A thermal recording medium

Abstract: A thermally sensitive recording medium which comprises, in order on a substrate, an undercoat layer, a thermally sensitive colour developing layer and a protective layer, wherein (a) an ultraviolet absorbent is included in at least one of the undercoat layer and the thermally sensitive colour developing layer, (b) an ultraviolet screening agent, which consists of calcined particles of a flaky pigment with a refractive index of 1.5 to 1.6 and a surface coating comprising an insoluble cerium compound and amorphous silica, is included in at least one of the thermally sensitive colour developing layer and the protective layer, and (c) a fluorescent dye is included in the protective layer.

Description: This invention relates to a thermally sensitive recording medium which is superior in light resistance, good in appearance of the unrecorded portion, and prevented from adherence of depositions and occurrence of sticking. A thermally sensitive recording medium having a thermally sensitive color developing layer containing a colourless or pale coloured dye precursor and a developer which develops color by a momentary chemical reaction caused by heating with the sensitizer is disclosed in JP-A-45-14035 and put to widespread practical use. Thermal printers having thermal sensitive heads and so on are used to obtain recordings on such recording media. Thermal recording methods of this type are used widely for facsimiles, computers, various measures, labels and so on, with the progress of the information industry. Their advantages over conventional recording methods are that no noise is made in recording, developing and fixing are unnecessary, there is no maintenance, the devices are comparatively cheap and compact and the developed color is very clear. However, thermal recording media using this kind of thermally sensitive recording material have defects in the light resistance of the image portions and the background portions. That is, if they are exposed to sun light for a long time, the density of image portions is lowered and discoloured, the colour of the background portions changes, and these cause damage to the impressions. The production of thermally sensitive color developing layers containing zinc oxide, which is an inorganic filler having an ultraviolet screening effect, for the purpose of improvement of light resistance is disclosed in JP-A-62-18626. However, although the ultraviolet screening effect of an inorganic filler is large, the permeability of the visible range is small, color developed portions are hidden, and the record density is low. Therefore, there is the problem that the ultraviolet screening effect is lowered. It becomes inconvenient and uneconomical if the amount of use is decreased so as to preserve a sufficient record density. A method of making a protective layer which contains cerium oxide particles is disclosed in JP-A-H6-64324. However, it is difficult to regard this as a suitable material for a thermal recording medium since cerium oxide itself is coloured . JP-A-H7-25147 discloses a method of making a protective layer containing an ultraviolet screener having a triple structure, in which kernel particles of an inorganic pigment are coated with cerium oxide and the cerium oxide surface is coated with an inorganic pigment which is the same as or different from the inorganic pigment of the kernal particles. It is difficult to obtain sufficient light resistance by this method since the ultraviolet screening effect of cerium oxide is prevented although the color of cerium oxide is partially suppressed. On the other hand, a method of making a protective layer containing a benzotriazole-based hydrophobic ultraviolet absorbent is disclosed in JP-A-61-193883. The purpose is the improvement of light resistance. However, it has the disadvantage that adherence of depositions and sticking occur during recording when the protective layer contains a hydrophobic ultraviolet absorbent. On the other hand, in the case of using a soluble ultraviolet absorbent, there is the disadvantage that the thermally sensitive head can be eroded electrochemically since ions such as sodium and so on are formed in the solubilisation of a soluble ultraviolet absorbent, as disclosed in JP-A-H7-17131. In JP-A-62-176879 a benzotriazole compound is contained in an undercoat layer and a protective layer and even ultraviolet radiation from the back side is absorbed, but this method does not result in a thermally sensitive recording medium having sufficient quality since an ultraviolet absorbent in the protective layer causes adherence of depositions and sticking during recording. A method of making a protective layer containing a fluorescent brightener is disclosed in JP-A-62-184880, but the effect of the fluorescent brightener is produced mainly in the appearance of the unrecorded portion, and this is insufficient to improve light resistance. An object of this invention is to provide a thermally sensitive recording medium which overcomes conventional problems and is superior in light resistance, wherein adherence of depositions and the occurrence of sticking is prevented. It has now been surprisingly found that a thermal recording medium achieving the above object can be obtained by including a specific ultraviolet screener as well as an ultraviolet absorbent. Accordingly, the present provides a thermally sensitive recording medium which comprises, in order on a substrate, an undercoat layer, a thermally sensitive colour developing layer and a protective layer, wherein (a) an ultraviolet absorbent is included in at least one of the undercoat layer and the thermally sensitive colour developing layer, (b) an ultraviolet screening agent which consists of calcined particles of a flaky pigment with a refractive index of 1.5 to 1.6 and the surface of which is coated with a surface coating comprising an insoluble cerium compound and amorphous silica, is included in at least one of the