HK1110842B - Thermosensitive recording medium - Google Patents

Description

Thermosensitive recording medium

Technical Field

The present invention relates to a thermosensitive recording medium utilizing a color-developing reaction between a leuco dye and a color-developing agent. In particular, the present invention relates to a thermosensitive recording medium in which a change in recording sensitivity due to a change in humidity of the environment of a thermosensitive recording medium used (hereinafter, also referred to as "environmental humidity change") is small.

Background

Conventionally, a thermosensitive recording medium utilizing a color development reaction by heat generated from a leuco dye and a color developer is known. The thermosensitive recording medium is relatively inexpensive, and the recording apparatus is compact and maintenance thereof is relatively easy. Therefore, the recording medium is used not only as a recording medium for facsimile machines, various computers, and the like, but also as a recording medium for printers of medical diagnostic apparatuses such as MRI image diagnosis and X-ray image diagnosis, and a recording medium for plotters and the like used in CAD.

However, the thermosensitive recording medium using the leuco dye and the color developer has the following disadvantages: when the moisture content of the thermosensitive recording layer changes with a change in humidity of the environment of the recording medium used, the recording sensitivity changes, and as a result, the recording density changes, and it is difficult to obtain a constant recording density particularly in the halftone region. Moreover, this drawback is particularly remarkable in the case of using a support such as a synthetic resin film.

The thermosensitive recording medium is desired to have a constant recording sensitivity, which does not vary depending on the humidity in the environment used in the season, region, country, etc. In particular, a thermosensitive recording medium for medical diagnosis is strongly required to be capable of obtaining an image similar to that of a silver salt system, capable of recording at a plurality of gradation levels, and capable of keeping a recording density of a halftone area constant without changing with humidity change of the environment. Therefore, a thermosensitive recording medium which improves the above-mentioned disadvantages is strongly demanded.

As a developer for thermosensitive recording media, 4, 4' -cyclohexylidenebisphenol has been known for a long time (see patent document 1). Further, a method of obtaining a thermosensitive recording medium with less background fogging and a large difference in transmission density between a recorded portion and an unrecorded portion by using the above developer has been proposed (patent document 2).

Further, a method of obtaining a thermosensitive recording medium is proposed: 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane is also known as a color-developer, and the use of this color-developer improves the oil resistance and plasticity resistance of a recorded image, thereby improving the long-term stability of the recorded image (see patent document 3).

In order to improve the storage stability of the recording portion of the thermal recording medium, the following methods are proposed: a method of using composite particles obtained by microencapsulating a leuco dye and a hydrophobic organic solvent using a wall film material formed of a hydrophobic resin (see patent documents 4 and 5); a method using composite particles obtained by polymerizing an acrylic hydrophobic resin on the surface of a leuco dye (see patent document 6); and a method of using composite particles obtained by including a leuco dye in a hydrophobic resin (see patent documents 7 and 8).

Patent document 1: japanese patent publication Showa No. 45-14039 (page 4)

Patent document 2: japanese patent laid-open publication No. 9-76641 (claim 1)

Patent document 3: japanese patent laid-open publication No. Hei 5-147357 (example 1)

Patent document 4: japanese patent laid-open publication No. Showa 60-244594 (claim 1)

Patent document 5: japanese patent laid-open publication No. Showa 61-86283 (example 1)

Patent document 6: japanese patent laid-open No. 2000-158822 (claims 1 and 7)

Patent document 7: japanese patent laid-open publication No. 9-263057 (claim 1)

Patent document 8: japanese patent laid-open publication No. 2003-266951 (claim 1)

Disclosure of Invention

The invention aims to provide a thermosensitive recording medium which has less change of recording sensitivity caused by the change of environmental humidity of the thermosensitive recording medium and has excellent image quality and heat-resistant background fogging property.

The present invention is a thermosensitive recording medium having a support and a thermosensitive recording layer,

the thermosensitive recording layer contains a leuco dye and a color developer,

the leuco dye is in the form of composite particles comprising the leuco dye and a hydrophobic resin,

the developer contains at least 4, 4 '-cyclohexylidenebisphenol and 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane.

That is, the present invention includes a thermosensitive recording medium,

item 1: a thermosensitive recording medium having a support and a thermosensitive recording layer,

the thermosensitive recording layer contains a leuco dye and a color developer,

the leuco dye is in the form of composite particles comprising the leuco dye and a hydrophobic resin,

the developer contains at least 4, 4 '-cyclohexylidenebisphenol and 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane.

Item 2: the thermosensitive recording medium according to item 1, wherein the color developer is obtained by wet-pulverizing a composition comprising 4, 4 '-cyclohexylidenebisphenol and 4, 4' -bis (N-p-toluenesulfonylaminocarboylamido) diphenylmethane.

Item 3: the thermosensitive recording medium according to item 1, wherein the 4, 4 '-bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane is 15 to 75% by mass with respect to the 4, 4' -cyclohexylidenebisphenol.

Item 4: the thermosensitive recording medium according to item 1, wherein the color developer further contains 4-hydroxy-4' -allyloxydiphenylsulfone.

Item 5: the thermosensitive recording medium according to item 4, wherein the color developer is obtained by wet-pulverizing a composition comprising 4, 4 ' -cyclohexylidenebisphenol, 4 ' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane, and 4-hydroxy-4 ' -allyloxydiphenylsulfone.

Item 6: the thermosensitive recording medium according to item 4, wherein the 4-hydroxy-4 '-allyloxydiphenesulfone is 10 to 50% by mass with respect to the 4, 4' -cyclohexylidenebisphenol.

Item 7: the thermosensitive recording medium according to item 1, wherein the color developer further contains N- (p-toluenesulfonyl) -N' -phenylurea.

Item 8: the thermosensitive recording medium according to item 7, wherein the N- (p-toluenesulfonyl) -N '-phenylurea is 25 to 150% by mass with respect to the 4, 4' -cyclohexylidenebisphenol.

Item 9: the thermosensitive recording medium according to item 1, wherein the hydrophobic resin forming the composite particles is a urea-based resin or a urea-urethane-based resin.

Item 10: the thermosensitive recording medium according to item 1, wherein the support is a transparent film.

Item 11: the thermosensitive recording medium as recited in item 1, wherein the thermosensitive recording medium further comprises a protective layer.

Item 12: the thermosensitive recording medium according to item 1, wherein the developer further contains 4-hydroxy-4 '-allyloxydiphenylsulfone and N- (p-toluenesulfonyl) -N' -phenylurea.

The present invention will be described in detail below.

I. Thermosensitive recording layer

The thermosensitive recording medium of the present invention contains a color-developer and a leuco dye in a thermosensitive recording layer.

I-1. color-developing agent

The developer of the present invention comprises at least 4, 4 '-cyclohexylidenebisphenol and 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane.

