Method for manufacturing strength-enhancing additives

Abstract

To provide a method for producing an easily handled cellulose nanofiber graft polymer with sufficiently evenly introduced graft chains, ensuring both productivity and efficiency.SOLUTION: Provided herein is a method for producing a cellulose nanofiber graft polymer having cellulose nanofibers with a grafted radical-polymerizable monomer by irradiating a dispersion liquid including the cellulose nanofibers and the radical-polymerizable monomer, and a dispersion medium, wherein the radiation is gamma rays; the concentration of the cellulose nanofibers in the dispersion liquid is less than 2 mass% relative to the total of the dispersion medium and the cellulose nanofibers; and the thickness of the dispersion liquid in the direction of gamma ray irradiation is 5 mm or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for producing a cellulose nanofiber graft polymer.

Background Art

[0002] Cellulose nanofibers are known to be useful as resin reinforcing materials and additives for various paints, and various proposals have been made. For example, modified cellulose nanofibers are known in which graft chains are bonded to cellulose nanofibers and a part of the hydroxyl groups (-OH groups) of the cellulose nanofibers are modified.

[0003] Regarding the bonding of graft chains to cellulose nanofibers, polymerization reactions using energy rays, polymerization reactions using polymerization initiators, etc. can be cited. Among these, from the viewpoint of a simple method, the polymerization reaction using a polymerization initiator is considered preferable (Patent Document 1). In this method, specifically, methyl methacrylate (MMA) and nitric acid are added to a cellulose nanofiber aqueous dispersion. However, when an acid or an alkali is used, there is a concern that the molecular chain of cellulose may be cleaved or the crystallinity may be reduced.

[0004] Also, a molded material mixture is known in which a hydrophobic polymer is chemically bonded to at least a part of the -OH groups of cellulose nanofibers, and the base material such as resin and cellulose nanofibers are pulverized or pelletized. The molded material mixture is produced through steps such as immersing or coating a cellulose nanofiber dispersion liquid on a base material sheet, irradiating with an electron beam, and then bringing it into contact with a solution or dispersion liquid containing a hydrophobic monomer to graft-polymerize the hydrophobic monomer onto the cellulose nanofibers (pre-irradiation) (Patent Document 2). This method has the advantage that the base material and cellulose nanofibers can be handled as pulverized materials or pellets. However, the grafted cellulose nanofibers are integrated with the base material sheet and cannot be handled alone.

[0005] Furthermore, as a material possessing bactericidal and antiviral properties, bactericidal and antiviral cellulose nanofibers having free acidic functional groups in at least a portion of them are known. In these bactericidal and antiviral cellulose nanofibers, polymerizable monomers and / or polymers thereof are bonded to the cellulose nanofiber as side chains, and at least the side chains have free acidic functional groups (Patent Document 3). The manufacturing process for these cellulose nanofibers involves, for example, irradiating them with radiation to form radicals, then grafting polymerizable monomers such as vinyl monomers onto these radicals, and finally contacting and fixing them with a substance containing the desired functional group (a substance containing a free acidic functional group). Specifically, glycidyl methacrylate is added to a cellulose nanofiber paste (a mixture of cellulose nanofibers and water), the mixture is prepared to a thickness of about 1 mm, and then irradiated with electron beams from both sides to obtain a cellulose nanofiber paste grafted with glycidyl methacrylate. In this case, the target material is obtained by irradiating a paste about 1 mm thick with an electron beam. However, if the electron beam irradiation device has a capacity of, for example, 30 cm x 30 cm x 1 mm thickness, the processing volume is 90 cm³. 3 Therefore, it is difficult to obtain the target product in large quantities.

[0006] In other words, because electron beams have low material penetration ability, when irradiating a dispersion containing cellulose nanofibers, radical polymerizable monomers, and a dispersion medium with an electron beam, increasing the thickness of the dispersion relative to the direction of electron beam irradiation prevents the electron beam from sufficiently penetrating the dispersion, thus preventing sufficient graft polymerization. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2021-59723 [Patent Document 2] Japanese Patent Publication No. 2018-16896 [Patent Document 3] Japanese Patent Publication No. 2017-14509 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] As described above, various methods have been proposed for producing cellulose nanofiber graft polymers by graft polymerization of radically polymerizable monomers onto cellulose nanofibers, but all of them have had problems in terms of graft polymerizability, productivity, and handling of the resulting graft polymers.

