Liquid-resistant multiply paper and paperboard

Abstract

The present invention relates to a liquid-resistant multiply paper or paperboard, comprising: a cellulose-based back ply, wherein said back ply comprises at least two different internal sizing agents in a total amount of 2-10 kg / ton, based on the total dry weight of the back ply; a cellulose-based top ply, wherein said top ply comprises a single internal sizing agent in a total amount of 0.1-5 kg / ton, based on the total dry weight of the top ply, and wherein the total amount of internal sizing agent in the top ply is lower than in the back ply; and a first surface sizing layer applied on the outermost surface of the back ply, wherein the first surface sizing layer has a coat weight of 0.5-3 g / m2 and comprises at least one hydrophobic surface sizing agent and at least one binder. The invention further relates to a method for manufacturing a liquid-resistant multiply paper or paperboard.

Description

[0001] LIQUID-RESISTANT MULTIPLY PAPER AND PAPERBOARD

[0002] Technical field

[0003] The present disclosure relates to repulpable and compostable liquid-resistant paper or paperboard suitable for use in disposable packaging containers, such as cups, boxes and trays, for food and beverages.

[0004] Background

[0005] Liquid-resistance, and particularly water-resistance, is an important property in many paper and paperboard applications. Important examples include disposable packaging containers, such as cups, boxes and trays, for food and beverages.

[0006] Coating of paper and paperboard with plastics is often employed to combine the mechanical properties of the paper and paperboard with the barrier and converting properties of a plastic film, such as heat-sealability and shape forming. However, plastics are undesirable from a sustainability and recycling point of view and negatively affects for example the repulping efficiency of pre- and post-consumer waste. Also, only few and less common plastics can be considered as compostable, especially home compostable. Furthermore, plastic coating must typically be performed offline, which adds significantly to production time and cost.

[0007] Paper and paperboard for use in food and beverage packaging applications are often hydrophobized using hydrophobizing agents, also commonly referred to as sizing agents, to increase the resistance or repellence to wetting and penetration of water and other liquids into the cellulose-based substrate. This hydrophobizing treatment, generally referred to as sizing, is important to maintain the function of the substrate in wet or damp conditions. There are two main types of sizing: internal sizing and surface sizing. When the sizing agent is added to the cellulose furnish it is referred to as internal sizing, and when the sizing agent is applied on the paper or paperboard surface, it is referred to as surface sizing. Common internal sizing agents include alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) and rosin. Common surface sizing components include starch and starch derivatives, styrene maleic anhydrides, polyurethanes, styrene acrylates (SA) and acrylic copolymers.

[0008] It is also known that the traditional sizing concept does not give any permanent water or liquid resistance, but that the technical effect is more commonly used to provide controlled wetting behavior of the substrate.

[0009] In order to achieve repulpable and compostable paper and paperboard suitable for use in disposable packaging containers, it would be desirable to replace plastic coatings by utilizing sizing methods. However, existing sizing concepts are not sufficient to meet the requirements in food and beverage packaging applications, including acceptable product performance, recyclability, and compostability, when removing the plastic coating. To improve product performance, an increased dosage of the sizing agent could be one approach, but the improvement will then typically come at the expense of decreased recyclability and / or compostability. Increased dosage alone will also not be sufficient to provide acceptable product performance for different types of foods and beverages.

[0010] Another problem associated with sizing agents is migration of the sizing chemicals which can result in deposits on the production machinery and / or finished products, limiting the technical feasibility and industrial applicability. Migration of sizing agents can also be detrimental from a food-safety point of view where each sizing agent has its own limitations of use.

[0011] The sizing agents may also affect the friction of the paper or paperboard, leading to too low friction. This may in turn result in difficulties with handling and / or converting of the paper and paperboard, and as an example in so-called telescope reels.

[0012] There remains a need for improved solutions to render paper and paperboard for food and beverage packaging applications liquid-resistant without losing the repulpability and compostability of the material. The solution should be industrially applicable, preferably avoiding issues with migration of sizing agents or too low friction, and the obtained paper or paperboard should be easy to convert and safe for food-contact usage.

[0013] Description of the invention

[0014] It is an object of the present disclosure to provide a liquid-resistant paper or paperboard with good repulpability and compostability.

[0015] It is a further object of the present disclosure to provide a liquid-resistant paper or paperboard with good repulpability and compostability, which is useful in food and beverage packaging applications.

[0016] It is a further object of the present disclosure to provide a liquid-resistant paper or paperboard for food and beverage packaging applications, which is free from plastic coatings.

[0017] 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.

[0018] According to a first aspect illustrated herein, there is provided a liquid-resistant multiply paper or paperboard, comprising: a cellulose-based back ply, wherein said back ply comprises at least two different internal sizing agents in a total amount of 2-10 kg / ton, based on the total dry weight of the back ply; a cellulose-based top ply, wherein said top ply comprises a single internal sizing agent in a total amount of 0.1 -5 kg / ton, based on the total dry weight of the top ply, and wherein the total amount of internal sizing agent in the top ply is lower than in the back ply; and a first surface sizing layer applied on the outermost surface of the back ply, wherein the first surface sizing layer has a coat weight of 0.5-3 g / m2and comprises at least one hydrophobic surface sizing agent and at least one binder.

[0019] Paper generally refers to a material manufactured in thin sheets or rolls from the pulp of wood or other fibrous substances comprising cellulose fibers, used for writing, drawing, or printing on, or as a packaging material.

[0020] Paperboard generally refers to strong, thick paper or cardboard used for example in containers such as cups, boxes and trays for food and beverages, and / or other types of packaging.

[0021] Paper or paperboard may generally be a single-ply material, or a multiply material comprising two or more plies. Paperboard can either be bleached or unbleached paperboard, pigment coated or uncoated paperboard, and produced in a variety of thicknesses, depending on the end use requirements.

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

[0023] The present disclosure is based on the realization that the aforementioned problems can be solved by differentiating the sizing process in different plies of a multiply paper or paperboard in a controlled manner to combine resistance to a wide range of different liquid and food types with good repulpability and compostability. The multiply paper or paperboard comprises at least two cellulose- based plies, a cellulose-based back ply and a cellulose-based top ply, and a surface sizing layer applied on the outermost surface of the back ply. The cellulose-based plies preferably comprise a sheet or web of material mainly formed from pulp of wood or other fibrous substances comprising cellulose fibers.

