JP3073198B1 - Counterfeit prevention method - Google Patents

Abstract

Abstract: PROBLEM TO BE SOLVED: To provide an irreversible forgery prevention method, in which only a hue of an irreversible thermal decoloring layer coated on an identification mark is lost by heat treatment to make the identification mark visible. SOLUTION: An identification mark provided in advance on an image forming medium comprises: (a) a coloring material colored by a leuco dye and a developer; and (b) water or an organic solvent which is insoluble or hardly soluble in an organic solvent. After covering and covering with an irreversible thermal decoloring layer containing a thermal decoloring ink composition containing a guanidine compound having a melting temperature in the range of 200 ° C., the medium is heated to heat the thermal decoloring layer. Wherein the identification mark is visualized by erasing the mark.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an anti-counterfeit image coated with an irreversible thermal decoloring layer, which is visualized by heating.
The present invention relates to an irreversible forgery prevention method.

[0002]

2. Description of the Related Art Presently, gift certificates (for example, gift cards,
Meal coupons, beer coupons, book coupons, etc.) are easy to obtain, use, and transfer, so they are not limited to the size and area of the store, and can be distributed at many merchandise stores. Many occurrences of gift certificates forged by copiers and the like have been found. Not only gift certificates, but also unsigned items such as banknotes, tickets and lotteries, and important documents such as bank passbooks, securities and passports,
Various measures have been taken to prevent forgery, and identification devices for determining their authenticity have been developed.

[0003] As means for preventing forgery, for example, a means in which a hologram is attached to a gift certificate is used, but these have a drawback that they are easily forged because the principle and configuration of the hologram layer are simple. A forgery prevention medium, a seal, a transfer foil or a transfer sheet exhibiting a color shift effect has also been developed (Japanese Patent Application Laid-Open No. 10-76746), and a ceramic thin film or a metal thin film or a composite thin film thereof having different optical properties on a substrate. It is disclosed that by providing a multilayer thin film layer in which two or more layers are stacked, when copying using a color copying machine, the multilayer thin film layer portion is copied in black, and it is easy to find fraudulent acts. Japanese Patent Application Laid-Open No. Hei 9-261418 discloses that when printing a document such as a gift certificate, an ink having a spectral characteristic having a sharp reflectance peak near a wavelength of 500 nm where color reproducibility is poor in image formation by a color copying machine is used. The disclosed image forming method is also disclosed. According to the inventions disclosed in these publications, it is easy to judge a counterfeit using a color copier, but the cost increases because a special device and additional steps are required in manufacturing. There is a problem.

[0004] Alternatively, forgery prevention printed matter described in Japanese Patent No. 2825166 is also required to irradiate light such as infrared irradiation or ultraviolet irradiation or cool at room temperature or lower (for example, 0 ° C or lower) in order to express forgery prevention information. Therefore, use of a specific device for authenticity determination is indispensable.

The present applicant has used a reversible thermo-decolorizable layer containing a reversible thermo-decolorizable coloring material for coating the identification mark,
A counterfeiting prevention method has been proposed in which the identification mark can be visualized by heating to remove the color and the color can be developed again upon cooling (Japanese Patent Application No. 11-28461). However,
This is characterized in that since the reversible heat decoloring agent is used, the coating layer develops color again when cooled.

As an example of a color ink composition capable of irreversible decolorization, when a desensitizer for pressure-sensitive paper is mixed with a color forming material containing a leuco dye and a developer, the color forming material is instantaneously mixed. Is irreversibly erased, but cannot be applied to the coating process because it cannot be made into ink. Alternatively, it is also known that currently known materials having relatively weak desensitizing effect, capable of being inked, and capable of undergoing thermal decolorization generally lose color over time when left at room temperature without heating. . Therefore, as for the counterfeiting prevention method using the irreversible thermal decoloring action, at present, its purpose has not yet been sufficiently achieved.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide an irreversible forgery prevention method which overcomes the above-mentioned problems in the prior art and makes the judgment method easier.

[0008]

According to a first aspect of the present invention, an identification mark provided in advance on an image forming medium is provided.
(a) a coloring material colored by a leuco dye and a developer and (b)
Insoluble or poorly soluble in water and organic solvents and 50 to 2
After covering and covering with a layer containing a thermal decoloring ink composition containing a guanidine compound having a melting temperature in the range of 00 ° C., the medium is heated to decolor the layer, and the identification mark is formed. And providing a forgery prevention method characterized by visualizing the information.

