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(11) | EP 0 410 410 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Silver halide emulsion comprised of grains of uniform shape and size |
| (57) The improved silver halide emulsion has a silver iodide content of 10 - 45 mol%,
is monodisperse and is chiefly comprised of twinned crystals. It is a high-speed,
high AgI content silver halide emulsion that is comprised of grains of a uniform shape
and size and which is suitable for use in the production of core/shell type emulsions
or epitaxially grown emulsions. |
BACKGROUND OF THE INVENTION
[0001] This invention relates to a photographic silver halide emulsion, more particularly to a high-speed silver halide emulsion suitable for use in the production of core/shell type silver halide emulsions or epitaxially grown silver halide emulsions.
[0002] The demand for increasing the sensitivity of silver halide photographic materials and for producing high-quality image is constantly growing and to meet this demand, a variety of techniques have been proposed for controlling the shape of silver halide crystals in light-sensitive silver halide emulsions, the size distribution of these grains, the halide compositional distribution in the grains, etc.
[0003] Unexamined Published Japanese Patent Application No. 69243/1979 describes a method for preparing a high-speed silver halide emulsion by permitting silver chloride or silver chlrobromide cryatals to grow on the surfaces of silver iodobromide crystals with high silver iodide content. Unexamined Published Japanese Patent Application No. 16124/1981 teaches a process for producing a highly monodisperse silver iodobromide emulsion of high AgI content that is suitable for the purpose of preparing such a high-speed emulsion. However, each of the silver iodobromide emulsions described in these prior patents is comprised of normal (regular) crystals and they provide no disclosure on twins.
[0004] Unexamined Published Japanese Patent Application No. 119344/1984 discloses a technique in which tabular silver iodide crystals are joined to epitaxially grown crystals of a dissimilar silver halide composition. However, this technique has not been practically feasible on account of the slow developability of silver iodide grains.
[0005] Examined Japanese Patent Publication No. 38692/1988 and Unexamined Published Japanese Patent Application No. 14636/1986 disclose a tabular grain emulsion of a core/shell type that has a high AgI content phase in the central portion. However, the emulsions described in the examples of these patents either have a core of low AgI content or, if they have a core of high AgI content, the grains have a broad size distribution.
[0006] Thus, although the utility of silver iodobromide emulsions with high AgI content, the importance of narrow grain-size distribution of silver halide emulsions (monodispersibility) and the desirability of twinned emulsions have been well recognized, no silver halide emulsion has yet been known that has high AgI content, that has a narrow grain-size distribution and that is comprised of twinned crystals.
SUMMARY OF THE INVENTION
[0007] An object, therefore, of the present invention is to provide a high-speed silver halide emulsion of high AgI content that is comprised of grains of a uniform shape and size and which is suitable for use in the production of core/shell type silver halide emulsions or epitaxially grown silver halide emulsions.
[0008] This object of the present invention can be attained by a silver halide emulsion that has a silver iodide content or 10 - 45 mol%, that is monodisperse and that is chiefly comprised of twinned crystals.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009]
Fig. 1 is an electron micrograph (X 20,000) showing the structure of grains in the emulsion of the present invention that was prepared in Example 2;
Fig. 2 is an X-ray diffraction scan for said emulsion at a (420) face; and
Fig. 3 shows schematically two kinds of twinned silver halide grains prepared in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0010] The silver halide emulsion of the present invention preferably has a silver iodide content of 15 - 40 mol%, more preferably 20 - 38 mol%, on grain surfaces, with the range of 25 - 35 mol% being particularly preferred. It is a also preferred that said emulsion contains a low AgI content phase in the central region, which AgI content of the central region is preferably not higher than 8 mol%, more preferably 0 - 5 mol%.
[0011] The silver halide emulsion of the present invention is preferably comprised of silver iodobromide grains but it may be comprised of silver chloroiodobromide grains.
[0012] The silver halide emulsion of the present invention is "monodisperse" in that the weight of silver halide grains whose size is within the range of ±20% of the mean value d is at least 70% of the total weight of all the silver halide grains present. Preferably, the relative weight is at least 80%, more preferably at least 90%, of the total weight of all the silver halide grains present.
