JPH01171981A - Heat-sensitive recording material - Google Patents

Abstract

PURPOSE:To obtain a heat-sensitive recording material being suitable for an overhead projector and multicolor recording, by a method wherein a basic dye precursor is dissolved by using an ester having at least a boiling point of a specific temperature or above, so as to be made into a transparent heat- sensitive layer having a higher heat sensitivity, when the basic dye precursor is contained in a microcapsule. CONSTITUTION:A microcapsule to be used is prepared preferably by emulsifying an oil phase to be a core substance containing a reactive substance such as a color-developing agent and then by forming a wall of a macromolecular substance around an oil droplet thus formed, and in this case a reactant forming the macromolecular substance is added to the inside or outside of the oil droplet. In the case when an ester having a high boiling point of 150 deg.C or above is used as an organic solvent for forming the oil droplet, the concentration of a basic dye precursor in the microcapsule can be increased since the solubility of the basic dye precursor to the ester is high, and thereby the concentration of a heat-sensitive recording material can be increased.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a heat-sensitive recording material with excellent transparency, and particularly to a heat-sensitive recording material having a heat-sensitive layer with high image density and excellent transparency.

(Prior art) The thermal recording method has three advantages: (1) no development is required; (2) when the support is paper, the quality of the paper is similar to that of ordinary paper; (3) it is easy to handle; and (4) color density is high. (5) The recording device is simple and inexpensive, and (6) There is no noise during recording, so it has rapidly become popular in the field of facsimiles and printers in recent years, and has become popular in the field of labels such as PO3 etc. The uses of thermal recording are also expanding.

Against this background, in recent years, in order to adapt to multicolor, overhead projectors (
It is desired to develop a transparent heat-sensitive recording material that can be directly recorded with a thermal head for use in OHP.

In order to meet such demands, the present inventors have prepared microcapsules containing a colorless or light-colored basic dye precursor on a support, and a color developer dissolved in an organic solvent that is sparingly soluble or insoluble in water. proposed a heat-sensitive recording material having a substantially transparent heat-sensitive layer formed by coating and drying a coating liquid consisting of an emulsified dispersion of .

(Problems to be Solved by the Invention) However, even in such special uses, it is desired to further increase thermal sensitivity in order to cope with the recent increase in speed of facsimiles and the like.

Therefore, a first object of the present invention is to provide a transparent heat-sensitive layer with higher and improved heat sensitivity.

A second object of the present invention is to provide a heat-sensitive recording material suitable for OHP and multicolor recording, which has a heat sensitivity that can be adapted to high-speed thermal recording devices.

Furthermore, a third object of the present invention is to provide a method for increasing the heat sensitivity of a heat-sensitive recording material having good transparency.

(Means for solving the problem) The above-mentioned aspects of the present invention include microcapsules containing a colorless or light-colored basic dye precursor, and a color developer dissolved in an organic solvent that is sparingly soluble or insoluble in water. In a heat-sensitive recording material having a heat-sensitive layer formed by emulsifying and dispersing a dye solution and coating a coating solution containing the obtained emulsified dispersion on a support and drying, the basic dye precursor is encapsulated in microcapsules. This has been achieved by a heat-sensitive recording material characterized in that the basic dye precursor is dissolved using an ester having a boiling point of at least 150'C or higher.

As the basic dye precursor used in the present invention, a colorless or light-colored one is appropriately selected from known compounds that develop color by donating electrons or accepting protons such as acids. Such compounds have partial skeletons such as lactones, lactams, sultones, spiropyrans, esters, and amides, and these partial skeletons are ring-opened or cleaved upon contact with a color developer. Preferred compounds include Examples include triarylmethane compounds, diphenylmethane compounds, xanthine compounds, thiazine compounds, and spiropyran compounds.

Particularly preferred compounds are those represented by the following general formula.

In the formula, R is an alkyl group having 1 to 8 carbon atoms, R2 is an alkyl group having 4 to 18 carbon atoms, an alkoxyalkyl group, or a tetrahydrofurfuryl group, and R5 is a hydrogen atom or an alkyl group having 1 to 15 carbon atoms. The group or halogen atom, R4, represents a substituted or unsubstituted aryl group having 6 to 20 carbon atoms.

