Oxygen barrier film

Abstract

The present invention relates to a process for improving the strechability of films comprising high amounts of microfibrillated cellulose (MFC) without negatively impacting the oxygen barrier properties. According to the present invention, a film is formed from a suspension comprising microfibrillated cellulose having a broad size distribution.

Claims

1. A method of manufacturing an oxygen barrier film comprising: providing an MFC suspension comprising at least 75 weight % microfibrillated cellulose (MFC), as calculated on the total solid content of said suspension, which MFC has a particle size distribution based on volume exhibiting a D50 value of between 25-40 μm, a D10 value of 5-15 μm and a D90 value of between 90-120 μm, wherein the MFC suspension has been formed by mixing a first suspension comprising MFC having a D50 value of between 26-35 μm and a suspension comprising MFC having a D50 value of between 1-25 μm, wherein the MFC suspension comprises 60-80 weight % of MFC from said first suspension and 40-20 weight % of MFC from said second suspension, forming a web of said MFC suspension, dewatering and/or drying said web to form a film.

2. The method according to claim 1, wherein said MFC suspension is free from long fibers.

3. The method according to claim 1, wherein said first suspension has been provided by a first process comprising mechanical treatment of cellulosic fibers at a first energy input and said second suspension has been provided by a second process comprising mechanical treatment of cellulosic fibers at a second energy input, wherein said first energy input is less than 50% of said second energy input.

4. The method according to claim 1, wherein said first suspension has been provided by a first process and said second suspension has been provided by a second process, wherein the first and second processes comprise enzymatic treatment of cellulosic fibers and wherein the enzymatic treatment in the first process to provide the first suspension is carried out with lower enzymatic activity and/or at a shorter time than the enzymatic treatment in the second process to provide the second suspension.

5. The method according to claim 1, wherein said MFC suspension is formed by providing a fiber suspension comprising cellulose fibers, dividing said fiber suspension into a first part and a second part, fibrillating the first part of said fiber suspension in a first number of mechanical fibrillation steps to provide the first suspension comrising MFC, fibrillating the second part of said fiber suspension in a second number of mechanical fibrillation steps to provide the second suspension comprising MFC, wherein said first number of steps comprises at least one more fibrillation step than said second number of steps.

6. The method according to claim 5, wherein the first part of the fiber suspension constitutes between 10-40 weight % of the fiber suspension as calculated on the total solid content of said suspension.

7. The method according to claim 5, wherein the first part of the fiber suspension is fibrillated in at least a first mechanical fibrillation step and thereafter mixed with the second part of the fiber suspension, which mixture is treated in a second mechanical fibrillation step.

8. The method according to claim 5, which method comprises pretreatment of the fiber suspension in an additional pre-treatment mechanical fibrillation step prior to the step of dividing the fiber suspension into a first and a second part.

9. The method according to claim 5, wherein the first part of the fiber suspension constitutes between 20-30 weight % of the fiber suspension as calculated on the total solid content of said suspension.

10. The method according to claim 1, wherein the film formed has a basis weight of less than 40 g/m.sup.2, an Oxygen Transmission Rate (OTR) value of below 10 ml/m.sup.2/per 24 h, measured by ASTM D-3985 at 50% RH, and a Strain at Break of at least 2%.

11. The method according to claim 1, wherein the web is formed by applying the suspension on a substrate, which web is further dried and/or dewatered to form the film.

12. The method according to claim 11, wherein the web is formed by applying the suspension on a non-porous substrate, which web is further dried to form the film.

13. The method according to claim 1, wherein the film formed has a basis weight of less than 35 g/m.sup.2, an Oxygen Transmission Rate (OTR) value of below 2 ml/m.sup.2/per 24 h, measured by ASTM D-3985 at 50% RH, and a Strain at Break of at least 4%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic process flow diagram of an embodiment of the present invention.

(2) FIG. 2 is a schematic process flow diagram of another embodiment of the present invention.

DETAILED DESCRIPTION

(3) 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).

(4) 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.

(5) 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.

(6) 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 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.

(7) 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.

(8) 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.

(9) 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.

(10) 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.

(11) Particle size distribution is defined by determining the D50 (the median), D10 and/or the D90-value.

(12) The median (D50) is defined as the size of the MFC in microns that splits the distribution with half above and half below this value.

