GB2246870A - Photographic materials with anti-static coatings - Google Patents

Abstract

A photographic material comprises at least one silver halide colloid layer and at least one anti-static layer either coated on the silver halide layer or on the side distal to the silver halide layer the said anti-static layer comprising a colloid binder having therein a mixture of a polyoxyalkylene compound and a polymeric salt. The binder is preferably gelatin and the polymeric salt may be the Li, Na, K, ammonium or triethylammonium salt of a polymer containing sulphonic acid units. It is thought that the polyoxyalkylene compound and the polymeric salt form a complex as has been suggested by P V Wright, British Polymer Journal I, 319 (1975). It has been found that this complex produces a much improved anti-static composition over the use of the equivalent amount of the two constituents used separately.

Description

Anti-Static Composition -- - This invention relates to anti-static compositions for use in photographic silver halide assemblies.

The build-up of static electricity on insulated plastic material webs is well known. This build-up can be discharged by unreeling the web so separating the coils of the web. Often when this happens an electric discharge occurs. If the plastic material web is coated with an unexposed silver halide emulsion then parts of the emulsion become exposed. These are observable on the exposed and processed web as light areas of various shapes. In order to reduce such static discharges various anti-static coatings for photographic materials coated on plastic material webs have been proposed. These include anti-block layers which are meant to prevent the layers of coiled web material from coming into close contact with each other. Also various so-called electrically conductive substances have been employed in an effort to cause the built-up charge to be dissipated.One such anti-static composition is described in USP 4 610 955, which comprises in a colloid binder a surface-active polymer having polymerised oxyalkylene monomers and an inorganic salt selected from inorganic tetrafluoroborates, perfluoroalkyl carboxylates, hexafluorophosphates and perfluoroalkyl sulphonates. Such compositions whilst acting as anti-static compositions exhibit a number of disadvantages. For example such inorganic salts when coated in admixture with gelatin and a polyether tend to crystallise on the surface of the coating, especially when the salts are used in a high enough concentration to provide good conductivity.Further these low molar mass salts do not contribute to the physical strength of the gelatin layer or other colloid layer and may indeed best be described as non-reinforcing fillers which can cause weakening and embrittlement of the assembly on which the composition is coated.

We have found an anti-static composition which is as effective as the composition described in USP 4 610 995 but which does not suffer from its disadvantages.

Therefore according to the present invention there is provided a photographic assembly which comprises at least one silver halide colloid layer and at least one anti-static composition layer either coated on the silver halide layer or on the side distal to the silver halide layer the said anti-static layer comprising a colloid binder having therein a mixture of a polyoxyalkylene compound and a polymeric salt.

It is thought that the polyoxyalkylene compound and the polymeric salt forms a complex as has been suggested by P V Wright, British Polymer Journal I, 319 (1975). It has been found that this complex produces a much improved anti-static composition over the use of the equivalent amount of the two constituents used separately.

Useful polyoxyalkylene compounds for use in the anti-static composition of use in the present invention are those of the Formula I: X (CH2CH20+L+m in which X, Y are end groups or may be linked to form a ring, L is a divalent linking group, or a single bond, n and m are integers and n has values 4 to 50 and m has values 1 to 50, with the proviso that when L is a single bond then m is 1.

Thus these polyoxyalkylene compounds are derivatives of polyethylene oxide.

These may for example include polyethylene oxide itself, its ethers and esters and block and graft copolymers thereof. Molecular weights may range from about 500 Dalton (one Dalton is one gram per mole) to about 100,000 Dalton but preferably about 500 Dalton to about 20,000 Dalton and most preferably 1,000 Dalton to about 10,000 Dalton. These such PEOs may be amorphous, partly amorphous or crystalline.

Particularly preferred polyoxyalkylene compounds for use in the present invention are derivatives of polyethylene oxide which contain segments of PEO linked together by short divalent sequences of atoms such that the final polymers are non-crystalline. An example of this type of polyether is the class of polymers known as poly((oxymethylene)-oligo (oxyethylene)) polymers. These are made by reacting a low molar mass polyethylene oxide with, for example, methylene chloride in the presence of base.

