SE1751540A1

SE1751540A1 - Method for oxidation of cellulose

Info

Publication number: SE1751540A1
Application number: SE1751540A
Authority: SE
Inventors: Adrianna Svensson
Original assignee: Stora Enso Oyj
Priority date: 2017-12-13
Filing date: 2017-12-13
Publication date: 2019-06-14
Also published as: SE542443C2; WO2019116245A1; EP3724233A1; 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

|\/IETHOD FOR OXIDATION OF CELLULOSE Technical field The present disclosure relates to methods for oxidation of cellulose to dialdehydecellulose (DAC) for use in the formation of DAC films, and particularly toregeneration and recycling of aqueous periodate solutions used for the oxidation insuch methods.

BackgroundFilms comprising microfibrillated cellulose (MFC), have proven to give excellent barrier properties. However, the gas barrier properties are very dependent on themoisture and the relative humidity in the surrounding environment. Therefore, it iscommon that l\/IFC films must for example be coated with a polymer film to prevent moisture or water vapor to swell and disrupt the l\/IFC film.

Another way to decrease the moisture sensitivity of cellulose is to chemicallymodify the cellulose with sodium periodate to obtain dialdehyde cellulose (DAC).By mechanically processing the dialdehyde fibers, it is possible to disintegratethem into fibrils. Periodate oxidation can be seen as the pretreatment to liberatemicrofibrils. Using fibrillated dialdehyde cellulose, barrier films with improvedmoisture resistance can be produced.

Unfortunately, periodate oxidants are environmentally harmful and also expensive.ln order to achieve a realistic industrial production, an efficient regeneration andrecycling of periodate is essential. However, oxidation of polysaccharides ingeneral, and cellulose in particular, using periodate oxidants are complexprocesses, involving many different reactions and possible byproducts.Periodate/iodate solutions obtained during the cellulose oxidation also typicallyinclude 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 lt is an object of the present disclosure to provide an improved manner forrecycling of periodate solution in a method for oxidation of cellulose to dialdehydecellulose. Another object, related to the above mentioned object, is to reduce thecosts of oxidation of cellulose to dialdehyde cellulose by an improved manner ofrecycling of a periodate solution in a method for oxidation of cellulose to dialdehyde cellulose. lt is a further object of the present disclosure to provide an improved method formanufacturing 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 benefitsof 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 foroxidation of cellulose to dialdehyde cellulose (DAC), said method comprising: a) oxidizing cellulose with an aqueous periodate solution having a pH in therange 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, optionallycombined with a make-up amount of fresh aqueous periodate solution, as the aqueous periodate solution in step a). 3 During the oxidation of cellulose at least a part of the periodate will be reduced toiodate. Accordingly, the aqueous periodate solution separated in step b) will alsocomprise iodate. ln 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 regenerationand reuse of the same aqueous periodate solution without the addition of chemicaloxidants in the regeneration step and without the need for time consuming and costly purification steps.

Electrolytic oxidation has previously been used for regeneration of periodatesolutions used in the oxidation of starch. However, oxidation of starch producesless byproducts than oxidation of cellulose. Oxidation byproducts can react furtherwith periodate in the solution leading to a decrease in periodate available for themain oxidation reaction and thereby inferior DAC products. Furthermore, Oxidationbyproducts in the used periodate solution may also interfere with the electrolyticoxidation process, leading to higher currents or reaction time required to reach adesired degree of regeneration. Therefore, previous methods using electrolyticoxidation for regeneration of periodate solutions for polysaccharide oxidation havealso included various purification steps, for example based on ion-exchangechromatography, 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 periodatesolution to the range of 3-5 following the electrolytic oxidation, formation ofbyproducts during the main oxidation reaction can be reduced. This, in turn, hasbeen found to allow for reusing the periodate solution in multiple cycles without theneed for time consuming and costly purification steps between each use. ln fact,the method according the present disclosure has been shown to successfully allowfor 6 oxidation and regeneration cycles, and it is envisaged that it would bepossible to increase the number of cycles even further, e.g. to 10 or more, without significant deterioration of the process or the obtained material. 4 lt 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 reductionis visible to the naked eye with yellow/orange filtrates being obtained after the 2“dand Sid oxidation cycles with the regenerated solutions. l\/lost likely, the reduction iscaused by the reaction of periodate and iodate with byproducts of celluloseoveroxidation. The byproducts are formed when cellulose reacts with periodate atvery low pH, i.e. lower than pH 2. As a consequence of iodine formation theperiodate 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 ofbyproducts and the best conditions for repeated recycling of the periodate solution.According to some embodiments, the aqueous periodate solution in step a) has apH in the range 3.5 to 4.5, preferably a pH of about 4.

