WO1995017702A1

WO1995017702A1 - Silver chlorobromide photographic emulsion, preparation and use in reversal colour processes

Info

Publication number: WO1995017702A1
Application number: PCT/EP1994/004130
Authority: WO
Inventors: Gérard Amédé Désiré FRIOUR; Sylvie Martine Danièle POURCHET
Original assignee: Kodak-Pathe; Eastman Kodak Company
Priority date: 1993-12-23
Filing date: 1994-12-13
Publication date: 1995-06-29
Also published as: FR2714493A1; FR2714493B1; JPH08508113A; EP0698229A1

Abstract

The present invention concerns a silver halide photographic emulsion for use in colour reversal photographic products. This emulsion comprises grains containing a silver bromide or bromoiodide core and a peripheral area with a high silver chlorobromide content. This emulsion has improved sensitivity characteristics.

Description

SILVER CHLOROBROMIDE PHOTOGRAPHIC EMULSION,

PREPARATION AND USE IN REVERSAL COLOUR PROCESSES

The present invention concerns a silver halide emulsion for colour photography containing chemically and spectrally sensitised silver halide grains and having a structure with a core and at least one peripheral area (also referred to hereinafter as core-shell structure). The present

invention also concerns a process for obtaining such silver halide emulsions and their use in reversal colour

processes.

Although silver halide photographic emulsions are widely known, there is a constant need for photographic products with improved photographic properties, such as greater sensitivity and better granularity.

One of the drawbacks of the known emulsions is that the sensitising dyes used to sensitise the emulsion spectrally, for example cyanine dyes, may cause a loss of the intrinsic sensitivity of the emulsion. This loss of sensitivity or desensitisation due to dyes is recognised in the prior art as being a problem and is mentioned, for example, in "The Theory of the Photographic Process", 4th Edition, Macmillan Publishing Co. Inc., page 265 et seq. Given the

considerable variety of sensitising dyes available, it might be thought that, if a particular dye produces

unacceptable desensitisation, a practical solution would consist of choosing another one having substantially the same sensitisation characteristics without desensitisation. However, optimum sensitisation of a silver halide emulsion is generally the result of a complex compromise between several factors, and there are cases where specific dyes are used in spite of their desensitising properties because these dyes have for example the desired adsorption on the grains or absorb light in the desired region of the

spectrum.

It is also known that increasing the chloride content of the silver halide grains may increase the developability of these grains and enable a fast processing of the emulsion. The emulsions with a high chloride content, distributed in one or more phases or areas of the grain, are described in the prior art, for example in US patents 4 590 155 and 3 935 014, or in UK patent 1 027 146. However, it is known that chloride emulsions suffer from various deficiencies, such as insufficient stability or reduced sensitivity, compared with bromide or bromoiodide emulsions. There exists a specific need for chloride-containing emulsions which are able to exhibit an optimum

sensitivity/grain position without the above-mentioned drawbacks associated with the use of certain sensitising dyes.

The present invention provides a means to eliminate the phenomenon of desensitisation generally caused by some sensitising dyes, and enables these dyes to be used without causing the undesirable effects which might have been expected.

Accordingly, one object of the present invention concerns a spectrally and chemically sensitised halide emulsion, comprising silver halide grains with a core and at least one shell, characterised in that these grains comprise from 1 to 50% molar chloride said chloride being localised in a peripheral area of the grains, said emulsion being further characterized in that which is practically free of iodide, a spectral sensitising dye being adsorbed on the said grains, the said spectral sensitizing dye corresponding to the formula: f

where

Z¹ and Z² represent independently the atoms required to complete a benzoxazole radical, a naphthoxazole radical, a benzothiazole radical, a naphthothiazole radical, a

benzimidazole radical or a naphthimidazole radical, in which the aromatic cycles may be substituted by groups such as the alkyl, aryl, aralkyl, alkaryl, halogeno or cyano groups;

L¹ and L² represent independently a methine group, such as -CH =, C(CH₃) =, -C(C₂H₅) = ; R¹ and R² represent independently a sulphoalkyl radical, such as β-sulphoethyl, 1-sulphobutyl; and n is 1 or 2. Examples of suitable dyes according to the above formula (I) are as follows:

Another object of the invention concerns a process for producing a chemically and spectrally sensitised silver halide photographic emulsion, the said process comprising the steps of a forming a silver halide core emulsion, (b) forming, at the periphery of this core emulsion, an

emulsion which is (I) substantially free of iodide and (II) contains from 1 to 50% molar chloride, (c) effecting the chemical sensitization of said emulsion and, (d) during or after the chemical sensitisation, adsorbing, on the surface of the grains of the emulsion obtained, a sensitising dye as in above formula (II).

