JP2023501001A - Crosslinked MFC - Google Patents

Abstract

1. A method for producing a crosslinked film comprising microfibrillated cellulose (MFC), comprising: a) a film formed from an aqueous suspension comprising MFC and having a moisture content of less than 15%, such as less than 10% A step of applying an aqueous composition comprising a cross-linking agent to the film, provided that when the cross-linking agent is a metal ion, the metal ion is a divalent metal whose concentration in the aqueous composition is less than 50 mM and b) at least 50% of the water absorbed by the film after application of step a) is A method is provided comprising drying the film from step a) such that it is removed within 5 minutes of starting a).

Description

The present disclosure relates to the production of films comprising microfibrillated cellulose (MFC).

Recent advances in science and technology have created an awareness of the environment and shifted the focus of society and industry toward environmentally friendly products and sustainable processes. This new approach was further strengthened by the shortage of petroleum reserves, which provided an incentive to replace petroleum-based polymeric materials with renewable and biodegradable materials. On the other hand, given the fact that petroleum-based polymer materials have established themselves in the market because of their superior material properties and ease of processing, the complete replacement of these materials with their natural counterparts is unlikely. It is expected that this will not be an easy task.

Good barrier properties and processability, along with resistance to different environments and transparency, are just a few of the desirable qualities of commonly used packaging materials, and petroleum-based polymer materials actually provide these qualities to a satisfactory degree. provide degree.

Cellulosic materials have received considerable attention due to their strength and stiffness coupled with low weight, biodegradability and renewability, in addition to being one of the most abundant biopolymers on earth. One of the promising new material streams for cellulosic materials is the production and use of microfibrillated cellulose (MFC) prepared from woody biomass. MFCs have desirable properties for applications such as packaging, such as the ability to form strong, transparent barrier films. A key feature of MFC is its hydrophilicity, which is very advantageous for processing in aqueous media. However, this affinity of MFC for water is also the reason why its barrier properties at high humidity are very poor.

Proposals on how to overcome this problem have already been presented, for example in EP 3333316, in which a fibrillated fiber sheet is soaked in a solution containing ions for several hours and the sheet is washed for several hours. By alternating and drying for several days, the fibers are cross-linked with a metal having a valence of 2 or higher to produce a transparent sheet containing ultrafine cellulose fibers with substituents introduced under high humidity conditions.

Similarly, JP 2016-210830 uses cellulose nanofibers oxidized with TEMPO to form ionically crosslinked films to withstand oxygen at high relative humidity (RH). . Crosslinking is done by soaking in an ionic solution for several hours, after which the film is washed for several hours and dried for several days.

JP 2016-089307 discloses a method of coating paper made of cellulose with an acid-form cellulose nanofiber dispersion crosslinked with a polyvalent metal compound. Crosslinking is taught to occur by soaking and washing for more than 35 minutes. After that, the paper is laminated with a heat-seal layer and evaluated as being suitable for a paper container that can be used under hot water treatment.

EP 2371892 discloses a laboratory method of spraying a cross-linking solution onto a nanocellulose film to cross-link it, the film being very thin (less than 1 μm) and the concentration of cross-linking agent being relatively high. .

Thus, industrial feasibility is an issue as existing processes are either very time consuming or difficult to combine with obtaining oxygen barrier properties even at high humidity.

An object of the present disclosure is to provide a method for manufacturing an MFC-based film with improved oxygen barrier properties, which method enables high productivity in full-scale production of the film on a paper machine.

To meet this objective, a method of making a crosslinked film comprising microfibrillated cellulose (MFC) comprising:
a) applying an aqueous composition comprising a cross-linking agent to a film comprising MFC, wherein the water content of the film is less than 15%, such as less than 10%, such as less than 8%;
provided, however, that when the cross-linking agent is a metal ion, the metal ion is a divalent metal ion whose concentration in the aqueous composition is less than 100 mM, such as less than 50 mM; and b) the application of step a) drying the film from step a) such that at least 50% of the water subsequently absorbed by the film is removed within 5 minutes of performing step a) or within 5 minutes of starting step a) A method is provided comprising the steps of:

When the crosslinker is a metal ion, the MFC preferably contains chargeable moieties and has a charge density of 500-1,800 μeq/g, measured according to SCAN-CM65:02.

