Corona generator

Abstract: A corona charger (10) for depositing negative charge on an imaging surface, comprises at least one elongated conductive metal corona discharge electrode (11,34) supported between insulating end blocks (12, 14) and being coated with a substantially continuous thin conductive dry film of aluminum hydroxide containing conductive particles. The corona electrode may be a thin metal wire or alternatively at least one linear array of pin electrodes, and the conductive particles in the coating are graphite particles.

Description: The present invention relates generally to charging devices and in particular to charging devices which produce a negative corona. In electrostatographic reproducing apparatus commonly used today, a photoconductive insulating member is charged to a negative potential, thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas, and creates an electrostatic latent image on the member which corresponds to the images on the original document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with a developing powder, referred to in the art as 'toner'. During development the toner particles are attracted from carrier particles by the charge pattern of the image areas on the photoconductive insulating area, to form a powder image on the photoconductive area. This image may be subsequently transferred to a support surface, such as copy paper, to which it may be permanently affixed by heating and or by the application of pressure. Following transfer of the toner image to the support surface, the photoconductive insulating surface is discharged and cleaned of residual toner to prepare for the next imaging cycle. Various types of charging devices have been used to charge or precharge photoconductive insulating layers. In commercial use, for example, are various types of corona-generating devices to which a high voltage of 5,000 to 8,000 volts may be applied, thereby producing a corona spray which imparts electrostatic charge to the surface of the photoreceptor. A particular device may take the form of a single bare wire, or an array of pins integrally formed from a sheet metal member, strung between insulating end blocks mounted on each end of a channel or shield. Another device which is frequently used to provide more uniform charging and to prevent overcharging, is a scorotron, which comprises two or more wires with a control grid or screen of parallel wires or apertures in a plate positioned between the wires and the photoconductor. A potential is applied to the control grid of the same polarity as the corona potential but at a much-lower potential (usually several hundred volts), which suppresses the electric field between the charged plate and the corona wires, and markedly reduces the ion current flow to the photoreceptor. While capable of performing satisfactorily, it has been observed that after prolonged use, for example in the process of making about 150,000 copies, difficulties are experienced for both thin metal wire corona electrodes and pin electrode arrays. These difficulties take the form of undeveloped streaks being formed in the copies produced, resulting in unpredictable images. While not wishing to be bound to any particular theory, this is believed to be caused by non-uniform corona generation, which in turn is believed to be caused in part by each of several corrosion and erosion mechanisms. The corona causes some sputtering of the metal from the electrode, whether it be a wire or pin electrode, which in the presence of air forms metal nitrates which deposit at various locations along the corona electrode. Furthermore, if there is any ammonia in the air, white whiskers or powder may also be observed building up at various locations on the corona electrode. These reactions are believed to take place within about 1 millimeter of the electrode, and the deposits formed on the corona electrode result in a non-uniformity of subsequent corona generated along the length of the electrode, producing hot spots, localized corona, in the location of the deposits. It is believed that these hot spots tend to create a higher electrostatic field, resulting in non-uniform charging. Furthermore, on a clean corona electrode, the hot spots tend to move along its length and are of a lower intensity than after an extended period of use. As the corona electrode ages, the hot spots become more intense and become fixed in location thereby accelerating further corrosion at their locations, resulting in increased non-uniformity of corona and thereby non-uniformity of charging of the imaging surface. In addition, in the pin-type electrode, the sputtering of metal from the pins results in a collar of deposits which build up around the pins and which eventually results in a periodic non-uniformity such that every other pin becomes ineffective. This results in an as yet unexplainable inactivation of corona generation at every other pin. Previous attempts to minimize the difficulties associated with the above-described erosion and corrosion processes have included physically periodically wiping each corona electrode with a cloth or foam pad. Alternatively, the corona electrodes have been coated with gold. This is effective although expensive, and difficulties are frequently experienced in the adhesion of the gold to the corona electrode since the gold tends to flake. Alternatively, fewer difficulties are experienced with platinum wire as the corona electrode which has a lower rate of degradation. US-A-4,585,321 discloses an electrode including a conductive linear member. This conductive linear member consists of a core of tungsten or molybdenum with a platinum layer covering the surface of the core. The platinum layer serves to enhance the uniform life and stability of the discharge effect. US-A-4,646,196 describes a corona generating device for depositing negative charge on an imaging surface wherein there is at least one element adjacent the corona discharge electrode capable of absorbing nitrogen oxide species generated by the corona device, which extra element has been coated with a substantially continuous thin conductive dry film of aluminum hydroxide which