Radiation imagery chemistry: process – composition – or product th – Thermographic process – Heat applied after imaging
Reexamination Certificate
1999-12-16
2001-03-20
Schilling, Richard L. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Thermographic process
Heat applied after imaging
C430S531000, C430S533000, C430S536000, C430S537000, C430S961000
Reexamination Certificate
active
06203970
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to photographic elements having a protective overcoat. The overcoat is present before exposure and processing and comprises a hydrophobic thermoplastic polymer that is present in the form of a porous membrane wherein the pores are occupied by a hydrophilic composition. During processing of the photographic element, the hydrophilic composition allows processing solution to pass through the membrane to the photosensitive layers in the element. During processing, the hydrophilic composition is removed from the overcoat. In the final step of processing, the element is heated so as to soften the hydrophobic thermoplastic polymer hereby forming a water resistance protective layer for the element.
DESCRIPTION RELATIVE TO THE PRIOR ART
Silver halide photographic elements contain light sensitive silver halide in a hydrophilic emulsion. An image is formed in the element by exposing the silver halide to light, or to other actinic radiation, and developing the exposed silver halide to reduce it to elemental silver.
In color photographic elements a dye image is formed as a consequence of silver halide development by one of several different processes. The most common is to allow a by-product of silver halide development, oxidized silver halide developing agent, to react with a dye forming compound called a coupler. The silver and unreacted silver halide is then removed from the photographic element, leaving a dye image.
In either case, formation of the image commonly involves liquid processing with aqueous solutions that must penetrate the surface of the element to come into contact with silver halide and, in the case of color elements, color forming compounds. Thus, gelatin, and similar natural or synthetic hydrophilic polymers, has proven to be the binder of choice for silver halide photographic elements. After processing, the photographic element, particularly where the element is a photographic print, is subjected to handling by the ultimate customer. Unfortunately, when gelatin, or similar polymers, are formulated so as to facilitate contact between the silver halide crystal and aqueous processing solutions, they are not as tough and mar-resistant as would be desired for something that is extensively handled after processing. Thus, fingerprints can easily mark the imaged element, it can be scratched or torn and it can swell or otherwise deform when it is contacted with liquids.
While numerous post-processing treatments have been proposed for photographic elements, it would be desirable to provide for a protective overcoat layer that could be incorporated as part of the photographic element as manufactured. This would eliminate the need for post-processing application of protective materials. Recently, there have been several proposals for protective overcoats that are part of the photographic element as manufactured. For example, in U.S. Pat. No. 5,856,051 there is described a protective overcoat applied during manufacture that comprises hydrophobic polymer particles with gelatin as a binder. Optionally, a relatively low molecular weight polymer is included in the composition to improve processing of the underlying photosensitive layers. In U.S. Pat. No. 5,853,926 there is described a similar overcoat including polymer particles in a soft polymer binder.
While the inventions in both the '051 and the '926 patents provide for desirable overcoats, continuous improvements were sought. These overcoat materials do not form porous membranes. For example, the materials used in the '051 patent are polymeric particles such as waxes (see col. 3 lines 13-25) which are extremely soft. Harder materials would be desirable to form more abrasion resistant overcoats. Further, during long term keeping, the low glass transition temperature of these materials causes some coalescence and this has a detrimental effect on processability.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a photosensitive photographic element comprising a support having a hydrophilic silver halide containing layer and an overcoat layer, the overcoat layer comprising a hydrophobic thermoplastic polymer that is present in the form of a porous membrane wherein the pores of said membrane are occupied by a hydrophilic composition. Preferably, the refractive index of the hydrophobic thermoplastic polymer and the refractive index of the hydrophilic composition are such that the porous layer is substantially transparent.
In another aspect of the present invention there is provided a method for producing an image in a photographic element comprising the steps of:
(1) exposing the photographic element described above so as to produce a photographic latent image in the silver halide layer,
(2) processing the photographic element in a manner such that the hydrophilic compound is removed from the overcoat membrane, and
(3) heating the processed photographic element so as to soften the hydrophobic thermoplastic polymer hereby closing the pores and producing a clear overcoat layer.
DETAILED DESCRIPTION OF THE INVENTION
Any solid hydrophobic thermoplastic polymer may be used in the formation of the membrane. Examples include ethyl cellulose, cellulose acetate, polyethylene, cellulose diacetate, polyvinyl chloride, cellulose nitrate, polysulfone, polypropylene, polystyrene, poly(ethylene terephthalate), polyamides, poly(butylene terephthalate), poly(methyl methacrylate), polycarbonate, poly(tetrafluoroethylene), poly(vinylidene fluoride). It is preferred that the material chosen has a Tg of 200° C. or less. The currently preferred material is ethyl cellulose.
The hydrophilic compound or composition may include any water-soluble organic or inorganic materials. It is preferred that said materials are of low molecular weight (below 1000). Preferred materials are low molecular weight surfactants or polymers that are compatible with the above hydrophobic components. Examples include sodium dodecyl sulfate, sodium bis(2 ethyl hexyl sulfosuccinate (Aerosol™ OT) from Cytec Industries (formerly American Cyanamid), sodium tetradecyl sulfate, sodium dodecyl benzene sulfonate, sorbitan monooleate(SPAN™ 80) from ICI Americas, poly(vinyl alcohol), poly(vinyl pyrrolidone), poly(ethylene oxide), polyoxyethylene sorbitan monooleate(TWEEN™ 80) from ICI Americas. Useful surfactants are also described in U.S. Pat. No. 3,378,507. The preferred membrane includes ethyl cellulose as the hydrophobic thermoplastic polymer and sodium bis(2-ethyl hexyl sulfosuccinate) as the hydrophilic composition because of the close match in refractive index.
Other common addenda, such as hardeners, spreading agents, charge control agents, dry scratch resistance compounds and lubricants can also be included in the formulation as needed.
The overcoats useful in the present invention can be highly porous. Highly porous membranes facilitate the processing of the underlying exposed photosensitive layers. However, it is well known that highly porous membranes are substantially opaque. Incorporating an index of refraction matched hydrophilic component renders the described membrane substantially transparent. Thus, the underlying photosensitive layers can be exposed through the unprocessed overcoat layer. It is preferred to make the membrane layer highly porous thereby making the hydrophilic component of the layer be in the majority. Preferably, the weight ratio of hydrophobic thermoplastic polymer to hydrophilic component is between about 1:0.5 and 1:5 and more preferably between about 1:1 and 1:3. Too much surfactant results in an unprocessed element with an undesirably tacky surface.
The described membranes are sometimes referred to as “phase inversion membranes”. Such membranes are described, for example, in “Synthetic Polymeric Membranes” by Robert E. Kesting, McGraw-Hill Book Company, New York, 1971 (see in particular Chapter 5: Porous Phase-inversion Membranes). In the terminology of that reference, the hydrophobic thermoplastic polymer is the matrix and has embedded therein the hydr
Eastman Kodak Company
Hawley J. Jeffrey
Schilling Richard L.
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