TECHNICAL FIELD

The present invention relates to methods for hydrophobic modification of nanocellulose films, as well as the hydrophobic modified nanocellulose films themselves so as to improve the barrier properties of said films.

BACKGROUND

Microfibrillated cellulose (MFC) which is a kind of nanocellulose comprises partly or totally fibrillated cellulose or lignocellulose fibers. 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 (see e.g . Chinga- Carrasco, G., Nanoscale research letters 2011, 6 :417), 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 pressure-drop disintegration process (see Fengel, D., Tappi J ., March 1970, Vol 53, No. 3.) . 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) .

The microfibrils in MFC provide a great number of surface OH-groups, making MFC hydrophilic. Films of MFC have excellent barrier properties, in particular oxygen and fat barrier properties, but are affected by water because of the hydrophilic nature. Above ambient temperatures and moisture conditions the dense fibrillar structure loses its integrity and the properties can deteriorate. Enhanced hydrophobicity will introduce water repellency to the material. Hydrophobicity can also help to maintain the integrity of the film at high humidity and thereby the low oxygen permeability can be maintained . By adding a layer of a thermoplastic polymer like PE onto a MFC film, water-vapor properties can be achieved .

There exist many modification routes to introduce hydrophobicity to MFC, where the modification can be done prior to defibrillation, i.e. on the fibers as a pretreatment method, or by treating the MFC dispersion. In the latter, time-consuming solvent exchange can be needed to perform the reaction in a non-aqueous state. If the modified MFC grade is utilized to form MFC films, irrespective of the fabrication methods, the quality is often poor since the modification generally occurs on the primary and secondary OH-groups, which then are blocked for hydrogen bonding that provides for the strong fibrillar network of films.

The strong network can render the MFC films more brittle than plastic films, and this can limit their production as well as post-handling in different applications. Plasticizers can be utilized but such components normally reduce the oxygen barrier properties.

Deep Eutectic Solvents (DESs) are classified as a new generation of ionic liquids but that also can be obtained from non-ionic species. Still, they have many similar physico-chemical properties to ionic liquids such as density, viscosity, conductivity, etc. A DES is a fluid generated by mixing two or three components that are capable of self-association through hydrogen bond interactions, to form a eutectic mixture having a melting point lower than each of its constituents, and can thereby be a solvent at ambient temperature. These solvents also have low vapour pressure, and hence low volatility. (Zhang et al., Chemical Society reviews 2012, 41, 7108-7146)

DESs have been applied as a pretreatment to produce MFC. (J. Sirvio et al., Green Chem 2015, 17, 3401-3406 and P. Li et al., ACS Appl mater. Interfaces 2017, 9, 2846-2855). They can also be utilized as solvents for further chemical modification of fibrils. (T. Selkala et al., Chem. Sus. Chem. 2016, 9, 1-11; J. Sirvio et al., J. Mater. Chem A. 2017, 5, 21828-21835). Also, crosslinking between DES component and polysaccharides have been proven by FT-IR. (C. Zdanowicz, Carbohydrate Polymers 2016, 151, 103-112).

It is reported in the literature that cellulose films have been made from cotton linters with DES and a plasticizing effect has been seen with very high strain as a result (34 %) for these regenerated cellulose films. The films where formed by soaking cellulose gels into the choline chloride/urea aqueous solutions for 12 h, followed by drying the gels in an oven at 55°C for 24 h to obtain ChCI/urea plasticized cellulose films. (S. Wang et al., Carbohydrate Polymers 117 (2015) 133- 139)

It is an object of the present technology to provide a nanocellulose film with improved hydrophobicity. It is also an object to improve oxygen barrier properties at high relative humidity (RH). It is further an objective to achieve an improved strainability of the modified nanocellulose film. SUMMARY

In a first aspect, a method for increasing the hydrophobicity of a nanocellulose film is provided . The method comprising exposing the nanocellulose film to (i) a deep eutectic solvent (DES) and (ii) a hydrophobic surface-modifying agent, wherein exposure to (i) and (ii) can take place sequentially, or simultaneously (preferably simultaneously) .

In another aspect, a nanocellulose film is provided, which has a hydrophobic surface with a water contact angle of at least 90°, suitably at least 100°. Typically, the hydrophobic surface is provided by hydrophobic chains, such as e.g. C ₂ -C ₂₈ hydrocarbon chains.

