JPH0753946A - Infrared absorber - Google Patents

Abstract

PURPOSE:To obtain a new IR absorber and an ink which absorb light only in an IR region and do not absorb visible light. CONSTITUTION:The absorber comprises YbPO4 crystallites having diameters of 5-200nm. This absorber is mixed with an ink vehicle to obtain an ink. By using this ink, an invisible non-information pattern having intense absorption in the range of 900-1,000nm, e.g. a detection mark or a code pattern, can be formed. The code pattern formed from this ink cannot be recognized with the naked eye and has excellent IR absorption. An optical detection mark formed from the ink on a clear sheet, e.g. OHP, is transparent and can be detected with IR rays.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is substantially invisible to the naked eye, but absorbs infrared rays to form optically identifiable code patterns and marks such as recognizable marks. The present invention relates to an infrared absorbing material used as a material. Further, the present invention relates to an ink and an invisible pattern using the infrared absorbing material.

[0002]

2. Description of the Related Art In recent years, bar codes as code patterns using optical reading have been widely used mainly for physical distribution management systems. For example, barcodes are widely used as optical data carriers such as JAN codes for POS (point of sale) systems, delivery slips, packing slips, and barcode tags for delivery.

Light source light for optical reading of these conventional bar codes is 650 nm, 800 nm or 950 nm.
A semiconductor laser or a light emitting diode having an emission wavelength in the vicinity is mainly used. Therefore, since the wavelength range of the light source is restricted, the bar code has an absorption characteristic in the ink using carbon black having an absorption band in the visible light range or in the red / infrared wavelength range of the cyan / green system. Printed or printed with ink.

The bar code printing method is letterpress, offset, flexo, gravure or silk printing, and is mainly applied to mass printing called source marking. Bar code printing methods are dot impact, thermal transfer, direct thermal, electrophotography, inkjet printing, etc., and are mainly applied to individual printing called in-store marking or manufacturing of small lot information code labels. .

However, there is a strong demand for eliminating such visible information codes because they impose design restrictions on printed matter. Therefore, an attempt has been made to make the information code transparent by printing or printing an ink having no absorption band in the visible light region to make visual judgment difficult.

As one of the attempts to make transparent, it is known to form an infrared pattern by using an ink which mainly absorbs infrared rays outside the visible light region [for example, JP-A-63-116286, JP-A-3-154187
Japanese Patent Application Laid-Open No. 3-227378, Japanese Patent Application Laid-Open No. 3-27538.
9, JP-A-4-70349, and JP-A-5-93160.
No.]).

[0007]

Dyes having an absorption region in the infrared region which have been conventionally used include cyanine dyes, phthalocyanine dyes, naphthoquinone dyes, anthraquinone dyes, zirol dyes, triphenylmethane dyes, and the like. There is. However, these are cyan colors because they have an absorption band in the wavelength region of 600 nm or more, or 30-40% in the visible region (380 nm-700 nm).
It has a slightly reddish cream color due to its absorption. Therefore, it was not possible to form a completely transparent barcode. Further, since these dyes are dyes, there is a drawback that light resistance as an ID carrier cannot be expected. In addition, there is also one that uses a cyan filter glass as an infrared absorbing pigment, but in this case, the glass contains Cu ²⁺ ions, and since the absorption starts from 550 nm, a cyan color is exhibited.

Under these circumstances, in order to provide a transparent invisible bar code, it is desired to provide a material that absorbs infrared rays but does not absorb visible rays. Therefore, an object of the present invention is to provide a new material that has absorption only in the infrared region and does not absorb in the visible light region. That is, the present invention is invisible to the naked eye, and infrared absorption characteristics, weather resistance, light resistance, inking characteristics, printability,
An object is to provide an infrared absorbing material having excellent printability and weather resistance, and an ink using the material.

By the way, there is a field in which an infrared absorbing material can be used also in a field other than the infrared absorbing code pattern. For example, when an image is formed on a transparent sheet for an over head projector (OHP) by a copying machine using an optical detection method, the transparent sheet for the transparent sheet is set in order to set the paper feed timing of the transparent sheet. A detection mark is provided at the edge. Optical detection is, for example, L
It is considered that an ED and a phototransistor are combined and an infrared absorbing material can be used as a detection mark.

