US 3684503 A
Description (OCR text may contain errors)
Aug. 15, 1972 w, HUMPHRISS ET AL 3,684,503
NOVEL ELECTROPHOTOGRAPHIC ELEMENTS CONTAINING ELECTRICALLY CONDUCTING SOLID DISPERSIONS Filed Jan. 13, 1971 PHOTOCONDUCTING LAYER CONDUCT|NG LAYER SUPPORT PHOTOCONDUCTING LAYER CONDUCTING LAYER SUPPORT PHOTOCONDUCTING LAYER S/CONDUCTING LAYER ./SUPPORT k-GROUNDING PLATE PHOTOCONDUCTING LAYER CONDUCTING LAYER SUPPORT I GROUNDING PLATE PHOTOCONDUCTING LAYER ADHESION LAYER CONDUCTING LAYER SUPPORT PHOTOCONDUCTING LAYER ADHESION LAYER CONDUCTING LAYER SUPPORT PHOTOCONDUCTING LAYER ADHESION LAYER CONDUCTING LAYER SUPPORT WESLEY D.HUMPHRISS LAWRENCE c. BARTLETT INVENTORS "FTP/V422 ATTORNEY United States Patent 3,684,503 NOVEL ELECTROPHOTOGRAPHIC ELEMENTS CONTAINING ELECTRICALLY CONDUCTING SOLID DISPERSIONS Wesley D. Humphriss, Cupertino, Calif., and Lawrence C. Bartlett, Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, NY. Continuation-impart of application Ser. No. 818,988, Apr. 24, 1969. This application Jan. 13, 1971, Ser. No. 106,069
Int. Cl. G03g 5/02 US. Cl. 961.5 17 Claims ABSTRACT OF THE DISCLOSURE A novel electrophotographic element having distributed through a non-recording portion thereof, an electrically conducting solid dispersion, is described. The dispersion extends from an external surface of the element to the electrically conducting layer, providing a novel method for grounding this layer while charging. Typical dispersions are formed from any electrically conducting material dispersed in a polymeric binder in the element.
This application is a continuation-in-part application of Ser. No. 818,988 filed Apr. 24, 1969, now abandoned.
This invention relates to electrophotography, and in particular to novel electrophotographic elements and to processes for making and using these elements.
The process of xerography employs an electrophotographic element usually having a support layer, an electrically conducting layer overlying the support layer and a photoconductive layer overlying the electrically conducting layer. The photoconductive layer contains a normally insulating material whose electrical resistance varies with the amount of incident electromagnetic radiation it receives during an imagewise exposure. The support layer can be any one of a wide variety of materials, but illustrative useful supports are paper supports and polymeric supports of film-forming resins such as poly- (ethyleneterephthalate) and cellulose acetate. The electrically conducting layer can be a separate layer, a part of the support layer or the support layer. There are many types of conducting layers, the most common being listed below:
(a) Metallic laminates such as an aluminum-paper laminate, V
(b) Metal plates e.g., aluminum, copper, zinc, brass, etc.,
(c) Metal foils such as aluminum foil, zinc foil, etc.,
(d) Vapor deposited metal layers such as silver, aluminum, nickel, etc.,
(e) Semiconductors dispersed in resins such as poly- (ethyleneterephthalate) as described in U.S. Pat. 3,- 245,833.
(f) Electrically conducting salts suchas described in US. 3,007,801 and 3,267,807.
Conducting layers ((1), (e) and (f) can be transparent and can be employed where transparent elements are required, such as in processes where the element is to be exposed from the back rather than the front or Where the electrophotographic element is to be used as a transparency.
The described electrophotographic element is first given a uniform surface charge, generally in the dark after a suitable period of dark adaptation. It is then exposed to a pattern of actinic radiation which has the effect of differentially reducing the potential of this surface charge in accordance with the relative energy contained in various parts of the radiation pattern. The differential surface charge or electrostatic latent image remaining on the electrophotographic element is then made visible by contacting the surface with a suitable electroscopic marking material. Such marking material or toner, whether contained in an insulating liquid or on a dry carrier, can be deposited on the exposed surface in accordance with either the charge pattern, or in the absence of charge pattern, as desired. Deposited marking material can then be either permanently fixed to the surface of the sensitive element by known means such as heat, pressure, solvent vapor, or the like, or transferred to a second element to which it can similarly be fixed. Likewise, the electrostatic latent image can be transferred to a second element and developed there.
