GB2196141A - Heat sensitive diazo recording material - Google Patents

Abstract

A support, e.g. of paper or polyester film, is coated with a composition containing (a) a diazonium salt <IMAGE> where Y is arylthio or substituted amino, R is hydrogen, halogen, alkyl, alkoxy, aryloxy or arylamino and (b) as coupling component, barbituric acid, which may have 1,3- (cyclo)alkyl substituents, cyclohexane-1,3- dione which may have 5,5-dialkyl and 4-phenyl or 4-ethoxy substituents, substituted 5-pyrazalones or a 1- hydroxy-2-naphthoic acid amide, and preferably (c) a basic substance slightly soluble or insoluble in water, such as a nitrogen-containing compound; at least one of (a), (b) and (c) is preferably microencapsulated in a polymeric wall, or otherwise they are each dispersed with a soluble high polymer. The separate constituents can also be coated in two adjacent layers. Local heating from a facsimile machine causes coupling to produce a bright red permanent image.

Description

SPECIFICATION Heat sensitive diazo recording material The present invention relates to a heat sensitive recording material, and, more particularly, to a fixable, red color-producing, diazo type heat sensitive recording material.

Heat sensitive recording materials of the type which can record images through coloration of leuco compounds have been used as recording materials in heat sensitive recording methods.

However, the heat sensitive recording materials of such type have a defect that they cause coloration in undesired areas by severe handling, heating, or adhesion of a solvent after recording to result in staining recorded images. With the intention of providing heat sensitive recording material free from the above-described defect, studies of heat sensitive recording materials of the type which can record images through coloration of diazo compounds have been actively made in recent years.For instance, a method in which a recording material utilizing a diazo compound, a coupling component and a basic component (including substances which can be rendered basic by heating) is subjected to heat recording, and then exposed to light to result in decomposition of the unreacted diazo compound and thereby, cessation of coloration, is disclosed in Japanese Patent Application (OPI) No. 123086/82 (the term "OPI" as used herein means an unexamined published application) and Gazo Denshi Gakkai-shi, vol. 11. p. 290 (1982).

According to this method, coloration in the areas where ho record is wanted can be stopped (that is, fixation can be effected) without fail. However, it happens in the above-described recording material that precoupling proceeds slowly during storage and therefrom, undesirable coloration (fog) results. Therefore, for the purpose of prevention of precoupling, it has been attempted to incorporate either of the color-forming constituents in the recording material in a condition of discrete particles (solid dispersion) and thereby to prevent the constituents from coming into contact with each other. However, this measure cannot impart satisfactory keeping quality (which is called freshness-keeping quality, hereinafter) to the recording material, and, what is worse, this measure causes lowering of thermal color-developability.As another measure to minimize the contact between the color-forming constituents, there has been known a method in which a diazo compound and a coupling component are incorporated in separate layers (for which, e.g., Japanese Patent Application (OPI) No. 123086/82, as described above, can be referred to). Although it can make successful improvement in freshness-keeping quality, such a method causes a decrease in thermal color-developability to such an extent that the recording material cannot respond to pulses of narrow width to be used in high-speed recording. Therefore, such a method is also impractical.In addition, as a method for satisfying both freshnesskeeping quality and thermal color-developability, a means of isolating either coupling component or basic substance from other constituents by encapsulating with a nonpolar waxy substance (Japanese Patent Applications (OPI) 4414/82 and 142636/82) or a hydrophobic macromolecular substance (Japanese Patent Application (OPI) 192944/82) has been proposed. The encapsulating method employed in the foregoing means consists in dissolving or dispersing a color-developing constituent in a solution prepared by dissolving a wax or a macromolecular substance in a proper solvent. Consequently, capsules formed therein differ in concept from usual ones -which consist of a core substance and a shell which covers the core.Such being the case, the colordeveloping constituent dissolved in an encapsulating substance does not become a core substance, but is mixed homogenously with the encapsulating substance throughout the capsule.

