EP3724233A1

EP3724233A1 - Method for oxidation of cellulose

Info

Publication number: EP3724233A1
Application number: EP18847219.5A
Authority: EP
Inventors: Adrianna SVENSSON
Original assignee: Stora Enso Oyj
Current assignee: Stora Enso Oyj
Priority date: 2017-12-13
Filing date: 2018-12-12
Publication date: 2020-10-21
Also published as: SE542443C2; SE1751540A1; WO2019116245A1; JP2021507016A

Abstract

A method for oxidation of cellulose to dialdehyde cellulose (DAC), said method comprising: a) oxidizing cellulose with an aqueous periodate solution having a pH in the range of 3 to 5 to form oxidized cellulose comprising DAC; b) separating the aqueous periodate solution from the oxidized cellulose; c) regenerating the separated aqueous periodate solution by electrolytic oxidation; d) adjusting of the pH of the regenerated aqueous periodate solution to a value in the range of 3 to 5; and e) reusing the pH adjusted regenerated aqueous periodate solution, optionally combined with a make-up amount of fresh aqueous periodate solution, as the aqueous periodate solution in step a).

Description

METHOD FOR OXIDATION OF CELLULOSE

Technical field

The present disclosure relates to methods for oxidation of cellulose to dialdehyde cellulose (DAC) for use in the formation of DAC films, and particularly to

regeneration and recycling of aqueous periodate solutions used for the oxidation in such methods.

Background

Films comprising microfibrillated cellulose (MFC), have proven to give excellent barrier properties. Flowever, the gas barrier properties are very dependent on the moisture and the relative humidity in the surrounding environment. Therefore, it is common that MFC films must for example be coated with a polymer film to prevent moisture or water vapor to swell and disrupt the MFC film.

Another way to decrease the moisture sensitivity of cellulose is to chemically modify the cellulose with sodium periodate to obtain dialdehyde cellulose (DAC).

By mechanically processing the dialdehyde fibers, it is possible to disintegrate them into fibrils. Periodate oxidation can be seen as the pretreatment to liberate microfibrils. Using fibrillated dialdehyde cellulose, barrier films with improved moisture resistance can be produced.

Unfortunately, periodate oxidants are environmentally harmful and also expensive. In order to achieve a realistic industrial production, an efficient regeneration and recycling of periodate is essential. Flowever, oxidation of polysaccharides in general, and cellulose in particular, using periodate oxidants are complex processes, involving many different reactions and possible byproducts.

Periodate/iodate solutions obtained during the cellulose oxidation also typically include a complex mixture of species that affect the regeneration processes.

Therefore, there exists a need in the field for improved regeneration methods for periodate solutions used in cellulose oxidation. Description of the invention

It is an object of the present disclosure to provide an improved manner for recycling of periodate solution in a method for oxidation of cellulose to dialdehyde cellulose. Another object, related to the above mentioned object, is to reduce the costs of oxidation of cellulose to dialdehyde cellulose by an improved manner of recycling of a periodate solution in a method for oxidation of cellulose to dialdehyde cellulose.

It is a further object of the present disclosure to provide an improved method for manufacturing a film comprising microfibrillated dialdehyde cellulose, which has improved barrier properties even at higher relative humidity in the surroundings.

Other objects may be to obtain environmental, health and/or economical benefits of reduced emission of chemicals used in a method for oxidation of cellulose to dialdehyde cellulose.

According to a first aspect illustrated herein, there is provided a method for oxidation of cellulose to dialdehyde cellulose (DAC), said method comprising: a) oxidizing cellulose with an aqueous periodate solution having a pH in the range of 3 to 5 to form oxidized cellulose comprising DAC; b) separating the aqueous periodate solution from the oxidized cellulose; c) regenerating the separated aqueous periodate solution by electrolytic oxidation; d) adjusting of the pH of the regenerated aqueous periodate solution to a value in the range of 3 to 5; e) reusing the pH adjusted regenerated aqueous periodate solution, optionally combined with a make-up amount of fresh aqueous periodate solution, as the aqueous periodate solution in step a). During the oxidation of cellulose at least a part of the periodate will be reduced to iodate. Accordingly, the aqueous periodate solution separated in step b) will also comprise iodate. In order to be made useful for further oxidation of cellulose, the formed iodate must again be converted to periodate.

