GB2160327A - Heat-sensitive recording material - Google Patents

Abstract

A heat-sensitive recording material comprises a paper sheet having a coated heat-sensitive recording layer containing, held in a polymeric binder, microcapsules containing a diazo compound dissolved in organic solvent, and organic coupling component within or outside the capsules, and a glass transition point reducer is present, preferably inside the capsules. The coupling component may include a basic substance, which may be within the capsules but with diazo and coupling components in the same capsules. The transition point reducer, preferably a hydroxy or organic sulfonamide compound, reduces the glass transition point of the capsule walls to 80 to 150 DEG C and it prevents fogging and improves shelf life. The capsules, e.g. formed from a polyvalent isocyanate and a polyol, are preferably of size less than 20 mu m. The material is usable in a high speed facsimile or computer printer.

Description

SPECIFICATION Heat-sensitive recording material The present invention relates to a heat-sensitive recording material, and particularly to a fixable, diazo tye heat sensitive recording material. More specifically, it is concerned with a heat sensitive recording material which is excellent in keeping quality before thermal recording, and which provides a developed image of high color density upon thermal recording and permits photofixation after thermal recording.

Heat-sensitive recording method has many advantages, in that (1) no particular liquid developing step is required, (2) if paper is used as a support, a recording material prepared in accordance with the method can have a quality akin to that of plain paper, (3) recording material usable in the method can be handled with ease, (4) the method can provide colored images of high density, (5) the method makes it feasible to develop a simple and cheap recording apparatus, and (6) no noise is produced during recording carried out in accordance with the method. Therefore, heat sensitive recording materials have recently enjoyed a markedly increasing demand, particularly in the arts of facsimile, printers, and the like.Heat sensitive recording materials of the type which can record images through coloration of leuco compounds have been used in such arts, mainly because of their high density of colored images and excellence in coloring speed. However, the heat sensitive recording materials of such a type have defects in that they can undergo coloration by careless handling, heating, or adhesion of a solvent after recording, to result in stained recorded images, and further suffer from a disadvantage in that their developed images are decolored by a plasticizer contained in cellulosic tape. Coloration due to careless handling is known to be preventable by addition of granulated wax, as described in Japanese Patent Publication No. 14531/75, and penetration of plasticizer is known to be hindered by formation of a coating layer as described in Japanese Utility Model Publication (unexamined) No. 125354/81.However, such measures as described above cannot yet produce sufficiently satisfactory effects, and to make matters worse, cannot be employed when prevention of alteration after recording is required. Therefore, it has been strongly desired to make improvements in the measures for obviating the foregoing defects. As a method for preventing undesired development of colors after thermal recording, on the other hand, a method has been known as disclosed in Japanese Patent Application (OPI) Nos. 123086/82 and 125092/82 (corresponding to U.S. Patent 4,411,979), U.S.Patent 3,281,244, and so on (the term "OPI" as used herein refers to a published unexamined Japanese patent application), wherein thermal recording is carried out using a heat sensitive recording material comprising a diazo compound, a coupling component, and an alkali-producing agent or a coloring aid, and, after recording, the material is exposed to light to decompose diazo compound molecules that have not taken part in the coupling reaction, whereby coloration is stopped.

However, it frequently happens in recording materials of this kind that precoupling proceeds slowly during storage, and thus undesirable coloration (fog) results. Accordingly, with the invention of preventing precoupling from occurring, it has been attempted to incorporate either of the color-forming constituents in the recording material in a condition of discrete particles (solid dispersion), to thereby prevent the constituents from coming into contact with each other.

However, this measure cannot impart satisfactory keeping quality (hereinafter referred to as shelf storage quality) to the recording material, and what is worse this measure causes lowering of thermal color-developability.

In another method, a diazo compound and a coupling component are incorporated in separate layers in order to minimize the contact between the color-forming constituents. Although it can make successful improvement in shelf storage quality, such a measure causes a decrease in thermal color-developability to such an extent that the recording material cannot respond to pulses of narrow width that are used in high-speed recording. Therefore, this measure is also impractical.

In addition, as a method for satisfying both shelf storage quality and thermal colordevelopability, the isolating of either a coupling component or basic substance from other constituents by encapsulating with a nonpolar waxy substance, as described in Japanese Patent Applications (OPi) Nos. 4414/82 and 142636/82, or with a hydrophobic polymeric substance, as described in Japanese Patent Application (OPI) No. 192944/82 has been proposed.

However, the encapsulating method employed in this method consists of dissolving or dispersing a color-forming constituent in a solution prepared by dissolving a wax or a polymeric substance in an appropriate solvent. Consequently, capsules formed using thins method differ in concept from conventional capsules consisting of a core substance and a shell which covers the core. Such being the case, the color-forming constituent dissolved in an encapsulating substance does not become a core substance, but is mixed homogeneously with the en apsulating substance throughout the capsule. This permits slow progress of precoupling at the interface of the capsule wall during storage, to result in failure with respect to the shelf storage quality.On the other hand, the color-forming constituent dispersed in an encapsulating substance does not undergo the color-forming reaction unless the capsule wall is melted by heat. Therefore, a drop in thermal color-developability occurs. Further, the solvent used for dissolving the wax or the polymer substance must be removed after formation of capsules, and the necessity of such a step is a problem from the point of view of production. Thus this encapsulating method also is not a measure which can achieve highly satisfactory results.

Moreover, an excellent method for solving these problems has been found, which consists of microencapsulating at least one constituent to take part in the color-forming reaction as a core substance through formation of a wall around the core using a polymerization process, as described in Japanese Patent Application (OPI) No. 190886/84, corresponding to U.S. Patent Application Serial No. 600,267.

However, it has been strongly desired to further enhance thermal color-developability of heatsensitive recording materials to which the foregoing microencapsulating method has been applied.

Therefore, a first object of the present invention is to provide a heat-sensitive recording material having both excellent shelf storage quality and high thermal color-developability.

A second object of the present invention is to provide a heat-sensitive recording material which can make it feasible to stop coloration in areas where no coloration is desired after thermal recording (that is to say, after fixation) by optically decomposing diazo compound molecules that have not taken part in the color-forming reaction.

A third object of the present invention is to provide a heat-sensitive recording material which is easily produced.

It has now been found out that the above-described objects can be attained with a heatsensitive recording material which comprises a support having provided thereon a recording layer containing a diazo compound and a coupling component, with said diazo compound being present in the recording layer in a microencapsulated condition, and said recording layer containing additionally a compound capable of lowering the glass transition point of a wall material of the microcapsules formed.

According to a preferred embodiment, the recording layer also contains a basic substance, or a substance capable of becoming basic when heated.

