Thermo-oxidative processes effect

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... 

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. 

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. 

It is strongly advisable for formulations of inhibited plastics to make allowance for the effect the Cl exerts on the polymer binder properties during processing and service life of the final products. Some Cl may activate thermo-oxidative destruction and atmospheric aging of polymers, while others vice versa, are able to retard these adverse processes. 

As with all organic polymers, exposure of aromatic polyesters to heat and oxygen can, especially over long time periods, result in degradation of the polymer. This thermo-oxidation manifests as discoloration of materials, loss of physical properties, and complete failure of the substrate. To prevent (or more likely control) such processes it is necessary to incorporate additives which can protect the host polyester against the effects of heat and oxygen antioxidants. 

Reflecting current concern over toxic hazards, determinations have been made of the amount of vinyl chloride evolved from poly(vinyl chloride) at typical processing temperatures it was less than 1 p.p.m. The effect of processing on subsequent thermo-oxidation has also been studied with unstabilized polymer reaction was immediate. Further papers with practical relevance report on the decomposition under smouldering, or combustion conditions, and the optimum... 

Processing stabilization of polyethylene is usually done by a combination of phenolic and phosphorous antioxidants. A phosphate stabilizer used in the absence of a phenolic antioxidant imparts very low oxidative stability to polyethylene. When hindered phenols are used in combination with phosphites or phosphonites, the melt flow behavior during processing and the thermo-oxidative stability of the polymer improve significantly. Fearon et al. [17] attributed the positive effect of phenolic antioxidants to their interaction with peroxides. The trivalent phosphorous additives often help to improve the color of polymers [18-21]. 

Addition of corn starch to LDPE has a stabilizing effect on the thermo-oxidation mode while the same additive destabilizes the LDPE to UV irradiation. This property would diminish the risk of degradation during the processing of such materials. 

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