[0009] The present invention aims to solve these problems. Specifically, the objective of the present invention is to provide a method for efficiently and productively producing cellulose nanofiber graft polymers that are easy to handle and have sufficiently uniformly distributed graft chains. [Means for solving the problem]

[0010] As a result of repeated studies to solve the above problems, the inventors have found that by using gamma rays, which have high material penetration ability, as the radiation irradiated onto a dispersion containing cellulose nanofibers, radical polymerizable monomers, and a dispersion medium, and by controlling the concentration of cellulose nanofibers in the dispersion, it is possible to generate radicals that serve as the starting point for graft polymerization in the cellulose nanofibers sufficiently, uniformly and effectively, even when the thickness of the dispersion is increased in the direction of gamma ray irradiation, thereby enabling the productive acquisition of cellulose nanofiber graft polymers.

[0011] The present invention is based on these findings and is summarized in the following [1] to [4].

[0012] [1] A method for producing a cellulose nanofiber graft polymer in which radical polymerizable monomers are grafted onto cellulose nanofibers by irradiating a dispersion containing cellulose nanofibers, a radical polymerizable monomer, and a dispersion medium with radiation, characterized in that the radiation is gamma rays, the concentration of cellulose nanofibers in the dispersion is less than 2% by mass relative to the total of the dispersion medium and the cellulose nanofibers, and the thickness of the dispersion in the direction of the gamma ray irradiation is 5 mm or more.

[0013] [2] The method for producing a cellulose nanofiber graft polymer according to [1], wherein the dose of gamma rays is 0.1 to 10 kGy.

[0014] [3] A method for producing a cellulose nanofiber graft polymer according to [1] or [2], wherein the water content in the dispersion medium is 80% by mass or more.

[0015] [4] A method for producing a cellulose nanofiber graft polymer according to any one of [1] to [3], wherein the radical polymerizable monomer is a (meth)acrylic acid ester compound. [Effects of the Invention]

[0016] According to the present invention, by using gamma rays with high material penetration ability and controlling the concentration of cellulose nanofibers in the dispersion, gamma rays can be sufficiently and uniformly irradiated even to relatively thick dispersions, allowing for the production of large quantities of cellulose nanofiber graft polymers and resulting in excellent production efficiency. Furthermore, gamma ray irradiation allows the use of large containers such as 1m x 1m x 1m, making it suitable for mass production. [Brief explanation of the drawing]

[0017] [Figure 1] This graph shows the relationship between the thickness of a water sample and its dose transmittance when exposed to cobalt-60 gamma rays. [Figure 2]It is a graph showing the relationship between the thickness of the water sample and the dose transmittance in an electron beam. [Figure 3] It is a graph showing the state in which methyl polymethacrylate is grafted onto cellulose. [Figure 4] It is a photograph of the MMA-grafted CNF dispersion obtained in Example 2. [Figure 5] It is a photograph of the MMA-grafted CNF dispersion obtained in Example 3. [Figure 6] It is a photograph of the MMA-grafted CNF dispersion obtained in Example 4. [Figure 7] It is a photograph of the MMA-grafted CNF dispersion obtained in Example 5. [Figure 8] It is a photograph of the MMA-grafted CNF dispersion obtained in Example 6. [Figure 9] It is a photograph of the MMA-grafted CNF dispersion obtained in Example 7. [Figure 10] It is a photograph of the MMA-grafted CNF dispersion obtained in Example 8.

Mode for Carrying Out the Invention

[0018] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the following description, and can be arbitrarily modified and implemented without departing from the gist of the present invention.

[0019] Unless otherwise specified, in this specification, a numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value. For example, "A~B" means A or more and B or less.

[0020] In the present invention, "mass%" indicates the content ratio of a predetermined component contained in the total amount of 100 mass%. In this invention, "graft" refers to a state in which one polymer is branched and connected to another polymer. The "cellulose nanofiber graft polymer" according to this invention refers to a cellulose nanofiber graft polymer in which a radical polymerizable monomer is applied to radicals generated on cellulose nanofibers by irradiating cellulose nanofibers with gamma rays, and the corresponding polymer is grafted onto the surface by graft polymerization, thereby forming a graft chain. The cellulose nanofiber graft polymer according to the present invention has polymer chains of radical polymerizable monomers that are covalently bonded to cellulose nanofibers as graft chains, and these graft chains are covalently bonded to the carbon atoms and / or oxygen atoms of the hydroxyl groups of the sugar chains of cellulose.

