Chemistry Reference
In-Depth Information
to 1,000 atm may be used. Radicals generated at temperatures between 50 and 100
C yield very high
molecular weight polymers.
Poly(vinyl fluoride) is moderately crystalline. The crystal melting point,
T
m
, is approximately
200
C. The high molecular weight polymers dissolve in dimethylformamide and in tetramethyl urea
at temperatures above 100
C. The polymer is very resistant to hydrolytic attack. It does, however,
loose HF at elevated temperatures.
6.17.5 Copolymers of Fluoroolefins
Mary different copolymers of fluoroolefins are possible and were reported in the literature. Commer-
cial use of fluoroolefin copolymers, however, is restricted mainly to elastomers. Such materials offer
superior solvent resistance and good thermal stability.
The elastomers that are most important industrially are vinylidine fluoride-chlorotrifluoroethylene
[
260
] and vinylidine fluoride-hexafluoropropylene copolymers [
282
]. These copolymers are amor-
phous due to irregularities in their structures and can range in properties from resinous to elastomeric,
depending upon composition [
283
]. Those that contain 50-70 mole percent of vinylidine fluoride are
elastomers. The
15
C, also depending upon vinylidine fluoride content [
284
].
They may be cross-linked with various peroxides, polyamines [
284
], or ionizing radiation. The cross-
linking reactions by peroxides take place through hydrogen abstraction by primary radicals:
T
g
ranges from 0 to
F
F
+
R
+
RH
n
n
F
F
F
F
F
2
n
m
m
F
FF
Copolymers of vinylidine fluoride with hexafluoropropylene are prepared in aqueous dispersions
using persulfate initiators. Hexafluoropropylene does not homopolymerize but it does copolymerize.
This means that its content in the copolymer cannot exceed 50%. Preferred compositions appear to
contain about 80% of vinylidine fluoride. The cross-linking reactions with diamines are not
completely understood. It is believed that the reaction takes place in two steps [
285
,
286
]. In the
first one, a dehydrofluorination occurs:
F
CF
3
F
F
CF
3
F
F
F
-HF
n
F
F
F
n
F
F
F
F
The above elimination is catalyzed by basic materials. These may be in the form of MgO, which
is often included in the reaction medium. In the second step the amine groups add across the
double bonds:
