Methods of Preparation of Precipitated Coupler Dispersions With Increased Photographic Activity

This invention relates to the formation of dispersions of photographic materials by precipitation from solution. It particularly relates to the formation of coupler dispersions by shift of pH. It particularly relates to precipitation in the presence of a coarse dispersion of a permanent solvent, which gets loaded into the precipitated coupler dispersion and results in a high activity photographic coupler dispersion.

It has been known in the photographic arts to precipitate photographic materials, such as couplers, from solvent solution. The precipitation of such materials can generally be accomplished by a shift in the content of a water miscible solvent and/or a shift in pH. The precipitation by a shift in the content of water miscible solvent is normally accomplished by the addition of an excess of water to a solvent solution. The excess of water, in which the photographic component is insoluble, will cause precipitation of the photographic component as small particles. In precipitation by pH shift, a photographic component is dissolved in a solvent that is either acidic or basic. The pH is then shifted such that acidic solutions are made basic or basic solutions are made acidic in order to precipitate particles of the photographic component which is insoluble at that pH.

United Kingdom Patent 1,193,349-Townsley et al discloses a process wherein an organic solvent, aqueous alkali solution of a color coupler is mixed with an aqueous acid medium to precipitate the color coupler. It is set forth that the materials can either be utilized immediately, or gelatin can be added to the dispersion and chilled and remelted for use at a later date.

In an article in Research Disclosure, December, 1977, entitled "Process for Preparing Stable Aqueous Dispersions of Certain Hydrophobic Materials", pages 75-80, by William J. Priest, it is disclosed that color couplers can be formed by precipitation of small particles from solutions of the couplers in organic auxiliary solvents.

Such precipitated dispersion particle formation processes have been successful in forming laboratory quantities of photographic materials. It is not believed that such dispersion particle formation of photographic materials has been successfully scaled up for commercial utilization. One difficulty with scaling up for commercial utilization is that the large quantities required do not successfully lend themselves to the batch techniques utilized in laboratory formation. A continuous technique would be desirable. Certain surfactants are potent in the formulation of such dispersions, but contain chemical linkages that are hydrolyzed by base in the high pH solution of the coupler. This causes problems with scaling up, in both batch and continuous processes where considerable loss of the surfactant by hydrolysis is encountered. This problem is particularly severe in commercial or large volume production where, because of the large volumes involved, the time of wait before neutralization of the micellar solution is very long (greater that 1/2 to 2 hours). The micellar solution is the basic coupler solution mixed with the aqueous surfactant solution, at highly alkaline pH, prior to neutralizing with acid. When the surfactant hydrolyzes, the particles from lack of enough stabilizer from larger particles that are, in many cases, less reactive and therefore undesirable. Time required in equipment preparation in pilot scale or full-scale manufacturing may make it necessary for such solutions to sit for periods of time up to several hours. It is necessary to adjust the pH of the basic coupler containing solution to slightly acid (about pH 6) to effect the formation of the dispersion. The addition of the neutralizing acid to large volumes of material cannot be performed rapidly enough to prevent formation of large particulate dispersions. If the micellar solution remains at high pH for a long enough time, such hydrolyzable surfactants undergo extensive hydrolysis and cause ther formation of large particles, due to lack of stabilizing surfactant, prior to neutralization with acid. Therefore, the particle sizes will not be uniform from batch to batch, as they will vary depending on how long the micellar solution was formed prior to utilization or neutralization. It will be necessary to discard large quantities of coupler dispersion that will not meet manufacturing specifications. It has been proposed in copending co-assigned U.S. Ser. No. 297,005 filed Jan. 17, 1989 that uniform small particle size coupler dispersions may be made by the process in which the particles are simultaneously formed and neutralized. While the process allows the formation of uniform, stable particles, it has been found that some of the coupler materials unexpectedly form particles that are not as photographically active as would be desirable. It had been assumed that small particles would unfailingly be more active than large particles. Therefore, there remains a need for a process that will allow the formation of such continuously precipitated dispersions of coupler materials that have adequate photographic activity.

In conventional photographic systems it has been the practice to mill polymer and/or gelatin, surfactant, and couplers with a mixture of solvents. The solvents consist of a permanent non-water soluble solvent normally having a high boiling temperature and sometimes a water miscible auxiliary solvent that is usually removed during film formation or removed by washing off from chilled gel noodles, or is distilled off. The coupler dissolved in the permanent solvent remains dispersed as a stable colloid in gelatin which is used in forming photographic products. Typical of such systems for polymeric couplers are those disclosed in U.S. Pat. No. 3,912,517--Van Poucke et al. The dispersion of couplers and solvents is also discussed at pages 348-351 of The Theory of Photographic Process, Fourth Edition, edited by T.H. James, MacMillan, New York, Copyright 1977.

