Paper or paperboard product with low liquid absorbency

Abstract

The invention relates to a method for producing a paper or paperboard product with improved liquid resistance is provided. The method comprises: providing a paper or paperboard substrate having specific property values including basis weight, thickness, density, air permeability, and Cobb values; applying a surface size composition comprising a polysaccharide binder and an sizing polymer, on the first surface of the paperboard; and drying the surface sized board at temperatures above 70°C; wherein the resulting surface sized paper or paperboard product has specific Cobb Unger and PTS reject properties.

Description

[0001] PAPER OR PAPERBOARD PRODUCT WITH LOW LIQUID ABSORBENCY

[0002] FIELD OF INVENTION

[0003] The present disclosure relates to a paper or paperboard product with improved surface properties, and more particularly to a method for producing a paper or paperboard product with low absorbency, and a paper or paperboard product obtainable by said method.

[0004] BACKGROUND

[0005] Paper and paperboard are widely used materials for disposable containers and food packaging items such as cups and trays. To provide the required liquid barrier performance for these applications, such items and products are typically coated with plastic. Most coating application methods, such as extrusion or dispersion coating require high coat weights of generally over 15-20 g / m2to form an effective continuous barrier layer. However, plastic coatings present several challenges from a sustainability perspective. They negatively impact recycling efficiency and traditional fossil-based plastics are not compostable. At the same time, reducing the plastic coat weight while keeping the required barrier functionality has proven to be challenging, and completely removing the plastic layer entirely significantly reduces or eliminates the barrier performance against liquids and oils, making the material unsuitable for food- and beverage-contact purposes. Additionally, producing a treated paperboard, laminate, or end products such as cups / trays that are repulpable (i.e. with a PTS reject of less than 10% or preferably less than 5%) remains difficult with current technologies. It has been appreciated that an improved paperboard with enhanced liquid resistance is needed that overcomes one or more of these problems.

[0006] SUMMARY

[0007] The present invention aims to provide a paper- or paperboard material suitable for manufacturing single-use liquid containers, including those for hot liquids such as paper cups. One object of the present invention is to enable production of such containers with a minimized plastic content while maintaining sufficient liquidresistance, thus enabling efficient fiber recyclability and fiber reuse and also reduce the use of plastics in disposable containers.

[0008] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. In a first aspect of the invention, a method for producing a paper or paperboard product with low liquid absorbency is provided. The method comprises the initial step of providing a paper or paperboard substrate having a first surface and a second surface. The paper or paperboard substrate provided in the initial phase of the method has certain product properties selected so that the material will function optimally for treatment according to the subsequent method steps, and so that the resulting paper or paperboard product will acquire the properties desired according to the invention, i.e. low liquid absorbency while enabling efficient repulping and recycling of the material. Thus, the provided paper or paperboard substrate comprises at least one of the following properties, preferably all of the following properties:

[0009] -a basis weight between 150 - 400 g / m2’ such as between 200 - 350 g / m2measured according to ISO 536:2019;

[0010] -a thickness between 190-400 pm measured according to ISO 534:2011 ; and / or -a density between 600-900 kg / m3.

[0011] Furthermore, the paper or paperboard substrate comprises a Bendtsen air permeability below 600 ml / min measured according to SCAN P 60:87; Cobb Unger value (15s) for the first surface below 60 g / m2measured according to SCAN-P 37:77; and a Cobb300 value for the first surface below 40 g / m2measured according to ISO 535:2023.

[0012] In a next step of the inventive method, a surface size composition is applied on at least the first surface of the paper or paperboard substrate, where said surface size composition comprises a polysaccharide binder and a sizing polymer. The surface sized paper or paperboard substrate is then dried at temperatures above 70°C measured as surface temperature. Drying at elevated temperatures promote film forming of the sizing composition on the first surface.After surface sizing and drying at elevated temperatures, a paper or paperboard product is obtained with improved surface properties, including reduced liquid absorbency and even exhibiting amphiphobic behavior, meaning the treated surface has acquired both hydrophobic and oleophobic characteristics. Thus, the paper or paperboard material comprises a low water absorbency with a Cobb300 value below 40 g / m2measured according to ISO 535:2023, and a low oil absorbency, with a Cobb-Unger (15 s) value below 20 g / m2, such as below 15 g / m2measured according to SCAN-P 37:77. The paper or paperboard product is also proven to be suitable for recycling, having a total reject of less than 10% or even less than 5% or less than 3% measured according to PTS RH 021 / 97 test method for Category II products. The paper or paperboard product may further have a KIT value above 4 measured according to ISO 16532-2:2007.

[0013] The term “first surface of the paper or paperboard substrate” refers to the side intended to face the food and / or liquid contents of a converted end product, such as a container for example, a paper cup. Accordingly, the “first surface” will form the interior surface of a final product.

[0014] Furthermore, in the present disclosure, a distinction is made between the terms “paper or paperboard substrate" and “paper or paperboard product’. The term “paper or paperboard substrate” refers to the starting material which serves as the base for the subsequent surface treatment in the form of surface sizing and drying according to the method of the invention. The term “paper or paperboard product” in its turn, herein refers to the material obtained after surface-sizing and drying.

