FIELD OF THE INVENTION
[0001] The present invention relates to a method for preparing homogeneously divided substantially
hexagonal (111) tabular grains rich in silver bromide.
BACKGROUND OF THE INVENTION
[0002] Tabular silver halide grains are grains possessing two parallel crystal faces with
a ratio between the diameter of a circle having the same area as these crystal faces,
and thickness, being the distance between the two major faces, of two or more.
[0003] Tabular grains are known in the photographic art for quite some time. As early as
1961 Berry et al. described the preparation and growth of tabular silver bromoiodide
grains in Photographic Science and Engineering, Vol 5, No 6. A discussion of tabular
grains appeared in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, p.
66-72.
[0004] Early patent literature includes Bogg US-A 4,063,951, Lewis US-A 4,067,739 and Maternaghan
US-A's 4,150,994; 4,184,877 and 4,184,878. However the tabular grains described herein
cannot be regarded as showing a high diameter to thickness ratio, commonly termed
aspect ratio. In a number of US-A's filed in 1981 and issued in 1984 tabular grains
with high aspect ratio and their advantages in photographic applications are described
as e.g. US-A's 4,434,226; 4,439,520; 4,425,425 and 4,425,426 and in Research Disclosure,
Volume 225, Jan 1983, Item 22534.
[0005] The anisotropic growth of the said tabular grains is known to be due to the formation
of parallel twin planes in the nucleation step of the precipitation.
[0006] The shape of the tabular grains may be variable: triangular, hexagonal, disc-shaped,
trapezoidal and even needle-shaped grains can be formed. The said shape can be regular
or irregular.
[0007] The appearance of triangular or hexagonal grains is mainly concerned with the number
of twin planes: it has been observed that an uneven number of twin planes leads to
a triangular shape of the grains, whereas an even number leads to a hexagonal shape,
whereas the appearance of trapezoidal and needle-shaped grains is related with the
coalescence phenomena or the formation of non-parallel twin planes. These topics have
been discussed in J. Imag. Sci.
31, 1987,p. 15-26 and p. 93-99.
[0008] Emulsion preparation of tabular grains by means of the methods well-known by a person
skilled in the art of photography leads to grain populations consisting of a mixture
of all shapes of crystals described hereinbefore.
[0009] 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 and 0 513 722 and US-A 4,797,354 are related with the preparation of monodisperse
hexagonal tabular crystals. In said US-A 4,797,354 the preparation has been described
of tabular emulsions having a high percentage of hexagonal, tabular crystals. accounting
for from 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, 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.
[0010] 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 US-A 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. With respect
to ecology it is thus of utmost importance to prepare tabular grains rich in silver
bromide having an enhanced covering power.
[0011] The desire to have morphologically homogeneous tabular crystals however doesn't match
with another desired feature: a high degree of homogeneity requires preparation of
tabular grains during a long time in diluted reaction vessels, which is undesirable
from an economical (waste of time) as well as from an ecological (waste of preparation
solutions) point of view. In order to manufacture emulsions in a cost-effective way
the yield should be maximized, meaning a minimum end volume of the precipitation mixture
for a maximum amount of precipitated silver halide. In US-A 4,334,012 a suitable way
has been disclosed of concentrating the reaction mixture volume in the reaction vessel
by applying as well-known emulsion washing technique ultrafiltration in a continuous
way during the precipitation steps. These references however do not include teachings
with respect to the preparation of monodisperse emulsions.
OBJECTS OF THE INVENTION
[0012] Therefore it is a first object of the present invention to provide a method for preparing
(111) tabular grains rich in silver bromide having a high degree of morphologic homogeneity.
More particularly hexagonal (111) tabular crystals are envisaged in a procentual amount
as high as possible versus other grain shapes that are leading to the presence of
redundant amounts of silver which do not contribute effectively to the desired photograpic
properties. Said desired properties are e.g. low coating amounts of silver nevertheless
showing a high covering power after processing.
[0013] A further object of the present invention is to prepare the said (111) tabular grains
rich in silver bromide accounting for an amount by number of the total amount of grains
as high as possible in order to make said tabular grains account for at least 70 %
of the total projective area of all grains, showing high morphologic homogeneity in
concentrated reaction vessels in order to improve the precipitation efficiency and
to make the preparation process more economically and ecologically acceptable.
[0014] Other objects will become apparent from the description hereinafter.
SUMMARY OF THE INVENTION
[0015] In accordance with the present invention a method is provided for preparing an emulsion
having grains rich in silver bromide in the presence of gelatin as a protective colloid
in a reaction vessel wherein a yield of more than 250 g of precipitated silver nitrate
per liter of reaction vessel mixture is attained,
wherein at least 70 % of a total projected area of all grains is provided by (111)
tabular grains having an average aspect ratio of more than 2:1 and an average thickness
of from 0.05 up to 0.30 µm and wherein a ratio by number of hexagonal tabular grains
to triangular tabular crystals present is more than 10:1,
said method comprising following steps:
- preparing in a reaction vessel a gelatinous dispersion medium containing an initial
amount of (oxidized) gelatin corresponding with less than 50 % of a total amount of
gelatin used in the said method, said initial amount of (oxidized) gelatin having
an average methionine content of less than 30 µmoles per mole and said dispersion
medium having a volume of less than 2 liter per 500 g of silver nitrate to be precipitated;
- precipitating therein silver halide crystal nuclei by double-jet precipitation of
an aqueous silver nitrate and an aqueous solution comprising halide ions, wherein
less than 10 % by weight of a total amount of silver nitrate used is consumed;
- adding to said reaction vessel gelatin in an amount of more than 50 % of a total amount
of gelatin used in the said method;
- growing said silver halide crystal nuclei by further precipitation of silver halide
by means of double-jet precipitation of an aqueous silver nitrate solution and an
aqueous solution comprising halide ions, wherein more than 90 % by weight of a total
amount of silver nitrate is consumed,
- concentrating by ultrafiltration the said reaction mixture volume in the said reaction
vessel obtained during precipitation growth steps.
[0016] Concentrating by ultrafiltration the said reaction mixture volume in the reaction
vessel during precipitation growth steps is applied at any moment when said ultrafiltration
is performed e.g. with an ultrafiltration flux equal to or higher than total flow
rates of silver salt and halide salt solutions.
[0017] By this method it is possible to prepare a gelatinous silver halide emulsion having
(111) tabular silver bromide, silver bromoiodide, silver bromochloride or silver bromochloroiodide
grains, wherein at least 70 % of the total projected area of all grains is provided
by tabular (111) grains having a preferred average aspect ratio of more than 2:1 and
an average thickness of from 0.05 to 0.30 µm, wherein a ratio by number of procentual
amounts of hexagonal tabular grains to triangular tabular crystals present is more
than 10:1
DETAILED DESCRIPTION OF THE INVENTION
[0018] In a reaction vessel a dispersion medium containing gelatin having less than 30 µmoles
of methionine per gram is preferably prepared in order to apply the method of the
present invention: an increased number of (111) tabular grains rich in silver bromide
in the total grain population is obtained if use is made in the preparation method
of the so-called "oxidized gelatin", characterized by the presence in the said gelatin
of amounts of methionine of less than 30 µmoles per gram of gelatin as set forth in
US-A 4,713,320 and in Research Disclosure 29945, published March 1989. A preparation
method of (111) tabular grain emulsions wherein in the grain growth process use is
made of gelatin derivatives with chemically modified NH
2-groups and wherein said gelatin has a specific methionine content has been described
in EP-A 0 697 618. Modification of the methionine content of a gelatinous dispersion
medium by means of an oxidizer which should be added to the reaction vessel immediately
before nucleation formation has been described in US-A 5,372,975, wherein seed grains
are further added. Seed grains formed in the presence of an oxidizing agent have been
described in JP-A 05-210187, in JP-A 06-003758 and in JP-A 06-003759. Processing a
gelatin solution by means of H
2O
2 has been described in JP-A 05-341415. Other oxidizing agents besides hydrogen peroxide
as e.g. ozone, peroxy acid salts, halogens, thiosulphonic acid salts, quinones and
organic peroxides have been used as disclosed in US-A 5,489,504. Further in order
to provide tabular grains having small twin-plane separations in tabular grains rich
in silver bromide a preparation method making use of oxidized gelatin has been described
in US-A 5,219,720.