The drying in step b) is preferably carried out by means of heated cylinders, such as steam-heated cylinders, and/or non-contact drying, such as by means of hot air and/or infrared radiation.

The reason for applying the crosslinker to the "dry" MFC film is that such films have regions of dense/crystalline MFC structure separated by regions of low density/amorphous MFC that are more accessible to the crosslinker. because it contains Without wishing to be bound by any scientific theory, we believe that the cross-linking agent cross-links regions of the low-density/amorphous MFC in the relatively short period of time provided by the above method, thereby migrating them. I'm thinking of lowering it. Furthermore, the inventors believe that the relatively short total time of steps a) and b) allows the cross-linking action to be concentrated in areas where it is needed most.

Also provided is a crosslinked film comprising MFC obtained by this method, exhibiting an oxygen permeability (OP) of less than 500 mlμmm ⁻² d ⁻¹ bar ⁻¹ at 80% RH according to standards ASTM D3985 and F1927. In this film, the cross-linking agent may be a divalent metal ion such as an ion selected from the group consisting of Zn2 ⁺ , ^{Ca2 +} , Cu2 ⁺ and Mg2+ and the MFC used to manufacture the film. may have a charge density of 500-1,800 μeq/g measured according to SCAN-CM65:02.

Description The present disclosure provides a method for making a crosslinked film comprising microfibrillated cellulose (MFC) comprising the steps of:
a) applying an aqueous composition comprising a cross-linking agent to a film formed from an aqueous suspension comprising MFC, wherein said film has a water content of less than 15%, such as less than 10%, such as less than 8%; Yes, provided that when the cross-linking agent is a metal ion, the metal ion is a divalent metal ion with a concentration of less than 50 mM in the aqueous composition, and the charge density of the MFC measured according to SCAN-CM65:02 is between 500 and is 1,800 μeq/g; and

b) drying the film from step a) such that at least 50% of the water absorbed by the film after application of step a) is removed within 5 minutes from the start of step a);
to provide a method comprising: In a variant, step b) is instead performed within 5 minutes of the start of step a), for example within 3 minutes of the start of step a), such that the moisture content of the film is less than 30%, such as less than 25%. Second, drying the film from step a).

The divalent metal ion concentration in the aqueous composition is less than 50 mM, typically greater than 0.1 mM, such as greater than 0.1 mM and less than 50 mM, such as 0.5-45 mM. A relatively low concentration is advantageous because if the concentration is too high, the fibrils may cross-link to each other too tightly, compromising the barrier. The divalent cations are preferably selected from the group consisting of Zn2 ⁺ , Ca2 ⁺ , Cu2 ⁺ and Mg2 ⁺ . Zn ²⁺ and Ca ²⁺ are particularly preferred. Ca ²⁺ can be considered the most preferred divalent cation because it is relatively small and penetrates amorphous regions better than larger ^ions .

When the crosslinker is a divalent metal ion, the charge density of the MFC measured according to SCAN-CM65:02 is 500-1,800 μeq/g, such as 500-1,000 μeq/g. Since the charge density is measured on chemically modified fibers and it is the total charge density that is measured, the charge density of the fiber before fibrillation is the charge density of the fibrils produced by fibrillating said fiber. Same as density. The charge density of the MFC in step a) arises from the chargeable moieties present on the MFC. The chargeable moieties are selected from carboxy, carboxymethyl, carboxyalkyl, sulfonyl, sulfoethyl and phosphoryl groups.

Chemical modifications that introduce chargeable moieties are preferably preceded by the introduction of aldehydes, for example by periodate oxidation, followed by TEMPO oxidation, alkoxylation, phosphorylation, sulfonation, sulfoethylation and chlorite oxidation. selected from the group consisting of Alkoxylation is preferably carboxymethylation.

