Some Industrially Important Polymers - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 128

make lower molecular weight polymers, molecular weight regulators, also
known as chain transfer agents, can be added to the polymerization. Examples
of chain transfer agents include hydrogen and compounds with a C - H bond
such as alkanes.
The PTFE chain adopts a slowly twisting helix with an outer sheath of
fluorine atoms encompassing a carbon-based core. There is mutual repulsion
of the fluorine atoms. These cylinders can slip past each other and this can
lead to cold flow or creep [32]. Creep is permanent deformation of a plastic
due to the prolonged application of stress. An example is a plastic strap that is
holding a weight and over time permanently stretches and is elongated even
after the weight is removed. PTFE is more susceptible to creep than many
other polymers.
Porous PTFE, also known as expanded PTFE (ePTFE) can be highly
porous while retaining excellent strength. Fluorocarbons are hydrophobic so
the combination of porosity and hydrophobicity makes ePTFE excellent for
waterproof clothing. This technology is the basis for the outdoor clothing
made from Gore-Tex ® [33 - 35].
There are several commercial copolymers of TFE such as the copolymer
of TFE with hexafluoropropylene (HFP), referred to as fluorinated ethylene
propylene (FEP). FEP has lower melt viscosity, enabling conventional melt
processing and is often used for wire insulation and molded parts. There
is a tradeoff with lower thermal stability and lower upper use temperature.
Another copolymer is TFE with perfluoro(propyl vinyl ether). This is com-
monly referred to as a perfluoroalkoxy (PFA) polymer. PFA polymers are
melt-processable and generally have better thermal stability and high temper-
ature properties than FEP polymers. PFA is commonly used as pipe liners,
tubing, and films. Copolymer side chains such as the perfluoropropoxygroup
and trifluoromethyl group increase the resistance of polymer chains sliding
past each other and therefore improve the creep resistance.
Other fluorinated polymers include polyvinylidenedifluoride (PVDF),
made by the polymerization of 1,1-difluorethylene. PVDF is known by its
tradenames, Kynar ® (Arkema) and Hylar ® (Solvay) and is less expensive. It
has excellent chemical resistance and is used as a coating for metals and for
wire jacketing.
8.10 POLYPHENYLENE SULFIDE
There have been several earlier attempts to make polyphenylene sulfide
(PPS), but in the late 1960s, workers at Phillips Petroleum developed the first
commercial process [36]. Sodium sulfide reacts with p-dichlorobenzene in a
polar organic solvent. Common trade names are Ryton ® (Phillips Petroleum)