JPH0664328A - Recording material - Google Patents

Abstract

PURPOSE:To enhance a protective layer or printing layer in adhesion and coloring density by a method wherein in a thermal color-forming layer or an intermediate layer of a thermal recording material, a copolymer made of a copolymerizable monomer having a photopolymerization initiating function and another specific monomer is used as a binder. CONSTITUTION:In a thermal recording material, a thermal color-forming layer containing a colorless or light-colored color-forming compound and a color- developing compound reacting with this to develop a color as main components is provided on a top surface of a substrate. On the top surface of the thermal color-forming layer, a protective layer or printing layer made of an ultraviolet- curing resin is provided directly or through an intermediate layer. In this invention, in the thermal color-forming layer or the intermediate layer, a copolymer made of a copolymerizable monomer having a photopolymerization initiating function and 1-3 kinds of monomers selected out of a styrene, a methacrylate, and an acrylate is used as a binder. In this manner, the adhesion of the protective layer or printing layer is enhanced, a background fog is eliminated, and a coloring density is raised.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-sensitive recording material, and more particularly to a heat-sensitive recording material having a UV-curable resin protective layer or printing layer formed on a heat-sensitive coloring layer, wherein the protective layer or printing layer is The present invention relates to a heat-sensitive recording material having excellent adhesion and good color image.

[0002]

2. Description of the Related Art A heat-sensitive recording material using a colorless or light-colored color-forming substance and a color-developing substance capable of coloring the color-forming substance under heat is disclosed in JP-B-43-4160 and JP-B-45-14039. Has been widely put into practical use. In general, a heat-sensitive recording material is prepared by separately dispersing a leuco dye and a color developing agent such as a phenolic substance in the form of fine particles, and then mixing the both with a binder,
Addition of additives such as sensitizers, fillers and lubricants to form a coating liquid, which is applied to a support such as paper, film, synthetic paper, etc., and one or both of the leuco dye and the developer can be heated by heating. Color recording is obtained by a chemical reaction caused by melting and contact, and a sheet-shaped heat-sensitive recording material is usually prepared. A thermal printer or the like having a built-in thermal head is used for coloring such a thermal recording sheet. Compared to other recording methods, this heat-sensitive recording method has (1) no noise during recording, (2) no need for developing and fixing, (3) maintenance-free, (4) machine relatively Due to its low cost, it is widely used in the fields of facsimile, computer output, printers such as calculators, medical measuring letterers, automatic ticket vending machines, and heat-sensitive recording labels.

[0003] The use of ultraviolet curable resins is increasing in order to perform printing on such a heat-sensitive recording sheet and to form a protective layer for imparting robustness. The main reason for using UV-curable resin is that it does not contain volatile solvents like those found in conventional water-based or solvent-based paints or inks, and therefore does not require a large amount of heat source for drying. is there.

However, since the UV-curable resin has a large shrinkage at the time of fixing and is hard, the adhesion of the protective layer or the printing layer obtained from the resin formed on the thermosensitive coloring layer is poor, and the resin is easily peeled off by friction or an adhesive tape. There is a drawback that it ends up.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks of the prior art. That is,
Provided is a thermosensitive recording material having a protective layer or a printing layer of an ultraviolet curable resin on a thermosensitive coloring layer, which has excellent adhesion of the protective layer or the printing layer and at the same time has no background fog and has a high coloring density. That is.

[0006]

Means for Solving the Problems As a result of various studies to solve the above-mentioned problems, the inventors of the present invention have found that a photopolymerizable initiation function is provided in a thermosensitive coloring layer or an intermediate layer on the thermosensitive coloring layer. The present invention has been completed by finding that the above-mentioned problems can be solved by using a copolymerizable monomer having the same and a copolymer obtained from another specific monomer. That is, the present invention provides a thermosensitive coloring layer mainly composed of a colorless or light-colored coloring compound and a color-developing compound that reacts with the coloring compound to develop a color on the support, and a thermosensitive coloring layer is directly or intermediately formed on the thermosensitive coloring layer. In a heat-sensitive recording material in which a protective layer or a printing layer formed via a layer is made of an ultraviolet curable resin, a copolymerizable monomer having a photopolymerization initiating function, styrene, a methacrylic acid ester, and an acryl in a thermosensitive coloring layer or an intermediate layer. 1 to 3 selected from acid esters
Provided is a heat-sensitive recording material characterized by using a copolymer obtained from a kind of a monomer as a binder.

