Manufacturing method for cellulose nanofiber gas barrier film

Abstract

To provide a production method of a gas barrier film capable of easily producing a gas barrier film with great thermal stability.SOLUTION: There is provided a production method of a cellulose nanofiber gas barrier film including following steps (a)-(c): (a) a step for dispersing a cellulose nanofiber in a disperse medium for preparing a suspension; (b) a step for removing the disperse medium from the suspension acquired in the step (a) and collecting a sheet-shaped residue; and (c) a step for performing hot press to the sheet-shaped residue obtained in the step (b) for acquiring a gas barrier film. In SEM observation after the suspension is subjected to freezing and drying, when viewing the suspension in the SEI image of one thousand times, a three-dimensional hollow network structure formed of fibers or fiber bundles having a diameter of 100 nm or smaller, can be observed in 50 area% or greater on the film observed.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a gas barrier film made of cellulose nanofibers. [Background technology]

[0002] In recent years, cellulose-based materials have been attracting attention as new gas barrier materials. Cellulose is found in the cell walls of plants and is the most abundant polysaccharide on Earth. In addition to being biodegradable, cellulose also has excellent physical properties such as strength, elastic modulus, dimensional stability, heat resistance, and crystallinity, making it expected to be applied to functional materials. An example of a cellulose-based material is cellulose nanofiber (hereinafter also referred to as "CNF"). Various technologies have been proposed for gas barrier materials using cellulose nanofiber.

[0003] On the other hand, membranes made from cellulose nanofibers are known to have problems such as a decrease in membrane strength due to moisture absorption and swelling of the cellulose under high humidity conditions.

[0004] In response to this problem, Patent Document 1 discloses an aqueous dispersion for forming a gas barrier film, which is capable of producing a gas barrier film containing cellulose nanofibers, and a method for producing a gas barrier film. More specifically, the aqueous dispersion for forming a gas barrier film of Patent Document 1 contains cellulose nanofibers, an inorganic layered compound, a water-soluble polymer, a crosslinking agent, and an auxiliary, and satisfies specific requirements (a) to (d). Furthermore, the method for producing a gas barrier film of Patent Document 1 involves coating a substrate with this aqueous dispersion for forming a gas barrier film, followed by heating and drying.

[0005] Patent Document 2 also discloses modified cellulose nanofibers that have high gas barrier properties even in high-humidity environments, and a method for producing a gas-barrier molded article. More specifically, the modified cellulose nanofibers in Patent Document 2 have an average fiber diameter of 200 nm or less, and the cellulose structural units have anionic groups, and the anionic groups have basic amino acids as counter ions. The method for producing a gas-barrier molded article in Patent Document 2 includes a step of adhering a gas-barrier material comprising pre-modified cellulose nanofibers to a substrate to obtain a gas-barrier molded article.

[0006] Patent Document 3 discloses cellulose nanofibers that enable the provision of high-performance sheet-like materials, a method for producing the same, and sheet-like materials obtained from the cellulose nanofibers. More specifically, the cellulose nanofibers in Patent Document 3 are bamboo-derived cellulose nanofibers characterized by a cellulose purity of 90% or more, a fiber diameter of 10 to 20 nm, and a crystallinity of 70% or more. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2020-143247 [Patent Document 2] Japanese Patent Publication No. 2020-132703 [Patent Document 3] International Publication No. 2019 / 138588 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the gas barrier film of Patent Document 1 requires, in addition to cellulose nanofibers, for example, an inorganic layered compound, a water-soluble polymer, a crosslinking agent, an auxiliary agent, etc., and also requires a substrate during production. Therefore, the production process of Patent Document 1 is complicated, and there are problems with the production costs.

[0009] Furthermore, Patent Document 2 uses modified cellulose nanofibers, which makes the production process complicated and also causes problems such as the low heat resistance of the gas barrier molded article produced.

[0010] Therefore, the present invention has been made to solve the above problems, and an object of the present invention is to provide a method for producing a gas barrier film that can easily produce a gas barrier film with high thermal stability. [Means for solving the problem]

[0011] The present inventors have found that the above problems can be solved by the following means.

