Patent Grant
7342227
This invention relates to electrostatic marking systems and more specifically to the corona charging components of these systems.
In an electrostatographic process, a system is used whereby a uniform electrostatic charge is placed upon a reusable photoconductive surface. The charged photoconductive surface is then exposed to a light image of an original to selectively dissipate the charge to form a latent electrostatic image of the original on the photoreceptor. The latent image is developed by depositing finely divided marking and charged particles (toner) upon the photoreceptor surface. The charged toner is electrostatically attached to the latent electrostatic image areas to create a visible replica of the original. The toned developed image is then transferred from the photoconductor surface to a final image support material, such as paper, and the toner image is fixed thereto by heat and pressure to form a permanent copy corresponding to the original.
In Xerographic systems of this type, a photoreceptor surface is generally arranged to move in an endless path through the various processing stations of the Xerographic process. The photoconductive or photoreceptor surface is generally reusable whereby the toner image is transferred to the final support material, and the surface of the photoreceptor is prepared to be used once again for another reproduction of an original. In this endless path, several stations of corona charging are traversed. These charging stations may involve one or a cluster of dicorotron or other corotron units.
Several methods are known for applying an electrostatic charge to the photosensitive member such as the use of electron-emitting pins, an electron-emitting grid, single corona-charging structures and single or multiple dicorotron wire assemblies. In recent development of high speed Xerographic reproduction machines where copiers can produce at a rate of or in excess of three thousand copies per hour, the need for several reliable dicorotron wire assemblies and good final copies are required.
Usually, in electrostatographic or electrostatic copy processes, as those above noted, a number of corotrons or dicorotrons are used at various stations around the photoreceptor. For example, the dicorotrons are used at the station that places a uniform charge on the photoreceptor, at a transfer station, at a cleaning station, etc. In today's high speed copiers, it is important that all corotrons (or dicorotrons) are somewhat controlled regarding effluents given off in the charging process. Generally, the structure of a dicorotron uses a thin, glass-coated wire mounted in an elongated U-shaped housing between two insulating anchors called “insulators”. These support the wire in the U-shaped housing in a spring-tensioned manner in a singular plane. This dicorotron unit or assembly, usually an aluminum, comprises housing, as above noted, is in one embodiment an elongated U-shaped shield. The wire or corona-generating electrode is typically a highly conductive, elongated wire situated in close proximity to the photoconductive surface to be charged.
As earlier noted, the charging of the photoreceptor is necessary for the proper operation of the Xerographic machine. A by-product of corona charging devices are several gasses (most notably NOx and ozone) which are referred to in this discussion as “effluents”. The effluents can interact with the surrounding atmosphere, which may include organic compounds like morpholine, and with the photoreceptor itself to produce substantial negative effects on the photoreceptor and the resulting copy. These are sometimes called lateral charge migration (LCM) and/or parking deletion. This can cause the output of a printed copy to appear blurry or have areas where the image is entirely missing (deleted). Currently, fans and special coatings are utilized to remove or neutralize the gasses to various degrees of success.
Nitric oxide deletions and other effluents have been a pervasive and persistent problem in these electrostatic copying systems. The embodiments of this invention are simple and effective ways to minimize these problems.
Charging devices historically come in three forms; corotrons, scorotrons and dicorotrons; all will be referred to in this disclosure as “corotrons” or a source of “corona” discharge. The charging devices use high voltages to create a corona. This corona can be thought of as a collection of ions (charged atoms or molecules) in a local area. In most cases, the corona is influenced to move towards the desired target by the opposite charge on a screen or grid-type device.
The different names of the charge device or corotrons denote different configurations. Corotrons are simply bare wires. A high DC potential is placed on the corotron to create the corona. To charge photoreceptors to a positive voltage, a large positive DC voltage is placed on the corotron wire. To charge negatively, a negative potential is placed on the wire. Dicorotrons are a wire device also. In this case, the wire is coated with a thick film of dielectric glass. Dicorotrons have an alternating voltage placed on them to create both positive and negative ions. A screen or shield with a DC bias directs the dicorotron's charge toward the photoreceptor. The grid or shield voltage determines the polarity and amplitude of the charge placed on the photoreceptor.
