Technical field

The present disclosure relates to mineral-coated cellulose-based substrates, such as paper or paperboard, for use in wet or damp conditions.

Background

Water-resistance is an important property in many paper or paperboard applications. Some examples include packaging, such as boxes, and other containers; fresh and aseptic liquid packaging; boxes, trays, or cups for hot, cold, dry, wet and frozen food and beverages; products for outdoor use such as boxes, signs and posters; pots, trays and covers for plants; packages for construction materials, and construction materials.

Coating of paper or paperboard with plastics is often employed to combine the mechanical properties of the paperboard with the barrier and sealing properties of a plastic film. A problem with the addition of plastics is that the repulpability of the material is severely reduced, which also can affect the recycling streams.

Therefore, as few different material types as possible is desirable in packaging materials. Also, to reduce the carbon footprint of the materials, particularly in packaging materials, there is a wish to reduce and replace fossil-based plastic films with renewable alternatives.

Paper or paperboard for use in wet or damp conditions is also usually treated with sizing agents to enhance certain qualities; and above all, to increase the resistance to penetration of water and other liquids into the cellulose-based substrate, which is important to maintain the integrity and/or function of the substrate. There are two main types of sizing: internal sizing and surface sizing. For internal sizing, chemicals are added to the pulp at the wet end, for example alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) or rosin sizing agent. Common surface-sizing agents include, e.g., modified starches and carboxymethyl cellulose. Plastic-coated paperboard is also often treated with a hydrophobic sizing agent to prevent so-called edge wick, i.e. absorption of liquid at the cut edges (or so-called raw edges) of the paperboard. Edge-wick resistance is an important parameter in many applications.

To improve the wet strength of the material, 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 between the fibers that does not break upon wetting. Common wet-strength agents include urea-formaldehyde (UF), melamine-formaldehyde (MF) and polyamide- epichlorohydrin (PAE). Other wet-strength agents can give wet strength by other mechanisms, and some of these wet-strength agents can also have a temporary wet-strength function.

A problem with the addition of wet-strength agents is that the repulpability of the material is severely reduced.

More recently, grafting with fatty acid halides has emerged as an alternative or complement to hydrophobic sizing. This technique utilizes fatty acid halides, preferably fatty acid chlorides, which are grafted covalently to hydroxyl groups on cellulose-based substrates, like paper and paperboard. The grafting is performed on the pre-formed and dried material. The fatty acid halide reagent may for example be applied to the surface of the substrate in liquid form using a roll- coating process, a curtain-coating process or a spray-coating process, or in gas form followed by condensation of the gaseous reagent on the substrate surface.

Many paper and paperboard products are provided with a mineral-coated surface to give desirable properties such as whiteness, brightness, gloss, and/or high- quality print. Typical coating components include pigments, binders, additives, and water. Commonly used pigments include calcium carbonate, talc, titanium dioxide, and/or kaolin clay. Binders can be fossil-based, such as styrene-butadiene latex, styrene-acrylate latex, vinylacetate latex, and/or vinylacetate-acrylate latex, or bio- based, such as carboxymethyl cellulose and/or a modified starch, where the latter group is preferred from an environmental perspective. Examples of other additives include insolubilizers, lubricants, defoamers, thickening agents, co-binders, stabilizers, and optical brightening agents (OBAs).

Mineral-coated substrates have not been considered compatible with fatty acid halide grafting as the main components of mineral coatings typically do not have a high incidence of hydroxyl groups. In fact, it was believed that in order to achieve an effective grafting of fatty acid halides on a mineral coating, the mineral coating must first be coated with another coating, such as a PVOH coating, having a high incidence of hydroxyl groups.

As the mineral coating covers at least one of the surfaces of the paper or paperboard, it limits the accessibility of the cellulose-based substrate for fatty acid halide grafting. Therefore, internal sizing of the cellulose-based substrate can be required.

There remains a need for improved solutions to obtain water resistant mineral- coated paper and paperboard products while reducing the need for plastics, which is beneficial both from a sustainability and recyclability perspective.

Description of the invention

It is an object of the present disclosure to provide an improved method for manufacturing a water-resistant mineral-coated cellulose-based substrate.

It is a further object of the present disclosure to provide an improved method for manufacturing a water-resistant mineral-coated cellulose-based substrate with maintained or improved grease resistance.

