The highest speed and therefore most commonly employed photographic elements are those which contain a radiation-sensitive silver bromide or bromoiodide emulsion layer coated on a support. Although other ingredients can be present, the essential components of the emulsion layer are radiation-sensitive silver bromide microcrystals, optionally containing iodide, commonly referred to as grains, which form the discrete phase of the photographic emulsion, and a vehicle, which forms the continuous phase of the photographic emulsion.
It is important to recognize that the vehicle encompasses both the peptizer and the binder employed in the preparation of the emulsion layer. The peptizer is introduced during the precipitation of the grains to avoid their coalescence or flocculation. Peptizer concentrations of from 0.2 to 10 percent, by weight, based on the total weight of emulsion as prepared by precipitation, can be employed.
It is common practice to maintain the concentration of the peptizer in the emulsion as initially prepared below about 6 percent, based on total emulsion weight, and to adjust the emulsion vehicle concentration upwardly for optimum coating characteristics by delayed binder additions. For example, the emulsion as initially prepared commonly contains from about 5 to 50 grams of peptizer per mole of silver, more typically from about 10 to 30 grams of peptizer per mole of silver. Binder can be added prior to coating to bring the total vehicle concentration up to 1000 grams per mole of silver. The concentration of the vehicle in the emulsion layer is preferably above 50 grams per mole of silver. In a completed silver halide photographic element the vehicle preferably forms about 30 to 70 percent by weight of the emulsion layer. Thus, the major portion of the vehicle in the emulsion layer is typically not derived from the peptizer, but from the binder that is later introduced.
While a variety of hydrophilic colloids are known to be useful peptizers, preferred peptizers are gelatin--e.g., alkali-treated gelatin (cattle bone or hide gelatin) or acid-treated gelatin (pigskin gelatin)--and gelatin derivatives--e.g., acetylated gelatin or phthalated gelatin. Gelatin and gelatin derivative peptizers are hereinafter collectively referred to as "gelatino-peptizers".
Materials useful as peptizers, particularly gelatin and gelatin derivatives, are also commonly employed as binders in preparing an emulsion for coating. However, many materials are useful as vehicles, including materials referred to as vehicle extenders, such as latices and other hydrophobic materials, which are inefficient peptizers. A listing of known vehicles is provided by Research Disclosure, Vol. 176, Dec. 1978, Item 17643, Section IX, Vehicles and vehicle extenders. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England.
It has been recognized that when the gelatin incorporated in an emulsion layer of a photographic element is oxidized, modification of emulsion photographic properties can result. Corben et al U.S. Pat. No. 2,890,215 discloses the desensitization of gelatin by treatment with a peracid. Komatsu et al Japanese Kokai No. 58(1983)-70221 discloses improved keeping stability for internal latent image forming silver halide emulsions when oxidized gelatin is employed. Komatsu et al Japanese Kokai No. 59(1984)-195232 discloses improved storage stability for silver halide emulsions having silver chloride grain surfaces prepared using oxidized gelatin.
Moll, "Investigations of Oxidized Gelatins", 2nd Photographic Gelatin Symposium, sponsored by the Royal Photographic Society, Oxford, United Kingdom, Sept. 6, 1985, discloses that the chemical and physical properties of oxidized gelatins, including luminescence of emulsions prepared therefrom, do not differ substantially from those of the native gelatin. The sensitometry and growth restraining properties, however, are reportedly changed by the oxidation treatment. It is stated that these changes cannot be attributed to oxidation of methionine.
Mifune et al EPO No. 0,144,990 A2 discloses a process for controlled ripening of a silver halide emulsion with a sulfur containing silver halide solvent. An oxidizing agent is relied upon to terminate ripening of the emulsion once the desired extent of ripening is accomplished.
Interest in silver halide photography has recently focused on tabular grain emulsions, particularly thin intermediate and high aspect ratio tabular grain emulsions. It has been shown that these emulsions can produce a variety of photographic advantages, including increased sharpness, improved speed-granularity relationships, increased blue and minus blue speed separations, more rapid developability, higher silver covering power when fully forehardened, reduced crossover in spectrally sensitized Duplitized.RTM. (two sided) radiographic formats, and various imaging advantages in dye image transfer film units. Research Disclosure, Vol. 225, January 1983, Item 22534, is considered representative of these teachings.
Dickerson U.S. Pat. No. 4,520,098, issued May 28, 1985, commonly assigned, discloses that dye stain reduction can be achieved in photographic elements containing a spectrally sensitized tabular grain emulsion by choosing the halide forming the tabular grains from among chloride and bromide and employing fine high iodide grains in combination. Dickerson teaches the use of conventional vehicles, including oxidizing agent treated gelatin.
One of the inefficiencies that has been encountered in the preparation of tabular grain silver bromide and bromiodide emulsions is the presence of unwanted grain shapes. In addition to unwanted nontabular grains, also in evidence are thick tabular grains, which have aspect ratios closely approaching those of nontabular grains.
In addition to low aspect ratio tabular grains and nontabular grains, these tabular grain emulsions, particularly silver bromide tabular grain emulsions, also contain a significant population of grains which are in the form of rods. Because of their length and limited projected arcos rods are of marginal photographic utility. Beyond this, their presence in emulsions is disadvantageous in conventional procedures for manufacturing photographic elements containing silver halide emulsion layers.
It is also known that the introduction of iodide ions during the precipitation of tabular grain emulsions results in thickening of the tabular grains. Thus, when tabular grain silver bromide and silver bromiodide emulsions precipitated under similar conditions and having similar mean grain diameters are compared, the tabular grain silver bromide emulsions exhibit higher average aspect ratios.
Finally, the precipitation of thin tabular grain silver bromide and bromoiodide emulsions requires control of bromide ion concentrations within a narrow range during initial tabular grain formation. Nontabular and thick tabular grains result when bromide ion concentrations are not maintained during precipitation.
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