Thermo-oxidative degradation mechanism

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

Romao W, Franco MF, Corilo YE, Eberlin MN, Spinace MAS, De Paoli MA. Poly (ethylene terephthalate) thermo-mechanical and thermo-oxidative degradation mechanisms. Polym Degrad Stab 2009 94 (10) 1849-1859. 

The thermo-oxidative degradation mechanism of TPEEs is proposed based on results of the analysis of model compounds (dibu-tyleneglycol dibenzoate and diethyleneglycol dibenzoate), as well as of the copolymers based on PBT and PEO or PTMO in an oxygen atmosphere [99,100]. The oxidation of polyfether ester)s is initiated in the polyether soft segment at the methylene carbon atom adjacent to the ether oxygen (a-carbon atom). Thermal oxidation mainly results in the formation of a stable ester with aliphatic and aromatic acids and an alcohol, beside CO and CO2. 

Figure 13.4 Thermo-oxidative degradation mechanism of the oils.

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... 

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. 

Protects against thermo-oxidative degradation in natural and synthetic rubbers as well as plastics. It is commonly used in applications such as tire carcasses, wire breaker retreads, apex, belts, hoses, seals, mechanical goods, footwear and wire. 

Degradation Reaction Mechanisms. A detailed discussion of a large number of reactions believed to take part in the thermo-oxidative degradation of PE at the tesq>eratures and oxygen concentrations encountered is given in ref. 2. In the following, only some main lines will be reviewed. 

The antioxidants added to the polyethers in the fabrication process, hindered phenols and diphenylamine types, act by a mechanism of chain breaking , by donation of hydrogen radicals to the very reactive radicals, formed during polyether thermo-oxidative degradation ... 

Decomposition temperatures, whether sharp or vague, are seldom actually reached in service life. Reinforced composites are preeminently load-bearing materials, and it is their temperature-dependent mechanical properties, such as T, or the closely related heat distortion temperature, that usually determine the maximum use temperature, at least for short or intermediate term use. Strength, yield stress and modulus all decline with increasing temperature, reflecting the increasing mobility of the molecular structure, and unacceptable levels of physical property loss will often occur well before the onset of thermal or thermo-oxidative degradation. 

The study of degradation processes of polymers and organic materials is a fundamental topic both from academic and industrial point of view and it can be expected that thermal and thermo-oxidative decomposition mechanisms are reasonably well established. [20,3 8] These complex reactions proceed via a typical free-radical chain as outlined in scheme 2. 

An experimental investigation directly on polymers in order to find out the degradation products is difficult to realize. However, both the study of degradation of model compounds like butylene dibenzoate [5-7], and the general mechanism of thermo-oxidation of organic molecules (scheme 2), can be very helpful to propose a specific mechanism for the thermo-oxidative degradation of PBT (scheme 6). In order to simplify the discussion, the contribution of thermal degradation has not been considered. 

Botelho and co-workers [25] made a comparative study of the thermo-oxidative degradation of poly(ethylene naphthalate) (PEN) and poly(butylene naphthalate) (PBN) The mechanism of the degradation of model compounds for these two polymers was similar in many ways to that noted for the terephthalate equivalents [13]. 

Significant degradation mechanisms for polymers include photo-oxidation and thermo-oxidation. Such mechanisms result in the formation of carbonylated... 

Thermogravimetric analyses of various polymer—silica composites including PDMS-sihca [61], polyamide-silica [33,34], polyimide-silica [15,38,47], polybenzoxazole-silica [16,17], polybenzobisthiazole-sihca [16,17] and PDMS -silica-titania [62] have been reported, in all cases indicating that the incorporation of silica raises the temperature of thermo oxidative degradation of the polymer. A possible mechanism for the improvements in degradation temperature could be due to silica inactivating the terminal OH groups... 

Material specifications for under-the-hood automotive applications require retention of mechanical properties for materials subjected to changes in ambient temperatures (-40-175°C) for prolonged periods of time. Besides temperature, the prolonged service lifetime of a molded part reflects material durability to extremes in environmental conditions that include moisture, thermo-oxidative degradation, exposure to sunlight, and creep resistance under high static stress loads. 

Polypropylene (PP) has wide acceptance for use in many application areas. However, low thermal resistance complicates its general practice. The new approach in thermal stabilization of PP is based on the synthesis of PP nanocomposites. This paper discusses new advances in the study of the thermo-oxidative degradation of PP nanocomposite. The observed results are interpreted by a proposed kinetic model, and the predominant role of the one-dimensional diffusion type reaction. According to the kinetic analysis, PP nanocomposites had superior thermal and fireproof behavior compared with neat PP. Evidently, the mechanism of nanocomposite flame retardancy is based on shielding role of high-performance carbonaceous-silicate char which insulates the underlying polymeric material and slows down the mass loss rate of decomposition products. 

Degradation processes are generally quite complex often more than one type of degradation is operational, e.g., thermo-oxidative degradation, thermo-mechanical degradation, etc. This situation is quite similar to wear in extruders, where usually more than one wear mechanism is operational at any one time. 

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