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Description  |
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This invention relates to a thermally processable imaging element
comprising a new overcoat that enables reduced release of volatile
components from the element during thermal processing and enables other
advantages.
Thermally processable imaging elements, including films and papers, for
producing images by thermal processing are known. These elements include
photothermographic elements in which an image is formed by imagewise
exposure to light followed by development by uniformly heating the
element. These elements also include thermographic elements in which an
image is formed by imagewise heating the element. Such elements are
described in, for example, Research Disclosure, June 1978, Item No. 17029;
U.S. Pat. No. 3,457,075; U.S. Pat. No. 3,933,508; and U.S. Pat. No.
3,080,254.
A problem exhibited by thermally processable imaging elements comprising
components that are volatile at thermal processing temperatures, such as
temperatures above 100.degree. C., is that the volatile components tend to
be released from the element during thermal processing. An example of this
is a silver halide photothermographic film as illustrated in following
comparative example A comprising a toner, such as succinimide, that has a
tendency to be released from the element upon thermal development and
comprising a poly(vinyl alcohol) overcoat. An example of such a poly(vinyl
alcohol) overcoat is described in, for example, U.S. Pat. No. 3,933,508,
U.S. Pat. No. 3,893,860, and Japanese published patent application No.
58/217930 published Dec. 19, 1983. As illustrated by comparative Example A
poly(vinyl alcohol) alone does not provide an answer to this problem
because it does not prevent release of the toner.
Other polymers which have been described or used as overcoats for such
elements also do not fully satisfy the requirements for an acceptable
overcoat. These other polymers do not satisfy one or more of the
requirements that the overcoat: (a) provide resistance to deformation of
the layers of the element during thermal processing, (b) prevent or reduce
loss of volatile components in the element during thermal processing, (c)
reduce or prevent transfer of essential imaging components from one or
more of the layers of the element into the overcoat layer during
manufacture of the element or during storage of the element prior to
imaging and thermal processing, (d) enable satisfactory adhesion of the
overcoat to a contiguous layer of the element, and (e) be free from
cracking and undesired marking, such as abrasion marking, during
manufacture, storage, and processing of the element. None of conventional
overcoats materials, such as cellulose acetate, gelatin and fully
hydrolyzed poly(vinyl alcohol) are fully satisfactory.
A continuing need has existed for an improved overcoat for a thermally
processable imaging element that satisfies all the described requirements.
It has been found that the described requirements are satisfied by a
thermally processable imaging element, particularly a photothermographic
element or thermographic element, comprising an overcoat layer comprising
poly(silicic acid). A preferred overcoat for such an element also contains
a water soluble hydroxyl containing polymer, such as water soluble
poly(vinyl alcohol) or water soluble cellulose derivative or monomer that
is compatible with poly(silicic acid).
The poly(silicic acid) is represented by the formula:
##STR2##
wherein X is an integer sufficient to provide a coatable aqueous solution
of poly(silicic acid), such as an integer within the range of at least 3
to about 600.
Poly(silicic acid) is prepared by methods known in the organic synthesis
art, such as by hydrolysis of tetraethyl ortho silicate. A typical method
of preparing poly(silicic acid) comprises mixing at room temperature
(20.degree. C.) distilled water with 1N p-toluenesulfonic acid and
absolute alcohol followed by mixing with tetraethyl ortho silicate. A
clear solution is obtained within several minutes. The resulting solution
of poly(silicic acid) is typically stable at 20.degree. C. for more than
30 days. A 1N aqueous solution of p-toluenesulfonic acid is typically
preferred in this preparation although a concentration of 0.1N to 1.0N
acid can be used. Stability of the poly(silicic acid) solution is often
less than optimum if a lower acid concentration is used in the
preparation. Acids which are useful in place of p-toluenesulfonic acid
include hydrochloric acid, sulfuric acid, and other mineral acids. A weak
organic acid, such as acetic acid, can provide the desired hydrolysis, but
the resulting poly(silicic acid) composition provides a gel within several
hours. This gel is not conveniently coated without added mixing and
preparation steps.
