SE544175C2

SE544175C2 - Water-resistant paper or paperboard

Info

Publication number: SE544175C2
Application number: SE2050753A
Authority: SE
Inventors: Isto Heiskanen; Kaj Backfolk; Raija Bådenlid; Susanne Hansson
Original assignee: Stora Enso Oyj
Priority date: 2020-06-24
Filing date: 2020-06-24
Publication date: 2022-02-22
Also published as: CN115917083A; WO2021260606A1; KR20230026353A; SE2050753A1; CA3186605A1; EP4172412A1; US20230243105A1

Abstract

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

Description

WATER-RESISTANT PAPER OR PAPERBOARD Technical field The present disclosure relates to water-resistant paper or paperboard for use inwet or damp environments. More specifically, the present disclosure relates to water-resistant paper or paperboard comprising a foam-formed cellulose-based Ply- BackgroundFoam-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 longerfibers than in traditional papermaking forming processes, improved control of webstructure, and manufacturing of paper and board having low density compared toconventionally formed paper and board. ln foam-forming technology an aqueous foam is used instead of water as a carriermedium in the manufacturing of fiber-based products. The presence of air bubblesin the aqueous medium changes the properties of the medium fundamentally. Thetechnology 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 asepticliquid 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 forconstruction 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 examplealkyl 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 tocombine the mechanical properties of the paperboard with the barrier and sealingproperties of a plastic film. Also in plastic coated paperboard, the board is oftentreated 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 thehydrogen bonding between the cellulose fibers, giving a debonding effect andhence a weaker material. To compensate for the weaker material, the grammageof paper and board is increased leading to higher carbon footprint due to overuseof wood fibers and higher transport weight at all stages downstream theproduction.

To improve the wet strength of the material, the internal sizing agent is oftencombined with a wet strength agent. A wet-strength agent improves the tensileproperties of the paper or paperboard in the wet state by for example covalentlybinding to the cellulose fibers and also by forming crosslinking network betweenthe fibers or fibers and other dry strength agents. Common wet strength agentsinclude urea-formaldehyde (UF), melamine-formaldehyde (MF) and polyamide-epichlorohydrin (PAE). Other wet strength agents can give wet-strength by othermechanisms, and some of these wet strength agents can also have a temporarywet-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 andwet-strength agents can cause unexpected effects on the foam structure or evenfoam collapse. Surface treatment of the foam-formed structure with hydrophobicagents is possible, but traditional concepts may not work since the high bulkmeans 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 paperboardcomprising a foam-formed cellulose-based ply water resistant, without weakeningthe material and without reducing the repulpability of the material.

Description of the inventionlt is an object of the present disclosure to provide a water-resistant paper orpaperboard comprising a foam-formed cellulose-based ply with high repulpability. lt is a further object of the present disclosure to provide a water-resistant paper orpaperboard comprising a foam-formed cellulose-based ply with improved wetstrength and similar repulpability as compared to a corresponding non water- resistant paper or paperboard. lt is a further object of the present disclosure to provide a water-resistant paper orpaperboard which is free from added hydrophobic sizing agents or which has onlya low amount of added hydrophobic sizing agents, for example alkyl ketene dimer(AKD), alkenyl succinic anhydride (ASA) and/or rosin-sizing agent. lt is a further object of the present disclosure to provide a water-resistant paper orpaperboard which is free from added wet strength agents or which has only a lowamount of added wet strength agents, particularly crosslink-forming wet strengthagents, 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 paperor 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 theskilled person in the light of the present disclosure, are achieved by the variousaspects of the present disclosure.

According to a first aspect illustrated herein, there is provided a water-resistant paper or paperboard comprising a foam-formed cellulose-based ply, wherein said foam-formed cellulose-based p|y has been subjected to grafting witha fatty acid ha|ide to render the foam-formed cellulose-based p|y water-resistant.

The paper or paperboard is preferably a sheet or web of material formed from apulp of wood or other fibrous substances comprising cellulose fibers. The paper orpaperboard can be used as such, or it can be provided with various coatingsand/or be used as a p|y in a multi-p|y laminate comprising one or more additionalcellulose-based or non cellulose-based plies.