This permits slow progress of precoupling at the surface of the capsule wall during storage, to result in a failure in ensuring satisfactory freshness-keeping quality. On the other hand, the colordeveloping constituent dispersed in an encapsulating substance does not undergo the colordeveloping reaction unless the capsule wall is fused by heat. Therefore, a drop in thermal colordevelopability is caused therein. Further, the solvent used for dissolving the wax or the macromolecular substance must be removed after formation of capsules, and addition of such a step is a problem in the production process. This encapsulating method also is not a measure which can achieve highly satisfactory results.

Still another method for solving these problems has been developed, in which at least one constituent to take part in the color-producing reaction is enmicrocapsulated as a core substance through formation of a wall around the core using a polymerization process (as described in Japanese Patent Application (OPI) 65043/83).

Amongst various recent requirements for heat sensitive materials, there has been the requirement to emphasise certain documents by posting mail envelope or various documents with red color-developed heat sensitive recording materials.

In particular, a fixable diazo-type heat sensitive recording material is used to advantage for the above-described purpose, specifically mail stamps, gold certificates, etc., because the material makes it impossible to tamper with records.

However, color-producing materials having fixability and excellence in both freshness keeping quality and red color developability are not known, and so it has not been possible to apply heat sensitive recording materials to the above-described uses.

Therefore, a first object of the present invention is to provide a heat sensitive recording material which has excellent freshness-keeping quality, and can give printed letters of red color excellent in hue.

A second object of the invention is to provide a heat sensitive recording material which can be fixed by irradiation of light, after formation of red images by thermal recording.

As a result of examining various heat sensitive recording materials utilizing microcapsules as described above, a combination of a diazonium salt and a coupler which barely cause any fog upon storage before recording and can produce red color or desirable quality has now been found.

Thus the invention comprises a heat sensitive recording material which comprises on a support a heat sensitive layer containing a diazonium salt of a kind which can produce blue color by coupling with Naphthol AS, and at least one coupling components selected from barbituric acid, derivatives thereof, a cyclohexane-1 ,3-dione and derivatives thereof.

Naphthol AS is ssoxynaphthoic acid anilide.

As the heat sensitive material of the present invention can not only provide printed letters having exceptionally excellent red hue, but also be fixed optically, the heat sensitive recording material can be used as a mail envelope or other various kinds of documents.

In particular, the freshness-keeping quality of the heat sensitive material and permanence of the records formed can be heightened by microencapsulating a color-producing agent. Therefore, the present invention is of important significance.

A diazo compound to be used in the present invention is one which produces blue color by coupling with Naphthol AS, and represented by the general formula, ArN2 +X- (wherein Ar represents an aryl group having an arylthio group or a substituted amino group in the position para to the N2+X- moiety, and the aryl group represents by Ar may be further substituted by one or more of a group, e.g., an alkyl group, an alkoxy group, an aryloxy group, an acryloxy group, a halogen atom, etc.; N+ represents diazonium group; and X represents an acid anion).

More specifically, those represented by the following general formula are preferred.

In the foregoing formula, Y represents a substituted amino group, or an arylthio group; and R represents a hydrogen atom, an alkyl group, an alkoxy group, an aryloxy group, an arylamino group, or a halogen atom (I, Br, CI or F).

Specific examples of a preferred arylthio group include phenylthio group, tolylthio group and naphthylthio group, and those of a preferred substituted amino group include dialkylamino groups, arylamino groups, morpholinio group, piperidino group, pyrrolidino group, acylamino groups, and so on.

Two or more of these diazonium salts can also be used together in the present invention.

Typical examples of diazoniums to constitute the salts include 4-diazo-1-dimethyl-aminobenzene, 4-diazo-1 -diethyl-aminobenzene, 4-diazo- 1 -dipropyl-aminobenzene, 4-diazo- 1 -methylbenzyl- aminobenzene, 4-diazo- 1 -dibenzylaminobenzene, 4-diazo- 1 -ethylhydroxyethl-aminobenzene, 4-di azo-1-diethylamino-3-methoxybenzene, 4-diazo-1 -dimethylamino-2-methylbenzene, 4-diazo- 1-ben- zoylamino-2,5-diethoxybenzene, 4-diazo- 1 -morpholinobenzene, 4-diazo- 1 -morpholino-2,5-diethoxy- benzene, 4-diazo- 1 -morpholino-2,5-dibutoxybenzene, 4-diazo-1 -anilinobenzene, 4-diazo-1 -toluyl- mercapto-2,5-diethoxybenzene and 4-diazo-1-(4-methoxybenzoylamino)-2,5-diethoxybenzene.

