EP0477772A1

EP0477772A1 - Photographic silver halide materials

Info

Publication number: EP0477772A1
Application number: EP91115913A
Authority: EP
Inventors: Michael Samuel Wharton; Alan Dobson
Original assignee: Ilford Ltd
Current assignee: Ilford Imaging UK Ltd
Priority date: 1990-09-26
Filing date: 1991-09-19
Publication date: 1992-04-01
Also published as: GB9020947D0

Abstract

Photographic silver halide material which comprises coated on a subbed film base a silver halide emulsion which is a blend of a silver halide emulsion of the twinned high-iodide core type as just described which comprises from 10 to 20 mole % iodide and of a silver iodiobromide emulsion.

Description

This invention relates to photographic silver halide material.
It is a continuing objective of manufacturers of photographic silver halide material to produce high speed film material which has a low granularity on development. One method of producing high speed emulsions is to employ silver halide grains of the core/shell type which have a relatively high iodide core. That is to say a core which can comprise up to 40 mole iodide. But it has proved difficult to incorporate iodide into the silver halide crystals in a controlled manner. However in British patent specifications 1520976; 1570581 and 1596602 a method is described for preparing twinned monosize crystal which have a desirable high iodide core and which exhibit a high photographic speed and wherein the iodide is incorporated into the core in a controlled manner.
The method described in these patent specifications can be summed up by the following steps:- a) forming in a colloid dispensing medium silver halide crystals being predominantly of the Agl hexagonal lattice structure, these are then used as seed crystals for step b) which comprises mixing in a dispersion medium which contains the seed crystals an aqueous solution of a silver salt and an aqueous solution of an alkali metal or ammonium bromide or chloride or mixtures thereof so forming twinned silver halide crystals containing the halide being added and iodide from the seed crystals and continuing the addition of the silver halide until dissolution of the seed crystals is substantially complete, then c) causing the twinned crystals formed in step b) to increase in size by adding to the colloidal dispersion further alkali metal or ammonium halide and aqueous silver salt solution and then d) removing the water-soluble salts and chemically sensitising the silver halide crystals.
Thus in this process a controlled high iodide containing core can be formed in step b) and a shell added in step c).
Photographic silver halide emulsion prepared by the above method are hereinafter referred to as twinned high iodide core emulsions.
Twinned high iodide core emulsions of this type have been used in the preparation of photographic material and they yield high speed photographic material. Moreover, the granularity of the processed negative is satisfactory. However, variable results are obtained when this material after exposure is developed in different types of developing solutions. Whilst the manufacturers of photographic materials often state in the instruction leaflet or on the film box what is the preferred developing solution it is not possible for the manufacturer to control this. The use by the film user of other developing solutions can lead to inferior results thus it is desirable that photographic materials should be able to produce optimum results in all the main classes of developing solutions.
Moreover it may be desirable that another emulsion is present in an assembly which comprises the twinned high iodide core emulsion in order to extend the useable exposure range of the material and to improve its tone scale. However drawbacks have arisen when the two emulsions are coated as a bipack with the high iodide core-shell above the other emulsion with regard to granularity of the developed image on development of such material in low-sulphite developers.
Therefore according to the present invention there is provided photographic silver halide material which comprises coated on a subbed film base a silver halide emulsion which is a blend of a silver halide emulsion of the twinned high-iodide core type as just described which comprises from 10 to 20 mole % iodide and of a silver iodobromide emulsion which comprises less than 10 mole % iodide and which has a smaller crystal size and a photographic speed of from 0.2 to 1.0 log E units less than the twinned high-iodide core emulsion, the amounts of the emulsions in the blend being from 20 to 70% by weight of the total silver present being derived from the emulsion of the twinned high-iodide core type and the remainder from the smaller crystal size silver iodobromide emulsion.
Preferably in the blend 30-50% by weight of the total silver present is derived from the twinned high-iodide core emulsion and most preferably 35 - 45% by weight.
By high iodide core is meant a core which comprises from 20 to 40 mole% iodide. Preferably the amount of iodide in the core is from 35 to 40 mole % iodide.
The volume of the core can be from 10% of the total volume of the crystal up to 50% of the total volume of the crystal. A useful volume of the core is from 25 to 45% of the total volume of the crystal.
Present on the core is at least one silver bromide shell which can comprise up to 10 mole % iodide overall. Often a plurality of shells are present. In one embodiment there is present a so-called development control layer which comprises up to 15 mole % iodide.
The silver iodobromide emulsion which is of smaller crystal size is of the type used in a number of commercially available black and white films. The Theory of the Photographic Process 4th Edition by J H James, published in 1977, chapter 3, pages 88-104 describes the preparation of such emulsions.
In one embodiment the iodobromide emulsion of smaller crystal size is of the ammonia ripening process type. In another embodiment the iodobromide emulsion of the smaller crystal size has been prepared by a controlled crystal growth method.
The range of crystal size for the silver halide of the twinned high iodide core type is from 0.6 to 1.2_/1.m.
The range of crystal size for the silver iodobromide emulsion which comprises less than 10 mole % iodide is from 0.4 to 0.9tim. There must be a difference of at least 0.1µm in the crystal size of the two emulsions in the blend.
Two types of developer which are used to develop exposed black and white films are so-called high sulphite developers often supplied as solid ingredients and which comprise when made up from 0.6 to 0.8 moles of sulphite per litre and so-called low sulphite developers which are often supplied as concentrated liquids and which comprise from 0.01 to 0.1 moles of sulphite per litre. Examples of both types of developing solutions are set forth in the example which follows.
The photographic material as shown in the examples which follow exhibits a high speed and low granularity in both types of developing solutions.
The water-soluble salts formed in the emulsification processes may be removed by any of the well-known methods. Such methods often involve flocculating the silver halide and colloid dispersing agent, removing this flocculate from the then aqueous medium, washing it and redispersing it in water. One other common method is ultrafiltration, in which the emulsion is passed over a membrane under pressure. The pore size of the membrane is such that the silver halide crystals and most of the colloid dispersing medium is retained, whilst water and solutes permeate through. Most of the well-known methods allow the emulsion to be concentrated as well as washed. This is important when weak reagent solutions are employed, particularly those with concentrations below 3M.
The silver halide crystals in the emulsion are preferably chemically sensitised on the surface by any of the well known means, for example by use of sulphur or selenium compounds and/or salts of the noble metals such as gold, iridium, rhodium, osmium, palladium or platinum. Chemical sensitisation is preferably carried out in the presence of sulphur-containing ripening agents such as thioethers or thiocyanate compounds. The fully grown crystals are sensitised in this manner, so that the products of chemical sensitisation are formed on the surface of the crystal so that such sensitised crystals become developable in a surface developer after exposure to light.
The silver halide crystals in the emulsion of the present invention are preferably spectrally sensitised by the addition of spectral sensitisers for example carbocyanine and merocyanine dyes to the emulsions.
The emulsions may contain any of the additives commonly used in photographic emulsions for example wetting agents, such as polyalkene oxides, stabilising agents, such as tetraazaindenes, metal sequestering agents, growth or crystal habit modifying agents commonly used for silver halide such as adenine and plasticisers such as glycerol to reduce the effect of mechanical stress.
Preferably the colloid dispersing medium is gelatin or a mixture of gelatin and a water-soluble latex for example or latex vinyl acrylate-containing polymer. Most preferably if such a latex is present in the final emulsion it is added after all crystal growth has occurred. However other water-soluble colloids for example casein, polyvinyl pyrrolidone or polyvinyl alcohol may be used alone or together with gelatin.
The silver halide emulsion assemblies of the present invention exhibit high speed and low granularity, thus they are of particular use in high speed camera film material.
The invention includes coated photographic materials containing the defined emulsion blends in at least one layer.
The following Examples will serve to illustrate the invention.