[0021] The present invention relates to a method for producing a cellulose nanofiber graft polymer, comprising irradiating a dispersion (hereinafter sometimes simply referred to as "dispersion") containing cellulose nanofibers, a radical polymerizable monomer, and a dispersion medium with radiation to produce a cellulose nanofiber graft polymer in which radical polymerizable monomers are grafted onto cellulose nanofibers, characterized in that the radiation is gamma rays, the concentration of cellulose nanofibers in the dispersion is less than 2% by mass relative to the total of the dispersion medium and the cellulose nanofibers, and the thickness of the dispersion (hereinafter sometimes simply referred to as "thickness of the dispersion") in the direction of the gamma ray irradiation is 5 mm or more.

[0022] According to the inventors' research, gamma rays, which have high material penetration capabilities, allow for sufficient graft polymerization even when the thickness of the dispersion in the direction of irradiation is increased, enabling the acquisition of a large quantity of the target product. However, it was found that if the concentration of cellulose nanofibers in the dispersion is too high, self-polymerization (crosslinking reaction) occurs between the cellulose nanofibers, leading to significant gelation and insufficient graft polymerization. On the other hand, it was found that when the concentration of cellulose nanofibers is appropriate, graft polymerization proceeds sufficiently even if gelation progresses, allowing for the acquisition of cellulose nanofiber graft polymers. In this invention, by setting the concentration of cellulose nanofibers in the dispersion to below a predetermined value, cellulose nanofiber graft polymers can be produced with high productivity.

[0023] <Cellulose nanofiber> The cellulose nanofibers used in this invention can be any type of cellulose fiber generally referred to as nanofibers. More specifically, it refers to nanofibers composed of cellulose (cellulose nanofibers) and / or nanofibers composed of lignocellulose (lignocellulose nanofibers), and is collectively referred to as "CNF". In this invention, CNF includes microfibrillated cellulose fibers and / or microfibrillated lignocellulose fibers. Examples of cellulose nanofibers include those obtained primarily by physical or mechanical defibration treatment, and those obtained primarily by chemical defibration treatment. Alternatively, cellulose nanofibers manufactured from raw materials (raw fibers) such as pulp may be used, or commercially available products may be used. These may be used individually or in combination of two or more types.

[0024] Various types of pulp can be cited as the main raw materials for cellulose nanofibers. Pulps can be categorized by material type, for example, wood pulp such as coniferous pulp and hardwood pulp; non-wood pulp such as rice pulp, kenaf pulp, hemp (linen) pulp, mulberry pulp, bagasse pulp, straw pulp, cotton pulp, bamboo pulp, fruit pulp, rag pulp, and linter pulp; recycled paper pulp; and synthetic fiber pulp. Pulps can also be categorized by manufacturing method, for example, mechanical pulp such as wood pulp (GP), refiner ground pulp (RGP), thermomechanical pulp (TMP), and chemothermetic pulp (CTMP); chemical pulp such as kraft pulp (KP), sulfide pulp (SP), and alkali pulp (AP); and unbleached pulp and bleached pulp. Furthermore, examples of raw materials for cellulose nanofibers include cellulose derived from animal materials (e.g., sea squirts), algae, microorganisms (e.g., acetic acid bacteria), and microbial products.

[0025] The size of cellulose nanofibers is not particularly limited as long as the fiber width is generally nanoscale (1000 nm or less). For example, ranges such as 1-1000 nm, 2-500 nm, and 3-300 nm can be used.

[0026] <Radical polymerizable monomers> The radical polymerizable monomers that can be used in the production method of the present invention are not particularly limited, but radical polymerizable monomers having one or more polymerizable vinyl groups in the molecule are preferred, and hydrophobic monomers are preferred from the viewpoint of being well dispersed in resins and paints and further enhancing the reinforcing effect, for example, the following a) to f).

