|
(11) | EP 0 691 569 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
|
|
|
|
|||||||||||||||||||
| (54) | Photographic emulsion with improved sensitivity |
| (57) The invention concerns a monodisperse emulsion with improved sensitivity. The emulsion
of the invention is sensitized with sulphur and gold, and a quantity of thiocyanate
of between 0.1 and 1.0 mole per mole of silver is added thereto. A sensitisation level
equivalent to that of a corresponding polydisperse emulsion can be obtained. |
[0001] The present invention concerns a silver halide photographic emulsion with improved sensitivity.
[0002] In order to obtain photographic products with high sensitivity, it is known that it is possible to use polydisperse silver halide emulsions, that is to say emulsions whose coefficient of variation, defined as the relationship 100 x (sigma/v), where sigma is the volume standard deviation and v the mean volume of the population of the grains, is greater than 35% and preferably greater than 25%. Silver halide emulsions comprising at least two halides are also known, one of which is present in a higher concentration in at least one region or phase of the grain. It is known in particular that silver bromoiodide emulsions of advantageous sensitivity can be obtained, when the iodide is localised in precise phases of the grains, as is the case for example with so-called "core/shell" emulsions. It is also desirable to use monodisperse emulsions when it is wished to modify the form of the sensitometric curve and especially the contrast, including by mixing two or more monodisperse emulsions. However, silver bromoiodide emulsions with localised iodide are more difficult to prepare than emulsions where the iodide is distributed throughout the grain. Consequently there is an advantage in obtaining monodisperse silver bromoiodide emulsions in which the iodide is uniformly distributed in the grains, since these emulsions can have a sensitivity at least equivalent to that of polydisperse silver bromoiodide emulsions.
[0003] The problem is that, in order to chemically sensitise these emulsions with sulphur and gold to the same level as polydisperse emulsions, large quantities of thiocyanate are required, which then cause an unacceptable reduction in the stability of the emulsions in storage.
[0004] The object of the present invention is a monodisperse silver bromoiodide emulsion, chemically sensitized to an optimum extent, but without its stability in storage being affected.
[0005] The photographic emulsion according to the invention is a monodisperse silver bromoiodide emulsion where the iodide is uniformly distributed in the grains (uniform intra-grain distribution), chemically sensitized with sulphur and gold, to the same level as a polydisperse silver bromoiodide emulsion, with the same speed/granularity compromise.
[0006] The emulsion according to the invention is sensitized with sulphur and gold and is characterised in that its sensitisation comprises the following steps:
(a) addition of a gold sensitiser and a sulphur sensitiser;
(b) addition of a thiocyanate in a proportion of 0.1 to 1.5 g/mole of silver;
(c) temperature stage at a temperature between 40 and 70°C.
[0007] According to one embodiment of the invention, the addition of the gold and sulphur sensitisers is effected at a temperature between 30 and 60°C and advantageously between 40 and 50°C.
[0008] Gold sensitisers are well known. These are notably the gold salts with a degree of oxidation of +1 or +3, described in US patents 2,399,083, 2,597,915 or 2,597,856. Suitable compounds are for example potassium chloroaurite, potassium chloroaurate, ammonium aurithiocyanate, potassium iodoaurate, potassium aurithiocyanate, auric triochloride, N-methyl-2-aurobenzosulphobenzothiazolium chloride, and bis (1,4,5-trimethyl-1,2,4-triazolium-3-thiolate)-Au(I)-tetrafluoroborate. Such sensitizers as illustrated in USP 5,219,721, USP 5,221,604, European Patent 521,612 or European Patent 541,104 can be used. Preferred gold sensitizers are the gold (I) compounds disclosed in USP 5,049,485. These compounds include those represented by the formula :
AuL₂⁺X⁻ or AuL(L₁)⁺X⁻
wherein
L is a mesoionic compound
X is an anion and
L₁ is Lewis acid donor.
