Grain growth process for the preparation of high bromide ultrathin tabular grain emulsions

A grain growth process is disclosed for providing an ultrathin tabular grain emulsion in which the average equivalent circular diameter of tabular grains is increased. An aqueous dispersion is provided containing high bromide grains having an average thickness of less or equal to that of the ultrathin tabular grains to be produced, the dispersion having a pH in the range of from 2 to 8 and a limited stoichiometric excess of bromide ions. An 8-hydroxyquinoline that contains at least one iodo substituent is introduced into the dispersing medium as a grain growth modifier. The aqueous dispersion containing the iodo-8-hydroxyquinoline grain growth modifier is held at 40.degree. C. or a convenient higher temperature until greater than 50 percent of total grain projected area is accounted for by ultrathin tabular grains having {111} major faces of a higher average equivalent circular diameter than the starting grains and an average aspect ratio of at least 5.

FIELD OF THE INVENTION 
The invention relates to a grain growth process for preparing ultrathin 
high bromide tabular grain emulsions for photographic use. 
BACKGROUND OF THE INVENTION 
The term "tabular grain" is employed to indicate a silver halide grain 
having an aspect ratio of at least 2, where "aspect ratio" is ECD/t, ECD 
being the equivalent circular diameter of the grain (the diameter of a 
circle having the same projected area as the grain) and t is the thickness 
of the grain. 
The term "ultrathin tabular grain" is employed to indicate a tabular grain 
of a thickness less than 0.07 .mu.m. 
The term "tabular grain emulsion" is employed to indicate an emulsion in 
which tabular grains account for at least 50 percent of total grain 
projected area. 
The term "high chloride" or "high bromide" as applied to a grain or 
emulsion is employed to indicate that the grain or the grains of the 
emulsions contain at least 50 mole percent chloride or bromide, 
respectively, based on total silver present in the grain or the grains of 
the emulsion. 
The term "{111} tabular grain" is employed to indicate an emulsion in which 
the parallel major faces of the tabular grain lie in {111} crystal planes. 
The first high chloride high aspect ratio (ECD/t&gt;8){111} tabular grain 
emulsion is disclosed in Wey U.S. Pat. No. 4,399,215. The grains were 
relatively thick. Maskasky U.S. Pat. No. 4,400,463 (hereinafter designated 
Maskasky I) obtained thinner high chloride {111} tabular grains by 
employing an aminoazaindene (e.g., adenine) in combination with a 
synthetic peptizer having a thioether linkage. Maskasky U.S. Pat. No. 
4,713,323 (hereinafter designated Maskasky II) produced thinner high 
chloride {111} tabular grains by employing the aminoazaindene grain growth 
modifier in combination with low methionine (&lt;30 micromole per gram) 
gelatin, also referred to as "oxidized" gelatin, since the methionine 
concentration is reduced by employing a strong oxidizing agent, such as 
hydrogen peroxide. 
High chloride ultrathin {111} tabular grain emulsions are disclosed in 
Maskasky U.S. Pat. No. 5,217,858 (hereinafter designated Maskasky III). 
Maskasky III discloses to be effective in preparing high chloride 
ultrathin {111} tabular grain emulsions triaminopyrimidine grain growth 
modifiers containing 4, 5 and 6 ring position amino substituents, with the 
4 and 6 position substituents being hydroamino substituents. The term 
"hydroamino" designates an amino group containing at least one hydrogen 
substituent--i.e., a primary or secondary amino group. The 
triaminopyrimidine grain growth modifiers of Maskasky III include both 
those in which the three amino groups are independent (e.g., 
4,5,6-triaminopyrimidine) and those in which the 5 position amino group 
shares a substituent with 4 or 6 position amino group to produce a 
bicyclic compound (e.g., adenine, 8-azaadenine, or 4-amino-7,8- 
dihydro-pteridine). 
The process which Maskasky III employs to prepare high chloride ultrathin 
{111} tabular grain emulsions is a double jet process in which silver and 
chloride ions are concurrently run into a dispersing medium containing the 
grain growth modifier. The first function of the grain growth modifier is 
to promote twinning while grain nucleation is occurring, so that ultrathin 
grains can form. Thereafter the same grain growth modifier or another 
conventional grain growth modifier can be used to stabilize the {111} 
major faces of the high chloride tabular grains. 
