Title: Silver halide photographic emulsion and photographic element comprising it
Abstract: A silver halide emulsion which comprises silver halide tabular grains showing an average thickness lower than 0.15 .mu.m, an average diameter of at least 1.20 .mu.m and an average aspect ratio of at least 8:1 dispersed in a hydrophilic colloid mixture comprising from 10% to 30% by weight of dextran, from 20% to 40% by weight of a hydrogenated polysaccharide having an average molecular weight equal to or lower than 10,000, and from 40% to 60% by weight of gelatin and photographic element comprising it.
Patent Number: 6,875,565 Issued on 04/05/2005 to Ceruti, et al.
| Inventors:
|
Ceruti; Luca (Savona, IT);
Fava; Simona (Genoa, IT)
|
| Assignee:
|
Ferrania, S.p.A. (Savona, IT)
|
| Appl. No.:
|
696873 |
| Filed:
|
October 30, 2003 |
Foreign Application Priority Data
| Oct 31, 2002[IT] | SV2002A0054 |
| Current U.S. Class: |
430/567; 430/631; 430/627; 430/642; 430/628; 430/634; 430/639 |
| Intern'l Class: |
G03C 001//00.5; G03C 001//49.4 |
| Field of Search: |
430/567,631,642,627,628,634,639
|
References Cited [Referenced By]
U.S. Patent Documents
| 3637389 | Jan., 1972 | Hofman | 96/66.
|
| 5302505 | Apr., 1994 | Delfino et al. | 430/567.
|
| 5370986 | Dec., 1994 | Lok et al. | 430/607.
|
| 5374509 | Dec., 1994 | Valentini | 430/536.
|
| 5529892 | Jun., 1996 | Massirio et al. | 430/523.
|
| 6235461 | May., 2001 | Sismondi et al. | 430/569.
|
| 6664038 | Dec., 2003 | Ceruti et al. | 430/642.
|
| Foreign Patent Documents |
| 0936201 | Aug., 1999 | EP | .
|
| 0950697 | Oct., 1999 | EP | .
|
| 0965880 | Dec., 1999 | EP | .
|
| 1300724 | Apr., 2003 | EP | .
|
| 55098745 | Jul., 1980 | JP | .
|
| 55098746 | Jul., 1980 | JP | .
|
| 08122956 | May., 1996 | JP | .
|
| WO 95/02614 | Jan., 1995 | WO | .
|
Other References
"Photographic Silver Halide Emulsions, Preparations, Addenda, Systems and
Processing," Research Disclosure No. 38597, Sep. 1996, Item IX.
|
Primary Examiner: Letscher; Geraldine
Attorney, Agent or Firm: Mark A. Litman & Associates, P.A.
Claims
What is claimed is:
1. A silver halide emulsion which comprises silver halide tabular grains
showing an average thickness lower than 0.15 .mu.m, an average diameter of
at least 1.20 .mu.m and an average aspect ratio of at least 8:1 dispersed
in a hydrophilic colloid mixture comprising from 10% to 30% by weight of
dextran, from 20% to 40% by weight of a hydrogenated polysaccharide having
an average molecular weight equal to or lower than 10,000, and from 40% to
60% by weight of gelatin.
2. The silver halide emulsion of claim 1, characterized in that said
hydrophilic colloid mixture comprises from 15% to 25% by weight of said
dextran, from 25% to 35% by weight of said hydrogenated polysaccharide,
and from 45% to 55% by weight of said gelatin.
3. The silver halide emulsion of claim 1, characterized in that said
hydrophilic colloid mixture comprises an amount of said dextran of from 5
to 100 grams per mole of silver, an amount of said hydrogenated
polysaccharide of from 10 to 100 grams per mole of silver, and an amount
of said gelatin of from 30 to 200 grams per mole of silver.
4. The silver halide emulsion of claim 1, characterized in that said
hydrophilic colloid mixture comprises an amount of said dextran of from 10
to 80 grams per mole of silver, an amount of said hydrogenated
polysaccharide of from 20 to 80 grams per mole of silver, and an amount of
said gelatin of from 50 to 150 grams per mole of silver.
5. The silver halide emulsion of claim 1, characterized in that said
hydrophilic colloid mixture comprises an amount of said dextran of from 20
to 40 grams per mole of silver, an amount of said hydrogenated
polysaccharide of from 40 to 60 grams per mole of silver, and an amount of
said gelatin of from 60 to 120 grams per mole of silver.
6. The silver halide emulsion of claim 1, characterized in that said
tabular grains have an average thickness within the range of from 0.05 to
0.15 .mu.m.
7. The silver halide emulsion of claim 1, characterized in that said
tabular grains have an average diameter of at least 1.40 .mu.m.
8. The silver halide emulsion of claim 1, characterized in that said
tabular grains have an average aspect ratio of from 8:1 to 50:1.
9. A silver halide photographic element comprising a support, at least one
silver halide emulsion layer coated on at least one side of said support,
and at least one protective layer coated over said emulsion layer, said
emulsion layer comprising a silver halide emulsion which comprises silver
halide tabular grains showing an average thickness lower than 0.15 .mu.m,
an average diameter of at least 1.20 .mu.m and an average aspect ratio of
at least 8:1 dispersed in a hydrophilic colloid mixture comprising from
10% to 30% by weight of dextran, from 20% to 40% by weight of a
hydrogenated polysaccharide having an average molecular weight equal to or
lower than 10,000, and from 40% to 60% by weight of gelatin.
10. The silver halide photographic element of claim 9, characterized in
that said hydrophilic colloid mixture comprises from 15% to 25% by weight
of said dextran, from 25% to 35% by weight of said hydrogenated
polysaccharide, and from 45% to 55% by weight of said gelatin.
11. The silver halide photographic element of claim 9, characterized in
that said hydrophilic colloid mixture comprises a dextran amount of from 5
to 100 grams per mole of silver, a hydrogenated polysaccharide amount of
from 10 to 100 grams per mole of silver, and a gelatin amount of from 30
to 200 grams per mole of silver.