Further details of the technology are presented in the following description and the dependent claims.

DETAILED DISCLOSURE

The present technology relates to using a deep eutectic solvent system combined with a hydrophobic surface-modifying agent (such as n-octylsuccinic anhydride or alkenyl succinic anhydride) to produce hydrophobic, moisture stable, and/or strainable MFC barrier films, where the modification takes place after the production of a nanocellulose film to not interfere with the hydrogen bonding between the fibrils.

To the best of our knowledge, the use of DES for nanocellulose film modification has not been reported in the literature before. In a typical method to produce a modified nanocellulose film, according to this invention, a pre-made nanocellulose film is exposed to a DES system (e.g . imidazole- triethylmethylammonium chloride) for 10 min to 24 h at 80 to 100 °C.

Shorter times can also be applied. The pre-made nanocellulose film can also be subjected to a pre-treatment prior to exposure to a DES system to create an all-cellulose composite film.

The contact angle of the modified film was clearly improved . The oxygen barrier properties are in the same range as for the reference nanocellulose film at high moisture conditions.

The enclosed experiments investigate whether it is possible to modify nanocellulose, especially MFC and all-cellulose composite films in deep eutectic solvents to improve their oxygen barrier properties in high relative humidity conditions. The chemistry behind the modification is to add functional groups through ester bonds to the cellulose chain by replacing free hydroxyl groups. The goal of the modification was that chemical reaction only occurs with the free hydroxyl groups on surface of the film, so that most of the inner hydrogen bond network will not be disturbed, keeping the required mechanical properties intact or further improving the flexibility of the film.

Therefore, a method for increasing the hydrophobicity of a nanocellulose film is provided. The method comprises exposing the nanocellulose film to (i) a deep eutectic solvent and (ii) a hydrophobic surface-modifying agent, wherein exposure to (i) and (ii) can take place sequentially, or simultaneously. Suitably, exposure of the nanocellulose film to (i) and (ii) takes place simultaneously. Alternatively, exposure of the nanocellulose film to (i) and (ii) takes place sequentially, with exposure to the deep eutectic solvent (i) taking place before exposure to the hydrophobic surface-modifying agent (ii). The hydrophobic surface-modifying agent (ii) may also be mixed with said deep eutectic solvent (i), and the nanocellulose film will be exposed to said mixture of (i) and (ii). The exposure of the film to the deep eutectic solventand the hydrophobic surface modifying agent is preferably done for a period of 30 seconds to 1500 minutes, preferably for a period of 1 minute to 60 minutes, even more preferably for a period of 1 minute to 15 minutes. The treatment is preferably done at a temperature of 20-100 °C, preferably between 60-100 °C and even more preferred at a temperature of 60-80 °C. However, the optimal temperature may differ depending on the DES system used. The dry content of the film being exposed is high, preferably above 85 % by weight, even more preferably above 95% by weight. If the film comprises too much water the reagents in the DES system will not react with the film but with the water instead.

Consequently, the dry content of the film needs to high in order for the treatments to be successful.

The deep eutectic solvent, the hydrophobic surface-modifying agent and the optionally aqueous solvent used in the pre-treatment may be added to the nanocellulose film by spraying, coating, spreading, vapour deposition and/or printing the solution onto the surface/s of the film. The film may also be submerged into the solutions.

If desired, the nanocellulose film may be subjected to a pre-treatment, prior to exposure to (i) and (ii). Pre-treatment may be carried out using any solvent that can interact with the hydroxyl groups on cellulose and partly dissolve the sample. In particular, pre-treatment of the nanocellulose film may take place using an aqueous solution of tetraalkylammonium hydroxide, such as tetraethylammonium hydroxide, tetraalkylammonium chloride, such as tetraethylammonium chloride or tetrabutylammonium chloride or mixtures of hydroxides of group I or II metals, such as sodium hydroxide and urea, thiourea or a zinc salt, preferably zinc oxide. After the pre-treatment, the nanocellulose film can be treated with a solvent to remove the aqueous solution and terminate the dissolution reaction and to precipitate the dissolved parts. The solvent is preferably a water miscible solvent, preferably an alcohol, such as ethanol, methanol, isopropanol and/or water. The nanocellulose film is treated with the solvent prior to the following surface-modification. Pre-treated nanocellulose films are referred to herein as "all-cellulose composite film". The pre-treatment with the aqueous solution may occur for a period of 1 second to 60 minutes and at a temperature between 20- 50°C, preferably at a temperature between 20-30°C.

