EP4735684A1

EP4735684A1 - A method for producing a microfibrillated cellulose web

Info

Publication number: EP4735684A1
Application number: EP24831153.2A
Authority: EP
Inventors: Otto NYLÉN; Anni KARPPINEN; Kaj Backfolk
Original assignee: Stora Enso Oyj
Current assignee: Stora Enso Oyj
Priority date: 2023-06-30
Filing date: 2024-06-11
Publication date: 2026-05-06
Also published as: SE2350820A1; WO2025003814A1; SE546696C2

Abstract

The present invention relates to a method for producing a microfibrillated cellulose (MFC) web. The method comprises providing an MFC suspension (2) comprising water as suspension medium. A wet MFC web (4) is formed of the MFC suspension by casting on a non-porous support (5). The wet MFC web (4) positioned on the non- porous support (5) is subjected to water removal by means of press dewatering and/or non-contact drying to form a dry MFC web (8). The water removal further comprises at least one induction heating step, wherein each induction heating step comprises heating said wet MFC web (4) by means of an induction heated device (10). The dry MFC web (8) is separated from the non-porous support (5).

Description

A METHOD FOR PRODUCING A MICROFIBRILLATED CELLULOSE WEB

Technical field

The present disclosure relates to a method for producing a microfibrillated cellulose (MFC) web, such as an MFC film, involving web formation by casting on a non- porous support.

Oxygen, grease, water vapor and/or aroma barrier properties are required in many uses of paper and paperboard packaging. However, paper and paperboard substrates do not have these properties inherently. Most commonly barrier characteristics of paper and paperboard substrates are created by adding one or more barrier coatings and/or laminated barrier layers which are based on plastics or other non-renewable materials. The disadvantage with these coatings and barrier layers is their non-renewable raw material basis that can increase the carbon dioxide footprint of the material as well as make the otherwise biodegradable paper or paperboard non-biodegradable and in some cases non-recyclable.

More recently, microfibrillated cellulose (MFC) webs have been developed, in which cellulosic fibrils, provided by fibrillation of cellulose fibers, have been suspended, e.g., in water and thereafter re-organized and re-bonded together to form a web. For example, MFC webs in the form of MFC films, which are dense films with barrier properties, such as oxygen, aroma and grease barrier properties, have been developed. MFC webs, such as MFC films, are recyclable and biodegradable as well as based on renewable raw material.

One approach to produce MFC webs, such as MFC films, is to use a wet laid technique, i.e., to apply a dilute MFC suspension comprising MFC and water as suspension medium on a dewatering wire or membrane in a forming section and dewater it by vacuum, gravitation, capillary dewatering, press dewatering or a combination of these on the wire or membrane followed by drying or liquid evaporation. When this approach is utilized, most of the water of the wet MFC web is removed through the wire or membrane in the forming section. Another approach to produce MFC webs, such as MFC films, is to use a casting method in which a wet MFC web is formed by casting of an MFC suspension comprising MFC and water as suspension medium on a non-porous support, such as a plastic or metal support, and thereafter dewatering and/or drying to remove water from the wet MFC web. This type of casting method has been shown to produce MFC films with very smooth surfaces with good barrier properties, such as oxygen barrier properties and/or water vapor barrier properties. However, when the wet MFC web is positioned on the non-porous support, removal of water from the wet MFC web needs to be substantially performed through the surface of the wet MFC web opposite the non-porous support. Thus, removal of water from the wet MFC web is more complex when the method involving casting on a non-porous support is utilized compared to when the wet laid technique with a dewatering wire or membrane is utilized. In particular, a significantly higher amount of water needs to be removed in the dewatering section (such as by press dewatering) and/or the drying section (such as by evaporation) when casting on a non-porous support is utilized compared to when the wet laid technique is utilized. In order to improve the efficiency of the water removal, when casting on a non-porous support is utilized, it is known to heat the wet MFC web by heating the non-porous support by steam. However, steam heating of the non-porous support is associated with limited heat transfer and, thus, energy waste. Also, steam heating of the non-porous support may be associated with challenges in connection with cross machine direction and/or machine direction moisture profile control. In addition, direct steam heating of the non-porous substrate is sensitive for steam leakages and consequent fluctuation of temperature. Steam heating of the non-porous substrate also creates condensate on the surface of the non-porous substrate, at the opposite side of the product, and condensate layer impairs heat transfer efficiency from steam to the substrate. Due to condensate generation, steam heating is not a practical solution in process configurations where the non-porous substrate is carrying the product on the bottom surface facing downwards and being heated from the top surface facing upwards.

Thus, there is still room for improvements of methods for producing an MFC web involving formation by casting on a non-porous support. Description of the invention

It is an object of the present invention to provide a method for producing an MFC web involving formation by casting on a non-porous support, which method eliminates or alleviates at least some of the disadvantages of the prior art methods.

The above-mentioned object, as well as other objects as will be realized by the skilled person in the light of the present disclosure, are achieved by the various aspects of the present disclosure.

The invention is defined by the appended independent claim. Embodiments are set forth in the appended dependent claims and in the following description.

According to a first aspect illustrated herein, there is provided a method for producing a microfibrillated cellulose (MFC) web, wherein the method comprises the steps of: providing an MFC suspension comprising between 50 weight-% to 100 weight-% MFC based on total dry weight and a suspension medium, wherein the suspension medium comprises water, wherein said MFC suspension has a dry content of 1-30 weight-%; forming a wet MFC web of said MFC suspension by casting on a non-porous casting support, subjecting said wet MFC web positioned on said non-porous casting support to water removal to form a dry MFC web, wherein the water removal comprises press dewatering of said wet MFC web and/or non-contact drying of said wet MFC web, wherein the water removal further comprises at least one induction heating step, wherein each induction heating step comprises heating said wet MFC web by means of an induction heated device, and separating said dry MFC web from said non-porous casting support.

