Abstract:
A light-sensitive silver halide reproduction material of high gradation stability which contains a disulfide compound substituted by two heterocyclic radicals, and a thiazolidine compound.

Description:
The invention relates to a light-sensitive silver halide reproduction material of high gradation stability and more particularly, one which contains a disulfide compound substituted by two heterocyclic radicals and a thiazolidine compound. 
     BACKGROUND OF THE INVENTION 
     Silver halide reproduction materials which contain disulfide compounds substituted by two heterocyclic radicals are known and are described, e.g., in U.S. Pat. Nos. 2,440,110 and 2,465,149. These serve to prevent fogging of silver halide emulsions and/or to improve the fog-lowering effect of sulfinic or seleninic acids. 
     Silver halide materials which contain thiazolidine carboxylic acids are likewise known and described, e.g., in U.S. Pat. No. 3,565,625. They are used here to improve the sensitivity-fogging ratio. Moreover, it is stated in P. Glafkides, Chimie et Physique Photographiques, 3d edition, Paris, 1967, p. 313, that thiazolidine carboxylic acids act as contrast-modifying desensitizers whereby negative emulsions are desensitized, while with positive emulsions gradation is elevated at the cost of sensitivity. 
     It is, moreover, known that in the development of a light-sensitive silver halide reproduction material the silver formed, and thus also gradation, which largely determines the image character, depends to a great extent on development conditions, especially on time and temperature of development. Therefore, it is endeavored to keep the time and temperature of development as constant as possible. In actual practice, however, this has not been possible to a satisfactory degree. 
     Gradation stability is of special significance in the development of x-ray films where for diagnostic reasons a high constancy of gradation is required. Today, as a result of the short development times used, these are very susceptible to deviations of development conditions, especially against overdevelopment. Thus, the task of the present invention is to provide a light-sensitive silver halide reproduction material which excels in high gradation stability during development under various conditions. 
     SUMMARY OF THE INVENTION 
     This task was solved by a light-sensitive silver halide reproduction material, which, if desired, is hardened and which consists of a support film, at least one light-sensitive silver halide emulsion layer, and, if desired, additional nonlight-sensitive auxiliary layers, characterized in that the reproduction material contains 
     (a) a disulfide compound of the formula ##STR1## and 
     (b) a thiazolidine compound of the formula ##STR2## wherein X represents the atoms necessary to complete, if desired, a substituted heterocyclic ring, and R, R 1 , R 2 , R 3 , and R 4  are the same or different and represent hydrogen, alkyl radicals with 1-4 carbon atoms, and aryl radicals. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Suitable heterocyclic rings for the disulfide compound are oxazol, thiazol, imidazol, pyrimidine, oxadiazol, thiadizol, triazol, tetrazol, etc. 
     For additional examples of suitable heterocycles, see E. J. Birr, Stabilization of Photographic Silver Halide Emulsions, &#34;The Focal Press&#34;, London and New York, 1974. The heterocyclic rings can also be substituted or can be parts of polynuclear ring systems. Examples of this, for example, are 2-aminothiadiazol, 1-phenyl-tetrazol, benzthiazol, etc. Disulfide compounds are added to the photographic silver halide emulsion in amounts of 10-30 mg/mole silver halide as ca. 0.5% solutions in aqueous or water-miscible organic solvents. Addition can be at any point in time, preferably after chemical ripening. It is also possible to add the disulfide compounds to a water-permeable auxiliary layer in direct contact with the photographic emulsion layer. 
     An especially suitable disulfide compound is bis-(5-mercapto-1,3,4-thiadiazol-2 yl-) disulfide, which, in addition to its good gradation stabilizing effect, further excels by providing especially good stability for the remaining sensitometric data. Anhydrous organic solvents have proven to be best as solvents for this disulfide, as for example, methanol, anhydrous ethanol, or dimethyl formamide. 
     The thiazolidine compounds, or, if desired, their salts, are added to the photographic silver halide emulsion in amounts of 5-20 mg/mole silver halide as ca. 0.5% aqueous solutions at a random point in time, preferably after chemical ripening. It is also possible to add the thiazolidine compounds to a water-permeable auxiliary layer in direct contact with the photographic emulsion. 
     In a special form of embodiment, suitable thiazolidine precursor compounds can also be added in place of the thiazolidine compounds. In this case, the photographic silver halide emulsion must be hardened by an aldehyde hardening agent, for example, by formaldehyde or glyoxal or glutardialdehyde, etc. which reacts with the precursor compound to form a thiazolidine compound. For example, cystein or penicillamine, etc. are suitable precursor compounds for this purpose. In addition to the disulfide compounds of the invention and the thiazolidine compounds, the photographic emulsions can contain conventional chemical and, if desired, optical sensitizers as well as conventional stabilizers and antifogging agents. Moreover, the emulsion may also contain additives elevating the covering power; also wetting agents, antistatic agents, hardeners, etc. 
    
    
     EXAMPLE 1 
     A highly sensitive silver bromoiodide emulsion with about 2 mole % AgI was prepared which contained 5 wt. % gelatin and 10 wt. % silver halide. This emulsion was divided into 4 parts. Part A contained no additives of the invention, while to Parts B, C and D were added the ingredients indicated in the table, shortly before coating. 
     
