US3512981A - Stable photographic developers containing ascorbic acid - Google Patents

Description

United States Pa'tent O 3,512,981 STABLE PHOTOGRAPHIC DEVELOPERS CON- TAINING ASCORBIC ACID Gordon L. Prchal and Richard W. Knlus, Minneapolis, Minn., assignors to Pako Corporation, Minneapolis, Minn., a corporation of Minnesota No Drawing. Continuation-impart of application Ser. No. 499,043, Oct. 20, 1965. This application Feb. 5, 1968, Ser. No. 702,797

Int. Cl. G03c 5/30 US. Cl. 96--66.4 8 Claims ABSTRACT OF THE DISCLOSURE An improved photographic developer particularly adapted to develop latent images present in photosensitive silver halide emulsions, the developer including a p-dihydroxybenzene such as hydroquinone, as an active developer ingredient, and further including dissociated sulfite, preferably derived from sodium sulfite as well as sodium formaldehyde bisulfite, the dissociated sulfite being present in an amount of between about 1 and 5 grams per liter of sodium sulfite, and further including from between about 0.02 and up to about 10 grams per liter of ascorbic acid.

The present application is a continuation-in-part of our co-pending application Ser. No. 499,043, filed Oct. 20,

1965 entitled Photographic Developers, which application is assigned to the assignee of this application, and is now abandned.

This invention relates in general to photographic developers, and more particularly to developers used to develop latent images present in those photosensitive silver halide photographic films employed in the graphic arts for making line or half-tone images and other films used for producing photographic reproductions having very high contrast and image sharpness. These developers are frequently referred to as lith type developers.

At the present time, the preferred photographic developer for producing high contrast and image sharpness for the graphic arts is a solution of a p-dihydroxybenzene developing agent. Normally hydroquinone is utilized as the developing agent for photographic silver halide emulsions. Working solutions containing such developing agents are very susceptible to aerial oxidation and consequently possess a very short tray life. After a few hours of exposure to ordinary atmospheric conditions, even without actual use, the response of the developer is greatly impaired and the solutions become generally ineffective. This loss of elfectiveness results in loss of density range and dot quality, and contrast is reduced or lost in the images being produced. Furthermore, this loss of effectiveness of developing solutions is extremely costly whenever large volumes of developer are being utilized as in automatic processing machines. One technique for reducing costs of developer in automatic processing machines is to utilize developer replenishers, these being commonly stored in large quantities within the processing machine, however, these replenishers are similarly subject to aerial oxidation and will accordingly become ineffective after limited periods of exposure.

In accordance with the present invention, an improved photographic developer is prepared which employs a combination of a p-dihydroxybenzene together with a combination of a water soluble sulfite and ascorbic acid. The p-dihydroxybenzene component is normally hydroquinone, the Water soluble sulfite normally being sodium sulfite, the ascorbic acid normally being either ascorbic acid or the sodium or other alkali salts of ascorbic acid as well 3,512,981 Patented May 19, 1970 as certain stereioisornetric forms of ascorbic acid. The ascorbic acids have the following general structure:

wherein R represents a hydroxy group when n represents a positive integer from 1 to 4 and R represents a hydrogen atom when n represents a positive integer of from 2 to 4. Derivatives of ascorbic acid including sodium or potassium ascorbate, 1 erythroascorbic acid, and d glucoascorbic acid have been found effective in this combination. the developing solutions employing a p-dihydroxybenzene such as hydroquinone, a synergistic effect is experienced when both sulfite and ascorbic acid are present, this synergistic effect being manifested in the preparation of a highly stable solution capable of preparing reproductions having sharp contrast with sharpness of image. While the exact mechanism of this synergistic effect upon the developing properties is not completely understood, it is believed to be due to a more effective degree of control of the reaction prior to the point in time that the developing rate curve reaches the typical knee zone in the ascending portion of the plot of the developing operation, this being coupled with close control of reaction rate subsequent to the reaching of this point in the plot of the reaction curve.

This synergistic effect, while enhancing the performance of the developing solutions, has also been found to drastically reduce the deterioration or oxidation of the solutions upon exposure at ordinary atmospheric conditions, and thereby significantly increase the tray life of the material. In addition, the shelf life of the substance is significantly increased.

