A set of printed material and ink composition having a latent image.

Abstract

To provide: a printed matter having a latent image that is unidentifiable with naked eyes and identifiable only in a specific temperature region and in a near-infrared region; and an ink composition set for forming the latent image.SOLUTION: Provided are: a printed matter having a latent image on at least one surface of a substrate, the latent image being formed with a specific pattern and other pattern, where the specific pattern and the other pattern respectively change near-infrared light absorption characteristics with temperature, the near-infrared light absorption characteristics in a specific temperature range differs between the specific pattern and the other pattern, and the specific pattern is identifiable only in the specific temperature region and in a near-infrared region: and ink compositions comprising a specific ink composition whose near-infrared light absorption characteristics change with temperature and another ink composition whose near-infrared light absorption characteristics change with temperature, where the specific ink composition and the other ink composition have different near-infrared light absorption characteristics in the specific temperature region.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a printed material having a latent image that is indistinguishable to the naked eye but discernible in the near-infrared light range, and a set of ink compositions for forming the latent image. [Background technology]

[0002] For purposes such as preventing counterfeiting, verifying authenticity, and preventing copying, latent image printing using ink compositions that are indistinguishable to the naked eye (hereinafter referred to as "stealth ink") is widely practiced. As stealth inks, inks are known that do not have an absorption band in the visible light range (wavelengths approximately 380 nm to 780 nm) but absorb or reflect light in the near-infrared range (wavelengths approximately 780 nm to 2500 nm) (see Patent Documents 1 and 2).

[0003] Patent Document 1 describes a stealth ink containing a colorant made of an aluminium compound, and Patent Document 2 describes a stealth ink containing composite tungsten oxide particles.

[0004] On the other hand, vanadium dioxide (VO2) or compounds obtained by adding other elements to vanadium dioxide to control the temperature at which the phase transition occurs is lower than that of vanadium dioxide are known as materials in which a reversible metal-insulator phase transition occurs with temperature changes, and the transmittance in the near-infrared region changes depending on the temperature (Patent Documents 3-5, Non-Patent Documents 1-10). As for the applications of these compounds, it is also known that coatings containing compound fine particles are used in dimming members that control the transmission and absorption of near-infrared light (see Patent Document 3, paragraph

[0026] , Patent Document 4, paragraphs

[0047] ,

[0048] , Patent Document 5, paragraphs

[0019] ,

[0020] , etc.).

[0005] In addition, Non-Patent Document 11 describes forming an image print in which the appearance in the visible light region changes depending on temperature, using a set of two types of ink sets with different temperatures at which the optical properties change depending on the presence or absence of ultraviolet light irradiation with a wavelength of 365 nm. Specifically, it is a triazobenzene-based ink that changes from red to transparent at 65°C and an ink that changes from blue to transparent at 80°C (see Fig. 4).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Non-Patent Documents

[0007]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Non-Patent Document 7

Non-Patent Document 8

Non-Patent Document 9

Non-Patent Document 10

Non-Patent Document 11

Summary of the Invention

Problems to be Solved by the Invention

[0008] The stealth-type ink having general near-infrared light absorption characteristics described in Patent Documents 1 and 2 is not completely invisible in the visible light region and exhibits, for example, a light yellowish-brown or blue color. Therefore, as shown in the prior art of FIG. 1, the latent image formed by the known ink may be visible depending on the way light hits it, and there is a problem that it is particularly easy to be visible when the background is white. Also, even if the latent image is extremely difficult to be visually recognized by the naked eye, if a third party knows the existence of the latent image, it can be easily identified by reading the reflection or transmission of near-infrared light using a general infrared camera, and there is a problem that the security level is low.

[0009] The vanadium dioxide and compounds of vanadium dioxide with other elements added, as described in Patent Documents 3-5 and Non-Patent Documents 1-10, allow for control of the temperature at which the metal-insulator phase transition occurs by adjusting the composition, thereby changing the absorption characteristics in the near-infrared region within a specific temperature range. Therefore, when a latent image is formed using a stealth ink containing a vanadium dioxide-based compound with an adjusted composition, a temperature condition is added as a necessary condition for recognizing the latent image, thus improving the security level. However, similar to the general stealth inks described above, some degree of coloration is unavoidable even when the vanadium dioxide-based compounds are atomized, making it difficult to completely conceal the latent image.

[0010] Non-patent document 11 describes forming a printed material using two types of ink sets whose appearance in the visible light range changes with temperature, and changing the temperature to alter the image recognizable in the visible light range. However, it does not suggest any latent images that are not recognizable in the visible light range but are recognizable in the near-infrared light range. The present invention has been made in view of these problems and aims to provide a printed material having a latent image that is indistinguishable to the naked eye and discernible only in a specific temperature range and in the near-infrared light range, and a set of ink compositions for forming said latent image. [Means for solving the problem]

[0011] The inventors of the present invention conducted studies to solve the above problems and found that the above problems can be solved by forming latent images having a specific pattern and other patterns, as shown in Figure 1 as the present technology, and by making the near-infrared light absorption characteristics in a specific temperature range different between the specific pattern and the other patterns, thus completing the present invention.

[0012] In other words, one aspect of the present invention for solving the above problem is, Formed by a specific pattern and other patterns, The aforementioned specific pattern and the aforementioned other patterns each have near-infrared light absorption characteristics that change with temperature. The near-infrared light absorption characteristics in a specific temperature range differ between the aforementioned specific pattern and the aforementioned other patterns. The aforementioned specific pattern is a latent image that is discernible only in the aforementioned specific temperature range and the near-infrared light region. It is a printed material having at least one surface on a substrate. Hereafter, this printed material will be referred to as a "latent image print." The near-infrared light absorption characteristics can be determined by measuring the light transmittance spectrum, light reflectance spectrum, or light absorptance spectrum in the near-infrared region.

