Details
Material hierarchy: Polymer - Engineering Thermoplastic
Fluoroplastics are also known as fluoropolymers. Polytetrafluoroethylene (PTFE), commonly known as Teflon, accounts for about 85% of the family of polymers called fluoropolymers, in which fluorene atoms replace hydrogen atoms in the hydrocarbon chain. PTFE is extremely resistant to chemical and environmental attack, is unaffected by water, possesses good electrical properties, good heat resistance, and a very low coefficient of friction. These latter two properties have promoted its use in nonstick household cookware. Other applications that rely on the same property include nonlubricating bearings and similar components. PTFE also finds applications in chemical equipment and food processing.
Outstanding properties of fluorocarbon polymers (or fluoroplastics, as they also are called) include inertness to most chemicals and resistance to high temperatures. Fluoroplastics have a rather waxy feel, extremely low coefficients of friction, and excellent dielectric properties that are relatively insensitive to temperature and power frequency. Their mechanical properties normally are low, but change dramatically when the fluorocarbons are reinforced with glass fibers or molybdenum disulfide fillers.
Numerous fluoroplastics are available, including polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), fluorinated ethylene propylene (FEP), ethylene chlortrifluoroethylene (ECTFE), ethylene tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF), polyvinylfluoride (PVF), and perfluoroalkoxy (PFA) resin.
PTFE is extremely heat-resistant (up to 500°F) and has outstanding chemical resistance, being inert to most chemicals. Its coefficient of friction is lower than that of any other plastic, and it can be used unlubricated. TFE has a tensile strength on the order of 1500 to 5000 psi and an impact strength of 2.5 to 3.0 ft-lb/in. of notch. TFE also has outstanding low-temperature characteristics and remains usable even at cryogenic temperatures.
PTFE is extremely difficult to process via melt extrusion and molding. The material usually is supplied in powder form for compression and sintering or in water-based dispersions for coating and impregnating.
CTFE has a heat resistance of up to 390°F, and is chemically resistant to all inorganic corrosive liquids, including oxidizing acids. It also is resistant to most organic solvents except certain halogenated materials and oxygen-containing compounds (which cause a slight swelling).
In terms of processibility, CTFE can be molded and extruded by conventional thermoplastic processing techniques. It can be made into transparent film and sheet with extremely low water vapor transmission.
FEP, a copolymer of TFE and hexafluoropropylene, shows some of the same properties as TFE (i.e., toughness, excellent dimensional stability, and outstanding electrical insulating characteristics over a wide range of temperatures and humidities), but it exhibits a melt viscosity low enough to permit it to be molded by conventional thermoplastic processing techniques. FEP is used as a pipe lining for chemical process equipment, in wire and cable applications, and for glazing in solar collectors.
ECTFE, an alternating copolymer of ethylene with chlorotrifluoroethylene, is significantly stronger and more wear-resistant than PTFE and FEP. Its ignition and flame resistance makes it suitable for wire and cable applications, including plenum cable jacketing. Other uses include tank linings, tower packings, and pump and valve components for the chemical process industry and corrosion-resistant film coatings.
ETFE is a related copolymer to ECTFE, consisting of ethylene and tetrafluoroethylene. Although its continuous-use temperature is somewhat higher (355°F) than that of ECTFE, it melts and decomposes when exposed to a flame.
PVDF also is a tough fluoroplastic with high chemical resistance and weatherability. It is self-extinguishing and has a high dielectric loss factor. A melt-processible material, it is used in seals and gaskets for chemical process equipment and in electrical insulation.
PVF, available only in film form, is used as a protective lamination on plywood, hardboard, and other panel constructions, as well as for interior truck linings.
Lastly, PFA is a family of melt-processible fluoropolymers, similar to FEP but wigh higher temperature resistance.
When reinforced with such materials as molybdenum disulfide, graphite, asbestos, or glass, the fluorocarbons show increased stiffness, hardness, and compressive strength, and reduced elongation and deformation under load.
Stress-strain data on fluorocarbons show these materials to be much better in compression than in tension. For example, depending on crystallinity, TFE has a tensile yield strength at 73°F of about 1800 psi (30% strain); in compression, however, this value can be as high as 3700 psi (25% strain).