According to another aspect of the present invention, there is provided a counterfeiting prevention method characterized by using an identification mark having the same hue as that of the thermal decoloring ink and not being erased by heating. In the method of this aspect, the image forming medium having the identification mark and the layer of the thermal decolorable ink covering the identification mark is subjected to heat treatment, whereby the layer of the thermal decolorable ink is decolorized, and the identification mark concealed thereby. The use mark can be raised and visualized.

In the method of the present invention, the guanidine compound (b) may be supplied in a form encapsulated in a microcapsule. In this case, the microcapsule contains a guanidine compound-containing content and a guanidine compound-containing content. And a wall material to be included therein, wherein the wall material contains at least one selected from the group consisting of polyurea, polyurethane and polyurea-polyurethane.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
As shown in FIG. 1, the method of the present invention comprises the steps of: I) hiding an identification mark 2 previously provided on a support 1 of an image forming medium 10 with an irreversible thermal decoloring layer 3; The method basically includes visualizing the identification mark 2 by heating the medium 10 to decolor the color of the irreversible thermal decoloring layer 3 and determining the authenticity.

I) Concealment of Identification Mark In the present invention, first, an irreversible thermal decoloring layer is formed so as to conceal an identification mark provided on an image forming medium in advance. Since the identification mark is physically hidden by the layer, the identification mark cannot be reproduced by a copying machine, thereby preventing forgery.

The irreversible heat-decoloring layer used in the method of the present invention comprises (a) a coloring material colored by a leuco dye and a developer, and (b) a water-insoluble or sparingly-soluble solvent. And a guanidine compound having a melting temperature in the range of 50 to 200 ° C.

The color-forming material (a) contained in the irreversible heat-decolorable ink composition of the present invention is a color formed from a leuco dye and a color developer. Leuco dyes and developers can be selected from materials generally used for pressure-sensitive papers and thermal papers. Leuco dyes include triphenylmethanephthalide, fluoran, phenothiazine, and indolephthalide. , Leuco auramines, spiropyrans, rhodamine lactams and the like. Hundreds of these leuco dyes have already been developed, their colors are aspirational, and any color scheme is possible. The following are examples of suitable leuco dyes for use in the present invention.
The parentheses in the following formula represent the colors at the time of coloration.

Triphenylmethanephthalides:

Embedded image Indole phthalides:

Embedded image Leuco auramines:

Embedded image Spiropyrans:

Embedded image Fluorans:

Embedded image Phenothiazines:

Embedded image Rhodamine lactams:

Embedded image

On the other hand, examples of the color developer contained in the coloring material (a) include 4-tert-butylphenol, 4-oxydiphenyl ether, α-naphthol, β-naphthol, p-naphthol
Methyl oxybenzoate, p-oxyacetophenone, 2,
2'-methylenebis (4-methyl-6-tert-isobutylphenol), p-phenylphenol, 4,4'-isopropylidenediphenol (so-called bisphenol A), 2,2'-methylenebis (4-chlorophenol) , Hydroquinone, 2-oxy-1-naphthoic acid, 2-oxy-p-toluic acid, salicylic acid, m-oxybenzoic acid, 4-oxyphthalic acid, phloroglucin carboxylic acid,
Examples include gallic acid, propyl gallate, phthalic acid and the like. These developers may be used alone or in combination of two or more.

The leuco dye and the developer are dissolved in a suitable printing ink varnish by heating, mixed and stirred to obtain a solution of the coloring material (a) in which the leuco dye has developed.
The color tone and density of the coloring material (a) can be adjusted by the type of leuco dye and the amount used. Instead of the printing ink varnish, for example, a volatile organic solvent such as toluene is used, and a leuco dye and a developer are dissolved and mixed in the solvent to form a color. May be prepared.

In the present invention, the compounding ratio of the color developer to the leuco dye in the color forming colorant (a) is usually in a weight ratio,
1:50 to 30:50, more preferably 3:50 to 1
0:50.

The thermal decoloring ink composition used in the present invention is insoluble or hardly soluble in water and an organic solvent as a second component and has a temperature in the range of 50 to 200 ° C., preferably 10 to 200 ° C.
It contains a guanidine compound (b) having a melting temperature in the range of 0 to 180 ° C. Such a guanidine compound (b) is
It functions as a decoloring agent in the thermal decoloring ink composition, and has the following formula:

Embedded image (Wherein, R ^{1 to} R ⁵ are each independently hydrogen, carbon atom 1
-7, an alkyl group having up to 10 carbon atoms, a cyclic alkyl group having up to 10 carbon atoms, an aryl group having up to 10 carbon atoms, an aralkyl group having up to 10 carbon atoms, and / or a phenyl group). It may be selected from guanidine compounds that are insoluble or hardly soluble in a non-volatile hydrophobic organic solvent. Preferred compounds include guanidine compounds such as diphenylguanidine and di-o-tolylguanidine.

