Thermo-oxidation

There have been many studies on the thermal and thermo-oxidative degradation of PMMA.23 24 It is well established that the polymer formed by radical polymerization can be substantially less stable than predicted by consideration of the idealized structure and that the kinetics of polymer degradation are dependent on the conditions used for its preparation. There is still some controversy surrounding the details of thermal degradation mechanisms and, in particular, the initiation of degradation.31... 

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... 

Phenolic networks are well known for their excellent thermal and thermo-oxidative stabilities. The mechanisms for high-temperature phenolic degradation include dehydration, thermal crosslinking, and oxidation, which eventually lead to char. 

Phenolic degradation, thermal and thermo-oxidative, 418-425 Phenolic-epoxy networks, 413 Phenolic monomers, second-order reaction rate constants of formaldehyde with, 403... 

Report 131 Failure of Polymer Products Due to Thermo-oxidation,... 

Table I Thermo-oxidative stability (at 150 C) of PP (ICI.HF22) without and with 30X glass fibre (6F) in the absence and presence of antioxidants. Soxhlet extraction was with chloroform and acetone...

Figure 5 Left thermo-oxidative carbonyl index vs oven ageing times at 90°C. Right photo-oxidative carbonyl index vs irradiation time after exposure to UV radiation (300-800 nm wavelength). In each case, the carbonyl index is based on the intensity of the mid-infrared absorption at 1,720 cm-1 normalised against sample film thickness, d, which was 290 gm. Reprinted from Corrales et al. [13]. Copyright 2002, with permission from Elsevier. 

Recently, these researchers, again using a combination of TGA and TGA-MS, published further studies on the thermo-oxidative ageing of POM, including determining various activation energies [108]. 

One important point should be stressed here the efficiency of any stabilizing system depends very much on the removal or depletion of the defect structures in the polymer. An example is shown in Figure 1 where the case 5 mechanism vide supra) of a synergistic mixture is theoretically depicted [7]. The line 2 shows that the effect of the synergistic mixture of two antioxidants on thermo-oxidation stability is negligible, if there occurs a relatively significant initiation of oxidation reaction in a way independent from the route taking place via hydroperoxides. 

The quality of the polymer, its photo-oxidation and thermo-oxidation history expressed in concentration of hydroperoxides, carbonyl groups or of other oxidized structures and terminal groups. The rate of an oxidative attack may then be related to the average molar mass and to its distribution, and to the ratio of amorphous/crystalline structures. Polymers cannot be simply ordered according to the intensity of light emission at a given temperature. The chemiluminescence-time patterns are related with the rate of sample oxidation, but they may differ from one to the next polymer. 

The effect of molar mass and stereoregularity on the thermo-oxidation of polymers and on the chemiluminescence-time patterns has been investigated only rarely [44-46]. 

Heat and temperature Thermo-oxidation, additive migration, crosslinking, crosslink loss (reversion)... 

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