Transparent thermal recording medium

Abstract: This invention provides a transparent thermal recording medium comprising: a thermal recording layer, which is provided on a transparent layer and comprises an electron-donating chromophoric compound, an electron-accepting compound and binder resin; and a further-provided protective layer having an approximately equal refractive index to the refractive index of the thermal recording layer, wherein the binder resin is a compound having a group selected from a hydroxyl group and a carboxyl group. The transparent thermal recording medium can be effectively used for a block copy film, on which an image is formed, for plate-making, particularly, in photogravure, offset printing and screen process printing, because the transparent thermal recording medium has a contrast of light transmission factors between a color-imaging portion and a non-imaging portion, in which the contrast is not less than 50% at a wavelength ranging from 370 nm to 450 nm.

Description: The present invention relates to a thermal recording medium based on a coloration reaction of an electron-donating chromophoric compound with an electron-accepting compound and, in particular, a transparent thermal recording medium which is useful for a sheet of a block copy film (for image forming) for plate-making in gravure printing, offset lithography and screen process printing, more particularly, screen process printing for dyeing, an image-forming film sheet for an overhead projector (hereinafter referred to as an "OHP"), an image forming film for a CAD system and a geologic map. The above-mentioned thermal recording medium, which is based on the coloration reaction of the electron-donating chromophoric compound (hereinafter also referred to as a "color-producing agent"), is well known in the art. Application of the thermal recording medium has been required to be expanded for various purposes such as the OHP, a sub origin in the diazo process and designing of drawings. Furthermore, the thermal recording medium has been required to be used for the block copy film for gravure printing, offset lithography and screen printing. General requirements for properties of the block copy film are as follows: (1) A light-shielding property at a wavelength corresponding to ultraviolet light is required to be achieved in one portion of the block copy film, where the ultraviolet light should be shielded, and a transparency towards the light is required to be obtained in another portion, where the film should be transparent (2) The light-shielding property and the transparency towards the light due to a change in temperature, moisture and light do not change too significantly during a desired interval (preservability). (3) A visibility for inspecting a positioning error and a misprint between some superimposed block copy films (inspectability) is required. (4) A precise dimensional accuracy is required. (5) A high resolution is required. (6) A physical strength capable of recycling is required. The known thermal recording medium for the block copy film does not yet achieve the above-mentioned requirements. Transparent thermal recording media are described in Japanese Patent Application No.61-121875 and JP-A-1-99873, in which an image can be recorded directly on the transparent thermal recording medium with a thermal head. However, it is a problem that a complicated process described below is required to produce such a transparent thermal recording medium. For example, the color-producing agent must be enclosed in a microcapsule, and application liquid, which comprises an emulsified dispersion material formed by emulsifying and dispersing a developer dissolved in an organic solvent which is slightly soluble or insoluble in water, must be applied on a transparent support. On one hand, the thermal recording medium formed in the above-mentioned way has an insufficient transparency. On the other hand, other transparent thermal recording media of good transparency have the disadvantage that the stability of a coloring-image formed by the thermal energy is low. The transparent thermal recording media, used for the block copy film for plate-making, have a low contrast between a color-imaging portion and a non-imaging portion at a wavelength range from 370 nm to 450 nm, so that the transparent thermal recording media cannot be used for the block copy film for photosensitive plate-making when the block copy film utilizes a lamp having a wavelength range from 370 nm to 450 nm. Furthermore, the conventional transparent thermal recording medium has another problem that an offset between images printed on the respective films can hardly be found during an inspection of the block copy film formed, for example, by an automatic tracer, since the conventional transparent thermal recording medium has a substantially black coloring tone when more than two block copy films are superimposed together on the inspection. In other words, the color-imaging portion of the block copy film has a high absorption of light at wavelengths ranging from 450 nm to 600 nm, which is particularly visible by visual inspection, and is deemed to be black, and thus results in a difficulty in determining whether the imaging portions of the superimposed block copy films are registered together. Accordingly, it is a general object of the present invention to provide a novel and useful transparent thermal recording medium based on a coloration reaction of an electron-donating chromophoric compound with an electron-accepting compound, in which the above-mentioned problems are overcome and the transparent thermal recording medium has a high enough difference between the light transmission factors of a color-imaging portion and a non-imaging portion and has an effective coloring tone for inspecting an image-formed block copy film to be useful for a block copy film sheet for plate-making. To this end, the present invention provides a transparent thermal recording medium as defined in claim 1. According to the present invention, a transparent thermal recording medium is further provided, wherein the refractive index of said binder resin and the refractive index of resin forming said protective layer range from 1.45 to 1.60 at ordinary temperature. A transparent thermal recording medium as defined above is also provided, wherein said electron-accepting compound is an organo phosphoric acid compound. Still further, according to the present invention, a transparent thermal recording medium as defined above is provided, wherein said organo phosphoric acid compound is selected from phosphonic acid compounds of the following general formulas (I) and (II): where R is selected from linear alkyl groups having from 16 to 24 carbon atoms; and where R' is selected from linear alkyl groups having from 13 to 23 carbon atoms. In the transparent thermal recording medium as defined above said electron-donating chromophoric compound may be selected from fluoran compounds of the following general formulas (III), (IV), (V), (VI), (VII) and (VIII): where R 1 is selected from alkyl groups having equal to or less than 8 carbon atoms, R 2 is selected from a hydrogen atom and an alkyl group having equal to or less than 4 carbon atoms, and X represents a halogen atom selected from a fluorine atom, a chlorine atom and a bromine atom; where R 3 is selected from a hydrogen atom and an alkyl group having equal to or less than 8 carbon atoms, and R 4 is selected from alkyl groups having equal to or less than 8 carbon atoms; where R 5 and R 6 are selected from alkyl groups having equal to or less than 8 carbon atoms, and R 7 is selected from a hydrogen atom, a lower alkyl group and a lower alkoxy group; where R 8 represents a hydrogen atom, R 9 represents an alkyl group having equal to or less than 8 carbon atoms, R 10 is selected from a hydrogen atom, a lower alkyl group and a lower alkoxy group, R 11 is selected from a hydrogen atom and an alkyl group having equal to or less than 8 carbon atoms, and R 12 is selected from an alkyl group having equal to or less than 8 carbon atoms, a phenyl group and a substituted phenyl group; where R 13 represents an alkyl group having equal to or less than 8 carbon atoms, R 14 is selected from a methyl group and an ethyl group, R 15 is selected from a hydrogen atom and an alkyl group having equal to or less than 4 carbon atoms, and Y and Z are selected from halogen atoms such as fluorine atoms, chlorine atoms and bromine atoms; and where R 16 represents an alkyl group having equal to or less than 8 carbon atoms, R 17 is selected from a methyl group and an ethyl group, R 18 is selected from an alkyl group having equal to or less than 4 carbon