Technical field

The present invention relates to a method for manufacturing at least one layer of a film.

Background

Today, films comprising microfibrillated cellulose (MFC), have proven to give excellent barrier properties (see e.g. Aulin et al., Oxygen and oil barrier properties of microfibrillated cellulose films and coatings, Cellulose (2010) 17:559-574, Lavoine et al., Microfibrillated cellulose - Its barrier properties and applications in cellulosic materials: A review, Carbohydrate polymers 90 (2012) 735-764, Kumar et al., Comparison of nano- and microfibrillated cellulose films, Cellulose (2014) 21 :3443-3456), whereas the gas barrier properties are very dependent on the moisture or the relative humidity in the surrounding environment. Therefore, it is quite common that MFC films have to be coated with a polymer film to prevent moisture or water vapor to swell and disrupt the MFC film.

The lack of gas barrier properties such as oxygen or air, at high relative humidity has been investigated and described but most of the suggested solutions are expensive and difficult to implement in industrial scale. One route is to modify the MFC or nanocellulose such as disclosed in

EP2554589A1 where MFC dispersion was modified with silane coupling agent. The EP2551 104A1 teaches the use of MFC and polyvinyl alcohol (PVOFI) and/or polyuronic acid with improved barrier properties at higher relative humidity (RFI). An alternative solution is to coat the film with a film that is non-water-soluble and/or having a low water vapor transmission rate. The JP2000303386A discloses e.g. latex coated on MFC film, while

US2012094047A teaches the use of wood hydrolysates mixed with

polysaccharides such as MFC that can be coated with a polyolefin layer. In addition to this chemical modification, the possibility of cross-linking fibrils or fibrils and copolymers has been investigated, and has been shown to improve water vapor transmission rates. EP2371892A1 , EP2371893A1 , claims cross- linking MFC with metal ions, glyoxal, glutaraldehyde and/or citric acid, respectively.

Another way to decrease the moisture sensitivity of cellulose is to chemically modify the cellulose with sodium periodate to obtain dialdehyde cellulose (DAC). By fibrillating dialdehyde cellulose, a barrier film with improved moisture resistant can be produced. The barrier properties at high relative humidity is better with an increased degree of oxidation (D.O.) of the DA-MFC, however the strain-at-break decreases with increased D.O., which is a disadvantage for barrier films e.g. since they are to form a stable, non- disrupted layer through a converting process.

Flence, there is a need to find a simple solution of producing films having good barrier properties even at high humidity, combined with improved convertibility.

Summary

It is an object of the present invention to provide an improved film comprising microfibri Hated dialdehyde cellulose, which has good barrier properties as well as improved strain at break.

The invention is defined by the appended independent claims.

Embodiments are set forth in the appended dependent claims and in the following description and drawings.

It has surprisingly been found that by mixing dialdehyde cellulose (DA- MFC) having a first degree of oxidation (D.O.) with dialdehyde cellulose having a second degree of oxidation, said second degree of oxidation being lower than the first, and subsequently microfibrillate the cellulose fibers to DA- MFC, a barrier film may be obtained which has improved ductility while maintaining its good oxygen barrier property, compared to known films comprising DA-MFC. Thus, thanks to the method and product according to the present invention, a fiber based barrier film is provided which combines good oxygen barrier properties with improved ductility and higher strain at break. As stated above, it is known that the oxygen transmission rate (OTR) of a barrier film improves with an increased amount of aldehyde groups in the cellulose content of the film suspension. (The skilled person understands that “improved OTR” means that less oxygen passes through the film.) However, it has also been found that barrier films comprising DA-MFC with a high D.O. leads to fragile films with a low strain at break, which is disadvantageous when using the film e.g. for producing packages for oxygen-sensitive food products. The present invention solves this problem by combining DA-MFC with a high D.O. with DA-MFC having a low D.O. in a mixture used for film forming, thereby achieving a film with good oxygen barrier properties and high strain at break.

In the context of the present invention, the term“high D.O.” is referring to a D.O. of between 30 - 60%, and the term“low D.O.” is referring to a D.O. of between 15 - 25%.

As explained above, the cellulose derivative“dialdehyde cellulose” (DAC) can be produced by chemically modifying the cellulose with sodium periodate thereby selectively cleaving the C2-C3 bond of the anhydroglucose unit in the cellulose chain, forming two aldehyde groups at said location. The term“degree of oxidation” is understood to refer to the portion of the total number of anhydroglucose units that undergo said reaction (forming the two aldehydes). The degree of oxidation is given in %.

