Multilayer film containing highly refined cellulose fibers

Abstract

The present invention relates to a method for producing a multilayer film comprising highly purified cellulose fibers, the method comprising the steps of: a) forming a first wet web by applying a first pulp suspension comprising highly purified cellulose fibers onto a first wire; b) partially dewatering the first wet web to obtain a first partially dewatered web; c) forming a second wet web by applying a foamed second pulp suspension comprising highly purified cellulose fibers and a foaming agent onto a second wire; d) partially dewatering the foamed second wet web to obtain a second partially dewatered web; e) bonding the first and second partially dewatered webs to obtain a multilayer web; and f) further dewatering and optionally drying the multilayer web to obtain a multilayer film comprising highly purified cellulose fibers.

Description

[Technical Field]

[0001] The present disclosure relates to gas barrier films, such as those useful in paper and paperboard-based packaging materials. More specifically, the present disclosure relates to methods for producing films comprising highly purified cellulose fibers, particularly films comprising microfibrillated cellulose (MFC). [Background technology]

[0002] Effective gas barriers, aroma barriers, and / or moisture barriers are needed in the packaging industry to shield sensitive products. In particular, oxygen-sensitive products require oxygen barriers to extend their shelf life. Oxygen-sensitive products include many foods, but also include pharmaceutical products and electronics industry products. Known packaging materials with oxygen barrier properties can consist of one or several polymer films, usually as part of a multi-layer coating structure, or fibrous paper or board coated with one or several layers of oxygen-barrier polymers. Another important property for food packaging is resistance to grease and oil.

[0003] More recently, microfibrillated cellulose (MFC) films have been developed in which defibrillated cellulosic fibrils are suspended, e.g., in water, reorganized, and recombined to form a continuous film. MFC films have been found to provide good gas barrier properties and good resistance to grease and oil.

[0004] MFC films can be made using a casting technique, which involves applying an MFC dispersion to a non-porous cast substrate, such as a polymer or metal substrate, and drying the film by evaporation. Advantages of this technique include uniform thickness distribution and a smooth surface. Publication EP 2771390 A4 describes the preparation of MFC films, in which an aqueous cellulose nanofiber dispersion is coated onto a paper or polymer substrate, dried, and finally peeled off as a nanofiber film sheet.

[0005] A problem associated with the casting process is that as the film forms during the drying step, the slow diffusion of water limits the drying rate. The diffusion of water vapor through the film is a slow process that has a negative impact on process efficiency. As the drying rate increases, pinholes can form in the film, compromising its barrier properties. A further problem with the casting process is the formation of shrinkage tension in the formed film, which can have a negative impact on its strength properties, such as failure strain or tensile strength.

[0006] Alternatively, a film can be made by applying an MFC suspension onto a porous substrate to form a web, followed by dewatering the web by draining water through the substrate to form a film. The porous substrate can be, for example, a membrane or wire fabric, or it can be a paper or paperboard substrate. Formation of the web can be achieved, for example, by using a paper machine or paperboard machine-type process. U.S. Patent Application US20120298319 A1 teaches a method for producing an MFC film by applying a furnish containing MFC directly onto a porous substrate, thus allowing the MFC to be dewatered and filtered.

[0007] The production of films and barrier substrates from highly refined cellulose or suspensions that drain very slowly is challenging on paper machines because it is difficult to create a good barrier due to the occurrence of pinholes. Pinholes are microscopic holes that can appear in the web during the forming process. Examples of reasons for the appearance of pinholes include irregularities in the pulp suspension, such as those caused by fibril aggregation or reagglomeration, rough dewatering of the fabric, uneven pulp distribution on the wire, or too low a web basis weight. Pinhole formation typically increases with increasing dewatering rate. However, in the pinhole-free region, oxygen transmission rates tend to be higher for basis weights between 20 and 40 g / m². 2 It is good when it exceeds

[0008] One approach to improving barrier properties has been to create a thin base substrate containing several pinholes and then coat the substrate with a polymer coating composition. However, this approach requires a coating concept and a coating formulation that is optimized for surface coverage while simultaneously providing a barrier. Coating thin webs is also difficult because the coating can cause web breakage. The number of times the substrate is rewetted and dried should also be kept to a minimum, as each additional step increases costs. Polymer coatings can also reduce the repulpability of the film, thereby reducing the recyclability of products containing the film.

[0009] Another possibility discussed in the prior art would be to have a very slow dewatering time, but this is not feasible for a fast and thorough drainage concept.

[0010] Another solution would be to increase the basis weight or roughness of the film, which would significantly increase the drain time and increase the risk of pinholes, respectively.

[0011] From a technical and economic point of view, it would be preferable to find a solution that allows fast dehydration and at the same time improves either the mechanical or barrier properties of the film, or both. Summary of the Invention

[0012] It is an object of the present disclosure to provide a method for producing films comprising highly purified cellulose fibers, such as microfibrillated cellulose (MFC), that alleviates at least some of the above-mentioned problems associated with prior art methods.

[0013] It is a further object of the present disclosure to provide a method for producing a film comprising highly refined cellulose fibers that exhibits reduced pinhole formation.

[0014] It is a further object of the present disclosure to provide an improved method for producing films comprising highly refined cellulose fibers in a paper machine or board machine type process.

[0015] It is a further object of the present disclosure to provide a film useful as a gas barrier in paper or paperboard based packaging materials that is based on renewable raw materials.

[0016] It is a further object of the present disclosure to provide a film useful as a gas barrier in paper or paperboard based packaging materials that is highly repulpable while providing high recyclability of packaging products containing the film.

[0017] The above objectives, as well as other objectives which will be realized by those skilled in the art in light of this disclosure, are accomplished by various aspects of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0018] According to a first aspect exemplified herein, there is provided a method for producing a multilayer film comprising highly purified cellulose fibers, the method comprising: a) forming a first wet web by applying a first pulp suspension containing highly refined cellulose fibers onto a first wire; b) partially dewatering the first wet web to obtain a first partially dewatered web; c) forming a second wet web by applying a foamed second pulp suspension containing highly refined cellulose fibers and a foaming agent onto a second wire; d) partially dewatering the foamed second wet web to obtain a second partially dewatered web; e) joining the first and second partially dewatered webs to obtain a multi-layer web; and f) further dewatering and optionally drying the multi-layer web to obtain a multi-layer film comprising highly purified cellulose fibers. A method is provided, comprising:

[0019] The term film as used herein generally refers to a thin continuous sheet-forming material. Depending on the composition of the pulp suspension, the film may also be considered a thin paper or even a membrane. The film is preferably made of a material having a thickness of 100 g / m 2 less than 20 to 100 g / m 2 The multilayer film is typically relatively dense. In some embodiments, the multilayer film has a basis weight in the range of 600 kg / m 3 above 900 kg / m 3 It has a density greater than

[0020] The method of the present invention allows for the production of films containing highly purified cellulose fibers in a paper machine type process. More importantly, the method allows for the production of films having very low pinhole incidence or being substantially pinhole-free, 20-100 g / m 2 Due to the highly refined cellulose fiber content, the resulting multilayer film typically has a basis weight of 600 kg / m 3above 900 kg / m 3 Such films have been found to be very useful as gas barrier films, for example in packaging applications. The films can be used to replace conventional barrier films, such as synthetic polymer films, which reduce the recyclability of paper or paperboard packaging products. The films of the present invention have high repulpability while providing high recyclability of the films and paper or paperboard packaging products containing the films.

