Chemistry Reference
In-Depth Information
6.17 Halogen-Bearing Polymers
The volume of commercial fluorine-containing polymers is not large when compared with other
polymers like, for instance, poly(vinyl chloride). Fluoropolymers, however, are required in many
important applications. The main monomers are tetrafluoroethylene, trifluorochloroethylene, vinyl
fluoride, vinylidine fluoride, and hexafluoropropylene.
6.17.1 Polytetrafluoroethylene
This monomer can be prepared from chloroform [
275
]:
50 -100
o
C
+
+
CHCl
3
CHCF
2
2HF
2HCl
up t o 39 at m
pressure
F
F
70
o
C
2 CHCF
2
+
HCl
F
F
76.3
C. It is not the only product from the above pyrolytic reaction of
difluorochloromethane. Other fluorine by-products form as well and the monomer must be isolated.
The monomer polymerizes in water at moderate pressures by free-radical mechanism. Various
initiators appear effective [
276
]. Redox initiation is preferred. The polymerization reaction is strongly
exothermic, and water helps dissipate the high heat of the reaction. A runaway, uncontrolled polymer-
ization can lead to explosive decomposition of the monomer to carbon and carbon tetrafluoride [
277
]:
Tetrafluoroethylene boils at
F
F
+
C
CF
4
F
F
Polytetrafluoroethylene is linear and highly crystalline [
278
]. Absence of terminal CF
2
= CF--
groups shows that few, if any, polymerization terminations occur by disproportionation but probably
all take place by combination [
279
]. The molecular weights of commercially available polymers
range from 39,000 to 9,000,000. Polytetrafluoroethylene is inert to many chemical attacks and is only
swollen by fluorocarbon oils at temperatures above 300
C. The
T
m
of this polymer is 327
C and the
100
C.
The physical properties of polytetrafluoroethylene depend upon crystallinity and on the molecular
weight of the polymer. Two crystalline forms are known. In both cases the chains assume helical
arrangements to fit into the crystalloids. One such arrangement has 15 CF
2
groups per turn and the
other has 13.
Polytetrafluoroethylene does not flow even above its melting point. This is attributed to restricted
rotation around the C-C bonds and to high molecular weights. The stiffness of the solid polymer is
also attributed to restricted rotation. The polymer exhibits high thermal stability and retains its
physical properties over a wide range of temperatures. The loss of strength occurs at about the
crystalline melting point. It is possible to use the material for long periods at 300
C without any
significant loss of its strength.
T
g
is below
