Evaluation of Oxidative Stability in Polyethylene Using Differential Scanning Calorimetry

Introduction

Polymer materials, such as Polyethylene (PE), are susceptible to oxidative degradation when exposed to oxygen, leading to a decline in mechanical strength and electrical properties. The oxidative stability of polymers is a critical factor in applications such as wire coating materials, where prolonged durability is required. Unlike thermal decomposition, which typically occurs around 400°C in an inert gas environment (e.g., nitrogen), oxidation-induced degradation can begin at significantly lower temperatures, often below 200°C, in the presence of oxygen.

To enhance polymer stability, antioxidant additives are incorporated into PE formulations to prevent premature degradation. The effectiveness of these additives can be evaluated using Oxidation Induction Time (OIT), a parameter measured via Differential Scanning Calorimetry (DSC). The DSC curve provides insights into temperature dependence, the impact of catalyst effects (e.g., copper influence), and polymer aging.

The OIT measurement follows a standardized process:

This study aims to assess the oxidative stability of PE by measuring its OIT at different isothermal temperatures, evaluating the catalyst effect of copper, and comparing different grades of PE.

Figure 1 Oxidation induction time measurement result for PE at 205°C

Experimental Method

A DSC200 instrument was used for all measurements. The experimental parameters were as follows:

Hitachi NEXTA DSC200

Results and Discussion

1. Temperature Dependence of Oxidation Induction Time

Figure 1 presents the DSC curve of PE at 205°C. Following the gas switch from nitrogen to oxygen, no significant changes were observed for 17 minutes, indicating the presence of antioxidant additives. However, beyond this point, an exothermic peak appeared due to oxidation, marking the OIT at 17.2 minutes.

Figure 2 Temperature dependence of oxidation induction time for PE

Figure 3 Cu influences on the oxidation induction time for PE
Isothermal temperature: 205°C

Figure 2 shows the OIT measurements at different isothermal temperatures (200°C, 205°C, 210°C, and 215°C). The results demonstrate that higher temperatures lead to shorter oxidation induction times, confirming the temperature dependence of the oxidation process.

2. Influence of Copper as a Catalyst

Figure 3 illustrates the catalyst effect of copper (Cu) on the oxidation induction time of PE. When Cu was in contact with PE, the OIT was significantly reduced from 17 minutes to 6 minutes, indicating that Cu accelerates oxidation reactions. This result is particularly relevant for wire coating materials, where Cu conductors may directly interact with the polymer insulation, affecting polymer aging and long-term polymer stability.

3. Comparison of Different Polyethylene Grades

Figure 4 compares the OIT of three different PE grades. The order of oxidative stability was found to be C > B > A, highlighting the variation in polymer stability across different formulations. This demonstrates the usefulness of OIT analysis for material evaluation and quality control in industrial applications.

Figure 4 Measurement results of three different grades of PE
Isothermal temperature: 205°C

Conclusion

The oxidation induction time (OIT) of Polyethylene (PE) was successfully measured using Differential Scanning Calorimetry (DSC). This method provides a reliable approach for assessing oxidative stability, the influence of temperature dependence, the catalyst effect of copper, and variations among different PE grades. The DSC curve serves as an essential tool for evaluating polymer aging, optimizing antioxidant additives, and ensuring the long-term performance of wire coating materials and other polymer-based products.

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Reference: Hitachi