Chemistry Reference
In-Depth Information
some of the hydrolytic depolymerization of PET is catalyzed by the carboxylic acid groups produced
during the reaction. The dependence of the rate constant on the reaction temperature was correlated
by the Arrhenius equation [
530
].
9.9.10 Oxidative Degradation of Polymers
This section deals not only with oxidative reactions of polymers at room temperature, but also with
thermo-oxidative degradation at elevated temperatures. Although this type of degradation resembles
photo-oxidative degradation, for the purpose of maintaining clarity, the latter is discussed separately
in the next section.
The simplest oxidative reactions occur in hydrocarbon polymers. This free-radical process gener-
ally follows the same path as the oxidation of low molecular weight compounds. The difference,
however, is in the propagation of the reaction. In oxidation of low-molecular weight hydrocarbons,
each step of chain propagation results in transfer of active center from on molecule to another. In
polymers, however, the probability of such a transfer is low. Instead, the oxidation propagates along
the polymer backbone [
520
].
9.9.11 Oxidation of Chain-Growth Polymers
Polymers that lack double bonds, like polyethylene, can be considered high molecular weight
paraffin. They are slow to oxidize in the absence of UV light, much like the low molecular weight
hydrocarbons. On the other hand, polymeric materials with double bonds oxidize rapidly. Neverthe-
less, polymers like polyethylene may oxidize rapidly as well when contaminated with metallic ions
because such ions catalyze the decomposition of peroxides.
The chemical structure of the polyolefins determines their susceptibility to oxidative degradation.
Linear polyethylene, in the absence of additives, is more resistant to oxidation that polypropylene that
oxidizes rather readily due to the presence of labile tertiary hydrogens. It was demonstrated, for
instance, that the molecular weigh of polypropylene sheets in a 138
C oven can drop from 250,000 to
approximately 10,000 in 3 h [
522
]. The process of oxidation was shown to take place according to the
following scheme [
522
]:
O
2
O
2
OO
termination
propagatio
n
OOH
O
OO
