# Patent

## Title
Systhetic whitener pigment

## Abstract

A heat-sensitive recording composition comprising agglomerates which comprise an aromatic isocyanate compound, an imino compound and a sensitizer and have an average diameter of 2-30 .mu.m; and a process for producing the composition are disclosed. This heat-sensitive recording composition is excellent in heat response and high in sensitivity. From the point of image stability, the agglomerates are preferably contained in microcapsules together with a polymer.

## Background
The present invention relates to a heat-sensitive recording composition excellent in heat response and having high sensitivity and a process for producing it.

Heat-sensitive recording materials generally comprise a substrate and a heat-sensitive recording layer coated thereon comprising a heat-sensitive recording composition mainly composed of an electron donating colorless dye precursor and an electron accepting color developer. The colorless dye precursor and the color developer instantaneously react with each other by heating them with a thermal head, thermal pen, laser beam and the like to form a record image. These are disclosed in Japanese Patent Kokoku Nos. 43-4160 and 45-14039, etc.

Such heat-sensitive recording materials have the advantages that record can be obtained by relatively simple devices, maintenance is easy and little noises is generated and are used in various fields such as recording instruments, facsimiles, printers, terminals of computers, labels, and automatic ticket vending machines for passenger tickets and the like.

Such heat-sensitive recording materials in which an electron donating colorless dye precursor and an electron-accepting color developer are used have various favorable characteristics such as good appearance, good feel, high density color image and images of various color hues, but also have defects in image stability. That is, if the color formed portion (record image portion) contacts with plastics such as polyvinyl chloride, the image disappears due to plasticizers or additives contained in the plastics, or if it contacts with chemicals contained in foods, cosmetics, etc., the image readily disappears, or readily fades if exposed to sunlight even for short period of time. Owing to these defects, they are restricted in use.

As heat-sensitive recording materials which can provide record images of high stability, heat-sensitive recording materials comprising an aromatic isocyanate compound and an imino compound which reacts with the aromatic isocyanate compound upon being heated to form a color have been proposed in, for example, Japanese Patent Application Kokai Nos. 58-38733, 58-54085, 58-104959, 58-149388, 59-115887, and 59-115888, and U.S. Pat. No. 4,521,793.

However, these heat-sensitive recording materials comprising an aromatic isocyanate compound and an imino compound are low in sensitivity and can hardly provide record images of sufficient density in high-speed printing, though stability of the record image is improved to some extent.

Furthermore, as another means for improving the stability of record image, heat-sensitive recording materials using microcapsules have been proposed. For example, Japanese Patent Application Kokai No. 59-19193 (Japanese Patent Application Kokoku No. 2-2440) of the inventors discloses a heat-sensitive recording paper which comprises a support and microcapsules coated thereon which contain at least a dye precursor, a color developer and a wax substance which is solid at room temperature, but melts upon heating. This relates to a heat-sensitive recording paper prepared using microcapsules containing a dye precursor, a color developer and a wax substance (a sensitizer) and color is formed inside the microcapsules without rupturing them.

In this patent publication, the following encapsulation methods are exemplified.

(1) A color forming colorless dye or a color developer is mixed and molten with a sensitizer. The respective mixtures are emulsified and the resulting emulsion of color forming colorless dye--sensitizer and emulsion of color developer--sensitizer are mixed and encapsulated.

This method (1) has a defect in that the concentration of the color forming colorless dye or the color developer in the sensitizer cannot be increased sufficiently because the dye and developer form deposition when their concentration is high. When an emulsion of each of said component is mixed and microencapsulated, capsules containing each alone are formed, so that mixture of them will make a heat-sensitive recording material of which colour development efficiency is poor.

(2) A color forming colorless dye or a color developer is mixed and molten with a sensitizer. The respective mixtures are emulsified and the resulting emulsion of color forming colorless dye--sensitizer and the emulsion of color developer--sensitizer are processed into quasi-capsules (very thinly walled capsules), respectively and these quasi-capsules are mixed and encapsuled.

(3) Finely dispersed color forming colorless dye and color developer are respectively encapsulated in the form of quasi-capsules and these quasi-capsules are mixed and dispersed in a molten sensitizer and then encapsulated.

The above methods (2) and (3) require the step of formation of quasi-capsules and hence are less efficient in productivity.

The object of the present invention is to provide a heat-sensitive recording composition high in sensitivity by use of heretofore used aromatic isocyanate compounds, imino compounds, and sensitizers.

According to the present invention, there are provided a heat-sensitive recording composition comprising agglomerates which have an average diameter of 2-30 .mu.m and comprise an aromatic isocyanate compound, an imino compound and a sensitizer; and a process for producing the composition.

The present invention will be explained in detail.

The heat-sensitive recording composition of the present invention contains agglomerates a an essential component and optionally a binder, a pigment and other additives.

A heat-sensitive recording material can be obtained by providing a heat-sensitive recording layer by coating a heat-sensitive recording composition on a substrate.

The agglomerates comprise an aromatic isocyanate compound, an imino compound and a sensitizer. The agglomerates contain the imino compound in an amount of 50-300, preferably 100-200 parts by weight, and the sensitizer in an amount of 10-300, preferably 30-200 parts by weight, based on 100 parts by weight of the aromatic isocyanate compound. When amount of each of the imino compound is less than 50 parts by weight, a large amount of unreacted aromatic isocyanate compound remains after use. When the amount is more than 300 parts by weight, a large amount of unreacted imino compound remains after use. Both cases are not economical.

The agglomerates have an average diameter of 2-30 .mu.m, preferably 3-20 .mu.m, more preferably 5-10 .mu.m.

Hitherto, each of the three components, the aromatic isocyanate compound, imino compound and sensitizer, has been ground and dispersed respectively, or in combination of the two, i.e. the aromatic isocyanate compound and sensitizer, or the imino compound and sensitizer, so that each of them was ground down to an average diameter of about 0.5 .mu.m and used as it was. It is considered that the smaller the diameter of the components the higher sensitivity would result. However, when paper is used for the substrate for a heat-sensitive recording material, its surface has irregularity portions due to pulp fibers, so that the thus finely ground particles of those components fill up recesses and the advantage of that fineness is not effectively utilized.

According to the present invention, the three components are agglomerated whereby the three components are prevented from filling up recesses of the substrate and are uniformly arranged on the surface of the substrate. Thus, high sensitivity can be attained. Moreover, since the finely dispersed three components are in the state of being close to one another in the agglomerates, color is very effectively formed upon transmission of heat of the thermal head to the agglomerates per se.

However, since thickness of the heat-sensitive recording layer of the heat-sensitive recording material is usually about 30 .mu.m, if the agglomerates have a diameter of more than 30 .mu.m, the agglomerates protrude beyond the heat-sensitive recording layer to result in deterioration of surface smoothness of the heat-sensitive material and to cause fogging with application of pressure. On the other hand, if average diameter is less than 2 .mu.m, sensitivity is insufficient.

The heat-sensitive recording materials of the present invention comprising a support and the heat-sensitive recording composition coated thereon has another advantage in that the coated side has a low surface gloss (matte). This is because since the fine three components are agglomerated they easily scatter light, and agglomerates per se have a large particle diameter and are interspersed on the substrate. In general, heat-sensitive recording materials are high in gloss and have a defect that printed letters thereon are difficult to read. In order to inhibit glare of the coated surface, a method to impart lower gloss like a plain paper by applying a matte coating on a heat-sensitive recording layer is employed recently. In the present invention, such effect can be obtained only by coating the heat-sensitive recording composition on the substrate without applying such a matte coating.

The aromatic isocyanate compound used in the present invention is a colorless or light colored aromatic isocyanate or heterocyclic isocyanate compound which is solid at room temperature and, for example, at least one of the following ones may be used; 2,6-dichlorophenyl isocyanate, p-chlorophenyl isocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate 1,3-dimethyl-benzene-4,6-diisocyanate, 1,4-dimethylbenzene-2,5-diisocyanate, 1-methoxybenzene-2,4-diisocyanate, 1-methoxybenzene-2,5-diisocyanate, 1-ethoxybenzene-2,4-diisocyanate, 2,5-dimethoxybenzene-1,4-diisocyanate, 2,5-diethoxybenzene-1,4-diisocyanate 2,5-dibutoxybenzene-1,4-diisocyanate, azobenzene-4,4'-diisocyanate, diphenylether-4,4'-diisocyanate, naphthalene-1,4-diisocyanate, naphthalene-1,5-diisocyanate, naphthalene-2,6-diisocyanate, naphthalene-2,7-diisocyanate, 3,3'- dimethyl-biphenyl-4,4'-diisocyanate, 3,3'-dimethoxy-biphenyl-4,4'-diisocyanate, diphenylmethane-4,4'-diisocyanate, diphenyldimethylmethane-4,4'-diisocyanate, benzophenone-3,3'-diisocyanate, fluorene-2,7-diisocyanate, anthraquinone-2,6-diisocyanate, 9-ethylcarbazole-3,6-diisocyanate, pyrene-3,8-diisocyanate, naphthalene-1,3,7-triisocyanate, biphenyl-2,4,4'-triisocyanate, 4,4',4"-triisocyanato-2,5-dimethoxytriphenylamine, p-dimethylaminophenylisocyanate, and tris(4-phenylisocyanato)thiophosphate.

If necessary, these isocyanate compounds may be used in the form of so-called block isocyanates which are addition compounds with phenols, lactams, oximes or the like or may be used in the form of dimers of diisocyanates such as a dimer of 1-methylbenzene-2,4-diisocyanate or in the form of isocyanurates which are trimers. Furthermore, they may be used as polyisocyanates which are adducts with various polyols.

The imino compounds used in the present invention are compounds which are represented by the formula ##STR1## (wherein .phi. is an aromatic compound residue which can form a conjugated system with an adjacent C.dbd.N) and which are colorless or light colored and are solid at room temperature. Examples of the imino compounds are shown below and these may be used singly or in combination of two or more depending on the objects.