12. The silver halide photographic element of claim 9, characterized in
that said hydrophilic colloid mixture comprises a dextran amount of from
10 to 80 grams per mole of silver, a hydrogenated polysaccharide amount of
from 20 to 80 grams per mole of silver, and a gelatin amount of from 50 to
150 grams per mole of silver.
13. The silver halide photographic element of claim 9, characterized in
that said hydrophilic colloid mixture comprises a dextran amount of from
20 to 40 grams per mole of silver, a hydrogenated polysaccharide amount of
from 40 to 60 grams per mole of silver, and a gelatin amount of from 60 to
120 grams per mole of silver.
14. The silver halide photographic element of claim 9, characterized in
that said tabular grains have an average thickness within the range of
from 0.05 to 0.15 .mu.m.
15. The silver halide photographic element of claim 9, characterized in
that said tabular grains have an average diameter of at least 1.40 .mu.m.
16. The silver halide photographic element of claim 9, characterized in
that said tabular grains have an average aspect ratio of from 8:1 to 50:1.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a silver halide photographic emulsion and
to a silver halide photographic element. More particularly, the present
invention relates to a silver halide photographic emulsion for use in a
radiographic element having improved sensitometric results and covering
power.
2. Background of the Art
In recent years, there has been a strong demand for high sensitivity, low
graininess and low fog in silver halide photographic elements as well as
for rapid processing in which development is accellerated. Recently,
demand for photographic performance of silver halide photographic light
sensitive materials has become more severe. In particular, demands for not
only basic performance such as high sensitivity, low fog and superior
graininess but also other performance such as rapid processing, mechanical
resistance and storage stability become stronger than those demands in the
past.
In general, silver halide photographic light sensitive materials are
subject to a variety of mechanical stresses. A photographic film is
subject to mechanical stresses in the manufacturing process thereof, or is
bent or abraded when being transported in the automatic processor. As well
known in the art, when mechanical stresses are applied to the silver
halide photographic material, changes in photographic performance are
produced, and a technique for enhancing resistance to these mechanical
stresses has been desired. The silver halide emulsions presently employed
in photographic elements are more sensitive to mechanical stresses during
automatic processing. There is the need to provide a photographic element
having increased mechanical resistance without negatively affecting the
underlying sensitometric properties.
Several approaches have been attempted to solve this problem. Hardening of
emulsion layers has been the more general approach described in a number
of patents and patent applications, such as, for example, in U.S. Pat.
Nos. 5,529,892 and 5,302,505. Another approach relates to the introduction
of an intermediate gelatin layer interposed between the support and the
emulsion layer, as described, for example, in U.S. Pat. No. 3,637,389.
Still another approach relates to the introduction of coating additives.
For example, methods in which polymer latexes or plasticizers are
included, methods in which the silver halide/gelatin ratio in the silver
halide emulsion layer is reduced, and methods in which a lubricant or
colloidal silica is added to the protective layer, are well known as means
of improving the mechanical resistance of photographic elements. A
description of useful coating additives can be found in Research
Disclosure No. 38597, September 1996, "Photographic Silver Halide
Emulsions, Preparations, Addenda, Systems and Processing", Item IX.
U.S. Pat. No. 5,374,509 describes a mixture of hydrophilic colloid, a
branched polysaccharide, a polyacrylamide, a polyvinylidine chloride and a
polyacrylate in a binder.
JP 08-0122956 describes a silver halide emulsion which contains a metal
chelating agent (type tartaric acids, ethylene diamine tetraacetates,
nitro triacetates, uramil diacetates) and a mono-, di- or poly-saccharide.
JP 55-098745, JP 55-098746, describes polysaccharides having glucose units
as main chain and mannose, fucose and glucoronic acids as side chain in
photographic solution preparation for high speed coating and improved
physical properties.
U.S. Pat. No. 5,370,986 describes the use of polyhydroxyalkyl stabilizer
compounds and a co-stabilizing agent in silver chloride photographic
element to prevent fog formation. The polyhydroxyalkyl stabiliser is a
non-reducing oligosaccharide or its alkyl-substituted glycoside of formula
R--(CHOH).sub.n (CHOR.sub.1).sub.m --Z with n=3-7, m=0-7, R=R.sub.1.dbd.H
or 1-3C alkyl, Z.dbd.COOR' or CONR'R' and R'=1-3C alkyl.
WO 95-02614, EP 950,697, and EP 936,201 describe the preparation and use of
hydrogenated polysaccharides for the preparation of mixtures with mineral
binders, fillers and/or pigments.
EP 965,880 describes the use of hydrogenated polysaccharides in combination
with an aryl compound having at least two hydroxyl groups to increase the
speed to Dmin ratio of a light-sensitive silver halide element.
When using a low molecular weight polysaccharide mixed with dextran in a
silver halide emulsion layer, it is advisable to use a lower amount of
hardener, to allow the optimum swelling of the layer in the processing
baths. In this way a high speed/Dmin ratio can be obtained as disclosed in
EP 1,300,724, filed on Sep. 17, 2002. A significant draw-back of this
technique is that the swollen layer has to be further hardened in the
processing chemicals, in order to avoid collapsing of developed silver
coils during drying, bringing to lower Dmax as a consequence. For this
reason the coating formulation described above is more suitable for
materials which are processed in processing chemistries containing a
hardener (typically glutaraldehyde or similar).
The present invention made clear that if the advantages of the coating
formulation have to be achieved in materials which undergo completely
hardener-free processing, for the sake of extending the use of the
material universally, not only silver halide tabular crystals should have
been used, but they also needed to be extremely thin, in order to get
acceptable sensitometric properties in all processing conditions and
chemistries.
SUMMARY OF THE INVENTION
One aspect of the present invention relates to a silver halide emulsion
which comprises silver halide tabular grains showing an average thickness
lower than 0.15 .mu.m, an average diameter of at least 1.20 .mu.m and an
average aspect ratio of at least 8:1 dispersed in a hydrophilic colloid
mixture comprising from 10% to 30% by weight of dextran, from 20% to 40%
by weight of a hydrogenated polysaccharide having an average molecular
weight equal to or lower than 10,000, and from 40% to 60% by weight of
gelatin.
In another aspect, the present invention relates to a silver halide
photographic element comprising a support, at least one silver halide
emulsion layer coated on at least one side of said support, and at least
one protective layer coated over said emulsion layer, said emulsion layer
comprising the above described silver halide emulsion.
