JP2006239971A - Thermosensitive recording medium and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermosensitive recording medium, in which the change of an image density and color tone with time is suppressed and by which a highly dense and high image quality image can be formed and which is especially suitable for visually diagnosing with a backlight by an X film illuminator method or for referencing, and an image forming method for forming an image on the thermosensitive recording medium. <P>SOLUTION: In the thermosensitive recording medium comprising a support, a thermosensitive recording layer formed on the support, and a protective layer formed on the thermosensitive recording layer, wherein the thermosensitive recording layer comprises a resin, a leuco dye, a developer, and an antoxidant. The developer includes general formula (1), wherein R<SB>1</SB>represents a 4-16C straight chained alkyl group or alkyl amino group. The antioxidant includes a substituent represented by 3-(3,5-di-t-butyl-4-hydroxy phenyl)propyonate. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a heat-sensitive recording medium particularly suitable in the medical field and an image forming method using the same.

  In the medical field, it has been practiced to visualize the state of the body seen by X-rays, MRI, CT, etc. on a silver salt film, and to visually diagnose and refer to the image with a backlight by the Schaukasten method. However, the wet process of silver salt film has a problem of waste liquid treatment, and the recent trend of digitization of images demands the emergence of a dry process as an alternative. Photo exposure thermal fixing system, thermal transfer system, thermal recording system Are already commercialized. Of these, the recording material used in the thermal recording system is one in which a conventional leuco-type thermal recording material is applied in the sense that a leuco dye and a developer react to form a colored body to obtain an image. It is.

  Conventional leuco-type heat-sensitive recording materials generally have a basic structure in which a recording layer containing a colorless or light-colored leuco dye and a developer is provided on paper, and heating means such as a thermal head By heating the recording layer in an image-like manner, the leuco dye and the developer react to form a color former to obtain an image. This leuco-type thermal recording material does not need to be subjected to frequent processing such as development and fixing, can be recorded in a short time with a relatively simple structure and a compact apparatus, has less noise, and is low in cost. However, it is widely used in various applications such as electronic computers, facsimiles, ticket vending machines, label printers, recorders, etc., but the object of recording is mainly character information.

  On the other hand, in general, heat-sensitive recording materials for medical use have a reflection type in which the formed image can be viewed by reflection of light and a light transmission property of the entire recording material. However, the image characteristic level required for the latter is much higher than the former because it is a transmissive type. This transmission type thermal recording material can be used for making a plate for lithographic printing using a thermal recording material on which an image is formed as a plate, in addition to medical treatment in which the formed image is examined by a backlight using the Schaukasten method. A transmission type heat-sensitive recording material formed with can be used for OHP. Especially in medical applications that use translucent heat-sensitive recording materials with high translucency and images that are examined by the backlight of the Schaukasten method, they are used for diagnosis. It is important that the shape information is the main feature and the image recognized by the backlight is excellent in terms of shading and contrast, and that the original shape information is accurately reflected, and in a high-temperature and high-humidity environment. The image is expected to be retained for more than 10 years. Therefore, transmission-type heat-sensitive recording materials, particularly medical applications, require a much higher level of performance than conventional leuco-type heat-sensitive recording materials. It is impossible to apply technology.

  The properties required for the transmission type heat-sensitive recording material and particularly for medical use will be described in more detail. The first required characteristic is that the obtained image has high light-shielding properties or high recognition accuracy even when illuminated by a backlight. Therefore, the color image density is high and reproducibility in a wide density range. It has excellent contrast, high recognizability, and high image storage in a high humidity environment. The level required for the transmissive type with respect to this image density characteristic is required to have no fog in the image area and a maximum color density of 2.5 or more, preferably 3.0 or more. Simply increasing the thickness of the layer to achieve a high concentration is undesirable because it increases the background concentration and deteriorates transparency.

  Secondly, in order to be applicable to the Schaukasten method, it is necessary to develop a color with a single color tone from a low density range to a high density range. In the case of the reflection type, this characteristic can be obtained relatively easily by using a kind of leuco dye. On the other hand, in the case of the transmissive type, since the support is translucent and the contribution of factors other than absorption by the color former is small, the unevenness of the absorbance curve of the thermosensitive layer is severe, and the color tone varies greatly depending on the density. In general, a color tone is adjusted by mixing a plurality of leuco dyes. However, when the storability of the density of the color former formed by each of the plurality of leuco dyes differs from that of the developer, particularly in a high-temperature and high-humidity environment, the recording material immediately after image formation and after storage for a certain period of time are used. When the recording materials are compared, the color tone of the two is greatly different. In particular, many of them have poor storage stability at low concentrations. Therefore, in order to prevent the color tone from changing, it is necessary that the leuco dyes that develop colors of different colors are equal and have high concentration preservability especially in high temperature and high humidity storage. Therefore, it is important to form a recording material.

  Furthermore, the third required property is required to have high transparency. When the transparency is low, image recognition is deteriorated because the light rays are diffused. As the transparency, the haze value of the non-image area is preferably 70% or less, more preferably 50% or less, and further preferably 20 to 40% equivalent to the silver salt film. In order to improve transparency, there is a method of making the particulate material added to the recording layer, the protective layer, etc. fine particles, or increasing the ratio of the adhesive, but that alone is not sufficient transparency, The current situation is highly dependent on materials.