The method according to the first aspect enables production of an MFC web involving casting on a non-porous support, wherein the water removal comprises press dewatering and/or non-contact drying and wherein the efficiency of the water removal is improved by heating the wet MFC web in at least one induction heating step. It has surprisingly been found that by utilization of heating by an induction heated device (e.g., part of the non-porous support or an induction heated roll) an efficient heating of the wet MFC web may be obtained, i.e., an efficient heat transfer to the wet MFC web is obtained. Compared to heating of the non-porous support with steam heating, it has been found that heating by an induction heated device is associated with an improved heating efficiency and less waste of energy. Also, the heating and temperature profile of the wet MFC web may be faster and more accurately controlled when utilizing heating by an induction heated device compared to steam heating.

Microfibri Hated cellulose (MFC) shall in the context of this patent application mean a cellulose particle, fiber or fibril having a width or diameter of from 20 nm to 1000 nm.

Various methods exist to make MFC, such as single or multiple pass refining, prehydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment steps is usually required in order to make MFC manufacturing both energy efficient and sustainable. The cellulose fibers of the pulp used when producing MFC may thus be native or pre-treated enzymatically or chemically, for example to reduce the quantity of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, wherein the cellulose molecules contain functional groups other (or more) than found in the original cellulose. Such groups include, among others, carboxymethyl (CM), aldehyde and/or carboxyl groups (cellulose obtained by oxidation, for example 2, 2', 6,6'- tetramethylpiperidin-N-oxyl (TEMPO) mediated oxidation), or quaternary ammonium (cationic cellulose). After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC.

MFC can be produced from wood cellulose fibers, both from hardwood and/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, including pulp from virgin fiber, e.g., mechanical, chemical and/or thermomechanical pulps. It can also be made from broke or recycled paper.

The term film as used herein refers generally to a thin continuous sheet formed material, such as a thin substrate with good gas, aroma and/or grease or oil barrier properties, e.g., oxygen barrier properties and/or water vapor barrier properties. Depending on the composition of the MFC suspension from which it is formed, the MFC film can also be considered as a thin paper (e.g., nanopaper or micropaper) or even as a membrane.

As mentioned above, the MFC suspension used in the method of the first aspect comprises between 50 weight-% to 100 weight-% MFC based on total dry weight. In some embodiments, the MFC suspension comprises between 60 weight-% to 100 weight-%, preferably between 70 weight-% to 100 weight-%, more preferably between 80 weight-% to 100 weight-% of MFC, based on total dry weight.

The MFC suspension used in the method of the first aspect has a dry content of 1-30 weight-%. In some embodiments, the MFC suspension has a dry content of 1-25 weight-%, such as 1-20 weight-% or 1-15 weight-%. Preferably, the MFC suspension has a dry content of 2-25 weight-%, such as 2-20 weight-% or 2-15 weight-% or 3-8 weight-%.

The MFC suspension used in the method of the first aspect comprises a suspension medium, which is water. Thus, the MFC suspension is an aqueous suspension.

The microfibrillated cellulose of the MFC suspension used in the method of the first aspect may comprise one or more fractions of microfibrillated cellulose. In some embodiments, the microfibrillated cellulose of the MFC suspension comprises one fraction of microfibrillated cellulose of a fine grade. In some embodiments, the microfibrillated cellulose of the MFC suspension comprises two or more fractions of microfibrillated cellulose of different fine grades. In some embodiments, the microfibrillated cellulose of the MFC suspension comprises one fraction of a fine grade and one fraction of a coarse grade, wherein the coarse grade for example may be an additive. Coarse MFC in this case has typically a Schopper-Riegler value of 80-100 SR°, such as 80-99 SR° or 90-99 SR° or 95-99 SR°, whereas fine MFC is fibrillated so measurement of the Schopper-Riegler value is not possible (theoretical value about or above 100 SR°) as determined by standard ISO 5267-1 .

In some embodiments, the MFC suspension used in the method of the first aspect comprises one or more further cellulose pulp fractions in addition to the microfibrillated cellulose, such as e.g. a cellulose pulp fraction having a Schopper- Riegler value of < 70 SR°, such as 15-70 SR° or 25-60 SR°, as determined by standard ISO 5267-1 and/or a further fraction of normal fibers. The MFC suspension may comprise, for example, 1-30 weight-%, more preferably 2-30 weight-%, most preferably 5-30 weight-%, of further cellulose pulp fraction(s), based on the total dry weight of microfibrillated cellulose and further cellulose pulp fraction(s) (i.e., based on the total dry weight of total amount of fibers in the MFC suspension).

By normal fibers is meant normal pulp fibers of a conventional length and fibrillation for papermaking. Normal fibers may include mechanical pulp, thermochemical pulp, pressure groundwood, chemical pulp such as sulphate (kraft) or sulphite pulp, dissolving pulp, recycled fiber, organosolv pulp or chemi-thermomechanical pulp (CTMP), or combinations thereof. The pulp may be bleached or unbleached. The normal fibers can be vegetable fibers, such as wood derived (e.g., hardwood or softwood) or agricultural sources including straw, bamboo, etc.

The normal fibers may have a beating degree, i.e., Schopper-Riegler value, in the range of 15 to 50 SR° or more preferably in the range of 18 to 40 SR° as determined by standard ISO 5267-1 . The normal fibers may preferably be chemical pulp, such as kraft pulp.

The normal fibers may have a mean length in the MFC suspension of 0.5 to 5 mm, preferably 1 mm to 5 mm, more preferably in the range of 2 mm to 4 mm, as determined using a FS5 fiber analyzer (Valmet). Mean fiber length as used herein refers to the mean length-weighted ISO fiber length measured according to the standard ISO 16065-2 using an FS5 fiber analyzer (Valmet).

