Radiation imagery chemistry: process – composition – or product th – Radiation sensitive product – Silver compound sensitizer containing
Reexamination Certificate
2001-07-11
2003-05-06
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Radiation sensitive product
Silver compound sensitizer containing
C430S567000, C430S966000, C430S967000
Reexamination Certificate
active
06558892
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of preparing light-sensitive ultrathin tabular grains rich in silver bromide, having {111} major faces.
BACKGROUND OF THE INVENTION
High aspect ratio tabular grains exhibit several pronounced photographic advantages as is well-known since their introduction into photographic applications in the eighties. Thanks to their particular morphology greater amounts of spectral sensitizers can be adsorbed per mole of silver halide if compared with classical globular (e.g. cubic or octahedral) grains. As a consequence such spectrally sensitized tabular grains show an improved speed-granularity relationship and a wide separation between their blue speed and minus blue speed. Sharpness of photographic images can be improved using tabular grains thanks to their lower light scattering properties, again if compared with conventionally known globular emulsion grains. In color negative materials e.g. the conventional sequence of the light-sensitive layers can be altered and the yellow filter layer can be omitted. In developed black-and-white images high covering power is obtained even at high hardening levels. Alternatively reduced silver halide coverages can be achieved if desired, which again results in improved sharpness. In duplitized radiographic materials the presence of tabular grains reduces the so-called cross-over which is the main factor for sharpness in such materials. Moreover coating amounts of silver can be reduced, further in favor of production cost and ecology.
An emulsion is generally understood to be a “tabular grain emulsion” when tabular grains account for at least 50 percent of total grain projected area. A grain is generally considered to be a tabular grain when the ratio of its equivalent circular diameter to its thickness is at least 2. The equivalent circular diameter of a grain is the diameter of a circle having an area equal to the projected area of the grain.
Early patent disclosures on high aspect tabular grains, e.g. U.S. Pat. Nos. 4,434,226; 4,439,520; 4,425,425; 4,425,426; 4,433,048 and Research Disclosure, Vol. 225, January 1983, Item 22534, are concerned with high sensitive silver bromide or silver iodobromide emulsions. In a lot of photographic applications high sensitivity remains important. As nowadays ultrathin tabular grains, in the range of from 0.05 up to 0.10 &mgr;m are envisaged as in EP-A's 0 650 085, 0 667 557, 0 701 164, 0 701 166, 0 706 079, 0 699 945 up to 0 699 951, 0 756 199, 0 758 758 and in U.S. Pat. Nos. 5,470,698; 5,641,618; 5,667,955 it is clear that a major role is dedicated to spectral sensitization of such thin tabular grains, making use of huge sensitizing amounts thereof, in order to provide high speed, high covering power, low cross-over and a low level of residual colour after (rapid) processing. As it remains important to provide a high a percentage of tabular grains as high as possible within the whole emulsion crystal population, in favor of “chemical homogneity”, reflected in sensitometric benifits (like desired higher contrast e.g.) as a result of “physical” or “morphological homogeneity” offered by the said tabular grains, it is clear that every improvement in that direction is highly appreciated. Without taking into account well-defined emulsion preparation parameters when preparing tabular grains, it is well-known by a person skilled in the art of photography that such a method leads to grain populations consisting of a mixture of all shapes of crystals.
As a consequence many attempts have been made in order to improve the degree of homogeneity of the size and shape of the crystals. In this context EP-A's 0 566 076; 0 506 947; 0 518 066; 0 513 722 and 0 911 687 and U.S. Pat. No. 4,797,354 are related with the preparation of monodisperse hexagonal tabular crystals. In said U.S. Pat. No. 4,797,354 the preparation has been described of tabular emulsions having a high percentage of hexagonal, tabular crystals accounting for 70 to 100% of the total projected area of the said crystals with an average aspect ratio of from 2.5/1 to 20/1. However the examples therein, and in the other references cited, except for the said EP-A 0 911 687 are illustrative for a low yield of silver halide emulsion in the reaction vessel mixture, said yield being defined as amount of silver nitrate precipitated per liter of the said reaction vessel mixture.
For radiographic applications photographic advantages of tabular grains, if compared with normal globular grains, are a high covering power at high forehardening levels as set forth in U.S. Pat. No. 4,414,304. Further a high developability and high sharpness especially in double side coated spectrally sensitized materials can be obtained. The thinner the tabular grains and the lower the number of non-tabular grains in the total grain population the greater these advantages are. To express it in another way: a high degree of homogeneity in grain morphology is desired, leading to a high covering power in order to further offer the possibility to coat lower amounts of silver. Moreover with respect to ecology it is of utmost importance to prepare tabular grains rich in silver bromide having an enhanced covering power. The present invention thus further extends the teachings on thin tabular {111} emulsion grains rich in silver bromide showing a high degree of homogeneity.
OBJECTS OF THE INVENTION
It is an object to reduce the amount of silver halide coated in black-and-white materials, and more particularly, those coated on one or both sides of a transparent support as used in radiographic applications, in order to have reduced production costs before selling those materials and environmental load after processing said materials after exposure.
As ultrathin grains having {111} major faces are envisaged to offer a valuable tool to reach the objects of the present invention it was a further object to provide a method to produce said ultrathin grains, preferably having an average grain thickness of less than 0.08 &mgr;m and even more preferably of less than 0.06 &mgr;m.
It was still a further object of the present invention to add at least one spectral sensitizer in amounts providing high speed, covering power and low cross-over.
Further objects will become apparent from the description hereinafter.
SUMMARY OF THE INVENTION
It has been found that the objects of the present invention are attained by a preparation method, within a temperature-controlled reaction vessel, of an ultrathin tabular grain emulsion rich in silver bromide, having {111} major faces, wherein tabular grains having a thickness of less than 0.08 &mgr;m exhibit an average aspect ratio of more than 5:1 and account for at least 80 percent of total grain projected area of said grains, said process comprising introducing into a temperature-controlled reaction vessel an aqueous dispersing medium and silver, bromide, and, optionally, iodide and/or chloride, and maintaining pBr values within said reaction vessel during grain nucleation and growth that favors the formation and growth of tabular grains, characterized in that at the time said tabular grains are being formed, (a) pH in said reaction vessel is maintained at a value ranging from 0.8 to 10; (b) a gelatino-peptizer is present in the reaction vessel in a concentration of from 0 to 50 g per liter of said dispersing medium, and pBr within said reaction vessel has a value of at least 1.8 is maintained during grain nucleation, wherein nucleation is performed in a reaction vessel at a temperature in the range from 10-60° C., more preferably in the range from 10-50° C., and even more preferably in the range from 40-50° C., and a pBr value within said reaction vessel is maintained at a value of less than 2.4 during growth, wherein growth is performed in a reaction vessel at a temperature in the range from 40-80° C. (in the range from 40-50° C. if an average grain thickness of 0.06 &mgr;m is envisaged), provided that a gelatino peptizer which is free from calcium ions and has a meth
Elst Kathy
Mans Ilse
Agfa-Gevaert
Baxter Janet
Guy Joseph T.
Nexsen Pruet Jacobs & Pollard LLC
Walke Amanda C.
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