Heat transfer sheet

Abstract: A heat transfer sheet comprising a lubricating layer provided on one surface of a substrate film and a dye layer formed on the other surface, wherein the elastic modulus in at least one of the sub-scanning direction (MD) and the main scanning direction (TD) in said heat transfer sheet is 280 kg/mm² or more, and the elastic modulus ratio MD/TD in the sub-scanning direction (MD) and the main scanning direction (TD) is within the range of from 0.8 to 1.3.

Description: BACKGROUND OF THE INVENTION This invention relates to a heat transfer sheet by use of a sublimable dye (heat migratable dye), more particularly, to a heat transfer sheet which has solved the both problems of printing wrinkle and image dislocation due to slippage generated during heat transfer printing. Various heat transfer methods have been known in the art, and among them, there has been proposed a method in which a sublimable dye is used as the recording material. The dye is carried on a substrate sheet such as a polyester film to form a heat transfer sheet. By using the transfer sheet, various full colors are formed on an image-receiving sheet having a dye receptive layer with a sublimable dye such as paper or plastic film. In this case, a thermal head of a printer is used as the heating means, and a large number of color dots of 3 or 4 colors are transferred onto the image-receiving sheet by heating for a very short time, thereby reproducing the full-color image of the original with the color dots of said multicolor. The thus formed image is very sharp, since the colorant used is a dye and also excellent in transparency, whereby the obtained image is excellent in reproducibility and gradation of the intermediate color, similar to the image according to the offset printing and gravure printing of the prior art, and further can form an image of high quality comparable with full-color photographic image. As the substrate film of the above heat transfer sheet, papers such as condenser paper may be sometimes employed, but such thin paper is lower in strength, particularly weak in bursting strength and therefore, it is desirable to use a film having a tough plastic such as polyester resin as the substrate film. However, in this case, the following problems will further ensue That is, the transfer sheet is thermally deformed due to the heat of a temperature of 250 to 300°C or higher being locally loaded from the thermal head to the heat transfer sheet during printing. Further, the heat transfer sheet is conveyed under pressurization of thermal head to be nonuniformly elongated, whereby a large number of wrinkles are generated on the sheet. As the result, not only running under the thermal head is obstructed, but also slippage and drop-out of the dots are generated in the obtained image, thus involving the problem that the resolution of the printed image is lowered, and also that color reproducibility is lowered in formation of full-color. Such problems become particularly conspicuous when a marked density difference is needed for the image to be formed, because the heat content imparted to the thermal head has a locally great difference. The problems as mentioned above can be alleviated by use of a substrate having relatively thicker thickness, but in this case, sensitivity of the heat transfer sheet is lowered to become practically useless. In another method, it has been proposed to provide a heat-resistant protective layer such as a thermosetting resin on the surface opposite to the dye layer. However, even by use of these methods, if the heat-resistant protective layer is made thick to the extent effective for prevention of printing wrinkle phenomenon, sensitivity of the heat transfer sheet and resolution of the printed image are lowered, and therefore they cannot be satisfactory measures of solution. EP-A-0 222 374 discloses a thermal transfer (printing) material comprising a biaxially oriented polyethylene 2,6-naphtalate film of Young's modulus in longitudinal direction and in transverse direction of not less than 600 Kg/mm² as the base material and a thermal transfer layer coated on one of the surface of said film, whereby the ratio of the Young's modulus in transverse direction