In some embodiments, the MFC suspension used in the method of the first aspect comprises 1-30 weight-%, preferably 2-30 weight-%, most preferably 5-30 weight-%, of reinforcement fibers based on the total dry weight of microfibrillated cellulose and further cellulose pulp fraction(s) (i.e., based on the total dry weight of total amount of fibers in the MFC suspension), wherein the reinforcement fibers have a mean diameter of >10 pm and a mean length of >1 .5 mm, as determined using a FS5 fiber analyzer (Valmet).

Thus, besides MFC, the prepared MFC suspension may also comprise longer fibers, either hardwood or softwood fibers, preferably kraft pulp softwood fibers. The MFC suspension used in the method of the first aspect may in addition to MFC and optional further pulp fraction(s) comprise any conventional paper making additives or chemicals such as film-forming agents, dispersants, fillers, pigments, wet strength chemicals, cross-linkers, plasticizers, softeners, humectants, adhesion primers, wetting agents, biocides, colorants, de-foaming chemicals, hydrophobizing chemicals such as alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), waxes, rosin resins, mineral additives (fillers) such as bentonite, kaolin, talcum, mica, montmorillonite, organoclays, graphene and graphene oxide, stearate, starch, silica, precipitated calcium carbonate, cationic polysaccharide, rheology modifiers, etc. These additives or chemicals may thus be process chemicals or web performance chemicals added to provide the end product web with specific properties and/or to facilitate production of the web.

In some embodiments, the MFC suspension is free, or at least essentially free of, cationic flocculation and drainage chemicals.

Preferably, the MFC suspension comprises no more than 50 weight-%, more preferably no more than 35 weight-%, most preferably no more than 30 weight-% or no more than 25 weight-%, of additives, based on total dry weight of the MFC suspension. For example, the MFC suspension may comprise 1-50 weight-% or 1-35 weight-% or 1-30 weight-% or 1-25 weight-% of additives, based on total dry weight of the MFC suspension.

In some embodiments, the MFC suspension comprises a water soluble polymer that can form a network, such as a film, and/or improve binding between cellulose fibrils. Typical examples of such polymers are natural gums or polysaccharides or derivatives thereof such as carboxymethylated cellulose (CMC), hemicellulose, starch, or polyvinyl alcohol (PVOH) or derivatives or analogues thereof. The PVOH may be a single type of PVOH, or it can comprise a mixture of two or more types of PVOH, differing, e.g., in degree of hydrolysis or viscosity. The PVOH may, for example, have a degree of hydrolysis in the range of 80-99 mol%, preferably in the range of 88-99 mol%. In some embodiments, the MFC suspension comprises 0-30 weight-% or 0.5-20 weight-% or 3-15 weight-% of one or more humectants and/or plasticizing agents based on total dry weight, such as a sugar alcohol (e.g., sorbitol), glycol, other polyol or a combination thereof.

In some embodiments, the MFC suspension comprises up to 20 weight-% of mineral fillers (regular filler or nanofiller) based on total dry weight, such as bentonite, kaolin, talcum, mica, montmorrillonite, organoclays, silica, graphene, graphene oxide or a combination thereof.

As mentioned above, the method of the first aspect comprises a step of forming a wet MFC web of the MFC suspension by casting on a non-porous casting support.

The term “casting”, when utilized in web-forming, such as film-forming, is a known term designating methods wherein a suspension is deposited by means of contact or non-contact deposition and levelling methods on a support to form a wet web. Examples of such a deposition and levelling method are curtain coating/application, slot die casting, or dosing the MFC suspension with spray or similar device and optionally leveling with, for example, a doctor-blade, rod, air knife or roll.

The non-porous casting support may be a metal (e.g., steel), rubber, plastic or polymer (e.g., polyurethane) support (e.g., belt or roll). The non-porous casting support may be induction-heatable or not. In some embodiments, the non-porous casting support needs to be induction-heatable (i.e., in embodiments in which at least a part of the non-porous casting support is induction heated for the induction heating), whereas in other embodiments the non-porous casting support does not need to be induction-heatable (i.e., in embodiments only using one or more separate induction heated devices for the induction heating). In some embodiments, the non- porous casting support is a metal belt (i.e., a belt made of metal) such as a steel belt, a polymer belt or a coated belt with a permanent or temporary coating such as a polymer coated belt, e.g., a polymer coated steel belt. For example, the belt can be coated with controlled adhesion/releasing agents (e.g., polytetrafluoroethylene). A metal belt may be coated, e.g., with ceramic material. The non-porous casting support may be a continuous or endless non-porous support, such as a conveyor belt. Thus, in some embodiments the non-porous casting support is a continuous or endless metal belt.

The MFC web can be a single or multilayer web, or single or multilayer ply. Thus, the formed wet MFC web may comprise a single wet web layer or two or more wet web layers on top of each other.

As mentioned above, the method of the first aspect comprises a step of subjecting the wet MFC web positioned on the non-porous casting support to water removal to form a dry MFC web. Thus, the wet MFC web is positioned on the non-porous casting support during the water removal. The dry MFC web formed by the water removal has a moisture content of 20 weight-% or less, preferably 10 weight-% or less, more preferably 5 weight-% or less. In some embodiments, the dry MFC web has a moisture content of 1-20 weight-%, preferably 1-10 weight-%, most preferably 1-5 weight-%. The moisture content may be measured under ambient conditions. For example, the moisture content may be measured using spectroscopy methods, such as infra-red (IR) spectroscopy, near infra-red (NIR) spectroscopy or Raman spectroscopy methods, in particular infra-red methods suitable for single side measurement. Alternatively, the dry content may be measured in order to determine the moisture content. For example, the dry content may be measured according to standard ISO 638-2 and the moisture content may be calculated based on the dry content measurement.