The cellulose fibers referred to in the present application may originate 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, cotton 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.

By“oxygen transmission rate” (OTR) means a measure of the amount of oxygen gas that passes through the film over a given time period, that is: cm ³ /m ² /24h. The mixture containing DA-MFC with higher and lower D.O.

respectively, for use in subsequent barrier film making can be achieved in different ways.

In one example, a batch process may be used, wherein at least two suspensions of cellulose fibers are pre-oxidized to dialdehyde cellulose having different degrees of oxidation (i.e. high and low D.O.) before being mixed together and then m icrofibril lated by means of mechanical treatment. Thus, according to one aspect of the invention, the method for manufacturing at least one layer of a film comprises the steps of:

providing a first suspension comprising dialdehyde cellulose fibers with a first degree of oxidation;

providing at least a second suspension comprising dialdehyde cellulose fibers with a second degree of oxidation which is lower than said first degree of oxidation;

mixing the first suspension with the second suspension to form a mixture;

mechanically treating the cellulose fibers in said mixture to

microfibrillated cellulose;

applying said mixture to a substrate to form a fibrous web; and drying said web to form at least one layer of said film.

It is understood that the above mentioned exemplary method according to the invention also comprises the mixing of more than two suspensions of dialdehyde cellulose having different degrees of oxidation.

The method according to the invention comprises mechanically treating the dialdehyde cellulose fibers in the mixture, whereby microfibrillated cellulose is obtained. It is to be understood that the microfibrillated cellulose resulting from the mechanical treatment and referred to above includes the microfibrillated dialdehyde cellulose, DA-MFC. Furthermore, the mechanical treatment may be carried out by means of a refiner, grinder, homogenizer, colloider, friction grinder, ultrasound sonicator or fluidizer. All conventional homogenizers and fluidizers available may be used, such as Gaulin

homogenizer, microfluidizer, macrofluidizer or fluidizer-type homogenizer. It is also understood that the term“microfibrillate” refers to a mechanical treatment whereby microfibrillated (cellulose) fibers are obtained. In addition to the batch process mentioned above, it is also

conceivable to use a continuous process for achieving a mixture of high- and low oxidized DA-MFC. In such a procedure, cellulose is continuously fed into a reactor wherein oxidation into cellulose dialdehyde is performed under conditions adapted for the yield of an emerging suspension comprising at least one portion of dialdehyde cellulose with high D.O., and at least one portion of dialdehyde cellulose of low D.O. Thus, according to another aspect of the invention, the method for manufacturing at least one layer of film comprises the steps of:

providing a mixer reaction chamber;

adding a suspension comprising cellulose fibers to the mixer reaction chamber;

adding an oxidizing agent into the mixer reaction chamber to oxidize the cellulose fibers in the suspension to dialdehyde cellulose;

continuously feeding new suspension comprising cellulose fibers into the mixer reaction chamber and mixing to form a mixture;

continuously withdrawing a certain volume of mixture from the mixer reaction chamber;

wherein the process parameters are adjusted such that the withdrawn mixture comprises at least one portion with a degree of oxidation of cellulose between 30 - 60%, and at least one second portion with a degree of oxidation between 15 - 25%;

mechanically treating the dialdehyde cellulose fibers in the mixture to microfibrillated cellulose;

applying said mixture to a substrate to form a fibrous web; and drying said web to form at least one layer of said film.

According to one aspect of the invention, said continuous process also comprises the step of continuously or stepwise feeding new oxidizing agent into the reaction chamber to compensate for the oxidizing agent that is consumed in the chemical reaction process, and also for replacing the oxidizing agent that is withdrawn from the reaction chamber along with the dialdehyde cellulose mixture.

According to an aspect of the present invention, the process

parameters of the continuous process comprise one or more of a) mixing intensity of the suspension inside the reaction chamber, b) the feeding rate measured in volume per time unit of new suspension into the chamber, c) the rate of withdrawal of mixture out of the chamber, d) temperature in the reactor, e) pulp consistency of pulp that is fed into the reactor and f) ratio of the concentration of oxidizing agent vs. pulp consistency in weight % (e.g. wt% sodium periodate/wt% pulp consistency).

