Multilayer film containing MFC

Abstract

The present invention relates to a method for manufacturing a multi-layer film containing microfibrillated cellulose (MFC) in a paper machine, the method comprising the steps of: a) forming a bottom web layer by applying a first pulp suspension onto a bottom web wire, the first pulp suspension comprising at least 50% by dry weight of a cellulose-based fiber material having a Schopper-Riegler (SR) value in the range of 18 - 75; b) forming or applying an intermediate web layer formed from a second pulp suspension onto the bottom web layer, the second pulp suspension comprising at least 50% by dry weight of MFC having an SR value in the range of 80 - 100; c) applying a top web layer formed from a third pulp suspension onto the intermediate web layer to form a multi-layer web, the third pulp suspension comprising at least 50% by dry weight of a cellulose-based fiber material having a Schopper-Riegler (SR) value in the range of 18 - 75; and d) dewatering and optionally drying the formed multi-layer web to obtain a multi-layer film containing MFC.

Description

Technical Field

[0001] The present disclosure relates to gas barrier films that can be used, for example, in paper and paperboard-based packaging materials. More specifically, the present disclosure relates to methods for manufacturing films comprising highly refined cellulose fibers, particularly films comprising microfibrillated cellulose (MFC). Background Art

[0002] In the packaging industry, effective gas, odor (aroma), and / or moisture barriers are needed to shield sensitive products. In particular, oxygen-sensitive products require oxygen barriers to extend their shelf life. Oxygen-sensitive products include many foods, as well as pharmaceutical and electronic industry products. Known packaging materials with oxygen barrier properties can comprise one or several polymer films, or fibrous paper or paperboard coated with one or several oxygen barrier polymer layers that are typically part of a multilayer coating structure. Another important property for food packaging is oil and grease resistance.

[0003] Recently, microfibrillated cellulose (MFC) films have been developed, in which defibrillated cellulose fibrils are suspended in, for example, water, reorganized, and re-bonded together to form a continuous film. It has been found that MFC films provide good gas barrier properties as well as good oil and grease resistance.

[0004] MFC films can be prepared by applying an MFC suspension onto a porous substrate to form a web, and then dehydrating the web by discharging water through the substrate to form a film. The formation of the web can be achieved, for example, by using a papermaking- or paperboard-making type process. The porous substrate can be, for example, a membrane or a screen fabric, or it can be a paper or paperboard substrate.

[0005] On a paper machine, it is difficult to manufacture films and barrier substrates from highly refined cellulose or MFC suspensions with very slow drainage because it is difficult to produce good barriers due to the occurrence of pinholes. Pinholes are micropores that may appear in the web during the forming process. Examples of the causes of pinholes include irregularities in the pulp suspension (such as those formed by the flocculation or re-flocculation of fibrils), a rough dewatering fabric, uneven pulp distribution on the screen, or too low a basis weight of the web. The formation of pinholes typically increases with an increase in the dehydration rate. However, in the pinhole-free regions, when the basis weight is higher than 20 - 40 g / m 2 the oxygen transmission rate values are good.

[0006] MFC membranes are typically relatively weak, and thus the membranes are usually formed or laminated with one or more additional support layers to improve mechanical strength. However, due to the shrinkage characteristics of MFC membranes, forming or laminating with other cellulose-based layers may generally result in curling problems in the formed multi-layer structure.

[0007] In addition, when forming a multi-layer structure, the high water retention rate and low water permeability of the MFC suspension and the wet web can lead to drainage problems. The low water permeability of the MFC membrane can prevent water from being removed from other layers of the multi-layer structure, which can lead to delamination or bubble formation.

[0008] One solution to overcome this problem is to form an MFC layer by coating a relatively dry substrate with an MFC suspension and then drying the substrate. Unfortunately, since the MFC suspension is typically relatively wet, this solution may lead to problems regarding rewetting of the substrate.

[0009] Another possibility is wet-on-dry lamination, where a sheet containing wet MFC is laminated onto a dry substrate. However, in this case, curling and asymmetric shrinkage must be controlled by other means, such as coating the back side with MFC. This results in additional rewetting without obtaining any additional barrier properties.

[0010] From a technical and economic perspective, it is preferable to find a solution that can quickly dehydrate and at the same time improve the mechanical properties or barrier properties of the membrane or both. Detailed Description

[0011] One object of the present disclosure is to provide a method for manufacturing a membrane comprising highly refined cellulose fibers (such as microfibrillated cellulose (MFC)), which alleviates at least some of the above-mentioned problems associated with prior art methods.

[0012] Another object of the present disclosure is to provide an improved method for manufacturing a membrane comprising highly refined cellulose fibers in a paper- or board-making process.

[0013] Another object of the present disclosure is to provide a membrane that can be used as a gas barrier in paper- or board-based packaging materials, and the membrane is based on renewable raw materials.

[0014] Another object of the present disclosure is to provide a membrane that can be used as a gas barrier in paper- or board-based packaging materials, and the membrane has a high repulpability, thereby providing a high recyclability for the packaging products containing the membrane.

[0015] The above-mentioned objects and other objects recognized by those skilled in the art according to the present disclosure are achieved through various aspects of the present disclosure.

[0016] The method of the present invention allows for the efficient manufacture of multilayer films containing microfibrillated cellulose in a papermachine-type process. 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 or aluminum foils, which reduce the recyclability of paper or cardboard packaging products. The films of the present invention have high repulpability, thus providing a higher recyclability for the films as well as for paper or cardboard packaging products containing the films.