[0024] What has been found important is to provide a back ply comprising a combination of at least two different internal sizing agents, and having a surface sizing layer comprising at least one hydrophobic surface sizing agent applied on the outermost surface thereof, intended for being in direct contact with the packed food or beverage. Using a combination of at least two different internal sizing agents in the back ply, combined with the surface sizing layer, allows for the combination of different sizing agents that provide resistance towards a wide range of different liquid and food types, for example hot and cold beverages, hot and cold foods, frozen foods, and complex food or liquid mixtures having different phases, such as water, grease, oil or emulsion phases. The combination of at least two different internal sizing agents in the back ply, combined with the surface sizing layer preferably applied by means of size press method, may also support retention of the sizing agents in the paper or paperboard and reduce the potential migration of sizing agents as the amount of each sizing agent can be reduced or kept at a relatively low level, and as the surface sizing layer may act as a barrier against migration of the internal sizing agents.

[0025] When different sizing agents are added at the same time to a cellulose furnish in the stock handling or wet-end section of the paper or paperboard manufacturing, competitive adsorption between the sizing agents may occur. Therefore, if the dosage of one sizing agent is increased, the retention of another sizing agent may become reduced. As adjustment of the retention system may not be sufficient to solve this problem, a surface sizing layer becomes one viable way to add additional hydrophobic sizing agents to the paper or paperboard, improving the retention and dosage amount of one specific hydrophobic sizing agent or giving the possibility to introduce another type of sizing agent with different functionality to further reduce penetration of different liquid and food types. In a top ply, which is not intended for being in direct contact with the packed food or beverage, a lower amount of a single internal sizing agent provides moderate liquid resistance, but good printability, high repulpability and compostability.

[0026] In addition to the back and top plies, the paper or paperboard may further comprise at least one cellulose-based mid ply arranged between said top ply and said back ply. In some embodiments, the paper or paperboard further comprises: a cellulose-based mid ply arranged between said top ply and said back ply, wherein said mid ply comprises a single internal sizing agent in a total amount of 0.1-5 kg / ton, based on the total dry weight of the mid ply, and wherein the total amount of internal sizing agent in the mid ply is lower than in the back ply, and preferably higher than in the top ply.

[0027] The additional mid ply, or plies, allows for further differentiation of the sizing process.

[0028] In addition to the first surface sizing layer applied on the outermost surface of the back ply, the paper or paperboard may further comprise a second surface sizing layer applied on the outermost surface of the top ply.

[0029] Thus, in some embodiments, the paper or paperboard further comprises: a second surface sizing layer applied on the outermost surface of the top ply, wherein the second surface sizing layer has a coat weight of 0.5-3 g / m2and comprises at least one hydrophobic surface sizing agent and at least one binder.

[0030] A second surface sizing layer applied on the outermost surface of the top ply can add further liquid resistance to the surface of the paper or paperboard which is not intended for being in direct contact with the packed food or beverage, often also referred to as the print side.

[0031] Each of the cellulose-based plies of the paper or paperboard can have a certain composition of pulp fibers, such as bleached and / or unbleached Kraft pulp, sulfite pulp, dissolving pulp, thermomechanical pulp (TMP), chemi-thermomechanical pulp (CTMP), high-temperature CTMP (HT-CTMP), pressurized groundwood pulp (PGW) and / or mixtures thereof. In some embodiments, the paper or paperboard comprises a substantial amount of recycled cellulose fibers, such as broke or fibers recycled from pre- or post-consumer waste. In some embodiments, the amount of recycled cellulose fibers in the paper or paperboard is in the range of 1- 50 wt%, preferably in the range of 5-45 wt%, and more preferably in the range of 10-35 wt%, based on the total dry weight of the paper or paperboard. It is also possible that the paper or paperboard product comprises only recycled fibers. It is especially preferred to use recycled cellulose in the mid ply. Thus, in some embodiments, the cellulose-based mid ply comprises a substantial amount of recycled cellulose fibers. In some embodiments, the amount of recycled cellulose fibers in the mid ply is in the range of 1 -50 wt%, preferably in the range of 5-45 wt%, and more preferably in the range of 10-35 wt%, based on the total dry weight of the mid ply. It is also possible that the mid ply comprises only recycled fibers.

[0032] In some embodiments, the paper or paperboard has a back ply comprising bleached Kraft pulp, a mid ply comprising of a mixture of bleached Kraft pulp and CTMP, and a top ply comprising bleached Kraft pulp, wherein the mid ply has a lower density than both the back and top plies, respectively. The lower density mid ply may typically have a density below 750 kg / m3, preferably below 700 kg / m3, or below 650 kg / m3, or below 600 kg / m3, or below 550 kg / m3or below 500 kg / m3. In an example, the lower density mid ply may have a density between 550-650 kg / m3. The higher-density back and top plies typically have a density that is at least 100 kg / m3higher than the mid ply, preferably at least 200 kg / m3higher than the mid ply. In some embodiments, the total weight of the liquid-resistant multiply paper or paperboard according to the invention is between 600-900 kg / m3, such as between 700-850 kg / m3.

[0033] The multiply paper or paperboard of the present disclosure is liquid resistant. The term “liquid resistant” generally means that the multiply paper or paperboard with the internal sizing agents and surface sizing layer has a higher resistance or repellence to liquid penetration and absorption than the same multiply paper or paperboard without said internal sizing agents and surface sizing layer. The multiply paper or paperboard of the present disclosure is preferably at least water-resistant. The term “water-resistant” generally means that the multiply paper or paperboard with the internal sizing agents and surface sizing layer has a higher resistance to water absorption (e.g. indicated by the difference between water Cobb300 and water Cobb30, as determined according to standard ISO 535:2014 for 300 and 30 seconds, respectively) than the same multiply paper or paperboard without said internal sizing agent and surface sizing layer.

[0034] The liquid-resistant multiply paper or paperboard substrate is preferably suitable for use as packaging material intended for contact with food and liquids. In some embodiments, the cellulose-based substrate is for use in disposable packaging containers, such as boxes, cups, lids, boxes and trays for food and beverages.

[0035] The back, top and optional mid plies of the paper or paperboard all comprise internal sizing agent. Internal sizing agents are often used in paper or paperboard in order to control wetting. An internal sizing agent is a hydrophobizing agent which is added to the cellulose furnish in the stock handling or wet-end section of the paper or paperboard manufacturing process. The most common internal sizing agents are alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) and rosin sizing agents, where the latter can be used as soap size or rosin emulsion or as cationic or anionic dispersions. However, other agents that increase the resistance to liquid penetration of water and other liquids into the cellulose-based substrate may also be used as internal sizing agents. Examples include natural waxes, fatty acids, fatty acid derivatives, and / or combinations thereof. The internal sizing agents are typically added to the cellulose furnish in emulsion form or as a dispersion.

