Oil-water emulsions for photographic applications comprising gelatin having high stability

Abstract

 An oil-water emulsion for photographic application said emulsion comprising oil, water, gelatin and a surfactant, said gelatin consisting of

a) a fraction of gelatin molecules with a molecular weight smaller than 70 kDa, said fraction forming 30-90% by weight on the basis of the total weight of gelatin in the emulsion,

b) a fraction of gelatin molecules with a molecular weight higher than 130 kDa, said fraction forming 5-38% by weight on the basis of the total weight of gelatin in the emulsion,

c) the fractions of gelatin molecules with a molecular weight higher than 130 kDa and with a molecular weight smaller than 70 kDa are further related by having as lower limit

and as upper limit

d) whereby in total

.

Description

Field of the invention

[0001] The present invention relates to a method of emulsifying small oil droplets into a hydrophilic colloid composition containing surfactant molecules and gelatin in such a manner as to stabilize the oil droplet size. Various highly functional additives (like coupler molecules, stabilisers, UV-absorbers etc) are only soluble in oil; these additives should remain exactly fixed in the photographic coating layers during the development process (diffusion of additives between the various photographic layers should be prevented). The silver halide compounds in the aqueous gelatin phase in the red, green and blue sensitive emulsion layers oxidize the developer molecules after light exposure after which the oxidized developer molecules diffuse towards the oil/water interface in order to react with the cyan, magenta and yellow couplers in respectively the red, green and blue sensitive emulsion layers. The resulting oil-water emulsions are applied in the manufacturing of photographic elements such as photographic paper and negative film applications.

Description of the prior-art

[0002] In the manufacture of a photographic material various oil-soluble photographic additives are dissolved in substantially water-insoluble solvents (for example high boiling organic oil solvents). The large oil droplets are then broken up and emulsified with a high shear mechanical method into a hydrophilic colloid aqueous solution. The smaller droplet size is stabilised by the adsorbed layers of the colloid and a surface active agent (called surfactant) both being present in the aqueous water phase. Surfactants are important compounds to improve the emulsification of the oil droplets in the aqueous water phase. Anionic surfactants are most popular which will interact with the protonated -NH₂ group of gelatin. Also interactions occur between the cationic surfactants and the deprotonated -COOH group of gelatin. Limited interactions between gelatin and non-ionic surfactants arise. Thus both anionic and cationic type surfactants are commonly applied.

[0003] The small oil droplets should remain finely emulsified and not exhibit growth into larger droplets by either coalescence and/or Ostwald ripening. A smaller droplet size is considered advantageous as the interface area will be expanded which will improve the reaction between for instance the coupler photographic additive (= dye precursor) and the oxidized developed molecules in the hydrophilic aqueous colloid solution during development.

[0004] Usually the oil droplet size is preserved at the end of the emulsification process by cooling down quickly followed by storage in a refrigerator at 5 - 7° C at which temperature the droplets remain stable due to gelation of the colloid. If the cooling down is however carried out slowly the ageing of the droplet size can start resulting in an oil droplet size which is too large. Also upon reuse of the oil-water emulsions after taking them out of the refrigerator and melting them at 40 - 60° C the ageing process will also commence. Accordingly it is important that the growth of the oil droplets during the melting process should be prevented. The majority of oil-water emulsions in photographic applications are usually made with polar oils as organic solvents. However these type of emulsions are much more unstable (i.e. exhibit faster ageing into larger oil droplets) than emulsions with apolar oils like n-dodecane etc. Thus it is particularly important to find a means of improving the stability of oil-water emulsions with regard to the ageing process in which polar oils are present. For this reason it is important to find out by which means the stabilization of the oil droplet size can be improved. The subject inventors addressed the problem of stability for gelatin containing oil-water emulsions for use in photographic applications. Gelatin compounds are well-known as colloids in the photographic industry for various applications. Obviously many modifications could possibly be carried out with the basic gelatin ingredient in order to obtain a more suitable colloid which stabilises this specific oil-water emulsion.

[0005] The interaction of the colloid phase and the surfactant molecules at the oil-water interface determines to a high extent the stability of the emulsified droplets during ageing. The complexation of gelatin and the anionic surface active agents at the interface of oil-water emulsions is well known in literature (e.g. T.H.Whiteside¹⁾). Interfacial tension profiles as a function of anionic surfactant concentration are reported by several authors (W.J.Knox²⁾,P.C.Griffith³⁾, E.Dickinson⁴⁾) for gelatin-free and gelatin containing oil-water systems.

