JP2005233990A - Electrophotographic toner, electrophotographic developer and method for forming image - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrophotographic toner characterized in that information in an invisible image formed on an image output medium surface can be read even when the toner amount (TMA) per unit area is small and that an invisible image in any figure can be hardly visually recognized. <P>SOLUTION: The electrophotographic toner contains an inorganic near-IR absorbent and further contains an organic near-IR absorbent. When an image is formed on a substrate by controlling the toner amount (TMA) per unit area to 3 (×10<SP>-4</SP>g/cm<SP>2</SP>), the invisible information recovery rate of the image is ≥80% which is the information recovery rate when the image is read by using near-IR light at 800 to 1,200 nm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  INDUSTRIAL APPLICABILITY The present invention is suitably used when forming an arbitrary near-infrared light absorption invisible image that can be read by a reading device or the like on the surface of an image output medium such as recording paper but is difficult to visually observe. The present invention relates to an electrophotographic toner, an electrophotographic developer, and an image forming method.

  Conventionally, there is an additional data embedding technique for embedding additional information in an image. In recent years, there has been an active movement to improve security by using this additional data embedding technology for copyright protection of digital works such as still images, prevention of unauthorized copying, and ID cards.

  When the additional data embedding technique is used for a digital work, generally, image data in which additional data such as a copyright ID and a user ID is embedded so as not to be visually noticeable is distributed. In order to prevent forgery of securities and the like, various measures are incorporated in the color image forming apparatus. As one of the techniques, there is a technique for superimposing a symbol unique to an image forming apparatus that is difficult to visually recognize on the image with a certain amount of modulation on the image information in order to identify the image forming apparatus used for copying or printing. is there.

  If this technology is used, even if securities are forged using the image forming apparatus, the image of the counterfeit is read by a reading device that can extract a specific wavelength range, and the above image formation is performed. Symbols unique to the device can be read. Therefore, since the image forming apparatus used for counterfeiting can be specified by reading this symbol, an effective clue for tracking the counterfeiter can be obtained.

  However, in the above technique, depending on the gradation characteristics of the image forming apparatus, even if a symbol specific to the image forming apparatus is superimposed in the low density range, it is not reflected in the image density and becomes unreadable, However, in the high density region, there is a problem that the superimposed symbols unique to the image forming apparatus can be easily recognized visually.

  Under such circumstances, as a technique for embedding additional information so as not to be visually noticeable, for example, JP-A-7-271081 (Patent Document 1) discloses phosphoric acid having copper ions as a near-infrared light absorber. The use of salt powder and aminium compounds is disclosed.

  Japanese Patent Laid-Open No. 2002-146254 (Patent Document 2) discloses an infrared absorption having a spectral absorption maximum wavelength at 750 nm to 1100 nm and having an absorbance at 650 nm of 5% or less of the absorbance at the spectral absorption maximum wavelength. In the infrared absorbing ink for forming an invisible pattern containing an agent, a squarylium compound and a croconium compound are disclosed as infrared absorbing agents.

Japanese Patent Laid-Open No. 7-271081 JP 2002-146254 A

  However, in the method of Patent Document 1, the phosphate powder having copper ions has a cyan color due to absorption generated from 550 nm, and the aminium compound also has absorption at around 400 or 800 nm, so that it has a dark blue color. It has a color and, in actual use, it is colored in the visible part and becomes visible.

  Further, in the method of Patent Document 2, the absorption amount of only 650 nm is defined in the invisible degree determination method, and the invisible degree of 380 to 780 nm that is the visible light region is not defined. When the indicated near-infrared light absorber is used, coloring in the visible region is inevitable.

  As described above, in the conventional technique, the invisible rule is ambiguous, so that it is actually invisible, but in practice, coloring can be easily confirmed. On the other hand, when a near-infrared light absorber such as a white inorganic material is used alone, the amount of fixing toner increases because of generally less absorption, resulting in a large amount of lamination on the document. The toner becomes a toner, and the problem that it is easy to visually confirm arises.

  In the present invention, information can be read from an invisible image provided on the surface of an image output medium even if the amount of toner (TMA) per unit area is small, and it is difficult to visually confirm an invisible image of an arbitrary shape. An electrophotographic toner, an electrophotographic developer, and an image forming method.

The present invention relates to an electrophotographic toner containing an inorganic near-infrared light absorber, wherein the toner further contains an organic near-infrared light absorber, and the toner amount (TMA) per unit area is determined. When an image is formed on a substrate as 3 (× 10 ⁻⁴ g / cm ² ), the invisible information restoration rate is an information restoration rate when the image is read using near-infrared light of 800 to 1200 nm. Is 80% or more.

  The present invention is also an electrophotographic developer containing the electrophotographic toner.

  Furthermore, the present invention is an image forming method using the electrophotographic toner.

  According to the present invention, when only a non-visible image or a visible image laminated on the non-visible image is viewed on the surface of the image output medium, even a small amount of TMA can be read without impairing the image quality of the visible image. An electrophotographic toner, an electrophotographic developer, and an image forming method capable of obtaining an invisible information pattern can be provided.

  Hereinafter, embodiments of the electrophotographic toner of the present invention will be described.

The electrophotographic toner containing the inorganic near-infrared light absorber according to this embodiment further contains an organic near-infrared light absorber, and the toner amount (TMA) per unit area is 3 (× 10 ^{− 4} g / cm ² ), when an image is formed on a substrate, the invisible information restoration rate, which is the information restoration rate when the image is read using near infrared light of 800 to 1200 nm, is 80% or more. is there.

In the electrophotographic toner, when an image is formed on a substrate with a toner amount (TMA) per unit area of 10 (× 10 ⁻⁴ g / cm ² ), the maximum of the toner at 380 to 780 nm is obtained. It is preferable that the absorption rate of the absorption wavelength λmax is 5% or less of the absorption rate of λmax at 800 to 1200 nm, and the near infrared light absorption rate of λmax at 800 to 1200 nm of the toner is 15% or more.

In the toner for electrophotography, the absorption rate of λmax at 380 to 780 nm of the inorganic near infrared light absorber is 5% or less of the absorption rate of λmax at 800 to 1200 nm, and the absorption of inorganic near infrared light The near-infrared light absorption rate of λmax at 800 to 1200 nm of the agent is 10% or more, and the absorption rate of λmax at 380 to 780 nm of the organic near-infrared light absorber is 20% of the absorption rate of λmax at 800 to 1200 nm. The molar absorption coefficient of λmax at 800 to 1200 nm of the organic near-infrared light absorber is preferably 1.0 × 10 ⁴ or more.

  In one embodiment of the electrophotographic toner of the present invention, the toner contains an inorganic near-infrared light absorber and an organic near-infrared light absorber. The inorganic system here refers to a compound containing no carbon and a simple carbon compound. Further, the organic type refers to all carbon compounds other than inorganic compounds, that is, a small number of simple compounds such as carbon compounds and metal carbonates. Further, it may contain a binder resin, and if necessary, it may contain a pigment, a release agent, and other components within a range not affecting the coloring.

