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Degradation and Protection
Published in Anil K. Bhowmick, Current Topics in ELASTOMERS RESEARCH, 2008
Rabin N. Datta, Nico M. Huntink
The protective film mechanism states that the rapid reaction of ozone with the antiozonant produces a film on the surface of the rubber, which prevents attack on the rubber, like waxes do [63]. This mechanism is based on the fact that the ozone uptake of elongated rubber containing a substituted p-phenylene diamine type of antiozonant is very fast initially and then decreases rather rapidly with time and eventually stops almost completely. The film has been studied spectroscopically and shown to consist of unreacted antiozonant and its ozonized products, but no ozonized rubber is involved [64]. Since these ozonized products are polar, they have poor solubility in the rubber and accumulate on the surface.
Protection Mechanism of Rubbers from Ozone Attack
Published in Ozone: Science & Engineering, 2019
The antiozonants are molecules which are able to migrate, i.e. to move from the bulk of rubber compound to the surface of the rubber compound in order to be ready to react with the incoming ozone. Thus, the antiozonants must not be very soluble in the rubber and indeed the N,N’-substituted PPDs are not. Moreover, the antiozonant molecules must be very reactive with ozone to be able to scavenge ozone before it has time to attack the double bonds. For example, the antiozonant 6PPD (see Figure 1 for the chemical structure) reacts with ozone 7 × 106 M−1s−1 (Cataldo 2018). This figure should be compared with the reaction speed with ozone of the double bonds of IR which is 1.4 × 105 M−1s−1 corresponding to 50 times less than 6PPD or with the double bonds of BR and SBR i.e. 6.0 × 104 M−1s−1 corresponding to 117 times less than 6PPD. The effect of an antiozonant is to scavenge the ozone available on the surface of the rubber before it has time to react with the rubber double bonds. As the antiozonant is continuously consumed through its reaction with ozone at the rubber surface, diffusion of new antiozonant from the inner parts to the surface replenishes the surface concentration to provide the continuous protection against ozone. Blends of different PPDs antiozonants are used in the industrial practice to ensure short- and long-term protection taking advantage from the different migration ability of the various PPDs shown in Figure 1.