Polymers Oxidation Kinetics

Isothermal Kinetics of Polymers Oxidation and Its Relation to the Concentration of Oxygen in Surrounding Atmosphere 487... 

Comparison of chemiluminescence isothermal runs with oxygen uptake and DSC measurements has been at the centre of interest since practical industrial applications of the chemiluminescence method were attempted. It is a fact that the best comparison may be achieved when studying polymers that give a distinct induction time of oxidation typical for autoaccelerating curves of a stepwise developing oxidation. This is the particular case of polyolefins, polydienes and polyamides. The theoretical justification for the search of a mutual relationship between the oxidation runs found by the various methods follows directly from the kinetic analysis of the Bolland-Gee scheme of polymer oxidation. 

ISOTHERMAL KINETICS OF POLYMERS OXIDATION AND ITS RELATION TO THE CONCENTRATION OF OXYGEN IN SURROUNDING ATMOSPHERE... 

CL accompanies many reactions of the liquid-phase oxidation of hydrocarbons, ketones, and other compounds. It was discovered in 1959 for liquid-phase ethylbenzene oxidation [219,220]. This phenomenon was intensively studied in the 1960s and 1970s, providing foundation for several methods of study of oxidation, decay of initiators, and kinetics of antioxidant action [12,17,221], Later this technique was effectively used to study the mechanism of solid polymer oxidation (see Chapter 13). 

When diffusion of oxygen in the polymer occurs rapidly (Ak 1), we observe the kinetic regime of polymer oxidation (see Equation [13.16]). When penetration of oxygen into polymer is slow, the reaction rate v KXp02. 

Like the oxidation of hydrocarbons, the autocatalytic oxidation of polymers is induced by radicals produced by the decomposition of the hydroperoxyl groups. The rate constants of POOH decomposition can be determined from the induction period of polymer-inhibited oxidation, as well as from the kinetics of polymer autoxidation and oxygen uptake. The initial period of polymer oxidation obeys the parabolic equation [12]... 

Oxidation kinetics of oriented PP was measured under conditions of external priming. The parameter specifying the oxidability of a polymer is slightly dependent on deformation. For instance, at 200°C it only decreases by 1.5 times with X changing from 0 to 10. This unambiguously clarifies that the main reason for increase in thermal-oxidative stability of deformed PP is a sharp drop in the escape of a branching agent (hydroperoxide), i.e. a decrease in hydroperoxide escape. 

The oxidations of secondary alcohols and sulfides by halamine polymers produce ketones and sulfoxides, respectively, with some sulfones and chlorosulfoxides produced in the latter case. A mechanism is proposed based on the oxidation kinetics. 

The results obtained by liquid-phase oxidation or co-oxidation of various hydrocarbons are reviewed, and new results are reported for new kinds of compounds such as alkyl-aromatics, alcohols, and ethers, which were also systematically studied by co-oxidation. Gathering all kinetic data and discussing them in connection with data on absolute termination constants, obtained by other groups through physical measurements, enables us to estimate the termination and propagation rate constants for about 40 compounds and to present characteristic values for some new classes of compounds. Examples demonstrate that co-oxidation studies make it possible to explain the behavior of complex compounds reacting by different kinds of bonds, and more particularly the behavior of polymers oxidized in solution. 

Oxygen is also a dopant for polyacetylene, but on exposure the conductivity rises to a maximum then rapidly declines as oxidation of the polymer backbone occurs, as shown in Fig. 21. We have no data on the diffusion coefficient as the process is rapid and is masked by the reaction of oxygen with the polymer. The kinetics are first-order, implying that the doping reaction is rapid, goes to less than 1 mol%, and is then followed by irreversible oxidation of the polymer. Based on the observa-... 

Because two or more radicals, R, are produced by the reaction of one initial polymer molecule, PH, the reaction speeds up as the reaction proceeds, as shown in Figure 15.8. (Note PH = A in this figure.) Autocatalytic oxidation kinetics are often prevalent with hydroperoxide and metal catalyzed oxidation of polymers. 

Relating Kinetic Models to Embrittlement in Polymer Oxidative Aging... 

One can consider that the kinetics carbonyl build-up is representative of the overall oxidation kinetics, at least when considered at the molecular scale (or monomer unit). It remains to establish a relationship between structural changes at this scale and molar mass changes. For the PE polymer understudy, random chain scission is predominant. It will be assumed that the main scission process is the rearrangement of alkoxyl radical (p scission). Then, every elementary reaction generating alkoxyl radicals will induce chain scission. In the chosen mechanistic scheme, both hydroperoxide decomposition processes and the nonterminating bimolecular peroxyl combination are alkoxyl sources. Thus, the number of moles of chain scissions per mass unit (s) is given by ... 

One of the obvious features of the oxidation of polypropylene is the formation of hydroperoxides (reaction (3) in Scheme 1.55) as a product. The initiation of the oxidation sequence is usually considered to be thermolysis of hydroperoxides formed during synthesis and processing (shown as the bimolecular reaction (1 ) in Scheme 1.55). The kinetics of oxidation in the melt then become those of a branched chain reaction as the number of free radicals in the system continually increases with time (ie the product of the oxidation is also an initiator). Because of the different stabilities of the hydroperoxides (e.g. p-, s- and t- isolated or associated) under the conditions of the oxidation, only a fraction of those formed will be measured in any hydroperoxide analysis of the oxidizing polymer. The kinetic character of the oxidation will change from a linear chain reaction, in which the steady-state approximation applies, to a branched-chain reaction, for which the approximation might not be valid since the rate of formation of free radicals is not... 

Jellinek et al. [312—314] discussed the role of diffusion in the kinetics of polymer oxidative degradation. It should be ascertained in the investigation of the oxidation of polymer films whether the observed oxidation rate is actually due to the chemical oxidation reaction and that it is not influenced by the relatively slow diffusion of oxygen into the film. When the polymer is very finely powdered, diffusion effects would be expected to be negligible. 

The kinetics of polymer oxidation are discussed in a number of excellent papers [Refs. 48—50, 146, 167, 311, 524, 530, 531, 597, 599, 600, 642]. The kinetics are thoroughly examined in accordance with the assumed reaction mechanism, the conditions of oxidation and the particular experimental results obtained. Kinetic equations that have been published are deduced from investigations of the following reactions and are frequently highly complicated. 

The kinetics of polymer oxidation in the solid phase (films) and in solutions of films, and of the polymers themselves, has been investigated. On account of the extremely large amount of theoretical and experimental data, only two fundamental cases, namely, autocatalytical oxidation in the presence of high and low concentrations of oxygen and the experimental principles in the investigation of degradation and cross-linking kinetics, are discussed in this chapter. 

The model compound, frans-4-chloro-4-octene, was chosen because it possessed the —C1C=CH— group of polychloroprene, but without the repeating 1,5-diene structure of the polymer. The autoxidation was similar to that observed for polychloroprene, although no induction period was observed (cf. hexachlorobutadiene oxidation above). The evolution of HC1 was proportional to the square of the time, but the oxidation kinetics were approximated more closely by the equation... 

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