Patent Application
20060254921
The present disclosure, in various exemplary embodiments, relates to a fast anodization process that produces a hard anodized barrier layer adapted for photoreceptor substrates.
Electrostatographic imaging systems, which are well known, involve the formation and development of electrostatic latent images on an imaging surface of an electrostatographic or photoreceptor. Xerographic photoreceptors can be prepared in either a single-layer or a multilayer configuration. Depending on the application, the photoreceptors can be prepared in several forms, such as flexible belts, cylindrical drums, plates, etc. Belts are usually prepared on polymer substrates, poly(ethylene terephthalate) being the most common. For drums, the substrate is typically a metal cylinder. Usually, hollow aluminum cylinders are widely used in low- and mid-volume applications. The drum configuration, however, has certain process limitations for high-volume and color applications.
Photoreceptors are prepared by the sequential application of various layers (i.e., charge generating layer, charge transport layer, etc.) onto the outer surface of a polymer or drum substrate. Many coating techniques (i.e., spraying, spinning, extrusion, dipping, blade coating, roll coating, etc.) may be utilized to produce these layer(s). Vapor deposition may also be used for metallization and application of some pigments.
At present, long life photoreceptors free of defects and carbon fiber problems cannot be made. An electrolytic barrier layer on the substrate however, can extend the life of a photoreceptor. However, such a barrier layer typically requires up to 15 minutes to apply thereby making it unattractive for commercial manufacture. Another disadvantage to an electrolytic barrier layer, relates to the barrier's insufficient hardness, and thus, poor carbon fiber roboustness.
Extended photoreceptor life, such as for example an increase in life of two to ten times, and robustness is presently achieved as a result of using, in part, an electrolytic barrier layer, and particularly one formed by low temperature controlled voltage and organic acid anodization. While the process appears to be substantially complete in less than one minute, mixed results regarding the extent of defects, thus, extended life are obtained if the process is terminated at the one, two, five, and ten-minute points. It has been determined that consistent results are obtained by continuing the application of voltage for times exceeding ten minutes. It is believed that additional impurities are removed from the aluminum photoreceptor substrate surface during the extended process time. When such impurities are left in place and not removed, these impurities act as sites for the initiation of defects. However, times in excess of one minute are not attractive to most commercial manufacturing operations.
Accordingly, it would be desirable to provide a process that reliably facilitates extended life but that could be performed in one minute or less.
The present disclosure concerns, in various exemplary embodiments, a process for forming a barrier layer on a photoreceptor substrate by organic acid anodization. The process comprises providing a photoreceptor substrate and providing an organic acid electrolyte. The process further comprises contacting the photoreceptor substrate with the electrolyte. The process also comprises applying a multi-step voltage profile to the photoreceptor substrate in contact with the electrolyte. The profile includes a first step in which a first voltage is applied for a first time period and a second step in which a second voltage, less than the first voltage, is applied for a second time.
In another exemplary embodiment, a process for forming a barrier layer on a photoreceptor substrate by organic acid anodization is provided. The process comprises providing a photoreceptor substrate and providing an organic acid electrolyte. The process comprises heating the photoreceptor substrate to a temperature of from about 450° C. to about 650° C. The process also comprises contacting the photoreceptor substrate with the electrolyte. And, the process comprises applying a voltage to the photoreceptor substrate in contact with the electrolyte for a period of time so as to form an anodized layer thereon.
Still further advantages and benefits of the present exemplary embodiments will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.
The exemplary embodiment provides a fast and low voltage anodization process to form an electrolytic barrier layer for substrates such as those for organic photo conductor photoreceptors. The process uses a reduced voltage profile, such as from about 24 volts to about 12 volts, to reduce the process time from 15 minutes to as short as 42 seconds. The resulting anodized barrier layer extends the functional life of organic photoconducting photoreceptors and prevents defects caused by carbon fiber penetration of the coated layers.
The exemplary embodiment provides a process for forming an electrolytic barrier layer using a multi-step voltage profile. A two-step profile can utilize a first step in which a first voltage is applied for a first time period and a second voltage, less than the first voltage is applied for a second time period. The second voltage is from about 50% to about 20% of the first voltage, and particularly, from about 40% to about 30% of the first voltage. Representative voltages for the first step can be from about 20 to about 24 volts, and for the second step can be from about 12 to about 17 volts. The total time for the two-step profile, i.e. the sum of the first and second time periods, is less than about 5 minutes, and particularly less than about 1 minute.
