At the present time, thermal transfer recording is widely used as a simple printing method. The thermal transfer recording can simply form various images and thus is utilized in printing wherein the number of prints may be relatively small, for example, in the preparation of ID cards, such as identification cards or photographs for business, or is utilized, for example, in printers of personal computers or video printers.
When a full-color gradational image such as a photograph-like image of a face is desired, the thermal transfer sheet used is such that, for example, colorant layers of yellow, magenta, and cyan and optionally black are provided as ink layers repeatedly in a large number in a face serial manner on a continuous substrate sheet.
Such thermal transfer sheets are classified roughly into thermal transfer sheets of the so-called “heat-fusion” or “thermal ink transfer” type wherein the colorant layer is melted and softened upon heating and as such is transferred onto an object, that is, an image-receiving sheet, and thermal transfer sheets of the so-called “sublimation dye transfer” or “thermal dye transfer” type wherein, upon heating, a dye contained in the colorant layer is sublimated to permit the dye to migrate onto the image-receiving sheet.
When the above thermal transfer sheet is used, for example, for preparing identification cards or documents, a method known for forming a protective layer on an image with a view to protecting the image is that a protective layer transfer sheet with a thermally transferable resin layer is stacked on an image formed by the thermal transfer of a heat-fusion colorant layer or thermally sublimable dye and the thermally transferable resin layer is transferred by means of a thermal head, a heating roll or the like to form a protective layer on the image.
The provision of the protective layer can improve abrasion resistance, chemical resistance, solvent resistance and the like of images, and, further, the addition of an ultraviolet absorber or the like to the protective layer can improve lightfastness of the images.
For example, Japanese Patent Laid-Open No. 240404/2002 discloses a thermal transfer sheet for a protective layer in which a thermally transferable protective layer is provided on at least a part of one side of a substrate sheet and the protective layer is a laminate having a structure of at least two layers, that is, comprises at least a layer composed mainly of an acrylic resin and a layer composed mainly of a polyester resin provided in that order on the substrate sheet.
The thermal transfer sheet for a protective layer disclosed in Japanese Patent Laid-Open No. 240404/2002, however, is disadvantageous in that when the formation of aqueous ink images, for example, stamps put at the joining of two leaves, or various stamp images, using an aqueous ink on a thermally transferred image, with a protective layer formed using the thermal transfer sheet for a protective layer, on a photographic paper is contemplated for use, e.g., in a photographic image of a face in a passport, the print cannot absorb and fix the aqueous ink.
To overcome the above problem, for example, Japanese Patent Laid-Open No. 324140/1996 discloses a thermal transfer film for a protective layer in which, for example, a water absorptive surface layer constituting the uppermost surface after transfer is a layer capable of absorbing and fixing an aqueous ink and the water absorptive surface layer is a substantially transparent porous layer or a partially water absorptive layer comprising at least water absorptive micro-regions and water resistant micro-regions.
In the thermal transfer film for a protective layer disclosed in Japanese Patent Laid-Open No. 324140/1996, however, the following facts should be noted. Specifically, when the water absorptive surface layer is a partially water absorptive layer comprising water absorptive micro-regions and water resistant micro-regions, the water absorptive layer is considered to have the so-called “islands-sea structure.” When the proportion of the constituents of the water absorptive micro-regions to the constituents of the water resistant micro-regions exceeds a predetermined value, that is, when the proportion of the constituents of the water absorptive micro-regions increases, a part of the components constituting the water absorptive micro-regions, which are not resistant to water, is disadvantageously separated to form a layer which does not form an islands-sea structure. As a result, after the thermal transfer of the protective layer, when water penetrates into the protective layer and reaches the interface, a part of the protective layer is disadvantageously separated from the interfacial part.
Further, as the island-sea structure region constituted by the water resistant micro-regions and the water absorptive micro-regions is reduced, the speed of the penetration of water, from the surface of a print, at which water reaches the stampable and writable protective layer becomes so low that the drying speed of an aqueous stamp or ink becomes low and the stampability of the aqueous stamp and ink and writability are disadvantageously low.
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