[0013] The term "mean grain size d˝ is defined as the size di of such grains that provide a maximum value of ni x d
where ni is the frequency of the grains having size di (calculation is made to three significant digits, with the least significant figure being rounded). The term "grain size" means the diameter of a circle equivalent in area to the projected image of a grain of interest. A typical method of grain size measurement consists of taking an electron micrograph of the grain of interest at a magnification of 1 - 5 x 10⁴ and then measuring the diameter of the grain as observed on the print or the area of the projected image (assuming that the measurement is performed on randomly selected 1,000 or more grains).
[0014] A highly monodisperse emulsion that is particularly preferred for the present invention is such that the width of distribution as defined by
is no more than 20%, preferably no more than 15%. The "mean grain size" and "standard deviation of grain size" are assumed to be measured by the method described above, with the "mean grain size" being a simple or arithmetic mean as defined below:
[0015] A preferred method for preparing the silver halide emulsion of the present invention consists of allowing a high AgI content phase to precipitate on the surface of monodisperse seed crystals. A particularly preferred method is to provide a step for growing a monodisperse seed emulsion comprised of spherical twins as described in Unexamined Published Japanese Patent Application No. 6643/1986. It is preferred that the silver halide emulsion of the present invention is chiefly composed of twinned crystals having at least two parallel twin planes, more preferably even-numbered twin planes, and most preferably two twin planes.
[0016] The expression "the silver halide emulsion is chiefly composed of twinned crystals having at least two parallel twin planes" means that the number of twinned grains having at least two parallel twin planes, as counted from the larger grains, is at least 30%, preferably at least 50%, more preferably at least 60%, of all the grains present.
[0017] The twins in the emulsion of the present invention may be composed of either (111) faces or (100) faces or both (111) and (100) faces, and they are preferably composed of (111) faces.
[0018] Two types of twinned crystal suitable for use in the present invention are shown schematically in Fig. 3, in which parallel twin planes are indicated by A-A′ and B-B′.
[0019] In twinned grains having at least two parallel twin planes, the ratio of the diameter of an equivalent circle of the image of a grain as projected in a direction perpendicular to the twin planes to the distance between a parallel twin plane and a grain surface parallel to it is preferably in the range of 1 - 20, more preferably 1 - 5.
[0020] The process for producing the silver halide photographic emulsion of the present invention comprises the basic step of supplying a solution of a water-soluble silver salt and a solution of a water-soluble halide in the presence of a protective colloid and is characterized by including the following steps:
(A) a step of producing nuclear grains in which the pBr of the mother liquor is held at a value between 2.0 and -0.7 for a period that is at least one first half of the period of the formation of silver halide precipitates having a AgI content of 0 - 5 mol%;
(B) a step of forming seed grains in which, subsequent to step (A), silver halide seed grains which are substantially monodisperse spherical twins are formed in the presence of a silver halide solvent in the mother liquor in an amount of 10⁻⁵ - 2.0 moles per mole of silver halide; and
(C) a growth step in which the seed grains are grown by adding a solution of a water-soluble silver salt and a solution of a water-soluble halide and/or fine silver halide grains.
[0021] The term "mother liquor" as used hereinabove means a liquor (inclusive of a silver halide emulsion) that serves as a silver halide emulsion preparation site unitl a complete photographic emulsion is obtained.
[0022] The silver halide grains formed in step (A) of producing nuclear grains are twins composed of silver iodobromide containing 0 - 5 mol% AgI.
[0023] A twin is a silver halide crystal having at least one twin plane in one grain. The morphology of twins is classified in details in E. Klein & E. Moisar, Photogr. Korresp., 99, 99 (1963) and ibid., 100, 57 (1964). The two or more twin planes in a twin may or may not be parallel to each other. The twinned crystal may be bounded by either (111) faces or (100) faces or both (100) and (111) faces.
[0024] In the present invention, twinned nuclear grains can be obtained by adding a water-soluble silver salt either alone or in combination with a water-soluble halide, with the concentration of bromide ions in an aqueous solution of protective colloid being held at 0.01 - 5 mol/L (i.e. equivalent to pBr = 2.0 to -0.7). preferably at 0.03 - 5 moles/L (pBr = 1/5 to =0.7) for a period that is at least one first half of the period of the step for producing nuclear grains.