The function of the electrically conducting layer in electriphotographic elements is to create a highly conducting reference plane which ideally is held at or near ground potential. During charging of the photoconductive layer with a corona charger, the potential of the conducting layer has a tendency to build up with respect to ground if it is not grounded. Typically, if the surface of the photoconductive layer is charged to 600 volts, the potential of an ungrounded conducting layer [of type (d), (e) or (f)] can vary from about 50 to about 450 volts or more. Thus, the differential between the conducting layer and the photoconductive layer may range from about volts to about 550 volts. In this situation, when the charging step is completed and the surface of the element is exposed to a pattern of actinic radiation, the photoconductive layer becomes conducting in the lightstruck regions and the potential of the surface of the photoconductive layer in these areas approaches that of the conducting layer. Because of the small difference in potentials which may exist between areas struck by light and those not struck, little or no latent image is produced.
Similarly poor results are obtained when the conducting layer is inefiiciently grounded. Conventional grounding methods, such as metal strips, rollers, etc., placed in electrical contact with the support may be satisfactory in those systems where the element is charged while stationary but are often ineffective when used in those systems wherein the element is charged while in motion. Direct electrical contact with the conducting layer for grounding purposes is very diflicult and inefficient when it is extremely thin, e.g., 0.001 inch or less (e.g., a few hundred angstroms) and creates wear problems if the element is contacted for grounding during charging while Conducting lacquers such as those described in US. Ser. No. 803,708 now US. Pat. 3,639,121, by W. C. York filed Mar. 3, 1969* entitled, Novel Conducting Lacquers for Electrophotographic Elements are used to aid in maintaining electrically conducting layers at ground potential. However, these materials should be applied directly to the electrically conducting layer, i.e., to an exposed portion of the layer. This is accomplished by either applying it to the edge of the element or, if the conducting layer is set off so that a portion of its surface is exposed, by applying the lacquer to the exposed surface. Thus, the modes of application are somewhat limited, particularly if the conducting layer is inaccessible.
It is therefore an object of this invention to provide novel electrophotographic elements having electrically conducting layers which are readily grounded.
It is also an object of this invention to provide a process for preparing these novel electrophotographic elements.
It is a further object to provide a process for using the novel electrophotographic elements of this invention.
These and other objects of the invention are accomplished with reusable electrophotographic elements containing at ieast three layers including a support layer,-
an electrically conducting layer overlying the support layer, a photoconductive layer overlying the electrically conducting layer and in a nonrecording section of the element a solid dispersion of a particulate electrically conducting material which extends from an external surface of the element through a portion of at least one of the layers overlying the conductive layer to electrically contact the conducting layer. Thus, if any overcoated or intervening layers are interposed between the electrically conducting and photoconductive layer such as an adhesion layer or barrier layer, the dispersion also penetrates these layers. Alternatively, the dispersion can extend into the element from an edge. The dispersion in either position serves as an electrical conduit having relatively low resistivity. The function of the dispersion is to establish electrical contact with the electrically conducting layer in a non-recording section of the electrophotographic element thus providing an electrical path from the conducting layer to an external ground. By using such a dispersion, the conducting layer is efliciently maintained at ground potential during charging.
As noted hereinabove, the solid dispersion of the particulate electrically conducting material ,is located in a non-recording section of the electrophotographic element. Obviously, this is simply due to the fact that no matter Where the solid dispersion of particulate electrical- 1y conducting material is located in the electrophotographic element; that section of the element, if previously possessing a recording capability, is thereby rendered non-recording, or if that section never possessed a recording capability, it remains non-recordin For example, if the particulate electrically conducting material is imbibed through a portion of the surface area of the photoconducting layer which is to receive an electrostatic charge pattern, that imbibed portion of the photoconductive layer is thereby rendered incapable of receiving and holding the electrostatic charge pattern since the electrostatic charge will rapidly dissipate through the electrical grounding circuit formed by the imbibed dispersion of particulate electrically conducting material which electrically connects the conducting layer to an external ground. Alternatively, if the dispersion extends into the element from an edge such that none of the particulate electrically conducting material is located in a portion of the surface of the photoconductive layer which is to receive an electrostatic charge pattern, then it will be apparent that the particulate electrically conducting material is again in a non-recording section of the element, in fact, a section of the element never actually possessing a recording capability.
The novel electrophotographic elements of this invention containing dispersions of particulate electrically conducting materials are resistant to abrasion. In a grounding system such as that described in the following examples, wherein the grounding means is a metal strip or a metal roller, that portion of the element which contains the dispersion and is in contact with the grounding means is not worn away by the metal contact produced while the element is in motion. Accordingly, the element may be reused in a number of subsequent electrophotographic exposure operations. Because of the low persquare surface resistivity of the dispersion, the conduct-' ing layer is effectively held at ground potential during charging for both stationary and moving elements.
