SE542054C2

SE542054C2 - Multilayer film comprising microfibrillated cellulose and a method of manufacturing a multilayer film

Info

Publication number: SE542054C2
Application number: SE1751637A
Authority: SE
Inventors: Isabel Knöös
Original assignee: Stora Enso Oyj
Priority date: 2017-12-22
Filing date: 2017-12-22
Publication date: 2020-02-18
Also published as: WO2019123405A1; EP3727849A1; JP2021507830A; SE1751637A1

Abstract

The invention relates to a multilayer, oxygen barrier film comprising:- a first layer (1) comprising at least 80 % by weight of microfibrillated cellulose (MFC) as calculated on the total solid content of said layer,- a second layer (2) comprising a polymer and optionally pigments, and - a third layer (3) comprising at least 40 %, preferably at least 50 %, by weight of chemically modified microfibrillated cellulose as calculated on the total solid content of said layer.The multilayer film of the invention has shown to provide excellent gas barrier properties, especially oxygen barrier properties, also at high humidity and also good strength properties.

Description

Multilayer film comprising microfibrillated cellulose and a method of manufacturing a multilayer film.

The present invention relates to a method of manufacturing a fibrous-based oxygen barrier film. The invention further covers films made by the method and uses thereof.

Background of the invention Today, films comprising microfibrillated cellulose (MFC), have proven to give excellent barrier properties, see e.g. Aulin et al., Oxygen and oil barrier properties of microfibrillated cellulose films and coatings, Cellulose (2010) 17:559-574, Lavoine et al., Microfibrillated cellulose - Its barrier properties and applications in cellulosic materials: A review, Carbohydrate polymers 90 (2012) 735-764, Kumar et al., Comparison of nano- and microfibrillated cellulose films, Cellulose (2014) 21 :3443-3456. However, the gas barrier properties are very dependent on the moisture or the relative humidity in the surrounding environment.

The problem with deteriorated gas barrier properties of MFC films at high relative humidity has been investigated and described in the art, but most of the suggested solutions are expensive and difficult to implement in industrial scale. One proposed solution is to modify the MFC or nanocellulose. This is e.g. described in publication EP2554589A1 where an MFC dispersion is modified with a silane coupling agent. The publication EP2551104A1 teaches the use of MFC and polyvinyl alcohol (PVOH) and/or polyuronic acid with improved barrier properties at higher relative humidity (RH). US2012094047A teaches the use of wood hydrolysates mixed with polysaccharides such as MFC. In addition to the disclosed chemical modification, the possibility of cross-linking fibrils or fibrils and copolymers has been investigated. This may improve the moisture resistance of the film but also increase water vapor transmission rates. The publications EP2371892A1 and EP2371893A1, disclose methods of cross-linking MFC with metal ions, glyoxal, glutaraldehyde and/or citric acid.

Another way to decrease the moisture sensitivity of cellulose is to chemical modify the cellulose with sodium periodate to obtain dialdehyde cellulose (DAC). By fibrillating dialdehyde cellulose a barrier film with improved moisture resistant can be produced. However, it has been shown that films made from dialdehyde microfibrillated cellulose are brittle and have low elongation at break.

There is thus a need for a film with high strength properties as well as good barrier properties even at high humidity and high temperatures.

Summary of the invention It is an object of the present invention to provide an improved film comprising microfibrillated cellulose, which has high strength properties and improved barrier properties even at higher relative humidity in the surroundings. Another object of the invention is to provide an improved film comprising microfibrillated cellulose which is able to remain good barrier properties even if the humidity fluctuates.

The invention is defined by the appended independent claims. Embodiments are set forth in the appended dependent claims and in the following description and drawings.

According to a first aspect, there is provided an oxygen barrier film comprising: - a first layer (1) comprising at least 80 % by weight of unmodified microfibrillated cellulose (MFC) as calculated on the total solid content of said layer, - a second layer (2) comprising a polymer chosen from the group consisting of polyvinyl alcohol (PVOH), latex, CMC, starch, guar gum and/or waxes, and - a third layer (3) comprising at least 40 %, preferably at least 50 %, by weight of dialdehyde microfibrillated cellulose (DA-MFC) as calculated on the total solid content of said layer, wherein said second layer (2) is arrange don said first layer (1) and said third layer (3) is arranged on said second layer (2).

