Electrically-Conductive Overcoat for Photographic Elements

This application relates to commonly assigned copending application Ser. No. 08/991,289, filed simultaneously herewith and hereby incorporated by reference for all that it discloses.

This invention relates generally to imaging elements comprising a support material, polymeric subbing layer, one or more image forming layers, and one or more electrically conductive layers. More specifically, this invention relates to improved imaging elements comprising electrically-conductive surface protective (overcoat) layer(s) overlying the image-forming layer comprising colloidal, electronically-conductive metal containing particles, a first charge control agent which imparts positive charging and a second charge control agent which imparts negative charging and a polymeric film-forming binder.

Problems associated with the generation and discharge of electrostatic charge during the manufacture and use of photographic film and paper products have been recognized for many years by the photographic industry. The accumulation of static charge on film or paper surfaces can cause irregular static marking fog patterns in the emulsion layer. The presence of static charge also can lead to difficulties in support conveyance as well as the attraction of dust which can result in fog, desensitization, and other physical defects during emulsion coating. The discharge of accumulated charge during or after the application of the sensitized emulsion layer(s) also can produce irregular fog patterns or "static marks" in the emulsion layer. The severity of static-related problems has been exacerbated greatly by increases in the sensitivity of new emulsions, increases in coating machine speeds, and increases in post-coating drying efficiency. The generation of electrostatic charge during the coating process results primarily from the tendency of webs to undergo triboelectric charging during winding and unwinding operations, during conveyance through the coating machines, and during finishing operations such as slitting and spooling. Static charge can also be generated during the use of the final photographic film product. In an automatic camera, the winding of roll film out of and back into the film cassette, especially in a low relative humidity environment, can result in static charging and marking. Similarly, high-speed automated film processing equipment can produce static charging resulting in marking. Sheet films are especially subject to static charging during use in automated high-speed film cassette loaders (e.g., x-ray, graphic arts films).

It is widely known and accepted that accumulated electrostatic charge can be dissipated effectively by incorporating one or more electrically conductive "antistatic" layers into the overall film structure. Antistatic layers can be applied to one or to both sides of the film support as subbing layers either underlying or on the side opposite to the sensitized emulsion layer. Alternatively, an antistatic layer can be applied as the outermost coated layer either over the emulsion layers (i.e., as an overcoat) or on the side of the film support opposite to the emulsion layers (i.e., as a backcoat) or both. For some applications, the antistatic function can be included in the emulsion layers or pelloid layers as an intermediate layer. A wide variety of electrically conductive materials can be incorporated in antistatic layers to produce a broad range of surface conductivities. Many of the traditional antistatic layers used for photographic applications employ materials which exhibit predominantly ionic conductivity. Antistatic layers containing simple inorganic salts, alkali metal salts of surfactants, alkali metal ion-stabilized colloidal metal oxide sols, ionic conductive polymers or polymeric electrolytes containing alkali metal salts and the like have been taught in Prior Art. The electrical conductivities of such ionic conductors are typically strongly dependent on the temperature and relative humidity of the surrounding environment. At low relative humidities and temperatures, the diffusional mobilities of the charge carrying ions are greatly reduced and the bulk conductivity is substantially decreased. At high relative humidities, an exposed antistatic backcoating can absorb water, swell, and soften. Especially in the case of roll films, this can result in a loss of adhesion between layers as well as physical transfer of portions of the backcoating to the emulsion side of the film (viz. blocking). Also, many of the inorganic salts, polymeric electrolytes, and low molecular weight surface-active agents typically used in such antistatic layers are water soluble and can be leached out during film processing, resulting in a loss of antistatic function.

One of the numerous methods proposed by prior art for increasing the electrical conductivity of the surface of photographic light-sensitive materials in order to dissipate accumulated electrostatic charge involves the incorporation of at least one of a wide variety of surfactants or coating aids in the outermost (surface) protective layer overlying the emulsion layer(s). A wide variety of ionic-type surfactants have been evaluated as antistatic agents including anionic, cationic, and betaine-based surfactants of the type described, for example, in U.S. Pat. Nos. 3,082,123; 3,201,251; 3,519,561; and 3,625,695; German Patent Nos. 1,552,408 and 1,597,472; and others. The use of nonionic surfactants having at least one polyoxyethylene group as antistatic agents has been disclosed in U.S. Pat. Nos. 4,649,102 and 4,891,307; British Patent No. 861,134; German Patent Nos. 1,422,809 and 1,422,818; and others. Further, surface protective layers containing nonionic surfactants having at least two polyoxyethylene groups have been disclosed in U.S. Pat. No. 4,510,233. In order to provide improved performance, the incorporation of an anionic surfactant having at least one polyoxyethylene group in combination with a nonionic surfactant having at least one polyoxyethylene group in the surface layer was disclosed in U.S. Pat. No. 4,649,102. A further improvement in antistatic performance by incorporating a fluorine-containing ionic surfactant having a polyoxyethylene group into a surface layer containing either a nonionic surfactant having at least one polyoxyethylene group or a combination of nonionic and anionic surfactants having at least one polyoxyethylene group was disclosed in U.S. Pat. Nos. 4,510,233 and 4,649,102. Additionally, surface or backing layers containing a combination of specific cationic and anionic surfactants having at least one polyoxyethylene group in each which form a water-soluble or dispersible complex with a hydrophilic colloid binder are disclosed in European Patent Appl. No. 650,088 and British Patent Appl. No. 2,299,680 to provide good antistatic properties both before and after processing without dye staining.