[0027] a) (meth)acrylic acid ester compounds: Specifically, methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, n-nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate, n-amyl (meth)acrylate, isoamyl (meth)acrylate, lauryl (meth)acrylate Examples include alkyl (meth)acrylates such as lyl; aralkyl (meth)acrylates such as benzyl (meth)acrylate; aryl (meth)acrylates such as phenyl (meth)acrylate and 2-naphthyl (meth)acrylate; cyclic alkyl (meth)acrylates such as cyclohexyl (meth)acrylate; and alkyl (meth)acrylates having substituents such as alkoxy groups and aryl groups, such as methoxyethyl (meth)acrylate, ethoxyethyl (meth)acrylate, and phenoxymethyl (meth)acrylate. Among these, alkyl (meth)acrylate esters with 1 to 8 carbon atoms in the alkyl group are preferred, and methyl (meth)acrylate is more preferred. Here, "(meth)acrylic acid" refers to either or both methacrylic acid and acrylic acid.

[0028] b) Aromatic vinyl compounds: Specifically, these include styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene, o-ethylstyrene, p-ethylstyrene, o-chlorostyrene, p-chlorostyrene, p-methoxystyrene, p-acetoxystyrene, α-vinylnaphthalene, and 2-vinylfluorene.

[0029] c) Unsaturated nitrile compounds: Specifically, these include acrylonitrile, α-chloroacrylonitrile, α-methoxyacrylonitrile, methacrylonitrile, and vinylidene cyanide.

[0030] d) Ethylene-unsaturated ether compounds: Specifically, these include methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, methyl allyl ether, and ethyl allyl ether.

[0031] e) Halogenated vinyl compounds: Specifically, these include vinyl chloride, vinylidene chloride, 1,2-dichloroethylene, vinyl bromide, vinylidene bromide, and 1,2-dibromoethylene.

[0032] f) Aliphatic conjugated diene compounds: Specifically, these include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-neopentyl-1,3-butadiene, 2-chloro-1,3-butadiene, 1,2-dichloro-1,3-butadiene, 2,3-dichloro-1,3-butadiene, 2-bromo-1,3-butadiene, and 2-cyano-1,3-butadiene.

[0033] These radical polymerizable monomers may be used individually or in combination.

[0034] Among these, (meth)alkyl acid ester compounds that do not dissolve in water and can maintain a good dispersion state are preferred in the production method of the present invention. In particular, alkyl (meth)acrylates having linear hydrocarbon groups in their side chains can be suitably used because their high monomer concentration can stabilize the rate of graft polymerization. Furthermore, methyl (meth)acrylate can be suitably used in particular because the molecular weight distribution is narrowed during graft polymerization, improving the uniformity of polymerization.

[0035] <Dispersion> The dispersion medium used in a dispersion containing cellulose nanofibers, radical polymerizable monomers, and a dispersion medium is usually water, but it may also contain hydrophilic organic solvents other than water, such as alcohols like ethanol or methanol. However, in terms of the risk of cellulose nanofibers agglomerating, the concentration of water in the dispersion medium is usually 80% by mass or more, preferably 90% by mass or more, and more preferably 95% by mass or more.

[0036] The dispersion is preferably prepared by first preparing a dispersion of cellulose nanofibers, and then adding and mixing a predetermined amount of radical polymerizable monomer or an aqueous dispersion of radical polymerizable monomer thereto.

[0037] Methods for preparing a dispersion of cellulose nanofibers include crushing cellulose fibers in a dispersion medium such as water, and then adding desalted water to adjust to a predetermined concentration. More specifically, the method described in Japanese Patent Publication No. 2009-299043 is one example.

[0038] The concentration of cellulose nanofibers in the dispersion before the addition and mixing of the radical polymerizable monomer, or the concentration of cellulose nanofibers relative to the total of the dispersion medium and cellulose nanofibers in the dispersion subjected to gamma irradiation, is less than 2% by mass, preferably 1% by mass or less, more preferably 0.5% by mass or less, and even more preferably 0.2% by mass or less. On the other hand, the concentration of cellulose nanofibers is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.1% by mass or more. As is related to the thickness of the dispersion described later, if the cellulose nanofiber concentration exceeds the above upper limit, the cellulose nanofibers will aggregate in the dispersion, and after gamma irradiation, the cellulose nanofibers will self-polymerize (crosslink) and gel, making it very difficult to separate the cellulose nanofibers. In particular, a cellulose nanofiber concentration of 0.2% by mass or less tends to facilitate the removal of water after graft polymerization, which is preferable. The concentration of cellulose nanofibers in the dispersion is (mass of cellulose nanofibers) / (mass of cellulose nanofibers + mass of dispersion medium) × 100.

