Heat transfer sheet

Abstract: The present invention relates to a heat transfer sheet comprising a substrate sheet provided on the same surface with a first heat transfer layer comprising a thermally migratable dye and an untransferable binder and a second heat transfer layer comprising a dyed or pigmented, heat-meltable binder, the substrate sheet comprising a polyester film and at least the surface having the heat transfer layers being made easily bondable. The heat transfer sheet enables excellent gray scale images for photographs etc. formed together with monochromic high-density images, such as verbal, numerical or other images.

Description: This is a divisional application to copending European patent application no. 90910943.1 (publication no. 0 487 727). The present invention relates to a heat transfer sheet. More particularly, the present invention relates to a heat transfer sheet enabling excellent gray scale images for photographs etc. formed together with monochromic, high-density images being improved in terms of such durability as rub resistance and allowing them to develop color and luster so well. The present invention also concerns a heat transfer process for making cards using the heat transfer sheet of the present invention. So far, heat transfer techniques have been widely used for simple and expeditious printing. Allowing various images to be produced expeditiously, these heat transfer techniques have incidentally been employed for prints usually made in a small number, e.g. for preparing ID or other cards. Where it is desired to obtain color images like photographs of face, another type of heat transfer technique is now available, making use of heat transfer films of continuous length comprising a continuous substrate film on which a number of heat transfer layers colored in yellow, magenta and cyan (and black, if necessary) are formed successively and repeatedly. Such heat transfer sheets are generally broken down into two types, one referred to as a so-called wax type of heat transfer film in which a heat transfer layer is thermally softened and transferred onto an image-receiving material in an imagewise manner and the other a so-called sublimation type of heat transfer film in which only a dye sublimes (migrates) thermally from within a heat transfer layer onto an image receiving sheet after an imagewise pattern. When ID or other cards are to be produced with such heat transfer films as mentioned above, the wax type of heat transfer film has the advantage of being capable of forming verbal, numerical or other images, but involves the disadvantage that such images are poor in durability, esp., rub resistance. With the sublimation type of heat transfer film, on the other hand, it is possible to obtain gray scale images, i.e., gradation pattern, like photographs of face. Unlike those obtained with ordinary ink, however, the formed images are less lustrous for lack of any vehicle and, by the same token, are poor in durability, e.g. rub resistance. Such items of information as characters, signs and bar codes carried on cards, e.g. ID cards are required to be recorded in black at high density rather than on a gray scale Thus such items of information are desired to be recorded with a heat meltable type of heat transfer sheet. With that purpose in mind, there has been proposed a mixed type of heat transfer sheet in which a sublimation type of dye layer and a heat meltable of ink layer are successively provided on the same substrate sheet (see Japanese Patent Laid-Open Publication (KOKAI) No. 63-9574). With this mixed type of heat transfer sheet, excellent gray scale images for photographs for faces, etc. are formed together with monochromic, high-density images for characters, signs and the like. In the case of such a mixed type of heat transfer sheet as aforesaid, it is required for the sublimation type of dye layer that only the dye migrate onto the image-receiving material while the binder remain on the substrate sheet. In other words, the dye layer is required to be well adhesive to the substrate sheet. For the wax type of ink layer, it is required that the ink layer be transferred onto the image-receiving material in its entirety. To put it another way, the ink layer should be well releasable from the substrate sheet. Such requirements may possibly be met by forming a heat meltable type of ink layer with a well-releasable substrate sheet and forming an adhesive layer on its region to be provided with a sublimation type of dye layer or, alternatively, providing a substrate sheet including an adhesive layer with a release layer and forming a heat meltable ink layer on that release layer. A problem with forming such an adhesive layer, however, is that the heat sensitivity of the sublimable dye layer is so decreased that no satisfactory gray scale image can be obtained, because more energy is generally required for the heat transfer of the sublimable dye layer than for the transfer of the heat meltable ink layer. To avoid this, the adhesive layer should be made as thin as possible. Still, some difficulty has been involved so far in providing an adhesive layer of the order of submicrons uniformly, thus offering such problems as unevenness of printing and unusual (or overall) transfer of dye layers. It is therefore an object of this invention to provide a heat transfer sheet enabling an image having an improved gray scale to be easily produced simultaneously with high-density verbal, numerical or other images. This object is achievable by the heat transfer sheet of the present invention, in which a substrate sheet is provided on the same surface with a first heat transfer layer comprising a thermally migratable dye and an untransferable binder and a second heat transfer layer comprising a dyed or pigmented, heat-meltable binder, characterized in that said substrate sheet is made of a polyester film treated on at least the surface having said heat transfer layers to be provided with an adhesive layer having a thickness of 0.001 to 1 µm. By using