[0006] Usually the gelatin-free oil-water system shows a uniform decrease of interfacial tension as the surfactant concentration is increased until a well-defined value, called cmc (critical micelle concentration of surfactant), is reached. Above this surfactant concentration the interface is saturated with the surfactant and free micelles of surfactant molecules are formed in the solution; hence the interfacial tension does not reduce anymore.

[0007] For a gelatin containing oil-water system (see fig. 1) a first break of the interfacial tension is observed at a low surfactant concentration, called cac (critical aggregation concentration). This point is indicative for the saturated adsorption of surfactant-gelatin complexes at the interface while surfactant micelle-like aggregates are adsorbed with gelatin above this concentration. At the end of the plateau the interfacial tension gradually decreases indicating that hydrophobic gelatin segments are displaced from the interface by surfactant molecules. These replacements result in a second break in the interfacial tension curve indicating that the interface is completely covered by only single surfactant molecules.

[0008] The reduction of interfacial tension by the addition of gelatin coincides with a higher adsorption capacity of surfactant molecules at the interface. Hence gelatin can influence the adsorption characteristics of surfactants at the interface by complexation between gelatin and the surfactants. The adsorption capabilities at the oil-water interface determines the stability of the oil droplet sizes during the photographic manufacturing process.

[0009] The interaction of the anionic surfactants and the gelatin at the interface is determined by 2 types of bonding:

a) electrostatic interaction: the anionic head of the surfactant is linked to the positive gelatin sites of residual aminoacids (like Lys and Mg)

b) hydrophobic interaction: linking between the hydrophobic residues of the surfactant and the aminoacids of gelatins

[0010] Accordingly it is important to find which molecular configuration of gelatin is preferable for the stability of oil-water emulsions in photographic applications. From the many options available to potentially alter the gelatin configuration in the required positive manner the subject inventors selected the molecular weight as parameter. The idea is that smaller (hydrolyzed) gelatin molecules have more electrostatic interactions with surfactant molecules than large (non-hydrolyzed) gelatin molecules. The complexes of the small (hydrolyzed) gelatin and the surfactant will be more hydrophobic and interfacially active which results in a higher gelatin density at the interface. The small (hydrolyzed) gelatin molecules have more conformation freedom due to less steric hindrance enabling easier accessability of the surfactant molecules to the cationic and anionic interaction sites of the gelatin molecules. Hence the adsorption capacity for small gelatin molecules per surface unit at the interface is higher than for large gelatin molecules.

Summary of the invention

[0011] The object of the present invention was to provide a specification for an oil-water emulsion containing gelatin which exhibited improved stability vis a vis the prior art oil-water photographic emulsions. Surprisingly particular distributions of the molecular weight of gelatin exhibit improved stability when small polar oil droplets are emulsified in a hydrophilic colloid composition. The invention prevents the small droplets growing into larger droplets under ageing conditions in contrast to prior art emulsions. The oil-water emulsions must comprise at least two fractions of gelatins with different size ranges. The 0-70 kDa MW fraction and the >130 kDa MW fraction of gelatin should lie between 30 and 90% resp. between 5 and 38%. The fractions of gelatin molecules with a molecular weight larger than 130 kDa and with a molecular weight smaller than 70 kDa are further related by having the following lower limit:

and the following upper limit:

, whereby in total

does not exceed 100%. Size stability does not vary a lot when a high percentage of large long gelatin molecules occupies the interface. When a low fraction of large long molecules is present (i.e. the > 130 kDa fraction is low), then a small variation will have a large influence on size stability. It is preferable to contain 55-90 % of the 0-70 kDa gelatin fraction and 5-24 % of the > 130 kDa gelatin fraction in order to obtain the best emulsion stability. The above objects of the present invention were achieved when a polar organic solvent was emulsified into a hydrophilic colloid composition containing an anionic surface active agent and a gelatin derivative. The highest emulsion stability is obtained when a gelatin is applied with an average molecular weight range between 20-100 kDa. An average MW range of gelatin between 50-80 kDa is most preferred. The average molecular weight must be at least 20 kDa with a preference for at least 50 kDa. Thus, a suitable range for average molecular weight is between 50-100 kDa. In another preferred embodiment the average molecular weight should not exceed 80 kDa.

[0012] The trend that size ageing performance is better with gelatins having an average MW ranging between 50-80 kDa than with the conventional gelatins having a MW of about 180 kDa is also observed when an oil-soluble photographic coupler is also dissolved in the polar oil. The stability difference for the various gelatin mixtures become less pronounced as the coupler itself is known to improve the emulsion stability. Quite surprisingly in addition to the improved size stability under ageing conditions the emulsions according to the invention exhibited another improved characteristic. Specifically the emulsions according to the invention exhibit improved ageing behavior over a range of pH with an optimum at pH=6. The emulsion stability of the 50-80 kDa MW type gelatins is only pH dependent in the range between 5.5<pH<6.5 in a minor manor, while the gelatins with a MW>100 kDa are much more dependent upon the pH. From an operational point of view in photographic element production a reduced dependence on pH is extremely desirable.

