TECHNICAL FIELD

[0001] The present disclosure relates to the production of films comprising microfibrillated cellulose (MFC).

BACKGROUND

[0002] Recent advances in science and technology has created an environmental consciousness that shifted the societal and industrial focus towards green products and sustainable processes. This new approach was further fuelled by the scarcity of oil reserves, which gave an incentive to replace oil-based polymeric materials with the renewable and biodegradable materials. On the other hand, considering the fact that it was the excellent material properties and good processability of oil-based polymeric materials that established their market position, it is foreseeable that completely replacing these materials with their natural counterparts will not be an easy task.

[0003] Good barrier properties and processability, together with resistance to different environments and transparency are just some of the desired qualities for commonly used packaging materials, and oil-based polymeric materials indeed provide these qualities to a sufficient extent.

[0004] Besides being one of the most abundant biopolymers on Earth, cellulosic materials attracts considerable attention due to strength and stiffness combined with low weight, biodegradability and renewability. One of the promising new material streams of cellulose-based materials is the production and use of microfibrillated cellulose (MFC) prepared from wood biomass. MFC have desired properties, such as ability to form strong and transparent barrier films for applications such as packaging. An important feature of MFC is its hydrophilicity, which is a great advantage for processing in aqueous media. However, this affinity for water of MFC is also the reason for very poor barrier properties at higher humidity.

[0005] Suggestions on how to overcome this issue have previously been presented in for example EP3333316, wherein a transparent sheet containing substituent-introduced ultrafme cellulose fibres under high-humidity conditions is produced by cross-linking of the fibres with a divalent or higher metal by alternating dipping the sheet of fibrillated fibres in a solution containing ions for hours and washing for the sheet hours prior to drying for days.

[0006] Similarly, in JP2016210830 TEMPO-oxidized cellulose nanofibres are used to form a film that is crosslinked with ions to withstand oxygen at increased relative humidity (RH). The crosslinking is performed by immersion in a solution of ions for hours and the film is thereafter washed for hours and dried for days.

[0007] JP2016089307 discloses a method of coating a paper made of cellulose with an acid-type cellulose nanofiber dispersion that is crosslinked with a polyvalent metal compound. The crosslinking is taught to be conducted by immersion and washing for more than 35 minutes. The paper is thereafter laminated with a heat-seal layer and evaluated as suitable for paper container that can be used under hot water treatment.

[0008] EP2371892 discloses a lab method of spraying cross-linking solution over a nanocellulose film to crosslink it, wherein the film is very thin (below 1 pm) and the concentration of crosslinker is relatively high.

[0009] One problem is thus the industrial feasibility as the existing processes are highly time consuming or difficult to combine with obtaining oxygen barrier properties, also at increased humidity.

SUMMARY

[0010] An objective of the present disclosure is to provide a method of producing an MFC-based film having improved oxygen-barrier properties, which method allows for high productivity in full-scale production of the film on a paper machine.

[0011] To meet the objective, there is provided a method for producing a crosslinked film comprising microfibrillated cellulose (MFC), wherein the method comprises the steps of: a) applying an aqueous composition comprising a crosslinking agent to a film comprising MFC, which film has a moisture content of less than 15 %, such as less than 10 %, such as less than 8 %, provided that if the crosslinking agent is a metal ion, the metal ion is a divalent metal ion having a concentration in the aqueous composition of below 100 mM, such as below 50 mM; and b) drying the film from step a) such that at least 50% of the water absorbed by the film following the application of step a) is removed within five minutes of the performance of step a) or within five minutes from the start of step a).

[0012] When the crosslinking agent is a metal ion, the MFC preferably comprises a chargeable moiety and has a charge density measured according to SCAN-CM 65:02 of 500-1800 peq/g.

[0013] The drying in step b) is preferably conducted by means of heated cylinders, such as steam-heated cylinders, and/or contactless drying, such as by hot air and/or infrared radiation.

[0014] The reason for applying the crosslinking agent to a “dry” MFC film is that such a film comprises regions of dense/crystallized MFC structures separated by regions of less dense/amorphous MFC, which is more accessible for the crosslinking agent. Without being bound by any scientific theory, the inventors believe that in the relatively short period of time available in the above method, the crosslinking agent crosslinks the regions of less dense/amorphous MFC and thereby reduces their mobility. Further, the inventors believe that the relatively short combined time period of step a) and b) may focus the crosslinking action to the regions where it is most needed.

