Chemistry Reference
In-Depth Information
O
hν
OH
+
O
OOH
The hydroperoxides apparently have no photo inductive effect on the overall oxidation of
polyethylene [
566
]. Photolysis of hydroperoxides, however, initiates new vinylidine oxidation.
It was shown that the kinetics of photo-oxidation of polyethylene is characterized by the superpo-
sition of two phenomena. The first corresponds to an exponential increase in the concentration of the
carbonyl groups with time and is observed when the kinetics are controlled by the diffusion of
oxygen. The second one is not controlled by diffusion, but corresponds to a linear increase of the
carbonyl concentration with time and takes place in degraded samples. This is explained in terms of
chain rupture in the amorphous regions of the polymer, allowing free access of oxygen [
567
].
Commercial
can photo-degrade rapidly. This is due to a presence of chromophore
groups that form during polymerization. In addition, chromophores may also form through oxidation by
singlet oxygen during extrusion in air. At temperatures of 250-300
C, light of 300 nm will initiate
oxidation by atmospheric oxygen [
568
]. The hydroperoxide groups that form are not stable and
decompose to alkoxy radicals to form ketones on the backbones and at the ends of the chains. These
ketones in turn undergo Norrish Type I and Type II reactions when irradiated with ultraviolet light [
568
]:
polypropylene
O
90
o
C
O
CH
2
+
hν
10%
+
OH
+