[0021] The manufacturing method involves the separate preparation and partial dewatering of two lower basis weight webs containing highly refined cellulose fibers. One of the webs is prepared from a foamed pulp suspension. The combination of the web formed from the unfoamed pulp suspension with the web formed from the foamed pulp suspension provides a multilayer film that combines the excellent gas barrier properties afforded by the unfoamed web with the combination of good gas barrier and strength properties enabled by the foamed web.

[0022] Foamed pulp suspensions allow for web formation at higher solids contents (lower water contents) compared to non-foamed pulp suspensions.

[0023] A further advantage of foamed pulp suspensions compared to non-foamed pulp suspensions is that it allows for the addition of higher amounts of unrefined or slightly refined cellulose fibers and other long fibers without causing problems of pinhole formation and loss of barrier properties.

[0024] Although it is possible to form a multilayer film from two webs formed from foamed pulp suspensions, it is typically preferred that one of the webs be formed from an unfoamed pulp suspension, as a film formed from an unfoamed pulp suspension will typically have better gas barrier properties. Thus, in some embodiments, the first pulp suspension is unfoamed.

[0025] The partially dewatered, but still wet, webs are bonded to form a multilayer web of higher basis weight, which is subsequently further dewatered and optionally dried to obtain a multilayer film containing highly refined cellulose fibers. Bonding the webs while they are still wet ensures good adhesion between the layers. In fact, if the compositions of the two layers are identical, the resulting multilayer film may even be difficult to distinguish from a single-layer film of corresponding thickness. It has been found that partially dewatering and laminating the partially dewatered webs virtually eliminates the occurrence of pinholes in the finished multilayer film while still allowing for high production rates. In the prior art, increased dewatering rates have sometimes been achieved by using large amounts of retention and drainage chemicals at the wet end of the process, sometimes causing increased clumping. However, retention and drainage chemicals can also cause a more porous web structure, thus minimizing the use of such chemicals is needed. The method of the present invention provides an alternative method for increasing dewatering rates that is less dependent on the addition of retention and drainage chemicals.

[0026] Although different configurations for carrying out the steps of the method of the present invention may be contemplated by those skilled in the art, the method of the present invention may be advantageously carried out on a paper machine, more preferably on a Fourdrinier paper machine.

[0027] A paper machine (or paper machine) is an industrial machine used in the pulp and paper industry to produce paper in large quantities at high speeds. Modern paper machines are typically based on the Fourdrinier machine principle, which uses a moving woven mesh "wire" to filter out fibers held in a pulp suspension, producing a continuous web by producing a continuously moving wet web of fibers. This wet web is dried within the machine to produce a strong paper web.

[0028] The forming, dewatering, and bonding steps of the method of the present invention are preferably carried out in the forming section of a paper machine, commonly referred to as the wet end. The wet web is formed on different wires within the forming section of the paper machine. A preferred type of forming section for use with the present invention includes two or three Fourdrinier wire sections combined with support wires. The wires are preferably endless wires. The wires used in the method of the present invention preferably have a relatively high porosity to allow for fast dewatering and high drainage capacity. The wire permeability is preferably 5000 m@2 at 100 Pa. 3 / m 2 / hour. The wire is preferably 1 cm long to reduce fiber markings. 2 At least 500 knuckles per cm, preferably 1 cm 2 Each may contain at least 1000 knuckles.

[0029] The first and second pulp suspensions comprise an aqueous suspension mixture of cellulose-based fibrous material and optional non-fibrous additives. The method of the present invention uses a pulp suspension containing highly refined cellulose fibers. Refining, or beating, cellulose pulp refers to the mechanical treatment and modification of cellulose fibers to impart desired properties to them. Highly refined cellulose fibers can be produced from different raw materials, such as softwood pulp or hardwood pulp. The highly refined cellulose fibers are preferably never-dried cellulose fibers.

[0030] The term highly refined cellulose fibers as used herein preferably refers to refined cellulose fibers having a Schopper-Riegler (SR) value of 65 or greater, preferably 70 or greater, as determined by ISO Standard 5267-1.

[0031] In some embodiments, the first and / or second pulp suspensions are formed from a cellulosic furnish having a Schopper-Riegler (SR) value in the range of 65-99, preferably in the range of 70-90.

[0032] In some embodiments, the foamed second pulp suspension is formed from a cellulosic furnish having an SR value that is less than the SR value of the cellulosic furnish of the second pulp suspension. The foamed second pulp suspension may, for example, be less highly refined than the first pulp suspension or may not contain a greater amount of unrefined or slightly refined cellulosic fibers. In some embodiments, the first pulp suspension may be formed from a cellulosic furnish having an SR value of 80 or greater, and the second pulp suspension may be formed from a cellulosic furnish having an SR value below 80.

[0033] One advantage of using a foamed pulp suspension is that it allows for web formation at a higher solids content (lower water content) compared to a non-foamed pulp suspension.

[0034] The dry solids content of the first pulp suspension is typically in the range of 0.1 to 0.7 wt%, preferably in the range of 0.15 to 0.5 wt%, more preferably in the range of 0.2 to 0.4 wt%.

[0035] The foamed second pulp suspension typically has a higher dry solids content than the first pulp suspension, typically in the range of 0.1 to 15 wt%, preferably 0.2 to 10 wt%, more preferably 1 to 8 wt%.

[0036] The second pulp suspension is foamed. The terms foam and foamed, as used herein, refer to a material created by trapping air or gas bubbles within a solid or liquid. Typically, the volume of gas is much greater than the volume of the liquid or solid, with a thin film separating the gas pockets. Three requirements must be met for foam to form. Mechanical work is required to increase the surface area. This can occur by agitation, dispersing a large volume of gas into the liquid, or injecting gas into the liquid. The second requirement is that a foam-forming agent, typically an amphiphile, surfactant, or surface-active component, must be present to reduce surface tension. Finally, foams must form more rapidly than they collapse.