Surface layers containing either non-ionic or anionic surfactants having polyoxyethylene groups often demonstrate specificity in their antistatic performance such that good performance can be obtained against specific supports and photographic emulsion layers but poor performance results when they are used with others. Surface layers containing fluorine-containing ionic surfactants of the type described in U.S. Pat. Nos. 3,589,906; 3,666,478; 3,754,924; 3,775,236; and 3,850,642; British Patent Nos. 1,293,189; 1,259,398; 1,330,356 and 1,524,631 generally exhibit negatively charged triboelectrification when brought into contact with various materials. Such fluorine-containing ionic surfactants exhibit variability in triboelectric charging properties after extended storage, especially after storage at high relative humidity. However, it is possible to reduce triboelectric charging from contact with specific materials by incorporating into a surface layer other surfactants which exhibit positively charged triboelectrification against these specific materials. The dependence of the triboelectrification properties of a surface layer on those specific materials with which it is brought into contact can be somewhat reduced by adding a large amount of fluorine-containing nonionic surfactants of the type disclosed in U.S. Pat. No. 4,175,969. However, the use of a large amount of said fluorine-containing surfactants results in decreased emulsion sensitivity, increased tendency for blocking, and increased dye staining during processing. Thus, it is extremely difficult to minimize the level of triboelectric charging against all those materials with which an imaging element may come to contact without seriously degrading other requisite performance characteristics of the imaging element.

The inclusion in a surface or backing layer of a combination of three kinds of surfactants, comprising at least one fluorine-containing nonionic surfactant, and at least one fluorine-containing ionic surfactant, and a fluorine-free nonionic surfactant has been disclosed in U.S. Pat. No. 4,891,307 to reduce triboelectric charging, prevent dye staining on processing, maintain antistatic properties on storage, and preserve sensitometric properties of the photosensitive emulsion layer. The level of triboelectric charging of surface or backing layers containing said combination of surfactants against dissimilar materials (e.g., rubber and nylon) is alleged to be such that little or no static marking of the sensitized emulsion occurs. The incorporation of another antistatic agent such as colloidal metal oxide particles of the type described in U.S. Pat. Nos. 3,062,700 and 3,245,833 into the surface layer containing said combination of surfactants was also disclosed in U.S. Pat. No. 4,891,307.

The use of a hardened gelatin-containing conductive surface layer containing a soluble antistatic agent (e.g., Tergitol 15-S-7), an aliphatic sulfonatetype surfactant (e.g., Hostapur SAS-93), a matting agent (e.g., silica, titania, zinc oxide, polymeric beads), and a friction-reducing agent (e.g., Slip-Ayd SL-530) for graphic arts and medical x-ray films has been taught in U.S. Pat. No. 5,368,894. Further, a method for producing such a multilayered photographic element in which the conductive surface layer is applied in tandem with the underlying sensitized emulsion layer(s) is also claimed in U.S. Pat. No. 5,368,894. A surface protective layer comprising a composite matting agent consisting of a polymeric core particle surrounded by a layer of colloidal metal oxide particles and optionally, conductive metal oxide particles and a nonionic, anionic or cationic surfactant has been disclosed in U.S. Pat. No. 5,288,598.

Antistatic layers incorporating electronic rather than ionic conductors also have been described extensively in the prior art. Because the electrical conductivity of such layers depends primarily on electronic mobilities rather than on ionic mobilities, the observed conductivity is independent of relative humidity and only slightly influenced by ambient temperature. Antistatic layers containing conjugated conductive polymers, conductive carbon particles, crystalline semiconductor particles, amorphous semiconductive fibrils, and continuous semiconductive thin films or networks are well known in the prior art. Of the various types of electronic conductors previously described, electroconductive metal-containing particles, such as semiconductive metal oxide particles, are particularly effective. Fine particles of crystalline metal oxides doped with appropriate donor heteroatoms or containing oxygen deficiencies are sufficiently conductive when dispersed with polymeric film-forming binders to be used to prepare optically transparent, humidity insensitive, antistatic layers useful for a wide variety of imaging applications, as disclosed in U.S. Pat. Nos. 4,275,103; 4,416,963; 4,495,276,; 4,394,441; 4,418,141; 4,431,764; 4,495,276; 4,571,361; 4,999,276; 5,122,445; 5,294,525; 5,368,995; 5,382,494; 5,459,021; and others. Suitable claimed conductive metal oxides include: zinc oxide, titania, tin oxide, alumina, indium oxide, zinc antimonate, indium antimonate, silica, magnesia, zirconia, barium oxide, molybdenum trioxide, tungsten trioxide, and vanadium pentoxide. Of these, the semiconductive metal oxide most widely used in conductive layers for imaging elements is a crystalline antimony-doped tin oxide, especially with a preferred antimony dopant level between 0.1 and 10 atom percent Sb (for Sb.sub.x Sn.sub.1-x O.sub.2) as disclosed in U.S. Pat. No. 4,394,441.

An electroconductive protective overcoat overlying a sensitized silver halide emulsion layer of a black-and white photographic element comprising at least two layers both containing granular conductive metal oxide particles and gelatin but at different metal oxide particle-to-gelatin weight ratios has been taught in Japanese Kokai A-63-063035. The outermost layer of said protective layer contains a substantially lower total dry coverage of conductive metal oxide (e.g., 0.75 g/m.sup.2 vs 2.5 g/m.sup.2 ) present at a lower metal oxide particle-to-gel weight ratio (e.g., 2:1 vs 4:1) than that of the innermost conductive layer.

