Conducting layer for organic photoconductive element
United States Patent Rasch Apr. 29, 1975 [54] CONDUCTING LAYER FOR ORGANIC 3,203,830 8/1965 Ostrandcr et al. 117/106 R PHOTOCONDUCTIVE ELEMENT 3.308.528 3/1967 Bullard et a1. 1. 117/106 R 3,498,832 3/1970 Wilson 1l7/l06 R [75] Inventor: Arthur A. Rasch, Kodak ark 3,677,816 7/1972 Hayashi et al. 96/15 Division, Rochester, NY. 14650 3,684,503 8/1972 Humphriss et a] 96/15 1 4 Y k 9 [73] Assignee: Eastman Kodak Company, H197 or 5 Rochester NY Primary Examiner Cameron K. Weiffenbach [22] Filed: July 8, 1974 Attorney, Agent, or FirmR. P. Hilst [21] Appl. No.1 486,284  
 [57] ABSTRACT 52 11.5. C1. 96/15; 117/217; 117/218; A unitary multilayer photoconductive element having 1 7/22 a support bearing a conducting layer and a hydropho- 51 1111. (:1. 844d l/l8; 003 5/00 bic Organic photoconductive layer Coated Over the [58] Field of Search 96/15; 1 17/217, 218, 22}, Conducting layer is disclosed. The conducting layer is 17/34 10 a binderless layer containing an electrically conducting metal such as chromium intermixed with a protec- 5 References Cited tive inorganic oxide such as silicon oxide or silicon di- UNITED STATES PATENTS Oxlde&#39; 2,808,351 10/1957 Colbcrtetal 117 107 12 Claims, No Drawings CONDUCTING LAYER FOR ORGANIC PHOTOCONDUCTIVE ELEMENT Reference is made to Rasch, copending U.S. Pat. application Ser. No. 255,487, filed May 22, I972.  
  This invention relates to a unitary multilayer electrophotographic element containing a hydrophobic organic photoconductive layer and more particularly to a composition suitable for use as the conducting layer in such a unitary multilayer photoconductive element.  
 BACKGROUND OF THE INVENTION One type of unitary photoconductive element known for use in electrophotography is a multilayer structure typically comprising a conducting support bearing a hydrophobic photoconductive insulating composition containing one or more organic photoconductors. Advantageously, one or more optional adhesive interlayers, may also be used in the multilayer element. As is known in the art, the support itself need not be conducting but may simply be overcoated with a suitable conducting coating or film. A variety of various con ducting coatings have been proposed for use in multilayer electrophotographic elements including such materials as substantially transparent conducting coatings, for example, cuprous iodide such as disclosed in Trevoy U.S. Pat. No. 3,245,833, issued Apr. 12, 1966; vacuum-deposited layers of metal such as copper, nickel, silver, aluminum, etc. as described in Research Disclosure, Vol. No. 109, page 62, May 1973; and the like.  
  Although unitary electrophotographic elements containmg conducting layers such as those noted above are useful, it has been found that many of these conducting layers are susceptible to the formation of certain defects when subjected to extended use such as occurs in reusable photoconductive elements which are subjected to thousands of imaging cycles in a plain paper electrophotographic copying machine. Such defects are especially prevalent with vacuum-deposited metallic conducting layers such as vacuum-deposited nickel conducting layers. Moreover, it has been found that the frequency and size of these defects tend to increase with the number of imaging cycles to which the electrophotographic element is subjected.  
  In a typical electrophotographic imaging sequence wherein a reusable electrophotographic element is employed, the above-described defects which occur in the conducting layer of the element lead to corresponding visible defects in the resultant electrophotographic copies or transfer prints which are produced from these reusable electrophotographic elements. Such defects which occur in the final print or copy produced from such a reusable photoconductive element appear as a white spot&#34; in the final print or copy. The socalled white spots become particularly evident when the reusable electrophotographic element is employed in a document copying machine under conditions of somewhat elevated temperatures (above C) and relatively high humidity (greater than 75% relative humidity).  
  Although the above-described vacuum-deposited metallic layers are susceptible to the defects noted above, these materials do exhibit other certain useful properties which are desirable in the conducting layer of reusable photoconductive elements. For example, these vacuum-deposited metallic layers tend to be substantially transparent so that the hydrophobic organic photoconductive layer of the element can be exposed either directly from the top or indirectly through the conducting layer, if desired. In addition, it has been found that many of these vacuum-deposited metallic layers are, not only useful conducting layers, but may also be used without a separate electrical barrier layer between the conducting layer and the overcoated photoconductive layer. Thus, it is apparent that the devel opment of a unitary multilayer electrophotographic element as described above having a conducting layer which retains and improves on the advantageous properties of present day vacuum-deposited metal layers but eliminates the various defects to which these layers are susceptible would represent a substantial advance in the art.  