By using 4, 4 '-cyclohexylidenebisphenol and 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamido tolylsulfonylaminocarboxamide) diphenylmethane together, a developer which suppresses variation in recording sensitivity due to changes in environmental humidity and has excellent image quality can be formed.

In the case where 4, 4' -cyclohexylidenebisphenol is not used, the recording sensitivity, particularly in the halftone region, is liable to vary depending on the humidity change in the environment of the thermal recording medium used.

On the other hand, in the case where 4, 4 '-bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane is not used, 4' -cyclohexylidenebisphenol forms crystals in the coating liquid for a thermosensitive recording layer, and the image quality of the obtained thermosensitive recording medium is degraded.

The developer obtained by dispersing the substance may be used. For example, it can also be obtained by adding a substance formed by dispersing 4, 4 '-bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane alone in 4, 4' -cyclohexylidenebisphenol.

However, a developer obtained by mixing and dispersing (co-dispersing) 4, 4 '-cyclohexylidenebisphenol and 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane, in other words, a developer obtained by preparing a composition containing 4, 4 '-cyclohexylidenebisphenol and 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane and wet-pulverizing the composition is preferable because the image quality of the obtained thermosensitive recording medium is more excellent.

Although the particle diameter of the developer dispersion is not particularly limited, the volume average particle diameter is preferably 0.1 to 1.0. mu.m, particularly preferably 0.2 to 0.5. mu.m.

The ratio of 4, 4 '-bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane is preferably 15 to 75% by mass, more preferably 20 to 65% by mass, based on 4, 4' -cyclohexylidenebisphenol. By setting the content in this range, crystallization of 4, 4' -cyclohexylidenebisphenol in the coating liquid for a thermosensitive recording layer can be suppressed, and the image quality of the thermosensitive recording medium can be further improved.

In the present invention, the color developer may further contain other compounds than 4, 4 '-cyclohexylidenebisphenol and 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane, depending on the purpose, within a range not hindering the desired effect of the present invention.

Other compounds may include, for example, 4 '-isopropylidenediphenol, 2-bis (4-hydroxyphenyl) -4-methylpentane, benzyl 4-hydroxybenzoate, 4' -dihydroxydiphenylsulfone, 2, 4 '-dihydroxydiphenylsulfone, 4-hydroxy-4' -isopropoxydiphenylsulfone, 4-hydroxy-4 '-allyloxydiphenylsulfone, bis (3-allyl-4-hydroxyphenyl) sulfone, 4' -bis [ (4-methyl-3-phenoxycarbonylaminophenyl) ureide]Phenol compounds such as diphenylsulfone, 4-hydroxy-4 ' -methyldiphenylsulfone, 3, 4-dihydroxyphenyl-4 ' -methylphenylsulfone, bis (4-hydroxyphenylthioethoxy) methane and bis (p-hydroxyphenyl) butyl acetate, N-p-toluenesulfonylcarbamic acid-p-cumylphenyl ester, N- (p-toluenesulfonyl) -N ' -phenylurea, and N-o-tolyl-p-tolylsulfonamide containing-SO in the molecule₂NHCO-bonded compound, zinc p-chlorobenzoate, 4- [ 2-p-methoxyphenoxyethoxy group]Zinc salicylate, 4- [ 3-p-toluenesulfonylpropyl ] hydroxy]Zinc salicylate, 5- [ p- (2-p-methoxyphenoxyethoxy) cumyl]Zinc salts of aromatic carboxylic acids such as zinc salicylate. The number of the compounds may be 1 or 2 or more.

Among them, 4-hydroxy-4' -allyloxydiphenylsulfone is preferable.

When 4-hydroxy-4 '-allyloxydiphenesulfone is used in combination with 4, 4' -cyclohexylidenebisphenol and 4, 4 '-bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane, a phenomenon in which 4, 4' -cyclohexylidenebisphenol as a color-developing component of a recording portion is crystallized and deposited on a thermosensitive recording medium, that is, a so-called whitening phenomenon, can be prevented when the thermosensitive recording medium after recording is stored in a high-humidity environment. The whitening phenomenon here means that 4, 4' -cyclohexylidenebisphenol as a developer component is crystallized on the surface of the recording portion in the recording portion, and appears as if white powder is scattered on the surface of the recording portion.

When 4-hydroxy-4 '-allyloxydiphenesulfone is used together, the use ratio thereof is preferably about 10 to 50% by mass, more preferably 20 to 40% by mass, based on 4, 4' -cyclohexylidenebisphenol.

When 4-hydroxy-4 '-allyloxydiphenylsulfone is used simultaneously, a compound in which 4-hydroxy-4' -allyloxydiphenylsulfone is dispersed alone can be added to a compound in which 4, 4 '-cyclohexylidenebisphenol and 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane are mixed and dispersed (co-dispersed) to obtain a developer.

However, a developer obtained by mixing and dispersing (co-dispersing) 4, 4 '-cyclohexylidenebisphenol, 4' -bis (N-p-toluenesulfonylaminocarboylamido) diphenylmethane and 4-hydroxy-4 '-allyloxydiphenylsulfone, in other words, a developer obtained by preparing a composition comprising 4, 4' -cyclohexylidenebisphenol, 4 '-bis (N-p-toluenesulfonylaminocarboylamido) diphenylmethane and 4-hydroxy-4' -allyloxydiphenylsulfone and wet-pulverizing the composition is preferable because it is more convenient in handling.

The particle diameter of the developer dispersion is not particularly limited, but is preferably 0.1 to 1.0. mu.m, more preferably 0.2 to 0.5. mu.m, as the volume average particle diameter.

And preferably, the color developer further comprises N- (p-toluenesulfonyl) -N' -phenylurea.

When N- (p-toluenesulfonyl) -N ' -phenylurea is used in combination with 4, 4 ' -cyclohexylidenebisphenol and 4, 4 ' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane, particularly in the halftone region, a very fine image gradation is obtained, and therefore, it is preferable. Here, the very fine image gradation property means that the change of the recording density with respect to the applied energy is small. The halftone area, in other words, the halftone recording density area, is, for example, an area having a recording density of 1.0 to 1.5 in a measurement using a MACBETH densitometer (product name: TR-927J, manufactured by MACBETH).

In the case of using N- (p-toluenesulfonyl) -N ' -phenylurea at the same time, the ratio of 4, 4 ' -cyclohexylidenebisphenol to N- (p-toluenesulfonyl) -N ' -phenylurea is preferably 80: 20-40: about 60, more preferably 75: 25-55: 45. in other words, the N- (p-toluenesulfonyl) -N '-phenylurea is preferably 25 to 150 mass%, more preferably 33 to 82 mass%, based on 4, 4' -cyclohexylidenebisphenol.