[0009] Sulphur sensitisers are also known. These are principally alkali metal thiosulphates, thiourea or thiourea derivatives, thioethers, etc. Useful compounds as well as the afore mentioned gold sensitizers are described in Research Disclosure, September 1994, publication 36544, Section IV, "Chemical Sensitisation". These gold and sulphur sensitisers are used in quantities which are well known and reported in the literature. For example, the gold compound is used in a proportion of 10⁻⁶ moles to 10⁻¹ moles per mole of silver and the sulphur compound in a proportion of 10⁻⁵ moles to 10⁻¹ moles per mole of silver.
[0010] The thiocyanate used in step (b) of the process defined above is preferably a thiocyanate of an alkali metal, sodium or potassium. The optimum quantity of thiocyanate depends on the composition of the emulsion and the desired characteristics, but is in general between 0.1 and 1.5 g/mole of silver and preferably between 0.2 and 1.0 g/mole of silver. The temperature stage is observed for a conventional period, usually lying between 5 and 60 minutes and advantageously between 10 and 30 minutes.
[0011] According to one embodiment, after the addition of thiocyanate and before the temperature stage, a modifying compound is added which is able to reduce fogging without reducing sensitivity. The invention therefore comprises the embodiment which consists of using such modifying agents. These agents are azaindenes, azapyrimidines, benzothiazolium salts and compounds with one or more heterocycles. Examples of these compounds are cited in US patents 2,131,038, 3,411,914, 3,554,754, 3,565,631 and 3,901,714, in Canadian patent 778,723 and in Photographic Emulsion Chemistry by Duffin, Focal Press (1966) New York, pages 138-143, or in Research Disclosure, September 1994, publication No 36544, section VII. It is also specifically contemplated to use phenylmercaptotetrazole antifogging agents. Any of the mercaptoheterocyclic nitrogen compounds, as described herein, will act as antifoggants in the practice of this invention. However, particularly good results are obtained with the mercaptotetrazoles, especially the 5-mercaptotetrazoles. 5-mercaptotetrazoles which can be employed include those having the structure :
where R is a hydrocarbon (aliphatic or aromatic) radical containing up to 20 carbon atoms. The hydrocarbon radicals comprising R can be substituted or unsubstituted. Suitable substituents include, for example, alkoxy, phenoxy, halogen, cyano, nitro, amino, amido, carbamoyl, sulfamoyl, sulfonamido, sulfo, sulfonyl, carboxy, carboxylate, ureido and carbonyl phenyl groups. Instead of an -SH group as shown in formula A-I, an -SM group can be substituted, where M represents a monovalent metal cation.
[0012] Various other conventional substances may be added to the emulsion, during or after precipitation. These are solvents for silver halides, maturation agents, such as in particular thioethers as described in US patents 3,062,646 or 3,271,157, dyes for spectral sensitisation, dopants, or other modifying agents. These compounds, according to circumstances and depending on their nature or function, may be adsorbed on the surface of the grains, or at certain predetermined places on this surface, or buried in the grains.
[0013] The additives may be introduced initially into the precipitation reactor, or added separately with one of the halides then introduced into the reactor, in accordance with normal techniques. Thus dopants, such as compounds of noble metals in group VIII, may be present during the precipitation.
[0014] The silver salts and halides may be introduced into the reactor by means of feed pipes arriving just below the surface of the contents of the reactor, whilst controlling the flow rate, pH and pAg. Suitable techniques are described in US patents 3,821,002 and 3,031,304, and by Claes et al in Photographisches Korrespondenz, Vol 102, No 10, page 162, 1967. Agitation arrangements can be provided to accelerate the homogenisation of the reagents in the reactor, as described in Research Disclosure, Vol 166, paragraph 16662, February 1978, or in US patents 2,996,287, 3,342,605, 3,415,650, 3,785,777, 4,147,551 and 4,171,224.
