mechanical properties of the paperboard with the barrier and sealing properties of a plastic film. Also in plastic coated paperboard, the board is often treated with a hydrophobic sizing agent to prevent so-called edge wick, i.e. absorption of liquid at the cut edges (or so-called raw edges) of the paperboard. Edge-wick resistance is an important parameter in many applications. A problem with internal sizing agents, such as AKD, is that they interfere with the hydrogen bonding between the cellulose fibers, giving a debonding effect and hence a weaker material. To compensate for the weaker material, the grammage of paper and board is increased leading to higher carbon footprint due to overuse of wood fibers and higher transport weight at all stages downstream the production. To improve the wet strength of the material, the internal sizing agent is often combined with a wet strength agent. A wet-strength agent improves the tensile properties of the paper or paperboard in the wet state by for example covalently binding to the cellulose fibers and also by forming crosslinking network between the fibers or fibers and other dry strength agents. Common wet strength agents include urea-formaldehyde (UF), melamine-formaldehyde (MF) and polyamide- epichlorohydrin (PAE). Other wet strength agents can give wet-strength by other mechanisms, and some of these wet strength agents can also have a temporary wet-strength function. A problem with the addition of wet strength agents is that the repulpability of the paperboard is severely reduced. Furthermore, in foam-forming technology the addition of internal sizing agents and wet-strength agents can cause unexpected effects on the foam structure or even foam collapse. Surface treatment of the foam-formed structure with hydrophobic agents is possible, but traditional concepts may not work since the high bulk means that the surface size uptake will be higher, and the risk of substrate collapse is greater. Thus, there remains a need for improved solutions to render paper or paperboard comprising a foam-formed fibrous ply water resistant, without weakening the material or destroying the foam structure, and without reducing the repulpability of the material. Description of the invention It is an object of the present disclosure to provide a water-resistant fibrous material, such as flexible sheets, paper or paperboard comprising a foam-formed fibrous ply with high repulpability. It is a further object of the present disclosure to provide a water-resistant fibrous material comprising a foam-formed fibrous ply with improved wet strength and similar repulpability as compared to a corresponding non water-resistant fibrous materials such as paper or paperboard. It is a further object of the present disclosure to provide a water-resistant fibrous material such as flexible sheets, paper or paperboard which is free from added hydrophobic sizing agents or which has only a low amount of added hydrophobic sizing agents, for example alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) and/or rosin-sizing agent. It is a further object of the present disclosure to provide a water-resistant fibrous material such as flexible sheets, paper or paperboard which is free from added wet strength agents or which has only a low amount of added wet strength agents, particularly crosslink-forming wet strength agents, for example urea-formaldehyde (UF), melamine-formaldehyde (MF) and/or polyamide-epichlorohydrin (PAE). Another object of the present disclosure is to provide a method for rendering fibrous material such as flexible sheets, paper or paperboard water resistant, without weakening the material and without reducing the repulpability of the material. 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. According to a first aspect illustrated herein, there is provided a water-resistant fibrous material comprising a foam-formed fibrous ply, wherein said foam-formed fibrous ply has been subjected to grafting with a fatty acid halide to render the foam-formed fibrous ply water-resistant. According to one aspect of the invention, the foam-formed fibrous ply is a flexible sheet, paper or paperboard which, it its turn, is preferably a flat, continuous or discontinuous piece of sheet or web of material formed from wood pulp or other fibrous substances comprising cellulose fibers. That is, the term “fibrous” is here intended to refer to materials originating from, or based on, natural fibers such as wood pulp, non-wood plant materials or combinations thereof. The flexible sheet, paper or paperboard can be used as such, or it can be provided with various coatings and/or be used as a ply in a multi-ply laminate comprising one or more additional cellulose-based or non cellulose-based plies. As used herein, the term “fibrous material” is intended to comprise at least said flexible sheet, paper or paperboard mentioned in this application. Herein, flexible sheet generally refers to a flat, continuous or discontinuous piece of fibrous sheet or web made from wood or other fibrous substances comprising cellulose fibers, comprising flexibility-, stretch- and strength-properties which makes the material suitable for use as e.g. lightweight carrier bags and sacks, shopping bags, refuse sacks and for