The ratio of the color-developing agent used in the thermosensitive recording layer is not particularly limited, and the color-developing agent is preferably about 10 to 60 mass%, particularly preferably about 20 to 50 mass%, based on the total solid components of the thermosensitive recording layer.

When the usage ratio of the color developer is 10% by mass or more, the recording sensitivity is less likely to be lowered. And when 60 mass% or less, the ratio of the leuco dye is reduced, so that the possibility that the maximum recording concentration is reduced becomes small.

I-2 leuco dyes

In the thermosensitive recording layer, the leuco dye is used in the form of composite particles containing the leuco dye and a hydrophobic resin.

The kind of the hydrophobic resin forming the composite particles is not particularly limited, and examples thereof include urea resins, urethane resins, urea-urethane resins, styrene resins, and acrylic resins. Among them, urea resins and urea-urethane resins are preferable because they are excellent in heat resistance, particularly in resistance to background fogging.

The morphology of the composite particle comprising a leuco dye and a hydrophobic resin in the present invention may include, for example:

(1) a particle form in which a film formed of a hydrophobic resin contains 1 or more kinds of leuco dyes and a hydrophobic organic solvent and is microencapsulated,

(2) a form in which a hydrophobic resin is polymerized on the surface of 1 or more kinds of leuco dye particles, and

(3)1 or more leuco dyes are dispersed in solid hydrophobic resin particles.

The particles of (1) above, which contain a leuco dye and a hydrophobic organic solvent in the film formed of a hydrophobic resin and are then microencapsulated, can be obtained by emulsifying the leuco dye and the organic solvent and forming a wall formed of a hydrophobic resin around oil droplets thereof. For example, in the case where a urea-urethane resin is used as a wall material, the composite particles are obtained by mixing a polyvalent isocyanate compound and a second substance (for example, a polyol compound and a polyamine compound) that reacts with the polyvalent isocyanate compound to form a capsule wall into an oily liquid in which a leuco dye and an organic solvent are dissolved, emulsifying and dispersing the mixture in water, and then raising the temperature to cause a polymer-forming reaction at an oil droplet interface to form a capsule wall.

The composite particles in which the hydrophobic resin is polymerized on the surface of the leuco dye particles of the above (2) can be produced, for example, as follows. Can be obtained by the following process: the surface of the leuco dye particles is covered by adding a compound having an unsaturated carbon bond to a dispersion of particles composed of a leuco dye, adding a polymerization initiator, and then heating the mixture as necessary to perform addition polymerization of the compound having an unsaturated carbon bond.

The form of the composite particles in which the leuco dye of the above (3) is dispersed in the hydrophobic resin is a form including 1 or more kinds of leuco dyes dispersed in solid hydrophobic resin particles. The composite particles can be produced as follows, for example, when the resin is a urea resin or a urethane-urea resin.

The leuco dye, and the polyurea or polyurea-polyurethane compound formed by reaction with water, are dissolved to form a solution. The aqueous medium containing a protective colloid substance such as polyvinyl alcohol is dissolved, and the solution is added, and if necessary, a surfactant is added to the solution to carry out emulsification and dispersion. To this emulsion dispersion, a reactive substance such as a water-soluble polyamine is further added as necessary. The emulsion dispersion is heated to polymerize the polymerization component, thereby obtaining composite particles composed of a leuco dye and a urea resin or a urea-urethane resin. Here, the compound that forms polyurea or polyurea-polyurethane by reacting with water may be only a polyisocyanate compound, or may be a mixture of a polyisocyanate compound and a polyol and a polyamine that react therewith. Also, addition products of a polyvalent isocyanate compound and a polyhydric alcohol or multimers of a polyvalent isocyanate compound, such as biuret, isocyanurate, etc., are also possible.

Specific examples of the leuco dye include, for example, 3- [2, 2-bis (1-ethyl-2-methylindol-3-yl) vinyl ] -3- (4-diethylaminophenyl) phthalide, 3-bis (p-dimethylaminophenyl) -6-dimethylaminophenylphthalide, 3- (4-diethylamino-2-methylphenyl) -3- (4-dimethylaminophenyl) -6-dimethylaminophenylphthalide, 3-cyclohexylamino-6-chlorofluoroalkane, 3-diethylamino-6-methyl-7-chlorofluorane, 3-diethylamino-6, 8-dimethylfluorane, 3-diethylamino-6, 7-dimethylfluoran, 3-diethylamino-7-chlorofluoran, 3- (N-ethyl-N-isopentyl) amino-6-methyl-7-anilinofluoran, 3-bis (N-butyl) amino-6-methyl-7-anilinofluoran, 3-bis (N-pentyl) amino-6-methyl-7-anilinofluoran, 3- (N-ethyl-p-toluylamino) -6-methyl-7-anilinofluoran, 3-bis (N-butyl) amino-6-chloro-7-anilinofluoran, 3-diethylamino-6-methyl-7- (3-toluylamino) fluoran, and a salt thereof, 3-pyrrolidinyl-6-methyl-7-anilinofluoran, 3-piperidino-6-methyl-7-anilinofluoran, 3-bis (4-diethylamino-2-ethoxyphenyl) -4-azaphthalide, 3-bis [1- (4-methoxyphenyl) -1- (4-dimethylaminophenyl) ethen-2-yl ] -4, 5, 6, 7-tetrachlorophthalide, 3-p- (p-dimethylaminoanilino) anilino-6-methyl-7-chlorofluoran, 3-p- (p-chloroanilino) anilino-6-methyl-7-chlorofluoran, 3- [1, 1-bis (1-ethyl-2-methylindol-3-yl) ] -3-p-diethylaminophenylphthalide, 3-bis (1-n-butyl-2-methylindol-3-yl) phthalide, 3, 6-bis (dimethylamino) fluorene-9-spiro-3 '- (6' -dimethylamino) phthalide, and the like. The number of the compounds may be 1 or 2 or more.

Among the polyvalent isocyanate compounds, there are included, for example, p-phenylene diisocyanate, 2, 6-xylylene diisocyanate, 2, 4-xylylene diisocyanate, naphthalene-1, 4-diisocyanate, dicyclohexylmethane-4, 4 '-diisocyanate, 1, 3-bis (isocyanatomethyl) cyclohexane, 3' -dimethyldiphenylmethane-4, 4 '-diisocyanate, xylylene-1, 4-diisocyanate, tetramethylxylylene diisocyanate, 4' -diphenylpropane diisocyanate, cyclohexane diisocyanate, butene-1, 2-diisocyanate, cyclohexylene-1, 4-diisocyanate, 4, 4' -triphenylmethane triisocyanate, toluene-2, 4, 6-triisocyanate, trimethylolpropane adduct of cyclohexane diisocyanate, trimethylolpropane adduct of 2, 4-benzylidene diisocyanate, trimethylolpropane adduct of xylylene diisocyanate, and the like. The number of the compounds may be 1 or 2 or more.