[0017] According to a first aspect as shown herein, there is provided a method for manufacturing a multilayer film containing microfibrillated cellulose (MFC) in a paper machine, the method comprising the steps of:

[0018] a) forming a base web layer by applying a first pulp suspension onto a base web wire, the first pulp suspension comprising at least 50% by dry weight of a cellulose-based fibrous material having a Schopper-Riegler (SR) value in the range of 18 - 75;

[0019] b) forming or applying an intermediate web layer formed from a second pulp suspension onto the base web layer, the second pulp suspension comprising at least 50% by dry weight of MFC having an SR value in the range of 80 - 100;

[0020] c) applying a top web layer formed from a third pulp suspension onto the intermediate web layer to form a multilayer web, the third pulp suspension comprising at least 50% by dry weight of a cellulose-based fibrous material having a Schopper-Riegler (SR) value in the range of 18 - 75; and

[0021] d) dewatering and optionally drying the formed multilayer web to obtain a multilayer film containing MFC.

[0022] As used herein, the term film generally refers to a thin continuous sheet-like material. Depending on the composition of the pulp suspension, the film may also be considered a thin paper or even a membrane.

[0023] The multilayer film can be used as such, or it can be combined with one or more other layers. The film can, for example, be used as a barrier layer in a paperboard-based packaging material. The film can also be or form a barrier layer in cellophane, greaseproof paper, or thin wrapping paper.

[0024] Although those skilled in the art may envision different arrangements for performing the steps of the method of the present invention, the method of the present invention can advantageously be carried out in a paper machine, more preferably in a Fourdrinier paper machine.

[0025] A paper machine (or paper-making machine) is an industrial machine used in the pulp and paper industry for high-speed and large-scale production of paper. Modern paper machines are typically based on the principle of the Fourdrinier paper machine, which uses a moving woven wire (i.e., "wire mesh") to produce a continuous web by filtering out the fibers contained in the pulp suspension and generating a continuously moving wet fiber web. This wet web is dried in the machine to produce paper or film.

[0026] The forming and dewatering steps of the method of the present invention are preferably carried out at the forming section (commonly referred to as the wet end) of a paper machine.

[0027] The wet web is formed on different wire meshes in the forming section of a paper machine. Preferred types of forming sections for use in the present invention include 2 or 3 Fourdrinier paper machine wire mesh sections in combination with a supporting wire mesh. The wire mesh is preferably an endless wire mesh. The wire mesh used in the method of the present invention preferably has a relatively high porosity to allow for rapid dewatering and high drainage capacity. At 100 Pa, the air permeability of the wire mesh is preferably above 5000 m 3 / m 2 / hour. The wire mesh used in the method of the present invention preferably has a relatively high porosity to allow for rapid dewatering and high drainage capacity. The wire mesh preferably has a relatively high fiber support index (F.S.I) typically above 190, such that fine particulate material does not penetrate into the structure and results in fewer wire mesh markings as well as a rough and open backside. The wire mesh section of a paper machine can have various dewatering devices, such as doctor blades, turntables, and / or foil elements, suction boxes, frictionless dewatering, ultrasonic-assisted dewatering, couch rolls, or dandy rolls.

[0028] In the method of the present invention, an intermediate web layer formed from a pulp suspension containing at least 50% dry weight of MFC with a high water retention value is formed between two outer layers formed from pulp suspensions containing less refined cellulose-based fiber materials with a lower water retention value.

[0029] The method of the present invention includes forming a bottom web layer by applying a first pulp suspension onto a bottom web wire mesh, the first pulp suspension containing at least 50% dry weight of a cellulose-based fiber material with an SR value in the range of 18 - 75.

[0030] The method of the present invention further includes forming or applying an intermediate web layer on the bottom web layer, the intermediate web layer formed from a second pulp suspension containing at least 50% dry weight of MFC with an SR value in the range of 80 - 100.

[0031] The first and second layers can be formed on different wire meshes respectively, or together on the same wire mesh.

[0032] The bottom web layer is preferably partially dewatered before forming or applying the intermediate web layer. Thus, in some embodiments, the method comprises the steps of:

[0033] a1) forming a bottom web layer by applying a first pulp suspension to a bottom web wire, the first pulp suspension comprising at least 50% by dry weight of a cellulosic fiber material having a Schopper - Riegler (SR) value in the range of 18 - 75; and

[0034] a2) partially dewatering the bottom web layer.

[0035] The intermediate web layer is formed from a second pulp suspension comprising at least 50% by dry weight of MFC having an SR value in the range of 80 - 100. The intermediate layer is formed or applied on the bottom web layer. This means that in some embodiments, the intermediate web layer is formed directly on the bottom web layer by applying the second pulp suspension to the wet or partially dried bottom web layer. In other embodiments, the intermediate web layer is formed separately, e.g., on a separate wire, partially dewatered, and then wet - laminated onto the bottom web layer.

[0036] In some embodiments, the intermediate web layer of step b) is formed by applying a second pulp suspension to the bottom web layer, the second pulp suspension comprising at least 50% by dry weight of MFC having an SR value in the range of 80 - 100. Various methods can be used to apply the second pulp suspension, including but not limited to spraying or curtain coating. When using these types of deposition techniques, the application can be carried out in a single deposition step or using multiple deposition steps in order to obtain a more uniform quality and not to interfere with the formation of the bottom web layer. The application of the second pulp suspension can be achieved, for example, using at least two consecutive spraying or curtain coating units that apply the same or substantially the same pulp suspension.