[0036] In some embodiments, each of the internal sizing agents in the back, top and optional mid plies of the paper or paperboard are selected from the group consisting of alkyl ketene dimer (AKD), alkenyl or alkyl succinic anhydride (ASA), rosin (soap or emulsion), stearic anhydride, and waxes, where said waxes are having a melting temperature above 50°C, or above 55°C or above 60°C. The waxes are preferably natural waxes, i.e. waxes derived from natural sources, such as plants, animals, or minerals. Non-limiting examples of natural waxes include beeswax, carnauba wax, and shellac wax. The melting temperatures of the waxes are determined according to standard ASTM D3418-21 unless otherwise stated.

[0037] In some embodiments each of the internal sizing agents in the back, top and optional mid plies of the paper or paperboard are selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), and rosin.

[0038] In some embodiments, each of the internal sizing agents in the back ply are selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin, stearic anhydride, and waxes with a melting temperature above 50°C or above 55°C or above 60°C. In some embodiments, the back ply comprises dual sizing selected from AKD and rosin, ASA and rosin, ASA and AKD, or AKD and stearic anhydride. These combinations of internal sizing agents are especially useful, but not limited to, since the liquid resistant properties of the different internal sizing agents complement each other.

[0039] Due to their different chemical and physical properties, the different internal sizing agents may be used in different amounts. The internal sizing agents are typically added to the cellulose furnish in emulsion or dispersion form. The internal sizing agents may be added to the cellulose furnish sequentially, simultaneously or as a pre-mixed blend. Preferably, sequential or simultaneous addition is utilized. If premixed, the emulsions or dispersions preferably have similar charges or net-charge sign and can be stabilized electrostatically, such that the mixture is colloidally stable when the emulsions are blended together.

[0040] In some embodiments, the back ply comprises the internal sizing agents in a total amount of 2-8 kg / ton, based on the total dry weight of the back ply.

[0041] In some embodiments, the back ply comprises AKD in an amount of 1 -4 kg / ton, based on the total dry weight of the back ply. In some embodiments, the back ply comprises ASA in an amount of 1-2 kg / ton, based on the total dry weight of the back ply.

[0042] In some embodiments, the back ply comprises rosin in an amount of 2-6 kg / ton, based on the total dry weight of the back ply.

[0043] In some embodiments, the internal sizing agent in the top ply is selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin, stearic anhydride, and waxes with a melting temperature above 50°C or above 55°C or above 60°C.

[0044] In some embodiments, the top ply comprises the internal sizing agent in a total amount of 1-6 kg / ton, and preferably 1-4 kg / ton, based on the total dry weight of the top ply.

[0045] In some embodiments, the top ply comprises AKD in an amount of 1-4 kg / ton, based on the total dry weight of the top ply.

[0046] In some embodiments, the top ply comprises ASA in an amount of 1-2 kg / ton, based on the total dry weight of the top ply.

[0047] In some embodiments, the top ply comprises rosin in an amount of 2-6 kg / ton, based on the total dry weight of the top ply.

[0048] In some embodiments, the internal sizing agent in the mid ply is selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin, stearic anhydride, and waxes with a melting temperature above 50 and more preferably above 55 and most preferably above 60 °C.

[0049] In some embodiments, the mid ply comprises the internal sizing agent in a total amount of 1-6 kg / ton, and preferably 1-4 kg / ton, based on the total dry weight of the mid ply. In some embodiments, the mid ply comprises AKD in an amount of 1-4 kg / ton, based on the total dry weight of the mid ply.

[0050] In some embodiments, the mid ply comprises ASA in an amount of 1-2 kg / ton, based on the total dry weight of the mid ply.

[0051] In some embodiments, the mid ply comprises rosin in an amount of 2-6 kg / ton, based on the total dry weight of the mid ply.

[0052] To improve the wet strength of the paper or paperboard, the internal sizing agent can be 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 a crosslinked network within the fiber network that does not break upon wetting and reduces swelling, thereby also reducing the liquid absorption. Common wet-strength agents include urea-formaldehyde (UF), melamine-formaldehyde (MF), polyamide-epichlorohydrin (PAE), glyoxal, cross-linked cationic polyalkylene amine, vinylformamide- vinylamine copolymers, dialdehyde starch and copolymers of acrylamide and diallylamine. 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 such as glyoxalated polyacrylamide (G-PAM). In some embodiments, the paper or paperboard comprises 0.1 -10 kg / ton of a wet-strength agent, preferably 0.1-4 kg / ton, based on the total dry weight of the paper or paperboard.

[0053] In some embodiments, the back ply is formed from a pulp having a higher Schopper Riegler (SR) value, as determined according to standard ISO 5267-1 , than the pulp used for the top ply, and the pulp used for the optional mid ply when present. A pulp having a higher Schopper Riegler value typically has a higher retention of internal sizing agents than a pulp having a lower Schopper Riegler value. The higher Schopper Riegler value of the pulp of the back ply may for example be obtained by refining or beating the pulp, or by adding fine cellulosic material, such as cellulose fines, highly refined cellulose, or m icrofibrillated cellulose, to the pulp. The fine cellulosic material may preferably be obtained from bleached or unbleached soft- or hardwood kraft pulp. The fine cellulosic material may preferably have a Schopper Riegler value in the range of 30-85, preferably in the range of 55-85, as determined according to standard ISO 5267-1. The fine cellulosic material may preferably be present in the back ply in an amount of 0.1- 15 wt%, based on the total dry weight of the back-ply. To prevent curling of the board material, also the top ply may be formed from a pulp having a higher Schopper Riegler (SR) value. The higher Schopper Riegler value of the pulp of the top ply may be obtained in the same way as for the back ply.

[0054] The multiply paper or paperboard comprises a first surface sizing layer applied on the outermost surface of the back ply, and optionally also a second surface sizing layer applied on the outermost surface of the top ply. Surface sizing enhances the surface properties of the paper or paperboard, and can, depending on the type of surface sizing layer applied, make the surface more resistant to water penetration, improve printability, and increase the overall surface strength and appearance of the material. The primary objective of the surface sizing layer(s) of the inventive paper or paperboard is to improve the liquid resistance of the paper or paperboard, particularly for use in contact with hot and cold food and beverages. Liquid resistance is improved both by increasing the hydrophobicity and by closing of the surface by reducing the size and number of pores in the surface of the fiber matrix. The closing of the surface also helps to prevent migration of compounds from the paper or paperboard plies to food or beverage in contact with the paper or paperboard surface, especially when hot food and beverages are utilized. The surface sizing layer may also facilitate subsequent converting of the multiply paper or paperboard by improving the adhesion to an adhesive utilized to form a container from the paper or paperboard. The surface strength provided by the surface sizing layer can facilitate application of the adhesive, and also reduce absorption of the adhesive into the paper or paperboard.