The microcapsules formed in the present invention are not of the type which have been employed in conventional recording materials, in which the microcapsule wall is ruptured by externally applied heat or pressure to allow contact between a reactive substance incorporated in the core of microcapsules and the other reactive substance present on the outside of the microcapsules, whereby the color-forming reaction is caused, but rather are of a kind wherein reactive substances present separately inside the core of microcapsules and on the outside of microcapsules come to acquire permeability into the microcapsule wall by heating, to result in occurrence of the color-forming reaction.

Compounds capable of lowering the glass transition point of the wall material for forming the microcapsules to be employed in the present invention (hereinafter described as a transition point controlling agent) can exceptionally enhance the thermal color-developability when used in combination with the microcapsules in the present invention, compared with generally used thermoplastic substances.

In the present invention, the term glass transition point is taken to be equal to the temperature at which the value of tan 8 defined by the following equation reaches a peak, and measured with a Vibron Model DDV-II (product of Toyo Baldwin). These dynamic modulus and dynamic loss modulus are described in L.E. Nielsen, Mechanical Property of Polymers, Marcel Dekker Inc., New York (1975).

Dynamic Loss Modulus tan 8= ~~~~~~~~~~~~~~~~~~~~~~ Dynamic Modulus Transition point controlling agents which can be preferably employed in the present invention are those capable of lowering glass transition points of wall materials used upon formation of microcapsules to from 80 to 1 50"C, and more preferably to from 100 to 1 30to.

As examples of transition point controlling agents which can be used herein, mention may be made of hydroxy compounds, carbamic acid ester ester compounds, aromatic methoxy compounds, organic sulfonamide compounds, and so on. Specific examples of hydroxy compounds include phenol compounds such as p-t-butylphenol, p-t-octylphenol, p-a-cumylphenol, p-tpentylphenol, m-xylenol, 2,5-dimethylphenol, 2,4, 5-trimethylphenol, 3-methyl-4-isopropylphenol, p-benzylphenol, o-cyclohexylphenol, p-(diphenylmethyl)phenol, p-(a,a-diphenylethyl)phenol, o-phenylphenol, ethyl p-hydroxybenzoate, propyl p-hydroxybenzoate, butyl p-hydroxybenzoate, benzyl p-hydroxybenzoate, p-methoxyphenol, p-butoxyphenol, p-heptyloxypheno, p-benzyloxyphenol, dimethyl 3-hydroxyphthalate, vanillin, 1,1-bis(4-hydroxyphenyl)dodecarlle, 1,1-bis(4- hydroxyphenyl)-2-ethylhexane, 1,1 -bis(4-hydroxyphenyl)-2-methylpentane, 2, 2-bis(4-hydroxyphenyl)heptane, 2-t-butyl-4-methoxyphenol, 2,6-dimethoxyphenol, 2,2'-dihydroxy-4-methoxybenzophenone and so on, and alcohol compounds such as 2,5-dimethyl-2,5-hexanediol, resorcinol di(2-hydroxyethyl) ether, resorcinol mono(2-hydroxyethyl)ether, salicyl alcohol, 1,4-di(hydroxyethoxy)benzene, p-xylylenediol, 1-phenyl-1,2-ethanediol, diphenylmethanol, 1,1 -diphenylethanol, 2-methyl-2-phenyl-l ,3-propanediol, 2,6-dihydroxymethyl-p-cresol benzyl ether, 2,6-dihydroxymethyl-p-cresol benzyl ether, 3-(o-methoxyphenoxy)-1,2-propanediol, and so on.Specific examples of carbamic acid ester compound include ethyl N-phenylcarbamate, benzyl Nphenylcarbamate, phenyl N-phenylcarbamate, benzyl carbamate, butyl carbamate and isopropyl carbamate. Specific examples of aromatic methoxy compouns include 2-methoxybenzoic acid, 3, 5-dimethoxyphenylacetic acid, 2-methoxynaphthalene, 1,3, 5-trimethoxybenzene, p-dimethoxybenzene and p-benzyloxymethoxybenzene.Specific examples of organic sulfonamide compounds include p-toluenesulfonamide, o-toluenesulfonamide, benzenesulfonamide, p-toluenesulfonanilide, N-(p-methoxyphenyl)-p-toluenesulfonamide, N-(o-methoxyphenyl)-p-toluenesulfonamide, N (p-ch lorophenyl)-p-toluenesulfonamide, N-(o-ch lorophenyl)-p-tol uenesulfonamide, N -(p-tolyl)-ptoluenesulfonamide, N-(o-tolyl)-p-toluenesulfonamide, N-(o-hydroxyphenyl)-p-toluenesulfonamide, N-benzyl-p-toluenesulfonamide, N-(2-phenetyl)-p-toluenesulfonamide, N-(2-hydroxyethyl)-p-toluenesulfonamide, N-(3-methoxypropyl)-p-toluenesulfonamide, methanesulfonanilide, N-(p-tolyl)sulfonamide, N-(o-tolyl)sulfonamide, N-(p-methoxyphenyl)sulfonamide, N-(o-methoxy)sulfonamide, N-(p-chlorophenyl)sulfonamide, N-(o-chlorophenyl)sulfonamide, N-(2,4-xylyl)sulfonamide, N-(pethoxyphenyl)sulfonamide, N-benzylmethanesulfonamide, N-(2-phenoxyethyl)methanesulfonamide, 1,3-bis(methanesulfonylamino)benzene and 1 ,3-bis(p-toluenesulfonylamino)propane, preferably p-toluenesulfonamide. However, the present invention is not limited to the compounds set forth above.

One or more compounds are selected from these compounds depending on the kind of wall material used in the microcapsules, and employed as the transition point controlling agent of the present invention.

In the foregoing compounds, those which can lower the glass transition point of the wall material of the microcapsules are included. Therefore, if such compounds are selected as the transition point controlling agent, the coloration temperature can be enhanced.

In general, it is preferable to employ hydroxy compounds and organic sulfonamide compounds as transition point controlling agents. Specific examples of such preferred hydroxy compounds and organic sulfonamide compounds include p-benzyloxyphenol, p-t-butylphenol, pxylylenediol, 2, 6-dimethylphenol, and p-toluenesulfonamide.

The transition point controlling agent of the present invention is used in an amount ranging, preferably, from 0.1 to 5 parts by weight per 1 part by weight of coupling component. The addition amount thereof is chosen so that the coloring reaction may be initiated at an intended temperature.