Typical examples of an acid anion include BF4-, PF6-, (C6Hs)4B-, CnF2n+1COO- (n=an integer of 3 to 9), CnF2n+lSO3- (n=an integer of 2 to 8). In addition, diazosulfonates of these diazo compounds may be employed.

Specific examples of diazo compounds (or diazonium salts) which can be used in the present invention are illustrated below.

A coupling component to be used in the present present invention is one which forms a red dye by coupling with such a diazo compound as illustrated above, with specific examples including barbituric acid derivatives, such as 1,3-dicyclohexylbarbituric acid, 1,3-didodecylbarbituric acid, 1-oxtyi-3-octadecylbarbituric acid, etc.; cyclohexane-1 ,3-dione derivatives, such as 5,5 dimethylcyclohexane- 1 3-dione, 4-ethoxycarbonyl-5 ,5-dimethylcyclohexane- 1 3-dione, 4-phenyl 5 5-dimethylcyclohexane- 1 3-dione, etc.; 5-pyrazolone derivatives, such as 3-heptadecyl-1-phenyi- 5-pyrazolone, 3-(4-chlorobenzoyl)amino-5-pyrazolone, etc.; and 1-hydroxy-2-naphthoic acid amides, such as 1-hydroxy-2-naphthoic acid (4-ethoxy)anilide, etc.

Of these compounds, barbituric acid derivatives and cyclohexane derivatives are superior to others in freshness-keeping quality and hue.

These cyclohexane derivatives and barbituric acid derivatives are represented by the following general formulae (I) and (II), respectively.

Herein, R1, R2 and R3 each represents a hydrogen atom, or an alky, aralkyl, aryl or alkoxycarbonyl group containing not more than 20 carbon atoms.

Herein, R4 and R5 each represents a hydrogen atom or an alkyl, aralkyl or aryl group containing not more than 20 carbon atoms.

Of such compounds, those illustrated below are particularly preferred.

These coupling components may be used as a mixture of two or more thereof, and further they may be used together with conventional couplers which develop colors differing in hue, e.g., naphthol couplers and so on.

For the purpose of making the system basic to accelerate the coupling reaction, it is to be desired that a basic substance should be added to red color-producing constituents in a recording layer of the heat sensitive recording material of the present invention.

As for the basic substance, basic substances slightly soluble or insoluble in water, and substances capable of producing an alkali by heating can be used.

As examples of such basic substances as described above, mention may be made of nitrogencontaining compounds, such as inorganic and organic ammonium salts, organic amines, thiazoles, pyrimidines, piperazines, guanidines, imidazolines, morpholines, piperidines, amidines, formamidines, pyridines. Specific examples of such nitrogen-containing compounds include ammonium acetate, tricyclohexylamine, tribenzylamine, octadecylbenylamine, stearylamine, 2-undecyl-imidazoline, 2,4,5-trifuryl-2-imidazoline, 1 ,2-diphenyl-4,4-dimethyl-2-imidazoline, 2-phenyl-2-imidazoline, 1 ,2,3-triphenylguanidine, 1 2-ditolylguanidine, 1 ,2-dicyclohexylguanidine, 1 ,2,3-ytricyclohexylguanidfine, guanidium trichloroacetate, N,N'-dibenzyl-piperazine, 4,4'-dithiomorpholine, morpholium trichloroacetate, 2-amino-benzothiazole, 2-benzoyhydraino-benzothiazole.These basic substances can also be used as a mixture of two or more thereof.

In the present invention, excellent freshness-keeping quality can be achieved by enmicrocapsulating at least one constituent selected from among the above-described color-producing agents and basic substance, preferably a diazonium salt or a coupling component, particularly preferably a diazonium salt.

In the heat sensitive recording material of the present invention, a compound capable of lowering the melting point of a coupling component or a basic substance, or a compound capable of raising the thermal permeability of a microcapsule wall, for example, phenol compounds, carbamic acid esters, aromatic alkoxy compounds, organic sulfonamide compounds, etc., can be incorporated for the purpose of enhancing thermal color-developability.