It is also possible to achieve the mixture comprising the dialdehyde cellulose of high and low D.O. respectively by means of controlling the time during which the cellulose is subjected to oxidation. One way of influencing the degree of oxidation is to vary the time during which cellulose is subjected to an oxidizing agent (such as sodium periodate, potassium periodate or other available agents). Thus, according to yet another aspect of the invention, the method for manufacturing at least one layer of a film comprises the steps of: providing a first suspension comprising cellulose fibers;

oxidizing the cellulose fibers of said first suspension to dialdehyde cellulose during a first oxidation time span;

at the end point of said first oxidation time span, adding a second suspension comprising cellulose fibers to the first suspension and mixing to form a mixture;

continuing oxidation of the mixture during continuous stirring/mixing for at least a second oxidation time span;

interrupting the oxidation of the cellulose fiber suspension at the end of said second oxidation time span, to acquire a mixture of dialdehyde cellulose fibers comprising at least two different degrees of oxidation;

mechanically treating the dialdehyde cellulose fibers in said mixture to microfibrillated dialdehyde cellulose;

applying said mixture to a substrate to form a fibrous web; and drying said web to form at least one layer of said film.

The skilled person hereby understands that the present invention is based on the idea of controlling oxidation of cellulose fibers in such a way that a suspension of dialdehyde cellulose is achieved which comprises at least a portion of dialdehyde cellulose having a high D.O. and at least another portion of dialdehyde cellulose having a low D.O. Thus, according to yet another aspect of the invention, the method for manufacturing at least one layer of a film comprises the steps of: providing a suspension comprising cellulose fibers;

oxidizing the cellulose fibers in said suspension to dialdehyde cellulose;

controlling the oxidation of cellulose so that said suspension after oxidation comprises at least one portion of dialdehyde cellulose with a first degree of oxidation, and at least a second portion of dialdehyde cellulose with a second degree of oxidation, wherein said second degree of oxidation is lower than said first degree of oxidation, thus acquiring a mixture of dialdehyde cellulose fibers comprising at least two different degrees of oxidation;

mechanically treating the dialdehyde cellulose fibers in said mixture to microfibrillated dialdehyde cellulose;

applying said mixture to a substrate to form a fibrous web; and drying said web to form at least one layer of said film.

According to another aspect of the invention said first degree of oxidation is between 30 - 60%, and said second degree of oxidation is between 15 - 25%.

According to yet another aspect of the invention, the method further comprises a step of adding native microfibrillated cellulose (MFC) to the mixture before applying said mixture to the substrate, preferably adding native MFC to the mixture before mechanically treating the dialdehyde cellulose fibers to microfibrillated dialdehyde cellulose. More specifically, according to this aspect of the invention, native MFC is added to the mixture of dialdehyde cellulose which comprises high and low D.O., so that the resulting mixture thereafter comprises dialdehyde cellulose of a first degree of oxidation, dialdehyde cellulose of a second degree of oxidation, and native

microfibrillated cellulose. The resulting mixture is subsequently mechanically treated to obtain microfibrillated dialdehyde cellulose, i.e. the dialdehyde cellulose fibers are fibrillated to DA-MFC in the presence of native MFC. Adding native MFC before homogenization (or other corresponding

mechanical treatment) contributes to improved runnability of mechanical treatment, and presence of native MFC also results in that the final suspension is more stable. It is understood that“native MFC” refers to MFC that is made from chemical, chemimechanical and/or mechanical pulp which is chemically unmodified. According to yet another aspect of the invention, the mixture comprises between 5 - 85 % by weight of cellulose fibers oxidized to the first degree of oxidation, between 5 - 85 % by weight of cellulose fibers oxidized to the second degree of oxidation, and 10 - 50 % of native microfibrillated cellulose (MFC), based on the total fiber weight of the mixture.

According to yet another aspect of the invention the dry content of the mixture applied to the substrate is between 1-10% by weight.

According to yet another aspect of the invention the substrate is a polymer or metal substrate onto which the mixture is casted. The cast coated fibrous web can be dried in any conventional manner and thereafter optionally peeled off from the substrate. It may be possible to cast or coat more than one layer onto the substrate forming a multilayer film. It is possible to produce a film comprising more than one layer wherein at least one of the layers comprises the mixture according to the invention. The substrate may also be a porous wire of a paper making machine, i.e. any kind of paper making machine known to a person skilled in the art used for making paper, paperboard, tissue or any similar products. The substrate may also be a paper or paperboard product to which the mixture is applied to form a coated product. In one aspect of the invention, the substrate is heated to 50 - 200°C upon applying said mixture thereon.

