MICROFIBRILLATED FILM

Abstract

The invention concerns a method of producing an MFC film comprising reinforcement fibers, which film show excellent oxygen barrier properties and is easy to handle. The method of manufacturing the film comprises the steps of: providing a suspension comprising a first microfibrillated cellulose (MFC) in an amount of at least 50 weight %, reinforcement fibers in an amount of at least 5 weight %, all percentages calculated on the total solid content of said suspension, and a formation aid, mixing said suspension to form a mixture, forming a fibrous web from the mixture, and dewatering and/or drying said fibrous web to form a film having a basis weight of less than 40 g/m.sup.2, a specific formation number of below 0.45 g.sup.0.5/m.sup.2, and an Oxygen Transmission Rate (OTR) value of below 100 ml/m.sup.2/per 24 hours, preferably of below 50 ml/m.sup.2 per 24 hours at 50% relative humidity. The invention further discloses a film and use of the film in food or liquid packaging applications.

Claims

1. A method of manufacturing a fibrous, oxygen barrier film comprising the steps of: providing a suspension comprising: i. a first microfibrillated cellulose (MFC) in an amount of at least 50 weight %, ii. reinforcement fibers, having a weighted fiber length of >0.8 mm, in an amount of at least 5 weight %, iii. a formation aid, wherein all percentages calculated on the total solid content of said suspension, mixing said suspension to form a mixture, forming a fibrous web from said mixture, dewatering and/or drying said fibrous web to form a film having a basis weight of less than 40 g/m.sup.2, a specific formation number of below 0.45 g.sup.0.5/m, and an Oxygen Transmission Rate (OTR) value of below 100 ml/m.sup.2/per 24 h at 50% RH determined at 50% relative humidity in accordance with ASTM D 3985-05.

2. A method according to claim 1, wherein said first MFC exhibits an SR value of at least 85.

3. A method according to claim 1, wherein the MFC is made from softwood fibers.

4. A method according to claim 1, wherein the reinforcement fibers exhibit an SR value of below 50.

5. A method according to claim 1, wherein the reinforcement fibers are hardwood kraft fibers.

6. A method according to claim 1, wherein the formation aid is added to the reinforcement fibers before these are mixed with the first MFC.

7. A method according to claim 1, wherein the formation aid is added to the first MFC at the formation thereof.

8. A method according to claim 1, wherein the reinforcement fibers have been mechanically treated before being added to the suspension.

9. A method according to claim 1, wherein the reinforcement fibers have been chemically treated before being added to the suspension.

10. A method according to claim 1, wherein the reinforcement fibers are never-dried fibers.

11. A method according to claim 1, wherein the formation aid is chosen from the group consisting of anionic polyelectrolytes, a second finer MFC having an SR value higher than that of the first MFC, modified starch, gum-like natural polymers or their synthetic equivalents, polyethylene oxides, metaphosphates and unmodified or modified PVA.

12. A method according to claim 11, wherein the formation aid is APAM and wherein the APAM is present in the suspension in an amount giving rise to a content of said APAM in the web in the range of 0.1 to 5 kg/metric ton of the web.

13. A method according to claim 11, wherein the formation aid is a second, finer MFC, and wherein the finer MFC is present in the suspension in an amount giving rise to a content of said second, finer in the web in the range of 20-100 kg/metric ton of the web.

14. A method according to claim 13, wherein the second finer MFC has a SR value and/or a viscosity higher than said first MFC.

15. A method according to claim 13, wherein the first MFC has a viscosity of below 4000 cP and said second MFC has a viscosity of above 4000 cP.

16. A method according to claim 13, wherein the fibers of said second finer MFC has a weighted average length smaller than said first MFC.

17. A method according to claim 1, wherein the method further comprises the steps of forming the web by applying the suspension mixture onto a porous wire, dewatering the web, drying the web and calendaring the web to form the film.

18. A method according to claim 1, wherein the method further comprises the step of applying a polymer layer onto the formed film, preferably a polyethylene layer.

19. A fibrous-based oxygen barrier film, which comprises at least a first layer comprising: a first microfibrillated cellulose (MFC) in an amount of at least 50 weight %, reinforcement fibers having a length of >0.8 mm, in an amount of at least 5 weight %, a formation aid, said film exhibiting a basis weight of less than 40 g/m.sup.2, a formation number of below 0.45 g.sup.0.5/m and an oxygen transmission rate (OTR) of below 100 ml/m.sup.2/per 24 hours at 50% RH determined at 50% relative humidity in accordance with ASTM D 3985-05.