Diazo compounds to be employed in the present invention are diazonium salts represented by the formula ArN2+X-, which are capable not only of forming colors by reacting with a coupling component, but also of being decomposed by exposure to light. In the foregoing formula, Ar represents a substituted or unsubstituted aromatic moiety, N2+ represents a diazonium group, and X represents an acid anion.

The preferred aromatic moiety is represented by the following formula:

wherein Y represents a substituted amino group, an alkoxy group, an arylthio group, an alkylthio group or an acylamino group; and R represents an alkyl group, an alkoxy group, an arylamino group, or a halogen (e.g., I, Br, Cl, or F).

Suitable examples of substituted amino groups represented by Y include monoalkylamino groups, dialkylamino groups, arylamino groups, a morpholino group, a piperidino group, a pyrrolidino group, and the like.

Specific examples of diazonium to form the diazonium salts include 4-diazo-1-dimethylaminobenzene, 4-diazo- 1 -diethylaminobenzene, 4-diazo- 1 -dipropylaminobenzene, 4-diazo- 1 -methylben- zylaminobenzene, 4-diazo- 1 -dibenzylaminobenzene, 4-diazo- 1 -ethylhydroxyethylaminobenzene, 4-diazo- 1 -diethylamino-3-methoxybenzene, 4-diazo- 1 -dimethylamino-2-methylbenzene, 4-diazo 1 -benzoylamino-2, 5-diethoxybenzene, 4-diazo- 1 -morpholinobenzene, 4-diazo-l -morpholino-2.5- diethoxybenzene, 4-diazo-1-morpholino-2,5-dibutoxybenzene, 4-diazo- 1 -anilinobenzene, 4-diazo1 -anilinobenzene, 4-diazo- 1 -toluylmercapto-2, 5-diethoxybenzene, 4-diazo- 1 , 4-methoxybenzoy- lamino-2, 5-diethoxybenzene, 4-diazo-1-pyrrolidino-2-ethylbenzene and the like.

Specific examples of acid anions forming salts together with the diazo compounds set forth above include Cn F2n+ COQ-, wherein N is an integer of 3 to 9, CmF2m+,SO3-, wherein m is an integer of 2 to 8, (C,F2,+1SO2)2CH-, wherein I is an integer of 1 to 18,

wherein n is an integer of 3 to 9,

wherein n is an integer of 3 to 9, BF4, PF6-, and so on.

Of the above-illustrated acid anions, those containing perfluoroalkyl groups, perfluoroalkenyl groups or PF6- are more desirable, because they can contribute to minimize fog upon storage prior to recording.

Specific examples of diazo compounds (diazonium salts) which can be preferably employed are illustrated below.

The coupling component to be used in the present invention is a compound which couples with the diazo compound (diazonium salt) to form a dye. Such coupling component includes a compound of the type wherein the coloration is accelerated depending upon the presence of a basic substance, and a compound of the type wherein the high coloration density is obtained regardless of the presence of a basic substance.The typical examples of the former type coupling component dependent upon the basic substance include resorcin, phloroglucin, sodium 2, 3-dihydroxynaphthalene-6-sulfonate, 1 -hydroxy-2-naphthoic acid-morpholinopropylamide, 1,5dihydroxynaphthalene, 2, 3-dihydroxynaphthalene, 2,3-dihydroxy-6-sulfanylnaphthalene, 2-hydroxy-3-naphthoic acid-anilide, 2-hydroxy-3-naphthoic acid-2'-methylanilide, 2-hydroxy-3-naphthoic acid-ethanolamide, 2-hydroxy-3-naphthoic acid-octylamide, 2-hydroxy-3-naphthoic acid N-dodecyloxypropylamide, 2-hydroxy-3-naphthoic acid-tetradecylamide, acetanilide, acetoacetanilide, benzoylacetanilide, 1 -phenyl-3-methyl-5-pyrazolone, 1-(2',4',6 '-trichlorophenyl)-3-benzam- ido-5-pyrazolone, 1 -(2',4',6'-trichlorophenyl)-3-anilino-5-pyrazolone, 1 -phenyl-3-phenylacetam- ido-5-pyrazolone, etc.Typical examples of the latter type of coupling component independent of the presence of a basic substance include an active methylene compounds, for example, fi-keto- carboxylic acid amides such as benzoylacetanilide, pivaloylacetanilide, 1 ,3-bis(benzoylacetami- no)toluene, 1,3-bis(pivaloylacetaminomethyl)benzene, etc., pyrazolones such as 3-methyl-1- phenylpyrazolone, 3-hexylcarbamoyl- 1 -phenylpyrazolone, 3-myristoylamino-(2,4,6-trichlorophe- nyl)pyrazolone, etc., barbituric acids such as 1 ,3-didodecylbarbituric acid, 1. 3-dicyclohexylbarbi- turic acid, 1-octyl-3-stearylbarbituric acid, etc., 1,3-cyclohexanediones such as 5,5-dimethyl-1,3cyclohexanedione, 5,5-dimethyl-4-phenyl-1 3-cyclohexanedione, etc.; an aromatic amine type compound, for example, a-naphthylamine, ssnaphthylamine, 1 -anilinonaphthalene, 2-anilinonaphthalene, 3-aminodiphenylamine, 4,4'-diaminodiphenylmethane, N, N-dicyclohexylaniline, 2aminocarbazole, 2-phenylindole, 1-phenyl-2-methylindole, and an organic or inorganic acid salt of an aromatic amine such as a p-toluenesulfonic acid salt of N,N-dimethylaniline and anaphthylamine hydrochloride, etc.; and an aromatic hydroxy compound having a basic group in the molecular structure thereof, for example, 2-hydroxy-3-naphthoic acid-3'-morpholinopropylamine, 2-hydroxy-3-naphthoid acid-2'-diethylaminoethylamide, 2-hydroxy-3-naphthoic acid-3piperidinopropylamide, 2-hydroxy-3-naphthoic acid-3'-peperidinopropylamide, 2-hydroxy-3-naphthoic acid-p-(3'-N '-cyanoguanidinopropyl)oxyanilide, salicylic acid-p-(3'-morphlinopropyl)oxy- anilide, 1-naphthol-4-su 1-naphthol-4-sulfonic acid-3'-diethylaminopropylamide, 8-hydroxyquinoline-4-sulfonic acid-2'-diethylaminoethylamide, and an aromatic hydroxy compound having a residue such as an organic carboxylic acid salt of amines capable of forming basic when heated, e.g., a trichloroacetic acid salt of 2-hydroxy-3-naphthoic acid-3'morpholinopropylamide, a phenylthioacetic acid salt of 1-naphthol-4-sulfonic acid-3'-diethylaminopropylamide, etc. An image of an optional color may be obtained by using two or more of these coupling components in combination.