[0037] The foamed second pulp suspension is prepared by incorporating a significant amount of gas, typically air, into a liquid, typically aqueous, pulp suspension. In some embodiments, foaming is achieved using a foam generator. The suspension may be pumped through the foam generator one or several times to reach the desired gas content or foam density. In some embodiments, the liquid pulp suspension is pumped through a high-shear mixer or refiner, which generates foam. The foam can be generated either offline or inline in the paper machine. The air content of the foamed second pulp suspension is typically in the range of 40-90% by volume. Depending on the composition and foam generator, different bubble sizes can be produced. The average bubble radius is preferably greater than 20 μm, such as in the range of 20-2000 μm. Foaming reduces the density of the pulp suspension compared to the unfoamed pulp suspension. Thus, in some embodiments, the density of the foamed second pulp suspension is greater than 800 kg / m. 3 preferably below 600 kg / m 3 and preferably below 400 kg / m 3 The pH of the foam is typically in the range of 4 to 10, preferably in the range of 6 to 8. The temperature of the foam is preferably kept constant, preferably below 60°C.

[0038] To enable foaming of the pulp, the foamed second pulp suspension includes a foaming agent. Typically, the foamed second pulp suspension includes a foamed aqueous mixture of highly refined cellulose fibers and a foaming agent.

[0039] The foaming agent may be any foaming agent suitable for facilitating foam formation in an aqueous pulp suspension and stabilizing the foam once formed. Foaming agents are generally anionic, nonionic, zwitterionic, or cationic surfactants. Surfactants reduce the surface tension of the liquid, thereby reducing the work required to create foam, and they increase the colloidal stability of the foam by inhibiting bubble coalescence.

[0040] In some embodiments, the foaming agent is a non-ionic surfactant.

[0041] Certain polymeric foaming agents have been found to be particularly useful for forming and stabilizing foam in aqueous pulp suspensions. In addition to acting as a foaming agent, polymeric foaming agents can also act as polymeric dispersants and / or rheology modifiers. Therefore, the use of polymeric foaming agents can reduce or entirely eliminate the addition of additional polymeric dispersants and / or rheology modifiers. Polymeric foaming agents can also improve the stability and mechanical properties of the web formed when the pulp suspension is dewatered on a wire. Another advantage of polymeric foaming agents in some applications is that they are inherently less likely to migrate from the finished product than low-molecular-weight surfactants or surface-active agents. Therefore, in some preferred embodiments, the foaming agent is a polymeric foaming agent.

[0042] In some embodiments, the blowing agent is an amphiphilic polymer.

[0043] In some embodiments, the foaming agent is selected from the group consisting of optionally hydrophobically modified polysaccharides, proteins, polyvinyl alcohol (PVOH), partially hydrolyzed polyvinyl acetate (PVOH / Ac), and mixtures thereof. The optional hydrophobic modifications typically include one or more hydrophobic groups, such as alkyl groups, covalently bonded to the foaming agent.

[0044] In some embodiments, the foaming agent is lignin or a lignin derivative, preferably lignin.

[0045] In some embodiments, the foaming agent is an optionally hydrophobically modified polysaccharide selected from the group consisting of cellulose, starch, hemicellulose, and mixtures thereof.

[0046] In some embodiments, the foaming agent is an optionally hydrophobically modified polysaccharide selected from the group consisting of optionally hydrophobically modified cellulose acetate (CA), ethyl(hydroxyethyl)cellulose (EHEC), methylcellulose (MC), ethylcellulose (EC), hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), sodium carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), sulfoethylcellulose, starch, and mixtures thereof.

[0047] In some embodiments, the blowing agent is selected from the group consisting of ethyl(hydroxyethyl)cellulose (EHEC), polyvinyl alcohol (PVOH), and partially hydrolyzed polyvinyl acetate (PVOH / Ac).

[0048] In some embodiments, the foaming agent is optionally hydrophobically modified methylcellulose.

[0049] In some embodiments, the blowing agent has a molecular weight greater than 5000 g / mol, preferably greater than 10000 g / mol.

[0050] The foamed second pulp suspension, and the finished multilayer film, are preferably free of low molecular weight surfactants or surface active agents that may migrate from the material. In some embodiments, the foamed second pulp suspension is free of surface active chemicals having a molecular weight below 1000 g / mol.

[0051] In some embodiments, the foamed second pulp suspension comprises in the range of 0.1 to 80 wt %, preferably in the range of 0.1 to 50 wt %, preferably in the range of 0.1 to 10 wt %, more preferably in the range of 0.1 to 5 wt % foaming agent based on the total dry weight of the foamed second pulp suspension.

[0052] The second pulp suspension is applied to the wire in foam form. Forming the second pulp suspension in a foamed state has several advantages, including the possibility of a higher solids content and the possibility of including a greater amount of long fibers in the pulp suspension. However, it is not necessarily desirable to retain the foam structure in the finished multilayer film. Rather, it may be desirable to collapse the foam during processing of the web, for example, to improve the barrier properties of the film. Partial dewatering on the wire also leads to at least partial collapse of the foam structure. Bonding the second partially dewatered web with the first partially dewatered web and further dewatering the formed multilayer web, optionally with the application of pressure or suction, leads to further collapse of the foam structure. The foam structure may be further collapsed using steam or radiant heating. Thus, the amount of gas trapped within the web typically decreases significantly as the web is processed. In some embodiments, the finished multilayer film is free or substantially free of residual gas bubbles from the foamed pulp suspension.

[0053] In some embodiments, a defoaming agent may be added to the second wet web or the second partially dewatered web to further accelerate foam collapse. In some embodiments, a defoaming agent may be applied between the first and second partially dewatered webs before joining the webs to obtain a multilayer web. The defoaming agent may be applied to the wet web or the partially dewatered web in liquid or solid form, for example, by spraying or curtain coating.

[0054] The defoaming agent may be added directly to the unfoamed first pulp suspension. When the first and second partially dewatered webs are joined, the defoaming agent from the first web will migrate into the second web. Thus, in some embodiments, the first pulp suspension further comprises a defoaming agent.

[0055] The dry solids content of the first and / or second pulp suspension may consist solely of highly purified cellulose fibers, or it may comprise a mixture of highly purified cellulose fibers with other components or additives. The first and / or second pulp suspension preferably comprises highly purified cellulose fibers as its major component based on the total dry weight of the pulp suspension. In some embodiments, the first and / or second pulp suspension comprises at least 50 wt%, preferably at least 70 wt%, more preferably at least 80 wt% or at least 90 wt% highly purified cellulose fibers based on the total dry weight of the pulp suspension.

[0056] In some embodiments, the highly refined cellulose fiber of the first and / or second pulp suspension is refined kraft pulp. Refined kraft pulp typically contains at least 10% hemicellulose. Thus, in some embodiments, the first and / or second pulp suspension contains at least 10% hemicellulose, such as in the range of 10-25% of the amount of highly refined cellulose fiber.

[0057] The first and / or second pulp suspensions may further comprise additives such as native starch or starch derivatives, cellulose derivatives such as sodium carboxymethylcellulose, fillers, retention and / or drainage chemicals, flocculation additives, deflocculation additives, dry strength additives, softeners, crosslinking aids, sizing chemicals, pigments and colorants, wet strength resins, fixatives, defoaming aids, microbial and slime control aids, or mixtures thereof. The first and / or second pulp suspensions may further comprise additives that will improve different properties of the mixture and / or the film produced, such as latex and / or polyvinyl alcohol (PVOH) to improve film ductility. The method of the present invention provides an alternative method for increasing dewatering rates that is less dependent on the addition of retention and drainage chemicals, although smaller amounts of retention and drainage chemicals may still be used.