[0037] The dry solids content of the second pulp suspension applied to the bottom web layer can vary within a wide range, depending on the technique used for depositing the suspension. The dry solids content of the second pulp suspension applied to the bottom web layer can generally be in the range of 0.1 - 5 wt%. When using a headbox to apply the second pulp suspension, the dry solids content can typically be lower. The dry solids content of the second pulp suspension applied to the bottom web layer is typically in the range of 0.1 - 0.7 wt%, preferably in the range of 0.15 - 0.5 wt%, more preferably in the range of 0.2 - 0.4 wt%.

[0038] The water of the second pulp suspension can be removed via drainage through the bottom web layer with less drainage resistance, or via drying, or via a combination thereof. The drainage and / or drying of the second pulp suspension results in the formation of the intermediate web layer on the bottom web layer.

[0039] The dewatering of the web on the wire can be carried out using methods and equipment known in the art. Examples include but are not limited to couch rolls and foils, frictionless dewatering, and ultrasonic-assisted dewatering.

[0040] Partial dewatering means that the dry solids content of the wet web has decreased 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 partially dewatered web is above 1 wt% but below 15 wt%. In some embodiments, partial dewatering of the wet web means that the dry solids content of the partially dewatered web is above 1 wt% but below 10 wt%. It has been found that the dry solids content of the partially dewatered web within this range is particularly suitable for joining the wet web into a multi-layer web. In some embodiments, the dry solids content of the partially dewatered web layer before lamination is in the range of 1.5 - 8 wt%, preferably in the range of 2.5 - 6 wt%, and more preferably in the range of 3 - 4.5 wt%.

[0041] In some embodiments, the intermediate web layer of step b) is formed simultaneously with the bottom web layer of step a), for example, using a multi-layer headbox or two headboxes arranged at the same wire. In some embodiments, the bottom web layer of step a) and the intermediate web layer of step b) are formed simultaneously using a multi-layer headbox. The lower drainage resistance of the bottom web layer allows water to be removed via drainage through the bottom web layer and the wire.

[0042] In an alternative embodiment, the bottom web layer and the intermediate web layer are formed separately on different wires and then joined by wet lamination. Thus, in some embodiments, step b) includes:

[0043] b1) forming an intermediate web layer by applying a second pulp suspension onto the intermediate web wire, the second pulp suspension comprising at least 50% dry weight of MFC with an SR value in the range of 80 - 100;

[0044] b2) partially dewatering the intermediate web layer; and

[0045] b3) applying the partially dewatered intermediate web layer onto the bottom web layer.

[0046] In some embodiments, the dry solids content of the partially dewatered intermediate web layer is in the range of 1.5 - 8 wt%, preferably in the range of 2.5 - 6 wt%, and more preferably in the range of 3 - 4.5 wt%.

[0047] In some embodiments, the bottom web layer is also partially dewatered. In some embodiments, the dry solids content of the partially dewatered bottom web layer ranges from 1.5 to 8 wt%, preferably from 2.5 to 6 wt%, and more preferably from 3 to 4.5 wt%.

[0048] The top web layer is preferably formed and partially dewatered on a top web wire separate from the bottom web layer and the middle web layer, and the partially dewatered top web layer is then applied to the middle web layer to form a multi-layer web. The partial dewatering of the top web layer reduces the problem of drainage through the low-permeability middle web layer. This prevents delamination or bubble formation in the multi-layer web.

[0049] Thus, in some embodiments, step c) of the method comprises:

[0050] c1) forming a top web layer by applying a third pulp suspension on a top web wire, the third pulp suspension comprising at least 50% by dry weight of a cellulose-based fibrous material having a Schopper-Riegler (SR) value in the range of 18 - 75;

[0051] c2) partially dewatering the top web layer; and

[0052] c3) applying the partially dewatered top web layer to the middle web layer to form a multi-layer web.

[0053] In some embodiments, the dry solids content of the partially dewatered top web layer ranges from 1.5 to 8 wt%, preferably from 2.5 to 6 wt%, and more preferably from 3 to 4.5 wt%.

[0054] The partially dewatered webs are preferably joined by wet lamination. When the pulp suspension is dewatered on the wire, a visible boundary line appears at the point where the web goes from having a reflected water layer until the reflected layer disappears. This boundary line between the reflected and non-reflected webs is called the water line. The water line indicates a specific solids content of the web. The webs are preferably joined after the water line. Joining the webs while they are still wet ensures good adhesion between the layers. The joining can be achieved by applying one of the partially dewatered webs on top of the other. The joining can be non-wire side to non-wire side, or wire side to non-wire side. The joining and further dewatering of the formed multi-layer web can be improved by various additional operations. In some embodiments, the joining also comprises pressing the partially dewatered webs together. In some embodiments, the joining also comprises applying suction to the joined partially dewatered webs. Applying pressure and / or suction to the formed multi-layer web improves the adhesion between the web layers.

[0055] Joining the webs while they are still wet ensures good adhesion between the layers. It has been found that partial dehydration and lamination of the webs in a partially dehydrated state substantially eliminates the occurrence of pinholes in the finished multi-layer film, while still allowing high production speeds. In the prior art, an increase in the dehydration rate has sometimes been achieved by using large amounts of retention and drainage chemicals at the wet end of the process, which results in increased flocculation. However, retention and drainage chemicals can also result in a more porous web structure, so the use of such chemicals needs to be minimized. The method of the present invention provides an alternative way to increase the dehydration rate, which is less dependent on the addition of retention and drainage chemicals. In some embodiments, the second pulp suspension does not contain added retention and drainage chemicals.