A common feature of the Maskasky high chloride {111} tabular grain emulsion 
precipitations is the presence of a grain growth modifier. The reason for 
this is that high chloride {111} tabular grains, unlike high bromide {111} 
tabular grains, cannot be formed or maintained in the absence of a grain 
growth modifier, but rather take nontabular forms, since {100} crystal 
faces are more stable in high chloride grains. 
The art has long recognized that distinctly different techniques are 
required for preparing high chloride {111} tabular grain emulsions and 
high bromide {111} tabular grain emulsions. For example, Maskasky III does 
not disclose the processes of preparing high chloride ultrathin {111} 
tabular grain emulsions to be applicable to the preparation of high 
bromide ultrathin {111} tabular grain emulsions. Further, since at low pBr 
the {111} major faces of high bromide tabular grains have no tendency to 
revert to {100} crystal faces, the precipitation of high bromide {111} 
tabular grain emulsions has not required the addition of compounds 
comparable to the grain growth modifiers of Maskasky. 
Daubendiek et al U.S. Pat. 4,914,014, Antoniades et al U.S. Pat. 5,250,403 
and Zola et al EPO 0 362 699 illustrate the preparation of high bromide 
ultrathin {111} tabular grain emulsions. Each of the Examples resulting in 
the formation of ultrathin tabular grain emulsions are replete with 
adjustments undertaken during precipitation. Typical complexities include 
(a) different pBr conditions for grain nucleation and growth, (b) 
interruptions of the silver and/or halide salt additions, (c) frequent 
modifications of the rate of silver and/or halide salt additions, (d) the 
use of separate reaction vessels for grain nucleation and growth, thereby 
at least doubling the complexity of reaction vessel and control equipment, 
(e) the variance in dispersing medium volume as precipitation progresses, 
which makes optimized reaction vessel sizing for all phases of 
precipitation impossible, (f) dilution of emulsion silver content as 
precipitation progresses toward completion, thereby creating a water 
removal burden and increasing the required capacity of the reaction 
vessel, and (g) when pBr is maintained at customary low (e.g., pBr&lt;1.5) 
values employed for precipitating high bromide {111} tabular grain 
emulsions, large excess amounts of soluble bromide salts must be 
discarded. Note that since pBr is the negative logarithm of bromide ion 
activity, bromide ion concentrations increase as pBr decreases. This is 
directly analogous to hydrogen ion activity increasing as pH decreases. 
None of Antoniades, Daubendiek et al and Zola et al suggest the use of any 
compound comparable to a grain growth modifier to prepare high bromide 
ultrathin {111} tabular grain emulsions. 
Verbeeck EPO 0 503 700 discloses reduction of the coefficient of variation 
(COV) of high bromide high aspect ratio {111} tabular grain emulsions 
through the presence of an aminoazaindene, such as adenine, 
4-aminopyrazolopyrimidine and substitutional derivatives, prior to the 
precipitation of 50 percent of the silver. Double jet precipitation 
techniques are employed. The minimum disclosed thickness of a tabular 
grain population is 0.15 .mu.m. 
Related Applications 
Maskasky U.S. Ser. No. 195,807, filed Feb. 14, 1994, titled GRAIN GROWTH 
PROCESS FOR THE PREATION OF HIGH BROMIDE ULTRATHIN TABULAR GRAIN 
EMULSIONS, commonly assigned, (Maskasky V) discloses a process for the 
preparation of ultrathin high bromide tabular grain emulsions by ripening 
in the presence of a 4,5,6-triaminopyrimidine. 
