WO2021074879A1

WO2021074879A1 - Mfc composition with phosphorylated cellulose fibers

Info

Publication number: WO2021074879A1
Application number: PCT/IB2020/059762
Authority: WO
Inventors: Kaj Backfolk; Isto Heiskanen; Katja LYYTIKÄINEN; Gisela CUNHA; Otto NYLEN
Original assignee: Stora Enso Oyj
Priority date: 2019-10-17
Filing date: 2020-10-16
Publication date: 2021-04-22
Also published as: SE543624C2; SE1951179A1

Abstract

The present invention relates to a microfibrillated cellulose (MFC) composition for preparation of transparent or translucent films or coatings, said composition comprising MFC at a concentration in the range of 50-99.9 wt%, and phosphorylated cellulose fibers at a concentration in the range of 0.1-50 wt%, based on the total dry weight of the MFC composition. The invention further relates to an MFC film and a multilayer material comprising the MFC composition.

Description

MFC COMPOSITION WITH PHOSPHORYLATED CELLULOSE FIBERS

Technical field

The present disclosure relates to microfibrillated cellulose (MFC) compositions for preparation of transparent or translucent films useful, for example, as gas and/or grease barrier films in paper and paperboard. The present invention further relates to multilayer materials comprising such MFC compositions and to methods for manufacturing such MFC compositions. Background

Films and coatings made from nanocellulosic materials such as microfibrillated 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. US2012298319A 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 ability 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. Flowever, 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. 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 to further improve the properties of MFC films.

Description of the invention

It is an object of the present disclosure to provide an improved microfibrillated cellulose (MFC) composition for preparation of transparent or translucent films or coatings, 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 a microfibrillated cellulose (MFC) composition which enables the manufacturing of an MFC film with good transparency or translucency, good gas barrier properties, and improved mechanical properties.

It is a further object of the present disclosure to enable the manufacturing of an MFC film, which shows high 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 phosphorylated cellulose fibers can be added to an MFC composition to improve the mechanical properties of MFC films formed from the composition, while still maintaining the good transparency or translucency and good gas barrier properties characteristic to MFC films without added fibers. Without wishing to be bound to any specific scientific theory, it is believed that the improved mechanical properties of MFC films with phosphorylated cellulose fibers may at least in part be derived from the anionic nature and functional groups of the fibers providing an increased reactivity and number of sites for cross-linking.

Additionally, as the phosphorylated cellulose fibers are highly charged (compared to most other fibers), the fibers repel each other. This in turn means that fibers become more even distributed in the composition, leading to better formation and improved evenness of formed sheets, visual appearance, evenness of mechanical strength properties, etc. The addition of phosphorylated cellulose fibers to the MFC composition may also reduce the need for other added formation aids. The phosphorylated fibers are also expected to facilitate redispersion and thereby re-use of the MFC material such as in the form of broke or pre- or post-consumer reject. According to a first aspect illustrated herein, there is provided a microfibrillated cellulose (MFC) composition for preparation of transparent or translucent films or coatings, said composition comprising:

MFC at a concentration in the range of 50-99.9 wt%, and phosphorylated cellulose fibers at a concentration in the range of 0.1-50 wt%, based on the total dry weight of the MFC composition.

The composition may be in the form of a solid composition, e.g. in the form of a dried or substantially dried film, coating or powder, or it can be in the form of a dispersion in a liquid medium, preferably water. A composition in the form of a dispersion can be used for the preparation of a film or coating.

In some embodiments, the composition is a solid composition. The solid composition preferably has a moisture content of 20 wt% or less, preferably 15 wt% or less, more preferably 10 wt% or less.

In some embodiments, the MFC composition 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 drainability of the MFC composition will depend on the type and amount of MFC and the phosphorylated cellulose fibers used, as well as other components added to the composition. In some embodiments, the MFC composition has a Schopper-Riegler (SR) number > 20, preferably > 30, and more preferably > 40, as measured according to the standard ISO 5267-1.