The water removal comprises press dewatering and/or non-contact drying the wet MFC web. The press dewatering and the non-contact drying, respectively, may be performed using any method known in the art that are suitable to remove water from the wet MFC web and provide the dry MFC web. The wet MFC web is positioned on the non-porous casting support during the press dewatering and/or non-contact drying.

The press dewatering may be performed in one or more press dewatering steps. Each press dewatering step may comprise application of a press fabric in direct contact or indirect contact (e.g., via a separate membrane) with the wet MFC web positioned on the non-porous casting support and conducting the wet MFC web, arranged between the press fabric and the non-porous casting support, through a pressing equipment to remove water from the wet MFC web by transferring water from the wet MFC web into the press fabric. With press fabric is meant a fabric that is permeable and allows water to be removed from the wet MFC web either by absorbing the water or by allowing the water to be removed through the fabric. The press fabric may be a press felt (dewatering felt). Any known suitable press fabric or press felt may be utilized. With pressing equipment is meant an equipment comprising one or more nip through which the wet MFC web is conducted and thus pressed and dewatered.

The non-contact drying may be performed in one or more non-contact drying steps. Each non-contact drying step may comprise hot gas (or air) impingement drying, microwave drying, ultraviolet drying, electron beam drying, infrared drying, near infrared drying or a combination thereof.

Furthermore, the water removal further comprises heating the wet MFC web in at least one induction heating step, i.e., the water removal comprises the heating of the wet MFC web in at least one induction heating step in addition to the press dewatering and/or non-contact drying. Each induction heating step comprises heating the wet MFC web by means of an induction heated device. The induction heated device may be any suitable induction heated device, such as an induction heated belt or induction heated roll or parts of a belt or roll being induction heated. The temperature of the induction heated device or the surface of the induction heated device arranged to be in direct or indirect contact with the wet MFC web may be set or controlled to a temperature or temperature range suitable for heating of the wet MFC web, such as temperatures between 50-350°C, preferably 60-300°C.

At least one induction heating step of the at least one induction heating step may be performed during at least a part of the press dewatering of the wet MFC web and/or at least one induction heating step of the at least one induction heating step may be performed during at least a part of the non-contact drying of the wet MFC web.

The phrase “an induction heating step performed during at least a part of the press dewatering” is herein intended to refer to the induction heating step being performed when the wet MFC web is press dewatered, i.e., when pressing equipment used for the press dewatering impacts the wet MFC web in order to remove water. The induction heating step may be performed during the complete dewatering or during a part (such as a step or part of a step) of the dewatering.

The phrase “an induction heating step performed during at least a part of the noncontact drying” is herein intended to refer to the induction heating step being performed when the wet MFC web is dried by means of non-contact drying, i.e., when non-contact drying equipment used for the non-contact drying impacts the wet MFC web in order to remove water. The induction heating step may be performed during the complete non-contact drying or during a part (such as a step or part of a step) of the non-contact drying.

In some embodiments, at least one induction heating step is performed during at least a part of the press dewatering. In these embodiments, the at least one induction heating step may be performed such that the wet MFC web is heated by an induction heated device during the complete dewatering or during a part or parts thereof.

In some embodiments, at least one induction heating step is performed during at least a part of the non-contact drying. In these embodiments, the at least one induction heating step may be performed such that the wet MFC web is heated by an induction heated device during the complete non-contact drying or during a part or parts thereof.

In some embodiments, at least one induction heating step is performed during at least a part of the press dewatering and at least one induction heating step is performed during at least a part of the non-contact drying. In these embodiments, the at least one induction heating step performed during at least a part of the press dewatering may be performed such that the wet MFC web is heated by an induction heated device during the complete press dewatering or during a part or parts thereof. Also, in these embodiments, the at least one induction heating step performed during at least a part of the non-contact drying may be performed such that the wet MFC web is heated by an induction heated device during the complete non-contact drying or during a part or parts thereof. In some embodiments, at least one induction heating step of the at least one induction heating step comprises heating the wet MFC web by means of at least a part of the non-porous casting support being induction heated. Thus, in these embodiments the at least a part of the non-porous casting support being induction heated constitutes or forms part of an induction heated device. The at least a part of the non-porous casting support being induction heated may be constituted by at least a surface part of the non-porous casting support being in contact with the wet MFC web during heating of the wet MFC web in the induction heating step. Thus, in these embodiments, the non-porous casting support, or at least a surface thereof, is induction heatable. The non-porous casting support may be a metal casting support, such as a metal casting belt, in these embodiments. Preferably, the non-porous casting support is an endless metal casting belt in these embodiments. The non- porous casting support may be induction heated by any suitable induction heating device such as one or more induction heating coils and/or one or more induction heated rolls positioned in contact with the surface of the non-porous casting support opposite the casting surface and/or one or more guiding rolls being induction heated, which guiding roll(s) is/are arranged for guiding movement of the non-porous casting support. Thus, in these embodiments, a heating of the wet MFC web by one or more parts of the non-porous casting support being induction heated is obtained. Induction heating step(s) comprising heating the wet MFC web by means of at least a part of the non-porous support being induction heated may be performed during at least a part of the press dewatering and/or during at least a part of the non-contact drying in these embodiments. Alternatively or additionally, induction heating step(s) comprising heating the wet MFC web by means of at least a part of the non-porous support being induction heated may be performed before press dewatering and/or between press dewatering steps and/or between press dewatering and non-contact drying and/or between non-contact drying steps and/or after non-contact drying in these embodiments. The one or more parts of the non-porous casting support being induction heated may be provided at such positions of the production line such that an intended temperature profile of the non-porous casting support, and thus of the wet MFC web, is obtained. Also, in these embodiments the induction heating of the parts of the non-porous casting support being induction heated may be controlled so as to control the temperature profile of the wet MFC web and thus, the moisture profile of the wet MFC web. In some embodiments, at least one induction heating step of the at least one induction heating step comprises heating of the wet MFC web by means of an induction heated roll which is provided in indirect or direct contact with the wet MFC web on the side opposite the non-porous casting support during heating of the wet MFC web by means of the induction heated roll. Thus, in these embodiments the induction heated roll constitutes or forms part of an induction heated device. The induction heated device may be a metal roll in these embodiments. Induction heating step(s) comprising heating of the wet MFC web by means of an induction heated roll may be performed during at least a part of the press dewatering and/or during at least a part of the non-contact drying in these embodiments. Alternatively or additionally, induction heating step(s) comprising heating of the wet MFC web by means of an induction heated roll may be performed before press dewatering and/or between press dewatering steps and/or between press dewatering and non-contact drying (such as by a smoothening roll being induction heated) and/or between noncontact drying steps and/or after non-contact drying in these embodiments. For example, induction heating step(s) comprising heating of the wet MFC web by means of an induction heated roll provided in direct contact with the wet MFC web may be performed during at least a part of the non-contact drying. Induction heating step(s) comprising heating of the wet MFC web by means of an induction heated roll provided in indirect contact with the wet MFC web may be performed during at least a part of the the press dewatering, wherein a press felt may be provided between the induction heated roll and the wet MFC.