After the reaction, the deep eutectic solventand any excess hydrophobic surface-modifying agent should be removed from the modified nanocellulose film. Suitably, the deep eutectic solvent is removed by washing the modified nanocellulose film with a solvent, e.g. alcohols or water, or combinations thereof, suitably water. It has been discovered that washing with ethanol forms pores in the film which are not formed when water is utilized.

Henriksson et al. (Biomacromolecules 9(6) : 1579-1585; 2008) studied making of porous films from nanocellulose films. Authors stated that films prepared from water had porosities up to 28%, when films prepared from less polar liquids (methanol, ethanol and acetone) showed reducing capillary forces and porosities up to 40%. This probably explains the problem:

ethanol loosens the inte rfibrillar network making the films more porous, therefore the oxygen molecules have a shorter way to pass through the film.

The treated film is preferably dried after being modified and the liquids have been removed. The film may be dried using any drying equipment known to a person skilled in the art.

In an alternative method for providing a hydrophobic-modified nanocellulose film, unmodified cellulose pulp is exposed to (i) a deep eutectic solvent and (ii) a hydrophobic surface modifying agent, wherein exposure to (i) and (ii) can take place sequentially, or

simultaneously, to provide hydrophobically-modified cellulose pulp; followed by the steps of: a. microfibrillation of said hydrophobic-modified cellulose pulp to provide hydrophobically modified microfibrillated cellulose (MFC), b. optionally, blending the hydrophobically modified nanocellulose with another grade of polysaccharide such as e.g. native nanocellulose, native microfibrillated cellulose or starch; and c. film formation of said hydrophobically modified microfibrillated cellulose (MFC) or said blend, to provide a hydrophobically modified nanocellulose film.

If the hydrophobically modified nanocellulose is blended with another grade of polysaccharide it is preferred that the mixture comprises 20-100% of hydrophobically modified MFC, preferably between 30-80%, even more preferred 40-60%. Nanocellulose

By nanocellulose is meant any one of nanofibrillated cellulose (NFC), microfibrillated cellulose (MFC), bacterial cellulose and/or nanocrystalline cellulose.

Nanocellulose Films

With film is meant a thin substrate with good gas, aroma or grease or oil barrier properties, preferably oxygen barrier properties. The film preferably has a basis weight of less than 100 g/m ² and a density in the range from 700-1400 kg/m ³ .

The film may be produced by applying a suspension comprising nanocellulose, for example microfibrillated cellulose onto a wire. The wire may be porous felt wire or made from polymer of metal. It may also be possible to apply the suspension comprising nanocellulose by casting the suspension onto a substrate. The substrate may be a polymer or metal substrate. The casted fibrous web can then be dried and optionally peeled off from the substrate. The applied nanocellulose suspension is thereafter dried to form a nanocellulose film.

The film preferably comprises 70-100% by weight of nanocellulose, for example

microfibrillated cellulose based on total dry weight of the film, preferably between 80-95% by weight of nanocellulose.

The film may further comprise other additives such as any one of a starch, carboxymethyl cellulose, a filler, retention chemicals, flocculation additives, deflocculating additives, wet strength additives, dry strength additives, plasticizers and polymers, such as polyvinyl acetate (PVAc), polyvinyl alcohol (PVOH), polyvinyl alcohol-acetate (PVOH/Ac), ethylene vinyl alcohol (EVOH), softeners or mixtures thereof.

MFC and MFC films

Microfibrillated cellulose (MFC) or so called cellulose microfibrils (CMF) shall in the context of the present invention mean a micro-scale cellulose particle fiber or fibril with at least one dimension less than 100 nm. MFC comprises partly or totally fibrillated cellulose or lignocellulose fibers. 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 300 m ² /g, such as from 1 to 200 m ² /g or more preferably 50-200 m ² /g when determined for a freeze-dried material with the BET method. Various methods exist to make MFC, such as single or multiple pass refining, pre-treatment 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 supplied may thus be 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, 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.