The periodate solution preferably comprises an aqueous solution of sodiumperiodate. Sodium periodate is the inorganic salt of periodic acid. lt is composed ofsodium, iodine and oxygen. Periodate can exists either as IO4' or IOGS". When DACis being produced, it is the metaperiodate form, IO4' that reacts with cellulose according to: Cellulose + NalO4 -> Dialdehyde cellulose + NalOs + H20 According to some embodiments, the cellulose in step a) is in the form of pulphaving a cellulose concentration in the range of 1-10 wt°/>, preferably in the rangeof 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 ml\/l,preferably in the range of 120-160 mM, more preferably of about 140 mM.

The reaction temperature of the cellulose oxidation is preferably selected so as toobtain a high reaction rate while not causing an unacceptable degree ofdegradation of the participating ingredients or byproduct formation. Notably, 5 periodate starts to decompose at temperatures above 55 °C, but temperatureslightly above 55 °C may still be used since the shorter reaction time can make upfor the periodate decomposition. According to some embodiments, the cellulose instep a) is contacted with the aqueous periodate solution at a temperature in therange of 30-70 °C, preferably in the range of 30-60 °C, more preferably of about50 °C.

The reaction time required for the cellulose oxidation will of course vary dependingon the reaction conditions. According to some exemplary embodiments, thecellulose in step a) is contacted with the aqueous periodate solution for a period inthe range of 0.5-5 hours, preferably in the range of 1-4 hours, more preferably inthe range of 2-3 hours. However, the skilled person realizes that longer or shorterreaction time may be required depending, e.g., on the pH, periodate concentration,temperature and desired oxidation degree. ln order to be useful in the formation of barrier films with suitable barrier andmoisture resistance, the oxidized cellulose obtained should preferably have anoxidation degree of at least 20 °/>, preferably at least 30 °/-.~. Accordingly, in someembodiments at least 20 °/>, preferably at least 30 °/>, of the cellulose in step a) isoxidized to DAC.

The method of the present disclosure is further advantageous since it allows forthe repeated regeneration and recycling of periodate solutions without the use ofadded chemical oxidants. According to some embodiments, the aqueousperiodate solution is regenerated by electrolytic oxidation without addition ofchemical oxidants. Less added chemicals results in less required work-up andpurification of the periodate solution, less waste and better process economy.

According to some embodiments, the electrolytic oxidation is performed in anelectrolytic cell comprising a cathode chamber and an anode chamber separatedby a cation exchange membrane, and wherein the cathode chamber comprises acathode, preferably made of stainless steel, and the anode chamber comprises ananode, preferably made of PbOz on a Ti substrate. 6 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 densityand the temperature. The skilled person understands that different combinations ofparameters can be used to achieve substantially the same result. According tosome exemplary embodiments, the electrolytic oxidation is performed at a currentdensity in the range of 100-2000 mA per dm2, such as in the range of 300 to 650mA per dm2.

According to some exemplary embodiments, the electrolytic oxidation is performedat 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 performedfor a period in the range of 5-30 hours.

The aqueous periodate solution to be used in step a) should preferably compriseperiodate ions at a starting concentration in the range of 100-230 ml\/l, preferablyin the range of 120-160 mM, more preferably of about 140 mM. Accordingly, in tosome embodiments the regenerated aqueous periodate solution comprisesperiodate ions at a concentration of at least 100 ml\/l, preferably at least 120 ml\/l,and more preferably at least 140 ml\/l.

