Process for the preparation of high chloride tabular grain emulsions (III)

A process of preparing a radiation sensitive high chloride high aspect ratio tabular grain emulsion is disclosed wherein silver ion is introduced into a gelatino-peptizer dispersing medium containing a stoichiometric excess of chloride ions with respect to the silver ions further characterized by a chloride ion concentration of less than 0.5 molar and a grain growth modifier of the formula: ##STR1## where Z.sup.8 is --C(R.sup.8).dbd. or --N.dbd.; PA0 R.sup.8 is H, NH.sub.2 or CH.sub.3 ; and PA0 R.sup.1 is hydrogen or a hydrocarbon of from 1 to 7 carbon atoms.

FIELD OF THE INVENTION 
The invention relates to the precipitation of radiation sensitive silver 
halide emulsions useful in photography. 
BACKGROUND OF THE INVENTION 
Radiation sensitive silver halide emulsions containing one or a combination 
of chloride, bromide and iodide ions have been long recognized to be 
useful in photography. Each halide ion selection is known to impart 
particular photographic advantages. Although known and used for many years 
for selected photographic applications, the more rapid developability and 
the ecological advantages of high chloride emulsions have provided an 
impetus for employing these emulsions over a broader range of photographic 
applications. As employed herein the term "high chloride emulsion" refers 
to a silver halide emulsion containing at least 50 mole percent chloride 
and less than 5 mole percent iodide, based on total silver. 
During the 1980's a marked advance took place in silver halide photography 
based on the discovery that a wide range of photographic advantages, such 
as improved speed-granularity relationships, increased covering power both 
on an absolute basis and as a function of binder hardening, more rapid 
developability, increased thermal stability, increased separation of 
native and spectral sensitization imparted imaging speeds, and improved 
image sharpness in both mono- and multi-emulsion layer formats, can be 
realized by increasing the proportions of selected tabular grain 
populations in photographic emulsions. 
The various photographic advantages were associated with achieving high 
aspect ratio tabular grain emulsions. As herein employed and as normally 
employed in the art, the term "high aspect ratio tabular grain emulsion" 
has been defined as a photographic emulsion in which tabular grains having 
a thickness of less than 0.3 .mu.m and an average aspect ratio of greater 
than 8:1 account for at least 50 percent of the total grain projected area 
of emulsion. Aspect ratio is the ratio of tabular grain effective circular 
diameter (ECD), divided by tabular grain thickness (t). 
Although the art has succeeded in preparing high chloride tabular grain 
emulsions, the inclusion of high levels of chloride as opposed to bromide, 
alone or in combination with iodide, has been difficult. The basic reason 
is that tabular grains are produced by incorporating parallel twin planes 
in grains grown under conditions favoring {111} crystal faces. The most 
prominent feature of tabular grains are their parallel {111} major crystal 
faces. 
To produce successfully a high chloride tabular grain emulsion two 
obstacles must be overcome. First, conditions must be found that 
incorporate parallel twin planes into the grains. Second, the strong 
propensity of silver chloride to produce {100} crystal faces must be 
overcome by finding conditions that favor the formation of {111} crystal 
faces. 
Wey U.S. Pat. No. 4,399,215 produced the first silver chloride high aspect 
ratio (ECD/t&gt;8) tabular grain emulsion. An ammoniacal double-jet 
precipitation technique was employed. The tabularity of the emulsions was 
not high compared to contemporaneous silver bromide and bromoiodide 
tabular grain emulsions because the ammonia thickened the tabular grains. 
A further disadvantage was that significant reductions in tabularity 
occurred when bromide and/or iodide ions were included in the tabular 
grains. 
Wey et al U.S. Pat. No. 4,414,306 developed a process for preparing silver 
chlorobromide emulsions containing up to 40 mole percent chloride based on 
total silver. This process of preparation has not been successfully 
extended to high chloride emulsions. 
Maskasky U.S. Pat. No. 4,400,463 (hereinafter designated Maskasky I) 
developed a strategy for preparing a high chloride, high aspect ratio 
tabular grain emulsion capable of tolerating significant inclusions of the 
other halides. The strategy was to use a particularly selected synthetic 
polymeric peptizer in combination with a grain growth modifier having as 
its function to promote the formation of {111} crystal faces. Adsorbed 
aminoazaindenes, preferably adenine, and iodide ions were disclosed to be 
useful grain growth modifiers. The principal disadvantage of this approach 
has been the necessity of employing a synthetic peptizer as opposed to the 
gelatino-peptizers almost universally employed in photographic emulsions. 
This work has stimulated further investigations of grain growth modifiers 
for preparing tabular grain high chloride emulsions, as illustrated by 
Takada et al U.S. Pat. No. 4,783,398, which employs heterocycles 
containing a divalent sulfur ring atom; Nishikawa et al U.S. Pat. No. 
4,952,491, which employs spectral sensitizing dyes and divalent sulfur 
atom containing heterocycles and acyclic compounds; and Ishiguro et al 
U.S. Pat. No. 4,983,508, which employs organic bis-quaternary amine salts. 
Maskasky U.S. Pat. No. 4,713,323 (hereinafter designated Maskasky II), 
continuing to use aminoazaindene growth modifiers, particularly adenine, 
discovered that tabular grain high chloride emulsions could be prepared by 
running silver salt into a dispersing medium containing at least a 0.5 
molar concentration of chloride ion and an oxidized gelatino-peptizer. An 
oxidized gelatino-peptizer is a gelatino-peptizer treated with a strong 
oxidizing agent to modify by oxidation (and eliminate or reduce as such) 
the methionine content of the peptizer. Maskasky II taught to reduce the 
methionine content of the peptizer to a level of less than 30 micromoles 
per gram. King et al U.S. Pat. No. 4,942,120 is essentially cumulative, 
differing only in that methionine was modified by alkylation. 
