Thermal recording sheet

Abstract: A thermal recording sheet comprises a substrate and a thermal color developing layer containing a color developer, a colorless or pale colored basic chromogenic dye, a specific epoxidized diphenylsulfone derivative as stabilizer, and, as sensitizer, a compound of Formula (II) and/or a compound of Formula (III)

Description: This invention relates to a thermal recording sheet. In general, in thermal recording sheets, a normally colorless or pale colored basic chromogenic dye and an organic color developer such as a phenolic substance are individually pulverized into fine particles, mixed, and a binder, a filler, a sensitivity improver, a slip agent, and other additives are added to obtain a coating color, which is coated on a substrate such as paper, synthetic paper, films, plastics, and the like. The thermal recording sheet enables color recording by a momentary chemical reaction caused by heating with a thermal pen, a thermal head, a hot stamp, laser light, or the like. These thermal recording sheets are applied in a variety of areas such as measurement recorders, computer terminal printers, facsimiles, automatic ticket vendors, and bar-code labels, however, with recent diversification and improvement of these recording devices, requirements to the thermal recording sheets have become stricter. For example, with increasing recording speed, it is required to obtain a high-concentration, sharp color image even with a small heat energy and, in addition, to have improved storage stability in terms of light resistance, weather resistance, and oil resistance. Prior art examples of thermal recording sheets include, for example, thermal recording materials disclosed in JP-B-43-4160 and 45-14039, however, these prior art thermal recording materials have been defective, among others, in that the thermal response is low and a sufficient color developing density is not obtained in high-speed recording. To improve such defects, high-sensitivity dyes such as using 3-N-methyl-N-cyclohexylamino-6-methyl-7-anilinofluorane (JP-A-49 10912) and 3-dibutylamino-6-methyl-7-anilinofluorane (JP-A-59-190891) have been developed, and technologies using 1,7-bis (hydroxyphenylthio)-3,5-dioxaheptane (JP-A-59-106456), 1,5-bis (4-hydroxyphenylthio)-3-oxaheptane (JP-A-59-116262), and 4-hydroxy-4'-isopropoxydiphenylsulfone (JP-B-63-46067) as color developers for higher speed and sensitivity have been disclosed. However, while these thermal recording sheets are high in sensitivity, they involve problems in heat resistance causing reduction in image density when stored at high temperatures. Furthermore, since the recording image is inferior in storage stability, disadvantages still remain in that water or oil components tend to adhere to the developed color image, and considerable reduction in image density or discoloration of the image occurs when contacting with plasticizers (DOP, DOA, etc.) contained in wrapping films such as PVC films. Therefore, it is an object of the present invention to provide a thermal recording sheet which is high in sensitivity and superior in heat resistance, water resistance, and oil resistance. In accordance with the present invention, there is provided a thermal recording sheet comprising a substrate provided with a thermal colour developing layer containing a colourless or pale coloured basic chromogenic dye and an organic colour developer as main ingredients, wherein said thermal colour developing layer contains as