may contain a conductive non-reactive filler such as graphite. In accordance with the present invention, the above disadvantages are overcome by providing a corona electrode coated with a thin, conductive dry film of aluminum hydroxide containing conductive particles. The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is an isometric view of an embodiment of a corona generator according to the present invention, wherein the corona electrode is a thin metal wire, and Figure 2 is an isometric view of another embodiment of the present invention, wherein the corona electrode comprises at least one linear array of pin electrodes. Referring to Figure 1, the corona generator 10 of this invention is seen to comprise a single wire electrode 11 supported between insulating end block assemblies 12 and 14. A conductive corotron shield 18, which is grounded, increases the ion intensity available for conduction. Since no charge builds up on the shield, the voltage between the shield and the wire remain constant and a constant density of ions is generated by the wire. The effect of the grounded shield is to increase the amount of current flowing to the plate. The corona wire 11 at one end is fastened to port 20 in an end block assembly 14, and at the other end is fastened to port 22 of the other end block assembly 12. The wire 11 at assembly 12 is connected to a potential source 24 by lead 26. Such a device might have utility as a precharge corona generator. The wire 11 may be made of any conventional conductive filler material, such as stainless steel, gold, aluminum, copper, tungsten, platinum or the like. The diameter of the wire is not critical, and may vary typically between 0.012 and 0.10 mm and preferably is about 0.05 mm. The wire 11 has a substantially continuous thin uniform conductive coating of aluminum hydroxide along its length, as will be described below. Figure 2 illustrates an alternative embodiment of to the present invention. In Figure 2, scorotron 30 includes two linear pin electrode arrays 32 and 34 supported between insulating end block assemblies 38 and 40. A conductive corona control grid 42 is placed on top of the linear pin arrays and anchored in place, by means of screw 44, to an external potential source by lead 46. Both of the linear pin electrode arrays 32 and 34 are connected to a potential source by conductor 48. Such a device might have utility as a negative corona generator wherein the potential from a high voltage DC power supply applied to the grid is about -800 volts or very close to the voltage desired on the imaging surface which is closely spaced therefrom. The potential applied to the two linear pin electrode arrays is in the range of from about -6,000 to about -8,000 volts. The entire assembly is supported by being clamped between three injection-molded plastics support strips. In this configuration the two linear pin coronodes, in the shape of a saw, provide vertically directional fields and currents because of their geometry providing a higher efficiency of current to the photoconductor versus the total current generated. The grid acts as a leveling device or reference potential limiting the potential on the substrate being charged. In accordance with the present invention, the linear pin electrode arrays 32 and 34 are coated with a substantially continuous thin conductive dry film of aluminum hydroxide containing conductive particles. In a preferred embodiment, the pins in the pin electrode array are made of beryllium copper alloy, in which the beryllium is present in amount of between about 0.1% to about 2.% by weight. Such an array is preferred because of relative ease of monufacturability and its resilient properties. The single corona wire 11 in Figure 1, and the pin arrays 32 and 34 in Figure 2, are coated with a substantially continuous thin conductive film of aluminum hydroxide containing conductive particles. Preferably the aluminum hydroxide is applied to the corona electrode in aqueous media providing a somewhat gelatinous coating which is subsequently readily dehydrated by driving off the water. The adherent film formed on drying is believed to exist as the unhydrated aluminum oxide, a hydrated oxide or aluminum hydroxide or mixtures thereof. The film-forming properties may be improved by the addition of small amounts of water-soluble binders, such as polyvinylpyrrolidone or polyvinyl alcohol. One percent by weight of solids may be adequate without impairing water resistance of the dry film. To impart the desired conductivity to the film, it also contains a conductive non-reactive filler such as graphite. Graphite is particularly preferred in this application since it functions as a conductor, it is chemically inert, forming only carbon dioxide, and provides lubricity to the coating. The particle size of the graphite is significant, particularly with the small-diameter wires. Typically, the filler such as graphite has a maximum dimension less than 5 micrometers. It is desired generally to provide a small diameter wire as the corona electrode, which enables the use of lower voltages with which to achieve the desired corona level and thereby enables the use of smaller and cheaper power supplies. Accordingly, when using small diameter wires, it is necessary to control the particle size of the graphite to ensure a substantially uniform continuous film. Typical formulations to be applied to the corona electrodes comprise aluminum oxide-hydrate and conductive filler such as graphite in a weight ratio of from about 1.5 to about 2.2 of aluminum oxide-hydrate to graphite dispersed in an aqueous medium, to provide from about 10% to 30% by weight solids. A particularly preferred formulation comprises 77.5 percent by weight water, about 14.5 percent aluminum oxide-hydrate, about 7 percent graphite and about 1 percent polyvinylpyrrolidone, and has a pH value of 7. The substantially continuous thin conductive dry film of aluminum hydroxide may be formed on the corona electrode by applying an aqueous solution or dispersion as a thin film thereto. Upon heating, the liquid film dehydrates to provide a strong rigid inorganic adhesive bond to the substrate. Typically, the