Alternatively, the MFC of step a) comprises a quaternary amine. The cross-linking agent in such cases may be a polyvalent anion such as a phosphate ion or a polycarboxylate ion. Introduction of the quaternary amine is preferably via a compound containing both a group that reacts with a hydroxyl group to form a covalent bond and a quaternary ammonium group. Introduction may also occur via a compound that contains both a group that reacts with a hydroxyl group to form a covalent bond and a group that can further react to attach an amine. Preferably, the groups reactive with hydroxyl groups are selected from either epoxies, halohydrins capable of forming epoxies, active halogens, isocyanates, active vinyls or methylols. Examples of compounds having groups that react with hydroxyl groups to form covalent bonds and quaternary ammonium groups are 2,3-epoxypropyltrimethylammonium chloride (EPTMAC), chlorocholine chloride (ClChCl), glycidyltrimethylammonium chloride. and 3-chloro-2-hydroxypropyltrimethylammonium chloride. Crosslinking cationic amine-functional MFCs would be advantageous because such functionalization chemistry is readily available on an industrial scale.

In addition to the ionically crosslinked MFC, the film may further comprise nanofillers, for example in an amount of 1-20% by weight. Nanofillers are particles characterized by high surface areas and high aspect ratios. A high surface area and high aspect ratio can be beneficial in barrier applications as these particles can make it more difficult for gas molecules to diffuse into the coating layer, thus improving barrier properties. . Said nanofillers are preferably selected from bentonite, kaolin or montmorillonite. Preferably, the cross-linking agent is not glutaraldehyde when the film contains nanofillers. Rather, Zn ²⁺ at a concentration of 0.1-5 mM or Ca ²⁺ at a concentration of 0.1-50 mM is preferred.

Cross-linking agents may form covalent bonds. In such cases the cross-linking agent is preferably selected from the group consisting of borax, glutaraldehyde, citric acid or polycarboxylic acids. Glutaraldehyde may be used with a catalyst such as zinc nitrate, and MFCs typically have a low charge, eg, charge densities of 10-200 μeq/g, eg, 10-100 μeq, measured according to SCAN-CM65:02. /g.

The method is suitably practiced on a full-scale paper machine, ie a paper machine operating at a speed of at least 300 m/min and having a trim width of at least 1,500 mm, such as at least 3,000 mm. As a result, the time available to complete each step of the process is very limited.

Steps a) and b) are preferably carried out in or after the drying section of the paper machine. A press section is usually arranged upstream of the drying section of the paper machine. A forming section such as a wire section is usually arranged upstream of the press section of the paper machine.

Step a) may be performed by a size press or a film press. Alternatively, step a) comprises spraying the aqueous composition onto the film.

The aqueous composition, if applied by size press or film press, is pressed according to Brookfield instructions at 100 rpm and 25° C. with spindle no. It typically has a viscosity of from 10 to 1,000 mPas, preferably from 10 to 300 mPas, when measured as dynamic viscosity with a Brookfield rotational viscometer using No. 4.

According to another embodiment, a curtain coater or direct rod coater is used to apply the aqueous composition. In such cases, the viscosity of the aqueous composition is determined by spindle no. It is typically 100-800 mPas when measured as dynamic viscosity with a Brookfield rotational viscometer using No. 4.

According to yet another embodiment, a blade coater is used to apply the aqueous composition. In such cases, the viscosity of the aqueous composition is determined by spindle no. It is typically 400 to 1,500 mPas when measured as dynamic viscosity with a Brookfield rotational viscometer using No. 4.

To facilitate application (and to obtain the desired viscosity), the aqueous composition may contain polymers such as starch, carboxymethylcellulose (CMC), polyvinyl alcohol (PVOH) or MFC. In addition to the polymer, the composition may contain rheology modifiers.

The drying in step b) is preferably carried out by non-contact drying using heated cylinders, such as steam-heated cylinders, and/or preferably hot air and/or infrared radiation.