and in longitudinal direction is about 1,0. But this document is concerned with a base film per se for a thermal transfer material, i.e., it does not disclose nor suggest that the conditions according to claim 1 of the heat transfer sheet are important for preventing wrinkles during thermal recording. Accordingly, an object of the present invention is to provide a heat transfer sheet capable of giving the image being excellent in sharpness and resolution having sufficient printing density without printing wrinkles and image slippage. SUMMARY OF THE INVENTION The above objects can be accomplished by the present invention as specified below. More specifically, the present invention is a heat transfer sheet comprising a lubricating layer provided on one surface of a substrate film and a dye layer formed on the other surface of the substrate film, characterized in that the elastic modulus in at least one of the sub-scanning direction (MD) and the main scanning direction (TD) in said heat transfer sheet is 280 kg/mm² or more, and the elastic modulus ratio MD/TD in the sub-scanning direction (MD) and the main scanning direction (TD) is within the range of from 0.8 to 1.3. By making the elastic modulus in at least one of the sub-scanning direction (MD) and the main scanning direction (TD) of the heat transfer sheet 280 kg/mm² or more, and the elastic modulus ratio MD/TD within the range of from 0.8 to 1.3, no fine wrinkle or no image slippage occurs in the heat transfer sheet during printing, whereby an image with excellent resolution and color reproducibility can be formed. DETAILED DESCRIPTION OF THE INVENTION Referring now to preferred embodiments, the present invention is described in more detail. As the substrate film to be used in the heat transfer sheet of the present invention, polyester films such as polyethylene terephthalate film, polyethylene naphthalene dicarboxylate film, etc. are particularly preferred, but otherwise, other plastic films such as polystyrene film, polypropylene film, polysulfone film, polycarbonate film, Aramide film, or polyether ether ketone film may be preferably used. Of course, in these films, any desired additive such as extender pigment, UV-ray absorber, antioxidant, or stabilizer may be contained. Also, an easily adherable film previously applied with easy adhesion treatment on one surface or both surfaces of the film may be used. Also, the above-mentioned film should be preferably stretched by use of a general method into a biaxially oriented film, but a substrate film strongly in either one direction of the MD direction or the TD direction is not desirable. If the thickness of the film is too thin, heat resistance is deficient, while if it is too thick, migration efficiency of dye is lowered. Therefore, its preferable thickness may be 0.5 to 50 µm, particularly 1 to 20 µm, and the shape may be a film shaped in sheet cut into predetermined dimensions, or a continuous or wind-up film, or further a tape-like film with a narrow width. The above-mentioned substrate film, when the adhesive force with the dye layer formed on its surface is poor, should be preferably applied with the primer treatment or the corona discharging treatment. The sublimable (heat migratable) dye layer to be formed on the substrate film as mentioned above is a layer having a dye carried with any desired resin. As the dye to be used, all of the dyes used in the heat transfer sheets known in the art are effective available for the present invention, and not particularly limited. For example, some preferable dyes may include, as red dyes, MS Red G, Macrolex Red Violet R, Ceres Red 78, Samaron Red HBSL, SK Rubin SEGL, Baymicron SN VP 2670, Resoline Red F3Bs, etc.; as yellow dyes, Foron Brilliant Yellow S-6GL, PTY-52, Macrolex Yellow 6G, Terasil Golden Yellow-2RS, etc.; as blue dyes Kayaset Blue 714, Waxoline Blue AP-FW, Foron Brilliant Blue S-R, MS Blue-100, Daito Blue No. 1, etc. As the binder resin for carrying the heat migratable dye as mentioned above, any of those known in the art can be used, and preferable examples may include cellulose resins such as ethyl cellulose, hydroxyethyl cellulose, ethylhydroxy cellulose, hydroxypropyl cellulose, methyl cellulose, cellulose