Near-infrared absorbing material, ink, coating material and molded article each using near-infrared absorbing material, display article, article, and method for determining authenticity of article

Abstract

The purpose of the present invention is to provide a novel near-infrared absorbing material which has sufficiently high near-infrared absorbing ability. A near-infrared absorbing material according to the present invention is characterized by being composed of copper-containing oxidized cellulose nanofibers that have a number average fiber diameter of 100 nm or less and contain copper in the form of salt. An ink and a coating material according to the present invention are characterized by including the near-infrared absorbing material according to the present invention. A molded article according to the present invention is characterized by being composed of the near-infrared absorbing material. A display article according to the present invention is characterized by using the ink, the coating material or the molded article according to the present invention. An article according to the present invention is characterized by being provided with the display article according to the present invention. A method for determining the authenticity of an article according to the present invention is characterized by using the near-infrared absorbing material according to the present invention.

Description

Near-infrared absorbing material, ink, paint and molded product using the near-infrared absorbing material, display material, article, and method for determining authenticity of article

[0001] The present invention relates to a near-infrared absorbing material, an ink, a paint and a molded product using the near-infrared absorbing material, an indicia, an article, and a method for discriminating the authenticity of an article.

[0002] In recent years, near-infrared absorbing materials have come to be used in a wide range of fields, such as optical recording media, near-infrared photosensitizers, photothermal conversion agents, near-infrared absorbing filters, image sensors, optical filters for optical sensors such as face authentication and fingerprint authentication, near-infrared absorbing inks, heat ray shielding materials, etc. In particular, there is a growing demand for the development of near-infrared absorbing materials with high near-infrared absorption ability for applications such as transparent inks used for security purposes.

[0003] As such near-infrared absorbing materials, various organic dyes such as phthalocyanine compounds, cyanine compounds, diimonium compounds, squarylium compounds, etc. For example, Patent Document 1 discloses a near-infrared absorbing material containing a squarylium compound having a predetermined structure.

[0004] Japanese Patent Application Laid-Open No. 2021-085024

[0005] However, in recent years, there has been a demand for the development of near-infrared absorbing materials other than organic dyes.

[0006] Therefore, an object of the present invention is to provide a novel near-infrared absorbing material having sufficiently high near-infrared absorbing ability. Another object of the present invention is to provide an ink and a paint containing the near-infrared absorbing material. Another object of the present invention is to provide a molded product made from the near-infrared absorbing material. Another object of the present invention is to provide an indication made using the ink, paint, or molded product. Another object of the present invention is to provide an article equipped with the indication. Still another object of the present invention is to provide a method for determining the authenticity of an article.

[0007] The present inventors have conducted extensive research with the aim of solving the above-mentioned problems, and have newly discovered that the above-mentioned problems can be solved by a near-infrared absorbing material made of copper-containing oxidized cellulose nanofibers that have a predetermined number-average fiber diameter and contain copper in the form of a salt, and have thus completed the present invention.

[0008] That is, the present invention aims to advantageously solve the above-mentioned problems, and the present invention provides [1] a near-infrared absorbing material comprising copper-containing oxidized cellulose nanofibers having a number-average fiber diameter of 100 nm or less and containing copper in the form of a salt. The near-infrared absorbing material comprising the above-mentioned specified copper-containing oxidized cellulose nanofibers has excellent near-infrared absorption ability. In this specification, the "number-average fiber diameter" of copper-containing oxidized cellulose nanofibers can be determined by measuring the fiber diameters of five or more copper-containing oxidized cellulose nanofibers using an atomic force microscope and calculating the number average of the measured fiber diameters. As the atomic force microscope, for example, a Dimension FastScan AFM (manufactured by BRUKER, Tapping mode) can be used.

[0009] [2] [1] In the near-infrared absorbing material, the number-average fiber length of the copper-containing oxidized cellulose nanofibers is preferably 50 nm or more and 2000 nm or less. When the number-average fiber length of the copper-containing oxidized cellulose nanofibers is equal to or greater than the lower limit, sufficiently high mechanical strength can be imparted to molded articles such as films made from the near-infrared absorbing material. On the other hand, when the number-average fiber length of the copper-containing oxidized cellulose nanofibers is equal to or less than the upper limit, the dispersibility of the copper-containing oxidized cellulose nanofibers is ensured, improving the transparency of the resulting molded article while imparting even more excellent near-infrared absorption ability to the near-infrared absorbing material. In this specification, the "number-average fiber length" of the copper-containing oxidized cellulose nanofibers can be determined by measuring the fiber lengths of five or more copper-containing oxidized cellulose nanofibers using an atomic force microscope and calculating the number average of the measured fiber lengths. As the atomic force microscope, for example, a Dimension FastScan AFM (manufactured by BRUKER, Tapping mode) can be used.

[0010] [3] In the near-infrared absorbing material of [1] or [2] above, the average degree of polymerization of the copper-containing oxidized cellulose nanofibers is preferably 100 or more and 2000 or less. When the average degree of polymerization of the copper-containing oxidized cellulose nanofibers is equal to or more than the above lower limit, a molded product such as a film can be imparted with sufficiently high mechanical strength. On the other hand, when the average degree of polymerization of the copper-containing oxidized cellulose nanofibers is equal to or less than the above upper limit, the dispersibility of the copper-containing oxidized cellulose nanofibers is ensured, and the transparency of the obtained molded product can be improved, while imparting even more excellent near-infrared absorption ability to the near-infrared absorbing material. In this specification, the "average degree of polymerization" of the copper-containing oxidized cellulose nanofibers is, for example, as described in "Isogai, A., Mutoh, N., Onabe, F., Usuda, M., "Viscosity measurements of cellulose / SO ２－ The measurement can be carried out in accordance with the method described in "Ammonia-dimethylsulfoxide solution", Sen'i Gakkaishi, 45, 299-306 (1989)."

[0011] [4] In the near-infrared absorbing material of any one of [1] to [3] above, the copper-containing oxidized cellulose nanofibers are preferably copper-containing carboxylated cellulose nanofibers. If the copper-containing oxidized cellulose nanofibers are copper-containing carboxylated cellulose nanofibers, the dispersibility of the copper-containing oxidized cellulose nanofibers is ensured, and the transparency of the resulting molded product can be improved, while imparting even better near-infrared absorbing ability to the near-infrared absorbing material.

[0012] Another object of the present invention is to advantageously solve the above problems, and [5] the present invention is an ink containing the near-infrared absorbing material of any one of [1] to [4] above. The ink containing the near-infrared absorbing material has excellent near-infrared absorbing ability.

[0013] Another object of the present invention is to advantageously solve the above-mentioned problems, and [6] the present invention is a coating material containing the near-infrared absorbing material of any one of [1] to [4] above. The coating material containing the near-infrared absorbing material has excellent near-infrared absorbing ability.

[0014] Another object of the present invention is to advantageously solve the above-mentioned problems, and [7] the present invention is a molded article made of the near-infrared absorbing material of any one of [1] to [4] above. The molded article containing the near-infrared absorbing material has excellent near-infrared absorbing ability.

[0015] Another object of the present invention is to advantageously solve the above problems, and the present invention [8] is a display made using the ink of the above [5], the paint of the above [6], or the molded product of the above [7]. The display made using the ink, paint, or molded product has excellent near-infrared absorbing ability.