[0019] It should be stressed that it is an essential feature that in the reaction vessel
said oxidized gelatin is present in an amount of less than 50 % of the total amount
of gelatin present in the emulsion at the end of the preparation and that the dispersion
medium present before starting precipitation has a volume of less than 2 liter per
500 g of silver nitrate to be precipitated. This means that the nucleation step proceeds
in a concentrated reaction vessel, more concentrated than has hitherto been disclosed.
[0020] According to the method of the present invention after preparing in a reaction vessel
a dispersion medium containing gelatin having less than 30 µmoles of methionine per
gram according to the method of this invention, a total amount of silver nitrate of
less than 10 % by weight, and more preferably 0.5 % to 5.0 %, is added during the
nucleation step which preferably consists of an approximately equimolecular simultaneous
addition of silver nitrate and halide salts at a pBr of 1.0 to 2.0.
[0021] The rest of the silver nitrate and halide salts is added during one or more consecutive
double jet growth step(s) after having added to said reaction vessel. Gelatin added
before and/or during the said growth can be oxidized gelatin, already defined hereinbefore,
or non-oxidized gelatin having 30 or more µmoles of methionine per gram so that the
total amount of gelatin may contain per gram an average amount of higher than 30 µmoles
of methionine, and even up to about 80 µmoles per gram.
[0022] In a preferred embodiment according to the method of the present invention, growing
said silver halide crystal nuclei proceeds by precipitation of silver halide by means
of double-jet precipitation of an aqueous silver nitrate solution and an aqueous solution
comprising halide ions, wherein more than 90 % and more preferably up to 95 % by weight
of the total amount of silver nitrate is consumed.
[0023] The different steps of the precipitation can be alternated by physical ripening steps
or by so called "neutralization steps", during which the pAg value is changed to a
value required in the next growth step by adding an amount of silver nitrate solution
or a water soluble halide salt within a well-defined time of addition by means of
the single-jet technique. Alternative ways to regulate the pAg to the desired value
before continuing the processing are diluting the emulsion present in the reaction
vessel, diafiltration or ultrafiltration and even flocculation and washing procedures,
the last techniques being preferred to concentrate the emulsion crystals in the reaction
vessel. Any combination or any choice of the mentioned techniques may be applied thereto.
[0024] At least two growth steps are commonly used. The ratio of the second growth step
to the first growth step and the pBr in this second growth step is such that the tabular
(111) grains rich in silver bromide at the end of the preparation according to the
method of the present invention exhibit an average aspect ratio of at least 2:1, more
preferably from 5:1 to 15:1, wherein tabular (111) grains rich in silver bromide account
for at least 70 %, and more preferably at least 90 % of the total projected area of
all grains. Further said tabular grains rich in silver bromide, prepared according
to the method of the present invention have an average thickness of from 0.05 to 0.30
µm and more preferably from 0.05 µp to 0.20 µm and a coefficient of variation of the
grain size distribution of tabular grains of less than 0.30 and more preferably between
0.10 and 0.20. In order to obtain such a high degree of homogeneity useful compounds
added to the reaction vessel are polyalkyleneoxides as in US-A's 5,252,442 and 5,147,771.
[0025] During the growth step(s) an increasing flow rate of silver and halide solutions
is preferably applied, e.g. a linearly increasing flow rate. Typically the flow rate
at the end is about 3 to 10 times greater then at the start of the growth step. For
a succesful preparation of emulsions having tabular grains rich in silver bromide
according to the method of the present invention the pBr before the start and during
the different stages of the precipitation is maintained at a well-defined value as
will become apparent from the examples hereinafter.
[0026] In another embodiment of the method of the present invention nuclei can be prepared
in a separate vessel, whereas growth of the said nuclei may proceed in another vessel.
[0027] According to the present invention, besides performing nuclation in a concentrated
reaction vessel, it is of utmost importance to concentrate the reaction mixture volume
obtained by ultrafiltration during the precipitation growth steps by applying during
said ultrafiltration process an ultrafiltration flux equal to (as preferred in steady-state
circumstances) or higher than total flow rates of silver salt and halide salt solutions,
thereby concentrating silver halide formed in the said reaction vessel to at least
250 g, expressed as an equivalent amount of silver nitrate, per liter, preferably
up to 300 g and even more preferred up to 450 g per liter. In order to get the preferred
volume of the reaction mixture in the reaction vessel it is however possible that
a temporary lower ultrafiltration flux is required. The practically applied ultrafiltration
or membrane flux further is a function of the total operative surface of the membrane
and the trans-membrane pressure. The right choice of the membrane used in order to
reach the desired volume of the reaction mixture in the reaction vessel is thus very
important. Preferably the ultrafiltration procedure is applied in a continuous way
during the precipitation steps, but, if required, it can be interrupted for short
periods as e.g. during physical ripening preferably no ultrafiltration is applied.
By applying the ultrafiltration procedure the total reaction mixture volume can be
lowered during the precipitation. Alternatively the reaction mixture volume can be
readjusted, e.g. kept constant by the application of an additional jet of water. By
the methods described it is possible to reduce the end precipitation volume and to
concentrate silver halide to values set forth hereinbefore. This achievement cannot
be attained by solely concentrating the silver ion and halide ion jets as in that
case a tremendous deviation from the required morphologic homogeneity and homogeneity
of the crystal size distribution is observed, but as already set forth hereinbefore,
by the presence in low amounts of oxidized gelatin in a low starting volume of the
reaction vessel. In a preferred embodiment the ultrafiltration module is conceived
in such a way that the total volume of the ultrafiltration module and of its connecting
means, is lower than 1/3 of the total precipitation volume. Moreover the circulation
flux through the ultrafiltration module is preferably high enough, in order to achieve
a delay time in the module of any liquid volume unit of lower than 60 seconds and,
most preferably lower than 30 seconds. Even delay times as low as 10 seconds can be
achieved.
[0028] A preferred ultrafiltration module for the practice of this invention is a ROMICON
HF2-20-PM10, provided with a pump. For a typical precipitation (see examples) the
flow rate of the silver ion jet during the growth step(s) is linearly increased to
an end rate of 25 ml/min per 500 g of silver nitrate to be precipitated and a linearly
increasing flux having an end rate of about 50 ml/min is applied. But in the case
of more strongly increasing flow rates, e.g. quadratically increasing flow rates,
a flux of about 200 ml/min can be established if required.
[0029] A gelatinous silver halide emulsion is thus prepared according to the method of the
present invention, wherein said emulsion has silver bromide, silver bromoiodide, silver
bromochloride or silver bromochloroiodide grains (wherein the halide present in the
highest amount as expressed in mole % is called first), and wherein at least 70 %
of the total projected area of all grains is provided by tabular {111} grains having
an average aspect ratio of more than 2:1 and an average thickness of from 0.05 to
0.30 µm, wherein a ratio by number of hexagonal tabular grains to triangular tabular
grains is more than 10:1, and more preferably more than 20:1. Said silver halide is
furthermore, according to the present invention, present in said emulsion in an amount
per liter of at least 250 g, more preferred up to 300 g and even up to 450 g, wherein
silver halide is expressed as an equivalent amount of silver nitrate.