The MFC composition 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 composition preferably includes MFC as its main component based on the total dry weight of the MFC composition. Specifically, the MFC composition comprises MFC at a concentration in the range of 50-99.9 wt%. In some embodiments, the MFC composition comprises in the range of 50-99.5 wt%, preferably in the range of 60-99.5 wt%, more preferably in the range of 65-98 wt% of MFC, based on the total dry weight of the MFC composition.

Microfibrillated cellulose (MFC) shall in the context of the patent application be understood to mean a nano scale cellulose particle fiber or fibril with at least one dimension less than 1000 nm. MFC comprises partly or totally fibrillated cellulose or lignocellulose fibers. The liberated fibrils have an average diameter less than 1000 nm, whereas the actual fibril diameter or particle size distribution and/or aspect ratio (length/width) depends on the source and the manufacturing methods.

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 fibers, i.e. protruding fibrils from the tracheid (cellulose fiber), and with a certain amount of fibrils liberated from the tracheid (cellulose fiber). 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 microfibril aggregates. The cellulose fiber is preferably fibrillated to such an extent that the final specific surface area of the formed MFC is from about 1 to about 400 m²/g, or more preferably 50-300 m²/g when determined for a solvent exchanged and freeze-dried material with the Nitrogen sorption (BET) method.

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 steps are usually required in order to make MFC manufacturing both energy efficient and sustainable. The cellulose fibers of the pulp to be utilized may thus be pre-treated, for example enzymatically or chemically, to hydrolyse or swell the fibers or to reduce the quantity of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, such that the cellulose molecules contain other (or more) functional groups than found in the native cellulose. Such groups include, among others, carboxymethyl (CMC), 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 or nanofibrils.

The nanofibrillar cellulose may contain some hemicelluloses, the amount of which is dependent on the plant source. Mechanical disintegration of the pre-treated fibers, e.g. hydrolysed, pre-swelled, or oxidized cellulose raw material is carried out with suitable equipment such as a refiner, grinder, homogenizer, colloider, 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 nanocrystalline cellulose, or other chemicals present in wood fibers or in papermaking process. The product might also contain various amounts of micron size fiber particles that have not been efficiently fibrillated.

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

The MFC composition comprises phosphorylated cellulose fibers at a concentration in the range of 0.1-50 wt%. In some embodiments, the MFC composition comprises in the range of 0.1-30 wt%, preferably in the range of 0.5- 20 wt%, more preferably in the range of 1-10 wt% of phosphorylated cellulose fibers, based on the total dry weight of the MFC composition.

The phosphorylated cellulose fibers may for example be phosphorylated cellulose fibers obtained from hardwood or softwood. The phosphorylated cellulose fibers may be phosphorylated fibers obtained from chemical, mechanical or chemimechanical pulp, such as kraft pulp or dissolving pulp.

The phosphorylated cellulose fibers are larger than the microfibrils of the MFC.

The MFC is typically made up of cellulose particle fibers or fibrils with at least one dimension in the range of 1-1000 nm and often less than 100 nm. The phosphorylated cellulose fibers of the MFC composition preferably have an average fiber diameter (fiber width) above 1000 nm. The size of the phosphorylated cellulose fibers may depend on the source of the fibers, e.g. hardwood, softwood and the pulp source. In some embodiments, the phosphorylated cellulose fibers of the MFC composition have a fiber width of more than 5 pm, preferably more than 10 pm, as measured using an FS5 optical fiber analyzer (Valmet).

The phosphorylated cellulose fibers of the MFC composition preferably have a length-weighted mean fiber length above 0.5 mm, i.e. above 500000 nm. In some embodiments, the phosphorylated cellulose fibers have a length-weighted mean fiber length (Lc(l)) of more than 0.2 mm and preferably more than 0.5 mm (as measured according to the standard ISO 16065-2).