[0056] When the partially dehydrated top web layer has been applied, the dry solids content of the multi-layer web typically further increases. The increase in the dry solids content may be due to the dehydration of the multi-layer web on the wire under the optionally applied pressure and / or suction to the web, and may also be due to drying operations performed during or shortly after joining, such as impulse drying or air or steam drying. Under the optionally applied pressure and / or suction, the dry solids content of the multi-layer web after joining is typically above 8 wt% but below 28 wt%. In some embodiments, the dry solids content of the multi-layer web before the further dehydration and optional drying steps is in the range of 8 - 28 wt%, preferably in the range of 10 - 20 wt%, and more preferably in the range of 12 - 18 wt%.

[0057] The formed multi-layer web is then further dehydrated and optionally dried to obtain a multi-layer film comprising MFC. In the dehydration and optional drying step d), the dry solids content of the multi-layer web further increases. The resulting multi-layer film preferably has a dry solids content of above 90 wt%.

[0058] Further dehydration typically comprises pressing the multi-layer web to squeeze out as much water as possible. Further dehydration may for example include passing the formed multi-layer web through the press section of a paper machine, where the web passes between large rolls loaded under high pressure to squeeze out as much water as possible. In some embodiments, further dehydration includes passing the web through one or more shoe presses. The removed water is typically received by a fabric or felt. In some embodiments, the dry solids content of the multi-layer film after further dehydration is in the range of 15 - 48 wt%, preferably in the range of 18 - 40 wt%, and more preferably in the range of 22 - 35 wt%.

[0059] Optional drying may for example comprise drying the multi-layer web by passing the multi-layer web around a series of heated cylinders. Drying typically removes the water content down to a level of about 1-15 wt%, preferably about 2-10 wt%. In some embodiments, drying comprises drying the web on a Yankee cylinder. The Yankee cylinder can also be used to create a glazed surface on the finished film.

[0060] It has been found that the combination of dewatering in one or more shoe presses followed by drying in a Yankee cylinder enables the multi-layer film to be dewatered and dried in a very efficient manner (i.e., at high speed and with good runnability) without destroying the barrier properties of the multi-layer film.

[0061] The dry solids content of the final multi-layer film can vary depending on the intended use of the film. For example, a multi-layer film used as a stand-alone product can have a dry solids content in the range of 85-99 wt%, preferably in the range of 90-98 wt%, while a film used for further lamination to form a paper or paperboard-based packaging material can have a dry solids content of less than 90 wt%, preferably less than 85 wt%, such as in the range of 30-85 wt%.

[0062] The first and third pulp suspensions are aqueous suspensions which are aqueous suspension mixtures comprising a cellulose-based fibrous material and optional non-fibrous additives. The pulp can be produced from different raw materials, such as selected from bleached or unbleached softwood or hardwood pulp, kraft pulp, pressure groundwood pulp (PGW), thermomechanical pulp (TMP), chemi-thermomechanical pulp (CTMP), neutral sulfite semi-chemical pulp (NSSC), broke or recycled fibers.

[0063] The pulp suspensions can be unrefined or refined. Refining or beating of cellulose pulp refers to the mechanical treatment and modification of cellulose fibers in order to provide them with the desired properties. The cellulose-based fibrous material of the first and third pulp suspensions has an SR (Schopper-Riegler) value in the range of 18-75. In some embodiments, the cellulose-based fibrous material of the first and third pulp suspensions has an SR value in the range of 18-70.

[0064] The dry solids content of the first and / or third pulp suspensions is typically in the range of 0.1-0.7 wt%, preferably in the range of 0.15-0.5 wt%, more preferably in the range of 0.2-0.4 wt%.

[0065] The dry solids content of the first and / or third pulp suspensions can consist only of the cellulose-based fibrous material, or it can comprise a mixture of the cellulose-based fibrous material and other components or additives.

[0066] Based on the total dry weight of the pulp suspension, the first and / or third pulp suspension preferably comprises a cellulosic fiber material as its main component. In some embodiments, the first and / or third pulp suspension comprises at least 50% dry weight, preferably at least 70% dry weight, more preferably at least 80% dry weight or at least 90% dry weight of cellulosic fiber material based on the total dry weight of the pulp suspension.

[0067] In some embodiments, the first and / or third pulp suspension is a kraft pulp suspension. Refined kraft pulp typically contains at least 10% dry weight of hemicellulose. Thus, in some embodiments, the first and / or third pulp suspension comprises at least 10% dry weight, such as an amount in the range of 10 - 25% dry weight, of hemicellulose based on the amount of cellulosic fiber material.

[0068] The first and / or third pulp suspension may further comprise additives such as natural starch or starch derivatives, cellulose derivatives such as sodium carboxymethyl cellulose, fillers, retention and / or drainage chemicals, flocculation additives, deflocculation additives, dry strength additives, softeners, crosslinking aids, sizing chemicals, dyes and colorants, wet strength resins, fixatives, antifoaming aids, microbial and slime (control, foulant) control aids, or mixtures thereof. The first and / or third pulp suspension may further comprise additives that improve different properties of the mixture and / or the resulting film, such as latex and / or polyvinyl alcohol (PVOH) for enhancing the ductility of the film. The method of the present invention provides an alternative way to increase the dehydration rate, which is less dependent on the addition of retention and drainage chemicals, but still allows the use of a smaller amount of retention and drainage chemicals.

[0069] In some embodiments, based on the total dry weight of the pulp suspension, the first and / or third pulp suspension comprises an amount of hydrophobic chemicals such as AKD, ASA or rosin sizing in the range of 0 - 10 kg / ton, preferably 0.1 - 5 kg / ton, and more preferably 0.2 - 2 kg / ton.