[0015] Thanks to the method according to the invention, a paper or paperboard product is obtained which has oleophobic as well as hydrophobic properties while maintaining recyclability offering improved sustainability due to possibly reduced plastic content while maintaining the necessary properties for food packaging and disposable container applications. The paper or paperboard product obtained by the inventive method is particularly suitable as food service board (FSB), meaning it is intended for food service applications such as cups for hot and cold beverages, bowls, trays, plates, and frozen food packaging. Food service board (FSB) generally refers to multi-ply paperboard, such as three-ply paperboard, specifically designed to be converted into various food-contact products.The surface size composition comprises a sizing polymer. The sizing polymer, which according to the invention may contribute to providing the first surface with an amphiphobic character, is preferably a hydrophobic polymer such as a latex. The latex may be selected from the group consisting of styrene acrylate latex, styrene butadiene latex, and carboxylated styrene-butadiene latex. The sizing polymer may also be selected from the group consisting of polyurethane, styrene maleic anhydride, styrene acrylic acid and polyhydroxyalkanoate (PHA). Using these specific hydrophobic polymers in the surface size composition improves film-forming properties - especially at elevated drying temperatures - and enhances the hydrophobicity of the obtained paper or paperboard product. Additionally, these polymers provide flexible and stretchable surface sizing, tolerating various downstream processing including calendering, folding, creasing and / or converting operations without rupturing or destroying the surface hydrophobicity.

[0016] The amount of sizing polymer in the surface size composition may be between 5 - 25 wt%, such as 5 - 20 wt% or 10 - 20 wt% based on total solids content in the surface size composition. This content range provides a balance between improved surface properties and maintaining the recyclability of the paper or paperboard product.

[0017] The polysaccharide binder of the surface size composition may be starch, preferably modified or depolymerized starch or a starch derivative, preferably cooked starch. Starch as a polysaccharide binder offers good binding properties, it is renewable, and it contributes to the overall sustainability of the product. The amount of starch in the surface size composition may be between 50 - 95 wt% based on total solids content in the composition.

[0018] The surface size composition, comprising the polysaccharide binder and the sizing polymer, is preferably based almost entirely on organic components, preferably at least 90 wt%, such as at least 95 wt% of the dry content in said surface size composition is made from organic components.

[0019] The surface size composition may be applied in amounts between 1 - 10 g / m2, such as between 2 - 7 g / m2. This enables efficient surface treatment, minimizing material usage while optimizing both cost and performance.

[0020] The paper or paperboard substrate provided at the initial phase of the inventive method may be a multi-ply paperboard, preferably comprising three plies. A multi-ply structure,such as a three-ply structure, enhances the strength and stiffness of the paperboard while allowing for tailored properties in different plies. The paper or paperboard substrate comprises a hydrophobic internal sizing agent, preferably selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), a rosin sizing agent, a fatty acid, a fatty acid derivative, and combinations thereof. Preferably, the internal sizing is added amounts between 0.5 - 8 kg / tn or between 0.5 - 5 kg / tn. Internal sizing improves the water resistance throughout the paper or paperboard structure, complementing the surface treatment for enhanced overall performance. Preferably, the paper or paperboard substrate has a Cobb300 value for the first surface below 40 g / m2according to ISO 535:2023.

[0021] In case of a multi-ply paper or paperboard substrate, such as a three-ply paperboard, at least one or both of the outer plies, such as at least the outer ply which forms the first surface of the substrate, may comprise kraft pulp, such as at least 70 wt%, preferably at least 80 wt%, such as 85 - 100 wt% bleached or unbleached hardwood kraft pulp based on the total fiber content. Preferably, bleached hardwood kraft pulp is used which has been refined to a Schopper Riegler Value of at least 24 or at least 28, such as between 24 - 45, or 24 - 40, measured according to ISO 5267-1:1999. Preferably, the average fiber length of the bleached hardwood kraft pulp used for the outer plies is between 0.7 - 1.5 mm such as 0.8 - 1.4 mm measured according to ISO 16065-2 e.g. with an FS5 fiber analyzer (Valmet). Preferably, the consistency of the pulp suspension when forming the outer plies is between 0.2 - 0.8 wt%, such as between 0.25 - 0.75 wt% calculated as dry weight of fibers of the total weight of the suspension, to optimize formation and promote creation of a smooth surface. Higher refining of the fibers used for forming the outer plies leads to increased densification and reduced air permeability of the paper or board material. It also improves the formation and smoothness of the paperboard surface, enhancing its printability and barrier properties. Furthermore, the pulp suspension used for the outer ply / plies may include further additives including organic fillers such as microcrystalline cellulose (MCC), cross-linked cellulose, starch particles and / or fines in amounts between 0.5 -10 wt% based on the total solids content in the pulp suspension.

[0022] In case of a multi-ply structure, the mid-ply may have a different composition compared to the outer plies. In some examples the mid-ply, located between two outer plies, may contain a combination of kraft pulp and chemithermomechanical pulp (CTMP). TheCTMP content in the mid-ply may be between 10 - 60 wt% of the total pulp composition. In addition to CTMP, the pulp composition for the mid-ply may also comprise 5 - 40 wt% broke and 0 - 50 wt% kraft pulp. The inclusion of CTMP in the mid-ply can provide additional bulk and stiffness to the paperboard while maintaining a cost-effective composition. In other examples, the mid-ply may contain 100% kraft pulp. In some other examples, the mid-ply may contain a combination of kraft pulp and broke, such as between 40 - 55 wt% kraft pulp, 40 - 55 wt% broke and 0 - 10 wt% CTMP. The mid-ply may optionally comprise < 5 wt% m icrofibrillated cellulose (MFC) or highly refined cellulose.

