Introduction
Degradation of a polymer goes hand in hand with worsening of its properties and shortening its lifetime, so that stabilizers are usually added to avoid oxidative decomposition or to slow it down. An easy and quick method of measuring the stability of polymers against oxidative stress is to perform oxidation induction tests, also called OIT, with a differential scanning calorimeter (DSC). These tests are internationally recognized and described in different standards, e.g., ASTM D3895-92, ASTM D6186, EN 728 and ISO 11357-6.
Measurement Conditions
In the following, OIT tests are carried out on two different polyethylene samples using the DSC 300 Caliris® Classic. In order to ensure good repeatability of the DSC curves, the samples were prepared exactly the same way and the measurements performed with crucibles without lid. In fact, this type of test is very sensitive to sample preparation and measurement parameters, and even the hole size in the lid would influence the results.
The test consists of three different steps:
- Heating to a temperature higher than the melting temperature of the sample under a dynamic nitrogen flow
- Isothermal segment for 3 minutes under nitrogen
- Switching the gas from nitrogen to synthetic air until degradation occurs. The end of the test is automatically detected via the Proteus® measurement and evaluation software
Table 1 summarizes the measurement conditions.
Table 1: Conditions of the OIT test
| Device | DSC 300 Caliris® Classic |
| Crucible | Concavus® (aluminum), open |
| Sample mass | 10.2 ± 0.2 mg |
| Temperature program | 25… 220°C, 20 K/min, nitrogen (40 ml/min) 220°C, 3 min, nitrogen (40 ml/min) 220°C, synthetic air (50 ml/min) |
Test Results
Figure 1 depicts the test results. The endothermal peak detected during heating is due to melting of the polyethylene. Oxidative degradation is characterized by an exothermal effect during the isothermal segment. Here, it takes place 35.9 to 36.1 min (tangential evaluation) after switching the gas to synthetic air for sample 1 (red and pink curves). Sample 2 exhibits a better oxidative stability with an OIT value of more than 10 min higher (blue and dark blue curves). The curves show the excellent repeatability of the measurements.
Conclusion
An OIT test allows for easy and fast characterization of the oxidative stability of polymers and comparison of their thermo-oxidative performance. Careful sample preparation combined with the outstanding measurement performance of the DSC 300 Caliris® Classic and the automatic evaluation feature of Proteus® ensure good repeatability of the results.