stabiliser at least one compound of formula (I): wherein A represents in which R 1 and R 2 individually represent hydrogen or methyl; α is 0 or an integer from 1 to 5; β and γ individually represent an integer from 1 to 5; B represents C and D individually represent chlorine, bromine, methyl, methoxy, or ethoxy; and m and n individually are 0, 1, or 2; and as sensitiser a compound of formula (II) and for a compound of formula (III) Examples of the compound of formula (I) include, for example, the following compounds. Of course, the epoxidized diphenylsulfone derivative is not limited to these compounds, and two or more compounds thereof may be used in combination as needed. In one aspect of the invention, the organic color developer is a diphenylsulphone of the formula: Further examples of the color developer include, for example, bisphenol A and its derivatives, 4-hydroxybenzoic esters, 4-hydroxyphthalic diesters, phthalic monoesters, bis(hydroxyphenyl) sulfides, 4-hydroxyarylsulfones, 4-hydroxyphenylarylsulfonates, 1,3-di[2-(hydroxyphenyl)-2-propyl]-benzenes, 4-hydroxybenzoyloxybenzoic ester, and bisphenolsulfones. Practical examples thereof are shown below: Bisphenol A and its derivatives 4,4'-Isopropylidenediphenol (bisphenol A) 4-4'-Cyclohexylidenediphenol p,p'-(1-Methyl-n-hexylidene) diphenol 1,7-Di(4-hydroxyphenylthio)-3,5-dioxaheptane. 4-Hydroxybenzoic esters Benzyl 4-hydroxybenzoate Ethyl 4-hydroxybenzoate Propyl 4-hydroxybenzoate Isopropyl 4-hydroxybenzoate Butyl 4-hydroxybenzoate Isobutyl 4-hydroxybenzoate Methylbenzyl 4-hydroxybenzoate 4-Hydroxyphthalic diesters Dimethyl 4-hydroxyphthalate Diisopropyl 4-hydroxyphthalate Dibenzyl 4-hydroxyphthalate Dihexyl 4-hydroxyphthalate Phthalic monoesters Monobenzyl phthalate Monocyclohexyl phthalate Monophenyl phthalate Monomethylphenyl phthalate Monoethylphenyl phthalate Monopropylbenzyl phthalate Monohalogenbenzyl phthalate Monoethoxybenzyl phthalate Bis-(hydroxyphenyl) sulfides Bis-(4-hydroxy-3-tert-butyl-6-methylphenyl) sulfide Bis-(4-hydroxy-2,5-dimethylphenyl) sulfide Bis-(4-hydroxy-2-methyl-5-ethylphenyl) sulfide Bis-(4-hydroxy-2-methyl-5-isopropylphenyl) sulfide Bis-(4-hydroxy-2,3-dimethylphenyl) sulfide Bis-(4-hydroxy-2,5-dimethylphenyl) sulfide Bis-(4-hydroxy-2,5-diisopropylphenyl) sulfide Bis-(4-hydroxy-2,3,6-trimethylphenyl) sulfide Bis-(2,4,5-trihydroxyphenyl) sulfide Bis-(4-hydroxy-2-cyclohexyl-5-methylphenyl) sulfide Bis(2,3,4-trihydroxyphenyl) sulfide Bis-(4,5-dihydroxy-2-tert-butylphenyl) sulfide Bis-(4-hydroxy-2,5-diphenylphenyl) sulfide Bis-(4-hydroxy-2-tert-octyl-5-methylphenyl) sulfide 4-Hydroxyphenylarylsulfones 4-Hydroxy-4'-isopropoxydiphenylsulfone 4-Hydroxy-4'-propoxydiphenylsulfone 4-Hydroxy-4'-n-butyloxydiphenylsulfone 4-Hydroxy-4'-n-propoxydiphenylsulfone 4 Hydroxyphenylarylsulfonates 4-Hydroxyphenylbenzenesulfonate 4-Hydroxyphenyl-p-tolylslfonate 4-Hydroxyphenylmethylenesulfonate 4-Hydroxyphenyl-p-chlorobenzenesulfonate 4-Hydroxyphenyl-p-tert-butylbenzenesulfonate 4-Hydroxyphenyl-p-isopropoxybenzenesulfonate 4-Hydroxyphenyl-1'-naphthalenesulfonate 4-Hydroxyphenyl-2'-naphthalenesulfonate 1,3-Di[2-(hydroxyphenyl)-2-propyl]benzenes 1,3-Di[2-(4-hydroxy-3-alkylphenyl)-2-propyl]benzene 1,3-Di[2-(2,4-dihydroxyphenyl)-2-propyl]benzene 1,3-Di[2-(2-hydroxy-5-methylphenyl)-2-propyl]benzene Resorcinols 1,3-Dihydroxy-6(α,α-dimethylbenzyl)benzene 4-Hydroxybenzoyloxybenzoic esters Benzyl 4-hydroxybenzoyloxybenzoate Methyl 4-hydroxybenzoyloxybenzoate Ethyl 4-hydroxybenzoyloxybenzoate Propyl 