This invention relates to a dip coating method, for use in fabricating for instance photosensitive members, wherein coating beads are subjected to partial drying to prevent contamination of the coating solution in the next dip coating vessel. The term bead refers to a coating buildup such as an excessively thick portion of the coating on the substrate.
Dip coating is a coating method typically involving dipping a substrate in a coating solution and taking up the substrate. In dip coating, the coating thickness depends on the concentration of the coating material and the take-up speed, i.e., the speed of the substrate being lifted from the surface of the coating solution. It is known that the coating thickness generally increases with the coating material concentration and with the take-up speed. A bead may be formed during dip coating on the bottom end region of the substrate, especially at the bottom edge, where the bead covers a portion of the outer and inner surface of the bottom end region. The bead can be quite large such as from about 100 to 250 microns in thickness (measured from the substrate surface) and from about 3 to 10 mm in width (measured along the length of the substrate). It has been found that the time required for ambient conditions to partially dry the bead to a tacky film, which is then sufficiently dry for the next coating solution without danger of contaminating that coating solution, may be in excess of about 90 minutes. Such a long time period may be needed because the bead is generally much thicker than the rest of the coated layer and because ambient air cannot freely circulate within the substrate interior to evaporate solvent from the portion of the wet coating solution bead on the inside surface of the substrate. This is a problem since there may be less than 90 minutes between dip coating cycles in certain production processes and thus the insufficiently dry bead will contaminate the coating solution in the next dip coating vessel. Consequently, there is a need, which the present invention addresses, for a coating method which decreases the time required to evaporate a sufficient amount of the solvent from a wet coating to avoid contaminating the coating solution in the next dip coating vessel.
The following documents disclose conventional dip coating methods, dip coating apparatus, and photosensitive members:
Miyake, U.S. Pat. No. 5,213,937, discloses a process for preparing an electrophotographic photoreceptor;
Takeda et al., U.S. Pat. No. 4,421,838, discloses processes for preparing photoconductive elements and electrophotosensitive materials;
Yashiki et al., U.S. Pat. No. 4,610,942, discloses an electrophotographic member having corresponding thin end portions of charge generation and charge transport layers;
Nozomi et al., U.S. Pat. No. 5,120,627, discloses an electrophotographic photoreceptor having a dip coated charge transport layer; and
Sumino et al., U.S. Pat. No. 5,279,916, discloses a process for producing an electrophotographic photosensitive member.