Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having image qualities closest to that of silver salt photography (see, for example, “Joho Kiroku (Hard Copy) to Sono Zairyo no Shintenkai (Information Recording (Hard Copy) and New Development of Recording Materials)” published by Toray Research Center Inc., 1993, pp. 241-285; and “Printer Zairyo no Kaihatsu (Development of Printer Materials)” published by CMC Publishing Co., Ltd., 1995, p. 180). Moreover, this system has advantages over silver salt photography: it is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like.
In this dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
General paper may be used as a support of an image-receiving sheet in this dye diffusion transfer recording system, and it enables the image-receiving sheet to be produced at low costs. In an image-receiving sheet using such paper as the support, a layer having high cushion properties, for example, a foam layer made of a resin and a foaming agent, is formed between the support and a receptor layer, to provide cushion properties, thereby improving the adhesion between an image-receiving sheet and an ink sheet. Also, an intermediate layer is further formed between this foam layer and the receptor layer, to prevent the foam layer from being broken (flatten) by heating during printing. However, there are caused certain problems in current image-receiving sheets because of the fact that this intermediate layer is formed using an organic-solvent-type resin coating solution. The problems are that this coating solution breaks down air cells and voids in the foam layer, and thus, desired cushion properties are not attained, resulting in voids and density unevenness in the formation of an image, and further reduction in the heat insulation property of the foam layer is caused, resulting in diffusion of the calories required to transfer dyes in the direction of the backside of the image-receiving sheet, bringing about reduction in sensitivity that is required for printing.
To solve the above problems, for example, JP-A-5-147364 (“JP-A” means unexamined published Japanese patent application) discloses that a resin layer including a dye receptor layer is made to contain a hollow capsule; and also JP-A-11-321128 discloses that an intermediate layer containing, as its major components, hollow particles and a polymer resistant to an organic solvent, is formed between a support and a receptor layer. In these methods, however, the receptor layer is applied after the resin layer and the intermediate layer are applied and dried under heating. Therefore, not only do many image defects arise from formation of unevenness on the surface of the receptor layer, but also there are problems of insufficient sensitivity and rise in costs. Further, for instance, JP-A-2004-9572 discloses providing, on a paper base that exhibits a specific air permeability, with a layer containing hollow particles and a dye-receiving layer in order of mention. However, even this method cannot satisfactorily reduce the number of occurrences of image defects ascribable to bubbles released from the hollow particles at the time of image formation. Therefore, not only do many image defects arise from formation of unevenness on the surface of the receptor layer, but also there are problems of an insufficient sensitivity and rise in costs.