Patent Application
20130344424
An organic photoconductor (OPC) is one of the components of an electrophotographic (or xerographic) process employed in many printing and/or photocopying devices. The lifetime of known organic photoconductors are limited by the degradation of print quality as defects arise within the surface of the OPC as a result of chemical, electrical and/or mechanical interactions between the OPC and the printing environment. As a result, an OPC is one of the most frequently replaced components of a printing device, thereby resulting in increased costs of using such printing devices.
Efforts have been made to fabricate cylindrical-shaped OPCs by sputtering or evaporating selected organic compounds either directly on the surface of a drum form usually made of metal, or on a rigid cylindrical sleeve subsequently mounted to the drum. However, such efforts have yielded poor quality photoconductors with mechanically weak, readily cracking, rough surfaces.
The shortcomings of known OPCs are particularly critical in the case of high speed digital printing that seeks to reduce printing costs in order to successfully compete with analog printing. Previous attempts of replacing an OPC with an inorganic photoconductor or coating the OPC with a hard inorganic protective layer have failed due to excessive cost, manufacturing problems, and/or poor performance of the resulting product.
Furthermore, many known OPCs are manufactured from surface layers formed as a flexible flat sheet and subsequently wound or wrapped around a cylindrical drum. Such an approach results in the presence of a seam along the surface of the OPC where the sheet ends meet. A seam may lead to limits on page length in printing, and/or re-printing of areas surrounding the seam to achieve a desired color efficacy. Furthermore, an OPC seam presents significant challenges for web-fed printing applications. Additionally, the process of wrapping a flat sheet of photoconductive material around a drum creates stress and/or strain within the photoconductive material that may undesirably affect the mechanical properties of the materials to become more susceptible to damage when placed in operation.
An organic coating for an OPC formed of damage resistant cross-linkable polymers and short chain polymeric charge transfer moieties (CTMs) can significantly increase the damage resistance and durability (i.e., useful life) of the OPC while maintaining high print quality. For example, controlled experimental scratch tests performed using flat sheets of the organic coating wrapped around an OPC drum show improvements of the mechanical damage resistance of approximately five to ten times over known OPCs while substantially maintaining print performance. However, while such a coating improves the durability of OPCs, using flexible a flat sheet of photoconductive material as the coating around a drum results in an undesirable seam and potentially undesirable stresses and/or strains in the surface coating. Example methods to manufacture seamless OPCs are disclosed herein which overcome these and other problems by cross-linking a matrix polymer species with embedded CTMs directly on the surface of a cylindrical OPC. In some examples, the CTMs include small molecules instead of, or in addition to, the short chain polymers described above.
The drum 104 of the illustrated example comprises a conductive material (e.g., aluminum). The conductive material serves as a substrate upon which the OPC layered surface 108 resides and provides an electrical path to ground. In some examples, the conductive substrate is incorporated directly into the drum 104. In other examples, the conductive substrate comprises a rigid cylindrical sleeve which is placed around the drum form 104. The layered surface 108 of the illustrated example includes a bottom charge generation layer (CGL) and a top charge transport layer (CTL). In the illustrated example, the layered surface 108 also comprises a protective layer or coating as will be described in greater detail below.
In the illustrated example, as the OPC 102 rotates during a printing process, it passes through several stations, including a charging station 110, an exposure or image-forming station 112, a development station 114, and a transfer station 116. At the charging station 110 of the illustrated example, a negative electrostatic charge is uniformly distributed over the surface of the OPC 102 and maintained as a result of the electrical characteristics of the OPC layered surface 108. In some examples, the charging is done by a corona charger. In other examples, the charging is accomplished via a charge roller.
At the exposure station 112 of the illustrated example, a document to be printed (e.g., electrophotographed), or an image of the document formed on a screen, is illuminated and either passed over a lens or is scanned by a moving light and lens, such that its image is projected onto and synchronized with the surface of the rotating OPC 102. Light from the projected image passes through the CTL (which is substantially transparent) and strikes the CGL resulting in the generation of free electrons and holes. Electrons are collected by the electrical ground of the photoreceptor (e.g., via the drum 104) and holes are driven by an applied electrical field or bias towards the top surface of the CTL. The CTL of the illustrated example is formed of a non-conductive organic material (e.g., a polymer) matrix with a dopant species (referred to as charge transport moieties (CTMs)) embedded within the non-conductive matrix. The CTMs within the CTL enable the hole transport towards the surface. At the surface the holes are used to neutralize negative surface ions deposited via the charging station 110. Accordingly, in the illustrated example, the area(s) where the document, or document image, contains content (e.g., text, pictures, etc.) the corresponding area(s) of the OPC 102 remains unlit. As a result, these area(s) do not pass light through the CTL to strike the CGL and, thus, do not generate an electrical charge to neutralize the electrostatic charge at such location(s). In contrast, the area(s) where the document, or document image, contains no content, the OPC 102 is illuminated and the electrostatic charge at the corresponding locations is dissipated by the holes as explained above. As a result, the area(s) of electrostatic charge remaining on the example OPC 102 of the illustrated example form a “latent” image that is a negative of the original document, or document image. In some examples, a reversed arrangement is employed in which a positive electrostatic charge is deposited on the surface of the OPC 102, and then selectively dissipated by photogenerated electrons rather than holes.
At the development station 114 of the illustrated example, the OPC 102 is presented with toner 117 such as black or colored electro-ink and/or any other marking particles. The toner 117 is electrically charged complementarily to the electrostatic charge on the OPC layered surface 108 to be attracted to the areas on the OPC 102 corresponding to the latent image of the document.
