Thermo-oxidation mechanism

Elastomeric materials undergo both thermal and oxidation degradation over time. Main chain scission and loss of sulfur crosslinks can occur with either factor or by both factors by a thermo-oxidative mechanism. 

However, it is very difficult to propose a mechanistic explanation of the obtained results, but a look to the general features of the accepted thermo-oxidation mechanism of organic materials and polymers can be helpful to have an idea about the processes involved during the degradation. This subject will be explored in the next sections. 

Keywords Polymer composites Thermo-oxidation Mechanical properties... 

If degradation occurs by a thermo-oxidative mechanism, air should be excluded from the extruder. This can be done by putting a nitrogen bianket on the feed hopper, vent port, or at the die, depending on where the air is introduced. If degradation occurs by hydrolysis, moisture has to be excluded from the process. If degradation occurs by a chemical reaction with the metal surfaces of screw and barrel, a nonreacting material of construction has to be selected for the screw and barrel. 

Based on thermo-oxidation mechanism of polymers (see Section 10.1), it is possible to say that macroradicals are formed on the surface of polymers according to the following reactions [13-17] ... 

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... 

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. 

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. 

PTT shares several similar thermo-oxidation degradation mechanisms with PET [32], Some of the more important ones are as follows ... 

Polyimides have experienced tremendous growth since their conception over 30 years ago. Whereas the initial goal of these materials was high thermo-oxidative stability along with excellent mechanical properties, more recent applications... 

Poly[2,2 -(m-phenylene-5,5 -benzimidazole)] (PBI) is a very high glass transition temperature (Tg 430°C), commercially available material. It possesses excellent mechanical properties, but is difficult to process into large parts and has high moisture regain and poor thermo-oxidative stability at temperatures above approximately 260 °C. Polyimides, especially the thermoplastic polyimides, offer attractive thermo-oxidative stability and processibility, but often lack the thermal and mechanical characteristics necessary to perform in applications such as the matrix for high use-temperature (over 300 °C) structural composites (for example, carbon fiber reinforced) for aerospace use. The attempt to mitigate... 

The dissociation process is described by a free radical chain mechanism. The thermo-oxidative dissociation is initiated by the oxidation of the aliphatic moieties by a subsequent cleavage of the hydroperoxides formed. With increasing time of oxidation the temperature of the onset of degradation is lower as compared with that for a purely thermal degradation. 

Le Bras, M., Bourbigot, S., Delporte, C., Siat C., and Le Tallec, Y. 1996. New intumescent formulations of fire retardant polypropylene Discussion about the free radicals mechanism of the formation of the carbonaceous protective material during the thermo-oxidative treatment of the additives. Fire Mater. 20 191-203. 

Besides organic polymers, inorganic polymers also are susceptible to thermo-oxidative degradation. Nielsen describes the stabilization of silicone fluids with redox systems of iron and cerium by a free radical mechanism. When stabilized, these fluids show oxidative stability up to 700°F. 

---