All SBR vulcanizates, when tested in a relaxed state, reacted with ozone of low concentrations to form a film of oxidized products which provided an effective barrier against further attack by gaseous ozone. SBR vulcanizates that were under stress, but which contained no effective antiozonant in their formulation, were readily attacked by ozone. The absorption of ozone appeared to begin with an initial rate of zero which was followed by rapidly increasing rates until a maximum had been reached, when ozone cracks were visible on the surface of the rubber. SBR vulcanizates that were under stress, but which contained an antiozonant, were protected from an attack by ozone to a degree that ranged from poor to excellent. The degree of protection depended (a) on the differential in the rates of reaction of the antiozonant and the rubber hydrocarbon with ozone, (b) on the initial concentration of the antiozonant on the surface of the vulcanizate which reacted with ozone to form a barrier of oxidized residues, and (c) on the rate of effusion of fresh antiozonant from within the rubber to the outer surface of the barrier. SBR vulcanizates containing a naphthenic processing oil as an extender were not so resistant to ozone as standard SBR vulcanizates. It is probable that the effectiveness of the antiozonants tested in these vulcanizates was reduced by their high solubility in the oil phase. SBR vulcanizates containing trioctyl phosphate as a plasticizer were vigorously attacked by ozone. It is possible that the gaseous ozone dissolved to some extent into the plasticizer phase, increasing the concentration of ozone in the area causing a more severe oxidation of the rubber. SBR vulcanizates that had been coated with an antiozonant by dipping the specimen several times into a solution of the antiozonant in a solvent were found to contain a higher concentration of the antioxonant directly on the surface of the vulcanizate than in the case where three parts of the antiozonant were added during vulcanization. A mechanism for the ozonization of SBR vulcanizates has been proposed which includes a possible mechanism for the protective action of antiozonants. A rate equation has been derived from this mechanism which was consistent with the experimental rate data.
All styrene-butadiene rubber (SBR) vulcanizates, when tested in a relaxed state, reacted with ozone (25 to 500 pphm (parts per hundred million) ozone in air) to form an oxidized film which provided an effective barrier against attack. SBR vulcanizates that were under stress, but which contained no effective antiozonant in their formulation, were readily attacked. The absorption of ozone began with an initial rate of zero but rapidly increased until a maximum rate had been reached when ozone cracks became visible. SBR vulcanizates that were under stress, but which contained an antiozonant, absorbed ozone with a high initial rate which depended on the concentration of the ozone-sensitive materials on the surface of the vulcanizate and on the partial pressure of the gaseous ozone. The rate of absorption declined during the exposure time until a steady low equilibrium rate was reached. The vulcanizates were protected from an attack by ozone (cracking) to a degree which ranged from poor to excellent, and depended (a) on the differential in the rates of reaction of ozone with the antiozonant and the rubber hydrocarbon, (b) on the initial concentration of the antiozonant at the surface of the vulcanizate, and (c) on the rate of effusion of fresh antiozonant from within the rubber to the outer surface of the oxidized barrier. A mechanism is proposed for the protective action of antiozonants in vulcanizates, and a rate equation has been derived from this mechanism which is consistent with the experimental data.
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