Degradation mechanisms underlying

Examination of a ftuoropolymer surface visually shows the following t5q)es of degradation. They are based on visual manifestations only and do not refer to the underlying degradation mechanism. Underlying degradation mechanisms are covered in the failure analysis section (Ch. 10). 

The aim of the first task of the work is to exploit the evidence contained in the available dataset with N=30 complete data to estimate the values of the parameters of the stochastic model describing the degradation mechanism under study. The assumption is that the transition from working to failed state obeys a two-parameter Weibull dis-... 

Teranishi K, Kawata K, Tsushima S, Hirai S. 2006. Degradation mechanism of PEMPC under open circuit operation. Electrochem Solid State Lett 9 A475-A477. 

Hydrolysis appears to be the most important abiotic degradative mechanism for organophosphate esters under basic pH conditions. Under neutral and acidic conditions, the reaction slows considerably and could become an insignificant removal mechanism. The hydrolysis proceeds by a stepwise mechanism in which one alcohol group is removed at a time. The first step is cleavage of a P-OR bond (where "R" is an aryl or alkyl group) to produce a diester of phosphoric acid, which, under basic conditions, becomes an anion. 

The purified sample C is not degradated under such conditions while degradation is measured before and after purification of sample H. This results confirms the difference of degradation mechanism according to the method of preparation of the polymer. 

Chand et al. [88] degraded phenol under sonication at different frequencies with H2O2 and O3 in presence of zero valent iron and copper. The effectiveness of 300 kHz sonochemical reactor was found to be the maximum for the generation of OH radicals among 20, 300 and 520 kHz. The complete removal of phenol was achieved when sonicated with 300 kHz frequency with zero valent iron compared to zero valent copper. In their mechanism it was supposed that at first iron metal was corroded in the presence of hydrogen peroxide, under acidic conditions, oxidizing Fe° to Fe2+, which on further reaction with H202 produce OH radicals and Fe3+. The Fe° then reduces the Fe3+ back to Fe2+ and the chain was maintained as... 

Bioavailability is also influenced by certain, albeit poorly understood, characteristics of bacteria. To degrade soil-sorbed molecules, bacteria must either use sorbed molecule directly or facilitate desorption in some manner. Mechanisms underlying the apparent availability of sorbed chemicals are complex due to the divergent properties of chemicals considered, the resultant sorption/desorption mechanisms, the metabolic diversity of microorganisms, and the heterogeneity of soils. Several microbial-based mechanisms have been proposed for the access of soil-sorbed organic chemicals (i) production of bio surfactants (Desai and Banat 1997 Alexander 1999) ... 

The thermal stability of PMI with additives is not changed by the introduction of clay, charcoal, AI2O3. These additives were quite stable over the temperature range under study (up 900°C). The lower thermal stability observed for the cases of ZnCl2 and NiC12 as additives may have resulted from a change in the degradation mechanism. 

It is a well-known fact that the mechanical properties of fluoropolymers, especially perfluoropolymers, degrade dramatically under irradiation. Nevertheless a considerable improvement of the mechanical properties of the final grafted copolymers was observed in comparison with mechanical properties of the initial irradiated fluoropolymer. Thus it is possible to minimize or completely avoid the degradation of mechanical properties of the final grafted composites in comparison with the initial fluoropolymers by choosing appropriate reaction conditions. 

Proof of the (3-oxidation mechanism was demonstrated in the LC-ESI-MS chromatogram of the FBBR sample taken on day 8, shown in Fig. 5.1.4 [34]. The C-even-SPC were identified as the major degradation intermediates. Under the applied chromatographic conditions several isomeric SPC, e.g. four phenyl isomers of C6-SPC, could be separated (Fig. 5.1.4). 

Serajuddin et al. [19] reported that the degradation pathway of prevastatin sodium was directly linked to the micro-pH environment within the formulation. Under neutral conditions (pH 6.5), the statin formed two degradation products, a cyclic lactone and an internal hydroxyl rearrangement product. However, as the pH was increased to 9.9 with the incorporation of magnesium oxide into the blend, the only degradation mechanism involved the formation of the cleavage product, 2-methylpropanoic acid. This latter approach of increasing the... 

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