While the above processes for making photographic materials have been successful, there is a continuing need for preparing them in a continuous mode for efficient process control in the production of very large volume products, such as photographic paper and motion picture print films.

Generally the invention is performed by providing a first flow of water, water immiscible activating permanent solvent, and surfactant agitated by a mixer to form an unstable coarse or crude dispersion of the permanent solvent in water, a second flow comprising water miscible solvent, base, and photographic material, bringing together said first and second flows and then either simultaneously or immediately following mixing, neutralizing said streams to precipitate particles. The precipitated particles containing activating permanent solvent are generally more active than percipitated particles from systems where the particles do not contain the activating permanent solvent. During and probably up to some time after the precipitation process, the permanent solvent forms the coarse droplets that carried by the water miscible auxiliary solvent, which is also miscible with the permanent solvent, into the precipitated coupler particles, to produce solvent swollen particles of the size of between about 5 and about 100 mm in diameter. The formed dispersions are stable, do not contain gelatin, and can be washed by dialysis or by diafiltration to remove the water miscible auxiliary solvent to produce a photographic dispersion containing the particles of permanent solvent and coupler for further processing to produce photographic coatings at a later time.

The invention is practiced in a semicontinuous mode by bringing a first flow of coupler solution in basic aqueous-auxiliary solvent solution into a vessel containing a crude dispersion of the permanent solvent in an aqueous surfactant solution, and immediately neutralizing it with an acid solution, with vigorous agitation. The reaction vessel is fitted with a temperature sensor and a pH sensor which senses the pH and drives the acid pump such that for a constant rate of delivery of the basic coupler solution, the correct amount of acid is always pumped in by a prcessor controlled pump to maintain a constant pH of 6.0.+-.0.2 in the reactor. In a continuous mode this invention can be practiced by having a third flow of the surfactant containing crude dispersion of the permanent solvent flow into the reactor at a pre-set rate. The dispersion is then dialyzed to remove the auxiliary solvent and processed for photographic use when necessary.

In preferred methods, for large scale preparation, the first stream of coupler solution in basic aqueous-auxiliary solvent and the second stream of the crude dispersion of the permanent solvent in aqueous surfactant may be brought together immediately prior to a centrifugal mixer with addition of acid directly into the mixer. In the alternative, the first and second flow, as well as the acid flow, may all be added simultaneously in the centrifugal mixer. The streams will have a residence time of about 1 to about 30 seconds in the mixer. When leaving the mixer, they may be diafiltered on line to remove the auxiliary solvent and immediately be processed for utilization in photographic materials. When the process is stopped, the mixer may be shut off with minimum waste of material, as it is only necessary to discard the material in the mixer and pipelines immediately adjacent to it when the process is reactivated after a lengthy shutdown.

In all the described procedures of practicing this invention the surfactant containing crude dispersion of the permanent solvent is in contact with the high pH environment of the coupler solution for a minimum period of time. Since pH neutralization is very rapid, the surfactant experiences a high pH environment for very short times. There are many surfactants that are excellent stabilizers for precipitated dispersions. However, some of them contain a chemical linkage such as an ester linkage that gets easily hydrolyzed by the base, causing the loss of the stabilizing ability of the surfactant. Utilization of the process of mixing with immediate neutralization by acid virtually eliminates the chance of hydrolysis of such hydrolyzable surfactants, which leads to cost savings in the need to use less surfactant.

The process of the invention produces particles of coupler that are present in water without gelatin. The gelatin free suspensions of the invention are stable in storage and may be stored at room temperature rather than chilled as are gelating suspensions.

FIG. 1 illustrates schematically the small scale device for the preparation of the dispersions of this invention in a continuous mode.

FIG. 2 illustrates schematically the small scale device for the preparation of the dispersions of this invention in a semicontinuous mode.

FIG. 3 illustrates schematically the pilot scale device for the preparation of the dispersions of this invention in a continuous mode.

FIG. 4 illustrates schematically an alternate pilot scale device for the preparation of the dispersions of this invention in a continuous mode.

FIGS. 5A, 5B, 6A, and 6B are sensitometric curves comparing the invention dispersions with control dispersions.