atoms and a halogen atom such as a fluorine atom, a chlorine atom and a bromine atom, and Ar is selected from a phenyl group and a benzyl group. Still further, according to the present invention, a transparent thermal recording medium is provided, which transparent thermal recording medium comprises: a thermal recording layer provided on a transparent support, wherein said thermal recording layer consists essentially of an electron-donating chromophoric compound as defined in claim 1, an organo phosphoric add compound, and binder resin having a refractive index ranging from 1.45 to 1.60 at ordinary temperature and including a hydroxyl group and/or a carboxyl group; and a protective layer provided on said thermal recording layer, said protective layer consisting essentially of resin having a refractive index ranging from 1.45 to 1.60 at ordinary temperature. Still further, a transparent thermal recording medium is provided, which transparent thermal recording medium comprises: a thermal recording layer provided on a transparent support, wherein said thermal recording layer consists essentially of an electron-donating chromophoric compound as defined in claim 1, an organo phosphoric acid compound, and binder resin having a refractive index ranging from 1.45 to 1.60 at ordinary temperature and including a hydroxyl group and/or a carboxyl group; and a protective layer provided on said thermal recording layer, said protective layer consisting essentially of resin having a refractive index ranging from 1.45 to 1.60 at ordinary temperature, wherein the difference in light transmission factors between a color-producing imaging portion formed on the transparent thermal recording medium by a thermal energy and a non-imaging portion is over 35% at a wavelength ranging from 380 nm to 440 nm or ranging from 350 nm to 470 nm, respectively. The difference of the light transmission factor (A%) is determined by the light transmission factor in a non-imaging portion (B%) and the light transmission factor in an imaging portion (C%) according to the following equation. A = B-C The above and other objects, features, and advantages of the present invention will be more apparent from the following detailed description. A description will now be given of embodiments of the transparent thermal recording medium according to the present invention. An electron-donating chromophoric compound as used in the present invention is per se an achromatic or pale dye precursor. Said color-producing agents can e.g. be selected from the following compounds. Embodiments of general formula (III) 2-(o-chlorophenylamino)-6-ethylamino-7-methylfluoran 2-(o-chlorophenylamino)-6-n-butylamino-7-methylfluoran 2-(o-fluorophenylamino)-6-ethylamino-7-methylfluoran 2-(o-chlorophenylamino)-6-n-butylaminofluoran 2-(o-chlorophenylamino)-6-n-hexylaminofluoran 2-(o-chlorophenylamino)-6-n-octylaminofluoran 2-(o-fluorophenylamino)-6-iso-amylaminofluoran 2-(o-fluorophenylamino)-6-n-octylaminofluoran Embodiments of general formula (IV) 2-(o-nitrophenylamino)-6-dietnylaminofluoran 2-(o-nitrophenylamiro)-6-di-butylaminofluoran 2-(o-nitrophenylamino)-6-(N-ethyl-N-n-butylamino)fluoran 2-(o-nitrophenylamino)-6-(N-ethyl-N-iso-amylamino) fluoran Embodiments of general formula (V) 2-amino-6-diethylaminofluoran 2-amino-6-di-n-butylaminofluoran 2-amino-3-methyl-6-diethylaminofluoran 2-amino-3-methyl-6-di-n-butylaminofluoran 2-amino-3-methyl-6-(N-ethyl-N-iso-amylamino)fluoran 2-amino-3-methoxy-6-diethylaminofluoran 2-amino-3-methoxy-6-di-n-butylaminofluoran Embodiments of general formula (VI) 2-methylamino-6-n-butylaminofluoran 2-n-butylamino-6-n-butylaminofluoran 2-n-octylamino-6-n-ethylaminofluoran 2-n-octylamino-3-methyl-6-n-butylaminofluoran 2-phenylamino-6-ethylaminofluoran 2-phenylamino-6-n-butylaminofluoran 2-phenylamino-6-n-octylaminofluoran 2-phenylamino-3-methyl-6-n-butylaminofluoran 2-phenylamino-3-methyl-6-ethylaminofluoran 2-phenylamino-3-methyl-6-n-hexylaminofluoran 2-phenylamino-3-methyl-6-n-amylaminofluoran 2-phenylamino-3-methyl-6-iso-amylaminofluoran 2-phenylamino-3-methyl-6-n-octylaminofluoran 2-phenylamino-3-methoxy-6-n-butylaminofluoran 2-phenylamino-3-methoxy-6-n-hexylaminofluoran Embodiments of general formula (VII) 2-(3',4'-dichlorophenylamino)-6-ethylamino-7-methylfluoran 2-(3',4'-dichlorophenylamino)-6-n-butylamino-7-methylfluoran 2-(3'-chloro-4'-fluorophenylamino)-6-ethylamino-7-methylfluoran 2-(N'-methyl-N-3'-chlorophenylamino)-6-ethylamino-7-methylfluoran 2-(N-ethyl-N-3'-chlorophenylamino)-6-ethylamino-7-methylfluoran 2-(N-methyl-N-4'-chlorophenylamino)-6-ethylamino-7-methylfluoran Embodiments of general formula (VIII) 2-phenylamino-3-methyl-6-ethylamino-7-methylfluoran 2-phenylamino-3-methyl-6-n-butylamino-7-methylfluoran 2-phenylamino-3-ethyl-6-ethylamino-7-methylfluoran 2-benzylamino-3-methyl-6-ethylamino-7-methylfluoran 2-phenylamino-3-chloro-6-ethylamino-7-methylfluoran 2-phenylamino-3-chloro-6-N-butylamino-7-methylfluoran 2-benzylamino-3-chloro-6-ethylamino-7-methyfluoran According to one embodiment of the present invention, a developer for coloring the above-described color-producing agent is preferably selected from a phenol compound and an organo phosphoric acid compound. For example, the phenol compound may be selected from a gallic acid compound, a protocatechuic acid compound and bis(hydroxyphenyl)acetic acid. The organo phosphoric acid compound may be selected from an alkylphosphonic acid compound and an α-hydroxyalkylphosphonate. The organo phosphoric acid is excellent in surface blushing and thermal sensitivity. The organo phosphoric acid is preferably selected from a phosphonate of the general formulas (I) and (II): where R is selected from linear alkyl groups having from 16 to 24 carbon atoms; and where R' is selected from linear alkyl groups having from 13 to 23 carbon atoms. The phosphonic acid compound of general formula (I) may be selected from hexadecylphosphonate, octadecylphosphonate, eicosylphosphonate, docosylphosphonate and tetracosylphosphonate. The phosphonic acid compound of general formula (II) may be selected from α-hydroxytetradecylphosphonate, α-hydroxyhexadecylphosphonate, α-hydroxyoctadecylphosphonate, α-hydroxyeicosylphosphonate and α-hydroxytetracosylphosphonate. According to the present invention, either one developer solely or a mixture of two or more developers can be employed. Either of one color-producing agent or a mixture of two or more color-producing agents can also be employed. The average particle size of the developer used according to the present invention is preferably equal to or less than 10 µm, and more preferably, the average particle size is equal to or less than 1 µm and the maximum particle size of the developer is not more than 1 µm, so that the thermal sensitivity and the resolution of the thermal recording medium can be improved. Conditions required for a binder included in a thermal recording layer are described hereinafter. When a coloration reaction of the color-producing agent with the developer is generated, for example, by thermal energy, protons from the developer may attack the color-producing agent so as to enrich the periphery of a dye coloring body, being colored by ring-opening, with the protons, thus allowing the coloring body to remain stable and preventing the coloring dye from fading. Therefore, the binder resin is selected from compounds including a hydroxyl group and/or a carboxylic acid group to satisfy the above-mentioned requirements, and preferably the compound has a refractive index (hereinafter also referred to as R.I.) ranging from 1.45 to 1.60 at ordinary temperature. This binder resin may be selected from poly(vinyl butyral): R.I. = 1.48 to 1.49, poly(vinyl acetal): R.I. = 1.50, epoxy resin: R.I. = 1.55 to 1.61, ethyl cellulose: R.I. = 1.46 to 1.49, cellulose acetate: R.I. = 1.46 to 1.50, cellulose acetate butyrate: R.I. = 1.46 to 1.49, cellulose acetate propionate: R.I. = 1.46 to 1.49, nitrocellulose: R.I. = 1.49 to 1.51 and styrene-maleic acid monoalkylester: R.I. = 1.50 to 1.51. Also, oxides as impurities included in the binder resin, and the ultraviolet absorbing agent and antioxidant agent having a hydroxyl group or a carboxyl group in the molecule can perform the same function as the above binder resin. An improvement of the light stability of the thermal recording medium according to the present invention can be achieved by including a light stabilizer in either the thermal recording layer or the protective layer. According to the present invention the light stabilizer may be selected from an ultraviolet absorber, an antioxidant, an anti-aging agent, an extinctive agent of a singlet enzyme and an extinctive agent of a superoxide