However, conventionally, for example, Japanese Patent Laid-Open No. 3-99
As described in Japanese Patent No. 878, the detection mark was formed of an opaque material. However, the opaque detection mark is displayed on the OHP together with the target image, which makes it difficult to view the displayed image. Therefore, if a detection mark that is transparent but can be optically detected is provided, such a defect is eliminated.

Therefore, another object of the present invention is to provide a transparent OHP.
The object is to provide a material that absorbs infrared radiation but not visible light in order to provide a transparent detection mark that can be applied to the sheet. That is, it is an object of the present invention to provide a material that can be optically detected by absorbing infrared rays and can provide a transparent detection mark by not absorbing visible rays, and an ink using the material.

[0012]

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an infrared absorbing material characterized by being composed of YbPO ₄ crystallites having a diameter of 5 to 200 nm.

The infrared absorbing material of the present invention comprises YbPO ₄ having a crystallite diameter of 5 to 200 nm. The crystallite diameter is preferably in the range of 10 to 100 nm. Y
The crystal type of bPO ₄ is tetragonal, and 900 to 10
It has strong absorption in the wavelength region of 00 nm. Furthermore, if the crystallite has a diameter of 200 nm or less, it does not absorb visible light. The diameter of the crystallite may be 200 nm or less from the viewpoint of transparency to visible light, and the smaller the diameter, the more difficult the production becomes. Therefore, practically, the diameter of the crystallite is 5 nm or more.

The infrared absorbing material of the present invention is prepared, for example, by mixing equimolar amounts of ytterbium oxide (Yb ₂ O ₃ ) fine powder and phosphorus pentoxide (P ₂ O ₅ ) powder and heating the resulting mixture. It can be manufactured by gradually cooling the resulting melt. The heating of the mixture can be performed at about 1300 ° C. Further, the size of the crystallite changes depending on the rate of slow cooling, and it is suitable to set it to 100 to 300 ° C./hr, for example. YbPO ₄ obtained by cooling is bulky and can be appropriately pulverized. However, the bulky YbPO ₄ is crushed so as to suppress changes in the crystal form of YbPO ₄ . Examples of such pulverization include a method of cooling a container and the like during pulverization, pulverization in an inert gas, pulverization in an organic solvent such as xylenetoluene, and the like.

The infrared absorbing material comprising YbPO _{4 of the} present invention may be produced by a metal alkoxide method or a hydrothermal method in which YbPO ₄ is produced from a ytterbium alkoxide as a raw material, in addition to the above method. Production of the YbPO ₄ infrared absorbing material of the present invention by the metal alkoxide method
For example, ytterbium tributoxide [Yb (OC ₄ H ₉ ) ₃ ] dissolved in an organic solvent such as benzene and an equimolar amount of anhydrous H ₃ PO ₄ in ethanol are mixed and stirred, and then stirred at room temperature for about 1 to 10 After being left for one day, an aqueous solution of ethanol is added, and the mixture is left at room temperature for about 1 to 10 days to hydrolyze ytterbium tributoxide. Then, the product can be obtained by drying to remove the alcohol and solvent produced by hydrolysis. For drying, for example, air-dry at room temperature, and then 100
It can be carried out by heating and drying at ˜200 ° C.

Yb ^{3+ in} YbPO ₄ particles has an absorption band in the infrared region in the ionic state based on the transition of ² F _7/2 → ³ F _5/2 of Yb ³⁺ . Further, this absorption is not colored because it does not become broad and does not have absorption in the visible region, unlike coloring due to a transition metal ion.

The infrared absorbing material of the present invention having such characteristics remarkably absorbs irradiation light in the infrared region having a peak at about 970 nm as shown in FIG.
It has no absorption in the visible region of 00 nm. Therefore, when a bar code is formed with ink or the like using the infrared absorbing material of the present invention, a bar portion (printing portion) that absorbs,
Between the reflection space and the non-printed area, the density of reflected light of absorption / reflection of irradiation infrared light is formed, and the barcode signal can be read, but it cannot be visually recognized with the naked eye. . Furthermore, when a detection mark is formed with ink or the like using the infrared absorbing material of the present invention, absorption / reflection of irradiation infrared light is prevented between the detection mark (printed portion) for absorption and the non-printed portion for reflection. The shade of reflected light is formed, and the detection mark is recognized, but it is not visible to the naked eye.