It is a further object of the present disclosure to provide an improved method for manufacturing a water-resistant mineral-coated cellulose-based substrate which can be performed at an industrial scale at a high speed. It is a further object of the present disclosure to provide an improved method for manufacturing a water-resistant mineral-coated cellulose-based substrate which can be implemented in existing manufacturing or converting facilities without adding spacious, time-consuming, and expensive treatment stages.

It is a further object of the present disclosure to provide a method for manufacturing a water-resistant mineral-coated cellulose-based substrate with similar repulpability as compared to a corresponding non-water-resistant mineral- coated cellulose-based substrate.

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.

The present invention is based on the inventive realization that unlike the processes conventionally used for grafting fatty acid halides to hydroxy-functional substrates such as cellulose and polyvinyl alcohol, in which the efficiency and rate of grafting is highly dependent on the reaction temperature, the treatment of a mineral-coated surface, without abundant hydroxyl groups, proceeds quickly and efficiently even at low temperatures. Accordingly, fatty acid halide treatment directly on a mineral-coated surface can be performed in an industrial scale at a high speed and without the requirement for spacious, time-consuming, and expensive heating stages typically associated with grafting of fatty acid halides. This allows for the inventive process to be integrated in existing conventional processes for manufacturing or converting mineral-coated cellulose-based paper or paperboard.

According to a first aspect illustrated herein, there is provided a method for manufacturing a water-resistant mineral-coated cellulose-based substrate, said method comprising: a) providing a mineral-coated cellulose-based substrate comprising a cellulose-based substrate, and a mineral-coating layer disposed on at least one surface of said substrate, said mineral-coating layer comprising

50-95 wt% of particulate calcium carbonate, and 5-50 wt% of a water-dispersible binder, based on the total dry weight of the mineral coating; and b) treating the dry mineral-coating layer with a solvent-free fatty acid halide composition at a temperature above the melting temperature of the fatty acid halide but below 100 °C to obtain a water-resistant mineral-coated cellulose-based substrate.

Many paper and paperboard products require a mineral-coated surface to give desirable properties such as whiteness, brightness, gloss, and/or high-quality print.

Typical mineral coating components include one or more particulate mineral(s) as the main component, and one or more water-dispersible binder(s) as the secondary component. Mineral coatings may also comprise optional additives, such as insolubilizers, lubricants, defoamers, thickening agents, co-binders, stabilizers, and/or optical brightening agents (OBAs). The additives are typically present at an amount of less than 10 wt%, based on the total dry weight of the mineral coating.

Grafting with fatty acid halide has been identified as an interesting alternative to internal sizing agents and wet-strength agents for rendering cellulose-based substrates hydrophobic or water resistant. However, mineral-coated substrates have not been considered compatible with fatty acid halide grafting as the main components of mineral coatings typically do not have a high incidence of hydroxyl groups. In fact, it was believed that in order to achieve an effective grafting of fatty acid halides on a mineral coating, the mineral coating must first be coated with another coating, such as a PVOH coating, having a high incidence of hydroxyl groups. As the mineral coating covers at least one of the surfaces of the paper or paperboard, it also limits the accessibility to the underlying cellulose-based substrate for fatty acid halide grafting.

The present inventors have now surprisingly found that conventional mineral coatings, formed mainly of low hydroxyl-containing components, can be effectively hydrophobized with fatty acid halides, without prior chemical modification or coating, to obtain a highly hydrophobic and water-resistant mineral-coated surface. As an example, the Cobb ₆₀ value of a mineral-coated surface of a cellulose-based substrate was reduced from 26 g/m ² to about 3 g/m ² after being subjected to treatment with palmitoyl chloride (C16). Furthermore, it has also been unexpectedly found that the treatment with a fatty acid halide can improve or at least maintain the grease resistance of the treated surface, even though the surface energy is reduced. As an example, a KIT value of 12 (according to Tappi T559) and an increase exceeding 100% in the time for break-through of grease (according to ASTM F119) was observed for a mineral-coated surface of a cellulose-based substrate after being subjected to treatment with palmitoyl chloride (C16). Thus, in some embodiments, the water-resistant mineral-coated cellulose- based substrate is both water resistant and grease resistant.

