Multiply paper substrate comprising a barrier mid-ply

Abstract

The present invention relates to a method for manufacturing a multiply paper substrate comprising a first outer ply, a second outer ply, and a barrier mid-ply in a paper-making machine, the method comprising the steps of: a) forming and dewatering a first web layer from a first pulp suspension comprising at least 70 wt% (based on dry weight) of Kraft pulp to obtain a first outer ply; b) forming and dewatering a second web layer from a second pulp suspension comprising a mixture of 40-90 wt% (based on dry weight) of highly refined cellulose (HRC) having a Schopper-Riegler (SR) value in the range of 70-100, and 10-60 wt% (based on dry weight) of refined broke pulp having an SR value in the range of 30-70, to obtain a barrier mid-ply; c) forming and dewatering a third web layer from a third pulp suspension comprising at least 70 wt% (based on dry weight) of Kraft pulp to obtain a second outer ply; wherein the HRC is obtained from virgin Kraft pulp, and wherein the refined broke pulp in the second pulp suspension comprises refined broke pulp obtained from the method for manufacturing a multiply paper substrate.

Description

[0001] MULTIPLY PAPER SUBSTRATE COMPRISING A BARRIER MID-PLY

[0002] Technical field

[0003] The present disclosure relates to paper substrates, e.g. useful in paper and paperboard-based packaging materials. More specifically, the present disclosure relates to methods for manufacturing paper substrates comprising highly refined cellulose or microfibrillated cellulose (MFC).

[0004] Background

[0005] Effective gas, aroma, and grease barriers are required in paper and paperboardbased packaging materials for food packaging. Paper and paperboard-based packaging materials should also have suitable mechanical properties allowing them to be used in converting lines for manufacturing packaging containers.

[0006] Coating of paper with plastics is often employed to combine the mechanical properties of the paper with the barrier and sealing properties of a plastic film or layer. Paper provided with even a relatively small amount of a suitable plastic material can provide the properties needed to make the paper suitable for many demanding applications, for example as liquid or food packaging. In liquid or food packaging, polyolefin coatings are frequently used as liquid barrier layers, heat sealing layers and adhesives. However, the recycling of such polymer coated paper is difficult since it is difficult to separate the polymers from the fibers. Also, in many cases the water vapor barrier properties of the polymer coated paper are still insufficient unless the coating layers are thick or combinations of different polymer coating layers are used. Therefore, in order to ensure high water vapor barrier properties, the polymer coated paper is often combined with one or more layers of aluminum foil. The aluminum foil is typically bonded to the laminate using one or more polymeric tie layers. However, the addition of polymer and aluminum foil add significant costs and the combination of polymeric layers and aluminum foils makes repulping and recycling of the materials more difficult. Also, due to its high carbon footprint there is a wish to replace aluminum foils in paper or paperboard based packaging materials. In the prior art, attempts have been made to replace the aluminum foil with more environmentally friendly and / or easier to recycle solutions. For example, microfibri Hated cellulose (MFC) films and coatings have been developed, in which cellulosic fibrils provided by fibrillation of cellulose fibers have been dispersed e.g. in water and thereafter re-organized and rebonded together to form a dense film or coating with excellent gas, aroma, and grease barrier properties. The MFC films are typically laminated to a paper or paperboard based substrate in the same manner as aluminum foils. However, challenges still remain in terms of providing sufficient barrier properties, mechanical strength, durability, repulpability and recyclability, at an acceptable cost, in order to effectively replace aluminum foils and plastic films with cellulose based alternatives.

[0007] MFC films are typically relatively weak, and the films are therefore often formed or laminated with paper or paperboard substrate to improve the mechanical strength. However, the MFC films in such laminates are also prone to cracking and delamination during converting of the laminates into a packaging container. Due to the shrinking properties of the MFC films, the forming or lamination with other cellulose based layers may also often result in problems with curling of the formed multilayer structures.

[0008] There remains a need for improved solutions to replace the combinations of plastic films and aluminum foils commonly used in paper and paperboard-based packaging materials, while maintaining acceptable gas, aroma, and grease barrier properties, as well as acceptable mechanical properties for converting into packaging containers. At the same time, there is a need to replace the combination of plastic films and aluminum foils with alternatives that facilitate repulping and recycling of the used packaging materials.

[0009] Description of the invention

[0010] It is an object of the present disclosure to provide a method for manufacturing a paper substrate comprising highly refined cellulose (HRC), such as microfibrillated cellulose (MFC), which alleviates at least some of the above-mentioned problems associated with prior art methods. It is a further object of the present disclosure to provide an improved method for manufacturing a paper substrate comprising highly refined cellulose in a paper- or paperboard machine type of process.

[0011] It is a further object of the present disclosure to provide a paper substrate with gas, aroma, and grease barrier properties useful in paper and paperboard-based packaging materials for food packaging.

[0012] 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.

[0013] The present invention is based on the realization that a barrier layer with excellent gas, aroma, and grease barrier properties can be formed in a multiply paper substrate by combining a fraction of highly refined cellulose (HRC) obtained from virgin Kraft pulp and having a Schopper-Riegler (SR) value in the range of 70-100 with a fraction of broke pulp refined to an SR value in the range of 30-70 in a specific weight ratio.

[0014] The inventors have surprisingly found that a high proportion in the range of 10-60 wt% (based on dry weight) of the broke pulp, although only refined to a relatively low SR value in the range of 30-70, can be added to the highly refined pulp while still maintaining good gas, aroma, and grease barrier properties in the formed barrier layer. The barrier layer is formed as a barrier mid-ply between two outer plies, which allows for a high proportion of broke pulp to be used and minimizes the stress on the barrier layer during transport, handling and converting of the multiply paper substrate. The barrier layer arranged as a mid-ply in the multiply paper substrate does not need to stretch as much during folding and converting as a barrier layer arranged as an outer ply in a multiply paper substrate. This reduces the risk of cracking the sensitive barrier layer during folding and converting.

[0015] The inventive method allows for efficient manufacturing of a multilayer paper substrate comprising highly refined cellulose, such as microfibri Hated cellulose in a paper machine type of process. Forming the barrier layer as a mid-ply in the multiply paper substrate allows dewatering and drying in both directions which helps to retain the barrier properties of the barrier mid-ply.

[0016] The method allows for a high degree of broke recycling which also contributes to the overall properties of the obtained product. The multiply paper substrates have been found to be very useful in packaging applications requiring acceptable gas, aroma, and grease barrier properties, as well as acceptable mechanical properties for converting into packaging containers. The inventive multiply paper substrates have high repulpability, providing for high recyclability of the paper substrates and packaging products comprising the paper substrates.

[0017] In addition, the barrier mid-ply of the multiply paper substrate also acts as a barrier for migration of internal contaminants between the outer ply on one side of the barrier mid-ply to the outer ply on the other side of the barrier mid-ply. Such internal contaminants may for example include saturated hydrocarbons (MOSH) and alkylated aromatic hydrocarbons (MOAH). This is advantageous as it may allow for using higher amounts of recycled fibers, which often contain elevated levels of MOSH and / or MOAH, in the outer plies of the multiply paper substrate.