Such compounds can be microencapsulated together with a core substance of the microcapsules to be formed, or can be used in such a condition that they are present outside the microcapsules by addition to a coating composition to form the heat sensitive recording material.

In particular, the combined use of microcapsules and an aqueous dispersion of the foregoing compound is preferred. In all case, the foregoing compound is used in an amount ranging from 0.01 to 10 parts by weight, preferably from 0.1 to 5 parts by weight, per 1 part by weight of coupling component. However, the addition amount can be properly chosen so as to control the density of the developed color to a desired value.

Microcapsules which can be preferably used in the present invention are obtained by dissolving or dispersing a reactive substance to be incorporated in a core material into a water-insoluble organic solvent, if necessary; emulsifying the dissolved or dispersed substance; and then forming a microcapsule wall around individual droplets in the resulting emulsion using a polymerization process. Organic solvents suitable for dissolving or dispersing reactive substances are those having boiling points higher than 180"C. Specifically, phosphoric acid esters, phthalic acid esters and other carboxylic acid esters aliphatic amides, alkylated biphenyls, alkylated phenols, chlorinated paraffins, alkylated naphthalenes, and diarylethanes are used.As specific examples of such compounds, mention may be made of tricresyl phosphate, trioxtyl phosphate, octyl diphenyl phosphate, tricyclohexyl phosphate, dibutyl phthalate, dioctyl phthalate, dilauryl phthalate, dicyclohexyl phthalate, butyl oleate, diethylene glycol dibenzoate, dioctyl sebacate, dibutyl sebacate, dioxtyl adipate, trioxtyl trimellitate, triethyl acetycitrate, octyl maleate, dibutyl maleate, isopropylbiphenyl, isoamylbiphenyl, chlorinated paraffin, diisopropynaphthalene, 1,1'-ditolylethane, 2,4-ditert-amylphenol, N,N-dibutyl-2-butoxy-5-tert-octylaniline, N,N-diphenylformamidine, N,N',N'-triphe nylbenzamidine and N,N'-diphenylbenzamidine.

Of these compounds, ester type solvents such as dibutylphthalate, tricresyl phosphate, diethyl phthalate, dibutylmaleate and the like are particularly preferred over others.

The microcapsules preferred in the present invention are prepared by emulsifying a core material in which a reactive substance is contained, and then forming a wall of a high polymer around individual oil droplets in the emulsion. Reactants to produce the high polymer may be added to the inside and/or the outside of the oil droplets. Specific examples of high polymers for forming a microcapsule wall include polyurethanes, polyureas, polyamides, polyester, polycarbonates, urea-formaldehyde resins, melamine resins, polystyrenes, styrene-methacrylate copolymers, styrene-acrylate copolymers, gelatin, polyvinylpyrrolidone, polyvinyl alcohol and so on.

These high polymers can also be used in combination of two or more thereof. Of these high polymers, polyurethanes, polyureas, polyamides, polyesters and polycarbonates can produce a more desirable effect. In particular, polyurethanes and polyureas are preferred over others.

Of methods usable for forming the microcapsule wall in the present invention, the microencapsulation method utilizing polymerization of reactants supplied to the inside of the oil droplets can product a particularly great effect. That is, capsules having a uniform particle size and excellent freshness keeping property, and thus, well suited for a recording material can be obtained in a short time.

Details of the above-described method and specific examples of compounds used therein are described in U.S. Patents 3,726,804 and 3,796,669.

For example, when it is desired to use polyurethane as a material for capsule wall, a polyisocyanate and a second substance which can react with the isocyanate as required to form a capsule wall (e.g., a polyol) are admixed with an oily liquid to be encapsulated, dispersed into water in an emulsified condition, and then heated to a prescribed temperature to cause a high polymer-producing reaction at the surface of the oil droplets, and thereby to form microcapsule wall. Therein, an auxiliary solvent having a low boiling point and a strong dissolving power (e.g., ethyl acetate, propyl acetate, butyl acetate, methylene chloride, etc.) can be added to the oily liquid.