According to the present invention, the term "substantially free of iodide" means that the peripheral area of the grain can contain only a very small quantity of iodide, generally not above about 0.5% M.

Compared with a reference emulsion using the same

sensitising dye and having similar characteristics, except that the chloride is replaced by bromide, the emulsion of the invention makes it possible to compensate for the loss of sensitivity resulting from the desensitisation due to the dye and even to obtain an increase in sensitivity.

According to one embodiment, the emulsion comprises a chlorohalide peripheral area or shell so that, the chloride is distributed on the periphery of the grain ; and in the peripheral area, the chloride content may vary abruptly or content, within the ranges previously mentioned. As indicated, the chlorobromide shell of an emulsion according to the invention comprises from 1 to 50% molar chloride ; the shell preferably contains from 1 to 30% molar chloride and more preferably from 2 to 20% molar chloride based on the total silver in the grain.

According to a preferred embodiment, the emulsion of the present invention comprises silver halide grains containing a core and one or more shells, the composition of the halides being different in the core and the shell or shells. The peripheral area containing the chloride preferably represents a thickness greater than 100 A and less than 60% of the radius of the grain. Advantageously, the thickness of the peripheral area containing the

chloride represents from 10 to 50% of the radius of the grain.

The silver halide grains have the crystal habit generally used in silver halide photography; these are cubic,

octahedral or cubo-octahedral grains. The core grains may consist of silver bromide, silver chloride, silver

bromoiodide, silver chloroiodide, silver chlorobromoiodide or mixtures of these. When the grains contain iodide in the core, the maximum possible quantity of iodide is the quantity which can be accepted by the cubic crystal

lattice. In a silver bromide cubic crystal lattice, up to about 40% molar iodide can be incorporaied, depending on the precipitation temperature. When iodide is used in the core, the grains preferably contain less than about 15% molar iodide. In conventional photographic applications, the iodide concentrations are generally below about 15% molar. The distribution of the iodide may be homogeneous (for example, resulting from precipitation by a continuous jet of iodide) or discrete, the structures resulting from a sudden addition of iodide during precipitation. The sudden addition of iodide during precipitation. The emulsions may comprise coarse, medium or fine silver halide grains, predominantly delimited by crystal planes [100] or [111], and they may be regular or irregular in shape, for example cubic or octahedral.

The core grain emulsions may be prepared using various techniques, for example single-jet, double-jet or

accelerated-flow precipitation techniques as described by Trivelli and Smith, The Photographic Journal. Vol. LXXIX, May 1939, pp. 330-338, by T H James, The Theory of the Photographic Process, 4th Edition, Macmillan, 1977, Chapter 3, by Niertz et al in US patent 2 222 264, by Wilgus in the German patent application No 2 107 118, by Lewis in UK patents 1 335 925, 1 430 465 and 1 469 480, by Irie et al in US patent 3 650 757, by Morgan in US patent 3 917 485, by Musliner in US patent 3 979 213, in Research Disclosure, Article 17643, Vol. 175, December 1978, and Research

Disclosure, Article 22434, Vol. 225, January 1983.

Research Disclosure and its predecessor. Product Licensing Index, are publications of Industrial Opportunities Ltd, Homewell, Havant, Hampshire, P09 1EF, United Kingdom.

Modifying compounds may be present during the precipitation of the grains. Such compounds may be present in the reaction vessel initially or they may be added at the same time as one or more salts, in accordance with conventional operating methods. The modifying compounds, such as the middle chalcogens (namely sulphur, selenium and tellurium), gold and the noble metals in Group VIII (for example, iridium), may be present during the precipitation of the halides, as described in Research Disclosure, Vol. 134, June 1975, Article 13452. An important characteristic of the invention is that the chloride is coprecipitated with another silver halide, so as to form a mixed phase of silver chlorohalide in the outer peripheral area of the grain. According to a

preferred embodiment, the process of the invention consists of precipitating an emulsion with a bromide or bromoiodide core, and then coprecipitating a chlorobromide emulsion on the core emulsion, or else the bromide or bromoiodide core emulsion can first be shelled with a bromide emulsion which will itself be shelled by coprecipitating a chlorobromide emulsion.

The parameters for these precipitation stages, such as the pAg, the flow rates, the flow profiles, the salt

concentrations, and the number of moles precipitated as a function of time and of temperature, largely depend on the type of emulsion in question and on the desired results, and will consequently be determined in each case by a person skilled in the art.

