Technical field

The present disclosure relates to m icrofibrillated cellulose (MFC) films useful, for example, as oxygen barrier films in paper and paperboard. The present invention further relates to multilayer materials comprising such MFC films and to methods for manufacturing such MFC films.

Background

Films and coatings made from nanocellulosic materials such as m icrofibrillated cellulose (MFC), have emerged as an interesting alternative to conventional gas barrier films, such as aluminum and synthetic polymer films and various laminates thereof. Nanocellulosic films have been developed, in which cellulosic fibrils have been dispersed and/or suspended in aqueous media and thereafter re-organized and rebonded together to form a dense film with high barrier properties.

In addition to providing excellent gas barrier properties, the MFC films and coatings are also inherently transparent or translucent to visible light, making them especially useful in applications where transparency or translucency of the film in the visible light spectrum (typically in the range of 380 to 740 nm) is required. An MFC coatings may for example be used as a varnish or overlay varnish.

MFC films can be made by applying an MFC suspension on a porous substrate, for example a membrane or wire, forming a web followed by dewatering of the web by draining water through the substrate to form the film. This can be accomplished e.g. by use of a paper- or paperboard machine type of process. LIS2012298319A teaches a method of manufacturing of an MFC film by applying a furnish comprising MFC directly on porous substrate thus allowing the MFC to be dewatered and filtered.

Alternatively, the film can be made by use of casting technologies, including applying an MFC dispersion onto a non-porous cast substrate, such as a polymeric or metal substrate, and drying said film by evaporation and/or wet pressing. Films made by casting technologies usually provide a more uniform thickness distribution and a smoother surface. The publication EP2771390 A4 describes preparation of MFC films, in which an aqueous cellulose nanofiber dispersion is coated on a paper or polymeric substrate, dried and finally peeled off as a nanofiber film sheet.

A problem with MFC films is that they may be brittle and provide low strain at break and tear resistance since the fiber network formed from short fibers will not have the ability to stretch in the same way as longer fibers. When forming MFC films of low grammage and thickness, the film may easily break during wet web forming, converting or handling. Also, the gas barrier properties of such MFC films tend to deteriorate at high temperatures and high humidity. Moreover, MFC is a relatively expensive material, making the cost of pure MFC films high.

Various additives have been considered in order to address the problems associated with improving the mechanical properties of MFC films. However, while the use of a given additive may solve one specific problem, it may not be able to solve or maintain other physical or mechanical requirements and may even cause new problems. As an example, the addition of longer cellulose fibers in the MFC films may improve the mechanical properties of the films but will at the same time impair the gas barrier properties and transparency or translucency of the film.

Adding a plasticizer such as sorbitol is a good way to get better strain at break but this also reduces the gas barrier properties of the films when increasing the humidity. Other additives may adversely affect the re-use of the material such as in the form of broke or pre- or post-consumer reject.

Thus, there remains a need for improved MFC films combining good oxygen barrier properties with an acceptable high strain at break.

Description of the invention

It is an object of the present disclosure to provide an improved m icrofibrillated cellulose (MFC) film, which eliminates or alleviates at least some of the problems in the prior art. It is a further object of the present disclosure to provide an MFC film, which shows good oxygen barrier properties, is easy to handle, easy to produce at higher speeds, easy to convert, and/or makes use of more cost- efficient raw materials.

The above-mentioned objects, 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 present disclosure is based on the inventive realization that including very small amounts of a low molecular weight polyethylene glycol (LMW-PEG) having a molecular weight in the range of 300-500 g/mol in a m icrofibrillated cellulose (MFC) composition allows for preparation of oxygen barrier films or coatings combining good oxygen barrier properties with an acceptable strain at break. The molecular weight of the PEG has been found to be important for achieving both good oxygen barrier properties and an acceptable strain at break. PEG having a molecular weight higher than 500 g/mol may give an even lower oxygen transmission rate (OTR), but at the cost of a lower strain at break. The amount of LMW-PEG should preferably be in the range of 2-8 wt% based on the total dry weight of the MFC film. The optimal combination of oxygen barrier properties and strain at break seems to be achieved with about 5 wt% of LMW-PEG having a molecular weight in the range of 300-500 g/mol. It is surprising that so small amounts of LMW-PEG can give these positive effects on strain at break and still not affect the OTR too much.

The LMW-PEG is also expected to facilitate redispersion and thereby re-use of the MFC film such as in the form of broke or pre- or post-consumer reject.

According to a first aspect illustrated herein, there is provided a m icrofibrillated cellulose (MFC) film, said film comprising:

MFC at a concentration in the range of 50-98 wt%, and a low molecular weight polyethylene glycol (LMW-PEG) having a molecular weight in the range of 300-500 g/mol at a concentration in the range of 2-8 wt%, based on the total dry weight of the MFC film.