[0016] Another object of the present invention is to advantageously solve the above-mentioned problems, and [9] the present invention is an article comprising the display material of the above-mentioned [8]. The article comprising the display material has excellent near-infrared absorbing ability.

[0017] The present invention also aims to advantageously solve the above-mentioned problems, and

[10] the present invention is a method for discriminating the authenticity of an article, comprising the steps of: applying a visually indistinguishable mark to an authentic article using any of the near-infrared absorbing materials [1] to [4] above; acquiring a near-infrared image of the article to be inspected; and determining that an article in which the mark appears in the near-infrared image is an authentic article, and determining that an article in which the mark does not appear in the near-infrared image is a counterfeit article. According to the authenticity discrimination method of the present invention, the authenticity of an article can be easily determined.

[0018] According to the present invention, a novel near-infrared absorbing material having sufficiently high near-infrared absorbing ability can be provided. Furthermore, according to the present invention, an ink and a paint containing the near-infrared absorbing material can be provided. Furthermore, according to the present invention, a molded product made of the near-infrared absorbing material can be provided. Furthermore, according to the present invention, an indication made using the ink, paint, or molded product can be provided. Furthermore, according to the present invention, an article equipped with the indication can be provided. Furthermore, according to the present invention, a method for determining the authenticity of an article can be provided.

[0019] 1A and 1B are schematic diagrams of an imaging device 1 used in Examples and Comparative Examples, and an imaging device 2 used in Examples and Comparative Examples.

[0020] Hereinafter, embodiments of the present invention will be described in detail. The near-infrared absorbing material of the present invention can be suitably used, for example, when producing the ink, coating material, and molded article of the present invention. The ink of the present invention can be suitably used, for example, as a security ink for preventing counterfeiting of printed materials. The coating material of the present invention can be suitably used, for example, as a heat-shielding coating applied to exterior walls and the like for the purpose of insulating heat, or as a security coating for preventing counterfeiting. The molded article of the present invention can be suitably used, for example, as a transparent film, as a near-infrared absorbing filter (near-infrared cut filter) attached to dye-sensitized solar cells, eyeglasses, sunglasses, etc. for the purpose of absorbing and / or cutting near-infrared rays. The display article of the present invention can be suitably used, for example, to identify an article equipped with the display article. The article of the present invention can be suitably used, for example, as a security article. The authenticity determination method of the present invention can be suitably used to determine the authenticity of an article.

[0021] (Near-infrared absorbing material) The near-infrared absorbing material of the present invention is composed of copper-containing oxidized cellulose nanofibers having a number-average fiber diameter of 100 nm or less and containing copper in the form of a salt. Such near-infrared absorbing materials have excellent near-infrared absorption properties. Note that, since the near-infrared absorbing material of the present invention is composed of specified copper-containing oxidized cellulose nanofibers, it does not usually contain any components other than impurities that are inevitably mixed in during production. Here, the content of copper-containing oxidized cellulose nanofibers in the near-infrared absorbing material of the present invention is usually 99% by mass or more, preferably 99.5% by mass or more, and more preferably 99.9% by mass or more.

[0022] <Copper-containing oxidized cellulose nanofibers containing copper in the form of a salt> The copper-containing oxidized cellulose nanofibers that constitute the near-infrared absorbing material of the present invention contain copper in the form of a salt. By using such copper-containing oxidized cellulose nanofibers, the near-infrared absorbing material can exhibit excellent near-infrared absorption properties. Hereinafter, copper-containing oxidized cellulose nanofibers that contain copper in the form of a salt may be simply referred to as "copper-containing oxidized cellulose nanofibers."

[0023] The copper-containing oxidized cellulose nanofibers are preferably copper-containing carboxylated cellulose nanofibers, which ensures the dispersibility of the copper-containing oxidized cellulose nanofibers, improves the transparency of the resulting molded product, and imparts even better near-infrared absorbing ability to the near-infrared absorbing material.

[0024] Here, the carboxylated cellulose nanofibers constituting the copper-containing carboxylated cellulose nanofibers are those in which the primary hydroxyl groups at the 6-position of the β-glucose units of the starting cellulose have been oxidized to carboxy groups via aldehyde groups. From the viewpoint of sufficiently imparting the desired properties to the copper-containing carboxylated cellulose nanofibers, it is preferable that the primary hydroxyl groups in the carboxylated cellulose nanofibers are oxidized to carboxy groups in an amount of preferably 50 mol% or more, more preferably 70 mol% or more, and even more preferably 90 mol% or more. The amount of carboxy groups in the copper-containing carboxylated cellulose nanofibers can be measured according to the method described in JP 2016-141777 A or JP 2019-199622 A.

[0025] The amount of copper in the copper-containing oxidized cellulose nanofibers is not limited as long as it can impart the desired near-infrared absorption ability to the near-infrared absorbing material. For example, in the copper-containing carboxylated cellulose nanofibers, copper is preferably present in a proportion of at least 1 / 3, and more preferably at least 1 / 2, of the molar amount of carboxy groups in the carboxylated cellulose nanofibers. The greater the copper content in the copper-containing oxidized cellulose nanofibers, the more the near-infrared absorbency of the near-infrared absorbing material can be improved. Note that copper in the copper-containing oxidized cellulose nanofibers can be qualitatively and quantitatively analyzed by ICP-AES, for example, according to the methods described in JP 2016-141777 A or JP 2019-199622 A.

[0026] The number-average fiber diameter of the copper-containing oxidized cellulose nanofibers must be 100 nm or less. By using such copper-containing oxidized cellulose nanofibers, it is possible to impart excellent near-infrared absorption ability to the near-infrared absorbing material. The number-average fiber diameter of the copper-containing oxidized cellulose nanofibers is preferably 50 nm or less, more preferably 10 nm or less, and even more preferably 5 nm or less. The number-average fiber diameter of the copper-containing oxidized cellulose nanofibers is, for example, 0.5 nm or more, and may be 1 nm or more.

[0027] The number-average fiber length of the copper-containing oxidized cellulose nanofibers is preferably 50 nm or more, more preferably 70 nm or more, even more preferably 100 nm or more, even more preferably 400 nm or more, and preferably 2000 nm or less, more preferably 1500 nm or less, even more preferably 1000 nm or less, and even more preferably 600 nm or less. When the number-average fiber length of the copper-containing oxidized cellulose nanofibers is equal to or greater than the above-mentioned lower limit, sufficiently high mechanical strength can be imparted to molded articles such as films made using the near-infrared absorbing material. On the other hand, when the number-average fiber length of the copper-containing oxidized cellulose nanofibers is equal to or less than the above-mentioned upper limit, the dispersibility of the copper-containing oxidized cellulose nanofibers is ensured, improving the transparency of the resulting molded article while imparting even more excellent near-infrared absorption ability to the near-infrared absorbing material. The number-average fiber length of the copper-containing oxidized cellulose nanofibers can be adjusted, for example, by changing the number-average fiber length of the natural cellulose used as a raw material, the oxidation treatment conditions, or the conditions for dispersing (defibrating) the carboxylated cellulose nanofibers after the oxidation treatment. Specifically, the number average fiber length can be shortened by extending the time of the dispersion treatment (fibrillation treatment).