  In the electrophotographic toner containing two types of inorganic near-infrared light absorber and organic near-infrared light absorber, the inorganic near-infrared light absorber absorbs at a maximum absorption wavelength λmax at 380 to 780 nm. The rate is preferably 5% or less, more preferably 3% or less, of the absorption rate of λmax at 800 to 1200 nm. In addition, the near-infrared light absorptance of λmax at 800 to 1200 nm of the inorganic near-infrared light absorber is preferably 10% or more, and more preferably 15% or more. Otherwise, the degree of invisibility in the visible light region becomes ambiguous, so the information pattern can be confirmed visually, or when reading, the absorption difference between the visible light region and the near-infrared light region. May not be obtained, and the function as a toner for an invisible information pattern may not be achieved.

  The maximum absorption wavelength λmax of the inorganic near-infrared light absorber is preferably in the wavelength range of 800 to 1200 nm, more preferably in the wavelength range of 800 to 1000 nm, and the optical sensitivity of the CCD is high from 900 nm to 900 nm. More preferably, it has a wavelength range of 1000 nm.

  The inorganic near-infrared light absorber has at least one selected from ytterbium oxide, ITO (Indium Tin Oxide), ATO (Antimony Tin Oxide), tin oxide, or the like, having the characteristics shown above. It is preferred to use one compound.

Further, the absorption rate of λmax at 380 to 780 nm of the organic near infrared light absorber is preferably 20% or less, more preferably 10% or less of the absorption rate of λmax at 800 to 1200 nm. In addition, the molar extinction coefficient of λmax at 800 to 1200 nm of the organic near-infrared light absorber is preferably 1.0 × 10 ⁴ or more, more preferably 1.0 × 10 ⁵ or more. preferable. If this is not satisfied, the degree of invisibility in the visible light region is likely to be ambiguous, making it easier to visually confirm the information pattern, and for reading, between the visible light region and the near infrared light region. Thus, there is a possibility that the function as the toner for the invisible information pattern cannot be performed.

  The maximum absorption wavelength λmax of the organic near-infrared light absorber is preferably in the wavelength region of 800 to 1200 nm, more preferably in the wavelength region of 800 to 1000 nm, and the optical sensitivity of the CCD is high from 900 nm to 900 nm. More preferably, it has a wavelength range of 1000 nm.

  As the organic near infrared light absorber, those having the characteristics shown above, phthalocyanine compounds, cyanine compounds, aminium compounds, imonium compounds, nickel complex compounds, anthraquinone compounds, squarylium compounds, It is preferable to use at least one compound selected from croconium compounds, polymethine compounds, pyrylium compounds, or naphthalocyanine compounds.

  “Absorptance” (near-infrared light absorptivity) at λmax in the near-infrared light region of organic and inorganic near-infrared light absorbers is a spectral reflectance measuring machine (Hitachi: U-4000), etc. And NIR (1) for the spectral reflectance at λmax in the near infrared region of the fixed image (paper + toner) and NM (1) for the spectral reflectance at λmax in the near infrared region of the paper. By measuring, it can obtain | require as shown in the following Formula (1).

  Absorptivity at λmax in the near infrared region = NIR (1) −NM (1) (1)

  Furthermore, by measuring in the visible light region in the same manner as described above, it is possible to obtain the “absorption rate” (visible light absorption rate) of λmax in the visible light region of organic and inorganic near-infrared light absorbers. . That is, by measuring the spectral reflectance of the fixed image (paper + toner) at λmax in the visible light region V (1) and the spectral reflectance of the paper VM (1), the following equation (2) is obtained. Can be asking.

  Absorptivity at λmax in the visible light region = V (1) −VM (1) (2)

The “maximum absorption wavelength λmax” used in inorganic near-infrared light absorbers and organic near-infrared light absorbers is the wavelength of the spectrum having the largest absorption in any spectral range as described above. Refers to that. The “molar extinction coefficient” represented by the organic near-infrared absorber is an attenuation coefficient related to absorption, and indicates that the larger the coefficient, the larger the amount of absorption. The molar extinction coefficient ε is measured by measuring the absorption of a solution obtained by dissolving a near-infrared light absorbing agent having an arbitrary concentration in acetone as a solvent, using the above-described spectral reflectance measuring device or the like, and the Lambert-Beer equation (A = (εlc) (where A: absorbance, ε: molar extinction coefficient [cm ⁻¹ · M ⁻¹ ], l: cell length [cm], c: concentration [M]).

  In the electrophotographic toner according to the exemplary embodiment, the content ratio of the inorganic near-infrared light absorber and the organic near-infrared light absorber is preferably 99: 1 to 90:10 by mass ratio, More preferably, it is 99: 1 to 96: 4. When the ratio of the inorganic near-infrared light absorber is smaller than 90:10, the ratio of the organic near-infrared light absorber increases and the absorption increases, but the image is easily recognized visually. In addition, the absorption difference between the near-infrared part and the visible part becomes small, and the invisible information restoration rate described later deteriorates. When the ratio of the organic near-infrared light absorber is less than 99: 1, the amount of absorption in the near-infrared portion is insufficient, and it becomes difficult to read information.

  Further, the total amount of each of the near-infrared light absorbers is preferably 10 to 70% by mass, more preferably 15 to 60% by mass in the toner. If it is less than 10% by mass, absorption capable of reading information cannot be obtained. Moreover, when it exceeds 70 mass%, coloring of an organic type near-infrared-light absorber is conspicuous, and there exists a possibility that it may become easy to recognize visually. In addition, when toner is formed by kneading, the amount of binder becomes relatively small, making it difficult to make toner, and inorganic near-infrared light absorber and organic near-infrared light absorber are uniformly dispersed. It is likely that it will be difficult.

  The average dispersion diameter of the inorganic near-infrared light absorber is preferably 3 μm or less, more preferably 1 μm or less. When it exceeds 3 μm, the specific surface area decreases, and thus the absorbency tends to decrease. The lower limit is preferably 0.05 μm or more. Less than that is practically impossible and undesirably lowers the handling properties.

  Further, the average dispersion diameter of the organic near infrared light absorber is preferably 1 μm or less, more preferably 0.5 μm or less. When it exceeds 1 μm, the coloring of the near-infrared light absorber is easily noticeable.

  The “average dispersion diameter” means the average particle diameter of individual near infrared absorbers dispersed in the toner. This average dispersion diameter is determined by TEM (transmission electron microscope: manufactured by JEOL Datum Co., Ltd., JEM-1010), with respect to 1000 particulate near infrared absorbers dispersed in the toner. The particle diameter is calculated from the cross-sectional area, and can be obtained from the average value.