The film of the invention has shown to provide excellent gas barrier properties, especially oxygen barrier properties, also at high humidity and also good strength properties.

Preferably, the second layer further comprises pigments. A second layer comprising a polymer and pigments decreases the moisture and oxygen transmission rate even further.

The film may have a basis weight of less than 50 g/m<2>, such as 10 - 50 g/m<2>and an Oxygen Transmission Rate (OTR) value of below 10 ml/m<2>/per 24h, preferably below 5 ml/m<2>/per 24h, at 23 °C, at 50% RH.

At 80 % RH, the film may exhibit an OTR value of below 20 ml/m<2>per 24h, at 23 °C. At 90 % RH, the film may exhibit an OTR value of below 100 ml/m<2>per 20h, at 38 °C.

The first layer, made from a composition comprising a high amount of MFC, provides strength and strain to the structure. The second layer, comprising a polymer and optionally pigments, provides protection of the third layer by decreasing the moisture and oxygen transmission rate through the film. The third layer, including DA-MFC, provides oxygen barrier properties even at high or fluctuating humidity. Said property of the third layer is further enhanced by the first and second layers. Thus, the inventive combination of the layers contribute to provide enhanced oxygen barrier properties at high humidity and improved strength properties.

Said third layer may further comprise at least 10 %, preferably at least 15 % by weight of unmodified microfibrillated cellulose. Thus, preferably, said third layer is made from a mixture of DA-MFC and un-modified microfibrillated cellulose. It has been shown that mixtures of DA-MFC and un-modified microfibrillated cellulose give rise to a stable particle size distribution after storage and consequently better barrier properties when formed to a film.

In one embodiment, the oxygen barrier film further comprises - a fourth layer (4) comprising a polymer and optionally pigments, and - a fifth layer (5) comprising at least 80 % by weight of microfibrillated cellulose as calculated on the total solid content of said layer.

In this embodiment, the third layer forms a middle layer whereby the first and second layers are arranged on one side of said middle layer and said fourth and fifth layers are arranged on the opposite side of said middle layer. Such a film provides equal barrier properties on both sides and can thus also advantageously be used as a self-standing film.

The polymer present in the second layer is chosen from the group consisting of polyvinyl alcohol (PVOH), latex, CMC, starch, guar gum and/or waxes. The pigments are preferably nanopigments. In one preferred embodiment, the nanopigments are platy and may have an aspect ratio of at least 90:1 or at least 100:1. The second layer, comprising polymers and such platy pigments, provides an efficient mechanical barrier to moisture. In addition, nanopigments swell in contact with moisture and thereby improves the barrier properties of the film even further.

According to a second aspect, there is provided a method of manufacturing a film as described above, said method comprising the steps of a) forming a first layer comprising at least 80 % by weight of unmodified microfibrillated cellulose as calculated on the total solid content of said layer, b) forming a second layer comprising a polymer chosen from the group consisting of polyvinyl alcohol (PVOH), latex, CMC, starch, guar gum and/or waxes, and optionally pigments on one side of said first layer, c) forming a third layer comprising at least 40 % by weight of dialdehyde microfibrillated cellulose (DA-MFC), as calculated on the total solid content of said layer, on said second layer The method may further comprise the steps of forming a fourth layer comprising a polymer and optionally pigments on the third layer and forming a fifth layer comprising MFC on said fourth layer.

Said layers may be formed by cast forming and/or by spraying.

Brief description of the drawings Figure 1 discloses a film according to one embodiment of the invention.

Figure 2 shows a film according to another embodiment of the invention Detailed description Microfibrillated cellulose (MFC) shall in the context of the patent 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 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 2011, 6:417), while it is common that the aggregated form of the elementary fibrils, also defined as microfibril ( Fengel, D., Ultrastructural behavior of cell wall polysaccharides, Tappi J., March 1970, Vo! 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 physicalchemical 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 300 m<2>/g, such as from 1 to 200 m<2>/g or more preferably 50-200 m<2>/g when determined for a freeze-dried material with the BET method.

