Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Radiation-sensitive composition or product
Reexamination Certificate
1999-01-27
2003-04-08
Chapman, Mark A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Radiation-sensitive composition or product
C430S058450, C430S058500, C430S059100, C430S083000, C430S970000
Reexamination Certificate
active
06544702
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to charge transport layers which comprise a hydrazone charge transport compound, and which further comprise Savinyl® yellow and an antioxidant. The invention is also directed to photoconductors including such charge transport layers.
BACKGROUND OF THE INVENTION
In electrophotography, a latent image is created on the surface of an imaging member which is a photoconducting material by first uniformly charging the surface and selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between those areas on the surface which are exposed to light and those areas on the surface which are not exposed to light. The latent electrostatic image is developed into a visible image by electrostatic toners. The toners are selectively attracted to either the exposed or unexposed portions of the photoconductor surface, depending on the relative electrostatic charges on the photoconductor surface, the development electrode and the toner.
Typically, a dual layer electrophotographic photoconductor comprises a substrate such as a metal ground plane member on which a charge generation layer (CGL) and a charge transport layer (CTL) are coated. The charge transport layer contains a charge transport material which comprises a hole transport material or an electron transport material. For simplicity, the following discussions herein are directed to the use of charge transport layer which comprises a hole transport material as the charge transport compound. One skilled in the art will appreciate that if the charge transport layer contains an electron transport material rather than a hole transport material, the charge placed on the photoconductor surface will be opposite that described herein.
When the charge transport layer containing a hole transport material is formed on the charge generation layer, a negative charge is typically placed on the photoconductor surface. Conversely, when the charge generation layer is formed on the charge transport layer, a positive charge is typically placed on the photoconductor surface. Conventionally, the charge generation layer comprises the charge generation compound or molecule alone and/or in combination with a binder. The charge transport layer typically comprises a polymeric binder containing the charge transport compound or molecule. The charge generation compounds within the charge generation layer are sensitive to image-forming radiation and photogenerate electron hole pairs therein as a result of absorbing such radiation. The charge transport layer is usually non-absorbent of the image-forming radiation and the charge transport compounds serve to transport holes to the surface of a negatively charge photoconductor. Photoconductors of this type are disclosed in the Adley et al U.S. Pat. No. 5,130,215 and the Balthis et al U.S. Pat. No. 5,545,499.
The Anderson et al U.S. Pat. No. 4,150,987 discloses a layered electrophotographic plate or element having a conventional charge generation layer and a charge transport layer containing p-type hydrazone. While the Anderson et al photoconductor is particularly good for use in electrophotography processes, it has been found that prolonged exposure to ambient light, and particularly to cool-white fluorescent light usually found in offices, may decrease the photosensitivity of the photoconductor. This is commonly referred to in the art as room light fatigue (RLF). Exposure of such photoconductors to cool-white ambient fluorescent lighting, even for just a few minutes, results in a significant shift in the residual voltage, commonly referred to as fatigue. This shift in residual potential means that factors such as print density and background density will be different on a print made from the fatigued drum when compared to the last print made before fatiguing this drum. Hence, when a machine is opened for the slightest reason, for example to clear a paper jam, ambient fluorescent light can enter and damage the photoconductor.
Typically, room light fatigue does not occur in high speed duplicators, since experienced, well-trained operators commonly service such devices and do not expose the photoconductor to ambient light for prolonged periods. However, room light fatigue typically occurs in low speed copiers since such copiers are often attended by operators having little or no training.
A number of experiments have suggested that room light fatigue is caused by a hydrazone transport compound undergoing syn-anti isomerization about the hydrazone C═N double bond. The preferred hydrazone molecule, p-diethylaminobenzaldehyde-(diphenylhydrazone) (DEH), represented by the structural formula (I), has been found to experience an undesirable change in light sensitivity when exposed to conventional cool-white fluorescent room light for 15 minutes or more.
The suggestion of a syn-anti isomerization has led to various approaches in the art to prevent this isomerization. One of the first approaches was the “sunblock” approach as disclosed by Nakasawa et al, Japanese Patent Reference No. 63-271453. Just as a sunscreen retards light absorption by human skin pigments, it was suggested that incorporating a molecule that absorbs at the cool-white fluorescent wavelength would prevent this isomerization. However, large amounts of the light-absorbing molecule were typically required in order to absorb most of the damaging radiation and resulted in a marked decrease in photosensitivity as charge generation molecule (CGM) and charge transport molecule (CTM) concentrations were correspondingly reduced. Hence, this was not a viable approach to an RLF-protected, yet fully functional, photoconductor.
Additional studies in the art have involved the addition of a molecule that could quench the excited singlet state of the hydrazone CTM, thereby preventing the syn-anti photoisomerization which retards RLF. The Maeda et al European Patent Publication No. 041338A1 discloses that addition of a fluorenone derivative compound at a 1-5% level has afforded some RLF protection, while not compromising electrical performance.
However, a need remains for hydrazone-containing photoconductors which exhibit reduced room light fatigue.
SUMMARY OF THE INVENTION
Accordingly, it is the object of the present invention to provide novel charge transport layers which overcome disadvantages of the prior art. It is a more specific object of the invention to provide charge transport layers which reduce or eliminate the room light fatigue exhibited in conventional hydrazone-containing organic photoconductors.
These and additional objects are provided by charge transport layers and photoconductors of the present invention. The charge transport layers comprise a hydrazone charge transport compound, Savinyl® yellow and an ester-containing antioxidant. In one embodiment of the invention, the antioxidant comprises octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl) proprionate or pentaerythritol tetrakis (3,5-di-tert-butyl-4-hydroxy hydrocinnamate). The photoconductors comprise a substrate, a charge generation layer, and a charge transport layer, wherein the charge transport layer comprises a hydrazone charge transport compound, Savinyl® yellow, and an ester-containing antioxidant.
The charge transport layers according to the present invention surprisingly reduce or eliminate the room light fatigue exhibited by conventional hydrazone based organic photoconductors. While not being limited by theory, it is believed that the charge transport layers containing the hydrazone charge transport compound, the Savinyl® yellow and an ester-containing antioxidant in combination preclude the undesired syn-anti photoisomerization of the hydrazone charge transport compound, and therefore substantially eliminate room light fatigue. These and additional objects and advantages will be more readily apparent in view of the following detailed description.
DETAILED DESCRIPTION
The charge transport layers according to the present invention are suitable for use in dual layer photoconductors. Such
Haggquist Gregory Walter
Levin Ronald Harold
Mosier Scott Thomas
Brady John A.
Chapman Mark A.
Lexmark International Inc.
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