Porous materials derived from tetrafluoroethylene and process for their production

Abstract

Abstract 1355373 Laminated products W L GORE & ASSOCIATES Inc 5 May 1971 [21 May 1970] 13178/71 Headings B5B and B5N [Also in Divisions B2 C3 and H1] A laminate comprises a first shaped article bonded to a second shaped porous polymeric article the polymer containing units derived from tetrafluoroethylene and the porous polymer having a microstructure (i.e. which cannot be resolved with the naked eye) characterized by nodes interconnected with fibrils and having a matrix tensile strength (i.e. max. tensile strength multiplied by the ratio of the specific gravity of the solid polymer to that of the expanded, porous polymer) of at least 7300 p.s.i. in at least one direction. The porous polymer is made by forming a shaped article of the polymer containing a lubricant by a pasteforming extrusion technique, removing the lubricant and stretching the shaped article at an elevated temperature and at a rate exceeding 10% per second. The expansion may be effected in one direction or biaxially. The polymer may be a homopolymer of tetrafluoroethylene, or a copolymer thereof with ethylene, chlorotrifluoroethylene or hexafluoropropylene and the laminate may be in the form of a film, tube, filament or rod. The porous polymer may be compressed, and may also contain asbestos, carbon black, mica, silica, titanium dioxide, potassium titanate or a polysiloxane. In the examples a paste of the tetrafluoroethylene polymer with mineral spirits or kerosene was extruded to form a rod, film or tube which were dried to remove the spirits or kerosene, the articles were then expanded by stretching while being heated and held in the extended condition while being cooled. Fig. 2 shows a machine for making continuous expanded film by feeding polytetrafluoroethylene film 13 from roll 14 on to heated roll 15 then to heated roll 16 which can run faster than 15, gap A is adjustable; the stretched film goes on to hot roll 17 (running at same peripheral speed as 16) where it is heated to 370‹ C. to produce amorphous locking (i.e. the amorphous content of the polymer is increased by the heating and this inhibits slip along the crystal axes of the crystallites); roll 18 controls the residence time of film on 17 which is then fed to water-cooled roll 19 and wound on roll 20. The film may be expanded instead by vacuum forming or moulding, and tubes may be expanded by feeding compressed gas therein. The laminate is formed in Example 9 by heating two films which have been expanded as above (the films being placed in contact with and at right angles to each other); and in Example 11 by impregnating one expanded film with epoxy resin and a second film with a solution of metaphenylene diamine, and heating the two films as above.

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