JP7764192B2 - Thermal recording medium and image forming method - Google Patents

Description

The present invention relates to a thermal recording medium and an image forming method using the same.

Conventionally, thermal recording media have been widely used, which utilize a mechanism for color development by reacting a leuco dye with a developer. Because thermal 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 thermal recording medium is manufactured by printing or coating a water-containing thermosensitive color-forming composition in liquid form onto a substrate, followed by drying to form a thermosensitive color-forming layer.

A method of forming a thermosensitive coloring layer by irradiating a UV-curable thermosensitive coloring composition with UV light is also being considered. For example, Patent Document 1 proposes a photosensitive and heat-sensitive recording material having a photosensitive and heat-sensitive 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. As a result, the cured portion does not develop color, even when heated after UV exposure.

Furthermore, Patent Document 2 proposes a polymerizable phenol derivative and a photopolymerizable composition containing the derivative, which have high polymerization sensitivity and excellent storage stability, and a recording material, in a method of converting a polymer image obtained by photocuring into a visible image by thermal development.

Furthermore, Patent Document 3 proposes a thermal recording medium that suppresses problems that occur during storage before image formation, such as background fogging, and enables image formation with excellent color development.

Japanese Patent Application Publication No. 3-72358 Japanese Patent Application Laid-Open No. 2003-012609 Japanese Patent Application Laid-Open No. 2020-142513

However, according to the investigations of the present inventors, it was found that the photosensitive and heat-sensitive recording materials proposed in Patent Documents 1 to 3 require a certain level of accumulated light intensity (approximately 1,000 mJ/ ^cm2 or more) in order to suppress color development after ultraviolet irradiation. On the other hand, from the viewpoint of miniaturizing the ultraviolet irradiation device and reducing power consumption, it was necessary to further reduce the amount of ultraviolet light irradiated.

Therefore, an object of the present invention is to provide a thermal recording medium that can suppress color development due to heating after UV irradiation, even when the amount of UV light irradiated is low. Another object of the present invention is to provide an image forming method using the above-mentioned thermal recording medium.

The above objectives are achieved by the following invention.

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, a photoradical polymerization initiator, and an electron-accepting compound having a radical polymerizable group, the thermosensitive recording medium comprising:
The thermosensitive recording medium is characterized in that the electron-accepting compound having a radical polymerizable group contains a compound A represented by the following formula ( 2 ):

(In formula ( 2 ), R ₂₁ and R ₂₂ each independently represent a hydrocarbon group having 1 to 8 carbon atoms;
R ₂₃ represents a hydrogen atom or a methyl group;
X2 represents a hydrocarbon group having 1 to 49 carbon atoms _;
a methylene group in the group _represented by X2 may be substituted with —NHCOO— , —NHCO—, —O—, —CO—, —COO—, or —NR ₂₄ —;
R ₂₄ represents a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms;
d and e each independently represent an integer of 0 to 4;
and f represents an integer of 1 to 10.

The present invention also provides an image forming method comprising the step of heating the above-mentioned thermal recording medium to form an image.

The present invention provides a thermal recording medium that can suppress color development due to heating after UV irradiation, even when the amount of UV light irradiated is low. The present invention also provides an image formation method using the thermal recording medium.

1 is a cross-sectional view showing one embodiment of a thermosensitive recording medium of the present invention. FIG. 2 is a cross-sectional view showing another embodiment of the thermosensitive recording medium of the present invention.

<Thermal Recording Medium>
The present invention will be described in detail below with reference to preferred embodiments, but is not limited to the following 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, a photoradical polymerization initiator, and an electron-accepting compound having a radical polymerizable group.

The electron-accepting compound having a radically polymerizable group includes compound A ((meth)acrylate compound A) represented by the following formula (1):

(In formula (1), R ₁₁ and R ₁₂ each independently represent a hydrocarbon group having 1 to 8 carbon atoms,
R ₁₃ represents a hydrogen atom or a methyl group;
_X1 represents a single bond or a hydrocarbon group having 1 to 50 carbon atoms;
a methylene group in the group represented by X ₁ may be substituted with a heterocycle, —NHCOO—, —NHCO—, —O—, —CO—, —COO—, or —NR ₁₄ —;
R ₁₄ represents a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms;
a and b each independently represent an integer of 0 to 4;
and c represents an integer of 1 to 10.

Note that "(meth)acrylate compound" means "acrylate" and "methacrylate."

In the present invention, the electron-accepting compound having a radical polymerizable group has, within the same molecule, both a structure that acts as an electron-accepting compound (developer) that develops color upon contact with an electron-donating dye precursor, and a structure that acts as a radical polymerizable compound that undergoes a polymerization reaction upon reaction with the radical polymerization initiator. This makes it easy to fix the electron-accepting compound having a radical polymerizable group within the UV-cured product. Therefore, even with low-intensity UV irradiation, the electron-accepting compound having a radical polymerizable group is less likely to move within the thermosensitive recording layer upon heating, thereby suppressing color development upon contact with the electron-donating dye precursor.

(Electron-donating dye precursor)
The thermosensitive color-forming layer contains an electron-donating dye precursor (leuco dye). The electron-donating dye precursor is usually colorless or light-colored. The electron-donating dye precursor has the property of developing color by donating electrons or accepting protons from an acid or the like. Specific examples of electron-donating dye precursors are listed below.

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, 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.

Further examples of 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. Examples include 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, and 2-methyl-6-(N-p-tolyl-N-ethylamino)-fluoran.

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, phthalide], 7-(N-ethyl-N-isoamylamino)-3-methyl-1-p-methylphenylspiro[(1,4-dihydrochromeno[2,3-c]pyrazole)-4,3'-phthalide], 7-(N-ethyl-N-n-hexylamino)-3-methyl-1-phenylspiro[(1,4-dihydrochromeno[2,3-c]pyrazole)-4,3'-phthalide], etc.

Further examples of electron-donating dye precursors that produce 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.

As an 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-(N-p-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.

Electron-donating dye precursors that produce 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. Examples include 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, and 3-diphenylamino-6-diphenylaminofluoran.

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.

Examples of electron-donating dye precursors that produce blue or cyan color 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 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.

Examples of electron-donating dye precursors that produce 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, 4-(2,6-diphenyl-4-pyridinyl)-N,N-dimethylbenzenamine, Examples 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.