The invention provides numerous advantages over prior processes of forming dispersions of photographic components. The invention provides continuous or semicontinuous methods of forming highly photographically active dispersions of couplers. Even though procedures for the preparation of precipitated dispersions have been well known, the method of incorporating coupler solvents into them in a single step during their formulation was unknown. Many precipitated coupler dispersions such as formed by the above-referenced U.S. Ser. No. 297,005--Bagchi et al filed Jan. 17, 1989, and hereby incorporated by reference do not have high photographic activity. It was discovered incorporation of permanent coupler solvents during precipitation in a manner of this invention produced coupler dispersion of desirable and high photographic activity. Methods have been discovered in which permanent coupler solvents can be incorporated in precipitated dispersion during its formation in a single step. Such permanent solvent-containing dispersions have been found to be much more active than the precipitated dispersions that did not contain any permanent coupler solvent. The activity of such dispersions are more than adequate for formation of photographic products. Since these permanent solvent-containing precipitated dispersions do not contain gelatin, they can be held at room temperature until photographic coatings are made. This is a cost saving advantage over conventional milled dispersions that contain gelatin, which need to be refrigerated.

FIGS. 1 and 2 describe respectively the continuous and the semicontinuous equipment to prepare such dispersions as those of this invention for small laboratory size preparation. The practice of the invention requires neutralization to be complete within not more than about two minutes from the time the basic auxiliary solvent coupler solution and the crude dispersion of permanent solvent and surfactant join. For obtaining small particle size it is preferred that neutralization be completed within much less than about one minute. The device of FIG. 1 was designed for continuous pH-controlled precipitation of dispersions of this invention. Container 92 is provided with a crude dispersion of the permanent coupler solvent, prepared by simple agitation in aqueous surfactant solution 94. The agitator 93 is used to form the crude dispersion. Container 96 is provided with an acid solution. Container 100 contains a basic coupler solution in the auxiliary solvent 102. Container 104 provides a mixing and reacting chamber where the dispersion formation takes place. Container 106 is a collector for the formed coupler dispersion 158. In operation the surfactant solution 94 is metered by pump 108 through line 110 into the reaction vessel 104. At the same time the basic coupler solution is metered by pump 112 through line 114 into the reactor 104 at a constant predetermined rate. The solutions are agitated by stirrer 116, and acid 98 is metered by pump 118 through line 121 into the reactor 104 to neutralize the solution. The pumping by metering pump 118 is regulated by controller 120. Controller 120 is provided with a pH sensor 122 that senses the pH of the dispersion 124 in reactor 104 and controls the amount and the rate of the addition of acid 98 added by pump 118 to neutralize the content of the reaction chamber. The drive for stirrer 116 is 126. The recorder 130 constantly records the pH of the solution to proivide a history of the dispersion 124. Metering pump 132 withdraws the dispersion solution from reactor 104 and delivers it to the container 106 using pump 132 and line 150 where it may exit from the outlet 134. In a typical precipitation there is a basic coupler solution 102 of solvent, sodium hydroxide solution, and the coupler. The surfactant is in water, and the neutralizing acid is an aqueous solution of acetic or propionic acid. The reaction chamber has a capacity of about 800 ml. The coupler solution tank 100, has a capacity of about 2500 ml. The surfactant solution tank 92, has a capacity of about 5000 ml. The acid solution tank has a capacity of about 2500 ml and the dispersion collection tank has a capacity of about 10,000 ml. The temperature is controlled by placing the four containers 92, 96, 104, and 100 in a bath 136 of water 138 whose temperature can be regulated to its temperature up to 100.degree. C. Usually precipitation is carried out at 25.degree. C. The temperature of the bath 138 is controlled by a steam and cold water mixer (not shown). The temperature probe 140 is to sense the temperature of the reactor. This is necessary for correct pH reading. The neutralization of the basic coupler solution in the reaction chamber 104 by the proportionally controlled pump 118 which pumps in acid solution 98 results in control of pH throughout the run to .+-.0.2 of the set pH value which is usually about 6.0. In the continuous mode similar volumes as pilot scale equipment (to be described next) have been made, except that the flow rates being about 20-30 times smaller than the pilot scale equipment of FIGS. 3 and 4, the preparation takes about 20-30 times longer.

FIG. 2 schematically illustrates a semicontinuous system for forming dispersions of coupler materials. Identical items are labeled the same as in FIG. 1. Because of reduced scale, the sizes of acid kettle 96 and the coupler kettle 100 are smaller (about 800 ml each). In the system of FIG. 2, the reactor 104 is initially provided with a crude aqueous surfactant dispersion of a permanent coupler solvent. Into this is pumped a basic solution of coupler and solvent 102 through pipe 114. pH sensor 122 that works through controller 120 to activate pump 118 and neutralize the dispersion to a pH of about 6 by pumping acetic acid 98 through metering pump 118 and lime 121 to the reactor 104. Reactor 104 must be removed, dumped, and refilled with the aqueous surfactant solution in order to start a subsequent run. However, the systems of FIGS. 1 and 2 do provide fast control of pH in order to produce photographically useful dispersions. Dispersions may be formulated and optimized using the semicontinuous process using this equipment before scale up for continuous running in continuous pilot scale equipment such as that of FIGS. 3 and 4.