The use of electroconductive antimony-doped tin oxide granular particles in combination with at least one fluorine-containing surfactant in a surface, overcoat or backing layer has been disclosed broadly in U.S. Pat. Nos. 4,495,276; 4,999,276; 5,122,445; 5,238,801; 5,254,448; and 5,378,577 and also in Kokai Nos. A-07-020,610 and B-91-024,656. The fluorine-containing surfactant is preferably located in the same layer as the electroconductive tin oxide particles to provide improved antistatic performance. A surface protective layer or a backing layer comprising at least one fluorine-containing surfactant, at least one nonionic surfactant having at least one polyoxyethylene group, and optionally one or both of electroconductive metal oxide granular particles or a conductive polymer or conductive latex is disclosed in U.S. Pat. No. 5,582,959. The addition of said electroconductive metal oxide particles to a subbing, backing, intermediate or anti-halation layer was disclosed in a particularly preferred embodiment. Further, the addition of a nonionic surfactant having at least one polyoxyethylene and a fluorine-containing surfactant each either singly or in combination to a surface protective layer or a backing layer was disclosed in another particularly preferred embodiment. However, the inclusion of electroconductive metal oxide particles in a surface protective layer was neither taught by examples nor claimed.

Similarly, a silver halide photographic material comprising an outermost layer overlying a sensitized silver halide emulsion layer containing an organopolysiloxane and a nonionic surfactant having at least one polyoxyethylene group, optionally combined with or replaced by one or more fluorine-containing surfactants or polymers, and a backing layer containing electroconductive metal oxide particles is disclosed in U.S. Pat. No. 5,137,802. The backing layer is located on the opposite side of the support from said outermost layer overlying the emulsion layer. The incorporation of an organopolysilane, a nonionic surfactant having a polyoxyethylene group and/or a fluorine-containing surfactant or polymer in said outermost layer was disclosed as providing excellent antistatic performance with a minimum degree of deterioration with storage time, and negligible occurrence of static marking.

As indicated herein above, the prior art for electrically-conductive overcoat layers containing ionic surfactants or combinations of ionic and nonionic surfactants and for antistatic layers containing electrically-conductive metal oxide particles useful for imaging elements discloses a wide variety of overcoat layer compositions. However, there is still a critical need in the art for a conductive overcoat which not only effectively dissipates accumulated electrostatic charge, but also minimizes triboelectric charging against a wide variety of materials with which the imaging element may come into contact. In addition to providing superior antistatic performance, the conductive overcoat layer also must be highly transparent, must resist the effects of humidity change, strongly adhere to the underlying layer, exhibit suitable mushiness, not exhibit ferrotyping or blocking, not exhibit adverse sensitometric effects, not impede the rate of development, not exhibit dusting, and still be manufacturable at a reasonable cost. It is toward the objective of providing such improved electrically-conductive, non-charging overcoat layers that more effectively meet the diverse needs of imaging elements, especially of silver halide photographic films, than those of the prior art that the present invention is directed.

The present invention is a multilayer imaging element which includes a support, one or more image-forming layers superposed on the support; and an outermost transparent electrically-conductive, non-charging, overcoat layer superposed on the support. The outermost transparent electrically-conductive, non-charging overcoat layer includes colloidal, electrically-conductive metalcontaining granular particles, dispersed in a film-forming binder at a volume percentage of conductive metal-containing particles of from 20 to 80 and a first charge control agent which imparts positive charging properties and a second charge control agent which imparts negative charging properties.

FIG. 1 shows an x-ray film structure using the overcoat of the present invention.

FIG. 2 shows the net charge density using a conductive rubber versus the net charge density using an insulating polyurethane for various overcoat layers.

FIG. 3 shows the net charge density using a conductive rubber versus the net charge density using an insulating polyurethane for various overcoat layers.

For a better understanding of the present invention together with other advantages and capabilities thereof, reference is made to the following description in connection with the above-described drawings.

This invention relates to improved imaging elements comprising electrically-conductive overcoat layers containing colloidal electronically-conductive metal-containing granular particles dispersed in a film forming binder, and a first charge control agent which imparts negative charging properties and a second charge control agent which imparts positive charging properties. The method for preparing the electrically conductive overcoat layers in accordance with this invention includes reducing the average primary particle size of selected metal-containing granular particles having small x-ray crystallite sizes by means of attrition milling or other suitable methods to obtain a stable aqueous colloidal dispersion. The colloidal dispersion is combined with a first charge control agent which imparts a positive charging property and a second charge control agent which imparts a negative charging property, a polymeric film-forming binder, optionally a thickener or viscosity modifier, and other additives, and applied to an imaging element in the form of a thin overcoat layer. The resulting imaging element exhibits improved electrostatic charging performance, without adversely impacting inter-layer adhesion, mushiness when compared to imaging elements of prior art.

The transparent, electrically-conductive, non-charging overcoat layer of the present invention serves to protect the silver halide sensitized emulsion layer(s) from the effects of accumulated electrostatic charge, such as dirt attraction, physical defects during manufacturing, uneven motion during conveyance, and irregular `fog` patterns resulting from triboelectric charging as well as from static marking resulting from the discharge of accumulated electrostatic charge. The electrically-conductive, non-charging overcoat layer includes both electrically-conductive metal-containing particles to provide superior dissipation of accumulated electrostatic charge and at least one and preferably a combination of charge control agents to minimize the level of triboelectric charging. Electrically-conductive metal-containing particles in accordance with this invention can be prepared by reducing the mean primary particle size of said particles having an x-ray crystallite size of less than 100 .ANG. by means of attrition milling or other suitable methods to obtain particles having an average equivalent circular diameter of less than about 0.02 .mu.m but not less than the x-ray crystallite size. Minimal triboelectric charging is achieved with a combination of charge control agents including a first charge control agent which imparts negative charging properties and a second charge control agent which imparts positive charging properties in low concentrations and at the desired relative proportions. The electrically-conductive, non-charging overcoat layer of the present invention provides superior antistatic protection relative to those conductive layers of prior art which contain only surfactants since in order to increase conductivity of such layers it is necessary to increase the surfactant concentration which also can increase the level of triboelectric charging. Further, the electrically-conductive overcoat layers of the present invention provide superior antistatic protection compared to conductive layers of prior art containing electrically-conductive metal oxide particles without charge control agents.