An important consideration is that there are many ways to charge photoreceptors. Some ways have a propensity for problems to occur while others have less of an issue. In relation to nitric oxide deletions, the AC devices (dicorotrons) and the negative DC devices have a higher probability of deletion problems.
As stated, the charge device is the originator of the nitric oxide parking deletion (or, for sake of clarity, deletion). The deletion process begins with the production of corona in normal atmosphere. Corona is a “cloud” of charged ions. Different types of corona contain different ions, H+ and N4+ are the major positive ions for both AC and DC devices. The negative ions NO3− and O3− (ozone) are the major ions in negative DC discharge and AC with airflow. AC devices (dicorotrons) also contain the following negative ions: O−, OH−, O2−, NO2−, CO3−.
The ozone (O3) and NOx (NO and NO2) occur in relatively large amounts. These compounds are also very reactive, chemically. NOx is known as Oxides of Nitrogen. While both gasses and morpholine can contribute to the deletion problem, NOx has been cited as the main culprit, hence the reference to Nitric Oxide Deletion.
Recent experiments show that the NOx output from a dicorotron operated at nominal voltage is entirely NO2. Charge device NO2 output is attributed to the presence of ozone in the charge device area. Ozone oxidizes NO to NO2.
The oxidation of NO to NO2 produces one photon of light at about 1200 nm. This occurs in about 20% of the oxidized NO2. As the molecule decays to a stable state, a photon is emitted with the peak excitation of 1200 nm. This is the basis for a Chemilumenesence Nitric Oxide detector sometimes used in the present embodiments to measure effluents.
Photoreceptors have been shown to be very sensitive to nitric acid-type compounds (HNO3 and HNO2). The nitric acid attacks certain molecules in the transport layer of the photoreceptor rendering them too conductive. This conductivity allows any developed charge on the photoreceptor to leak to ground in the area of the attack or spread in what is sometimes (mistakenly) called lateral charge migration. Lateral charge migration is a separate issue involving the deposit of conductive salts on the photoreceptor through the interaction of corona and atmospheric contaminants, such as morpholine. In Nitric Oxide deletions, in the worst cases, areas near the acid attack appear blank on a copy because toner is not developed to the photoreceptor in those areas. In lesser extent cases, the problem manifests itself as a blurring of the image. Some volatile organic compounds, such as morpholine and organic nitrates are effluents also detrimental to the photoreceptor.
Nitric oxide deletions are often termed parking deletions. This nomenclature arises from the way in which nitric oxide deletions are most prevalent. When charging devices are run for a long period of time (during a long print run) a relatively large amount of NOx and O3 (as above indicated, collectively known as effluents) are built up. The effluents become adsorbed on the surface of nearby solids. When the machine is shut down, the photoreceptor stops rotation and becomes “parked” with a small area directly adjacent to the charge device. Over a short period of time, the adsorbed effluents are released from the charge device in a process known as outgassing. Since the photoreceptor is parked in very close proximity to the charge device, a small local area of the photoreceptor becomes damaged.
The embodiments of the present invention provide strategies employed to combat and minimize these deletions.
It is known to use Titanium to help chemically reduce effluents around a photoreceptor. (By virtue of its native oxide surface layer). It is also known to use Titanium Dioxide (TiO2) to remove nitric oxides from the environment via titanium dioxide coatings. See articles “Reactive Oxygen Species inhibited by Titanium Dioxide Coatings” (Suzuki et al.)) R. Suzuki, J. Muyco, J. McKittrick, J. McKitrick, J. Frangos. J. Biomed. Mater. Res. A, 2003, Aug. 1 vol. 66 No. 2: pg. 396-402), and “Titanium Dioxide: Environmental White Knight”, L. Frazer Environmental Health Perspectives Vol. 109, No. 4, April 2001. It is important to make the distinction between the metal Titanium and the ceramic titanium dioxide. Neither Suzuki nor Frazer suggests use of their process on electrostatic marking system nor suggest the problems of effluents in said systems. Suzuki et. al does not use a thermal plasma spray process; therefore, only provides a titanium coating of from 100-200 nanometers thick. Suzuki's thin coating of elemental titanium was produced by a process that would take an inordinate amount of time (days) to build a coating, as in the present embodiments, of about 20-100 microns and his process must be done in a vacuum. L. Frazer's article does suggest the use of a thermal plasma spray; his process is incapable of producing a coating of TiO2 that is relatively thick (at least 20 microns) and has a porosity of at least 1% of the coating. Also, a pore size of 0.1 micron would be difficult with Frazer's process. An important process difference between the present invention and the process of Frazer is that this invention uses UV energy supplied by the dicorotron as the photo source in the photocatalytic reaction, Frazer uses the UV energy from the sun.