While Maskasky II overcame the synthetic peptizer disadvantage of Maskasky 
I, the requirement of a chloride ion concentration of at least 0.5 molar 
in the dispersing medium during precipitation presents disadvantages. At 
the elevated temperatures typically employed for emulsion precipitations 
using gelatino-peptizers, the high chloride ion concentrations corrode the 
stainless steel vessels used for the preparation of photographic 
emulsions. Additionally, the high chloride ion concentrations increase the 
amount of emulsion washing required after precipitation, and disposal of 
the increased levels of chloride ion represents increased consumption of 
materials and an increased ecological burden. 
Tufano et al U.S. Pat. No. 4,804,621 disclosed a process for preparing high 
aspect ratio tabular grain high chloride emulsions in a gelatino-peptizer. 
Tufano et al taught that over a wide range of chloride ion concentrations 
ranging from pCl 0 to 3 (1 to 1.times.10.sup.-3 M) 4,6-diaminopyrimidines 
satisfying specific structural requirements were effective growth 
modifiers for producing high chloride tabular grain emulsions. Tufano et 
al specifically required that the following structural formula be 
satisfied: 
##STR2## 
wherein Z is C or N; R.sub.1, R.sub.2 and R.sub.3, which may be the same 
or different, are H or alkyl of 1 to 5 carbon atoms; Z is C, R.sub.2 and 
R.sub.3 when taken together can be --CR.sub.4 .dbd.CR.sub.5 --or 
--CR.sub.4 .dbd.N--, wherein R.sub.4 and R.sub.5, which may be the same or 
different are H or alkyl of 1 to 5 carbon atoms, with the proviso that 
when R.sub.2 and R.sub.3 taken together form the --CR.sub.4 .dbd.N-- 
linkage, --CR.sub.4 .dbd. must be joined to Z. Tufano et al also 
contemplated salts of the formula compound. Tufano et al demonstrated the 
failure of adenine as a growth modifier. Thus, Tufano et al discourages 
the selection of heterocycles for use as grain growth modifiers that lack 
two primary or secondary amino ring substituents in the indicated 
relationship to the pyrimidine ring nitrogen atoms and those compounds 
that contain a nitrogen atom linked to the 5-position of the pyrimidine 
ring. 
RELATED PATENT APPLICATIONS 
Maskasky U.S. Ser. No. 763,382, concurrently filed, now abandoned, and 
commonly assigned, titled IMPROVED PROCESS FOR THE PREATION OF HIGH 
CHLORIDE TABULAR GRAIN EMULSIONS (I), (hereinafter designated Maskasky 
III) discloses a process for preparing a high chloride tabular grain 
emulsion in which silver ion is introduced into a gelatino-peptizer 
dispersing medium containing a stoichiometric excess of chloride ions of 
less than 0.5 molar, a pH of at least 4.5, and a 
4,6-di(hydroamino)-5-aminopyrimidine grain growth modifier. 
Maskasky U.S. Ser. No. 762,971, concurrently filed and commonly assigned, 
titled IMPROVED PROCESS FOR THE PREATION OF HIGH CHLORIDE TABULAR GRAIN 
EMULSIONS (II), (hereinafter designated Maskasky IV) discloses a process 
for preparing a high chloride tabular grain emulsion in which silver ion 
is introduced into a gelatino-peptizer dispersing medium containing a 
stoichiometric excess of chloride ions of less than 0.5 molar and a grain 
growth modifier of the formula: 
##STR3## 
where 
Z.sup.2 is --C(R.sup.2).dbd. or --N.dbd.; 
Z.sup.3 is --C(R.sup.3).dbd. or --N.dbd.; 
Z.sup.4 is --C(R.sup.4).dbd. or --N.dbd.; 
Z.sup.5 is --C(R.sup.5).dbd. or --N.dbd.; 
Z.sup.6 is --C(R.sup.6).dbd. or --N.dbd.; 
with the proviso that no more than one of Z.sup.4, Z.sup.5 and Z.sup.6 is 
--N.dbd.; 
R.sup.2 is H, NH.sub.2 or CH.sub.3 ; 
R.sup.3, R.sup.4 and R.sup.5 are independently selected, R.sup.3 and 
R.sup.5 being hydrogen, hydrogen, halogen, amino or hydrocarbon and 
R.sup.4 being hydrogen, halogen or hydrocarbon, each hydrocarbon moiety 
containing from 1 to 7 carbon atoms; and 
R.sup.6 is H or NH.sub.2. 
Maskasky U.S. Ser. No. 763,030, concurrently filed and commonly assigned, 
titled ULTRATHIN HIGH CHLORIDE TABULAR GRAIN EMULSIONS, (hereinafter 
designated Maskasky V) discloses a high chloride tabular grain emulsion in 
which greater than 50 percent of the total grain projected area is 
accounted for by ultrathin tabular grains having a thickness of less than 
360 {111} crystal lattice planes. A {111} crystal face stabilizer is 
adsorbed to the major faces of the ultrathin tabular grains. 
