WO1993005441A1

WO1993005441A1 - Method of producing a photographic emulsion, and a photographic emulsion produced by this method

Info

Publication number: WO1993005441A1
Application number: PCT/US1992/007362
Authority: WO
Inventors: Barry L. Bragger
Original assignee: Aquotech, Inc.
Priority date: 1991-08-28
Filing date: 1992-08-28
Publication date: 1993-03-18

Abstract

In the preparation of a photographic emulsion there is formed a solution and/or suspension of a photosensitive material such as silver halide in water. According to the invention, this water is treated with an electrical AC signal at a selected frequency such as 50 MHz.

Description

METHOD OF PRODUCING A PHOTOGRAPHIC EMULSION, AND A PHOTOGRAPHIC EMULSION PRODUCED BY THIS METHOD

BACKGROUND OF THE INVENTION

The present invention relates to a method for producing a photographic emulsion, and to a photographic emulsion produced by this method.

Photographic emulsions of the type which are spread onto film or paper to produce black and white or color photographic images have been known and produced for many, many years. A typical emulsion used for black and white images includes minute particles of silver halide in solution and/or suspension in an aqueous gel. The silver halide particles are precipitated within an aqueous solution containing gelatine by mixing a silver salt, such as silver nitrate, with a soluble halide such as sodium chloride, sodium bromide or sodium iodide.

This process of manufacturing photographic media is extremely well known and is described, for example, in the following reference texts:

(1) "Photographic Gelatin", Proceedings of the 2nd. Symposium on Photographic Gelatin, Trinity college, Cambridge, England; ed. R.J. Cox, 1972, Academic Press, London, ISBN:0-12-194450-6.

(2) History of Photography, Helmut Gernsheim, 1969, McGraw-Hill, New York, LCCCN: 69-18762. (3) Photographic Emulsion Techni ue. T. Thome Baker, 1941 (1948 2nd. printing) , American Photographic Publishing Co. , Boston , no ID #.

(4) Chemistry for Photographers, Allen R. Greenlea , 1941 (1942 2nd. printing) , American Photographic Publishing Co., Boston, no ID #.

(5) "Photographic Sensitivity", Proceedings of the Symposium on Photographic Sensitivity, Caius College & Little Hall, Cambridge, England, 1973, Academic Press, London, ISBN: 0-12-1904465-4.

(6) Photochemistry, S.E. Sheppard, University College, 1914, Longmans, Green & Co., London, no ID #.

The production of photographic emulsions requires the production of an aqueous intermediate which includes a gel- forming material, such as gelatin, as well as either the precursors of, or the final photosensitive material, such as silver halide. This aqueous intermediate, which contains some or all of the ingredients for a photographic emulsion, will hereinafter be referred to as "photographic process water".

The objective, in producing photographic process water is to generate photosensitive material in finely divided particulate form which will either enter into solution or remain in suspension as the emulsion is laid down or deposited on a substrate such as a celluloid film or a paper web. Normally such application is accomplished, after the emulsion is allowed to age, by passing the substrate beneath a doctor blade which limits the thickness of the emulsion on the substrate to approximately 3/4 microns. After application, the emulsion is allowed to harden on the substrate.

Finally, the substrate with the hardened emulsion is washed with water to remove loose and excess emulsion material.

The principal measure of the speed and effectiveness of a photographic emulsion as a photographic medium is its "reduction potential". This reduction potential may be determined as the reduction time during which the silver composition (or other photosensitive material) in solution and/or suspension is reduced to the metallic state. The lower the reduction potential (reduction time of the test) the faster will be the "photographic speed" of the emulsion.

As is well known, the reduction potential can be decreased by reducing the physical size of the particles of photosensitive material. In order to keep these particles small during their formation, certain chemical means, called "matrix modifiers" have been added to the photographic process water. These chemicals are expensive both to produce and to eliminate. Not only does their use in connection with photographic process water require that these chemicals be purchased or synthesized, but their removal during the final washing process requires the elimination of toxic waste.

