Improved oil-in-water emulsions

Abstract: This invention provides an oil-in-water emulsion, particularly a photographic dispersion, having a reduced droplet size in the oil phase by an increase in the viscosity of the aqueous phase prior to homogenisation of the oil and aqueous phases; methods of forming thereof and to the use of the emulsion in a silver halide based photographic system.

Description: The present invention relates to oil-in-water emulsions and in particular to improved photographic dispersions which conventionally comprise an aqueous gel continuous phase and a dispersed oil phase which comprises droplets of image dye-forming coupler dissolved in one or more organic coupler solvents. In particular, the present invention relates to a method of decreasing the droplet size of the organic discontinuous phase. Whilst the invention is particularly described herein with reference to photographic dispersions, it will be appreciated by a person skilled in the art that the invention can be applied in other fields of technology in which oil-in-water emulsions are required to have a discontinuous phase with a small droplet size. With reference to the photographic art, it has been found that the reactivity of image-dye-forming coupler in a photographic dispersion is normally inversely related to the droplet size of the organic disperse phase of the dispersion. It is desirable therefore to provide photographic dispersions having as small a droplet size as possible. Ullmans Enzyklopädie der technischen Chemie, 4 ea., vol 10, 1975, Verlag Chemie, Weinheim, pages 450-455 and 461-465, disclose that an increase in viscosity of an emulsion and a decrease of droplet size are factors leading to a more stable emulsion. There is no disclosure however of an increase in viscosity of the aqueous phase leading to a reduction in droplet size in the oil phase. US-A-5 419 848 teaches that unstable 2-phase agglomerates can be transformed into stable oil-in-water emulsions having a viscosity of at least 500cP and a droplet size of 0.1 to 3 millimicrons by the addition to the organic solvent of a viscosifying thickener. US-A-5 380 628 describes a fine particle coupler dispersion prepared from a coupler and coupler solvents in an aqueous gelatin medium containing at least 1% by weight of an anionic surfactant, with final washing of the dispersion to remove at least a quarter of the surfactant. It states that when evaporation techniques are employed with an excess of surfactant, the viscosity of the resulting dispersion after homogenisation is too high, rendering the dispersion useless for coating. According to one aspect of the present invention there is provided an oil-in-water emulsion comprising a viscosity enhancer other than a surfactant or gelatin and having an average droplet size in the oil phase in the range 0.1µm to 2µm wherein the initial viscosity of the aqueous phase is increased by the use of said viscosity enhancer, prior to homogenisation of the oil and aqueous phases. In another aspect there is provided a method of forming an oil-in-water emulsion, comprising homogenising the oil and aqueous phases of the emulsion and characterised by increasing the initial viscosity of the aqueous phase, prior to homogenisation of the oil and aqueous phases, thereby reducing the average droplet size of the oil phase to be in the range 0.1µm to 2µm. In a particular aspect of the present invention, the oil-in-water emulsion is a photographic dispersion wherein the oil phase comprises droplets of image-dye-forming coupler or of filter agent dissolved in one or more organic coupler solvents and in which the aqueous phase comprises gelatin. Where required, for example for coating, the photographic dispersion may be diluted with water after homogenisation with no apparent effect upon droplet size or stability. After coating, the photographic dispersion is solid or substantially solid under the conditions of pressure and temperature which are normal for the use of photographic film or photographic paper. The oil and aqueous dispersion may be homogenised at an elevated temperature at which the two phases are sufficiently fluid for homogenising. The viscosity of the oil-in-water emulsion and particularly a photographic dispersion may be increased by the addition of a viscosity enhancer, or 'thickener'. For this purpose, a proprietary viscosity enhancer may be used such as, for example, Viscofas™ of ICI or Gantrez™ of GAF Chemicals Corp. Other viscosity increasing agents which may be used in accordance with the present invention may include polyvinylpyridine, polyvinylpyrrolidone (GAF), polyvinylacetate, polyvinylalcohol, Viscalex™ (Messrs. Allied Colloids), polyethyleneglycol (BDH), polyethylene oxide (Aldrich) and Natrosol™ (hydroxyethylcellulose) (Hercules Inc.). Yet other