anion. The ultraviolet absorber, for example, may be selected from a benzophenone ultraviolet absorber such as 2,4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone, 2-hydroxy-4-n-octoacybenzophenone, 4-dodecyloxy-2-hydroxybenzophenone, 2,2'-dihydroxy-4-methoxybenzophenone, 2,2'-dihydroxy-4,4'-dimethoxybenzophenone, 2,2',1,4'-tetrahydroxybenzophenone, 2-hydroxy-4-methoxy-2'-carboxybenzophenone, 2-hydroxy-4-oxybenzylbenzophenone, 2-hydroxy-4-chlorobenzophenone, 2-hydroxy-5-chlorobenzophenone, 2-hydroxy-4-methoxy-4'-mehhylbenzophenone, 2-hydroxy-4-n-heptoxybenzophenone, 2-hydroxy-3,6-dichloro-4-methoxybenzophenone, 2-hydroxy-3,6-dichloro-4-ethoxybenzophenone and 2-hydroxy-4-(2-hydroxy-3-methacryloxy)propoxybenzophenone; a benzotriazol ultraviolet absorber such as 2-(2'-hydroxy-5'-methylphenyl)benzotriazol, 2-(2'-hydroxy-3',5'-ditertiary-butylphenyl)benzotriazol, 2-(2'-hydroxy-3'-tertiary-butyl-5'-methylphenyl)benzotriazol, 2-(2'-hydroxy-4'-octoxy)benzotriazol, 2-(2'-hydroxy-3',5'-ditertiary-butylphenyl)5-chlorobenzotriazol, 2-(3'-tertiary-butyl-2'-hydroxy-5'-methylphenyl)-5-chlorobenzotriazol and 2-(2'-hydroxy-5-ethoxyphenyl) benzotriazol; a salicylic acid phenyl ester ultraviolet absorber such as phenyl salicylate, p-octylphenyl salicylate, p-tertiary-butylphenyl salicylate, carboxyphenyl salicylate, methylphenyl salicylate and dodecylphenyl salicylate; p-methoxybenzylidene malonic add dimethyl ester; 2-ethylhexyl-2-cyano-3,3'-diphenylacrylate; ethyl-2-cyano-3,3'-diphenylacrylate; 3,5-ditertiary-butyl-p-hydroxybenzoic acid; resorcinol monobenzoate; 24-ditertiary-butylphenol; 3,5-ditertiary-butyl-4-hydroxybenzoate; and the like. The antioxidant and the anti-aging agent may be selected, for example, from 2,6-ditertiary-butyl-4-methylphenol, 2,4,6-tritertiarybutylphenol, styrene-modified phenol, 2,2'-methylenebis(4-methyl-6-tertiarybutylphenol), 4,4'-isopropylidenebisphenol, 2,6-bis(2'-hydroxy-3'-tertiarybutyl-5'-methylbenzyl)-4-methylphenol, 4,4'-thiobis-(3-methyl-6-tertiarybutylphenol), tetrakis- {methylene(3,5-ditertiarybutyl-4-hydroxyhydrocinnamate)} methane, para-hydroxyphenyl-3-naphthylamine, 2,2,4-trimethyl-1,2-dihydroquinoline, thiobis(β-naphthol), mercaptobenzothiazole, mercaptobenzimidazole, aldol-2-naphthylamine, bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate, 2,2,6,6-tetramethyl-4-piperidylbenzoate, dilauryl-3,3'-thiodipropionate, distearyl-3,3'-thiodipropionate, tris(4-nonylphenol)phosphate, and the like. The extinctive agent of the singlet enzyme may be selected from a carotene class, a pigment class, an amine class, a phenol class, a nickel complex group and a sulfide class. The extinctive agent of the singlet enzyme may be, for example, selected from 1,4-diazabicyclo(2.2.2)octane, β-carotene, 1,3-cyclohexadiene, 2-diethylaminomethylfuran, 2-phenylaminomethylfuran, 9-diethylaminomethylanthracene, 5-diethylaminomethyl-6-phenyl-3,4-dihydroxypyran, nickeldimethyldithiocarbamate, nickeldibutyldithiocarbamate, nickel-3,5-di-t-butyl-4-hydroxybenzyl-O-ethylphosphonate, nickel-3,5-di-t-butyl-4-hydroxybenzyl-O-butylphosphonate, nickel {2,2'-thiobis(4-t-octylphenolate)} (n-butylamine), nickel-{2,2'-thiobis(4-t-octylphenolate)} (2-ethylhexylamine), nickelbis {2,2'-thiobis(4-t-octylphenolate)}, nickelbis-{2,2'-sulfonebis(4-octylphenolate)}, nickelbis(2-hydroxy-5-methoxyphenyl-N-n-butylaldimine), nickelbis(dithiobenzyl), nickelbis(dithiobisacetyl) and so on. A non-limiting example of the extinctive agent of the superoxide anion according to the present invention may be selected from superoxide dismutase, a cobalt[III] complex and a nickel[II] complex. These are used solely or in a mixture of two or more thereof. A substrate of the thermal recording medium according to the present invention is a transparent support, which preferably has a refractive index ranging from 1.45 to 1.60 at ordinary temperature. For example, the transparent support can be generally selected from a polyester film such as poly(ethylene terephthalate) and poly(butylene terephthalate); a cellulose derivative film such as cellulose triacetate; a polyolefin film such as polypropylene and polyethylene; a polystyrene film; and a laminate thereof. It is preferable that an adhesive layer is inserted between the thermal recording layer and the transparent support. The adhesive layer may be generally formed of acryl resin, saturated polyester resin and hardened resin thereof. In the case of a thermal recording medium having no protective layers, the thermal recording layer contains fine particles of the developer dispersed in the binder resin, so that the surface and the inside of the thermal recording layer are inhomogeneous. Since this inhomogeneity results in the presence of air in an unevenness or vacancy of the thermal recording layer and a difference of the refractive index in the thermal recording layer, light thus being scattered, the thermal recording layer appears to be opaque or semitransparent. However, according to the thermal recording medium of the present invention, the unevenness and the vacancy of the thermal recording layer is removed by applying and drying (hardening) some resin on the opaque or semitransparent recording layer, in which the resin has the same refractive index at ordinary temperature as that of the binder resin of the thermal recording layer, and thus the thermal recording layer remains homogeneous. This eliminates the light scattering and improves the transparency of the thermal recording medium. The resulting protective layer not only contributes to transparency of the medium, but also effectively improves chemical resistance, water resistance, abrasion resistance, light fastness and a head matching property. Therefore, the protective layer is an essential component of the high performance transparent thermal recording medium. The protective layer according to the present invention includes a coating formed principally of water-soluble resin or hydrophobic resin as well as a coating formed principally of ultraviolet curable resin or electron beam curable resin. Due to the formation of such a protective layer, a thermal recording medium with no practical problems can be achieved even if an organic solvent, a plasticiser, oil, sweat and water contact the thermal recording medium. Furthermore, an inclusion of an organic or inorganic filler and a slip agent results in a thermal recording medium of high reliability and high head matching quality while preventing, for example, the medium from sticking when contacting the thermal head. A detailed description of the protective layer according to the present invention will be given hereinafter. The protective layer of the present invention comprises resin having substantially the same refractive index as that of the binder resin forming the thermal recording layer. An acceptable difference between those refractive indexes, which are substantially equal to each other, ranges from approximately -5% to +5%. The resin preferably has the refractive index ranging from 1.45 to 1.60 at ordinary temperature. The resin satisfying the above-mentioned requirement can be selected from water-soluble resin, aqueous resin emulsion, hydrophobic resin, ultraviolet curable resin and electron beam curable resin. The water-soluble resin may be selected from polyvinyl alcohol, denatured polyvinyl alcohol, cellulose derivatives (methylcellulose, methoxycellulose, hydroxyethylcellulose and so on), casein, gelatin, polyvinyl pyrrolidone, styrene-maleic anhydride copolymer, diisobutylene-maleic anhydride copolymer, polyacrylamide, modified polyacrylamide, methylvinyl ether-maleic anhydride copolymer, carboxy modified polyethylene, polyvinyl alcohol/acrylamide block copolymer, melamine-formaldehyde resin, urea-formaldehyde resin and so on. The aqueous resin emulsion and the hydrophobic resin may be selected from polyvinyl acetate, polyurethane, styrene/butadiene-copolymer, styrene/butadiene/acryl-copolymer, polyacrylic acid, polyacrylate, vinyl chloride/vinylacetate-copolymer, polybutyl methacrylate, ethylene/vinylacetate-copolymer and so on. These resins can be used individually or mixed together, and a hardner may also be added to the resin to harden the resin. A detailed description of the ultraviolet curable resin and the electron beam curable resin, which are the most preferred embodiments of the protective layer according to the present invention, is given hereinafter. Various well-known monomers and oligomers (prepolymers), which are polymerised and hardened by ultraviolet light so as to form resin and which are non-limiting examples, can be used for the ultraviolet curable resin for forming the protective layer. The monomer or oligomer may be selected