  Various recording materials using the thermal coloring process have been proposed, but in general, a coating solution in which a colorless or light leuco dye and a developer are dissolved or dispersed is applied on paper. Manufactured. Particularly in recent years, mainly in the medical field, there is a demand for a transparent dry film system that can be easily output from the waste liquid treatment problem caused by the wet process of silver salt X-ray film and the trend of digitization of images. The current medical dry process includes three systems: a light exposure thermal fixing system, a thermal transfer system, and a direct thermal recording system.

  Thermal recording material, one of them, has been used as various recording materials for electronic computers, facsimiles, ticket vending machines, label printers, recorders, etc., and needs to be processed frequently such as development and fixing. In recent years, the market has been rapidly expanding due to advantages such as being capable of recording in a short time with a compact apparatus having a relatively simple structure, less noise generation, and low cost.

  In Patent Document 1 (Japanese Patent Application Laid-Open No. 10-211767), a reversible feeling provided with a reversible thermosensitive recording layer containing a reversible color developer that develops color by heating and recolors the dye precursor by heating. In a thermal recording material, 3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate or 3,5-di-t-butyl-4-hydroxy-benzylphosphine is contained in the reversible thermosensitive recording layer. It has been proposed to contain an antioxidant containing a substituent represented by phonate. This reversible thermosensitive recording material has a good light fastness that makes it less noticeable as an erasure mark after erasing the color image and reducing the density change of the decolored part due to light exposure, but the transparency cannot be obtained. There is.

Patent Document 2 (Japanese Patent Laid-Open No. 11-58959) proposes that a thermosensitive recording material comprising a leuco dye, a developer, and a binder resin as a binder contains an organic amine and an antioxidant. This heat-sensitive recording material provides a highly transparent heat-sensitive recording material and good light resistance, but there is a problem that the color tone of the black image is green and a good black image cannot be obtained and the color density is low.
JP-A-10-211767 Japanese Patent Laid-Open No. 11-58959

  The present invention suppresses temporal changes in image density and color tone, and can form high-density, high-quality images, and is particularly suitable for visual diagnosis and reference using a backlight based on the Schaukasten method. It is an object to provide a recording medium and an image forming method for forming an image on the thermosensitive recording medium.

（１）

（式中、Ｒ₁は炭素数４〜１６の直鎖のアルキル基又はアルキルアミノ基を表す。）
を含み、前記酸化防止剤は３−（３，５−ジ−ｔ−ブチル−４−ヒドロキシフェニル）プロピオネートで表される置換基を含むことを特徴とする感熱記録媒体。
［２］ 前記３−（３，５−ジ−ｔ−ブチル−４−ヒドロキシフェニル）プロピオネートで表される置換基を含む酸化防止剤の融点が、１１０℃以上であることを特徴とする［１］に記載の感熱記録媒体。
［３］ 前記酸化防止剤が、該ロイコ染料に対して５重量％以上８０重量％以下であることを特徴とする［１］または［２］に記載の感熱記録媒体。
［４］ 前記記録層樹脂が、ポリエステル樹脂であることを特徴とする［１］〜［３］のいずれかに記載の感熱記録媒体。
［５］ 前記ロイコ染料は、赤発色染料及び橙発色染料の少なくとも一方、近赤外発色染料並びに一般式（２）

（２）
（式中、Ｒ₂は、水素原子、ハロゲン原子、炭素数が１以上４以下のアルキル基又は炭素数が１以上４以下のアルコキシ基であり、Ｒ₃は、炭素数が１以上４以下のアルキル基を表す。）
で示される染料を含有することを特徴とする［１］〜［４］のいずれかに記載の感熱記録媒体。
［６］ ［１］〜［５］のいずれか一項に記載の感熱記録媒体に画像を形成することを特徴とする画像形成方法。
に関する。 As a result of intensive studies, the present inventor has found that it is effective to select specific compounds for the developer and the antioxidant used in the heat-sensitive recording layer of the heat-sensitive recording medium, and has led to the present invention. .
That is, the present invention
[1] In a thermosensitive recording medium having a support, a thermosensitive recording layer formed on the support, and a protective layer formed on the thermosensitive recording layer, the thermosensitive recording layer comprises a resin, a leuco dye, a developer, and an antioxidant. And the developer is represented by the following general formula (1)

(1)

(In the formula, R ₁ represents a linear alkyl group or alkylamino group having 4 to 16 carbon atoms.)
And the antioxidant contains a substituent represented by 3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate.
[2] The melting point of the antioxidant containing a substituent represented by 3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate is 110 ° C. or higher [1] ].
[3] The thermal recording medium according to [1] or [2], wherein the antioxidant is 5% by weight or more and 80% by weight or less based on the leuco dye.
[4] The thermal recording medium according to any one of [1] to [3], wherein the recording layer resin is a polyester resin.
[5] The leuco dye is at least one of a red coloring dye and an orange coloring dye, a near-infrared coloring dye, and a general formula (2)

(2)
(In the formula, R ₂ is a hydrogen atom, a halogen atom, an alkyl group having 1 to 4 carbon atoms or an alkoxy group having 1 to 4 carbon atoms, and R ₃ has 1 to 4 carbon atoms. Represents an alkyl group.)
The heat-sensitive recording medium according to any one of [1] to [4], which comprises a dye represented by the formula:
[6] An image forming method comprising forming an image on the heat-sensitive recording medium according to any one of [1] to [5].
About.