Polyisocyanates and polyols or polyamines to react with the polyisocyanates in the capsule wall formation are disclosed in U.S.Patents 3,281,383, 3,773,695 and 3,793,268, Japanese Patent Publications 40347/73 and 24159/74, and Japanese Patent Applications (OPI 80191/73 and 84,086/73, and those compounds can be also used in the present invention.

In addition, for the purpose of accelerating the urethanation reaction, tin salts and like can be used with the reactants.

It is preferable to use a polyisocyanate as the first substance for forming a wall film, and water or a polyol as the second substance for forming the wall film, because the wall film formed can ensure excellent freshness-keeping quality to the recording material. In addition, thermal permeabilities of reactive substances can be arbitrarily changed by combining these compounds properly.

Specific examples of preferred polyols include 4,4'-dihydroxy-diphenylmethane, 2-(p,p'-dihydroxydiphenylmethyl)benzyl alcohol, an addition product of bisphenol A and ethylene oxide, an addition product of bisphenol A and propylene oxide, and so on. Polyols can produce a desirable effect when they have hydroxyl group in a ratio of from 0.02 to 2 mols to 1 mole of isocyanate group. On the other hand, when polyurea is employed as a capsule wall material, the abovedescribed procedures are carried out without using polyols. Also, biocapsuies described in Japanese Patent Application (OPI) No. 54978/86 may be employed as the microcapsules.

Upon microencapsulation, water-soluble high polymers can be used. Suitable examples of water-soluble high polymers include water-soluble anionic high polymers, nonionic high polymers and amphoteric high polymers. As for the anionic high polymers, both natural and synthetic ones can be employed, in which -COO-, 503 or the like is contained. Specific examples of natural anionic high polymers include gum arabic and alginic acid, and those of semisynthetic anionic high polymers include carboxymethyl cellulose, phthaloylated gelatin, sulfonated starch, sulfonated cellulose and lignin sulfonate. In addition, specific examples of synthetic anionic high polymers include maleic anhydride copolymers (including hydrolysis products) and acrylic acid homo- and co-polymers (including those of methacrylic acid), carboxyl-denatured polyvinyl alcohol.

Polyvinyl alcohol, hydroxyethyl cellulose, methyl cellulose and the like can be employed as nonionic high polymer.

Gelatin and the like can be employed as amphoteric high polymer.

These water-soluble high polymers are used in a form of an aqueous solution having a concentration of 0.01 to 10 wt%.

The particle size of the microcapsules is controlled to 20 microns or less. When the particle size is increased beyond 20 microns, printed letter images tend generally to be inferior in quality.

In particular, when heating with a thermal head is carried out from the coated layer side, it is desirable that the particle size is controlled to 8 microns or less in order to avoid occurrence of pressure marks.

As for the diazo compound and the coupling component to be used in the heat sensitive material of the present invention, it is to be desired that at least one of them should be used as the core substance of microcapsules. When both are employed as core substance, they may be incorporated in either the same microcapsule or separate microcapsules. Other constituents which are not contained in the core substance of the microcapsules are used in the recording layer on the outside of the microcapsules.

The basic substance in the present invention cannot be incorporated together with both diazo compound and coupling component in the core of microcapsules.

Microcapsules can be prepared from an emulsion containing a constituent to be enmicrocapsulated in a concentration of 0.2 wt% or more.

A diazo compound, a coupling component and a basic substance to be employed in the present invention may be incorporated in either interior or exterior of microcapsules, and a content of the coupling component is 0.1 to 30 parts by weight, and that of the basic substance is 0.1 to 30 parts by weight, per 1 part by weight of diazo compound. It is desirable that the diazo compound is used at a coverage of 0.05 to 5.0 g/m2.

When a diazo compound, a coupling component, a thermal developability improver, and a basic substance are not microencapsulated in the present invention, it is desirable to use them together with a water-soluble high polymer in the state of a solid dispersion prepared with a sand mill or the like. Suitable water-soluble high polymers which can be used in preparing the solid dispersion include those usable in forming microcapsules. In preparing the solid dispersion, a concentration of the water-soluble high polymer solution is controlled to 2 to 30 wt%, and a diazo compound, a coupling component and a basic substance are added into the water-soluble high polymer solution in such amounts as to correspond to concentrations ranging from 5 to 40 wt%, respectively.