The pH of the aqueous periodate solution after the electrolytic oxidation is typicallybelow 2, such as below 1.5. The pH value is then adjusted to a value in the rangeof 3 to 5. According to some embodiments, the pH of the regenerated aqueousperiodate 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 about4 gives the least formation of byproducts and the best conditions for repeatedrecycling 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 toadjust the pH does not appear to affect the main oxidation reaction or theregeneration reaction adversely, at least not in the envisaged range ofregeneration cycles, e.g. 6-10 cycles. ln order to avoid excessive dilution of theperiodate solution, it is preferred to add a smaller volume of concentrated NaOH 7 solution, or even solid NaOH, rather than a larger volume of a more dilute NaOHsolution. According to some embodiments, the pH is adjusted by addition of solid NaOH or aqueous NaOH having a concentration of at least 0.1 l\/l.

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

Naturally, each oxidation and regeneration cycle will involve a certain loss ofperiodate solution. ln order to maintain a sufficient volume of periodate solution thesolution can be supplemented with a make-up amount of fresh aqueous periodatesolution. The fresh aqueous periodate solution may preferably have aconcentration of periodate ions which is the same or slightly higher than theconcentration of the aqueous periodate solution first used in the cellulose oxidationstep. A slightly higher concentration allows for compensation of decreasingperiodate concentration e.g. due to less than 100 °/> efficiency in the electrolyticoxidation. According to some embodiments, the make-up amount of fresh aqueousperiodate 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 volumeand concentration of the aqueous periodate solution may also be adjusted byaddition of water.

The inventor has surprisingly found that using the inventive method, the sameaqueous periodate solution can be effectively regenerated and reused at least fivetimes without additional work-up and purification steps. According to someembodiments, the same aqueous periodate solution is regenerated and reused atleast 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 preparedusing periodate solution regenerated according to the inventive method up to atleast five times retained their barrier properties as compared to DAC films formedfrom DAC prepared using fresh (i.e. not previously used and regenerated) periodate solution. 8 ln order to be useful in the formation of barrier films with suitable barrier andmoisture 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 togetherwith microfibrillated cellulose (MFC), to fibrillation to obtain microfibrillated DAC ora microfibrillated mixture of DAC and MFC.

According to a second aspect i||ustrated herein, there is provided a method formanufacturing at least one layer of a film wherein the method comprises the stepsof:providing a suspension comprising (i) a mixture of microfibrillateddialdehyde cellulose (DAC) and microfibrillated cellulose (MFC), or (ii) amicrofibrillated 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. lt has been found that it is possible to produce a very stable suspension or mixtureby mixing a first suspension comprising MFC and a second suspension comprisingmicrofibrillated DAC, or by using a microfibrillated mixture of DAC and MFC. lt iscrucial that a suspension used for the production of barrier materials is stablesince uneven distribution of the fibrils will lead to deteriorated barrier properties.Furthermore, it was found that the use of both MFC and microfibrillated DACmakes it possible to produce at least one layer of a film that gives the filmimproved 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. Dependingon the end use and the properties of the MFC and microfibrillated DAC, the amount of microfibrillated DAC may vary. 9 According to some embodiments, the suspension comprises between 5-80 °/> ofl\/IFC based on the total fiber weight of the mixture.