[0028] The average degree of polymerization of the copper-containing oxidized cellulose nanofibers (the average number of glucose units contained in a cellulose molecule) is preferably 100 or more, more preferably 300 or more, even more preferably 500 or more, and preferably 2000 or less, more preferably 1500 or less, even more preferably 1000 or less, and even more preferably 700 or less. When the average degree of polymerization of the copper-containing oxidized cellulose nanofibers is equal to or greater than the above-mentioned lower limit, sufficiently high mechanical strength can be imparted to molded articles such as films. On the other hand, when the average degree of polymerization of the copper-containing oxidized cellulose nanofibers is equal to or less than the above-mentioned upper limit, the dispersibility of the copper-containing oxidized cellulose nanofibers is ensured, improving the transparency of the resulting molded article while imparting even better near-infrared absorption ability to the near-infrared absorbing material. The average degree of polymerization of the copper-containing oxidized cellulose nanofibers can be adjusted by changing the average degree of polymerization of the natural cellulose used as a raw material, the oxidation treatment conditions, the conditions for dispersing (defibrating) the carboxylated cellulose nanofibers after the oxidation treatment, the conditions for dispersing (defibrating) the carboxylated cellulose nanofibers after the copper substitution step, etc.

[0029] The copper-containing oxidized cellulose nanofibers described above can be produced, for example, according to the methods described in JP-A-2016-141777 and JP-A-2019-199622. The copper-containing oxidized cellulose nanofibers produced as described above are usually obtained as a dispersion in a dispersion medium such as water.

[0030] <Properties> The near-infrared absorbing material of the present invention preferably has a light transmittance of 70% or more, more preferably 80% or more, and even more preferably 85% or more for light with a wavelength of 550 nm. If the light transmittance of light with a wavelength of 550 nm is equal to or greater than the above-mentioned lower limit, the visual transparency of the near-infrared absorbing material can be improved. On the other hand, the light transmittance of light with a wavelength of 550 nm in the near-infrared absorbing material is, for example, 99% or less, or may be 95% or less, or may be 90% or less.

[0031] The near-infrared absorbing material of the present invention preferably has a light transmittance of 83% or less, more preferably 80% or less, even more preferably 66.3% or less, and particularly preferably 60% or less, for light with a wavelength of 800 nm. If the value is equal to or less than the upper limit, the near-infrared absorbing material has excellent near-infrared absorption ability. On the other hand, the light transmittance of the near-infrared absorbing material for light with a wavelength of 800 nm is, for example, 10% or more, or may be 20% or more, or may be 30% or more.

[0032] The near-infrared absorbing material of the present invention is difficult to see with the naked eye, but it absorbs infrared rays well and is visible in near-infrared images, and therefore can be used for various purposes such as determining the authenticity of articles.

[0033] In the near-infrared absorbing material of the present invention, the value obtained by subtracting the light transmittance of light with a wavelength of 800 nm from the light transmittance of light with a wavelength of 550 nm is preferably 5% or more, more preferably 10% or more, and even more preferably 20% or more. If this value is equal to or greater than the lower limit, the light absorption selectivity of the near-infrared absorbing material can be improved. On the other hand, the value in the near-infrared absorbing material is, for example, 80% or less, or may be 60% or less, or may be 50% or less.

[0034] The light transmittance of the near-infrared absorbing material for light with a wavelength of 800 nm can be measured by the method described in the examples.

[0035] (Ink) The ink of the present invention contains a near-infrared absorbing material and a solvent (dispersion medium), and may optionally further contain at least one component selected from the group consisting of a colorant, a pigment dispersant, and other components.

[0036] <Near-infrared absorbing material> The near-infrared absorbing material of the present invention may be the near-infrared absorbing material of the present invention described above. The concentration of the near-infrared absorbing material (copper-containing oxidized cellulose nanofiber) in the ink is preferably 0.20% by mass or more, more preferably 0.25% by mass or more, even more preferably 0.28% by mass or more, and preferably 0.40% by mass or less, more preferably 0.35% by mass or less. When the concentration of the near-infrared absorbing material in the ink is within the above range, the dispersibility of the copper-containing oxidized cellulose nanofiber can be ensured. Furthermore, the change in visibility of the display due to the incorporation of the near-infrared absorbing material (i.e., the difference in color between the portion containing the near-infrared absorbing material of the present invention and the portion not containing the near-infrared absorbing material of the present invention) can be suppressed. For example, when the ink of the present invention is a transparent ink containing no colorant, the coloring caused by the incorporation of the near-infrared absorbing material can be suppressed, thereby reducing the visibility of the resulting display with the naked eye. Therefore, the ink of the present invention is even more suitable for use as a security ink.

[0037] <Solvent> Examples of the solvent contained in the ink of the present invention include water and organic solvents. The solvent is a component that dissolves or disperses the solid components (near-infrared absorbing material and other components) contained in the ink of the present invention.

[0038] The organic solvent is not particularly limited, and examples thereof include alcohols such as ethanol, propanol, isopropyl alcohol, ethylene glycol, and propylene glycol; hydrocarbons such as hexane, cyclohexane, toluene, and xylene; halogenated hydrocarbons such as methylene chloride and chloroform; ethers such as dimethyl ether, diethyl ether, dioxane, and tetrahydrofuran; esters such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, and ethyl butyrate; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, cyclopentanone, and N-methyl-2-pyrrolidone; and cellosolves such as methyl cellosolve, ethyl cellosolve, and butyl cellosolve. Examples of suitable organic solvents include: sorbents; carbitols such as methyl carbitol, ethyl carbitol, and butyl carbitol; propylene glycol monoalkyl ethers such as propylene glycol monomethyl ether, propylene glycol monoethyl ether, and propylene glycol mono-n-butyl ether; glycol ether esters such as ethylene glycol monomethyl ether acetate, propylene glycol monomethyl ether acetate, and ethyl diglycol acetate (EDGAC); amides such as N,N-dimethylformamide and N,N-dimethylacetamide; sulfoxides such as dimethyl sulfoxide; and nitriles such as acetonitrile and benzonitrile. Among these, preferred organic solvents are alcohols, with ethanol being more preferred. The solvent preferably contains water, and water is more preferred, as this allows for favorable dispersion of copper-containing oxidized cellulose nanofibers in the ink. When water is used as the solvent, the water contained in the ink may be, for example, water derived from an aqueous dispersion obtained by preparing the copper-containing oxidized cellulose nanofibers.

[0039] The solvent may be used alone or in any combination of two or more in any ratio. When a mixture of water and an organic solvent is used as the solvent, the mass ratio of water to the organic solvent (water / organic solvent) is preferably more than 1 / 1, more preferably 2 / 1 or more, and more preferably 4 / 1 or less, from the viewpoint of ensuring the dispersibility of the near-infrared absorbing material (copper-containing oxidized cellulose nanofibers).

