Polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. The best known brand name of PTFE-based formulas is polymer by Chemours. Melting point: 620.3°F (326.8°C);Formula: (C2F4)n;Density: 2.2 g/cm³;IUPAC ID: poly(1,1,2,2-tetrafluoroethylene).
PTFE is produced by free-radical polymerization mechanism in an aqueous media via addition polymerization of TFE in a batch process. The initiator for the polymerization is usually a water-soluble peroxide such as ammonium persulfate or disuccinic peroxide. A redox catalyst is used for low temperature polymerization. PTFE is produced by suspension (or slurry) polymerization without a surfactant to obtain granular resins or with a perfluorinated surfactant (emulsion polymerization) such as ammonium perfluorooctanoate to produce fine powder and dispersion products. Polymerization temperature and pressure usually range from 0 to 100 C and 0.7 to 3.5 MPa.
Granular PTFE is produced by polymerizing TFE alone or by using trace amounts of comonomers. A peroxide initiator, little or no surfactant, and other additives may be present in the aqueous polymerization medium that is vigorously stirred and sometimes buffered by an alkaline solution. Most of the polymer is formed in the gas phase in the shape of stringy and irregularly shaped particles. The particles are comminuted to different sizes, depending on the powder properties required by the fabrication process. For example, a smoother surface part requires smaller particle size while good flow is improved by larger particle size.
Fine powder PTFE is produced by polymerization of TFE in an aqueous medium in the presence of an initiator and surfactant. The polymerization does not follow a conventional emulsion mechanism but some of the principles, which apply. The stability of the dispersion during the polymerization, to avoid premature coagulation, is balanced against the need to break the emulsion to recover the PTFE. Low shear rate agitation is maintained during the polymerization using surfactant levels below the critical micelle concentration. The rate of polymerization and particle shape and size are affected by the concentration of the surfactant. Majority of the particles is generated in the early part of polymerization and grows as the cycle proceeds. Molecular weight and composition within the particle can be controlled using the polymerization ingredients and conditions.
The same polymerization process makes aqueous dispersions of PTFE as fine powder. The dispersion is concentrated and stabilized using a variety of ionic and nonionic surfactants. Several concentration methods have been reported including electrodecantation, evaporation, and thermal concentration. Chemical additives to match them with the fabrication process or part property requirements can modify the final PTFE dispersion.
Filled compounds are produced from all three forms of PTFE using fillers such as glass fiber, graphite, metal powder, carbon fiber, and others.
Post time: Sep-24-2018