EP0613047A2

EP0613047A2 - Photographic assembly

Info

Publication number: EP0613047A2
Application number: EP94102209A
Authority: EP
Inventors: John Leslie Cawse; John Charles Clarke; Steven Leslie Jones; Michael Philip Odalami Davidson
Original assignee: Ilford Ltd
Current assignee: Ilford Imaging UK Ltd
Priority date: 1993-02-23
Filing date: 1994-02-14
Publication date: 1994-08-31
Also published as: GB9303591D0; EP0613047A3

Abstract

A photographic assembly which comprises an opaque photographic base on which is coated at least one photographic emulsion layer, there being coated on the topmost emulsion layer a gelatin supercoat layer which comprises from 5 to 35% by weight of colourless polymer particles which have a glass transition temperature (Tg) of at least 40°C and have a particle size of not more than 90nm and have substantially the same refractive index as gelatin.

The object of the coating is to produce a high gloss and reduce the haze.

Description

This invention relates to photographic assemblies which after exposure and processing yield a black and white print.
There is a constant desire to produce black and white prints in which the black areas exhibit a high black gloss and little or no haze.
When black and white material after exposure and processing is infra-red dried a remarkable increase in black gloss is observed. However most black and white print material after processing is cold air or hot air dried. We have found a black and white material which after exposure and processing, followed by air drying exhibits a remarkable increase in black gloss and a diminution in the haze exhibited. This is obtained in a way not fully understood by incorporating a polymer latex having defined properties in the gelatin supercoat layer of the assembly.
In the past photographic assemblies have been described for photographic assemblies which comprise polymer latexes of different properties in gelatin supercoat layer in order to overcome other disadvantages but not to increase air-dried black gloss. Such assemblies are described for example in British Patent Specification No 1228828, British Patent Specification 1384967 and in British Patent Specification 1401768
According to the present invention there is provided a photographic assembly which comprises an opaque photographic base on which is coated at least one photographic emulsion layer, there being coated on the topmost emulsion layer a gelatin supercoat layer which comprises from 5 to 35% by weight of colourless polymer particles which have a glass transition temperature (T.g.) of at least 40°C and have a mean particle size of not more than 90nm and have substantially the same refractive index as gelatin. Preferably not more than 10% of the particles should be larger than 90nm.
It is to be understood that the photographic assembly of the present invention relates to the prepared assembly ready for use. Thus the gelatin supercoat layer is a dried gelatin layer. Preferably the particle size of the polymer particles is less than 65nm. The coating weight of the supercoat layer is preferably from 2.0 to 3.0g/m² of binder (i.e. gelatin and latex).
Preferably the T.g. of the polymer particles is at least 70°C.
Dried gelatin has a refractive index of 1.5. Thus the polymer particles should have a refractive index between 1.3 and 1.7 but most preferably as close to 1.5 as possible.
Examples of monomers and monomer mixtures which can be used to prepare the polymer particles for use in the present invention are styrene, methacrylate esters and in particular styrene/methylmethacrylate. A 50:50 mixture by weight of styrene/methylmethacrylate has a refractive index of 1.54.
The photographic assembly of the present invention after exposure and processing exhibits greatly improved air-dried black gloss. However, if the material after processing is infrared dried no improvement of black gloss is obtained but no diminution of gloss is observed either. Nevertheless most photographic prints after processing are either cold air or hot air dried. There is also obtained a diminution in the image haze.
Black gloss is measured as the percentage of specular light reflected from the surface of the photographic print. This is measured for angles of incidence and reflection of 60° on a Hunterlab D48D gloss meter.
Haze is measured as the percentage of diffuse reflected light from the photographic print surface with normal specular illumination. This is measured on a Gardener Hazemeter. Furthermore there is also a diminution in the water-uptake of the supercoat layer during processing so less drying of the processed material is required.
It is not clear why the provision of what is in effect a hard polymer layer next to an emulsion layer in which the silver image is formed should increase the air-dried black gloss. However, it is believed by the inventors that the hard latex particles restrain the growth of silver filaments during development, and thereby reduce the distortion of the interface between the non-stress layer and the emulsion layer. This leads to a reduction in light scatter and therefore improves the gloss as shown in the examples which follow, if the polymer particles have a T.g. of less than 40°C, that is to say they are soft particles, this increase in air-dried black gloss is not obtained. Further, if the particle size is greater than 90nm the increase in air-dried black gloss is not obtained and an increase in haze is observed.
If the polymer particles have a refractive index greatly different from dried gelatin the air-dried gloss improvement is not great and the haze is increased.
The supercoat layer of the assembly of the present invention is conveniently prepared by forming a latex which comprises polymer particles having the attributes set forth above, at least one surfactant and water. A latex which comprises about 60-80% by weight of water, 1-5% by weight of surfactant and 20-35% by weight of the polymer particles is then mixed in the correct proportions with an aqueous gelatin solution. This aqueous gelatin and latex layer is then coated on the top-most silver halide emulsion layer and dried.
The aqueous latex comprising the polymer particles, surfactant and water is conveniently prepared by an emulsion polymerisation process at a temperature of 60°C to 70°C.
Preferably the polymer particles in the latex have an average particle size of about 65nm or less, as determined by light scattering techniques.
It is preferred that the total amount of surfactant used is at least 10% by weight of the monomers present. If less is used, as in the process described in GB No. 1333663, the average particle size of the copolymers in the latex is significantly greater. The preferred amount of total surfactant to be present during the polymerisation reaction is from 10 to 20% by weight of the monomers present.
Both anionic and/or nonionic surfactants can be used but cationic or amphoteric surfactants are not preferred.
Particularly suitable anionic surfactants for use in the process are sulphosuccinate compounds, for example disodium ethoxylated nonyl phenol half ester of sulphosuccinic acid, tetrasodium N-(1,2-dicarboxyethyl)-N-octadecylsulphosuccinamate, or a mixture of either of these or similar surfactants with other conventional anionic surfactants.
Other useful anionic surfactants are sulphated or sulphonated polyethylene oxide compounds. Certain surfactants, although capable of producing latexes with a very small particle size, may, however, also have deleterious photographic effects. Thus the preferred polyethylene oxide surfactants have ethylene oxide chains of 30 ethylene oxide units or fewer.
A particularly suitable anionic surfactant is sodium alkyl aryl poly ether sulphate.
Conveniently a non-ionic surfactant is used together with an anionic surfactant. A particularly useful non-ionic surfactant is of the alkyl phenyl glycidyl type such as nonyl phenyl polyglycidyl ether with a variable glycidyl chain length. A commercially available surfactant of this type is marketed by the Olin Chemical Company under the name of Olin 10G. This has an average of 10 glycidyl units in the chain.
In the assembly of the present invention the base should be white. Examples of such a base are paper pigmented with baryta, white paper covered on both sides with a thin polyethylene layer, white pigmented film base and voided polyester base.
Preferably there is only one gelatino silver halide emulsion layer and the gelatin/polymer supercoat layer is coated on this gelatino silver halide emulsion layer. This silver halide emulsion layer may have some gelatin replaced by a polymer latex. Preferably the polymer in this latex has a T.g. below 40°C.
The silver halide emulsion may be any of the usual silver halide used in photographic materials but is usually silver chloride or silver bromochloride. This emulsion may contain any of the usual additives present in silver halide emulsions.