As an 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-dimethylbenzenamine, 3,6-dimethoxyfluoran, and 1-(4-n-dodecyloxy-3-methoxyphenyl)-2-(2-quinolyl)ethylene.

Examples of electron-donating dye precursors that produce green tones include 3-(N-ethyl-N-n-hexylamino)-7-anilinofluoran, 3-diethylamino-7-dibenzylaminofluoran, 3-pyrrolidino-7-dibenzylaminofluoran, 3,3-bis(4-diethylamino-2-ethoxyphenyl)-4-azaphthalide, 3-(N-ethyl-N-p-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.

As an 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.

Electron-donating dye precursors that produce 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-( N-n-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, etc.

As an 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 lightfastness.

Examples of electron-donating dye precursors that absorb 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. Examples include 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-(p-n-butylaminoanilino)-6-methyl-7-chlorofluoran, and 2-mesidino-8-diethylamino-benz[C]fluoran.

It is preferable that the content of the electron-donating dye precursor in the thermosensitive color-forming layer is 0.01 g/m ² or more and 2.00 g/m ² or less, since an image with a sufficient optical density can be formed.

(Photoradical polymerization initiator)
The thermosensitive coloring layer contains a photoradical polymerization initiator. The photoradical polymerization initiator may be any compound capable of generating radicals by the action of light. As the photoradical polymerization initiator, various known compounds such as radical generators, radical polymerization initiators, and photoradical polymerization initiators can be used.

Examples of photoradical polymerization initiators that efficiently carry out a curing reaction under ultraviolet light include oxime ester compounds, aromatic ketone compounds, acylphosphine oxide compounds, benzoin alkyl ether compounds, benzoin ether compounds, thioxanthone compounds, benzophenone compounds, benzoate compounds, aromatic onium salt compounds, organic peroxides, thio compounds (such as thiophenyl group-containing compounds), α-aminoalkylphenone compounds, hexaarylbiimidazole compounds, borate compounds, azinium compounds, metallocene compounds, active ester compounds, compounds with a carbon-halogen bond, and alkylamine compounds. Radical generators described in JP 2018-35369 A, JP 2018-39265 A, and the like can also be used. Among these, aromatic ketone compounds, oxime ester compounds, acylphosphine oxide compounds, benzoin alkyl ether compounds, benzoin ether compounds, thioxanthone compounds, benzophenone compounds, and benzoate compounds are preferred, with oxime ester compounds being more preferred.

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 color-forming layer is preferably 10% by mass or more and 200% by mass or less, and more preferably 25% by mass or more and 100% by mass or less, based on the content of the electron-accepting compound having a radically polymerizable group.

Examples of oxime ester compounds include 1,2-octanedione, 1-[4-(phenylthio)phenyl]-, 2-(O-benzoyloxime), ethanone, 1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl]-, 1-(O-acetyloxime), (9-ethyl-6-nitro-9H-carbazol-3-yl)-(4-((1-methoxypropan-2-yl)oxy)-2-methylphenyl)methanone-o-acetyloxime, and 1-[4-[[4-(2-hydroxyethoxy)phenyl]thio]phenyl-1]-1,2-propanedione-2-(O-acetyloxime).

Aromatic ketone compounds include acetophenone, 2-hydroxy-2-methyl-1-phenyl-1-propanone, 2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone, 4-methylbenzophenone, 2,2'-phenyl p-tert-butyltrichloroacetophenone, p-tert-butyldichloroacetophenone, benzophenone, 4-phenylbenzophenone, methylbenzoyl formate, 4-[(4-methylphenyl)thio]benzophenone, 4,4'-bis(diethylamino)benzophenone, N,N'-tetramethyl-4,4'-diaminobenzophenone (Michler's ketone), Examples include 1-hydroxycyclohexyl phenyl ketone, 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methylpropane, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,2-methyl-[4-(methylthio)phenyl]-2-morpholino-1-propane, 2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]-1-butanone, and 2-hydroxy-1-{4-[4-(2-hydroxy-2-methyl-propionyl)-benzyl]-phenyl}-2-methyl-propan-1-one.

Examples of acylphosphine oxide compounds include 2,4,6-trimethylbenzoyldiphenylphosphine oxide and bis(2,4,6-trimethylbenzoyl)-diphenylphosphine oxide.

Examples of benzoin alkyl ether compounds include benzoin methyl ether, benzoin ethyl ether, benzoin butyl ether, and benzoin isopropyl ether.

Examples of benzoin ether compounds include methyl benzoin and ethyl benzoin.

Examples of thioxanthone compounds include 2-chlorothioxanthone, 2,4-diethylthioxanthone, isopropylthioxanthone, and 2-methylthioxanthone.

Examples of benzophenone compounds include benzophenone, 4-methylbenzophenone, 4-phenylbenzophenone, 4-(4-methylphenylthio)benzophenone, and 4,4'-bis(diethylamino)benzophenone.

Examples of benzoate compounds include ethyl 4-(dimethylamino)benzoate, ethylhexyl 4-dimethylaminobenzoate, methyl o-benzoylbenzoate, and 3-methylbutyl p-(dimethylamino)benzoate.

Among the above, commercially available oxime ester compounds that are preferred photoradical polymerization initiators include, for example, Irgacure OXE01 (manufactured by BASF Japan Ltd.), Irgacure OXE02 (manufactured by BASF Japan Ltd.), Irgacure OXE03 (manufactured by BASF Japan Ltd.), Irgacure OXE04 (manufactured by BASF Japan Ltd.), ADEKA ARCLES N-1919 (manufactured by ADEKA Corporation), ADEKA ARCLES NCI-831 (manufactured by ADEKA Corporation), and ADEKA ARCLES NCI-930 (manufactured by ADEKA Corporation).

(Electron-accepting compound having a radically polymerizable group)
The thermosensitive color-forming layer contains an electron-accepting compound having a radical polymerizable group. The electron-accepting compound having a radical polymerizable group has, in the same molecule, both a structure that acts as an electron-accepting compound (developer) that develops color upon contact with an electron-donating dye precursor, and a structure that acts as a radical polymerizable compound that undergoes polymerization upon reaction with the radical polymerizable initiator.

The mechanism is explained in detail below.