as a substrate sheet a polyester film made readily bondable to heat transfer layers, it is possible to provide a heat transfer sheet enabling a clear gray scale image and a clear verbal or other image to be made at the same time. Such a heat transfer sheet as described above is especially useful for forming the images required to have a cover film. For that purpose, this heat transfer sheet may also have a transparent layer for such a cover film. By using this heat transfer sheet in combination with the aforesaid heat transfer cover film, it is possible to obtain high-quality image representations. Such a heat transfer cover film is characterized by an ionizing radiation-cured resin layer releasably formed on a substrate film. By forming an ionizing radiation-cured resin layer on a substrate film in a releasable manner and transferring that layer onto the surface of a transfer image, it is possible to provide expeditious production of an excellent, curl-free image representation which is improved in terms of such properties as durability, esp. rub resistance, gloss and color development. In a particularly preferable embodiment, a relatively large amount of transparent particles may be incorporated in the ionizing radiation-cured resin layer, whereby a protective layer having a much more improved rub resistance is heat transferable, because the film can be well cut during heat transfer. Another heat transfer cover film is characterized by a wax-containing transparent resin layer releasably formed on a substrate film. By forming a wax-containing resin layer on a substrate film in a releasable manner and transferring it onto the surface of a transfer image, it is possible to provide expeditious production of an excellent, curl-free image representation which is improved in terms of such properties as durability, esp. rub resistance, gloss and color development, since that layer can be easily transferred onto the image by the heat used for printing. Still another heat transfer cover film is characterized by a silicone-modified transparent resin layer releasably formed on a substrate film. By forming a silicone-modified transparent resin layer on a substrate film in a releasable manner and transferring it onto the surface of a transfer image, it is possible to provide expeditious production of an image representation which is improved in terms of such properties as durability, esp. rub resistance, chemical resistance and solvent resistance, since the transparent resin layer is easily transferable onto the image by the heat used far printing. Still another heat transfer cover film includes a substrate film having a transparent resin layer releasably formed thereon, said resin layer being further provided on its surface with a heat-sensitive adhesive layer, characterized in that said heat-sensitive adhesive layer is made of a resin having a glass transition temperature or Tg lying between 40°C and 75°C. By constructing from a resin with a Tg of 40-75°C a heat-sensitive adhesive layer provided on the surface of a transparent resin layer, the transparent resin layer can be well transferred onto an image through a thermal head while it is kept in good "foil cutting" condition. Thus the transparent resin layer is so easily transferred on the image by the heat of the thermal head that an image representation improved in terms of such properties as durability, esp. rub resistance, chemical resistance and solvent resistance can be obtained expeditiously. The aforesaid heat transfer sheet will now be explained more illustratively with reference to its preferred embodiments. In the present disclosure, the "polyester film made easily bondable" refers to a polyester film provided thereon with a very thin, uniform adhesive layer. In order to obtain such an adhesive layer, it is preferred that heat-, catalyst- and ionizing radiation-curable type of crosslinked resins, for instance, polyurethane, acrylic, melamine or epoxy resins are first dispersed in water or dissolved in organic solvents to prepare coating solutions. They may then be coated on the aforesaid polyester film by any desired coating means, for instance, blade coating, gravure coating, rod coating, knife coating, reverse roll coating, spray coating, offset gravure coating or moss coating, followed by drying. Of importance in this case is the thickness of the adhesive layer formed. At too large a thickness the heat sensitivity of the sublimation type of dye layer drops, whereas at too small a thickness such unusual transfer of dye layers as mentioned above takes place. Thus the adhesive layer should have a thickness lying in the range of 0.001 to 1 µm, preferably 0.05 to 0.5 µm. It is particularly preferred that the adhesive layer formed be of uniform thickness. For instance, this is achieved by forming a few-µm thick adhesive layer before stretching the polyester film and then biaxially stretching that film, whereby the adhesive layer can be made uniform and reduced to as thin as 1 µm or less in thickness. Particularly preferable as the aforesaid polyester film is a film of polyethylene terephthalate or polyethylene naphthalate, which is commercially available or may be prepared by known methods (see, for instance, Japanese Patent Laid-Open Publication Nos. 62-204939 and 62-257844). Such a substrate sheet as aforesaid may have a thickness enough to assure some heat resistance and strength, say, 0.5 to 50 µm, preferably about 3 µm to about 10 µm. The sublimation type of dye layer that is the first heat transfer layer formed on the surface of the substrate sheet contains a sublimable dye carried by any desired binder resin. Any dye so far used for conventional known heat transfer sheets may be effectively applied to this end without exception. By way of example alone, use may be made of dye reds such as MS® Red G, Macrolex® Red Violet R, Ceres® Red 7B, Samaron® Red HBSL and Resolin® Red F3BS; yellow dyes such as Foron® Brilliant Yellow 6GL, PTY-52 and Macrolex® Yellow 6G; and blue dyes such as Kayaset® Blue 714, Vacsolin® Blue