The present invention relates to a thermal transfer sheet and more particularly to a thermal transfer image-receiving sheet capable of forming a record image excellent in the color density, sharpness and various types of fastness, particularly durability such as light fastness, fingerprint resistance and plasticizer resistance.
Various thermal transfer printing processes are known in the art. One of them is a transfer printing process which comprises supporting a sublimable dye as a recording agent on a substrate sheet, such as a polyester film, to form a thermal transfer sheet and forming various full color images on an image-receiving sheet dyeable with a sublimable dye, for example, an image-receiving sheet comprising paper, a plastic film or the like and, formed thereon, a dye-receiving layer.
In this case, a thermal head of a printer is used as heating means, and a number of color dots of three or four colors are transferred to the image-receiving material, thereby reproducing a full color image of an original by means of the multicolor dots.
Since the color material used is a dye, the image thus formed is very clear and highly transparent, so that the resultant image is excellent in the reproducibility and gradation of intermediate colors. Therefore, according to this method, the quality of the image is the same as that of an image formed by the conventional offset printing and gravure printing, and it is possible to form an image having a high quality comparable to a full color photographic image.
Not only the construction of the thermal transfer sheet but also the construction of an image-receiving sheet for forming an image are important for usefully practicing the above-described thermal transfer process.
For example, Japanese Patent Laid-Open Publication Nos. 1639370/1982, 207250/1982 and 25793/1985 disclose prior art techniques applicable to the above-described thermal transfer image-receiving sheet, wherein the dye-receiving layer is formed by using vinyl resins such as a polyester resin, a polyvinyl chloride, a polycarbonate resin, a polyvinyl butyral resin, an acrylic resin, a cellulose resin, an olefin resin and a polystyrene resin.
In the above-described thermal transfer image-receiving sheet, the dyeability of the dye-receiving layer and various types of durability and storage stability of an image formed thereon greatly depend upon the kind of the resin constituting the dye-receiving layer.
The dyeing capability of the dye which is transferred can be improved by improving the diffusivity of the dye at the time of the thermal transfer through the formation of the dye-receiving layer from a resin having a good dyeability or the incorporation of a plasticizer in the dye-receiving layer. In the dye-receiving layer comprising the above-described resin having a good dyeability, the formed image blurs during storage. Therefore, the storage stability is poor or the the fixability of the dye is poor, so that the dye bleeds out on the surface of the image-receiving sheet, which causes other articles in contact with the surface of the sheet to be liable to staining.
The above-described problems of storage stability and staining can be solved by selecting such a resin that the dye transferred to the dye-receiving layer is less liable to migration within the dye-receiving layer. In this case, however, the dyeing property of the dye is so poor that it is impossible to form an image having a high density and a high sharpness.
There are other large problems such as the light fastness of transferred dye, fading of the formed image due to sweat or sebum migrated to the image surface when the hand touches the image portion, swelling or cracking of the image-receiving layer per se, fingerprint resistance, bleeding of the dye when the dye is in contact with a substance containing a plasticizer, such as an eraser or a soft vinyl chloride resin, that is, a plasticizer resistance.
Examples of the resin having an excellent light fastness include polycarbonate resins, and various polycarbonate resins are disclosed in Japanese Patent Laid-Open Nos. 19138/1985, 169694/1987, 202791/1987 and 301487/1990. However, conventional polycarbonate resins are poor in the fingerprint resistance, and the solubility of the bisphenol A polycarbonate resin described as a favorable resin in the above-described documents is so poor that it is necessary for the coating to be conducted through the use of a chlorinated hydrocarbon solvent such as methylene chloride or chloroform, which is unfavorable from the viewpoint of the work environment.
Examples of the polycarbonate resin having a good solubility and coatable in the form of a solution thereof in a non-halogenated hydrocarbon solvent, such as a ketone solvent, a toluene solvent or a mixture thereof include polycarbonate resins represented by the following structural formula: ##STR2##
These polycarbonate resins, however, are disadvantageously poor in the fingerprint resistance and plasticizer resistance.
Accordingly, an object of the present invention is to provide a thermal transfer image-receiving sheet which can form an image excellent in the coloring density, sharpness and various types of fastness, particularly durability such as light fastness, fingerprint resistance and plasticizer resistance according to a thermal transfer printing process wherein use is made of a sublimable dye, and can be easily produced by conventional coating equipment through the use of a non-halogenated hydrocarbon solvent, such as a ketone solvent, a toluene solvent or a mixture thereof.
The above-described object can be attained by the following present invention. According to the first aspect of the present invention, there is provided a thermal transfer image-receiving sheet comprising a substrate sheet and a dye-receiving layer formed on at least one surface of the substrate sheet, wherein said dye-receiving layer comprises a random copolycarbonate resin having structural units represented by the following general formulae (1) and (2), the molar ratio of the structural unit represented by the general formula (1) to the structural unit represented by the general formula (2) being 30:70 to 70:30 ##STR3## wherein R.sup.1 to R.sup.8 stand for hydrogen, a halogen or an alkyl group having 1 to 4 carbon atoms, A stands for a straight-chain, branched or cyclic alkylidene group having 1 to 10 carbon atoms, an aryl-substituted alkylidene group, an aryl group or a sulfonyl group and B stands for an oxygen atom or a sulfur atom.
The formation of the dye-receiving layer through the use of a polycarbonate resin having the above-described particular structure can provide a thermal transfer image-receiving sheet which can form an image excellent in the coloring density, sharpness and various types of fastness, particularly durability such as light fastness, fingerprint resistance and plasticizer resistance according to a thermal transfer printing process wherein use is made of a sublimable dye, and can be easily produced by conventional coating equipment through the use of a non-halogenated hydrocarbon solvent, such as a ketone solvent, a toluene solvent or a mixture thereof.
According to the second aspect of the present invention, there is provided a thermal transfer image-receiving sheet comprising a substrate sheet and a dye-receiving layer formed on at least one surface of the substrate sheet, wherein said dye-receiving layer comprises at least a polycarbonate resin having the above structure and an aromatic polyester resin.
The formation of the dye-receiving layer through the use of a polycarbonate resin and an aromatic polyester resin can provide a thermal transfer image-receiving sheet which can form an image excellent in the coloring density, sharpness and various types of fastness, particularly durability such as light fastness, fingerprint resistance and plasticizer resistance according to a thermal transfer printing process wherein use is made of a sublimable dye, and can be easily produced by conventional coating equipment through the use of a non-halogenated hydrocarbon solvent, such as a ketone solvent, a toluene solvent or a mixture thereof. Since the dyeing capability of the dye can be further improved by mixing the polycarbonate resin with the aromatic polyester resin, the thermal transfer image-receiving sheet according to the second aspect of the present invention can further improve the coloring density, fingerprint resistance and plasticizer resistance as compared with the thermal transfer image-receiving sheet according to the first aspect of the present invention.