thermally sensitive colour developing layer and the protective layer, and (c) a fluorescent dye is included in the protective layer. In the thermally sensitive recording medium of this invention, as the ultraviolet absorbent included in the undercoat layer or thermally sensitive colour developing layer, various well-known absorbents can be used. In a preferred aspect the recording medium of the invention comprises, in order on a substrate, an undercoat layer, a thermally sensitive colour developing layer and a protective layer, wherein (a) an ultraviolet absorbent is included in at least one of the undercoat layer and the thermally sensitive colour developing layer, (b) an ultraviolet screening agent, which consists of calcined particles of a flaky pigment with a refractive index of 1.5 to 1.6 and the surface of which is coated with a surface coating comprising an insoluble cerium compound and amorphous silica, is included in the thermally sensitive colour developing layer, and (c) a fluorescent dye is included in the protective layer. Examples of the hydrophobic ultraviolet absorbent are as follows: Benzophemone-based ultraviolet absorbent such as 2,4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone, 2-hydroxy-4-octyloxybenzophenone, 2-hydroxy-4-dodecyloxybenzophenone, 2,2'-dihydroxy-4-methoxybenzophenone, 2,2'-dihydroxy-4,4'-dimethoxybenzophenone, and 2-hydroxy-4-methoxy-5-sulfobenzophenone ; benzotriazole based ultraviolet absorbent such as 2-)2'-hydroxyphenyl)benzotriazole. 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, 22-(2'-hydroxy-5'-tert-butylphenyl)benzotriazole, 2-(2'-hydroxy-3',5'-di-tert-butylphenyl)benzotriazole, 2-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chlorobenzo triazole, 2-(2'-hydroxy-3',5'-di-tert-butylphenyl)-5-chlorobenzo triazole, 2-(2'-hydroxy-3',5'-di-tert-aminophenyl)benzotriazole, 2-(2'-hydroxy-3',5'-di-tert-butylphenyl)-5-tert-butylbenzo triazole, 2-(2'-hydroxy-3'-dodecyl-5'-methylphenyl)benzotriazole, 2-[2'-hydroxy-4'-(2"-ethylhexyl)oxyphenyl]benzotriazole, condensation product with methyl-3-[3-tert-butyl-5-(2H-benzotriazole-2-yl)-4-hydroxyphenyl]propionate-polyethyleneglycol (molecular weight is about 300), 5-tert-butyl-3-(5-chloro-2H-benzotriazole-2-yl)-4-hydroxy benzene-propionate octyl, 2, 2-methylenebis [4- (1, 1, 3, 3, -tetramethylbutyl)-6-(2H-benzo triazole-2-yl)phenol], 2-(2'-hydroxy-3'-sec-butyl-5'-tert-butylphenyl)-5-tert-butylbenzotriazole and so on, Salicylic acid-based ultraviolet absorbents such as phenylsalicylate, p -tert-butylphenylsalicylate, p -octylphenylsalicylate and so on, Cyanoacrylate-based ultraviolet absorbents such as 2-ethylhexyl-2-cyano-3,3'-diphenylacrylate, ethyl-2-cyano-3,3'-diphenylacrylate and so on, and hindered amine-based ultraviolet absorbents such as bis(2,2,6,6,-tetramethyl-4-piperidyl)sebacate, succinate-bis(2, 2, 6, 6, -tetramethyl-4-piperidyl)ester, 2-(3,5-di-tert-butyl)malonate-bis (1, 2, 2, 6, 6, -pentamethyl-4-piperidyl)ester and so on. Examples of the soluble ultraviolet absorbent are as follows: benzophenone-based ultraviolet absorbents such as 2-hydroxy-4-methoxybenzophenone-5-sulfonate, 2-hydroxy-4-methoxybenzophenone-5-sulfonate sodium, 2-hydroxy-4-methoxybenzophenone-5-sulfonate kalium, 2, 2'-dihydroxy-4, 4'-dimethoxybenzophenone-5-sulfonate sodium, 2,4-dihydroxybenzophenone-5-sulfonate sodium, 2,2'-dihydroxy-4,4'-dimethoxybenzophenone-5,5'-disulfonate sodium, 2,4-dihydroxybenzophenone-5'-sulfonate sodium, 2, 2', 4, 4'-tetrahydroxybenzophenone-5, 5'-disulfonate sodium and so on, and benzotriazole-based ultraviolet absorbents such as 2-(2'-hydroxy-4'-methoxy-5'-sulfophenyl)benzotriazole sodium salt, 2-(2'-hydroxy-4'-butoxy-5'-sulfophenyl)benzotriazole sodium salt and so on. Benzotriazole-based ultraviolet absorbents amongst these having the most effective light resistance are preferably used. They may, if desired, be used as mixtures of two or more, in accordance with requirement. The use of 2,2-methylenebis[4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazole-2-yl)phenol] is particularly preferred since colouring is slight and heat resistance of the thermal recording mass is favourable. The amount of ultraviolet absorbent to be used is typically about 0.1 wt.% to 15 wt.% based on the dry mass of the undercoat layer or thermally sensitive color developing layer, and more desirably 1 wt.% to 10 wt.% based on the dry mass of the undercoat layer and 2 wt.% to 10 wt.% based on the dry mass of the thermally sensitive color developing layer. When the amount of ultraviolet absorbent to be used in the undercoat layer or thermally sensitive developing layer is less than 0.1 wt.% the effect in light resistance is low, and when it is more than 15 wt.% the sensitivity for color developing and image shelf life become low. In the thermal recording medium of this invention, the ultraviolet screener to be included in the thermally sensitive color developing layer or in the protective layer should desirably be high in transparency and small in hiding nature in order to achieve good color developing density. Calcined particles of a flaky pigment with a refractive index of 1.5 to 1.6 and the surfaces of which are coated with an insoluble cerium compound and with amorphous silica, and which have triple structure, are therefore used. Such an ultraviolet screener is disclosed in JP-A-H6-145645 and is well-known. To produce this ultraviolet screener, 1 to 30 wt.% of cerium salt water solution based on the pigment as CeO 2 is dropped into a water dispersion of a flaky pigment under heating, the pH value is adjusted to 7 to 9, and the insoluble cerium compound deposits on the surface of said pigment and forms a coated pigment. Second, silicate solution is added to the water dispersion of this cerium coated pigment under heating, the pH value is adjusted to 6 to 8, amorphous silica is deposited and covered on the surface of the said pigment, and it is then calcined at a high temperature of more than 200°C. The amount of deposited amorphous silica is 2 to 40 wt.