A three-step profile can utilize a first step in which a first voltage is applied for a first time period, a second voltage, less than the first voltage, is applied for a second time period, and a third voltage, less than the second voltage, is applied for a third time period. The second voltage is from about 40% to about 30% of the first voltage. The third voltage is about 30% to about 20% of the second voltage. Representative voltages for the first step can be from about 22 to about 26 volts, for the second step from about 14 to about 18 volts, and for the third step from about 10 to about 12 volts.
By starting the process at higher voltages and stepping down the voltage during the process, it is possible to quickly obtain the same barrier layer as one formed from a lower voltage applied for a longer time. For example, starting at 24 volts for 10 seconds then stepping down to 16 volts for another 12 seconds followed by another 20 seconds at 12 volts produces the same barrier layer that is obtained using 12 volts for 15 minutes but only takes 42 seconds. Thus, the process is now compatible with a manufacturing process step window of one minute.
Barrier layers having thicknesses of twenty nanometers formed at 12 volts and thirty nanometers (formed at 17 volts) were produced in a 1% w/w citric acid electrolyte at 14° C. in one minute or less by starting the barrier layer process at 20 and 24 volts respectively for 15 seconds followed by 25 seconds at 12 and 17 volts respectively. These barrier layers were tested and found to be similar to barrier layers produced when 12 and 17 volts were used for 15 minutes.
In addition to these two-step voltage processes, three-step voltage processes also produce layers with excellent characteristics. Indeed, while undemonstrated, it is believed that a continued or gradual ramping reduction of the voltage would produce comparable results.
Note that just maintaining a higher voltage for a shorter time produces a barrier layer that is too thick for use with many photoreceptors, thus producing a residual voltage in excess of 100 volts.
Although not wishing to be limited to any particular theory, it is believed that the more aggressive starting voltage is deep cleaning the substrate surface during the first few seconds, without sufficient time to form a barrier layer that is too thick, followed by the production of a useable barrier layer at the reduced voltage.
The exemplary embodiment also provides an anodization process, specifically using a heat pretreatment that produces a hard anodized barrier layer on a substrate such as used with organic photoconductor photoreceptors. The resulting hard layer extends photoreceptor life and reduces the carbon fiber penetration problem. The exemplary embodiment can utilize 550° C. heat pretreatment. The heat treatment is believed to create crystalline oxide “seeds” that propagate during the electrolytic (anodizing) step, causing the formation of a harder anodized barrier layer.
By subjecting the naturally occurring aluminum oxide on a photoreceptor substrate to elevated temperatures before the member is anodized, the barrier layer becomes a crystalline versus the normal amorphous form. This crystalline variant is much harder than the amorphous form.
More specifically, the exemplary embodiment process utilizing a heating pretreatment can be utilized in conjunction with conventional processes for forming an electrolytic barrier layer, and more particularly, with the accelerated processes described herein. The exemplary embodiment heat treatment features subjecting a member to receive a barrier layer, such as a photoreceptor, to temperatures from about 450° C. to about 650° C., and particularly about 500° C. The time period for such heating can vary, however, times of from about 10 seconds to about 60 seconds, and particularly about 30 seconds can be utilized.
The cleaned photoreceptor substrate is first subjected to a 30 second heat treatment at 550° C. then anodized at 10 volts for 15 minutes in a 1% citric acid electrolyte at 14° C. Alternatively, the exemplary embodiment includes the use of 16 volts for 15 seconds followed by 10 volts for 25 seconds. These processes will produce a crystalline barrier layer that has an equivalent capacitance to the barrier layer produced at 12 volts in the same system after 15 minutes.
While this heat treatment described herein does not cause the natural oxide to grow very much, it is believed that the heat treatment causes the formation of “seeds” of crystalline oxide that propagate during the subsequent electrolytic (anodizing) step.