EXAMPLE 1

Preparation of silver iodobromide emulsion which is of the twinned high-iodide core type. This is emulsion A.

Step A - Preparation of monosized silver iodide emulsion

The silver iodide seed emulsion was grown as described in Example 1 of British Patent Specification No. 1596602 except that the crystals had an average size of 0.65tim.

Step B - Recrystallisation

16.9 litres of 4.1 % gel solution was added to a quantity of the emulsion prepared in step A containing 35 moles of silver at a temperature of 70 ° C and with constant stirring. 1.5m solutions of silver nitrate and sodium bromide were added by double jetting until 35 litres of silver nitrate had been added. The initial jetting rate was 199 cm³/minute. The potential was maintained at -58mV at 70 C.

Step C - Growth

3.6% of the emulsion produced in step B was taken, and .35 litres of a 29% w/v inert gel solution added with thorough agitation at 65 °C. 0.2 cm³ of tri-n-butyl phosphate was added as an antifoam. The volume of each crystal was then increased by double jetting 1.5 m solutions of silver nitrate and sodium bromide while maintaining the potential at -33 mV at 65 °C. The first 355 cm³ was added as a jetting rate of 10 cm³ minute, a second 355 cm3 was added at a jetting rate of 15 cm3/minute and then 533 cm3 was added at 20 cm3/minute.

