The Behaviour of Substituted Diaryl Nitroxyl Radicals and their Derivatives as Antifatigue Agents for Rubbers

Abstract

A variety of arylnitroxyl radicals and their precursors have been evaluated as antifatigue agents, antioxidants and antiozonants in natural rubber. Nitroxyl radicals and their derived hydroxyl amines were shown to be the active species during fatiguing and mechanistic studies using ESR have shown that these stable nitroxyl radicals are regenerated in a cyclical mechanism, N-nitroso diaryl amines were shown to be highly effective antifatigue agents due in part to the formation and regeneration of the diaryl nitroxyl radicals. Diaryl nitrones were also shown to produce the corresponding nitroxyl radical during processing, although their antifatigue activity was low. However inclusion of a semi-hindered phenolic moiety in the molecule markedly improved activity. The antiozonant and antioxidant activity of the nitroxyl radicals is generally less than the corresponding amines or hydroxyl amines. Although all the nitrones have similar antioxidant and antiozonant activity, those containing a semi-hindered phenolic group were effective antiozonant and they also synergised with the bisphenol antioxidant WSP. The N-nitroso amines too were effective antiozmants and antioxidants. Although the aramatic amines, nitroxyls and hydroxyl amines were shown to be staining and discolouring, the effect was reduced by branched chain substitution on the para position of the ring system. The N-nitroso amines behaved similarly but in contrast the nitrones were relatively non-discolouring and non-staining. Diaryl nitroxyls and their corresponding hydroxyl amines did not affect the vulcanisation process significantly but nitrones and N-nitroso compounds retarded cure, This work has shown that with the appropriate chemical modification, the antidegradant performance of the commercial stabilisers can be improved significantly.