When a thermosensitive recording medium is heated, the electron-donating dye precursor contained in the thermosensitive color-forming layer comes into contact with and reacts with the electron-accepting compound having a radically polymerizable group, causing the heated areas to develop color and form an image. However, if a thermosensitive recording medium with an image formed on it is heated again, the uncolored areas may develop color. Therefore, a preferred method for forming an image on a thermosensitive recording medium is one that includes the steps of applying heat pulses to the thermosensitive recording medium using a thermal head to form an image, and irradiating the thermosensitive recording medium with ultraviolet light on the image formed thereon to fix the thermosensitive color-forming layer. Furthermore, when using this image-forming method, the thermosensitive color-forming layer in the thermosensitive recording medium preferably contains a photoradical polymerization initiator and an electron-accepting compound having a radically polymerizable group. In this image-forming method, the thermosensitive color-forming layer in the area where color is desired is first heated with a thermal head.

On the other hand, areas not heated by the thermal head do not develop color. An image is formed on the thermal recording medium depending on whether or not this thermosensitive coloring layer develops color. Next, to maintain the state of whether or not this thermosensitive coloring layer develops color, the thermal recording medium on which the image has been formed is irradiated with ultraviolet light. This causes the photoradical polymerization initiator in the thermosensitive coloring layer to cleave, generating radicals.

These generated radicals then propagate the electron-accepting compound with radical polymerizable groups in the thermosensitive coloring layer, forming a cured product with a three-dimensional network structure in which polymer chains are crosslinked. Because the electron-accepting compound with radical polymerizable groups is fixed within this cured product, the opportunity for contact between the electron-donating dye precursor and the electron-accepting compound with radical polymerizable groups is reduced. Because the electron-accepting compound with radical polymerizable groups is a single molecule, it can be assumed that it is virtually unable to move within the thermosensitive recording layer after curing, further reducing the opportunity for contact with the electron-accepting compound. This prevents the thermosensitive coloring layer from developing color even when heated after UV irradiation, maintaining the coloration or non-coloration state of the thermosensitive coloring layer prior to UV irradiation.

In the present invention, the electron-accepting compound having a radically polymerizable group includes compound A represented by the following formula (1):

(In formula (1), R ₁₁ and R ₁₂ each independently represent a hydrocarbon group having 1 to 8 carbon atoms,
R ₁₃ represents a hydrogen atom or a methyl group;
_X1 represents a single bond or a hydrocarbon group having 1 to 50 carbon atoms;
a methylene group in the group represented by X ₁ may be substituted with a heterocycle, —NHCOO—, —NHCO—, —O—, —CO—, —COO—, or —NR ₁₄ —;
R ₁₄ represents a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms;
a and b each independently represent an integer of 0 to 4;
and c represents an integer of 1 to 10.

Specific examples of compound A represented by formula (1) include the following compounds (D-1) to (D-5).

When the case where the methylene group in the group represented by X ₁ in formula (1) is substituted with a heterocycle, -NHCOO-, -NHCO-, -O-, -CO-, -COO-, or -NR ₁₄ - is explained using compound (D-5) as an example, X ₁ in compound (D-5) is a hydrocarbon group having two carbon atoms, in which one methylene group is substituted with -O- and the other methylene group is substituted with a pyrrole ring.

Furthermore, from the viewpoint of carrying out the ultraviolet curing reaction more efficiently, it is preferable that compound A represented by formula (1) above be represented by formula (2) below. As represented by formula (2) below, the presence of an -NHCOO- (urethane bond) within the molecule of (meth)acrylate compound A improves molecular flexibility and accelerates the radical polymerization reaction, which is presumed to result in more efficient curing reaction.

(In formula (2), R ₂₁ and R ₂₂ each independently represent a hydrocarbon group having 1 to 8 carbon atoms,
R ₂₃ represents a hydrogen atom or a methyl group;
_X2 represents a hydrocarbon group having 1 to 49 carbon atoms;
a methylene group in the group represented by X ₂ may be substituted with —NHCOO—, —NHCO—, —O—, —CO—, —COO—, or —NR ₂₄ —;
R ₂₄ represents a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms;
d and e each independently represent an integer of 0 to 4;
and f represents an integer of 1 to 10.

Specific examples of compound A represented by formula (2) include the following compounds (D-6) to (D-38).

Furthermore, from the viewpoint of more efficiently carrying out the ultraviolet curing reaction, it is preferable that the compound A represented by the formula (1) is represented by the following formula (3): As represented by the formula (3), when _X3 in the molecule of the (meth)acrylate compound A is a methylene group or a methylene group substituted with —O— (ether bond), it is presumed that the flexibility of the molecule is improved and the radical polymerization reaction is further accelerated, so that the curing reaction proceeds more efficiently.

(In formula (3), R ₃₁ and R ₃₂ each independently represent a hydrocarbon group having 1 to 8 carbon atoms,
R ₃₃ represents a hydrogen atom or a methyl group;
_X3 represents a hydrocarbon group having 1 to 49 carbon atoms;
The methylene group in the group represented by _X3 may be substituted with —O—,
g and h each independently represent an integer of 0 to 4;
and i represents an integer from 1 to 10.

Specific examples of compound A represented by formula (3) include the following compounds (D-39) to (D-48).

Next, the form of the electron-accepting compound having a radical polymerizable group in the thermosensitive color-forming layer will be described. The form of the electron-accepting compound having a radical polymerizable group in the thermosensitive color-forming layer is not particularly limited, and examples include particles and layers. For example, when the electron-accepting compound having a radical polymerizable group is contained in the thermosensitive color-forming layer in the form of a layer, the thermosensitive color-forming layer preferably has a first layer containing an electron-donating dye precursor and a second layer containing an electron-accepting compound having a radical polymerizable group. Hereinafter, the first layer containing the electron-donating dye precursor will also be referred to as the "electron-donating dye precursor layer" or "leuco layer." Furthermore, the second layer containing the electron-accepting compound having a radical polymerizable group will also be referred to as the "electron-accepting compound layer having a radical polymerizable group" or "developer layer."

The thermosensitive coloring layer preferably contains particles containing an electron-accepting compound having a radical polymerizable group and a photoradical polymerization initiator. The method for preparing particles containing an electron-accepting compound having a radical polymerizable group and a photoradical polymerization initiator is not particularly limited, but an O/W emulsion method is preferred. In this case, the photoradical polymerization initiator is preferably premixed with the electron-accepting compound having a radical polymerizable group. The particle diameter of the particles containing the electron-accepting compound having a radical polymerizable group and the photoradical polymerization initiator is preferably 10 nm to 1,000 nm, and more preferably 50 nm to 300 nm. Having particle diameters of 10 nm or more, or even 50 nm or more, enhances radical polymerization reactivity and improves image storage stability. Having particle diameters of 1,000 nm or less, or even 300 nm or less, reduces unnecessary light scattering in the thermosensitive coloring layer and increases image density. In this specification, particle size refers to the 50% particle size (D50) based on volume distribution.

