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Description  |
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FIELD OF THE INVENTION
The present invention concerns a method for forming dispersed particles of
base degradable photographic components for photographic systems. It
particularly relates to the dispersion of photographic coupler materials
that would be degraded if present in highly alkaline solution.
PRIOR ART
The art of precipitation of hydrophobic components of photographic systems,
starting from a solution state, to a stable fine particle colloidal
dispersion is known. This is generally achieved by dissolving the coupler
in a water-miscible solvent aided by addition of base to ionize the
coupler, addition of a surfactant with subsequent precipitation of the
photographic component by lowering the pH, or by shift in concentration of
&he two or more miscible solvents, such that the photographic component is
no longer soluble in the continuous phase and precipitates as a fine
colloidal dispersion.
In United Kingdom Patent No. 1,193,349, Townsley et al discloses a process
whereby a color coupler is dissolved in a mixture of water-miscible
organic solvent and aqueous alkali. The solution of color coupler is then
homogeneously mixed with an aqueous acid medium including a protective
colloid. Thus was formed a dispersion of precipitated color coupler by
shift of PH, and this dispersion of color coupler when mixed with a
dispersion of an aqueous silver halide emulsion and coated on a support,
was incorporated into a photographic element.
In an article in Research Disclosure 16468, December 1977, pages 75-80
entitled "Process for Preparing Stable Aqueous Dispersions of Certain
Hydrophobic Materials" by W. J. Priest, a method of forming stable aqueous
dispersions of hydrophobic photographic material was disclosed. The
process of Priest involves the formation of an alkaline aqueous solution
of an alkali soluble color-forming coupler compound in the presence of a
colloid stabilizer or polymeric latex. The alkali solution is then made
more acidic in order to precipitate the hydrophobic protonated
color-forming coupler compounds. The droplets of color-forming coupler
compounds are stabilized against excessive coagulation by adsorption of a
colloid stabilizer.
U.S. Pat. No. 4,388,403--Helling et al discloses a process of preparation
of dispersions of hydrophobic substances in water. In Helling et al the
dispersions of hydrophobic substances in water are prepared by dissolving
the hydrophobic substance together with an ionic polyaddition or
condensation product in an organic, water-miscible solvent or a mixture of
such a solvent with water, diluting the solution with water and removing
the organic solvent. This Process is a preparation that causes the
particle formation by solvent shift of the solution. Helling et al
suggests utilization of the Process for preparation of photographic
recording materials.
Japanese Patent No. 53 139532 (1978)--Iwao et al discloses a method for
preparing dispersions in which a dye image precursor is dissolved in a
water-miscible organic solvent, and then said solution is mixed with an
aqueous solution containing a surfactant in order to precipitate the
water-insoluble dye image precursor as a dispersion. It is further
required in Iwao et al that the process be carried out in the "virtual
absence" of a polymer with a number average molecular weight of 2,000 or
above.
While the above processes have been somewhat successful with some color
photographic materials, there still remain difficulties in obtaining
dispersions of couplers that are degradable in base solution. Generally
use of polymeric steric stabilizers, such as polyvinyl pyrrolidone (PVP),
leads to reduction of activity of the formed coupler dispersions. The
process of using pH shift to cause particle precipitation by changing an
alkaline solution to an acidic solution is clearly unsatisfactory for
photographic materials that will degrade in basic solutions. Those
processes dealing with particle formation by shift of the concentration of
the solvent have difficulty in the excessive use of surfactants, uncertain
control of particle growth, and inability of many water-miscible solvents
to successfully dissolve large proportions of the couplers. The procedure
of the Iwao et al patent, which involves dissolving in only an organic
solvent, will be unable to achieve high loading of the hydrophobic
component in the solvent, thereby dramatically increasing Production cost,
as a large amount of solvent must be handled in order to achieve the
dispersion of a small amount of hydrophobic component particles.