The method of the present disclosure allows for multiple cycles of regeneration and reuse of the same aqueous periodate solution without the addition of chemical oxidants in the regeneration step and without the need for time consuming and costly purification steps.

Electrolytic oxidation has previously been used for regeneration of periodate solutions used in the oxidation of starch. However, oxidation of starch produces less byproducts than oxidation of cellulose. Oxidation byproducts can react further with periodate in the solution leading to a decrease in periodate available for the main oxidation reaction and thereby inferior DAC products. Furthermore, Oxidation byproducts in the used periodate solution may also interfere with the electrolytic oxidation process, leading to higher currents or reaction time required to reach a desired degree of regeneration. Therefore, previous methods using electrolytic oxidation for regeneration of periodate solutions for polysaccharide oxidation have also included various purification steps, for example based on ion-exchange chromatography, to remove impurities such as added ionic species or oxidation byproducts before the periodate solution is reused.

The inventor has found that by adjusting the pH of the regenerated periodate solution to the range of 3-5 following the electrolytic oxidation, formation of byproducts during the main oxidation reaction can be reduced. This, in turn, has been found to allow for reusing the periodate solution in multiple cycles without the need for time consuming and costly purification steps between each use. In fact, the method according the present disclosure has been shown to successfully allow for 6 oxidation and regeneration cycles, and it is envisaged that it would be possible to increase the number of cycles even further, e.g. to 10 or more, without significant deterioration of the process or the obtained material. It appears that if the pH of the filtrate is not adjusted after the electrolytic oxidation, iodine is formed due to unwanted periodate reduction. The effect of this reduction is visible to the naked eye with yellow/orange filtrates being obtained after the 2^nd and 3^rd oxidation cycles with the regenerated solutions. Most likely, the reduction is caused by the reaction of periodate and iodate with byproducts of cellulose overoxidation. The byproducts are formed when cellulose reacts with periodate at very low pH, i.e. lower than pH 2. As a consequence of iodine formation the periodate regeneration efficiency is decreased, and in practice it is not possible to perform more than 3 oxidation and regeneration cycles.

The inventors have found that a pH value of about 4 gives the least formation of byproducts and the best conditions for repeated recycling of the periodate solution. According to some embodiments, the aqueous periodate solution in step a) has a pH in the range 3.5 to 4.5, preferably a pH of about 4.

The periodate solution preferably comprises an aqueous solution of sodium periodate. Sodium periodate is the inorganic salt of periodic acid. It is composed of sodium, iodine and oxygen. Periodate can exists either as IO4 or IOQ⁵ . When DAC is being produced, it is the metaperiodate form, IO4 that reacts with cellulose according to:

Cellulose + Nal04 ® Dialdehyde cellulose + Nal03 + H2O

According to some embodiments, the cellulose in step a) is in the form of pulp having a cellulose concentration in the range of 1 -10 wt%, preferably in the range of 1 -5 wt%, more preferably in the range of 1 -3 wt%.

According to some embodiments, the aqueous periodate solution in step a) comprises periodate ions at a starting concentration in the range of 100-230 mM, preferably in the range of 120-160 mM, more preferably of about 140 mM.

According to some embodiments, the aqueous periodate solution in step a) has a molar ratio of periodate ion to cellulose from 0.30 - 1 .14. Fine tuning of pH and of the molar ratio of periodate to cellulose ensures a balance between optimized yield and minimized overoxidation.

The reaction temperature of the cellulose oxidation is preferably selected so as to obtain a high reaction rate while not causing an unacceptable degree of

degradation of the participating ingredients or byproduct formation. Notably, periodate starts to decompose at temperatures above 55 °C, but temperature slightly above 55 °C may still be used since the shorter reaction time can make up for the periodate decomposition. According to some embodiments, the cellulose in step a) is contacted with the aqueous periodate solution at a temperature in the range of 30-70 °C, preferably in the range of 30-60 °C, more preferably of about 50 °C.

The reaction time required for the cellulose oxidation will of course vary depending on the reaction conditions. According to some exemplary embodiments, the cellulose in step a) is contacted with the aqueous periodate solution for a period in the range of 0.5-5 hours, preferably in the range of 1 -4 hours, more preferably in the range of 2-3 hours. However, the skilled person realizes that longer or shorter reaction time may be required depending, e.g., on the pH, periodate concentration, temperature and desired oxidation degree.