[0040] <Colorant> The colorant optionally contained in the ink of the present invention is not particularly limited, and conventionally known pigments and dyes used as colorants (coloring materials) for various inks can be used. Both organic and inorganic pigments can be used as pigments. Examples of organic pigments include azo pigments, polycyclic pigments, dye chelates, nitro pigments, nitroso pigments, and aniline black. Examples of inorganic pigments include titanium oxide, iron oxide, calcium carbonate, barium sulfate, aluminum hydroxide, barium yellow, cadmium red, chrome yellow, and carbon black. These colorants can be used alone or in combination of two or more. Examples of dyes include azo dyes, benzoquinone dyes, naphthoquinone dyes, anthraquinone dyes, cyanine dyes, squarylium dyes, croconium dyes, merocyanine dyes, stilbene dyes, diarylmethane dyes, triarylmethane dyes, fluoran dyes, spiropyran dyes, phthalocyanine dyes, indigo dyes such as indigoid, fulgide dyes, nickel complex dyes, and azulene dyes.

[0041] The content of the colorant is not particularly limited and can be 0.05% by mass or more and 20% by mass or less relative to the total amount of the ink. If the content of the colorant is within the above range, a good balance between the dispersion stability and coloring power of the colorant can be achieved.

[0042] <Pigment Dispersant> The pigment dispersant optionally contained in the ink of the present invention is not particularly limited, and for example, a conventionally known surfactant used as a pigment dispersant for various inks can be used. The pigment dispersant is a substance that has the function of improving the dispersibility of the pigment in the ink by adhering to part of the pigment surface.

[0043] Examples of surfactants include cationic, anionic, nonionic, amphoteric, silicone, and fluorine-based surfactants, and for example, those described in Japanese Patent No. 7366311 can be used.

[0044] The content of the pigment dispersant is not particularly limited, and can be set to 0.1% by mass or more and 15% by mass or less with respect to the total amount of the ink.

[0045] <Other Components> Examples of other components that may be optionally contained in the ink of the present invention include a foam inhibitor (antifoaming agent), a pH adjuster, an antiseptic / antifungal agent, a chelating agent, a rust inhibitor, an antioxidant, an ultraviolet absorber, an oxygen absorber, and a light stabilizer. For example, those described in JP-A-2023-067770 can be used.

[0046] <Ink Properties> The surface tension of the ink is preferably 20 mN / m or more, more preferably 30 mN / m or more, and preferably 40 mN / m or less, more preferably 35 mN / m or less. If the surface tension of the ink is equal to or greater than the above lower limit, dripping can be suppressed. If the surface tension of the ink is equal to or less than the above upper limit, the coatability of the ink on a substrate can be improved. In the present invention, the surface tension can be measured by the method described in the examples.

[0047] <Uses> The ink of the present invention can be used as an ink for inkjet printers, an ink for thermal printers, or an ink for letterpress printing, an ink for offset printing, an ink for flexographic printing, an ink for gravure printing, an ink for silk printing, an ink for writing instruments, etc. In particular, the ink of the present invention has excellent near-infrared absorption ability and can therefore be suitably used as a security ink that uses a near-infrared image to prevent counterfeiting.

[0048] <Production Method> The ink of the present invention is not particularly limited, and can be produced, for example, by mixing the above-mentioned components.

[0049] (Paint) The paint of the present invention comprises a near-infrared absorbing material, a resin, and a solvent, and may optionally further comprise at least one component selected from the group consisting of a colorant and other components.

[0050] <Near-infrared absorbing material> The near-infrared absorbing material of the present invention can be the near-infrared absorbing material contained in the coating material of the present invention. The concentration of the near-infrared absorbing material (copper-containing oxidized cellulose nanofiber) in the coating material is preferably 0.20% by mass or more, more preferably 0.25% by mass or more, and preferably 0.40% by mass or less, more preferably 0.35% by mass or less. When the concentration of the near-infrared absorbing material in the coating material is within the above range, the dispersibility of the copper-containing oxidized cellulose nanofiber can be ensured. Furthermore, changes in the visibility of the display object due to the incorporation of the near-infrared absorbing material (i.e., the difference in color between the portion containing the near-infrared absorbing material of the present invention and the portion not containing the near-infrared absorbing material of the present invention) can be suppressed. For example, when the coating material of the present invention is used as a transparent coating material without a colorant, coloration due to the incorporation of the near-infrared absorbing material can be suppressed, thereby reducing the visibility of the resulting display object (coated object) to the naked eye. Therefore, the coating material of the present invention can be more suitably used as a security coating material.

[0051] <Resin> The resin blended into the coating material of the present invention is a component that forms a coating film together with the near-infrared absorbing material when the coating material is dried, and any conventionally known resin that can be used in coating materials can be used. The resin is not particularly limited, and examples include acrylic resin, polyester resin, alkyd resin, urethane resin, epoxy resin, vinyl chloride resin, vinyl chloride / vinyl acetate copolymer resin, polyamide resin, melamine resin, olefin resin, and cyclic olefin resin. One type of resin may be used alone, or two or more types may be used in combination. The content of the resin (solid content) in the coating material can be 5 to 70 parts by mass per 100 parts by mass of the solid content of the coating material.

[0052] <Solvent> The solvent contained in the coating material of the present invention is not particularly limited, and the solvents described above in the section "Ink of the present invention" can be used.

[0053] <Colorant> The colorant optionally contained in the coating material of the present invention is not particularly limited, and the colorants described above in the section "Ink of the present invention" can be used. The content of the colorant is not particularly limited, and can be 1% by mass or more and 20% by mass or less with respect to the total amount of the coating material. If the content of the colorant is within the above range, a good balance can be achieved between the dispersion stability and coloring power of the colorant.

[0054] <Other Components> Examples of other components that may be optionally contained in the coating material of the present invention include drying accelerators, curing agents, curing accelerators, thickeners, anti-settling agents, anti-cracking agents, anti-drooping agents, flow-extending agents, anti-foaming agents, ultraviolet absorbers, light stabilizers, etc. The content of other components is not particularly limited, and can be 1% by mass or more and 50% by mass or less relative to the total amount of the coating material.

[0055] <Production Method> The coating material of the present invention is not particularly limited, and can be produced, for example, by mixing the above-mentioned components.

[0056] (Molded product) The molded product of the present invention is obtained by molding the near-infrared absorbing material of the present invention described above into any shape. Therefore, the molded product of the present invention has excellent near-infrared absorbing ability. The molded product of the present invention is not particularly limited, but is preferably a film.

[0057] <Properties> The preferred attributes (light transmittance, etc.) that the molded product of the present invention may have are the same as those described above in the section "Near-infrared absorbing material."

[0058] Furthermore, when the molded product of the present invention is a film, the thickness of the film is preferably 10 μm or more, more preferably 20 μm or more, and even more preferably 30 μm or more, and is preferably 100 μm or less, more preferably 70 μm or less, and even more preferably 55 μm or less. If the thickness of the film is equal to or greater than the above lower limit, the near-infrared absorption properties of the film can be improved. On the other hand, if the thickness of the film is equal to or less than the above upper limit, the transparency of the film can be improved. In this specification, the "thickness" of the film can be measured according to the method described in the examples.

[0059] <Manufacturing Method> The shaped product of the present invention can be obtained, for example, by a method including a dispersion step in which copper-containing oxidized cellulose nanofibers, which have a number average fiber diameter of 100 nm or less and contain copper in the form of a salt, are dispersed in a dispersion medium to obtain a dispersion, and a drying step in which the dispersion is dried to obtain a shaped product. The above method may optionally further include steps other than the dispersion step and the drying step (hereinafter, sometimes referred to as "other steps"). Examples of other steps include a degassing step in which dissolved gas is removed from the dispersion obtained in the dispersion step, and a coating step in which the dispersion is coated prior to the drying step to obtain a coating film.