It is therefore the principal object of the present invention to provide a developer composition and technique which is capable of producing high contrast photo graphic images, the composition having an extended tray life and shelf life, this enhanced action being retained over prolonged periods of use and exposure to atmospheric conditions.

'It is a further object of the present invention to provide a photographic developer replenisher that has an extended tray and shelf life.

Other and further objects of the present invention will become apparent from a review of the following specification and appended claims.

In the preferred modification of the present invention, an exposed photographic silver halide emulsion is processed or developed in a solution of a p-dihydroxybenzene developing agent, preferably hydroquinone, the solution containing ascorbic acid or its derivatives in an amount preferably in the range of about .5 to 10 grams to liter of solution (depending upon the degree of extended tray life desired) and between about V2 gram and 5 grams of a dissociated sulfite per liter of solution. The term dissociated is employed herein in a comprehensive sense, and accordingly is intended to include ionized, as well. It has been found that the synergestic effect as pointed out hereina'bove is diminished when less than about A.

gram of dissociated sulfite (determined as sodium sulfite) per liter of solution is utilized, and it has been further found that the elfect is lost when amounts greater than about 5 grams of dissociated sulfite are present per liter of solution. In this connection, the typical peppering of images developed in p-dihydroxybenzene developing agents, including hydroquinone, is eliminated. This peppering eifect is acknowledged as a highly undesirable effect in the graphic arts industry, and images possessing this effect are normally considered undesirable.

This phenomenon defined as peppering may be described as the formation of one or more randomly arranged generally round high or low density spots occurring in areas of reciprocal developed density. While the phenomenon is not entirely understood, it appears to be the result of an accelerated development of certain areas of the photosensitive silver halide emulsion film which areas have received only a minimal or marginal amount of exposure, and which exposure is below that lower threshold limit necessary to obtain a maximum or high density. This phenomena occurs more frequently with those photosensitive silver halide emulsions which are normally regarded as having the highest ability to render high contrast over prolonged developing time ranges.

In order to minimize the problem with peppering, re-touching operations may be undertaken to restore the product. Unfortunately, re-touching is exceedingly expensive and it is accordingly deemed preferable to eliminate the problem during the developing operation if at all possible. The developer of the present invention is capable of providing rapid developing of latent images present in photosensitive silver halide emulsion films, this rapid development being possible without the introduction of areas of peppering in the product.

Excessive quantities of a water soluble sulfite in the developing solutions will normally inhibit the phenomena known as infectious development of the latent images present in the photosensitive silver halide emulsions. In order to exercise greater control in the solutions of the present invention, the sulfite is kept at a level concentration of between about /2 and grams per liter determined as sodium sulfite, and formaldehyde bisulfite complexes are utilized as sulfite buffers to provide a solution having a constant and controllable low sulfite concentration within the desired range. This range provides a working solution having a sulfite concentration of from between about 0.004 molar and 0.04 molar. Solutions with a sulfite concentration of 0.002 molar may be useful in some systems. The formaldehyde bisulfite complexes control the sulfite concentration through replenishing of the sulfite consumed during the developing reactions,

and which may otherwise be lost due to oxidation reactions and the like.

The role of formaldehyde bisulfite in the solution is disclosed in detail in an article by Yule, Formaldehyde- Hydroquinone Developers and Infectious Development, Journal of the Franklin Institute, vol. 239, pages 221- 230, 1945. In his paper, Yule suggests that the main function of the formaldehyde bisulfite is that of a sulfite buffer which performs a control on the concentration of free (dissociated) sulfite ions present in the solution.

Photographic emulsions designed for producing high contrast images including lith type emulsions are commercially available. One such type is available under the designation Kodalith films from Eastman Kodak Company of Rochester, N.Y., various similar types being available commercially from the E. I. du Pont de Nemours Company of Wilmington, Del., as well as others. While these various photographic emulsions have the desired high contrast and dot quality when developed in known high contrast developers, it is of course desirable to increase the useful life of the developer solution by increasing the resistance of the developing solution to aerial oxidation upon exposure to ordinary atmospheric conditions.