As prior art, Japanese Patent Publication No. 51-29202
No. 4, JP-B-51-37542 and JP-A-50-147736, among the various guanidine compounds, those which are insoluble or hardly soluble in an organic solvent and 50 to 20.
Those having a melting temperature in the range of 0 ° C. can also be preferably used in the present invention.

The guanidine compound (b) may be a reaction product of guanidine and a polyisocyanate. Examples of the polyvalent isocyanate compound capable of reacting with guanidine include, for example, m-phenylene diisocyanate,
p-Phenylene diisocyanate, 2,6-tolylene diisocyanate, 2,4-tolylene diisocyanate, naphthalene-1,5-diisocyanate, 4,4'-diphenylmethane-diisocyanate, 3,3'-dimethyldiphenylmethane-4,4 ' -Diisocyanate, xylylene-1,4-diisocyanate, 4,4'-diphenylpropane diisocyanate, trimethylene diisocyanate, hexamethylene diisocyanate, propylene-1,2-diisocyanate, butylene-1,2-diisocyanate, cyclohexylene-1,2 -Diisocyanate, cyclohexylene-
1,4-diisocyanate, 4,4 ', 4''-triphenylmethane triisocyanate, toluene-2,4,6-triisocyanate, 4,4'-dimethyldiphenylmethane-2,2',
5,5′-tetraisocyanate, adduct of hexamethylene diisocyanate with trimethylolpropane,
Adducts of 4-tolylene diisocyanate with trimethylolpropane, adducts of xylylene diisocyanate with trimethylolpropane and the like can be mentioned. Furthermore, a mixture of two or more of the above compounds can be used, and it is particularly preferable to use a mixture of an aliphatic isocyanate compound and an aromatic isocyanate.

Examples of the non-volatile hydrophobic organic solvent used in the reaction between guanidine and the polyvalent isocyanate compound include phosphates such as tricresyl phosphate and octyl diphenyl phosphate; dibutyl phthalate and dioctyl phthalate; Phthalates; maleic esters such as dibutyl maleate and dioctyl maleate; alkylated naphthalenes such as monoisopropyl naphthalene and diisopropyl naphthalene; alkylated biphenyls such as diisopropyl biphenyl and diisobutyl biphenyl; 1-phenyl-1- Xylylethane, 1-methyl-1-
Diarylalkanes such as dimethylphenyl-1-phenylmethane, 1-ethyl-1-dimethylphenyl-1-phenylmethane, 1-propyl-1-dimethylphenyl-1-phenylmethane; chlorinated paraffins, and mixtures thereof No. The organic solvent used for preparing the guanidine compound as the component (b) may be a low-boiling organic solvent such as ethyl acetate, isopropyl acetate, or butyl acetate as a solubilizing agent, if necessary. It may be contained in an amount of not more than weight percent.

In the present invention, as the guanidine compound (b), it is preferable to use solid fine particles of the reaction product of guanidine and a polyvalent isocyanate compound.

Alternatively, the guanidine compound (b) may be supplied in the form of microcapsules containing the guanidine compound. Such microcapsules may be prepared by a conventionally known method, for example, by dispersing the guanidine compound in a nonvolatile hydrophobic organic solvent by first stirring the same in a nonvolatile hydrophobic organic solvent, and then dispersing the guanidine compound therein. Add the polyvalent isocyanate compound and stir until uniform (e.g., at 100-1,000 rpm at room temperature).
At a stirring speed of 3-10 minutes). Before the reaction of the guanidine compound with the polyvalent isocyanate compound is completed, a wall-forming material capable of reacting with the remaining isocyanate is added in a solution state, preferably, from about 5,000 to about 10,000.
Under a stirring condition of 00 rpm for 10 to 60 minutes, a diameter of 0.1
Emulsify and disperse to 10 μm Thereafter, for example, the stirring speed is reduced to 3,000 to 5,000 rpm, optionally heated to 40 to 80 ° C., and the stirring is continued for 60 to 180 minutes to obtain the residual polyvalent isocyanate in the nonvolatile hydrophobic organic solvent. By the interfacial polymerization reaction between the compound and water, microcapsules having walls formed around the emulsified and dispersed (guanidine compound / isocyanate compound) reaction product are obtained. By enclosing the guanidine compound in the microcapsules in this manner, the decolorability and the stability over time of the heat-decolorable ink composition can be increased.