AP-FW, Foron® Brilliant Blue S-R and MS Blue 100. Known resins may all be used as the binders for carrying such dyes as aforesaid. By way of example, preferable-are cellulosic resins such as ethylcellulose, hydroxyethylcellulose, ethylhydroxycellulose, hydroxypropylcellulose, methylcellulose, cellulose acetate and cellulose acetate butyrate; vinylic resins such as polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, polyvinyl acetal, polyvinyl pyrrolidone and polyacrylamide; polyester; and the like. Of these resins, preference is given to resins based on cellulose, acetal, butyral and polyester in consideration of such properties as heat resistance and dye migration. Such a dye layer may preferably be formed by dissolving or dispersing the aforesaid sublimable dye and binder resin as well as other components, e.g. releasants in suitable solvents to prepare a coating or ink material for forming the dye layer and coating it on the aforesaid substrate sheet, followed by drying. The dye layer formed in this manner may have a thickness of 0.2 to 5.0 µm, preferably about 0.4 to about 2.0 µm, and the sublimable dye may preferably account for 5 to 90% by weight, preferably 10 to 70% by weight of the dye layer. When it is desired to obtain a monochromic image, the dye layer may be made from one selected from the group consisting of the aforesaid dyes. When it is desired to obtain a full-color image, on the other hand, the dye layer may be formed choosing suitable cyan, magenta and yellow (and, if necessary, black) dyes. In this invention, the heat meltable ink layer is located in parallel to the aforesaid sublimable dye layer or layers. In what order these dye layers are arranged is not critical. For instance, yellow, magenta and cyan dye layers and a heat-meltable, black ink layer may be successively formed according to an A4 size. The aforesaid ink layer comprises a dyed or pigmented, heat-meltable binder. A preferable colorant is carbon black, but other dyes or pigments of different hues may be used as well. The binder used may be a thermoplastic resin or wax having a relatively low melting point or their mixture, but care should preferably taken of its adhesion to the associated image-receiving material. For instance, when the image-receiving material is a vinyl chloride resin often used for ID cards, thermoplastic resins such as (meth)acrylic ester, vinyl chloride/vinyl acetate copolymer resin, ethylene/vinyl acetate copolymer resin and polyester resin are preferable. In order to form the heat meltable ink layer on the substrate sheet, the aforesaid ink materials may be coated thereon by not only hot melt coating but also a number of other coating means as well, inclusive of hot melt coating, hot lacquer coating, gravure coating, gravure reverse coating and roll coating. Required to be determined with harmony between the required density and heat sensitivity in mind, the ink layer formed preferably lies in the range of 0.2 to 3.0 µm. At too small a thickness the reflection density of the transfer image is insufficient, whereas at too large a thickness the "foil cutting" at the time of printing degrades, resulting in a drop of the sharpness of the printed image. In this invention, the substrate sheet has preferably included a release protective layer on its surface before forming the aforesaid ink layer. This release protective layer serves to improve the releasability of the ink layer and is transferred along with the ink layer, giving a surface protective layer on the transfer image and thereby improving its rub resistance, etc. Such a release protective layer may be made of (meth)acrylic resin, silicone base resin, fluorine base resin, cellulosic resin such as cellulose acetate, epoxy base resin, polyvinyl alcohol and the like, which contain waxes, organic pigments, inorganic pigments and the like, and may preferably have a thickness of 0.2 to 2.5 µm. At too small a thickness it fails to produce sufficient protective effects such as scratch resistance, whereas at too large a thickness the "foil cutting" at the time of printing goes worse. The heat transfer film used in this invention is made of a polyester film made easily bondable, and a water soluble polymer may be used as a release layer. As such a water soluble polymer, use is preferably made of polyvinyl alcohol, polyvinyl pyrrolidone, gelatin, carboxymethylcellulose, methylcellulose, polyethylene oxide, gum arabic, water soluble butyral, water soluble polyester, water soluble polyurethane, water soluble polyacrylic and water soluble polyamide, which may be used in combination of two or more to control releasability. The release layer may then have a thickness of about 0.01 µm to about 5 µm. In this invention, it is preferred that a heat-sensitive adhesive layer be additionally provided on the aforesaid ink layer. This adhesive layer should again be chosen in consideration of its adhesion to the associated image-receiving material. Such an adhesive layer, for instance, may be formed by coating on that surface resins of improved hot adhesiveness such as acrylic resin, vinyl chloride resin, vinyl chloride/ vinyl acetate copolymer resin and polyester resin, followed by drying. For instance, when the image-receiving material is a card material made of a resin based on vinyl chloride, it is preferable to use such a well-adhesive thermoplastic resin as aforesaid. The adhesive layer formed should preferably have a thickness lying in the range of 0.05 to 1.0 µm. At too small a thickness no desired adhesion is obtained, whereas at too large a thickness the "foil cutting" at the time of printing goes worse. The aforesaid heat transfer sheet may also include a cover film. In the present invention, it is further preferred that the aforesaid substrate sheet be provided on its back surface with a heat-resistant slip layer adapted to prevent a thermal head from sticking to it and improve its slip properties. The image-receiving material used to make images with such a heat transfer sheet as aforesaid may be made of any