  According to the present invention, when visually diagnosing and referring to the backlight by the Schaukasten method, the temporal change in image density and color tone is suppressed, and a thermal recording medium capable of forming a high-density, high-quality image and the recording medium An image forming method for forming an image on a thermal recording medium can be provided.

（１）
で示される化合物を含有する。ここで、Ｒ₁は、炭素数が４以上１６以下の直鎖アルキル基又は炭素数が４以上１６以下の直鎖アルキルアミノ基である。これにより、画像濃度の経時変化を抑制し、高濃度、高画質で、耐湿保存性に優れる画像を形成することができる。 Next, the best mode for carrying out the present invention will be described.
The heat-sensitive recording medium of the present invention is provided with a heat-sensitive recording layer on a support, and the heat-sensitive recording layer contains a leuco dye, a developer and an antioxidant, and the developer is represented by the general formula (1).

(1)
The compound shown by these is contained. Here, R ₁ is a linear alkyl group having 4 to 16 carbon atoms or a linear alkylamino group having 4 to 16 carbon atoms. As a result, it is possible to suppress an image density change with time, and to form an image having high density, high image quality, and excellent moisture storage stability.

  Specific examples of such a developer include 4- (n-pentanoylamino) salicylic acid, 4- (n-hexanoylamino) salicylic acid, 4- (n-octanoylamino) salicylic acid, 4- (hexadeca Noylamino) salicylic acid, 4- (N′-n-butylcarbamoylamino) salicylic acid, 4- (N′-n-hexylcarbamoylamino) salicylic acid, 4- (N′-n-octylcarbamoylamino) salicylic acid, 4- ( N'-hexadecylcarbamoylamino) salicylic acid and the like.

（３）
で示される顕色剤を用いると、地肌カブリの発生が少なく、画像保存性が優れる透過型感熱記録媒体が得られる。ここで、Ｒ₄は、炭素数が６以上１２以下の直鎖アルキル基である。なお、顕色剤は、単独又は２種以上混合して用いることができ、従来公知の顕色剤を混合することもできる。 In particular, the general formula (3)

(3)
Is used, a transmission-type heat-sensitive recording medium with less background fogging and excellent image storage stability can be obtained. Here, R ₄ is a linear alkyl group having 6 to 12 carbon atoms. In addition, a color developer can be used individually or in mixture of 2 or more types, A conventionally well-known color developer can also be mixed.

  Specific examples of the antioxidant containing a substituent represented by 3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate used in the present invention include triethylene glycol-bis- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate], 1,6-hexanediol-bis- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate], Pentaerythrityl-tetrakis [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate], 2,2-thio-diethylenebis [3- (3,5-di-t-butyl-4 -Hydroxyphenyl) propionate], octadecyl-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate, pentadecyl-3- (3,5-di) t-butyl-4-hydroxyphenyl) propionate, dodecyl-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate, N, N′-hexamethylenebis (3,5-di-t- Butyl-4-hydroxy-hydroxynamamide), 3,5-di-tert-butyl-4-hydroxy-benzylphosphonate-diethyl ester, 3,5-di-tert-butyl-4-hydroxy-benzylphosphine Phonate-dimethyl ester etc. are mentioned.

The addition amount of the antioxidant is usually 5% by weight to 80% by weight and preferably 20% by weight to 50% by weight with respect to the leuco dye. When the amount is less than 5% by weight, the effect of the antioxidant is insufficient, and when the amount is more than 80% by weight, side effects such as a decrease in image density tend to occur.
Normally, the developer dispersed in the heat-sensitive recording layer melts by heating and reacts with the leuco dye dissolved in the resin to form a color and form an image. However, if the antioxidant has a low melting point, the color develops. The effect of the later antioxidant becomes insufficient. The melting point of the antioxidant is preferably 110 ° C. or higher.

  As the binder resin used in the heat-sensitive recording layer in the present invention, various known organic solvent-soluble resins can be used. For example, polyethylene, polyvinyl acetate, polyacrylamide, maleic acid copolymer, polyacrylic acid and its ester, polymethacrylic acid and its ester, vinyl chloride / vinyl acetate copolymer, styrene copolymer, polyester, polyurethane, Polyvinyl butyral, ethyl cellulose, polyvinyl acetal, polycarbonate, epoxy resin, polyamide, and the like can be used. These resins can be used alone or in combination of two or more.

The amount of binder resin used in the heat-sensitive recording layer is not particularly limited, but in order to ensure gradation and transparency, it is 15% by weight or more based on the total solid content of the coating liquid. A more preferable range in consideration of adhesiveness to the support, color developability, etc. is preferably adjusted in the range of 30 to 60% by weight.
In the present invention, it is preferable to use a resin having a glass transition temperature (Tg) of 50 ° C. or more in the binder resin of the heat-sensitive recording layer at 20% or more of the total resin. More preferably, a glass transition temperature (Tg) of 50 ° C. to 90 ° C. is used. When the glass transition point is less than 50 ° C., the change in concentration with time increases, and when it exceeds 90 ° C., the adhesion to the support deteriorates. If the content is less than 20%, the concentration change with time increases.