[0012] <Aspect 1> The method includes the following steps (a) to (c): (a) dispersing cellulose nanofibers in a dispersion medium to prepare a suspension; (b) removing the dispersion medium from the suspension obtained in step (a) to recover a sheet-like residue; and (c) hot-pressing the sheet-like residue obtained in step (b) to obtain a gas barrier film; and In the suspension, when the suspension is freeze-dried and then observed under an SEM at a magnification of 10,000, a three-dimensional hollow mesh structure of fibers or fiber bundles having a diameter of 100 nm or less can be observed in 50% or more of the area of ​​the film observed. A method for manufacturing a gas barrier film made of cellulose nanofiber. <Aspect 2> The method according to aspect 1, wherein the cellulose nanofibers are derived from bamboo and have a cellulose purity of 80% or more, a fiber diameter of 10 to 20 nm, and a crystallinity of 70% or more. <Aspect 3> 3. The method of claim 1, wherein the dispersion medium is water, an alcohol, or a mixture thereof. <Aspect 4> the dispersion medium is water, and The Brookfield viscosity of the suspension is 0.24 s at 25°C and a shear rate of 0.24 s -1 is 6000 mPa·s or more, The method of manufacturing according to embodiment 3. <Aspect 5> Aspect 5. The production method according to any one of Aspects 1 to 4, wherein in step (b), the dispersion medium is removed by suction filtration or freeze-drying, and the residue is recovered. [Effects of the Invention]

[0013] According to the present invention, a gas barrier film made of cellulose nanofibers with high thermal stability can be easily produced. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a photograph of the gas barrier film 1 produced in Example 1. [Figure 2] FIG. 2 is a photograph of the gas barrier film 2 produced in Example 2. [Figure 3] FIG. 3 is a photograph of the gas barrier film 3 produced in Comparative Example 1. [Figure 4a] FIG. 4a is a 10,000-magnification image of the SEI observed by SEM after freeze-drying the suspension prepared in Example 1. [Figure 4b] FIG. 4b is a 10,000-magnification image of the SEI observed by SEM after freeze-drying the suspension prepared in Example 1. [Figure 4c] FIG. 4c is a 10,000-magnification image of the SEI observed by SEM after freeze-drying the suspension prepared in Example 1. [Figure 4d] FIG. 4d is a 10,000-magnification image of the SEI observed by SEM after freeze-drying the suspension prepared in Example 1. [Figure 4e] FIG. 4e is a 10,000-magnification image of the SEI observed by SEM after freeze-drying the suspension prepared in Example 1. [Figure 4f] FIG. 4f is a 10,000-magnification image of the SEI observed by SEM after freeze-drying the suspension prepared in Example 1. [Figure 4g] FIG. 4g is a 10,000-magnification image of the SEI observed by SEM after freeze-drying the suspension prepared in Example 1. [Figure 4h] FIG. 4h is a 10,000-magnification image of the SEI observed by SEM after freeze-drying the suspension prepared in Example 1. [Figure 5] FIG. 5 is a 10,000-magnification image of the SEI observed by SEM after freeze-drying the suspension prepared in Example 2. [Figure 6a] FIG. 6a is a 10,000-magnification SEI image of the suspension prepared in Comparative Example 1 after freeze-drying, observed by SEM. [Figure 6b] FIG. 6b is a 10,000-magnification image of the SEI observed by SEM after freeze-drying the suspension prepared in Comparative Example 1. [Figure 6c] FIG. 6c is a 10,000-magnification image of the SEI observed by SEM after freeze-drying the suspension prepared in Comparative Example 1. [Figure 7] FIG. 7 is a graph showing the Brookfield viscosities of the suspensions prepared in Examples 1 and 2 and Comparative Example 1. [Figure 8] FIG. 8 is a graph showing the thermal stability of the gas barrier film 1. DETAILED DESCRIPTION OF THE INVENTION

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the preferred embodiments. However, the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the present invention.

[0016] <<Method for manufacturing a cellulose nanofiber gas barrier film>> The method for producing a gas barrier film made of cellulose nanofibers of the present invention (hereinafter also simply referred to as the "production method of the present invention") includes: The method includes the following steps (a) to (c): (a) dispersing cellulose nanofibers in a dispersion medium to prepare a suspension; (b) removing the dispersion medium from the suspension obtained in step (a) to recover a sheet-like residue; and (c) hot-pressing the sheet-like residue obtained in step (b) to obtain a gas barrier film; and In the suspension, when the suspension is freeze-dried and then observed under an SEM at a magnification of 10,000, a three-dimensional hollow mesh structure of fibers or fiber bundles having a diameter of 100 nm or less can be observed in 50% or more of the area of ​​the film observed. Manufacturing method for cellulose nanofiber gas barrier film is.

