Thermal recording medium and image forming method

Abstract

To provide a thermosensitive recording medium which can suppress color development due to heating after ultraviolet irradiation even when irradiated with a low quantity of ultraviolet rays.SOLUTION: The thermosensitive recording medium includes a thermosensitive color developing layer containing an electron-donating dye precursor, an electron-accepting compound, a radically polymerizable compound, and a photoradical polymerization initiator. The photoradical polymerization initiator contains an oxime ester compound having a group represented by the following formula (1).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a thermosensitive recording medium and an image forming method using the same. [Background technology]

[0002] Conventionally, thermosensitive recording media have been widely used, which utilize a mechanism for color development by reacting a leuco dye with a color developer. Because thermosensitive recording media do not require consumables such as ink or toner and are relatively inexpensive, they are widely used as recording media for fax machines, receipts, and other applications. A typical thermosensitive recording medium is manufactured by printing or coating a water-containing thermosensitive color-developing composition in the form of a coating liquid onto a substrate, followed by drying to form a thermosensitive color-developing layer.

[0003] A method of forming a thermosensitive coloring layer by irradiating a UV-curable thermosensitive coloring composition with UV rays has also been investigated. For example, Patent Document 1 proposes a photosensitive and thermosensitive recording material having a photosensitive and thermosensitive layer containing (1) a photocurable composition containing an electron-accepting and polymerizable vinyl monomer and a photopolymerizable compound, and (2) microcapsules containing an electron-donating colorless dye. According to Patent Document 1, UV exposure slows the diffusion of the electron-accepting and polymerizable vinyl monomer, thereby preventing contact with the electron-donating dye precursor. Therefore, the cured portion does not develop color even when heated after UV exposure. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 3-72358 Summary of the Invention [Problem to be solved by the invention]

[0005] However, according to the investigations of the present inventors, the photosensitive and heat-sensitive recording material proposed in Patent Document 1 requires a certain level of accumulated light (approximately 1000 mJ / cm ) to suppress color development after ultraviolet irradiation. 2 On the other hand, in order to reduce the size of the ultraviolet irradiation device and power consumption, it was necessary to further reduce the amount of ultraviolet light to be irradiated.

[0006] Therefore, an object of the present invention is to provide a thermosensitive recording medium that can suppress color development due to heating after ultraviolet irradiation even when the amount of ultraviolet light irradiated is low. Another object of the present invention is to provide an image forming method using the above thermosensitive recording medium. [Means for solving the problem]

[0007] The above object can be achieved by the present invention as follows.

[0008] That is, according to the present invention, there is provided a thermosensitive recording medium having a thermosensitive color-forming layer containing an electron-donating dye precursor, an electron-accepting compound, a radical polymerizable compound, and a photoradical polymerization initiator, The thermal recording medium is characterized in that the photoradical polymerization initiator contains an oxime ester compound having a group represented by the following formula (1):

[0009] [ka]

[0010] (In formula (1), R1 and R2 each independently represent a hydrogen atom, a halogen atom, a nitro group, a cyano group, a hydrocarbon group having 1 to 20 carbon atoms, or a group containing a heterocycle and having 2 to 20 carbon atoms; a hydrogen atom in the groups represented by R1 and R2 may be substituted with a halogen atom, a nitro group, a cyano group, a hydroxyl group, an amino group, a carboxyl group, a methacryloyl group, an acryloyl group, an epoxy group, a vinyl group, a vinyl ether group, a mercapto group, an isocyanate group, or a group containing a heterocycle and having 2 to 20 carbon atoms; a methylene group in the groups represented by R1 and R2 may be substituted by -O-, -CO-, -COO-, -OCO-, -NR3-, -NR3CO-, -S-, -CS-, -SO2-, -SCO-, -COS-, -OCS-, -CH=CH-, -CC-, or -CSO-, and R3 represents a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms; m represents 0 or 1; * represents a bonding site to another structure, and the bonding site may form a ring together with the group represented by R1.

[0011] According to the present invention, there is also provided an image forming method comprising the steps of: applying a heat pulse to the above-mentioned thermal recording medium using a thermal head to form an image; and irradiating the thermal recording medium on which the image has been formed with ultraviolet light to fix the thermosensitive coloring layer. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a thermosensitive recording medium that can suppress color development due to heating after ultraviolet irradiation even when the amount of ultraviolet light irradiated is low. Also, according to the present invention, it is possible to provide an image forming method using the above thermosensitive recording medium. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a cross-sectional view showing one embodiment of a thermosensitive recording medium of the present invention. [Figure 2] FIG. 2 is a cross-sectional view showing another embodiment of the thermosensitive recording medium of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] <Thermal recording medium> The present invention will be described in detail below with reference to preferred embodiments, but is not limited to these embodiments. The thermosensitive recording medium of the present invention is a thermosensitive recording medium having a thermosensitive color-forming layer containing an electron-donating dye precursor, an electron-accepting compound, a radically polymerizable compound, and a photoradical polymerization initiator. The photoradical polymerization initiator contains an oxime ester compound having a group represented by the following formula (1):

[0015] [ka]

[0016] (In formula (1), R1 and R2 each independently represent a hydrogen atom, a halogen atom, a nitro group, a cyano group, a hydrocarbon group having 1 to 20 carbon atoms, or a group containing a heterocycle and having 2 to 20 carbon atoms; a hydrogen atom in the groups represented by R1 and R2 may be substituted with a halogen atom, a nitro group, a cyano group, a hydroxyl group, an amino group, a carboxyl group, a methacryloyl group, an acryloyl group, an epoxy group, a vinyl group, a vinyl ether group, a mercapto group, an isocyanate group, or a group containing a heterocycle and having 2 to 20 carbon atoms; a methylene group in the groups represented by R1 and R2 may be substituted by -O-, -CO-, -COO-, -OCO-, -NR3-, -NR3CO-, -S-, -CS-, -SO2-, -SCO-, -COS-, -OCS-, -CH=CH-, -CC-, or -CSO-, and R3 represents a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms; m represents 0 or 1; * represents a bonding site to another structure, and the bonding site may form a ring together with the group represented by R1.

[0017] In the configuration of the present invention, the NO bond of the oxime ester compound having the group represented by formula (1) is efficiently cleaved upon UV irradiation to generate a radical. Therefore, even with low-intensity UV irradiation, the polymerization reaction of the radical-polymerizable compound proceeds sufficiently, and the electron-donating dye precursor and the electron-accepting compound can be fixed in the UV-cured product.

[0018] As a result, even if the layer is heated after ultraviolet irradiation, the electron-donating dye precursor and the electron-accepting compound are less likely to move within the thermosensitive color-forming layer, and color development due to contact between the electron-donating dye precursor and the electron-accepting compound can be suppressed.

[0019] Furthermore, when an oxime ester compound having a group represented by formula (1) was used, the electron-donating dye precursor tended to be more easily encapsulated in the thermosensitive coloring layer. While the detailed mechanism is unclear, the inventors speculate that one of the following two points may be involved. First, the oxime ester compound having a group represented by formula (1) has good compatibility with the electron-donating dye precursor. This allows the electron-donating dye precursor to be uniformly present in the thermosensitive coloring layer without localization. Second, a portion of the oxime ester compound having a group represented by formula (1) is localized on the surface of the thermosensitive coloring layer. This allows the electron-donating dye precursor to be encapsulated without being exposed to the surface of the thermosensitive coloring layer.

[0020] Therefore, the present inventors speculate that the ease with which the oxime ester compound having a group represented by formula (1) generates radicals and the ease with which the electron-donating dye precursor is encapsulated in the thermosensitive color-forming layer may have contributed to the inhibition of color development due to contact between the electron-donating dye precursor and the electron-accepting compound.

[0021] (Electron-donating dye precursor) The thermosensitive color-forming layer contains an electron-donating dye precursor (also called a leuco dye). Electron-donating dye precursors are usually colorless or pale in color. Electron-donating dye precursors have the property of developing color by donating electrons or accepting protons from acids, etc. Specific examples of electron-donating dye precursors are listed below.

[0022] Examples of electron-donating dye precursors that develop red or vermilion colors include 3,6-bis(diethylamino)fluoran-γ-anilinolactam, 3,6-bis(diethylamino)fluoran-γ-(p-nitro)anilinolactam, 3,6-bis(diethylamino)fluoran-γ-(o-chloro)anilinolactam, 3-dimethylamino-7-bromofluoran, 3-diethylaminofluoran, 3-diethylamino-6-methylfluoran, 3-diethylamino-7-methylfluoran, and 3- Examples include diethylamino-7-chlorofluoran, 3-diethylamino-7-bromofluoran, 3-diethylamino-7,8-benzofluoran, 3-diethylamino-6,8-dimethylfluoran, 3-diethylamino-6-methyl-7-chlorofluoran, 3-diethylamino-7-tert-butylfluoran, 3-(N-ethyl-N-tolylamino)-7-ethylfluoran, and 3-(N-ethyl-N-isobutylamino)-6-methyl-7-chlorofluoran.

