Chemistry Reference
In-Depth Information
appears to be greatly enhanced by imperfections in the
polymer structure that form during polymerization and processing. Formation of peroxide and
carbonyl groups is very difficult to prevent. The initiation of photo-oxidation probably results from
the free-radical that forms as these groups decompose upon irradiation [
568
]. In addition, double
bonds that form from loss of HCl will sensitize formation of free radicals. These radicals are further
oxidized and also promote cross-linking that results in gels.
Photo-oxidation of
Photo-oxidation of
poly(vinyl chloride)
was also shown to involve singlet oxygen [
570
]. Attacks by
singlet oxygen on double bonds with formation of allylic hydroperoxides and shifts of these double
bonds according to the
cis-polybutadiene
ene
reaction were confirmed by several other studies [
571
]. On the other hand,
a study of model compounds for photo-oxidation of polyisoprene failed to show formation of endo
peroxides by 1,4-cycloaddition [
572
].
Aromatic polyurethanes exhibit a strong tendency to yellow in sunlight. This may be accounted for
by two mechanisms based on formation of different photoproducts. The first one was found to be a
photo-Fries reaction caused by a short wave length light [
572
]. The second one, caused by longer
wave length light, is formation of quinonoid structures [
573
]:
O
O
R
hν/O
2
O
N
NO
H
H
O
O
R
N
O
O
N
H
+
O
O
R
O
N
N
O
Photo-oxidation of Nylon 6 was also investigated [
574
]
The subject of
is seldom raised because these polymers are known for
their stability to oxidation. Nevertheless, an investigation of photo-oxidative degradation of these
materials showed that these materials can be affected by combined attacks of oxygen and ultraviolet
irradiation [
585
]. The groups that are affected are the functional groups that are more fragile than the
backbones. Thus, the silicon hydride group was found to be readily photo-oxidized, perhaps due to its
low bond energy. The reaction can be illustrated as follows [
585
]:
photo-oxidation of silicones
O
2
SiH
Si
H
Si
Si
OO
Si
OOH
The main process that occurs in silicones that contain vinyl groups is photo-scission. Also, the
vinyl groups are photo-oxidized into silanols, carboxylic acids, and esters. Part of the silanols are
condensed into high molecular weight silicones [
585
].