Paper generally refers to a material manufactured in sheets from the pulp of woodor 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 comprisingcellulose fibers used for example in boxes, bags and other types of packaging.Paperboard can be comprised of one or more plies. Paperboard can either bebleached or unbleached, coated or uncoated, and produced in a variety ofthicknesses, depending on the end-use requirements.

The paper or paperboard useful in the present disclosure may typically have abasis weight in the range of 15-500 g/m2. Thanks to the foam-formed cellulose-based ply, the paper or paperboard may typically have a density below 700 kg/m3.ln some embodiments, the density of the paper or paperboard is below 600 kg/m3, preferably below 500 kg/m3, or below 400 kg/m3. Although in thinner papers or paperboards the density can be higher, as long as the basis weight is still not toohigh. ln thinner papers, having a thickness below about 100 pm, the density mayfor example be in the range of 600-1400 or in the range of 700-1200 kg/m3. Higherdensity and thickness, resulting in a higher basis weight, prevent grafting throughthe entire thickness of the foam-formed cellulose-based ply.

The thickness of the paper or paperboard may range from thin to very thick. lnsome embodiments, the thickness of said paper or paperboard is above 30 pm,preferably above 100 pm. ln some embodiments, the thickness of the paper orpaperboard is in the range of 30 pm to 10 mm. ln some embodiments, the thickness of the paper or paperboard is in the range of 100 pm to 1 mm. ln some embodiments, the basis weight of said paper or paperboard is in therange of 15-500 g/m2, preferably in the range of 100-400 g/m2. ln some embodiments, the air permeance of the paper or paperboard is in the range of 10-100 pm/(Pa-s) as measured using the Bendtsen method (ISO 5636-3).

The inventive water-resistant paper or paperboard comprises a foam-formedcellulose-based ply. Foam-forming technology enables improving paper propertiesand manufacturing high porosity and light weight products. lt also offers apossibility to decrease raw material and production costs remarkably compared toconventional paper forming methods.

Existing paper and board machines can be modified for foam-forming technologyto make high quality bulkier sheet structures with excellent formation and while also reducing energy and chemical consumption. ln foam-forming, a large amount of air is added to an aqueous pulp suspension inthe 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 isneeded to increase the surface area. This can occur by agitation, dispersing alarge volume of gas into a liquid, or injecting a gas into a liquid. The secondrequirement is that a foam forming agent, typically an amphiphilic substance, asurfactant or surface active component, must be present to decrease surface tension. Finally, the foam must form more quickly than it breaks down. ln some embodiments, the foaming required for the foam-forming is achievedusing a foam generator. The pulp suspension may be pumped through a foamgenerator one or several times in order to reach a desired gas content or foamdensity. ln some embodiments, the pulp suspension is pumped via a high shearmixer or refiner which generates the foam. Foam can be generated either offline or inline at the paper machine. ln 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 tofoaming 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 pulpsuspension. The surfactant may be a small molecule surfactant or a polymericsurfactant or a mixture thereof. The amount of surfactant in the foam may typicallybe in the range of 0.005 to 30 wt%, but will be easily determinable by a skilledperson. An example of a small molecule surfactant useful for the foam forming issodium dodecyl sulfate (SDS). The amount of SDS in the foam may typically be inthe range of 0.005 to 10 wt%, for example about 0.02 wt%. Examples of apolymeric surfactant useful for the foam forming include polyvinyl alcohol (PVOH)and partially hydrolyzed polyvinyl acetate (PVOH/Ac). The amount of polyvinylalcohol (PVOH) or partially hydrolyzed polyvinyl acetate (PVOH/Ac) in the foammay 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) isof particular interest as a surfactant in the foam-formed cellulose-based 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 polymericsurfactants, such as PVOH can contribute to the mechanical properties and mayalso be less prone to leaching or migration from the finished foam-formedcellulose-based ply, which may be valuable in certain applications. Thus, in somepreferred embodiments the foaming agent is a polymeric foaming agent. ln someembodiments, the polymeric foaming agent is selected from the group consistingof optionally hydrophobically modified polysaccharides, proteins, po|yviny| alcohol(PVOH), partially hydrolyzed po|yviny| acetate (PVOH/Ac), and mixtures thereof.The optional hydrophobic modification typically comprises one or morehydrophobic groups, e.g. alkyl groups, covalently attached to the foaming agent. lna preferred embodiment, the foaming agent is po|yviny| alcohol (PVOH) or partiallyhydrolyzed po|yviny| acetate (PVOH/Ac).