 SUMMARY OF THE INVENTION In accord with the present invention there is provided a unitary multilayer photoconductive element comprising a dielectric support, a conducting layer overcoating said support, and a hydrophobic organic photoconductive composition overcoating said conducting layer. The conducting layer has a surface resistivity of less than about 10 ohms/sq. and is a binderless layer which consists essentially of an electrically conducting metal intimately intermixed with a protective inorganic oxide. The inorganic oxide is present in the conducting layer in an amount sufficient to retard oxidation of the metal. In a particularly useful embodiment of the invention, the conducting layer is substantially transparent having an optical density of about 0.30 or less.  
  In accord with the invention, it has been found quite surprisingly that by incorporating a protective inorganic oxide in the conducting layer, the problem of white spots discussed hereinabove, which is now believed to be related to the oxidation ofthe metal, is substantially reduced.  
  Moreover, it has been found that the conducting layer used in the multilayer photoconductive element of the invention provides a relatively stable level of electrical conductivity which is equal to or better than the stability which previously could be obtained with vacuum-deposited metal layers only by using relatively thick vacuum-deposited metal conducting layers. As a result, the improved electrically stable multilayer photoconductive elements of the present invention which contain relatively thin conducting layers exhibit a smaller loss in exposure radiation intensity when exposed through the conducting layer as compared to electrically stable multilayer photoconductive elements having a conductive layer composed solely of a vacuum-deposited metal. The latter type of multilayer element requires a higher intensity of exposure radiation when exposure is made through the conducting layer because of the higher optical density of the relatively thicker conducting layer which is needed to achieve electrical stability when a conducting layer composed solely of a vacuum-deposited metal is used.  
  In an especially useful embodiment of the present invention there is provided a multilayer photographic element composed of a polyester support, an adhesive polymeric subbing layer deposited on said polyester support, a conducting layer as described herein deposited on said subbing layer, and a hydrophobic organic photoconductive composition coated over said conducting layer. An optional adhesive polymeric subbing layer may also be inserted between the conducting layer and the overcoated hydrophobic organic photoconductive composition in the aforementioned unitary element. Advantageously, the adhesive subbing layer(s) is a vinylidene chloride-containing polymer, including copolymers, terpolymers and tetrapolymers thereof. Such vinylidene chloride-containing polymeric layers have been found especially useful for obtaining good adhesion to a polyester support. Nevertheless, when subjected to extended storage conditions and to conditions of high temperature and humidity, it has been found in the past that these vinylidene chloridecontaining polymeric materials tend to degrade and attack conventional vacuum-deposited metallic conducting layers. It has been found, however, that such chemical degradation is relatively harmless using the conducting layers described herein because these layers are substantially resistant to chemical attack by the degradation products of the vinylidene chloridecontaining polymeric materials.  
 DESCRlPTlON OF THE PREFERRED EMBODIMENTS The conducting layer used in the present invention exhibits a surface resistivity of less than about l ohms per square. To insure that in all localized areas a surface resistivity of less than about ohms per square is attained, it is preferred that the conducting layer have an overall surface resistivity of less than about 10 ohms per square. With conducting layers having overall surface resistivity of less than about 10 ohms per square it has been observed that electrophotographic reproductions can be uniformly and reliably obtained from the multilayer photoconductive element of the invention with no evidence of optical alterations attributable to localized discharge of static electrical charge.  
  As is well understood by those skilled in the art, surface resistivity is determined by measuring the resistance between two parallel electrodes of a given length spaced apart by the same distance along a surface. Since an increase in the length of the electrodes tends to decrease the resistance observed by an amount equal to that by which the resistance would be increased by lengthening the spacing between the electrodes by a like increment, it is apparent that the electrode length and spacing is not material so long as they are identical, Hence, the surface resistivity expressed in ohms per square is a resistance measurement taken for the special case in which electrode length and spacing are identical and therefore mutually canceling parameters. Specifically, surface resistivity measurements referred to herein are made by placing the surface to be measured face up on a flat, hard substrate. Gold plated brass electrodes 5 mm. wide by mm. long mounted 35 mm. apart are placed on the surface to be measured and are weighted with 350 grams to establish good electrical contact with the surface to be measured. Resistance between the electrodes is then measured using a Keithley Model 610 C electrometer.  