According to some embodiments, the dry content of the mixture applied to thesubstrate 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 asubstrate to form a fibrous web and drying said web to form at least one layer ofsaid 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 therange of from 0.1 to 300 cc/m2/24h according to ASTM D-3985, at a relativehumidity of 50 °/> at 23 °C and/or at a relative humidity of 90 % at 38 °C. By thepresent invention it is possible to produce at least one layer of a film that has verygood oxygen barrier properties at high humidity. lt has especially been found thatthe 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. lt ispreferred that the mixture is cast coated onto said substrate. The cast coatedfibrous web can be dried in any conventional manner and thereafter optionallypeeled off from the substrate. lt may be possible to cast or coat more than onelayer onto the substrate forming a multilayer film. lt is possible to produce a filmcomprising more than one layer wherein at least one of the layers comprises themixture according to the invention. lt may also be possible that more than onelayer of the film comprises the mixture according to the invention. lt may also bepossible that one or more layers of the film only comprises microfibrillatedcellulose, 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 kindof 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. lt hasbeen shown that the barrier properties of the film is increased if the film issubjected to increased pressure after drying. The pressure applied in the pressingis preferably above 40kN/m2 (over pressure), more preferably between 100-900kN/m2. The pressing may last for a period of less than 10 minutes, preferablybetween 1 second to 10 minutes. lt is preferred that the pressing is done atelevated temperatures. The temperature is preferably increased to between 50-200 °C, preferably between 100-150°C during pressing of the film. The pressingmay be done in any conventional equipment such as presses or calenders. Bycombining the use of pressing, preferably hot pressing of the formed film thebarrier 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 degreeof at least 20 °/-.~. ln some embodiments, the microfibrillated DAC has an oxidationdegree of between 25-75 °/>.

According to some embodiments, the mixture further comprises any one of astarch, carboxymethyl cellulose, a filler, retention chemicals, flocculation additives, deflocculating additives, dry strength additives, softeners, or mixtures thereof. 11 The microfibrillated cellulose is microfibrillated cellulose produced frommechanical, thermomechanical or chemical pulp. The microfibrillated cellulose ispreferably produced from kraft pulp. The microfibrillated cellulose preferably has aSchopper Riegler value (SR°) of more than 90. According to another embodimentthe l\/IFC may have a Schopper Riegler value (SR°) of more than 93. According toyet another embodiment the l\/IFC may have a Schopper Riegler value (SR°) ofmore than 95. The Schopper-Riegler value can be obtained through the standardmethod 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 afilm or started e.g. hornification. The dry solid content of this kind of web, beforedisintegrated and measuring SR, is less than 50 °/> (w/w). To determine theSchopper Riegler value it is preferable to take a sample just after the wire sectionwhere the wet web consistency is relatively low. The skilled person understandsthat paper making chemicals, such as retention agents or dewatering agents, havean impact on the SR value. The SR value specified herein, is to be understood asan indication but not a limitation, to reflect the characteristics of the l\/IFC materialitself.

The microfibrillated dialdehyde cellulose should in this context mean a dialdehydecellulose treated in such way that it is microfibrillated. The production of themicrofibrillated dialdehyde cellulose is done by treating dialdehyde cellulose forexample by a homogenizer or in any other way such that fibrillation occurs toproduce microfibrillated dialdehyde cellulose. The microfibrillated dialdehydecellulose preferably has an oxidation degree between 25-75 °/-.~, preferablybetween 30-65 °/>, even more preferably between 30-50 °/> or most preferredbetween 35-45 °/>. The degree of oxidation was determined according to thefollowing description: after the dialdehyde cellulose reaction, the amount of C2-C3bonds in the cellulose that are converted to dialdehydes are measured. Thedegree of oxidation is the amount of C2-C3 bonds that are converted compared toall 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 PolyaldehydeDexran by the Hydroxylamine Hydrochloride Method”, Pharmaceutical Research,vol. 8, pp. 400-402, 1991, where the available aldehyde groups reacts withhydroxylamine hydrochloride. This forms oxime groups and releases hydrochloric 12 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 formulabelow. The received aldehyde content is divided by two to get the value of thedegree of oxidation, since an oxidized anhydroglucose unit has two aldehyde groups.

X íxlgg DIO[%] I msampleXMw 2VNaoH = the amount of sodium hydroxide needed to reach pH 4 (I)CNaoH = 0.1 mol/Imsampie = dry weight of the analysed DAC sample (g)lVlw = 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. ltmay be possible to add additives that will improve different properties of themixture and/or the produced film. lt may be possible to add the additive to the firstsuspension, the second suspension and/or to the mixture. lt has been shown thatthe use of a softener, such as sorbitol, glycerol, polyethylene glycol, sorbic acid,propylene glycol, erythritol, maltitol or polyethylene oxides, will modify and improvesome of the mechanical properties of the film, especially the stretch at breakproperties. The amount of sorbitol used is preferably between 1-20 °/> by dryweight of the film.