At the transfer station 116 of the illustrated example, the toner 117 on the OPC 102 is transferred to a print medium 118 (e.g., paper) that is moving relative to the speed and direction of the rotating OPC 102. In some examples, the polarities of this process may be reversed depending on the initial document or image being copied (e.g., photocopying a white on black microfilm to black on white paper). Following the toner transfer, the example OPC 102 is prepared for a new imaging cycle (e.g., by scraping off any remaining toner 117 via a doctor blade).
To achieve high quality printing using an electrophotographic process, the example OPC layered surface 108 of
In the illustrated examples of
In some examples, the solution also includes other additives or species, such as, for example, an initiator and/or cross-linker to facilitate the cross-linking of the matrix polymer species. Further, in some examples, the solution contains additives such as for example, wetting and/or viscosity-controlling agents to provide the desired rheological properties of the material, and/or other auxiliary species and/or additives to provide other desired properties to the material (e.g., hard inorganic nanoparticles to increase resistance against mechanical damage).
In the illustrated examples, the solvent used as the base for the solution is to dissolve both the matrix polymer species and the dopant species to provide a substantially uniform dispersion of the dopant species and other moieties or auxiliary additives within the solution. For example, if the solvent is an alcohol (e.g., isopropyl alcohol), the matrix polymer species may be a polyimide while the dopant species may be a polyvinylcarbazole (to transport holes) or a polythiophene (to transport electrons). This and other examples of matrix polymer species and dopant species with a corresponding solvent are outlined in Table 1.
Furthermore, in the illustrated examples, the solvent is inert or non-reactive with the material of the layer beneath the coatings 206, 304. That is, in the illustrated examples, the solvent used in the solution to form the coating 206 of
As previously described, in known methods of manufacturing OPC drums with a durable coating, the structured photoconductive layers and/or the coating are formed as flat sheets and subsequently wrapped around a cylindrical drum. The result is an OPC with a seam that presents limitations to its use in certain printing applications. However, in addition to the limitations mentioned above, manufacturing flat sheets of photoconductive layers presents several other challenges. For example, known manufacturing processes for the photoconductive layers of an OPC are limited in their ability to form thin layers, thereby resulting in greater quantities of material used to make the OPC, and increasing costs of production. Additionally, known methods of manufacturing such flat sheets of desired photoconductive layers are limited in how consistent and/or evenly distributed the thickness of the sheets are over their surface area thereby resulting in more uneven OPCs and lowering the quality of the resulting printing. Such obstacles are overcome by applying the coating to an OPC in accordance with the teachings disclosed herein.
In the example shown in
Once the liquid film 400 covers the entire peripheral surface of the OPC 102, the solvent is allowed to evaporate. However, unlike known methods where the solvent evaporates while the solution rests on a flat surface to form flat sheets, in the illustrated examples, the solvent evaporates as the OPC 102 continues to rotate. Consequently, the resulting residue containing the matrix polymer species, the dopant species, and any other additives is distributed with a substantially uniform thickness about the OPC 102 without creating seams (e.g., a seamless protective coating is formed). The speed of rotation, along with other factors based on the properties of the materials involved (e.g., concentrations and types of matrix polymer species, dopant species, and any other additives in the solution), can be used to substantially control the thickness of the protective coating. In some examples, the thickness of the protective coating ranges from approximately 0.1 μm to approximately 20 μm. In other examples, the thickness ranges from approximately 0.2 μm to approximately 2 μm.
In the illustrated examples, after the solvent has evaporated, the residue is heated and/or exposed to UV irradiation to cross-link the matrix polymer species along with any other polymerizable species contained in the residue (e.g., a cross-linker). In some thermally activated examples, the annealing temperature of the matrix polymer species ranges from approximately 70 degrees Celsius to 150 degrees Celsius. In some examples, the temperature is maintained in the range of approximately 80 degrees Celsius to approximately 100 degrees Celsius. Regardless of the activation method, in some examples, the OPC 102 is rotated during the polymerization process to maintain greater uniformity in the cross-linking of the materials. Such rotation may be especially beneficial when a directional heat and/or UV source is used. Furthermore, the speed of rotation in such situations, along with controlling other factors in the cross-linking process (e.g., varying time, UV exposure and/or temperature), can be used to tune the mechanical strength of the protective layers 206, 304 of the illustrated examples.
Fabricating OPCs with coatings in this manner provide OPCs that are both durable and do not have seams, thereby overcoming challenges faced in the prior art. In particular, a seamless OPC has no limitation on page length and can, therefore, be adapted to web-fed printing processes. Additionally, seamless printing allows for increased speed in printing and, thus, reduced cost. Furthermore, the matrix polymer species are cross-linked after being placed on the round surface of the OPC such that no internal stress or strain is present, thereby increasing the mechanical strength of the coating. Further still, the use of organic photoconductive materials enables recycling and/or reusing the drum form by removing old and/or damaged photoconductive materials (e.g., by dissolving them in an organic solvent) to prep the drum for the reapplication of a new layered surface using the methods described herein.
The example process of
The liquid solution applied to the surface of the substrate in the example of
Once the liquid solution has been applied to the substrate, the example process of
After the solvent has evaporated, the example process of
Although the foregoing description has described methods of applying a protective coating to organic photoconductors, the teachings disclosed herein may be suitably adapted to applying a coating to an inorganic photoconductor or an OPC with one or more layers of inorganic materials on its surface. Furthermore, although certain example methods and apparatus have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus and articles of manufacture fairly falling within the scope of the claims of this patent either literally or under the doctrine of equivalents.