[0018] According to a first aspect illustrated herein, there is provided a method for manufacturing a multiply paper substrate comprising a first outer ply, a second outer ply, and a barrier mid-ply in a paper-making machine, the method comprising the steps of: a) forming and dewatering a first web layer from a first pulp suspension comprising at least 70 wt% (based on dry weight) of Kraft pulp to obtain a first outer ply of the multiply paper substrate; b) forming and dewatering a second web layer from a second pulp suspension comprising a mixture of 40-90 wt% (based on dry weight) of highly refined cellulose (HRC) having a Schopper-Riegler (SR) value in the range of 70-100, and 10-60 wt% (based on dry weight) of refined broke pulp having an SR value in the range of 30-70, to obtain a barrier mid-ply of the multiply paper substrate; c) forming and dewatering a third web layer from a third pulp suspension comprising at least 70 wt% (based on dry weight) of Kraft pulp to obtain a second outer ply of the multiply paper substrate; wherein the HRC is obtained from virgin Kraft pulp, and wherein the refined broke pulp in the second pulp suspension comprises refined broke pulp obtained from the method for manufacturing a multiply paper substrate.

[0019] The term “web” or “web layer” as used herein refers to a sheet formed cellulose- based material obtained by applying a suspension comprising a cellulose-based fibrous material and / or highly refined cellulose on a surface, preferably a porous surface, and dewatering the applied suspension to increase the dry solids content of the suspension until a cellulose-based web layer is formed on the surface.

[0020] The term “ply” as used herein refers generally to the dewatered and dried web layer which may form part of a multiply paper substrate.

[0021] The multiply paper substrate as used herein refers generally to a multilayer sheet formed material obtained by co-formation or wet lamination of two or more cellulose-based web layers. Depending on the thickness and composition of the multiply paper substrate, it can be considered as a multiply paper or a multiply paperboard.

[0022] The multiply paper substrate can be used as such, or it can be combined with one or more other layers. The multiply paper substrate may for example be useful as a barrier layer in a paperboard-based packaging material. The multiply paper substrates may optionally be coated or laminated with one or more polymer layers to provide additional resistance to liquids, gases, aromas, and grease. The multiply paper substrates of the present disclosure provided with one or more polymer layers may be especially suited as substrates for packaging laminates, such as liquid packaging board (LPB).

[0023] Although different arrangements for performing the steps of the inventive method could be contemplated by the skilled person, the inventive method may advantageously be performed in a paper machine, more preferably in a Fourdrinier type paper machine, i.e. a paper machine based on the principles of the Fourdrinier Machine.

[0024] A paper machine (or paper-making machine) is an industrial machine which is used in the pulp and paper industry to create paper or fiber-based substrates in large quantities at high speed. Modern paper-making machines are typically based on the principles of the Fourdrinier machine, which uses a moving dewatering fabric or woven mesh, commonly referred to as a “wire”, to create a continuous web by filtering out the fibers held in a pulp suspension and producing a continuously moving wet web of fiber. This wet web is dried in the machine to produce paper or film. Paper machines for manufacturing a multiply paper substrate in accordance with the present disclosure are well known and available to the skilled person. The pulp suspension is preferably applied to the wire using a so-called headbox. The wires are preferably endless wires. The dewatering fabric of the wires can be a single ply or multiply fabric, made of plastic, non-woven, composite, or metal. The wire section of a paper machine may have various dewatering devices such as blade, table and / or foil elements, suction boxes, friction less dewatering, ultra-sound assisted dewatering, couch rolls, or a dandy roll.

[0025] In some embodiments, at least two of the web layers are formed on the same wire in the forming section of the paper machine using two or more headboxes, or the same multilayer headbox, to form the different web layers. In some embodiments, all three of the web layers are formed on the same wire in the forming section of the paper machine using three headboxes, or the same multilayer headbox, to form the different web layers.

[0026] In some embodiments, the web layers are formed on different wires in the forming section of the paper machine. The preferred type of forming section for use with the present invention includes at least two wires.

[0027] The second pulp suspension is an aqueous suspension comprising a water- suspended mixture of highly refined cellulose-based material, referred to herein as highly refined cellulose (HRC), refined broke pulp, and optionally other non-fibrous additives. The second pulp suspension comprises 40-90 wt% (based on dry weight) of HRC having an SR value in the range of 70-100, and 10-60 wt% (based on dry weight) of refined broke pulp having an SR value in the range of 30-70.

[0028] The HRC of the second pulp suspension is obtained from virgin Kraft pulp. Virgin Kraft pulp refers to a type of pulp made from wood chips that have not been previously used or recycled. Virgin Kraft pulp has a consistent chemical composition free from contaminants and impurities, such as additives and fillers, that are often found in recycled pulp. This purity ensures that the HRC produced is of high quality, with consistent properties. This consistency is important for producing HRC with predictable and reliable properties. Virgin Kraft pulp contains long, strong fibers that are ideal for producing HRC. The strength of the fibers contributes to the mechanical strength and durability of webs and plies formed of the HRC and the consistent properties of the formed HRC contributes to improved barrier properties of the webs and plies formed of the HRC.

[0029] The HRC of the second pulp suspension can be produced from wood cellulose fibers, both from hardwood and softwood fibers or a combination thereof. The HRC is preferably produced from wood cellulose fibers sourced from trees such as pine, spruce, and other softwoods, although hardwoods can also be used. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources.

[0030] HRC may be obtained from bleached or unbleached Kraft pulp. In some embodiments, the HRC of the second pulp suspension comprises or consists of HRC obtained from bleached Kraft pulp. In some embodiments, the HRC of the second pulp suspension comprises or consists of HRC obtained from unbleached Kraft pulp.

[0031] In some embodiments, the HRC of the second pulp suspension is obtained from never-dried virgin Kraft pulp, i.e. a virgin Kraft pulp that has not undergone any drying process after its initial production and washing stages. Never-dried HRC maintains its high mechanical strength, flexibility, and reinforcing abilities important for mechanical performance and barrier properties, of webs and plies formed from the HRC. Dried HRC typically exhibits reduced performance in these areas due to the loss of its original m icrofibril lated structure.

[0032] The HRC of the second pulp suspension is preferably more refined than the refined broke pulp of the second pulp suspension and the second pulp suspension comprises 40-90 wt% (based on dry weight) of the HRC. Accordingly, the pulp will have a relatively low drainage rate and high water retention. The drainage rate is expressed as a Schopper-Riegler (SR) value, as determined by standard ISO 5267-1. The term “highly refined cellulose” as used herein preferably refers to a refined cellulose-based material having an SR value in the range of 70-100, as determined by standard ISO 5267-1.

[0033] The HRC of the second pulp suspension has an SR value in the range of 70-100. In some embodiments, the HRC in the second pulp suspension has an SR value in the range of 80-100, and preferably in the range of 90-100. In some embodiments, the HRC of the second pulp suspension has an SR value in the range of 80-98. In some embodiments, the HRC of the second pulp suspension has an SR value in the range of 85-98. The water retention of the HRC is expressed as the water retention value (WRV), as determined by standard ISO 23714:2014. In some embodiments, the HRC of the second pulp suspension has a WRV of >200%, preferably >250%.