In the present invention, examples of the substance suitably used for the wall forming material of the microcapsules include polyurea, polyurethane, and polyurethaneurea. The wall forming material may be added in an amount of 15 to 45% by weight based on the total weight of the guanidine compound or the reaction product of the guanidine compound and the polyvalent isocyanate compound. If an amount of the wall-forming material below or above the above range is mixed, the guanidine compound will not be well encapsulated and microcapsules of the desired diameter will not be obtained.

The above-mentioned guanidine compound, a reaction product of guanidine and a polyhydric alcohol compound or a guanidine compound-encapsulated microcapsule is described in detail in Japanese Patent Application Laid-Open No. Hei 10-109477 filed earlier by the present applicant. And insert its contents here.

In the present invention, the microcapsules obtained by the above method are heat-decolorized in the form of finely dispersed in water or, if necessary, in the form of dry fine powder by subjecting them to means such as spray drying. It is preferably supplied as component (b) of the ink composition.

The irreversible heat-decoloring ink composition comprises a color ink (a) and a guanidine compound and / or a guanidine compound-encapsulated microcapsule (b) in the form of a solution and / or a dry powder. It can be prepared by mixing and dispersing with a varnish for use, an extender and the like.
The coloring density, thermal decolorability, printability, and the like of the thermal decolorable ink can be appropriately adjusted by the amount of each component and other additives.

The heat-decolorable ink composition preferably used in the present invention comprises a coloring material (a) based on the total weight of the composition.
8 to 40% by weight, preferably 15 to 20% by weight; guanidine compound and / or guanidine microcapsule
(b) 5 to 30% by weight, preferably 10 to 20% by weight; for example, 40 to 60% by weight of a varnish for printing ink composed of mineral oil, linseed oil, rosin-modified phenolic resin and the like; and, for example, calcium carbonate; It may contain about 10 to 15% by weight of an extender pigment such as magnesium carbonate.

The image forming medium used in the present invention may be provided on a support usually used for gift certificates, banknotes, tickets and lottery tickets, bank passbooks, securities, passports and the like, with desired patterns and prints, and According to the above, an uneven pattern including storage means capable of magneto-optical recording may be formed.

Suitable supports for use in the image forming medium include paper, various nonwoven fabrics, woven fabrics, plastic films such as polyethylene terephthalate and polypropylene, and film-laminated paper in which synthetic resins such as polyethylene, polypropylene and polyethylene terephthalate are laminated. , Synthetic paper, metal foil of aluminum or the like, or a composite sheet combining these can be used arbitrarily according to the purpose, but is not limited thereto.
The support may be opaque, transparent, or translucent, but it is essential that the identification mark formed thereon is not visible from the surface opposite to the surface on which the mark is formed through the support. .

The identification mark provided on the support in advance is used for authenticity discrimination for the purpose of preventing forgery, and is a printing method known in the field of manufacturing gift certificates, passbooks, securities, credit cards and the like. May be a printed coloring character or pattern, a character or pattern stamped using a stamp, a hologram, or the like. However, the identification mark must not be sublimated, decomposed, or erased by heating for erasing the irreversible thermal erasing layer described later. The hue of the identification mark is not particularly limited, and is completely hidden by the irreversible heat-decolorable layer, and when the irreversible heat-decolorable layer is almost completely decolored, the hue of the identification mark is visually confirmed. Anything can be used as long as it can be easily distinguished and clearly recognized. That is, the identification mark may be a coating layer or the like that contains a colorant such as a dyeing material and does not change color by the action of applying and heating the thermal decoloring ink. Further, the identification mark may have the same hue as the thermal decoloring ink layer, or may have an image form of a special pattern.

In the present invention, particularly suitable identification marks are, for example, coloring pigments such as carbon black, navy blue, phthalocyanine blue and lake red, and linseed oil,
Using a common printing ink composed of a varnish containing mineral oil, rosin-modified phenolic resin and the like, the ink is provided on a suitable support selected from the above.