[0017] In the present invention, the "cellulose nanofiber gas barrier film" may be composed essentially of cellulose nanofibers alone, and therefore may be substantially free of polymer binder components that bind the cellulose nanofibers together. Specifically, for example, the "cellulose nanofiber gas barrier film" of the present invention may contain 10% by mass or less, 7% by mass or less, 5% by mass or less, 3% by mass or less, or 1% by mass or less of a polymer binder component such as an ethylene-vinyl alcohol copolymer, and may in particular be substantially free of a polymer binder component. Thus, the "cellulose nanofiber gas barrier film" of the present invention has a low polymer binder component content, and in particular is substantially free of a polymer binder component, thereby exhibiting high heat resistance.

[0018] The manufacturing method of the present invention allows for the easy production of a gas barrier film made of cellulose nanofibers. Furthermore, the gas barrier film produced by the present invention (i.e., the "gas barrier film of the present invention") is a self-supporting film that can maintain its shape without the use of a substrate or the like. The gas barrier film of the present invention has high gas barrier properties and thermal stability.

[0019] Through intensive research by the present inventors, it has been found that the state of the suspension obtained in step (a) particularly affects the production of a gas barrier film. More specifically, it has been found that the desired gas barrier film can be obtained when the suspension is freeze-dried and then observed with a scanning electron microscope (SEM) to obtain a secondary electron image (SEI image) at 10,000 magnification, and a three-dimensional hollow network structure of fibers or fiber bundles with a diameter of 100 nm or less is observed over 50% or more of the area of ​​the film (for example, the cases shown in Figures 4a to 4h).

[0020] In other words, when this suspension is freeze-dried and then observed under SEM in an SEI image at 10,000 magnification, if fibers or fiber bundles with a diameter of more than 100 nm are observed over 50% or more of the area of ​​the film observed, or if fibers or fiber bundles with a diameter of 100 nm or less are observed over 50% or more of the area of ​​the film observed but no three-dimensional hollow mesh structure is observed (for example, the cases shown in Figures 6a to 6c), then it is highly likely that the desired gas barrier film will not be obtained even if the steps of the present invention are followed.

[0021] In the present invention, the term "three-dimensional hollow mesh structure" refers to a mesh structure containing air, such as cotton or cotton candy. Specifically, this is a structure such as that shown in Figures 4a to 4h.

[0022] Furthermore, the reason for freeze-drying the suspension for SEM observation is that it tends to retain the original structure of the cellulose nanofibers contained in the suspension, allowing for reliable observation results. Therefore, the "fiber bundles" in the observation results (which are clearly thicker than the other fibers) naturally include some fibers that have aggregated together due to freeze-drying, but the majority of them are thought to represent the original structure of the cellulose nanofibers that were present in the suspension.

[0023] In the present invention, when such "fiber bundles" are present in the suspension, the diameter of the fiber bundles may be 100 nm or less in 50% or more of the area of ​​the observed film in the above-mentioned SEM observation, and more specifically, may be less than 100 nm, 95 nm or less, 90 nm or less, 85 nm or less, 80 nm or less, 75 nm or less, 70 nm or less, 65 nm or less, 60 nm or less, 55 nm or less, 50 nm or less, 45 nm or less, or 40 nm or less, or may be 20 nm or more, 30 nm or more, or 40 nm or more.

[0024] Furthermore, in the above SEM observation, the fiber diameter may be 100 nm or less over 50% or more of the area of ​​the observed film, and more specifically, may be less than 100 nm, 95 nm or less, 90 nm or less, 85 nm or less, 80 nm or less, 75 nm or less, 70 nm or less, 65 nm or less, 60 nm or less, 55 nm or less, 50 nm or less, 45 nm or less, 40 nm or less, 35 nm or less, 30 nm or less, 25 nm or less, 20 nm or less, or 15 nm or less, or may be 10 nm or more, or 20 nm or more.

[0025] Furthermore, the SEI image obtained by SEM observation may be only one image as long as the object is photographed at a good angle, but preferably two or more, three or more, four or more, five or more, six or more, seven or more, eight or more, nine or more, or ten or more images may be used for comprehensive evaluation.

[0026] The manufacturing method of the present invention will be described in detail below.

[0027] <Process (a)> In the present invention, step (a) involves dispersing cellulose nanofibers in a dispersion medium to prepare a suspension.