A useful poly(silicic acid) overcoat composition as coated does not
adversely flow, smear or distort at the processing temperatures of the
element, typically within the range of 100.degree. C. to 200.degree. C.
The optimum concentration of poly(silicic acid) in the overcoat will vary
depending upon the components in the overcoat, the particular
photothermographic element and processing conditions. Concentrations of
poly(silicic acid) below 50% by weight when poly(vinyl alcohol) is present
in the overcoat do not provide the desired degree of reduction of release
of volatile components from the thermally processable element. Preferably
when poly(vinyl alcohol) is present in the overcoat the concentration of
poly(silicic acid) is within the range of 50% to 90% by weight of the
overcoat. The optimum concentration of poly(silicic acid) can vary,
depending upon such factors as the particular imaging element, thermal
processing conditions, components used in combination with the
poly(silicic acid) and the like.
Useful overcoat compositions comprising the poly(silicic acid) are
typically transparent and colorless. If the overcoat is not transparent
and colorless, then it is necessary, if the element is a
photothermographic element, that is be at least transparent to the
wavelength of radiation employed to provide and view the image. The
overcoat does not significantly adversely affect the imaging properties,
such as the sensitometric properties in the case of a photothermographic
element, such as minimum density, maximum density or photographic speed.
Other components, particularly other polymers, can be useful with the
poly(silicic acid) in the overcoat. Other components than can be useful in
combination with poly(silicic acid) in the overcoat include, for example,
other polymers, such as water soluble hydroxyl containing polymers or
monomers that are compatible with poly(silicic acid), for example,
acrylamide polymers, water soluble cellulose derivatives, such as water
soluble cellulose acetate, and hydroxy ethyl cellulose acetate and the
like. It is important that the water soluble polymer must be compatible
with poly(silicic acid).
Imaging elements, particularly photothermographic and thermographic
elements according to the invention can comprise, if desired, multiple
polymer containing layers, particularly multiple overcoat layers. For
example, an imaging element according to the invention can comprise a
first overcoat layer comprising a polymer other than poly(silicic acid),
such as a water soluble cellulose derivative, for example, water soluble
cellulose acetate, and a second overcoat layer comprising poly(silicic
acid) and another polymer.
The overcoat according to the invention is useful on any thermally
processable element, particularly any photothermographic element or
thermographic element, that is compatible with poly(silicic acid). The
thermally processable element can be a black and white imaging element or
a dye-forming thermally processable imaging element. The overcoat is
particularly useful on a silver halide photothermographic element designed
for dry physical development. Useful silver halide elements on which the
overcoat is useful are described in, for example, U.S. Pat. Nos.
3,457,075; 4,459,350; 4,264,725 and Research Disclosure, June 1978, Item
No. 17029. The overcoat is particularly useful on, for example, a
photothermographic element comprising a support bearing, in reactive
association, in a binder, (a) photographic silver halide, prepared ex situ
and/or in situ, (b) an image forming combination comprising (i) an organic
silver salt oxidizing agent, preferably a silver salt of a large chain
fatty acid, such as silver behenate, with (ii) a reducing agent for the
organic silver salt oxidizing agent, preferably a phenolic reducing agent,
and (c) an optional toning agent.
A preferred embodiment of the invention comprises a photothermographic
element comprising a support bearing, in reactive association, in a
binder, particularly a poly(vinyl butyral) binder, (a) photographic silver
halide, prepared in situ and/or ex situ, (b) an image-forming combination
comprising (i) silver behenate, with (ii) a phenolic reducing agent for
the silver behenate, (c) a toning agent, such as succinimide, and (d) an
image stabilizer, such as 2-bromo-2-(4-methylphenylsulfonyl) acetamide,
and having an overcoat according to the invention, preferably an overcoat
comprising (A) poly(silicic acid) and (B) water soluble poly(vinyl
alcohol) which is 80% to 99% hydrolyzed, wherein the ratio of (A) to (B)
is at least 1, such as 1 to 1.5.