3-Iminoisoindoline-1-one, 3-imino-4,5,6,7- tetrachloroisoindoline-1-one, 3-imino-4,5,6,7-tetrabromoisoindoline-1-one, 3-imino-4,5,6,7-tetrafluoroisoindoline-1-one, 3-imino-5,6-dichloroisoindoline-1-one, 3-imino-4,5,7-trichloro-6-methoxyisoindoline-1-one, 3-imino-4,5,7-trichloro-6-methylmercaptoisoindoline-1-one, 3-imino-6-nitroisoindoline-1-one, 3-imino-isoindoline-1-spirodioxolan, 1,1-dimethoxy-3-imino-isoindoline, 1,1-diethoxy-3-imino-4,5,6,7-tetrachloroisoindoline, 1-ethoxy-3-iminoisoindoline, 1,3-diiminoisoindoline, 1,3-diimino-4,5,6,7-tetrachloroisoindoline, 1,3-dimino-6-methoxyisoindoline, 1,3-diimino-6-cyanoisoindoline, 1,3-diimino-4,7-dithia-5,5,6,6-tetrahydroisoindoline, 7-amino-2,3-dimethyl-5-oxopyrrolo[3,4b]pyrazine, 7-amino-2,3-diphenyl-5-oxopyrrolo[3,4b]pyrazine, 1-iminonaphthalic acid imide, 1-iminodiphenic acid imide, 1-phenylimino-3-iminoisoindoline, 1-(3'-chlorophenylimino)-3-iminoisoindoline, 1-(2',5'-dichlorophenylimino)-3-iminoisoindoline, 1-(2',4',5'-trichlorophenylimino)-3-iminoisoindoline, 1-(2'-cyano-4'-nitrophenylimino)-3-iminoisoindoline, 1-(2'-chloro-5'-cyanophenylimino)-3-iminoisoindoline, 1-(2',6' -dichloro-4'-nitrophenylimino)-3-iminoisoindoline, 1-(2',5'-dimethoxyphenylimino)-3-iminoisoindoline, 1-(2',5'-diethoxyphenylimino)-3-iminoisoindoline, 1-(2'-methyl-4'-nitrophenylimino)-3-iminoisoindoline, 1-(5'-chloro-2'-phenoxyphenylimino)-3-iminoisoindoline, 1-(4'-N,N-dimethylaminophenylimino)-3-iminoisoindoline, 1-(3'-N,N-dimethylamino-4'-methoxyphenylimino)-3-inimoisoindoline, 1-(2'-methoxy-5'-N-phenylcarbamoylphenylimino)-3-iminoisoindoline, 1-(2'-chloro-5'-trifluoromethylphenylimino)-3-iminoisoindoline, 1-(5',6'-dichlorobenzothiazolyl-2'-imino)iminoisoindoline, 1-(6'-methylbenzothiazolyl-2'-imino)-3-iminoisoindoline, 1-(4'-phenylaminophenylimino)-3-iminoisoindoline, 1-(p-phenylazophenylimino)-3-iminoisoindoline, 1-(naphthyl-1'-imino)-3-iminoisoindoline, 1-(anthraquinone-1'-imino)-3-iminoisoindoline, 1-(5'-chloroanthraquinone-1'-imino)-3-iminoisoindoline, 1-(N-ethylcarbazolyl-3'-imino)-3-iminoisoindoline, 1-(naphthoquinone-1'-imino)-3iminoisoindoline, 1-(pyridyl-4'-imino)-3-iminoisoindoline, 1-(benzimidazolone-6'-imino)-3-iminoisoindoline, 1-(1'-methylbenzimidazolone-6'-imino)-3-iminoisoindoline, 1-(7'-chlorobenzimidazolone-5'-imino)-3-iminoisoindoline, 1-(benzimidazolyl-2'-imino)-3-iminoisoindoline, 1-(benzimidazolyl-2'-imino)-3-imino-4,5,6,7-tetrachloroisoindoline, 1-(2',4'-dinitrophenylhydrazone)-3-iminoisoindoline, 1-(indazolyl-3'-imino)-3-iminoisoindoline, 1-(indazolyl-3'-imino)-3-imino-4,5,6,7-tetrabromoisoindoline, 1-(indazolyl-3'-imino)-3-imino-4,5,6,7-tetrafluoroisoindoline, 1-(benzimidazolyl-2'-imino)-3 -imino-4,7-dithiatetrahydroisoindoline, 1-(4',5'-dicyanoimidazolyl-2'-imino)-3-imino-5,6-dimethyl-4,7-pyrazisoindo line, 1-(cyanobenzoylmethylene)-3-iminoisoindoline, 1-(cyanocarbonamidemethylene)-3-iminoisoindoline, 1-(cyanocarbomethoxymethylene)-3-iminoisoindoline, 1-(cyanocarboethoxymethylene)-3-iminoisoindoline, 1-(cyano-N-phenylcarbamoylmethylene)-3-iminoisoindoline, 1-[cyano-N-(3'-methylphenyl)-carbamoylmethylene]-3-iminoisoindoline, 1-[cyano-N-(4'-chlorophenyl)carbamoylmethylene]-3-iminoisoindoline, 1-[cyano-N-(4'-methoxyphenyl)carbamoylmethylene]-3-iminoisoindoline, 1-[cyano-N-(3'-chloro-4'-methylphenyl)carbamoylmethylene]-3-iminoisoindoli ne, 1-(cyano-p-nitrophenylmethylene)-3-iminoisoindoline, 1-(dicyanomethylene)-3-iminoisoindoline, 1-(cyano-1',2',4'-triazolyl-(3')carbamoylmethylene)-3-iminoisoindoline, 1-(cyanothiazoyl(2')-carbamoylmethylene)-3-iminoisoindoline, 1-(cyanobenzimidazolyl-(2')-carbamoylmethylene)-3-iminoisoindoline, 1-(cyanobenzothiazolyl-(2')-carbamoylmethylene)-3iminoisoindoline, 1-[(cyanobenzimidazolyl-2')-methylene]-3-iminoisoindoline, 1-[(cyanobenzimidazolyl-2')methylene]-3-imino-4,5,6,7-tetrachloroisoindoli ne, 1-[cyanobenzimidazolyl-2')-methylene]-3-imino-5-methoxyisoindoline, 1-[(cyanobenzimidazolyl-2')-methylene]-3-imino-6-chloroisoindoline, 1-[(1'-phenyl-3'-methyl-5-oxo)-pyrazolidene-4']-3-iminoisoindoline, 1-[(cyanobenzimidazolyl-2')-methylene]-3-imino-4,7-dithiatetrahydroisoindo line, 1-[(cyanobenzimidazolyl-2')methylene]-3-imino-5,6-dimethyl-4,7-pyrazisoind oline, 1-[(1'-methyl-3' -n-butyl)barbituric acid-5']-3-iminoisoindoline, 3-imino-1-sulfobenzoic acid imide, 3-imino-1-sulfo-6-chlorobenzoic acid imide, 3-imino-1-sulfo-5,6-dichlorobenzoic acid imide, 3-imino-1-sulfo-4,5,6,7-tetrachlorobenzoic acid imide, 3-imino-1-sulfo-4,5,6,7-tetrabromobenzoic acid imde, 3-imino-1-sulfo-4,5,6,7-tetrafluorobenzoic acid imide, 3-imino-1-sulfo-6-nitrobenzoic acid imide, 3-imino-1-sulfo-6-methoxybenzoic acid imide, 3-imino-1-sulfo-4,5,7-trichloro-6-methylmercaptobenzoic acid imide, 3-imino-1-sulfonaphthoic acid imide, 3-imino-1-sulfo-5-bromonaphthoic acid imide,and 3-imino-2-methyl-4,5,6,7-tetrachloroisoindoline-1-one.

Examples of the sensitizers used in the present invention are waxes such as N-hydroxymethylstearic acid amide, stearic acid amide, palmitic acid amide, oleic acid amide, ethylene.bisstearic acid amide, ricinoleic acid amide, paraffin wax, microcrystalline wax, polyethylene wax, rice wax, and carnauba wax; naphthol derivatives such as 2-benzyloxynaphthalene; biphenyl derivatives such as p-benzylbiphenyl and 4-allyloxybiphenyl; polyether compounds such as 1,2-bis(3-methylphenoxy)ethane, 2,2'-bis(4-methoxyphenoxy)diethyl ether, and bis(4-methoxyphenyl) ether; and carbonic acid or oxalic acid diester derivatives such as diphenyl carbonate, dibenzyl oxalate, and di(p-furolbenzyl) oxalate. These sensitizers may be used singly or in combination of two or more.

The heat-sensitive recording composition of the present invention usually contains binders.

As examples of the binders, mention may be made of water-soluble binders such as starches, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, gelatin, casein, polyvinyl alcohol, modified polyvinyl alcohol, sodium polyacetate, acrylic acid amide/acrylic acid ester copolymer, acrylic acid amide/acrylic acid ester/methacrylic acid terpolymer, alkali salts of styrene/maleic anhydride copolymer, and alkali salts of ethylene/maleic anhydride copolymer; and latexes of polymers such as polyvinyl acetate, polyurethane, polyacrylic acid esters, styrene/butadiene copolymer, acrylonitrile/butadiene copolymer, methyl acrylate/butadiene copolymer, and ethylene/vinyl acetate copolymer.

The heat-sensitive recording composition of the present invention may further contain pigments such as diatomaceous earth, talc, kaolin, calcined kaolin, calcium carbonate, magnesium carbonate, titanium oxide, zinc oxide, silicon oxide, aluminum hydroxide, and ureaformalin resin.

Moreover, for inhibition of wear of a thermal head and inhibition of sticking, if necessary, there may be added to the heat-sensitive recording composition metallic salts of higher fatty acids such as zinc stearate and calcium stearate, waxes such as paraffin, oxidized paraffin, polyethylene, polyethylene oxide, stearic acid amide, and castor wax; there may be further added a dispersing agent such as sodium dioctylsulfosuccinate, an ultraviolet absorber such as benzophenone type and benzotriazole type, a surfactant, and a fluorescent dye.

As the substrate on which the heat sensitive recording composition is to be coated, paper is mainly used, but there may also be used nonwoven fabrics, plastic films synthetic papers, metallic foils and composite sheets comprising combination of them. Furthermore, there may also be used such substrate on which an undercoat layer containing inorganic pigments, organic pigments or the like has been coated.