According to yet another aspect of the invention said method further comprises the step of pressing the film after drying. It has been shown that the barrier function of the film is improved if the film is subjected to increased pressure after drying. The pressure used is preferably between 40-900kPa and the pressing may last for a period of less than 10 minutes, preferably between 1 second to 10 minutes. It is preferred that the pressing is done at elevated temperatures. Temperatures used during pressing may be between 50-200°C, preferably between 100-150°C. The pressing may be done in any conventional equipment such as presses or calenders. By combining the use of pressing, preferably hot pressing of the formed film the barrier of the film is strongly increased. According to yet another aspect of the invention, said mixture further comprises any one of a starch, carboxymethyl cellulose, a filler, retention chemicals, flocculation additives, deflocculating additives, dry strength additives, softeners, or mixtures thereof.

The invention also comprises a film obtainable by means of any one of the previously mentioned methods. According to one aspect of the invention the film has a basis weight of less than 50 g/m ² , preferably between 10-50 g/m ² .

According to yet another aspect of the invention, the at least one layer of the film has an oxygen transmission rate in the range of from 0.1 to 300 cc/m ² /24h according to ASTM F-1927, at a relative humidity of 50 % at 23 ^° C and/or at a relative humidity of 90% at 38 ^° C.

According to yet another aspect of the invention, the at least one layer of film has a strain at break of at least 1.5%, preferably 4%, more preferably 6%.

It is possible to produce a film comprising more than one layer wherein at least one of the layers comprises the mixture according to the invention. It may also be possible that more than one layer of the film comprises the mixture according to the invention. It may also be possible that one or more layers of the film only comprises native microfibrillated cellulose, i.e. which does not comprise microfibrillated dialdehyde cellulose (DA-MFC). The film may comprise two, three, four, five, six, seven, eight, nine, ten or more layers.

The film according to the invention may be used in a package material e.g. intended for food stuff. Such a package material may e.g. comprise a base material and a barrier film according to the present invention laminated thereto. The base material may include, but is not limited to, paper, cardboard, paperboard, fabric, plastic, polymer film, metal, composites and the like. The package material may be in the form of a multilayer laminate which, in addition to the base layer and the barrier film comprising DA-MFC, also includes one or more laminate layers such as polyethylene coating, a polyvinyl alcohol coating and/or a metallized film layer. The present invention also comprises the use of a film according to any one of the previously mentioned aspects, as an oxygen barrier film.

Description of Embodiments

The method according to the present invention relates to a method for manufacturing at least one layer of a barrier film having at least oxygen barrier properties, said method comprising providing a suspension with microfibrillated dialdehyde cellulose having a first degree of oxidation (D.O.) mixed with microfibrillated dialdehyde cellulose having at least a second D.O., wherein the second D.O. is lower than said first D.O., applying said mixture to a substrate to form a fibrous web and drying said web to form at least one layer of film. It has surprisingly been found that by providing a suspension of dialdehyde cellulose which comprises at least a portion with DA-MFC having a high D.O. and at least another portion with DA-MFC having a low D.O., a film can be formed which has improved ductility while maintaining a good oxygen barrier property. A mixture in accordance with the present invention (i.e. comprising at least one portion with DA-MFC having a high D.O., and at least one portion with DA-MFC having a low D.O.) can be achieved in different ways. In one example, two suspensions comprising cellulose fibers are firstly oxidized separately to different D.O. (high and low respectively), whereafter they are mixed together to form a mixture. The dialdehyde cellulose in the mixture is then mechanically treated in such way that it is microfibrillated, for example by a homogenizer or in any other way such that fibrillation occurs to produce microfibrillated dialdehyde cellulose. It is thus understood that the term “microfibrillated dialdehyde cellulose” in this context means a dialdehyde cellulose treated in such way that it is microfibrillated. Flereby fibrils in a width of less than 200 pm are obtained, such as fibrils in the width range of between 1 nm - 200 pm, and the at least one layer of the film is then formed by applying said microfibrillated mixture to a substrate to form a fibrous web, and drying said web to form at least one layer of said film. The drying of said web may be done in any conventional way known to the skilled person. The dry content of the at least one layer of the film after drying is preferably above 95% by weight. Another way of acquiring the suspension in accordance with the present invention is to oxidize the cellulose fibers of the same suspension to different degrees (high and low respectively). This can be done by means of performing the oxidation step of the method in a continuous process as will now be described in a non-limiting way. A volume of suspension comprising cellulose fibers is added to a continuous stirred-tank reactor (also called a flow reactor tank or a mixer reaction chamber), and oxidation is initiated for instance by means of subjecting said suspension to sodium periodate in a conventional manner known to the skilled person. Oxidation of the