SUMMARY OF THE INVENTION It is a principal object of the present invention, therefore, to provide a non-chemical method for reducing the particle size of photosensitive material in a photographic emulsion.

It is a further object of the present invention to provide a non-chemical method for manufacturing photographic process water which results in increased hardness of the gelatin after the photographic emulsion is deposited on a substrate.

These objects, as well as further objects which will become apparent from the discussion that follows, are achieved, in accordance with the present invention, by- treating water which is, or which will subsequently be used in photographic process water, with an electrical AC signal at a selected frequency.

More particularly, the present invention involves applying an electrical AC signal to the water in the manner disclosed in the U.S. Patents Nos. 4,865,748 and 4,963,268 to D.E. Morse, which disclosure is incorporated herein by reference. Equipment for treating water with an electrical AC signal at a selected frequency is available commercially from Aqua Dynamics Group Corp. in Adamsville, Tennessee. Such devices are sold or leased under the name "QED" for "Quantum Electronic Device".

It has been discovered, remarkably, that the treatment of water with an electrical AC signal at one or a plurality of frequencies by means of the aforementioned device reduces the average particle size of suspended materials. Accordingly, the treatment according to the present invention can be used to create a finer photosensitive material particle, such as a silver halide particle, in a photographic emulsion, thereby creating a photographic film of finer grain character and of higher speed.

Since the water treatment method according to the present invention decreases the amount of energy needed to reduce the silver halide to the metallic state, the process according to the invention can be used to create a faster photographic film for low light applications.

In addition, or in the alternative, the water treatment method according to the invention allows less silver to be used to create a photographic emulsion of comparable or improved quality with respect to the prior known photographic emulsions. The method according to the present invention is therefore capable of producing a high speed, low grain photographic film with a minimum of silver.

While it is not known with certainty why the water treatment according to the present invention results in reducing the particle size of photosensitive material such as silver halide, the following theory is offered as a possible or partial explanation:

The application of an electrical AC signal to water results in the introduction of energy into the water if the AC signal is at a proper frequency or frequencies. This energy appears to disrupt the normal bonding energies in either the solvent (the water) or the solute (the particulates, such as silver halide) . As a result, the water is better able to dissolve or hydrate the species and either to increase its solubility or reduce the apparent particle size.

According to a preferred feature of the present invention, the AC signal is applied at a frequency in the range of 0.1 KHz to 1 GHz. For example, in a preferred embodiment, the frequency is preferably about 50 MHz.

The optimum frequency of the AC signal may be easily determined by varying the AC signal frequency or frequencies and selecting that frequency at which the energy absorption by the water is a maximum.

The Quantum Electronic Device available from Aqua Dynamics Group Corp. is capable of delivering approximately 10 watts of power to water. Once the water has been treated, it retains its advantages for the production of a photographic emulsion for a number of hours. The typical "memory" of water after this type of treatment is about 12 hours. Another remarkable advantage of the water treatment according to the present invention in the production of a photographic emulsion is an increase in the surface hardness of the photographic emulsion after it has been applied to a substrate and subsequently dried and aged. As a result of this increased gelatin hardness, the photographic film product is less prone to surface defects and scratches.

Still another advantage of the water treatment according to the present invention is an increased wettability of the photographic emulsion when it is deposited on a substrate (celluloid film or paper web) . This increased wettability is believed to result from a reduction in surface tension of the water as a result of the water treatment.

The preferred embodiments of the present invention will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTIONS OF THE DRAWINGS Fig. 1 is a cross-sectional view of a water tank on which are mounted means for applying an electrical AC signal to the water at a prescribed frequency.

Fig. 2 is a cross-sectional view of the water tank of Fig. 1 taken along the line 2-2.

Fig. 3 is a detailed view of an electrode employed in the system of Figs. 1 and 2.

Fig. 4 is a cross-sectional view of a section of a water pipe including means for applying an electrical AC signal at a prescribed frequency.

Fig. 5 is a cross-sectional view of the water pipe of Fig. 4 taken along the line 5-5.

Fig. 6 is a cross-sectional view showing a detail of an electrode employed in the electrical system of Figs. 4 and 5.