material with the recording surface showing dye receptivity with respect to the aforesaid dye. When made of a dye receptivity-free material such as paper, metals, glass or synthetic resin, it may have been provided with a dye-receiving layer on at least its one surface. The heat transfer sheet of this invention is particularly fit for the preparation of cards made of polyvinyl chloride resin. With no need of forming any special dye-receiving layer, a gray scale image comprising the sublimable dye layer and characters, signs, bar codes, etc. comprising the meltable ink layer may be printed directly on these card materials. In this invention, a particularly preferable card material contains a plasticizer in an amount of 0.1 to 10 parts by weight, preferably 1 to 5 parts by weight per 100 parts by weight of polyvinyl chloride. Moreover, it should be well receptible with respect to the sublimable dye and well adhesive to the meltable ink. In a more preferred embodiment, the card material contains, in addition to the aforesaid plasticizer, a slip agent in an amount of 0.1 to 5 parts by weight per 100 parts by weight of polyvinyl chloride. According to that embodiment, it is found that even when a relatively large amount, e.g. 1 to 5 parts by weight of the plasticizer is incorporated in the polyvinyl chloride, the card material offers no blocking problem with respect to the heat transfer sheet, and is improved in terms of its receptivity with respect to the sublimable dye. Such a polyvinyl chloride card material as aforesaid may be obtained by blending together the required components and forming the blend into a sheet of, e.g. about 0.05 mm to about 1 mm in thickness by known means such as calendering or extrusion, and may be in the form of either a card or a sheeting which will be cut into card size. Also, the card material may be of a monolayer or multilayer structure, in which latter case, for instance, a white pigment-containing center core is provided with a transparent resin layer on at least its one surface. It is understood that the heat transfer sheet of this invention is never limited to preparing polyvinyl chloride cards. For instance, it is not only suited for making image-receiving materials other than cards, e.g. passports, to say nothing of polyester cards, but is also useful for producing various prints inclusive of less sophisticated catalogs, for which gray scale images and monochromic images for characters, signs, bar codes, etc. are required at the same time. Energy applicator means so far known in the art may all be used to apply heat energy to carry out heat transfer with such heat transfer sheet and image-receiving material as mentioned above. For instance, the desired images may be obtained by the application of a heat energy of about 5 mJ/mm 2 to about 100 mJ/mm 2 for a time controlled by recording hardware such as a thermal printer (e.g. Video Printer VY-l00 made by Hitachi, Ltd.) According to this invention wherein the substrate sheet used is a polyester film made easily bondable, as described above, there is provided a heat transfer sheet capable of forming clear gray scale images and clear verbal or other images at the same time. With this heat transfer sheet, it is possible to provide an excellent card. The present invention will now be explained more illustratively with reference to the reference examples, examples, application examples and comparative examples, wherein unless otherwise stated, the "parts" and "%" are given by weight. Preparation Example A1 Three ink compositions containing sublimable dyes of different colors were prepared with the components mentioned just below. Yellow Ink Disperse dye (Macrolex® Yellow 6G made by Bayer Co., Ltd.) 5.5 parts Polyvinyl butyral resin (Eslec® BX-l made by Sekisui Chemical Co., Ltd.) 4.5 parts Methyl ethyl ketone/toluene (at a weight ratio of 1:1) 89.5 parts Magenta Ink This ink was similar to the yellow ink with the exception that a magenta disperse dye (Disperse Red 60) was used. Cyan Ink This ink was similar to the yellow ink, provided that a cyan disperse dye (Solvent Blue 63) was used. Provided as a substrate film was a 6.0-µm thick polyester film Lumirror® made by Toray Industries, Ltd.) having on its back surface a heat-resistant slip layer (of 1 µm in thickness) and on its front surface a primer layer (of 0.5 µm in thickness) comprising a polyurethane base resin. Using gravure coating, the aforesaid ink compositions were successively and repeatedly coated on the front surface of the substrate film in the order of yellow, magenta and cyan, at a width of 15 cm and to a coverage of about 3 g/m 2 . Subsequent drying gave a sublimation type of heat transfer sheet containing sublimable dye layers of three different colors. Preparation Example A2 The following wax ink composition, heated at a temperature of 100°C, was coated on the same substrate film as used in Preparation Ex. A1 but including no primer layer, to a coverage of about 4 g/m 2 by hot melt roll coating, thereby preparing a wax type of heat transfer sheet. Wax Ink Ester wax 10 parts Wax oxide 10 parts Paraffin wax 60 parts Carbon black 12 parts Preparation Example A3 Using gravure coating, the following ink composition was coated on the same substrate film as used in Preparation Ex. A2 at a ratio of 1 g/m 2 on dry solid basis. Subsequent drying gave a release layer. Ink for Release Layer Silicone base resin 10 parts Vinyl chloride/vinyl acetate copolymer 10 parts Methyl ethyl ketone 100 parts Toluene 100 parts Then, the following ink was coated on the surface of the aforesaid release layer at a ratio of 10 g/m 2 on dry solid basis. Subsequent drying gave an ionizing radiation-curable resin layer. Ink for Ionizing Radiation-Curable Resin Layer Dipentaerythritol hexacrylate 40 parts Hydrophobic colloidal silica 40 parts Polymethyl methacrylate 20 parts Polyethylene wax 3 parts Methyl ethyl ketone 250 parts Toluene 250 parts Then, the following ink composition was coated on the surface of the aforesaid resin layer at a ratio of 