viscosity increasing agents that may be used include latexes and hardeners. Alternatively, a viscosity increasing surfactant may be used such as Alkanol XC™ of Du Pont. It is well known that the addition of a surfactant will decrease droplet size through an interfacial tension-decreasing effect. However, it has been found that the concomitant increase in viscosity also correlates with a decrease in droplet size. The viscosity of the aqueous gel phase may also be increased by increasing the concentration of the gelatin. The conventional gelatin used in the photographic art is commercially available inter alia from Messrs. Eastman (e.g. Eastman 852 gelatin or Eastman 890 gelatin) or from Messrs. Croda. The aqueous gel phase of a commercial photographic film paper typically comprises 8-14% weight gelatin, more typically about 11% weight. In order to increase the viscosity of the aqueous the phase concentration of gelatin may be increased by as much as a factor of 2. After homogenisation, the dispersion may be rediluted by the addition of water to reduce the concentration of gelatin to the normal amount. In yet another aspect of the invention, the viscosity of the oil-in-water emulsion and particularly a photographic dispersion may be increased by replacing at least part of the water in the aqueous gel phase with a polyhydric alcohol such, for example, as glycerol. In some embodiments glycerol may be substituted for water such that 50-80% of the total gel volume is constituted by glycerol, typically say 62%. By way of example, the initial viscosity of the aqueous gel phase, for the purposes of the present invention, which may be in the range of 20cP to 2500cP, may typically be increased by at least 100cP as measured at 50°C. The viscosity may preferably be increased by a factor of from 3 to 50 inclusive. The final viscosity may be in the range of 100 to 2500cP, frequently 1000cP to 2500cP. The average droplet size of the oil dispersed phase in accordance with the invention is in the range 0.1µm to 2.0µm. Preferably, the average droplet size may be 0.1µm to 0.5µm. Normally, the homogenising step will comprise mixing the oil and aqueous phases to form a substantially uniform emulsion or dispersion. In yet another aspect of the present invention there is provided a photographic element comprising a photographic dispersion in which a dispersed oil phase comprises an image-dye-forming coupler or filter agent in one or more organic solvents contained within an aqueous gel as the continuous phase, in association with a silver halide emulsion layer comprising a viscosity enhancer other than a surfactant or gelatin, wherein the average droplet size of the dispersed oil phase is in the range of 0.1µm to 2µm and wherein the initial viscosity of the aqueous phase is increased by the use of said viscosity enhancer, prior to homogenisation of the oil and aqueous phases. The photographic element may be a single colour element or a multicolour element. Multicolour elements contain image-dye-forming units sensitive to each of the three primary regions of the visible range of the electromagnetic spectrum. Each unit may comprise a single emulsion layer or a plurality of emulsion layers sensitive to a given region of the spectrum. The layers of the element, including the layers of the image-forming units, may be arranged in various orders as known in the art. In an alternative format, the emulsions sensitive to each of the three primary regions of the spectrum may be disposed as a single segmented layer. A typical multicolour photographic element comprises a support bearing a cyan image-dye-forming unit comprising a red-sensitive silver halide emulsion layer and a cyan dye-forming coupler; a magenta dye image-forming unit comprising at least one green-sensitive silver halide emulsion layer and a magenta dye-forming coupler; a yellow image-dye-forming unit comprising at least one blue-sensitive silver halide emulsion layer and a yellow dye-forming coupler. In particular with regard to the present invention the element may contain one or more UV absorbing layers incorporated into oil-in-water dispersions. Each image-dye-forming unit comprises a dispersed oil phase comprising an image-dye-forming coupler in one or more organic solvents, all contained within an aqueous gel as the continuous phase, wherein the emulsion layer comprises a viscosity enhancer other than a surfactant or gelatin and has an average droplet size of the dispersed oil phase in the range 0.1µm to 2µm and wherein the initial viscosity of the aqueous phase is increased by the use of said viscosity enhancer, prior to homogenisation of the oil and aqueous phases. The element may contain additional layers, such as for example filter layers, interlayers, overcoat layers and subbing layers. If desired, the