BACKGROUND OF THE INVENTION

The production of unitary conductive elements which ar particularly useful in such areas as electrophotography and electrography have been extensively described in patents and other literature. Many of these conductive elements have multilayer structures and are prepared by coating a substrate layer with a conductive material. A further coating may then be added, as for example, in an electrophotographic element, wherein a layer of photoconductive composition is coated over the conductive material. If desired, a barrier layer may be imposed between the conducting material and the photoconductive compositional layer.

One of the many problems encountered in the process of producing conductive elements, and particularly those useful in electrophotography and electrography, is that there is a difficulty in obtaining good adhesion between the various layers. Moreover, the uniform dispersions of conducting material used in producing the conductive elements usually include a polymeric binder. Solvents for the polymeric binder and conductivity material are often difficult to obtain since conducting materials are often insoluble in the polymer solvent and vice versa.

The foregoing disadvantages for producing conductive elements, and in particular

electrophotographic elements, are even more problematic when the conductive layer is a cuprous iodide or Cul coating. For instance, in U.S. Patent No. 3,245,833, a volatile organic solvent is used to solubilize the binding material and to dissolve the solubilized semiconductor compound. However, in some instances in order to solubilize the semiconductor compound a complexing agent must also be added. This complexing agent is usually a chelating agent, and in some cases it can be the solvent. After the coating of the solution of Cul, the solvent is then evaporated and Cul particles are formed in-situ in the coating after drying.

U.S. Patent Nos. 3,597,272 and 3,740,217 suggest another method of achieving, specifically, an electrophotographic element while overcoming the problems of layer adhesion and mutual solvents. An imbibition procedure is disclosed. The conductive layer is formed by imbibing a binder-free solution of volatile solvent and a metal-containing semiconductor into an electrically insulating polymeric subcoating carried on a support, and then evaporating the solvent.

Many of the examples illustrate the use of a solution of cuprous iodide in acetonitrile as the volatile solvent.

The use of acetonitrile as a solvent for the coating process of Cul is well known. However, when using acetonitrile, the uniformity of the conductive coating is difficult to control. East German Patent NOS. DD223,550, DD220,155, DD201,527, DD157,369, DD157,368, and DD149,721 illustrate the preparation of conductive layers containing Cul from organic solutions, including acetonitrile solutions, or the preparation of opaque conductive stripes for the purposes of annotation using a dispersion of Cul in a binder.

Other attempts to produce electrically conductive support elements that display a uniformity of the

conductive coating layer have been made. For instance, in U.S. Patent No. 4,416,963, an electrically conductive transparent support with an electrically conductive metal oxide of average grain size of 0.05μ or less is disclosed. However, the patent does not address the problems of using Cul in the conductive layer.

As can be seen from the prior art, the use of cuprous iodide for conductive coatings is known. However, coating cuprous iodide from solution is a problem due to the poor solubility characteristics encountered by the use of most coating solvents. Furthermore, in many solvent systems such as acetonitrile, the uniformity of the conductive coating is difficult to control. As well, environmental impact and safety hazards are items that must be considered whenever solvents are used in coating processes. For example, acetonitrile is known to produce HCN under thermal degradation at high temperatures.

SUMMARY OF THE INVENTION

It is accordingly, an object of the present invention to provide a conductive coating material that provides a uniform coating on a support layer, e.g., a polymeric or paper support layer. Yet another object of the present invention is to provide a dispersion of the conductive material and the binder resin so that there is realized an enhanced adhesion between the polymeric support layer and the conductive coating. Still another object of the present invention is to provide a conductive coating material that yields a transparent, low optical density coating on a support.

Yet another object of the present invention is to provide a method that does not involve the handling of

noxious chemicals in order to provide a ground plane for a polymeric support layer.

Another object of the present invention is to provide a novel and efficient process for forming a transparent semi-conductive coating on a support.

Still another object of the present invention is to provide such a process which permits a flexibility in the conductivity exhibited by the coating.