[0070] In some embodiments, the first and / or third pulp suspension comprises thermoplastic particles or fibers, such as PLA or PVOH fibers, to provide heat sealability. In some embodiments, the first and / or third pulp suspension comprises an amount of thermoplastic particles or fibers in the range of 5 - 50% dry weight, preferably 10 - 50% dry weight, more preferably 15 - 50% dry weight based on the total dry weight of the pulp suspension.

[0071] In some embodiments, the first and / or third pulp suspension comprises mechanical pulp to impart a natural appearance to the film.

[0072] To prevent the formed multi-layer web from curling during further dewatering and drying, the bottom and top web layers should preferably exhibit the same or similar shrinkage rates during dewatering or drying. In a preferred embodiment, the same or similar shrinkage rates can be achieved by using the same pulp and basis weight for the bottom and top web layers. Of course, the same or similar shrinkage rates can also be achieved by using different pulps but adjusting the basis weight or including additives to obtain the same or similar shrinkage rates.

[0073] In some embodiments, the first and / or third pulp suspensions are the same. In some embodiments, the SR values of the first and / or third pulp suspensions differ by less than 30%, preferably less than 25% and more preferably less than 20%.

[0074] In some embodiments, the bottom and top web layers have the same or similar composition and basis weight.

[0075] In some embodiments, the dry basis weight of the bottom and top web layers is in the range of 20 - 120 gsm, preferably in the range of 20 - 100 gsm, and more preferably in the range of 20 - 80 gsm.

[0076] The second pulp suspension is an aqueous suspension which is a water suspension mixture containing a cellulose-based fibrous material and optionally a non-fibrous additive. The pulp can be produced from different raw materials, such as softwood pulp or hardwood pulp.

[0077] The second pulp suspension is more refined than the first and third pulp suspensions and contains at least 50% dry weight of microfibrillated cellulose (MFC). The MFC of the second pulp suspension has an SR (Schopper - Riegler) value in the range of 80 - 100. In some embodiments, the MFC of the second pulp suspension has an SR value in the range of 80 - 98. In some embodiments, the MFC of the second pulp suspension has an SR value in the range of 85 - 98.

[0078] The SR value of the second pulp suspension is significantly higher than the SR values of the first and third pulp suspensions. More specifically, the SR value of the second pulp suspension is preferably at least 10 SR units higher, more preferably at least 20 or at least 30 SR units higher than the SR values of the first and third pulp suspensions.

[0079] The dry solids content of the second pulp suspension applied to the bottom web layer can vary within a wide range, depending on the technique used for depositing the suspension. The dry solids content of the second pulp suspension applied to the bottom web layer can generally be in the range of 0.1 - 5 wt%. When using a headbox to apply the second pulp suspension, the dry solids content can typically be lower. The dry solids content of the second pulp suspension is typically in the range of 0.1 - 0.7 wt%, preferably in the range of 0.15 - 0.5 wt%, and more preferably in the range of 0.2 - 0.4 wt%.

[0080] The dry solids content of the second pulp suspension can consist only of MFC, or it can contain a mixture of MFC and other components or additives.

[0081] Preferably, based on the total dry weight of the pulp suspension, the second pulp suspension includes MFC as its main component. Having a higher dry content of MFC in the second pulp suspension ensures good barrier properties in the finished film. In some embodiments, the second pulp suspension contains at least 50 dry wt%, preferably at least 70 dry wt%, more preferably at least 80 dry wt% or at least 90 dry wt% of MFC, based on the total dry weight of the pulp suspension. In some embodiments, the second pulp suspension contains MFC in the range of 50 - 99 dry wt%, preferably in the range of 70 - 99 dry wt%, more preferably in the range of 80 - 99 dry wt%, and even more preferably in the range of 90 - 99 dry wt%, based on the total dry weight of the pulp suspension.

[0082] In some embodiments, the second pulp suspension is a highly refined kraft pulp suspension. Refined kraft pulp typically contains at least 10 dry wt% of hemicellulose. Thus, in some embodiments, the first and / or third pulp suspension contains hemicellulose in an amount of at least 10% dry wt, such as in the range of 10 - 25% dry wt, based on the amount of MFC.

[0083] The second pulp suspension can also contain additives such as natural starch or starch derivatives, cellulose derivatives such as sodium carboxymethyl cellulose, fillers, flocculation additives, deflocculation additives, dry strength additives, softeners, crosslinking aids, sizing chemicals, dyes and colorants, wet strength resins, fixatives, antifoaming aids, microbial and slime control aids, or mixtures thereof. The second pulp suspension can also contain additives that improve different properties of the mixture and / or the resulting film, such as latex and / or polyvinyl alcohol (PVOH) for enhancing the ductility of the film. The method of the present invention provides an alternative way to increase the dehydration rate, which is less dependent on the addition of retention and drainage chemicals, but still allows the use of smaller amounts of retention and drainage chemicals. In some embodiments, the second pulp suspension does not contain added retention and drainage chemicals.

[0084] The MFC with a high dry content in the second pulp suspension ensures good barrier properties in the finished film. Thus, the second pulp suspension preferably contains a total additive of not more than 20 dry weight % based on the total dry weight of the pulp suspension. More preferably, the second pulp suspension preferably contains a total of not more than 10 dry weight % of additives based on the total dry weight of the pulp suspension.

[0085] In some embodiments, the second pulp suspension contains up to 20 dry weight %, preferably up to 10 dry weight % of a filler, such as bentonite, based on the total dry weight of the pulp suspension.