One class of electronically-conductive metal-containing granular particles particularly useful for the electrically-conductive overcoat layers of this invention are semiconductive metal oxide granular particles. Other examples of useful electrically-conductive, metal-containing granular particles include selected metal carbides, nitrides, silicides, and borides. Examples of suitable semiconductive metal oxides include: zinc oxide, titania, tin oxide, alumina, indium sesquioxide, zinc antimonate, indium antimonate, silica, magnesia, zirconia, barium oxide, molybdenum trioxide, tungsten trioxide, and vanadium pentoxide. Suitable semiconductive metal oxide particles are those which exhibit a specific (volume) resistivity of less than 1.times.5.sup.5 ohm-cm, preferably less than 1.times.10.sup.3 ohm-cm, and more preferably, less than 1.times.10.sup.2 ohm-cm. Such semiconductive metal oxides are typically doped with donor heteroatoms or exhibit an oxygen atom deficiency. Another physical property used to characterize metal oxide granular particles is the average x-ray crystallite size. The concept of x-ray crystallite size is described in detail in U.S. Pat. No. 5,484,694 and references cited therein. Transparent conductive layers containing semiconductive antimony-doped tin oxide granular particles exhibiting a crystallite size less than 10 nm are taught in U.S. Pat. No. 5,484,694 to be particularly useful for imaging elements. Similarly, photographic elements comprising antistatic layers containing conductive granular metal oxide particles with average x-ray crystallite sizes ranging from 1 to 20 nm, preferably from 1 to 5 nm, and more preferably from 1 to 3.5 nm are claimed in U.S. Pat. No. 5,459,021. Advantages to using metal oxide particles with small crystallite size are disclosed in U.S. Pat. Nos. 5,484,694 and 5,459,021 and include the ability to be milled to a very small size without degradation of electrical performance, the ability to produce a specified level of conductivity at lower weight coverages, as well as decreased optical density, brittleness, and cracking of conductive layers containing such particles.

The semiconductive metal oxide that has been most widely used in electrically-conductive layers for photographic imaging elements is antimony-doped tin oxide. A variety of semiconductive, crystalline, antimony-doped tin oxide powders are commercially available from various manufacturers (e.g., Keeling & Walker Ltd., Ishihara Sangyo Kaisha Ltd., Dupont Performance Chemicals, Mitsubishi Metals, Nissan Chemical Industries Ltd., etc.). Antimony-doped tin oxide particles in accordance with this invention have antimony dopant levels less than about 20 atom % Sb. These commercial electroconductive tin oxide powders can be prepared by a variety of manufacturing processes including traditional ceramic, hybrid ceramic, sol-gel, coprecipitation, spray pyrolysis, hydrothermal precipitation processes, as well as other unspecified processes. In the traditional ceramic process, finely ground powders of tin oxide and an antimony oxide are intimately mixed, heat treated at elevated temperatures (>700.degree. C.) for various periods of time, and subsequently remilled to a fine powder. In one variation of the ceramic process (See British Pat. No. 2,025,915) an insoluble tin-containing precursor powder is prepared by precipitation from aqueous solution, treated with a solution of a soluble antimony compound, the slurry dried, and the resulting powder heat-treated as in the ceramic process. This method is said to achieve a more homogeneous distribution of the antimony dopant throughout the bulk of the particles. It is possible to prepare even more homogeneously doped particles by means of a variety of other chemical coprecipitation processes, including steps with heat treatment temperatures lower than those used for typical ceramic processes. In some of the coprecipitation processes, the separate heat treatment step is eliminated altogether (e.g., hydrothermal precipitation). Such powders also can be prepared by means of a variety of other chemical coprecipitation processes including steps with heat treatment temperatures lower than those used for typical ceramic processes.

Antimony-doped tin oxide particles suitable for use in this invention exhibit a very small primary particle size, typically, less than 0.01 .mu.m. A small particle size minimizes light scattering which would result in reduced optical transparency of the conductive coating. The relationship between the size of a particle, the ratio of its refractive index to that of the medium in which it is incorporated, the wavelength of the incident light, and the light scattering efficiency of the particle is described by Mie scattering theory (G. Mie, Ann. Physik., 25, 377(1908)). A discussion of this topic as it is relevant to photographic applications has been presented (See T. H. James, "The Theory of the Photographic Process", 4th ed, Rochester: EKC, 1977). In the case of Sb-doped tin oxide particles coated in a thin layer employing a typical gelatin-based binder system, it is necessary to use powders with an average particle size less than about 100 nm in order to limit the scattering of light at a wavelength of 550 nm to less than about 10%. For shorter wavelength light, such as ultraviolet light used to expose daylight insensitive graphic arts films, particles less than about 0.08 .mu.m in size are preferred. In addition to ensuring transparency of thin conductive layers, a small average particle size is needed to form a multiplicity of interconnected chains or a network of conductive particles which provide multiple electrically-conductive pathways. Suitable antimony-doped tin oxide colloidal dispersions exhibit a very small average agglomerate size. In the case of the preferred commercially available Sb-doped tin oxide bulk powders, the average particle size (typically 0.5-0.9 .mu.m) must be reduced substantially by various attrition milling processes, such as small media milling, well known in the art of pigment dispersion and paint making. However, not all commercial Sb-doped tin oxide powders are sufficiently chemically homogeneous to permit the extent of size reduction required to ensure both optical transparency and the formation of multiple conductive pathways and still retain sufficient particle specific conductivity to form conductive thin coated layers. Average primary particle sizes (determined from TEM micrographs) of less than about 0.01 .mu.m for the preferred Sb-doped tin oxides permit extremely thin (i.e., <0.05 .mu.m) conductive layers to be coated. Such layers can exhibit comparable conductivity to much thicker layers containing larger size particles (e.g., >0.05 .mu.m) of other nonpreferred Sb-doped tin oxides.