photographic element can be used in conjunction with an applied magnetic layer as described in Research Disclosure , November 1992, Item 34390 published by Kenneth Mason Publications, Ltd., Dudley Annex, 12a North Street, Emsworth, Hampshire, P010 7DQ, England. When it is desired to employ the inventive materials in a small format film, Research Disclosure , June 1994, Item 36230, provides suitable embodiments. In the following discussion of suitable materials for use in the emulsions and elements of this invention, reference will be made to Research Disclosure , September 1994, Item 36544, available as described above, which will be identified hereafter by the term "Research Disclosure". The contents of the Research Disclosure, including the patents and publications referenced therein and the Sections hereafter referred to are Sections of the Research Disclosure. The silver halide emulsions employed in the elements of this invention can be either negative-working or positive-working. Suitable emulsions and their preparation as well as methods of chemical and spectral sensitization are described in Sections I through V. Various additives such as UV dyes, brighteners, antifoggants, stabilizers, light absorbing and scattering materials, and physical property modifying addenda such as hardeners, coating aids, plasticizers, lubricants and matting agents are described, for example, in Section II and VI through IX. Colour materials are described in Sections X through XIII. Scan facilitating is described in Section XIV. Supports, exposure, development systems, and processing methods and agents are described in Sections XV to XX. Certain desirable photographic elements and processing steps are described in Research Disclosure , Item 37038, February 1995. With negative working silver halide a negative image may be formed. Optionally a positive (or reversal) image may be formed. The colour developing agent may be selected from p-phenylenediamines; typically the agent may be selected from:- 4-amino N,N-diethylaniline hydrochloride, 4-amino-3-methyl-N,N-diethylaniline hydrochloride, 4-amino-3-methyl-N-ethyl-N-(2-methanesulfonamido-ethyl)aniline sesquisulfate hydrate, 4-amino-3-methyl-N-ethyl-N-(2-hydroxyethyl)aniline sulfate, 4-amino-3-(2-methanesulfonamido ethyl)-N,N-diethylaniline hydrochloride and 4-amino-N-ethyl-N-(2-methoxyethyl)-m-toluidine di-p-toluene sulfonic acid. Said cyan, magenta and yellow dye-forming couplers may be used in combination with other classes of image couplers such as 3-acylamino- and 3-anilino-5-pyrazolones and heterocyclic couplers (e.g. pyrazoloazoles) such as, for example, those described in EP 285,274, U.S. Patent 4,540,654 and EP 119,860; and other 5-pyrazolone couplers containing different ballasts or coupling-off groups such as, for example, those described in U.S. Patent 4,301,235, U.S. Patent 4,853,319 and U.S. Patent 4,351,897. Yellow or cyan coloured couplers (e.g. to adjust levels of interlayer correction) and/or masking couplers such as, for example, those described in EP 213,490, Japanese Published Application 58-172,647, U.S. Patent 2,983,608, German Application DE 2,706, 117C, U.K. Patent 1,530,272, Japanese Application A-113935, U.S. Patent 4,070,191 and German Application DE 2,643,965 may also be used. Said masking couplers may be shifted or blocked. Photographically useful coupling-off groups ("PUGs") are well-known in the art. Such groups can determine the equivalency of the coupler, i.e., whether it is a 2-equivalent or a 4-equivalent coupler, or modify the reactivity of the coupler. Such groups can advantageously affect the layer in which the coupler is coated, or other layers in the photographic recording material, by performing, after release from the coupler, functions such as dye formation, development acceleration or inhibition, bleach acceleration or inhibition, electron transfer facilitation and colour correction. Representative classes of coupling-off groups include halo, alkoxy, aryloxy, heteryloxy, sulfonyloxy, acyloxy, acyl, heterocyclyl, sulfonamido, mercaptotetrazole, mercaptopropionic acid, phosphonyloxy, arylthio, and arylazo. These coupling-off groups are described in the art, for example, in U.S. Pat. Nos. 2,455,169, 3,227,551, 3,432,521, 3,476,563, 3,617,291, 3,880,661, 4,052,212 and 4,134,766; and in U.K. Patents and published application Nos. 1,466,728, 1,531,927, 1,533,039, 2,006,755A and 2,017,704A. Thus, the couplers may be used in association with materials that accelerate or otherwise modify the processing steps e.g. of bleaching or fixing to improve the quality of the image. Bleach accelerators described in EP 193,389; EP 301,477; U.S. 4,163,669; U.S. 4,865,956; and U.S. 4,923,784 are particularly useful. Also contemplated is use of the coupler in association with nucleating agents, development