As a method of forming images using thermal transfer, a thermal diffusion dye transfer method (sublimation dye transfer printing method) of superimposing a thermal diffusion type thermal transfer sheet in which a thermal diffusion dye (sublimation dye) as a recording material is supported on a base of a plastic filmor the like on a thermal transfer image-receiving sheet in which a layer receiving the dye is provided on another base of paper, a plastic film or the like to form a full color image.
With respect to a thermal transfer recording method based on sublimation transfer, a problem that conventional thermal transfer sheets cannot attain sufficient print densities has arisen as a printing speed of a thermal transfer printer are becoming increasingly high.
As a thermal transfer sheet in which the print density is improved, a thermal transfer sheet in which an intermediate layer is provided between a base sheet and a dye layer is known.
As the thermal transfer sheet provided with the intermediate layer, there are known, for example, a thermal transfer sheet in which a hydrophilic barrier consisting of polyvinylpyrrolidone and polyvinyl alcohol is provided between a dye layer and a base sheet as an under coat layer, and a thermal transfer sheet in which an intermediate layer containing a sublimation dye having a diffusion coefficient smaller than that of a sublimation dye contained in a recording layer is provided between a base film and the recording layer containing a sublimation dye (See, for example, Japanese Kokai Publication Hei5-131760 and Japanese Kokai Publication Sho60-232996). However, there is a problem that a printed substance having an adequately high print density cannot be obtained in any thermal transfer sheet.
In Japanese Kokai Publication Sho59-78897, a thermal transfer sheet, in which a layer formed by vapor deposition of metal or metal oxide is formed on a base and on this layer, a thin layer of dye is provided, is described. However, there was a problem that this thermal transfer sheet cannot attain a printed substance having an adequately high print density, and it requires special equipment in vapor deposition and a production cost becomes high.
In Japanese Kokai Publication 2003-312151, a thermal transfer sheet, in which a good adhesive layer containing a homopolymer of N-vinylpyrrolidone or a copolymer of N-vinylpyrrolidone and another component is provided between the base and the dye layer, is described. This good adhesive layer may be a substance formed by mixing alumina, silica and like in addition to the polymers described above, but it is not essential to contain these compounds. In the thermal transfer sheet of Japanese Kokai Publication 2003-312151, there is a problem that the efficiency of dye transfer is insufficient.
In Japanese Kokai Publication Sho63-135288, an example, in which an ethanol solution or a 1-propanol solution of aminopropyltrialkoxysilane is applied to an interface as an under coat layer between the base of a thermal transfer sheet and the dye layer, is described. However, there is a problem that transfer sensitivity in printing at high speed is low since a relatively thick base is used.
In Japanese Kokai Publication Hei5-155150, a under coat layer formed by reacting a polymer having an inorganic primary chain comprising oxide of Group IVb metal with a copolymer such as acryloxyalkoxysilane is described. The under coat layer in Japanese Kokai Publication Hei5-155150 has a problem that it is low in heat resistance since it is an organic chain derived from the above copolymer and that it is prone to hydrolysis and unstable since it has the above inorganic primary chain.
In thermal diffusion type thermal transfer sheets, there was further a problem that when a plastic film is used as a base, a base is deteriorated and print wrinkles are produced due to heating and tension received during printing.
In order to solve this problem, in Japanese Kokai Publication Hei8-230032 and Japanese Kokai Publication Hei11-188791, a highly stretched base by a stretching method in which a draw ration in a machine direction (lengthwise) is enhanced, for example, a method of re-stretching in a machine direction in which the biaxially stretched film stretched lengthwise and crosswise is further stretched lengthwise again in processing a thin film base is described as a plastic film base.
However, since this highly stretched base requires a special film formation step, there is a problem that an increase in the cost cannot be avoided. Further, in recent years, there are tendencies that thermal damages to a base is increasing as a printing speed in a thermal transfer printer becomes higher, and a problem that conventional thermal transfer sheets are low in heat resistance and strength is arising.
On the other hand, it is performed that for the main purpose of imparting durability to images obtained by a thermal transfer method, a thermal transfer sheet, in which a protective layer is provided in advance for providing a protective layer on images later, is used, and this protective layer is transferred on images formed by a thermosensitive printer. However, there was a problem that when the protective layer is peeled off from the thermal transfer sheet, a large amount of static electricity is produced, and this causes carrying defects of a body on which the dye is transferred or a thermal transfer sheet in the thermosensitive printer.
In order to solve this problem, in Japanese Kokai Publication Hei11-105437, it is proposed that a protective layer (protection transfer layer) installed in a thermal transfer sheet includes an antistatic layer containing a surfactant, quaternary ammonium salt, and an antistatic agent of conductive metal oxide and the like such as zinc antimonite and the like and the antistatic agent may be contained in the protective layer composing the protection transfer layer or an adhesive layer. However, there are problems that when this antistatic agent is quaternary ammonium salt surfactant, quaternary ammonium salt is bled out to the outermost surface of the protection transfer layer with time to impair a transferring property and plasticizer resistance is deteriorated.
For the purpose of solving a problem of quaternary ammonium salt surfactant, a protective layer thermal transfer sheet, which is formed by providing a conductive protective layer containing a conductive inorganic substance obtained by treating a needle crystal of potassium titanate and the like with a conductive agent such as SnO2/Sb, is proposed in Japanese Kokai Publication 2003-145946.
However, there is a problem that when a conductive agent is inorganic particles of metal oxides, if an amount of the conductive agent to be added is too much, the transparency of the protective layer is lost and opacity is produced.
All of the antistatic agents described above (conductive agents) need to form a layer together with a binder resin. However, an antistatic layer comprising a conductive agent using a binder resin has a problem (1) that since a mixing ratio have to be set in consideration of adhesion to a base sheet or another layer and an amount of the conductive agent to be added has a restraint, a certain amount of coating is required for achieving a desired antistatic power, and a problem (2) that a combination of the conductive agent with the binder has a restraint because the compatibility of the conductive agent with the binder have to be considered.
As a protective layer transfer film, a substance provided with a thermal transferring resin layer composed of a layered body prepared by forming a transparent resin layer, a plasticizer resistance resin layer, and a thermally adhesive resin layer in this order from a base film side is proposed. In Japanese Kokai Publication Hei11-156567 (claim 1, paragraph 31), it is said that when as the plasticizer resistance resin layer among the above substances, a resin formed by introducing ammonium salt, sulfonate salt, and acetate salt into an acrylic copolymerized resin as a polar group is used, this film is superior in an antistatic property. However, the plasticizer resistant resin layer in which a polar group is introduced into an acrylic copolymerized resin is inadequate in some cases.
Japanese Kokai Publication Hei5-131760
Japanese Kokai Publication Sho60-232996
Japanese Kokai Publication Sho59-78897
Japanese Kokai Publication 2003-312151
Japanese Kokai Publication Sho63-135288
Japanese Kokai Publication Hei5-155150
Japanese Kokai Publication Hei8-230032
Japanese Kokai Publication Hei11-188791
Japanese Kokai Publication Hei11-105437
Japanese Kokai Publication 2003-145946
Japanese Kokai Publication Hei11-156567
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