[0015] To form the emulsions, a dispersion medium is first introduced into the reactor, this normally consisting of a mixture of water and a vehicle which is a binder and/or a peptiser. This vehicle is usually a hydrophilic colloid, notably a proteinic one, such as gelatin, for example bone gelatin (treated with an alkali), pigskin gelatin (treated with an acid), modified gelatin, for example acetylated gelatin, phthalylated gelatin, etc, optionally associated with hydrophobic substances. The vehicle may also comprise synthetic substances, for example acrylamide polymers, cellulose derivatives, polyvinyl alcohol, etc. These substances do not necessarily have to be present at the time of precipitation. They may be added at the time of coating.
[0016] The emulsion is preferably washed in order to eliminate the soluble salts therefrom. This may be achieved by decantation, filtration, solidification, coagulation, centrifugation or flocculation, as described for example in US patents 2,316,845, 3,396,027, 2,618,556, 2,614,928, 2,565,418, 3,241,969, 2,489,341, 2,996,287, 3,498,454. The emulsions can be dried and kept before use, as described in Research Disclosure, Vol 101, paragraph 10152, September 1972.
[0017] Apart from the agents and additives already mentioned, the emulsion may contain yet other substances such as light-absorbent agents, surfactants, tanning agents, etc. These agents may also be incorporated in auxiliary layers. Information concerning these agents are given in Research Disclosure, September 1994, sections VIII and IX.
[0018] The grain sizes of the monodisperse emulsions are measured as described by A Holland and A Feinerman in J Applied Photo. Engin, 8, 165 (1982). This method makes it possible to determine an equivalent diameter of the sphere, or ESD, equal to:
where V is the mean volume.
[0019] The standard deviation (sigma) and the coefficient of variation (COV) can then be determined.
where Vi is the volume of a given grain and N the number of grains counted
[0020] The emulsion according to the invention can be spectrally sensitized by the addition of one or more sentising dyes. The sentising dye can be added at different stages in the preparation of the emulsion. The most usual operating methods consisting of adding it just before the sensitisation or chemical maturation, during this maturation or afterwards.
The usual sensitising dyes are well known; these are for example cyanines or merocyanines described with their bibliographic references in Research Disclosure, December 1989, publication No 308119, Section IV, "Spectral Sensitisation and Desensitisation".
EXAMPLE 1
[0021] An emulsion of silver bromoiodide was prepared, containing 3% molar silver iodide, by the double jet technique.
[0022] In a 20 litre reactor, 4 litres of demineralised water and 57.8 g/l of deionised gelatin were introduced. This mixture was heated to 60°C. A maturation agent (280 mg/mole Ag of thioether) and an anti-foaming agent were added. The pH was adjusted to 5.1 and the pAg to 9.3 with a solution of NaBr. By double jet method, 0.035 moles of AgNO₃ and 0.035 moles of NaBr were added in 1.17 minutes; for this nucleation step, 0.5 M solutions of AgNO₃ and NaBr were used. An excess of bromide was maintained so as to adjust the pAg to 9.3, at a temperature of 60°C.
[0023] After the nucleation step, an initial growth step was effected by introducing, by the accelerated double jet method, the following reagents:
AgNO₃ (2 M/l)
NaBr (1.94 M/l)
KI (0.06 M/l)
duration 27.2 minutes
pAg 9.3
temperature 60°C
[0024] A second growth step was effected with the following parameters and the following reagents, introduced by double jet method at constant flow rates:
AgNO₃ (2M/l)
NaBr 1.94 M/l)
KI (0.06 M/l)
36.7 minutes
pAg 9.3
temperature 60°C
[0025] The emulsion obtained by flocculation was washed. An AgBr(97)I(3) monodisperse octahedral emulsion comprising 50 g of gelatin per mole of Ag is obtained.
[0028] The preceding procedure is repeated except that the precipitation is carried out in the presence of 95 mg of thioether per Ag mole ; samples of the resulting emulsion B are sensitized as follows (amounts in mg/Ag mole).
