Oxygen barrier film and laminate and methods of manufacturing the same

Abstract

The invention concerns a fibrous-based oxygen barrier film, which comprises (i) a microfibrillated cellulose (MFC) film, (ii) an oxygen barrier polymer layer, (iii) an optional tie layer, and (iv) an outermost polyolefin layer. The invention even concerns a laminate, in which said multilayer film is combined with a fibrous paper or board base by means of an intermediate polyethylene layer. The oxygen barrier polymerlayer, which preferably is EVOH, and the polyolefin layer are brought by coextrusion onto the MFC film. The film and the laminate according to the invention aim at an improved oxygen barrier in high humidity conditions and have use in oxygen-sensitive food packaging.

Claims

1. A fibrous-based oxygen barrier film, which comprises: i. a premade microfibrillated cellulose (MFC) film having a weight of 20 to 40 g/m.sup.2, and an oxygen transmission rate (OTR) above 50 ml/m.sup.2, at 50% relative humidity, ii. a co-extruded oxygen barrier polymer layer in direct contact with the MFC film, wherein the oxygen barrier polymer is selected from a group consisting of ethyl vinyl alcohol copolymer (EVOH), polyamide (PA), polyvinyl alcohol copolymer (PVOH) and polyvinyl chloride (PVC), iii. an optional co-extruded tie layer, and iv. a co-extruded outermost polyolefin layer.

2. The film of claim 1, wherein said oxygen barrier polymer is EVOH.

3. The film of claim 1, wherein said polyolefin is polyethylene (PE).

4. The film of claim 1, wherein there is an extruded polyolefin layer on the reverse side of the MFC film.

5. A laminate comprising comprising: i. a fibrous paper or board base, ii. a polyethylene (PE) layer, iii. a premade microfibrillated cellulose (MFC) film having a weight of 20 to 40 g/m.sup.2, and an oxygen transmission rate (OTR) above 50 ml/m.sup.2, at 50% relative humidity, iv. a co-extruded oxygen barrier polymer layer in direct contact with the MFC film, wherein said oxygen barrier polymer is selected from a group consisting of ethyl vinyl alcohol copolymer (EVOH), polyamide (PA), polyvinyl alcohol copolymer (PVOH) and polyvinyl chloride (PVC), v. an optional co-extruded tie layer, and vi. a co-extruded outermost polyolefin layer.

6. The laminate of claim 5, wherein the reverse side of the fibrous paper or board base is provided with a heat-sealable polyolefin layer.

7. The laminate of claim 5, wherein the fibrous base (i) comprises paperboard of a weight of 130 to 250 g/m.sup.2.

8. The laminate of claim 5, wherein the fibrous base (i) comprises paper of a weight of 40 to 130 g/m.sup.2.

9. A method for manufacturing a fibrous-based oxygen barrier film comprising co-extruding an oxygen barrier polymer layer, an optional tie layer, and a polyolefin layer onto a premade MFC film having a weight of 20 to 40 g/m.sup.2, and an oxygen transmission rate (OTR) above 50 ml/m.sup.2, at 50% relative humidity, wherein the oxygen barrier polymer layer is in direct contact with the MFC film, and wherein the oxygen barrier polymer is selected from a group consisting of ethyl vinyl alcohol copolymer (EVOH), polyamide (PA), polyvinyl alcohol copolymer (PVOH) and polyvinyl chloride (PVC).

10. The method of claim 9, wherein the oxygen barrier polymer is EVOH, and the polyolefin is a polyethylene or polypropylene.

11. The method of claim 9 further comprising dry-laminating the fibrous-based oxygen barrier film to a paper or board base coated with a PE layer.

12. The film of claim 1, wherein the MFC film has an oxygen transmission rate (OTR) in the range of 100 to 8000 ml/m.sup.2 (ASTM D 3985-05), at 50% relative humidity.

13. The laminate of claim 5, wherein the fibrous base (i) comprises paperboard of a weight of 200 to 250 g/m.sup.2.

14. The film of claim 1, wherein the oxygen barrier film has an OTR below 50 ml/m.sup.2, in 24 h at 50% relative humidity (RH).

15. The film of claim 1, wherein the oxygen barrier film has an OTR below 15 ml/m.sup.2, in 24 h at 50% relative humidity (RH).

16. The laminate of claim 5, wherein the laminate has an OTR below 50 ml/m.sup.2, in 24 h at 50% relative humidity (RH).

17. The laminate of claim 5, wherein the laminate has an OTR below 15 ml/m.sup.2, in 24 h at 50% relative humidity (RH).

18. The method of claim 9, wherein the oxygen barrier film has an OTR below 50 ml/m.sup.2, in 24 h at 50% relative humidity (RH).

Description

DETAILED DESCRIPTION

(1) 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).

(2) There are different acronyms for MFC such as cellulose microfibrils, fibrillated cellulose, nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibers, cellulose nanofibrils, cellulose microfibers, cellulose fibrils, microfibrillar cellulose, microfibril aggregrates and cellulose microfibril aggregates. MFC can also be characterized by various physical or physical-chemical properties such as large surface area or its ability to form a gel-like material at low solids (1-5 wt %) when dispersed in water. The cellulose fiber is preferably fibrillated to such an extent that the final specific surface area of the formed MFC is from about 1 to about 200 m.sup.2/g, or more preferably 50-200 m.sup.2/g when determined for a freeze-dried material with the BET method.

(3) 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.

(4) 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.

(5) 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.

(6) The above described definition of MFC includes, but is not limited to, the new proposed TAPPI standard W13021 on cellulose nanofibril (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.

