Ionic liquids as addenda in photothermographic systems

Radiation imagery chemistry: process – composition – or product th – Thermographic process – Heat applied after imaging

Reexamination Certificate

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C502S508000, C502S522000, C502S522000, C502S522000, C502S522000, C502S320000

Reexamination Certificate

active

06586166

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the use of ionic liquids as addenda in photothermographic systems.
BACKGROUND OF THE INVENTION
Ionic liquids are salts characterized by their unusually low melting points, which salts can be molten even at room temperature. Ionic liquids were disclosed early on by Hurley and Wier in a series of U.S. Patents (U.S. Pat. Nos. 2,446,331, 2,446,339; 2,446,350). These patents disclosed room temperature melts, comprised of AlCl
3
and a variety of n-alkylpyridinium halides, which afforded an advantageous conducting bath, free of volatile solvents, for aluminum electroplating.
Over the past 15 years, work in room-temperature melts has been dominated by the use of varying proportions of AlCl
3
and 1-ethyl-3-methylimidazolium (EMI) chloride, as discussed in separate review articles by Wilkes and Osteryoung (Osteryoung, Robert A., (p. 329) and Wilkes, John S., (p. 217) in Molten Salt Chemistry, G. Mamantov and R. Marassi, eds., (D. Reidel Publishing, Dordrecht, Holland, 1987) and in Japanese patent Nos. 0574656 (Endo, 1993) and 0661095 (Kakazu, 1994). A disadvantage of these first molten salts, and a serious problem with any solvent-free ionic liquid containing a strong Lewis acid such as AlCl
3
, is the liberation of toxic gas when exposed to moisture. Additionally, the highly reactive nature of Lewis acids used to form room temperature melts limits the kinds of organic and inorganic compounds which are stable in these media.
Ionic liquids typically exhibit mixed organic and inorganic character. The cation is usually a heterocyclic cation such as 1-butyl-3-methyl imidazolium or n-butylpyridinium. These organic cations, which are relatively large compared to simpler organic or inorganic cations, account for the low melting point of the salts. The anions, on the other hand, determine to a large extent the chemical properties of the system. Tetrafluoroborate and hexafluorophosphate are among the types of anions that are attracting the interest of ionic-liquid research groups. These ions do not combine with their corresponding Lewis acids and therefore are not potentially acidic. They are air and water stable.
U.S. Pat. No. 5,827,602 to Koch et al. discloses ionic liquids having improved properties for application in batteries, catalysis, chemical separations, and other uses. The ionic liquids described in Koch et al. are hydrophobic in nature, being poorly soluble in water, and contain only non-Lewis acid anions. When fluorinated, they were found to be particularly useful as inert liquid diluents for highly reactive chemicals.
Ionic liquids are discussed, for example, by Freemantle, M.
Chem. Eng. News
1998, 76 [March 30], 32; Carmichael, H.
Chem. Britain
, 2000, [January], 36; Seddon, K. R. J.
Chem. Tech. Biotechnol
. 1997, 68,351, Welton, T.
Chem. Rev
. 1999, 99, 2071; Bruce, D. W., Bowlas, C. J., Seddon, K. R.
Chem. Comm
. 1996, 1625; Merrigan, T. L., Bates, E. D., Dorman, S. C., Davis, J. H.
Chem. Comm
. 2000, 2051; Freemantle, M.
Chem. Eng. News
, 2000, 78 [May 15], 37. See also the following reviews of ionic liquids: Holbrey, J. D.; Seddon, K. R.
Clean Products and Processes
1999, 1, 223-236; and Dupont, J., Consorti, C. S. Spencer, J. J
Braz. Chem. Soc
. 2000, 11, 337-344.
Ionic liquids have generally been disclosed for use as solvents for a broad spectrum of chemical processes. These ionic liquids, which in some cases can serve as both catalyst and solvent, are attracting increasing interest from industry because they promise significant environmental benefits, since they are nonvolatile and therefore do not emit vapors. Hence they have been used, for example, in butene dimerization processes.
PCT publication WO 01/25326 to Lamanna et al. discloses an antistatic composition comprising at least one ionic salt consisting of a nonpolymeric nitrogen onium cation and a weakly coordinating fluoroorganic anion, the conjugate acid of the anion being a superacid, in combination with thermoplastic polymer. The composition was found to exhibit good antistatic performance over a wide range of humidity levels.
U.S. Pat. No. 6,048,388 to Schwarz et al. discloses an ink composition for inkjet printing which comprises water, a colorant and an ionic liquid material. In a preferred embodiment, the ink is substantially free of organic solvents.
In contrast to ink-jet media, such as disclosed in Schwarz et al. U.S. Pat. No. 6,048,388, photographic color images are typically obtained by a coupling reaction between the development product of an incorporated developing agent (e.g., oxidized aromatic primary amino developing agent) and a color forming compound commonly referred to as a coupler. The dyes produced by coupling are typically indoaniline, azomethine, indamine or indophenol dyes, depending upon the chemical composition of the coupler and the developing agent. In multicolor photographic elements, the subtractive process of color formation is ordinarily employed and the resulting image dyes are usually cyan, magenta and yellow dyes which are formed in or adjacent silver halide layers sensitive to radiation complementary to the radiation absorbed by the image dye, i.e. silver halide emulsions sensitive to red, green and blue radiation.
PROBLEM TO BE SOLVED BY THE INVENTION
Photothermographic systems involve heat processable photosensitive elements that are constructed so that after exposure, they can be processed in a substantially dry state by applying heat. Achieving adequate dye density and image discrimination has been a recurrent problem in photothermographic systems. Although black and white photothermographic systems, particularly in the areas of health imaging and microfiche, are commercially available, dye-forming color systems offer much greater challenges. Light-sensitive imaging elements that form color records of comparable density-forming ability and consistent stability in all color records in a photothermographic system can be difficult.
SUMMARY OF THE INVENTION
This invention involves use of ionic liquids as addenda in a color or monochrome photothermographic systems. The presence of ionic liquids in photothermographic systems has been found to provide, after processing, improvement in the amount of density and dye formed; In particular, the use of ionic liquids as addenda in the imaging layer of a photothermographic element improves the image discrimination after thermal development.
Various ionic liquids can be used in the present invention, preferably a compound comprising an organic cation and a suitable anion. Examples of anions include, for example, tetrafluoroborate, hexafluorophosphate, toluenesulfonate, methanesulfonate, or nitrate. Examples of cations include, for example, imidazolium, tetraalkylphosphonium or tetraalkylammonium cations. Many other combinations of suitable anions and cations can be used.
These and other objects are achieved in accordance with the invention which comprises a photothermographic element comprising a support having thereon at least one silver halide emulsion layer having associated therewith an ionic liquid dispersed alone or mixed with another ingredients and/or solvents.
DETAILED DESCRIPTION OF THE INVENTION
This invention involves the use of ionic liquids as photothermographic addenda to boost imaging performance such as image discrimination and, in color systems, dye formation. When ionic liquids are used as addenda at 5-50 mg/ft
2
with either red-sensitized, green-sensitized, or blue-sensitized photographic emulsions in a thermally processable color format, it was found that higher levels of image discrimination were seen, in a level-dependent fashion. Any suitable combination of cation and anion that does not have adverse photographic properties, yet results in an ionic liquid, can be used.
The use of ionic liquids has been found to provide more D-max, with little or no penalty in D-min. Alternately, the use of ionic liquids allows a lower processing temperature to be used.
Typically, the ionic liquid is incorporated in a silver halide em

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