Step D - Further Growth

48% of the emulsion prepared in step C was taken and the crystal volume was increased by double jetting 1.5 Molar solultions of silver nitrate and sodium bromide maintaining the potential at -33mV at 65C. A total of 1070 cm3 of each solution was added at 20 cm3/minute.
At this stage the emulsion was composed of AglBr in which 15 mole % of the halide was iodide and the median crystal size was 1.0 um (based on equivalent circular diameter).
The emulsion was then adjusted at 40 °C to pH 6 and pAg 8.2. It was then digested at 52 °C with sodium thiosulphate and sodium tetrachloroaurate to chemically sensitise it. After optimised chemical sensitisation a tetrazindene stabiliser was added.
The mole % iodide in the core was 35% and the volume of the core was 43% of the whole crystal. The average mean size of the crystals was 1.0µm.
Preparation of silver iodobromide emulsion which is prepared by a double-jetting and ammonia ripening process. This is emulsion B.
5.26 kg of gelatin was dissolved in 262 litres of distilled water.
A mixed halide solution which comprised 90.8% ammonium bromide and 9.2% potassium iodide 266 litres with a molar concentration of 1.75 was then jetted in to the gelatin solution.
Shortly after the commencement of the jetting of the mixed halide solution into the gelatin solution 228 litres of a 2.31 M silver nitrate solution was jetted slowly into the gelatin solution.
Then after a short period 34 litres of a mixed ammonium bromide/ammonia solution was added which comprised 20% NH₃ and 2.73 mole % ammonium bromide.
Then after a short period 52 litres of a 20% ammonium solution is added to cause Ostwald ripening.
Then after another short period 61 litres of a 7.2% N sulphuric acid is added to the gelatin solution to stop the Ostwald ripening process. The average mean size of the crystals thus prepared was 0.6µm.
The emulsion as thus prepared was then desalinated and then chemically digested using sodium thiosulphite and a gold salt as with emulsion A. After chemical digestion was complete a tetraazindene stabiliser was added to the emulsion. An optimum quantity of a carbocyanine sensitising dye was added to both emulsions A and B.
Both emulsions A and B were coated on to a film base, exposed and developed and their photographic speeds were compared at full development.
Emulsion A had speed of 5.92 log E units and Emulsion B had a speed of 5.42 log E units.
Preparation of photographic materials.
Material 1. Sufficient of emulsion A was then coated on a cellulose triacetate base to yield a coating weight of 7.0g Ag/_m ².
Material 2. A blended emulsion was then prepared by mixing emulsions A and B so that 40% by weight of the silver was derived from emulsion A and 60% by weight of the silver was derived from emulsion B.
Then sufficient of this blended emulsion was coated on a cellulose triacetate base to yield a coating weight of 7g Ag/_m ².
Samples of material 1 and 2 were then light exposed through a step-wedge of 1/30 second by a light source of 250 lux.
Samples of materials 1 and 2 were then developed to the same contrast both in developer 1 and developer 2.
Developer 1 was a high sulphite developer of the following formula.
Developer 2 was a low sulphite developer in which the sulphite was present as diethanolamine bisulphite.
Made up to 1 litre and then diluted 1 + 31 for use.
Both sets of material were then fixed in an ammonium thiosulphate fixing bath, washed and then sensitometric results were measured.
Both materials 1 and 2 were found to exhibit a photographic speed of approximately 400 I.S.O.. Both materials exhibited low fog. The granularity of material 1 was acceptable but the image in the high lights was noticeably grainy. However in material 2 the granularity was low and even in the high lights very little if any grain was visible.

EXAMPLE 2

Emulsion Growth Scheme

Preparation of silver iodobromide emulsion which is of the twinned high iodide core type and which has a development control layer. This is emulsion C.

Step A - Preparation of monosized iodide emulsion

The silver iodide seed emulsion was grown as described in Example 1 of British Patent Specification No. 1596602 except that the crystals had an average size of 0.65µm.

Step B - Recrystallisation

16.9 litres of 4.1% gel solution was added to a quantity of the emulsion prepared in step A containing 35 moles of silver at a temperature of 70 °C and with constant stirring. 1.5m solutions of silver nitrate and sodium bromide were added by double jetting until 35 litres of silver nitrate had been added. The initial jetting rate was 199 cm³/minute at the end of addition. The potential was maintained at -58mV at 70 C. The mole % iodide in the core was 35% and the volume of the core was 43% of the whole crystal.