% based on the cerium coated pigment as SiO 2 . The pigment to be used as the kernel is flaky, and has a refractive index of 1.5 to 1.6. As such a pigment, mica, talc, sericite, aluminium hydroxide, calcium carbonate, kaoline, calcium hydroxide, aluminium silicate, polyethylene powder, polystyrene, latex and so on can be mentioned. A thermally sensitive recording medium whose color developing density is particularly favourable can be obtained, for instance, by using mica, talc and sericite. Cerium chloride, cerium nitrate, cerium sulfate and so on can be used as the cerium compound, and organic silicate or an inorganic salt such as sodium silicate and so on can be used as silicate. The amount of ultraviolet screener to be used in the protective layer is desirably 5 wt.% to 40 wt% based on its dry mass. When it is less than 5 wt%, the light resistance is low, and when it is more than 40 wt.% the barrier nature and sensitivity for color developing of the protective layer becomes lower. The amount of ultraviolet screener to be used in the thermally sensitive color developing layer desirably is from 10 wt% to 40 wt% based on its dry mass. When it is less than 10 wt%, the effect in light resistance is low, and when it is more than 40 wt%, sensitivity for color developing decreases. Furthermore, in the thermal recording medium of this invention the fluorescent dye included in the protective layer also effects an improvement in the light resistance. As the fluorescent dye various well-known ones can be used, such as derivatives of stilbene, derivatives of coumarin, derivatives of pyrazoline, derivatives of bisstyrylbiphenyl, derivatives of naphthalimide, derivatives of bisbenzooxazolyl and so on. Derivatives of diaminostilbene disculfonate are particularly desirably used since their light resistance is high. The amount of fluorescent dye to be used is desirably 0.01 to 3 wt.% based on the dry mass of the protective layer, and more desirably 0.1 to 2 wt.%. When the amount of fluorescent dye in the protective layer is less than 0.01 wt.%, the light resistance decreases and when it is more than 3 wt.% the coloring of the protective layer intensifies and this leads to a fall in the background color of the recording medium. The ultraviolet screener used in this invention has a characteristic triple structure and can retain sufficient brightness. Undesired characteristics such as background fogging, desensitization and others are not observed if the screener is contained in the thermally sensitive color developing layer. Furthermore, when included in the protective layer it allows the thermally sensitive recording medium to have a writing nature. The said ultraviolet screener can also be included in the undercoat layer. However, the required light resistance cannot be obtained if the thermally sensitive recording medium includes only the said ultraviolet screener. This is probably because the cerium compound is coated with amorphous silica which suppresses the ultraviolet screening effect of the cerium compound. The present invention therefore includes an ultraviolet absorbent in addition to the said ultraviolet screener and fluorescent dye, resulting in a thermally sensitive recording medium having superior light resistance. Specifically, in the case of including an ultraviolet absorbent in the undercoat layer and an ultraviolet absorbent and ultraviolet screener in the thermally sensitive color developing layer, high light resistance can be obtained. In the case of including an ultraviolet absorbent and ultraviolet screener in the thermally sensitive color developing layer, it is preferred that the ultraviolet absorbent and ultraviolet screener be contained in a ratio of 1:4 to 4:1. In this invention it is believed that including an ultraviolet absorbent in the undercoat layer or thermally sensitive color developing layer not only absorbs ultraviolet from the reverse side of the thermally sensitive recording medium but also enlarges the ultraviolet absorbability by synergy with the ultraviolet screener and fluorescent dye in the thermally sensitive color developing layer or protective layer. In the case of including an ultraviolet absorbent in the protective layer in place of the undercoat layer, for example, the melting point of the benzotriazole-based ultraviolet absorbent has a low value such as 120 to 150°C, it melts and causes depositions and so on in heating with a thermally sensitive head. Consequently the inclusion of an ultraviolet absorbent in the undercoat the layer or thermally sensitive color developing layer and the inclusion of an ultraviolet screener in the thermally sensitive color developing layer or protective layer is an effective means of improving the light resistance and preventing the adherence of depositions and sticking. To obtain the thermally sensitive recording medium of this invention, for example, the ultraviolet absorbent consisting of the benzotriazole compound is dispersed with binder and filler, undercoat layer coating fluid is prepared, coated on a substrate and dried, and an undercoat layer is formed. Secondly, a dispersion containing dye and color developer is mixed, additives such as filler and so on are added, thermally sensitive color developing layer coating fluid is prepared, coated and dried on the above undercoat layer, and a thermally sensitive color developing layer is formed. A protective layer coating fluid including necessary additives such as the ultraviolet screener as defined above, fluorescent dye, soluble high molecule, filler and so on are prepared, coated and dried on the above thermally sensitive color developing layer, and a protective layer is formed. In the undercoat layer used in this invention, an emulsion of a soluble high molecule or a hydrophobic high molecule can be used as binder. For example emulsions of a soluble high molecule such as polyvinyl alcohol, polyvinylacetal, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose and so on, starch, its derivatives, polyacrylate soda, polyvinylpyrrolidone, acrylate amide/acrylate ester copolymer, acrylate amide/acrylate ester/methacrylic acid copolymer, styrene/maleic anhydride copolymer alkali salt, isobutylene/maleic anhydride copolymer alkali salt, polyacrylamide, alginate soda, gelatin, casein and so on, and hydrophobic high molecule such as polyvinyl acetate, polyurethane, styrene/butadiene copolymer, polyacrylate, polyacrylate ester, vinyl chloride/vinyl acetate copolymer, polybutyl methacrylate, ethylene/vinyl acetate copolymer, styrene/butadiene/acrylic copolymer, and so on can be used. As a filler in the undercoat layer, well-known conventional organic fillers such as styrene-methacryl copolymer resin, urea-formaldehyde resin, polystyrene, and so on in addition to inorganic fillers such as calcium carbonate, silica, zinc oxide, titanium oxide, aluminium hydroxide, magnesium hydroxide, baked kaolin, clay, talc, and so on can be used. In the protective layer used in this invention, various well-known fillers can be used. Inorganic pigments such as kaolin, clay, calcium carbonate, baked clay, baked kaolin, aluminium hydroxide, titanium oxide, diatomaceous earth, fine silica anhydride, activated clay, and so on, organic pigment such as styrene microball, nylon powder, polyethylene powder, urea-formaldehyde, resin filler, raw starch particle, and so on are examples. As a binder in the protective layer, any high molecular compound which is conventional in the field of thermal recording can be used. For example, starch, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, gelatin, casein, gum arabic, polyvinyl alcohol, carboxy-denatured polyvinyl alcohol, acetoacetyl group denatured polyvinyl alcohol, silicon denatured polyvinyl alcohol, polyvinyl acetate, vinyl chloride/vinyl acetate copolymer, ethylene/vinyl acetate copolymer, acrylonitrile-butadiene-styrene copolymer, vinyl acetate-acrylic acid copolymer, ethylene/acrylic acid copolymer, styrene/acrylic acid copolymer, acrylate resin, acrylemulsion, diisobutylene/maleic anhydride copolymer, styrene/maleic anhydride copolymer, styrene/butadiene copolymer emulsion, methyl methacrylate/butadiene copolymer, methyl methacrylate-styrene-butadiene copolymer, styrene polymer, isoprene polymer, butadiene polymer, vinyl chloride polymer, vinylidene chloride polymer, urea resin, melamine resin, amide resin, polyurethane resin, and so on can be mentioned. In the protective layer it is also possible to add, as occasion demands, various auxiliaries. Examples include lubricants such as zinc stearate, calcium stearate, polyethylene wax, carnauba wax, parafin wax, ester wax and so on, surface active agents (dispersant, humectant) such as dioctylsulfosuccinate sodium and so on, anti-foaming agents, and soluble multivalent metal salt such as potassium alum, aluminium acetate and so on. It is also possible to use a hardener such as glyoxal, boric acid, dialdehyde starch, or an epoxy based compound for greater water resistance. In the thermal recording medium of this invention, as the colorless or pale colored dye precursor contained in the thermally sensitive color developing layer, various well-known ones can be used, for example, blue color developing dye such as 3, 3-bis ( p -dimethylaminophenyl)-6-dimethylaminophthalide, 3-(4-diethylamino-2-methylphenyl)-3-(4-dimethylaminophenyl)-6-dimethylaminophthalide, 3-diethylamino-7-dibenzylamino-benzo[a]fluoran and so on, green color developing dye such as 3-(N-ethyl-N- p -tolyl)amino-7-N-methylanilinofluoran, 3-diethylamino-7-anilinofluoran, 3-diethylamino-7-dibenzylaminofluoran and so on, red color developing dye such as 3,6-bis(diethylamino)fluoran-γ-anilinolactam, 3-cyclohexylamino-6-chlorofluoran, 3-diethylamino-7-chlorofluoran, rhodamine( o -chloroanilino)lactam, rhodamine( p -chloroanilino)lactam, 3-diethylamino-7,8-benzofluoran, 3-(N-ethyl- p -toluidino)-7-methylfluoran, 3-diethylamino-6,8-dimethylfluoran and so on, black color developing dye such as 3-(N-ethyl-N-isoamyl)amino-6-methyl-7-anilinofluoran, 3-(N-methyl-N-cyclohexyl)amino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-anilinofluoran, 3-di( n -butyl)amino-6-methyl-7-anilinofluoran, 3-di( n -pentyl)amino-6-methyl-7-anilinofluoran, 3-diethylamino-7-( o -chlorophenylamino)fluoran, 3-di( n -butyl)amino-7-( o -chlorophenylamino)fluoran, 3-diethylamino-7-( o -fluorophenylamino)fluoran, 3-di( n -butyl)amino-7-( o -fluorophenylamino)fluoran, 3-(N-ethyl- p -toluidino)-6-methyl-7-anilinofluoran, 3-(N-ethyl- p -toluidino)-6-methyl-7-( p -toluidino)fluoran, 3-diethylamino-6-chloro-7-anilinofluoran, 3-(N-methyl-N- n -propylamino)-6-methyl-7-anilinofluoran, 3-dimethylamino-6-methyl-7-anilinofluoran, 3-dibutylamino-6-methyl-7- m -toluidinofluoran, 3-(N- n -hexyl-N-ethyl)amino-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isobutyl)amino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7- p -ethoxyanilinofluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-piperidino-6-methyl-7-anilinofluoran, 2,2-bis{4-[6'-(N-cyclohexyl-N-mehtylamino)-3'-methylspiro [phthalide-3,9'-xanthene-2'-ylamino]phenyl}propane, 2, 4-dimethyl-6-[(4-dimethylamino)anilino]-fluoran, 3-diethylamino-7-(3'-trifluoromethylphenyl)aminofluoran, 3-dipentylamino-7-(3'-trifluoromethylphenyl)anilinofluoran and so on, and dye having an absorption wavelength in near infrared ranges such as 3, 3-bis[1-(4-methoxyphenyl)-1-(4-dimethylaminophenyl) ethylene-2-yl]-4, 5, 6, 7-tetrachlorophthalide, 3, 3-bis[1-(4-methoxyphenyl)-1-(4-pyrrolidinophenyl)ethylene-2-yl]-4, 5, 6, 7-tetrachlorophthalide, 3, 3-bis [1, 1-bis (4-pyrrolidinophenyl) ethylene-2-yl]-4,5,6,7-tetrabromophthalide, 3- p -( p -dimethylaminoanilino)anilino-6-methyl-7-chloro fluoran, 3- p -( p -chloroanilino)anilino-6-methyl-7-chlorofluoran, and 3, 6-bis(dimethylamino)fluorene-9-spiro-3'-[6'-dimethylamino] phthalide. Such dyes may be used in mixtures of two or more, in accordance with requirements, but 