In some embodiments, the phosphorylated cellulose fibers of the MFC composition have a length-weighted mean fiber length in the range of 0.2-4 mm, preferably in the range of 0.3-2 mm, more preferably in the range of 0.5-2 mm. Length-weighted mean fiber length as used herein refers to the length-weighted mean fiber length (Lc(l)) measured according to the standard ISO 16065-2 using an FS5 optical fiber analyzer (Valmet).

In some embodiments, the phosphorylated cellulose fibers of the MFC composition comprise less than 10 wt%, preferably below 7.5 wt%, more preferably below 5 wt%, of fibers having a length in the range of <0.2 mm, as measured using an FS5 optical fiber analyzer (Valmet).

The phosphorylated cellulose fibers include cellulose fibers or fragments of cellulose fibers or a mixture thereof, in which at least a portion of the hydroxyl groups have been chemically modified by phosphorylation. In some embodiments, the phosphorylated cellulose fibers have a degree of phosphorylation of 0.1 mmol/g or higher, preferably 0.3 mmol/g or higher.

Phosphorylated pulp or phosphorylated cellulose fibers can for example be obtained by reacting a pulp of cellulose fibers soaked in a solution of NH4H2PO4, water and urea and optionally neutralizing and washing the phosphorylated pulp. Preferably, the pulp should not be subjected to mechanical disintegration which leads high degree of fibrillation. Mixing and gentle shear can be applied on the suspension in order to create a homogenous suspension. Other suitable phosphorylating agents include phosphoric acid, phosphorus pentaoxide, phosphorus oxychloride, diammonium hydrogen phosphate and sodium dihydrogen phosphate.

In the reaction to form phosphorylated pulp, alcohol functionalities (-OH) in the cellulose are converted to phosphate groups (-OPO3^2'). In this manner, crosslinkable functional groups (phosphate groups) are introduced to the pulp fibers. Typically, the phosphorylated pulp is in the form of its sodium salt. Phosphorylated pulp or phosphorylated cellulose fibers can be tailored with different degree of substitution. In some embodiments, the degree of substitution is in the range of 0.1 -4.0, preferably in the range of 0.2 - 3.8, more preferably in the range of 0.3-3.0, or most preferably in the range of 0.4 to 2.0 mmol/g of phosphate groups as measured by a titration method or by using elemental analysis described in the prior art.

The phosphorylated fibers are preferably washed but the present inventors have found that relatively high amounts of residual chemicals or salts or impurities from the manufacturing process can be present in the material without significant adverse effects on the barrier properties or the optical or mechanical properties. The purity of phosphorylated cellulose fibers may typically be in the range of 50- 100%, wherein the remaining portion may include salts or electrolytes or neutralization or washing chemicals from the phosphorylation process, as well as dissolved hemicellulose. The phosphorylated fibers can also be phosphorylated pulp or a reject fraction from a phosphorylated fiber fibrillation plant.

In some embodiments, the phosphorylated cellulose fibers used in the MFC composition have a Schopper-Riegler (SR) number < 80, preferably < 60, and more preferably < 40, as measured according to the standard ISO 5267-1. These values refer to the phosphorylated fibers as such or the actual fiber content of the phosphorylated pulp. Preferably, the SR value is determined for a washed fraction The formulation of the MFC composition may vary depending on the intended use of the MFC composition and on the intended mode of application or formation of a film or coating of the MFC composition. The MFC composition may include a wide range of ingredients in varying quantities to improve the end performance of a film or coating of the MFC composition.

In some embodiments, the MFC composition further comprises a metal salt. A metal salt may assist crosslinking of the phosphorylated fibers, further improving the mechanical properties of coatings or films formed of the MFC composition. The metal salt may preferably be added during or after coating or film formation.

The MFC composition 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 composition may for example comprise additives that will improve different properties of the MFC composition and/or a film or coating formed thereof, such as latex and/or polyvinyl alcohol (PVOFI) for enhancing the ductility of the coating. In some embodiments, the MFC composition further comprises a water-soluble polymer selected from the group consisting of a starch, a polyvinyl alcohol (PVOFI), a cellulose derivative, a hemicellulose, a polyacrylamide, a polydiallyldimethylarnmonium chloride (PDADMAC), a polyvinylamine (PVAm), a polyethyleneimine (PEI), a protein or a mixture thereof, preferably a PVOFI.