[0023] Further examples of the electron donating dye precursors that develop red or vermilion colors include 3-cyclohexylamino-6-chlorofluoran, 3-di(n-butyl)amino-6-methyl-7-bromofluoran, 3-di(n-butyl)amino-7,8-benzofluoran, 3-tolylamino-7-methylfluoran, 3-tolylamino-7-ethylfluoran, 2-(N-acetylanilino)-3-methyl-6-di(n-butyl)aminofluoran, 2-(N-propionylanilino)-3-methyl-6-di(n-butyl)aminofluoran, and 2-(N-benzoylanilino)-3-methyl-6-di(n-butyl)aminofluoran. fluoran, 2-(N-carbobutoxyanilino)-3-methyl-6-di(n-butyl)aminofluoran, 2-(N-formylanilino)-3-methyl-6-di(n-butyl)aminofluoran, 2-(N-benzylanilino)-3-methyl-6-di(n-butyl)aminofluoran, 2-(N-allylanilino)-3-methyl-6-di(n-butyl)aminofluoran, 2-(N-methylanilino)-3-methyl-6-di(n-butyl)aminofluoran, 3-diethylamino-7-phenoxyfluoran, 2-methyl-6-(Np-tolyl-N-ethylamino)-fluoran, and the like.

[0024] Examples of electron-donating dye precursors that produce magenta colors include 3,3-bis(1-ethyl-2-methylindol-3-yl)phthalide, 3,3-bis(1-n-octyl-2-methylindol-3-yl)phthalide, 7-(N-ethyl-N-isoamylamino)-3-methyl-1-phenylspiro[(1,4-dihydrochromeno[2,3-c]pyrazole)-4,3'-phthalide], 7-(N-ethyl-N-isoamylamino)-3-methyl-1-p-methylphenylspiro[(1,4-dihydrochromeno[2,3-c]pyrazole)-4,3'-phthalide], and 7-(N-ethyl-Nn-hexylamino)-3-methyl-1-phenylspiro[(1,4-dihydrochromeno[2,3-c]pyrazole)-4,3'-phthalide].

[0025] Further examples of electron-donating dye precursors that develop magenta color tones include 3-(N-ethyl-N-isoamylamino)-7,8-benzofluoran, 3,3-bis(1-n-butyl-2-methylindol-3-yl)phthalide, and 3-(N-ethyl-N-isoamylamino)-7-phenoxyfluoran.

[0026] As the electron-donating dye precursor that develops a red, vermilion, or magenta color tone, it is preferable to use at least one selected from the group consisting of 3-diethylamino-7-chlorofluoran, 3-diethylamino-6,8-dimethylfluoran, 3-(N-ethyl-N-isoamylamino)-7,8-benzofluoran, 2-methyl-6-(Np-tolyl-N-ethylamino)-fluoran, 3-di(n-butyl)amino-6-methyl-7-bromofluoran, and 3,3-bis(1-n-butyl-2-methylindol-3-yl)phthalide.

[0027] Examples of electron-donating dye precursors that develop blue tones include 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide, 3-(4-diethylamino-2-methylphenyl)-3-(4-dimethylaminophenyl)-6-dimethylaminophthalide, 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide, and 3-(1-ethyl-2-methylindol-3-yl)-3-(4-diethylaminophenyl)phthalide. amide, 3-(1-ethyl-2-methylindol-3-yl)-3-(2-methyl-4-diethylaminophenyl)-4-azaphthalide, 3-(1-ethyl-2-methylindol-3-yl)-3-(2-ethoxy-4-diethylaminophenyl)-4-azaphthalide, 3-(1-ethyl-2-methylindol-3-yl)-3-(2-n-hexyloxy-4-diethylaminophenyl)-4-azaphthalide, 3-diphenylamino-6-diphenylaminofluoran, and the like can be mentioned.

[0028] Examples of electron-donating dye precursors that develop cyan color tones include 3-(1-ethyl-2-methylindol-3-yl)-3-(4-diethylamino-2-methylphenyl)-4-azaphthalide, 3-[1,1-bis(p-diethylaminophenyl)ethylene-2-yl]-6-dimethylaminophthalide, 3,3-bis(4-diethylamino-2-ethoxyphenyl)-4-azaphthalide, and 3,3'-bis(4-diethylamino-2-ethoxyphenyl)-4-azaphthalide.

[0029] Examples of electron donating dye precursors that develop blue or cyan colors include 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide, 3-(4-diethylamino-2-methylphenyl)-3-(4-dimethylaminophenyl)-6-dimethylaminophthalide, 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide, and 3-(1-ethyl-2-methylindol-3-yl)-3-(4 It is preferable to use at least one selected from the group consisting of 3-(1-ethyl-2-methylindol-3-yl)-3-(2-n-hexyloxy-4-diethylaminophenyl)-4-azaphthalide, 3-[1,1-bis(p-diethylaminophenyl)ethylene-2-yl]-6-dimethylaminophthalide, and 3,3'-bis(4-diethylamino-2-ethoxyphenyl)-4-azaphthalide.

[0030] Examples of electron-donating dye precursors that develop yellow tones include 4-[2-[2-(butoxy)phenyl]-6-phenyl-4-pyridinyl]-N,N-dimethylbenzenamine, 4-[2-[2-(octyloxy)phenyl]-6-phenyl-4-pyridinyl]-N,N-dimethylbenzenamine, 4-[2-[2-(ethoxy)phenyl]-6-phenyl-4-pyridinyl]-N,N-dimethylbenzenamine, 4-[2,6-bis(2-ethoxyphenyl)-4-pyridinyl]-N,N-dimethylbenzenamine, and 4-(2,6-diphenyl-4-pyridinyl)-N,N-dimethylbenzenamine. Examples thereof include 4-[2,6-bis(2-butoxyphenyl)-4-pyridinyl]-N,N-dimethylbenzenamine, 4-[2,6-bis(2-octyloxyphenyl)-4-pyridinyl]-N,N-dimethylbenzenamine, 4-[2-[2-(hexyloxy)phenyl]-6-phenyl-4-pyridinyl]-N,N-dimethylbenzenamine, 4-[2,6-bis(2-hexyloxyphenyl)-4-pyridinyl]-N,N-dimethylbenzenamine, 3,6-dimethoxyfluoran, and 1-(4-n-dodecyloxy-3-methoxyphenyl)-2-(2-quinolyl)ethylene.

[0031] As the electron-donating dye precursor that develops a yellowish color tone, it is preferable to use at least one selected from the group consisting of 4-[2-[2-(octyloxy)phenyl]-6-phenyl-4-pyridinyl]-N,N-dimethylbenzeneamine, 3,6-dimethoxyfluoran, and 1-(4-n-dodecyloxy-3-methoxyphenyl)-2-(2-quinolyl)ethylene.

[0032] Examples of electron-donating dye precursors that develop green tones include 3-(N-ethyl-Nn-hexylamino)-7-anilinofluoran, 3-diethylamino-7-dibenzylaminofluoran, 3-pyrrolidino-7-dibenzylaminofluoran, 3,3-bis(4-diethylamino-2-ethoxyphenyl)-4-azaphthalide, 3-(N-ethyl-Np-tolylamino)-7-(N-phenyl-N-methylamino)fluoran, 3-[p-(p-anilinoanilino)anilino]-6-methyl-7-chlorofluoran, and 3,6-bis(dimethylamino)fluorene-9-spiro-3'-(6'-dimethylamino)phthalide.

[0033] As the electron-donating dye precursor that develops a greenish color tone, it is preferable to use at least one selected from the group consisting of 3-diethylamino-7-dibenzylaminofluoran and 3-pyrrolidino-7-dibenzylaminofluoran.

[0034] Examples of electron-donating dye precursors that develop black tones include 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-diethylamino-6-methyl-7-(m-methylanilino)fluoran, 3-(N-isoamyl-N-ethylamino)-7-(o-chloroanilino)fluoran, and 3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran. Nilinofluoran, 3-(N-ethyl-N-2-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3-diethylamino-6-chloro-7-anilinofluoran, 3-di(n-butyl)amino-6-methyl-7-anilinofluoran, 3-di(n-amyl)amino-6-methyl-7-anilinofluoran, 3-(N-isoamyl-N-ethylamino)-6-methyl-7-anilinofluoran, 3-( Nn-hexyl-N-ethylamino)-6-methyl-7-anilinofluoran, 3-[N-(3-ethoxypropyl)-N-ethylamino)-6-methyl-7-anilinofluoran, 3-[N-(3-ethoxypropyl)-N-methylamino)-6-methyl-7-anilinofluoran, 3-diethylamino-7-(2-chloroanilino)fluoran, 3-di(n-butyl)amino-7-(2-chloroanilino)fluoran , 3-diethylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-(2,6-dimethylanilino)fluoran, 3-diethylamino-6-methyl-7-(2,4-dimethylanilino)fluoran, 2,4-dimethyl-6-(4-dimethylaminoanilino)fluoran, 3-(N-cyclohexyl-N-methylamino)-6-methyl-7-anilinofluoran, and the like can be mentioned.

[0035] As the electron-donating dye precursor that develops a black color tone, it is preferable to use at least one selected from the group consisting of 3-di(n-butyl)amino-6-methyl-7-anilinofluoran, 3-di(n-amyl)amino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-(2,6-dimethylanilino)fluoran, 3-diethylamino-6-methyl-7-(2,4-dimethylanilino)fluoran, and 2,4-dimethyl-6-(4-dimethylaminoanilino)fluoran, which have relatively excellent light resistance.

