A Study of the Reaction of Ozone with Polybutadiene Rubbers

Abstract

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.