  It is a significant feature of this invention that the metallic electrical conductor incorporated in the conducting layer need not itself be resistant to oxidation under the electrophotographic processing conditions in which it is used. That is, it is noted that metals employed according to this invention in combination with inorganic oxides are protected against excessive oxidation in use even where layers formed entirely by like quantities of the same metals have been observed to be highly oxidized. The metals that are generally preferred as metallic conductors are those yielding a low level of optical density for a given resistance value. Chromium is observed to be outstandingly suited to the practice of this invention because of its low optical densities at given levels of layer conductivity and because of its exceptional resistance to oxidation when used in combination with a protective inorganic oxide. Other metals that are noted to be highly useful in the practice of this invention are silver, copper and nickel. Still other metals which are not objectionably reactive with the dielectric support and the hydrophobic organic photoconductive composition can be used, depending upon the specific parameters, such as initial cost, optical density, conductance, short and long term oxidation resistance, etc., that may be operative for any particular application.  
  The inorganic oxide component of the conducting layer functions to protect the metal against oxidation. Typically inorganic oxides are dielectric materials, and it is a surprising feature of this invention that they are capable of protecting the metal without increasing the electrical resistance of the subbing layer beyond useful levels. The protective inorganic oxides are characteristically water insoluble and substantially chemically inert under (a) typical electrophotographic processing conditions and (b) to hydrophobic organic photoconductive coatings. Preferred metal oxides are those which exhibit a low level of optical density and, advantageously, are substantially transparent. Oxides of silicon, such as silicon monoxide and silicon dioxide, are preferred oxides for the practice of this invention because (a) they are substantially water insoluble and chemically inert under conventional electrophotographic processing conditions and (b) they are essentially transparent. Silicon oxides are also preferred because they can be vapor codeposited with metals by heating to vaporization temperatures that are low as compared to those required for vaporizing other protective oxides. Metal oxides such as aluminum oxide, magnesium oxide, tantalum oxide, boro-silicon oxide (e.g., borosilicate) and titanium oxide are also recognized to be particularly suited to the practice of this invention. The protective oxides are usable in both crystalline and amorphous forms. It is specifically contemplated that glasses may be used, particularly glass forming mixtures of oxides. The use of crystalline mineral mixed oxides is also contemplated.  
  The unexpected combination of conductance and oxidation resistance properties observed in the conducting layer are attributable to the intimate intermixture of the metal and protective inorganic oxide. While various techniques are known that may be employed to achieve an intermixture of metal and protective inorganic oxide, it is preferred that the mixture be formed by vacuum codeposition of metal and protective inorganic oxide vapors onto a support. The metal and protective oxide mixture can be blended at a molecular level as taught by Colbert et. al. in US. Pat. No. 2,808,35l issued Oct. 1, 1957, or can be a mixture of metal particles of up to 200 angstroms in diameter in a continuous matrix of protective oxide as noted, for example, by Milgram and Lu, 39, 4,219-24, Journal of Applied Physics; such useful mixtures of metal and protective oxide are commonly referred to as cermets. Generally, the most intimate physical intermixture obtainable of metal and protective oxide is preferred.  
  The proportions of metal and protective oxide can be varied as required to yield the desired balance of conductance and oxidation resistance properties. The minimum metal content of the subbing layer is determined by its maximum acceptable surface resistivity. Generally, it is preferred to incorporate at least about 30 per cent by weight and, preferably at least about 40 percent by weight, metal, based on total conducting layer weight, in the layer. On the other hand, in providing metal-rich conducting layers according to this invention having sufficient oxidation resistance to improve resistance to white spotting, it is necessary that the conducting layer be comprised of at least about 30 percent by weight and, preferably, at least about 40 percent by weight, inorganic oxide, based on the total weight of the conducting layer. For vacuum codeposited conducting layers of chromium and silicon oxides designed (a) to exhibit conducting layer surface resistivities of less than about ohms per square and (b) to optimize resistance to white spotting, chromium concentrations of from about 70 to about 30 percent by weight and silicon oxide concentrations of from about 30 to about 70 percent by weight (each percentage being based on the total weight of the conducting layer) are fully satisfactory.  