1 g/m 2 on dry solid basis, followed by drying which gave an adhesive layer. After that, the product was exposed to electron beams of 180 KV at a dose of 5 Mrad in a nitrogen atmosphere of 10 -7 Torr with an electron beam irradiator made by Nisshin High Voltage Co., Ltd. to cure the ionizing radiation-curable resin layer, thereby obtaining a heat transfer cover film used in this invention. Ink for Adhesive Layer Vinyl chloride/vinyl acetate copolymer 10 parts Methyl ethyl ketone 100 parts Toluene 100 parts Preparation Example A4 The procedures of Preparation Example A3 were followed with the exception that the following ionizing radiation-curable ink was used, thereby obtaining a heat transfer cover film used in this invention. Ink for Ionizing Radiation-Cured Resin Layer Trimethylolpropane triacrylate 60 parts Talc (Microace® L-l made by Nippon Talc Co., Ltd.) 10 parts Polymethyl methacrylate 30 parts Fluorine base surfactant (Flow Lard® 432 made by Sumitomo 3M Co., Ltd.) 3 parts Methyl ethyl ketone 200 parts Toluene 200 parts Preparation Example B1 Three ink compositions containing sublimable dyes of different colors were prepared with the components mentioned just below. Yellow Ink Disperse dye (Macrolex® Yellow 6G made by Bayer Co., Ltd.) 5.5 parts Polyvinyl butyral resin (Eslec® BX-l made by Sekisui Chemical Co., Ltd.) 4.5 parts Methyl ethyl ketone/toluene (at a weight ratio of 1:1) 89.0 parts Magenta Ink This ink was similar to the yellow ink with the exception that a magenta disperse dye (Disperse Red 60) was used. Cyan Ink This ink was similar to the yellow ink, provided that a cyan disperse dye (Solvent Blue 63) was used. Provided as a substrate film was a 6.0-µm thick polyester film (Lumirror® made by Toray Industries, Ltd.) having on its back surface a heat-resistant slip layer (of 1 µm in thickness) and on its front surface a primer layer (of 0.5 µm in thickness) comprising a polyurethane base resin. Using gravure coating, the aforesaid ink compositions were successively and repeatedly coated on the front surface of the substrate film in the order of yellow, magenta and cyan, at a width of 15 cm and to a coverage of about 3 g/m 2 . Subsequent drying gave a sublimation type of heat transfer sheet containing sublimable dye layers of three different colors. Preparation Example B2 The following wax ink composition, heated at a temperature of 100°C, was coated on the same substrate film as used in Preparation Ex. B1 but including no primer layer, to a coverage of about 4 g/m 2 by hot melt roll coating, thereby preparing a wax type of heat transfer sheet. Wax Ink Acrylic/vinyl chloride/vinyl acetate copolymer resin 20 parts Carbon black 10 parts Toluene 35 parts Methyl ethyl ketone 35 parts Preparation Example B3 Using gravure coating, the following ink composition was coated on the same substrate film as used in Preparation Ex. B2 at a ratio of 1 g/m 2 on dry solid basis. Subsequent drying gave a release layer. Ink for Release Layer Acrylic resin 20 parts Methyl ethyl ketone 100 parts Toluene 100 parts Then, the following ink was coated on the surface of the aforesaid release layer at a ratio of 3 g/m 2 on dry solid basis. Subsequent drying gave a transparent resin layer. Ink for Transparent Resin Layer Acrylic resin 20 parts Polyethylene wax 1 part Methyl ethyl ketone 50 parts Toluene 50 parts Then, the following ink composition was coated on the surface of the aforesaid resin layer at a ratio of 1 g/m 2 on dry solid basis, followed by drying which gave an adhesive layer. In this way, a heat transfer cover film used in this invention was prepared. Ink for Adhesive Layer Acrylic resin 10 parts Vinyl chloride/vinyl acetate copolymer 10 parts Methyl ethyl ketone 100 parts Toluene 100 parts Preparation Example B4 The procedures of Preparation Example B3 were followed with the exception that the following ink for the transparent resin layer was used, thereby obtaining a heat transfer cover film used in this invention. Ink for Transparent Resin Layer Aqueous emulsion of acrylic resin (with a solid matter content of 30 %) 20 parts Aqueous emulsion of paraffin wax (with a solid matter content of 30 %) 3 parts Water 20 parts Isopropyl alcohol (Drying was carried out at 50 to 55°C). 10 parts Preparation Example C1 Three ink compositions containing sublimable dyes of different colors were prepared with the components mentioned just below. Yellow Ink Disperse dye (Macrolex® Yellow 6G made by Bayer Co., Ltd.) 5.5 parts Polyvinyl butyral resin (Eslec® BX-l made by Sekisui Chemical Co., Ltd.) 4.5 parts Methyl ethyl ketone/toluene (at a weight ratio of 1:1) 89.5 parts Magenta Ink This ink was similar to the yellow ink with the exception that a magenta disperse dye (Disperse Red 60) was used. Cyan Ink This ink was similar to the yellow ink, provided that a cyan disperse dye (Solvent Blue 63) was used. Provided as a substrate film was a 6.0-µm thick polyester film (Lumirror® made by Toray Industries, Ltd.) having on its back surface a heat-resistant slip layer (of 1 µm in thickness) and on its front surface a primer layer (of 0.5 µm in thickness) comprising a polyurethane base resin. Using gravure coating, the aforesaid ink compositions were successively and repeatedly coated on the front surface of the substrate film in the order of yellow, magenta and cyan, at a width of 15 cm and to a coverage of about 3 g/m 2 . Subsequent drying gave a sublimation type of heat transfer sheet containing sublimable dye layers of three different colors. Preparation Example C2 The following wax ink composition, heated at a temperature of 100°C, was coated on the same substrate film as used in Preparation Ex. C1 but including no primer layer, to a coverage of about 4 g/m 2 by hot melt roll coating, thereby preparing a wax type of heat transfer sheet. Wax Ink Acrylic/vinyl chloride/vinyl acetate copolymer resin 20 parts Carbon black 10 parts Toluene 35 parts Methyl ethyl ketone 35 parts Preparation Example C3 Using gravure coating, the following ink composition was coated on the same substrate