CELLULOSE

Technical field

The present invention relates to a method for manufacturing at least one fibrous barrier layer comprising microfibrillated dialdehyde cellulose. The present invention also relates to a fibrous barrier layer, such as a barrier film, having good mechanical properties and low oxygen transmission rate at high relative humidity.

Background

Microfibrillated cellulose (MFC) is widely used to manufacture barrier films for instance in multilayered paper laminates. However, the gas barrier properties of MFC based films are dependent on the moisture or the relative humidity in the surrounding environment. Therefore, it is quite common that MFC films are coated with a polymer film to prevent moisture or water vapor to swell and disrupt the MFC film. The EP2551104A1 teaches the use of MFC and polyvinyl alcohol (PVOH) and/or polyuronic acid with improved barrier properties at higher relative humidity (RH). Another solution is to coat the film with a film that has high water fastness and/or low water vapor transmission rate. The JP2000303386A discloses e.g. latex coating 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, different chemical modification routes have been investigated to give the possibility to cross-link of fibrils with each other or fibrils with other compounds. This improves water fastness of the films but also water vapor transmission rates. Another way to decrease the moisture sensitivity of cellulose is to chemically modify the cellulose with sodium periodate to obtain dialdehyde cellulose (DAC). Thanks to being based on a renewable material, use of DAC allows for designing barrier structures with a high renewable content. By fibrillating DAC into dialdehyde microfibrillated cellulose (DA- MFC), a barrier film with improved moisture resistant can be produced. This is presented e.g. in WO2015034426. However, a dispersion comprising DA- MFC is very unstable since the DA-MFC sediments and spontaneously crosslinks to a certain degree already in the dispersion, leading to that the microfibrils are bound or entangled. The crosslinks further render the DA- MFC barrier film brittle, which restrain the processing and handling. For instance, if the film is part of a paper-based laminate structure, undesired cracks may appear in the film upon converting.

Hence, there is a need to find a way of manufacturing a barrier film which presents good gas barrier properties even at high relative humidity and also having a good mechanical properties.

Summary

It is an object of the present invention to provide an improved barrier film or barrier layer comprising microfibrillated dialdehyde cellulose, which has good barrier properties at high relative humidity as well as improved mechanical properties.

The invention is defined by the appended independent claims.

Embodiments are set forth in the appended dependent claims.

The present invention relates to a method for manufacturing at least one fibrous barrier layer wherein the method comprises the steps of:

providing a suspension comprising intermixed microfibrillated cellulose (MFC) and microfibrillated dialdehyde cellulose (DA-MFC);

adding polyvinyl alcohol (PVOH) to said suspension and mix to form a mixture;

applying said mixture to a substrate to form a wet fibrous web; and drying said wet web on said substrate to form a fibrous barrier layer. It is to be understood that said PVOH can be mixed firstly with MFC and thereafter DA-MFC is added to form said mixture. It is equally possible to firstly mix the PVOFI with DA-MFC and thereafter adding MFC to form said mixture. It is understood that“fibrous barrier layer” may refer to a film (e.g. a free-standing film) which in its turn can be laminated e.g. onto a base layer such as paper board and/or paper, and it may also refer to a layer in a multilayer structure which has been created by wet coating directly onto a substrate and subsequently dried thereon.

It has been found that addition of polyvinyl alcohol (PVOFI) to a mixture comprising MFC and DA-MFC in making of films leads to films with improved mechnical properties compared to corresponding films without PVOFI, and at the same time the gas barrier function at high humidities is preserved.

According to one aspect of the invention, the PVOFI to be added preferably has a degree of hydrolysis between 80 - 99 mol%, more preferably between 88 - 99 mol%. Furthermore, the PVOFI preferably has a viscosity above 5 mPaxs in a 4 % aqueous solution at 20 °C DIN 53015 / JIS K 6726.

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 (AGU) in the cellulose chain, forming two aldehyde groups at said location. The term “degree of oxidation” (D.O.) 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 skilled person understands that the DA-MFC added in the suspension can have different degrees of oxidation, defined as the portion of AGUs that have a dialdehyde, as explained above. The amount of DA-MFC needed to be added in order to achieve a specific D.O. in the final mixture depends on the degree of oxidation in the corresponding suspension. For instance, a mixture of DA- MFC and native MFC naturally has an average degree of oxidation that is lower than that of the DA-MFC part of the mixture. In that sense, such a mixture containing both MFC and DA-MFC could theoretically be interpreted as a DA-MFC with low degree of oxidation.

