An Infrared Spectrometric Study of the Oxidation of Natural Rubber

The kinetics of the oxidation of natural rubber by atmospheric oxygen has been studied by various methods, for example, by volumetric and manometric measurements of the absorption of oxygen, by measurements of the increase of weight of the rubber, and by changes in the mechanical properties as a function of time. However, the oxidation of rubber is a complex phenomenon, and none of these techniques offers a means of following the oxidation in detail. In principle, it is possible, by infrared absorption spectrometry, to detect the presence of different groups which are present in a given sample of rubber and to determine their concentrations. Hence, it was hoped that this technique would offer a means of studying the various phenomena involved in the oxidation of rubber. In fact, this technique has already been employed in studies of the structure of rubber as well as in the qualitative and quantitative analysis of synthetic products. In addition to these already published studies, various authors have reported that the oxidation of natural rubber results in changes in its infrared spectrum, but none of the investigators has followed these changes quantitatively. Nevertheless, the possible interest of such a quantitative study, when carried out in parallel with a study of the changes in mechanical properties, has been pointed out by different authors. The authors of the present work decided, as a first approach to the problem, to prepare samples of rubber which could be studied by infrared absorption. The progress of oxidation could then be followed quantitatively by the disappearance of the principal groups present in rubber which had not undergone oxidation and the formation of characteristic groups in the oxidation products. Finally, from the data obtained, it was hoped, if not to draw conclusions as to the mechanism or mechanisms of oxidation, at least to obtain some indications of the precise kinetics of this oxidation.