Oxidative Stability of Polymers: The OIT Test

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 reactionA decomposition reaction is a thermally induced reaction of a chemical compound forming solid and/or gaseous products. decomposition or to slow it down. An easy and quick method of measuring the stability of polymers against oxidative StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress is to perform OxidationOxidation can describe different processes in the context of thermal analysis.oxidation induction tests, also called Oxidative-Induction Time (OIT) and Oxidative-Onset Temperature (OOT)Oxidative Induction Time (isothermal OIT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition. Oxidative-Induction Temperature (dynamic OIT) or Oxidative-Onset Temperature (OOT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition.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, Oxidative-Induction Time (OIT) and Oxidative-Onset Temperature (OOT)Oxidative Induction Time (isothermal OIT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition. Oxidative-Induction Temperature (dynamic OIT) or Oxidative-Onset Temperature (OOT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition.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 Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).melting temperature of the sample under a dynamic nitrogen flow 
• IsothermalTests at controlled and constant temperature are called isothermal.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 Oxidative-Induction Time (OIT) and Oxidative-Onset Temperature (OOT)Oxidative Induction Time (isothermal OIT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition. Oxidative-Induction Temperature (dynamic OIT) or Oxidative-Onset Temperature (OOT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition.OIT test

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 ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermal effect during the IsothermalTests at controlled and constant temperature are called isothermal.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 Oxidative-Induction Time (OIT) and Oxidative-Onset Temperature (OOT)Oxidative Induction Time (isothermal OIT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition. Oxidative-Induction Temperature (dynamic OIT) or Oxidative-Onset Temperature (OOT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition.OIT value of more than 10 min higher (blue and dark blue curves). The curves show the excellent repeatability of the measurements.

Conclusion

An Oxidative-Induction Time (OIT) and Oxidative-Onset Temperature (OOT)Oxidative Induction Time (isothermal OIT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition. Oxidative-Induction Temperature (dynamic OIT) or Oxidative-Onset Temperature (OOT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition.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.