THE INVENTION
The invention provides a method of forming a dispersion of base degradable
hydrophobic component for a photographic system. The invention is
accomplished by mixing the component, solvent, and surfactant, then
metering the mixture of solvent, surfactant, and hydrophobic component
into an excess of water with agitation so as to precipitate from the
water-solvent solution small particles of the hydrophobic component. After
washing by diafiltration or by dialysis to remove the water-miscible
solvent, the small particles form a stable dispersion that will not
agglomerate during storage Prior to use. The preferred photographic
component for use in the system of the invention is an ester-terminated
photographic coupler that would hydrolyze in alkaline solutions used in
prior art procedures.
BRIEF DESCRIPTION OF THE DRAWING
FIGS. 1 and 2 illustrate schematically apparatus for carrying out the
process of the invention.
MODES OF PREFORMING THE INVENTION
The invention has numerous advantages over the prior art, particularly the
process of Japanese Patent No. 53-139532 (1978)-Iwao et al. Addition of
the surfactant to the solvent increases the solubility of the coupler in
the solvent by greater than 100%. Therefore, a less potent solvent can be
utilized, preventing ripening or particle growth after precipitation of
the coupler in fine dispersion state. The particles formed by the system
of the invention are more uniform in size than those formed by milling
processes. The process of the invention may be formed generally in the
ambient conditions and does not require special cooling in order to
maintain control of the process. These and other advantages will become
apparent from the detailed description below.
In the drawing, apparatus 10 may be utilized in performing the process of
the invention. To container 12 is added a solvent, surfactant (preferably
in aqueous solution), and a dry photographic system component, from inlet
devices 14, 16, and 18. The dry photographic system component is stored in
container 38. The mixture of the solvent, surfactant, and photographic
component are mixed by agitator 22 to form a clear solution when heated to
the desired temperature in container 12. After mixing, the materials are
pumped by pump 24 into tank 26 through inlet 28. Tank 26 contains water
fed through inlet 30. As the mixture of solvent, surfactant, and
photographic component is added via inlet 28 to the tank 26, and agitated
by stirrer 34, particles precipitate out in stable, fine dispersion form.
The dispersion of these particles may be removed through outlet 36 for
washing to remove the solvent by ultrapiltration or dialysis and then for
treatment to form a photographic element.
The above description is that of a semicontinuous batch process. This
process can be very easily converted to a continuous manufacturing process
using apparatus 50 described in FIG. 2, using the Example 1 of Table I. In
Example 1, the density of the coupler solution, which is made up of 20 g
of coupler, 80 g of propanol, and 15 g of 33% Aerosol A102 solution, is
about 0.8 g/ml. Its total weight being 115 g, its volume is 115/0.8=144
ml. Total volume of the final dispersion in the formulation is, therefore,
600 ml+144 ml=744 ml. Tube 52 is inserted into the reaction kettle 26 to a
level such that its end touches the liquid level when the dispersion
volume in the kettle is 744 ml. This is the residence volume of the
reaction kettle 26. When a ratio of 600 ml of water to 144 ml of coupler
solution and a coupler flow rate of 20 ml/min. is used, a water flow rate
of (600.times.20)/144=83 mg/min. is necessary. Total dispersion formation
rate in the kettle is 20+83=103 mg/min. Therefore, to maintain a constant
volume in the reactor, the dispersion withdrawal pump 60 is set at 103
mg/min to remove the formed dispersion from the reaction kettle 26.
To start the process, the reaction vessel 26 is emptied by opening valve in
outlet 36, which is then closed. The coupler solution pump 24 at 20
ml/min. and the water pump 31 at 83 ml/min. are started simultaneously.
Dispersion is formed in the reaction kettle 26 as described before. The
dispersion is withdrawn via pump 60 started soon after the start of the
process. The moment the dispersion volume in the reaction kettle reaches
744 ml, as determined by the level of outlet 52 in the reaction vessel,
the formed dispersion is pumped out via outlet 52 for diafiltration and
concentration to .about.15% coupler suitable for subsequent formulation
into an element of a photographic system. In this concept a residence
volume of 744 ml is used. It can be chosen to be any convenient volume to
suit production conditions.