[0030] In order to determine the methionine content of gelatin many references from literature
are available as e.g. in J.Phot.Sc., Vol. 28(1980), p.111-118, wherein as most obvious
reducing substances in gelatin methionine residues of the macromolecule are determined
in reaction with Au(III)-ions. The so-called "gold number" permits determination of
amounts of methionine in the gelatin following the rule that 1 mmole of Au corresponds
with 1.6 mmole of methionine. In J.Phot.Sc., Vol. 33(1989), p.10-17 the methionine
content was determined using the gaschromatographic procedure developed by Apostolatos
and Hoff (Anal. Biochem. Vol. 118(1981), p.126) and applied to gelatin by Rose and
Kaplan. In this article calorimetry is used in a quantitative procedure for determining
methionine (constant over initial pH range examined: 3.0 - 8.0). In J.Phot.Sc., Vol.
40(1992), p.149-151, amounts of methionine, methionine sulphoxide and methionine sulphone
are determined by a chromato-graphic technique for amino acids (Hitachi Amino Acid
Analyser), whereas in J.Phot.Sc., Vol. 41(1993), p.172-175, these compounds are determined
by HPLC. In J.Phot.Sc., Vol. 39(1995), p. 367-372, it has been established that a
good correlation between methionine content determined by Rose and Kaplan making use
of gas chromatographic techniques (4th IAG Conference , Fribourg 1985, Amman-Brass
& Pouradier) and the Scatchard technique (described in J.Phot.Sc., Vol. 42(1994),
p.117-119) can be found. In the said technique the interaction at pH = 3.0 of Ag
+ and gelatin is determined by means of potential measurements of free Ag
+-ions.
[0031] In a preferred embodiment of the presentinvention, gelatin differing from the initial
amount of gelatin present in the reaction vessel and which is added after ending nucleation,
is added in an amount of more than 80 % by weight of the total amount of gelatin used,
wherein said gelatin differing from said initial amount of gelatin contains methionine
in an amount of more than 30 µmoles per gram. As an advantage thereof the homogeneity
of the diameter of the formed crystals is still further improved in that the standard
deviation thereof is decreased.
[0032] Preferably according to the method of the present invention in said silver silver
bromoiodide or silver bromochloroiodide iodide ions are present in an amount of up
to 3 mole % and in a preferred embodiment iodide ions are provided by means of an
iodide releasing agent. Patent applications referring to methods wherein iodide releasing
agents are used are e.g. EP-A's 0 563 701, 0 563 708, 0 561 415 and 0 651 284.
[0033] Preparation of silver bromo(chloro)iodide emulsion crystals can be achieved by mixing
a soluble bromide or bromochloride mixture and a soluble iodide salt in one or more
of the halide solutions up to the desired concentrations, expressed in mol %, required
in each preparation step by double jet or by a triple jet technique by separate addition
of an iodide containing aqueous solution. Due to the lower solubility of silver iodide
in comparison with silver bromide, said iodide ions are able to displace bromide and
chloride ions from the grain, a technique known in the art as conversion. Iodide ions
may also be incorporated into the silver halide crystal lattice by the addition of
a previously prepared silver iodide micrate emulsion, composed of either pure silver
iodide or mixed halide ultrafine crystals, but as has already set forth hereinbefore
in a preferred embodiment iodide releasing agents are used, at least partially, e.g.
in one or more conversion steps during or at the end of the precipitation. Even bromide
releasing agents are not excluded in the precipitation steps according to the method
of this invention.
[0034] Silver chloride, if present as in silver bromochloride or silver bromochloroiodide
emulsions, takes about 5 mole % up to 20 mole % in the composition of the silver halide
grains rich in silver bromide.
[0035] Two or more types of tabular silver halide emulsions that have been prepared differently
can be mixed for forming a photographic emulsion for use in photographic materials
according to the present invention, depending on the desired specifications.
[0036] The size distribution of the {111} tabular silver halide particles of the photographic
emulsions prepared according to the method of the present invention is thus monodisperse
as it is not desirable to have a low contrast, especially in the higher densities
of the sensitometric curve, characteristic for heterodisperse emulsions with a coefficient
of variation of the tabular grains between 0.20-0.40 which show a lower covering power.
As set forth in the objects of the present invention a higher covering power is preferred,
in order to coat less silver in the emulsion layers of suitable silver halide photographic
materials and therefore the more homodisperse emulsions, prepared according to the
method of the present invention are preferred with coefficients of variation being
lower than 0.20 and more preferred from 0.10 to less than 0.20.
[0037] Tabular silver halide emulsions rich in silver bromide, prepared by the method of
the present invention, can be chemically sensitized as has been described e.g. in
"Chimie et Physique Photographique" by P. Glafkides, in "Photographic Emulsion Chemistry"
by G.F. Duffin, in "Making and Coating Photographic Emulsion" by V.L. Zelikman et
al, and in "Die Grundlagen der Photographischen Prozesse mit Silberhalogeniden" edited
by H. Frieser and published by Akademische Verlagsgesellschaft (1968). Chemical sensitization
has e.g. also been described in Research Disclosure N° 38957 (September 1996), Chapter
IV. As described in said literature chemical sensitization can be carried out by effecting
the ripening in the presence of small amounts of compounds containing sulphur e.g.
thiosulphate, thiocyanate, thioureas, sulphites, mercapto compounds, and rhodamines.
Said compounds containing sulphur can also be, at least partially, replaced by compounds
containing selenium and/or tellurium. The emulsions may be sensitized also by means
of gold-sulphur, gold-sulphur-selenium, gold-selenium ripeners or by means of reductors
as e.g. tin compounds described in GB-Patent 789,823, amines, hydrazine derivatives,
formamidine-sulphinic acids, and silane compounds.
[0038] The tabular silver halide emulsions may be spectrally sensitized with methine dyes
such as those described by F.M. Hamer in "The Cyanine Dyes and Related Compounds",
1964, John Wiley & Sons and in Research Disclosure N° 38957 (1994), Chapter V. Dyes
that can be used for the purpose of spectral sensitization include cyanine dyes, merocyanine
dyes, complex cyanine dyes, complex merocyanine dyes, hemicyanine dyes, styryl dyes
and hemioxonol dyes. Particularly valuable dyes are those belonging to the cyanine
dyes, merocyanine dyes and complex merocyanine dyes. A survey of useful chemical classes
of spectral sensitizing dyes and specific useful examples in connection with tabular
grains is given in the already cited Research Disclosure Item 22534. Oxacarbocyanines
have been described e.g. in US-P 5,434,042. Especially preferred green sensitizers
in connection with the present invention are anhydro-5,5'-dichloro-3,3'-bis(n.sulfobutyl)-9-ethyloxacarbo-cyanine
hydroxide and anhydro-5,5'-dichloro-3,3'-bis(n.sulfopropyl)-9-ethyl-oxacarbo-cyanine
hydroxide. Imidacarbocyanines as e.g. those described in Research Disclosure N° 37312
(1995) may be useful as well as combinations of oxacarbocyanines and imidacarbocyanines
as in EP-A 0 590 593 from the viewpoint of sensitivity as well as from the viewpoint
of decolouring properties and stain removal in the processing of materials containing
spectrally sensitized tabular grains rich in silver bromide as in this invention.
