METHOD OF MANUFACTURING HEAT SENSITIVE RECORDING MATERIAL

Abstract: A composition containing an emulsified dispersion prepared by dispersing a color developer dissolved in an organic solvent slightly soluble or insoluble in water and microcapsules containing a colorless or light colored electron donating dye precursor (core material), is coated onto a support (1) and dried, the refractive index of the core material and that of the oil phase of the color developer dispersion being selected in the range of 0.97 to 1.03. By providing a transparent heat sensitive layer (3) obtained by the above method on a printed support, printed matter (2) can be seen through the heat sensitive layer (1).

Description: The present invention relates to a method of manufacturing heat-sensitive recording material having excellent transparency and being convenient for a special use. A heat-sensitive recording method, described, for example, in GB-A-2 158 958, has many advantages in that (1) no particular developing step is required, (2) if paper is used as a support, the recording material can have a quality akin to that of plain paper, (3) handling of the recording material used is easy, (4) the images recorded have high color density, (5) this method can be effected using a simple and cheap apparatus and (6) no noise is caused during recording. Therefore, heat-sensitive recording materials have recently enjoyed a markedly increasing demand, particularly for use with a facsimile or printer, and have come to be used for many purposes such as a pass, a label or a score card. Moreover, it has been desired to devise transparent heat-sensitive recording materials which enable direct recording with a thermal head in order to adapt them for multicolor development, or to make them usable for an overhead projector (hereinafter abbreviated as OHP). The possibility of providing such transparent heat-sensitive recording materials depends on the possibility of providing a transparent heat-sensitive layer, and it is easily foreseen that the demand for heat-sensitive recording material will be increased if said transparent heat-sensitive recording material is effectively provided. For example, in the case where a heat-sensitive recording material is used for the afore-mentioned label, a heat-sensitive layer is usually coated on a support, then the required format such as a ruled line, a trade name or quantity, is thermally printed on the heat sensitive layer. In this case, however, a stain which is caused by unexpected coloring occurs when an organic solvent etc. is adsorbed on the heat-sensitive layer. Therefore, a protective layer comprised of a material which is not damaged by the organic solvent should be provided on the heat-sensitive layer to prevent the above mentioned staining. Moreover, on these labels water such as rain is often adsorbed so that water-soluble printing ink cannot then be used. Therefore, a special ink which contains selected organic solvents should be employed, but in this case, these organic solvents are often harmful to health, manufacturing costs become higher, and moreover a blocking phenomenon is apt to occur in the manufacturing process since a long time is necessary to dry the printed labels. These disadvantages are avoided if it is possible to set a transparent heat-sensitive layer on a support already bearing desired printed matter. However, the heat-sensitive layer of a conventional heat-sensitive recording material which can be recorded by a thermal head is not transparent, so that desired transparency of the heat-sensitive recording material cannot be realized even if the conventional heat-sensitive layer is provided on a transparent support. A transparent heat-sensitive recording material which is known so far cannot fulfil the above mentioned new needs, since the transparent heat-sensitive recording material is of a type which is used by contacting with the original document and then exposing the recording material to light; the temperature of an image part is increased by absorption of infrared light by the image part of the original and the recording material is then colored imagewisely. The objects of the present invention are to provide a method of producing a transparent heat-sensitive material having a transparent heat-sensitive layer, and of controlling the transparency of the transparent heat-sensitive layer; to provide a heat-sensitive recording material comprising a transparent heat-sensitive layer through which matter printed on a support is visible; and to provide a heat-sensitive recording label comprising a transparent heat-sensitive layer coated on a support on which desired matter is already printed. It has been found that when adopting a combination of a colorless or a light colored electron-donating dye precursor and a color developer as coloring agents, and microencapsulating the former and