         ______________________________________  Disulfide Compound                  Thiazolidine Compound  per mole silver per mole silverSample halide          halide______________________________________A      --              --B      --              17 mg                  thiazolidine-4-car-                  boxylic acidC      17 mg           --  Bis-(1-phenyl-  tetrazolyl-5-  disulfideD      17 mg           17 mg  Bis-(1-phenyl-  thiazolidine-4-  tetrazolyl-5-   carboxylic acid  disulfide______________________________________ 
    
     The emulsions were coated according to known processes onto a polyethylene glycol terephthalate film support and were dried. These light-sensitive materials were exposed, developed, and subjected to a normal sensitometric evaluation, wherein a developer of the following composition was used: 
     
         ______________________________________Hydroquinone                  30.00 g1-Phenyl-3-pyrazolidone        1.00 gNa.sub.2 SO.sub.3 (anhydrous) 60.00 gKOH                           23.00 gNaBO.sub.2 . 4H.sub.2 0       20.00 gWater                to       1 liter______________________________________ 
    
     15, 22 and 51 s were selected as times of development. The results are summarized in the following table. 
     
         __________________________________________________________________________UNDER-          NORMAL     OVERDEVELOPMENT     DEVELOPMENT                      DEVELOPMENT34° C./15 s           34° C./22 s                      34° C./51 s%          %Sensi-    Grada- Sensi-               Grada- Sensi-                          Grada-SAMPLEtivity    tion        Fog           tivity               tion                   Fog                      tivity                          tion                              Fog__________________________________________________________________________A    74  3.5 0.16           100 3.3 0.20                      170 2.5 0.31B    98  3.4 0.23           129 3.3 0.24                      195 2.6 0.34C    66  3.4 0.16            77 3.5 0.17                      126 2.9 0.19D    98  3.3 0.20           129 3.3 0.20                      190 3.2 0.31__________________________________________________________________________ 
    
     Evaluation points out the gradation-stabilizing effect of Sample D of the invention against comparative Sample A and Samples B and C containing the individual components. 
     EXAMPLE 2 
     The silver emulsion described in Example 1 was prepared and divided into 3 parts. To the individual parts, shortly before coating, were added the disulfide- and thiazolidine compounds indicated in the following table. 
     
         ______________________________________   Disulfide Compound                  Thiazolidine Compound   per mole silver                  per mole silverSample  halide         halide______________________________________A       --             --B       13 mg          11 mg   Bis-(5-acetyl- 2-methyl-thiazolidine-   mercapto-1,3,4-                  4-carboxylic acid   thiadiazolyl-2-)   disulfideC       13 mg          11 mg   Bis-(5-mercapto-)                  Thiazolidine-4-   thiadiazolyl-2-                  carboxylic acid   disulfide______________________________________ 
    
     The emulsions were coated, dried, exposed and developed as indicated. Evaluation indicated the following values: 
     
         ______________________________________   Gradation   Gradation   Gradation   with under- with normal with over-   development development developmentSample  34° C./15 s               34° C./22 s                           34° C./51 s______________________________________A       3.4         3.2         2.3B       3.4         3.4         2.9C       3.4         3.2         3.1______________________________________ 
    
     The gradation-stabilizing effect of the additives of the invention with longer development times is clearly illustrated. 
     EXAMPLE 3 
     A silver halide emulsion as described in Example 1 was prepared and divided into 4 parts. To the individual parts, shorting before casting, were added the disulfide- and thiazolidine compounds indicated in the following table. 
     
         ______________________________________  Disulfide Compound                  Thiazolidine Compound  per mole silver per mole silverSample halide          halide______________________________________A      --              --B      27 mg           7 mg  Bis-(1-phenyl-  2,2&#39;-dimethyl-  tetrazolyl-5-)  thiazolidine-4-  disulfide       carboxylic acidC      11 mg           10 mg  Bis-(benz-      Thiazolidine-4-  thiazolyl-2-)   carboxylic acid  disulfide       ethyl esterD      11 mg           10 mg  Bis-(5-amino-   Thiazolidine-4-  thiadiazolyl-2-)                  carboxylic acid  disulfide       ethyl ester.______________________________________ 
    
     The emulsions were coated and dried as indicated. After exposure, development took place at 30°, 34° and 38° C. Evaluation yielded the following values: 
     
         ______________________________________    Gradation   Gradation   Gradation    with under- with normal with over-    development development developmentSample   30° C./22 s                34° C./22 s                            38° C./22 s______________________________________A        3.2         3.2         2.1B        3.3         3.3         2.8C        3.2         3.1         2.7D        3.4         3.3         2.9______________________________________ 
    
     As seen from the above table, the same gradation-stabilizing effect is also evident upon variation of the temperature of development. 
     EXAMPLE 4 
     The effect of the invention is also seen when using thiazolidine precursor compound in combination with a disulfide compound. 
     A silver halide emulsion as described in Example 1 was prepared and hardened using formaldehyde. The emulsion was divided into 3 parts to which were added a disulfide compound and, in place of the thiazolidine compound, a precursor compound from the following table: 
     
         ______________________________________  Disulfide Compound                  Thiazolidine Precursor  per mole silver Compound per moleSample halide          silver halide______________________________________A      --              --B      --              7 mg                  CysteinC      13 mg           7 mg  Bis(-1-phenyl-  Cystein  tetrazolyl-5-)  disulfide______________________________________ 
    
     The emulsions were coated, dried, exposed and developed as indicated. Evaluation yielded the following values: 
     
         ______________________________________   Gradation   Gradation   Gradation   with under- with normal with over-   development development developmentSample  34° C./15 s               34° C./22 s                           34° C./51 s______________________________________A       3.3         3.2         2.6B       3.2         2.9         2.4C       3.0         3.0         3.0______________________________________