# Aging characterization of different nitrile butadiene rubbers for sealing in a pneumatic system: Linking the change of the physicochemical state to the mechanical properties

Adrien Redon, Jean-Benoit Le Cam, Eric Robin, Mathieu Miroir, Jean-Charles Fralin

<figure><img src="image_1.png" /></figure>

## ► To cite this version:

Adrien Redon, Jean-Benoit Le Cam, Eric Robin, Mathieu Miroir, Jean-Charles Fralin. Aging characterization of different nitrile butadiene rubbers for sealing in a pneumatic system: Linking the change of the physicochemical state to the mechanical properties. Journal of Applied Polymer Science, 2023, pp.e54068. 10.1002/app.54068. hal-04115327

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Submitted on 1 Jan 2026

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# Aging characterization of different nitrile butadiene rubbers for sealing in a pneumatic system: Linking the change of the physicochemical state to the mechanical properties

Adrien Redon<fnref n="1" />,<fnref n="2" /> ID

Jean-Benoît Le Cam<fnref n="1" /> ID

Eric Robin<fnref n="1" />

Mathieu Miroir<fnref n="1" /> ID

Jean-Charles Fralin<fnref n="2" />

## Correspondence

Jean-Benoît Le Cam, IPR (Institut de Physique de Rennes) – UMR 6251, Univ Rennes, CNRS, Rennes F-35000, France.
Email: jean-benoit.lecam@univ-rennes1.fr

## Funding information

SNCF Voyageurs

## Abstract

Nitrile butadiene rubbers (NBR) are widely used in sealing applications, such as O-rings inside the pneumatic system of the French high-speed train (TGV). In this application, a strong hardening of the NBRs alters the sealing function. Predicting the evolution of the mechanical properties during service life, especially the material hardening, is therefore a strategic issue to optimize time in maintenance operations of the TGV's pneumatic system. The main goal of this study is to reproduce the aging observed on a train's pneumatic system by performing thermo-oxidative accelerated aging tests with different commercial nitrile rubbers at several temperatures and up to 2016 h. After achieving similar degradation to specimens aged on trains, aging mechanisms and effects have been investigated through different characterization techniques: infrared spectroscopy, swelling tests, X-ray fluorescence spectrometry, differential scanning calorimetry, thermo-gravimetric analysis, and micro-hardness measurements. The results obtained enabled us to identify and to relate aging mechanisms to aging conditions and to precisely determine and quantify the effects of physicochemical state evolution on the mechanical properties of each NBR considered. Extra crosslinks and oxidative functionalities form in the different elastomers, making them hard and brittle, and thus impacting the sealing function.

## KEYWORDS

aging, micro-hardness, nitrile butadiene rubber, thermo-oxidative degradation

## 1 | INTRODUCTION

Nitrile butadiene rubbers (NBR) are among the most used elastomeric materials for sealing applications with O-rings or lip seals, due to excellent oil resistance,

good mechanical properties and gas impermeability, as well as moderate production costs.<fnref n="1" />⁻³ Nevertheless, several aging processes alter their mechanical behavior, which decreases the lifetime of the parts made with them.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2023 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals LLC.

<footnote n="1">¹IPR (Institut de Physique de Rennes) – UMR 6251, Univ Rennes, CNRS, Rennes, France</footnote>

<footnote n="2">²Ingénierie du Matériel (CIM OMF), SNCF Voyageurs, Rennes, France</footnote>
<figure><img src="image_3.png" /><figcaption>**FIGURE 1** Cross-section of nitrile rubber O-rings aged inside a train's pneumatic system. When critical aging is achieved (right), specimens significantly harden and present a permanent deformation, which leads to a loss of sealability. A distinction is made between the side exposed to higher pressures (side a) and the side where the seal is made (side b). [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

The present study focuses on commercial NBR O-rings used for the sealing function in the pneumatic system of the French high-speed train (TGV). During their lifetime, these O-rings are exposed to prolonged contact with oxygen and to temperatures ranging between –30 and 50°C. This induces a significant hardening, as well as permanent deformation (see Figure 1), which leads to a loss of sealability, since the distorted shape loses its elasticity. This forces Société Nationale des Chemins de fer Français (SNCF, France's national railway company), to multiply the number of maintenance operations, to replace only the hardened O-rings, whereas all the other components of the pneumatic system are still functional.