The microfibrillar cellulose may contain some hemicelluloses; the amount 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, single - or twin-screw extruder, 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 e.g. other chemicals present in wood fibers or other lignocellulosic fibers used in papermaking processes. The product might also contain various amounts of micron size fiber particles that have not been efficiently fibrillated.

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 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 above described definition of MFC includes, but is not limited to, the proposed TAPPI standard W13021 on cellulose nano- or microfibril (CNF or CMF, respectively) defining a cellulose nanofiber material containing multiple elementary fibrils with both crystalline and amorphous regions, having a high aspect ratio with width of 5-30 nm and aspect ratio usually greater than 50.

Deep eutectic solvent (DES)

The deep eutectic solvent (component i) activates the nanocellulose film, and provides a (non-aqueous) environment in which the hydrophobic surface-modifying agent (component ii) is stable and can react with the nanocellulose film. According to the present invention, DESs are a particular class of ionic liquids, but DESs can also be obtained from non-ionic species. They comprise a eutectic mixture of compounds having a melting point much lower than either of the individual components. In the present invention, the DES may be selected from imidazole - triethylmethylammonium chloride, lithium chloride-urea, malic acid - choline chloride, malic acid - proline, AlCh - l-ethyl-3- methylimidazolium chloride, AlCh - urea as well as combinations of imidazole or choline chloride with one or more salts selected from AgCI, CuCI, LiCI, CuCh, SnCh, ZnCI ₂ , LaCh,

YCI ₃ , and SnCU-

An advantage with the use of deep eutectic solvent (DES) compared to many ionic liquids is that it is less toxic to use.

Hydrophobic surface-modifying agent

The hydrophobic surface-modifying agent (component ii) reacts with the activated OH-groups on the nanocellulose or MFC surface. The hydrophobic surface-modifying agents for use herein may therefore comprise at least one hydrophobic moiety and at least one OH-reactive moiety.

The hydrophobic moiety of said hydrophobic surface-modifying agent(s) may be a

hydrophobic chain, suitably a hydrocarbon chain, preferably a C ₂ -C ₂₈ hydrocarbon chain, more preferably a C ₅ -C ₂₂ hydrocarbon chain.

In one aspect, the OH-reactive moiety is selected from an epoxy, a silicon halide, a silazane, a silane, a chlorosilane, an organic acid, an organic acid ester, an organic acid anhydride, an organic acid halide, an organic amide, or a combination thereof.

In one aspect, the hydrophobic surface-modifying agent is selected from the group of C ₂ -C ₂₈ fatty acids, C ₂ -C ₂₈ fatty acid esters, C ₂ -C ₂₈ fatty acid anhydrides, C ₂ -C ₂₈ fatty acid amides and C ₂ -C ₂₈ fatty acid halides such as C ₂ -C ₂₈ fatty acid chlorides.

Particular examples of the hydrophobic surface-modifying agent are acetic anhydride, alkenyl succinic anhydrides (ASA), n-octyl succinic anhydride, tetradecenyl succinic anhydride (TDSA), iso-octadenyl succinic anhydride (iso-ODSA), acetyl chloride, ethyl acetate, 1- acetylimidazole, isopropenyl acetate, palmitic acid, stearic acid, palmitoyl chloride, octadecanoyl chloride, hexadecyltrimethoxysilane (HMDS), (3-aminopropyl)triethoxysilane or bis(trimethylsilyl)acetamide (BSA), preferably acetic anhydride, alkenyl succinic anhydrides (ASA), n-octyl succinic anhydride, palmitic acid, stearic acid, and (3-aminopropyl) triethoxysilane. Modified nanocellulose film

The method described herein provides a route to modify nanocellulose films and especially microfibrillated cellulose films to be more hydrophobic.

A hydrophobically-modified nanocellulose film is also provided, which has a water contact angle of at least 90°, suitably at least 100°. The hydrophobic modification is suitably provided by hydrophobic chains, such as e.g. C ₂ -C ₂₈ hydrocarbon chains. Such hydrophobic chains are bonded to the cellulose fibrils of the nanocellulose, preferably at least partly at the surface of the nanocellulose film.