[0028] The amount of cellulose nanofibers dispersed in the dispersion medium is not particularly limited as long as it does not impair the effects of the present invention. For example, the content of cellulose nanofibers relative to the total amount of the dispersion medium and cellulose nanofibers (i.e., the amount of the obtained suspension) may be, for example, 0.05% by mass or more, 0.1% by mass or more, 0.2% by mass or more, 0.3% by mass or more, 0.4% by mass or more, 0.5% by mass or more, 0.6% by mass or more, 0.7% by mass or more, 0.8% by mass or more, 0.9% by mass or more, or 1.0% by mass or more, and may be 5.0% by mass or less, 4.0% by mass or less, 3.0% by mass or less, 2.0% by mass or less, or 1.0% by mass or less. The amount of cellulose nanofibers dispersed in the dispersion medium may be adjusted appropriately depending on the Brookfield viscosity range of the suspension (described below) and the desired size of the gas barrier film. For example, from the perspective of easily maintaining fluidity, the content of cellulose nanofibers relative to the total amount of the dispersion medium and cellulose nanofibers may be 3.0% by mass or less.

[0029] The means for dispersing cellulose nanofibers in a dispersion medium is not particularly limited, and may be, for example, stirring with a mixer, dispersion by ultrasonic waves, etc. Here, the type of mixer or ultrasonic device is not particularly limited, and may be appropriately selected so as to obtain the desired treatment effect.

[0030] (Cellulose nanofiber) In the present invention, the cellulose nanofibers are not particularly limited as long as they can produce the desired suspension. For example, the cellulose nanofibers may be cellulose nanofibers derived from plants, such as those derived from wood pulp or bamboo. Of these, the cellulose nanofibers are preferably cellulose nanofibers derived from bamboo, having a cellulose purity of 80% or more, a fiber diameter of 10 to 20 nm, and a crystallinity of 70% or more. More specifically, the cellulose purity may be, for example, 80% or more, 85% or more, 90% or more, or 94% or more.

[0031] More specifically, the cellulose nanofibers used in the present invention may have a cellulose purity of 90% or more, 91% or more, 92% or more, 93% or more, or 94% or more, or may have a cellulose purity of 100% or less, 99% or less, 98% or less, 97% or less, 96% or less, 95% or less, or 94% or less. In the present invention, cellulose purity refers to the total content of α-cellulose and β-cellulose contained in the cellulose nanofibers.

[0032] Furthermore, the cellulose nanofibers used in the present invention may contain γ-cellulose, hemicellulose, and / or lignin. For example, when lignin is contained, the content thereof may be 0.1% by mass or more, 0.2% by mass or more, 0.3% by mass or more, 0.4% by mass or more, 0.5% by mass or more, 0.6% by mass or more, 0.7% by mass or more, 0.8% by mass or more, 0.9% by mass or more, or 1.0% by mass or more, relative to the total mass of the cellulose nanofibers, or 10.0% by mass or less, 9.0% by mass or less, 8.0% by mass or less, 7.0% by mass or less, 6.0% by mass or less, 5.0% by mass or less, 4.0% by mass or less, 3.0% by mass or less, or 2.0% by mass or less.

[0033] The cellulose nanofibers used in the present invention may have a fiber diameter of 10 nm or more, 11 nm or more, 12 nm or more, 13 nm or more, 14 nm or more, or 15 nm or more, or may have a fiber diameter of 20 nm or less, 19 nm or less, 18 nm or less, 17 nm or less, 16 nm or less, or 15 nm or less. The fiber diameter of the cellulose nanofibers may be calculated, for example, by SEM observation, by randomly selecting fibers at 100 locations from an SEI image at 100,000 magnification, and averaging the diameters.

[0034] Cellulose nanofibers have a high aspect ratio (for example, 100 or more), and their fiber length may be, for example, 5 μm or more, although it is difficult to measure a specific numerical value in reality.

[0035] The cellulose nanofibers used in the present invention may have a crystallinity of 70% or more, 71% or more, 72% or more, 73% or more, 74% or more, 75% or more, 76% or more, 77% or more, 78% or more, 79% or more, 80% or more, 81% or more, or 82% or more, and may have a crystallinity of 95% or less, 90% or less, or 85% or less. The crystallinity can be measured by the method described in the Examples.

[0036] In the present invention, commercially available cellulose nanofibers may be used as the cellulose nanofibers, or those produced, for example, according to the following steps (1) to (6): (1) A step of alkaline treating bamboo with a 0.01 to 0.50 M aqueous sodium hydroxide solution; (2) A process of mechanically treating the alkaline-treated bamboo to break it down into bamboo fibers; (3) delignifying the bamboo fiber obtained in step (2); (4) mechanically untwining the delignified bamboo fibers; (5) removing hemicellulose from the untwisted bamboo fibers; and (6) A process for removing metal components from the bamboo fiber after hemicellulose removal.

[0037] As for details of the above steps (1) to (6), the entirety of Patent Document 3 can be referred to, and therefore a detailed explanation will be omitted here.

