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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to processes for the preparation of photographic
emulsions and more particularly to said processes wherein the photographic
emulsion contains mixed crystal silver halide grains.
2. Description of the Prior Art
Various processes for the precipitation of silver halide grains are known
in the art of preparing photographic emulsions, including double jet
processes and stream processes which are described in The Theory of The
Photographic Process, Third Edition, Mees-James, Macmillan Co., New York
(1966) at 31 and 40 respectively. In the double-jet process, the silver
nitrate solution and halide solution are simultaneously added to a mixing
vessel containing a gelatin solution. In the stream process, the silver
nitrate solution and halide solution are fed into a constant volume
chamber wherein precipitation of all grains takes place in the same
environment.
U.S. Pat. No. 3,415,650, issued to Frame, discloses a process wherein the
silver nitrate and halide solutions are fed into a centrifugal mixing
chamber submerged in a ripening vessel containing gelatin. The gelatin is
drawn into the mixing chamber wherein precipitation of silver halide is
accomplished within the confines of the mixing chamber and the silver
grains are dispersed in the gelatin. The dispersion is forced through
slots in the mixing chamber by centrifugal force into the ripening vessel.
British Pat. No. 1,243,356 discloses a similar process wherein the
precipitation chamber is located outside the ripening chamber and at least
some of the dispersion in the ripening chamber is recycled to the
precipitation chamber.
The objectives of the various processes of the prior art generally include
the preparation of reliable and reproducible emulsions at practical rates
of throughput. The preparation of such emulsions, and the resulting
quality of photographic film made therefrom, is highly dependent on the
uniformity of the silver halide grains of the emulsions. Indeed, even with
careful ingredient selection, maintaining a photographic environment, and
the most skillful subsequent tailoring of the emulsions, significant film
quality variations can result from irreproducibilities in the
precipitation and ripening steps of the emulsion preparation. Accordingly,
it is the object of the present invention to provide a process for
preparing a photographic emulsion having silver halide grains of
controlled grain size, structure and size distribution.
SUMMARY OF THE INVENTION
The invention comprises a process for preparing a photographic emulsion,
having a controlled silver halide grain size, structure and size
distribution, comprising the steps,
(1) adding silver nitrate to a stream, supplied from a conversion vessel,
containing gelatin solution and a first soluble halide salt to initially
precipitate first silver halide grains in said stream and form a
dispersion,
(2) recycling said stream, containing a dispersion of silver halide grains,
to said vessel,
(3) adding one or more aqueous solutions of said first soluble halide salt
and a second soluble halide salt to substitute said first silver halide
grains with a second less soluble silver halide by halide conversion in
said vessel, thereby forming mixed crystal silver halide grains, and
(4) recycling the contents of said vessel into said stream wherein
additional silver halide grains are precipitated by said first soluble
halide salt and grown on the surfaces of said mixed crystal silver halide
grains, to form mixed crystal silver halide grains of controlled
structure, size and size distribution.
The process of the invention provides advantages over either the
conventional double-jet or stream processes of the prior art in that
silver halide grains are precipitated separately in a controlled
environment from the environment of the conversion vessel. The conversion
vessel initially contains a single first soluble halide salt, but after
the initial precipitation and addition of the second halide, it provides
the environment for halide conversion. The environment of the conversion
vessel can be controlled to facilitate halide conversion, and to
substantially exclude the presence of the second soluble halide during
precipitation in the recycle stream. Avoiding direct precipitation of
silver nitrate with the second halide provides more uniform control of
grain structure, size and size distribution. Additionally, flexibility is
inherent in the process by providing control of the addition flow rates of
silver and halide solutions and the recycle flow rate of the dispersion.
Addition of the one or more aqueous solutions of halide salts may be by
injection into the recycle stream, after the initial precipitation, either
upstream or downstream of the addition of silver nitrate to the stream, or
it may be by direct addition to the conversion vessel.
Addition of silver nitrate and halide salt solutions and recycling of the
dispersion may all be either continuous or individually intermittent.
