JP5470940B2 - Thermal recording medium - Google Patents

Description

  The present invention relates to a heat-sensitive recording medium utilizing a color developing reaction between an electron-donating color-forming compound and an electron-accepting compound, and more particularly to a heat-sensitive recording medium suitable as a medical image forming sheet.

Conventionally, a heat-sensitive recording layer containing an electron-donating color-forming compound (hereinafter also referred to as a color former) and an electron-accepting compound (hereinafter also referred to as a developer) is formed on a support such as paper. Thermal recording media using a color development reaction are widely known. This thermal recording medium recording system has advantages such as a compact and inexpensive recording apparatus and easy maintenance, and is used for facsimile printing, automatic ticket vending machines, scientific measurement printers, POS barcode printing, etc. This printer is used in a wide range of applications such as printers for CRTs and printers for CRT medical measuring instruments.
In this recording method, heat is directly transferred from the thermal head and recording is performed by heating. Therefore, the surface layer of the thermal recording medium that is in direct contact with the thermal head (generally called an over layer or a protective layer) is used as a resin for the protective layer. Various inventions that devise lubricants, fillers and the like have been issued.

Further, in recent years, there has been a demand for higher recording speed and higher definition, and accordingly, there is a tendency to increase the applied power to one element of the thermal head.
In addition, since a thermal recording medium for medical use is used for diagnosis and reference, the recording target is mainly structural information or shape information such as a human internal organ or bone. Therefore, it is important for the image to be recognized to accurately reflect the original shape information, and it is expected to have excellent blackness, high gradation, high glossiness, and excellent contrast, contrast, etc. Has been.
Therefore, in medical applications, in order to achieve higher gradation than the conventional leuco dye-type thermal recording medium, higher output recording is required, and the outermost surface of the thermal recording medium and the thermal head are heated. The mold release function and slip function are very important.
In order to meet these demands, various inventions have been made, and many inventions have been made that include a lubricant in the surface layer. As lubricants, metal salts of fatty acids are widely known, and among them, zinc stearate is used in many fields. (Patent Documents 1 to 4)

Furthermore, in medical applications, uniform image quality from low density to high density is required.
Therefore, by including a pigment and a lubricant present in a liquid at the time of printing, the protective layer aimed to solve the problems of image quality from low density to high density, head fixation, and white powder on the film surface after printing. The invention is known. (Patent Document 5)
However, in recent medical applications, further higher density, higher gradation, and higher image quality are required, and it is required that the head is not fixed on the head heating element even when continuously printing at high output. It has been found that adhesion of a melt (printing white powder) to a portion where a heat-sensitive recording medium comes into contact becomes a serious problem in a field requiring high definition and high gradation.
In consideration of these new problems, the above-described known metal salts of fatty acids cannot be used to prevent both the sticking residue generation and the printing white powder generation that accompany the head fixing, and therefore a new improvement is required. As far as the present inventors know, there is no literature regarding the technology that can prevent the adhesion of printed white powder even in a halftone image that is particularly important in medical applications.

  The present invention is a heat-sensitive recording medium utilizing a color developing reaction between a color former and a developer, maintaining high gloss, free from image defects due to head adhering debris, and generating white powder during printing. In addition, the present invention has an object to provide a thermal recording medium in which sticking by printing is good and high gradation in halftone can be realized.

As a result of intensive studies, the present inventors have simultaneously satisfied the prevention of head sticking, which tends to be in a trade-off relationship, and the prevention of printing white powder, while maintaining high glossiness and good head matching of the image. Has found that a zinc salt of a specific fatty acid may be contained as a lubricant in the uppermost layer of the thermosensitive recording medium provided with the thermosensitive recording layer and the uppermost layer.
That is, the above-mentioned problems are solved by the following inventions 1) to 6 ).
1) A heat-sensitive recording medium having at least a heat-sensitive recording layer and an uppermost layer on a support, wherein the heat-sensitive recording layer contains a binder, a color former and a developer, and the uppermost layer has a general formula ( A heat-sensitive recording medium comprising a fatty acid zinc salt represented by 1).
(N is an integer of _{11~13) (C n H 2n +} 1 COO) 2 -Zn ... (1)
2 ) The heat-sensitive recording medium according to 1), wherein the zinc salt of the fatty acid is n = 11 zinc laurate.
3 ) The thermosensitive recording medium according to 1) or 2) , wherein the uppermost layer contains paraffin wax as a lubricant.
4 ) The thermal recording medium according to any one of 1) to 3 ), wherein the uppermost layer contains an inorganic pigment, and at least one of the inorganic pigments is calcium carbonate.
5 ) The thermal recording medium according to any one of 1) to 4 ), wherein the lubricant has a volume average particle diameter of 1.0 μm or less.
6 ) The thermal recording medium according to any one of 1) to 5 ), wherein the inorganic pigment has a volume average particle size of 0.3 to 0.6 μm.

  According to the present invention, while maintaining high glossiness, there is no image defect due to head sticking residue, no white powder is generated during printing, and sticking by printing is excellent, and an excellent high gradation thermal recording medium. Can provide. In particular, it can be used for diagnosis and reference of images such as X-ray, MRI, CT, etc. and has the same high image quality, glossiness, high density, and high gradation as silver halide film, and excellent head matching. A reflection type heat-sensitive recording medium suitable for medical use can be provided.

Hereinafter, the present invention will be described in detail.
In the present invention, the zinc salt of the fatty acid represented by the general formula (1) is added as a lubricant to the uppermost layer in order to satisfy the prevention of head sticking and the prevention of white print generation while maintaining high gloss.
Here, the uppermost layer is a layer generally called a protective layer or an over layer, and usually contains a binder resin, a lubricant, a pigment (filler), and the like.
Among known lubricants composed of metal salts of fatty acids, it is known that zinc stearate is very good at preventing head sticking. It is also generally known that for higher printing energy, the effect is increased by increasing the content of these lubricants.
However, it has been found that when the metal salt of stearic acid is melted, the melt is easily cooled and solidified, and white powder due to printing is likely to be generated. This printed white powder is not a major problem for general facsimiles, automatic ticket vending machines, scientific measurement printers, POS barcode printers, etc., but requires high-definition and high-gradation images. In such applications, the halftone image area is affected and improvement is required.
However, the prevention of head sticking and the prevention of printing white powder are contradictory matters, and it is impossible to achieve both with the known techniques so far.