A preferred size of dispersed particles as described above is 10 microns or less.

The heat sensitive recording material of the present invention can contain a pigment such'as silica, barium sulfate, titanium oxide, aluminium hydroxide, zinc oxide, or calcium carbonate, styrene beads, and fine powder of urea-melamine resin or the like, for the purposes of preventing the material from sticking to a thermal head and making an improvement in writing quality.

Similarly, metallic soaps also can be used in the heat sensitive recording material of the present invention for the purpose of prevention of a sticking phenomenon. These additives are used at a coverage of 0.2 to 7 g/m2.

The heat sensitive recording material of the present invention can further contain a heat fusible substance in order to heighten the density of thermally recorded image. The heat fusible substance is one which is solid at ordinary temperature, can be fused by heating with a thermal head since it has a melting point ranging from 50 to 150 C, and can dissolve diazo compounds, coupling components or basic substances. The heat fusible substance is dispersed in a form of particles having a size ranging from 0.1 to 10 microns, and used in an amount of 0.2 to 7 g/m2 on a solid basis. Specific examples of the heat fusible substances include fatty acid amides, Nsubstituted fatty acid amides, ketone compounds, urea compounds, esters and so on.

Substances constituting the heat sensitive recording material of the present invention can be coated using a suitable binder together therewith. Specific examples of such binders include polyvinyl alcohol, methyl cellulose, carboxymethyl cellulose, hydroxypropyl cellulose, gum arabic, gelatin, polyvinyl pyrrolidone, casein, styrene-butadiene latex, acrylonitrile-butadiene latex, and various kinds of emulsions of polymers, such as polyvinyl acetate, polyacrylate and ethylene/vinyl acetate copolymer. Such a binder is used at a coverage of 0.5 to 20 g/m2, preferably 0.5 to 5 g/m2, on a solid basis.

In addition to the above-described ingredients, citric acid, tartaric acid, oxalic acid, boric acid, phosphoric acid, pyrophosphoric acid or the like can be added as an acid stabilizer in the present invention also.

A process for producing the heat sensitive recording material of the present invention comprises preparing a coating composition containing a diazo compound and a coupling component as main constituents, and further a basic substance and other additives, and coating the thus prepared composition on a support made of paper, a synthetic resin film or so on using a coating method such as bar coating, blade coating, air knife coating, gravure coating, roll coating, spray coating, dip coating or so on, and drying the coated composition to provide a heat sensitive recording layer at a coverage of 2.5 to 25 g/m2 on a solids basis.Another process which can be employed in the present invention comprises providing on a support a pre-coat layer having a coverage of 2 to 10 g/m2 on a solids basis by coating a composition prepared through the step of adding a main constituent, e.g., a coupling component, and a basic substance and other additives as a core substance of microcapsules, or dissolving them in a condition of finely divided solids, or dissolving them separately in the form of a water solution and then mixing them; and providing on the pre-coat layer the other layer having a solids content of 1 to 30 g/m2 by coating a composition prepared adding a diazo compound which is a a main constituent, and other additive as a core substance, or dispersing them in a condition of finely divided solids, or dissolving them separately in the form of a water solution and mixing them and then, by drying the coated composition. Thereby, an integral unit type of recording layer can be obtained. The arrangement order of the laminated layers in the above-described integral unit type heat sensitive recording material can also be reversed. Such layers may be provided either by coating them one after another or by using a simultaneous coating technique.

These heat sensitive recording materials of integral unit type can exhibit excellent freshness keeping quality over a long period of time.

EXAMPLE The present invention will now be illustrated in greater detail by reference to the following examples, wherein all parts are by weight unless otherwise indicated.

EXAMPLE 1 Preparation of Capsule Solution A: 50 parts of Compound A-1 (the diazonium compound illustrated below) was mixed homogene ously with 150 parts of methylene chloride, 50 parts of tricresyl phosphate, 150 parts of trimethylolpropane trimethacrylate, and 200 parts of a 75 % ethyel acetate solution of 3:1 addition product of m-xyienediisocyanate and trimethylolpropane (Takenate DIION, trade name of Takeda Chemical Industries, Ltd.) to prepare an oil phase liquid.