accelerators or their precursors (U.K. Patent 2,097,140; U.K. Patent 2,131,188; electron transfer agents (U.S. 4,859,578; U.S. 4,912,025); antifogging and anti colour-mixing agents such as derivatives of hydroquinones, aminophenols, amines, gallic acid; catechol; ascorbic acid; hydrazides; sulfonamidophenols; and non colour-forming couplers. The couplers may be used in combination with filter dye layers comprising colloidal silver sol or yellow and/or magenta filter dyes or UV absorbing layers, either as oil-in-water dispersions as hereindescribed, latex dispersions or as solid particle dispersions. Additionally, they may be used with "smearing" couplers (e.g. as described in U.S. 4,366,237; EP 96,570; U.S. 4,420,556; and U.S. 4,543,323). Also, the couplers may in some embodiments be blocked or coated in protected form as described, for example, in Japanese Application 61/258,249 or U.S. 5,019,492. The couplers may further be used in combination with image-modifying compounds such as "Developer-Inhibitor-Releasing" compounds (DIRs); DIRs useful in conjunction with said couplers are known in the art and examples are described in U.S. Patent Nos. 3,137,578; 3,148,022; 3,148,062; 3,227,554; 3,384,657; 3,379,529; 3,615,506; 3,617,291; 3,620,746; 3,733,201; 4,049,455; 4,095,984; 4,126,459; 4,149,886; 4,150,228; 4,211,562; 4,248,962; 4,259,437; 4,362,878; 4,409,323; 4,477,563; 4,782,012; 4,962,018; 4,500,634; 4,579,634; 4,579,816; 4,607,004; 4,618,571; 4,678,739; 4,746,600; 4,746,601; 4,791,049; 4,857,447; 4,865,959; 4,880,342; 4,886,736; 4,937,179; 4,946,767; 4,948,716; 4,952,485; 4,956,269; 4,959,299; 4,966,835; 4,985,336 as well as in patent publications GB 1,560,240; GB 2,007,662; .GB 2,032,914; GB 2,099,167; DE 2,842,063, DE 2,937,127; DE 3,636,824; DE 3,644,416 as well as the following European Patent Publications: 272,573; 335,319; 336,411; 346,899; 362,870; 365,252; 365,346; 373,382; 376,212; 377,463; 378,236; 384,670; 396,486; 401,612; 401,613. Such compounds are also disclosed in "Developer-Inhibitor-Releasing (DIR) Couplers for Colour Photography", C.R. Barr. J. R. Thirtle and P.W. Vittum in Photographic Science and Engineering, Vol. 13, p.174 (1969). Generally, the developer inhibitor-releasing (DIR) couplers may include a coupler moiety and an inhibitor coupling-off moiety (IN). The inhibitor-releasing couplers may be of the time-delayed type (DIAR couplers) which also include a timing moiety or chemical switch which produces a delayed release of inhibitor. Examples of typical inhibitor moieties are: oxazoles, thiazoles, diazoles, triazoles, oxadiazoles, thiadiazoles, oxathiazoles, thiatriazoles, benzotriazoles, tetrazoles, benzimidazoles, indazoles, isoindazoles, mercaptotetrazoles, selenotetrazoles, mercaptobenzothiazoles, selenobenzothiazoles, mercaptobenzoxazoles, selenobenzoxazoles, mercaptobenzimidazoles, selenobenzimidazoles, benzodiazoles, mercaptooxazoles, mercaptothiadiazoles, mercaptothiazoles, mercaptotriazoles, mercaptothiatriazoles, mercaptotriazoles, mercaptooxadiazoles, mercaptodiazoles, mercaptooxathiazoles, telleurotetrazoles or benzisodiazoles. The following is a description by way of example only and with reference to the accompanying drawings of methods of carrying the present invention into effect. Note that in the examples all viscosities were measured 50°C unless otherwise stated. In the drawings:- Figure 1 shows the effect of gel phase viscosity (altered by changing gel concentration) on droplet size. Figure 2 shows the effect of gel phase viscosity (modified by adding Viscofas™) on droplet size. Figure 3 shows the effect of gel phase viscosity (varied by adding Gantrez™ AN 149) on droplet size. Figure 4 shows the effect of gel phase viscosity (altered by adding surfactant) on droplet size and distribution, in which the oil phase contained only solvent and no coupler. Figure 5 shows the effect of gel phase viscosity (altered by addition of glycerol) on droplet size. Figure 6 shows the effect of gel phase viscosity (altered by addition of Gantrez™S 97) on droplet size. Figure 7 shows the effect of gel phase viscosity (altered by various methods) on coated D max for five magenta-forming dispersions. Example 1 A photographic dispersion, consisting of an oil phase of tricresyl phosphate solvent and 1-(2,4,6-trichlorophenyl)-3-{-[α-(3-tertbutyl-4-hydroxyphenoxy)tetradecane amido]-2-chloroanilino}-5-pyrazolone as dye-forming coupler, in the ratio of 1:1, was made in a "rotor-stator" high speed mixer, with a gel phase consisting of an Eastman gelatin (15.2cP) solution plus surfactant (a constant 0.5% Alkanol XC™). The gelatin concentration was varied so the overall viscosity varied from below 100cP to above 1000cP. After a constant homogenizing time, the "peak" droplet size of the resulting dispersions was measured with a