[0036] Examples of electron-donating dye precursors having absorption in the near infrared region include 3,3-bis[1,1-bis(4-pyrrolidinophenyl)ethylene-2-yl]-4,5,6,7-tetrabromophthalide, 3,3-bis[1-(4-methoxyphenyl)-1-(4-dimethylaminophenyl)ethylene-2-yl]-4,5,6,7-tetrachlorophthalide, 3,3-bis[1-(4-methoxyphenyl)-1-(4-pyrrolidinophenyl)ethylene-2-yl]-4,5,6,7-tetrachlorophthalide, 3-[p-(p-anilinoanilino)anilino]-6-methyl-7-chlorofluoran, and 3-[p-(p-dimethylaminoanilino)anilino]-6-methyl-7-chlorofluoran. fluoran, 3,6-bis(dimethylamino)fluorene-9-spiro-3'-(6'-dimethylamino)phthalide, bis(p-dimethylaminostyryl)-p-tolylsulfonylmethane, 3-[p-(p-dimethylaminoanilino)anilino]-6-methylfluoran, 3-di(n-pentyl)amino-6,8,8-trimethyl-8,9-dihydro-(3,2,e)pyridofluoran, 3-di(n-butyl)amino-6,8,8-trimethyl-8,9-dihydro-(3,2,e)pyridofluoran, 3-(pn-butylaminoanilino)-6-methyl-7-chlorofluoran, 2-mesidino-8-diethylamino-benz[C]fluoran, and the like can be mentioned.

[0037] The electron-donating dye precursor is preferably contained in the thermosensitive coloring layer in a state where it is encapsulated in particles comprising a radical polymerizable compound and a photoradical polymerization initiator. 2 More than 2.00g / m 2 If it is less than this, an image with a sufficient optical density can be formed, which is preferable.

[0038] (Electron-accepting compounds) The thermosensitive color-forming layer contains an electron-accepting compound (also called a color developer) that causes the electron-donating dye precursor to develop color upon contact. The electron-accepting compound is preferably a compound that liquefies or dissolves upon temperature increase. Examples of the electron-accepting compound include organic acidic substances such as phenolic compounds, aromatic carboxylic acids, and polyvalent metal salts of these compounds.

[0039] Examples of electron-accepting compounds include 4-tert-butylphenol, 4-acetylphenol, 4-tert-octylphenol, 4,4'-sec-butylidenediphenol, 4-phenylphenol, 4,4'-dihydroxydiphenylmethane, 4,4'-isopropylidenediphenol, 4,4'-dihydroxydiphenyl ether, 4,4'-cyclohexylidenediphenol, 1,1-bis(4-hydroxyphenyl)ethane, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, 4,4'-dihydroxydiphenyl sulfide, 4,4'-thiobis(3-methyl-6-tert-butylphenol), 4,4'-dihydroxydiphenyl sulfone, 2,4'-dihydroxydiphenyl sulfone, 4- ...4,4'-dihydroxydiphenyl ether, 4,4'-cyclohexylidenediphenol, 4,4'-dihydroxydiphenyl ether, 4,4'-cyclohexylidenediphenol, 4,4'-dihydroxydiphenyl ether, 4,4'-cyclohexylidenediphenol, 4,4'-dihydroxydiphenyl ether, 4,4'-cyclohexylidenediphenol, 4,4'-dihydroxydiphenyl ether, 4,4'-cyclohexylidenediphenol, 4,4'-dihydroxydiphenyl ether, 4,4'-cyclohexylidenediphenol, 4,4'-dihydroxydiphenyl Examples of the alkyl ester include 4'-isopropoxydiphenyl sulfone, 4-hydroxy-4'-n-propoxydiphenyl sulfone, 4-hydroxy-4'-allyloxydiphenyl sulfone, bis(3-allyl-4-hydroxyphenyl)sulfone, 4,4'-bis[(4-methyl-3-phenoxycarbonylaminophenyl)ureido]diphenyl sulfone, 4-[4'-(1'-methylethyloxy)phenyl]sulfonylphenol, N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyloxyphenyl)urea, Np-tolylsulfonyl-p-butoxycarbonylphenylurea, N-(p-toluenesulfonyl)-N'-phenylurea, and 4,4'-bis(3-tosylureido)diphenylmethane.

[0040] Further examples of the electron-accepting compound include phenolic compounds such as 4-hydroxybenzophenone, dimethyl 4-hydroxyphthalate, methyl 4-hydroxybenzoate, propyl 4-hydroxybenzoate, sec-butyl 4-hydroxybenzoate, phenyl 4-hydroxybenzoate, benzyl 4-hydroxybenzoate, tolyl 4-hydroxybenzoate, chlorophenyl 4-hydroxybenzoate, and 4,4'-dihydroxydiphenyl ether; aromatic carboxylic acids such as benzoic acid, p-tert-butylbenzoic acid, trichlorobenzoic acid, terephthalic acid, salicylic acid, 3-tert-butylsalicylic acid, 3-isopropylsalicylic acid, 3-benzylsalicylic acid, 3,5-(α-methylbenzyl)salicylic acid, and 3,5-di-tert-butylsalicylic acid; and organic acidic substances such as salts of these compounds with polyvalent metals such as zinc, magnesium, aluminum, and calcium.

[0041] (Radical polymerizable compound) The thermosensitive coloring layer contains a radically polymerizable compound, such as a radically polymerizable monomer, a radically polymerizable oligomer, or a radically polymerizable polymer. Among these, a radically polymerizable monomer is preferred in terms of efficient curing by ultraviolet light.

[0042] The radical polymerizable compound is preferably at least one compound selected from the group consisting of pentaerythritol derivatives, isocyanurate derivatives, and trimethylolpropane derivatives, and the proportion of the at least one compound selected from the group consisting of pentaerythritol derivatives, isocyanurate derivatives, and trimethylolpropane derivatives among the radical polymerizable compounds is preferably 80% by mass or more and 100% by mass or less.

[0043] Examples of pentaerythritol derivatives include pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, ethylene oxide-modified pentaerythritol tri(meth)acrylate, propylene oxide-modified pentaerythritol tri(meth)acrylate, ethylene oxide-modified pentaerythritol tetra(meth)acrylate, propylene oxide-modified pentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and tripentaerythritol (meth)acrylate. Among these, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and tripentaerythritol (meth)acrylate are preferred. Note that "(meth)acrylate" refers to methacrylate or acrylate. Commercially available pentaerythritol derivatives include, for example, Light Acrylate PE-3A manufactured by Kyoeisha Chemical Co., Ltd., Light Acrylate PE-4A manufactured by Kyoeisha Chemical Co., Ltd., Light Acrylate DPE-6A manufactured by Kyoeisha Chemical Co., Ltd., SR295 manufactured by Sartomer Co., Ltd., Viscoat #300 manufactured by Osaka Organic Chemical Industry Co., Ltd., MT-3549 manufactured by Toagosei Co., Ltd., NK Oligo U-6LPA manufactured by Shin-Nakamura Chemical Co., Ltd., U-15HA manufactured by Shin-Nakamura Chemical Co., Ltd., KAYARAD D-310 manufactured by Nippon Kayaku Co., Ltd., KAYARAD D-310 manufactured by Nippon Kayaku Co., Ltd., KAYARAD D-330 manufactured by Nippon Kayaku Co., Ltd., A-DPH manufactured by Shin-Nakamura Chemical Co., Ltd., and A-TMMT manufactured by Shin-Nakamura Chemical Co., Ltd.

[0044] Examples of isocyanurate derivatives include tris(2-hydroxyethyl)isocyanurate di(meth)acrylate, tris(2-hydroxyethyl)isocyanurate tri(meth)acrylate, and urethane acrylate isocyanurate derivatives obtained by reacting an isocyanurate polyisocyanate with a polyfunctional pentaerythritol derivative or a polyfunctional trimethylol derivative having a hydroxyl group to form a polyfunctional isocyanurate via a urethane bond. Among these, tris(2-hydroxyethyl)isocyanurate tri(meth)acrylate and urethane acrylate isocyanurate derivatives are preferred. Commercially available isocyanurate derivatives include Fancryl FA-731A (manufactured by Hitachi Chemical Co., Ltd.), SR368 (manufactured by Sartomer Co., Ltd.), Aronix M-315 (manufactured by Toagosei Co., Ltd.), and U-15HA (manufactured by Shin-Nakamura Chemical Co., Ltd.).

[0045] Examples of trimethylolpropane derivatives include trimethylolpropane tri(meth)acrylate, propoxylated trimethylolpropane tri(meth)acrylate, ethylene oxide-modified trimethylolpropane triacrylate, propylene oxide-modified trimethylolpropane, ditrimethylolpropane tetraacrylate, etc. Among these, trimethylolpropane tri(meth)acrylate and ditrimethylolpropane tetraacrylate are preferred as trimethylolpropane derivatives. Commercially available trimethylolpropane derivatives include, for example, Sunester TMP (manufactured by Sanshin Chemical Industry Co., Ltd.), Viscoat #295 (manufactured by Osaka Organic Chemical Industry Co., Ltd.), Lightester TMP (manufactured by Kyoeisha Chemical Co., Ltd.), Acryester TMP (manufactured by Mitsubishi Chemical Holdings Corporation), Miramar M410 (manufactured by Toyo Chemicals Co., Ltd.), Miramar M300 (manufactured by Toyo Chemicals Co., Ltd.), Miramar M301 (manufactured by Toyo Chemicals Co., Ltd.), EBECRYL140 (manufactured by Daicel-Okunex Co., Ltd.), EBECRYL1142 (manufactured by Daicel-Okunex Co., Ltd.), SR355 (manufactured by Sartomer Co., Ltd.), AD-TMP (manufactured by Shin-Nakamura Chemical Co., Ltd.), and A-TMPT (manufactured by Shin-Nakamura Chemical Co., Ltd.).