A particularly preferred binder is a copolymer obtained from a copolymerizable monomer having a photopolymerization initiation function, styrene and a methacrylic acid ester.

The present invention will be described in detail.

In preparing the heat-sensitive recording material of the present invention, the heat-sensitive color-forming layer contains a color-forming compound (A), a color-developing compound (B), a binder (C) and, if necessary, a filler, a heat-fusible compound. A compound, a surfactant or the like is used. Examples of these are given below.

(A) Examples of color-forming compounds Fluoran compounds, triarylmethane compounds, spiropyran compounds, diphenylmethane compounds, thiazine compounds, lactam compounds, fluorene compounds and the like. For example, the following compounds may be mentioned.

Examples of the fluorane compound include 2-anilino-3-methyl-6-diethylaminofluorane,
2-anilino-3-methyl-6- (N-methyl-N-cyclohexylamino) -fluorane, 2-anilino-3
-Methyl-6- (N-ethyl-N-isopentylamino) fluorane, 2-anilino-3-methyl-6-dibutylaminofluorane, 2- (p-chloroanilino)-
3-methyl-6-diethylaminofluorane, 2- (p
-Fluoroanilino) -3-methyl-6-diethylaminofluorane, 2-anilino-3-methyl-6- (p-
Toluidinoethylamino) fluorane, 2- (p-toluidino) -3-methyl-6-diethylaminofluorane, 2- (o-chloroanilino) -6-dibutylaminofluorane, 2- (o-fluoroanilino- 6-diethylaminofluorane, 2- (o-fluoroanilino)-
6-dibutylaminofluorane, 2-anilino-3-methyl-6-piperidinofluorane, 2-anilino-3-
Methyl-6-pyrrolidinofluorane, 2-ethoxyethylamino-3-chloro-6-diethylaminofluorane, 2-anilino-3-chloro-6-diethylfluorane, 2-chloro-6-diethylaminofluorane, 2 −
Methyl-6-diethylaminofluorane, 2-anilino-3-methyl-6- (N-ethyl-N-ethoxypropylamino) fluorane, 2-anilino-3-methyl-6
-Dipentylaminofluorane, etc .; triarylmethane compounds such as 3,3-bis (p-dimethylaminophenyl) -6-dimethylaminophthalide (also known as crystal violet lactone), 3,3-bis (p- Dimethylaminophenyl) phthalide, 3- (p-
Dimethylaminophenyl) -3- (1,2-dimethylaminoindole-3-) inphthalide, etc .; Examples of spiropyran-based compounds include 3-methyl-3-spirodinaphthopyran, 1,3,3-trimethyl-6-nitro −
8'-methoxyspiro (indoline 2,2'-benzopyran) and the like; diphenylmethane compounds such as N-halophenyl-leuco-olamine; thiazine compounds such as benzoylleucomethylene blue; lactam compounds such as rhodamine- B
-Anilinolactam, etc .; as a fluorene-based compound,
For example, 3,6-bis (dimethylamino) fluorene spiro (9,3 ′)-6-dimethylaminophthalide, [3,
6-bis (dimethylamino) fluorene spiro (9,
3 ')-6'-pyrrolidinophthalide], [3-dimethylamino-6-diethylaminofluorenspiro (9,
3 ')-6'-pyrrolidinophthalide] and the like. These color forming compounds may be used alone or as a mixture.