Prior art attempts involved depositing elemental Titanium onto aluminum using physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods such as RF plasma deposition. The titanium metal would then form a native oxide layer by exposure to ambient environment. Unfortunately, these coatings were too thin and were quickly destroyed by the aggressive corona environment. Coatings made by this method are typically 100-200 nm thick. Titanium sheet metal was also tried. Using titanium sheet metal to control effluents was very costly and it was difficult to form titanium sheet metal into the complex shapes required. Methods for the deposition of titanium dioxide ceramic exist (such as sol-gel deposition), but are either considered too expensive or unreliable. Further, the sintering temperatures of titanium dioxide ceramic are too high to allow the material to be directly formed on aluminum. Plasma spray is the most effective alternative when one wishes to place a ceramic coating, such as titanium dioxide on a metal substrate, such as aluminum.
The embodiments of the present invention provide a process and apparatus whereby an aluminum corona housing is at least partially thermally sprayed by a plasma spray method, this plasma spray raises the electrical conductivity of the TiO2 coating and makes it far more effective in reducing effluents. Generally, ceramics like TiO2 are non-conductive. However, in the present embodiments, reduction of TiO2 in the plasma renders the TiO2 electrically conductive. In the plasma, a structure is formed creating a coating with holes. These holes allow for the movement of electrons which create electronic conduction. Plasma spray is the only known practical method of applying a thick layer of TiO2 to aluminum. The purity of TiO2 used in the present embodiments is at least 99% pure. Any suitable ceramic mixtures containing TiO2 may be used, if desirable. In embodiments, pure titanium dioxide or titanium dioxide having from about 99 percent to about 100 percent by weight titanium dioxide, is used. This plasma spray is described as follows:
Plasma spray coating technology is known and described in, for example, “Plasma-spray Coating”, Scientific American, September 1988, pp. 112-117. In a plasma spray procedure, a heat source converts metallic and ceramic materials into a molten spray which is then deposited on a prepared substrate (in this case the corona housing) to form a porous coating. Plasma is a partially-ionized gas containing ions, electrons, free radicals and various neutral species at many different excitement levels. Plasma is created by dissociation and ionization of atomic species. Recombination gives off energy such as heat.
In plasma spray, gases include at least two primary purposes: (1) add heat/energy for material melting; (2) propel molten material to substrate.
A plasma spray gun is a water-cooled device containing an anode and a cathode across which an electric arc is struck. As the gasses pass through this arc, a plasma is formed. With an internal working temperature of about 10,000 degrees centigrade, high-refractory materials such as TiO2 ceramics melt and are propelled to the substrate by gas velocities. Adhesion to the substrate corona housing is primarily through mechanical bonding of the coating, the molten material forms splats on the substrate housing. Grit blasting of the corona housing with aluminum oxide or glass beads improves adhesion of the coating to the substrate by increasing the roughness of the substrate. The porosity of the coating is substantially high, there may be some unmelts which are ceramic particles which did not melt due to unoptimized spray parameters. These unmelts do not adversely affect the intended function of the TiO2 coating.
In exemplary embodiments, a bond coat can be applied on the corona housing to enhance adhesion of the coating on the corona housing. The bond coat can also increase the resistance of the coating to cracking or other defects during cooling after the coating process of the coating. The bond coat can comprise any suitable material, such as a mixture of chrome-aluminum-yttrium-cobalt, or a mixture of nickel-aluminum powder.