acetate, cellulose acetate butyrate, etc., vinyl resins such as polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, polyvinyl acetoacetal, polyvinyl pyrrolidone, polyacrylamide, etc., polyesters and others, and among them, cellulose type, acetal type, butyral type and polyester type are particularly preferred. The dye layer is formed along each predetermined pattern by selecting any desired one color from among the above-mentioned dyes when the image to be formed is a mono-color, while it is formed in any desired combination of those of predetermined hues selected from among appropriate cyan, magenta, yellow, black, etc. when the image to be formed is a multi-color image. The dye layer of the heat transfer sheet of the present invention is formed basically of the materials as described above, but otherwise can also include various similar additives known in the art, if necessary. Such dye layer is preferably prepared by adding the above-mentioned sublimable dye, binder resin and other optional components to dissolve or disperse the respective components to prepare a coating material or an ink for formation of dye layer, and coating and drying this on the substrate film as described above. The dye layer thus formed has a thickness of about 0.2 to 5.0 µm, preferably 0.4 to 2.0 µm, and the sublimable dye in the dye layer should be suitably exist in an amount of 5 to 90% by weight, preferably 10 to 70% by weight, of the weight of the dye layer. Also, in the present invention, a primer layer may be also provided between the substrate film and the dye layer, if necessary. The primer layer is provided for improvement of adhesion between the substrate film and the dye layer, protection of the substrate film, etc. For example, when a hydrophilic resin is used as the primer layer, it plays a role of the barrier layer which prevents migration of the dye from the dye layer to the substrate film. As the material for forming the primer layer, there may be effectively used those having smaller diffusion coefficient of the dye in the dye layer, such as polyester resins, polyurethane resins, acrylic polyol resins, vinyl chloride-vinyl acetate copolymer resins, cellulose resins such as cellulose acetate, methyl cellulose, etc., polyvinyl alcohol, gelatin, etc. In the present invention, it is preferable to improve the lubricating characteristic between the thermal head and the substrate film by providing a lubricating layer on the surface of the substrate opposite to the dye layer. As the material for forming such lubricating layer, phosphoric acid ester, silicone oil, graphite powder, etc. may be included. Also, it is preferable to impart heat resistance to the above-mentioned lubricating layer. As the heat-resistant lubricating layer, those known in the art may be available, including polyvinyl butyral resin, polyvinyl acetoacetal resin, polyester resin, vinyl chloride/vinyl acetate copolymer, polyether resin, polybutadiene resin, styrene/butadiene copolymer, acrylic polyol, polyurethane acrylate, polyester acrylate, polyether acrylate, epoxy acrylate, prepolymer of urethane or epoxy, nitrocellulose resin, cellulose nitrate resin, cellulose acetopropionate resin, cellulose acetate propionate resin, cellulose acetate butyrate resin, cellulose acetate hydrogen phthalate resin, cellulose acetate resin, aromatic polyamide resin, polyimide resin, polycarbonate resin, chlorinated polyolefin resin, etc. As the lubricity imparting agent to be added to or coated on these heat-resistant layers, there may be included phosphoric acid ester, silicone oil, graphite powder, silicon type graft polymer, fluorine type graft polymer, acrylic silicon graft polymer, silicone polymers such as acrylic siloxane, aryl siloxane, etc., but preferably a layer comprising a polyol, such as a polyalcohol polymeric compound, a polyisocyanate compound and a phosphoric acid ester type compound, and further it is more preferable to add a filler. Such polyalcohol polymeric compound should be desirably selected from among polyvinyl butyral resin having hydroxyl group, polyester resin, vinyl chloride/vinyl acetate copolymer, polyether resin, polybutadiene resin, acrylic polyol, prepolymer of urethane or epoxy, or nitrocellulose