As the substituent for R4, an alkyl group having 1 to 5 carbon atoms, an alkoxy group, a halogenated alkyl group, and a halogen atom are preferable.

In the present invention, by encapsulating the above-mentioned coloring agent in microcapsules, it is possible to prevent fogging during the production of heat-sensitive recording materials, and at the same time, to improve the raw and record storage properties of the heat-sensitive recording materials. can. In this case, by selecting the wall material and manufacturing method of the microcapsules, it is possible to increase the image density during recording. The amount of color former used is preferably 0.05 to 5.0 g/rd.

Examples of the wall material of the microcapsule include polyurethane, polyurea, polyester, polycarbonate, urea-formaldehyde resin, melamine resin, polystyrene, styrene methacrylate copolymer, styrene-acrylate copolymer, gelatin, polyvinylpyrrolidone, polyvinyl alcohol, etc. . In the present invention, two or more of these polymeric substances can also be used in combination.

In the present invention, among the above-mentioned polymeric substances, polyurethane, polyurea, polyamide, polyester, polycarbonate, etc. are preferable, and polyurethane and polyurea are particularly preferable.

After that, it is preferable to form a wall of a polymeric substance around the oil droplet to form a microcapsule, in which case a polymeric substance is formed or an actant is added to the inside of the oil droplet and/or the outside of the oil droplet. . For details about microcapsules that can be preferably used in the present invention, such as a preferred method for producing microcapsules, see, for example, JP-A-59-2227.
It is described in No. 16.

Here, boiling point 15 is used as an organic solvent for forming oil droplets.
When esters with a high boiling point of 0°C or higher are used, it is difficult to increase the concentration of the basic dye precursor in the microcapsules because the basic dye precursor has a high solubility in these esters. This makes it possible to increase the density of the heat-sensitive recording material of the present invention.

Examples of high-boiling esters that can be preferably used in the present invention include phosphoric esters (e.g., triphenyl phosphate, tricresyl phosphate, butyl phosphate, octyl phosphate, cresyl diphenyl phosphate), phthalic esters (dibutyl phthalate, 2-ethylhexyl phthalate, ethyl phthalate, octyl phthalate, butylbenzyl phthalate) dioctyl tetrahydrophthalate, benzoic acid ester (ethyl benzoate, propyl benzoate, butyl benzoate, isopentyl benzoate, benzyl benzoate), Abietate esters (ethyl abietate, benzyl abietate), dioctyl adipate, isodecyl succinate, ethyl succinate, dioctyl azelaate, oxalate esters (dibutyl oxalate, dipentyl oxalate)
, diethyl malonate, maleate esters (dimethyl maleate, diethyl maleate, dibutyl maleate)
, tributyl citrate, sorbate esters (methyl sorbate, ethyl sorbate, butyl sorbate), sebacate esters (dibutyl sebacate, dioctyl sebacate), ethylene glycol esters (formic acid monoesters and diesters, butyric acid monoesters and diesters, lauric acid monoesters and diesters, palmitic acid monoesters and diesters, stearic acid monoesters and diesters, oleic acid monoesters and diesters), triacetin, diethyl carbonate, diphenyl carbonate, ethylene carbonate, propylene carbonate, boric acid esters tributyl acid, tripentyl borate), etc. Among these, succinic acid lower alkyl esters such as ethyl succinate and butyl succinate, and maleic acid lower alkyl esters such as ethyl maleate are particularly preferred.

These esters may be used alone or in combination.

In the present invention, in addition to the above-mentioned esters, an oil appropriately selected from those used as pressure-sensitive oils can be used in combination. Examples of such pressure-sensitive oils include compounds represented by the following general formulas (1) to (■), triallylmethane (e.g. tritolylmethane, tolyldiphenylmethane), terphenyl compounds (e.g. phenyl), alkylated diphenyl ethers (eg, propyl diphenyl ether, hydrogenated terphenyl (eg, hexahydroterphenyl), diphenyl ether, etc.).

(I) In the formula, R" represents hydrogen or an alkyl group having 1 to 18 carbon atoms,
R2 represents an alkyl group having 1-18 carbon atoms hT, p',
q' represents an integer from 1 to 4, and the total number of alkyl groups is 4
No more than 100 pieces.

