JP2013010891A - Cellulose nanofiber composite and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cellulose nanofiber composite which improves mechanical properties.SOLUTION: The cellulose nanofiber composite 10 contains cellulose nanofibers 11 in which at least one part of a hydroxy group positioned on the microfibril surface of cellulose is oxidized to a carboxyl group, and plate-like nanoparticles 12.

Description

  The present invention relates to a cellulose nanofiber composite and a method for producing the same.

  In recent years, research on high-strength materials using cellulose nanofibers has been promoted, and those obtained by forming a dispersion of cellulose nanofibers into a sheet or the like by molding and drying are known. In addition, for the purpose of imparting a function such as strength improvement, composites of cellulose nanofibers with other materials have been studied. For example, the following Non-Patent Document 1 shows an example in which a composite of microfibrillated cellulose nanofibers and montmorillonite (clay) is produced.

Sehaqui, H., Liu, A., Zhou, Q., and Berglund, L.A. (2010) .Fast preparation procedure for large, flat cellulose and cellulose / inorganic nanopaper structures.Biomacromolecules, 11, 2195-2198.

  However, in the example described in Non-Patent Document 1, when montmorillonite is compounded with microfibrillated cellulose nanofiber, the strength, elongation, and transparency of the molding material are all deteriorated, and an advantageous effect by the compounding is found at all. It wasn't.

  The present invention has been made in view of the above problems of the prior art, and an object thereof is to provide a cellulose nanofiber composite having improved mechanical properties and a method for producing the same.

  The cellulose nanofiber composite of the present invention comprises cellulose nanofibers in which at least a part of hydroxyl groups located on the surface of cellulose microfibrils are oxidized to carboxyl groups, and plate-like nanoparticles.

  The plate-like nanoparticles are montmorillonite, sodium montmorillonite, calcium montmorillonite, magnesium montmorillonite, nontronite, bedellite, vorconskite, laponite, hectorite, saponite, soconite, magadite, kenyaite, sobokite, subindulite, steven The composition may be at least one selected from the group consisting of sight, vermiculite, halloysite, aluminate oxide, hydrotalcite, illite, rectolite, talosobite, ladykite, kaolinite, and mixtures thereof.

  In the method for producing a cellulose nanofiber composite of the present invention, a cellulose nanofiber dispersion is prepared by dispersing cellulose nanofibers in which at least a part of hydroxyl groups located on the surface of cellulose microfibrils are oxidized to carboxyl groups in a dispersion medium. Mixing the plate-like nanoparticles in a dispersion medium to prepare a plate-like nanoparticle dispersion, and mixing the cellulose nanofiber dispersion and the plate-like nanoparticle dispersion to prepare a mixed dispersion And a step of preparing the cellulose nanofiber composite by drying and solidifying the mixed dispersion.

ADVANTAGE OF THE INVENTION According to this invention, the cellulose nanofiber composite_body | complex which improved the mechanical characteristic by providing the structure disperse | distributed uniformly, without both a cellulose nanofiber and a plate-shaped nanoparticle aggregating is provided.
Moreover, according to this invention, the method of manufacturing said cellulose nanofiber composite_body | complex easily is provided.

The schematic block diagram of the cellulose nanofiber composite_body | complex of embodiment. Action | operation explanatory drawing of a cellulose nanofiber composite_body | complex. Explanatory drawing of the conventional composite_body | complex. The stress strain curve of the TOCN / clay composite film which concerns on an Example. The stress strain curve of the TOCN / clay composite film which concerns on an Example. The graph which showed the measurement result of the oxygen permeability which concerns on an Example. The transmittance | permeability curve of the TOCN / clay composite film which concerns on an Example. The transmittance | permeability curve of the TOCN / clay composite film which concerns on an Example.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

"Method for producing cellulose nanofiber composite"
The cellulose nanofiber composite includes (1) a step of preparing a cellulose nanofiber dispersion, (2) a step of preparing a plate-like nanoparticle dispersion, and (3) a cellulose nanofiber dispersion and a plate-like nanoparticle. It can be produced by a method comprising mixing a particle dispersion to prepare a mixed dispersion, and (4) drying and solidifying the mixed dispersion to produce a cellulose nanofiber composite.