resin, cellulose acetate propionate resin, cellulose acetate butyrate resin or cellulose acetate resin, etc. The above resin may be, in addition, those having hydroxyl groups in their polymer units or those having unreacted hydroxyl groups at the terminal ends or in the side chains thereof. A particularly polyalcohol polymer compounds is a polyvinyl butyral resin which forms a reaction product excellent in heat resistance. As the polyvinyl butyral resin, one having a high molecular weight and also containing many hydroxyl groups which are the reaction sites with polyisocyanates. Particularly preferred of the polyvinyl butyral resin are those having a molecular weight of 60,000 to 200,000, a glass transition temperature of 60 to 110°C, and a content of the vinyl alcohol moiety of 15 to 40% by weight. As the polyisocyanates to be used during formation of the above-mentioned heat-resistant lubricating layer, polyisocyanates such as diisocyanates, triisocyanates, etc. may be included and these may be used single or as a mixture. Specifically, there may be included: p-phenylene diisocyanate, 1-chloro-2,4-phenyldiisocyanate, 2-chloro-1,4-phenyldiisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, hexamethylene diisocyanate, 4,4′-biphenylene diisocyanate, triphenylmethane triisocyanate, 4,4′,4″-trimethyl-3,3′,2′-triisocyanate, 2,4,6-triphenylcyanurate, etc. The isocyanates may be used relative to the polyalcohol polymer compound in amounts generally of 1 to 400 parts by weight, preferably 5 to 300 parts by weight, based on 100 parts by weight of the polyalcohol polymer compound. The phosphoric acid ester type compound imparts lubricity to the heat-resistant layer, and specifically, GAFAC RD720 manufactured by Toho Kagaku, Japan, Plysurf A-208S manufactured by Daiichi Kogyo Seiyaku, Japan, may be employed. Such phosphoric acid ester type compound may be used at a ratio of 1 to 150 parts by weight, preferably 5 to 100 parts by weight, per 100 parts by weight of the polyalcohol polymer compound. As the filler to be added in the heat-resistant lubricating layer, there may be included inorganic fillers or organic fillers having heat resistance such as clay, talc, zeolite, aluminosilicate, calcium carbonate, Teflon® powder, lead oxide, titanium oxide, magnesium oxide, silica, carbon, condensates of benzoguanamine and formaldehyde, etc. The mean grain size of such fill may be 3 µm or less, desirably 0.1 to 2 µm. The filler may be used in an amount of 5 to 60% by weight, preferably 10 to 40% by weight, based on the polyalcohol polymer compound. By use of such filler in the heat-resistant lubricating layer, there is no fusion between the thermal head and the heat transfer sheet, whereby the so called sticking phenomenon will not be recognized at all. The heat-resistant lubricating layer may have a film thickness of 0.05 to 5 µm, preferably 1 to 2 µm. If the film thickness is thinner than 0.05 µm, the effect as the heat-resistant lubricating layer is not sufficient, while if it is thicker than 5 µm, the heat transmission from the thermal head to the dye layer becomes poorer, whereby there ensues the drawback that printing density is lowered. The heat transfer sheet in the present invention may also have an adhesion improving layer between the heat-resistant lubricating layer and the substrate film. As the adhesion improving layer, one which can consolidate the adhesion between the substrate film and the heat-resistant may be employed, as exemplified by polyester type resin, polyurethane type resin, acrylic polyol type resin, vinyl chloride-vinyl acetate copolymer type resin, etc., which may be used either singly or in a mixture by coating. Also, if necessary, a reactive curing agent such as polyisocyanate, etc. may be added. Further, a titanate and silane type coupling agent may be used. Also, if necessary, two or more layers may be laminated. The heat transfer sheet in the present invention may also substantially contain an antistatic agent, and as the antistatic agent, there can be employed cationic type surfactants (e.g. quaternary ammonium salt, polyamine derivative, etc.), anionic type surfactants (e.g. alkyl phosphate, etc.), amphoteric type surfactants (e.g. those of the betaine