As a result of extensive studies, it was first found that a zinc salt of a fatty acid is effective in preventing head sticking due to heat of printing. Further, in order to improve the white powder generation of the print while preventing the sticking of the print, it is effective to reduce the molecular weight of the fatty acid. If the zinc salt of the fatty acid represented by the general formula (1) is used, compatibility can be achieved. I found it possible. Particularly preferred is zinc laurate (n = 11). n needs to be in the range of 11-13 .
The volume average particle size of the fatty acid zinc salt is preferably in the range of 1.0 μm or less. If it exceeds 1.0 μm, the glossiness of the surface tends to be lowered, and a high gloss image quality and a tendency to impair the high density are observed. On the other hand, if the particle size becomes too small, the effect of preventing head sticking may be reduced. However, the current miniaturization technique has a limit of about 0.01 μm, and there is no practical problem if the particle size is this level. .

In particular, in the recording of medical images where high gradation is required, it is necessary to cope with various thermal energy from the thermal head depending on the image to be recorded, and two or more kinds of lubricant fine particles having different melting points should be used in combination. Is preferred. Examples of the lubricant used together with the fatty acid zinc salt include paraffin wax, polyethylene wax, carnauba wax, mondanic acid wax, rice wax, and beeswax. By melting and using these lubricants, for all images from low printing rate to high printing rate, head sticking prevention, printing white powder generation prevention, sticking prevention, and image high glossiness, regardless of printing rate, Pure blackening of the image can be achieved.
When paraffin wax is used, the melting point is preferably 40 to 100 ° C. If the melting point is less than 40 ° C., blocking occurs when the thermal recording medium is stored in a high temperature environment, or the thermal recording medium is heated after the thermal recording of the thermal head. There arises a problem that a low melting point lubricant bleeds on the surface and precipitates such as white powder are generated. Further, when the melting point exceeds 100 ° C., it is difficult to melt particularly in an energy part having a low halftone, and therefore there is a problem that the effect of releasability of sticking is small.
In addition, the volume average particle diameter of the paraffin wax is preferably in the range of 0.01 to 1.0 μm in order to achieve the anti-sticking effect and high gloss, like the zinc salt of the fatty acid.

In order to refine the lubricant to a volume average particle size of 0.01 to 1.0 μm, various known methods such as an emulsification method and a pulverization method using various beads can be used.
When miniaturizing with an aqueous medium, it is difficult to miniaturize only with a lubricant alone, and it is preferable to add a known water-soluble resin or surfactant to make the micronized. In particular, metal salts and amides of higher fatty acids have a poor affinity with water, and many of them are softer, and problems with the stability of the liquid when miniaturized are likely to occur.
For these problems, it is considered to add a surfactant as a dispersant or an emulsifier. As surfactants, the use of anionic surfactants is conceivable. In particular, sulfonic acid-based and carboxylic acid-based surfactants often used in other fields for dispersion miniaturization are dispersible. And excellent in liquid stability. However, when an aziridine compound is used as the cross-linking agent of the uppermost layer of the present invention, these surfactants are reactive, so that the cross-linking reaction proceeds in the liquid over time after the addition of the cross-linking agent. As a result, problems such as increase in viscosity of the liquid and reduction in gloss due to aggregation of the lubricant fine particle dispersion occur. Further, it is known that cationic surfactants cause problems such as corrosiveness to the thermal head and fogging of the thermal recording material.

Therefore, it is preferable to use a nonionic surfactant in the lubricant dispersion used in the present invention. As the nonionic surfactant, those generally known can be used. The surfactant can be selected in consideration of the compatibility between water and the dispersion from the HLB value and the like.
Furthermore, ether type surfactants have a good effect on metal salts and amide compounds of higher fatty acids, which have been particularly difficult to refine, and further, phenyl ether type surfactants have a desired particle size. It was suitable for miniaturization.
Regarding these surfactants, two or more surfactants may be used in combination in consideration of wettability, dispersibility, foam suppression and antifoaming properties. Further, a water-soluble resin may be used in consideration of the dispersion stability of fine particles when miniaturized, and various known resins can be used as these water-soluble resins.
Further, in addition to the lubricant fine particles, a known lubricant such as a silicone oil, a silicone resin, a fluororesin, a silane coupling agent, or a liquid lubricant or a lubricant resin can be used in the uppermost layer.

Examples of the pigment used for the uppermost layer include various known inorganic pigments. Specific examples include zinc oxide, calcium carbonate, barium sulfate, titanium oxide, lithopone, talc, wax, kaolin, aluminum hydroxide, and calcined kaolin. These include organic pigments such as urea formalin resin and polyethylene powder. Can also be used together.
As the pigment, those having an oil absorption of 100 cc / 100 g or less and a specific surface area of 100 m ² / g or more are preferable. Further, when the effect of preventing head sticking was examined, calcium carbonate was excellent.
Moreover, when the volume average particle diameter of the pigment was set to 0.3 to 0.6 μm, the effect of preventing head sticking was further improved without reducing the surface gloss. When the volume average particle diameter exceeds 0.6 μm, not only the surface glossiness is lowered, but also the uniformity of the image is impaired. On the other hand, when the volume average particle size is less than 0.3 μm, the adhesion between the surface and the thermal head is prevented, the head screening effect by the surface is lowered, and the head sticking prevention effect tends to be lowered.

As the uppermost layer pigment, various commonly known organic materials such as polystyrene resin, polyethylene resin, polypropylene resin, urea-formalin resin, silicone resin, polymethyl methacrylate resin, melamine-formaldehyde resin, polyester, polycarbonate, etc. It is also possible to use a pigment. Since these pigments do not impair the surface glossiness that is the object of the present invention, they can be added in a volume ratio of less than 1.5 times the resin used for the uppermost layer.
The binder resin used for the uppermost layer is not particularly limited. However, in order to minimize sticking to the thermal head and sticking to the thermal head as much as possible while maintaining high gloss, the resin itself preferably has high heat resistance, and the degree of polymerization and molecular weight are high. A high resin or a resin having improved heat resistance by crosslinking is preferred.
The coating method for the uppermost layer is not particularly limited, and a conventionally known method can be adopted.
The preferred film thickness of the uppermost layer is 0.1 to 20 μm, more preferably 0.5 to 10 μm. If the film thickness is too thin, the storage material storage function and the function as the uppermost layer such as head matching are insufficient, and if it is too thick, the thermal sensitivity of the heat-sensitive recording material is lowered, which is disadvantageous in terms of cost.

In the present invention, from the viewpoint of high gloss, it is preferable that the uppermost layer has high gloss and the adjacent lower layer also has high gloss. When the surface glossiness based on JIS-P-8142 of the adjacent lower layer is 30 {GS (75 °)} or more, further excellent glossiness can be achieved.
As a specific means, when the adjacent lower layer is a heat-sensitive recording layer, the volume average particle diameter of particles such as color formers, color developers, pigments and additives contained in the heat-sensitive recording layer is set to 0. By setting the thickness to 3 to 1.0 μm, high glossiness can be achieved. It can also be achieved by setting the ratio of the binder resin contained in the heat-sensitive recording layer to 30 to 80% by weight of the whole heat-sensitive recording layer and imparting smoothness.

