Chemistry Reference
In-Depth Information
This scheme explains the formation of carboxyl groups, acetaldehyde, benzaldehyde and
also considerable quantity of CO
2
during PETP thermal destruction.
Water, appearing in the system during forming polyene structures from the product of
PETP thermal destruction - acetaldehyde, causes hydrolysis of ester bonds, increasing the
rate of carboxyl groups formation. Besides, there takes place decrease of polymer relative
viscosity [202].
On the basis of mass - spectroscopic method there was made a supposition that the break
takes place mainly in carboxyl groups in three states [203]:
-CH
2
-CH
2
-/-CO-/-O-/-C
6
H
4
-
Mass - spectroscopic investigations are proved by pyrograms [204].
Kinetic parameters of thermal destruction were defined by DTA method in the work
[205]. Obtained data correlate well with the results received from the curves of kinetics of
carboxyl groups concentration increase.
Presence of CH
2
- unit in aliphatic groups leads to the decrease of thermal stability.
Attack during thermal destruction is directed onto α-CH
2
- group, as this group in
diethyleneoxide fragment differs by particular vulnerability [206].
Time of temperature effect on the sample influences the process of PETP destruction.
During short effect sharp decrease of polymer specific viscosity takes place while polymer
mass does not change. Drop of viscosity value slows down during the increase of temperature
effect time and polymer mass loss sharply increases. Viscosity change is caused by thermal
destruction of macromolecule, and mass change - by the destruction of chain end groups
[207].
Thermal destruction begins at lower temperatures in the air than in the inert medium.
This occurs because of the presence of oxygen [208].
Rate of thermal destruction is 10 times higher in the presence of oxygen than in the inert
medium [209]. At relatively low temperatures (up to 140
0
C) PETP fibre is quite resistant to
thermooxidation, but at the temperatures above 220-250
0
C oxidation processes flow with
considerable rate [210]. Thermal destruction in the presence of oxygen goes under the
complex mechanism including thermooxidation.
During thermooxidative destruction of PETP-fibre at 280
0
C [211] there were found the
same products of decomposition as during thermal destruction (in atmosphere of helium) but
in much larger quantities.
A great number of investigations are devoted to the study of thermooxidative destruction
by the method of differential thermal analysis (DTA) [212, 213]. During DTA curves analysis
it has been found out that resistance to PETP thermooxidation decreases with the increase of
its molecular mass [212].
All temperature transitions characteristic for thermooxidative destruction were well
observed on DTA curves. On DTA curves it is clearly seen that exothermic peak of oxidation
is being observed in the presence of oxygen in the melting region. Besides, DTA curves show
that PETP thermooxidation is slowing down as the temperature is approaching melting point,
and this is connected with the softening of the sample and further decrease of its surface.
DTA method was used to define kinetic parameters of thermooxidative destruction.
Calculated parameters correlate quite well with the results obtained from the curves of
kinetics of carboxyl groups concentration increase at isothermal oxidation of PETP [213].
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