Maskasky U.S. Ser. No. 281,500, filed Jul. 27, 1994, titled A NOVEL

SUMMARY OF THE INVENTION 
In one aspect the invention is directed to a grain growth process for 
providing a tabular grain emulsion in which the average equivalent 
circular diameter of tabular grains is increased while maintaining their 
average thickness at less than 0.07 .mu.m comprising introducing silver 
and halide ions into a dispersing medium in the presence of a grain growth 
modifier wherein tabular grains having an average thickness of less than 
0.07 .mu.m and a bromide content of greater than 50 mole percent are 
formed by (1) providing an aqueous dispersion containing at least 0.1 
percent by weight silver in the form of silver halide grains containing at 
least 50 mole percent bromide having an average thickness of less than 
0.06 .mu.m, the dispersion having a pH in the range of from 2 to 8 and a 
stoichiometric excess of bromide ions to silver ions limited to a pBr of 
at least 1.5, (2) introducing into the dispersing medium as the grain 
growth modifier an iodo-substituted 8-hydroxyquinoline, and (3) holding 
the aqueous dispersion containing the grain growth modifier at a 
temperature of at least 40.degree. C. until the average equivalent 
circular diameter of the grains in the dispersing medium is at least 0.1 
.mu.m greater than the average equivalent circular diameter of the grains 
provided in step (1) and greater than 50 percent of total grain projected 
area is accounted for by tabular grains having {111} major faces, an 
average aspect ratio of at least 5, and an average thickness of less than 
0.07 .mu.m. 
The high bromide ultrathin {111} tabular grain emulsions prepared by the 
process of the invention produce high bromide ultrathin {111} tabular 
grain emulsions in which the tabular grains, as demonstrated in the 
Examples below, can account for &gt;95 percent of total grain projected area. 
At the same time, the process itself offers significant advantages over 
the double jet processes heretofore reported for preparing high bromide 
ultrathin {111} tabular grain emulsions. All of the silver, halide and 
growth modifier can be present in the dispersing medium from the outset of 
grain growth. The volume of the reaction vessel can be constant and is 
almost always near constant throughout the growth process. The silver 
concentration levels can be relatively high. Water build up in the 
dispersing medium during the growth process does not occur and bromide ion 
concentration increases remain relatively small. A single reaction vessel 
can be employed for the growth process. Compared to the double jet 
procedures employed to prepare previously reported high bromide ultrathin 
{111} tabular grain emulsions it is apparent that the growth process of 
the invention is advantageous in allowing the use of simpler equipment, 
fewer controls, fewer and simpler manipulations, and the maintenance of 
higher silver concentrations in the dispersing medium, and in reducing 
halide ion effluent. Stated another way, all of the complexities (a) 
through (g) noted above can be either entirely obviated or significantly 
ameliorated. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
To satisfy the objective of a high bromide ultrathin {111} tabular grain 
emulsion with an average tabular grain aspect ratio of at least 5 as an 
end product the grain growth process of the invention can be practiced 
starting with any conventional high bromide silver halide emulsion in 
which the average grain thickness is less than 0.06 .mu.m. The starting 
emulsion can be either a tabular grain emulsion or a nontabular grain 
emulsion. 
In one application of the grain growth process of the invention a high 
bromide {111} tabular grain emulsion having a mean grain thickness of less 
than 0.06 .mu.m is chosen as a starting material. One practical incentive 
for discontinuing whatever conventional precipitation process that was 
employed to originate the starting tabular grain emulsion is that there 
are numerous conventional techniques for producing ultrathin tabular 
grains while the mean ECD of the grain population remains quite small, 
but, unfortunately, if grain growth is continued, the discrimination 
between surface and edge growth is insufficient to prevent tabular grain 
thickening beyond the ultrathin region. The grain growth process of the 
invention offers the advantage, demonstrated in the Examples below, that 
tabular grain ECD can be increased at a much higher rate than the 
thickness of the tabular grains. Under even the most adverse conditions an 
incremental increase in the ECD of the tabular grains at least 10 times 
greater than the incremental increase of their thickness can be realized. 
That is, at least a 0.1 .mu.m increase in ECD can be realized by the 
growth process of the invention before a 0.01 .mu.m increase in tabular 
grain thickness occurs. In fact, as demonstrated in the Examples below, 
extremely large increases in mean ECD in starting tabular grains can be 
realized while maintaining thickness increases well below 0.01 .mu.m. From 
these demonstrations it is apparent that, if the starting tabular grains 
have an average thickness of less than 0.06 .mu.m, it is possible to 
increase their mean ECD to any useful size. That is, mean ECD can be 
increased to 5 .mu.m or even to the 10 .mu.m commonly accepted maximum 
mean ECD useful limit for photographic purposes without exceeding the 
ultrathin average thickness limit of &lt;0.07 .mu.m. Since the grain growth 
process of the invention has the effect of increasing the percentage of 
total grain projected area accounted for by tabular grains, any high 
bromide tabular grain starting emulsion can be employed that satisfies the 
minimum projected area to satisfy the tabular grain emulsion definition 
(i.e., tabular grains accounting for at least 50 percent of total grain 
projected area). 