The cooling rate of the coating can be controlled to reduce the thermal differential between the corona housing and the coating, to thereby reduce the generation of thermal stresses in the coating. Cooling can be controlled by directing a gas flow onto the corona housing during the coating process. In addition, the corona housing can be preheated to a suitable temperature to reduce the thermal differential between the corona housing and the coating. Preheating the corona housing also promotes the adhesion of the coating. Typically, the temperature of the corona housing and the coating are maintained below about 300 degrees F. to achieve a suitable thermal differential and good coating adhesion.
This plasma sprayed coating on the corona housing offers several substantial advantages over known systems. It is very desirable to have this coating at least on the inside of the U-shaped housing. First, a plasma sprayed coating can be made relatively thick in comparison to the thin prior art coatings that degraded easily in an aggressive corona charging environment. Secondly, TiO2 coatings are hard and substantially chemically inert. This prevents damage and makes the coating impervious to effluent attack. Third, plasma sprayed ceramics such as TiO2 have a natural porosity that makes them well suited to capturing chemical effluents. A highly porous TiO2 coating provides a much larger surface area exposed to the effluents ozone and NOx for reaction to take place.
The plasma spray raises the electrical conductivity of TiO2 by 12-16 orders of magnitude from normal TiO2 compositions. This is important because dicorotron housings must be substantially electrically conductive for proper function of corona generation. The coatings of TiO2 must, therefore, be hard, highly porous and impervious to attack by the effluents. Coatings of TiO2 of from 10-200 microns thick have shown very desirable results. Large pore sizes of at least 0.1 microns also showed good results as did a coating of at least 1% porosity.
Several tests conducted using TiO2 coatings on a corotron aluminum housing compared to uncoated aluminum housings showed that there was no statistically significant difference in charging performance of the coated and non-coated corotrons.
To summarize, a by-product of corona charging devices are several gasses (mostly notably NOx (nitrogen and oxygen compounds or impurities) and Ozone) and morpholine, which are referred to as “effluents” which can negatively affect the photoreceptor and resulting copies. The present embodiments prevent or substantially minimize deletions by introducing a thick and porous TiO2 coatings on at least part of the corona housing by a plasma spray.
This plasma spray provides a thicker much more porous and larger surface area coating of TiO2 than previously used; therefore, a much more effective means to reduce effluents.
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In summary, an embodiment of this corona charging device is useful in an electrostatic marking system and comprises, in an operative arrangement, an aluminum housing, at least one corona generating wire electrode positioned within said housing and a substantially porous TiO2 coating on at least one portion of the housing. A bond coat is placed on the aluminum housing prior to coating the housing with TiO2. The wire electrode is enabled to deposit an electrical charge on a photoreceptor surface used in the marking system. The porous coating is enabled to capture, reduce and neutralize amounts of any effluents created by the corona generating electrode that are located in an environment of the corona charging electrode, the corona charging device and the photoreceptor. The porous coating is a thermally plasma sprayed TiO2 coating which has a thickness of at least from 20-40 microns and a pore size of at least 0.1 microns.
The TiO2 coating being thermal-plasma sprayed coating provides sufficient porosity and conductivity to the TiO2 to substantially increase its ability to remove effluents from the area around the corona charging device, the photoreceptor and other parts of the system. Other thermal sprays that accomplish the same result as plasma sprays are included within the scope of this invention. Such sprays include High Velocity Oxygen Fuel (HVOF) processes. However, plasma spray is the preferred thermal spray since it is proved to consistently produce the most effective and desirably thick and porous TiO2 coatings.
The preferred and optimally preferred embodiments of the present invention have been described herein and shown in the accompanying drawings to illustrate the underlying principles of the invention, but it is to be understood that numerous modifications and ramifications may be made without departing from the spirit and scope of this invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5153618 | Frank et al. | Oct 1992 | A |
| 5510879 | Facci et al. | Apr 1996 | A |
| 5539205 | Reale | Jul 1996 | A |
| 7264752 | Drappel | Sep 2007 | B2 |