[0058] The methods of the present invention are particularly useful for producing films of so-called microfibrillated cellulose (MFC). Thus, in some embodiments, the highly purified cellulose fibers are MFC.

[0059] Microfibrillated cellulose (MFC) in the context of this patent application shall be understood to mean nanoscale cellulose particle fibers or fibrils having at least one dimension less than 100 nm. MFC includes partially or fully fibrillated cellulose or lignocellulose fibers. While free fibrils have diameters less than 100 nm, the actual fibril diameter or particle size distribution and / or aspect ratio (length / width) depend on the source and production method. The smallest fibrils, called elementary fibrils, have a diameter of approximately 2 to 4 nm (see, e.g., Chinga-Carrasco, G., Cellulose fibers, nanofibrils, and microfibrils: The morphological sequence of MFC components from a plant physiology and fiber technology point of view, Nanoscale research letters 2011, 6:417), while aggregated forms of elementary fibrils, also defined as microfibrils (Fengel, D., Ultrastructural behavior of cell wall polysaccharides, Tappi J., March 1970, Vol. 53, No. 3), are typically the main product obtained when producing MFCs, for example, by using extended refining or pressure-drop cracking processes. Depending on the source and production process, the length of the fibrils can vary from approximately 1 micrometer to more than 10 micrometers. Coarse MFC grades may contain a significant proportion of fibrillated fibres, i.e. fibrils protruding from the tracheids (cellulose fibres), along with a certain amount of fibrils free from the tracheids (cellulose fibres).

[0060] Different acronyms exist for MFC, including cellulose microfibrils, fibrillated cellulose, nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibers, cellulose nanofibrils, cellulose microfibers, cellulose fibrils, microfibrillated cellulose, microfibril aggregates, and cellulose microfibril aggregates. MFC can also be characterized by various physical or physicochemical properties, such as its large surface area or its ability to form a gel-like material at low solids (1-5 wt%) when dispersed in water.

[0061] Various methods exist for producing MFC, including single-pass refining or multiple-pass refining, prehydrolysis followed by refining or high-shear degradation or fibril liberation. To make MFC production energy-efficient and sustainable, one or several pretreatment steps are usually required. Thus, the cellulose fibers of the utilized pulp may be pretreated, for example, enzymatically or chemically, to hydrolyze or swell the fibers or reduce the amount of hemicellulose or lignin. Cellulose fibers may be chemically modified before fibrillation so that the cellulose molecules contain (more) functional groups than those found in native cellulose. Such groups include, among others, carboxymethyl (CMC), aldehyde and / or carboxyl groups (cellulose obtained by N-oxyl-mediated oxidation, e.g., "TEMPO"), quaternary ammonium (cationic cellulose), or phosphoryl groups. After being modified or oxidized in one of the above methods, it is easier to decompose the fibers into MFC or nanofibrils.

[0062] Nanofibrillar cellulose may contain several types of hemicellulose, the amount of which depends on the plant source. Mechanical decomposition of pretreated fibers, such as hydrolyzed, preswollen, or oxidized cellulose raw materials, is carried out using suitable equipment, such as refiners, grinders, homogenizers, colloidal grinders, attrition grinders, ultrasonic sonicators, and fluidizers, such as microfluidizers, macrofluidizers, or fluidizer-type homogenizers. Depending on the MFC production method, the product may also contain fine fibers, nanocrystalline cellulose, or other chemicals present in wood fibers or during the papermaking process. The product may also contain varying amounts of micron-sized fiber particles that are not efficiently fibrillated.

[0063] MFC is produced from wood cellulose fibers, both hardwood and softwood. It can also be made from microbial sources, agricultural fibers such as straw pulp, bamboo, bagasse, or other non-wood fiber sources. It is preferably made from virgin fiber pulp, including mechanical, chemical, and / or thermomechanical pulp. It can also be made from broke or recycled paper.

[0064] The dry solids content of the first and / or second pulp suspension may consist solely of MFC, or it may include a mixture of MFC with other ingredients or additives. The first and / or second pulp suspension preferably comprises MFC as its major component, based on the total dry weight of the pulp suspension. In some embodiments, the first and / or second pulp suspension comprises 50-99 wt%, preferably at least 70-99 wt%, and more preferably at least 80-99 wt% MFC, based on the total dry weight of the pulp suspension.

[0065] In some embodiments, at least a portion of the MFC is obtained from MFC broke.

[0066] In addition to highly purified cellulose fibers, the first and / or second pulp suspensions may also contain a certain amount of unrefined or slightly refined cellulose fibers. As used herein, the term unrefined or slightly refined fibers preferably refers to cellulose fibers having a Schopper-Riegler (SR) value of less than 30, preferably less than 28, as determined by ISO Standard 5267-1. Unrefined or slightly refined cellulose fibers are useful for facilitating dewatering and can also improve the strength and fracture toughness of multilayer films. In some embodiments, the first and / or second pulp suspensions contain 0.1 to 50 wt%, preferably 0.1 to 30 wt%, and more preferably 0.1 to 10 wt%, of unrefined or slightly refined cellulose fibers, based on the total dry weight of the pulp suspension. Unrefined or slightly refined cellulose fibers can be obtained, for example, from bleached or unbleached, mechanical or chemimechanical pulps, or other high-yield pulps. The unrefined or slightly refined cellulose fibers are preferably never-dried cellulose fibers.

[0067] A foamed pulp suspension is advantageous in that it allows for the addition of larger amounts of unrefined or slightly refined cellulose fibers and other long fibers, such as regenerated cellulose fibers or thermoplastic fibers such as PLA or PVOH fibers, without surprisingly causing problems with pinhole formation and impaired barrier properties. The term "long fiber" as used herein refers to fibers having an average fiber length greater than 0.5 mm, preferably greater than 1 mm, more preferably greater than 2 mm or greater than 3 mm, greater than 4 mm, or greater than 5 mm. Long fibers typically have an average diameter in the range of 10 to 50 μm, preferably in the range of 10 to 40 μm, more preferably in the range of 10 to 30 μm, or in the range of 10 to 20 μm. Unrefined or slightly refined cellulose fibers and other long fibers can improve the mechanical properties of the film. In some embodiments, the foamed second pulp suspension contains 1 to 50 wt %, preferably 1 to 30 wt %, and more preferably 1 to 10 wt % of long fibers, based on the total dry weight of the pulp suspension.

[0068] Preferably, the foamed second pulp suspension contains a greater amount of unrefined or slightly refined cellulose or other long fibers than the unfoamed first pulp suspension.

[0069] In some embodiments, the long fibers of the foamed second pulp suspension are selected from the group consisting of unrefined or slightly refined cellulose fibers, regenerated cellulose fibers, and thermoplastic polymer fibers.