The method is
i) providing an aqueous suspension comprising MFC; and ii) forming a web from the aqueous suspension, dewatering and drying the web to apply an aqueous composition comprising a cross-linking agent in step a). The step of forming a film may be further included.

In one embodiment, steps i) and ii) are performed on a paper machine, such as the same paper machine used for steps a) and b). Thereby the overall efficiency of the method can be improved. Alternatively, steps a) and b) are performed offline, meaning that they are not performed on the paper machine used for steps i) and ii). Steps a) and b) can even be performed at a different location than steps i) and ii).

Step ii) is preferably carried out in the forming section of the paper machine. The width of the wires of the forming part may be at least 1,500 mm, for example at least 3,000 mm.

In one embodiment, the method further comprises fibrillating the cellulosic fiber to provide the MFC of step i). This step may involve mechanical, enzymatic and/or chemical sub-steps known to those skilled in the art. As discussed further below, the chemical sub-step may introduce chargeable moieties that facilitate cross-linking.

In the context of this disclosure, MFC means nanoscale cellulose particle fibers or fibrils with at least one dimension less than 100 nm. MFC comprises partially or fully fibrillated cellulose or lignocellulose fibers. The free fibril diameter is less than 100 nm, but the actual fibril diameter or particle size distribution and/or aspect ratio (length/width) depend on the raw material source and manufacturing method. The smallest fibrils, called elementary fibrils, are about 2-4 nm in diameter, but aggregated forms of elementary fibrils, also defined as microfibrils, can be produced, for example, by using an extended purification process or a pressure drop disaggregation process. It is generally the main product obtained when manufacturing MFC. Depending on the raw material source and manufacturing process, fibril lengths can vary from about 1 to over 10 microns. Coarse-grained MFCs may contain a significant proportion of fibrillated fibers, i.e. fibrils (cellulose fibers) protruding from tracheids, and a certain amount of fibrils liberated from tracheids.

MFCs include cellulose microfibrils, fibrillated cellulose, nanofibrillated cellulose (NFC), fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibers, cellulose nanofibrils (CNF), cellulose microfibers (CMF), cellulose fibrils, There are different synonyms such as microfibril cellulose, microfibril aggregates and cellulose microfibril aggregates. MFCs are also characterized by various physical or physicochemical properties such as high surface area or their ability to form gel-like materials with low solids content (1-5 wt%) when dispersed in water. can also The cellulose fibers are preferably such that the final specific surface area of the formed MFC is determined according to the BET method (Brunauer, Stephen, Paul Hugh Emmett, and Edward Teller. Adsorption of gases in multimolecular layers. Journal of the American 60 chemical 2 (1938):309-319) to the extent of about 1 to about 200 m ² /g, or more preferably 50 to 200 m ² /g, as determined for the lyophilized material. Nitrogen (N ₂ ) gas adsorption isotherms are recorded using an ASAP 2020 (Micromeritics, USA) apparatus. The measurements were made at liquid nitrogen temperature (ie 77 K) and the BET method was used to obtain the specific surface area of the sample from the isotherm.

Various methods exist for making MFCs, such as single-pass or multi-pass purification, prehydrolysis and subsequent purification, or high shear dissociation or liberation of fibrils. One or more pretreatment steps are usually required to make MFC production energy efficient and sustainable. Thus, the cellulose fibers of the supplied pulp can be pretreated enzymatically or chemically, for example to hydrolyze or swell the fibers, or to reduce the amount of hemicellulose or lignin. Cellulose fibers may be chemically modified prior to fibrillation. Such chemical modification usually makes it easier to defibrate the fibers into MFC or nanofibrillar size or NFC.