[0046] The radical polymerizable compound is preferably contained in the thermosensitive coloring layer in a premixed state with the electron-donating dye precursor. The form of the radical polymerizable compound in the thermosensitive coloring layer is not particularly limited, and examples include particles and layers. For example, when the radical polymerizable compound is contained in the thermosensitive coloring layer in the form of a layer, the thermosensitive coloring layer preferably has a first layer containing a radical polymerizable compound mixed with the electron-donating dye precursor and a second layer containing an electron-accepting compound. Hereinafter, the first layer containing a radical polymerizable compound mixed with the electron-donating dye precursor will also be referred to as the "electron-donating dye precursor layer" or "leuco dye layer." The second layer containing the electron-accepting compound will also be referred to as the "electron-accepting compound layer" or "developer layer."

[0047] The radical polymerizable compound can also be incorporated into the thermosensitive coloring layer in the form of particles. The method for preparing the radical polymerizable compound in the form of particles is not particularly limited, but is preferably prepared by the O / W emulsion method, and preferably prepared by pre-mixing an electron-donating dye precursor and a photoradical polymerization initiator (described below) with the radical polymerizable compound. Alternatively, the electron-donating dye precursor may be replaced with an electron-accepting compound. That is, particles containing either the electron-donating dye precursor or the electron-accepting compound, the radical polymerizable compound, and the photoradical polymerization initiator are preferably incorporated into the thermosensitive coloring layer. The particle size of the radical polymerizable compound particles is preferably 10 nm or more and 1,000 nm or less, and more preferably 50 nm or more and 300 nm or less. Having particle sizes of 10 nm or more, or even 50 nm or more, enhances radical polymerization reactivity and improves image storability. On the other hand, when the particle diameter is 1000 nm or less, or even 300 nm or less, unnecessary light scattering in the thermosensitive coloring layer is reduced, thereby increasing image density. Note that the particle diameter in this specification refers to the 50% particle diameter (D50) based on volume distribution.

[0048] The content of the radical polymerizable compound in the thermosensitive coloring layer is preferably 500% by mass or more and 2000% by mass or less, and more preferably 600% by mass or more and 1500% by mass or less, based on the content of the electron-donating dye precursor. A content of 500% by mass or more reduces background fogging, and a content of 600% by mass or more further reduces background fogging. On the other hand, a content of 2000% by mass or less reduces the color development of the image, and a content of 1500% by mass or less further improves the color development of the image.

[0049] (Photoradical polymerization initiator) The thermosensitive color developing layer contains a photoradical polymerization initiator, which includes an oxime ester compound having a group represented by the following formula (1):

[0050] [ka]

[0051] (In the formula, R1 and R2 each independently represent a hydrogen atom, a halogen atom, a nitro group, a cyano group, a hydrocarbon group having 1 to 20 carbon atoms, or a group containing a heterocycle and having 2 to 20 carbon atoms, a hydrogen atom in the groups represented by R1 and R2 may be substituted with a halogen atom, a nitro group, a cyano group, a hydroxyl group, an amino group, a carboxyl group, a methacryloyl group, an acryloyl group, an epoxy group, a vinyl group, a vinyl ether group, a mercapto group, an isocyanate group, or a group containing a heterocycle and having 2 to 20 carbon atoms; a methylene group in the groups represented by R1 and R2 may be substituted by -O-, -CO-, -COO-, -OCO-, -NR3-, -NR3CO-, -S-, -CS-, -SO2-, -SCO-, -COS-, -OCS-, -CH=CH-, -CC-, or -CSO-, and R3 represents a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms; m represents 0 or 1; * represents a bonding site to another structure, and the bonding site may form a ring together with the group represented by R1.

[0052] The oxime ester compound preferably has a structure represented by the following formula (2):

[0053] [ka]

[0054] (In formula (2), R1, R2, and m are the same as R1, R2, and m in formula (1), respectively. R4 and R5 each independently represent a hydrogen atom, a nitro group, a cyano group, an alkyl group having 1 to 20 carbon atoms, an aryl group having 6 to 20 carbon atoms, an arylalkyl group having 7 to 20 carbon atoms, or a group containing a heterocycle and having 2 to 20 carbon atoms; X1 represents -O-, -S-, -Se-, -CR6R7-, -CO-, -NR8-, or -PR9-; X2 represents a single bond, a hydrocarbon group having 1 to 20 carbon atoms, -CO-, or a hydrogen atom that is not bonded but is bonded to separate aromatic rings; R6 to R9 each independently represent a hydrogen atom, a hydrocarbon group having 1 to 20 carbon atoms, or a group containing a heterocycle and having 2 to 20 carbon atoms; A hydrogen atom in the groups represented by R4 to R9 may be substituted with a halogen atom, a nitro group, a cyano group, a hydroxyl group, a carboxyl group or a heterocyclic group; The methylene group in the groups represented by R4 to R9 may be substituted with —O— or —CO— in a state where the oxygen atoms are not adjacent to each other, R4 to R9 may each independently form a ring together with an adjacent benzene ring, g represents an integer of 0 to 4; h represents an integer of 0 to 3.

[0055] When the oxime ester compound has the structure represented by the formula (2), the polymerization reaction of the radical polymerizable compound proceeds more efficiently.

[0056] Furthermore, from the viewpoint of ease of production, the oxime ester compound more preferably has a structure represented by the following formula (3) or (4).

[0057] [ka]

[0058] (In formulas (3) and (4), R1, R2, R4, R5, R8, g, h, and m are the same as R1, R2, R4, R5, R8, g, h, and m in formula (2), respectively.)

[0059] Specific examples of the oxime ester compound having a group represented by the formula (1) include the following compounds (I-1) to (I-20).

[0060] [ka]

[0061] [ka]

[0062] [ka]

[0063] Commercially available oxime ester compounds having a group represented by formula (1) include, for example, IRGACURE OXE01, IRGACURE OXE02, IRGACURE OXE03, and IRGACURE OXE04 (all manufactured by BASF), and ADEKA ARCLES N-1919, NCI-831, and NCI-930 (all manufactured by ADEKA).

[0064] The photoradical polymerization initiator can be used alone or in combination of two or more. The content of the photoradical polymerization initiator in the thermosensitive coloring layer is preferably 4% by mass or more, and more preferably 10% by mass or more, based on the content of the radical polymerizable compound, from the viewpoint of improving reactivity. There is no particular upper limit on the content of the photoradical polymerization initiator, but it is preferably, for example, 40% by mass or less, based on the content of the radical polymerizable compound.

[0065] (Other ingredients) The thermosensitive coloring layer may contain a storage stability improver. By incorporating a storage stability improver into the thermosensitive coloring layer, the storage stability of the colored image can be further improved. Examples of storage stability improvers include 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1-bis(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 4,4'-[1,4-phenylenebis(1-methylethylidene)]bisphenol, and 4,4'-[1,3-phenylenebis(1-methylethylidene)]bisphenol. phenol compounds such as 4-benzyloxyphenyl-4'-(2-methyl-2,3-epoxypropyloxy)phenyl sulfone, 4-(2-methyl-1,2-epoxyethyl)diphenyl sulfone, and 4-(2-ethyl-1,2-epoxyethyl)diphenyl sulfone; and isocyanuric acid compounds such as 1,3,5-tris(2,6-dimethylbenzyl-3-hydroxy-4-tert-butyl)isocyanuric acid.

[0066] The thermosensitive coloring layer may contain a thermal sensitizer, which can increase the recording sensitivity. Examples of thermal sensitizers include stearic acid amide, methoxycarbonyl-N-stearic acid benzamilide, N-benzoylstearic acid amide, N-eicosanoic acid amide, ethylene bisstearic acid amide, behenic acid amide, methylene bisstearic acid amide, N-methylol stearic acid amide, dibenzyl terephthalate, dimethyl terephthalate, dioctyl terephthalate, diphenyl sulfone, benzyl p-benzyloxybenzoate, phenyl 1-hydroxy-2-naphthoate, 2-naphthyl benzyl ether, m-terphenyl, p-benzyl biphenyl, oxalic acid di-p-chlorobenzyl ester, oxalic acid di-p-methylbenzyl ester, oxalic acid dibenzyl ester, p-tolyl biphenyl ether, di(p-methoxyphenoxyethyl) ether, 1,2-di(3-methylphenoxy) ether, Examples of suitable benzene derivatives include benzene, 1,2-di(4-methylphenoxy)ethane, 1,2-di(4-methoxyphenoxy)ethane, 1,2-di(4-chlorophenoxy)ethane, 1,2-diphenoxyethane, 1-(4-methoxyphenoxy)-2-(3-methylphenoxy)ethane, p-methylthiophenylbenzyl ether, 1,4-di(phenylthio)butane, p-acetotoluidide, p-acetophenetidide, N-acetoacetyl-p-toluidine, 1,2-diphenoxymethylbenzene, di(β-biphenylethoxy)benzene, p-di(vinyloxyethoxy)benzene, 1-isopropylphenyl-2-phenylethane, di-o-chlorobenzyl adipate, 1,2-bis(3,4-dimethylphenyl)ethane, 1,3-bis(2-naphthoxy)propane, diphenyl, and benzophenone. The content of the thermal sensitizer in the thermosensitive coloring layer should be an amount effective for thermal sensitization, specifically, preferably from 2 to 40% by mass, and more preferably from 5 to 25% by mass, of the total solid content of the thermosensitive coloring layer.

[0067] Auxiliaries such as shelf-life improvers and heat sensitizers may be mixed in the form of fine particles dispersed in water (solid-dispersed fine particles) into the coating solution for forming the thermosensitive color-developing layer. These auxiliaries may also be dissolved in a solvent and emulsified using a water-soluble polymer compound as an emulsifier. Furthermore, shelf-life improvers and heat sensitizers may be incorporated into particles containing an electron-donating dye precursor or an electron-accepting compound.

