Some Industrially Important Polymers - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 127

Consumers know PTFE because it is used as a coating material in non-stick
cookware. The excellent chemical resistance makes it suitable for coating
reactors and piping that are exposed to aggressive chemicals. The extremely
low coefficient of friction makes it useful for bearings and gears. Specialty
bullets are coated with PTFE to reduce wear on a rifle barrel.
PTFE is available either as the homopolymer or modified with small
amounts (typically less than 1% by weight) of a comonomer. The comonomer
can have the benefit of improving processability by decreasing the molecular
weight. Crystallization is also inhibited. The modifier is used at a low enough
level that the desirable PTFE properties are maintained.

F F F F

+

PTFE

F F F R

TFE

Optional modifying modifier

R = CF 3 , OCF 2 CF 2 CF 3 , OCF 2 CF 3 , etc.

As with PVC, PTFE is made by a radical polymerization and can be made
by a suspension process or an emulsion process. The emulsion polymeriza-
tion gives an aqueous dispersion of PTFE and is sometimes referred to as
a dispersion polymerization. The TFE is dispersed in an agitated aqueous
medium, using a surfactant and a water soluble free radical initiator. Often a
hydrocarbon such as molten paraffin wax is present to retard coagulation of
the PTFE [29]. Typically, the surfactant has been a perfluoroalkanoic acid or
salt, commonly ammonium salts, such as the ammonium salt of perfluorooc-
tanoic acid (PFOA). PFOA has also been used in the textile industry to impart
water and oil repellency. PFOA is persistent in the environment and has been
found in blood samples of the general United States population so the industry
has been working toward elimination of plant emissions and product content.
Other surfactants, many of them based upon ethers have been described.
For example, CF 3 O(CF 2 ) 3 OCHFCF 2 CO 2 − NH 4 + has been described as a
TFE polymerization surfactant that in rat studies had a much lower tendency
to bioaccumulate than the ammonium salt of PFOA [30]. PTFE can also
be prepared by a suspension process [31]. This usually involves vigorous
agitation of an aqueous medium and yields granulated particles.
The carbon - luorine bond is much stronger than the carbon - hydrogen
bond (485 kJ/mol for C - F; 413 kJ/mol for C - H). Because of this, the C - F
bond is much less likely to be broken by homolytic cleavage. This is a major
contributor to the excellent UV, thermal, and oxidative stability of PTFE. It
also means that in the radical polymerization, chain termination reactions are
less likely. Therefore, PTFE tends to polymerize to high molecular weights
which in turn causes high viscosity and problems with melt processing. To