In some preferred embodiments, the water-soluble polymer is a PVOFI. The PVOFI may be a single type of PVOFI, or it can comprise a mixture of two or more types of PVOFI, differing e.g. in degree of hydrolysis or viscosity or different functional groups. The PVOFI 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 PVOFI 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 K6726. In some embodiments, the MFC composition comprises in the range of 0.1-20 wt%, preferably in the range of 0.5-15 wt%, more preferably in the range of 1-10 wt% of the water-soluble polymer, based on the total dry weight of the MFC composition. In some embodiments, the MFC composition further comprises a pigment. The pigment may for example comprise inorganic particles of talcum, silicates, carbonates, alkaline earth metal carbonates and ammonium carbonate, or oxides, such as transition metal oxides and other metal oxides. The pigment may also comprise nano-size pigments such as nanoclays and nanoparticles of layered mineral silicates, for instance selected from the group comprising montmorillonite, bentonite, kaolinite, hectorite and hallyosite.

In order to retain the transparency or translucency and the gas barrier properties of the films or coatings formed of the MFC composition, the particle size and the concentration of the pigments should preferably be low. In some embodiments, the pigment is selected from the group consisting of nanoclays and nanoparticles of layered mineral silicates, more preferably bentonite.

In some embodiments, the MFC composition comprises in the range of 0.1-20 wt%, preferably in the range of 0.5-15 wt%, more preferably in the range of 1-10 wt% of the pigment, based on the total dry weight of the MFC composition.

The MFC composition of the first aspect described above is useful in the preparation of MFC films, particularly transparent or translucent MFC films. Thus, according to a second aspect illustrated herein, there is provided an MFC film comprising an MFC composition as described above with reference to the first aspect. 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 phosphorylated cellulose fibers can be used to improve the mechanical properties and flexibility of MFC films, while still maintaining the good transparency or translucency and good gas barrier properties characteristic to MFC films without added natural or synthetic fibers with high scattering efficiency and which cause reduced barrier properties.

The addition of the phosphorylated cellulose fibers in the MFC film improves the mechanical strength of the film as compared to a corresponding MFC film without the phosphorylated cellulose fibers, i.e. in which the phosphorylated cellulose fibers are replaced by MFC. In some embodiments, the MFC film has a tear index at least 5% higher, preferably at least 10% higher, more preferably at least 15% higher, than the tear index of a corresponding film without the phosphorylated cellulose fibers, as measured according to the standard ISO 1974.

In some embodiments, the MFC film has an oxygen transfer rate (OTR), as measured according to the standard ASTM F-1927-98 at 50% relative humidity and 23 °C, of less than 30 cc/m²/24h/atm, and preferably less than 20 cc/m²/24h/atm.

In some embodiments, the MFC film has a transparency of at least 75%, preferably at least 80%, as measured according to the standard DIN 53147.

The MFC films or coatings are often used as a barrier layer in a multilayer material, such as a paper or paperboard comprised of two or more plies.

According to a third aspect illustrated herein, there is provided a multilayer material comprising at least: a substrate layer and an MFC layer comprising an MFC composition as described above with reference to the first aspect. In some embodiments, the MFC layer is an MFC film as described above with reference to the second 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/m²), preferably in the range of 80-400 gsm.

In some embodiments, the multilayer material is a paper or paperboard comprised of two or more plies, wherein at least one ply comprises an MFC composition as described above with reference to the first aspect.

In some embodiments, the at least one ply comprising an MFC composition is an MFC film as described above with reference to the second aspect.

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

According to a fourth aspect illustrated herein, there is provided a method of preparing an MFC film, said method comprising: a) preparing an aqueous MFC composition as described above with reference to the first aspect; b) forming a film of the aqueous MFC composition; and c) allowing the film to dry to obtain the MFC film.