[0070] The pH value of the first and / or second pulp suspension may typically be in the range of 4 to 10, preferably in the range of 5 to 8, and more preferably in the range of 5.5 to 7.5.

[0071] The temperature of the first and / or second pulp suspension may typically be in the range of 30 to 70°C, preferably in the range of 40 to 60°C, and more preferably in the range of 45 to 55°C.

[0072] The first and second pulp suspensions are preferably fed onto two different wires from two different headboxes. The foamed second pulp suspension may be applied by a curtain coating arrangement instead of a headbox. The headboxes and wires are preferably adapted to handle the different characteristics of the different pulp suspensions. That is, the first headbox and first wire are adapted to form the first unfoamed pulp suspension, while the second headbox and second wire are adapted to form the foamed second pulp suspension. The headboxes can be operated differently, for example, with different consistencies, headbox jet angles, or jet-to-wire ratios.

[0073] The wire used in the method of the present invention preferably has a relatively high porosity to allow for fast dewatering and high drainage capacity. The wire preferably has an air permeability of 5000 m at 100 Pa. 3 / m 2 / More than hours.

[0074] The wire preferably has a high Fiber Support Index (FSI), typically above 190, to prevent fine material from getting into the structure and to result in less wire marking and a rough and open backside.

[0075] The basis weight of each of the first and / or second wet webs, based on the total dry weight of the webs, is preferably 50 g / m 2 less than 30 g / m 2 Less than 50g / m 2 Less than or equal to 30 g / m 2 It has been found that a basis weight of less than 5 g / m allows for rapid partial dewatering of the wet web with little pinhole formation. The basis weight of the first and / or second wet web, based on the total dry weight of the web, is preferably at least 5 g / m 2Thus, in some embodiments, the basis weight of the first and / or second wet webs, based on the total dry weight of the webs, is 5 to 50 g / m 2 within the range of 5 to 30 g / m 2 is within the range.

[0076] After formation, the first and second wet webs are partially dewatered. Dewatering of the webs on the wire may be carried out using methods and equipment known in the art, including, but not limited to, table roll and foil, frictionless dewatering, and ultrasonically assisted dewatering. Partial dewatering means that the dry solids content of the wet web is reduced compared to the dry solids content of the pulp suspension, but the dewatered web still contains a significant amount of water. In some embodiments, partial dewatering of the wet web means that the dry solids content of the first and second partially dewatered webs is greater than 1 wt% but less than 25 wt%. In some embodiments, partial dewatering of the wet web means that the dry solids content of the first and second partially dewatered webs is greater than 1 wt% but less than 20 wt%. The solids content of the first web may be less than the solids content of the second web after partial dewatering. In some embodiments, the dry solids content of the first partially dewatered web is greater than 1 wt% but less than 15 wt%. In some embodiments, partially dewatering the wet web means that the dry solids content of the first partially dewatered web is greater than 1 wt% but less than 10 wt%. Dry solids contents of the first and second partially dewatered webs within these ranges have been found to be particularly suitable for bonding the first and second wet webs into a multi-layer web. In some embodiments, the dry solids content of the first and second partially dewatered webs prior to the bonding step is in the range of 1.5 to 15 wt%, preferably 2.5 to 10 wt%, and more preferably 3 to 8 wt%. The dry solids content of the first partially dewatered web prior to the bonding step may be in the range of 1.5 to 8 wt%, preferably 2.5 to 6 wt%, and more preferably 3 to 4.5 wt%.

[0077] The partially dewatered, but still wet, webs are bonded to form a multilayer web of higher basis weight. The dry solids content of the first and second partially dewatered webs when they are bonded is preferably greater than 1 wt% but less than 25 wt%, more preferably greater than 1 wt% but less than 20 wt%. In some embodiments, the dry solids content of the first partially dewatered web when the webs are bonded is greater than 1 wt% but less than 15 wt%. In some embodiments, the dry solids content of the first and second partially dewatered webs before the bonding step is in the range of 1.5 to 15 wt%, preferably in the range of 2.5 to 10 wt%, more preferably in the range of 3 to 8 wt%. In some embodiments, the dry solids content of the first partially dewatered web when the webs are bonded is in the range of 1.5 to 8 wt%, preferably in the range of 2.5 to 6 wt%, more preferably in the range of 3 to 4.5 wt%. The partially dewatered webs are preferably bonded by wet lamination.

[0078] As the pulp suspension is dewatered on the wire, a visible boundary line may appear, from the point where the web advances with a reflective water layer to the point where the reflective layer disappears. This boundary line between the reflective and non-reflective webs is called the water line. The water line indicates a certain solids content of the web. The webs are preferably bonded after the water line. Bonding the webs while they are still wet ensures good adhesion between the layers. Bonding can be achieved by applying one of the partially dewatered webs on top of the other. The webs may be bonded in any order. The first web can be used as the bottom layer, with the second web applied on top, or vice versa. Bonding may be performed non-wire side to non-wire side, or wire side to non-wire side. Bonding and further dewatering of the formed multilayer web can be improved by various additional operations. In some embodiments, bonding further comprises pressing the first and second partially dewatered webs together. In some embodiments, bonding further comprises applying suction to the bonded first and second partially dewatered webs. The application of pressure and / or suction to the formed multi-layer web improves adhesion between the web layers. The wire section of the paper machine may have various dewatering devices such as blades, table and / or foil elements, suction boxes, frictionless dewatering, ultrasonically assisted dewatering, couch rolls, or dandy rolls.

[0079] The surface of the web facing the wire is called the wire side, and the surface of the web facing away from the wire is called the non-wire side.

[0080] It has been discovered that when highly refined cellulose fibers, particularly MFC, are dehydrated on a wire, a difference in fines content occurs between the non-wire side and the wire side. Fines are typically concentrated on the non-wire side, while more fines are washed off the wire side where dehydration occurs. This difference or imbalance in web composition can cause problems with curling in the finished film due to humidity changes. Forming a multilayer film according to the present invention can solve or ameliorate this problem by reducing the imbalance in web composition.

[0081] Bonding of the webs may preferably be performed non-wire-side to non-wire-side or wire-side to non-wire-side. Bonding the webs non-wire-side to non-wire-side or wire-side to non-wire-side provides an additional advantage in that a greater proportion of the fine fibers are concentrated toward the center of the multilayer film. This concentration of fine fibers contributes to both the adhesion between the layers and the gas barrier properties of the film. Fine fibers can also contribute to the self-healing phenomenon, where they redistribute and fill voids in the felted sheet on the wet wire, thus making the resulting film less porous.

[0082] Bonding the web non-wire side to non-wire side is preferred because i) the fines will be concentrated in the center, ii) the film structure will be symmetrical, reducing curling problems, iii) the high concentration of fines at the contact surface will ensure good bonding between the layers, and iv) the more porous outer surface (wire side) will allow for more efficient dewatering and faster drying in the press section.