BRIEF DESCRIPTION OF THE DRAWINGS
The drawing is a diagram of an apparatus for carrying out the process of
the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference to the FIGURE, an apparatus for practicing the invention
comprises a vessel 10 having a jacket for circulating heating and cooling
water for receiving the initial charge of gelatin solution and a first
soluble halide. An agitator 20 is provided in said vessel. At least two
additional vessels 22 and 23 are provided for storing and supplying
aqueous silver salt solution and aqueous soluble halide solution,
respectively, to the process. A recycle stream 11 is provided to recycle
the dispersion formed in the stream 11 to the vessel 10. Mixers 14 and 15
are provided in the recycle stream for adding silver salt solution to the
process from vessel 22 through stream or conduit 12 and for adding aqueous
halide solution from vessel 23 through stream or conduit 13. Conduit or
stream 13' is provided for direct addition of aqueous halide solution to
vessel 10. Rotameters 17 are provided in the recycle stream 11 and in
streams 12 and 13 for measuring flow rates in the process. Control valves
19 are provided in all three streams. Recycling pump 18 is provided in
line 11 for recycling and controlling the contents of vessel 10. Metering
pumps 16 are provided for controlling the flow rates of silver salt
solution and aqueous halide solution added to the process. A three-way
valve 24 is provided in stream 13 for selecting addition of aqueous halide
solution to stream 11 or directly to the vessel 10. Similarly three-way
valves (not shown) may be provided in stream 11 from vessel 10 for
recycling the contents to the vessel or for feeding the contents to
another vessel. Also a three-way valve may be provided in line 12 for
direct addition of aqueous silver salts to vessel 10. A pAg meter 21 is
provided in stream 11 for monitoring the silver ion concentration in the
recycle stream. Additional pAg meters (not shown) may be provided for
monitoring the silver ion concentration at other locations in the process.
The rotameters and pAg meters may be used to generate control signals for
controlling flow rates in the process and controlling pAg or excess halide
concentration at various points in the process. A heat exchanger 25 may be
provided for controlling temperature in the recycle stream by circulation
of hot and cold water.
The mixers 14 and 15 are preferably tee-mixers, although other types of
static or dynamic mixers may be used. Using a conventional side tee-mixer
the main stream of the mixer is used for the recycle stream of the process
and the side stream of the mixer is used as the condition stream for
aqueous silver salts and aqueous halide solutions. The side tee-mixer
provides highly efficient mixing and precipitation in a reasonably short
length of the recycle stream e.g. substantially 100% mixing and
precipitation of silver halide is accomplished within 3 to 7 stream
diameters of the silver solution tee addition when the mass velocity ratio
of the side stream to the main stream is 2.7. The optimum mass velocity
ratio of 2.7 must be maintained for efficient mixing and precipitation of
the components in a tee-mixer. However, mixing times may be controlled by
varying the recycle flow rate and proportionally changing the side stream
flow rate.
The recycle ratio for the process is defined as the ratio of the recycle
flow rate to the silver solution flow rate. Although any recycle ratio may
be used, it is preferred to maintain the recycle ratio greater than or
equal to 10.
In order to reduce Ostwald ripening and coalescence and accomplish
substitution by halide conversion, an upper limit of 1.50 moles per liter
excess halide is maintained in the conversion vessel, however a limit of
substantially less than this is preferred, e.g. 0.03 moles per liter
excess halide. The start of addition of the aqueous halide solution is
preferably delayed or lags the start of the addition of silver solution by
a prescribed lag time, e.g. 0-5 minutes.
The process of the invention is useful for preparing mixed crystal silver
halide emulsion for photographic films having controlled grain structure,
size and size distribution over a wide latitude, for example cubic, mixed
cubic and octahedral or octahedral grains, having median particle sizes in
the range 0.2 microns to 2.0 microns and geometric standard deviations in
the range 1.1-1.6 with skew .+-.0.01-.+-.0.40. Temperature and pAg may be
varied over a wide range in the recycle stream and conversion vessel to
achieve the desired grain structure, size and size distribution, for
example, temperature at precipitation in the stream, and at conversion in
the vessel may be separately controlled within the range 100.degree.
F.-160.degree. F., and pAg may be controlled within the range 6-11 at
precipitation to achieve the grain structure desired.
The process of the invention has been found particularly useful for
producing fine grain, cubic structure, chlorobromide emulsions useful for
lithographic films as exemplified below.
EXAMPLE I
A gel-halide solution (A) was prepared and digested in the conventional
manner, comprising:
24,000 ml. distilled water
600 g. gelatin
42 g. Sodium Chloride
and was placed in the conversion vessel heated to 150.degree. F. and
agitated.