Note that the alkyl groups for R' and R2 are preferably alkyl groups having 1 to 8 carbon atoms.

(II) (R3)P''(R')(1'' In the formula, R2 is a hydrogen atom or an atom having 1 to 12 carbon atoms.
R4 represents an alkyl group having 1 to 12 carbon atoms, and n represents 1 or 2.

p 2 and q Z represent integers of 1 to 4. When n=1, the total number of alkyl groups is 4 or less, and when n=1, the total number of alkyl groups is 6 or less.

In the formula, R5, R' is a hydrogen atom or a carbon number of 1 to 18,
Represents the same or different alkyl groups.

m represents an integer from 1 to 13. P' and Q3 represent an integer of 1 to 3, and the total number of alkyl groups is 3 or less.

Note that the alkyl groups for R' and R'' are particularly preferably alkyl groups having 2 to 4 carbon atoms.

Examples of the compound represented by formula (1) include dimethylnaphthalene, diethylnaphthalene, diisopropylnaphthalene, and the like.

Examples of the compound represented by formula (II) include dimethylbiphenyl, diethylbiphenyl, diisopropylbiphenyl, diisobutylbiphenyl, and the like.

Examples of the compound represented by formula (III) include 1-methyl-1-dimethylphenyl-1-phenylmethane, 1-
ethyl-1-dimethylphenyl-1-phenylmethane,
Examples include 1-propyl-1-dimethylphenyl-1-phenylmethane.

Furthermore, as a co-solvent, ethyl acetate, isopropyl acetate,
A low boiling point organic solvent such as butyl acetate or methylene chloride may be used in combination.

The preferred microcapsules produced as described above are not destroyed by heat or pressure as used in conventional recording materials, and the reactive substances contained in the core and outside of the microcapsules are not destroyed by the walls of the microcapsules. can pass through and react.

In the present invention, the wall material of the microcapsule is selected,
If necessary, a glass transition point adjusting agent (for example, Japanese Patent Application No. 1983)
-119862) to prepare microcapsules consisting of walls with different glass transition points, and by selecting combinations of basic colorless dye precursors and their color developers with different hues. It is possible to realize multiple intermediate colors. Therefore, the present invention is not limited to monochrome thermal paper, but can also be applied to two-color or multi-color thermal paper and thermal paper suitable for recording images with gradation.

Further, if necessary, a photofading inhibitor described in, for example, JP-A-61-283589, JP-A-61-283590, and JP-A-61-283591 can be appropriately added.

The color developer that causes a color-forming reaction when melted with the basic colorless dye used in the present invention can be appropriately selected from known ones and used.

For example, color developers for leuco dyes include phenolic compounds, sulfur-containing phenolic compounds, carboxylic acid compounds, sulfone compounds, urea or thiourea compounds, etc. Times (1985), pages 49-54 and 65-70. Among these, especially those with a melting point of 50°C to 25°C
0''C is preferable, and 60'C to 200C and phenol and organic acid which are poorly soluble in water are particularly preferable.When two or more types of color developers are used in combination, the solubility increases, so it is preferable.

Among the color developers used in the present invention, particularly preferred ones are represented by the following general formulas (IV) to [■].

m=O~2, n=2~11 (V) R7 is an alkyl group, an aryl group, an aryloxyalkyl group, or an aralkyl group, with methyl and butyl groups being particularly preferred.

(Vl) R9 Re is an alkyl group, especially a butyl group, a pentyl group,
Heptyl and octyl groups are preferred.

R9 is a hydrogen atom or a methyl group, and n is 0-2.

[■] R'' is an alkyl group, an aralkyl group or an aryloxyalkyl group.

In the present invention, a color developer is dissolved in an organic solvent that is sparingly soluble or insoluble in water, and then mixed with an aqueous phase containing a surfactant and having a water-soluble polymer as a protective colloid. It is used in the form of an emulsified dispersion.

The organic solvent for dissolving the color developer can be appropriately selected from organic solvents that are sparingly soluble or insoluble in water, but in particular, when an organic solvent with a boiling point of 150°C or lower is used, it has good thermal sensitivity. Therefore, it is preferable. Examples of these organic solvents include ethyl acetate, isopropyl acetate, butyl acetate, and methylene chloride.

