GB1504293A - Method of forming magnetic media - Google Patents

Abstract

1504293 Magnetic recording media EMI Ltd 12 Feb 1975 [1 March 1974] 9282/74 Heading B2E A magnetic material 20, Fig. 2, comprises a support coated with a coating comprising a first layer 22 containing a first magnetisable material and overlying second layer 24 which merges with the first layer and contains a second magnetisable material of a different coercivity than the first. Fig. 2 also shows what is believed to be a typical coercivity distribution in such a coating. The layers may be applied by simultaneous extrusion of the magnetisable materials, each in a binder, or by applying the first layer, then applying the second layer which includes a solvent for the binder of the first layer. In Examples (1) a first layer comprises gamma-Fe 2 O 3 having a coercivity of 300 oersteds dispersed in a binder of vinyl acetate/vinyl chloride copolymer, butadiene/ acrylonitrile coplymer plasticiser and lecithin dispersing agent is applied by knife coatings; a second layer comprises cobalt doped iron oxide (480 oersteds) in the same binder and the support is a polyester web: (2) a first layer containing gamma-Fe 2 O 3 (250 oersteds) and a second layer containing cobalt doped iron oxide (480 oersteds) each in the binder of Example 1, are applied to the web simultaneously from the nozzle of a trough having an internal dividing wall separating the compositions; and (3) a first layer of gammairon oxide (240 to 350 oersteds) suspended in partially hydrolised vinyl acetate/vinyl chloride copolymer was applied to biaxially orientated polyethylene terephthalate film, and a second layer of chromium dioxide (400 to 550 oersteds) in the same binder is extruded onto the first layer before it dries.