suspension is performed in the reactor tank, and in accordance with the principles of a continuous process, a certain volume of cellulose-containing suspension is continuously fed into the tank and mixed with the suspension already present therein to form a mixture. A certain volume of the mixed suspension is also continuously withdrawn from the tank, and oxidation of the withdrawn mixture is interrupted by means of washing away the oxidizing agent. This procedure allows for adjustment of process parameters including e.g a) mixing intensity of the suspension inside the reaction chamber, b) the feeding rate of new suspension into the chamber, c) the rate of withdrawal of mixture out of the chamber, d) temperature in the reactor, e) pulp consistency of pulp that is fed into the reactor and f) ratio of the concentration of oxidizing agent vs. pulp consistency in wt%. By adjusting said parameters, it is possible to control the spread of the D.O. -range of the cellulose fibers that are withdrawn from the tank. This is due to that the degree of oxidation is depending on the time during which cellulose fibers is subjected to the oxidizing agent, i.e. cellulose fibers that has been in the presence of such oxidizing for a long time span will be get a higher D.O., and vice versa. Using a continuous stirred-tank reactor for oxidizing cellulose leads to that some fibers will stay in the tank for a longer time span, while other fibers will be withdrawn therefrom almost instantly upon having entered, and thereby the resulting dialdehyde cellulose will have varying degrees of oxidation - from higher to lower D.O.

In a continuous process, fresh oxidizing agent is fed into the reaction chamber to compensate for the oxidizing agent that is consumed in the chemical reaction process, and also for replacing the oxidizing agent that is withdrawn from the reaction chamber along with the dialdehyde cellulose mixture. This leads to that the spread of D.O. in the oxidized mixture becomes very wide (from low to high D.O.) because a portion of the cellulose fibers will become withdrawn from the reactor only a short time after having entered (i.e. D.O. equaling zero), while another portion will become oxidized to a high degree (i.e. D.O. between 30-60%) by the newly added, and highly reactive sodium periodate. Hence, a continuous process including oxidation of cellulose fibers results in a mixture of dialdehyde cellulose with a wide spread of D.O. According to the present invention, adjusting a continuous process for oxidation of cellulose fibers so that the mean D.O. is between 10 - 50% provides a mixture with the desired spread of D.O. for forming a film with good barrier properties and with improved strain at break. By such an adjustment of process parameters (e.g. the ones mentioned above, but not limited thereto), the mixture withdrawn from the continuous reactor tank comprises at least one portion with a degree of oxidation of cellulose between 30 - 60%, and at least one second portion with a degree of oxidation between 15 - 25%.

An example of carrying out a continuous process according to the present invention is as follows. A continuous stirred tank reactor is provided with an inflow of sodium periodate and cellulose pulp. An outflow of

suspension is set up, said outflow comprising a mixture of dialdehyde cellulose with a wide spread of degree of oxidation; unreacted cellulose fibers; periodate and water. The inlet pulp consistency may be between 1 - 30% by weight, preferably between 2 - 5% by weight. The pulp may also be dry, i.e. above 90% by weight. Depending on pulp consistency, the cellulose fibers are fed into the reactor tank by means of a pump or screw or other suitable technology known to the skilled person. Preferably, the ratio of periodate/cellulose consistency in wt% is 0.5 - 5 kg/kg, preferably 1 - 2 kg/kg. The temperature in the reactor is preferably between 20 - 70°C, preferably 40 - 60°C and even more preferably 45 - 50°C. The continuous stirred reactor tank has design criteria such as dwell time, volume and feed based on reaction rate, known to the person skilled in the art.