[0013] Another advantage exhibited by emulsions according to the invention is the possibility to reduce the cost of photographic element production. This can occur by applying less stringently purified gelatin than previously assumed to be required. A certain fluctuation in molecular sizes of the gelatin has been found not only permissible but advantageous for these emulsions. The use of gelatin with a relatively high peptide quotient i.e, size of gelatin below 70 kDa has now been found to be advantageous. This is in marked contrast to e.g. silver halide emulsions which require more uniform gelatin for optimised results.

Detailed description of invention

[0014] The surface active agents which can be used in the present invention include any conventially known anionic surface active agents. Of these compounds, a compound having a hydrophobic moiety containing 8 to 30 C-atoms and an -SO₃M or -OSO₃M group (M is a cation capable of forming a salt with sulfuric acid or sulfonic acid) are particularly preferred. In a suitable embodiment of this invention we applied SDBS (sodium dodecylbenzene sulfonate) as anionic surfactant as illustrated in our examples. However the invention is not only valid for anionic surfactants, but is also appropriate for cationic surfactants.

[0015] In most photographic oil water emulsion applications a high boiling organic polar solvent ist is used. High boiling implies a boiling temperature of at least 160°C. Preferably, a boiling temperature of at least 240°C and most preferably, a boiling temperature of at least 340°C for the solvent is desired. The high boiling point of the oil is necessary in order to prevent e.g. that the important coupler compounds will crystallize out (which causes deterioration of the quality). In the present invention suitably a phosphoric acid ester is used as polar oil having a high di-electric constant (we applied in our examples tricresyl phosphate (TCP) with ε=7.3). Most preferably polar oils can be applied having a di-electric constant which varies between 3.5 and 7.5 (see also the reference table 1 of polar oils).
In our productions OW-emulsions are made with TCP but also with trihexyl phosphate, trioctyl phosphate, triisopropylphenyl ester of phosphoric acid etc.

Reference table 1 for polar oils:

Boiling points and di-electric constant (which has been applied as measure for the polarity of the oil):

[0016] 

CA index name	chemical name	Molecular formula	Boiling point at 1 atm	Di-electric constant ε
phosphoric acid, tris(methylphenyl)ester	tricresyl phosphate	(2-Me-C6H5-O-)3-P=O	420	7.33
	trihexylphosphate
phos.acid, trihexylester		(nC6H13-O)3-P=O		5.86
	trioctylphosphate
phos.acid, trioctylester				4.8
	trixylenyl-phosphate
phenol, dimethyl, phosphate			402
	tris(chloroethyl) phosphate
ethanol,2-chloro-phosphate			338
	tris(2-butoxyethyl) phosphate
ethanol,2-butoxy-phosphate			418
di-n-octylphtalate	= apolar oil			3.96
n-dodecane		C12H26	216	2.05

[0017] Other organic polar solvents may however also suitably be applied. Such other solvent examples comprise phthalate esters, citric acid esters,benzoic acid esters, fatty acid esters and amides etc. Suitable phosphoric acid esters are trixylelyl phosphate, trihexyl phosphate, trioctyl phosphate, tridecyl phosphate, tris (butoxy ethyl) phosphate, tris (chloroethyl) phosphate, tris (dichloropropyl) phosphate etc. In our examples tricresyl phosphate TCP is applied.

[0018] Oil-soluble photographic additives are usually added in the emulsification recipes. Such additives can be selected from one or more of the categories of compounds consisting of couplers, UV light absorbing agents, fade preventing agents, stabilisers, antioxidants, dyes etc.

[0019] In the examples illustrating the invention the effect of a specific molecular weight gelatin on the emulsion stability is illustrated in the simplest recipe without all these photographic additives. In practising the present invention one merely has to dissolve the desired additives in the polar solvent oil in order to improve the emulsion stability. Due to the improved emulsion stability less additional stabilising compounds are required to obtain an emulsion with the same stability as the conventional oil-water gelatin containing emulsions.

[0020] In order to determine which molecular weight fraction of gelatin has a positive influence on the emulsion stability, three gelatin fractions were determined by the GPC analytical method:

• the gelatin fraction (% < 70 kDa) consists of gelatin molecules having a molecular weight range between 0 and 70 kDa
• the gelatin fraction (% 70-130 kDa) consists of gelatin molecules having a molecular weight range between 70 and 130 kDa
• the gelatin fraction (% > 130 kDa) consists of gelatin molecules having a molecular weight higher than 130 kDa.