DETAILED DESCRIPTION OF THE INVENTION
Accordingly, in one aspect the present invention relates to a silver halide
emulsion which comprises silver halide tabular grains showing an average
thickness lower than 0.15 .mu.m, an average diameter of at least 1.20
.mu.m and an average aspect ratio of at least 8:1 dispersed in a
hydrophilic colloid mixture comprising from 10% to 30% by weight of
dextran, from 20% to 40% by weight of a hydrogenated polysaccharide having
an average molecular weight equal to or lower than 10,000, and from 40% to
60% by weight of gelatin.
Silver halide tabular grains contained in the silver halide emulsion of the
present invention have an average diameter:thickness ratio (often referred
to in the art as aspect ratio) of at least 8:1, preferably 8:1 to 50:1,
more preferably 8:1 to 30:1, and most preferably 8:1 to 20:1. Average
diameter of the silver halide tabular grains suitable for use in this
invention preferably ranges from 1.20 to 5.00 .mu.m, more preferably from
1.40 to 3.00 .mu.m, and most preferably from 1.50 to 2.00 .mu.m. The
silver halide tabular grains suitable for use in this invention have a
thickness of less than 0.15 .mu.m, more preferably within 0.05 to 0.15
.mu.m. Copending U.S. patent application Ser. No. 10/697,513, filed the
same date as this application is incorporated herein by reference for its
disclosure of one technique for the formation of high quality tabular
grains with those specific parameters.
The silver halide tabular grain dimensions and characteristics described
above can be readily ascertained by analytical procedures otherwise
well-known to those skilled in the art. The term "diameter" means the
diameter of a circle having an area equal to the projected area of the
grain. The term "thickness" means the distance between two substantially
parallel main planes constituting the silver halide tabular grains. From
the measure of diameter and thickness of each grain the diameter:thickness
ratio of each grain can be calculated, and the diameter:thickness ratios
of all tabular grains can be averaged to obtain their average
diameter:thickness ratio. In practice, it is simpler to obtain the average
diameter and the average thickness of the tabular grains and to calculate
the average diameter:thickness ratio as the ratio of these two averages.
Whatever the method used, the obtained values of average
diameter:thickness ratio do not greatly differ.
The projected area of silver halide tabular grains in the emulsion of the
present invention accounts for at least 50%, preferably at least 80% and
more preferably at least 90% of the projected area of all the silver
halide grains of the emulsion.
The coefficient of diameter variation COVd of the tabular grains of the
emulsion of the present invention is preferably higher than 30%, more
preferably within the range of from 31% to 44%, and most preferably from
34% to 41%.
According to a preferred aspect of the present invention, the hydrophilic
colloid mixture comprises from 15% to 25% by weight of dextran, from 25%
to 35% by weight of a hydrogenated polysaccharide having an average
molecular weight equal to or lower than 10,000, and from 45% to 55% by
weight of gelatin.
Dextran is the generic name denoting many high molecular weight glucans
predominantly composed of alpha-1.fwdarw.6 bonds as derivatized from
sucrose by Leuconostoc mesenteroides and other organisms. Dextran is
commercially available in a range of average molecular weight of from
3,000 to 500,000. Preferred range of average molecular weight to be used
in the practice of the present invention is comprised between 5,000 and
50,000, more preferably from 10,000 to 25,000. Dextran derivatives include
(1) carboxyalkyl dextrans (such as carboxymethyl dextran), (2) dialkyl
aminoalkyl dextrans (such as diethyl aminoethyl dextran), and (3) amino
dextrans.
For the purposes of the present invention, dextran is typically added in an
amount of from 5 to 100 grams per mole of silver, preferably in the range
of from 10 to 80 grams per mole of silver, more preferably from 20 to 40
grams per mole of silver in the coating composition used to formulate a
specific silver halide layer in the photographic element. Such amounts can
be expressed in terms of grams per square meter per side of the resulting
silver halide radiographic element, wherein the amounts above correspond
to an amount of from about 0.1 to 2.0, preferably in the range of from 0.2
to 1.6, more preferably from 0.4 to 0.8 grams per square meter per side,
respectively.
Hydrogenated polysaccharides having a recurring unit comprising five or six
carbon atoms are preferably used in the present invention. Preferred
recurring units include, for example, adonitol, arbitol, xylitol,
dulcitol, iditol, mannitol, rabittol, sorbitol, and the like. The average
molecular weight of the hydrogenated polysaccharide derivatives used in
the present invention is equal to or lower than 10,000, preferably lower
than 8,000, and most preferably in the range of from 6,000 to 1,000.
Hydrogenated polysaccharides are commercially available, for example, under
the trade designation POLYSORB.TM., from Roquette, Lille, France. The
preparation of hydrogenated polysaccharides usually starts from natural
products (like starch, agar, tragacanth gum, xanthan gum, guar gum, and
the like) by means of enzymatic processes (to reduce the average molecular
weight) and of reducing processes (to saturate the molecule). POLYSORB.TM.
hydrogenated polysaccharides useful in the present invention are listed
below together with their respective CAS registration number.
Commercial Name CAS Registration Number
POLYSORB .TM. AN 221-10/80 111092-73-0
POLYSORB .TM. P 39317-78-7
POLYSORB .TM. RA 1000 204866-68-2
POLYSORB .TM. SI 134633-92-4
POLYSORB .TM. 05/60 153130-95-1
POLYSORB .TM. 70/12/12 167140-15-0
POLYSORB .TM. 10B 25086-67-3
POLYSORB .TM. 15/100 134688-45-2
POLYSORB .TM. 2 60976-39-8
POLYSORB .TM. 2-6 90955-13-8
POLYSORB .TM. 30/100 78690-76-3
POLYSORB .TM. 4 105287-50-1
POLYSORB .TM. 40/100 78690-77-4
POLYSORB .TM. 5 138726-47-3
POLYSORB .TM. 6 125148-17-6
POLYSORB .TM. 60/100 122525-49-9
POLYSORB .TM. 80/55 77466-50-3
POLYSORB .TM. 9 66593-04-2
For the purposes of the present invention, the hydrogenated polysaccharides
described above are typically added in an amount of from 10 to 100 grams
per mole of silver, preferably in the range of from 20 to 80 grams per
mole of silver, more preferably from 40 to 60 grams per mole of silver.