[0013] Furthermore, another aspect of the present invention is, It consists of a specific ink composition whose near-infrared light absorption properties change with temperature, and other ink compositions whose near-infrared light absorption properties change with temperature. The aforementioned specific ink composition and the aforementioned other ink compositions differ in their near-infrared light absorption characteristics in a specific temperature range. This is a set of ink compositions for forming a latent image that is discernible only within the aforementioned specific temperature range and in the near-infrared light region. [Effects of the Invention]

[0014] According to the present invention, it is possible to reliably form a highly secure latent image that is indistinguishable to the naked eye but identifiable within a specific temperature range and in the near-infrared light range. [Brief explanation of the drawing]

[0015] [Figure 1] A conceptual diagram showing an example of a latent image related to the present invention and the prior art. [Figure 2] A diagram showing an example of a latent image printed material of the present invention. [Figure 3] Light transmission spectra of sample A and sample B at 25°C according to Example 1 [Figure 4] Light transmission spectra of sample A and sample B at 50°C according to Example 1 [Figure 5] Light transmission spectra of sample A and sample B at 75°C according to Example 1 [Figure 6] Temperature dependence of light transmittance at a wavelength of 960 nm for Sample A and Sample B in Example 1 [Figure 7] A latent image was created by combining the L-shaped pattern from sample A and the L-shaped pattern from sample B in Example 1. [Figure 8] Infrared camera image of the sample in Figure 7 at a temperature of 25°C and in the near-infrared region (wavelength 940nm-960nm). [Figure 9] Infrared camera image of the sample in Figure 7 at a temperature of 50°C and in the near-infrared region (wavelength 940nm-960nm). [Figure 10] Infrared camera image of the sample in Figure 7 at a temperature of 75°C and in the near-infrared region (wavelength 940nm-960nm). [Figure 11] Temperature dependence of light transmittance at a wavelength of 960 nm for samples C and D in Example 2 [Figure 12] Temperature dependence of light transmittance at a wavelength of 960 nm for samples E and F in Example 3 [Figure 13] Light transmission spectra of sample A' and sample B' at 55°C according to Example 4 [Modes for carrying out the invention]

[0016] One embodiment for carrying out the present invention (hereinafter referred to as "this embodiment") will be described, but the present invention is not limited to this embodiment and includes various other embodiments as long as they are within the scope of the technical idea described in the claims.

[0017] One embodiment of the present invention includes the following embodiments [1] to

[14] . [1] Formed by certain patterns and other patterns, The aforementioned specific pattern and the aforementioned other patterns each have near-infrared light absorption characteristics that change with temperature. The near-infrared light absorption characteristics in a specific temperature range differ between the aforementioned specific pattern and the aforementioned other patterns. The aforementioned specific pattern is a latent image that is discernible only in the aforementioned specific temperature range and the near-infrared light region. A printed material having at least one surface on a substrate. [2] The print of [1], wherein the specific pattern and the other patterns include a metal oxide whose crystal structure changes between monoclinic and tetragonal systems with temperature, causing a metal-insulator phase transition and altering the absorption characteristics in the near-infrared region. [3] The printed material according to [2], wherein the metal oxide is selected from vanadium dioxide or composite vanadium dioxide obtained by adding one or more other elements to vanadium dioxide. [4] The composite vanadium dioxide is Vanadium in which some of the elements are replaced by other elements. A form in which some of the oxygen is replaced by other elements. A vanadium dioxide crystal lattice in which other elements are inserted between the crystal lattices, or This is a form in which granular precipitates of other elements are formed at the grain boundaries of vanadium dioxide. The printed material mentioned in [3] above. [5] The printed material of [2] wherein the metal oxide is in the form of fine particles. [6] When the absolute value of the difference in light transmittance expressed as a percentage (%) is expressed in points, Between the aforementioned specific pattern and the aforementioned other patterns, The number of points in the wavelength range of 380 nm to 700 nm is less than 5 points. A printed material according to any of the above [1] to [5], wherein the number of points in the specified temperature range and at any of the wavelengths of 950 nm, 1200 nm, or 1400 nm within the wavelength range of 800 nm to 2500 nm is 5 or more. [7] Any of the print materials described in [1] to [6] above, wherein the specific temperature range is a specific range between -20°C and 20°C. [8] Any of the print materials described in [1] to [6] above, wherein the specific temperature range is a specific range between 30°C and 80°C. [9] Any of the printed materials [1] to [8], wherein both the specific pattern and the other pattern contain the same type of coloring agent that transmits 75% or more of light at any of the wavelengths of 950 nm, 1200 nm, or 1400 nm within the wavelength range of 800 nm to 2500 nm.

[10] Any of the prints described in [1] to [8] above, wherein the specific pattern and the other patterns are covered with a colored layer that transmits 75% or more of light at a wavelength of 950 nm, 1200 nm, or 1400 nm within the wavelength range of 800 nm to 2500 nm.

[11] A particular ink composition whose near-infrared light absorption properties change with temperature, and other ink compositions whose near-infrared light absorption properties change with temperature, The aforementioned specific ink composition and the aforementioned other ink compositions differ in their near-infrared light absorption characteristics in a specific temperature range. A set of ink compositions for forming a latent image that is discernible only within the aforementioned specific temperature range and in the near-infrared light region.

[12] The set of ink compositions of

[11] , wherein the specific ink composition and the other ink compositions include fine particles of metal oxide whose crystal structure changes between monoclinic and tetragonal systems with temperature, and which undergo a metal-insulator phase transition, thereby changing the absorption characteristics in the near-infrared region.

[13] The set of ink compositions according to

[12] , wherein the metal oxide is selected from vanadium dioxide or composite vanadium dioxide obtained by adding one or more other elements to vanadium dioxide.

[14] The composite vanadium dioxide is Vanadium in which some of the elements are replaced by other elements. A form in which some of the oxygen is replaced by other elements. A vanadium dioxide crystal lattice in which other elements are inserted between the crystal lattices, or This is a form in which granular precipitates of other elements are formed at the grain boundaries of vanadium dioxide. A set of the ink compositions described in

[13] above.

[15] A set of any of the ink compositions from

[11] to

[14] , wherein each of the specific ink composition and the other ink compositions contains a colorant that transmits 75% or more of light at a wavelength of 950 nm, 1200 nm, or 1400 nm within the wavelength range of 800 nm to 2500 nm. The following describes each component in this embodiment in order.

[0018] [Latent Image Print] The latent image print according to this embodiment is a print having a latent image on at least one surface of a substrate that is identifiable only in a specific temperature range and in the near-infrared light range, wherein the latent image is formed by a specific pattern and other patterns (hereinafter, "specific pattern and other patterns" may be referred to as "multiple patterns"), and each of the multiple patterns has absorption characteristics in the near-infrared light range that change with temperature, and the near-infrared light absorption characteristics in the specific temperature range differ among the multiple patterns. In this specification, "printed material" means a material on which an image (including text) has been formed on a substrate. Therefore, in addition to materials on which an image has been formed using a printing plate, this also includes materials on which an image has been formed using, for example, an inkjet method, a transfer method, etc.

[0019] (Specific patterns and other patterns) In this embodiment, the specific pattern and other patterns show no difference in optical properties in the visible light range, but their near-infrared light absorption characteristics differ in a specific temperature range. By recognizing the specific pattern only in the specific temperature range and the near-infrared light range, the latent image becomes recognizable. Examples of recognizable latent images include figures, characters and symbols, and barcodes and QR codes (registered trademarks). Figure 2 is a schematic diagram illustrating the latent image print in this embodiment. (1) shows the state in which a latent image is printed on the substrate. In the visible light range, it is not possible to distinguish between a specific pattern and other patterns. (2) shows that at a specific temperature, the near-infrared light absorption characteristics differ between the two patterns. (3) shows a specific pattern that can only be recognized in a specific temperature range and in the near-infrared light range. In this case, the specific pattern may be either the left or right pattern.