In some embodiments, at least one induction heating step of the at least induction heating step comprises heating the wet MFC web by means of at least a part of the non-porous support being induction heated and heating of the wet MFC web by means of an induction heated roll which is provided in indirect or direct contact with the wet MFC web on the side opposite the non-porous casting support during heating of the wet MFC web by means of the induction heated roll.

In some embodiments, the water removal comprises the press dewatering, wherein at least one induction heating step of the at least one induction heating step is performed during at least a part of the press dewatering of the wet MFC web and comprises heating the wet MFC web by means of a part of the non-porous casting support being induction heated. In some embodiments, the water removal comprises the press dewatering, wherein at least one induction heating step of the at least one induction heating step is performed during at least a part of the press dewatering of the wet MFC web and comprises heating the wet MFC web by means of an induction heated roll which is provided in indirect or direct contact with the wet MFC web on the side opposite the non-porous casting support during heating of the wet MFC web by means of the induction heated roll.

In some embodiments, the water removal comprises the press dewatering, wherein at least one induction heating step of the at least one induction heating step is performed during at least a part of the press dewatering of the wet MFC web and comprises heating the wet MFC web by means of a part of the non-porous casting support being induction heated and by means of an induction heated roll which is provided in direct or indirect contact with the wet MFC web on the side opposite the non-porous casting support during heating of the wet MFC web by means of the induction heated roll.

In some embodiments, the water removal comprises the non-contact drying, wherein at least one induction heating step of the at least one induction heating step is performed during at least a part of the non-contact drying of the wet MFC web and comprises heating the wet MFC web by means of a part of the non-porous casting support being induction heated.

In some embodiments, the water removal comprises the non-contact drying, wherein at least one induction heating step of the at least one induction heating step is performed during at least a part of the non-contact drying of the wet MFC web and comprises heating the wet MFC web by means of an induction heated roll which is provided in direct contact with the wet MFC web on the side opposite the non-porous casting support during heating of the wet MFC web by means of the induction heated roll.

In some embodiments, the water removal comprises the non-contact drying, wherein at least one induction heating step of the at least one induction heating step is performed during at least a part of the non-contact drying of the wet MFC web and comprises heating the wet MFC web by means of a part of the non-porous casting support being induction heated and by means of an induction heated roll which is provided in direct contact with the wet MFC web on the side opposite the non-porous casting support during heating of the wet MFC web by means of the induction heated roll.

In some embodiments, the water removal comprises the press dewatering, wherein at least one induction heating step of the at least one induction heating step is performed after the press dewatering. The at least one induction heating step performed after the press dewatering may comprise heating the wet MFC web by means of a part of the non-porous casting support being induction heated and/or by means of an induction heated roll which is provided in direct contact with the wet MFC web on the side opposite the non-porous casting support during heating of the wet MFC web by means of the induction heated roll.

In some embodiments, the water removal comprises the non-contact drying, wherein at least one induction heating step of the at least one induction heating step is performed before the non-contact drying and/or wherein at least one induction heating step of the at least one induction heating step is performed after the noncontact drying. Each of the at least one induction heating step performed before the non-contact drying and the at least one induction heating step performed after the non-contact drying may comprise heating the wet MFC web by means of a part of the non-porous casting support being induction heated and/or by means of an induction heated roll which is provided in direct contact with the wet MFC web on the side opposite the non-porous casting support during heating of the wet MFC web by means of the induction heated roll.

As mentioned above, the method of the first aspect comprises a step of separating (such as peeling off) the dry MFC web from the non-porous casting support.

Typically, the dry MFC web is wound onto a core to form a reel of the dry MFC web after separation from the non-porous support. Thus, a reel of a free-standing continuous dry MFC web is then formed. Possibly, a further drying of the dry MFC web may be performed after the release of the dry MFC web from the non-porous casting support but before or in connection with the winding of the dry MFC web onto the core to form the reel. The MFC web produced in the above described embodiments of the method of the present disclosure may be an MFC film. Thus, in these embodiments, a wet MFC film is formed in the forming step and a dry MFC film is formed in the step of subjecting the wet MFC film to water removal.

