Synthesis and Effectiveness of Rubber-Bound Antioxidants

Abstract

The known facility of nitrones to undergo 1,3-dipolar cycloaddition reactions with olefins has been used to prepare rubber-bound antioxidants from suitably substituted nitrones and synthetic diene rubbers. No modification of processing or fabrication techniques is necessary and the reaction takes place conveniently during vulcanisation. The effect of substituents on the efficiency of conversion of nitrone into rubber-bound isoxazolidine has been investigated and the percentage of antioxidant bound at optimum cure has been determined. The effect of nitrones on vulcanisation behaviour has been investigated using the Monsanto oscillating-disc rheometer. Some substituted nitrone phenols were safe when used with a sulphenamide accelerator, but reduced scorch resistance in the presence of a thiuram disulphide. These effects are discussed in the light of the present understanding of the chemistry of vulcanisation. Oxygen absorption and stress relaxation measurements have been used to evaluate antioxidant activity. The activity of bound antioxidants was determined after extraction with an organic azeotrope. Comparison of their performance with commercial antioxidants has shown that the latter are completely removed by azeotropic extraction, whereas the protection given by the bound antioxidants is only reduced. In cis-1,4-polyisoprene vulcanisates, antioxidant activity is less than would be expected on the basis of bound antioxidant yield; best retention of activity is obtained in peroxide vulcanisate. In cis-1,4-polybutadiene vulcanisates, the slight increase in activity of some bound antioxidants is believed to be due to removal of a pro-oxidant species not identified. Nitrones are shown to be effective in stabilising polyethylene and polypropylene towards thermal oxidative degradation, but do not become bound during processing. Despite containing no recognised antioxidant function, simple nitrones are shown to be effective inhibitors for hydrocarbon oxidation catalysed by hydroperoxides, but do not inhibit hydrocarbon oxidation catalysed by azobis-isobutyronitrile. A stoichiometric decomposition of hydroperoxide in combination with alkoxy radical trapping is postulated to account for the experimental results.