4-hydroxybenzoyloxybenzoate Butyl 4-hydroxybenzoyloxybenzoate Isopropyl 4-hydroxybenzoyloxybenzoate tert-Butyl 4-hydroxybenzoyloxybenzoate Hexyl 4-hydroxybenzoyloxybenzoate Octyl 4-hydroxybenzoyloxybenzoate Nonyl 4-hydroxybenzoyloxybenzoate Cyclohexyl 4-hydroxybenzoyloxybenzoate β-Phenethyl 4-hydroxybenzoyloxybenzoate Phenyl 4-hydroxybenzoyloxybenzoate α-Naphthyl 4-hydroxybenzoyloxybenzoate β-Naphthyl 4-hydroxybenzoyloxybenzoate sec-Butyl 4-hydroxybenzoyloxybenzoate Biphenolsulfones (I) Bis-(3-1-butyl-4-hydroxy-6-methylphenyl)sulfone Bis-(3-ethyl-4-hydroxyphenyl)sulfone Bis-(3-propyl-4-hydroxyphenyl)sulfone Bis-(3-methyl-4-hydroxyphenyl)sulfone Bis-(2-isopropyl-4-hydroxyphenyl)sulfone Bis-(2-ethyl-4-hydroxyphenyl)sulfone Bis-(3-chloro-4-hydroxyphenyl)sulfone Bis-(2,3-dimethyl-4-hydroxyphenyl)sulfone Bis-(2,5-dimethyl-4-hydroxyphenyl)sulfone Bis-(3-methoxy-4-hydroxyphenyl)sulfone 4-Hydroxyphenyl-2'-ethyl-4'-hydroxyphenylsulfone 4-Hydroxyphenyl-2'-isopropyl-4'-hydroxyphenylsulfone 4-Hydroxyphenyl-3'-isopropyl-4'-hydroxyphenylsulfone 4-Hydroxyphenyl-3'-sec-butyl-4'-hydroxyphenylsulfone 3-Chloro-4-hydroxyphenyl-3'-isopropyl-4'-hydroxyphenylsulfone 2-Hydroxy-5-t-butylphenyl-4'-hydroxyphenylsulfone 2-Hydroxy-5-t-aminophenyl-4'-hydroxyphenylsulfone 2-Hydroxy-5-t-isopropylphenyl-4'-hydroxyphenylsulfone 2-Hydroxy-5-t-octylphenyl-4'-hydroxyphenylsulfone 2-Hydroxy-5-t-butylphenyl-3'-chloro-4'-hydroxyphenylsulfone 2-Hydroxy-5-t-butylphenyl-3'-methyl-4'-hydroxyphenylsulfone 2-Hydroxy-5-t-butylphenol-3'-isopropyl-4'-hydroxyphenylsulfone 2-Hydroxy-5-t-butylphenyl-3'-chloro-4'-hydroxyphenylsulfone 2-Hydroxy-5-t-butylphenyl-3'-methyl-4'-hydroxyphenylsulfone 2-Hydroxy-5-t-butylphenyl-3'-isopropyl-4'-hydroxyphenylsulfone 2-Hydroxy-5-t-butylphenyl-2'-methyl-4'-hydroxyphenylsulfone Bisphenolsulfonic acids (II) 4,4'-Sulfonyldiphenol 2,4'-Sulfonyldiphenol 3,3'-Dichloro-4,4'-sulfonyldiphenol 3,3'-Dibromo-4,4'-sulfonyldiphenol 3,3',5,5'-Tetrabromo-4,4'-sulfonyldiphenol 3,3'-Diamino-4,4'-sulfonyldiphenol Others p-tert-Butylphenol 2,4-Dihydroxybenzophenone Novolac type phenolic resin 4-Hydroxyacetophenone p-Phenylphenol Benzyl-4-hydroxyphenylacetate p-Benzylphenol In the present invention, it is also possible to use in combination with other fluorane-based leuco dyes as much as the effect of the present invention is not impaired, and some practical examples are shown below: 3-Diethylarmino-6-methyl-7-anilinofluorane 3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluorane 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluorane 3-Diethylamino-6-methyl-7-(o,p-dimethylanilino)fluorane 3-Pyrrolidino-6-methyl-7-anilinofluorane 3-Piperidino-6-methyl-7-anilinofluorane 3-(N-cyclohexyl-N-methylamino)-6-methyl-7-anilinofluorane 3-Diethylamino-7-(m-trifluoromethylanilino)fluorane 3-Dibutylamino-6-methyl-7-anilinofluorane 3-Diethylamino-6-chloro-7-anilinofluorane 3-Dibutylamino-7-(o-chloroanilino)fluorane 3-Diethylamino-7-(o-chloroanilino)fluorane. Furthermore, as a sensitizer, fatty acid amides such as stearamide, palmitamide, or the like; ethylene-bisamide, montan wax, polyethylene wax, dibenzyl terephthalate, benzyl p-benzyloxybenzoate, Di-p-tolylcarbonate, p-benzylbiphenyl, phenyl-α-naphthylcarbonate, 1,4-diethoxynaphthalene, phenyl-1-hydroxy-2-naphthoate, 1,2-di-(3-methylphenoxy) ethane, di(methylbenzyl)oxalate, β-benzyloxynaphthalene, 4-biphenyl-p-tolylether, or the like can be added as much as the effect of the present invention is not impaired. The binder used in the present