[0060] <Dispersion Step> In the dispersion step, copper-containing oxidized cellulose nanofibers having a number average fiber diameter of 100 nm or less and containing copper in the form of a salt are dispersed in a dispersion medium to obtain a dispersion liquid.

[0061] The copper-containing oxidized cellulose nanofibers containing copper in the form of a salt can be those described above. In the preparation of the copper-containing oxidized cellulose nanofibers, if the copper-containing oxidized cellulose nanofibers are obtained in the form of a dispersion, the dispersion can be used as is in the production of the molded article of the present invention.

[0062] Here, the solids concentration of the copper-containing oxidized cellulose nanofibers in the dispersion is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, preferably 2.0% by mass or less, more preferably 1.0% by mass or less, and even more preferably 0.5% by mass or less. If the solids concentration of the copper-containing oxidized cellulose nanofibers in the dispersion is equal to or greater than the above-mentioned lower limit, a molded product can be efficiently obtained. On the other hand, if the solids concentration of the copper-containing oxidized cellulose nanofibers in the dispersion is equal to or less than the above-mentioned upper limit, the dispersion state of the dispersion is improved, and dimensional unevenness of the obtained molded product can be effectively suppressed. The solids concentration of the copper-containing oxidized cellulose nanofibers in the dispersion can be adjusted by the amount of dispersion medium used, as described below, or by concentrating a dispersion with a low solids concentration using an evaporator, etc.

[0063] The dispersion medium used in the dispersion is not particularly limited as long as it can disperse copper-containing oxidized cellulose nanofibers, and for example, the solvents (dispersion mediums) described above in the section "Ink of the Invention" can be used, but water is preferred because it allows copper-containing oxidized cellulose nanofibers to be well dispersed in the dispersion and improves the productivity of molded products. Here, when water is used as the dispersion medium, the water contained in the dispersion may be, for example, water derived from an aqueous dispersion obtained when copper-containing oxidized cellulose nanofibers are prepared.

[0064] <Drying Step> In the drying step, the dispersion obtained in the dispersing step is dried to obtain a molded product. The molded product obtained in this step may be the molded product of the present invention. The drying of the dispersion may be carried out, for example, after the dispersion is placed in a container having a predetermined shape according to the shape of the molded product to be obtained.

[0065] The drying temperature of the dispersion is not particularly limited as long as it is a temperature at which the dispersion medium evaporates, but is preferably 20° C. or higher, more preferably 30° C. or higher, and even more preferably 35° C. or higher. On the other hand, the drying temperature of the dispersion is preferably 60° C. or lower, more preferably 50° C. or lower, and even more preferably 45° C. or lower, in order to prevent rapid drying, effectively prevent discoloration of the resulting molded product, and improve smoothness.

[0066] Here, the relative humidity during drying is preferably 50% or more, more preferably 60% or more, even more preferably 70% or more, and preferably 95% or less, more preferably 90% or less, and even more preferably 85% or less. If the relative humidity during drying is above the above lower limit, the smoothness of the resulting molded product can be improved. On the other hand, if the relative humidity during drying is below the above upper limit, the drying speed can be increased, and the productivity of the molded product can be improved.

[0067] The drying time is not particularly limited, but is usually from 1 day to 5 days.

[0068] The drying may be performed using a dryer that can simultaneously control the temperature and humidity, such as a thermo-hygrostat, although there is no particular limitation.

[0069] (Display) The display of the present invention is formed using the ink, paint, or molded article of the present invention described above. The display of the present invention contains the near-infrared absorbing material of the present invention described above, and therefore has excellent near-infrared absorbing ability. Therefore, the display of the present invention can be suitably used for articles such as security prints. Here, preferred attributes (light transmittance, etc.) that the display of the present invention can have are the same as those described above in the section "Near-infrared absorbing material."

[0070] <Displays Using Ink> When using the ink of the present invention described above, the display of the present invention is a dried product of the ink, and can be obtained, for example, by applying the ink to a desired substrate such as paper or film by any method and then drying the ink. The display is not particularly limited and can be, for example, numbers, letters, pictures, or a combination thereof. The substrate is not particularly limited and can be, for example, a conventionally known recording medium such as paper or plastic film. The display is not particularly limited and can be formed using a printing method such as offset printing, relief printing, flexographic printing, gravure printing, or inkjet printing, or using a writing instrument such as a pen or brush. The thickness of the display using the ink of the present invention is not particularly limited and can be 0.01 μm or more and 100 μm or less.

[0071] <Display using paint> When the above-described paint of the present invention is used, the display of the present invention is a dried product of the paint, and can be obtained, for example, as a coating film (coating film) formed on a substrate by applying the paint to a substrate by any method and then drying the paint. The display is not particularly limited and can be, for example, numbers, letters, pictures, or a combination thereof. The substrate is not particularly limited and can be, for example, any conventionally known substrate to which a paint can be applied, such as paper, wood board, gold plate, plastic plate, or glass. The display (coating film) is not particularly limited and can be formed using any method, such as brush painting. The thickness of the coating film is not particularly limited and can be 0.01 μm or more and 100 μm or less.

[0072] <Display using molded product> When the molded product of the present invention described above is used, it may be the molded product itself, or a molded product further processed into a predetermined shape. For example, when a film is produced as the molded product, the film can be processed into a desired shape (numbers, letters, pictures, or a combination thereof, etc.) by cutting or the like, and then optionally attached to a substrate, thereby obtaining a display. The substrate is not particularly limited, and the above-mentioned "display using ink" and "display using paint" can be used. The thickness of the display using the molded product of the present invention is not particularly limited, and can be 0.1 μm or more and 100 μm or less.

[0073] (Article) The article of the present invention comprises the display material of the present invention described above. Specifically, when the display material of the present invention described above has a substrate, the display material corresponds to the article of the present invention. Here, the substrate is the same as that described above in the section "Display material of the present invention." Since the article of the present invention contains the near-infrared absorbing material of the present invention described above, it has excellent near-infrared absorbing ability. Therefore, the article of the present invention can be suitably used as a security article, etc. Here, preferred attributes (light transmittance, etc.) that the display material provided in the article of the present invention may have are the same as those described above in the section "Near-infrared absorbing material."

[0074] (Method for determining the authenticity of an article) The method for determining the authenticity of an article of the present invention includes a step of applying a visually indistinguishable mark to an authentic article using the near-infrared absorbing material of the present invention described above (mark application step), a step of acquiring a near-infrared image of the article to be inspected (near-infrared image acquisition step), and a step of determining that an article in which the mark is displayed in the near-infrared image is an authentic article and that an article in which the mark is not displayed in the near-infrared image is a counterfeit article (determination step). According to the method for determining the authenticity of an article of the present invention, it is possible to easily determine the authenticity of an article.

[0075] <Marking Step> In the marking step, a visually indistinguishable mark is formed on at least a portion of an authentic article using the near-infrared absorbing material of the present invention. Here, the visually indistinguishable mark can be a transparent and indistinguishable mark (invisible to the naked eye) or a mark that cannot be visually distinguished (distinguished) from indicia provided on the article (such as numbers, letters, pictures, or combinations thereof, composed of materials that do not contain the near-infrared absorbing material of the present invention). The visually indistinguishable mark is not particularly limited and can be numbers, letters, pictures, or combinations thereof. Furthermore, the article is not particularly limited and examples include printed materials such as passports, securities, identification cards, cards, and travel tickets. In particular, a transparent and indistinguishable mark can be applied without impairing the design of the article.