In the present invention, these organic solvents can be mixed with the above-mentioned high-boiling esters and pressure-sensitive oils, but in particular, esters are preferably used from the viewpoint of stability of the color developer emulsion dispersion. is preferred.

Among these, it is particularly preferable to use tricresyl phosphate in combination because the emulsion dispersion stability of the color developer is good.

The water-soluble polymer to be contained as a protective colloid in the aqueous phase mixed with the oil phase in which the color developer is dissolved can be appropriately selected from known anionic polymers, nonionic polymers, and amphoteric polymers. In particular, polyvinyl alcohol, gelatin, cellulose derivatives, etc. are preferred.

Further, as the surfactant to be contained in the aqueous phase, one can be appropriately selected from anionic or nonionic surfactants that do not cause precipitation or aggregation by interacting with the above-mentioned protective colloid. .

Preferred surfactants include sodium alkylbenzenesulfonate (for example, sodium dodecylbenzenesulfonate), sodium alkylsulfate, dioctyl sodium sulfosuccinate, polyalkylene gelicol (for example, polyoxyethylene nonylphenyl ether), and the like. can.

The emulsified dispersion of a color developer in the present invention is produced by mixing an oil phase containing a color developer and an aqueous phase containing a protective colloid and a surfactant by means commonly used for fine particle emulsification, such as high-speed stirring and ultrasonic dispersion. It can be easily mixed and dispersed using.

A melting point depressant for a color developer may be added to this emulsified dispersion as appropriate. Some of these melting point depressants are
It also functions as a glass transition point regulator for the capsule wall. Examples of such compounds include hydroxy compounds, carbamate ester compounds, sulfonamide compounds, aromatic methoxy compounds, etc. For details, see
For example, it is described in Japanese Patent Application No. 59-244190.

These melting point depressants can be used in an amount of 0.1 to 2 parts by weight, preferably 0.5 to 1 part by weight, per 1 part by weight of the color developer that lowers the melting point. It is preferable that the color developer and the color developer whose melting point is lowered thereby are used at the same location. When adding to different locations, it is preferable to add 1 to 3 times the above amount.

The heat-sensitive recording material of the present invention can be coated using a suitable binder.

As a binder, polyvinyl alcohol, methylcellulose, carboxymethylcellulose, sodium alginate, hydroxypropylcellulose, gum arabic, gelatin, polyvinylpyrrolidone, casein, styrene-butadiene latex, acrylonitrile-butadiene latex, polyvinyl acetate, polyacrylic acid ester, ethylene- Various emulsions such as vinyl acetate copolymers can be used. The amount used is 0.0 as solid content.
It is 5 to 5 g/po.

The heat-sensitive recording material of the present invention is produced by preparing a coating liquid containing microcapsules containing a color former, main components of a dispersion in which at least a color developer is emulsified and dispersed, and other additives such as a binder. Bar coating, blade coating, air knife coating, gravure coating,
Apply by coating methods such as roll coating, spray coating, dip coating, etc. and dry to a solid content of 2.5 to 25%.
It is manufactured by providing a heat sensitive layer of g/rrf.

The heat-sensitive layer of the heat-sensitive recording material thus produced has extremely good transparency.

In the heat-sensitive recording material of the present invention, at least one protective layer is preferably provided on the heat-sensitive layer by a known method for the purpose of preventing sticking to a thermal head and improving writing properties. Details about the protective layer are described, for example, in "R Paper and Pulp Technology Times" (September issue, 1985), pages 2 to 4.

It is preferable to add a filler to the protective layer in order to improve film strength and/or prevent sticking during thermal recording.

Examples of such fillers include inorganic pigments such as zinc oxide, calcium carbonate, barium sulfate, titanium oxide, lithopone, talc, waxite, kaolin, aluminum hydroxide, amorphous silica, and colloidal silica, polystyrene, and vinyl acetate. Resin, polymethyl methacrylate, styrene
Organic pigments such as methacrylate copolymers, vinylidene chloride, polystyrene, polyurea, and melamine-formaldehyde, metal soaps such as zinc stearate, calcium stearate, and aluminum stearate, paraffin wax, microcrystalline wax, carnauba wax, and stearic acid amide. , stearic acid ethylene bisamide, methylol stearamide, polyethylene wax, silicone, and other waxes.