[0039] In classical emulsion preparation spectral sensitization traditionally follows the
completion of chemical sensitization. However, in connection with tabular grains,
it is specifically considered that spectral sensitization may occur simultaneously
with or may even precede completely the chemical sensitization step: the chemical
sensitization after spectral sensitization is believed to occur at one or more ordered
discrete sites of tabular grains. This may also be done with the emulsions prepared
according to the present invention, wherein the chemical sensitization proceeds in
the presence of one or more phenidone and derivatives, a dihydroxy benzene as hydroquinone,
resorcinol, catechol and/or a derivative(s) therefrom, one or more stabilizer(s) or
antifoggant(s), one or more spectral sensitizer(s) or combinations of said ingredients.
Especially 1-p-carboxyphenyl, 4,4' dimethyl-pyrazolidine-3-one may be added as a preferred
auxiliary agent.
[0040] The gelatinous silver halide emulsion rich in silver bromide of the present invention,
characterized by a specific gelatin composition as set forth hereinbefore is further
coated in hydrophilic layer(s) which may, just as non-light-sensitive layers of the
photographic material according to this invention, comprise compounds preventing the
formation of fog or stabilizing the photographic characteristics during the production
or storage of the photographic elements or during the photographic treatment thereof.
Many known compounds can be added as fog-inhibiting agent or stabilizer to the silver
halide emulsion layer or to other coating layers in water-permeable relationship therewith
such as an undercoat or a protective layer. Suitable examples are e.g. the heterocyclic
nitrogen-containing compounds such as benzothiazolium salts, nitroimidazoles, nitrobenzimidazoles,
chlorobenzimidazoles, bromobenzimidazoles, mercaptothiazoles, mercaptobenzothiazoles,
mercaptobenzimidazoles, mercaptothiadiazoles, aminotriazoles, benzotriazoles (preferably
5-methyl-benzotriazole), nitrobenzotriazoles, mercaptotetrazoles, in particular 1-phenyl-5-mercaptotetrazole,
mercaptopyrimidines, mercaptotriazines, benzothiazoline-2-thione, oxazoline-thione,
triazaindenes, tetrazaindenes and pentazaindenes, especially those described by Birr
in Z. Wiss. Phot. 47 (1952), pages 2-58, triazolopyrimidines such as those described
in GB 1,203,757, GB 1,209,146, JP-A 75/39537, and GB-A 1,500,278, and 7-hydroxy-s-triazolo-[1,5-a]-pyrimidines
as described in US-A 4,727,017, and other compounds such as benzenethiosulfonic acid,
benzenethiosulfinic acid and benzenethiosulfonic acid amide. Other compounds that
can be used as fog-inhibiting compounds are described in Research Disclosure N° 17643
(1978), Chapter VI and in RD N° 38957 (1996), Chapter VII. Many of these fog-inhibiting
compounds may have been already added during the chemical ripening of the tabular
silver halide crystals rich in silver bromide.
[0041] It is clear that additional gelatin is added in a later stage of the emulsion preparation,
e.g. after washing, in order to establish optimal coating conditions and/or to establish
the required thickness of the coated emulsion layer. Preferably a gelatin to silver
halide ratio ranging from 0.3 to 1.0 is then obtained, wherein extra gelatin added
is not required to have a composition as specific as in the preparation step of the
grains according to the method of the present invention. Another binder may also be
added instead of or in addition to gelatin or gelatin derivatives as e.g. phthalated
gelatin. Useful vehicles, vehicle extenders, vehicle-like addenda and vehicle related
addenda have been described e.g. in Research Disclosures Nos 36544 (1994) and 38957
(1996), Chapter II.
[0042] The gelatin binder of the photographic material having at least one gelatinous emulsion
according to the present invention can be forehardened with appropriate hardening
agents such as those of the epoxide type, those of the ethylenimine type, those of
the vinylsulfone type e.g. 1,3-vinylsulphonyl-2-propanol, chromium salts as e.g. chromium
acetate and chromium alum, aldehydes e.g. formaldehyde, glyoxal, and glutaraldehyde,
N-methylol compounds e.g. dimethylol-urea and methyloldimethylhydantoin, dioxan derivatives
e.g. 2,3-dihydroxy-dioxan, active vinyl compounds e.g. 1,3,5-triacryloyl-hexahydro-s-triazine,
active halogen compounds e.g. 2,4-dichloro-6-hydroxy-s-triazine, and mucohalogenic
acids e.g. mucochloric acid and mucophenoxychloric acid. These hardeners can be used
alone or in combination. The binder can also be hardened with fast-reacting hardeners
such as carbamoylpyridinium salts as disclosed in US-A 4,063,952 and with the onium
compounds as disclosed in EP-A 0 408 143.
[0043] In a preferred embodiment the hydrophilic layer package of silver halide photographic
materials comprising in one or more light-sensitive layers one or more (111) tabular
emulsions rich in silver bromide crystals prepared according to the method of the
present invention, has a swelling degree of not more than 200 %. Said swelling degree
is determined by means of the following procedure: a sample of the coated material
is incubated at 57 °C and 34% RH for 3 days, whereafter the thickness (a) of the layer
assemblage is measured. Thereafter the sample is immersed in distilled water at 21°C
for 3 minutes and the thickness (b) of the swollen layer is measured. The swelling
ratio is then calculated as:

.
[0044] The gelatinous emulsions comprising tabular grains rich in silver bromide of the
present invention can be used in various types of photographic elements, e.g. black
and white silver halide photographic materials, like materials used for X-ray diagnostic
purposes, or colour sensitive materials.
[0045] In a preferred embodiment according to the present invention said photographic element
or material comprises a support and on one or on each side thereof one or more silver
halide emulsion layer(s) coated from a gelatinous emulsion according to the present
invention. More specifically said photographic material is a single-side or double-side
coated X-ray material.
[0046] The single-side coated X-ray material may contain one single emulsion layer, as it
is the case for many applications, or it can be built up by two or even more emulsion
layers. In X-ray photography a material with a single or a duplitized emulsion layer
coated on one or both sides of the support thus contains at least one gelatinous silver
halide emulsion according to the present invention. By using duplitized emulsions
differing in photographic speed by at least 0.15 log E a gain in cross-over exposure
in double side coated materials can be obtained. In the case of colour photography
the material contains blue, green and red sensitive layers each of which can be single
coated, but merely consists of double or even triple layers. Besides the light sensitive
emulsion layer(s) the photographic material may contain several light-insensitive
layers, e.g. a protective layer, one or more backing layers, one or more subbing layers,
one or more intermediate layers e.g. filter layers and even an afterlayer containing
e.g. the hardening agent(s), the antistatic agent(s), filter dyes for safety-light
purposes, etc..
[0047] The photographic element of the present invention may further comprise various kinds
of coating physical property modifying addenda as described in RD's Nos 36544 (1994)
and 38957 (1996), Chapter IX, wherein coating aids, plasticizers and lubricants, antistats
and matting agents have been described. Development acceleration can be accomplished
by incorporating in the emulsion layer or adjacent layers various compounds, preferably
polyalkylene derivatives having a molecular weight of at least 400 such as those described
in e.g. US-A's 3,038,805; 4,038,075 and 4,292,400 as well as in EP-A's 0 634 688 and
0 674 215.
[0048] The photographic element of the present invention may further comprise various other
additives such as e.g. compounds improving the dimensional stability of the photographic
element, UV-absorbers, spacing agents and plasticizers.
