Aliphatic Nitroxyl Radicals as Antifatigue Agents for Rubbers

Abstract

The antifatigue activity of hindered nitroxyl radicals and their precursors have been studied. It was demonstrated that hindered alicyclic nitroxyl radicals and their precursors have weak antifatigue activities. However the trend increased in the order: amine < nitroxyl radical < hydroxylamine. The activity was improved by attaching PD-C and CB-D antioxidant functions to the alicyclic nitroxyl radicals. On the other hand N-alkyl aldonitrones containing semi-hindered phenolic antioxidant functions were good antifatigue agents. Optimum activity was achieved when they were premilled with the rubber for two minutes on an open two-roll mill prior to the compounding operations. Mechanistic studies showed that the role played by Oalkylated hydroxylamines during the antifatigue activity of the hindered alicyclic nitroxyl radicals is small. It was also shown that the nitrones probably function in part as antifatigue agents by a chain repair mechanism. The hindered piperidines and hydroxylamines did not greatly affect the curing characteristics of the vulcanisation process. However their nitroxyl radicals were found to retard cure. The nitrones did not particularly affect the curing characteristics either, except for the N-primary - alkyl nitrones. These were found to activate cure. None of the hindered piperidine nitroxyl radicals nor their precursors possessed any antiozonant or antioxidant activity. The nitrones on the other hand, were found to be weak antioxidants but very good static antiozonants. All the compounds were non-discolouring and non-staining. Combinations of the nitrones with other non-discolouring/ non-staining stabilisers did not impair their antidegredant activities very much. Furthermore the nitrones were shown to be better static antiozonants than the commercially available agents. Moreover they represent a new class of compounds which can potentially be used to protect light coloured rubber goods against fatigue and ozone deterioration.