[0015] There is also provided a crosslinked film comprising MFC obtained by the method exhibiting an oxygen permeability (OP) of less than 500 ml pm nr ² d ¹ bar ¹ at 80% RH according to the standards ASTM D3985 and F1927. In this film, the crosslinker may be a divalent metal ion, such as an ion selected from the group consisting of Zn ²⁺ , Ca ²⁺ , Cu ²⁺ and Mg ²⁺ , and the MFC used to produce the film may have a charge density measured according to SCAN-CM 65:02 of 500-1800 peq/g.

DESCRIPTION

[0016] The present disclosure provides a method for producing a crosslinked film comprising microfibrillated cellulose (MFC), wherein the method comprises the steps of: a) applying an aqueous composition comprising a crosslinking agent to a film formed from an aqueous suspension comprising MFC, which film has a moisture content of less than 15 %, such as less than 10 %, such as less than 8 %, provided that if the crosslinking agent is a metal ion, the metal ion is a divalent metal ion having a concentration in the aqueous composition of below 50 mM and the MFC has a charge density measured according to SCAN-CM 65:02 of 500-1800 peq/g; and

[0017] b) drying the film from step a) such that at least 50% of the water absorbed by the film following the application of step a) is removed within five minutes from the start of step a). In a variation, step b) is instead drying the film from step a) such that the moisture content of the film is less than 30%, such as less than 25% within five minutes from the start of step a), such as within three minutes from the start of step a).

[0018] The divalent metal ion concentration in the aqueous composition is below 50 mM, typically above 0.1 mM, such as being above 0.1 mM and below 50 mM, such as 0.5-45 mM. A relatively low concentration is advantageous since a too high concentration may cause the fibrils to crosslink too tight to each other yielding an impaired barrier. The divalent cation is preferably selected from the group consisting of Zn ²⁺ , Ca ²⁺ , Cu ²⁺ and Mg ²⁺ . Zn ²⁺ and Ca ²⁺ are particularly preferred. Ca ²⁺ may be considered to be the most preferred divalent cation since it is relatively small and better penetrates the amorphous regions than larger ions.

[0019] In the case where the crosslinker is a divalent metal ion, the MFC has a charge density measured according to SCAN-CM 65:02 of 500-1800 peq/g, such as 500-1000 peq/g. Charge density is measured on fibres that are chemically modified, and as it is the total charge density being measured, the charge density of a fibre prior to fibrillation is the same as the charge density of the fibrils produced by fibrillating said fibre. The charge density of the MFC of step a) emanates from a chargeable moiety present on the MFC. The chargeable moiety is selected from carboxy, carboxymethyl, carboxyalkyl, sulfonyl, sulfoethyl and phosphoryl groups.

[0020] The chemical modification introducing the chargeable moiety is preferably selected from the group consisting of TEMPO oxidation, alkoxylation, phosphorylation, sulfonation, sulfoethylation and chlorite oxidation preceded by introduction of aldehydes, for example through periodate oxidation. The alkoxylation is preferably carboxymethylation.

[0021] Alternatively, the MFC of step a) comprises quaternary amines. The crosslinking agent may in such case be a multivalent anion, such as a phosphate ion or a polycarboxylate ion. The introduction of quaternary amines is preferably conducted via a compound that both contains a group reacting with hydroxyl groups to form covalent bonds as well as a quaternary ammonium group. The introduction may also be conducted via a compound that both contains a group reacting with hydroxyl groups to form covalent bonds and a group that can further react to attach an amine. Preferably, the group reacting with hydroxyl groups is selected from any of epoxy, halohydrin capable of forming epoxy, active halogen, isocyanate, active vinyl or methylol. Examples of compounds bearing a group reacting with hydroxyl groups to form covalent bonds as well as a quaternary ammonium group are 2,3-epoxypropyl trimethylammonium chloride (EPTMAC), chlorocholine chloride (CIChCl), glycidyl trimethylammonium chloride and 3-chloro-2-hydroxypropyl trimethyl ammonium chloride. It may be advantageous to crosslink cationic amine-functional MFC since such functionalization chemistry is readily available on an industrial scale.

[0022] In addition to MFC that is crosslinked by an ion, the film may further comprise nanofiller, e.g. in an amount of 1-20 wt.%. Nanofillers are particles characterized by high surface areas and high aspect ratios. The high surface areas and aspect ratios may be beneficial in barrier applications, since the particles can make the diffusion of the gas molecules through the coating layer more difficult, thus improving the barrier properties. Said nanofiller is preferably chosen from bentonite, kaolin or montmorillonite. Preferably, the crosslinker is not glutaraldehyde when the film comprises nanofiller. Instead, it maybe Zn ²⁺ in a concentration of 0.1-5 mM or Ca ²⁺ in a concentration of 0.1-50 mM.

