Method for producing a film comprising microfibrillated cellulose and a film comprising microfibrillated cellulose

Abstract

The present invention relates to a method for producing a film comprising microfibrillated cellulose, the method comprising providing a suspension comprising 30-100% by weight of microfibrillated cellulose based on total dry weight; forming a wet web of the suspension by casting on a substrate, which is a non-porous substrate, paper, or paperboard; wet-pressing the wet web to form a dewatered web, comprising applying a press fabric in direct contact with the wet web and passing the wet web disposed between the press fabric and the non-porous substrate through a press apparatus; smoothing the dewatered web by applying at least one smoothing press to the dewatered web disposed on the non-porous substrate to form a smoothed web, wherein a pressure of 0.1-25 MPa is applied to each smoothing press, and when applying the smoothing press(es), the dewatered web has a dry content of 20-60% by weight; and drying the smoothed web to form the film. The present invention further relates to a film.

Description

[Technical field]

[0001] The present invention relates to a method for producing a film comprising microfibrillated cellulose. In addition, the present invention relates to a film comprising microfibrillated cellulose obtainable by said method. [Background technology]

[0002] Films containing high amounts of microfibrillated cellulose (MFC) are known to have excellent strength and oxygen barrier properties. 2 have produced MFC films with basis weights between 100 and 200 mm and investigated their strength and barrier properties, see Syverud, “Strength and barrier properties of MFC films”, Cellulose 2009 16:75-85.

[0003] There are various techniques for producing barrier papers and films containing high amounts of MFC. For example, methods for producing barrier papers or films may include dewatering on a wire or cast coating on a non-porous carrier substrate. In either case, regardless of the concept of a forming section, the wet web is further conveyed to a press section. In the press section, more water is squeezed out of the wet web by passing it through one or more press nips and / or by contacting at least one side with a press fabric. Typically, the material or film is not well fixed at the point of entering the press section, since the solids content is quite low. This means that the material will be further shaped upon application of pressure or suction. When the wet web is dewatered against the press fabric, the characteristic texture of the press fabric will be transferred to the web surface. Thus, a porous or textured press fabric will leave a mark on the wet web.

[0004] In some applications, the mark left on the wet web may be preferred, since it gives the film a special texture. However, in many applications, this mark is not preferred or is not desirable. For example, this is due to the fact that the mark formed on the wet web during press dewatering gives it a two-sided appearance, leading to small thickness differences that can be optically detected, increasing the risk of losing barrier properties. The latter is even more evident when producing thin sheets (lower basis weight).

[0005] The gloss and smoothness of paper and board can be improved by calendering the material in the dry state, i.e. after drying. The number of nips, nip pressure, material of the rolls, hardness of the rolls, temperature, speed, moisture content, and paper composition are some of the main variables that affect the results of calendering. One important effect of calendering is that the fibers and structure are broken down and the surface is plasticized. However, for paper-like substrates with high density or high content of MFC, or transparent or translucent materials / films, the effect of calendering is less obvious. One reason for this is that, for example, hard nip calendering mainly affects the density profile and does not significantly affect the lateral movement and distribution of the material, which is considered to be more important for reducing the effect of wire and press fabric marks. Another reason for this is that substrates with high density or high MFC content have a different fiber network, especially a very different 3D structure due to the lack of normal fiber lumens and other natural pores.

[0006] One possible solution to improve the smoothness of films containing high amounts of MFC is to increase the line load in multiple hard nip calendering, however the drawback of this solution is the increased risk of darkening, cracking and wrinkling, especially when the film contains higher amounts of fiber.

[0007] Therefore, there is still room for improvement in the manufacturing method of MFC films with improved smoothness. Summary of the Invention

[0008] It is an object of the present invention to provide an improved process for producing a film comprising microfibrillated cellulose with improved smoothness, which eliminates or mitigates at least some of the disadvantages of the prior art processes.

[0009] The above objectives, as well as other objectives which will be realized by those skilled in the art upon consideration of the present disclosure, are accomplished by various aspects of the present disclosure.

[0010] The invention is defined by the accompanying independent claims. Embodiments are set out in the accompanying dependent claims and the following description. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] According to a first aspect exemplified herein, there is provided a method for producing a film comprising microfibrillated cellulose, the method comprising the steps of: - providing a suspension comprising between 30% and 100% by weight of microfibrillated cellulose based on the total dry weight, - forming a wet web of said suspension by casting onto a substrate, said substrate being a non-porous substrate, a paper substrate, or a paperboard substrate, said formed wet web having a dry content of from 1 to 25% by weight; - wet-pressing the wet web to form a dewatered web having a dry content of 15 to 80% by weight, the wet-pressing comprising applying a press fabric in direct contact with the wet web and passing the wet web disposed between the press fabric and the support through a press apparatus; - smoothing the dewatered web by applying at least one smoothing press to the dewatered web disposed on the support to form a smoothed web, wherein a pressure of 0.1-25 MPa, preferably 0.1-15 MPa, most preferably 0.2-10 MPa is applied to each smoothing press, and when the at least one smoothing press is applied, the dewatered web has a dry content of 20-60% by weight, preferably 25-60% by weight, most preferably 30-60% by weight, and - drying the smoothed web to form the film. Includes.

[0012] Surprisingly, it has been found that it is possible to apply a smoothing press at a pressure of 0.1-25 MPa, preferably 0.1-15 MPa, most preferably 0.2-10 MPa to a dewatered web in a semi-wet state, comprising a large amount of MFC, such as MFC, present on a support that has been dewatered in contact with the press fabric and cast thereon at a dry content of 20-60 wt%, preferably 25-60 wt%, most preferably 30-60 wt%, in order to smooth imperfections such as marks or textures from the press fabric transferred to the wet web during dewatering (i.e. transferred to the side of the wet web in contact with the press fabric) and to obtain a film with improved smoothness. In particular, it has been found that it is possible to remove rather strong press fabric marks / imprints of the semi-wet MFC web by applying a smoothing press at a rather low pressure, as described above. The smoothing according to the present disclosure smooths out small scale thickness variations of the dewatered web (i.e. the side of the wet web in contact with the press fabric) and should not remove or essentially not remove moisture / water. Smoothing according to the present disclosure can also eliminate / reduce variations in thickness / basis weight or quality caused by vibration or pulsation effects. In addition, smoothing according to the present disclosure can be important for cross direction (CD) and machine direction (MD) tension or shrinkage control.

