Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Radiation-sensitive composition or product
Reexamination Certificate
2002-07-23
2003-11-11
Goodrow, John (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Radiation-sensitive composition or product
C430S058600, C430S058650
Reexamination Certificate
active
06645686
ABSTRACT:
FIELD
The present invention relates generally to electrophotographic imaging members. The invention relates particularly to electrophotographic imaging members in which crystallization of charge transport molecules is eliminated or minimized.
BACKGROUND
Typical electrophotographic imaging members (for example, photoreceptors) comprise a photoconductive layer comprising a single layer or composite layers. One type of composite photoconductive layer used in xerography is illustrated, for example, in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. The disclosure of this patent is incorporated herein in its entirety. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer the photogenerating layer is sandwiched between the contiguous charge transport layer and the supporting conductive layer, the outer surface of the charge transport layer is normally charged with a uniform electrostatic charge. The photosensitive member is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in illuminated areas of the photosensitive member 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 electrostatic toner particles on the surface of the photosensitive member. The resulting visible toner image can be transferred to a suitable receiving material such as paper. This imaging process may be repeated many times with reusable photosensitive members.
As more advanced, complex, highly sophisticated, electrophotographic copiers, duplicators and printers were developed, greater demands were placed on the photoreceptor to meet stringent requirements for the production of high quality images.
One type of multi-layered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a charge blocking layer a charge generating layer, and a charge transport layer. The charge transport layer often comprises an activating small molecule dispersed or dissolved in a polymeric film forming binder. Generally, the polymeric film forming binder in the transport layer is electrically inactive by itself and becomes electrically active when it contains the activating molecule. The expression “electrically active” means that the material is capable of supporting the injection of photogenerated charge carriers from the material in the charge generating layer and is capable of allowing the transport of these charge carriers through the electrically active layer in order to discharge a surface charge on the active layer. The multi-layered type of photoreceptor may also comprise additional layers such as an anti-curl backing layer, required when layers possess different coefficient of thermal expansion values, an adhesive layer, and an overcoating layer. Commercial high quality photoreceptors have been produced which utilize an anti-curl coating. Although excellent toner images may be obtained with multi-layered belt photoreceptors that are developed with dry developer powder (toner), it has been found that these same photoreceptors become unstable when employed with liquid development systems. These photoreceptors suffer from cracking, crazing, crystallization of active compounds, phase separation of activating compounds and extraction of activating compounds caused by contact with the organic carrier fluid, isoparaffinic hydrocarbons e.g. ISOPAR™, commonly employed in liquid developer inks which, in turn, markedly degrade the mechanical integrity and electrical properties of the photoreceptor. More specifically, the organic carrier fluid of a liquid developer tends to leach out activating small molecules, such as the arylamine containing compounds typically used in the charge transport layers. Representative of this class of materials are: N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine; bis-(4-diethylamino-2-methylphenyl)-phenylmethane; 2,5-bis-(4′-dimethylaminophenyl)-1,3,4-oxadiazole; 1-phenyl-3-(4′-diethylaminostyryl)-5-(4″-diethylaminophenyl)pyrazoline; 1,1-bis-(4-(di-N,N′-p-methylphenyl)-aminophenyl)-cyclohexane; 4-diethylaminobenzaldehyde-1,1-diphenylhydrazone; 1,1-diphenyl-2(p-N,N-diphenylamino phenyl)-ethylene; N-ethylcarbazole-3-carboxaldehyde-1-methyl-1-phenylhydrazone. In embodiment, the transport layer for typical photoreceptors, such as those described in U.S. Pats. Nos. 6,190,818 and 6,214,514, herein incorporated by reference, comprise mainly m-TBD (N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine) transport molecules and MAKROLON™ (biphenyl A polycarbonate) binders. Other additives such as antioxidant IRGANOX™ have been added for lateral charge migration control. This formulation was chosen because m-TBD has superior hole transport characteristics and MAKROLON™ are m-TBD compatible binders. Both the superior hole transport characteristics of the transport molecules and the compatibility of the binders are required for optimum photoreceptor functioning and manufacturing feasibility. A disadvantage of this specific transport layer formulation is that, as a result of using such a high concentration of m-TBD to provide optimum mobility in the transport layer, the m-TBD is more susceptible to crystallization. This has potential adverse effects on photoreceptor performance, such as photoreceptor sensitivity, stability, cracking, wear and other printable defects. Moreover, MAKROLON™ is in short supply and, therefore, other alternative binders are deemed necessary. New challenges for longer life photoreceptors and better print quality have made it necessary to continue improving photoreceptors.
Photoreceptors have been developed, such as those described in U.S. Pat. No. 5,055,366, which comprise a further protective overcoating layer that inhibits crystallization of the charge transport material.
Photoreceptors have been developed which comprise charge transfer complexes prepared with polymeric molecules. For example, charge transport complexes formed with polyvinyl carbazole are disclosed in U.S. Pat. Nos. 4,047,948, 4,346,158 and 4,388,392. Photoreceptors utilizing polyvinyl carbazole layers, as compared with current photoreceptor requirements, exhibit relatively poor xerographic performance in both electrical and mechanical properties. Polymeric arylamine molecules prepared from the condensation of di-secondary amine with a di-iodo aryl compound are disclosed in European patent publication 34,425, published Aug. 26, 1981 and issued May 16, 1984. Since these polymers are extremely brittle and form films which are very susceptible to physical damage, their use in a flexible belt configuration is precluded. Thus, in advanced imaging systems utilizing multi-layered belt photoreceptors exposed to liquid development systems, cracking and crazing have been encountered in critical charge transport layers during belt cycling. Cracks developing in charge transport layers during cycling can be manifested as print-out defects adversely affecting copy quality. Furthermore, cracks in the photoreceptor pick up toner particles which cannot be removed in the cleaning step and may be transferred to the background in subsequent prints. In addition, crack areas are subject to delamination when contacted with blade cleaning devices thus limiting the options in electrophotographic product design. It should also be noted that the presence of an anti-curl back coating will exacerbate the propagation of cracks in brittle polymers.
Recently photoreceptors having charge transport layers containing charge transporting arylamine polymers have been described in the patent literatur
Carmichael Kathleen M.
Evans Kent J.
Fu Min-Hong
Helbig Colleen A.
Schneider June E.
Goodrow John
Palazzo E. O.
Xerox Corporation
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