The electron-accepting compound having a radical polymerizable group can be used alone or in combination of two or more. The content of the electron-accepting compound having a radical polymerizable group in the thermosensitive color-forming layer is preferably 500% by mass or more, and more preferably 1500% by mass or more, based on the content of the electron-donating dye precursor, in order to efficiently cure with ultraviolet light. There is no particular upper limit to the content of the electron-accepting compound having a radical polymerizable group, but it is preferably 5000% by mass or less, based on the content of the electron-donating dye precursor.

[Method for synthesizing compound A represented by formula (1)]
Compound A represented by formula (1) is available as a commercially available product. Compound A that is not commercially available can also be obtained by a known synthesis method.

For example, the above compounds (D-6) to (D-38) can be obtained by selecting and reacting at least one of the following compounds (a) to (c):

Compound (a): A polyfunctional acrylate compound containing at least one OH group or -COOH group in the molecule. Commercially available products include SR295 (manufactured by Sartomer Co., Ltd.), SR399 (manufactured by Sartomer Co., Ltd.), and Aronix M510 (manufactured by Toagosei Co., Ltd.).

Compound (b): Diisocyanate compounds such as HDI (hexamethylene diisocyanate), MDI (diphenylmethane diisocyanate), PDI (1,5-pentamethylene diisocyanate), IPDI (isophorone diisocyanate), and TDI (toluene diisocyanate).

Compound (c): An electron-accepting compound having a diphenyl sulfone skeleton and containing two or more OH groups in the molecule. Commercially available products include TGSH(H) manufactured by Nippon Kayaku Co., Ltd. and BPS-24C manufactured by Nicca Chemical Co., Ltd.

Furthermore, the above compounds (D-39) to (D-48) can be obtained, for example, by the following scheme of method (i).

(In formulas (3) to (5), R ₃₁ and R ₃₂ each independently represent a hydrocarbon group having 1 to 8 carbon atoms,
R ₃₃ represents a hydrogen atom or a methyl group;
_X3 represents a hydrocarbon group having 1 to 49 carbon atoms;
The methylene group in the group represented by _X3 may be substituted with —O—,
g and h each independently represent an integer of 0 to 4;
and i represents an integer from 1 to 10.

The compound represented by formula (3) is obtained by adding the compound represented by formula (5) to the compound represented by formula (4) and stirring for a predetermined period of time. During this process, a condensation reaction occurs between the -OH group in the compound represented by formula (4) and the -NCO group in the compound represented by formula (5).

The amount of the compound represented by formula (5) added is preferably 100 mol % or more and 150 mol % or less, based on the compound represented by formula (4), from the viewpoint of reacting approximately 50% of the -OH groups in the compound represented by formula (4).

While method (i) can be performed without a solvent, it is preferable to perform it in the presence of a solvent to prevent the reaction from proceeding too rapidly. The solvent is not particularly limited as long as it does not inhibit the reaction. Examples include esters such as methyl acetate, ethyl acetate, and propyl acetate; ethers such as diethyl ether, tetrahydrofuran, and dioxane; hydrocarbons such as benzene, toluene, xylene, hexane, and heptane; and halogenated hydrocarbons such as dichloromethane, dichloroethane, and chloroform. Two or more of the above solvents can be mixed and used, and the mixing ratio can be determined arbitrarily depending on the solubility of the solute. The amount of the solvent used can be determined arbitrarily, but from the perspective of reaction rate, it is preferable that the amount be in the range of 100% by mass to 1000% by mass of the compound represented by formula (4) above.

Method (i) is typically carried out at a temperature ranging from 20°C to 100°C and is typically completed within 48 hours.

Next, compounds represented by formulas (4) and (5) will be described. Commercially available products of the compound represented by formula (4) include TGSH(H) manufactured by Nippon Kayaku Co., Ltd. and BPS-24C manufactured by Nicca Chemical Co., Ltd. Commercially available products of the compound represented by formula (5) include Karenz BEI manufactured by Showa Denko K.K., Karenz AOI manufactured by Showa Denko K.K., Karenz MOI manufactured by Showa Denko K.K., and Karenz MOI-EG manufactured by Showa Denko K.K.

The molecular structure of the synthesized compound represented by formula (1) can be identified using NMR (nuclear magnetic resonance), IR (infrared spectrophotometer), MS (mass spectrometer), etc.

(Other ingredients)
The thermosensitive coloring layer may contain a storage stability improver. By containing the storage stability improver in 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.

The thermosensitive coloring layer can contain a thermal sensitizer. By containing a thermal sensitizer in the thermosensitive coloring layer, recording sensitivity can be increased. Examples of the thermal sensitizer include stearic acid amide, methoxycarbonyl-N-stearic acid benzamide, 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)ethanol, and the like. 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, it is preferably 2% by mass or more and 40% by mass or less, and more preferably 5% by mass or more and 25% by mass or less, of the total solid content of the thermosensitive coloring layer.

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 used to form the thermosensitive color-developing layer. Alternatively, these auxiliaries may 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.

The thermosensitive color-forming layer may contain a polymerization accelerator. Examples of the polymerization accelerator include benzoate compounds and amine compounds.

Examples of benzoate compounds and amine compounds 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.

The thermosensitive coloring layer may contain a sensitizer. Any sensitizer may be used as long as it 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 10% by mass or more and 1000% by mass or less, and more preferably 100% by mass or more and 500% by mass or less, based on the content of the photoradical polymerization initiator.

To improve the electron transfer efficiency or energy transfer efficiency between the sensitizer and the photoradical polymerization initiator, it is preferable to incorporate a sensitization aid into the thermosensitive color-forming layer. Examples of sensitization 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 sensitization aid in the thermosensitive color-forming layer is preferably 10% by weight or more and 1,000% by weight or less, and more preferably 50% by weight or more and 500% by weight or less, based on the content of the sensitizer.