[0025] The "step of producing nuclear grains" may be defined as any step that precedes the step of forming seed grains and said step includes not only the period that starts with the addition of a water-soluble silver salt to the solution of protective colloid and that ends when the production of new crystal nuclei substantially ceases but also a subsequent period of grain growth.
[0026] The size distribution of the nuclear grains is not limited in any way and they may be monodisperse or polydisperse. "Polydisperse" nuclear grains are such that the coefficient of variation in grain size (having the same definition as the "width of distribution") is at least 25%. The nuclear grains for use in the present invention preferably contain twinned grains whose number is at least 50%, more preferably at least 70%, and most preferably at least 90%, of the total number of the nuclear grains formed in step (A).
[0027] Subsequent step (B) is one intended for ripening the nuclear grains in the presence of a silver halide solvent to form seed grains that are monodisperse and spherical in shape.
[0028] Ripening in the presence of a silver halide solvent (which is hereinafter referred to simply as "ripening") would be different from "Ostwald ripening" in which, if grains of different sizes are present, the larger grains grow in preference over the smaller grains which are dissolved to eventually provide a broad grain-size distribution in most cases. The condition of ripening seed grains from the nuclear grains produced in step (A) is such that the mother liquor of emulsion resulting from step (A) in which twinned nuclear grains are produced using silver halide grains with a AgI content of 0 - 5 mol% is ripened in the presence of a silver halide solvent in an amount of 10⁻⁵ to 2.0 moles per mole of silver to form substantially monodisperse spherical seed grains. The seed grains are "substantially monodisperse" if the width of their size distribution as already defined hereinabove is less than 25%.
[0029] The seed grains are "substantially spherical" if they are rounded to such an extent that, when examined with an electron microscope, crystal faces such as (111) or (100) faces are not clearly discernable, and if threedimensional axes that cross at right angles are assumed at a point in the neighborhood of the center of gravity of grains under consideration, the ratio C = L/ℓ (where L is the maximum grain diameter in the directions of length, width and height of the plane image of the grain, and ℓ is the minimum grain diameter) is in the range of 1.0 - 2.0, preferably 1.0 - 1.5.
[0030] The spherical grains described above generally assume at least 60, preferably at least 80%, more preferably almost 100%, of the total number of the seed grains present.
[0031] Examples of the silver halide solvent that can be used in step (B) for forming seed grains include: (a) the organic thioethers described in U.s. Patent Nos. 3,271,157, 3,531,289, 3,574,628, Unexamined Published Japanese Patent Application Nos. 1019/1979, 158917/1979 and Examined Japanese Patent Publication No. 30571/1983; (b) the thiourea derivatives described in Unexamined Published Japanese Patent Application Nos. 82408/1978, 29829/1980 and 77737/1980; (c) an AgX solvent having a thiocarbonyl group between an oxygen or sulfur atom and a nitrogen atom as described in Unexamined Published Japanese Patent Application No. 144319/1978; (d) the imidazoles described in Unexamined Published Japanese Patent Application No. 100717/1979; (e) sulfites; (f) thiocyanates; (g) ammonia; (h) ethylenediamines substituted by hydroxylalkyl as described in Unexamined Published Japanese Patent Application No. 196228/1982; (i) the substituted mercaptotetrazoles described in Japanese Patent Application No. 202531/1982; (j) water-soluble bromides; and (k) the benzimidazoles described in Unexamined Published Japanese Patent Application No. 54333/1983.
[0033] These solvents can be used either on their own or as admixtures. Preferred solvents are thioethers, thiocyanates, thioureas, ammonia and bromides, and combinations of ammonia and bromides are particularly preferred.
[0035] Step (B) is preferably performed at a pH of 3 - 13 and at a temperature of 30 - 70°C, with the pH of 6 - 12 and the temperature of 35 - 50°C being particularly preferred.
[0036] In a preferred embodiment of the present invention, an emulsion containing advantageous seed grains could be obtained by ripening at a pH of 10.8 - 11.2 and at a temperature of 35 - 45°C for a period of 30 seconds to 5 minutes using ammonia in an amount of 0.4 - 1.0 mole/L in combination with potassium bromide in an amount of 0.03 - 0.5 moles/L.