The pulp suspension, and consequently also the foam-formed cellulose-based plycan be made from the pulp of wood or other fibrous substances comprisingcellulose fibers. The foam-formed cellulose-based ply can comprise softwood orhardwood pulp. The foam forming process also allows addition of longer syntheticof natural fibers, for example reinforcement pulp, polylactic acid (PLA) fibers,po|yviny| alcohol (PVOH) fibers or man-made cellulosic fibers such as regeneratedcellulose, such as viscose or lyocell fibers. The longer fibers may be present in anamount of 3-20 wt%, based on the total dry weight of the foam-formed cellulose- based ply.

The foam-forming can be performed at an industrial scale on a running formingfabric of a paper or paperboard machine, wherein a web of the foam is applied tothe forming fabric, the web is dewatered by suction through the web and theforming fabric, and finally dried in a drying section of the paper or paperboard machine. ln some embodiments, the paper or paperboard consists of the foam-formed cellulose-based ply. ln some embodiments, the water-resistant paper or paperboard may further comprise one or more additional cellulose-based or non cellulose-base plies. ln some embodiments, the paper or paperboard comprises the foam-formed cellulose-based ply, and one or more additional cellulose-based plies.

The additional cellulose-based plies may be foam-formed or non foam-formed.

An important property of paperboard is the bending stiffness. The bendingstiffness in paperboard is often built up by having outer plies with high tensilestiffness and one or more bulky plies in between, so that the outer plies are placedat a desired distance from each other. The bulky plies are often referred to asmiddle layer or middle layers. ln some embodiments, the foam-formed cellulose-based ply constitutes a middle ply between two or more additional cellulose-basedplies, preferably the foam-formed cellulose-based ply constitutes a middle ply between two or more non foam-formed cellulose-based plies. ln embodiments, wherein the foam-formed cellulose-based ply constitutes amiddle ply between two or more additional cellulose-based plies, the thickness,density and/or grammage of at least one of the additional plies should be lowenough to allow grafting of the foam-formed cellulose-based ply though theadditional ply. This typically means that the thickness of the foam-formedcellulose-based ply is significantly greater than the thickness of the one or more additional cellulose-based plies. ln embodiments, wherein the paper or paperboard comprises the foam-formedcellulose-based ply and one or more additional cellulose-based plies, the foam-formed cellulose-based ply preferably constitutes at least 50 wt%, more preferablyat least 60 wt%, at least 70 wt%, at least 80 wt%, or at least 90 wt%, of the totalthickness of the foam-formed cellulose-based ply. ln embodiments, wherein the paper or paperboard comprises the foam-formedcellulose-based ply and one or more additional cellulose-based plies, the foam-formed cellulose-based ply preferably constitutes at least 10 wt%, more preferablyat least 30 wt%, or at least 50 wt%, of the total basis weight of the paper or paperboard.

The inventive paper or paperboard is preferably for use in wet or dampenvironments. ln some embodiments, the paper or paperboard is for use inpackaging, such as boxes, bags, corrugated board and other containers; fresh andaseptic liquid packaging; boxes, bags, corrugated board, trays, or cups for hot,cold, dry, wet and frozen food and beverages; products for outdoor use such asboxes, bags, signs and posters; pots, trays and covers for plants; packages forconstruction materials, and construction material.