A composition in the form of a dispersion can be used for the preparation of a film or coating. In some embodiments, the MFC composition used to prepare the MFC film is a dispersion, preferably an aqueous dispersion. The consistency of the dispersion is preferably in the range of 0.1-50 wt%, preferably in the range of 0.2- 30 wt%, and more preferably in the range of 0.3-20 wt%.

The MFC film may be comprised solely of a mixture of MFC and phosphorylated cellulose fibers, or it can comprise the mixture of MFC and phosphorylated cellulose fibers combined with other ingredients or additives. The MFC film preferably includes MFC as its main component based on the total dry weight of the MFC film. Specifically, the MFC film comprises MFC at a concentration in the range of 50-98 wt%. In some embodiments, the MFC film comprises MFC in the range of 60-98 wt%, preferably in the range of 70-98 wt%, more preferably in the range of 80-98 wt% of MFC, based on the total dry weight of the MFC film.

M icrofibri Hated cellulose (MFC) shall in the context of the 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 N-oxyl mediated oxidation, for example "TEMPO"), 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 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.

In some embodiments, the MFC comprises less than 50 wt%, preferably less than 30 wt%, and more preferably less than 20 wt%, of MFC fibers having a diameter above 1000 nm.

In some embodiments, the MFC comprises less than 10 wt%, preferably below 7.5 wt%, more preferably below 5 wt%, of fibers having a length of > 0.2 mm, as measured using an FS5 optical fiber analyzer (Valmet).

There are different synonyms for MFC such as cellulose microfibrils, fibrillated cellulose, nanocellulose, nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibers, cellulose nanofibrils, cellulose microfibers, cellulose fibrils, microfibrillar cellulose, microfibril aggregates and cellulose m icrofibril aggregates.

The MFC of the MFC film may be unmodified MFC or chemically modified MFC, or a mixture thereof. In some embodiments, the MFC is an unmodified MFC.

Unmodified MFC refers to MFC made of unmodified or native cellulose fibers. The unmodified MFC may be a single type of MFC, or it can comprise a mixture of two or more types of MFC, differing e.g. in the choice of cellulose raw material or manufacturing method.

Chemically modified MFC refers to MFC made of cellulose fibers that have undergone chemical modification before, during or after fibrillation. In some embodiments, the MFC is a chemically modified MFC. The chemically modified MFC may be a single type of chemically modified MFC, or it can comprise a mixture of two or more types of chemically modified MFC, differing e.g. in the type of chemical modification, the choice of cellulose raw material or the manufacturing method.

In some embodiments, the MFC of the MFC film has a Schopper-Riegler (SR) number > 70, preferably > 80, and more preferably > 90, as measured according to the standard ISO 5267-1.

The MFC film comprises the LMW-PEG at a concentration in the range of 2-8 wt%, based on the total dry weight of the MFC film. In some embodiments, the MFC film comprises in the range of 2-7 wt%, preferably in the range of 3-6 wt%, more preferably in the range of 4-6 wt% of LMW-PEG, based on the total dry weight of the MFC film.

The LMW-PEG of the inventive film has a molecular weight in the range of 300 - 500 g/mol. In some embodiments, the LMW-PEG has a molecular weight in the range of 350 - 450 g/mol. One preferred LMW-PEG is PEG 400, which typically has a molecular weight in the range of 380 - 420 g/mol. Unless otherwise specified, the term “molecular weight” as used herein with reference to PEG refers to the number average molecular weight Mn.

In some embodiments, the MFC film further comprises a water-soluble polymer selected from the group consisting of a starch, a polyvinyl alcohol (PVOH), a cellulose derivative, a hemicellulose, a polyacrylamide, a polydiallyldimethylammonium chloride (PDADMAC), a polyvinylamine (PVAm), a polyethyleneimine (PEI), a protein or a mixture thereof, preferably a PVOH.

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 or different functional groups. 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%. Furthermore, the PVOH may preferably have a viscosity above 5 mPaxs in a 4 % aqueous solution at 20 °C as measured according to the standard DIN 53015 / JIS K 6726. In some embodiments, the MFC film comprises in the range of 0.1-20 wt%, preferably in the range of 1-15 wt% of the water-soluble polymer, based on the total dry weight of the MFC film.

In some embodiments, the MFC film further comprises a pigment that promotes oxygen barrier function of a barrier film, preferably a pigment selected from the group consisting of clays and nanoclays, talcum, silicates, carbonates, alkaline earth metal carbonates, ammonium carbonate, metal oxides and transition metal oxides.

In some embodiments, the MFC film comprises in the range of 0.1-20 wt%, preferably in the range of 1-10 wt% of the pigment, based on the total dry weight of the MFC film.