[0037] A water-soluble silver salt may be added during step (B) for the purpose of adjusting the degree of ripening.
[0038] Step (C) for growing the silver halide seed grains can be performed by controlling the following factors during the precipitation of silver halide grains and the Ostwald ripening of the same: pAg, pH, temperature, the concentration of a silver halide solvent and the composition of silver halide, and the rates of addition of solutions of a silver salt and a halide.
[0039] A typical condition for growing the seed grains obtained in step (B) is described in Unexamined Published Japanese Patent Application Nos. 39027/1976, 142329/1980, 113928/1983, 48521/1979 and 49938/1983; namely, a solution of a water-soluble silver salt and a solution of a water-soluble halide are added by a double-jet method, with the rates of their addition being slowly changed in such a way that neither formation of new nuclei nor Ostwald ripening will occur as the growth of seed grains proceeds. Another condition for growing the seed grains is to add fine silver halide grains, dissolve them and perform re-crystallization as described on page 88 of A Summary of the Proceedings of the 1983 Annual Meeting of Society of Photographic Science and Technology of Japan. The former method is preferred.
[0040] In producing a silver halide emulsion of high AgI content in accordance with the present invention, the silver halide seed grains are preferably grown at a pAg of 5 - 11, a temperature of 40 - 85°C and at a pH of 1.5 - 12, with the pH of 1.5 - 7.0 being particularly preferred.
[0041] In the growth of silver halide seed grains, an aqueous solution of silver nitrate and an aqueous solution of a halide are preferably added by a double-jet method. Iodine may be supplied as silver iodide into the system. The rates of addition of those aqueous solutions are preferably such that no new nuclei will form while preventing the grain size distribution from spreading on account of Ostwald ripening, namely, at rates that are between 30% and 100% of the rate at which new nuclei will be produced.
[0042] The condition of stirring the mother liquor is of extreme importance to the production of the silver halide emulsion of the present invention. A particularly preferred stirrer is of such a type as described in Unexamined Published Japanese Patent Application No. 160128/1987 that has nozzles for supplying the necessary solutions provided in the mother liquor at a position close to the intake port on the stirrer. A preferred rotational speed of the stirrer is in the range of 400 - 1,200 rpm.
[0043] The following examples are provided for the purpose of further illustrating the present invention but are in no way to be taken as limiting.
Example 1
Preparation of spherical seed emulsion:
[0044] A seed emulsion comprised of monodisperse spherical grains was prepared by the following procedure.
[0045] To the vigorously stirred solution A₁ at 40°C, solution B₁ was added over a period of 20 seconds to produce nuclei. Fortyfive seconds later, solution C₁ was added over a period of 60 seconds. After an additional 45 seconds, solution D₁ was added over a period of 20 seconds and ripening was performed for 2 minutes. The concentrations of ammonia and potassium bromide during the ripening were 0.55 moles/L and 0.060 moles/L, respectively.
[0047] Examination by electron microscopy showed that the resulting seed emulsion was comprised of monodisperse spherical grains having an average size of 0.32 µm.
Example 2
Preparation of the emulsion of the present invention:
[0048] An emulsion within the scope of the present invention having an average AgI content of 28.1% was prepared by the following procedure.
[0049] An apparatus of the same type as described in Unexamined Published Japanese Patent Application No. 160128/1987 was used, with 6 nozzles being set to supply each of the solutions B₂ and C₂ beneath the agitating propeller in the mixing section.
[0050] To solution A₂ being stirred at 1,000 rpm at 75°C, solutions B₂ and C₂ were added by a double-jet method with the flow rates being gradually increased from the initial 11.42 ml/min to the final 39.62 ml/min. During the double-jet addition, the pAg and pH of the system were held at 7.5 and 5.8, respectively.
[0051] The structure of grains obtained upon completion of the double-jet addition was examined by electron microscopy and is shown in Fig. 1. The resulting emulsion was monodisperse and within the scope of the present invention; the content of twins with two parallel twin planes was 60%, and the width of grain-size distribution was 13%. An X-ray diffraction scan for the grains at a (420) face is shown in Fig. 2, from which one can see that the emulsion had a uniform composition with a peak observed for the 30 mol% AgI containing phase. The peak on the right side was due to the seed emulsion.