[0013] Thus, the application of smoothing according to the present disclosure allows the roughness of the wet-pressed web to be significantly reduced (smoothened) at a relatively low pressure. In addition, it is also possible to control the variation in smoothness. Improving the smoothness or controlling the smoothness variation is very important for ensuring a higher level of smoothness, for example after applying one or more additional coatings, i.e., improving the quality of the coating. A smoother film may mean that the number of pinholes is reduced after the subsequent coating. Without being bound by any theory, it is also believed that the smoothing press affects, for example, the drying shrinkage of the web, and thus the profile and sheet properties. It also makes the sheet denser and more uniform in density, thus improving the thermal conductivity and improving the drying of the film.

[0014] In addition, improving smoothness and controlling variability in smoothness can also be very important to reduce the risk of losing barrier properties and reduce bi-sidedness.

[0015] When using calendering to level out film defects according to the prior art, calendering is performed at high pressures, such as about 10-30 MPa, on dry films, i.e. after drying of the dewatered web. One of the important effects of calendering is that the fibers and structure are disintegrated and the surface is plasticized. However, for paper-like substrates with high density or high content of MFC, or transparent or translucent materials / films, the effect of calendering is less obvious. One reason for this is that, for example, hard nip calendering mainly affects the density profile and does not significantly affect the lateral movement and distribution of the material, which is considered to be more important for reducing the effects of wire and press fabric marks. Another reason for this is that substrates with high density or high MFC content have different fiber networks, especially 3D structures, due to the lack of normal fiber lumens and other natural pores. Therefore, calendering cannot be utilized, or at least cannot be utilized efficiently and easily, to remove defects such as texture and marks obtained by press dewatering with press fabrics.

[0016] A certain amount of residual moisture is necessary for smoothing to have an effect of leveling out small thickness variations, and the moisture content must be high enough to avoid damage to the film and to avoid the film surface sticking to the smoothing press.

[0017] The term film as used herein generally refers to a thin continuous sheet-forming material, such as a thin substrate, having good gas, aroma or grease or oil barrier properties, e.g. oxygen barrier properties. Depending on the composition of the suspension, the film can also be considered as a thin paper (e.g. nano- or micro-paper) or a membrane. The film is preferably thicker than 100 g / m 2 Less than 10 to 100 g / m 2 , or 10 to 60 g / m 2In some embodiments, the film has a thickness, when dry, of 8 to 500 μm, preferably 10 to 200 μm, and most preferably 15 to 100 μm. The film is usually relatively dense. In some embodiments, the film has a basis weight, when dry, of 700 to 1500 kg / m 3 , preferably 800 to 1500 kg / m 3 , and most preferably 900 to 1500 kg / m 3 The film has a density of preferably 15 cm 3 / m 2 / Less than 24 hours, preferably 10cm 3 / m 2 / Less than 24 hours, most preferably 5cm 3 / m 2 / Has an oxygen transmission rate (OTR) value of less than 24 hours.

[0018] The film can be used as is or in combination with one or more other layers. The film is useful, for example, as a barrier film / layer in paper or paperboard based packaging materials. The film may also be or constitute a barrier layer in a multi-layer product that includes a base such as glassine, greaseproof paper, barrier paper, or bioplastic film. Alternatively, the film can be included in at least one layer in a multi-sheet such as a liquid packaging board.

[0019] Paper generally refers to a material made from wood pulp or other fibrous substances containing cellulose fibers into thin sheets and used for writing, drawing, or printing, or as a packaging material.

[0020] Paperboard generally refers to a strong, thick paper or cardboard containing cellulose fibers used for boxes and other types of packaging. Paperboard can be produced in a variety of thicknesses, either bleached or unbleached, coated or uncoated, depending on the end-use requirements.

[0021] A paper or paperboard based packaging material is a single or multi-ply packaging material formed primarily or entirely from paper or paperboard. In addition to paper or paperboard, paper or paperboard based packaging materials may contain additional layers or coatings designed to improve the performance and / or appearance of the packaging material.

[0022] As described above, the method according to the first aspect of the present disclosure comprises providing a suspension comprising between 30% and 100% by weight of microfibrillated cellulose, based on total dry weight. The suspension is an aqueous suspension comprising an aqueous suspension mixture of cellulosic fibrous material and optionally non-fibrous additives.

[0023] Microfibrillated cellulose (MFC) is intended in the context of the patent application to mean cellulose particles, fibres or fibrils having a width or diameter of between 20 nm and 1000 nm.

[0024] There are various methods for the production of MFC, such as single-pass or multi-pass purification, prehydrolysis followed by purification, or shear dispersion or fibril release. One or several pretreatment steps are usually required to make the production of MFC both energy-efficient and sustainable. The cellulose fibers of the pulp used in the production of MFC may be natural or may have been pretreated, for example enzymatically or chemically, to reduce the amount of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, so that the cellulose molecules contain other (or more) functional groups than those found in the original cellulose. Such groups include, among others, carboxymethyl (CM), aldehyde and / or carboxyl groups (cellulose obtained by N-oxyl-mediated oxidation, e.g. "TEMPO"), or quaternary ammonium (cationic cellulose). After modification or oxidation by any of the above methods, the degradation of the fibers into MFC becomes easier.

[0025] MFC can be made from wood cellulose fibers, both from hardwood or softwood fibers. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It can be made from pulp from virgin fibers, including mechanical pulp, chemical pulp, and / or thermomechanical pulp. It can also be made from shredded or recycled paper.

[0026] In some embodiments, the suspension used in the method of the first aspect comprises between 40% and 100% by weight, preferably between 50% and 100% by weight, more preferably between 60% and 100% by weight, even more preferably between 70% and 100% by weight, and most preferably between 80% and 100% by weight, based on the total dry weight of the microfibrillated cellulose. Thus, the film produced from the dewatered and smoothed web in these embodiments comprises between 40-100% by weight of microfibrillated cellulose, or a higher amount of MFC, such as between 50% and 100% by weight, which relates to the amount of microfibrillated cellulose in the film itself before the final coating layer is added.

[0027] The microfibrillated cellulose of the suspension may comprise one or more fractions of microfibrillated cellulose. In some embodiments, the microfibrillated cellulose of the suspension comprises a fraction of fine microfibrillated cellulose. In some embodiments, the microfibrillated cellulose of the suspension comprises two or more fractions of microfibrillated cellulose of different fine sizes. In some embodiments, the microfibrillated cellulose of the suspension comprises a fraction of fine and a fraction of coarse, for example, the coarse size may be additive. The coarse MFC in this case typically has a Shopper-Rigler value of 80-100 SR°, for example 80-99 SR°, or 90-99 SR°, or 95-99 SR°, while the fine MFC is fibrillated and therefore no Shopper-Rigler value can be measured (theoretical value is about 100 SR° or more), as determined by ISO standard 5267-1.

