(a) Field of the Invention
The present invention relates to a heat sensitive transfer medium for use in thermographic recording which comprises combining a color former-containing transfer sheet with a developer-containing receiving sheet.
(b) Description of the Prior Art
As the heat sensitive transfer medium for use in thermographic recording, there have hitherto be known the one which comprises a transfer sheet formed by providing a heat-sublimable dye layer on a substrate and a receiving sheet which receives a dye image sublimed and transferred from said transfer sheet by heat-printing from the back (the substrate surface) of the transfer sheet, and the one which comprises combining a transfer sheet formed by providing a heat-fusible material and pigment or dye-containing transfer layer on a substrate with a receiving sheet which receives an image fused and transferred from said transfer sheet by heat-printing from the back of the transfer sheet. The former transfer medium is defective in that due to the use of the heat-sublimable dye, the dye image on the receiving sheet is inferior in preservability and consequently there is a necessity of overcoating the transferred image, while the latter transfer medium is defective in that because of having the transfer layer formed by dispersing pigment or dye in the heat-fusible material, when a large amount of pigment is contained in said layer in order to obtain a high density image, the transfer efficiency deteriorates, whereby it becomes difficult to obtain a high density image, and when using a large amount of heat-fusible material in order to improve the heat sensitivity, furthermore, as the result of said large amount of heat-fusible material transferring on the side of the receiving sheet, the separation of the transfer sheet from the receiving sheet can not be done smoothly to cause the fine line image area to lack definition.
On the other hand, a heat sensitive transfer medium (for instance, such as the medium disclosed in British Pat. No. 1,053,905), which comprises supporting on a substrate separately two kinds of materials, for instance the color forming agent such as benzoindolinospyropyran and the developer such as phenolic compound, which form color by their mutual thermal reaction, and contacting these supporting layers (transfer layer and receiving layer) facing each other to effect heat-printing, is also known, but the medium of this type is observed to be defective in that since the medium of this kind is of reaction type, if the transfer layer transfers only toward the receiving layer at the time of facing contact, the color formation is not effected to the full, thereby producing a low density image, while if the heating conditions are effected at higher temperature and for longer times in order to promote the sufficient reaction, the image on the receiving sheet surely becomes a high density image, but the color forming reaction proceeds also on the transfer sheet to thereby cause image formation.
We previously proposed, as the heat sensitive transfer medium which had improved the above mentioned drawbacks, the one prepared by incorporating a large amount of oil-absorptive porous filler in the receiving layer formed on the receiving sheet. The heat sensitive transfer medium of this type surely can obtain a high density image by a large number of transfer, but is still not satisfactory in respect of the uniformity of image density.
In addition, it was difficult for the above mentioned conventional heat sensitive transfer mediums to obtain images which are superior in the mechanical strength such as friction resistance, abrasion resistance and the like.