A 12,000 ml. aqueous solution of 1.5 molar silver nitrate (B) was prepared
in a conventional manner, placed in the silver solution storage and supply
vessel and heated to a temperature of 136.degree. F.
An aqueous solution of soluble halide (C) was prepared in a conventional
manner, comprising:
11,580 ml. distilled water
780 g. Sodium Chloride
556 g. Sodium Bromide
and was placed in the halide solution storage and supply vessel and heated
to a temperature of 138.degree. F.
The digested gel-halide solution (A) was fed into the recycle stream at a
flow rate of 9 gallons per minute (gpm). Silver nitrate solution (B) was
added to the recycle stream at a flow rate of 0.11 gpm through a 0.083
inch diameter side stream of a tee-mixer having a 0.67 inch main stream.
The aqueous solution of soluble halides (C) was added directly to the
conversion vessel at a flow rate of 0.11 gpm. The dispersion of initially
precipitated silver chloride grains was recycled to the vessel. The
dispersion in the vessel was continuously recycled at a recycle ratio of
82. Silver nitrate solution was continuously added at the 0.11 gpm flow
rate to the recycle stream. Aqueous soluble halide solution was
continuously added directly to the vessel at the 0.11 gpm flow rate. The
addition of aqueous silver and halide solutions was completed in 30
minutes. The dispersion of mixed crystal silver halide grains was
immediately quenched by addition of 22,000 ml of distilled water at
approximately 72.degree. F. and by circulating chill water at 55.degree.
F. in the jacket of the vessel till the temperature of the contents of the
vessel was lowered to 84.degree. F. The contents of the vessel were
coagulated and washed in a conventional manner to produce emulsion curds.
Subsequently the curds were redispersed, sensitized and coated on a
photographic support, as is well known in the art of manufacturing
photographic films, to produce a lithographic film having high sensitivity
and good dot quality. The grain structure determined by electron
micrograph of the mixed crystal silver halide grains was cubic. The grain
size distribution determined by mass settling had a median particle size
of 0.32.mu., with a geometric standard deviation of 1.24 and skew of
-0.05.
EXAMPLE II
Gel-halide, aqueous silver nitrate and aqueous soluble mixed halide
solutions were prepared and placed in the conversion and storage vessels
as in Example I.
Aqueous silver nitrate was added to the recycle stream at a flow rate of
0.106 gpm as in Example I. The aqueous solution of mixed halides was added
to the recycle stream through a 0.67 inch diameter side stream of a
tee-mixer upstream from the silver addition at a flow rate of 0.106 gpm.
The recycle ratio of the dispersion in the conversion vessel was 78, and
the contents of the vessel were continuously recycled for 30 minutes,
while the aqueous silver and aqueous halide solutions were continuously
added.
The contents of the vessel were quenched, coagulated and washed, and
subsequently redispersed and coated as in Example I to produce a
lithographic film having high sensitivity and good dot quality. The
structure of the mixed crystal silver halide grains was substantially
cubic, although some twin crystals were present. The grain size
distribution had a median particle size of 0.33.mu. with a geometric
standard deviation of 1.34 and skew of -0.03.
EXAMPLE III
A gel-halide solution was prepared in the manner of Example I, comprising
306,000 ml distilled water
900 g. Sodium Chloride
7,500 g. gelatin
placed in the conversion vessel and heated to 150.degree. F.
A 150,000 ml aqueous solution of 1.5 molar silver nitrate was prepared in
the manner of Example I placed in a silver storage vessel and maintained
at a temperature of 130.degree. F.
An aqueous solution of soluble mixed halides was prepared as in Example I,
comprising
152,000 ml distilled water
9,650 g. Sodium Chloride
6,960 g. Sodium Bromide
placed in a halide solution storage vessel and maintained at a temperature
of 130.degree. F.
Gel-halide solution was fed from the conversion vessel to the recycle
stream at a flow rate of 93 gpm. Silver nitrate solution was continuously
added to the recycle stream at a flow rate of 1.36 gpm through a 0.151
inch diameter side stream of a tee-mixer having a 1.875 inch diameter main
stream. The aqueous solution of soluble halides was continuously added
after a 30 second delay following the start of the addition of silver
solution directly to the conversion vessel as in Example I at a flow rate
of 1.40 gpm. The contents of the conversion vessel were continuously
recycled at a recycle ratio of 68 for 32 minutes. pAg in the recycle
stream was maintained at approximately 6.4.
