Chemistry Reference
In-Depth Information
Photoinitiator X, primary concentration of which in PA depends on prehistory of samples
and exceeds, as a rule, stationary value, achieved under light action, is practically the only
source of PA fluorescence.
Suggested mechanism allowed to describe quantitatively PA photooxidation at ultra-
violet and long-wave irradiation; there were got equations allowing to determine
concentrations of P
1
O
2
*X [27, 28, 29].
Thus, analysis of literary data allows to come to a conclusion about presence of PA
luminescence, related to the product of keto-imide structure, which is formed and consumed
in chain processes, performing here the function of photoinitiator. Unlike traditional schemes
of photoinitiation intermediate hydroperoxides does not play great role at photooxidative PA
destruction.
1.1.2. Photooxidation of Dyed Aliphatic Polyamides
Sensibilized mechanism of PA oxidation in the presence of dyes, pigments and other
additives is interesting from the point of view of practical use. Knowledge of this mechanism
allows to select effective additives to increase polymer photooxidative stability. Sensibilizing
action of TiO
2
is shown in the work [29]. TiO
2
is used for dulling PA fibers, which leads to
decrease of physical-mechanical strength of yarn from PA-66 at light action with wave-light
λmax=360 nm. Sensibilizing activity of TiO
2
is displayed at generation of singlet oxygen
1
O
2
and anion-radical O
2
in the presence of water - radical O*H and HO
2
*, which, in their
turn, are able to sensibilize PA oxidation. Dyes and pigments may have similar effect.
While investigating some xanthene, thiazine, azo and antraquinone dyes it has been
shown that they sensibilize photodestruction of aliphatic PA [30]. At studyng reaction
between α, β - nonsaturated carbonye groups of PA-66 and photoactive dye “Dispersed
yellow 13” it has been found that the process of triplet-singlet energy transfer takes place at
the distance of 4-6nm between donor and acceptor. This process is the reason of low light
stability of the dye and its photodestructive effect on polymer [31].
Determination of primary processes is very important for solving problems of dyed fibers
photodestruction. Today there are several theories trying to explain photodestruction of dyed
PA materials. They suppose that excited molecule of a dye transfers energy to molecular
oxygen, forming singlet oxygen
1
O
2
, which, in its turn, causes destruction of polymer
according to the following scheme:
К
р
hν
→ К
р
*
К
р
*
+ О
2
→ К
р
+
1
О
2
1
О
2
+ Р
1
Н → products of oxidation (in the absence of water)
1
О
2
+ Н
2
О
→ 2Н
2
О
2
Н
2
О
2
+ Р
1
Н → products of oxidation (in the presence of water)
According to another theory [32] the first stage of destruction is hydrogen atom (or
electron) breaking off the polymer by the excited molecule of a dye:
К
р
hν
→ К
р
К
р
+ О
*
Н
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