[0086] In addition to MFC, the second pulp suspension may also contain a specific amount of unrefined or lightly refined cellulose fibers. As used herein, the term unrefined or lightly refined fibers preferably refers to cellulose fibers having a Schopper - Riegler (SR) value of below 30, preferably below 28, as measured by standard ISO 5267 - 1. Unrefined or lightly refined cellulose fibers can be used to enhance dehydration and can also improve the strength and fracture toughness of the multilayer film. In some embodiments, the second pulp suspension contains 0.1 - 50% dry weight, preferably 0.1 - 30% dry weight, and more preferably 0.1 - 10% dry weight of unrefined or lightly refined cellulose fibers based on the total dry weight of the pulp suspension. Unrefined or lightly refined cellulose fibers can be obtained, for example, from bleached or unbleached or mechanical or chemimechanical pulp or other high - yield pulp.

[0087] The pH value of the second pulp suspension can typically be in the range of 4 - 10, preferably in the range of 5 - 8, and more preferably in the range of 5.5 - 7.5.

[0088] The temperature of the second pulp suspension can typically be in the range of 30 - 70 °C, preferably in the range of 40 - 60 °C, and more preferably in the range of 45 - 55 °C.

[0089] In the context of the present patent application, microfibrillated cellulose (MFC) should be understood to mean at least one nanoscale cellulose particle fiber or fibril with a size less than 1000 nm. MFC comprises partially or fully fibrillated cellulose or lignocellulosic fibers. The released fibrils typically have a diameter of less than 100 nm, while the actual fibril diameter or particle size distribution and / or aspect ratio (length / width) depend on the source and manufacturing method. The smallest fibrils are called primary fibrils and have a diameter of approximately 2 - 4 nm (see, for example, Chinga-Carrasco, G., Cellulose fibres, nanofibrils and microfibrils,: The morphological sequence of MFC components from a plant physiology and fibre technology point of view, Nanoscale research letters 2011, 6:417), and it is common that the aggregated form of primary fibrils (also defined as microfibrils) (Fengel, D., Ultrastructural behavior of cell wall polysaccharides, Tappi J., March 1970, Vol. 53, No. 3) is the main product obtained, for example, when preparing MFC by using an extended refining process or a pressure-drop disintegration process. Depending on the source and manufacturing process, the length of the fibrils can vary from about 1 micrometer to greater than 10 micrometers. Crude MFC grades may contain a significant fraction of fibrillated fibers, i.e., protruding fibrils (cellulose fibers) from tracheids, as well as a specific amount of fibrils (cellulose fibers) released from tracheids.

[0090] MFC has different acronyms, such as cellulose microfibril, fibrillated cellulose, nanofibrillated cellulose, fibril aggregate, nanoscale cellulose fibril, cellulose nanofiber, cellulose nanofibril, cellulose microfiber, cellulose fibril, microfibrillated cellulose, microfibril aggregate, and cellulose microfibril aggregate. 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.

[0091] There are various methods for preparing MFC, such as single-pass or multi-pass refining, pre-hydrolysis, followed by refining or high-shear disintegration or fibrillation release. In order to make MFC manufacturing both energy-efficient and sustainable, one or several pretreatment steps are usually required. Therefore, the cellulose fibers of the pulp utilized can be pretreated, for example, enzymatically or chemically, to hydrolyze or swell the fibers or reduce the amount of hemicellulose or lignin. The cellulose fibers can be chemically modified before fibrillation so that the cellulose molecules contain functional groups different from (or more than) those present in native cellulose. Such groups especially include carboxymethyl (CMC), aldehyde and / or carboxyl (cellulose obtained by N-oxyl-mediated oxidation, such as "TEMPO"), quaternary ammonium (cationic cellulose) or phosphoryl groups. After modification or oxidation in one of the above methods, it is easier to disintegrate the fibers into MFC or nanofibrils.

[0092] Nanofibrillated cellulose may contain some hemicellulose, the amount of which depends on the plant source. Mechanical disintegration of the pretreated fibers (e.g., hydrolyzed, pre-swollen, or oxidized cellulose raw materials) is carried out using suitable equipment such as refiners, grinders, homogenizers, colloiders, attrition mills, ultrasonicators, fluidizers such as microfluidizers, macro (large)-fluidizers or fluidizer-type homogenizers. Depending on the MFC manufacturing method, the product may also contain fines, or nanocrystalline cellulose, or other chemicals present in wood fibers or papermaking processes. The product may also contain various amounts of micron-sized fiber particles that are not effectively fibrillated.

[0093] MFC is produced from wood cellulose fibers from both hardwood and softwood fibers. It can also be made from microbial sources, crop fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It is preferably made from pulp, including pulp from virgin fibers, such as mechanical, chemical, and / or thermomechanical pulp. It can also be made from broke or recycled paper.

[0094] The middle web layer preferably has a lower grammage than the top and bottom web layers. In some embodiments, the dry basis weight of the middle web layer is in the range of 5 - 60 gsm (grams per square meter), preferably in the range of 10 - 40 gsm, and more preferably in the range of 20 - 40 gsm.

[0095] In some embodiments, the dry basis weight of the formed multi-layer web and multi-layer film is in the range of 45 - 300 gsm, preferably in the range of 50 - 200 gsm, more preferably in the range of 50 - 150 gsm.

[0096] In this text, the present invention is described mainly with reference to embodiments of a multilayer film formed of three web layers. However, it should be understood that the multilayer film may also include additional web layers. Thus, the multilayer film formed may also be formed of three or more web layers, such as three, four, five, six, or seven web layers.

[0097] In some embodiments, the geometric tearing index of the multilayer film is above 7 mN·m 2 / g, preferably above 8.5 mN·m 2 / g, and more preferably above 9.5 mN·m 2 / g. For comparison, a single-layer film made of 100% MFC may typically have a geometric tearing index in the range of 4 - 5.5 mN·m 2 / g.

