Chemistry Reference
In-Depth Information
9.8.5 Thermal Degradation of Chlorocarbon and Fluorocarbon Polymers
The thermal instability of
is a cause for concern commercially and has, therefore,
stimulated extensive investigations. The overall process of degradation is complex and still not
completely resolved. Some of the questions that remain are:
poly(vinyl chloride)
1. Is the overall process of degradation ionic or free radical in nature?
2. What is the exact mechanism of initiation?
It is easilly observable that the degradation is accompanied by evolution of HCl and blackening of
the polymer. Also, at elevated temperatures, poly(vinyl chloride) not only gives off hydrogen
chloride, but this dehydrochlorination is also accompanied by rapid depolymerization [
483
]. The
rate of decomposition decreases with increasing molecular weight and is highest in oxygen and
lowest in helium atmosphere. It was claimed that HCl exerts no catalytic effect upon the rate of
decomposition [
483
]. This contrasts with evidence that was presented earlier that HCl does accelerate
thermal degradation [
484
]. The earlier evidence was obtained from autocatalytic decmposition of
films. Based on that eveidence, it was hypothesized that the chloride ion acts as a base (supporting an
ionic mechanism of decomposition) [
484
]:
HCl
H
+
Cl
2
HCl
H
+
H
Cl
2
Cl
+
+
HCl
+
Cl
Cl
Cl
+
+
HCl
Cl
+
n
n
Cl
In a more recent study, HCl was also found to be essential for the initiation of the unzipping
process, but may or may not be essential to the depolymerization reaction itself, once the process has
been started [
485
]. The above shown ionic mechanism was disputed in a study where ESR signals
were recorded during thermal decomposition of the polymer at elevated temperatures [
486
]. This
strongly supports a free-radical mechanism. A free-radical mechanism was proposed earlier by
Bamford and Feuton [
487
]. This mechanism illustrates the formation of hydrochloric acid and is
based on rupture of carbon-chlorine bonds:
Initiation:
Δ
+
Cl
Cl
Cl
+
HCl
+
Cl
Cl