Nanofibrillar cellulose may contain some hemicellulose, but the amount depends on factors such as plant origin and pulping process. Mechanical defibration of pretreated fibers, such as hydrolyzed, pre-swollen, or oxidized cellulose feedstocks, can be performed using refiners, grinders, homogenizers, colliders, friction grinders, ultrasonic sonicators, microfluidizers, and the like. It is carried out using a suitable apparatus such as a fluidizer, macrofluidizer type or fluidizer type homogenizer. Depending on the MFC manufacturing method, the product may also contain fines or other chemicals such as those present in the wood fiber or in the papermaking process. The product may also contain varying amounts of micron-sized fiber particles that have not been efficiently fibrillated. MFC is made from wood cellulose fibers derived from both hardwood or softwood fibers. Products can also be made from agricultural fibers such as straw pulp, bamboo, bagasse, or other non-wood fiber sources. The product is preferably made from virgin fiber pulp, such as mechanical, chemical and/or thermomechanical pulp, preferably never dried fiber.

The MFC definition above defines a cellulose nanofiber material comprising a plurality of elementary fibrils with both crystalline and amorphous regions that are 5-30 nm wide and have a high aspect ratio, typically greater than 50. Including, but not limited to, TAPPI standard W13021 proposed for cellulose nanofibrils (CNF).

The amount of MFC in the film is preferably at least 50% by dry weight, such as at least 70% by dry weight, such as at least 80% by dry weight.

The basis weight of the film measured according to ISO 536:2012 is typically 5-100 g/m 2 , such as 5-70 g/m ² , such as 10-70 g/m ² , such as 10-60 g/m ² , such as 20 g/m ² . ~60 g/m ² , such as 30-60 g/m ² , preferably 45-60 g/m ² , more preferably 50-60 g/m ² .

The density of crosslinked films measured according to ISO 534:2011 is typically 0.7-1.4 g/cm ³ , eg 0.8-1.2 g/cm ³ .

The thickness of the crosslinked film measured according to ISO 534:2011 is typically 1-100 μm, such as 5-70 μm, such as 10-50 μm. Having a thickness of a few micrometers allows the cross-linking action of the film to be concentrated in the areas where it is needed most, which is beneficial for barrier properties.

In one embodiment of the method, at least 75% of the water absorbed by the film after application of step a) is removed within 5 minutes from the start of step a).

In another embodiment, 50% of the water absorbed by the film after application of step a) is removed within 3 minutes of starting step a), for example within 2 minutes of starting step a). This increases the efficiency of the method.

Further, the drying of step b) is carried out such that the moisture content of the film is less than 30%, such as less than 25%, within 5 minutes of performing step a), such as within 3 minutes of performing step a). obtain.

The crosslinked film produced by the method of the present disclosure is an oxygen barrier,

It exhibits an oxygen permeability (OP) of less than 500 mlμmm ⁻² d ⁻¹ bar ⁻¹ at 80% RH according to standards ASTM D3985 and F1927.

Crosslinked films are typically crosslinked with divalent metal ions and the charge density of the MFC used to make the films is typically between 500 and 1,800 μeq as measured according to SCAN-CM65:02. /g. Divalent metal ions are typically selected from the group consisting of Zn2 ⁺ , Ca2 ⁺ , Cu2 ⁺ and Mg2 ⁺ .

Accordingly, in one embodiment, a method for making a crosslinked film comprising MFC comprises:
- The fiber is chemically modified by one of the group consisting of TEMPO oxidation, alkoxylation, phosphorylation, sulfonation, sulfoethylation, chlorite oxidation, followed by aldehyde introduction and quaternary amine introduction. selectively modifying the
- fibrillating the fibers into MFCs,
- providing an aqueous suspension comprising MFC;
- forming a web from an aqueous suspension, dewatering and drying the web to form a film;
- applying an aqueous composition comprising a cross-linking agent to a film comprising MFC having a water content of less than 15%, such as less than 10%, provided that when the cross-linking agent is a metal ion, the metal ion is present in the aqueous composition; divalent metal ions at a concentration of less than 50 mM and the charge density of the MFC measured according to SCAN-CM65:02 is between 500 and 1,800 μeq/g;
- drying the film such that at least 50% of the water absorbed by the film after application of the aqueous composition comprising the cross-linking agent is removed within 5 minutes from the start of application of the aqueous composition comprising the cross-linking agent, and - thereby obtaining a crosslinked film comprising MFC,
including.