[0098] In some embodiments, the burst index of the multilayer film is above 1 kPa·m 2 / g, preferably above 1.5 kPa·m 2 / g, and more preferably above 2 kPa·m 2 / g.

[0099] Pinholes are micropores that may appear in the web during the forming process. Examples of the causes of pinholes include, for example, non-uniformities in the pulp suspension formed by flocculation or re-flocculation of fibrils, a rough dewatering fabric, non-uniform pulp distribution on the wire, or too low a basis weight of the web. In some embodiments, as measured according to standard EN13676:2001, the multilayer film contains less than 10 pinholes / m 2 , preferably less than 8 pinholes / m 2 , and more preferably less than 2 pinholes / m 2 . The measurement involves treating the multilayer film with a coloring solution (e.g., dye E131 blue, in ethanol) and examining the surface with a microscope.

[0100] The multilayer film typically exhibits good grease and oil resistance. The grease resistance of the multilayer film is evaluated by the KIT test according to standard ISO16532-2. This 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 the continuous mixture more difficult to withstand. The performance is rated by the highest-numbered solution that does not blacken the sheet after 15 seconds. The highest-numbered solution (the most aggressive) that remains on the paper surface without causing failure is reported as the "kit value" (maximum value of 12). In some embodiments, the KIT value of the multilayer film is at least 8, preferably at least 10, as measured according to standard ISO 16532-2.

[0101] 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 standard ISO 5636 / 6.

[0102] In some embodiments, the multilayer film has an oxygen transmission rate (OTR) of less than 100 cc / m 2 / 24 h / atm, preferably less than 50 cc / m 2 / 24 h / atm, more preferably less than 20 cc / m 2 / 24 h / atm, as measured according to standard ASTM D-3985 at 50% relative humidity and 23 °C.

[0103] The multilayer film preferably has high repulpability. In some embodiments, when tested as a Class II material according to the PTS-RH021 / 97 test method, the multilayer film exhibits less than 30%, preferably less than 20%, and more preferably less than 10% reject (tailings).

[0104] According to a second aspect as shown herein, there is provided a multilayer film comprising MFC, wherein the multilayer film can be obtained by the method of the present invention.

[0105] When coated or laminated with one or more thermoplastic polymer layers, the multilayer film of the present invention is particularly suitable as a thin packaging film. Thus, the multilayer film can preferably be coated or laminated with one or more polymer layers.

[0106] The multilayer film can be provided with polymer layers on one or both sides. The polymer layers may of course interfere with repulpability, but may still be required or desirable in some applications. The polymer layers can be applied, for example, by extrusion coating, film lamination, or dispersion coating.

[0107] The polymer layers can comprise any thermoplastic polymer commonly used in paper or paperboard-based packaging materials, or in particular polymers used in liquid packaging paperboard. Examples include polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polyhydroxyalkanoates (PHA), polylactic acid (PLA), polyglycolic acid (PGA), starch, and cellulose. Polyethylene, especially low-density polyethylene (LDPE) and high-density polyethylene (HDPE), are the most commonly used and versatile polymers in liquid packaging paperboard.

[0108] Thermoplastic polymers are useful because they can be conveniently 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 a preferred embodiment, the polymer layer comprises polyethylene, more preferably LDPE or HDPE.

[0109] The polymer layer may comprise one or more layers formed from the same polymer resin or different polymer resins. In some embodiments, the polymer layer comprises a mixture of two or more different polymer resins. In some embodiments, the polymer layer is a multi-layer structure comprising two or more layers, wherein the first layer is composed of a first polymer resin and the second layer comprises a second polymer resin different from the first polymer resin.

[0110] In some embodiments, the polymer layer is formed by extrusion coating the polymer onto the surface of a multi-layer film. Extrusion coating is a process of applying a molten plastic material to a substrate to form a very thin, smooth and uniform layer. The coating can be formed from the extruded plastic itself, or the molten plastic can be used as an adhesive to laminate a solid plastic film to the substrate. Common plastic resins used for extrusion coating include polyethylene (PE), polypropylene (PP) and polyethylene terephthalate (PET).

[0111] The basis weight of each polymer layer of the multi-layer film is preferably less than 50 g / m 2 . To achieve a continuous and substantially defect-free film, it is typically required that the basis weight of the polymer layer is at least 8 g / m 2 , preferably at least 12 g / m 2 . In some embodiments, the basis weight of the polymer layer is in the range of 8 - 50 g / m 2 , preferably in the range of 12 - 50 g / m 2 .

[0112] Generally speaking, although products, polymers, materials, layers and processes are described in the form of "comprising" various components or steps, products, polymers, materials, layers and processes can also "consist essentially of various components and steps" or "consist of various components and steps".

[0113] Although the present invention has been described with reference to various exemplary embodiments, those skilled in the art will understand that various changes can be made and elements can be replaced with equivalents without departing from the scope of the present invention. In addition, many modifications can be made to adapt a particular situation or material to the teachings of the present invention without departing from the basic scope of the present invention. Therefore, the present invention is not intended to be limited to the specific embodiments disclosed as the best mode for carrying out the present invention, but the present invention will include all embodiments falling within the scope of the appended claims.

[0114] Example

[0115] The tests were carried out on a pilot paper / paperboard machine. A two- or three-layer structure was produced at a speed of 20 m / min. The web layers were formed on separate wire meshes and then combined before pressing and drying. The grease barrier behavior of the resulting structures (flat as well as wrinkled and folded) was evaluated using the standard ASTM F119-82 method with palm kernel oil.