In addition, it is effective to provide an intermediate layer mainly composed of a resin between the heat-sensitive recording layer and the uppermost layer, and it is possible to achieve very high glossiness by using a layer having a high resin ratio.
When providing the intermediate layer, it is preferable that a water-soluble resin and / or a water-dispersible resin are mainly used in order to achieve high glossiness. Further, it is desirable to use a crosslinking agent in combination in order to provide a barrier function against water and solvents.
There is no restriction | limiting in particular in the coating method of an intermediate | middle layer, A conventionally well-known method is employable.
The thickness of the intermediate layer is preferably from 0.1 to 20 μm, more preferably from 0.5 to 10 μm. If the intermediate layer is too thin, the functions of glossiness, water resistance and solvent resistance are insufficient, and if it is too thick, the thermal sensitivity of the heat-sensitive recording material is lowered, which is disadvantageous in terms of cost.

(Coloring agent)
Various known color formers can be used in the heat-sensitive recording layer of the present invention, but leuco dyes are the most common.
A leuco dye is a compound exhibiting an electron donating property, and is used alone or in combination of two or more, but is itself a colorless or light-colored dye precursor. Examples include triphenylmethanephthalide, triallylmethane, fluorane, phenothiocyan, thiofluorane, xanthene, indophthalyl, spiropyran, azaphthalide, chromenopyrazole, methine, rhodamine anilinolactam. And leuco compounds such as rhodamine lactam, quinazoline, diazaxanthene, and bislactone. Particularly preferred are fluorane-based and phthalide-based leuco dyes. Specific examples thereof include, but are not limited to, the following.

  2-anilino-3-methyl-6-diethylaminofluorane, 2-anilino-3-methyl-6- (di-n-butylamino) fluorane, 2-anilino-3-methyl-6- (Nn-propyl) -N-methylamino) fluorane, 2-anilino-3-methyl-6- (N-isopropyl-N-methylamino) fluorane, 2-anilino-3-methyl-6- (N-isobutyl-N-methylamino) Fluorane, 2-anilino-3-methyl-6- (Nn-amyl-N-methylamino) fluorane, 2-anilino-3-methyl-6- (N-sec-butyl-N-ethylamino) fluorane, 2-anilino-3-methyl-6- (Nn-amyl-N-ethylamino) fluorane, 2-anilino-3-methyl-6- (N-iso-amyl-N-ethylamino) Fluorane, 2-anilino-3-methyl-6- (Nn-propyl-N-isopropylamino) fluorane, 2-anilino-3-methyl-6- (N-cyclohexyl-N-methylamino) fluorane, 2- Anilino-3-methyl-6- (N-ethyl-Np-toluidino) fluorane, 2-anilino-3-methyl-6- (N-methyl-Np-toluidino) fluorane, 3-diethylamino-7, 8-benzofluorane, 1,3-dimethyl-6-diethylaminofluorane, 1,3-dimethyl-6-di-n-butylaminofluorane, 3-diethylamino-7-methylfluorane, 3-diethylamino-7 -Chlorofluorane, 3-diethylamino-6-methyl-7-chlorofluorane, 10-diethylamino-2-ethylbenzo [1,4 Thiazino [3,2-b] fluorane, 3,3-bis (1-n-butyl-2-methylindol-3-yl) phthalide, 3,3-bis (4-diethylamino-2-ethoxyphenyl) -4 -Azaphthalide, 3- [2,2-bis (1-ethyl-2-methyl-3-indolyl) vinyl] -3- (4-diethylaminophenyl) phthalide, 3- [1,1-bis (4-diethylaminophenyl) ) Ethylene-2-yl] -6-dimethylaminophthalide and the like.

上記式中、Ｒ^４は、水素原子、ハロゲン原子、炭素数１〜４のアルキル基又はアルコキシ基を表し、Ｒ^５は、炭素数１〜４のアルキル基を表す。 In particular, in the case of a thermal recording medium for medical use, it is preferable to use three or more kinds of leuco dyes together in order to obtain a single color tone.
Examples thereof include a black coloring leuco dye represented by the following general formula (2) [leuco dye (2)], a red coloring leuco dye and / or an orange coloring leuco dye, and a near infrared coloring leuco dye. These are used as a mixture of one or more. That is, 3 or more types of leuco dyes are mixed, and 4-6 types are mixed as needed. The red coloring leuco dye, the orange coloring leuco dye, and the near infrared coloring leuco dye are those in which the color tone that develops color by heating indicates the absorption wavelength region of each color. The reason why the above leuco dyes are used in combination is that the colored body obtained by using the leuco dye (2) has two absorption bands in the visible range, but absorptions seen at around 450 to 600 nm and around 650 to 700 nm. This is because the absorption in the visible region is made flat like silver salt by filling the valley portion of the spectrum.

In the above formula, R ⁴ represents a hydrogen atom, a halogen atom, an alkyl group having 1 to 4 carbon atoms or an alkoxy group, and R ⁵ represents an alkyl group having 1 to 4 carbon atoms.

The degree of blackening of the image can be roughly expressed by the minimum / maximum ratio of absorbance in the 430 to 650 nm portion of the absorption spectrum, and when this ratio is 0.65 or more, it is practically used on at least the Schaukasten. Satisfactory black color. Moreover, if it is 0.75 or more, since the influence by types of fluorescent lamps, such as daylight color and daytime white, can also be reduced, it is preferable.
As the mixing ratio of each of the above dyes, it is preferable to increase the black-colored leuco dye having a large absorption from the viewpoint of high density, color tone adjustment, and storage stability, and the content of the leuco dye (2) It is preferable that the amount is in the range of 40 to 80% by weight, and the red coloring leuco dye, the orange coloring leuco dye, and the near infrared coloring leuco dye are each 10 to 30% by weight. When the content of the leuco dye (2) is more than 80% by weight, it becomes difficult to blacken the image area, and when it is less than 40% by weight, it is difficult to ensure the maximum density.

