Page 68 - Handbook of Plastics Technologies

THERMOPLASTICS

2.8 CHAPTER 2

stant, low friction properties, and low gas permeability. Its impact strength is better than
56
PTFE, but the other mechanical properties are similar to PTFE. FEP may be processed
by injection, compression, or blow molding. FEP may be extruded into sheets, films, rods
or other shapes. Typical processing temperatures for injection molding and extrusion are
57
in the range of 300 to 380°C. Extrusion should be done at low shear rates because of the
polymer’s high melt viscosity and melt fracture at low shear rates. Applications for FEP
58
include chemical process pipe linings, wire and cable, and solar collector glazing. A ma-
terial similar to FEP, Hostaflon TFB (Hoechst), is a terpolymer of tetrafluoroethylene,
hexafluoropropene, and vinylidene fluoride.
Ethylene chlorotrifluoroethylene (ECTFE) is an alternating copolymer of chlorotrifluo-
roethylene and ethylene. It has better wear properties than PTFE along with good flame re-
sistance. Applications include wire and cable jackets, tank linings, chemical process valve
59
and pump components, and corrosion-resistant coatings.
Ethylene tetrafluoroethylene (ETFE) is a copolymer of ethylene and tetrafluoroethyl-
ene similar to ECTFE but with a higher use temperature. It does not have the flame resis-
60
tance of ECTFE, however, and will decompose and melt when exposed to a flame. The
polymer has good abrasion resistance for a fluorine containing polymer, along with good
impact strength. The polymer is used for wire and cable insulation, where its high temper-
ature properties are important. ETFE finds application in electrical systems for computers,
aircraft and heating systems. 61
2.2.4.2 Polychlorotrifluoroethylene (PCTFE). Polychlorotrifluoroethylene (PCTFE) is
made by the polymerization of chlorotrifluoroethylene, which is prepared by the dechlorina-
62
tion of trichlorotrifluoroethane. The polymerization is initiated with redox initiators. The
replacement of one fluorine atom with a chlorine atom, as shown in Fig. 2.5, breaks up the
symmetry of the PTFE molecule, resulting in a lower melting point and allowing PCTFE to
be processed more easily than PTFE. The crystalline melting point of PCTFE at 218°C is
lower than PTFE. Clear sheets of PCTFE with no crystallinity may also be prepared.

FIGURE 2.5 Structure of PCTFE.

PCTFE is resistant to temperatures up to 200°C and has excellent solvent resistance,
with the exception of halogenated solvents or oxygen containing materials, which may
63
swell the polymer. The electrical properties of PCTFE are inferior to PTFE, but PCTFE
is harder and has high tensile strength. The melt viscosity of PCTFE is low enough that it
64
may be processing using most thermoplastic processing techniques. Typical processing
temperatures are in the range of 230 to 290°C. 65
PCTFE is higher in cost than PTFE, somewhat limiting its use. Applications include
gaskets, tubing, and wire and cable insulation. Very low vapor transmission films and
66
sheets may also be prepared.
2.2.4.3 Polytetrafluoroethylene (PTFE). Polytetrafluoroethylene (PTFE) is polymer-
67
ized from tetrafluoroethylene by free radical methods. The reaction is shown below in
Fig. 2.6. Commercially, there are two major processes for the polymerization of PTFE,

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.