As the binder resin having a glass transition point of 50 ° C. or higher, polyvinyl butyral, polyvinyl acetal, polyester resin and the like are preferably used. Polyester resin is preferable in terms of adhesion to the support and change with time, and bisphenol A is particularly preferred in the skeleton. Among the changes in density over time, a polyester resin containing is more preferable in the density change in a state where the image is heated by exposure to Saucusten.
Further, the binder resin of the heat-sensitive recording layer may contain an acrylic polyol resin having a hydroxyl value of 30 or more or a polyester polyol resin having a hydroxyl value of 30 or more and an isocyanate-based crosslinking agent. It is more preferable in the concentration change in the heated state.

  The leuco dye used in the present invention is a colorless or light-colored dye precursor, which is a conventionally known triphenylmethanephthalide, triallylmethane, fluorane, phenothiocyan, thiofluorane, xanthene, indophthalyl, spiropyran. Type, azaphthalide type, chromenopyrazole type, methine type, rhodamine anilinolactam type, rhodamine lactam type, quinazoline type, diazaxanthene type, bislactone type leuco dyes are preferred, and fluorane type and phthalide type leuco dyes are particularly preferred preferable.

  Specific examples of such leuco dyes include 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-di Tylamino-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. In addition, a leuco dye can be used individually or in mixture of 2 or more types.

  In the present invention, the ratio of the weight of the developer to the total weight of the leuco dye is preferably 0.5 or more and 2.5 or less, and particularly preferably 1.0 or more and 2.0 or less. When the weight of the developer is within this range, the halftone image storage stability is particularly improved. At this time, since the coloring efficiency is improved, a high-density image can be formed with a thin film. In gradation media, the advantage of thinning is in controlling the film thickness during coating and in reducing residual moisture and residual solvent in the drying process. Furthermore, since the application amount can be reduced, the cost can be reduced.

  In addition, the dispersion component in the case of coating using an organic solvent is only the developer. Therefore, the smaller the developer content, the more advantageous the light transmission of the coating layer, and the transmission type thermosensitive. The recording medium has an effect of improving contrast and improving image recognition. In addition, when using the developer represented by the general formula (1), fastness is imparted to the image by allowing the leuco dye to enter the network of the developer formed through hydrogen bonding. If the weight ratio is greater than 2.5, the leuco dye will not easily enter the network, and if it is less than 0.5, it will not be possible to construct an appropriate network, resulting in a decrease in coloring efficiency. I think that.

（２）
で示される染料（以下、黒発色染料という）を含有することが好ましい。ここで、Ｒ₂は、水素原子、ハロゲン原子、炭素数が１以上４以下のアルキル基又は炭素数が１以上４以下のアルコキシ基であり、Ｒ₃は、炭素数が１以上４以下のアルキル基である。
これにより、低濃度部分及び高濃度部分のいずれにおいても、黒色画像を得ることができる。 When the thermal recording medium of the present invention is used as a transparent thermal recording medium, the leuco dye is at least one of a red coloring dye and an orange coloring dye, a near-infrared coloring dye, and a general formula (2).

(2)
It is preferable to contain the dye shown below (henceforth black coloring dye). Here, R ₂ is a hydrogen atom, a halogen atom, an alkyl group having 1 to 4 carbon atoms or an alkoxy group having 1 to 4 carbon atoms, and R ₃ is an alkyl group having 1 to 4 carbon atoms. It is a group.
Thereby, a black image can be obtained in both the low density portion and the high density portion.

  Specific examples of black coloring dyes include 2-anilino-3-methyl-6- (N-ethyl-p-tolylamino) fluorane, 2-anilino-3-methyl-6- (N-methyl-p-tolylamino) fluorane. Is mentioned. In the case of a reflective thermal recording medium, the color tone obtained by the reaction between the black color developing dye and the developer represented by the general formula (1) is green in the low density portion and black in the high density portion. However, in the case of a transmissive thermosensitive recording medium, it looks green in the entire area from the low density portion to the high density portion. That is, a transmissive thermosensitive recording medium obtained by using a black coloring dye reflects or transmits light in the visible range from 450 nm to 600 nm and from 650 nm to 700 nm. Therefore, by adding at least one of a red coloring dye and an orange coloring dye and a near-infrared coloring dye, the absorption spectrum in the visible region can be made flat like a silver salt. This is because the red coloring dye and the orange coloring dye have absorption in the range of 450 nm to 600 nm, and the near infrared coloring dye has absorption in the vicinity of 650 nm to 700 nm. That is, it is preferable to mix 3 or more types of leuco dyes, and if necessary, 4 to 6 types may be mixed. The red coloring dye, the orange coloring dye, and the near infrared coloring dye mean leuco dyes whose color tones when heated are red, orange, and near infrared, respectively.