  Examples of red-colored or orange-colored leuco dyes used in combination with leuco dye (2) include rhodamine-B orthochloroanilinolactam, 3,6-bis (diethylamino) fluorane-γ- (4′-nitro) anilino Lactam, 1,3-dimethyl-6-diethylaminofluorane, 1,3-dimethyl-6-dibutylaminofluorane, 2-chloro-3-methyl-6-diethylaminofluorane, 2-chloro-6-diethylaminofluorane 3-chloro-6-N-cyclohexylaminofluorane, 6-diethylaminobenzo [α] fluorane, 6- (N-ethyl-N-isopentylamino) benzo [α] fluorane, 3,3-bis (1- n-Butyl-2-methylindol-3-yl) phthalide, 3,3-bis (1-n-octyl-2-methylindole) 3-yl) phthalide, spiro {chromeno [2,3C] pyrazole-4 (H) -1'-phthalane} -7- (N-ethyl-N-isoamylamino) -3-methyl-1-phenyl-3 ' -ON and the like.

(Developer)
In the heat-sensitive recording layer of the present invention, various electron-accepting substances that react with a color former upon heating to develop a color are used as a developer. For example, when the color former is the leuco dye, the following phenolic substances, organic or inorganic acidic substances, esters or salts thereof are used.
Gallic acid, salicylic acid, 3-isopropylsalicylic acid, 3-cyclohexylsalicylic acid, 3,5-di-tert-butylsalicylic acid, 3,5-di-α-methylbenzylsalicylic acid, 4,4′-isopropylidenediphenol, 1,1'-inopropylidenebis (2-chlorophenol), 4,4'-isopropylidenebis (2,6-dibromophenol), 4,4'-isopropylidenebis (2,6-dichlorophenol), 4,4'-isopropylidenebis (2-methylphenol), 4,4'-isopropylidenebis (2,6-dimethylphenol), 4,4'-isopropylidenebis (2-tert-butylphenol), 4, 4'-sec-butylidene diphenol, 4,4'-cyclohexylidene bisphenol, 4,4'-cycl Hexylidenebis (2-methylphenol), 4-tert-butylphenol, 4-phenylphenol, 4-hydroxydiphenoxide, α-naphthol, β-naphthol, 3,5-xylenol, thymol, methyl-4-hydroxybenzoate, 4 -Hydroxyacetophenone, novolac type phenol resin, 2,2'-thiobis (4,6-dichlorophenol), catechol, resorcin, hydroquinone, pyrogallol, phloroglysin, phloroglysin carboxylic acid, 4-tert-octylcatechol, 2,2 '-Methylenebis (4-chlorophenol), 2,2'-methylenebis (4-methyl-6-tert-butylphenol), 2,2-dihydroxydiphenyl, ethyl p-hydroxybenzoate, p-hydroxybenzoic acid Lopyr, butyl p-hydroxybenzoate, benzyl p-hydroxybenzoate, p-hydroxybenzoic acid-p-chlorobenzyl, p-hydroxybenzoic acid-o-chlorobenzyl, p-hydroxybenzoic acid-p-methylbenzyl, p -Hydroxybenzoic acid-n-octyl, benzoic acid, zinc salicylate, 1-hydroxy-2-naphthoic acid, 2-hydroxy-6-naphthoic acid, zinc 2-hydroxy-6-naphthoate, 4-hydroxydiphenylsulfone, 4 -Hydroxy-4'-chlorodiphenylsulfone, bis (4-hydroxyphenyl) sulfide, 2-hydroxy-p-toluic acid, zinc 3,5-di-tert-butylsalicylate, 3,5-di-tert-butylsalicylic acid Tin, tartaric acid, oxalic acid, maleic acid, citric acid, succinic acid, stear Phosphoric acid, 4-hydroxyphthalic acid, boric acid, thiourea derivative, 4-hydroxythiophenol derivative, bis (4-hydroxyphenyl) acetic acid, bis (4-hydroxyphenyl) acetic acid ethyl, bis (4-hydroxyphenyl) acetic acid -N-propyl, bis (4-hydroxyphenyl) acetic acid-m-butyl, bis (4-hydroxyphenyl) acetic acid phenyl, bis (4-hydroxyphenyl) acetic acid benzyl, bis (4-hydroxyphenyl) acetic acid phenethyl, bis ( 3-methyl-4-hydroxyphenyl) acetic acid, bis (3-methyl-4-hydroxyphenyl) acetic acid methyl, bis (3-methyl-4-hydroxyphenyl) acetic acid-n-propyl, 1,7-bis (4- Hydroxyphenylthio) -3,5-dioxaheptane, 1,5-bis (4-hydroxyphenyl) Nylthio) -3-oxaheptane, dimethyl 4-hydroxyphthalate, 4-hydroxy-4'-methoxydiphenylsulfone, 4-hydroxy-4'-ethoxydiphenylsulfone, 4-hydroxy-4'-isopropoxydiphenylsulfone, 4 -Hydroxy-4'-propoxydiphenylsulfone, 4-hydroxy-4'-butoxydiphenylsulfone, 4-hydroxy-4'-isobutoxydiphenylsulfone, 4-hydroxy-4'-butoxydiphenylsulfone, 4-hydroxy-4 ' -Tert-butoxydiphenylsulfone, 4-hydroxy-4'-benzyloxydiphenylsulfone, 4-hydroxy-4'-phenoxydiphenylsulfone, 4-hydroxy-4 '-(m-methylbenzyloxy) diphenylsulfone, 4- Dorokishi -4 '- (p-methyl benzyloxycarbonyl) diphenyl sulfone, 4-hydroxy -4' - (o-methyl benzyloxycarbonyl) diphenyl sulfone, 4-hydroxy -4 '- (p-chloro-benzyloxycarbonyl) diphenyl sulfone.

  The proportion of the developer in the heat-sensitive recording layer is preferably in the range of 0.5 to 5.0 parts by weight, particularly preferably in the range of 2.0 to 4.0 parts by weight, with respect to 1 part by weight of the leuco dye. When the weight of the developer is within this range, halftone image storability is improved. In addition, since the coloring efficiency is increased, the highest density can be obtained with a thin film. The advantage of thinning the gradation media is to control the film thickness during coating, to reduce residual moisture and residual solvent in the drying process, and to lead to cost reduction by reducing the coating amount.

When the leuco dye and the developer are bound and supported on the support in order to form the heat-sensitive recording layer, various conventional binders can be appropriately used. Specific examples thereof include the following. It is done.
Polyvinyl alcohol, starch and derivatives thereof, cellulose derivatives such as hydroxymethylcellulose, hydroxyethylcellulose, carboxymethylcellulose, methylcellulose, ethylcellulose, polyacrylic acid soda, polyvinylpyrrolidone, acrylamide / acrylic acid ester copolymer, acrylamide / acrylic acid ester / methacrylic acid In addition to water-soluble polymers such as terpolymers, styrene / maleic anhydride copolymer alkali salts, isobutylene / maleic anhydride copolymer alkali salts, polyacrylamide, sodium alginate, gelatin, casein, polyvinyl acetate, Emulsions of polyurethane, polyacrylic acid, polyacrylic acid ester, vinyl chloride / vinyl acetate copolymer, polybutyl methacrylate, ethylene / vinyl acetate copolymer ® emissions and styrene / butadiene copolymer, styrene / butadiene / latex such as acrylic copolymer.
These binders may be used in combination with a surfactant, a crosslinking agent, an auxiliary agent, and the like. By using a crosslinking agent that reacts with the binder, adhesion to the support, water resistance, and solvent resistance are improved. Various conventional crosslinking agents can be used.