Thanks to the anionicity of the phosphorylated cellulose fibers, the aqueous MFC composition may be prepared in a number of different ways. The aqueous MFC composition may for example be prepared by mixing dry MFC and dry phosphorylated cellulose fibers and dispersing the dry mixture in water, or by adding phosphorylated cellulose fibers, e.g. in the form of phosphorylated cellulose pulp, to an aqueous MFC dispersion, preferably a high consistency MFC dispersion. The phosphorylated cellulose fibers, e.g. in the form of phosphorylated cellulose pulp, can be added dry or in high consistency form. High consistency form means that the consistency of the pulp is higher than 5 wt% preferably higher than 10 wt%, and more preferably higher than 15 wt%.

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 phosphorylated cellulose fibers having a Schopper-Riegler (SR) number < 80, preferably < 60, and more preferably < 40, as measured according to the standard ISO 5267-1. 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. The drying source might may have a much higher temperature than the actual film. Drying at elevated temperature may assist crosslinking of the phosphorylated fibers, further improving the mechanical properties of coatings or films formed of the MFC composition. Drying means that 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.

The present disclosure is based on the inventive realization that phosphorylated cellulose fibers can be used to improve the mechanical properties of MFC films, while still maintaining the good transparency or translucency and good gas barrier properties characteristic to MFC films without added fibers.

According to a fifth aspect illustrated herein, there is provided the use of phosphorylated cellulose fibers as a strength enhancement agent in an MFC composition for preparation of transparent or translucent films or coatings.

The phosphorylated cellulose fibers preferably have an average fiber diameter (fiber width) above 1000 nm. In some embodiments, the phosphorylated cellulose fibers of the MFC composition have a fiber width of more than 5 pm, preferably more than 10 pm, as measured using an FS5 optical fiber analyzer (Valmet).

The phosphorylated cellulose fibers preferably have a length-weighted mean fiber length above 0.5 mm, i.e. above 500000 nm. In some embodiments, the phosphorylated cellulose fibers have a length-weighted mean fiber length in the range of 0.2-4 mm, preferably in the range of 0.3-2 mm, more preferably in the range of 0.5-2 mm. Length-weighted mean fiber length as used herein refers to the length-weighted mean fiber length (Lc(l)) as measured according to the standard 16065-2 using an FS5 optical fiber analyzer (Valmet).

In some embodiments, the phosphorylated cellulose fibers have a Schopper- Riegler (SR) number < 80, preferably < 60, and more preferably < 40, as measured according to the standard ISO 5267-1.

In some embodiments, the MFC composition of the fifth aspect is an MFC composition 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.

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.

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

Microfibri Hated cellulose (MFC):

The microfibri Hated cellulose used in the experiments was prepared from pre- treated pine kraft pulp and obtained through mechanical disintegration using a microfluidizer. The sample was run through the microfluidizer 3 times, using 2 passes through 400 and 200 micron chambers and 1 pass through 200 and 100 micron chambers. Phosphorylated pulp (p-pulp):

Kraft pulp fibers were treated with a mixture of phosphoric acid, urea and water (Table 1). The treated fibers were then dried at 80 °C until the moisture content was below 1%. The phosphorylation reaction was conducted by increasing the temperature to 150 °C and allowing the treated fibers to react for 10 min. The phosphorylated pulp fibers (referred to herein as unwashed (UW) phosphorylated pulp fibers) were subsequently dispersed in distilled water and washed in a porous fabric. The washed pulp was then re-dispersed in distilled water and the pH was adjusted to 12.5 with 50% NaOH and thereafter re-washed until pH was below 10. The phosphorylated pulp was subsequently allowed to drain inside the porous fabric to obtain a dry matter content of ca. 10%. The degree of substitution (DS) was determined by acid titration. Table 1. Conditions used to prepare the different phosphorylated pulp fibers and respective degree of substitution (DS) as determined by acid titration and fiber properties.