According to one embodiment the film, comprising microfibrillated cellulose andmicrofibrillated dialdehyde cellulose, has an oxygen transmission rate in the rangeof 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 between5-80 wt% by total dry weight of the film, preferably between 10-60 wt% by total dry 13 weight of the film and even more preferred between 10-40 wt% by total dry weightof the film. The amount of microfibrillated dialdehyde cellulose in the produced filmis preferably between 20-95 wt% by total dry weight of the film, preferably between40-90 wt% by total dry weight of the film and even more preferred between 60-90wt% by total dry weight of the film.

According to one embodiment the film may have a basis weight of less than 50g/m2, or less than 35 g/m2, or less than 25 g/m2. The basis weight is preferably atleast 10 g/m2, preferably between 10-50 g/m2, even more preferred between 10-35g/m2 and most preferred between 10-25 g/m2. l\/licrofibrillated cellulose (l\/IFC) shall in the context of the patent application meana nano scale cellulose fiber or fibril with at least one dimension less than 100 nm.l\/IFC comprises partly or totally fibrillated cellulose or lignocellulose fibers. Theliberated fibrils have a diameter less than 100 nm, whereas the actual fibrildiameter or particle size distribution and/or aspect ratio (length/width) depends onthe source and the manufacturing methods. The smallest fibril is called elementaryfibril and has a diameter of approximately 2-4 nm (see e.g. Chinga-Carrasco, G.,Cellulose fibres, nanofibrils and microfibrils,: The morphological sequence of l\/IFCcomponents from a plant physiology and fibre technology point of view, Nanoscaleresearch letters 2011, 6:417), while it is common that the aggregated form of theelementary fibrils, also defined as microfibril (Fengel, D., Ultrastructural behaviorof cell wall polysaccharides, Tappi J., l\/larch 1970, Vol 53, No. 3.), is the mainproduct that is obtained when making l\/IFC e.g. by using an extended refiningprocess or pressure-drop disintegration process. Depending on the source andthe manufacturing process, the length of the fibrils can vary from around 1 to morethan 10 micrometers. A coarse l\/IFC grade might contain a substantial fraction offibrillated fibers, i.e. protruding fibrils from the tracheid (cellulose fiber), and with acertain amount of fibrils liberated from the tracheid (cellulose fiber).

There are different acronyms for l\/IFC such as cellulose microfibrils, fibrillatedcellulose, nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils,cellulose nanofibers, cellulose nanofibrils, cellulose microfibers, cellulose fibrils,microfibrillar cellulose, microfibril aggregrates and cellulose microfibril aggregates. 14 l\/IFC can also be characterized by various physical or physical-chemicalproperties such as large surface area or its ability to form a gel-like material at lowsolids (1 -5 wt°/>) when dispersed in water. The cellulose fiber is preferablyfibrillated to such an extent that the final specific surface area of the formed l\/IFCis from about 1 to about 200 m2/g, or more preferably 50-200 m2/g whendetermined 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 l\/IFCmanufacturing both energy efficient and sustainable. The cellulose fibers of thepulp to be supplied may thus be pre-treated enzymatically or chemically, forexample to hydrolyse or swell fiber or reduce the quantity of hemicellulose orlignin. The cellulose fibers may be chemically modified before fibrillation, whereinthe cellulose molecules contain functional groups other (or more) than found in theoriginal cellulose. Such groups include, among others, carboxymethyl (GMC),aldehyde and/or carboxyl groups (cellulose obtained by N-oxyl mediatedoxydation, for example "TEl\/lPO"), 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 l\/IFC or nanofibrillar size or NFC.