dispersing the latter under special conditions, a transparent heat-sensitive layer can be obtained by coating a mixed composition of a vehicle containing the microcapsules and the dispersion of the developer. It has also been found that the transparency of the heat-sensitive layer can be adjusted by selecting the ratio of two refractive indexes, one refractive index being that of the microencapsulated component and the other that of the oily component contained in the developer emulsion. According to the present invention we provide a method of manufacturing a heat-sensitive recording material which has a heat-sensitive layer formed by coating and drying on a support at least a heat-sensitive composition comprising microcapsules, whose core contains at least a colorless or light colored electron-donating dye precursor, and an emulsified dispersion constituted by a non-volatile oily phase comprising a color developer and an organic solvent slightly soluble or insoluble in water and an aqueous phase, said light-sensitive layer being rendered transparent by selecting said microcapsule core and said non-volatile oily phase so that the ratio between the refractive indices thereof is within the range of 0.97 to 1.03. The invention also includes the recording material thus made. An excellent label, pass etc. can be easily obtained by providing the transparent heat-sensitive layer on a suitable printed support. Details of the transparent heat-sensitive layer and the heat-sensitive materials used to manufacture said layer will now be described. Electron-donating dye precursors to be employed in the present invention are selected suitably from known colorless or light colored compounds of the kind which can develop their colors by donating an electron or accepting a proton of an acid or the like. These compounds have a skeleton such as that of lactone, lactam, sultone, spiropyran, ester and amide, as a part of their structures, and these skeletons undergo ring-opening or bond cleavage upon contact with a color developer. Preferred examples of such compounds include triarylmethane compounds, diphenylmethane compounds, xanthene compounds, thiazine compounds and spiropyran compounds. Particularly preferred compounds are those represented by the following general formula: In the above formula, R₁ represents an alkyl group containing 1 to 8 carbon atoms; R₂ represents an alkyl or alkoxyalkyl group containing 4 to 18 carbon atoms, or a tetrahydrofuryl group; R₃ represents a hydrogen atom, an alkyl group containing 1 to 15 carbon atoms, or a halogen atom; and R₄ represents a substituted or unsubstituted aryl group containing 6 to 20 carbon atoms. As substituent group for R₄, alkyl, alkoxy and halogenated alkyl groups containing 1 to 5 carbon atoms, and halogen atoms are preferred. Microencapsulation of the above-described color former in the present invention can prevent generation of fog during production of a heat-sensitive material and, at the same time, can improve the keeping qualities of the heat-sensitive material and the keeping qualities of the record formed. Therein, the image density at the time of recording can be heightened by suitably selecting a material and a method for forming a microcapsule wall. A preferred amount of the color former used is 0.05 to 5.0 g per square meter. Suitable examples of wall materials for microcapsules include polyurethane, polyurea, polyester, polycarbonate, urea/formaldehyde resin, melamine resin, polystyrene, styrene/methacrylate copolymer, styrene/acrylate copolymer, gelatin, polyvinyl pyrrolidone and polyvinyl alcohol. These macromolecular substances can be used in combination of two or more thereof in the present invention. Of the above-cited macromolecular substances, polyurethane, polyurea, polyamide, polyester, and polycarbonate are preferred in the present invention. In particular, polyurethane and polyurea can bring about good results. Microcapsules to be employed in the present invention are preferably prepared by emulsifying a core material containing a reactive substance like a color former, and then forming a wall of a macromolecular substance around the droplets of the core material to microencapsulate the core material. Therein, reactants to produce a macromolecular substance are added to the inside and/or the outside of the oily droplets. For details of microcapsules which can be preferably employed in the present invention, e.g., for production methods of microcapsules which can be preferably used, the description in Japanese Patent Application (OPI) No. 242094/85 (the term "OPI" as used herein means an "unexamined published application"), can be referred to. An organic solvent to constitute the above-described oil droplets can be suitably selected from those used generally for pressure sensitive material. Some desirable oils are compounds represented by the following general