DETAILED DESCRIPTION OF THE INVENTION Figs. 1-3 and Figs. 4-6 illustrate two preferred embodiments, respectively, of electrical systems for treating water with an electrical AC signal at a select frequency in accordance with the present invention. The embodiment of Figs. 1-3 provides a treatment for a quantity of water stored in a tank, for batch processing applications, whereas the embodiment of Figs. 4-6 applies the electrical AC signal to water which is continuously flowing through a pipe.

Referring first to Figs. 1-3, there is shown a water tank 10 having inlet and outlet pipes 12 and 14, respectively. The tank may, for example, be made of non- corrosive stainless steel.

Situated at opposite sides of the tank are pairs of electrodes 16, 18 and 20, 22 which are connected, in turn, via wires 28 and 30 to a signal generator 32. This signal generator is of the type disclosed in the aforementioned U.S. Patents Nos. 4,865,748 and 4,963,268.

The electrodes 16-22 are each mounted on an insulating base 34 which is preferably made of Teflon. As shown in Fig. 3, each electrode 36 is cylindrical and has a dome shaped top.

A signal generator suitable for use with the present invention is available commercially from the Aqua Dynamics Group Corp. in Adamsville, Tennessee. Such a device, called a "Quantum Electronic Device" has a power output of between 5 and 10 watts.

The output frequency of the device is variable to permit selection of the optimum frequency of operation. The power output of the signal generator can be monitored as the frequency is varied so as to select that frequency at which the energy absorption by the water is a maximum. This frequency will probably be in the range of 0.1 KHz to 1 GHz, and most likely will be approximately 50 MHz.

The embodiment of Figs. 4-6 is very similar to that of Figs. 1-3, with the exception that the pairs of electrodes 38, 40 and 42, 44 are embedded in a 3 to 5 inch pipe made of insulating material such as Teflon. The electrodes, which are shown in detail in Fig. 6, are hemispherical and are imbedded such that their planar surfaces are substantially flush with the inner surface of the pipe 46.

As in the case of the embodiment of Figs. 1-3, the pairs of electrodes are connected to a signal generator 48. The electrodes, in whatever configuration they may be, are made of a substantially inert metal such as stainless steel or platinum. The surfaces of the electrodes are preferably polished smooth.

Assuming adequate pressure, a 3 inch pipe is capable of conducting up to 1000 gallons per minute of water on a continuous basis. A QED signal generator producing 5 to 10 watts of power is capable of treating 1000 gallons of water per minute.

Example

Two liquid samples were tested both before and after subjecting them to the electrical treatment according to the present invention. The first sample was purified, deionized water? the second sample was photographic process water containing silver halide particles in suspension and solution.

After the samples were tested in their original state, they were subjected to a 50 MHz electrical signal at a power of approximately 10 watts applied through two platinum 1/8 inch wires for 6 hours and then tested, as before, after the treatment was stopped.

The following tests were run in triplicate:

1) Surface Tension/Capillary Activity: Measured by placing a cleaned, precision-bore 0.6mm Pyrex glass capillary tube a fixed distance into the sample and measuring the final, stable height of rise in the tube from the liquid's surface in mm.

2) Reduction Potential: The second sample only was treated with ammonium hydroxide and dextrose, and the rate of reduction of any soluble or suspended silver to a shiny mirror (similar to Fehling's test for reducing alcohols) was measured in seconds.

3) Rate of Evaporation: Weighed 50 gram aliquots of both samples were placed in 100 ml. graduated cylinders in a vacuum oven set at 150°C an 20 mm Hg vacuum, and the rate of evaporation was measured in mis. per time interval. Only one set of readings was performed here.

The following table lists the average value for the measured properties tested in each sample. The relative standard deviation for the six data points (triplicate readings on duplicate preparations) is listed under the data value in brackets. Test Procedure First Liquid Sample Second Liquid Sample

Before After Before After

Surface Tension 21.7mm 24.7mm 36.1mm 40.0mm [7%] [8%] [7%] [5%]

Reduction n/a n/a 121 sees. 105 see Potential [8%] [5%]

Rate of Evap.