[0049] Suitable UV-absorbers are e.g. aryl-substituted benzotriazole compounds as described
in US-A 3,533,794, 4-thiazolidone compounds as described in US-A 3,314,794 and 3,352,681,
benzophenone compounds as described in JP-A 2784/71, cinnamic ester compounds as described
in US-A's 3,705,805 and 3,707,375, butadiene compounds as described in US-A 4,045,229,
and benzoxazole compounds as described in US-A 3,700,455 and those described in RD's
Nos. 36544 (1994) and 38957 (1996), Chapter VI, wherein also suitable optical brighteners
are mentioned. UV-absorbers are especially useful in colour materials where they prevent
the fading by light of the colour images formed after processing.
[0050] Spacing agents can be present of which, in general, the average particle size is
comprised between 0.2 and 10 µm Spacing agents can be soluble or insoluble in alkali.
Alkali-insoluble spacing agents usually remain permanently in the photographic element,
whereas alkalisoluble spacing agents usually are removed therefrom in an alkaline
processing bath. Suitable spacing agents can be made e.g. of polymethyl methacrylate,
of copolymers of acrylic acid and methyl methacrylate, and of hydroxypropylmethyl
cellulose hexahydrophthalate. Other suitable spacing agents have been described in
US-A 4,614,708.
[0051] Suitable additives for improving the dimensional stability of the photographic element
are e.g. dispersions of a water-soluble or hardly soluble synthetic polymer e.g. polymers
of alkyl(meth)acrylates, alkoxy(meth)acrylates, glycidyl (meth)acrylates, (meth)acrylamides,
vinyl esters, acrylonitriles, olefins, and styrenes, or copolymers of the above with
acrylic acids, methacrylic acids, α,β-unsaturated dicarboxylic acids, hydroxyalkyl
(meth)acrylates, sulphoalkyl (meth)acrylates, and styrene sulphonic acids.
[0052] The photographic material may contain several non-light sensitive layers, e.g. an
antistress topcoat layer, one or more backing layers, and one or more intermediate
layers eventually containing filter dyes or antihalation dyes that absorb scattering
light and thus promote the image sharpness. Suitable light-absorbing dyes used in
these intermediate layers are described in e.g. US-A's 4,092,168 and 4,311,787, in
DE-A 2,453,217, and in GB-A 7,907,440. Situated in such an intermediate layer between
the emulsion layers and the support there will be only a small negligable loss in
sensitivity but in rapid processing conditions decolouration of the filter dye layers
may form a problem. Therefor it should be recommended to decrease the thickness of
the whole coated layer packet resulting in shorter drying times after washing in the
processing cycle. Alternatively the use of intermediate layers situated between emulsion
layer(s) and support, reflecting the fluorescent light emitted by the screens may
bring a solution. As the light emitted from the screens by the phosphors incorporated
therein is a very important source of light-scattering the addition of appropriate
filter dyes to the screens may be recommended. In the presence in the screens of e.g.
green light-emitting phosphors use may be made of specific dyes as MAROLEX ORANGE
G or GG, trademarked products of BAYER AG.
[0053] One or more backing layers can be provided at the non-light sensitive side of the
support of materials coated with at least one emulsion layer at only one side of the
support. These layers which can serve as anti-curl layer can contain e.g. matting
agents like silica particles, lubricants, antistatic agents, light absorbing dyes,
opacifying agents, e.g. titanium oxide and the usual ingredients like hardeners and
wetting agents.
[0054] The support of the photographic material may be opaque or transparent, e.g. a paper
support or resin support. When a paper support is used preference is given to one
coated at one or both sides with an α-olefin polymer, e.g. a polyethylene layer which
optionally contains an anti-halation dye or pigment. It is also possible to use an
organic resin support e.g. cellulose nitrate film, cellulose acetate film, poly(vinyl
acetal) film, polystyrene film, poly(ethylene terephthalate) or poly(ethylene naphthalate)
film, polycarbonate film, polyvinylchloride film or poly-α-olefin films such as polyethylene
or polypropylene film. The thickness of such organic resin film is preferably comprised
between 0.07 and 0.35 mm. These organic resin supports are preferably coated with
a subbing layer which can contain water insoluble particles such as silica or titanium
dioxide.
[0055] The photographic material containing tabular grains prepared according to the present
invention can be image-wise exposed by any convenient radiation source in accordance
with its specific application.
[0056] Of course processing conditions and composition of processing solutions are dependent
from the specific type of photographic material in which the tabular grains prepared
according to the present invention are applied. For example, in a preferred embodiment
of materials for X-ray diagnostic purposes said materials may be adapted to rapid
processing conditions. Preferably an automatically operating processing apparatus
is used provided with a system for automatic regeneration (replenishment) of the processing
solutions.
[0057] The forehardened material may be processed using one-part package chemistry or three-part
package chemistry, depending on the processing application determining the degree
of hardening required in said processing cycle. Applications within total processing
times of 30 seconds and lower up to 90 seconds, known as common praxis, are possible.
From an ecological point of view it is e.g. possible to use sodium thiosulphate instead
of ammonium thiosulphate.
[0058] By the method of this invention it is thus possible to provide a high covering power
for different hardening levels of the layer material, wherein the substantially hexagonal
{111} tabular grains rich in silver bromide are coated in gelatinous emulsion form,
accounting for at least 70 % of the total projective area of all grains.
[0059] While the present invention will hereinafter in the examples be described in connection
with a preferred embodiment thereof, it will be understood that it is not intended
to limit the invention to that embodiment. On the contrary, it is intended to cover
all alternatives, modifications and equivalents as may be included in the spirit and
scope of the invention as defined by the claims.
EXAMPLES
[0060] All tabular grains were precipitated using the double jet technique with control
of the pAg value, said value being defined as the negative logarithm of the silver
ion concentration.
[0061] After precipitation, every example was analysed using shadowed carbon replicas obtained
with an electron microscope. For each example a minimum of hundred grains were measured
and the following characteristics were then calculated :
- the number of tabular grains were calculated, a tabular grain being defined as a grain
with two parallel main planes and a ratio between the diameter and the thickness of
the grains of at least 2, with
- the diameter being the diameter of a circle having an equivalent projective surface
area as the grain and
- the thickness being the distance between the main planes of the flat tabular crystals.
[0062] A characterization of the crystal population of an emulsion was given by:
- average diameter size dTab: calculated as the average (by number) from the diameters of the tabular grains;
- average standard deviation of the average diameter size sdTab;
- average thickness tTab: calculated as the average (by number) from the distance between the main planes
measured for all crystals
- procentual amount of hexagonal tabular crystals present in the total population of
tabular crystals: Thex., expressed as percentage of the total silver coverage (= volume);
- procentual amount of triangular tabular crystals present in the total population of
tabular crystals: Ttriang., expressed as percentage of the total silver coverage (= volume).
EXAMPLE 1
Comparative Examples
[0063] As comparative Examples the Emulsions Nos. 1-4 were prepared according to the Examples
described in EP-A 0 577 886.
Comparative emulsion No. 1 EP-A 0 577 886
[0064] The following solutions were prepared :
- a dispersion medium (C) consisting of 750 ml demineralized water, 4.04 g of inert
gelatin and 12.7 ml of a 2.94 molar potassium bromide solution; the temperature was
established at 45 °C and pH was adjusted to 4.5; the pAg corresponded to an electrochemical
potential of -63 mV measured with a silver electrode versus standard calomel;
- 1000 ml of a 2.94 molar silver nitrate solution (A);
- a mixture of a solution of 2.94 molar potassium bromide and 2.94 molar potassium iodide
at a ratio of 99/1 (B).
