Moisture- and liquid-resistant paper or paperboard comprising renewable polymers and having low total reject

Abstract

The present invention relates to a moisture- and liquid-resistant paper or paperboard, comprising an internally-sized cellulose-based substrate with a basis weight between 100-500 g / m2 and a density between 400-850 kg / m3, said substrate having a first top side and a second back side, at least one moistureresistant layer applied on said first top side of the substrate protecting the substrate against moisture penetration, applied on said moisture-resistant layer, at least one liquid-resistant layer protecting the substrate against liquid penetration, wherein said moisture-resistant layer and liquid-resistant layer are made up by unmodified and / or modified renewable polymers in an amount of 75-100 wt%, more preferably 85-100 wt% and even more preferably 95-100 wt% as calculated on the total dry weight of said barrier layers, and wherein the moisture- and liquidresistant paper- or paperboard has a total renewable content >90 wt% as calculated on the total dry weight of the paper or paperboard.

Description

The present disclosure relates to renewable and compostable moisture- and liquid resistant paper or paperboard suitable for use in disposable packaging containers, such as cups, boxes and trays, for food and beverages.BackgroundFiber-based material used in containers, such as cups, for liquids (hot and cold), fatty or frozen food is usually provided with barrier coatings both on the inside (facing the packed item) and on the outside (print-side). The barrier coating applied on the inside makes the material resistant against e.g. liquids, grease, and / or aroma and enables it to withstand the influence of the packed item on the packing material. The barrier on the outside is applied to protect the packed item from the surrounding, especially from water vapor and condensation that is formed on the surface due to temperature fluctuations or temperature differences between container outside vs container inside.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 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. The most common coating materials for creating barrier- and heat-sealable properties on fiber-based packaging materials include polyolefins, such as polyethylene (PE) or polypropylene (PP). Paperboard intended for e.g. cups is often provided with a polyolefin coating to provide barrier and heat sealability both on the inside and on the outside.Environmental concerns and recent directives and legislations have created a demand that disposable fiber-based containers, such as cups, should contain no or very low amounts of plastics manufactured from biobased, fossil or synthetic starting substances. Barrier coatings based on natural polymers have been tested in the art, but many of these have not been considered to provide high enough barrier properties.Therefore, it remains a need for a renewable and compostable container for liquid, fatty- and / or frozen food that provides enough barrier properties but contains no or low amounts of plastics.Description of the inventionIt is an object of the invention to provide a moisture- and liquid-resistant paper or paperboard made from mainly renewable resources and that exhibits good repulpability and compostability.It is a further object of the invention to provide a moisture- and liquid-resistant paper or paperboard with good repulpability and compostability, which is useful in food and beverage packaging applications.It is a further object of the invention to provide a moisture- and liquid-resistant paper or paperboard for food and beverage packaging applications, which is free from plastic-barrier coatings.It is a further object of the invention to provide a compostable container, such as a cup, which provides barrier functionality against moisture- and liquid penetration, particularly against hot beverages such as coffee or tea.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.Accordingly, the present invention pertains to a paper or paperboard that is resistant to both moisture and liquid. The inventive material comprises an internalsized cellulose-based substrate with a basis weight between 100-500 g / m2and density between 400-850 kg / m3. The substrate has a first top side and a second back side. To protect the cellulose substrate from moisture penetration, at least one moisture-resistant layer is applied on the first top side of the substrate. A moisture-resistant layer is a composition applied to at least one side of the substrate material, rendering it substantially impervious to moisture. As a result, the reduced moisture permeability through this layer does not significantly affect the initial properties of the board within the intended usage timeframe of the product. The function of a moisture-resistant layer can be evaluated by measuring water vapor transmission rate (WVTR).Further protection against liquid penetration is provided by applying at least one liquid-resistant layer on top of the existing moisture-resistant layer. According to the present invention, a key feature of these layers (i.e. the moisture-resistant layer and the liquid-resistant layer) lies in their composition and their combination as they cannot function on their own for the intended usage and removal of a plastic-barrier layer. They are made up predominantly, if not entirely, by unmodified or modified renewable and compostable polymers. This contributes significantly towards the overall renewable content of the paper or board, which exceeds 90 wt% as calculated on its total dry weight.Another important aspect about this invention is its performance under testing conditions. The material exhibits high resistance to water with a Cobb300 value less than 30 g / m2 according to ISO 535:2023 standard, preferably less than 25 g / m2. Furthermore, the Cobb-Unger (15 s) value is less than 10 g / m2, measured according to SCAN-P 37:77. Additionally, said material displays significant resistance against water vapor transmission rate (WVTR) with values less than 35g / m2 / h at 38°C and 80% RFI, measured according to standard ASTM E96 but where the value is recorded after 1 h (one hour) instead of the usual 24h.In a preferred embodiment, the paper or paperboard does not possess heatsealable properties, meaning it is free from any heat sealable barrier layer such as a plastic layer.In some embodiments, at least one moisture-resistant layer protecting the substrate against moisture penetration is applied also on said second back side of the substrate. Preferably, also a liquid-resistant layer is applied on top of the moisture-resistant layer at said second back side of the substrate, resulting in that a core fiber-based substrate is sandwiched between outer moisture- and liquidresistant layers, i.e. both the top- and back sides of the substrate are covered with moisture- and liquid-resistant