FT-IR Imaging of the Thermal Oxidation of Polyisoprene (PI) Rubber at High Temperature

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 auto-catalytic and the induction period is approximately two hours.