[0068] The thermosensitive color developing layer may contain a polymerization accelerator, such as a benzoate compound or an amine compound.

[0069] Examples of the benzoate compound and amine compound include ethyl 4-(dimethylamino)benzoate, ethylhexyl 4-dimethylaminobenzoate, methyl o-benzoylbenzoate, 3-methylbutyl p-(dimethylamino)benzoate, N,N-dimethylaminobenzoic acid ethyl ester, N,N-dimethylaminobenzoic acid isoamyl ester, pentyl 4-dimethylaminobenzoate, triethylamine, and triethanolamine.

[0070] The thermosensitive coloring layer may contain a sensitizer. The sensitizer may be any sensitizer that sensitizes the photoradical polymerization initiator via an electron transfer mechanism or an energy transfer mechanism. Examples of sensitizers include aromatic polycondensed ring compounds such as anthracene, 9,10-dialkoxyanthracene, pyrene, and perylene; aromatic ketone compounds such as acetophenone, benzophenone, thioxanthone, and Michler's ketone; and heterocyclic compounds such as phenothiazine and N-aryloxazolidinone. The content of the sensitizer in the thermosensitive coloring layer is preferably 0.1 to 10 parts by weight, more preferably 1 to 5 parts by weight, per part by weight of the photoradical polymerization initiator.

[0071] To improve the electron transfer efficiency or energy transfer efficiency between the sensitizer and the photoradical polymerization initiator, it is preferable to incorporate a sensitizing aid into the thermosensitive color-forming layer. Examples of sensitizing aids include naphthalene compounds such as 1,4-dihydroxynaphthalene, 1,4-dimethoxynaphthalene, 1,4-diethoxynaphthalene, 4-methoxy-1-naphthol, and 4-ethoxy-1-naphthol; and benzene compounds such as 1,4-dihydroxybenzene, 1,4-dimethoxybenzene, 1,4-diethoxybenzene, 1-methoxy-4-phenol, and 1-ethoxy-4-phenol. The content of the sensitizing aid in the thermosensitive color-forming layer is preferably 0.1 to 10 parts by weight, and more preferably 0.5 to 5 parts by weight, per part by weight of the sensitizer.

[0072] The thermosensitive coloring layer may contain a radical polymerization inhibitor. The photoradical polymerization initiator decomposes slightly during storage of the thermosensitive recording medium to form a radical compound. This radical compound may cause polymerization, so it is preferable to contain a radical polymerization inhibitor in the thermosensitive coloring layer to prevent this polymerization.

[0073] Examples of the radical polymerization inhibitor include phenolic hydroxyl group-containing compounds, quinones such as methoquinone (hydroquinone monomethyl ether), hydroquinone, and 4-methoxy-1-naphthol, hindered amine antioxidants, 1,1-diphenyl-2-picrylhydrazyl free radicals, N-oxyl free radical compounds, nitrogen-containing heterocyclic mercapto compounds, thioether antioxidants, hindered phenol antioxidants, ascorbic acids, zinc sulfate, thiocyanates, thiourea derivatives, various sugars, phosphoric acid antioxidants, nitrites, sulfites, thiosulfates, hydroxylamine derivatives, aromatic amines, phenylenediamines, imines, sulfonamides, urea derivatives, oximes, polycondensates of dicyandiamide and polyalkylenepolyamine, sulfur-containing compounds such as phenothiazine, complexing agents based on tetraazaannulene (TAA), and hindered amines.

[0074] Among these, preferred radical polymerization inhibitors are phenols, N-oxyl free radical compounds, 1,1-diphenyl-2-picrylhydrazyl free radicals, phenothiazine, quinones, and hindered amines. N-oxyl free radical compounds are even more preferred. The content of the radical polymerization inhibitor in the thermosensitive color-forming layer is preferably 1 ppm or more and 5,000 ppm or less by mass relative to the content of the radical polymerizable compound.

[0075] The thermosensitive coloring layer can contain a highly white pigment with an average particle diameter of 10 μm or less. The inclusion of such a pigment improves the whiteness of the thermosensitive coloring layer and the uniformity of the image. Examples of pigments include inorganic pigments such as calcium carbonate, magnesium carbonate, kaolin, clay, talc, calcined clay, silica, diatomaceous earth, synthetic aluminum silicate, zinc oxide, titanium oxide, aluminum hydroxide, barium sulfate, and surface-treated calcium carbonate or silica; and organic pigments such as urea-formaldehyde resin, styrene-methacrylic acid copolymer resin, and polystyrene resin. The pigment content in the thermosensitive coloring layer is preferably an amount that does not reduce the color density of the image. Specifically, it is preferably 50% by weight or less of the total solids content of the thermosensitive coloring layer.

[0076] A binder can be used as a component of the thermosensitive coloring layer. Optionally, crosslinkers, waxes, metal soaps, colored dyes, colored pigments, fluorescent dyes, etc. can also be added. Examples of binders include polyvinyl alcohol and its derivatives; starch and its derivatives; cellulose derivatives such as hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, and ethyl cellulose; water-soluble polymer materials such as sodium polyacrylate, polyvinylpyrrolidone, acrylamide-acrylic acid ester copolymers, acrylamide-acrylic acid ester-methacrylic acid ester copolymers, styrene-maleic anhydride copolymers, isobutylene-maleic anhydride copolymers, casein, gelatin, and their derivatives; emulsions of polyvinyl acetate, polyurethane, polyacrylic acid, polyacrylic acid esters, vinyl chloride-vinyl acetate copolymers, polybutyl methacrylate, and ethylene-vinyl acetate copolymers; and latexes of water-insoluble polymers such as styrene-butadiene copolymers and styrene-butadiene-acrylic copolymers.

[0077] The inclusion of a crosslinking agent in the thermosensitive coloring layer can improve the water resistance of the thermosensitive coloring layer. Examples of crosslinking agents include organic compounds such as aldehyde compounds such as glyoxal, polyamine compounds such as polyethyleneimine, epoxy compounds, polyamide resins, melamine resins, glyoxylates, dimethylol urea compounds, aziridine compounds, and blocked isocyanate compounds; inorganic compounds such as ammonium persulfate, ferric chloride, magnesium chloride, sodium tetraborate, and potassium tetraborate; and boric acid, boric acid triesters, boron-based polymers, hydrazide compounds, and glyoxylates. The content of the crosslinking agent in the thermosensitive coloring layer is preferably 1 to 10 parts by weight per 100 parts by weight of the total solids content of the thermosensitive coloring layer.

[0078] Examples of waxes include waxes such as paraffin wax, carnauba wax, microcrystalline wax, polyolefin wax, and polyethylene wax; higher fatty acid amides such as stearic acid amide and ethylene bisstearic acid amide; higher fatty acid esters and their derivatives; and examples of metal soaps include polyvalent metal salts of higher fatty acids such as zinc stearate, aluminum stearate, calcium stearate, and zinc oleate.

[0079] When the thermosensitive recording medium is a two-color thermosensitive recording medium, it is preferable to incorporate a color dye or color pigment in the thermosensitive coloring layer, which has a color tone complementary to the low-temperature color tone. By incorporating such a color dye or color pigment in the thermosensitive coloring layer, the color tone of the thermosensitive recording medium can be adjusted before and after image formation. Furthermore, various auxiliary agents such as oil repellents, antifoaming agents, and viscosity adjusters can be incorporated into the thermosensitive coloring layer as needed.

[0080] The thermosensitive coloring layer can be formed, for example, by applying a coating solution for the thermosensitive coloring layer containing the components constituting the thermosensitive coloring layer to a support using water as a dispersion medium to form a coating layer, and then drying the coating layer. The coating amount of the coating solution is 2 g / m2 in terms of dry mass. 2 More than 20g / m 2 It is preferable that the density is 2 g / m or less. 2 More than 15g / m 2 It is more preferable that the concentration is 2 g / m or less. 2 More than 10g / m 2 It is particularly preferred that:

[0081] It is preferable to use a surfactant to prepare the first particles and the second particles. Examples of the surfactant include anionic surfactants such as sodium alkyl sulfonate, sodium alkyl benzene sulfonate, sodium dialkyl sulfosuccinate, and sodium alkyl carboxylate; nonionic surfactants such as polyoxyethylene alkyl ether, polyoxyethylene alkyl ester, polyoxyethylene polyoxypropylene glycol, sorbitan alkyl ester, polyoxyethylene sorbitan alkyl ester, glycerin alkyl ester, and polyoxyethylene hydrogenated castor oil; cationic surfactants such as alkyl trimethyl ammonium chloride, dialkyl dimethyl ammonium chloride, and alkyl benzyl dimethyl ammonium chloride; and amphoteric surfactants such as alkyl betaine and alkyl dimethyl amine oxide. Furthermore, polymeric surfactants such as sodium salt of naphthalene sulfonate-formalin condensate and sodium polyacrylate can be used.

[0082] It is also possible to use radical polymerizable compounds to which an ionic group such as a sulfonic acid group, a carboxylic acid group, or an amino group, or a hydrophilic nonionic group such as a polyoxyethylene group or a polyglyceryl group is bonded to impart surfactant properties.