In addition to the leuco dye and the developer, if necessary, a filler, a thermofusible substance, a surfactant, and the like, which are auxiliary additives commonly used in this type of heat-sensitive recording material, can be used in combination.
Examples of the filler include calcium carbonate, silica, zinc oxide, titanium oxide, aluminum hydroxide, zinc hydroxide, barium sulfate, clay, talc, surface-treated inorganic fine powders such as calcium and silica, urea- Examples thereof include organic fine powders such as formalin resin, styrene / methacrylic acid copolymer, and polystyrene resin.
Examples of the heat-fusible substance include higher fatty acids or esters thereof, amides, metal salts, various waxes, condensation products of aromatic carboxylic acids and amines, benzoic acid phenyl esters, higher linear glycols, 3, Examples thereof include 4-epoxy-dihydroalkyl hexahexaphthalate, higher ketones, p-benzylbiphenyl, and other thermofusible organic compounds having a melting point of about 50 to 200 ° C.
The method for applying the heat-sensitive recording layer is not particularly limited, and a conventionally known method can be employed.
The preferred thickness of the thermosensitive recording layer is 1 to 30 μm, more preferably 3 to 20 μm. If the thickness is too thin, the image density becomes insufficient. If the thickness is too thick, the thermal sensitivity of the recording material is reduced, the background density is increased, and the cost is disadvantageous.

As the support, those used in conventional heat-sensitive recording media can be applied, and examples thereof include plastic film, paper, plastic resin laminated paper, and synthetic paper.
Among these, considering the contrast of the image quality in the application of the reflected image, those having high opacity and whiteness are preferable. In consideration of surface gloss, reproducibility of printing, high definition, and the like, it is preferable that the support surface is smooth and has high gloss.
In the case of a transmissive thermosensitive recording medium, it is necessary to use a transparent support. Specific examples of the transparent support include, but are not limited to, cellulose derivatives such as cellulose triacetate, polyolefins such as polypropylene and polyethylene, those made of polystyrene, and films obtained by bonding them together. Preferable is a material mainly composed of polypropylene resin (for example, synthetic paper).

The surface glossiness based on JIS-P-8142 on the heat-sensitive recording layer side of the support is preferably 50 {GS (75 °)}% or more. When this condition is satisfied, not only the surface glossiness of the heat-sensitive recording medium is excellent, but also the adhesiveness with the thermal head is excellent, so that it is effective in improving the fineness during recording, preventing omission of images, and increasing the sensitivity.
In addition, generally, the quality of glossiness and high sensitivity is improved by smoothing the thermal recording medium using a super calender or the like, but by using a support that satisfies the above surface glossiness conditions. Also, the effect of simplifying by omitting these steps can be obtained.
Further, in order to improve the adhesion when the thermal recording layer coating solution is applied, at least one surface of the support can be surface-modified by corona discharge treatment, oxidation reaction treatment (chromic acid or the like), etching treatment or the like.
In addition, for use in the medical field, a polypropylene-based synthetic paper having a thickness of about 50 to 250 μm is easy to use because of ease of handling.

Further, a back layer may be provided on the side opposite to the side having the thermosensitive recording layer. The back layer can be provided with an antistatic function, an anti-curl function, an adhesion preventing function by overlapping, and the like.
A wide variety of antistatic agents for imparting an antistatic function are known, but the antistatic function of the back layer requires a surface resistance value of 10 ¹⁰ Ωcm or less.
Antistatic agents capable of imparting this level of conductivity are broadly classified into those using a surfactant, those using a conductive polymer, and those using a conductive metal oxide.
First, those using surfactants account for the majority of current antistatic agents. These surfactants can be classified into four types, anionic, cationic, nonionic and amphoteric. As antistatic agents, cationic or amphoteric surfactants are superior in terms of antistatic properties and durability. . These surfactant types are relatively inexpensive, have a wide variety of types, and have good performance, but many of them achieve conductivity by adsorbing moisture from the surfactant itself. It is easily affected by humidity, and the antistatic property under low humidity tends to decrease.

In addition, the conductive polymer is a material that has been developed in recent years, and examples thereof include a material obtained by doping an organic polymer with an electron donor. Organic polymers include aliphatic groups such as polyacetylene, aromatic groups such as polyparaphenylene, heterocyclic systems such as polypyrrole, conjugated polymers such as aromatic amines such as polyaniline, Even if it is not a conjugated system, what has a cyclic | annular (pi) conjugated group in a side chain etc. are mentioned.
This conductive polymer, like the conductive metal oxide, is not a conductive function due to moisture, and thus exhibits a conductive function even under low humidity. In addition, although it varies depending on the organic polymer and the electron donor, the conductive function can be made extremely high, and a sufficient antistatic function can be imparted even with a thin film.

On the other hand, those using conductive metal oxides are fewer and more expensive than those using a surfactant. However, since the metal oxide itself has electrical conductivity, the electrical conductivity is high, and excellent transparency can be maintained even with a low adhesion amount, so that high transparency can be maintained. In addition, the antistatic property is excellent even under low humidity without being affected by humidity.
As the conductive metal oxide, for example, SnO ₂ , In ₂ O ₃ , ZnO, TiO ₂ , MgO, Al ₂ O ₃ , BaO, MoO ₃ or the like alone or P, Sb, Sn, Zn or the like is added to these. And composite oxides. Many of these conductive metal oxides are colored, and the transparency is impaired. Therefore, the amount used is preferably as small as possible. Therefore, you may use together a conductive metal oxide, surfactant, a conductive polymer, etc.
Further, the finer the powder of the antistatic agent, the better the transparency and the higher the antistatic effect. From the viewpoint of transparency, it is desirable that the volume average particle size of the antistatic agent is 0.2 μm or less.
The blending ratio of the antistatic agent is about 0.05 to 0.9 parts by weight, preferably 0.1 to 0.5 parts by weight, in 1 part by weight of the back layer material. If the amount is less than 0.05 parts by weight, a sufficient antistatic function cannot be exhibited. On the other hand, if it exceeds 0.9 parts by weight, the function of bonding to the support may not be sufficient.