[0076] A visually indistinguishable mark can be formed, for example, using the ink, paint, or molded product of the present invention described above. When the ink of the present invention is used, the mark can be formed by applying the ink of the present invention in a predetermined pattern (e.g., numbers, letters, pictures, or a combination thereof) to at least a portion of an article by an inkjet method or the like, followed by drying. When the paint of the present invention is used, the mark can be formed by applying the paint of the present invention in a predetermined pattern (e.g., numbers, letters, pictures, or a combination thereof) to at least a portion of an article by any method, followed by drying. Furthermore, when the molded product of the present invention is used, the mark can be obtained by forming the molded product of the present invention itself described above, or a molded product processed into a predetermined pattern (e.g., numbers, letters, pictures, or a combination thereof), on at least a portion of the article.

[0077] <Near-infrared image acquisition step> In the near-infrared image acquisition step, a near-infrared image of the object to be inspected is acquired. The near-infrared image is not particularly limited and can be obtained by photographing the object using a near-infrared camera or the like. From the viewpoint of easily determining the presence or absence of the mark in the near-infrared image in the determination step, the image may be photographed while further irradiating the image with light containing near-infrared rays. Furthermore, the photographing may be performed in the dark or under fluorescent light, but from the viewpoint of easily determining the presence or absence of the mark in the determination step, it is preferable to perform the photographing in the dark.

[0078] <Determination step> In the determination step, an article that displays the above mark in the near-infrared image obtained in the near-infrared image acquisition step is determined to be a genuine article, and an article that does not display the above mark in the near-infrared image is determined to be a counterfeit article. Such determination can be made visually.

[0079] In this way, the method for determining the authenticity of an article of the present invention can suitably determine the authenticity of articles such as security printed materials (anti-counterfeit printed materials) such as passports, securities, identification cards, cards, and travel tickets.

[0080] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" representing amounts are based on mass unless otherwise specified. Furthermore, in a polymer produced by copolymerizing multiple types of monomers, the proportion of a structural unit formed by polymerizing a certain monomer in the polymer usually coincides with the ratio (feed ratio) of the certain monomer to all monomers used in the polymerization of the polymer, unless otherwise specified.

[0081] <Surface Tension of Dispersion> The surface tension of the dispersions prepared in Examples 1-1 to 1-8 and Comparative Examples 1-1 to 1-4 was calculated by the Young-Laplace method using a portable contact angle analyzer PCA-11 manufactured by Kyowa Interface Science Co., Ltd., after 5 minutes. <Dispersibility of Dispersion> The dispersibility of the dispersions prepared in Examples 1-1 to 1-8 and Comparative Examples 1-1 to 1-4 was measured by the following method and evaluated according to the following criteria. [Method for Measuring Dispersibility] After shaking the dispersion well by hand, the transparency of the dispersion was confirmed visually. A: The dispersion is transparent. B: The dispersion is slightly cloudy. C: Precipitates are present in the dispersion. <Film Thickness> The thickness of the films produced in the examples and comparative examples was measured using a Digimatic Thickness Gauge (ID-C112XBS: manufactured by Mitutoyo Corporation). <Naked Eye Observation of Letters> The letters produced in Examples 1-1 to 1-8 and Comparative Examples 1-1 to 1-4 were observed with the naked eye and evaluated based on the following criteria. A: The letters were almost invisible, both as they were and when the sample was placed on white copy paper. B: The letters were invisible as they were, but were faintly visible when the sample was placed on white copy paper. C: The letters were clearly visible. <Near-infrared Absorption Ability (Near-infrared Transmittance)> The transmittance of light with a wavelength of 800 nm for the letters and films produced in the Examples and Comparative Examples was measured using an ultraviolet-visible-near-infrared spectrophotometer (V-570: manufactured by JASCO Corporation). The lower the transmittance for light with a wavelength of 800 nm, the better the near-infrared absorption ability. <Near-infrared Images of Letters> Near-infrared images of the letters in Examples 1-1 to 1-8 and Comparative Examples 1-1 to 1-4 were taken using the imaging device 1 in FIG. 1 or the imaging device 2 in FIG. 2, and evaluated based on the following criteria. A: Characters are clearly recognizable B: Characters are faint but recognizable C: Characters are not recognizable <Near-infrared images of film> Near-infrared images of the films of Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-9 were taken using the imaging device 1 of Figure 1 and evaluated according to the following criteria: A: Black and clearly recognizable B: Lightly black and recognizable C: Transparent and the same color as the back

[0082] Example 1-1 Preparation of an aqueous dispersion of sodium-containing TEMPO-oxidized cellulose nanofibers 1 g (dry weight equivalent) of bleached softwood kraft pulp, 5 mmol of sodium hypochlorite, 0.1 g (1 mmol) of sodium bromide, and 0.016 g (1 mmol) of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) were dispersed in 100 mL of water, gently stirred at room temperature for 4 hours, and washed with distilled water to obtain TEMPO-catalyzed oxidized pulp (oxidized cellulose). The carboxyl group content of the obtained TEMPO-catalyzed oxidized pulp was 1.4 mmol / g. Distilled water was then added to the undried TEMPO-catalyzed oxidized pulp to prepare an aqueous dispersion with a solids concentration of 0.1%. The aqueous dispersion was then defibrated using a homogenizer (Microtec Nichion, Hiscotron) at 7.5 x 1000 rpm for 2 minutes, and then using an ultrasonic homogenizer (Nissei, Ultrasonic Generator) with V-LEVEL 4 and TIP26D for 4 minutes while cooling the container with ice. This resulted in an aqueous dispersion containing TEMPO carboxylated cellulose nanofibers as TEMPO-oxidized cellulose nanofibers. The undefibrated components were then removed from the aqueous dispersion by centrifugation (12000 G (120 x 100 rpm / g), 10 minutes, 12°C) using a centrifuge (SAKUMA, M201-1VD, angle rotor 50F-8AL) to obtain a clear aqueous dispersion of TEMPO carboxylated cellulose nanofibers with a concentration of 0.1% by mass. The TEMPO-carboxylated cellulose nanofibers contained sodium derived from the co-oxidant in the form of a salt. That is, the above procedure yielded an aqueous dispersion of sodium-containing TEMPO-oxidized cellulose nanofibers (hereinafter sometimes simply referred to as "TOCN-Na").

[0083] <Preparation of Hydrogen-Substituted TEMPO-Oxidized Cellulose Nanofiber Aqueous Dispersion> To 100 mL of the TOCN-Na aqueous dispersion, 1 mL of 1 M hydrochloric acid was added under stirring to adjust the pH to 1. Stirring was then continued for 60 minutes. The TEMPO-carboxylated cellulose nanofibers, which had gelled due to the addition of hydrochloric acid, were recovered by centrifugation (12,000 G), and the recovered TEMPO-carboxylated cellulose nanofibers were washed sequentially with 1 M hydrochloric acid and a large amount of distilled water. Next, 100 mL of distilled water was added to obtain an aqueous dispersion of hydrogen-substituted TEMPO-carboxylated cellulose nanofibers (hereinafter sometimes simply referred to as "TOCN-H") at a concentration of 0.1% by mass, in which hydrogen-substituted TEMPO-carboxylated cellulose nanofibers were dispersed. When the carboxyl groups on the surface of TOCN-H were measured by FT-IR (manufactured by JASCO Corporation, FT / IR-6100) according to Biomacromolecules (2011, Vol. 12, pp. 518-522), 90% or more of the groups were substituted with carboxylic acid groups.

