Thermo-oxidative processes

In die presence of oxygen, more complex thermo-oxidative processes occur in polyesters containing aliphatic moieties. They result in crosslinked products and in the formation of compounds such as aldehydes, carboxylic acids and vinyl esters, as reported in the case of PET.93,94 On the other hand, the presence of oxygen has little effect on the thermal resistance of wholly aromatic polyesters below 550°C. Above this temperature a char combustion process takes place.85... 

Possible causes of radical formation in the polymer chain are residues of transition metals and peroxides used to catalyze polymerization and thermo-oxidative processes. Whatever causes homolytic bond scission to produce free radicals (eq 8.1 and 8.2), oxygen reacts with the resultant radical to generate hydroperoxide moieties. Key reactions involved in the initiation and propagation of radicals are illustrated in eq 8.1-8.4, where is a polymeric fragment ... 

The thermal stability of PAs was investigated in a series of papers by Lan-skd. The method of preparation influences the thermal stability of PAs." The side reactions are dependent on temperature. Thus, the polymers are containing different concentrations of structures which accelerate or retard the thermo-oxidation process. However, at polymerization temperatures below 250°C, the reaction time dos not influence the subsequent oxidation of the polymer. 

Generally [17] starch is added at fairly low concentrations (6-15%) the overall disintegration of these materials is achieved by the use of transition metal compounds, soluble in the thermoplastic matrix, as pro-oxidant additives that catalyse the photo and thermo-oxidative process [18-21]. 

Both this study and later work by Botelho and co-workers [13] showed that, unlike polyolefins, the thermo-oxidative degradation of PET involves non-oxidative thermal degradation processes, especially in the early stages. The overall thermo-oxidative process in PET is therefore extremely complex. Recent studies [19,20] using state-of-the-art mass spectroscopic techniques have provided further evidence for the role of hydroxylated terephthalate fragments in PET discoloration. 

At the same time, the IR spectrum of recycled PS almost coincides with that of the virgin polymer because commercial PS is stable, and it is not subject to prolonged exposure to operating factors involving photochemical and thermo-oxidation processes. Because wood contains polar -OH, -OOH and -COOH groups, it is natural to assume, that PWC components are capable of specific (for example, hydrogen bonding) chemical interactions... 

Crystalline starch beads can be used as a natural filler in traditional plastics [3]. They have been particularly used in polyolefins. When blended with starch beads, polyethylene films biodeteriorate upon exposure to a soil environment. The microbial consumption of the starch component leads to increased porosity, void formation, and loss of integrity of the plastic matrix. Generally, starch is added at fairly low concentrations (6-15 wt%). The total disintegration of these materials is obtained using transition metal compounds, soluble in the thermoplastic matrix, used as pro-oxidant additives to catalyze the photo and thermo-oxidative processes [4]. These products belong to the first generation of degradable polymers that biodeteriorate more than mineralize to CO2 and H2O in a time... 

Results showed the same induction period for all three systems (55-60 min) whereas isothermal heating for 6 h at 150°C caused no changes in the mechanical properties of the material. Therefore, the resistance of all three PP formulations to the thermo-oxidation process is substantially the same. Therefore, the same stablizing effect of these additives during PP processing can be assumed. 

A wide and increasing range of types of polymers are being used in these critical applications, and an understanding of thermo-oxidative processes will be very important in developing new applications in the future. 

In the post-irradiation oxidation process, the rate of oxidation decreases by more than one order of magnitude in the first 100 hours, although alkyl macroradicals are continuously formed along with the formation of hydroperoxides (Scheme 9, Reaction 23). The termination reaction of thermo-oxidative processes is generally described as a Russell reaction between two peroxy species. The relative inunobility and the stability of the peroxy radical makes the Russell bimolecular termination strongly disfavored in the solid state at room temperature [22, 23]. More likely. 

Although there are several compounds (Ascorbic acid (vitamin C) or alpha-tocopherol (vitamin E) which their antioxidant effect is proved to be effective in reducing the oxidation of UHMWPE [60], there are several studies that support the antioxidant capacity of carbon nanotubes (CNTs). For example, in the study of Zeynalov et al. [61] was conducted a simulation of thermo-oxidative processes which take place in the polymer chains, and the results showed an inhibition of oxidation of the polymer when carbon nanotubes are present. Also, P. Castell et al. [62] founded that the incorporation of low concentration of arc-discharged multiwall carbon nanotubes (MWCNTs) can act as inhibitors of the oxidative process on irradiated UHMWPE, proving the radical scavenger effect of this reinforcing material. Also, this study shows that the presence of MWCNTs enhances the chemical stability of the polymer. 

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