The MFC film may further comprise additives such as starch, fillers, retention aids, flocculation additives, deflocculating additives, dry strength additives, softeners, lubricants, wet strength agents, colorants or dyes, defoamers, fixatives, biocides, pH regulators, UV blocking agents, or mixtures thereof. The MFC film may for example comprise additives that will improve different properties of the MFC film, such as latex and/or polyvinyl alcohol (PVOH) for enhancing the ductility of the coating.

The MFC film of the first aspect described above is preferably an oxygen barrier film. The MFC film may be a free standing MFC film, an MFC coating on a substrate, or an MFC layer of a multilayer material. The film may for example be prepared by casting, coating or wet laid process.

In some embodiments, the MFC film has a grammage in the range of 10-100 gsm, preferably in the range of 12-50 gsm, more preferably in the range of 15-40 gsm. Correspondingly, in some embodiments, a coating can have a grammage in the range of 1-20 gsm, or more preferably in the range of 1-10 gsm. The present disclosure is based on the inventive realization that including very small amounts of a low molecular weight polyethylene glycol (LMW-PEG) having a molecular weight in the range of 300-500 g/mol in a m icrofibrillated cellulose (MFC) composition allows for preparation of oxygen barrier films or coatings combining good oxygen barrier properties with an acceptable strain at break.

The addition of the LMW-PEG in the MFC film improves the mechanical strength, particularly the strain at break, of the film as compared to a corresponding MFC film without the LMW-PEG. In some embodiments, the MFC film has a strain at break at least 10% higher, preferably at least 15% higher, more preferably at least 20% higher, than the tear index of a corresponding film without the LMW-PEG, as measured according to the standard ISO-1924-3, modified with clamped length of 20 mm and a speed of 2 mm/min.

In some embodiments, the film has a strain at break of at least 2.5%, preferably at least 3%, as measured according to the standard ISO-1924-3, modified with clamped length of 20 mm and a speed of 2 mm/min.

Reference material for MFC-film with sorbitol has shown that a strain at break of 3% is preferred for handling of the film in converting.

In some embodiments, the film has an oxygen transfer rate (OTR) less than 20 cc/rn ² /24h/atm higher, preferably less than 10 cc/m ² /24h/atm higher, than the OTR of a corresponding film without the LMW-PEG, as measured according to the standard ASTM F-1927-98 at 80% relative humidity and 23 °C.

In some embodiments, the film has an oxygen transfer rate (OTR) of less than 20 cc/m ² /24h/atm, preferably less than 10 cc/m ² /24h/atm, as measured according to the standard ASTM F-1927-98 at 80% relative humidity and 23 °C.

In some embodiments, the film has a transparency of at least 75%, preferably at least 80%, as measured according to the standard DIN 53147. In some embodiments, the film further comprises a metallization layer disposed on at least one surface thereof. A metallization layer is formed by vapor deposition of a metal or metal oxide on the surface, preferably by physical vapor deposition (PVD) or chemical vapor deposition (CVD), more preferably by physical vapor deposition (PVD). The metallization layer may for example comprise aluminum or an aluminum oxide. Metallization layers are normally merely nanometer-thick, i.e. have a thickness in the order of magnitude of nanometers. The metallization layer may for example have a thickness in the range of from 1 to 500 nm. In some embodiments, the metallization layer has a layer thickness in the range of 10-100 nm, preferably in the range of 20-50 nm.

The MFC films or coatings may preferably be used as an oxygen barrier layer in a multilayer material, such as a paper or paperboard comprised of two or more plies.

The MFC films or coatings may preferably be used as an oxygen barrier layer in a paper or paperboard based packaging material, particularly in a liquid packaging board (LPB), for use in the packaging of liquids or liquid containing products. The improved strain at break of the inventive MFC films, makes them suitable for conversion. Combined with good oxygen barrier properties, this makes the films interesting for use as a replacement for an aluminum foil layers commonly used in liquid packaging board.

According to a second aspect illustrated herein, there is provided a multilayer material comprising at least: a substrate layer and an MFC film as described above with reference to the first aspect.

In the multilayer material, the MFC layer may be attached to the substrate layer directly, or via one or more intermediate layers. For example, the MFC layer may be coated or wet laid directly onto the substrate layer, or an MFC film may be laminated to the substrate layer using an intermediate adhesive layer.

In some embodiments, the substrate layer is paper or paperboard. In some embodiments, the paper or paperboard has a basis weight in the range of 20-500 gsm (g/m ² ), preferably in the range of 80-400 gsm.

In some embodiments, the multilayer material is a paper or paperboard comprising an MFC film as described above with reference to the first aspect.

In some embodiments, the multilayer material comprises one or more heat sealable layers, such as one or more polyethylene layers.