film as used in Preparation Ex. C2 at a ratio of 1 g/m 2 on dry solid basis. Subsequent drying gave a transparent resin layer. Ink for Transparent Resin Layer Acrylic silicone resin (US3l0 made by Toa Gosei K.K.) 60 parts Microsilica 20 parts Methyl ethyl ketone 20 parts Toluene 20 parts Then, the following ink was coated on the surface of the aforesaid resin layer at a rate of 0.5 g/m 2 on dry solid basis. Subsequent drying gave an adhesive layer. In this way, a heat transfer cover film used in this invention was obtained. Ink for Adhesive Layer Nylon (FS-l75SVl6 made by Toa Gosei K.K.) 50 parts Microsilica 0.4 parts Modified ethanol 50 parts Preparation Example C4 The procedures of Preparation Example C3 were followed with the proviso that the following ink for the transparent resin layer was used, thereby obtaining a heat transfer cover film used in this invention. Ink for Transparent Resin Layer Acryl silicone resin (US350 made by Toa Gosei K.K.) 60 parts Microsilica 0.4 parts Methyl ethyl ketone 20 parts Toluene 20 parts Preparation Example D1 Three ink compositions containing sublimable dyes of different colors were prepared with the components mentioned just below. Yellow Ink Disperse dye (Macrolex® Yellow 6G made by Bayer Co., Ltd.) 5.5 parts Polyvinyl butyral resin (Eslec® BX-l made by Sekisui Chemical Co., Ltd.) 4.5 parts Methyl ethyl ketone/toluene (at a weight ratio of 1:1) 89.5 parts Magenta Ink This ink was similar to the yellow ink with the exception that a magenta disperse dye (Disperse Red 60) was used. Cyan Ink This ink was similar to the yellow ink, provided that a cyan disperse dye (Solvent Blue 63) was used. Provided as a substrate film was a 6.0-µm thick polyester film (Lumirror® made by Toray Industries, Ltd.) having on its back surface a heat-resistant slip layer (of 1 µm in thickness) and on its front surface a primer layer (of 0.5 µm in thickness) comprising a polyurethane base resin. Using gravure coating, the aforesaid ink compositions were successively and repeatedly coated on the front surface of the substrate film in the order of yellow, magenta and cyan, at a width of 15 cm and to a coverage of about 3 g/m 2 . Subsequent drying gave a sublimation type of heat transfer sheet containing sublimable dye layers of three different colors. Preparation Example D2 The following wax ink composition, heated at a temperature of 100°C, was coated on the same substrate film as used in Preparation Ex. D1 but including no primer layer, to a coverage of about 4 g/m 2 by hot melt roll coating, thereby preparing a wax type of heat transfer sheet. Wax Ink Acrylic/vinyl chloride/vinyl acetate copolymer resin 20 parts Carbon black 10 parts Toluene 35 parts Methyl ethyl ketone 35 parts Preparation Example D3 Using gravure coating, the following ink composition was coated on the same substrate film as used in Preparation Ex. D2 at a ratio of 1 g/m 2 on dry solid basis. Subsequent drying gave a transparent resin layer. Ink for Transparent Resin Layer Acrylic silicone graft resin (XSA-l00 made by Toa Gosei K.K.) 60 parts Methyl ethyl ketone 20 parts Toluene 20 parts Then, the following ink was coated on the surface of the aforesaid resin layer at a rate of 0.7 g/m 2 on dry solid basis. Subsequent drying gave an adhesive layer. In this manner, a heat transfer cover film used in this invention was obtained. Ink for Adhesive Layer Vinyl chloride/vinyl acetate copolymer (VYLF made by UCC; Tg=68°C and polymerization degree = 220) 30 parts Microsilica 0.4 parts Methyl ethyl ketone 35 parts Toluene 35 parts Preparation Example D4 The procedures of Preparation Ex. D3 were followed with the exception that a vinyl chloride/vinyl acetate copolymer (Denka Lac® #2lZA made by Denki Kagaku Kogyo K.K.; and with Tg=62°C and a polymerization degree of 240) was used as the adhesive, thereby obtaining a heat transfer cover film used in this invention. Preparation Example D5 The procedures of Preparation Ex. D3 were followed with the exception that a vinyl chloride/vinyl acetate copolymer (VYHH made by UCC; and with Tg=72°C and a polymerization degree of 450) was used as the adhesive, thereby obtaining a heat transfer cover film used in this invention. According to the cover film used in the present invention as aforesaid, wherein the heat-sensitive adhesive layer formed on the surface of the transparent resin layer is made of a resin whose Tg lies in the range of 40 to 75°C, the transparent resin layer can be well transferred on an image, while it can be well cut, by means of a thermal head. Thus, since the transparent resin layer is easily transferable onto the image by the heat of the thermal head, it is possible to provide expeditious production of an image representation improved in terms of such properties as durability, esp. rub resistance, chemical resistance and solvent resistance. Preparation Example E1 Polyvinyl butyral resin (Eslec® BX-l made by Sekisui Chemical Co., Ltd.) 5.0 parts Disperse dye (PTY-52 made by Mitsubishi Chemical Industries, Ltd.) 2.0 parts Silicone-modified acrylic resin (XS-3l5 made by Toa Gosei K.K.) 0.2 parts Methyl ethyl ketone/toluene (at a weight ratio of 1:1) 60.0 parts By gravure coating, the aforesaid coating solution was coated on one surface of a 6.0-µm thick polyester film having a heat-resistant slip layer on the other surface (S-PET made by Toyobo Co., Ltd.) to a coverage of about 3 g/m 2 on dry solid basis. Subsequent drying gave a heat transfer sheet. Vinyl chloride/vinyl acetate copolymer (Denka® lOOOA made by Denki Kagaku Kogyo K.K.) 20.0 parts Dimethylsiloxane (KF-96 made by The Shin-Etsu Chemical Co., Ltd.) 0.2 parts Methyl ethyl ketone/toluene (at a weight ratio of 1:1) 80.0 parts With a Miya bar #20, the aforesaid coating solution was coated on the surface of a white polyethylene terephthalate film (PETE-20 made by Toray Industries, Inc.; and with a thickness of 188 µm) at a rate of 5 g/m 2 on dry solid basis. Subsequent drying gave a heat transfer sheet. Nought decimal five (0.5) g/m 2 of a release layer (an acrylic resin TP-64 