[0020] For a latent image to be discernible, it means that, when the absolute value of the difference in light transmittance expressed as a percentage (%) is expressed as points, the number of points (absolute value of the difference in light transmittance (%)) for a specific pattern and other patterns in the near-infrared light region is approximately 5 points or more. Specifically, when using a general CMOS type or InGaAs type infrared camera, this means that the value is approximately 3 points or more, preferably 5 points or more, and more preferably 7 points or more, at any of the wavelengths of 950nm, 1200nm, or 1400nm, where detection sensitivity is high. Furthermore, for a difference in optical characteristics to be unrecognizable in the visible light region, it means that the number of points is approximately less than 5 points. These multiple patterns can be formed by incorporating a material whose near-infrared light absorption characteristics change with temperature, and which has a small difference in optical properties in the visible light range and a large difference in near-infrared light absorption characteristics in a specific temperature range.

[0021] (Materials included in the pattern) The materials contained in the specific patterns and other patterns according to this embodiment are not limited as long as their near-infrared light absorption characteristics change with temperature, but it is preferable that they consist of metal oxides whose crystal structure changes with temperature, causing a metal-insulator phase transition and thus changing their absorption characteristics in the near-infrared region. In the following description, the temperature at which a metal-insulator phase transition occurs due to a change in crystal structure may be simply referred to as the "phase transition temperature". As the aforementioned metal oxide, it is preferable to select two or more compounds from vanadium dioxide, whose crystal structure changes between monoclinic and tetragonal systems with temperature changes, and composite vanadium dioxide obtained by adding one or more other elements to vanadium dioxide (hereinafter, both compounds are collectively referred to as "vanadium dioxide-based compounds").

[0022] The phase transition temperature of vanadium dioxide-based compounds can be altered depending on the presence or absence of additive elements, or the type and amount of additive elements. The forms of composite vanadium dioxide to which other elements are added include: (1) Vanadium in which some of the elements are replaced by other elements, (2) In which some of the oxygen is replaced by other elements, (3) Vanadium dioxide in which other elements are inserted between the crystal lattices, (4) Vanadium dioxide with granular precipitates of other elements formed at the grain boundaries. It includes.

[0023] Other elements in the embodiment of (1) include W, Mo, Fe, Ni, Ti, Ru, Mg, Ca, Al, Cr, Mn, Co, Nb (Patent Documents 3-5, Non-Patent Document 1), Ga, Re, Ir, Os, Ru, Ge, Ta (Non-Patent Document 2), Zr (Non-Patent Document 3), Sn (Non-Patent Document 4), Tb (Non-Patent Document 5), Eu (Non-Patent Document 6), as well as Si, Ca, Sc, Zn, Sr, Y, Sb, Ba, Hf, La, Ce, etc. Examples of other elements in the embodiment of (2) include F, P, N (Patent Documents 3, 4, Non-Patent Documents 7, 8), etc. Examples of other elements in the embodiment of (3) include H (Non-Patent Document 9), B, Be, etc. Examples of other elements in the embodiment of (4) include Au (Non-Patent Document 10), Cu, Ag, etc.

[0024] Among the vanadium dioxide-based compounds included in multiple patterns, those that exhibit nearly identical optical properties in the visible light range, even with different compositions, and that show almost no difference in absorption characteristics between the insulating and metallic states, are preferred. Among such vanadium dioxide-based compounds, the difference in optical properties is small in the visible light range even when compared at different temperatures. Therefore, it is practically impossible to distinguish the differences in optical properties between multiple patterns containing these compounds with the naked eye.

[0025] On the other hand, vanadium dioxide compounds exhibit significantly different transmittances in the near-infrared region between their insulating and metallic states. Specifically, in the metallic state, the transmittance in the near-infrared region is significantly lower compared to the insulating state. Furthermore, the phase transition temperature can be varied depending on the composition. Therefore, by selecting vanadium dioxide compounds with different compositions as materials included in multiple patterns, it becomes possible to identify specific patterns by utilizing the difference in near-infrared light transmittance between patterns in the temperature range where one compound is in an insulating state and the other compound is in a metallic state.

[0026] In this case, a specific pattern and other patterns may each contain compounds with different compositions and different phase transition temperatures. Alternatively, a specific pattern and other patterns may each contain multiple compounds with different compositions and different phase transition temperatures, and the proportions of each compound may differ. In either case, in the temperature range where compounds in an insulating state and compounds in a metallic state coexist, a specific pattern can be identified by the difference in near-infrared light transmittance between the patterns.

[0027] (Specific temperature range) In this embodiment, a specific temperature range in which a particular pattern can be identified only in the near-infrared light region can be selected by selecting the material contained in the pattern. For example, by preparing multiple types of vanadium dioxide-based compounds with different phase transition temperatures, each having a phase transition temperature of 30°C or higher (higher than room temperature), and combining a specific pattern containing compounds with low phase transition temperatures with other patterns containing compounds with high phase transition temperatures, it is possible to provide a latent image that is indistinguishable in the visible light range, but can be identified in the near-infrared range only when the specific temperature is above room temperature. To give a specific example, Example 1, described later, shows the pattern of sample (A) containing vanadium dioxide nanoparticles with a phase transition temperature of approximately 70°C, and W with a phase transition temperature of approximately 50°C. 0.005 V 0.995 By combining the pattern from sample (B) containing O2 fine particles with a general infrared camera equipped with a CMOS image sensor, it was possible to distinguish the pattern from (B) based on the difference in light transmittance between the two patterns (see Figure 4) in the temperature range of 42°C to 61°C and the near-infrared light region of 940nm to 960nm, which is the detection wavelength of the CMOS sensor.

[0028] Furthermore, in vanadium dioxide-based compounds, if a relatively large amount of elements that lower the phase transition temperature are added, the metal-insulator phase transition can be made to occur at temperatures lower than room temperature. Therefore, for example, by selecting multiple types of vanadium dioxide-based compounds, all of which have phase transition temperatures below room temperature (20°C or lower), and which have different phase transition temperatures, it is possible to provide a latent image that is indistinguishable in the visible light range, and in which a specific pattern can be identified in the infrared range only when cooled to a specific temperature at which at least one of the compounds becomes an insulator.