In order to be useful in the formation of barrier films with suitable barrier and moisture resistance, the oxidized cellulose obtained should preferably have an oxidation degree of at least 20 %, preferably at least 30 %. Accordingly, in some embodiments at least 20 %, preferably at least 30 %, of the cellulose in step a) is oxidized to DAC.

The method of the present disclosure is further advantageous since it allows for the repeated regeneration and recycling of periodate solutions without the use of added chemical oxidants. According to some embodiments, the aqueous periodate solution is regenerated by electrolytic oxidation without addition of chemical oxidants. Less added chemicals results in less required work-up and purification of the periodate solution, less waste and better process economy. According to some embodiments, the electrolytic oxidation is performed in an electrolytic cell comprising a cathode chamber and an anode chamber separated by a cation exchange membrane, and wherein the cathode chamber comprises a cathode, preferably made of stainless steel, and the anode chamber comprises an anode, preferably made of Pb02 on a Ti substrate.

The reaction rate of the regeneration step depends on a number of parameters, including the type, configuration and size of the electrolytic cell, the current density and the temperature. The skilled person understands that different combinations of parameters can be used to achieve substantially the same result. According to some exemplary embodiments, the electrolytic oxidation is performed at a current density in the range of 100-2000 mA per dm², such as in the range of 300 to 650 mA per dm².

According to some exemplary embodiments, the electrolytic oxidation is performed at a temperature in the range of 10-30 °C, preferably in the range of 20-30 °C. According to some exemplary embodiments, the electrolytic oxidation is performed for a period in the range of 5-30 hours.

The aqueous periodate solution to be used in step a) should preferably comprise periodate ions at a starting concentration in the range of 100-230 mM, preferably in the range of 120-160 mM, more preferably of about 140 mM. Accordingly, in to some embodiments the regenerated aqueous periodate solution comprises periodate ions at a concentration of at least 100 mM, preferably at least 120 mM, and more preferably at least 140 mM.

The pH of the aqueous periodate solution after the electrolytic oxidation is typically below 2, such as below 1.5. The pH value is then adjusted to a value in the range of 3 to 5. According to some embodiments, the pH of the regenerated aqueous periodate solution in step d) is adjusted to a value in the range of 3.5 to 4.5, preferably to a pH of about 4.0. The inventors have found that a pH value of about 4 gives the least formation of byproducts and the best conditions for repeated recycling of the periodate solution. According to some embodiments, the pH is adjusted by addition of NaOH.

Contrary to indications in the prior art, the base, such as NaOH, added in order to adjust the pH does not appear to affect the main oxidation reaction or the regeneration reaction adversely, at least not in the envisaged range of

regeneration cycles, e.g. 6-10 cycles. In order to avoid excessive dilution of the periodate solution, it is preferred to add a smaller volume of concentrated NaOH solution, or even solid NaOH, rather than a larger volume of a more dilute NaOH solution. According to some embodiments, the pH is adjusted by addition of solid NaOH or aqueous NaOH having a concentration of at least 0.1 M.

According to some embodiments, the regenerated aqueous periodate solution is reused directly after regeneration and adjustment of the pH, without further purification.

Naturally, each oxidation and regeneration cycle will involve a certain loss of periodate solution. In order to maintain a sufficient volume of periodate solution the solution can be supplemented with a make-up amount of fresh aqueous periodate solution. The fresh aqueous periodate solution may preferably have a

concentration of periodate ions which is the same or slightly higher than the concentration of the aqueous periodate solution first used in the cellulose oxidation step. A slightly higher concentration allows for compensation of decreasing periodate concentration e.g. due to less than 100 % efficiency in the electrolytic oxidation. According to some embodiments, the make-up amount of fresh aqueous periodate solution constitutes 1 -30 %, preferably 1 -20 %, more preferably 1 -10 %, of the total volume of the aqueous periodate solution used in step a). The volume and concentration of the aqueous periodate solution may also be adjusted by addition of water.

The inventor has surprisingly found that using the inventive method, the same aqueous periodate solution can be effectively regenerated and reused at least five times without additional work-up and purification steps. According to some embodiments, the same aqueous periodate solution is regenerated and reused at least three times, preferably at least four times, more preferably at least five times. The inventor has further shown that the DAC films formed from DAC prepared using periodate solution regenerated according to the inventive method up to at least five times retained their barrier properties as compared to DAC films formed from DAC prepared using fresh (i.e. not previously used and regenerated) periodate solution.