The use of fatty acid halide treatment directly on the mineral coating surprisingly makes it possible to obtain a hydrophobic and water-resistant mineral-coated surface with a single process step and a low amount of reagent. The highly hydrophobic and water-resistant surface could be obtained with a very low amount of fatty acid halides, typically less than 1 g/m ² .

The use of fatty acid halide treatment directly on the mineral coating can also reduce the need for a water-resistant polymer coating on top of the mineral coating, which would reduce the need for an additional coating step, facilitating the processability and giving savings in material, time, and cost.

The mineral-coating layer comprises a particulate mineral as main component. More specifically the mineral-coating layer comprises 50-95 wt% of particulate calcium carbonate, and 5-50 wt% of a water-dispersible binder, based on the total dry weight of the mineral coating. The amount of particulate calcium carbonate in the mineral-coating layer is at least 50 wt%, and preferably above 50 wt%, based on the total dry weight of the mineral-coating layer. The amount of particulate calcium carbonate in the mineral-coating layer may for example be in the range of 60-95 wt%, 70-95 wt%, or 80-95 wt%, based on the total dry weight of the mineral- coating layer.

In some embodiments, the mineral-coating layer may further comprise up to 20 wt% of a particulate mineral selected from the group consisting of kaolin, talc, bentonite, and combinations thereof.

The mineral-coating layer further comprises a water-dispersible binder, e.g. a fossil-based latex, a biobased latex, carboxymethyl cellulose, and/or a modified starch. The fossil-based latex may for example be a styrene-butadiene latex, styrene-acrylate latex, vinylacetate latex, and vinylacetate-acrylate latex. The biobased latex may for example be a latex based on a modified starch or a modified cellulose. The amount of binder in the mineral-coating layer is preferably lower than the amount of particulate calcium carbonate. The amount of binder is 50 wt% or less, and preferably less than 50 wt%, based on the total dry weight of the mineral-coating layer. The amount of binder in the mineral-coating layer may for example be in the range of 5-40 wt%, 5-30 wt%, or 5-20 wt%, based on the total dry weight of the mineral-coating layer.

In some embodiments, the water-dispersible binder is a latex binder. The latex binder may be a fossil-based polymer latex or a biobased latex or a combination thereof. In some embodiments, the water-dispersible binder is a latex binder selected from the group consisting of a styrene-butadiene latex, styrene-acrylate latex, and a biobased latex, and combinations thereof. In some embodiments, the water-dispersible binder is a fossil-based polymer latex binder, preferably selected from the group consisting of a styrene-butadiene latex and styrene-acrylate latex, and combinations thereof.

The mineral-coating layer preferably comprises no more than an additive amount of polyvinyl alcohol (PVOH), i.e. less than 10 wt% based on the total dry weight of the mineral-coating layer. Higher amounts do not seem to give any further benefits and may also undesirably increase the hydrophilicity and/or the viscosity of the coating. In some embodiments, the mineral-coating layer comprises 0.1-3 wt%, preferably 0.1 -1 wt%, polyvinyl alcohol (PVOH), based on the total dry weight of the mineral-coating layer. In some embodiments, the mineral-coating layer comprises no PVOH.

The mineral-coating layer may further comprise one or more additives to facilitate the application and forming of the mineral-coating layer or to impart various properties to the finished coating. In some embodiments, the mineral-coating layer further comprises at least one additive selected from the group consisting of an insolubilizer, a lubricant, a defoamer, a thickening agent, a co-binder, a stabilizer, and/or an optical brightening agent (OBA), and combinations thereof. The concentration of each additive may be in the range of 0.01-10 wt% based on the total dry weight of the mineral-coating layer.

The mineral-coating layer may have a basis weight in the range of 5-50 g/m ² , preferably in the range of 8-30 g/m ² .

The mineral-coated cellulose-based substrate may for example be provided by: i) providing a cellulose-based substrate; and ii) applying an aqueous mineral-coating dispersion to the cellulose-based substrate and drying the coating dispersion to form a mineral-coating layer, wherein the aqueous mineral-coating dispersion is an aqueous dispersion comprising 50-95 wt% of particulate calcium carbonate and 5-50 wt% of a water- dispersible binder based on the total dry weight of the mineral coating dispersion.

The mineral-coated cellulose-based substrate is preferably dry when the fatty acid halide treatment is performed. The term “dry” as used herein means that the mineral-coated cellulose-based substrate has a dry content above 85 %, preferably above 88 %, and more preferably above 92 % by weight.