from (poly)ester acrylate, (poly)urethane acrylate, epoxy acrylate, polybutadiene acrylate, silicone acrylate and melamine acrylate. (Poly) ester acrylate is a reaction product of polyhydric alcohol such as 1,6-hexanediol, propylene glycol (in the form of propylene oxide) and diethylene glycol; polybasic acid such as adipic acid, phthalic anhydride and trimellitic acid; and acrylic acid. Formulas of the above-mentioned reaction products may be written as follows. (a) Adipic acid/1,6-hexanediol/acrylic acid: CH 2 =CHCOO(CH 2 ) 6 [O-CO-(CH 2 ) 4 COO(CH 2 ) 6 ] n OCOCH=CH 2 where n represents an integer varying from 1 to 10. (b) Phthalic anhydride/propylene oxide/acrylic acid: where I represents an integer varying from 1 to 10; m represents an integer varying from 1 to 10; and n represents an integer varying from 1 to 10. (c) Trimellitic acid/diethylene glycol/acrylic acid: (Poly)urethane acrylate is a reaction product of a compound having an isocyanate group such as tolylene diisocyanate (TDI) with acrylate having a hydroxy group. A formula of the reaction product is written as follows. (d) HEA/TDI/HDO/ADA/HDO/TDI/HEA HEA represents 2-hydroxyethylacrylate; HDO represents 1,6-hexanediol; and ADA represents adipic acid: where n represents an integer varying from 1 to 10. Epoxy acrylate is generally categorized into bisphenol type, novolac type and alicyclic type, in which an epoxy group of epoxy resin is acryl-modified with acrylic acid so that a functional group thereof is modified to an acryloyl group. Formulas of the epoxy acrylate are shown as follows. (e) Bisphenol A-epichlorohydrin type/acrylic acid: where n represents an integer varying from 1 to 15. (f) Phenol novolac-epichlorohydrin type/acrylic acid: where n represents an integer varying from 0 to 5. (g) Alicylic type/acrylic acid: where R represents -(CH 2 ) n -; and n represents an integer varying from 1 to 10. Polybutadiene acrylate is, for example, a reaction product of 1,2-polybutadiene acrylate including an OH end group with isocyanate or 1,2-mercaptoethanol which is further reacted with acrylic acid and so on. Silicone acrylate is, for example, prepared by a condensation reaction (demethanolization reaction) of an organic functional trimethoxysilane with a polysiloxane including a silanol group so as to be methacryl-modified. A formula (i) of silicone acrylate is given as follows: where n represents an integer varying from 10 to 14. When the ultraviolet curable resin is used, a solvent is sometimes used with the resin. The solvent is, for example, selected from organic solvents such as tetrahydrofuran, methyl ethyl keton, methyl isobutyl keton, chloroform, carbon tetrachloride, ethanol, isopropyl alcohol, ethyl acetate, butyl acetate, toluene, benzene and so on. Alternatively, a photopolymerizable monomer can be used as a reactive diluent to achieve an easy treatment. The photopolymerizable monomer may be selected from 2-ethylhexyl acrylate, cydohexyl acrylate, butoxyethyl acrylate, neopentylglycol diacrylate, 1,6-hexanediol diacrylate, polyethyleneglycol diacrylate, trimethylolpropane triacrylate, pentaerythrite acrylate and so on. Next a detailed description of the electron beam curable resin will be given. Various non-limiting examples of the electron beam curable resin are available. In particular, a preferred embodiment of the electron beam curable resin comprises a branched molecular structure having more than 5 functional groups of a polyester skeleton (hereinafter referred to as "electron beam curable acryl-modified polyurethane resin"), and another preferred embodiment is one which essentially consists of silicone-modified electron beam curable resin. The electron beam curable acryl-modified polyurethane resin, for example, can be produced as follows. First, polyester diol of a reaction product of 1,4-butanediol with adipic acid or another reaction product of propyleneglycol with adipic acid (both of them corresponding to the polyester skeleton) is mixed with polyether triol to achieve a mixture. Then diisocyanate and a compound having an acrylic double bond are added to the mixture to react with the mixture, so as to produce the electron beam curable acryl-modified polyurethane resin. A mixture of polyester diol with polyether triol, a mixture of polyester diol with polyester triol or polyether diol with polyester triol can be employed to prepare the electron beam curable acryl-modified polyurethane resin as an alternative to the mixture of the polyester diol with the polyether triol. For example, the diisocyanate may be selected from 2,4-tolylenediisocyanate, 2,6-tolylenediisocyanate, 1,6-hexamethylenediisocyanate, xylenediisocyanate, isophoronediisocyanate, methylenebis(4-phenylisocyanate) and so on. The compound having the acrylic double bond, for example, can be selected from 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, 3-hydroxypropyl(meth)acrylate and so on. Polyester diol is commercially available, for example, in the form of ADECANEWACE® Y4-30 (produced by ASAHI DENNKAKOGYO Corp.) and polyether triol is also commercially available, for example, in the form of SUNNIX® TP-400 or SUNNIX® GP-3000 (produced by SANYO KASEI Corp.). The molecular weight of the polyester portion of the electron beam curable acryl-modified polyurethane resin preferably ranges from 2000 to 4000 in order to achieve a desired flexibility and robustness in a heat resistant slip layer. Further, the total molecular weight of the electron beam curable acryl-modified polyurethane resin preferably ranges from 20000 to 50000 for the same reason as described above. A resin having not less than 5 functional groups, and preferably 7 to 13 functional groups, can effectively cause a progress for hardening and an improvement of hardness. The silicone-modified electron beam curable resin may be written as the following formula: where R represents -(CH 2 ) n -, where n represents an integer varying from 0 to 3; TDI represents 2,4-tolylenediisocyanate; and HEM represents 2-hydroxyethyl acrylate, x ranges from 50 to 100 and y ranges from 3 to 6. This silicone-modified electron beam curable resin has a superior covering property to form a uniform thin coating fairly well and has an effective slip property due to a silicone functional group. In simultaneous use of the electron beam curable acryl-modified resin and the electron beam curable silicone-modified resin, it is preferable that 30 parts by weight, and more preferably 5 to 20 parts by weight, of electron beam curable silicone-modified resin may be added to 100 parts by weight of electron beam curable acryl-modified resin. In the protective layer according to the present invention, it is preferable that a multi-functional electron beam curable monomer is employed simultaneously in order to promote the progress of the hardening while forming the layer and to improve the heat resistance of the layer. This monomer acts as a cross-linking stimulator and has the advantage of forming a complicated and high-density cross-linked structure. The above-mentioned monomer can be selected from trimethylolpropaneacrylate, tetramethylolmethanetetraacrylate, pentaerythritoltriacrylate, dipentaerythritolhexatriacrylate and so on. It is preferable that less than 50 parts by weight of monomer, more preferably 20 to 50 parts by weight, are added to 100 parts by weight of electron beam curable acryl-modified polyurethane resin. More than 50 parts of monomer result in a weakness of lubricant hardening and a degradation of the slip effect. Another embodiment of the protective layer according to the present invention is phosphazene resin having repeat units including a phosphazene skeleton of the following formula, and having significant heat resistance. (P = N) A more particular and non-limiting example of the phosphazene resin is written as the following formula: [NP(A) a (B) b] n where a and b represent real numbers satisfying the following equations: a > 0, b ≧ 0 and a + b = 2 ; A represents a polymerization curable group of the following formula such as a methacryloyloxyethyl group: where R 1 , R 2 , R 3 , R 4 and R 5 are selected from a hydrogen atom, a chlorine atom, a bromine atom and a halogenated alkyl group having from 1 to 4 carbon atoms; M is selected from an oxygen atom, a sulfur atom and an imino group. One of the above-mentioned phosphazene resins, where A is a methacryloyloxyethyl group and b is equal to 0, can be prepared by a ring-opening polymerization of a compound of the following formula: If the resin has the polymerization curable group as is the case with the phosphazene resin, mechanical strength, hardness and heat resistance of the resin can be improved by hardening with ultraviolet