Example Example 1a
Crosslinking of MFC films made from low-charge MFC

A suspension of low-charge MFC (0.1 wt%; total charge 40 μmol/g) was filtered and then dried (50°C; 4 h) into a film. Films were immersed for 5 seconds in a solution containing either borax or glutaraldehyde to act as a cross-linking agent. The latter chemical was used in combination with zinc nitrate, which acts as a catalyst for the reaction of glutaraldehyde with hydroxyl groups. The film was then dried (50°C; 4 hours). Films with basis weights of 30 g/m ² and 60 g/m ² were produced. No washing was done between soaking and drying.

Example 1b
Crosslinking of MFC films made from low-charge MFC and nanofillers

Using the same procedure as in Example 1a, but replacing 10% by weight of the MFC with Cloisite Na+ (bentonite nanofiller), a crosslinked film comprising nanofiller and MFC was made.

Example 2a
Crosslinking of MFC films made from carboxymethylated MFC

A suspension of carboxymethylated MFC (0.1 wt%; total charge 800 μmol/g) was filtered and then dried (50°C; 4 hours) into a film. The film was immersed for 5 seconds in a solution containing either zinc nitrate hexahydrate, calcium chloride dihydrate, iron chloride hexahydrate, or aluminum chloride hexahydrate, which acted as a cross-linking agent. The film was then dried (50°C; 4 hours). Films with basis weights of 20 g/m ² , 30 g/m ² and 60 g/m ² were produced. No washing was done between soaking and drying.

Example 2b
Using the same procedure as in Example 2a, but replacing 10% by weight of the MFC with Cloisite Na+ (bentonite nanofiller), a crosslinked film comprising nanofiller and MFC was made.

表１は、２価の金属イオンを１００ｍＭ未満の濃度で加えた場合の５０％及び８０％ＲＨでの酸素透過度の低下を示す。これに対して、高濃度（すなわち１００ｍＭ）の３価の金属イオン及び２価の金属イオンは、酸素透過度を低下させることができなかった。８０％ＲＨでは、ナノフィラーを含むフィルムにＣａ^２＋（１又は１０ｍＭ）又はＺｎ^２＋（１ｍＭ）を加えた場合に、最良の酸素透過値が得られた。
また表１は、共有結合を形成する架橋剤を加えたとき、８０％ＲＨで酸素透過度が低下することも示している。
Oxygen Transmission Rate Transmittance (OTR) was measured on 5 cm ² samples using MOCON OX-TRAN 2/21 according to ASTM D3985 and ASTM F1927 standards. OTR measurements were made at 23° C. and 50% RH or 80% RH using the same relative humidity on both sides of the sample. Multiplying the OTR by the film thickness (obtained by SEM) gives the oxygen permeability (OP). Table 1 shows the results.

Table 1 shows the decrease in oxygen permeability at 50% and 80% RH when divalent metal ions are added at concentrations less than 100 mM. In contrast, high concentrations (ie, 100 mM) of trivalent and divalent metal ions were unable to reduce oxygen permeability. At 80% RH, the best oxygen permeation values were obtained when Ca ²⁺ (1 or 10 mM) or Zn ²⁺ (1 mM) was added to films containing nanofillers.
Table 1 also shows that oxygen permeability decreases at 80% RH when crosslinkers that form covalent bonds are added.

