1. Field of the Invention
This invention relates to heat developable and heat stabilizable materials and processes for developing an image employing the described materials. In one of its aspects it relates to a heat developable and heat stabilizable photographic element comprising the described combination of components. In another aspect it relates to a heat developable and heat stabilizable photographic composition comprising the described imaging combination. A further aspect of the invention relates to a process of developing and stabilizing an image in a heat developable photographic material containing the described imaging combination.
2. Description of the State of the Art
It is known to obtain an image in an imaging material, especially a photographic imaging material, by what is known as dry processing with heat. These materials are sometimes described as heat developable photographic materials or photothermographic materials. Such heat developable photographic materials are imagewise exposed to provide a latent image. They are then heated to provide a developed image in the absence of separate processing solutions or baths. Typical heat developable imaging materials or photothermographic materials are described, for example, in U.S. Pat. No. 3,152,904 of Sorenson et al, issued Oct. 13, 1964; U.S. Pat. No. 3,457,075 of Morgan et al, issued July 22, 1969; U.S. Pat. No. 3,152,903 of Shepard et al, issued Oct. 13, 1964; U.S. Pat. No. 3,392,020 of Yutzy et al, issued July 9, 1968; and British Specification No. 1,161,777 published Aug. 20, 1969.
Most heat developable photographic materials or photothermographic materials comprising photosensitive silver compounds have required a separate post-processing image stabilizer or stabilizer precursor to enable a stable image after processing. Typically, the post-processing stabilizer or stabilizer precursor has been incorporated in the photothermographic material and is a sulfur-containing compound. Upon heating the image stabilizer or stabilizer precursor forms a stable silver mercaptide or silver complex with the silver compounds in the non-image areas of the photographic material. This provides post-processing image stabilization. Sulfur-containing stabilizers or stabilizer precursors of this class are described, for example, in U.S. Pat. No. 3,301,678 of Humphlett et al, issued Jan. 31, 1967; U.S. Pat. No. 3,506,444 of Haist et al, issued Apr. 14, 1970; and U.S. Pat. No. 3,669,670 of Haist et al, issued June 13, 1972. Typical sulfur-containing image stabilizer precursors are isothiouronium compounds which provide stabilization of a developed image upon heating of the material containing the described stabilizer precursors. It has been advantageous to provide a material which enables a stabilized image in the absence of these image stabilizers or stabilizer precursors or other compounds designed as post-processing stabilizers. However, none of the above patents suggest a solution to this problem.
It has also been typical to incorporate what is known as an activator or activator precursor in a heat developable and heat stabilizable photographic material. Such activators or activator precursors upon heating provide activation of the imaging process. Usually these compounds enable activation of the developing agent or developing agent precursor to provide development of the latent image in the heat developable material. These activators or activator stabilizers are typically alkali-release or base-release compounds which provide the desired increase in pH in the photographic material upon heating of the activator or activator precursor. Examples of activators or activator precursors in heat developable photographic materials or photothermographic materials are described, for instance, in U.S. Pat. No. 3,531,285 of Haist et al, issued Sept. 29, 1970. An example of an activator precursor which has been used in a heat developable material is guanidinium trichloroacetate which provides an increase in pH in the described heat developable photographic material. It has been desirable in these heat developable photographic materials to provide an increased development efficiency of the latent image. Typically the development efficiency of the latent image is far less than 90% and typically within the range of about 30 to 50% development efficiency. Accordingly, a significant concentration of the photographic silver salt remains unused in the development process. The mass of silver developed in these heat developable materials has been dependent upon the degree of imagewise exposure provided. It has been desirable to provide a material which is less dependent upon exposure and more dependent upon the development process to provide the desired development efficiency. The described activator precursors have not provided the desired increased development efficiency or enabled post-processing image stabilization in the absence of a separate image stabilizer or stabilizer precursor.
Further, it has been desirable to provide heat developable photographic materials which enable the stabilization of a processed image and provide development efficiency greater than 90% and still enable the use of conventional silver salt developing agents, especially silver halide developing agents, known to be useful in heat developable photographic materials. Many heat developable photographic materials used commercially employ unconventional reducing agents which require costly processes of preparation. It has been desirable to eliminate this problem with a heat developable photographic material which provides the described advantages and also enables use of conventional developing agents or developing agent precursors which can be incorporated in photographic materials without adversely affecting the desired properties of the photographic material. Commercially available photothermographic materials have used such a reducing agent as 2,2'-methylene bis(4-methyl-6-tertiary butylphenol) which is not a conventional, easily prepared silver halide developing agent. This reducing agent also does not provide the desired development efficiency nor does it enable stabilization of an image in the absence of a separate post-processing image stabilizer.
It has been desirable to provide a heat developable photographic material which enables the use of a reduced concentration of silver in the imaging process and material. The heat developable materials described have not enabled the desired degree of reduction of silver in a heat developable material.
One of the advantages of a photographic material which is heat developable and employs no separate post-processing image stabilizer or stabilizer precursor is that improved processing temperature latitude can be provided due to the fact that no stabilization reaction is required to compete with the image development reaction. Also, light stability of a processed material in the absence of a separate stabilizer can be superior to that of heat developable materials comprising a sulfur-containing stabilizer because no silver mercaptide is formed which can photolytically form silver sulfide over an extended time. Also, in the absence of the silver mercaptide, better light absorption characteristics can be observed to provide improvement in image contrast when observed with near ultraviolet radiation. This is advantageous in the graphic arts area involving reproduction of materials which are sensitive to ultraviolet radiation.
Another property which has been desirable in heat developable materials as described has been the property of non-volatility of the activators or activator precursors which are useful in heat developable materials. Unfortunately, heat developable photographic materials or photothermographic materials containing such an activator or activator precursor as guanidinium trichloroacetate do not provide this advantage.
Photographic materials which provide for development of a latent image by conventional processing solutions or baths are well known. It is also known to provide development of an image in such materials by what is known as covering power imaging. A fundamental difference exists between conventional silver development processes and what are known as covering power imaging processes. In most conventional photographic silver materials, changes in optical density with exposure arise primarily from differences in the amount of silver reduced at the latent image sites. In silver covering power imaging the amount of silver reduced does not vary greatly with exposure. In such covering power imaging, density variations at image areas are due, for the most part, to differences in silver covering power at various exposure levels. Silver covering power imaging as used herein is intended to mean imaging in a photographic material in which variations in optical density of the developed image derive primarily from variations in silver covering power as a function of exposure. Covering power as used herein is intended to mean the measure of the silver opacity in the developed image and is derived arithmetically by dividing (a) the optical density by (b) the grams of developed silver per square decimeter in the layer of the material containing the developed image. Covering power and covering power imaging are known in conventional photographic materials which provide development with processing solutions or baths. This is described, for example, in the article by K. Murofushi, J. Soc. Sci. Phot., Japan, Volume 30 (4), 193-198 (1967), Canadian Pat. No. 808,585 and "The Theory of the Photographic Process," 3rd Ed., by Mees, 1966, pages 74-75 and 419-420. None of the photographic materials described have used covering power imaging in heat developable and heat stabilizable materials.
It has been desirable to provide a heat developable and heat stabilizable photographic material which provides development efficiency of an image of at least 90% and enables a developed and stabilized image to be provided in the absence of a separate post-processing image stabilizer or stabilizer precursor.