SUMMARY OF THE INVENTION 
In one aspect, this invention is directed to a process of preparing a 
radiation sensitive high aspect ratio tabular grain emulsion, wherein 
tabular grains of less than 0.3 .mu.m in thickness and an average aspect 
ratio of greater than 8:1 account for greater than 50 percent of the total 
grain projected area, the tabular grains containing at least 50 mole 
percent chloride, based on silver, comprising introducing silver ion into 
a gelatino-peptizer dispersing medium containing a stoichiometric excess 
of chloride ions a chloride ion concentration of less than 0.5 molar and a 
grain growth modifier of the formula: 
##STR4## 
where 
Z.sup.8 is --C(R.sup.8).dbd. or --N.dbd.; 
R.sub.8 is H, NH.sub.2 or CH.sub.3 ; and 
R.sup.1 is hydrogen or a hydrocarbon containing from 1 to 7 carbon atoms. 
It has been discovered quite unexpectedly that a novel class of grain 
growth modifiers are capable of producing high chloride tabular grain 
emulsions at unexpectedly low stoichiometric levels of excess chloride 
ion. The lowered stoichiometric excess of chloride ion avoids the 
corrosion, increased washing, materials consumption and ecological burden 
concerns inherent in the Maskasky II process. The disadvantage of Maskasky 
I of requiring a synthetic peptizer is also avoided. At the same time, 
xanthines and 8-azaxanthines, a whole new class of grain growth modifiers 
are recognized to be useful. Thus, the process of the invention provides a 
practical and attractive preparation of high chloride tabular grain 
emulsions.

In FIG. 1 the emulsion is viewed perpendicular to the support, and in FIG. 
2 the emulsion is viewed at a declination of 60.degree. from the 
perpendicular and at high level of magnification. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In preferred embodiments the processes of preparing high chloride high 
aspect ratio tabular grain emulsions of this invention employ a novel 
class of grain growth modifiers satisfying the formula: 
##STR5## 
where 
Z.sup.8 is --C(R.sup.8).dbd. or --N.dbd.; 
R.sup.8 is H, NH.sub.2 or CH.sub.3 ; and 
R.sup.1 is hydrogen or a hydrocarbon of from 1 to 7 carbon atoms. 
The grain growth modifiers of formula I are hereinafter referred to 
generically as xanthine and 8-azaxanthine grain growth modifiers. 
When the grain growth modifier is chosen to have a xanthine nucleus, the 
structure of the grain growth modifier is as shown in the following 
formula: 
##STR6## 
When the grain growth modifier is chosen to have an 8azaxanthine nucleus, 
the structure of the grain growth modifier is as shown in the following 
formula: 
##STR7## 
No substituents of any type are required on the ring structures of formulae 
I to III. Thus, each of R.sup.1 and R.sup.8 can in each occurrence be 
hydrogen. R.sup.8 can in addition include a sterically compact hydrocarbon 
substituent, such as CH.sub.3 or NH.sub.2. R.sup.1 can additionally 
include a hydrocarbon substituent of from 1 to 7 carbon atoms. Each 
hydrocarbon moiety is preferably an alkyl group--e.g., methyl, ethyl, 
n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, etc. , although other 
hydrocarbons, such as cyclohexyl or benzyl, are contemplated. To increase 
grain growth modifier solubility the hydrocarbon groups can, in turn, be 
substituted with polar groups, such as hydroxy, sulfonyl or amino groups, 
or the hydrocarbon groups can be substituted with other groups that do not 
materially modify their properties (e.g., a halo substituent), if desired. 
An aqueous gelatino-peptizer dispersing medium is present during 
precipitation. Gelatinopeptizers include gelatin--e.g., alkali-treated 
gelatin (cattle bone and hide gelatin) or acid-treated gelatin (pigskin 
gelatin) and gelatin derivatives--e.g., acetylated gelatin, phthalated 
gelatin, and the like. 
The process of the invention is not restricted to use with 
gelatino-peptizers of any particular methionine content. That is, 
gelatinopeptizers with all naturally occurring methionine levels are 
useful. It is, of course, possible, though not required, to reduce or 
eliminate methionine, as taught by Maskasky II or King et al, both cited 
above and here incorporated by reference. 
During the precipitation of photographic silver halide emulsions there is 
always a slight stoichiometric excess of halide ion present. This avoids 
the possibility of excess silver ion being reduced to metallic silver and 
resulting in photographic fog. It is a significant advantage of this 
invention that the stoichiometric excess of chloride ion in the dispersing 
medium can be maintained at a chloride concentration of less than 0.5 M 
while still obtaining a high aspect ratio tabular grain emulsion. It is 
generally preferred that the chloride ion concentration in the dispersing 
medium be less than 0.2 M and, optimally, equal to or less than 0.1 M. 
The advantages of limiting the stoichiometric excess of chloride ion 
present in the reaction vessel during precipitation include (a) reduction 
of corrosion of the equipment (the reaction vessel, the stirring 
mechanism, the feed jets, etc.), (b) reduced consumption of chloride ion, 
(c) reduced washing of the emulsion after preparation, and (d) reduced 
chloride ion in effluent. It has also been observed that reduction in the 
chloride ion excess contributes to obtaining thinner tabular grains. 