invention can be fully-saponified polyvinylalcohol with a polymerization degree of 200 to 1,900, partially-saponified polyvinylalcohol, carboxy-modified polyvinylalcohol, amide-modified polyvinylalcohol, sulfonic acid-modified polyvinylalcohol, and other modified polyvinylalcohols, hydroxyethylcellulose, methylcellulose, carboxymethylcellulose, styrene-maleic anhydride copolymer, styrene-butadiene copolymer, cellulose derivatives such as ethylcellulose and acetylcellulose, polyvinylchloride, polyvinylacetate, polyacrylamide, polyacrylic esters, polyvinylbutyral, polystyrene and its copolymers, polyamide resins, silicone resins, petroleum resins, terpene resins, ketone resins, and coumarone resins. These polymeric substances can be dissolved in water, and solvents such as alcohols, ketones, esters, hydrocarbons, and the like, or emulsified or dispensed in water or other media, or can be used in combination according to the quality requirements. In the present invention, it is also possible to add known stabilizers based on metal salts (Ca, Zn) of p-nitrobenzoic acid or metal salts (Ca, Zn) of monobeznzylphthalate in amounts not to impair the effect of the present invention. Fillers used in the present invention can be inorganic or organic fillers such as silica, calcium carbonate, kaolin, calcinated kaolin, diatomaceous earth, talc, titaniumim oxide, aluminum hydroxide, or the like. In addition to the above, it is possible to use release agents such as fatty acid metal salts, slip agents such as wax, benzophenone- or triazole-based ultraviolet absorbers, water resistant agents such as glyoxal, dispersants, defoamers, and the like. The amounts of the stabilizer and the basic colorless dye used in the present invention and the types and amounts of other constituents are determined according to the required properties and recording adaptability, and are not specifically limited. In one aspect of the invention the thermal color developing layer contains 0.25 to 2.5 parts of the stabilizer and 3 to 12 parts of the sensitizer per 1 part by weight of the colorless or pale colored basic chromogenic dye. It is furthermore preferable to use 1 to 8 parts of organic color developer and 1 to 20 parts of fillers to 1 part of the basic colorless dye. The binder is preferably used in an amount of 10 to 25% of the total solid. The solution of the above composition can be coated on any type of substrate such as paper, synthetic paper, films, plastics, or the like to obtain the objective thermal recording sheet. In a further aspect of the invention, an overcoating layer is provided on the thermal color developing layer. The overcoating layer may be of polymeric substance or the like. It improves the storage stability. In a yet further aspect of the invention, in order to improve the storage stability and sensitivity, an undercoating layer is provided under the thermal color developing layer. Typically the undercoating layer contains an organic or inorganic filler. The organic color developer, the basic colorless dye, and the materials which are added as needed are pulverized by a pulverizing machine such as a ball mill, an attriter, a sand grinder, or the like, or by an appropriate emulsifying apparatus to a particle diameter of several microns or less, and mixed with the binder and various additives according to the purpose to obtain a solution. In the present invention, the reason why a combination of a specific stabilizer with a specific sensitizer gives the effect