Various methods exist to make MFC, such as single or multiple pass refining, pre-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.

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 fluid izer-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 nanofbril (CNF) defining a cellolose nanofbire 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.

“Chemically modified microfibrillated cellulose” as used throughout the description and in the claims shall in this context mean microfibrillated cellulose fibers that are chemically modified before or after fibrillation so that they 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).

“Un-modified microfibrillated cellulose” shall in this context mean microfibrillated cellulose fibers that have not been chemically modified to contain functional groups other (or more) than found in the original cellulose.

The oxygen transmission rate (OTR) as used in the patent claims and in the description is measured in accordance with ( ASTM D 3985-05), 24 hours at 23 °C, at the RH specified (50% or 80% RH). At 90% RH, the measurement is made after 20h at 38 °C.

In the following the invention will be described more in detail with reference to the schematic figures 1 and 2.

Figure 1 discloses a film according to one embodiment of the invention. The multilayer film (10) disclosed in fig. 1 comprises a first layer (1) comprising microfibrillated cellulose, a second layer (2) comprising a polymer and optionally pigments and a third layer (3) comprising chemically modified microfibrillated cellulose. The second layer is arranged on (and preferably in direct contact with) the first layer and the third layer is arranged on (and preferably in direct contact with) the second layer.

Preferably, the first layer (1) has a thickness of between 5 - 25 ?m, preferably 15 - 25 ?m, the second layer (2) has a thickness of between 0.1 - 15 ?m, most preferably of between 2 - 10 ?m, and the third layer (3) has a thickness of between of between 5 - 25 ?m, most preferably of between 5 - 15 ?m.

The first layer (1) comprises microfibrillated cellulose in an amount of at least 80 % by weight, preferably in an amount of at least 90 % by weight or even at least 95 % by weight, as calculated on the total solid content of said layer. Preferably, said MFC is unmodified MFC. Further additives may be e.g. fillers, pigments, wet strength chemicals, retention chemicals, cross-linkers, softeners or plasticizers, adhesion primers, wetting agents, biocides, optical dyes, fluorescent whitening agents, defoaming chemicals, hydrophobizing chemicals such as AKD, ASA, waxes, resins etc.

The second layer (2) comprises a polymer and optionally pigments. Said polymer may be chosen from the group consisting of polyvinylalcohol (PVOH), latex, starch, guar gum and/or wax. Preferred polymer is PVOH. The second layer is preferably formed from a dispersion comprising pigments and the polymer. The said layer may preferably comprise 0 - 30 wt% pigments, or 5 -30 wt%, more preferably 10 - 20 wt% pigments as calculated on the total dry weight of said second layer, the remains being polymer such as PVOH. The pigments are preferably nanopigments. In this context, nanopigment is defined as inorganic pigments having a weigh median diameter (d50) of less than 100 nm. The pigment may e.g. be clay such as kaolin clay, calcium carbonate, talc. Preferably, the pigment is montmorillonite clay, kaolinite Said third layer may comprise at least 75 wt% or at least 80 wt%, or at least 95 wt% of microfibrillated cellulose, whereof at least a part is chemically modified microfibrillated cellulose. In one embodiment, the third layer comprises at least 40 wt%, or at least 50 wt%, at least 60 or even at least 75 wt% of chemically modified cellulose. In one embodiment, the third layer further comprises unmodified microfibrillated cellulose in an amount of at least 10wt% or at least 20 wt%. In one embodiment, the third layer comprises between 40 - 80 wt% chemically modified microfibrillated cellulose and between 30 - 10 wt% unmodified microfibrillated cellulose and between 30 - 10 wt% of an additive. All percentages calculated based on the total solid content of said layer. Said additive may e.g. be any one of a starch, carboxymethyl cellulose, a filler, retention chemicals, flocculation additives, deflocculating additives, dry strength additives, softeners or mixture thereof.