Prolonged exposition to heat and oxygen are known aggravating factors in aging of NBRs,4–7 and more generally of elastomers. The past decades, this thermo-oxidative aging has been the subject of various studies.8,10,16 A mechanism was proposed in the mid-1940s by Bolland, Gee, Bateman et al.11–14 and is today commonly invoked to explain aging in most polymers. This oxidation scheme is based on a sequence of free radical reactions (basic auto-oxidation scheme), and is initiated when additional energy is transferred to the material (heat, mechanical load), resulting in the production of an unstable carbon radical from the polymer backbone.5,15 This unstable carbon radical reacts with oxygen, forming intermediate products (per-oxy radicals then hydroperoxide) and different oxygenated functionalities,16 which can be detected and quantified to follow the aging process. Hydroperoxides further decompose into two radicals, namely alkoxy and hydroxy radicals, resulting in an autocatalytic process. The cyclic reaction will continue with two competing processes, occurring simultaneously: chain transfer and chain scission will produce new active radicals, repeating the whole cycle; whereas oxidation induced crosslinking will form additional crosslinks between the polymer's chains.5 Generally, one of the two becomes dominant, resulting in different consequences on the mechanical behavior: the degradation

of the macromolecular network by chain scission causes a softening of the elastomer while the additional oxidative induced crosslinking causes its stiffening.4,6

As abovementioned, numerous studies investigate aging of NBR based elastomers. Nevertheless, comparing existing results is complicated, since material formulations and aging conditions generally differ from one study to another. The main interest of the present study is to compare different NBR formulations, submitted to the same aging conditions, that aim at reproducing ex-situ the aging observed on trains after several years. The comparison is based on various physical, chemical and mechanical characterizations, which enable us to correlate aging to variations in mechanical properties, particularly hardness. Establishing this relationship is needed to perform a reliable prediction of the loss in sealability of O-rings with time.

In the first part of this article, the different commercial NBRs considered in this study are presented, as well as the physicochemical characterization techniques. The second part focuses on the results obtained, before and after aging, at different aging times and temperatures. Accelerated aging tests were performed in order to reproduce ex-situ the aging phenomenon observed on O-rings aged on trains. Finally, a comparison between laboratory aged specimens and specimens aged on trains is made. This discussion lays the foundation for a better understanding of the aging mechanism of O-rings used in the pneumatic system of the French high-speed train (TGV).

## 2 | MATERIALS AND METHODS

### 2.1 | Materials and specimen geometries

NBRs are butadiene ($C_4H_6$) and acrylonitrile ($C_2H_3CN$) copolymers. The butadiene monomer polymerizes into three distinct isomers, namely *trans*-1,4, *cis*-1,4 and *vinyln*-1,2 (see Figure 2). The acrylonitrile (ACN) monomer is
<figure><img src="image_4.png" /><figcaption>**FIGURE 2** Chemical structure of NBR showing the main structural units in the chain backbone. From left to right: *trans*-1,4 (blue), acrylonitrile (gray), *vinyl*-1,2 (green) and *cis*-1,4 (red). [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

<table><caption>**TABLE 1** NBR considered in this study.</caption><thead><tr><td>Specimen code</td><td>T<sub>g</sub> (°C)</td><td>Hardness</td><td>Symbol</td></tr></thead><tbody><tr><td>A1-4070</td><td>-54</td><td>70 IRHD</td><td>▲</td></tr><tr><td>A2-4050</td><td>-56</td><td>50 IRHD</td><td>▲</td></tr><tr><td>A3-2070</td><td>-28</td><td>70 IRHD</td><td>★</td></tr><tr><td>A4-4050</td><td>-44</td><td>50 IRHD</td><td>★</td></tr><tr><td>B1-4070</td><td>-49</td><td>70 IRHD</td><td>■</td></tr><tr><td>B2-4050</td><td>-55</td><td>50 IRHD</td><td>■</td></tr><tr><td>C1-4070</td><td>-48</td><td>70 IRHD</td><td>◆</td></tr><tr><td>C2-4050</td><td>-45</td><td>50 IRHD</td><td>◆</td></tr></tbody></table>

statistically inserted within the macromolecular chain. ACN content confers to NBR its main properties, such as oil resistance and influences the glass transition temperature (T<sub>g</sub>).<sup>16</sup> It can be adjusted in a range generally going from 20% to 45%. For instance, for low temperature applications, ACN content is found around 20% in order to have lower values of T<sub>g</sub>, e.g. less than -40°C.

In the present study, eight types of commercial NBRs are considered, provided by three different suppliers, denoted A, B and C. They are classified according to their glass transition temperature and their original hardness (IRHD) and presented in Table 1.

Three different geometries are used: O-rings, with an internal diameter $\emptyset d_1 = 10.00$ mm and a tore diameter $\emptyset d_2 = 2.50$ mm (NF ISO 3601-1), compression set type A compression disks, with a diameter of 29.0 mm and a thickness of 12.5 mm (NF ISO 815-1) and sheets (150 × 150 × 2 mm<sup>3</sup>) in order to carry out the different tests. Each specimen is prepared from the same material batch by the suppliers.