[0055] Surface sizing is typically achieved through the application of a sizing composition, which comprises chemicals that form a thin film on the surface of the paper or paperboard fibers. The two main methods for applying surface sizing layers are the size press method and the film press method, but surface sizing layers may also be applied using a wet-end applicator, or by any other surface sizing or coating or spraying method known in the paper industry. The size press method involves applying the surface sizing composition directly onto the surface of the paper or paperboard as it passes through a set of rollers. The paper or paperboard is first coated with a liquid mixture comprising the surface sizing composition. The excess liquid mixture is then removed by passing the paper or paperboard through a nip between two rollers, leaving a thin, even sizing layer on the surface of the paper or paperboard fibers. Another option is to apply a flooded nip. In the film press method, a film of the surface sizing composition is formed separately, typically using a film press machine wherein a rod, blade or metering roll is used to create a thin film on a transfer roll. This film is then transferred onto the surface of the paper or paperboard using pressure. In addition to layers obtained by traditional surface sizing methods, the term surface sizing as used herein is also intended to encompass other methods for providing a thin water-resistant layer on a paper or paperboard surface, without forming a plastic coating. Accordingly, the term surface sizing as used herein is also intended to encompass impregnation of the surface with a hydrophobizing agent as well as wet on wet application of a hydrophobizing or sizing agent.

[0056] The formulation of the surface sizing composition and surface sizing layer depends on the specific requirements of the paper or paperboard, such as the desired level of water resistance, printability, and strength. The selection of hydrophobic surface sizing agent(s) and binder(s), and optional other additives, can be tailored to meet the requirements of different paper or paperboard grades and end-use applications.

[0057] The first surface sizing layer has a coat weight of 0.5-3 g / m2and comprises at least one hydrophobic surface sizing agent and at least one binder. In some embodiments, the first surface sizing layer comprises 1-80 wt% hydrophobic surface sizing agent and 20-99 wt% binder, based on the total dry weight of the first surface sizing layer. In some embodiments, the first surface sizing layer comprises 1-50 wt% hydrophobic surface sizing agent and 50-99 wt% binder, based on the total dry weight of the first surface sizing layer. In some embodiments, the first surface sizing layer comprises 1-15 wt% hydrophobic surface sizing agent and 75-99 wt% binder, based on the total dry weight of the first surface sizing layer.

[0058] In some embodiments, the first surface sizing layer comprises at least two hydrophobic surface sizing agents, such as two or three hydrophobic surface sizing agents.

[0059] In some embodiments, each hydrophobic surface sizing agent is selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin, stearic anhydride, latex dispersions, styrene-maleic anhydride copolymers (SMA), styrene-acrylic ester copolymers (SAE), modified polyurethane dispersions, and combinations thereof.

[0060] In some embodiments, each hydrophobic surface sizing agent is selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin, stearic anhydride, and latex dispersions, and combinations thereof.

[0061] In some embodiments, each binder is selected from the group consisting of water- soluble polymers, preferably selected from the group consisting of polyvinyl alcohol (PVOH), water soluble polysaccharides and water-soluble polysaccharide derivatives, more preferably selected from the group consisting of water-soluble starch, water-soluble starch derivatives, and water-soluble cellulose derivatives. In some embodiments, each binder is selected from the group consisting of water- soluble starch and water-soluble starch derivatives. In some embodiments, the binder is a hydrophobically modified polysaccharide, such as n-octenyl succinic anhydride modified starch (n-OSA starch), or an ethyl- or methyl-modified polysaccharide, such as an ethyl- or methyl-modified cellulose.

[0062] In some embodiments, the first surface sizing layer further comprises 0.1-20 wt% filler, based on the total dry weight of the first surface sizing layer.

[0063] In some embodiments, the first surface sizing layer further comprises 0.1-10 wt% fine cellulosic material, preferably selected from the group consisting of cellulose fines, highly refined cellulose, microfibri Hated cellulose (MFC), cellulose microcrystals, and cellulose nanocrystals, based on the total dry weight of the first surface sizing layer.

[0064] In some embodiments, the first surface sizing layer further comprises 0.1-20 wt% crosslinker, based on the total dry weight of the first surface sizing layer. In some embodiments, the crosslinker is selected from the group consisting of polyamideepichlorohydrin (PAE), and polyfunctional organic acids or aldehydes, such as citric acid, glyoxal, and glutaraldehyde.

[0065] In some embodiments, the first surface sizing layer comprises 0.1-10 wt% of a wet-strength agent, preferably 0.1-4 wt%, based on the total dry weight of the first surface sizing layer.

[0066] In some embodiments, the first surface sizing layer further comprises 0.1-5 wt% of other additives, based on the total dry weight of the first surface sizing layer. The other additives may include, but are not limited to, fixation agents, such as polyvinylamine (PVAm), polyaluminum chloride (PAC), and alum.

[0067] The second surface sizing layer may be further defined as described with reference to the first surface sizing layer. The coat weight and composition of the second surface sizing layer may be the same or different from the first surface sizing layer.

[0068] In a more specific embodiment, the liquid-resistant multiply paper or paperboard comprises: a cellulose-based back ply, wherein said back ply comprises AKD and rosin in a total amount of 2-10 kg / ton, preferably 5-8 kg / ton, based on the total dry weight of the back ply; a cellulose-based mid ply, wherein said mid ply comprises rosin in a total amount of 0.2-5 kg / ton, preferably 2-4 kg / ton, based on the total dry weight of the mid ply, and wherein the total amount of internal sizing agent in the mid ply is lower than in the back ply; and a cellulose-based top ply, wherein said top ply comprises a single internal sizing agent in a total amount of 0.1-5 kg / ton, preferably 1-3 kg / ton, based on the total dry weight of the top ply, and wherein the total amount of internal sizing agent in the top ply is lower than in the mid ply; and a first surface sizing layer applied on the outermost surface of the back ply, wherein the first surface sizing layer has a coat weight of 0.5-3 g / m2and comprises SA latex and at least one binder, and optionally a second surface sizing layer applied on the outermost surface of the top ply, wherein the second surface sizing layer has a coat weight of 0.5-3 g / m2and comprises SA latex and at least one binder.

[0069] In some embodiments, the surface-sized back ply of the liquid-resistant multiply paper or paperboard has a difference between water Cobb300 and water Cobb30, as determined according to standard ISO 535:2014 for 300 and 30 seconds, respectively, below 30 g / m2, preferably below 25 g / m2, more preferably below 20 g / m2.

[0070] In some embodiments, the surface-sized back ply of the liquid-resistant multiply paper or paperboard has a water penetration depth as determined optically from a cross-section of a sample treated according to standard ISO 535:2014 with dyed water for 300 seconds, below 40 pm, preferably below 30 pm, and more preferably below 20 pm.

