Radiation imagery chemistry: process – composition – or product th – Retrieving image made using radiation imagery
Reexamination Certificate
2000-12-06
2002-09-24
Schilling, Richard L. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Retrieving image made using radiation imagery
C430S352000, C430S353000, C430S339000, C430S343000, C430S517000, C430S519000, C430S507000, C430S522000, C430S964000
Reexamination Certificate
active
06455210
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to colored, aqueous heat-bleachable compositions that can undergo a change in electromagnetic absorption characteristics upon application of heat. These compositions are useful as antihalation or filter components of photothermographic elements.
BACKGROUND OF THE INVENTION
Photographic materials usually contain various layers and components, including antihalation or filter layers, overcoats and radiation sensitive layers. The antihalation layer of an imaging element helps to prevent light that has passed through the radiation sensitive layer(s) from reflecting back into those layers. If reflection is not prevented, the resulting image is less sharp. In wet processes, the antihalation layer is generally removed or rendered colorless during wet-chemical processing. A filter layer is used to absorb light of a color not completely absorbed by a color layer or color layer unit above the filter layer, while transmitting light of a color intended to be absorbed by a color layer or a color layer below the filter layer. In other words, a filter layer is used to selectively absorb light not used for image capture. An antihalation layer can be viewed as a type of filter layer positioned below all the color layers, wherein no light needs to be transmitted to any color layer below the antihalation layer, but reflection of light back through the antihalation unit is prevented or minimized. Both an antihalation layer and a filter layer will typically employ a filter dye which absorbs, or filters out, light not intended to be absorbed by a color layer.
Imaging elements that can be processed, after imagewise exposure, simply by heating the element are referred to as photothermographic elements. It is often desired that such elements include an antihalation or filter layer. In most cases, the antihalation layer must be rendered substantially transparent upon heat processing in order to avoid unwanted absorption of light during scanning, which would undesirably result in a higher level of minimum density (an increased “D
min
”). Particularly in the case of a color film, bleaching to transparency and avoiding or minimizing any tint is desirable.
A variety of antihalation compositions have been reported in the literature for use in photothermographic systems which avoid the use of processing solutions. Such compositions generally include heat bleachable antihalation dyes or incorporated addenda that act as bleaching agents. In particular, the use of radicals from biimidazoles as bleaching agents in antihalation compositions are known, being described, for example, in U.S. Pat. No. 4,196,002 (Levinson et al) and U.S. Pat. No. 4,201,590 (Levinson et al). The compositions contain filter dyes in reactive association with certain hexaarylbiimidazole (HABI) compounds. These antihalation compositions become colorless upon exposure to heat for a given time. Other patents that disclose similar antihalation compositions and photothermographic elements are: U.S. Pat. No. 5,652,091 (Perry et al.)U.S. No. Pat. 5,693,589 (Goswami et al.) and U.S. Pat. No. 5,705,323 (Perry et al.) and British patent GB 2,004,380 (Levinson et al.).
Most of the bleachable antihalation compositions in the prior art were designed for solvent systems in which the dyes and the bleaching agents were soluble as individual molecules. Since HABIs disclosed in the prior art have not been designed for use in aqueous systems, it is not predictable whether such systems would function when aqueous coated or in a hydrophilic layer comprising water. Furthermore, most of the bleachable antihalation compositions in the prior art have been directed to health imaging or graphic arts, as compared to photothermographic color film for consumer use. In the latter context, the dark keeping of a thermally bleachable dye composition would be a challenge. For such compositions to be useful, it would be crucial that they have the least amount of dark keeping loss, and at the same time undergo almost complete bleaching at higher temperatures.
There is a need for antihalation compositions that can be permanently and quickly bleached at lower temperatures in aqueous systems. Particularly in the field of color photothermographic film for consumer use, the requirements in terms of bleaching and keeping are high. Bleaching of an antihalation layer or filter layer is desired to improve scanning of the film.
SUMMARY OF THE INVENTION
The present invention relates to a photothermographic element comprising a support, at least one photothermographic layer, and at least one antihalation layer or a filter layer, wherein the antihalation or filer layer comprises a heat-bleachable composition comprising at least one light-absorbing filter dye (not derived from a latent image) and at least one hexaarylbiimidazole compound dispersed within the layer in the form of solid or liquid particles. The filter dye encompasses dyes used in filter layers or antihalation layers and excludes dyes resulting from developing agents or coupling agents. In one embodiment of the invention, the particles are dispersed in a matrix comprising a hydrophilic polymer or water-dispersible hydrophobic polymer. In one embodiment, the hydrophilic polymer is oxidized gelatin, as a binder for the antihalation or filter layer.
The present invention is also directed to a composition in the form of an aqueous dispersion comprising at least one antihalation or filter dye and at least one hexaarylbiimidazole compound, which aqueous dispersion comprises an aqueous phase and a dispersed organic phase, the aqueous phase comprising a soluble hydrophilic polymer or dispersed hydrophobic polymer and the dispersed organic phase comprising solid or liquid particles comprising said hexaarylbiimidazole compound, and wherein the hexaarylbiimidazole compound in said particles is capable of bleaching said dye when the dispersion is coated, dried and heated to a temperature of at least 90° C. for at least 0.5 seconds. The dye may be in the aqueous phase or in a separate dispersed organic phase. In one embodiment, solid particles comprising said hexaarylbiimidazole compound are employed. In another embodiment, an oil-in-water emulsion is employed.
The invention is also directed to a method of making a photothermographic element and the use of the photothermographic element, wherein the antihalation or filter layer becomes at least 40%, preferably at least 50%, more preferably at least 90%, colorless within about 20 minutes, preferably within about 5 minutes, more preferably within about 0.5 minutes, upon heating to a temperature of at least about 90° C. (according to controlled tests of such a layer essentially alone on the same support used in the product). The described antihalation or filter layer is especially advantageous because of the speed with which the layer becomes at least 40% colorless upon heating and its good shelf life storage stability. The invention is also directed to a method of forming an image in the multicolor photothermographic element, including scanning the developed image.
DETAILED DESCRIPTION OF THE INVENTION
As indicated above, a feature of the invention is the use, in a photothermographic element of a filter or antihalation layer comprising particles comprising a hexaarylbiimidazole compound dispersed in an aqueous matrix or an aqueous coated matrix. This hexaarylbiimidazole is used, in reactive association with a dye, to bleach the dye after image capture and upon suitable heating of the photothermographic element, advantageously during development of the image. A variety of hexaarylbiimidazole compounds are useful in an antihalation or filter layer according to the invention. These hexaarylbiimidazole compounds, also referred to as oxidative dimers of triarylimidazoles, are known compounds and can be prepared by methods known in the art. For instance, hexaarylbiimidazoles can be prepared by means of an interfacial oxidation of the parent triarylimidazole using potassium ferricyanide as an oxidant. For example, typical hexaarylbiimidazole compound
Goswami Ramanuj
Irving Mark E.
Kilminster Kenneth N.
Eastman Kodak Company
Schilling Richard L.
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