[0023] As mentioned previously, the pulp suspensions, including the ones used for forming the outer plies, may include internal sizing agents such as AKD, ASA and / or rosin resin at dosages between 0.5 - 8 kg / tn calculated on the total fiber dry weight. Internal sizing in combination with fine fiber properties will enable good sizing retention and both mechanical as well as positive physicochemical effects on further surface sizing penetration.

[0024] The paper or paperboard substrate comprises a Bendtsen air permeability between 50 - 600 ml / min, preferably between 50 - 550 ml / min such as below 550 ml / min or below 500 ml / min measured according to ISO 5636-3:2013. This air permeability range ensures good printability and coating holdout while maintaining sufficient porosity for processing the material. “Coating holdout” refers to the ability of the paper surface to prevent coating from penetrating into the sheet.

[0025] The paper or paperboard substrate comprises a Cobb Unger (15 s) value between 20-60 g / m2measured according to SCAN-P 37:77. This Cobb Unger value range indicates a suitable level of surface density for further surface treatment including surface sizing. The paper or paperboard substrate may have a moisture content between 4 - 11 wt% according to ISO 287:2017. Lower moisture content in the board improves the efficiency of the drying process, allowing the board to be heated more effectively. A lower moisture content results in a higher board temperature and more efficient drying. As previously explained, according to the invention, a surface size composition comprising a polysaccharide binder and a sizing polymer, such as a hydrophobic sizing polymer, is applied at least on the first surface of the paper or paperboard substrate, where said paper or paperboard substrate comprises specific property values asdescribed above. The surface sized board is then dried at temperatures above 70°C, such as above 75°C, preferably above 80°C. After this surface treatment, the resulting paper or paperboard product may have a Cobb Unger (15 s) below 20 g / m2, such as below 15 g / m2measured according to SCAN-P 37:77. Together with also having a Cobb300 value below 40 measured according to ISO 535:2023, the paper or paperboard product obtained by means of the method according to the invention comprises amphiphobic properties, i.e. being both oleophobic and hydrophobic. The surface sized paper or paperboard product may further have a total reject value or amount of less than 10%, or even less than 5%, such as less than 3% according to PTS RH 021 / 97 test method for Category II products. This low PTS reject value indicates efficient disintegration and pulping and thus good recyclability, contributing to the product's sustainability and circular economy potential.

[0026] Another advantage associated with the present invention is that the obtained paper or paperboard product presents surface properties that are particularly suitable for further application of additional on-line barrier coating, for instance in the form of a dispersion-coated barrier layer. The densified surface with low oil absorbency prevents applied polymer coatings and / or varnishes from penetrating into the paperboard and cause undesired fiber swelling. Instead, the coatings rather form a layer on top of the board’s surface, reducing the amount needed to create a barrier layer and resulting in source reduction and thereby also cost savings. Thus, the method according to the invention further comprises applying a barrier layer, e.g. in the form of a polymer coating, on the surface-sized first surface of the paper or paperboard product. Said barrier layer may comprise a grammage below 6 g / m2, preferably below 5 g / m2. This comparably thin barrier layer provides additional protection against moisture penetration while maintaining the product's overall low plastic content and good recyclability. The barrier layer may be applied by means of e.g. dispersion coating or printing. These application methods allow for precise and uniform application of the thin barrier layer, optimizing material use and performance.

[0027] In a second aspect, a paper or paperboard product with densified surface and improved liquid resistance, obtainable by the method according to any one of the previous aspects, is provided. This product combines enhanced surface properties with sustainability features, meeting the increasing demand for eco-friendly packaging solutions with reduced plastic content.The foregoing general description of the illustrative embodiments and the following detailed description thereof are merely exemplary aspects of the teachings of this disclosure and are not restrictive.

[0028] DETAILED DESCRIPTION

[0029] The following description sets forth exemplary aspects of the present disclosure. It should be recognized, however, that such description is not intended as a limitation on the scope of the present disclosure. Rather, the description also encompasses combinations and modifications to those exemplary aspects described herein.

[0030] As used herein, the terms “paper or paperboard substrate” and “paper or paperboard product” respectively refer to sheet material made from cellulose fibers, which may be derived from wood, recycled paper, or other plant-based sources. These terms encompass both paper and paperboard, where paperboard generally refers to a thicker and more rigid material than paper. The “paper or paperboard substrate” and “paper or paperboard product” respectively may be single-ply or multi-ply structures, and may include additional components such as coatings, fillers, and / or sizing agents. In a preferred example, the paper or paperboard substrate in the present invention is a three-ply paperboard.

[0031] A paper or paperboard substrate to be used as the base material in the method of the invention comprises at least one of the following properties, preferably all of the following properties:

[0032] -a basis weight between 150 - 400 g / m2, such as between 200 - 350 g / m2measured according to ISO 536:2019;

[0033] -a thickness between 180 - 400 pm measured according to ISO 534:2011 ; and / or -a density between 600 - 900 kg / m3.

