Patent GB-914702-A (GB)
Title: Improvements in electrostatic printing
IPC: G03G13/01
Filed: 1959-03-13
Abstract: 914,702. Colour electroph tography. RADIO CORPORATION OF AMERICA. March 13, 1959 [March 20, 1958; Nov. 25, 1958], No. 8877/59. Drawings to Specification. Class 98 (2). [Also in Groups XVI and XXXVI] A process for producing multi-coloured prints by an electrophotographic process comprises the following steps. A xerographic plate comprising a photo-conductive insulating layer such as zinc oxide in a polysiloxane binder on a paper base is placed on an earthed plate and uniformly charged by friction or corona discharge. The surface is then exposed to a first light image to form an electrostatic image, which is developed by means of a powder consisting of photoconductive zinc oxide particles coated with insulating electroscopic material (see below and also Group XXXVI). The powder may be applied by dusting or cascading or by a brush consisting of the powder and magnetic particles held in the field of a magnet. After fixing the image by heat the plate is recharged and a second light image projected on it to form a second electrostatic image. This image may overlap the previously-developed image since the latter acts merely as an extension of the main photoconductive insulator surface. It is developed and fixed in the same way as the first image. Further overlapping images may be formed by repeating the process, any developing powder being suitable for developing the final image. The above process may be used to produce a print in natural colours from a transparency. In this case light is projected through the transparency and thence through red, yellow and blue filters respectively to form the successive images, which are developed using red, yellow and blue powders respectively. In a modified process the heating in each fixing step is so controlled that only the parts of the powder particle coatings adjacent the underlying surface stick to it, the remaining parts and the zinc oxide particles being removed by brushing, wiping or by an air blast before the plate is recharged. When it is desired to produce powder images which cannot be overprinted in subsequent electrostatic printing steps, the image is uniformly flooded with light after fixing and before the next charging step. The developer powder comprises 50 to 85 parts by weight of French Process zinc oxide having a very high dark resistance and 50 to 15 parts by weight of insulating electroscopic coating material having a melting-point of 60 to 250 C. and a viscosity just above its melting-point of 45 to 10,000 c.p.s., such as natural and synthetic resins and waxes. The spectral response of the zinc oxide which has a natural peak in the near ultra-violet may be altered to give an additional peak in the orange-red by adding Brilliant Green, in the orange by adding Patent Blue, in the green by adding Rose Bengal or Eosin Y, in the bluegreen by adding Fluorescein Sodium, and in the blue by adding Thioflavin TG. The electroscopic coating may be carnauba wax, microcrystalline petroleum wax (which may be chemically modified), a thermoplastic hydrocarbon terpene resin, a mixture of this resin and microcrystalline petroleum wax, or an octadecenamide synthetic wax alone or in combination with a solid silicone resin or polythene of m.p. 130 C. Examples are given of white, blue, green, and red developers. For a blue developer Cyan Blue Toner GT or Brilliant Oil Blue BMA is added to the mix, for a green developer Benzidine Yellow is added as well as the blue colorant, and for a red developer there is added Benzidine Yellow or Sudan III Red and Oil Yellow 2G. Each developer powder is prepared by dispersing finely-divided zinc oxide in a melt of the coating material, adding the sensitizing and colouring agents, and, if desired, modifiers such as plasticizers, hardeners, dispersing agents, e.g. calcium stearate, and toughening agents, and ball-milling the solidified melt from which particles of less than 200 mesh (74 microns) are sorted for use as the developer.