(13) The D90 value is defined as the size in microns that splits the distribution so that 90% of the distribution lies below said value.

(14) The D10 value is defined as the size in microns that splits the distribution so that 10% of the distribution lies below said value.

(15) The Particle size distribution including the D50, D10 and D90- values throughout the application are measured by laser diffraction and are thus based on a volume distribution. In this application, these values are measured by use of Mastersizer 3000 (Malvern Instrument Ltd, UK).

(16) Strain at break was determined from stress strain-curves in a universal testing machine (Zwick) with a clamping length of 20 mm, a width of 15 mm and a speed of 2 mm/min.

(17) The term “plasticizers” as used herein is meant additives that increase the plasticity of the film. Plasticizers used in the process of the invention can e.g. be chosen from the group of sugar alcohols such as sorbitol, polyols, such as glycerol, polyethers, such as polyethylene glycol (PEG), cellulose derivatives, such as carboxy methyl cellulose (CMC), or a combination of any of these.

(18) The invention discloses a method of manufacturing an MFC film from an MFC suspension with a broad and optimized size distribution. Preferably, the MFC in the suspension has a particle size distribution based on volume exhibiting a D50 value of between 25-40 μm, preferably of between 25-35 μm, a D10 value of 5-15 μm, preferably of between 10-12 μm and a D90 value of between 90-120 μm, preferably of between 100-110 μm.

(19) Preferably, the MFC suspension to form the web comprises microfibrillated cellulose in an amount of at least 75 weight %, preferably at least 90 weight %, as calculated on the total solid content of said suspension. The MFC suspension may comprise 95 weight % or even 100 weight % of MFC. The remainder may be conventional additives such as e.g. fillers (such as clay), binders, such as PVOH or PVAC, dispersing agents or softeners etc. The consistency of the MFC suspension to be applied onto the substrate is preferably 1-10%, preferably 2-5%.

(20) Said MFC suspension may be formed by mixing of a first and a second microfibrillated cellulose of different particle size distributions. Said first microfibrillated cellulose may have a D50 value of 26-35 μm. It may further have a D10 value of between 10-15 μm and a D90 value of 110-130 μm. Said second microfibrillated cellulose may have a D50 value of 1-25 μm, preferably between 15-25 μm. It may further have a D10 value of between 8-10 μm and a D90 value of 35-80 μm. Aqueous suspensions comprising such MFC are, in accordance with the invention, mixed to form an MFC suspension to be formed to a web.

(21) According to one embodiment, the MFC suspension may be provided by treating a part of a fiber suspension in a first number of mechanical fibrillation steps, and treating a second part of the fiber suspension in a second number of mechanical fibrillation steps, which first number of steps comprise at least one more step than said second number of steps. The first part of the fiber suspension, which has been treated a first number of steps, may form the first suspension and the second part of the fiber suspension, which has been treated a second number of steps, may form the second suspension.

(22) This embodiment is further illustrated in the schematic FIG. 1. In accordance with the embodiment shown in FIG. 1, a fiber suspension (10), comprising e.g. at least 75 weight % cellulose fibers as calculated on the total solid content of said suspension, is divided into a first (1) and a second (2) part. The first part (1) is treated in at least one mechanical fibrillation step (3). In accordance with the embodiment shown FIG. 1, the first part (1) is treated in two mechanical fibrillation steps (3) and (4), but it is apparent to the skilled person that the first part may be treated any number of steps, depending on the mechanical energy input in each step. Said first part is thereafter combined or mixed with the second part of the fiber suspension, which mixture is treated in at least one additional mechanical fibrillation step (5) to form an MFC suspension (20).

(23) FIG. 2 shows a slight variation of the embodiment shown in FIG. 1, including the same reference numbers but with the difference that it comprises an additional mechanical fibrillation step (6), wherein the fiber suspension (10) is pretreated before it is divided into a first and a second part.

(24) The schematic illustrations of the embodiments shown in FIGS. 1 and 2. show the separate steps as separate units, it should however be understood that the different steps can be carried out using one single treatment device.

(25) The MFC suspension may in an alternative embodiment be provided by a continuous fibrillation process in which a fiber suspension is treated in a fibrillation step, and wherein a part of the thus formed MFC is recirculated back to be fibrillated at least a second time.