[0023] The crosslinking agent may form covalent bonds. In such case, the crosslinking agent is preferably selected from the group consisting of borax, glutaraldehyde, citric acid or polycarboxylic acid. Glutaraldehyde may be used together with a catalyst, such as zinc nitrate, and the MFC typically has a low charge, such as a charge density measured according to SCAN-CM 65:02 of 10-200 peq/g, such as 10-100 peq/g.

[0024] The method is suitably carried out in a full-scale paper machine, i.e. a paper machine running at a speed of at least 300 m/min and having a trim width of at least 1500 mm, such as at least 3000 mm. Consequently, only a very limited period of time is available for the completion of each step of the process.

[0025] Steps a) and b) are preferably carried out in or after a drying section of the paper machine. Upstream the drying section of the paper machine, there is typically arranged a press section. Upstream the press section of the paper machine, there is typically arranged a forming section, such as a wire section.

[0026] Step a) may be carried out by means of a size press or a film press. Alternatively, step a) comprises the spraying the aqueous composition onto the film.

[0027] If the aqueous composition is added by means of a size press or film press, is typically has a viscosity of 10-1000 mPas, preferably 10-300 mPas, when measured as dynamic viscosity with a Brookfield rotational viscometer using spindle no.4 at 100 rpm and 25 °C according to the Brookfield instruction sheet.

[0028] According to another embodiment, a curtain coater or a direct rod coater is used for the application of the aqueous composition. In such case, the viscosity of the aqueous composition is typically 100-800 mPas when measured as dynamic viscosity with a Brookfield rotational viscometer using spindle no.4 at 100 rpm and 25 °C according to the Brookfield instruction sheet.

[0029] According to yet another embodiment, a blade coater is used for the application of the aqueous composition. In such case, the viscosity of the aqueous composition is typically 400-1500 mPas when measured as dynamic viscosity with a Brookfield rotational viscometer using spindle no.4 at 100 rpm and 25 °C according to the Brookfield instruction sheet.

[0030] To facilitate the application (and to obtain the desired viscosity), the aqueous composition may comprise a polymer, such as starch, carboxymethyl cellulose (CMC), polyvinyl alcohol (PVOH) or MFC. In addition to the polymer, the composition may comprise a rheology modifier.

[0031] The drying in step b) is preferably conducted by means of heated cylinders, such as steam-heated cylinders, and/or contactless drying, preferably using hot air and/or infrared radiation.

[0032] The method may further comprise the steps of: i) providing the aqueous suspension comprising MFC; and ii) forming a web from the aqueous suspension and dewatering and drying the web to form the film to which the aqueous composition comprising a crosslinking agent is applied in step a). [0033] In one embodiment, steps i) and ii) are carried out in a paper machine, such as the same paper machine as the one used for steps a) and b). Thereby, the overall efficiency of the method may be improved. Alternatively, steps a) and b) are carried out offline, which means that they are not carried out in the paper machine used for steps i) and ii). Steps a) and b) may even be carried out at a different location than steps i) and ii).

[0034] Step ii) is preferably carried out in the forming section of a paper machine. The width of the wire of the forming section maybe at least 1500 mm, such as at least 3000 mm.

[0035] In one embodiment, the method further comprises a step of fibrillating cellulosic fibres to provide the MFC of step i). This step may comprise mechanical, enzymatic and/ or chemical sub-steps known to the skilled person. As further discussed below, a chemical sub-step may introduce a chargeable moiety that facilitates the crosslinking.

[0036] In the context of the present disclosure, MFC means nano-scale cellulose particle fibres or fibrils with at least one dimension less than 100 nm. MFC comprises partly or totally fibrillated cellulose or lignocellulose fibres. The liberated fibrils have a diameter less than 100 nm, whereas the actual fibril diameter or particle size distribution and/or aspect ratio (length/width) depends on the source and the manufacturing methods. The smallest fibril is called elementary fibril and has a diameter of approximately 2-4 nm, while it is common that the aggregated form of the elementary fibrils, also defined as microfibril is the main product that is obtained when making MFC e.g. by using an extended refining process or a pressure-drop disintegration process. Depending on the source and the manufacturing process, the length of the fibrils can vary from around 1 to more than 10 micrometers. A coarse MFC grade might contain a substantial fraction of fibrillated fibres, i.e. protruding fibrils from the tracheid (cellulose fibre), and with a certain amount of fibrils liberated from the tracheid.