The overcoat is preferably applied to the thermally processable element at
the time of manufacture of the element; however, the overcoat can
optionally be applied to the element at any stage after preparation of the
element if desired. The overcoat can, for example, optionally be applied
to the element after exposure and before thermal processing.
The optimum overcoat layer thickness depends upon various factors, such as
the particular element, processing conditions, thermal processing means,
desired image and the particular overcoat. A particularly useful overcoat
layer thickness is within the range of 1 to 10 microns, preferably 1 to 3
microns.
The photothermographic elements comprise a photosensitive component which
consists essentially of photographic silver halide. In the
photothermographic materials it is believed that the latent image silver
from the silver halide acts as a catalyst for the described
oxidation-reduction image-forming combination upon processing. A preferred
concentration of photographic silver halide is within the range of about
0.01 to about 10 moles of photographic silver halide per mole of organic
silver salt oxidizing agent, such as per mole of silver behenate, in the
photothermographic material. Other photosensitive silver salts are useful
in combination with the photographic silver halide if desired. Preferred
photographic silver halides are silver chloride, silver bromide, silver
bromoiodide, silver chlorobromoiodide and mixtures of these silver
halides. Very fine grain photographic silver halide is especially useful.
The photographic silver halide can be prepared by any of the procedures
known in the photographic art. Such procedures for forming photographic
silver halide and forms of photographic silver halide are described in,
for example, Research Disclosure, June 1978, Item No. 17029 and Research
Disclosure, December 1978, Item No. 17643. Tabular grain photosensitive
silver halide is also useful, as described in, for example, U.S. Pat. No.
4,435,499. The photographic silver halide can be unwashed or washed,
chemically sensitized, protected against the production of fog and
stabilized against loss of sensitivity during keeping as described in the
above Research Disclosure publications. The silver halide can be prepared
in situ as described in, for example, U.S. Pat. No. 3,457,075.
The photothermographic elements typically comprise an oxidation-reduction
image-forming combination which contains an organic silver salt oxidizing
agent, preferably a silver salt of a long-chain fatty acid. Such organic
silver salt oxidizing agents are resistant to darkening upon illumination.
Preferred organic silver salt oxidizing agents are silver salts of
long-chain fatty acids containing 10 to 30 carbon atoms. Examples of
useful organic silver salt oxidizing agents are silver behenate, silver
stearate, silver oleate, silver laurate, silver hydroxystearate, silver
caprate, silver myristate and silver palmitate. Combinations of organic
silver salt oxidizing agents are also useful. Examples of useful silver
salt oxidizing agents which are not silver salts of long-chain fatty acids
include, for example, silver benzoate and silver benzotriazole.
The optimum concentration of organic silver salt oxidizing agent in a
photothermographic material will vary depending upon the desired image,
particular organic silver salt oxidizing agent, particular reducing agent
and particular photothermographic element. A preferred concentration of
organic silver salt reducing agent is preferably within the range of about
0.1 to about 100 moles of organic silver salt reducing agent per mole of
Ag. When combinations of organic silver salt oxidizing agents are present,
the total concentration of organic silver salt oxidizing agents is
preferably within the described concentration range.