Claims (21)

A method for making a crosslinked film comprising microfibrillated cellulose (MFC) comprising:
a) applying an aqueous composition comprising a cross-linking agent to a film formed from an aqueous suspension comprising MFC and having a water content of less than 15%, such as less than 10%;
However, when the cross-linking agent is a metal ion, the metal ion is a divalent metal ion having a concentration of less than 50 mM in the aqueous composition, and the charge density of the MFC measured according to SCAN-CM65:02. is between 500 and 1,800 μeq/g; and b) a step such that at least 50% of the water absorbed by the film after application of step a) is removed within 5 minutes from the start of step a) drying the film from a). 2. The method of claim 1, wherein at least 75% of the water absorbed by the film after application of step a) is removed within 5 minutes from the start of step a). 3. According to claim 1 or 2, wherein at least 50% of the water absorbed by the film after application of step a) is removed within 3 minutes from the start of step a), such as within 2 minutes from the start of step a). described method. 4. According to any one of claims 1 to 3, wherein the drying of step b) is carried out such that the moisture content of the film is less than 30%, such as less than 25% within 5 minutes from the start of step a). described method. 5. The method of any one of claims 1-4, wherein the MFC of step a) comprises a chargeable moiety selected from carboxy, carboxymethyl, carboxyalkyl, sulfonyl, sulfoethyl and phosphoryl groups. 6. Any of claims 1-5, wherein the cross-linking agent is a divalent cation, preferably a divalent metal ion, such as an ion selected from the group consisting of Zn ²⁺ , Ca ²⁺ , Cu ²⁺ and Mg ²⁺ . The method according to item 1. 7. The method of claim 6, wherein the film further comprises nanofillers in an amount of, for example, 1-20% by weight. A method according to any one of claims 6-7, wherein the concentration of said divalent ions in said cross-linking solution is 0.1-50 mM, such as 0.1-40 mM, preferably 0.1-30 mM. A method according to any one of claims 1 to 5, wherein said cross-linking agent forms covalent bonds and is preferably selected from the group consisting of borax, glutaraldehyde, citric acid and polycarboxylic acids. A process according to any one of claims 1 to 4, wherein the MFC of step a) comprises a quaternary amine and the crosslinker is a polyvalent anion such as a phosphate ion or a polycarboxylate ion. 11. A method according to any one of claims 1 to 10, wherein the application of step a) is done by film press or size press. 12. A method according to any one of the preceding claims, wherein steps a) and b) are carried out in a paper machine, preferably in or after the drying section of said paper machine. 13. The method of any one of claims 1-12, further comprising:
i) providing said aqueous suspension comprising MFC;
ii) forming a web from said aqueous suspension, dewatering and drying said web to form a film to which said aqueous composition comprising a cross-linking agent is applied in step a);
method including. 14. The method of claim 13, further comprising fibrillating fibers to provide the MFC of step i). The fiber is chemically modified to have a charge density of 500-1,000 μeq/g measured according to SCAN-CM65:02, and the chemical modification is preceded by the introduction of aldehydes, followed by TEMPO oxidation, alkoxylation 15. The method of claim 14, which can be selected from the group consisting of oxidization, phosphorylation, sulfonation, sulfoethylation and chlorite oxidation. A method according to any one of the preceding claims, wherein the film has a basis weight of 5 to 100 g/m ² measured according to ISO 536:2012. A method according to any one of the preceding claims, wherein the thickness of the crosslinked film measured according to ISO 534:2011 is between 1 and 100 µm, such as between 5 and 70 µm, such as between 10 and 50 µm. 18. A method according to any one of the preceding claims, wherein the drying of step b) is performed by heating cylinders, such as steam-heated cylinders, and/or by non-contact drying, such as hot air and/or infrared radiation. 18. A crosslinked film comprising MFC obtained by the method according to any one of claims 1 to 17, said film having a capacity of 500 ml μmm ⁻² d ⁻¹ bar at 80% RH according to said standards ASTM D3985 and F1927. Crosslinked films exhibiting an oxygen permeability (OP) of less than ^-1 . 20. The method of claim 19, wherein the cross-linking agent is a divalent metal ion and the MFC used to make the film has a charge density of 500-1,800 μeq/g as measured according to SCAN-CM65:02. A crosslinked film as described. 21. The crosslinked film of claim 20, wherein said divalent metal ions are selected from the group consisting of Zn2 ⁺ , Ca2 ⁺ , Cu2 ⁺ and Mg2 ⁺ .