(26) The mechanical mfibrillation of cellulosic fibers used in the embodiments described above may be carried by use of a refiner, defibrator, beater, friction grinder, high shear fibrillator, disperger, homogenizator (such as a micro fluidizer) and/or other mechanical treatment devices known in the art. Such mechanical treatment is usually carried out at a consistency of between 2-40 wt %, or preferably 15-40 wt %. The MFC is preferably native MFC. Enzymatic treatment of the fibers may further be performed as a pre-treatment before the mechanical treatment or simultaneously with the mechanical treatment. The enzyme used in the treatment can be any wood degrading enzymes which decompose cellulosic fibres, such as cellulose, xylanase and or mannanase.

(27) The MFC film may be formed by use of casting technologies by applying the MFC suspension onto a non-porous web or by use of a paper machine or similar wet laid techniques. After applied onto a substrate, the web is dewatered and/or dried using conventional techniques.

(28) The MFC film formed by the method described has preferably a basis weight of 10-40 g/m2, more preferably of 20-40 g/m2, or 20-30 g/m.sup.2. It may further have a thickness of below 50 μm or below 40 μm, preferably in the range of 20-40 μm. According to one embodiment of the invention, the density of the film may be in the range of from 750 kg/m.sup.3 to 1550 kg/m.sup.3. According to one embodiment the density is higher than 750 kg/m.sup.3, according to an alternative the density is higher than 950 kg/m.sup.3, and according to yet an alternative embodiment the density is higher than 1050 kg/m.sup.3. The film may thus be a so called dense film. The film may further have an Oxygen Transmission Rate (OTR) value of below 10 ml/m.sup.2/per 24 h at 50% RH, or below 5 ml/m.sup.2/per 24 h at 50% RH and a strain at break value of at least 3,5%, preferably at least 4%

(29) The film as described above is as such useful for packaging foods or liquids.

(30) The film may alternatively be used as an MFC film layer in a multilayer laminate. Thus, the film may be applied onto a fibrous base, such as a paper, paperboard or cardboard made of chemical or (chemi-) mechanical pulp. Preferably the fibrous base is paperboard of a weight of 130 to 300 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

(31) A first MFC aqueous suspension with a consistency of 3% comprising 100 wt % MFC by total solids was produced with enzymatic and mechanical treatment. Said suspension had measured PSD values: D50 of 34 μm, D10 of 11 μm and D90 of 124μm.

(32) A second MFC aq. suspension with a consistency of 3% comprising 100 wt % by total solids was produced with higher level of enzymatic and mechanical treatment . Said second suspension had measured PSD values: D50 of 22 μm, D10 of 9.6 μm and a D90 of 49 μm.

(33) The first and the second MFC suspension were mixed at a ratio of 3:1 to provide a third MFC aq. suspension. Said third suspension had measured PSD values: D50 of 32 μm, D10 of 11.6 μm and a D90 of 104 μm. This third MFC aq. suspension was mixed using a magnetic stirrer for one hour under vacuum.

(34) TABLE-US-00001 MFC aq. suspension PSD D10 PSD D50 PSD D90 1 10.7 34.4 124 2 9.64 21.8 49 3 11.6 32.2 104

(35) After mixing the suspension was coated on a heated metal surface using a wire-wound metering rod. The rod was drawn by hand. An 1 mm frame was used as a distance on top of the metal surface to provide a film with thickness of 30-35 μm. The heated metal surface was holding 95° C. at the start of the drying and 80° C. when the suspension had dried to a film.

(36) Films were produced from all three MFC aq. suspensions. All films were tested for mechanical strength seen as stress strain-curves in a universal testing machine (Zwick) with a clamping length of 20 mm, a width of 15 mm and a speed of 2 mm/min. Special attention was taken to the clamps to avoid slippage between the film and the clamps. A mean value from 5 measurements was reported.

(37) The barrier level seen as oxygen barrier was measured according to ASTM D-3985 at a relative humidity of 50% at 23° C. The best of 2 samples was reported. The samples were masked to 5 cm.sup.2 before measurement.

(38) TABLE-US-00002 Tensile MFC stress, Strain at Thickness, OTR, 23° C., aq. suspension MPa break, % μm 50% RH 1 11695 2.6 31 1.4 2 6830 1.5 36 1.3-2.2 (several measurements) 3 7870 3.4 30 2.2

(39) 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.