It is desirable for acceleration of the coupling reaction to add a basic substance to the recording layer, to thereby render the layer basic. Suitable examples of a basic substance used for such a purpose include those which are slightly soluble or insoluble in water, and substances capable of producing an alkali by heating. More specifically, inorganic and organic ammonium salts, organic amines, amides, ureas and their derivatives, thioureas and their derivatives, thiazoles, pyrroles, pyrimidines, piperazines, guanidines, indoles, imidazoles, imidazolines, triazoles, morpholines, piperidines, amidines, formamidines, pyridines and like nitrogen-containing compounds can be used as preferable basic substance.Specific examples thereof include ammonium acetate, tricyclohexylamine, tribenzylamine, octadecylbenzylamine, stearylamine, allylurea, thiourea, methylthiourea, allylthiourea, ethylenethiourea, 2-benzxylimidazole, 4-phenylimidazole, 2-phenyl-4-methyl-imidazole, 2-undecyl-imidazoline, 2,4,5-trifuryl-2-imidazoline, 1,2diphenyl-4,4-dimethyl-2-imidazoline, 2-phenyl-2-imidazoline, 1,2, 3-triphenylguanidine, 1 , 2-ditol- lylguanidine, 1, 2-dicyclohexylguanidine, 1,2, 3-tricyclohexylguanidine, guanidine trichloroacetate, N,N'-dibenzylpyperazine, 4,4'-dithiomorpholine, morpholinium trichloroacetate, 2-aminobenzothiazole and 2-benzoylhydrazino-benzothiazole. These basic substance can be used in combinations of two or more thereof.

For use in the present invention, the diazo compound incorporated in a core material of microcapsules is dissolved in a water-insolube organic solvent, and emulsified. Around individual drops in the resulting emulsion, a microcapsule wall is formed using a polymerization process.

To microencapsulate the diazo compound in this manner is effective in improving the shelf storage property, and in heightening the coloring speed and color density of the developed image. Organic solvents preferable for dissolving the diazo compound are those having boiling points higher than 180"C, and phosphoric acid esters, phthalic acid esters and other carboxylic acid esters, fatty acid am ides, alkylated biphenyls, alkylated terphenyls, chlorinated paraffins, alkylated naphthalenes, diarylethanes, and so on can be used as the organic solvent.Specific examples of these compounds include tricresyl phosphate, trioctyl phosphate, octyl diphenyl phosphate, tricyclohexyl phosphate, dibutyl phthalate, dioctyl phthalate, dilauryl phthalate, dicyclohexyl phthalate, butyl oleate, diethylene glycol dibenzoate, dioctyl sebacate, dibutyl sebacate, dioctyl adipate, trioctyl trimellitate, triethyl acetylcitrate, octyl maleate, dibutyl maleate, isopropylbiphenyl, isoamylbiphenyl, chlorinated paraffin, diisopropylnaphthalene, 1 , 1 '- ditolylethane, 2,4-di-t-amylphenol and N, N-dibutyl-2-butoxy-5-t-octylaniline.

Of these compounds, ester type solvents such as dibutyl phthalate, tricresyl phosphate, diethyl phthalate, dibutyl maleate and the like are preferred over others. The preferred amount of the organic solvent contained in the liquid droplets emulsified as a core substance is 10 to 70 wt%, more preferably 20 to 55 wt%.

The microcapsules of the present invention are prepared by emulsifying a core material in which a diazo compound is incorporated, and then forming a polymeric wall around the individual oil droplets in the emulsion. A reactant for producing the polymer may be added to the inside and/or the outside of the oil droplets. Specific examples of polymers for forming a microcapsule wall include polyurethanes, polyureas, polyamides, polyesters, polycarbonates, urea-formaldehyde resins, melamine resins, polystyrenes, styrene-methacrylate copolymers, styrene-acrylate copolymers, gelatin, polyvinylpyrrolidone and polyvinyl alcohol.

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

In the recording layer in the present invention, a diazo compound and a coupling component are incorporated as main constituents and a basic substance is also preferably contained therein, and not only can the diazo compound be employed alone as a core material, but also both the diazo compound and the coupling component, both the diazo compound and the basic substance, or all of these constituents may be employed as core materials. When two kinds of constituents are employed as core materials, they may be incorporated in either the same microcapsule or in separate microcapsules. On the other hand, when three kinds of constituents are employed as core materials, they may be incorporated in microcapsules in various combinations, although they should not all be incorporated together in the same microcapsule.

Other constituents which are not used as core materials are incorporated outside of the microcapsules in the heat sensitive layer.

The transition point controlling agent can be used not only as a core material of the microcapsules, but can also be incorporated in the heat sensitive layer on the outside of microcapsules. However, it is preferred to use the agent as a core material, because a desired effect can be achieved even if the agent is added in a reduced amount. Even if the agent is added as a core material, some portion thereof is expected to pass into the microcapsule wall.

Of methods usable for forming the microcapsule wall of the present invention, a microencapsulation method utilizing polymerization of reactants supplied to the inside of the oil droplets can produce particularly desirable effects. That is, capsules having a uniform particle size and an excellent shelf storage property, in other words, well suited for a recording material, can be obtained in a short time.

Details of this method and specific compounds that can be used therein are described in U.S.

Patents 3,726,804 and 3,796,669.

For instance, when it is desired to use polyurethane as a material for the capsule wall, a polyvalent isocyanate and a second substance which can react with the isocyanate to form the 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 case a polymer-producing reaction at the surface of the oil droplets and thereby, to form a microcapsule wall. In this situation, an auxiliary solvent having a low boiling point and a strong dissolving power can be added to the oily liquid.

Polyisocyanates, and polyols and polyamines to be reacted with the polyisocyanates in the above-described reaction are disclosed in U.S. Patents 3,281,383, 3,773,695 and 3,793,268, Japanese Patent Publication Nos. 40347/73 and 24159/74 (corresponding to British Patent 1,127,338 and U.S. Patent 3,723,363, respectively), and Japanese Patent Application (OPI) Nos. 80191/73 and 84086/73 (corresponding to U.S. Patent 3,838,108 and British Patent 1,416,224, respectively), and such compounds can also be used in the present invention.

The process of preparing a capsule wall for the present invention is not limited to the abovedescribed processes. All polymeric substances formed by the reaction of polyvalent isocyanate and polyol are preferably used as a capsule wall in the present invention. The glass transition point of the capsule wall may be varied appropriately by the combination of these substances.