(1) Preparation of Cellulose Nanofiber Aqueous Dispersion This step includes an oxidation step of oxidizing cellulose as a raw material and a dispersion step of defibrating oxidized cellulose to form a dispersion.

[Oxidation process of cellulose]
The cellulose oxidation step is a step of preparing an aqueous dispersion of cellulose having a carboxylate type group in which at least a part of hydroxyl groups located on the surface of cellulose microfibrils is oxidized to a carboxyl group.
If an aqueous cellulose dispersion having the above-mentioned constitution is obtained, the method for oxidizing the cellulose is not particularly limited, but the cellulose oxidizing treatment using TEMPO catalytic oxidation already proposed by the present inventors is used. It is preferable.

  That is, natural cellulose is used as a raw material, an N-oxyl compound such as TEMPO (2,2,6,6-tetramethyl-1-piperidine-N-oxyl) is used as an oxidation catalyst in an aqueous solvent, and an oxidizing agent is allowed to act. It is preferable to prepare a cellulose aqueous dispersion oxidized by a production method including an oxidation treatment step of oxidizing natural cellulose.

By the oxidation treatment, primary hydroxyl groups (about 1.7 groups / nm ² ) exposed on the surface of the microfibrils constituting the natural fiber pulp fibers are oxidized into carboxyl groups.

  In the oxidation treatment step, first, a dispersion in which natural cellulose is dispersed in water is prepared. Natural cellulose is purified cellulose isolated from cellulose biosynthetic systems such as plants, animals, and bacteria-producing gels. Specifically, it is isolated from softwood pulp, hardwood pulp, cotton pulp such as cotton linter and cotton lint, non-wood pulp such as straw pulp and bagasse pulp, bacterial cellulose, cellulose isolated from sea squirt, seaweed A cellulose etc. can be illustrated.

  Moreover, you may give the process which expands surface areas, such as beating, to the natural cellulose isolated and refine | purified. Thereby, reaction efficiency can be improved and productivity can be improved. In addition, it is preferable to use natural cellulose that has been stored in an undried state after isolation and purification. By storing it in an undried state, it is possible to keep the microfibril bundle in an easily swellable state, so that the reaction efficiency is improved and cellulose nanofibers having a small fiber diameter are easily obtained in the dispersion step described later.

  In the oxidation treatment step, water is typically used as a dispersion medium for natural cellulose in the reaction solution. The natural cellulose concentration in the reaction solution is not particularly limited as long as the reagent (oxidant, catalyst, etc.) can be sufficiently dissolved. Usually, the concentration is preferably about 5% or less with respect to the weight of the reaction solution.

An N-oxyl compound is used as a catalyst added to the reaction solution. As the N-oxyl compound, TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) and a TEMPO derivative having various functional groups at the C4 position can be used. Examples of the TEMPO derivative include 4-acetamido TEMPO, 4-carboxy TEMPO, 4-phosphonooxy TEMPO, and the like. In particular, TEMPO and 4-acetamido TEMPO have obtained favorable results in the reaction rate.
A catalytic amount is sufficient for the addition of the N-oxyl compound. Specifically, it may be added in a range of 0.1 to 4 mmol / L with respect to the reaction solution. Preferably, it is the addition amount range of 0.1-2 mmol / L.

  Further, depending on the type of oxidizing agent, a catalyst component in which a bromide or iodide is combined with an N-oxyl compound may be used. For example, ammonium salt (ammonium bromide, ammonium iodide), bromide or alkali metal iodide (bromide such as lithium bromide, potassium bromide, sodium bromide, lithium iodide, potassium iodide, sodium iodide, etc. Iodide), bromide or alkaline earth metal iodides (calcium bromide, magnesium bromide, strontium bromide, calcium iodide, magnesium iodide, strontium iodide, etc.) can be used. These bromides and iodides can be used alone or in combination of two or more.

