Patent GB-999405-A (GB)
Title: Electrophotographic materials
IPC: G03G13/22
Filed: 1963-08-30
Abstract: 999, 405. Electrophotographic processes. EASTMAN KODAK CO. Aug. 30, 1963 [Sept. 5, 1962], No. 34361/63. Drawings to Specification. Heading G2H. [Also in Division HI] An electrophotographic material has two superposed continuous and contiguous photoconductive layers which are carried by a thin, electrically insulating, transparent support, the spectral sensitivities of the two layers being different. The material is used for bireflex copying of an original-the material, which is positively changed at some stage prior to exposure, is placed with its transparent support in contact with the original, and is illuminated from its free surface with light of a selected frequency range, the frequency range being chosen so that the light is absorbed only by the image areas of the original and by the photoconductive layer nearest the support. It is then cascade developed with any developer containing negatively charged toner particles. Photoconductive layers may contain inorganic photoconductors such as zinc oxide, zinc sulphide and cadmium sulphide. These are dispersed in binders such as styrene-butadiene copolymers, silicone resins, silicone-alkyd resins, styrene-alkyd resins, soyaalkyd resins, polyvinyl chloride, polyvinyl acetate, mixtures of these materials, or paraffin or mineral waxes. Organic photoconductors such as polyvinyl carbozole may also be used, preferably without a binder. Dyestuff sensitizers may be used to obtain the different spectral sensitivities of the layers and may be added in the normal way to dispersions of binder and photoconductor, the dispersions being applied to the support by conventional methods. The preferred photoconductor is zinc oxide. Different spectral sensitivities of layers containing zinc oxide may be achieved by having in the upper layer "white" zinc oxide which is sensitive only in the U.V. region and by having in the lower layer "pink" zinc oxide which has an extended spectral sensitivity reaching the red region of the spectrum, or alternatively by using "white" zinc oxide in the upper layer and by using dye-sensitized zinc oxide in the layer nearest the support. Preferred support materials are films of cellulose acetate and polyethylene terephthalate. In the two specific embodiments the base is a 0. 006 thick film of polyethylene terephthalate and the upper photoconductive layer consists of French process zinc oxide in equal weight proportion with a binder comprising 80 wt. per cent of a styrenebutadiene copolymer and 20 wt. per cent of a silicone resin. The photoconductive layers nearest the support consist for the two embodiments respectively of white zinc oxide, Rose Bengal, and 20 wt. per cent of a binder comprising 90% of the above styrene-butadiene copolymer and 10% of a high styrene copolymer of styrene and butadiene cross-linked with a rare earth naphthenate; and of white zinc oxide, Rose Bengal, and 50 wt. per cent of the styrene-butadiene copolymer used in the upper layer. These two materials are used with exposure sources consisting of tungsten lamps in conjunction with filters passing only wavelengths longer than about 470 mÁ.