The DA-MFC used for manufacturing of the fibrous layer is

mechanically treated to obtain microfibrillated dialdehyde cellulose. 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.

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.

According to one aspect of the invention, the dried fibrous barrier layer is a free-standing film.

According to another aspect of the invention, the substrate is a paper or paperboard substrate and the mixture is applied onto the substrate as a coating to form said wet fibrous web, wherein after drying said fibrous barrier layer and said substrate forms two layers of a multilayer structure. The coating as such can be applied and dried in one or more layers.

In one embodiment, the mixture comprises between 20-90wt% of microfibrillated cellulose and 10-80wt% microfibrillated dialdehyde cellulose based on the total dry fiber weight of the mixture. To this fiber-based mixture, the added PVOFI is 50-300 kg/ton dry fiber. In another embodiment, the mixture comprises 50-70wt%

microfibri Hated cellulose and 30-50 wt% m i crof i brill ated dialdehyde cellulose. To this fiber-based mixture, the added PVOH is 100-200 kg/ton dry fiber content.

According to another aspect of the invention, the dry content of the mixture applied to the substrate is between 1 -15% by weight.

According to another aspect of the invention, the fibrous barrier layer has an oxygen transmission rate in the range of from 0.1 to 100 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 80% at 23°C at a barrier layer thickness 10-70 pm.

According to another aspect of the invention, the fibrous barrier layer has a strain at break of at least 1 %, preferably at least 3%.

According to another aspect of the invention, the substrate is a polymer or metal substrate.

According to another aspect of the invention, said method further comprises the step of pressing the film upon and/or after drying. The temperature may be increased to 70-150°C during such pressing of the film.

It is within the scope of the invention to add further additives to the mixture, including one or more of a starch, carboxymethyl cellulose, a filler, clay, retention chemicals, flocculation additives, deflocculating additives, dry strength additives, softeners, cellulose nanocrystals or mixtures thereof.

According to another aspect of the invention, the m icrof ibri I lated dialdehyde cellulose in the suspension has an oxidation degree between 15- 50%.

Furthermore, the present invention relates to a fibrous barrier layer having an oxygen transmission rate in the range of from 0.1 to 100 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 80% at 23°C, and at a barrier layer thickness 10-70 pm, and wherein at least one fibrous barrier layer comprises a mixture of a m i crof i bri I lated dialdehyde cellulose and PVOFI. According to one aspect of the invention, the fibrous barrier layer comprises a mixture of microfibrillated cellulose, microfibrillated dialdehyde cellulose and PVOH.

According to one aspect of the invention, the fibrous barrier layer has a basis weight of less than 55 g/m ² , preferably between 10-50 g/m ²

According to one aspect of the invention, said fibrous barrier layer is a film, preferably comprising more than one layer.

According to one aspect of the invention, the fibrous barrier layer is a multilayer film wherein at least one layer of the film is a water vapor barrier film comprising any one of polyethylene (PE), polypropylene (PP), polyamide, polyethylene terephthalate (PET), polylactic acid (PLA), or ethylene vinyl alcohol (EVOH).

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 or more layers.

The present invention further relates to a packaging material e.g.

intended for food stuff comprising a base material and at least one fibrous barrier layer as described above. The base material may include, but is not limited to, paper, cardboard, paperboard, fabric, plastic, polymer film, metal, composites and the like.

The present invention further relates to the use of a fibrous barrier layer comprising a mixture of a microfibrillated dialdehyde cellulose, microfibrillated cellulose and PVOH 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 comprising intermixed microfibrillated cellulose (MFC) and microfibrillated dialdehyde cellulose (DA-MFC); adding polyvinyl alcohol (PVOFI) to said suspension and mix to form a mixture; applying said mixture to a substrate to form a wet fibrous web; and drying said wet web on said substrate to form a fibrous barrier layer.

It has been found that by providing a suspension of microfibrillated dialdehyde cellulose and microfibrillated cellulose which also comprises at least a portion of polyvinyl alcohol, a film can be formed which has improved ductility while maintaining a good oxygen barrier property.