Particularly preferred are such polymers with an overall molar mass of less than 10,000 Dalton and comprising polyethylene oxide sequences of up to 2000 Dalton.

Other suitable polyoxyalkylene compounds are those derivatives of polyethylene oxide with end groups other than hydroxyl, for example; chloro-, amino-, or carboxy-terminated polyethylene oxide and copolymers of ethylene oxide and propylene oxide. Examples of such compounds are:i) ClCH2CH2o(CH2CH2O)22 CH2CH2 ii) H2N-CH2CH2O(CH2CH2O)45-CH2CH2NH2

Examples of polymers of structure (ii) and (iii) above are the Jeffamine range of amino-terminated polyethers manufactured by Texaco. For instance, Jeffamine ED6000 is a polyalkylene oxide with terminal amino groups and molar mass approximately 6,000 Dalton.

Polyethers having these structures, in which there are present polar or reactive groups at the terminals of the polymer chain, are more substantive to gelatin and therefore have a reduced tendency to wash out of the layer during processing. This reduces the likelihood of contamination of processing baths as well as avoiding deleterious changes in the dimensional stability of the coated assembly.

Compounds of this type are amorphous and such amorphous polyoxyalkylene compounds provide improved conductivity to polyoxyalkylene compounds which are crystalline.

The preferred polymeric salts beneficially comprise the salts of strong polymeric acids. These polymeric acids may be formed by any of the processes well known in polymer science. Thus the polymeric acid may be formed by the polymerisation of a neutral monomeric species to give a neutral polymer, followed by the reaction of the neutral polymer with a mineral acid derivative to give an acidic polymer. An example of this would be the reaction of polystryene with sulphuric or chlorosulphonic acid to produce poly(styrene sulphonic acid). The acid polymer may then be treated with base to produce its salt. Polymers of this type are well known and a range of them are available under the trade name VERSA, manufactured by National Polymer Company. These are water soluble polystyrene sulphonate homopolymers and copolymers.

A second way of producing the polymeric salts is to effect the polymerisation of a monomeric, ethylenically unsaturated acidic substance or salt thereof by the standard polymerisation techniques well known in the prior art. For example, there is described in "Functional Monomers", Volume 1, editors R H Yocum and E B Nyquist, published by Marcel Dekker Inc, NY, 1973, Chapter 4, the polymerisation of sulphonic acid monomers and their salts. Such monomers include for example vinyl sulphonic acid, styrene sulphonic acid, 2-sulphoethylmethacrylate, 3-sulphopropyl acrylate, allyl sulphonic acid, methallyl sulphonic acid, vinyl toluene sulphonic acid and N-acryloyl taurine. Copolymers of these monomers with other ethylenically unsaturated monomers may be used.

Mixtures of two or more of the above mentioned acidic monomers may be used.

Further methods for forming similar polymers include for example condensation reactions such as that between toluene and formaldehyde which lead to linear or branches hydrocarbon resins which may then be treated with sulphuric acid or other sulphonylating reagents to give rise to polysulphonic acids.

In general the structure of the above polymeric acids is of the form:

where A and B are end groups; L1 is a repeat unit of a polymeric chain; X is a group containing an acid unit which may be for example -SO3H, -O-SO2H, -PO3H or -OPO2H; p is an integer greater than about 5, such that the molar mass of the polymeric acid is greater than 500 Dalton. Preferably the molar mass should be greater than 500 Dalton and less than about 100,000 Dalton.

The acidic polymers described above are to be used in the form of their salts with metals or organic amines. For example, salts may be obtained in which the cation is lithium, sodium, potassium, ammonium, triethylammonium and so on. Preferred are the simple metal salts since it is often found that organic bases can interfere with photographic sensitometry. The salts of these polymers with multivalent cations are not preferred since the polymeric nature of the acids means that insoluble, gel-like salts will be formed on addition of multivalent cations, and such gel-like polymers will be very difficult to incorporate in the aqueous coating formulation.