The present disclosure is based on the inventive realization that a paper orpaperboard comprising a foam-formed cellulose-based p|y can be convenientlyrendered water-resistant by subjecting the foam-formed cellulose-based p|y or thepaper or paperboard comprising the foam-formed cellulose-based p|y to graftingwith a fatty acid halide. Fatty acid ha|ide grafting has been found to be veryefficient for hydrophobization of paper or paperboard comprising a foam-formedcellulose-based p|y, possibly due to the relatively high porosity of the foam-formedcellulose. Unlike internal hydrophobic sizing agents, the fatty acid ha|ide graftingmaintains, and may even improve, the strength properties of the paper orpaperboard. Furthermore, unlike paper or paperboard comprising internalhydrophobic sizing agents and wet strength agents, the fatty acid ha|ide graftedpaper or paperboard can be efficiently repulped and recycled or re-used. ln embodiments wherein the paper or paperboard consists of the foam-formedcellulose-based p|y, the fatty acid ha|ide can be applied to the foam-formed cellulose-based p|y directly. ln embodiments wherein the paper or paperboard further comprises one or moreadditional cellulose-based or non cellulose-base plies, the fatty acid ha|ide can beapplied to the paper or paperboard such that the paper or paperboard, includingthe foam-formed cellulose-based p|y, is rendered water-resistant. Thus, in someembodiments, the paper or paperboard has been subjected to grafting with a fattyacid ha|ide to render the foam-formed cellulose-based p|y water-resistant. ln order to render the entire paper or paperboard water-resistant, the grafting ispreferably performed such that grafting of fatty acids to the cellulosic material isachieved through the entire thickness of the foam-formed cellulose-based ply orthe paper or paperboard comprising the foam-formed cellulose-based ply. Thus, insome embodiments, the foam-formed cellulose-based ply has been subjected tografting with a fatty acid ha|ide through the entire thickness of the foam-formed cellulose-based ply.

The inventive paper or paperboard is water-resistant. The term "water-resistant" asused herein generally means that the paper or paperboard with grafted fatty acidha|ide has a higher resistance to water absorption (e.g. indicated by the Cobbeovalue as determined according to standard ISO 535:2014 after 60 seconds) than the same paper or paperboard without grafted fatty acid ha|ide. ln some embodiments, the fatty acid ha|ide grafted on the paper or paperboardhas an aliphatic chain length of 8-22 carbon atoms. Examples of fatty acid halidesinclude octanoyl chloride (C8), lauroyl chloride (C12), myristoyl chloride (C14),palmitoyl chloride (C16), and stearoyl chloride (C18), and/or a mixture thereof. lnsome embodiments, the fatty acid ha|ide grafted on the paper or paperboard is aC16 or C18 fatty acid ha|ide, or a mixture thereof. ln some preferred embodiments,the fatty acid ha|ide grafted on the paper or paperboard is palmitoyl chloride or stearoyl chloride.

The paper or paperboard is preferably dry when the fatty acid ha|ide grafting isperformed. The term "dry" as used herein means that the paper or paperboard hasa dry content above 80 wt%, preferably above 90 wt%, and more preferably above95 wt%.

Grafting of the fatty acid ha|ide to the paper or paperboard having availablehydroxyl groups can be achieved by applying a fatty acid ha|ide to the surface ofthe paper or paperboard, followed by penetration of the reagent upon heating,which also promotes the formation of covalent bonds between the fatty acid ha|ideand the hydroxyl groups of the foam-formed cellulose-based ply. The reaction between the fatty acid ha|ide, e.g. fatty acid chloride, and the hydroxyl groups of 11 the cellulosic material results in ester bonds between the reagent and the foam-formed cellulose-based ply. Ungrafted and thereby unbound fatty acids may alsobe present to a certain extent. Upon the reaction with the hydroxyl groups in thefoam-formed cellulose-based ply, and/or with water in the foam-formed cellulose-based ply and/or in the air, hydrohalic acid, e.g. hydrochloric acid, is formed as areaction byproduct. The grafting may preferably be followed by removal of theformed hydrohalic acid, and optionally by removal of the ungrafted residues. Oneexample 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 theinternational patent application WO2012066015A1. Another example of a graftingprocess, which could be used in production of the water-resistant paper orpaperboard in the present disclosure, is described in detail in the internationalpatent application WO2017002005A1.