The developing solutions of the present invention contain a p-dihydroxybenzene developing agent such as hydroquinone, although toluhydroquinone, 2,5-dimethylhydroquinone and the like have been found useful. The definition of p-dihydroxybenzene is intended to include materials such as 1,2,4-trihydroxybenzene and the like. These solutions are, as indicated, substantially free of aminobenzene developing agents such as for example p-aminophenol, n-methyl-p-aminophenol, p-phenylenediamine, etc. It will be appreciated, of course, that trace quantities in the range of, for example, less, th n 0.05 gram per liter of these developing agents may be present in some solutions, greater quantities having been found to reduce contrast.

The ascorbic acids and the isomers and their derivatives, which are used in the developer solution are acids of the structural formula set forth hereinabove. Ascorbic acids are commonly referred to as Vitamin C and their preparation, sources and derivatives are described in the Encyclopedia of Chemical Technology, vol. 2, pages 150 to 163, published by the Interscience Encyclopedia, Inc. These ascorbic acids and the isomers and their derivatives are most readily obtained in pharmaceutical grade, however, pharmaceutical grade is not necessary. Good results are achieved Whenever developer solutions generally free of impurities are employed which provide extended tray life and shelf life of the working solutions of the present invention. The inclusion of ascorbic acid in the sulfite containing solution has been found to result in very little, if any, loss in speed of developing, and no loss in contrast (gamma) even after extended times of exposure of the developer solution to atmospheric air.

The invention is more specifically described by reference to the following examples.

{EXAMPLE A Three Strips of photographic film coated with a high contrast gelatino-silver chloro-bromide emulsion designed for use in the graphic arts for line or half-tone images were exposed identically on a Process Control Sensitom- ,eter, Model 5. One of the strips was developed for two minutes in a fresh developer of the following composition at F. and was thereafter fixed and washed.

Constituent: Grams/liter Hydroquinone 12 Sodium sulfide (desiccated) 3 Sodium formaldehyde bisulfite 25 Potassium bromide 2.6 Sodium carbonate (monohydrate) 23 Ascorbic acid 5 Water to make 1 liter. pH 10.05.

The second and third strips of the same exposed film were processed under identical conditions in the same developer solution, the second strip being processed after 4 hours and the third strip being processed after 6 hours.

During this time, the same developer solution was left uncovered in the original tray. The following results were obtained:

Aerial Relative speed In the table, the sensitometric results show that the relative speed and high image contrast (gamma reading) were maintained at substantially constant levels, irrespective of the increased aeration time.

EXAMPLE B Aerial Relative speed oxidation 10/1 at D=2.0 Strip tlme (hours) over fog Gamma In the table, the degradation of the relative speed and contrast range is shown in the sensitometric results of the second and third strips which were processed after 4 and 6 hours, respectively.

The results obtained in these experiments indicate that the presence of ascorbic acid in a developer solution containing dissociated alkali sulfite is stable and resistant to oxidation over extended periods of time, this being indicated by the consistency of results obtained with the material after extended exposure to the atmospheric air. This stability is achieved without experiencing loss in speed or loss in contrast (gamma), thus making the composition particularly adaptable for use in connection with automatic processing equipment.

EXAMPLE C A working solution of a developer composition was prepared in accordance with the following:

Constituent: Grams/liter Hydroquinone 12 Sodium sulfite (desiccated) 3 Potassium bromide 2.6 Ascorbic acid 3 Sodium carbonate (monohydrate) to adjust A similar solution was prepared as a control and having the following composition- Grams/ Constituent: liter Hydroquinone 12 Sodium sulfite 3 Potassium bromide 2.6 Sodium carbonate (monohydrate) 14 A developing solution was prepared having the following composition Grams/ Constituent: liter 1,2,4-trihydroxybenzene 12 Sodium sulfite (desiccated) 3 Potassium bromide 2.6 Sodium carbonate (monohydrate) to adjust pH Water to make 1 liter.

This composition utilized a multi-functional hydroxybenzene as a substituted hydroquinone, this being 1,2,4-trihydroxybenzene. Good results were achieved with this composition.