The hydrophobic nature of modified and unmodified MFC films can be tested by placing a water droplet on said film. A hydrophobically modified nanocellulose film has a water contact angle (CA) of at least 90°, suitably at least 100°, measured by contact angle measurement. The contact-angle measurement is based on the ISO standard TC 6/SC 2/WG 41 : Paper and board - Measurement of water contact angle by optical methods.

When the unmodified nanocellulose film was tested, the water droplet was absorbed directly into the film. However, tests on water droplet placed on top of a hydrophobic modified nanocellulose film showed that the droplet typically stood during the measurement.

The extent and nature of the hydrophobic modification can be established by e.g. infrared spectroscopy. Indeed, infrared spectroscopy on the nanocellulose film modified in the mixture of DES and hydrophobic surface-modifying agent showed that the dissolved agent reacted with the nanocellulose film.

Also provided is a hydrophobically-modified nanocellulose film obtained or obtainable via the methods described herein.

The hydrophobically-modified nanocellulose films have uses as liquid barriers in e.g. liquid- or food- packaging material. In one aspect, therefore, a liquid or food packaging material is provided which is a laminate of one or more layers of a hydrophobically-modified

nanocellulose film as described herein, with one or more base layers of paper or paperboard and/or one or more layers of a polymer. The polymer may preferably be polyethylene (PE), polypropylene (PP) and/or polyethylene terephthalate (PET). The film may be coated or laminated with one or more layers of polymer/s on one or both sides of the film. The polymer laminate can for example be used as a pouch in food packaging. All details of the methods described above are also relevant for the modified nanocellulose film, as described herein.

EXAMPLES

Materials

Pre-Masuko ground dissolving pulp in aqueous solution (consistency of 1.66 %-w) was used as a cellulose raw material for MFC production, and birch pulp for modified MFC production.

1. Microfibrillation of cellulose

The pre-Masuko ground dissolving pulp was diluted to a consistency of 0.5%-w and then mixed at 10000 rpm with an Ultra-Turrax mixer (IKA T25, Germany) for 1 min to achieve a homogeneous suspension. The suspension was microfibrillated through microfluidizer (Microfluidics M-110EH-30, USA) 5 times: one time through 400 pm and 200 pm chambers at a pressure of 1000 bar, and 4 times through 400 pm and 100 pm chambers at a pressure of 1500 bar. After microfibrillation, two samples were taken from the suspension and dried overnight at 100°C to determine the dry-matter content.

2. Preparation of MFC films

MFC films were prepared by measuring 0.265 g abs. (grammage of the film 60 g/m ² ) of dry microfibril suspension into a decanting glass and then dilution with water to total weight of 100 g. After this, the sample was degassed by ultrasonic treatment for 10 min. Next the sample was vacuum filtered on top of the membrane (Durapore DVPP 0,65 pm, Merck Millipore Ltd., Ireland) using a negative pressure of approximately 800 mbar. When the film was formed and the excess water was removed, the film was dried by vacuum drier (Karl Schroder KG, Germany) for 10 min at temperature of 93°C and negative pressure of 930 mbar. Finally, the weight of the film was measured.

3. Preparation of deep eutectic solvent and MFC film modification

Imidazole-TEMACI DES (molar ratio of 7 :3) was prepared by weighing the components into a decanting glass (total of 120 g) and heating it in the oil bath at 80°C or 100°C, depending on the sample. When approximately half of the liquid was formed, the magnetic stirrer was enabled and kept constantly mixing to speed until formation of the DES.

The hydrophobic surface-modifying agent was added to the DES when a clear liquid was observed. Before the MFC film was put into the DES/agent system, the film was washed with acetone to remove possible impurities that could disturb the chemical reaction. The film was submerged in the DES/reagent system horizontally and a film protection device prevented the magnetic stirrer touching the film. The stirrer was kept on throughout the reaction. The ratio between reagent and film was 10: 1 in weight and the amount of DES was selected so that there was enough liquid around the film. After the desired reaction time was achieved, the film was washed three times (approximately 5 min at a time) with ethanol or water (depending on the sample) to remove the DES and the reagent from the film. The washing liquid was changed between washing steps. The film was dried by vacuum drier for 10 min using the same apparatus and conditions as in the preparation of MFC film.

