The thermal aging of PI rubber in hot air (140 °C) has been studied using the FT-IR imaging technique. From the analysis of the FT-IR images of the absorbance of the carbonyl (C = O) groups, it was found that the most significant thermal oxidation occurred at the surface of the rubber with a decreasing gradient of oxidation from the surface to the interior of the system. The hydroxyl (OH) groups followed the same pattern as the carbonyl groups. The C = C double bonds decreased from the interior of rubber to the surface, demonstrating the existence of a highly cross-linked oxidative coating, which functions as a protective layer blocking further invasion of the oxygen. The depth of the oxidation coating was constant at approximately 160 μm and the most significant oxidation layer was ~90 μm after 10 h oxidation. Based on the FT-IR images as a function of time, the oxidation process is interpreted and classified in terms of four time periods: an induction period, a rapid oxidation period, a slow oxidation period, and a diffusion-forbidden period. Both the thickness of the oxidation coating and the degree of oxidation at different depths show that the oxidation process is autocatalytic and the induction period is approximately two hours.
Gui-Yang Li and J. L. Koenig, "FT-IR Imaging of the Thermal Oxidation of Polyisoprene (PI) Rubber at High Temperature," Appl. Spectrosc. 56, 1390-1396 (2002)