formulae (I) to (III), triarylmethanes (such as tritoluylmethane, toluyldiphenylmethane), terphenyl compounds (such as terphenyl), alkylated diphenyl ethers (such as propyldiphenyl ether), hydrogenated terphenyl compounds (such as hexahydroterphenyl), diphenyl ethers, chlorinated paraffins and so on. In the above formula, R¹ represents a hydrogen atom, or an alkyl group containing 1 to 18 carbon atoms; R² represents an alkyl group containing 1 to 18 carbon atoms; and p¹ and q¹ each represents an integer of 1 to 4, provided that the total number of alkyl groups therein is 4 or less. Preferred alkyl groups represented by R¹ and R² are those containing 1 to 8 carbon atoms. In the above formula, R³ represents a hydrogen atom, or an alkyl group containing 1 to 12 carbon atoms; R⁴ represents an alkyl group containing 1 to 12 carbon atoms; and n is 1 or 2. p² and q² each represents an integer of 1 to 4. The total number of alkyl groups is 4 or less in case of n=1, while it is 6 or less in case of n=2. In the above formula, R⁵ and R⁶, which may be the same or different, each represents a hydrogen atom, or an alkyl group containing 1 to 18 carbon atoms. m represents an integer of 1 to 13. p³ and q³ each represents an integer of 1 to 3, provided that the total number of alkyl groups is 3 or less. Of alkyl groups represented by R⁵ and R⁶, those containing 2 to 4 carbon atoms are particularly preferred. Specific examples of the compounds represented by the formula (I) include dimethylnaphthalene, diethylnaphthalene and diisopropylnaphthalene. Specific examples of the compounds represented by the formula (II) include dimethylbiphenyl, diethylbiphenyl, diisopropylbiphenyl and diisobutylbiphenyl. Specific examples of the compounds represented by the formula (III)include 1-methyl-1-dimethylphenyl-1-phenylmethane, 1-ethyl-1-dimethylphenyl-1-phenylmethane and 1-propyl-1-dimethylphenyl-1-phenylmethane. The above-cited oils can be used as a mixture of two or more thereof, or in combination with other oils. A preferred size of microcapsules to be employed in the present invention is 4 microns or less, particularly 2, 5 micron or less, on a volume average basis according to the evaluation method described, e.g., in Japanese Patent Application (OPI) No. 214990/85. Desirable microcapsules which are produced in the above-described manner are not those of the kind which are disrupted by heat or pressure, but those of the kind which have a microcapsule wall through which reactive substances present inside and outside the individual microcapsules respectively can permeate at high temperature to react with each other. Multicolored neutral tints can be effected by preparing various microcapsules having walls differing in glass transition point through suitable selection of wall materials, and optional addition of glass transition point controlling agents (e.g., plasticizers described in Japanese Patent Application (OPI)No. 277490/86) to the wall materials, respectively, and further by combining selectively colorless electron-donating dye precursors differing in hue with their respective color developers. Therefore, the present invention is not limited to a monochromatic heat-sensitive recording material but can be applied to a two-color or multicolor heat-sensitive recording material and a heat-sensitive recording material suitable for recording of a graded image. In addition, a photodiscoloration inhibitor as described, e.g., in Japanese Patent Applications (OPI) Nos. 283589/86, 283590/86 and 283591/86 can be added, if desired. Color developers to be employed in the present invention, which undergo the color development reaction with electron-donating colorless precursors when heated, can be those selected suitably from known color developers. For instance, suitable examples of color developers to be combined with leuco dyes include phenol compounds, sulfur-contained phenolic compounds, carboxylic acid compounds, sulfone compounds, urea or thiourea compounds. Details of the color developers are described, e.g., in "Kami Pulp Gijutsu Times," pp. 49-54, and pp. 65-70 (1985). Of such color developers, those having melting points of 50 to 250°C, particularly phenols and organic acids which have melting points of 60 to 200°C and are barely soluble in water are preferred over others. Combined use of two or more of color developers is desirable because of increase in solubility. Color developers preferred particularly in the present invention are represented by the following general formulae (IV) to (VII): R⁷ is an alkyl group, an aryl group, an aryloxy group, or an aralkyl group. In particular, a methyl group, ethyl group or butyl group is preferred as R⁷. R⁸ is an alkyl group. In particular, a butyl group, pentyl group, heptyl group or octyl group is preferred as R⁸. R⁹ is a hydrogen atom or methyl group and n is 0-2. R¹⁰ is an alkyl group, an aryloxy