Examples of the polyol compound include ethylene glycol, 1, 3-propanediol, 1, 4-butanediol, 1, 7-heptanediol, 1, 8-octanediol, propylene glycol, 1, 3-dihydroxybutane, 2-dimethyl-1, 3-propanediol, 2, 5-hexanediol, 3-methyl-1, 5-pentanediol, 1, 4-cyclohexanedimethanol, dihydroxycyclohexane, diethylene glycol, phenylglycol, pentaerythritol, 1, 4-bis (2-hydroxyethoxy) benzene, 1, 3-bis (2-hydroxyethoxy) benzene, p-xylylene glycol, m-xylylene glycol, 4 '-isopropylidenediphenol, 4' -dihydroxydiphenylsulfone and the like. The number of the compounds may be 1 or 2 or more.

Examples of the polyamine compound include ethylenediamine, propylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, p-phenylenediamine, m-phenylenediamine, 2, 5-dimethylpiperazine, triethylenetriamine, triethylenetetramine, and diethylaminopropylamine. These may be used in 1 kind, or 2 or more kinds.

Specific examples of the leuco dye, the polyisocyanate compound, the polyol compound, and the polyamine compound are merely illustrative, and are not limited to the above compounds.

The content of the leuco dye in the composite particles is about 10 to 70 mass%, preferably about 35 to 60 mass%, based on the total solid content of the composite particles.

The volume average particle diameter of the composite particles is about 0.5 to 3.0 μm, and particularly preferably about 0.5 to 1.0 μm.

The composite particles may further contain a sensitizer for improving recording sensitivity, an ultraviolet absorber for improving light resistance, and a stabilizer.

The sensitizer contains, for example, an aromatic organic compound having a melting point of about 40 to 150 ℃.

The stabilizer contains, for example, a hindered phenol, a hindered amine, and the like.

The ultraviolet absorber contains, for example, a benzotriazole compound, a benzophenone compound, and the like.

The content of the composite particles in the thermosensitive recording layer is about 10 to 60 mass%, preferably about 20 to 50 mass% with respect to the total solid content of the thermosensitive recording layer.

The ratio of the composite particles to the color-developing agent in the thermosensitive recording layer is about 50 to 300 parts by mass, preferably about 100 to 200 parts by mass, per 100 parts by mass of the composite particles.

When the leuco dye is in the form of composite particles containing the leuco dye and the hydrophobic resin, background fogging due to heat and humidity is reduced, and discoloration of a developed image is reduced.

When the leuco dye is dissolved in the isocyanate compound or the organic solvent, the heat-sensitive recording layer is more excellent in transparency than when the leuco dye is used in a solid fine particle state.

I-3 formation of thermosensitive recording layer

The thermosensitive recording layer is generally formed by using water as a medium and compounding the particlesA coating liquid for a thermosensitive recording layer prepared by mixing and stirring a specific developer, a binder and optionally an auxiliary agent, wherein the coating liquid is applied to a support so that the amount of the coating liquid after drying is 3 to 25g/m²Left and right, and then dried to form.

Examples of the binder to be mixed with the coating liquid for a thermosensitive recording layer include partially or completely saponified polyvinyl alcohol, polyvinyl alcohols such as carboxyl-modified polyvinyl alcohol, acetoacetyl-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol and silicon-modified polyvinyl alcohol, starch and derivatives thereof, cellulose derivatives such as hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose and ethyl cellulose, polyvinyl pyrrolidone, acrylamide-acrylate copolymers, acrylamide-acrylate-methacrylate copolymers, styrene-maleic anhydride copolymers, isobutylene-maleic anhydride copolymers, water-soluble adhesives such as casein and gelatin, and vinyl acetate latex, urethane latex, acrylic latex, styrene-butadiene latex, and the like.

The binder content is preferably about 10 to 50% by mass, particularly preferably about 15 to 40% by weight, based on the total solid content in the thermosensitive recording layer.

Examples of the auxiliary agent include surfactants, waxes, lubricants, pigments, water-resistant agents, antifoaming agents, fluorescent dyes, and coloring dyes.

Among the surfactants, there may be included, for example, dioctyl sodium sulfosuccinate, sodium dodecylbenzenesulfonate, lauryl sulfate, sodium salt, and the like.

Waxes may include polyethylene wax, palm wax, paraffin wax, ester wax, and the like.

The lubricant may include higher fatty acid metal salts such as zinc stearate and calcium stearate.

Pigments may include, for example, kaolin, clay, talc, calcium carbonate, calcined kaolin, titanium oxide, amorphous silica, aluminum hydroxide, and the like.

Water-resistant agents may include, for example, glyoxal, formalin, glycine, glycidyl esters, glycidyl ethers, dimethylol urea, diketene, dialdehyde starch, melamine resins, polyamide-epichlorohydrin resins, ketone-aldehyde resins, borax, boric acid, zirconium ammonium carbonate, epoxies, and the like.

The heat-sensitive recording layer may contain a storage stability improver as long as the desired effects of the present invention are not impaired.

The storage stability improver may include, for example, 1, 3-tris (2-methyl-4-hydroxy-5-cyclohexylphenyl) butane, 1, 3-tris (2-methyl-4-hydroxy-5-tert-butylphenyl) butane, 1-bis (4-hydroxyphenyl) -1-phenylethane, 2 ' -ethylidenebis (4, 6-di-tert-butylphenyl), 1, 4-bis [ d-methyl- α - (4 ' -hydroxyphenyl ] ethyl ]) benzene, 1, 3-bis [ α -methyl- α - (4-hydroxyphenyl ] ethyl ]) benzene, 4 ' -thiobis (2-methyl-6-tert-butylphenol), 1, 3, 5-tris (4-tert-butyl-3-hydroxy-2, 6-dimethylbenzyl) isocyanuric acid, and the like.

When the storage stability improver is contained, the amount of the storage stability improver is about 10 to 40% by mass, preferably about 15 to 30% by mass, based on the total amount of the color-developing agents.

The thermosensitive recording layer may contain a sensitizer which can be contained in the composite particles, an ultraviolet absorber for improving light resistance, and a hindered phenol, a hindered amine, or the like as a stabilizer, as necessary.

The thermosensitive recording layer may have a structure of a multi-layer color-developing layer in which a plurality of leuco dyes are arranged in different layers, or may have a structure of a single-layer color-developing layer in which a plurality of leuco dyes are contained in the same layer.

In the case where the support is a transparent thermosensitive recording medium, a single-layer color-developing layer structure in which a plurality of leuco dyes are contained in the same layer is preferable from the viewpoint of haze.