Such amounts can be expressed in terms of grams per square meter per side
of the resulting silver halide radiographic element, wherein the amounts
above correspond to an amount of from 0.2 to 2.0, preferably in the range
of from 0.4 to 1.6, more preferably from 0.8 to 1.2 grams per square meter
per side, respectively.
Gelatin is a hydrophilic colloid derived from animal collagen. Any gelatin
made from animal collagen can be used, but gelatin made from pig skin, cow
skin or cow bone collagen is preferable. The kind of gelatin is not
specifically limited, but several kinds of gelatins, such as, for example,
lime-processed gelatin, acid processed gelatin, amino group inactivated
gelatin (such as acetylated gelatin, phthaloylated gelatin, malenoylated
gelatin, benzoylated gelatin, succinoylated gelatin, methyl urea gelatin,
phenylcarbamoylated gelatin, and carboxy modified gelatin), or gelatin
derivatives, such as, for example, gelatin derivatives disclosed in JP
Patent Publications 38-4854/1962, 39-5514/1964, 40-12237/1965,
42-26345/1967 and 2-13595/1990, U.S. Pat. Nos. 2,525,753, 2,594,293,
2,614,928, 2,763,639, 3,118,766, 3,132,945, 3,186,846 and 3,312,553 and GB
Patents 861,414 and 103,189 can be used singly or in combination.
Preferably, gelatin derivatives include highly deionized gelatin,
acetylated gelatin and phthalated gelatin.
For the purposes of the present invention, gelatin is typically added in an
amount of from 30 to 200 grams per mole of silver, preferably in the range
of from 50 to 150 grams per mole of silver, more preferably from 60 to 120
grams per mole of silver. Such amounts can be expressed in terms of grams
per square meter per side of the resulting silver halide radiographic
element, wherein the amounts above correspond to an amount of from about
0.6 to 3.6, preferably in the range of from 0.9 to 2.7, more preferably
from 1.1 to 2.2 grams per square meter per side, respectively.
The silver halide emulsion of the present invention can be prepared either
directly conducting the formation and growth of silver halide grains into
the above described hydrophilic colloid mixture or, preferably, by first
conducting the formation and growth of silver halide grains in gelatin and
then adding the proper amounts of dextran and hydrogenated saccharide to
get the silver halide emulsion of the present invention. In the latter
case, the addition of dextran and hydrogenated saccharide can be done at
any time before the coating of the silver halide emulsion. The term "any
time before the coating" means to specially include at least after the
emulsion-making step, before, during or after the chemical and optical
sensitization step, or just before coating step. More preferably, the
addition of dextran and hydrogenated saccharide is conducted just before
the coating step.
Silver halide emulsions according to the present invention can be prepared
using conventional methods, including a single-jet method, a double-jet
method, or a combination of these methods and can be ripened using, for
instance, an ammonia method, a neutralization method, or an acid method.
Parameters which may be adjusted to control grain growth include pH, pAg,
temperature, shape and size of reaction vessel, and the reaction method
(e.g., accelerated or constant flow rate precipitation, interrupted
precipitation, ultrafiltration during precipitation, reverse mixing
processes and combinations thereof). A silver halide solvent, such as
ammonia, thioethers, thioureas, etc., may be used, if desired, for
controlling grain size, grain structure, particle size distribution of the
grains, and the grain-growth rate. Methods for preparing silver halide
emulsions are generally known to those skilled in the art and can be found
in references such as Trivelli and Smith, The Photographic Journal, Vol.
LXXIX, May 1939, pp. 330-338, T. H. James, The Theory of The Photographic
Process, 4th Edition, Chapter 3, Chimie et Physique Photographigue, P.
Glafkides, Paul Montel (1967), Photographic Emulsion Chemistry, G. F.
Duffin, The Focal Press (1966), Making and Coating Photographic Emulsions,
V. L. Zelikman, The Focal Press (1966), in U.S. Pat. Nos. 2,222,264;
2,592,250; 3,650,757; 3,917,485; 3,790,387; 3,716,276; and 3,979,213;
Research Disclosure, September 1994, Item 36544 "Photographic Silver
Halide Emulsions, Preparations, Addenda, Systems and Processing."
In the present invention, commonly employed halogen compositions of the
silver halide grains can be used. Typical silver halides include silver
chloride, silver bromide, silver iodide, silver chloroiodide, silver
bromoiodide, silver chlorobromoiodide and the like. However, silver
bromide and silver bromoiodide are preferred silver halide compositions
for silver halide tabular grains with silver bromoiodide compositions
containing less than 10 mol % silver iodide, preferably less than 5 mol %
silver iodide, and more preferably less than 1.5% mol silver iodide. The
halogen composition of individual grains may be homogeneous or
heterogeneous.
The preparation process of a silver halide emulsion generally comprises a
nucleation step, in which silver halide grain seeds are formed, followed
by one or more growing steps, in which the grain seeds achieve their final
dimension, and a washing step, in which all soluble salts are removed from
the final emulsion. A ripening step is usually present between the
nucleation and growing step and/or between the growing and the washing
steps.
In the preparation of the silver halide emulsion of the present invention,
an aqueous solution of a dispersing medium is put in a reaction vessel
together with a bromide salt aqueous solution.
The dispersing medium initially present in the reaction vessel can be
chosen among those conventionally employed in the silver halide emulsions.
Preferred dispersion media include hydrophilic colloids, such as proteins,
protein derivatives, cellulose derivatives (e.g. cellulose esters),
gelatin (e.g. acid or alkali treated gelatin), gelatin derivatives (e.g.
acetylated gelatin, phthalated gelatin and the like), polysaccharides
(e.g. dextran), gum arabic, casein and the like. It is also common to
employ said hydrophilic colloids in combination with synthetic polymeric
binders and peptizers such as acrylamide and methacrylamide polymers,
polymers of alkyl and sulfoalkyl acrylates and methacrylates, polyvinyl
alcohol and its derivatives, polyvinyl lactams, polyamides, polyamines,
polyvinyl acetates, and the like. The bromide salt is typically a water
soluble salt of alkaline or alkaline earth metals, such as, for example,
sodium bromide, potassium bromide, ammonium bromide, calcium bromide, or
magnesium bromide.