The heat-sensitive recording composition of the present invention may be formulated into an ink comprising the agglomerates, a pigment, an organic solvent and a binder soluble in the organic solvent. Such an ink can be used for a spot printing by means of a printing machine such as flexographic press, rotogravure press or offset press.

In the first embodiment of the present invention, the heat-sensitive recording composition comprises agglomerates formed using a cationic dispersing agent.

In this case, the heat-sensitive recording composition is obtained by a process comprising the following steps.

(1) Each of the aromatic isocyanate compound, the imino compound and the sensitizer is ground alone, or the aromatic isocyanate compound and mixture of the imino compound and sensitizer, or the imino compound and mixture of the sensitizer and aromatic isocyanate compound, are ground separately, until means particles diameter comes down to 0.5-1.0 .mu.m under presence of an anionic dispersing agent;

(2) The resulting dispersions are mixed; and

(3) A cationic dispersing agent is added to the mixture, which is stirred to form agglomerates having a mean diameter of 2-30 .mu.m and comprising the said three components.

The reason why the agglomerates are obtained by the above process is considered as follows. In the above step (1), the three components become negatively charged particles due to the presence of the anionic dispersing agent. In the above step (3), the negatively charged particles bond to the positively charged cationic dispersing agent to form an electrically neutral complex. As a result, the three components agglomerate one another, resulting in agglomerates comprising the three components.

The cationic dispersing agent includes cationic surface active agents, cationic polymers and the like.

Examples of the cationic surface active agents are amine salts, quaternary ammonium salts, phosphonium salts, sulfonium salts, and combinations thereof.

Examples of the cationic polymers are polyaminoalkyl methacrylate, aminoalkyl methacrylate-acrylamide copolymer, polyvinylpyridinium halides, polydiallylammonium halides, polyaminomethylacrylamide, polyvinylimidazoline, Mannich modified products of polyacrylamide, polyethyleneiminepolydiallylamine, polypyridinium halide chitosan, cationized starch, cationized cellulose, cationized polyvinyl alcohol, ionene condensates, epoxyamine condensates, cationized polymethacrylate resin, alkylenediamine-epichlorohydrin polycondensates, and combination thereof.

In view of stability of records (e.g., chemical resistance), the agglomerates are preferably microencapsulated. When the agglomerates are microencapsulated, discoloration of printed portion or color formation of unprinted portion hardly occurs even if the heat-sensitive recording material contacts with chemicals such as organic solvents.

Average diameter of the microcapsules is nearly the same as that of the agglomerates and hence is 2-30 .mu.m, preferably 3-20 .mu.m, more preferably 5-10 .mu.m. When the average diameter exceeds 30 .mu.m, there occur falling off of the microcapsules from the heat-sensitive recording material, roughening of the surface of the material and undesired color formation by scratching or frictional heat. The average diameter of less than 2 .mu.m is impossible since size of the agglomerates to be microencapsulated is 2-30 .mu.m as aforesaid.

The wall material of the microcapsules is preferably a thermocurable resin such as melamine-formaldehyde resin or urea-formaldehyde resin. Use of a thermocurable resin prevents rupture of the microcapsules when the heat-sensitive recording material is imaged by heat, so that occurrence of sticking of the material to a thermal head or piling on a thermal head is inhibited.

In the second embodiment of the present invention, the agglomerates formed using a cationic dispersing agent are microencapsulated. In this case, the heat-sensitive recording composition is obtained by a process comprising the following steps.

(1) Each of the aromatic isocyanate compound, the imino compound and the sensitizer is ground alone, or the aromatic isocyanate compound and mixture of the imino compound and sensitizer, or the imino compound and mixture of the sensitizer and aromatic isocyanate compound, are ground separately, until mean particles diameter comes down to 0.5-1.0 .mu.m under presence of an anionic dispersing agent;

(2) The resulting dispersions are mixed; and

(3) A cationic dispersing agent is added to the mixture, which is stirred to form agglomerates having a mean diameter of 2-30 .mu.m and comprising the said three components.

(4) The thus prepared agglomerates are added to an anionic protective colloid solution and emulsified or dispersed; and

(5) A wall forming material is added to the emulsion or dispersion to perform microencapsulation of the agglomerates.

According to the above process for production of the heat-sensitive recording composition, the three components can be microencapsulated more efficiently as compared to that attained according to conventional processes in terms of aspects explained in the following. After the three components are dispersed with the anionic dispersing agent in the step (1), the three components are agglomerated one another by adding the cationic dispersing agent in the step (3). In the thus formed agglomerates, the three components are gathered to a mass, which is stable with the lapse of time and can be handled in the same manner as for ordinary emulsified particles. In the step (4), the thus formed agglomerates are introduced into an anionic protective colloid solution for being dispersed or emulsified. It is considered that the surface of the agglomerates is converted from cationic state to anionic state or electrically neutral state by the protective colloid material. Thereafter, the microcapsule wall material is added thereto to carry out microencapsulation. The thus formed microcapsules apparently have a similar shape to that of the agglomerates since the wall is formed conforming to natural contour of the agglomerate. Since core material is the solid agglomerate, the microcapsules hardly rupture even when external pressure is applied, for example, by supercalender to the heat-sensitive recording material made by coating the microcapsules on a substrate. The agglomerates formed in the course of the production are solid, therefore can withstand pressure by themselves. Besides, they are microencapsulated, so that they are protected against permeation of an organic solvent or the like, which causes undesirable color formation.

As the cationic dispersing agents, those referred to in the first embodiment can be used.

The microencapsulation methods may be any known in the prior art, for example, complex coacervation method, in situ method, and interfacial polymerization method, of which preferred is the in situ method.

Use of a melamine-formaldehyde polymer or urea-formaldehyde polymer as the wall material is especially preferred for the in situ method, but there is no limitation about selection of the wall materials.

As the anionic protective colloid materials, mention may be made of, for example, carboxymethyl cellulose, sulfonated cellulose, sulfonated starch, carboxy-modified polyvinyl alcohol, polyacrylic acid, ethylene-maleic anhydride copolymer, methyl vinyl ether-maleic anhydride copolymer, vinyl acetate-maleic anhydride copolymer, and styrene-maleic anhydride copolymer.

As mentioned above, when the agglomerates are formed using a cationic dispersing agent, the step of emulsification or dispersion using anionic protective colloid is required.

In the third embodiment of the present invention, agglomerates formed using an alkali metal salts or ammonium salt of a copolymer of maleic anhydride and a monomer copolymerizable therewith are microencapsulated.

In this case, the heat-sensitive recording composition is obtained by a process comprising the following steps.

(1) Each of the aromatic isocyanate compound, the imino compound and the sensitizer is ground alone, or the aromatic isocyanate compound and mixture of the imino compound and sensitizer, or the imino compound and mixture of the sensitizer and aromatic isocyanate compound, are ground separately, until mean particles diameter comes down to 0.5-1.0 .mu.m under presence of an anionic dispersing agent;

(2) The resulting dispersions are mixed;

(3) An alkali metal salt or an ammonium salt of a copolymer of maleic anhydride and a monomer copolymerizable therewith is added to the mixture, which is stirred to form agglomerates having a mean diameter of 2-30 .mu.m and comprising the said three components; and

(4) A wall forming material is added to the emulsion or dispersion to perform microencapsulation of the agglomerates.

The three components negatively charged in the above step (1) bond with the alkali metal salt or ammonium salt of copolymer of maleic anhydride and a monomer copolymerizable therewith to form a complex in the above step (3). As a result, the three components are combined into agglomerates. Since the alkali metal salt or ammonium salt of the copolymer exerts an emulsification or dispersing action, an emulsion or a dispersion of the agglomerates is obtained in the step (3). In the subsequent step (4), a wall material for microencapsulation is introduced and the agglomerates are microencapsulated. Therefore, addition of anionic protective colloid required in the second embodiment is not required in this embodiment and thus, the production process is simplified a compared with that in the second embodiment.

Amount of the alkali metal salt or ammonium salt of the copolymer of maleic anhydride and a monomer copolymerizable therewith used above is 5-45 parts by weight, preferably 7.5-25 parts by weight based on 100 parts by weight of the three components (core materials) of the aromatic isocyanate compound, the imino compound and the sensitizer. When the amount of the alkali metal salt or ammonium salt of the copolymer is less than 5 parts by weight, anionic portion in the core materials is somewhat excessive to cause incomplete formation of the agglomerates. Moreover, this amount is insufficient to perform emulsification and dispersion of the core material and hence, microencapsulation is also incomplete. When the amount is more than 45 parts by weight, the balance between the anionic portion in the core material and the cationic portion of the alkali metal salt or ammonium salt of the copolymer is lost and the cationic portion becomes excessive and as a result, agglomerates are hardly formed and particles composed of one of the above components alone are liable to be formed.

As the copolymers of maleic anhydride and a monomer copolymerizable therewith, there may be used, for example, ethylene-maleic anhydride copolymer, methyl vinyl ether-maleic anhydride copolymer, propylene-maleic anhydride copolymer, butadiene-maleic anhydride copolymer, isobutylene-maleic anhydride copolymer, isobutene-maleic anhydride copolymer, styrene-maleic anhydride copolymer, vinyl acetate-maleic anhydride copolymer, methacrylamidemaleic anhydride copolymer, and mixtures thereof.

Formation of the microcapsules is carried out in the same manner as in the second embodiment.

For further improvement of image stability, the microcapsules preferably contain a polymer in addition to the agglomerates.

In general, agglomerates are amorphous and have voids therein and depressions on the surface. When such agglomerates are, as they are, contained in microcapsules, the microcapsules become amorphous and thickness of the wall is liable to become nonuniform. For this reason, microcapsules may be ruptured by application of Pressure, and chemicals such as organic solvents may permeate into the microcapsules.

When the voids or depressions of the agglomerates are filled with a polymer and thereafter the agglomerates are microencapsulated, the microcapsules become nearly spherical or fusiform and thickness of the wall becomes more uniform. Accordingly, strength of the microcapsules increases and besides, permeation of organic solvents into microcapsules can be more effectively inhibited.