The chemically modified microfibrillated cellulose used in the invention is dialdehyde microfibrillated cellulose (DA-MFC). The microfibrillated dialdehyde cellulose should in this context mean a dialdehyde cellulose treated in such way that it is microfibrillated. The microfibrillated dialdehyde cellulose may be produced by treating dialdehyde cellulose for example by a homogenizer or in any other way such that fibrillation occurs to produce microfibrillated dialdehyde cellulose. Preferably, the microfibrillated dialdehyde cellulose has an oxidation degree between 20-75%, 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 Flydroxylamine Flydrochloride 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.

Image available on "Original document" VNaOH= the amount of sodium hydroxide needed to reach pH 4 (I) CNaOH= 0,1 mol/I msample= dry weight of the analysed DAC sample (g) Mw= 160 g/mol, which is the molecular weight of the dialdehyde cellulose unit There are several method for preparing a film of MFC, including wire forming and cast forming technologies. In wire forming, a suspension, comprising microfibrillated cellulose, is dewatered on a porous surface to form a fibrous web. A suitable porous surface is e.g. a wire in a paper machine. The fibrous web can then be dried in a drying section in a paper machine to form the MFC film.

In cast forming, the suspension, comprising MFC, is applied on a supporting medium with a non-porous surface. The non-porous surface may e.g. be a plastic or metal belt on which the suspension is evenly distributed through a slot or sprayed and the MFC film is formed during drying. The MFC film is then peeled off from the supporting medium in order to form a stand-alone film.

The multilayer film of the invention can be manufactured by use of cast forming, wire forming and/or spraying. In one embodiment, the first layer is formed by cast forming a suspension comprising microfibrillated cellulose and possible additives at a consistency of between 3 - 30 %, preferably between 5 - 20 %, onto a non-porous surface to form a web. Said web is thereafter at least partly dried to form a first film layer having a first and a second side. A dispersion comprising a polymer and optionally pigments is casted or sprayed onto the first side of said first film to form a second film having a first and a second side. Thereafter, the third layer is formed by casting or spraying a suspension comprising chemically modified microfibrillated cellulose onto the first side of said second film, which is finally dried to form a multilayer film.

The multilayer film formed by the method described has preferably a basis weight of less than 50 g/m2, such as between 10 - 50 g/m2, more preferably of 20 - 40 g/m2, or 20 - 30 g/m<2>and a thickness preferably of below 70 pm or below 50 pm, preferably in the range of 20 - 40 pm. According to one embodiment of the invention, the density of the film may be in the range of from 750 kg/m<3>to 1550 kg/m<3>. According to one embodiment the density is higher than 750 kg/m<3>, according to an alternative the density is higher than 950 kg/m<3>, and according to yet an alternative embodiment the density is higher than 1050 kg/m<3>. The film may thus be a so called dense film.

The film, comprising said three layers can be applied on a substrate, such as a paper or board substrate to form a barrier. A final paper- or board product comprising the film according to the present invention may comprise several layers. In one embodiment, the product has the following structure: board or paper/third layer/second layer/first layer, i.e. the layers are the following: a layer of a conventional board or paper material, a layer comprising DA-MFC, a layer comprising polymers and pigments and a layer comprising MFC. In one embodiment, the film may be used as a layer on a paperboard to provide a barrier layer on a liquid packaging board (instead of the al layer).

Figure 2 shows another embodiment of the invention, wherein the reference numbers (1), (2) and (3) correspond to the reference numbers described in connection with Figure 1. This embodiment differs from the one in figure 1 in that the multilayer film (20) further comprises a forth layer (4) comprising a polymer and optionally pigments and a fifth layer (5) comprising microfibirllated cellulose. The forth layer (4) may be identical to the second layer (2) and said fifth layer (5) may be identical to the first layer (1). In the manufacturing process, the forth layer can be formed by spraying or casting a dispersion comprising a polymer and pigments onto the formed third layer (3) with subsequent drying to form a fourth layer. Thereafter a suspension comprising microfibrillated fibers may be casted or sprayed onto the forth layer, which is subsequently dried to form the multilayer film.

The multilayer film disclosed in this embodiment may be used as a selfstanding film, e.g. in packaging of food, but may also be applied on a paper or board substrate.

Further layers, e.g. polymer layer such as polyethylene (PE), may be applied in-between the paper- or board substrate and said film comprising said three or five layers, as well as on the outer, opposite surface of the film.