As the oxidizing agent, hypohalous acid or a salt thereof (hypochlorous acid or a salt thereof, hypobromous acid or a salt thereof, hypoiodous acid or a salt thereof, etc.), a halogenous acid or a salt thereof (chlorous acid or a salt thereof) Salts thereof, bromous acid or salts thereof, iodic acid or salts thereof, perhalogen acids or salts thereof (perchloric acid or salts thereof, periodic acid or salts thereof), halogens (chlorine, bromine, iodine, etc.) ), Halogen oxides (ClO, ClO ₂ , Cl ₂ O ₆ , BrO ₂ , Br ₃ O ₇ etc.), nitrogen oxides (NO, NO ₂ , N ₂ O ₃ etc.), peracids (hydrogen peroxide, hydrogen peroxide, Acetic acid, persulfuric acid, perbenzoic acid, etc.). These oxidizing agents can be used alone or in combination of two or more. Further, it may be used in combination with an oxidase such as laccase. The content of the oxidizing agent is preferably in the range of 1 to 50 mmol / L.

  As hypohalite, in the case of hypochlorous acid, alkali metal salts such as lithium hypochlorite, potassium hypochlorite, sodium hypochlorite, calcium hypochlorite, hypochlorous acid, etc. Examples include magnesium, alkaline earth metal salts such as strontium hypochlorite, and ammonium hypochlorite. Moreover, hypobromite and hypoiodite corresponding to these can also be used.

  Examples of halous acid salts include, in the case of chlorous acid, alkali metal salts such as lithium chlorite, potassium chlorite, and sodium chlorite, calcium chlorite, magnesium chlorite, and strontium chlorite. Examples thereof include alkaline earth metal salts and ammonium chlorite. In addition, bromite and iodate corresponding to these can also be used.

  As perhalogenates, for example, in the case of perchlorates, alkali metal salts such as lithium perchlorate, potassium perchlorate, sodium perchlorate, calcium perchlorate, magnesium perchlorate, strontium perchlorate Examples thereof include alkaline earth metal salts such as ammonium perchlorate. Moreover, perbromate and periodate corresponding to these can also be used.

As a preferable oxidizing agent in the present invention, an alkali metal hypohalite or an alkali metal halite can be mentioned, and an alkali metal hypochlorite or an alkali metal chlorite is more preferably used. .
The catalyst described above may be appropriately selected according to the kind of the oxidizing agent. For example, when an alkali metal hypochlorite is used as the oxidizing agent, an N-oxyl compound, bromide or iodide, It is preferable to use a catalyst component in combination with N, and when an alkali metal chlorite is used as the oxidizing agent, it is preferable to use an N-oxyl compound alone as the catalyst component.

[Dispersion process]
Next, in the dispersion step, the oxidized cellulose obtained in the oxidation treatment step or the oxidized cellulose that has undergone the purification step is dispersed in the medium.
As a medium (dispersion medium) used for dispersion, an aqueous solvent is used. The aqueous solvent in the present embodiment is a solvent that is only water except for components that are inevitably mixed, or a mixed solvent of water and an organic solvent such as an alcohol compatible with less than 20% by weight of water. In this embodiment, water is used as the dispersion medium.

  Through the dispersion step, oxidized cellulose is fibrillated, and a cellulose nanofiber dispersion in which cellulose nanofibers are dispersed in a medium is obtained. In the cellulose nanofiber aqueous dispersion prepared in this dispersion step, cellulose nanofibers in which a part of C6 primary hydroxyl group of cellulose is oxidized to sodium carboxylate (sodium salt of carboxyl group) are contained in an aqueous solvent. It is uniformly dispersed.

  In the case of this embodiment, the concentration of the cellulose nanofiber aqueous dispersion is preferably in the range of 0.05 wt% to 4 wt%. More preferably, it is 0.1 weight% or more and 2 weight% or less.

  Various devices can be used as a dispersion device (defibration device) used in the dispersion step. For example, a household mixer, an ultrasonic homogenizer, a high-pressure homogenizer, a biaxial kneading device, a defibrating device such as a stone mill can be used. In addition to these, a dispersion of cellulose nanofibers can be easily obtained with a defibrating apparatus generally used for household use or industrial production. In addition, if a defibrating apparatus having a strong and beating ability such as various homogenizers and various refiners is used, cellulose nanofibers having a small fiber diameter can be obtained more efficiently.