[0034] In some embodiments, the SR value of the HRC is at least 10 SR degrees, more preferably at least 20 or at least 30 SR degrees higher than the SR value of the pulp of the refined broke pulp of the second pulp suspension.

[0035] In some embodiments the highly refined cellulose is formed from a fractionated pulp from which a fraction of the finest particulate material has been removed. Such a fractionated pulp provides a highly refined cellulose having a high content of long fibrils and fibrillated fibers, whereas a large part of the smallest fibrils would have been removed. In this way, both suspension rheology and retention on the wire can be significantly improved. The content of long fibrils and fibrillated fibers in a sample can be determined using the L&W Fiber tester Plus instrument (L&W / ABB). The L&W Fiber tester Plus instrument determines the content of fibers having a length >0.2 mm (including long fibrils and fibrillated fibers having a length >0.2 mm). A known sample weight of 0.100 g is used for each sample and the content of fibers having a length >0.2 mm (million fibers per gram) is calculated using the following formula: Million fibers per gram = (No. fibers in sample) / (Sample weight) / 1 000 000 = (Property ID 3141) / property ID 3136) / 1 000 000.

[0036] In some embodiments, the HRC of the second pulp suspension has a content of fibers having a length >0.2 mm of at least 8 million fibers per gram based on dry weight. In some embodiments, the HRC of the second pulp suspension has a content of fibers having a length >0.2 mm of at least 10 million fibers per gram based on dry weight, preferably at least 12 million fibers per gram based on dry weight, and more preferably at least 14 million fibers per gram based on dry weight, as determined using the L&W Fiber tester Plus instrument (L&W / ABB).

[0037] The term “broke” as used herein refers to paper that is discarded at any point in the manufacturing process. This can include dry or wet web layer, multilayer web, or paper trimmings, off-spec dry or wet web layer, multilayer web, or paper, or any dry or wet web layer, multilayer web, or paper that is damaged or defective during production, as well as combinations thereof. The term “broke pulp” as used herein refers to pulp obtained from broke. Essentially, broke encompasses all forms of paper waste generated within a paper mill. Accordingly, the broke pulp may comprise pulp obtained from all plies of the multiply paper substrate.

[0038] The refined broke pulp of the second pulp suspension comprises, and preferably consists of, refined broke pulp obtained from the method for manufacturing a multiply paper substrate.

[0039] Recycling broke within the paper manufacturing process typically involves several steps to reintegrate it back into the production cycle. Broke is collected from various points in the manufacturing process. This includes trimmings from paper cutting, rejected rolls, and damaged sheets. It is sorted based on its quality and type. The sorted broke is sent to a repulper, where it is mixed with water and agitated to break it down into its fibrous components. This creates a broke pulp suspension. The broke pulp suspension can be screened and cleaned to remove any contaminants, such as filler, adhesives, ink, or debris. This step ensures that the broke pulp is of high quality and suitable for reuse. Preferably the broke pulp comprises less than 10 wt%, more preferably less than 5 wt%, of filler. The optionally cleaned broke pulp suspension is then refined to an SR value in the range of 30-70. The refined broke pulp is mixed and preferably co-refined with the HRC and optionally other components to obtain a pulp suspension with the desired quality and characteristics for the final product. The pulp suspension comprising the mixture is then fed back into the papermaking machine, where it is formed and dewatered to obtain the barrier mid-ply of the multiply paper substrate.

[0040] The broke pulp of the second pulp suspension is refined to an SR value in the range of 30-70. Refining, or beating, of cellulose-based fibrous materials refers to mechanical treatment and modification of the cellulose fibers in order to provide them with desired properties. In some embodiments, the refined broke pulp in the second pulp suspension has an SR value in the range of 30-60, preferably in the range of 35-55, and more preferably in the range of 40-50. This degree of refining has been found to allow for a relatively high proportion of the broke pulp to be added to the highly refined pulp while still maintaining good gas, aroma, and grease barrier properties in the formed barrier layer. By recycling broke in this manner waste can also be reduced, production costs lowered, and the environmental impact minimized.

[0041] In some embodiments, the refined broke pulp in the second pulp suspension is subjected to treatment with defibrator or deflaker at a consistency in the range of 3-7 wt%, and more preferably in the range of 4-6 wt%.

[0042] In some embodiments, the refined broke pulp in the second pulp suspension is subjected to low consistency (LC) refining at a consistency in the range of 3-5 wt%. In some embodiments, the broke pulp is refined in the presence of a polysaccharide, such as sodium carboxymethyl cellulose, starch, or alginate, or in the presence of a protein.

[0043] The refining process preferably results in a refined broke pulp having a fiber length Lc(l) of less than 1.5 mm, preferably less than 1.3 mm, and more preferably less than 1.2 mm, as measured according to ISO 16065-2 using an FS5 fiber analyzer (Valmet Oyj).

[0044] In preferred embodiments, the refined broke pulp of the second pulp suspension consists of refined broke pulp obtained from the method for manufacturing a multiply paper substrate.

[0045] The second pulp suspension comprises 40-90 wt% (based on dry weight) of the highly refined cellulose (HRC). In some embodiments, the second pulp suspension comprises 50-80 wt%, preferably 60-80 wt%, (based on dry weight) of the HRC.

[0046] The second pulp suspension comprises 10-60 wt% (based on dry weight) of the refined broke pulp. In some embodiments, the second pulp suspension comprises 20-50 wt%, preferably 20-40 wt%, (based on dry weight) of the refined broke pulp.

[0047] The mixture of HRC and refined broke pulp is preferably subjected to co-refining to increase uniformity and enhance the bonding between the different fiber types in the mixture. Thus, in some embodiments the mixture of the second pulp suspension is a co-refined mixture. In some embodiments, the HRC and refined broke pulp are subjected to co-refining at a consistency in the range of 2-5 wt%, preferably in the range of 2.5-4 wt%. The co-refining may preferably be performed with a specific energy consumption in the range of 20-200 kWh / tn, and preferably in the range of 30-150 kWt / tn.

[0048] In some embodiments, the pulp mixture of the second pulp suspension has an SR value in the range of 50-95, preferably in the range of 55-90, and more preferably in the range of 60-90. One advantage of using a combination of highly refined cellulose (HRC) obtained from virgin Kraft pulp, and refined broke pulp, according to the inventive method is that the amount of retention or flocculation chemicals or fixatives commonly used to improve wire retention of the highly refined cellulose can be reduced. These types of additives usually have a negative impact on the barrier properties of the resulting ply or layer.

[0049] The dry solids content of the second pulp suspension may consist solely of the mixture of HRC and refined broke pulp, or it can further comprise other ingredients or additives.

[0050] The second pulp suspension preferably includes the HRC as its main component based on the total dry weight of the suspension. Having a high content of the HRC obtained from virgin Kraft pulp in the second pulp suspension ensures good barrier properties in the finished multiply paper substrate. The second pulp suspension comprises at least 40 wt% (based on dry weight) of the HRC. In some embodiments, the second pulp suspension comprises 50-80 wt% (based on dry weight), preferably 60-80 wt% (based on dry weight), of the HRC.