wrappings of goods. The flexible sheet described in the present disclosure may have a grammage of 2- 500 g/m², preferably 10-500 g/m², which may be adjusted according to the end- use. The flexible sheet further comprises a stretch in the range of 5-45%, measured according to the method ISO 1924-2:2008. Paper generally refers to a sheet or web material made from pulp of wood or other fibrous substances comprising cellulose fibers, used for e.g. writing, drawing, or printing on, or as packaging material. Paperboard generally refers to strong, thick paper or cardboard comprising cellulose fibers used for example in boxes, bags and other types of packaging. Paperboard can be comprised of one or more plies. Paperboard can either be bleached or unbleached, coated or uncoated, and produced in a variety of thicknesses, depending on the end-use requirements. The paper or paperboard useful in the present disclosure may typically have a basis weight in the range of 15-500 g/m². Thanks to the foam-formed fibrous ply, the flexible sheet, paper or paperboard may typically have a density below 700 kg/m³. In some embodiments, the density of the fibrous material is below 600 kg/m³, preferably below 500 kg/m³, or below 400 kg/m³. Although in thinner fibrous materials the density can be higher, as long as the basis weight is still not too high. In thinner flexible sheets or papers, having a thickness below about 100 μm, the density may for example be in the range of 600-1400 or in the range of 700-1200 kg/m³. Higher density and thickness, resulting in a higher basis weight, prevent grafting through the entire thickness of the foam-formed fibrous ply. The thickness of the fibrous material may range from thin to very thick. In some embodiments, the thickness of said fibrous material is above 30 μm, preferably above 100 μm. In some embodiments, the thickness of the fibrous material is in the range of 30 μm to 10 mm. In some embodiments, the thickness of the fibrous material is in the range of 100 μm to 1 mm. In some embodiments, the basis weight of said fibrous material is in the range of 15-500 g/m², preferably in the range of 100-400 g/m². In some embodiments, the air permeance of the fibrous material is in the range of 10-100 μm/(Pa∙s) as measured using the Bendtsen method (ISO 5636-3). The inventive water-resistant flexible sheet, paper or paperboard comprises a foam-formed fibrous ply. Foam-forming technology enables improving paper properties and manufacturing high porosity and light weight products. It also offers a possibility to decrease raw material and production costs remarkably compared to conventional paper forming methods. Existing paper and board machines can be modified for foam-forming technology to make high quality bulkier sheet structures with excellent formation and while also reducing energy and chemical consumption. In foam-forming, a large amount of air is added to an aqueous pulp suspension in the presence of a surfactant. The air bubbles formed prevent fiber flocculation, enhance dewatering, and enables production of light weight structures. The terms foam and foamed, as used herein, refers to a substance made by trapping air or gas bubbles inside a solid or liquid. Typically, the volume of gas is much larger than that of the liquid or solid, with thin films separating gas pockets. Three requirements must be met in order for foam to form. Mechanical work is needed to increase the surface area. This can occur by agitation, dispersing a large volume of gas into a liquid, or injecting a gas into a liquid. The second requirement is that a foam forming agent, typically an amphiphilic substance, a surfactant or surface active component, must be present to decrease surface tension. Finally, the foam must form more quickly than it breaks down. In some embodiments, the foaming required for the foam-forming is achieved using a foam generator. The pulp suspension may be pumped through a foam generator one or several times in order to reach a desired gas content or foam density. In some embodiments, the pulp suspension is pumped via a high shear mixer or refiner which generates the foam. Foam can be generated either offline or inline at the paper machine. In some embodiments, the foam is brought to an air content of 60 to 70 vol% before being applied to the forming fabric. The consistency of the pulp subjected to foaming may typically be in the range of from 0.1 to 2 wt% based on the amount of water. The foam is formed and stabilized using a surfactant present in the pulp suspension. The surfactant may be a small molecule surfactant or a polymeric surfactant or a mixture thereof. The amount of surfactant in the foam may typically be in the range of 0.005 to 30 wt%, but will be easily determinable by a skilled person. An example of a small molecule surfactant useful for the foam forming is sodium dodecyl sulfate (SDS). The amount of SDS in the foam may typically be in the range of 0.005 to 10 wt%, for example about 0.02 wt%. Examples of a polymeric surfactant useful for the foam forming include polyvinyl alcohol (PVOH) and partially hydrolyzed polyvinyl acetate (PVOH/Ac) or other modifications of PVOH. The amount of polyvinyl alcohol (PVOH) or partially hydrolyzed polyvinyl acetate (PVOH/Ac) in the foam