  In addition, in the oxidation treatment and dispersion treatment in the above process, since the chemical treatment that cleaves the main chain skeleton of cellulose is not performed, the cellulose nanofiber to be produced is the length of the microfibril of natural cellulose used as a raw material. It will have. Accordingly, the cellulose nanofibers to be produced have dimensions of microfibril units reaching a length of several μm while the width is, for example, several nm.

  The cellulose nanofibers contained in the aqueous dispersion obtained by the above steps have a maximum fiber diameter of 1000 nm or less and a number average fiber diameter of 2 nm or more and 150 nm or less, and at least a part of hydroxyl groups located on the microfibril surface of cellulose. Can be identified as cellulose nanofibers oxidized to carboxyl groups.

The maximum fiber diameter and the number average fiber diameter of the cellulose nanofiber can be analyzed by the following method.
First, a 0.05 to 0.1% by weight cellulose nanofiber dispersion is prepared. This dispersion is cast on a lyophilic carbon film-coated grid to obtain a sample for TEM observation. Thereafter, this sample is observed with an electron microscope at a magnification of 5000 times, 10000 times, or 50000 times. At this time, when an axis of arbitrary vertical and horizontal image width is assumed in the obtained image, a sample (concentration etc.) and an observation condition (magnification etc.) such that this axis intersects with 20 or more fibers are used. .
Then, with respect to the observation image satisfying this condition, two random axes are drawn vertically and horizontally per image, and the fiber diameter of the fiber intersecting with the axis is visually read. In this way, the fiber diameter value is read for at least three non-overlapping region images. Thereby, information on the fiber diameter of at least 20 fibers × 2 (axis) × 3 (sheets) = 120 fibers is obtained.
From the fiber diameter data obtained as described above, the maximum fiber diameter (maximum value) and the number average fiber diameter can be calculated.
In the above description, the TEM observation is performed. However, when a fiber having a large fiber diameter is included, the TEM observation may be performed.

  In the present embodiment, when the maximum fiber diameter of the cellulose nanofiber is larger than 1000 nm or the number average fiber diameter is larger than 150 nm, it is difficult to obtain characteristics such as transparency and strength described below. As the cellulose nanofibers, the maximum fiber diameter is 500 nm or less and the number average fiber diameter is 2 nm or more and 100 nm or less, more preferably, the maximum fiber diameter is 30 nm or less and the number average fiber diameter is as follows. It is 2 nm or more and 10 nm or less.

In particular, a cellulose nanofiber having a maximum fiber diameter of 30 nm or less and a number average fiber diameter of 2 nm or more and 10 nm or less, the dispersion is transparent, and a structure such as a film obtained by drying the dispersion Also have excellent transparency.
More specifically, the cellulose nanofibers obtained by the production method of the present invention have a very narrow width of 3 nm to 10 nm (3 to 4 nm if wood cellulose is used, and about 10 nm if cotton cellulose is used), and the length is 500 nm or more. (Normally 1 μm or more) and longer than “cellulose nanowhiskers (length: 500 nm or less)” obtained by conventional acid hydrolysis, high strength is exhibited.

(2) Preparation of plate-like nanoparticle aqueous dispersion In this step, a plate-like nanoparticle aqueous dispersion is prepared by dispersing plate-like nanoparticles in a dispersion medium.

  As the plate-like nanoparticles, natural or synthesized clay materials can be used. Specifically, smectite clay (montmorillonite, sodium montmorillonite, calcium montmorillonite, magnesium montmorillonite, nontronite, bedellite, vorconskite, laponite, hectorite, saponite, sauconite, magadite, kenyaite, sobokite, subindulite, Typical examples include stevensite, vermiculite, halloysite, aluminate oxide, hydrotalcite, or mixtures thereof. Examples of other clay materials include illite, rectolite, tarosovite, ladykite, and kaolinite. A mixture of illite, rectolite, etc. and smectite clay may be used.