Any solvent that will dissolve the photographic component without
degradation of the component and that is miscible with water is suitable
for the invention. Typical of such solvents are acetone, methyl alcohol,
ethyl alcohol, isopropyl alcohol, tetrahydrofuran, dimethylformamide,
dioxane, N-methyl-2-pyrrolidone, acetonitrile, ethylene glycol, ethylene
glycol monobutyl ether, diacetone alcohol, etc. A preferred solvent is
n-propanol because n-propanol allows the particles to stay dispersed
longer after formation.
The surfactant suitable in the process may be any surfactant that allows
increased solubility of the photographic component in the solvent, as well
as adding stability to the final dispersion of particles. Generally the
surfactant contains a hydrocarbon chain comprising about 6 to about 24
carbon atoms and at least 3 oxyethylene groups and preferably having two
negative charges at the hydrophilic end of the surfactant. A preferred
group of surfactants has been found to be the disodium ethoxylated C-10 to
C-12 alcohol half esters of sulfosuccinic acid, as these surfactants give
dispersions that are stable and allow a higher concentration of the
photographic component to be dissolved in the solvent. A particularly
preferred surfactant has been found to be the surfactant available as
Aerosol A102 from Cyanamid that has the formula as set forth below.
##STR1##
Other suitable surfactants are Aerosol A103 from Cyanamid and Polystep B23
from Stepan Chemical having the following formulas:
##STR2##
The hydrophobic photographic components that are suitable for the
dispersion formation method of the invention are those having groups that
are destroyed in base solutions. Typical of such materials are those
photographic components that contain terminal ester groups, such as those
illustrated in the structures below. The method of the invention has been
found to be particularly preferred for ester-terminated photographic
couplers and image modifying (DIR and DIAR) couplers. The base
hydrolyzable group that will decompose in a base solution is underlined.
##STR3##
These ester-terminated compounds are not suitable for pH shift particle
formation processes as they will degrade in base compositions.
The following examples are representative of the process of the invention
and are not intended to be exhaustive of the possibilities of the
invention. Parts and percentages are by weight unless otherwise indicated.
EXAMPLES 1-4
Preparation of Dispersions of a Yellow Dye-Forming Coupler
Four dispersions of Coupler 2 above were prepared by the following
procedure: To 20 g Coupler 2 was added 80 g n-propanol and the amount
shown in Table I of a 33% active solution of Aerosol A102 (Disodium
ethoxylated C-10 to C-12 alcohol half ester of sulfosuccinic acid, made by
Cyanamid). The mixture was heated to 65.degree. C., with stirring to
dissolve the coupler. The hot coupler solution was then pumped at 20
ml/min. into 600 g of water in the reactor with stirring. The
precipitation chamber was kept at room temperature.
Solvent was removed from the dispersions by continuous dialysis against
distilled water. The particle sizes shown in Table I were determined by
photon correlation spectroscopy (PCS), an analytical method described in
"Laser Light Scattering" by B. Chu, Academic Press, N.Y. (1974). The
repeat makes are reproducible in terms of composition and particle size.
It appears that increase in the surfactant concentration leads to slight
increase in particle size probably due to ripening by the solubilizing
surfactant. A high pressure liquid chromatography of the Example 1
dispersion indicated no decomposition compared to the pure coupler.
TABLE I
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Preparation Conditions and Physical
Properties of Coupler 2 Dispersions
Coupler Solution
Components (Grams) Final PCS*
Cou- 33% Cou- Al02 Particle
Exam- pler normal aqueous pler 2 conc.
Diameter
ple 2 propanol Al02 soln.
conc. %
% (nm)
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1 20 80 15 1.7 0.17 212
2 20 80 15 1.7 0.17 234
3 20 80 30 1.9 0.25 237
4 20 80 45 1.7 0.35 250
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*Photon Correlation on Spectroscopy
EXAMPLES 5-10
Photographic Testing of Experimental Dispersion
Photographic elements were prepared using the following coating format in
which the no-solvent dispersion of the invention was compared with a
conventional coupler solvent dispersion of the same coupler prepared by
the method of Fierke and Chechak described in U.S. Pat. No. 2,801,171
(components are given in mg/m.sup.2).