To provide a specific illustration of how the grain growth process of the 
invention can be applied, attention is directed to Tsaur et al U.S. Pat. 
No. 5,210,013, which discloses the preparation of high bromide {111} 
tabular grain emulsions in which the COV is less than 10 percent and 
substantially all of the grain projected area is accounted for by tabular 
grains. Unfortunately, the process of preparation employed by Tsaur et al 
thickens the tabular grains. A minimum mean tabular grain thickness of 
0.08 .mu.m is disclosed. By initiating tabular grain emulsion preparation 
employing the process of Tsaur et al and then completing grain growth with 
the process of the present invention it is possible to initiate tabular 
grain preparation as taught by Tsaur et al while still obtaining an 
ultrathin tabular grain emulsion. 
Another preferred approach that, together with the approach above, 
illustrates the breadth of the invention is to choose as a starting 
emulsion for the grain growth process a high bromide Lippmann emulsion. 
The term "Lippmann emulsion" has historically been applied to emulsions in 
which the grain sizes are too small to scatter visible light. Thus, the 
emulsions are visually identifiable in coatings as being nonturbid. A 
typical Lippmann emulsion grain size is around 500.ANG. or less. The grain 
population is, of course, entirely nontabular. The Examples below 
demonstrate the practice of the invention starting with the precipitation 
of a Lippmann emulsion. 
Having demonstrated the extremes of the starting grain populations to which 
the grain growth process can be applied, it is apparent that the grain 
growth process of the invention can also be practiced with intermediate 
starting emulsions. That is, so long as mean grain thickness remains less 
than 0.06 .mu.m, it is immaterial whether the grains in the starting 
emulsion are entirely nontabular (all grains having aspect ratios of less 
than 2), entirely tabular or a mixture of both. Conventional emulsion 
preparation processes that produce fine nontabular grains or ultrathin 
tabular grains can be employed without modification while precipitation 
processes that would otherwise produce grains exceeding the 0.06 .mu.m 
grain mean thickness parameter can simply be brought to an earlier 
termination to stay within this grain size limit. 
The grains provided by the starting emulsion can be pure bromide or can 
contain minor amounts of chloride and/or iodide. Silver chloride can be 
present in the high bromide starting grains in any concentration up to, 
but less than 50 mole percent. The incorporation of chloride in high 
bromide starting grains can be used to reduce native blue sensitivity and 
to increase photographic development rates. Preferred chloride ion 
concentration levels in the starting grains are less than 25 mole percent. 
The solubility limit of iodide ions in silver bromide varies, depending 
upon precipitation conditions, but is rarely greater than 40 mole percent, 
while typical iodide concentrations in photographic emulsions are less 
than 20 mole percent. Extremely low levels of iodide in silver bromide, as 
low as 0.01 mole percent, can produce detectable increases in photographic 
sensitivity. Since iodide slows photographic processing rates and is not 
required in high concentrations to enhance photographic sensitivity, it is 
usually preferred to limit the iodide content of the starting grains to 
less than 10 mole percent and, for rapid processing applications, to less 
than 5 mole percent. The starting grains can be silver bromide, silver 
iodobromide, silver chlorobromide, silver iodochlorobromide or silver 
chloroiodobromide grains, where halides are named in order of ascending 
concentrations. It is also possible to introduce each different halide in 
a separate grain population. For example, the iodide ions can be supplied 
by introducing with silver bromide grains a separate silver iodide 
Lippmann emulsion. As grain growth occurs grains emerge that contain the 
desired mixture of halides. By timing the addition of a separate halide it 
is also possible to control the profile of that halide within the grains 
being grown. 
The starting grains, apart from the required features described above, can 
take any convenient conventional form. 
Starting with a conventional high bromide emulsion of the type described 
above an aqueous dispersion is prepared containing at least 0.1 percent by 
weight silver, based on total weight, supplied by the starting emulsion. 
The weight of silver in the dispersing medium can range up to 20 percent 
by weight, based on total weight, but is preferably in the range of from 
0.5 to 10 percent by weight, based on the total weight of the dispersion. 