[0083] A dispersing aid may be used to prepare the first and second particles. Examples of the dispersing aid include water-soluble polymers such as polyvinyl alcohol and modified polyvinyl alcohol, polyacrylic acid amide and derivatives thereof, ethylene / vinyl acetate copolymer, styrene / maleic anhydride copolymer, ethylene / maleic anhydride copolymer, isobutylene / maleic anhydride copolymer, polyvinylpyrrolidone, ethylene / acrylic acid copolymer, vinyl acetate / acrylic acid copolymer, carboxymethylcellulose, methylcellulose, casein, gelatin, starch derivatives, gum arabic, and sodium alginate.

[0084] The amount of surfactant or dispersing aid added is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 0.5% by mass or more and 5% by mass or less, based on the mass of each of the first particles and the second particles.

[0085] (middle class) When the thermosensitive coloring layer has an electron-donating dye precursor layer (leuco dye layer) and an electron-accepting compound layer (developer layer), an intermediate layer can be provided between these layers. Materials constituting the intermediate layer can be water-soluble polymeric materials or water-insoluble polymers used in known thermosensitive recording media. Specific examples of materials constituting the intermediate layer include those similar to the binders that constitute the thermosensitive coloring layer. Furthermore, the intermediate layer may contain, as an auxiliary agent, particles with high porosity such as silica or calcined kaolin, plastic pigments, hollow particles, foams, or organic compounds such as polyethylene wax having a glass transition point or melting point.

[0086] The intermediate layer can be formed, for example, by applying a coating liquid for the intermediate layer containing the components constituting the intermediate layer using water as a dispersion medium to form a coating layer, and then drying the coating layer. The coating amount of the coating liquid is 1 g / m2 in terms of dry mass. 2 More than 40g / m 2 It is preferable that the density is 2 g / m or less. 2 More than 10g / m 2 It is more preferable that:

[0087] An intermediate layer may also be provided between the two thermosensitive coloring layers. The intermediate layer disposed between the thermosensitive coloring layers preferably contains an ultraviolet absorber. By incorporating an ultraviolet absorber into this intermediate layer, the ultraviolet transmittance of the intermediate layer can be controlled to a desired value.

[0088] As the ultraviolet absorber, a benzotriazole-based ultraviolet absorber, a triazine-based ultraviolet absorber, a benzophenone-based ultraviolet absorber, a cyanoacrylate-based ultraviolet absorber, a salicylic acid-based ultraviolet absorber, or titanium oxide can be used.

[0089] (protective layer) It is preferable to have a protective layer on the thermosensitive coloring layer. The protective layer may be a protective layer used in known thermosensitive recording media. For example, it is preferable to provide a protective layer containing a water-soluble polymer material and particles. The water-soluble polymer material and particles may be the same as those that can be contained in the thermosensitive coloring layer. It is also preferable to add a crosslinking agent to impart water resistance to the protective layer.

[0090] The inclusion of microcapsules encapsulating UV absorbers or solid dispersion particles of UV absorbers in the protective layer can significantly improve light resistance. Microcapsules with walls made of polyurethane-polyurea resins or aminoaldehyde resins are particularly preferred because they have excellent heat resistance and exhibit additional benefits, such as suppressing sticking to thermal heads. Furthermore, microcapsules with walls made of polyurethane-polyurea resins or aminoaldehyde resins have a lower refractive index than microcapsules with walls made of other resins. Furthermore, because of their spherical shape, even if a large amount of these microcapsules is added to the protective layer, they are less likely to experience a decrease in density due to diffuse reflection of light.

[0091] Furthermore, it is preferable to include particles in the protective layer, since this can prevent adhesion of dirt and sticking to the thermal head. The oil absorption of the particles is preferably 50 mL / 100 g or more. The content of particles in the protective layer is preferably an amount that does not reduce the color density, and specifically, it is preferably 60 mass % or less of the total solid content of the protective layer.

[0092] The protective layer can be formed, for example, by applying a coating solution for the protective layer containing the components constituting the protective layer using water as a dispersion medium onto the thermosensitive color-developing layer to form a coating layer, and then drying the coating layer. The coating amount of the coating solution is 0.1 g / m2 in terms of dry mass. 2 More than 15g / m 2 It is preferable that the density is 0.5 g / m or less. 2More than 8g / m 2 It is more preferable that:

[0093] (resin layer) A resin layer composed of a resin cured by electron beams or ultraviolet light can be provided on each of the thermosensitive coloring layer, intermediate layer, and protective layer. Examples of resins cured by electron beams include those described in JP-A-58-177392. The resin constituting the resin layer may contain additives such as non-electron beam curable resins, particles, antifoaming agents, leveling agents, lubricants, surfactants, and plasticizers. Adding particles such as calcium carbonate and aluminum hydroxide, or lubricants such as waxes and silicones is particularly preferred because it can prevent sticking to the thermal head.

[0094] (Other layers) By processing a thermal recording medium to impart enhanced functionality, the added value of the thermal recording medium can be increased. For example, by applying a pressure-sensitive adhesive, rewettable adhesive, delayed-tack pressure-sensitive adhesive, or the like to the back surface, it can be made into adhesive paper, rewettable adhesive paper, or delayed-tack paper. Furthermore, by imparting functionality such as thermal transfer paper, inkjet recording paper, carbonless paper, electrostatic recording paper, or xeography paper to the back surface, it can be made into a recording paper capable of double-sided recording. Furthermore, by providing a thermal color-developing layer on the back surface, it can be made into a double-sided thermal recording medium. A back layer can also be provided on the back surface of the thermal recording medium to suppress penetration of oil or plasticizer from the back surface, or to control curl or prevent static electricity.

[0095] (Layer structure of the thermal recording medium) Fig. 1 is a cross-sectional view showing one embodiment of a thermosensitive recording medium of the present invention. The thermosensitive recording medium 100 shown in Fig. 1 includes a sheet-like support 101. An electron-donating dye precursor layer 102, an intermediate layer 103, an electron-accepting compound layer 104, and a protective layer 105 are laminated in this order on one surface of the support 101. In a thermosensitive recording medium according to an embodiment of the present invention, the order of the electron-donating dye precursor layer 102 and the electron-accepting compound layer 104 may be reversed, and the protective layer 105 shown in Fig. 1 may not be provided.

[0096] Fig. 2 is a cross-sectional view showing another embodiment of the thermosensitive recording medium of the present invention. The thermosensitive recording medium 200 shown in Fig. 2 comprises a sheet-like support 201, and an electron-donating dye precursor layer 202, an electron-accepting compound layer 203, and a protective layer 204 laminated in this order on one side of the support 201. In the thermosensitive recording medium according to the embodiment of the present invention, the order of the electron-accepting compound layer 202 and the electron-donating dye precursor layer 203 may be reversed, and the protective layer 204 shown in Fig. 2 may not be provided.

[0097] The supports 101 and 201 may be made of any material capable of forming a coating film using a coating liquid for a thermosensitive coloring layer (thermosensitive coloring composition). Examples of materials for the supports 101 and 201 include paper, synthetic paper, and various plastics. Examples of plastics include PET (polyethylene terephthalate) and OPP (oriented polypropylene). The surfaces of the supports 101 and 201 are preferably subjected to corona discharge treatment, sandblasting, primer treatment (lamination of a primer layer), or the like, as needed. These treatments can improve the wettability of the surfaces of the supports 101 and 201, roughen them, or make them more adhesive, thereby enhancing the formability of a coating film from the thermosensitive coloring composition.

[0098] A coating film can be formed by applying or printing a thermosensitive color-forming composition onto the support 101 or 201. Examples of methods for applying or printing the thermosensitive color-forming composition include blade coaters, rod coaters, reverse roll coaters, die coaters, offset printing machines, gravure printing machines, flexographic printing machines, letterpress printing machines, and silk screen printing machines. Intermediate layers (including protective intermediate layers) and protective layers can be formed using intermediate layer compositions and overcoat compositions prepared by methods similar to those used to prepare the thermosensitive color-forming composition. Coating films can be formed by applying these intermediate layer compositions and overcoat compositions to predetermined locations. Each coating film can be formed and then dried to form the desired layer, thereby obtaining the desired thermosensitive recording medium. The coating films can be applied and dried one layer at a time, or the same coating solution can be applied and dried two or more times. Simultaneous multilayer coating, in which two or more coating solutions are applied simultaneously, can also be performed. After forming each layer or after forming all layers, it is preferable to smooth the surface using known methods such as supercalendering or soft calendering. By carrying out the surface smoothing treatment, it is possible to improve the recording sensitivity and also to increase the uniformity of the formed image.

[0099] <Image forming method> Next, the image forming method of the present invention will be described. The image forming method of the present invention comprises a step of applying a heat pulse to the above-mentioned thermosensitive recording medium using a thermal head to form an image (image forming step), and a step of irradiating the thermosensitive recording medium with the image formed thereon with ultraviolet light to fix the thermosensitive coloring layer (fixing step).

[0100] The temperature of the heat pulse applied to the thermosensitive recording medium in the image formation process can be, for example, 80°C or higher and 120°C or lower. A desired image can be formed by applying a heat pulse to the thermosensitive coloring layer of the thermosensitive recording medium while a thermal head is in contact with the thermosensitive recording medium. Specifically, the application of a heat pulse causes heating to dissolve the radical polymerizable compound contained in the thermosensitive coloring layer. When the radical polymerizable compound dissolves, the electron-donating dye precursor and the electron-accepting compound come into contact with each other, causing the thermosensitive coloring layer to color, forming an image.