[0051] In addition to the HRC and the refined broke pulp, the second pulp suspension may also comprise a certain amount of unrefined or slightly refined cellulose fibers. The term “unrefined or slightly refined cellulose fibers” as used herein preferably refers to cellulose fibers having a Schopper-Riegler (SR) value below 30, preferably below 28, as determined by standard ISO 5267-1. Unrefined or slightly refined cellulose fibers are useful to enhance dewatering and may also improve strength and fracture toughness of the multiply paper substrate. In some embodiments, the second pulp suspension comprises 0.1-40 wt% (based on dry weight), preferably 0.1-25 wt% (based on dry weight), and more preferably 0.1-10 wt% (based on dry weight) of unrefined or slightly refined cellulose fibers. The unrefined or slightly refined cellulose fibers may for example be obtained from bleached or unbleached Kraft pulp, bleached or unbleached or mechanical or chemi-mechanical pulp, or other high yield pulps. The HRC of the second pulp suspension is a highly refined kraft pulp suspension. Kraft pulp will typically comprise at least 10 wt% (based on dry weight) hemicellulose. Thus, in some embodiments the second pulp suspension comprises hemicellulose at an amount of at least 10 wt%, such as in the range of 10-25 wt%, based on dry weight of HRC.

[0052] The second pulp suspension may further comprise additives such as fillers, deflocculating additives, dry strength additives, latexes, softeners, cross-linking aids, sizing chemicals, dyes and colorants, wet strength resins, de-foaming aids or foaming aids, microbe and slime control aids, or mixtures thereof.

[0053] In some embodiments, the combined content of the HRC and the refined broke pulp in the second pulp suspension is at least 70 wt% (based on dry weight), preferably at least 80 wt% (based on dry weight), and more preferably at least 90 wt% (based on dry weight). Having a high total content of the HRC and the refined broke pulp in the second pulp suspension ensures good barrier properties in the finished multiply paper substrate. Thus, the second pulp suspension preferably comprises no more than 30 wt% (based on dry weight), preferably no more than 20 wt% (based on dry weight), and more preferably no more than 10 wt% (based on dry weight), of additives in total.

[0054] In some embodiments, the second pulp suspension comprises up to 20 wt% (based on dry weight), preferably up to 10 wt% (based on dry weight), of a filler, e.g. a phyllosilicate such as bentonite. The filler is preferably a platy filler. In some embodiments, the filler has a shape factor higher than 20, preferably higher than 30, and more preferably higher than 40. "Shape factor" as used herein is a measure of an average value (on a weight average basis) of the ratio of mean particle diameter to particle thickness for a population of particles of varying size and shape, as measured using the electrical conductivity method and apparatus described in, for example, patent publications US 5 128606 and US 5 576617.

[0055] The pH value of the second pulp suspension may typically be in the range of 4-10 preferably in the range of 5-8, and more preferably in the range of 5.5-7.5. The temperature of the second pulp suspension may typically be in the range of 40-80 °C, preferably in the range of 50-80 °C, and more preferably in the range of 60-80 °C.

[0056] In some embodiments, the HRC in the second pulp suspension is microfibrillated cellulose (MFC). MFC shall in the context of the patent application mean a cellulose particle, fiber or fibril having a width or diameter of from 20 nm to 1000 nm. Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment steps is usually required in order to make MFC manufacturing both energy efficient and sustainable. The cellulose fibers of the pulp used when producing MFC may thus be native or pre-treated enzymatically or chemically, for example to reduce the quantity of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, wherein the cellulose molecules contain functional groups other (or more) than found in the original cellulose. Such groups include, among others, carboxymethyl (CM), aldehyde and / or carboxyl groups (cellulose obtained by N-oxyl mediated oxidation, for example "TEMPO"), or quaternary ammonium (cationic cellulose). After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC.

[0057] The density of the barrier mid-ply in the formed multiply paper substrate is preferably in the range of 700-1300 kg / m3, more preferably in the range of 750- 1300 kg / m3, and more preferably in the range of 800-1300 kg / m3.

[0058] The density of the barrier mid-ply is typically significantly higher than the density of the first and second outer plies in the formed multiply paper substrate. In some embodiments, the density of the barrier mid-ply is at least 100 kg / m3higher, and more preferably at least 150 kg / m3higher, than the density of the first and second outer plies in the formed multiply paper substrate.

[0059] In some embodiments, the grammage of the second web layer is in the range of 20-180 g / m2, preferably in the range of 20-140 g / m2, more preferably in the range of 20-80 g / m2. In some embodiments the second web layer preferably has a lower grammage than the first web layer.

[0060] The first pulp suspension is an aqueous suspension comprising a water- suspended mixture of cellulose-based fibrous material, or pulp, and optionally non- fibrous additives. The first pulp suspension comprises at least 70 wt% (based on dry weight) of Kraft pulp. The Kraft pulp may be softwood pulp or hardwood Kraft pulp. The Kraft pulp may be bleached or unbleached Kraft pulp. In a preferred embodiment, the Kraft pulp of the first pulp suspension is unbleached Kraft pulp. The pulp of the first pulp suspension can further include pressurized groundwood pulp (PGW), thermomechanical (TMP), chemi-thermomechanical pulp (CTMP), neutral sulfite semi chemical pulp (NSSC), broke, or recycled fibers, or combinations thereof. The pulp of the first pulp suspension may comprise up to 30 wt% (based on dry weight), such as in the range of 5-30 wt% (based on dry weight), of recycled fibers. In a preferred embodiment, the pulp of the first pulp suspension comprises or consists of unbleached Kraft pulp.

[0061] The pulp of the first pulp suspension can be unrefined or refined. Refining, or beating, of cellulose-based fibrous materials refers to mechanical treatment and modification of the cellulose fibers in order to provide them with desired properties. The pulp of the first pulp suspension is preferably unrefined or only slightly refined, such that the pulp will have a relatively high drainage rate and low water retention. The drainage rate is expressed as a Schopper-Riegler (SR) value, as determined by standard ISO 5267-1. In some embodiments, the pulp of the first pulp suspension has an SR value in the range of 10-50, preferably in the range of IQ- 40 and more preferably in the range of 10-30. In some embodiments, the pulp of the first pulp suspension has an SR value in the range of 18-50. In some embodiments, the pulp of the first pulp suspension has an SR value in the range of 20-35. The water retention of the pulp is expressed as the water retention value (WRV), as determined by standard ISO 23714:2014. In some embodiments, the pulp of the first pulp suspension has a WRV in the range of 100-220%, preferably in the range of 120-190%. The dry solids content of the first pulp suspension is typically in the range of 0.1- 1.5 wt%, preferably in the range of 0.1-1 wt%, more preferably in the range of 0.1- 0.5 wt%.

[0062] The dry solids content of the first pulp suspension may be comprised solely of the pulp, or it can comprise a mixture of pulp and other ingredients or additives.

[0063] The first pulp suspension includes the Kraft pulp as its main component, based on the total dry weight of the suspension. In some embodiments, the first pulp suspension comprises at least 80 wt% (based on dry weight) or at least 90 wt% (based on dry weight), of the Kraft pulp.

[0064] Kraft pulp will typically comprise at least 10 wt% (based on dry weight) of hemicellulose. Thus, in some embodiments the first pulp suspension comprises hemicellulose at an amount of at least 10 wt%, such as in the range of 10-25 wt%, based on the dry weight of Kraft pulp.

