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 second pulp suspension comprising highly purified cellulose fibers onto a second wire; d) partially dewatering the second wet web to obtain a second partially dewatered web; e) applying a binder between the first and second partially dewatered webs and 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 second pulp suspension containing highly refined cellulose fibers onto a second wire; d) partially dewatering the second wet web to obtain a second partially dewatered web; e) applying a binder between the first and second partially dewatered webs and bonding 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 2Due to the highly refined cellulose fiber content, the resulting multilayer film typically has a basis weight of 600 kg / m 3 above 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. The partially dewatered, but still wet, webs are then bonded to form a higher basis weight multilayer web, 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 typically 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 the corresponding thickness.

[0022] It has been discovered that partial dewatering and lamination of the partially dewatered web substantially 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 been achieved by using large amounts of retention and drainage chemicals in the wet end of the process, sometimes causing increased agglomeration. However, retention and drainage chemicals can also cause a more porous web structure, and therefore there is a need to minimize the use of such chemicals. The method of the present invention provides an alternative method of increasing dewatering rates that is less dependent on the addition of retention and drainage chemicals.

[0023] The inventors have also found that various functional additives can be advantageously applied between the partially dewatered webs before they are joined. The additives can be applied to one or both of the surfaces of the webs to be joined.

[0024] Sandwiching additives between partially dewatered webs in this manner has several advantages compared to adding them directly to the pulp suspension before forming, including less interference with the forming process and better retention of the additives within the web.

[0025] As mentioned, joining the webs while they are still wet typically ensures good adhesion between the layers. However, in some cases, it may be desirable to further improve contact and adhesion between the layers. The inventors have found that applying a binder, or adhesive, between the first and second partially dewatered webs before joining the webs to obtain the multilayer web results in better and more uniform adhesion. The binder can also further improve the barrier properties of the finished multilayer film. The binder may be applied to the wet or partially dewatered web in liquid or solid form and may be applied by spray coating, curtain coating, or foam coating.

[0026] Examples of possible binders include fine nanocellulose and nonionic, anionic, or weakly amphoteric water-soluble polymers. The problem when such materials are added directly to the pulp suspension in the headbox is that, although they affect discharge, their retention within the formed web is poor.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] The first and second pulp suspensions are aqueous suspensions containing a water-suspended 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.

[0031] 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.

[0032] 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 to 99, preferably in the range of 70 to 90. 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 to 75.

[0033] The dry solids content of the first and / or second 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%.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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).

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] In some embodiments, at least a portion of the MFC is obtained from MFC broke.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] The compositions of the first and second pulp suspensions may be the same or different. For example, in some embodiments, one of the pulp suspensions may contain unrefined or slightly refined cellulose fibers, while the other pulp suspension does not contain unrefined or slightly refined cellulose fibers. One possibility is to have a first pulp suspension containing less highly refined cellulose fibers and / or a greater amount of unrefined or slightly refined cellulose fibers with a lower SR value to provide faster dewatering, and a second pulp suspension containing more highly refined cellulose fibers and / or a smaller amount of unrefined or slightly refined cellulose fibers with a higher SR value to provide good barrier properties or a surface with very high smoothness. The first web formed from the first pulp suspension may have a density of, for example, 15 to 20 g / m. 2 a slightly higher basis weight than the second web formed from the second pulp suspension, e.g., 25-30 g / m 2 The sheet may have a basis weight in the range of 1000 to 15000.

[0049] In some embodiments, the first and second pulp suspensions are supplied from two different headboxes, which can be advantageous because the headboxes can be operated slightly differently, for example with different consistencies, headbox jet angles, or jet-to-wire ratios.

[0050] In some embodiments, the first and second pulp suspensions have the same composition. This can simplify the process because only one source of pulp suspension is needed. Also, having the same composition can reduce curling issues in the finished film, where two layers of a multilayer film have the same composition.

[0051] 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 2less than 30 g / m 2 Less than 50g / m 2 Less than or 30g / 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 2 Thus, 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.

[0052] 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 15 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 10 wt%. Dry solids contents of the first and second partially dewatered webs within this range have been found to be particularly suitable for joining 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 may be in the range of 1.5 to 8 wt%, preferably in the range of 2.5 to 6 wt%, and more preferably in the range of 3 to 4.5 wt%.

[0053] A binder is applied to one or both of the partially dewatered, but still wet, webs. The binder can be any substance or material that acts to increase adhesion between the layers of the multilayer film being formed. The binder can be in liquid or solid form and can be applied to the partially dewatered webs by, for example, spraying or curtain coating.

[0054] In some embodiments, the binder is a polymer binder. The binder is typically a natural or synthetic polymer. The polymer is preferably water-soluble or water-dispersible (e.g., latex), allowing it to be applied in liquid form as an aqueous solution or dispersion. Examples of binders that may be useful in the present invention include, but are not limited to, cellulose derivatives, natural and modified starches, polyvinyl alcohol or partially hydrolyzed polyvinyl alcohol and their derivatives, and hemicellulose. The solution or dispersion containing the binder may also contain other functional additives, such as one or more crosslinkers, softeners, and / or humectants.