[0029] As a specific example, Example 3, described later, is a W phase transition that occurs from approximately -5°C to approximately 25°C. 0.025 V 0.975 Pattern from sample (E) containing O2 fine particles, and W, where a phase transition occurs from approximately -10°C to approximately 15°C. 0.035 V 0.965 By combining the pattern produced by sample (F) containing O2 fine particles, it was possible to distinguish the pattern produced by (F) based on the difference in transmittance between the two patterns (see Figure 12) in the near-infrared light region with wavelengths between 940 nm and 960 nm, within a temperature range of approximately 5°C to 15°C.

[0030] In this embodiment, the specific temperature range naturally includes room temperature. However, if a third party is aware of or can infer the existence of a latent image, the latent image will be immediately recognized if it is captured using a general infrared camera under room temperature conditions. Therefore, latent images where the specific temperature is other than room temperature are preferable in terms of enhancing the security level.

[0031] (Particle size of the material included in the pattern) In order to avoid the existence of latent images on the substrate being known to third parties, each pattern according to this embodiment preferably includes a highly transparent material. When the aforementioned material is in particulate form, if the particle size is approximately the same as the wavelength in the visible light range, Mie scattering occurs with little wavelength dependence of scattering intensity, resulting in a color close to white. However, if the particle size is sufficiently smaller than the wavelength in the visible light range, it enters the Rayleigh scattering region where light scattering decreases inversely proportional to the sixth power of the particle size, improving transparency and generally resulting in improved transparency in the visible light range. Therefore, when the material included in the pattern according to this embodiment is particulate, the particle size is preferably 200 nm or less, and more preferably 100 nm or less, which is smaller than the wavelength of visible light, in order to enhance transparency in the visible light range. Furthermore, in the vanadium dioxide-based compound according to this embodiment, having a particle size of 200 nm or less is preferable in that it ensures the identification of specific patterns that appear in a specific temperature range and in the near-infrared light region. Because the particle size is 200 nm or less, the effect of surface plasmon absorption in the metallic state in the infrared light region is increased, and the range of change in near-infrared light absorption characteristics associated with the metal-insulator phase transition is increased, so identification in the infrared light wavelength band by forming a latent image using multiple patterns can be performed more effectively.

[0032] Even when the vanadium dioxide compound is used as nanoparticles with a particle size of 100 nm or less, if an ink composition obtained by kneading these nanoparticles with a highly transparent resin is applied to a transparent substrate, some absorption in the visible light range may occur, resulting in a transparent yellowish-brown color, and it may be possible for specific patterns and other patterns to be recognized in the visible light range. However, as will be described later, by arranging specific patterns and other patterns so that they overlap or have no gaps, multiple patterns can be seen as a single unit in the visible light range, thus avoiding the situation where only specific patterns are visible in the visible light range.

[0033] (Material content included in the pattern) The material content in each pattern according to this embodiment can be changed depending on the intended application, but in order for a significant difference in transmittance in the near-infrared region to be shown, 1 m 2For the pattern, it is preferably 0.05 g (0.05 g / m 2 ) or more. Also, although the upper limit of the content is not particularly limited, when it exceeds 4 g / m 2 , light in the visible light region is significantly absorbed, so when it is necessary to maintain transparency, it is preferably less than 4 g / m 2 than .

[0034] (Colorant) The plurality of patterns in this embodiment may contain the same type of colorant that transmits 75% or more of near-infrared light together with the material. Due to the inclusion of such a colorant, all of the plurality of patterns exhibit the color of the colorant and are visually recognized, so it becomes even more difficult to distinguish between patterns in the visible light region. On the other hand, in the near-infrared light region of a specific temperature range and a specific wavelength, a latent image in which a specific pattern can be identified can be formed due to the difference in transmittance between the materials included in each pattern. The colorant may have a transmittance of 75% or more at all wavelengths (780 nm or more and 2500 nm or less) in the near-infrared light region, or may have a transmittance of 75% or more at a specific wavelength in the near-infrared light region. For a colorant having a transmittance of 75% or more at the specific wavelength, it is possible to detect the difference in transmittance of the materials included in each pattern at the specific temperature range and the specific wavelength. In addition, the wavelength range with high detection sensitivity by a general CMOS type infrared camera is 940 nm or more and 960 nm or less, and the wavelength range with high detection sensitivity by a general InGaAs type infrared detection camera is 950 nm or more and 1500 nm or less. Therefore, the specific wavelength is preferably, for example, any one of 950 nm, 1200 nm, and 1400 nm.

[0035] Also, for example, in addition to the plurality of patterns containing the colorant together with the material, a pattern that does not contain the material but contains the colorant may be provided as a dummy. Since all patterns exhibit the same color in the visible light region, the identification of the patterns constituting the latent image becomes more complicated, and a higher security level can be obtained.

[0036] (Layering of colored layers) The multiple patterns according to this embodiment may be covered with a colored layer containing a colorant that transmits 75% or more of near-infrared light. The optical properties of this colorant may be the same as those of the colorant that may be included in the multiple patterns together with the material. In the visible light spectrum, the area covered by the colored layer exhibits the color of the coloring agent and is therefore perceived. However, within that area, characters or symbols that can only be read in the near-infrared light spectrum or at specific wavelengths within a particular temperature range are hidden and printed, thus enabling the creation of highly secure latent image printed materials.

[0037] (Pattern arrangement) The positional relationship between a specific pattern and other patterns according to this embodiment is not particularly limited, but if the difference in near-infrared light absorption characteristics between patterns in a specific temperature range is small, it is preferable that the patterns are arranged so that they do not overlap, so that the specific pattern is easily recognizable after development. Furthermore, as mentioned above, if a particular pattern exhibits visible coloration in the visible light range, the patterns may be arranged to overlap in order to make it more difficult to identify the pattern. More preferably, as shown in the present technology in Figure 1, a specific pattern with a large difference in near-infrared light absorption characteristics and other patterns are formed without any gaps or overlaps, and furthermore, they have the same thickness. In this configuration, only a unified colored image can be recognized in the visible light range, while in the near-infrared light range, the specific pattern can be clearly recognized due to the significant difference in absorption characteristics. In addition, it is possible to avoid the recognition of the shape of the specific pattern due to the difference in the thickness of the ink layer.

[0038] (base material) The substrate on which the latent image according to this embodiment is formed on at least one surface is not particularly limited, as long as it is a substrate on which a pattern containing the material according to the present invention can be printed. It may be a metal, inorganic material, or organic material, and examples include metal foil, glass, ceramics, cloth, paper, etc. It may also be in a planar or three-dimensional shape.

[0039] [Set of ink compositions] The set of ink compositions according to this embodiment consists of a specific ink composition whose near-infrared light absorption characteristics change with temperature, and other ink compositions whose near-infrared light absorption characteristics change with temperature, wherein the specific ink composition and the other ink compositions have different near-infrared light absorption characteristics in a specific temperature range. Therefore, by using this set of ink compositions, it is possible to form a latent image that is identifiable only in the specific temperature range and a specific infrared light range.