EXAMPLE E A developing solution was prepared having a composition similar to that in Example A, with the exception of toluhydroquinone being substituted for hydroquinone, the quantity remaining at 12 grams per liter. This solution was not as readily balanced as the solution in Example A, however developer activity level was considered satisfactory.

It will be appreciated, therefore, that these exemplary compositions are set forth for purposes of illustration only, and are not to be construed as limitations upon the reasonable spirit of this invention.

What is claimed is:

1. A stable photographic developer solution consisting of an aqueous solution of:

(a) a p-dihydroxybenzene developing agent;

(b) an ascorbic acid analogue selected from the group 6 consisting of those certain substances having the structural formula wherein R represents a hydroxy group when n represents a positive integer from 1 to 4 and R represents a hydrogen atom when n represents a positive integer of from 2 to 4, and the alkali salts of said certain substances; and

(c) a certain predetermined amount of dissociated sulfite, the said amount being suificient to provide a solution with a dissociated sulfite concentration of between about 0.004 molar and 0.04 molar.

2. The photographic developer solution as set forth in claim 1 being particularly characterized in that said ascorbic acid analogue is l-ascorbic acid.

3. The photographic developer solution as set forth in claim 2 being particularly characterized in that said l-ascorbic acid is present in said solution in an amount ranging from between about 0.5 gram per liter and 10 grams per liter.

4. The photographic developer solution as set forth in claim 1 being particularly characterized in that said dissociated sulfite is derived from the combined presence of an alkali sulfite and formaldehyde bisulfite in said solution.

5. The photographic developer solution as set forth in claim 1 being particularly characterized in that said p-dihydroxybenzene developing agent is selected from the group consisting of hydroquinone, toluhydroquinone, 2,5-dimethylhydroquinone, 2,5-diethylhydroquinone, and 1,2,4-trihydroxybenzene.

6. A stable photographic developer solution consisting of an aqueous solution of:

(a) a p-dihydroxybenzene developing agent;

(b) ascorbic acid in an amount ranging from between 0.5 gram per liter and 10 grams per liter;

(c) dissociated sulfite in a concentration of between about 0.004 molar and 0.04 molar, said sulfite being derived from sodium sulfite and formaldehyde bisulfite, the sodium sulfite being present in an amount of between about 2 grams per liter and 4 grams per liter, the formaldehyde bisulfite being present in an amount ranging from between about 20 grams per liter and 50 grams per liter, the pH of said developing solution being between about 9.5 and 10.5.

7. The method of developing latent images in photosensitive silver -halide emulsions which comprises the steps of subjecting said emulsions to an aqueous solution of:

(a) a p-dihydroxybenzene developing agent;

(b) an ascorbic acid analogue selected from the group consisting of those certain substances having the structural formula wherein R represents a hydroxy group when n represents a positive integer from 1 to 4 and R represents a hydrogen atom when n represents a positive integer of from 2 to 4, and the alkali salts of said certain substances; and

(c) a certain predetermined amount of dissociated sulfite, the said amount being sufficient to provide a solution with a dissociated sulfite concentration of between about 0.004 molar and 0.04 molar.

8. The method as set forth in claim 7 being particularly characterized in that said solution is:

(a) a p-dihydroxybenzene developing agent;

(b) ascorbic acid in an amount ranging from between 0.5 gram per liter and 10 grams per liter;

(0) dissociated sulfite in a concentration of between References Cited about molar and molar, Said Sulfite being derived from the combined presence of sodium sulfite and formaldehyde bisulfite in said solution, the 2,415,666 2/1947 wFlssberger et sodium sulfite being present in an amount of be- 3,022,168 2/1962 Stlamkvlst 9666'3 tween about 2 grams per liter and 4 grams per liter, 5 3,128,180 4/1964 et 9627 the formaldehyde bisulfite being present in an amount ranging from between about 20 grams per liter and NORMAN TORCHIN Pnmary Exammer 50 grams per liter, the pH of said developing solu- M. F KELLEY, A i ta t Examiner tion being between about 9.5 and 10.5. 10