may typically be in the range of 0.01 to 30 wt%, for example about 5 wt%. Polyvinyl alcohol (PVOH) and partially hydrolyzed polyvinyl acetate (PVOH/Ac) is of particular interest as a surfactant in the foam-formed fibrous ply of the invention since in addition to acting as a surfactant, the hydroxyl groups will also act as a substrate for grafting with a fatty acid halide. The long chains of polymeric surfactants, such as PVOH can contribute to the mechanical properties and may also be less prone to leaching or migration from the finished foam-formed fibrous ply, which may be valuable in certain applications. Thus, in some preferred embodiments the foaming agent is a polymeric foaming agent. In some embodiments, the polymeric foaming agent is selected from the group consisting of optionally hydrophobically modified polysaccharides, proteins, polyvinyl alcohol (PVOH), partially hydrolyzed polyvinyl acetate (PVOH/Ac), and mixtures thereof. The optional hydrophobic modification typically comprises one or more hydrophobic groups, e.g. alkyl groups, covalently attached to the foaming agent. In a preferred embodiment, the foaming agent is polyvinyl alcohol (PVOH) or partially hydrolyzed polyvinyl acetate (PVOH/Ac). The foaming agent may also comprise polysaccharides or polymers or derivatives thereof. The pulp suspension, and consequently also the foam-formed fibrous ply can be made from the pulp of wood or other fibrous substances comprising cellulose fibers. The foam-formed fibrous ply can comprise softwood or hardwood pulp. The foam-formed fibrous ply can also comprise a mix of softwood and hardwood pulp. The foam-formed fibrous ply can also comprise recycled fiber, e.g. recycled used beverage carton. The foam forming process also allows addition of longer synthetic or natural fibers, for example reinforcement pulp, polylactic acid (PLA) fibers, polyvinyl alcohol (PVOH) fibers or man-made cellulosic fibers such as regenerated cellulose, such as viscose or lyocell fibers. The longer fibers may be present in an amount of 0- 50wt%, preferably 1-20wt%, or 3-20 wt%, based on the total dry weight of the foam-formed fibrous ply. The foam-forming can be performed at an industrial scale on a running forming fabric of a paper or paperboard machine, wherein a web of the foam is applied to the forming fabric, the web is dewatered by suction through the web and the forming fabric, and finally dried in a drying section of the paper or paperboard machine. In some embodiments, the flexible sheet, paper or paperboard consists of the foam-formed fibrous ply. In some embodiments, the water-resistant flexible sheet, paper or paperboard may further comprise one or more additional fibrous or non cellulose-based plies. In some embodiments, the flexible sheet, paper or paperboard comprises the foam-formed fibrous ply, and one or more additional fibrous plies. The additional fibrous plies may be foam-formed or non foam-formed. An important property of paperboard is the bending stiffness. The bending stiffness in paperboard is often built up by having outer plies with high tensile stiffness and one or more bulky plies in between, so that the outer plies are placed at a desired distance from each other. The bulky plies are often referred to as middle layer or middle layers. In some embodiments, the foam-formed fibrous ply constitutes a middle ply between two or more additional fibrous plies, preferably the foam-formed fibrous ply constitutes a middle ply between two or more non foam-formed fibrous plies. In embodiments, wherein the foam-formed fibrous ply constitutes a middle ply between two or more additional fibrous plies, the thickness, density and/or grammage of at least one of the additional plies should be low enough to allow grafting of the foam-formed fibrous ply though the additional ply. This typically means that the thickness of the foam-formed fibrous ply is significantly greater than the thickness of the one or more additional fibrous plies. In embodiments, wherein the flexible sheet, paper or paperboard comprises the foam-formed fibrous ply and one or more additional fibrous plies, the foam-formed fibrous ply preferably constitutes at least 50 wt%, more preferably at least 60 wt%, at least 70 wt%, at least 80 wt%, or at least 90 wt%, of the total thickness of the foam-formed fibrous ply. In embodiments, wherein the flexible sheet, paper or paperboard comprises the foam-formed fibrous ply and one or more additional fibrous plies, the foam-formed fibrous ply preferably constitutes at least 10 wt%, more preferably at least 30 wt%, or at least 50 wt%, of the total basis weight of the paper or paperboard. The inventive flexible sheet, paper or paperboard is preferably for use in wet or damp environments. In some embodiments, the flexible sheet, paper or paperboard is for use in packaging, such as boxes, bags, corrugated board and other containers; fresh and aseptic liquid packaging; boxes, bags, corrugated board, trays, or cups for hot, cold, dry, wet and frozen food and beverages; products for outdoor use such as boxes, bags, signs and posters; pots, trays and covers for plants; packages for construction materials, and construction material. The present disclosure is based on the inventive realization that a