[0083] The dry solids content of the multilayer web typically increases further during the bonding step. This increase in dry solids content can be due to dewatering of the multilayer web on the wire and optional pressure and / or suction applied to the web, as well as drying operations such as impingement drying or air or steam drying performed during or immediately after bonding. The dry solids content of the multilayer web after bonding and the optional application of pressure and / or suction is typically greater than 8 wt% but less than 35 wt%. In some embodiments, the dry solids content of the multilayer web prior to further dewatering and optional drying steps is in the range of 8 to 35 wt%, preferably in the range of 10 to 20 wt%, and more preferably in the range of 12 to 18 wt%.

[0084] The basis weight of multilayer webs and multilayer films, based on the total dry weight of the web, is typically 100 g / m 2 Less than 60 g / m 2 less than 40 g / m 2 In some embodiments, the basis weight of the multilayer web and multilayer film, based on the total dry weight of the web, is from 10 to 100 g / m 2 within the range of 10 to 60 g / m 2 in the range of 10 to 40 g / m 2 Pinhole-free films having basis weights within these ranges have been found to have good oxygen barrier properties.

[0085] The present invention is described herein primarily with reference to an embodiment in which a multilayer film is formed from two web layers comprising highly purified cellulose fibers. However, it is understood that the multilayer film may also include additional web layers comprising highly purified cellulose fibers. Thus, the multilayer film formed may be formed from more than two web layers comprising highly purified cellulose fibers, such as three, four, five, six, or seven layers. The formation, composition, and structure of each additional layer may be further characterized as described above with reference to the first and second web layers. Thus, in some embodiments, a method for producing a multilayer film comprises: c2) forming a third wet web by applying a third pulp suspension containing highly refined cellulose fibers onto a third wire; d2) partially dewatering the third wet web to obtain a third partially dewatered web; e2) joining the first, second and third partially dewatered webs to obtain a multi-layer web. Further includes:

[0086] The first, second, and third partially dewatered webs may be joined in any order, for example, the foamed second partially dewatered web may be arranged as a middle layer sandwiched between the first and third webs, or as an outer layer.

[0087] In the further dewatering and optional drying step f), the dry solids content of the multilayer web is typically further increased. The resulting multilayer film preferably has a dry solids content of greater than 90 wt%.

[0088] Further dewatering typically involves pressing the web to squeeze out as much water as possible. Further dewatering may include, for example, passing the formed multilayer web through the press section of a papermaking machine, where the web passes between large rolls loaded under high pressure to squeeze out as much water as possible. The removed water is typically received by a fabric or felt. In some embodiments, the dry solids content of the multilayer film after further dewatering is in the range of 15 to 48 wt%, preferably in the range of 18 to 40 wt%, and more preferably in the range of 22 to 35 wt%.

[0089] Optional drying may include drying the multi-layer web, for example, by passing the multi-layer web around a series of heated drying cylinders. Drying can typically remove the water content to a level of about 1-15 wt %, preferably about 2-10 wt %.

[0090] The dry solids content of the final multilayer film may vary depending on the intended use of the film, for example, films for use as stand-alone products may have a dry solids content in the range of 85-99 wt%, preferably 90-98 wt%, while films for use in further lamination to form paper or paperboard-based packaging materials 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%.

[0091] Pinholes are microscopic holes that can appear in the web during the forming process. Examples of reasons for the appearance of pinholes include irregularities in the pulp suspension, such as caused by aggregation or reagglomeration of fibrils, rough dewatering of the fabric, uneven pulp distribution on the wire, or too low a web basis weight. In some embodiments, the multilayer film has less than 10 pinholes / m2 as measured according to EN Standard 13676:2001. 2 Less than 8 pinholes / m 2Less than 2 pinholes / m 2 The measurement involves treating the multilayer film with a coloring solution (e.g., the dye E131 Blue in ethanol) and examining the surface microscopically.

[0092] Multilayer films typically exhibit good resistance to grease and oil. The grease resistance of multilayer films was evaluated by the KIT test according to ISO standard 16532-2. The test uses a series of mixtures of castor oil, toluene, and heptane. As the ratio of oil to solvent decreases, the viscosity and surface tension also decrease, making subsequent mixtures more difficult to tolerate. Performance is rated by the highest numbered solution that does not darken the sheet after 15 seconds. The highest numbered solution (most aggressive) that remains on the paper surface without causing failure is reported as the "KIT rating" (maximum 12). In some embodiments, the KIT value of the multilayer film is at least 6, preferably at least 8, when measured according to ISO standard 16532-2.

[0093] In some embodiments, the multilayer film has a Gurley-Hill value of at least 10,000 s / 100 ml, preferably at least 25,000 s / 100 ml, and more preferably at least 40,000 s / 100 ml, as measured according to ISO standard 5636 / 6.

[0094] The multilayer film preferably has high repulpability. In some embodiments, the multilayer film exhibits less than 30%, preferably less than 20%, and more preferably less than 10% residue when tested as a Category II material according to the PTS-RH 021 / 97 test method.

[0095] Films that contain large amounts of highly refined cellulose fibers are typically transparent or translucent to visible light. Thus, in some embodiments, the multilayer film is transparent or translucent to visible light.

[0096] In a more particular embodiment, the method of the present invention comprises: i) preparing a first furnish from a fiber mix containing 5-15 wt% unrefined or slightly refined bleached softwood or hardwood kraft pulp having a Schopper-Riegler (SR) value in the range of 15-25, preferably 20-25, and 95-85% highly refined bleached softwood or hardwood kraft pulp in the form of MFC having an SR value of at least 90. All of the cellulosic materials can be prepared from the same kraft pulp source, where the highly refined fiber is obtained by extensive fiber refining and / or homogenization and optional enzymatic pretreatment. The pH of the first furnish is between 6.5 and 8.5.

[0097] The Water Retention Value (WRV) of the mixture is about 300-350%. The SR value of the mix (without any additional chemicals added) is at least 80, preferably at least 85. The mixture therefore exhibits high drainage resistance.

[0098] The first furnish is prepared to a consistency of 0.15-0.35 wt% and a temperature of 35-45°C. To the furnish are added process chemicals such as retention aids (single or two or multi-component), forming aids (non-ionic or anionic water soluble polymers such as CMC), and optionally other additives such as fillers (<10 wt%), wet strength additives, hydrophobizing chemicals (<5 kg / tn).

[0099] ii) preparing a second furnish by forming an aqueous suspension of a fiber mix containing 20% ​​unrefined or slightly refined bleached softwood or hardwood kraft pulp having a Schopper-Riegler (SR) value in the range of 15 to 25, preferably in the range of 20 to 25, and 80% highly refined bleached softwood or hardwood kraft pulp in the form of MFC having an SR value of at least 90, and adding ethyl (hydroxyethyl) cellulose (EHEC) in an amount of 1 to 20 kg / tn of the fiber mix. The cellulosic material used is the same as in the first furnish.