[0040] Each ink composition preferably contains fine particles of a metal oxide that have a phase transition temperature and whose near-infrared light absorption properties change with the phase transition. Preferred particle size of metal oxide, and pattern forming latent image 1m 3 The preferred content per unit is as described in the section on latent image prints. The metal oxide is more preferably a vanadium dioxide-based compound. Each ink composition may contain vanadium dioxide-based compounds with different compositions, or it may contain multiple types of vanadium dioxide-based compounds with different compositions, each in varying proportions.

[0041] Furthermore, in the set of ink compositions according to this embodiment, each ink composition may further contain the same type of coloring agent that transmits 75% or more of near-infrared light. By printing a specific pattern and other patterns using each ink composition containing such a coloring agent, multiple patterns appear the same color without distinction in the visible light range, but in the near-infrared light range, a latent image can be formed in which a specific pattern can be distinguished due to the difference in transmittance between the ink compositions.

[0042] Examples of ink compositions according to this embodiment include printing inks such as gravure inks, screen inks, and offset inks, as well as inkjet inks, transfer inks, and handwriting inks.

[0043] The ink composition according to this embodiment can be prepared by dispersing the aforementioned materials, or a colorant added as needed, in water or an organic solvent. Examples of organic solvents include alcohols such as ethanol, ketones such as methyl ethyl ketone, toluene, and xylene. The method for dispersing the aforementioned materials and colorants is not particularly limited, but using ultrasound or a media stirring mill is preferable because it can finely atomize the particles. Furthermore, the solvent may contain an organic binder. The organic binder is not particularly limited and includes, for example, acrylic, urethane, epoxy, fluorine, vinyl, and rosin-based resin binders. By curing the organic binder, adhesion to the substrate is improved, and a printed film with high surface strength can be obtained. Furthermore, various common additives such as plasticizers, antioxidants, thickeners, and waxes can be added depending on the intended use of the ink. [Examples]

[0044] The present invention will be described in more detail below based on examples, but the present invention is not to be construed as being limited thereto.

[0045] <Example 1> (Synthesis of vanadium dioxide (VO2) nanoparticles) The following procedure was used to synthesize vanadium dioxide (VO2) nanoparticles without the addition of other elements. 1450 mg of vanadium pentoxide (V2O5, manufactured by Fujifilm Wako Pure Chemical Industries, special grade), 15 mL of 10% sulfuric acid aqueous solution (10% H2SO4, manufactured by Fujifilm Wako Pure Chemical Industries), and 4750 mg of a 5% diluted aqueous solution of hydrazine monohydrate (N2H4·H2O, manufactured by Fujifilm Wako Pure Chemical Industries, Wako special grade) were mixed in 10 mL of distilled water, and the mixture was stirred and held at a liquid temperature of 60°C for 30 minutes. When the solution was observed while being stirred, it changed from orange to a clear blue color. This is due to oxovanadium(IV) ions (VO2). 2+ This is understood to be due to the generation of ). Next, 10% aqueous ammonia (10% NH3, manufactured by Fujifilm Wako Pure Chemical Industries) was added to the resulting blue transparent solution to adjust the pH to 7, yielding a café au lait-colored suspension. This is understood to be due to the precipitation and suspension of vanadium oxyhydroxide. Next, this café au lait-colored suspension was centrifuged at 300 rpm for 5 minutes to allow the suspension to settle. After removing the supernatant, the resulting precipitate was resuspended in 50 mL of distilled water and then centrifuged at 300 rpm for 5 minutes, repeating this process three times to wash the precipitate. The precipitate (vanadium oxyhydroxide) after washing was suspended in distilled water to obtain 50 g of raw material solution. The raw material liquid was placed in a commercially available hydrothermal reaction autoclave (manufactured by San-ai Kagaku, 100 mL rotary reaction decomposition vessel set (pressure-resistant stainless steel outer cylinder RDVS-100, PTFE inner cylinder HUTc-100)), and the hydrothermal reaction autoclave was then placed inside a commercially available roller oven (manufactured by San-ai Kagaku, RDV-TM2), and hydrothermal synthesis was carried out at 270°C for 24 hours. After the synthesis was complete, the oven temperature was confirmed to be below 50°C, and the temperature of the autoclave outer cylinder surface was confirmed to be equivalent to room temperature before opening the autoclave. The solution was placed in a centrifuge tube and centrifuged at 12,000 rpm for 10 minutes, and the supernatant was removed. Furthermore, distilled water was added to the reaction product precipitated at the bottom of the centrifuge tube and mixed by shaking. The mixture was then centrifuged again, the supernatant water was removed, and ethanol was added to the reaction product precipitated at the bottom of the centrifuge tube and mixed by shaking. The mixture was then centrifuged again, and the supernatant ethanol was removed to wash the reaction product. The reaction product, washed in this manner, was dried overnight in a constant-temperature dryer at 70°C to obtain a particulate sample.

[0046] Scanning electron microscopy observation of the particulate sample revealed that rod-shaped microparticles with a particle size of approximately 30 nm to 80 nm in the long axis direction and a particle size of approximately 10 nm to 50 nm in the short axis direction were formed. X-ray diffraction measurements of the obtained particulate samples revealed that only the diffraction peak of the VO2 crystal monoclinic phase was observed, suggesting that all the particles undergo a metal-insulator phase transition.

