1. Field of the Invention
The present invention relates to a heat-developable photosensitive material having photosensitivity, which may be referred to as a “heat-developable photosensitive material” hereinafter, and an image-forming process using the same. More specifically, the present invention relates to a heat-developable photosensitive material which has a high development activity, a high sensitivity and an excellent image-keeping property, and causes less fogging in non-image portions; a heat-developable photosensitive material having a low Dmin and an excellent image-keeping property by including characteristic organic silver salt particles; and an image-forming process using the heat-developable material.
2. Description of the Related Art
In recent years, a reduction in the waste amount of processing solution has been intensely demanded from the standpoints of environment preservation and saving space in the fields of films for medical diagnosis and films for photoengraving. Thus, techniques about heat-developable photosensitive materials are made necessary for medical diagnosis films and photoengraving films which can be effectively exposed to light with a laser image setter or a laser imager, and can form a black image having high resolution and sharpness. According to these heat-developable photosensitive materials, a simpler heat-developable processing system which requires no solution-type processing chemicals and gives no damage to environment can be supplied to clients.
Similar matters are demanded in the field of ordinary image-medical forming materials. Particularly for images for diagnosis, high image quality superior in sharpness and granularity is necessary because fine depiction is required. Moreover, images of cool black tone are preferred from the standpoint of easiness of diagnosis. At present, various hard copy systems using pigment or dye, such as an ink-jet printer or an electrophotographic apparatus, are in circulation as ordinary image-forming systems. However, satisfactory systems are not made practicable as systems for outputting medical images.
Incidentally, a thermal image-forming system using an organic silver salt is described in, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075, and “Thermally Processed Silver Systems” (Imaging Processes and Materials) Neblette, the 8th edition, written by D. Klosterboer, and edited by J. Sturge, V. Walworth and A. Shepp (Chapter 9, p. 279 (1989)).
In particular, a heat-developable photosensitive material has a photosensitive layer wherein in general a catalytically-active amount of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and an optional color tone adjuster for controlling the color tone of silver are dispersed in a matrix made of a binder.
The heat-developable photosensitive material is imagewise exposed to light, and then heated at a high temperature (for example, 80° C. or more), so as to cause redox reaction between the reducible silver salt (which functions as an oxidizer) and the reducing agent. In this way, a silver image in black is formed. The redox reaction is accelerated by catalytic effect of a latent image of the silver halide generated by the exposure. Therefore, the silver image in black is formed in the exposed area. Such technique is disclosed in many literatures including U.S. Pat. No. 2,910,377 and Japanese Patent Application Publication (JP-B) No. 43-4924.
In the heat-developable photosensitive material, it is preferable that the redox reaction between the reducible silver salt and the reducing agent advances at a realistic temperature for a realistic time, thereby giving a sufficient image density. Thus, in the present situation, it is desired to make further progress with heat-developable photosensitive materials having a high sensitivity and a high development ability and causing the reaction to advance rapidly.
In a heat-developable photosensitive material using an organic silver salt, the organic silver salt and so on are not fixed. For this reason, after a silver image is formed by heat, there is a probability that a further silver image is caused to appear by light, heat or the like. Of course, under ordinary use conditions, such a phenomenon is not caused. However, for example, in the case that a processed film is put in a car in summer for the purpose of the carriage thereof or the like, or in the case that a film is preserved under conditions severe for heat-developable photosensitive material, there arises a trouble such as discoloration of the whole of the film or transfer of characters on a bag in which the film is preserved onto the film, that is, a problem that fogging is caused at the time of the preservation thereof.
Technique of raising the content of silver behenate in the organic silver salt is described in Japanese Patent Application Laid-Open (JP-A) No. 11-271920. However, a reducing agent for organic silver salt particles described in this publication does not produce a remarkable effect upon development ability, image-keeping property, and prevention of fogging at the time of preservation.
Furthermore, it is found out that image-keeping property is improved by raising the silver behenate content. However, problems that development-advance is delayed and poor sensitivity is caused arise.