The temperature of the reaction vessel content is preferably in the range
of from 30.degree. C. to 80.degree. C., more preferably from 40.degree. C.
to 70.degree. C. The pH of the starting solution ranges from 2 to 7,
preferably from 3 to 6. The pBr of the starting solution ranges from 0 to
2, preferably from 0.5 to 1.5.
During the nucleation step (a), a soluble silver salt aqueous solution
(usually a silver nitrate solution) and a soluble bromide salt aqueous
solution (usually a sodium or potassium bromide solution) are added by
double jet method to the reaction vessel at a constant flow rate ranging
from 10 to 40 ml/min, preferably from 15 to 30 ml/min, by maintaining the
temperature constant. During the nucleation step, the amount of silver
nitrate added is lower than 5% by weight of total silver nitrate.
According to the present invention, the term "total silver nitrate" means
the amount of silver nitrate employed during the overall emulsion making
process, that is, from step (a) to (d).
At the end of the nucleation step, the addition of silver nitrate solution
is stopped and the obtained silver halide grain seeds are subjected to the
ripening step (b). The silver halide seeds are allowed to ripen at a
temperature of from 30.degree. to 80.degree. C., preferably from
50.degree. to 80.degree. C., for a period of time ranging from 1 to 20
minutes, preferably from 5 to 15 minutes, in the presence of a silver
halide solvent. The silver halide solvent is chosen amongst any
conventionally known silver halide solvents, e.g., thiourea, ammonia,
thioether, thiosulfate or thiocyanate. The concentration of the silver
halide solvent into the reaction vessel after the addition can range from
0.002 to 0.3N, preferably form 0.02 to 0.2N. According to a preferred
embodiment, the silver halide solvent is an ammonia aqueous solution. At
the end of the ripening step, the pH of the reaction vessel content is
adjusted to a value of from 4.5 to 6.5, preferably at about 6.
After that, the silver halide grain seeds are subjected to a growth step
(c) by double jet addition of a silver nitrate aqueous solution and a
halide salt aqueous solution at accelerated flow rate, with a linear ramp
starting from within 10 to 50 ml/min and rising to within 40 to 120
ml/min. The halide salt aqueous solution added during this step can either
comprise bromide ions or a mixture of bromide and iodide ions. The pBr of
the reaction vessel content is kept under control at a value of from 1.0
to 2.0, preferably from 1.0 to 1.5. During this growth step (c), the
amount of silver nitrate added is from 55 to 90% by weight of the total
silver nitrate.
The final step (d) is performed by a double jet addition of silver nitrate
and halide salt aqueous solutions at a constant flow rate of from 20 to 70
ml/min. The halide salt aqueous solution added during this step can either
comprise bromide ions or a mixture of bromide and iodide ions. During this
step, the amount of silver nitrate added is from 10 to 40%, preferably
from 25 to 35% by weight of total silver nitrate. During this step, the
pBr is kept under control at a value of from 1.0 to 2.0, preferably from
1.0 to 1.5.
If during the growing step and/or the final step, a soluble iodide salt is
added together with the bromide salt the amount of the iodide present in
the final emulsion ranges from 0.01 to 10% mol, preferably from 0.05 to
5%mol based on the total halide content.
At the end of the final step (d), the tabular grains can optionally be
further ripened for a period of time of from 1 to 20 minutes.
At the end of the silver halide grain formation, water soluble salts are
removed from the emulsion by procedures known in the art. Suitable washing
processes are those wherein the dispersing medium and soluble salts
dissolved therein can be removed from the silver halide emulsion on a
continuous basis, such as, for example, a combination of dialysis or
electrodialysis for the removal of soluble salts or a combination of
osmosis or reverse osmosis for the removal of the dispersing medium.
Among the known techniques for removing the dispersing medium and soluble
salts while retaining silver halide grains in the remaining dispersion,
ultrafiltration is a particularly advantageous washing process. Typically,
an ultrafiltration unit comprising membranes of inert, non-ionic polymers
is used as a washing process. Since silver halide grains are large in
comparison with the dispersing medium and the soluble salts or ions,
silver halide grains are retained by the membranes while the dispersing
medium and the soluble salts dissolved therein are removed.
Prior to use, silver halide grain emulsions are generally fully dispersed
and subjected to any of the known methods for achieving a desired
sensitivity. A wide description of methods and compounds useful in
chemical and optical sensitization can be found in Research Disclosure No.
38597, September 1996, "Photographic Silver Halide Emulsions,
Preparations, Addenda, Systems and Processing", Items IV and 5.
Chemical sensitization is performed by adding chemical sensitizers and
other additional compounds to the silver halide emulsion, followed by the
so-called chemical ripening at high temperature for a predetermined period
of time. Chemical sensitization can be performed by various chemical
sensitizers such as gold, sulfur, reducing agents, platinum, selenium,
sulfur plus gold, and the like. Silver halide tabular grains, after grain
formation and desalting, are preferably chemically sensitized by at least
one gold sensitizer and at least one sulfur sensitizer. During chemical
sensitization other compounds can be added to improve the photographic
performances of the resulting silver halide emulsion, such as, for
example, antifoggants, stabilizers, optical sensitizers, supersensitizers,
and the like.
Gold sensitization is performed by adding a gold sensitizer to the emulsion
and stirring the emulsion at high temperature of preferably 40.degree. C.
or more for a predetermined period of time. As a gold sensitizer, any gold
compound which has an oxidation number of +1 or +3 and is normally used as
gold sensitizer can be used. Preferred examples of gold sensitizers are
chloroauric acid, the salts thereof and gold complexes, such as those
described in U.S. Pat. No. 2,399,083. Specific examples of gold
sensitizers include chloroauric acid, potassium chloroaurate, auric
trichloride, sodium aurithiosulfate, potassium aurithiocyanate, potassium
iodoaurate, tetracyanoauric acid, 2-aurosulfobenzothiazole methochloride
and ammonium aurothiocyanate.