[0021] In the following examples it is shown that the oil-water emulsion stability (defined as the droplet stability after 4 hours of ageing) improves surprisingly (at the optimum surfactant SDBS concentration of 0.487 mMol/5 g gelatin per liter) when the gelatin fraction with a MW between 0-70 kDa increases and the gelatin fraction with a MW > 130 kDa decreases in comparison to the usually applied gelatin fractions of the state-of-the art conventional gelatins. This conventional type comprises < 30% for the gelatin fraction 0-70 kDa > 38 % for the gelatin fraction >130 kDa. This has been verified in our statistical variations of droplet stability with conventional gelatins having an average molecular weight of 177 kDa. The average droplet size of 227.2 nm was measured with a standard deviation of 6.6 nm.

[0022] The improved oil-water size stability can be realised if the gelatin % of the fraction < 70 kDa ranges between 30 and 90 % together with a gelatin % of the fraction >130 kDa varying between 5 and 38 %. The best emulsion stability is obtained when the most preferred gelatin concentrations are applied like 55-90 % for the < 70 kDa fraction and 5-24 % for 130 kDa fraction. It is clear from the examples that merely applying a smaller gelatin molecule per se does not provide the required result. This is apparent from the lack of improvement exhibited upon application of only a 23 kDa fraction as the sole gelatin component in an oil/water water emulsion.

[0023] Further by varying the pH of the emulsions between 5 and 7 we found unexpectedly how insensitive the emulsion stability of the emulsions according to the invention is against pH variations. We found the optimum pH of the emulsion stability of the emulsions according to the invention at pH 6. This is desirable for photographic applications. In our attempts to emulsify the TCP oil droplets into the hydrophilic colloid composition at a pH between 5 and 7 we noticed the emulsion stability gradually dropped for the large gelatin mixtures (with MW> 100 kDa) after decreasing the pH from 7 to 5; but in contrast discovered an optimum high emulsion stability for the lower MW gelatins as is shown in figure 4 for all gelatin mixtures with a MW below 100 kDa specifically between 20 and 80 kDa. The best emulsion stability is obtained at pH=6 for the gelatin mixtures with a low MW between 50 and 80 kDa where the broad emulsion stability over a wide pH range is most beneficial for good operational control of the size stability.

[0024] The effect of the preferred MW gelatin in which the 0-70 kDa fraction is more than 30 % but less than 90 % and the >130 kDa fraction varies between 5 and 38 % is also observed in the attached example where the emulsion also contains a cyan coupler in the recipe. The emulsion stability differences become smaller with the coupler compounds which can be expected as the emulsion stability is usually already stabilised to a degree by these coupler compounds.

[0025] The emulsifying apparatus used to practise the present invention should preferably be such that a high shear is accomodated inside the liquid to be treated. Suitable apparatus include a colloid mill, a homogenizer, a microporous emulsifier/fluidizer, an electro magnetic strain type ultrasonic generator etc.

[0026] Among the various types of gelatin, one can use alkaline processed gelatin, the hydrolysed product therefrom or the peptized product thereof after an enzymatic treatment or the acid processed gelatin. Recombinant gelatin or gelatin fragments of the required length can also be used. The process steps to arrive at such forms are general common knowledge for a person skilled in the art.

[0027] A suitable amount of the gelatin derivative and the surface active agent is the amount used in the present invention. It will however be apparent to the skilled person that the optimum amounts to be used depend on the type of oil-water application, the type and the amount of the solvent and the type of the resulting color photographic product as well as the type of surfactant. Suitable amounts of surfactant are 0.01-10.00 mM/5 grammes of gelatin per liter. Suitably a narrower range of 0.20-1.00 mM/5 grammes of gelatin per liter can be applied. In the examples it is also illustrated that the range 0.45-0.50 mM/5 grammes of gelatin per liter is suitable.

[0028] By practising the present invention one can emulsify a polar solvent oil with a hydrophilic colloid composition containing an anionic or cationic surface active agent and a gelatin derivative; the emulsion stability is best at pH=6 for the gelatin mixtures with a MW ranging between 50 and 100 kDa. The increased adsorption capability of the surfactant by complexation with the small gelatin molecules results in a reduced size ageing effect of the small oil droplets.

[0029] The prominent features and effects of the present invention will now be explained in more detail in the examples below.

EXAMPLES

Materials

[0030] Sodium dodecyl benzene sulfonate SDBS was applied as anionic surfactant. Deionized alkali-processed ossein gelatin is used which has an isoelectric point IEP of 5.0 and a weighted average molecular weight of 177 kDa (= comparative # 1).