One specific example of a suitable Sb-doped tin oxide is the electroconductive tin oxide powder described in Japanese Kokai No. 04-079104 and available under the tradename "SN-100D" from Ishihara Techno Corporation. The tin oxide powder includes granular particles of single phase, crystalline tin oxide doped with about 5-10 weight percent antimony. The specific (volume) resistivity of the antimony-doped tin oxide powder is about 1-10 ohm-cm when measured as a packed powder using a DC two-probe test cell similar to that described in U.S. Pat. No. 5,236,737. The average equivalent circular diameter of primary particles of the Sb-doped tin oxide powder as determined by image analysis of transmission electron micrographs is approximately 0.01-0.015 .mu.m. An x-ray powder diffraction analysis of this Sb-doped tin oxide has confirmed that it is single phase and highly crystalline. The typical mean value for x-ray crystallite size determined in the manner described in U.S. Pat. No. 5,484,694 is about 35-45 .ANG. for the as-supplied dry powder.

The small primary particle size of metal-containing granular particles in accordance with this invention permits the use of lower volume fractions of conductive particles in coated conductive layers to obtain suitable levels of surface electrical conductivity than is possible using larger particles of the prior art. This effectively increases the volume fraction of the polymeric binder which improves various binder-related properties of the overcoat layer such as adhesion to underlying layers, cohesion of the overcoat layer, and retention of optional matte particles (resulting in lower dusting). The volume fraction of metal-containing particles is preferably in the range of from about 20 to 80% of the volume of the overcoat layer. The use of significantly less than about 20 volume percent conductive metal-containing granular particles in the overcoat layer of this invention will not provide a useful level of surface electrical conductivity. The amount of metal-containing particles in the overcoat layer is defined in terms of volume percent rather than weight percent because the densities of suitable conductive particles may vary widely. For the antimony-doped tin oxide particles described hereinabove, this corresponds to tin oxide to binder weight ratios of from about 3:2 to 24:1. The optimum ratio of conductive particles to binder varies depending on particle size, binder type, and conductivity requirements of the particular imaging element.

The choice of the particular combination of charge control agents to be used with the conductive metal-containing granular particles in the overcoat layer is extremely important to the method of this invention. The combination of charge control agents and metal-containing particles must be optimized so as to provide a minimum (preferably zero) level of triboelectric charging and a maximum efficiency of electrostatic charge dissipation under typical handling and transport conditions including exposure and processing equipment Typically, a suitable concentration of a first charge control agent which imparts negative charging properties to the overcoat surface is used in combination with a second charge control agent which imparts positive charging properties to the overcoat surface. Combinations of charge control agents/coating aids useful in conducting overcoats of this invention comprise at least one of each of the following two groups of compounds, group (i) and (ii):

(i) a positive charging anionic compound represented by the following formulas (1) and (2),

where R represents an alkyl or alkenyl group (preferably an alkyl group having 10 to 18 carbon atoms or alkenyl group having 14 to 18 carbon atoms) or alkyl aryl group (preferably an alkyl aryl group having 12-18 carbon atoms, such as C.sub.8 H.sub.17 --(C.sub.6 H.sub.4)-- or C.sub.9 H.sub.19 --(C.sub.6 H.sub.4)--); A represents a single covalent bond or --O-- or --(OCH.sub.2 CH.sub.2).sub.m --O.sub.n --, wherein m is an integer from 1 to 4 and n is zero or 1; and M represents an alkali metal cation such as sodium, potassium or an ammonium group, or an alkyl-substituted ammonium group.

Formula (2) is a sulfosuccinate compound ##STR1## where R.sub.2 and R.sub.3 represent the same or different alkyl or alkyl-aryl groups and wherein the preferred alkyl groups contain 6 to 10 carbon atoms, and alkyl-aryl groups contain 7 to 10 carbon atoms; where M is a cation as defined above for formula (1).

ii) a negative charging fluorine-containing anionic or nonionic compound having a fluoroalkyl or fluoroalkenyl group and a hydrophilic group, which is represented by the formula (3), (4), (5) or (6) ##STR2## where R.sub.f represents a perfluorinated alkyl or alkenyl group having 6 to 12 carbon atoms; R.sub.4 represents a methyl or ethyl group or a hydrogen atom; n has a value of 0 or 1; a has a value of 0, 1, 2 or 3, when n is zero or a value of 1, 2 or 3, when n is one: and B represents an anionic hydrophilic group such as --SO.sub.3 M, --OSO .sub.3 M or --CO.sub.2 M, where M is a cation as defined above for formula (1), or a nonionic hydrophilic group such as --O(CH.sub.2 CH.sub.2 O).sub.y --D, where y is 4 to 16 and D is --H or --CH.sub.3.

Formula 4 is: ##STR3## where R'.sub.f and R".sub.f represent the same or different fluorinated alkyl group having 4 to 10 carbon atoms and at least 7 fluorine atoms, including 3 fluorine atoms on the end carbon atom; M is a cation defined above for formula (1).

Formula 5 is the following compound: ##STR4## where R'".sub.f represents a mixture of perfluorinated alkyl groups having 6,8 and 10 carbon atoms, and X is --CONH(CH.sub.2).sub.3 N(CH.sub.3).sub.2. Formula 6 is the following compound:

where R.sub.f is defined in Formula (3), and Y is a suitable nonionic hydrophilic group such as --(CH.sub.2 CH.sub.2 O).sub.b -- where b is 6 to 20, or --(CH.sub.2 CH(OH)CH.sub.2 O).sub.d -- where d is 6 to 16 and where D is --H or --CH.sub.3.

