1. Field of the Invention
The present invention relates to photothermographic films, i.e., silver-containing photographic films which develop upon being heated, and in particular, to heat-developable films containing silver sulfinates as physical developers.
2. State of the Art
Historically, photographic silver halide films have been processed by immersing the exposed film into an aqueous developer. In order to simplify the image-forming process, newer dry-processable (photothermographic) films have been produced. These silver-containing films are heat-developable, i.e., produce an image upon being heated. These films enjoy an obvious advantage over the wet development and drying steps required for conventional films. Even so, many disadvantages in terms of sensitivity and stability exist for dry-processable films, and the image produced cannot be employed in as many applications as those from conventional films. Also, the silver salts most widely used in dry-processable films must be coated from an organic solvent, whereas conventional silver films can be applied from aqueous coating compositions.
Heat-developable or photothermographic films conventionally contain a photocatalyst, a silver salt, a developer, and a binder as major components. The silver salt serves as a physical developer to supply the silver for producing the image in these films, in combination with a photocatalyst such as silver bromide. Among the numerous silver salts disclosed for use in these films, silver behenate and silver benzotriazole have been most frequently cited in patent examples.
While silver sulfinates are listed in U.S. Pat. No. 4,258,127 as suitable organic silver salts for a photothermographic layer, this and other references contain no disclosure of how to prepare a useful composition. The reference lists three specific aromatic salts, i.e., silver p-toluene sulfinate, silver p-acetoaminobenzene sulfinate, and silver benzene sulfinate. The latter compound is also disclosed in U.S. Pat. No. 3,152,904 as a light-sensitive photocatalyst for use in a photothermographic composition. German Pat. No. 2,511,361 and Japanese Patent Application J 51054-428 disclose that sulfinic acids and their salts prevent fog and improve image color in dry-processable films. U.S. Pat. No. 4,173,482, 4,212,937, and 4,234,679 all contain a broad disclosure of silver salts of sulfinic acids for use in compositions coated from an organic solvent.
The present invention provides novel light-insensitive silver sulfinates useful as physical developers for photothermographic compositions.