rays , electron rays or heat. The improvement of light stability of the protective layer according to the present invention is also achieved by the protective layer containing the same light stabilizer as that contained in the thermal recording layer as described above. The light stabilizer can be selected from the ultraviolet absorber, the antioxidant, the anti-aging agent, the extinctive agent of the singlet enzyme and the extinctive agent of the superoxide anion, which are all the same as those employed with the thermal recording layer. The inclusion of an organic or inorganic filler and a slip agent, to the extent that the transparency of the protective layer is not be degraded, results in an improvement of the head matching property. The organic filler employed in the present invention may be selected from polyolefin particles, polystyrene particles, urea-formaldehyde resin particles and plastic fine hollow spherical particles; and the inorganic filler may be selected from aluminium hydroxide, heavy and light calcium carbonate, zinc oxide, titanium oxide, barium sulfate, silica gel, colloidal silica (from 10 to 50 nm), alumina sol (from 10 to 200 nm), activated clay, talc, clay titanium white, kaolinite, calcined kaolinite, diatomaceous earth, synthetic kaolinite, zirconium compounds and glass fine hollow spheres. In particular, a spherically shaped filler having the same slip property as that of Si resin or fluorine resin is preferably employed. A slip additive may be selected from a slip agent such as silicone oil, a surfactant, an organic salt and a class of waxes; and a slip filler. The silicone oil may be selected from dimethylpolysiloxane, methylphenylpolysiloxane, methylhydrodienepolysiloxane, alkyl-modified polysiloxane, carbon-modified polysiloxane and alcohol-modified polysiloxane. The surfactant may be selected from a commercially available carboxylate, sulfate ester salt of higher alcohol, sulfonate, phosphate of higher alcohol and salt thereof. Non-limiting embodiments of the surfactant are sodium laurate, sodium stearate, sodium oleate, lauryl alcohol sodium sulfate ester, myristyl alcohol sodium sulfate ester, cetyl alcohol sodium sulfate ester, stearyl alcohol sodium sulfate ester, oleyl alcohol sodium sulfate ester, sodium sulfate ester of an ethylene oxide adduct of higher alcohol, sodium octylsulfonate, sodium decylsulfonate, sodium dodecylsulfonate, sodium octylbenzene sulfonate, sodium dodecylbenzene sulfonate, potassium dodecylbenzene sulfonate, sodium non-ylnaphthalene sulfonate, sodium dodecylnaphthalene sulfonate, potassium dodecylnaphthalene sulfonate, sodium N-oleyl-N-methyltaurine, tetraethoxylaurylalcohol acid ester, sodium monostearylester phosphate and sodium distearylesterphosphate. The organic salts may be selected from metal soaps such as zinc stearate, aluminium stearate, calcium stearate, magnesium stearate; and a class of salts such as hexylammoniumchloride, sodium sulfosalicylate, sodium succinate, potassium succinate, potassium benzonate and potassium adipate. The wax may be selected from natural wax such as candelilla wax, carnauba wax, rice wax, bees wax, lanolin wax, montan wax, paraffin wax and microcrystalline wax; and synthetic wax such as polyethylene wax, hydrogenated castor oil and derivatives thereof and fatty acid amide. An appropriate amount of the slip agent in the protective layer ranges from 0.001 to 15.0% by weight. If the amount of the slip agent exceeds the appropriate range, the mechanical strength of the protective layer degrades, and if the amount is less than the appropriate one, an effect of the slip agent can not be achieved. The transparent thermal recording medium according to the present invention can be prepared with one of the following methods. First the coating liquid is prepared in accordance with each of the following methods. In the first method, solely the developer is homogeneously dispersed in the organic solvent, and then the color-producing agent and the binder resin successively are homogeneously mixed with the solvent to prepare the coating liquid for the thermal recording layer. In the second method, the developer is homogeneously dispersed in a solution of the binder resin, in which the binder resin is dissolved in the organic solvent, and the coating liquid for the thermal recording layer is prepared by homogeneously mixing the color-producing agent and so on with the solution. In the third method, the color-producing agent and the developer are dispersed in the organic solvent with the binder resin to prepare the coating liquid for the thermal recording layer. Then the coating liquid having been dispersed homogeneously by one of the above-mentioned ways is applied and dried on one side or both sides of the transparent support so as to provide the thermal recording layer on the support, and then the protective layer consisting essentially of resin is provided on the thermal recording layer. The organic solvent for dissolving the binder resin can be selected from ethers such as dibutylether, isopropylether, dioxane and tetrahydrofuran; ketones such as acetone, diethylketone, methylethylketone, methylisobutylketone and methylpropylketone; esters such as ethyl acetate, isopropyl acetate and n-propyl acetate; and aromatic hydrocarbons such as benzene, toluene and xylene. One of those compounds solely or a mixture of several of the compounds can be employed. There are no limitations of the available method for coating the protective layer and the amount of the applied material. However, in consideration of performance and economy, the protective layer requires the thickness of the applied layer on the thermal recording medium to be from 0.1 to 20 µm, and preferably from 0.5 to 10 µm,so as to achieve enough performance of the protective layer and keep a capacity of the thermal recording medium. Also, it is preferred that an antistatic layer is provided on the back side of the recording medium for easy handling thereof, preventing dust from being attached to the recording medium and improving image quality. As electrostatic agent suitable even at low temperature, electrically conductive metal oxide compounds can be mentioned. Generally speaking, an antistatic agent including electrically conductive metal oxide is expensive. However, since the metal oxide compound itself is electrically conductive, even a small amount of metal oxide compound affords great antistatic characteristics. Also, a metal oxide compound does not prevent the production of a transparent recording medium. As the electrically conductive metal oxide, SnO 2 , In 2 O 3 , ZnO, TiO 2 , MgO, Al 2 O 3 , BaO or MoO 3 can be used solely or these compounds can be used with P, Sb, Sn or Zn. However, the electrically conductive metal oxide is not limited to those listed above. It is preferred that particles of the electrically conductive metal oxide are fine to realize a transparent recording medium. In this invention, the average particle size is less than 0.2 µm to realize a transparent recording medium. As binders to be used with the above metal oxides, hydrophilic resin, hydrophilic emulsion, hydrophobic resin, ultraviolet curable resin and electron beam curable resin can be mentioned. As the hydrophilic resin, polyvinylalcohol, cellulose derivative, casein, gelatin, styrene-maleic acid anhydride, carboxy-denatured polyethylene resin can be mentioned. As the hydrophilic emulsion and the hydrophobic resin, polyvinylacetate, polyurethane, vinyl chloride/vinyl acetate-copolymer, polyester, polybutylacrylate, polyvinylacetal, ethylene/vinylacetate-copolymer can be mentioned. One of those compounds solely or a mixture of several of the compounds can be employed. Also, a hardener can be used with those compounds if necessary. An image to be recorded on the transparent thermal recording medium according to the present invention can be formed in various ways by using, for example, a thermal pen, a thermal head, laser heating, or thermal etching with light, according to a purpose of image usage. In practice it is preferable that the thermal head is employed to form the image. The transparent thermal recording medium is suitable for a thermal recording medium for a block copy. Further, a thermal recording medium for a block copy, comprising, a transparent supporting member, and a thermal recording layer provided on said transparent supporting member, said thermal recording layer including an electron-donating chromophoric compound, an organic phosphoric compound and a binder resin having a refractive index ranging from 1.45 to 1.60, said binder resin including a hydroxyl group and/or a carboxyl group in a molecule thereof, and a protective layer provided