[0071] In some embodiments, the surface-sized back ply of the liquid-resistant multiply paper or paperboard has a water contact angle above 90°, preferably above 100°. In some embodiments, the difference in contact angle between the top ply and the back ply is >10°, more preferably >15°. Water contact angles were determined after 10 seconds according to standard TAPPI T558 om-15 method using 4 pl droplets and the Young-Laplace Model as water droplet model, unless otherwise specified. In some embodiments, the surface-sized back ply of the liquid-resistant multiply paper or paperboard has a Cobb-Unger 15 s value, as determined according to SCAN-P 37:77, of <25 g / m2, more preferably <15 g / m2, or even more preferably <10 g / m2.

[0072] In preferred embodiments, the multiply paper or paperboard does not comprise a plastic layer, such as a plastic coating-layer or a plastic film-layer formed by extrusion coating or dispersion coating, having a grammage above 5 g / m2, often closer to 8-15 g / m2, or higher. The term “plastic” as used herein generally refers to water insoluble synthetic polymers, typically made from hydrocarbons like ethylene, propylene, and other similar compounds. These plastics can be categorized into various types based on their chemical composition and properties, including but not limited to polyethylene (PE), such as low-density polyethylene (LDPE) and high-density polyethylene (HDPE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), and polyvinyl chloride (PVC). Examples of common polymeric compounds that can be utilized in above- mentioned dispersion coatings are polyvinylidene chlorides, poly(vinyl acetate), styrene-acrylic latex, and styrene-butadiene latex, polyethylene, and these or others are utilized at least 50 wt% of the dispersion coating.

[0073] In some embodiments, the multiply paper or paperboard can contain additional polymeric compounds that can be defined as plastics, such as process chemical, additives, binders, and / or retention agents, to a total content below 5 wt%, preferably below 1 wt%, and more preferably below 0.5 wt%, based on the total dry weight of the paper or paperboard.

[0074] The multiply paper or paperboard preferably has high repulpability, which facilitates recycling. Repulping involves breaking down paper and paperboard products into pulp to create new products thereof. In some embodiments, the multiply paper or paperboard has a total reject according to PTS RH 021 / 97 test method for Category II products below 5 %, preferably below 1 %, and more preferably below 0.5 %. The multiply paper or paperboard is preferably compostable, and more preferably also home compostable. Compostable materials are those that can biodegrade in a composting system, which is a controlled environment designed to facilitate the decomposition of organic matter. Composting involves the breakdown of organic materials, such as food waste and certain biodegradable products, into nutrientrich compost that can be used to enrich soil and promote plant growth. Home compostable materials are designed to break down at lower temperatures and shorter timeframes compared to industrial compostable materials, making them suitable for household composting systems. In some embodiments, the paper or paperboard is compostable as determined according to standard ASTM D6868-11 .

[0075] In some embodiments, the paper or paperboard has a basis weight above 150 g / m2. In some embodiments, the paper or paperboard has a basis weight above 200 g / m2. In some embodiments, the paper or paperboard has a basis weight in the range of 200-400 g / m2. In some embodiments, the paper or paperboard has a basis weight in the range of 220 - 320 g / m2. It has been observed that a grammage above 220 g / m2leads to good rigidity which is advantageous when using the material for food and / or beverage packaging containers.

[0076] The basis weight of the back, top and mid plies can be the same or different. In some embodiments, the basis weight of the back ply and the top ply are in the range of 20-100 g / m2, respectively. In some embodiments, the basis weight of the mid ply is in the range of 50-250 g / m2. In some embodiments, the back ply has higher density but lower grammage than the top ply. This allows for a denser back ply, while at the same time preventing curling of the multiply paper or paperboard due to differences in density between the plies.

[0077] According to a second aspect illustrated herein, there is provided a method for manufacturing a liquid-resistant multiply paper or paperboard, said method comprising: a) forming a cellulose-based back ply from a first cellulose furnish, wherein said first cellulose furnish comprises at least two different internal sizing agents in a total amount of 2-10 kg / ton, based on the total dry weight of the first cellulose furnish; b) forming a cellulose-based top ply from a second cellulose furnish, wherein said second cellulose furnish comprises a single internal sizing agent in a total amount of 0.1-5 kg / ton, based on the total dry weight of the second cellulose furnish, and wherein the total amount of internal sizing agent in the second cellulose furnish is lower than in the first cellulose furnish; and c) forming a first surface sizing layer on the outermost surface of the back ply, wherein the first surface sizing layer has a coat weight of 0.5-3 g / m2and comprises at least one hydrophobic surface sizing agent and at least one binder.

[0078] In some embodiments, the method further comprises: forming a cellulose-based mid ply from a third cellulose furnish between said top ply and said back ply, wherein said third cellulose furnish comprises a single internal sizing agent in a total amount of 0.1-5 kg / ton, based on the total dry weight of the third cellulose furnish, and wherein the total amount of internal sizing agent in the third cellulose furnish is lower than in the first cellulose furnish, and preferably higher than in the second cellulose furnish.

[0079] In some embodiments, the method further comprises: forming a second surface sizing layer on the outermost surface of the top ply, wherein the second surface sizing layer has a coat weight of 0.5-3 g / m2and comprises at least one hydrophobic surface sizing agent and at least one binder.

[0080] In some embodiments, each of the internal sizing agents in the first cellulose furnish are selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin, stearic anhydride, and waxes with a melting temperature above 50°C or above 55°C or above 60°C. In some embodiments, the first cellulose furnish comprises AKD and rosin, ASA and rosin, ASA and AKD, or AKD and stearic anhydride.

[0081] In some embodiments, the first cellulose furnish comprises the internal sizing agents in a total amount of 2-8 kg / ton, based on the total dry weight of the first cellulose furnish.

[0082] In some embodiments, the first cellulose furnish comprises AKD in an amount of

[0083] 1-4 kg / ton, based on the total dry weight of the first cellulose furnish.

[0084] In some embodiments, the first cellulose furnish comprises ASA in an amount of 1- 2 kg / ton, based on the total dry weight of the first cellulose furnish.

[0085] In some embodiments, the first cellulose furnish comprises rosin in an amount of

[0086] 2-6 kg / ton, based on the total dry weight of the first cellulose furnish.

[0087] In some embodiments, the internal sizing agent in the second cellulose furnish is selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin, stearic anhydride, and waxes with a melting temperature above 50°C or above 55°C or above 60°C.

[0088] In some embodiments, the second cellulose furnish comprises the internal sizing agent in a total amount of 1-6 kg / ton, and preferably 1-4 kg / ton, based on the total dry weight of the second cellulose furnish.

