Radiation imagery chemistry: process – composition – or product th – Radiation sensitive product – Identified backing or protective layer containing
Reexamination Certificate
1997-06-18
2001-05-08
Chea, Thorl (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Radiation sensitive product
Identified backing or protective layer containing
C430S523000, C430S533000, C430S534000, C430S536000, C430S619000
Reexamination Certificate
active
06228571
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a photothermographic material capable of forming an image through heat development and more particularly, to a photothermographic material having a non-photosensitive layer on the back surface, that is, a back layer which can be coated at a low cost without a need for a harmful organic solvent. The photothermographic material is often referred to as a photosensitive material.
There are known many photosensitive materials comprising a photosensitive layer on a support which are exposed imagewise to form images. Among them, a process of forming an image through heat development is known as an environment friendly system capable of simplifying image forming means.
The process of forming an image through heat development is disclosed, for example, in U.S. Pat. No. 3,152,904 and 3,457,075, and D. Morgan and B. Shely, “Thermally Processed Silver Systems” in “Imaging Processes and Materials,” Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969. These photosensitive materials generally contain a reducible non-photosensitive silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), and a reducing agent for silver, typically dispersed in an (organic) binder matrix. Photosensitive materials are stable at room temperature. When they are heated at an elevated temperature (e.g., 80° C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, forming a black and white image.
In conjunction with photothermographic materials, it is well known to form a back layer by applying a coating solution of a binder in an aqueous solvent and drying the coating. Such a back layer is referred to as an aqueous back layer, hereinafter. For example, JP-A 254443/1990 discloses the use of gelatin as the binder and JP-A 129220/1976 discloses the use of polyvinyl alcohol as the binder.
As compared with back layers formed by applying a coating solution of a binder in an organic solvent and drying the coating, the aqueous back layers have environmental and economical advantages that they eliminate the detrimental influence of organic solvents on the environment and human body as well as the recovery of organic solvents.
Photosensitive materials having such aqueous back layers, however, suffer from drawbacks that when sheets of photosensitive material are stored in a humid atmosphere, fog increases and sheets stick to each other. It is then desirable to provide a photosensitive material with a back layer which is free of a fog increase, a sticking phenomenon, and detrimental influence on the environment and human body and is economically advantageous.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a novel and improved black and white photothermographic material capable of forming a black and white image through heat development. The material has a non-photosensitive layer on the back surface, that is, a back layer which is free of a fog increase and a sticking phenomenon even when sheets of the material are stored in a humid atmosphere, and wherein the back layer can be formed without a need for organic solvents which are harmful to the environment and human body and relatively expensive.
According to the invention, a photothermographic material forming an image through heat development is provided comprising a support having a pair of opposed surfaces and at least one photosensitive layer on one surface thereof. The photosensitive layer contains (i) a photosensitive silver halide, (ii) a non-photosensitive silver salt, and (iii) a reducing agent for the silver salt. The photothermographic material further includes a non-photosensitive or back layer on the other or back surface of the support. The non-photosensitive layer is formed by dispersing a binder containing at least 50% by weight of a polymer latex in a solvent containing at least 30% by weight of water to form a coating solution, applying the coating solution, and drying the coating. The polymer latex is typically of a polymer having an equilibrium moisture content of up to 2% by weight at 25° C. and RH 60%.
In one preferred embodiment, the non-photosensitive silver salt is an organic silver salt, more preferably a silver salt of an aliphatic carboxylic acid having at least 10 carbon atoms.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Back Layer
According to the invention, the photothermographic material has a photosensitive layer on one surface and a non-photosensitive layer on the other surface of a support. Since the other surface is a back surface, the non-photosensitive layer is also designated a back layer. The back layer is formed by dispersing a binder containing at least 50% by weight of a polymer latex in a solvent (or dispersing medium) containing at least 30% by weight of water to form a coating solution, applying the coating solution, and drying the coating.
With respect to the polymer latex used herein, reference should be made to Okuda & Inagaki Ed., “Synthetic Resin Emulsion,” Kobunshi Kanko-kai, 1978; Sugimura, Kataoka, Suzuki & Kasahara Ed., “Applications of Synthetic Latex,” Kobunshi Kanko-kai, 1993; and Muroi, “The Chemistry of Synthetic Latex,” Kobunshi Kanko-kai, 1970. The dispersed particles preferably have a mean particle size of about 1 to 50,000 nm, more preferably about 5 to 1,000 nm. The particle size distribution of the dispersed particles is not critical. Either a wide particle size distribution or a monodisperse particle size distribution is acceptable.
The polymer latex used herein encompasses polymer latices of both the conventional uniform structure and the core/shell structure. Polymer latices of the core/shell structure wherein the core and the shell have different glass transition temperatures are sometimes preferred.
Preferably the polymer latices used herein have a minimum film-forming temperature (MFT) of about −30° C. to 90° C., more preferably about 0° C. to 70° C. Film-forming assistants may be added in order to control the minimum film-forming temperature. The film-forming assistants also known as plasticizers are organic compounds (typically organic solvents) for lowering the minimum film-forming temperature of polymer latices, examples of which are described in the above-referred Muroi, “The Chemistry of Synthetic Latex,” Kobunshi Kanko-kai, 1970.
Included in the polymers used in the polymer latices are acrylic resins, vinyl acetated resins, polyester resins, polyurethane resins, rubbery resins, vinyl chloride resins, vinylidene chloride resins, polyolefin resins, and copolymers thereof.
The polymer of the polymer latex which is used in the non-photosensitive or back layer should preferably have an equilibrium moisture content of less than 2% by weight, preferably 0.1 to 2.0% by weight, more preferably 0.2 to 1.0% by weight at 250° C. and RH 60%.
The equilibrium moisture content of a polymer which is used as the binder is the moisture content (% by weight) that the polymer possesses when equilibrium is reached while the polymer is kept at a temperature of 25° C. and a relative humidity of 60%. With respect to the definition and measurement of the equilibrium moisture content, reference should be made to Japanese Polymer Society Ed., “Polymer Engineering Lecture No. 14—Polymeric Material Test Methods,” Chijin Shokan, for example. More specifically, the equilibrium moisture content of a polymer is determined as follows. A polymer film of 5 &mgr;m thick is conditioned in an atmosphere of 25° C. and RH 60% for 48 hours whereupon the weight (W
1
grams) of the moist film is measured. The moist film is then conditioned in an absolute dry condition (for example, in a desiccator with a solid phosphorus pentoxide fill) at 25° C. for 48 hours whereupon the w
Birch & Stewart Kolasch & Birch, LLP
Chea Thorl
Fuji Photo Film Co. , Ltd.
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