flexible sheet, paper or paperboard comprising a foam-formed fibrous ply can be conveniently rendered water-resistant by subjecting the foam-formed fibrous ply or the paper or paperboard comprising the foam-formed fibrous ply to grafting with a fatty acid halide. Fatty acid halide grafting has been found to be very efficient for hydrophobization of paper or paperboard comprising a foam-formed fibrous ply, possibly due to the relatively high porosity of the foam-formed cellulose. Foam- formed fibrous plies comprise an inherent open structure leading to that a higher contact angle towards water is needed compared to less porous structures to prevent water penetration. Surprisingly, fatty acid halide grafting has proven to provide such water-resistance to these porous materials. Furthermore, unlike internal hydrophobic sizing agents, the fatty acid halide grafting maintains, and may even improve, the strength properties of the paper or paperboard. Also, unlike paper or paperboard comprising internal hydrophobic sizing agents and wet strength agents, the fatty acid halide grafted paper or paperboard can be efficiently repulped and recycled or re-used. Using grafting for rendering the foamed fibrous material water-resistant leads to process-related advantages. Thanks to the invention, there is no need (or nearly no need) for addition of internal sizing agents and/or wet-strength agents when producing the foamed material. Instead, the water-resistance property is acquired on an already foamed dry material. Thus, problems with additive chemicals causing foam collapse is avoided. In embodiments wherein the flexible sheet, paper or paperboard consists of the foam-formed fibrous ply, the fatty acid halide can be applied to the foam-formed fibrous ply directly. In embodiments wherein the flexible sheet, paper or paperboard further comprises one or more additional fibrous or non cellulose-base plies, the fatty acid halide can be applied to the paper or paperboard such that the paper or paperboard, including the foam-formed fibrous ply, is rendered water-resistant. Thus, in some embodiments, the paper or paperboard has been subjected to grafting with a fatty acid halide to render the foam-formed fibrous ply water-resistant. In order to render the entire flexible sheet, paper or paperboard water-resistant, the grafting is preferably performed such that grafting of fatty acids to the cellulosic material is achieved through the entire thickness of the foam-formed fibrous ply or the paper or paperboard comprising the foam-formed fibrous ply. Thus, in some embodiments, the foam-formed fibrous ply has been subjected to grafting with a fatty acid halide through the entire thickness of the foam-formed fibrous ply. In some embodiments, it is enough to perform grafting of the fibrous material with a fatty acid halide such that only a surface of said material is rendered water- resistant. The inventive flexible sheet, paper or paperboard is water-resistant. The term “water-resistant” as used herein generally means that the flexible sheet, paper or paperboard with grafted fatty acid halide has a higher resistance to water absorption (e.g. indicated by the Cobb₆₀ value as determined according to standard ISO 535:2014 after 60 seconds) than the same flexible sheet, paper or paperboard without grafted fatty acid halide. In some embodiments, the fatty acid halide grafted on the flexible sheet, paper or paperboard has an aliphatic chain length of 8-22 carbon atoms. Examples of fatty acid halides include octanoyl chloride (C8), lauroyl chloride (C12), myristoyl chloride (C14), palmitoyl chloride (C16), and stearoyl chloride (C18), and/or a mixture thereof. In some embodiments, the fatty acid halide grafted on the flexible sheet, paper or paperboard is a C16 or C18 fatty acid halide, or a mixture thereof. In some preferred embodiments, the fatty acid halide grafted on the flexible sheet, paper or paperboard is palmitoyl chloride or stearoyl chloride. The flexible sheet, paper or paperboard is preferably dry when the fatty acid halide grafting is performed. The term “dry” as used herein means that the paper or paperboard has a dry content above 80 wt%, preferably above 90 wt%, and more preferably above 95 wt%. Grafting of the fatty acid halide to the flexible sheet, paper or paperboard having available hydroxyl groups can be achieved by applying a fatty acid halide to the surface of the flexible sheet, paper or paperboard, followed by penetration of the reagent upon heating, which also promotes the formation of covalent bonds between the fatty acid halide and the hydroxyl groups of the foam-formed fibrous ply. The reaction between the fatty acid halide, e.g. fatty acid chloride, and the hydroxyl groups of the cellulosic material results in ester bonds between the reagent and the foam-formed fibrous ply. Ungrafted and thereby unbound fatty acids may also be present to a certain extent. Upon the reaction with the hydroxyl groups in the foam-formed fibrous ply, and/or with water in the foam-formed fibrous ply and/or in the air, hydrohalic acid, e.g. hydrochloric acid, is formed as a reaction byproduct. The grafting may preferably be followed by removal of the formed hydrohalic acid, and optionally by removal of the ungrafted residues. One