[0039] It is preferable to add the radical polymerizable monomer in an amount of 0.1 to 30 times the mass of the cellulose nanofiber, particularly 0.5 to 20 times, even more preferably 0.75 to 10 times, and especially 1 to 2.5 times the mass of the cellulose nanofiber. If the amount of radical polymerizable monomer is above the lower limit, the graft polymerization effect is easily obtained, and if the amount of radical polymerizable monomer is below the upper limit, the proportion of self-polymerization among the radical polymerizable monomers without grafting is reduced, making it more efficient. Furthermore, by setting the amount of radical polymerizable monomer within an appropriate range relative to the cellulose nanofiber concentration, it is possible to obtain cellulose nanofiber graft polymers that have been grafted without gelation.

[0040] <Graft polymerization> In the method for producing cellulose nanofiber graft polymers of the present invention, graft polymerization is carried out by irradiating a dispersion containing cellulose nanofibers prepared as described above, a radical polymerizable monomer, and a dispersion medium with gamma rays. The gamma ray dose during graft polymerization is preferably 0.1 to 10 kGy. A gamma ray dose of 0.1 kGy or higher tends to yield sufficient graft polymerization reactivity. A gamma ray dose exceeding 10 kGy may cause crosslinking and gelation of the cellulose nanofibers. A more preferable dose is 1 to 8 kGy. The duration of gamma ray irradiation depends on the type and distance of the irradiation source. However, when using gamma rays emitted from a cobalt-60 source, it is preferable to place the dispersion to be treated at a position where the dose rate is 1 kGy / h to 10 kGy / h, particularly 1 kGy / h to 8 kGy / h, and irradiate it every 0.1 to 2 hours, for example, every hour.

[0041] The dispersion is placed in a container that allows gamma rays to pass through, such as a glass container, and irradiated with gamma rays.

[0042] In this invention, the thickness of the dispersion in the direction of gamma ray irradiation is set to 5 mm or more, preferably 10 mm or more, and more preferably 15 mm or more. A thicker thickness is preferable as it yields a larger amount of graft polymer, but as the thickness increases, the transmittance of gamma rays gradually decreases. From this viewpoint, the upper limit of the dispersion thickness is 1000 mm or less, preferably 300 mm or less.

[0043] Furthermore, when irradiating a dispersion in a large container (a dispersion with a large thickness) with gamma rays, it is preferable to irradiate while stirring in order to make the dose distribution within the container uniform. In other words, although gamma rays have high material penetration, when irradiating a thick dispersion, a dose distribution occurs in the direction of the thickness. In this case, stirring can alleviate the limitations on the thickness of the container. The stirring speed depends on the size of the container, and a particularly fast speed is not necessary. For a container measuring 1m x 1m x 1m, it is sufficient for the dispersion inside to circulate slowly over one minute, and if the sample is irradiated while stirring for one hour, the entire sample will be irradiated uniformly.

[0044] Graft polymers obtained by graft polymerization induced by gamma radiation can be easily separated and recovered by removing water through filtration, freeze-drying, etc., and then washing with an organic solvent to remove unreacted radical polymerizable monomers and polymers not grafted onto cellulose nanofibers.

[0045] <Comparison of gamma rays and electron beams> Gamma rays are electromagnetic waves with high material penetration capabilities, allowing for the uniform application of radicals to the entire irradiated object (a dispersion in this invention), even if it is thick. Therefore, they are an ideal irradiation source for large-scale synthesis.

