1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member. More particularly, the present invention relates to an electrophotographic photosensitive member having a surface layer containing a resin of a specified structure. The present invention further relates to an electrophotographic apparatus and a device unit employing the above electrophotographic photosensitive member.
2. Related Background Art
In recent years, many proposals have been presented for electrophotographic photosensitive members employing an organic photoconductive material because of non-pollution, high productivity, ease of material design, and future prospect thereof. Some of them are practically used.
The electrophotographic photosensitive member is required to have electric, mechanical, and optical properties satisfying the electrophotographic process to which the member is applied. In particular, for the photosensitive member for repeating use what is required is durability against external electric or mechanical force applied to the surface of the member during the process of corona charging, toner development, image transfer onto paper, and surface cleaning. Specifically, the photosensitive member is required not to undergo deterioration, due to ozone evolved during corona charging, such as sensitivity reduction, rise of residual potential, and decrease of chargeability and to be resistant to abrasion and scratching on the surface caused by sliding friction during image transfer and surface cleaning process.
Since the surface of the photographic member is a layer composed mainly of a resin, the properties of the resin greatly affect the properties of the surface of the photosensitive member. Conventionally, a polycarbonate resin which has a skeleton unit of bisphenol A (hereinafter referred to as "bisphenol A type polycarbonate") is employed as the resin satisfying the above requirements.
The bisphenol A type polycarbonate resin, however, does not satisfy all the requirements for the resin for photosensitive members. The bisphenol A type polycarbonate resin involves the problems below:
(1) The resin is poor in solubility, and is soluble in practice only in limited solvents, namely some halogenated aliphatic hydrocarbons such as dichloromethane and 1,2-dichloroethane. Since such halogenated aliphatic hydrocarbons have a low boiling point, the coating layer is liable to whiten when a photosensitive member is produced by use of a coating solution prepared with such a solvent, and the control of the process is troublesome in adjustment of the solid content of the coating solution. PA0 (2) The resin is partly soluble in a few solvents other than the halogenated aliphatic hydrocarbons, such as tetrahydrofuran, dioxane, cyclohexanone, and the mixed solvents thereof. However, these solutions are unstable in storage, turning into gels within a few days and are not suitable for production of photosensitive members. PA0 (3) A polycarbonate having a main chain skeleton consisting only of bisphenol A or a bisphenol A derivative is liable to cause solvent cracking. PA0 (4) A film formed from a conventional polycarbonate resin does not have sufficient lubricity, and is liable to be scratched which causes image defects. The film tends to give insufficient cleaning caused by rapid deterioration of the cleaning blade or turn-over of the cleaning blade to shorten the life of the photosensitive member. Further, the resin has relatively high surface free energy, which induces sticking of a developing agent onto the surface of the photosensitive member causing spot-like image defects.
Hitherto, the problems of solution stability mentioned in the above items (1) and (2) are solved by use of a polycarbonate Z resin having a bulky cyclohexylene group as the polymer structure unit. However, as indicated in the above item (3), the polycarbonate Z resin and the polycarbonate A resin exhibit relatively large shrinkage on forming a coating film with the solution by casting, which sometimes causes residual stress in the interior of the coating film, rendering the above resins less resistant to stress corrosion. To solve this problem, Japanese Laid-Open Patent No. 61-62040 discloses the use of a mixture of the polycarbonate A resin with the polycarbonate Z resin to suppress stress cracking, and Japanese Laid-Open Patent No. 61-62039 discloses copolymerization of bisphenol A and bisphenol Z to suppress the cracking. These methods, however, are still unsatisfactory to give the resin sufficient resistance against solvent cracking.
As mentioned in the above item (4), an ordinary polycarbonate resin exhibits relatively low lubricity to the cleaning blade used in the electrophotographic process. The low lubricity causes turning-over of the cleaning blade due to the deterioration of the cleaning blade, thereby leading to insufficient cleaning. Further the low lubricity often results in scratch formation on the photosensitive member surface owing to formation on strong force applied thereto. To solve this problem, silicone oil is applied, or as disclosed in Japanese Laid-Open Patent No. 61-132954, polydimethylsiloxane blocks are incorporated into the polycarbonate resin by copolymerization.
The addition of the silicone oil has disadvantages in that electric characteristics, specifically sensitivity and residual potential characteristics are impaired by the silicone oil, and the silicone oil is lost during repeated use and thereby the lubricity decreases. The aforementioned copolymerization of the polydimethylsiloxane block gives relatively good lubricity to the surface layer of the photosensitive member. However, conventional polydimethylsiloxane copolymers, when dissolved in a solvent, tend to become turbid and white, or tend to turn into a gel, and do not give satisfying durability for the surface layer of an electrophotographic photosensitive member.
Furthermore, in recent years, a low molecular compound such as a charge-transporting substance is often incorporated in the photosensitive layer in a large amount to obtain the high sensitivity required for the organic electrophotographic photosensitive member. The low molecular substance may deposit during a long term of storage of such photosensitive member to cause layer separation.
The present invention solves the above problems of conventional electrophotographic photosensitive members.