Plastic Part Failure Analysis - Using Thermal Analysis (DSC) to Estimate the Anti-Oxidant Level in Polymers

Hello and welcome to a new blog post. Let me start today with the following question: How do you ensure the long-term performance of polyolefin materials in demanding applications?

Understanding and measuring oxidative stability is key. The following post explores why oxidative stability matters for polyolefins like polypropylene, and how Differential Scanning Calorimetry (DSC) provides valuable insights into material durability, processing effects, antioxidant performance, and ultimately prevent plastic part failure. Dive in to learn how this classic yet often overlooked test method can help you make informed decisions about material selection and process optimization.

DSC Testing for Oxidative Stability

DSC measures the heat absorbed or released by a material as temperature or time changes. While commonly used for phase transitions (melting, recrystallization, glass transition), it is also effective for detecting exothermic events like oxidation.

How does a typical test procedure for oxidative stability look like?

• A sample (raw material or molded part) is placed in the DSC.
• The sample is heated in a nitrogen atmosphere to a set temperature (commonly 200°C/392°F, which melts PE or PP).
• After reaching the target temperature, air or oxygen is introduced.
• The antioxidant in the polymer protects it until it is depleted; then, oxidation occurs, shown by a sharp increase in the DSC baseline.
• The time from oxygen introduction to oxidation onset is called the Oxidation Induction Time (OIT).
• Also, the test can be used to access the oxidation onset temperature (OOT).

Example PP raw material with standard antioxidant package vs. PP raw material with improved antioxidant package

Figure 1 shows the result for a tested polypropylene (PP) raw material and the OIT was measured at 2.29 minutes. A second PP raw material, which contains an improved antioxidant package, showed a higher OIT (6.42 minutes), indicating better resistance to oxidation under the same test conditions [1].

Figure 1: Using DSC to estimate the anti-oxidant level in Polyolefins - Example PP [1].

Interpreting OIT Results

The OIT value alone is not meaningful, but comparing OITs of materials with similar antioxidant chemistries provides a relative measure of oxidative stability. AS a rule of thumb, higher OIT indicates better oxidative stability and, typically, higher antioxidant content.

DSC offers a quick and practical way to assess oxidative stability compared to more complex antioxidant quantification methods.

Applications of OIT measurements

Raw Material vs. Molded Part:

• Processing (molding) consumes some antioxidants, so molded parts usually have a lower OIT than raw materials. 
• Changes in processing conditions (temperature, screw speed, backpressure) affect OIT and thus the remaining antioxidant content.

Post-Processing and Environmental Effects:

• Sterilization (gamma or E-beam) can significantly reduce OIT, leading to loss of material toughness.
• Long-term heat aging also reduces OIT over time.

Conclusion

DSC-based OIT testing, despite its limitations in perfectly simulating real-world conditions, remains a valuable and practical method for comparing the oxidative stability of polyolefins. It is particularly useful for evaluating the impact of processing and post-processing on antioxidant depletion and for comparing materials with similar stabilizer chemistries.

Thanks for reading & #findoutaboutplastics

Greetings, 

Herwig Juster

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Literature:

[1] https://www.ptonline.com/articles/the-importance-of-oxidative-stability-in-polyolefins-part-2