The schematic of FIG. 3 illustrates apparatus 10 for performing the process of the invention in a pilot scale continuously. The apparatus is provided with high purity water delivery line 12. Tank 14 contains a crude emulsion of the permanent solvent in aqueous surfactant. Jacket 15 on tank 14 regulates the temperature of the tank. Surfactant enters the tank through line 16. Line 9 provides the permanent solvent and agitator 13 produces a crude dispersion of the permanent solvent in water in tank 14. Line 16 is also used to feed the surfactant. Tank 18 contains the basic coupler solution 19. Jacket 17 controls the temperature of materials in tank 18. In tank 18 the coupler enters through manhole 20, a base material such as aqueous sodium hydroxide solution entering through line 22, and solvent such as n-propanol entering through line 24. The solution is maintained under agitation by the mixer 26. Tank 81 contains acid solution 25 such as propionic acid entering through line 30. The tank 81 is provided with a heat jacket 28 to control the temperature, although with the acids normally used, it is not necessary. In operation, the acid is fed from tank 81 through line 32 to mixer 34 via the metering pump 86 and flow meter 88. A pH sensor 40 senses the acidity of the dispersion as it leaves mixer 34 and allows the operator to adjust the acid pump 86 to maintain the proper pH in the dispersion exiting the mixer 34. The photographic component 19 passes through line 42, metering pump 36, flow meter 38, and joins the surfactant solution in line 44 at the T fitting 46. The particles are formed in mixer 34 and exit through pipe 48 into the ultrafiltration tank 82. In tank 82 the dispersion 51 is held while it is washed by ultrafiltration membrane 54 to remove the solvent and salt from solution and adjust the material to the proper water content for makeup as a photographic component. The source of high purity water is purifier 56. Agitator 13 agitates the surfactant solution in tank 14. Agitator 27 agitates the acid solution in tank 81. The impurities are removed during the ultrafiltration process through permeate (filtrate) stream 58.

The apparatus 80 schematically illustrated in FIG. 4 is similar to that illustrated in FIG. 3 except that the acid solution in pipe 32, the crude emulsion of a permanent solvent in aqueous surfactant solution in pipe 44, and the basic coupler solution in an auxiliary in pipe 42 are directly led to mixing device 34. Corresponding items in FIG. 3 and FIG. 4 have the same numbers. In this system all mixing takes place in the mixer 34 rather than joining of the surfactant solution and the photographic component in the T connection immediately prior to the mixer as in the FIG. 3 process.

The surfactants of the invention may by any surfactant that will aid in formation of stable dispersions of particles. Typical of such surfactants are those that have a hydrophobic portion to anchor the surfactant to the particle and a hydrophilic part that acts to keep the particles separated either by steric repulsion (see, for example, P. Bagchi, J. Colloid and Interface Science, Vol. 47, page 86, and 110, 1974, Vol. 41, page 380, 1972, and Vol. 50, page 115, 1975) or by charge repulsion. Many classes of surfactants can be utilized to perform this invention. There can, in general, be clarified in the following classes:

Class I: Surfactants with single, double, or triple C.sub.5 to C.sub.25 hydrocarbon chain terminated with one or more charged head groups. Additional polymeric or oligomeric steric stabilizers could be used with such surfactants.

Examples of this class of surfactants are as follows: ##STR1##

Use of additional polymeric or oligomeric steric stabilizers with in addition to such surfactants can provide additional colloidal stability of such dispersions and can be added if necessary. Polymeric materials for such use are water soluble, homo-, or co-polymers such as polyvinyl pyrrolidone, dextran, and derivatized dextrans polyvinyl alcohol and poly(vinyl pyrrolidone-co-vinyl alcohol) of various ratios. Other types of oligomeric co-stabilizers that can be used are block oligomeric compounds comprising hydrophobic polyoxypropylene blocks A and hydrophilic polyoxyethylene blocks B joined in the manner of A--B--A, B--A--B, A--B, (A--B).sub.n .tbd.G.tbd.(B--A), or (B--A).sub.n .tbd.G.tbd.(A--B), wherein G is a connective organic moiety and n is between 1 and 3. Examples of such surfactants are shown in Table A.

Class II: Surfactants comprising between 6 to 22 carbon atom hydrophobic tail with one or more attached hydrophilic chains comprising at least 4 oxyethylene and/or glycidyl ether groups that may or may not be terminated with a negative charge such as a sulfate group.