Polymeric film-forming binders useful in conductive overcoat layers prepared by the method of this invention include: water-soluble, hydrophilic polymers such as gelatin, gelatin derivatives, maleic acid anhydride copolymers; cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, cellulose acetate butyrate, diacetyl cellulose or triacetyl cellulose; synthetic hydrophilic polymers such as polyvinyl alcohol, poly-N-vinylpyrrolidone, acrylic acid copolymers, polyacrylamide, their derivatives and partially hydrolyzed products, vinyl polymers and copolymers such as polyvinyl acetate and polyacrylate acid ester; derivatives of the above polymers; and other synthetic resins. Other suitable binders include aqueous emulsions of addition-type polymers and interpolymers prepared from ethylenically unsaturated monomers such as acrylates including acrylic acid, methacrylates including methacrylic acid, acrylamides and methacrylamides, itaconic acid and its half-esters and diesters, styrenes including substituted styrenes, acrylonitrile and methacrylonitrile, vinyl acetates, vinyl ethers, vinyl and vinylidene halides, and olefins and aqueous dispersions of polyurethanes or polyesterionomers. Gelatin and gelatin derivatives are the preferred binders.

Solvents useful for preparing dispersions of conductive metal-containing particles by the method of this invention include: water; alcohols such as methanol, ethanol, propanol, isopropanol; ketones such as acetone, methylethyl ketone, and methylisobutyl ketone; esters such as methyl acetate, and ethyl acetate; glycol ethers such as methyl cellusolve, ethyl cellusolve; and mixtures thereof. Preferred solvents include water, alcohols, and acetone.

In addition to binders and solvents, other components that are well known in the photographic art also can be included in the conductive overcoat layer of this invention. Other addenda, such as polymer matte beads, polymer lattices to improve dimensional stability, thickeners or viscosity modifiers, hardeners or cross linking agents, soluble and/or solid particle dyes, antifoggants, lubricating agents, and various other conventional additives optionally can be present in any or all of the layers of the multilayer imaging element.

Colloidal dispersions of conductive, metal-containing, granular particles formulated with the preferred combination of charge control agents, polymeric binder, and additives can be applied to imaging elements coated onto a variety of supports. Typical photographic film supports include: cellulose nitrate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, poly(vinyl acetal), poly(carbonate), poly(styrene), poly(ethylene terephthalate), poly(ethylene naphthalate), poly(ethylene terephthalate) or poly(ethylene naphthalate) having included therein a portion of isophthalic acid, 1,4-cyclohexane dicarboxylic acid or 4,4-biphenyl dicarboxylic acid used in the preparation of the film support; polyesters wherein other glycols are employed such as, for example, cyclohexanedimethanol, 1,4-butanediol, diethylene glycol, polyethylene glycol; ionomers as described in U.S. Pat. No. 5,138,024, incorporated herein by reference, such as polyester ionomers prepared using a portion of the diacid in the form of 5-sodiosulfo-1,3-isophthalic acid or like ion containing monomers, polycarbonates, and the like; blends or laminates of the above polymers. Supports can be either transparent or opaque depending upon the application. Transparent film supports can be either colorless or colored by the addition of a dye or pigment. Film supports can be surface-treated by various processes including corona discharge, glow discharge, UV exposure, flame treatment, electron-beam treatment, as described in co-pending U.S. patent application Ser. No. 08/662,188 (filed Jun. 12, 1996) assigned to the same assignee as the present Application or treatment with adhesion-promoting agents including dichloro- and trichloro-acetic acid, phenol derivatives such as resorcinol and p-chloro-m-cresol, solvent washing or overcoated with adhesion promoting primer or tie layers containing polymers such as vinylidene chloride-containing copolymers, butadiene-based copolymers, glycidyl acrylate or methacrylate-containing copolymers, maleic anhydride-containing copolymers, condensation polymers such as polyesters, polyamides, polyurethanes, polycarbonates, mixtures and blends thereof, and the like. Other suitable opaque or reflective supports are paper, polymer-coated paper, including polyethylene-, polypropylene-, and ethylene-butylene copolymer-coated or laminated paper, synthetic papers, pigment-containing polyesters, and the like. Of these support materials, films of cellulose triacetate, poly(ethylene terephthalate), and poly(ethylene naphthalate) prepared from 2,6-naphthalene dicarboxylic acids or derivatives thereof are preferred. The thickness of the support is not particularly critical. Support thicknesses of 50 .mu.m to 254 .mu.m (2 to 10 mils) are suitable for photographic elements in accordance with this invention.

Aqueous dispersions of conductive metal-containing granular particles can be prepared in the presence of appropriate levels of optional dispersing aids, colloidal stabilizing agents or polymeric co-binders by any of various mechanical stirring, mixing, homogenization or blending processes well-known in the art of pigment dispersion and paint making. Alternatively, stable colloidal dispersions of suitable conductive metal-containing particles can be obtained commercially, for example, a stabilized dispersion of electroconductive antimony-doped tin oxide particles at nominally 30 weight percent solids is available under the tradename "SN-100D" from Ishihara Sangyo Kaisha Ltd. Formulated dispersions containing colloidal conductive metal-containing granular particles and the preferred combination of charge control agents, polymeric binder, and additives can be applied to the aforementioned film or paper supports by any of a variety of well-known coating methods. Hand coating techniques include using a coating rod or knife or a doctor blade. Machine coating methods include air doctor coating, reverse roll coating, gravure coating, curtain coating, bead coating, slide hopper coating, extrusion coating, spin coating and the like, and other coating methods well known in the art.