A variety of reducing agents are useful in the photothermographic
materials. Examples of useful reducing agents include substituted phenols
and naphthols such as bis-.beta.-naphthols; polyhydroxybenzenes, such as
hydroquinones, including hydroquinone, alkyl-substituted hydroquinones,
such as tertiarybutylhydroquinone, methylhydroquinone,
2,5-dimethylhydroquinone and 2,6-dimethylhydroquinone; catechols and
pyrogallols; aminophenol reducing agents, such as 2,4-diaminophenols and
methylaminophenols; ascorbic acid reducing agents, such as ascorbic acid,
ascorbic acid ketals and other ascorbic acid derivatives; hydroxylamine
reducing agents; 3-pyrazolidone reducing agents, such as
1-phenyl-3-pyrazolidone and
4-methyl-4-hydroxymethyl-1-phenyl-3-pyrazolidone; sulfonamidophenols and
other organic reducing agents described in U.S. Pat. No. 3,933,508 and
Research Disclosure, June 1978, Item No. 17029, the description of which
is incorporated herein by reference. Combinations of organic reducing
agents are also useful.
Preferred organic reducing agents in photothermographic materials are
sulfonamidophenol reducing agents, such as described in U.S. Pat. No.
3,801,321. Examples of useful sulfonamidophenol reducing agent include
2,6-dichloro-4-benzenesulfonamidophenol; benzenesulfonamidophenol;
2,6-dibromo-4-benzenesulfonamidophenol and mixtures thereof.
An optimum concentration of reducing agent in a photothermographic material
varies depending upon such factors as the particular photothermographic
element, desired image, processing conditions, the particular organic
silver salt oxidizing agent and the particular stabilizer precursor. A
preferred concentration of reducing agent is within the range of about 0.2
mole to about 2.0 moles of reducing agent per mole of silver in the
photothermographic material. When combinations of reducing agents are
present, the total concentration of reducing agent is preferably within
the described concentration range.
The photothermographic materials preferably comprise a toning agent, also
known as an activator-toning agent or a toner-accelerator. Combinations of
toning agents are useful in photothermographic materials. An optimum
toning agent or toning agent combination depends upon such factors as the
particular photothermographic material, particular components in the
photothermographic material, desired image and processing conditions.
Examples of useful toning agents and toning agent combinations are
described in, for example, Research Disclosure, June 1978, Item No. 17029
and U.S. Pat. No. 4,123,282. Examples of useful toning agents include, for
instance, phthalimide, N-hydroxyphthalimide, N-potassium phthalimide,
succinimide, N-hydroxy-1,8-naphthalimide, phthalazine 1-(2H)-phthalazinone
and 2-acetylphthalazinone.
Stabilizers which are useful in photothermographic materials include
photolytically active stabilizers and stabilizer precursors as described
in, for example, U.S. Pat. No. 4,459,350, and include, for instance, azole
thioethers and blocked azolinethione stabilizer precursors and carbamoyl
stabilizer precursors such as described in U.S. Pat. No. 3,877,940.
Photothermographic materials according to the invention preferably contain
various colloids and polymers alone or in combination as vehicles, binding
agents and in various layers. Useful materials are hydrophobic or
hydrophilic. They are transparent or translucent and include both
naturally occuring substances such as proteins, for example gelatin,
gelatin derivatives, cellulose derivatives, polysaccharides, such as
dextran, gum arabic and the like; and synthetic polymeric substances, such
as water-soluble polyvinyl compounds like poly(vinylpyrrolidone) and
acrylamide polymers. Other synthetic polymeric compounds which are useful
include dispersed vinyl compounds such as in latex form and particularly
those which increase dimensional stability of photographic materials.
Effective polymers include water insoluble polymers of alkylacrylates and
methacrylates, acrylic acid, sulfoalkylacrylates and those which have
cross-linking sites which facilitate hardening or curing. Preferred high
molecular weight materials and resins include poly(vinylbutyral),
cellulose acetate butyrate, poly(methylmethacrylate),
poly(vinylpyrrolidone), ethyl cellulose, polystyrene, poly(vinylchloride),
chlorinated rubbers, polyisobutylene, butadiene-styrene copolymers, vinyl
chloride-vinylacetate copolymers, copolymers of vinylacetate and
vinylchloride, poly(vinyl alcohol) and polycarbonates.