It is to be understood that the term ground as used herein is relative and merely represents a relative potential to which other positive or negative potentials are referred. For example, the term +600 volts is to be interpreted as meaning 600 volts above a reference ground potential. For convenience, ground-potential as used hereinafter is arbitrarily assigned a value of zero volts.
The term surface resistivity conventionally refers to measurement of electrical leakage across an insulating surface. Inthe present specification, however, the term is used with reference to resistance of conducting films forming the conducting means of this invention that apparently behave as conductors transmitting currents through the body of film. Resistivity is the usually accepted measurement for the conductive property of conducting and semiconducting materials. However, in the case of thin conductive coatings, measurement of the conductive property in terms of surface resistivity provides a value that is useful in practice and involves a direct method of measurement. It should be pointed out that the dimensional units for specific resistance (ohmcm.) and the unit for surface resistivity ohms per square) as used herein are not equivalent and the respective measurements should not be confused. For an electrically conducting material whose electrical behavior is ohmic, the calculated resistance per square of a film of such material would be the specific resistance of the material divided by the film thickness, but this calculated resistance for a given material will not always coincide with measured surface resistivity. Surface resistivity (ohms per square) of the coating is measured by placing a set of l-cm. long stainless steel electrodes along opposite sides of a l-cm. square sample cut from the coated surface. In the instant system, resistance is measured with an RCA Senior Volt Ohrnyst. The resistivity of the conducting dispersion of this invention is generally 10 ohms per square or less.
The particulate electrically conducting material which is dispersed through the photoconductive and conducting layers to establish electrical contact between the conducting layer and an external ground can be any finely divided particulate material having good electrical conducting properties. Typical conducting materials include graphite, carbon black, nickel, silver, aluminum, copper, tin, etc. and mixtures thereof all of which are particulate and have good electrical conducting properties. The particle size of these conducting materials can vary depending on the particular material used but generally ranges from about 0.001; to about p. It is to be understood that the optimum particle size is readily determinable by methods employed by those skilled in the art. Graphite has been found to be very satisfactory based on its property of being a good lubricant as well as a conductor. When graphite conducting coatings are used, less wear is encountered in those non-recording regions of the element which are in contact with the metal grounding devices.
In preparing the novel electrophotographic elements of this invention, a liquid, dispersion of the particulate electrically conducting material in a solvent is applied to either the edge of the electrophotographic element or to a portion of the exposed surface of the photoconductive layer. The solvent is one which is capable of impregnating (i.e., by swelling, cracking or dissolving) the polymeric binders contained in the photoconductive layer, barrier layer, adhesion layer and any other intervening layer present between the electrically conducting and photoconducting layers. Suitable solvents having those characteristics include aliphatic alcohols having 1 to 8 carbon atoms such asmethanol, ethanol, isopropanol, etc., ketones having 3 to 10 carbon atoms such as acetone, methylethyl ketone, etc. and chlorinated alkanes having 1 to 8 carbon atoms such as methylene chloride, propylene chloride, chloroform, etc. Mixtures of these solvents may also be used. The particular solvent or mixture employed is somewhat dependent upon the polymer to be impregnated and the selection of the optimum solvent to be used is apparent to those skilled in the art. A particularly useful solvent which is capable of impregnating most of the more common hydrophobic film-forming resin binders employed in the various layers of electrophotographic elements, i.e., polyesters including polycarbonates, comprises a mixture of a ketone such as acetone or methyl ethyl ketone with a chlorinated hydrocarbon such as -methylene chloride or propylene chloride. The solvent and particulate electrically conducting material are thoroughly mixed, e.g., with a ball mill or blender, so as to create a uniform dispersion of the conducting material in the solvent. Frequently, in order to obtain a uniform stable dispersion of solids in liquid, it is necessary to employ a small amount of a polymeric binder. The added binder aids primarily in the creation of a more uniform dispersion. When such a binder is employed, the ratio of conducting material to binder ranges from 0.5 to 10 parts by weight and preferably 1.5 to 2.5 parts by weight of conducting material for each part by weight of binder. Enough solvent is added to bring the solids content to at least 5% and not more than 90% of the liquid d1spersion. The liquid dispersion is then applied to the edge of the electrophotographic element or to a minor portion (i.e., less than about 50%) of the exposed surface of the uppermost layer of the element, usually the photoconductive layer. Preferably from about 0.001% to about 10% of the exposed surface is impregnated with the liquid dispersion. After the liquid dispersion penetrates the topmost layer, it then penetrates the same area of the next layer in juxtaposition with the topmost layer, etc. until all layers overlying the conducting layer have been impregnated. The liquid dispersion, prepared in the manner described, is applied by any