Examples of such surfactants are as follows: ##STR5## Class III: Sugar surfactants, comprising between one and three 6 to 22 carbon atom hydrophobic tails with one or more attached hydrophilic mono, di, tri or oligosaccharidic chains that may or may not be terminated by a negatively charged group such as a sulfate group.

Examples of such surfactants are as follows: ##STR6##

The invention may be practiced with any hydrophobic photographic component that can be solubilized by base and solvent. Typical of such materials are colored dye-forming couplers, development inhibitor release couplers, development inhibitors, filter dyes, UV-absorbing dyes, development boosters, development moderators, and dyes. Suitable for the process of the invention are the following compounds which have been utilized to form precipitated dispersions: ##STR7##

All of the above compounds are amenable to the described process of the invention. Many of the precipitated dispersions of the above list are photographically very active and some are substantially more active compared to their conventional milled dispersions. However, some of the examples of the above list such as, for example, compounds C-3 and C-4, are extremely inactive as precipitated dispersions. These are the compounds that need to have permanent solvent incorporated in them to produce photographically active dispersions that can be used in viable photographic systems. The couplers that are typically suitable for the process are those that are without many polar or ionizable groups, as such couplers are less reactive unless in the presence of an activating solvent.

The mixing chamber, where neutralization takes place, may be of suitable size that has a short residence time and provides high fluid shear without excessive mechanical shear that would cause excessive heating of the particles. In a high fluid shear mixer, the mixing takes place in the turbulence created by the velocity of fluid streams impinging on each other. Typical of mixers suitable for the invention are centrifugal mixers, such as the "Turbon" centrifugal mixer available from Scott Turbon, Inc. of Van Nuys, California. It is preferred that the centrifugal mixer be such that in the flow rate for a given process the residence time in the mixer will be of the order of 1-30 seconds. Preferred residence time is 10 seconds to prevent particle growth and size variation. Mixing residence time should be greater than 1 second for adequate mixing.

The volatile water miscible solvents suitable for dissolving the photographic component may be any suitable solvent that may be utilized in the system in which precipitation takes place by solvent shift and/or pH shift. Typical of such materials are the solvents acetone, methyl alcohol, ethyl alcohol, isopropyl alcohol, tetrahydrofuran, dimethylformamide, dioxane, N-methyl-2-pyrrolidone, acetonitrile, ethylene glycol, ethylene glycol monobutyl ether, diacetone alcohol, ethyl acetate and cyclohexanone. A preferred solvent is n-propanol because n-propanol provides a very stable supersaturated basic coupler solution that is used for this precipitation process.

The activating permanent water immiscible high boiling coupler solvents are compounds as shown below. These are chosen for their compatibility and activity in general with large numbers of couplers and for their competitive price advantages. ##STR8##

The acid and base may be any materials that will cause a pH shift and not significantly decompose the photographic components. The acid and base utilized in the invention are typically sodium hydroxide as the base and propionic acid or acetic acid as the acid, as these materials do not significantly degrade the photographic components and are low in cost.

The process of this invention leads to gelatin free, fine particle colloidal dispersions of photographic materials that are stable from precipitation at least for six weeks at room temperature. This is a cost saving feature as conventional milled dispersions need to be stored under refrigerated conditions. Under refrigerated conditions dispersions prepared by the method of this invention photographically useful lives anywhere up to two months.

All particle sizes of the precipitated dispersions were made by photon correlation spectroscopy (PCS) as described by B. Chu, Laser Light Scattering, Academic Press, 1974, New York. Unless otherwise mentioned, all photographic development we carried out by the standard C-41 color development process as described in the British Journal of Photography Annual of 1988, pages 196-198. Solution reactivity rates of the dispersions were determined using an automated dispersion reactivity analysis (ADRA) method. A sample of the dispersion is mixed with a carbonate buffer and a solution containing CD-4 developer.

Potassium sulfite is added as a competitor. The carbonate buffer raises the pH of this reaction mixture to a value close to the normal processing pH (10.0). An activator solution containing the oxidant potassium ferricyanide is then added. The oxidant generates oxidized developer which reacts with the dispersed coupler to form image dye and with sulfite to form side products. After the addition of a clarifier (solution of Triton X-100), the dye density is read using a flow spectrometer system. The concentration of dye is derived from the optical density and a known extinction coefficient.

A kinetic analysis is carried out by treating the coupling reaction as a homogeneous single phase reaction. It is also assumed that the coupling reaction and the sulfonation reaction (sulfite with oxidized developer) may be represented as second-order reactions. Furthermore, the concentrations of reagents are such that the oxidant and coupler are in excess of the developer. Under these conditions, the following expression is obtained for the rate constant of the coupling reaction:

where k' is the sulfonation rate constant, a is the concentration of coupler, b is the concentration of sulfite, c is the concentration of developer, and x is the concentration of the dye. The rate constant k is taken as a measure of dispersion reactivity. From an independently determined or known value of k' and with this knowledge of all of the other parameters, the rate constant k (called the automated dispersion reactivity analysis, ADRA, rate) is computed.