  The particle size distributions of the inorganic near-infrared light absorber and the organic near-infrared light absorber in the toner are each preferably 1 to 1.5, and more preferably 1 to 1.2. When the particle size distribution exceeds 1.5, the presence of coarse particles tends to reduce the near infrared light absorption.

  The “particle size distribution” was calculated from the 16% diameter and the 84% diameter in the “average dispersion diameter” obtained above.

  In particular, in the use of an inorganic near-infrared light absorber and an organic near-infrared light absorber, an inorganic near-red light whose λmax absorption rate at 380 to 780 nm is 5% or less of the λmax absorption rate at 800 to 1200 nm. An external light absorber is preferable for reading information of an invisible information pattern. However, although the inorganic near-infrared light absorber has a white appearance, since it absorbs little in the near-infrared light region, it alone requires a large amount of toner. It becomes easy to recognize visually.

Therefore, an organic material having an absorption factor of λmax at 380 to 780 nm that is 20% or less of the absorption factor of λmax at 800 to 1200 nm, and a molar extinction coefficient of λmax at 800 to 1200 nm is 1.0 × 10 ⁴ or more. By further adding a system near infrared light absorber, there is almost no absorption at 380 to 780 nm while there is absorption at 800 to 1200 nm, and as a result, it is substantially invisible while having a small toner amount (TMA). It is preferable because a certain information pattern can be formed.

  Here, TMA is the amount of toner per unit fixed on the substrate. Specifically, it is the weight (g) of the toner placed in 1 × 1 cm, and the weight (g) of the toner multiplied by 10,000 times is expressed as TMA. As a toner weight measuring method, the toner weight can be obtained from the weight of the paper after toner fixing-the weight of only the paper before toner fixing. Further, TMA can be calculated by multiplying the weight by 10,000 times.

Specifically, when an image is formed on a substrate with a toner amount (TMA) per unit area of 3 (× 10 ⁻⁴ g / cm ² ), near infrared light of 800 to 1200 nm is applied to the image. The invisible information restoration rate, which is the information restoration rate when used and read, is 80% or more, preferably 85% or more. By setting the invisible information restoration rate to 80% or more, an information pattern capable of reading information in the near infrared light region of 800 to 1200 nm can be formed.

  Here, the “invisible information restoration rate” means that an image forming surface of a recorded material is irradiated with a light source including light in the near infrared wavelength region of 800 to 1200 nm, and in this state, a wavelength component of 800 nm or less is applied. An invisible image is obtained by reading the image forming surface with a CCD camera having a light receiving sensitivity in a wavelength range of 800 nm to 1000 nm with a filter to be cut mounted on the lens unit, and binarizing with a certain contrast (threshold) as a boundary. Extract the data, decrypt it with software, evaluate whether the information can be accurately restored, and when this evaluation is performed a predetermined number of times, the number of times that the information can be restored correctly is calculated as the invisible information restoration rate. (%). For the measurement of the invisible information restoration rate, P paper was used as a substrate.

Further, in order to form an information pattern that is substantially invisible and capable of reading information in the near infrared light region of 800 to 1200 nm, the toner amount (TMA) per unit area is 10 (× 10 ⁻⁴ g). / Cm ² ), when the image is formed on the substrate, the absorption rate of the maximum absorption wavelength λmax at 380 to 780 nm of the toner is 5% or less of the absorption rate of λmax at 800 to 1200 nm, and 800 of the toner. The near infrared light absorptance of λmax at ˜1200 nm is preferably 15% or more. At this time, it is more preferable that the absorption rate of the maximum absorption wavelength λmax at 380 to 780 nm of the toner is 3% or less of the absorption rate of λmax at 800 to 1200 nm. Further, it is more preferable that the near-infrared light absorptance of λmax at 800 to 1200 nm of the toner is 30% or more. If the near-infrared light absorptivity of λmax at 800 to 1200 nm of the toner is 15% or more, it is preferable from the viewpoint of ensuring reading intensity and decoding accuracy by a reading device such as a CCD.

From the above, even when an image is formed on the substrate with the toner amount per unit area (TMA: × 10 ⁻⁴ g / cm ² ) being 3 to 10 in actual use, in the range of 800 to 1200 nm, An invisible information pattern capable of reading information can be formed. When TMA is less than 3, it becomes difficult to read information and it is difficult to recognize an accurate information pattern. On the other hand, when the TMA exceeds 10, toner lamination is conspicuous and the toner itself is easily colored.

  Further, when a high-definition image incorporating higher-density information is formed and this is read by a CCD, the absorption peak (maximum absorption rate) of the toner is preferably in the wavelength range of 800 to 1200 nm, It is more preferable to have in the wavelength range of 800 to 1000 nm, and it is more preferable to have in the wavelength range of 900 nm to 1000 nm where the optical sensitivity of the CCD is high.

  The λmax of the toner can be measured in the same manner as in the case of the near infrared light absorber. Further, the absorption rate of λmax (near infrared light absorption rate) in the near infrared light region of the toner is the same as in the case of the above-mentioned near infrared light absorber, a spectral reflectance measuring machine (Hitachi: U-4000). ) And the like, the spectral reflectance in the near-infrared light region of the image formed with toner is measured as NIR (2), and the spectral reflectance in the near-infrared light region of the image output medium is measured as NM (2). Is obtained as shown in the following expression (3).

  Near-infrared light absorption rate of toner = NIR (2) −NM (2) (3)

  Further, similarly to the above, by measuring in the visible light region, the absorptance (visible light absorptance) of the toner in the visible light region can also be obtained. That is, by measuring the spectral reflectance of the visible light region of the image formed with toner as V (2) and the spectral reflectance of the image output medium as VM (2), the following equation (4) is obtained. Desired.

  Visible light absorption rate of toner = V (2) −VM (2) (4)

  Moreover, a banknote, a stock certificate, a passport, etc. can be used as a base material in this embodiment.

  As the binder resin used in one embodiment of the electrophotographic toner of the present invention, when prepared as an electrophotographic toner, the absorptivity in the visible light region, the average dispersion diameter, and the particle size distribution are as described above. Although it will not specifically limit if it is the inorganic material particle which satisfy | fills, For example, the material which is enumerated below can be used.

  For example, styrenes such as styrene and chlorostyrene; monoolefins such as ethylene, propylene, butylene, and isoprene; vinyl esters such as vinyl acetate, vinyl propionate, vinyl benzoate, and vinyl acetate; methyl acrylate, ethyl acrylate, acrylic Α-methylene aliphatic monocarboxylic acid ester such as butyl acid, dodecyl acrylate, octyl acrylate, phenyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, dodecyl methacrylate; vinyl methyl ether, vinyl ethyl ether, Homopolymers or copolymers of vinyl ethers such as vinyl butyl ether, vinyl ketones such as vinyl methyl ketone, vinyl hexyl ketone, and vinyl isopropenyl ketone;

  Particularly representative binder resins include polystyrene, styrene-alkyl acrylate copolymer, styrene-alkyl methacrylate copolymer, styrene-acrylonitrile copolymer, styrene-butadiene copolymer, styrene-maleic anhydride copolymer. Examples thereof include a polymer, polyethylene, and polypropylene. Further examples include polyester, polyurethane, epoxy resin, silicon resin, polyamide, modified rosin, paraffin, waxes and the like.