The heat-sensitive recording layer can be formed by a known appropriate coating method without limitation to the method for forming the heat-sensitive recording layer. Examples of the known coating method include air knife coating, adjustable bar coating, blade coating, bar coating, short dwell coating, curtain coating, die coating, and gravure coating.

II. support body

The support used for the thermosensitive recording medium of the present invention is not particularly limited, and for example, a wood-free paper, a paper subjected to lamination processing, a synthetic paper, a film base material such as polyethylene terephthalate, polypropylene, polyethylene, or the like can be used. The film substrate may be transparent, and may also be a colored transparent film.

The thickness of the support is about 20 to 200 μm, preferably about 75 to 200 μm.

When the support is a transparent film, the effect of the present invention of sufficiently suppressing the fluctuation of the recording sensitivity due to the change in the environmental humidity and exhibiting excellent image quality can be exhibited, and therefore, the present invention is preferable.

A surface of the support may be provided with a anchor coat layer or subjected to corona discharge treatment in order to improve adhesiveness to the thermosensitive recording layer. A conductive treatment based on a conductive agent may be further performed.

Thermosensitive recording Medium

The thermosensitive recording medium of the present invention can be produced by applying a coating liquid for a thermosensitive recording layer on at least one side of a support, drying the coating liquid, and forming a thermosensitive recording layer.

In addition, an undercoat layer may be provided between the support and the thermosensitive recording layer, if necessary.

A coating liquid for an undercoat layer containing a pigment and a binder is applied to a support and dried to form the undercoat layer. Various pigments can be used as the pigment, and inorganic pigments such as calcined clay and organic pigments such as hollow or dense plastic pigments are generally exemplified. As the binder, the binder used for the thermosensitive recording layer and the like can be used.

When the undercoat layer is provided, the amount of the coating is 1 to 30g/m in terms of the mass after drying²About, preferably 5 to 20g/m²Left and right.

In the thermosensitive recording medium of the present invention, a protective layer may be provided on the thermosensitive recording layer.

The protective layer is generally obtained by applying a coating liquid for a protective layer, which is obtained by mixing and stirring a binder and, if necessary, a pigment or various auxiliary agents with water as a medium, to the thermosensitive recording layer and drying the coating liquid.

As the binder, the same one as that used for the thermosensitive recording layer can be cited. Examples thereof include polyvinyl alcohols such as partially or completely saponified polyvinyl alcohol, carboxyl-modified polyvinyl alcohol, acetoacetyl-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, and silicon-modified polyvinyl alcohol, starch and its derivatives, cellulose derivatives such as hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, and ethyl cellulose, polyvinylpyrrolidone, acrylamide-acrylate copolymer, acrylamide-acrylate-methacrylate copolymer, styrene-maleic anhydride copolymer, isobutylene-maleic anhydride copolymer, water-soluble binders such as casein and gelatin, and vinyl acetate latex, urethane latex, acrylic latex, and styrene-butadiene latex.

The amount of the binder is not particularly limited, and is preferably about 30 to 90 mass%, and particularly preferably about 40 to 80 mass% with respect to the total solid content of the protective layer.

Examples of the pigment include kaolin, clay, talc, calcium carbonate, calcined kaolin, titanium oxide, amorphous silica, and aluminum hydroxide, and the pigment having a volume average particle diameter of 1 μm or less is preferable as the particle diameter.

When the pigment is contained, the content is not particularly limited, and is preferably about 10 to 60 mass%, more preferably about 15 to 50 mass% with respect to the entire solid content of the protective layer.

Examples of the auxiliary agents include surfactants, waxes, lubricants, water-resistant agents, antifoaming agents, fluorescent dyes, and coloring dyes.

Surfactants may include, for example, dioctyl sodium sulfosuccinate, sodium dodecylbenzene sulfonate, lauryl sulfate-sodium salt, perfluoroalkyl ethylene oxide addition products, and the like.

The wax may include, for example, paraffin wax such as polyethylene wax and palm wax, and ester wax such as potassium stearyl phosphate.

The lubricant may include higher fatty acid amides such as stearic acid amide and ethylene bis-stearic acid amide, higher fatty acid metal salts such as zinc stearate and calcium stearate, and the like.

The water-resistant agent includes glyoxal, formalin, glycine, glycidyl ester, glycidyl ether, dimethylol urea, diketene, dialdehyde starch, melamine resin, polyamide-epichlorohydrin resin, ketone-aldehyde resin, adipic dihydrazide, borax, boric acid, zirconium ammonium carbonate, epoxy compounds, and the like.

In the case where the auxiliary is contained, the amount thereof to be used may be appropriately set from a wide range.

The method of applying the coating liquid for a protective layer is not particularly limited, and a known method can be appropriately used. For example, air knife coating, adjustable bar coating, blade coating, bar coating, short dwell coating, curtain coating, die coating, gravure coating, and the like may be used.

The coating amount of the protective coating liquid is 0.5 to 10g/m in terms of dry weight²About, preferably 2 to 5g/m²Left and right.

The thermosensitive recording medium of the present invention may be subjected to smoothing treatment by supercalendering after various layers are formed or all layers are formed.

The thermosensitive recording medium of the present invention may be added with various known techniques in the field of manufacturing other thermosensitive recording media as necessary.

The thermosensitive recording medium of the present invention thus obtained has little change in recording sensitivity due to changes in environmental humidity, and is excellent in thermal resistance to background fogging and image quality.

Effects of the invention

The heat-sensitive recording medium of the present invention comprises a support and a heat-sensitive recording layer, wherein the heat-sensitive recording layer contains a leuco dye in the form of composite particles comprising the leuco dye and a hydrophobic resin, and a color-developing agent comprising 4, 4 '-cyclohexylidenebisphenol and 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane, and is characterized in that the recording sensitivity is less fluctuated by changes in ambient humidity, and the image quality and heat-resistant background fogging properties are excellent.

In particular, when the developer is obtained by wet-pulverizing a composition containing 4, 4 '-cyclohexylidenebisphenol and 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane, the image quality is further improved.

Further, the color developer, when further containing 4-hydroxy-4' -allyloxydiphenylsulfone, has a feature of preventing a whitening phenomenon of the thermosensitive recording medium.

Further, when the developer further contains N- (p-toluenesulfonyl) -N' -phenylurea, it is characterized in that particularly fine image gradation can be obtained in a halftone area.

The thermosensitive recording medium of the present invention having such characteristics is suitably used for medical use, particularly for a transparent thermosensitive recording medium for image diagnosis, etc.

Drawings

Fig. 1 is a graph illustrating the relationship between applied energy and recording density in the thermosensitive recording media obtained in example 5 and example 7.

Detailed Description

The present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.