[0065] A nucleation step was performed by introducing solution A and solution B simultaneously
in dispersion medium C both at a flow rate of 25 ml/min during 28 seconds. After a
physical ripening time of 15 minutes during which the temperature was risen to 70
°C, 13.02 g of phtalated gelatin, dissolved in 250 ml of water, was added and the
mixture was stirred for an additional 5 minutes. Then a first growth step was performed
by introducing simultaneously during 564 seconds solution (A) at a flow rate of 5
ml/min and solution B in such a way that a constant silver potential of -33 mV is
maintained. Then a second growth step was performed by introducing by a double jet
during 3763 seconds solution A starting at a flow rate of 5 ml/min and linearly increasing
the flow rate to an end value of 25 ml/min, and solution B at an increasing flow rate
in order to maintain a constant silver potential value of -33 mV.
[0066] Ultrafiltration was applied during the growth steps. The circulation rate of the
kettle mixture through the ultrafiltration module was 2 liter/min. The dead volume
was 250 ml. In this way a precipitation efficiency of approximately 500 g of AgNO
3/liter was achieved.
Comparative emulsion No. 2
[0067] The precipitation scheme was identical to emulsion No. 1 with the exception that
during the two growth steps the silver potential was maintained at -3 mV instead of
-33 mV. The end volume was likewise about 1 1.
Comparative emulsion No. 3
[0068] The following solutions were prepared :
- a dispersion medium (C) consisting of 750 ml of demineralized water, 4.04 g of inert
gelatin and 12.7 ml of a 2.94 molar potassium bromide solution; the temperature was
established at 45 °C and pH was adjusted to 4.5; the pAg corresponded to an electrochemical
potential of -63 mV measured with a silver electrode versus standard calomel;
- 1000 ml of a 2.94 molar silver nitrate solution (A);
- a mixture of a solution of 2.94 molar potassium bromide and 2.94 molar potassium iodide
at a ratio of 99/1 (B).
[0069] A nucleation step was performed by introducing solution A and solution B simultaneously
in dispersion medium C both at a flow rate of 25 ml/min during 28 seconds. After a
physical ripening time of 15 minutes during which the temperature was risen to 70
°C, 13.02 g of phtalated gelatin, dissolved in 250 ml of water, was added and the
mixture was stirred for an additional 5 minutes. Then a first growth step was performed
by introducing simultaneously during 425 seconds solution A starting at a flow rate
of 5 ml/min and linearly increasing the flow rate to an end value of 25 ml/min, and
solution B at an increasing flow rate as to maintain a constant silver potential value
of -33 mV. A second growth step was performed by introducing simultaneously during
440 seconds solution A starting at a flow rate of 25 ml/min and linearly increasing
the flow rate to an end value of 56 ml/min, and solution B at an increasing flow rate
in order to maintain a constant silver potential value of -33 mV. A third growth step
was performed by introducing simultaneously during 445 seconds solution A starting
at a flow rate of 56 ml/min and linearly increasing the flow rate to an end value
of 100 ml/min, and solution B at an increasing flow rate as to maintain a constant
silver potential value of -33 mV.
[0070] By applying continuous ultrafiltration during precipitation the end volume of the
reaction mixture was reduced to about 1 l.
Comparative emulsion No. 4
[0071] The precipitation scheme was identical to that of emulsion No.3 with the exception
that during the three growth steps the silver potential was maintained at -3 mV instead
of -33 mV. The end volume was likewise about 1 liter.
Inventive Emulsions prepared according to the method of the present invention:
Inventive emulsion No. 1
[0072] The following solutions were prepared :
- a dispersion medium (C) consisting of 900 ml of demineralized water, 2.5 g of oxidized gelatine and 4.26 ml of a 6 N sulfuric acid solution; the temperature
was established at 51 °C; the pAg corresponded to a value of 8.77 (UAg= 38 mV vs.
a Ag/AgCl reference electrode)
- a 2.40 molar solution of silver nitrate solution (A);
- a 2.40 molar solution of potassium bromide (B1);
- a mixture of a solution of 2.36 molar potassium bromide and 0.037 molar potassium
iodide (B2);
- a solution of 460 ml of demineralized water and 20 g of phthalated gelatin (G).
[0073] A nucleation step was performed by introducing solution A and solution B simultaneously
in dispersion medium C both at a flow rate of 16 ml/min during 46 seconds. After a
physical ripening time of 29 minutes during which the temperature was risen to 70
°C, pH was adjusted to a value of 5.8 and 3 minutes later solution G was added and
the mixture was stirred for an additional 6 minutes.
[0074] A neutralization step was introduced by adding solution (B1) at an addition rate
of 3.75 ml/min. during 80 seconds, followed by a first growth step by introducing
simultaneously during 200 seconds solution (A) at a flow rate of 3.75 ml/min and solution
(B1) in such a way that a constant pAg value of 8.58 (silver potential of 18 mV vs.
a silver/silver chloride reference electrode) was maintained. Then a second growth
step was performed by introducing by a double jet during 2588 seconds solution A starting
at a flow rate of 3.75 ml/min and linearly increasing the flow rate to an end value
of 15 ml/min, and solution (B1) at an increasing flow rate in order to maintain a
constant pAg of 8.58 (silver potential of 18 mV).
[0075] Then a second growth step was performed by introducing by a double jet during 2391
seconds solution A starting at a flow rate of 15 ml/min and linearly increasing the
flow rate to an end value of 25 ml/min, and solution (B1) at an increasing flow rate
in order to maintain a constant pAg of 8.58 (silver potential of 18 mV).
Ultrafiltration was applied during the precipitation growth steps.
The circulation rate of the vessel mixture through the ultrafiltration module was
2 liter/min. The dead volume was 250 ml. In this way a precipitation efficiency of
360 g AgNO
3/liter is achieved.
Inventive emulsion No. 2
[0076] The precipitation scheme was identical to that of emulsion 1 except for the presence
in of solution (G) of 42.5 g instead of 20 g of phthalated gelatine.
Inventive emulsion No. 3
[0077] The precipitation scheme was identical to that of emulsion 1 except for the presence
in solution (G) of 20 g of an inert low viscous gelatin instead of 20 g of phthalated
gelatine.
Inventive emulsion No. 4
[0078] The precipitation scheme was identical to that of emulsion 1 except for the presence
in solution (G) of 42.5 g of an inert low viscous gelatine instead of 20 g of phthalated
gelatine.
Table 1
Emulsion No. |
dVol. (µm) |
dTab. (µm) |
tTab. (µm) |
%Thex. |
%Ttriang |
Thex/Ttriang. |
Comp.-1 |
0.59 |
1.36 |
0.19 |
63 |
37 |
1.8 |
Comp.-2 |
0.60 |
1.25 |
0.19 |
69 |
31 |
2.2 |
Comp.-3 |
0.59 |
1.36 |
0.23 |
72 |
28 |
2.6 |
Comp.-4 |
0.33 |
0.75 |
0.23 |
68 |
32 |
2.1 |
Inv.-1 |
0.53 |
0.99 |
0.18 |
93 |
7 |
13.3 |
Inv.-2 |
0.56 |
1.01 |
0.18 |
95 |
5 |
19.0 |
Inv.-3 |
0.54 |
0.93 |
0.21 |
96 |
4 |
24.0 |
Inv.-4 |
0.55 |
0.85 |
0.21 |
96 |
4 |
24.0 |
[0079] As is clear from the data in Table 1 the emulsions prepared according to the present
invention are containing a ratio by number of of hexagonal tabular grains to triangular
tabular crystals of more than 10:1 and even of more than 20:1, opposite to the comparative
emulsions as disclosed in EP-A 0 577 886. It can thus be concluded that the {111}
tabular grains rich in silver bromide prepared according to the method of the present
invention are substantially
hexagonal grains.