Thus, in one preferred embodiment, the product has the following structure: Board or paper substrate/PE/third layer/second layer/first layer/PE, or PE/board or paper substrate/PE/third layer/second layer/first layer/PE. In another preferred embodiment, the product has the following structure: Boardor paper substrate/PE/fifth layer/forth layer/third layer/second layer/first layer/PE, or PE/board- or paper substrate/PE/fifth layer/forth layer/third layer/second layer/first layer/PE. In the manufacturing of such a product, the paperboard may first be provided with a PE layer on both a first side and on a second side, preferably by extrusion coating. Thereafter, the film according to the first embodiment (including the first, second and third layer) or the second embodiment (including the first, second, third, forth and fifth layer) may be applied, e.g. by lamination, onto the first side of the paperboard which preferably is to form the inner side of a package made by the paperboard.

Thereafter, the paperboard may be provided with an additional PE layer ontop of the laminated film. This structure is particularly suitable for use in liquid packaging. In liquid packaging board, the film of the invention may thus replace an aluminum foil layer normally applied as liquid barrier on the inner side of the packaging board.

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

Claims

1. An oxygen barrier film comprising - a first layer (1) comprising at least 80 % by weight of unmodified microfibrillated cellulose as calculated on the total solid content of said layer, characterized in that the film further comprises; - a second layer (2) comprising polymers chosen from the group consisting of polyvinyl alcohol (PVOH), latex, CMC, starch, guar gum and/or waxes, - and a third layer (3) comprising at least 40 % by weight of dialdehyde microfibrillated cellulose (DA-MFC) as calculated on the total solid content of said layer, wherein said second layer (2) is arranged on said first layer (1) and said third layer (3) is arranged on said second layer (2)

2. An oxygen barrier film according to claim 1, wherein the second layer (2) further comprises pigments.

3. An oxygen barrier film according to anyone of claims 1 or 2, wherein the film has a basis weight of less than 50 g/m<2>and an Oxygen Transmission Rate (OTR) value of below 10 ml/m<2>/per 24h, preferably below 5 ml/m<2>/per 24h, at 23 °C, at 50% RH.

4. An oxygen barrier film according to any one of claims 1 - 3, wherein said film exhibits an OTR value of below 20 ml/m<2>per 24h, at 23 °C at 80 % RH.

5. An oxygen barrier film according to any one of claims 1 -4, wherein said film exhibits an OTR value of below 100 ml/m<2>per 20h, at 38 °C at 90 % RH.

6. An oxygen barrier film according to any one of claims 1 - 5, wherein said third layer further comprises at least 10 % by weight of unmodified microfibrillated cellulose.

7. An oxygen barrier film according to any one of claims 1 - 6, wherein the film further comprise: - a fourth layer (4) comprising a polymer and optionally pigments, and - a fifth layer (5) comprising at least 80 wt% microfibrillated cellulose.

8. An oxygen barrier film according to claim 7, wherein said polymer in the fourth layer is chosen from the group consisting of polyvinyl alcohol (PVOH), latex, CMC, starch, guar gum and/or waxes.

9. An oxygen barrier film according to any one of claims 1 - 8, wherein the second layer (2) comprises pigments and said pigments are nanopigments.

10. An oxygen barrier film according to claim 9, wherein the nanopigments are platy.

11.A method of manufacturing a film according to any one claims 1 - 10, which method is characterized by the steps of: a) forming a first layer comprising at least 80 % by weight of unmodified microfibrillated cellulose as calculated on the total solid content of said layer, b) forming a second layer comprising a polymer chosen from the group consisting of polyvinyl alcohol (PVOH), latex, CMC, starch, guar gum and/or waxes and optionally pigments on one side of said first layer, c) forming a third layer comprising at least 40 % by weight of dialdehyde microfibrillated cellulose (DA-MFC), as calculated on the total solid content of said layer, on said second layer

12. A method according to claim 11, wherein the method further comprising the steps of d) forming a fourth layer comprising a polymer and optionally pigments on the opposite side of said first layer e) forming a fifth layer comprising MFC on said fourth layer.

13. A method according to any one of claimsl 1 and 12, , wherein the layers are formed by cast forming and/or by spraying.