GB800551A - Improvements in or relating to electrical contact elements - Google Patents

Abstract

800,551. Impregnated electric contact brushes. NOBRAC CARBON, Ltd. June 6, 1955 No. 11777/54. Class 140. An electrical contact element of carbon or graphite or mixtures of carbon and/or graphitewith a metal or metals includes a proportion of between 10 and 50 per cent by weight of polymerized tetrafluoroethylene or mono-halogen substituted derivatives of tetrafluoroethylene and the requisite porosity is provided by incorporating into the moulding mixture, before moulding, a substance which either vaporizes or sublimes within the mixture to provide the required porosity or alternatively is a substance or substances which, during curing, is unaffected itself and is subsequently removed after curing by leaching or the like, and the mixture thus prepared is first impregnated with a dispersion or suspension of the polymer and subsequently impregnated with an aqueous solution of silver nitrate which is then reduced to metallic silver. Impregation of the porous brush element with the polymer may be effected by vacuum and pressure means using a suspension of particles of the polymer in water or a suspension of polymerized monochlorotrifluoroethylene dispersed in butyl-glycollate. The impregnated brush element is then heated to sinter the impregnant. Examples describe the following means of rendering the brush mixture suitably porous: (i) graphite, ammonium chloride and phenol-formaldehyde resin powder are mixed, moulded under pressure and then baked in a non-oxidizing atmosphere whereby the ammonium chloride is removed by subblimation and the resinous binder is carbonized; (ii) petroleum coke, pitch and sodium chloride are mixed in a heated mixer, moulded under pressure and baked in a non-oxidizing atmosphere after which the sodium chloride is removed by forcing water through the structure or by leaching and the resultant body is then heated in an electrographitizing furnace. After impregnation with the polymer the brush may be impregnated with silver nitrate solution, followed by reduction, one or more times. Suitable reducing agents are gaseous hydrogen or formaldehyde. In an example a block of porous electrographite, previously impregnated with the polymer, is boiled in an aqueous solution of silver nitrate, dried in a current of warm air, then heated in a current of hydrogen in a sealed container and finally raised to an elevated temperature to complete the reduction. The block is then used for making contact brushes.