Infrared analysis of isomerized, vulcanized and oxidized natural rubber

The purpose of this study was to follow by i.r. analysis double bond shifts and chemical modification of natural rubber. Accurate determination of the original rubber spectrum is facilitated by the use of an internal standard. Isorubber, prepared by thermal dissociation of rubber hydrochloride, contains about 50% of double bonds in the side groups. Although a similar double bond shift is observed in the cyclization of rubber, part of the absorption, in the 11 μ region, may here be due to ring formation. Vulcanization with accelerated sulfur mixtures does not produce changes in the i.r. spectrum, unless the rubber is transformed into a derivative by overcuring with S 2 Cl 2 or during hard rubber formation. At least three types of oxidation reactions can be distinguished spectroscopically: ( 1 ) degradation of the rubber (by oxidation of a rubber solution with air) with the ultimate formation of only volatile oxidation products; ( 2 ) gradual formation of oxygenated derivatives (oxidation with ozone), with a distinguishable structure in the spectrum; ( 3 ) gradual formation of an oxidized and more saturated rubber (oxidation of solid rubber with air), with less characteristic broad absorption bands.