The dispersion of mixed crystal silver halide grains was quenched by the
addition of 416,000 ml of water at approximately 60.degree. F. and by
circulating chill water in the jacket of the conversion vessel till the
temperature of the contents of the vessel was lowered to 84.degree. F.
The contents of the conversion vessel were coagulated and washed, as in
Example I, and subsequently redispersed and coated on a photographic
support to produce a high sensitivity lithographic film having good dot
quality.
The structure of the mixed crystal silver halide grains was cubic. The
grain size distribution had a mean particle size of 0.36.mu., with a
geometric standard deviation of 1.18 and skew -0.09.
EXAMPLE IV
Gel-halide, aqueous silver nitrate and aqueous soluble halide solutions
were prepared, and placed in the conversion vessel and storage vessels as
in Example III.
The gel-halide solution was fed into the recycle stream at a flow rate of
101 gpm. Silver nitrate solution was continuously added to the stream
through a 0.136 inch diameter side stream of a tee-mixer having a 1.875
inch diameter main stream at a flow rate of 1 gpm. After a 1 second delay
following the start of addition of silver solution, aqueous halide
solution was continuously added to the recycle stream, 51.5 inches
downstream of the silver solution addition tee-mixer, through the side
stream of an identical tee-mixer at a flow rate of 1.22 gpm. The contents
of the conversion vessel were continuously recycled for 40 minutes until
the addition of silver and mixed halide solutions was complete. The
recycle ratio was 101. pAg in the recycle stream was maintained at 7.1-7.3
and was 7.15 in the conversion vessel when recycling was stopped.
The contents of the conversion vessel were coagulated and washed and
subsequently redispersed and coated on a photographic support to produce a
high sensitivity lithographic film having good dot quality.
The structure of the mixed crystal silver grains was generally cubic, with
some twins. The grain size distribution had a mean particle size of
0.34.mu. with a geometric standard deviation of 1.37 and skew -0.04.
EXAMPLE 5
Two gel-halide solutions (A) were prepared and digested in a conventional
manner, comprising:
14,280 grams distilled water
350 grams gelatin
42 grams sodium chloride
Two aqueous solutions of 3 molar silver nitrate (B) were prepared in a
conventional manner, comprising:
3,940 grams distilled water
5,220 grams silver nitrate
and two aqueous solutions of soluble halides (C) were prepared in a
conventional manner, comprising:
7,580 grams distilled water
480 grams sodium chloride
350 grams sodium bromide
Solution (A) was placed in a conversion vessel, heated to 150.degree. F.
and agitated. Solution (B) was placed in a silver solution storage and
supply vessel and heated to a temperature of 127.degree. F. Solution (C)
was placed in a halide solution supply and storage vessel and heated to a
temperature of 130.degree. F.
In Method 1 the digested gel halide solution (A) was fed into the recycle
stream at a flow rate of 0.7 gallons per minute (gpm). Silver nitrate
solution (B) was continuously added to the recycle stream at a flow rate
of 0.07 gpm through a 0.187 inch side stream of a tee-mixer having a 0.857
inch diameter main stream. The aqueous solution of soluble halides (C) was
continuously added directly to the conversion vessel as in Example 1, at a
flow rate of 0.07 gpm. The contents of the conversion vessel were
continuously recycled at a recycle ratio of 10 for 29 minutes. pAg in the
conversion vessel was maintained at approximately 6.6-6.0.
The dispersion of mixed crystal silver halide grains was pumped to a
separate vessel and quenched with water to lower the temperature of the
mixture to 85.degree. F.
The contents were then coagulated and washed, as in Example 1 and
subsequently redispersed, sensitized and coated on a photographic support
to produce a silver halide film having high sensitivity. The grain
structure determined by electron micrographs of the mixed crystal silver
halide grains was cubic. The particle size distribution was determined
using a particle size analyzer and the results are tabulated in Table 1.
Sensitometric results are listed in Table 2.
In Method 2, which serves as a control, Solution A is fed from the
conversion vessel to the recycle stream at a flow rate of 0.7 gpm, similar
to that used in Example 1. Solutions B and C were added together and
continuously to the recycle stream through a 3000 revolutions per minute
(rpm) centrifugal pump mixer in which the solutions were thoroughly mixed.
