As a color or monochrome image forming technology, known has been an image forming technology in which an ink sheet containing thermally transferable dyes featuring diffusion transition characteristics by application of heat, were an image is transferred onto an image receiving layer, facing the ink sheet to the image receiving layer of a image receiving sheet, using a heat printing means such as a thermal head or a laser. This thermal transfer method enables image formation from digital data, and has a reputation for high image quality comparable to silver salt photography, without using processing solutions such as a developing solution.
However, regarding image storage stability and durability of the obtained images, there are drawbacks such as being inferior to conventional silver salt photography. Specifically, the following inferior characteristics are known.
The problems are:
1) During long period storage, discoloration and bleeding of images are caused by light, heat, oxygen and ambient moisture, that is, unacceptable light stability and heat resistance.
2) In cases when images are contacted with a material having high dyeing properties or containing a plasticizer such as album sheets, clear file sheets and plastic pencil erasers, dyes are reverse transferred to these materials or bleed upon contact during long term storage, that is, they exhibit low plasticizer resistance.
3) When water, juices, alcohol beverages, or coffee is spilled on formed images and wiped away, dyes are also wiped off whereby discoloration occurs, exhibiting lack of water resistance and solvent resistance.
4) Discoloration also occurs due to skin oil when fingers touch the images, that is, low sebum resistance.
5) The images are damaged when rubbed by an eraser, that is, lack of abrasion resistance.
6) When the images are laminated with a commercially available laminate material, specifically with a low temperature laminating material, dyes diffuse to the laminate material and cause bleeding during long term storage, that is, poor laminatability.
The reasons for these problems are assumed to be that the dyes employed in a thermal transfer recording material are primarily dispersed in a binder, and tend to be directly affected by external influences, contrary to the dyes employed in silver salt photography which are protected by high boiling point solvents and UV absorbing agents.
As a means to overcome the foregoing drawbacks, several image forming methods to react with compounds in an ink layer with compounds in a image receiving layer by thermal transfer are proposed, to be an image forming method employing a so-called reactive dye. At this point, a compound contained in an ink layer is defined as a dye precursor, and a compound contained in an image receiving layer is defined to be a dye fixing agent. For example, proposed are image forming methods with re-protonation of a cationic dye by thermal transfer, employing a deprotonated cationic dye as a dye precursor and an organic polymer acid or an oligomer acid capable of protonating the cationic dye as a dye fixing agent, as described in Unexamined Japanese Patent Application Publication (hereinafter, referred to as JP-A) 9-327976, and U.S. Pat. Nos. 4,880,769 and 5,534,479. Further, in JP-A 5-221151, employing a certain structured dye having a reactive group as a dye precursor and a reactive hydrogen compound as a dye fixing agent, proposed are image forming methods to make them react by thermal transfer.
Further, employing a thermally diffusive dye capable of chelating as a dye precursor and a metal ion containing compound as a dye fixing agent, image forming methods to make them react to form a metal chelate are proposed (see, for example, Patent Documents 1–3).
The formed images employing this method rarely cause discoloration and bleeding of dyes even though the image receiving material, carrying images, are stored at high temperature and high humidity. Light stability of the images is superior to that of images formed using existing thermally transferable dyes. However, there have been problems that color of the images changes over time due to unreacted dyes which remain due to incomplete reaction between the dyes and the dye fixing agents in high image density areas.
The means to overcome this problem is to increase the added amount of the dye fixing agents in an image receiving layer to increase reactivity. However, this results in another problem of coloring of white background due to the coloring within the dye fixing agents. Another proposed counter method is heating the images again after thermal transfer (for example, please refer to Patent Document 4), however, this results in a problem of lowered density due to reverse transfer of the dyes in the images to an ink layer during re-heating since no dye is contained in the ink layer between the thermal head and the images.
Further, another proposed method is to provide a protective layer on the images, in which a protective layer transfer sheet having a thermally transferable protective layer is applied onto the image forming layer of an image receiving sheet, and then the protective layer is transferred using a heating means such as a thermal head or a heated roller (for example, please refer to Patent Document 5). In cases when a protective layer is provided on images, physical resistance such as the foregoing abrasion resistance, water resistance, solvent resistance, and sebum resistance of images can be enhanced. However, in order to adhere the protective layer onto the image receiving layer, it is requested to decrease the amount of dye fixing agents in the image receiving layer, resulting in lowered reactivity between the dyes and the dye fixing agents. When transfer energy of protective layer transfer is raised to counter this, the protective layer is adversely lowered in quality due to heat, resulting in surface roughness and yellowing.
Patent Document 1: JP-A 59-78893
Patent Document 2: JP-A 59-109394
Patent Document 3: JP-A 60-2398
Patent Document 4: JP-A 11-70746
Patent Document 5: JP-A 2001-246845
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