Chemistry Reference
In-Depth Information
Gupper et al. used micro Raman spectroscopy to study thermal degradation of poly(vinyl chloride)
containing various additives [
586
]. They observed a linear increase in conjugated sequences in the
process of dehydrochlorination.
Carty and coworkers investigated thermal decomposition of chlorinated poly(vinyl chloride)
[
496
]. The thermal decomposition of pure chlorinated poly(vinyl chloride) (without stabilizer or
lubricant) was studied by dynamic thermogravimetric analysis at heating rates from 5 to 100
C/min in
atmospheres of nitrogen, air, and oxygen. In each case, a two-step decomposition was observed,
similar to that for poly(vinyl chloride) where dehydrochlorination is followed by pyrolysis/oxidation
of the carbonaceous residue. The rate of dehydrochlorination was dependent on atmosphere, occur-
ring slightly slower in nitrogen than in air, and slightly more quickly in oxygen than in air. The
decomposition of the residual char was clearly dependent on the conditions that it formed in. Under
dynamic conditions, chars formed at high heating rates appeared more resistant to oxidative degra-
dation than those formed more slowly. However, when chars were formed by heating at different rates
and then held at 500
C, the char formed at the slowest heating rate was the slowest to be oxidized. The
uptake of oxygen by the char appears to be rate limiting. At low heating rates, char oxide is similar in
both air and oxygen. As the heating rate is raised, the rate of mass loss of char in air becomes
progressively closer to that in nitrogen until at 100
C/mm they are almost identical.
The rates of thermal decompositions of
were shown to depend upon the
method by which the polymers were prepared [
497
]. Those that were formed from very pure
monomers by mass polymerization are most stable. Polymers prepared by emulsion polymerization,
on the other hand, degrade fastest. The mechanism of degradation of poly(vinylidine chloride) was
proposed to be as follows [
498
-
500
]:
poly(vinylidine chloride)s
1. Hydrochloric acid is eliminated in a chain reaction.
2. Conjugated sequences condense to form cross-linked structures.
Some support for the above mechanism came from stepwise heating studies of poly(vinylidine
chloride) [
501
].
Although
has the reputation for being quite stable thermally, it does
degrade at elevated temperatures. The polymer upon pyrolysis yields almost 100% of monomer.
The mechanism is believed to be free-radical unzipping of the chains until the entire chain is
consumed. This can be illustrated as follows [
457
]. Initially, the chain ruptures:
polytetrafluoroethylene
FF
F
F
FF
FF
FF
F
F
2
FF
F
F
FF
FF
this is followed by formation of monomers:
F
F
FF
F
F
F
F
+
FF
F
F
is less stable thermally than polytetrafluoro-ethylene. It yields as
much as 86.0% volatiles in 30 h at 331.8
C[
11
]. These volatiles contain large amounts of monomer.
A mechanism that resembles the postulated degradation mechanism of polytetrafluoroethylene was
proposed [
457
].
Polychlorotrifluoroethylene