The grafting process may preferably be repeated, in order to increase the amountof grafted and free fatty acids in the paper or paperboard. ln order to achieve fattyacid halide grafting through the entire thickness of the paper or paperboard, it hasbeen found that the paper or paperboard may preferably be subjected to graftingwith a fatty acid halide at least twice. Thus, in some embodiments, the paper orpaperboard has a front surface and a back surface and said paper or paperboardhas been subjected to grafting with a fatty acid halide on both the front surface andthe back surface or subjected to grafting with a fatty acid halide at least twice onthe same surface. ln some embodiments, the total amount of grafted and free fatty acids in the paperor paperboard is in the range of 0.1-4 g/m2 of the total dry weight of the paper orpaperboard. ln some embodiments, the total amount of grafted and free fatty acidsin the paper or paperboard is in the range of 0.1 -1 .5 g/m2 of the total dry weight ofthe paper or paperboard. The method for analyzing the amount of free and graftedfatty acids in the treated substrate is based on the method for AKD analysis. Freefatty acids are extracted from the board sample with an organic solvent andanalyzed with GC-FID after silylation. The same board sample is subsequentlysubmitted to alkaline hydrolysis for breaking the ester bonds to cellulose and the released fatty acids are thereafter extracted and analyzed with GC-FID after 12 silylation. The sum of the analyzed free and bound fatty acids constitutes the totalamount of fatty acid halide. ln some embodiments, a surface of said paper or paperboard subjected to graftingwith a fatty acid halide has a water contact angle above 90°, preferably above100°. The method for measurement of contact angle (CA) is based on thestandard ISO TC 6/SC 2/WG 41: Paper and board - Measurement of water contactangle by optical methods.

The fatty acid halide grafting typically results in a paper or paperboard having aCobbeo value below 30 g/m2. ln some embodiments, a surface of said paper orpaperboard subjected to grafting with a fatty acid halide has a Cobbeo value (asdetermined according to standard ISO 535:2014 after 60 seconds) below 30 g/m2,preferably below 20 g/m2, more preferably below 10 g/m2. ln some embodiments, the paper or paperboard subjected to grafting with a fattyacid 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 morepreferably below 0.4 kg/m2h.

The fatty acid halide grafting through the entire thickness of the foam-formedcellulose-based ply removes the need for a hydrophobic sizing agent. Thus, insome embodiments, the paper or paperboard is free from added hydrophobicsizing agents, or has only a low amount (e.g. less than 30% of the amountnormally used) of added hydrophobic sizing agents, for example alkyl ketenedimer (AKD), alkenyl succinic anhydride (ASA) and/or rosin sizing agent. lnpreferred embodiments, the paper or paperboard is free from added hydrophobicsizing agents.

The fatty acid halide grafting through the entire thickness of the foam-formedcellulose-based ply also removes the need for an added wet strength agent toimprove the wet strength of the foam-formed cellulose-based ply. Thus, in someembodiments 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 13 wet strength agents, for example urea-formaldehyde (UF), melamine-formaldehyde (MF) and polyamide-epichlorohydrin (PAE). ln 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-resistantpolymer 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 wetstrength agents, the fatty acid halide grafted paper or paperboard can be efficientlyrepulped and recycled. ln some embodiments, the paper or paperboard has arepulpability characterized by a reject rate (as determined according to the PTSRH 021/97 test method) below 20%, preferably below 10%, more preferably below5%, and most preferably below 1%.

According to a second aspect illustrated herein, there is provided a method formanufacturing a water-resistant paper or paperboard, comprising: a) providing a paper or paperboard comprising a foam-formed cellulose-basedply, and b) subjecting said foam-formed cellulose-based ply to grafting with a fatty acidhalide to render the foam-formed cellulose-based ply water-resistant. ln some embodiments, the step a) comprises providing a paper or paperboardwherein 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 cellulose-based ply. 14 ln some embodiments, the paper or paperboard consists of the foam-formed cellulose-based ply. ln some embodiments, the water-resistant paper or paperboard may furthercomprise one or more additional cellulose-based or non cellulose-based plies. ln some embodiments, the paper or paperboard comprises the foam-formedcellulose-based ply, and one or more additional cellulose-based plies.

The additional cellulose-based plies may be foam-formed or non foam-formed.