A general description of the electrolytic process now follows. In the electrolytic cell, the working electrode is the photoreceptor substrate (Anode). The counterelectrode can be concentric, generally surrounding the exterior of the substrate. To simultaneously clean the interior of the substrate in the case of a cylindrical substrate, a concentric counter electrode may be disposed in the interior of the substrate. The counterelectrode or electrodes may be a noble metal such as gold, silver, platinum, palladium; an inert material such as graphite; or a strongly passive material such as titanium, lead, tantalum, or alloys thereof. The cell voltage is modulated with a power source capable of delivering direct voltage.
The electrolyte used may be any of several acids. These include citric acid monohydrate, oxalic acid dihydrate, and d-tartaric acid. Preferably, citric acid is used at 0.5 and 1.0 w/v % (pH 2). Oxalic acid is used at 0.5, 0.62, and 1.0 w/v % (pH 1). D-tartaric acid is used at 0.5 and 1.0 w/v % (pH 2.5). The temperatures used are from 13 to 18° C. These baths are not generally sensitive to concentration variability and only slightly sensitive to temperature changes in these ranges. Note that these characteristics make these baths attractive as well for manufacturing (low concentrations and robust to operating parameter changes). In addition to these organic acids, inorganic acids commonly used for anodizing like sulfuric acid, chromic acid, et cetera can also be used. Generally, citric acid is preferred as it is the most environmentally friendly. The voltages used are generally in the range from 8 to 24 volts.
In embodiments, there is formed a metal oxide layer on the substrate surface wherein the metal oxide layer added by the exemplary embodiments has a thickness ranging from about 50 to about 200 angstroms, and more particularly from about 70 to about 150 angstroms. The metal oxide may be for example aluminum oxide. The actual thickness is difficult to measure. Hence, in an alternate, the so-called, Vlow of the finished photoreceptor is measured and adjusted in the process to keep the Vlow to less than 100 Volts. The final current passing at the end of the anodizing process is used as a surrogate to know that a good barrier layer has been obtained. Note that this final current is dependent on several factors in addition to the thickness of the barrier layer. These other factors are, therefore, preferably kept constant and include: the number and size of the parts being anodized, the rack (holds the parts) configuration, and to a lesser extent the temperature of the electrolyte. Note also, that sufficient electrolyte movement (mixing) to insure uniform temperature (+/−1° C.) is preferred.
The substrate preferably is a hollow cylinder and defines a top non-imaging portion, a middle imaging portion, and a bottom non-imaging portion. The precise dimensions of these three substrate portions vary in embodiments. As illustrative dimensions, the top non-imaging portion ranges in length from about 10 to about 50 mm, and particularly from about 20 to about 40 mm. The middle imaging portion may range in length from about 200 to more than 1000 mm, and particularly from about 250 to about 300 mm. The bottom non-imaging portion may range in length from about 10 to about 1 mm, and particularly from about 5 to about 10 mm. The substrate may be bare of layered material or may be coated with a layered material prior to immersion of the substrate into the coating solution.
The substrate can be formulated entirely of an electrically conductive material, or it can be an insulating material having an electrically conductive surface. The substrate can be opaque or substantially transparent and can comprise numerous suitable materials having the desired mechanical properties. The entire substrate can comprise the same material as that in the electrically conductive surface or the electrically conductive can merely be a coating on the substrate. Any suitable electrically conductive material can be employed. Typical electrically conductive materials include metals like copper, brass, nickel, zinc, chromium, stainless steel; and conductive plastics and rubbers, aluminum, semitransparent aluminum, steel, cadmium, titanium, silver, gold, paper rendered conductive by the inclusion of a suitable material therein or through conditioning in a humid atmosphere to ensure the presence of sufficient water content to render the material conductive, indium, tin, metal oxides, including tin oxide and indium tin oxide and the like. The coated or uncoated substrate can be flexible or rigid, and can have any number of configurations such as a cylindrical drum, an endless flexible belt, and the like. This (ground strip) should preferably be anodizable (Ti, Al, et cetera) or added after the anodize process.
The layers of the substrate member can vary in thickness over substantially wide ranges depending on the desired use of the photoconductive member. Generally, the conductive layer ranges in thickness of from about 50 Angstroms to 10 centimeters, although the thickness can be outside of this range. If desired, a conductive substrate can be coated onto an insulating material. In addition, the substrate can comprise a metallized plastic, such as titanized or aluminized MYLAR® (available from DuPont). The coated or uncoated substrate can be flexible or rigid, and can have any number of configurations. The substrates preferably have a hollow, cylindrical configuration.