  The overall thickness of the conducting layer can be varied. To the extent that the layer is formed so thin that it exhibits undue surface resistivity i.e. does not fully cover the support surface the advantages of this invention may be at least partially diminished. At the same time, beyond a certain thickness, there is so little additional increase in conductivity that further increases in conducting layer thickness are unnecessary. In most photoconductive elements, layer thicknesses of from about 5 to about 1,000 A (angstroms) are useful, with thicknesses of from about 10 to about 150 A being preferred for optimum optical transparency properties. In accord with those particularly useful embodiments of the invention where a transparent conducting layer is desired, it is useful to employ a conducting layer sufficiently thin so that the optical density of the conducting layer is less than about 0.30, and advantageously less than about 0.15.  
  The conducting layers utilized in the practice of this invention may be advantageously applied to conventional dielectric supports for photoconductive elements, that is, supports which exhibit a surface resistivity in excess of about 10 ohms per square and, most commonly, in excess of 10 ohms per square, and, particularly to those dielectric supports which present a hydrophobic bonding surface.  
  The conducting layer can be coated on any conventional dielectric supporting surfaces and is particularly effective as a conducting interlayer between a hydrophobic organic photoconductive layer and a hydrophobic dielectric supporting surface. Typical hydrophobic polymers which form supporting surfaces according to this invention include cellulose esters such as cellulose nitrate and cellulose acetate; poly(vinyl acetal) polymers, polycarbonates, polyesters such as polymeric, linear polyesters of bifunctional saturated and unsaturated aliphatic and aromatic dicarboxylic acids condensed with bifunctional polyhydroxy organic compounds such as polyhydroxy organic compounds such as polyhydroxy alcohols-eg. polyesters of alkylene glycol and/or glycerol with terephthalic, isophthalic, adipic, maleic, fumaric and/or azelaic acid; polyhalohydrocarbons such as polyvinyl chloride; and polymeric hydrocarbons, such as polystyrene and polyolefms, particularly polymers of olefins having from 2 to 20 carbon atoms. The above polymers may be used in the form of flexible films or other unitary dielectric supports or may be used as coatings on glass, paper and polymer dielectric supports. An especially useful class of coated dielectric supports is polymeric film supports, preferably polyester film supports such as poly(ethylene terephthalate), bearing a vinylidene chloride&#39;containing adhesive polymeric subbing layer.  
  Typical vinylidene chloride-containing adhesive polymeric subbing layers for use in the elements of the present invention are two, three, and four component hydrosol addition type copolymers prepared by aqueuos emulsion co-polymerization of a monomer blend containing over percent by weight vinylidene chloride, such as a terpolymer of methyl acrylate, vinylidene chloride, and itaconic acid as disclosed in U.S. Pat. No. 3,143,421. Various vinylidene chloride containing hydrosol tetrapolymers which may be used include tetrapolymers of vinylidene chloride, methyl acrylate, acrylonitrile, and acrylic acid as disclosed in U.S. Pat. No. 3,640,709. A partial listing of other useful vinylidene chloride-containing copolymers includes poly(vinylidene chloride-methyl acrylate), poly(vinylidene chloride-methacrylonitrile), poly(vinylidene chloride-acrylonitrile), and poly(vinylidene chlorideacrylonitrile-methyl acrylate). Other useful subbing materials include the so-called tergels which are described in Nadeau et. a1. U.S. Pat. No. 3,501,301. Of course, other types of adhesive polymeric subbing layers may also be used in the present invention.  