The infrared absorbing material of the present invention is used as an offset ink for offset printing, thermal transfer printing, inject printing, electrophotographic printing, which is a printing method suitable for printing a code pattern among marks.
When used as a pigment of a thermal transfer ribbon ink, an inkjet ink, or a toner ink, the infrared absorbing material of the present invention is a powder having an average particle diameter of 0.01 μm to 0.1 μm and a maximum particle diameter of 1 μm or less. Preferably there is. The printed film thickness or the printed film thickness obtained by the above method is usually about 1 to 2 μm, and the maximum is 3 μm.
This is because, by setting the average particle diameter of the infrared absorbing material to the submicron order within the above range, it is possible to suppress printing unevenness. The same applies to the gravure ink for gravure printing the detection mark.

Further, in consideration of the ink characteristics, for offset inks, thermal transfer ribbon inks, and toner inks whose binder component is non-polar, a lipophilic coating is applied to the surface of the powder of the infrared absorbing material to form an ink binder. It is preferable to improve the dispersibility of the infrared absorbing material particles. By improving the dispersibility, the formed mark can be read well.

That is, in the offset printing, it is possible to prevent the infrared absorbing material powder from being deposited from the ink during printing, or the occurrence of a missing state where the infrared absorbing portion is not printed. Further, in the case of the thermal transfer ribbon, it is possible to obtain a uniform ribbon coating of the coating layer, and it is possible to prevent transfer failure during printing. Furthermore, even for toner for electrophotographic prints,
The infrared ray absorbing material powder content of the toner ink can be kept uniform, and a mark having a stable absorption level can be obtained.

Typical examples of coating methods for modifying the surface of the infrared absorbing material are listed below. (A) Coating The coating functions as a surfactant. As a specific example, a dispersant such as fatty acid (low molecular weight / high molecular chain), fatty acid salt, and wax can be used.
In particular, when an infrared absorbing material coated with a coating agent using a fluorophosphate fatty acid is made into an ink and printed, uneven printing on the infrared absorbing pattern printed portion and the detection mark printed portion can be remarkably improved. Also,
Coating can also be performed by dispersing the coating agent in the ink vehicle and mixing the infrared absorbing material therein, and the dispersion in the vehicle is also possible.

(B) Coupling Agent The coupling agent is firmly bound to the infrared absorbing material and also reacts with the polymer. As a specific example, a silane compound,
Examples thereof include titanium compounds and metal chelate compounds. (C) Polymerizable monomer A low molecular weight monomer or oligomer is reacted with the surface of the infrared absorbing material to form an irreversible layer. Specific examples include polymerizable organic acids and reactive oligomers.

In the offset and letterpress inks of the present invention, the resin constituting the vehicle is generally a natural resin such as protein, rubber, celluloses, shellac, copal, starch, rosin, etc., vinyl resin, acrylic resin. Thermoplastic resins such as resin, styrene resin, polyolefin resin, novolac type phenol resin, etc., thermosetting resin such as resol type phenol resin, urea resin, melamine resin, polyurethane resin, epoxy, unsaturated polyester, etc. . Further, in the vehicle, if necessary, a plasticizer for stabilizing the flexibility and strength of the printed film, a solvent for adjusting viscosity and drying property, and an auxiliary agent for drying, viscosity, dispersibility, various reaction agents, etc. Can be added as appropriate.

However, since it is not desirable that the formed mark adsorb contaminants by the oil and fat component, it is preferable to use a photopolymerization curable or electron beam curable ink that does not use the oil and fat component that is liquid at room temperature. To form. The main component of the cured product of these inks is an acrylic resin. Therefore, the ink contains an alkyl monomer, and specific examples thereof include the following acrylic monomers that are commercially available.

As the monofunctional acrylate, 2-ethylhexyl acrylate, 2-ethylhexyl EO adduct acrylate, ethoxydiethylene glycol acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, caprolactone adduct of 2-hydroxyethyl acrylate, 2- Phenoxyethyl acrylate, phenoxydiethylene glycol acrylate, nonylphenol EO adduct acrylate,
Acrylate obtained by adding caprolactone to nonylphenol EO adduct, 2-hydroxy-3-phenoxypropyl acrylate, tetrahydrofurfuryl acrylate, furofuryl alcohol caprolactone adduct acrylate, acryloylmorpholine, dicyclopentenyl acrylate, dicyclopentanyl acrylate, dicyclopentyl acrylate Pentenyloxyethyl acrylate, isobornyl acrylate, acrylate of caprolactone adduct of 4,4-dimethyl-1,3-dioxolane, acrylate of caprolactone adduct of 3-methyl-5,5-dimethyl-1,3-dioxolane, etc. Can be used.