[0065] The first pulp suspension may further comprise additives such as native starch or starch derivatives, cellulose derivatives such as sodium carboxymethyl cellulose, a filler, retention and / or drainage chemicals, flocculation additives, deflocculating additives, dry strength additives, softeners, cross-linking aids, sizing chemicals, dyes and colorants, wet strength resins, fixatives, de-foaming aids, microbe and slime control aids, or mixtures thereof.

[0066] In some embodiments, the first pulp suspension comprises a hydrophobizing chemical such as an alkyl ketene dimer (AKD), an alkenyl succinic anhydride (ASA), or a rosin size in an amount of 0-10 kg / ton, preferably 0.1-5 kg / ton and more preferably 0.2-2 kg / ton based on the total dry weight of the suspension.

[0067] In some embodiments, the first pulp suspension comprises unbleached pulp to give the multiply paper substrate a natural look. In some embodiments, the first pulp suspension comprises less than 15 wt% (based on dry weight), and more preferably less than 10 wt% (based on dry weight), of refined broke pulp.

[0068] In some embodiments, the grammage of the first web layer is in the range of 20- 180 g / m2, preferably in the range of 20-140 g / m2, more preferably in the range of 20-100 g / m2.

[0069] The density of the first outer ply in the formed multiply paper substrate is preferably in the range of 400-800 kg / m3, more preferably in the range of 400-750 kg / m3, and more preferably in the range of 400-700 kg / m3.

[0070] The third pulp suspension is an aqueous suspension comprising a water- suspended mixture of cellulose-based fibrous material, or pulp, and optionally non- fibrous additives. The third pulp suspension comprises at least 70 wt% (based on dry weight) of Kraft pulp. The Kraft pulp may be softwood pulp or hardwood Kraft pulp. The Kraft pulp may be bleached or unbleached Kraft pulp. In a preferred embodiment, the Kraft pulp of the third pulp suspension is unbleached Kraft pulp. The pulp of the third pulp suspension can further include pressurized groundwood pulp (PGW), thermomechanical (TMP), chemi-thermomechanical pulp (CTMP), neutral sulfite semi chemical pulp (NSSC), broke, or recycled fibers, or combinations thereof. The pulp of the third pulp suspension may comprise up to 30 wt% (based on dry weight), such as in the range of 5-30 wt% (based on dry weight), of recycled fibers. In a preferred embodiment, the pulp of the third pulp suspension comprises or consists of unbleached Kraft pulp.

[0071] The pulp of the third pulp suspension can be unrefined or refined. Refining, or beating, of cellulose-based fibrous materials refers to mechanical treatment and modification of the cellulose fibers in order to provide them with desired properties. The pulp of the third pulp suspension is preferably unrefined or only slightly refined, such that the pulp will have a relatively high drainage rate and low water retention. The drainage rate is expressed as a Schopper-Riegler (SR) value, as determined by standard ISO 5267-1. In some embodiments, the pulp of third pulp suspension has an SR value in the range of 10-50, preferably in the range of 10- 40 and more preferably in the range of 10-30. In some embodiments, the pulp of the third pulp suspension has an SR value in the range of 18-50. In some embodiments, the pulp of the third pulp suspension has an SR value in the range of 20-35. The water retention of the pulp is expressed as the water retention value (WRV), as determined by standard ISO 23714:2014. In some embodiments, the pulp of the third pulp suspension has a WRV in the range of 100-220%, preferably in the range of 120-190%.

[0072] The dry solids content of the third pulp suspension is typically in the range of 0.1- 1.5 wt%, preferably in the range of 0.1-1 wt%, more preferably in the range of 0.1- 0.5 wt%.

[0073] The dry solids content of the third pulp suspension may be comprised solely of the pulp, or it can comprise a mixture of pulp and other ingredients or additives.

[0074] The third pulp suspension includes the Kraft pulp as its main component, based on the total dry weight of the suspension. In some embodiments, the third pulp suspension comprises at least 80 wt% (based on dry weight) or at least 90 wt% (based on dry weight), of the Kraft pulp.

[0075] Kraft pulp will typically comprise at least 10 wt% (based on dry weight) of hemicellulose. Thus, in some embodiments the third pulp suspension comprises hemicellulose at an amount of at least 10 wt%, such as in the range of 10-25 wt%, based on the dry weight of Kraft pulp.

[0076] The third pulp suspension may further comprise additives such as native starch or starch derivatives, cellulose derivatives such as sodium carboxymethyl cellulose, a filler, retention and / or drainage chemicals, flocculation additives, deflocculating additives, dry strength additives, softeners, cross-linking aids, sizing chemicals, dyes and colorants, wet strength resins, fixatives, de-foaming aids, microbe and slime control aids, or mixtures thereof.

[0077] In some embodiments, the third pulp suspension comprises a hydrophobizing chemical such as an alkyl ketene dimer (AKD), an alkenyl succinic anhydride (ASA), or a rosin size in an amount of 0-10 kg / ton, preferably 0.1-5 kg / ton and more preferably 0.2-2 kg / ton based on the total dry weight of the suspension.

[0078] In some embodiments, the third pulp suspension comprises unbleached pulp to give the multiply paper substrate a natural look.

[0079] In some embodiments, the third pulp suspension comprises less than 15 wt% (based on dry weight), and more preferably less than 10 wt% (based on dry weight), of refined broke pulp.

[0080] In some embodiments, the grammage of the third web layer is in the range of 20- 180 g / m2, preferably in the range of 20-140 g / m2, more preferably in the range of 20-100 g / m2.

[0081] The density of the second outer ply in the formed multiply paper substrate is preferably in the range of 400-800 kg / m3, more preferably in the range of 400-750 kg / m3, and more preferably in the range of 400-700 kg / m3.

[0082] The multiply structure of the formed multiply paper substrate may preferably be at least partially symmetrical. This means that the first outer ply may have the same or similar composition and / or properties as the second outer ply. This symmetrical structure helps to prevent curling of the formed multilayer structures.

[0083] The term “similar properties” as used herein generally means that the value of a property of the first outer ply differs from the corresponding value of the property of the second outer ply by less than 10 %, and preferably less than 5 %, when the values are measured by the same method.

[0084] In some embodiments, the Kraft pulp in the first and third pulp suspensions have the same or similar SR values. The term "similar SR values” as used herein means that the difference between the SR values of the Kraft pulp in the first and third pulp suspensions is less than 10 %, and preferably less than 5 %. Having the same or similar SR values of the Kraft pulp in the first and third pulp suspensions leads to the same or similar shrinkage during dewatering or drying of the first and the third web layers.

[0085] In some embodiments, the first and third web layers exhibit the same or similar shrinkage during dewatering or drying. The term "similar shrinkage” as used herein means that the difference between the shrinkage of the first and the third web layer is less than 10 %, preferably less than 5 % in both the machine direction and in the cross direction.