Both the identification mark and the thermal decolorable ink layer formed in advance can be provided on the support by a usual printing method such as offset, letterpress, flexo, gravure or the like. In the method of the present invention, the thermal decolorable ink layer may be formed so as to completely cover the identification mark, for example, may be formed to have the same size or more as the size of the identification mark, and And / or may be formed in the same manner as the identification mark. The coating amount of each of the identification mark and the thermal decolorable ink layer is controlled by the hue of the identification mark and the decoloring characteristics of the thermal decolorable ink layer. 50 ~ 2.50g /
m ² , the thermal decolorizable ink layer is preferably from about 0.50
4.00 g / m ² , more preferably from 1.0 to 4.00 g / m ²
It is printed coating to a range of m ^2. Further, the thermal decolorizable ink layer is usually formed as a printing coating layer by the above-described method, but may be coated on a transparent support such as a thin plastic film for the purpose of protecting the identification mark. After that, the coating layer surface may be supplied so as to adhere to the identification mark.

II) Heat Decoloring of Thermal Decoloring Layer In the method of the present invention, the image forming medium is then heated to decolor the hue of the irreversible heat decoloring layer, thereby providing The mark is visualized, and the authenticity of the image forming medium is determined.

The heat treatment temperature applied to the thermal decoloring layer in this step should be set to a temperature at which the decoloring of the thermal decoloring layer does not occur during normal handling and the identification mark is not exposed. And preferably in the range of 50 to 200 ° C, more preferably in the range of 100 to 180 ° C. This color erasing temperature range can be adjusted by appropriately selecting the melting temperature of the guanidine compound (b) to be used. Alternatively, it is also possible to further lower the decolorizing temperature of the thermal decoloring layer by adding a conventionally known sensitizer usually blended with thermal paper to the thermal decoloring ink composition used in the present invention. It is.

No special equipment is required for these heat treatments, and hair dryers, hot plates, irons,
Conventionally known heating means such as a thermal printer can be used. For the purpose of the present invention, it is particularly preferable to use a thermal printer set in the above heating temperature range.

In the present invention, the color of the irreversible thermal decoloring layer is decolorized through the above-mentioned steps I) and II), thereby exposing the identification mark provided thereunder, thereby forming an image. The authenticity of the medium can be determined.

According to a second aspect of the method of the present invention, the color of the thermal decoloring layer is erased in the same manner as described above by making the hue of the identification mark and the color of the thermal decoloring layer the same or similar. In addition, the identification mark is expressed so as to stand out visually.

As described above, the forgery prevention method of the present invention basically uses the identification mark and the thermal decoloring layer in combination, but both are not essential, and the thermal decoloring layer is provided on the support. It should be understood that a similar effect can be obtained even when only one is formed. That is, in the latter case, the authenticity can be determined based on the presence or absence of the decoloration of the thermal decoloring layer or the appearance of the pattern formed on the support.

[0041]

The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the invention is limited thereto. Example 1 This example is an example of the first aspect of the forgery prevention method of the present invention, in which an identification mark which is not erased by heating in advance in a manufacturing process, for example, a red mark is hidden by a thermal decoloring layer. The following shows how to prevent counterfeiting of securities.

FIGS. 2 (a) and 2 (b) show that the imprint (12) imprinted on the paper support (11) on which the necessary printing (200) has been made is performed with the heat dissipation of the composition shown in Table 1 below. Coating layer of color red ink composition
The securities (110) covered by (13) are shown. Here, the mark (12) corresponds to the identification mark in the present invention.

TABLE 1 Red thermal decolorable ink composition (parts by weight) Printing ink varnish (1) 40 Calcium carbonate 10 Guanidine microcapsules 25 Red printing ink varnish (2) 20 Mineral oil 5 Total 100 3 By kneading with this roll, a red heat decolorable ink composition was prepared. In the composition of Table 1 above, the printing ink varnish (1) was linseed oil 5
It is obtained by dissolving 0 parts by weight, 10 parts by weight of mineral oil and 40 parts by weight of rosin-modified phenol resin in a nitrogen stream at 250 ° C. and stirring for 60 minutes.
In Table 1, the red printing ink varnish (2) is 30 parts by weight of the printing ink varnish (1), 20 parts by weight of mineral oil, 42 parts by weight of 4-tert-butylphenol, and 2-chloro-6-diethylamino. 8 parts by weight of fluoran was prepared by dissolving in a nitrogen stream at 200 ° C. and stirring for 60 minutes.

