Stock material or miscellaneous articles – Structurally defined web or sheet – Physical dimension specified
Patent
1995-06-07
1998-09-08
Chapman, Mark
Stock material or miscellaneous articles
Structurally defined web or sheet
Physical dimension specified
428364, 428401, 428421, 264127, 2642884, D02G 300, B32B 2700, B27J 500, B29C 4908
Patent
active
058042903
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a monoaxially stretched moulded article made of polytetrafluorethylene (PTFE) which may be shaped as a film, tape, yarn or fibre, and relates to a process for its production.
Because of its thermal stability and its chemical inertness, PTFE is a valued material. AT-B 370,674 discloses monoaxially stretched films made of sintered PTFE which in the stretch direction have values for strength of between 50N/mm.sup.2 and 140N/mm.sup.2. These films are produced by first pressing PTFE powder to form a cylindrical moulded article. Subsequently the moulded article is sintered, whereupon films are peeled off, heated to temperatures of at least 327.degree. C. and stretched.
GB-A-2 025 835 describes the production of porous PTFE moulded articles by the paste-extrusion process, a pasty composition, containing essentially PTFE powder and a lubricant (hydrocarbon), being forced through dies, whereupon the lubricant is removed by drying. Then the moulded article is heated to above the crystallite melting point of the PTFE (327.degree. C.) and stretched during heating.
AT-B 391.473 discloses a process for the production of monoaxially stretched moulded articles made of PTFE, a pasty PTFE composition being continually processed to form a moulding which is led over a plurality of rollers or rolls and is heated and stretched, the moulding, before application of the stretching, being heated to a temperature between 327.degree. and 450.degree. C. and sintered. This process permits the production of monoaxially stretched moulded articles from PTFE with strength values in the stretching direction of at least 22 cN/tex (500 N/mm.sup.2).
It is known that PTFE multifilament yarns produced by matrix spinning according to U.S. Pat. Nos. 2,893,105 and 2,933,154 and those produced by the gap peel process of AT-B-340,561 can be used to produce plait cords for packing glands. From AT-B-347,003 it is further known how to coat such yarns with PTFE and graphite to improve the sliding characteristics.
It is also known that oriented but not sintered film tapes produced by means of paste extrusion, impregnated with PTFE dispersant and lubricated with paraffin oil are used for the manufacture of packing glands. These film tapes are also commercially available when containing graphite fillings.
All these products filled with graphite have the drawback that the graphite filling, which is provided to increase the sliding characteristics, is of black colour, whereby these products cannot be used in certain areas of the industry. In addition, these products exhibit a high tendency to flow (cold flow) which naturally increases wear. It is a further drawback that the moulded articles monoaxially stretched and filled with graphite solely have strength values of less than 10 cN/tex.
SUMMARY OF THE INVENTION
The present invention aims to eliminate these drawbacks, to broaden the applicability of the PTFE material and to provide a monoaxially stretched moulded article having a light colour and a higher strength and a lower cold flow tendency than a moulded PTFE article filled with graphite. It should further be feasible to produce said moulded PTFE article having different sliding characteristics.
The monoaxially stretched moulded article made of polytetrafluorethylene (PTFE) according to the present invention is characterized in that it contains talc and/or mica and/or a high temperature resistant polyimide and/or whiting as filler, and has strength values of at least 10 cN/tex and preferably more than 16 cN/tex. As high temperature resistant polyimides those disclosed in EP-A-0 119 185 and in U.S. Pat. No. 3,985,934 are particularly suited.
It has surprisingly been successful to mix PTFE powder by means of paste extrusion with said fillers and to process it to e.g. films, to sinter these films and to stretch them subsequently, whereby very high strength values can be achieved despite the presence of the filler. The strength values of e.g. an unfilled PTFE yarn (produced according to AT-B-370,674) a
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Bachmair Josef
Marini Ingo
Wimmer Adalbert
Chapman Mark
Lenzing Aktiengesellschaft
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