Sulfur sensitization is performed by adding a sulfur sensitizer to the
silver halide emulsion and stirring the emulsion at a high temperature of
40.degree. C. or more for a predetermined period of time. Useful examples
of sulfur sensitizer include thiosulfonates, thiocyanates, sulfinates,
thioethers, and elemental sulfur.
The amounts of the gold sensitizer and the sulfur sensitizer change in
accordance with the various conditions, such as activity of the gold and
sulfur sensitizer, type and size of silver halide grains, temperature, pH
and time of chemical ripening. These amounts, however, are preferably from
1 to 20 mg of gold sensitizer per mole of silver, and from 1 to 100 mg of
sulfur sensitizer per mole of silver. The temperature of chemical ripening
is preferably 45.degree. C. or more, and more preferably 50.degree. C. to
80.degree. C. The pAg and pH may take arbitrary values.
During chemical sensitization, addition times and order of gold sensitizer
and sulfur sensitizer are not particularly limited. For example, gold and
sulfur sensitizers can be added at the initial stage of chemical
sensitization or at a later stage either simultaneously or at different
times. Usually, gold and sulfur sensitizers are added to the silver halide
emulsion by their solutions in water, in a water-miscible organic solvent,
such as methanol, ethanol and acetone, or as a mixture thereof.
A stabilizer is preferably added at any time before the addition of the
sulfur sensitizer. While not intending to be bound by any particular
theory, it is believed that it acts as a digest stabilizer and a site
director for the sulfur sensitizer. Preferably, the stabilizer is added
before the addition of sulfur chemical sensitizer in an amount of from 1
to 500 milligrams per mole of silver, preferably, from 10 to 300
milligrams per mole of silver.
Specific examples of useful stabilizers include thiazole derivatives;
benzothiazole derivatives; mercapto-substituted heterocyclic compounds
(e.g., mercaptotetrazoles, mercaptotriazoles, mercaptodiazoles,
mercaptopyrimidines, mercaptoazoles); azaindenes, (e.g., triazaindenes and
tetrazaindenes); triazoles; tetrazoles; and sulfonic and sulfinic benzene
derivatives. Azaindenes are preferably used, more preferably,
tetraazaindenes.
A silver halide grain emulsion may be optically sensitized to a desired
region of the visible spectrum. Suitable methods for spectral
sensitization are known. For example, optical sensitization may be
achieved by using an optical sensitizer, such as a cyanine dye, a
merocyanine dye, complex cyanine and a merocyanine dye, an oxonol dye, a
hemioxonol dye, a styryl dye and a streptocyanine dye, or a combination
thereof. Useful optical sensitizers include cyanines derived from
quinoline, pyridine, isoquinoline, benzindole, oxazole, thiazole,
selenazole, imidazole. Particularly useful optical sensitizers are the
dyes of the benzoxazole-, benzimidazole- and benzothiazole-carbocyanine
type. Typically, the addition of the spectral sensitizer is performed
after the completion of chemical sensitization. Alternatively, spectral
sensitization can be performed concurrently with chemical sensitization,
before chemical sensitization, or even prior to the completion of silver
halide precipitation. When the spectral sensitization is performed before
the chemical sensitization, it is believed that the preferential
absorption of spectral sensitizing dyes on the crystallographic faces of
the tabular grains allows chemical sensitization to occur selectively at
unlike crystallographic surfaces of the tabular grains. In a preferred
embodiment, the spectral sensitizers produce J aggregates, if adsorbed on
the surface of the silver halide grains, and a sharp absorption band
(J-band) with a bathochromic shift with respect to the absorption maximum
of the free dye in aqueous solution.
It is known in the art of radiographic photographic elements that the
intensity of the sharp absorption band (J-band) shown by the spectral
sensitizing dye absorbed on the surface of the light-sensitive silver
halide grains will vary with the quantity of the specific dye chosen as
well as the size and chemical composition of the grains. The maximum
intensity of J-band has been obtained with silver halide grains having the
above described sizes and the chemical compositions absorbed with J-band
spectral sensitizing dyes in a concentration of from 25 to 100 percent or
more of monolayer coverage of the total available surface area of the
silver halide grains. Optimum dye concentration levels can be chosen in
the range of 0.5 to 20 millimoles per mole of silver halide, preferably,
in the range of 2 to 10 millimoles.
Spectral sensitizing dyes producing J aggregates are known in the art, such
as described by F. M. Hamer, Cyanine Dyes and Related Compounds, John
Wiley and Sons, 1964, Chapter XVII and by T. H. James, The Theory of the
Photographic Process, 4th Edition, MacMillan, 1977, Chapter 8.
In a preferred form, J-band exhibiting dyes are cyanine dyes. Such dyes
comprise two basic heterocyclic nuclei joined by a linkage of methine
groups. The heterocyclic nuclei preferably include fused benzene rings to
enhance J aggregation. The heterocyclic nuclei are preferably quinolinium,
benzoxazolium, benzothiazolium, benzoselenazolium, benzimidazolium,
naphthoxazolium, naphthothiazolium and naphthoselenazolium quaternary
salts.
Suitable cyanine dyes, which are joined by a methine linkage, include two
basic heterocyclic nuclei, such as pyrrolidine, oxazoline, thiazoline,
pyrrole, oxazole, thiazole, selenazole, tetrazole and pyridine and nuclei
obtained by fusing an alicyclic hydrocarbon ring or an aromatic
hydrocarbon ring to each of the above nuclei, such as indolenine,
benzindolenine, indole, benzoxazole, naphthoxazole, benzothiazole,
naphthothiazole, benzoselenazole, benzimidazole and quinoline. These
nuclei can have substituent groups.
Suitable merocyanine dyes, which are joined by a methine linkage, include a
basic heterocyclic nucleus of the type described above and an acid
nucleus, such as a 5- or 6-membered heterocyclic nucleus derived from
barbituric acid, 2-thiobarbituric acid, rhodanine, hydantoin,
2-thiohydantoin, 4-thiohydantoin, 2-pyrazolin-5-one, 2-isoxazolin-5-one,
indan-1,3-dione, cyclohexane-1-3-dione, and isoquinolin-4-one.
