Reaction of Ozone with Natural Hevea and Acrylonitrile-Butadiene Rubbers

Abstract

The cracking of rubber vulcanizates when exposed to the atmosphere in a state of stress is a well-known phenomenon which has been studied extensively. A comprehensive review of the literature on this subject has been published by Newton. Following the lines of earlier work Newton showed conclusively that the cracking is produced by traces of ozone present in the atmosphere. The simultaneous presence of both stress and ozone is required for cracking to occur and the cracks that form are oriented with their lengths perpendicular to the direction of the stress. It has been reported, however, that similar cracking is produced when stretched rubber is exposed to free radicals produced by the thermal breakdown of organic peroxides. Unstretched vulcanizates do not crack when exposed to ozone, nor do they crack when stretched after exposure to ozone. Apart from the frosting effect described by Tuley, which is stated to require the presence of moisture and which may require the presence of small stresses, there seems to be no record in the literature of a visible effect being produced by the action of ozone on unstretched vulcanizates. The effect of ozone on unstretched vulcanizates has been investigated more completely and the results are reported in the present paper. Little is known of the mechanism of the reactions that lead to cracking of vulcanizates and this paper describes experiments designed to throw some light on this. The products of the reaction between ozone and solid rubber have been investigated by means of infrared spectroscopy. Allison and Stanley have previously described the infrared spectra of products formed from solutions of Hevea and various synthetic rubbers and Salomon and van der Schee have recently reported changes which occur in the infrared spectrum of a film of Hevea rubber when exposed to ozonized air.