Unless otherwise stated, "part" and "%" represent "part by mass" and "% by mass", respectively. The volume average particle diameter is a value measured by using a laser diffraction particle size distribution meter SALD2000 (manufactured by shimadzu corporation).

Example 1

Preparation of solution A (composite particle Dispersion)

A leuco dye and an ultraviolet absorber were dissolved in a solvent comprising a polyisocyanate compound at 150 ℃ under heating to prepare a solution.

As the leuco dye, 11 parts of 3-diethylamino-6-methyl-7- (3-toluylamino) fluoran, 6 parts of 3-diethylamino-6, 8-dimethylfluoran, and 5 parts of 3, 3-bis (4-diethylamino-2-ethoxyphenyl) -4-azaphthalide were used.

10 parts of 2-hydroxy-4-octoxybenzophenone is used as the ultraviolet absorber. 2 parts of dicyclohexylmethane-4, 4' -diisocyanate (デスモジュ - ル W, manufactured by Sumitomo バイエルウレタン Co.) and 12 parts of m-tetramethylxylylene diisocyanate (TMXDI, manufactured by Mitsui Wuta chemical Co., Ltd.) were used as the polyvalent isocyanate compound.

This solution was slowly added to 100 parts of an aqueous solution containing 8.5 parts of polyvinyl alcohol (ポバ - ル PVA-217EE, manufactured by クラレ Co.) and 1.5 parts of an ethylene oxide addition product of acetylene alcohol (オルフイン 31010, manufactured by Nissan chemical Co., Ltd.) as a surfactant, and emulsified and dispersed by stirring at 10000rpm using a homogenizer.

To this emulsion dispersion, an aqueous solution prepared by dissolving 2.5 parts of a polyamine compound (Epicure T, manufactured by shell international petroleum co., ltd.) in 22.5 parts of water and 30 parts of water were added to make uniform the mixture.

The obtained emulsion dispersion was heated to 75 ℃ and subjected to polymerization reaction for 7 hours, thereby obtaining a leuco dye-containing composite particle dispersion having a volume average particle diameter of 0.8 μm. The obtained liquid was diluted with water to prepare a leuco dye-containing composite particle dispersion containing 25% of leuco dye.

Preparation of solution B (developer Dispersion)

A composition comprising 42 parts of 4, 4' -cyclohexylidenebisphenol, 60 parts of a 10% aqueous solution of sulfone-modified polyvinyl alcohol and 18 parts of water was pulverized with an ultrahigh-viscosity mill to a volume average particle diameter of 0.28. mu.m, to obtain a developer dispersion.

Preparation of solution C (developer Dispersion)

A composition comprising 42 parts of 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane, 60 parts of a 10% aqueous solution of sulfone-modified polyvinyl alcohol and 18 parts of water was pulverized with an ultrahigh-viscosity mill to a volume average particle diameter of 0.30. mu.m, to obtain a developer dispersion.

(preparation of coating solution for thermosensitive recording layer)

A coating liquid for a thermal recording layer was obtained by mixing and stirring a composition comprising 100 parts of the liquid a, 100 parts of the liquid B, 20 parts of the liquid C, 60 parts of a butadiene-styrene latex (Smartex PA-9280, manufactured by japan a & L) having a solid content concentration of 48%, and 30 parts of water.

(preparation of coating solution for protective layer)

100 parts of ionomer-type urethane-based latex [ manufactured by Dainippon ink chemical industry Co., Ltd., HYDRAN (registered trademark) AP-30F, solid concentration 20% ] and acetoacetyl-modified polyvinyl alcohol [ manufactured by Nippon synthetic chemical industry Co., Ltd., GOHSEFIMER (registered trademark) Z-410, polymerization degree: about 2300, degree of saponification: about 98 mol% ] of an 8% aqueous solution 500 parts, a polyamidoamine-epichlorohydrin 25% aqueous solution 5 parts, kaolin (UW-90, manufactured by Angel corporation) of 60% slurry having a volume average particle diameter of 0.8 μm 50 parts, stearic acid amide (HyMICRON L-271, manufactured by Zhongjing grease Co., Ltd., a solid concentration of 25%) 26 parts, stearyl potassium phosphate (WOOPOL 1800, manufactured by Songbu grease pharmaceuticals Co., Ltd., a solid concentration of 35%) 4 parts, a perfluoroalkyl ethylene oxide adduct [ manufactured by Qing beauty chemical Co., Ltd., SURLON S-145], 10% aqueous solution 15 parts, and water 300 parts were mixed and stirred to obtain a coating liquid for a protective layer.

Production of thermosensitive recording Medium

A coating liquid for a heat-sensitive recording layer was applied to one surface of a blue transparent polyethylene terephthalate film (メリネックス 912, 175 μm thick, Dupont, Techni) by means of a slit die coater, and dried so that the amount of the coating liquid applied after drying became 20g/m²A thermosensitive recording layer is provided. On the surface, the coating liquid for the protective layer was applied by a slit die coater and dried so that the amount of the coating after drying was 3.5g/m²A protective layer was provided, thereby obtaining a thermosensitive recording medium.

Example 2

A thermosensitive recording medium was obtained in the same manner as in example 1, except that 50 parts of the liquid B and 50 parts of the liquid D described below were used in place of 100 parts of the liquid B in the preparation of the coating liquid for a thermosensitive recording layer in example 1.

Preparation of solution D (developer Dispersion)

A composition comprising 42 parts of N- (p-toluenesulfonyl) -N' -phenylurea, 60 parts of a 10% aqueous solution of sulfone-modified polyvinyl alcohol and 18 parts of water was pulverized to a volume average particle diameter of 0.30 μm using an ultrahigh-viscosity mill to obtain a developer dispersion.

Example 3

A thermosensitive recording medium was obtained in the same manner as in example 1, except that 45 parts of the liquid B, 45 parts of the liquid D, and 10 parts of the liquid E described below were used in place of 100 parts of the liquid B in the preparation of the coating liquid for a thermosensitive recording layer in example 1.

Preparation of E solution (developer Dispersion)

A composition comprising 42 parts of 4-hydroxy-4' -allyloxydiphenesulfone, 60 parts of a 10% aqueous solution of sulfone-modified polyvinyl alcohol and 18 parts of water was pulverized to a volume average particle diameter of 0.30 μm using an ultrahigh-viscosity mill to obtain a color former dispersion.

Example 4

A thermosensitive recording medium was obtained in the same manner as in example 1, except that 120 parts of the following F liquid was used in place of 100 parts of the B liquid and 20 parts of the C liquid in the preparation of the coating liquid for a thermosensitive recording layer in example 1.

Preparation of F liquid (developer Mixed Dispersion)

A composition comprising 26 parts of 4, 4-cyclohexylidenebisphenol, 16 parts of 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane, 60 parts of a 10% aqueous solution of sulfone-modified polyvinyl alcohol and 18 parts of water was pulverized with an ultrahigh-viscosity mill to a volume average particle diameter of 0.28 μm to obtain a developer mixed dispersion.