20. A fibrous-based oxygen barrier film according to claim 19, further comprising a second layer comprising a polyolefin, preferably polyethylene.

21. (canceled)

22. A method according to claim 1, wherein said first MFC exhibits an SR value of at least 90.

23. A method according to claim 1, wherein the MFC is made from pine fibers.

24. A method according to claim 1, wherein the reinforcement fibers exhibit an SR value of below 40.

25. A method according to claim 11, wherein the formation aid is APAM and wherein the APAM is present in the suspension in an amount giving rise to a content of said APAM in the web in the range of 0.1-1 kg/metric ton of the web.

26. A method according to claim 11, wherein the formation aid is a second, finer MFC, and wherein the finer MFC is present in the suspension in an amount giving rise to a content of said second, finer in the web in the range of 30-80 kg/metric ton of the web.

Description

DETAILED DESCRIPTION

[0042] 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, Vol 53, No. 3.), is the main product that is obtained when making MFC e.g. by using an extended refining process or pressure-drop disintegration process. Depending on the source and the manufacturing process, the length of the fibrils can vary from around 1 to more than 10 micrometers. A coarse MFC grade might contain a substantial fraction of fibrillated fibers, i.e. protruding fibrils from the tracheid (cellulose fiber), and with a certain amount of fibrils liberated from the tracheid (cellulose fiber).

[0043] There are different acronyms for MFC such as cellulose microfibrils, fibrillated cellulose, nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibers, cellulose nanofibrils, cellulose microfibers, cellulose fibrils, microfibrillar cellulose, microfibril aggregrates and cellulose microfibril aggregates. MFC can also be characterized by various physical or physical-chemical properties such as large surface area or its ability to form a gel-like material at low solids (1-5 wt %) when dispersed in water.

[0044] 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.sup.2/g, such as from 1 to 200 m.sup.2/g or more preferably 50-200 m.sup.2/g when determined for a freeze-dried material with the BET method.

[0045] Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment step is usually required in order to make MFC manufacturing both energy efficient and sustainable. The cellulose fibers of the pulp to be supplied may thus be pre-treated enzymatically or chemically, for example to reduce the quantity of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, wherein the cellulose molecules contain functional groups other (or more) than found in the original cellulose. Such groups include, among others, carboxymethyl (CMC), aldehyde and/or carboxyl groups (cellulose obtained by N-oxyl mediated oxydation, for example TEMPO), or quaternary ammonium (cationic cellulose). After being modified or oxidized in one of the above-described methods, it is easier to disintegrate the fibers into MFC or nanofibrillar size or NFC.

[0046] The nanofibrillar cellulose may contain some hemicelluloses; the amount is dependent on the plant source. Mechanical disintegration of the pre-treated fibers, e.g. hydrolysed, pre-swelled, or oxidized cellulose raw material is carried out with suitable equipment such as a refiner, grinder, homogenizer, colloider, friction grinder, ultrasound sonicator, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer. Depending on the MFC manufacturing method, the product might also contain fines, or nanocrystalline cellulose or e.g. other chemicals present in wood fibers or in papermaking process. The product might also contain various amounts of micron size fiber particles that have not been efficiently fibrillated.

[0047] 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.

[0048] The above described definition of MFC includes, but is not limited to, the new proposed TAPPI standard W13021 on cellulose nanofbril (CMF) defining a cellolose nanofbire 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.

[0049] The oxygen transmission rate (OTR) as used in the patent claims and in the description is measured in accordance with (ASTM D 3985-05), in 24 hours at 23, 50% RH.

[0050] The term formation aid as used herein, also sometimes referred to as dispersant or dispersion agent, is a substance or polymer added to a suspension to separate particles/fibers from each other and to prevent flocculation.

[0051] The Schopper-Riegler value (SR), as used herein, can be obtained by use of the standard method defined in EN ISO 5267-1.

[0052] The specific formation number is measured by use of Ambertec Beta Formation instrument according to standard SCAN-P 92:09. Specific formation value is calculated as formation divided by the square root of the film grammage.

[0053] The viscosity, as used herein, is measured in accordance to the VTT Brookfield standard for CNF (cellulose nanofibers) by use of Brookfield rheometer, 100 rpm rotational speed, spindle vane-73, temperature 20 C., consistency 1.5%.

[0054] To practice the invention the MFC film is preferably formed in a paper or paperboard making machine or according to a wet laid production method, by providing a MFC suspension onto a wire and dewatering the web to form a film.