Preparation 1

Preparation of polymer latexes.

Water, potassium persulphate and a portion of the surfactant are added to the reaction vessel which is purged with nitrogen, and the temperature raised to 70°C. A pre-emulsified mixture is added portionwise to the reaction vessel using a metering pump over the course of approximately two hours. The reaction mixture is maintained under nitrogen atmosphere at approximately 70°C during addition. At the end of the addition the mixture is stirred for a further three hours at 70°C. The mixture is allowed to cool to room temperature with continued stirring before filtering through muslin and bottling up.
A surfactant of the following formula was used in this preparation:-

Supplied as 30% surfactant dissolved in 47% water and 23% isopropanol. This is referred to as surfactant 1. This surfactant is marketed as 'Triton' by Rohm and Haas.
Surfactant 2 was Olin 10G supplied by the Olin Chemical Company. Supplied as 50% Surfactant dissolved in 50% water.

The following latex polymers and copolymers were produced using this method.

Latex	1	2	3	4	5	6
Materials
Styrene	260.0	521.7	-	-	253.0	-
Methylmethacrylate	260.0	-	521.7	-	-	253.0
Butylacrylate	-	-	-	521.7	268.7	268.7
Water	1269.6	1269.6	1269.6	1269.6	1269.6	1269.6
Olin 10G	78.3	78.3	78.3	78.3	78.3	78.3
Triton 770	130.5	130.5	130.5	130.5	130.5	130.5
Potassium persulphate	3.5	3.4	3.5	2.7	6.3	6.3
Sodium metabisulphite	1.2	1.2	1.2	1.0	2.2	2.2
Tg (°C)	100	100	100	-54	3	3
Particle size (nm)	38	43	60	53	40	50
Refractive index	1.54	1.59	1.49	1.46	1.52	1.47

Preparation 2

The following gelatin based coating solutions were prepared:-
Into 411g of gelatin solution which comprised 3.5% by weight of gelatin there was added 12g of the latex.
Thus 20% by weight of the gelatin in the coating solution was replaced by latex.

Solution 1 comprised 12g of the latex 1
Solution 2 comprised 12g of the latex 2
Solution 3 comprised 12g of the latex 3
Solution 4 comprised 12g of the latex 4
Solution 5 comprised 12g of the latex 5
Solution 6 comprised 12g of the latex 6
Solution 7 comprised gelatin solution which comprised 4.2% by weight of gelatin and no latex as control.

Example

The following seven photographic assemblies were prepared. In each case they were coated on a piece of polyethylene coated base silver bromochloride emulsion at a coating weight of 83g/m² and a silver coating weight of 1.6g/m².

Assembly 1. Solution 1 at a coating weight of 55g/m² was coated on the silver halide emulsion layer.
Assembly 2. Solution 2 at a coating weight of 55g/m² was coated on the silver halide emulsion layer.
Assembly 3. Solution 3 at a coating weight of 55g/m² was coated on the silver halide emulsion layer.
Assembly 4. Solution 4 at a coating weight of 55g/m² was coated on the silver halide emulsion layer.
Assembly 5. Solution 5 at a coating weight of 55g/m² was coated on the silver halide emulsion layer.
Assembly 6. Solution 6 at a coating weight of 55g/m² was coated on the silver halide emulsion layer.
Assembly 7. (Control) Solution 7 at a coating weight of 55g/m² was coated on the silver halide emulsion layer.

The coatings on each of the assemblies was then air dried in a dark room at 25°C. After drying each of the assemblies were overall exposed for sufficient time to expose fully all the silver halide grains.

Each assembly was then developed for one minute in a photographic developer of the following formula:-

Sodium Sulfite	13g
Hydroquinone	3.1g
Potassium Carbonate	10g
1-phenyl-3-pyrazolidinone	0.13g
Sodium Hydroxide	0.55g
Potassium Bromide	0.5g
Benzotriazole	0.06g
Water	to 1 litre
at 20°C

and then fixed for 3 minutes in a fixing solution of the following formula:-

Ammonium Thiosulphate (60% w/w)	138g
Boric Acid	7g
Sodium Hydroxide	2.5g
Sodium Metabisulphite	5g
Sodium Sulphite	6g
Acetic Acid (Glacial)	9cm³
Water	to 1 litre
at 20°C

Each assembly was then cold air dried to fully remove all the water from the assembly. The amount of polymer in the dried down layer was 20% by weight of the gelatin.
Each assembly was then tested in a Hunterlab D48D meter to determine its air-dried black gloss reading. The higher the figure the better the black gloss, and the results were recorded. The Hunterlab measures the percentage of light that is reflected specularly. Thus the higher the percentage the greater the gloss.
Each assembly was then tested in a Gardener Haze meter to determine its haze. In this case the lower the figure the lower the haze, and the results were recorded. The Gardener Haze meter measures the percentage of light that is scattered from the normal. Thus the lower the percentage the lower the haze. Table 1

Air-dried black gloss (as%) Haze (as %)

Assembly 1 75 6.1

Assembly 2 76 6.4

Assembly 3 73 6.2

Assembly 4 70 6.7

Assembly 5 70 7.5

Assembly 6 66 7.4

Assembly 7 (control) 71 7.1
These figures show that assemblies 1, 2 and 3 all of which comprised polymer particles in the supercoat which had a T.g. greater than 40°C had a substantially better air-dried black gloss than the control which comprised no polymer particles in the supercoat. They also showed less haze than the control assembly.
On the other hand assemblies 4, 5 and 6 all of which comprised polymer particles in the supercoat which had a T.g. less than 40°C had air-dried black gloss which was worse than the control assembly. Also their haze was worse than the control.

Claims

A photographic assembly which comprises an opaque photographic base on which is coated at least one photographic emulsion layer, the assembly being characterised in that there is coated on the topmost emulsion layer a gelatin supercoat layer which comprises from 5 to 35% by weight of colourless polymer particles which have a glass transition temperature (T.g.) of at least 40°C and have a particle size of not more than 90nm and have substantially the same refractive index as gelatin.
A photographic assembly according to claim 1 characterised in that the polymer particles have a glass transition temperature of at least 70°C.
A photographic assembly according to claim 1 characterised in that the particle size of the polymer particles is less than 65nm.
A photographic assembly according to claim 1 characterised in that the refractive index of the polymer particles is between 1.3 and 1.7.
A photographic assembly according to claim 1 characterised in that the polymer particles are comprised of styrene, methylmethacrylate or a copolymer thereof.
A method of preparing an assembly according to claim 1 which is characterised in that it comprises forming a latex which comprises 60-80% by weight of water, 1-5% by weight of a surfactant and 20-35% by weight of polymer particles as defined in claim 1, mixing this latex in the correct proportions with an aqueous gelatin solution, coating this solution on the topmost emulsion layer of an assembly which comprises at least photographic emulsion layer coated on an opaque base and then drying the assembly.