It is to be understood that the anti-static compositions of use in the present invention may be coated as a supercoat on a photographic emulsion or as a coating on the reverse side of the assembly to the silver halide emulsion. Preferably the anti-static composition is the uppermost layer on the side on which it is coated.

The preferred colloid binder for use in the anti-static composition is gelatin but other natural colloids such as albumen or casein may be used.

Other hydrophilic colloids which may be used include carboxylmethylcellulose, hydroxyethylcellulose, polyvinyl alcohol, polyacrylic acid and poly-N-vinylpyrrolidone or mixtures thereof including mixtures with gelatin.

An anti-static composition of use in the present invention may usefully comprise from 60 to 80% by weight of the colloid binder, from 10 to 30% by weight of the polyoxyakylene compounds and from 5 to 20% by weight of the polymeric salt.

Polymers containing sequences of ethylene oxide units linked by methoxy units were prepared as follows.

PREPARATION To a stirred suspension of 50g finely divided powdered potassium hydroxide (kept in a darkened reaction flask) in dichloromethane (some) there was added a solution of PEG 200 (lOg), PEG 400 (30.5g) and PEG 350 methyl ether (9.5g) in dichloromethane (50ml) and the mixture was kept stirred at 100C overnight. After filtering off the inorganic sediment the viscous solution was extracted successively with dilute HC1 and water and dried over Na2S04, then concentrated under vacuum at < 300C to yield a colourless viscous oil, 49g. The molar mass (Mn) of this product was determined to be 2900 by gel permeation chromatography using a PEO calibration, and Mw was 5700. NMR confirmed the presence of the -CH2O- linkage.

This product is referred to as Polymer V in the Table. In a similar way, Polymer VI was prepared from PEG 200, PEG 600 and PEG 350 methyl ether.

The polymers were characterised for melting point by differential scanning calorimetry (DSC) using a Perkin-Elmer DSC7 instrument at a heating rate of 20"/minute. Polymer V showed a melting point at 9.50C and Polymer VI showed a melting point at 180 C.

The term PEG is commonly used to refer to lower molecular weight polyethylene oxides.

The following Example will serve to illustrate the invention.

EXAMPLE To determine the effect of blends of polyethers and polymeric salts described in the above pages on the static discharge and resistivity behaviour of gelatin, a series of coating solutions was prepared by mixing 10% aqueous solutions of deionised ossein gelatin, the polyether and the polymeric salt. The gelatin was allowed to swell before melting out at 400 C. The polyether and polysalt solutions were added slowly to the gelatin with stirring. The solution thus prepared was coated on to polyester sheets which had been coated with a photographic emulsion, chilled at 40C and the chilled coatings dried in a forced air stream at ambient temperature. The dried coatings were conditioned at 27% relative humidity and 200C before their resitivity and charge decay characteristics were measured.

Surface resitivity was measured using an EIL surface resistivity Megohmeter, model 29A, in which the two electrodes were 5cm in length and separated by 1cam. The applied voltage was 500V.

Charge decay rate was measured using a John Chubb Charge Decay Test Instrument. A surface charge of 6kV was applied and the time taken for the surface charge to fall to 1/e of its original retained charge value was recorded.

The values recorded for the different combinations are listed in the Table.

PREPARATION OF PHOTOGRAPHIC ELEMENT A photographic silver halide iodobromide emulsion, comprising 4% iodide and having an average particle size of 0.5 microns was coated on a subbed polyethylene terephthalate film base. Separate strips of this were coated with the nine aqueous coating solutions set forth in the Table below, by dip-coating. These coatings were dried to provide antistatic layers on the silver halide emulsion.

