Ozone attack natural rubber compounds

Like NR, SBR is an unsaturated hydrocarbon polymer. Hence unvulcanised compounds will dissolve in most hydrocarbon solvents and other liquids of similar solubility parameter, whilst vulcanised stocks will swell extensively. Both materials will also undergo many olefinic-type reactions such as oxidation, ozone attack, halogenation, hydrohalogenation and so on, although the activity and detailed reactions differ because of the presence of the adjacent methyl group to the double bond in the natural rubber molecule. Both rubbers may be reinforced by carbon black and neither can be classed as heat-resisting rubbers. 

The effect of ozone is both greatly delayed and reduced when various antiozonants, such as NJ -di-alkyl-p-phenylene-diamines (X) or similar compounds, are incorporated in the rubber. They may react directly with ozone or with the ozone-olefin reaction products in such a way as to prevent chain scission. SBR is much more receptive to protection than either the nitrile or natural rubbers. Of the latter two the nitrile is more readily inhibited. If a wax is also added in small quantities with the antiozonant, the retardation of the attack of ozone is in certain instances enhanced several fold. 

As mentioned in section 20.2.3, natural rubber (in common with other diene elastomers) is readily attacked by ozone. The mechanism of the reaction is probably the same as that established for simple olefins [10]. In this case, the initial product is a -complex which cleaves to form an aldehyde or ketone and a zwitterion. Several subsequent reactions may then occur, depending on the nature of the reactants and conditions. The zwitterion may dimerize or polymerize or react with the carbonyl compound to form an ozonide ... 

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