The thermosensitive coloring layer can contain a radical polymerization inhibitor. Photoradical polymerization initiators decompose slightly during storage of the thermosensitive recording medium, turning into radical compounds. These radical compounds can cause polymerization, so it is preferable to include a radical polymerization inhibitor in the thermosensitive coloring layer to prevent this polymerization.

Radical polymerization inhibitors include phenolic hydroxyl group-containing compounds, methoquinone (hydroquinone monomethyl ether), hydroquinone, quinones such as 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, phosphate 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, tetraazaannulene (TAA)-based complexing agents, and hindered amines.

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. Furthermore, 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 radically polymerizable compound.

The thermosensitive coloring layer can contain a highly white pigment with an average particle size 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 preferable that the pigment content be 50% by weight or less of the total solids content of the thermosensitive coloring layer.

A binder can be used as a component of the thermosensitive coloring layer. If necessary, 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 copolymer, acrylamide-acrylic acid ester-methacrylic acid ester copolymer, styrene-maleic anhydride copolymer, isobutylene-maleic anhydride copolymer, casein, gelatin, and their derivatives; emulsions of polyvinyl acetate, polyurethane, polyacrylic acid, polyacrylic acid ester, vinyl chloride-vinyl acetate copolymer, polybutyl methacrylate, and ethylene-vinyl acetate copolymer; and latexes of water-insoluble polymers such as styrene-butadiene copolymer and styrene-butadiene-acrylic copolymer.

The inclusion of a crosslinking agent in the thermosensitive coloring layer can improve the layer's water resistance. 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% by weight, based on the total solids content of the thermosensitive coloring layer.

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

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

The thermosensitive coloring layer can be formed, for example, by applying a coating liquid 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 amount of coating liquid applied is preferably 2 g/ ^m2 to 20 g/ ^m2 , more preferably 2 g/ ^m2 to 15 g/ ^m2 , and particularly preferably 2 g/ ^m2 to 10 g/ ^m2, in terms of dry mass.

A surfactant is preferably used to prepare particles containing an electron-accepting compound having a radically polymerizable group and a photoradical polymerization initiator. Examples of surfactants include anionic surfactants such as sodium alkyl sulfonate, sodium alkylbenzene 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 alkyltrimethylammonium chloride, dialkyldimethylammonium chloride, and alkylbenzyldimethylammonium chloride; and amphoteric surfactants such as alkylbetaine and alkyldimethylamine oxide. Furthermore, polymeric surfactants such as the sodium salt of naphthalenesulfonic acid formalin condensate and sodium polyacrylate can be used.

Radically polymerizable compounds that have been given surfactant properties by bonding ionic groups such as sulfonic acid groups, carboxylic acid groups, and amino groups, or hydrophilic nonionic groups such as polyoxyethylene groups and polyglyceryl groups, can also be used.

Dispersing aids can also be used to prepare the aforementioned particles. Examples of dispersing aids include water-soluble polymers such as polyvinyl alcohol and its modified products, polyacrylic acid amide and its derivatives, 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.

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 the particles.

(middle class)
When the thermosensitive coloring layer has an electron-donating dye precursor layer (leuco layer) and an electron-accepting compound layer (developer layer) having a radically polymerizable group, 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.

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 amount of coating liquid applied is preferably 1 g/m2 or ^more and 40 g/m2 ^{or less} , and more preferably 2 g/m2 ^{or more} and 10 g/m2 or ^less , in terms of dry mass.

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

Usable UV absorbers include benzotriazole-based UV absorbers, triazine-based UV absorbers, benzophenone-based UV absorbers, cyanoacrylate-based UV absorbers, salicylic acid-based UV absorbers, and titanium oxide.

(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.

By incorporating microcapsules containing UV absorbers or solid-dispersed particles of UV absorbers into the protective layer, light resistance can be significantly improved. Microcapsules with walls made of polyurethane-polyurea resin or aminoaldehyde resin are particularly preferred, as they have excellent heat resistance and also exhibit the additional benefit of suppressing sticking to thermal heads. Furthermore, microcapsules with walls made of polyurethane-polyurea resin or aminoaldehyde resin have a lower refractive index than microcapsules with walls made of other resins. Furthermore, because they are spherical, adding a large amount to the protective layer is less likely to result in a decrease in density due to diffuse reflection of light, making them preferable.

It is also preferable to include particles in the protective layer, as this helps prevent adhesion of dirt and sticking to the thermal head. The oil absorption capacity of the particles is preferably 50 mL/100 g or more. The particle content in the protective layer is preferably an amount that does not reduce the color density; specifically, it is preferably 60 mass % or less of the total solid content of the protective layer.

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-forming layer to form a coating layer, and then drying the coating layer. The coating amount of the coating solution is preferably 0.1 g/m2 or ^more and 15 g/m2 or ^less , and more preferably 0.5 g/m2 ^{or more} and 8 g/m2 ^or less, in terms of dry mass.

(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.

(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.

(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 having a radical polymerizable group, and a protective layer 105 are laminated in this order on one side of the support 101. In a thermosensitive recording medium that is an embodiment of the present invention, the order of the electron-donating dye precursor layer 102 and the electron-accepting compound layer 104 having a radical polymerizable group may be reversed, and the protective layer 105 shown in Fig. 1 may not be provided.

Figure 2 is a cross-sectional view showing another embodiment of a thermal recording medium of the present invention. The thermal recording medium 200 shown in Figure 2 comprises a sheet-like support 201, and on one side of the support 201, an electron-donating dye precursor layer 202, an electron-accepting compound layer 203 having a radical polymerizable group, and a protective layer 204, which are layered in this order. In a thermal recording medium that is an embodiment of the present invention, the order of the electron-donating dye precursor layer 202 and the electron-accepting compound layer 203 having a radical polymerizable group may be reversed, and the protective layer 204 shown in Figure 2 may not be provided.

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

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 desired locations. Each coating film can be formed and then dried to form the desired layer, resulting in 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 multi-layer coating, in which two or more coating solutions are applied simultaneously, is also possible. 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. Surface smoothing treatment can improve recording sensitivity and increase the uniformity of the formed image.

<Image forming method>
Next, the image forming method of the present invention will be described. The image forming method of the present invention includes a step of heating the above-mentioned thermosensitive recording medium to form an image (image forming step). The method for heating the thermosensitive recording medium is not particularly limited, and any known heating method can be used. Among these, from the viewpoint of miniaturizing the image forming apparatus, it is preferable to heat the thermosensitive recording medium using a thermal head. Specifically, it is preferable to heat the thermosensitive recording medium by applying a heat pulse to the thermosensitive recording medium using a thermal head.