  As the binder resin and the near-infrared light absorber constituting one embodiment of the electrophotographic toner of the present invention, the materials as described above are preferably used, but the following materials are particularly used. preferable. That is, when the electrophotographic toner according to the present embodiment is produced by kneading, the binder resin is a resin mainly composed of polyester, the inorganic near infrared light absorber is ytterbium oxide, It is at least one compound selected from ITO, ATO, tin oxide, etc., and the organic near infrared light absorber is a phthalocyanine compound, cyanine compound, aminium compound, imonium compound, nickel complex compound, anthraquinone It is preferably at least one compound selected from a compound, a squarylium compound, a croconium compound, a polymethine compound, a pyrylium compound, a naphthalocyanine compound, and the like.

  When a toner is formed by a hot melt kneading and pulverization method containing a near infrared light absorber, the dispersion uniformity of the near infrared light absorber in the toner particles, the degree of freedom in setting the concentration, and the near red From the viewpoint of securing the mechanical strength of the outer toner particles, it is preferable to use polyester as the binder resin.

  As the polyester resin, a polyester resin synthesized by polycondensation from a polyol component and a polycarboxylic acid component is preferably used as a binder resin. For example, bisphenol A and a polyvalent aromatic carboxylic acid are mainly used as a single monomer. A linear polyester resin comprising a polycondensate as a body component can be preferably used.

  Examples of the polyol component used for the synthesis of the polyester resin include ethylene glycol, propylene glycol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, diethylene glycol, triethylene glycol, and 1,5-butane. Examples include diol, 1,6-hexanediol, neopentyl glycol, cyclohexanedimethanol, hydrogenated bisphenol A, bisphenol A-ethylene oxide adduct, bisphenol A-propylene oxide adduct, and the like.

  As the polycarboxylic acid component, maleic acid, fumaric acid, phthalic acid, isophthalic acid, terephthalic acid, succinic acid, dodecenyl succinic acid, trimellitic acid, pyromellitic acid, cyclohexanetricarboxylic acid, 2,5,7-naphthalenetricarboxylic acid, 1,2,4-naphthalenetricarboxylic acid, 1,2,5-hexanetricarboxylic acid, 1,3-dicarboxyl-2-methylenecarboxypropanetetramethylenecarboxylic acid, and the like, and anhydrides thereof may be mentioned.

  Furthermore, as these polyester-based binder resins, the softening point is 90 ° C to 150 ° C, the glass transition point is 50 ° C to 80 ° C, the number average molecular weight is 2000 to 7000, the mass average molecular weight is 8000 to 15000, and the acid value is A resin having a hydroxyl value of 5 to 40 and a hydroxyl value of 5 to 50 is particularly preferable from the viewpoint of imparting glossiness to an invisible information image region formed by a toner capable of exhibiting fixing properties and anti-counterfeiting effects.

  Examples of the wax include the following. For example, paraffin wax and derivatives thereof, montan wax and derivatives thereof, microcrystalline wax and derivatives thereof, Fischer-Tropsch wax and derivatives thereof, polyolefin wax and derivatives thereof, and the like. This derivative includes an oxide, a polymer with a vinyl monomer, and a graft modified product. In addition, alcohols, fatty acids, plant waxes, animal waxes, mineral waxes, ester waxes, acid amides, and the like can be used.

  The amount of the wax added to the toner is preferably in the range of 1% by mass to 10% by mass, and more preferably in the range of 3% by mass to 10% by mass. When the addition amount of the wax is less than 1% by mass, sufficient fixing latitude (temperature range of the fixing roll that can be fixed without toner offset) cannot be obtained. On the other hand, when it is more than 10% by mass, the dispersion uniformity of the near-infrared light absorber is impaired. Further, the powder fluidity of the toner is deteriorated, and free wax adheres to the surface of the photosensitive member on which the electrostatic latent image is formed, so that the electrostatic latent image cannot be formed accurately. Moreover, it is preferable that all are used in the range which does not affect coloring.

  Furthermore, in order to further improve the long-term storability, fluidity, developability, and transferability of the toner, inorganic powders, resin powders are used as additives in a range that does not affect the spectrum from the visible light region to the near infrared light region Etc. may be used alone or in combination. Examples of the inorganic powder include silica, alumina, titania, zinc oxide; examples of the resin powder include spherical particles such as PMMA, nylon, melamine, benzoguanamine, and fluorine-based powder, and amorphous powders such as vinylidene chloride and fatty acid metal salts. Can be mentioned. The addition amount of these additives is preferably in the range of 0.1% by mass to 5% by mass and more preferably in the range of 0.5-3% by mass with respect to the toner particles.

  In particular, when an image is formed on the surface of an image output medium having a high whiteness using the toner, it is preferable to use a white additive for the purpose of further increasing the invisibility of the image. Preferably uses the titania particles described above.

  The titania particles can be added and dispersed inside the toner and / or added to the surface, and can exhibit the effect of increasing the invisibility. It is preferably smaller than the average dispersion diameter. When the titania particle size is larger than the average dispersed particle size of the near-infrared light absorber, the whiteness of the toner is increased, but on the other hand, the light hiding property is increased and the near-infrared light absorption ability is inhibited. There is a case.

  As a method of adding the internal additive to the inside of the toner particles, a known method can be used, and a hot melt kneading process is particularly preferably used. The kneading at this time can be performed using various heating kneaders. Examples of the heat kneader include a three roll type, a single screw type, a twin screw type, and a Banbury mixer type.

  Further, the method for producing the toner particles is not particularly limited, and a known method can be used. When the toner particles are produced by pulverizing the kneaded product, for example, a micronizer, Ulmax, JET-O -It can be carried out by means of a mizer, KTM (krypton), turbome jet or the like. Furthermore, as a subsequent process, the toner shape after grinding by applying mechanical external force using a hybridization system (Nara Machinery Co., Ltd.), mechano-fusion system (Hosokawa Micron Corp.), kryptron system (Kawasaki Heavy Industries Ltd.), etc. Can be changed. In addition, spheronization with hot air can also be mentioned. Further, classification processing may be performed to adjust the toner particle size distribution.