The aqueous dispersion also receives the water and peptizer that are 
present with the grains in the starting emulsion. The peptizer typically 
constitutes from about 1 to 6 percent by weight, based on the total weight 
of the aqueous dispersion. In the simplest mode of practicing the 
invention, the grain growth process of the invention is undertaken 
promptly upon completing precipitation of the starting grain emulsion, and 
only minimum required adjustments of the dispersing medium of the starting 
grain emulsion are undertaken to satisfy the aqueous dispersion 
requirements of the grain growth process. This is particularly 
advantageous where the starting grains are susceptible to ripening, as in 
a Lippmann emulsion. Where the stability of the precipitated starting 
grain population permits, intermediate steps, such as washing, prior to 
commencing the grain growth process are not precluded. 
The pH of the aqueous dispersion employed in the grain growth process is in 
the range of from 2 to 8, preferably 3 to 7. Adjustment of pH, if 
required, can be undertaken using a strong mineral base, such as an alkali 
hydroxide, or a strong mineral acid, such as nitric acid or sulfuric acid. 
If the pH is adjusted to the basic side of neutrality, the use of ammonium 
hydroxide should be avoided, since under alkaline conditions the ammonium 
ion acts as a ripening agent and will increase grain thickness. 
To minimize the risk of elevated minimum densities in the emulsions 
prepared, it is common practice to prepare photographic emulsions with a 
slight stoichiometric excess of bromide ion present. At equilibrium the 
following relationship exists: 
EQU -log K.sub.sp =pBr+pAg (I) 
where 
K.sub.sp is the solubility product constant of silver bromide; 
pBr is the negative logarithm of bromide ion activity; and 
pAg is the negative logarithm of silver ion activity. 
The solubility product constant of silver bromide emulsions in the 
temperature range of from 0.degree. to 100.degree. C. has been published 
by Mees and James The Theory of the Photographic Process, 3th Ed., 
Macmillan, N.Y., 1966, page 6. The equivalence point, pBr=pAg=-log 
K.sub.sp +2, which is the point at which no stoichiometric excess of 
bromide ion is present in the aqueous dispersion, is known from the 
solubility product constant. By employing a reference electrode and a 
sensing electrode, such as a silver ion or bromide ion sensing electrode 
or both, it is possible to determine from the potential measurement of the 
aqueous dispersion its bromide ion content (pBr). Lin et al U.S. Pat. No. 
5,317,521 is cited to show electrode selections and techniques for 
monitoring pBr. To avoid unnecessarily high bromide ion concentrations in 
the aqueous dispersion (and hence unnecessary waste of materials) the pBr 
of the aqueous dispersion is adjusted to at least 1.5, preferably at least 
2.0 and optimally greater than 2.6. Soluble bromide salt (e.g. alkali 
bromide) addition can be used to decrease pBr while soluble silver salt 
(e.g. silver nitrate) additions can be used to increase pBr. 
To the aqueous dispersion, either before, during or following the pBr and 
pH adjustments indicated, is added an 8-hydroxyquinoline containing at 
least one iodo substituent, hereinafter also referred to as 
iodo-substituted 8 hydroxyquiline or iodo-8hydroxyquinoline. 
The required iodo substituent can occupy any synthetically convenient ring 
position of the 8-hydroxyquinolines. When the 8-hydroxyquinoline ring is 
not otherwise substituted, the most active sites for introduction of a 
single iodo substituent are the 5 and 7 ring positions, with the 7 ring 
position being the preferred substitution site. Thus, when the 
8-hydroxyquinoline contains two iodo substituents, they are typically 
located at the 5 and 7 ring positions. When the 5 and 7 ring positions 
have been previously substituted, iodo substitution can take place at 
other ring positions. 
Further ring substitutions are not required, but can occur at any of the 
remaining ring positions. Strongly electron withdrawing substituents, such 
as other halides, pseudohalides (e.g., cyano, thiocyanato, isocyanato, 
etc.), carboxy (including the free acid, its salt or an ester), sulfo 
(including the free acid, its salt or an ester), .alpha.-haloalkyl, and 
the like, and mildly electron withdrawing or electron donating 
substituents, such as alkyl, alkoxy, aryl and the like, are common at a 
variety of ring positions on both of the fused rings of the 
8-hydroxyquinolines. 