The size of the emulsion grains is determined using a grain volume analyser and the determination is based on the principle of direct electrolytic reduction. Such an apparatus is described by A Holland and A Feinerman in J. Applied Photo. Eng. 8, 165 (1982). Such an apparatus makes it possible to obtain a volume distribution of the size of the grains from which a mean distribution volume (V) can be calculated, and then an equivalent spherical diameter (ESD) and a standard deviation (SIGMA), Vi being the volume of a given grain and N being the number of grains counted.

The coefficient of variation (COV) is defined as

representing 100 times the ratio of the volume standard deviation to the mean volume of the grain population

These emulsions are preferably monodisperse, that is to say they have a narrow size distribution characterised, for example, by a coefficient of variation of less than 20% and preferably less than 10%.

The emulsions according to the invention are intended for reversal colour processes. The chromogenic products generally comprise a support covered with at least three layers of silver halide emulsion, with which are associated one or more dye-forming couplers. Colour reversal

processes are well known and generally comprise a first black and white development, a fogging stage, a chromogenic development and a bleaching/fixing stage.

These emulsions can be chemically sensitised by any

conventional technique or using any conventional

sensitiser, such as those indicated in Research Disclosure No 308, December 1989, paragraph III. These sensitisers comprise, for example, active gelatin, as described by T H James, The Theory of the Photographic Process, 4th Edition, Macmillan; 1977, pages 67-76, or sulphur, selenium,

tellurium, gold, platinum, palladium, iridium, osmium, rhodium, rhenium or phosphorus sensitisers or combinations of these sensitisers, at pAg values between 5 and 10, pH levels between 5 and 8 and temperatures between 30° and 80°C, as described in Research Disclosure, Vol 120, April 1974, Article 12008, Research Disclosure, Vol 134, June 1975, Article 13452, by Sheppard et al in US patent 1 623 499, by Matthies et al in US patent 1 673 522, by Waller et al in US patent 2 339 083, by Damschroder et al in US patent 2 642 361, by McVeigh in US patent 3 297 447, by Dunn in US patent 3 297 446, by McBride in UK patent 1 315 755, by Berry et al in US patent 3 772 031, by Gilman et al in US patent 3 761 267, by Ohi et al in US patent 3 857 711, by Klinger et al in US patent 3 565 633, by Oftedahl in US patents 4 901 714 and 3 904 415 and by

Simons in UK patent 1 396 696; the chemical sensitisation may optionally be effected in the presence of thiocyanates, preferably at concentrations between 2 × 10^-3 and 2% molar with respect to the total silver content, as described by Damschroder in US patent 2 642 361; sulphur-containing compounds of the type described in the US patents 2 521 926 of Lowe et al, 3 021 215 of Williams et al and 4 054 457 of Bigelow. Specifically, it is considered that chemical sensitisation can be effected in the presence of compounds which modify chemical sensitisation, that is to say

compounds known to eliminate fogging and increase

sensitivity when they are present during chemical

sensitisation, such as the azaindenes, azapyridazines, azapyrimidines and salts of benzothiazolium, and the sensitisers comprising one or more heterocyclic rings.

Examples of finishing modifiers are described in the US patents 2 131 038 of Brooker et al, 3 411 914 of Dostes, 3 554 757 of Kuwabara et al, 3 565 631 of Oguchi et al and 3 901 714 of Oftedahl, in the Canadian patent 778 723 of Walworth and in Duffin, Photographic Emulsion Chemistry, Focal Press (1966), New York, pages 138-143. In addition, the emulsions may be sensitised by reduction - for example, with hydrogen, as described by Janusonis in US patent

3 891 446 and by Babcock et al in US patent 3 984 249, by a process using a low pAg (for example below 5) and/or a high pH (for example above 8) or by using reducing agents, such as stannous chloride, thiourea dioxide, polyamines and amine boranes, as described by Allen et al in US patent 2 983 609, Oftedahl et al in Research Disclosure, Vol. 136, August 1975, Article 13654, by Lowe et al in US patents 2 518 698 and 2 739.060, by Roberts et al in US patents

2 743 182 and 2 734 183, by Chambers et al in US patent

3 026 203 and by Bigelow et al in US patent 3 361 564. The superficial chemical sensitisation, including sub-surface sensitisation, described by Morgan in US patent 3 917 485 and by Becker in US patent 3 966 476, is specifically considered.