The monochrome bilayer coating format used for the photographic evaluations of the coupler dispersions was as follows:

Layer 1 (TOP):

2.691 g/m.sup.2 of gelatin overcoat.

0.0113 g/m.sup.2 of bis(vinylsulfonyl)methane hardener.

Layer 2 (BOTTOM):

Indicated amounts of image, development inhibitor releasing (DIR) or colored couplers, with or without indicated amounts of permanent coupler solvent.

1.641 g/m.sup.2 of silver in a green-sensitized, medium speed, three-dimensional, 320 nm diameter AgBr(I) 12 mole percent Iodine crystal. 3.767 g/m.sup.2 of gelatin.

Support:

Clear ester subbed with a thin polymer layer for the adhesion of the gelatin coatings.

Coatings were made in slide hopper coating and drying machine in two passes.

The following examples are intended to be illustrative and not exhaustive of the invention. Parts and percentages are by weight unless otherwise specified.

This example utilizes a process and apparatus generally as schematically illustrated in FIG. 3. The coupler solution, surfactant solution, and acid solution are prepared as follows:

Above ingredients were mixed together and heated to 50.degree. C. to dissolve the coupler and then cooled to 30.degree. C. before use.

The description of the apparatus setup for this example is as follows:

Temperature-controlled, open-top vessels

Gear pumps with variable-speed drives

A high fluid shear centrifugal mixer operated with a typical residence time of about 2 sec.

A SWAGE-LOC "T" fitting where surfactant and coupler streams join

Residence time in pipe between T-fitting and mixer <1 sec.

In-line pH probe used to monitor pH in the pipe exiting the mixer

Positive displacement pump for recirculation in batch ultrafiltration

Ultrafiltration membrane OSMONICS 20K PS 3' by 4" spiral-wound permeator

The three solutions are continuously mixed in the high-speed mixing device in which the ionized and dissolved coupler is reprotonated causing precipitation. The presence of the surfactant stabilizes the small particle size dispersion. The salt byproduct of the acid/base reaction is sodium propionate. Ultrafiltration is used for constant-volume washing with distilled water to remove the salt and the solvent (n-propanol) from the crude dispersion. The recirculation rate is approximately 20 gal/min. with 50 psi back pressure which gives a permeate rate of about 1 gal/min. The washed dispersion is also concentrated by ultrafiltration to the desired final coupler concentration of about 10-15 weight percent. The time to perform the ultrafiltration and produce the final coupler concentration is about 1 hour. Average particle size is about 66 nanometers as measured by Photon Correlation Spectroscopy.

This example utilizes a process and apparatus generally as schematically illustrated in FIG. 3. The coupler solution, surfactant solution, and acid solution are prepared as follows:

Above ingredients were mixed together and heated to 50.degree. C. to dissolve the coupler and then cooled to 30.degree. C. before use.

The description of the apparatus setup and the process for this example is similar to that in Example 1. Average particle size of the dispersion as measured by Photon Correlation Spectroscopy was 39 nm. The solution ADRA reactivity rate of the dispersion was 1390 l/(mole sec).

This example utilizes a process and apparatus generally as schematically illustrated in FIG. 3. The coupler solution, surfactant solution, and acid solution are prepared as follows:

Above ingredients were mixed together and heated to 60.degree. C. to dissolve the coupler and then cooled to 30.degree. C. before use.

The description of the apparatus setup and the process for this example is similar to that in Example 1. Average particle size of the dispersion as measured by Photon Correlation Spectroscopy was 13 nm. The solution ADRA reactivity rate of the dispersion was 18500 l/(mole sec).

This example utilizes the process of this invention and the apparatus schematically illustrated in FIG. 2. The coupler solution, the crude coupler solvent/aqueous surfactant emulsion, and the acid solution are prepared as follows:

Above ingredients were mixed together and heated to 60.degree. C. to dissolve the coupler and then cooled to 30.degree. C. before use.

A crude emulsion of the above ingredients was prepared by placing the mixture in vessel 104 of FIG. 2 and agitating it with mixer 116.

Acid solution: 15% propionic acid, placed in vessel 96 of FIG. 2.