Polar substituents, such as the carboxy and sulfo groups, can perform the 
advantageous function of increasing the solubility of the iodo-substituted 
8-hydroxyquinoline in the aqueous dispersing media employed for emulsion 
precipitation. 
In one specifically preferred form the iodo-8-hydroxyquinolines satisfy the 
following formula: 
##STR1## 
where R.sup.1 and R.sup.2 are chosen from among hydrogen, polar 
substituents, particularly carboxy and sulfo substituents, and strongly 
electron withdrawing substituents, particularly halo and pseudohalo 
substituents, with the proviso that at least one of R.sup.1 and R.sup.2 is 
iodo. 
The following constitute specific illustrations of iodo-substituted 
8-hydroxyquinoline grain growth modifiers contemplated for use in the 
practice of the invention: 
______________________________________ 
GGM-1 5-Chloro-8-hyroxy-7-iodoquinoline 
GGM-2 8-Hydroxy-7-iodo-2-methylquinoline 
GGM-3 4-Ethyl-8-hydroxy-7-iodoquinoline 
GGM-4 5-Bromo-8-hydroxy-7-iodoquinoline 
GGM-5 5,7-Diiodo-8-hydroxyquinoline 
GGM-6 8-Hydroxy-7-iodo-5-quinolinesulfonic 
acid 
GGM-7 8-Hydroxy-7-iodo-5-quinolinecarboxylic 
acid 
GGM-8 8-Hydroxy-7-iodo-5-iodomethylquinoline 
GGM-9 8-Hydroxy-7-iodo-5-trichloromethyl- 
quinoline 
GGM-10 .alpha.-(8-Hydroxy-7-iodoquinoline)acetic 
acid 
GGM-11 7-Cyano-8-hydroxy-5-iodoquinoline 
GGM-12 8-Hydroxy-7-iodo-5-isocyanatoquinoline 
______________________________________ 
It is believed that the effectiveness of the grain growth modifier is 
attributable to its preferential absorption to the major faces of {111} 
tabular grains and its ability to preclude additional silver halide 
deposition on these surfaces. Actual observations indicate that the 
interactions between the various grain surfaces present in the aqueous 
dispersion and the grain growth modifier are, in fact, complex. For 
example, it is not understood why double jet precipitations employing the 
grain growth modifier are less effective than the grain growth process of 
the invention. Contemplated concentrations of the grain growth modifier 
for use in the grain growth process of the invention range from 0.1 to 500 
millimoles per silver mole. A preferred grain growth modifier 
concentration is from 0.4 to 200 millimoles per silver mole, and an 
optimum grain growth modifier concentration is from 4 to 100 millimoles 
per silver mole. 
Once the grain growth modifier has been introduced into the aqueous 
dispersion a high bromide ultrathin {111} tabular grain emulsion having an 
average tabular grain aspect ratio of at least 5 is produced by holding 
the aqueous dispersion at any convenient temperature known to be 
compatible with grain ripening. This can range from about 40.degree. C. up 
to the highest temperatures conveniently employed in silver halide 
emulsion preparation, typically up to about 90.degree. C. A preferred 
holding temperature is in the range of from about 40.degree. to 80.degree. 
C. 
The holding period will vary widely, depending upon the starting grain 
population, the temperature of holding and the objective sought to be 
maintained. For example, starting with a high bromide ultrathin {111} 
tabular grain emulsion to provide the starting grain population with the 
objective of increasing mean ECD by a minimum 0.1 .mu.m, a holding period 
of no more than a few minutes may be necessary in the 50.degree. to 
60.degree. C. temperature range, with even shorter holding times being 
feasible at increased holding temperatures. In this instance virtually all 
of the tabular grains present in the starting emulsion act as seed grains 
for further grain growth and survive the holding period. On the other 
hand, if the starting grain population consists entirely of fine grains 
and the intention is to continue the growth process until no fine grains 
remain as such in the emulsion, holding periods can range from few minutes 
at the highest contemplated holding temperatures to overnight (16 to 24 
hours) at 40.degree. C. In this instance a small fraction of the fine 
grains present in the starting emulsion act as seed grains for the growth 
of tabular grains while the remainder of the grains are ripened out onto 
the seed grains. The holding period is generally comparable to run times 
employed in preparing high bromide ultrathin {111} tabular grain emulsions 
by double jet precipitation techniques when the temperatures employed are 
similar. The holding period can be shortened by the introduction into the 
aqueous dispersion of a ripening agent of a type known to be compatible 
with obtaining thin (less than 0.2 .mu.m mean grain thickness) tabular 
grain emulsions, such as thiocyanate or thioether ripening agents. 