The dry mineral-coating layer is treated with a solvent-free fatty acid halide composition at a temperature above the melting temperature of the fatty acid halide but below 100 °C to obtain a water-resistant mineral-coated cellulose-based substrate. The fatty acid halide treatment increases the hydrophobicity and water resistance of the mineral-coated surface.

The use of fatty acid halide treatment directly on the mineral coating surprisingly makes it possible to obtain a hydrophobic and water-resistant mineral-coated surface with a single process step and a low amount of reagent.

Unlike the processes conventionally used for grafting fatty acid halides to hydroxy- functional substrates such as cellulose and polyvinyl alcohol, in which the efficiency and rate of grafting is highly dependent on the reaction temperature, the present inventors have surprisingly found that the treatment of a mineral-coated surface, without abundant hydroxyl groups, proceeds quickly and efficiently even at low temperatures. Accordingly, fatty acid halide treatment directly on a mineral- coated surface can be performed in an industrial scale at a high speed and without the requirement for spacious, time-consuming, and expensive heating stages typically associated with grafting of fatty acid halides. Additionally, excessive heating may lead to over-drying of the mineral-coated cellulose-based substrate, making it less flexible and more prone to cracking.

The fatty acid halide treatment of the inventive method comprises applying a solvent-free fatty acid halide to the dry mineral-coating layer at a temperature above the melting temperature of the fatty acid halide but below 100 °C.

The application temperature is preferably selected such that the fatty acid halide is in a liquid state, i.e. in the melt phase. This allows for the fatty acid halide to be applied using conventional liquid coating techniques without adding a solvent. Using a solvent-free fatty acid halide facilitates the coating and reduces the environmental impact of the process.

The temperature at which the fatty acid halide is applied is below 100 °C but depending on the melting temperature of the fatty acid halide used the temperature may also be significantly lower. In some embodiments, the temperature is above the melting temperature of the fatty acid halide but below 90 °C, below 80 °C, below 70 °C, below 60 °C, below 50 °C, below 40 °C, or below 30 °C. In preferred embodiments, the temperature is below 80 °C, below 70 °C, below 60 °C, or below 50 °C. As a reference, the melting temperature of the preferred fatty acid halide palmitoyl chloride is 9-14 °C and the melting temperature of the preferred fatty acid halide stearoyl chloride is 21-22 °C.

Preferably, the temperature of the mineral-coated cellulose-based substrate remains below 100 °C during the treatment. In some embodiments, the temperature of the mineral-coated cellulose-based substrate remains below 90 °C, below 80 °C, below 70 °C, below 60 °C, below 50 °C, below 40 °C, or below 30 °C, during the treatment. In preferred embodiments, the temperature of the mineral-coated cellulose-based substrate remains below 80 °C, below 70 °C, below 60 °C, or below 50 °C, during the treatment.

The inventive manufacturing method preferably does not include any heat treatment of the water-resistant mineral-coated cellulose-based substrate. Preferably, the inventive manufacturing method preferably does not include any heat treatment which involves heating the water-resistant mineral-coated cellulose-based substrate at a temperature above 100 °C. Specifically, the inventive manufacturing method preferably does not include any heat treatment which involves heating the water-resistant mineral-coated cellulose-based substrate at a temperature above 150 °C.

Since no heat treatment is required, fatty acid halide treatment directly on a mineral-coated surface can be performed in an industrial scale at a high speed and without the requirement for spacious, time-consuming, and expensive heating stages. This allows for the inventive process to be integrated in an existing conventional process for manufacturing or converting a mineral-coated cellulose- based paper or paperboard. Thus, in some embodiments, the inventive method is performed on-line in a machine for manufacturing or converting a mineral-coated cellulose-based substrate. In some embodiments, the treatment of the inventive method is performed at a treatment speed higher than 200 m/min, preferably higher than 300 m/min, and more preferably higher than 400 m/min. The highly hydrophobic and water-resistant surface could be obtained with a very low amount of fatty acid halides, such as less than 1 g/m ²

In some embodiments, the amount of fatty acid applied on the mineral-coating layer is less than 1 g/m ² , preferably less than 0.85 g/m ² .