Joyce-Loebel disc centrifuge. The results are shown in Figure 1. It will be seen that as the viscosity increased in the range 100-1000cP the droplet size decreased from 1.2µm to 0.4µm. Using the same conditions but with a Croda (4.8cp)gelatin, it appeared that the viscosity/droplet size relationship was even steeper. Example 2 Using a similar dispersion to that described in Example 1, the gel phase viscosity was varied by adding a commercial viscosity enhancer, Viscofas™ of Messrs. ICI. As the gel viscosity was increased from about 20cP to about 1000cP, the peak droplet diameter fell from 2.4 µm to 0.5µm; the results are shown in Figure 2. Example 3 Using a similar dispersion to that described in Example 1, the gel phase viscosity was varied by adding a different commercial viscosity enhancer, viz Gantrez™ AN 149 (from GAF Great Britain Ltd). This time, as the gel phase viscosity increased over a similar range, there was an initial plateau, but above about 100cP, the droplet size again fell, from about 1.9µm to about 0.7µm at 2000cP; the results are shown in Figure 3. Example 4 Using a "blank" dispersion i.e one in which the oil phase contained only tricresyl phosphate solvent and no dye-forming coupler, the viscosity of the continuous phase (7% gel solution) was increased by adding increasing volumes of surfactant solution (10% aqueous Alkanol XC™). In this case, not only was the mean droplet size measured (by disc centrifuge), but the size distribution was also measured (as the width of the distribution curve at the position of half the peak height). Again, as the gel phase viscosity increased, from 20 to 300cP, the droplet size decreased from 2.2µm to 0.4µm. The size spread also fell - from 2.6µm to 0.2µm. The results are shown in Figure 4. Example 5 Using a yellow-forming photographic dispersion (dispersion A described hereinafter) on a pilot scale high pressure orifice plant, it was found (from a designed experiment having over 100 varying runs) that, with all other conditions constant, the droplet size was inversely proportional to 0.02 times the % gelatin concentration (for concentrations between 8 and 11%), i.e. a gel multiplying factor of -0.02. This means that for a 0.1% increase in gelatin concentration, there will be an expected 0.002µm decrease in droplet size. Dispersion A Oil phase 2-(2'-butoxyethoxy)ethyl acetate 32.9g dibutylphthalate 30.9g α-[4-4-benzyloxyphenylsulphonyl)-phenoxy]-α-(pivaloyl)-2-chloro-5-[γ(2,4-ditertamylphenoxy)butyramido]acetanilide 129.0g Aqueous phase dry gelatin (Eastman 852) 87.7g demineralised water 640ml sodium propionate solution (14.8%) 10.4ml 2N propionic acid 4.6ml 10% Alkanol XC solution 90.0ml Example 6 Using a similar analysis for a UV filter dispersion (Dispersion B as described hereinafter) a similar result to that described in Example 5 was obtained. This time, the gel multiplying factor predicting droplet size was -0.07. This result was obtained on a research-scale plant, and was subsequently confirmed on a larger scale. Dispersion B Oil phase 2,4-di-t-butyl-6-benzotriazol-2-ylphenol 1.000kg 6-t-butyl-2-(5-chlorobenzotriazol-2-yl)-4 methylphenol 0.1765kg 1,4-cyclohexylene-dimethylene bis(2-ethylhexanoate) 0.3918kg dioctylhydroquinone 0.1341kg Aqueous phase dry gelatin (Eastman 890) 0.8329kg demineralised water 5.877kg sodium propionate 0.0280kg 2N propionic acid 0.0693kg 10% Alkanol XC™ in water 0.667kg Example 7 Using the basic formula of Dispersion B above, the aqueous phase was modified by serially substituting glycerol for water up to a maximum of glycerol 62% of the total gel volume. The total gel volume was maintained constant in all samples. The effect on the droplet size as a result of changing the viscosity (measured at 500 sec -1 shear rate) in this way is shown in Figure 5. Example 8 Similarly Gantrez™ S 97 (as a 4% aqueous solution) was serially substituted in the same basic formula up to a maximum of 0.8% by total gel volume. The effect on the droplet size as a result of changing the viscosity (measured at 500 sec -1 shear rate) in this way is shown in Figure 6. In Examples 7 and 8 it appears that a minimum droplet size is reached when the viscosity has reached a certain value; in both cases, under the conditions cited for the given dispersions, this value is around 230 cP. Example 9 The above Examples show that it is possible to decrease droplet size by increasing the viscosity of the gel phase. A number of experiments have shown that smaller droplets lead to an increase in D max in a range of coatings. To check if a variation in gel phase viscosity was carried through to D max four magenta-forming dispersions (Dispersions C-F as described hereafter) were made in which the viscosity was altered by a range of methods, (increasing surfactant concentration, changing surfactant, adding