The grain growth modifiers of the invention are effective over a wide range 
of pH levels conventionally employed during the precipitation of silver 
halide emulsions. It is contemplated to maintain the dispersing medium 
within conventional pH ranges for silver halide precipitation, typically 
from 3 to 9, while the tabular grains are being formed, with a pH range of 
4.5 to 8 being in most instances preferred. Within these pH ranges optimum 
performance of individual grain growth modifiers can be observed as a 
function of their specific structure. A strong mineral acid, such as 
nitric acid or sulfuric acid, or a strong mineral base, such as an alkali 
hydroxide, can be employed to adjust pH within a selected range. When a 
basic pH is to be maintained, it is preferred not to employ ammonium 
hydroxide, since it has the unwanted effect of acting as a ripening agent 
and is known to thicken tabular grains. However, to the extent that 
thickening of the tabular grains does not exceed the 0.3 .mu.m thickness 
limit, ammonium hydroxide or other conventional ripening agents (e.g., 
thioether or thiocyanate ripening agents) can be present within the 
dispersing medium. 
Any convenient conventional approach of monitoring and maintaining 
replicable pH profiles during repeated precipitations can be employed 
(e.g., refer to Research Disclosure Item 308,119, cited below). 
Maintaining a pH buffer in the dispersing medium during precipitation 
arrests pH fluctuations and facilitates maintenance of pH within selected 
limited ranges. Exemplary useful buffers for maintaining relatively narrow 
pH limits within the ranges noted above include sodium or potassium 
acetate, phosphate, oxalate and phthalate as well as 
tris(hydroxymethyl)aminomethane. 
In forming high chloride high aspect ratio tabular grain emulsions, tabular 
grains containing at least 50 mole percent chloride, based on silver, and 
having a thickness of less than 0.3 .mu.m must account for greater than 50 
percent of the total grain projected area. In preferred emulsions the 
tabular grains having a thickness of less than 0.2 .mu.m account for at 
least 70 percent of the total grain projected area. 
For tabular grains to satisfy the projected area requirement it is 
necessary first to induce twinning in the grains as they are being formed, 
since only grains having two or more parallel twin planes will assume a 
tabular form. Second, after twinning has occurred, it is necessary to 
restrain precipitation onto the major {111} crystal faces of the tabular 
grains, since this has the effect of thickening the grains. The grain 
growth modifiers employed in the practice of this invention are effective 
during precipitation to produce an emulsion satisfying both the tabular 
grain thickness and projected area parameters noted above. 
It is believed that the effectiveness of the grain growth modifiers to 
induce twinning during precipitation results from the spacing of the 
required nitrogen atoms in the fused five and six membered heterocyclic 
rings and their ability to form silver salts. This can be better 
appreciated by reference to the following structure: 
##STR8## 
C. Cagnon et al, Inorganic Chem., 16:2469 (1977) reports a silver salt 
satisfying the nitrogen atom and silver pairing arrangement of formula IV 
and provides bond lengths establishing the spacing between the adjacent 
silver atoms of the formula. Based on the crystal structure of silver 
chloride revealed by X-ray diffraction it is believed that the resulting 
spacing between the silver ions is much closer to the nearest permissible 
spacing of silver ions in next adjacent {111} silver ion crystal lattice 
planes separated by a twin plane than the nearest spacing of silver ions 
in next adjacent {111} silver ion crystal lattice planes not separated by 
a twin plane. Thus, when one of the silver ions shown above is positioned 
during precipitation in a {111} silver ion crystal lattice plane, assuming 
a sterically compatible location (e.g., an edge, pit or coign position) is 
occupied, the remaining of the silver ions shown above favors a position 
in the next {111} silver ion crystal lattice plane that is permitted only 
if twinning occurs. The remaining silver atom of the growth modifier 
(together with other similarly situated growth modifier silver ions) acts 
to seed (enhance the probability of) a twin plane being formed and growing 
across the {111} crystal lattice face, thereby providing a permanent 
crystal feature essential for tabular grain formation. 
It is, of course, also important that the ring substituents next adjacent 
the ring nitrogen shown in formula IV be chosen to minimize any steric 
hindrance that would prevent the silver ions from having ready access to 
the {111} crystal lattice planes as they are being formed. A further 
consideration is to avoid substituents to the ring positions next adjacent 
the ring nitrogen shown that are strongly electron withdrawing, since this 
creates competition between the silver ions and the adjacent ring position 
for the .pi. electrons of the nitrogen atoms. When Z.sup.8 is --N.dbd. or 
--CH.dbd., an optimum structure for silver ion placement in the crystal 
lattice exists. When Z.sup.8 is --C(R.sup.8).dbd. and R.sup.8 is a compact 
substituent, as described above, twin plane formation is readily realized. 
In formula IV the ring positions separated from the ring nitrogen by an 
intervening ring position are not shown, these ring positions and their 
substituents are not viewed as significantly influencing twin plane 
formation. 
In addition to selecting substituents for their role in twin plane 
formation, they must also be selected for their compatibility with 
promoting the formation of {111} crystal faces during precipitation. By 
selecting substituents as described above the emergence of {100}, {110} 
and higher index crystal plane faces of the types described by Maskasky 
U.S. Pat. Nos. 4,643,966, 4,680,254, 4,680,255, 4,680,256 and 4,724,200, 
is avoided. In those instances in which a second grain growth modifier is 
relied upon to assure emergence of {111} crystal faces during 
precipitation, a broadened selection of substituents not affecting twin 
plane formation is specifically contemplated. 
It is generally recognized that introducing twin planes in the grains at a 
very early stage in their formation offers the capability of producing 
thinner tabular grains than can be achieved when twinning is delayed. For 
this reason it is usually preferred that the conditions within the 
dispersing medium prior to silver ion introduction at the outset of 
precipitation be chosen to favor twin plane formation. To facilitate twin 
plane formation it is contemplated to incorporate the grain growth 
modifier in the dispersing medium prior to silver ion addition in a 
concentration of at least 2.times.10.sup.-4 M, preferably at least 
5.times.10.sup.-4 M, and optimally at least 7.times.10.sup.-4 M. Generally 
little increase in twinning can be attributed to increasing the initial 
grain growth modifier concentration in the dispersing medium above 0.01 M. 