Preferably the outer peripheral area containing the

chloride or chlorobromide is sensitised using gold and/or sulphur. In addition to being sensitised chemically, the silver halide emulsions of the present invention are also

sensitised spectrally by means of spectral sensitising dyes, as defined above, which have maximum absorption levels in the blue and minus blue portions, ie green and red in the visible spectrum. In addition, in specialised applications, spectral sensitising dyes which improve the spectral response beyond the visible spectrum can be used.

One or more spectral sensitising dyes can be used. Dyes are known which have maximum sensitisation at various wavelengths in the visible spectrum and a great variety of spectral sensitivity curve forms. The choice and the relative proportions of the dyes depend on the region of the spectrum where it is desired to obtain the sensitivity and on the desired spectral sensitivity curve form. Dyes with overlapping spectral sensitivity curves often make it possible to obtain, in combination, a curve in which the sensitivity at each wavelength of the area of overlap is approximately equal to or even greater than

(supersensitisation) the sum of the sensitivities of the individual dyes. It is also possible to use combinations of the previously defined dyes and other conventional sensitising dyes.

The invention is described in greater detail in the

following examples. The indicated potentials of silver are measured by means of a saturated calomel electrode, unless otherwise indicated.

EXAMPLE 1 - (Control)

4.156 litres of distilled water and 57.8 g/l of deionised phthalated gelatin are introduced into a 20 litre

precipitation vessel and heated to 60°C. An anti-foaming agent and a thioether ripening agent are added before adjusting the pAg to 9.1 using a solution of NaBr, and the pH is adjusted to 5.1. The nucleation of a stable

population of microcrystals of AgBr is effected by

introducing 0.035 moles of AgNO₃ and NaBr by double-jet precipitation over a period of 70 seconds, using 0.5 M/l of reagents. An excess of bromide is maintained so as to obtain a pAg of 9.1 at a temperature of 60°C.

This nucleation is followed by a hold period of 120

seconds. This hold period is followed by a period of initial growth for 33.3 minutes, in which 2.0 M/l of AgNO₃, 1.82 M/l of NaBr and 0.18 M/l of KI are introduced, by accelerated double-jet precipitation, so as to produce a mixed phase of AgBrI while maintaining the temperature and the pAg at the same values as before. 3.36 moles of silver halides are precipitated in this initial growth stage. The precipitation is ended by a final growth segment lasting 29 minutes in which 2.0 M/l of AgNO₃ and 2.0 M/l of NaBr are introduced by double-jet precipitation, while maintaining the temperature and the pAg at the same values as before. 6.66 moles of silver halides are precipitated during this second growth stage. The emulsion is then washed using a conventional

flocculation process. The final emulsion consists of octahedral grains having an equivalent spherical diameter of 1.135 micrometres and a total iodide content of 3% molar.

This emulsion is sensitised chemically and spectrally at a point determined as being the optimum position.

The chemical sensitisation is carried out for 20 minutes at 70°C using:

50 mg of sodium thiocyanate per mole of Ag

5.71 mg of sulphur in the form of Na₂S₂O₃,5H₂O per mole of Ag

5.71 mg of gold in the form of potassium tetrachloroaurate per mole of Ag, and then 198.4 mg of sensitising dye (formula A) per mole of Ag is added for green sensitisation.

The emulsion obtained is an octahedral sensitised bromide core-shell emulsion, having an equivalent spherical

diameter (used as a mean dimension of the grains) of 0.639 μm and a volume coefficient of variation of 10.8%. The total iodide content corresponds to 3% molar and the iodide content of the core corresponds to 9% molar.

This bromide shell emulsion (emulsion E1) is used as a control.

Example 2 (Control)

4.156 litres of distilled water and 57.8 g/1 of deionised phthalated gelatin are introduced into a 20 litre

precipitation vessel and heated to 70°C. An anti-foaming agent and a thioether ripening agent are added before adjusting the pAg to 8.93 using a solution of NaBr, and the pH is adjusted to 5.1. The nucleation of a stable

population of microcrystals of AgBr is effected by

introducing 0.035 moles of AgNO₃ and NaBr by double-jet precipitation over a period of 70 seconds, using 0.5 M/l of reagents. An excess of bromide is maintained so as obtain a pAg of 8.93 at a temperature of 70°C.

This nucleation is followed by a period of inactivity of 180 seconds. This period of inactivity is followed a period of initial growth lasting 34.3 minutes, during which 2.0 M/l of AgNO₃, 1.82 M/l of NaBr and 0.18 M/l of KI are introduced, by accelerated double-jet precipitation, so as to produce a mixed phase of AgBrI while maintaining the temperature and the pAg at the same values as before. A total of 3.36 moles of silver halides are precipitated during this initial growth stage. Following this initial growth stage the pAg is modified to a value of 7.0 using a solution of 2.0 M/l of AgNO₃ introduced over a period of 152 seconds.