film can be applied to a previously-degreased electrode by spraying or brushing as with a paint or by dip coating so as to provide a uniform coherent film on the electrode. Typically, the film is applied in a thickness of from 0.007 to about 0.025 mm and preferably 0.012 mm as a substantially uniform continuous layer without pores. It has been found that a very uniform layer may improve the geometry of the device, since the film may tend to level off any irregularities such as burrs formed during stamping of the array. The manner in which the aluminum hydroxide film functions to minimize the erosion and corrosion is not fully understood. However, it is believed that a non-reactive coating similar to glass is provided, which is much more inert than the bare metal of the corona electrode, and that a high binding energy coating is provided which adheres to the substrate without flaking off. In addition, in the preferred embodiment with the presence of graphite in the coating an electrode is provided which is relatively easy to clean because of the lubricity imparted by the graphite. To test the efficiency of the films of aluminum hydroxide, a pin scorotron as used in the Xerox 1065, and similar to that shown in Figure 2, was tested. One-half of the pin scorotron was coated with an aluminum hydroxide film according to the present invention, and one-half was not so coated. The previously degreased pin scorotron having 188 beryllium copper alloy pins 2mm apart was coated with an aqueous dispersion of semicolloidal graphite in an organic binder, which cures at 350°C in one hour to form a hard conductive coating and which is available from Acheson Colloid Company. The dispersion, which is believed to contain 77.5 percent by weight water, 14.5 percent aluminum oxide hydrated, 7 percent by weight graphite and about 1% by weight polyvinylpyrrolidone, was applied to one half of the scorotron by dip coating followed by drying in air. The pin scorotron was placed in a Xerox 1065 duplicator and a uniform gray test pattern was placed on the platen. The initial copies produced of the uniform gray test pattern showed no difference between the two halves corresponding to the coated and uncoated areas of the pin scorotron. The pin scorotron was removed from the Xerox 1065 and placed in a text fixture for a life test during which it was turned on and off, occasionally being observed, and left on for a total time equivalent to that necessary to form 250,000 copies, after which it was returned to the Xerox 1065 for additional reproduction of the uniform gray test pattern on the platen. The copies produced showed severe streaking in the area corresponding to the bare half section of the pin array, with the formation of a large number of white lines in the developed gray area. The area on the copiers corresponding to the coated half of the pin scorotron showed minimal evidence of streaking. In addition, the uncoated section of the pin scorotron showed an oxidized discolored appearance with white powder formation, while there was negligible change in the appearance of the coated side of the pin scorotron from the initial test. Furthermore, when observing the pin scorotron during corona generation, alternate pin shutdown was observed as a periodic change in corona intensity along the length of the uncoated section of the pin array, which causes non-uniform charging, thereby creating a streaking problem. On the coated side of the pin array, there was no pin shutdown and charging was substantially uniform, with only minimal streaking observed. Thus, according to the present invention, a substantial extension in the useful life of a corona generator for depositing negative charge has been achieved. According to the present invention, the presence of streaks of undeveloped areas in copies is avoided by the application of a substantially continuous, thin, conductive dry film of aluminum hydroxide containing conductive particles. Further, more uniform charging of an imaging surface is obtained. This coating is inexpensive, easily applied, has a high voltage resistance, high corrosive chemical resistance and provides an excellent conductive coating for a negative charging corona generating device. The invention has been illustrated as useful in making prints from a copying device, it will be understood that it has equal application to the making of prints in printer applications wherein the images are created electronically.

Claims: A corona generator (10) for depositing a negative charge on an imaging surface carried on a conductive substrate held at a reference potential, comprising at least one elongated conductive corona discharge electrode (11) which is made of metal and is supported between insulating end blocks (12, 14) and, means for connecting the corona discharge electrode to a voltage source, characterized in that the corona discharge electrode (11) is coated with a substantially continuous thin conductive dry film of aluminum hydroxide containing conductive particles. The corona generator of Claim 1, wherein the film is from 0.0015 to 0.025 mm in thickness. The corona generator of Claim 1 or 2, wherein the film also comprises unhydrated aluminium oxide and/or hydrated aluminium oxide. The corona generator of any preceding Claim, wherein the corona discharge electrode comprises a thin metal wire from about 0.0125 to 0.10 mm in diameter. The corona generator of Claim 1 or 3, wherein the or each corona discharge electrode (32, 34) comprises at least one linear array of pin electrodes. The corona generator of claim 5, wherein the pins are of beryllium copper alloy. The corona generator of claim 6, wherein the beryllium copper alloy comprises from about 0.1% to 2.0% by weight of beryllium. The corona generator of any preceding claim, wherein the conductive particles are of graphite having a maximum dimension less than five micrometers. The corona generator of claim 8 wherein the aluminum oxide-hydrate to graphite weight ratio is from about 1.5 to about 2.2.

Inventors: REALE LOUIS, REALE, LOUIS
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1989
Main classification: G03G15
IPC codes: G03G15, H01T19