1. Field of the Invention
The present invention relates to an electrophotographic photoconductor that is used in electrophotographic printers, copiers, fax machines and the like (hereafter also referred to simply as “photoconductor”), to a method for producing the electrophotographic photoconductor, and to an electrophotographic apparatus. More particularly, the present invention relates to an electrophotographic photoconductor that, by containing a polymer of specific structure, exhibits excellent contamination resistance, electrical characteristic stability, and ozone resistance, to a method for producing the electrophotographic photoconductor, and to an electrophotographic apparatus.
2. Background of the Related Art
The electrophotographic photoconductor has a basic structure wherein a photoconductive layer having a photoconductive function is disposed on a conductive support. Ongoing research and development are actively carried out on organic electrophotographic photoconductors that utilize organic compounds as functional components for charge generation and transport, given the advantages that organic electrophotographic photoconductors afford, such as material diversity, high productivity, safety and the like. The use of organic electrophotographic photoconductors in copiers, printers and the like is thus becoming more widespread.
Generally, a photoconductor must fulfill the function of holding surface charge in the dark, generating charge when receiving light, and transporting the charge thus generated. Such photoconductors encompass so-called single layer-type photoconductors that are provided with a single photoconductive layer that combines the above functions, and so-called multilayer-type (of function-separated type) photoconductors provided with a photoconductive layer that is a stack of layers functionally separated into a charge generation layer that fulfils mainly the function of charge generation upon reception light, and a charge transport layer that fulfils the function of transporting the charge that is generated in the charge generation layer upon reception of light.
The above photoconductive layers are generally formed by coating a conductive support with a coating solution in which a charge generation material, a charge transport material and a binder resin are dissolved or dispersed in an organic solvent. In many instances a polycarbonate is used as the binder resin in organic electrophotographic photoconductors, in particular in the outermost surface layer, since polycarbonates are resistant to friction with paper and with blades for toner removal, and boast excellent flexibility and good exposure transparency. Among the foregoing, bisphenol Z polycarbonates are widely used as the binder resin. Technologies in which such polycarbonates are used as a binder resin include, for instance, Japanese Patent Application Publication No. S61-62040 (Patent literature 1).
Electrophotographic printing devices are required to possess ever higher durability and sensitivity, and faster responses, to cope with, for instance, increases in the number of prints to be printed in a networked office, and with the rapid development of lightweight electrophotographic printing machines. These devices, moreover, are held to strict requirements in terms of being little affected by gases, such as ozone and NOx that are generated in the device, and exhibiting little fluctuation in image characteristics arising from variations in the usage environment (room temperature and humidity).
Recent developments in color printers, and the growing prevalence of the latter, have been accompanied with a need for higher printing speeds, smaller equipment and fewer constituent members, as well as the need for accommodating various usage environments. Under such circumstances, there is a pressing demand for photoconductors that exhibit little variation in image characteristics and electrical characteristics caused by repeated use and/or derived from fluctuations in the usage environment (room temperature and environment). Conventional technologies have thus far failed to meet these requirements simultaneously to a sufficient degree.
Ozone is a widely known example of a gas that is generated in equipment. Ozone is generated by a charger or roller charger that triggers corona discharge. The photoconductor becomes thus exposed to residual ozone or dwelling ozone within the equipment. It is found that the organic substances that make up the photoconductor become oxidized thereby, and, as a result, the original structure of the photoconductor breaks down, and the photoconductor characteristics are significantly impaired. Moreover, it is found that ozone oxidizes the nitrogen in air into NOx, which in turn alters the organic substances that make up the photoconductor.
It is deemed that not only does characteristic deterioration elicited by such gases extend to the outermost layer of the photoconductor, but also adverse effects arise when the gas flows into the interior of the photoconductive layer. It is found that the outermost layer itself of the photoconductor is scraped off, though slightly, on account of friction with the above-described various members. When a harmful gas flows into the interior of the photoconductive layer, the organic substances in the photoconductive layer may undergo structural breakdown. Suppressing the inflow of such harmful gas is thus an issue to be addressed. In tandem-type color electrophotographic apparatuses that rely on a plurality of photoconductors, in particular, variation in color tone occurs as a result of differences in the degree of influence of the gas, depending on, for instance, the position at which drums are disposed in the device. Such variations are deemed to constitute an impediment to forming adequate images. Therefore, it is found that characteristic deterioration caused by gas is a particularly important issue in tandem-type color electrophotographic apparatuses.
The photoconductor surface may also be contaminated by ozone, nitrogen oxides and the like that are generated during charging of the photoconductor. Problems that arise in such a case include, for instance, image smearing by those contaminants themselves, as well as lowered surface lubricity caused by the adhered substances, greater likelihood of adhesion of paper dust and toner, and likelier occurrence of blade squealing, curling, surface scratches and the like.