## 2.2 | Accelerated aging

Accelerated aging tests were performed at 70, 85, and 100°C in a Memmert (ULP 500) air-circulating oven for

336, 672, 1176, and 2016 h (2, 4, 7, and 12 weeks, respectively), according to NF ISO 188 and NF ISO 23529. It should be noted that these times are longer than those of protector consumption, such as antioxidants (roughly 7 days<sup>17</sup>). Temperature measurements are made by a 4-wire Pt100 sensor, with a thermal resolution of 1 K. A fan is used for forced air circulation. For each aging time and temperature, three specimens per material were tested.

## 2.3 | Micro-hardness measurements

Micro-hardness (NF ISO 48-2) is measured with an automatic IRHD tester Compact III from Bareiss, equipped with a centering device for O-rings. For each specimen, five measures are taken.

## 2.4 | Infrared spectroscopy

Infrared spectroscopy is employed to determine the main chemical functional groups in the elastomer, and to monitor their changes during aging. The infrared spectrometer used in the study is a Fourier Transform Infrared (FTIR) spectrometer from PerkinElmer (Spectrum Two), equipped with an Attenuated Total Reflectance (ATR) module and a germanium crystal. The measurements are taken in absorbance mode, with 16 scans at a resolution of 4 cm<sup>-1</sup> and a scan range from 4000 to 700 cm<sup>-1</sup>. Reported spectra are an average of three measures per material tested.

## 2.5 | Swelling tests in solvent

The swelling measurements were carried out on unaged and accelerated aged O-rings. The samples are weighted before and after immersion using a Mettler Toledo micro balance (XP56), with an accuracy of 1 µg. The initial mass ($m_i$) is taken and the sample placed in individual glass containers, before being immersed in toluene for 72 h at room temperature, allowing the test specimens to expand under equilibrium conditions.<sup>18</sup> After swelling, the samples are patted dry with paper towels in order to remove excess toluene on the surface, before being weighted to obtain the swollen mass ($m_s$). Samples are then placed in an oven at 70°C for 24 h, before being weighted to obtain the dry mass ($m_d$). The molar mass between crosslinks ($M_c$), was estimated by using the modified Flory-Rehner equation as follows<sup>20,21</sup>:
$$M_c = \frac{\rho_r V_s \left( V_r^{\frac{1}{3}} - \frac{V_r}{2} \right)}{\ln(1 - V_r) + V_r + \chi V_r^2} \quad (1)$$

with $V_r = \frac{m_d}{\frac{m_d}{\rho_r} + \frac{m_s - m_d}{\rho_s}}$ the local volume fraction of swollen elastomer in the solvent (toluene), $\rho_r$ the density of the rubber, $\rho_s$ the density of the solvent ($\rho_s = 0.867 \text{ g cm}^{-3}$), $V_s$ the solvent molar volume ($V_s = 106.27 \text{ cm}^3 \text{ mol}^{-1})^{22,23}$ and $\chi$ the Flory-Huggins polymer-solvent interaction parameter.²⁴,²⁵ The latter is based on the Hildebrand solubility parameters ($\chi = \chi_\beta + \frac{V_s}{RT}(\delta_s - \delta_r)^2$).²⁶,²⁷ Considering the solubility parameter of the solvent ($\delta_s = 18.2 \text{ MPa}^{1/2}$), the solubility parameter of NBR ($\delta_r = 19.3 \text{ MPa}^{1/2}$) and the polymer-solvent lattice parameter ($\chi_\beta = 0.34$),²³ a value of $\chi = 0.393$ was used. The reported values of $M_c$ are the average of three samples. Crosslinking density $\nu$ in mol g⁻¹, can then be calculated as follows²¹:

$$\nu = \frac{1}{2M_c} \quad (2)$$

Here, it should be noted that the materials are filled. This means that a correction has to be done on the volume effectively swollen.²² As the filler amount is not given by the suppliers, it is not possible to correct the formula²⁸,²⁹ and to obtain $M_c$. Therefore, in the present case $\nu$ is not the crosslinking density and is denoted $\nu^*$ in the following. Nevertheless, $\nu^*$ is affected by aging, which is the reason why the ratio between current and initial values of $\nu^*$ ($\frac{\nu^*}{\nu_0^*}$) is used as an indicator of aging.