example of a grafting process which could be used in production of the water- resistant paper or paperboard of the present disclosure is described in detail in the international patent application WO2012066015A1. Another example of a grafting process, which could be used in production of the water-resistant paper or paperboard in the present disclosure, is described in detail in the international patent application WO2017002005A1. The grafting process may preferably be repeated, in order to increase the amount of grafted and free fatty acids in the paper or paperboard. In order to achieve fatty acid halide grafting through the entire thickness of the paper or paperboard, it has been found that the paper or paperboard may preferably be subjected to grafting with a fatty acid halide at least twice. Thus, in some embodiments, the paper or paperboard has a front surface and a back surface and said paper or paperboard has been subjected to grafting with a fatty acid halide on both the front surface and the back surface or subjected to grafting with a fatty acid halide at least twice on the same surface. In some embodiments, the total amount of grafted and free fatty acids in the flexible sheet, paper or paperboard is in the range of 0.1-4 g/m² of the total dry weight of the paper or paperboard. In some embodiments, the total amount of grafted and free fatty acids in the paper or paperboard is in the range of 0.1-1.5 g/m² of the total dry weight of the paper or paperboard. The method for analyzing the amount of free and grafted fatty acids in the treated substrate is based on the method for AKD analysis. Free fatty acids are extracted from the board sample with an organic solvent and analyzed with GC-FID after silylation. The same board sample is subsequently submitted to alkaline hydrolysis for breaking the ester bonds to cellulose and the released fatty acids are thereafter extracted and analyzed with GC-FID after silylation. The sum of the analyzed free and bound fatty acids constitutes the total amount of fatty acid halide. In some embodiments, a surface of said flexible sheet, paper or paperboard subjected to grafting with a fatty acid halide has a water contact angle above 90°, preferably above 100°. The method for measurement of contact angle (CA) is based on the standard ISO TC 6/SC 2/WG 41: Paper and board - Measurement of water contact angle by optical methods. The fatty acid halide grafting typically results in a flexible sheet, paper or paperboard having a Cobb60 value below 30 g/m². In some embodiments, a surface of said paper or paperboard subjected to grafting with a fatty acid halide has a Cobb₆₀ value (as determined according to standard ISO 535:2014 after 60 seconds) below 30 g/m², preferably below 20 g/m², more preferably below 10 g/m². In some embodiments, the flexible sheet, paper or paperboard subjected to grafting with a fatty acid halide has an edge wick index (Lactic acid 1% solution, 1 h at 23 °C and 50 % relative humidity) below 1 kg/m²h, preferably below 0.7 kg/m²h, and more preferably below 0.4 kg/m²h. The fatty acid halide grafting through the entire thickness of the foam-formed fibrous ply removes the need for a hydrophobic sizing agent. Thus, in some embodiments, the paper or paperboard is free from added hydrophobic sizing agents, or has only a low amount (e.g. less than 30% of the amount normally used) of added hydrophobic sizing agents, for example alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) and/or rosin sizing agent. In preferred embodiments, the paper or paperboard is free from added hydrophobic sizing agents. The fatty acid halide grafting through the entire thickness of the foam-formed fibrous ply also removes the need for an added wet strength agent to improve the wet strength of the foam-formed fibrous ply. Thus, in some embodiments the paper or paperboard is free from added wet strength agents or has only a low amount (e.g. less than 30% of the amount normally used) of added wet strength agents, for example urea-formaldehyde (UF), melamine-formaldehyde (MF) and polyamide-epichlorohydrin (PAE). In preferred embodiments, the paper or paperboard is free from added wet-strength agents. The use of fatty acid halide grafting can also reduce the need for a water-resistant polymer coating on the paper or paperboard, which would reduce the need for an additional coating step, facilitating the processability and savings in material, time, and cost. Unlike paper or paperboard comprising internal hydrophobic sizing agents and wet strength agents, the fatty acid halide grafted flexible sheet, paper or paperboard can be efficiently repulped and recycled. In some embodiments, the flexible sheet, paper or paperboard has a repulpability characterized by a reject rate (as determined according to the PTS RH 021/97 test method) below 20%, preferably below 10%, more preferably below 5%, and most preferably below 1%. According to a second aspect illustrated herein, there is provided a method for manufacturing a water-resistant fibrous material, comprising: a) providing a flexible sheet, paper or paperboard comprising a foam-formed fibrous ply, and b) subjecting said foam-formed fibrous ply to grafting with a fatty acid halide to render the foam-formed fibrous