[0028] In some embodiments, the suspension comprises, in addition to the microfibrillated cellulose, one or more further cellulose pulp fractions, such as, for example, a cellulose pulp fraction having a Shopper-Rigler value of ≦70 SR°, e.g., 15-70 SR°, or 25-60 SR°, as determined by ISO standard 5267-1, and / or a further fraction of normal fibres. The aqueous suspension may, for example, comprise 1-30% by weight, more preferably 2-30% by weight, most preferably 5-30% by weight of the further cellulose pulp fraction, based on the total dry weight of the microfibrillated cellulose, and the further cellulose pulp fraction(s) (i.e. based on the total dry weight of the total amount of fibres in the aqueous suspension).

[0029] By conventional fibers is meant conventional pulp fibers of conventional length and fibrillation for papermaking. Conventional fibers may include mechanical pulps, thermochemical pulps, chemical pulps such as sulfate (kraft) or sulfite pulps, dissolving pulps, recycled fibers, organosolv pulps or chemithermomechanical pulps (CTMP), or combinations thereof. Pulps may be bleached or unbleached. Conventional fibers may be plant fibers such as wood-derived (e.g., hardwood or softwood), or agricultural sources including straw, bamboo, etc.

[0030] Conventional fibres may have a freeness, or Shopper-Rigler value, as determined by ISO standard 5267-1, in the range of 15 to 50 SR°, or more preferably in the range of 18 to 40 SR°. Chemical pulps, such as kraft pulp, may be preferred as conventional fibres.

[0031] Typical fibres may have an average length in suspension ranging from 1 mm to 5 mm, more preferably from 2 to 4 mm.

[0032] In some embodiments, the suspension comprises 1 to 30 wt.-%, preferably 2 to 30 wt.-%, most preferably 5 to 30 wt.-%, based on the total dry weight of the microfibrillated cellulose and the further cellulose pulp fraction(s) (i.e. based on the total dry weight of the total amount of fibers in the aqueous suspension), of reinforcing fibers having a diameter of >10 μm and a length of >1.5 mm.

[0033] Thus, in addition to the MFC, the suspension may also contain longer fibres, either hardwood fibres or softwood fibres, preferably kraft pulp softwood fibres.

[0034] In addition to the MFC and optional further pulp fraction(s), the suspension may contain any conventional papermaking additives or chemicals such as fillers, pigments, wet strength agents, retention aids, crosslinkers, softeners or plasticizers, adhesion primers, wetting agents, biocides, optical dyes, colorants, optical brighteners, defoamers, hydrophobizing agents such as AKD and ASA, waxes, resins, bentonite, stearates, wet end starches, silica, precipitated calcium carbonate, cationic polysaccharides, etc. These additives or chemicals may be process chemicals or film performance chemicals added to impart specific properties to the final film and / or to facilitate the manufacture of the film.

[0035] Preferably, the suspension comprises no more than 35% by weight, more preferably no more than 30% by weight, most preferably no more than 25% by weight of the additive, based on the total dry weight of the suspension. For example, the suspension may comprise 1-35% by weight, or 1-30% by weight, or 1-25% by weight of the additive, based on the total dry weight of the suspension.

[0036] In some embodiments, the suspension comprises a water-soluble polymer capable of forming a film and / or improving the bonds between the cellulose fibrils. Typical examples of such polymers are natural gums or polysaccharides or their derivatives, such as CMC, starch, or PVOH, or their analogues.

[0037] In some embodiments, the suspension comprises 0.5 to 20% by weight of a plasticizer, such as sorbitol, glycols, or other polyols, based on the total dry weight.

[0038] In some embodiments, the suspension contains up to 20% mineral filler, such as bentonite, kaolin, talcum, or montmorillonite.

[0039] As described above, the wet web is formed from the suspension on the substrate, the wet web passes through a press apparatus on the substrate, and the dewatered web passes through a smoothing press(es) on the substrate. The wet web is formed on the substrate by casting the suspension onto the substrate, such as by cast coating.

[0040] The term "casting" as applied to film formation is a known term that refers to a process in which a suspension is deposited on a support by contact or non-contact deposition and leveling methods to form a wet web. Examples of such deposition and leveling methods are spray deposition, curtain coating / application, or slot die casting. After casting, the wet web is dewatered and dried to form a film.

[0041] It is important that the suspension is applied to the support in such a way that a uniform wet web is formed, meaning that the wet web must be as uniform as possible and have as uniform a thickness as possible. The thickness of the applied wet web may be, for example, 40-6000 μm, or 60-3000 μm, or 70-2000 μm, or 100-2000 μm, at the time of application. The formed wet web has, at the time of formation (i.e. during application to the support or immediately after application to the support), a dry content of 1-25 wt.-%, preferably 2-20 wt.-%, most preferably 3-15 wt.-% or 3-8 wt.-%.

[0042] As noted above, the substrate upon which the wet web is formed is a non-porous substrate, a paper substrate, or a paperboard substrate.

[0043] The non-porous support (substrate) on which the wet web can be formed preferably has a smooth surface and can be a polymer / plastic support or a metal support. In some embodiments, the support is a metal support, i.e. the support is made of metal, e.g. steel. Preferably, the non-porous support is a metal belt. The metal support is preferably heated to a temperature above 30° C., preferably between 30 and 150° C., more preferably between 45 and 150° C., even more preferably between 60 and 100° C., before or immediately after the web is applied to the support. It has been found that by increasing the temperature of the support and thus the temperature of the applied web, it is possible to further increase the efficiency of dewatering the web in the press apparatus.

[0044] In embodiments where the substrate on which the wet web is formed is a paper or paperboard substrate, the MFC film is formed on the paper or paperboard substrate, i.e., a coated paper or paperboard product, or a paper or paperboard laminate is formed.

[0045] The formed wet web can be a monolayer or multilayer web, or a single ply or multi-ply web, produced in one or several casting units. Thus, in some embodiments, the wet web comprises a single web layer, or two or more web layers formed on top of each other.

[0046] As described above, the method of the first aspect includes a step of wet pressing the wet web to form a dewatered web having a dry content of 15-80% by weight. For example, the step of wet pressing the wet web can be performed to form a dewatered web having a dry content of 20-60% by weight, or 25-60% by weight, or 30-60% by weight. The wet pressing includes applying a press fabric in direct contact with the wet web and passing the wet web through a press apparatus disposed between the press fabric and the support. Thus, in an embodiment where the support is a non-porous support, the wet web formed during casting on the non-porous support remains on the non-porous support in the wet pressing step. In an embodiment where the support is a paper or paperboard substrate, the paper or paperboard substrate with the cast wet web is provided on the non-porous wet pressing support in the wet pressing step.