Example 3
Preparation of spherical seed emulsion:
[0053] To the vigorously stirred solution A₃ at 40°C, solutions B₃ and C₃ were added by a double-jet method over a period of 30 seconds to produce nuclei. The pBr was maintained at 1.09 - 1.15 during the double-jet addition. One and a half minute later, solution C₃ was added over a period of 20 seconds and ripening was performed for 5 minutes. The concentrations of KBr and ammonia during the ripening were 0.071 mole/L and 0.63 moles/L, respectively.
[0054] Thereafter, the pH was adjusted to 6.0, immediately followed by desalting and washing. Examination by electron microscopy showed that the resulting seed emulsion was comprised of monodisperse spherical grains having an average size of 0.36 µm.
Example 4
Preparation of the emulsion of the present invention:
[0055] Another emulsion within the scope of the present invention having an average AgI content of 26.1% was prepared by the following procedure.
[0056] An apparatus of the same type as in Example 2 was used, with 6 nozzles being set to supply each of the solutions B₄ and C₄ beneath the agitating propeller in the mixing section.
[0057] To solution A₄ being stirred at 1,000 rpm at 75°C, solutions C₄ were added by a double-jet method with the flow rates being gradually B₄ and increased from the initial 8.57 ml/min to the final 24.16 ml/min. During the double-jet addition, the pAg and pH of the system were held at 7.5 and 2.0, respectively (pH control was effected with nitric acid).
[0058] The structure of grains obtained upon completion of the double-jet addition was examined by election microscopy. The grains were totally composed of twins and were monodisperse: the content of twins having two parallel twin planes was 72%, and the width of grain-size distribution was 13%. An X-ray diffraction scan for the grains at a (420) face showed that the emulsion had a uniform composition with a peak observed for the 30 mol% AgI containing phase.
Example 5
[0059] An additional emulsion within the scope of the present invention was prepared by repeating the procedure of Example 4 except that the pAg during the double-jet addition was changed to 8.5. Examination by electron microscopy showed that the grains obtained upon completion of the double-jet addition were monodisperse; the content of twins with two parallel twin planes was 68% and the width of grain-size distribution was 17%. The twins having parallel twin planes had an average diameter to thickness ratio of 2.2.
1. A silver halide emulsion that has a silver iodide content of 10 - 45 mol%, that
is monodisperse and that is chiefly comprised of twinned crystals.
2. A silver halide emulsion according to claim 1 which is a core/shell type emulsion
with the silver iodide content being 15 - 40 mol% in the surface layer and being no
more than 8 mol% in the central portion.
3. A silver halide emulsion according to claim 1 or 2 which is composed of silver
iodobromide.
4. A silver halide emulsion according to claim 1 whose monodispersity is expressed
by a width of grain-size distribution that is no more than 20%.
5. A silver halide emulsion according to claim 1 which is chiefly composed of twinned
crystals having at least two parallel twin crystal planes.
6. A silver halide emulsion according to claim 5 wherein the twinned crystals comprise
(111) faces.
7. A silver halide emulsion according to claim 5 wherein the ratio of the diameter
of an equivalent circle of the image of a twinned grain as projected in a direction
perpendicular to the twin planes to the distance between a parallel twin plane and
a grain surface parallel to it is in the range of 1 - 20.
8. A silver halide emulsion according to claim 1 which is produced by a process that
comprises the basic step of supplying a solution of a water-soluble silver salt and
a soluton of a water-soluble halide in the presence of a protective colloid, which
process further comprises:
(A) a step of producing nuclear grains in which the pBr of the mother liquor is held at a value between 2.0 and -0.7 for a period that is at least one first half of the period of the formation of silver halide precipitates having an AgI content of 0 - 5 mol%;
(B) a step of forming seed grains in which, subsequent to step (A), silver halide seed grains which are substantially monodisperse spherical twins are formed in the presence of a silver halide solvent in the mother liquor in an amount of 10⁻⁵ - 2.0 moles per mole of silver halide; and
(C) a growth step in which the seed grains are grown by adding a solution of a water-soluble silver salt and a solution of a water-soluble halide and/or fine silver halide grains.