In accordance with the foregoing objectives, there is provided by the present invention a colloidal dispersion of cuprous iodide in a resin solution, which is generally utilized as a novel coating in the preparation of a ground plane for electrophotographic and electrographic imaging elements, as well as in the application of antistatic coatings. The size of the cuprous iodide particles in the dispersion is most preferably submicron. A novel and efficient process for the production of such a coating material and its coating onto a substrate are also provided herewith.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graphic representation of various amounts of cuprous iodide pigment coated onto a film and the equilibration time of these pigments in various amounts.

Figure 2 graphically represents the surface resistivity versus the amount of milling time and also exhibits the percent haze level as measured with a Gardner Hazemeter.

Figure 3 schematically illustrates the formation of the cuprous iodide onto a support base and the formation of a transparent electrophotographic film.

Figure 4 photographically represents a conducting layer formed by using methods known in the prior art.

Figure 5 photographically represents the cuprous iodide coating of the present invention on a polymeric support.

Figure 6 graphically represents the response of a film element made in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION According to the present invention, novel conductive coatings are provided which comprise a metal- containing semiconductor compound dispersed in a resin solution. Such coatings preferably have a surface resistivity of from 10 ⁴ to 10 ⁹ ohms per square, which surface resistivity can be modified by changing the pigment to binder ratio.

The semiconductor compound is generally dispersed in the binder resin solution as particles of a sub- micron size. This dispersion is a colloidal dispersion and results in a uniform coating on the polymeric support. A colloidal dispersion, as is understood in the art, is a suspension of finely divided particles in a continuous medium. The particles themselves are called the disperse phase, or the colloid, and the medium is the dispersing medium. The colloidal dispersion differs from an ordinary solution or dispersion in that the size of the particles is in the range between 1 and .001 micron. While it is preferred that the dispersion of the present invention contain only particles of submicron size, the present invention is intended to also cover those instances where the dispersion does contain some particles having a size greater than one micron, and which would not be normally considered "colloidal" in nature.

In various embodiments of the invention, conductive coatings will contain semiconductor compounds in amounts ranging from about 10-99 percent by weight, and most preferably from 70-99 percent by weight. Depending on the surface resistivity one wants. to achieve with these coatings, various amounts of semiconductor compounds can be utilized. For example, Figure 1 demonstrates the surface resistivity of variable semiconductor compounds over a period of weeks. Useful conductive coatings generally have a surface resistivity of less than 10 ⁹ ohms per square as measured by procedures well known in the art.

Cuprous iodide is the preferred metal-containing semiconductor compound utilized in the preferred embodiment of this invention. However, the invention contemplates the use of other semiconductor compounds, such as copper halides other than cuprous iodide, silver halides including silver iodide, halides of bismuth, gold, indium, iridium, lead, nickel, palladium, rhenium, tin, tellurium and tungsten; cuprous, cupric and silver thiocyanates, and iodomercurates, and other metal-containing semiconductor compounds.

The term "semiconductor", as used herein, encompasses those metal-containing compounds having a specific resistance in the range of 10~3 to 10 ⁹ ohm-cm, as measured by standard procedures. In contrast to specific resistance in ohm-cm, the term "surface resistivity" generally refers to measurement of electrical leakage across an insulating surface. However, in the present specification, the term is used with reference to resistance or conductivity of films that behave as conductors transmitting currents through the body of the coating of electrically conducting and semiconducting materials. Moreover, in the case of thin conductive coatings, measurement of the conductive

property in terms of surface resistivity in units of ohms per square provides a value that is useful in measurement and practice.

