Scavenger theory

Several theories have appeared in the Hterature regarding the mechanism of protection by -PDA antiozonants. The scavenger theory states that the antiozonant diffuses to the surface and preferentially reacts with ozone, with the result that the mbber is not attacked until the antiozonant is exhausted (25,28,29). The protective film theory is similar, except that the ozone—antiozonant reaction products form a film on the surface that prevents attack (28). The relinking theory states that the antiozonant prevents scission of the ozonized mbber or recombines severed double bonds (14). A fourth theory states that the antiozonant reacts with the ozonized mbber or carbonyl oxide (3) in Pig. 1) to give a low molecular weight, inert self-healing film on the surface (3). 

Disharmonies in the Conception of the Direct 0 /Antiozonant Reaction Importance. Four antiozonant theories have been formulated within the last 25 years. Ozone scavenging theory suggests a preferential direct reaction of an antiozonant with ozone on the rubber surface as a decisive process (26-27). As the antiozonant is depleted via direct ozonation on the surface, fresh antiozonant diffuses rapidly from the rubber bulk to reestablish the equilibrium surface concentration. At a comparable additive concentration and migration rate, the antiozonant efficiency of an additive should be therefore dependent on its ozonation rate and the vulcanizate will be protected until the antiozonant is depleted below the lowest critical concentration. From this point of view, the ozonation rate seems to be a more important factor than the total amount of ozone scavenged by one mole of an antiozonant (this latter phenomenon may be called ozonation factor). Relations between antiozonant efficiency in vulcanizate and antiozonant ozonation rate or antiozonant surface concentration have been indeed reported in some papers and an appreciable higher ozonation rate of PD in comparison with rubber unsaturation, a preferential consumption of an antiozonant in model olefin solution or in rubber were observed. The rubber surface was not attacked by ozone until the antiozonant was almost completely consumed (28). ... 

Toxicity rules out selenium and tellurium compounds, which have also been reported to have antiozonant activity. Several theories have appeared in the literature regarding the mechanism of protection by p-PDA antiozonants. The scavenger theory states that the antiozonant diffuses to the surface and preferentially reacts with ozone, with the result that the rubber is not attacked until the antiozonant is exhausted. 

The scavenger theory in which it is proposed that the antiozonants diffuse to the surface and somehow neutralize, or scavenge, the ozone present, by a chemical mechanism. 

Objections to the protective film theory are less clear cut. Murray (1972) has stated that if the film were to be composed only of antiozonant or ozonized antiozonant, then some rather unrealistic rates of diffusion would have to be assumed in order to form such a film in the time period observed . On the other hand the results of Andries et al (1975), who studied the surface of ozonized natural rubber compounds by attenuated total reflectance spectroscopy obtained results consistent with this theory (and for that matter the scavenger theory) but not with the re-linking and diversionary theories. 

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). 

This interceptor theory does not seem to be the only protective mechanism in operation. Inhibition of cytochrome P450 enzymes related to the bioactivation of mutagens and toxic radical scavenger activities have been proposed to integrate the different modes of action. Other investigations have reported the involvement of chlorophyUin in inducing apoptosis in human colon cells, which may be important in limiting cancer cell invasion and metastasis. ... 

Rather scanty evidence exists for the participation of free radicals in Alzheimer s disease and Down s syndrome. However, more recendy, reports have appeared that suggest possible free-radical involvement in the pathogenesis of these two conditions. Zemlan et al. (1989) repotted that the activity of the free-radical scavenging enzyme, SOD, was significantly increased in fibroblast cell lines derived from familial Alzheimer s and Down s patients. They hypothesized that the elevation in SOD activity observed in the Alzheimer patients supports the theory that paired helical filaments are formed by free-radical hydroxylation of proline residues. They further su ested that SOD levels might also be increased in the brains of Alzheimer s and Down s patients, and that the increase in SOD may reflect an enhanced generation of free radicals. 

In theory, one assumes the formation of radicals before the chemical stage begins (see Sect. 2.2.3). These radicals interact with each other to give molecular products, or they may diffuse away to be picked up by a scavenger in a homogeneous reaction to give radical yields. The overlap of the reactive radicals is more on the track of a high-LET particle. Therefore, the molecular yields should increase and the radical yields should decrease with LET. This trend is often observed, and it lends support to the diffusion-kinetic model of radiation-chemical reactions. 

Samuel and Magee (1953) employed a 1-radical model to find the relative forward yield in water radiolysis as a function of radiation quality. In such models, no distinction is made between reactive radicals or molecular products. The products of radiolysis are called forward (F) to denote observable molecular yield or radical (R), denoting yield of scavenger reaction at small concentration. The aim of the theory is to calculate the relative forward yield G(F)/[G(F) + G(R)], where the G values refer to the respective yields for 100 eV energy absorbed in... 

The latter reaction is widely used as a proving ground for new theories. Particles B (called also scavengers) are unsaturable energy sinks and thus after rapid light emission can absorb it anew. In this case the reaction scheme is A + B —> B (concentration of B s remains constant raB) which could be described by the following kinetic equation... 

DCB, and 2,6-DCB] as the river flowed over contaminated upstream sediments. Thus, they hypothesized that the composition of the PCBs available for deposition in the downstream sediments had already undergone major changes. This theory led to the conclusion that the alterations in PCB residue patterns found in bottom sediments were not likely to be the result of in situ weathering processes. Instead they were seen as caused by the scavenging from the water column of previously altered mixtures of PCBs by suspended particles or water-filtering macrophytes growing in the sediments. 

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