Radiation imagery chemistry: process – composition – or product th – Thermographic process – Heat applied before imaging
Reexamination Certificate
2002-11-26
2003-11-11
Chea, Thorl (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Thermographic process
Heat applied before imaging
C430S350000, C430S353000, C430S354000, C430S501000, C430S503000, C430S543000, C430S619000, C430S620000, C430S935000
Reexamination Certificate
active
06645704
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process of making a color photothermographic capture films. In particular, it has been found that the use of an annealing treatment reduces speed loss upon keeping and, hence, the performance of photothermographic systems.
BACKGROUND OF THE INVENTION
Photothermographic imaging elements that can be processed, after imagewise exposure, simply by heating the element are referred to as photothermographic elements. Preferably, photothermographic films do not require any processing solutions and instead contain within them all the chemistry required for development of a photographic image. Subsequent processing steps may employ liquid processing. These film chemistries are designed so that at room temperature they are inactive, but at elevated temperatures (greater than 120° C.), the film chemistries become functionally active.
The conventional manufacture of conventional films typically involves drying of the imaging element at elevated temperatures. The intent in such a manufacturing step is not for the film material itself to reach a high temperature which is prevented by evaporative cooling of the film, such that the film is substantially less than the temperature of the heat applied, for example, by convective air under carefully controlled temperature and humidity. Heat treatments in combination with very high humidity has also been proposed for promoting hardening prior to rolling the film sheet.
It is also known that in conventional films the gelatin contained therein is affected by temperature and humidity. The reason for this is that when the gelatin in a conventional film is contacted with an environment containing moisture, such as air at a given relative humidity, the gelatin can come to equilibrium with the environment. In that process, water will be incorporated or lost from the gelatin in order to obtain this equilibrium condition. For a given sample of gelatin, a given environmental relative humidity will correspond to a given water content in the gelatin. This is the principle of a moisture content isotherm. Thus, the extent of water content in gelatin for a given environmental relative humidity can vary. It can depend upon the type of gelatin, the additional components incorporated into the gelatin, and the environmental history of the gelatin.
PROBLEM TO BE SOLVED
While conventional film does not appear to be much affected by humidity or variations in the moisture content of the product, it has been found that the storage stability of a color photothermographic film may be significantly affected by the temperature or humidity of the film during storage. It has been observed that the film properties of aqueous-based color photothermographic films, on storage, may degrade over time and exhibit a loss of photographic speed. The reasons for this speed loss may be due to several factors. Without wishing to be bound by theory, one reason may be that such films involve chemistries that are significantly more sensitive to environmental changes and which can be adversely affected by the conditions of storage. For example, in some cases, greater moisture content can be obtained in such films upon exposure to lower relative humidity, compared to conventional films.
Compared to conventional film or black-and-white film, photothermographic film requires at least three color records and all the components needed for development and image formation may be incorporated into the imaging element. Thus, there are a greater number of potentially reactive components that can prematurely react during storage. Color photothermographic films involve new chemical systems, including new and complicated combinations of components that may be subject to unpredictable or undesirable interactions, incompatibilities, and side reactions. In particular, the imaging chemistry must be designed to provide fast, high-quality latent image formation during image capture, but must not interact prematurely to any significant degree. Similarly, the film must be capable of fast development and high quality image formation during thermal processing, but the same components must not prematurely interact before the processing step. This problem is particularly acute in photothermographic films, since the components of a photothermographic film are in intimate proximity before development.
A problem in developing or designing color photothermographic films is, therefore, to obtain good raw stock keeping. It is desirable that photothermographic elements be capable of maintaining their imaging properties, including speed, during storage periods (after manufacture). This is sometimes referred to as “raw stock keeping”. Ideally, film should be storage stable, under normal conditions, for 12 months, more preferably 24 months. Most ideally, films will not degrade over substantially greater time periods, involving many years, allowing great variability in the distribution of such films as well as ensuring optimal image quality for consumers. If a film unduly loses speed during storage, poor or unacceptable image formation can occur.
In summary of the above, there remains a need for a color photothermographic film that does not exhibit any significant loss of speed or other performance characteristics during raw stock keeping.
SUMMARY OF THE INVENTION
The present invention relates to a method of making an improved photothermographic element comprising a support and at least one photothermographic layer comprising a silver halide emulsion in association with at least one organic silver salt (herein meant to include both salts and ligands). In particular, it has been found that speed loss on raw stock keeping in color photothermographic film can be prevented or minimized by the use of an annealing step subsequent to coating and drying of all the imaging layers in the film and prior to image capture and development. In particular, the present invention involves treatment of a dried imaging element by heating the element such that the temperature of at least the surface thereof is 50° C. to 90° C. degrees for at least 2 seconds, resulting in a significant reduction in speed loss in the product during to raw stock keeping. Preferably a substantial portion, or at least the imaging element, or even all of the imaging element, reaches the indicated temperature.
As used herein, the “temperature of annealing” refers herein to the temperature of the means used to heat the imaging element. With a drum heater, for example, it is the drum temperature, which is about the same as the temperature of the surroundings under unwet or substantially dry conditions. In contrast to conventional drying of film, the present invention involves heating of an already dry film, although the annealing treatment, during manufacture, can occur either separately from, or in the same unit, as the final drying of the imaging element. Because, the film has been substantially dried, the difference in temperature between the surroundings and the film surface is relatively narrow, typically less the 5° C., preferably less the 3° C.
Since photothermographic films are developed by applying heat to the film, it would be expected that photothermographic films should avoid heating prior to development. In fact, photothermographic film sold in the market is often maintained under refrigeration in storage to maintain keeping. In some embodiments, a blocked developing agent is unblocked by heating and, if prematurely unblocked, can deleteriously react with silver imaging centers in the imaging layer. Therefore, it would not be expected that an annealing process involving the application of higher amounts of heat than thought necessary for drying or manufacture would actually improve keeping.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a method of making a color photothermographic element comprising at least three imaging layers comprising a blocked developer, a gelatin binder, a coupler, silver halide, and at least one organic silver salt. As indicated above, the present method invo
Chea Thorl
Konkol Chris P.
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