Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Radiation-sensitive composition or product
Reexamination Certificate
2001-03-27
2003-02-11
Rodee, Christopher (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Radiation-sensitive composition or product
C430S066000, C430S067000
Reexamination Certificate
active
06517984
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to electrophotography and, more particularly, to photoreceptors having silsesquioxane overcoats that contain hydroxysubstituted hole transport agents.
BACKGROUND OF THE INVENTION
In charge generating elements, incident light induces a charge separation across various layers of a multiple layer device. In an electrophotographic charge generating element, also referred to herein as an electrophotographic element, an electron-hole pair produced within a charge generating layer separate and move in opposite directions to develop a charge between an electrically conductive layer and an opposite surface of the element. The charge forms a pattern of electrostatic potential, also referred to as an electrostatic latent image. The electrostatic latent image can be formed by a variety of means, for example, by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is developed by contacting it with an electrographic developer to form a toner image, which is then fused to a receiver. If desired, the latent image can be transferred to another surface before development, or the toner image can be transferred before fusing.
The requirements of the process of generating and separating charge place severe limitations on the characteristics of the layers in which charge is generated and holes and/or electrons are transported. For example, many such layers are very soft and subject to abrasion. This places severe constraints on the design of charge generating elements. Some configurations cannot provide a reasonable length of service unless an abrasion resistant overcoat layer is provided over the other layers of the element. This presents its own problems, since charge must be able to pass through the overcoat.
The resistivity of an overcoat has major consequences in an electrophotographic system. If the overcoat has high resistivity, the time constant for voltage decay will be excessively long relative to the processing time for the electophotographic element, and the overcoat will retain a residual potential after photodischarge of the underlying photoreceptor. The magnitude of the residual potential depends upon the initial potential, the dielectric constants of the various layers, the thickness and the charge transport characteristics of each layer. A solution has been to reduce the thickness of the overcoat layer. Another solution is to provide an overcoat that is conductive. The overcoat must, however, not be too conductive. The electrophotographic element must be sufficiently electrically insulating in the dark that the element neither discharges excessively nor allows an excessive migration of charge along the surface of the element. An excessive discharge (“dark decay”) would prevent the formation and development of the latent electrostatic latent image. Excessive migration causes a loss of resolution of the electrostatic image and the subsequent developed image. This loss of resolution is referred to as “lateral image spread.” The extent of image degradation will depend on the processing time for the electrophotographic element and the thicknesses and dielectric constants of the layers. It is thus desirable to provide an overcoat that is neither too insulating nor too conductive.
The triboelectric properties of the overcoat must be matched to the triboelectric characteristics of the electrophotographic toner used to develop the electrostatic latent image. If the triboelectric properties are not matched, the electrophotographic element will triboelectrically charge against the electrophotographic toner. This causes disruption of the charge pattern of the electrostatic latent image and results in background in the resulting toner image. For example, an overcoat can triboelectrically match a particular negatively charging toner, but not triboelectrically match another toner that charges positively.
In an electrophotographic process, an organic photoreceptor is subjected to a variety of physical and chemical abuses that may limit its productive lifetime. As already noted, the surface of an organic photoreceptor is relatively soft, so that cleaning, by blade or brush, causes scratches and abrasive wear. Unintended contacts of the surface with sharp objects may result in scratches that necessitate immediate photoreceptor replacement. The photoreceptor surface is also relatively permeable and its components are reactive towards the ozone and nitrogen oxides generated during corona charging. After extended exposure to such chemicals, the electrophotographic characteristics may degrade to the point where image defects become objectionable and the photoreceptor must be replaced. Organic photoreceptors are also susceptible to photochemical damage from ultraviolet radiation emitted from the corona discharge or from exposure to room light. As a result of these factors, the lifetime limit of an organic photoreceptor is on the order of one hundred thousand cycles. By contrast, a lifetime of one million cycles is typical of the much harder amorphous selenium and arsenic triselenide photoreceptors. Extensive efforts have therefore been made to protect organic photoreceptors from physical, chemical, and radiation damage, as disclosed, for example, in U.S. Pat. Nos. 5,204,201; 4,912,000; 4,606,934; 4,595,602; 4,439,509; and 4,407,920. The protection of organic photoconductors using an overcoat comprising various polysiloxane mixtures in a polycarbonate resin is described in U.S. Pat. No. 6,030,736.
Silsesquioxanes are a class of silicone polymers that are useful as abrasion resistant overcoats, including overcoats for organic photoreceptors. Overcoating an organic photoreceptor with a silsesquioxane layer can provide protection from physical, chemical, and radiation damage. Silsesquioxane layers are harder than organic photoreceptors and less permeable to chemical contaminants. Silsesquioxanes can be imbibed with acid scavengers to keep contaminants, such as acids, from reaching the photoreceptor surface. Also, dyes can be added to silsesquioxane layers to protect the photoreceptor from photofatigue, especially from room lights.
A silsesquioxane layer would also be expected to increase the efficiency of particle transfer from the photoreceptor surface. The surface energies of silsesquioxane layers are lower than those of organic polymers and, in addition, are typically smooth and hard, as measured by the higher moduli than those of polyesters and polycarbonates. These factors combine to make silsesquioxanes good release coatings, which should aid in toner transfer, an increasingly significant consideration as toner particle size decreases to meet the demands of higher image resolution. Silsesquioxane overcoats for organic photoreceptors are disclosed in, for example, U.S. Pat. Nos. 5,731,117; 5,693,442; 5,874,018; and 6,066,425.
Charge transport materials (CTMs) are generally added to polymeric layers to transport charge in organic photoreceptors. These layers are generally insulators that carry charge when either holes or electrons are injected into them. U.S. Pat. No. 3,542,544 discloses triphenylmethanes and tetraphenylmethanes substituted with dialkylamines as CTMs that are incorporated into photoconductive elements. Triphenylmethane CTMs containing hydroxyaniline groups to facilitate incorporation into polymer structures such as polyamide film-forming overcoats arylamines are described in U.S. Pat. No. 5,368,967. Electrophotographic photoreceptors in which triarylamine compounds with dihydroxy substituents are covalently bonded into polycarbonate resins are disclosed in U.S. Pat. No. 5,747,204. The incorporation of triarylamines in a functional subunit of a composition that also includes an inorganic glassy network subunit and a flexible organic subunit is discussed in U.S. Pat. No. 5,116,703. Imaging members containing hole transporting polysilylene ceramers are described in U.S. Pat. No. 4,917,980.
The incorporation of tertiary arylamines into silsesquioxane p
Cowdery-Corvan Jane Robinson
Ferrar Wayne Thomas
Gruenbaum William Tod
Kaeding Jeanne Ellen
Molaire Michel Frantz
Heidelberger Druckmaschinen AG
Rodee Christopher
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