[0038] (dispersion medium) In the present invention, the dispersion medium may be water, an alcohol (e.g., methanol, ethanol, or propanol), or a mixture thereof. From the viewpoint of liquid stability, the dispersion medium is preferably water.

[0039] (suspension) The viscosity of the suspension obtained in step (a) is not particularly limited, and may have a certain level of viscosity or higher.

[0040] More specifically, the Brookfield viscosity of this suspension is, for example, 25°C at a shear rate of 0.24 s -1 In this case, the viscosity may be 6000 mPa·s or more, 7000 mPa·s or more, 8000 mPa·s or more, 9000 mPa·s or more, 10000 mPa·s or more, 11000 mPa·s or more, 12000 mPa·s or more, 13000 mPa·s or more, 14000 mPa·s or more, or 15000 mPa·s or more, and may be 25000 mPa·s or less, 20000 mPa·s or less, or 18000 mPa·s or less.

[0041] The Brookfield viscosity of this suspension is, for example, 25°C at a shear rate of 0.75 s -1 The viscosity may be 2000 mPa·s or more, 3000 mPa·s or more, 4000 mPa·s or more, 5000 mPa·s or more, 6000 mPa·s or more, 7000 mPa·s or more, 8000 mPa·s or more, 9000 mPa·s or more, or 10000 mPa·s or more, and may be 25000 mPa·s or less, 20000 mPa·s or less, or 18000 mPa·s or less.

[0042] The Brookfield viscosity of this suspension is, for example, 25°C at a shear rate of 1.25 s -1In this case, the viscosity may be 1500 mPa·s or more, 2000 mPa·s or more, 2500 mPa·s or more, 3000 mPa·s or more, 3500 mPa·s or more, 4000 mPa·s or more, 4500 mPa·s or more, 5000 mPa·s or more, 5500 mPa·s or more, 6000 mPa·s or more, 6500 mPa·s or more, 7000 mPa·s or more, 7500 mPa·s or more, 8000 mPa·s or more, or 8500 mPa·s or more, and may be 25000 mPa·s or less, 20000 mPa·s or less, or 18000 mPa·s or less.

[0043] In the present invention, the Brookfield viscosity can be measured using, for example, a Brookfield (registered trademark) viscometer. For more specific measurement methods, see the methods described in the Examples.

[0044] <Step (b)> In the present invention, in step (b), the dispersion medium is removed from the suspension obtained in the above-mentioned step (a) to recover a sheet-like residue.

[0045] Here, the dispersion medium can be removed by, for example, suction filtration or freeze-drying, and is preferably removed by suction filtration.

[0046] When the dispersion medium is removed using suction filtration in step (b), the type of filter is not particularly limited and may be appropriately selected so as to achieve the desired treatment effect. For example, an aspirator such as an MDA-015A (manufactured by ULVAC KIKO, Inc.) may be used. The pressure during suction filtration is also not particularly limited and may be appropriately adjusted so that the dispersion medium is slowly sucked into the filter. Slowly sucking the dispersion medium into the filter is preferred because it makes it easier for the cellulose nanofiber fibers to gather together and form a uniform sheet-like residue.

[0047] In the case of suction filtration, the dispersion in step (a) and the removal of the dispersion medium in step (b) can be carried out using a single device, such as a stirring type Ultraholder (manufactured by ADVANTEC), which is convenient in terms of operation.

[0048] In addition, when the dispersion medium is removed by freeze-drying in step (b), the suspension can be placed in a flat container such as a petri dish, frozen, and then freeze-dried using a vacuum freeze-drying apparatus. The type of vacuum freeze-drying apparatus is not particularly limited, and it may be appropriately selected so as to obtain the desired treatment effect.

[0049] Here, the obtained sheet-like residue may be dried, or the next step (c) may be carried out without drying. From the viewpoint of easily forming a cleaner film, it is preferable to dry the sheet-like residue to a certain extent without completely drying it, and then carry out the next step (c).

[0050] <Process (c)> In the present invention, in step (c), the sheet-like residue obtained in step (b) is hot-pressed to obtain a gas barrier film.

[0051] The hot press device is not particularly limited and may be appropriately set so as to obtain a desired processing effect. For example, a small hot press machine (AH-2003 manufactured by AS ONE Corporation) may be used.

[0052] The hot pressing temperature is not particularly limited and may be, for example, 50°C or higher, 60°C or higher, 70°C or higher, 80°C or higher, 90°C or higher, 100°C or higher, or 120°C or higher, and may be 150°C or lower, 120°C or lower, or 100°C or lower.