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

It is also preferable that the image forming method further includes a step of irradiating the thermosensitive recording medium on which the image has been formed with ultraviolet light to fix the thermosensitive coloring layer (fixing step).

In the fixing process, the thermosensitive recording medium on which the image has been formed is irradiated with ultraviolet light. The wavelength of the irradiated ultraviolet light should be a wavelength that can cause a reaction with the radical polymerization initiator contained in the thermosensitive coloring layer, for example, 365 nm or more and 425 nm or less. Irradiating with ultraviolet light causes the radically 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 that reaches the color development initiation temperature is subsequently applied, allowing the color development of the formed image to be maintained 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 means that the color development state of the thermosensitive coloring layer is fixed.

The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited in any way by the following examples as long as it does not deviate from the gist of the invention. "Parts" and "%" used to describe component amounts are by weight unless otherwise specified.

<Production of Thermosensitive Recording Material (1)>
Example 1
[Production of Electron Accepting Compound Having a Radical Polymerizable Group]
Compound (D-39), which is an exemplary compound of the electron-accepting compound having the above-mentioned radically polymerizable group, was produced by the following method.

First, 55 parts of TGSH(H) (manufactured by Nippon Kayaku Co., Ltd.) were dissolved in 202 parts of ethyl acetate, and 43 parts of Karenz BEI (manufactured by Showa Denko K.K.) were added. The mixture was stirred at 55°C for 24 hours to obtain compound (D-39).

[Analysis of Electron-Accepting Compounds Having Radical Polymerizable Groups]
In order to confirm that the obtained compound had the structure of compound (D-39), the compound was subjected to an appropriate pretreatment and then to the following analyses (1) to (4).

(1) ¹ H-NMR measurement: 600 MHz, CDCl ₃ , room temperature δ [ppm] = 7.2 (a), 5.1 (b)

(2) ¹³ C-NMR measurement: 600 MHz, CDCl ₃ , room temperature δ [ppm] = 152 (c)

The results of analyses (1) and (2) confirmed the formation of -NHCO- (urethane bond).

(3) LC-MS measurement:
LC (liquid chromatography) measurement conditions: Unison UK-Phenyl (particle size 3 μm×150 mm) column, 40° C., water/methanol=40/60
MS (mass spectrometry) measurement conditions: ESI, positive, 380°C
From the results of the analysis of (3), it was confirmed that the molecular weight was consistent with that of compound (D-39).

(4) FT-IR measurement: ATR method, germanium, room temperature. From the results of the analysis in (4), it was confirmed that the -NCO (isocyanate group) peak in Karenz BEI (manufactured by Showa Denko KK) decreased due to the reaction.

The above analysis confirmed that the obtained compound had the structure of compound (D-39).

[Preparation of raw material composition]
Solutions [A] to [D] were prepared as follows.
Solution [A]: a composition containing an electron-accepting compound having a radical polymerizable group; 30 parts of compound (D-39); and a photoradical polymerization initiator (Irgacure OXE01; manufactured by BASF Japan Ltd.).
8 parts Ethyl acetate 62 parts The above materials were mixed and dissolved to obtain solution [A].
Solution [B]: Dispersion of particles containing electron-donating dye precursor: 3 parts electron-donating dye precursor (BLUE 220, manufactured by Fukui Yamada Chemical Co., Ltd.); 0.3 parts dispersant (Pelex NBL, manufactured by Kao Corporation); 96.7 parts water. The above materials were mixed, 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 dispersion of particles containing electron-donating dye precursor was about 700 nm.
Solution [C]: intermediate layer coating solution: 10 parts of polyvinyl alcohol (Kuraray Poval 5-88; manufactured by Kuraray); 90 parts of water. The above materials were mixed and dissolved to obtain solution [C].
Solution [D]: Kaolin dispersion: Kaolin (HYDRAGLOSS90, manufactured by KaMin, LLC) 59.5 parts, dispersant (Aron T-50, manufactured by Toagosei Co., Ltd., solids concentration 40%) 0.5 parts, water 40 parts. The above materials were dispersed for 1 hour using a Coles disperser to obtain Solution [D].

[Formation of thermosensitive coloring layer]
Using a printability tester, Solution [B] was applied to a 130 μm thick synthetic paper (Yupo, manufactured by Yupo) and then dried with a dryer. The coating amount of Solution [B] after drying was 40.0 g/ ^m² , and the coating amount of BLUE 220 was 1.2 g/ ^m² . Next, Solution [C] was applied using a printability tester and then dried with a dryer. The coating amount of Solution [C] after drying was 0.2 g/ ^m² . Next, Solution [A] was applied using a printability tester and then dried with a dryer to form a thermosensitive coloring layer. The coating amount of Solution [A] after drying was 60.0 g/ ^m² , and the coating amount of Compound (D-39) was 18.0 g/ ^m² .

[Formation of protective layer]
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 solution [D], 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 liquid 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).

The obtained protective layer coating liquid was applied to the thermosensitive color-developing layer so that the coating amount after drying would be 1.5 g/ ^m2, and then dried to form a protective layer. The surface was then smoothed with a supercalender to obtain a thermosensitive recording medium.

(Examples 2 to 14 and Reference Examples 1 and 2 )
The electron-donating dye precursor, photoradical polymerization initiator, and electron-accepting compound having a radical polymerizable group used in Example 1 were changed as shown in Table 1 to obtain the thermosensitive recording materials of Examples 2 to 14 and Reference Examples 1 and 2 .