  Hydrophobic organic photoconductive layers useful in the electrographic element typically comprise an organic photoconductor and optionally a binder and/or a sensitizer. Typically, this layer has a thickness in the range of about 1 micron to about 500 microns after drying. Useful results can be obtained where the photoconductor is present in an amount ranging from about 1 weight percent to about 99 weight percent of the coating composition. A wide variety of photoconductors can be used in electrophotographic elements. Useful organic photoconductors can also include organometallic photoconductive compounds as well as solely organic compounds. Examples of various photoconductors include the following:  
 A. Arylamine photoconductors including substituted and unsubstituted arylamines, diarylamines, nonpolymeric triarylamines and polymeric triarylamines such as those described in Fox U.S. Pat. No. 3,240,597, issued Mar. 15, 1966 and Klupfel et. al. U.S. Pat. No. 3,180,730, issued Apr. 27, 1965;  
 B. Polyarylalkane photoconductors of the types described in Noe et. al. U.S. Pat. No. 3,274,000, issued Sept. 20, 1966, Wilson U.S. Pat. No. 3,542,547, issued Nov. 24, 1970 and in Seus et. al. U.S. Pat. No. 3,542,544, issued Nov. 24, 1970;  
 C. 4-Diarylamiho-substituted chalcones of the types described in Fox U.S. Pat. No. 3,526,501, issued Sept. 1, 1970;  
 D. Non-ionic cycloheptenyl compounds of the types described in Looker U.S. Pat. No. 3,533,786, issued Oct. 13, 1970;  
 E. Compounds containing an )mar:  
 nucleus, as described in Fox U.S. Pat. No. 3,542,546, issued Nov. 24, 1970;  
 F. Organic compounds having a 3,3&#39;-bis-aryl-2- pyrazoline nucleus, as described in Fox et. al. U.S. Pat. No. 3,527,602, issued Sept. 8, 1970;  
 G. Triarylamines in which at least one of the aryl radicals is substituted by either a vinyl radical or a vinylene radical having at least one active hydrogen containing group, as described in Brantly et. al. U.S. Pat. 3,567,450, issued Mar. 2, 1971;  
 H. Triarylamines in which at least one of the aryl radicals is substituted by an active hydrogencontaining group, as described in Brantly et. al. Belgian Pat. No. 728,563, dated Apr. 30, 1969;  
 l. Organo-metallic compounds having at least one aminoaryl substituent attached to a Group lVa or Group Va metal atom, as described in Goldman et. al. Canadian Pat. No. 818,539, dated July 22, 1969;  
 J. Organo-metallic compounds having at least one aminoaryl substituent attached to a Group 111a metal atom, as described in Johnson Belgian Pat. No. 735,334, dated Aug. 29, 1969;  
 K. Charge transfer combinations, e.g., those comprising a photoconductor and a Lewis acid, as well as photoconductive compositions involving complexes of non-photoconductive material and a Lewis acid, such as described, for example, in Jones U.S. Defensive Publication T881,002, dated Dec. 1, 1970 and Mammino U.S. Pat. Nos. 3,408,181 through 3,408,190, all dated Oct. 29, 1968 and lnami et. al. US Pat. No. 3,418,116, dated Dec. 24, 1968.  
  The binder materials useful in forming hydrophobic organic photoconductive compositions include a wide variety of film-forming resinous materials. Typical binders for use in preparing the photoconductive layers are film-forming hydrophobic polymeric materials having a fairly high dielectric strength and which are good electrically insulating film-forming vehicles. Materials of this type include styrene-butadiene copolymers; si silicone resins; styrene-alkyd resins; silicone-alkyd resins; soya-alkyd resins; poly(vinyl chloride); poly(vinylidene chloride); vinylidene chloride-acrylonitrile copolymers; poly(vinyl acetate); vinyl acetate-vinyl chloride copolymers; poly(vinyl acetals), such as poly(vinyl butyral); polyacrylic and methacrylic esters, such as poly(methyl methacrylate), poly (n-butyl methacrylate), poly(isobutyl methacrylate), etc; polystyrene; nitrated polystyrene; polymethylstyrene; isobutylene polymers; polyesters, such as poly[ethylene-coa1kylenebis(alkyleneoxyarylene) phenylenedicarboxylate]; phenolformaldehyde resins; ketone resins; polyamides; polycarbonates; polythiocarbonates; poly[ethylene-co-isopropylidene-2,2-bis- (ethyleneoxyphenylene) terephthalate]; copolymers of vinyl haloarylates and vinyl acetate such as poly(vinylm-bromobenzoate-co-vinyl acetate); etc. Methods of making resins of this type have been described in the prior art, for example, styrene-alkyd resins can be prepared according to the method described in Gerhart U.S. Pat. No. 2,361,019, issued Oct. 24, 1944 and Rust U.S. Pat. No. 2,258,423, issued Oct. 7, 1941. Suitable resins of the type contemplated for use in the photoconductive layers of the invention are sold under such tradenames as VlTEL PE-ll, CYMAC, Piccopale 100, Saran F-220. and LEXAN 145. Other types of binders which can be used in photoconductive layers include such materials as paraffin, mineral waxes, etc.  