  The size of the plate-like nanoparticles varies depending on the clay material used. For example, the plate-like nanoparticles are flat plate-like fine particles having a width of 0.01 μm to 10 μm and a thickness of about 0.5 nm to 10 nm. More specifically, for example, montmorillonite is an aluminosilicate plate-like nanoparticle having a thickness of about 1 nm, and the surface of the nanoparticle is substituted with a metal cation. The plate-like nanoparticles form a multilayer laminated structure having a thickness of about 10 μm.

  As a medium (dispersion medium) used for dispersion, an aqueous solvent is used. The aqueous solvent in the present embodiment is a solvent that is only water except for components inevitably mixed in, or a mixed solvent of water and an organic solvent such as an alcohol compatible with less than 20% by weight of water. In this embodiment, water is used as the dispersion medium. In addition, a modifier, a surfactant, an emulsifier, a stabilizer, a release agent, and the like may be added to the dispersion.

  Various devices can be used as a dispersion device used in the dispersion step. For example, a household mixer, an ultrasonic homogenizer, a high-pressure homogenizer, a biaxial kneader, a mixing / stirring device such as a stone mortar can be used. In the present embodiment, it is preferable that the plate-like nanoparticles forming a laminated structure in the clay material are peeled off and dispersed in the form of individual plate-like nanoparticles, and the dispersion in such a form is used. Select the resulting device.

  The concentration of the aqueous dispersion of plate-like nanoparticles varies depending on the type of the plate-like nanoparticles, but if the concentration is too high, gelation occurs due to the formation of a card house structure, and the concentration of the aqueous dispersion of cellulose nanofibers increases. Since the dispersibility may decrease, the concentration should be such that the viscosity does not increase excessively. For example, when montmorillonite or saponite is used as the clay material, the range is preferably 0.05% by weight or more and less than 3% by weight. More preferably, it is 0.1 weight% or more and 2 weight% or less.

(3) Preparation of mixed dispersion In this step, the cellulose nanofiber aqueous dispersion prepared in (1) and (2) above and the plate-like nanoparticle aqueous dispersion are mixed at a predetermined ratio, and the mixed dispersion is obtained. Get.
For example, a mixed dispersion in which cellulose nanofibers and plate-like nanoparticles are dispersed in water is prepared by stirring while dropping the plate-like nanoparticle aqueous dispersion little by little with respect to the cellulose nanofiber aqueous dispersion.

  A conventionally well-known thing can be used for the stirring apparatus used for preparation of a mixed dispersion liquid. In this embodiment, the cellulose nanofiber aqueous dispersion and the plate-like nanoparticle aqueous dispersion to be mixed are obtained by uniformly dispersing cellulose nanofibers or plate-like nanoparticles in advance, and both are mixed relatively easily. It is possible to integrate. Therefore, when preparing the mixed dispersion, a normal stirring device that rotates a stirrer such as a rod, plate, or propeller is sufficient.

(4) Drying and solidifying step of mixed dispersion In this step, cellulose nanofibers comprising cellulose nanofibers and plate-like nanoparticles are dried and solidified by removing water from the mixed dispersion prepared in (3) above. A complex is obtained. As the drying means used in this step, conventionally known means can be used, and for example, natural drying, vacuum drying, heat drying, suction drying, and the like can be used.

  By removing water in a state where the mixed dispersion is put in a mold having a desired shape, a cellulose nanofiber composite having a desired shape can be easily produced. For example, when producing a sheet-like cellulose nanofiber composite, the mixed dispersion is poured onto the surface of a membrane filter, etc., and the moisture is removed by permeating the filter, so that a sheet-like solid can be easily obtained. Can do. Alternatively, the mixed dispersion can be cast on a supporting substrate such as a glass substrate to form a sheet.

  The cellulose nanofiber composite of this embodiment can be obtained by the above steps (1) to (4).

"Cellulose nanofiber composite"
The cellulose nanofiber composite according to this embodiment is a cellulose nanofiber composite including cellulose nanofibers in which at least a part of hydroxyl groups located on the surface of cellulose microfibrils are oxidized to carboxyl groups, and plate-like nanoparticles. is there.

FIG. 1 is a schematic configuration diagram of a cellulose nanofiber composite 10 of the present embodiment.
As shown in FIG. 1, the cellulose nanofiber composite 10 of the present embodiment has plate-like nanoparticles 12 in a state of being individually separated in the tissue of cellulose nanofibers 11 separated into almost one. It is uniformly dispersed without agglomeration.