[0053] <Gas barrier film> The gas barrier film of the present invention obtained through the above steps is made of cellulose nanofibers and does not require a polymer binder, etc., and therefore has the advantage of being less susceptible to shrinkage, i.e., less dimensional deformation. Therefore, the gas barrier film of the present invention is a self-supporting film and can be used alone, or, of course, can be used in combination with other functional layers, laminates, etc.

[0054] The gas barrier film of the present invention has high gas barrier properties, and for example, the permeabilities and permeability coefficients of various gases obtained by a method in accordance with JIS K7126-1 (differential pressure method) are as follows. For more detailed measurement methods, please refer to the methods described in the Examples.

[0055] The gas barrier film of the present invention has an oxygen permeability of 1×10 -15 mol / (m 2 ·s·Pa) or less, and the oxygen permeability coefficient is 5×10 -20 mol m / (m 2 ·s·Pa).

[0056] The gas barrier film of the present invention has a nitrogen permeability of 5×10 -16 mol / (m 2 ·s·Pa) or less, and the nitrogen permeability coefficient is 1×10 -20 mol m / (m 2 ·s·Pa).

[0057] The gas barrier film of the present invention has gas barrier properties against oxygen, nitrogen, hydrogen, carbon dioxide, etc., but interestingly, it allows water vapor to pass through. The gas barrier film of the present invention has a water vapor permeability of, for example, 50 g / m 2 ·Over 24 hours, 100g / m 2 ·Over 24 hours, 150g / m 2 ·24h or more, 200g / m 2 ·24h or more, 250g / m 2 ·Over 24 hours, 300g / m 2 ·Over 24 hours, 350g / m 2 ·Over 24 hours, 400g / m 2 ·Over 24 hours, 450g / m2 24 hours or more, or 500g / m 2 24 hours or more. Without being limited by theory, it is believed that this is because cellulose has hydroxyl groups (-OH groups), and these hydroxyl groups have a high affinity for water, allowing water vapor to pass through.

[0058] The gas barrier film of the present invention also has high thermal stability. For example, the temperature at which the gas barrier film of the present invention loses 1% by mass of its own weight in air, as measured by differential thermogravimetry (TG-DTA), is 250°C or higher, 260°C or higher, 270°C or higher, 280°C or higher, 290°C or higher, or 300°C or higher. [Example]

[0059] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the preferred embodiments. However, the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the present invention.

[0060] Example 1: Production of Gas Barrier Film 1 (Preparation of cellulose nanofibers) Cellulose nanofibers were prepared according to the following procedure: (1) The inner and outer skins of the bamboo were physically removed using a blade. (2) The bamboo pieces obtained in (1) above were treated with a 0.1 mol / L aqueous solution of sodium hydroxide at 120°C for 3 hours to obtain primary bamboo fibers. (3) The primary bamboo fibers obtained in (2) above were treated with a peracetic acid solution at 80°C for 6 hours. After that, they were immersed in a 1.18 mol / L potassium hydroxide solution for 12 hours and a 0.01 mol / L hydrochloric acid solution for 24 hours, and then mechanically treated with water to obtain cellulose nanofibers.

[0061] In the above (3), the peracetic acid solution can be prepared with a molar ratio of acetic acid to hydrogen peroxide (acetic acid:hydrogen peroxide) in the range of 1:0.1 to 1:1. The mechanical treatment may be carried out using a mixer, an emulsifying disperser, or the like.

[0062] The obtained cellulose nanofibers were analyzed as follows.

[0063] (Cellulose purity) The total content of α-cellulose and β-cellulose in the cellulose nanofibers was determined as the cellulose purity based on the method described in paragraphs 0062 to 0074 of Patent Document 3. As a result, the cellulose purity of the cellulose nanofibers obtained in Example 1 was 94% or higher.

[0064] (fiber diameter) The fiber diameter of the cellulose nanofibers was calculated by randomly selecting 100 fibers from a 100,000x SEI image using SEM observation and averaging the results. As a result, the average fiber diameter of the cellulose nanofibers obtained in Example 1 was approximately 17 nm.

[0065] (crystallinity) The crystallinity of the obtained cellulose nanofibers was evaluated using an X-ray diffractometer (XRD (Rigaku Denki Co., Ltd., RINT-Ultima III)). The detailed measurement conditions were the same as those in Table 13 of Patent Document 3. The crystallinity of the cellulose nanofibers was evaluated at 2θ=15 o The intensity (IA) of the 10-1 diffraction line of cellulose from the baseline subtracted at 2θ=10° to 80° and the intensity (IB) of the 10-1 diffraction line of cellulose from the baseline subtracted at 2θ=10° to 20° were calculated using the following formula: Crystallinity=(IA / IB)×100

[0066] As a result, the crystallinity of the cellulose nanofibers obtained in Example 1 was found to be approximately 82%.