suitable method such as with a brush, sprayer, hopper, etc. After its application, the solvent portion of the dispersion penetrates into the binder of the photoconductive layer and other intervening layers causing them to swell, crack or dissolve. The penetrating action is aided by heating the element to elevated temperatures, e.g., 30 C. to 100 C. However, temperatures as low as C. and as high as 150 C. can be employed during the heating operation. The heat also helps to create additional voids and cracks in the binder and also removes any excess solvent. The time for drying is dependent upon the temperature, i.e., higher temperatures require lower times and vice versa. Generally, times ranging from 1 second to 10 minutes are suitable with 2 seconds to 5 minutes being preferred. It is believed that the conducting particles are dispersed through the penetrated layers in such a manner that an electrically conducting conduit is established between the internal conducting layer and an external surface of the element by the thus formed solid dispersion. The liquid dispersion can be applied in any one of several ways and in any one of several locations. It can be applied as a point, line or area on the surface of the element, or the edge of a reel containing a roll of the photoconductive sheet can be sprayed or painted with the dispersion. In a typical operation wherein a line application is used, a thin stripe of the liquid dispersion is applied to the surface of the photoconductive layer adjacent and parallel to each end and edge. A quantity sufl'lcient to reduce the electrical conductivity to a maximum of ohms per square is applied. This measurement is carried out by attaching the leads of an ohmmeter to each stripe. The resistivity of the circuit, comprised of (1) one of the conducting stripes and dispersion on one end, (2) the conducting layer and (3) the other conducting stripe and dispersion on the other end, is measured. Thus, after drying, there is formed within the photoconductive layer a solid dispersion of particulate electrically conducting material adjacent and parallel to the ends and the edge of the photoconductive layer extending from the top surface of the photoconductive layer into electrical contact with the conducting layer.
A particular advantage residing in the preparation and use of the elements of this invention over other methods of grounding electrically conducting layers is that no binder is necessary for the coating composition such as is present in conducting lacquers. The contact obtained and the resulting resistivities are just as efiicient. Also, the ease of fabrication of such elements over other types is an additional advantage. However, in certain instances, the use of a polymeric binder is desirable but not necessary to aid in the preparation of the liquid dispersion. The use of such binders has been explained previously. Other advantages resulting from the use of the elements of this invention includes the relative ease of grounding the element while in motion, the good electrical contact obtained with the conducting layer and the low level of wear obtained when a graphite dispersion is used. All of these advantages have been explained previously.
The novel electrophotographic elements having conducting dispersions as defined in this invention generally contain several layers as described previously. Overlying a support layer, which is usually a transparent insulator, is a conducting layer. This layer may be coated on the support layer, evaporated onto the support, or imbibed into the support layer. However, it is to be understood that the terms support layer, conducting support and conducting layer overlying a support layer include those instances where the conducting layer is coated on the support as well as where the conducting layer is imbibed into or evaporated onto the support. The materials useful in the support layer and conducting layer have been described above.
Other layers may be interposed between the conducting layer and the photoconductive layer such as a barrier layer or a layer to promote adhesion between any other layers which may be present. Additionally, a protective overcoat layer overlying the photoconductive layer may also be employed.
A photoconductive layer containing an organic photoconductor in a polymeric binder overlies the conducting layer. A sensitizer for the photoconductor may optionally be present to change the spectral sensitivity or electrophotosensitivity of the element. Any organic photoconductor is useful in the electrophotographic elements of this invention. Typical ones are described in copending application Ser. No. 772,370 filed Oct. 31, 1968, now abandoned, in the name of Stewart H. Merrill including certain arylamines, polyarylalkanes, chalcones, bis-pyraz0 lines, triarylamines, organometallic compounds.
Sensitizing compounds useful in the photoconductive layers described herein can be selected from a wide variety of mate-rials, including such materials as pyryliums, including thiapyrylium and selenapyrylium dye salts, disclosed in Van Allan et a1. U.S. Pat. 3,250,615; fluorenes, such as 7,l2-dioxo-l3-dibenzo(a,h)fiuorene, 5,10-dioxo- 4a,l l-diazabenzo(b)fluorene, 3,13 dioxo 7 oxadibenzo (b,g)fluorene, and the like; aromatic nitro compounds of the kinds described in U.S. Pat. 2,610,120; anthrones like those disclosed in U.S. Pat. 2,670,284; quinones, U.S. Pat. 2,670,286; benzophenones U.S. Pat. 2,670,287; thiazoles U.S. Pat. 2,732,301; mineral acids; carboxylic acids, such as maleic acid, dichloroacetic acid, and salicyclic acid; sulfonic and phosphoric aids; and various dyes, such as cyanine (including carbocyanine), mercocyanine, diarylmethane, thiazine, azine, oxazine, Xanthene, phthalein, acridine, azo, anthraquinone dyes and the like and mixtures thereof. The sensitizing dyes preferred for use with this invention are selected from pyrylium, selenapyrylium and thiapyrylium salts, and cyanines, including carbocyanine dyes.