Higher initial grain growth modifier concentrations up to 0.05 M, 0.1 M or 
higher are not incompatible with the twinning function. The maximum growth 
modifier concentration in the dispersing medium is often limited by its 
solubility. It is contemplated to introduce into the dispersing medium 
growth modifier in excess of that which can be initially dissolved. Any 
undissolved growth modifier can provide a source of additional growth 
modifier solute during precipitation, thereby stabilizing growth modifier 
concentrations within the ranges noted above. It is preferred to avoid 
quantities of grain growth modifier in excess of those observed to control 
favorably tabular grain parameters. 
Once a stable multiply twinned grain population has been formed within the 
dispersing medium, the primary, if not exclusive, function the grain 
growth modifier is called upon to perform is to restrain precipitation 
onto the major {111} crystal faces of the tabular grains, thereby 
retarding thickness growth of the tabular grains. In a well controlled 
tabular grain emulsion precipitation, once a stable population of multiply 
twinned grains has been produced, tabular grain thicknesses can be held 
essentially constant. 
The amount of grain growth modifier required to control thickness growth of 
the tabular grain population is a function of the total grain surface 
area. By adsorption onto the {111} surfaces of the tabular grains the 
grain growth modifier restrains precipitation onto the grain faces and 
shifts further growth of the tabular grains to their edges. 
The benefits of this invention can be realized using any amount of grain 
growth modifier that is effective to retard thickness growth of the 
tabular grains. It is generally contemplated to have present in the 
emulsion during tabular grain growth sufficient grain growth modifier to 
provide a monomolecular adsorbed layer over at least 25 percent, 
preferably at least 50 percent, of the total {111} grain surface area of 
the emulsion grains. Higher amounts of adsorbed grain growth modifier are, 
of course, feasible Adsorbed grain growth modifier coverages of 80 percent 
of monomolecular layer coverage or even 100 percent are contemplated. In 
terms of tabular grain thickness control there is no significant advantage 
to be gained by increasing grain growth modifier coverages above these 
levels. Any excess grain growth modifier that remains unadsorbed is 
normally depleted in post-precipitation emulsion washing. 
Prior to introducing silver salt into the dispersing medium at the outset 
of the precipitation process, no grains are present in the dispersing 
medium, and the initial grain growth modifier concentrations in the 
dispersing medium are therefore more than adequate to provide the 
monomolecular coverage levels noted above as grains are initially formed. 
As tabular grain growth progresses it is a simple matter to add grain 
growth modifier, as needed, to maintain monomolecular coverages at desired 
levels, based on knowledge of amount of silver ion added and the 
geometrical forms of the grains being grown. If, as noted above, grain 
growth modifier has been initially added in excess of its solubility 
limit, undissolved grain growth modifier can enter solution as dissolved 
grain growth modifier is depleted from the dispersing medium by adsorption 
on grain surfaces. This can reduce or even eliminate any need to add grain 
growth modifier to the reaction vessel as grain growth progresses. 
The grain growth modifiers described above are capable of use during 
precipitation as the sole grain growth modifier. That is, these grain 
growth modifiers are capable of influencing both twinning and tabular 
grain growth to provide high chloride high aspect ratio tabular grain 
emulsions. 
It has been discovered that improvements in precipitation can be realized 
by employing a combination of grain growth modifiers in which the more 
tightly adsorbed of the grain growth modifiers is employed for tabular 
grain thickness growth reduction while the less tightly adsorbed of the 
grain growth modifiers is employed for twinning. Different grain growth 
modifiers of this invention can be employed in combination on this basis, 
with the less tightly adsorbed grain growth modifier being employed during 
grain twinning and the more tightly adsorbed grain growth modifier being 
present during grain growth following twinning. 
Instead of employing a grain growth modifier of this invention to perform 
each of the twinning and tabular grain thickness control functions, it is 
possible to employ another growth modifier to perform one of these two 
functions. 
It is specifically contemplated to employ during twinning or grain growth a 
grain growth modifier of the following structure: 
##STR9## 
wherein Z is C or N; R.sub.1, R.sub.2 and R.sub.3, which may be the same 
or different, are H or alkyl of 1 to 5 carbon atoms; Z is C, R.sub.2 and 
R.sub.3 when taken together can be --CR.sub.4 .dbd.CR.sub.5 -- or 
--CR.sub.4 .dbd.N--, wherein R.sub.4 and R.sub.5, which may be the same or 
different are H or alkyl of 1 to 5 carbon atoms, with the proviso that 
when R.sub.2 and R.sub.3 taken together form the --CR.sub.4 .dbd.N-- 
linkage, --CR.sub.4 .dbd. must be joined to Z. Grain growth modifiers of 
this type and conditions for their use are disclosed by Tufano et al, 
cited above, the disclosure of which is here incorporated by reference. 