[0046] Figure 1 shows the relationship between the thickness of the irradiated object (water) and the dose transmittance when exposed to cobalt-60 gamma rays. As shown in Figure 1, the transmittance of gamma rays gradually decreases as the thickness of the irradiated object increases, but radicals can still be generated even in an irradiated object with a thickness of 1000 mm. The relationship between thickness and gamma ray transmittance is as follows: Thickness: 82mm, Transmittance: 90% Thickness: 167mm, Transmittance: 75% Thickness: 299mm, Transmittance: 50% Thickness: 481mm, Transmittance: 25% Thickness: 1020mm, Transmittance: 2% Thickness: 1156mm, Transmittance: 1%

[0047] On the other hand, electron beams are particle beams and have low material penetration capabilities. Therefore, they cannot uniformly impart radicals to the entire surface of a thick object being irradiated. Consequently, even when irradiated with an electron beam, radicals are generated only on the surface of the object being treated. Figure 2 shows the relationship between the thickness of the irradiated object (water) and the dose transmittance when using an electron beam. As shown in Figure 2, the higher the acceleration voltage, the deeper the electrons can penetrate, but even at a relatively high acceleration voltage of 2 MeV, electrons can only penetrate up to a thickness of 12 mm. For example, the thickness is 11.6 mm and the transmittance is 2%.

[0048] It should be noted that commercially available electron accelerators are generally low-energy types with acceleration voltages of 300 keV or less. For example, in an electron accelerator with an acceleration voltage of 250 keV, electrons can only penetrate to a thickness of 0.5 mm.

[0049] Based on a transmittance of 2%, gamma rays can handle dispersions 88 times thicker than those for electron beams (2 MeV) and 2000 times thicker than those for electron beams (250 keV), as follows: Gamma rays: Thickness: 1020 mm, Transmittance: 2% Electron beam (2 MeV): Thickness: 11.6 mm, Transmittance: 2% Electron beam (250 keV): Thickness: 0.5 mm, Transmittance: 2% [Examples]

[0050] The manufacturing method of the present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0051] [Manufacturing Example 1: Preparation of CNF Aqueous Dispersion] <Purification process of wood flour raw materials> Douglas fir wood flour (Miyashita Lumber Co., Ltd., average maximum length 250 μm, average minimum length 50 μm) was added to a 2% by mass sodium carbonate aqueous solution and degreased at 80°C for 6 hours. After washing with desalinated water, lignin was removed by immersion in a 0.66% by mass sodium chlorite and 0.14% by mass acetic acid aqueous solution at 80°C for 5 hours. After washing with desalinated water and filtering, the recovered purified cellulose was washed with desalinated water and then immersed in a 5% by mass potassium hydroxide aqueous solution for 16 hours to remove hemicellulose. After washing again with desalinated water, it was prepared as a 1% by mass aqueous dispersion.

[0052] <Fibrillation process using an ultra-high pressure homogenizer> Water was added to the aqueous dispersion obtained above to adjust it to 0.5% by mass. The defibrillation treatment was repeated five times using a Sugino Machine ultra-high pressure homogenizer (Ultimizer (Starburst Lab HJP-24005), pore diameter 150 μm) at a jet pressure of 245 MPa.

[0053] The resulting high-pressure homogenizer-treated cellulose aqueous dispersion (average fiber diameter of cellulose fibers in the aqueous dispersion is 220 nm, cellulose concentration is 0.504 mass%) was then subjected to an SMT UH-600S ultrasonic homogenizer (frequency 20 kHz, effective power density 22 W / cm²). 2 Ultrasonic treatment was performed using ). A 36mm diameter straight tip (made of titanium alloy) was used, and the output volume was tuned to 8. Ultrasonic treatment was then performed for 60 minutes at the optimal tuning position with 50% intermittent operation. 50% intermittent operation means that ultrasonic waves are emitted for 0.5 seconds, followed by a 0.5-second pause. The cellulose dispersion was cooled from the outside of the processing container with 5°C cold water, and the dispersion temperature was maintained at 15°C ± 5°C during processing. The process was also carried out while stirring with a magnetic stirrer. The average fiber diameter of the CNF in the obtained aqueous CNF dispersion (as observed by TEM) was 10 nm.