[0047] (Tungsten composite vanadium dioxide (W 0.005 V 0.995 O2) Synthesis of fine particles) Next, composite vanadium dioxide (W) with approximately 0.5 at% of W added as a heterogeneous element. 0.005 V 0.995 O2) Fine particles were synthesized using the following procedure. 1450 mg of vanadium pentoxide (V2O5, manufactured by Fujifilm Wako Pure Chemical Industries, special grade), 15 mL of 10% sulfuric acid aqueous solution (10% H2SO4, manufactured by Fujifilm Wako Pure Chemical Industries), and 4750 mg of a 5% aqueous solution of hydrazine monohydrate (N2H4·H2O, manufactured by Fujifilm Wako Pure Chemical Industries, Wako special grade) were mixed in 10 mL of distilled water, and the mixture was stirred and held at a temperature of 60°C for 30 minutes. (Solution a) Prepare another beaker and add an appropriate amount of ammonium tungstate-para-pentahydrate ((NH4)) to 5 mL of distilled water. 10 W 12 O 41 (Solution b) was mixed with 5H2O (manufactured by Fujifilm Wako Pure Chemical Industries) and stirred at a liquid temperature of 60°C for 30 minutes. Solution a and solution b were mixed, and the mixture of solution a and solution b was stirred and held at room temperature for 10 minutes. Then, 10% aqueous ammonia (10% NH3, manufactured by Fujifilm Wako Pure Chemical Industries) was added to adjust the pH to 7, and a café au lait-colored suspension was obtained. Next, this café au lait-colored suspension was centrifuged at 300 rpm for 5 minutes to precipitate the suspended matter. After removing the supernatant, the resulting precipitate was resuspended in 50 mL of distilled water and then centrifuged at 300 rpm for 5 minutes, repeating this process three times to wash the precipitate. The precipitate after washing (tungsten-doped vanadium oxyhydroxyoxide) was suspended in distilled water to obtain 50 g of raw material solution. The raw material liquid was placed in a commercially available hydrothermal reaction autoclave (manufactured by San-ai Kagaku, 100 mL rotary reaction decomposition vessel set (pressure-resistant stainless steel outer cylinder RDVS-100, PTFE inner cylinder HUTc-100)), and the hydrothermal reaction autoclave was then placed inside a commercially available roller oven (manufactured by San-ai Kagaku, RDV-TM2), and hydrothermal synthesis was carried out at 270°C for 24 hours. After the synthesis was complete, the oven temperature was confirmed to be below 50°C, and the temperature of the autoclave outer cylinder surface was confirmed to be equivalent to room temperature before opening the autoclave. The solution was then placed in a centrifuge tube and centrifuged at 12,000 rpm for 10 minutes, and the supernatant was removed. Furthermore, distilled water was added to the reaction product precipitated at the bottom of the centrifuge tube and mixed by shaking. The mixture was then centrifuged again, the supernatant water was removed, and ethanol was added to the reaction product precipitated at the bottom of the centrifuge tube and mixed by shaking. The mixture was then centrifuged again, and the supernatant ethanol was removed to wash the reaction product. The reaction product, washed in this manner, was dried overnight in a constant-temperature dryer at 70°C to obtain a particulate sample.

[0048] Scanning electron microscopy observation of this particulate sample revealed that rod-shaped microparticles with a particle size of approximately 30 nm to 60 nm in the short axis direction and a particle size of approximately 50 nm to 100 nm in the long axis direction were formed. X-ray diffraction measurements were performed on the obtained particulate samples. Analysis software revealed that only diffraction peaks attributed to VO2 with a monoclinic structure were observed, suggesting that all the particles undergo a metal-insulator phase transition. The content of V and W in the particulate sample was measured using a wavelength-dispersive X-ray fluorescence analyzer (Rigaku Supermini 200), and the molar percentage of W relative to V was calculated to be 0.52%.

[0049] (Manufacturing of ink compositions) Two g of VO2 fine particles synthesized according to the above procedure were mixed with a silane coupling agent in eight g of ethanol to prepare a dispersion. Two g of polyvinyl butyral was then mixed into the dispersion to prepare an ink composition containing dispersed VO2 fine particles (hereinafter referred to as "VO2 dispersed ink"). Furthermore, using the same method as for VO2 nanoparticles, composite vanadium dioxide (W) synthesized by the procedure described above was obtained. 0.005 V 0.995 Regarding O2) fine particles, an ink composition in which these fine particles are dispersed (hereinafter referred to as "W-doped ink") was also prepared.

[0050] (Preparation of sample (A) and sample (B)) A sample (A) with a printed film made of VO2 dispersion ink was prepared by coating a PET (polyethylene terephthalate) film measuring 10 cm wide, 20 m long, and 50 μm thick with a wire bar coat to a thickness of approximately 4 μm, and then drying it for 1 minute using a hot air dryer at approximately 60°C. The thickness of the resulting dried printed film was approximately 200 nm. For the W-doped ink, a sample (B) having a printed film made of the W-doped ink on a PET film was prepared in the same manner as for the VO2-dispersed ink.

[0051] (Temperature dependence of the optical properties of sample (A) and sample (B)) The optical properties of sample (A) and sample (B) were evaluated by measuring the light transmission spectra at various sample temperatures using a UV-Vis-Near-Infrared spectrophotometer (JASCO V-770) incorporating a Peltier-type variable sample temperature unit. Figure 3 shows the light transmission spectra of sample (A) and sample (B) at a sample temperature of 25°C in the range of wavelengths from 200 nm to 2500 nm. In the visible light range with wavelengths between 380 nm and 800 nm, the light transmittance of sample (A) and sample (B) was almost the same. Furthermore, in the near-infrared light range with wavelengths between 800 nm and 2500 nm, the light transmittance of sample (A) and sample (B) was also almost the same. The light transmittance in the near-infrared region with wavelengths between 800 nm and 2500 nm is higher than the light transmittance around 700 nm in the visible light region. This suggests that in both sample (A) and sample (B), the vanadium dioxide and composite vanadium dioxide fine particles contained in the printed film are in an insulating state.

[0052] Figure 4 shows the light transmission spectra of sample (A) and sample (B) at a sample temperature of 50°C in the wavelength range from 200 nm to 2500 nm. In the visible light range with wavelengths between 380 nm and 800 nm, the light transmittances of sample (A) and sample (B) were almost the same. However, in the near-infrared light range with wavelengths between 800 nm and 2500 nm, the light transmittance of sample (B) was lower than that of sample (A). In this case, sample (A) is understood to have contained vanadium dioxide nanoparticles in an insulating state, while sample (B) is understood to have contained composite vanadium dioxide nanoparticles that were either in a metallic state or in a transitional state from an insulating state to a metallic state.

[0053] Figure 5 shows the light transmission spectra of sample (A) and sample (B) at a sample temperature of 75°C in the wavelength range from 200 nm to 2500 nm. In the visible light range with wavelengths between 380 nm and 800 nm, the light transmittance of sample (A) and sample (B) was almost the same. Furthermore, in the near-infrared light range with wavelengths between 800 nm and 2500 nm, the light transmittance of sample (A) and sample (B) was also almost the same. The light transmittance in the near-infrared region with wavelengths between 800 nm and 2500 nm was lower than the light transmittance around 700 nm in the visible light region. This suggests that in both sample (A) and sample (B), the vanadium dioxide and composite vanadium dioxide nanoparticles contained within were in a metallic state.

[0054] Next, using a UV-Vis-Near-Infrared spectrophotometer incorporating a Peltier-type variable sample temperature unit, the light transmittance of sample (A) and sample (B) at a wavelength of 960 nm was measured while raising the sample temperature from 10°C to 75°C at a rate of 10°C / min. The results are shown in Figure 6. From the measurement results, it is understood that in sample (A), within a temperature range of approximately 50°C to 70°C, as the sample temperature increases, the vanadium dioxide nanoparticles contained in the sample change from an insulating state to a metallic state, and the light transmittance decreases accordingly. On the other hand, in sample (B), it is understood that in the temperature range of approximately 35°C to approximately 55°C, as the sample temperature increases, the contained composite vanadium dioxide nanoparticles change from an insulating state to a metallic state, and the light transmittance decreases accordingly.