[0089] In some embodiments, the second cellulose furnish comprises AKD in an amount of 1-4 kg / ton, based on the total dry weight of the second cellulose furnish.

[0090] In some embodiments, the second cellulose furnish comprises ASA in an amount of 1-2 kg / ton, based on the total dry weight of the second cellulose furnish.

[0091] In some embodiments, the second cellulose furnish comprises rosin in an amount of 2-6 kg / ton, based on the total dry weight of the second cellulose furnish. In some embodiments, the internal sizing agent in the third cellulose furnish is selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin, stearic anhydride, and waxes with a melting temperature above 50°C or above 55°C or above 60°C.

[0092] In some embodiments, the third cellulose furnish comprises the internal sizing agent in a total amount of 1-6 kg / ton, and preferably 1-4 kg / ton, based on the total dry weight of the third cellulose furnish.

[0093] In some embodiments, the third cellulose furnish comprises AKD in an amount of 1-4 kg / ton, based on the total dry weight of the third cellulose furnish.

[0094] In some embodiments, the third cellulose furnish comprises ASA in an amount of

[0095] 1-2 kg / ton, based on the total dry weight of the third cellulose furnish.

[0096] In some embodiments, the third cellulose furnish comprises rosin in an amount of

[0097] 2-6 kg / ton, based on the total dry weight of the third cellulose furnish.

[0098] In some embodiments, the first cellulose furnish has a higher Schopper Riegler value than the second furnish and the optional third furnish, as determined according to standard ISO 5267-1.

[0099] The first surface sizing layer has a coat weight of 0.5-3 g / m2and comprises at least one hydrophobic surface sizing agent and at least one binder. In some embodiments, the first surface sizing layer comprises 1-80 wt% hydrophobic surface sizing agent and 20-99 wt% binder, based on the total dry weight of the first surface sizing layer. In some embodiments, the first surface sizing layer comprises 1-50 wt% hydrophobic surface sizing agent and 50-99 wt% binder, based on the total dry weight of the first surface sizing layer. In some embodiments, the first surface sizing layer comprises 2-30 wt% hydrophobic surface sizing agent and 70-98 wt% binder, based on the total dry weight of the first surface sizing layer. In some embodiments, the first surface sizing layer comprises at least two hydrophobic surface sizing agents, such as two or three hydrophobic surface sizing agents.

[0100] In some embodiments, each hydrophobic surface sizing agent is selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin, stearic anhydride, latex dispersions, styrene-maleic anhydride copolymers (SMA), styrene-acrylic ester copolymers (SAE), modified polyurethane dispersions, and combinations thereof. The latex may for example be styreneacrylate (SA) latex, styrene-butadiene (SB) latex, polyvinyl acetate (PVAc) latex, and is preferably SA latex.

[0101] In some embodiments, each hydrophobic surface sizing agent is selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin, stearic anhydride, and latex dispersions, and combinations thereof.

[0102] In some embodiments, each binder is selected from the group consisting of water- soluble polymers, preferably selected from the group consisting of polyvinyl alcohol (PVOH), water-soluble polysaccharides and water-soluble polysaccharide derivatives, more preferably selected from the group consisting of water-soluble starch, water-soluble starch derivatives, and water-soluble cellulose derivatives. In some embodiments, each binder is selected from the group consisting of water- soluble starch and water-soluble starch derivatives.

[0103] In some embodiments, the first surface sizing layer further comprises 0.1-20 wt% filler, based on the total dry weight of the first surface sizing layer.

[0104] In some embodiments, the first surface sizing layer further comprises 0.1-10 wt% fine cellulosic material, preferably selected from the group consisting of cellulose fines, highly refined cellulose, microfibri Hated cellulose (MFC), cellulose microcrystals, and cellulose nanocrystals, based on the total dry weight of the first surface sizing layer. In some embodiments, the first surface sizing layer further comprises 0.1-5 wt% crosslinker, based on the total dry weight of the first surface sizing layer. In some embodiments, the crosslinker is selected from the group consisting of polyamideepichlorohydrin (PAE), and polyfunctional organic acids or aldehydes, such as citric acid, glyoxal, dialdehyde starch and glutaraldehyde.

[0105] In some embodiments, the first surface sizing layer comprises 0.1-10 kg / ton of a wet-strength agent, preferably 0.1-4 kg / ton, based on the total dry weight of the first surface sizing layer.

[0106] In some embodiments, the first surface sizing layer further comprises 0.1-5 wt% of other additives, based on the total dry weight of the first surface sizing layer. The other additives may include, but are not limited to, fixation agents, such as polyvinylamine (PVAm), polyaluminum chloride (PAC), and alum.

[0107] The second surface sizing layer may be further defined as described with reference to the first surface sizing layer. The coat weight and composition of the second surface sizing layer may be the same or different from the first surface sizing layer.

[0108] When different sizing agents are added at the same time to a cellulose furnish in the stock handling or wet-end section of the paper or paperboard manufacturing process competition between the sizing agents may occur. Therefore, if the dosage of one sizing agent is increased, the retention of another sizing agent may become reduced. As adjustment of the retention system may not be sufficient to solve this problem, a surface sizing layer becomes one viable way to add additional hydrophobic sizing agents to the paper or paperboard, improving the retention of one specific hydrophobic sizing agent or giving the possibility to introduce another type of sizing agent with different functionality to further improve resistance and reduce penetration of different liquid and food types.

[0109] Different internal sizing agents are cured in different ways depending on the functionality of the sizing chemical. The term "curing" refers to the process of allowing the sizing agent to adequately react, anchor, spread and / or orient on the fibers within the paper or fiber matrix to develop the desired sizing effect. The on- machine curing typically occurs at the drying section of a paper or board machine, i.e. prior to the surface sizing unit. ASA and rosin are examples of sizing agents that mainly show on-machine curing, whereas AKD sizing typically shows a significant post-curing feature, i.e. the sizing effect continues to develop in storage reels even after the production, e.g even weeks after the production. Curing of at least one of the internal sizing agents prior to surface sizing is preferred, meaning that the sizing effect is fully or almost fully developed for at least one sizing agent before the surface sizing is applied to reduce the penetration of the surface sizing solution to the inner parts of the board.

[0110] In some embodiments, at least one of the internal sizing agents in the back ply is cured before the surface sizing layer is applied. More specifically, ASA or rosin can be utilized in combination with AKD as a part of the dual sizing in the back ply, with the intention to both reduce the surface sizing adsorption into the board as well as provide for increased hydrophobicity of the final product. In some embodiments, all of the internal sizing agents are cured before the surface sizing layer is applied.

[0111] According to a third aspect illustrated herein, there is provided a container formed from liquid-resistant multiply paper or paperboard according to the first aspect or obtained by the method according to the second aspect, wherein the surface sized outermost surface of the back ply forms an inside surface of the container.