The electrically-conductive overcoat layer of this invention can be applied to the support at any suitable coverage depending on the specific requirements of a particular type of imaging element. For example, for silver halide photographic films, dry coating weights of the preferred antimony-doped tin oxide in the conductive overcoat layer are preferably in the range of from about 0.01 to about 2 g/m.sup.2. More preferred dry coverages are in the range of about 0.02 to 0.5 g/m.sup.2. The conductive overcoat layer of this invention typically exhibits a surface resistivity (20% RH, 20.degree. C.) of less than 1.times.10.sup.10 ohms/square, preferably less than 1.times.10.sup.9 ohms/square, and more preferably less than 1.times.10.sup.8 ohms/square.

The imaging elements of this invention can be of many different types depending on the particular use for which they are intended. Such imaging elements include, for example, photographic, thermographic, electrothermographic, photothermographic, dielectric recording, dye migration, laser dye-ablation, thermal dye transfer, electrostatographic, and electrophotographic imaging elements. Details with respect to the composition and function of this wide variety of imaging elements are provided in co-pending U.S. patent application Ser. Nos. 08/746,618 and 08/747,480 (both filed Nov. 12, 1996) assigned to the same assignee as the present Application and incorporated herein by reference. Suitable photosensitive image-forming layers are those which provide color or black and white images. Such photosensitive layers can be image-forming layers containing silver halides such as silver chloride, silver bromide, silver bromoiodide, silver chlorobromide and the like. Both negative and reversal silver halide elements are contemplated. For reversal films, the emulsion layers described in U.S. Pat. No. 5,236,817, especially examples 16 and 21, are particularly suitable. Any of the known silver halide emulsion layers, such as those described in Research Disclosure, Vol. 176, Item 17643 (December, 1978) and Research Disclosure, Vol. 225, Item 22534 (January, 1983), and Research Disclosure, Item 36544 (September, 1994), Research Disclosure, Item 37038 (February, 1995) and Research Disclosure, Item 38957 (September, 1996) and the references cited therein are useful in preparing photographic elements in accordance with this invention.

In a particularly preferred embodiment, imaging elements comprising electrically-conductive overcoat layers of this invention are photographic elements which can differ widely in structure and composition. For example, said photographic elements can vary greatly with regard to the type of support, the number and composition of the image-forming layers, and the number and types of auxiliary layers that are included in the elements. In particular, photographic elements can be still films, motion picture films, x-ray films, graphic arts films, paper prints or microfiche. It is also specifically contemplated to use the conductive overcoat layer of the present invention in small format films as described in Research Disclosure, Item 36230 (June 1994). Photographic elements can be either simple black-and-white or monchrome elements or multilayer and/or multicolor elements adapted for use in a negative-positive process or a reversal process. Generally, the photographic element is prepared by coating one side of the film support with one or more layers comprising a dispersion of silver halide crystals in an aqueous solution of gelatin and optionally one or more subbing layers. The coating process can be carried out on a continuously operating coating machine wherein a single layer or a plurality of layers are applied to the support. For multicolor elements, layers can be coated simultaneously on the composite film support as described in U.S. Pat. Nos. 2,761,791 and 3,508,947. Additional useful coating and drying procedures are described in Research Disclosure, Vol. 176, Item 17643 (December, 1978).

Conductive overcoat layers of this invention can be incorporated into multilayer photographic elements in any of various configurations depending upon the requirements of the specific application. A conductive overcoat layer can be applied directly over the sensitized emulsion layer(s), on the side of the support opposite the emulsion layer(s), as well as on both sides of the support. When a conductive overcoat layer containing conductive, metal-containing granular particles is applied over a sensitized emulsion layer, it is not necessary to apply any intermediate layers such as barrier layers or adhesion-promoting layers between the overcoat layer and the sensitized emulsion layer(s), although they can optionally be present. Alternatively, a conductive overcoat layer can be applied as part of a multi-component curl control layer (i.e., pelloid) on the side of the support opposite to the sensitized emulsion layer(s). In the case of photographic elements for direct or indirect x-ray applications, the conductive overcoat layer can be applied on either side or both sides of the film support. In one type of photographic element, the conductive overcoat layer is present on only one side of the support and the sensitized emulsion coated on both sides of the film support. Another type of photographic element contains a sensitized emulsion on only one side of the support and a pelloid layer containing gelatin on the opposite side of the support. Conductive overcoat layers of this invention can be applied so as to overlie the sensitized emulsion layer(s) or alternatively, the pelloid layer or both.

The conductive overcoat layer of this invention also can be incorporated in an imaging element comprising a support, an imaging layer, and a transparent magnetic recording layer containing magnetic particles dispersed in a polymeric binder. Such imaging elements are well-known and are described, for example, in U.S. Pat. Nos. 3,782,947; 4,279,945; 4,302,523; 4,990,276; 5,147,768; 5,215,874; 5,217,804; 5,227,283; 5,229,259; 5,252,441; 5,254,449; 5,294,525; 5,335,589; 5,336,589; 5,382,494; 5,395,743; 5,397,826; 5,413,900; 5,427,900; 5,432,050; 5,457,012; 5,459,021; 5,491,051; 5,498,512; 5,514,528 and others; and in Research Disclosure, Item No. 34390 (November, 1992) and references cited therein. Such elements are particularly advantageous because they can be employed to record images by the customary imaging processes while at the same time additional information can be recorded into and read from a transparent magnetic layer by techniques similar to those employed in the magnetic recording art. The transparent magnetic recording layer comprises a film-forming polymeric binder, magnetic particles, and other optional addenda for improved manufacturability or performance such as dispersants, coating aids, fluorinated surfactants, crosslinking agents or hardeners, catalysts, charge control agents, lubricants, abrasive particles, filler particles, plasticizers and the like. The magnetic particles include ferromagnetic oxides, complex oxides including other metals, metal alloy particles with protective oxide coatings, ferrites, hexagonal ferrites, etc. and can exhibit a wide variety of shapes, sizes, and aspect ratios. The magnetic particles also can contain a variety of metal dopants and optionally can be overcoated with a shell of particulate inorganic or polymeric materials to decrease light scattering as described in U.S. Pat. Nos. 5,217,804 and 5,252,444. The preferred ferromagnetic particles for use in transparent magnetic recording layers used in combination with the electrically-conductive overcoat layers of this invention are cobalt surface-treated .gamma.-Fe.sub.2 O.sub.3 or magnetite with a specific surface area (BET) greater than 30 m.sup.2 /g. The transparent, conductive overcoat layer of this invention can be applied so as to overlie the emulsion layer(s).