Separately, 600 parts of a 7 % solution of polyvinyl alcohol (PVA 217E, saponification degree: 88-89 %, polymerization degree: 1700, trade name of Kurare Co., Ltd.) was prepared as a water-soluble high polymer solution.

A dissolver was mounted in the 5 liter stainless steel pot equipped with a warming bath, and therein was placed a protective colloid aqueous solution. Thereto, the oil phase liquid was added with stirring by the dissolver. The stirring was continued till an emulsified dispersion having a mean droplet size of about 1.5 microns was obtained under observation with a microscope. At the conclusion of the dispersion, the stirring was slackened, and 420C water was passed through the warming bath to keep the inside temperature at 400C. Thereby, the encapsulation reaction was completed in 3 hours. To the thus obtained solution, was added 25 cc of ion exchange resin MB-3 (trade name of Japan Organo Co., Ltd.). After stirring, the ion exchange resin was removed by filtration. Thus, the capsule solution A was obtained.

Preparation of Capsule Solution B: A capsule solution B was prepared in the same manner as the capsule solution A, except that a a 9:1 (by weight) mixture of Compound A-1 and Compound A-2 was used in place of Compound A-1.

Preparation Of Dispersions A, B and C: Each of the following mixtures was previously dispersed with the dissolver, and further dispersed with a Dyno mill (produced by Willy A Bachofen AG) to obtain a dispersion having a mean particle size of 2 microns.

Raw Material Dispersion A Dispersion B Dispersion C, 15% PVA-205 aq.

solon. (trade 33 parts 33 parts 33 parts name,Kurarey Co., Ltd.) Compound B-l 4.3 parts Compound B-2 --- 4.3 parts Compound B-3 --- --- 4.3 parts Compound C 5.0 parts 5.0 parts 5.0 parts Compound D 30 parts 30 parts 30 parts Water 70 parts 70 parts 70 parts Preparation Of Coating Compositions A, B, and C; Raw Solutions Composition A Composition B Composition C Capsule Solution A 20 parts --- 9 parts Capsule Solution B --- 15 parts Capsule Solution C --- --- 6 parts Dispersion A 27 parts Dispersion B --- 27 parts Dispersion C --- --- 27 parts Nissan Rapidol B-90, 20 % aq.

soln. (made by Nippon Yushi K.K.) 1.5 parts 1 part 0.7 part Kayahol S, 50 % aq. soln. (made by Nippon Kayaku K.K.) 0.7 part 0.5 part 0.4 part Hydorin 2-7 (made by Chukyo Yushi K.K.) 2 parts 1.5 parts 2 parts Hydorin D-757 (made by Chukyo Yushi K.K.) 2 parts 1.5 parts 1.5 parts Preparation Of Coating Composition D: Raw Material Composition D PVA R-2105, 10 % aq. soln.

(made by Kuraray K.K.) 6 parts Water 15 parts Dispersion D 3 parts Hydorin 2-7 0.4 parts Cellosol D-130 (made by Chukyo Yushi K.K.) 0.3 parts Nissan Rapidol B-90, 2 % aq. soln. 0.2 parts Compound A-1: Diazonium Salt

Compound A-2: Diazonium Salt

Compound B-1: Coupler

Compound B-2: Coupler

Compound B-3: Coupler

Compound C: Organic Base Compound

Compound D: Color Developability Improver

Examples Each coating composition was coated on a support using a bar coater under the following condition, and further thereon was coated the coating composition D in a dry thickness of 0.5 micron using a bar coater, followed by drying at 50 C.

Coating Thickness Drying Example Composition of coat Temperature Support 1 A 10 room temp. A 2 2 A 10 room temp. B 3 B 9 40 C A 4 B 9 40 0C C 5 5 5 C' 15 60 C A 6 C 15 60 C D Support A: Commercial wood free paper.

Support B: Commercial art paper.

Support C: 100g-thick polyester film provided with a subbing layer containing styrene/butadi ene copolymer as a main component.

