Roll or roller – Concentric layered annulus – Specific composition
Reexamination Certificate
1999-10-26
2002-09-24
Hughes, S. Thomas (Department: 3726)
Roll or roller
Concentric layered annulus
Specific composition
C492S053000, C428S035900
Reexamination Certificate
active
06454688
ABSTRACT:
This application is based on Japanese Patent Application No. 10-310146 filed Oct. 30, 1998, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a charging roll for use in an image forming apparatus such as an electrophotographic copying machine or printer.
2. Discussion of the Related Art
There is known a charging roll which is installed in an image forming apparatus such as an electrophotographic copying machine or printer, such that the charging roll is held in rolling contact with a photosensitive drum for charging the circumferential surface of the photosensitive drum. More specifically described, such a charging roll is used in a so-called “roll charging” method which is one of the known methods for charging a photosensitive drum on which an electrostatic latent image is formed. In the roll charging method, the charging roll to which a charging voltage is applied is held in pressing contact with the outer circumferential surface of the photosensitive drum. The charging roll and the photosensitive drum are rotated together so that the outer circumferential surface of the photosensitive drum is evenly charged by the charging roll.
Since the charging roll is held in rolling contact with the outer circumferential surface of the photosensitive drum, the charging roll is required to have a low hardness (high softness) and high flexibility. Further, the charging roll is required to have a suitable degree of electric conductivity, so as to effectively charge the photosensitive drum.
In recent years, there is an increasing demand for higher image forming capability and excellent energy-saving characteristics of the image forming apparatus, as well as higher process speed and excellent durability of the image forming apparatus. In an attempt to improve the energy-saving characteristics for reducing the electric power to be consumed by the image forming apparatus, the melting point of the toner is lowered for the purpose of fixing the toner on a recording medium at a lower temperature. Further, for improving the performance of the image forming apparatus to provide sufficiently high image quality, the size of the toner particles is made smaller.
However, when the melting point of the toner is lowered or the size of the toner particles is made smaller, the toner undesirably tends to adhere to the charging roll which is inevitably heated in the image forming apparatus during its operation. The toner which adheres or clings to the charging roll causes deterioration of the image forming capability of the image forming apparatus. Described more specifically, with an increase in the number of copying or printing operations, in other words, with an increase in the number of operations to develop the electrostatic latent images into visible toner images on the outer circumferential surface of the photosensitive drum, the electric resistance of the charging roll tends to be raised as a whole due to the adhesion of the toner to the charging roll. Further, uneven adhesion of the toner to local portions of the roll causes a variation in the electric resistance of the roll at the local portions. As a result, the image to be formed or reproduced by the image forming apparatus is undesirably deteriorated.
Recently, there is an increasing demand for an image forming apparatus capable of reproducing a color image. In the reproduction of the color image, a non-magnetic one-component toner is generally used to establish a desired hue or color. The non-magnetic one-component toner tends to be easily charged in the absence of a magnetic component such as magnetite or ferrite therein, which serves as a black-color pigment. Thus, for controlling the amount of charging of the toner, a large amount of finely particulate additives such as SiO
2
, TiO
2
or Al
2
O
3
are added to the toner.
Conventionally, a suitable amount of carbon black or other electrically conductive agent is added to a material which constitutes a surface layer (outermost layer) of the charging roll, so that the surface layer has a desired value of electric resistance. In general, the surface layer of the charging roll is required to have a volume resistivity within a range of about 10
7
-10
15
&OHgr;·cm. For controlling the volume resistivity within such a limited range, there is proposed the use of an electrically conductive agent which does not excessively influence or reduce the electric resistance, so that the electric resistance does not greatly change with a change of the amount of the conductive agent to be added to the material of the surface layer. Thus, the use of such an electrically conductive agent permits the volume resistivity to be finely controlled by adding a large amount of the electrically conductive agent to the material of the surface layer, without necessity of a delicate adjustment of the amount of the conductive agent to be added. However, the carbon black or the large amount of the conductive agent added to the material of the surface layer leads to an increased size of the secondary particles of the carbon black and an increased amount of the conductive agent exposed on the surface of the charging roll. Thus, the surface of the charging roll is activated and considerably roughened with a multiplicity of minute recesses and projections, due to the increased size of the secondary particles and the increased amount of the exposed conductive agent, whereby the toner and additives included in the toner are more likely to stick or adhere to the activated surface of the exposed conductive agent, or to accumulate in the recesses of the rough surface of the charging roll, causing the above-described variation in the electric resistance at the local portions of the surface of the roll, and accordingly making it difficult to establish an even distribution of the electric resistance over the entire surface area of the charging roll. As a result, the image formed or reproduced by the image forming apparatus is likely to be problematically deteriorated.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to minimize a local variation of the electric resistance value of the surface layer of a charging roll, by preventing adhesion or accumulation of toner and additives to or on the outer surface of the charging roll, for preventing deterioration of an image formed by an image forming apparatus on which the charging roll is installed.
It is another object of this invention to provide a charging roll which has an improved ease of control of the electric resistance value, a sufficiently even distribution of the electric resistance value of the surface layer of the charging roll, and improved smoothness of the outer surface of the charging roll.
The above objects may be achieved according to the principle of the present invention which provides a charging roll that is held in rolling contact with a photosensitive drum for charging the photosensitive drum. The charging roll includes an outermost layer formed of a resin composition containing a grafted carbon as an electrically conductive agent. The grafted carbon includes a carbon black and a polymer which is grafted on the carbon black so as to cover a surface of the carbon black.
In a conventional charging roll, a carbon black is used as an electrically conductive agent which is a constituent of the resin composition forming the outermost layer. In the charging roll constructed according to the present invention, on the other hand, the electrically conductive agent is constituted by the grafted carbon which is obtained by grafting the suitable polymer on the carbon black. The carbon black is coated with the grafted polymer, and active points or spots on the surface of the carbon black are accordingly covered by the grafted polymer. This covering of the active spots by the grafted polymer advantageously provides improved dispersibility of particles of the carbon black and also a minimized agglomeration or cohesion of the carbon black particles, leading to a su
Kato Hiroyasu
Maruyama Yuichiro
Okuda Hirofumi
Sugiura Hiroki
Burr & Brown
Hughes S. Thomas
Jimenez Marc
Tokai Rubber Industries Ltd.
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