US2141354A - Method of treating photographic film - Google Patents

Description

Patented Dec. 27, 1938 UNITED STATES PATENT OFFICE.

Alan M. Gundelfinger, Los Angeles, Calif., as-

signor to Cinecolor, Inc., Los Angeles, Calif., a corporation of California No Drawing. Application April 23, 1937, Serial No. 138,536

8 Claims.

My invention relates broadly to that type of film treatment commonly known in the photographic art as bleaching, and more particularly to bleaching by the use of iodine and/or iodide compounds. The so-called iodide bleach consisting of a solution of potassium iodide, or other soluble alkali metal iodide and free iodine, is well known as an agent which will bleach or cause a silver image to become substantially transparent 10 and at the same time produce an image which will mordant basic dyes.

Iodide bleaching has been a part of, and the basis for, many systems of color photography, but has never -found favor commercially because there has always been either an over-all stain or a lack of transparency in the finished picture, and it is therefore a major object of my invention to provide a method of iodide bleaching which will entirely eliminate this over-all stain heretofore obtained, without in any way sacrificing transparency.

It is a further object of my invention to provide a method of bleaching which is easy to control and operate, and which is therefore eminently practical for commercial laboratory use.

It is a further object of my invention to provide a bleaching process which can be used continuously over long periods of time during which large quantities of film are treated with consistently good results.

Generally speaking, I accomplish these objects and advantages by a close regulation of the concentration of the ingredients of the bleaching solution and a correlation of those concentrations 1 with the concentration of iodide in the emulsion at various stages of the process.

Inasmuch as my invention is applicable to many and varied color processes, I will not attempt to describe the color treatment of a film in detail,

other than to mention that the usual type of process involves bleaching one or more images and then dyeing or dyetoning them the desired color; whether the film has one or more imagebearing emulsions on one or both sides is immaterial to the practice of my invention.

A typical iodide bleach as known in the art usually comprises a solution of an alkali metal iodidesuch as potassium iodide, free iodine, a salt such as sodium acetate, and a relatively weak, preferably organic, acid such as acetic acid. The usual type of film treated with a bleach of this nature, as for instance ordinary motion picture film has a light sensitive emulsion on one or both sides which usually is in the form of a gelatin layer containing a silver halide, such as bromide or chloride. It has long been known that films bleached with an, iodide bleach and subsequently dyed, often are not sufliciently Y transparent for satisfactory projections; and further, that if the bleach is varied to get adequate transparency the film usually .has stained'highlights, which of course render it unfit for commercial use. In addition to these objections, it has been found that a bleach which at first gives good results, soon ceases to give clear images 9 after a quantity of film has been run through it, although an analysis of the constituents of the bleach may show no appreciable change in their concentrations. So faras I am aware no one has ever successfully overcome these obstacles, 15 although it is generally admitted that when the iodide bleach is just righ excellent color pictures are produced, and it may therefore be said that the major object and accomplishment of the present invention is the solution of the foregoing i problems by providing definite limits of concentration of certain of the ingredients of an iodide bleach which insures the production of satisfactory colored images even after great quantities of film have been processed by the bleach.

While it is to be understood that I do not mean to be bound by the following theoretical discus sion of what I believe happens in the film when it is bleached, it is thought that a brief summary of my theory of reactions, arrived at as the result of many tests and a consideration of known chemical and physical laws, will be helpful in understanding the full scope of my invention as hereinafter set forth in detail.

The first and most obvious reaction that takes 35 place when a film bearing silver images is immersed in an iodide bleach is probably between the silver in the film and the iodine in the bleach to form silver iodide according to the equation: 2 A+Iz=2 Ag I and the rate of this 4 reaction will depend upon the iodine concentration of the bleach. This silver iodide is'relatively opaque and tends to form in large grains, so that if it is allowed to precipitate in the film,

the resulting images will be very grainy and rela- 45 tively lowin transparency, two very serious objections where the images are to be greatly enlarged by projection on a motion picture screen. However, if the above reaction takes place in the presence of an adequate amount of alkali 50 iodide, such as potassium iodide for instance, the

