1. Field of the Invention
The present invention relates to a thermosensitive recording material (suitable for recording medical images), which uses a coloring reaction between an electron-donating coloring compound and an electron-accepting compound, and has excellent keeping quality of colored recorded images including half-tone images, and also relates to a method for producing such thermosensitive recording material.
2. Description of the Related Art
A thermosensitive recording matter has a thermosensitive recording layer on one surface of a base, where the thermosensitive recording layer contains a colorless or slightly colored coloring matter, such as an electron-donating leuco dye, an organic acid color developer such as an electron-accepting phenol compound, and a binder as main component, and the base is generally formed of paper, synthesized paper, or a plastic film. The coloring dye and the color developer are allowed to react by an application of thermal energy to thereby form a colored recording image.
Such thermosensitive recording matter has advantages such that a recording device used therefore is a compact and inexpensive, and maintenance thereof is easy. For these reasons, it has been widely used in the various fields, such as a recording material of electronic computers, facsimiles, ticket venders, scientific computers, CAD printers, plotters, and printers for CRT-based medical measurements.
Moreover, because of problems in processing of waste liquid cased by a wet process of a silver X-ray film as well as digitalization of images, in recent years, there have been demands for a dry film system which easily output, especially in the medical field. Along with such trend, a thermosensitive recording film usable for the medical field has been also desired because of easiness of the thermosensitive recording process.
A thermosensitive recording material for medical use is generally classified into a transmitting thermosensitive recording material which is translucent and uses such translucency, and a reflective thermosensitive recording material which has extremely low or no translucency, and uses the reflected light to view a formed image thereon.
In the case where the thermosensitive recording material for medical use is the reflective type, a synthesized paper having a multilayer structure is used because such thermosensitive recording material needs to achieve the required properties, such as water resistance, sufficient tensile strength, uniformity of recorded images, high resolution, size stability, and thin line recording ability.
Medical images have images of low to high color density ranges, and especially the keeping quality of a coloring image of half tone is important. Moreover, it is also desired that a recording color density is high, coloring tone is in pure black in all density ranges, and image do not discolor or fade by influences of light, heat, and moisture.
In Japanese Patent Application Laid-Open (JP-A) No. 05-318937, for stably preserving recording images of half tone, there is proposed a thermosensitive recording material, which contain, on a base, a thermosensitive recording layer containing a colorless or slightly tinted base dye and a coloring agent configured to develop a color in contact with the dye, and an intermediate layer containing a water-soluble resin or water-dispersible resin, and an overcoat layer containing an ionizing radiation-curable resin, where the thermosensitive recording layer contains, as the coloring agent, at least one salicylic acid derivative expressed by the following general formula (1) or a polyvalent metal salt thereof, and at least one phenol derivative expressed by the following general formula (2):

However, this thermosensitive recording material has a problem such that materials usable as a coloring agent are limited.
In JP-A No. 2004-314329, there is proposed a thermosensitive recording material which contains, on a base, a color reversible thermosensitive recording layer containing generally a colorless or slightly tinted dye precursor and a reversible color developer capable of reversibly changing a color tone of the dye precursor, an oxygen-barrier layer containing at least one water-soluble polymer selected from the group consisting of alkoxysilane having an amino group and/or imino group, alkoxysilane having an epoxy group, and an aziridine compound, and a protective layer, disposed in this order.
However, it is not sure whether such thermosensitive recording material has an effect on keeping quality of half tone or not.
In JP-A No. 2007-203608, a thermosensitive recording material having improved water resistance and the like is proposed. In this thermosensitive recording material, a polyamine or polyamide resin in combination with an epichlorohydrin resin is added to an intermediate layer provided between a recording layer and a protective layer.
However, this thermosensitive recording material requires both the epichlorohydrin resin and the polyamine or polyamide resin having functional groups interactive with the pichlorohydrin resin. In addition, whether or not this thermosensitive recording material provides an effect of preserving half tone images is uncertain.
Moreover, such thermosensitive recording material is proposed in JP-A Nos. 05-024350, 05-169811, 05-185721, and 05-330232 that it contains a thermosensitive recording layer to which certain aziridine is added for improving keeping quality. However, there is a possibility that aggregations of aziridine may be caused if the reactivity of the aziridine is excessively high. Therefore, this thermosensitive recording material has a problem such that aziridine usable therein is limited to the aziridine compound having a low reactivity can be used.