It is also contemplated to employ during grain twinning or grain growth 
following twinning a grain growth modifier of the type disclosed by 
Maskasky III, cited above. These grain growth modifiers are effective when 
the dispersing medium is maintained at a pH in the range of from 4.6 to 9 
(preferably 5.0 to 8) and contains a stoichiometric excess of chloride 
ions of less than 0.5 molar. These grain growth modifiers are 
4,6-di(hydroamino)-5-aminopyrimidine grain growth modifiers, with 
preferred compounds satisfying the formula: 
##STR10## 
where 
N.sup.4, N.sup.5 and N.sup.6 are amino moieties independently containing 
hydrogen or hydrocarbon substituents of from 1 to 7 carbon atoms, with the 
proviso that the N.sup.5 amino moiety can share with each or either of 
N.sup.4 and N.sup.6 a common hydrocarbon substituent completing a five or 
six member heterocyclic ring. The grain growth modifiers of this formula 
when present during grain twinning are capable of producing ultrathin 
tabular grain emulsions. 
It is also contemplated to employ during grain twinning or growth a grain 
growth modifier of the type disclosed by Maskasky IV, cited above. These 
grain growth modifiers are effective when the dispersing medium is 
maintained at a pH in the range of from 3 to 9 (preferably 4.5 to 8) and 
contains a stoichiometric excess of chloride ions of less than 0.5 molar. 
These grain growth modifiers satisfy the formula: 
##STR11## 
where 
Z.sup.2 is --C(R.sup.2)=or N.dbd.; 
Z.sup.3 is --C(R.sup.3)=or --N.dbd.; 
Z.sup.4 is --C(R.sup.4)=or --N.dbd.; 
Z.sup.5 is --C(R.sup.5)=or --N.dbd.; 
Z.sup.6 is --C(R.sup.6)=or --N.dbd.; 
with the proviso that no more than one of Z.sup.4, Z.sup.5 and Z.sup.6 is 
--N.dbd.; 
R.sup.2 is H, NH.sub.2 or CH.sub.3 ; 
R.sup.3, R.sup.4 and R.sup.5 are independently selected, R.sup.3 and 
R.sup.5 being hydrogen, hydroxy, halogen, amino or hydrocarbon and R.sup.4 
being hydrogen, halogen or hydrocarbon, each hydrocarbon moiety containing 
from 1 to 7 carbon atoms; and 
R.sup.6 is H or NH.sub.2. 
Still another type of grain growth modifier contemplated for use during 
grain growth is iodide ion. The use of iodide ion as a grain growth 
modifier is taught by Maskasky I, the disclosure of which is here 
incorporated by reference. 
In Maskasky U.S. Ser. No. 623,839, filed Dec. 7, 1990, AN IMPROVED PROCESS 
FOR THE PREATION OF HIGH CHLORIDE TABULAR GRAIN EMULSIONS, commonly 
assigned, (hereinafter referred to as Maskasky VII) it is taught to 
maintain a concentration of thiocyanate ions in the dispersing medium of 
from 0.2 to 10 mole, based on total silver introduced, to produce a high 
chloride tabular grain emulsion. It is here contemplated to utilize 
thiocyanate ion in a similar manner to control tabular grain growth. 
However, whereas Maskasky VII employs a 0.5 M concentration of chloride 
ion in the dispersing medium, the presence of the xanthine or azaxanthine 
grain growth modifier in the dispersing medium at the outset of 
precipitation allows lower chloride ion levels to be present in the 
dispersing medium, as described above. The thiocyanate ion can be 
introduced into the dispersing medium as any convenient soluble salt, 
typically an alkali or alkaline earth thiocyanate salt. When the 
dispersing medium is acidic (i.e., the pH is less than 7.0) the counter 
ion of the thiocyanate salt can be ammonium ion, since ammonium ion 
releases an ammonia ripening agent only under alkaline conditions. 
Although not preferred, an ammonium counter ion is not precluded under 
alkaline conditions, since, as noted above, ripening can be tolerated to 
the extent that the 0.3 .mu.m thickness limit of the tabular grains is not 
exceeded. 
In addition to or in place of the preferred growth modifiers for use in 
combination with any of the growth modifiers of this invention it is 
contemplated to employ other conventional growth modifiers, such any of 
those disclosed by Takada et al, Nishikawa et al, and Ishiguro et al, 
cited above and here incorporated by reference. 
Since silver bromide and silver iodide are markedly less soluble than 
silver chloride, it is appreciated that bromide and/or iodide ions, if 
introduced into the dispersing medium, are incorporated into the grains in 
the presence to the chloride ions. The inclusion of bromide ions in even 
small amounts has been observed to improve the tabularities of the 
emulsions. Bromide ion concentrations of up to 50 mole percent, based on 
total silver are contemplated, but to increase the advantages of high 
chloride concentrations it is preferred to limit the presence of other 
halides so that chloride accounts for at least 80 mole percent, based on 
silver, of the completed emulsion. Iodide can be also incorporated into 
the grains as they are being formed. It is preferred to limit iodide 
concentrations to 2 mole percent or less based on total silver. Thus, the 
process of the invention is capable of producing high chloride tabular 
grain emulsions in which the tabular grains consist essentially of silver 
chloride, silver bromochloride, silver iodochloride or silver 
iodobromochloride, where the halides are designated in order of ascending 
concentrations. 
Either single-jet or double-jet precipitation techniques can be employed in 
the practice of the invention, although the latter is preferred. Grain 
nucleation can occur before or instantaneously following the addition of 
silver ion to the dispersing medium. While sustained or periodic 
subsequent nucleation is possible, to avoid polydispersity and reduction 
of tabularity, once a stable grain population has been produced in the 
reaction vessel, it is preferred to precipitate additional silver halide 
onto the existing grain population. 