on said thermal recording layer, said protective layer consisting essentially of a resin having a refractive index similar to that of said binder resin at ordinary temperature, wherein the difference in light transmission factors between a color-producing imaging portion formed by thermal energy and a non-imaging portion is over 35% at a wavelength ranging from 350nm to 470nm, can be used. Moreover, a thermal recording medium for a block copy, comprising a transparent supporting member, and a thermal recording layer provided on said transparent supporting member, said thermal recording layer including an electron-donating chromophoric compound, an organic phosphoric compound and a binder resin having a refractive index ranging from 1.45 to 1.60, said binder resin including a hydroxyl group and/or a carboxyl group in a molecule thereof, and a protective layer provided on said thermal recording layer, said protective layer consisting essentially of a resin having a refractive index similar to that of said binder resin at ordinary temperature, wherein the difference in light transmission factors between a color-producing imaging portion formed by thermal energy and a non-imaging portion is over 35% at a wavelength ranging from 380nm to 440nm. However, the present invention is not limited to the above-described media. A detailed description of the present invention will be given hereinafter by referring to non-limiting examples. The terms "parts" and "%" in the following examples are based on weight. Example 1 A coating liquid for the recording layer was prepared by dispersing the following composition with a desk-top type ball mill so as to yield a 0.3µm average particle size of octadecylphosphonic acid. [coating liquid for recording layer] 2-(o-chlorophenylamino)-6-ethylamino-7-methylfluoran 10 parts Octadecylphosphonic acid 30 parts Polyvinylbutyral [refractive index 1.49] (Denkabutyral #3000-2 produced by Denka Kagaku Kogyo Corp.) 15 parts Mixed liquid of toluene/methylethylketone (ratio 1/1) 285 parts A coating liquid for the protective layer was prepared by dispersing the following composition homogeneously. [coating liquid for protective layer] 75% of urethane acrylate ultraviolet curable resin solution in n-butylacetate [refractive index 1.56] (Unidick C7-157 produced by Dainihon Ink Kagaku Corp.) 100 parts Solution of 52% silicone resin in xylene (Byk-344 produced by Bic Chemy Japan Corp.) 4 parts Ethylacetate 50 parts [Production of transparent thermal recording medium] The coating liquid for the recording layer was applied and dried on a 100µm HPJ polyester film (produced by Teijin Corp.) by a wire bar so as to yield an 8.0µm thickness of the applied coating layer, and thus forming the thermal recording layer. Further, the coating liquid for the protective layer was applied and dried on the thermal recording layer with a wire bar, and then hardened with an 80W/cm ultraviolet ray lamp to form the protective layer of about 6.0µm in thickness. Thus a transparent thermal recording medium was produced. Example 2 A coating liquid for the recording layer was prepared by dispersing the following composition with a desk-top type ball mill so as to yield a 0.3µm average particle size of eicosylphosphonic acid. [coating liquid for recording layer] 2-(o-chlorophenylamino)-6-n-octylaminofluoran 10 parts Eicosylphosphonic add 30 parts Polyvinylbutyral [refractive index 1.49] (Denkabutyral#3000-2 produced by Denka Kagaku Kogyo Corp.) 15 parts Mixed liquid of toluene/methylethylketone (ratio 1/1) 285 parts A coating liquid for the protective layer was prepared by dispersing the following composition homogeneously. [coating liquid for protective layer] 75% of urethane acrylate ultraviolet curable resin solution in n-butylacetate [refractive index 1.56] (Unidick C7-157 produced by Dainihon Ink Kagaku Corp.) 100 parts Solution of 52% silicone resin in xylene (Byk-344 produced by Bic Chemy Japan Corp.) 4 parts Ethylacetate 50 parts [Production of transparent thermal recording medium] The coating liquid for the recording layer was applied and dried on a 100µm HPJ polyester film (produced by Teijin Corp.) by a wire bar so as to yield an 8.0µm thickness of the applied coating layer, and thus forming the thermal recording layer. Further, the coating liquid for the protective layer was applied and dried on the thermal recording layer with a wire bar, and then hardened with an 80W/cm ultraviolet ray lamp to form a protective layer of about 6.0 µm in thickness. Thus a transparent thermal recording medium was produced. Example 3 A coating liquid for the recording layer was prepared by dispersing the following composition with a desk-top type ball mill so as to yield a 0.3µm average particle size of eicosylphosphonic acid. [coating liquid for recording layer] 2-(o-nitrophenylamino)-6-diethylaminofluoran 10 parts Eicosylphosphonic acid 30 parts Polyvinylbutyral [refractive index 1.49] (Denkabutyral #3000-2 produced by Denka Kagaku Kogyo Corp.) 15 parts Mixed liquid of toluene/methylethylketone(ratio 1/1) 285 parts A coating liquid for the protective layer was prepared by dispersing the following composition homogeneously. [coating liquid for protective layer] 75% of urethane acrylate ultraviolet curable resin solution in n-butylacetate [refractive index 1.56] (Unidick C7-157 produced by Dainihon Ink Kagaku Corp.) 100 parts Solution of 52% silicone resin in xylene (Byk-344 produced by Bic Chemy Japan Corp.) 4 parts Ethylacetate 50 parts [Production of transparent thermal recording medium) The coating liquid for the recording layer was applied and dried on a 100 µm HPJ polyester film (produced by Teijin Corp.) by a wire bar so as to yield an 8.0µm thickness of the applied coating layer, and thus forming the thermal recording layer. Further, the coating liquid for the protective layer was applied and dried on the thermal recording layer with a wire bar, and then hardened with an 80W/cm ultraviolet ray lamp to form a protective layer of about 6.0 µm in thickness. Thus a transparent thermal recording medium was produced. Example 4 A coating liquid for the recording layer was prepared by dispersing the following composition with a desk-top type ball mill so as to yield a 0.3 µm average particle size of octadecylphosphonic acid. [coating liquid for recording layer] 2-amino-3-methyl-6-di-n-butylaminofluoran 10 parts Octadecylphosphonic acid 30 parts Polyvinylbutyral [refractive index 1.49] (Denkabutyral #3000-2 produced by Denka Kagaku Kogyo Corp.) 15 parts Mixed liquid of toluene/methylethylketone (ratio 1/1) 285 parts A coating liquid for the protective layer was prepared by dispersing the following composition homogeneously. [coating liquid for protective layer] 75% of urethane acrylate ultraviolet curable resin solution in n-butylacetate [refractive index 1.56] (Unidick C7-157 produced by Dainihon Ink Kagaku Corp.) 100 parts Solution of 52% silicone resin in xylene (Byk-344 produced by Bic Chemy Japan Corp.) 4 parts Ethylacetate 50 parts [Production of transparent thermal recording medium] The coating liquid for the recording layer was applied and dried on a 100 µm HPJ polyester film (produced by Teijin Corp.) by a wire bar so as to yield an 8.0µm thickness of the applied coating layer, and thus forming the thermal recording layer. Further, the liquid for the protective layer was applied and dried on the thermal recording layer with the wire bar, and then hardened with an 80-W/cm ultraviolet ray lamp to form a protective layer of about 6.0µm in thickness. Thus a transparent thermal recording medium was produced. recording layer. Further, the coating liquid for the protective layer was applied and dried on the thermal recording layer with a wire bar, and then hardened with a 80W/cm ultraviolet ray lamp to form a protective layer of about 6.0µm in thickness. Thus a transparent thermal recording medium was produced. Example 5 A coating liquid for the recording layer was prepared by dispersing the following composition with a desk-top type ball mill so as to yield a 0.3 µm average particle size of octadecylphosphonic acid. [coating liquid for recording layer] 2-(N-methyl-N-3'-chlorophenylamino)-6-ethylamino-7-methylfluoran 10 parts Octadecylphosphonic acid 30 parts Polyvinylbutyral [refractive index 1.49] (Denkabutyral #3000-2 produced by Denka Kagaku Kogyo Corp.) 15 parts Mixed liquid of toluene/methylethylketone (ratio 1/1) 285 parts A coating liquid for the protective layer was prepared by dispersing the following composition homogeneously. [coating liquid for protective layer] 75% of urethane acrylate ultraviolet curable resin solution in n-butylacetate [refractive index 1.56] (Unidick C7-157 produced by