group, or an aralkyl group. In the present invention, such a color developer is used in the form of an emulsified dispersion. The dispersion can be prepared by dissolving the color developer in an organic solvent slightly soluble or insoluble in water, and mixing the resulting solution with an aqueous phase which contains a surface active agent, and a water-soluble high polymer as a protective colloid to emulsify and to disperse the solution in the aqueous phase. An organic solvent to be used for dissolving the color developers can be suitably selected from known oils. In the present invention, esters having a high boiling point or the before-mentioned oils used for pressure-sensitive materials are preferable. In particular, esters are more preferable from the viewpoint of the stability of the color developer emulsion. Specific examples of esters include phosphates (e.g.,triphenyl phosphate, tricresyl phosphate, butyl phosphate, octyl phosphate, cresyl-bi-phenyl phosphate), phthalates (e.g., dibutyl phthalate, 2-ethylhexyl phthalate, ethyl phthalate, octyl phthalate, butylbenzyl phthalate, tetrahydro dioctyl phthalate), benzoates (e.g., ethyl benzoate, propyl benzoate, butyl benzoate, isopentyl benzoate, benzyl benzoate), abietates(e.g., ethyl abietate, benzyl abietate ), dioctyl adipate, diethyl succinate, isodecyl succinate, dioctyl azelate, oxalates (e.g., dibutyl oxalate, dipentyl oxalate), diethyl malonate, maleates (e.g., dimethyl maleate, diethyl maleate, dibutyl maleate), tributyl citrate, sorbic esters (methyl sorbate, ethyl sorbate, butyl sorbate), sebacic esters (dibutyl sebacate, dioctyl sebacate), ethyleneglycol esters (e.g., formic acid monoesters and diesters, butyric acid monoesters and diesters, lauric acid monoesters and diesters, palmitic acid monoesters and diesters, stearic acid monoesters and diesters, oleic acid monoesters and diesters), triacetin, diethylcarbonate, diphenylcarbonate, ethylenecarbonate, propylenecarbonate, boric acid esters (e.g., tributyl borate, tripentyl borate ). Of these esters, it is particularly preferred to use tricresyl phosphate from the standpoint of stabilization of emulsified dispersion of the color developers. Organic solvents having a boiling point lower than 150 °C can be added to the foregoing organic solvents. Some of these organic solvents are ethylacetate, isopropyl acetate, butyl acetate, methylene chloride, and the like. Water soluble high polymers to be contained as a protective colloid in an aqueous phase, which is to be mixed with an oily phase wherein color developers are dissolved, can be selected suitably from known anionic, nonionic or amphoteric high polymers. Of these high polymers, polyvinylalcohol, gelatin, cellulose derivatives and the like are preferred. Surface active agents to be contained additionally in the aqueous phase can be selected suitably from anionic or nonionic surface active agents of the kind which do not cause any precipitation or condensation by interaction with the above-described protective colloids. As examples of surface active agents which can be preferably used, mention may be made of sodium alkylbenzenesulfonates (such as sodium laurylbenzenesulfonate), sodium dioctylsulfosuccinates, and polyalkylene glycols (such as polyoxyethylene nonylphenyl ether). An emulsified dispersion of color developer to be used in the present invention can be prepared with ease by mixing an oil phase containing the color developers and an aqueous phase containing a protective colloid and a surface active agent, using a customary means for preparing a fine grain emulsion, such as a high speed stirrer or an ultrasonic disperser, to disperse the former phase into the latter phase. To the emulsified dispersion thus obtained, a melting point depressant for the color developer can be added, if desired. Some of these melting points depressants have such a function as to control glass transition points of the capsule walls described hereinbefore, also specific examples of such melting point depressants include hydroxy compounds, carbamate compounds, sulfonamide compounds and aromatic methoxy compounds. Details of these compounds are described in Japanese Patent Application NO. 244190/84. The melting point depressant can be used in an amount of 0.1 to 2 parts by weight, preferably 0.5 to 1 part by weight, per 1 part by weight of color developer whose melting point is to be depressed. It is desirable that the melting point depressant and the color developer, whose melting point can be depressed thereby, should be added at the same time. When they are added at different times, a preferred addition amount of the melting point depressant is 1 to 3 times that of the color developer. The heat-sensitive recording material of the present invention is produced by forming a heat-sensitive layer on a support, such as paper or a synthetic resin film, by coating and