<Production of thermosensitive recording medium (2)>
(Example 17)
[Preparation of raw material composition]
Solutions [E] to [I] were prepared as follows.
Solution [E]: a composition containing an electron-accepting compound having a radical polymerizable group; 30 parts of compound (D-39); and a photoradical polymerization initiator (Irgacure OXE01; manufactured by BASF Japan Ltd.).
8 parts Ethyl acetate 62 parts The above materials were mixed and dissolved to obtain solution [E].
Solution [F]: Dispersion of particles containing an electron-donating dye precursor: 3 parts of electron-donating dye precursor (BLUE 220, manufactured by Fukui Yamada Chemical Co., Ltd.); 0.3 parts of dispersant (Pelex NBL, manufactured by Kao Corporation); 96.7 parts of water. The above materials were mixed, and then pulverized and dispersed using a bead mill 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 particle dispersion was approximately 700 nm.
Liquid [G]: Dispersant-containing liquid: Dispersant (Pelex NBL, manufactured by Kao Corporation) 10 parts; Water 100 parts. The above materials were mixed and dissolved to obtain Liquid [G].
Solution [H]: Dispersion of particles containing an electron-accepting compound having a radical polymerizable group. 50 parts of solution [E] and 50 parts of solution [G] were mixed and then emulsified using an ultrasonic homogenizer (UH-600S, manufactured by SMT). Ethyl acetate was then removed under reduced pressure using a rotary evaporator to obtain solution [H], a dispersion of particles containing an electron-accepting compound having a radical polymerizable group. The particle diameter (D50) of the particles in the dispersion of particles containing an electron-accepting compound having a radical polymerizable group, as measured using a particle size distribution analyzer (Nanotrac, manufactured by Microtrac), was 160 nm.
Solution [I]: Kaolin dispersion: Kaolin (HYDRAGLOSS 90, manufactured by KaMin, LLC) 59.5 parts, dispersant (Aron T-50, manufactured by Toagosei, solids concentration 40%) 0.5 parts, water 40 parts. The above materials were dispersed for 1 hour using a Coles disperser to obtain Solution [I].

[Formation of thermosensitive coloring layer]
Using a printability tester, the [F] solution was applied to a 130 μm thick synthetic paper (Yupo, manufactured by Yupo) and then dried with a dryer. The coating amount of the [F] solution after drying was 40.0 g/ ^m² , and the coating amount of BLUE 220 was 1.2 g/ ^m² . Thereafter, the [H] solution was applied using a printability tester and then dried with a dryer to form a thermosensitive coloring layer. The coating amount of the [H] solution after drying was 60.0 g/ ^m² , and the coating amount of compound (D-39) was 18.0 g/ ^m² .

[Formation of protective layer]
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 solution [I], 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 liquid 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" (solids concentration 40%, manufactured by Mitsui Chemicals). The obtained protective layer coating liquid was applied to the thermosensitive coloring layer so that the coating amount after drying would be 1.5 g/ ^m2 , and then dried to form a protective layer. The surface was then smoothed with a supercalender to obtain a thermosensitive recording medium.

The obtained protective layer coating liquid was applied to the thermosensitive color-developing layer so that the coating amount after drying would be 1.5 g/ ^m2, and then dried to form a protective layer. The surface was then smoothed with a supercalender to obtain a thermosensitive recording medium.

(Comparative Examples 1 to 4)
The electron-donating dye precursor, the photoradical polymerization initiator, and the electron-accepting compound having a radical polymerizable group used in Example 1 were changed as shown in Table 1 to obtain thermosensitive recording materials of Comparative Examples 1 to 4.

<Production of thermosensitive recording medium (3)>
(Comparative Example 5)
[Preparation of raw material composition]
Solutions [J] to [M] were prepared as follows.
Solution [J]: Composition containing electron-donating dye precursor: 3 parts of electron-donating dye precursor (BLUE 220, manufactured by Fukui Yamada Chemical Co., Ltd.); 30 parts of radically polymerizable compound (8KX-078, manufactured by Taisei Fine Chemical Co., Ltd.); 9 parts of photoradical polymerization initiator (Omnirad TPO, manufactured by IGM Resins); 58 parts of ethyl acetate. The above materials were mixed and dissolved to obtain Solution [J].
Solution [K]: Dispersion of particles containing an electron-accepting compound: 40 parts of electron-accepting compound (TGSH(H), manufactured by Nippon Kayaku Co., Ltd.); 4 parts of dispersant (Pelex NBL, manufactured by Kao Corporation); 56 parts of water. The above materials were mixed and then pulverized and dispersed using a bead mill to obtain Solution [K]. When measured using a particle size distribution analyzer (Nanotrac, manufactured by Microtrac), the particle diameter (D50) of the dispersion of particles containing an electron-accepting compound was found to be approximately 700 nm.
Liquid [L]: intermediate layer coating liquid Polyvinyl alcohol (Kuraray Poval 5-88, manufactured by Kuraray) 10 parts Water 90 parts The above materials were mixed and dissolved to obtain liquid [L].
Solution [M]: Kaolin dispersion: Kaolin (HYDRAGLOSS 90, manufactured by KaMin, LLC) 59.5 parts, dispersant (Aron T-50, manufactured by Toagosei, solids concentration 40%) 0.5 parts, water 40 parts. The above materials were dispersed for 1 hour using a Coles disperser to obtain Solution [M].

[Formation of thermosensitive coloring layer]
Using a printability tester, the [J] solution was applied to a 130 μm thick synthetic paper (Yupo, manufactured by Yupo), and then dried with a dryer to evaporate the ethyl acetate. After drying, the coating amount of the [J] solution was 40.0 g/ ^m² , and the coating amount of the BLUE 220 was 1.20 g/ ^m² . Next, the [L] solution was applied using a printability tester, and then dried with a dryer. After drying, the coating amount of the [L] solution was 0.2 g/ ^m² . Next, the [K] solution was applied using a printability tester, and then dried with a dryer. After drying, the coating amount of the [K] solution was 30.0 g/ ^m² , and the coating amount of the TGSH(H) was 12.0 g/ ^m² .

[Formation of protective layer]
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 [M] 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 liquid 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).

The obtained protective layer coating liquid was applied to the thermosensitive color-developing layer so that the coating amount after drying would be 1.5 g/ ^m2, and then dried to form a protective layer. The surface was then smoothed with a supercalender to obtain a thermosensitive recording medium.

<Image formation and evaluation>
(Color development)
Using a thermal head (KPE type, manufactured by Kyocera), a 2 cm x 2 cm area was heated on the surface of the thermal recording medium obtained in Examples 1 to 15, Reference Examples ¹ and 2, and Comparative Examples 1 to 5 to form an image. The applied power and pulse width of the thermal head were set so that the recording energy per unit area was 150 mJ/mm2. The optical reflection density of the formed image was measured using a reflection densitometer (trade name "Xrite 530", manufactured by Sakata Inx Engineering). The measurement results are shown in Table 2.

