Various electrophotographic processes have been developed and have achieved wide acceptance. Generally, these processes utilize a photoconductive surface, typically on a drum, to transfer the image of the subject matter to the copy medium. The photoconductive surface is employed to convert the optical image to an electrical image. See, for example, "A Comparison of Some Electrophotographic Processes Based on Photoconductor-Insulator Layer Combinations," in the Volume 18, No. 3, May-June issue of Photographic Science and Engineering.
A great deal of effort has been directed towards producing photoconductive surfaces which are sensitive, durable and highly uniform in their response to the optical image. For example, see U.S. Pat. No. 3,151,982 issued on Oct. 6, 1964 to Lester Corrsin, and entitled XEROGRAPHIC PLATE and also U.S. Pat. No. 3,837,906 issued on Sept. 24, 1974 to Robert N. Jones and entitled METHOD OF MAKING A XEROGRAPHIC BINDER LAYER AND LAYER SO PREPARED.
Some attempts to increase the uniformity of the application of the photoconductive material to the surface of the drum have been directed towards the use of an enamel slip. See, for example, the above-mentioned patent to Corrsin. Basically, these techniques tried in the prior art have emulated techniques used in the enamelling industry. These prior art enamel slips were water-based and employed relatively low photoconductor concentrations, e.g., 4% to 20% of the weight of the solids. These enamel slips exhibited several disadvantages when they were used to prepare a photoconductive surface. One of these disadvantages was the requirement of critical drying and firing schedules to avoid cracking of the surface during the curing process. Another disadvantage of prior art enamel slips was the deleterious effect the water contained therein had on the photoconductive material. Also, using these water-based compounds, it was not possible to produce an enamel slip with the high pigment concentrations desired for electrophotographic uses, e.g., 50% by weight of the solids. Prior art attempts to produce enamel slips with such high concentrations of photoconductive material resulted in severe cracking and other poor surface characteristics. For example, see the above-mentioned U.S. Pat. No. 3,151,982 wherein it is stated that in photoconductive surfaces prepared using an aqueous base enamel slip wherein the pigment concentration exceeded 15%, the result was a rough surface with poor adhesion and poorly suited for electrophotography. There have been some suggestions in the literture that this can be overcome by adding elements such as water-glass, i.e. sodium silicate, to hold the coating together, but this would have detrimental effects on the performance of the photoconductor material due to the conductivity of the sodium silicate and by contamination from Group I metals. Similarly, miscellaneous chemical additives generally used in enamel either as adhesive or suspension agents contain Group I metals such as sodium and potassium which are electrically active and interfere with the operation of the photoconductor particles in the coating. Hence, there were no prior art solutions to making satisfactory photoconductive coatings by spraying an enamel slip. Other prior art techniques used to prepare photoconductive surfaces involved the reverse roller coating of discontinuous surfaces, but these techniques resulted in unstable and fragile coatings.
In accordance with the preferred embodiment of the present invention, a photoconductive surface coating suitable for use with the various methods of electrophotography is provided by spraying an enamel slip on a surface to provide a continuous film coating of photoconductive material. The enamel slip comprises a non-aqueous carrier vehicle, a photoconductive powder, a glass enamel frit, and miscellaneous additives. The technique is particularly useful in making films which are resistant to moisture under conditions which might tend to abrade the surface of the coatings. The coatings produced eliminate any seam characteristic of some prior art techniques and provide highly stable, photoconductive surfaces for repetitive use in electrophotography. The spray techniques described are also much simpler than other techniques under development such as the dipping of substrates.