Antioxidants and Metal Ion Deactivating Agents
In 1897 Engler and Bach [1] reported one of the earliest studies on thermooxidative degradation of materials. They developed a theory of oxidation leading to peroxides.
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Antioxidants and Metal Ion Deactivating Agents
10.1 Thermo-oxidative Degradation In 1897 Engler and Bach [1] reported one of the earliest studies on thermooxidative degradation of materials. They developed a theory of oxidation leading to peroxides. Very important results from studies on oxidations in the gas phase have also been obtained. They have shown that oxidation is a radical chain reaction, and this has been confirmed more recently by electron paramagnetic resonance (EPR) [1].
The mechanism of oxidation in the liquid phase is more difficult to study . In spite of numerous works published in this domain, it is not yet possible to define exactly the basic processes or their relative importance in oxidation reactions of liquid organic substances. However, the chemiluminescence method offers some promising applications in this field [2]. It has been used to show the participation of R· and ROi radicals in these oxidation reactions [3]. This method measures the intensity of chemiluminescence produced by the combination of ROi radicals . Absolute rates of different oxidizing reactions, efficiencies of initiators, and rate constants of radical reactions involving some antioxidants have been measured with this experimental technique [4]. A stepwise mechanism based on results from many works [1,5,6] can be suggested for the oxidation of hydrocarbons in the liquid phase . The same mechanism can be applied to thermal oxidation and to light-induced oxidation of polymers [7,8]: the main steps are as follows: Initiation: RH + O2 ~ R· + HOi
activation energy (Ea) = 126-189 kJ mol- l [9,10,11]. J. Štěpek et al., Additives for Plastics © Springer Science+Business Media New York 1983
(1)
168
10: Antioxidant s and Metal Ion Deactivat ing Agents
Free radicals R· derived from RH might also be formed by a bimolecular reaction according to 2RH- 2R· + Hz
(2)
which is thermodynamically possible [12]. Howe ver , E; for step 2 is 280-410 kJ mol" [13], which is twice as large as that for the much more probable step (1). A trimolecular reaction is also possible [14]: 2RH + Oz -
2R· + HzOz
(3)
Propagation: k,
R· + Oz RO i
RO i k:
(E a
= 0)
+ RH - ROOH + R·
(4) (rate-determining step)
(5)
The abstraction of a hydrogen atom from a polyolefin is done by an intramolecular reaction:
or by an intermolecular reaction [15]:
It must be recalled that the C-H bond scission energy varies relatively accord-
ing to the following series: -CH 3 > -CH z- > -t-H.
I
Termination :
The termination of the chain reaction is produced by recombination of radicals: RO i
+
ROi ~]
R· + R· ~
R· + ROi~
nonreactive products
(6)
169
to. I: Thermo-oxidative Degradation
Peroxides decompose presumably by a secondary monomolecular reaction: k.
ROOH - RO' + 'OH
(7)
or a secondary bimolecular reaction [16,17]: ROOH + RH- RO' + R· + H20
(8)
A polyolefin alkoxy radical may decompose according to its structure into formaldehyde, acetone, or acetaldehyde [1]. CH 3
I I
.
Formaldehyde: RCCH 20 · - RCHCH 3 + CH 20 H
CH 3
I I o
Aceton
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