The precipitation was started by setting the pH controller at pH 6.0 and starting the coupler solution pump 112. As the basic coupler solution entered the reaction vessel 104, the pH of the mixture increased. This was sensed by the pH probe which then caused the activation of the acid pump 118 to pump in acid into the stirred reaction chamber 104 to lower the pH and cause precipitation of the coupler in the form of a fine particle stable dispersion. In the presence of the water miscible auxiliary solvent n-propanol the water immiscible high boiling permanent solvent, tricresyl phosphate, was solubilized and transported into the formed coupler dispersion particles to produce a permanent solvent loaded coupler dispersion. The dispersion was dialyzed against distilled water for 24 hours to remove the formed salts and the auxiliary solvents. The average particle diameter of the dispersion particle as measured by Photon Correlation Spectroscopy was 114 nm, and the ADRA reactivity rate was determined to be 3760 l/(mole sec.). It is to be noted that compared to the comparison in Example 2, the particle size of this solvent loaded coupler dispersion of the invention has about three times the particle size, but its reaction rate with color developer to form image dye is about three times larger. In other words, this single step incorporation of the coupler solvent during precipitation increased its coupling propensity drastically. The coupler content of this dispersion was analyzed by high pressure liquid chromotography and was found to be around 2%. Such dilute dispersion could be diafiltered and concentrated, but was held as such for further processing to form a photographic coating.

This example utilizes the process of this invention and the apparatus schematically illustrated in FIG. 2. The coupler solution, the crude coupler solvent/aqueous surfactant emulsion, and the acid solutions were prepared as follows:

Above ingredients were mixed together and heated to 50.degree. C. to dissolve the coupler and then cooled to 30.degree. C. before use.

A crude emulsion of the above ingredients was prepared by placing the mixture in vessel 104 of FIG. 2 and agitating it with mixer 116.

Acid Solution: 15% propionic acid, placed in vessel 96 of FIG. 2.

The precipitation was started by setting the pH controller at pH 6.0 and starting the coupler solution pump 112. As the basic coupler solution entered the reaction vessel 104, the pH of the mixture increased. This was sensed by the pH probe which then caused the activation of the acid pump 118 to pump in acid into the stirred reaction chamber 104 to lower the pH to cause precipitation of the coupler into a fine particle stable dispersion. In the presence of the water miscible auxiliary solvent n-propanol the water immiscible high boiling permanent solvent, tricresyl phosphate, was solubilized and transported into the formed coupler dispersion particles to produce a permanent solvent loaded coupler dispersion. The dispersion was dialyzed against distilled water for 24 hours to remove the formed salts and the auxiliary solvents. The average particle diameter of the dispersion particle as measured by Photon Correlation Spectroscopy was 109 nm, and the ADRA reactivity rate was determined to be 52200 l/(mole sec). It is to be noted that compared to the comparison in Example 3, the particle size of this solvent loaded coupler dispersion of the invention has about eight times the particle size, but its reaction rate with color developer to form image dye is about three times larger. In other words, this single step incorporation of the coupler solvent during precipitation increased its coupling propensity drastically. The coupler content of this dispersion was analyzed by high pressure liquid chromotography and was found to be around 2%. Such dilute dispersion could be diafiltered and concentrated, but was held as such for further processing to form a photographic coating.

The comparison dispersion of Example 2 and the dispersion of the invention Example 4 were evaluated in a coating format as described earlier with the precipitated image coupler dispersion of coupler C-7 of Example 1. The description of the various coatings are indicated in Table B. The coating melts were prepared just prior to coating in order to minimize coupler solvent transport to the image coupler dispersion. The coatings were given a stepwise exposure with green light and then processed by the C41 processing as described in British Journal of Photography Annual of 1988, page 196 to 198. The formed magenta images were then read in green light which gave the sensitometric curves shown in FIGS. 5A and 5B. The sensitometric results of coatings 1 through 5 are also listed in Table B.

FIG. 5A is a sensitometric curve for control coatings 1, 2, and 3.

FIG. 5B is a sensitometric curve for control coatings 1 and 2, and coating 3 (invention).

FIG. 5A shows that when a precipitated dispersion of coupler C-7 containing no permanent coupler solvent, is coated with a similar precipitated no permanent solvent dispersion of DIR coupler C-3 at levels 0 (coating #1), 0.0323 g/m.sup.2 (coating #2) and 0.0646 g/m.sup.2 the sensitometric curves are virtually identical with no change in the contrast of this image. This indicates that even though the no solvent precipitated dispersion of the image coupler of C-7 was very active, the similar no solvent precipitated dispersion of the DIR coupler of C-3 was extremely inactive compared to the similar experiment performed with the precipitated DIR coupler dispersion containing a permanent solvent. According to the method of this invention, the results in FIG. 5B and Table B show that with increased laydown of the DIR coupler, the contrast and the D.sub.max of the recorded image decreased progressively. This clearly demonstrates that the permanent solvent containing precipitated dispersion of the invention is definitely much more active than that of the comparison where no permanent solvent was incorporated into the precipitated dispersion of C-3. It is also to be noted in Table B that in spite of the larger particle size of the permanent solvent containing DIR dispersion of C-3, it has about three times larger ADRA reactivity rate compared to that of the no solvent containing precipitated dispersion, indicating again that the incorporation of the permanent solvent into the dispersion particles of C-3 in the manner of this invention caused them to be highly reactive.