Where a sensitizing compound is employed with the binder and organic photoconductor to form a sensitized electrophotographic element, it is suitable to mix an amount of the sensitizing compound with the coating composition so that, after thorough mixing, the sensitizing compound is uniformly distributed in the coated element. Other methods of incorporating the sensitizer or the effect of the sensitizer may, however, be employed consistent w1th the practice of this invention. In preparing the photoconductive layers, no sensitizing compound is required to give photoconductivity in the layers which contain the photoconducting substances, therefore, no sensitizer is required in a particular photoconductive layer. However, since relatively minor amounts of sensitizing compound give substantial improvement in speed in such layers, the
sensitizer is preferred. The amount of sensitizer that can be added to a photoconductor-incorporating layer to give effective increases in speed can vary widely. The optimum concentration in any given case will vary with the specific photoconductor and sensitizing compound used. In general, substantial speed gains can be obtained where an appropriate sensitizer is added in a concentration range from about 0.0001 to about 30 percent by weight based on the weight of the film-forming coating composition. Normally, a sensitizer is added to the coating composition in an amount by weight from about 0.005 to about 5.0 percent by weight of the total coating composition.
Solvents useful for preparing the photoconductive coating compositions include a wide variety of organic solvents for the components of the coating composition. For example, benzene; toluene; acetone; Z-butanone; chlorinated hydrocarbons such as methylene chloride; ethylene chloride; and the like; ethers, such as tetrahydrofnran and the like, or mixtures of such solvents can advantageously be employed in the practice of this invention.
In preparing the coating compositions for the photoconductive layer, useful results are obtained where the photoconductive substance is present in an amount equal to at least about 1 weight percent of the coating composition. The upper limit in the amount of photoconductive material present can be Widely varied in accordance with usual practice. It is normally required that the photoconductive material be present in an amount ranging from about 1 weight percent of the coating composition to about 99 Weight percent of the coating composition. A preferred Weight range for the photoconductive material in the coating composition is from about weight percent to about 60 weight percent.
Coating thicknesses of the photoconductive composition on a support can vary widely. Normally, a wet coating thickness in the range of about 0.001 inch to about 0.01 inch is useful in the practice of the invent-ion. A preferred range of coating thickness is from about 0.002 inch to about 0.006 inch before drying although such thicknesses can vary widely depending on the particular application desired for the electrophotographic element.
The novel electrophotograp'hic elements containing the electrically conducting dispersions of this invention are useful in the xerographic process. In this process, the electrophotographic element, while held in the dark, is given a blanket electrostatic charge by placing it under a corona discharge to give a uniform charge to the surface of the photoconductive layer. During this charging step, the electrically conducting layer is maintained at ground potential by electrically connecting the exposed portion of the electrically conducting dispersion on the photoconductive element to ground. In the absence of grounding in this manner, the difference in potential between the photoconduotive layer and the conducting layer is not large enough to produce a suitable latent image. The charge is retained on the surface of the photoconductive layer because of the substantial dark insulating property of the layer, i.e., the low conductivity of the layer in the dark. The charging operation'can be performed while the element is stationary or in motion. It is in the latter case wherein the benefits of the instant invention are particularly noticeable. When using an element containing an electrically conducting dispersion as described herein that is charged while in motion, the potential of the conducting layer is maintained at ground potential as efficiently as when the element is charged while stationary. 'In other words, the electrically conducting dispersion permits exceptionally good contact to be made between the conducting layer and the grounding means while the element is in motion. The electrostatic charge formed on the surface of the photoconductive layer is then selectively dissipated from the surface of the layer :by imagewise exposure to light by means of a conventional exposure operation such as for example, by a contact-printing technique, or by lens projection of an image, or reflex or bireflex techniques and the like, to thereby form a latent image in the photoconductive layer. Exposing the surface in this manner forms a pattern of electrostatic charge by virtue of the fact that light energy striking the photoconductor causes the electrostatic charge in the light struck areas to be conducted away from the surface in proportion to the intensity of the illumination in a particular area.