Imaging elements incorporating conductive overcoat layers of this invention useful for other specific imaging applications such as color negative films, color reversal films, black-and-white films, color and black-and-white papers, electrographic media, dielectric recording media, thermally processable imaging elements, thermal dye transfer recording media, laser ablation media, and other imaging applications should be readily apparent to those skilled in photographic and other imaging arts.

The method of the present invention is illustrated by the following detailed examples of its practice. However, the scope of this Invention is by no means restricted to these illustrative examples.

A coating mixture comprising 0.47% lime treated ossein gelatin in water and various additives including a combination of a positively-charging sodium-bis(2-ethylhexyl) sulfosuccinate (Cytec Ind.) charge control agent/coating aid (A) and a negatively-charging perfluorooctyl sulfonate, tetraethylammonium salt (Bayer AG), charge control agent/coating aid (B). Other additives included 0.011% chrome alum hardener, 0.42% bis-vinylsulfonylmethyl ether (BVSME), and 0.0023% polymethylmethacrylate matte particles (1-2 .mu.m diameter). The concentration of charge control agent/coating aid A was 0.42 g/kg mixture and the concentration of charge control agent/coating aid B was 0.042 g/kg mixture.

This coating mixture was applied using a coating hopper to both sides of a moving web of 178 .mu.m (7 mil) thick polyethylene terephthalate film support 10 that had been previously coated with: a vinylidene chloride/acrylonitrile/itaconic acid terpolymer undercoat layer 11; a gelatin subbing layer 12; a sensitized TMAT G/RA silver halide emulsion (Eastman Kodak Company) layer 13; and an all-gelatin intermediate layer 14, producing the x-ray film structure shown in FIG. 1. The wet laydown of the overcoat coating solution applied to the previously coated layers was 2.0 ml/ft.sup.2. The overcoat layer is shown by 15 in FIG. 1.

The surface electrical resistivity (SER) of the conductive overcoat was measured after conditioning for 24 hours at 20% RH, 20.degree. C. using a two-probe parallel electrode method as described in U.S. Pat. No. 2,801,191 incorporated herein by reference.

The net surface charge density (Q) present on a film after contact with and separation from insulating polyurethan or conductive EPDM (ethylene propylene diene monomer) rubber was measured at 20% RH, 20.degree. C. The values obtained for SER, Q.sub.poly and Q.sub.epdm are reported in Table 1. Antistatic performance for a given overcoat layer formulation is represented by its charging location in the Q.sub.poly -Q.sub.epdm charging space (FIG. 2), with the "0,0" location being most desirable, as can be demonstrated by testing in exposure and processing equipment.

Coating compositions were prepared and characterized as described in Example 1 except that concentrations of charge control agents/coating aids A and B were varied as listed in Table 1. The range of values for net charge density representing sensitivity to concentration(s) of charge control agent(s) is shown in FIG. 2. The number labels for the points in FIG. 2 correspond to the Example numbers indicated in Table 1.

A coating mixture comprising colloidal electroconductive SN-100b-doped tin oxide granular particles (Ishihara Sangyo Kaisha Ltd.) with 0.47% lime-treated ossein gelatin, (85/15 SnO.sub.2 to gelatin weight ratio) and various additives was prepared. Other additives included 0.011 % chrome alum hardener, 0.42 % BVSME hardener, and 0.0023 % polymethylmethacrylate matte particales (1-2 .mu.m diameter). The concentration of charge control agent/coating aid A was 0.10 g/kg mixture and the concentration of charge control agent/coating aid B was 0.010 g/kg mixture. Overcoat layers were prepared and characterized as ed in Example 1.

Coating mixtures were prepared as described in Example 11 except that the concentrations of SN-100D tin oxide dispersion and gelatin were varied as listed in Table 2. Overcoat layers were prepared and characterized as described in Example 1. The concentration of charge control agent/coating aid A was 0.10 g/kg mixture and the concentration of charge control agent/coating aid B was 0.010 g/kg mixture. These concentrations were selected as having the lowest changing values as shown in FIG. 2. The values obtained for SER, Q.sub.poly, and Q.sub.epdm are reported in Table 2. Antistatic performance for overcoat layer formulations 11-17 is represented by their relative locations in the Q.sub.poly -Q.sub.epdm charging space (FIG. 3), with the 0,0 location being the most desirable.

The range of charge density values representing sensitivity to tin oxide coverage (i.e., conductivity) is shown in FIG. 3. The numbers associated with the points in the figure correspond to example numbers identified in Table 2. As shown in FIG. 3, the use of an electrically-conductive overcoat comprising an optimized combination of charge control agents and electronically-conductive metal-containing particles provides for robust antistatic protection performance and minimizes triboelectric charging against various roller materials used in exposure and processing equipment.

The effect of a tin-oxide containing overcoat similar to Examples 11-17 on an x-ray film sensitometric response was evaluated by routine testing procedures, and no adverse sensitometric response was observed. Thus, the present invention provides overcoat layers that have no effect on the sensitometry of an x-ray film.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.