1. Field of the Invention
The present invention relates to an electrophotographic photoconductor comprising an electroconductive support and a photoconductive layer formed thereon, containing lignin.
2. Discussion of Background
Conventionally, inorganic materials such as selenium, cadmium sulfide and zinc oxide are used as photoconductive materials for electrophotographic photoconductors for use in electrophotography. Electrophotography is one of the image formation processes, through which the surface of a photoconductor is charged uniformly in the dark to a predetermined polarity, for instance, by corona charge. The uniformly charged photoconductor is exposed to a light image to selectively dissipate the electric charges of the exposed areas, so that a latent electrostatic image is formed on the photoconductor. The thus formed latent electrostatic image is developed into a visible image with a developer comprising a coloring agent such as a dye or a pigment, and a binder agent such as a polymeric material.
Fundamental characteristics required for the photoconductor for use in such electrophotography are: (1) chargeability to an appropriate potential in the dark, (2) minimum dissipation of electric charges in the dark, and (3) rapid dissipation of electric charges when exposed to light.
However, while the above-mentioned inorganic materials have many advantages over other materials, they have several shortcomings from the viewpoint of practical use.
For instance, a selenium photoconductor, which is widely used at present, satisfies the above-mentioned requirements (1) to (3) completely, but it has the shortcomings that its manufacturing conditions are difficult and, accordingly, its production cost is high. In addition, it is difficult to work it into the form of a belt due to its poor flexibility. Furthermore, the selenium photoconductor is so vulnerable to heat and mechanical shocks that it must be handled with the utmost care.
A cadmium sulfide photoconductor and a zinc oxide photoconductor can be respectively produced by dispersing cadmium sulfide particles and zinc oxide particles in a binder resin. However, they are poor in mechanical properties such as surface smoothness, hardness, tensile strength and wear resistance. Therefore, they cannot be used repeatedly, as they are.
To solve the problems of such inorganic materials, various electrophotographic photoconductors employing organic materials are recently proposed. For example, there are known a photoconductor comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluorenone-9-on as described in U.S. Pat. No. 3,484,237; a photoconductor prepared by sensitizing poly-N-vinylcarbazole with a pyrylium-salt-based pigment as described in Japanese Patent Publication 48-25658; a photoconductor comprising as a main component an organic pigment as described in Japanese Laid-Open Patent Application 47-37543; a photoconductor comprising as a main component a eutectic crystal complex composed of a dye and a resin as described in Japanese Laid-Open Patent Application 47-10735; a photoconductor prepared by sensitizing a triphenylamine compound with a sensitizer pigment as described in U.S. Pat. No. 3,180,730; a photoconductor comprising an amine derivative as a charge transporting material as described in Japanese Laid-Open Patent Application 57-195254; a photoconductor comprising poly-N-vinylcarbazole and an amine derivative as charge transporting materials as described in Japanese Laid-Open Patent Application 58-1155; and a photoconductor comprising as a photoconductive material a polyfunctional tertiary amine compound, in particular, a benzidine compound, as described in U.S. Pat. No. 3,265,496, Japanese Patent Publication 39-11546 and Japanese Laid-Open Patent Application 53-27033.
These organic electrophotographic photoconductors are still unsatisfactory for use in practice, especially with respect to the durability thereof. As the demand for a photoconductor with higher durability is increasing year by year, ensuring the charging stability in the photoconductor has become a requirement that cannot be ignored. When a photoconductor with a decreased charging stability is employed, in the case of an electrophotographic copying machine, the lowering of image density is caused; and in the case of a laser printer employing the reversal development method, image quality is lowered, for instance, with the occurrence of toner deposition on the background of printed images. In order to solve these problems, it is proposed to provide an intermediate layer between an electroconductive support and a photoconductive layer. However, in the case where a material with high resistivity, having high barrier properties, is employed for the intermediate layer, the photosensitivity of the photoconductor decreases and the residual potential thereof increases although the charging characteristics are improved. When a material with relatively low resistivity is employed for the intermediate layer to prevent the increase of the residual potential, the charging stability of the photoconductor is still insufficient.
When a photoconductor is incorporated into a copying machine in practice, the photoconductor is exposed to ozone generated from a corona charger. By the ozone thus generated, organic materials such as a charge transporting material contained in a photoconductive layer of the photoconductor are oxidized, so that there are caused the problems that the photosensitivity is decreased, the residual potential is increased, and the charging potential is decreased.
To solve such problems, the addition of an antioxidant to the photoconductive layer is proposed as in Japanese Laid-Open Patent Applications 57-122444, 50-33857, 63-18355, 63-18356 and 3-172852; and the provision of a gas-barrier resin layer on the charge transport layer is also proposed as in Japanese Laid-Open Patent Application 63-135955.
The above-mentioned countermeasures, however, have not yet solved the problems of the conventional photoconductors, such as the increase of the residual potential, the lowering of the photosensitivity, and the insufficient improvement of the durability thereof.