Specific examples of polyvalent isocyanate include diisocyanates such as m-phenylenediisocyanate, p-phenylenediisocyanate, 2, 6-tolylenediisocyanate, 2, 4-tolylenediisocyanate, naphthalene 1 ,4-diisocyanate, diphenylmethane-4,4'-diisocyanate, 3, 3'-dimethoxy-4,4'-biphenyl-diisocyanate, 3,3'-dimethyidiphenylmethane-4,4'-diisocyanate, xylylene- 1 ,4-diisocyanate, 4,4'-diphenylpro panediisocyanate, trimethylenediisocyanate, hexamethylenediisocyanate, propylene- , 2-diisocy- anate, butylene-1 ,2-diisocyanate, cyclohexylene-1 ,2-diisocyanate, cyclohexylene-1 ,4-diisocyanate and so on;; triisocyanates such as 4,4',4"-triphenylmethanetriisocyanate, toluene-2,4,6-triisocyanate and so on; tetraisocyanates such as 4,4'-dimethyldiphenylmethane-2,2',5,5-tetraisocyan- ate and the like: and isocyanate prepolymers such as a hexamethylenediisocyanate-trimethylolpropane addition product, a 2,4-tolylenediisocyanate-trimethylolpropane addition product, a xylylenediisocyanate-trimethylolpropane addition product and a tolylenediisocyanate-hexanetriol addition product.

Specific examples of polyols include those made of aliphatic and aromatic polyhydric alcohols, hydroxypolyesters, hydroxypolyalkylene ethers, and the like. Preferred polyols are polyhydroxy compounds including the partial structure (I), (if), (III), or (IV) between two hydroxy groups in the molecular structure, and having a molecular weight of 5,000 or less.

(I) an aliphatic hydrocarbon residue containing from 2 to 8 carbon atoms (II) I I -C-O-Ar-O-C (Ill) I I -C-Ar-C I I (IV) -O-Ar-C-Ar-O In structures 11, III, and IV, Ar represents a substituted or unsubstituted aromatic moiety, and the aliphatic hydrocarbon residue of (I) contains a moiety of the formula -CnH2n- as a basic skeleton, a hydrogen of which may be substituted with another element.

Specific examples of polyols of type (I) include ethylene glycol, 1,3-propanediol, 1,4 butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, propylene glycol, 2, 3-dihydroxybutane, 1 ,2-dihydroxybutane, 1 , 3-dihydroxybutane, 2,2-d i 2-dimethyl-1 , 3-propanediol, 2,4-pentanediol, 2, 5-hexanediol, 3-methyl- 1 , 5-pentanediol, 1 ,4-cyclohexanedimethanol, dihydroxycyclohexane, diethylene glycol, 1,2,6-trihydroxyhexane, phenylethylene glycol, 1,1,1trimethylolpropane, hexanetriol, pentaerythritol, and glycerine.

Specific examples of polyols of type (11) include condensation products of aromatic polyhydric alcohols, such as 1 ,4-di(2-hydroxyethoxy)benzene, resorcinol dihydroxyethyl ether, etc., and alkylene oxides.

Specific examples of polyols of type (III) include p-xylylene glycol, m-xylylene glycol, a,a' dihydroxy-p-diisopropylbenzene, and the like.

Specific examples of polyols of type (IV) include 4,4'-dihydroxydiphenylmethane, 2-(p,p'dihydroxydiphenylmethyl)benzyl alcohol, a bisphenol A-ethylene oxide addition product and a bisphenol A-propylene oxide addition product.

It is desirable to use polyols in such an amount that 0.02 to 2 moles of hydroxy group per 1 mole of isocyanate group may be present in starting the reaction.

Specific examples of the polyamine, which can be used instead of the polyol, include ethylenediamine, trimethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, p-phenylenediamine, m-phenylenediamine, piperazine, 2-methylpiperazine, 2,5 dimethylpiperazine, 2-hydroxytrimethylenediamine, diethylenetriamine, triethylenetriamine, triethylenetetramine, diethylaminopropylamine, tetraethylenepentamine, an amine adduct of epoxy resin, etc.

Further, the polyvalent isocyanate is able to form a polymeric substance as a preferred capsule wall by the reaction with water.

If other conditions are maintained unchanged, the wall thickness can be varied by changing an additive amount of polyisocyanate, polyol, or polyamine. If other conditions are maintained unchanged, the particle size, and therefore the wall thickness, can be varied by changing the state of agitation in emulsifying/dispersing.

Water-soluble polymers can be used for the microencapsulation. Preferred examples of watersoluble polymers include water-soluble anionic polymers non ionic polymers, and amphoteric high polymers. As for the anionic polymers, both natural and synthetic can be employed, in which -COO-, -S03-, or the like is contained. Specific examples of natural anionic polymers include gum arabic, alginic acid, and so on, and semisynthetic anionic polymers include carboxymethyl cellulose, phthaloylated gelatin, sulfonated starch, sulfonated cellulose, lignin sulfonate, and so on.In addition, specific examples of synthetic anionic polymers include maleic anhydride copolymers (including hydrolysis products), acrylic acid homo- and co-polymers (including those of methacrylic acid), vinylbenzenesulfonic acid homo- and co-polymers, carboxyl-denatured polyvinyl alcohol, and so on.

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

Gelatin and the like can be employed as amphoteric polymers.

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

The particle size (average diameter) of the microcapsules is controlled to 20 m or less. If the particle size is increased beyond 20,us, the quality of the images recorded tends generally to suffer deterioration. 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 lim or less in order to avoid occurrence of pressure marks.

Microcapsules can be prepared from an emulsion containing a diazo compound to be microencapsulated in a concentration of 0.2 wt% or more.

A coupling component to be employed in the present invention, and the basic substance which can be employed herein, if desired, may be contained either inside of microcapsules or in the heat sensitive layer on the outside of the microcapsules, and their amounts are preferably from 0.1 to 10 parts by weight (preferably 0.3 to 4 parts by weight) and from 0 to 20 parts by weight (preferably 0.1 to 4 parts by weight), respectively, per 1 part by weight of diazo compound. It is desirable that the diazo compound be used at a coverage of from 0.05 to 2.0 g/m2. However, the basic substance cannot be incorporate together with both diazo compound and coupling component in the core of microcapsules.

When the coupling component, the basic substance, and/or the transition point controlling agent are not microencapsulated in the material of the present invention, it is desirable to use them in a condition of a solid dispersion, prepared by using a sand mill or the like. Preferred water soluble polymers which can be used in preparing the solid dispersion include those usable in forming microcapsules. In preparing the solid dispersion, the concentration of the watersoluble polymer solution is generally controlled to be from 2 to 30 wt%, preferably 2 to 5 wt%, and a coupling component, a basic substance and a transition point controlling agent are incorporated into the water-soluble polymer solution in amounts so as to have concentrations ranging from 5 to 40 wt% each, with respect to the solution as a whole, preferably 20 to 40 wt%. The transition point controlling agent may be dispersed together with the coupling component or the basic substance, or it may be dispersed alone.