In some embodiments, the multilayer material is a liquid packaging board (LPB). In some embodiments, the liquid packaging board comprises the layers:

Extruded polyolefin/Paperboard/Extruded polyolefin/MFC film/Extruded polyolefin

The laminate may further include tie layer/s comprising a tie resin applied between the extruded polyolefin layer/s and the MFC film.

In some embodiments, the extruded polyolefin is a polyethylene (PE), preferably low density polyethylene (LDPE).

According to a third aspect illustrated herein, there is provided method of preparing an MFC film, said method comprising: a) preparing an aqueous dispersion comprising:

MFC at a concentration in the range of 50-98 wt%, and a low molecular weight polyethylene glycol (LMW-PEG) having a molecular weight in the range of 300- 500 g/mol at a concentration in the range of 2-8 wt%, based on the total dry weight of the aqueous dispersion; b) forming a wet film of the aqueous dispersion; and c) drying the wet film to obtain the MFC film. Thanks to the high solubility of the LMW-PEG, the aqueous dispersion may be prepared in a number of different ways. The aqueous dispersion may for example be prepared by mixing dry MFC and dry LMW-PEG and dispersing the dry mixture in water, or by adding LMW-PEG, in dry or dissolved form, to an aqueous dispersion.

In some embodiments of the method, the step a) comprises mixing MFC having a Schopper-Riegler (SR) number > 70, preferably > 80, and more preferably > 90, as measured according to the standard ISO 5267-1 , with the LMW-PEG.

In some embodiments, the drying in step c) is performed at a temperature above 50 °C, preferably above 70 °C, more preferably above 90 °C. The drying temperature refers to the temperature in the film during the drying. The drying source may have a much higher temperature than the actual film. Drying at elevated temperature may further improve the mechanical properties of coatings or films formed of the aqueous dispersion. Drying means that the solid content of the MFC film is at least 80 wt%, preferably at least 85 wt% and more preferably at least 90% after the drying.

According to a fourth aspect illustrated herein, there is provided the use of a low molecular weight polyethylene glycol (LMW-PEG) having a molecular weight in the range of 300-500 g/mol at a concentration in the range of 2-8 wt% as a strength enhancement agent in an MFC film.

The MFC film comprises the LMW-PEG at a concentration in the range of 2-8 wt%, based on the total dry weight of the MFC film. In some embodiments, the MFC film comprises in the range of 2-7 wt%, preferably in the range of 3-6 wt%, more preferably in the range of 4-6 wt% of LMW-PEG, based on the total dry weight of the MFC film.

The LMW-PEG of the inventive film has a molecular weight in the range of 300 - 500 g/mol. In some embodiments, the LMW-PEG has a molecular weight in the range of 350 - 450 g/mol. One preferred LMW-PEG is PEG 400, which typically has a molecular weight in the range of 380 - 420 g/mol. In some embodiments, the MFC film of the fourth aspect is an MFC film as described above with reference to the first aspect.

The term “wt%” as used herein (e.g. with reference to pulp compositions or pulp fractions) refers to weight percent based on the total dry weight of the composition or film.

The term “consistency” as used herein (e.g. with reference to pulp compositions or pulp fractions) refers to weight percentage of dry solid substances in the composition based on the total weight of the composition.

Unless otherwise specified, the term “molecular weight” as used herein with reference to PEG refers to the number average molecular weight Mn.

While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

EXAMPLES

Materials

The PVOH grade had a viscosity of 12.5-17.5 mPa*s of a 4% aqueous solution at 20 °C, DIN 53015 / JIS K 6726 and a hydrolysis degree of 99%. The bentonite was Na-Cloisite. The mixture for manufacturing the films were prepared as follows. Polyvinyl alcohol was jet cooked for 2 h at a solids content of 14%. The bentonite clay was mixed with high shear rate for 2 h at a solids content of 8% and the left at least 48 h for swelling with no mixing.

Unmodified MFC, seen as 100%, at a solids content of 3 wt% was mixed with 10 wt% PVOH, 5% of Bentonite clay and a changed amount of either PEG 400 or PEG 600. The suspension was mixed and deaerated in a vacuum assisted mixing using a magnetic stirrer. The film was produced by rod coating the dispersion on a heated metallic surface. The film was dry after 5 to 10 minutes. The estimated drying temperatures was 90 degrees at the start and around 60 degrees when dry.

The reference film was prepared in the same way as the samples but only using native MFC and adding 15% sorbitol instead of the PVOH, bentonite clay and PEG. Sorbitol was added as a 50% solution.

The films referred to as Reference and Sample 1 to 7 were after drying separated from the metallic substrate. The obtained films had a thickness of 35-45 pm and a grammage of about 50 g/m ² .

Analysis

The films were characterized with strain at break according to the standard ISO- 1924-3, modified with clamped length of 20 mm and a speed of 2 mm/min.

The oxygene transmission rate was measured according to the standard ASTM F- 1927-98 at 80% relative humidity and 23 °C.