[0080] The emulsions were further redispersed and chemically ripened to an optimal fog-sensitivity
relationship after addition of compounds providing sulfur and gold as chemical sensitizers.
Anhydro-5,5'-di-chloro-3,3'-bis-(n.sulfobutyl)-9-ethyloxacarbocyanine hydroxide was
added as a green sensitizer.
[0081] Each emulsion was stabilized with 4-hydroxy-6-methyl-1,3,3a,7-tetra-azaindene and
after addition of the normal coating additives the solutions were coated simultaneously
together with a protective layer containing 1.1 g of gelatin per m
2 and per side on both sides of a polyethylene terephthalate film support having a
thickness of 175 µm.
[0082] Hardening of the layers was performed with formaldehyde.
[0083] The resulting photographic material was containing per side of the support an amount
of silver halide corresponding to 3.90 grams of AgNO
3 per m
2.
Exposure,sensitometric and densitometric data:
[0084] samples of these coatings were exposed with green light of 540 nm during 0.1 second
using a continuous wedge and were processed during the 90 seconds cycle described
below. The density as a function of the light dose was measured and therefrom were
determined the following parameters:
- fog level F (with an accuracy of 0.001 density),
- sensitivity (speed) S at a density of 1 above fog (in log(Exposure): a decrease with
a factor of 0.30 is indicative for an increase of sensitivity with a factor of 2),
- the contrast C, calculated between densities 1.0 and 2.5 above fog.
[0085] Development processing occurred in a glutaraldehyde containing hydroquinone/1-phenyl-3-pyrazolidinone
developer marketed by Agfa-Gevaert N.V. under the trade name G138.
[0086] Fixation was carried out in fixer G334, also marketed by Agfa-Gevaert N.V..
[0087] Processing conditions and developers used are given hereinafter.
- processing machine : CURIX 402 (Agfa-Gevaert trade name) with the following time (in
seconds (sec.)) and temperature (in °C) characteristics for a total processing time
of 98.0 sec:
- loading :
- 3.4 sec.
- developing :
- 23.4 sec./ 35°C high or low activity developer
- cross-over :
- 3.8 sec.
- fixing :
- 15.7 sec./ 35°C in fixer AGFA G334 (trade name)
- cross-over :
- 3.8 sec.
- rinsing :
- 15.7 sec./ 20°C.
- drying :
- 32.2 sec. (cross-over time included)
[0088] Table 2 hereinafter summarizes as sensitometric characteristics for the comparative
material (MC-1) and for the inventive materials (MI-1 to MI-4) the fog F, sensitivity
(speed) S and contrast (gradation) C of the samples after processing and the covering
power (CP) calculated from the ratio of maximum density and grams of coated silver
before processing.
Table 2
Material |
F |
S |
C |
CP |
MC-1 |
0.20 |
1.52 |
2.99 |
50 |
MI-1 |
0.20 |
1.74 |
3.42 |
61 |
MI-2 |
0.20 |
1.74 |
3.61 |
60 |
MI-3 |
0.20 |
1.77 |
3.32 |
59 |
MI-4 |
0.20 |
1.75 |
3.63 |
58 |
[0089] It can be concluded that
gradation and covering power are remarkably increased for the emulsions prepared according to the method of the present invention.
EXAMPLE 2
Inventive emulsion No. 5
[0090] In order to prepare the inventive Emulsion No. 5 following solutions were prepared
:
- a dispersion medium (C) consisting of 900 ml of demineralized water, 2.50 g of oxidized
gelatin having a methionine content of 13 µmole per mole of gelatin and 4.26 ml of
a 6 molar solution of sulfuric acid; the temperature was established at 51 °C and
a pAg value of 8.77 was measured, corresponding with an electrochemical potential
of 38 mV, measured with a silver electrode versus a silver/silver chloride reference
electrode;
- 1000 ml of a 2.40 molar of silver nitrate solution (A);
- a solution of 2.40 molar of potassium bromide (B1);
- a mixture of a solution of 2.36 molar of potassium bromide and 0.037 molar of potassium
iodide (B2).
[0091] A nucleation step was performed by introducing solution A and solution B1 simultaneously
in dispersion medium C both at a flow rate of 16 ml/min during 46 seconds. After a
physical ripening time of 25 minutes during which the temperature was risen to 70
°C, 42.5 g of gelatin having a methionine content of 13 µmole/mole of gelatin, dissolved
in 460 ml of water, was added and the mixture was stirred for an additional 6 minutes.
After a neutralization step by addition of solution B1 at a rate of 3.75 ml/min. during
80 seconds a first growth step was performed by introducing simultaneously during
200 seconds solution (A) at a flow rate of 3.75 ml/min and solution B in such a way
that a constant pAg of 8.58 (silver potential of 18 mV) is maintained. Then growth
was further performed by introducing by a double jet during 2588 seconds solution
A starting at a flow rate of 3.75 ml/min and linearly increasing the flow rate to
an end value of 15 ml/min. Solution B1 was added at an increasing flow rate in order
to maintain a constant pAg value of 8.58.
[0092] A second growth step was performed after a physical ripening time of 5 minutes by
introducing by a double jet during 2391 seconds solution A starting at a flow rate
of 15 ml/min and linearly increasing the flow rate to an end value of 25 ml/min. Solution
B1 was added at an increasing flow rate in order to maintain the same constant pAg
value of 8.58. An average methionine content of 13 µmole/mole of gelatin was measured.
[0093] Ultrafiltration was applied during the growth steps. The reaction mixture volume
was maintained at a constant level of 1.39 liter. The circulation rate of the kettle
mixture through the ultrafiltration module was 2 liter/min. The dead volume was 250
ml. In this way a precipitation efficiency of approximately 360 g AgNO
3/liter is achieved.
Inventive emulsion No. 5'
[0094] The same preparation procedure was followed with the same solutions, except for the
presence of methionine in the aqueous gelatinous solution added after the nucleation
step in an amount of 50 µmole/mole of gelatin. An average methionine content of 48
µmole/mole of gelatin was measured at the end of the emulsion preparation.
Table 3
Emulsion No. |
dVol. (µm) |
dTab. (µm) |
sdTab. |
tTab (µm) |
%Thex. |
%Ttriang |
Thex/Ttriang |
Comp.-5 |
0.51 |
0.91 |
0.30 |
0.21 |
96 |
4 |
24.0 |
Inv.-5 |
0.50 |
0.90 |
0.22 |
0.20 |
97 |
3 |
24.0 |
[0095] As is clear from the data in Table 3 representing grain characteristics as in Table
1 hereinbefore the inventive emulsion No. 5' not only shows the same degree of morphologic
homogeneity with respect to inventive emulsion No. 5, but moreover shows a significantly
higher degree of homogeneity on the diameter (s
dTab. defined hereinbefore as standard deviation on average crystal diameter).
[0096] This effect is specifically related with the methionine content of the gelatin added
after nucleation.
EXAMPLE 3
Comparative emulsions Nos. 5 and 6.