A preferred size of dispersed particles as described above is 10 cm 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, calcium carbonate, or so on; and styrene beads, and fine powder of urea-melamine resin, or the like for the purpose of preventing the material from sticking to a thermal head and improvement in writing quality.

Similarly, metallic soaps also can be incorporated in the heat sensitive recording material of the present invention for the purpose of prevention of sticking. These additives are used at a coverage of from 0.2 to 7 g/m2, preferably 0.2 to 2 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 means those which are solid at ordinary temperature, can be fused by heating with a thermal head as they have 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,um, 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, N-substituted 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 various kinds of emulsions such as a colloidal dispersion of polyvinyl alcohol, methyl cellulose, carboxymethyl cellulose, hydroxypropyl cellulose, gum arabic, gelatin, polyvinylpyrrolidone, casein, styrenebutadiene latex, acrylonitrile-butadiene latex, polyvinyl acetate, polyacrylate, ethylene-vinyl acetate copolymer, etc. Such a binder is used in a solid amount of 0.5 to 5 g/m2, preferably 0.5 to 3 g/m2.

In addition to the above-described constituent substances, citric acid, tartaric acid, oxalic acid, boric acid, phosphoric acid, pyrophosphoric acid, etc., can be added as acid stabilizers in the present invention.

A process for producing the heat sensitive recording material for the present invention comprises preparing a coating composition by the step of adding constituents such as a coupling component, a basic substance and a transition point controlling agent, and other additives as a core substance for the microcapsules, or dispersing them in a condition of finely divided solids, or dissolving them in water, and then mixing the resulting solutions; the thus prepared composition is coated on a support made of paper, a synthetic resin film, etc. using a coating method such as bar coating, blade coating, air knife coating, gravure coating, roll coating, spray coating, dip coating, etc.; and the coated composition is dried to provide a heat sensitive recording layer at a dry coverage of from 2.5 to 25 g/m2, preferably from 4 to 1 5 g/m2.Another process which can be employed in the present invention comprises providing a precoated layer having a solids content of from 2 to 10 g/m2, preferably from 5 to 10 g/m2, by coating on a support a coating composition prepared by the step of adding constituents including a coupling component, a basic substance, and a transition point controlling agent, and other additives as core substance of microcapsules, or dispersing them as finely divided solids, or dissolving them in the form of an aqueous solution, and mixing the solutions; then, on the precoated layer, another layer is provided having a solids content of 1 to 1 5 g/m2, preferably 4 to 1 5 g/m2, by coating a composition prepared adding a diazo compound, which is a main constituent, and other additives as core substance and then, by drying the coated composition.

Therein, an integral unit type recording layer is obtained.

The arrangement order of the laminated layers in the above-described integral until 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 an excellent shelf storage property over a long period of time.

It is advantageous from the standpoint of the acquisition of excellent shelf storage quality that neutralized paper sized with a neutral sizing agent such as an alkylketene dimer or the like, and manifesting a pH of from 6 to 9 upon thermal extraction, as described in Japanese Patent Application (OPI) No. 14281/80 (corresponding to U.S. Patent 4,255,491), is employed as a paper support.

In addition, for the purpose of preventing a coating composition from permeating into a paper support and creating desirable contact between a thermal head and a heat sensitive recording layer upon recording, it is favorable to use papers satisfying the following condition: Stöckigt sizing degree 13 x 10-3 Base weight in grams per square meter (m2) and having a Beck smoothness of 90 seconds or more, as described in Japanese Patent Application (OPI) No. 116687/82 (corresponding to U.S. Patent 4,416,939).

Further, papers that can also be used to advantage include the following: papers having an optical surface roughness of 8 ym or less and a thickness of 40 to 75 pm as described in Japanese Patent Application (OPI) No. 136492/83; paper having a density of 0.9 g/cm3 and an optical contact rate of 15% or more as described in Japanese Patent Application (OPI) No.

69091/83 (corresponding to U.S. Patent Application Serial No. 436,083); paper made from pulp which has been beaten so as to have a Canadian standard freeness (JIS P8121) of 400 cc or more and thereby, rendered resistant to permeation of a coating composition thereinto as described in Japanese Patent Application (OPI) No. 69097/83 (corresponding to U.S. Patent Application Serial No. 435,803); paper made using a Yankee machine whose gloss surface is used as the face on which a coating composition is to be coated to result in improvement upon color density and resolution of developed images as described in Japanese Patent Application (OPI) No. 65695/83 (corresponding to U.S. Patent 4,466,007); paper which has received a corona discharge treatment to improve coating aptitude, as described in Japanese Patent Application (OPI) No. 35985/84; and so on.Moreover, any supports which has been conventionally employed in the art of heat sensitive recording materials can be used as a support of the present invention.

The heat sensitive recording material of the present invention can be used as printing paper for high-speed facsimile and electronic computers, and records printed can be fixed by exposing them to light at the conclusion of thermal printing to decompose the diazo compound molecules that have not taken part in the coupling reaction. In addition, it can also be employed as heat developable copying paper.

The present invention is illustrated in greater detail by reference to the following examples.

Unless otherwise indicated, all parts are by weight.

EXAMPLE 1 Two parts of the following diazo compound and 18 parts of an xylylenediisocyanatetrimethylolpropane (3/1 in molar ratio) addition product were added to a mixed solvent composed of 24 parts of dibutyl phthalate and 5 parts of ethyl acetate, and dissolved therein.

The resulting solution of the diazo compound was admixed with an aqueous solution prepared by dissolving 3.5 parts of polyvinyl alcohol and 1.7 parts of gelatin in 58 parts of water, and dispersed in an emulsified condition at 20"C. Thus, an emulsion having an average particle size of 3 ym was obtained. To the emulsion was added 100 parts of water, and the resulting emulsion was heated up to 60"C with stirring. After two hours of heating and stirring, a capsule solution containing the diazo compound as a core material was obtained.

The diazo compound used is represented by the formula

Then, 20 parts of 2-hydroxy-3-naphthoic acid anilide was added to 100 parts of a 5% water solution of polyvinyl alcohol, and dispersed using a sand mill for about 24 hours to prepare a dispersion of the coupling component having an average particle size of 3,us.

Separately, 20 parts of triphenylguanidine was added to 100 parts of a 5% water solution of polyvinyl alcohol, and dispersed using a sand mill for about 24 hours to prepare a dispersion of triphenylguanidine having an average particle size of 3 lzm.