3-di( n -butyl)amino-6-methyl-7-anilinofluoran is preferred for reasons of economy. As the color developer used to make the dye precursor contained in the thermally sensitive color developing layer of this invention, well-known color developers can be mentioned as follows. Inorganic acidic substances, such as activated clay, attapulgite, colloidal silica, aluminium silicate and so on, phenolic compounds, such as 4, 4'-isopropylidenediphenol, 1, 1-bis (4-hydroxyphenyl) cyclohexane, 2, 2-bis (4-hydroxyphenyl)-4-methylpentane, 4,4'-dihydroxydiphenylsulfide, hydroquinone monobenzylether, 4-hydroxybenzylbenzoate, 4,4'-dihydroxydiphenylsulfone, 2,4'-dihydroxydiphenylsulfone, 4-hydroxy-4'-isopropoxydiphenylsulfone, 4-hydroxy-4'- n -propoxydiphenylsulfone, bis(3-allyl-4-hydroxyphenyl)sulfone, 4-hydroxy-4'-methyldiphenylsulfone, 4-hydroxyphenyl-4'-benzyloxyphenylsulfone, 3,4-dihydroxyphenyl-4'-methylphenylsulfone, bis(4-hydroxyphenylthioethoxy)methane, 1,5-di(4-hydroxyphenylthio)-3-oxapentane, bis( p -hydroxyphenyl)butylacetate, bis( p -hydroxyphenyl)methylacetate, 1,1-bis(4-hydroxyphenyl)-1-phenylethane. 1,4-bis [α-methyl-α-(4'-hydroxyphenyl)ethyl]benzene, 1,3-bis[α-methyl-α-(4'-hydroxyphenyl)ethyl]benzene, di(4-hydroxy-3-methylphenyl)sulfide, 2,2'-thiobis(3-tert-octylphenol), 2,2'-thiobis(4-tert-octylphenol) and so on, thiourea compound such as N,N'-di- m -chlorophenylthiourea and so on, aromatic carboxylic acid such as p -chlorobenzoate, 4-[2-( p -methoxyphenoxy)ethyloxy]salicylate, 4-[3-( p -tolylsulfonyl)propyloxy]salicylate, 5-[ p -(2- p -methoxyphenoxyethoxy)cumyl]salicylate and so on, salts of these aromatic carboxylic acids with multivalent metals such as zinc, magnesium, aluminium, calcium, titanium, manganese, tin, nickel and so on, and organic acidic substances such as the antipyrine complex of zinc thiocyanate, and the compound zinc salt of tetraphthalaldehyde acid with other aromatic carboxylic acids, 4-hydroxy-4'-isopropoxydiphenylsulfone and bis(3-allyl-4-hydroxyphenyl)sulfone are particularly preferred since a recording image which is superior in general preservability can be obtained. The amount of dye precursor and color developer to be used is selected according to the kind of dye precursor and color developer used, but 1 to 50 parts, preferably about 2 to 10 parts of color developer based on 1 part of dye precursor is typically used. A sensitizer may be added to the thermally sensitive color developing layer depending on the purpose. Examples of the sensitizer include amide stearate, methoxycarbonyl-N-benzamide stearate, N-benzoylamide stearate, N-amide eicosanoate, ethylenebisamide stearate, amide behenate, methylenebisamide stearate, methylol amide, N-mehtylol amide stearate, dibenzyl terephthalate, dimethyl terephthalate, dioctyl terephthalate, p -benzyloxybenzyl benzoate, 1-hydroxy-2-phenyl naphthoate, oxalic acid dibenzyl, oxalic acid-di- p -methylbenzyl, oxalic acid-di- p -chlorobenzyl, 2-naphthylbenzylether, m -tarphenyl, p -benzylbiphenyl, tolylbiphenylether, di( p -methoxyphenoxyethyl)ether, 1,2-di (3-methylphenoxy) ethane, 1,2-di (4-methylphenoxy)ethane, 1,2-di(4-methoxyphenoxy)ethane, 1,2-di (4-chlorophenoxy) ethane, 1,2-diphenoxyethane, 1-(4-methoxyphenoxy)-2-(2-methylphenoxy)ethane, p -methylthiophenylbenzylether, 1,4-di(phenylthio)butane, p -acetotoluidide, p -acetophenetidide, N-acetoacetyl- p -toluidine, di( β -biphenylethoxy)benzene, p -di(biphenyloxyethoxy)benzene, and 1-isopropylphenyl-2-phenylethane. It is possible to include various pigments in the thermally sensitive color developing layer. Examples include inorganic pigments such as kaolin, clay, calcium carbonate, baked clay, baked kaolin, aluminium hydroxide, titanium oxide, diatomaceous earth, fine silica anhydride and activated clay, and organic pigments such as styrene microball, nylon powder, polyethylene powder, urea-formaldehyde resin filler and raw starch particles. Various auxiliaries can be added to the thermally sensitive color developing layer in accordance with requirement, for example dispersants such as dioctylsulfosuccinate sodium, dodecylbenzenesulfonate sodium, lauryl alcohol sulfuric ester sodium and fatty acid metal salts, wax such as zinc stearate, calcium stearate, polyethylene wax, carnauba wax, paraffin wax and ester wax, anti-foaming agents and colored dyes. It is also possible to add shelf life improvement agents to the thermally sensitive color developing layer. As such shelf life improvement agents examples include hindered phenol compounds such as 2,2'-methylenebis(4-methyl-6-tert-butylphenol), 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), 2,2'-ethylidene bis(4,6-di-tert-butylphenol), 4, 4'-thiobis(2-methyl-6-tert-butylphenol), 4, 4'-butylidene bis(6-tert-butyl- m -cresol), 1-[α-methyl-α-(4'-hydroxyphenyl)ethyl]-4-[α',α'-bis(4"-hydroxyphenyl) ethyl] benzene, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenol)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 4,4'-thiobis(3-methylphenol), 4,4'-dihydroxy-3,3',5,5'-tetrabromodiphenylsulfone, 4,4'-dihydroxy-3,3'-5,5'-tetramethyldiphenylsulfone, 2,2-bis(4-hydroxy-3,5-dibromophenyl) propane, 2,2-bis(4-hydroxy-3,5-dichlorophenyl)propane, and 2,2-bis(4-hydroxy-3,5-dimethylphenyl)propane, epoxy compounds such as 1,4-diglycidyloxybenzene, 4,4'-diglycidyloxydiphenylsulfone, 4-benzyloxy-4'-(2-methylglycidyloxy)diphenylsulfone, diglycidyl terephthalate, cresolnovolac type epoxy resin, phenolnovolac type epoxy resin, bisphenol A type epoxy resin, N, N'-di-2-naphthyl- p -phenylenediamine, salts of sodium or a multivalent metal with 2,2'-methylenebis (4, 6-di-tert-butylphenol) phosphate, and bis(4-ethyleneiminocarbonylaminophenyl)methane. In the production of the thermally sensitive recording medium of this invention, the coat layer, thermally sensitive color developing layer and protective layer are produced by a method wherein a coating fluid for the recording layer is coated and dried on a substrate by a suitable coating method, for instance