[0116] Example 1

[0117] A three-layer structure was successfully produced using the pilot machine. The bottom web layer was formed from 100% birch pulp with an SR value of 28 and had a dry basis weight of 40 gsm. The middle web layer was formed from 100% MFC and had a dry basis weight of 32 gsm. The top web layer was formed from 100% birch pulp with an SR value of 28 and had a dry basis weight of 60 gsm.

[0118] The solids content after the wire section and before the press section was 20%. After three pressing steps, the solids content was 32%. The adhesion between the different layers was excellent. Surprisingly, no curling was observed in the three-layer structure. For the flat structure, the grease resistance was measured to be 2 - 5 hours, and for the wrinkled and folded structures it was 5 - 6 hours, which was considered good since the web was not coated.

[0119] Example 2 (comparative)

[0120] As a comparative example, the grease barrier behavior of an uncoated commercial three-layer sheet paperboard with a basis weight of 247 gsm was tested using the same method. The grease resistance of both the flat and folded samples was less than 15 minutes.

Claims

1. A method for manufacturing a multi-layer film containing microfibrillated cellulose (MFC) in a papermaking machine, the method comprising the following steps: a) forming a bottom web layer by applying a first pulp suspension onto a bottom web wire, the first pulp suspension comprising at least 50% dry weight of a cellulose-based fiber material having a Schopper-Riegler (SR) value in the range of 18 - 75; b) forming or applying an intermediate web layer formed from a second pulp suspension onto the bottom web layer, the second pulp suspension comprising at least 50% dry weight of MFC having an SR value in the range of 80 - 100; c) applying a top web layer formed from a third pulp suspension onto the intermediate web layer to form a multi-layer web, the third pulp suspension comprising at least 50% dry weight of a cellulose-based fiber material having a Schopper-Riegler (SR) value in the range of 18 - 75; and d) dewatering and optionally drying the formed multi-layer web to obtain a multi-layer film containing MFC, wherein step a) comprises: a1) forming a bottom web layer by applying a first pulp suspension onto a bottom web wire, the first pulp suspension comprising at least 50% dry weight of a cellulose-based fiber material having a Schopper-Riegler (SR) value in the range of 18 - 75; and a2) partially dewatering the bottom web layer; wherein step b) comprises: b1) forming an intermediate web layer by applying a second pulp suspension onto an intermediate web wire, the second pulp suspension comprising at least 50% dry weight of MFC having an SR value in the range of 80 - 100; b2) partially dewatering the intermediate web layer; and b3) applying the partially dewatered intermediate web layer onto the bottom web layer.

2. The method according to claim 1, wherein step b) comprises forming an intermediate web layer by applying the second pulp suspension onto the bottom web layer, the second pulp suspension comprising at least 50% dry weight of MFC having an SR value in the range of 80 - 100.

3. The method according to claim 1, wherein the bottom web layer of step a) and the intermediate web layer of step b) are formed simultaneously using a multi-layer headbox.

4. The method according to any one of claims 1 to 3, wherein step c) comprises: c1) forming a top web layer by applying a third pulp suspension onto a top web wire, the third pulp suspension comprising at least 50% dry weight of a cellulose-based fiber material having a Schopper-Riegler (SR) value in the range of 18 - 75; c2) partially dewatering the top web layer; and c3) applying the partially dewatered top web layer onto the intermediate web layer to form the multi-layer web.

5. The method according to claim 4, wherein the dry solids content of the partially dewatered top web layer is in the range of 1.5 - 8 wt%.

6. The method according to claim 5, wherein the dry solids content of the partially dewatered top web layer is in the range of 2.5 - 6 wt%.

7. The method according to claim 6, wherein the dry solids content of the partially dewatered top web layer is in the range of 3 - 4.5 wt%.

8. The method according to any one of claims 1 to 3, wherein the cellulose-based fibrous material of the first and third pulp suspensions has an SR value in the range of 18 - 70.

9. The method according to any one of claims 1 to 3, wherein the MFC of the second pulp suspension has an SR value in the range of 85 - 98.

10. The method according to any one of claims 1 to 3, wherein the bottom and top web layers exhibit the same or similar shrinkage rates during dewatering or drying.

11. The method according to any one of claims 1 to 3, wherein the bottom and top web layers have the same or similar composition and basis weight.

12. The method according to any one of claims 1 to 3, wherein the dry basis weight of the bottom and top web layers is in the range of 20 - 120 gsm.

13. The method according to claim 12, wherein the dry basis weight of the bottom and top web layers is in the range of 20 - 100 gsm.

14. The method according to claim 13, wherein the dry basis weight of the bottom and top web layers is in the range of 20 - 80 gsm.

15. The method according to any one of claims 1 to 3, wherein the dry basis weight of the intermediate web layer is in the range of 5 - 60 gsm.

16. The method according to claim 15, wherein the dry basis weight of the intermediate web layer is in the range of 10 - 40 gsm.

17. The method according to claim 16, wherein the dry basis weight of the intermediate web layer is in the range of 20 - 40 gsm.

18. The method according to any one of claims 1 to 3, wherein the geometric tearing index of the membrane > 7 mN m 2 / g.

19. The method according to claim 18, wherein the geometric tear index of the film > 8.5 mNm 2 / g.

20. The method according to claim 19, wherein the geometric tearing index of the film > 9.5 mN m 2 / g.

21. The method according to any one of claims 1 to 3, wherein the bursting strength index of the film > 1 kPam 2 / g.

22. The method according to claim 21, wherein the bursting strength index of the film > 1.5 kPam 2 / g.

23. The method according to claim 22, wherein the bursting strength index of the film > 2 kPam 2 / g.