[0047] By press fabric is meant a fabric that is permeable and capable of removing water from a web by absorbing the water or by removing the water through the fabric. The press fabric can be a press felt (dewatering felt). Press fabrics and press felts are commonly used today in dewatering paper and paperboard webs. Any known press fabric or press felt can be utilized.

[0048] It may be preferable to use multiple press fabrics, i.e., two or more press fabrics that are consecutive to one another in the machine direction. When two or more press fabrics are utilized, the press fabrics may have the same or different constructions and / or properties. For example, a first press fabric may be utilized that has a low basis weight and low water permeability that will prevent fines from passing through the press fabric, and a second press fabric that has high water absorption properties. Using different fabrics with different coarsenesses may improve the dewatering rate.

[0049] Each press fabric may include one fabric layer or two or more fabric layers. The fabric layers may have the same or different properties. In addition, each press fabric may include one or more layers of batt material. The layers of press fabrics having multiple layers may be woven or arranged into a laminated or composite structure.

[0050] Additionally, a support structure can be disposed on a second surface of each press fabric opposite a first surface of the press fabric that is disposed in contact with the wet web. The support structure can be disposed on the second surface of the press fabric before, after, or during application of the press fabric in contact with the wet web. In some embodiments, the support structure is attached to the second surface of the press fabric, such as in a laminated or composite structure. The support structure can include one or more layers of batt material, and / or a support fabric, and / or one or more press felts. The support fabric(s) can be comprised of any suitable fabric. When multiple support fabrics are present, they can be the same or different.

[0051] In one embodiment, the support arrangement comprises a press felt, i.e., the press felt is disposed on or attached to the second surface of the press fabric, In one embodiment, the support arrangement comprises a support fabric and a press felt, the press felt being preferably disposed as the outermost layer.

[0052] In some embodiments, a press fabric is utilized that includes at least a first fabric layer that is a woven fabric, i.e. a press fabric that includes a woven first fabric layer. The first fabric layer is intended to be in contact with the wet web to be dewatered. The woven first fabric layer does not include a batt (i.e. a filling material) or other filling material. The woven structure of the first fabric layer is therefore a woven structure that does not include a filling material or other filling material. The woven first fabric layer is therefore arranged so that it constitutes a layer of the press fabric that provides a web-side surface structure, i.e. one of its surfaces constitutes a web-side first surface, which is the outer surface of the press fabric that is arranged to contact the wet web. However, one or more batt surface layers (or other surface layers) may be arranged on a surface of the first fabric layer opposite the web-side first surface. The press fabric that includes at least a first fabric layer may include one or more further fabric layers having the same or different properties as the first fabric layer. The one or more additional fabric layers may be woven layers, but may alternatively have a woven or nonwoven base with a batt of synthetic filling material. If the one or more additional fabric layers are woven, they may have different properties than the woven first fabric layer, such as a different weave pattern and / or of different materials. In some embodiments, the press fabric consists of the woven first fabric layer.

[0053] The woven first fabric layer is a woven fabric of multiple threads, which may include or consist of polymer threads. It may therefore be woven with multiple polymer threads, i.e. may comprise or consist of a textile structure woven with multiple polymer threads. Furthermore, the polymer threads may be threads of one or more synthetic polymers. The synthetic polymer(s) may be any known suitable synthetic polymer used in paper machine fabric threads. Alternatively, cotton or rayon may be used as thread material. The thickness of the woven first fabric layer may be, for example, 0.05 to 2 mm, or preferably 0.1 to 1 mm. For example, the basis weight of the woven first fabric layer may be 30 to 1900 g / m 2 Or 60~1400g / m 2 It is possible.

[0054] The press fabric is preferably applied to the wet web at least 20 cm before it is passed through the press, i.e. in direct contact with the wet web. It is preferred that the press fabric is applied to the wet web at a distance between 20 cm and 5 meters, more preferably between 50 cm and 3 meters, before it is passed through the press. It is preferred that no external pressure is used on the press fabric when it is applied to the wet web before it is passed through the press. It may be possible to wind the support, the wet web and the press fabric on a roll and thus create a small dewatering pressure, but it is important not to use too high a pressure, and pressure from the use of nip roll(s) cannot be used. The combined use of a press fabric at a remote location before intensifying the dewatering in the press improves the dewatering of the web and further prevents clogging of the press fabric due to the migration of the microfibrillated cellulose fibrils into the press fabric. In addition, it may be possible to increase the pressure used in the press and speed up the dewatering process.

[0055] Each press fabric is preferably washed and dewatered after passing through the press apparatus.

[0056] By press device is meant a device forming a nip through which the wet web passes and is then pressed and dewatered. According to the present disclosure, the wet web passes through a press device arranged between a press fabric and a support. The press device may have a metal backing, which may be, for example, a hard roller. An external load element may be utilized to press the non-porous support and the backing against each other to generate pressure. The press device preferably comprises an extended nip, and the press device is preferably a belt press. The belt press includes a metal belt (i.e., the non-porous support on which the wet web is cast, or the non-porous wet press support) and a roll, and the dewatering of the web is performed by applying the web and press fabric between the metal belt and the roll. It may be preferable to increase the length of the nip by treating the wet web in the belt press at a distance of at least 20% of the diameter of the roll of the belt press. The press device may comprise multiple nips. In some embodiments, the press device comprises a low pressure press nip followed by at least one high pressure press nip.

[0057] The pressure used in the press is preferably between 0.01 and 15 MPa, preferably between 0.05 and 10 MPa, more preferably between 0.1 and 6 MPa, even more preferably between 0.1 and 5 MPa. It may be preferable to gradually increase the pressure in the press. The initial pressure of the press is 0.05-1 MPa, preferably gradually increasing the pressure to 0.5-2 MPa, then optionally further increasing the pressure to 1-2 MPa, and then optionally increasing the pressure to between 2-5 MPa. The increase in pressure may be performed in the same pressure nip, for example an extended nip, or the press may comprise several nips.

[0058] In wet pressing, the web is preferably passed through the pressing apparatus at a speed of at least 20 m / min, preferably at a speed of more than 100 m / min, more preferably at a speed of more than 200 m / min.