drying a heat-sensitive coating composition of microcapsules and a dispersion as defined above as main components, and optionally a binder and other additives are incorporated, by a conventional coating method, such as bar coated, blade coating, air knife coating, gravure coating, roll coating, spray coating or dip coating. The coverage of the heat-sensitive layer is preferably controlled to 2.5 to 25 g/m² on a solid basis. It was surprising to find that the prepared heat-sensitive layer had very excellent transparency. The transparency can be estimated by measuring the Haze (%) using a HTR meter (integrating - sphere photometer) manufactured by Nippon Seimitsu Kogyo K.K. However, the transparency of a heat-sensitive layer of a test sample is remarkably affected by light-scattering caused by minute roughness of a surface of the heat-sensitive layer; therefore, a transparent adhesive tape is stuck on the surface of the heat-sensitive layer to prevent the light scattering when measurement is carried out to estimate the intrinsic transparency of the heat-sensitive layer. Now, it is known that the closer the refractive indexes of materials on each side of a boundary layer becomes, the larger the transmittance obtained when light passes through the boundary layer. The present inventors discovered that this also applied to the refractive index of a component contained in the microcapsules (core material), and that of a nonvolatile oil phase of a dispersion comprising a developer and an organic solvent, even though a number of components are contained in the heat-sensitive layer. In fact, haze % of the heat-sensitive layer can be reduced to less than 30% when the refractive index of the former is 0.97 to 1.03 times that of the latter. Especially, haze % can be reduced to less than 20% by making said ratio in the range of 0.99 to 1.01. This means that the transparency of the heat-sensitive layer can easily be controlled by adjusting the refractive index of the component contained in the microcapsules (core material) and that of the aforementioned oil phase in the emulsified dispersion. Above refractive indexes are measured with an Abbe Refractometer manufactured by Atago Co., Ltd. For the purpose of prevention of sticking to a thermal head, and improvement in writing quality, pigments such as silica, barium sulfate, titanium oxide, aluminium hydroxide, zinc oxide, calcium carbonate, etc., styrene beads, or fine particles of urea/melamine resin can be added to the heat-sensitive recording material of the present invention. Also, metal soaps can be added for the purpose of prevention of the sticking phenomenon. They are used at a coverage of 0.2 to 7 g/m². The heat-sensitive recording material of the present invention can be formed using a coating technique with the aid of an appropriate binder. As for the binder, water soluble polymers and various kinds of emulsions, such as a polyvinyl alcohol, a methyl cellulose, a carboxymethyl cellulose, a hydroxypropyl cellulose, a gum arabic, a gelatin, a polyvinyl pyrrolidone, a casein, a styrene-butadiene latex, an acrylonitrile-butadiene latex, a polyvinyl acetate emulsion, a polyacrylate emulsion, and an ethylene-vinyl acetate copolymer emulsion can be employed. The amount of the binder used is 0.2 to 5 g per square meter on a solids basis. In order to prevent sticking to a thermal head and scratching of the heat-sensitive layer or to add various qualities such as a water-resisting property, flatness and antistatic property, it is preferable to provide at least one protective layer on the heat-sensitive layer. Some of the polymers used in the protective layer are a methylcellulose, a carboxymethylcellulose, a hydroxymethylcellulose, a starch, a gelatin, a gum arabic, a casein, a hydrolyzed product of styrene-maleic anhydride copolymer, a hydrolyzed half-ester product of styrene-maleic anhydride copolymer, hydrolyzed product of isobutylene-maleic anhydride copolymer, a polyvinylalcohol, a modified polyvinylalcohol with silicon, a modified polyvinyl alcohol with a carboxyl group, a polyacrylamide derivative, a polyvinyl pyrrolidone, a polystyrene sodium sulfate, a metal salt of alginic acid, styrene-butadiene rubber latex, acrylonitrile-butadiene-butadiene rubber latex, methylacrylate-butadiene rubber latex, polyvinylacetate emulsion. Particularly, a modified polyvinyl alcohol with silicon is preferable. These binders can be used singly or in combination. When a modified polyvinyl alcohol with silicon is used together with other polymer, a preferred amount of the latter is from 0.01 to 0.5 part by weight per 1 part by weight of modified polyvinyl alcohol with silicon. In the protective layer, a pigment, metal soap, wax or cross-linking agent, etc. can be added in order to improve matching of the heat-sensitive material with a thermal head when thermal recording is performed or to improve