Degradation of polymer oxidative

Oxidative degradation of polymers is initiated by radicals (R ) generated in the polymer by heat or mechanical shear during processing or by exposure to UV light. These radicals, in turn, react with 02 to form peroxy and hydroperoxide radicals that promote radical reactions. 

A useful concise source to a collective description of the processes of oxidative degradation of polymers and coverage of the analytical tools used to investigate and measure them is Chapter 14 in Ref. [1]. This reference also discusses the limitations of many of the techniques, and contains a more extensive list of references. 

XPS and SIMS are both surface sensitive and surface-specific techniques that may be used to probe surface oxidation/degradation of polymers. They use X-rays and ions, respectively, to characterise surface species. Both XPS and SIMS are operated under ultra-high vacuum, UHV, conditions. 

Simulation programs for the ESR line shapes of peroxy radicals for specific models of dynamics have been developed for the study of oxidative degradation of polymers due to ionizing radiation [66]. The motional mechanism of the peroxy radicals, ROO, was deduced by simulation of the temperature dependence of the spectra, and a correlation between dynamics and reactivity has been established. In general, peroxy radicals at the chain ends are less stable and more reactive. This approach has been extended to protiated polymers, for instance polyethylene and polypropylene (PP) [67],... 

Abstract The oxidation of polymers such as polypropylene and polyethylene is accompanied by weak chemiluminescence. The development of sensitive photon counting systems has made it comparatively easy to measure faint light emissions and polymer chemiluminescence has become an important method to follow the initial stages in the oxidative degradation of polymers. Alternatively, chemiluminescence is used to determine the amount of hydroperoxides accumulated in a pre-oxidised polymer. Chemiluminescence has also been applied to study how irradiation or mechanical stress affects the rate of polymer oxidation. In recent years, imaging chemiluminescence has been established as a most valuable technique offering both spatial and temporal resolution of oxidation in polymers. This technique has disclosed that oxidation in polyolefins is non-uniformly distributed and proceeds by spreading. 

The principal effects of oxidative degradation of polymers are the decay of good mechanical properties (strength, elongation, resilience, etc.) and discoloration (mainly yellowing). 

The role of singlet oxygen in the oxidative degradation of polymers has been pointed out by several authors,e.g.for polyethylene (36.56),polypropylene (59)>polystyrene (57>60), poly(vinyl chloride)(58,61) cmd rubber (62-66) and reviewed by Rabek cmd R nby (54> 55) ... 

The processing of polymers should occur with dry materials and with control of the atmosphere so that oxidative reactions may be either avoided, to maintain the polymer s molar mass, or exploited to maximize scission events (in order to raise the melt-flow index). The previous sections have considered the oxidative degradation of polymers and its control in some detail. What has not been considered are reactions during processing that do not involve oxidation but may lead to scission of the polymer chain. Examples include the thermal scission of aliphatic esters by an intramolecular abstraction (Scheme 1.51) (Billingham et al., 1987) and acid- or base- catalysed hydrolysis of polymers such as polyesters and polyamides (Scheirs, 2000). If a polymer is not dry, the evolution of steam at the processing temperature can lead to physical defects such as voids. However, there can also be chemical changes such as hydrolysis that can occur under these conditions. 

Mueller, H.G. and Leidigkeit, G. (1979) Thermal and oxidative degradation of polymers in multiviscosity oils (in German). Schmiertech. Tribal. 26 (6) 201-204. 

Formation of singlet oxygen in a polymer matrix. 2.Initation of oxidative degradation of polymers. 

We have demonstrated three techniques that measure the chemical changes associated with the oxidative degradation of polymers. Each of these techniques has the potential to measure very low concentrations of chemical change. The ability to measure slow oxidation rates enable the degradation process to be monitored at the low temperatures more closely related to the material s service conditions. Thus, more accurate service life predictions can be made without relying on extrapolation of high temperature behavior to low temperatures. 

The UV radiation affects aldehyde groups in polymers in the same way as ketone groups that is, Norrish reactions of type I and II are involved in the photochemical process [116]. These reactions are not very important in the photo-oxidative degradation of polymers, because the aldehyde groups are exclusively found at chain ends namely,... 

When hydrogen peroxide is formed in the photo-oxidative degradation of polymers it is also necessary to take into account the secondary reactions due to its photolysis. 

Various chromophore groups, colouring the polymer yellow to brown, are formed during the oxidative degradation of polymers. The production of such groups was identified in the case of polystyrene [2, 255], but their structure is not yet entirely elucidated. Achhammer et al. [2] suggested that the colour of oxidized polystyrene is due to quinomethane formed by the reaction of polystyryl radicals ... 

Photo-oxidized degradation of polymers in solution is highly complicated for the following reasons ... 

Dyes are widely known to be active photo-initiators and photosensitizers of the polymerization reaction [489, 503], but the Complete elucidation of their influence on the photo-oxidative degradation of polymers still requires extensive fundamental investigation. 

The above discussion illustrates the great importance of ozone in the autocatalytic oxidation process and the oxidative degradation of polymers. 

Following Ashby s first observation (7) of chemiluminescence from the oxidative degradation of polymers, a number of papers have appeared dealing with oxidative chemiluminescence from a variety of polymers (8-16). In this chapter we continue the 1,4-polyisoprene work with a study of the low-temperature chemiluminescence emitted in the autoxidation of three additional elastomers, cis-1,4-polybutadiene, amorphous 1,2-polybutadiene, and fmns-polypentenamer. We also report the chemiluminescence obtained from singlet-oxygenated samples of cis-1,4-polybutadiene and trans-polypentenamer, as well as rate data for singlet oxygen reactions with the 1,4-polyisoprene, 1,4-polybutadiene, and model compounds in solution. 

Photo-oxidative degradation of polymers, especially in the presence of U V radiation. Presence of hydroxyperoxides strongly accelerates this process for PP, PE, PS, etc. Such polymers as PEST, PC, PA, POM sufficiently absorb light to start the degradative process. 

While many excellent reviews and books on the photo-oxidative degradation of polymers have been published, in this book, we concentrate on reactions... 

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