[0055] (Formation of specific patterns and other patterns) Next, samples (A) and (B) were cut with a utility knife to create two L-shaped pieces from each, and a sample according to Example 1 was formed, having an image that combines the L-shaped pattern from sample (A) and the L-shaped pattern from sample (B), as shown in Figure 7.

[0056] (Temperature dependence of optical properties of specific patterns and other patterns) All four L-shaped patterns constituting the sample in Figure 7 exhibited a yellowish-brown color, making it impossible to distinguish between sample (A) and sample (B) with the naked eye. Furthermore, when the sample shown in Figure 7 was placed on a Peltier-type plate and the sample temperature was changed from 10°C to 80°C, all four L-shaped patterns constituting the image in Figure 7 appeared yellowish-brown across the entire temperature range, making it impossible to distinguish between sample (A) and sample (B) with the naked eye.

[0057] Next, the sample shown in Figure 7 was placed on a Peltier-type plate, the sample temperature was set to 25°C, and a near-infrared image was observed using an infrared microscope (Hozan, L-KIT649, detection wavelength range: 940nm~960nm, image sensor: 1 / 2.5-inch CMOS image sensor). As shown in Figure 8, all four L-shaped patterns constituting the image appeared light gray, making it impossible to distinguish between sample (A) and sample (B).

[0058] Next, the sample temperature was set to 50°C, and the infrared image was observed in the same manner as in the case of 25°C. As shown in Figure 9, among the four L-shaped patterns that make up the image, sample (A) appeared light gray, and sample (B) appeared a slightly darker gray, allowing us to distinguish between sample (A) and sample (B). Next, when the sample temperature was set to 75°C, the infrared image was observed in the same manner as in the cases of 25°C and 50°C. As shown in Figure 10, all four L-shaped patterns constituting the image appeared dark gray, making it impossible to distinguish between sample (A) and sample (B).

[0059] Next, when the sample temperature was increased from 10°C to 75°C at a rate of 10°C / min, infrared images were observed, and it was possible to distinguish between sample (A) and sample (B) in the temperature range of approximately 42°C to 61°C.

[0060] <Example 2> A composite vanadium dioxide (W) was prepared using the same procedure as in Example 1, except that the amount of ammonium tungstate-para-pentahydrate added was changed, with approximately 1.0% W added as a heterogeneous element. 0.01 V 0.99 We synthesized O2 fine particles. The particle size of this fine particle sample is the same as that of Example 1. 0.005 V 0.995 It was almost identical to O2 particles. When the wtol content of the particulate sample was measured using wavelength-dispersive X-ray fluorescence analysis, the wtol / vitrified (mol%) value was found to be 1.06%.

[0061] Next, using the same procedure as in Example 1, except that the amount of ammonium tungstate-para-pentahydrate mixed was changed, a composite vanadium dioxide (W) with approximately 1.5% W added as a heterogeneous element was prepared. 0.015 V 0.985 We synthesized O2 fine particles. The particle size of this fine particle sample is the same as that of Example 1. 0.005 V 0.995 It was almost identical to O2 particles. When the wtaldehyde content of the particulate sample was measured using wavelength-dispersive X-ray fluorescence analysis, the wtaldehyde / vitrification (mol%) value was found to be 1.55%.

[0062] The above-mentioned obtained composite vanadium dioxide (W 0.01 V 0.99 O2) fine particles, and the obtained composite vanadium dioxide (W 0.015 V 0.985 For the O2) fine particles, an ink composition in which each fine particle was dispersed was prepared using the same procedure as in Example 1.

[0063] Each ink composition was coated onto a PET film measuring 10 cm wide, 20 m long, and 50 μm thick using a wire bar coat to a thickness of approximately 4 μm to form a film. The film was then dried for 1 minute in a dryer at approximately 60°C to prepare samples (C) and (D).

[0064] Using the same spectrophotometer as in Example 1, the light transmittance of sample (C) and sample (D) at a wavelength of 960 nm was measured while the sample temperature was increased from 10°C to 75°C at a rate of 10°C / min. The results are shown in Figure 11. From the measurement results, it is understood that in sample (C), within a temperature range of approximately 10°C to approximately 50°C, as the sample temperature increases, the contained composite vanadium dioxide nanoparticles change from an insulating state to a metallic state, and the light transmittance decreases accordingly. Sample (D) is understood to exhibit a change in light transmittance as the sample temperature increases, from an insulating state to a metallic state, within a temperature range of approximately 5°C to 45°C, as the contained composite vanadium dioxide nanoparticles change accordingly.

[0065] Next, samples (C) and (D) were cut with a utility knife to create two L-shaped pieces from each, and these were combined in the same manner as in Figure 7 to form a sample according to Example 2 having a pattern from sample (C) and a pattern from sample (D). When observing infrared images while raising the sample temperature from 5°C to 50°C at a rate of 10°C / min, it was possible to distinguish between sample (C) and sample (D) in the temperature range of approximately 29°C to 41°C.

[0066] <Example 3> A composite vanadium dioxide (W) was prepared using the same procedure as in Example 1, except that the amount of ammonium tungstate-para-pentahydrate added was changed, with approximately 2.5% W added as a heterogeneous element. 0.025 V 0.975 We synthesized O2 fine particles. When the wtaldehyde content of the particulate sample was measured using wavelength-dispersive X-ray fluorescence analysis, the wtaldehyde / vitrified (W / V) value was found to be 2.48%. Next, using the same procedure as in Example 1, except that the amount of ammonium tungstate-para-pentahydrate mixed was changed, a composite vanadium dioxide (W) with approximately 3.5% W added as a heterogeneous element was prepared. 0.035 V 0.965 We synthesized O2 fine particles. The particle size of this fine particle sample is the same as that of Example 1. 0.005 V 0.995 It was almost identical to O2 particles. When the wtaldehyde content of the particulate sample was measured using wavelength-dispersive X-ray fluorescence analysis, the wtaldehyde / vitrification (mol%) value was found to be 3.65%.

[0067] Ink compositions containing the obtained composite vanadium dioxide fine particles were prepared using the same procedure as in Example 1. Each of the aforementioned ink compositions was coated onto a PET film measuring 10 cm wide, 20 m long, and 50 μm thick using a wire bar coat to a coating thickness of approximately 4 μm to form a film. The film was then dried for 1 minute in a dryer at approximately 70°C to prepare samples (E) and (F).