Analysis

The MFC can be characterized with different means. One way to estimate the particle size dimension is by laser diffraction. By using high resolution microscope such as Scanning Electron Microscope, it is possible to evaluate the thickness and thickness distribution. Another method to estimate MFC fineness is to determine the drainage resistance using the Schopper-Riegler method according to the standard ISO 5267-1. The SR value of the unwashed phosphorylated fibers were 11 measured according to ISO 5267-1 and the SR value of the low DS phosphorylated pulp were 11.5 measured according to ISO 5267-1.

Dimensional analysis of the p-pulps was carried out using a FS5 optical fiber analyzer (Valmet). The fiber length analysis was done according to the standard ISO 16065-2. The zero-span analysis of unwashed (UW) p-pulp was done according to the standard ISO 15361:2000.

The oxygen barrier properties of the films were measured according to the standard ASTM F 1927-98 at 50% RH and 23 °C.

The water vapor barrier properties were measured according to the standard ASTM F 1249-90 at 50 % RH and 23 °C. The optical properties of the films were determined according to the standard ASTM D1003-13 which measures the haze and luminous transmittance of transparent materials. In addition, the total light transmittance of the films was determined using Cary 100 UV/VIS spectrophotometer with DRA CA301 Integrating Sphere in the transmission port in the wavelength area of 200-890 nm. In addition, the transparency of the films was determined according to the standard DIN 53147.

The determination of the tensile strength properties was done according to the standard ISO 1924 with the exception that the draw cap was 20 mm, the sample width was 15 mm, clamp pressure was 6 bar and the draw speed was 0.1 mm/s. The tear strength of the films was determined according to the standard ISO 1974.

Example 1 (Comparative) - MFC film prepared by cast forming 20 or 30 gsm (g/m²) films comprising 100% microfibrillated cellulose were prepared by dispersing a non-chemically modified microfibrillated cellulose in RO water to a consistency of 0.25 wt%. The suspension was then cast coated on a Petri dish in order to simulate a cast film forming process. The sample was dried in 23 °C / 50 % RH. The mechanical properties were determined for the 30 gsm films, and the barrier and optical properties were determined for the 20 gsm films. The results are presented in Table 2.

Example 2 - 10 % of Low PS (degree of substitution) phosphorylated pulp (p-pulp) Non-chemically modified MFC and low DS p-pulp were weighed in a given ratio of 90/10 w/w and dispersed in RO water to a total consistency of 0.25 wt%. A sample film was cast coated on a Petri dish as in Example 1. The results are presented in Table 2.

Example 3 - 20% of Low DS p-pulp Similar setup as in Example 2 but with 20% of low DS p-pulp. Non-chemically modified MFC and low DS p-pulp were weighed in a given ratio of 80/20 w/w and dispersed in RO water to a total consistency of 0.25 wt%. A sample film was cast coated on a Petri dish as in Example 1. The results are presented in Table 2. The higher content of low DS p-pulp shows that transparency is improved and barrier properties on same level or even slightly lower compared to the reference (Example 1). A significant increase in tear strength was seen.

Example 4 - 10% of high PS o-oulo 10% of a high DS p-pulp was added to an MFC suspension prior to film formation.

A sample film was cast coated on a Petri dish as in Example 1. The results are presented in Table 2. In particular, stretch was significantly improved when comparing to the reference (Example 1) and to the p-pulp having lower DS (Example 2).

Example 5 - 20% of high DS p-pulp

20% of a high DS p-pulp was added to the MFC suspension prior to film formation. A sample film was cast coated on a Petri dish as in Example 1. The results are presented in Table 2. The effects of 20% addition of this high DS p-pulp included increased stretch and tear index, but also transmittance was improved especially at 200 nm. Despite the high concentration pf p-pulp, the oxygen barrier (OTR) was maintained on a very good level.