In some embodiments, the fatty acid halide has an aliphatic chain length of 8-22 carbon atoms. Examples of fatty acid halides include octanoyl chloride (C8), lauroyl chloride (C12), myristoyl chloride (C14), palmitoyl chloride (C16), and stearoyl chloride (C18), and/or a mixture thereof. In some preferred embodiments, the fatty acid halide is palmitoyl chloride or stearoyl chloride.

The treatment typically involves contacting the mineral-coating layer with a fatty acid halide in a liquid or gas state. The fatty acid halide reagent may for example be applied to the surface of the substrate in liquid form using a roll-coating process, a curtain-coating process or a spray-coating process, or in gas form followed by condensation of the gaseous reagent on the substrate surface. In preferred embodiments, the treatment involves contacting the mineral-coating layer with a fatty acid halide in a liquid state. In some embodiments, the treatment involves applying the fatty acid halide in a liquid state to the mineral-coating layer using a roll-coating process, a curtain-coating process or a spray-coating process. In a preferred embodiment the treatment involves applying the fatty acid halide in a liquid state to the mineral-coating layer using a roll-coating process.

The fatty acid halide treatment of the mineral-coated cellulose-based substrate renders the surface of the mineral-coating layer water-resistant. Preferably, the fatty acid halide treatment of the mineral-coating layer results in a surface having a Cobb ₆₀ value below 10 g/m ² . In some embodiments, the surface of the mineral- coating layer subjected to treatment with a fatty acid halide has a Cobb ₆₀ value (as determined according to standard ISO 535:2014 after 60 seconds) below 10 g/m ² , preferably below 8 g/m ² , more preferably below 4 g/m ² .

The treatment with a fatty acid halide can improve or at least maintain the grease resistance of the treated surface. Preferably, the fatty acid halide treatment of the mineral-coated cellulose-based substrate improves the grease resistance of the surface of the mineral-coating layer. In some embodiments, the fatty acid halide treatment of the mineral-coating layer results in a surface having same or higher KIT value (as determined according to Tappi T559) than the untreated surface. In some embodiments, the fatty acid halide treatment of the mineral-coating layer results in a surface having KIT value of at least 8, preferably at least 10, and more preferably at least 12 (as determined according to Tappi T559).

In some embodiments, the fatty acid halide treatment of the mineral-coating layer results in a surface having a higher time for break-through of grease (as determined according to ASTM F119) than the untreated surface. In some embodiments, the fatty acid halide treatment of the mineral-coating layer results in a surface having at least 100% higher time for break-through of grease (as determined according to ASTM F119) than the untreated surface.

The cellulose-based substrate is preferably a web or sheet having a first main surface and a second main surface. The cellulose-based substrate can be mineral coated on one or both of its two main surfaces, depending on the intended use of the substrate. Depending on the need for printability and water-resistance, one or both of the mineral-coated surfaces may be subjected to treatment with a fatty acid halide.

In some embodiments, the mineral coating is disposed on the first main surface of the substrate.

In many cases, the cellulose-based substrates will have a mineral-coating layer on one of its two main surfaces, whereas the other of the two main surfaces is uncoated. Such one-side mineral-coated substrates can advantageously be subjected to treatment with a fatty acid halide on the coated surface as well as on the uncoated surface. Thus, in some embodiments, wherein no mineral-coating layer is disposed on the second main surface of the substrate, the uncoated surface is also subjected to treatment with a fatty acid halide. Fatty acid halide applied to an uncoated surface will typically permeate into the substrate to a greater extent than fatty acid halide applied to a mineral-coated surface. Fatty acid halide treatment of the uncoated surface can reduce or completely eliminate the need for an added internal hydrophobic sizing agent in the cellulose-based substrate.

In some embodiments, the water-resistant mineral-coated cellulose-based substrate further comprises a mineral-coating layer disposed on the second main surface of the substrate, said mineral-coating layer comprising

50-95 wt% of particulate calcium carbonate, and 5-50 wt% of a water-dispersible binder, based on the total dry weight of the mineral coating, wherein the fatty acid halide treatment is performed on both of the main surfaces of the substrate.

Mineral coating on both of the main surfaces provides a coated cellulose-based substrate where both sides of the substrate have the combination of improved water-resistance, and possibly improved or maintained grease resistance, as well as properties such as whiteness, brightness, gloss, and/or high-quality print.