[0054] [Example 1] Water was added to the CNF dispersion obtained in Production Example 1 to obtain a CNF aqueous dispersion with a CNF concentration of 0.4% by mass. 5 g of this CNF aqueous dispersion was placed in an 8 mL glass vial (manufactured by Nichiden Rika Glass, model: SV-8, size: diameter 21 mm, height 5 mm, wall thickness 1.3 mm), and MMA (product name: methyl methacrylate monomer, manufactured by Kanto Chemical Co., Ltd.) was added to a concentration of 5.0% by mass (12.5 times the mass of CFN). The mixture was then thoroughly stirred to ensure that the MMA was uniformly dispersed in the CNF aqueous dispersion. At a gamma-ray irradiation facility, the above-mentioned CNF / MMA aqueous dispersion contained in a vial was irradiated with gamma rays to carry out a graft polymerization reaction. The thickness of the dispersion at this time was 21 mm. Cobalt-60 was used as the gamma ray source, and the dose rate was adjusted to 5 kGy / h, and irradiation was performed for 1 hour (gamma dose: 5 kGy). As a result, in the 0.4 mass% CNF / 5.0 mass% MMA dispersion, the irradiated material (polymer) gelled, but it was possible to obtain CNF (cellulose nanofiber graft polymer) with MMA grafts. Subsequently, water was removed by freeze-drying, and then unreacted radical polymerizable monomers and ungrafted polymers were removed by washing with tetrahydrofuran (THF). XRD analysis of the obtained cellulose nanofiber graft polymer confirmed that polymethyl methacrylate was grafted onto the cellulose. (See Figure 3). The XDR analysis conditions were as follows: using an X-ray diffractometer (PANalytical, X'Pert Pro MPD), with a focusing optical system, a CuKα source, and a scanning range of 5°<2θ<50°.

[0055] [Example 2] In an aqueous dispersion of CNF with a CNF concentration of 0.4% by mass, MMA was added to a concentration of 2.0% by mass (5 times the mass of CNF), and the gamma ray dose rate was set to 1 kGy / h. The procedure was carried out in the same manner as in Example 1. In the 0.4% by mass CNF / 2.0% by mass MMA dispersion, the irradiated material (polymer) gelled, but CNF with MMA grafted onto it was obtained. Figure 4 shows a photograph of the vial containing the obtained MMA graft CNF dispersion.

[0056] [Example 3] In an aqueous dispersion of CNF with a CNF concentration of 0.2% by mass, an MMA concentration of 1.0% by mass (5 times the mass of CNF) was added, and the gamma ray dose rate was set to 1 kGy / h. The procedure was carried out in the same manner as in Example 1. In the 0.2% by mass CNF / 1.0% by mass MMA dispersion, the irradiated material (polymer) gelled, but CNF with MMA grafted onto it was obtained. Figure 5 shows a photograph of the vial containing the obtained MMA graft CNF dispersion.

[0057] [Example 4] In the same procedure as in Example 1, except that MMA was added to a CNF aqueous dispersion with a CNF concentration of 0.4% by mass (1x the mass of CNF) at a concentration of 0.4% by mass, the irradiated material (polymer) gelled in the 0.4% by mass CNF / 0.4% by mass MMA dispersion, but CNF with MMA grafted onto it was obtained. Figure 6 shows a photograph of the vial containing the obtained MMA graft CNF dispersion.

[0058] [Example 5] In the same procedure as in Example 1, except that MMA was added to a CNF aqueous dispersion with a CNF concentration of 0.2% by mass to a concentration of 1.0% by mass (5 times the mass of CNF), the irradiated material (polymer) gelled in the 0.2% by mass CNF / 1.0% by mass MMA dispersion, but CNF with MMA grafted onto it was obtained. Figure 7 shows a photograph of the vial containing the obtained MMA graft CNF dispersion.

[0059] [Example 6] In the same procedure as in Example 1, except that MMA was added to a CNF aqueous dispersion with a CNF concentration of 0.2% by mass (1x the mass of CNF) at a concentration of 0.2% by mass, in the 0.2% by mass CNF / 0.2% by mass MMA dispersion, the CNF grafted with MMA, which is the irradiated substance (polymer), remained in a liquid state without gelling, and the grafted polymer of CNF could be easily isolated by the subsequent filtration step. Figure 8 shows a photograph of the vial containing the obtained MMA graft CNF dispersion.

[0060] [Example 7] In a CNF aqueous dispersion with a CNF concentration of 0.2% by mass, MMA was added to a concentration of 0.2% by mass (1x the mass of CNF), the dose rate was adjusted to 5 kGy / h, and the procedure was carried out in the same manner as in Example 1, except that irradiation was performed for 2 hours (gamma dose: 10 kGy). In the 0.2% by mass CNF / 0.2% by mass MMA dispersion, the CNF grafted with MMA, which is the irradiated material (polymer), remained in a liquid state without gelling, and the grafted polymer of CNF could be easily isolated by the subsequent filtration step. Figure 9 shows a photograph of the vial containing the obtained MMA graft CNF dispersion.