The methine spectral sensitizing dyes are generally known in the art, such
as those described in U.S. Pat. Nos. 2,503,776; 2,912,329; 3,148,187;
3,397,060; 3,573,916; and 3,822,136 and FR Pat. No. 1,118,778. Also their
use in photographic emulsions is known, wherein they are used in optimum
concentrations corresponding to desired values of sensitivity to fog
ratios. Optimum or near optimum concentrations of spectral sensitizing
dyes generally go from 10 to 500 mg per mole of silver, preferably, from
50 to 200, and more preferably, from 50 to 100.
Spectral sensitizing dyes can be used in combinations which result in
supersensitization, i.e., spectral sensitization which is greater in a
spectral region than that from any concentration of one dye alone or which
would result from an additive effect of the dyes. Supersensitization can
be obtained with selected combinations of spectral sensitizing dyes and
other addenda, such as stabilizers and antifoggants, development
accelerators and inhibitors, optical brighteners, surfactants and
antistatic agents, as described by Gilman, Photographic Science and
Engineering, 18, pp. 418-430, 1974 and in U.S. Pat. Nos. 2,933,390;
3,635,721; 3,743,510; 3,615,613; 3,615,641; 3,617,295; and 3,635,721.
Other additives can be added to the silver halide emulsion before or during
coating, such as, stabilizers or antifoggants (i.e., azaindenes,
triazoles, tetrazoles, imidazolium salts, polyhydroxy compounds and
others); developing promoters (e.g., benzyl alcohol, polyoxyethylene type
compounds, etc.); image stabilizers (i.e., compounds of the chromane,
cumaran, bisphenol type, etc.); and lubricants (i.e., wax, higher fatty
acids glycerides, higher alcohol esters of higher fatty acids, etc.).
Also, coating aids, modifiers of the permeability in the processing
liquids, defoaming agents, antistatic agents and matting agents may be
used. Other useful additives are disclosed in Research Disclosure, Item
17643, December 1978 in Research Disclosure, Item 18431, August 1979, in
Research Disclosure, Item 308119, Section IV, December 1989, and in
Research Disclosure Item 36544, September 1994.
The silver halide emulsion is then coated on a support to form the
photographic element of the present invention. Suitable supports include
glass, paper, polyethylene-coated paper, metals, polymeric film such as
cellulose nitrate, cellulose acetate, polystyrene, polyethylene
terephthalate, polyethylene, polypropylene and the like. A preferred
support is polyethylene terephthalate.
Preferred light-sensitive silver halide photographic elements are
radiographic light-sensitive elements employed in X-ray imaging comprising
a silver halide emulsion layer(s) coated on both surfaces of a support.
The silver halide emulsions are preferably coated on the support at a
silver coverage in the range of 1.5 to 3 g/m.sup.2 per side, more
preferably of from 1.5 to 2.5 g/m.sup.2 per side.
Usually, the radiographic light-sensitive elements are associated with
intensifying screens so as to be exposed to radiation emitted by the
screens. Preferable intensifying screens are made of relatively thick
phosphor layers which transform the X-rays into more imaging-effective
radiation such as light (e.g., visible light and ultraviolet light). In
operation, the screens absorb a larger portion of X-rays than the
light-sensitive elements do and are used to reduce the X-ray dose
necessary to obtain a useful image. Intensifying screens absorbing more
than 25% of the total X-radiation are preferably used. Depending on their
chemical composition, the phosphors can emit radiation in the ultraviolet,
blue, green or red region of the visible spectrum and the silver halide
emulsions are sensitized to the wavelength region of the radiation emitted
by the screens. Sensitization is performed by using spectral sensitizing
dyes absorbed on the surface of the silver halide grains as described
above.
Photographic elements of the present invention can include other layers and
additives such as subbing layers, surfactants, filter dyes, intermediate
layers, protective layers, anti-halation layers, barrier layers, dye
underlayers, development inhibiting compounds, speed-increasing agents,
stabilizers, plasticizers, chemical sensitizers, UV absorbers and the
like. Dye underlayers are particularly useful to reduce the cross-over of
the double coated silver halide photographic element. Reference to
well-known dye underlayer can be found in U.S. Pat. Nos. 4,900,652;
4,855,221; 4,857,446; and 4,803,150. In a preferred embodiment, a dye
underlayer is coated on at least one side of the support, more preferably,
on both sides of the support, before the coating of at least two silver
halide emulsion.
The silver halide photographic elements of the present invention are
preferably forehardened. Typical examples of organic or inorganic
hardeners include chrome salts (e.g., chrome alum, chromium acetate),
aldehydes (e.g., formaldehyde and glutaraldehyde), carbamoyl pyridinium
compounds (1-(N,N-Diethyl carbamoyl)-4-(2-sulfoethyl)pyridine), isocyanate
compounds (hexamethylene diisocyanate), active halogen compounds (e.g.,
2,4-dichloro-6-hydroxy-s-triazine), epoxy compounds (e.g., tetramethylene
glycol diglycidylether), N-methylol derivatives (e.g., dimethylolurea,
methyloldimethyl hydantoin), aziridines, mucohalogeno acids (e.g.,
mucochloric acid), active vinyl derivatives (e.g., vinylsulfonyl and
hydroxy-substituted vinylsulfonyl derivatives) and the like. Other
references to well known hardeners can be found in Research Disclosure,
December 1989, Vol. 308, Item 308119, Section X, and Research Disclosure,
September 1994, Vol. 365, Item 36544, Section 11(b). The more useful
hardeners have a quick action and migrate easily through the several
layers of the photographic element during its coating. The hardener can be
added to any layer of the photographic element of the present invention.
The hardener is preferably added to the protective layer in an amount
effective to fore-harden the resulting photographic element. Typical
amounts of hardener added to the photographic element of the present
invention are in the range of from 10 to 100 mg/m.sup.2, the specific and
preferred amounts also depending on the chemical nature of the hardener.