In the fourth embodiment of the present invention, the above polymer has a form of microemulsion having an average diameter of 0.2 .mu.m or less.

In this case, the heat-sensitive recording composition is obtained by a process comprising the following steps.

(1) Each of the aromatic isocyanate compound, the imino compound and the sensitizer is ground alone, or the aromatic isocyanate compound and mixture of the imino compound and sensitizer, or the imino compound and mixture of the sensitizer and aromatic isocyanate compound, are ground separately, until mean particles diameter comes down to 0.5-1.0 .mu.m under presence of an anionic dispersing agent;

(2) The resulting dispersions are mixed, then a microemulsion having an average emulsified particles diameter of 0.2 .mu.m or less is added;

(3) An alkali metal salt or an ammonium salt of a copolymer of maleic anhydride and a monomer copolymerizable therewith is added to the mixture, which is stirred to form agglomerates having a mean diameter of 2-30 .mu.m and comprising the said three components; and

(4) A wall forming material is added to the emulsion or dispersion to perform microencapsulation of the agglomerates.

The microemulsion used here has an average diameter of 0.2 .mu.m or less, preferably 0.1 .mu.m or less, more preferably 0.05 .mu.m or less. When the average diameter is more than 0.2 .mu.m, the voids or depressions of the agglomerates are not sufficiently filled and image stability cannot be improved.

Addition amount of the microemulsion is 25-200 parts by weight, preferably 50-150 parts by weight, more preferably 75-125 parts by weight based on 100 parts by weight of total of the aromatic isocyanate compound, the imino compound and the sensitizer. When the amount of the microemulsion is less than 25 parts by weight, voids in the agglomerates remain and this is not preferred. In other words, voids in the agglomerates are not sufficiently filled with the microemulsion and chemical resistance tends to be insufficient. On the other hand, when the amount is more than 250 parts by weight, proportions of the dye precursor and the color developer which take part in color formation reaction decrease, resulting in reduction of image density. Besides, coating amount must be increased and this is not economical.

The microemulsion includes a carboxylated emulsion, a solubilized emulsion and the like.

The carboxylated emulsion (this may be called "carboxylated latex", but is consistently referred to as "carboxylated emulsion" in this specification) comprises a copolymer of a principal monomer and an unsaturated carboxylic acid. In general, it is difficult to reduce the average particle diameter of an emulsion (a latex) to less than 0.1 .mu.m. However, the carboxylated emulsion is produced by adding an unsaturated carboxylic acid to a principal monomer to effect emulsion-polymerization, heating and dissolving the resulting emulsion in the presence of an alkali, and then cooling and neutralizing the emulsion and the thus produced carboxylated emulsion has an average particle diameter of 0.1 .mu.m or less and is excellent in various properties such as mechanical stability, freeze stability, and adhesion.

Examples of the unsaturated carboxylic acid are acrylic acid, methacrylic acid, maleic acid, fumaric acid, crotonic acid, itaconic acid, maleic acid esters, fumaric acid esters, and itaconic acid esters. Examples of the principal monomer are, acrylonitrile, styrene, vinyl chloride, vinyl acetate, methyl acrylate, ethyl acrylate, butyl acrylate, 2-hexyl acrylate, butadiene, and ethylene.

As examples of the carboxylated emulsion, mention may be made of styrene-ethylhexyl acrylate copolymer, methyl metahcrylate-ethylhexyl acrylate copolymer, methyl methacrylate-ethyl acrylate copolymer, methyl methacryalte-butadiene copolymer, styrene-ethyl acrylate compolymer, styrene-butyl acrylate copolymer, styrene-butadiene copolymer, styrene-butadiene-acrylic acid terpolymer, styrene-acrylic acid copolymer, vinyl acetate-ethylene copolymer, vinyl acetate-ethyl acrylate copolymer, vinyl acetate-butyl acrylate copolymer, vinyl acetate-butyl maleate copolymer, ethyl acrylate-acrylic acid copolymer, acrylonitrile-butadiene copolymer, ethylene-ethyl acrylate copolymer, and vinyl chloride-acrylic acid copolymer. These may be used singly or in combination of two or more.

The solubilized emulsion is obtained by emulsifying a heat meltable material with a solubilizing agent.

As examples of the solubilizing agents, mention may be made of surface active agents such as polyglycerine fatty acid esters, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene castor oil, hardened castor oil, polyoxyethylene alkyl ether, polyoxyethylene phytosterol.phytostanol, polyoxyethylenepolyoxypropylenealkyl ether, polyoxyethylenealkylphenyl ether, polyoxyethylenelanolin.lanolin alcohol.bees wax derivatives, polyoxyalkylamine.fatty acid amide, and polyoxyalkyl ether phosphoric acid.phosphate.

As examples of the heat meltable materials, mention may be made of waxes such as bees wax, spermaceti, Chinese wax, wool wax, candelilla wax, carnauba wax, Japan wax, ouricury wax, sugar cane wax, montan wax, ozocerite, ceresine, lignite wax, paraffin wax, microcrystalline wax, petrolatum, low molecular weight polyethylene wax and derivatives thereof, castor wax, opal wax, oleic amide, lauric acid amide, erucic amide, behenic amide, palmitic amide, stearic amide, hydroxystearic amide, acrylamide, methylolstearic amide, methylolbehenic amide, ethylenebisstearic amide, ethylenebisoleic amide, and ethylenebislauric amide. These heat meltable materials may be used singly or in combination of two or more. The heat meltable materials include those which have an action as a sensitizer. However, the heat meltable materials are limited to those which can form microemulsion having an average diameter of 0.2 .mu.m or less as mentioned above.

Dispersing of the three components and formation of microcapsules are carried out in the same manner as in the second embodiment.

In the fifth embodiment of the present invention, a water-soluble polymer is used in place of the microemulsion used in the fourth embodiment.

As examples of the water-soluble polymer, mention may be made of synthetic polymers such as polyvinyl alcohol, polyethylene glycol, polyacrylamide, polyacrylic acid esters, polymethacrylic acid esters, and polyesters; semisynthetic polymers such as methyl cellulose, ethyl cellulose, carboxyethyl cellulose, and hydroxyethyl cellulose; and natural polymers such as gelatin, gum arabic, and pullulan. These may be used singly or in combination of two or more.

When voids or depressions of the agglomerates are filled with the water-soluble polymer, the filling may often not proceed rapidly depending on conditions such as kind of the water-soluble polymer, temperature and stirring rate.

However, it has been found that the filling can be carried out rapidly by adding ammonia solution to at least one of the steps of production of the heat-sensitive recording composition.

That is, in the fifth embodiment of the present invention, the heat-sensitive recording composition is obtained by a process comprising the following steps.

(1) Each of the aromatic isocyanate compound, the imino compound and the sensitizer is ground alone, or the aromatic isocyanate compound and mixture of the imino compound and sensitizer, or the imino compound and mixture of the sensitizer and aromatic isocyanate compound, are ground separately, until mean particles diameter comes down to 0.5-1.0 .mu.m under presence of an anionic dispersing agent;

(2) The resulting dispersions are mixed, then a water-soluble polymer is added;

(3) An alkali metal salt or an ammonium salt of a copolymer of maleic anhydride and a monomer copolymerizable therewith is added to the mixture, which is stirred to form agglomerates having a mean diameter of 2-30 .mu.m and comprising the said three components; and

(4) A wall forming material is added to the emulsion or dispersion to perform microencapsulation of the agglomerates.

Wherein, ammonia solution is added in at least one of the above steps in an amount of 0.75-15.0 parts by weight (in terms of NH.sub.3 content) based on 100 parts by weight of the components enclosed in the microcapsules.

The reason for the filling of voids or depressions of the agglomerates being rapidly attained by adding ammonia solution in at least one of the above steps has not yet been sufficiently elucidated, but can be presumed as follows. The water-solubilization phenomenon of the alkali metal salt or ammonium salt of the maleic anhydride copolymer which has the actions to form agglomerates and to perform emulsification and dispersion is further promoted by addition of ammonia solution. As a result, with progress of water-solubilization of the maleic anhydride copolymer, viscosity of the copolymer decreases. Therefore, this maleic anhydride copolymer having a reduced viscosity agglomerates the mixture of the above-mentioned three components and the water-soluble polymer to form agglomerates and in addition surrounds the agglomerates, resulting in gelling state to show a phase separation phenomenon in the aqueous medium. The respective agglomerates are surrounded with the maleic anhydride copolymer in the form of gel and are in stabilized state. Subsequently, with progress of microencapsulation, inside of the agglomerates is in the concentrated state and is completely filled with the water-soluble polymer. Furthermore, the excess water-soluble polymer fills the depressions on the surface of the agglomerates. Thus, the ammonia solution accelerates water-solubilization of the maleic anhydride copolymer and affects inside and outside of the formed agglomerates.

The ammonia solution may be added in any of the above four steps, but preferably is added in the step (2) or (3) because in these steps the effect of the ammonia solution on the maleic anhydride copolymer is more direct. Moreover, the ammonia solution may be added at one time or dividedly at several times without loss of the effect as far as the amount of the solution is within the range mentioned above.

The heat-sensitive recording composition is produced in the same manner as in the fourth embodiment, except that the water-soluble polymer and the ammonia solution are added.

The present invention is illustrated by the following examples, but they should not be construed as limiting the invention in any manner. In these examples, "part" and "%" represent "part by weight" and "% by weight", respectively unless otherwise notified.