The charge pattern produced by exposure is then developed or transferred to another surface and developed there, i.e., either the charge or uncharged areas are rendered visible, by treatment with a medium comprising electrostatically responsive particles having optical density. The developing electrostatically responsive particles can be in the form of a dust, or powder and generally comprise a pigment in a resinous carrier called a toner. A preferred method of applying such a toner to a latent electrostatic image for solid area development is by the use of a magnetic brush. Methods of forming and using a magnetic brush toner applicator are described in the following US. Pats. 2,786,439; 2,786,440; 2,786,441; 2,811,465; 2,874,063; 2,984,163; 3,040,704; 3,117,884; and reissue Re. 25,779. Liquid development of the latent electrostatic image may also be used. In liquid development the developing particles are carried to the imagebearing surface in an electrically insulating liquid carrier. Methods of development of this type are widely known and have been described in the patent literature, for example, U.S. Pat. 2,297,691 and in Australian Pat. 212,315. In dry developing processes the most Widely used method of obtaining a permanent record is achieved by selecting a developing particle which has as one of its components a low-melting resin. Heating the powder image then causes the resin to melt or fuse into or on the element. The powder is, therefore, caused to adhere permanently to the surface of the photoconductive layer. In other cases, a transfer of the charge image or powder image formed on the p hotoconductive layer can be made to a second support such as paper which would then become the final print after developing and fusing or fusing respectively. Techniques of the type indicated are well known in the art and have been described in a number of U.S. and foreign patents, such as US. Pats. 2,297,691 and 2,551,582 and in RCA Review, vol. 15. It is frequently necessary during development to maintain the electrically conducting layer at a given potential in order to obtain a clean background. The conducting dispersions contained in the elements of this invention enables one to easily maintain the potential of the electrically conducting layer at a given potential.
The drawings described below illustrate typical embodiments of the invention.
FIG. I represents an electrophotographic element having support layer 12 and electrically conducting layer 11. The electrically conducting layer overlies the support layer. Overlying the conducting layer is photoconductive layer 10 which generally contains a photoconductor, a polymeric binder and optionally an optical sensitizer for the photoconductor. Impregnated into the surface of the element is a region containing a solid dispersion 13 of an electrically conducting material in particulate form in a polymeric binder. The electrically conducting dispersion 13 is also in electrical contact with grounding means 14. The electrically conducting layer 11 is maintained at ground potential during the charging process by electrically conducting dispersion 13 and grounding means 14. After the charging step is completed, there is a uniform surface charge on the surface of the photoconductive layer and the electrically conducting layer is at ground potential. Thus, there is a. potential difierence between the photoconductive layer and the electrically conducting layer after charging is completed. FIG. la is the same asFIG. I except-that the electrically conducting dispersion 15 is coated on the edge of the element.
FIG. II shows the use of metal plate 24 as the electrically grounding means. Electrically conducting dispersion 23 is in contact with both electrically conducting layer 21 and grounding plate 24. The grounding plate can be made of any suitable material, stainless steel being one of the preferred materials. Photoconductive layer 20 and support layer 22 are the same as described in FIG. I. FIG. Ila is the same as FIG. II except the electrically conducting dispersion 25 is coated on the edge of the element.
Frequently, photoconductive layer 30 does not adhere readily to electrically conducting layer 31. Adhesion layer 35 is applied between these two surfaces to improve the adhesion. This layer comprises any material which has good adhesive properties yet does not interfere with the electrical properties of either photoconductive layer 30 or conducting layer 31. An element having this configuration is shown in FIG. III. Electrically conducting dispersion 33 provides an electrical connection between the electrically conducting layer and ground means 34. The support layer is 32. FIG. IIIa is the same as FIG. III except the electrically conducting dispersion 36 is coated on the edge of the element. FIG. IIIb is the same as FIG. Illa except the electrically conducting dispersion 37 does not contact any portion of the photoconductive layer.
The conducting compositions of the present invention can be used with electrophotographic elements having many structural variations. For example, the photoconductive layer composition can be coated in the form of single layers or multiple layers on a suitable opaque or transparent conducting support. Likewise, the layers can be contiguous or spaced having layers of insulating material or other photoconductive material between layers or over coated or interposed between the photoconductive layer or sensitizing layer and the conducting layer. Configuration diflFering from those contained in the examples and drawings can be useful or even preferred for the same or different application for the electrophotographic element. In all configurations, it is necessary, in order to achieve the advantages of this invention, to establish electrical contact between the grounding means and the conducting layer by using elements having the electrically conducting dispersion described above.
The following examples are included for a further understanding of the invention.