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Overcoat Layer: Gelatin - 2691, Hardener at 1.8% of
total gelatin
Emulsion Layer: Gelatin - 2691, Blue-sensitized
silver bromoidide emulsion (as silver) - 646,
Coupler 2 and DIR Compound 3 as shown in Table II
Support: Gelatin - 4887 on Cellulose Triacetate
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Strips of each element were step-exposed to white light and processed in a
Kodacolor C41 process as described in the British Journal of Photography
Annual, 1982, pp. 209-211. Reading the processed strips with blue light
gave the results reported in Table II, where G (gamma) is the contrast of
the straight-line portion of the sensitometric curve and % G is a measure
of the effectiveness of the DIR Compound 3 in repressing this contrast. It
is calculated as % G=100.times. (1-G.sub.DIR /G.sub.NONE), where G.sub.DIR
is the contrast with the DIR compound present and G.sub.NONE is the
contrast using the same coupler dispersion in the absence of DIR compound.
TABLE II
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Coupler 2/
Example DBP* DIR-3/DBP* G % G
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5 Comparison
646/323 -- 1.90 0
6 Comparison
646/323 65/65 1.60 16
7 Invention
646/0 -- 1.55 0
8 Invention
646/0 32/32 1.41 9
9 Invention
646/0 65/65 1.21 22
10 Invention
646/0 97/97 1.07 31
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*Amounts are in mg/m.sup.2 : DIR Compound 3 was dispersed in an equal
weight of dibutyl phthalate (DBP); Coupler 2 (comparison) was dispersed i
half its weight of dibutyl phthalate.
It can be seen from the results in Table II that although the dispersion
according to the invention is slightly less active than the conventional
dispersion, it is still very active for a non-solvent dispersion. It is
noteworthy that the same amount of DIR Produces a gamma suppression of 22%
with the Coupler 2 dispersion of the invention (Example 9), while it gives
only 16% with the conventional dispersion (Example 6). The advantage is
that less DIR compound is needed: an estimated 50 mg/m.sup.2 of the DIR
using the inventive dispersion should produce the effect of 65 mg/m.sup.2
of the same DIR compound in the comparison coupler dispersion.
EXAMPLES 11 and 12
Demonstration of Decomposition in High pH Solution of a Photographic
Compound Containing an Ester Terminal Group
The Procedure of Priest described in Research Disclosure 16468, cited
above, requires that the photographic component be dissolved, often with
heating, in an alkaline solution with pH as high as 13.5. The following
experiment demonstrates that a photographic component containing an ester
terminal group will decompose under such harsh conditions and, therefore,
should be dispersed under the milder conditions described in this
invention:
Example 11 (basic solution): a 1 g sample of Coupler 2 was dissolved in a
solution containing 0.1 g NaOH, 15 g n-propanol, and 5 g distilled water.
Example 12 (no base): a 1 g sample of Coupler 2 was dissolved in 15 g
n-propanol and 5 g distilled water. Each solution was held at 60.degree.
C. for 3 hours and then analyzed by high pressure liquid chromatography
(HPLC). The HPLC trace of Example 12 showed the same two major peaks (at
about 17 and 19 min. retention times) as a fresh sample of Coupler 2, but
the HPLC trace of Example 11 showed severe decomposition had occurred,
since the characteristic peaks for this compound were gone and replaced by
a multiplet of peaks at lower retention times indicating the presence of
many smaller fragments.
The above examples are intended to be illustrative of the process of the
invention. For instance, while only illustrated with yellow couplers, the
invention could be utilized with magenta or cyan couplers that are ester
or peptide terminated. The invention has been described in detail with
particular reference to preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within the
spirit and scope of the invention that is intended to be referred by the
scope of the claims attached hereto.
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Description |
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