The nanofibrillar cellulose may contain some hemicelluloses; the amount isdependent on the plant source. l\/lechanical disintegration of the pre-treated fibers,e.g. hydrolysed, pre-swelled, or oxidized cellulose raw material is carried out withsuitable equipment such as a refiner, grinder, homogenizer, colloider, frictiongrinder, ultrasound sonicator, fluidizer such as microfluidizer, macrofluidizer orfluidizer-type homogenizer. Depending on the l\/IFC manufacturing method, theproduct might also contain fines, or nanocrystalline cellulose or e.g. otherchemicals present in wood fibers or in papermaking process. The product mightalso contain various amounts of micron size fiber particles that have not been efficiently fibrillated. l\/IFC is produced from wood cellulose fibers, both from hardwood or softwood fibers. lt can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. lt ispreferably made from pulp including pulp from virgin fiber, e.g. mechanical,chemical and/or thermomechanical pulps. lt can also be made from broke or recycled paper.

The above described definition of l\/IFC includes, but is not limited to, the newproposed TAPPI standard W13021 on cellulose nanofibril (CNF) defining acellulose nanofiber material containing multiple elementary fibrils with bothcrystalline 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 exemplaryembodiments, it will be understood by those skilled in the art that various changesmay be made and equivalents may be substituted for elements thereof withoutdeparting from the scope of the invention. ln addition, many modifications may bemade to adapt a particular situation or material to the teachings of the inventionwithout departing from the essential scope thereof. Therefore, it is intended thatthe invention not be limited to the particular embodiment disclosed as the bestmode 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 drawinos Figure 1 is a photograph of sample bottles containing periodate solution filtratesobtained in Example 2, where pH was not adjusted after the regeneration. Thesamples were collected after one oxidation and one regeneration (left), and threeoxidations and two regenerations (middle and right).

Examplesl\/laterials and equipment: Bleached sulphate pulp refined to SF¶° 30 and l\/IFC was supplied from Stora Enso Kaukopää mill in lmatra. Sodium periodate waspurchased from Sigma-Aldrich and from AJAY Europe Sarl. Sodium hydroxide in 16 pellets was purchased from Sigma-Aldrich. Electro l\/IP Cell was purchased fromElectroCell.

Example 1 - Cellulose oxidation and periodate regeneration with pH adiustmentThe pulp was oxidized using sodium periodate in aqueous solution. The pulpconcentration, in tap water, was 2 °/-.~ and the pH value was adjusted to pH 4.0 with0.5 l\/l H2SO4 at the beginning.

A series of six subsequent oxidations followed by periodate electroregenerationwere performed as set out in Table 1. Each oxidation was done at 50 °C for 2 h 45min. 100% fresh sodium periodate (140 ml\/I in aqueous solution) was used onlyfor the first oxidation, i.e. KV1-T1. After each oxidation periodate concentration decreased from 140 ml\/I to less than 60 ml\/I or even less than 40 ml\/l. lodate produced during the oxidation was electrochemically regenerated toperiodate during the electrolysis of the filtrate. Electroregeneration was performedin a two compartment Electro lVlP Cell with a lead dioxide anode (area 1dm2),stainless steel cathode and Nafion membrane N324. The regenerated filtrate wascirculated through the anodic compartment of the cell at room temperature. NaOHsolution was used as catholyte. The applied current density was between 400 and650 mA/dm2.

Periodate concentration was monitored using a UV-Vis spectrophotometer - tofollow 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 absorbancewas measured in the range between 400 and 200 nm. The measurement wasrepeated 2 times and the obtained absorbances for the peaks at ca 219 nm (ABS.1and Abs.2) were used for calculating the periodate concentration: cperiodate _ - 0,07%) /0,00s7 [m|\/|] 17 The calibration curve was made for the solutions containing periodate and iodate indifferent rations, but the sum of their concentrations was always 140 ml\/l. Solutionscontaining from 70 ml\/I to 140 ml\/I periodate were used.