4. Preparation of all-cellulose composite film (pre-treatment)

All-cellulose composite film was fabricated by submerging the MFC film in

tetraethylammonium hydroxide for 30 s in room temperature, subsequently washing with ethanol was performed 4 times for approximately 5 min each time. Fresh ethanol was used in every wash. After washing with ethanol, the all-cellulose composite film was washed with acetone before being modified in the DES/reagent system. The same procedure (above step 3) was used to modify the all-cellulose composite film as in MFC film modification.

5. Summary of experiments

An overview of the samples prepared is given in Table 1, with an unmodified MFC reference (Reference), samples A-P prepared according to the method steps 1-3 above, and sample Q according to method steps 1-4 (i.e. with pre-treatment).

Characterization of MFC films

Oxygen transmission rate

The oxygen transmission rate (OTR) of the films was measured using a MOCON Ox-Tran 2/22 (Minneapolis, MN, USA). The film was exposed to 100% oxygen gas on one side and to 100% nitrogen gas on the other side. The oxygen permeability (OP) was calculated by multiplying the OTR by the thickness of the film and dividing it by the difference in the oxygen gas partial pressure between the two sides of the film. Before the OTR measurement, the thickness of the film was determined as an average of three random measuring points using Precision Thickness Gauge (FT3, Hanatek Instruments, United Kingdom). The measurements were carried out at 23°C or at 38°C, normal atmospheric pressure, and relative humidity of 50%, 80% and 90% with a specimen are of 5 cm ² . The OTR were measured according to ASTM F 1927-98.

Fourier-transform infrared spectroscopy

Fourier-transform infrared spectroscopy (FTIR) was used to see if the chemical reactions had occurred. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) was used to determine the spectrum for which a little piece of the film was cut. The spectra were obtained by Bruker Vertex 80v (USA) with a wavelength range of 400-4000 cm ¹ and with the total of 40 scans at a resolution of 4 cm ¹ for each sample.

Contact angle measurements

The method for measurement of contact angle (CA) is based on standard ISO TC 6/SC 2/WG 41 : Paper and board - Measurement of water contact angle by optical methods. Contact angle was measured for 10 seconds and values for each second and at 0.1 s were noted. The values are an average from 3 drops. The liquid used was Milli-Q water, drop size was 4 pi and drops were evaluated by the software calculation program, evaluation method Circle.

Tensile strength

The tensile strength of the films was measured by a universal testing machine (Instron 5544, USA). The films were kept in a constant temperature and humidity conditions (temperature 23 ± 0.5°C, RH 50 ± 2%) 18 h before the measurements, during the sample preparations and during the measurements. Before the sample preparation, the thickness of the films was measured by taking an average value of five random points on the film using the same device as used above. The samples were prepared by cutting 3-4 strips from the film with a width of 5 mm and length of 70 mm. For the tensile testing, the gauge length was set to 40mm and the strain was controlled at 5 mm/min with a pre-strain value of 1 MPa, until the sample broke.

Results DRIFT spectroscopy

In all samples A-Q, except Reference, IR signal at wavenumbers of 1735-1750 cm ¹ were seen, indicating that the esterification reaction between the reagent and hydroxyl groups of the cellulose occurred in DES. The C=0 stretch of the ester signal at wavenumbers 1735- 1750 cm ^-1 was the indication that the modification had succeeded. Samples which are modified with the same reagent had essentially the same DRIFT profile, regardless of the temperature, reaction time or washing liquid used.

Oxygen transmission rate (OTR) and oxygen permeability (OP)

OTR measurements were carried out on certain water-washed films, and the OTR and OP results are presented in Table 2. The OTR results at 38°C and 90% RH are the average of two different samples, except for sample Q. Best oxygen barrier properties are seen at low OTR and OP values. The OP value takes the thickness of the film into account.

Table 2. CA, OTR and OP values for chosen films. OTR values are expressed in ml/(m ² -day); OP values in ml^m/(m ² day kPa).

*Over-range; **Not analyzed

In Table 2, sample B, E, and F have the same reagent - isopropenyl acetate - but the reaction temperature is lower for F compared to E, which seems to give a slight higher OTR at 23/80 for F. The reason for the high value for sample B is due to washing in ethanol and not in water as for E and F.