In order to impart an adhesion preventing function to the back layer, it is effective to add a filler. Examples of fillers include phosphate fiber, potassium titanate, acicular magnesium hydroxide, whiskers, talc, mica, glass bead flakes, calcium carbonate, platy carbon cal, aluminum hydroxide, silica, clay, kaolin, calcined clay, hydro Spherical organics such as spherical inorganic fillers such as taruisite, polystyrene resins, polyethylene resins, polypropylene resins, urea-formalin resins, silicone resins, polymethylmethacrylate resins, melamine-formaldehyde resins, polyesters, polycarbonates, etc. Although a filler is mentioned, it is not restricted to these.
These fillers are preferably those having a volume average particle size of 6 to 25 μm from the viewpoint of adhesion prevention, and further, spherical fillers are preferable because they can efficiently form convex portions on the surface.
Examples include phosphate fiber, potassium titanate, acicular magnesium hydroxide, whiskers, talc, mica, glass bead flakes, calcium carbonate, platy carbon, aluminum hydroxide, silica, clay, kaolin, talc, calcined clay. Spherical inorganic fillers such as hydrotalicite, spherical resins such as polystyrene resins, polyethylene resins, polypropylene resins, urea-formalin resins, silicone resins, polymethyl methacrylate acrylate resins, melamine-formaldehyde resins, polyesters, polycarbonates, etc. However, the organic filler is not limited to these.

  Furthermore, various known resins can be used as the binder for the back layer, such as polyethylene, polyvinyl acetate, polyacrylamide, maleic acid copolymer, polyacrylic acid and esters thereof, polymethacrylic acid and esters thereof, Examples thereof include vinyl chloride / vinyl acetate copolymer, styrene copolymer, polyester, polyurethane, polyvinyl butyral, ethyl cellulose, polyvinyl acetal, polycarbonate, epoxy resin, polyamide, polyvinyl alcohol, starch, and gelatin. These can be used alone or in combination of two or more, and can be selected in consideration of the affinity with the support or antistatic agent used. Further, those having a Tg of 80 ° C. or more are preferable, and this is very effective when curling easily toward the heat-sensitive recording layer side.

The heat-sensitive recording medium as in the present invention is usually a long product immediately after production, but the form as a product is the one that is rolled and hardened, and cut into a predetermined size and a predetermined number of sheets. Some are in a bag. In any case, it is preferable to store and distribute the product by wrapping it in a light-shielding packaging material because of the nature of the product. When used, the product is opened and taken out from the bag and mounted on the image forming apparatus.
In order to form an image using the heat-sensitive recording medium of the present invention, a heating means is prepared, and the heat-sensitive recording medium may be heated based on character and / or shape information. Examples of the heating means include a thermal pen, a thermal head, and laser heating, and are not particularly limited. However, the thermal recording medium of the present invention prints a high-definition and high-gradation image such as a medical image. It is most preferable to use a thermal head from the viewpoints of being suitable for the above, and cost of the apparatus, output speed, and compactness.
Further, in the case of medical use where the gradation of the image is regarded as important, a pulse control method or a voltage control method may be adopted.

  EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated further more concretely, this invention is not restrict | limited at all by these Examples. The following “parts” and “%” are based on weight. The volume average particle size was measured with a laser diffraction particle size measuring device LA-920 manufactured by Horiba.

Example 1
First, a [Liquid A] leuco dye dispersion and a [Liquid B] developer dispersion were prepared as follows, and a [Liquid C] thermosensitive recording layer coating solution was prepared using them.

[Liquid A] Dye dispersion A dye dispersion was prepared by pulverizing and dispersing the following formulation with a magnetic ball mill so that the volume average particle diameter was 1.5 μm or less.
・ 20 parts of 2-Alinino-3-methyl-6-dibutylaminofluorane ・ 20 parts of modified polyvinyl alcohol aqueous solution (Kuraray K Polymer KL-318, solid content 10%)
・ 60 parts of water

[Liquid B] Developer dispersion A developer dispersion was prepared by pulverizing and dispersing the following formulation with a magnetic ball mill so that the volume average particle size was 0.8 μm or less.
4-hydroxy-4'-isopropoxydiphenylsulfone 12 parts Silica (P-603 made by Mizusawa Chemical) 4 parts Stearamide 4 parts Modified polyvinyl alcohol aqueous solution 20 parts (Kuraray K Polymer KL-318 solid content 10% )
・ 60 parts of water

[C liquid] Thermal recording layer coating solution The following formulation was mixed and stirred to prepare a thermal recording layer coating solution.
[Liquid A] Dye dispersion 12.5 parts [Liquid B] Developer dispersion 62.5 parts ・ Modified polyvinyl alcohol aqueous solution 25 parts (Kuraray K Polymer KL-318, solid content 10%)

Next, the above-mentioned [Liquid C] was coated on a synthetic paper having a thickness of 170 μm (Nyanya PX170 surface glossiness 60%) using a wire bar, passed through a dryer maintained at a temperature of 70 ° C. for 3 minutes and dried. Then, a thermosensitive recording layer (surface glossiness 25%) having a thickness of 8.5 g / m ² was formed.

Subsequently, [Liquid D] pigment dispersion and [Liquid E] lubricant dispersion were prepared as follows, and [Liquid F] uppermost layer coating liquid was prepared using them.

[Liquid D] Pigment Dispersion The following formulation was pulverized and dispersed with a magnetic ball mill to prepare a dye dispersion having a volume average particle size of 0.3 μm.
Pigment: Aluminum hydroxide (H43M) 5 parts Modified polyvinyl alcohol aqueous solution 5 parts (Kuraray K Polymer KL-318 solid content 10%)
・ 17.5 parts of water

[Liquid E] Lubricant dispersion The following composition was pulverized and dispersed with a magnetic ball mill to prepare a lubricant dispersion having a volume average particle size of 1.9 μm.
・ Lubricant: Zinc myristate (n = 13) (Powder Base M manufactured by NOF Chemical) 30 parts ・ Anionic surfactant (Newcol 290M Nippon Emulsifier) 3 parts ・ Modified polyvinyl alcohol aqueous solution 30 parts (Kuraray K Polymer KL-318) 10% solids)
・ 177 parts of water

[F solution] Uppermost layer coating solution The following composition was mixed and stirred to prepare the uppermost layer coating solution.
-Modified polyvinyl alcohol aqueous solution 10 parts (Kuraray K polymer KL-318 solid content 10%)
[Liquid D] Pigment dispersion (volume average particle size 0.3 μm) 2.75 parts [Liquid E] Lubricant dispersion (volume average particle size 1.9 μm) 2.4 parts ・ Polyamide epichlorohydrin (paper strength agent WS -525 25%) 0.8 part ・ Water 5.15 parts

Next, [F liquid] was coated on the heat-sensitive recording layer previously formed on the synthetic paper using a wire bar, passed through a drier maintained at a temperature of 70 ° C. for 3 minutes, and dried, 3 g / The uppermost layer of m ² was formed to obtain the thermal recording medium of Example 1.