silver iodide immediately forms a complex salt and probably several complex salts of the general nature of KxAgyLH-y, although the exact number of these complexes and their precise structural 2 formulas are not definitely known at this time, and I will therefore refer to them collectively as the iodide complexes. These complexes unlike the silver iodide, are soluble and capable of diffusing into the surrounding gelatin if not properly 7 of formation, of the iodide complexes, is properly I controlled there will be a diffusion of the iodide complexes into the surrounding gelatin, and since the resulting gelatin complex is a mordant for basic dyes, the subsequent dye treatment will give a fuzzy image. Furthermore, since the iodide complexes are soluble, a certain amount of them is bound to diffuse into the surrounding bleach and as more film is treated and more iodide complexes are formed, the bleach will gradually build up in soluble complexes. Obviously some of the iodide complexes in the bleach are going to contact gelatin in the unexposed parts of the picture, i. e. the parts which are to remain clear in the finished picture. This contact of course forms gelatin complex in those portions, which subsequently mordants dye to give what is known as an over-all stain.

If the foregoing reasoning is correct it follows that the amount of visible stain on the film, which, of course, is a direct function of the gelatin complex present, will vary directly as the amount of iodide complex in the bleach, and therefore that the only way to minimize over-all stain is to reduce the amount of complex in the bleach to a minimum.

I have found that there is a difinite saturation point for the iodide complexes in potassium iodide, and further, that the amount of iodide complexes soluble in potassium iodide increases as the fourth power of the potassium iodide concentration. In other words, the amount of visible stain in the film increases as the fourth power of the alkali iodide concentration in the bleach, which means that relatively small increases in the alkali iodide concentration in the bleach cause relatively large increases in the amount of stain in the film. Therefore, it follows that if the amount of stain is to be kept within allowable limits, the concentration of alkali iodide in the bleach must be kept low enough to prevent excessive amounts of iodide complexes from going into solution in the bleach. In this connection, I have found that the maximum concentration of alkali metal iodide in the bleach which can be used with a normal treating time without getting excessive stain is 40 grams per liter of solution or a 4% solution, and that since the stain increases as the fourth power of the iodide concentration this limit is very critical.

As previously mentioned the silver iodide first formed is relatively opaque and if permitted to deposit on the film, produces images of too low a transparency to allow satisfactory projection. My experiments have shown that the transparency of, the image and its ability to mordant basic dyes increase as the concentration of the alkali iodide in the bleach increases, which results coincide with the foregoing theory, since an excess of alkali iodide in the bleach insures all of the silver iodide being converted to the transparent iodide complexes. I have found, however, that sufficiently transparent images can be produced with relatively low concentrations of alkali iodide in the bleach if the rate of formation of the silver iodide is not too rapid, for if the silver' iodide forms much more rapidly than it is converted into the complex there is bound to be a deposition of silver iodide in the image resulting in low transparency. The rate of formation of silver iodide obviously depends upon the amount of free iodide present in the bleach, and by keeping this amount low, say below 1 gram per liter of solution, it is pomible to get a very transparent image if the film is bleached until substantially all of the silver is converted to the complexes. However, even with an excessively long treating time such as 45 or 50 minutes a sufficiently transparent image cannot be obtained if the alkali concentration in the bleach is dropped below 1%.

Restating the foregoing facts in general termi nology, I have found first, that the alkali iodide concentration of the bleach must be kept low enough to prevent excessive iodide complex concentrations in the bleach in order to keep stain below the allowable limit, and second, that the alkali iodide concentration of the bleach must be high enough to insure substantially complete conversion of the silver iodide first formed, to the 1 iodide complex, and ultimately to the gelatin complex, to insure adequate transparency and mordanting, and finally-that the permissible concentrations of the alkali iodide in the bleach in conformity with these conditions are between 1% and 4%.