In order not to reduce transparency, the particle size of these fillers must be 3 microns or less, and more preferably 1.5 microns or less.

These fillers may be used alone or in combination of two or more.

The total amount of these fillers added to the protective layer is usually 0.
It is preferably 1 to 100%, particularly 10 to 50%.

As the support for the heat-sensitive recording material of the present invention, both a transparent support and an opaque support such as paper can be used depending on the purpose and use.

By using a transparent support, the recorded matter can be kept free of O
It can be used for HP sheets, etc.

The transparent support mentioned here includes polyester films such as polyethylene terephthalate and polybutylene terephthalate, cellulose triacetate films, polypropylene films, polyolefin films such as polyethylene, etc., and these can be used alone or in combination. can.

The thickness of the transparent support used is 20 to 200 .mu.m, particularly preferably 50 to 100 .mu.m.

・The paper used for the support is thermally extracted pH sized with a neutral sizing agent such as an alkyl ketene dimer.
Use of neutral paper No. 6 to No. 9 (described in JP-A-55-14281) is advantageous in terms of storage stability over time.

In order to prevent the coating from penetrating into the paper and to improve the contact between the thermal recording head and the thermal recording layer, Japanese Patent Laid-Open No. 57-1166
The paper described in No. 87 and having a Bekk smoothness of 90 seconds or more is advantageous.

Also, paper having an optical surface roughness of 8μ or less and a thickness of 40 to 75μ, described in JP-A-58-136492, JP-A-58-136492.
Density 0.-69097. Paper with an optical contact rate of 9 g/cm' or less and an optical contact rate of 15% or more, JP-A-58-690
Canadian standard freeness described in No. 97 (JIS P812
Paper made from valves beaten to 400 cc or more in 1) to prevent penetration of coating liquid, JP-A-58-656
No. 95, the glossy side of base paper made by a Yankee machine is used as the coating surface to improve color density and resolution, and JP-A No. 59-35985 describes a method in which the base paper is subjected to corona discharge treatment. Papers with improved coating suitability may also be used in the present invention with good results. In addition to these, any support commonly used in the field of heat-sensitive recording paper can be used as the support of the present invention.

In the present invention, an undercoat layer can be provided between the transparent support and the heat-sensitive layer in order to enhance adhesion between the two layers. Gelatin, synthetic polymer latex, nitrocellulose, etc. are used as the material for the undercoat layer. The coating amount of the undercoat layer is 0.
It is preferably in the range of 1 g/m" to 2.0 g/m2, particularly 0.2 g/m" to 1.0 g/m" to 1.0 g/m"
A range of is preferred.

If it is less than 0.1 g/m", the adhesion between the support and the heat-sensitive layer is insufficient, and even if it is increased to 2.0 g/m" or more, the adhesion between the support and the heat-sensitive layer has reached saturation. Therefore, it is disadvantageous in terms of cost.

The undercoat layer is coated with a hardening agent because the image quality of the heat-sensitive layer may deteriorate if the undercoat layer swells due to the water contained in the heat-sensitive layer when the heat-sensitive layer is coated on top of it. It is desirable to cure it.

As hardeners that can be used in the present invention, the following can be mentioned.

■Divinylsulfone-N,N'-ethylenebis(vinylsulfonylacetamide), 1,3-bis(vinylsulfonyl)-2-propatol, methylenebismaleimide, 5-acetyl-1,3-diacryloyl- Hexahydro-3-triazine, 1.3.5-triacryloyl-
Active vinyl compounds such as hexahydro-s-triazine, 1,3.5-)rivinylsulfonyl-hexahydro-3-triazine.

■2.4-dichloro-6-hydroxy-5-t-riazine sodium salt, 2.4-dichloro-6-nodoxy-
S-triazine, 2.4-dichloro-6-(4-sulfoanilino)-s-) riazine sodium salt, 2.4-
Active halogen compounds such as dichloro-6-(2-sulfoethylamino)-S-)riazine and N-N'-bis(2-chloroethylcarbamyl)piperazine.