a commercial viscosity enhancer Gantrez™ AN 149 and increasing gelatin concentration); a control sample (Dispersion G) was also included. Dispersions C-G The basic formula of the magenta-forming Dispersions C-G was as follows:- Oil phase 1-(2,4,6-trichlorophenyl)-3-{-[α-(3-tertbutyl-4-hydroxyphenoxy) tetradecane amido]-2-chloroanilino}-5-pyrazolone 94g tricresyl phosphate 47g 2-(2'-butoxyethoxy) ethylacetate 14g dioctylhydroquinone 9.4g 7-t-octyl-2,2,4-trimethylchroman-6-ol 37g Aqueous phase dry gelatin 94g demineralised water 643g 10% Alkanol XC 47ml 2N propionic acid 10ml 2N sodium hydroxide 5ml Dispersion C included an additional 47ml 10% Alkanol XC™ (i.e a total of 94ml). In Dispersion D sodium dodecylbenzene sulfonate was substituted for the Alkanol XC™; sodium dodecylbenzene sulfonate was known from previous experiments to provide a significantly higher gel phase viscosity than the normal Alkanol XC™ surfactant. In Dispersion Example E the demineralised water was replaced with a 1.5% aqueous solution of Gantrez™ AN 149 viscosity enhancer. In Dispersion F a total of 188g dry gelatin and 549g water was used in place of the amounts cited above (94g and 645g respectively). Dispersion G was the control formulated as above. Each of Dispersions C-G was homogenised under identical conditions. PHOTOGRAPHIC EVALUATION OF MAGENTA-FORMING DISPERSIONS The magenta-forming dispersions were incorporated into photographic coatings containing a silver bromoiodide emulsion, on a transparent support, according to the following coating diagram: Gel Supercoat Gelatin 1.50 gm -2 Emulsion Layer (as above) Support Cellulose acetate The 2-(2'-butoxyethoxy)ethylacetate was included as an auxiliary solvent to aid in dispersion preparation and was removed by washing the dispersion for 6 hours at 4°C and pH 6.0. Sensitometric testing The experimental photographic coatings prepared in this way were slit and chopped into 30cm x 35mm test strips. After hardening the strips were exposed (1.0 sec) through a 0-4.0 neutral density step wedge (0.2 ND step increments) and Daylight V. Wratten 35 + 38A filters and 0.3 ND filter then processed through a standard C-41 process as described in the British Journal of Photography Annual (1988) 196-198 using the following steps and process times: Developer 2.5 minutes Bleach 4.0 minutes Wash 2.0 minutes Fix 4.0 minutes Wash 2.0 minutes For each test strip, Status M densities were measured as a function of exposure using a spectral array automatic transmission densitometer. Measurements of maximum density (D max ) were obtained from plots of density vs. log exposure (DlogE curves). The results are shown, for a single layer coating on film base, in Figure 7. It is apparent that by whatever means the gel phase viscosity is increased, an increase in viscosity generated an increase in D max . In accordance with the present invention as hereinbefore described therefore the droplet size of an oil-in-water emulsion such as a photographic dispersion may be reduced by increasing the gel phase viscosity. In the photographic art, this represents a considerable advantage, as a reduced droplet size in the dispersed organic phase comprising coupler solvent and dye-forming coupler leads to an increased reactivity between the dye-forming coupler and oxidised developer. The invention can be used to produce commercial photographic film or paper having the same or a similar droplet size as compared with films or papers which are commercially available at the time of writing, by employing a more economical manufacturing process, as fewer passes through a homogeniser may be required to produce the same droplet size. Alternatively, the invention can be used to form new photographic paper or film products which have a significantly smaller droplet size as compared with films or papers which are available at the time of writing. The viscosity of the emulsion or dispersion can be increased by using proprietary viscosity enhancers or suitable surfactants. Alternatively, an increased concentration of gelatin in the aqueous phase can be employed. It is well known to those skilled in the art that diluting photographic dispersions with moderate amounts of water has no detectable effect upon the droplet size or stability. The concentrated aqueous gel phase can therefore be rediluted after homogenising, without affecting the droplet size produced in the homogenisation step. Since most production dispersions have a "melt-making" step after the homogenisation step, the present invention can be readily incorporated in existing manufacturing processes to increase the reactivity (reduce the droplet size) of the dispersions. The viscosity may be reduced even more efficiently by adding suitable aqueous solutions of proteins and non-ionic surfactants as described in WO-A-92/01971.

