1. Field of the Invention
The present invention relates to semiconductive rubber compositions in which the electrical resistance is easily set, is uniform and stable, and is not likely to be influenced by environmental changes in temperature, humidity, and the like, and more particularly, relates to a semiconductive rubber composition preferably forming an elastic layer of a conductive member in pressure-contact with a photosensitive member in the steps of charging, developing, transferring, and the like performed in an electrophotographic image forming apparatus, such as a copying machine, a printer, or a facsimile machine.
2. Description of the Related Art
In an image forming apparatus, that is, in an electrostatic recording apparatus, an electrophotographic apparatus, and the like, such as a copying machine and an optical printer, as an apparatus for charging the surface of an image supporting member, such as a photosensitive member, a dielectric member, or the like, a corona discharge apparatus has been widely used.
The corona discharge apparatus is effective for uniformly charging the surface of the member to be charged, such as an image supporting member, to a predetermined potential; however, there have been problems in that the apparatus is large since an expensive high-voltage power source is required, a large amount of corona products such as ozone is formed during discharge, the surface of the member to be charged is damaged by abnormal discharge, and the like.
In addition to the corona discharge apparatus described above, recently, a contact charging method tends to be employed. In the contact charging method, a charging member to which a voltage is applied (hereinafter referred to as a charging member) is placed in contact with or is placed in the vicinity of the surface of the member to be charged so as to perform a charging treatment on the surface thereof. Compared to the corona discharge method, the advantages of the contact charging method are that the formation of the corona products such as ozone is suppressed, cost reduction or miniaturization of the apparatus can be achieved due to the simple structure thereof, damage of the surface of the member to be charged caused by abnormal discharge is decreased, and the like. In general, a charging member in the form of a rubber roller composed of a semiconductive elastic layer formed around a metal shaft is used.
In order to prevent leakage caused by pinholes or damages on the surface of the members to be charged such as a photosensitive member, an elastic layer of the charging member used in the contact charging method must have an appropriate electrical conductivity. In addition, in order to uniformly charge the member to be charged, it is important that the charging member has uniform semiconductive properties having an intrinsic volume resistivity of approximately 1xc3x97103 to 1xc3x97109 xcexa9xc2x7cm. In order to realize the electrical properties described above, heretofore, an electron conductive rubber material containing electron conductive particles such as an electron conductive carbon black has been used for forming an elastic layer having semiconductive properties.
The electrical conductivity can be controlled by adjusting the amount of conductive particles such as a conductive carbon black added to a raw rubber for forming the electron conductive rubber material described above; however, an electrical resistance in a semiconductive region in which the intrinsic volume resistance is in the range of from 1xc3x97103 to 1xc3x97109 xcexa9xc2x7cm may be considerably varied by a small change in addition amount of the conductive particles in some cases. In the case described above, it becomes difficult to form an elastic rubber layer having a uniform and a predetermined electrical resistance in the semiconductive region, and as a result, the electrical resistance in the charging member and between the charging members is easily varied.
In addition, in the electron conductive rubber material, the electrical conductivity largely depends on the distance between the conductive particles. Accordingly, since the charge between the conductive particles is more easily transferred due to the electric field effect when an applied voltage is increased, the voltage dependence of the electrical resistance is increased, and as result, a uniform image may not be obtained in some cases even when a stable current flows.
A contact charging member provided with a conductive layer containing a conductive pigment and a polymeric elastomer is disclosed in U.S. Pat. No. 2,705,780, in which the conductive layer contains two polymeric elastomers therein, i.e., a polymeric elastomer A and a polymeric elastomer B having an affinity to the conductive pigment lower that that of the polymeric elastomer A, and in which the amount of the conductive pigment contained in an area formed of the polymeric elastomer A is larger than that in an area formed of the polymeric elastomer B.
In this rubber material described above, according to the publication described above, two types of polymeric elastomer portions uniformly coexist. One polymeric elastomer portion, which is obtained by uniformly dispersing the conductive pigment in a polymeric elastomer having a high affinity thereto, has a low resistance but has a small variation thereof, and the other polymeric elastomer portion has a high resistance and a low affinity to the conductive pigment and contains no conductive pigment or a relatively small amount thereof. Accordingly, as a whole, a contact charging member can be obtained having a small variation in charging properties in an intermediate resistance region.
However, in the structure in which the conductive pigment is primarily contained in a polymeric elastic portion forming domain portions, since a polymeric elastomer forming a continuous phase portion has a high resistance, the electrical conductivity between domains depends on a voltage applied thereto, and hence, the voltage dependence of the resistance of the charging member tends to increase.
In addition, as a method for forming a rubber composition having a uniform resistance and a small voltage dependence of the resistance, it is known that an elastic layer is formed of a polar rubber having its own semiconductive properties, such as an epichlorohydrine rubber, and an ion conductive rubber material such as a rubber composition containing an ion conductive agent so as to have semiconductive properties. However, in the case described above, since the intrinsic volume resistivity of the elastic layer is varied in accordance with the environmental change in temperature or humidity, the electrical properties may be varied depending on environmental conditions in some cases.
In addition, in the ion conductive rubber material, even though a large amount of ion conductive agent is contained, an intrinsic volume resistivity of 1xc3x97105 xcexa9xc2x7cm or less may be difficult to obtain in some cases, and an image defect may occur by contamination caused by the ion conductive agent transferred to the photosensitive member in some cases.
Accordingly, an object of the present invention is to provide a semiconductive rubber composition comprising a polymeric dispersed phase containing conductive particles, that is, is to provide a semiconductive rubber composition containing a polymeric dispersed phase having electron conductive properties. The semiconductive rubber composition has superior electrical properties, such as a small variation in electrical resistance in a voltage application, uniform electrical properties, little change in electrical properties due to an environmental change in temperature, humidity, and the like, and stable electrical properties with time. In addition, the semiconductive rubber composition described above can be used for forming a charging member which prevents contamination of the member to be charged, such as a photosensitive member.
In addition, another object of the present invention is to provide an electrophotographic apparatus and a process cartridge, which are provided with the charging member described above.
To these ends, a semiconductive rubber composition according to the present invention has a dispersed domain structure, in which the dispersed domain structure comprises a polymeric continuous phase comprising an ion conductive rubber material and a polymeric dispersed phase comprising an electron conductive rubber material. In the semiconductive rubber composition described above, the ion conductive rubber material primarily contains a raw rubber A having an intrinsic volume resistivity of 1xc3x971012 xcexa9xc2x7cm or less, and the electron conductive rubber material contains a raw rubber B and conductive particles mixed therewith.
According to the present invention, a charging member to which a voltage is applied and which is brought into contact with a member to be charged so as to charge the member comprises an elastic layer, wherein the elastic layer comprises the semiconductive rubber composition described above.
In addition, an electrophotographic apparatus according to the present invention comprises a charging member and an electrophotographic photosensitive member, wherein the charging member is the charging member described above.
Furthermore, a process cartridge according to the present invention comprises an electrophotographic photosensitive member and a charging member, wherein the electrophotographic photosensitive member is assembled integrally with the charging member so as to form a cartridge which is detachably mountable to a main image forming apparatus, and the charging member is the charging member described above.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments (with reference to the attached drawings).