[0067] (Preparation of suspension and collection of sheet residue) The cellulose nanofibers obtained above were dispersed in 15 mL of water so that the content was 1.0 mass %. The water was then removed to produce a circular sheet-like residue, which was then collected.

[0068] The dispersion and water removal were carried out using a stirring type ultra holder (UHP-6K, manufactured by ADVANTEC).

[0069] A portion of the prepared suspension was taken out for SEM observation and Brookfield viscosity measurement, which will be described later.

[0070] (Preparation of Gas Barrier Film 1) The sheet-like residue prepared above was heat-pressed at 20 MPa and 120°C using a small heat press (AH-2003, manufactured by AS ONE Corporation) to produce a circular gas barrier film 1 (diameter 60 mm).

[0071] A photograph of the obtained gas barrier film 1 is shown in Figure 1. As shown in Figure 1, the gas barrier film 1 was found to be a uniform film in a clean state.

[0072] Example 2: Production of Gas Barrier Film 2 A gas barrier film 2 was produced in the same manner as in Example 1, except that commercially available cellulose nanofibers (manufactured by Sugino Machine Ltd.; BiNFi-s standard fiber length type) were used.

[0073] A photograph of the obtained gas barrier film 2 is shown in Figure 2. As shown in Figure 2, the gas barrier film 2 was found to be a uniform film in a clean state.

[0074] Comparative Example 1: Production of Gas Barrier Film 3 An attempt was made to produce a gas barrier film 3 in the same manner as in Example 1, except that commercially available cellulose nanofibers (manufactured by Sugino Machine Ltd.; BiNFi-s extra long fiber type) were used.

[0075] However, as shown in FIG. 3, the resulting gas barrier film 3 was not uniform, and cracks and the like were observed on the surface and periphery.

[0076] Measurement and Evaluation <SEM observation of the state of the suspension after freeze-drying> The suspension prepared in Example 1 was freeze-dried and then subjected to SEM observation (using a field emission scanning electron microscope (FE-SEM)). Eight SEI images taken from different directions at 10,000 magnifications are shown in Figures 4a to 4h, respectively.

[0077] Furthermore, an SEI image was taken at 10,000 magnification for the suspension prepared in Example 2 in the same manner as above, and is shown in FIG.

[0078] Furthermore, for the suspension prepared in Comparative Example 1, three SEI images were taken at 10,000 magnifications in the same manner as above, and are shown in FIGS. 6a to 6c.

[0079] As is clear from the above-mentioned Figures 4a to 4h and Figure 5, in Examples 1 and 2, when the suspensions were freeze-dried and then observed under SEM, it was found that when viewed in SEI images at 10,000 magnifications, a three-dimensional hollow mesh structure made of fibers or fiber bundles having a diameter of 100 nm or less was observed in 50% or more of the area of ​​the observed film.

[0080] On the other hand, as is clear from Figures 6a to 6c, in Comparative Example 1, when the suspension was freeze-dried and then observed under SEM, it was found that when the SEI image was viewed at 10,000 magnification, a three-dimensional hollow mesh structure made of fibers or fiber bundles with a diameter of 100 nm or less was not observed in 50% or more of the area of ​​the observed film.

[0081] The results of each observation are summarized in Table 1 below.

[0082] [Table 1]

[0083] Brookfield Viscosity Measurement The Brookfield viscosity of each of the suspensions of Examples 1 and 2 and Comparative Example 1 was measured.

[0084] More specifically, a Brookfield (registered trademark) DV2T viscometer was used. A predetermined amount of each suspension was placed in a chamber, and the viscometer was set up and measured at 25°C. The shear rate was 0.24 s -1 , 0.75s -1 , 1.25s -1 , 2.50s -1 , 5.00s -1 , 10.00s -1 , 20.00s -1 , 20.50s -1 The measurement was performed at 100°C, and the data was recorded 5 minutes after the start of the measurement.

[0085] In Example 1, the viscosity was measured five times at each shear rate and the average value was calculated. Table 2 shows the Brookfield viscosity results for Example 1 (average value of five measurements).