[0059] One or more press sections with a press apparatus may be utilized. Thus, multiple press fabrics as described above may be utilized, such as two press fabrics in different press sections. When multiple press fabrics are utilized, the different press fabrics may be the same or different. For example, a first press fabric may have low water permeability, while a second press fabric may have high water absorption properties.

[0060] The wet web is preferably heated before contacting the press fabric. In this way, the temperature and solids content of the web are increased, further improving the subsequent dewatering of the web. The wet web preferably has a temperature between 10 and 99°C, preferably between 50 and 95°C, when it enters the wet press. Increasing the temperature of the wet web can reduce the viscosity of the water and accelerate the dewatering process. The increased heat can be applied using any known method. By increasing the solids content of the wet web before the wet press section, the surface of the wet web in contact with the web-side surface of the press fabric, or the wet web as a whole, becomes more viscous and the penetration (i.e. of fine materials) into the pores of the press fabric can be reduced or avoided. However, a higher solids content usually means that more pressure is applied, which means that the risk of marking on the press fabric is evident.

[0061] In some embodiments, the method includes a further step of pre-drying the formed wet web on a support prior to wet pressing. In some embodiments, the step of pre-drying the wet web includes a step of drying the wet web by heating, such that the dry content of the wet web increases by at least 1% by weight by evaporation, prior to the step of directly contacting the press fabric with the wet web. For example, heating can be performed by heating the support, i.e., a heated support can be utilized in the pre-drying step. Thus, in these embodiments, the wet web is pre-dried after forming the wet web on the support and prior to applying the press fabric. For example, a pre-drying step may need to be performed when the dry content is between 1 and 25% by weight, or between 3 and 15% by weight, or between 3 and 10% by weight. For example, pre-drying can be performed by evaporation, impingement drying with hot air, IR, microwave, thermal heating, or any other method well known in the art.

[0062] The dry content of the wet web when it enters the wet press is preferably 3-25% by weight, more preferably 4-20% by weight, most preferably 5-15% by weight. The dry content of the wet web after dewatering in the press is preferably 15-80% by weight, or 20-60% by weight, or 25-60% by weight, or 30-60% by weight, or 30-55% by weight, or 30-50% by weight.

[0063] In some embodiments, the method comprises the further step of subjecting the dewatered web to at least one pre-wetting step prior to said smoothing step. The dewatered web remains disposed on the support during the pre-wetting step. The pre-wetting step may comprise steaming or evaporation. Pre-wetting may be performed using steam or water, with or without the use of chemicals. In some embodiments, the pre-wetting step may comprise a rate of 1 to 15 g / m 2 , preferably 2 to 10 g / m 2 , and most preferably 2.5 to 8 g / m 2of steam or water is applied. In some embodiments, during the steam or water pre-wetting, the temperature of the dewatered web may be increased by at least 10° C., or at least 20° C., thereby making the material more susceptible to plasticization and restructuring during smoothing.

[0064] In some embodiments, the method includes the further step of intermediately drying the dewatered web before the smoothing step. Thus, in these embodiments, the dewatered web is dried on a support after the dewatering step and before the smoothing step. For example, intermediate drying can be performed by evaporation, impingement drying with hot air, IR, microwave, thermal heating, steam or electrical heating of the support, or any other method well known in the art. Different drying techniques can also be used in combination.

[0065] As described above, the method of the first aspect of the present disclosure includes a step of smoothing the dewatered web by applying at least one smoothing press to the dewatered web disposed on a support to form a smoothed web. Thus, the dewatered web is smoothed with one or more smoothing presses if it remains on the support after dewatering and optional intermediate drying and optional pre-wetting steps. Thus, the dewatered web is smoothed with at least one smoothing press by passing / passing the dewatered web disposed on the support on which it was cast through at least one smoothing press. Thus, in an embodiment in which the support is a non-porous support, the wet web formed during casting on the non-porous support remains on the non-porous support in the wet pressing and smoothing steps. In an embodiment in which the support is a paper or paperboard substrate, the paper or paperboard substrate with the cast wet web is provided on a non-porous wet press support in the wet pressing step and provided on a non-porous wet press support or non-porous smoothing support in the smoothing step. The non-porous wet-pressing and non-porous smoothing supports can be polymeric / plastic supports or metal supports such as metal belts.

[0066] A pressure of 0.1 to 25 MPa, preferably 0.1 to 15 MPa, more preferably 0.2 to 10 MPa is applied (used) in each smoothing press, i.e. the dewatered web is smoothed at a pressure of 0.1 to 25 MPa, preferably 0.1 to 15 MPa, more preferably 0.2 to 10 MPa. In some embodiments, a pressure of 0.1 to 20 MPa, or 0.5 to 15 MPa, or 0.5 to 10 MPa, or 1 to 10 MPa is applied in each smoothing press.

[0067] When at least one smoothing press is applied to the dewatered web, the dewatered web has a dry content of 20-60% by weight, preferably 25-60% by weight, most preferably 30-60% by weight, i.e., when entering each nip of the at least one smoothing press, the dewatered web has a dry content of 20-60% by weight, preferably 25-60% by weight, most preferably 30-60% by weight. When applying a smoothing press, a certain wet web strength is required, i.e., a wet web with too low dry content will not withstand smoothing. In addition, some water / liquid is required to make the web reformable and reshapeable.

[0068] In some embodiments, when at least one smoothing press is applied to the dewatered web, the dewatered web has a dry content of 30-55% by weight or 30-50% by weight.

[0069] The above-mentioned dry content of the dewatered web when at least one smoothing press is applied may be provided in the dewatering step or may be essentially provided (i.e. it may occur after the dewatering step). Alternatively, the above-mentioned dry content may be provided or may be essentially provided as a result of the dewatering step and the optional intermediate drying step (i.e. after these steps). Still alternatively, the above-mentioned dry content may be provided or may be essentially provided as a result of the dewatering step and the optional pre-wetting step (i.e. after these steps). In another alternative, the above-mentioned dry content may be provided or may be essentially provided as a result of the dewatering step, the optional intermediate drying step, and the optional pre-wetting step (i.e. after these steps).

[0070] In some embodiments, the smoothing is performed by applying one smoothing press to the dewatered web. In some embodiments, the smoothing is performed by applying two or more successive smoothing presses to the dewatered web.

[0071] By applying at least one smoothing press at a pressure of 0.1-25 MPa, preferably 0.1-15 MPa, more preferably 0.2-10 MPa to a dewatered web containing a large amount of MFC, such as MFC present on a support that has been dewatered in contact with a press fabric and cast thereon at a dry content of 20-60 wt%, preferably 25-60 wt%, most preferably 30-60 wt%, imperfections such as marks or textures from the press fabric transferred to the wet web during dewatering can be smoothed. Thereby, an MFC film with improved smoothness can be obtained. It is also possible to control the variation in smoothness. The smoothing according to the present disclosure is to level out small scale thickness variations of the dewatered web and should not remove or essentially remove moisture / water.