In order to be useful in the formation of barrier films with suitable barrier and moisture resistance, the oxidized cellulose comprising DAC must first be fibrillated. Thus, according to some embodiments the method further comprises the step e) subjecting the separated oxidized cellulose comprising DAC, optionally together with microfibrillated cellulose (MFC), to fibrillation to obtain microfibrillated DAC or a microfibrillated mixture of DAC and MFC.

According to a second aspect illustrated herein, there is provided a method for manufacturing at least one layer of a film wherein the method comprises the steps of:

providing a suspension comprising (i) a mixture of microfibrillated

dialdehyde cellulose (DAC) and microfibrillated cellulose (MFC), or (ii) a microfibrillated mixture of DAC and MFC,

applying said suspension to a substrate to form a fibrous web, and drying said web to form at least one layer of said film. wherein the DAC is obtained according to the method of the first aspect described herein.

It has been found that it is possible to produce a very stable suspension or mixture by mixing a first suspension comprising MFC and a second suspension comprising microfibrillated DAC, or by using a microfibrillated mixture of DAC and MFC. It is crucial that a suspension used for the production of barrier materials is stable since uneven distribution of the fibrils will lead to deteriorated barrier properties. Furthermore, it was found that the use of both MFC and microfibrillated DAC makes it possible to produce at least one layer of a film that gives the film improved barrier properties at high humidity, especially at fluctuating humidity. According to some embodiments, the suspension comprises between 20-95 wt% of microfibrillated DAC based on the total fiber weight of the mixture. Depending on the end use and the properties of the MFC and microfibrillated DAC, the amount of microfibrillated DAC may vary.

According to some embodiments, the suspension comprises between 5-80 % of MFC based on the total fiber weight of the mixture.

According to some embodiments, the dry content of the mixture applied to the substrate is between 1 -10 wt%. Depending on the substrate onto which the mixture is applied the dry content of the mixture may vary.

The at least one layer of the film is produced by 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 drying of said web may be done in any conventional way. The dry content of the at least one layer of the film after drying is preferably above 95 wt%.

The at least one layer of the film preferably has an oxygen transmission rate in the range of from 0.1 to 300 cc/m²/24h according to ASTM D-3985, at a relative humidity of 50 % at 23 ^°C and/or at a relative humidity of 90 % at 38 ^°C. By the present invention it is possible to produce at least one layer of a film that has very good oxygen barrier properties at high humidity. It has especially been found that the film according to the present invention is more resistant towards fluctuations in humidity, i.e. the film still has good barrier properties even if the humidity varies.

The substrate for the film formation is preferably a polymer or metal substrate. It is preferred that the mixture is cast coated onto said substrate. 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. 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 microfibrillated cellulose, i.e. it does not comprise microfibrillated dialdehyde cellulose. The film may comprise two, three, four, five, six, seven, eight, nine, ten or more layers.

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.

The method may further comprise the step of pressing the film after drying. It has been shown that the barrier properties of the film is increased if the film is subjected to increased pressure after drying. The pressure applied in the pressing is preferably above 40kN/m² (over pressure), more preferably between 100-900 kN/m². 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. The temperature is preferably increased to between 50- 200°C, preferably between 100-150°C during pressing of the film. 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.

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. Additive may be added to the first suspension, the second suspension and/or to the mixture.

According to some embodiments, the microfibrillated DAC has an oxidation degree of at least 20 %. In some embodiments, the microfibrillated DAC has an oxidation degree of between 25-75 %. According to some embodiments, the 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 microfibrillated cellulose is microfibrillated cellulose produced from

mechanical, thermomechanical or chemical pulp. The microfibrillated cellulose is preferably produced from kraft pulp. 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.

The microfibrillated dialdehyde cellulose should in this context mean a dialdehyde cellulose treated in such way that it is microfibrillated. The production of the microfibrillated dialdehyde cellulose is done by treating dialdehyde cellulose for example by a homogenizer or in any other way such that fibrillation occurs to produce microfibrillated dialdehyde cellulose. The microfibrillated dialdehyde cellulose preferably has an oxidation degree between 25-75 %, preferably between 30-65 %, even more preferably between 30-50 % or most preferred between 35-45 %. The degree of oxidation was 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 Hydroxylamine 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

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

Mw = 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 the film, comprising microfibrillated cellulose and microfibrillated dialdehyde cellulose, has an oxygen transmission rate in the range of from 0.1 to 300 cc/m²/24h measured according to the standard ASTM D-3985, at a relative humidity of 50 % at 23 ^°C and/or at a relative humidity of 90 % at 38 ^°C.