[0034] In one example of the invention, the paper or paperboard substrate comprises at least the property of comprising a density between 600 - 900 kg / m3.

[0035] In one example of the invention, the paper or paperboard substrate to be used as a base material in the method of the invention is a multi-ply paperboard comprising a basis weight between 150 g / m2- 350 g / m2, such as between 200 - 310 g / m2; a thickness between 180 - 400 pm; and a density between 650 - 900 kg / m3.As used herein, the term “amphiphobic” refers to the ability of repelling both water and oil, thus comprising both hydrophobic and oleophobic character simultaneously. In other words, an amphiphobic surface resists wetting by polar liquids like water as well as it resists absorption of non-polar liquids like oil.

[0036] As used herein, the term “hydrophobic” refers to the ability of repelling water, and the term “oleophobic” refers to the ability of repelling oil and / or grease.

[0037] As used herein, the term “liquid” refers to a non-gaseous fluid composition, compound material or the like, which may be readily flowable at the temperature of use, such as room temperature, and with low compressibility. One non-limiting example of a polar liquid is water, and another example of a non-polar liquid is oil.

[0038] As used herein, the term “absorbency” refers to the ability of a material to take in and retain a liquid (such as water, oil, or other fluids) within its structure. A material with high absorbency soaks up liquid easily whereas a material with low absorbency resists liquid penetration.

[0039] As used herein, “surface sizing” refers to a treatment applied to the surface of a paper or paperboard, typically at or after the drying section of a paper machine, in which a thin layer of size composition is applied to the sheet. The purpose of surface sizing is to modify and improve surface properties without necessarily forming a distinct external layer.

[0040] As used herein, "barrier layer" or “barrier coating” refers to a distinct external coating layer on top of the surface of a paperboard providing barrier properties to the paperboard substrate by reducing or eliminating the permeability of gases through the material and / or the absorption of liquids in the fiber structure. Barrier coatings prevent the egress of flavors, aromas or other ingredients out of the packaged product, and especially to prevent the ingress into the package of oxygen, moisture, grease and oil, especially mineral oil, and other contaminants that might deteriorate the quality of the packaged product. As used herein, "dispersion barrier layer" refers to a layer that has been brought by dispersion coating onto a paperboard substrate.

[0041] The smoothness of the paper or paperboard substrate, particularly of the first surface, may be characterized by a Bendtsen smoothness below 700 ml / min measured according to ISO 8791 -2:2013. A substrate with appropriate smoothness may allow formore uniform application of subsequent surface sizing and also reduced penetration of the surface size composition into the substrate, i.e. lowered absorbency.

[0042] The paper or paperboard substrate of the present invention also has a dense structure with low air permeability. This helps prevent or at least minimize problems with uneven barrier coating and / or pinhole formation. More porous fiber substrates tend to absorb large amounts of applied material, leading to variations in barrier thickness and an increased risk of pinholes.

[0043] The “air permeability” referred to herein is sometimes also called the “Bendtsen porosity” or “Bendtsen air permeability” measured in accordance with ISO 5636-3:2013, which corresponds to the rate of airflow passing perpendicularly through a known area under a prescribed air pressure differential between the two surfaces of a material. According to the invention, the air permeability of the paper or paperboard substrate before surface sizing is below 600 ml / min such as below 550 ml / min measured according to ISO 5636-3:2013. In some examples, the Bendtsen air permeability may be between 50-600 ml / min, or more specifically, between 50-500 ml / min or between 50-200 ml / min. A low air permeability means a less porous and more sealed structure.

[0044] As explained above, the paper or paperboard substrate serving as the base material is selected to exhibit specific material properties making the material particularly suitable for use as food service board, as well as specific surface properties, such as low air permeability, indicating a dense surface structure. Surface densification of a paper or paperboard can be accomplished in various ways known to the skilled person. For example, the surface of the paper or paperboard may be pre-treated with a surface sizing composition e.g. applied under pressure. Refining the fibers used for the outer plies is another way to achieve a dense board surface. Refined fibers become more flexible, creating a denser and less porous surface, especially when combined with wet pressing.

[0045] The liquid absorption properties of the paper or paperboard substrate may be described by Cobb values. The Cobb Unger value (15s) for the first surface of the substrate may be below 60 g / m2according to SCAN-P 37:77. Additionally, the Cobb300 value for the first surface of the substrate may be below 40 g / m2according to ISO 535:2023. In some examples, the paper or paperboard substrate may have a Cobb Unger value (15s) between 20-60 g / m2.The moisture content of the paper or paperboard substrate may be between 4-11 wt% according to ISO 287:2017.

[0046] Further properties of the paper or paperboard substrate include stretch in cross direction (CD) which preferably is above 5%, such as between 6 - 11%, or 6 - 9% measured according ISO 1924-3; and / or Scott bond which preferably is above 180 J / m2, preferably above 200 J / m2or between 200 - 700 J / m2measured according to TAPPI 569. In one example, the paper or paperboard substrate has a geometric SCT index above 22 Nm / g, preferably above 24 Nm / g, measured according to ISO 9895. The paper or paperboard substrate includes additional treatments to enhance its properties and performance in the form of incorporation of hydrophobic internal sizing agents. Additives in the form of internal sizing agents improve water resistance and control liquid penetration. Examples of hydrophobic internal sizing agents that may be used include alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin sizing agents, fatty acids, fatty acid derivatives, and combinations thereof. These sizing agents may be incorporated into the paper or paperboard substrate during the papermaking process, helping to reduce the substrate's affinity for water and improve its overall liquid resistance. A preferred combination of internal sizing agents is rosin and ASA, i.e. a combination whereof at least one internal sizing additive provides a sizing effect early in the production process, such as before the size press.