Comparative Example 1
A thermosensitive recording layer and an uppermost layer were formed in the same manner as in Example 1 except that the uppermost layer coating solution was changed to the following [G solution], and a thermosensitive recording medium of Comparative Example 1 was obtained.

[Liquid G] Uppermost layer coating solution The following composition was mixed and stirred to prepare the uppermost layer coating solution.
-Modified polyvinyl alcohol 10 parts (Kuraray K polymer KL-318 solid content 10%)
[Liquid D] Pigment dispersion (volume average particle size 0.25 μm) 2.75 parts Lubricant: Zinc stearate (n = 17) dispersion 0.75 parts (F930 Solid content 40% Volume average particle made by Chukyo Oil & Fats) Diameter 0.9μm)
・ Polyamide epichlorohydrin (paper strength agent WS-525, solid content 25%) 0.8 part ・ Water 5.15 parts

Example 2
[Liquid E] Zinc myristate was changed to wet-synthesized zinc laurate (n = 11) Zinclanate GP (manufactured by NOF Chemical Co., Ltd.), and a lubricant dispersion [Volume J-1] having a volume average particle size of 2.1 μm Was prepared.
Except that the following [K-1 solution] was prepared using this [J-1 solution], the thermosensitive recording layer and the uppermost layer were formed in the same manner as in Example 1, and the thermosensitive recording medium of Example 2 was formed. Got.

[K-1 solution] Uppermost layer coating solution-10 parts of modified polyvinyl alcohol aqueous solution (Kuraray K Polymer KL-318, solid content 10%)
[Liquid D] Pigment dispersion (volume average particle diameter 0.25 [mu] m) 2.75 parts [J-1 liquid] Lubricant: Zinc laurate dispersion (volume average particle diameter 2.1 [mu] m) 2.4 parts Epichlorohydrin (paper strength agent WS525 solid content 25%) 0.8 part ・ Water 5.15 parts

Example 3
[Liquid E] Zinc myristate was changed to dry-synthesized zinc laurate (n = 11) Zinclanate GP (manufactured by NOF Chemical), and a lubricant dispersion with a volume average particle size of 2.1 μm [J-2 solution] Was prepared.
Using this [J-2 solution], except that the following [K-2 solution] was prepared, the thermosensitive recording layer and the uppermost layer were formed in the same manner as in Example 1, and the thermosensitive recording medium of Example 3 was used. Got.

[Liquid K-2] Top layer coating solution 10 parts of modified polyvinyl alcohol aqueous solution (Kuraray K Polymer KL-318, solid content 10%)
[Liquid D] Pigment dispersion (volume average particle size 0.25 [mu] m) 2.75 parts [J-2 liquid] Lubricant dispersion (volume average particle size 2.1 [mu] m) 2.4 parts-Polyamide epichlorohydrin (paper strength) Agent WS525 solid content 25%) 0.8 part water 5.15 parts

Example 4
[Liquid D] aluminum hydroxide was changed to calcium carbonate (Brilliant 15, manufactured by Shiraishi Kogyo Co., Ltd.) to prepare a [L liquid] pigment dispersion having a volume average particle size of 0.3 μm.
Using this [Liquid L], except that the following [Liquid N] was prepared, the thermosensitive recording layer and the uppermost layer were formed in the same manner as in Example 3 to obtain the thermosensitive recording medium of Example 4.

[N solution] Top layer coating solution-10 parts of modified polyvinyl alcohol aqueous solution (Kuraray K Polymer KL-318 solid content 10%)
[Liquid L] Pigment: Calcium carbonate dispersion (volume average particle size 0.2 μm) 2.75 parts [J-2 solution] Lubricant: Zinc laurate dispersion (volume average particle size 2.1 μm) 2.4 Part ・ Polyamide epichlorohydrin (paper strength agent WS525 solid content 25%) 0.8 part ・ Water 5.15 parts

Example 5
In the same manner as in Example 4 except that paraffin wax (melting point: 54 ° C., volume average particle size: 0.9 μm, solid content: 30%) was added to [N solution] and the following [O solution] was prepared. A heat-sensitive recording layer and an uppermost layer were formed to obtain a heat-sensitive recording medium of Example 5.

[Liquid O] Uppermost layer coating solution ・ Denatured polyvinyl alcohol aqueous solution 10 parts (Kuraray K Polymer KL-318 solid content 10%)
[Liquid L] Pigment: Calcium carbonate dispersion (volume average particle size 0.2 μm) 2.75 parts [J-2 solution] Lubricant: Zinc laurate dispersion (volume average particle size 2.1 μm) 2.4 Part ・ Paraffin wax aqueous dispersion (Made by Chukyo Oil and Fat, 54 ° C.) 0.167 part ・ Polyamide epichlorohydrin (paper strength agent WS525 solid content 25%) 0.8 part ・ Water 5.15 parts

Example 6
In the same manner as in Example 4 except that paraffin wax (melting point: 54 ° C., volume average particle size: 0.9 μm, solid content: 30%) was added to [N solution] and the following [O solution] was prepared. A heat-sensitive recording layer and an uppermost layer were formed to obtain a heat-sensitive recording medium of Example 5.

[Liquid O] Uppermost layer coating solution ・ Denatured polyvinyl alcohol aqueous solution 10 parts (Kuraray K Polymer KL-318 solid content 10%)
[Liquid L] Pigment: Calcium carbonate dispersion (volume average particle size 0.2 μm) 2.75 parts [J-2 solution] Lubricant: Zinc laurate dispersion (volume average particle size 1.1 μm) 2.4 Part ・ Paraffin wax aqueous dispersion (Made by Chukyo Oil and Fat, 54 ° C.) 0.167 part ・ Polyamide epichlorohydrin (paper strength agent WS525 solid content 25%) 0.8 part ・ Water 5.15 parts

Example 7
The volume average particle size of [J-2 liquid] was changed to 0.7 μm, and the following [P liquid] lubricant dispersion was prepared.
Using this [P solution], except that the following [Q solution] was prepared, the thermosensitive recording layer and the uppermost layer were formed in the same manner as in Example 5 to obtain the thermosensitive recording medium of Example 6.

