STABILIZATION AND MODIFICATION OF POLYCAPROAMIDE - Photochemical Reactions in Heterochain Polymers - page 6

Р 1 * + О 2 → Р 1 О 2 * (in the absence of water)
Р 1 О 2 + К р → Р 1 ООН + К р
Effect of water is connected with the electron transfer from ion OH − to excited molecule
of a dye and, in such a way, forming radical O*H:
К р * + ОН - → К р + О * Н
О * Н + Р 1 Н → Н 2 О + Р 1 * (in the presence of water)
Since polyamides are classified as compounds weakly reacting to O 2 , then the second
mechanism is more probable for them, though this question is still open because
photodestruction of dyed PA depends also on medium, dye properties and conditions for
carrying out experiments.
1.1.3. Thermooxidative Destruction of Polyamides
Macromolecule of PA consists of methylene chains connected by amide groups, which
cause vulnerability of these polymers at thermooxidative destruction.
Thermooxidation of PA-fibers becomes visible at temperatures 100-150 0 C. PCA - fiber
nonreversibly loses 58-65% of initial strength after warming-up in the air at the temperature
180˚C for 2 hours, and after 14 hours heating - 75%. PA-fiber loses a great part of molecular
weight at high temperatures (270-350 0 C) [33].
That is why, destruction processes go on at temperatures of PA synthesis and processing.
And these processes lead to the change of PA structure, decline of phisico-chemical
properties of PA-products, which influence their thermostability during usage.
Detailed survey on thermal destruction of a wide range of polymers and PA, specifically,
is presented in monograph of Kovarskaya [34]. There are a few works in literature devoted to
investigation of the kinetics of oxidation at relatively low temperatures. Kinetics of PCA
thermooxidation has been studied in the work [35] having the aim to determine the possibility
of forecasting of shelf lives and usage of PA-materials and products on their basis, proceeding
from the temperature dependence of their “induction periods of oxidation”.
Pakhomov and his assistants [36] note, that loss of strength is defined by kinetic
destruction of molecular chains in amorphous regions of PA. Low values of activation energy
of thermooxidative destruction may be explained by PCA water receptivity and hydrolysis of
amide bonds at the time of warming up. So, for dry PCA-E act =43 Kcal/mole, and in the
presence of moisture - E act = (20-30) Kcal/mole. Last conclusions agree with the results of
investigation [37] quite of satisfactory.
Kinetic model of thermooxidation of aliphatic PA is given later works [24], model
including formation of azomethine structures during the interaction of aldehyde and ketone
groups with end NH 2 - groups of PA and further oligomerization of azomethine structures,
accompanied by the appearance of conjugated links and PA dyeing. It is noted that calculated
values of the change of functional groups (CO; COOH; NH 2 ) concentrations and also