Example 5

A thermosensitive recording medium was obtained in the same manner as in example 1, except that 100 parts of the G liquid and 20 parts of the D liquid described below were used in place of 100 parts of the B liquid and 20 parts of the C liquid in the preparation of the coating liquid for a thermosensitive recording layer in example 1.

G liquid Dispersion (preparation of developer Mixed Dispersion)

A composition comprising 23 parts of 4, 4-cyclohexylidenebisphenol, 14 parts of 4, 4-bis (N-p-toluenesulfonylaminocarboylamido) diphenylmethane, 5 parts of 4-hydroxy-4' -allyloxydiphenylsulfone, 60 parts of a 10% aqueous solution of sulfone-modified polyvinyl alcohol and 18 parts of water was pulverized with an ultrahigh-viscosity mill to a volume average particle diameter of 0.28 μm to obtain a developer mixed dispersion.

Example 6

A thermal recording medium was obtained in the same manner as in example 1, except that 66 parts of the liquid B, 29.5 parts of the liquid C, and 24.5 parts of the liquid E were used instead of 100 parts of the liquid B and 20 parts of the liquid C in the preparation of the coating liquid for a thermal recording layer in example 1.

Example 7

A thermosensitive recording medium was obtained in the same manner as in example 1, except that 120 parts of the following H liquid was used in place of 100 parts of the B liquid and 20 parts of the C liquid in the preparation of the coating liquid for a thermosensitive recording layer in example 1.

H liquid Dispersion (preparation of developer Mixed Dispersion)

A composition comprising 23 parts of 4, 4-cyclohexylidenebisphenol, 10 parts of 4, 4-bis (N-p-toluenesulfonylaminocarboylamido) diphenylmethane, 9 parts of 4-hydroxy-4' -allyloxydiphenylsulfone, 60 parts of a 10% aqueous solution of sulfone-modified polyvinyl alcohol and 18 parts of water was pulverized with an ultrahigh-viscosity mill to a volume average particle diameter of 0.28 μm to obtain a developer mixed dispersion.

Comparative example 1

A thermosensitive recording medium was obtained in the same manner as in example 1, except that 20 parts of the C liquid was not used in the preparation of the coating liquid for a thermosensitive recording layer in example 1.

Comparative example 2

A thermosensitive recording medium was obtained in the same manner as in example 1, except that 50 parts of the B liquid and 50 parts of the D liquid were used instead of 100 parts of the B liquid and 20 parts of the C liquid were not used in the preparation of the coating liquid for a thermosensitive recording layer in example 1.

Comparative example 3

A thermosensitive recording medium was obtained in the same manner as in example 1, except that 100 parts of the following liquid I was used instead of 100 parts of the liquid B, and 20 parts of the liquid C was not used in the preparation of the coating liquid for a thermosensitive recording layer in example 1.

I liquid Dispersion (preparation of developer Dispersion)

A composition comprising 42 parts of 3, 3 '-diallyl-4, 4' -dihydroxydiphenyl sulfone, 60 parts of a 10% aqueous solution of sulfone-modified polyvinyl alcohol and 18 parts of water was pulverized to a volume-average particle diameter of 0.30 μm using an ultrahigh-viscosity mill to obtain a developer dispersion.

Comparative example 4

A thermosensitive recording medium was obtained in the same manner as in example 1, except that 100 parts of the following J liquid was used instead of 100 parts of the B liquid, and 20 parts of the C liquid was not used in the preparation of the coating liquid for a thermosensitive recording layer in example 1.

Preparation of J liquid (developer Dispersion liquid)

A composition comprising 42 parts of 4-hydroxy-4' -isopropoxydiphenyl sulfone, 60 parts of a 10% aqueous solution of sulfone-modified polyvinyl alcohol and 18 parts of water was pulverized to a volume average particle diameter of 0.30 μm using an ultrahigh-viscosity mill, to obtain a developer dispersion.

Comparative example 5

A thermosensitive recording medium was obtained in the same manner as in example 1, except that 120 parts of the following K liquid was used instead of 100 parts of the B liquid and 20 parts of the C liquid was not used in the preparation of the coating liquid for a thermosensitive recording layer in example 1.

Preparation of K Dispersion (developer Dispersion)

A composition comprising 42 parts of 4, 4' -dihydroxydiphenyl sulfone, 60 parts of a 10% aqueous solution of sulfone-modified polyvinyl alcohol and 18 parts of water was pulverized to a volume average particle diameter of 0.30 μm using an ultrahigh-viscosity mill to obtain a developer dispersion.

Comparative example 6

A thermosensitive recording medium was obtained in the same manner as in example 1, except that 35 parts of the following L liquid and 55 parts of water were used instead of 100 parts of the a liquid and 30 parts of water in the preparation of the coating liquid for a thermosensitive recording layer in example 1.

Preparation of L liquid (leuco dye dispersion)

A composition comprising 11 parts of 3-diethylamino-6-methyl-7- (3-toluylamino) fluoran, 6 parts of 3-diethylamino-6, 8-dimethylfluoran, 5 parts of 3, 3-bis (4-diethylamino-2-ethoxyphenyl) -4-azaphthalide, 10 parts of 2-hydroxy-4-octyloxybenzophenone, 20 parts of a 10% aqueous solution of sulfone-modified polyvinyl alcohol and 46 parts of water was pulverized to a volume average particle diameter of 0.6. mu.m using an ultrahigh-viscosity mill.

(evaluation)

The thermosensitive recording medium thus obtained was evaluated as follows.

(recording Density)

The thermosensitive recording media obtained in examples and comparative examples were left to stand in an environment of 23 ℃ x 50% RH for 24 hours, and then recorded in a grayscale mode using a thermal printer UP-DF500 (manufactured by sony corporation). Next, the concentration of the recorded portion was measured in a visible mode using a Macbeth densitometer (trade name: model TU-927J, manufactured by Macbeth corporation). The recording densities of the 9 th and 16 th gradation levels (levels) in all the 16 gradation levels (levels) are shown in table 1. In addition, the recording densities of all 16 gradations (levels) of the thermosensitive recording medium obtained in examples 5 and 7 are graphically shown in fig. 1.

Similarly, after the thermosensitive recording medium was left to stand in an environment of 23 ℃ x 20% RH for 24 hours, recording was performed in a grayscale mode using a thermal printer UP-DF500 (manufactured by sony corporation). Subsequently, the concentration of the recorded portion was measured using the visible mode of the Macbeth densitometer described above. The 9 th recording density is shown in table 1 for all 16 gradations.

