ASTM D 1418 Designation: FEP

ASTM D 2000, SAE J200 Type / Class: None

STANDARD COLOR: White

TRADE NAMES:
• Algoflon® (Ausimont USA, Ltd.)
• Polyflon® (Daikin Industries, Ltd.)
• Teflon® (DuPont)

RELATIVE COST: High

GENERAL TEMPERATURE RANGE: -300° to +500° F

Polytetrafluoroethylene (PTFE) is a completely fluorinated polymer produced when the monomer tetrafluoroethylene (TFE) undergoes free radical vinyl polymerization. As a monomer, TFE is made up of a pair of double-bonded carbon atoms, both of which have two fluorine atoms covalently bonded to them. Thus the name: “tetra” means there are four atoms bonded to the carbons, “fluoro” means those bonded atoms are fluorine, and “ethylene” means the carbons are joined by a double bond as in the classic ethylene structure. (Ethylene has hydrogen atoms attached to the carbons, as in Figure 50, but TFE has fluorine in place of the hydrogen, as in Figure 51.) When TFE polymerizes into PTFE, the carbon-to-carbon double bond becomes a single bond and a long chain of carbon atoms is formed, as in Figure 52. This chain is the polymer’s backbone.

With a ratio of four fluorine atoms to every two carbon atoms, the backbone is essentially shielded from contact. It’s almost impossible for any other chemical to gain access to the carbon atoms. Even if an agent could gain access, the carbon-to-fluorine bonds have high bond disassociation energy, so they’re almost unbreakable. This makes PTFE the most chemically resistant thermoplastic polymer available. PTFE is inert to almost all chemicals and solvents, allowing PTFE parts to function well in acids, alcohols, alkalis, esters, ketones, and hydrocarbons. There are only a few substances harmful to PTFE, notably fluorine, chlorine trifluoride, and molten alkali metal solutions at high pressures.

PTFE is also very slippery. By its very nature, the fluorine in PTFE repels everything. As part of a molecule, fluorine is decidedly “anti-social.” Anything getting close is repelled, and repelled molecules can’t stick to the PTFE surface. This makes PTFE perfect for applications requiring a low coefficient of friction. The only thing slicker than PTFE is ice! Because they are essentially self-lubricating, PTFE parts are ideal for applications in which external lubricants (such as oils and greases) can’t be used.

PTFE can withstand a wide range of temperatures (-300° to 500° F, -184° to 260° C). Because it’s non-flammable and doesn’t dissipate heat, PTFE is often used as a thermal insulator (as in welding equipment). At the other extreme, PTFE is widely used in very cold environments (such as space). Other important properties include resistance to both weathering and water absorption. PTFE can also act as an electrical insulator.

Because of its chemical inertness, PTFE cannot be cross-linked like an elastomer. Therefore it has no memory and is subject to creep (also known as cold flow). Creep is the increasing deformation of a material under a constant compressive load. This can be both good and bad. A little bit of creep allows PTFE seals to conform to mating surfaces better than most other plastic seals. Too much creep, however, and the seal is compromised. Compounding fillers are used to control unwanted creep, as well as to improve wear, friction, and other properties.

Keep in mind that PTFE fillers don’t act like elastomer fillers, which become chemically bonded to the elastomer. With polytetrafluoroethylene, the high shear modulus fillers are encapsulated and bound by the low shear modulus PTFE. Because it does not possess a good elastic memory at or below normal temperatures, PTFE may need to be heated to facilitate installation. PTFE has poor cut resistance, so extra care must be taken not to damage seals during installation.

“The inability of other materials to stick to PTFE makes it perfect for applications requiring a low coefficient of friction.”

Figures 50-52