Step C - Growth

3.6% of the emulsion produced in step B was taken, and 0.35 litres of a 29% w/v inert gel solution added with thorough agitation at 65 °C. 0.2 cm³ of tri-n-butyl phosphate was added as an antifoam. The volume of each crystal was then increased by double jetting 1.5m solutions of silver nitrate and sodium bromide while maintaining the potential at -33 mV at 65 ° C. The first 355 cm³ was added as a jetting rate of 10cm³ minute, a second 355cm³ was added at jetting rate of 15cm³/minute and then 553 cm³ was added at 20cm³/minute.

Step D Formation of development control layer and final growth

48% of the emulsion prepared in step C was taken and the crystal volume was increased by double jetting 1.5 Molar solutions of silver nitrate and a 95 : 5 mole percent mixture of sodium bromide and potassium iodide maintaining the potential at -33mV at 65C. A total of 713 cm3 of each solution was added at 20cm3/minute. Finally, 357 cm3 of solutions of 1.5 molar silver bromide and 1.5 molar sodium bromide were added maintaining the same temperature and potential at 20cm³/minute.
At this stage the emulsion was composed of AglBr in which 16.3 mole % of the halide was iodide and the median crystal was 1.0 um (based on equivalent circle diameter).
After the completion of crystal growth, the emulsion was desalinated and redispersed with a solution of limed ossein gelatin.
The emulsion was then chemically digested using sodium thiosulphate and gold. After chemical sensitisation was complete a tetraazindine stabiliser was added to the emulsion.
A sample of emulsion B as described in Example 1 was also prepared.
An optimum quantity of a carbcyamine optical sensitising dye was added to both emulsion B and C.
A sample of Emulsion C was coated on a film base, exposed and fully developed. It was found to have a speed of 5.90 log E units. This is to be compared with the speed of 5.42 log E units for Emulsions B as tested in Example 1.
Preparation of photographic materials.
Material 1. Sufficient of emulsion B was then coated on a cellulose triacetate base to yield a coating weight of 2.8g Ag/m². Over this layer was coated sufficient of emulsion C to yield a coating weight of 4.2 Ag/_m2.
Material 2. A blended emulsion was then prepared by mixing emulsions C and B so that 40% by weight of the silver was derived from emulsion C and 60% by weight of the silver was derived from emulsion B.
Then sufficient of this blended emulsion was coated on a cellulose triacetate base to yield a coating weight of 7g Ag/_m ².
Samples of material 1 and 2 were then light exposed through a step-wedge of 1/30 second by a light source of 250 lux.
These samples of materials 1 and 2 were then developed in developer 1 and developer 2 to equal contrast.
Developer 1 was a high sulphite developer of the following formula.
Developer 2 was a low sulphite developer in which the sulphite was present as diethanolamine bisulphite.
Made up to 1 litre and then diluted 1 + 31 for use.
Both sets of material were then fixed in an ammonium thiosulphate fixing bath, washed and then sensitometric results were measured.
In developer 1 both materials were developed to a contrast of G1.5 = 0.62.
In developer 2 both materials were developed to a contrast of G1.5 = 0.62.
The granularity measured was the root mean square granularity which was measured at a density of 1.0 above Dmin using a 21µ square aperture. The density of the scale was specular.

Claims

1. Photographic silver halide material which comprises coated on a subbed film base a silver halide emulsion which is characterised in that it is a blend of a silver halide emulsion of the twinned high-iodide core type as just described which comprises from 10 to 20 mole % iodide and of a silver iodiobromide emulsion which comprises less than 10 mole % iodide and which has a smaller crystal size and a photographic speed of from 0.2 to 1.0 log E units less than the twinned high-iodide core emulsion, the amounts of the emulsions in the blend being from 20 to 70% by weight of the total silver present being derived from the emulsion of the twinned high-iodide core type and the remainder from the smaller crystal size silver iodobromide emulsion.

2. Photographic silver halide material according to claim 1 characterised in that the blend comprises 30-50% by weight of the total silver present derived from the twinned high-iodide core emulsion.

3. Photographic silver halide material according to claim 1 characterised in that the blend comprises 35-45% by weight of the total silver present derived from the twinned high-iodide core emulsion.

4. Photographic silver halide material according to claim 1 characterised in that the twinned high-iodide core emulsion the core comprises from 20 to 40 mole % iodide.

5. Photographic silver halide material according to claim 1 characterised in that in the twinned high iodide core emulsion the core comprises from 35 to 40 mole % iodide.

6. Photographic silver halide material according to claim 1 characterised in that the twinned high iodide core emulsion comprises a development control layer which comprises up to 15 mole % iodide.