At least about 90 °/> and up to about 97 °/> of periodate in the filtrate waselectroregenerated and used for next oxidation after the addition of a make-upamount of sodium periodate and water, and pH adjustment to 4.0. The make-upamount varied between 12 and 29 °/>, depending mostly on how much filtrate wasleft after the regeneration and on the eletroregeneration efficiency - see Error!Reference source not found.. pH adjustment was done with NaOH. The solutionused at the beginning of each oxidation contained 140 ml\/I periodate and 2 °/>pulp. The filtrate obtained after the 6"" oxidation step was still essentiallyuncolored, like the sample container on the left in Figure 1.

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

Sample m dry V regenerated Regenerated Make-up Water D.O.name pulp (g) filtrate (I) periodate (%) (%) added (g) (%)KV1-T1 50.00 - - - - 39.8KV1-T2 17.59 0.9007 90.4 29.0 91.65 35.7KV1-T3 21.50 0.9781 97.2 12.4 17.91 36.1KV1-T4 21.50 0.9541 93.3 18.0 40.09 NOdataKV1-T5 21.50 0.8914 91.6 24.8 100.61 34.4KV1-T6 21.50 0.8926 87.3 28.3 98.27 37.6 Example 2 (Comparative example) - Cellulose oxidation and periodate regeneration without pH adiustment Example 1 was repeated as set out in Table 2, except no pH adjustment of the regenerated filtrate was performed. 18 Table 2. Selected parameters of the subsequent oxidations and calculateddegrees of oxidation of obtained DAC. The pH was not adjusted before oxidations.

Sample m dry V regenerated Regenerated Make-up D.O.name pulp (g) filtrate (I) periodate (%) (%) (%) A-T1 5.37 - - - 37.3A-T2 5.37 247.4 73.7 29.5 39.5A-T3 5.37 231.5 92.9 41.8* 45.0 *This high make-up was made to compensate the loss of periodate ions whichaccrued when the regenerated solution was kept in the fridge for a few daysbetween the oxidations A-T2 and A-T3.

When the pH of the filtrate is not adjusted after the regeneration, iodine is formeddue to unwanted periodate reduction. The effect of this reduction can be observedas a yellow/orange coloration of the regenerated filtrates obtained after the 2“d and3fd oxidation. The filtrates obtained after the 3fd oxidation are shown as the middleand right sample container in Figure 1. The reduction is most likely the caused bythe 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 wasdecreased. l\/loreover, it proved impossible in practice to perform more than 3oxidation 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 followingmethod was used 6 times, for each DAC product. 3 °/> DAC was mixed with 3 °/>l\/IFC 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 hotpressed at 100 °C for 10 seconds under a pressure of 10 kPa. 19 The OTR value for films comprising mixtures of l\/IFC and DA-l\/IFC were firstmeasured at a humidity of 50 °/-.~ at 23 °C (23/50) and then at a humidity of 90 °/> at38 °C (38/90) at two different cycles. The OTR values were measured according tostandard ASTM D-3985, except the fact that not all values have reached the 5 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 wasonce 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. 10 Table 3. OTR values (cc/m2/24h at 749 mmHg) for the films made from six DAC samples obtained in the reactions with the regenerated periodate solutions.

Sample OTRname _______ ___________________ ____________________________ _____________________________ ______________________ ____________________ ____________________ __________ ._t|me time in t|me(h) (h) 23/50 (h)for24h KV1-T1 4.6 22.42 76.4 30.69 / 80.2 22.05KV1-T2 6.0 18.58 65.8 27.66 / 89.2 22.74KV1-T3 3.0 23.62 54.0 23.32 / 89.6 27.12KV1-T4 3.2 21.44 53.2 30.00 / 70.0 30.00KV1-T5 3.2 21.75 48.4 23.20 / 112.8* 24.15KV1-T6 1.0 24.19 14.4 18.68 / 75.6 18.68 *This sample was conditioned in 23/50 (humidity of 50 °/> at 23 °C) for a few days,not for 24h. l\/loreover, the relative humidity reached 98% due to an uncontrolled15 error.

Claims

CLAIIVIS

1. A method for oxidation of cellulose to dialdehyde cellulose (DAC), saidmethod 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, optionallycombined 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 instep a) has a pH in the range 3.5 to 4.5, preferably a pH of about 4.