A suitable binder resin for use in the present invention may be selected from, among the following binders and classes of resins: gelatin, polyvinyl alcohol, polyvinyl acetate, carboxylated polyvinyl acetate, polyvinyl acetal, polyvinyl chloride, polyvinyl phthalate, polyvinyl methyl ether maleic anhydride, polymethylmethacrylate, polyvinyl acetal phthalate, polystyrene-butadiene-acrylonitrile, polyvinyl butyral, polystyrene-maleic acid, polyvinylidene chloride- acrylonitrile, polymethylmethacrylate-methacrylic acid, polybutyl methacrylate-methacrylic acid, cellulose acetate, cellulose acetate-butyrate, cellulose acetate- phthalate, cellulose ethylether phthalate, methylcellulose, ethylcellulose, polymethylacrylate- vinylidene chloride-itaconic acid, poly-2-vinyl pyridine, celluloseacetate diethylamino-acetate, polyvinyl methyl ketone, polyvinyl acetophenone, polyvinyl benzophenone, polyvinylmethyl-acrylate- methacrylic acid, polyvinyl acetate maleic anhydride, polyacrylonitrile-acrylic acid, polystyrene-butadiene, polyethylene-maleic acid, poly-4-vinyl pyridine, carboxylic esters of rosin lactones, polystyrene, cellulose nitrate, polyurethane resins, polyamide resins, phenolic resins, urea resins, melamine resins, ethyl cellulose diethylamino acetate, other basic polymers, polybasic acid polymers, polyesters, epoxy resins, alkyds, etc. However, the transparency of the semiconductive coating films can be affected by the binder, as evaluated by the optical density of these films and Gardner Hazemeters. It is, therefore, preferable to use a polyvinyl alcohol, copolymer of polyvinylidene chloride and acrylonitrile, polyvinyl

butyral or ethyl cellulose binder, since the transparency of the semiconductive coating film appears to be enhanced.

Generally, about 1.0 to 30 wt percent of the binder is utilized in this invention, with from 1.0 to 20 wt percent being most preferred. The more conductive one desires the coating, the less binder that is used. The desired conductivity, of course, will depend upon the ultimate application. The present invention advantageously permits one to easily control the conductivity of the coating such that the specific conductivity necessary for the alternate applications e.g., electrophotography, electrography and antistatic applications is achieved. Such flexibility and ability to obtain the narrow allowable ranges of conductivity required in some applications, i.e., 1 to 3 x 10 ⁶ ohms/square for certain electrographic applications, has heretofore never been possible.

The binder is usually dissolved in a solvent system that is comprised of toluene and volatile ketone solvents. Water can also be used as the sole solvent with the appropriate binders, e.g., polyvinyl alcohol. Suitable volatile ketone solvents include acetone, methylethylketone, 2-pentanone, 3-pentanone, 2-hexanone, 2-heptanone, 4-heptanone, methylisopropylketone, methylisobutylketone, methyl t-butylketone, diacetyl, acetyl acetone, acetonyl acetone, diacetone alcohol, mesityloxide, chloroacetone, cyclopentanone, cyclohexanone, acetophenone, and benzophenone. A mixture of these ketone solvents may be used or a single ketone solvent may be used. It is preferable, however, to dissolve the binder in a solvent system of toluene, or toluene with methylethylketone. Generally, in the system with methylethylketone, the toluene is present in the range of 70-90 weight percent, while the ketone

solvent is generally present in the amount of 10-30 weight percent. However, other polar solvents can be used in combination with toluene, e.g., ethyl alcohol.

The colloidal dispersion is generally formed by use of a surfactant material. Any surfactant may be utilized that is known in the art such as fluorosurfactants, organic titanates, lauryl sulfate, hydroxylated polymers, Triton-X (Rohm and Haas), phospholipids, etc. It is, however, preferable to use lecithin, which is normally present in the range of 0.05 to 0.5 weight percent of the dispersion, or from about 0.05 to 5 wt % of the pigment.

The colloidal coating material is prepared by adding the above-identified ingredients, e.g., cuprous iodide, binder, solvent and surfactant, and milling these ingredients over a period of time, generally under ambient conditions. Any type of milling is generally appropriate, e.g., ball milling, sand milling or media milling. Media milling using horizontal media mills such as those manufactured by Netzsch are efficient, and dispersions prepared using the horizontal media mill have been found to produce excellent coatings. Thus, media milling is the preferred method of milling.

The transparency of the films that are formed by this method can also be affected by the duration of milling. For instance, Figure 2 illustrates that seven days of ball milling can significantly decrease the haze level of the films to less than 10 percent.