The amount of microfibrillated cellulose in the produced film is preferably between 5-80 wt% by total dry weight of the film, preferably between 10-60 wt% by total dry weight of the film and even more preferred between 10-40 wt% by total dry weight of the film. The amount of microfibrillated dialdehyde cellulose in the produced film is preferably between 20-95 wt% by total dry weight of the film, preferably between 40-90 wt% by total dry weight of the film and even more preferred between 60-90 wt% by total dry weight of the film.

According to one embodiment the film may have a basis weight of less than 50 g/m², or less than 35 g/m², or less than 25 g/m². The basis weight is preferably at least 10 g/m², preferably between 10-50 g/m², even more preferred between 10-35 g/m² and most preferred between 10-25 g/m².

Microfibrillated cellulose (MFC) shall in the context of the patent application mean a nano scale cellulose fiber or fibril with at least one dimension less than 100 nm. MFC 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., Cellulose fibres, nanofibrils and microfibrils, : The morphological sequence of MFC components from a plant physiology and fibre technology point of view, Nanoscale research letters 201 1, 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 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 (CMC), aldehyde and/or carboxyl groups (cellulose obtained by N-oxyl mediated oxydation, 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 nanofibrillar size or NFC.

The nanofibrillar 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, 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 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 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.

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.

Brief description of the drawings

Figure 1 is a photograph of sample bottles containing periodate solution filtrates obtained in Example 2, where pH was not adjusted after the regeneration. The samples were collected after one oxidation and one regeneration (left), and three oxidations and two regenerations (middle and right). Examples

Materials and equipment: Bleached sulphate pulp refined to SR° 30 and MFC was supplied from Stora Enso Kaukopaa mill in Imatra. Sodium periodate was purchased from Sigma-Aldrich and from AJAY Europe Sari. Sodium hydroxide in pellets was purchased from Sigma-Aldrich. Electro MP Cell was purchased from ElectroCell.

Example 1 - Cellulose oxidation and periodate regeneration with pH adjustment The pulp was oxidized using sodium periodate in aqueous solution. The pulp concentration, in tap water, was 2 % and the pH value was adjusted to pH 4.0 with 0.5 M H2SO4 at the beginning. A pulp concentration of 2% corresponds to 123 mM, and Periodate concentration is 140 mM which in its turn corresponds to a molar ratio of periodate ion to cellulose of 1 .14.

A series of six subsequent oxidations followed by periodate electroregeneration were performed as set out in Table 1 . Each oxidation was done at 50 °C for 2 h 45 min. 100% fresh sodium periodate (140 mM in aqueous solution) was used only for the first oxidation, i.e. KV1 -T 1 . After each oxidation periodate concentration decreased from 140 mM to less than 60 mM or even less than 40 mM. lodate produced during the oxidation was electrochemically regenerated to periodate during the electrolysis of the filtrate. Electroregeneration was performed in a two compartment Electro MP Cell with a lead dioxide anode (area 1 dm²), stainless steel cathode and Nation membrane N324. The regenerated filtrate was circulated through the anodic compartment of the cell at room temperature. NaOH solution was used as catholyte. The applied current density was between 400 and 650 mA/dm².

Periodate concentration was monitored using a UV-Vis spectrophotometer - to follow the regeneration steps. UV-Vis spectrophotometer (Evolution 201 , UV-visible, Thermo Scientific, US) and a quartz cuvette were used for the periodate analysis. The sample was diluted 2500x in deionized water, in two steps and the absorbance was measured in the range between 400 and 200 nm. The measurement was repeated 2 times and the obtained absorbances for the peaks at ca 219 nm (ABS.1 and Abs.2) were used for calculating the periodate concentration:

The calibration curve was made for the solutions containing periodate and iodate in different rations, but the sum of their concentrations was always 140 mM. Solutions containing from 70 mM to 140 mM periodate were used.

At least about 90 % and up to about 97 % of periodate in the filtrate was

electroregenerated and used for next oxidation after the addition of a make-up amount of sodium periodate and water, and pH adjustment to 4.0. The make-up amount varied between 12 and 29 %, depending mostly on how much filtrate was left after the regeneration and on the eletroregeneration efficiency - see Error! Reference source not found.. pH adjustment was done with NaOH. The solution used at the beginning of each oxidation contained 140 mM periodate and 2 % pulp. The filtrate obtained after the 6^th oxidation step was still essentially

uncolored, like the sample container on the left in Figure 1.