Photothermographic materials can contain development modifiers that
function as speed increasing compounds, sensitizing dyes, hardeners,
antistatic layers, plasticizers and lubricants, coating aids, brighteners,
absorbing and filtered dyes, such as described in Research Disclosure,
June 1978, Item No. 17029 and Research Disclosure, December 1978, Item No.
17643.
The thermally processable elements according to the invention comprise a
variety of supports. Examples of useful supports include poly(vinylacetal)
film, polystyrene, film, poly(ethyleneterephthalate) film, polycarbonate
film and related films or resinous materials, as well as glass, paper,
metal and other supports which can withstand the thermal processing
temperatures.
The layers, including the overcoat, of thermally processable elements
according to the invention are coated on a support by coating procedures
known in the photographic art, including dip coating, air knife coating,
curtain coating or extrusion coating using hoppers. If desired, two or
more layers are coated simultaneously.
Spectral sensitizing dyes are useful in the described photothermographic
materials to confer additional sensitivity to the elements and
compositions. Useful sensitizing dyes are described in, for example,
Research Disclosure, June 1978, Item No. 17029 and Research Disclosure,
December 1978, Item No. 17643.
A photothermographic material preferably comprises a thermal stabilizer to
help stabilize the photothermographic material prior to imagewise exposure
and thermal processing. Such a thermal stabilizer aids improvement of
stability of the photothermographic material during storage. Preferred
thermal stabilizers are:
(a) 2-bromo-2-arylsulfonylacetamides, such as
2-bromo-2-p-tolysulfonylacetamide,
(b) 2(tribromomethyl sulfonyl)benzothiazole and
(c) 6-substituted-2,4-bis(tribromomethyl)-s-triazines, such as 6-methyl or
6-phenyl-2,4-bis(tribromomethyl)-s-triazine.
The thermally processable elements according to the invention are imagewise
exposed by means of various forms of energy in the case of silver halide
photothermographic elements. Such forms of energy include those to which
the photosensitive silver halide is sensitive and encompass the
ultraviolet, visible and infrared regions of the electromagnetic spectrum
as well as electron beam and beta radiation, gamma ray, x-ray, alpha
particle, neutron radiation and other forms of corpuscular wave-like
radiant energy in either non-coherent (random phase) forms or coherent (in
phase) forms as produced by lasers. Exposures are monochromatic,
orthochromatic, or panchromatic copending upon the spectral sensitization
of the photographic silver halide. Imagewise exposure is preferably for a
sufficient time and intensity to produce a developable latent image in the
photothermographic material. After imagewise exposure of the
photothermographic material, the resulting latent image is developed
merely by overall heating the element to moderately elevated temperatures.
This overall heating merely involves heating the photothermographic
element to a temperature within the range of about 90.degree. C., to about
150.degree. C., until a developed image is produced, such as within about
0.5 to about 60 seconds. By increasing or decreasing the length of time of
heating, a higher or lower temperature within the described range is
useful depending upon the desired image, the particular components of the
photothermographic material and heating means. A preferred processing
temperature is within the range of about 100.degree. C. to about
130.degree. C.
In the case of thermographic elements, the thermal energy source and means
for imaging purposes can be any imagewise thermal exposure source and
means that are known in the thermographic imaging art. The imagewise
heating means can be, for example, an infrared heating means, laser,
microwave heating means or the like.
Heating means known in the photothermographic and thermographic art are
useful for providing the desired processing temperature range. The heating
means is, for example, a simple hot plate, iron, roller, heated drum,
microwave heating means, heated air or the like.
Thermal processing is preferably carried out under ambient conditions of
pressure and humidity. Conditions outside normal atmospheric pressure and
humidity are useful if desired.