the water-resisting property of the protective layer. Some of the pigments usable are zinc oxide, calcium carbonate, barium sulfate, titanium oxide, lithopone, talc, agalmatolite, kaolin, aluminum hydroxide and amorphous silica; an amount added of 0.05 - 2 times the total weight of the polymer, especially 0.1 - 0.5 times, is preferable. An amount less than 0.05 times cannot improve the matching of the heat-sensitive recording material with the thermal head, on the other hand an amount more than 2 times reduces both the transparency and sensitivity of the heat-sensitive recording material remarkably, which causes damage to its commercial value. Some of the metal soaps usable are an emulsion of a metal salt of a higher fatty acid (e.g., zinc stearate, calcium stearate, aluminum stearate) etc., and its amount to be added is 0.5 - 20 weight %, preferably 1 - 10 weight % of the total weight of the protective layer. Some of the waxes are a paraffin wax, a microcrystalline wax, a carnauba wax, a methylol stearoamide, a polyethylene wax, an emulsion of silicone etc., and an amount thereof to be added is 0.5 - 40 weight %, preferably 1 - 20 weight % of the total weight of the protective layer. To the coating solution for the protective layer a surface active agent may be added to ensure that the protective layer is uniformly received on the heat-sensitive layer. Some of the active agents usable are an alkali metal salt of sulfosuccinic acid group and an active surface agent containing fluorine atoms etc., specifically the former may be a sodium salt or an ammonium salt etc., of a di-(2-ethylhexyl) sulfosuccinic acid or di-(n-hexyl) sulfosuccinic acid. Other surface active agents, polymer electrolytes or metal oxides can also be added to the protective layer as an antistatic agent. A preferable amount of the protective layer to be coated is usually 0.2 - 5 g/m², particularly 1 g - 3 g/m² at the solids coverage. As a support, an opaque base such as a paper or an undercoated paper as well as a known transparent polymer film can be used in the present invention. When a heat-sensitive recording material of the present invention is intended to be used for OHP, A polyethyleneterephthalate film (PET) and cellulosetriacetate film (TAc) are preferably used as the support from a view point of dimensional stability and strength etc. As for the paper to be used as a support, neutralized paper which is sized with a neutral sizing agent like an alkylketene dimer and shows pH 6-9 upon hot extraction (Japanese Patent Application (OPI) No. 14281/′80) is employed to advantage in the respect of long-range preservation. In order to prevent the penetration of a coating composition into paper, and in order to effect a close contact between a heat recording head and a heat sensitive recording layer, paper described in Japanese Patent Application (OPI) No. 116687/82, which is characterized by Stökigt sizing degree/(meter basis weight)²≧3×10⁻³ and Beck smoothness of 90 seconds or more, is used to advantage. In addition, paper having optical surface roughness of 8 microns or less and a thickness of 40 to 75 microns, as described in Japanese Patent Application (OPI) No. 136492/83; paper having a density of 0.9 g/cm³ or less and optical contact rate of 15 % or more, as described in Japanese Patent Application (OPI) No. 69097/83; paper which is prepared from pulp having received a beating treatment till its freeness has come to 400 cc or more on a basis of Canadian Standard Freeness (JIS P8121) to prevent permeation of a coating composition thereinto, as described in Japanese Patent Application (OPI) No. 69097/83; raw paper made with a Yankee paper machine, which is to be coated with a coating composition on the glossy side and thereby, improvements on developed color density and resolution are intended, as described in Japanese Patent Application (OPI) No. 65695/83; raw paper which has received a corona discharge processing and thereby, its coating aptitude has been enhanced, as described in Japanese Patent Application (OPI) No. 35985/84; and so on can be employed in the present invention, and can bring about good results. In addition to the above-described papers, all supports which have so far been used for general heat sensitive recording papers can be employed as the support of the present invention. The transparency of the heat sensitive recording layer of the present invention can easily be adjusted by controlling the refractive index of the component contained in the microcapsules and that of the oily phase in a color developer emulsified dispersion. Since the transparency of the heat-sensitive layer of the present invention is quite excellent, printed matter can be seen through the heat-sensitive layer. Moreover not only is the reproducibility of multi-colors improved, but also when it is applied on a transparent support to use for an OHP, the