The layer of a photosensitive member including such layers as a subbing layer, a charge barrier layer, an adhesive layer, a charge transport layer, and a charge generating layer, such materials and amounts thereof being illustrated for instance in U.S. Pat. No. 4,265,990, U.S. Pat. No. 4,390,611, U.S. Pat. No. 4,551,404, U.S. Pat. No. 4,588,667, U.S. Pat. No. 4,596,754 and U.S. Pat. No. 4,797,337, the disclosures of which are totally incorporated by reference. These layers may be apparent by known coating processes.
In certain embodiments, the coating solution may be formed by dispersing a charge generating material (CGL) selected from azo pigments such as Sudan Red, Dian Blue, Janus Green B, and the like; quinine pigments such as Algol Yellow, Pyrene Quinone, Indanthrene Brilliant Violet RRP, and the like; quinocyanine pigments; perylene pigments; indigo pigments such as indigo, thioindigo, and the like; bisbenzoimidazole pigments such as Indofast Orange toner, and the like; phthalocyanine pigments such as copper phthalocyanine, aluminochlorophthalocyanine, and the like; quinacridone pigments; or azulene compounds in a binder resin such as polyester, polystyrene, polyvinyl butyral, polyvinyl pyrrolidone, methyl cellulose, polyacrylates, cellulose esters, and the like.
The average particle size of the pigment particles is between about 0.05 micrometer and about 0.10 micrometer. Generally, charge generating layer dispersions for immersion coating mixture contain pigment and film forming polymer in the weight ratio of from 20 percent pigment/80 percent polymer to 80 percent pigment/20 percent polymer. The pigment and polymer combination are dispersed in solvent to obtain a solids content of between 3 and 6 weight percent based on total weight of the mixture. However, percentages outside of these ranges may be employed so long as the objectives of the process of this disclosure are satisfied. A representative charge generating layer coating dispersion comprises, for example, about 2 percent by weight hydroxy gallium phthalocyanine; about 1 percent by weight of terpolymer of vinyl acetate, vinyl chloride, and maleic acid (or a terpolymer of vinylacetate, vinylalcohol and hydroxyethylacrylate); and about 97 percent by weight cyclohexanone.
In other embodiments, the coating solution may be formed by dissolving a charge transport material (CTL) selected from compounds having in the main chain or the side chain a polycyclic aromatic ring such as anthracene, pyrene, phenanthrene, coronene, and the like, or a nitrogen-containing hetero ring such as indole, carbazole, oxazole, isoxazole, thiazole, imidazole, pyrazole, oxadiazole, pyrazoline, thiadiazole, triazole, and the like, and hydrazone compounds in a resin having a film-forming property. Such resins may include polycarbonate, polymethacrylates, polyacrylate, polystyrene, polyester, polysulfones, styrene-acrulonitrile copolymer, styrene-methyl methacrylate copolymer, and the like.
An illustrative charge transport layer coating solution contains, for example, about 10 percent by weight N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′diamine; about 14 percent by weight poly(4,4′-diphenyl-1,1′-cyclohexane carbonate) (400 molecular weight); about 57 percent by weight tetrahydrofuran; and about 19 percent by weight monochlorobenzene.
Furthermore, the charge generating layer, charge transport layer, and/or other layers may be applied in any suitable order to produce either positive or negative photoreceptors.
The photoreceptors produced by the present disclosure can be utilized in an electrophotographic imaging process by, for example, first uniformly electrostatically charging the photoreceptor, then exposing the charged photoreceptor to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoreceptor while leaving behind an electrostatic image in the non-illuminated areas. This electrostatic latent image may then be developed at one or more developing stations to form a visible image by depositing finely divided electroscopic toner particles, for examples, from a developer composition, on the surface of the photoreceptor. The resulting visible toner image can be transferred to a suitable receiving member, such as paper. The photoreceptor is then typically cleaned at a cleaning station prior to being recharged for formation of subsequent images.
While particular embodiments have been described, alternatives, modifications, variations, improvements, and substantial equivalents that are or may be presently unforeseen may arise to applicants or others skilled in the art. Accordingly, the appended claims as filed and as they may be amended are intended to embrace all such alternatives, modifications variations, improvements, and substantial equivalents.