[0097] In order to prepare the
comparative Emulsion No. 5 following solutions were prepared:
- a dispersion medium (C) consisting of 3000 ml of demineralized water, 10 g of oxidized gelatin having a methionine content of 11 µmole per mole of gelatin
and 14.2 ml of a 6 molar solution of sulfuric acid; the temperature was established
at 51 °C and a pAg value of 8.77 was measured, corresponding with an electrochemical
potential of 38 mV, measured with a silver electrode versus a silver/silver chloride
reference electrode;
- 1000 ml of a 2.40 molar of silver nitrate solution (A);
- a solution of 2.40 molar of potassium bromide (B1);
- a mixture of a solution of 2.36 molar of potassium bromide and 0.037 molar of potassium
iodide (B2).
[0098] A nucleation step was performed by introducing solution A and solution B1 simultaneously
in dispersion medium C both at a flow rate of 16 ml/min during 46 seconds. After a
physical ripening time of 2 minutes the temperature was risen to 70 °C in a time interval
of 25 minutes. 2 minutes later the pH value was adjusted to a value of 5.8. 4 g of
gelatin having a methionine content of 50 µmole/mole of gelatin, dissolved in 460
ml of water, was added and the mixture was stirred for an additional 6 minutes.
[0099] After a neutralization step by addition of solution B1 at a rate of 5.2 ml/min. during
90 seconds a first growth step was performed by introducing simultaneously during
180 seconds solution (A) at a flow rate of 5.2 ml/min and solution B in such a way
that a constant pAg of 8.58 (silver potential of 18 mV) was maintained. Then growth
was further performed by introducing by a double jet during 3189 seconds solution
A starting at a flow rate of 5.2 ml/min and linearly increasing the flow rate to an
end value of 9.9 ml/min. Solution B1 was added at an increasing flow rate in order
to maintain a constant pAg value of 8.58.
[0100] A second growth step was performed after a physical ripening time of 5 minutes by
introducing by a double jet during 2391 seconds solution A starting at a flow rate
of 9.9 ml/min and linearly increasing the flow rate to an end value of 15.4 ml/min.
Solution B1 was added at an increasing flow rate in order to maintain the same constant
pAg value of 8.58. An average methionine content of 22 µmole/mole of gelatin was measured.
The procentual amount of gelatin in the nucleation step was 71 %. As
no ultrafiltration step was performed during precipitation the maximum amount of silver halide, expressed
as an equivalent amount of silver nitrate, was 83 g/l at the end of the precipitation.
In order to prepare the
comparative Emulsion No. 6 ultrafiltration was performed during growth keeping a constant volume of 1390 ml in the reaction
vessel. An average methionine content of 22 µmole/mole of gelatin was measured and
the procentual amount of gelatin in the nucleation step was 71 %. The only difference
with comparative emulsion No. 6 was the maximum amount of silver halide, expressed
as an equivalent amount of silver nitrate, which was
145 g/l instead of 83 g/l for comparative emulsion No. 5, at the end of the precipitation. Ultrafiltration thus brings about a higher yield
of silver nitrate in the reaction vessel, but not the yield of more than 250 g/liter
as required, according to the present invention.
Comparative emulsion No. 7.
[0101] As a starting point a dispersion medium (C) was prepared consisting of
900 ml of demineralized water, 10 g of oxidized gelatin having a methionine content of 11 µmole per mole of gelatin
and 4.26 ml of a 6 molar solution of sulfuric acid; the temperature was established
at 51 °C and a pAg value of 8.77 was measured, corresponding with an electrochemical
potential of 18 mV, measured with a silver electrode versus a silver/silver chloride
reference electrode.
[0102] This comparative emulsion No. 7 was further prepared in the same way as emulsion
No. 6, except for the differences in amounts of demineralized water in the starting
vessel as indicated above.
[0103] An average methionine content of 22 µmole/mole of gelatin was measured at the end
of the preparation of the silver bromoiodide emulsion crystals and the procentual
amount of gelatin in the nucleation step was 71 %. At the end of the precipitation
the only difference with comparative emulsion No. 6 was the maximum amount of silver
halide, expressed as an equivalent amount of silver nitrate, which was 360 g/l instead
of 145 g/l for comparative emulsion No. 6, thanks to the lower starting volume in
the reaction vessel.
Inventive emulsion No.6
[0104] The same preparation procedure was followed as for the comparative emulsion No. 6,
except for the composition of the dispersion medium (C), which was prepared with
900 ml of demineralized water,
2.5 g of oxidized gelatin having a methionine content of 11 µmole per mole of gelatin and 4.26 ml of a 6 molar
solution of sulfuric acid; the temperature was established at 51 °C and a pAg value
of 8.77 was measured, corresponding with an electrochemical potential of 18 mV, measured
with a silver electrode versus a silver/silver chloride reference electrode.
[0105] Moreover after the nucleation step, when after a physical ripening time of 2 minutes
the temperature was risen to 70 °C in a time interval of 25 minutes and 2 minutes
later the pH value was adjusted to a value of 5.8, 11.5 g of gelatin instead of 4
g of gelatin having a methionine content of 50 µmole/mole of gelatin, dissolved in
460 ml of water, was added whereafter the mixture was stirred for an additional 6
minutes.
[0106] An average methionine content of 43 µmole/mole of gelatin was measured at the end
of the preparation of the silver bromoiodide emulsion crystals and the procentual
amount of gelatin in the nucleation step was 18 % instead of 71 % as for the comparative
examples hereinbefore. Just as for the comparative emulsion No. 7 the maximum amount
of silver halide, expressed as an equivalent amount of silver nitrate, which was 360
g/l at the end of the precipitation, thanks to the low starting volume in the reaction
vessel and to on-line ultrafiltration which was also applied during the two growth
steps.
[0107] In Table 4 data have been summarized about average amounts of methionine, about
procentual amounts of gelatin in the nucleation step of the emulsion crystal preparation and about maximum amounts of silver nitrate per
liter present at the end of the precipitation as volume of the vessel. Moreover volume
percentages (vol.%) of hexagoral {111} tabular grains (hex.tabs) have been given as
well as d
Vol., the average diameter of all crystals calculated from the volume of all spheres,
having the same volume as the crystals.
Table 4
Emulsion No. |
av. amt. of methionine (µmol/mole) |
% amt. of gel in nucl. step |
max. amt. of AgNO3/l |
vol% (hex.tabs) |
dVol. (µm) |
Comp.-5 |
22 |
71 |
83 |
....95 |
0.69 |
Comp.-6 |
22 |
71 |
145 |
....90 |
0.67 |
Comp.-7 |
22 |
71 |
360 |
-- * |
----* |
Inv.-6 |
43 |
18 |
360 |
93 |
0.72 |
* impossible to determine |
[0108] As can be concluded from the data in the Table 4, high amounts of {111} tabular hexagonal
grains are obtained in a reaction vessel concentrated from the start of the precipitation
(in the nucleation step wherein
oxidized gelatin is present in low amounts (less than 50% versus the total amount used during growth precipitation)), wherein
ultrafiltration on-line is applied during the precipitation growth steps
[0109] As becomes clear from the comparative Examples 5-7 if compared with the inventive
Example 6 in the Table 4 hereinbefore, application of
ultrafiltration-on-line as such during precipitation in order to concentrate the reaction
vessel mixture is an insufficient measure in order to fully reach the objects of the present invention, in particular, to get
monodisperse substantially hexagonal {111} tabular grains: more than 250 g of silver
nitrate per liter to be precipitated is required, apart from amounts of oxidized gelatin
in the nucleation step versus amounts present during growth.
[0110] Having described in detail preferred embodiments of the current invention, it will
now be apparent to those skilled in the art that numerous modifications can be made
therein without departing from the scope of the invention as defined in the following
claims.