  As a measure of blackening of the image, the ratio of the minimum absorbance to the maximum absorbance in the region of 430 nm or more and 650 nm or less of the absorption spectrum can be used, and when this ratio is 0.65 or more, at least practical on the Schaukasten. Black color can be satisfied. Moreover, if it is 0.75 or more, the influence by the kind of fluorescent lamp with which daytime color, daytime white, etc. are mounted | worn with shaukasten can also be reduced. As the mixing ratio of the above dyes, it is preferable to increase the black coloring dye having high absorption from the viewpoints of high density, color tone adjustment, and storage stability. The content of the black coloring dye is preferably in the range of 40 wt% to 80 wt% of the total dye, and the content of at least one of the red coloring dye and the orange coloring dye and the near infrared coloring dye is 10%, respectively. It is preferable that it is 30 to 30 weight%. When the content of the black coloring dye is more than 80% by weight, it is difficult to blacken the image, and when it is less than 40% by weight, it is difficult to form a high density image.

  Specific examples of the red coloring dye and the orange coloring dye used in the present invention include rhodamine B-o-chloroanilinolactam, 3,6-bis (diethylamino) fluorane-γ- (4′-nitro) anilinolactam, 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-methylindol-3-yl) Phthalides, spiro {chromeno [2,3C] pyrazole-4 (H) -1′-phthalan} -7- (N-ethyl-N-isoamylamino) -3-methyl-1-phenyl-3′-one, etc. Can be mentioned.

（４）
で示される橙発色染料が挙げられる。ここで、Ｒ₅及びＲ₆は、それぞれ炭素数が１以上５以下のアルキル基、フェニル基、トリル基、シクロヘキシル基及びエトシキプロピル基からなる群より選ばれる官能基である。このような橙発色染料としては、１，３−ジメチル−６−ジエチルアミノフルオラン、１，３−ジメチル−６−ジブチルアミノフルオランが挙げられる。 Among these, when a black coloring dye is used, an orange coloring dye suitable for adjusting the absorption spectrum is represented by the general formula (4).

(4)
An orange coloring dye represented by Here, R ₅ and R ₆ are each a functional group selected from the group consisting of an alkyl group having 1 to 5 carbon atoms, a phenyl group, a tolyl group, a cyclohexyl group, and an ethoxypropyl group. Examples of such orange coloring dyes include 1,3-dimethyl-6-diethylaminofluorane and 1,3-dimethyl-6-dibutylaminofluorane.

（５）
で示される３，３−ビス（４−ジエチルアミノ−２−エトキシフェニル）−４−アザフタリドが挙げられる。 Specific examples of the near infrared coloring dye used in the present invention include 6-diethylamino-2-ethylbenzo [1,4] thiazino [3,2-b] fluorane, 3,3-bis (4-diethylamino-2- Ethoxyphenyl) -4-azaphdalide, 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. Among these, as a near-infrared coloring dye suitable for adjusting the absorption spectrum of the black coloring dye, structural formula (5)

(5)
3,3-bis (4-diethylamino-2-ethoxyphenyl) -4-azaphthalide represented by the formula:

  In the present invention, a flat absorption spectrum can be formed in the same manner as the silver salt by combining the black coloring dye with at least one of the red coloring dye and the orange coloring dye and the near infrared coloring dye. At this time, if a commonly used leuco dye in which the amino group at the 6-position of the general formula (3) is an alkyl-substituted amino group is used instead of the black coloring dye, the leuco dye is preferentially colored at a low concentration portion. Therefore, the low density portion cannot be made black. For this reason, when the developer and the black coloring dye represented by the general formula (1) are used, the image can be specifically blackened. This is considered to be due to the affinity between the black coloring dye and the developer.

  In the present invention, the commonly used leuco dyes can be mixed as long as they do not affect the color tone adjustment. Specific examples of such leuco dyes include 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 and the like. In addition, conventionally known triphenylmethane phthalide, triallyl methane, fluoran, phenothiocyan, thiofluorane, xanthene, indophthalyl, spiropyran, azaphthalide, chromenopyrazole, methine, rhodamine anilinolactam Leuco dyes such as rhodamine lactam, quinazoline, diazaxanthene, and bislactone can also be mixed.

  As the support used in the present invention, a support used in a conventional leuco-type thermal recording medium can be applied, but in the case of a transmission type thermal recording medium, it is necessary to use a transparent support. Specific examples of the transparent support include cellulose derivatives such as cellulose triacetate, polyolefins such as polypropylene and polyethylene, polyester resins such as polystyrene, polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate, and films obtained by bonding these. And a resin selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate and polyethylene naphthalate. Further, in order to obtain a heat-sensitive recording medium having high transparency, the haze defined by JISK7105 of the support is preferably 10% or less (including 0%). Polyethylene terephthalate is particularly preferred. In order to improve the adhesion of the heat-sensitive recording layer, at least one surface of the support may be subjected to surface modification by corona discharge treatment, oxidation reaction treatment with chromic acid, etching treatment, or the like.

  In the present invention, a protective layer is preferably provided on the thermosensitive recording layer. Thereby, chemical resistance, water resistance, friction resistance, light resistance and head matching properties can be improved. From the viewpoint of transparency, the protective layer is preferably a resin-only layer, but the smoothness is too high, and printing defects due to sticking and dust drag are likely to occur. In particular, when a plastic film is used as a support, it tends to be smoother than when a paper is used as a support, and therefore there is a tendency for head matching to be reduced and dust to be easily dragged. In the case of a general thermoplastic resin, since the glass transition temperature is lower than the heating temperature by the thermal head, the protective layer of the resin alone may cause surface alteration, recording layer exposure, and the like. As a method for improving the head matching property, a method of adding a filler to the protective layer is common. However, in the case of a heat-sensitive recording medium having high transparency, if a filler used in a conventional reflective recording material is added to the protective layer, the transparency is often lowered. For this reason, in order to maintain transparency by adding a filler, there are a method of adding a filler having a small particle size, a method of adding a small amount of a filler having a large particle size, and the like. In the present invention, the protective layer can be formed by combining these methods as necessary. In addition, it is preferable that the friction coefficient of the surface of a protective layer is 0.07 or more and 0.14 or less.