It has also been found quite surprisingly that improved transparency is obtained when a separation step is employed in the process of the present invention. The separation step is preceded by a milling of the ingredients until a sub-micron particle distribution of the cuprous iodide particles is achieved. A portion of this distribution, e.g., which consists of particles

having a size of 500 nm or less, can be separated from the rest of the distribution by conventional means, e.g., centrifugation or filtration (microfiltration) . Centrifugation is the preferred method of separation due to its effectiveness and efficiency in separating particles of small size. The dispersion of smaller particles is then preferably used in the coating step that follows. This separation step has been found to be most useful when employing water as the solvent, and is therefore preferred for use in conjunction with aqueous coatings. An application of the separation step in conjunction with organic solvents can also yield beneficial results.

After the colloidal dispersion has been milled for the appropriate time, and optionally subjected to the separation step described above, an electrophotographic film, for example, can then be made by layering the colloidal dispersion onto a support layer of choice, e.g., by a Mayer rod. Any support or substrate layer may be utilized to produce a ground plane for a conductive element such as an electrophotographic imaging element. These supports generally consist of polymer films such as polyethylene terephthalate films (PET) , polyethylene films, polypropylene films, bond- coated polyester films, as well as any other support utilized in the art. Other supports, however, such as paper supports, can also be appropriately used. The support materials may be properly selected according to the use and purpose of the electrophotographic imaging element.

Furthermore, the present invention is not limited to any particular means for applying the colloidal dispersion onto the substrate or support layer and any suitable means may be used such as coating by a Mayer rod, roll coating, gravure, offset gravure, whirl

coating, dip coating, spray coating, etc. The means for applying the colloidal dispersion is only limited by the fact that a colloid may be difficult to apply with various forms of instrumentation. The colloidal dispersion can generally be coated onto the support in any desired thickness. It is preferred, however, that the coating be of an amount in the range of from about 1 mg/dm ² to about 50 mg/dm ² . The coating thickness or amount can also be used to vary the conductivity of the coating.

After forming the conductive layer on the support, a barrier layer may be further coated over the conductive layer with any known coating that is available in the art, such as a polymer of polyvinyl alcohol or polyamide.

A further coating of an electrophotographic layer may then be coated on top of the barrier layer. Figure 3 is illustrative of the formation of such electrophotographic films using the layering method. In this figure, the polymeric support 1, is coated with a cuprous iodide colloidal dispersion layer 2, a barrier layer 3, may be applied thereafter, and then a photosensitive layer 4 is added to complete the formation of an electrophotographic film. The invention will now be more fully explained by the following examples. However, the scope of the invention is not intended to be limited to these examples.

EXAMP1-E I

A transparent semi-conductive coating was formed on a support by the following procedure.

To a 4.5", 440 stainless steel specimen jar (ball mill) , 6 lbs of 3/16" 440 stainless steel balls were charged. To these stainless steel balls 300 grams of

toluene, 0.5 grams of lecithin (Alcolec S, purchased from American Lecithin) , 80 grams of polyvinyl butyral solution consisting of 58 grams of toluene, 16 grams methyl ethyl ketone, and 6 grams of polyvinyl butyral and 80 grams of cuprous iodide (technical grade) were added.

The above formulation was ball milled for about 168 hours at a revolving rate of 97 rp (114 fpm) at 16°C and under ambient conditions. After the milling procedure was completed, the mill was discharged by replacing a grinding cover with a screen and pouring the contents into a clean, dry amber pint jar.

The resultant solution comprising a fine insolubilized colloidal dispersion of cuprous iodide in a solubilized resin solution was then hand coated with a #18 wire-wound Mayer rod to a dry coating weight of 49 mg/dm ² on a suitable support, such as bond coated polyester (PET) film. This coating was then further dried at 110"C for three minutes.

The coating was transparent and the surface resistivity was 6.3 X 10 ⁵ ohms per square at 55% RH and 72°F. The same surface resistivity of the coating was measured after seven weeks and was 1.3 X 10 ⁴ ohms per square at 55% RH and 72°F.

Figure 5 is a photomicrograph of this novel conductive layer as compared to the prior art conductive layer, pictured in Figure 4. One can clearly see the distinguishing characteristics of the present conducting layer over the prior art, i.e., the conductive layer of the present invention has a more porous surface, thus more conductive material is incorporated therein.

EXAMPLE II

To form a transparent electrophotographic film, the following procedure was followed.

The polyester-based support with the said conductive layer applied thereto, as described in Example I, was barrier coated with a hole injection inhibiting polymer of polyvinyl alcohol (75% hydrolyzed, mw=2,000) .