Each of the mixtures having the following compositions was ground and dispersed by a sand mill until average particle diameter reached about 0.7 .mu.m. [Liquor A] Dispersion of aromatic isocyanate compound 4,4',4"-Triisocyanato-2,5- 100 parts dimethoxyphenylamine 2.5% Aqueous anionic polyvinyl 400 parts alcohol solution [Liquor B] Co-dispersion of imino compound-sensitizer 1,3-Diimino-4,5,6,7-tetra- 150 parts chloroisoindoline Benzyloxynaphthalene 150 parts 2.5% Aqueous anionic polyvinyl 700 parts alcohol solution ______________________________________

Liquor A and liquor B obtained in the above (1) were mixed at the following ratio until the mixture became homogeneous and then, 10% aqueous cationized polyvinyl alcohol solution as a cationic dispersing agent was gently added to the resulting mixture with stirring. After stirring for 1 hour, the resulting dispersion was sampled and inspected under an optical microscope to monitor that agglomerates of 10 .mu.m in average particle diameter were formed. Liquor A (20%)     250 parts Liquor B (30%)     500 parts Cationic dispersing agent 150 parts ______________________________________

A heat-sensitive coating composition of the following formulation was prepared using the agglomerates having an average particle diameter of 10 .mu.m obtained in the above (2). Agglomerates (20%)    200 parts 40% Aqueous dispersion of zinc 25 parts stearate 10% Aqueous polyvinyl alcohol 100 parts solution 40% Aqueous dispersion of calcium 125 parts carbonate Water                 100 parts ______________________________________

The thus obtained coating composition was coated on a base paper of 40 g/m.sup.2 in basis weight at a coating amount (solid) of 6 g/m.sup.2 by a Meyer bar, dried and then supercalendered to obtain a heat-sensitive recording material.

Color density of the resulting heat-sensitive recording material was measured using GIII facsimile tester. The tester used was TH-PMD manufactured by Ohkura Denki K.K. and printing was carried out using a thermal head of 8 dots/mm in dot density, 1300 .OMEGA. in head resistance at a head voltage of 22 V, and current duration of 1.0 ms. The color density of the printed image was measured by Macbeth RD-918 reflective densitometer.

A coating composition was prepared using the dispersions comprising the liquor A and the liquor B of Example 1 as they were with the following formulation (without forming agglomerates). Liquor A (20%)         50 parts Liquor B (30%)        100 parts 40% Aqueous dispersion of zinc 25 parts stearate 10% Aqueous polyvinyl alcohol 100 parts solution 40% Aqueous dispersion of calcium 125 parts carbonate Water                 150 parts ______________________________________

The resulting coating composition was coated on a base paper of 40 g/m.sup.2 in basis weight at a coating amount (solid) of 6 g/m.sup.2 by a Meyer bar, dried and then supercalendered to obtain a heat-sensitive recording material.

This heat-sensitive recording material was subjected to printing and evaluated in the same manner as in Example 1. Color density ______________________________________ Example 1         1.25 Comparative Example 1 1.03 ______________________________________

As can be seen from the above results, the heat-sensitive recording material prepared using the agglomerates in Example 1 shows higher color density than the heat-sensitive recording material prepared without forming agglomerates in Comparative Example 1.

Upon observation of the color formed portion of the heat-sensitive recording materials under an optical microscope, it was found that the color formed portion of the material of Example 1 retained the form of agglomerates while that of the material of Comparative Example 1 was in the state of fine dots as a whole.

In agglomeration of the three components in Example 1, amount of the 10% aqueous cationized polyvinyl alcohol solution was increased to 500 parts and agglomerates of 33 .mu.m in average particle diameter were prepared. The resulting agglomerates were coated on a base paper of 40 g/m.sup.2 in basis weight by a Meyer bar in the same manner as in Example 1. However, the surface of the coated side was observed to have roughness due to the agglomerates and this material was not preferred as a heat-sensitive recording material.

Heat-sensitive recording materials were prepared in the same manner as in Example 1, except that a 15% aqueous polyaminomethylacrylamide solution was used in place of the cationic dispersing agent in Examples 2-4 while the cationic dispersing agent was eliminated in Comparative Examples 3-5. Moreover, ratio of the three components was varied as shown in Table 1. In Examples 2-4, diameter of the agglomerates was 5 .mu.m, 10 .mu.m, and 25 .mu.m, respectively and in Comparative Examples 4-6, no agglomerates were formed. Color density was measured in the same manner as in Example 1 and the results are shown in Table 1. ______________________________________ A    B        C      Color density ______________________________________ Example 2   100    200      300  1.26 Example 3   100    100      100  1.22 Example 4   100     50      100  1.18 Comparative 100    200      300  1.05 Example 3 Comparative 100    100      100  1.01 Example 4 Comparative 100     50      100  0.80 Example 5 ______________________________________ In Table 1, A, B and C are as follows: A: Aromatic isocyanate compound (part by weight) B: Imino compound (part by weight) C: Sensitizer (part by weight)

As can be seen from Table 1, the recording materials of Examples 2-4 showed high color density and thus were high in sensitivity. On the other hand, amounts of the three components used in Comparative Examples 3-5 correspond to those of Examples 2-4, respectively, but the recording materials of comparative Examples 3-5 showed low color density and were low in sensitivity because the three components formed no agglomerates.

Each of the mixtures having the following compositions was ground and dispersed by a sand mill until average particle diameter reached about 0.7 .mu.m. [Liquor A] Dispersion of aromatic isocyanate compound 4,4',4"-Triisocyanato-2,5- 100 parts dimethoxyphenylamine 2.5% Aqueous anionic polyvinyl 400 parts alcohol solution [Liquor B] Co-dispersion of imino compound-sensitizer 1,3-Diimino-4,5,6,7-tetra- 150 parts chloroisoindoline Benzyloxynaphthalene 150 parts 2.5% Aqueous anionic polyvinyl 700 parts alcohol solution ______________________________________

Liquor A and liquor B obtained in the above (1) were mixed with each other at the following ratio using a 10% aqueous cationized polyvinyl alcohol solution as a cationic dispersing agent to prepare agglomerates which had an average particle diameter of 10 .mu.m and comprised the three components. Liquor A (20%)     250 parts Liquor B (30%)     500 parts Cationic dispersing agent 150 parts ______________________________________ components

To 100 parts of a 5% aqueous solution having pH of 4.0 and containing styrene-maleic anhydride copolymer and a small amount of sodium hydroxide, was gradually added 130 parts of the 27% dispersion of the three components prepared in the above (2) and was dispersed and emulsified.

Separately, a mixture comprising 10 parts of melamine, 25 parts of 37% aqueous formaldehyde solution and 65 parts of water was adjusted to pH 9.0 with sodium hydroxide and was heated at 60.degree. C. with stirring to perform dissolution to obtain a transparent melamine-formaldehyde precondensate.

To 200 parts of the emulsion of the three components was added 230 parts of the melamine-formaldehyde precondensate and reaction was allowed to proceed for 4 hours with stirring in a thermostat set at 60.degree. C. Then, the product was cooled to room temperature to prepare microcapsules.

The resulting microcapsules had an average particle diameter of 10 .mu.m and its shape was almost the same as that of the agglomerates. Solid content of the microcapsules containing liquor was 18%.

A heat-sensitive coating composition was prepared with the following formulation using the aqueous dispersion of the microcapsules having an average particle diameter of 10 .mu.m prepared in the above (3). Microcapsules (18%)     200 parts 10% Aqueous polyvinyl alcohol solution 84 parts Calcium carbonate       20 parts Water                   18 parts ______________________________________

The thus obtained 20% coating composition was coated on a base paper of 40 g/m.sup.2 in basis weight at a coating amount (solid) of 8.5 g/m.sup.2 using a Meyer bar, dried and then supercalendered to obtain a heat-sensitive recording material. The surface of the coated side was observed under an optical microscope to find that the microcapsules were damaged quite a little by the pressing treatment by the supercalender.

The resulting heat-sensitive recording material was measured for color density using G III facsimile tester. The tester used was TH-PMD manufactured by Ohkura Denki K.K. and printing was carried out using a thermal head of 8 dots/mm in dot density and 1300 .OMEGA. in head resistance at a head voltage of 22 V and current duration of 1.0 ms. The color density of the printed image was measured by Macbeth RD-918 reflective densitometer. Moreover, 75.degree. gloss of the coated surface of the heat-sensitive material was measured.

The liquor A and the liquor B prepared in Example 5 were used as they were (without forming agglomerates) to prepare a heat-sensitive coating composition in the following mixing ratio. Liquor A (20%)          25 parts Liquor B (30%)          50 parts 10% Aqueous polyvinyl alcohol solution 60 parts Calcium carbonate       20 parts Water                   75 parts ______________________________________

The resulting 20% coating composition was coated on a base paper of 40 g/m.sup.2 in basis weight at a coating amount (solid) of 6 g/m.sup.2 by a Meyer bar, dried and then supercalendered to obtain a heat-sensitive recording material.

This heat-sensitive recording material was subjected to printing and evaluated in the same manner as in Example 5. Moreover, 75.degree. gloss of the coated surface of the heat-sensitive recording material was measured.

Results of evaluation: Color density 75.degree. gloss ______________________________________ Example 5        1.24       13 Comparative Example 6 0.95       36 ______________________________________

As can be seen from the above results, the heat-sensitive recording material prepared using the microcapsules in Example 5 showed higher color density than the heat-sensitive recording material prepared without forming agglomerates in Comparative Example 6. Furthermore, the recording material obtained in Example 5 had a low 75.degree. gloss of 13, which is the same as that of plain papers while the recording material of Comparative Example 6 had a high gloss of 36.

Observation of the color formed portion of the heat-sensitive recording materials under an optical microscope showed that color was formed inside the microcapsules in the color formed portion of the material of Example 5. On the other hand, in the color formed portion of the material of Comparative Example 6, the coating composition penetrated into the substrate to show no shade in color.

As another evaluation, chemical resistance was evaluated by putting droplets of acetone on the coated surface (unprinted portion) of the heat-sensitive recording material obtained in Example 5 and Comparative Example 6, and observing that portion.

As a result, no change was seen on the surface of the material of Example 5, i.e. the surface remained white, while in the material of Comparative Example 6, the color forming component was dissolved with acetone to result in a black spot. Thus, it was confirmed that in the material of Example 5, the color forming components were covered with the microcapsule wall.

In agglomeration of the three components in Example 5, amount of the 10% aqueous cationized polyvinyl alcohol solution was increased to 500 parts and agglomerates having an average particle diameter of 35 .mu.m were prepared. And then microcapsules were prepared in the same manner as in Example 5. The resulting microcapsules were coated on a base paper of 40 g/m.sup.2 in basis weight by a Meyer bar in the same manner as in Example 5. However, the surface of the coated side was observed to have roughness due to the microcapsules and this material was not preferred as a heat-sensitive recording material.