3. A method according to any one of the preceding claims, wherein the aqueousperiodate solution in step a) comprises periodate ions at a starting concentration inthe range of 100-230 ml\/l, preferably in the range of 120-160 ml\/l, more preferablyof about 140 ml\/l.

4. A method according to any one of the preceding claims, wherein the cellulosein step a) is contacted with the aqueous periodate solution at a temperature in therange of 30-70 °C, preferably in the range of 30-60 °C, more preferably of about50 °C. 21

5. A method according to any one of the preceding claims, wherein the cellulosein step a) is contacted with the aqueous periodate solution for a period in therange of 0.5-5 hours, preferably in the range of 1-4 hours, more preferably in therange of 2-3 hours.

6. 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.

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

8. A method according to any one of the preceding claims, wherein theelectrolytic oxidation is performed in an electrolytic cell comprising a cathodechamber and an anode chamber separated by a cation exchange membrane, andwherein the cathode chamber comprises a cathode, preferably made of stainlesssteel, and the anode chamber comprises an anode, preferably made of PbOz on aTi substrate.

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

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

11. 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.

12. A method according to any one of the preceding claims, wherein theregenerated aqueous periodate solution comprises periodate ions at aconcentration of at least 100 ml\/l, preferably at least 120 ml\/l. 22

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

14. A method according to any one of the preceding claims, wherein the pH ofthe 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.

15. A method according to any one of the preceding claims, wherein the pH isadjusted by addition of NaOH.

16. A method according to any one of the preceding claims, wherein the pH isadjusted by addition of solid NaOH or aqueous NaOH having a concentration of atleast 0.1 l\/l.

17. A method according to any one of the preceding claims, wherein theregenerated aqueous periodate solution is reused directly after regeneration andadjustment of the pH, without further purification.

18. 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 solutionused in step a).

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

20. A method according to any one of the preceding claims, further comprisingthe step e) subjecting the separated oxidized cellulose comprising DAC, optionallytogether with microfibrillated cellulose (MFC), to fibrillation to obtain microfibrillatedDAC or a microfibrillated mixture of DAC and MFC. 23

21. A method for manufacturing a least one layer of a film wherein the methodcomprises the steps of: providing a suspension comprising (i) a mixture of microfibrillateddialdehyde cellulose (DAC) and microfibrillated cellulose (MFC), or (ii) amicrofibrillated mixture of DAC and l\/IFC, applying said suspension to a substrate to form a fibrous web, anddrying said web to form at least one layer of said film. wherein the DAC is obtained according to the method of to any one of the preceding claims.

22. The method according to claim 21, wherein the suspension comprisesbetween 20-95 wt% of microfibrillated DAC based on the total fiber weight of the mixture.

23. The method according to any one of claims 21-22, wherein suspensioncomprises between 5-80 °/> of microfibrillated cellulose based on the total fiber weight of the mixture.

24. The method according to any one of claims 21-23, wherein the dry content of the mixture applied to the substrate is between 1-10 wt°/-.~.

25. The method according to any one of claims 21-24, wherein the at least onelayer of the film has an oxygen transmission rate in the range of from 0.1 to 300cc/m2/24h according to ASTM D-3985, at a relative humidity of 50 °/> at 23 °Cand/or at a relative humidity of 90 °/> at 38 °C.

26. The method according to any one of claims 21-25, wherein the substrate is a polymer or metal substrate.

27. The method according to any one of claims 21-26, wherein said method further comprises the step of pressing the film after drying. 24

28. The method according to claim 27, wherein the temperature is increased to 100-150°C during pressing of the film.

29. The method according to any one of c|aims 21-28, wherein said mixturefurther comprises any one of a starch, carboxymethyl cellulose, a fi||er, retentionchemicals, flocculation additives, defloccuiating additives, dry strength additives, softeners, or mixtures thereof.

30. The method according to any one of c|aims 21-29, wherein themicrofibrillated DAC has an oxidation degree of at least 20 °/>.