[0031] The same gelatin was hydrolyzed until a weighted average molecular weight of 23 kDa was obtained with an IEP of 5.2 (= comparative # 2).

[0032] Other gelatin fractions with different molecular weights were obtained by mixing the large gelatin fraction with the average MW of 177 kDa and the small gelatin fraction with the average MW of 23 kDa in the proper ratio; the following molecular weight gelatins were prepared after mixing: 53.9 kDa (= invention #4), 74.5 kDa (= invention #3), 100.2 kDa (= invention #2) and 125.9 kDa (= invention #1). A commercially available enzymatic gelatin with a weighted average molecular weight of 85 kDa (= invention #5) was used. The same enzymatic gelatin was processed in our microfluidiser for 6 minutes at 6 bar air pressure in order to shift between the various gelatin fractions; the molecular weight dropped from 85 to 54 kDa (= comparative #3). (Table 4 provides details)

[0033] As solvent oil tricresyl phosphate (TCP) was applied. TCP was chosen for its relatively high polarity.

[0034] The pH was adjusted by addition of either 1N NaOH or 1N HCl analytical grade agents.

Testing methods

[0035] Interfacial surface tension measurements were performed with the drop volume method using a Lauda KG TVT-1 tensiometer. The measurements were performed at 40 °C with a 0.5 % gelatin solution; the gelatin pH was adjusted to pH 6.0 (or in example 4 to pH=5 or pH=7) and the SDBS concentration was varied in the range between 0.01 - 200 mmol SDBS per 5 gram gelatin.

[0036] Prior to the measurements, the apparatus was cleaned thorougly, rinsed with methanol and dried up. A beaker containing the gelatin/SDBS solution was placed into a water-bath at 40 °C. A cuvet or a syringe was flushed and finally filled with the TCP solvent oil.

[0037] The Lauda drop volume equipment measures the interfacial tension at different time periods (=t) which can be plotted as a linear relationship between interfacial tension and 100/t^1/2. The equilibrium interfacial tension value of the measured system is obtained by extrapolation to t=∞. Emulsification experiments were carried out with a microfluidizer M-110 Y (Microfluidics International Corp. in USA). A gelatin solution was prepared and adjusted to the desired pH. SDBS was added and varied between 0.024 - 20.4 mmol SDBS per 5 gram gelatin. The TCP oil was manually added to the gelatin/SDBS solution and the temperature of the emulsion adjusted to 40 °C before the emulsification experiment at 4 bar air pressure was initiated. An emulsion volume of about 0.5 Ltr was emulsified 6 times for about 1.5 minutes in a batch mode in order to manufacture sufficiently fine emulsions. The initial average droplet size was about 140 nm. The stability of the emulsion is evaluated by measuring the droplet size ageing for 4 hours after the emulsification is stopped and the emulsion has remained in the 40 °C water-bath without agitation.

[0038] The turbidimetric method⁵⁾ was applied in order to determine the oil droplet size by measuring the turbidity at λ=500 and λ=600 nm in a standard spectrophotometer. With the refractive index of TCP (=1.552) and the ratio of the turbidities at λ=600 nm and λ=500 nm the average droplet size is calculated based upon the theory of Mie (described in the same reference⁵⁾).

[0039] The droplet size turbidity measurements were carried out at 0, 1, 2, 3, 4 hour time intervals after the emulsification was finished. The GPC method applied is disclosed in detail in Example 2.

Example 1: Effect of low MW gelatin mixtures on interfacial tension

[0040] For the interfacial tension experiments various solutions were prepared in a beaker glass which was placed in the 40 °C waterbath and contained gelatins with various average MW sizes and a SDBS surfactant. A volume of 50 ml of a 1 % gelatin solution was mixed in the beaker with a SDBS surfactant volume of x ml of which the concentration varied between 0.01 - 200 mmol SDBS per 5 gram gelatin per liter. The TCP solvent oil was present in a cuvette or syringe above the glass beaker. The following MW sizes of the gelatin mixtures were obtained by mixing the two starting gelatin materials (23 kDa and 177 kDa): 53.9 kDa, 74.5 kDa, 100.2 kDa and 125.9 kDa.

[0041] The equilibrium interfacial tension data for the gelatin mixtures are plotted as function of the SDBS additions in figure 1. The gelatin mixtures with a smaller MW size resulted in a continuous decrease of the interfacial tension. Hence a higher adsorption capability at the interface occurs with gelatins having a lower MW. Whether this also results in a better performance of the emulsion stability will be discussed in Example 2.