Heat-sensitive recording materials were prepared in the same manner as in Example 5, except that a 15% aqueous polyaminomethylacrylamide solution was used in place of the cationic dispersing agent in Examples 6-8 while the cationic dispersing agent was eliminated in Comparative Examples 8-10. Moreover, ratio of the three components was varied as shown in Table 2. In Examples 6-8, diameter of the agglomerates was 5 .mu.m, 10 .mu.m, and 25 .mu.m, respectively and in Comparative Examples 8-10, no agglomerates were formed. Color density was measured in the same manner as in Example 5 and the results are shown in Table 2. ______________________________________ A     B       C       Color density Gloss ______________________________________ Example 6 100     200     300   1.23     16 Example 7 100     100     100   1.20     14 Example 8 100      50     100   1.14     11 Comparative 100     200     300   1.00     37 Example 8 Comparative 100     100     100   0.96     35 Example 9 Comparative 100      50     100   0.77     38 Example 10 ______________________________________ In Table 2, A, B and C are as follows: A: Aromatic isocyanate compound (part by weight) B: Imino compound (part by weight) C: Sensitizer (part by weight)

As can be seen from Table 2, the recording materials of Examples 6-8 showed high color density and thus were high in sensitivity and besides, they showed low 75.degree. gloss. On the other hand, though amounts of the three components used in Comparative Examples 8-10 corresponded to those of Examples 6-8, respectively, the recording materials of Comparative Examples 8-10 showed low color density and were low in sensitivity because dispersion was used as it was. Besides, they were high in 75.degree. gloss.

Each of the mixtures having the following compositions was ground and dispersed by a sand mill until average particle diameter reached about 0.7 .mu.m using anionic polyvinyl alcohol. [Liquor A] Dispersion of aromatic isocyanate compound 4,4',4"-Triisocyanato-2,5- 100 parts dimethoxyphenylamine 2.5% Aqueous polyvinyl 400 parts alcohol solution [Liquor B] Co-dispersion of imino compound-sensitizer 1,3-Diimino-4,5,6,7-tetra- 150 parts chloroisoindoline Benzyloxynaphthalene 150 parts 2.5% Aqueous polyvinyl 700 parts alcohol solution ______________________________________

50 parts of 20% liquor A (dispersion of aromatic isocyanate compound) and 100 parts of 30% liquor B (co-dispersion of imino compound-sensitizer) obtained in the above (1) were mixed with each other until a homogeneous mixture was obtained. 150 parts of the mixture of liquor A and liquor B was gradually added with 120 parts of a 5% aqueous solution of sodium salt of styrene-maleic anhydride copolymer adjusted to pH 4.0 with stirring. Stirring was carried out for about 30 minutes to obtain agglomerates having an average particle diameter of 10 .mu.m and it was simultaneously confirmed that the agglomerates were emulsified and dispersed. Separately, a mixture comprising 11.4 parts of melamine, 28.5 parts of a 37% aqueous formaldehyde solution and 74.1 parts of water was adjusted to pH 9.0 with sodium hydroxide and then was heated at 60.degree. C. with stirring to perform dissolution to obtain 114 parts of a transparent melamine-formaldehyde precondensate.

114 parts of this melamine-formaldehyde precondensate was added gently to 270 parts of the above emulsified and dispersed liquor and reaction was allowed to proceed for 4 hours with stirring in a thermostat set at 60.degree. C. Then, the product was cooled to room temperature to prepare microcapsules. The resulting microcapsules had an average particle diameter of 10 .mu.m which was almost the same as that of the agglomerates and solid content in the aqueous dispersion of the microcapsules was 18%.

A heat-sensitive coating composition was prepared with the following formulation using the aqueous dispersion of the microcapsules having an average particle diameter of 10 .mu.m prepared in the above (2). Microcapsule aqueous dispersion (18%) 200 parts 10% Aqueous polyvinyl alcohol solution 84 parts Calcium carbonate       20 parts Water                   18 parts ______________________________________

The thus obtained 20% coating composition was coated on a base paper of 40 g/m.sup.2 in basis weight at a coating amount (solid) of 6 g/m.sup.2 by a Meyer bar, dried and then supercalendered to obtain a heat-sensitive recording material. The surface of the coat was observed under an optical microscope to find that the microcapsules were damaged quite a little by the pressing treatment by the supercalender.

The resulting heat-sensitive recording material was measured for of color density using G III facsimile tester. The tester used was TH-PMD manufactured by Ohkura Denki K.K. and printing was carried out using a thermal head of 8 dots/mm in dot density, 1300 .OMEGA. in head resistance at a head voltage of 22 V and current duration of 1.0 ms. The color density of the printed image was measured by Macbeth RD-918 reflective densitometer.

The liquor A and the liquor B prepared in Example 9 were used as they were (without forming agglomerates) to prepare a heat-sensitive coating composition in the following mixing ratio. Liquor A (dispersion of aromatic 25 parts isocyanate compound) Liquor B (co-dispersion of imino 50 parts compound-sensitizer) 10% Aqueous polyvinyl alcohol 60 parts solution Calcium carbonate    20 parts Water                75 parts ______________________________________

The resulting 20% coating composition was coated on a base paper of 40 g/m.sup.2 in basis weight at a coating amount (solid) of 4.6 gm/.sup.2 by a Meyer bar, dried and then supercalendered to obtain a heat-sensitive recording material.

This heat-sensitive recording material was subjected to printing and evaluation in the same manner as in Example 9. Moreover, 75.degree. gloss of the coated surface of the heat-sensitive recording material was measured.

The results are shown in Table 3. ______________________________________ Color density 75.degree. gloss ______________________________________ Example 9        1.22         12 Comparative Example 11 0.88         33 ______________________________________

As can be seen from the above Table 3, the heat-sensitive recording material prepared using the microcapsules in Example 9 showed higher color density than the heat-sensitive recording material prepared without forming agglomerates in Comparative Example 11. Furthermore, the recording material obtained in Example 9 had a low 75.degree. gloss of 12, which is almost the same as that of plain papers while the recording material of Comparative Example 11 had a high gloss of 33. Observation of the color formed portion of the heat-sensitive recording materials under an optical microscope showed that color was formed inside the microcapsules and this portion of the substrate was interspersed with these microcapsules in the material of Example 9. On the other hand, in the color formed portion of the material of Comparative Example 11 the coating composition penetrated into the substrate to show less tinctorial power.

As another evaluation, chemical resistance was evaluated by putting a droplet of acetone on the coated surface (unprinted portion) of the heat sensitive recording materials obtained in Example 9 and Comparative Example 11, and observing that portion. As a result, no change was seen on the surface of the material of Example 9, i.e. color remained white, while in the material of Comparative Example 11, the color forming component was dissolved with acetone to result in a black spot. Thus, it was confirmed that in the material of Example 9, the color forming components were covered with the microcapsule wall.

Microcapsules containing therein the three components were prepared in the same manner as in Example 9, except that amount of the 5% aqueous solution of sodium salt of styrene-maleic anhydride copolymer used was 44 parts in place of 100 parts in preparation of microcapsules containing therein the three components. This amount of sodium salt of 5% styrene-maleic anhydride copolymer corresponds to 4 parts based on 100 parts of the three components. As a result, agglomeration of the three components was insufficient since the amount of the sodium salt of styrene-maleic anhydride copolymer was too small. Moreover, microencapsulation was not sufficiently attained because formation of the microcapsule wall was incomplete.

Microcapsules containing therein the three components were prepared in the same manner as in Example 9, except that amount of the 5% aqueous solution of sodium salt of styrene-maleic anhydride copolymer used was 550 parts in place of 100 parts in preparation of microcapsules containing therein the three components. This amount of sodium salt of 5% styrene-maleic anhydride copolymer corresponds to 50 parts based on 100 parts of the three components. As a result, since the amount of the sodium salt of styrene-maleic anhydride copolymer was too large and cationic property imparted with the sodium salt was excessive, agglomerates were collapsed in the course of addition of the three components and returned to the particles of each component. Therefore, though microencapsulation was attained, most of the microcapsules contained the particles of each component alone.

Each of the mixtures having the following compositions was ground and dispersed by a sand mill until average particle diameter reached about 0.7 .mu.m. [Liquor A] Dispersion of aromatic isocyanate compound 4,4',4"-Triisocyanato-2,5- 100 parts dimethoxyphenylamine 2.5% Aqueous anionic polyvinyl 400 parts alcohol solution [Liquor B] Co-dispersion of imino compound-sensitizer 1,3-Diimino-4,5,6,7-tetra- 150 parts chloroisoindoline Benzyloxynaphthalene 150 parts 2.5% Aqueous anionic polyvinyl 700 parts alcohol solution ______________________________________

Previously, 50 parts of 20% liquor A (dispersion of aromatic isocyanate compound) and 100 parts of 30% liquor B (co-dispersion of imino compound-sensitizer) obtained by grinding and dispersing in the above (1) were mixed with each other until a homogeneous mixture was obtained. The resulting homogeneous mixture of liquor A and liquor B was mixed with 85 parts of a 47% carboxylated styrenebutadiene rubber latex (average emulsified particle diameter: 0.016 .mu.m) as a microemulsion and the mixture was homogenized to prepare a core material. Then, 235 parts of the mixture of the liquor A, liquor B and microemulsion was gradually added to 160 parts of a 5% aqueous solution of sodium salt of styrene-maleic anhydride copolymer adjusted to pH 4.0. Stirring was effected for about 30 min to obtain agglomerates having an average particle diameter of 10 .mu.m and it was simultaneously confirmed that the agglomerates were able to be emulsified and dispersed.