Varnish made by DIC K.K.), 3.0 g/m 2 of a transparent protective layer (an acrylic resin BR-53 made by Mitsubishi Rayon Co., Ltd. and 0.5 g/m 2 of a heat-sensitive adhesive layer (a vinyl chloride/vinyl acetate copolymer Denka® l000A made by Denki Kagaku Kogyo K.K.) were successively coated on the surface of a polyethylene terephthalate film (S-PET made by Toyobo Co., Ltd.; and with a thickness of 9 µm). Subsequent drying gave a heat transfer cover film. The heat transfer sheet was overlaid on the heat transfer image-receiving sheet while the former's dye layer was in opposition to the latter's dye-receiving layer. With a thermal sublimation type of transfer printer (VY50 made by Hitachi, Ltd.), a printing energy of 90 mJ/mm 2 was then applied to the back side of the heat transfer sheet through the thermal head to make an image. Finally, the transparent protective film was transferred from the heat transfer cover film onto the image under similar conditions. In consequence, the transparent protective layer could be easily transferred onto the image. They remained so well bonded to each other that they could hardly be separated from each other. Preparation Example E2 The transfer of the transparent protective layer was performed with a laminator made by Meiko Shokai K.K. As a result, that layer could be easily transferred onto the image. They remained so well bonded to each other that they could hardly be separated from each other. Preparation Example E3 Experimentation was carried out by following the procedures of Preparation Example E1 with the proviso that the dye layer was made from the following coating solution. As a result, the transparent protective layer could be easily transferred onto the image. They remained so well bonded to each other that they could hardly be separated from each other. Polyvinyl butyral resin (Eslec® BX-l made by Sekisui Chemical Co., Ltd.) 5.0 parts Disperse dye (KST-B-l36 made by Nippon Kayaku K.K.) 0.5 part Fluorine-modified silicone (FLl00 made by The Shin-Etsu Chemical Co., Ltd.) 0.2 parts Methyl ethyl ketone/toluene (at a weight ratio of 1:1) 60.0 parts Preparation Example E4 The procedures of Ex. E1 were followed with the exception that the dye-receiving layer was made from the following coating solution. In consequence, the transparent protective layer could be easily transferred onto the image. They remained so well bonded to each that they could hardly be separated from each other. Polyester resin (Vylon® 600 made by Toyobo Co., Ltd.) 20.0 parts Epoxy-modified silicone (KF-393 made by The Shin-Etsu Chemical Co., Ltd.) 0.5 parts Methyl ethyl ketone/toluene (at a weight ratio of 1:1) 80.0 parts Example A1 Provided as a substrate film was a 6-µm thick polyethylene terephthalate film having a O.1-µm thick, easily bondable layer on one surface and a heat-resistant slip layer on the other surface. A toluene solution of an acrylic resin comprising 10 parts of TR-64 Varnish (made by Dainippon Ink & Chemicals, Inc.) and 40 parts of toluene was coated on said one surface of the polyethylene terephthalate film, while leaving three regions of A4 size, to a dry thickness of 0.7 µm, followed by drying which resulted in a releasable protective layer being formed on such regions. Subsequently, a black ink comprising 10 parts of MSF (made by Toyo Ink Mfg. Co., Ltd.) and 40 parts of toluene was coated on the surface of that layer to a dry thickness of 2 µm, followed by drying which gave a heat-meltable ink layer. Further, a toluene solution of an acrylic resin comprising 10 parts of TR-64 varnish (made by Dainippon Ink & Chemicals, Inc.) and 40 parts of toluene was coated on the surface of that ink layer to a dry thickness of 0.5 µm, followed by drying which gave a heat-sensitive adhesive layer. Moreover, three ink compositions of different colors forming the dye layer were successively gravure printed between the aforesaid ink layers to a dry thickness of 1.0 g/m 2 in the order of yellow, magenta and cyan. Subsequently drying gave a heat transfer sheet of this invention in the form of a continuous film. Yellow Ink PTY-52 (C.I. Disperse Yellow l4l made by Mitsubishi Chemical Industries, Ltd.) 5.50 parts Polyvinyl butyral resin (Eslec® BX-l made by Sekisui Chemical Co., Ltd.) 4.80 parts Methyl ethyl ketone 55.00 parts Toluene 34.70 parts Releasant 1.03 parts Magenta Ink MS® Red G (C.I. Disperse Red 60 made by Mitsui Toatsu Chemicals, Inc.) 2.60 parts Macrolex® Red Violet R (C.I. Disperse Violet 26 made by Bayer Co., Ltd.) 1.40 parts Polyvinyl butyral resin (Eslec® BX-l) 3.92 parts Methyl ethyl ketone 43.34 parts Toluene 43.34 parts Releasant 0.40 parts Cyan Ink Kayaset® Blue 7l4 (C.I. Solvent Blue 63 made by Nippon Kayaku K.K.) 5.50 parts Polyvinyl butyral resin (Eslec® BX-l) 3.92 parts Methyl ethyl ketone 22.54 parts Toluene 68.18 parts Releasant 0.94 parts Example A2 A heat transfer sheet was obtained by following the procedures of Example A1 with the exception that the releasable protective layer having a dry thickness of 0.5 µm was made from an acrylic/vinylic resin solution comprising 10 parts of MCS-5065 (made by Dainippon Ink & Chemicals, Inc.) and 40 parts of toluene. Example A3 A heat transfer sheet was obtained by following the procedures of Example A1 with the exception that the releasable protective layer having a dry thickness of 0.5 µm was made from a chlorinated polyolefinic resin solution comprising 10 parts of TR-l5 varnish (made by Dainippon Ink & Chemicals, Inc.) and 40 parts of toluene. Example A4 A heat transfer sheet according to this invention was obtained by following the procedures of Example A1 with the exception that the substrate film used was a polyethylene naphthalate film (6 µm in thickness) including an easily bondable layer (of 0.2 µm in thickness) made of a heat-curable epoxy resin. Comparative Example A1 A heat transfer sheet according to this invention was obtained by following the procedures of Example A1 with the proviso that the substrate film used was the same polyethylene terephthalate film as used