Patent GB-864474-A (GB)
Title: Improvements in electrophotography
IPC: G03G17/02
Filed: 1957-03-28
Abstract: 864,474. Photographic processes. MINNESOTA MINING &,MANUFACTURING CO. March 28, 1957 [March 30, 1956], No. 10195/57. Class 98(2) A visible image is formed on the surface of a photoconductive coating comprising a photoconductor having a photoconductivity of at least 10-7 mho per cm. on exposure to light and bonded to an electrically conductive base, by exposing the coating imagewise and either during or after exposure electrolytically developing the image by applying a potential between the conductive base and an electrolyte over the exposed surface of the photoconductive coating in the presence of a developer material which effects a visible and permanent colour change on the photoconductive coating during the electrolysis. In Fig. 1, the zinc oxide layer 16 is exposed through its transparent conductive base and either during or after exposure is placed in the electrolytic cell 13 having a copper anode and copper sulphate electrolyte, whereby copper is deposited on the exposed areas of the zinc oxide coating. The source of potential may be A.C. or D.C. An opaque support 17 may be used and exposure made from the right in Fig. 1 by using an electrode 19 in the form of a frame or grid. Instead of copper sulphate, the electrolyte may be tartaric acid, silver nitrate or nickelous chloride (preferably also containing sodium thiosulphate). Instead of using a cell, the electrolytic developer solution may be brushed on the exposed zinc oxide coating by means of a brush whose bristles are connected to the source of potential by means of a wire or applied by means of a copper bar or drum. In Fig. 6, such a roller 65 has a metal core connected to the source of potential and an absorbent surface which is impregnated with electrolytic developer solution or supplied with solution from rollers 67, 68 dipping into a bath 66 of the electrolyte. Alternatively, nickel acetate may be included in the zinc oxide material and the image developed using a roller moistened with water and connected to the source of potential. In another embodiment, the developing electrolyte is contained in a separate sheet which may comprise copper or aluminium foil coated with a mixture of gelatin, glycerine and copper sulphate or silver nitrate. Development is effected by pressing the exposed zinc oxide layer against the gelatin layer of the developer sheet and applying a potential across the conductive backing layers. The gelatin layer may be replaced by a sponge or porous paper containing the electrolytic solution. In a further embodiment, an electrolyte such as nickel chloride is coated in a binder such as polyethylene glycol wax over the zinc oxide layer. Development takes place by drawing a heated metal rod over the surface while the rod and conductive backing are connected to a source of potential. The heat melts the outer layer sufficiently to permit electrolysis to take place. Other heat liquifiable coatings such as polyacrylic acid, carboxymethyl cellulose or gelatin may be used. Other methods of forming the image are; (i) using an electrolyte containing a diazonium salt and a coupler in acid medium, . e.g. a solution of tartaric acid, phloroglucinol and the zinc chloride salt of p-diazo-N-ethyl-N-benzyl aniline. Such a solution may be applied by the above brush technique. Electrolysis reduces the pH and permits coupling to take place; (ii) coating the zinc oxide layer with a mixture of a diazotizable amino and a coupler and using sodium nitrite as electrolyte, whereby electrolysis causes diazotization of the amino followed by coupling; (iii) coating a dye which is easily reduced by electrolysis over the zinc oxide layer, such that its colour is destroyed on electrolysis; a suitable dye is methylene blue and the electrolyte may be water containing tartaric acid. Alternatively a diazonium dye may be used in which case zinc chloride and sodium bisulphate are included in the coating; (iv) a variation of (iii) uses methylene blue and zinc oxide in a resin binder on a metallized paper support and an overcoat of gelatin containing zinc chloride. After exposure, development is by the heated metal rod technique; (v) colloidal charged particles such as Prussian blue may be deposited on the zinc oxide coating from a suspension in an electrolyte. Specification 864,475 is referred to.