ply water-resistant. In some embodiments, the step a) comprises providing a flexible sheet, paper or paperboard wherein at least one ply is formed by: i) bringing an aqueous pulp suspension comprising cellulosic material, a foaming agent, and optional additives, into a foam, ii) forming a web of the foam, and iii) dewatering and drying the web to obtain a foam-formed fibrous ply. In some embodiments, the flexible sheet, paper or paperboard consists of the foam-formed fibrous ply. In some embodiments, the water-resistant paper or paperboard may further comprise one or more additional fibrous or non fibrous plies. In some embodiments, the flexible sheet, paper or paperboard comprises the foam-formed fibrous ply, and one or more additional fibrous plies. The additional fibrous plies may be foam-formed or non foam-formed. An important property of paperboard is the bending stiffness. The bending stiffness in paperboard is often built up by having outer plies with high tensile stiffness and one or more bulky plies in between, so that the outer plies are placed at a desired distance from each other. The bulky plies are often referred to as middle layer or middle layers. In some embodiments, the foam-formed fibrous ply constitutes a middle ply between two or more additional fibrous plies, preferably the foam-formed fibrous ply constitutes a middle ply between two or more non foam-formed fibrous plies. In embodiments, wherein the flexible sheet, paper or paperboard comprises the foam-formed fibrous ply and one or more additional fibrous plies, the foam-formed fibrous ply preferably constitutes at least 50 wt%, more preferably at least 60 wt%, at least 70 wt%, at least 80 wt%, or at least 90 wt%, of the total thickness of the foam-formed fibrous ply. In embodiments, wherein the flexible sheet, paper or paperboard comprises the foam-formed fibrous ply and one or more additional fibrous plies, the foam-formed fibrous ply preferably constitutes at least 10 wt%, more preferably at least 30 wt%, or at least 50 wt%, of the total basis weight of the paper or paperboard. In embodiments wherein the flexible sheet, paper or paperboard consists of the foam-formed fibrous ply, the fatty acid halide can be applied to the foam-formed fibrous ply directly. In embodiments wherein the flexible sheet, paper or paperboard further comprises one or more additional fibrous or non cellulose-base plies, the fatty acid halide can be applied to the paper or paperboard such that the flexible sheet, paper or paperboard, including the foam-formed fibrous ply, is rendered water-resistant. Thus, in some embodiments, the flexible sheet, paper or paperboard is subjected to grafting with a fatty acid halide to render the foam-formed fibrous ply water- resistant. In order to render the entire flexible sheet, paper or paperboard water-resistant, the grafting is preferably performed such that grafting of fatty acids to the cellulosic material is achieved through the entire thickness of the foam-formed fibrous ply or the paper or paperboard comprising the foam-formed fibrous ply. Thus, in some embodiments, the foam-formed fibrous ply is subjected to grafting with a fatty acid halide through the entire thickness of the foam-formed fibrous ply. The grafting may be performed as described above with reference to the first aspect. The flexible sheet, paper or paperboard is preferably dry when the fatty acid halide grafting is performed. The term “dry” as used herein means that the paper or paperboard has a dry content above 80 wt%, preferably above 90 wt%, and more preferably above 95 wt%. In some embodiments, the grafting comprises contacting at least one surface of the flexible sheet, paper or paperboard with a fatty acid halide in a liquid, spray and/or vapor state. The fatty acid halide grafting in step b) results in a paper or paperboard having a Cobb₆₀ value below 30 g/m². In some embodiments, the paper or paperboard subjected to grafting with a fatty acid halide has a Cobb60 value below 20 g/m², preferably below 15 g/m². Using a paper or paperboard comprising a foam-formed fibrous ply which has a higher permeability for fatty acid halides can facilitate grafting through the entire thickness of the foam-formed fibrous ply. Thus, in some embodiments the grafting is only performed once. In order to achieve fatty acid halide grafting through the entire thickness of the foam-formed fibrous ply, it has been found that the foam-formed fibrous ply should preferably be subjected to grafting with a fatty acid halide at least twice. In some embodiments, the paper or paperboard has a front surface and a back surface and said substrate has been subjected to grafting with a fatty acid halide on both the front surface and the back surface, or subjected to grafting with a fatty acid halide at least twice on the same surface. According to a third aspect illustrated herein, there is provided a carton blank comprising a water-resistant paper or paperboard according to the first aspect. According to a fourth aspect illustrated herein, there is provided a container, comprising a water-resistant paper or paperboard according to the first aspect. While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not 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.

