Patent GB-917223-A (GB)
Title: Homogeneous solid materials
Inventor: PRATT GEORGE CHRISTOPHER
IPC: C08K3/00
Filed: 1959-04-30
Abstract: General classes of filler for use in polytetrafluoroethylene compositions are as follows:-  (a) Copper or a copper-base alloy, e.g. tin-bronze, or an oxide of copper in a proportion between 10 and 50% by volume of the whole.  (b) Graphite in a proportion between 10 and 50% by volume of the whole.  (c) Copper or a copper-base alloy, e.g. tin-bronze, or an oxide of copper and graphite or lead or an oxide of lead such that the total per cent filler is 10-50% by volume of the whole.  (d) Asbestos or glass fibres or other fibrous materials for giving increased creep resistance.  (e) An oxide of copper, e.g. cupric oxide, in a proportion between 10 and 60% by volume of the whole.  (f) Graphite 5-35% and an alloy of copper hardened by the addition of tin, zinc, aluminium or silicon 5-35%, the total per cent by volume being not more than 60% of the whole. Three specific examples of fillers are as follows:-  Example 1. Tin-bronze 20% by volume, graphite 20%, the remainder being polytetrafluoroethylene.  Example 2.  Graphite 17-23% by volume, a copper alloy (8-12% tin) 17-23%, lead or an oxide of lead 0-10%, the remainder being polytetrafluoroethylene.  Example 3.  Tin bronze 10% by volume, lead 30%, the remainder being polytetrafluoroethylene. Incorporation of the filler in the p.t.f.e. is effected by intimately mixing the powdered filler with a dispersion of p.t.f.e.; coagulating the p.t.f.e.; drying the coagulate; compacting the dried mixture under pressure; powdering the prepressed material; and recompacting the powder to the desired article which is then sintered (see Group V).  Articles specified are bearings.  Specifications 870,117 and 917,222 are referred to.ALSO:Homogenous solid articles, e.g. bearings, consisting of polytetrafluoroethylene (p.t.f.e.) and one or more fillers are made by intimately mixing the fillers in powdered form with a dispersion of p.t.f.e; coagulating the p.t.f.e; drying the coagulations; compacting the dried mixture under pressure to form a self-sustaining solid; powdering this pre-pressed material; recompacting it to form the desired article; and finally sintering the article.  The dried coagulation, the pre-pressed material, or the powdered pre-pressed material may be pre-heated in an atmosphere of nitrogen or other inert gas within the temperature range 250 DEG  to 327 DEG C. to cause volatile matter from wetting agents used to stabilise the p.t.f.e. dispersion and from the p.t.f.e. itself to be emitted before the final sintering in order to avoid cracking.  The pre-pressed material is ground into a powder of a size such as to pass a 10-mesh screen but to be caught by a 300-mesh screen.  The recompacting step is carried out in a mould at a pressure of at least 5,000 psi, the final moulded article being sintered at 340 DEG  to 360 DEG C. in an atmosphere of nitrogen or other inert gas while it is appropriately supported to avoid distortion.  Suitable fillers are copper, copper-base alloys such as tin-bronze, copper oxide, graphite, lead, lead oxide, asbestos or glass fibre.  A filler particularly suitable for use in a bearing material with good wear resistance comprises tinbronze and graphite, each present 20% by volume, the remainder being p.t.f.e.  Specifications 870,117 and 917,222 are referred to.