layers.Thanks to the invention, there is achieved a material which is plastic-layer free, yet providing moisture- and liquid resistant functions that meet the requirements for containers used to hold liquid content, such as hot liquid content (i.e. with temperatures above 75°C) for the time of utilization of such product. Moreover, the material is made mainly from renewable resources, and it is repulpable as well as compostable.The innovative combination of moisture and liquid resistant layers described in this invention further results in a significantly reduced Cobb-Unger value (i.e. oil Cobb). This has several benefits when it comes to additional processes like coating, lamination, or printing on the material surface.As used herein, the term “renewable polymer” refers to a polymer that is derived from a renewable resource. Furthermore, the term “renewable resource” refers to a resource that is produced via a natural process at a rate comparable to its rate of consumption (e.g., within a 100-year time frame). The resource can be replenished naturally, or via agricultural techniques, and are reusable, recyclable, and naturally degradable. Non-limiting examples of renewable resources include plants (e.g., woody plants, lignocellulose, hemicellulose, cellulosic waste sugar cane, beets, corn, potatoes, citrus fruit), bacteria, fungi, and forestry products. These resources can be naturally occurring, hybrids, or genetically engineered organisms. Natural resources, such as crude oil, coal, natural gas, and peat, which take longer than 100 years to form, are not considered renewable resources.The moisture- and liquid-resistant multiply paper or paperboard substrate according to the invention is preferably suitable for use as packaging material intended for contact with food and liquids, such as hot liquids. In some embodiments, the cellulose-based substrate is for use in disposable packaging containers, such as cups, lids, boxes and trays for food and beverages.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.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.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.The paper or paperboard can be manufactured in a paper or paperboard machine. Typically, the machine layout comprises a stock handling section, a wet-end section, a press section, a drying section, a calendaring and / or coating section, and optionally a surface sizing unit.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), hightemperature CTMP (HT-CTMP) 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.In some embodiments, the paper or paperboard has a multilayer structure comprising a back ply comprising bleached Kraft pulp, a mid ply comprising 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 multiply paper or paperboard of the present disclosure is liquid repellent. The term “liquid repellent” generally means that the multiply paper or paperboard with the internal sizing agents and barrier layers has a higher resistance or repellence to liquid penetration and absorption than the same multiply paper or paperboard without said internal sizing agents and barrier layers.The substrate of the paper or paperboard preferably comprises internal sizing agent / s. 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 surface- and raw-edge penetration of water and other liquids into the cellulosebased 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.The moisture-resistant layer is arranged to slow down unwanted absorption of atmospheric or incidental moisture into the substrate during the intended use of the product. Said moisture-resistant layer comprises at least one renewable polymer as main component selected from: microfibrillated cellulose (MFC), waxes, water-soluble polysaccharides, modified polysaccharides, hydrophobically modified polysaccharides, and cellulose derivatives, such as hydroxypropyl starch, n-octenyl succinic anhydride (n-OSA) starch, dialdehyde starch, oxidized starch, sodium carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose and / or chitosan.In some embodiments of this invention, there can be addition of a crosslinker in the moisture-resistant layer to improve the functionality. In some embodiments, the moisture-resistant layer comprises 0.1-20 wt% crosslinker, based on the total dry weight of the moisture-resistant layer. In some embodiments, the crosslinker is selected from the group consisting of polyamide-epichlorohydrin (PAE), zirconium carbonates, and / or polyfunctional organic acids or aldehydes, such as citric acid, tartaric acid, glyoxal, sodium trimetaphosphate and glutaraldehyde or combinations thereof.In some embodiments, the moisture-resistant layer also comprises particles that are impermeable to moisture to slow down the transmission rate. In some embodiments, the moisture-resistant layer comprises 0.1-20 wt% particles, based on the total dry weight of the moisture-resistant layer. In some embodiments, the impermeable particles are selected from the group consisting of clay, silica, calcium carbonate, cellulose nanocrystals and / or combinations thereof.In some embodiments, the moisture-resistant layer also comprises at least one hydrophobic agent. In some embodiments, the moisture-resistant layer comprises 0.1-10 wt% of at least hydrophobic agent, based on the total dry weight of the moisture-resistant layer. In some embodiments, the hydrophobic agents are preferably selected from the group consisting of rosin, wax, and / or SA-latex.In some embodiments, the moisture-resistant layer may also comprise additional polymers, preferably water-soluble polymers, preferably polyvinyl alcohol (PVOH). Preferably, the content of PVOH is <15 wt% or <11wt% of the total dry weight of the moisture-resistant layer.The moisture-resistant layer can be created by means of applying a surface sizing layer, said surface sizing layer comprising one or more of said renewable polymers. 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 and increase the overall strength and appearance of the material. The primary objective of the surface sizing layer(s) of the inventive paper or paperboard is to create a moisture-resistant layer on the paper or paperboard, particularly for use in contact with hot and cold food and beverages. To improve the moisture resistance of the paper or paperboard material, a moisture-resistant layer is needed to prevent or at least delay the moisture from reaching the underlying substrate. The moisture-resistant layer should thereby have a structural design with low permeability towards water vapor and sufficient hydrophobicity to minimize the wetting as a result of liquid condensation. By the addition of different additives, the moisture resistance of the layer can be further improved, e.g. impermeable particles that creates a tortuous pathway for the moisture molecules and / or crosslinker that prevents the structure from swelling.Surface sizing is typically achieved through the application of a sizing composition, which comprises chemicals on a paper web. 