(B) Color-developing compound α-naphthol, β-naphthol, p-octylphenol, 4-t-octylphenol, pt-butylphenol, p-phenylphenol, 1,1'-bis (p-hydroxyphenyl) ) Propane, 2,2'-bis (p-hydroxyphenyl) propane, 2,2'-bis (p-hydroxyphenyl) butane, 1,1'-bis (p-hydroxyphenyl) cyclohexane, 4,4 '
-Thiobisphenol, 4,4'-cyclo-hexylidene diphenol, 4,4'-sulfonyldiphenol,
4,4'-sulfonyl-bis (2-allylphenol), 4-hydroxy-4'-isopropoxy-diphenylsulfone, 1,1,3-tris (3-t-butyl-)
4-hydroxy-6-methylphenyl) butane, 2,
2'-bis (2,5-dibromo-4-hydroxyphenyl) propane, 4,4'-isopropylidene bis (2-
t-butylphenol), 2,2'-methylenebis (4
-Chlorophenol) 1,1,3-tris (3-t-butyl-4-hydroxy-6-cyclohexylphenyl)
Phenolic compounds such as butane, methylbis (4-hydroxyphenyl) acetate, butylbis (4-hydroxyphenyl) acetate, benzylbis (4-hydroxyphenyl) acetate, benzyl p-hydroxybenzoate, ethyl p-hydroxybenzoate, 4- Dibenzyl hydroxyphthalate, dimethyl 4-hydroxyphthalate, ethyl 5-hydroxyisophthalate, 3,5-di-
Examples thereof include aromatic carboxylic acid derivatives such as t-butylsalicylic acid and 3,5-di-α-methylbenzylsalicylic acid, aromatic carboxylic acids or polyvalent metal salts thereof.

(C) Binder Methylcellulose, methoxycellulose, hydroxyethylcellulose, carboxymethyl, sodium carboxymethylcellulose, cellulose, polyvinyl alcohol (PVA), carboxy group-modified polyvinyl alcohol, sulfonic acid group-modified polyvinyl alcohol, polyvinylpyrrolidone, polyacrylamide, Polyacrylic acid,
Water-soluble substances such as starch and its derivatives, casein, gelatin water-soluble isoprene rubber, alkali salts of styrene / maleic anhydride copolymers, alkali salts of iso (or diiso) butylene / maleic anhydride copolymers, or polyacetic acid Vinyl, vinyl chloride / vinyl acetate copolymer, polystyrene, polyacrylic acid ester, polyurethane, styrene / butadiene (SB) copolymer, carboxylated styrene / butadiene (SB) copolymer, styrene / butadiene / acrylic acid copolymer Polymers, composite particles of colloidal silica and acrylic resin, and aqueous emulsions thereof are used.

Examples of other additives Examples of fillers include calcium carbonate, magnesium carbonate, magnesium oxide, silica, white carbon, talc, clay, alumina, magnesium hydroxide, aluminum hydroxide, aluminum oxide, barium sulfate and polystyrene. Resin, urea-formalin resin and the like.

Examples of the heat-fusible compound include waxes such as animal and vegetable wax, polyethylene wax and synthetic wax, and higher fatty acids, higher fatty acid amides, higher fatty acid metal salts, acetylated aromatic amines, aromatic ether compounds and biphenyls. Examples thereof include derivatives that are solid at room temperature and have a melting point of about 80 ° C or higher.

In addition, lubricants such as zinc stearate, calcium stearate and aluminum stearate, various surfactants, antifoaming agents and the like are added as required.

Next, in the present invention, 1 to 3 kinds of monomers selected from styrene, methacrylic acid ester and acrylic acid ester as a binder or a component of the thermosensitive coloring layer or the intermediate layer and a copolymerizability having a photopolymerization initiation function. A copolymer composed of monomers is used. Particularly preferred copolymer is a copolymer obtained from a copolymerizable monomer having a photopolymerization initiation function, styrene and a methacrylic acid ester, or a copolymerizable monomer having a photopolymerization initiation function, styrene and an acrylate ester. The resulting copolymer. Examples of the copolymerizable monomer having a photopolymerization initiation function include photopolymerization initiators having a radically polymerizable functional group such as an acrylic group, an aryl group, and a vinyl group in a side chain, and specific examples thereof include the following formula ( Examples thereof include compounds represented by the formulas 1) to (6).

[0018]

[Chemical 1]

[0019]

[Chemical 2]

[0020]

[Chemical 3]

[0021]

[Chemical 4]

[0022]

[Chemical 5]

[0023]

[Chemical 6] The monomer component ratio of the copolymer used in the present invention is 0.1 to 20% by weight, preferably 1 to 10% by weight of the copolymerizable monomer having a photopolymerization initiation function, and 0 to 90% by weight of styrene, preferably 30 to 70% by weight, methacrylic acid ester and acrylic acid ester are in the range of 0 to 90% by weight, preferably 30 to 70% by weight, and the molecular weight of the copolymer is 10,000 to 500,000, preferably 50,000 to 250,000.

