Polytetrafluoroethylene (PTFE) is polymerized in mainly two types of processes: suspension polymerization and aqueous dispersion (emulsion) polymerization. In latex obtained by aqueous dispersion (emulsion) polymerization, polymer fine particles are aggregated and dried to obtain powder (fine powder), which is then processed by paste extrusion after being compounded mainly with a liquid lubricant. By paste extrusion, the compound is extruded into a form of a rod or a tube from an extruder. The extruded articles are used as sealing materials without being subjected to further processing or by being rolled into a sheet without sintering (calendering), or as filter materials by being stretched into porous films, or are further sintered to be used as molded articles such as tubes, and wire-coatings.
When a high shear force is applied to PTFE aqueous dispersions, primary particles are aggregated and secondary particles having average particle size of 100 to 1000 μm are formed (coagulation). It is known that in order to promote coagulation an additive such as acids, alkalis, organic solvents is added. Secondary particle powder obtained by coagulation of PTFE aqueous dispersions is dried to give PTFE fine powder.
PTFE fine powder is characterized by its fibrillating properties. By paste extrusion or calendering as described above, molded articles such as sealing materials, filters, tubes, pipes, wire-coatings can be produced.
Although the above properties of PTFE fine powder may be advantageous, some problems may also be caused such that the secondary particle powder is tend to be aggregated due to fibrillation and that the formed aggregates may not restore the original secondary particle powder. Because of these problems, PTFE fine powder is tend to be aggregated or aggregated to be consolidated during production, transport or molding processes thereof, impairing handling characteristics thereof.
The aggregates may lead to reduced molding properties and deteriorated appearance of molded articles. Thus, it is usually necessary to remove the aggregates with a sieve during production or before molding. They may lead to decreased working efficiency or productivity.
Patent Document 1 discloses a process for producing a fine powder having a high apparent density and excellent flowability by adding a fluorine-containing anionic surfactant to a PTFE aqueous dispersion.    Patent Document 1: WO 97/17382
According to the process disclosed in Patent Document 1, polytetrafluoroethylene (PTFE) fine powder have an increased apparent density, so that PTFE fine powder has increased flowability as well as improved handling characteristics. However, molding PTFE fine powder having a high apparent density by paste extrusion requires high extrusion pressure, which may impair molding properties.
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