The grain growth process of the present invention is capable of providing 
high bromide ultrathin {111} tabular grain emulsions having precisely 
selected mean ECD's and average tabular grain aspect ratios. The emulsions 
produced by the process of the invention typically have average aspect 
ratios of greater than 8 and, in specifically preferred forms, at least 
12. The emulsions can also exhibit high levels of grain uniformity. 
Attaining emulsions in which the tabular grains account for greater than 
70 percent of total grain projected area can be readily realized and, with 
typical starting grain populations, tabular grain projected areas 
accounting for greater than 90 percent of total grain projected area have 
been realized. 
During their preparation and subsequently conventional adjustments of the 
photographic emulsions can be undertaken. Convenional features are 
summarized in Research Disclosure, Vol. 308, December 1989, Item 308119, 
the disclosure of which is here incorporated by reference. Research 
Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 
12 North St., Emsworth, Hampshire P010 7DQ, England. 
Examples 
The invention can be better appreciated by reference to the following 
specific embodiments. 
Example 1 
AgBr Ultrathin Tabular Grain Emulsion 
To a vigorously stirred reaction vessel containing 50 g oxidized gelatin 
and 2L distilled water at 25.degree. C. were added 300 mL of 2M AgNO.sub.3 
solution at a rate of 300 mL per min using two pumps and a 12-hole ring 
outlet. A 2M NaBr solution was simultaneously added at a rate needed to 
maintain a pBr of 3.82 using two pumps and a 12-hole ring outlet. The 
silver and bromide introducing ring outlets were mounted above and below a 
rotated stirring head, respectively. 
To 90 g of the resulting emulsion at 25.degree. C. were added 2 mL of a 
dimethylformamide solution containing a total of 4 mole per mole silver of 
5,7- diiodo-8-hydroxyquinoline. The temperature was increased to 
40.degree. C. Then the pH was adjusted to 5.0 and the pBr to 3.38. The 
mixture was heated to 60.degree. C. and the pH was adjusted to 5.0 and the 
pBr to 3.08. The emulsion was heated for 4 hr at 60.degree. C., resulting 
in an AgBr ultrathin {111} tabular grain emulsion. 
The resulting emulsion contained tabular grains having an average ECD of 
2.5 .mu.m, an average thickness of 0.05 .mu.m, and an average aspect ratio 
of 50. The tabular grains accounted for greater than 95% of the total 
projected area of the emulsion grains. The emulsion is shown in FIG. 1. 
The emulsion is listed in Table I for ease of comparison. 
Example 2 
AgBr Ultrathin Tabular Grain Emulsion 
This example was made similar to that of Example 1, except that the pH of 
the heated fine grains was maintained at 4.0. 
The resulting emulsion contained tabular grains having an average ECD of 
1.8 .mu.m, an average thickness of 0.045 .mu.m, and an average aspect 
ratio of 40. Tabular grains accounted for approximately 90% of the total 
grain projected area. The emulsion is listed in Table I for ease of 
comparison. 
Example 3 
AgBr Ultrathin Tabular Grain Emulsion 
To a vigorously stirred reaction vessel containing 50 g oxidized gelatin 
and 2 L distilled water at 25.degree. C. was added 300 mL of 2M AgNO.sub.3 
solution at a rate of 300 mL per min. using two pumps and a 12-hole ring 
outlet. A 2M NaBr solution was simultaneously added at a rate needed to 
maintain a pBr of 3.82 using two pumps and a 12-hole ring outlet. The ring 
outlets were mounted above and below a rotated stirring head as in Example 
1. 
To 90 g of the resulting fine grain emulsion at 25.degree. C. were added 8 
mL of an aqueous solution containing 4 mmole per mole Ag of 
8-hydroxy-7-iodo-5-quinolinesulfonic acid. The temperature of the emulsion 
was increased to 40.degree. C., then the pH was adjusted to 4.0 and the 
pBr to 3.38. The mixture was heated to 60.degree. C., and the pH and pBr 
were adjusted to 4.0 and 3.08, respectively. The emulsion was then held 
for 4 hours, resulting in a tabular grain emulsion. 
