Effects of high oxygen pressure and temperature on the aging of polypropylene

Thin films of polypropylene were aged in air and in pure oxygen at pressures ranging from 0.051 to 4.24 MPa (7.4 to 614 psi) and temperatures from 60 to 90°C. Carbonyl formation resulting from polypropylene oxidation was monitored using infrared spectroscopy. The induction time for spectroseopically detectable oxidation coincided with a drastic reduction in tensile elongation for similarly aged tensile specimens. The induction time decreased monotonically with increasing oxygen pressures, while the maximum rate of carbonyl formation increased rapidly at lower oxygen pressures and increased slightly thereafter. The maximum rate of carbonyl formation as a function of oxygen pressure was shown to be consistent with a kinetic rate equation proposed by Stivala, et al. Nearly identical activation energies for the initiation of polypropylene oxidation were determined from reciprocal induction times for samples aged in air and in 4.24 MPa of oxygen (23.9 and 23.8 kcal/g‐mole, respectively). The similarity of activation energies suggests that the same mechanism is involved in the initiation step. Samples aged 3.6 times faster in 4.24 MPa of oxygen than in air at the same aging temperature.