In one approach silver ion is first introduced into the dispersing medium 
as an aqueous solution, such as a silver nitrate solution, resulting in 
instantaneous grain nuclei formation followed immediately by addition of 
the growth modifier to induce twinning and tabular grain growth. Another 
approach is to introduce silver ion into the dispersing medium as 
preformed seed grains, typically as a Lippmann emulsion having an ECD of 
less than 0.05 .mu.m. A small fraction of the Lippmann grains serve as 
deposition sites while the remaining Lippmann grains dissociate into 
silver and halide ions that precipitate onto grain nuclei surfaces. 
Techniques for using small, preformed silver halide grains as a feedstock 
for emulsion precipitation are illustrated by Mignot U.S. Pat. No. 
4,334,012; Saito U.S. Pat. No. 4,301,241; and Solberg et al U.S. Pat. No. 
4,433,048, the disclosures of which are here incorporated by reference. In 
still another approach, immediately following silver halide seed grain 
formation within or introduction into a reaction vessel, a separate step 
is provided to allow the initially formed grain nuclei to ripen in the 
presence of a grain growth modifier. During the ripening step the 
proportion of untwinned grains can be reduced, thereby increasing the 
tabular grain content of the final emulsion. Also, the thickness and 
diameter dispersities of the final tabular grain population can be reduced 
by the ripening step. Ripening can be performed by stopping the flow of 
reactants while maintaining initial conditions within the reaction vessel 
or increasing the ripening rate by adjusting pH, the chloride ion 
concentration, and/or increasing the temperature of the dispersing medium. 
The pH, chloride ion concentration and grain growth modifier selections 
described above for precipitation can be first satisfied from the outset 
of silver ion precipitation or during the ripening step. 
Except for the distinguishing features discussed above, precipitation 
according to the invention can take any convenient conventional form, such 
as disclosed in Research Disclosure Vol. 225, January, 1983, Item 22534; 
Research Disclosure Vol. 308, December, 1989, Item 308,119 (particularly 
Section I); Maskasky I, cited above; Wey et al, cited above; and Maskasky 
II, cited above; the disclosures of which are here incorporated by 
reference. It is typical practice to incorporate from about 20 to 80 
percent of the total dispersing medium into the reaction vessel prior to 
nucleation. At the very outset of nucleation a peptizer is not essential, 
but it is usually most convenient and practical to place peptizer in the 
reaction vessel prior to nucleation. Peptizer concentrations of from about 
0.2 to 10 (preferably 0.2 to 6) percent, based on the total weight of the 
contents of the reaction vessel are typical, with additional peptizer and 
other vehicles typically be added to emulsions after they are prepared to 
facilitate coating. 
Once the nucleation and growth steps have been performed the emulsions can 
be applied to photographic applications following conventional practices. 
The emulsions can be used as formed or further modified or blended to 
satisfy particular photographic aims. It is possible, for example, to 
practice the process of this invention and then to continue grain growth 
under conditions that degrade the tabularity of the grains and/or alter 
their halide content. It is also common practice to blend emulsions once 
formed with emulsions having differing grain compositions, grain shapes 
and/or tabular grain thicknesses and/or aspect ratios. 
EXAMPLES 
The invention can be better appreciated by reference to the following 
examples. 
The mean thickness of tabular grain populations was measured by optical 
interference for mean thicknesses &gt;0.06 .mu.m measuring more than 1000 
tabular grains. 
The terms ECD and t are employed as noted above; r.v. represents reaction 
vessel; GGM is the acronym for grain growth modifier; TGPA indicates the 
percentage of the total grain projected area accounted by tabular grain of 
less than 0.3 .mu.m thickness. 
EXAMPLE 1 
AgCl High Aspect Ratio Tabular Grain Emulsions Precipitated at pH 6.2 
To a stirred reaction vessel containing 300 mL of a solution at 75.degree. 
C. that was 2.7% in bone gelatin, 0.053 M in NaCl, and 2.7 M in sodium 
acetate was added 100 mL of 12 mM basic xanthine solution. The pH of the 
resulting solution was adjusted to 6.2. A 4M AgNO.sub.3 solution and a 4M 
NaCl solution were added. The AgNO.sub.3 solution was added at 0.25 mL/min 
for 4 min then its flow was stopped for 15 minutes then resumed at 0.25 
mL/min for 2 min. The flow rate was then accelerated over an additional 
period of 30 min (20 X from start to finish) and finally held constant at 
5 mL/min until 0.4 mole of AgNO.sub.3 was added. The NaCl solution was 
added at a similar rate as needed to maintain a constant pAg of 6.65. When 
the pH dropped 0.2 units below the starting value of 6.2, the flow of 
solutions was momentarily stopped and the pH was adjusted back to the 
starting value. The results are shown in Table I and in FIGS. 1 and 2. 
EXAMPLE 1B 
This emulsion was prepared similar to that of Example 1A, except that the 
precipitation was stopped after 0.27 mole of AgNO.sub.3 had been added. 
The results are given in Table I. 
EXAMPLE 1C 
This emulsion was prepared similar to that of Example 1, except that the 
precipitation was stopped after 0.13 mole of AgNO.sub.3 had been added. 
The results are given in Table I. 