In some embodiments in which the MFC web is an MFC film, a dry MFC film having an oxygen transmission rate (OTR), measured according to the standard ASTM F1927-20 at 50% relative humidity and 23 °C, of less than 10 cc/m²/24h, preferably less than 7 cc/m²/24h, and more preferably less than 5 cc/m²/24h, is formed.

In some embodiments in which the MFC web is an MFC film, a dry MFC film having a water vapor transmission rate (WVTR), measured according to the standard ASTM F1249-20 at 50% relative humidity and 23 °C, of less than 100 g/m²/24h, preferably less than 50 g/m²/24h, and more preferably less than 20 g/m²/24h, is formed.

In some embodiments in which the MFC web is an MFC film, a dry MFC film having a dry grammage of 10-70 g/m², preferably 10-60 g/m² or 10-50 g/m² or 15-40 g/m², as measured according to ISO 536, is formed.

In some embodiments in which the MFC web is an MFC film, a dry MFC film having an average film thickness of 5-60 pm, preferably 10-50 pm, 15-45 pm or 20-40 pm, is formed. The average film thickness may be defined as an average thickness of the film across the entire width. Thickness of the MFC film may be measured using, as non-limiting examples, white light interferometry, laser profilometry, or optically by cutting a sample in cross-machine directional line (either cast in resin or not) and microscopic imaging (e.g., scanning electron microscopy or other applicable method) of the cut section in thickness direction.

In some embodiments in which the MFC web is an MFC film, a dry MFC film having a width of 0.3-4 m, preferably 0.5-4 m, 1 -4 m or 2-4 m, is formed.

In some embodiments in which the MFC web is an MFC film, a dry MFC film having a density of 700-1500 kg/m³, preferably 800-1500 kg/m³, most preferably 900-1500 kg/m³, as measured according to ISO 534:2011 , is formed. In some embodiments the MFC web produced in the method of the present disclosure is an absorbent layer, a membrane, a separation medium or a prefabricate.

A free-standing MFC film provided by the method of the present disclosure may be applied to the surface of any one of a paper product and a paperboard product so as to form a laminate, such as a paper or paper-based packaging material laminate.

Paper generally refers to a material manufactured in thin sheets from the pulp of wood or other fibrous substances comprising cellulose fibers, used for writing, drawing, or printing on, or as packaging material.

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

A free-standing MFC film provided by the method of the present disclosure may be utilized in a laminate together with one or more polymer layers, such as termoplastic polymer layers. For example, the one or more additional polymer layers may be constituted by any suitable polyolefin or polyester. The additional polymer layer(s) can be provided e.g. by extrusion coating, film coating or lamination 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 (PHA) and polybutylene succinate (PBS).

A free-standing MFC film provided by the method of the present disclosure may be used as a packaging material or in a packaging material, such as a food or liquid packaging material, and may be incorporated into any type of package, such as a box, bag, a wrapping film, cup, container, tray, bottle etc.

According to a second aspect illustrated herein, there is provided an MFC web such as an MFC film obtainable by the method of the first aspect. The MFC web according to the second aspect may be further defined as set out above with reference to the method of the first aspect.

Brief description of the drawings

In the following, the invention will be further illustrated by description of exemplified embodiments with reference to the accompanying drawings, wherein:

Fig. 1 shows a schematic overview of a first embodiment of the method according to the present disclosure;

Fig. 2 shows a schematic overview of a second embodiment of the method according to the present disclosure;

Fig. 3 shows a schematic overview of a third embodiment of the method according to the present disclosure;

Fig. 4 shows a schematic overview of a fourth embodiment of the method according to the present disclosure;

Fig. 5 shows a schematic overview of a fifth embodiment of the method according to the present disclosure;

Fig. 6 shows a schematic overview of a sixth embodiment of the method according to the present disclosure, and

Fig. 7 shows a schematic overview of a seventh embodiment of the method according to the present disclosure;

Detailed description of the drawings

Fig. 1 shows a schematic overview of a first embodiment of the method according to the first aspect of the present disclosure performed in a system 1 for MFC web production. In the first embodiment illustrated in Fig. 1 , an MFC suspension 2 is provided to a casting unit 3, such as a slot die applicator. The MFC suspension 2 comprises between 50 weight-% to 100 weight-% MFC based on total dry weight and has a dry content of 1-30 weight-%. The MFC suspension 2 comprises water as suspension medium. Optionally, one or more additives (e.g., paper making additives and/or chemicals) and/or one or more other suitable components (e.g., further pulp fraction(s)) may also be comprised in the MFC suspension 2. A wet MFC web 4 is formed of the MFC suspension 2 by casting a layer of the MFC suspension 2 by the casting unit 3 on a non-porous casting support in the form of an endless metal belt 5. In the first embodiment illustrated in Fig. 1 , the wet MFC web 4 positioned on the metal belt 5 is subjected to water removal comprising press dewatering in a press dewatering device 6, and drying in a non-contact drying device 7, to form a dry MFC web 8. Furthermore, in the first embodiment illustrated in Fig. 1 , the water removal comprises further an induction heating step performed during the press dewatering and an induction heating step performed during the non-contact drying. The induction heating step performed during the press dewatering and the non-contact drying, respectively, comprises heating the wet MFC web 4 by means of a respective part 10a of the metal belt 5 being induction heated by an induction heating device 9. The part 10a of the metal belt 5 being induction heated forms an induction heated device 10. After non-contact drying, the dry MFC web 8 is separated from the metal belt 5 and wound onto a core to form a reel 11 of the dry MFC web 8.

Fig. 2 shows a schematic overview of a second embodiment of the method according to the first aspect of the present disclosure. The second embodiment differs from the first embodiment in the inductivon heating steps. In the second embodiment illustrated in Fig. 2, the water removal comprises two induction heating steps performed during a respective part of the press dewatering. Each induction heating step comprises heating the wet MFC web 4 by means of a respective part 10a of the metal belt 5 being induction heated by an induction heating device 9. Each part 10a of the metal belt 5 being induction heated forms an induction heated device 10.

