Antiozonant theories ozone scavenging 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). 

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). 

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). ... 

In spite of many objections, ozone scavenger and protective film theories cannot be neglected because of serious experimental evidence. They should be considered as an important part of the overall antiozonant mechanism. Their role prevails in rubber solution or in very thin rubber films. It is very probable that a part of an antiozonant is wastefully depleted just because of direct ozonation. 

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 relinking and self-healing film theories require chemical interaction between the antiozonant, or ozonized antiozonant, and the rubber or ozonized rubber. The evidence for these interactions is sparse in the literature. The products of the ozone-antiozonant reaction are soluble in acetone. Thus if only the scavenger and protective film mechanisms are operative, no nitrogen from the antiozonant should be left in the rubber after ozonation and subsequent acetone extraction. Nitrogen analyses of extracted rubber showed, however, that some of the nitrogen was unextractable this nitrogen was presumably attached chemically to the rubber network. 

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. 

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... 

Many theories have been proposed to explain the mechanism of how antiozo-nants protect elastomeric articles from the effects of ozone. Most support the scavenger model, and indicate that an ideal antiozonant should be capable of migrating in a mbber matrix to the surface whenever the equilibrium of the antiozonant concentration in the compound is upset by the formation of ozonized antiozonant at the surface, but yet not freely migrate to the surface and volatilize out of the elastomer without first reacting with ozone at the surface. On the... 

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