This pAg shift is followed by a hold period of 120 seconds. The hold period is followed by a final growth segment.

In the final growth segment, 2.0 M/l of AgNO₃ and 2.0 M/l of NaBr are introduced over a period of 20 minutes by accelerated double-jet precipitation, while maintaining the temperature and the pAg at the same values as before. A total of 6.69 moles of silver halides are precipitated during this second growth stage.

The emulsion is then washed by a conventional flocculation process. The final emulsion consists of cubo-octahedral grains having an equivalent spherical diameter of 0.580 μm and a total iodide content of 3% molar.

This emulsion is sensitised chemically and spectrally at a point determined as being the optimum position. The chemical sensitisation is carried out for 20 minutes at 70°C using:

50 mg of sodium thiocyanate per mole of Ag

5.97 mg of sulphur in the form of Na₂S₂O₃,5H₂O per mole of Ag

3.98 mg of gold in the form of potassium tetrachloroaurate per mole of Ag, and then

217.0 mg of sensitising dye A per mole of Ag is added for green sensitisation. The emulsion obtained is a sensitised cubo-octahedral core-shell emulsion having an equivalent spherical diameter (used as a mean dimension of the grains) of 0.580 μm, and a volume coefficient of variation of 11.9%. The total iodide content corresponds to 3% molar and the iodide content of the core corresponds to 9% molar.

This bromide shell emulsion (emulsion E2) is used as a control.

EXAMPLE 3 (Invention)

population of microcrystals of AgBr is effected by

introducing 0.035 moles of AgNO₃ and NaBr by double-jet precipitation over a period of 70 seconds, using 0.5 M/l of reagents. An excess of bromide is maintained so as to obtain a pAg of 8.93 at a temperature of 70°C.

This nucleation is followed by a hold period of 180

seconds. This hold period is followed by a period of initial growth lasting 34.3 minutes, during which 2.0 M/l of AgNO₃, 1.82 M/l of NaBr and 0.18 M/l of KI are

introduced, by accelerated double-jet precipitation, so as to produce a mixed phase of AgBrI while maintaining the temperature and pAg at the same values as before. A total of 3.36 moles of silver halides are precipitated during this initial growth stage.

After this initial growth the pAg is modified to a value of 7.0 using a solution of 2.0 M/l of AgNO₃ introduced over a period of 153 seconds. This pAg shift is followed by a hold period of 120 seconds. The hold period is followed by a final growth segment.

For the final growth segment, 2.0 M/l of AgNO₃, 1.82 M/l of NaBr and 0.18 M/l of NaCl are introduced over a period of 20 minutes, by carrying out double-jet accelerated

precipitation, while maintaining the temperature and pAg at the same values as before. A total of 6.69 moles of silver halides are precipitated in the course of this second growth stage.

The emulsion is then washed by a conventional flocculation process. The final emulsion consists of cubo-octahedral grains having an equivalent spherical diameter of 0.606 micrometres and a total iodide content of 3% molar.

This emulsion is sensitised chemically and spectrally at a point determined as being the optimum position. The chemical sensitisation is carried out over a period of 20 minutes at 70°C using:

50 mg of sodium thiocyanate per mole of Ag

4.60 mg of sulphur in the form of Na₂S₂O₃,5H₂O per mole of Ag

2.30 mg of gold in the form of potassium tetrachloraurate per mole of Ag, and then, 313.9 mg of sensitising dye A per mole of Ag is added for green sensitisation.

The emulsion obtained (E3) is a cubo-octahedral core-shell emulsion containing 9% molar chloride in the shell, where the silver chloride and silver bromide are coprecipitated, and having an equivalent spherical diameter of 0.606 μm and a volume coefficient of variation of 11.6%.

EXAMPLE 4 (Invention)

precipitation vessel and heated to 70°C. An anti-foaming agent and a maturation agent consisting of thioether ripening agent are added before adjusting the pAg to 8.93 using a solution of NaBr, and the pH is adjusted to 5.1. The nucleation of a stable population of AgBr microcrystals is effected by introducing 0.035 moles of AgNO₃ and NaBr by double-jet precipitation over a period of 70 seconds, using 0.5 M/l of reagents. An excess of bromide is maintained so as to obtain a pAg of 8.93 at a temperature of 70°C.