[0061] [Example 8] In an aqueous dispersion of CNF with a CNF concentration of 1.9% by mass, an MMA concentration of 0.4% by mass (0.21 times the mass of CNF) was added, and the gamma ray dose rate was set to 1 kGy / h. The procedure was carried out in the same manner as in Example 1. In the 1.9% by mass CNF / 0.4% by mass MMA dispersion, the irradiated material (polymer) gelled, but CNF with MMA grafted onto it was obtained. Figure 10 shows a photograph of the vial containing the obtained MMA graft CNF dispersion.

[0062] In other words, a method for producing a cellulose nanofiber graft polymer can be provided, preferably characterized in that the cellulose nanofiber graft polymer produced by gamma-ray irradiation maintains a liquid state. Furthermore, as conditions for the produced cellulose nanofiber graft polymer to maintain a liquid state, preferably characterized in that the concentration of cellulose nanofibers is 0.2% by mass or less, the concentration of radical polymerizable monomers is 0.2% by mass or less, and the gamma-ray dose is 10 kGy or less.

[0063] [Comparative Example 1] In Example 1, the procedure was carried out similarly except that an electron beam was used instead of gamma rays. As a result, when the acceleration voltage was 250 keV or less, the electron beam could not penetrate the glass vial, and therefore no radicals that serve as the starting point for graft polymerization were generated in the cellulose nanofibers in the CNF / MMA aqueous dispersion. Therefore, when an electron beam with an acceleration voltage of 250 keV or less was used, no MMA was grafted onto the cellulose nanofibers at any CNF / MMA concentration ratio. On the other hand, when the acceleration voltage was 0.5 MeV or higher, although the electron beam could penetrate the glass vial, the electron beam was not uniformly irradiated to the entire cellulose nanofiber in the CNF / MMA aqueous dispersion, and therefore, radicals that serve as the starting point for graft polymerization were not generated in more than half of the cellulose nanofibers. Therefore, when an electron beam with an acceleration voltage of 0.5 MeV or higher was used, although the graft polymerization reaction proceeded in some of the cellulose nanofibers contained in the dispersion, the graft polymerization reaction did not proceed in more than half of the cellulose nanofibers, and it was not possible to obtain CNF with uniformly grafted MMA. [Industrial applicability]

[0064] The cellulose nanofiber graft polymer obtained by this invention exhibits improved affinity to various resins and paints due to the effect of graft chains introduced on the surface of the cellulose nanofibers, resulting in excellent reinforcing effects. This enhances the strength of the resulting composite material and the strength of the coating film. For example, a gel-like cellulose nanofiber graft polymer can be mixed with various resins and the dispersion medium removed by pressing or other means to obtain a high-strength composite material. Alternatively, a liquid cellulose nanofiber graft polymer can be extracted as a powder by removing the dispersion medium and kneading it with various resins to obtain a high-strength composite material. These composite materials are suitable for use in reinforcing resins and paints in lighting materials, building materials, vehicle materials, and other applications.

Claims

1. A method for producing a strength-enhancing additive consisting of a cellulose nanofiber graft polymer in which radical polymerizable monomers are grafted onto cellulose nanofibers, by irradiating a dispersion containing cellulose nanofibers, a radical polymerizable monomer, and a dispersion medium with radiation, The radical polymerizable monomer is a (meth)acrylic acid ester compound, The aforementioned radiation is gamma rays, The concentration of cellulose nanofibers in the dispersion is less than 2% by mass relative to the total of the dispersion medium and the cellulose nanofibers. A method for producing a strength-enhancing additive, characterized in that the thickness of the dispersion liquid present in the direction of gamma ray irradiation is 5 mm or more.

2. A method for producing a strength-enhancing additive according to claim 1, wherein the dose of the gamma rays is 0.1 to 10 kGy.

3. A method for producing a strength-improving additive according to claim 1 or 2, wherein the water content in the dispersion medium is 80% by mass or more.

4. A method for producing the strength-improving additive according to claim 1 or 2, which is a strength-improving additive for resins or paints.

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