  Sensitizing compounds useful in electrophotographic elements can be selected from a wide variety of materials, including such materials as pyrylium dye salts including thiapyrylium dye salts and selenapyrylium dye salts disclosed in VanAllan et. al. U.S. Pat. No. 3,250,615, issued May 10, 1966; fluorenes, such as 7,1- 2-dioxol 3-dibenzo(a,h)fluorene, 5,l0-dioxo-4a,1 1- diazabenzo(b)fluorene, 3,13-dioxo-7- oxadibenzo(b,g)fluorene, and the like; aggregate-type sensitizers of the type described in Light Belgian Pat. No. 705,1 17, dated Apr. 16, 1968; aromatic nitro compounds of the kind described in Minsk et. al. U.S. Pat. No. 2,610,120, issued Sept. 9, 1952; anthrones like those disclosed in Zvanut U.S. Pat. No. 2,670,284, issued Feb. 23, 1954; quinones, Minsk et. al. U.S. Pat. No. 2,670,286, issued Feb. 23, 1954; benzophenones, Minsk et. al. US. Pat. No. 2,670,287, issued Feb. 23, 1954; thiazoles, Robertson et. al. US. Pat. No. 2,732,301, issued Jan. 24, 1956; mineral acids; carboxylic acids, such as maleic acid, diand trichloroacetic acids, and salicylic acids; sulfonic and phosphoric acids; and other electron acceptor compounds as disclosed by H. Hoegl, J. Phys. Chem., 69, No. 3, 755-766 Mar., 1965), and Hoegl et. al. U.S. Pat. No. 3,232,755, issued Feb. 1, 1966.  
  Electrophotographic elements can be overcoated with an outer protective layer if desired. This overcoat layer can function as a protective overcoat to prevent damage from abrasion or from chemical attack of solvents such as those used in liquid developing procedures, etc. Suitable overcoats can be selected from a wide variety of materials which are typically insulating such as waxes, insulating resins, e.g., polystyrene, ureaphenoland melamine-formaldehyde resins, vinyl resins, cellulose esters, silicone resin poly(vinyl acetals). etc. Various overcoats are described, for example, in Dessauer U.S. Pat. 2,901,348, issued Aug. 25, 1959; Kinsella U.S. Pat. 3,146,145 issued Aug. 25, 1965; and Deubner U.S. Pat. 2,860,048, issued Nov. 11, 1958. In addition, certain inorganic materials can be used as described, for example, in Corrsin U.S. Pat. No. 3,288,604, issued Nov. 29, 1966 and Kaiser U.S. Pat. No. 3,092,493, issued June 4, 1963.  
  The practice of this invention is illustrated by the following examples.  
 EXAMPLE 1 To illustrate the improved stability of the conducting layers used in the invention, several different conducting layers were made on rolls of poly(ethylene terephthalate) support. Each roll of polyester support was loaded into a Model 440-10 vacuum roll coater manufactured by the Stokes Equipment Division of Pennwalt, Inc. A mixture composed of a 1:1 ratio by weight of chromium to silicon monoxide (a cermet manufactured by Cerac, Inc.) was placed in the crucible of a Model 40-2029-180 electron beam heated vapor source manufactured by Airco Temescal Division of Air Reduction Company, Inc. The vacuum chamber was closed and pumped down to a pressure of 2.2 X 10 torr. The cermet was heated in the electron beam of the vapor source to a point where it was subliming at a high rate. Shutters protecting the support from the vapor were opened and the support was drawn through the vapor beam at a rate such that the cermet that condensed on the support formed a film approximately 70 A thick and exhibited an optical density of about 0.10. A second roll of polyester support was also coated with a cermet conducting layer as described above. except that the resultant cermet layer was about 25 A thick and exhibited an optical density of about 0.05.  
  In subsequent experiments, the above-described chromiumsilicon monoxide mixture in the vapor source was replaced with nickel and again with chromium, and thin films 25 A thick were deposited on the support under the same conditions described above. Samples of all the coatings were subjected to an accelerated keeping test in which they were stored for 14 weeks under conditions of 50 percent relative humidity at 50C. The results of these tests are shown in Table I.  
 TABLE 1 After 14 Weeks Storage Fresh Composition of Optical Resistivity Optical Resistivity Coating Density (ohms/sq) Density (ohms/sq) 507! Cr-SOV: SiO 0.l 1.4 X l0&#34; 0.09 2.2 X I0 5071 Cr-50/r SiO 0.05 3.3 X l0&#34; 0.04 L8 X l0 Chromium (Control) 0.l2 9.6 X l0 0.09 2 2 X l0 Nickel (Control) 0.12 840 0.05 l X 10&#39;&#34; The improved stability of the chromium-silicon mon&#39; oxide mixture over pure metal films is clearly demonstrated in this experiment.  