The mineral-coating layer disposed on the first and second main surface of the substrate may be identical or different. Each of the mineral-coating layers may independently be defined as described above with reference to the mineral-coating layer disposed on the first main surface.

In some embodiments, the mineral-coating layer disposed on the second main surface of the substrate is further defined as the mineral-coating layer disposed on the first main surface.

The cellulose-based substrate (also referred to herein as “the substrate”) is preferably a sheet or web of material mainly formed from pulp of wood or other fibrous substances comprising cellulose fibers. The cellulose-based substrate is preferably paper or paperboard. Paper generally refers to a material manufactured in sheets or rolls from the pulp of wood or other fibrous substances comprising cellulose fibers, used for e.g. writing, drawing, or printing on, or as packaging material. Paper can either be bleached or unbleached and produced in a variety of thicknesses, depending on the end-use requirements.

Paperboard generally refers to strong, thick paper or cardboard comprising cellulose fibers used for example as flat substrates, trays, boxes and/or other types of packaging. Paperboard can either be bleached or unbleached and produced in a variety of thicknesses, depending on the end-use requirements.

In some embodiments, the cellulose-based substrate is comprised of two or more cellulose-based plies. Each of the cellulose-based plies 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) and/or mixtures thereof. The different plies can have different grammages and/or thicknesses and may contain different amounts of internal sizing agent and/or fatty acids.

As an example, the substrate can be built up of one top-ply consisting of bleached Kraft pulp, a mid-ply consisting of a mixture of bleached Kraft pulp and CTMP, and a bottom-ply consisting of bleached Kraft pulp, wherein the mid-ply has a higher thickness than both the top and bottom plies, respectively.

In some embodiments, the basis weight of the cellulose-based substrate is in the range of 20-800 g/m ² .

The inventive mineral-coated cellulose-based substrate is water-resistant. The term “water-resistant” generally means that the mineral-coated cellulose-based substrate treated with fatty acid halide has a higher resistance to water absorption (e.g. indicated by the Cobb ₆₀ value as determined according to standard ISO 535:2014 after 60 seconds) than the same mineral-coated cellulose-based substrate without fatty acid halide. The water-resistant mineral-coated cellulose-based substrate is preferably suitable for use in wet or damp conditions. The wet or damp conditions may be due to water or moisture present in the environment where the substrate or products comprising the substrate are to be used, including condensation formed on the substrate surface. In some embodiments, the water-resistant mineral-coated cellulose-based substrate is for use in packaging, such as boxes, and other containers; fresh and aseptic liquid packaging; boxes, trays, or cups for hot, cold, dry, wet and frozen food and beverages; products for outdoor use such as boxes, signs and posters; pots, trays and covers for plants; packages for construction materials, and construction materials.

The cellulose-based substrate has preferably been rendered hydrophobic, either by fatty acid halide treatment of the cellulose-based substrate through the entire thickness of the substrate or by an internal sizing agent comprised in the cellulose- based substrate, or by a combination thereof.

In some embodiments, the water-resistant mineral-coated cellulose-based substrate has a repulpability characterized by a reject rate (as determined according to the PTS RH 021/97 test method) below 20%, preferably below 10%, and more preferably below 5%.

One advantage of the present invention is that the fatty acid halide treated mineral coating can reduce or fully eliminate the need for a polymer layer to achieve water- resistance. However, the treated mineral coating may also further be coated, laminated or printed with a protective polymer-layer such as a varnish to improve surface properties. Varnishes or lacquers are utilized to improve the surface performance after printing. The most common type of varnish is the over print varnishes (OPV) that improve and reduce smearing of printing colors, but many different types of varnishes exist, such as barrier varnishes and sealing varnishes, where the name reveals what they are intended to improve. As known by the skilled person, varnish is applied in the print press at one of the printing units, normally at a low coat weight, e.g., between 3-8 g/m ² A varnish can chemically be very similar to a dispersion coating, i.e., consist of a binder polymer, such as a latex, in combination with other component depending on the intended utilization. In some applications, it may be desirable to provide the opposite side of the treated mineral-coated surface with a protective polymer-layer. The protective polymer-layer may comprise a thermoplastic polymer or a water-dispersed polymer. The polymer layer may for example comprise any of the polymers commonly used in paper-based or paperboard-based packaging materials in general. Examples include polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polylactic acid (PLA) and polyvinyl alcohol (PVOH). Polyolefins, especially low-density polyethylene (LDPE) and high-density polyethylene (HDPE), are the most common and versatile polymers used.

