ASTM D 1418 Designation: FEP

ASTM D 2000, SAE J200 Type / Class: None

RELATIVE COST: High

GENERAL TEMPERATURE RANGE: -65° to +325° F

Polytetrafluoroethylene (PTFE) is a completely fluorinated polymer produced when the monomer tetrafluoroethylene (TFE) undergoes free radical vinyl polymerization. Because of PTFE’s unique properties, it is used for high pressure, high speed, and non-lubricated applications, as well as in applications involving aggressive chemicals.

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 40, but TFE has fluorine in place of the hydrogen, as in Figure 41.) 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 42. This chain is the polymer’s backbone. PTFE powders are combined with a variety of fillers to improve strength, wear resistance, and lubricity. PTFE is often used in composite high-pressure seals as a backup/support ring.

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 structure 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, so PTFE parts function well in acids, alcohols, alkalis, esters, ketones, and hydrocarbons. Only a few substances harm 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’s inherent anti-stick characteristics do, however, make it difficult to bond to other materials (such as the metal of a shaft seal case). For this reason, the PTFE lips in PTFE shaft seals may sometimes need to be clamped in place. Some seals bond the PTFE to rubber, which is then bonded to the case.

PTFE can withstand a wide range of temperatures (-65° to 325° F, -54° to 163° C). On the down side, PTFE has poor cut resistance, so extra care must be taken during installation so as not to damage shaft seal lips made of PTFE. PTFE also has poor flexibility, and special designs are required when it is used as a shaft seal lip material. Sealability decreases noticeably in the presence of shaft vibrations or eccentricities.

PTFE PERFORMS WELL IN:
• Petroleum oils & fuels
• Ozone
• Chemical applications
• Solvents (MEK, acetone, xylene)
• Weather

PTFE DOES NOT PERFORM WELL IN:
• Highly fluorinated greases & oils

“Because of PTFE’s unique properties, it is used for high pressure, high speed, and non-lubricated applications, as well as in applications involving aggressive chemicals.”

Figures 40-42