[0112] Generally, while the products, polymers, materials, plies, layers and processes are described in terms of “comprising” various components or steps, the products, polymers, materials, plies, layers and processes can also “consist essentially of” or “consist of” the various components and steps.

[0113] Detailed description of the invention

[0114] Figure 1 shows a cross-sectional view of an example of a paper or paperboard substrate according to the invention. It is to be understood that the drawing in Fig.

[0115] 1 is schematic and not to scale. Herein is seen a multiply material 1 comprising three layers: a top ply 3, a back ply 4 and a mid ply 5. The material has a first print side 2 and a second inner side 6. According to the invention, the cellulose-based top ply 3 of the multiply material 1 comprises internal sizing agent, for instance in the form of rosin sizing. Furthermore, the cellulose-based back ply 4 of the multiply material 1 comprises dual internal sizing agents, for instance in the form of AKD and rosin. In this embodiment, the material 1 also comprises an internally sized mid ply 5. Both the first print side 2 and the second inner side 6 have been surface sized for instance with starch and SA-latex.

[0116] EXAMPLES

[0117] According to a first example, a cellulose-based top ply of a multiply material comprises internal sizing agent in the form of rosin sizing at the amount of 4 kg / ton. Furthermore, a cellulose-based back ply of the multiply material comprises dual internal sizing agents in the form of modified AKD at the amount of 5 kg / ton and rosin at 2kg / ton. In this first example, the material also comprises a mid-ply comprising rosin sizing at the amount of 6 kg / ton. Both the first print side and the second inner side have been surface sized with a mixture of starch and SA-latex, where the content of SA latex was 10 wt% based on the solid content of starch. The surface size coat weight was 0.9 g / m2 / side. The basis weight of the multiply paperboard in the first example was 255 g / m2

[0118] According to a second and comparable reference example, a cellulose-based top ply of a multiply material comprises internal sizing agent in the form of rosin sizing at the amount of 4 kg / ton. Furthermore, the cellulose-based back ply of the multiply material comprises only one internal sizing agent in the form of rosin at 2 kg / ton. In this example, the material also comprises a mid-ply comprising rosin sizing at the amount of 6 kg / ton. Neither the first print side nor the second inner side had been surface sized. The basis weight of the multiply paperboard in the second example was 266 g / m2.

[0119] According to a third example, a single ply material comprises internal sizing agent in the form of AKD sizing at the amount of 2.5 kg / ton. No surface sizing has been applied. The basis weight of the single ply material in the third example was 120 g / m2

[0120] According to a fourth example, a single ply material comprises internal sizing agent in the form of AKD sizing at the amount of 2.5 kg / ton. Both sides have been surface sized with a mixture of starch and SA-latex, where the content of SA latex was 7.5 wt% based on the solid content of starch. The surface size coat weight was 1 .45 g / m2 / side. The basis weight of the single ply material in the fourth example was 120 g / m2.

[0121] According to a fifth example, a single ply material comprises internal dual-sizing system in the form of ASA at the amount of 2 kg / ton and rosin at the amount of 6 kg / ton. No surface sizing has been applied. The basis weight of the single ply material in the fifth example was 120 g / m2.

[0122] According to a sixth example, a single ply material comprises internal dual-sizing system in the form of ASA at the amount of 2 kg / ton and rosin at the amount of 6 kg / ton. Both sides have been surface sized with a mixture of starch and SA-latex, where the content of SA latex was 7.5 wt% based on the solid content of starch. The surface size coat weight was 0.6 g / m2 / side. The basis weight of the single ply material in the sixth example was 120 g / m2.

[0123] According to a seventh example, a single ply material comprises internal dualsizing system in the form of AKD at the amount of 2.5 kg / ton and rosin at the amount of 6 kg / ton. No surface sizing has been applied. The basis weight of the single ply material in the seventh example was 120 g / m2.

[0124] According to an eighth example, a single ply material comprises internal dualsizing system in the form of AKD at the amount of 2.5 kg / ton and rosin at the amount of 6 kg / ton. Both sides have been surface sized with a mixture of starch and SA-latex, where the content of SA latex was 7.5 wt% based on the solid content of starch. The surface size coat weight was 0.5 g / m2 / side. The basis weight of the single ply material in the eighth example was 120 g / m2. Results

[0125] The Cobb values for the 3-ply structures (examples 1-2) have been measured according to standard ISO 535:2014 (determination of water absorptiveness) but with a usage of a blotting paper underneath the test sample to prevent the condensing water from wetting the sample from the reverse side.

[0126] The Cobb measurements for the 1 -ply structures (examples 3-8) have been measured according to standard ISO 535:2014 but with free space underneath the test sample by removing the bottom plate in the Cobb device. Furthermore, different liquids have been tested to show the impact on different characteristics of the test liquids and the need for the multi-sized board that this invention protects.

[0127] Table 1. Sizing dosages and Cobb values measured at the back-ply surface (unit is g / m2)

[0128] The results in form of Cobb measurements are presented in Table 1 . The three-ply comparable reference Example 2, without internal dual sizing nor surface sizing, showed very poor performance compared to Example 1 with dual sizing combined with hydrophobized surface sizing. It is also evident that Example 1 provides for significant improvement towards not only water but also hot creme coffee which can be considered as the harshest conditions a board can be subjected to as it is both acidic and fatty as well as having high temperature (i.e.80°C).

[0129] The results for the single-ply structures in Examples 3-8 showed similar trends (Table 1 ). For examples 3 and 4, which do not contain any dual sizing but only AKD, the additional surface sizing with the SA-latex does not provide any improvement of the Cobb values for hot water and creme coffee Cobb.

[0130] Comparatively, the other examples 5-8 with dual-sizing, showed a significant improvement towards hot water and with a special improvement for the Creme Coffee Cobb when additionally surface sized with SA-latex (Ex. 6 and 8). This is believed to be related to the ability of rosin to provide sufficient sizing effect already before the surface size application, which effect cannot be obtained by the use of AKD alone. However, the presence of AKD as a part of the sizing concept is also desired as it is considered to provide greater sizing effect when properly cured over time, since AKD sizing typically shows a significant post-curing feature. This observed effect is significant for this invention and the material’s performance, as the inadequate on-machine curing before te size-press leads to an increased penetration of the surface sizing into the board structure, and thus to poorer results.

