The Effect of Hindered Phenol Stabilizers on Oxygen Induction Time (OIT) Measurements, and The Use of OIT Measurements to Predict Long Term Thermal Stability

The Oxygen Induction Time (OIT) test is an accelerated aging test that is often used to predict the long term stability of hydrocarbon materials including plastics, rubbers, and adhesives. The OIT test can also be used to measure the level of effective antioxidant present or remaining in a polymeric material after some environmental exposure. Since thermal-oxidatative degradation requires the presence of oxygen at elevated temperatures, the OIT test is designed to accelerate this degradation in order to achieve meaningful comparisons in a short period of time. The standard OIT test is performed according to ASTM D3895, and uses a Differential Scanning Calorimeter (DSC). A few mg of the material to be tested are introduced into the DSC at room temperature, and the sample is heated to about 200 C under a nitrogen atmosphere. When 200 C is reached the cell is maintained in an isothermal condition, and the gas is changed from nitrogen to oxygen. The pressure and flow rate of the oxygen are maintained at 35 kPa and 50 cc/min, respectively. Under these conditions the stabilizer is consumed over time until it is totally depleted. At this point the polymer sample catastrophically degrades or oxidizes liberating additional heat (exotherm). The time it takes for this exotherm to appear from the time that the oxygen is introduced is reported as the OIT time, and is a measure of the thermal stability of the material. A typical OIT curve for polyethylene is illustrated in Figure 1.