[0055] The binder is preferably based on renewable raw materials. The binder is preferably selected so as not to adversely affect the repulpability and recyclability of the multilayer film. In some embodiments, the binder comprises a polymer derived from wood or other plant-based raw materials.

[0056] In some embodiments, the binder is selected from the group consisting of nanocellulose and lignin, and their derivatives. These binders are useful because they are based on renewable raw materials and do not adversely affect the repulpability and recyclability of the multilayer film. Nanocellulose is advantageous because it resembles the highly refined cellulose fibers of the multilayer film and uses a high surface area to improve contact and adhesion. Lignin is advantageous because, in addition to providing improved adhesion, it also acts as a UV light barrier within the multilayer film.

[0057] The binder can be a single binder or a mixture of two or more binders, for example a mixture of nanocellulose and a water-soluble polymer in a ratio of 100:1 to 1:100.

[0058] In some embodiments, the binder further comprises nanopigment particles, preferably in an amount of 0 to 50 wt % of the amount of the binder.

[0059] In some embodiments, the binder further comprises a cross-linking agent, with a preferred cross-linking agent being citric acid.

[0060] The binder is preferably applied at a basis weight, on a dry weight basis, less than the basis weight of the multilayer film, and preferably less than the basis weight of each of the web layers. In some embodiments, the binder is applied at a basis weight of 20 g / m based on the total dry weight of the binder. 2 Less than 15 g / m 2 less than 10 g / m 2 Less than or 5g / m 2 In some embodiments, the binder is applied at a basis weight of 0.1 to 20 g / m based on the total dry weight of the binder. 2 within the range of 0.3 to 15 g / m 2 in the range of 0.5 to 5 g / m 2 It is applied at a basis weight within the range of

[0061] The binder may be applied, for example, in the form of a liquid solution or suspension, preferably an aqueous solution or suspension.

[0062] In some embodiments, the binder is applied in the form of a suspension, the solids content of which may be in the range of 0.5 to 50 wt%, preferably in the range of 1 to 30 wt%.

[0063] The liquid solution or suspension may be applied using a non-contact deposition technique, such as, for example, preferably spray coating or curtain coating (e.g., slot or die). In some embodiments, the binder is applied in the form of a foam formed from a liquid solution or suspension containing the binder and a foaming agent. The foaming agent may be any suitable foaming agent. PVOH is a preferred foaming agent. The foam may be applied to one or both of the partially dewatered, but still wet, webs using a foam application device.

[0064] In a more specific embodiment, the binder is applied in the form of an aqueous suspension. The suspension is preferably applied at a temperature in the range of 30 to 95° C., more preferably in the range of 40 to 80° C. The pH of the suspension may vary depending, for example, on the raw material composition and the use of additives.

[0065] 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 15 wt%, more preferably greater than 1 wt% but less than 10 wt%. In some embodiments, the application of a binder may increase the solids content of the web to which it is applied, such that the binder-coated web may have a solids content greater than 1 wt% but less than 20 wt%. In some embodiments, the dry solids content of the first and second partially dewatered webs when they are bonded is in the range of 1.5 to 8 wt%, preferably 2.5 to 6 wt%, more preferably 3 to 4.5 wt%. The partially dewatered webs are preferably bonded by wet lamination. When the pulp suspension is dewatered on the wire, a visible boundary line with a reflective water layer will appear from the point where the web advances to the point where this reflective layer disappears. This boundary between the reflective and non-reflective webs is called the waterline. The waterline indicates a certain solids content of the web. The webs are preferably bonded after the waterline. Bonding the webs while they are still wet ensures good adhesion between the layers. The presence of a binder on one or both of the surfaces of the bonded webs further improves contact and adhesion between the layers. Bonding can be achieved by applying one of the partially dewatered webs over the other. 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. Applying pressure and / or suction to the formed multilayer web improves adhesion between the web layers.The wire section of the paper machine may have a variety of dewatering devices such as blades, table and / or foil elements, suction boxes, frictionless dewatering, ultrasonically assisted dewatering, couch rolls, or dandy rolls.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] Applying a binder between the partially dewatered webs will further improve the contact and adhesion between them when they are bonded. This may be particularly interesting when it is preferable to bond the webs wire-side to wire-side, since these surfaces may contain fewer fines than the non-wire-side, and therefore may result in lower adhesion. Bonding the webs wire-side to wire-side may be interesting, for example, to obtain a multilayer film with low surface roughness.

[0071] 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 28 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 28 wt%, preferably in the range of 10 to 20 wt%, and more preferably in the range of 12 to 18 wt%.

[0072] 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 2In 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.

[0073] 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:

[0074] 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 %.

[0075] 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%.

[0076] 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 %.

[0077] 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%.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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 70, preferably at least 75. The mixture therefore exhibits high drainage resistance.

[0085] 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).

[0086] ii) Prepare a second furnish according to the same recipe as the first furnish but with 5-20% less MFC.

[0087] iii) forming and partially dewatering a first web layer on a first wire using a Fourdrinier technique, the first layer having a basis weight of 20 g / m 2 is.

