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1.5.
M
ECHANISM OF
L
IGHT
-S
TABILIZING
A
CTION OF
B
IS
-A
ROILENBENZIMIDAZOLES
Breaks of macromolecule main chain, being the results of complex set of processes
taking place in polymer, following primary acts of light and oxygen absorption, cannot
characterize the process of oxidative destruction on the whole, measure of which may be only
oxygen absorption [132]. Method of determining oxygen absorption during photooxidation of
polymers is highly sensitive and allows to estimate the efficiency of stabilizers action at early
stages of photooxidation at small polymer weights, at low intensities of irradiation, which
gives the possibility to create conditions of artificial ageing, approximate to natural.
1.5.1. Absorption of Oxygen by Polycaproamide Stabilized by
Aroilenbenzimidazole Derivatives
Taking into account the most important role of oxidative processes at PCA light ageing
(Section 1.1) it was reasonable to investigate oxygen absorption by initial and stabilized
PCA-fibre during long-wave irradiation by filtered light of the lamp DRSH-1000, because
under the action of long-wave light, which is not absorbed by chromophore amide group of
the polymer, photooxidation of PCA is defined by initiation action of impurities, products of
oxidation and also special additives (pigments, dyes and so on) [16].
Curves of the dependence of absorbed oxygen amount per unit of polymer mass on the
irradiation period at the given temperature are given in Figures 1.7-1.9. Analysis of kinetic
curves of oxygen absorption shows that at initial stage photooxidation is going on at a great
speed but after some time this process slows down and at last goes into stationary mode.
Probably at the initial stage of irradiation there takes place oxidation of impurities or by-
products being formed in the polymer during the process of its production and processing. It
may be supposed that formation and determination of stationary concentration of ketoimide
product [26] responsible for the process of PCA oxidative destruction plays a great role. It
should be noted that in our experiments stationary mode is set up for 3-5 hours depending on
the nature of irradiated samples. After the light source is turned off the process of oxidation
gradually inhibits up to complete stoppage. However, repeated turn on of the light leads to
setting stitionary mode up already in 0,5 hour, moreover this pause in irradiation may be
rather long - a day or more.
All this shows that photooxidation of impurities in PCA at the initial stage does not
influence further course of the process of PCA oxidation. That is why the rate of oxidation in
stationary mode may be used for quantitative evaluation of photooxidative stability of PCA
fibres.
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