[0100] The aqueous suspension is foamed by pumping the suspension several times through a foam generator until an air content of 40-90% by volume is reached.

[0101] The second furnish is prepared to a consistency of about 0.5 wt%. The foamed second furnish is fed to a tank and then pumped further to the headbox. The pH and temperature are the same as the first furnish.

[0102] iii) forming and dewatering a first web layer of a first furnish using a Fourdrinier technique on a first wire, the first layer having a basis weight of 20 g / m 2 is.

[0103] iv) forming and dewatering a second web layer of foamed second furnish using the Fourdrinier technique on a second wire, the second layer having a basis weight of 20 g / m 2 is.

[0104] v) transferring the first web layer onto the second web layer, bonding the first web layer to the second web layer, and pressing the two layers together to ensure contact and adhesion between the layers and to further dewater the formed multi-layer web. The dry solids content of the first and second web layers before the bonding step is in the range of 1.5 to 8 wt%. The solids content of the second layer is slightly less than that of the first layer.

[0105] vi) further dewatering and optionally drying the multilayer web to obtain a multilayer film.

[0106] The resulting product is pinhole-free and has good oxygen barrier properties (15 cc / m when measured according to ASTM standard D-3985 at 50% relative humidity and 23°C). 2 / 24hr / atm) and grease barrier properties (KIT>11). The uncalendered base PPS10 has a surface roughness greater than 3.0 and a density of 600-900 kg / m 3 The Gurley-Hill value of the film is 42300 s / 100 ml.

[0107] According to a second aspect exemplified herein, there is provided a multilayer film comprising highly purified cellulose, the multilayer film being obtainable by the method of the invention as described with reference to the first aspect.

[0108] The multilayer films of the present invention are particularly suitable as thin packaging films when coated or laminated with one or more layers of thermoplastic polymer. Thus, the multilayer films may preferably be coated or laminated with one or more polymer layers.

[0109] Multilayer films may be provided with polymer layers on one or both sides.

[0110] The polymer layer may comprise any of the thermoplastic polymers commonly used in paper or paperboard-based packaging materials in general, or polymers used in liquid packaging boards in particular. Examples include polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polylactic acid (PLA), polyglycolic acid (PGA), starch, and cellulose. Polyethylene, particularly low-density polyethylene (LDPE) and high-density polyethylene (HDPE), is the most common and versatile polymer used in liquid packaging boards.

[0111] Thermoplastic polymers are useful because they can be easily processed by extrusion coating techniques to form very thin and uniform 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.

[0112] The polymer layer may include one or more layers formed of the same or different polymer resins. In some embodiments, the polymer layer includes a mixture of two or more different polymer resins. In some embodiments, the polymer layer is a multilayer structure consisting of two or more layers, where a first layer is made of a first polymer resin and a second layer is made of a second polymer resin, which is different from the first polymer resin.

[0113] In some embodiments, the polymer layer is formed by extrusion coating a polymer onto the surface of the multilayer film. 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. The coating can be formed by the extruded plastic itself, or the molten plastic can be used as an adhesive to laminate a solid plastic film onto the substrate. Common plastic resins used in extrusion coating include polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET).

[0114] The basis weight of each polymer layer of the multilayer film is preferably 50 g / m 2 At least 8 g / m² to achieve a continuous and substantially defect-free film. 2 , preferably at least 12 g / m 2 A polymer layer basis weight of 8 to 50 g / m is typically required. In some embodiments, the polymer layer basis weight is 8 to 50 g / m 2within the range of 12 to 50 g / m 2 is within the range.

[0115] The multilayer film of the present invention may preferably be used as a gas barrier layer in paper or paperboard based packaging materials, such as liquid packaging boards (LPBs) for use in packaging liquids or liquid containing products. Thus, according to a third aspect exemplified herein: a paper or paperboard substrate; and Multilayer films obtainable by the process of the present invention A paper or paperboard based packaging material is provided comprising:

[0116] Paper generally refers to a material made in sheets or rolls from wood pulp or other fibrous substances containing cellulose fibers, used, for example, for writing, drawing, or printing on, or as a packaging material. Paper can be bleached or unbleached, coated or uncoated, and can be produced in a variety of thicknesses depending on end-use requirements.

[0117] Paperboard generally refers to strong, thick paper or cardboard containing cellulose fibers, used, for example, as flat substrates, trays, boxes, and / or other types of packaging. Paperboard can be bleached or unbleached, coated or uncoated, and can be produced in a variety of thicknesses depending on end-use requirements.

[0118] The multilayer film of paper or paperboard based packaging material according to the second aspect may be further defined as above with reference to the first aspect.

[0119] In some embodiments, the multilayer film is bonded directly to the paper or paperboard substrate, for example, when the multilayer film is wet-laminated onto the substrate. Thus, in some embodiments, the multilayer film is in direct contact with the substrate.

[0120] In other embodiments, the multilayer film is indirectly bonded to the paper or paperboard substrate, for example, when the multilayer film is laminated onto the substrate using an adhesive layer disposed between the substrate and the multilayer film. Thus, in some embodiments, the paper or paperboard-based packaging material further comprises an adhesive layer disposed between the substrate and the multilayer film.

[0121] In some embodiments, the paper or paperboard based packaging material has a mass of 200 g / m 2 measured according to ISO standard 15106-2 / ASTM standard F1249 at 50% relative humidity and 23° C. 2 / Has a water vapor transmission rate (WVTR) of less than 24 hours.

[0122] In some embodiments, the paper or paperboard based packaging material has a viscosity of 1000 cc / m as measured according to ASTM standard D-3985 at 50% relative humidity and 23° C. 2 / 24 hours / atm or less, preferably 500cc / m 2 / 24 hours / atm or less, preferably 100cc / m 2 / 24 hours / atm or less, most preferably 50cc / m 2 It has an oxygen transmission rate (OTR) of less than / 24 hours / atm.

[0123] Generally, products, polymers, materials, layers, and processes are described in terms of "comprising" various components or steps, but the products, polymers, materials, layers, and processes can also "consist essentially of" or "consist of" various components and steps.

[0124] While the present 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 essential scope thereof. Therefore, it is to be understood that the invention is not to be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but rather, the invention is intended to include all embodiments falling within the scope of the appended claims. [Example]

[0125] Experiments were carried out in a pilot Fourdrinier machine to show that using two Fourdrinier wire sections the dewatering speed, and consequently the running speed, can also be increased.

[0126] Driving conditions Pulp mixture: 100% MFC Water retention value:>350% SR:>90 Additives: Cationic starch, cationic retention aid, anionic retention aid, hydrophobic sizing, wet strength agent pH: 7.5 Temperature: 45℃ Wet pressure: 3 nips 10 / 15 / 15kN / m

[0127] Reference trial point As a standard, 30 g / m 2 The web was run at a running speed of 30 m / min on wire section 1. Wire retention was 99%. Since the waterline was very slow, increasing the running speed was not possible with this setup.