  In the present invention, as the resin used for the protective layer, in addition to the same water-soluble resin as that used as the binder resin of the thermosensitive recording layer, an aqueous emulsion, a hydrophobic resin, an ultraviolet curable resin, an electron beam curable resin, etc. Can be used, and these can be used together as necessary. From the viewpoint of transparency, the ratio of the refractive index of the resin used in the recording layer and the protective layer to the refractive index of the support is preferably from 0.8 to 1.2. Specific examples of the resin include polyacrylate resin, polymethacrylate resin, polyurethane resin, polyester resin, polyvinyl acetate resin, styrene acrylate resin, polyolefin resin, polystyrene resin, and polyvinyl chloride resin. Examples thereof include resins, polyether resins, polyamide resins, polycarbonate resins, polyethylene, polypropylene, and polyacrylamide resins. Moreover, a crosslinking agent can be used with resin, As a crosslinking agent, well-known compounds, such as an isocyanate compound and an epoxy compound, can be used. Specific examples of isocyanate compounds include compounds having two or more isocyanate groups in the molecule, such as toluylene diisocyanate, dimers thereof, polyisocyanates such as diphenylmethane diisocyanate, polymethylene polyphenyl isocyanate, hexamethylene diisocyanate, and derivatives thereof. Is mentioned. Specific examples of the epoxy compound include ethylene glycol diglycidyl ether, butyl glycidyl ether, polyethylene glycol diglycidyl ether, and epoxy acrylate.

  Specific examples of fillers include phosphate fiber, potassium titanate, acicular magnesium hydroxide, whisker, talc, mica, glass flake, calcium carbonate, plate calcium carbonate, aluminum hydroxide, plate aluminum hydroxide, silica , Clay, calcined clay, kaolin, hydrotalcite and other inorganic fillers and crosslinked polystyrene particles, urea-formalin copolymer particles, silicone resin particles, crosslinked polymethyl methacrylate resin particles, guanamine-formaldehyde copolymer particles, melamine -Organic fillers such as formaldehyde copolymer particles. In the present invention, from the viewpoint of head wear, crosslinked organic polymethyl methacrylate resin particles and melamine-formaldehyde copolymer particles are preferable for organic fillers, and calcium carbonate, kaolin, talc, and aluminum hydroxide are preferable for inorganic fillers. However, the present invention is not limited to this, and a plurality of fillers may be used at the same time in order to impart various characteristics.

  Furthermore, in order to improve the head matching property, by adding waxes or oils to the protective layer, mixing and using a resin modified with silicone, or adjusting the ratio of resin to filler The friction coefficient can be adjusted. The waxes that can be used here include stearic acid amide, palmitic acid amide, oleic acid amide, lauric acid amide, ethylene bisstearyl amide, methylene bisstearyl amide, methylol stearyl amide, paraffin wax, polyethylene, carnauba wax, oxidation Examples include paraffin and zinc stearate. Silicone oil etc. can be used as oils.

  The protective layer can be applied by a conventionally known method. The thickness of the protective layer is preferably from 0.1 μm to 20 μm, and more preferably from 0.5 μm to 10 μm. If the thickness of the protective layer is less than 0.1 μm, the functions of the recording material as a protective layer such as storage stability and head matching properties are insufficient, and if it is thicker than 20 μm, the thermal sensitivity of the recording material is lowered. It is also disadvantageous in terms of cost.

  In the heat-sensitive recording medium of the present invention, it is preferable that at least one of the support, the heat-sensitive recording layer, and the protective layer is colored blue. Thereby, it has an anti-glare effect and can improve image recognizability. Examples of the blue coloring method include a method of kneading a blue pigment on a support, a method of adding a blue dye or a blue pigment to a heat-sensitive recording layer or a protective layer, and the blue pigment and the blue dye are not particularly limited. A known material can be used. The blue density level is preferably 0.15 to 0.25 in terms of transmission density, and the color tone is a * in the CIE-LAB color system of −15 to −4 and b * is It is preferably -15 or more and -5 or less. The color tone was measured by d / 0 (diffuse illumination vertical light receiving method), the field of view was 10 degrees, and the absorbance was measured every 10 nm using the light source D65.

  Although the heat-sensitive recording medium of the present invention is usually a long product immediately after production, the form as a product is a roll-shaped one, and is cut into a predetermined size and a predetermined number of sheets is put into a bag. There is something that I put in. Both are preferably stored and distributed in a light-shielding packaging material because of the nature of the product. A transmission type heat-sensitive recording medium which is opened at the time of use and taken out of the bag is mounted on the image forming apparatus.