On the other hand, as the polyfunctional acrylate, hexanediol diacrylate, neopentyl glycol diacrylate, polyethylene glycol diacrylate, tripropylene glycol diacrylate, hydroxypivalic acid neopentyl glycol ester diacrylate, hydroxypivalic acid neopentyl glycol ester. Diacrylate of caprolactone adduct of
Acrylic acid adduct of diglycidyl ether of 1,6-hexanediol, diacrylate of acetal compound of hydroxypivalaldehyde and trimethylolpropane, 2,2-bis [4- (acryloyloxydiethoxy) phenyl] propane, 2,2-bis [4- (acryloyloxydiethoxy, phenyl] methane, hydrogenated bisphenol A ethylene oxide adduct diacrylate, tricyclodecane dimethanol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, Trimethylol propane propylene oxide adduct triacrylate, glycerin propylene oxide adduct triacrylate, dipentaerythritol hexaacrylate / pentaacrylate mixture, dipentaery Lower fatty acids and esters of acrylic acid Lithol, dipentaerythritol caprolactone adduct acrylate, tris (acryloyloxyethyl) isocyanurate, and 2-acryloyloxyethyl phosphate may be used.

Inks composed of these resins or monomers are solvent-free and cure by causing a chain polymerization reaction by irradiation with electromagnetic waves or electron beams. Among these, for the ultraviolet irradiation type, a photopolymerization initiator and, if necessary, a polymerization inhibitor, a chain transfer agent and the like as a sensitizer and an auxiliary agent are added.

As the photopolymerization initiator, 1) direct photolysis type arylalkylketone, oxime ketone, acylphosphine oxide, etc., 2) radical polymerization type benzophenone derivative, thioxanthone derivative, etc.
3) There are aryl diazonium salts, aryl iodonium salts, aryl sulfonium salts, aryl acetophenones, etc. as the cationic polymerization reaction type, and in addition, 4) energy transfer type, 5) photoredox type, and 6) electron transfer type are used. obtain.

For the electron beam curable type, the same resins or monomers as those used in the ultraviolet ray irradiation type described above may be used without the need for a photopolymerization initiator, and various auxiliaries may be added as necessary.

The ink-jet ink may contain water and an aqueous organic solvent in addition to the infrared absorbing material powder of the present invention and the vehicle. The water may have a purity of ion-exchanged water or higher.

The water-soluble organic solvent is used for the purpose of preventing the ink from drying and imparting penetrability, for example, ethylene glycol,
Polyhydric alcohols such as diethylene glycol, polyethylene glycol and glycerin: N-alkylpyrrolidones: esters such as ethyl acetate and amyl acetate: lower alcohols such as methanol, ethanol, propanol and butanol: ethylene oxide of methanol, butanol and phenol Alternatively, glycol ethers such as propylene oxide adducts can be used. These water-soluble organic solvents are not limited to the above-mentioned solvent examples, and may be a single solvent or a plurality thereof in consideration of hygroscopicity, moisture retention, dye solubility and penetrability of the solvent, ink viscosity and freezing point. Used in. The amount of the water-soluble organic solvent used is preferably in the range of 0.1 to 70% by weight of the ink.

In order to satisfy various conditions required for the system of the ink jet recording apparatus, it is possible to add an additive conventionally known as a component of the ink, if necessary. Examples of these additives include alcohol amines as pH adjusters, ammonium salts, metal hydroxides: organic salts as resistivity adjusters, inorganic salts: antioxidants: antiseptics: fungicides: sequestering agents. Examples of the chelating agent and the like.

In addition to the above composition, polyvinyl alcohol, polyvinylpyrrolidone, carboxymethyl cellulose, styrene acrylic acid resin, styrene maleic acid resin, etc. are added to the extent that clogging of the jetting nozzle portion and change in ink ejection direction do not occur. A water-soluble resin can also be added.

