1. Field of the Invention
The present invention relates to a heat transfer image-receiving sheet. More particularly, the present invention relates to a heat transfer image-receiving sheet which is highly releasable from a heat transfer sheet without being thermally fused therewith when an image is formed and on which a deeply-dyed image can be formed.
2. Background Art
Heretofore, a variety of heat transfer printing methods have been known. One of them is a method in which a sublimable dye is thermally transferred from a heat transfer sheet in which the sublimable dye is supported as a recording agent on a substrate sheet such as a polyester film to an image-receiving sheet capable of being dyed with the sublimable dye, prepared, for example, by providing a dye-receiving layer on paper or a plastic film, thereby producing various full-colored images on the image-receiving sheet.
In the above printing method, the thermal head of a printer is employed as a heat application means, and a large number of dots in three or four colors, the amount of heat applied thereto being properly controlled, are transferred to the image-receiving sheet in an extremely short heat application time. A full-colored original image can thus be successfully reproduced by the multicolored dots on the image-receiving sheet.
The above-obtained image is very sharp and excellent in transparency because a dye is used as the recording agent. The printing method of this type can thus produce an image excellent in the reproduction of half-tones and also in gradation, comparable to an image obtainable by conventional offset printing or gravure printing. Moreover, the quality of the image is as high as that of a full-colored photograph.
Upon effectively conducting the above heat transfer printing, not only the structure of the heat transfer sheet but also that of the image-receiving sheet on which an image is formed is important. Conventionally-known image-receiving sheets are those disclosed, for instance, in Japanese Laid-Open Patent Publications Nos. 169370/1982, 207250/1982 and 25793/1985, in which image-receiving sheets are prepared by providing a dye-receiving layer, using a resin selected from polyester resins, vinyl resins such as polyvinyl chloride resin, polycarbonate resins, polyvinyl butyral resins, acrylic resins, cellulose resins, olefin resins, polystyrene resins and the like.
In the case where the above-described heat transfer image-receiving sheets have an image-receiving surface which is poor in release properties, they are thermally fused with a heat transfer sheet due to heat which is applied by a thermal head when an image is formed. A loud noise is therefore made when the image-receiving sheet is separated from the heat transfer sheet. Moreover, troubles called abnormal transfer printing are caused; for instance, the dye layer of the heat transfer sheet is entirely transferred to the image-receiving sheet, and the dye-receiving layer of the image-receiving sheet is separated from the substrate
A method in which various releasing agents are incorporated into the dye-receiving layer, and a method in which a releasing layer Is separately provided on the dye-receiving layer have been conventionally known as the methods for solving the above-described problems in the release properties. Among various releasing agents, silicone compounds such as silicones and silicone resins are effective for imparting release properties, and modified silicones which can be cured are particularly effective.
Among curing-type silicones, addition-polymerizable silicones, condensation-polymerizable silicones and active-energy-ray-curing silicones are presently known well. These silicones are added as silicone oils, and cured by the application of heat or active energy rays. Release properties can thus be obtained.
However, these cured silicones have, depending upon the type thereof, a problem in mar resistance, that is, during the transportation of a heat transfer image-receiving sheet having a dye-receiving layer into which any of the above silicones is incorporated, the dye-receiving layer is rubbed with the back surface of another piece of the heat transfer image-receiving sheet and marred, so that the image-receiving sheet cannot fully exhibit release properties when heat transfer printing is conducted; and a problem of heavy separation, that is, it becomes heavy to separate the image-receiving sheet from a heat transfer sheet when an image is formed.
Further, a cured product which is obtained by the reaction between an amino-modified silicone and an epoxy-modified silicone has also been known well. However, the separation of an image-receiving sheet comprising such a cured product from a heat transfer sheet is heavier then that of an image-receiving sheet comprising a cured product of an addition-polymerizable silicone, a condensation-polymerizable silicone or an active-energy-ray-curing silicone. In addition, it takes time to cure amino- and epoxy-modified silicones.
In order to obtain satisfactory release properties by the use of a curing-type modified silicone oil selected from the above-described addition-polymerizable silicones, condensation-polymerizable silicones, active-energy-ray-curing silicones, and amino- and epoxy-modified silicones, it is effective to add a large amount of the silicone oil to a dye-receiving layer, or to provide a thick releasing layer. However, in either of these cases, the dyeability of a dye used is impaired, and, as a result, an image having a high density cannot be obtained. Further, even if the amount of the silicone oil used is made large, or even if the releasing layer is made thick, the above-described mar resistance or anti-heavy separation properties cannot be improved depending upon the type of the resin used for forming the dye-receiving layer.
In order to improve the mar resistance, it is preferable to obtain release properties by reacting a silicone oil having active hydrogen such as a hydroxyl-modified silicone, a carboxyl-modified silicone or an amino-modified silicone with a curing agent such as an isocyanate compound or an organometallic compound, thereby curing the silicone.
However, In the above reaction, it is necessary to conduct baking at a high temperature for many hours, and also to conduct aging for a long time after the step of drying. The above reaction thus requires many hours, so that it is disadvantageous from the viewpoint of productivity. On the other hand, when baking is conducted at a low temperature for a short time, there may be a case where release properties cannot be fully obtained even if aging is conducted thereafter. Further, if the amount of the curing agent or that of a catalyst is increased to such an extent that release properties can be fully obtained, the pot life of a coating liquid becomes extremely short. The coating liquid is therefore gelled before it is coated, or the coatability thereof becomes worse.
Accordingly, an object of the present invention is to solve the aforementioned problems, thereby providing a heat transfer image-receiving sheet which is highly releasable from a heat transfer sheet and on which a high-density image can be recorded.