  The image forming method of the present invention forms an image on the thermal recording medium of the present invention. As a result, when visually diagnosing and referring to the backlight by the Schaukasten method, it is possible to suppress changes over time in image density and color tone, and to form a high-density, high-quality image, and an image forming method with excellent image storage stability Is obtained. When an image is formed, the heat-sensitive recording medium is heated like an image by a heating unit based on character and shape information. Examples of the heating means include a thermal pen, a thermal head, and laser heating depending on the purpose of use, but it is preferable to use a thermal head from the viewpoint of the cost of the apparatus, the output speed, compactness, and the like.

EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated in more detail, this invention is not restrict | limited at all by these Examples. In the following, parts and% are based on weight.
Example 1
The following composition was pulverized and dispersed with a ball mill until the volume average particle size became 0.5 μm or less to prepare [Liquid A] having a solid content of 20%. The particle size was measured with a laser diffraction particle size measuring device LA-700 manufactured by Horiba Ltd.
[Liquid A]
MEK 28 parts Toluene 24 parts 4- (N′-n-hexylureido) salicylic acid 15 parts 15% polyvinyl butyral MEK solution 33 parts (BM-1, manufactured by Sekisui Chemical Co., Tg = 62 ° C.)

Next, the following composition was sufficiently stirred to prepare a recording layer coating solution [B solution] having a solid content of 18%.
[Liquid B]
1,6-hexanediol-bis- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate] (melting point 94-108 ° C.)
0.1 parts Developer dispersion [liquid A] 40 parts leuco dye A 3 parts leuco dye B 0.2 parts leuco dye C 0.8 parts 15% polyvinyl butyral MEK solution 40 parts (Sekisui Chemical Co., Ltd.) BM-1, Tg = 62 ° C)
MEK 1 part TOL 14.9 parts

ロイコ染料Ｂ：ＧＮ２

ロイコ染料Ｃ：Ｏｒａｎｇｅ１００

Leuco dye A: ETAC

Leuco dye B: GN2

Leuco dye C: Orange 100

The following composition was pulverized and dispersed to a volume average particle size of 0.3 μm with a ball mill to prepare a filler dispersion [liquid C] having a solid content of 18%.
[C liquid]
Silica 15 parts 10% polyvinyl acetal MEK solution (KS-1 manufactured by Sekisui Chemical Co., Ltd.) 30 parts MEK 55 parts The following composition was pulverized and dispersed with a ball mill to a volume average particle size of 0.3 μm, and a lubricant dispersion having a solid content of 18%. [Liquid D] was prepared.
[Liquid D]
Zinc stearate 15 parts 10% polyvinyl acetal MEK solution (KS-1 manufactured by Sekisui Chemical Co., Ltd.) 30 parts MEK 55 parts Next, the following composition is sufficiently stirred, and a protective layer coating liquid having a solid content of 18% [E liquid] Was prepared.
[E liquid]
Filler dispersion [Liquid C] 60 parts Lubricant dispersion [Liquid D] 6 parts 10% polyvinyl acetal MEK solution (KS-1 manufactured by Sekisui Chemical Co., Ltd.) 30 parts 75% hexamethylene isocyanate ethyl acetate solution 4 parts (Japan) Polyurethane Coronate HL)

  The recording layer coating solution [Liquid B] thus prepared was applied onto a transparent polyester film (having a value of 3%) having a thickness of 175 μm using a wire bar, and the recording layer coating solution [B] was kept at a temperature of 100 ° C. After drying for 15 minutes to form a heat-sensitive recording layer having a thickness of 15 μm, the protective layer coating solution [E solution] was applied using a wire bar and dried for 1 minute with a dryer maintained at a temperature of 80 ° C. A protective layer having a thickness of 3 μm was formed to produce a thermosensitive recording medium.

Example 2
Except that 0.4 part of 1,6-hexanediol-bis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] in Example 1 [Liquid B] was used in the same manner. A thermosensitive recording medium was produced.

Example 3
Thermal recording in the same manner except that 1 part of 1,6-hexanediol-bis- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate] in Example 1 [Liquid B] was used. A medium was made.

Example 4
Thermal recording in the same manner as in Example 1 [Liquid B] except that 4 parts of 1,6-hexanediol-bis- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate] was used. A medium was made.

Example 5
0.1 part of 1,6-hexanediol-bis- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate] in Example 1 [Liquid B] was added to pentaerythrityl-tetrakis [3 A thermosensitive recording medium was prepared in the same manner except that 1 part of-(3,5-di-t-butyl-4-hydroxyphenyl) propionate] (melting point: 110 to 125 ° C) was used.

Example 6
The following composition was pulverized and dispersed with a ball mill until the volume average particle size became 0.5 μm or less to prepare [F solution] having a solid content of 20%. The particle size was measured with a laser diffraction particle size measuring device LA-700 manufactured by Horiba Ltd.
[F liquid]
MEK 32.5 parts Toluene 32.5 parts 4- (N′-n-hexylureido) salicylic acid 15 parts 25% polyester solution (MEK / TOL = 1/1) 20 parts (Unitika UE-3600, Tg = 75 ℃)
Next, the following composition was sufficiently stirred to prepare a recording layer coating solution [G solution] having a solid content of 18%.