In addition to the infrared ray absorbing material powder of the present invention, a synthetic resin, a wax, and, if necessary, a solvent and a colorant are added to the thermal transfer ribbon ink, the gravure ink and the screen ink for printing the detection mark as a vehicle. And prepare. As the synthetic resin, an appropriate one is used alone or as a mixture in consideration of the voltage and melting point of the thermal head. Specific examples include polyethylene, polystyrene, polypropylene, polybutyne, petroleum resin,
Examples thereof include vinyl chloride resin, polyvinyl alcohol, vinylidene chloride resin, methacrylic resin, polyamide, polycarbonate, fluororesin, polyvinyl formal, polyvinyl butyral, acetyl cellulose plastic, nitrocellulose, and polyacetal. The wax can be appropriately selected and used from beeswax, touch wax, ivowa wax, wool wax, shellac wax, carnauba wax, montan wax, paraffin wax, candelilla wax, betorolactam, microcrystalline wax and the like.

The solvent is used when the thermal transfer ribbon ink composition is made into an ink which can be applied by a usual printing method. Benzene, xylene, toluene, trichlene, white spirit, ethyl acetate, n-butyl acetate, methanol,
Examples are ethanol, isopropanol, n-butanol, ethylcyclohexane, methyl ethyl ketone, ethyl cellosolve, butyl cellosolve, cyclohexanone. In particular, methyl ethyl ketone, ethyl acetate, methanol, ethanol, xylene and toluene are preferably used.

The thermal transfer ribbon ink may be a thermal transfer sheet provided on a base film. As a material for the base film, a commonly used material may be used. Specifically, polyester, polypropylene, cellophane,
Films of plastics such as acetate and polycarbonate, and papers such as condenser paper and paraffin paper can be used.

In the case of the electrophotographic system, the constituent components of the toner ink are the infrared absorbing material powder of the present invention, a vehicle, and if necessary, a charge control agent, an offset preventing agent, and an external additive (fluidizing agent). Examples of the vehicle include thermoplastic resins such as polystyrene resins, styrene-acrylic copolymers, styrene-butadiene copolymers, polyester resins, epoxy resins and polyolefin resins. Regarding the contact charging property, a compound containing an electron-donating substituent such as an amino group is likely to be positively charged, and a compound having an electron-accepting substituent such as fluorine or a carboxyl group is likely to be negatively charged.

As the charge control agent, a nigrosine dye, a quaternary ammonium compound or the like is used for positive charge, and a metal complex of alkylsalicylic acid or an azo metal-containing complex is used for the negative charge toner. As other additives, low molecular weight polyethylene, low molecular weight polypropylene, etc. are used as offset preventing agents for heat roll fixing.

Further, the ink of the present invention may contain an irreversible decolorizable colorant in the composition. The decolorizable colorant in this case is a colorant that maintains a visible state in the visible light range and irreversibly changes to an invisible state by an operation for decoloring, for example, an operation such as irradiation of near infrared rays. When a code pattern is formed with an ink composition containing such a color erasable colorant, the printed image can be visually identified and the printing accuracy can be improved. After that, the code pattern can be changed to the invisible state by performing the erasing operation.

As a specific example, the coexistence of an erasable colorant IR820B (manufactured by Showa Denko) represented by the following structural formula, a cyanine dye and an organic boron ammonium salt such as tetrabutylammonium / butyltriphenylborate. There are some that absorb near-infrared light and couple with each other to become irreversibly transparent.

[0041]

[Chemical 1]

The infrared absorbing glass of the present invention shows a sharp absorption at about 970 nm as shown in FIG. Therefore, a code pattern or a detection mark formed by using this infrared absorbing glass is used as an irradiation light source, for example, light of 900 nm or less and 1000 nm or more with respect to pulsed infrared light of a semiconductor laser or infrared emission of a light emitting diode. If a lens coated with a bandpass filter that absorbs light is attached to the light receiving sensor side and infrared rays are irradiated, it can be identified as a sharp absorption signal.

The present invention includes an invisible pattern and an invisible information pattern using the infrared absorbing material. Here, the pattern includes a non-information pattern and an information pattern. The non-information pattern may be a detection mark or the like. The information pattern may be a code pattern. As a code pattern,
A barcode can be exemplified, and the barcode may be a two-dimensional code or the like other than the one-dimensional barcode. In particular, the present invention is effective for a two-dimensional code because a high resolution can be obtained.