(7) To practice the invention the MFC film (nanopaper) is preferably made at a paper machine adapted for film production from a MFC suspension of 0.1 to 1.0 wt-% consistency (for MFC, excluding eventual solid additives such as fillers) and 100 nm to 200 μm, preferably 100 nm to 10 μm, fibre length of the microfibrils (as indicated by a Schopper-Riegler drainability number of at least 70 SR, preferably at least 90 SR). The machine speed may be 100 m/min, preferably 300 m/min or even more. With such speeds the web is quickly dewatered and dried, and a large-scale production can be achieved economically. The weight of the MFC film would be about 20 to 40 g/m.sup.2. The film has an OTR in 24 h at 23° C. and 50% RH in the range of 100 to 8000 ml/m.sup.2, preferably 500 to 2000 ml/m.sup.2. For an oxygen barrier these transmission rates are not remarkable as such, but the cost of producing such “second rate” MFC film is low.

(8) Alternatively the MFC film could be prepared by casting an aqueous MFC suspension of a 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.

(9) The next step is coextrusion of an oxygen barrier polymer layer and a polyolefin layer onto the dried MFC film. Preferably the oxygen barrier polymer is EVOH and the polyolefin is heat-sealable LDPE. LDPE has a role in supporting the EVOH layer in the coextrusion. In order to better adhere LDPE to EVOH a tie layer of an ethylene copolymer may be included as a middle layer in the coextrusion. Suitable extruded polymer layer weights are 3 to 30 g/m.sup.2 of EVOH, 0.5 to 15 g/m.sup.2 of the tie polymer, and 10 to 50 g/m.sup.2 of LDPE. A heat-sealable layer of 10 to 40 g/m.sup.2 of LDPE may be extruded onto the reverse side of the MFC film layer.

(10) In the coextrusion process the EVOH layer adapts itself to the tiniest irregularities of the MFC film surface, achieving a snug fit which is believed to produce a unitary crystallised layer forming an oxygen barrier greatly superior to that achieved by the MFC film and EVOH layers separately, and well above the mere sum of the two.

(11) The multilayer film as described above is as such useful for packaging oxygen-sensitive foods, which are stored and marketed in high humidity conditions.

(12) The alternative embodiment of the invention, namely a laminate incorporating a MFC film layer, may be based on 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/m.sup.2, preferably of 200 to 250 g/m.sup.2, or paper of a weight of 40 to 130 g/m.sup.2. At a first step the fibrous base is extrusion coated with a 10 to 20 g/m.sup.2 layer of polyethylene, preferably LDPE. The second step then is dry or extrusion lamination of the MFC film based multilayer film as described above to the PE coated fibrous base, which includes adhering an uncoated or LDPE coated reverse side of the MFC film to the PE coat of the fibrous base. A heat sealable LDPE layer may be added by extrusion onto the reverse side of the fibrous base. The laminate is useful e.g. for heat-sealable carton or box packages of dry foods such as coffee, tea, chocolate, biscuits etc., which must be shielded against oxygen and high humidity.

EXAMPLE

(13) A first multilayer laminate was prepared with a structure (i) an LDPE layer of 15 g/m.sup.2, (ii) a paper layer of 70 g/m.sup.2 (“Lumiflex 70”), (iii) an LDPE layer of 12 g/m.sup.2, (iv) a MFC film layer of 30 g/m.sup.2, (v) an EVOH layer of 6 g/m.sup.2, (vi) an ethylene copolymer tie layer of 5 g/m.sup.2, and (vii) an LDPE layer of 45 g/m.sup.2.

(14) A second multilayer laminate was prepared, which corresponded to the first multilayer laminate as described above, except that for (ii) a paperboard layer of 200 g/m.sup.2 replaced the “Lumiflex 70” paper layer.

(15) Results from measurements of oxygen and water vapour transmission rates at different temperatures and relative humidities are presented in Table 1. As the temperature of 23° C. and RH of 50% represent normal room circumstances, the temperature of 38° C. and RH of 85% are typical for tropical high humidity circumstances that are challenging to packaging industry and that the invention is specifically aimed at dealing with.

(16) The measured low OTRs thus stem from paper and board based laminates according to the invention. However, results not markedly inferior to these would be expected from films lacking the fibrous paper or board base, as the surplus PE and base layers comprised in the laminates do not substantially contribute to the oxygen barrier, which is predominantly dependent on the MFC/EVOH interface structure.

(17) TABLE-US-00001 TABLE 1 O2tr 23° O2tr 23° C./50% RH C./50% RH 30 min 1 h measurement measurement SuperBarr Paper <5 <5 2.4 2.6 (PE/Lumiflex70/PE/MFC/EVOH/tie/PE) SuperBarr Board 749 6.8 ~2 ~2 (PE/board/PE/MFC/EVOH/tie/PE) O2tr 38° C./ 85% RH SuperBarr Paper 23 21 (PE/Lumiflex70/PE/MFC/EVOH/tie/PE) SuperBarr Board ~20 ~15 (PE/board/PE/MFC/EVOH/tie/PE) WVTR 23° C./ 50% RH SuperBarr Paper 2.4 2.3 (PE/Lumiflex70/PE/MFC/EVOH/tie/PE) SuperBarr Board 2.5 2.3 (PE/board/PE/MFC/EVOH/tie/PE) WVTR 38° C./ 85% RH SuperBarr Paper 9.2 9.6 (PE/Lumiflex70/PE/MFC/EVOH/tie/PE) SuperBarr Board 7.7 8.1 (PE/board/PE/MFC/EVOH/tie/PE)