[0047] In some examples, the paper or paperboard substrate may be a multi-ply paperboard. For instance, the substrate may comprise three plies, and have a basis weight between 150 g / m2- 350 g / m2, such as between 200 - 310 g / m2; a thickness between 180 -400 pm; and a density between 650 - 900 kg / m3The use of multiple plies may contribute to the overall strength and performance characteristics of the substrate. In a preferred example of the present invention, the paper or paperboard substrate to be used as a base material in the method of the invention is a multi-ply paperboard with a basis weight between 150 g / m2- 350 g / m2, a thickness between 180 - 400 pm; and a density between 650 - 900 kg / m3. The multi-ply paperboard comprises at least two outer plies and a mid-ply positioned between the outer plies. In this example, the outer plies contain 85-100 wt% kraft pulp, preferably bleached hardwood kraft pulp, having a Schopper-Riegler value of at least 24, or at least 27, such as within the range of 24-40, measured according to ISO 5267-1:1999.Furthermore, in this example, the mid-ply comprises 40-55 wt% kraft pulp, 40-55 wt% broke, and 0-10 wt% CTMP. The mid-ply may also optionally include < 5 wt% m icrofibrillated cellulose (MFC) or highly refined cellulose. The weight percentages are calculated based on the total fiber weight of the respective ply.

[0048] According to the invention, the paper or paperboard substrate comprising the above-mentioned properties is subjected to a surface treatment by means of applying a surface size composition on at least the first surface thereof. By applying the specific surface size composition and performing a controlled drying process at elevated temperatures, the method produces a paper or paperboard product with an even denser surface compared to before surface treatment, resulting in reduced absorbency and improved performance characteristics such as acquired amphiphobic surface properties.

[0049] The surface size composition applied to the paper or paperboard substrate comprises a polysaccharide binder and a sizing polymer, preferably a hydrophobic sizing polymer such as latex. This composition is designed to further densify the surface of the substrate, reducing absorbency and e.g. improving the substrate's suitability for subsequent barrier layer application.

[0050] The polysaccharide binder in the surface size composition may be starch. Starch is a widely available and renewable resource that can effectively bind the components of the surface size composition and contribute to the densification of the substrate surface. The selected starch may be depolymerized or modified, including derivatives aimed at surface sizing or coating. Oxidized starch or dextrin, with high solids content and low viscosity (Brookfield viscosity less than 100 mPas, when determined at 100 rpm, at 10 % solids content and 23 °C), are especially suitable for use in a pond size press. The starch is preferably cooked and can be nonionic, anionic, cationic, or amphoteric. The amount of starch in the surface size composition may be between 50 - 95 wt% based on total solids content.

[0051] The surface size composition may include sizing polymer in anionic, cationic, or amphoteric emulsion. The choice of polymer emulsion type may depend on the specific properties desired for the final product and the compatibility with other components of the surface size composition and the substrate. For example, the sizing polymer of the surface size composition may be selected from the group comprising latex,polyurethane, styrene maleic anhydride, styrene acrylic acid and polyhydroxyalkanoate (PHA). The sizing polymer is preferably a polymeric chemical, such as a hydrophobic polymeric chemical, that provides hydrophobicity directly upon drying.

[0052] In a preferred example of the invention, the sizing polymer is latex. The latex component may be selected from various types of latex emulsions. In some examples, the latex may be selected from the group consisting of styrene acrylate latex, styrene butadiene latex, and carboxylated styrene-butadiene latex. These latex types can provide effective film-forming properties and contribute to the desired surface characteristics of the treated substrate.

[0053] The amount of the sizing polymer in surface size composition may be between 5 - 50 wt% such as 5 - 20 wt% or 10 - 20 wt% based on total solids content.

[0054] This range can provide effective film-forming properties while maintaining a balance with the polysaccharide binder and other components of the surface size composition. The surface size composition may include further additives including organic fillers such as nanocellulose and / or m icrofibrillated cellulose (MFC) in amounts between 0.1 -15wt% based on the total solids content in the surface size composition. Also, a fixative such as polyaluminum chloride (PAC) may be added to the surface size composition in amounts between 1 - 5 wt% based on the total solids content.

[0055] The surface size composition, comprising the polysaccharide binder and the sizing polymer, is preferably based almost entirely on organic components, preferably at least 90 wt%, such as at least 95 wt% of the dry content in said surface size composition is made from organic components.

[0056] The surface size composition may be applied to the paper or paperboard substrate in amounts between 1 - 10 g / m2, such as between 2 - 7 g / m2. This range allows for effective surface treatment while maintaining a relatively low addition of material to the substrate.