Further, 20 parts of p-t-butylphenol was added to 100 parts of a 5% water solution of polyvinyl alcohol, and dispersed using a sand mill for about 24 hours to prepare a dispersion of p-t-butylphenol having an average particle size of 3,us.

A coating composition was prepared by admixing 50 parts of the thus obtained capsule solution of diazo compound, 1 5 parts of the thus obtained dispersion of the coupling component, 1 5 parts of the thus obtained dispersion of triphenylguanidine, and 30 parts of the thus obtained dispersion of p-t-butylphenyl. This coating composition was coated on smooth, wood-free paper (having a basis weight of 50 g/m2) at a dry coverage of 20 g/m2 according to a bar coating method using a coating rod, and dried at 45"C for 30 minutes to produce a heatsensitive recording material.

The thus obtained heat-sensitive recording material was examined for color density of a developed image therein and the temperature at which the coloring reaction starts according to the following testing methods, respectively. The results obtained are shown in Table 1.

On the other hand, 20 parts of a xylylenediisocyanate-trimethylolpropane (3/1 in molar ratio) addition product was dissolved in 30 parts of ethyl acetate, coated on a polyethylene sheet using a bar coating method, and heated to 40 to 60"C in water to undergo the reaction. The coated layer was peeled off, and air-dried at 24"C and 64% RH for one day to form a film having a thickness of 10 to 20 u. This film was soaked in a 20% methanol solution of p-tbutylphenol for 30 hours and then, air-dried at 24"C and 64% RH for one day. Thus, a sample for measuring a glass transition point of a capsule wall material was prepared.

A glass transition point of the thus obtained sample film was measured according to the following testing method. The result obtained is also shown in Table 1.

On the other hand, a 1/1 mixture of triphenylguanidine and p-t-butyl phenol was examined for melting point, and the result is also shown in Table 1.

The testing methods used are described below.

(1) Measurement of Image Density and Coloring Initiation Temperature: Images were thermally recorded on the heat-sensitive recording materials using a GIll Mode facsimile (Hifax 700, product of Hitachi, Ltd.) and then, fixed by overall exposure using a Ricopy Super Dry (product of Ricoh Company, Ltd.).

Blue color densities of the recorded images were measured with a Macbeth reflex densitometer.

On the other hand, the heat-sensitive recording materials were heated to a temperature ranging from 60 to 150"C by pressing a hot stamp with a load of 300 g/m2 against the recording materials, whereby the temperature at which the coloring reaction is initiated was determined.

(2) Measurement of Glass Transition Point: Dynamic modulus (E') and Dynamic loss modulus (E") of the sample film were measured with a Vibron Model DDV-II (products of Toyo Baldwin), and the temperature dependence of tan 8= = E"/E' was examined. The temperature at which the value of tan 8 reached a peak was taken as the glass transition point of the sample film. In the foregoing measurement, the heating speed was 2"C/min.

(3) Measurement of Melting Point: The mixture was examined for melting point using a scanning type differential calorimeter DSC-II (products of Perkin-Elmer Corp.) at a heating speed of 1 0'C/min. The result obtained is shown in Table 1.

EXAMPLES 2 TO 6 Heat-sensitive recording materials were prepared in the same manner as in Example 1 except that p-oxybenzylphenol, p-xylylenediol, benzyl p-oxybenzoate, 2,6-dimethylphenol and 1,4dimethoxybenzyl were employed in place of p-t-butylphenol, respectively. The thus obtained, five kinds of heat sensitive recording materials were examined for coloring initiation temperature and color density of recorded image using the same methods as in Example 1.

Next, samples for measuring glass transition point of the capsule wall were prepared using in place of p-t-butylphenol the foregoing transition point controlling agents, respectively, in a similar manner as in Example 1, and then the glass transition point of these samples was measured in the same manner as in Example 1.

Further, the melting point of 1/1 mixture of triphenylguanidine and one of the transition point controlling agents set forth above was measured using the same method as in Example 1.

The thus obtained results are also shown in Table 1.

COMPARATIVE EXAMPLE 1 A heat sensitive recording material and a sample for measuring a glass transition point of a capsule wall material were prepared in a similar manner as in Example 1, except that p-t-butyl phenol was not incorporated therein, and examined using the same method as in Example 1. As for the melting point measurement, that of p-t-butylphenol alone was determined. Results obtained are shown in Table 1.

COMPARATIVE EXAMPLE 2 A heat sensitive recording material and a sample for measuring a glass transition point of the capsule wall material were prepared in the same manner as in Example 1, except that 2,4-di-tbutyl-5-methylphenol was used in place of p-t-butylphenol, and examined using the same method as in Example 1. Further, the melting point of 1/1 mixture of triphenylguanizine and 2,4-di-t-butyl-5-methylphenol was measured. The results obtained are shown in Table 1.

Table 1 High Sensitive Coloring Glass Recording Initiation Image Transition Melting Material Temserataure Densitv Point Point ("c) ( 0C) ( 0C) Example 1 80-90 1.18 106 65 Example 2 80-90 1.25 121 93 Example 3 90 1.16 121 90 Example 4 90 1.13 128 100 Example 5 80-90 1.03 118 96 Example 6 90 1.06 130 105 Comparative Example 1 120 0.98 157 144 Comparative Example 2 110-120 0.96 152 45 It can be seen from the data set forth in Table 1 that the heat sensitive recording materials produced in Examples 1, 2, 3, 4, 5, and 6 wherein compounds having the effect of sharply lowering the glass transition point of the capsule wall material were incorporated, respectively, started the coloring reaction at low temperatures and all provided images of high densities, compared with the recording material produced in Comparative Example 1, wherein no such compound was incorporated and the recording material produced in Comparative Example 2 wherein a compound incapable of lowering the glass transition point of the capsule wall material (though capable of lowering the melting point of triphenylguanidine) was incorporated. Thus, it is seen that the heat-sensitive recording material of the present invention has excellent qualities.

EXAMPLE 7 3.45 parts of the following diazo compound and 18 parts of an xylylenediisocyanatetrimethylolpropane (3/1 in molar ratio) addition product were added to a mixed solvent composed of 24 parts of tricresyl phosphate and 5 parts of ethyl acetate, and dissolved therein.

The resulting solution of the diazo compound was admixed with an aqueous solution prepared by dissolving 5.2 parts of polyvinyl alcohol in 58 parts of water, and dispersed in an emulsified condition at 20"C. Thus, an emulsion having an average particle size of 2.5 m was obtained.