Example 6 - 10% of DP (dissolving pulp) p-pulp 10% of a p-pulp made of dissolving pulp was added to an MFC suspension prior to film formation. A sample film was cast coated on a Petri dish as in Example 1. The results are presented in Table 2. The use of 10% p-pulp made from dissolving pulp resulted in a tear index comparative to the reference samples but the stretch was significantly higher.

Example 7 -20% of DP (dissolving pulp) p-pulp

20% of a p-pulp made of dissolving pulp was added to an MFC suspension prior to film formation. A sample film was cast coated on a Petri dish as in Example 1. The results are presented in Table 2. The use of 20% p-pulp made from dissolving pulp resulted in a tear index comparative to the reference samples but the stretch was significantly higher. Also, the oxygen barrier properties (OTR) was significantly improved. The transparency or translucency was on same level as obtained when using 20% of the high DS p-pulp. Example 8 - 10% of unwashed p-pulp

10% of an unwashed p-pulp was added to an MFC suspension prior to film formation. The unwashed p-pulp refers to a Low DS pulp of Table 1 , but where the pulp was not washed after the reaction phosphorylation. A sample film was cast coated on a Petri dish as in Example 1. The results are presented in Table 2.

The pulp was washed with RO water and filtrate analyzed after which the organic and ash content were determined. It was estimated that the total dissolved material content was about 40 wt%, whereas the ash content was 34 % when determined at 525 °C according to the standard ISO 1762.

Example 9 - 20% of unwashed p-pulp

20% of an unwashed p-pulp was added to an MFC suspension prior to film formation. The unwashed p-pulp refers to a Low DS pulp of Table 1 , but where the pulp was not washed after the reaction phosphorylation. A sample film was cast coated on a Petri dish as in Example 1. The results are presented in Table 2. The film showed very low OTR value and high transmittance.

Example 10 (Comparative) - Reference film made with wet laid method In this example, the same MFC as used in Example 1 was used. The 20 and 30 gsm films were prepared using a vacuum dewatering method. The chemically non- modified microfibrillated cellulose was dispersed in 0.1 wt% consistency. The pH of the suspension was not adjusted. Vacuum filtration was done using 0.65 pm DVPP filters as filtration media. The films were couched and wet pressed prior to drum drying at 80 °C for 90 minutes. The mechanical properties of the films were determined for the 30 gsm films, and the barrier and optical properties were determined for the 20 gsm films. The results are presented in Table 2.

Example 11 - 10% Low DS p-pulp

The same experimental setup as in Example 10 was used, but with a mixture of MFC and 10% low DS p-pulp as prepared in Example 2. The results are presented in Table 2. A small improvement in tear index was observed while, stretch and barrier decreased slightly as compared to the corresponding reference (Example 10). Example 12 - 20% Low PS p-pulp The same experimental setup as in example 10 was used, but with a mixture of MFC and 20% low DS p-pulp as prepared in Example 3. The results are presented in Table 2. A small increase in tear index and better OTR oxygen barrier properties were observed as compared to the corresponding reference (Example 10).

Table 2.

Claims

1. A microfibrillated cellulose (MFC) composition for preparation of transparent or translucent films or coatings, said composition comprising: MFC at a concentration in the range of 50-99.9 wt%, and phosphorylated cellulose fibers at a concentration in the range of 0.1-50 wt%, based on the total dry weight of the MFC composition.

2. The MFC composition according to claim 1 , wherein the MFC composition is a dispersion.

3. The MFC composition according to any one of claims 1-2, wherein the consistency of the dispersion is 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%.

4. The MFC composition according to any one of claims 1-3, wherein the MFC composition has a Schopper-Riegler (SR) number number > 20, preferably > 30, and more preferably > 40, as measured according to the standard ISO 5267-1. 5. The MFC composition according to any one of claims 1 -4, wherein the MFC composition comprises in the range of 50-99.5 wt%, preferably in the range of 60- 99.

5 wt%, more preferably in the range of 65-98 wt% of MFC, based on the total dry weight of the MFC composition.

