Antiozonants, rubber scavenger-protective film mechanism

The most effective antiozonants are the substituted PPDs. Their mechanism of protection against ozone is based on the scavenger-protective film mechanism [68-70]. The reaction of ozone with the antiozonant is much faster than the reaction with the C=C bond of the rubber on the rubber surface [56]. The rubber is protected from the ozone attack tUl the surface antiozonant is depleted. As the antiozonant is continuously consumed through its reaction with ozone at the mbber surface, diffusion of the antiozonant from the inner parts to the surface replenishes the surface concentration to provide the continuous protection against ozone. A thin flexible film developed from the antiozonant/ozone reaction products on the mbber surface also offers protection. 

Chemical antiozonants have been developed to protect rubber against ozone under such dynamic conditions. Several mechanisms have been proposed to explain how chemical antiozonants protect rubber. The scavenging mechanism, the protective film mechanism, or a combination of both are nowadays the most accepted mechanisms. 

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. 

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. 

Formation of the protective film from ozonized rubber and AOZ creates a barrier against penetration of 03 into rubber [3,4]. Reactions of AOZ with ozonides 3, zwitterions 4 and aldehydic fragments 8 are envisaged. The respective reactivities are lower than the direct ozonation and the relevant contribution to the antiozonant mechanism is therefore inferior to the ozone scavenging. The reactivity of the ozonized rubber with AOZ accounts for rubber chain repairing, classified also as chain relinking or selfhealing mechanisms [4,21,247-249]. This contributes to formation of relaxed surface films. 

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