This invention relates to a display material which is capable of reversibly changing its coloring condition by application of heat and an electric field, and more particularly to a novel display material which is useful as electrochromic display element material and thermosensitive recording material.
For example, in the field of thermosensitive recording paper, there has been used a display material which has a leuco-dye dispersed in the form of crystallite in a binder together with a developer or activator so that, when thermally fused, the pigment and the developer are contacted with each other on a molecular level to develop a color. More specifically, the conventional thermosensitive recording paper which is in wide use has crystallites of a leuco dye and phenolic compound as the developer dispersedly coated together with a hydrophilic binder on substrate paper (a support body) such as synthetic paper or the like.
However, in consideration of the manufacturing process, the conventional thermosensitive recording paper of such a construction unavoidably suffers from the problem of fogging (a slight degree of coloration before printing). Namely, it is difficult to avoid a certain degree of reaction between the crystallites of the leuco dye and phenolic compound as the developer during the manufacturing process in the stages of kneading them into a hydrophylic binder, coating and drying, as a result coloring the heat sensitive paper as a whole in grey to a certain degree before serving for printing. Thus, at the present moment it is difficult to obtain thermosensitive recording paper which is satisfactory in whiteness. In addition, the above-mentioned conventional thermosensitive recording paper is susceptible to colorations by inadvertent heating, for example, colorations by application of heat during storage or after printing, making the printed information illegible or inviting considerable degradations in the quality of picture images.
Nevertheless, due to the steady increase in the amount of documents which are handled in the office work, there have been strong demands for thermosensitive recording paper which can be used repeatedly, from a viewpoint of saving space and resources. Besides, polychromic coloration of thermosensitive paper is considered for improving visibility of documents or the like.
However, the demands cannot be fulfilled by the conventional thermosensitive recording paper, including the sublimation transfer type and the thermal fusion type which are also unsatisfactory in case of high-speed full-color image production by a thermal head and are almost impossible to use repeatedly for reproduction.
On the other hand, in the field of the display, researches have been made for development of a display material which is suitable for the so-called flat display of a flat plate-like shape, including development of electrochromic display elements.
In this connection, the conventional electrochromic display elements are mostly designed for monochromic coloration and decoloration, and scarcely capable of coloration and decoloration in three primary colors such as RGB (the three primary colors in the additive color method) and YMC (the three primary colors in the subtractive color method) suitable for the full-color display.
Since, as mentioned hereinbefore, the conventional display materials used in the fields of the thermosensitive recording paper and display invariably lack stability and are difficult to apply to repeated use and polychromic coloration and insufficient in color vividness, the development of a novel display material which can eliminate these problems is greatly expected.