[0086] [Table 2]

[0087] In Example 2, the viscosity was measured three times at each shear rate and the average value was calculated. Table 3 shows the Brookfield viscosity results for Example 2 (average of the three measurements). [Table 3]

[0088] In Comparative Example 1, the viscosity was measured three times at each shear rate and the average value was calculated. Table 4 shows the Brookfield viscosity results (average value of three measurements) of the suspension in Comparative Example 1. [Table 4]

[0089] For comparison, the Brookfield viscosities of the suspensions of the Examples and Comparative Examples obtained above are summarized in FIG.

[0090] As is clear from FIG. 7 and the above Tables 2 to 4, the viscosities of the suspensions of Examples 1 and 2 are all higher than that of Comparative Example 1. For example, at 25° C. and a shear rate of 0.24 s -1 It was found that all of the values ​​were 6000 mPa·s or higher.

[0091] <Gas barrier properties> The gas barrier properties of the gas barrier films 1 and 2 of Examples 1 and 2 were measured by the following method. Gas barrier film 3 of Comparative Example 1 had many cracks and was clearly unusable as a gas barrier film, so its gas barrier properties were not measured.

[0092] Each gas barrier film was evaluated and confirmed by a gas permeability test method, more specifically, a method conforming to JIS K7126-1 (differential pressure method).

[0093] The test gas was dry air (0%RH) for separation analysis, the differential pressure was 1 atm (partial pressure of air [oxygen: 15.2 cmHg, nitrogen: 60.8 cmHg]), and the temperature was 23°C. The permeation area was 15.2 x 10 -4 m 2 (Transmission part diameter φ4.4×10 -2 m).

[0094] The gas permeability coefficient was calculated using the following formula: Gas permeability coefficient = gas permeation amount (volume) x membrane thickness / (pressure difference x permeation area x time)

[0095] The water vapor permeability was measured using a method conforming to JIS K7129 (differential pressure method). The measurement conditions were a water vapor atmosphere, 40±2°C, 90±5% RH, and a differential pressure of 1 atm. The permeation area was 15.2×10 -4 m 2 (Transmission part diameter φ4.4×10 -2 m).

[0096] The gas permeability and permeability coefficient results obtained are shown in Table 5.

[0097] [Table 5]

[0098] 《Thermal stability》 The gas barrier film 1 of Example 1 was measured by differential thermogravimetry (TG-DTA) to determine the temperature at which the film lost 1% by mass of its own weight in air. The results are shown in FIG.

[0099] As is clear from Figure 8, the temperature at which the gas barrier film 1 loses 1% by mass of its own weight in air is at least 260°C or higher. In other words, it was found that the gas barrier film 1 has high heat resistance and can withstand temperatures of 260°C or higher.

Claims

1. A method for producing a cellulose nanofiber gas barrier film, comprising the following steps (a) to (c): (a) dispersing the cellulose nanofibers in a dispersion medium to prepare a suspension; (b) removing the dispersion medium from the suspension obtained in step (a) to recover a sheet-like residue; and (c) hot-pressing the sheet-like residue obtained in step (b) to obtain a gas barrier film; and the dispersion medium is water, alcohol, or a mixture thereof; In the suspension, when the suspension is freeze-dried and observed under an SEM at a magnification of 10,000, a three-dimensional hollow mesh structure of fibers or fiber bundles having a diameter of 100 nm or less can be observed in 50% or more of the area of ​​the film observed, and The Brookfield viscosity of the suspension is 6000 mPa·s or more at 25°C and a shear rate of 0.24 s −1 . Manufacturing method.

2. The manufacturing method according to claim 1, wherein the cellulose nanofibers are cellulose nanofibers derived from bamboo and have a cellulose purity of 80% or more, a fiber diameter of 10 to 20 nm, and a crystallinity of 70% or more.

3. The dispersion medium is water. The method according to claim 1 or 2.

4. The method according to claim 1 or 2, wherein in the step (b), the dispersion medium is removed by suction filtration or freeze-drying, and the residue is recovered.

5. A manufacturing method described in claim 1 or 2, wherein the cellulose nanofiber gas barrier film does not contain a polymer binder component or contains it at a concentration of 10 mass% or less.

6. The method according to claim 1, wherein the cellulose nanofiber gas barrier film has an oxygen permeability of 1×10 −15 mol / (m 2 ·s·Pa) or less.

7. The method according to claim 1, wherein the cellulose nanofiber gas barrier film has a nitrogen permeability of 5×10 −16 mol / (m 2 ·s·Pa) or less.

8. The manufacturing method according to claim 1 or 2, wherein the cellulose nanofiber gas barrier film has a water vapor permeability of 50 g / m 2 ·24 h or more.

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