This nucleation is followed by a hold period of 180

seconds. This hold period is followed by a period of initial growth lasting 34.3 minutes, during which 2.0 M/l of AgNO₃, 1.82 M/l of NaBr and 0.18 m/1 of KI are

This initial growth is followed by an adjustment of the pAg to a value of 7.0, using a solution of 2.0 M/l of AgNO₃ introduced over a period of 153 seconds.

This pAg shift is followed by a hold period of 120 seconds. The hold period of inactivity is followed by a final growth segment. A final growth segment is precipitated over a period of 20 minutes, during which 2.0 M/l of AgNP₃, 1.70 M/l of NaBr and 0.3 M/l of NaCl are introduced by accelerated double-jet precipitation, while maintaining the temperature and the pAg at the same values as before. A total of 6.69 moles of silver halides are precipitated during this second growth stage.

The emulsion is then washed by a conventional flocculation process. The final emulsion consists of octahedral grains having an equivalent spherical diameter of 0.630

micrometres and a total iodide content of 3% molar.

This emulsion is sensitised chemically and spectrally at a point determined as being the optimum position. The chemical sensitisation is effected for 20 minutes at 70°C using:

50 mg of sodium thiocyanate per mole of Ag

4.50 mg of sulphur in the form of Na₂S₂O₃,5H₂O per mole of Ag

3.0 mg of gold in the form of potassium tetrachloroaurate per mole of Ag, and then

299.0 mg of sensitising dye A (formula A) per mole of Ag is added for green sensitisation. The emulsion obtained (E4) is a cubo-octahedral core-shell emulsion containing 15% molar chloride in the shell, in which silver chloride and silver bromide have been

coprecipitated, and having an equivalent spherical diameter of 0.630 μm and a volume coefficient of variation of 10.9%. EXAMPLE 5 - (Invention)

population of AgBr microcrystals is effected by introducing 0.035 moles of AgNO₃ and NaBr by double-jet precipitation over a period of 70 seconds, using 0.5 M/l of reagents. An excess of bromide is maintained so as to obtain a pAg of 8.93 at a temperature of 70°C. This nucleation is followed by a hold period of 180

introduced by accelerated double-jet precipitation, so as to produce a mixed phase of AgBrI while maintaining the temperature and pAg at the same values as before. A total of 3.36 moles of silver halides are precipitated during this initial growth stage. After this initial growth the pAg is adjusted to a value of 7.0 using a solution of 2.0 M/l of AgNO₃ introduced over a period of 153 seconds.

For the final growth segment, 2.0 M/l of AgNO₃, 1.50 M/l of NaBr and 0.50 M/l of NaCl are introduced over a period of 20 minutes by accelerated double-jet precipitation, while maintaining the temperature and the pAg at the same values as before. 6.69 moles of silver halides are precipitated during this second growth stage.

The emulsion is then washed by a conventional flocculation process. The final emulsion consists of cubo-octahedral grains having an equivalent spherical diameter of 0.620 micrometres and a total iodide content of 3% molar.

50 mg of sodium thiocyanate per mole of Ag

3.50 mg of sulphur in the form of Na₂S₂O₃,5H₂O per mole of Ag

2.33 mg of gold in the form of potassium tetrachloroaurate per mole of Ag, and then

306.9 mg of sensitising dye A per mole of Ag is added for green sensitisation. The emulsion obtained (E5) is a cubo-octahedral core-shell emulsion containing 25% molar chloride in the shell, in which silver chloride and silver bromide have been

coprecipitated, and having an equivalent spherical diameter of 0.620 μm and a volume coefficient of variation of 10.4%.

EXAMPLE 6

Samples of emulsions E1, E2, E3, E4 and E5 are applied using proportions of 8.07 mg/dm² of silver, 23.68 mg/dm² of gelatin and 16.15 mg/dm² of a magenta coupler (compound F) on a triacetate support. Each sample is then exposed for 1/100 of a second to a colour temperature equivalent to 5500° K through a Wratten^R #9 filter and a neutral density filter, and the samples are then subjected to the Kodak E-6^R colour reversal process.

The sensitometric results are given in Table I below. The parameters used in Table I are defined as follows:

- shoulder sensitivity - measured at a density Dl lower than the maximum density for which a variation in the sensitivity of 0.60 log E is observed when going from D1 to D2 = 3 × D1, the sensitivity index (SI) being determined by SI = 100 × (1 - log E_D1), - sensitivity at D = 1 : measured at a density of 1, the sensitivity index (SI) being determined by SI = 100 × (1 -log E₁),

- contrast at mid-scale : maximum absolute value of the first derivative of the curve D = f (log E),

- toe contrast : measured between two points of density Dl = Dmin + 0.1 and D2 = Dmin + 0.3, the value being

determined by the following equation: contrast at the foot = 0.2 / (log E₁ - log E₂)

- granularity at D = 1: measured at a density of 1 using a microdensitometer with an aperture of 48 μm.