[0084] <Preparation of Copper-Containing TEMPO-Oxidized Cellulose Nanofiber Aqueous Dispersion> 50 g of the 0.1% by mass TOCN-H aqueous dispersion was stirred, and 18 g of a 0.1% by mass copper (II) acetate aqueous solution was added thereto. Stirring was continued for 3 hours at room temperature. The carboxylated cellulose nanofibers gelled by the addition of the copper (II) acetate aqueous solution were then recovered by centrifugation (12,000 G (120 × 100 rpm / g), 10 minutes, 12°C) using a centrifuge (SAKUMA, M201-1VD, angle rotor 50F-8AL). The recovered cellulose nanofibers were then washed with a 0.1% by mass copper (II) acetate aqueous solution and then with a large amount of distilled water. Subsequently, 50 mL of distilled water was added, and the copper ion-substituted TEMPO-carboxylated cellulose nanofibers were dispersed by ultrasonic treatment (2 minutes) using an ultrasonic homogenizer (Nissei Ultrasonic Generator) with V-LEVEL 4 and TIP26D while cooling the container with ice. Subsequently, undefibrated components were removed from the aqueous dispersion of copper ion-substituted TEMPO-carboxylated cellulose nanofibers by centrifugation (12,000 G (120 x 100 rpm / g), 10 minutes, 12°C) using a centrifuge (SAKUMA M201-1VD, angle rotor 50F-8AL). This resulted in an aqueous dispersion of copper-containing TEMPO-oxidized cellulose nanofibers (hereinafter sometimes simply referred to as "TOCN-Cu") with a solids concentration of 0.1% by mass. The aqueous dispersion was then concentrated using an evaporator to obtain a TOCN-Cu aqueous dispersion with a solid content of 0.34% by mass. The surface tension and dispersibility of the aqueous dispersion as an ink were evaluated. The results are shown in Table 1.

[0085] The aqueous dispersion was placed in a dropper, and letters were written on a PET film (G2R manufactured by Toyobo Co., Ltd.: double-sided easy-adhesion treatment). This PET film was dried in an oven at 80°C for 1 hour, and a PET film with letters composed of the dried aqueous dispersion was obtained as a sample. These letters were then observed with the naked eye and evaluated for near-infrared absorption. Near-infrared images of the displayed object were also captured and evaluated using an imaging device 1 having the following configuration, as shown schematically in Figure 1. The results are shown in Table 1. In Figure 1, reference numeral 10 denotes the camera, reference numeral 11 denotes the lens, reference numeral 20a denotes the LED surface light, and reference numeral 30 denotes the sample. <Imaging device 1> LED surface light: TMN150x180-22IR850D-4 manufactured by ITEC Systems Co., Ltd. Wavelength range (750 nm to 900 nm) Peak at 850 nm Illuminance 15.23 mW / cm ２ Camera: Basler a2A1920-160umBAS Wavelength range: 400nm to 1000nm Peak at 600nm Lens: Basler EMVL-MP420

[0086] (Example 1-2) The same operations as in Example 1-1 were performed, except that near-infrared images were captured using an imaging device 2 having the following configuration, as outlined in FIG. 2, instead of the imaging device 1. The results are shown in Table 1. In FIG. 2, reference numeral 10 denotes a camera, reference numeral 11 denotes a lens, reference numeral 20b denotes an LED bar light, and reference numeral 30 denotes a sample. <Imaging device 2> LED bar light: TLWA325x41-221R850D-4 manufactured by ITEC Systems Co., Ltd. Wavelength range (750 nm to 900 nm): Peak at 850 nm Illuminance: 23.16 mW / cm ２ Two cameras: Basler a2A1920-160umBAS, wavelength range 400nm to 1000nm, peak at 600nm; Lens: Basler EMVL-MP420

[0087] Example 1-3 The same procedure as in Example 1-1 was carried out, except that 0.25 g of ethanol was added to 1.25 g of the TOCN-Cu aqueous dispersion of Example 1 and the mixture was stirred to prepare a TOCN-Cu dispersion with a concentration of 0.28 mass % in a mixed solvent of water / ethanol = 4 / 1 (mass ratio). The results are shown in Table 1.

[0088] Example 1-4 The same operations as in Example 1-3 were carried out, except that the near-infrared image was captured using the imaging device 2 instead of the imaging device 1. The results are shown in Table 1.

[0089] Example 1-5 The same procedure as in Example 1-1 was carried out, except that 0.73 g of ethanol was added to 1.25 g of the TOCN-Cu aqueous dispersion of Example 1-1 and the mixture was stirred to prepare a TOCN-Cu dispersion with a concentration of 0.24 mass % in a mixed solvent of water / ethanol = 2 / 1 (mass ratio). The results are shown in Table 1.

[0090] Example 1-6 The same operations as in Example 1-5 were carried out, except that the near-infrared image was captured using the imaging device 2 instead of the imaging device 1. The results are shown in Table 1.

[0091] Example 1-7 The same procedure as in Example 1-1 was carried out, except that white synthetic paper (SDI80 manufactured by Yupo Co., Ltd.) was used instead of the PET film. The results are shown in Table 1.

[0092] Example 1-8 The same operations as in Example 1-7 were carried out, except that the near-infrared image was captured using the imaging device 2 instead of the imaging device 1. The results are shown in Table 1.

[0093] Comparative Example 1-1 The same procedure as in Example 1 was carried out, except that a TOCN-Na dispersion prepared as described below was used instead of the TOCN-Cu aqueous dispersion. The results are shown in Table 1. <Preparation of TOCN-Na Dispersion> The 0.1% by mass TOCN-Na aqueous dispersion prepared in Example 1-1 was concentrated using an evaporator to a solids concentration of 0.34%, yielding a TOCN-Na aqueous dispersion with a solids concentration of 0.34%. 0.25 g of ethanol was added to 1.25 g of this TOCN-Na aqueous dispersion and the mixture was stirred to obtain a TOCN-Na dispersion with a concentration of 0.28% by mass in which TOCN-Na was dispersed in a mixed solvent of water / ethanol = 4 / 1 (mass ratio).

[0094] Comparative Example 1-2 The same operations as in Comparative Example 1-1 were carried out, except that the near-infrared image was captured using imaging device 2 instead of imaging device 1. The results are shown in Table 1.

[0095] Comparative Example 1-3 The same procedure as in Comparative Example 1-1 was carried out, except that white synthetic paper (SDI80 manufactured by Yupo Co., Ltd.) was used instead of the PET film. The results are shown in Table 1.

[0096] Comparative Example 1-4 The same operations as in Comparative Example 1-3 were carried out, except that the near-infrared image was captured using imaging device 2 instead of imaging device 1. The results are shown in Table 1. In Table 1, CNF means cellulose nanofiber.