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 or indirectly onto the surface of the paper or paperboard as it passes through a nip or a set of rollers. 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 traditional surface sizing methods, other methods can also be applied for providing a thin moisture-resistant layer on a paper or paperboard surface, without forming a plastic coating. Accordingly, for example impregnation as well as weton-wet application of a composition as described above can also be utilized and the invention is not limited to the specific application method to form the moistureresistant layer, but preferably, the application method should be included as an online application in the paperboard machine.In some embodiments of this invention, the moisture-resistant layer has a coat weight of at least 0.2 g / m2 such as at least 0.3 g / m2. Furthermore, to ensure optimal functionality while keeping material usage minimalistic, the moistureresistant layer has a coat weight of less than 5 g / m2, preferably less than 3 g / m2, and comprises at least one of said renewable polymers. In some embodiments, the first surface-sizing layer comprises at least 55 wt% renewable polymers, preferably at least 70 wt% renewable polymers and 1-45 wt% of additional compounds based on the total dry weight of the first surface sizing layer.In some embodiments, the moisture-resistant layer has a moisture content of less than 10wt%, preferably less than 8wt% such as between 3-7wt% based on the total weight of the layer.In some embodiments, the moisture-resistant layer comprises at least two renewable polymers, such as two or three renewable polymers.In some embodiments, the moisture-resistant layer comprises a hydrophobic agent to reduce the wetting from condensed moisture and / or upon application of the subsequent liquid-resistant layer.In some embodiments, the moisture-resistant layer can be calendered prior to application of further layer / s.As previously stated, at least one liquid-resistant layer is applied on top of the existing moisture-resistant layer for further protection against liquid penetration e.g. in case of direct and persistent water contact. The liquid-resistant layer comprises at least one hydrophobizing agent selected from various options including: rosin resin, ASA, wax, fatty acids and fatty acid derivatives, waterdispersible latex such as styrene-acrylate latex, silane, cationic modified silanes, hydrophobic particles and organosilicons among others for performance against liquids, preferably SA-latex. In some embodiments, the hydrophobizing agent is further combined with a binder or carrier. The hydrophobicity can also be induced by application of various varnishes, water-based or organic solvent-based compositions. Varnish is also referring to dispersions, i.e. not fully solved compounds. It can further include cellulose-based derivatives.Said liquid-resistant layer may be applied in any conventional way, on-line as well as off-line, in one or multiple layers such as 2-3 layers on top of each other.Examples of conceivable coating methods include surface sizing preferably by using film size press, curtain coaters or spray coaters, and various printing or offline coaters such as gravure, reverse gravure and flexography.In some embodiments of this invention, there might be addition of a binder in the liquid-resistant layer with optional inclusion of crosslinker and / or fixative / flocculant for enhanced functionality.Like its counterpart protecting against moisture penetration, the liquid-resistant layer’s coat weight is maintained between 0.2 - 5 g / m2, preferably lower than 3 g / m2.One advantageous aspect about the invention lies in its environmental sustainability credentials since the inventive paper or paperboard does not have a plastic coating-layer and it 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 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 %. Furthermore, the 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. In some embodiments, the paper or paperboard is compostable as determined according to standard EN 13432. In embodiments, the container is home compostable, preferably according to the Australian standard AS 5810.The invention also extends to a method for manufacturing paper or paperboard according to the invention. This involves forming a cellulose-based substrate from a cellulose furnish, followed by application of at least one moisture-resistant layer on the first top side of the substrate to protect against moisture penetration. Next, at least one liquid-resistant layer is applied on top of the existing moisture-resistant layer to protect against liquid penetration.The innovative paper or paperboard can be used for manufacturing containers wherein the outermost surface of its barrier layered first top side forms an inside surface of the container offering resistance against both moisture and liquid ingress. One example of a container formed from paper or paperboard according to the first aspect is a paper cup for containing hot liquid such as hot tea or coffee.ExampleIn the following example, reference is made to Fig. 1, which is a schematic crosssectional view of a paper or paperboard according to one example of the invention. Herein is seen a moisture- and liquid-resistant paper or paperboard 1 comprising an internal sized cellulose-based substrate 2 with a basis weight between 100-500 g / m2 and a density between 400-850 kg / m3. The substrate comprises a first top side 20 and a second back side 21. A moisture-resistant layer 3 protecting the substrate against damaging moisture penetration is applied on said top side 20 of the substrate 2. The moisture-resistant layer provides the function of at least substantially reducing penetration of moisture into the substrate 2. On top of said moisture-resistant layer 3, there is applied a liquid-resistant layer 4 protecting the substrate 2 against liquid penetration. According to the invention, both the moisture- and liquid-resistant layers 3, 4 are made from unmodified and / or modified renewable polymers, preferably in an amount of 50-100 wt% as calculated on the total dry weight of said barrier layers. In addition, the paper- or paperboard 1 has a total renewable content >90 wt% as calculated on the total dry weight of the paper- or paperboard.The moisture- and liquid-resistant paper or paperboard 1 is suitable for use as packaging material intended for contact with food and liquids, such as hot liquids. Thanks to the combined moisture- and liquid-resistant layers, the material provides a functioning barrier without use of any plastic barrier layer, and thus may offer a solution for replacing products which today require >5 wt% plastic content, for example hot liquid cups.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