of the present invention is considered as follows. First, the superior dynamic color developing ability is due to a high melt diffusion rate and a high saturation solubility of the sensitizer to the stabilizer of the present invention, thereby instantaneously forming a recording image by a momentary contact with a high-temperature thermal head. The reason why the recording image has an extremely high stability in terms of heat resistance, water resistance, and oil resistance is explained as follows. In general, a thermal recording paper uses a basic colorless dye as an electron donor, and an organic acid substance such as a phenolic compound, an aromatic carboxylic acid, an organic sulfonic acid, or the like as an electron acceptor. Heat melting reaction of the basic colorless dye and the color developer is an acid-base reaction based on electron donation and acceptance, which forms a metastable "charge transfer complex", thereby obtaining a color image. It is hypothesized that, in this case, by containing an epoxidized diphenylsulfone derivative in the system, the epoxy ring opens during the heat melting reaction, reacts with the sensitizer, the leuco dye, and the organic color developer to stabilize the recording image. In this reaction process, when a specific epoxidized diphenyllsulfone derivative and a specific sensitizer are combined, the ring-opening reaction of the epoxy ring actively takes place, and as a result, stability of the color image is maintained even if the recording image is exposed to environmental conditions under which it is affected by water, oil, and heat for an extended period of time. Best Mode for Practicing the Invention The present invention will now be described with reference to the embodiments. In the description, part means part by weight. Examples 1 (Test Nos.1-8) Solution A (color developer dispersion) Part Color developer (Table 1) 6.0 10% aqueous polyvinylalcohol solution 18.8 Water 11.2 Solution B (stabilizer dispersion) Diphenylsulfone derivative (Table 1) 4.0 10% aqueous polyvinylalcohol solution 5.0 Water 3.0 Solution C (sensitizer dispersion) Sensitizer (Table 1) 4.0 10% aqueous polyvinylalcohol solution 5.0 Water 3.0 Solution D (dye dispersion) 3-n-Dibutylamino-6-methyl-7-anilinofluorane 2.0 10% aqueous polyvinylalcohol solution 4.6 Water 2.6 The above solutions were individually ground by a sand grinder to an average particle diameter of 1 micron. Then, the dispersions were mixed in the following ratio to obtain a coating color. Solution A 36.0 parts Solution B 12.0 Solution C 12.0 Solution D 9.2 Kaolin clay (50% dispersion) 12.0 The above coating color was coated on one side of a 50 g/m 2 base paper to an amount of 6.0 g/m 2 and dried, and the sheet was treated by a super-calender to a flatness of 500-600 seconds to obtain a thermal recording sheet. Comparative Example 1 (Test Nos.9-11) Solution E (color developer dispersion) Part 4-Hydroxy-4'-isopropoxydiphenylsulfone 6.0 10% aqueous polyvinylalcohol solution 18.8 Water 11.2 Solution F (dye dispersion) 3-n-Dibutylamino-6-methyl-7-anilinofluorane 2.0 10% aqueous polyvinylalcohol solution 4.6 Water 2.6 Solution G (stabilizer dispersion) Diphenylsulfone derivative (Table 1) 4.0 10% aqueous polyvinylalcohol solution 5.0 Water 3.0 Solution H (sensitizer dispersion) Sensitizer (Table 1) 4.0 10% aqueous polyvinylalcohol solution 5.0 Water 3.0 The above solutions were individually ground by a sand grinder to an average particle diameter of 1 micron. Then, the