[0055] The MFC content of the suspension may be above 50 weight %, or above 70 weight % or above 80 weight %, based on the weight of solids of the suspension. Preferably the MFC content is in the range of from 50 to 95 weight-% based on the weight of solids of the suspension. In one embodiment, the microfibrillated cellulose content of the suspension may be in the range of 70 to 95 weight-%, in the range of 70 to 90 weight-%, or in the range of from 75 to 90 weight-%. According to the invention, the suspension further comprises fibers in an amount of at least 5%, or in the range of from 5-25 weight %, 10-25%, or most preferably in the range of 10-15 weight %, as calculated on the total solid content of said suspension. The suspension further comprises a formation aid.

[0056] The suspension may also comprise small amounts of other process or functional additives, such as fillers, pigments, wet strength chemicals, dry strength chemicals, retention chemicals, cross-linkers, softeners or plasticizers, adhesion primers, wetting agents, biocides, optical dyes, fluorescent whitening agents, de-foaming chemicals, hydrophobizing chemicals such as AKD, ASA, waxes, resins etc. Further additives can also be added to the formed web using a size press.

[0057] According to the invention, the suspension comprising the MFC, the reinforcement fibers and the formation aids is mixed before being formed as a web. The mixing may be done in a fibrillator or in a refiner. It has surprisingly been found that the forming the film in such a manner so that the film exhibits a formation number of below 0.45 g0.5/m, preferably below 0.4, or even below 0.3 g0.5/m, gives rise to superior oxygen barrier- and strength properties. The film formed according to the invention may further function as a barrier against other gases, grease, mineral oils and/or aromas.

[0058] The suspension may be applied onto the wire at a consistency of 0.1 to 1.0 wt-% consistency. Subsequent to the wet web being placed onto the wire, it is dewatered to form a film.

[0059] The dewatering on wire may, according to one embodiment be performed by using known techniques with single wire or twin wire system, frictionless dewatering, membrane-assisted dewatering, vacuum- or ultrasound assisted dewatering, etc. After the wire section, the wet web is further dewatered and dried by mechanical pressing including shoe press, hot air, radiation drying, convection drying, etc. The film might also be dried or smoothened by soft or hard nip (or various combinations) calenders etc.

[0060] Alternatively the MFC film could be prepared by casting the above described mixed MFC suspension, at consistency of 5 to 25 wt-%, onto a polymeric substrate to form a coating film, followed by drying and finally separating the film by peeling if off from the substrate.

[0061] The MFC film formed by the method described has preferably a basis weight of 10-40 g/m2, more preferably of 20-30 g/m2, and a thickness of below 50 m or below 40 m, preferably in the range of 20-40 m.

[0062] The film as described above is as such useful for packaging foods or liquids.

[0063] The film may alternatively be used as a MFC film layer in a multilayer laminate. In this embodiment, the film be applied onto a fibrous paper, paperboard or cardboard made of chemical or wood pulp. Preferably the fibrous base is paperboard of a weight of 130 to 250 g/m2, preferably of 200 to 250 g/m2, or paper of a weight of 40 to 130 g/m2. The laminate may further comprise polymer layers, e.g. of polyethylene, or further barrier layers. Such laminates are useful e.g. for is useful e.g. for heat-sealable packages of food or liquids.

Example 1

[0064] The aim of this trial was to clarify the effect of long fibers and improved formation (by addition of formation aids and mixing) on MFC web dewatering and runnability as well as on resulting product properties, especially barrier properties. In addition to MFC, retention system comprising of wet end starch (4 kg/t), galactomannan (1 kg/t), silica (5 kg/t), and wet-strength chemical (5 kg/t) was used. In addition, hydrophobic sizing agent AKD (1.5 kg/t) was applied into the wet end. Test point P11_1 was the reference containing 100% MFC as fiber source.

TABLE-US-00001 TABLE 1 Test points P11_1 P11_2 P11_3 P11_5 P20_5 P20_6 Fiber source, % MFC 100 MFC 85 MFC 70 MFC 85 MFC 85 MFC 100 Birch 15* Birch 30* Pine 15* Birch 15* Wet end starch, kg/t 4 4 4 4 4 4 Silica, kg/t 5 5 5 5 5 5 PAE, kg/t 5 5 5 5 5 5 Galactomannan, kg/t 1 1 1 1 1 1 AKD, kg/t 1.5 1.5 1.5 1.5 1.5 1.5 Other additives, kg/t Fine MFC A-PAM 0.5 kg/t*** 50 kg/t** O-water temp., C. 50 50 50 50 50 50 Machine speed, m/min 15 15 15 15 15 15 Target grammage, 30 30 30 30 30 30 g/m.sup.2 SR 96.5 94.5 92.0 94.0 97.0 97.0 *long fibers added to pulper, mixing together with MFC with fiberizer **Fine MFC added to pulper with long fibers, followed by mixing with fiberizer ***High Mw A-PAM added to pulper, mixing together with MFC with fiberizer