The dialdehyde cellulose fibers of the mixture withdrawn from the reactor tank are thereafter mechanically treated (i.e. microfibrillated) to obtain microfibrillated dialdehyde cellulose (DA-MFC). Said mixture is applied to a substrate to form a fibrous web which is dried to form at least one layer of film. It is within the scope of the invention to add native MFC to the mixture before microfibrillation, so that such native MFC is present when the dialdehyde cellulose fibers are mechanically treated to DA-MFC.

The degree of oxidation of the dialdehyde cellulose may be determined according to the following description: after the dialdehyde cellulose reaction, the amount of C2-C3 bonds in the cellulose that are converted to dialdehydes are measured. The degree of oxidation is the amount of C2-C3 bonds that are converted compared to all C2-C3 bonds. This is measured with a method by H. Zhao and N.D. Heindel,“Determination of Degree of Substitution of Formyl Groups in Polyaldehyde Dexran by the Flydroxylamine Hydrochloride

Method”, Pharmaceutical Research, vol. 8, pp. 400-402, 1991 , where the available aldehyde groups reacts with hydroxylamine hydrochloride. This forms oxime groups and releases hydrochloric acid. The hydrochloric acid is titrated with sodium hydroxide until pH 4 is reached, and the degree of oxidation is thereafter calculated from according to the formula below. The received aldehyde content is divided by two to get the value of the degree of oxidation, since an oxidized anhydroglucose unit has two aldehyde groups.

VNaOH = the amount of sodium hydroxide needed to reach pH 4 (I)

CNaOH = 0, 1 mol/l

msample = dry weight of the analysed DAC sample (g)

M _w = 160 g/mol, which is the molecular weight of the dialdehyde cellulose unit

The mixture may further comprise additives, preferably any one of a starch, carboxymethyl cellulose, a filler, retention chemicals, flocculation additives, deflocculating additives, dry strength additives, softeners, or mixtures thereof. It may be possible to add additives that will improve different properties of the mixture and/or the produced film. It may be possible to add the additive to the first suspension, the second suspension and/or to the mixture. It has been shown that the use of a softener, such as sorbitol, glycerol, polyethylene glycol, sorbic acid, propylene glycol, erythritol, maltitol or polyethylene oxides, will modify and improve some of the mechanical properties of the film, especially the stretch at break properties. The amount of sorbitol used is preferably between 1 -20% by dry weight of the film. According to one embodiment of the invention, native microfibrillated cellulose (MFC) is added to the mixture before the microfibrillation step. The native microfibrillated cellulose is microfibrillated cellulose produced from mechanical, thermomechanical or chemical pulp, preferably produced from Kraft pulp. The native MFC is chemically unmodified, i.e. non-oxidized. The microfibrillated cellulose preferably has a Schopper Riegler value (SR°) of more than 90. According to another embodiment the MFC may have a Schopper Riegler value (SR°) of more than 93. According to yet another embodiment the MFC may have a Schopper Riegler value (SR°) of more than 95. The Schopper-Riegler value can be obtained through the standard method defined in EN ISO 5267-1. This high SR value is determined for a pulp, with or without additional chemicals, thus the fibers have not

consolidated into a film or started e.g. hornification. The dry solid content of this kind of web, before disintegrated and measuring SR, is less than 50 % (w/w). To determine the Schopper Riegler value it is preferable to take a sample just after the wire section where the wet web consistency is relatively low. The skilled person understands that paper making chemicals, such as retention agents or dewatering agents, have an impact on the SR value. The SR value specified herein, is to be understood as an indication but not a limitation, to reflect the characteristics of the MFC material itself.

According to one embodiment the film may have a basis weight of less than 50 g/m ² , or less than 35 g/m ² , or less than 25 g/m ² The basis weight is preferably at least 10 g/m ² , preferably between 10-50 g/m ² , even more preferred between 10-35 g/m ² and most preferred between 10-25 g/m ² .

