Patent GB-1026634-A (GB)
Title: Electrophotographic recording element
IPC: G03G5/022
Filed: 1963-08-23
Abstract: 1,026,634. Electrophotographic processes. RADIO CORPORATION OF AMERICA. Aug. 23, 1963 [Sept. 12,1962], No. 33552/63. Heading G2H. [Also in Division H1] An electrophotographic material consists basically of a thermoplastic photoconductive layer having one surface deformed to correspond to a half-tone screen pattern. The material is produced by forming a layer of thermoplastic photoconductive material, applying a uniform electrostatic charge, exposing the layer through a positive or negative half-tone screen or through a line screen, and by subsequently or simultaneously heating the layer to its thermoplastic softening point upon which the layer will collapse and decrease in thickness in the areas which have retained charge. The material is used to produce a - screened replica of a continuous tone image. The .charge pattern corresponding to this image may be produced on the layer by an electron beam but it is preferred to repeat the electrophotographic process so that after development the deformations of the layer correspond to the screen form of the original used in the second step. Many specific examples are given of the preferred compositions for the photoconductive layer. These consist of a dye intermediate and one or more resinous materials, the dye intermediate being a diaryl or triaryl methane or a diaryl ketone, and suitable resinous materials being chlorinated paraffins, polyvinyl chloride, copolymers of vinyl chloride, polystyrene, copolymers of styrene and butadiene, hydrocarbon resins, thermoplastic hydrocarbon terpene resins, acrylates and acrylic copolymers, and epoxy resins. A self-supporting film may be formed by applying a solution of the components to a mirror-polished metal plate and subsequently stripping off the dried film. Supported layers are generally used, the preferred supports being of glass with a conductive tin oxide coating or of polyethylene terephthalate film provided with a vacuum deposited copper or aluminium surface layer. Plasticisers may be included in the photoconductive layers to obtain greater flexibility. Examples given are butyl phthalyl-butyl-glycolate, tris-(2 : 3-dibromopropyl)- phosphate, and di-(2-ethyl-hexyl)-phthalate. In cases where the molecules of the binder used have no halogen content increased photosensitivity is obtained by including in the layer a non-volatile halogen-containing compound such as tris-(2 3- dibromo-propyl)-phosphate. When it is necessary to avoid colour formation in the layer a small quantity of a stabilizer for the dye intermediate is included. Examples given are (C 8 H 17 ) 2  Sn(S. CH 2 . C 8 H 17 ) 2 , pyrocatchecol, 2-hydroxy-4-methoxybenzophenone, and 2 : 2<SP>1</SP>-di-hydroxy-4-methoxybenzophenone. Pattern formation is illustrated with a specific example-The photoconductive layer consists of a mixture of polystyrene, a chlorinated paraffin, and bis- (p-dimethylaminophenyl)-phenylmethane. A layer of this composition is provided on one of the preferred supports and after the coating solvent has been removed the material is heated at about 180‹C for 2-3 minutes to produce a faint green colour in the clear layer. The material thus produced has a photoconductive response peak at 6300Š and a lesser peak at 4300Š. It has a relatively sharp thermoplastic transition point at about 50‹C. To form the surface pattern the material is corona charged with either polarity and exposed to a half-tone or line original by contact or projection methods. The exposed material is placed on a hot-plate at 140‹C and observed using low angle illumination from a yellow safelight. When the deformations are seen to form the material is removed from the hot-plate and allowed to cool. A cold metal plate which acts as a heat-sink may be used to speed the process. Alternatively exposure and development may take place simultaneously using a source providing both light to which the material is sensitive and infra-red radiation. Filters may be used to ensure that the correct balance between exposure and heating rate is obtained. Separate light and I. R. sources may be used simultaneously and are provided with appropriate pre-balancing filters. An electrostatic image corresponding to a continuous tone pattern may now be produced by electron beam scanning or by electrophotographic methods. Development is carried out by again raising the photoconductive layer to its thermoplastic transition point and then allowing it to cool. The developed material viewed by glancing illumination or in a schlieven projection system. During the development, areas of the layer corresponding to lighttransmissive areas of the original will be unaffected and will retain their original screen pattern, and when projected in the schlieven system will appear white. Light-blocking areas of the original will produce developed areas in which the screen pattern has been completely obliterated. Such areas will appear black when projected. "Grey" areas of the original will result in partial obliteration of the screen pattern and give projected greys.