Example 2: Effect of low MW gelatins on emulsion size stability

[0042] The various MW gelatins were applied in the microfluidiser emulsification tests which are described above. The recipe of each emulsion batch contained 15 gram of gelatin, 43 gram of TCP oil, 435 gram of water and a varying amount of SDBS (0.024 - 20.4 mmol SDBS per 5 grammes of gelatin per liter). A total volume of about 500 ml was emulsified in each batch (at 4 bar air pressure).

[0043] The droplet size after 4 hours ageing without agitation was compared with the initial droplet size at the end of the emulsification process. This droplet size difference (0-4 hrs) is plotted in figure 2 against the SDBS variations for each MW gelatin mixture.

[0044] For all MW gelatins an optimum size stability was found at a SDBS-concentration of 0.5 mmol SDBS per 5 grammes of gelatin per liter; the most stable emulsions were obtained for the gelatin mixtures with a MW range between 50 - 100 kDa. If the gelatin became too small (23 kDa) the emulsion stability deteriorated again.

[0045] The emulsion size stability with the various gelatin mixtures are also plotted (at the optimum SDBS conc. of 0.487 mMol/5 g of gelatin per liter) against the gelatin fractions with a MW-range of < 70 kDa and of > 130 kDa (see figure 3). Compared with the stability of the state-of-the-art conventional gelatins the MW fractions of the mixed gelatins can be claimed as being superior in stability when the < 70 kDa fraction varies between 30 and 90 % together with the variation of the > 130 kDa fraction between 5 and 38 %. The gelatin composition was determined with GPC analytical equipment; three fractions were defined with a different molecular weight range:
   0-70 kDa, 70-130 kDa and > 130 kDa.
The GPC analysis of the different molecular weight fractions was carried out at 214 nm while the separation was performed over a 300 * 7.8 mm column (TOSO Haas) loaded with the TSK-gel 4000 SWXL. The eluent consisted of 1 wt.% SDS, 0.1 mol/l Na₂SO₄ and 0.01 mol/l NaH₂PO₄ at a flow rate of 0.5 ml/min.

[0046] As reference for a gelatin type containing small molecules a hydrolysed gelatin with an average molecular weight of 23 kDa has been included. When the two reference gelatins with an average MW of 23 and 177 kDa were mixed in a specific ratio the following MW of the gelatin mixtures were obtained:

Table 2

Gelatin types	Sample number	% conventional - % hydrolysed	Average MW (kDa)
Conventional	Comparative #1	100 - 0	177
Hydrolysed	Comparative #2	0 - 100	23
Mix 1	Invention #1	67 - 33	126
Mix 2	Invention #2	50 - 50	100
Mix 3	Invention #3	33 - 67	75
Mix 4	Invention #4	20 - 80	54

[0047] The influence of both gelatin fractions (> 130 kDa and < 70 kDa) on the droplet size stability is shown in figure 3 while the data for the composition of the gelatin fractions are shown in table 4. As the gelatin fraction % < 70 kDa increases and the gelatin fraction % > 130 kDa decreases the droplet stability of the emulsion improves for the prepared gelatin mixtures (155 - 160 nm) but the droplet stability deteriorates again to 188 nm for the small hydrolysed gelatin with an average MW of 23 kDa. When the conventional gelatin with an average MW of 177 kDa is treated for 60 min at 6 bar in our microfluidiser, the large gelatin fraction % (> 130 kDa) dropped from 47 to 33.3 % while the small fraction % (0-70 kDa) increased from 16.8 to 24.3 % without a noticeable effect on droplet size stability (228 nm). Another enzymatically processed gelatin with an average MW of 85 kDa correlates reasonably well with the 0-70 and > 130 kDa gelatin fractions of the previously discussed gelatin mixtures concerning the droplet stability. However the same fluidiser treatment of 60 min at 6 bar with the enzymatically processed gelatin shows in table 3 the same trend of a shift between the gelatin fractions as is shown above for the conventional gelatin:

Table 3

Gelatin fraction	enzymatic processed gelatin	same gelatin after extra microfluidiser treatment
> 130 kDa	18.6 %	5.8 %
0-70 kDa	64.3 %	74.2 %

[0048] A significant deterioration of the size stability is observed (from 188 to 237 nm) by the extra microfluidiser treatment as is shown in figure 3 and table 4. In this case the large gelatin fraction (> 130 kDa) has become too small (5.8 %) which results in a poorer stability performance. For the microfluidiser treatment of the conventional gelatin of 177 kDa the reduction of large gelatin fraction (> 130 kDa) has no effect on the droplet stability as the solution contains an excess of these large gelatin molecules which is not the case for the enzymatically treated gelatins. Therefor upper and lower limits are shown for the fraction % > 130 kDa as function of the 0-70 kDa fraction in figure 3 which limits become smaller when the 0-70 kDa fraction increases as the effect of the reduced large molecules (> 130 kDa) becomes more relevant for the droplet stability. The band between the upper and lower limit for the large gelatin fraction (> 130 kDa) is selected by drawing two lines out of the 0-70 kDa fraction point = 100%. As long as the 0-70 and > 130 kDa fractions are within the following limits:

the droplet stability of the invented gelatins will be better than the reference gelatin types as is known from prior-art (see figure 3 and table 4).