Fig. 3 shows a schematic overview of a third embodiment of the method according to the first aspect of the present disclosure. The third embodiment differs from the first embodiment in the induction heating steps. In the third embodiment illustrated in Fig. 3, the water removal comprises one induction heating step performed during the non-contact drying. The induction heating step performed during the non-contact drying comprises heating the wet MFC web 4 by means of an induction heated roll 10b being induction heated by an induction heating device 9. Thus, the induction heated roll 10b forms an induction heated device 10. In the embodiment shown in Fig. 3 the induction heated roll 10b is provided in direct contact with the wet MFC web 4.

Fig. 4 shows a schematic overview of a fourth embodiment of the method according to the first aspect of the present disclosure. The fourth embodiment differs from the first embodiment in the induction heating steps. In the fourth embodiment illustrated in Fig. 4, the water removal comprises one induction heating step performed during the press dewatering and one induction heating step performed during the noncontact drying. The induction heating step performed during the press dewatering and the non-contact drying, respectively, comprises heating the wet MFC web 4 by means of a respective part 10a of the metal belt 5 being induction heated by an induction heating device 9. Each part 10a of the metal belt 5 being induction heated forms an induction heated device 10. In addition, the induction heating step performed during the non-contact drying comprises heating the wet MFC web 4 by means of an induction heated roll 10b being induction heated by an induction heating device 9. Thus, the induction heated roll 10b forms an induction heated device 10. In the embodiment shown in Fig. 4 the induction heated roll 10b is provided in direct contact with the wet MFC web 4.

Fig. 5 shows a schematic overview of a fifth embodiment of the method according to the first aspect of the present disclosure. The fifth embodiment differs from the fourth embodiment in the induction heating steps. In the fifth embodiment illustrated in Fig. 5, the water removal comprises one induction heating step performed during the press dewatering and one induction heating step performed during the non-contact drying. The induction heating step performed during the press dewatering and the non-contact drying, respectively, comprises heating the wet MFC web 4 by means of a respective part 10a of the metal belt 5 being induction heated by an induction heating device 9. Each part 10a of the metal belt 5 being induction heated forms an induction heated device 10. In addition, the induction heating steps performed during the press dewatering and the non-contact drying, respectively, comprise heating the wet MFC web 4 by means of an induction heated roll 10b being induction heated by an induction heating device 9. Thus, the induction heated roll 10b forms an induction heated device 10. In the embodiment shown in Fig. 5 the induction heated roll 10b used during non-contact drying is provided in direct contact with the wet MFC web 4, whereas the induction heated roll 10b used during the press dewatering is provided in contact with a press felt 12 used in the press dewatering and, thus, provided in indirect contact with the wet MFC web 4.

Fig. 6 shows a schematic overview of a sixth embodiment of the method according to the first aspect of the present disclosure. The sixth embodiment differs from the third embodiment illustrated in Fig. 3 in the water removal. In the sixth embodiment illustrated in Fig. 6, the non-contact drying is omitted. Thus, the water removal comprises press dewatering in the press dewatering device 6 and one induction heating step performed after the press dewatering. The induction heating step performed after the press dewatering comprises heating the wet MFC web 4 by means of an induction heated roll 10b being induction heated by an induction heating device 9. Thus, the induction heated roll 10b forms an induction heated device 10. In the embodiment shown in Fig. 6 the induction heated roll 10b is provided in direct contact with the wet MFC web 4.

Fig. 7 shows a schematic overview of a seventh embodiment of the method according to the first aspect of the present disclosure. The seventh embodiment differs from the first embodiment illustrated in Fig. 1 in the water remvoal. In the seventh embodiment illustrated in Fig. 7, the press dewatering and the induction heating step during the non-contact drying are omitted. Thus, the water removal comprises non-contact drying by the non-contact drying device 7 and one induction heating step performed before the non-contact drying. The induction heating step performed before the non-contact drying comprises heating the wet MFC web 4 by means of a part 10a of the metal belt 5 being induction heated by an induction heating device 9. The part 10a of the metal belt 5 being induction heated forms an induction heated device 10.

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

In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.

Claims

1 . A method for producing a microfibrillated cellulose (MFC) web, wherein the method comprises the steps of: providing an MFC suspension (2) comprising between 50 weight-% to 100 weight-% MFC based on total dry weight and a suspension medium, wherein the suspension medium comprises water, wherein said MFC suspension (2) has a dry content of 1-30 weight-%; forming a wet MFC web (4) of said MFC suspension (2) by casting on a non- porous casting support (5); subjecting said wet MFC web (4) positioned on said non-porous casting support (5) to water removal to form a dry MFC web (8), wherein the water removal comprises press dewatering of said wet MFC web (4) and/or noncontact drying of said wet MFC web (4), wherein the water removal further comprises at least one induction heating step, wherein each induction heating step comprises heating said wet MFC web (4) by means of an induction heated device (10, 10a, 10b), and separating said dry MFC web (8) from said non-porous casting support (5).

2. The method according to claim 1 , wherein at least one induction heating step of said at least one induction heating step comprises heating said wet MFC web (4) by means of at least a part (10a) of said non-porous casting support (5) being induction heated.

3. The method according to claim 1 or 2, wherein at least one induction heating step of said at least one induction heating step comprises heating said wet MFC web (4) by means of an induction heated roll (10b) which is provided in indirect or direct contact with said wet MFC web (4) on the side opposite the non-porous casting support (5) during heating of said wet MFC web (4) by means of the induction heated roll (10b).