Microfibrillated cellulose (MFC) (including microfibrillated dialdehyde cellulose) shall in the context of the patent application mean a nano scale cellulose/dialdehyde cellulose particle fiber or fibril with at least one dimension less than 200 nm or less than 100 nm. MFC comprises partly or totally fibrillated cellulose or lignocellulose fibers. The liberated fibrils have a diameter less than 200 nm, or 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., Cellulose fibres, nanofibrils and microfibrils, : The morphological sequence of MFC components from a plant physiology and fibre technology point of view, Nanoscale research letters 2011, 6:417), while it is common that the aggregated form of the elementary fibrils, also defined as microfibril ( Fengel , D., Ultrastructural behavior of cell wall polysaccharides, Tappi J., March 1970, Vol 53, No. 3.), is the main product that is obtained when making MFC e.g. by using an extended refining process or pressure- drop disintegration process. Depending on the source and the manufacturing process, the length of the fibrils can vary from around 1 to more than 10 micrometers. A coarse MFC grade might contain a substantial fraction of fibrillated fibers, i.e. protruding fibrils from the tracheid (cellulose fiber), and with a certain amount of fibrils liberated from the tracheid (cellulose fiber).

There are different acronyms for MFC such as cellulose microfibrils, fibrillated cellulose, nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibers, cellulose nanofibrils, cellulose

microfibers, cellulose fibrils, microfibrillar cellulose, microfibril aggregrates and cellulose microfibril aggregates. MFC can also be characterized by various physical or physical-chemical properties such as large surface area or its ability to form a gel-like material at low solids (1 -5 wt%) when dispersed in water. 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 200 m2/g, or more preferably 50-200 m2/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-hydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment step is 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 hydrolyse or swell fiber or reduce the quantity of hemicellulose or lignin. The nanofibrillar cellulose may contain some hemicelluloses; the amount is dependent on the plant source.

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

Brief description of the figures

Figure 1 : describes the OTR value for films having different average degree of oxidation in the mixture.

Figure 2: describes the strain at break of films having different average degrees of oxidation in the mixture.

Example

Strain at break and OTR at different average D.O.

Dialdehyde cellulose with a degree of oxidation of 20% was mixed with dialdehyde cellulose with a degree of oxidation of 40%, and with standard homogenized native MFC, forming a mixture. The native MFC content was 20wt-% by total dry weight of the mixture. The mixture was mechanically treated in a homogenizer to form microfibrillated dialdehyde cellulose mixed with native MFC. Films were prepared on a heated metal plate. A first layer of 20 g/m ² of 100% standard MFC was firstly applied, and then a second layer of 20 g/m ² of the DA-MFC mixture was applied on top of the first layer.

Two tests with mixed D.O. DA-MFC were performed, corresponding to trial numbers 1 and 2 shown in Table 1. For comparison, two tests with

suspensions comprising DA-MFC with one D.O. were also performed, corresponding to trial numbers 3 and 4.

For each film resulting from trial numbers 1 - 4 respectively, the Oxygen Transmission Rate (OTR) and strain at break were measured, the results of which are shown in Table 1.

The strain at break was measured by means of a standard tensile test (ISO 1924-2 with a span length of 20 mm), wherein the film to be tested was stretched with test speed of 2 mm/minute until a point where it ruptured. The strain at break then corresponds to the percent elongation when rupturing, i.e. to what extent in % the film deforms without breaking upon being subjected to stretching.

Table 1 below shows the formulas tested in this example. Fig. 1 shows a graph of the results of the OTR measurements at 23°C and 80% RH, where the OTR is plotted against average D.O. of the mixtures used. Furthermore, Fig. 2 shows a graph of the results of the tensile tests, where the strain is plotted against average D.O. of the mixtures used.

In trial 1 , the mixture comprised 20wt-% by total dry weight of the mixture of native MFC; 60wt-% of DA-MFC with a D.O. of 20%; and 20wt-% of DA-MFC with a D.O. of 40%.

In trial 2, the mixture comprised 20wt-% by total dry weight of the mixture of native MFC; 40wt-% of DA-MFC with a D.O. of 20%; and 40wt-% of DA-MFC with a D.O. of 40%.

In trial 3, the mixture comprised 20wt-% by total dry weight of the mixture of native MFC; and 80wt-% of DA-MFC with a D.O. of 20%.

In trial 4, the mixture comprised 20wt-% by total dry weight of the mixture of native MFC; and 80wt-% of DA-MFC with a D.O. of 40%.

It is evident from Figure 2 that the suspensions comprising a mixture of DA- MFC with two different D.O., (with high and low degrees of oxidation respectively), provides a higher strain at break compared to suspensions comprising DA-MFC with only one D.O. It is further evident from Fig. 1 that the suspensions comprising a mixture of DA-MFC with two different D.O. prove to be good oxygen barriers, also when compared to suspensions comprising DA-MFC with only one D.O.

Table 1 In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.