air knife coating, baribar blade coating, pure blade coating, rod blade coating, short dwell coating, curtain coating or dry coating, and a coating fluid for the protective layer is coated and dried on the recording layer. Examples of suitable substrate materials include paper, plastic film, synthetic paper, non-woven cloth and depositing metal. The amount of coating fluid for the undercoating layer to be coated is about 2 to 12 g/m 2 , preferably 3 to 10g/m 2 in dry mass. The amount of coating fluid for undercoating layer to be coated is about 2 to 12g/m 2 , preferably 3 to 10g/m 2 in dry mass. The amount of coating fluid for the recording layer to be coated is about 2 to 12 g/m 2 , preferably 3 to 10 g/m 2 in dry mass. The amount of coating fluid for the protection layer to be coated is about 0.1 to 20 g/m 2 , preferably 0.5 to 10 g/m 2 . The present invention is further illustrated by following Examples. EXAMPLES In the Examples and Comparative Examples, the term "parts" means "parts by weight". <Producing of ultra violet screener> 500 g of flaky pigment that is a kernel substance of ultra violet screener used in this invention is dispersed to 10l of water, and 264 g of cerium nitrate solution is dropped into it with agitation and heating at 80°C. And, pH value is adjusted to 7 to 9 by aqueous solution of sodium hydroxide, cerium hydroxide is deposited on the pigment surface, then the pigment coated with the cerium compound is obtained by drying and crushing it. Secondly, after dispersing this pigment coated with cerium into 10l of water, 348g of No.3 sodium silicate is added with agitation and by heating to more than 80°C, pH value is adjusted to 6 to 8 with sulfuric acid, and the pigment coated with the cerium compound and silica is obtained by drying and crushing it. Further, it is calcinated at 500°C for 2 hours to obtain the ultra violet screener. <Producing of thermal recording medium> Example 1 (1) Forming of an undercoat layer Solution A (dispersion of ultra violet absorbent 1) 2-(2'-hydroxyphenyl-5'-methylphenyl) benzotriazol 5 parts 3% polyvinyl alcohol water solution 1 part water 20 parts Each solution of above-mentioned compound is ground to average particle diameter of 1µm with a sand grinder. And these dispersion are mixed together by following ratio and the coating fluid is obtained. Solution A 26 parts 10% polyvinyl alcohol water solution 150 parts calcinated kaolin (40% dispersion) 250 parts Above-mentioned coating fluid is coated on one surface of 50g/m 2 of substrate in 5.0g/m 2 amount and form an undercoat layer. (2) Forming of a thermal sensitive color developing layer. Solution B (dispersion of color developer) 4-hydroxy-4'-isopropoxydiphenylsulfone 6 parts 10% polyvinyl alcohol water solution 18.8 parts water 11.2 parts Each solution of above-mentioned compound is ground to average particle diameter of 1µm with a sand grinder. Solution C (dispersion of dye) 3-di(n-butyl)amino-6-methyl-7-anilinofluoran 2.0 parts 10% polyvinyl alcohol water solution 4.6 parts water 2.6 parts Each solution of above-mentioned compound is ground to average particle diameter of 1µm with a sand grinder. And these dispersion are mixed together by following ratio and the coating color is obtained. Solution B 36.0 parts Solution C 9.2 parts kaolin clay (50% dispersion) 12.0 parts Above-mentioned coating fluid is coated on one surface of 50g/m 2 of substrate in 6.0g/m 2 amount and form an undercoat layer. (3) Forming of a protective layer Dispersion are mixed together by following ratio and the coating color is obtained. 10% polyvinyl alcohol water solution 60.0 parts aluminium hydroxide (50% dispersion) 30.0 parts zinc stearate 10.0 parts 30% dispersion of ultra-violet screener (kernel pigment is flaky cericite whose refrective index is 1.556) 30.0 parts water 50.0 parts fluorescent dye (diaminostilbene disulfonate derivative ; Chinopal ABP liq. Chiba Gaigie) 0.5 parts After each above-mentioned coating color is coated and dried on the recording layer in 4.0g/m 2 of coating amount, processed by a super calender to obtain thermal recording medium (this is an example which includes ultra violet absorbent in an undercoating layer, and includes ultra violet screener and fluorescent dye in a protective layer). Example 2 In the preparation of a protective layer of Example 1, using a same procedure as disclosed in Example 1 except that the ultra violet screener is replaced with a compound of which, kernel pigment is flaky talc having 1.549 refractive index, a thermal recording medium is obtained. Example 3 In the preparation of a protective layer of- Example 1, using a same procedure as disclosed in Example 1 except that the ultra violet screener is replaced with a compound of which kernel pigment is flaky mica having 1.578 reflactive index, a thermal recording medium is obtained. Example 4 In the preparation of an undercoat layer of Example 1, using a same procedure as disclosed in Example 1 except that solution A is replaced with solution D, a thermal recording medium is obtained. Solution D (dispersion of ultra violet absorbent 2) 2-(2'-hydroxyphenyl-5'-methylphenyl) benzotriazol 15 parts 3% polyvinyl alcohol water solution 3 part water 60 parts Each solution of above-mentioned compound is ground to average particle diameter of 1µm with a sand grinder. And these dispersion are mixed together by following ratio and the coating color is obtained. Solution D 78 parts 10% polyvinyl alcohol water solution 150 parts calcinated kaolin (40% dispersion) 250 parts Example 5 In the preparation of a protective layer of Example 1, using a same procedure as disclosed in Exampl-e 1 except altering the amount of ultra violet screener to 60 parts, a thermal recording medium is obtained. Example 6 In the preparation of a protective layer of Example 1, using a same procedure as disclosed in Example 1 except altering the amount of fluorescent dye 1 part, a thermal recording medium is obtained. Example 7 In the preparation of a