Oxidative-Induction Time (OIT) Measurements of Polyolefins and Thermoplastic Elastomers by Using DSC

OIT measurements enable characterization of the long-term stability of hydrocarbons such as fats and oils as well as plastics such as polyolefins, especially polyethylene and polypropylene. The oxidative stability can be determined by using Differential Scanning Calorimetry (DSC) in standardized test methods. The OIT tests can be easily performed and provide useful stability data. Moreover, they can help predict the thermo-oxidative performance of materials and achieve failure prevention. DSC systems used for OIT determination include the DSC 404 F1/F3 Pegasus, DSC 200 F3 Maia and DSC 204 F1 Phoenix from Netzsch.

DSC 200 F3 Maia

Figure 1. DSC 200 F3 Maia

Test Procedure

The test procedure involves heating of the sample to a temperature beyond its melting point under a protective gas. The sample atmosphere is transformed from inert to oxidative at a consistent temperature. The amount of time elapsing until the onset of the exothermal oxidation of the sample is the OIT. The following are the measurement conditions:

Atmosphere: Oxygen or nitrogen
Crucible Material: Aluminum, open
Isothermal temperature: 210 °C, 190 °C
Purge gas rate: 50 ml/min

OIT Analysis of PP T20 from Different Producers

In this test, the oxidative stability of two highly heat-resistant polypropylene samples from different producers is determined. Here, there is only a very small difference in the melting behavior of the two materials. However, the OIT test is able to clearly show the difference between the samples. The oxidative stability of the sample from Producer A is identified at 15 min. On the other hand, the sample from Producer B exhibits a very high stability and deterioration begins after 122 min, as shown in Figure 2.

OIT tests on PP from different producers

Figure 2. OIT tests on PP from different producers

OIT Analysis of PE-HD Samples Varying in Grade

Here again, the melting behavior of two PE-HD samples differing in grade is nearly identical, as shown in Figure 3. However, the OIT test is able to clearly show the difference between the samples. The oxidative stability of Sample Grade 1 is determined at 43 min, while that of Sample Grade 2 is much lower, as shown in Figure 4. This example clearly shows that it is possible to acquire in-depth data from DSC curves by simply changing the temperature program.

Samples PE-HD Grade 1 and Grade 2, melting

Figure 3. Samples PE-HD Grade 1 and Grade 2, melting

Oxidation behavior of PE-HD at 210 °C

Figure 4. Oxidation behavior of PE-HD at 210 °C

OIT Tests on PE Granulate, Extruded Tube and Aged Tube

This test involved the analysis of materials PE-HD, PE-RT Type 1 and PE-RT Type 2, each as an aged tube, an extruded tube and a granulate. All underwent a temperature change treatment and their melting behavior and behavior in an oxidizing atmosphere are illustrated in Figures 5, 6, 7 and 8.

PE-HD, melting

Figure 5. PE-HD, melting

PE-HD, oxidation behavior at 210 °C

Figure 6. PE-HD, oxidation behavior at 210 °C

PE-RT Type 1, melting

Figure 7. PE-RT Type 1, melting

PE-RT Type 1, oxidation behavior at 190 °C

Figure 8. PE-RT Type 1, oxidation behavior at 190 °C

Dynamic Temperature Program to Determine Oxidative Stability of PE Granulate, Aged Tube and Extruded Tube

If the samples to be investigated vary greatly with their resistance to oxygen, then comparison at an identical isothermal temperature will not be possible. An alternative temperature program depicted in Figure 9 ensures that the samples are completely molten and allowing for a change of atmosphere at a temperature at which point most reactive sample does not react instantly subsequent to the gas change.

Dynamic temperature program for an improved comparison of the OIT

Figure 9. Dynamic temperature program for an improved comparison of the OIT

The melting and oxidation behavior of PE-RT Type 2 samples are shown in Figures 10 and 11. It is not possible to select the isothermal temperature below 180 °C as the melting point of one component is roughly 180 °C. Hence, a dynamic temperature program can be used to significantly differentiate between strongly varying oxidation behaviors.

PE-RT Type 2, melting; second phase at 180 °C, deteced in the new and aged tube

Figure 10. PE-RT Type 2, melting; second phase at 180 °C, deteced in the new and aged tube

PE-RT Type 2, oxidation behavior, dynamic OIT determination

Figure 11. PE-RT Type 2, oxidation behavior, dynamic OIT determination

Failure Analysis of TPE Parts through Dynamic OIT or Oxidative Onset Temperature (OOT)

As outlined in ASTM E2009-08, the OOT is a relative measure of the level of oxidative stability of a material analyzed at a specified heating rate and oxidative environment. This test method may be useful in the determination of the presence or efficacy of antioxidants. In this example, DSC measurements were performed on two TPE parts (poor and good) and the results are illustrated in Figures 12 and 13.

During the first heating, the good (blue curve) and poor sample (green curve) exhibited the identical thermal behavior. Nevertheless, subsequent to the environment change with increasing temperature, the DSC curves showed variations which can be observed from the deviating oxidation behavior of the two samples. The OOT of the good part took place above 241 °C, while that of the poor sample was 229 °C.

First heating curves of two TPE parts

Figure 12. First heating curves of two TPE parts

OOT determination of a good and a poor part

Figure 13. OOT determination of a good and a poor part

Influence of Crucibles on OIT

Standard aluminum or open copper crucible can be used to determine the OIT as outlined in ASTM D3895. The OIT measurements on HDPE performed in an open copper (red) and aluminum (black) crucible, respectively, are depicted in Figure 14. From the results, it is evident that the onset of HDPE oxidation under isothermal conditions is 23 min earlier in the copper crucible when compared to the aluminum crucible.

Comparison of the oxidative-induction time in open copper and aluminum crucibles

Figure 14. Comparison of the oxidative-induction time in open copper and aluminum crucibles

Moreover, the bottom of these crucibles can be shaped using the stamping tool kit of the sealing press as shown in Figure 15. These crucibles are specially designed to determine the OIT of lubricants and grease as outlined in ASTM D5483-5.

Copper (left) and aluminum crucibles (right), especially for OIT determination

Figure 15. Copper (left) and aluminum crucibles (right), especially for OIT determination

This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.

For more information on this source, please visit NETZSCH-Gerätebau GmbH.

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