  The volume average particle size of the toner is preferably in the range of 3 μm to 12 μm, and more preferably in the range of 5 μm to 10 μm. When the volume average particle diameter is smaller than 3 μm, the electrostatic adhesion force is larger than that of gravity, and there is a high possibility that it is difficult to handle as a powder. On the other hand, if the volume average particle size is larger than 12 μm, there is a high possibility that it is difficult to record high-definition invisible information.

  The electrophotographic toner according to this embodiment is preferably used for forming an invisible information pattern capable of reading information at 800 to 1200 nm.

(Image forming method)
As an embodiment of the image forming method of the present invention, the electrophotographic toner according to the embodiment is used. In the image forming method according to the present embodiment, an invisible image is provided on the surface of an image output medium, or a visible image is laminated on an invisible image, and at least one of the invisible images is formed of a two-dimensional pattern. Preferably, the invisible image is formed using the electrophotographic toner according to the embodiment.

  An “invisible image” is an image that can be recognized by a reading device such as a CCD in the near-infrared light region, and the toner that forms the invisible image is caused by absorption of a specific wavelength in the visible light region. It means an image that cannot be visually recognized (that is, is invisible) in the visible light region because it does not have the coloring property.

  A “visible image” is an image that cannot be recognized by a reading device such as a CCD in the near-infrared light region, and the toner that forms the visible image absorbs a specific wavelength in the visible light region. It means an image that can be visually recognized (that is, is visible) in the visible light region because of the resulting color developability.

  The invisible image formed by the image forming method according to the present embodiment is formed using the electrophotographic toner, so that machine reading / decoding processing is stably performed over a long period of time by irradiation with near infrared light. Yes, information can be recorded with high density. Further, since the invisible image does not have color developability in the visible light region and is invisible, any image forming surface can be used regardless of whether or not the visible image is provided on the image forming surface of the image output medium. It can be formed in the region.

  Further, when a part or all of the visible image area formed on the image forming surface and the invisible image area overlap, the invisible image is formed in the area formed by overlapping the visible image and the invisible image. The image is preferably formed between the visible image and the image output medium surface. In such a case, only a visible image can be recognized even when the image forming surface is viewed from the front, but when viewed from an oblique direction, a visible image is generated due to a difference in gloss between the region where the invisible image is formed and the other region. The presence of the invisible image can be confirmed without impairing the quality of the image. On the other hand, when an invisible image is formed on the surface of the visible image formed on the surface of the image output medium, the visible light concealment by the invisible image hinders the coloring of the visible image, resulting in an image defect.

  Moreover, the invisible image is protected by the visible image by forming the invisible image between the surface of the image output medium and the visible image. For this reason, since the invisible image is not easily deteriorated due to wear of the image forming surface on which the visible image and the invisible image are formed on the image output medium, the machine reading / decoding is stably performed by near infrared light irradiation for a longer period of time. Can be processed.

  Furthermore, in confidential documents and securities that are highly likely to suffer a great disadvantage due to the distribution of counterfeits, information recorded as an invisible image to identify authenticity is protected by the visible image. It becomes extremely difficult to remove or rewrite information, and an excellent anti-counterfeiting effect can be obtained.

  Visual recognition of invisible images caused by such gloss differences is not limited to the real recognition / anti-counterfeiting effect. For example, image output by a handy type reader such as a barcode is possible. When reading information of an invisible image formed at a specific position on the surface of the medium, it can be widely used for other purposes as a mark for recognizing the position where the invisible information is recorded.

  In the image forming method according to the present embodiment, the visible image may be formed with at least one of yellow, magenta, and cyan toners having an absorption rate of 5% or less in the near infrared region. preferable.

  In this embodiment, when electrophotography is also used for visible image formation, a known toner can be used for visible image formation, but the absorptance (near infrared light) in the near infrared light region can be used. The use of yellow, magenta and / or cyan toners (absorption rate) of 5% or less (hereinafter sometimes abbreviated as “visible toner”) is preferable from the viewpoint of ensuring the reading accuracy of invisible information. .

  The visible toner may be other than yellow, magenta, and cyan, and may be a toner of a desired color such as red, blue, green, etc. It is preferable that external light absorptance is 5% or less.

  When the near-infrared light absorption rate of the visible toner is 5% or more, the image forming surface on which the invisible image and the visible image are formed on the surface of the image output medium is mechanically read by near-infrared light irradiation. In some cases, a visible image may be mistaken for an invisible image. In particular, in the case of machine reading without specifying the area where the invisible image is formed on the image forming surface, or when forming the invisible image between the visible image and the image output medium surface, only the information of the invisible image is used. There is a high possibility that it will be difficult to accurately decode and read.

  The near-infrared light absorptance of this visible toner is the spectral reflectance of the near-infrared light region of the visible image formed by the visible toner using a spectral reflectance measuring device or the like as in the case of the toner described above. Is determined as VNIR (1) and the spectral reflectance of the image output medium is M (1), as shown in the following equation.

  Near-infrared light absorption rate of visible toner = VNIR (1) −M (1) (5)

  Coloring agents used to obtain the visible toner as described above include aniline blue, calcoyl blue, chrome yellow, ultramarine blue, dupont oil red, quinoline yellow, methylene blue chloride, phthalocyanine blue, malachite green oxalate, and lamp black. Rose Bengal, C.I. I. Pigment red 48: 1, C.I. I. Pigment red 122, C.I. I. Pigment red 57: 1, C.I. I. Pigment yellow 97, C.I. I. Pigment yellow 12, C.I. I. Pigment blue 15: 1, C.I. I. Pigment Blue 15: 3 can be exemplified as a representative example.

  In order to increase the reading accuracy of the invisible image, the near infrared light absorption rate of the toner forming the invisible image at λmax of 800 to 1200 nm is the same as the near infrared light absorption of the visible toner forming the visible image at λmax of 800 to 1200 nm. It is preferably 15% or more larger than the absorptance, more preferably 30% or more.

  When the difference in near-infrared light absorptance between the invisible image and the visible image is less than 15%, the absorptance region between the near-infrared absorptance of the invisible image and the near-infrared absorptance of the visible image , When it is difficult to recognize and read only the invisible image by binarizing with a certain contrast (threshold) as a boundary in order to identify and read whether or not the image is invisible when machine reading There is. In such a case, the visible image may become an obstacle when reading the invisible image and further correctly decoding information recorded in the invisible image.

  The difference between the near-infrared light absorption rate at λmax of 800 to 1200 nm of the toner forming an invisible image and the near-infrared light absorption rate at λmax of 800 to 1200 nm of the visible toner forming the visible image. (Hereinafter, simply referred to as “near-infrared light absorptance difference”) is a spectral reflectance NIR (invisible image (solid image) formed on the surface of the image output medium using a spectral reflectance measuring machine. 2) and the spectral reflectance VNIR (1) of the visible image (solid image) formed on the surface of the image output medium, and are obtained as shown in the following equation.