thermal sensitive color developing layer of Example 1, dispersions are mixed together by following ratio and the coating color is obtained. Solution B 36.0 parts Solution C 9.2 parts ultra violet screener of Example 1 (30% dispersion) 13.3 parts kaolin clay (50% dispersion) 12.0 parts In the preparation of a protective layer of Example 1, using a same procedure as disclosed in Example 1 except not mixing the dispersion ultra violet screener, a thermal recording medium is obtained (includes ultra violet absorbent in an undercoating layer, ultra violet screener in thermal sensitive color developing layer and fluorescent dye in a protective layer). Example 8 In the preparation of a thermal sensitive color developing layer of Example 7, Solution E (ultra violet absorbent dispersion 3) 22'-methylenebis[4-(1,1,3,3,-tetramethylbutyl) -6-(2H-benzotriazol-2-yl)phenol] 1.5 parts 10% polyvinyl alcohol water solution 3.0 parts water 6.0 parts further, except to add 7.8 parts of above-mentioned Solution E, using a same procedure as disclosed in Example 7, a thermal recording medium is obtained (includes ultra violet absorbent in an undercoating layer, ultra violet absorption and screener in thermal sensitive color developing layer and fluorescent dye in a protective layer). Comparative Example 1 In the preparation of an undercoat layer of Example 1, using a same procedure as disclosed in Example 1 except not using solution A (dispersion of ultra violet absorbent 1) and not using dispersion of ultra violet screener and fluorescent dye, a thermal recording medium is obtained (nothing is included). Comparative Example 2 In the preparation of an undercoat layer of Example 1, using a same procedure as disclosed in Example 1 except not mixing dispersion of ultra violet screener and fluorescent dye, a thermal recording medium is obtained (just ultra violet absorbent alone is included in an undercoat layer). Comparative Example 3 In the preparation of an undercoat layer of Example 1, using a same procedure as disclosed in Example 1 except not mixing solution A (dispersion of ultra violet absorbent 1), mixing 7.8 parts of Solution E of Example 8 (ultra violet absorbent dispersion 3) in the preparation of a thermal sensitive color development layer and not mixing dispersion of ultra violet screener and fluorescent dye in the preparation of a protective layer a thermal recording medium is obtained (just ultra violet absorbent alone is included in a thermal sensitive color developing layer). Comparative Example 4 In the preparation of an undercoat layer of Example 7, using a same procedure as disclosed in Example 7 except not mixing solution A (dispersion of ultra violet absorbent 1), and not mixing fluorescent dye in the preparation of a protective layer a thermal recording medium is obtained (just ultra violet screener alone is included in a thermal sensitive color developing layer). Comparative Example 5 In the preparation of an undercoat layer of Example 1, using a same procedure as disclosed in Example 1 except not mixing solution A (dispersion of ultra violet absorbent 1), and not mixing fluorescent dye in the preparation of a protective layer a thermal recording medium is obtained (just ultra violet screener alone is included in a protective layer). Comparative Example 6 In the preparation of an undercoat layer of Example 1, using a same procedure as disclosed in Example 1 except not mixing solution A (dispersion of ultra violet absorbent 1), and not mixing ultra violet screener in the preparation of a protective layer a thermal recording medium is obtained (just fluorescent dye alone is included in a protective layer). Comparative Example 7 In the preparation of a protective layer of Example 1, using a same procedure as disclosed in Example 1 except not mixing ultra violet screener and mixing 48 parts of dispersion of aluminium hydroxide a thermal recording medium is obtained (ultra violet absorbent is included in an undercoat layer and fluorescent dye is included in a protective layer). Comparative Example 8 In the preparation of a protective layer of Example 1, using a same procedure as disclosed in Example 1 except not mixing fluorescent dye a thermal recording medium is obtained (ultra violet absorbent is included in an undercoat layer and ultra violet screener is included in a protective layer). Comparative Example 9 In the preparation of an undercoat layer of Example 1, using a same procedure as disclosed in Example 1 except not mixing solution A (dispersion of ultra violet absorbent 1) a thermal recording medium is obtained (ultra violet absorbent and fluorescent dye is included in a protective layer). Comparative Example 10 In the preparation of an undercoat layer of Example 1, except mixing 26 parts of Solution A (dispersion of ultra violet absorbent 1) instead of solution of ultra violet screener and 48 parts of dispersion of aluminium hydroxide, a thermal recording medium is obtained (ultra violet absorbent is included in an undercoat layer, and ultra violet absorbent and fluorescent dye is included in a protective layer). Comparative Example 11 In the preparation of an undercoat layer of Example 1, except mixing dispersion of zinc oxide (30%) instead of solution of ultra violet screener, a thermal recording medium is obtained (ultra violet absorbent is included in an undercoat layer, and zinc oxide and fluorescent dye is included in a protective layer). Note (1) color developing feature : The density of an image recorded by thermal sensitive facsimile KB-4800 (manufactured by Toshiba) by 18.03 impressive energy and 3. 