[0131] 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 will not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

CLAIMS1 . A liquid-resistant multiply paper or paperboard, comprising: a cellulose-based back ply, wherein said back ply comprises at least two different internal sizing agents in a total amount of 2-10 kg / ton, based on the total dry weight of the back ply; a cellulose-based top ply, wherein said top ply comprises a single internal sizing agent in a total amount of 0.1 -5 kg / ton, based on the total dry weight of the top ply, and wherein the total amount of internal sizing agent in the top ply is lower than in the back ply; and a first surface sizing layer applied on the outermost surface of the back ply, wherein the first surface sizing layer has a coat weight of 0.5-3 g / m2and comprises at least one hydrophobic surface sizing agent and at least one binder.

2. The paper or paperboard according to claim 1 , further comprising: a cellulose-based mid ply arranged between said top ply and said back ply, wherein said mid ply comprises a single internal sizing agent in a total amount of 0.1 -5 kg / ton, based on the total dry weight of the mid ply, and wherein the total amount of internal sizing agent in the mid ply is lower than in the back ply, and preferably higher than in the top ply.

3. The paper or paperboard according to any one of the preceding claims, further comprising: a second surface sizing layer applied on the outermost surface of the top ply, wherein the second surface sizing layer has a coat weight of 0.5-3 g / m2and comprises at least one hydrophobic surface sizing agent and at least one binder.

4. The paper or paperboard according to any one of the preceding claims, wherein each of the internal sizing agents are selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin, stearic anhydride, and waxes, where said waxes are having a melting temperature above 50°C or above 55°C or above 60°C.

5. The paper or paperboard according to any one of the preceding claims, wherein the back ply comprises AKD and rosin, ASA and rosin, ASA and AKD, or AKD and stearic anhydride.

6. The paper or paperboard according to any one of the preceding claims, wherein the back ply comprises the internal sizing agents in a total amount of 2-8 kg / ton, based on the total dry weight of the back ply.

7. The paper or paperboard according to any one of the preceding claims, wherein the top ply comprises the internal sizing agent in a total amount of 1-6 kg / ton, and preferably 1-4 kg / ton, based on the total dry weight of the top ply.

8. The paper or paperboard according to any one of claims 2-7, wherein the mid ply comprises the internal sizing agent in a total amount of 1-6 kg / ton, and preferably 1-4 kg / ton, based on the total dry weight of the mid ply.

9. The paper or paperboard according to any one of the preceding claims, wherein the first surface sizing layer comprises 1-80 wt% hydrophobic surface sizing agent and 20-99 wt% binder, preferably 1 -50 wt% hydrophobic surface sizing agent and 50-99 wt% binder, and more preferably 1-15 wt% hydrophobic surface sizing agent and 75-99 wt% binder, based on the total dry weight of the first surface sizing layer.

10. The paper or paperboard according to any one of the preceding claims, wherein each hydrophobic surface sizing agent is selected from the groupconsisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin, stearic anhydride, and latex dispersions, and combinations thereof.11 . The paper or paperboard according to any one of the preceding claims, wherein each binder is selected from the group consisting of water-soluble polymers, preferably selected from the group consisting of water-soluble polysaccharides and water-soluble polysaccharide derivatives.

12. The paper or paperboard according to any one of the preceding claims, wherein the surface sized outermost surface of the back ply has a difference between water Cobb300 and water Cobb30, as determined according to standard ISO 535:2014 for 300 and 30 seconds, respectively, below 30 g / m2, preferably below 25 g / m2, more preferably below 20 g / m2.

13. The paper or paperboard according to any one of the preceding claims, wherein the surface sized outermost surface of the back ply has a water penetration depth as determined optically from a cross-section of a sample treated according to standard ISO 535:2014 with dyed water for 300 seconds, below 40 pm, preferably below 30 pm, and more preferably below 20 pm.

14. The paper or paperboard according to any one of the preceding claims, wherein the surface sized outermost surface of the back ply has a water contact angle above 90°, preferably above 100°.

15. The paper or paperboard according to any one of the preceding claims, wherein the paper or paperboard has a total reject according to PTS RH 021 / 97 test method for Category II products below 5 %, preferably below 1 %, and more preferably below 0.5 %.

16. The paper or paperboard according to any one of the preceding claims, wherein the paper or paperboard is compostable as determined according to standard ASTM D6868-11 .

17. The paper or paperboard according to any one of the preceding claims, wherein the paper or paperboard has a basis weight above 150 g / m2such as above 200 g / m2, and below 400 g / m2.

18. A method for manufacturing a liquid-resistant multiply paper or paperboard, said method comprising: a) forming a cellulose-based back ply from a first cellulose furnish, wherein said first cellulose furnish comprises at least two different internal sizing agents in a total amount of 2-10 kg / ton, based on the total dry weight of the first cellulose furnish; b) forming a cellulose-based top ply from a second cellulose furnish, wherein said second cellulose furnish comprises a single internal sizing agent in a total amount of 0.1-5 kg / ton, based on the total dry weight of the second cellulose furnish, and wherein the total amount of internal sizing agent in the second cellulose furnish is lower than in the first cellulose furnish; and c) forming a first surface sizing layer on the outermost surface of the back ply, wherein the first surface sizing layer has a coat weight of 0.5-3 g / m2and comprises at least one hydrophobic surface sizing agent and at least one binder.

19. The method according to claim 18, said method further comprising: forming a cellulose-based mid ply from a third cellulose furnish between said top ply and said back ply, wherein said third cellulose furnish comprises a single internal sizing agent in a total amount of 0.1-5 kg / ton, based on the total dry weight of the third cellulose furnish, and wherein the total amount of internal sizing agent in the third cellulose furnish is lower than in the first cellulose furnish, and preferably higher than in the second cellulose furnish.

20. The method according to any one of claims 18-19, said method further comprising:forming a second surface sizing layer on the outermost surface of the top ply, wherein the second surface sizing layer has a coat weight of 0.5-3 g / m2and comprises at least one hydrophobic surface sizing agent and at least one binder.21 . The method according to any one of claims 18-20, wherein each of the internal sizing agents are selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin, stearic anhydride, and waxes, where said waxes are having a melting temperature above 50°C or above 55°C or above 60°C.

22. The method according to one of claims 18-21 , wherein the first cellulose furnish has a higher Schopper Riegler value than the second furnish and the optional third furnish, as determined according to standard ISO 5267-1 .

23. The method according to one of claims 18-22, wherein each hydrophobic surface sizing agent is selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin, stearic anhydride, and latex dispersions, and combinations thereof.

24. The method according to one of claims 18-23, wherein each binder is selected from the group consisting of water-soluble polymers, preferably selected from the group consisting of water-soluble polysaccharides and water-soluble polysaccharide derivatives.

25. A container formed from liquid-resistant multiply paper or paperboard according to any one of claims 1-17 or obtained by the method according to claim 18-24, wherein the surface sized outermost surface of the back ply forms an inside surface of the container.

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