Obviously the exact concentration used between the foregoing limits will depend on a number of factors, but generally speaking it may be said that it is most advisable to operate with as high an alkali iodide content as possible commensurate with allowable stain, since this gives a transparent image with a much shorter treating time. In commercial laboratories where thousands of feet of film are processed every day and the profit is often dependent upon quantity production, this gelatin therewith. When the film goes into the.

bleach it then has an excess quantity of alkali iodide available to react with the silver iodide formed, even before the alkali iodide from the bleach can penetrate the gelatin. By utilizing this predip I find that I can materially increase the concentration of iodine in the bleach and thus speed up the formation of the silver iodide and its conversion to iodide complex. The amount of alkali iodide that the film will absorb and carry over to the bleach is ample to effect this result, yet is so small that practically all of it in the images is used up in the conversion of the silver iodide, and the amount in the clear gelatin which diiiuses into the bleach does not materially raise the alkali iodide concentration of the bleach, which may therefore be kept well below the upper allowable stain limit. If after several hundred thousand feet of film have been processed, the alkali iodide concentration of the bleach is found to have raised one or two tenths of a per cent, the bleach may be diluted slightly and is then good for many more thousands of feet.

mg a 2% alkali iodide content it will be. found that a 11% or 12% predip is the most satisfactory, whereas with a bleach containing 4% alkali iodide, the predip should preferably be kept down to around 8%. In other words, as the concentration of the alkali iodide in the bleach is raised, the ratio of alkali concentration in the predip to that in the bleach is lowered.

It will be understood however, that irrespective of whether a predip is used or 'not, that the permissible alkali iodide concentration in the bleach must be kept within the limits heretofore given, if satisfactory results are to be obtained. Likewise, it is to be understood that while the theoretical discussion previously given fits the facts as I have found them by extensive experiments, I do not mean to limit myself to the theory of reactions herein set forth, but merely ofier them as a satis- I factory explanation oi the phenomenal results obtained by me in the practice of my invention as defined by the appended claims.

I claim as my invention:

1. The method of bleaching photographic im- 35 ages which includes treating them with a solution of potassium iodide and then treating them with a solution containing iodine and from 1% to 4% potassium iodide.

2. The method of bleaching photographic im- 40 ages which includes treating them with a solution of alkali metal iodide and then treating them with Patent iio 2,11 1 ,3 L.

a solution containing from .02% to .6% iodine and from 1% to 4% of an alkali metal iodide.

3. The method of bleaching photographic images which includes treating them with a somtion of potassium iodide andthen treating them withasolutioncontaining from 92% to .6% iodine and from 1% to 4% potassium iodide, the iodide concentration of said first solution being between 7% and 14%.

4. The method of bleaching photographic images which includes first treating them with a solution of alkali metal iodide and then treatin them with a solution containing iodine and from 1% to 4% of said alkali metal iodide.

5. The method of bleaching photographic images which includes treating them with a solution of alkali iodide, and then treating them with a solution containing from .02% to .6% iodine and from 1% to 4% of said alkali iodide, the iodide concentration of said first solution being between 7 and 14%.

6. The method of bleaching photographic images which includes treating them with a solution of potassium iodide and then treating them with a solution containing iodine and from 1% to 4% potassium iodide, the iodide concentration of said first solution being between 7% and 14%.

7. [the method of bleaching photographic images which includes treating them with a solution of an alkali iodide and then treating them with a solution containing iodine and from 1% to 4% of said alkali iodide, the iodide concentraiiiim of said first solution being between 7% and 8. The method of bleaching photographic images which includes treating them with a solution of potassium iodide and then treating them with a solution containing i'rom .02% to .6% iodine and from 1% to 4% potassium iodide.

- ALAN M. GUNDEL FINGER:

December 27-, 1958-;

ALAN n. GUHDELFINGER. it is hereby certified that error appears in the printed specificationof the above numbered patent requiring-correction as follows: Page 2, first column, line 1 2, for 'd ifinit e read definite; and 'second eolm nni, line'l2, for theword "iodide" read iodine; and that the said Letters iatent should 'be readwith-this correction therein that the samegnay conform .to the record of the casein the Patent Office.

' [Signed and sealed this 28th day of March, A. D. 1939;.

(Seal) Henry Van Arsdale Acting Commissioner of Ifatentsa