1. A moisture- and liquid-resistant paper or paperboard, comprising:-an internally-sized cellulose-based substrate (2) with a basis weight between 100-500 g / m2 and a density between 400-850 kg / m3, said substrate having a first top side (20) and a second back side (21);-at least one moisture-resistant layer (3) applied on said first top side (20) of the substrate (2) protecting the substrate against moisture penetration;-applied on said moisture-resistant layer (3), at least one liquid-resistant layer (4) protecting the substrate against liquid penetration;wherein;-said moisture-resistant layer (3) and liquid-resistant layer (4) are made up by unmodified and / or modified renewable polymers in an amount of 75-100 wt%, more preferably 85-100 wt% and even more preferably 95-100 wt% as calculated on the total dry weight of said barrier layers;-the moisture- and liquid-resistant paper- or paperboard has a total renewable content >90 wt% as calculated on the total dry weight of the paper or paperboard; and wherein the moisture- and liquid-resistant paper or paperboard (1) has:-a water Cobb300 value <30 g / m2, preferably <25 g / m2 according to ISO 535:2023;-a modified water Cobb300 measurement at 80°C with blotting paper <30 g / m2, preferably <25 g / m2 according to ISO 535:2023;-a Cobb-Unger (15s) value <10 according to SCAN-P 37:77; and-a water vapor transmission rate (WVTR) <35 g / m2 / h at 38°C and 80% RH according to standard ASTM E96.