■Bis(2,3-epoxypropyl)methylpropylammonium p-)luenesulfonate, 1,4-bis(2°,3'-epoxypropyloxy)butane, 1
, 3.5-1 liglycidyl isocyanurate, 1.3-
Epoxy compounds such as diglycidyl-5-(γ-acetoxyβ-oxypropyl)isocyanurate.

■2.4.6-) lyethylene-s-) riazine, 1.6
Ethyleneimino compounds such as -hexamethylene-N,N'-bisethyleneurea and bis-β-ethyleneiminoethylthioether.

■1.2-di(methanesulfonoxy)ethane, 1.4
- Methanesulfonic acid ester compounds such as di(methanesulfonoxy)butane and 1°5-di(methanesulfonoxy)pentane.

■Dicyclohexylcarbodiimide, 1-cyclohexyl-8-(3-)dimethylaminopropyl)carbodiimide-T)-)riensulfonate, 1-ethyl-3-
Carbodiimide compounds such as (3-dimethylaminopropyl)carbodiimide hydrochloride.

■2.5-dimethylisoxazole perchlorate, 2
Isoxapole-based compounds such as -ethyl-5-phenylisoxazole-3'-sulfonate and 5,5'-(paraphenylene)bisisoxazole.

■Inorganic compounds such as chromium alum, chromium acetate, zirconium salts and boric acid.

■N-carboethoxy-2-isopropoxy-1,2-
Dihydroquinoline, N-(1-morpholinocarboxy)
Dehydrated condensation type peptide reagents such as -4-methylpyridinium chloride, active ester compounds such as N,N'-aziboyldioxydisuccinimide, and N,N''-terephthaloyldioxydisuccinimide.

[Phase] Toluene-2.4-diisocyanate, 1°6-
Isocyanates such as hexamethylene diisocyanate.

■Dialdehydes such as glutaraldehyde, glyoxal, dimethoxyurea, and 2,3-dihydroxy-1,4-dioxane.

Among these, dialdehydes such as glutaraldehyde and 2°3-dihydroxy-1,4-dioxane, and boric acid are particularly preferred.

The amount of these hardeners added is based on the weight of the base coat material.
An appropriate addition amount can be selected in the range of 0.20% by weight to 3.0% by weight depending on the coating method and desired degree of curing.

If the amount added is less than 0.2% by weight, the degree of curing will be insufficient no matter how long it is allowed to last, and the undercoat layer will swell when the heat-sensitive layer is applied. If the amount is too high, the degree of curing will proceed too much, and the adhesion between the undercoat layer and the support will deteriorate, resulting in the disadvantage that the undercoat layer will become film-like and peel off from the support.

Depending on the hardening agent used, if necessary, the pH of the solution can be made alkaline by adding caustic soda or the like, or the pH of the solution can be made acidic by adding citric acid or the like.

It is also possible to add an antifoaming agent to eliminate foam generated during coating, or to add an activator to improve liquid leveling and prevent coating streaks.

Furthermore, before applying the undercoat layer, it is desirable to activate the surface of the support by a known method.

Activation treatment methods include acid etching treatment,
Flame treatment using a gas burner, corona discharge treatment, glow discharge treatment, etc. are used, but from the point of view of cost or simplicity, U.S. Pat. Same No. 3,549,406,
The corona discharge treatment described in Patent No. 3,590.107 and the like is most preferably used.

The heat-sensitive recording material of the present invention only needs to have the above-described substantially transparent heat-sensitive layer formed on at least one side of the support,
In addition to providing one layer of the heat-sensitive layer on one side of a transparent or opaque support, a double-sided heat-sensitive recording material may be prepared by providing one heat-sensitive layer of the same structure on both sides of the opaque support, and Substantially transparent heat-sensitive layers capable of developing different colors may be provided on each side of the transparent support, or furthermore, known heat-sensitive layers may be laminated, depending on the use and purpose. Various embodiments are possible.

(Effects of the Invention) In the heat-sensitive recording material of the present invention, not only is the heat-sensitive layer itself substantially transparent, but also the image density is high and the sensitivity and recording stability are good. It is suitable for directly obtaining OHP or multicolor recording.

(Examples) Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited thereto.