quality of the OHP is the same as that of a known OHP. The present invention is illustrated in greater detail by reference to the following Examples. However, the invention should not be construed as being limited to the Examples. EXAMPLE 1 (Preparation of Capsule Solution) 14 g of Crystal Violet lactone (leuco dye), 60 g of Takenate D 110N (Trade name of capsule wall material, produced by Takeda Yakuhin K.K. ) and 2 g of Sumisoap 200 (Trade name of ultraviolet absorbent, produced by Sumitomo Kagaku K.K.) were added to a mixed solvent consisting of 55 g of 1-phenyl-1-xylylethane and 55 g of methylene chloride, and dissolved therein. The solution of the above-described leuco dye was mixed with an aqueous solution consisting of 100 g of a 8 % water solution of polyvinyl alcohol, 40 g of water and 1.4 g of a 2 % water solution of sodium dioctylsulfosuccinate (dispersant), and emulsified by stirring at 10,000 r.p.m. for 5 minutes using an Ace Homogenizer made by Nippon Seiki K.K.. Then, the resulting emulsion was diluted with 150 g of water, and allowed to stand at 40 °C for 3 hours to conduct the microencapsulation reaction therein. Thus, a solution containing microcapsules having a size of 0.7 micron was obtained. (Preparation of Color Developer-emulsified Dispersion) The color developers (a), (b) and (c) represented by the structural formulae illustrated below were added in amounts of 8 g, 4 g and 30 g, respectively, to a solvent mixture of 2.0 g of 1-phenyl-1-xylylethane, 6.0 g of dibutylphthalate and 30 g of ethyl acetate, and dissolved therein. The thus obtained solution of the color developers was mixed with 100 g of a 8 % water solution of polyvinyl alcohol, 150 g of water and 0.5 g of sodium dodecylbenzensulfonate, and emulsified by stirring at 10,000 r.p.m. for 5 minutes at room temperature using Ace Homogenizer made by Nippon Seiki K.K. to prepare an emulsified dispersion having a grain size of 0.5 micron. Production of Heat-Sensitive Material A 5.0 g portion of the foregoing capsule solution, a 10.0 g portion of the foregoing color developer-emulsified dispersion and 5.0 g of water were mixed by stirring, coated on a 70 micron-thick transparent polyethylene terephthalate (PET) film support at a coverage of 15 g/m² on a solids basis, and dried. Thereon, a 2 micron -thick protective layer having the following composition was further provided to produce a transparent heat-sensitive film. (Composition of Protective Layer) 10 % water solution of polyvinyl alcohol 20 g Water 30 g Sodium salt of 2 % dioctyl sulfosuccinate 0.3 g Kaolin dispersion of 3 g of polyvinyl alcohol, 100 g of water and 35 g of Kaolin dispersed by ball mill. 3 g Zinc stearate 0.5 weight part (Hidolin Z-7: manufactured by Chukyo Yushi K.K.) (solid basis) On the thus obtained heat sensitive recording material, thermal recording was carried out using a G III-mode thermal printer (Mitsubishi Melfas 600 (trade name) manufactured by Mitsubishi Denki K.K.) and a blue image was obtained. A transmittal image density was measured as 0.7 using a McBeth densitometer and the image could be seen by OHP. EXAMPLES 2 - 12 AND COMPARATIVE EXAMPLE 1 A transparent black image was obtained by the same procedure as in Example 1 except using the following oil cited in Table 1 instead of the 1-phenyl-1-xylylethane and the dibutyl phthalate used for the preparation of a color developer emulsified dispersion. Each of the thus obtained color developer-emulsified dispersions was diluted by adding 0.5 parts of water, stirred for 6 hours with a stirrer, and then coated on a PET base. The surface condition of each coat was observed visually and thereby comparison of the extents of emulsification stability were made. The above results were shown in Table 1 together with the McBeth transmission density. EXAMPLE 13 Figure 1 of the accompanying drawings is a cross section through the present heat-sensitive recording material which is used for labels. In the figure, 1 is a support, 2 represents an image printed on the support and 3 is the present transparent heat-sensitive layer as prepared in Example 1. Since the transparency of the heat-sensitive layer is excellent, images printed on the support are able to be seen through the heat-sensitive layer. This enables desired matter to be printed on a support before a heat-sensitive layer is coated on the support. Therefore, a paper, which is able to absorb ink easily, can be used as a support and printing on the support using a cheap aqueous ink is possible, while a blocking phenomenon can be prevented. It is easily foreseen that if necessary, a transparent film or the like can be used as the support. EXAMPLES 14 - 16, COMPARATIVE EXAMPLES 2, 3 AND 4 Preparation of Capsule Solution Compounds A shown in Table 2 and 20 g of Takenate D-110N (capsule wall material produced by TAKEDA YAKUHIN K.K.) were added to 25 g of methylenechloride