The production method of the copolymer used in the present invention will be described by way of example, the suspension polymerization method. Styrene and methyl methacrylate and 4- (2-acryloxyethyl) -phenyl- (2-hydroxy-2-). Propyl) ketone (compound of formula (3) above) under N ₂ gas filling PV
80-90 using organic peroxide as a polymerization catalyst in aqueous solution A
A spherical copolymer is obtained by reacting at ℃.
This is filtered, dried, and then dissolved in a solvent (for example, benzene, toluene, xylene, ethyl acetate, methyl isobutyl ketone, isopropyl alcohol, etc.) that can be used in the intended system.

When the above-mentioned copolymer is used as a binder in the thermosensitive coloring layer, the proportion thereof is in the range of 1 to 90% by weight, preferably 10 to 50% by weight, based on the solid component of the thermosensitive coloring layer.
In addition, if necessary, the above-mentioned other additives and heat-fusible compounds may be added, and the (C) binder may be used in combination.

When the above copolymer is used in the intermediate layer, other conventional fillers (eg clay,
Talc, kaolin, titanium oxide, zinc oxide, calcium carbonate, aluminum oxide, fine particle resin (for example, urea-
Fine particles of formalin resin, polystyrene, benzoguanamine resin, phenol resin, etc.) and surfactants (eg, fatty acid salt, aromatic, sulfonate, polycarboxylate, dialkylsulfosuccinate, etc.) may be used in combination. The coating amount of the intermediate layer is not particularly limited, but the solid content weight is preferably 1 to 6 g / m ² .

The ultraviolet curable resin used in the protective layer contains a resin component such as a photopolymerizable monomer, prepolymer or polymer and a photopolymerization initiator.

Examples of prepolymers include poly (meth) acrylates of aliphatic, alicyclic, araliphatic polyhydric alcohols having 2 to 6 valences and polyalkylene glycols, poly (meth) acryloyloxyalkyl phosphates,
Polyester poly (meth) acrylate, epoxy (meth) acrylate, polyurethane poly (meth) acrylate, polyamide (meth) polyacrylate, polysiloxane poly (meth) acrylate side chain and / or vinyl having metha (acryloyloxy group) at the end Examples thereof include prepolymers such as a system or diene low polymer. As the monomer, an alkyl ester of an ethylenically unsaturated carboxylic acid, a mono (meth) acrylate of an alkylene oxide addition polymer of a compound containing active hydrogen, an alkylene oxide addition polymer of a compound having active hydrogen and (meth) Amide group-containing monomers typified by vinyl lactams such as bifunctional monomers N-vinylpyrrolidone composed of diesters with acrylic acid, alkylene oxide addition polymers of compounds having active hydrogen, and (meth) acrylic acid And a polyfunctional monomer made of polyester. Examples of the photopolymerization initiator include benzophenone and 2-
Hydroxy-2-methyl-propiophenone, 1-hydroxy-cyclohexyl phenyl ketone, 2,4-diethylthioxanthone and the like can be mentioned, and they are added and used in a proportion of 0.5 to 10% by weight with respect to the prepolymer and the monomer.

Inorganic and organic fillers, if necessary,
Lubricants, surfactants and the like can be added. The coating amount is preferably 1 to 6 g / m ² in terms of solid content weight.

The printing ink used in the printing layer is a composition containing the UV-curable resin used in the protective layer as a main component, a colored pigment or an extender pigment, and other waxes and stabilizers if necessary. . Examples of commercially available products of this UV curable ink include Die Cure
Ink (manufactured by Dainippon Ink and Chemicals), best cure ink (manufactured by Toka Dye Chemicals Industry), UV REX ink (manufactured by Dainichi Seika), flash dry ink (manufactured by Toyo Ink) are listed. Used for printing by a printing machine equipped with an ultraviolet curing device.

The heat-sensitive recording material of the present invention is prepared by using the above-mentioned material by the following method, for example. That is, according to a conventional method, the color-forming compound and the color-developing compound are separately pulverized in the copolymer solution of the present invention (for example, a toluene solution) separately, if necessary, together with other additives. This is mixed before use to form a coating solution having a solid content concentration of 10 to 50% by weight, and a solid content weight of ² to 40 g / m ² preferably 3 to 10 g / m on a support such as paper and plastic sheet. ^It is coated so as to be ² and dried to obtain a support having a thermosensitive coloring layer.