The fibrous barrier layer 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 film or coating. The drying of said web may be done in any conventional way, preferably in combination with heat treatment and increased pressure. The dry content of the at least one layer of the film after drying is preferably above 90% by weight.

Microfibrillated cellulose (MFC) or so called cellulose microfibrils (CMF) shall in the context of the present application mean a nano-scale cellulose particle fiber or fibril with at least one dimension less than 100 nm. MFC comprises partly or totally fibrillated cellulose or lignocellulose fibers. The cellulose fiber is preferably fibrillated to such an extent that the final specific surface area of the formed MFC is from about 1 to about 300 m ² /g, such as from 1 to 200 m ² /g or more preferably 50-200 m ² /g when determined for a freeze-dried material with the BET method. The term“native MFC” refers to MFC that is made from conventional chemical, chemo-mechanical and/or mechanical pulp without further chemical treatment, e.g. said native MFC is lacking special functional groups.

Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment steps are usually required in order to make MFC manufacturing both energy-efficient and sustainable. The cellulose fibers of the pulp to be supplied may thus be pre-treated

enzymatically or chemically. For example, 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, carboxym ethyl, aldehyde and/or carboxyl groups (cellulose obtained by N-oxyl mediated oxidation, for example

"TEMPO"), or quaternary ammonium (cationic cellulose). After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC or 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, single - or twin- screw extruder, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer. Depending on the MFC manufacturing method, the product might also contain fines, or nanocrystalline cellulose or e.g. other chemicals present in wood fibers or in papermaking process. The product might also contain various amounts of micron size fiber particles that have not been efficiently fibrillated.

MFC can be produced from wood cellulose fibers, both from hardwood or softwood fibers. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It is preferably made from pulp including pulp from virgin fiber, e.g. mechanical, chemical and/or thermomechanical pulps. It can also be made from broke or recycled paper.

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

Dialdehyde cellulose (DA-MFC) is typically obtained by reacting cellulose with an oxidising agent such as sodium periodate. During the periodate oxidation, selective cleavage of the C2-C3 bond of the

anhydroglucose unit (AGU) of cellulose takes place, with concurrent oxidation of the C2- and C3-OH moieties to aldehyde moieties. In this manner, crosslinkable functional groups (aldehyde groups) are introduced to the cellulose. The microfibrillated dialdehyde cellulose in the suspension 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 so that fibrillation occurs to produce microfibrillated dialdehyde cellulose. The microfibrillated dialdehyde cellulose in the suspension preferably has an oxidation degree between 15-50%, preferably between 20- 40%. 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 is 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.

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

CNaOH = 0,1 mol/l

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

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

The PVOH to be added to the suspension preferably has a degree of hydrolysis between 80 - 99 mol%, more preferably between 88 - 99 mol%. Furthermore, the PVOH preferably has a viscosity above 5 mPaxs in a 4 % aqueous solution at 20 °C DIN 53015 / JIS K 6726.

The mixture may further comprise additives, preferably any one of a starch, carboxymethyl cellulose, a filler, retention chemicals, clay, talcum, 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 MFC, the DA-MFC, the mix of MFC and DA-MFC, or to the mixture of MFC, DA-MFC and PVOH.

Example - PVOH as additive to DA-MFC barrier film for improving mechanical properties

DA-MFC with D.O. 40% was mixed with native MFC in the proportion 60 wt-% / 40 wt-%. The solids content of the mixture was 1 wt-%. To this reference mixture of fibrillar material, PVOFI (Poval 15-99, Kuraray) was added in an amount of 100 kg dry/ton dry solids.

Both mixtures were stirred with a magnetic stirrer in a vacuum desiccator, to get rid of air bubbles, until no air bubbles could be seen. This took up to 2 h. Films were cast in polystyrene Petri dishes with an amount of mixture corresponding to 30 g dry/m ² . The films were dried in 23 °C, 50% RFI for 5 days. The mechanical properties were measured with a material tester (Zwick Z010) at a testing speed of 2 mm/min and a span length of 20 mm. Strips with a width of 15 mm were used in this measurement.

The results are shown in Table 1 below. Table 1 : Mechanical properties of DA-MFC barrier films

As can be noted from the results displayed in Table 1 , adding PVOFI to a suspension comprising DA-MFC and MFC provides a way to make barrier films with improved strain at break, E-modulus and strength.