Table 1. Selected parameters of the subsequent oxidations and calculated degrees of oxidation (D.O.) of obtained DAC. The pH was adjusted to 4.0 at the beginning of each oxidation.

Example 2 (Comparative example) - Cellulose oxidation and periodate

regeneration without pH adjustment Example 1 was repeated as set out in Table 2, except no pH adjustment of the regenerated filtrate was performed. Table 2. Selected parameters of the

subsequent oxidations and calculated degrees of oxidation of obtained DAC. The pH was not adjusted before oxidations.

^*This high make-up was made to compensate the loss of periodate ions which accrued when the regenerated solution was kept in the fridge for a few days between the oxidations A-T2 and A-T3. When the pH of the filtrate is not adjusted after the regeneration, iodine is formed due to unwanted periodate reduction. The effect of this reduction can be observed as a yellow/orange coloration of the regenerated filtrates obtained after the 2^nd and 3^rd oxidation. The filtrates obtained after the 3^rd oxidation are shown as the middle and right sample container in Figure 1. The reduction is most likely the caused by the reaction of periodate and iodate with byproducts of cellulose overoxidation.

The overoxidation takes place when cellulose reacts with periodate at very low pH, i.e. lower than 2.

As a consequence of iodine formation periodate regeneration efficiency was decreased. Moreover, it proved impossible in practice to perform more than 3 oxidation cycles. No films were made with DAC from oxidations A-T1 to A-T3.

Example 3 - DAC film formation

DAC films were made from each of the DAC products of Example 1. The following method was used 6 times, for each DAC product. 3 % DAC was mixed with 3 % MFC in the ratio 3:2. The obtained suspension was fluidized 3 times and vacuum- filtrated to get the round films at a grammage of about 40 gsm. The films were hot pressed at 100 °C for 10 seconds under a pressure of 10 kPa. The OTR value for films comprising mixtures of MFC and DA-MFC were first measured at a humidity of 50 % at 23 °C (23/50) and then at a humidity of 90 % at 38 °C (38/90) at two different cycles. The OTR values were measured according to standard ASTM D-3985, except the fact that not all values have reached the steady state. The films were stored at room temperature (humidity of 50 % at 23 °C) for 24 hours between two measurements in (38/90) and the OTR value was once again measured at a high humidity of 90 % at 38 °C. As shown in Table 3, the films obtained after each oxidation retained their barrier properties.

Table 3. OTR values (cc/m²/24h at 749 mmFIg) for the films made from six DAC samples obtained in the reactions with the regenerated periodate solutions.

^*This sample was conditioned in 23/50 (humidity of 50 % at 23 °C) for a few days, not for 24h. Moreover, the relative humidity reached 98% due to an uncontrolled error.

Claims

1. A method for oxidation of cellulose to dialdehyde cellulose (DAC), said method comprising: a) oxidizing cellulose with an aqueous periodate solution having a pH in the range of 3 to 5 to form oxidized cellulose comprising DAC; b) separating the aqueous periodate solution from the oxidized cellulose; c) regenerating the separated aqueous periodate solution by electrolytic oxidation; d) adjusting of the pH of the regenerated aqueous periodate solution to a value in the range of 3 to 5; e) reusing the pH adjusted regenerated aqueous periodate solution, optionally combined with a make-up amount of fresh aqueous periodate solution, as the aqueous periodate solution in step a).

2. A method according to claim 1 , wherein the aqueous periodate solution in step a) has a pH in the range 3.5 to 4.5, preferably a pH of about 4.

3. A method according to claim 1 or 2, wherein the molar ratio of periodate ion to cellulose in step a) is between 0.3 - 1.14.

4. A method according to any one of the preceding claims, wherein the aqueous periodate solution in step a) comprises periodate ions at a starting concentration in the range of 100-230 mM, preferably in the range of 120-160 mM, more preferably of about 140 mM.

5. A method according to any one of the preceding claims, wherein the cellulose in step a) is contacted with the aqueous periodate solution at a temperature in the range of 30-70 °C, preferably in the range of 30-60 °C, more preferably of about 50 °C.