4. The method according to any one of the preceding claims, wherein at least one induction heating step of said at least one induction heating step comprises heating said wet MFC web (4) by means of at least a part of said non-porous casting support (5) being induction heated and heating said wet MFC web (4) by means of an induction heated roll (10b) which is provided in indirect or direct contact with said wet MFC web (4) on the side opposite the non-porous casting support (5) during heating of said wet MFC web (4) by means of the induction heated roll (10b).

5. The method according to any one of the preceding claims, wherein at least one induction heating step of said at least one induction heating step is performed during at least a part of said press dewatering of said wet MFC web (4) and/or at least one induction heating step of said at least one induction heating step is performed during at least a part of said non-contact drying of said wet MFC web (4).

6. The method according to any one of the preceding claims, wherein said water removal comprises said press dewatering, wherein at least one induction heating step of said at least one induction heating step is performed during at least a part of said press dewatering of said wet MFC web (4) and comprises heating said wet MFC web (4) by means of a part (10a) of said non-porous casting support (5) being induction heated.

7. The method according to any one of the preceding claims, wherein said water removal comprises said press dewatering, wherein at least one induction heating step of said at least one induction heating step is performed during at least a part of said press dewatering of said wet MFC web (4) and comprises heating said wet MFC web (4) by means of an induction heated roll (10b) which is provided in indirect or direct contact with said wet MFC web (4) on the side opposite the non-porous casting support (5) during heating of said wet MFC web (4) by means of the induction heated roll (10b).

8. The method according to any one of the preceding claims, wherein said water removal comprises said press dewatering, wherein at least one induction heating step of said at least one induction heating step is performed during at least a part of said press dewatering of said wet MFC web (4) and comprises heating said wet MFC web (4) by means of a part (10a) of said non-porous casting support (5) being induction heated and by means of an induction heated roll (10b) which is provided in direct or indirect contact with said wet MFC web (4) on the side opposite the non-porous casting support (5) during heating of said wet MFC web (4) by means of the induction heated roll (10b).

9. The method according to any one of the preceding claims, wherein said water removal comprises said non-contact drying, wherein at least one induction heating step of said at least one induction heating step is performed during at least a part of said non-contact drying of said wet MFC web (4) and comprises heating said wet MFC web (4) by means of a part (10a) of said non-porous casting support (5) being induction heated.

10. The method according to any one of the preceding claims, wherein said water removal comprises said non-contact drying, wherein at least one induction heating step of said at least one induction heating step is performed during at least a part of said non-contact drying of said wet MFC web (4) and comprises heating said wet MFC web (4) by means of an induction heated roll (10) which is provided in direct contact with said wet MFC web (4) on the side opposite the non-porous casting support (5) during heating of said wet MFC web (4) by means of the induction heated roll (10b).

11 . The method according to any one of the preceding claims, wherein said water removal comprises said non-contact drying, wherein at least one induction heating step of said at least one induction heating step is performed during at least a part of said non-contact drying of said wet MFC web (4) and comprises heating said wet MFC web (4) by means of a part of said non- porous casting support (5) being induction heated and by means of an induction heated roll (10b) which is provided in direct contact with said wet MFC web (4) on the side opposite the non-porous casting support (5) during heating of said wet MFC web (4) by means of the induction heated roll (10b).

12. The method according to any one of the preceding claims, wherein said water removal comprises said press dewatering, wherein at least one induction heating step of said at least one induction heating step is performed after said press dewatering.

13. The method according to claim 12, wherein said at least one induction heating step performed after said press dewatering comprises heating said wet MFC web (4) by means of a part of said non-porous casting support (5) being induction heated and/or by means of an induction heated roll (10b) which is provided in direct contact with said wet MFC web (4) on the side opposite the non-porous casting support (5) during heating of said wet MFC web (4) by means of the induction heated roll (10b).

14. The method according to any one of the preceding claims, wherein said water removal comprises said non-contact drying, wherein at least one induction heating step of said at least one induction heating step is performed before said non-contact drying and/or wherein at least one induction heating step of said at least one induction heating step is performed after said non-contact drying.

15. The method according to claim 14, wherein each of said at least one induction heating step performed before said non-contact drying and said at least one induction heating step performed after said non-contact drying comprises heating said wet MFC web (4) by means of a part of said non- porous casting support (5) being induction heated and/or by means of an induction heated roll (10b) which is provided in direct contact with said wet MFC web (4) on the side opposite the non-porous casting support (5) during heating of said wet MFC web (4) by means of the induction heated roll (10b).

16. The method according to any one of the preceding claims, wherein said non- porous casting support (5) is a metal belt.

17. The method according to any one of the preceding claims, wherein said press dewatering comprises at least one press dewatering step, wherein each press dewatering step comprises applying a press fabric (12) into direct or indirect contact with said wet MFC web (4) positioned on said non-porous casting support (5) and conducting said wet MFC web (4), arranged between said press fabric (12) and said non-porous casting support (5), through a pressing device (6) to remove water from said wet MFC web (4) by transferring water from said wet MFC web (4) into the press fabric (12).

18. The method according to any one of the preceding claims, wherein said noncontact drying comprises at least one non-contact drying step, wherein each non-contact drying step comprises hot gas impingement drying, microwave drying, ultraviolet drying, electron beam drying, infrared drying, near infrared drying or a combination thereof.

19. The method according to any one of the preceding claims, wherein said formed wet MFC web (4) comprises a single web layer or two or more web layers formed on top of each other.

20. The method according to any one of the preceding claims, wherein said MFC web is an MFC film.

21 . The method according to claim 20, wherein said dry MFC film (8) has an average thickness of 5-60 pm and a dry grammage of 2-70 g/m² as measured according to ISO 536.

22. An MFC web obtainable by the method as claimed in any one of claims 1 -21 .