type, etc.) or nonionic surfactants (e.g. fatty acid ester, etc.), and further those of the polysiloxane type. The heat transfer sheet of the first embodiment, in such a constitution as described above, is characterized by making the modulus in at least one of the sub-scanning direction (MD) and the main scanning direction (TD) in the heat transfer sheet 280 kg/mm² or more, and, the modulus ratio MD/TD within the range of from 0.8 to 1.3. If the modulus of either one of MD and TD is less than 280 kg/mm², or the modulus ratio MD/TD is outside the above range, fine wrinkles will be generated during heat transfer, whereby the objects of the present invention cannot be accomplished. Further preferable modulus in the MD or TD direction is 300 kg/mm² or more, and further preferable modulus ratio of MD/TD is in the range of from 0.9 to 1.1, and in this case, it is more preferable that the strength balance should be better in both the MD and TD direction. The heat transfer sheet having the modulus characteristics as described above can be obtained by taking care about the preparation conditions in the preparation steps, such as drying, etc. of the above heat transfer sheet so that the time residing at a high temperature of 100°C or higher may be suppressed within 90 seconds at the maximum, desirably within 60 seconds. The modulus in the present invention does not concern the substrate film alone, but in the state of the completed heat transfer sheet, and its measurement was conducted for a sample strip of 50 mm × 15 mm under the conditions of normal temperature and normal pressure by means of Tensilon (UCT-100, Orientech K.K.). Measurement was conducted under the conditions of an initial gauge length of 33 mm, a drawing speed of 50 mm/min, and within the range where the sample exhibits elastic deformation, elongation was measured for every certain load (every 50 g from 250 g to 750 g of weight), the slope was determined from the load difference and the elongation difference, and the modulus was determined by linearization according to the method of least squares. Also, in a preferred embodiment of the present invention, by controlling the heating shrinkage in the MD and TD direction of the heat transfer sheet (150°C and 30 minutes) within the range of 0 to 2.5%, further excellent wrinkle generation prevention effect can be obtained. The heat transfer sheet having the above thermal characteristics can be obtained by taking care about the preparation conditions in the preparation steps, such as drying, etc. of the above heat transfer sheet so that the time residing at a high temperature of 100°C or higher may be suppressed within 90 seconds at the maximum, desirably within 60 seconds. It should be noted that the heating shrinkage in the present invention does not concern the substrate film itself, but it is a value measured under the state of the completed heat transfer sheet. According to the present invention as described above, by making at least one modulus in the sub-scanning direction (MD) and the main scanning direction (TD) of the heat transfer sheet comprising a dye layer formed on the surface of a substrate film having a lubricating layer on the back 280 kg/mm² or higher, and also the modulus ratio of MD/TD within the range from 0.8 to 1.3, no wrinkle or image slippage is generated during printing, whereby it becomes possible to form an image excellent in resolution and color reproducibility. Referring now to Examples and Comparative examples, the present invention is described in more detail. In the sentences, parts or % are based on parts by weight, unless otherwise particularly noted. Example A-1 On one surface of a polyethylene terephthalate film with a thickness of 4.5 µm (5AF53, Toray) was provided a polyester type subbing layer, and on its surface was coated an ink composition for formation of heat-resistant lubricating layer by a gravure coater, followed by drying under the conditions of a drying temperature of 100 to 110°C, a residence time in the drying hood of 10 seconds. Ink composition: Polyvinyl butyral resin (Ethlec BX-1) 2.2 parts Toluene 35.4 parts Methyl ethyl ketone 53.0 parts Isocyanate (Barnock D-750, Dainippon Ink Kagaku) 6.8 parts Phosphoric acid ester (Plysurf