[0057] The combination of the polysaccharide binder and latex in the surface size composition, when applied to the paper or paperboard substrate, results in a paper or paperboard product with densified surface and low liquid absorbency.The surface size composition may be applied to the paper or paperboard substrate in various ways. In some examples, the surface size composition is applied by means of pond size press or film size press, or by using gravure coaters, curtain coating applicators, roll applicators or size press. It is also possible to apply the surface sizing by other available application methods in the paper and board machine, such as rod coating, blade coating, or spraying. Surface sizing can be performed through direct or indirect transfer, where indirect transfer involves first applying the sizing to a transfer roll, which then transfers it to the board substrate. Surface sizing can be applied as a single or double layer, and both sides of the paper or paperboard board can be treated. Additionally, the recipe can be added in the form of foam using for example n-OSA starch as foamable binder. In case of a size press, e.g. using driven rolls, may apply a nip pressure with a line load below 40 kN / m, such as below 35 kN / m, preferably between 10 - 25kN / m. These nip pressures are selected to promote the coating-hold out of the applied surface sizing and result in a better barrier. These application methods allow for controlled and uniform application of the surface size composition to the substrate surface.

[0058] After application of the surface size composition onto the substrate, the surface sized paper or paperboard undergoes a drying process. The drying process may be carried out at elevated temperatures to promote film formation and achieve the desired surface properties. In some examples, the surface sized board is dried at above glass transition temperatures (Tg), such as above 70°C, more preferably above 75°C such as between 78 - 95°C measured at the surface of the board. Drying at elevated temperatures may be performed using drying cylinders, metal belts and / or combination of drying curing via IR and drying cylinders, optionally also using air dryers.

[0059] The elevated drying temperature may contribute to the densification of the substrate surface and the formation of a continuous film from the surface size composition. This process can result in reduced absorbency of the treated surface, which may improve the substrate's suitability for subsequent barrier layer application.

[0060] The surface size composition, comprising a polysaccharide binder and a latex, interacts with the substrate surface to create a densified layer. The polysaccharide binder, such as starch, may penetrate the surface fibers and form bonds between them, while the latex component may form a film on the surface. This combination of penetration and film formation may result in a more closed and uniform surfacestructure. Furthermore, during the drying process at elevated temperatures, the components of the surface size composition undergo physical and chemical changes. The elevated temperature may promote the film-forming properties of the latex, leading to a more continuous and cohesive surface layer. Simultaneously, the polysaccharide binder may undergo gelatinization and form stronger bonds with the substrate fibers. The surface sizing treatment in combination with drying process at elevated temperatures affect the final properties of the surface sized paper or paperboard product. One significant property of the resulting surface sized paper or paperboard product is the oleophobicity, demonstrated by the low Cobb Unger value, which measures the amount of liquid in the form of oil absorbed by the paper or paperboard surface over a specific time period. The paper or paperboard product of the invention has a Cobb Unger 15s value below 20 g / m2according to SCAN-P 37:77, preferably below 15 g / m2or below 10 g / m2. The low Cobb Unger value of the paper or paperboard product is achieved by sealing the paper and paperboard surface as described above, which results in reduced oil absorption into the substrate. The low Cobb Unger value indicates a significant reduction in oil absorption compared to untreated substrates, demonstrating the effectiveness of the surface treatment according to the method of the invention. Additionally, oil and grease resistance properties in the surface sized paper or paperboard product are also improved. In some examples, the paper or paperboard product also exhibits a KIT value above 4 according to ISO 16532-2:2007, indicating resistance to oil and grease penetration. These properties collectively contribute to the improved performance of the paper or paperboard product. The low Cobb Unger value and high KIT value demonstrate enhanced oil resistance, which may be particularly beneficial in food packaging and disposable container applications. Another significant property of the resulting surface sized paper or paperboard product is the hydrophobicity, demonstrated by the low Cobb60 and low Cobb300 values respectively, which measures the amount of water in the form of water absorbed by the paper or paperboard surface over a specific time period. The paper or paperboard product of the invention a Cobb300 value for at least the first surface below 35 g / m2measured according to ISO 535:2023; and a Cobb60 value for at least the first surface below 20 g / m2measured according to ISO 535:2023.The product's ability to resist both water- and oil penetration, thus demonstrating amphiphobic character, while maintaining recyclability addresses the challenges associated with traditional plastic-coated paper products.

[0061] Another advantageous feature of the product according to the invention is recyclability. The recyclability of the paper or paperboard product is assessed by using the RH 021 / 97 test method for Category II products. The surface sized paper or paperboard product may have a total reject of less than 10%, or even less than 5%, such as less than 3% according to PTS RH 021 / 97 test method for Category II products. This low PTS reject percentage indicates good recyclability characteristics, suggesting that the product can be effectively recycled and the fibers reused through standard paper recycling processes. In some examples, the paper or paperboard product may achieve a PTS reject below 5%, further enhancing its recyclability profile.

[0062] The low absorbency of the paper or paperboard product according to the invention is advantageous for example when applying additional barriers onto the board’s surface, such as a dispersion coating. The film formation of a barrier coating onto a paper or paperboard product according to the invention, and hence also the resulting barrier performance, can be improved even at lower coat weights. This means that the invention enables for application of thin barrier layers, which in its turn can provide moisture resistance and / or sealing properties also at reduced coat weight compared to conventional methods such as extrusion coating. This leads to potential reduction of the overall plastic content of the final product while maintaining desired performance properties. Also, the combination of improved liquid resistance and good recyclability may allow for the reduction or elimination of plastic coatings in certain applications, potentially leading to more environmentally friendly packaging solutions. These characteristics may also contribute to extended shelf life for packaged products and improved performance in various end-use scenarios.