[0072] Thus, by applying at least one smoothing press according to the present disclosure, it is possible to significantly improve the roughness level (smoothness) of the wet-pressed web at a relatively low pressure. The improvement in smoothness can be very important, for example, to ensure a higher level of smoothness after applying additional coating(s). The smoothing press also makes the sheet denser, thus improving the thermal conductivity and improving the drying of the film.

[0073] In addition, improving smoothness and controlling smoothness variation can also be very important to reduce the risk of losing barrier properties and reduce two-sidedness.Since the web is on the support during wet pressing, the surface of the web positioned adjacent to the support is free of defects such as marks and textures caused by the press fabric (i.e., the surface after wet pressing will be much smoother than the surface of the web provided in direct contact with the press fabric).Smoothing according to the present disclosure can reduce the difference in smoothness between the two surfaces of the web, i.e., two-sidedness can be reduced.

[0074] Each smoothing press comprises a smoothing element arranged to contact the dewatered web and compress the dewatered web between the smoothing element and the support. The smoothing element may comprise an abrasive or non-abrasive metal surface arranged to contact the dewatered web. The smoothing element may be a smoothing roll or a smoothing belt. The smoothing roll or the smoothing belt is applied in contact with the dewatered web, which is present on the support with a pressure as described above and has a dry content as described above. Thus, the smoothing roll or the smoothing belt is applied in contact with the side (first side) of the dewatered web that was in contact with the press fabric during dewatering, i.e., the smoothing roll or the smoothing belt is applied in contact with the side of the dewatered web opposite the side (second side) that is in contact with the support.

[0075] In some embodiments, at least one of the at least one smoothing press includes a smoothing roll that is a soft roll. In some embodiments, smoothing is performed by one smoothing press that includes a soft roll. In some embodiments, smoothing is performed by two or more smoothing presses, each smoothing press including a soft roll. Alternatively, or additionally, at least one smoothing press includes a smoothing roll that is a hard roll.

[0076] In some embodiments, at least one of the at least one smoothing press includes an extended nip, for example, the residence time in the extended nip may be greater than 5 milliseconds.

[0077] Any suitable inclination angle of the dewatering web entering and exiting each of the at least one smoothing press may be utilized.

[0078] In some embodiments, at least one of the at least one smoothing presses includes a counter element (loading element) disposed on an opposite side of the support relative to the dewatered web. In some embodiments, each smoothing press includes a counter element.

[0079] In some embodiments, at least one of the at least one smoothing presses includes a smoothing roll and a counter roll, the counter roll being positioned on the opposite side of the support relative to the smoothing roll (and relative to the dewatering web). In some embodiments, smoothing is performed by two or more smoothing presses, each smoothing press including a smoothing roll and a counter roll.

[0080] In some embodiments, each smoothing press has a temperature of 40-200° C., preferably 40-150° C., more preferably 60-150° C., and most preferably 70-150° C. In some embodiments, each smoothing press has a temperature of 40-99° C. As the temperature increases, the viscosity of the MFC in the web decreases, making it easier to mold.

[0081] In some embodiments, each smoothing press has a temperature at least 1° C., preferably at least 5° C., and most preferably at least 10° C. higher than the dewatered web being smoothed.

[0082] In some embodiments, at least one of the at least one smoothing press includes a cleaning washer and / or a doctor blade.

[0083] In some embodiments, at least one of the at least one smoothing presses includes a heat applicator and / or a steam applicator and / or a spray applicator for applying a release agent or adhesive, and / or a spreader roll for applying a release agent or adhesive. In some embodiments, at least one smoothing element / roll can be permanently coated with a substance that facilitates release of the smoothed web from the smoothing element / roll surface, such as a PTFE coating.

[0084] After smoothing with at least one smoothing press, the smoothed web is dried to form a film. Drying can be done by non-contact drying and / or contact drying. Drying of the smoothed web can include drying by any conventional method, such as by further pressing the web or by contacting the web with a hot or warm cylinder or metal belt, by using a vacuum, by irradiative drying, and / or by using hot air (such as using impingement drying), and / or by heating the support from below with steam or any other medium, to obtain a suitable dry content. The moisture content of the dry film is preferably 0.5-15% by weight, more preferably 1-12% by weight, and most preferably 1.5-10% by weight.

[0085] In some embodiments, the method includes the further step of calendering the film after the drying step. In these embodiments, any suitable calender may be utilized, such as, for example, a soft nip calender.

[0086] The method of the present disclosure may further comprise a step of peeling the formed film from the non-porous support after the drying step, preferably at a dryness of <20% by weight. This forms a free-standing film. Thus, the method of the present disclosure may be a method for producing a free-standing MFC film.

[0087] In some embodiments, the first side of the produced MFC film, i.e. the side of the MFC film in contact with the press fabric and smoothed with at least one smoothing press, has a Bendtsen roughness of 300 ml / min or less, preferably 250 ml / min or less, or 150 ml / min or less, as measured by ISO 8791-2:2013. The second, opposite side of the produced MFC film, i.e. the side of the MFC film in contact with the support, may also have a Bendtsen roughness of 300 ml / min or less, preferably much lower (such as 0-100 ml / min), depending on the support used, but alternatively may have a Bendtsen roughness of more than 300 ml / min. In some embodiments, the first side of the produced film has a roughness greater than the second side of the film. In some embodiments, the ratio of the Bendtsen roughness of the first side to the Bendtsen roughness of the second side is less than 6, preferably less than 4, most preferably less than 3. In some embodiments, the second side of the produced film has a greater roughness than the first side of the produced film.

[0088] According to a second aspect of the present disclosure, there is provided an MFC film obtainable by the method of the first aspect. Preferably, both the first side and the opposite second side of the film have a Bendtsen roughness of 300 ml / min or less, preferably 250 ml / min or less, or 150 ml / min or less, as measured by ISO 8791-2:2013. The first side of the film may have a higher roughness than the second side of the film. In some embodiments, the ratio of the Bendtsen roughness of the first side to the Bendtsen roughness of the second side is less than 6, preferably less than 4, and most preferably less than 3. Alternatively, the second side of the film may have a higher roughness than the first side of the film. The MFC film, when dry, preferably has a surface roughness of 10 to 100 g / m 2 The thickness of the MFC film is preferably 8 to 500 μm, preferably 10 to 200 μm, and most preferably 15 to 100 μm when dry. The density of the MFC film is preferably 700 to 1500 kg / m when dry. 3 , preferably 800 to 1500 kg / m 3 , and most preferably 900 to 1500 kg / m 3 The film may have a transparency of more than 80% according to DIN 53147. If uncoated, the film may have a 1 m 2 It is preferred to have no more than one pinhole per lens.