The intermediate layer formed on the thermosensitive coloring layer has a solid content of 5 to 20 by adding the above-mentioned copolymer solution (for example, a toluene solution) alone or if necessary by adding a filler, a surfactant and the like. The coating liquid for forming the intermediate layer is used in the range of%. Then, the intermediate layer is obtained by coating and drying the heat-sensitive color-developing layer so that the solid content becomes 1 to 6 g / m ² .

Next, a UV-curable printing ink and / or protective layer is formed on the thermosensitive coloring layer or the intermediate layer provided on the thermosensitive coloring layer formed by a known method by using a screen printer or an offset printer. Print or apply,
Ultraviolet irradiation is carried out to fix the print layer and / or the protective layer to obtain the heat-sensitive recording material of the present invention. UV irradiation is for example
Line speed 5m / at a distance of 10cm under an 80W high pressure mercury lamp
A method of passing 1 to 3 times under the condition of minutes is adopted. The heat-sensitive recording material of the present invention is characterized by excellent adhesion to the protective layer or the printing layer, and good background and color density.

[0034]

EXAMPLES The present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

In the examples, "part" means part by weight.

Example 1 Formation of Thermosensitive Coloring Layer A mixture [A] and a solution [B] were prepared by pulverizing a mixture having the following composition for 1 hour using a sand grinder.

Liquid [A]: 2-o-fluoroanilino-6-diethylaminofluorane 25 parts 20% Copolymer of styrene, methyl methacrylate and the compound of formula (1) (monomer ratio, styrene: methyl methacrylate: Compound of formula (1) = 3: 6: 1, molecular weight about 50,000) in toluene solution 25 parts toluene 50 parts [B] liquid: 4,4-sulfonyl-bis (2-allylphenol)
25 parts Toluene solution of the same copolymer used in solution [A]
25 parts Toluene 50 parts Then [A] liquid: [B] liquid: same copolymer solution as used in [A] and [B] liquid = 1 part: 3 parts: 3 parts by mixing and heat sensitive A coloring layer coating solution was prepared, coated on a white polyester film so that the solid content was 8 g / m ^2, and dried to obtain a polyester film with a thermosensitive coloring layer.

Formation of Protective Layer Next, an ultraviolet curable ink (Kayarad DPHA (97 parts) manufactured by Nippon Kayaku, Irgacure 184 (3 parts) manufactured by Ciba-Geigy) was applied onto the thermosensitive coloring layer at 2 g / m ^2. The thermosensitive recording material of the present invention was obtained by curing with an ultraviolet irradiation device (manufactured by Nippon Battery Co., Ltd., under an 80 W high-pressure mercury lamp, 10 cm, twice at a line speed of 5 m / min).

The peeling (adhesion) of this recording material with an adhesive tape and the color development were evaluated. The results are shown in Table 1.
(The same applies to each of the following examples) Example 2 Styrene used in the preparation of the thermosensitive coloring layer in Example 1,
Instead of the copolymer of methyl methacrylate and the compound of formula (1), the monomer component (ratio) is a copolymer of styrene: methyl methacrylate: compound of formula (3) (= 18: 1: 1) (molecular weight of about 10). The heat-sensitive recording material of the present invention was prepared in the same manner as in Example 1 except that

Example 3 Styrene used in the preparation of the thermosensitive coloring layer in Example 1,
Instead of the copolymer of methyl methacrylate and the compound of formula (1), the monomer component (ratio) is a copolymer of styrene: methyl methacrylate: the compound of formula (6) (= 11: 8: 1) (molecular weight about 20). The heat-sensitive recording material of the present invention was prepared in the same manner as in Example 1 except that

Example 4 Formation of Thermosensitive Coloring Layer A mixture of the following compositions was pulverized with a sand grinder for 1 hour to prepare solutions [A] and [B], respectively.

Liquid [A]: 2-o-fluoroanilino-6-dibutylaminofluorane 25 parts 10% methylmethacrylate polymer (molecular weight about 500,000) in xylene 25 parts xylene 50 parts Liquid [B]: bisphenol A 25 parts 10% methyl methacrylate polymer (molecular weight of about 500,000) in xylene 25 parts xylene 50 parts Then [A] liquid: [B] liquid: 10% methyl methacrylate copolymer xylene solution = 1 part: 3 parts A thermosensitive color developing layer coating liquid was prepared by mixing in a ratio of 3 parts, coated on a white polyester film so that the solid content was 6 g / m ^2, and dried to form a thermosensitive color developing layer.