A detailed description of photographic elements and of various layers and
additives can be found in Research Disclosure 17643 December 1978,
Research Disclosure 18431 August 1979, Research Disclosure 18716 November
1979, Research Disclosure 22534 January 1983, Research Disclosure 308119
December 1989, and Research Disclosure 36544, September, 1994. The present
invention will be now described in greater detail with reference to the
following but not limiting examples. Various modifications and alterations
of this invention will become apparent to those skilled in the art without
departing from the scope and spirit of this invention. It should be
understood that this invention is not to be unduly limited to the
illustrative embodiments set forth herein.
All the amounts referred to in the following examples are relative to one
mole of silver in the resulting silver halide emulsion, unless differently
specified. All amounts are referred to one side.
EXAMPLES
Five silver bromide emulsions were prepared using a double jet method. All
emulsions contained one mole of silver. The emulsions were chemically and
spectrally sensitized using conventional sulfur, gold, and palladium
sensitizers plus a triethyl ammonium salt of
5,5'-dichloro-9-ethyl-3,3'-di-(3-sulfopropyl) oxacarbocyanine as spectral
sensitization dye. The digest was performed for about 120 to 150 minutes
at 60.degree. C. The emulsions were successively stabilized with 200 mg of
potassium iodide and 1366 mg of 5-methyl-7-hydroxy-2-3-4-triazoindolizine
(4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene) before chilling and kept in
cold storage until needed for coating. The following Table 1 summarizes
the grain characteristics and gelatin content of the resulting emulsions
at the end of sensitization step.
TABLE 1
Average Average
Emulsion Diameter Thickness COVd Gelatin (g)
A 1.20 0.22 40 58.0
B 1.80 0.11 37 67.0
C 1.80 0.11 37 67.0
D 1.80 0.11 37 67.0
E 1.80 0.11 37 67.0
The sensitized silver bromide emulsions were melted at 45.degree. C. and
subjected to coating finals. As coating auxiliaries were added 1.37 g of
calcium nitrate, 50 mg of azodicarboxylic dimorpholide, 19.87 g of
polyethylacrylate (in dispersion at 30% in water), 260 mg of Colanyl blue
and 53 mg of Flexonyl violet as chromatic correctors, the amount of
gelatin reported in Table 2, the amount of dextran (CAS No. 9004-54-0)
reported in Table 2, and the amount of hydrogenated polysaccharide
commercially available under the trade designation POLYSORB.TM. 70/12/12
(CAS No. 167140-15-0), from Roquette Freres, Lille, France reported in
Table 2. Finally, the pH was corrected to 6.5.
The resulting silver bromide emulsions A to E were immediately coated on
the two sides of blue 7 mil polyester base with a conventional antistatic
protective layer to give radiographic films 1 to 5. The protective layer
was based on gelatin (1.19 g/m.sup.2) and contained surfactants (11
mg/m.sup.2 of Niaproof, the trade name of an anionic surfactant of the
alkane sulfate type, 42 mg/m.sup.2 of ZONYL.TM. FSN100, the trade name of
a non-ionic perfluoroalkylpolyoxyethylene surfactant, and 12 mg/M.sup.2 of
lauric acid diethanolamide), matting agents (75 mg/m.sup.2 of
polymethylmethacrylate particles), and an amount of hardening agent
(1,3-bisvinylsulfonyl-2-propanol) as reported in Table 2. The resulting
covering weight was adjusted around about two grams of silver per square
meter.
TABLE 2
POLYSORB .TM. Dextran Gelatin Hardener
Film Emulsion (g) (g) (g) (mg/m.sup.2)
1 (C) A 57.3 36.0 9.0 19
2 (C) B 5.2 72.0 9.0 61
3 (I) C 57.3 36.0 9.0 32
4 (C) D 5.2 72.0 9.0 61
5 (I) E 57.3 36.0 18.0 61
The following Table 3 summarizes the percentages of POLYSORB.TM., dextran
and gelatin in the emulsion layer of the resulting radiographic films.
TABLE 3
Film POLYSORB .TM. % Dextran % Gelatin %
1 (C) 35.0 22.5 42.5
2 (C) 3.4 47.0 49.6
3 (I) 33.8 21.2 45.0
4 (C) 3.4 47.0 49.6
5 (I) 32.0 20.0 48.0
The fresh film samples were kept 3 days at 38.degree. C. before being
subjected to X-ray exposure using an X-ray tube at 70 KVp and 160
Milliampere for 0.1 second with two green emitting screens commercially
available under the trade designation LIFERAY.TM. Medium Screen
manufactured by Ferrania S.p.A., Italy.
The exposed films were processed through a 90 second dry to dry process in
a automatic processor XP-515 (manufactured by Ferrania S.p.A., Italy) with
commercially available processing chemistry (LIFERAY.TM. XAD-3 developer
and LIFEARY.TM. XAF-3 fixer, both from Ferrania S.p.A., Italy or Kodak RP
X-OMAT.TM. developer and fixer).
The sensitometric results and covering power are reported in the following
Table 4, wherein, under the "RP" columns are reported the values obtained
by using the above mentioned Kodak RP chemistry, and under the "APS"
columns are reported the values obtained by using the above mentioned
LifeRay chemistry. The Kodak RP chemistry comprises a hardener, the
LIFERAY.TM. chemistry is hardener-free.
TABLE 4
Shoulder Covering
D.max Speed Contrast Power
Film D. min RP APS RP APS RP APS RP APS
1 (C) 0.240 3.53 3.21 2.46 2.45 3.25 2.96 0.87 0.80
2 (C) 0.260 3.97 3.70 2.48 2.50 3.85 3.32 1.09 1.01
3 (I) 0.260 3.71 3.44 2.54 2.56 3.50 2.88 1.02 0.95
4 (C) 0.260 3.85 3.80 2.49 2.48 3.90 3.40 1.02 0.99
5 (I) 0.250 3.70 3.54 2.53 2.52 3.40 3.24 0.95 0.91
Comparison radiographic films showed a low speed value (samples 1, 2 and
4), a bad covering power (sample 1), and a strong decrease of maximum
density (Dmax) in LIFERAY.TM. chemistry (sample 1). Radiographic films of
the present invention, comprising the thin tabular grains and the coating
formulation described above, showed the best results in terms of speed,
with a still good covering power and a more limited decrease of Dmax in
LIFERAY.TM. chemistry.
*