[0097] The results shown in Table 1 show that in Examples 1-1 to 1-6, which used displays (letters) prepared by drying a dispersion prepared using copper-containing oxidized cellulose nanofibers having a number-average fiber diameter of 100 nm or less and containing copper in the form of a salt, the characters had excellent near-infrared absorption ability and were clearly recognizable in near-infrared images. The reason for the significantly low near-infrared transmittance in Examples 1-7 to 1-8 and Comparative Examples 1-3 to 1-4 is presumably due to the high near-infrared absorptivity of the synthetic paper used as the substrate.

[0098] Example 2-1 Preparation of Copper-Containing TEMPO-Oxidized Cellulose Nanofiber Aqueous Dispersion A TOCN-Cu aqueous dispersion with a solids concentration of 0.34% by mass was obtained in the same manner as in Example 1-1. Film Production 20 g of the TOCN-Cu aqueous dispersion obtained above was placed in a 60 mm untreated tissue culture dish manufactured by AGC Technoglass. This untreated tissue culture dish was then placed in a thermo-hygrostat chamber at a temperature of 40°C and a relative humidity of 80% for 4 days to obtain a transparent film with a smooth surface. The obtained film was used as a sample for various measurements and evaluations. The results are shown in Table 2.

[0099] Examples 2-2 to 2-4 The same procedure as in Example 2-1 was carried out except that the film thickness was changed as shown in Table 2. The results are shown in Table 2.

[0100] Comparative Example 2-1 The same procedure as in Example 2-1 was carried out, except that a 23 μm thick film was produced using a TOCN-Na aqueous dispersion prepared as follows instead of the TOCN-Cu aqueous dispersion. The results are shown in Table 2. <Preparation of TOCN-Na aqueous dispersion> The 0.1 mass % TOCN-Na aqueous dispersion prepared in Example 1-1 was concentrated using an evaporator to a solids concentration of 0.34%, yielding a TOCN-Na aqueous dispersion with a solids concentration of 0.34%.

[0101] Comparative Examples 2-1 to 2-3 The same procedure as in Comparative Example 2-1 was carried out except that the film thickness was changed as shown in Table 2.

[0102] (Comparative Example 2-4) The same procedure as in Example 2-1 was carried out, except that an aluminum-containing TEMPO-oxidized cellulose nanofiber aqueous dispersion prepared as follows was used instead of the TOCN-Na aqueous dispersion. The results are shown in Table 2. <Preparation of aluminum-containing TEMPO-oxidized cellulose nanofiber aqueous dispersion> The same procedure as in Example 1-1 was carried out, except that 50 g of a 0.1 mass % TOCN-H aqueous dispersion prepared in the same manner as in Example 1-1 was stirred, and 26 g of a 0.1% aluminum chloride (III) hexahydrate aqueous solution was added thereto, and the mixture was stirred at room temperature for 3 hours.

[0103] (Comparative Example 2-5) The same procedure as in Example 2-1 was carried out, except that a lithium-containing TEMPO-oxidized cellulose nanofiber aqueous dispersion prepared as follows was used instead of the TOCN-Na aqueous dispersion. The results are shown in Table 2. <Preparation of lithium-containing TEMPO-oxidized cellulose nanofiber aqueous dispersion> The same procedure as in Example 1-1 was carried out, except that 50 g of a 0.1% by mass TOCN-H aqueous dispersion prepared in the same manner as in Example 1-1 was stirred, to which 4 g of 0.1% lithium acetate was added, and the mixture was stirred at room temperature for 3 hours.

[0104] (Comparative Example 2-6) The same procedure as in Example 2-1 was carried out, except that a zinc-containing TEMPO-oxidized cellulose nanofiber aqueous dispersion prepared as follows was used instead of the TOCN-Na aqueous dispersion. The results are shown in Table 2. <Preparation of zinc-containing TEMPO-oxidized cellulose nanofiber aqueous dispersion> 50 g of a 0.1% by mass TOCN-H aqueous dispersion prepared in the same manner as in Example 1-1 was stirred, to which 20 g of zinc acetate at a concentration of 0.1% was added, and the mixture was stirred at room temperature for 3 hours. The same procedure as in Example 1-1 was carried out, except that

[0105] Comparative Example 2-7 The same procedure as in Example 2-1 was carried out, except that a 85 μm thick TAC (triacetyl cellulose) film was used instead of the film produced in Example 2-1. The results are shown in Table 2.

[0106] Comparative Example 2-8 The same procedure as in Example 2-1 was carried out, except that a 53 μm thick PET (polyethylene terephthalate) film was used instead of the film produced in Example 2-1. The results are shown in Table 2.

[0107] Comparative Example 2-9 The same procedure as in Example 2-1 was carried out, except that a 53 μm thick COP (cyclic olefin polymer) film was used instead of the film produced in Example 2-1. The results are shown in Table 2. In Table 2, CNF means cellulose nanofiber.

[0108] The results shown in Table 2 show that the films of Examples 2-1 to 2-4, which used near-infrared absorbing materials made of copper-containing oxidized cellulose nanofibers having a number average fiber diameter of 100 nm or less and containing copper in the form of a salt, have excellent near-infrared absorbing ability and can be clearly recognized in near-infrared images.

[0109] According to the present invention, a novel near-infrared absorbing material having sufficiently high near-infrared absorbing ability can be provided. Furthermore, according to the present invention, an ink and a paint containing the near-infrared absorbing material can be provided. Furthermore, according to the present invention, a molded product made of the near-infrared absorbing material can be provided. Furthermore, according to the present invention, an indication made using the ink, paint, or molded product can be provided. Furthermore, according to the present invention, an article equipped with the indication can be provided. Furthermore, according to the present invention, a method for determining the authenticity of an article can be provided.

[0110] 10 Camera 11 Lens 20a LED surface lighting 20b LED bar lighting 30 Sample

Claims

1. A near-infrared absorbing material consisting of copper-containing oxidized cellulose nanofibers having a number-average fiber diameter of 100 nm or less and containing copper in the form of a salt.

2. The near-infrared absorbing material according to claim 1, wherein the number-average fiber length of the copper-containing oxidized cellulose nanofibers is 50 nm or more and 2000 nm or less.

3. The near-infrared absorbing material according to claim 1, wherein the average degree of polymerization of the copper-containing oxidized cellulose nanofibers is 100 or more and 2000 or less.

4. The near-infrared absorbing material according to any one of claims 1 to 3, wherein the copper-containing oxidized cellulose nanofibers are copper-containing carboxylated cellulose nanofibers.

5. An ink comprising the near-infrared absorbing material according to claim 1.

6. A paint comprising the near-infrared absorbing material according to claim 1.

7. A molded article made of the near-infrared absorbing material according to claim 1.

8. A display product made using the ink according to claim 5, the paint according to claim 6, or the molded product according to claim 7.

9. An article comprising the display material according to claim 8.

10. A method for determining the authenticity of an article, comprising the steps of: applying a visually indistinguishable mark to an authentic article using the near-infrared absorbing material described in claim 1; acquiring a near-infrared image of the article to be inspected; and determining that an article having the mark displayed in the near-infrared image is an authentic article, and determining that an article having the mark not displayed in the near-infrared image is a counterfeit article.

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