Each of the thus obtained heat sensitive recording material was subjected to heat recording using a GIll Mode facsimile (Hifax 700, produced by Hitachi, k.k.), and then to overall exposure using Ricopy Super Dry 100 (manufactured by Ricoh Co., Ltd.) to effect fixation. Visual densities of the records obtained were measured with a Macbeth reflection densitometer. The results obtained are shown in Table 1. Further, as the result of another attempting to carry out heat recording in the fixed area, it was confirmed that no image was recorded in every case, that is, fixation was complete. In addition, observation results regarding hues of the printed letter image are also shown in Table 1.

Table 1 Color Density Background after 24 of Developed Hue of Devel- hours' storage at Example Image oped Image. 400C and 90 8 RH 1 0.41 excellent red excellent 2 0.43 excellent red excellent 3 0.40 excellent red excellent 4 0.40 excellent red excellent 5 0.51 good red excellent 6 0.53 good red excellent The results shown in Table 1 demonstrate that the heat sensitive recording materials of the present invention produce developed images of high color density and excellent red hue, and have excellent freshness-keeping quality.

Claims (16)

1. A heat sensitive recording material which comprises on a support a heat sensitive layer or layers containing a diazonium salt of a kind which can produce blue color by coupling with Naphthol AS, and at least one coupling component selected from barbituric acid, derivatives thereof, cyclohexane-1,3-dione, derivatives thereof, 5-pyrazolone derivatives and 1-hydroxy-2naphthoic amides.

2. A heat sensitive recording material as claimed in Claim 1, wherein said diazonium salt is at least one which is selected from those represented by the following formula:

wherein Y represents a substituted amino group or an arylthio group; R represents a hydrogen atom, an alkyl group, an alkoxy group, an aryloxy group, an arylamino group or a halogen atom and X represents an acid anion.

3. A heat sensitive recording material as claimed in Claim 2, wherein said substituted amino group is a monoalkylamino group, diakylamino group, arylamino group, morpholino group, piperidino group, pyrrolidino group or acylamino group.

4. A heat sensitive recording material as claimed in Claim 2 or 3, wherein said arylthio group is a phenylthio group, tolylthio group or naphthylthio group.

5. A heat sensitive recording material as claimed in any preceding claim, wherein said coupling component is represented by the following general formula:

wherein R1, R2 and R3 each represents a hydrogen atom or an alkyl, aralkyl, aryl or alkoxycarbonyl group containing not more than 20 carbon atoms.

6. A heat sensitive recording material as claimed in any of Claims 1 to 4, wherein said coupling component is represented by the following general formula:

wherein R4 and R5 each represents a hydrogen atom or an alkyl, aralkyl or aryl group containing not more than 20 carbon atoms.

7. A heat sensitive recording material as claimed in any preceding claim, wherein a basic substance is included in the heat sensitive layer.

8. A heat sensitive recording material as claimed in any preceding claim, wherein at least the diazonium salt or the coupling component is contained in microcapsules.

9. A heat sensitive recording material as claimed in Claim 7, wherein at least one component selected from the diazonium salt, coupling component and basic substance is contained in microcapsules.

10. A heat sensitive recording material as claimed in Claim 7 or 8, wherein at least the diazonium salt is contained in the microcapsules.

11. A heat sensitive recording material as claimed in any of Claims 8 to 10, wherein said microcapsules were prepared by emulsifying a core material in which a reactive substance is contained, and then forming a wall of a high polymer around individual oil droplets in the emulsion.

12. A heat sensitive recording material as claimed in any preceding claim, wherein the amount of said compound is 0.01-10 parts by weight per part by weight of coupling component.

13. A heat sensitive recording material as claimed in any preceding claim, wherein the coupling component and diazonium salt are present in separate adjacent layers coated on the support.

14. A heat sensitive recording material as claimed in any preceding claim, wherein said coupling component is any of compounds (1) to (6) shown hereinbefore.

15. A heat sensitive recording material as claimed in Claim 1, substantially as hereinbefore described in any of Examples 1 to 6.

16. A colored image made by local application of heat to a recording material as claimed in any preceding claim.