The detection mark is a mark provided for setting the paper feed timing of a transparent sheet which is not optically detected when an image is formed by a copying machine using an optical detection method. . The shape of the detection mark and the position of the detection mark on the transparent sheet are not particularly limited. For example, JP-A-58-106550 and JP-A-58-1
No. 05157, 59-7367 and JP-A-3-99.
A detection mark as described in Japanese Patent No. 878 can be used.

[0045]

EXAMPLES The present invention will be further described below based on examples.

Example 1 A mixture of 100 parts by weight of ytterbium oxide (Yb ₂ O ₃ ) powder and 100 parts by weight of phosphorus pentoxide (P ₂ O ₅ ) powder was placed in a platinum crucible and heated to 1300 ° C. It was melted by heating for 2 hours. After that, 200 ° C /
It was annealed at a rate of hr to obtain a solid. An X-ray diffraction test was performed using this solid as a sample. As a result, this solid was tetragonal YbPO ₄ . Furthermore, the crystallite diameter was determined by the Scherrer method, and was 30 nm.

The YbPO ₄ solid obtained above was pulverized to obtain a powder having an average particle diameter of 0.9 μm. The result of measuring the spectral reflectance of this powder is shown in FIG. The measurement was carried out using a self-recording spectrophotometer UV3101PC manufactured by Shimadzu Corporation, with the reflectance of barium sulfate being 100%.

Example 2 4 parts by weight of the powder obtained in Example 1 was added to an offset vehicle consisting of 6 parts by weight of an acrylate monomer, 3 parts by weight of a wax and 0.5 part by weight of a sensitizer, and mixed to prepare an offset ink. Was prepared. This ink was offset-printed on a coated paper with a bar code by a conventional method.

The bar code obtained was not visible to the naked eye. An infrared light emitting diode (SHARP, GL480) was used as a light source for this bar code, and a CCD linear sensor (SONY, ILX503) was used as a light receiving unit to perform a reading test. As a result, the barcode information could be read. Furthermore, the obtained bar code was subjected to a UV irradiation deterioration test using an arc lamp for 100 hours,
Further, the bar code information could be read even after the chemical deterioration test with weak acid and weak alkali, and the weather resistance was excellent.

Example 3 An ink ribbon was obtained by coating 3 parts by weight of the powder obtained in Example 1 with 5 parts by weight of wax and stirring the resulting ink ribbon ink on a PET film having a thickness of 4 μm by means of grapia coating. It was A bar code was printed on a coated paper by a conventional method using this ink ribbon.

The obtained bar code was invisible to the naked eye, as in Example 4. Furthermore, infrared light emitting diodes (SHARP, GL
480) and a CCD linear sensor (S
A read test was performed using ONY, ILX503). As a result, the barcode information could be read.

Example 4 5.0 parts by weight of the powder obtained in Example 1 was added to 2.4 parts by weight of acrylic resin, 6.4 parts by weight of nitrified cotton, 13.0 parts by weight of isopropyl alcohol, and 15.9 parts by weight of denatured ethanol. Parts, 31.9 parts by weight of ethyl acetate and 5.4 parts by weight of propylene glycol monopropyl ether to obtain a gravure ink. The obtained ink was gravure printed on the edge of a PET sheet having a thickness of 100 μm to obtain a transparent OHP sheet having an optical detection mark. This OHP
The part of the gravure printed detection mark on the sheet was also transparent.

The transparent OHP sheet thus obtained was subjected to a print test with a copying machine equipped with a light-sensitive type sheet detection device. As a result, both the sheet detection property and the paper feeding property were good. Further, since the detection mark portion of the image-formed sheet is transparent, the image when used in OHP was easy to see.

[0054]

According to the present invention, it has no absorption in the visible light region and has a sharp absorption peak in the infrared region, and is used for forming an infrared absorption mark such as an infrared absorption code pattern or an infrared absorption detection mark. It is possible to provide excellent materials and ink materials.

[Brief description of drawings]

1 shows a spectrum of spectral reflectance of YbPO ₄ powder obtained in Example 1. FIG.

Claims (4)

[Claims] 1. YbPO ₄ having a diameter of 5 to 200 nm.
An infrared-absorbing material comprising a crystallite of 2. An ink containing the powder of the infrared absorbing material according to claim 1 and a vehicle. 3. An invisible pattern containing the infrared absorbing material according to claim 1. 4. An invisible information pattern containing the infrared absorbing material according to claim 1.