  Near-infrared light absorption difference = NIR (2) −VNIR (1) (6)

(Specific examples of invisible images)
Next, the image configuration of the invisible image formed by the image forming method according to the present embodiment, the visual recognition of the invisible image, the machine reading of the invisible image, and the like will be specifically described.

  The invisible image is formed using the electrophotographic toner, and is not particularly limited as long as it is machine-readable by near-infrared light irradiation, but is not limited to letters, numbers, symbols, patterns, pictures, photographs. Of course, it may be a two-dimensional pattern such as a known barcode called JAN, standard ITF, Code 128, Code 39, NW-7, or the like.

  When the invisible image is composed of a two-dimensional pattern such as a barcode, the serial number for identifying the image forming apparatus that has formed the image on the image output medium, or the visible image formed on the surface of the image output medium together with the invisible image. It can be used as a copyright certification number for images. Moreover, when the visible image formed with an invisible image takes the form of a confidential document, securities, a license, a personal ID card, etc., it can also be used effectively to detect the identification of these counterfeits.

  In addition to the above barcode example, in the present embodiment, the two-dimensional pattern is not particularly limited as long as it is a known recording method that has been used as a visible and recognizable image. For example, as a method of forming a two-dimensional pattern in which minute area cells are geometrically arranged, a two-dimensional barcode called a QR code can be cited. In addition, as a method of forming a two-dimensional pattern in which minute line bitmaps are geometrically arranged, a code forming method using a plurality of patterns with different rotation angles, which is a technique described in Japanese Patent Laid-Open No. Hei 4-233683 Etc.

  By forming such an invisible image consisting of a two-dimensional pattern on the surface of the image output medium, it is possible to embed large-capacity information, for example, music information, electronic files of text application software, etc. in an image that cannot be visually understood. Therefore, it is possible to provide a technique for creating a more advanced confidential document or digital / analog information sharing document.

  Hereinafter, although an example and a comparative example are given and the present invention is explained more concretely in detail, the present invention is not limited to the following examples. Here, the near-infrared light absorbent used for toner preparation, toner preparation, image formation by an image forming apparatus, evaluation of invisible and visible images formed on a recorded material, and evaluation of absorption rate will be described.

<Example 1>
55 parts by mass of linear polyester 1 as a binder resin, ytterbium oxide (manufactured by Shin-Etsu Chemical) as an inorganic near-infrared light absorber (λmax 800-1200 = 950 nm, (absorption rate at λmax 380-780) / (absorption rate at) λmax800-1200) = 0.02 (2%), absorptance at λmax800-1200 = 10%) and 39 parts by mass of fluorine-based phthalocyanine TX-EX-910B (made by Nippon Shokubai Co., Ltd.) as an organic near infrared light absorber. ) (Λmax800-1200 = 913 nm, (absorption rate at λmax380-780) / (absorption rate at λmax800-1200) = 0.1 (10%), molar extinction coefficient at λmax800-1200 = 10.7 × 10 ⁴ ) A mixture of toner raw materials consisting of 1 part by mass and 5 parts by mass of wax (long chain linear fatty acid long chain linear saturated alcohol; stearyl bebenate) as an additive is kneaded with an extruder, pulverized, and then wind-powered Fine and coarse particles are classified by a classifier, and the body Particles having a product average particle diameter (average particle diameter D50) of 8.6 μm were obtained.

  The linear polyester 1 was synthesized using terephthalic acid, bisphenol A / ethylene oxide adduct, and cyclohexanedimethanol as raw materials, glass transition point Tg = 60 ° C., number average molecular weight Mn = 4600, mass. Average molecular weight Mw = 29000, acid value = 13 mgKOH / g, and hydroxyl value = 24 mgKOH / g.

  Further, when the cross section of the obtained particles was observed with a TEM at a magnification of about 30,000 times, the average dispersion diameter of the inorganic near-infrared light absorbent dispersed in the particles was 1.5 μm, The average dispersion diameter of the organic near infrared light absorber is 0.7 μm, the particle size distribution of the inorganic near infrared light absorber is 1.2, and the particle size distribution of the organic near infrared light absorber is 1. .2.

<Example 2>
The same as Example 1 except that 39.5 parts by mass of ytterbium oxide as an inorganic near infrared light absorber and 0.5 parts by mass of the fluorinated phthalocyanine as an organic near infrared light absorber were used. It was created by the method. Volume average particle size (average particle size D50) was 9.0 μm.

  Further, when the cross section of the obtained particles was observed with a TEM at a magnification of about 30,000 times, the average dispersion diameter of the inorganic near-infrared light absorbent dispersed in the particles was 1.5 μm, The average dispersion diameter of the organic near-infrared light absorber is 0.5 μm, the particle size distribution of the inorganic near-infrared light absorber is 1.2, and the particle size distribution of the organic near-infrared light absorber is 1. .18.

<Example 3>
As an organic near-infrared light absorber, a pyrylium compound (manufactured by Hayashibara Biochemical Research Institute, NK-2673), (λmax 800-1200 = 943 nm, (absorption rate at λmax 380-780) / (absorption rate at λmax 800-1200) = 0.2 (20%), molar extinction coefficient at λmax 800-1200 = 13.0 × 10 ⁴ ) was used in the same manner as in Example 1 except that 1 part by mass was used. Volume average particle size (average particle size D50) was 9.0 μm.

  Further, when the cross section of the obtained particles was observed with a TEM at a magnification of about 30,000 times, the average dispersion diameter of the inorganic near-infrared light absorbent dispersed in the particles was 1.5 μm, The average dispersion diameter of the organic near infrared light absorber is 0.7 μm, the particle size distribution of the inorganic near infrared light absorber is 1.25, and the particle size distribution of the organic near infrared light absorber is 1 .2.

<Example 4>
As an organic near-infrared light absorber, an imonium compound CIR-1080 (manufactured by Nippon Carlit), (λmax800-1200 = 1065 nm, (absorption rate at λmax380-780) / (absorption rate at λmax800-1200) = 0.2 ( 20%), and molar extinction coefficient at λmax 800-1200 = 9.0 × 10 ⁴ ), except that 1 part by mass was used. Volume average particle diameter (average particle diameter D50) was 8.6 μm.

  Further, when the cross section of the obtained particle was observed with a TEM at a magnification of about 30,000 times, the average dispersion diameter of the inorganic near-infrared light absorbent dispersed in the particle was 1.6 μm, The average dispersion diameter of the organic near-infrared light absorber is 0.8 μm, the particle size distribution of the inorganic near-infrared light absorber is 1.3, and the particle size distribution of the organic near-infrared light absorber is 1 .2.

<Example 5>
It was prepared in the same manner as in Example 1 except that 65 parts by mass of the linear polyester 1 was used as the binder resin and 29 parts by mass of ytterbium oxide as the inorganic near infrared light absorber. Volume average particle diameter (average particle diameter D50) was 8.8 μm.