An 8 wt. percent aqueous polyvinyl alcohol solution was coated by means of a #18 wire-wound Mayer rod to form a layer after three minutes of drying at 110 ^β C.

The dry coat weight of the layer is equal to about 22 mg/dm .

The barrier layer was then overcoated with a photosensitive layer comprising a substituted phenylene diamine organic photoconductor, a triphenylmethane sensitizing dye, and a resinous binder via a standard procedure to form a dry 90 mg/dm ² coating. The binder was comprised of a mixture of vinylchloride- vinylacetate and acrylic polymers.

The light exposure necessary to reduce the applied surface voltage by 200 volts from an initial surface voltage of ± 1000 volts was 103 and 85 ergs/cm ² for negative and positive charging, respectively. The visual density was 0.18 and the haze was 8% of the same film.

A sample of the film was charged to 1000 volts with a scorotron, patternwise exposed with a tungsten source through a Kodak 2A step tablet and then developed using liquid toner. The D-log E response is graphically exhibited in Figure 6.

The method of the present invention provides an alternative to the handling of noxious chemicals to make ground planes for electrophotographic imaging.

EXAMPLE III

A transparent conductive coating was prepared on a subbed polyester support using the following procedure: A Netzsch LME-4 Molinex horizontal media mill was set up with alumina agitator discs and charged to 90% of the chamber volume with 0.4-0.6 mm SAZ (Zirconium alumina silicate) media. A pre-mix was prepared by slurrying 23.26 parts by weight of technical grade Cul with 76.74 parts by weight of vehicle made up of 2.0 wt percent Hercules N-50 grade ethylcellulose dissolved in toluene. The slurry was cycled through the Netzsch mill using a residence time of 5 minutes per cycle for a total of 9 cycles. Power consumption by the 5 HP motor was kept in the range of 4.9-5.2 amperes at 480 volts. Product temperature at the outlet was maintained at less than 40°C.

The resulting dispersion was coated on subbed PET film at a dry coating weight of 0.5-0.8g/m . The air dried coating yielded a surface resistivity of 5 x 10 ⁵ ohm per square and a Gardner Hazemeter value of 20%.

EXAMPLE IV An electrographic coating was constructed on a subbed polyester support by successively coating first a conductive layer, then a barrier layer, followed by overcoating with a functional dielectric layer.

The conductive coating was applied as described in Example 1. The pigment to resin ratio was adjusted to yield a surface resistivity of 5 x 10 ⁶ ohms per square. The barrier layer was prepared using a 4 wt percent solution of Dupont Elvamide 8061, a polyamide resin, in ethanol and applied at 1.5 g/m ² dry coating weight. The dielectric overcoat layer was applied at 5 g/m ² and dried at 210"F for 3 minutes. The dielectric coating

was prepared by dispersing 0.34 wt percent Syloid 74, amorphous silica, and 1.02 wt percent micronized polyfluo 190 fluorocarbon wax in 98.64 wt percent vehicle made by dissolving 23.68 wt percent Desoto E-326 resin (a styrene-acrylic resin), and 2.0 wt percent Piccolastic A-5 resin (a polystyrene resin) in 74.32 wt percent toluene.

The resulting construction, when processed in a Versatec V-80F electrographic plotter, yielded an acceptable image.

EXAMPLE V 845 gms. of cuprous iodide were dispersed in 1,000 gms. of 2% solution of polyvinyl alcohol (fully hydrolyzed grade of PVA available from Air Products) , and ground in a laboratory Eiger Mill. The resulting dispersion was subjected to separation by centrifugation in order to separate particles of a size of 250 nm or less. The separated dispersion of small particles was then applied with a Mayer Rod #2 on a polyester support to give a conductive coating exhibiting a surface resistivity of about 2 megohms per square, and a transparency of about 83% in the visual spectrum, and about 53% in the reprographic range of the ultraviolet spectrum. The resulting ground plane was overcoated with a dielectric coating. When processed in a Versatec V-80F electrographic plotter, an image having a reflectance density of 1.3 was observed.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made without departing from the spirit and scope thereof.