[Q liquid] Top layer coating liquid-10 parts of modified polyvinyl alcohol aqueous solution (Kuraray K Polymer KL-318, solid content 10%)
[Liquid L] Pigment: 2.75 parts of calcium carbonate dispersion (volume average particle size 0.2 μm) [Liquid P] Lubricant: Zinc laurate dispersion (volume average particle size 0.7 μm) 2.4 parts Paraffin wax aqueous dispersion (Made by Chukyo Yushi Co., Ltd., melting point 54 ° C.) 0.167 parts ・ Polyamide epichlorohydrin (paper strength agent WS525 solid content 25%) 0.8 parts ・ Water 5.15 parts

Example 8
The [R liquid] pigment dispersion was prepared by changing the volume average particle size of [L liquid] to 0.35 μm.
Using this [R solution], except that the following [S solution] was prepared, the thermosensitive recording layer and the uppermost layer were formed in the same manner as in Example 6 to obtain the thermosensitive recording medium of Example 7.

[S solution] Uppermost layer coating solution-10 parts of denatured polyvinyl alcohol aqueous solution (Kuraray K Polymer KL-318, solid content 10%)
[Liquid R] Pigment: 2.75 parts of calcium carbonate dispersion (volume average particle size 0.35 μm) [Liquid P] Lubricant: Zinc laurate dispersion (volume average particle size 0.7 μm) 2.4 parts Paraffin wax aqueous dispersion (Made by Chukyo Yushi Co., Ltd., melting point 54 ° C.) 0.167 parts ・ Polyamide epichlorohydrin (paper strength agent WS525 solid content 25%) 0.8 parts ・ Water 5.15 parts

Example 9
The [R liquid] pigment dispersion was prepared by changing the volume average particle size of [L liquid] to 0.35 μm.
Using this [R solution], except that the following [S solution] was prepared, the thermosensitive recording layer and the uppermost layer were formed in the same manner as in Example 6 to obtain the thermosensitive recording medium of Example 7.

[S solution] Uppermost layer coating solution-10 parts of denatured polyvinyl alcohol aqueous solution (Kuraray K Polymer KL-318, solid content 10%)
[Liquid R] Pigment: Calcium carbonate dispersion (volume average particle size 0.65 μm) 2.75 parts [Liquid P] Lubricant: Zinc laurate dispersion (volume average particle size 0.7 μm) 2.4 parts Paraffin wax aqueous dispersion (Made by Chukyo Yushi Co., Ltd., melting point 54 ° C.) 0.167 parts ・ Polyamide epichlorohydrin (paper strength agent WS525 solid content 25%) 0.8 parts ・ Water 5.15 parts

The heat-sensitive recording materials of the above Examples and Comparative Examples were evaluated for head fixing residue, printed white powder, sticking, and surface gloss. The results are summarized in Table 1.
(1) Head fixing residue Using a variable energy horizon (manufactured by CODONICS) equipped with a gradation head with a resolution of 300 dpi, the energy in the width direction is + 20%, + 10%, std. , −10%, −20%, and −40% printing patterns were created, and 50 sheets were continuously printed in A4 size. A halftone having a printing rate of about 40 to 80% was printed before and after the test, and the influence on the image was visually determined. The evaluation criteria are as follows.
5: Energy is not more than + 20% and there is no change in the front and rear halftone images.
4: Although the energy is + 20%, the halftone image density is slightly changed.
There is no change in the halftone images before and after the -20% to + 10% portion.
3: Change in halftone image density can be seen in the area where the energy is + 10%.
-20% to std. There is no change in the halftone image before and after the part.
2: Energy is std. The part of can see a little change in the halftone image density,
There is no change in the halftone images before and after the -20% to -10% portion.
1: Energy is std. Halftone images are faint at ~ 20%.

(2) Printed white powder Using a variable energy horizon (manufactured by CODONICS) equipped with a gradation head with a resolution of 300 dpi, the energy in the width direction is + 20%, + 10%, std. , -10%, -20%, and -40% of six-level printing patterns were created, and 50 sheets of A4 size were printed continuously. A halftone having a printing rate of about 40 to 80% was printed before and after the test, and the influence on the thermal head and the image was visually determined. The evaluation criteria are as follows.
5: No white powder adheres to the rear of the heating element and the thermal head nose, and there is no effect on the image.
4: Slightly in part of energy part, behind heating element and thermal head nose part
White powder adhesion is observed. Only a part of the image is slightly affected.
3: Energy level of 2-3 levels out of 6 levels, slightly behind heating element and thermal
White powder adheres to the head nose, and the image is slightly affected.
2: Energy level of 2-3 levels out of 6 levels, heating element rear and thermal head
The white powder is clearly attached to the nose, and the image is clearly affected.
1: A large amount of white powder is observed in all energy parts, and the image is clearly affected.

(3) Sticking Using a variable energy horizon (manufactured by CODONICS) equipped with a gradation head with a resolution of 300 dpi, printing was performed with a pattern for printing gradation in the flow direction, and the influence on the image was visually determined. The evaluation criteria are as follows.
5: No stick is generated.
4: Although the stick line is slightly visible, the print length is not a problem.
3: Although the stick line is visible, the print length is not a problem.
2: The stick line is clearly visible and the image length is short.
1: The stick line is clearly visible and the image length is very short

(4) Surface glossiness Using a gloss meter Model VG-2000 75 ° manufactured by Nippon Denshoku Industries Co., Ltd. before printing, std. Each Dmax part after printing with energy was measured. The larger the value, the higher the gloss.

JP 2006-062189 A Japanese Patent Laid-Open No. 06-135148 Japanese Patent Laid-Open No. 10-035103 JP 2006-256274 A JP 2003-2111845 A

Claims (6)

A thermosensitive recording medium having at least a thermosensitive recording layer and an uppermost layer on a support, wherein the thermosensitive recording layer contains a binder, a color former and a developer, and the uppermost layer has a general formula (1) as a lubricant. A heat-sensitive recording medium comprising a fatty acid zinc salt represented by the formula:
(N is an integer of _{11~13) (C n H 2n +} 1 COO) 2 -Zn ... (1) 2. The heat-sensitive recording medium according to claim 1, wherein the zinc salt of the fatty acid is n = 11 zinc laurate. 3. The heat-sensitive recording medium according to claim 1, wherein the uppermost layer contains paraffin wax as a lubricant. The thermosensitive recording medium according to any one of claims 1 to 3 , wherein the uppermost layer contains an inorganic pigment, and at least one of the inorganic pigments is calcium carbonate. The thermosensitive recording medium according to any one of claims 1-4, wherein the volume average particle diameter of the lubricant is 1.0μm or less. The thermosensitive recording medium according to any one of claims 1 to 5, wherein a volume average particle diameter of the inorganic pigment is characterized in that it is a 0.3 to 0.6 .mu.m.