Antiozonant theories

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). ... 

Products Studies in Antiozonant Mechanisms. Experimental proof of the individual antiozonant theories based on product studies are scarce. Using a very sophisticated instrumental analytical approach, the composition of a very complicated mixture of ozonation products of two technically important antiozonants, i.e., DOPPD and HPPD has been revealed and the reactivity pathways with ozone have been established (24, 40). Influence of the character of N-substituents on the ozonation mechanism has been evidenced. Some important... 

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). 

This paper shows that Fukui s theory is plausible for interpreting the presence of the optimum oxidation potentials for amines to be effective as antioxidants or antiozonants. 

The influence of molecular structures and substituents on the antiozonant properties of a series of related aromatic diamine compounds was studied. The relative effectiveness of the compounds was determined by viscometric techniques and by comparison of the rate of degradation of protected vuicanizates. Results indicate that unsymmetrical p-phenylenediamine derivatives are less effective than analogous symmetrical compounds as antiozonants. The protective capacity of the antiozonants decreases as the size or number of the N-hydrogen substituents, or the distance between the amine groups, increases. The comparative stability of the free radicals of aryl diamines, in terms of the theory of resonance, is utilized to explain the relative inhibiting properties of the chemicals examined. 

Because it is generally accepted that resonance influences the chemical properties of substances in which it occurs, consideration of the comparative stability of the free radicals, in terms of theory of resonance (P, 10), provides substantiation of the compounds relative efficiencies as antiozonants. 

Nitroxides and benzoquinonediimines are formed from aromatic amines and diamines respectively as a consequence of amine involvement in antioxidant and/or antiozonant processes. Their participation in antioxidant regenerative mechanisms is suggested. Features of phenylenediamine involvement in antiozonant processes are discussed in relation to contemporary theories. 

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. 

Involvement of the Ozonized Rubber Moieties in Antiozonant Mechanism. The rubber chain relinking theory (30) is consistent in part with the self-healing film formation theory (37) a reaction between an antiozonant or some of its transformation products and ozonized elastomer is considered. Either scission of ozonized rubber is prevented in this way or severed parts of the rubber chain are recombined (i.e., relinked). A "self-healing" film resistant to ozonation is formed on the rubber surface. Such a film formed by the contribution of nonvolatile and flexible fragments of the rubber matrix should be more persistent than any film suggested in the protective film theory. 

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. 

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 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. 

Mechanisms for Protection against the Effects of Ozone Attack. The mechanism of antiozonant protection is still not fuUy understood. However, there are several theories, which detail the mechanism of protection by chemical antiozonants inert barrier, competitive reaction, reduced critical stress, and polymer back-bone chain repair. 

According to the reduced critical stress theory, certain materials migrate to or near the rubber surface and modify the internal stress of the polymer such that cracks do not appear. Although this phenomenon is poorly understood, it is easy to observe. The use of increasingly higher levels of antiozonant raises the critical stress level required for cracks to form. 

The chain repair theories suggest that severed polymer chains (terminated by carboxy or aldehyde groups) can be relinked by reaction with the antiozonant or that the antiozonant reacts with the ozonide or zwitterion (carbonyl oxide) to give a low-molecular-weight, inert, self-healing film. Either way, the antiozonant would be chemically linked to the rubber. However, the chain repair or self-healing film theories do not appear to be as strongly supported as the other theories. 

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. 

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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