Thermoplastic polymers, and particularly polyolefins are useful since they can be conveniently processed by extrusion coating techniques to form very thin and homogenous films with good barrier properties. In preferred embodiments, the polymer layer comprises a polyethylene, more preferably LDPE or HDPE.

The protective polymer-layer is preferably made of a polymer obtained from renewable resources.

The basis weight of the protective polymer layer is preferably less than 50 g/m ² In order to achieve a continuous and substantially defect free film, a basis weight of the polymer layer of at least 2 g/m ² , preferably at least 4 g/m ² , is typically required, depending on the polymer used. If the protective polymer layer is applied by extrusion coating, a basis weight of the polymer layer of at least 8 g/m ² , is typically preferred. In some embodiments, the basis weight of the polymer layer is in the range of 2-15 g/m ² or in the range of 4-30 g/m ² .

According to a second aspect illustrated herein, there is provided a water-resistant mineral-coated cellulose-based substrate manufactured according to the method of the first aspect.

According to a third aspect illustrated herein, there is provided a carton blank comprising a water-resistant mineral-coated cellulose-based substrate manufactured according to the method of the first aspect. According to a fourth aspect illustrated herein, there is provided a container, comprising a water-resistant mineral-coated cellulose-based manufactured according to the method of the first aspect.

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

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 is not 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.

Examples

Example 1. Treatment at different temperatures

A bleached board of 200 g/m ² was coated on one side with 20 g/m ² mineral layer consisting of calcium carbonate (79 wt%), clay (7 wt%), latex (11 wt%), and additives (3 wt%, whereof PVOH 0.9 wt%), named B220.

The mineral-coated side of the board was treated with palmitoyl chloride heated to 60 °C (i.e. in liquid form), and thereafter subjected to a surface for 2.2 s with three different temperatures: 30, 100, or 190 °C. Cobb ₆₀ measurements, presented in Table 1 , showed that the temperature did not impact the results. Cobb ₆₀ values were determined according to standard ISO 535:2014

Table 1.

Example 2. Treatment at different speeds

A bleached board of 210 g/m ² was coated on one side with 20 g/m ² mineral layer consisting of calcium carbonate (80 wt%), clay (9 wt%), latex (10 wt%), and additives (1 wt%, whereof PVOH 0.4 wt%), named B230.

The mineral-coated side of the board was treated with palmitoyl chloride at 60 °C (i.e. in liquid form) at different speeds (50, 100, 200, or 400 m/min), and thereafter subjected to a surface with a temperature of 190 °C. Cobb ₆₀ measurements in Table 2 showed that the speed did not impact the results.

Table 2. Example 3. Oil and grease resistance

The exemplified board in Example 2 above was analyzed with regards to oil and grease resistance. The KIT Test (Tappi T559) gave both the reference and treated boards the highest and best possible value to achieve: 12, suggesting that the treatment did not impact the oil and grease resistance (Table 3).

With the standard ASTM test F119 method, the time for transmission is determined when the board has been exposed to grease or oil. As can be seen in Table 3, the time for break-through of grease was prolonged after being treated with a fatty acid chloride.

Table 3.

Example 4. Varnish coating

The board B230 referred to in Example 2, both ungrafted (Ref.) and grafted (T4), was coated with approximate 5 g/m ² of two different commercial varnishes:

1 . Sun Chemicals OPV Offset FJ49 and

2. Flint Group DecaCode Barrier 83; Matt, WP3R-083.

Cobb600 measurements presented in Table 4, show how the varnish greatly improved the water resistance of the uncoated reference when applying the barrier varnish 2: from 67.5 to 5.6 g/m ² , whereas the over print varnish only reduced it slightly. The grafting lowered the Cobb value down to 25 g/m ² by itself, but after applying varnish 1 , the value is further dropped to 9.9 g/m ² despite that varnish 2 did not give much water resistance by its own, suggesting that there is a synergy between the two technologies. The Cobb value was further reduced for the grafted and varnished sample when applying the barrier varnish 2, resulting in the lowest value of 2.8 g/m ² . Table 4.