Dainihon Ink Kagaku Corp.) 100 parts Solution of 52% silicone resin in xylene (Byk-344 produced by Bic Chemy Japan Corp.) 4 parts Ethylacetate 50 parts [Production of transparent thermal recording medium] The coating liquid for the recording layer was applied and dried on a 100 µm HPJ polyester film (produced by Teijin Corp.) by a wire bar so as to yield an 8.0µm thickness of the applied coating layer, and thus forming the thermal recording layer. Further, the applied liquid for the protective layer was applied and dried on the thermal recording layer with a wire bar, and then hardened with a 80W/cm ultraviolet ray lamp to form a protective layer of about 6.0µm in thickness. Thus a transparent thermal recording medium was produced. Example 6 A coating liquid for the recording layer was prepared by dispersing the following composition with a desk-top type ball mill so as to yield a 0.3 µm average particle size of eicosylphosphonic acid. [coating liquid for recording layer] 2-phenylamino-3-methyl-6-ethylamino-7-methylfluoran 10 parts Eicosylphosphonic acid 30 parts Polyvinylbutyral [refractive index 1.49] (Denkabutyral #3000-2 produced by Denka Kagaku Kbgyo Corp.) 15 parts Mixed liquid of toluene/methylethylketone (ratio 1/1) 285 parts A coating liquid for the protective layer was prepared by dispersing the following composition homogeneously. [coating liquid for protective layer] 75% of urethane acrylate ultraviolet curable resin solution in n-butylacetate [refractive index 1.56] (Unidick C7-157 produced by Dainihon Ink Kagaku Corp.) 100 parts Solution of 52% silicone resin in xylene (Byk-344 produced by Bic Ghemy Japan Corp.) 4 parts Ethylacetate 50 parts [Production of transparent thermal recording medium] The coating liquid for the recording layer was applied and dried on a 100µm HPJ polyester film (produced by Teijin Corp.) by a wire bar so as to yield a 8.0µm thickness of the applied coating layer, and thus forming the thermal recording layer. Further, the coating liquid for the protective layer was applied and dried on the thermal recording layer with a wire bar, and then hardened with a 80W/cm ultraviolet ray lamp to form a protective layer of about 6.0µm in thickness. Thus a transparent thermal recording medium was produced. Example 7 A coating liquid for the recording layer was prepared by dispersing the following composition with a desk-top type ball mill so as to yield a 0.3µm average particle size of octadecylphosphonic acid. [coating liquid for recording layer] 2-benzylamino-3-chloro-6-ethylamino-7-methylfluoran 10 parts Octadecylphosphonic acid 30 parts Polyvinylbutyral [refractive index 1.49] (Denkabutyral #3000-2 produced by Denka Kagaku Kogyo Corp.) 15 parts Mixed liquid of toluene/methylethylketone (ratio 1/1) 285 parts A coating liquid for the protective layer was prepared by dispersing the following composition homogeneously. [coating liquid for protective layer] 75% of urethane acrylate ultraviolet curable resin solution in n-butylacetate [refractive index 1.56] (Unidick C7-157 produced by Dainihon Ink Kagaku Corp.) 100 parts Solution of 52% silicone resin in xylene (Byk-344 produced by Bic Chemy Japan Corp.) 4 parts Ethylacetate 50 parts [Production of transparent thermal recording medium] The coating liquid for the recording layer was applied and dried on a 100 µm HPJ polyester film (produced by Teijin Corp.) by a wire bar so as to yield a 8.0µm thickness of the applied coating layer, and thus forming the thermal recording layer. Further, the coating liquid for the protective layer was applied and dried on the thermal recording layer with a wire bar, and then hardened with a 80W/cm ultraviolet ray lamp to form a protective layer of about 6.0µm in thickness. Thus a transparent thermal recording medium was produced. Example 8 A coating liquid for the recording layer was prepared by dispersing the following composition with a desk-top type ball mill so as to yield a 0.3µm average particle size of octadecylphosphonic acid. [coating liquid for recording layer] 2-(3',4'-dichlorophenylamino)-6-ethylamino-7-methylfluoran 10 parts Octadecylphosphonic acid 30 parts Polyvinylbutyral [refractive index 1.49] (Denkabutyral #3000-2 produced by Denka Kagaku Kogyo Corp.) 15 parts Mixed liquid of toluene/methylethylketone (ratio 1/1) 285 parts A coating liquid for the protective layer was prepared by dispersing the following composition homogeneously. [coating liquid for protective layer] 75% of urethane acrylate ultraviolet curable resin solution in n-butylacetate [refractive index 1.56] (Unidick C7-157 produced by Dainihon Ink Kagaku Corp.) 100 parts Solution of 52% silicone resin in xylene (Byk-344 produced by Bic Chemy Japan Corp.) 4 parts Ethylacetate 50 parts [Production of transparent thermal recording medium] The coating liquid for the recording layer was applied and dried on a 100µm HPJ polyester film (produced by Teijin Corp.) by a wire bar so as to yield a 80 µm thickness of the applied coating layer, and thus forming the thermal recording layer. Further, the coating liquid for the protective layer was applied and dried on the thermal recording layer with the wire bar, and then hardened with the 80W/cm ultraviolet ray lamp to form a protective layer of about 6.0µm in thickness. Thus a transparent thermal recording medium was produced. Example 9 A coating liquid for the recording layer was prepared by dispersing the following composition with a desk-top type ball mill so as to yield a 0.3 µm average particle size of eicosylphosphonic acid. [coating liquid for recording layer] 2-(o-chlorophenylamino)-6-n-octylaminofuran 10 prats Eicosylphosphonic acid 30 parts Styrene/maleic acid monoisobutyl ester-copolymer [refractive index 1.57, produced by Gifu Cerac Corp.] 15 parts Mixed liquid of toluene/methylethylketone (ratio 1/4) 285 parts A coating liquid for the protective layer was prepared by dispersing the following composition homogeneously. [coating liquid for protective layer] 75% urethane acrylate ultraviolet curable resin [refractive index 1.56] in n-butyl acetate solution (Unidick C7-157 produced by Dainihon Ink Kagaku Corp.] 100 parts Xylene solution of 52% silicone resin (Byk-344 produced by Byk Chemy Japan Corp.) 4 parts Colloidal silica gel (Mizucasil® P-527 produced by Mizusawa Kagaku Corp.) 20 parts Ethylacetate 50 parts [Production of transparent thermal recording medium] The coating liquid for the recording layer was applied and dried on a 75µm Melinex® 705 polyester film (produced by ICI Japan Inc.) by a wire bar so as to yield a 8.0µm thickness of the applied coating layer, and thus forming the thermal recording layer. Further, the coating liquid for the protective layer was applied and dried on the thermal recording layer with a wire bar, and then hardened with a 80W/cm ultraviolet ray lamp to form a protective layer of about 4.0 µm in thickness. Thus, a transparent thermal recording medium was produced. Control 1 A coating liquid for the recording layer was prepared by dispersing the following composition with a desk-top type ball mill so as to yield a 1.3 µm average particle size of octadecylphosphonic acid. [coating liquid for recording layer] 3-diethylamino-6-methyl-7-anilinofluoran 10 parts Octadecylphosphonic acid 30 parts Polyvinylbutyral [refractive index 1.49] (Denkabutyral #3000-2 produced by Denka Kagaku Kogyo Corp.) 15 parts Mixed liquid of toluene/methylethylketone (ratio 1/1) 285 parts [Production of thermal recording medium] The coating liquid for the recording layer was applied and dried on the 100 µm HPJ polyester film (produced by Teijin Corp.) by a wire bar, and thus a thermal recording medium was produced. Control 2 A transparent thermal recording medium according to Control 2 was prepared similarly to Example 1 except that polyvinylbutyral was replaced by a polyvinyl chloride-vinyl acetate copolymer [refractive index: 1.54] (UYHH: produced by Union Carbide Corp.). Control 3 A transparent thermal recording medium according to Control 3 was prepared similarly to the Example 1 except that polyvinylbutyral was replaced by saturated polyester Byron® 300 [refractive index: 1.56] (produced by Toy-obo Corp.). Control 4 A transparent thermal recording medium according to Control 4 was prepared similarly to Example 1 except that polyvinylbutyral was replaced by acryl resin Dianal® BR-85 [refractive index: 1.49] (produced by Mitsubishi Kasei Corp.). An energy of 0.7 W/dot and a pulse width of 0.5 msec was applied to the thermal recording media, which had been produced in the above-mentioned ways, by a printer using a thermal head of 8 dot/mm so as to record images on the media. Then the recorded images were evaluated by the following tests. [Color