Protection effects theory

The finding of an inverted-U function is consistent with the biobehavioral theory, which predicts this biologically protective effect of... 

Surface spectroscopy also supports preferential attack of ozone on the antiozonant. Although all antiozonants must react rapidly with ozone, not all highly reactive materials are antiozonants. Something else in addition to the scavenging effect is required. The protective film theory contends that ozonized products, to... 

The relinking (14) and self-healing film (3) theories require chemical interaction between the antiozonant and ozonized mbber. The evidence for these interactions is meager (35,36). Overall, there seems to be no clear evidence in the Hterature for PDA derivatives becoming attached to mbber chains as a result of ozone attack. Much fundamental work in this area remains to be done, however. It seems clear at this point that any antiozonant—mbber interaction must be much less important than the scavenging effect of the antiozonant. In summary, the scavenger model is beheved to be the principal mechanism of antiozonant action. Ozone—antiozonant reaction products form a surface film that provides additional protection (37). 

The philosophy to assume the impedance of the source of supply (generator or a transformer) as the impedance of the faulty circuit may be far from reality and may give a very high fault current. In actual operation, the fault intensity may be far less, as every device and component connected in the circuit will tend to add to the effective impedance of the faulty circuit and limit the magnitude of the fault current. Figure 13.15 also subscribes to this theory. But it is customary to design the systems for the worst fault conditions which, in all likelihood, may not arise, and decide the protective scheme and the current settings of the protective relays for the minimum possible fault current. 

The theory has been advanced that the rapid growth of marine fouling in the tropics may provide a protective shield which counteracts the effect of the greater activity of the hotter water, and LaQue" has pointed out that in flowing sea water, when no fouling organisms became attached to small fully immersed specimens, corrosion of steel at 11° C proceeded at 0-18 mm/y compared with 0-36 mm/y at 21° C. This increase corresponds with what would be expected from chemical kinetics, where the rate of reaction is approximately doubled for a rise of 10° C. 

Hurst (19) discusses the similarity in action of the pyrethrins and of DDT as indicated by a dispersant action on the lipids of insect cuticle and internal tissue. He has developed an elaborate theory of contact insecticidal action but provides no experimental data. Hurst believes that the susceptibility to insecticides depends partially on the cuticular permeability, but more fundamentally on the effects on internal tissue receptors which control oxidative metabolism or oxidative enzyme systems. The access of pyrethrins to insects, for example, is facilitated by adsorption and storage in the lipophilic layers of the epicuticle. The epicuticle is to be regarded as a lipoprotein mosaic consisting of alternating patches of lipid and protein receptors which are sites of oxidase activity. Such a condition exists in both the hydrophilic type of cuticle found in larvae of Calliphora and Phormia and in the waxy cuticle of Tenebrio larvae. Hurst explains pyrethrinization as a preliminary narcosis or knockdown phase in which oxidase action is blocked by adsorption of the insecticide on the lipoprotein tissue components, followed by death when further dispersant action of the insecticide results in an irreversible increase in the phenoloxidase activity as a result of the displacement of protective lipids. This increase in phenoloxidase activity is accompanied by the accumulation of toxic quinoid metabolites in the blood and tissues—for example, O-quinones which would block substrate access to normal enzyme systems. The varying degrees of susceptibility shown by different insect species to an insecticide may be explainable not only in terms of differences in cuticle make-up but also as internal factors associated with the stability of oxidase systems. 

The effects of QMT at cryogenic temperatures can be quite spectacular. At extremely low temperatures, even very small energy barriers can be prohibitive for classical overbarrier reactions. For example, if = Ikcal/mol and A has a conventional value of 10 s for a unimolecular reaction of a molecule, Arrhenius theory would predict k = 2 X 10 ° s , or a half-life of 114 years at lOK. But, many tunneling reactions of reactive intermediates have been observed to occur at measurable rates at this and lower temperatures, even when energy barriers are considerably higher. Reactive intermediates can, thus, still be quite elusive at extremely low temperatures if protected only by small and narrow energy barriers. 

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