Table 4

Gelatin types	Sample number	0-70 kDa fraction(%) from GPC	70-130 kDa fraction (%) from GPC		>130 kDa fraction (%) from GPC
Conventional (A)	Comp. #1	16.8	35.9		47
hydrolysed (B)	Comp. #2	98	2.9		-
mix gelatin	Invention #1	43.8	24.9		31.3
mix gelatin	Invention #2	57.5	19.4		23.5
mix gelatin	Invention #3	70.9	13.9		15.7
mix gelatin	Invention #4	81.8	9.5		9.4
enzymatic (C)	Invention #5	64.3	17.1		18.6
(C) treated for 6 min at 6 bar in microfluidiser	Comp. #3	74.2	20.4		5.8

Gelatin types	Sample number	lower limit % of> 130 kDa fraction (*)	upper limit % of> 130 kDa fraction (**)	Average MW (kDa)	Droplet stability (nm)
Conventional (A)	Comp. #1	37.8	50.4	177	227
hydrolysed (B)	Comp. #2	0.9	1.2	23	188
mix gelatin	Invention #1	25.6	34	126	215
mix gelatin	Invention #2	19.3	25.8	100	160
mix gelatin	Invention #3	13.2	17.6	75	155
mix gelatin	Invention #4	8.3	11	54	155
enzymatic (C)	Invention #5	16.2	21.6	85	188
(C) treated for 6 min at 6 bar in microfluidiser	Comp. #3	11.7	15.6	54	237

(*)

(**)

Example 3: Effect of addition of cyan coupler in above recipe on emulsion size stability

[0049] The above MW gelatin mixtures were also applied in the microfluidiser stability tests with the addition of a cyan coupler (chemical name: 3',5'-dichloro-4'-ethyl-2'-hydroxypentadecananilia). The recipe of each emulsion batch contained 30 gram gelatin, 121 gram TCP oil, 300 gram water, 10 gram cyan coupler and 10 ml 10 % SDBS-solution. This mixture was premixed for 15 mm at 10000 rpm. Subsequently 470 ml water was added and again premixed. This mix was finally emulsified in the standard microfluidizer test(at 4 bar air pressure).

[0050] The initial droplet size after the emulsification was comparable for the various gelatin/coupler/TCP mixtures; the droplet size ageing was followed for 168 hours at a storage temperature of 40°C which is shown below:

Table 5

Average MW of gelatin (in kDa)	Droplet size increase (in nm) after 168 hrs ageing
177 =state-of-the-art comparative #1	27
54=invention #4	19

[0051] Also when the cyan coupler was added to the oil-water recipe besides the TCP and the colloid gelatin compounds, the droplet size ageing was improved for the gelatin mixtures with the low MW of 54 and 100 kDa vis a vis the conventional state-of-the-art gelatin. The emulsion stability differences are smaller in the presence of the cyan coupler as expected because the emulsion is strongly stabilised by the coupler compounds.

Example 4: Effect of pH on emulsion stability

[0052] The emulsions with the various gelatin mixtures were also tested at different pH's (pH varied between 5 and 7); the droplet size stability improved for the gelatin mixtures with high MW of 100-177 kDa when the pH decreased from 7 to 5 (see figure 4). However the most stable emulsions were obtained with the low MW gelatin mixtures (54-75 kDa)at the optimum pH=6. The low pH dependency wound pH=6 for these smaller MW gelatin mixtures is most preferable from operational point of view.

Literature references

[0053] 

1) T.H.Whiteside, D.D.Miller, Langmuir 10, 2899-2909 (1994)

2) W.J.Knox, T.O.Parshall, J.Colloid Interface Sci. 33,16 (1970)

3) P.C.Griffith, P.Stilbs, A.M.Howe, T.Cosgrove, Langmuir 12,2884 (1996)

4) E.Dickinson, C.M.Woskett, Special publication R.Soc.Chem. 75 (Food and Colloids), 74(1989)

5) D.H.Melik, H.S.Fogler, J.Colloid Interface Sci. 92, 161 (1983)

Claims

1. An oil-water emulsion for photographic application said emulsion comprising oil, water, gelatin and a surfactant, said gelatin consisting of

a) a fraction of gelatin molecules with a molecular weight smaller than 70 kDa, said fraction forming 30-90% by weight on the basis of the total weight of gelatin in the emulsion,

b) a fraction of gelatin molecules with a molecular weight higher than 130 kDa, said fraction forming 5-38% by weight on the basis of the total weight of gelatin in the emulsion,

c) the fractions of gelatin molecules with a molecular weight higher than 130 kDa and with a molecular weight smaller than 70 kDa are further related by having as lower limit

and as upper limit

d) whereby in total

.