[0037] There are different synonyms for MFC such as cellulose microfibrils, fibrillated cellulose, nanofibrillated cellulose (NFC), fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibres, cellulose nanofibrils (CNF), cellulose microfibres (CMF), cellulose fibrils, microfibrillar cellulose, microfibril aggregrates and cellulose microfibril aggregates. MFC can also be characterized by various physical or physical- chemical properties such as large surface area or its ability to form a gel-like material at low solids content (1-5 wt.%) when dispersed in water. The cellulose fibre is preferably fibrillated to such an extent that the final specific surface area of the formed MFC is from about 1 to about 200 m ² /g, or more preferably 50-200 m ² /g when determined for a freeze-dried material with the BET method (Brunauer, Stephen, Paul Hugh Emmett, and Edward Teller. "Adsorption of gases in multimolecular layers." Journal of the American chemical society 60.2 (1938): 309-319.). Nitrogen (N2) gas adsorption isotherms are recorded using an ASAP 2020 (Micromeritics, USA) instrument. Measurements are performed at liquid nitrogen temperatures (i.e., 77 K), and the specific surface areas of the samples were obtained from the isotherms using the BET method.

[0038] Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment step(s) is/are usually required in order to make MFC manufacturing both energy efficient and sustainable. The cellulose fibres of the pulp to be supplied may thus be pre-treated enzymatically or chemically, for example to hydrolyse or swell fibre or reduce the quantity of hemicellulose or lignin. The cellulose fibres may be chemically modified before fibrillation. After such chemical modification, it is typically easier to disintegrate the fibres into MFC or nanofibrillar size or NFC.

[0039] The nanofibrillar cellulose may contain some hemicelluloses; the amount is dependent on factors such as plant source and pulping process. Mechanical disintegration of the pre-treated fibres, e.g. hydrolysed, pre-swelled, or oxidized cellulose rawmaterial is carried out with suitable equipment such as a refiner, grinder, homogenizer, collider, friction grinder, ultrasound sonicator, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer. Depending on the MFC manufacturing method, the product might also contain fines or e.g. other chemicals present in wood fibres or in papermaking process. The product might also contain various amounts of micron size fibre particles that have not been efficiently fibrillated. MFC is produced from wood cellulose fibres, both from hardwood or softwood fibres. It can also be made from, agricultural fibres such as wheat straw pulp, bamboo, bagasse, or other non-wood fibre sources. It is preferably made from pulp of virgin fibre, e.g. mechanical, chemical and/or thermomechanical pulps, preferably never-dried fibres. [0040] The above described definition of MFC includes, but is not limited to, the proposed TAPPI standard W13021 on cellulose nanofibril (CNF) defining a cellulose nanofibre material containing multiple elementary fibrils with both crystalline and amorphous regions, having a high aspect ratio with width of 5-3 onm and aspect ratio usually greater than 50.

[0041] 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.

[0042] The film typically has a grammage measured according to ISO 536:2012 of 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 20-60 g/m ² , such as 30-60 g/m ² , preferably 45-60 g/m ² , more preferably 50-60 g/m ² .

[0043] The crosslinked film typically has a density of o.7-1.4 g/cms, such as o.8-1.2 g/cms measured according to ISO 534:2011.

[0044] The crosslinked film typically has a thickness of 1-100 pm, such as 5-70 pm, such as 10-50 pm measured according to ISO 534:2011. By having a thickness of several micrometres, the crosslinking action of the film can be focused to the regions where it is most needed, which is beneficial for the barrier properties.

[0045] In one embodiment of the method, at least 75% of the water absorbed by the film following the application of step a) is removed within five minutes from the start of step a).

[0046] In another embodiment, 50% of the water absorbed by the film following the application of step a) is removed within three minutes from the start of step a), such as within two minutes from the start of step a). Thereby, the efficiency of the method is increased.

[0047] Further, the drying of step b) maybe carried out such that the moisture content of the film is less than 30%, such as less than 25% within five minutes of the performance of step a), such as within three minutes of the performance of step a).

[0048]

[0049] The crosslinked film produced by the method of the present disclosure is an oxygen barrier, exhibiting an oxygen permeability (OP) of less than 500 ml pm nr ² d ¹ bar ¹ at 80% RH according to the standards ASTM D3985 and F1927. [0050] The crosslinked film is typically crosslinked with a divalent metal ion and the MFC used to produce the film typically has a charge density measured according to SCAN-CM 65:02 of 500-1800 peq/g. The divalent metal ion is typically selected from the group consisting of Zn ²⁺ , Ca ²⁺ , Cu ²⁺ and Mg ²⁺ .

