1. Field of Invention
The invention is directed to self-regenerative, polymeric coatings and methods of using the coatings in xerography to increase the life and effectiveness of catalytic surfaces, such as, for example, charging device surfaces, by neutralizing ozone and nitrogen oxide species.
2. Description of Related Art
In electrophotographic printing, also known as electrophotography or xerography, a photoreceptor containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The photoreceptor is then exposed to a pattern of activating electromagnetic radiation, such as light or a scanning laser beam. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided toner particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the photoconductor to a support, such as a transparency or paper. This imaging process may be repeated many times.
The charging device uses high voltages to create a corona (a collection of ions (charged atoms or molecules) in a local area). In most cases, the corona is influenced to move towards a desired target by the opposite charge on a screen or grid type device. Generally, charging devices require a high voltage of about 5,000 volts to about 8,000 volts to produce a corona spray that imparts the electrostatic charge to the surface of the photoreceptor.
Charging devices are known as corotrons, dicorotrons, or scorotrons. 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 corotron wire. Dicorotrons are also wire devices. In dicorotrons, the wire is coated with a thick film of dielectric glass. Dicorotrons have an alternating voltage placed on the wires to create both positive and negative ions. A screen or shield with a DC bias directs the dicorotron charge towards the photoreceptor. A positive bias on the shield allows the photoreceptor to be charged positively. A negative bias on the shield allows the photoreceptor to be charged negatively. Dicorotrons often contain dicorotron shields made of aluminum coated with dispersed aqueous graphite (DAG) to generate the high voltage required to charge the photoreceptor. The aluminum material comprising a first layer of the dicorotron shield and the DAG coating comprising a second layer of the dicorotron shield combine to form a shield for containing the high voltage generated by the dicorotron and for directing the charge to the photoreceptor.
The final form of charging device is a scorotron. The scorotron can come in many configurations, such as a pin array, which is a concentration of jagged pins. The corona is created around the tips of the pins by a large negative DC charge. The corona is actually created by the pins by stripping electrons from the surrounding air molecules, thus creating positive ions. The charge is directed and regulated by a scorotron grid, which is generally a photoetched piece of sheet metal that has a bias placed on it.
Certain problems have been observed when using charging devices that produce a negative corona, such as the production of various noxious gases, including nitrogen species and ozone. For example, the nitrogen output from a dicorotron operated at nominal voltage is entirely NO2, which is attributed to the presence of ozone in the corona atmosphere. Ozone oxidizes NO to NO2 by the following reaction:NO+O3NO2+O2+0.2 hvThis reaction produces one photon of light when about 20% of the oxidized NO2 is in the excited state. As the molecule decays to a stable state, a photon is emitted with a peak excitation of about 1200 nm, which leads to the next reaction (and ultimately causes “parking deletions”):2NO2+H2OHNO3+HNO2 (Nitric+Nitrous Acid)This reaction is called acid hydrolysis. It requires outside energy to proceed, which, in the case of dicorotrons, is readily available as free energy put out by the corona.
Photoreceptors have been shown to be very sensitive to the resulting nitric acid 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 the ground in the area of the attack or spread in what is sometimes (mistakenly) called lateral charge migration. (Lateral charge migration is actually a separate issue involving the deposit of conductive salts on the photoreceptor.) In the worst case, areas near the acid attack appear blank or blurred on a copy because the toner does not develop properly to the photoreceptor in those areas, thus forming parking deletions.
Parking deletions generally occur when charging devices are run for a long period of time (during a long print run) when relatively large amounts of NO2 and NO3 (collectively known as effluents) are built up. The effluents become adsorbed on the surface of nearby solids. When the machine is shutdown, the photoreceptor stops rotation and becomes “parked” with a small area directly adjacent to the charging device. Over a short period of time, the adsorbed effluents are released from the charging device in a process known as “outgassing.” Since the photoreceptor is parked in very close proximity to the charging device, a small local area of the photoreceptor becomes damaged. The nitric and nitrous acids produced deteriorate and weaken the photoreceptor surface, which eventually results in uneven charging of the photoreceptor. Once damaged, the photoreceptor must be replaced, posing significant operating costs.
To reduce the parking deletion problem associated with negative corona charging, dispersed active graphite (DAG or electrodag) coatings have been applied to catalytic surfaces. Such coatings typically include a catalytic metal base as an active component, such as nickel, lead, copper, or zinc, or mixtures thereof, which tend to absorb or form harmless compounds with nitrogen oxide species, thus neutralizing the harmful chemicals. For example, U.S. Pat. No. 4,585,320 describes the adsorption of nitrogen oxide species using a thin layer of lead.
DAG coatings only work if the active component is exposed to the atmosphere. However, DAG coatings generally contain the active component in a nonfunctional matrix that supports the active component and adheres it to the substrate surface, but only permits the active component to interact with the atmosphere at the surface of the coating. When the active component at the surface of the coating is depleted, the coating is no longer functional.
As an alternative to conventional DAG coatings, aluminum, chromium, titanium, stainless steel, and refractory metals, e.g., tungsten, molybdenum, etc., have been found to desorb the problem gases that cause parking deletions. For example, U.S. Pat. No. 4,585,322 describes the use of an alkali metal silicate coating capable of adsorbing and neutralizing nitrogen oxide species and U.S. Pat. No. 4,646,196 describes the use of a conductive dry film of aluminum hydroxide as a coating capable of absorbing and neutralizing nitrogen oxide species. Similarly, U.S. Pat. No. 4,920,266 discloses a corona generating device including at least one element adjacent to the corona discharge electrode capable of absorbing nitrogen oxide species generated when the electrode is energized, and capable of desorbing the nitrogen oxide species once the electrode is no longer energized. The element is coated with a thin, conductive, dry film of aluminum hydroxide containing graphite and powdered nickel.
Alternative parking deletion remedies are described in U.S. Pat. No. 5,257,073, which discloses a corona generating device wherein a control screen adjacent to the corona generating electrode regulates the charge flow. The control screen is coated with a substantially continuous layer of boron electronless nickel, which serves to extend the effective life of the device by preventing line image deletions. U.S. Pat. No. 4,792,680 discloses a scorotron screen for use in a negative corona charging device. The device includes a beryllium copper alloy, which reduces the problems associated with line image deletions.
While some success has been found using these various approaches, parking deletions continue to be a problem due to the failure of the known coatings and screens to continue to absorb or form harmless compounds with the ozone and nitrogen oxide species over time. Thus, a need exists for a xerographic machine that can operate efficiently while continuing to neutralize the hazardous gases that cause parking deletions and other printing/imaging problems.