(fluctuation in recording sensitivity due to changes in ambient humidity)

From the recording density of the 9 th tone (level) recorded in the environment of 23 ℃ x 50% RH and the recording density of the 9 th tone (level) recorded in the environment of 23 ℃ x 20% RH obtained in the above recording density measurement, the variation percentage was obtained by the following formula 1, and this was evaluated as the variation in recording sensitivity due to the change in the ambient humidity. The results are shown in Table 1.

Formula 1

Percent change of [ (a-B)/B ] x 100

(A represents a recording concentration value in an environment of 23 ℃ C. times.20% RH; B represents a recording concentration value in an environment of 23 ℃ C. times.50% RH.)

Among them, when the absolute value of the percentage change is 25% or more, there is a practical problem.

(evaluation of fogging resistance against Heat)

After the thermosensitive recording medium was subjected to a treatment at 60 ℃ C.. times.20% RH for 24 hours, the unrecorded portion was measured in a visible mode using a Macbeth densitometer (trade name: model TR-927J, manufactured by Macbeth corporation).

(evaluation of image quality)

The coating liquids for thermal recording medium layers of examples 1 to 7 and comparative examples 1 to 6 were left to stand in an environment of 23 ℃ x 50% RH for 3 days, and then thermal recording media were produced. After each of the thermosensitive recording media thus obtained was left to stand in an environment of 23 ℃ x 50% RH for 24 hours, 10 sheets (17 x 14 inches/1 sheet) were recorded in a full-scale gradation pattern using a thermal printer UP-DF500 (manufactured by sony corporation), and the number of black clumps (number of defects) generated in the thermosensitive recording medium was visually measured and evaluated for image quality as follows. The results are shown in Table 1. When the number of defects is more than 5, there is a practical problem.

Very good: 0 number of

O: 1 to 4

And (delta): 5 to 10

X: more than 11

(evaluation of whitening prevention)

After the thermosensitive recording medium was left to stand in an environment of 23 ℃ x 50% RH for 24 hours, recording was performed in a grayscale mode using a thermal printer UP-DF500 (manufactured by sony corporation), a treatment was performed for 24 hours in an environment of 40 ℃ x 90% RH, and the degree of whitening of the recording portion after the treatment was measured visually and evaluated as follows. The results are shown in Table 1.

Very good: no whitening occurs.

O: whitening slightly occurs.

X: the whitening is serious, and the density of the recorded portion is reduced, which causes practical problems.

TABLE 1

As is clear from table 1, the heat-sensitive recording media of the examples of the present application showed a small change in recording density due to a change in ambient humidity, with a percentage change of 20% or less. Further, the heat resistance fogging resistance was 0.3 or less, and it was found that the heat resistance fogging resistance was also excellent. Further, the image quality may be evaluated to be not less than o, indicating that the image quality is also excellent.

In particular, it was found that the developer was a thermal recording medium obtained by wet-pulverizing 4, 4 '-cyclohexylidenebisphenol and 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamino) diphenylmethane (examples 4, 5 and 7), and the image quality was excellent as evaluated as "x" or more.

Further, it was found that the heat-sensitive recording media (examples 3 and 5 to 7) in which the color developer further contained 4-hydroxy-4' -allyloxydiphenesulfone were excellent in the evaluation of the whitening prevention as excellent as or better than that.

As shown in fig. 1, it was found that the thermal recording medium further containing N- (p-toluenesulfonyl) -N '-phenylurea (example 5) suppressed the change in recording sensitivity with respect to the applied energy and the fluctuation in recording density in the halftone region, as compared with the thermal recording medium not containing N- (p-toluenesulfonyl) -N' -phenylurea (example 7). From this, it was found that by using N- (p-toluenesulfonyl) -N' -phenylurea together, a very fine gradation property particularly in a halftone region can be achieved.

Industrial applicability of the invention

The thermosensitive recording medium of the present invention is characterized by less change in recording sensitivity to changes in environmental humidity and excellent image quality and thermal resistance to background fogging.

The thermosensitive recording medium of the present invention having such a feature can be suitably used for various applications requiring a constant recording density at all times.

The thermosensitive recording medium of the present invention is suitable for a thermosensitive recording medium for medical use, which is capable of recording in a large number of pairs at a gradation level and is particularly required to have a recording density that is always constant even when the humidity of the environment changes, and is particularly suitable for use as a thermosensitive recording medium used for image diagnosis such as MRI image diagnosis and X-ray image diagnosis.

Claims (12)

1. A thermosensitive recording medium having a support and a thermosensitive recording layer, wherein,

the thermosensitive recording layer contains a leuco dye and a color developer,

the leuco dye is in the form of composite particles comprising the leuco dye and a hydrophobic resin,

the developer comprises 4, 4 '-cyclohexylidenebisphenol and 4, 4' -bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane.

2. The thermosensitive recording medium according to claim 1, wherein the color developer is obtained by wet-pulverizing a composition comprising 4, 4 '-cyclohexylidenebisphenol and 4, 4' -bis (N-p-toluenesulfonylaminocarboylamido) diphenylmethane.

3. The thermosensitive recording medium according to claim 1, wherein the 4, 4 '-bis (N-p-toluenesulfonylaminocarbonylamido) diphenylmethane is 15 to 75 mass% with respect to the 4, 4' -cyclohexylidenebisphenol.

4. The thermosensitive recording medium according to claim 1, wherein the developer further contains 4-hydroxy-4' -allyloxydiphenesulfone.

5. The thermosensitive recording medium according to claim 4, wherein the developer is obtained by wet-pulverizing a composition comprising 4, 4 ' -cyclohexylidenebisphenol, 4 ' -bis (N-p-toluenesulfonylaminocarboylamido) diphenylmethane, and 4-hydroxy-4 ' -allyloxydiphenylsulfone.

6. The thermosensitive recording medium according to claim 4, wherein the 4-hydroxy-4 '-allyloxydiphenesulfone is 10 to 50% by mass with respect to the 4, 4' -cyclohexylidenebisphenol.

7. The thermosensitive recording medium according to claim 1, wherein the color developer further contains N- (p-toluenesulfonyl) -N' -phenylurea.

8. The thermosensitive recording medium according to claim 7, wherein the N- (p-toluenesulfonyl) -N '-phenylurea is 25 to 150% by mass with respect to 4, 4' -cyclohexylidenebisphenol.

9. The thermosensitive recording medium according to claim 1, wherein the hydrophobic resin forming the composite particles is a urea-based resin or a urea-urethane-based resin.

10. The thermosensitive recording medium according to claim 1, wherein the support is a transparent film.

11. The thermosensitive recording medium according to claim 1, further comprising a protective layer.

12. The thermosensitive recording medium according to claim 1, wherein the developer further contains 4-hydroxy-4 '-allyloxydiphenylsulfone and N- (p-toluenesulfonyl) -N' -phenylurea.