EXAMPLE I 1.4 grams of poly(4,4' isopropylidenebisphenoxyethylcoe-thylene terephthalate) binder containing 0.5 gram of 4,4 benzylidine bis (N,N-diethyl-m-toluidine) photoconductor and .04 gram of 2,4-(4ethoxyphenyl)-6-(4-namyloxystyryl) pyrylium fiuoroborate sensitizer are dissolved in 15.6 grams of methylene chloride by stirring the solids in the solvent for one hour at room temperature. The resulting solution is hand coated at a wet coating thickness of 0.004 inch on a conducting layer comprising the sodium salt of a carboxyester lactone, such as described in US. 3,120,028, which in turn is coated on a cellulose acetate film base. The coating block is maintianed at a temperature of 90 F. After drying, a small portion (approximately of the surface of the photoconductive layer of the electrophotographic element is impregnated with a liquid dispersion having the following composition Graphite (Dixon #635 Methyl alcohol 10 The dispersion is applied so that the initial coating thickness is 0.0005 inch. This composition is prepared by ball milling the above formulation for 16 hours with %-inch stainless steel balls. The element is stored for 10 minutes at 35 C. to allow the composition to penetrate through the photoconductive layer to the electrically conducting layer and to remove residual solvent, After the impregnation treatment is completed, the element contains a solid electrically conducting dispersion of graphite particles in a polymeric binder which is in contact with the conducting layer. The element is then charged under a positive corona source until the surface potential, as measured by an electrometer probe, reaches about 600 volts. Charging is accomplished by passing the corona over the surface of the element while it is held in a stationary position. In the charging process, the electrically conducting dispersion holds the electrically conducting layer at ground potential by providing an electrical path from the electrically conducting layer to the grounding means. In this case, the grounding means is a copper strip which is in electrical contact with the electrically conducting dispersion on the surface of the photoconductive element. The photoconductive layer is then covered with a transparent sheet bearing a pattern of opaque and light transmitting areas and exposed to the radiation from an incandescent lamp with an illumination intensity of about 75 meter-candles for 12 seconds. The resulting electrostatic latent image is developed in the usual manner by cascading over the surface of the layer a mixture of negatively charged thermoplastic toner particles and glass beads. A good reproduction of the pattern results. When the electrically conducting dispersion is omitted, the reproduction has poor contrast and is generally of inferior quality.
EXAMPLE II Example I is again repeated except that the surface of the element is charged while in motion at 30 feet per minute and the corona charger is stationary. In this case, the grounding means is a metal roller which is in electrical contact with the conducting dispersion. Again a good reproduction is obtained.
EXAMPLE IV Example II is repeated except that the liquid dispersion is coated on an edge of the electrophotographic element. A satisfactory reproduction of the pattern is obtained. In the absence of the dispersion, a poor quality image is obtained.
EXAMPLE V Example I is repeated using three electrically conducting dispersions containing copper, nickel and silver respectively in place of the graphite. Similar results are obtained in each instance.
EXAMPLE VI Example II is repeated except that the element contains an adhesion layer interposed between the photoconductive layer and the electrically conducting layer. The adhesion layer is comprised of a terpolymer of 2% itaconic acid, 14% methyl acrylate and 84% vinylidene chloride. Similarly good results are obtained.
EXAMPLE VII Example II is repeated except that the following liquid dispersion is used:
Grams Binder (Butvar B-76, a polyvinylbutyral of Shawinigan resins) 20 Carbon black (Vulcan XC-72, Cabot Corp.) 40 Acetone 200 Methylene chloride 200 The composition is prepared by dissolving the binder in the mixture of solvents in a blender. The carbon black is then added and the resulting dispersion is mixed rapidly for 10 minutes. 'I wo thin stripes are applied with a coating hopper along the top surface of the photoconductive element in an amount such that the resistivity of the circuit created by (1) the two stripes, (2) the corresponding conducting dispersions created by the stripes within the element and (3) the conducting layer is less than 10 ohms/square. The width of the two coated stripes together covers approximately 1% of the surface of the element. The element itself contains a photoconductive layer, adhesion layer and conducting layer. The dispersion permeates all of these layers and good electrical contact is attained. An excellent image is obtained after the element is exposed and developed.
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.
1. An electrophotographic element having at least three layers comprising:
(a) a support layer,
(b) an electrically conducting layer overlying said support layer, and
(c) a photoconductive layer overlying said electrically conducting layer and having a top surface opposite said conducting layer, said element having in a nonrecording portion thereof a solid dispersion of particulate electrically conducting material extending through the top surface of said photoconductive layer and into electrical contact with said conducting layer.
2. The element according to claim 1 wherein an adhesion layer is interposed between said electrically conducting layer and said photoconducting layer.
3. The element of claim 1 wherein said conducting layer comprises a material selected from the group consisting of conducting metal salts and vapor deposited metals.