The area covered with the thermal decoloring layer was heated at 150 ° C.
By heating on a hot plate heated to 160 ° C., the red color of the thermal decoloring layer was instantaneously and completely decolored, and the mark below it was confirmed.

Embodiment 2 This embodiment shows an example of the second embodiment of the forgery prevention method of the present invention.
The gift certificate which can judge the authenticity by the heat decoloring of the heat decoloring layer is shown. As shown in FIGS. 3 (a) and (b), first, on a paper support (11 ′) on which various prints (200 ′) representing the value as a gift certificate (120) have been made, the following Table 2 The layer (13 ') of the blue heat-decolorable ink composition prepared by the formulation shown in (1) was printed using an offset part of a Miyakoshi business form printing machine.

TABLE 2 Blue thermal decolorable ink composition (parts by weight) Printing ink varnish (1) 50 Calcium carbonate 10 Guanidine microcapsule 15 Blue printing ink varnish (2) 20 Mineral oil 5

The blue thermal decolorable ink composition was prepared by kneading each of the above compositions with three rolls. In Table 2,
Printing ink varnish (1) was prepared in the same manner as in Example 1. The blue printing ink varnish (2) was prepared by subjecting 30 parts by weight of the printing ink varnish (1), 20 parts by weight of mineral oil, 42 parts by weight of 4-tert-butylphenol and 8 parts by weight of crystal violet lactone to 200 ° C. in a nitrogen stream. And then stirred for 60 minutes. When the heat decoloring layer (13 ') part of the manufactured gift certificate (120) is heated to 150 ° C using an iron, the blue color of the heat decoloring layer is completely and quickly decolorized, and the paper is supported. The skin of the body was exposed, confirming that it was a true gift certificate.

[0046]

According to the present invention, the identification mark provided in advance on the image forming medium has storage stability without being deteriorated by heating or the like. Therefore, when the image forming medium is subjected to a heat treatment, only the hue of the thermal decoloring layer portion coated on the identification mark is irreversibly disappeared, and the identification mark appears, thereby indicating whether the image is a forgery. Can be easily checked.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of an example of a forgery prevention mark for authenticity identification of the present invention.

FIG. 2 shows a schematic top view and a cross-sectional view of a security that has been subjected to the forgery prevention method according to the first embodiment of the present invention.

FIG. 3 shows a schematic top view and a cross-sectional view of a gift certificate that has been subjected to the forgery prevention method according to the second embodiment of the present invention.

[Explanation of symbols]

1, 11, 11 '... (paper) support, 2, 12 ... identification mark, 3, 13, 13' ... irreversible thermal decoloring layer, 10 ... image forming medium, 110 ... securities, 120 ... goods Ticket, 20
0, 200 '... printing section.

Continued on the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) B41M 3/14 B41M 5/26 B41M 5/28 B41M 5/36 B42D 15/10 501

Claims (6)

(57) [Claims] 1. An identification mark provided in advance on an image forming medium comprises: (a) a coloring material colored by a leuco dye and a developer; and (b) insoluble or hardly soluble in water and an organic solvent.
After covering and covering with an irreversible heat-decoloring layer containing a heat-decoloring ink composition containing a guanidine compound having a melting temperature in the range of from 200 ° C. to 200 ° C., the heat decoloring is performed by heating the medium. A forgery prevention method, wherein the identification mark is visualized by erasing a layer. 2. The forgery prevention method according to claim 1, wherein the identification mark has the same or similar hue as the thermal decoloring layer, and does not decolor by heating. 3. A guanidine compound (b) having the formula: (Wherein, R ^{1 to} R ⁵ are each independently hydrogen, carbon atom 1
-7, an alkyl group having up to 10 carbon atoms, an aryl group having up to 10 carbon atoms, an aralkyl group having up to 10 carbon atoms, and / or a phenyl group). The anti-counterfeiting method according to claim 1 or 2, wherein the anti-counterfeiting method is selected from guanidine compounds which are insoluble or hardly soluble in a neutral organic solvent. 4. The forgery prevention method according to claim 3, wherein the guanidine compound (b) is a reaction product of guanidine and a polyvalent isocyanate. 5. The forgery prevention method according to claim 1, wherein the guanidine compound (b) is supplied in a form encapsulated in microcapsules. 6. The microcapsule is composed of a content containing a guanidine compound and a wall material enclosing the content, wherein the wall material is one selected from the group consisting of polyurea, polyurethane and polyurea-polyurethane. 6. The forgery prevention method according to claim 5, comprising the above.