[0089] According to a third aspect of the present disclosure, there is provided a film comprising between 30% and 100% by weight of microfibrillated cellulose based on total dry weight, wherein both the first side and the opposite second side of the film have a Bendtsen roughness of 300 ml / min or less, preferably 250 ml / min or less, or 150 ml / min or less, as measured by ISO 8791-2:2013. The first side of the film may have a higher roughness than the second side of the film. In some embodiments, the ratio of the Bendtsen roughness of the first side to the Bendtsen roughness of the second side is less than 6, preferably less than 4, and most preferably less than 3. Alternatively, the second side of the film may have a higher roughness than the first side of the film. The MFC film, when dry, preferably has a roughness of 10 to 100 g / m 2 The thickness of the MFC film is preferably 8 to 500 μm, preferably 10 to 200 μm, and most preferably 15 to 100 μm when dry. The density of the MFC film is preferably 700 to 1500 kg / m when dry. 3 , preferably 800 to 1500 kg / m 3 , and most preferably 900 to 1500 kg / m 3 The film may have a transparency of more than 80% according to DIN 53147. Preferably, the film, when uncoated, has a transparency of more than 1 m according to EN 13676:2001. 2 There is no more than one pinhole per sample.

[0090] The film according to the third aspect may be further defined as described above with reference to the method of the first aspect.

[0091] The free-standing MFC films according to the present disclosure can be applied to the surface of either paper or paperboard products to form laminates, such as paper or paper-based packaging laminates.

[0092] The free-standing MFC film according to the present disclosure can also be utilized in a laminate with one or more polymer layers, such as a thermoplastic polymer layer. For example, the one or more additional polymer layers can be comprised of any suitable polyolefin or polyester. The additional polymer layer(s) can be provided, for example, by extrusion coating, film coating, or laminate or dispersion coating. Common plastic resins used in extrusion coating include polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polylactic acid (PLA), polyglycolic acid (PGA), polyhydroxyalkanoates (PHAs), and polybutylene succinate (PBS).

[0093] The MFC film can also be part of a flexible packaging material, such as a transparent or translucent free-standing pouch or bag. Thus, the MFC film according to the present disclosure can be used as a box bag material when packaging dry foods such as cereals. Furthermore, the MFC film according to the present disclosure can be used as a packaging substrate, as a laminate material for paper, paperboard, or plastic, and / or as a substrate for disposable electronic devices. The MFC film can also be included in, for example, a closure, lid, or label. The MFC film can be incorporated into any type of package, such as a box, a bag, a packaging film, a cup, a container, a tray, a bottle, etc.

[0094] According to a fourth aspect of the present disclosure, there is provided a laminate comprising a film comprising microfibrillated cellulose laminated to a paper or paperboard material obtainable by the method according to the present disclosure.

[0095] According to a fifth aspect of the present disclosure there is provided a laminate comprising a film comprising between 30% and 100% by weight, based on total dry weight, of microfibrillated cellulose laminated with a paper or paperboard material, wherein a first side (opposite a second side of the film laminated with the paper or paperboard material) has a Bendtsen roughness of 300 ml / min or less, preferably 250 ml / min or less, or 150 ml / min or less, as measured by ISO 8791-2:2013.

[0096] The laminate according to the fifth aspect may be further defined as described above with reference to the method of the first aspect.

[0097] In view of the above detailed description of the invention, other modifications and variations will be apparent to those skilled in the art, but it will be apparent that such other modifications and variations are possible without departing from the spirit and scope of the invention. EXAMPLES

[0098] A number of examples were performed to demonstrate the effectiveness of smoothing according to the present disclosure, and the example data and results are shown in Tables 1a-b below.

[0099] Example 1 - (Comparative) - Low solids content after pressing, no smoothing In Example 1, an MFC film containing 13 wt. % sorbitol and 87 wt. % MFC (enzyme treated kraft pulp fluidized and microfibrillated) based on total dry weight was prepared using a cast coating method. The furnish was cast coated onto a metal belt, which initially served as a forming section and then as a web carrier for the press section. The press section was located between the metal belt (cast substrate), the press fabric, and a metal surface in contact with the press fabric. There was a loading element under the metal belt cast substrate as the opposing surface of the press section. The press section had a first low pressure press nip and a high pressure press nip with the same press configuration. In the low pressure press nip, a pressure of 0.2 MPa was used with a residence time of 1000 ms. In the high pressure press nip, a pressure of 6.2 MPa was used with a residence time of 1000 ms. The press fabric was a two-layer press fabric, the first layer in contact with the film surface was a smooth woven layer, and the second layer in contact with the metal surface was a needle press felt. After casting on the metal belt and before pressing, the film was pre-dried to increase the solids content from 3% to 5.6%. The pre-drying step was performed using hot air impingement. The solids content (dry content) after pressing was 31%. Final drying after pressing was performed by hot air impingement until the solids content reached 95%. No smoothing press was used and the Bendtsen roughness (ISO 8791-2:2013) of the press fabric side of the film after drying was 1350 ml / min. The solids content after wet pressing was low, i.e. 31%. The Bendtsen roughness of the belt side of the film after drying was 20 ml / min. Oxygen transmission rate (OTR) values ​​were measured at 23 °C and 50% relative humidity according to ASTM D-3985.

[0100] Example 2 - (Comparative) - Medium solids content after pressing, no smoothing Example 2 was carried out in a similar manner to Example 1, but with improved (increased) solids content after wet pressing. Roughness was slightly improved, reaching a value of 810 ml / min on the pressed fabric side of the film after drying.

[0101] Example 3 - (Comparative) - High solids content after pressing, no smoothing Example 3 was carried out in a similar manner to Example 1, but the solids content after wet pressing was improved (increased) to 52%. The roughness was slightly improved, reaching a value of 680 ml / min on the press fabric side of the film after drying.