Tone] The color tone of each of the recorded images was visually inspected immediately after being recorded. [Transmission Density] The image density and the non-printed surface density for each of the recorded images were measured by a transparent densitometer X-Rite 310TR (produced by X-RITE COMPANY) operating in VISUAL mode. [Spectral Transmission Factor] Spectral transmission factors for a colored imaging portion and a non-imaging portion (non-printed surface) of the thermal recording media were measured by a spectrophotometer UV-3100 produced by Simazu Seisakusyo at spectral wavelengths of 380 nm, 440 nm and 550 nm. [Continuous Heat Resistance] After the thermal recording media were kept at 60° C in a dry environment for 24 hours, transmission rates for the color-imaged portion and the non-imaged portion of the thermal recording media were measured. Results of the above-mentioned tests are given in the following Table 1. Applications The films produced in the above-mentioned examples, in which the images were formed thereon with the thermal head were used for positive films (block copy films) for screen process printing, and thus blocks for the screen process printing were produced. Images were printed on the blocks with an easy mimeograph machine and the block copy films were evaluated as regards capability for printing. Furthermore, two block copy films, on which the same image had been formed, were superimposed and the capability for visual inspection of the superimposed images was evaluated. The following Table 2 illustrates results of the applications. Positive Film Sample Print Inspection Application 1 Example 1 YES YES Application 2 Example 2 YES YES Application 3 Example 3 YES YES Application 4 Example 4 YES YES Application 5 Example 5 YES YES Application 6 Example 6 YES YES Application 7 Example 7 YES YES Application 8 Example 8 YES a little bad Application 9 Example 9 YES YES Application 13 Control 1 NO NO Application 14 Control 2 NO NO Application 15 Controls 3 NO NO Application 16 Control 4 NO NO Therefore, the transparent thermal recording medium according to the present invention can be effectively used for the block copy film, on which images are formed, for plate-making, particularly, in photogravure, offset printing and screen process printing, because the transparent thermal recording medium has a difference in light transmission factors between the color-imaging portion and the non-imaging portion of not less than 50% at the wavelength ranging from 370 nm to 450 nm.

Claims: A transparent thermal recording medium comprising a thermal recording layer which is provided on a transparent layer and comprises an electron-donating chromophoric compound, an electron-accepting compound and a binder resin; and a further-provided protective layer having a refractive index approximately equal to the refractive index of said thermal recording layer and comprising a binder resin having a refractive index approximately equal to the refractive index of the binder resin forming said thermal recording layer, wherein said binder resin forming said thermal recording layer is a compound having a hydroxyl group and/or a carboxyl group and said electron-donating chromophoric compound is selected from the following fluoran compounds: wherein R 1 is selected from alkyl groups having equal to or less than 8 carbon atoms, R 2 is selected from a hydrogen atom and an alkyl group having equal to or less than 4 carbon atoms, and X represents a halogen atom selected from a fluorine atom, a chlorine atom and a bromine atom; wherein R 3 is selected from a hydrogen atom and an alkyl group having equal to or less than 8 carbon atoms, and R 4 is selected from alkyl groups having equal to or less than 8 carbon atoms; wherein R 5 and R 6 are selected from alkyl groups having equal to or less than 8 carbon atoms, and R 7 is selected from a hydrogen atom, a lower alkyl group and a lower alkoxy group: wherein R 8 represents a hydrogen atom, R 9 represents an alkyl group having equal to or less than 8 carbon atoms, R 10 is selected from a hydrogen atom, a lower alkyl group and a lower alkoxy group, R 11 is selected from a hydrogen atom and an alkyl group having equal to or less than 8 carbon atoms, and R 12 is selected from an alkyl group having equal to or less than 8 carbon atoms, a phenyl group and a substituted phenyl group: wherein R 13 represents an alkyl group having equal to or less than 8 carbon atoms, R 14 is selected from a methyl group and an ethyl group, R 15 is selected from a hydrogen atom and an alkyl group having equal to or less than 4 carbon atoms, and Y and Z are selected from halogen atoms such as fluorine atoms, chlorine atoms and bromine atoms; wherein R 16 represents an alkyl group having equal to or less than 8 carbon atoms, R 17 is selected from a methyl group and an ethyl group, R 18 is selected from an alkyl group having equal to or less than 4 carbon atoms and a halogen atom such as a fluorine atom, a chlorine atom and a bromine atom, and Ar is selected from a phenyl group and a benzyl group; 2-(N'-methyl-N-3'-chlorophenylamino)-6-ethylamino-7-methylfluoran, 2-(N-ethyl-N-3'-chlorophenylamino)-6-ethylamino-7-methylfluoran and 2-(N-methyl-N-4'-chlorophenylamino)-6-ethylamino-7-methylfluoran. The transparent thermal recording medium as claimed in claim 1, wherein the refractive indices of the binder resin forming said thermal recording layer and the binder resin forming said protective layer range from 1.45 to 1.60 at ordinary temperature. The transparent thermal recording medium as claimed in claim 1 or 2, wherein said electron-accepting compound is an organo phosphoric acid compound. The transparent thermal recording medium as claimed in claim 3, wherein said organic phosphoric acid compound is selected from phosphonic acid compounds of the following general formulae (I) and (II): wherein R is selected from linear alkyl groups having from 16 to 24 carbon atoms; and wherein R' is selected from linear alkyl groups having from 13 to 23 carbon atoms. A transparent thermal recording medium comprising a thermal recording layer provided on a transparent support, wherein said thermal recording layer consists essentially of an electron-donating chromophoric compound selected from fluoran compounds as defined in claim 1, an organo phosphoric acid compound and a binder resin having a refractive index ranging from 1.45 to 1.60 at ordinary temperature and including a hydroxyl group and/or a carboxyl group; and a protective layer provided on said thermal recording layer, said protective layer consisting essentially of a resin having a refractive index ranging from 1.45 to 1.60 at ordinary temperature. A thermal recording medium for a block copy, comprising: a transparent supporting member, and a thermal recording layer provided on said transparent supporting member, said thermal recording layer including an electron-donating chromophoric compound selected from fluoran compounds as defined in claim 1, an organic phosphoric compound and a binder resin having a refractive index ranging from 1.45 to 1.60, said binder resin including a hydroxyl group and/or a carboxyl group in a molecule thereof, and a protective layer provided on said thermal recording layer, said protective layer consisting essentially of resin having a refractive index similar to that of said binder resin at ordinary temperature, wherein the difference in light transmission factors between a color-producing imaging portion formed by thermal energy and a non-imaging portion is over 35% at a wavelength ranging from 350 nm to 470 nm. A thermal recording medium for a block copy, comprising: a transparent supporting member, and a thermal recording layer provided on said transparent supporting member, said thermal recording layer including an electron-donating chromophoric compound selected from fluoran compounds as defined in claim 1, an organic phosphoric compound and a binder resin having a refractive index ranging from 1.45 to 1.60, said binder resin including a hydroxyl group and/or a carboxyl group in a molecule thereof, and a protective layer provided on said thermal recording layer, said protective layer consisting essentially of resin having a refractive index similar to that of said binder resin at ordinary temperature, wherein the difference in light transmission factors between a color-producing imaging portion formed by thermal energy and a non-imaging portion is over 35% at a wavelength ranging from 380 nm to 440 nm.

Inventors: EMA HIDEAKI, GOTO HIROSHI, SAKAI KIYOSHI, EMA, HIDEAKI, GOTO, HIROSHI, SAKAI, KIYOSHI
Applicants: RICOH KK, RICOH COMPANY, LTD
Application year: 1995
Main classification: B41M5
IPC codes: B41M5