Chemistry Reference
In-Depth Information
was studied at 280
C[
535
]. The oxidation is a
radical chain reaction with the initiation consisting of hydrogen abstraction from a methyl group:
Thermal-oxidative degradation of
polysulfones
O
O
CH
2
+
HOO
S
+
O
2
S
O
O
The process then continues with formation of alkyl peroxide radicals, isomerization, breakdown,
and formation of oxygen-containing compounds like aldehydes, and ketones.
In the oxidative pyrolysis of
the oxygen attacks methylene groups first to form
hydroperoxides [
536
]. This is followed by chain cleavage:
poly-p-xylene,
OH
O
O
O
2
O
H
2
C
+
OH
-H
O
O
O
O
2
OO
H
+
OOH
+
CO
+
CO
2
+H
2
O
9.9.13 Photo-Degradation of Macromolecules
The quantum energies associated with sunlight in the violet and near ultraviolet portions of the
spectrum are of the magnitude sufficient to rupture chemical bonds present in most polymers [
537
].
The mechanism of photo-degradation, however, is more complex than would be visualized by the
simple bond rupture, because photo-absorptions are complicated by various factors, including
crystallinity that causes scattering of light. In addition, it was demonstrated that little light is absorbed
above the wavelength of 2,800
˚
. Yet, this wavelength represents the lower limit of sunlight reaching
the earth. On the other hand, presence of impurities that can act as photosensitizes can markedly
accelerate the degradation process.
Polymers that are in current commercial use on a large scale fall roughly into three categories,
depending upon their ability to withstand photo-degradation [
538
]:
1. Polymers that are resistant to sunlight attacks outdoors, like polyethylene and poly(methyl
methacrylate).
2. Moderately stable polymers, like poly(ethylene terephthalate) and polycarbonate.
3. Polymers that are unstable in sunlight and require ultraviolet light stabilizer, like poly(vinyl
chloride), polypropylene, nylons, rubbers, and cellulose.
