Patent GB-1092081-A (GB)
Title: Process for making paper and air-pervious cardboard or boardlike structures predominantly of polytetrafluoroethylene
IPC: C08J5/04
Filed: 1964-10-14
Abstract: Air-pervious sheets are made by dispersing in a liquid whose surface tension at 25 DEG C is less than 40 dynes/cm. a polytetrafluoroethylene fibrous powder, forming a web from the dispersion and sintering it.  The fibrous powder must have an average fibre length of 100 to 5000 microns, an average shape factor (ratio of length to width) of not less than 10 and an anisotropic expansion factor (a measure of change of shape on sintering) of 1.3 to 7.0. Suitable liquids are aliphatic hydrocarbons, aromatic hydrocarbons, alcohols, ethers, aldehydes, acetal, ketones, halogen derivatives such as chloroform, carbon tetrachloride, allyl iodide, ethylene dibromide,, chloral, dichloracetic acid, acetyl chloride, monochlorobenzene and benzyl chloride, fluoro-derivatives such as fluorochlorohydrocarbons, mixtures of the above, and water whose surface tension has been lowered by the addition of a surfactant, e.g. an alkylaryl-ethylene glycol.  A synthetic resin powder may be incorporated by adding it after the fibrous powder has been dispersed in the liquid, by mixing it dry with the fibrous powder, or by dispersing fibrous powder in liquid that has the synthetic resin powder already dispersed in it.  The synthetic resin powder may be of polytetrafluoroethylene, a copolymer of tetrafluoroethylene with another perfluorolefine, e.g. 6 to 20% of hexafluoropropylene, polyethylene, polyvinyl chloride, polypropylene, polychlorotrifluoroethylene, polyacetal, polyamide, copolymer of tetrafluoroethylene and vinylidene fluoride, polyvinylidene chloride, vinyl chloridevinyledene chloride copolymer, or ethylenepropylene copolymer.  The sintering then takes place at a temperature above the melting point of the synthetic resin.  The web may be formed on a layer of p  liquid-pervious reinforcing material which retains its original form when heated, e.g. wire mesh, metallic wool, glass cloth or glass wool.  The sheets obtained may be thinner than 200 g/m2 or may be pliable cardboard or boardlike structures. They may be made water-pervious by wetting with a hydrophilic organic solvent and displacing the solvent with water.ALSO:A water-insoluble liquid, e.g. gasoline, can be separated from a mixture with water by filtering the mixture through a sheet made by dispersing in a liquid whose surface tension at 25 DEG C is less than 40 dynes/cm. a polytetrafluorethylene fibrous powder, forming a web from the dispersion and sintering it.  The fibrous powder must have an average fibre length of 100 to 5000 microns, an average shape factor (ratio of length to width) of not less than 10 and an anisotropic expansion factor (a measure of change of shape on sintering) of 1.3 to 7.0 and may be of polytetrafluoroethylene, a copolymer of tetrafluoroethylene with another perfluorolefine, e.g. 6 to 20% of hexafluoropropylene, polyethylene, polyvinyl chloride, polypropylene, polychlorotrifluoroethylene, polyacetal, polyamide, copolymer of tetrafluoroethylene and vinylidene fluoride, polyvinylidene chloride, vinyl chloride-vinylidene chloride copolymer, or ethylene-propylene copolymer.  The sintering then takes place at a temperature above the melting point of the synthetic resin.  The web may be formed on a layer of liquid-pervious reinforcing material which retains its original form when heated, e.g. wire mesh, metallic wool, glass cloth or glass wool.  The sheets obtained may be thinner than 200 g/m2 or may be pliable cardboard or boardlike structures.