[0101] In the fixing process, ultraviolet light is irradiated onto the thermosensitive recording medium on which the image has been formed. The wavelength of the irradiated ultraviolet light may be any wavelength capable of inducing a reaction of the photoradical polymerization initiator contained in the thermosensitive coloring layer, for example, 365 nm to 425 nm. The irradiation of ultraviolet light causes a radical polymerizable compound to polymerize, thereby fixing the thermosensitive coloring layer. Once the thermosensitive coloring layer has been fixed, the thermosensitive coloring layer will not develop color even if thermal energy reaching the coloring initiation temperature is subsequently applied, thereby maintaining the color development of the formed image for a long period of time. Note that the wavelength of ultraviolet light in this specification refers to the peak wavelength of the irradiated ultraviolet light. Fixing the thermosensitive coloring layer refers to fixing the color development state of the thermosensitive coloring layer. [Example]

[0102] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples as long as the gist of the invention is not exceeded. The terms "parts" and "%" used to describe the amounts of components are based on mass unless otherwise specified.

[0103] <Production of thermal recording media (1)> Example 1 [Preparation of raw material composition] Solutions [A] to [D] were prepared as follows: Solution [A]: a composition containing an electron-donating dye precursor Electron-donating dye precursor (BLUE220, manufactured by Fukui Yamada Chemical Co., Ltd.) 3 parts Radical polymerizable compound (SR368, manufactured by Arkema Co., Ltd.) 30 parts Photoradical polymerization initiator (compound (I-15)) 9 parts 58 parts ethyl acetate The above materials were mixed and dissolved to obtain solution [A]. [B] Liquid: Dispersion liquid of particles containing electron-accepting compounds Electron-accepting compound (TGSH(H), manufactured by Nippon Kayaku Co., Ltd.) 40 parts Dispersant (Pelex NBL, manufactured by Kao Corporation) 4 parts ·Wednesday 56 parts The above materials were mixed and then pulverized and dispersed using a bead mill to obtain solution B. When measured using a particle size distribution analyzer (Nanotrac, manufactured by Microtrac), the particle diameter (D50) of the electron-accepting compound-containing particles in solution B was approximately 700 nm. [C] Liquid: Coating liquid for intermediate layer 10 parts polyvinyl alcohol (Kuraray Poval 5-88, manufactured by Kuraray) ·Wednesday 90 parts The above materials were mixed and dissolved to obtain solution [C]. [D] Liquid: Kaolin dispersion liquid 59.5 parts kaolin (HYDRAGLOSS90, manufactured by KaMin, LLC) ·Dispersant (Aron T-50, manufactured by Toagosei, solid content concentration 40%) 0.5 part ·Water 40 parts The above materials were dispersed for 1 hour using Coles to obtain solution [D].

[0104] [Formation of thermosensitive coloring layer] Using a printability tester, liquid [A] was applied to a 130 μm thick synthetic paper (Yupo, manufactured by Yupo Co., Ltd.), and then dried with a dryer to evaporate the ethyl acetate. After drying, the amount of liquid [A] applied was 40.0 g / m 2 The coating amount of BLUE220 was 1.20 g / m 2 Next, the liquid [C] was applied using a printability tester and then dried with a dryer. After drying, the amount of liquid [C] applied was 0.2 g / m 2After that, the liquid [B] was applied using a printability tester and then dried with a dryer. After drying, the amount of liquid [B] applied was 30.0 g / m 2 The coating amount of TGSH(H) was 12.0 g / m 2 It was.

[0105] [Protective layer formation] 210 parts of a 10% aqueous solution of acetoacetyl-modified polyvinyl alcohol A, 80 parts of a 20% aqueous solution of acetoacetyl-modified polyvinyl alcohol B, 100 parts of [D] solution, 5.6 parts of an aqueous dispersion of zinc stearate, and 2.5 parts of polyethylene wax emulsion were prepared. These components were mixed and stirred to obtain a coating solution for a protective layer. As the acetoacetyl-modified polyvinyl alcohol A, "GOHSEFIRMER Z-200" (saponification degree 99.4 mol%, average polymerization degree 1,000, modification degree 5 mol%, manufactured by Nippon Synthetic Chemical Industry Co., Ltd.) was used. As the acetoacetyl-modified polyvinyl alcohol B, "GOHSEFIRMER Z-100" (saponification degree 99.4 mol%, average polymerization degree 500, modification degree 5 mol%, manufactured by Nippon Synthetic Chemical Industry Co., Ltd.) was used. As the aqueous dispersion of zinc stearate, "HYDRIN Z-8-36" (manufactured by Chukyo Yushi, solids concentration 36%) was used. The polyethylene wax emulsion used was "Chemipearl W-400" (solid content concentration 40%, manufactured by Mitsui Chemicals).

[0106] The obtained coating liquid for the protective layer was applied to the heat-sensitive color developing layer in a coating amount of 1.5 g / m after drying. 2 After forming a protective layer by coating and drying the coating, the surface was smoothed by a supercalender to obtain a thermal recording medium.

[0107] (Examples 2 to 31 and 33) The photoradical polymerization initiator, radical polymerizable compound, electron donating dye precursor, and electron accepting compound used in Example 1 were changed as shown in Table 1, and thermosensitive recording materials of Examples 2 to 31 and 33 were obtained.

[0108] <Production of thermal recording media (2)> Example 32 [Preparation of raw material composition] [Oil phase E] liquid, [F] liquid, [aqueous phase G] liquid, [H] liquid and [D] liquid were prepared in the following manner. [Oil phase E] solution: composition containing electron-donating dye precursor Electron-donating dye precursor (BLUE220, manufactured by Fukui Yamada Chemical Co., Ltd.) 3 parts Radical polymerizable compound (SR368, manufactured by Arkema Co., Ltd.) 30 parts Photoradical polymerization initiator (compound (I-15)) 9 parts 58 parts ethyl acetate The above materials were mixed and dissolved to obtain an [oil phase E] liquid. [F] Liquid: Dispersion liquid of particles containing electron-accepting compounds Electron-accepting compound (TGSH(H), manufactured by Nippon Kayaku Co., Ltd.) 40 parts Dispersant (Pelex NBL, manufactured by Kao Corporation) 4 parts ·Wednesday 56 parts The above materials were mixed to obtain solution [F]. When measured using a particle size distribution analyzer (Nanotrac, manufactured by Microtrac), the particle diameter (D50) of the electron-accepting compound-containing particles in solution [F] was found to be approximately 700 nm. [Aqueous phase G] liquid: Dispersant (Pelex NBL, manufactured by Kao Corporation) 15 parts ·Wednesday 110 copies The above materials were mixed and dissolved to obtain an [aqueous phase G] liquid. [H] Solution: Dispersion of particles containing electron-donating dye precursor After mixing 40 parts of [oil phase E] liquid and 50 parts of [aqueous phase G] liquid, the mixture was emulsified using an ultrasonic homogenizer (UH-600S, manufactured by SMT). Ethyl acetate was then removed under reduced pressure using a rotary evaporator to obtain [H] liquid, a dispersion of electron-donating dye precursor-containing particles. The particle diameter (D50) of the electron-donating dye precursor-containing particles in [H] liquid was measured using a particle size distribution analyzer (Nanotrac, manufactured by Microtrac) and found to be 160 nm. [D] Liquid: Kaolin dispersion liquid 59.5 parts kaolin (HYDRAGLOSS90, manufactured by KaMin, LLC) ·Dispersant (Aron T-50, manufactured by Toagosei, solid content concentration 40%) 0.5 part ·Water 40 parts The above materials were dispersed for 1 hour using Coles to obtain solution [D].

[0109] [Formation of thermosensitive coloring layer] The [H] solution was applied to a 130 μm thick synthetic paper (Yupo, manufactured by Yupo) using a printability tester, and then dried with a dryer. The applied amount of [H] solution after drying was 40.0 g / m 2 The coating amount of BLUE220 was 1.20 g / m 2 After that, the [F] liquid was applied using a printability tester and then dried with a dryer to form a thermosensitive coloring layer. The coating amount of [F] liquid after drying was 30.0 g / m 2 The coating amount of TGSH(H) was 12.0 g / m 2 It was.

[0110] [Protective layer formation] 210 parts of a 10% aqueous solution of acetoacetyl-modified polyvinyl alcohol A, 80 parts of a 20% aqueous solution of acetoacetyl-modified polyvinyl alcohol B, 100 parts of [C] solution, 5.6 parts of an aqueous dispersion of zinc stearate, and 2.5 parts of polyethylene wax emulsion were prepared. These components were mixed and stirred to obtain a coating solution for a protective layer. As the acetoacetyl-modified polyvinyl alcohol A, "GOHSEFIRMER Z-200" (saponification degree 99.4 mol%, average polymerization degree 1,000, modification degree 5 mol%, manufactured by Nippon Synthetic Chemical Industry Co., Ltd.) was used. As the acetoacetyl-modified polyvinyl alcohol B, "GOHSEFIRMER Z-100" (saponification degree 99.4 mol%, average polymerization degree 500, modification degree 5 mol%, manufactured by Nippon Synthetic Chemical Industry Co., Ltd.) was used. As the aqueous dispersion of zinc stearate, "HYDRIN Z-8-36" (manufactured by Chukyo Yushi, solids concentration 36%) was used. The polyethylene wax emulsion used was "Chemipearl W-400" (solid content: 40%, manufactured by Mitsui Chemicals). The obtained coating liquid for the protective layer was applied to the thermosensitive color developing layer in a coating amount of 1.5 g / m2 after drying. 2 After forming a protective layer by coating and drying the coating, the surface was smoothed by a supercalender to obtain a thermal recording medium.

[0111] The obtained coating liquid for the protective layer was applied to the heat-sensitive color developing layer in a coating amount of 1.5 g / m after drying. 2 After forming a protective layer by coating and drying the coating, the surface was smoothed by a supercalender to obtain a thermal recording medium.