It is also found that human sebum and the like becomes adhered to the photoconductor surface during repair of the electrophotographic apparatus and during the operation of replacing photoconductor units. Thus far, however, the durability of photoconductors against such contaminant adhesion has been not necessarily sufficient, and surface cracks, as well as image defects such as white spots and black spots occur in some instances when human nose fat or scalp sebum is left adhered to the surface of the photoconductor over long periods of time.
Various methods for improving the outermost surface layer of photoconductors have been proposed in order to solve the above problems. Specifically, various polycarbonate resin structures have been proposed in order to enhance the durability of photoconductor surfaces. For instance, Japanese Patent Application Publication No. 2004-354759 (Patent literature 2) and Japanese Patent Application Publication No. H04-179961 (Patent literature 3) propose polycarbonate resins that comprise a specific structure, but not enough consideration is given to compatibility with various charge transport agents and various additives, or to resin solubility. For instance, Japanese Patent Application Publication No. 2004-85644 (Patent literature 4) proposes a polycarbonate resin that comprises a specific structure; however, a resin having a highly bulky structure includes large spaces between polymers, and thus substances released during charging, as well as contact members and foreign matter, permeate readily into the photoconductive layer, and it is accordingly difficult to achieve sufficient durability. Japanese Patent Application Publication No. H03-273256 (Patent literature 5) proposes a polycarbonate having a special structure, in order to enhance printing durability and coatability, but does not sufficiently disclose additives or charge transport materials that are combined with the polycarbonate. Patent literature 5 is problematic in that maintaining stable electrical characteristics over long periods of time is difficult.
Japanese Patent Application Publication No. 2010-276699 (Patent literature 6) proposes the feature of adding a highly branched polymer and a polymerizable charge transport agent to a surface protective layer, to enhance thereby abrasion properties and transfer properties, but coating solution stability is still an issue. Japanese Patent Application Publication No. 2003-255580 (Patent literature 7) proposes the feature of incorporating, into the surface layer of the photoconductor, a binder resin and a linear vinyl polymer having long-chain alkyl groups at side chains. However, it is found that upon polymerization of a vinyl polymer in a solution in the presence of another binder resin, it is difficult to control the molecular weight and the resin skeleton, due to presence of that other resin. Regarding improvements in wear resistance by a surface protective layer, Japanese Patent Application Publication No. 2011-64734 (Patent literature 8) proposes a technology that involves configuring a surface protective layer that contains a cured product having a three-dimensional crosslinked structure and formed out of a predetermined radically polymerizable compound, a trifunctional or higher functional radically polymerizable monomer, and a radically polymerizable compound having a charge transporting structure, but this configuration is problematic in terms of productivity, since the photoconductive layer has a multilayer structure. Improvements derived from the charge transport layer are an important issue herein.
Transfer current tends to increase in color printers on account of toner color overlap and/or the use of transfer belts. When printing on paper of various sizes, a difference in transfer fatigue arises between portions with paper and portions without paper. This in turn exacerbates differences in image density, which is problematic. In case of frequent printing on small-sized paper, bare photoconductor portions over which the paper does not pass (paper non-passage sections) are continuously and directly affected by transfer, and exhibit greater transfer fatigue than bared photoconductor portions over which paper does pass (paper passage sections). As a result, when printing is subsequently performed on large-size paper, the above discrepancy in transfer fatigue between paper passage sections and paper non-passage sections gives rise to a potential difference in the developed area, which translates into observable differences in density. This trend becomes yet more pronounced as transfer current increases. Under such circumstances, the demand has intensified for photoconductors that exhibit little fluctuation in image characteristics and electric characteristics as a result of repeated use, or on account of fluctuations in the usage environment (room temperature and environment), and that exhibit excellent transfer resiliency, particularly in color printer, as compared to monochrome printers. Conventional technologies have thus far failed to meet these requirements simultaneously to a sufficient degree.
Various additives, such as hindered phenol compounds, phosphorus compounds, sulfur compounds, amine compounds, hindered amine compounds and the like have been proposed to enhance gas resistance. The current situation, however, is that these technologies fail to provide sufficient gas resistance, or, even if satisfactory characteristics are exhibited in terms of gas resistance, no satisfactory results are achieved, through a combination of resins and charge transport materials, regarding electrical characteristics, for instance, responsiveness, image memory and potential stability in endurance printing. The applicants had proposed diester compounds in WO 2011/108064 (Patent literature 9) and Japanese Patent Application Publication No. 2007-279446 (Patent literature 10), but have since made further progress in the study of combinations of more appropriate binder resins and high-mobility charge transport materials.
Various conventional technologies have been proposed pertaining to improvement of the surface layer of photoconductors. However, these technologies as disclosed in the citations above were not all sufficient as regards electrical characteristics such as light response, and also contamination resistance towards sebum, photoconductor productivity and the like.
Therefore, it is an object of the present invention to provide an electrophotographic photoconductor that has excellent contamination resistance and stable electrical characteristics and so forth upon repeated use, and superior transfer resistance and gas resistance, and to provide a method for producing the electrophotographic photoconductor, and an electrophotographic apparatus.