As in the evaluation of the solid samples, the application of the electrical AC signal energy to the solubilized components in these liquid samples appeared to effect some physical and chemical constants of these species. Both sample solutions exhibited a marked reduction in surface tension (as a function of capillary action) . Such a change is usually induced by surfactants or polymeric materials which increase the "wetness" of the water. It is theorized that the electrical energy changes the normal "clustering" or coordination of the species present, which would manifest itself as a change in surface tension.

The second liquid sample showed a significant and reproducible decrease in reduction time (as shown by the modified Fehling's test) after the treatment with an AC electrical signal according to the invention. This is evidence that the silver in solution is more easily reduced to the metal (as seen by the more rapid appearance of the "mirror" film) .

The vastly different rates of evaporation on both liquid samples, before and after treatment, cannot be easily explained. This physical characteristic is usually determined by the interactions of the solute and solvent species, based on their relative bonding energies. For such a change in evaporation, without the addition of chemical agents, there was probably some kind of change in these bonding interactions.

As may be seen, the reduction potential test was carried out only on the second liquid sample. The reduction time was substantially (approximately 10%) less after the treatment according to the present invention than before. Consequently, it can be expected that the photographic speed of an emulsion will be increased by approximately 10% as a result of the treatment according to the invention.

There has thus been shown and described a novel method of producing a photographic emulsion which fulfills all the objects and advantages sought therefor. Many changes, modifications, variations and other uses and applications of the subject invention will, however, become apparent to those skilled in the art after considering this specification and the accompanying drawings which disclose the preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention,- which is to be limited only by the claims which follow.

Claims

C L A I M S What is Claimed is:

1. A method of producing a photographic emulsion, said method comprising the steps of:

(a) treating water with an electrical AC signal;

(b) preparing a partially dissolved and partially suspended photosensitive material in said water.

2. The method defined in claim 1, wherein said water treated in step (a) is photographic process water.

3. The method defined in claim 2, wherein said photographic process water contains a gel-forming material.

4. The method defined in claim 2, wherein said photographic process water contains a soluble silver salt.

5. The. method defined in claim 4 , wherein said preparing step includes of the step of mixing an alkaline halide solution with said photographic process water to form silver halide particles.

6. The method defined in claim 1, wherein said AC signal is at a frequency in the range of 0.1 KHz to 1 GHz.

7. The method defined in claim 6, wherein said frequency is about 50 MHz.

8. The method defined in claim 6, wherein said frequency is that at which the energy absorption by said water is a maximum.

9. A photographic emulsion produced by the method of claim 1.

10. An aqueous intermediate in the production of photographic emulsions comprising finely divided silver halide particles dissolved and suspended in water which has been treated with an electrical AC signal.

11. The intermediate defined in claim 10, wherein said AC signal is at a frequency in the range of 0.1 KHz to 1 GHz.

12. The intermediate defined in claim 11, wherein said frequency is about 50 MHz.

13. The intermediate defined in claim 11, wherein said frequency is that at which the energy absorption by said water is a maximum.

14. In the preparation of a photographic emulsion wherein there is formed a solution and suspension of a photosensitive material in water, the improvement which comprises treating the water with an electrical AC signal.

15. The improvement defined in claim 14, wherein said solution contains a gel-forming material.

16. The improvement defined in claim 15, wherein said solution contains a gelatin.

17. The improvement defined in claim 14, wherein said photosensitive material includes a photosensitive salt.

18. The improvement defined in claim 14, wherein said photosensitive material includes an alkaline halide.

19. The improvement defined in claim 14, wherein said photosensitive material includes a silver salt.

20. The improvement defined in claim 19, wherein said photosensitive material includes a silver halide.

21. The improvement defined in claim 14, wherein said AC singal is at a frequency in the range of 0.1 KHz and 1 GHz.

22. The improvement defined in claim 21, wherein said frequency is about 50 MHz.

23. The improvement defined in claim 21, wherein said frequency is that at which the energy absorption by said water is a maximum.