EXAMPLE 2 
AgCl High Aspect Ratio Tabular Grain Emulsions Precipitated at pH 7.0 
A reaction vessel, equipped with a stirrer, was charged with 5600 g of 
distilled water containing 50 g of oxidized gelatin containing &lt;4 .mu.mole 
methionine per gram gelatin, 2 grams of xanthine, 2.5 g of NaCl and 1 mL 
of an antifoamant. The pH was adjusted to 7.0 at 80.degree. C. and 
maintained at that value throughout the precipitation by additions of NaOH 
or HNO.sub.3. A 4M AgNO.sub.3 solution was added over a period of 2.5 min 
at a rate consuming 1.0% of the total Ag used. The flow was stopped for 40 
min and followed by addition of 120 g of 4M NaCl solution. Then 4M 
AgNO.sub.3 and 4M NaCl solutions were added simultaneously with linearly 
accelerated addition rates over a period of 40 minutes (5X from start to 
finish) during which time the remaining 99% of silver was consumed. The 
pAg of the emulsion was maintained at 6.28 during the last 40 minutes of 
the precipitation. The total silver precipitated was 3.88 moles. The 
results are presented in Table I. 
EXAMPLE 3 
AgCl High Aspect Ratio Tabular Grain Emulsions Precipitated at pH 5.3 
The precipitation conditions of this example were the same as those of 
Example 2, except that 5 g of xanthine was used, the reaction vessel was 
maintained at pH 5.3 and at 75.degree. C., the pAg during growth was 
maintained at 6.61, and the total silver precipitated was 4.11 moles. The 
results are summarized in Table I. 
EXAMPLE 4 
AgCl High Aspect Ratio Tabular Grain Emulsions Precipitated at pH 6.0 and 
40.degree. C. 
The precipitation conditions of this example were the same as those of 
Example 2, except that 5 g of xanthine were used, the reaction vessel was 
maintained at pH 6.0 and at 40.degree. C., and the pAg during growth was 
maintained at 7.74. The results are presented in Table I. 
EXAMPLE 5 
AgBrCl (.apprxeq.10 Mole% Br) High Aspect Ratio Tabular Grain Emulsions 
EXAMPLE 5A 
(10.2 M% Br) 
To a stirred reaction vessel containing 300 mL of a solution at 75.degree. 
C. that was 2.7% in bone gelatin, 0.040 M in NaCl, 2.7 mM in NaBr and 2.7 
M in sodium acetate were added 100 mL of a 12 mM basic xanthine solution. 
The pH of the resulting solution was adjusted to 6.2. A solution 4 M in 
AgNO.sub.3, a salt solution 3.6 M in NaCl, and 0.4 M in NaBr were added to 
the reaction vessel at 75.degree. C. The AgNO.sub.3 solution was added at 
0.25 mL/min for 1 min then its flow rate was accelerated at 0.158 
mL/min/min until 0.27 mole of AgNO.sub.3 was added, requiring a total of 
29 min. The salt solution was added at a similar rate, but as needed to 
maintain a constant pAg of 6.65. When the pH dropped 0.2 units below the 
starting value of 6.2, the flow of solutions was momentarily stopped, and 
the pH was adjusted back to the starting value. The results are presented 
in Table I. 
EXAMPLE 5B 
(10.8 Mole% Br) 
This emulsion was prepared similar to that of Example 5A, except that the 
precipitation was stopped after 0.13 mole of AgNO.sub.3 had been added. 
The results are summarized in Table I. 
CONTROL 6 
Attempt to use Uric Acid to form High Aspect Ratio AgCl Tabular Grain 
Emulsions 
##STR12## 
CONTROL 6A 
(pH 6.2) 
This emulsion was prepared similar to that of Example 1A, except that 100 
mL of a 12 mM basic uric acid solution was added to the reaction vessel in 
place of the xanthine solution. A nontabular grain emulsion resulted. 
CONTROL 6B 
(pH 4.5) 
This emulsion was prepared similar to that of Control 6A, except that the 
pH was maintained at 4.5. A nontabular grain emulsion resulted. 
CONTROL 7 
Attempt to use Guanine to form a High Aspect Ratio AgCl Tabular Grain 
Emulsion 
##STR13## 
This emulsion was prepared similar to that of Example 1A, except that 100 
mL of a 12 mM acidic guanine solution was added to the reaction vessel in 
place of the xanthine solution. A nontabular grain emulsion resulted. 
CONTROL 8 
Attempt to use Hypoxanthine to form a High Aspect Ratio AgCl Tabular Grain 
Emulsion 
##STR14## 
The emulsion was prepared similar to that of Example 1A, except that the 
xanthine solution was replaced with 100 mL of a 12 mM basic hypoxanthine 
solution. A nontabular grain emulsion resulted. 
TABLE I 
__________________________________________________________________________ 
AgNO.sub.3 
Final GGM 
Projected area 
Tabular Grain Population 
Temp 
added 
per Ag as fine grains 
Mean ECD 
Mean t 
Mean Aspect 
Example 
pH (.degree.C.) 
(mole) 
(mmole/mole) 
* (%) (.mu.m) 
(.mu.m) 
ratio % TPGA 
__________________________________________________________________________ 
1A 6.2 
75 0.40 3.0 2 2.87 0.170 
16.9 85 
1B 6.2 
75 0.27 4.4 10 2.40 0.125 
19.2 80 
1C 6.2 
75 0.13 9.2 20 2.07 0.093 
22.3 70 
2 7.0 
80 3.90 3.4 0 3.20 0.15 
21.3 85 
3 5.3 
75 4.10 8.0 10 2.30 0.25 
9.2 85 
4 6.0 
40 3.90 8.5 10 1.10 0.087 
12.6 90 
5A 6.0 
75 0.27 4.4 10 2.40 0.120 
20.0 85 
5B 6.0 
75 0.13 9.2 20 1.83 0.091 
20.1 75 
__________________________________________________________________________ 
* ECD &lt; 0.2 .mu.m 
5A = 10.2 mole % AgBr; 
5B = 10.6 mole % AgBr 
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.