Sequential degradation mechanisms

Although many authors insist upon the sequential degradation of phyllosilicates, i.e., taking the same initial material and transforming it into the various types of expandable minerals in the weathering process, (Heaver and Jackson, notably) this is undoubtedly not the only mechanism by which these clay minerals are formed in soils and possibly not the dominant one. Studies on the weathering of granites and more basic rocks... 

The kinetics of these reactions are such that their role becomes significant only above 1500°C. Consequently, diffusion flames are particularly prone to higher levels of thermal NO production because of their higher peak flame temperatures. Besides the Zeldovich mechanism, NO formation can also occur via the prompt-NO mechanism and from fuel nitrogen sources. In the prompt-NO mechanism, the reactions of CH radicals, produced by the sequential degradation of hydrocarbon fuels, with N2 are responsible for NO production ... 

Moreover, if two degradation mechanisms occur sequentially (e g., UV-radiation followed by mechanical degradation), then the two mechanisms are analyzed separately and the predicted lifetimes simply added. Further difficulties arise when extrapolating the short lifetime obtained from accelerated tests at increased load frequency/intensity and increased temperature to predict longer lifetime at service conditions. Power laws are often used for extrapolation. For more details, refer to Greenwood and Friday. ... 

The qualification of materials by the evaluation of kinetic parameters and attributes is based on the degradation mechanisms, which combine the contribution of each sequential step to the product accumulation and the modification of material behavior [45-48]. 

If degradation is due to sequential or synergistic mechanisms, then a simple extrapolation may not be reasonable. For example, in accelerating the rate of oxidation of polyethylene it may be necessary to identify the induction time and subsequent degradation time separately and to produce Arrhenius diagrams for each. The total time to failure is the sum of the two times. Figure 9.3 shows an instance of where unknowing extrapolation of the short-term results in tests on polyaramid fibres could have led to overestimates of lifetime and premature failure. 

Biological. Four Pseudomonas sp., including Pseudomonas putida (ATCC culture 29607) isolated from soil, degraded chloropicrin by sequential reductive dechlorination. The proposed degradative pathway is chloropicrin -> nitrodichloromethane nitrochloromethane nitromethane + small amounts of carbon dioxide. In addition, a highly water soluble substance tentatively identified as a peptide was produced by a nonenzymatic mechanism (Castro et al., 1983). 

In order to apply ZVI-based dechlorination for site remediation, we have to be sure that no toxic reaction products are produced in the end, and toxic intermediates, if there are any, are degradable. Product distribution is controlled by reaction mechanisms. For the reduction of chlorinated methanes such as carbon tetrachloride (CT), hydrogenolysis, where chlorines are sequentially replaced by hydrogen, takes place in the presence ofZVI (Vogel etal., 1987 Matheson andTratnyek, 1994 Glod etal., 1997) ... 

In cases where no additional oxygen is present, polystyrene can undergo nearly pure thermal degradation. The two prevalent mechanisms are sequential elimination of monomer units, which is called unzipping or depolymerization. In this case, styrene monomer is formed. Random chain scission can also occur. It is sometimes combined with unzipping at the reactive broken chain ends. At temperatures approaching 300 °C, up to 40 % of a polystyrene molecule can be converted to styrene monomer. 

PE catalytic degradation over zeolite NaY was also investigated by Ishihara et al 6 in a batch reactor. The authors propose a sequential mechanism similar... 

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