Chemistry Reference
In-Depth Information
9.9.2 Thermal Degradation of Polyesters
Poly(ethylene terephthalate)
decomposes upon heating through a series of different reactions. These run
either concurrently or consecutively. The result is a complex mixture of volatile and nonvolatile
products. It was found that when poly(ethylene terephthalate) is maintained in molten condition under
an inert atmosphere at 282-323
C, it slowly converts to a mixture of gaseous low molecular weight
fragments [
581
]. The major products from pyrolysis of poly(ethylene terephthalate) are carbon dioxide,
acetaldehyde and terephthalic acid. In addition, there can be detected trace amounts of anhydrides,
benzoic acid,
-acetylbenzoic acid, acetophenone, vinyl benzoate, water, methane, ethylene, acetylene,
and some ketones [
505
]. The following mechanism of degradation was postulated [
505
]:
p
O
O
Δ
O
O
O
O
O
+
O
O
O
O
The vinyl end groups that form fromcleavage of the ester groups decompose further in a number of ways:
OH
+
O
O
+
CO
2
O
+
OH
O
O
O
O
O
O
O
O
OO
O
+
poly(butylene terephthalate)
was examined with the aid of a laser
microprobe and mass spectrometry [
506
]. A complex multistage decomposition mechanism was
observed that involves two reaction paths. The initial degradation takes place by an ionic mechanism.
This results in an evolution of tetrahydrofuran. This is followed by concerted ester pyrolyses reactions
that involve intermediate cyclic transition states and result in formation of 1,3-butadiene. Simulta-
neous decarboxylations occur in both decomposition paths. The latter stages of decomposition are
The thermal degradation of