In Table 2, sample L, M, N, P and Q have the same reagent - n-octyl succinic anhydride - and lower reaction times have been applied : 10-60 minutes. The best samples, with the lowest OTR at 23/80 and OP, are M, N and P, and they are in the same range as the reference. Compared to L, a much shorter reaction time was employed : 10-15 vs 60 min, and it seems like a shorter reaction time gives a better barrier in this DES system. P was run at a higher reaction temperature, 100 compared to 80 °C for the other samples, but this did not improve the barrier to any larger extent. Sample M, showed the best OP at the higher climate of 38 C and 90 % relative humidity and outperformed also the reference.

Sample Q, which was pretreated, shows a similar OP as for the other, but a significant increase of almost 20° in CA can be seen compared to the other samples (Table 2).

All samples show a clear improvement in hydrophobicity, i.e. water-repellency, compared to the unmodified MFC film. These results show that hydrophobic MFC films can be obtained which have OP and OTR values comparable to or slightly better than the reference films.

Mechanical properties

The mechanical properties of three of the selected films are presented in Table 3. Young's modulus is improved for the modified films compared to the reference. Sample L shows a higher improvement in both the ultimate strength and tensile strain compared to the reference sample and sample M.

Table 3, Mechanical properties of selected films.

Alternative method: Modification of cellulose in deep eutectic solvent, followed by

microfibrillation and film formation

Imidazole-TEMACI DES (molar ratio 7 :3) was prepared by weighing the components into a round bottom flask, mixing it with magnetic stirrer and heating it in the oil bath at 80°C until the clear liquid was formed. Next, pulp was torn by hand and added into the DES and hydrophobic surface-modifying agent was added into the mixture after the pulp was dissolved into the DES. The reaction continued for 24 hours while being continuously mixed by magnetic stirrer. Then the suspension was washed with water and filtered by Buhner funnel. The modified cellulose was dried over night at 100°C. In total, three cellulose modifications were made: two modifications where n-octylsuccinic anhydride was reagent with molar ratios of 3: 1 and 2 : 1 between reagent and pulp, and one modification where isopropenyl acetate worked as reagent with a molar ratio of 2 : 1 between reagent and pulp. The weight ratio between DES and pulp was 25: 1, so the total amount of DES and pulp was 40 g and 1.6 g in the case of pulp modified with n-octylsuccinic anhydride (molar ratio 3: 1) and acetylated cellulose, and 25 g and 1 g in pulp modified with n-octylsuccinic anhydride (molar ratio 2: 1).

Microfibrillation of this modified cellulose was done from two samples. The modified dry pulp was diluted to consistency of 0.5 %-w and the suspension was mixed at 10000 rpm with an Ultra-Turrax mixer for 3 min before microfibrillation, which was conducted by microfluidizer (Microfluidics M-110EH-30, USA). The pulp modified with n-octylsuccinic anhydride (molar ratio 2: 1) was passed three times through microfluidizer: two times through 400 pm and 200 pm chambers and one time through 400 pm and 100 pm chambers. The acetylated cellulose was passed six times through microfluidizer: three times through 400 pm and 200 pm chambers and three time through 400 pm and 100 pm chambers. After microfibrillation, two samples from each suspension were taken and dried over night at 100°C to determine the dry matter content.

Four different types of MFC films were made from the modified MFC using different weight ratios. The films were made from octylsuccinylated MFC, acetylated MFC, octylsuccinylated and unmodified MFC with weight ratio of 1 : 1, and acetylated and unmodified MFC with weight ratio of 1 : 1. The suspensions were weighed into a decanting glass so that the dry cellulose content was 0.265 g and then the mixture was diluted with water to total weight of 100 g. After that the same procedure was used to make the films as in the preparation of MFC films.

The films made from modified pulp were brittle, felt and looked more like paper. The films gave over-range values in the OTR measurements, but the samples with octylsuccinylated MFC (50 or 100 wt%) exhibited contact angles above 90, see Table 4, suggesting improved hydrophobicity Table 4. Contact angle (CA) values of the films made from MFC modified in two different DES systems.

While the invention has been described with reference to a number of embodiments and examples, the skilled person can freely amend and combine these embodiments within the scope of the claims. The scope of the invention is defined by the appended claims.