Separately, a mixture of 21.3 parts of melamine, 53.3 parts of a 37% aqueous formaldehyde solution and 138.4 parts of water was adjusted to pH 9.0 with sodium hydroxide and then was heated at 60.degree. C. with stirring to perform dissolution to obtain a transparent melamineformaldehyde precondensate. 213 parts of this melamineformaldehyde precondensate was added gently to 395 parts of the above emulsified and dispersed liquid and reaction was allowed to proceed for 4 hours with stirring in a thermostat set at 60.degree. C. Then, the product was cooled to room temperature to prepare microcapsules. It was confirmed that the resulting microcapsules had an average particle diameter of 10 .mu.m which was almost the same as that of the agglomerates. Solid concentration of the aqueous dispersion of the microcapsules was 21%.

A heat-sensitive coating composition was prepared with the following formulation using the aqueous dispersion of the microcapsules having an average particle diameter of 10 .mu.m prepared in the above (2). Microcapsule aqueous dispersion (18%) 200 parts 10% Aqueous polyvinyl alcohol solution 84 parts Calcium carbonate       20 parts Water                   18 parts ______________________________________

The thus obtained 20% coating composition was coated on a base paper of 40 g/m.sup.2 in basis weight at a coating amount (solid) of 11.5 g/m.sup.2 by a Meyer bar, dried and then treated by a supercalender to obtain a heat-sensitive recording material. The surface of the coated side was observed under an optical microscope to find that the microcapsules were not damaged by the pressing treatment by the supercalender.

The resulting heat-sensitive recording material was measured for color density using G III facsimile tester. The tester used wa TH-PMD manufactured by Ohkura Denki K.K. and printing was carried out using a thermal 1 head of 8 dots/mm in dot density, 1300 .OMEGA. in head resistance at a heat voltage of 22 V and current duration of 1.0 ms. The color density of the printed image was 1.20 measured by Macbeth RD-918 reflective densitometer. Moreover, according to observation under an optical microscope, in the color formed portion the microcapsules were not ruptured and color was formed inside the microcapsules.

As another evaluation, chemical resistance was evaluated by putting a droplet of acetone on the coated surface (unprinted portion) of the heat-sensitive recording material and observing the portion. As a result, no change was seen on the surface, i.e. color remained white. In addition, acetone was put in the same manner on the color formed portion to find no decrease in color density. Therefrom, the effect was recognized that the color forming components were completely covered with the microcapsule wall.

The liquor A and the liquor B prepared in Example 10 were used as they were (without forming agglomerates) to prepare a heat-sensitive coating composition at the following mixing ratio. Liquor A (dispersion of aromatic 25 parts isocyanate compound) Liquor B (co-dispersion imino 50 parts compound-sensitizer) 10% Aqueous polyvinyl alcohol 60 parts solution Calcium carbonate    20 parts Water                75 parts ______________________________________

The resulting 20% coating composition was coated on a base paper of 40 g/m.sup.2 in basis weight at a coating amount (dry solid content) of 4.6 g/m.sup.2 by a Meyer bar, dried and then supercalendered to obtain a heat-sensitive recording material.

This heat-sensitive recording material was subjected to printing and evaluation in the same manner as in Example 10 to obtain a color density of 0.86 which was lower than the value obtained in Example 10. Observation of the color formed portion under an optical microscope showed that the reaction product penetrated into the substrate, resulting in a color of less tinctorial power.

As another evaluation, chemical resistance was evaluated by putting a droplet of acetone on the coated surface (unprinted portion) of the heat-sensitive recording material obtained above and observing that portion. As a result, the color forming components were dissolved in acetone and reacted with each other to result in a black spot. Similarly, acetone was put on the color formed portion to find that the density decreased and chemical resistance was insufficient.

Each of the mixtures having the following compositions was ground and dispersed by a sand mill until average particle diameter reached about 0.5 .mu.m. [Liquor A] Dispersion of aromatic isocyanate compound 4,4',4"-Triisocyanato-2,5- 100 parts dimethoxyphenylamine 2.5% aqueous anionic polyvinyl 400 parts alcohol solution [Liquor B] Co-dispersion of imino compound-sensitizer 1,3-Diimino-4,5,6,7-tetra- 100 parts chloroisoindoline Benzyloxynaphthalene 100 parts 10% aqueous anionic polyvinyl 300 parts alcohol solution ______________________________________

Previously, 50 parts of 20% liquor A (dispersion of aromatic isocyanate compound) and 50 parts of 40% liquor B (co-dispersion of imino compound-sensitizer) obtained by grinding and dispersing in the above (1) were mixed with each other until a homogeneous mixture was obtained. The resulting homogeneous mixture of liquor A and liquor B was mixed with 37.5 parts of a 40% solubilized emulsion (average particle diameter 0.05 .mu.m) comprising microcrystalline wax having a melting point of 75.degree. C. as a microemulsion and the mixture was homogenized to obtain a core material. Then, 137.5 parts of the liquor A-liquor B-microemulsion mixture was gradually added to 90 parts of a 5% aqueous solution of sodium salt of styrenemaleic anhydride copolymer adjusted to pH 4.0 with stirring. Stirring was continued for about 30 minutes to obtain roundish agglomerates having an average particle diameter of 10 .mu.m and it was also found that the agglomerates were emulsified and dispersed. Separately, a mixture of 12 parts of melamine, 30 parts of a 37% aqueous formaldehyde solution and 78 parts of water was adjusted to pH 9.0 with sodium hydroxide and was heated at 60.degree. C. with stirring to perform dissolution to obtain 120 parts of a transparent melamine-formaldehyde precondensate. Then, 120 parts of this melamine-formaldehyde precondensate was added gently to 227.5 parts of the above emulsified and dispersed liquid and reaction was allowed to proceed for 4 hours with stirring in a thermostat set at 60.degree. C. Then, the product was cooled to room temperature to prepare microcapsules. It was confirmed that the resulting microcapsules had an average particle diameter of 10 .mu.m which was almost the same as that of the agglomerates and had a roundish fusiform shape. Solid concentration of the aqueous dispersion of the microcapsules was 21%. Amount of the solubilized emulsion used here corresponds to 50 parts by weight based on 100 parts by weight of the three components (aromatic isocyanate compound, imino compound and sensitizer) in total. thereof

A heat-sensitive coating composition was prepared with the following formulation using the aqueous dispersion of the microcapsules having an average particle diameter of 10 .mu.m prepared in the above (2). Microcapsule aqueous dispersion (20%) 200 parts 10% Aqueous polyvinyl alcohol solution 50 parts Calcium carbonate       10 parts Water                   15 parts ______________________________________

The thus obtained 20% coating composition was coated on a based paper of 40 g/m.sup.2 in basis weight at a coating amount (dry solid content) of 6.0 g/m.sup.2 by a Meyer bar, dried and then supercalendered to obtain a heat-sensitive recording material. The surface of the coated side was observed under an optical microscope to find that the microcapsules were not damaged by the pressing treatment by the supercalender. Results of evaluation are shown in Table 4.

Microcapsules were prepared in the same manner as in Example 11, except that amount of the 40% solubilized emulsion (average particle diameter: 0.05 .mu.m) used was varied. Using the resulting microcapsules, heat-sensitive coating compositions were produced and then heat-sensitive recording materials were prepared in the same manner as in Example 11. Amounts of the microemulsion based on 100 parts by weight of the three components in total and the coating amount (dry solid content) of the coating composition are shown in Table 4. Moreover, evaluation of the heat-sensitive recording materials was conducted in the same manner as in Example 10. Evaluation of chemical resistance was carried out by putting a droplet of acetone on the colored and unprinted portions, and density of the color formed spot after volatilization of the solvent was measured by Macbeth RD-918 reflective densitometer. __________________________________________________________________________ Coating amount Amount of of coating    Chemical solubilized composition   resistance emulsion (dry solid) Color Colored Unprinted (Part by weight (g/m.sup.2) density portion portion __________________________________________________________________________ Example 11 50       6.0      1.25 1.25  0.07 Example 12 25       5.0      1.27 1.25  0.08 Example 13 150      10.0     1.19 1.19  0.07 Example 14 200      12.0     1.16 1.16  0.07 Example 15 20       4.8      1.27 1.15  0.13 Comparative 225      13.0     1.02 1.02  0.07 Example 14 __________________________________________________________________________

As can be seen from the results shown in Table 4, high color density was obtained in Examples 11-14. With reference to the chemical resistance, color density of the color formed portion showed no or little change as compared with the initial density in Examples 11-14. However, in Example 15, color density decreased in the color formed portion and chemical resistance was somewhat inferior because the amount of solubilized emulsion used was small. Furthermore, it is recognized that in Comparative Example 14, color density was low and sensitivity was inferior because amount of the solubilized emulsion was large.

Each of the mixtures having the following compositions was ground and dispersed by a sand mill until average particle diameter reached about 0.7 .mu.m. [Liquor A] Dispersion of dye precursor [Liquor A] Dispersion of aromatic isocyanate compound 4,4',4"-Triisocyanato-2,5- 100 parts dimethoxyphenylamine 2.5% Aqueous anionic polyvinyl 400 parts alcohol solution [Liquor B] Co-dispersion of imino compound-sensitizer 1,3-Diimino-4,5,6,7-tetra- 150 parts chloroisoindoline Benzyloxynaphthalene 150 parts 2.5% Aqueous anionic polyvinyl 700 parts alcohol solution ______________________________________

Previously, 50 parts of 20% liquor A (dispersion of aromatic isocyanate compound) and 100 parts of 30% liquor B (co-dispersion of imino compound-sensitizer) obtained by grinding and dispersing in the above (1) were mixed with each other until a homogeneous mixture was obtained. The resulting homogeneous mixture of liquor A and liquor B was mixed with 100 parts of a 40% aqueous solution of a polyacrylate ester copolymer as a water-soluble polymer and the mixture was homogenized to obtain a core material. To the core material was added 14 Parts of a 28% aqueous ammonia solution (corresponding to 5 parts by weight based on 100 parts by weight of the components contained in the microcapsules) to obtain a homogeneous mixture. Then, 424 parts of the mixture of the liquor A-liquor B-water-soluble polymer modulated with ammonia was gradually added to 160 parts of a 5% aqueous solution of sodium salt of styrene-maleic anhydride copolymer adjusted to pH 4.0 with stirring. Stirring was continued for about 30 minutes to obtain roundish agglomerates having an average particle diameter of 10 .mu.m and it was also found that the agglomerates were emulsified and dispersed. Separately, a mixture of 21.3 parts of melamine, 53.3 parts of a 37% aqueous formaldehyde solution and 138.4 parts of water was adjusted to pH 9.0 with sodium hydroxide and was heated at 60.degree. C. with stirring to perform dissolution to obtain 213 parts of a transparent melamine-formaldehyde precondensate. Then, 213 parts of this melamine-formaldehyde precondensate was gently added to 424 parts of the above emulsified and dispersed liquid and reaction was allowed to proceed for 4 hours with stirring in a thermostat set at 60.degree. C. Then, the product was cooled to room temperature to prepare microcapsules. It was confirmed that the resulting microcapsules had an average particle diameter of 10 .mu.m which was almost the same as that of the agglomerates and had a roundish fusiform shape. Solid concentration of the aqueous dispersion of the microcapsules was 21%.

