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# Investing thermal oxidative aging mechanisms in EPDM

Chloé Jost, Jihane Sahyoun, Maxime Lacuve, Xavier Colin, Eliane Espuche

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Chloé Jost, Jihane Sahyoun, Maxime Lacuve, Xavier Colin, Eliane Espuche. Investing thermal oxidative aging mechanisms in EPDM. MoDesT 2024, Sep 2024, Palermo, Italy. pp.224-225. hal-04979078

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Distributed under a Creative Commons Attribution 4.0 International License
# INVESTING THERMAL OXIDATIVE AGING MECHANISMS IN EPDM

C. Jost<fnref n="1" />, J. Sahyoun<fnref n="2" />, M. Lacuve<fnref n="2" />, X. Colin<fnref n="3" /> and E. Espuche<fnref n="1" />

## INTRODUCTION

The terpolymer Ethylene Propylene Diene Monomer (EPDM) is particularly used in the field of electrical wiring as insulating materials, for accessories, due to its excellent dielectric properties and thermomechanical stability. The environmental solicitations endured by underground electric power distribution lines in service can be diverse: heat, oxygen, humidity, mechanical stress, and can lead to irreversible degradation of their functional properties, thus potentially leading to premature failures [1]. One of the main stresses to which insulating materials are exposed in service is temperature, generally in the 50-70 °C range. Thermal oxidation causes a reduction of the material's lifetime. Understanding this reaction is therefore a major challenge for the electrical wiring industry.

Two cross-linking modes are possible for this polymer: sulphur and peroxide cross-linking [2]. Sulfur-crosslinked EPDMs (denoted EPDMs XL-S) are generally employed when looking for materials with higher mechanical strength [3], whereas peroxide-crosslinked EPDMs (EPDMs XL-P) are used when needing for materials that maintain high mechanical properties throughout aging processes. In this study, two EPDMs crosslinked according to these two modes were manufactured then subjected to thermal aging in air in order to examine the impact of the cross-linking route on the thermal oxidation mechanisms and kinetics.

## EXPERIMENTAL

### Materials preparation and thermal aging

The starting linear EPDM is composed of 50 wt% in ethylene, 45 wt% in propylene and 5 wt% in ethylidene norbornene (ENB) as diene monomer. EPDM plates of about 0.6-0.8 mm thick were press-moulded and crosslinked under a compression of 2.0 MPa. EPDM XL-S was crosslinked using 0.40 phr of sulfur compounds at 170 °C for 15 min, whereas EPDM XL-P was crosslinked with 0.56 phr of dicumyl peroxide at 180 °C for 11 min. In both cases, the optimal crosslinking conditions (temperature and duration) were previously determined using rheometry according standard ISO 3417 [4]. The duration is defined as the time required for the torque to reach 90% of its maximum value plus 5 min. A lower temperature was chosen for EPDM XL-S in order to avoid reversion.

Before the aging experiments, rectangular samples of size 12x38 mm were degassed for 72h to remove moisture and residual vulcanization products. The samples are then thermally aged in an air-ventilated oven at 90, 110 and 130 °C for defined periods of time. The temperatures were chosen in order to accelerate the thermal oxidation without changing the mechanism involved in this reaction [5].

### Characterization

Oxidized chemical functions were observed in a reflexion mode with a Spectrometer IR-TF Nicolet™ iS™10 equipped with a diamond crystal. Density measurements were performed through hydrostatic weighing at room temperature using a high-precision balance from Mettler Toledo. The samples were first weighed in air, then in immersion in ethanol, and their density was determined by applying the Archimedes' principle. Swelling measurements were performed in cyclohexane at room temperature to determine the crosslinking density

<footnote n="1">¹ Université Claude Bernard Lyon 1, CNRS UMR 5223, Ingénierie des Matériaux Polymères F-69622 Villeurbanne Cedex, France chloe.jost@univ-lyon1.fr</footnote>

<footnote n="2">² Nexans Research Center, 69353 Lyon Cedex 07, France</footnote>

<footnote n="3">³ Arts et Métiers Institut of Technology, PIMM, CNRS UMR 8006, 75013 Paris, France</footnote>
using the common Flory-Rehner's equation. Finally, changes in hydrophilicity throughout thermal aging were determined on few carefully chosen samples by dynamic water sorption (DVS).

## RESULTS AND DISCUSSION

Monitoring the formation of oxidized functions by IRTF spectroscopy highlights the different mechanisms involved during thermal aging. At 130 °C for both EPDMs, two major bands are observed at 1730 and 1710 cm⁻¹, corresponding to the formation of carbonyl groups. In addition, in the case of EPDM XL-S (Fig. 1a), double bonds are formed during thermal aging, as shown by the shoulder at around 1640 cm⁻¹. In the case of EPDM XL-P (Fig. 1c), however, very few double bonds are formed. The presence of polysulfide junctions in an EPDM has clearly a protective effect against thermal oxidation.

<figure><img src="image_3.png" /><figcaption>Fig. 1 Changes in the ATR spectrum during thermal aging (in hours) of EPDM XL-S in air at 130 °C (a) and 90 °C (b), and EPDM XL-P in air at 130 °C (c) and 90 °C (d)</figcaption></figure>

The comparison of the agings at 130 °C (Fig. 1a) and 90 °C (Fig. 1b) for EPDM XL-S reveals that the double bonds band (at 1590 cm⁻¹) becomes more intense than the carbonyl band (at 1730 cm⁻¹) when decreasing the temperature. There is thus a competition between the thermal oxidation mechanisms, which is clearly temperature dependent. In light of these findings, one may question the reliability of lifetime predictions for industrial applications, knowing that accelerated aging tests are generally conducted at temperatures above 90 °C [6].

## References

[1] S. Hettal, S.V. Suraci, S. Roland, D. Fabiani and X. Colin, Polymers, 13:24, 4427 (2021)

[2] M. Akiba, Progress in Polymer Science, 22:13, 475-521 (1997)

[3] C. M. Kok and V. Yee, European Polymer Journal, 22:14, 341-345 (1986)

[4] ISO 3417, Rubber - Measurement of vulcanization characteristics with the oscillating disc curemeter

[5] M. Gardette, A. Perthue, J.-L. Gardette, T. Janecska, E. Földes, B. Pukánszky and S. Therias, Polymer Degradation and Stability, 98:111, 2383-2390 (2013)

[6] Camille Blivet, Jean-François Larché, Yaël Israëli and Pierre-Olivier Bussière, Polymer Degradation and Stability, 201, 109978 (2022)