 EXAMPLE 2 Several thin film conducting layer coatings were prepared by deposition in vacuum as described in Example 1. The films were deposited on poly(ethylene terephthalate) support which had been coated with a vinylidene chloride-methyl methacrylate-itaconic acid terpolymer subbing layer. The thin conducting films were overcoated with another terpolymer subbing layer and an aggregate&#34; hydrophobic organic photoconductive layer which was prepared and applied in the manner described in U.s. Pat. No. 3,615,414. The resultant multilayer photoconductive elements are described in Table ll.  
 TABLE ll Type of Thin Film Conducting Layer Net Optical Density Multilayer Photoeonductive Element Resistivity ohms/ sq The multilayer photoconductive elements of Table ll were preconditioned at 21C, 80 percent relative humidity for 16 hours, hermetically sealed in packages and stored for 2 weeks at 50C. After this, the conduct- 10 ing film layer of each element was examined from the rear through the support. The results are summarized in Table III.  
 TABLE III Multilayer Photocon- Thin Film Keeping Effects ductive Containing (2 weeks storage at Element Layer 50C RH) l Nickel Corrosion spots in (Control) metal film 2 Chromium-SiO No visible corrosion cermet (lovv optical density) 3 Chromium-SiO No visible corrosion cermet (higher optical density) 4 Chromium l\&#39;o visible corrosion.  
 (Control) but structure changes in the metal film are apparent 5 Monel No visible corrosion (Control) spots but overall large increase in resistivity suggests uniform attack of film 6 Nickel No visible corrosion platinum overcoat (Control) 7 Nickel- Many small corrosion molybdenum spots alloy (Control) As can be seen from the results of Table III the chromium-silicon monoxide films are significantly more resistant to corrosion and other changes than pure metal or alloy films.  
  Multilayer photoconductive element Nos. 1, 2, 3, 4, and 6 of Tables II and [H which had the more stable conducting layers were then used as a reusable endless web photoconductive element in a plain paper xerographic document copying machine using magnetic brush development under a process environment in which ambient conditions were maintained at 24C and percent relative humidity. The results of these tests are given in Table IV. In this case the developed, plain paper xerographic prints made from each of photoconductive web element Nos. 1, 2, 3, 4, and 6 were evaluated on the basis of the maximum number of prints obtainable from a given web exposure zone before reaching a moderate&#34; population of white spot defects in the prints (caused by areas of corrosion in the conducting layer of the photoconductive element). A moderate population is considered to be approximately the maximum number of white spot defects that could be tolerated in the prints from the standpoint of acceptable print quality. (The prints consist of a large, uniform area having an optical density of 0.7.).  
 TABLE IV Multilayer Photoconduc- Thin Film tive Element Conducting No. Layer Print Count (White Spots Moderate) As can be seen from Table IV the chromium-silicon monoxide films in multilayer photoconductive elements Nos. 2 and 3 provide a significant improvement in process life of the element in comparison to the use of photoconductive elements Nos. 1 and 6 containing conducting films composed solely of nickel or nickelplatinum alloys. Another improvement that is not obvious from the data given in Table IV is that in the case of photoconductive elements Nos. 2 and 3, not only is.  
 there a lower frequency of white spot defect areas, but there is a substantially lower average size for each individual white spot defect. For example, from Table IV it would appear that photoconductive element No. 4.  
 containing a conducting chromium layer is comparable to photoconductive elements Nos. 2 and 3 containing the chromium-silicon monoxide conducting layer. However, an actual inspection of prints made from elements Nos. 2-4 shows that the average size of the individual white spot defects in the prints made from element No. 4 is appreciably larger than the average size of the individual white spot defects in prints made from elements Nos. 2 and 3 of the present invention. Moreover. it has been observed that conducting layer defects that appear in elements Nos. 1,4, 5, and 6 tend to grow in size as the element is reused, while this is not evident in the case of elements Nos. 2 and 3.  
  The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.  
 I claim:  
  1. A unitary multilayer photoconductive element comprising a support, a conducting layer overcoating said support. and a hydrophobic organic photoconductive layer overcoating said conducting layer, said conducting layer having (a) an electrical resistivity less than about ohm per square and (b) a composition consisting essentially of an intimate physical mixture of at least one protective inorganic oxide and from about to about 70 weight percent of at least one electrically conducting metal.  