Claims: An oil-in-water emulsion comprising a viscosity enhancer other than a surfactant or gelatin and having an average droplet size in the oil phase in the range 0.1µm to 2µm wherein the initial viscosity of the aqueous phase is increased by the use of said viscosity enhancer, prior to homogenisation of the oil and aqueous phases. An emulsion as claimed in claim 1 comprising a photographic dispersion. An emulsion as claimed in claim 2 wherein the oil phase comprises droplets of image-dye-forming coupler or a filter agent dissolved in one or more organic coupler solvents and in which the aqueous phase comprises gelatin. A method of forming an oil-in-water emulsion comprising homogenising the oil and aqueous phases of the emulsion and characterised by increasing the initial viscosity of the aqueous phase prior to homogenisation of the oil and aqueous phases, thereby reducing the average droplet size of the oil phase to be in the range 0.1µm to 2µm. A method as claimed in claim 4 wherein a viscosity enhancer is used for increasing the viscosity of the aqueous phase. A method as claimed in claim 4 wherein the viscosity of the aqueous phase is increased by replacing at least part of the water therein with a polyhydric alcohol, such as glycerol. A method as claimed in claim 4 wherein the viscosity of the aqueous phase is increased by increasing the concentration of gelatin. A method as claimed in claim 4 wherein the viscosity of the aqueous phase is increased by the addition of a surfactant. A method as claimed in any one of claims 4 to 8 wherein the initial viscosity of the aqueous phase is in the range 20cP to 2500cP and wherein the increase in viscosity is by a factor of from 3 to 50 inclusive. A method as claimed in any one of the preceding claims wherein the dispersion is homogenised at an elevated temperature at which the aqueous and oil phases are sufficiently fluid for homogenising. A method as claimed in any one of claims 4 to 10 in which the dispersion is rediluted after the homogenisation. A method as claimed in any one of claims 4 to 11 wherein the emulsion is a photographic dispersion. A photographic element comprising a photographic dispersion in which a dispersed oil phase comprises an image-dye-forming coupler or filter agent in one or more organic solvents contained within an aqueous gel as the continuous phase, in association with a silver halide emulsion layer comprising a viscosity enhancer other than a surfactant or gelatin, wherein the average droplet size of the dispersed oil phase is in the range 0.1µm to 2µm and wherein the initial viscosity of the aqueous phase is increased by the use of said viscosity enhancer, prior to homogenisation of the oil and aqueous phases. A multi-colour photographic material comprising a support bearing yellow, magenta and cyan image dye-forming units comprising at least one blue, green or red sensitive silver halide emulsion layer having associated therewith at least one yellow, magenta or cyan dye-forming coupler respectively or one or more UV absorbing layers; wherein each image-dye-forming unit comprises a dispersed oil phase comprising an image-dye-forming coupler in one or more organic solvents, all contained within an aqueous gel as the continuous phase, wherein the emulsion layer comprises a viscosity enhancer other than a surfactant or gelatin and has an average droplet size of the dispersed oil phase in the range 0.1µm to 2µm and wherein the initial viscosity of the aqueous phase is increased by the use of said viscosity enhancer, prior to homogenisation of the oil and aqueous phases.

Inventors: YOUNG DAVID JOHN, YOUNG, DAVID JOHN, Young, David John, c/o Kodak Ltd.
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1996
Main classification: B01F17
IPC codes: G03C1, G03C7