These results show that emulsions E3, E4 and E5 have a higher sensitivity that emulsion E2 (+ 0.10 log E; + 0.23 log of E; + 0.32 log of E, respectively), with practically the same contrast and the same granularity. These results also show that similar improvements can be obtained in comparison with a reference octahedral coreshell emulsion (E1). EXAMPLE 7 - (Control)

population of AgBr microcrystals is effected by introducing 0.035 moles of AgNO₃ and NaBr by double-jet precipitation over a period of 70 seconds, using 0.5 M/l of reagents. An excess of bromide is maintained so as to obtain a pAg of 9.1 at a temperature of 60°C.

This nucleation is followed by a hold period of 120

seconds. This hold period is followed by a period of initial growth lasting 33.3 minutes, during which 2.0 M/l of AgNO₃, 1.82 M/l of NaBr and 0.18 M/l of KI are

introduced by accelerated double-jet precipitation, so as to produce a mixed phase of AgBrI while maintaining the temperature and pAg at the same values as before. A total of 3.35 moles of silver halides are precipitated during this initial growth stage.

A final growth segment is brought about over a period of 29 minutes, during which 2.0 M/l of AgNO₃ and 2.0 M/l of NaBr are introduced by double-jet precipitation while

maintaining the temperature and the pAg at the same values as before. 6.66 moles of silver halide are precipitated during this second growth stage. The emulsion is then washed by a conventional flocculation process. The final emulsion consists of cubo-octahedral grains having an equivalent spherical diameter of 1.135 micrometres and a total iodide content of 3% molar.

200 mg of sodium thiocyanate per mole of Ag

3.08 mg of sulphur in the form of Na₂S₂O₃,5H₂O per mole of Ag

2.05 mg of gold in the form of potassium tetrachloroaurate per mole of Ag.

The green spectral sensitisation is carried out during the chemical sensitisation stage using a mixture of 125.7 mg of sensitising dye A per mole of Ag and 26.3 mg of sensitising dye B per mole of Ag.

The emulsion obtained is an octahedral sensitised bromide core-shell emulsion having an equivalent spherical diameter (used as the mean dimension of the grains) of 1.135 μm, and a volume coefficient of variation of 7.1%. The total iodide content corresponds to 3% molar and the iodide content of the core corresponds to 9% molar.

This bromide shell emulsion (emulsion E6) is used as a reference.

EXAMPLE 8 - (Invention)

This nucleation is followed by a hold period of 120

After this initial growth the pAg is adjusted to 7.0 using a solution of 2.0 M/l of AgNO₃ introduced over a period of 156 seconds. This pAg shift is followed by a hold period of 60 seconds. The hold period is followed by a final growth segment.

For the final growth segment, 2.0 M/l of AgNO₃, 1.70 M/l of NaBr and 0.30 M/l of NaCl are introduced over a period of 39 minutes by double-jet precipitation, while maintaining the temperature and the pAg at the same values as before. 6.66 moles of silver halides are precipitated during this second growth stage.

The emulsion is then washed by a conventional flocculation process. The final emulsion consists of octahedral grains having an equivalent spherical diameter of 1.107

micrometres and a total iodide content of 3% molar.

200 mg of sodium thiocyanate per mole of Ag

2.0 mg of sulphur in the form of sodium thiosulphate pentahydrate per mole of Ag

1.33 mg of gold in the form of potassium tetrachloroaurate per mole of Ag.

A mixture of 166.0 mg of sensitising dye A per mole of Ag and 48.0 mg of sensitising dye B per mole of Ag is added during the chemical sensitisation stage for effecting green sensitisation.

The emulsion obtained (E7) is a cubo-octahedral core-shell emulsion containing 15% molar chloride in the shell, in which silver chloride and silver bromide have been coprecipitated, and having an equivalent spherical diameter of 1.107 μm and a volume coefficient of variation of 6.2%.

EXAMPLE 9

The emulsions of Examples 7-8 are applied as a coating, exposed to light and process as described in Example 6. Similar results are observed to those of Example 6, ie emulsion E7 has greater sensitivity than emulsion E6, without having any appreciable effect on the contrast and granularity.