2. An oil-water emulsion for photographic application according to claim 1 wherein the fraction of gelatin molecules with a molecular weight smaller than 70 kDa forms 55-90% by weight on the basis of the total weight of gelatin in the emulsion, and/or wherein the fraction of gelatin molecules with a molecular weight higher than 130 kDa forms 5-24% by weight on the basis of the total weight of gelatin in the emulsion.

3. An oil-water emulsion for photographic application according to any of the preceding claims wherein the total average molecular weight of the gelatin molecules is between 20-100 kDa., suitably the total average molecular weight of the gelatin molecules is smaller than 80 kDa and/or higher than 20 kDa, more suitably higher than 50 kDa.

4. An oil-water emulsion for photographic application according to any of the preceding claims wherein the total average molecular weight of the gelatin molecules is between 50-100 kDa, suitably the total average molecular weight of the gelatin molecules is between 50-80 kDa.

5. An oil-water emulsion for photographic application according to any of the preceding claims wherein the various gelatin fractions consist of gelatin molecules selected from the group consisting of natural gelatin, alkaline processed gelatin, acid processed gelatin, hydrolysed gelatin, peptised gelatin resulting from enzymatic treatment, recombinant gelatin and recombinant gelatin fragment.

6. An oil-water emulsion for photographic application according to any of the preceding claims wherein the oil component is a polar organic solvent with one or more of the following characteristics:

- a di-electric constant ε between 3.5 and 7.5.

- a high boiling point, wherein high boiling point implies a boiling point of at least 160°C, preferably at least 240°C and more preferably at least 340°C under standard conditions,

- the oil component is selected from the group consisting of phosphoric acid esters, phtalate esters, citric acid esters, benzoic acid esters, fatty acid esters and amides.

- the oil component is a phosphoric acid ester, suitably the oil component is selected from the group of compounds consisting of tricresyl phosphate, trixylelyl phosphate, trihexyl phosphate, trioctyl phosphate, tridecyl phosphate, tris(butoxy ethyl) phosphate, tris (chloro ethyl) phosphate, tris (dichloro propyl) phosphate.

7. An oil-water emulsion for photographic application according to any of the preceding claims wherein the surfactant is an anionic surfactant, suitably the surfactant is an anionic surfactant with a hydrophobic moiety of 8-30 carbon atoms and a group -SO₃M or -OSO₃M, whererin M is a cation capable of forming a salt with sulphuric or sulphonic acid e.g. the surfactant is the anionic surfactant sodium dodecylbenzene sulphonate.

8. An oil-water emulsion for photographic application according to any of the preceding claims wherein anionic surfactant is present in an amount of 0.01-10.00 mMol per 5 grammes of gelatin per liter, suitably 0.20-1.00 mMol surfactant per 5 grammes of gelatin per liter, more suitably 0.45-0.50 mMol per 5 grammes of gelatin per liter.

9. An oil-water emulsion for photographic application according to any of the preceding claims further comprising an oil soluble photographic additive selected from one or more of the group consisting of couplers, UV light absorbing agents, fade preventing agents, stabilisers, antioxidants and dye developers e.g one or more of

- an oil soluble photographic additive selected from red layer couplers e.g. cyan couplers

- an oil soluble photographic additive selected from green layer couplers e.g. magenta couplers

- an oil soluble photographic additive selected from blue layer couplers e.g. yellow couplers.

10. An oil-water emulsion according to any of the preceding claims exhibiting optimum emulsion stability at pH 6 i.e. between pH 5.5 and 6.5.

11. A process for producing a photographic element comprising applying an oil-water emulsion according to any of the preceding claims in a manner known per se for producing photographic elements using an oil-water emulsion, suitably the process is for producing a photographic element comprising applying an oil-water emulsion of the red, green or blue layer type according to any of the preceding claims in a manner known per se for producing photographic elements using a red, green or blue layer oil-water emulsion containing respectively the cyan, magenta or yellow coupler compounds.

12. A photographic element obtained from a process according to claim 10 or 11.

Drawing