and dissolved. This solution containing the leuco dye was mixed with an aqueous solution consisting of 50 g of a 8 % water solution of polyvinyl alcohol, 15 g of water and 0.2g of 2% water solution of sodium dioctylsulfosuccinate (dispersant), and emulsified by stirring at 10,000 r.p.m. for 5 minutes using an Ace Homogenizer made by Nippon Seiki K.K.. Then, the resulting emulsion was diluted with 150 g of water and allowed to stand at 40 °C for 3 hours to conduct the microencapsulation reaction therein. Thus a solution containing microcapsules having a size of 0.7 micron was obtained. Preparation of Color Developer-emulsified Dispersion Compounds B shown in the Table 2 were dissolved in 10 g of ethylacetate. The obtained solution of the color developers was mixed with 50 g of a 8 % water solution of polyvinylalcohol, 50 g of water and 0.5 g of sodium dodecylbenzensulfonate, and emulsified by stirring at 10,000 r.p.m. for 5 minutes at ordinary temperature using an Ace Homogenizer made by Nippon Seiki K.K. to prepare an emulsified dispersion having a grain size of 0.5 micron. Production of Heat Sensitive Material A 5.0 g portion of the foregoing capsule solution, a 10.0 g portion of the foregoing color developer-emulsified dispersion and 5.0 g of water were mixed by stirring, coated on a 70 micron-thick transparent polyethylene terephthalate (PET) film support providing a coverage of 15 g/m² on a solid basis, and dried. Thereon, a 2 micron-thick protective layer having the following composition was further provided to produce a transparent heat sensitive film. Composition of Protective Layer Modified Polyvinylalcohol with silicon 1 weight part (PVA R2105: manufactured by Kurare K.K.) (solid basis) Colloidal silica 1.5 weight part (Snowtex 30: manufactured by Nissan Kagaku K.K.) (solid basis) Zinc stearate 0.02 weight part (Hidolin Z-7: manufactured by Chukyo Yushi K.K.) (solid basis) Paraffin wax 0.01 weight part (Hidolin P-7: manufactured by Chukyo Yushi K.K.) (solid basis) On each sample produced by using each material shown in Table 2, the refractive index of a component contained in a microcapsule (core material) and that of the nonvolatile component oil phase of the color developer dispersion were measured with an Abee refractometer. The measurement of the refractive indexes were carried out on solutions obtained as follows; the core material or the nonvolatile component of the developer dispersion was heated together with a small amount of ethyl acetate to give a solution, then the ethyl acetate was distilled off. The results of above measurements are shown in Table 3 together with the Haze % measured using a HTR meter (integrating-sphere photometer) manufactured by Nippon Seimitsu Kogyo K.K.. EXAMPLE No. Ra Rb Ra/Rb Haze(%) 14 1.568 1.553 1.010 16 15 1.534 1.553 0.987 16 16 1.568 1.576 1.001 15 Comparative (2) 1.578 1.531 1.031 30 Comparative (3) 1.568 1.520 1.031 31 Comparative (4) 1.499 1.553 0.965 32 Ra: Refractive Index of core material of microcapsule Rb: Refractive Index of nonvolatile component of Oil Phase in emulsified dispersion The results in Table 3 indicate that the Haze % of a heat-sensitive recording material of the present invention is small and the transparency of the heat-sensitive recording material is quite excellent.

Claims: A method of manufacturing a heat-sensitive recording material which has a heat-sensitive layer formed by coating and drying on a support at least a heat-sensitive composition comprising microcapsules, whose core contains at least a colorless or light colored electron-donating dye precursor, and an emulsified dispersion constituted by a non-volatile oily phase comprising a color developer and an organic solvent slightly soluble or insoluble in water and an aqueous phase, said light-sensitive layer being rendered transparent by selecting said microcapsule core and said non-volatile oily phase so that the ratio between the refractive indices thereof is within the range of 0.97 to 1.03. A method as claimed in Claim 1, wherein said range is from 0.99 to 1.01. A method as claimed in Claim 1, wherein said support is printed prior to coating of said heat-sensitive composition. A heat-sensitive recording material manufactured by the method as claimed in Claim 1, 2 or 3. Heat-sensitive recording material as claimed in Claim 4, wherein said support is made of paper. Heat-sensitive recording material as claimed in CLaim 4 or 5, wherein the support has been printed with one or more aqueous inks. Heat-sensitive recording material as claimed in any of Claims 4 to 6, wherein said transparent heat-sensitive layer has sufficient heat-sensitivity to enable recording to be performed using a thermal head.

Inventors: HATAKEYAMA, SEIJI, SHIMOMURA, AKIHIRO, TATSUTA, SUMITAKA, USAMI, TOSHIMASA
Applicants: FUJI PHOTO FILM CO., LTD.
Application year: 1987
Main classification: B41M5
IPC codes: B41M5