Polytetrafluoroethylene service temperature

Major applications of unsintered polytetrafluoroethylene are as tape in thread sealing and wrapping electrical cables, and as rod and tape in packings. Important properties of PTFE like chemical resistance, broad service temperature, low friction, flexibility, high machine direction strength, and deformability in the cross direction make unsintered fine powder PTFE ideal for these applications. 

The part may need to be in contact with service fluids such as mineral and vegetable based oils. The selection of the correct polymer depends on the exact nature of the fluid and the service temperature. For mineral oils a polychloroprene or acrylonitrile -butadiene copolymer based compound may be appropriate but small variations in lubricant constituents make it worthwhile to measure the changes that can occur at operating temperatures to properties such as modulus and tear resistance. For solvents it may be more viable to use a physical sheath of an impervious material such as polytetrafluoroethylene. Swelling or shrinkage is strongly influenced by the nature of fillers and oils used to compound the rubber. 

The commercial copolymers have physical properties similar to those of polytetrafluoroethylene but with somewhat greater impact strength. Comparative values for some properties are given in Table 7.1. The copolymers also have similar excellent electrical insulation properties and chemical resistance. The maximum service temperature for the copolymers is about 60 deg C lower than that for the homopolymer under equivalent conditions. The temperature range of useful performance is from about —80°C to 200°C. Tetrafluoro-ethylene-hexafluoropropylene copolymers are used for various electrical and corrosion resistant mouldings, coatings and wire covering. 

Polytetrafluoroethylene has a decomposition temperature above 400 °C. Its service temperature ranges between -200 and 300 °C. Short-term upper service temperature without mechanical load in air is 300 °C, for continuous load it is 250 °C [86]. 

Perfluroalkoxy polymer or PFA is one of the most important meltprocessible fluoroplastics due to its relative ease of processing and high service temperature equivalent to polytetrafluoroethylene (260°C). It also has the same excellent chemical resistance and low friction properties as PTFE. Perfluroalkoxy polymers are prepared by copolymerization of a perfluoroalkylvinyl ethers (Rf—O—CF=CF2, where Rj is a perfluorinated alkyl group) with tetrafluoroethylene. Examples of commercially utilized ethers include perfluoromethyl-vinyl ether (CFg—O—CF=CF2), perfluoroethylvinyl ether (C2F5—O—CF=CF2) and perfluoropropylvinyl ether (C3F7—O—CF=CF2). Several percent of ether is incorporated in a copolymer. 

Dry chlorine has a great affinity for absorbing moisture, and wet chlorine is extremely corrosive, attacking most common materials except HasteUoy C, titanium, and tantalum. These metals are protected from attack by the acids formed by chlorine hydrolysis because of surface oxide films on the metal. Tantalum is the preferred constmction material for service with wet and dry chlorine. Wet chlorine gas is handled under pressure using fiberglass-reinforced plastics. Rubber-lined steel is suitable for wet chlorine gas handling up to 100°C. At low pressures and low temperatures PVC, chlorinated PVC, and reinforced polyester resins are also used. Polytetrafluoroethylene (PTFE), poly(vinyhdene fluoride) (PVDE), and... 

D. Fluorocarbon Polymers. Four different fluorocarbons account for the bulk of the laboratory applications polytetrafluoroethylene, Teflon PTFE po-ly(chlorotrifluoroethylene), KEL-F tetrafluoroethylene-hexafluoropropylene copolymer, Teflon FEP and tetrafluoroethylene-perfluorovinyl ether copolymer, PFA. These polymers are inert with most chemicals and solvents at room temperature and exceptionally inert with oxidizing agents. They also have an exceptional resistance to temperature extremes. However, they are decomposed by liquid alkali metals, solutions of these metals in liquid ammonia, and carban-ion reagents such as butyllithium. Teflon retains some of its compliance at liquid hydrogen temperature. The maximum temperature which is recommended for continuous service is 260°C for Teflon PTFE and PFA, and about 200°C for Kel-F and Teflon FEP. 

Seal manufactures develop their own rubber compounds suitable for seals, which possess the chemical, physical and swelling properties to match the functional requirements and working conditions of the application. The compounds used in the manufacture of seals are derived from base rubbers such as natural rubber, nitriles, neoprenes, butyls, styrene butadiene, carboxylated nitriles, viton, silicones and polytetrafluoroethylene. Of all the properties exhibited by the various types of rubber compounds, the most critical ones pertain to how they change when they are installed as seals and while in service. All physical properties change with age, and exposure to variations in temperature, fluid type, pressure, and other factors which can include corrosive chemicals and fumes and gases. Compounds with the smallest tendency to change their properties, whether chemical or physical, are easier to work with. More adaptable and versatile seals can be produced with these compounds. 

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