2 second pulse width is measured by Macbeth densitometer (RD-914 with umber filter). Note (2) light resistance survival rate ; The thermal recording medium printed by the above-mentioned condition is left in a fade-o-meter (BH type manufactured by Toyo Seiki) for 12 hours and the density of printed portion is measured by Macbeth densitometer, and survival rate is culculated by following numerical formula. survival rate = Da Dn × 100% wherein, Da=density of image portions after light resistant test Dn=density of image portions not processed Note(3) Light resistance ground color ; The thermal recording medium printed in the above-mentioned condition is 12H processed, and the density of not printed portions is measured by Macbeth densitometer (blue filter). Note(4) Adherence of depositions ; Using Panafax UF-60 (manufactured by Panasonic) by copy mode, vertical striped manuscript of B4 is printed and judged visually. o ○ Very small O Small Δ Considerable Note(5) Sticking ; Using Panafax UF-22 (manufactured by Panasonic) by sending mode, printed is judged visually. o ○ Quiet O High noise × Sticking pattern appears in the image Note(6) appearance of unrecorded portion ; The whiteness of not printed portions is judged visually. o ○ Pale white O White Δ Yellowish white × Yellowish It is clearly understood from Table 1 that good quality thermal recording medium having high light resistance, prevention of deposition adherence and sticking, and good appearance of unrecorded portion is obtained in Examples 1 to 8 of this invention. On the other hand, light resistance is low in Comparative Examples 1 to 6 wherein ultraviolet absorbent, ultraviolet screener and fluorescent dye are not contained or one of them is singly contained. In comparative Example 7 wherein aluminium hydroxide is used in spite of ultraviolet screener, light resistance is very bad. Further, not only appearance of unrecorded portion but also light resistance is inferior in comparison with Example 1, in Comparative Example 8 lacking fluorescent dye in protective layer, and Comparative Example 9 lacking ultraviolet absorbent in undercoat layer is inferior in light resistance, too. On the other hand, in Comparative Example 10 wherein ultraviolet absorbent is contained in protective layer in spite of ultraviolet screener, deposition adherence and sticking occur conspicuously and appearance of unrecorded portion is inferior, too. And, in Comparative Example 11 wherein zinc oxide is combined in spite of ultraviolet screener, light resistance and appearance of unrecorded portion is inferior and deposition adherence and sticking occur. Therefore, thermal recording medium having objective ability cannot be obtained if any one of condition of this invention is lacked. As mentioned above, excellent thermal recording medium wherein light resistance is improved, discoloring and yellowing of ground portions by light do not occur, and fall of record density and aggravation of appearance by containing ultraviolet absorbent, ultraviolet screener and fluorescent dye simultaneously. And, in the thermal recording medium of this invention, recording with thermal sensitive head can be performed smoothly since deposition adherence and sticking are prevented, and that is very useful.

Claims: A thermally sensitive recording medium which comprises, in order on a substrate, an undercoat layer, a thermally sensitive colour developing layer and a protective layer, wherein (a) an ultraviolet absorbent is included in at least one of the undercoat layer and the thermally sensitive colour developing layer, (b) an ultraviolet screening agent which consists of calcined particles of a flaky pigment with a refractive, index of 1.5 to 1.6 and the surface of which is coated with a surface coating comprising an insoluble cerium compound and amorphous silica, is included in at least one of the thermally sensitive colour developing layer and the protective layer, and (c) a fluorescent dye is included in the protective layer. A recording medium according to claim 1, wherein the ultraviolet screening agent is included in the thermally sensitive colour developing layer. A recording medium according to claim 1 or 2, wherein the ultraviolet absorbent is a benzotriazole compound. A recording medium according to any one of the preceding claims, wherein the amount of ultraviolet absorbent in the undercoat layer is 0.1 wt% to 15 wt % based on the dry mass of the undercoat layer. A recording medium according to any one of the preceding claims wherein the amount of ultraviolet absorbent in the thermally sensitive colour developing layer is 0.1 wt% to 15 wt% based on the dry mass of the thermally sensitive colour developing layer. A recording medium according to any one of the preceding claims wherein the amount of ultraviolet screening agent in the thermally sensitive colour developing layer is 10 wt% to 40 wt% based on the dry mass of the thermally sensitive colour developing layer. A recording medium according to any one of claims 1 and 3 to 6 wherein the amount of ultraviolet screening agent in the protective layer is 5 wt% to 40 wt% based on the dry mass of the protective layer. A recording medium according to any one of the preceding claims wherein the amount of fluorescent dye included in the protective layer is 0.01 to 3 wt% based on the dry mass of the protective layer. A recording medium according to any one of the preceding claims wherein the flaky pigment used in the ultraviolet screening agent is selected from mica, talc, sericite, aluminium hydroxide, calcium carbonate, kaolin, calcium hydroxide, aluminium silicate, polyethylene powder, polystyrene and latex. A recording medium according to any one of the preceding claims wherein the insoluble cerium compound used in the coating of the flaky pigment is cerium chloride, cerium nitrate or cerium sulphate.

Inventors: MIDORIKAWA YOSHIMI, MIYAKE SUMIO, OGINO NAOMI, UEDA HIROSHI, WAKITA YUTAKA, MIDORIKAWA, YOSHIMI, MIYAKE, SUMIO, OGINO, NAOMI, UEDA, HIROSHI, WAKITA, YUTAKA, Midorikawa, Yoshimi, c/o Nippon Paper Ind Co, Ltd., Miyake, Sumio, c/o Nippon Paper Ind. Co., Ltd., Ogino, Naomi, c/o Nippon Paper Ind. Co., Ltd., Ueda, Hiroshi, c/o Nippon Paper Ind. Co., Ltd., Wakita, Yutaka, c/o Nippon Paper Ind. Co., Ltd.
Applicants: JUJO PAPER CO LTD, NIPPON PAPER INDUSTRIES CO., LTD.
Application year: 1996
Main classification: B41M5
IPC codes: B41M5