Example - Treatment of foam-formed board with fatty acid chloride A foam-formed 1-ply board made of 100% bleached pine was grafted by subjecting both sides of the board to palmitoyl chloride (C16) followed by heat treatment at 190 °C. The grammage of the foamed board was 150 g/m², the thickness was 300 μ and the density was 500kg/m³. Air permeance for the foamed board was 45 μm/(Pa∙s). Air permeance was measured using the Bendtsen method (ISO 5636-3). As shown in Table 1 the untreated reference sample showed very poor surface hydrophobicity with a high Cobb 60 value and not measurable contact angle. After grafting the surface hydrophobicity was greatly improved showing a low Cobb 60 value and a very high contact angle. The untreated reference showed no protection against edge penetration but after grafting very low LA wick values were reached. Table 1 Cobb 60s contact angle and edge wick values Cobb₆₀ analyses were performed by ISO 535:2014 for 60 seconds. The method for measurement of contact angle (CA) is based on the standard ISO TC 6/SC 2/WG 41: Paper and board - Measurement of water contact angle by optical methods. Edge-wick penetration testing was performed with a lactic acid (LA) solution (1 %) for 1 h at 23 °C and 50% RH.

Claims

CLAIMS 1. Water-resistant fibrous material comprising a foam-formed fibrous ply, wherein said foam-formed fibrous ply has been subjected to grafting with a fatty acid halide to render the foam-formed fibrous ply water-resistant.

2. Water-resistant fibrous material according to any one of the preceding claims, wherein said foam-formed fibrous ply has been subjected to grafting with a fatty acid halide through the entire thickness of the foam-formed fibrous ply.

3. Water-resistant fibrous material according to any one of the preceding claims, wherein the foam-formed fibrous ply consists of a flexible sheet for use as lightweight carrier bags and sacks, shopping bags, refuse sacks and for wrappings of goods, said flexible sheet comprising a stretch in the range of 5-45%, measured according to the method ISO 1924-2:2008.

4. Water-resistant fibrous material according to any one of the preceding claims, wherein said foam-formed fibrous ply consists of paper or paperboard.

5. Water-resistant fibrous material according to claim 4, wherein said paper or paperboard comprises the foam-formed fibrous ply, and one or more additional fibrous plies.

6. Water-resistant fibrous material according to any one of the preceding claims, wherein the foam-formed fibrous ply constitutes a middle ply between two or more additional fibrous plies.

7. Water-resistant fibrous material according to any one of the preceding claims, wherein said flexible sheet, paper or paperboard has been subjected to grafting with a fatty acid halide to render the foam-formed fibrous ply water-resistant.

8. Water-resistant fibrous material according to any one of the preceding claims, wherein said foam-formed fibrous ply has a front surface and a back surface and said foam-formed fibrous ply has been subjected to grafting with a fatty acid halide on both the front surface and the back surface or subjected to grafting with a fatty acid halide at least twice on the same surface.