A heat-sensitive coating composition was prepared with the following formulation using the aqueous dispersion of the microcapsules having an average particle diameter of 10 .mu.m prepared in the above (2). Microcapsule aqueous dispersion (20%) 200 parts 10% Aqueous polyvinyl alcohol solution 90 parts Calcium carbonate       20 parts Water                   35 parts ______________________________________

The thus obtained 20% coating composition was coated on a base paper of 40 g/m.sup.2 in basis weight at a coating amount (dry solid) of 11.5 g/m.sup.2 by a Meyer bar, dried and then supercalendered to obtain a heat-sensitive recording material. The surface of the coated side was observed under an optical microscope to find that the microcapsules were not damaged by the pressing treatment by the supercalender.

The resulting heat-sensitive recording material was measured for color density using G III facsimile tester. The tester used was TH-PMD manufactured by Ohkura Denki K.K. and printing was carried out using a thermal head of 8 dots/mm in dot density, 1300 .OMEGA. in head resistance at a head voltage of 22 V and current duration of 10 ms. The color density of the printed image was 1.21 measured by Macbeth RD-918 reflective densitometer. Moreover, according to observation under an optical microscope, in the color formed portion the microcapsules were not ruptured and color was formed inside the microcapsules.

As another evaluation, chemical resistance was evaluated by putting a droplet of acetone on the coated surface (unprinted portion) of the heat-sensitive recording material and observing that portion. As a result of measurement of whiteness of the coated surface (background) and the portion on which acetone was put by Macbeth RD-918 reflective densitometer, both of the portions had a whiteness of 0.06. Moreover, acetone was also put on the color formed portion and as a result, color density of the color formed portion was 1.21 and that of the acetone-treated portion was 1.21. This shows the effect that the color forming components were completely covered with microcapsule wall.

In Examples 17-19, microcapsules were prepared in the same manner as in Example 16, except that the 28% aqueous ammonia solution was respectively used in the amounts of 0.75 parts by weight, 10 parts by weight and 15 parts by weight based on 100 parts by weight of the components contained in the microcapsules in place of the amount thereof in (2) of Example 16 (corresponding to 5 parts by weight based on 100 parts by weight of the components contained in the microcapsules). In the same manner as in Example 16, heat-sensitive recording compositions were produced and then heat-sensitive recording materials were prepared using the resulting microcapsules. In Example 20 and Comparative Example 15, microcapsules were prepared adding the aqueous ammonia solution in an amount of 0 part by weight and 16 parts by weight, respectively and a heat-sensitive recording composition and then a heat-sensitive recording material were prepared in the same manner as in Example 16. Amount of the aqueous ammonia solution based on 100 parts by weight of the components contained in the microcapsules and coating amount (dry solid content) of the heat-sensitive coating composition are shown in Table 5.  Evaluation of the thus obtained heat-sensitive recording materials was conducted in the same manner as in Example 16, namely, by subjecting them to color formation using GIII facsimile tester and putting acetone on the color formed portion and the unprinted portion, volatilizing acetone, and thereafter, measuring density by Macbeth RD-918 reflective densitometer. __________________________________________________________________________ Amount of Coating amount aqueous ammonia of coating Color formed Unprinted portion solution  composition portion      (background) (part by weight) g/m.sup.2 Untreated Treated Untreated Treated __________________________________________________________________________ Example 17 0.75      11.0     1.22   1.20  0.06   0.07 Example 18 10        12.0     1.22   1.22  0.06   0.06 Example 19 15        12.0     1.20   1.20  0.06   0.06 Example 20 0         11.0     1.21   1.06  0.06   0.13 Comparative 16        12.0     1.14   0.97  0.06   0.18 Example 15 __________________________________________________________________________

As can be seen from the above Table 5, when the water-soluble polymer was used for internal filling of the agglomerates, both the color formed portion and the unprinted portion (background portion) retained the initial density and showed substantially no decrease in Examples 17-19 in which aqueous ammonia solution was added.

On the other hand, in Example 19 in which aqueous ammonia solution was not added, density of the color formed portion decreased from 1.21 to 1.06 (desensitized) and density of the unprinted portion (background) increased from 0.06 to 0.13 which showed occurrence of fogging in the background. Since aqueous ammonia solution was not used in microencapsulation, wall of the microcapsules was not uniform and somewhat inferior in chemical resistance.

In Comparative Example 15, aqueous ammonia solution was added in excess, namely, in an amount of 16 parts by weight based on 100 parts by weight of the components contained in the microcapsules. Owing to the influence of the excessive aqueous ammonia solution, the agglomerates once formed were separated in microencapsulation and microencapsulation was incomplete. Moreover, emulsified particles of melamine which was a wall material were singly formed and were in the state of admixture with microcapsules. Therefore, color density was low although the coating composition was coated in the proper amount. It was found that the color formed portion and the unprinted portion on which acetone was put showed decrease in color density (desensitization) and fogging occurred in the background.

## Claims

1. A heat-sensitive recording composition comprising agglomerates which comprise an aromatic isocyanate compound, an imino compound and a sensitizer, and have an average diameter of 2-30 .mu.m.

2. A composition according to claim 1, wherein the agglomerates are formed using a cationic dispersing agent.

3. A composition according to claim 1, wherein the agglomerates are microencapsulated using a thermocurable wall material.

4. A composition according to claim 3, wherein the agglomerates are formed using a cationic dispersing agent.

5. A composition according to claim 3, wherein the agglomerates are formed using an alkali metal salt or ammonium salt of a copolymer of maleic anhydride and a monomer copolymerizable therewith.

6. A composition according to claim 3, wherein a polymer is further enclosed in the microcapsules.

7. A composition according to claim 6, wherein the polymer is in the form of a microemulsion having an average emulsified diameter of 0.2 .mu.m or less.

8. A composition according to claim 6, wherein the polymer is water-soluble.

9. A process for producing the composition of claim 2 which comprises the following steps: and the sensitizer alone; or grinding separately the aromatic isocyanate compound and a mixture of the sensitizer and the imino compound, or the imino compound and a mixture of the sensitizer and the aromatic isocyanate compound, in the presence of an anionic dispersing agent until average particle diameter comes down to 0.5-1.0 .mu.m, stirring to form agglomerates having an average diameter of 2-30 .mu.m and comprising the above three components.

10. A process for producing the composition of claim 4 which comprises the following steps: and the sensitizer alone; or grinding separately the aromatic isocyanate compound and a mixture of the sensitizer and the imino compound, or the imino compound and a mixture of the sensitizer and the aromatic isocyanate compound, in the presence of an anionic dispersing agent until average particle diameter comes down to 0.5-1.0 .mu.m, stirring to form agglomerates having an average diameter of 2-30 .mu.m and comprising the above three components, solution, and emulsifying or dispersing the agglomerates therein, and resulting emulsion or dispersion, wherein the resulting composition is subjected to heat-curing in order to perform microencapsulation of the agglomerates.

11. A process for producing the composition of claim 5 which comprises the following steps: and the sensitizer alone; or grinding separately the aromatic isocyanate compound and a mixture of the sensitizer and the imino compound, or the imino compound and a mixture of the sensitizer and the aromatic isocyanate compound, in the presence of an anionic dispersing agent until average particle diameter comes down to 0.5-1.0 .mu.m, salt of a copolymer of maleic anhydride and a monomer copolymerizable therewith with stirring to form an emulsion or dispersion containing agglomerates having an average diameter of 2-30 .mu.m and comprising the above three components, and resulting emulsion or dispersion, wherein the resulting composition is subjected to heat-curing in order to perform microencapsulation of the agglomerates.

12. A process for producing the composition of claim 7 which comprises the following steps: and the sensitizer alone; or grinding separately the aromatic isocyanate compound and a mixture of the sensitizer and the imino compound, or the imino compound and a mixture of the sensitizer and the aromatic isocyanate compound, in the presence of an anionic dispersing agent until average particle diameter comes down to 0.5-1.0 .mu.m, mixture a microemulsion having an average diameter of 0.2 .mu.m or less, salt of a copolymer of maleic anhydride and a monomer copolymerizable therewith with stirring to form an emulsion or a dispersion containing agglomerates having an average particle diameter of 2-30 .mu.m and comprising the above three components, and resulting emulsion or dispersion, wherein the resulting composition is subjected to heat-curing in order to perform microencapsulation of the agglomerates.

13. A process for producing the composition of claim 8 which comprises the following steps: and the sensitizer alone; or grinding separately the aromatic isocyanate compound and a mixture of the sensitizer and the imino compound, or the imino compound and a mixture of the sensitizer and the aromatic isocyanate compound, in the presence of an anionic dispersing agent until average particle diameter comes down to 0.5-1.0 .mu.m, polymer to the resulting mixture, salt of a copolymer of maleic anhydride and a monomer copolymerizable therewith with stirring to form an emulsion or a dispersion containing agglomerates having an average particle diameter of 2-30 .mu.m and comprising the above three components, and resulting emulsion or dispersion, wherein the resulting composition is subjected to heat-curing in order to perform microencapsulation of the agglomerates, parts by weight (in terms of NH.sub.3 content) based on 100 parts by weight of the components contained in the microcapsules is added in at least one of the above steps.