This invention relates to the use of aqueous silver halide emulsions precipitated in the presence of cationic starch peptizing agent to formulate imaging forming compositions that are useful in aqueous photothermographic imaging elements.
Photothermographic materials are well known in the photographic art. Photothermographic materials are also known as heat developable photographic materials. The photothermographic materials, after imagewise exposure, are heated to moderately elevated temperatures to produce a developed image in the absence of separate processing solutions or baths. The heat development can provide a developed silver image in the photothermographic material.
An example of a known photothermographic silver halide material comprises (a) a hydrophilic photosensitive silver halide emulsion containing a gelatino peptizer with (b) an organic solvent mixture, (c) a hydrophobic binder and (d) an oxidation-reduction image-forming composition. The oxidation-reduction imaging forming composition typically comprises (i) an organic silver compound that can be a silver salt of a long-chain fatty acid, such as silver behenate or silver stearate, in combination with (ii) an organic reducing agent, such as a phenolic reducing agent. It has been desirable to have hydrophilic photosensitive silver halide emulsion containing a gelatino peptizer in such a photothermographic material because of the higher photosensitivity of these silver halide emulsions and the ease of control in preparation of the emulsion based on conventional aqueous silver halide gelatino emulsion technology.
A problem that has been solved in the prior art relates to the preparation of these photothermographic silver halide materials. This problem involves the mixing of a hydrophilic photosensitive silver halide emulsion containing a gelatino peptizer with an oxidation-reduction imaging forming composition. The imaging forming composition contains hydrophobic components including a hydrophobic binder, such as poly(vinyl butyral), and a silver salt of a long-chain fatty acid, such as a silver salt of behenic acid. Typically, when the hydrophilic photosensitive silver halide emulsion is mixed with the hydrophobic imaging forming materials and then coated on a suitable support to produce a photothermographic element, the resulting element produces a less than desired degree of photosensitivity, contrast and maximum density upon exposure and heat processing. This problem has been encountered in photothermographic silver halide materials, as described in, for example, U.S. Pat. No. 3,666,477 of Goffe, issued May 30, 1972. Goffe proposed addition of alkylene oxide polymers and a mercaptotetrazole derivative to the photothermographic material to help provide increased photosensitivity. This problem has been substantially solved by formulating aqueous based photothermographic elements.
In addition, a variety of organic solvents have been proposed in order to help prepare a photothermographic silver halide composition containing the described image-forming components. The organic solvents that have been proposed include isopropanol, acetone, toluene, methanol, 2-methoxyethanol, chlorinated solvents, acetone-toluene mixtures and certain non-aqueous polar organic solvents. The described individual solvents, such as isopropanol, have not provided the desired improved properties. There has been a continuing need to reduce fog (Dmin) and to prevent fog growth on raw stock keeping.
The use of cationic starches as a peptizer for the precipitation of high bromide {111} tabular grain emulsions is taught by Maskasky U.S. Pat. No. 5,604,085, issued Feb. 18, 1997, U.S. Pat. No. 5,620,840, issued Apr. 15, 1997, U.S. Pat. No. 5,667,955, issued Sep. 16, 1997, U.S. Pat. No. 5,691,131, issued Nov. 25, 1997, and U.S. Pat. No. 5,733,718, issued Mar. 31, 1998. Oxidized cationic starches are advantageous in exhibiting lower levels of viscosity than gelatino-peptizers. This facilitates mixing. Under comparable levels of chemical sensitization higher photographic speeds can be realized using cationic starch peptizers. Alternatively, speeds equal to those obtained using gelatino-peptizers and lower Dmin densities can be achieved at lower precipitation and/or sensitization temperatures, thereby avoiding unwanted grain ripening. Furthermore, cationic starch peptized emulsions have been disclosed for use in gelatin based elements that are processed in aqueous processing solutions.
There is a continuing need for aqueous based photothermographic elements that exhibit good image discrimination while maintaining good keeping properties and low fog.
In one aspect of the invention, there is provided an aqueous photothermographic composition comprising a) a photosesitive silver halide emulsion containing a cationic starch peptizer and b) an oxidation-reduction imaging forming composition comprising (i) an aqueous dispersion of organic silver compound particles and (ii) an organic reducing agent. The described photothermographic composition can be coated on a support to provide a useful photothermographic element.