[0088] iv) forming and partially dewatering a second web layer on a second wire using the Fourdrinier technique, the second layer having a basis weight of 20 g / m 2 is.

[0089] v) spraying a binder in the form of an aqueous nanocellulose suspension onto the surface of the partially dewatered first web layer that is to be contacted with the partially dewatered second web layer, the binder having a basis weight of 5 g / m based on the total dry weight of the binder; 2 is.

[0090] vi) transferring and bonding a second web layer to the first web layer so that the binder is sandwiched between the two layers, 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 prior to 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.

[0091] vii) Further dewatering and optionally drying the multilayer web to obtain a multilayer film.

[0092] 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 is within the range.

[0093] According to a second aspect exemplified herein, there is provided a multilayer film comprising highly purified cellulose fibers, the film comprising: a first layer comprising highly refined cellulose fibers; and A second layer containing highly refined cellulose fibers wherein the first and second layers are bonded together by a binder applied between the first and second layers.

[0094] In some embodiments, the binder is a polymeric binder.

[0095] In some embodiments, the binder is selected from the group consisting of nanocellulose and lignin, and derivatives thereof.

[0096] In some embodiments, the basis weight of the binder is 20 g / m 2 Less than 15 g / m 2 less than 10 g / m 2 is less than.

[0097] In some embodiments, multilayer films can be obtained by the methods of the present invention.

[0098] 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.

[0099] Multilayer films may be provided with polymer layers on one or both sides.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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).

[0104] 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 2 within the range of 12 to 50 g / m2 is within the range.

[0105] The multilayer film of the present invention may preferably be used as a gas barrier layer in paper or paperboard based packaging materials, for example in 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:

[0106] 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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] 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]

[0115] 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.

[0116] 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

[0117] 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.

[0118] 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.

[0119] 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 clear that even higher running speeds would have been possible.

[0120] 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 of 65 or greater 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 second pulp suspension comprising highly refined cellulose fibers having a Schopper-Riegler (SR) value of 65 or greater as determined by ISO Standard 5267-1; d) partially dewatering the second wet web to obtain a second partially dewatered web; e) applying a binder between the first and second partially dewatered webs by spray coating, curtain coating, or foam coating, and bonding 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 containing highly purified cellulose fibers. wherein the dry solids content of the first and / or second pulp suspension is in the range of 0.1 to 0.7 wt %.

2. 10. The method of claim 1, wherein the first and / or second pulp suspension comprises at least 50 wt% highly refined cellulose fibers based on the total dry weight of the pulp suspension.

3. 3. The method of claim 1 or 2, wherein the first and / or second pulp suspension is formed from a cellulosic furnish having a Schopper-Riegler (SR) value in the range of 65 to 99 as determined by ISO standard 5267-1.

4. 4. The method according to claim 1, wherein the highly purified cellulose fibers are microfibrillated cellulose (MFC).

5. The method of claim 4, wherein the first and / or second pulp suspension comprises 50 to 99 wt% MFC based on the total dry weight of the pulp suspension.

6. 6. The method of any one of claims 1 to 5, wherein the first and / or second pulp suspension comprises less than 50 wt. % unrefined or slightly refined cellulose fibers having a Schopper-Riegler (SR) value below 30 as determined by ISO standard 5267-1, based on the total dry weight of the pulp suspension.

7. 7. The method of claim 1, wherein the first and second pulp suspensions have the same composition.

8. 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 The method of any one of claims 1 to 7, wherein the

9. 9. The method of claim 1, wherein the binder is a polymeric binder.

10. 10. The method of any one of claims 1 to 9, wherein the binder is selected from the group consisting of nanocellulose and lignin, and derivatives thereof.

11. The binder is 0.1 to 20 g / m ２ 11. The method of claim 1, wherein the coating is applied at a basis weight in the range of

12. 12. The method of any one of claims 1 to 11, 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 8 wt%.

13. 13. The method of any one of claims 1 to 12, wherein the bonding is performed by wet lamination of the first and second partially dewatered webs.

14. 14. The method of any one of claims 1 to 13, wherein the bonding further comprises pressing the first and second partially dewatered webs together.

15. 15. The method of any one of claims 1 to 14, wherein bonding further comprises applying suction to the bonded first and second partially dewatered webs.

16. 16. The method of any one of claims 1 to 15, wherein the dry solids content of the multi-layer web before further dewatering and optional drying steps is in the range of 8 to 28 wt%.

17. The basis weight of the multilayer web, based on the total dry weight of the web, is 10 to 100 g / m ２ 17. The method of claim 1, wherein the solubility of the solubility of the solubility of the

18. 18. The method of any one of claims 1 to 17, wherein the multilayer film is transparent or translucent to visible light.

19. 19. The method of any one of claims 1 to 18, wherein the multilayer film has a KIT value of at least 6 when measured according to ISO standard 16532-2.

20. 10 pinholes / m when the multilayer film is measured according to EN standard 13676:2001 ２ 20. The method of any one of claims 1 to 19, comprising less than

21. 21. The method of any one of claims 1 to 20, wherein the multilayer film has a Gurley-Hill value of at least 10,000 s / 100 ml when measured according to ISO standard 5636 / 6.