Example 1 [Preparation of capsule liquid] 10 g of the following leuco dye was mixed with 10 g of diethyl succinate, 1
-Phenyl-1-xylylethane 2g and ethyl acetate 1
It was added to 5 g of a mixed solvent and heated to 60°C to dissolve. After cooling the melt to 30°C, Takenate D-11O
16 g of N (capsule wall material manufactured by Takeda Pharmaceutical ■) was added. This leuco dye solution was mixed with 8% polyvinyl alcohol (
45g of Kuraray PVA 17E) aqueous solution and 33g of water (
7) Mix with aqueous solution, emulsify with Nippon Seiki's Ace homogenizer at 10.00 rpm for 5 minutes, and add 1 liter of water.
50g was added and reacted at 40°C for 3 hours to produce a capsule liquid with a capsule size of 0.7μ.

[Preparation of developer emulsion dispersion]

Color developer (a) 8 g, Φ) represented by the following structural formula
4g and 30g of (C) were dissolved in 8.0g of 1-phenyl-1-xylylethane and 30g of ethyl acetate. The obtained developer solution was mixed with 8% polyvinyl alcohol aqueous solution 1
00g, water 150g, and sodium dodecylbenzenesulfonate 0.5g aqueous solution and emulsified using Nippon Seiki's Ace homogenizer at 10.00 rpm for 5 minutes at room temperature to obtain an emulsified dispersion with a particle size of 0.5μ. I got something.

Color developer (a) Color developer ('b) [Preparation of heat-sensitive recording material] 5.0 g of the above capsule liquid, 10.0 g of color developer emulsion dispersion
and 5 g of water were stirred and mixed, and the mixture was coated on one side of a transparent polyethylene terephthalate (PET) support with a thickness of 70 μm to a solid content of 10 g/m” and dried, followed by a 2 μm protective layer having the following composition. A transparent thermosensitive film was prepared.

[Composition of protective layer] Silicon-modified polyvinyl alcohol 1 part by weight (PVA R2105 manufactured by Kuraray ■) (solid content) Colloidal silica 1.5 parts by weight (Snowtex 30 manufactured by Nissan Chemical ■) (solid content) Paraffin wax
0.4 parts by weight (Cellosol 42B manufactured by Middle East Oil & Fat ■)
(Solid content) Zinc stearate 0.0
2 parts by weight (Hydrin Z-7 manufactured by Middle East Oil Co., Ltd.) (Solid content) The heat-sensitive recording material thus obtained was copied and printed using a commercially available G mode facsimile machine, and the color density was measured using a Macbeth transmission densitometer. The color density was 1.2.
It was an extremely high value of 0, which was a good result.

Example 2 The same good results as in Example 1 were obtained in exactly the same manner as in Example 1, except that 10 g of diethyl maleate was used in place of 10 g of diethyl succinate used in Example 1.

Comparative Example 1 10 g of dibutyl succinate used in Example 1 was not used, and 10 g of 1-phenyl-1-xylylethane was used instead.
Encapsulation was attempted in the same manner as in Example 1 except that g was added. In this case, precipitation of the leuco dye occurred during the process of cooling the leuco dye-dissolved solution to 30° C., making it impossible to encapsulate the leuco dye.

Comparative Example 2゜Completely the same as Example 1 except that the diethyl succinate log used in Example 1 was not used, and instead, 10 g of leuco dye was reduced to 6 g, and 12 g of 1-phenyl-1-xylylethane was increased to 16 g. When the same procedure was performed, the color density was 0.8, which was insufficient.

From the results of the above examples and comparative examples, the image density of the heat-sensitive recording material can be improved by dissolving the leuco dye and using an organic solvent with a boiling point of 150°C or higher as the organic solvent that becomes the core material of the microcapsules. It has been proven that it can be improved.

Claims (1)

[Claims] 1) Contains an emulsified dispersion obtained by emulsifying and dispersing microcapsules containing a colorless or light-colored basic dye precursor and a color developer solution dissolved in an organic solvent that is sparingly soluble or insoluble in water. In a heat-sensitive recording material having a heat-sensitive layer formed by coating and drying a coating solution on a support, when the basic dye precursor is encapsulated in microcapsules, an ester having a boiling point of at least 150° C. or higher is used to encapsulate the base. A heat-sensitive recording material characterized by dissolving a dye precursor.