The resulting emulsion consisted of tabular grains having an average 
diameter of 2.0 .mu.m, an average thickness of 0.05 .mu.m, and an average 
aspect ratio of 40. The tabular grains accounted for 90 percent of total 
grain projected area. The emulsion is listed in Table I for ease of 
comparison. 
Comparative Example 4 
Emulsion A. Fine Grain AgBr Emulsion 
To a stirred reaction vessel containing 2 L of 5 wt % gelatin at 35.degree. 
C. were added 2M AgN.sub.3 solution and 2M NaBr solution. The AgNO.sub.3 
solution was added at 300 mL/min, and the NaBr solution was added as 
needed to maintain a pBr of 3.63. A total of 0.6 mole of AgNO.sub.3 was 
added. 
Emulsion B. AgBr Tabular Seed Grain Emulsion 
To a stirred reaction vessel containing 7.5 g of oxidized gelatin, 1.39 g 
NaBr, and distilled water to 2 L at 35.degree. C. and pH 2.0, 10 mL of 2M 
AgN.sub.3 solution were added at 50 mL/min. Concurrently, 2M NaBr solution 
was added to maintain a pBr of 2.21. The temperature was increased to 
60.degree. C. at a rate of 5.degree. C. per 3 min. Then 150 mL of a 33% by 
weight oxidized gelatin solution at 60.degree. C. were added. The pH was 
adjusted to 6.0, and 14 mL of a 2M NaBr solution were added. At 60.degree. 
C. and pH 06.0, 500 mL of a 2M AgNO.sub.3 solution were added at 20 
mL/min. Concurrently, 2M NaBr solution was added to maintain a pBr of 
1.76. The resulting tabular grain seeds were 1.3 .mu.m in diameter and 
0.04 .mu.m in thickness. 
Testing Potential Tabular Grain Growth Modifiers 
At 40.degree. C. to 0.021 mole Emulsion A was added with stirring 0.0032 
mole Emulsion B. The pBr was adjusted to 3.55. A solution of the potential 
tabular grain growth modifier was added in the amount of 7.0 mmole/mole 
Ag. The mixture was adjusted to a pH of 6.0 then heated to 70.degree. C. 
and the pH was again adjusted to 6.0. After heating for 17 hr at 
70.degree. C., the resulting emulsions were examined for ultrathin tabular 
grains by optical and electron microscopy to determine mean grain diameter 
and thickness. The compounds tested for utility as grain growth modifiers 
in the production of ultrathin grains and the results are provided in 
Table I. 
TABLE I 
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Average {111} 
% Projected Area 
% Projected Area 
Potential Tabular Grain 
Tabular Grain 
as Nontabular 
as {111} Tabular 
Emulsion 
Growth Modifier 
Dimensions (.mu.m) 
Grains Grains 
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Example 1 
5,7-diiodo-8-hydroxy 
2.5 .times. 0.05 
&lt;5% &gt;95% 
quinoline 
Example 2 
5,7-diiodo-8-hydroxy 
1.8 .times. 0.045 
10% 90% 
quinoline 
Example 3 
8-hydroxy-7-iodo-5- 
2.0 .times. 0.05 
10% 90% 
quinolinesulfonic acid 
Control 4A 
none 1.7 .times. 0.18 
40% 60% 
Control 4B 
adenine None 100% 0% 
Control 4C 
4,5,6-triaminopyrimidine 
4.3 .times. 0.042 
&lt;5% &gt;95% 
Control 4D 
xanthine 1.3 .times. 0.20 
60% 40% 
Control 4E 
4-aminopyrazolo [3,4-d]- 
2.0 .times. 0.20 
10% 90% 
pyrimidine 
__________________________________________________________________________ 
As the above results show, only Example emulsions 1, 2 and 3 and Control 
Emulsion 4C (4,5,6-triaminopyrimidine) yielded an ultrathin tabular grain 
emulsion. Control Emulsion 4A, with no added tabular grain growth 
modifier, resulting in only minor lateral growth and significant thickness 
growth. Control 4B (adenine) yielded nontabular grains, including large 
grains lacking {111} major faces, shown in FIG. 2. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.