Patent GB-1069426-A (GB)
Title: Information recording
IPC: G11C13/04
Filed: 1963-09-30
Abstract: 1,069,426. Electrophotographic materials. GENERAL ELECTRIC CO. Sept. 30, 1963 [Oct. 17, 1962], No. 38495/63. Heading H1K. [Also in Division .G2] A recording material has an active layer consisting of a dielectric material in which is dissolved or dispersed a photo-sensitive substance with or without optical sensitizers and other additives. The nature of the photo-sensitive substance is such that when the recording material is exposed to a light pattern a corresponding electrical charge pattern is directly produced in the active layer. Development may take place by normal methods such as those involving the deposition of charged powders, inks and dyes, but in the preferred method materials are used which have thermoplastic active layers which when heated to their softening point undergo surface deformation in accordance with charge patterns previously produced in the layers. Suitable photo-sensitive substances include inorganic ionic compounds such as metal salts, covalent compounds such as organo metallics, co-ordination compounds such as chelates, and organic materials such as phthalocyanines, chlorophyll derivatives, unstabilized polyvinylidene chloride, polyhalogenated polymers and the preferred compounds (which are soluble in thermoplastic polymers):-fluorescein, iodofluorescein, tetraiodoethylene, methylene iodide, 1 - iodo - 1 - nitrobenzene, 2 - iodo - 2-methyl-propane, diiodobenzene, iodopropane, iodobenzene, iodoform, polyvinyl iodide, and polyvinyl bromide. Suitable dielectric materials include polymers selected from the following classes:-acetals, acrylics, polyesters, silicones, and vinyl resins. Mixtures of polymers are also satisfactory, for example 70% polystyrene, 2% m-terphenyl, and 28% copolymer of 95% butadiene and 5% styrene. Typical compositions for active layers are. that having 5 parts iodoform dissolved in 100 parts polystyrene, and that having a small amount of methylene iodide disposed in a blend of polydiphenyl siloxane and poly (2: 6 - dimethyl - 1: 4 - phenylene) - ether. Optical sensitizers identified are diphenylamine, triphenylamine, and p-phenylazoaniline. The active layers of heat deformable materials are supported on base layers of relatively temperature stable materials such as paper, synthetic organic polymers, metals, ceramics, or glass. Specifically mentioned are aromatic polyesters and polyamides with fusion points > 150‹ C. Polyethylene glycol terephthalate, acetals, silicones and polycarbonates are suggested if the support is to be transparent. If sufficiently thick the base layer will act as a heat sink for cooling the active layer after development. It is convenient to provide a conductive layer between the base layer and the active layer. Heat produced in this layer by the passage of current or by microwave heating may be used to effect development or erasure of the image as subsequently described. Transparent conductive layers may be formed of chromium, iron, nickel, indium oxide or of copper iodide. (Copper iodide layers may be formed on substrates by direct evaporation of the iodide or by vapour or electrolytic deposition of copper to form a layer which is then exposed to iodine vapour). It is stated that the presence of the conductive layer is also beneficial to. the operation of the active layer. For many purposes transparent recording materials are used but if an opaque material is required an opaque base layer may be provided or particles of reflective solids may be included in the active layer. Although the examples described are intended for exposure to visible light patterns it is stated that with suitable materials U.V., I.R., X-rays, γ-rays &c. may be used. The recording material may be exposed by projection, by contact methods using negatives or by contact methods using a cathode-ray tube with glass fibres extending through its face-plate. The change pattern is formed within the material and is not removed by earthing the surfaces. Thermoplastic materials are developed by heating the active layer to its softening point and cooling to retain the resultant deformations. With some materials development may take place up to a day after exposure. The information stored may be retrieved by a light-scattering reflective system or by transmissive or reflective Schlieren systems. If the original exposure takes place through a suitable grid the Schlieren image shows a grey scale comparable to the original image. With some materials the deformation pattern may be erased by heating the active layer to its fusing point for some time, erasure being facilitated by uniform exposure of the layer before heating. With other materials this process does not remove the internal charge pattern and the deformation image is restored on cooling the material and reheating it to the softening point-the " erasure " process here will remove the effects of an initial random surface charge on the material.