[0086] In some embodiments, the first and third web layers have the same or similar composition and grammage. The term "similar composition” as used herein means that the first and third web layers are composed of the same or equivalent components, such as Kraft pulp and optional additives. The amounts of the components preferably differ by less than 10 %, and preferably less than 5 %. In some embodiments, the amount of Kraft fibers in the first and third web layers (based on dry weight) differ by less than 10 %, and preferably less than 5 %. In some embodiments, the same pulp suspension is used for the first and third web layers. In other words, in some embodiments the first and third pulp suspensions are the same. The term "similar grammage” as used herein means that the grammages of the first and third web layers differ by less than 10 %, and preferably less than 5 %. In some embodiments, the first and third web layers have the same composition. In some embodiments, the first and third web layers have the same grammage. In some embodiments, the first and third web layers have the same density.

[0087] In some embodiments, each of the first and third pulp suspensions comprise less than 15 wt% (based on dry weight), and more preferably less than 10 wt% (based on dry weight), of refined broke pulp.

[0088] In some embodiments, the grammage of each of the first and third web layers is in the range of 20-180 g / m2, preferably in the range of 20-140 g / m2, more preferably in the range of 20-100 g / m2. The method of the present disclosure comprises forming and dewatering a first web layer from a first pulp suspension to obtain a first outer ply, forming and dewatering a second web layer from a second pulp suspension to obtain a barrier mid-ply, and forming and dewatering a third web layer from a third pulp suspension to obtain a second outer ply of the multiply paper substrate.

[0089] Dewatering of the webs on the wire may be performed using methods and equipment known in the art. Examples include but are not limited to table roll and foils, friction less dewatering and ultra-sound assisted dewatering.

[0090] In some embodiments, the second web layer is subjected to two-sided dewatering.

[0091] Dewatering means that the dry solids content of the web layers is reduced compared to the dry solids content of the pulp suspensions, but the dewatered web layer may still comprise some water. In some embodiments, dewatering of the web layer means that the dry solids content of the dewatered web layer is above 1 wt% but below 15 wt%. In some embodiments, dewatering of the web layer means that the dry solids content of the dewatered web layer is above 1 wt% but below 10 wt%. A dry solids content of the dewatered webs in this range has been found to be especially suitable for joining the wet webs into a multilayer web.

[0092] The first web layer, the second web layer and the third web layer can be formed separately, on different wires, or together, on the same wire.

[0093] In some embodiments, the second web layer is formed directly on the first web layer by applying the second pulp suspension on the wet or dewatered first web layer. The first web layer is preferably dewatered before the second web layer is formed or applied. In some embodiments, the second web layer is formed simultaneously with the first web layer, e.g. using a multilayer headbox or two headboxes arranged at the same wire. In some embodiments, the first web layer and the second web layer are formed simultaneously using a multilayer headbox. The water of the second pulp suspension can be removed by dewatering through the less drainage resistant first web layer, or by drying, or by a combination thereof. The dewatering and / or drying of the second web layer results in the formation of the barrier mid-ply on the first outer ply.

[0094] In other embodiments, the second web layer is formed separately, e.g. on a separate wire, dewatered and subsequently wet laminated onto the first web layer.

[0095] The dewatered web layers are preferably joined by wet lamination. When the pulp suspension is dewatered on the wire a visible boundary line will appear at a point where the web layer goes from having a reflective water layer to where this reflective layer disappears. This boundary line between the reflective and non- reflective web layer is referred to as the waterline. The waterline is indicative of a certain solids content of the web. The webs are preferably joined after the water line. Joining the web layers while they are still wet ensures good adhesion between the layers. The joining can be achieved by applying one of the dewatered web layers on top of the other. The joining may be done non-wire side against non-wire side, or wire-side against non-wire side. Joining and further dewatering of the formed multilayer web may be improved by various additional operations. In some embodiments, the joining further comprises pressing the dewatered webs together. In some embodiments, the joining further comprises applying suction to the joined dewatered web layers. Applying pressure and / or suction to the formed multilayer web improves adhesion between the web layers.

[0096] In some embodiments, step a) comprises: a 1 ) forming a first web layer by applying the first pulp suspension on a wire, and a2) dewatering said first web layer to obtain a a first outer ply of the multiply paper substrate.

[0097] In some embodiments, step b) comprises: b1) forming a second web layer by applying the second pulp suspension on a wire or on the first outer ply, and b2) dewatering said second web layer to obtain a barrier mid-ply of the multiply paper substrate.

[0098] In some embodiments, step c) comprises: c1) forming a third web layer by applying a third pulp suspension on a wire or on the barrier mid-ply, and c2) dewatering said third web layer to obtain a second outer ply of the multiply paper substrate.

[0099] In some embodiments, the third web layer is formed directly on the second web layer by applying the second pulp suspension on the wet or dewatered second web layer. In other embodiments, the third web layer is formed separately, e.g. on a separate wire, dewatered and subsequently wet laminated onto the second web layer. The third web layer is preferably formed and dewatered on a separate wire and subsequently applied to the dewatered second web layer to form the multilayer web. The dewatering of the third web layer reduces the problems of draining water through the low permeability second web layer. This prevents delamination or bubble formation of the multilayer web.

[0100] In some embodiments, at least the first web layer and the second web layer are formed using the same multilayer headbox.

[0101] In some embodiments, the method further comprises the step: d) laminating the dewatered second web layer to the dewatered first web layer and / or the dewatered third web layer to obtain a multilayer web, and

[0102] In some embodiments, the method further comprises the step: e) further dewatering, and optionally drying, the formed multilayer web.

[0103] In the further dewatering and optional drying step, the dry solids content of the multilayer web is further increased. The resulting multiply paper substrate preferably has a dry solids content above 90 wt%.

[0104] The further dewatering typically comprises pressing the multilayer web to squeeze out as much water as possible. The further dewatering may for example include passing the formed multilayer web through a press section of a paper machine, where the web passes between large rolls loaded under high pressure to squeeze out as much water as possible. In some embodiments the further dewatering comprises passing the web through one or more shoe presses. The removed water is typically received by a fabric or felt. In some embodiments, the dry solids content of the multilayer web after the further dewatering is in the range of 15-48 wt%, preferably in the range of 18-40 wt%, and more preferably in the range of 22- 35 wt%.

[0105] The optional drying may for example include drying the multilayer web by passing the multilayer web around a series of heated drying cylinders. Drying may typically remove the water content down to a level of about 1-15 wt%, preferably to about 2-10 wt%.

[0106] The dry solids content of the final multiply paper substrate may vary depending on the intended use of the multiply paper substrate. For example a multiply paper substrate for use as a stand-alone product may have a dry solids content in the range of 85-99 wt%, preferably in the range of 90-98 wt%, whereas a multiply paper substrate for use in further lamination to form paper or paperboard based packaging material may have a dry solids content in the range of less than 90 wt%, preferably less than 85 wt%, such as in the range of 30-85 wt%.

[0107] In some embodiments, the grammage of the formed multilayer web and multiply paper substrate is in the range of 50-300 g / m2, preferably in the range of 50-200 g / m2, more preferably in the range of 50-150 g / m2.