The components of the thermally processable element according to the
invention can be in any location in the element according to the invention
which provides the desired image. If desired, one or more components of
the photothermographic element according to the invention are in one or
more layers of the element. For example, in some cases, it is desirable to
include certain percentages of the reducing agent, toner, stabilizer
precursor and/or other addenda in the overcoat layer over the
photothermographic layer of the element. This, in some cases, reduces
migration of certain addenda in the layers of the photothermographic
element.
It is necessary that the components of the imaging combination be "in
association" with each other in order to produce the desired image. The
term "in association" herein means that in a photothermographic element
the photosensitive silver halide and the image-forming combination are in
a location with respect to each other which enables the desired processing
and produces a useful image.
Thermographic elements on which the described overcoat is useful include
any that are compatible with poly(silicic acid). Such thermographic
elements include those described in, for example, U.S. Pat. Nos.
2,663,657; 2,910,377; 3,028,254; 3,031,329 and 3,080,254, the disclosure
of which are incorporated herein by reference. An example of a useful
thermographic element comprises a support bearing a thermographic layer
comprising materials designed for electrically activated recording and
thermography known in the imaging arts, and an overcoat layer comprising
at least 50% by weight poly(silicic acid).
The term water soluble herein means at least 2 grams of the compound or
compositions dissolves in one liter of water within 2 hours at 90.degree.
C.
The following examples further illustrate the invention.
EXAMPLES 1-3
I. Preparation of Control:
A control photothermographic element was prepared having the following
composition:
______________________________________
mg/ft.sup.2
______________________________________
Overcoat:
Photographic gelatin 161.0
Matte 10.0
Formaldehyde 4.2
Surfactant (Surfactant 10G which is -p-
4.7
isononylphenoxypolyglycidol, a trademark
of and available from the Olin Corp., U.S.A.)
Photothermographic Layer:
Silver Behenate (Ag) 80.0
HgBr.sub.2 (Hg) 0.1
AgBr (Ag) 40.0
NaI 3.5
Succinimide toner/development modifier
42.0
Surfactant (SF-96 which is a polysiloxane
1.5
fluid and is available from and a trademark
of General Electric Co., U.S.A.)
Monobromo stabilizer: 6.0
##STR3##
Naphthyltriazine stabilizer:
6.0
##STR4##
Poly(vinyl butyral) binder (Butvar B-76 a
400.0
trademark of the Monsanto Co., U.S.A.)
Sensitizing dye 0.5
Benzenesulfonamidophenol developing agent:
100.0
##STR5##
MIBK solvent 30.0
______________________________________
Support
4 mil blue poly(ethylene terephthalate) film
In the following examples only the compositions of the overcoats will be
specified. The composition of the photothermographic layer used throughout
the examples is as described above.
II. Hydrolysis of tetraethyl orthosilicate (TEOS) to form poly(silicic
acid) (PSA)
The following components were mixed in the following order:
______________________________________
Distilled Water 144 g
1N-- -p-Toluenesulfonic Acid
36 g
Ethyl Alcohol 200 g
TEOS 208 g
______________________________________
A clear solution of PSA was obtained in less than 10 minutes.
III. Solution of Poly(vinyl alcohol)(PVA)
An aqueous solution of 8% by weight poly(vinyl alcohol) in water was
prepared. (8% by weight ELVANOL 52/22 in water. ELVANOL 52/22 is a
trademark of E. I. duPont deNemours U.S.A.)
IV. The following PSA/PVA solutions were prepared:
______________________________________
A B
______________________________________
##STR6## 0.75 1.25
8% PVA, Elvanol 52/22
125.0 g 125.0 g
Distilled Water 79.0 g 48.5 g
PSA solution 46.0 g 76.5 g
TOTAL 250.0 g 250.0 g
______________________________________
V. The following POLY(VINYL ALCOHOLS) were used:
______________________________________
Viscosity
% Soln.
(CPS) Hydrolysis pH
______________________________________
*ELVANOL 71/30 27-33 99.0-99.8 5.0-7.0
*ELVANOL 5 | | |
