Currently, thermal transfer recording methods are widely used as a convenient printing method. Since thermal transfer methods allow to form various images, they are used in printing materials which may have a relatively few printing pages, for example, in making ID cards such as identification papers and business photography, or printing machines for personal computers, and video printers.
When roughly classifying thermal transfer sheets used in thermal transfer recording methods, they are classified by a so-called fusion transfer type, in which a heat-fusion colored layer fusion-softens and the heat-fusion colored layer is transferred to an object, i.e. a receiving sheet, and a so-called sublimation transfer type, in which dyes in a colorant layer sublimate and transfer to a receiving sheet by sensing heat. When in making IDs such as identification papers, and particularly when in forming uniform images such as texts and figures, heat-fusion type thermal transfer sheets are used. A thermal transfer sheet by stacking in the order a substrate, a release layer comprising an acrylic-styrene-based resin and a heat fusion colored layer is known as an example of such heat-fusion type thermal transfer sheet. However, when an acrylic-styrene-based resin is used as the release layer of the thermal transfer sheet, the release layer softens due to the heat conveyed from a thermal head and the peel force between the release and the transfer layers becomes high, thus problems may occur such as transfer failure, where the transfer layer is not transferred, whitening of an image due to roughening of a peel interface, and abnormal noise at peeling.
Further, when IDs such as identification papers are made by using the above mentioned thermal transfer sheet, a method is known for forming a protective layer on the image by superimposing a protective layer thermal transfer sheet stacked by, in the order, a substrate, a release layer consisting of a polyvinyl alcohol (PVA) and a water-dispersible urethane, and a protective layer, on the image obtained by the thermal transfer of the heat fusion colored layer, and transferring the protective layer by using such as a thermal head or a heating roll for protection of images. Furthermore, a method for forming a lamination material is known by using the heat roll after forming the protective layer on the image for improving image durability. However, a water-based release layer (a release layer comprising a polyvinyl alcohol and a water-dispersible urethane) is unstable to moisture and especially under high moisture environment; problems may occur such as whitening of an image due to roughening of a peel interface, and abnormal noise at peeling.
In order to solve the problems as described above, it is proposed to provide a thermal transfer sheet comprising a release layer containing polyamide based resins to reduce the heat applied to the thermal head and the effect of moisture environment and improve the peeling (refer to patent document 1). Further, a thermal transfer sheet containing a combination of specific resin components on a substrate sheet through a release layer is proposed (refer to patent document 2). Furthermore, a thermal transfer sheet is proposed that provides in a serial manner, a peel layer and a colored adhesive layer on a substrate, and in which the colored adhesive layer does not contain a heat-fusion material but contain specific particles in specific proportions (refer to patent document 3).
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