[0030] The emulsion samples were coated at 8.07 x 100 mg of Ag, 32.3 x 100 mg gelatin and 10.5 x 100 mg of coupler per m. The coatings were exposed 1/100 sec at a tungsten source of 5500°K through a Wratten filter No 9, plus a color conversion filter DL5A.
[0031] The ESD is 1.085 µm for sample A, and 0.66 µm for sample B. The COV (by volume) is 6.1% for A and 9.6% for B.
[0032] The two samples were developed for 3 minutes 15 seconds in an AUTOPAN machine using the Kodak C41 process for colour negative development.
[0033] The results obtained are set out in Table 1. A mixture of the following sensitising dyes wes used:
EXAMPLE 2
[0035] The operating method of Example 1 was repeated, except that the sensitisation was effected as follows (quantities in mg/mole of silver) and the holding stage is 20 minutes at 60°C.
[0037] The gold compound is bis(1,4,5-trimethyl-1,2,4-triazolium-3-thiolate) Au(I) tetrafluororborate.
EXAMPLE 3
[0040] A polydisperse silver bromoiodide emulsion with 12% molar iodide was prepared, sensitized with sulphur and gold with thiocyanate digestion and spectrally sensitized by means of the mixture of dyes I and III. This emulsion had an equivalent diameter, of 0.84 µm and was coated at 8.07 x 100 mg of silver, 32.3 x 100 mg of gelatin and 10.5 x 100 mg of coupler per m.
[0042] The product is exposed and developed as in Example 1. The results obtained, appearing in Table 1, show that the monodisperse emulsions according to the invention give sensitometric results comparable to those of the reference emulsion of Example 3.
- RELATIVE sensitivity:
- sensitivity at the bottom of the H & D sensitometric curve
- INERT sensitivity:
- inertial sensitivity corresponding to the intersection of the slope of the H &D curve and of the line parallel to the axis of the Log H values for D = Dmin
- Contrast:
- maximum value of the first derivative of the H & D curve: dD/d Log H in the part corresponding to the intermediate densities
- GNG:
- minimum value of the granularity standardised by the contrast
1. Monodisperse silver halide photographic emulsion, with a uniform intra-grain distribution
of iodide, characterised in that it has been sensitized by :
a) addition of a sulphur sensitiser and a gold sensitiser;
b) addition of a thiocyanate in the range of from 0.1 to 1.5 g per mole of silver;
c) heating at a temperature in the range of from 40 to 70°C.
2. The photographic emulsion of Claim 1, wherein the addition of thiocyanate is in the
range of from 0.2 to 1.0 g per mole of silver.
3. The photographic emulsion of Claim 2, wherein thiocyanate is an alkali metal or ammonium
thiocyanate.
4. The photographic emulsion of any of Claims 1-3, characterised in that the emulsion
is sulphur sensitized by means of a thiosulphate.
5. The photographic emulsion of any of Claims 1-4, which is gold sensitized with a gold
(I) sensitizer
6. The photographic emulsion of any of Claims 1-4, characterised in that the emulsion
is gold sensitized by means of a compound selected in the class consisting of an alkali
metal or ammonium aurithiocyanate, an alkali metal chloroaurate or an alkali metal
chloroaurite.
7. The photographic emulsion of any of Claims 1-6, which is spectrally sensitized.
8. The photographic emulsion of any of Claims 1-7, which consists mainly of silver bromoiodide.
9. The photographic emulsion of any of Claims 1-8, which has a coefficient of variation
of less than 35%.
10. The photographic emulsion of Claim 9, which has a coefficient of variation of less
than 25%.
11. The photographic emulsion of any of Claims 1-10, which comprises an anti-fogging modifying
agent.
12. The photographic emulsion of Claim 11, which comprises a phenylmercaptotetrazole antifogging
modifying agent.
13. The photographic emulsion of Claim 11, which comprises a benzothiazolium antifogging
modifying agent.