Formation of Intermediate Layer Copolymer of styrene, methyl methacrylate and the compound of formula (1) (monomer ratio of styrene: methyl methacrylate: compound of formula (1) = 6: 3) on the thermosensitive coloring layer. A 10% toluene solution having a molecular weight of about 50,000) was applied with a bar coater to a solid content of 2 g / m ² and dried to form an intermediate layer.

Formation of Printing Layer Next, an ultraviolet curable ink (best cure No.2 UV thermal 118 red G manufactured by Toka Dye and Chemical Co., Ltd.) is formed on the intermediate layer.
X) is printed by using an offset printing machine (Hamada 611 XL, manufactured by Hamada Printing Co., Ltd.) so that the ink layer thickness is about 0.5 μm, and then it is cured by the above-mentioned ultraviolet irradiation device to obtain the heat-sensitive recording material of the present invention. Got

Example 5 Instead of the copolymer of styrene, methyl methacrylate and the compound of formula (1) in the intermediate layer in Example 4, the monomer component (ratio) was styrene: methyl methacrylate: the compound of formula (3) ( The thermal recording material of the present invention was prepared in the same manner as in Example 4 except that the copolymer (18: 1: 1) (molecular weight: about 100,000) was used.

Example 6 Instead of the copolymer of styrene, methyl methacrylate and the compound of the formula (1) in the intermediate layer in Example 1, the monomer component (ratio) was styrene: methyl methacrylate: the compound of the formula (6) ( The thermal recording material of the present invention was prepared in the same manner as in Example 1 except that the copolymer (11: 8: 1) (molecular weight: about 200,000) was used.

Comparative Example 1 Styrene used in the preparation of the thermosensitive coloring layer in Example 1,
Instead of the copolymer of methyl methacrylate and ethyl acrylate, the monomer component (ratio) is a copolymer of styrene: methyl methacrylate (= 3: 1) (molecular weight about 60,000)
A thermal recording material for comparison was prepared in the same manner as in Example 1 except that was used.

Comparative Example 2 A thermosensitive recording material for comparison was prepared in the same manner as in Example 4 except that the intermediate layer was not provided and the printing layer was provided directly on the thermosensitive coloring layer.

[0049]

[Table 1] As is apparent from the table, the recording material of the present invention is characterized in that it has excellent adhesion to the printing ink or protective layer and good color development, as compared with the heat-sensitive recording material for comparison.

[0050]

EFFECT OF THE INVENTION When forming a protective layer or a printing layer made of an ultraviolet curable resin, the thermosensitive coloring layer or the intermediate layer has styrene and / or methacrylic acid ester and / or acrylic acid ester and a radically polymerizable functional group. By using a copolymer composed of a photopolymerizable monomer having a photopolymerization initiation function, a heat-sensitive recording material having excellent adhesion to a protective layer or a printing layer composed of an ultraviolet curable resin and good color development Has come to be obtained.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroaki Tsugawa 563 Oyama, Matsudo City, Chiba Prefecture (72) Inventor ▲ Yoshi ▼ Kenichiro Oka 1039 Kamiochiai, Yono City, Saitama Prefecture

Claims (2)

[Claims] 1. A thermosensitive coloring layer comprising, as a main component, a colorless or light-colored coloring compound and a color-developing compound which reacts with the coloring compound to form a color on a support, and a direct or intermediate layer is provided thereon. In a heat-sensitive recording material in which a protective layer or a printing layer formed via a layer is made of an ultraviolet curable resin, a copolymerizable monomer having a photopolymerization initiating function, styrene, a methacrylic acid ester, and an acryl in a thermosensitive coloring layer or an intermediate layer. A heat-sensitive recording material comprising a copolymer obtained from 1 to 3 kinds of monomers selected from acid esters as a binder. 2. The heat-sensitive recording material according to claim 1, wherein the copolymer is a copolymer obtained from a copolymerizable monomer having a photopolymerization initiation function, styrene and a methacrylic acid ester.