2. The paper or paperboard according to claim 1, wherein the moistureresistant layer (3) comprises unmodified or modified polysaccharides.

3. The paper or paperboard according to anyone of claims 1 or 2, wherein the moisture-resistant layer (3) comprises at least one renewable polymer selected from the group comprising: MFC, wax, modified starch such as hydroxypropyl starch, cellulose derivatives such as CMC and methyl cellulose, and chitosan.

4. The paper or paperboard according to claim 2, wherein the moistureresistant layer (3) further comprises a crosslinker selected from the group consisting of polyamide-epichlorohydrin (PAE), and polyfunctional organic acids or aldehydes, such as citric acid, tartaric acid, glyoxal, glutaraldehyde and sodium trimetaphosphate.

5. The paper or paperboard according to claim 2, wherein the moistureresistant layer (3) further comprises 0.1-20 wt% of at least one impermeable particle selected from the group consisting of clay, silica, calcium carbonate, cellulose nanocrystals and / or combinations thereof.

6. The paper or paperboard according to anyone of the previous claims, wherein the moisture-resistant layer (3) has a coat weight between 0.2 - 5 g / m2, preferably 0.5 - 3 g / m2.

7. The paper or paperboard according to anyone of the previous claims, wherein the liquid-resistant layer (4) comprises at least one hydrophobizing agent selected from the group comprising: rosin resin, ASA, wax, water-dispersible latex, such as styrene-acrylate latex, silane, cationic modified silanes, varnish, laquer and organosilicons.

8. The paper or paperboard according to claim 7, wherein the liquidresistant layer (4) further comprises a binder and optionally a crosslinker and / or a fixator / flocculant.

9. The paper or paperboard according to anyone of the previous claims, wherein the liquid-resistant layer (4) has a coat weight between 0.2 - 5 g / m2, preferably 0.2 - 3 g / m2 or 0.3 - 3g / m2.

10. The paper or paperboard according to anyone of the previous claim, wherein the substrate (2) comprises more than one ply.

11. 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 %.

12. 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.

13. The paper or paperboard according to any one of the preceding claims, wherein the paper or paperboard has a basis weight above 150 g / m2.

14. The paper or paperboard according to any one of the previous claims, wherein at least one moisture-resistant layer protecting the substrate against moisture penetration applied on said second back side (21) of the substrate (2).

15. The paper or paperboard according to claim 14, wherein at least one liquid-resistant layer protecting the substrate against liquid penetration is applied on top of said moisture-resistant layer at said second back side (21) of the substrate (2).

16. The paper or paperboard according to any one of the previous claims, wherein the paper or paperboard is void of any heat-sealable barrier layer.

17. A method for manufacturing a moisture-resistant and liquid-resistant paper or paperboard according to anyone of claims 1 -16, said method comprising:a) forming a cellulose-based substrate (2) from a cellulose furnish;b) applying on said first top side (20) of the substrate (2), at least one moistureresistant layer (3) protecting the substrate against moisture penetration; andc) apply on said moisture-resistant layer (3), at least one liquid-resistant layer (4) protecting the substrate against liquid penetration.

18. A container formed from liquid-resistant multiply paper or paperboard according to any one of claims 1-16 or obtained by the method according to claim 15, wherein the outermost surface of the barrier layered first top side (20) of the substrate (2) forms an inside surface of the container.