EXAMPLE 10 The operating method of Example 8 is used again, except that dyes A and B are replaced by dye C.

Dye C has the formula:

By exposure and processing as in Example 6, similar results are obtained.

EXAMPLE 11

The operating method of Example 8 is used again, except that dyes A and B are replaced by dye D. Dye D has the formula:

By exposure and processing as in Example 6, similar results are obtained.

EXAMPLE 12

The operating method of Example 8 is used again, except that dyes A and B are replaced by dye E.

Dye E has the formula:

By exposure and processing as in Example 8, similar results are obtained.

TABLE I

Emulsion sample Relative sensitivity Contrast at Contrast at Granularity at shoulder at D=1 mid-scale the foot at D=1.0*

Example 1 E1 (Control) 100 100 2.32 1.32 24.6

Example 2 E2 (control) 104 110 2.53 1.01 24.3

Example 3 E3 (Invention) 114 120 3.26 1.41 26.5

Example 4 E4 (Invention) 127 136 3.01 1.25 26.8

Example 5 E5 (Invention) 136 136 2.50 1.27 26.6

Claims

1. Silver halide photographic emulsion comprising silver halide grains with a bromide or bromoidide core,

characterised in that these grains comprise from 1 to 50% molar chloride based on total silver in the grains, said chloride being localised in a peripheral area of the grains, said emulsion being further characterized in that a spectral sensitising dye being adsorbed on the said

spectral sensitizing grains, the said dye corresponding to the formula:

where

Z¹ and Z² represent independently the atoms required to complete a benzoxazole, naphthoxazole, benzothiazole, naphthothiazole, benzimidazole or naphthimidazole radical, in which the aromatic cycles may be substituted by groups such as alkyl, aryl, aralkyl, alkaryl, halogeno or cyano groups;

L¹ and L² represent independently a methine group, such as -CH =, C(CH₃) =, -C(C₂H₅) = ;

R¹ and R² represent independently a sulphoalkyl radical, such as ß-sulphoethyl, ω-1-sulphobutyl; and n is 1 or 2

2. The photographic emulsion of Claim 1, which is

monodisperse.

3. The photographic emulsion of Claim 1, wherein the grain core contains up to 40% molar iodide based on total silver present in the grain core.

4. The photographic emulsion of Claim 1, wherein the grains contain from about 1 to about 30% molar chloride based on total silver present in the grains.

5. The photographic emulsion of Claim 4, wherein the grains contain from about 2 to about 20% molar chloride, based on total silver present in the grains.

6. The photographic emulsion of Claim 1, wherein the peripheral area in which the chloride is localised has a thickness greater than about 100 A representing less than about 40% of the radius of the grain.

7. The photographic emulsion of Claim 6, characterised in that the peripheral area in which the chloride is located has a thickness representing from about 10 to about 35% of the radius of the grain.

8. The photographic emulsion of Claim 1, wherein the grains have a bromoiodide core containing up to 15% molar iodide based on total silver in the core, and the

peripheral area of chlorobromide, the thickness of which is greater than about 100 Å, represents less than 60% of the radius of the grain.

9. The photographic emulsion of Claim 8, characterised in that the chloride content of the grain is in the range of from about 2 to about 20% molar based on total silver in the grain.

10. The photographic emulsion of Claim 1, characterised in that the grains comprise, from the center a core, a first silver bromide peripheral area and a second outer

peripheral area in which chloride is localized.

11. The photographic emulsion of Claim 1, characterised in that it is chemically sensitised to the optimum.

12. The photographic emulsion of Claim 11, characterised in that the peripheral area containing chloride is

chemically sensitized to the optimum.

13. Colour reversal photographic product, containing an emulsion according to Claim 1.

14. Process for preparing a silver halide colour reversal photographic emulsion, comprising the following stages: a) precipitation of a core emulsion; b) precipitation at the peripheral of this core emulsion, of a shell emulsion which is substantially free of iodide and contains from 1 to 50 % molar chloride; c) chemical sensitisation. characterised in that, during or after the chemical

sensitisation stage, at least one dye represented by the following formula is added:

where

Z¹ and Z² represent independently the atoms required to complete a benzoxazole, naphthoxazole, benzothiazole, naphthothiazole, benzimidazole or naphthimidazole radical, in which the aromatic cycles may be substituted by groups such as alkyl, aryl, aralkyl, alkaryl, halogeno or cyano groups; L¹ and L² represent independently a methine group, such as -CH =, C(CH₃) =, -C(C₂H₅) = ;