[G liquid]
Pentaerythrityl-tetrakis [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate]
1 part Developer dispersion [F liquid] 32 parts leuco dye A 3 parts below leuco dye B 0.2 parts leuco dye C 0.8 parts 25% polyester solution (MEK / TOL = 1/1) 24 Part (UE-3600 manufactured by Unitika, Tg = 75 ° C.)
MEK 20 parts TOL 20 parts

The following composition was sufficiently stirred to prepare a back layer coating solution [H solution] having a solid content of 16%.
[H liquid]
74.5 parts of 20% polyester MEK solution (GK880 manufactured by Toyobo Co., Ltd.)
Silicone filler (TOSpearl 2000B manufactured by Toshiba Silicone Co., Ltd., average particle size 6 μm)
0.1 part MEK 25.4 parts Example on a transparent polyester film having a thickness of 175 μm and bluing to a background penetration density of 0.22, a * = − 7, b * = − 9 (CIE-LAB color system) After forming the recording layer and the protective layer using the recording layer liquid [G liquid] and the protective layer liquid [E liquid] in the same manner as in No. 1, apply the back layer coating liquid H liquid on the back surface using a wire bar. And dried for 1 minute with a drier kept at a temperature of 80 ° C. to form a back layer having a thickness of 4 μm.

Comparative Example 1
A thermal recording medium was prepared in the same manner as in Example 5 [Liquid A] except that 4- (N′-n-hexylureido) salicylic acid was changed to octadecylphosphonic acid.
Comparative Example 2
A thermosensitive recording medium was prepared in the same manner except that 1,6-hexanediol-bis- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate] in Example 1 [Liquid B] was omitted. Produced.
Comparative Example 3
Example 2 [Liquid B] 1,6-hexanediol-bis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] was converted to tris (2,4-di-t-butyl). A thermosensitive recording medium was prepared in the same manner except that it was phenyl) phosphite.
Comparative Example 4
Example 2 [Liquid B] 1,6-hexanediol-bis- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate] was converted to distearyl-3,3-thiodipropionate A thermosensitive recording medium was prepared in the same manner except that.

For the heat-sensitive recording materials of Examples and Comparative Examples produced as described above, a gray scale is printed by a variable energy printer (manufactured by Oaks) equipped with a gradation head with a resolution of 300 dpi, and the transmission density and color tone. The light resistance was evaluated.
Evaluation methods and evaluation criteria are shown below.
Transmission density Measured with 309TR manufactured by X-Rite.
○: Transmission density 3.0 or more Δ: Transmission density 2.0 or more ×: Transmission density lower than 2.0

Color tone The evaluation was carried out by visual observation with a daylight color fluorescent lamp mounted on an X-ray photograph observer ICH-3K (manufactured by Moriyama X-ray Supplies Co., Ltd.).
Light fastness Immediately after printing, the transmission density and color value are measured. Immediately after the measurement, it is attached to an X-ray photograph observer ICH-3K (manufactured by Moriyama X-ray Supplies Co., Ltd.), and the daylight color fluorescent lamp is turned on for 72 hours continuously. The transmission density and color value of the image after mounting were measured, and the change in density and the color difference were evaluated over time. Judgment was carried out from the portion of the largest change in each gradation.
The transmission density was measured using 309TR manufactured by X-Rite.
○: Transmission density variation 0.2 or less Δ: Transmission density variation 0.40 or less ×: Transmission density variation 0.40 or higher

The color value (L * a * b * color system) was measured using a colorimeter CM-3700d manufactured by Minolta. The color difference was calculated by the following formula.
Color difference = [(immediately after printing a * −a * after storage) ² + (b * immediately after printing−b * after storage) ² ] ^1/2
○: Color difference 2 or less △: Color difference 3 or less ×: Background color difference greater than 3 = [(L * immediately after printing * L * after storage) ² + (a * immediately after printing * a * after storage) ² + (b immediately after printing b * -After storage b *) ² ] ^1/2
○: Color difference 1 or less Δ: Color difference 1.5 or less ×: Color difference greater than 1.5 Table 1 shows the evaluation results.

Claims (6)

In a thermosensitive recording medium having a support, a thermosensitive recording layer formed on the support, and a protective layer formed on the thermosensitive recording layer, the thermosensitive recording layer comprises a resin, a leuco dye, a developer, and an antioxidant. The developer is represented by the following general formula (1)

(1)

(In the formula, R ₁ represents a linear alkyl group or alkylamino group having 4 to 16 carbon atoms.)
And the antioxidant contains a substituent represented by 3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate.   The melting point of the antioxidant containing the substituent represented by the 3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate is 110 ° C. or higher. Thermal recording media.   3. The heat-sensitive recording medium according to claim 1, wherein the antioxidant is 5% by weight to 80% by weight with respect to the leuco dye.   The thermal recording medium according to claim 1, wherein the recording layer resin is a polyester resin. The leuco dye is at least one of a red coloring dye and an orange coloring dye, a near infrared coloring dye, and a general formula (2)

(2)
(In the formula, R ₂ is a hydrogen atom, a halogen atom, an alkyl group having 1 to 4 carbon atoms or an alkoxy group having 1 to 4 carbon atoms, and R ₃ has 1 to 4 carbon atoms. Represents an alkyl group.)
The heat-sensitive recording medium according to claim 1, comprising a dye represented by the formula:   An image forming method comprising forming an image on the heat-sensitive recording medium according to claim 1.