[0128] Trial point 1 The first web was run on wire section 1 and the second web was run on wire section 2. Each web was run at a 20 g / m 2The web was run at a running speed of 30 m / min. The wire retention on each wire was 99.6%. The web was wet bonded and 40 g / m 2 The multi-layer web was further dewatered to form a multi-layer web having a combined weight of 1.000 lb. Based on the waterline position, it was clear that much higher running speeds would have been possible.

[0129] Trial point 2 The first web was run on wire section 1 and the second web was run on wire section 2. Each web was run at a 15 g / m 2 The web was run at a running speed of 45 m / min. The wire retention on each wire was 98.8%. The web was wet bonded and 30 g / m 2 The multi-layer web was further dewatered to form a multi-layer web having a combined weight of 1.0001 lb. Based on the waterline position, it was apparent that even higher running speeds would have been possible.

[0130] The results show that using two Fourdrinier wire sections can increase the dewatering rate, and consequently the running speed. All three films obtained had high Gurley-Hill values ​​(42,300 s / ml, measured according to ISO standard 5636 / 6, which was the maximum value for the instrument), indicating that higher running speeds did not significantly affect the barrier properties of the films.

Claims

1. 1. A method for producing a multilayer film comprising highly purified cellulose fibers, comprising: a) forming a first wet web by applying onto a first wire a first pulp suspension comprising highly refined cellulose fibers having a Schopper-Riegler (SR) value greater than 90 as determined by ISO Standard 5267-1; b) partially dewatering the first wet web to obtain a first partially dewatered web; c) forming a second wet web by applying onto a second wire a foamed second pulp suspension comprising highly refined cellulose fibers having a Schopper-Riegler (SR) value greater than 90 as determined by ISO Standard 5267-1 and a foaming agent; d) partially dewatering the foamed second wet web to obtain a second partially dewatered web; e) joining the first and second partially dewatered webs to obtain a multi-layer web; and f) further dewatering and optionally drying the multilayer web to obtain a multilayer film comprising highly refined cellulose fibers, the multilayer film having a Gurley-Hill value of at least 10,000 s / 100 ml and a viscosity of at least 600 kg / m when measured according to ISO standard 5636 / 6; ３ further dewatering and optionally drying the multi-layer web having a density greater than Including, The method, wherein the highly purified cellulose fibers are microfibrillated cellulose (MFC), and the first pulp suspension and the foamed second pulp suspension comprise 80 to 99 wt % MFC based on the total dry weight of the pulp suspensions.

2. 2. The method of claim 1, wherein the dry solids content of the first pulp suspension is in the range of 0.1 to 0.7 wt %.

3. 3. The method of claim 1, wherein the dry solids content of the foamed second pulp suspension is in the range of 0.1 to 15 wt %.

4. The density of the foamed second pulp suspension is 800 kg / m ３ The method according to any one of claims 1 to 3, wherein the

5. 5. The method of any one of claims 1 to 4, wherein the foamed second pulp suspension comprises a foamed aqueous mixture of MFC and foaming agent.

6. 6. The method of any one of claims 1 to 5, wherein the blowing agent is a polymeric blowing agent.

7. 7. The method of claim 1, wherein the blowing agent is an amphiphilic polymer.

8. 8. The method of any one of claims 1 to 7, wherein the foaming agent is selected from the group consisting of optionally hydrophobically modified polysaccharides, proteins, polyvinyl alcohol, partially hydrolyzed polyvinyl acetate and mixtures thereof.

9. 9. The method of any one of claims 1 to 8, wherein the foaming agent is an optionally hydrophobically modified polysaccharide selected from the group consisting of cellulose, starch, hemicellulose and mixtures thereof.

10. 10. The method of any one of claims 1 to 9, wherein the foaming agent is an optionally hydrophobically modified polysaccharide selected from the group consisting of optionally hydrophobically modified cellulose acetate (CA), ethyl(hydroxyethyl)cellulose (EHEC), methylcellulose (MC), ethylcellulose (EC), hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), sodium carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), sulfoethylcellulose, starch, and mixtures thereof.

11. 11. The method of any one of claims 1 to 10, wherein the foaming agent is optionally hydrophobically modified methylcellulose.

12. 12. The method of any one of claims 1 to 11, wherein the blowing agent has a molecular weight greater than 5000 g / mol.

13. 13. The method of any one of claims 1 to 12, wherein the foamed second pulp suspension does not contain surfactant chemicals having a molecular weight below 1000 g / mol.

14. 14. The method of any one of claims 1 to 13, wherein the first pulp suspension further comprises a defoamer.

15. The basis weight of the first and / or second wet webs, based on the total dry weight of the webs, is 50 g / m ２ 15. The method of claim 1, wherein the solubility is less than 100%.

16. 16. The method of any one of claims 1 to 15, wherein the dry solids content of the first and second partially dewatered webs prior to the bonding step is in the range of 1.5 to 15 wt%.

17. 17. The method of any one of claims 1 to 16, wherein the bonding is performed by wet lamination of the first and second partially dewatered webs.

18. 18. The method of any one of claims 1 to 17, wherein the bonding further comprises pressing the first and second partially dewatered webs together.

19. 19. The method of any one of claims 1 to 18, wherein the bonding further comprises applying suction to the bonded first and second partially dewatered webs.

20. 20. The method of any one of claims 1 to 19, wherein the dry solids content of the multi-layer web before further dewatering and before the optional drying step is in the range of 8 to 35 wt%.

21. The basis weight of the multilayer web, based on the total dry weight of the web, is 10 to 100 g / m ２ 21. The method of claim 1, wherein the solubility of the solubility of the solubility of the

22. 22. The method of any one of claims 1 to 21, wherein the multilayer film is transparent or translucent to visible light.

23. 23. The method of any one of claims 1 to 22, wherein the multilayer film has a KIT value of at least 6 when measured according to ISO standard 16532-2.

24. 10 pinholes / m when the multilayer film is measured according to EN standard 13676:2001 ２ 24. The method of any one of claims 1 to 23, comprising less than

25. 25. The method of any one of claims 1 to 24, wherein the multilayer film has a Gurley-Hill value of at least 25000 s / 100 ml when measured according to ISO standard 5636 / 6.

26. A method for producing a paper or paperboard based packaging material, the method comprising: a paper or paperboard substrate; and A multilayer film obtainable by the method according to any one of claims 1 to 25. A method comprising:

27. ​​The paper or paperboard based packaging material has a modulus of elasticity of 200 g / m2 measured according to ISO standard 15106-2 / ASTM standard F1249 at 50% relative humidity and 23°C. ２ 27. The method of claim 26, wherein the composition has a water vapor transmission rate (WVTR) of less than 24 hours.

28. The paper or paperboard based packaging material has a moisture content of 1000 cc / m2 measured according to ASTM standard D-3985 at 50% relative humidity and 23°C. ２ 27. The method of claim 26, wherein the oxygen transmission rate (OTR) is less than 1 / 24 hr / atm.