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.

Date of Award	1983
Original language	English