[0112] (Comparative Examples 1 to 12) The photoradical polymerization initiator, radical polymerizable compound, electron donating dye precursor, and electron accepting compound used in Example 1 were changed as shown in Table 1 to obtain thermosensitive recording materials of Comparative Examples 1 to 12.

[0113] <Image formation and evaluation> (UV fixability) The thermal recording media obtained in Examples 1 to 33 and Comparative Examples 1 to 12 were irradiated once with ultraviolet light using an ultraviolet irradiation device (GC77 (irradiation wavelength: 365 nm), manufactured by HAMAMATSU) equipped with a linear irradiation type UV-LED, at conveyor speeds of 100 m / min and 10 m / min. Separately, the integrated light amount under the same conditions was measured using an ultraviolet integrating actinometer (C9536-01, manufactured by HAMAMATSU), and was found to be 100 mJ / cm2. 2 and 1000mJ / cm 2 It was.

[0114] Then, a thermal head (KPE type, manufactured by Kyocera) was used to record the data with a recording energy of 150 mJ / mm 2 The applied power and pulse width were set so that the image was 2 cm x 2 cm, and a 2 cm x 2 cm image was formed on the thermal recording medium after UV irradiation. The optical reflection density of the formed image was measured using a reflection densitometer (product name "Xrite530", manufactured by Sakata Inx Engineering). The difference from the optical reflection density of the synthetic paper (Yupo, manufactured by Yupo) described above was taken as the "Δ optical reflection density," and UV fixability (difficulty in color development due to heating after UV irradiation) was evaluated based on this Δ optical reflection density. The evaluation results are shown in Table 2.

[0115] As shown in Table 2, in Examples 1 to 33, the cumulative light amount was 100 mJ / cm 2 and 1000mJ / cm 2From the absolute value of the optical reflection density of the image after irradiation, it was confirmed that all of the images were sufficiently fixed. 2 It was confirmed that even when the amount of ultraviolet light irradiation was low, color development due to heating after ultraviolet light irradiation was suppressed.

[0116] On the other hand, in Comparative Examples 1 to 4, the photoradical polymerization initiators used were acylphosphine oxide compounds (I-21) and (I-22). 2 The Δ optical reflection density at 100 mJ / cm 2 was 0.15 or more, and it was found that there was slight color development compared to Examples 1 to 33. Furthermore, at an integrated light intensity of 100 mJ / cm 2 2 The Δoptical reflection density at 100 mJ / cm 2 was 0.58 or more, which indicates that the color was more pronounced than in Examples 1 to 33. 2 When the amount of ultraviolet light irradiation was low as in the example above, color development due to heating after ultraviolet light irradiation could not be suppressed.

[0117] In addition, in Comparative Examples 5 and 6, an α-aminoalkylphenone compound (I-23) was used, in Comparative Examples 7 and 8, a benzophenone compound (I-24), in Comparative Examples 9 and 10, a thioxanthone compound (I-25), and in Comparative Examples 11 and 12, a ketocoumarin compound (I-26) were used. In Comparative Examples 5 to 12, as in Comparative Examples 1 to 4, the cumulative light dose was 100 mJ / cm. 2 It can be seen that in the case of ultraviolet irradiation, color development due to heating after ultraviolet irradiation could not be suppressed.

[0118] Furthermore, when Examples 1 to 16 and 33 are compared with Examples 17 to 20, when a pentaerythritol derivative, an isocyanurate derivative, or a trimethylolpropane derivative is used as the radical polymerizable compound, the Δ optical reflection density is smaller than when a glycerin derivative is used. In particular, in Examples 1 to 16 and 33, the Δ optical reflection density is lower at low light intensities (100 mJ / cm 2) and then heated, color development was suppressed.

[0119] Furthermore, when Examples 17 to 20 are compared with Examples 21 to 27, the Δ optical reflection density is smaller when the content of the photoradical polymerization initiator is 4% by mass or more than when it is 3% by mass.

[0120] Furthermore, when Examples 21 to 27 are compared with Examples 28 to 29, the Δ optical reflection density is smaller when an oxime ester compound represented by the formula (3) or (4) is used as the photoradical polymerization initiator than when an oxime ester compound other than the formula (3) or (4) is used.

[0121] Furthermore, when Examples 28 to 29 are compared with Examples 30 to 31, the Δ optical reflection density is smaller when the oxime ester compound represented by the formula (2) is used as the photoradical polymerization initiator than when the oxime ester compound represented by the formula (1) other than the formula (2) is used.

[0122] [Table 1]

[0123] [Table 2]

[0124] In Table 1, (I-1), (I-2), (I-4), (I-5), (I-8), (I-9), (I-11), (I-13), (I-15), (I-16), and (I-17) are the same as the exemplified compounds (I-1), (I-2), (I-4), (I-5), (I-8), (I-9), (I-11), (I-13), (I-15), (I-16), and (I-17) of the photoradical polymerization initiator described above.

[0125] Tables 3 to 6 show the details of each component used in the examples and comparative examples.

[0126] [Table 3]

[0127] [Table 4]

[0128] [Table 5]

[0129] [Table 6] [Explanation of symbols]

[0130] 100, 200 Thermal recording medium 101, 201 Support 102, 202 Electron-donating dye precursor layer 103 Middle Class 104, 203 Electron-accepting compound layer 105, 204 protective layer

Claims

1. A thermosensitive recording medium having a thermosensitive color-forming layer containing an electron-donating dye precursor, an electron-accepting compound, a radical polymerizable compound, and a photoradical polymerization initiator, A thermosensitive recording medium, wherein the photoradical polymerization initiator contains an oxime ester compound having a group represented by the following formula (1): 【Chemical 1】 (In formula (1), R １ and R ２ each independently represents a hydrogen atom, a halogen atom, a nitro group, a cyano group, a hydrocarbon group having 1 to 20 carbon atoms, or a group containing a heterocycle and having 2 to 20 carbon atoms; R １ and R ２ a hydrogen atom in the group represented by the formula (I) may be substituted with a halogen atom, a nitro group, a cyano group, a hydroxyl group, an amino group, a carboxyl group, a methacryloyl group, an acryloyl group, an epoxy group, a vinyl group, a vinyl ether group, a mercapto group, an isocyanate group, or a group containing a heterocycle and having 2 to 20 carbon atoms; R １ and R ２ The methylene group in the group represented by the formula: ３ -, -NR ３ CO-, -S-, -CS-, -SO ２ -, -SCO-, -COS-, -OCS-, -CH=CH-, -CC-, or -CSO-; R ３ represents a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms, m represents 0 or 1; * represents a binding site with another structure, and the binding site is R １ may form a ring together with the group represented by

2. 2. The thermosensitive recording material according to claim 1, wherein the oxime ester compound has a structure represented by the following formula (2): 【Chemistry 2】 (In formula (2), R １ , R ２ , and m are R in the formula (1), respectively. １ , R ２ , and m, R ４ and R ５ each independently represents a hydrogen atom, a nitro group, a cyano group, an alkyl group having 1 to 20 carbon atoms, an aryl group having 6 to 20 carbon atoms, an arylalkyl group having 7 to 20 carbon atoms, or a group containing a heterocycle and having 2 to 20 carbon atoms, X １ is -O-, -S-, -Se-, -CR ６ R ７ -, -CO-, -NR ８ - or -PR ９ represents -, X ２ represents a single bond, a hydrocarbon group having 1 to 20 carbon atoms, —CO—, or a hydrogen atom that is not bonded but is bonded to separate aromatic rings, R ６ ~R ９ each independently represents a hydrogen atom, a hydrocarbon group having 1 to 20 carbon atoms, or a heterocyclic ring-containing group having 2 to 20 carbon atoms, R ４ ~R ９ a hydrogen atom in the group represented by the formula (I) may be substituted with a halogen atom, a nitro group, a cyano group, a hydroxyl group, a carboxyl group, or a heterocyclic group; R ４ ~R ９ The methylene group in the group represented by the formula: may be substituted with —O— or —CO— in a state where the oxygen atoms are not adjacent to each other, R ４ ~R ９ may each independently form a ring together with an adjacent benzene ring, g represents an integer of 0 to 4; h represents an integer of 0 to 3.

3. 3. The thermosensitive recording material according to claim 2, wherein the oxime ester compound has a structure represented by the following formula (3) or (4): 【Chemistry 3】 (In formulas (3) and (4), R １ , R ２ , R ４ , R ５ , R ８ , g, h, and m are R in the formula (2), respectively. １ , R ２ , R ４ , R ５ , R ８ , g, h, and m.)

4. 4. The thermosensitive recording medium according to claim 1, wherein the content of the photoradical polymerization initiator is 4% by mass or more based on the content of the radical polymerizable compound.

5. 5. The thermosensitive recording medium according to claim 1, wherein the radical polymerizable compound comprises at least one compound selected from the group consisting of pentaerythritol derivatives, isocyanurate derivatives, and trimethylolpropane derivatives.

6. The thermosensitive recording medium according to any one of claims 1 to 5, wherein particles containing either the electron-donating dye precursor or the electron-accepting compound, the radical polymerizable compound, and the photoradical polymerization initiator are contained in the thermosensitive coloring layer.

7. a step of applying a heat pulse to the thermal recording medium according to any one of claims 1 to 6 using a thermal head to form an image; a step of irradiating the thermosensitive recording medium on which the image has been formed with ultraviolet light to fix the thermosensitive coloring layer; An image forming method comprising the steps of:

Images