[0051] Accordingly, in one embodiment the method for producing a crosslinked film comprising MFC comprises the steps of:

- Chemically modifying fibres through any of the modifications in the group consisting of TEMPO oxidation, alkoxylation, phosphorylation, sulfonation, sulfoethylation, chlorite oxidation preceded by introduction of aldehydes and introduction of a quaternary amine;

- Fibrillating the fibres to MFC;

- Providing an aqueous suspension comprising the MFC;

- Forming a web from the aqueous suspension and dewatering and drying the web to form a film;

- Applying an aqueous composition comprising a crosslinking agent to the film comprising MFC, which film has a moisture content of less than 15 %, such as less than 10 %, provided that if the crosslinking agent is a metal ion, the metal ion is a divalent metal ion having a concentration in the aqueous composition of below 50 mM, and the MFC has a charge density measured according to SCAN-CM 65:02 of 500-1800 peq/g;

- drying the film such that at least 50% of the water absorbed by the film following the application of the aqueous composition comprising a crosslinking agent is removed within five minutes from the start of the application of the aqueous composition comprising a crosslinking agent; and

- Thereby, obtaining the crosslinked film comprising MFC.

EXAMPLES Example la

[0052] Crosslinking an MFC film produced from low-charged MFC

[0053] A suspension of low-charged MFC (0.1 wt%; total charge 40 pmol/g) was filtrated and thereafter dried (50 °C; 4 h) into a film. The film was dipped for 5 seconds in a solution containing either borax or glutaraldehyde acting as crosslinker. The latter chemical was employed in combination with zinc nitrate, which acts as catalyst for the reaction of glutaraldehyde with hydroxyl group. The film was thereafter dried (50 °C; 4 h). Films of grammages of 30 g/m ² and 60 g/m ² were produced. No washing was performed between the dipping and the drying.

Example lb

[0054] Crosslinking an MFC film produced from low-charged MFC and nanofiller

[0055] Using the same procedure as for Example la, but replacing 10 wt% of the MFC with Cloisite Na+ (bentonite nanofiller), a cross-linked film containing nanofiller and MFC was produced.

Example 2 a

[0056] Crosslinking an MFC film produced from carboxymethylated MFC

[0057] A suspension of carboxymethylated MFC (0.1 wt%; total charge 800 pmol/g) was filtrated and thereafter dried (50 °C; 4 h) into a film. The film was dipped for 5 seconds in a solution containing either zinc nitrate hexahydrate, calcium chloride dihydrate, iron chloride hexahydrate or aluminum chloride hexahydrate acting as crosslinkers. Subsequently the film was dried (50 °C; 4 h). Films of grammages of 20 g/m ² , 30 g/m ² and 60 g/m ² were produced. No washing was performed between the dipping and the drying.

Example 2b

[0058] Using the same procedure as for Example 2a, but with replacing 10 wt% of the MFC with Cloisite Na+ (bentonite nanofiller) a cross-linked film containing nanofiller and MFC was produced.

Oxygen permeability

[0059] The transmission rate (OTR) was measured on 5 cm ² samples using a MOCON OX-TRAN 2/21 according to the ASTM D3985 and ASTM F1927 standards. The OTR measurements were performed at 23 ⁰ C and 50% RH or 80% RH, using the same relative humidity on both sides of the sample. By multiplying OTR with film thickness (obtained by SEM) the oxygen permeability (OP) is obtained. The results are presented in table 1.

Table 1.

N.M. means not measured.

* The weight ratio of glutaraldehyde to zinc nitrate was 1:0.76 and the concentration of glutaraldehyde in the solution was iwt.%.

** The weight ratio of glutaraldehyde to zinc nitrate was 2:0.76 and the concentration of glutaraldehyde in the solution was 2 wt.%.

Table 1 shows reduction of oxygen permeability at 50% and 80% RH when divalent metal ions are applied at a concentration below 100 mM. In contrast, the trivalent metal ions and divalent metal ions in a higher concentration (i.e. 100 mM) failed to reduce oxygen permeability. For 80% RH, the best oxygen permeability values were obtained when Ca ²⁺ (1 or 10 mM) or Zn ²⁺ (1 mM) was added to a film comprising nanofiller.

Table 1 also shows reduction of oxygen permeability at 80% RH when crosslinkers forming covalent bonds are applied.