Yellow colored magenta coupler C-4 is a color correction coupler that is usually incorporated in the magenta layer of color negative products along with the image coupler and a DIR coupler.

The comparison dispersion of coupler C-4 of Example 3 and the permanent solvent incorporated precipitated dispersion of Example 5 were evaluated in a coating format described earlier. The description of the two coatings are shown in Table C. The yellow colored magenta coupler dispersion of C-4 was coated at 0.646 g/m.sup.2 with the indicated green sensitized emulsion to evaluate their comparative reactivities. The coatings were given a stepwise exposure with green light and then processed by the C-41 processing as described in British Journal of Photography Annual of 1988, pages 196 to 198 for two minutes. The formed magenta images were then read using green and blue lights which gave the sensitometric results of coatings 1 and 2 as listed in Table C and shown in FIGS. 6A and 6B respectively.

FIG. 6A is a sensitometric curve for coating 1 (control) of Example 7.

Ag.fwdarw.1.614 g/m.sup.2

No solvent precipitated yellow colored magenta coupler (Dispersion of Example3) C-4.fwdarw.0.646 g/m.sup.2

FIG. 6B is a sensitometric curve for coating 2 (invention) of Example 7.

Ag.fwdarw.1.614 g/m.sup.2

2.times.S-13 permanent solvent containing precipitated yellow colored magenta coupler (Dispersion of Example 5) C-7.fwdarw.0.646 g/m.sup.2

In the images of FIGS. 6A and 6B, it is seen with the yellow colored magenta coupler that as exposure is increased magenta dye is formed imagewise and yellow dye is at the same time consumed imagewise. It is also seen that the coupler solvent incorporated precipitated dispersion of the invention (FIG. 6B) showed greater D.sub.max, higher contrast, and larger ADRA reactivity (Table C) compared to the no solvent precipitated control of FIG. 6A, indicating the usefulness and efficacy of this invention.

In this example a coupler solvent incorporated precipitated codispersion of the image coupler C-7 (79-90%), the DIR Coupler C-3 (3.5%), and the yellow colored magenta Coupler C-4 (16.5%) were prepared by the method of this invention containing 1 X permanent solvent S-13 (at a weight equal to total couplers present) in the continuous apparatus schematically illustrated in FIG. 1. The coupler solution, the crude coupler solvent/agueous surfactant emulsion, and the acid solutions were prepared as follows:

Above ingredients were mixed together and heated to 50.degree. C. dissolve the coupler and then cooled to 30.degree. C. before use.

A crude emulsion of the above ingredients was prepared by simple agitation in a vessel and 500 ml placed in vessel 104 of the continuous equipment shown in FIG. 1 and the rest in vessel 92 which was stirred with stirrer 93 to maintain the crude emulsion. Stirrer 116 was started.

Acid solution 15% propionic acid solution was placed in vessel 96.

To start the continuous precipitation, the coupler solution pump 112 was started at a constant flow rate of 17.5 g/min. As the coupler solution entered the reaction chamber 104, the pH of the reaction chamber increased. This was sensed by the pH electrodes and signal sent to the controller. The controller then produced a proportional signal compared to the set pH of 6.0 to the acid pump 118, which pumped acid into the reaction chamber to neutralize the base and induce precipitation of the coupler. The precipitated coupler in the presence of the water miscible auxiliary solvent prepared absorbed the permanent solvent from the crude emulsion of S-13, the permanent solvent and then the permanent solvent loaded precipitated codispersion was formed. The formed dispersion was pumped out of the reaction vessel 104 via line 114 by the pump 132 set at 20 g/min. The line 116 maintained a constant head in the reaction vessel at a volume of about 500 ml, such that pump 132 being on during the run, the formed dispersion was only pumped into the reservoir 158 when the dispersion volume in the reaction vessel was greater than 500 ml. At the end of the precipitation, the dispersion in vessel 158 was dialyzed against distilled water to remove the salt and the auxiliary solvent propanol. The codispersion had a particle diameter of 195 nm as measured by PCS.

The dispersion was prepared to demonstrate that a permanent coupler solvent containing co-dispersion of all of the couplers in a photographic layer can be prepared by the method of this invention.

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.