[0063] According to the invention, in addition to internal sizing and surface treatment in the form of surface sizing for densifying the surface, a barrier layer may be applied to the surface sized paper or paperboard product to further enhance its barrier properties like providing low Water Vapor Transition Rate (WVTR) and low oxygen transmission rate (OTR) and / or providing heat sealability. This barrier layer may be applied to the first surface of the surface sized paper or paperboard product using various methods, including dispersion coating or printing like Flexographic printing technology or use ofanilox applicators. As previously mentioned, thanks to the surface performance of the paper or paperboard product, it is possible to apply a very thin barrier layer while still retaining the required barrier function; even having a grammage below 6 g / m2, and in some examples, below 5 g / m2such as between 1.5 - 5 g / m2. The use of a thin barrier layer may contribute to reducing the overall plastic content of the final product.

[0064] The coating composition used for the barrier layer may have a high renewable content. In some examples, the coating composition may have a renewable content of greater than 80%. In other examples, the renewable content may be greater than 90%. By “renewable” means “derived from renewable biomass sources”, such as from plants, where renewable material and substances are designed to be more sustainable and environmentally friendly compared to substances made from fossil fuels.

[0065] The use of renewable materials in the coating composition may contribute to the overall sustainability profile of the converted end product such as a paper cup.

[0066] Internal sizing agents work in conjunction with the surface treatment to provide improved resistance to liquid penetration throughout the substrate structure. A thin barrier layer applied to the surface-treated substrate may build upon the improved surface characteristics to provide additional protection against moisture and also provide heat sealability. The combination of these elements may result in a paper or paperboard product with enhanced oil resistance, as demonstrated by the improved Cobb Unger and KIT values. The low absorbency of the material’s surface may also contribute to improved print quality and reduced ink or coating penetration in subsequent coating and converting processes.

[0067] Examples

[0068] Example 1 - Comparative

[0069] In a comparative example, a three-ply 232 gsm paperboard substrate was used for surface sizing trial. The paperboard substrate was internally sized and had a Cobb300 of 35 g / m2, a Cobb Unger (15 s) of 79 g / m2and air permeability of 806 ml / min (Bendtsen). This comparative paperboard substrate is referred to as “Board 1” in Table 1 below.The paperboard substrate was surface sized with a surface sizing recipe comprising 6.7 wt% styrene / acrylate latex based on total solids content; 3.9 wt% of cationic fixative polyaluminum chloride (PAC) based on total solids content; and 89.4 wt% of a starch polysaccharide, i.e. percentage of starch of the total solids content in the surface size recipe. The amount of surface sizing applied onto the paperboard substrate was 2.8 g / m2The surface sizing was performed using a rod coater and then dried with hot air at temperatures of about 130°C measured at the board surface.

[0070] Example 2 - Invention

[0071] A paperboard substrate of the same grade as in Comparative Example 1 was provided and is referred to as “Board 2” in Table 1 below. Board 2 had been treated to achieve different material properties compared to the paperboard in Example 1 , specifically lower air permeability, a smoother surface, and improved liquid resistance, particularly in the form of lower oil absorbency. For Board 2, the water resistance measured with Cobb300 was 34 g / m2, the air permeability was 489 mL / min (Bendtsen), and the oil absorbency determined with Cobb Unger (15 s) was 57 g / m2.

[0072] This paperboard substrate, i.e., Board 2, was subjected to surface sizing using the same recipe as in Example 1, applied at a level of 2.8 g / m2, and dried with hot air at temperatures of approximately 130°C, measured at the board surface.

[0073] It was shown that surface sizing of Board 2 gives a significant improvement in especially reduced oil absorbency upon subjecting the board having lower air permeability to the above-described surface treatment. The Cobb-Unger value (15s) was lowered from 57 g / m2to 7 g / m2, which will have great impact on coating holdout and reduction in absorption or penetration of applied liquids such as for example dispersion barrier, lacquers, varnish, inks and adhesives.Example 1: Example 2:

[0074] Standard Board 1 Board 2 (Surface sized (Surface sized Board 1) Board 2) Paperboard

[0075] Board 1 Board 2 substrate

[0076] Basis weight

[0077] ISO 536:2019 231 235

[0078] (g / m2)

[0079] Density (kg / m3) ISO 534:2011 628 699

[0080] Thickness ISO 534:2011 368 336

[0081] Cobb60 (g / m2) ISO 535:2023 18 18 17 16

[0082] Cobb300 (g / m2) ISO 535:2023 35 34 31 31

[0083] Air permeability, ISO 5636- 806 489 581 314 Bendtsen (ml / min) 3:2013

[0084] Bendtsen surface

[0085] smoothness ISO 8791-2 834 644

[0086] (ml / min)

[0087] Cobb Unger 15s

[0088] SCAN-P 37:77 79 57 30 7

[0089] (g / m2)

[0090] Stretch in cross

[0091] ISO 1924-3 5.6 5.9

[0092] direction (%)

[0093]

[0094] Table 1

[0095] In summary, the interaction between the substrate properties, surface treatment steps, and optional additional treatments creates a synergistic effect that results in a paper or paperboard product with improved oil resistance, in particular for Example 2 where the Cobb Unger (15s) value is significantly decreased compared to Example 1. It is also observed that the water resistance measured as Cobb60 and Cobb300 is slightly improved in both Examples 1 and 2 compared to before surface sizing treatment.