[0068] Using the same spectrophotometer as in Example 1, the light transmittance of sample (E) and sample (F) at a wavelength of 960 nm was measured while the sample temperature was increased from 10°C to 75°C at a rate of 10°C / min. The results are shown in Figure 12. From the measurement results, it is understood that in sample (E), within a temperature range of approximately -5°C to approximately 25°C, as the sample temperature increases, the contained composite vanadium oxide fine particles change from an insulating state to a metallic state, and the light transmittance decreases accordingly. It is understood that in sample (F), within a temperature range of approximately -10°C to approximately 15°C, as the sample temperature increases, the contained composite vanadium oxide fine particles change from an insulating state to a metallic state, and consequently, the light transmittance decreases.

[0069] Next, similar to Example 1, sample (E) and sample (F) were cut with a utility knife to create two L-shaped pieces from each sample, which were then combined to form a sample having an image similar to that shown in Figure 7 of Example 1. When infrared images were observed while the sample temperature was increased from 5°C to 50°C at a rate of 10°C / min, it was possible to distinguish between sample (E) and sample (F) in the temperature range of approximately 10°C or below. In this example, infrared imaging observations below 5°C were not performed, but it is considered that the lower temperature limit at which sample (E) and sample (F) can be distinguished is lower than 5°C. Table 1 shows the temperature range in which latent images can be recognized in the near-infrared region for Examples 1 to 3.

[0070] [Table 1]

[0071] <Example 4> Using sample (A) prepared in Example 1, the side of the PET film substrate that was not coated with VO2 dispersion ink was colored with a commercially available black oil-based pen (Zebra Macky® extra fine, product number MO-120-MC-BK, JAN code 4901681 503513) to obtain sample (A'). Similarly, the side of sample (B) prepared in Example 1 that was not coated with W-doped ink was colored with the same commercially available black oil-based pen to obtain sample (B'). In addition, a sample was prepared as a reference sample by coloring one side of a transparent PET film substrate with the same black oil-based pen as described above. Sample (A'), Sample (B'), and the reference sample were all completely black in appearance and were confirmed to be indistinguishable to the naked eye.

[0072] Figure 13 shows the light transmission spectra of sample (A'), sample (B'), and reference sample at a sample temperature of 55°C, with wavelengths ranging from 200 nm to 2500 nm. In the wavelength range of 300 nm to 600 nm, sample (A'), sample (B'), and the reference sample all exhibited a light transmittance of less than 5%. However, in the wavelength range of 900 nm to 2500 nm, the light transmittances of sample (A'), sample (B'), and the reference sample differed, and it was found that the reference sample showed almost no absorption of infrared light. From the above, it was found that although sample (A'), sample (B'), and the reference sample are indistinguishable to the naked eye, sample (A') and sample (B') can be distinguished by observing near-infrared images at a sample temperature of 55°C. [Industrial applicability]

[0073] According to the present invention, it is possible to reliably form a highly secure latent image that is indistinguishable to the naked eye but identifiable within a specific temperature range and in the near-infrared light range. Therefore, the present invention can be used for preventing counterfeiting of banknotes and documents, determining their authenticity, and preventing copying. Furthermore, the present invention can also be used as a means of transmitting confidential information without it being known to third parties.

Claims

1. Formed by a specific pattern and other patterns, The aforementioned specific patterns and other patterns include metal oxides whose crystal structure changes between monoclinic and tetragonal systems with temperature, causing a metal-insulator phase transition and altering their absorption characteristics in the near-infrared region. The near-infrared light absorption characteristics in a specific temperature range differ between the aforementioned specific pattern and the aforementioned other patterns. The aforementioned specific pattern is a latent image that is discernible only in the aforementioned specific temperature range and the near-infrared light region. A printed material having at least one surface on a substrate.

2. The printed article according to claim 1, wherein the metal oxide is selected from vanadium dioxide or composite vanadium dioxide obtained by adding one or more other elements to vanadium dioxide.

3. The aforementioned composite vanadium dioxide is Vanadium in which some of the elements are replaced by other elements. A form in which some of the oxygen is replaced by other elements. A vanadium dioxide crystal lattice in which other elements are inserted between the crystal lattices, or This is a form in which granular precipitates of other elements are formed at the grain boundaries of vanadium dioxide. The printed material according to claim 2.

4. The printed material according to claim 1, wherein the metal oxide is in the form of fine particles.

5. When the absolute value of the difference in light transmittance, expressed as a percentage (%), is expressed in points, Between the aforementioned specific pattern and the aforementioned other patterns, The number of points in the wavelength range of 380 nm to 700 nm is less than 5 points. The printed material according to claim 1, wherein the number of points in the specified temperature range and at any of the wavelengths of 950 nm, 1200 nm, and 1400 nm within the wavelength range of 800 nm to 2500 nm is 5 or more.

6. The printed material according to claim 1, wherein the specified temperature range is a specific range between -20°C and 20°C.

7. The printed material according to claim 1, wherein the specified temperature range is a specific range between 30°C and 80°C.

8. The printed material according to any one of claims 1 to 7, wherein both the specific pattern and the other pattern contain the same type of coloring agent that transmits 75% or more of light at any wavelength of 950 nm, 1200 nm, or 1400 nm within the wavelength range of 800 nm to 2500 nm.

9. The printed material according to any one of claims 1 to 7, wherein the specific pattern and the other patterns are covered with a colored layer that transmits 75% or more of light at a wavelength of 950 nm, 1200 nm, or 1400 nm within the wavelength range of 800 nm to 2500 nm.

10. It consists of a specific ink composition whose near-infrared light absorption properties change with temperature, and other ink compositions whose near-infrared light absorption properties change with temperature. The aforementioned specific ink composition and the aforementioned other ink compositions contain fine particles of metal oxide whose crystal structure changes between monoclinic and tetragonal systems with temperature, causing a metal-insulator phase transition, thereby changing their near-infrared light absorption characteristics, and thus differing in near-infrared light absorption characteristics in a specific temperature range. A set of ink compositions for forming a latent image that is discernible only within the aforementioned specific temperature range and in the near-infrared light region.

11. The set of ink compositions according to claim 10, wherein the metal oxide is selected from vanadium dioxide or composite vanadium dioxide obtained by adding one or more other elements to vanadium dioxide.

12. The aforementioned composite vanadium dioxide is Vanadium in which some of the elements are replaced by other elements. A form in which some of the oxygen is replaced by other elements. A vanadium dioxide crystal lattice in which other elements are inserted between the crystal lattices, or This is a form in which granular precipitates of other elements are formed at the grain boundaries of vanadium dioxide. A set of ink compositions according to claim 11.

13. A set of ink compositions according to any one of claims 10 to 12, wherein both the specific ink composition and the other ink compositions contain the same type of coloring agent that transmits 75% or more of light at a wavelength of 950 nm, 1200 nm, or 1400 nm within the wavelength range of 800 nm to 2500 nm.

Images