6. The MFC composition according to any one of claims 1 -5, wherein the MFC composition comprises in the range of 0.1-30 wt%, preferably in the range of 0.5- 20 wt%, more preferably in the range of 1-10 wt% of phosphorylated cellulose fibers, based on the total dry weight of the MFC composition.

7. The MFC composition according to any one of claims 1-6, wherein the MFC is unmodified MFC or chemically modified MFC, or a mixture thereof.

8. The MFC composition according to any one of claims 1-7, wherein the MFC has a Schopper-Riegler (SR) number > 70, preferably > 80, and more preferably > 90, as measured according to the standard ISO 5267-1.

9. The MFC composition according to any one of claims 1 -8, wherein said phosphorylated cellulose fibers have a length-weighted mean fiber length in the range of 0.2-4 mm, preferably in the range of 0.3-2 mm, more preferably in the range of 0.5-2 mm.

10. The MFC composition according to any one of claims 1-9, wherein said phosphorylated cellulose fibers have a degree of phosphorylation of 0.1 mmol/g or higher, preferably 0.3 mmol/g or higher.

11. The MFC composition according to any one of claims 1 -10, wherein said phosphorylated cellulose fibers have a Schopper-Riegler (SR) number < 80, preferably < 60, and more preferably < 40, as measured according to the standard ISO 5267-1.

12. The MFC composition according to any one of claims 1-11, wherein the MFC composition further comprises a water-soluble polymer selected from the group consisting of a starch, a polyvinyl alcohol (PVOFI), 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 PVOFI.

13. The MFC composition according to claim 12, wherein the MFC composition comprises in the range of 0.1-20 wt%, preferably in the range of 0.5-15 wt%, more preferably in the range of 1-10 wt% of the water-soluble polymer, based on the total dry weight of the MFC composition.

14. The MFC composition according to any one of claims 1-13, wherein the MFC composition further comprises a pigment, preferably a pigment selected from the group consisting of nanoclays and nanoparticles of layered mineral silicates, more preferably bentonite.

15. The MFC composition according to claim 14, wherein the MFC composition comprises in the range of 0.1-20 wt%, preferably in the range of 0.5-15 wt%, more preferably in the range of 1-10 wt% of the pigment, based on the total dry weight of the MFC composition.

16. An MFC film comprising an MFC composition according to any one of claims

1-15.

17. The MFC film according to claim 16, wherein the 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.

18. The MFC film according to any one of claims 16-17, wherein the film has a tear index at least 5% higher, preferably at least 10% higher, more preferably at least 15% higher, than the tear index of a corresponding film without the phosphorylated cellulose fibers, as measured according to the standard ISO 1974.

19. The MFC film according to any one of claims 16-18, wherein the film has an oxygen transfer rate (OTR), as measured according to the standard ASTM F-

1927-98 at 50% relative humidity and 23 °C, of less than 30 cc/m²/24h/atm, and preferably less than 20 cc/m²/24h/atm.

20. The MFC film according to any one of claims 16-19, wherein the film has a transparency of at least 75%, preferably at least 80%, as measured according to the standard DIN 53147.

21. A multilayer material comprising at least: a substrate layer and an MFC layer comprising an MFC composition according to any one of claims 1-15.

22. The multilayer material according to claim 21 , wherein the substrate layer is paper or paperboard.

23. The multilayer material according to claim 22, wherein the paper or paperboard has a basis weight in the range of 20-500 gsm, preferably in the range of 80-400 gsm.

24. A method of preparing an MFC film, said method comprising: a) preparing an aqueous MFC composition according to any one of claims 2-15; b) forming a film of the aqueous MFC composition; and c) allowing the film to dry to obtain the MFC film.

25. The method according to claim 24, wherein 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 phosphorylated cellulose fibers having a Schopper-Riegler (SR) number < 80, preferably < 60, and more preferably < 40, as measured according to the standard ISO 5267-1.

26. Use of phosphorylated cellulose fibers as a strength enhancement agent in an MFC composition for preparation of transparent or translucent films or coatings.