[0096] The dimensions and values disclosed herein are not to be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, eachsuch dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a value disclosed as “10 wt.%” is intended to mean “about 10 wt.%”. A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other implementations are within the scope of the following claims.

Claims

CLAIMS1. A method for producing a paper or paperboard product with improved liquid resistance, comprising:-providing a paper or paperboard substrate having a first surface and a second surface and comprising at least one of the following properties:-a basis weight between 150 - 400 g / m2’ such as between 200 - 350 g / m2measured according to ISO 536:2019;-a thickness between 190 - 400 pm measured according to ISO 534:2011; and / or-a density between 600 - 900 kg / m3measured according to ISO 534:2011 ; wherein said paper or paperboard substrate further comprises a Bendtsen air permeability below 600 ml / min measured according to ISO 5636-3:2013; a Cobb Unger value (15s) for the first surface below 60 g / m2measured according to SCAN-P 37:77; and a Cobb300 value for the first surface below 40 g / m2measured according to ISO 535:2023;-applying a surface size composition on at least the first surface of the paperboard, said surface size composition comprising a polysaccharide binder and a sizing polymer, and-drying the surface sized board at temperatures above 70°C;-wherein the resulting surface sized paper or paperboard product has a Cobb Unger (15s) below 20 g / m2, such as below 15 g / m2, measured according to SCAN-P 37:77, and a total reject of less than 10% measured according to PTS RH 021 / 97 test method for Category II products.

2. The method of claim 1 , wherein the sizing polymer of the surface size composition is latex selected from the group consisting of styrene acrylate latex, styrene butadiene latex, and carboxylated styrene-butadiene latex.

3. The method of claim 1 , wherein the sizing polymer of the surface size composition is selected from the group consisting of polyurethane, styrene maleic anhydride, styrene acrylic acid and polyhydroxyalkanoate (PHA).

4. The method according to any one of the previous claims, wherein the polysaccharide binder is starch, preferably modified or depolymerized starch or a starch derivative, wherein the amount of starch in the surface size composition is between 50 - 95 wt% based on total solids content.

5. The method according to any one of the previous claims, wherein the surface size composition is applied in amounts between 1 - 10 g / m2.

6. The method according to any one of the previous claims, wherein the amount of sizing polymer in the surface size composition is between 5 - 50 wt%, preferably 5 -20 wt% based on total solids content.

7. The method according to any one of the previous claims, wherein the paper or paperboard substrate is a multi-ply paperboard, preferably comprising at least two outer plies and a mid-ply located between said outer plies, wherein the outer plies comprise 85 - 100 wt% kraft pulp.

8. The method according to any one of the previous claims, wherein the paper or paperboard substrate comprises a hydrophobic internal sizing agent, preferably selected from the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), a rosin sizing agent, a fatty acid, a fatty acid derivative, and combinations thereof.

9. The method according to any one of the previous claims, wherein the paper or paperboard substrate comprises a Bendtsen surface smoothness for at least the first surface below 700 ml / min measured according to ISO 8791-2.

10. The method according to any one of the previous claims, wherein the paper or paperboard substrate has a Bendtsen air permeability between 50 - 550 ml / min, preferably between 50 - 500 ml / min according to ISO 5636-3:2013.

11. The method according to any one of the previous claims, wherein the paper or paperboard substrate has a Cobb Unger (15 s) value for at least the first surface between 20 - 60 g / m2according to SCAN-P 37:77.

12. The method according to any one of the previous claims, wherein the paper or paperboard substrate has a moisture content between 4 - 11 wt% according to ISO 287:2017.

13. The method according to any one of the previous claims, wherein the paper or paperboard substrate has a stretch in cross direction (CD) above 5% measured according to ISO 1924-3.

14. The method according to any one of the previous claims, wherein the paper or paperboard product has a Cobb300 value for at least the first surface below 35 g / m2measured according to ISO 535:2023; and a Cobb60 value for at least the first surface below 20 g / m2measured according to ISO 535:2023.

15. The method according to any one of the previous claims, wherein the paper or paperboard product has a total reject of less than 5%, such as less than 3% according to PTS RH 021 / 97 test method for Category II products.

16. The method according to any one of the previous claims, wherein the paper or paperboard product has a KIT value above 4 measured according to ISO 16532-2:2007.

17. The method according to any one of the previous claims, comprising applying a barrier layer on the first surface of the surface sized paper or paperboard product, said barrier layer having a grammage below 6 g / m2, preferably below 5 g / m2.

18. The method according to claim 17, wherein the barrier layer is applied by means of dispersion coating or printing.

19. A paper or paperboard product with densified surface and improved liquid resistance, obtainable by the method according to any one of the previous claims, comprising a Cobb Unger (15s) below 20 g / m2measured according to SCAN-P 37:77 and a total reject of less than 10% measured according to PTS RH 021 / 97 test method for Category II products.

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