6. A method according to any one of the preceding claims, wherein the cellulose in step a) is contacted with the aqueous periodate solution for a period of less than 3 hours, preferably in the range of 1 -3 hours.

7. A method according to any one of the preceding claims, wherein at least 20 %, preferably at least 30 %, of the cellulose in step a) is oxidized to DAC.

8. A method according to any one of the preceding claims, wherein the aqueous periodate solution is regenerated by electrolytic oxidation without addition of chemical oxidants.

9. A method according to any one of the preceding claims, wherein the electrolytic oxidation is performed in an electrolytic cell comprising a cathode chamber and an anode chamber separated by a cation exchange membrane, and wherein the cathode chamber comprises a cathode, preferably made of stainless steel, and the anode chamber comprises an anode, preferably made of Pb02 on a Ti substrate.

10. A method according to any one of the preceding claims, wherein the electrolytic oxidation is performed at a current density in the range of 300 to 650 mA per dm².

1 1. A method according to any one of the preceding claims, wherein the electrolytic oxidation is performed at a temperature in the range of 10-30 °C, preferably in the range of 20-30 °C.

12. A method according to any one of the preceding claims, wherein the electrolytic oxidation is performed for a period in the range of 5-30 hours.

13. A method according to any one of the preceding claims, wherein the regenerated aqueous periodate solution comprises periodate ions at a

concentration of at least 100 mM, preferably at least 120 mM.

14. A method according to any one of the preceding claims, wherein the pH of the aqueous periodate solution after the electrolytic oxidation is below 2, such as below 1.5.

15. A method according to any one of the preceding claims, wherein the pH of the regenerated aqueous periodate solution in step d) is adjusted to a value in the range of 3.5 to 4.5, preferably to a pH of about 4.0.

16. A method according to any one of the preceding claims, wherein the pH is adjusted by addition of NaOH.

17. A method according to any one of the preceding claims, wherein the pH is adjusted by addition of solid NaOH or aqueous NaOH having a concentration of at least 0.1 M.

18. A method according to any one of the preceding claims, wherein the regenerated aqueous periodate solution is reused directly after regeneration and adjustment of the pH, without further purification.

19. A method according to any one of the preceding claims, wherein the make up amount of fresh aqueous periodate solution constitutes 1 -30 %, preferably 1 -20 %, more preferably 1 -10 %, of the total volume of the aqueous periodate solution used in step a).

20. A method according to any one of the preceding claims, wherein the same aqueous periodate solution is regenerated and reused at least three times, preferably at least four times, more preferably at least five times.

21. A method according to any one of the preceding claims, further comprising the step e) subjecting the separated oxidized cellulose comprising DAC, optionally together with microfibrillated cellulose (MFC), to fibrillation to obtain microfibrillated DAC or a microfibrillated mixture of DAC and MFC.

22. A method for manufacturing at least one layer of a film wherein the method comprises the steps of:

providing a suspension comprising (i) a mixture of microfibrillated

applying said suspension to a substrate to form a fibrous web, and drying said web to form at least one layer of said film. wherein the DAC is obtained according to the method of any one of claims

1 - 21 .

23. The method according to claim 22, wherein the suspension comprises between 20-95 wt% of microfibrillated DAC based on the total fiber weight of the mixture.

24. The method according to any one of claims 22-23, wherein suspension comprises between 5-80 % of microfibrillated cellulose based on the total fiber weight of the mixture.

25. The method according to any one of claims 22-24, wherein the dry content of the mixture applied to the substrate is between 1 -10 wt%.

26. The method according to any one of claims 22-25, wherein the at least one layer of the film has an oxygen transmission rate in the range of from 0.1 to 300 cc/m²/24h according to ASTM D-3985, at a relative humidity of 50 % at 23 ^°C and/or at a relative humidity of 90 % at 38 ^°C.

27. The method according to any one of claims 22-26, wherein the substrate is a polymer or metal substrate.

28. The method according to any one of claims 22-27 , wherein said method further comprises the step of pressing the film after drying.

29. The method according to claim 28, wherein the temperature is increased to

100-150°C during pressing of the film.

30. The method according to any one of claims 22-29, wherein 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.

31. The method according to any one of claims 22-30, wherein the

microfibrillated DAC has an oxidation degree of at least 20 %.

32. Use of microfibrillated dialdehyde cellulose (DAC) obtained according to the method of any one of claims 1 - 21 for manufacturing at least one layer of film.