  Further, when the cross section of the obtained particles was observed with a TEM at a magnification of about 30,000 times, the average dispersion diameter of the inorganic near-infrared light absorbent dispersed in the particles was 1.3 μm, The average dispersion diameter of the organic near-infrared light absorber is 0.5 μm, the particle size distribution of the inorganic near-infrared light absorber is 1.2, and the particle size distribution of the organic near-infrared light absorber is 1. .2.

<Example 6>
55 parts by mass of linear polyester 2 as binder resin, ITO (manufactured by Fuji Titanium) as inorganic near infrared light absorber (λmax800-1200 = 1200 nm, (absorption rate at λmax380-780) / (absorption rate at λmax800) -1200) = 0.05 (5%), absorptance at λmax 800-1200 = 10%) and 1 part by mass of the phthalocyanine as an organic near infrared light absorber, The same method as in Example 1 was used.

  The linear polyester 2 is synthesized from terephthalic acid, bisphenol A / ethylene oxide adduct, and cyclohexanedimethanol, and has a glass transition point Tg = 75 ° C., a number average molecular weight Mn = 5000, and a mass. The average molecular weight Mw was 35000, the acid value was 12 mgKOH / g, and the hydroxyl value was 21 mgKOH / g. Particles having a volume average particle diameter (average particle diameter D50) of 9.2 μm were obtained.

  Further, when the cross section of the obtained particle was observed with a TEM at a magnification of about 30,000, the average dispersion diameter of the inorganic near-infrared light absorber dispersed in the particle was 2.0 μm, The average dispersion diameter of the organic near infrared light absorber is 1.0 μm, the particle size distribution of the inorganic near infrared light absorber is 1.35, and the particle size distribution of the organic near infrared light absorber is 1. .2.

<Example 7>
It was prepared in the same manner as in Example 1 except that 36 parts by mass of ytterbium oxide as an inorganic near infrared light absorber and 4 parts by mass of the phthalocyanine as an organic near infrared absorber were used. Volume average particle size (average particle size D50) was 9.0 μm.

  Further, when the cross section of the obtained particles was observed with a TEM at a magnification of about 30,000 times, the average dispersion diameter of the inorganic near-infrared light absorbent dispersed in the particles was 1.4 μm, The average dispersion diameter of the organic near-infrared light absorber is 0.9 μm, the particle size distribution of the inorganic near-infrared light absorber is 1.19, and the particle size distribution of the organic near-infrared light absorber is 1. .3.

<Comparative Example 1>
It was prepared in the same manner as in Example 1 except that 40 parts by mass of the phthalocyanine was used as an organic near infrared light absorber without adding an inorganic near infrared light absorber. Volume average particle diameter (average particle diameter D50) was 8.6 μm.

  Further, when the cross section of the obtained particles was observed with a TEM at a magnification of about 30,000 times, the average dispersion diameter of the organic near-infrared light absorbent dispersed in the particles was 0.8 μm. In addition, the particle size distribution of the organic near-infrared light absorber was 1.61.

<Comparative example 2>
An organic near infrared light absorber is not added, and 40 parts by mass of the ITO is used as an inorganic near infrared light absorber, and the organic near infrared absorber is not added. Created by the method. Volume average particle size (average particle size D50) was 9.0 μm.

  Further, when the cross section of the obtained particle was observed with a TEM at a magnification of about 30,000 times, the average dispersion diameter of the organic near-infrared light absorbent dispersed in the particle was 1.0 μm. In addition, the particle size distribution of the organic near-infrared light absorber was 1.4.

<Comparative Example 3>
Other than using 85 parts by mass of linear polyester 1 as a binder resin, 34 parts by mass of ytterbium oxide as an inorganic near infrared light absorber, and 6 parts by mass of the phthalocyanine as an organic near infrared absorber. Was prepared in the same manner as in Example 1. Volume average particle diameter (average particle diameter D50) was 8.8 μm.

  Further, when the cross section of the obtained particles was observed with a TEM at a magnification of about 30,000 times, the average dispersion diameter of the inorganic near-infrared light absorbent dispersed in the particles was 1.0 μm, The average dispersion diameter of the organic near infrared light absorber is 0.8 μm, the particle size distribution of the inorganic near infrared light absorber is 1.19, and the particle size distribution of the organic near infrared light absorber is 1. .6.

<Comparative Example 4>:
75 parts by mass of linear polyester 1 as a binder resin and a copper ion-containing glass frit (manufactured by Asahi Glass Co., Ltd.) as an inorganic near infrared light absorber (λmax 800-1200 = 880 nm, (absorption rate at λmax 380-780) / ( Absorption rate at λmax 800-1200) = 0.2 (20%), Absorption rate at λmax 800-1200 = 30%) is 39 parts by mass, and 1 part by mass of the phthalocyanine is used as an organic near infrared light absorber. The method was the same as in Example 1 except that. Particles having a volume average particle diameter (average particle diameter D50) of 8.2 μm were obtained.

  Further, when the cross section of the obtained particle was observed with a TEM at a magnification of about 30,000, the average dispersion diameter of the inorganic near-infrared light absorbent dispersed in the particle was 1.7 μm, The average dispersion diameter of the organic near infrared light absorber is 0.9 μm, the particle size distribution of the inorganic near infrared light absorber is 1.5, and the particle size distribution of the organic near infrared light absorber is 1. .2.

  Here, λmax, near-infrared light absorptivity, and visible light absorptivity of the organic near-infrared light absorber and the inorganic near-infrared light absorber were measured as follows.

45 parts by weight of an organic or inorganic near-infrared light absorber, 5 parts by weight of a wax (long-chain straight-chain fatty acid long-chain straight-chain saturated alcohol; stearyl bebenate), and the linear shape used in Example 1 as a binder 50 parts by mass of polyester 1, with an extruder, screw rotation speed 200 rpm (circumferential speed 0.5 m / s), barrel set temperature 80 ° C., kneaded kneading temperature (temperature at discharge) 130 ° C., feed rate After kneading and pulverizing under the condition of 100 kg / hr, fine particles and coarse particles were classified by an air classifier, and particles having a volume average particle size (average particle size D50) of 8.6 μm were prepared. Fixing was performed using a modified DocuColor 1250 manufactured by Fuji Xerox Co., Ltd. as an image forming apparatus so that the toner amount (TMA) was 3 (× 10 ⁻⁴ g / cm ² ). As an image output medium used for the image forming test, A4 size white paper (Fuji Xerox, P? A4 paper, width: 210 mm, length: 297 mm) was used. Using the spectral reflectance measuring device (Hitachi: U-4000) for the surface of the image output medium on which the fixed image and no image are formed, λmax at 380 to 780 nm and 800 to 1200 nm is obtained. It was measured. Further, the spectral reflectance at λmax was measured, and the near infrared light absorptance and visible light absorptance were determined by the above formulas (1) and (2).