therein, but including no easily bondable layer. Comparative Example A2 A heat transfer sheet according to this invention was obtained by following the procedures of Example A4 with the proviso that the substrate film used was the same polyethylene terephthalate film as used therein, but including no easily bondable layer. Application Example A With the following components, a white card substrate core (of 0.2 µm in thickness and 30 × 30 cm in size) was prepared. Compound of polyvinyl chloride having a polymerization degree of 800 and containing about 10% of such additives as a stabilizer 100 parts White pigment (titanium oxide) 15 parts Then, transparent sheets of 0.15 mm in thickness) were formed of the following components, and were in turn thermally pressed onto both sides of the aforesaid white core to prepare a card substrate. Compound of polyvinyl chloride having a polymerization degree of 800 and containing about 10% of such additives as a stabilizer 100 parts Plasticizer (DOP) 3 parts Slip agent (amide stearate) 0.5 parts Each of the heat transfer sheets according to this invention and for comparative purposes was overlaid on the surface of the aforesaid card substrate, and heat energy was in turn applied thereto through a thermal head connected to electrical signals of the cyan component obtained by the chromatic separation of a photograph of face. Then, the sublimation transfer of magenta and yellow images was carried out to make a full-color image thereof. Moreover, such pieces of information as name and address and bar codes were formed with a wax type of ink layer. Finally, examination was made of whether the unusual transfer of the sublimable dye layers took place and the resolution of the resulting images. The results are set out in Table 5. Heat Transfer Sheets Unusual Transfer Resolution Example A1 Not found Good A2 Not found Good A3 Not found Good A4 Not found Good Comp. Ex. A1 found Bad A2 found Bad Preparation Example F1 A heat transfer cover sheet was prepared by following the procedures of Preparation Example A3 with the proviso that the following water soluble polymer composition was used as the ink for the release layer. Ink for Release Layer Polyvinyl alcohol AH-26 (made by Nippon Gosei Kagaku K.K.) 2.0 parts Ethyl alcohol 49.0 parts Pure water 49.9 parts Preparation Example F2 A heat transfer cover sheet was prepared by following the procedures of Preparation Example A3 with the proviso that the following water soluble polymer composition was used as the ink for the release layer. Ink for Release Layer Polyvinyl alcohol C-500 (made by Nippon Gosei Kagaku K.K.) 2.0 parts Ethyl alcohol 49.0 parts Pure water 49.9 parts Preparation Example F3 A heat transfer cover sheet was prepared by following the procedures of Preparation Example A3 with the proviso that the following water soluble polymer composition was used as the ink for the release layer. Ink for Release Layer Polyvinyl alcohol KL-05 (made by Nippon Gosei Kagaku K.K.) 2.0 parts Polyvinyl alcohol L-5407 (made by Nippon Gosei Kagaku K.K.) 1.8 parts Ethyl alcohol 49.0 parts Pure water 49.9 parts INDUSTRIAL APPLICABILITY The present invention may find wide applications in preparing objects on which prints or images are formed by heat transfer techniques, for instance, ID cards.

Claims: A heat transfer sheet comprising a substrate sheet provided side by side on the same surface with a first heat transfer layer comprising a thermally migratable dye and an untransferable binder and a second heat transfer layer comprising a dyed or pigmented, heat-meltable binder, the substrate sheet comprising a polyester film and at least the surface having the heat transfer layers being provided with an adhesive layer having a thickness of 0.001 to 1 µm. A heat transfer sheet as claimed in Claim 1, wherein the polyester film is a polyethylene terephthalate or polyethylene naphthalate film. A heat transfer sheet as claimed in Claims 1 or 2, wherein the adhesive layer is drawn simultaneously with the substrate sheet. A heat transfer sheet as claimed in anyone of Claims 1 to 3, wherein a release protective layer is interleaved between the second heat transfer layer and the substrate sheet. A heat transfer sheet as claimed in anyone of Claims 1 to 4, wherein the second heat transfer layer is provided with a heat-sensitive adhesive layer on its surface. A heat transfer sheet as claimed in anyone of Claims 1 to 5, in which the second heat transfer layer or the heat-sensitive adhesive layer formed thereon is well adhesive to a vinyl chloride base resin. A heat transfer sheet as claimed in anyone of Claims 1 to 6, which is provided with a heat-resistant slip layer on its back surface. A process for making cards, comprising: Forming a gray scale image and/or non-gray scale image on the surface of a card substrate made of a vinyl chloride resin using the heat transfer sheet as claimed in anyone of Claims 1 to 7. A process as claimed in Claim 8, wherein a transparent protective layer is laminated on the surface of the resulting image in a heat transfer manner. A process as claimed in Claim 9, wherein the lamination of the transparent protective layer is carried out with a heat transfer cover film comprising a substrate film and, releasably formed thereon, either an ionizing-radiation-cured resin layer or a wax-containing transparent resin layer or a silicone-modified transparent resin layer or a transparent resin layer and a heat-sensitive adhesive layer further provided on the transparent resin layer, said heat-sensitive adhesive layer being made of a resin having a glass transition temperature lying in the range of 40 to 75°C. A heat transfer sheet as claimed in Claim 4, wherein the release layer comprises a water soluble polymer.

Inventors: ANDO JITSUHIKO, OSHIMA KATSUYUKI, TORII MASANORI, ANDO, JITSUHIKO, OSHIMA, KATSUYUKI, TORII, MASANORI, ANDO, JITSUHIKO, C/O DAI NIPPON INSATSU K. K., OSHIMA, KATSUYUKI, C/O DAI NIPPON INSATSU K. K., TORII, MASANORI, 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, B41M7