The silver solution and halide solution were added at flow rates similar
to those used in Method 1. The contents of the conversion vessel were
continuously recycled at a recycle ratio of 10 for 29 minutes. pAg of the
vessel contents was maintained at 7.1-6.5.
The dispersion of mixed crystal silver halide grains was quenched,
coagulated, washed and subsequently redispersed, sensitized and coated on
a photographic support as described in Method 1.
The grain structure determined by electron micrographs of the mixed crystal
silver halide grains was a mixture of cubes and twins. The particle size
distribution determined with a particle size analyzer was broad compared
to the narrower size distribution of the grains produced by Method 1. The
results are shown in Table 1.
Table 1
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Particle Size Analyzer
Median Distribution Width
Volume Alpha Geom.
Emulsion Cu. Microns Std. Dev. Skew
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Method 1 0.014 0.79 1.27 -0.079
Electron Microscope
Edge Length
Shape of Range
Grains Microns
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99.9% cubic
0.1% Twins 0.2 to 0.4
Particle Size Analyzer
Median Distribution Width
Volume Alpha Geom.
Emulsion Cu. Microns Std. Dev. Skew
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Method 2 0.012 1.69 1.69 -0.005
Control)
Electron Microscope
Edge Length
Shape of Range
Grains Microns
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50% cubic 0.2 to 0.5
50% Twins Cubes
Pyramid Twins
0.96 to 0.64
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Results of the sensitometric tests are listed below in Table 2.
Table 2
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Top
Emulsion Speed Gradient Density
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Method 1 255.60 3.10 3.80
Method 2 243.00 2.60 3.30
(Control)
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Films made with the emulsion prepared by Method 1 had a 16% better
gradient, 13% better top density and a 5% better speed than films made
with the emulsion prepared by Method 2 (control).
EXAMPLE 6
Two silver chlorobromide emulsions were prepared by Method 1 and Method 2
respectively as described in Example 5. The emulsions were coagulated and
washed, as in Example 1 and subsequently redispersed, chemically
sensitized with conventional sulfur and gold sensitizers and
panchromatically sensitized with a mixture of a merocyanine and two
carbocyanine type dyes, and coated on photographic supports to produce
lithographic films. The grain structure determined by electron micrographs
of the mixed crystal silver halide grains prepared by Method 1 was cubic
whereas that of the grains prepared by Method 2 (control) was a mixture of
cubes and twins. Sensitometric results of the exposed and developed
samples are listed in Table 3.
TABLE 3
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Speed
Emulsion Red Green Blue
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Method 1 76 55 65
Method 2 46 30 30
(Control)
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Although the process of the invention is exemplified with reference to the
preparation of a photographic emulsion containing mixed crystal silver
chlorobromide having fine grain and cubic structure, such exemplification
is merely illustrative and not limiting. The process is equally useful for
preparation of photographic emulsions having other mixed cyrstal silver
halide grains, e.g. silver iodobromide, silver iodochloride and silver
iodochlorobromide, by precipitation and substitution by halide conversion.
Similarly other soluble halides are suitable for use in the precipitation
and substitution in the process, e.g. ammonium halides and other alkali
metal halides which are well known in the preparation of photographic
emulsions.
Additionally, the recycling of the contents of the conversion vessel and
addition of silver salt and soluble halide solution may be intermittent
rather than continuous. Also a plurality of mixers may be used in the
recycle stream for addition of the silver salt and soluble mixed halide
solutions if desired.
Auxiliary conventional ripening techniques which are well known in the art
of manufacturing photographic emulsions may be employed in the conversion
vessel if desired in conjunction with intermittent recycling.
Control of flow rates of silver salt and soluble halide solution may be
accomplished by using flow valves or variable speed pumps. Changes in flow
rates must be maintained within limits to insure the mass velocity ratio
of the main stream and side stream within limits of the 2.7.+-.10%
required for efficient mixing. Variations in flow rates may also be
accomplished by interchanging tee-mixers or using other mixers well known
in the mixing arts.
The recycle stream may be divided into a plurality of individual streams to
which silver nitrate and halide salt solutions are individually added.
Thus, silver nitrate or equivalent silver salt solution may be added to
one stream and halide salts may be added to another.
The process of the invention is particularly useful for making
monodisperse, cubic structure, silver chlorobromide grains of controlled
size, structure and size distribution.
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