9. Water-resistant fibrous material according to any one of the preceding claims, wherein the density of said foam-formed fibrous ply is below 700 kg/m³, preferably below 600 kg/m³, below 500 kg/m³, or below 400 kg/m³.

10. Water-resistant fibrous material according to any one of the preceding claims, wherein the thickness of said foam-formed fibrous ply is above 30 μm, preferably above 100 μm.

11. Water-resistant fibrous material according to any one of the preceding claims, wherein the basis weight of said foam-formed fibrous ply is in the range of 15-500 g/m², preferably in the range of 100-400 g/m².

12. Water-resistant fibrous material according to any one of the preceding claims, wherein the foam-formed fibrous ply constitutes at least 50 wt%, more preferably at least 60 wt%, at least 70 wt%, at least 80 wt%, or at least 90 wt%, of the total thickness of the foam-formed fibrous ply.

13. Water-resistant fibrous material according to any one of the preceding claims, wherein the foam-formed fibrous ply constitutes at least 10 wt%, more preferably at least 30 wt%, or at least 50 wt%, of the total basis weight of the foam-formed fibrous ply.

14. Water-resistant fibrous material according to any one of the preceding claims, wherein the total amount of grafted and free fatty acids in the foam-formed fibrous ply is in the range of 0.1-1.5 g/m² of the total dry weight of the foam-formed fibrous ply.

15. Water-resistant fibrous material according to any one of the preceding claims, wherein a surface of said foam-formed fibrous ply subjected to grafting with a fatty acid halide has a water contact angle above 90°, preferably above 100°.

16. Water-resistant fibrous material according to any one of the preceding claims, wherein a surface of said foam-formed fibrous ply subjected to grafting with a fatty acid halide has a Cobb60 value (as determined according to standard ISO 535:2014 after 60 seconds) below 30 g/m², preferably below 20 g/m², more preferably below 10 g/m².

17. Water-resistant fibrous material according to any one of the preceding claims, wherein said foam-formed fibrous ply subjected to grafting with a fatty acid halide has an edge wick index (Lactic acid 1% solution, 1 h at 23 °C and 50 % relative humidity) below 1 kg/m²h, preferably below 0.7 kg/m²h, and more preferably below 0.4 kg/m²h.

18. Water-resistant fibrous material according to any one of the preceding claims, wherein said foam-formed fibrous ply is free from added hydrophobic sizing agents, for example alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) and/or rosin sizing agent.

19. Water-resistant fibrous material according to any one of the preceding claims, wherein said foam-formed fibrous ply is free from added wet-strength agents, for example urea-formaldehyde (UF), melamine-formaldehyde (MF) and polyamide-epichlorohydrin (PAE).

20. Water-resistant fibrous material according to any one of the preceding claims, wherein said foam-formed fibrous ply has a repulpability characterized by a reject rate (as determined according to the PTS RH 021/97 test method) below 20%, preferably below 10%, more preferably below 5%, and most preferably below 1%.

21. Water-resistant fibrous material according to any one of the preceding claims, wherein said foam-formed fibrous ply is for use in wet or damp environments.

22. A method for manufacturing a water-resistant fibrous material, said method comprising: a) providing a fibrous material comprising a foam-formed fibrous ply, and b) subjecting said foam-formed fibrous ply to grafting with a fatty acid halide to render the foam-formed fibrous ply water-resistant.

23. A method according to claim 22, wherein step a) comprises providing a fibrous material in the form of flexible sheet, paper or paperboard wherein at least one ply is formed by: i) bringing an aqueous pulp suspension comprising cellulosic material, a foaming agent, and optional additives, into a foam, ii) forming a web of the foam, and iii) dewatering and drying the web to obtain a foam-formed fibrous ply.

24. A method according to any one of claims 22-23, wherein step a) comprises subjecting said foam-formed fibrous ply to grafting with a fatty acid halide to render the foam-formed fibrous ply water-resistant.

25. A method according to any one of claims 22-24, wherein the grafting comprises contacting at least one surface of the foam-formed fibrous ply with a fatty acid halide in a liquid, spray and/or vapor state.

26. A method according to any one of claims 22-25, wherein the grafting is only performed once.