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Thermally developable light-sensitive layers containing photobleachable sensitizers
| Details |
Inventors: Rossman, Mitchell A.; Sher, Frank T.;
Assignee: Minnesota Mining and Manufacturing Company (St. Paul, MN)
Primary Examiner: Bowers, Jr.; Charles L.
Assistant Examiner: Chea; Thorl
Attorney, Agent or Firm: Griswold; Gary L., Kirn; Walter N., Litman; Mark A.
Negative-acting photothermographic imaging systems with improved color fidelity and light fastness are disclosed comprising a photobleachable sensitizer, a nitrate salt, an initiator, a leuco dye, a binder, and an optional organic acid. These systems may be used in a variety of applications comprising single or multiple layers in either single or multiple sheet constructions to provide color imaging elements. |
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DETAILED DESCRIPTION OF THE INVENTION Thermally developable imaging systems described in U.
S.
Pat.
Nos.
4,460,677 and 4,386,154 comprise a leuco dye, nitrate ion, and either (1) a spectrally sensitized organic compound having photolyzable halogen atom; or (2) a spectrally sensitized diaryliodonium salt.
Each of these systems has two disadvantages: (1) aesthetically undesirable residual sensitizer stain, and (2) relatively unstable images due to absorption of light causing subsequent changes in color.
Accordingly, there is a need for the light-sensitive, thermally developable imaging layers of this invention which provide cleaner and more stable images.
There are a minimum of five components to the imaging systems of the present invention.
The five required ingredients are (1) a photobleachable spectral sensitizing dye (spectral sensitizer), (2) a nitrate salt, (3) a leuco dye, (4) an initiator, and (5) polymeric resin (binder).
An acidic material constitutes a preferred sixth ingredient.
PHOTOBLEACHABLE SENSITIZER Any dye which is both a sensitizer for initiators of the present invention and which is also photobleachable is useful in the present invention.
A particular class of dyes useful as photobleachable sensitizers of this invention is the class of o-nitro-substituted arylidene dyes comprising an o-nitro-substituted aryl group joined through a methine chain linkage, said methine chain having a minimum of 2 methine carbon atoms, to an electron donor; said donor comprising either (1) a basic heterocyclic nucleus containing an electron-donating atom, typically a nitrogen, oxygen, or sulfur electron-donating atom, or (2) an alkylamino group, or (3) a dialkylamino group.
The number of atoms joining the basic heterocyclic nucleus and the aryl group can be an even or odd number.
As employed herein the term "arylidene" refers to a group formed by an aryl group and a methine linkage (e.
g.
, benzylidene, cinnamylidene, etc.
) The term photobleachable means that upon exposure to actinic radiation between about 350 nm and about 1100 nm the dye is converted to a colorless or nearly colorless form (i
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