4. The element of claim 1 wherein said particulate eletrically conducting material is selected from the group consisting of nickel, silver, conducting carbon black, graphite, aluminum, tin, copper and mixtures thereof.
5. An electrophotographic element having at least three layers comprising (a) a support layer,
(1:) an electrically conducting layer overlying said sup port layer and (c) an organic photoconductor-containing layer overlying said electrically conducting layer and having a top surface opposite said conducting layer, said element having in a non-recording portion thereof a solid dispersion of particulate electrically conducting material extending through the top surface of said organic photoconductor-containing layer to electrically contact said conducting layer, said particulate electrically conducting material selected from the group consisting of nickel, silver, conducting carbon black, graphite, aluminum, tin, copper and mixtures thereof.
6. The element according to claim 5 wherein the organic photoconductor-containing layer is the top layer of said element and the particulate electrically conducting material is dispersed therein to form a solid dispersion of particulate conducting material adjacent and parallel to at least one edge ofthe photoconductor-containing layer extending from the top surface of the photoconductorcontaining layer into electrical contact with the conducting layer.
7. The element according to claim 5 wherein the particulate electrically conducting material is graphite and wherein the top surface area of the photoconductor-containing layer from which said dispersion of particulate electrically conducting graphite extends comprises not more than of the surface area of said layer.
8. The process for preparing an electrophotographic element comprising the steps of (a) providing a support layer,
(b) applying an electrically conducting layer over said support layer,
(c) applying a polymeric binder-containing photoconductive layer over said conducting layer, and
(d) impregnating in an non-recording section of the element comprising a minor portion of a surface of said photoconductive layer opposite said conducting layer a liquid dispersion of a particulate electrically conducting material in a solvent capable of penetrating the polymeric binder contained in said photoconductive layer.
9. The process as defined in claim 8 wherein said solvent is selected from the group consisting of acetone, methyl ethyl ketone, methyl alcohol, methylene chloride, propylene chloride and mixtures thereof.
10. The process as defined in claim 8 wherein said particulate electrically conducting material is selected from the group consisting of nickel, silver, conducting carbon black, graphite, aluminum, tin, copper and mixtures thereof.
11. The process as defined in claim 8 wherein said dispersion is applied to not more than 10% of the surface of said photoconductive layer.
12. The process for preparing an electrophotographic element comprising the steps of (a). providing a support layer,
(b) applying an electrically conducting layer over said support layer,
(0) applying over said conducting layer a polymeric binder-containing organic photoconductor-containing layer having a top surface opposite said conducting layer, and
(d) impregnating in a non-recording section of the element comprising from about 0.001 to about 10% of the top surface area of said organic photoconductorcontaining layer a dispersion of a particulate, electrically conducting graphite in methyl alcohol, said methyl alcohol capable of penetrating the polymeric binder contained in said photoconductive layer.
13. A process for forming an electrostatic charge pattern on an electrophotognaphic element having a support layer, an electrically conducting layer, a polymeric bindercontaining photoconductive layer overlying said conducting layer and having a top surface opposite said conducting layer, and a particulate electrically conducting material dispersed in a non-recording section of the element extending through the top surface of said photoconductive layer and electrically connecting said electrically conducting layer to a grounding means comprising the steps of (a) charging said photoconductive layer and (b) exposing the charged photoconductive layer to a pattern of activating radiation and thereby at least partly discharging said photoconductive layer in areas of exposure.
14. The process of claim 13 wherein said particulate electrically conducting material is selected from the group consisting of nickel, silver, conducting carbon black, graphite, aluminum, tin, copper and mixtures thereof.
15. The process as defined in claim 13 wherein the electrically conducting layer comprises a material selected from the group consisting of conducting metal salts and vapor deposited metals.
16. The process for preparing an electrophotographic element comprising the steps of (a) providing a support layer,
(b) applying an electrically conducting layer over said support layer,
(c) applying over said conducting layer a polymeric binder-containing photoconductive layer having a top surface opposite said conducting layer, and
(d) impregnating a non-recording section of the element comprising from about 0.001 to about 10% 13 14 of the top surface area of said photoconductive References Cited layer a liquid dispersion comprising a conducting UNITED STATES PATENTS carbon black, acetone, poly(vinylbutyra1) and methylene chloride, the mixtures of acetone and methylene garlson chloride contained in said dispersion capable of penc- 5 3552957 1/1971 gf trating the polymeric binder contained in said photo- 3574615 4/1971 Morse conductive layer. 17. The process of claim 16 wherein said liquid (liS- JOHN C COOPER III, Primary Examiner persion is applied as a thin stripe adjacent and parallel to at least one edge of the top surface of the photocon- 10 US. Cl. X.R. ductive layer. 961 PC