TABLE COATING POLYETHER, % POLYMERIC GELATIN SURFACE CHARGE DECAY SALT, X X RESISTIVITY TIME sec A - . 100 2.0 X 1012 - 1.0 X 1013 5.0 to 45.0 B PEO 14,000; 30 - 70 2.0 x 1012 3.0 C PEO 14,000; 22.5 1. 7.5 70 1.0 x 1011 0.04 D IV, 22.5 II, 7.5 70 2.1 x 1011 2.52 E JEFFAMINE ED 6000, 22.5 III, 7.5 70 8.1 x 10 0.09 F V, 20 - 80 1.0 x 1013 23.2 G V, 15 I, 15 70 2.0 x 1010 0.045 H Vl, 20 I, 10 70 2.6 x 1010 0.089 J - 111,20 80 2.0 x 1012 6.57 I Sodium salt of sulphonated copolymer of styrene/maleic anhydride, Mn # 3,000 II Sodium salt of poly(ethene sulphonic acid), Mn# 1,000 III Lithium salt of poly(styrene sulphonic acid), Mn # 70,000 IV Ethoxylated nonyl phenol, 40 ethylene oxide units V Polymer from PEG 200, (208); PEG 400 (61%); PEG 350 Me ether (19%) and dichloromethane - see Synthesis Example VI Polymer from PEG 200 (24%); PEG 600 (72%);PEG 350 Me ether (4%) and dichloromethane As can be seen from the Table, the use of polyether or polymeric salt alone has little or no effect on the surface resistivity or the charge dissipative ability of the gelatin layer, under conditions of low relative humidity. Under such circumstances there is insufficient moisture present in the layer to enable charge transport to occur. In addition, when a polymeric salt alone is present in the layer (coating J) the physical properties of the layer are extremely poor and the material is very brittle. For this reason it was not possible to exceed a level of 20% by weight of lithium polystyrene sulphonate in the coated layer.

By comparison, the Table shows clearly that a combination of a polyether with a polymeric salt reduces the surface resitivity and charge decay time very significantly with the greatest effects being achieved using the methoxy-linked PEGs.

PHOTOGRAPHIC TEST Samples of each of the nine coatings were light exposed and developed in a black and white film developer. The sensitometric characteristics of the nine samples were then compared and they all exhibited substantially the same characteristics. This showed that none of the anti-static coating caused adverse photographic characteristics to occur.

Claims (10)

CLAIMS:-

1. A photographic assembly which comprises at least one silver halide colloid layer and at least one anti-static composition layer either coated on the silver halide layer or on the side distal to the silver halide layer the said anti-static layer comprising a colloid binder having therein a mixture of a polyoxyalkylene compound and a polymeric salt.

2. A photographic assembly according to Claim 1 wherein the colloid binder of the anti-static composition is gelatin.

3. A photographic assembly according to Claim 1 wherein the polyoxylakylene compound used in the anti-static composition is of the Formula I: Xs (CH2 CH2# LtY in which X and Y are end groups or may be linked to form a ring, L is a divalent linking group, or a single bond, n and m are integers and n has values 4 to 50 and m has values 1 to 50, with the proviso that when L is a single bond then m - 1-.

4. A photographic assembly according to Claim 3 wherein the polyoxyalkylene compound is a polyether having a molecular weight of 200 to 20,000 and L in Formula I is a direct linkage, X is -OH and Y is H, and m is 1.

5. A photographic assembly according to Claim 3 wherein the polyoxyalkylene compound is a polyether having a molecular weight of 200 to 20,000 and L in Formula I is a direct linkage, X contains a halogen, amino or a carboxylic end group, and Y is the same as X or is H; and m is 1.

6. A photographic assembly according to Claim 3 where in the polyoxyalkylene compound of Formula I, L is a -CH20- link, Y is hydrogen, X is OH, halogen, amino or a carboxylic end group, or each of X and Y may comprise alkyl or aryl end groups, n is from 4 to 50 and m is from 1 to 50.

7. A photographic assembly according to Claim 1 wherein the polymeric salt in the anti-static composition is a salt of polystyrene sulphonic acid.

8. A photographic assembly according to Claim 1 wherein the polymeric salt in the anti-static composition is a salt of the sulphonated copolymer of styrene and maleic anhydride.

9. A photographic assembly according to either Claim 7 or 8 wherein the polymeric salt is a lithium salt.

10. A photographic assembly according to either Claim 7 or 8 wherein the polymeric salt is a sodium salt.