<Image formation by image forming apparatus>
In the image formation test using the toner prepared in each example and comparative example, a DocuColor 1250 remodeling machine manufactured by Fuji Xerox Co., Ltd. was used as an image forming apparatus. As an image output medium used for the image formation test, A4 size white paper (Fuji Xerox, P-A4 paper, width: 210 mm, length: 297 mm) was used.

  The recorded matter obtained by forming an image on the surface of the image output medium by the image forming apparatus using the developer described above has a visible image and an invisible image formed on the image forming surface. It consists of a document composed entirely of characters, pictures and the like.

  On the other hand, the invisible image is composed of a two-dimensional pattern that can be machine-read / decoded formed by two kinds of minute line bitmaps having different rotation angles, and the invisible image composed of the two-dimensional pattern is glossy. When macroscopic recognition is possible, 150-byte copyright information is repeatedly arranged so that the characters “XEROX” can be seen in order to exert an anti-counterfeit effect when visually observed.

  In the image formation test, as a reference for evaluating the quality of the visible image, a recorded matter (hereinafter, abbreviated as “recorded matter 1”) in which the invisible image and the visible image are formed on the surface of the image output medium. In addition, a recorded matter (hereinafter referred to as “recorded matter 2”) in which only the same visible image as the recorded matter 1 was formed on the surface of the image output medium was imaged simultaneously.

  The recorded materials 1 and 2 are obtained by fixing toner (and visible toner) to a size of 5 cm × 5 cm so as to be an arbitrary TMA. In particular, the recorded matter 2 refers to a toner in which toner is first fixed, and visible toner is fixed on the same size with an arbitrary TMA.

<Evaluation of Invisible Image and Visible Image Formed on Recorded Material>
The evaluation of the invisible image and the visible image formed on the image forming surface of the recorded material 1 was evaluated with respect to the visible image quality for the visible image with respect to the invisible information restoration rate and the forgery prevention effect for the invisible image. . These specific evaluation methods and evaluation criteria will be described below.

(Evaluation of invisible information restoration rate)
The evaluation of the invisible information restoration rate is based on a ring-shaped LED light source including light in the near-infrared wavelength region of 800 to 1200 nm, in which the image forming surface of the recorded material 1 is placed at a position approximately 10 cm directly above the image forming surface. (Kyoto Denki, LEB-3012CE). In this state, a CCD camera (manufactured by KEYENCE, having a light receiving sensitivity in a wavelength region of 800 nm to 1000 nm, which is installed at a position approximately 15 cm directly above the image forming surface and which has a filter for cutting a wavelength component of 800 nm or less mounted on the lens unit. CCD TL-C2) reads the image forming surface, binarizes with a certain contrast (threshold) as a boundary, extracts an invisible image, decodes it with software, and copyright information We evaluated whether it was possible to restore correctly. And when this evaluation was implemented 500 times, the frequency | count that the information was able to be correctly restored was shown in Table 2 as an invisible information restoration rate (%). If the invisible information restoration rate (%) is 80% or more, the level is practically acceptable.

(Evaluation of anti-counterfeit effect)
The forgery prevention effect is evaluated when the image forming surface of the recorded matter 1 is viewed from a substantially vertical direction (front) of the image forming surface and when viewed from an angle with respect to the vertical direction of the image forming surface. Whether or not the characters of “XEROX” formed as an invisible image can be read was determined as the invisible pattern degree according to the following criteria. The evaluation results are shown in Table 2.

A: The characters “XEROX” cannot be read from any direction, and a sufficient anti-counterfeit effect can be obtained.
◯: The character “XEROX” is not known when viewed from the front, but can be clearly read when viewed from an oblique direction, but provides a sufficient anti-counterfeit effect in practice.
Δ: The character “XEROX” is not known when viewed from the front, but when viewed from an oblique direction, the presence of an invisible image can be confirmed as image noise, and although it is practically weak, a counterfeit suppression effect is obtained. Can do.
X: The character “XEROX” cannot be discriminated whether viewed from the front or obliquely, and it cannot be confirmed as image noise, and therefore no counterfeit prevention effect can be obtained.

(Evaluation of visible image quality)
The visual image quality was evaluated by visual comparison between the visible image of the recorded product 1 and the visible image of the recorded product 2 and the following criteria. The evaluation results are shown in Table 2.

○: There is no difference in the image quality of the visible images of the recorded matter 1 and the recorded matter 2 and there is no practical problem.
Δ: Compared with the visible image of the recorded matter 2, although the image quality of the visible image of the recorded matter 1 is slightly observed, there is almost no problem in practical use.
X: Compared with the visible image of the recorded matter 2, a clear image quality noise is confirmed in the visible image of the recorded matter 1, which causes a practical problem.

<Evaluation of absorption rate>
The absorptance in the visible light region of the toners used in the examples and comparative examples, and the evaluation of the near-infrared light absorptivity difference between the toner and the visible toner were performed as described below.

(Evaluation of absorption rate of toner in the near-infrared and visible light regions)
As described above, a solid image of toner is formed on the image output medium used in the examples, and the area where the solid image is formed and the surface of the image output medium on which no image is formed are spectrally described. Measured with a reflectance measuring instrument, and the spectral reflectance of each is substituted into equations (3) and (4) to obtain the near infrared light absorption rate and visible light absorption rate of the toner. Table 2 shows the maximum visible light absorptance and (absorbance at λmax 380-780) / (absorption rate at λmax 800-1200) in the wavelength range.

(Evaluation of near infrared light absorption difference)
As described above, the difference in the near-infrared light absorption rate between the toner and the visible toner is the difference in spectral reflectance between the invisible image (solid image) and the visible image (solid image) created using these toners. Measured at a wavelength of 900 nm using a spectral reflectometer, and determined by Equation 6. The results are shown in Table 2.

  As described above, according to the present invention, for example, when an invisible image alone or a visible image laminated on the invisible image is formed on the surface of the image output medium, the image quality of the visible image can be reduced even with a small amount of TMA. To provide an electrophotographic toner, an electrophotographic developer, and an image forming method capable of obtaining an invisible image that can be stably read over a long period of time by irradiation with near-infrared light without impairing image quality. It is extremely useful.

Claims (3)

An electrophotographic toner containing an inorganic near infrared light absorber,
The toner further contains an organic near infrared light absorber,
When an image was formed on a substrate with the toner amount (TMA) per unit area set to 3 (× 10 ⁻⁴ g / cm ² ), the image was read using near infrared light of 800 to 1200 nm. A toner for electrophotography characterized by having an invisible information restoration rate of 80% or more as an information restoration rate.   An electrophotographic developer comprising the electrophotographic toner according to claim 1.   An image forming method using the electrophotographic toner according to claim 1.