An important property of paperboard is the bending stiffness. The bendingstiffness in paperboard is often built up by having outer plies with high tensilestiffness and one or more bulky plies in between, so that the outer plies are placedat a desired distance from each other. The bulky plies are often referred to asmiddle layer or middle layers. ln some embodiments, the foam-formed cellulose-based ply constitutes a middle ply between two or more additional cellulose-basedplies, preferably the foam-formed cellulose-based ply constitutes a middle ply between two or more non foam-formed cellulose-based plies. ln embodiments, wherein the paper or paperboard comprises the foam-formedcellulose-based ply and one or more additional cellulose-based plies, the foam-formed cellulose-based ply preferably constitutes at least 50 wt%, more preferablyat least 60 wt%, at least 70 wt%, at least 80 wt%, or at least 90 wt%, of the total thickness of the foam-formed cellulose-based ply. ln embodiments, wherein the paper or paperboard comprises the foam-formedcellulose-based ply and one or more additional cellulose-based plies, the foam-formed cellulose-based ply preferably constitutes at least 10 wt%, more preferablyat least 30 wt%, or at least 50 wt%, of the total basis weight of the paper or paperboard. ln embodiments wherein the paper or paperboard consists of the foam-formedcellulose-based ply, the fatty acid halide can be applied to the foam-formed cellulose-based p|y directly. ln embodiments wherein the paper or paperboard further comprises one or moreadditional cellulose-based or non cellulose-base plies, the fatty acid halide can beapplied to the paper or paperboard such that the paper or paperboard, includingthe foam-formed cellulose-based ply, is rendered water-resistant. Thus, in someembodiments, the paper or paperboard is subjected to grafting with a fatty acidhalide to render the foam-formed cellulose-based p|y water-resistant. ln order to render the entire paper or paperboard water-resistant, the grafting ispreferably performed such that grafting of fatty acids to the cellulosic material isachieved through the entire thickness of the foam-formed cellulose-based p|y orthe paper or paperboard comprising the foam-formed cellulose-based p|y. Thus, insome embodiments, the foam-formed cellulose-based p|y is subjected to graftingwith a fatty acid halide through the entire thickness of the foam-formed cellulose- based p|y.

The grafting may be performed as described above with reference to the first aspect.

The paper or paperboard is preferably dry when the fatty acid halide grafting isperformed. The term "dry" as used herein means that the paper or paperboard hasa dry content above 80 wt%, preferably above 90 wt%, and more preferably above95 wt%. ln some embodiments, the grafting comprises contacting at least one surface ofthe paper or paperboard with a fatty acid halide in a liquid, spray and/or vaporstate.

The fatty acid halide grafting in step b) results in a paper or paperboard having aCobbeo value below 30 g/m2. ln some embodiments, the paper or paperboard 16 subjected to grafting with a fatty acid halide has a Cobbeo value below 20 g/m2,preferably below 15 g/m2.

Using a paper or paperboard comprising a foam-formed cellulose-based ply whichhas a higher permeability for fatty acid halides can facilitate grafting through theentire thickness of the foam-formed cellulose-based ply. Thus, in some embodiments the grafting is only performed once. ln order to achieve fatty acid halide grafting through the entire thickness of thefoam-formed cellulose-based ply, it has been found that the foam-formedcellulose-based ply should preferably be subjected to grafting with a fatty acidhalide at least twice. ln some embodiments, the paper or paperboard has a frontsurface and a back surface and said substrate has been subjected to grafting witha fatty acid halide on both the front surface and the back surface, or subjected tografting with a fatty acid halide at least twice on the same surface.

According to a third aspect illustrated herein, there is provided a carton blankcomprising 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 exemplaryembodiments, it will be understood by those skilled in the art that various changesmay be made and equivalents may be substituted for elements thereof withoutdeparting from the scope of the invention. ln addition, many modifications may bemade to adapt a particular situation or material to the teachings of the inventionwithout departing from the essential scope thereof. Therefore, it is intended thatthe invention not be limited to the particular embodiment disclosed as the bestmode contemplated for carrying out this invention, but that the invention willinclude all embodiments falling within the scope of the appended claims. 17 Example - Treatment of foam-formed board with fatty acid chloride A foam-formed 1-ply board made of 100% bleached pine was grafted bysubjecting both sides of the board to palmitoyl chloride (C16) followed by heattreatment at 190 °C.