[0108] In an exemplary embodiment, the first outer ply has a grammage of 35 g / m2and comprises 90 wt% (based on dry weight) unbleached Kraft pulp and 10 wt% (based on dry weight) refined broke pulp (SR 50) obtained from the method for manufacturing the multiply paper substrate. The barrier mid-ply has a grammage of 35 g / m2and comprises 30 wt% (based on dry weight) of refined broke pulp (SR 50) obtained from the method for manufacturing the multiply paper substrate and 70 wt% (based on dry weight) of MFC prepared from unbleached Kraft pulp. The second outer ply has a grammage of 35 g / m2and comprises 90 wt% (based on dry weight) unbleached Kraft pulp and 10 wt% (based on dry weight) refined broke pulp (SR 50) obtained from the method for manufacturing the multiply paper substrate. The obtained multiply paper substrate will typically exhibit good resistance to grease and oil. Grease resistance of the multiply paper substrate can be determined by the KIT-test according to standard ISO 16532-2. The test uses a series of mixtures of castor oil, toluene, and heptane. As the ratio of oil to solvent is decreased, the viscosity and surface tension also decrease, making successive mixtures more difficult to withstand. The performance is rated by the highest numbered solution which does not darken the sheet after 15 seconds. The highest numbered solution (the most aggressive) that remains on the surface of the paper without causing failure is reported as the "kit rating" (maximum 12). In some embodiments, the KIT value of the obtained multiply paper substrate is at least 6, preferably at least 8, as measured according to standard ISO 16532-2.

[0109] In some embodiments, the obtained multiply paper substrate has a Gurley Hill value of at least 10 000 s / 100ml, preferably at least 25000 s / 100ml, and more preferably at least 40 000 s / 100ml, as measured according to standard ISO 5636 / 6.

[0110] In addition to barrier properties, the obtained multiply paper substrate also has a high compressive strength. In some embodiments, the obtained multiply paper substrate has a compressive strength (SOT index in cross direction) of at least 15 Nm / g, preferably at least 18 Nm / g, and more preferably at least 20 Nm / g or at least 22 Nm / g, as measured according to according to ISO 9895.

[0111] The obtained multiply paper substrate preferably also has high repulpability. In some embodiments, the obtained multiply paper substrate exhibits less than 30 %, preferably less than 20 %, and more preferably less than 10 % residues, when tested as a category II material according to the PTS-RH 021 / 97 test method.

[0112] The multiply paper substrates of the present disclosure are especially suited as substrates for packaging laminates, such as liquid packaging board (LPB), when coated or laminated with one or more polymer layers. Thus, the multiply paper substrate may preferably be coated or laminated with one or more polymer layers. In some embodiments one or more polymer layer(s) are applied on one side or on both sides of the multiply paper substrate. The one or more polymer layer(s) may of course interfere with repulpability but may still be required or desired in some applications. Polymer layers may for example be applied by extrusion coating, film lamination or dispersion coating.

[0113] In some embodiments, the one or more polymer layer(s) comprises a PVOH coating layer. The PVOH coating layer improves the barrier properties of the coated multiply paper substrate. The PVOH coating layer may also provide an additional barrier against migration of low molecular weight substances, such as MOAH and MOSH from the multiply paper substrate. The PVOH coating layer may also improve the adhesion of a further polymer coating layer applied on the PVOH coating layer.

[0114] The PVOH coating layer may be applied by any suitable method known in the art. The PVOH coating layer may for example be applied as a solution or dispersion in an aqueous or organic solvent carrier using liquid coating methods known in the art, in melt form using extrusion coating, or in the form of a solid film by lamination.

[0115] The PVOH coating layer is preferably formed by means of a liquid film coating process, i.e. in the form of a solution or dispersion which, on application, is spread out to a thin, uniform layer on the substrate and thereafter dried. The liquid phase of the solution or dispersion is preferably water or an aqueous solution, but organic solvents or mixtures of water or aqueous solutions and organic solvents may also be used. The one or more polymers may be present in the solution or dispersion in dissolved form or in the form of polymer particles, such as a latex. The PVOH coating layer can be applied by contact or non-contact coating methods. Examples of useful coating methods include, but are not limited to rod coating, curtain coating, film press coating, cast coating, transfer coating, size press coating, impregnation, flexographic coating, gate roll coating, twin roll HSM coating, blade coating, such as short dwell time blade coating, jet applicator coating, spray coating, gravure coating or reverse gravure coating. To minimize the risk of pinholes in the PVOH coating layer, the PVOH coating layer may preferably be applied in at least two different coating steps with drying of the coated film between the steps.

[0116] In some embodiments, the PVOH coating layer comprises at least 50 wt% of a PVOH, preferably at least 70 wt% of a PVOH, based on the total dry weight of the PVOH coating layer.

[0117] In some embodiments, the PVOH coating layer comprises at least 50 wt% of a water-soluble PVOH based on dry weight. The water-soluble PVOH of the PVOH coating layer is soluble in cold water or soluble in hot water, e.g. at a temperature below 100 °C or even above 100 °C, for a given period of time. The water-soluble PVOH coating layer in addition to providing barrier properties and improving the adhesion of a further polymer coating layer applied on the PVOH coating layer, can also facilitate separation of a further polymer coating layer applied on the PVOH coating layer during repulping.

[0118] In some embodiments, the PVOH has a degree of hydrolysis in the range of 80-99 mol%, preferably in the range of 85-99 mol%.

[0119] In some embodiments, the PVOH coating layer further comprises a crosslinking agent capable of crosslinking the water-soluble polymer. The crosslinking agent may advantageously be applied together with the water-soluble polymer, and then activated, e.g. by heat or radiation, when the PVOH coating layer is in contact with the inorganic thin film coating. Crosslinking improves the water vapor barrier properties of the PVOH coating layer. Suitable crosslinking agents include but are not limited to polyfunctional organic acids or aldehydes, such as citric acid, glyoxal, and glutaraldehyde. In some embodiments, the crosslinking agent is an organic acid, and more preferably citric acid. The concentration of the crosslinking agent may for example be 1-20 wt%, preferably 1-15 wt%, based on the total dry weight of the PVOH coating layer. In some embodiments, the grammage of the PVOH coating layer is in the range of 1-20 g / m2, preferably in the range of 2-15 g / m2, more preferably in the range of 2- 12 g / m2, based on dry weight.

[0120] The one or more polymer layer(s) may also comprise a thermoplastic polymer layer comprising any of the thermoplastic polymers commonly used in paper or paperboard based packaging materials in general or polymers used in liquid packaging board in particular. Examples include polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polyhydroxyalkanoates (PHA), polylactic acid (PLA), polyglycolic acid (PGA), starch and cellulose. Polyethylenes, especially low density polyethylene (LDPE) and high density polyethylene (HDPE), are the most common and versatile polymers used in liquid packaging board.

[0121] Thermoplastic polymers are useful since they can be conveniently processed by extrusion coating techniques to form very thin and homogenous films with good liquid barrier properties. In some embodiments, the polymer layer comprises polypropylene or polyethylene. In preferred embodiments, the polymer layer comprises polyethylene, more preferably LDPE or HDPE.

[0122] In some embodiments, the polymer layer is formed by extrusion coating of the polymer onto a surface of the multiply paper substrate. Extrusion coating is a process by which a molten plastic material is applied to a substrate to form a very thin, smooth, and uniform layer. Common plastic resins used in extrusion coating include polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET).