A-208S) 1.6 parts Phosphoric acid ester sodium salt (Gafac RD720, Toho Kagaku Kogyo) 0.6 part Talc (Microace L-1, Nippon Talc) 0.4 part Amine type catalyst (Desmorapid, PP, Sumito Bayern Urethane) 0.02 part The above film was subjected to the curing treatment by heating in an oven at 60°C for 3 days. The amount of the ink coated after drying was found to be about 1.2 g/m². Next, on the surface of the above film opposite to the heat-resistant lubricating layer, a polyester type subbing layer was provided, and an ink composition for formation of dye layer having the composition shown below was coated by a gravure coater to a dry coated amount of 1.2 g/m² thereon, followed by drying under the conditions of a drying temperature of 100 to 110°C and a residence time in the drying hood of 30 seconds, to form a dye layer. Yellow ink: Foron Brilliant Yellow S-6GL (Sandoz) 2.7 parts Polyvinyl acetal resin (Sekisui Kagaku) 3.3 parts Polyvinyl butyral resin (Ethlec BX-1, Sekisui Kagaku) 2.7 parts Methyl ethyl ketone 45.65 parts Toluene 45.65 parts Magenta ink: MS RED G (Disperse Red 60, Mitsui Toatsu) 2.4 parts Microlex Red Violet R (Disperse Violet 26, Bayer) 1.29 parts Polyvinyl acetal resin (Sekisui Kagaku) 3.85 parts Hydrocarbon type wax (Microfine MG-8F, Dura) 0.11 part Methyl ethyl ketone 46.22 parts Toluene 46.22 parts Cyan ink: Kayaset Blue 714 (Solvent Blue 63, Nippon Kayaku) 4.55 parts Polyvinyl acetal resin (Sekisui Kagaku) 3.85 parts Hydrocarbon type wax (Microfine MF-8F, Dura) 0.12 part Methyl ethyl ketone 45.8 parts Toluene 45.8 parts Example A-2 A heat transfer sheet was obtained in the same manner as in Example A-1 except for changing the drying conditions after coating of the back heat-resistant lubricating layer to a drying temperature of 100 to 110°C and a residence time in the drying hood to 40 seconds. Example A-3 A heat transfer sheet was obtained in the same manner as in Example A-1 except for using a polyethylene terephthalate film with a thickness of 6 µm (6CF53, Toray) as the substrate film. Comparative Example A-1 A heat transfer sheet was obtained in the same manner as in Example A-1 except for changing the drying conditions after coating of the heat-resistant lubricating layer to a drying temperature of 140°C and a residence time in the drying hood of 120 seconds. Comparative Example A-2 A heat transfer sheet was obtained in the same manner as in Example A-3 except for using a polyethylene terephthalate film of 6 µm having a stretching degree in the MD direction increased to great extent as the substrate film. Comparative Example A-3 A heat transfer sheet was obtained in the same manner as in Example A-3 except for using a polyethylene terephthalate film of 6 µm having a stretching degree in the TD direction increased to great extent as the substrate film.

Claims: A heat transfer sheet comprising a lubricating layer provided on one surface of a substrate film and a dye layer formed on the other surface of the substrate film, characterized in that the elastic modulus in at least one of the sub-scanning direction (MD) and the main scanning direction (TD) in said heat transfer sheet is 280 Kg/mm² or more, and the elastic modulus ratio MD/TD in the sub-scanning direction (MD) and the main scanning direction (TD) is within the range of from 0.8 to 1.3. A heat transfer sheet according to claim 1, wherein the heating shrinkages in the sub-scanning direction (MD) and the main scanning direction (TD) are 0 to 2.5 % under the conditions of 150°C and 30 minutes. A heat transfer sheet according to claim 1 or 2, wherein said lubricating layer has heat resistance. A heat transfer sheet according to claim 1 or 2, wherein at least one of said dye layer, substrate film and lubricating layer contains an antistatic agent.

Inventors: FURUSE MINORU C O DAI NIPPON I, KUTSUKAKE MASAKI C O DAI NIPPO, SAITO HITOSHI C O DAI NIPPON I, YAMAUCHI MINEO C O DAI NIPPON, FURUSE, MINORU, C/O DAI NIPPON INSATSU K.K., KUTSUKAKE, MASAKI, C/O DAI NIPPON INSATSU K.K., SAITO, HITOSHI, C/O DAI NIPPON INSATSU K.K., YAMAUCHI, MINEO, C/O DAI NIPPON INSATSU K.K.
Applicants: DAINIPPON PRINTING CO LTD, DAI NIPPON INSATSU KABUSHIKI KAISHA
Application year: 1990
Main classification: B41M5
IPC codes: B41M5