The grammage of the foamed board was 150 g/m2, the thickness was 300 u andthe density was kg/m3. Air permeance for the foamed board was 45 um/(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 surfacehydrophobicity with a high Cobb 60 value and not measurable contact angle. Aftergrafting 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 Sample Cobb 60s Contact angle, LA wick index (9/m2) 58 (°) (kg/mf h)Untreated Foam board reference 385 N/A SoakedTreated Foam board 15 135 0.22 Cobbeo analyses were performed by ISO 535:2014 for 60 seconds.

The method for measurement of contact angle (CA) is based on the standard ISOTC 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

1. Water-resistant paper or paperboard comprising a foam-formed cellulose-based ply, wherein said foam-formed cellulose-based p|y has been subjected to grafting witha fatty acid halide to render the foam-formed cellulose-based p|y water-resistant.

2. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein said foam-formed cellulose-based p|y has been subjected tografting with a fatty acid halide through the entire thickness of the foam-formed cellulose-based ply.

3. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein said paper or paperboard consists of the foam-formed cellulose- based ply.

4. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein said paper or paperboard comprises the foam-formed cellulose-based ply, and one or more additional cellulose-based p|ies.

5. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein the foam-formed cellulose-based p|y constitutes a middle p|ybetween two or more additional cellulose-based p|ies.

6. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein said paper or paperboard has been subjected to grafting with afatty acid halide to render the foam-formed cellulose-based p|y water-resistant.

7. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein said paper or paperboard has a front surface and a back surfaceand 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 witha fatty acid halide at least twice on the same surface.

8. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein the density of said paper or paperboard is below 700 kg/ms,preferably below 600 kg/ms, below 500 kg/m3, or below 400 kg/m3.

9. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein the thickness of said paper or paperboard is above 30 um,preferably above 100 um.

10. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein the basis weight of said paper or paperboard is in the range of 15-500 g/m2, preferably in the range of 100-400 g/m2.

11. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein the foam-formed cellulose-based ply constitutes at least 50 wt%,more preferably at least 60 wt°/>, at least 70 wt°/>, at least 80 wt°/-.~, or at least 90wt°/-.~, of the total thickness of the foam-formed cellulose-based ply.

12. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein the foam-formed cellulose-based ply 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.

13. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein the total amount of grafted and free fatty acids in the paper orpaperboard is in the range of 0.1 -1 .5 g/m2 of the total dry weight of the paper orpaperboard.

14. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein a surface of said paper or paperboard subjected to grafting with afatty acid halide has a water contact angle above 90 °, preferably above 100 °.

15. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein a surface of said paper or paperboard subjected to grafting with afatty acid halide has a Cobbeo value, as determined according to standard ISO535:2014 after 60 seconds, below 30 g/m2, preferably below 20 g/m2, morepreferably below 10 g/m2.

16. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein said paper or paperboard subjected to grafting with a fatty acidhalide 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 morepreferably below 0.4 kg/m2h.

17. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein said paper or paperboard is free from added hydrophobic sizingagents, for example alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) and/or rosin sizing agent.

18. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein said paper or paperboard is free from added wet-strength agents,for example urea-formaldehyde (UF), melamine-formaldehyde (l\/IF) andpolyamide-epichlorohydrin (PAE).

19. Water-resistant paper or paperboard according to any one of the precedingclaims, wherein said paper or paperboard has a repulpability characterized by areject rate, as determined according to the PTS RH 021/97 test method, below20%, preferably below 10%, more preferably below 5%, and most preferablybelow 1%

20. A method for manufacturing a water-resistant paper or paperboard, said method comprising: a) providing a paper or paperboard comprising a foam-formed cellulose-basedply, and b) subjecting said foam-formed cellulose-based p|y to grafting with a fatty acid halide to render the foam-formed cellulose-based p|y water-resistant.

21. A method according to claim 20, wherein step a) comprises providing a paper or paperboard wherein at least one p|y 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 cellulose-based ply.

22. A method according to any one of c|aims 20 or 21, wherein stepcomprises subjecting said paper or paperboard to grafting with a fatty acid halideto render the foam-formed cellulose-based p|y water-resistant.

23. A method according to any one of c|aims 21-22, wherein the graftingcomprises contacting at least one surface of the paper or paperboard with a fatty acid halide in a liquid, spray and/or vapor state.

24. A method according to any one of c|aims 21-23, wherein the grafting is only performed once.