dispersions were mixed in the following ratio to obtain a coating color, which was treated as in Example 1 to prepare a thermal recording sheet. Solution E 36.0 parts Solution F 9.2 Solution G 12.0 Solution H 12.0 Kaolin clay (50% dispersion) 12.0 Comparative Example 2 (Test Nos.12-13) Solution E (color developer dispersion) Part 4-Hydroxy-4'-isopropoxydiphenylsulfone 5.0 10% aqueous polyvinylalcohol solution 18.8 Water 11.2 Solution I (sensitizer dispersion) Sensitizer (Table 1) 4.0 10% aqueous polyvinylalcohol solution 5.0 Water 3.0 Solution J (dye dispersion) 3-n-Dibutylamino-6-methyl-7-anilinofluorane 2.0 10% aqueous polyvinylalcohol solution 4.6 Water 2.6 The above solutions were individually ground by a sand grinder to an average particle diameter of 1 micron. Then, the dispersions were mixed in the following ratio to obtain a coating color, which was treated as in Example 1 to prepare a thermal recording sheet. Solution E 36.0 parts Solution I 12.0 Solution J 9.2 Kaolin clay (50% dispersion) 12.0 The thermal recording sheets obtained in the above Example and Comparative Examples were tested for quality and properties. The test results are summarized in Table 1. In Table 1, sensitizers A to E are the following. The effects of the present invention are as follows: (1) With superior heat response, a sharp, high-density image can be obtained even in high-speed, high-density recording (high sensitivity). (2) Almost no discoloration occurs in the printed portion (color developed portion) even when contacts with plasticizers, salad oil, vinegar, and the like (oil resistance). (3) Almost no discoloration occurs in the printed portion even when contacts with water (water resistance). (4) Image is stable at high temperatures (heat resistance).

Claims: A thermal recording sheet comprising a substrate provided with a thermal color developing layer containing a colorless or pale colored basic chromogenic dye and an organic color developer as main ingredients, wherein said thermal color developing layer contains as stabilizer at least one compound of Formula (1) wherein A represents in which R 1 and R 2 individually represent hydrogen or methyl; α is 0 or an integer from 1 to 5; β and γ individually represent an integer from 1 to 5; B represents C and D individually represent chlorine, bromine, methyl, method or ethoxy; and m and n individually are 0, 1 or 2 and as sensitizer a compound of Formula (II) and/or a compound of Formula (III) A thermal recording sheet according to claim 1 wherein said organic color developer is a diphenylsulfone of the formula: (wherein R represents propyl, isopropyl or butyl.). A thermal recording sheet according to claim 1 or 2 wherein the thermal color developing layer contains 0.25 to 2.5 parts by weight of said stabilizer and 3 to 12 parts by weight of said sensitizer per 1 part by weight of said colorless or pale colored basic chromogenic dye. A thermal recording sheet according to claim 1, 2 or 3 wherein an overcoating layer is provided on said thermal color developing layer. A thermal recording sheet according to any one of the preceding claims wherein an undercoating layer is provided under said thermal color developing layer.

Inventors: KANEKO TOSHIO, MINAMI TOSHIAKI, KANEKO, TOSHIO, MINAMI, TOSHIAKI, Kaneko, Toshio, c/o New Products Research Lab., Minami, Toshiaki, c/o New Products Research Lab.
Applicants: JUJO PAPER CO LTD, NIPPON PAPER INDUSTRIES CO., LTD.
Application year: 1993
Main classification: B41M5
IPC codes: B41M5