[0123] In some embodiments, the polymer layer is formed by laminating a plastic film to a surface of the multiply paper substrate using an adhesive tie layer. The adhesive tie layer may for example comprise PVOH. The adhesive tie layer may be further defined as described for the PVOH coating layer.

[0124] In some embodiments, the polymer layer is vacuum deposition coated, preferably metallized. Vacuum deposition coating refers to a family of processes used to deposit layers of metals, metal oxides and other inorganic and organic compositions, typically atom-by-atom or molecule-by-molecule, on a solid surface. Vacuum deposition of a metal or metal oxide may also be referred to as metallization. In some embodiments, the vacuum deposition coating comprises a metal or metal oxide selected from the group consisting of aluminum, magnesium, silicon, copper, aluminum oxides, magnesium oxides, silicon oxides, and combinations thereof, preferably aluminum or an aluminum oxide. Multiple layers of the same or different materials can be combined. The process can be further specified based on the vapor source; physical vapor deposition (PVD) uses a liquid or solid source and chemical vapor deposition (CVD) uses a chemical vapor. Vacuum deposition coating typically results in very thin coatings. In some embodiments, the vacuum deposition coating has a thickness in the range of IQ- 600 nm, preferably in the range of 10-250 nm, and more preferably in the range of 50-250 nm. This should be compared to conventional aluminum foils used in packaging laminates, which foils typically have thickness in the range of about 3- 12 pm.

[0125] In some embodiments, the polymer layer is formed by laminating a vacuum deposition coated, preferably metallized, plastic film to a surface of the multiply paper substrate using an adhesive tie layer. The adhesive tie layer may for example comprise PVOH. The adhesive tie layer may be further defined as described for the PVOH coating layer.

[0126] The polymer layer may comprise one or more layers formed of the same polymeric resin or of different polymeric resins. In some embodiments the polymer layer comprises a mixture of two or more different polymeric resins. In some embodiments the polymer layer is a multilayer structure comprised of two or more layers, wherein a first layer is comprised of a first polymeric resin and a second layer is comprised of a second polymeric resin, which is different from the first polymeric resin. For example, in some embodiments, the multiply paper substrate comprises a PVOH coating layer and a polymer layer applied by extrusion coating or a plastic film or a metallized plastic film applied by lamination onto the PVOH coating layer. When a plastic film or a metallized plastic film is applied by lamination, the PVOH coating layer may advantageously be used as the adhesive tie layer. The grammage of each polymer layer of the multiply paper substrate is preferably less than 50 g / m2. In order to achieve a continuous and substantially defect free film, a grammage of the polymer layer of at least 8 g / m2, preferably at least 12 g / m2is typically required. In some embodiments, the grammage of the polymer layer is in the range of 8-50 g / m2, preferably in the range of 12-50 g / m2.

[0127] Unless otherwise specified, parameters discussed in the present disclosure are measured according to the following standards:

[0128] Grammage ISO 536

[0129] Density ISO 534

[0130] Drainability (SR) ISO 5267-1

[0131] Water retention (WRV) ISO 23714:2014 Fiber length (Lc(l)) ISO 16065-2 Grease resistance (KIT) ISO 16532-2 Viscosity SCAN-P 50:84

[0132] Porosity (Gurley Hill) ISO 5636 / 6 SGT (CD) ISO 9895

[0133] Repulpability (PTS) PTS-RH 021 / 97

[0134] Generally, while the products, polymers, materials, layers, and processes are described in terms of “comprising” various components or steps, the products, polymers, materials, layers, and processes can also “consist essentially of’ or “consist of” the various components and steps.

[0135] 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 not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

CLAIMS1 . A method for manufacturing a multiply paper substrate comprising a first outer ply, a second outer ply, and a barrier mid-ply in a paper-making machine, the method comprising the steps of: a) forming and dewatering a first web layer from a first pulp suspension comprising at least 70 wt% (based on dry weight) of Kraft pulp to obtain a first outer ply of the multiply paper substrate; b) forming and dewatering a second web layer from a second pulp suspension comprising a mixture of 40-90 wt% (based on dry weight) of highly refined cellulose (HRC) having a Schopper-Riegler (SR) value in the range of 70-100, and 10-60 wt% (based on dry weight) of refined broke pulp having an SR value in the range of 30-70, to obtain a barrier mid-ply of the multiply paper substrate; c) forming and dewatering a third web layer from a third pulp suspension comprising at least 70 wt% (based on dry weight) of Kraft pulp to obtain a second outer ply of the multiply paper substrate; wherein the HRC is obtained from virgin Kraft pulp, and wherein the refined broke pulp in the second pulp suspension comprises refined broke pulp obtained from the method for manufacturing a multiply paper substrate.

2. The method according to claim 1 , wherein the refined broke pulp in the second pulp suspension consists of refined broke pulp obtained from the method for manufacturing a multiply paper substrate.

3. The method according to any one of the preceding claims, wherein the refined broke pulp in the second pulp suspension has an SR value in the range of 30-60, preferably in the range of 35-55, and more preferably in the range of 40-50.

4. The method according to any one of the preceding claims, wherein the HRC in the second pulp suspension has an SR value in the range of 80-100, and preferably in the range of 90-100.

5. The method according to any one of the preceding claims, wherein the HRC in the second pulp suspension is microfibrillated cellulose (MFC).

6. The method according to any one of the preceding claims, wherein the second pulp suspension comprises 50-80 wt%, preferably 60-80 wt%, (based on dry weight) of the highly refined cellulose (HRC).

7. The method according to any one of the preceding claims, wherein the second pulp suspension comprises 20-50 wt%, preferably 20-40 wt%, (based on dry weight) of the refined broke pulp.

8. The method according to any one of the preceding claims, wherein the mixture of the second pulp suspension is a co-refined mixture.

9. The method according to any one of the preceding claims, wherein the pulp mixture of the second pulp suspension has an SR value in the range of 50-95, preferably in the range of 55-90, and more preferably in the range of 60-90.

10. The method according to any one of the preceding claims, wherein the grammage of the second web layer is in the range of 20-180 g / m2, preferably in the range of 20-140 g / m2, more preferably in the range of 20-80 g / m2.11 . The method according to any one of the preceding claims, wherein the first and third pulp suspensions comprise less than 15 wt% (based on dry weight), and more preferably less than 10 wt% (based on dry weight), of refined broke pulp.

12. The method according to any one of the preceding claims, wherein the pulp of the first and third pulp suspensions has an SR value in the range of 10-50, preferably in the range of 10-40 and more preferably in the range of 10-30.

13. The method according to any one of the preceding claims, wherein the grammage of the first and third web layers is in the range of 20-180 g / m2, preferably in the range of 20-140 g / m2, more preferably in the range of 20-10014. The method according to any one of the preceding claims, wherein the first and third web layers exhibit the same or similar shrinkage during dewatering or drying.

15. The method according to any one of the preceding claims, wherein the first and third web layers have the same or similar composition and grammage.

16. The method according to any one of the preceding claims, further comprising applying a polymer layer to the multiply paper substrate by laminating a vacuum deposition coated, preferably metallized, plastic film to a surface of the multiply paper substrate using an adhesive tie layer.

17. The method according to claim 16, wherein the adhesive tie layer comprises polyvinyl alcohol (PVOH).