  FIG. 2 is an explanatory diagram of the action of the cellulose nanofiber composite. Fig.2 (a) is a figure which shows the cellulose nanofiber used by this embodiment, FIG.2 (b) is explanatory drawing regarding interaction with a cellulose nanofiber and plate-shaped nanoparticle.

  As described above, a carboxyl group exists on the surface of the cellulose nanofiber 11 used in the present embodiment. In such an aqueous dispersion of cellulose nanofibers, carboxyl groups are ionized in water, and negative charges 11 a are distributed on the surface of the cellulose nanofibers 11, so repulsive force Fr acts between the cellulose nanofibers 11. Thereby, aggregation of the cellulose nanofiber 11 is suppressed, and each cellulose nanofiber 11 can be stably dispersed in a separated state.

  On the other hand, as shown in FIG. 2B, the plate-like nanoparticles 12 such as montmorillonite have a positive charge 12a distributed on the surface and are dispersed one by one in the dispersion. When such a cellulose nanofiber aqueous dispersion and a plate-like nanoparticle dispersion are mixed, the negative charge 11a of the cellulose nanofiber 11 and the plus charge 12a of the plate-like nanoparticle 12 are ionically bonded in the region A shown in the figure. . When the mixed dispersion in such a state is dried and solidified, a solid body of a composite of cellulose nanofibers 11 and plate-like nanoparticles 12 can be obtained while maintaining the above dispersion state.

The cellulose nanofiber composite 10 of the present embodiment has excellent transparency and mechanical properties by having a structure in which both the cellulose nanofibers 11 and the plate-like nanoparticles 12 are uniformly dispersed without aggregation as described above. In addition, excellent oxygen barrier properties are also obtained.
That is, since neither the cellulose nanofiber 11 nor the plate-like nanoparticle 12 is aggregated, the transparency of the nano-sized structure is not impaired, and as a result, a transparent composite can be formed. Also. The cellulose nanofiber 11 and the plate-like nanoparticle 12 are ion-bonded by charges on the respective surfaces, so that the strength can be greatly improved. Further, since the plate-like nanoparticles 12 are uniformly dispersed without agglomeration, the plate-like nanoparticles 12 effectively block the oxygen diffusion path in the cellulose nanofiber composite 10, and the entire composite Excellent oxygen barrier properties can be obtained.

  The composite described in Non-Patent Document 1 is also a composite made of cellulose nanofibers and montmorillonite, but is significantly different from that of this embodiment. This reason is thought to be due to the difference in the cellulose nanofibers used. Hereinafter, it will be described in detail with reference to FIG.

FIG. 3 (a) is a view showing microfibrillated cellulose, and FIG. 3 (b) is a view showing a composite of microfibrillated cellulose and plate-like nanoparticles.
Conventional microfibrillated cellulose is mechanically defibrated to the order of microfibrils by applying shearing force or impact force to kraft pulp and the like, as shown in FIG. The book B is not separated, but the site B where the microfibrils 110 are finely separated and the site C that remains agglomerated are mixed and exist as a network (cobweb-like) structure having a large void D. Yes.

  When a composite with plate-like nanoparticles (for example, montmorillonite) is formed using such microfibrillated cellulose, the plate-like nanoparticles 12 are formed in the voids D of the network structure as shown in FIG. It is in a state where a multi-layer structure is formed. This is because no carboxyl group is introduced on the surface of the microfibril 110. That is, since the plate-like nanoparticles 12 and the microfibril 110 are not bonded together, the plate-like nanoparticles 12 aggregate in the process of drying and solidifying the dispersion, thereby forming a multilayer laminated structure. In the composite having such a structure, the light is scattered by the aggregated portion of the microfibrillated cellulose and the aggregated plate-like nanoparticles 12, so that the transparency is lowered. Moreover, since the microfibril 110 and the plate-shaped nanoparticle 12 do not couple | bond together and the plate-shaped nanoparticle 12 exists as a foreign material between the microfibrils 110, intensity | strength will fall rather. Furthermore, the oxygen-barrier property is hardly expected in the aggregated plate-like nanoparticles 12.