  2. A unitary multilayer photoconductive element as defined in claim 1 wherein said oxide is selected from the group consisting of silicon monoxide, silicon dioxide, aluminum oxide, magnesium oxide, tantalum oxide. boro-silicon oxide, and titanium oxide and wherein said conducting layer thickness is less than about 1,000 A.  
 3. A unitary multilayer photoconductive element as defined in claim 1 wherein said metal is selected from the group consisting of chromium, silver, nickel, copper, and gold.  
  4. A unitary multilayer photoconductive element as defined in claim 1 wherein said metal is chromium, said inorganic oxide is selected from the group consisting of silicon monoxide, silicon dioxide, and mixtures thereof, and said conducting layer thickness is within the range of from about 10 A to about l50 A.  
  5. A unitary multilayer photoconductive element comprising a support, a conducting layer overcoating said support, and a hydrophobic organic photoconductive layer overcoating said conducting layer. said conducting layer having a. an optical density less than about 0.30,  
 b. an electrical resistivity less than about 10 ohms per square and c. a composition consisting essentially of an inorganic oxide selected from the group consisting of silicon monoxide, silicon dioxide, and mixtures thereof and from about 40 to about 60 weight percent of chromium.  
  6. A unitary multilayer photoconductive element comprising a support; a first polymeric adhesive subbing layer on said support; a conducting layer on said first subbing layer; a second polymeric adhesive subbing layer on said conducting layer; and a hydrophobic organic photoconductive layer on said second subbing layer, said conducting layer having a. an optical density less than about 0.30,  
 b. an electrical resistivity less than about 10 ohms per square, and  
 c. a composition consisting essentially of an intimate physical mixture of at least one protective inorganic oxide and from about 30 to about weight percent of at least one electrically conducting metal.  
  7. A unitary multilayer photoconductive element as defined in claim 6 wherein said oxide is selected from the group consisting of silicon monoxide, silicon dioxide, aluminum oxide, magnesium oxide, tantalum oxide, boro-silicon oxide, and titanium oxide and wherein said conducting layer thickness is less than about 1,000 A.  
  8. A unitary multilayer photoconductive element as defined in claim 6 wherein said metal is selected from the group consisting of chromium, silver, nickel, copper, and gold.  
  9. A unitary multilayer photoconductive element as defined in claim 6 wherein said metal is chromium, said inorganic oxide is selected from the group consisting of silicon monoxide. silicon dioxide, and mixtures thereof, and said conducting layer thickness is within the range of from about l0 A to about A.  
  10. A unitary multilayer photoconductive element as defined in Claim 6 wherein said first and second polymeric adhesive subbing layers comprise a vinylidene chloride-containing polymer.  
  11. A unitary multilayer photoconductive element comprising a support; a first adhesive polymeric subbing layer comprising a copolymerized blend of monomers containing over 50 percent by weight of vinylidene chloride on said support&#34;. a conducting layer on said first subbing layer; a second adhesive polymeric subbing layer comprising a copolymerized blend of monomers containing over 50 percent by weight of vinylidene chloride on said conducting layer; and a hydrophobic organic photoconductive layer on said second subbing layer, said conducting layer having a. an optical density less than about 0.15,  
 b. an electrical resistivity less than about 10 ohms per square, and  
 c. a composition consisting essentially of an inorganic oxide selected from the group consisting of silicon monoxide, silicon dioxide, and mixtures thereof and from about 40 to about 60 weight percent of chromium.  
  12. In an electrophotographic copying process wherein a reusable photoconductive element is subjected to multiple imaging cycles, each cycle comprising image-forming and imagetransfer steps, the improvement which comprises using in said process a reusable photoconductive element comprising a support,  
 b. a composition consisting essentially of an intimate physical mixture of at least one protective inorganic oxide and from about 30 to about weight percent of at least one electrically conducting metal.  
 UNITED sTAT s PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 880, 657 DATED April 29 1975 INVENTOR(S) Arthur A. Rasch it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 Column 6, line 16, &#34;aqueuos&#34; should read ---aqueous-- Column 6, line E L, &#34;3,6 iO,7O9&#34; should read ---3,6 iO,7O8-.  
 Column 8, lines 9-10, that part of formula reading &#34;7,1-2&#34; should read -&#39;T,l2---.  
 Column 8, line 41, &#34;1965&#34; should read -l96 t-.  
 Erigned and Sealed this third Day of February 1976 [SEAL] A ttes t:  
  C. MARSHALL DANN Commissioner ufPatents and Trademarks RUTH C. MASON Arresting Officer