[0065] In test points P11_2 and P11_3, 15 wt-% and 30 wt-%, of hardwood fibers were mixed with MFC in the pulper, respectively, followed by mixing with fiberizer of the fibers and MFC. In test point P11_5 15 wt-% of softwood fibers were mixed with MFC in the pulper followed by mixing with fiberizer of the fibers and MFC. In test point P20_5 15 wt-% of hardwood fibers were mixed with addition of 50 kg/t of fine MFC to the pulper and the MFC, fine MFC and hardwood fibers were further mixed with fiberizer. In test point P20_6 high molecular weight (Mw) A-PAM was added to the pulper, followed by mixing with fiberizer of the high Mw A-PAM and MFC. Table 1 summarizes the test points.

TABLE-US-00002 TABLE 2 Results for the test points P11_1 P11_2 P11_3 P11_5 P20_5 P20_6 Fiber source MFC MFC 85 MFC 70 MFC 85 MFC 85 MFC 100 Birch 15 Birch 30 Pine 15 Birch 15 Other additives, kg/t Fine MFC A-PAM 0.5 kg/t 50 kg/t Grammage, g/m.sup.2 35.7 32.4 32.3 31.9 31.5 30.1 Thickness, m 49 48 48 50 42 40 Density, kg/m.sup.3 733 669 667 640 759 752 Specific formation, 0.45 0.45 0.38 0.43 0.27 0.28 g.sup.0.5/m OTR, cc/(m.sup.2-day)* 30.4 6604 fail** fail** 7 8 *determined at 50% RH, 23 C. **fail is over 10 000 cc/(m.sup.2-day)

[0066] Addition of 15 wt-% of hardwood fibers to MFC film (P11_2) gave improved barrier properties (measured as OTR, cc/m.sup.2*day) compared to addition of 15 wt-% of softwood fibers (P11_5). With the addition of 50 kg/t of fine MFC together with 15 wt-% of hardwood fibers (P20_5) the dispersion of these long fibers in the MFC film was improved, as indicated by the lower specific formation value and higher density of the film. At the same time the oxygen barrier properties of the MFC film (P20_5) were improved compared to test point with 15 wt-% of hardwood fibers without addition of fine MFC (P11_2). Addition of high Mw A-PAM to the pulper, followed by mixing with fiberizer of the high Mw A-PAM and MFC (P20_6) improved specific formation of the MFC film and OTR compared to test point without addition of dispersion aid (P11_1). Table 2 summarizes the test point results.

Example 2

[0067] The MFC films containing 15-50 wt-% of hardwood fibers (P11_1-P11_4) and 15-wt % of softwood fibers (P11_5) were extrusion PE-coated with 25 g/m.sup.2 of LDPE or 25 g/m.sup.2 of HDPE/LDPE co-extrusion.

[0068] The oxygen transmission rate (OTR) of the PE-coated MFC films was measured in 23 C. and 50% relative humidity (RH) conditions. Based on the results with 15 wt-% addition of hardwood fibers (P11_2) to MFC film the OTR of PE-coated film, either LDPE or HDPE/LDPE coated, is approximately on the same level as with PE-coated film having 100% of MFC as fiber source (P11_1). Furthermore, with 30 wt-% addition of hardwood fibers (P11_3) the OTR values are better compared to 15 wt-% addition of softwood fibers (P11_5) to MFC film after LDPE or HDPE/LDPE coating. Results of PE-coated films are summarized in Table 1.

TABLE-US-00003 TABLE 1 Results for the PE-coated test points P11_1 P11_2 P11_3 P11_5 Fiber source MFC MFC 85 MFC 70 MFC 85 Birch 15 Birch 30 Pine 15 PE-coating, 25 g/m.sup.2 LDPE HDPE/ LDPE HDPE/ LDPE HDPE/ LDPE HDPE/ LDPE LDPE LDPE LDPE OTR, cc/(m.sup.2-day)* 1.5 1.5 3.0 2.7 80.1 90.4 fail 362 *determined at 50% RH, 23 C. **fail is over 10 000 cc/(m.sup.2-day)