[0102] Example 4 - Smoothing Press: High Starting Solids Content, High Pressure Example 4 utilized the same recipe and forming and pressing section as Example 1, but now a smoothing press was applied after the pressing section. The smoothing press was arranged such that the web was compressed between the metal belt cast substrate and the polished smooth metal surface that was the top of the smoothing press. The temperature of the metal belt and the top of the smoothing press was 50° C. A loading element was present under the metal belt cast substrate as the counter face of the press. The solids content after the pressing section (and therefore the solids content when the smoothing press was applied) was 43% and drying was achieved by hot air impingement. The smoothing pressure applied was 5.3 MPa. A significant reduction (60 ml / min) in the Bentdsen roughness (ISO 8791-2:2013) of the press fabric side of the dried film was obtained, indicating successful smoothing. The Bendtsen roughness of the belt side of the dried film was 20 ml / min.

[0103] Example 5 - Smoothing Press: High Starting Solids Content, Medium Pressure Example 5 was carried out in a similar manner to Example 4, but using a lower smoothing press pressure, i.e., 3.5 MPa, which gave similarly good results in terms of Bendtsen roughness (40 ml / min) on the press fabric side of the film after drying.

[0104] Example 6 - Smoothing Press: High Starting Solids Content, Low Pressure Example 6 was carried out in a similar manner to Example 4, but now using a lower smoothing press pressure, i.e., 1.5 MPa. The smoothness of the press fabric side of the dried film is significantly better than the corresponding standard (90 ml / min).

[0105] Example 7 - Smoothing Press: Very high starting solids content, high pressure Example 7 was carried out in a similar manner to Example 4, but using a higher solids content after the press section, i.e., >66%. The smoothing press pressure was 6.7 MPa. This was a failure, showing that smoothing with the smoothing press does not work on films that are too dry. Samples with these conditions have high surface roughness on the press fabric side of the film.

[0106] Example 8 - Smoothing Press: High Starting Solids Content, Very High Pressure Example 8 was carried out in a similar manner to Example 4, but using a higher smoothing press pressure, i.e. 11.4 MPa. This was a failure example, showing that smoothing with too high a smoothing press pressure destroys the film. All samples with these conditions have pinholes / holes.

[0107] Example 9 - Smoothing Press: High Starting Solids Content, Very Low Pressure Example 9 was carried out in a similar manner to Example 4, but using a lower smoothing press pressure, i.e. 0.5 MPa. The roughness was at the same level as the previous sample.

[0108] Example 10 - Smoothing Press: Low Starting Solids Content, High Pressure Example 10 was carried out in a similar manner to Example 4, but with a lower solids content after the press section, i.e., 29%. The smoothing press pressure was 5 MPa. The smoothing effect was somewhat similar to Example 4.

[0109] Example 11 - Smoothing Press: Very low starting solids content, low pressure Example 11 was carried out in a similar manner to Example 4, but with a lower solids content after the press section, i.e., 26 percent. The smoothing press pressure was 1.8 MPa. The smoothing effect was somewhat similar to Example 4.

[0110] Example 12 - Different smoothing press surfaces: high starting solids content, high pressure Example 12 was carried out in the same manner as Example 4, but a non-polished ground metal plate was used as the upper surface of the smoothing press. The smoothing press pressure was 5.3 MPa. The smoothing effect was somewhat similar to that of Example 4.

[0111] Example 13 - Smoothing Press: High Starting Solids Content (Intermediate Drying), High Pressure Example 13 was carried out in a similar manner to Example 4, but with intermediate drying before the smoothing press. The smoothing press pressure was 5.1 MPa. The roughness was at the same level as the previous sample. TIFF2024539547000001.tif95170TIFF2024539547000002.tif98170

Claims

1. 1. A method for producing a film comprising microfibrillated cellulose, comprising: - providing a suspension comprising between 30% and 100% by weight of microfibrillated cellulose, based on the total dry weight; - forming a wet web of said suspension by casting it onto a substrate, said substrate being a non-porous substrate, a paper substrate, or a paperboard substrate, said formed wet web having a dry content of 1 to 25% by weight; - wet-pressing the wet web to form a dewatered web having a dry content of 15 to 80% by weight, said wet-pressing comprising applying a press fabric in direct contact with said wet web and passing said wet web disposed between said press fabric and said support through a press apparatus; - smoothing the dewatered web by applying at least one smoothing press to the dewatered web disposed on the support to form a smoothed web, wherein a pressure of 0.1 to 25 MPa, preferably 0.1 to 15 MPa, most preferably 0.2 to 10 MPa is applied to each smoothing press, and wherein when the at least one smoothing press is applied, the dewatered web has a dry content of 20 to 60% by weight, preferably 25 to 60% by weight, most preferably 30 to 60% by weight; and drying the smoothed web to form the film. A method comprising the steps of:

2. 10. The method of claim 1, wherein said smoothing is accomplished by applying two or more successive smoothing presses to said dewatered web.

3. 3. The method of claim 1 or 2, wherein each smoothing press comprises a smoothing roll or a smoothing belt.

4. 3. The method of claim 1 or 2, wherein at least one smoothing press comprises a smoothing roll that is a soft roll.

5. The method of claim 1 or 2, wherein at least one smoothing press comprises an extended nip.

6. The method of claim 1 or 2, wherein the non-porous support is a metal belt.

7. 3. The method according to claim 1 or 2, wherein the support is the paper substrate or the paperboard substrate, the paper substrate or the paperboard substrate is provided on a non-porous wet-press support in the wet-pressing step, and the paper substrate or the paperboard substrate is provided on the non-porous wet-press support or the non-porous smoothing support in the smoothing step.

8. 3. The method of claim 1 or 2, wherein the suspension comprises between 50% and 100% by weight of microfibrillated cellulose, based on total dry weight.

9. The method of claim 1 or 2, wherein the formed wet web comprises a single web layer or two or more web layers formed on top of each other.

10. 3. The method of claim 1 or 2, wherein the dewatered web is subjected to at least one pre-wetting step prior to the smoothing step.

11. 3. The method of claim 1 or 2, wherein the film is calendered after the drying step.

12. The method of claim 1 or 2, wherein the method includes the step of pre-drying the formed wet web prior to the wet-pressing step.

13. 3. The method of claim 1, wherein the method includes intermediate drying of the dewatered web prior to the smoothing step.

14. 3. A film comprising microfibrillated cellulose obtainable by the method according to claim 1 or 2.

15. 1. A film comprising between 30% and 100% by weight of microfibrillated cellulose based on a total dry weight, wherein both a first surface and an opposite second surface of the film have a Bendtsen roughness of 300 ml / min or less.

16. 16. The film of claim 15, wherein the first side of the film has a greater roughness than the second side of the film, and the ratio of the Bendtsen roughness of the first side to the Bendtsen roughness of the second side is less than 6, preferably less than 4, and most preferably less than 3.