  As described above in detail, the cellulose nanofiber composite 10 of the present embodiment is completely different from the conventional composite in its structure, and is exceptionally different from the conventional one based on the structural difference. There is an effect.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.

(Production of composite sample)
A cellulose nanofiber composite was produced using the production method of the above embodiment.

[TEMPO catalytic oxidation]
TEMPO (0.1 mmol) and sodium bromide (1 mmol) were dissolved in 1 g of a softwood kraft pulp that had drifted well dispersed in water (deionized water, 100 mL) (1% by weight slurry). . Thereafter, 2M sodium hypochlorite (3.8 mmol / g based on the amount of pulp) was added to start the reaction. Since the pH continued to decrease during the reaction, 0.5 M sodium hydroxide was added dropwise to keep the pH at 10.
The point in time when the pH did not decrease was regarded as the end of the reaction (ca. 60 min), and filtered and washed thoroughly with distilled water to obtain oxidized pulp.
The amount of introduced carboxyl groups determined by the conductivity titration method was 1.2 mmol per 1 g of pulp.

[Distributed processing]
A TEMPO oxidized pulp slurry (20 mL) having a weight to volume ratio of 0.12% was treated with a double cylindrical homogenizer (7500 rpm, 20 mm diameter shaft, Physcotron NS-56, Microtec Nition) for 1 minute, followed by an ultrasonic homogenizer (300 W). 19.5 kHz, 7 mm diameter chip, UT-300, Nissei) for 4 minutes. Thereafter, coarse particles such as undefibrated pulp were removed by centrifugation (12000 g, 20 min) to obtain a TEMPO oxidized cellulose nanofiber (hereinafter, TOCN) dispersion. The concentration of the obtained dispersion was about 0.1% by weight.

[Clay dispersion]
As the clay material, montmorillonite (Kunipia F, Kunimine Industries) and saponite (Smecton SA, Kunimine Industries), which are smectite-type layered clays, were prepared.
Clay powder (0.6 g) was added little by little to stirring water (30 mL) to obtain a 2.0% by weight clay slurry. This clay slurry was treated with a double cylindrical mechanical homogenizer (Physcotron NS-56, MicrotecNition) for 1 minute and with an ultrasonic homogenizer (UT-300, Nissei) for 4 minutes to prepare a clay dispersion.

[TOCN / Clay dispersion]
The 2% clay dispersion was dropped into the 0.1% TOCN dispersion and stirred so that the blending weight ratio of TOCN and clay was 50:50 or 99: 1. Liquid).

[Formation of complex]
The TOCN / clay dispersion (25 mL) was poured into a polystyrene petri dish (50 mm diameter) and dried at 40 ° C. for 3 days to prepare a TOCN / clay composite film (thickness˜7 μm).
In addition, as a sample for evaluating oxygen barrier properties, the above TOCN / clay dispersion was cast-dried on a PET film hydrophilized by plasma irradiation (DSDE-AF, Meiwafosis), and a two-layer film (TOCN / clay composite) was obtained. Film thickness: ˜1 μm).

(Evaluation of composite sample)
Using the samples prepared above (TOCN / clay composite film, two-layer film), mechanical strength, oxygen barrier properties, and transparency were evaluated.

[Mechanical strength]
The mechanical strength was evaluated by carrying out a tensile test on four types of TOCN / clay composite films produced by changing the type of clay (montmorillonite, saponite) and the mixing ratio (99: 1, 50:50). The tensile test was performed under the conditions of a tensile tester (Shimadzu EZ-TEST), a tensile speed: 1.0 mm · min ⁻¹ , a span length: 10 mm, and a specimen width: 2 mm.

  FIG. 4 is a stress strain curve of a TOCN / clay composite film using montmorillonite (MTM) as a clay material. FIG. 5 is a stress strain curve of a TOCN / clay composite film using saponite (SPN) as a clay material. 4 and 5 show, for comparison, a stress strain curve of a film manufactured using a non-composite TOCN simple substance. Table 1 below summarizes the analysis results of the tensile test. In Table 1, “TOCN / MTM” is a composite film composed of TOCN and montmorillonite, “TOCN / SPN” is a composite film composed of TOCN and saponite, and “TOCN” is a film composed only of uncombined TOCN.