As shown in Table 2, when the results of Examples 1 to 12 and 15 are compared with those of Examples 13 to 14 and Reference Examples 1 and 2 , it can be seen that the optical reflection density is higher when the content of the electron-accepting compound having a radical polymerizable group is 1500% by mass or more, based on the content of the electron-donating dye precursor, than when it is less than 1500% by mass.

(UV Fixability)
The thermal recording media obtained in Examples 1 to 15, Reference Examples 1 and 2, and Comparative Examples 1 to 5 were irradiated once with ultraviolet light using three ultraviolet irradiation devices (ME12-L61, manufactured by Eye Graphics) equipped with metal halide lamps (120 W/cm) at conveyor speeds of 100 m/min and 10 m/min. Separately, the integrated light dose under the same conditions was measured using an ultraviolet integrating actinometer (C9536-01, manufactured by HAMAMATSU), and was found to be 100 mJ/ ^cm² and 1,000 mJ/ ^cm² , respectively.

Then, using a thermal head (KPE type, manufactured by Kyocera), the applied power and pulse width were set so that the recording energy per unit area was 150 mJ/ ^mm2 , and ultraviolet light was applied to form a 2 cm x 2 cm image on the thermal recording medium obtained in Examples 1 to 15, Reference Examples 1 and 2, and Comparative Examples 1 to 5. The optical reflection density of the formed image was measured using a reflection densitometer (trade name "Xrite 530", manufactured by Sakata Inx Engineering). The difference from the optical reflection density of the aforementioned synthetic paper (Yupo, manufactured by Yupo) was taken as "Δ optical reflection density," and the results are shown in Table 2.

As shown in Table 2, it was confirmed that the images were sufficiently fixed in Examples 1 to 15 and Reference Examples 1 and 2 ^from the absolute values of the Δoptical reflection densities of the images after irradiation with cumulative light amounts of 100 mJ/cm2 and 1,000 mJ/ ^cm2 . In other words, it was confirmed that color development due to heating after ultraviolet irradiation was suppressed even when the ultraviolet irradiation amount was as low as 100 mJ/ ^cm2 .

On the other hand, in Comparative Examples 1 to 4, phenolic compounds (D-49) and (D-50) were used as the electron-accepting compound having a radical polymerizable group, and the Δ optical reflection density at an integrated light amount of 1,000 mJ/ ^cm2 was 0.17 or more, indicating color development compared to Examples 1 to 15 and Reference Examples 1 and 2. Furthermore, the Δ optical reflection density at an integrated light amount of 100 mJ/ ^cm2 was 0.45 or more, indicating significant color development compared to Examples 1 to 15 and Reference Examples 1 and 2. In other words, when the integrated light amount was as low as 100 mJ/ ^cm2 , color development due to heating after ultraviolet irradiation could not be significantly suppressed.

In Comparative Example 5, an electron-accepting compound having no radical polymerizable group and a radical polymerizable compound were used instead of an electron-accepting compound having a radical polymerizable group. However, the Δ optical reflection density at an integrated light amount of 1,000 mJ/ ^cm2 was 0.18, and the Δ optical reflection density at an integrated light amount of 100 mJ/ ^cm2 was 0.20, which means that the color developed slightly more than in Examples 1 to 15 and Reference Examples 1 and 2 .

The types of electron-accepting compounds having a radical polymerizable group (D-39) to (D-44) in Table 1 are the same as the exemplary compounds (D-39) to (D-44) of the electron-accepting compounds having a radical polymerizable group described above.

The structures of electron-accepting compounds having radical polymerizable groups other than (D-39) to (D-44) in Table 1 are shown in Table 3 below.

Details of the types of electron-donating dye precursors in Table 1 are shown in Table 4 below.

Details of the types of photoradical polymerization initiators in Table 1 are shown in Table 5 below.

Details of the types of electron-accepting compounds in Table 1 that do not have a radically polymerizable group are shown in Table 6 below.

Details of the types of radically polymerizable compounds in Table 1 are shown in Table 7 below.

100, 200 Thermosensitive recording medium 101, 201 Support 102, 202 Electron-donating dye precursor layer 103 Intermediate layer 104, 203 Electron-accepting compound layer having a radical polymerizable group 105, 204 Protective layer

Claims (8)

A thermosensitive recording medium having a thermosensitive color-forming layer containing an electron-donating dye precursor, a photoradical polymerization initiator, and an electron-accepting compound having a radical polymerizable group,
A thermosensitive recording medium, wherein the electron-accepting compound having a radical polymerizable group contains a compound A represented by the following formula ( 2 ):

(In formula ( 2 ), R ₂₁ and R ₂₂ each independently represent a hydrocarbon group having 1 to 8 carbon atoms;
R ₂₃ represents a hydrogen atom or a methyl group;
X2 represents a hydrocarbon group having 1 to 49 carbon atoms _;
a methylene group in the group _represented by X2 may be substituted with —NHCOO— , —NHCO—, —O—, —CO—, —COO—, or —NR ₂₄ —;
R ₂₄ represents a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms;
d and e each independently represent an integer of 0 to 4;
and f represents an integer of 1 to 10. The thermosensitive recording material according to claim 1 , wherein the compound A is represented by the following formula (3):

(In formula (3), R ₃₁ and R ₃₂ each independently represent a hydrocarbon group having 1 to 8 carbon atoms,
R ₃₃ represents a hydrogen atom or a methyl group;
_X3 represents a hydrocarbon group having 1 to 49 carbon atoms;
The methylene group in the group represented by _X3 may be substituted with —O—,
g and h each independently represent an integer of 0 to 4;
and i represents an integer from 1 to 10. 3. The thermosensitive recording medium according to claim 1, wherein the content of the electron-accepting compound having a radical polymerizable group is 1500% by mass or more based on the content of the electron-donating dye precursor. 4. The thermosensitive recording medium according to claim 1, wherein the photoradical polymerization initiator comprises an oxime ester compound. 5. The thermosensitive recording medium according to claim 1 , wherein the thermosensitive coloring layer comprises particles containing the electron-accepting compound having the radical polymerizable group and the photoradical polymerization initiator. 6. An image forming method comprising a step of heating the thermosensitive recording medium according to claim 1 to form an image. 7. The image forming method according to claim 6 , wherein the heating is performed by applying a heat pulse to the thermosensitive recording medium using a thermal head. 8. The image forming method according to claim 6 , further comprising the step of irradiating the thermosensitive recording medium on which the image has been formed with ultraviolet light to fix the thermosensitive coloring layer.