To the emulsion was added 100 parts of water, and the resulting emulsion was heated to 60"C with stirring. After two hours of heating and stirring, a capsule solution containing the diazo compound as a core material was obtained.

The diazo compound used is represented by the formula

Then, 10 parts of 2-hydroxy-3-naphthoic acid anilide and 10 parts of triphenylguanidine were added to 100 parts of 5% aqueous solution of polyvinyl alcohol, and dispersed using a sand mill for about 24 hours to prepare a dispersion of the coupling component and triphenylguanidine which had an average particle size of 3 lim.

Separately, 20 parts of p-toluenesulfonamide was added to 100 parts of a 4% aqueous solution of polyvinyl alcohol and thereto, 100 parts of water was further added. The resulting mixture was dispersed for 2 hours using a paint shaker to prepare a dispersion of p toluenesulfonamide having an average particle size of 3 cm.

A coating composition was prepared by admixing 50 parts of the thus obtained capsule solution of diazo compound, 24 parts of the thus obtained dispersion of triphenylguanidine, and 28 parts of the thus obtained dispersion of p-toluenesulfonamide. This coating composition was coated on smooth, wood-free paper (having a basis weight of 50 g/m2) at a dry coverage of 10 g/m2 using a coating bar, and dried at 25"C for 30 minutes to produce a heat-sensitive recording material.

EXAMPLE 8 A heat-sensitive recording material was prepared in the same manner as in Example 7, except that N-(o-chlorophenyl)sulfonamide was employed in place of p-toluenesulfonamide.

EXAMPLE 9 Another heat-sensitive recording material was prepared in the same manner as in Example 7, except that N-(p-methoxyphenyl)sulfonamide was employed in place of p-toluenesulfonamide.

COMPARATIVE EXAMPLE 3 A further heat-sensitive recording material was prepared in the same manner as in Example 7, except that p-toluenesulfonamide was excluded.

COMPARATIVE EXAMPLE 4 A still another heat-sensitive recording material was prepared in the same manner as in Example 7, except that stearic acid amide was employed in place of p-toluenesulfonamide.

Measurement of image density for Examples 7, 8 and 9 and Comparative Examples 3 and 4 was conducted in the same manner as described for Example 1, etc., hereinbefore. The results are set forth in Table 2 below.

In addition, background densities (fog) of the heat-sensitive materials described above were measured with a Macbeth reflex densitometer just after preparation and after the conclusion of a forced deterioration test in which the recording materials were stored for 24 hours in the dark under the condition of 40"C and 90% RH (relative humidity). The shelf storage quality was evaluated by the thus observed changes in fog density. Data obtained by these experiments are also shown in Table 2.

Table 2 Heat Sensitive Fog Before Fog After Recording Image Deterioration Deterioration Material Density Test Test Example 7 1.21 0.09 0.10 Example 8 1.15 0.09 0.11 Example 9 1.16 0.09 0.11 Comparative Example 3 0.98 0.09 0.13 Comparative Example 4 0.97 0.10 0.14 As can be seen from the data in Table 2, all of the heat sensitive recording materials prepared in Examples 7, 8, and 9, in which a diazo compound was present in a microencapsulated condition, and wherein a sulfonamide compound was additionally incorporated, provided images of high densities and had small increases in fog after receiving the forced deterioration test, compared with the recording material of Comparative Example 1 in which no sulfonamide compound was incorporated. That is, the recording materials of the present invention are superior to conventional materials that have been utilized in a microencapsulating process.

Moreover, the recording materials of the present invention have proved to be remarkably excellent in thermal color-developability, compared with the recording material of Comparative Example 2, to which a heat-fusible substance was added.

Further, improvements in whiteness of the background and sticking resistance were noted by use the recording materials (Examples 7, 8 and 9) of the present invention.

Claims (11)

1. A heat-sensitive recording material, comprising a support having provided thereon a recording layer containing a diazo compound and a coupling component, with the recording layer containing said diazo compound in a microencapsulated condition, and also containing a compound capable of lowering the glass transition point of a wall material of the microcapsules, in an amount of from 0.01 to 10 parts by weight per 1 part by weight of said coupling component.

2. A heat-sensitive recording material as claimed in Claim 1, wherein the glass transition point of the wall material of the microcapsules is from 80 to 150"C.

3. A heat-sensitive recording material as claimed in Claim 2, wherein said glass transition point is from 100 to 130"C.

4. A heat-sensitive recording material as claimed in Claim 1, 2 or 3, wherein the compound capable of lowering the glass transition point is a hydroxy compound, a carbamic acid ester compound, an aromatic methoxy compound or an organic sulfonamide compound.

5. A heat-sensitive recording material as claimed in any of Claims 1 to 4, wherein the capsule wall formed in the microencapsulation is a polymeric substance selected from polyurethane, polyurea, polyamide, polyester and polycarbonate.

6. A heat-sensitive recording material as claimed in any of Claims 1 to 4, wherein the capsule wall formed in the microencapsulation is a polymeric substance obtained by the reaction of polyvalent isocyanate and polyol.

7. A heat-sensitive recording material as claimed in any of Claims 1 to 4, wherein the capsule wall formed in the microencapsulation is a polymeric substance obtained by the reaction of polyvalent isocyanate and water.

8. A heat-sensitive recording material as claimed in any preceding claim, wherein the solvent for the diazo compound is an organic solvent having a boiling point of 180"C or more.

9. A heat-sensitive recording material as claimed in Claim 6, wherein the polyol is used in an amount of 0.02 to 2 mols in terms of hydroxy group per mol of isocyanato group.

10. A heat-sensitive recording material as claimed in any preceding claim, wherein the recording layer contains a basic substance or a substance capable of becoming basic when heated.

11. A heat-sensitive recording material as claimed in any preceding claim, wherein said recording material also contains a binder selected from the emulsions of polyvinyl alcohol, methyl cellulose, carboxymethyl cellulose, hydroxypropyl cellulose, gum arabic, gelatin, polyvinyl pyrrolidone, casein, styrenebutadiene latex, acrylonitrile-butadiene latex, polyvinylacetate, polyacrylic esters and ethylenevinyl acetate copolymers.

1 2. A heat-sensitive recording material as claimed in any preceding claims, wherein the diazo compound is coated in an amount of from 0.05 to 2.0 g/m2.

1 3. A heat-sensitive recording material as claimed in any preceding claim, wherein the coupling component is used in an amount of from 0.1 to 10 parts by weight per 1 part by weight of diazo compound.

1 4. A heat-sensitive recording material, substantially as hereinbefore described with reference to any of Examples 1 to 9.

1 5. A visible image formed by locally heating a recording material as claimed in any preceding claim.