The following knowledge was obtained from the results of the tensile test.
By combining 1% of clay with TOCN, the strength can be improved 1.5 times while maintaining the Young's modulus.
When 50% of saponite is combined with TOCN, the strength can be improved by 1.5 times and the Young's modulus can be improved by 2 times.
When 50% of montmorillonite is combined with TOCN, both strength and Young's modulus can be improved by a factor of two. The composite of TOCN and montmorillonite recorded a maximum Young's modulus of 25 GPa and a breaking strength of 441 MPa.

[Oxygen barrier properties]
Oxygen barrier properties were evaluated for four types of bilayer films (PET-TOCN / clay composite film) produced by changing the clay type (montmorillonite, saponite) and the mixing ratio (99: 1, 50:50). This was done by performing rate measurements. The oxygen transmission rate was measured under the conditions of a testing machine (MOCON ML & SL), a test environment: 23 ° C., 0% RH, and a test area: 50 cm ² .

FIG. 6 is a graph showing measurement results of oxygen permeability for each sample.
As shown in the graph, it was found that by combining 50% of clay with TOCN, the oxygen transmission rate can be greatly reduced (1 to 2 digits). When montmorillonite and saponite were compared under conditions where the oxygen permeability was greatly reduced, it was found that montmorillonite was more effective in reducing oxygen permeability.

[transparency]
The evaluation of transparency is performed by carrying out light transmittance measurement on four types of TOCN / clay composite films prepared by changing the type of clay (montmorillonite, saponite) and the mixing ratio (99: 1, 50:50). It was. The light transmittance was measured under the conditions of a tester (Shimadzu UV-1700) and a measurement wavelength range: 350 nm to 850 nm.

  FIG. 7 is a transmittance curve of a TOCN / clay composite film using montmorillonite (MTM) as a clay material. FIG. 8 is a transmittance curve of a TOCN / clay composite film using saponite (SPN) as a clay material. 7 and 8 show the transmittance curves of a film produced using a non-composite TOCN simple substance for comparison.

  As shown in the figure, in the sample in which saponite was combined with TOCN, even when saponite was added up to 50%, the transmittance was not substantially lowered and the transparency was maintained. Even in a sample in which montmorillonite was combined with TOCN, the transmittance was hardly lowered under the condition of the addition amount of 1%, and the transparency was maintained. On the other hand, when the amount of montmorillonite added was 50%, the transparency was lowered.

  DESCRIPTION OF SYMBOLS 10 ... Cellulose nanofiber composite, 11 ... Cellulose nanofiber, 12 ... Plate-shaped nanoparticle, 110 ... Microfibril

Claims (3)

  A cellulose nanofiber composite comprising cellulose nanofibers in which at least a part of hydroxyl groups located on the microfibril surface of cellulose are oxidized to carboxyl groups, and plate-like nanoparticles.   The plate-like nanoparticles are montmorillonite, sodium montmorillonite, calcium montmorillonite, magnesium montmorillonite, nontronite, bedellite, vorconskite, laponite, hectorite, saponite, sauconite, magadite, kenyaite, sobokite, subindulite, steven The at least one selected from the group consisting of sight, vermiculite, halloysite, aluminate oxide, hydrotalcite, illite, rectolite, talosobite, ladykite, kaolinite, and mixtures thereof. Cellulose nanofiber composite. Dispersing cellulose nanofibers in which at least a part of hydroxyl groups located on the microfibril surface of cellulose are oxidized to carboxyl groups in a dispersion medium, and preparing a cellulose nanofiber dispersion;
A step of dispersing plate-like nanoparticles in a dispersion medium to prepare a plate-like nanoparticle dispersion;
Mixing the cellulose nanofiber dispersion and the plate-like nanoparticle dispersion to prepare a mixed dispersion;
Drying and solidifying the mixed dispersion to produce a cellulose nanofiber composite;
A method for producing a cellulose nanofiber composite, comprising:

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