## 2.6 | X-ray fluorescence spectrometry

X-ray fluorescence (XRF) is a non-destructive technique used to obtain elemental information on the eight NBR specimens. A Bruker S8 TIGER 1 kW sequential spectrometer with a rhodium X-ray tube is employed. Specimens were cut from the NBR sheets to fit disks with a diaphragm diameter of 18 mm, before being placed in a chamber with helium atmosphere and irradiated by the X-ray tube. The emitted fluorescent X-rays coming from each individual element present in the NBR specimens tested are separated using analyzing crystals (LiF 200, PET and XS-55). The intensities of characteristic emission spectral lines, such as $K_\alpha$, are measured for 15 min by a gas flow proportional counter and a scintillation counter.³⁰ They are directly related to the amount of elements present in the material. Finally, when extracting analytical results, C, H and N elements are used as a

matrix to be more representative of an actual NBR formulation.

## 2.7 | Differential scanning calorimetry

Differential Scanning Calorimetry (DSC) measurements are performed in order to measure heat exchanges between a tested specimen and a reference specimen. It allows the determination of phase transitions, such as the glass transition temperature (Tg), melting and crystallization temperatures and enthalpies of reaction of various polymers.¹⁷,³¹ Here, Tg is measured for the different specimens using a Q20 calorimeter from TA Instruments, according to the NF T 46-050 standard. Pieces of tested specimens are cut, weighted (between 5 and 20 mg) and fitted in an aluminum capsule inside the calorimeter. Tg values are determined using the tangent method on curves of heat flux versus temperature (cf. Figure 3).

## 2.8 | Thermogravimetric analysis

Compositional analysis of the elastomer's main components (volatile substances, carbon black, mineral fillers) is monitored using the Q50 thermogravimetric analyzer (TGA) from TA Instruments. 6 to 10 mg pieces of specimens are cut, weighted and heated according to the NF T 46-047 standard and their composition is calculated by monitoring their weight during heating. Characteristic temperatures at which components volatize or decompose are tracked.

<figure><img src="image_5.png" /><figcaption>FIGURE 3 Differential scanning calorimetry thermograms of A1-4070, B1-4070 and C1-4070 specimens, highlighting Tg differences for each specimens. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>
<table><caption>TABLE 2 Elemental chemical composition of the different NBR samples using XRF analyzes before aging.</caption><thead><tr><th>Elements (%)</th><th>A1-4070</th><th>A2-4050</th><th>A3-2070</th><th>B1-4070</th><th>B2-4050</th><th>C1-4070</th><th>C2-4050</th></tr></thead><tbody><tr><th>CHN</th><td>97</td><td>94</td><td>95</td><td><strong>85</strong></td><td><strong>91</strong></td><td>96</td><td>95</td></tr><tr><th>S</th><td>1.6</td><td>2.4</td><td>1.6</td><td><strong>0.4</strong></td><td><strong>0.3</strong></td><td>1.7</td><td>1.7</td></tr><tr><th>Zn</th><td>1.5</td><td>3.0</td><td>2.5</td><td>2.1</td><td>2.3</td><td>1.3</td><td>2.7</td></tr><tr><th>Si</th><td>&lt; 1</td><td>0.1</td><td>0.5</td><td><strong>9.1</strong></td><td><strong>3.1</strong></td><td>0.1</td><td>&lt; 1</td></tr><tr><th>Al</th><td>&lt; 1</td><td>0.1</td><td>0.3</td><td><strong>3</strong></td><td><strong>3.2</strong></td><td>&lt; 1</td><td>&lt; 1</td></tr></tbody></table>

Note: Major compositional differences are highlighted in bold for the peroxide-cured NBRs (B specimens).

<table><caption>TABLE 3 Mean content of the different levels of the NBR samples from TGA analyzes before aging.</caption><thead><tr><th>Content per phase (%)</th><th>A1-4070</th><th>A2-4050</th><th>A3-2070</th><th>A4-4050</th><th>B1-4070</th><th>B2-4050</th><th>C1-4070</th><th>C2-4050</th></tr></thead><tbody><tr><th>P1</th><td>10.9</td><td>15.5</td><td>7.4</td><td>15.3</td><td>11.4</td><td>18.8</td><td>9.3</td><td>15.0</td></tr><tr><th>P2</th><td>40.2</td><td>51.3</td><td>47.3</td><td>50.8</td><td>42.7</td><td>38.6</td><td>43.4</td><td>51.7</td></tr><tr><th>P3</th><td>2.1</td><td>4.0</td><td>5.9</td><td>2.8</td><td>2.8</td><td>3.3</td><td>1.6</td><td>2.2</td></tr><tr><th>P5</th><td>45.1</td><td>25.3</td><td>34.7</td><td>25.2</td><td><strong>14.5</strong></td><td><strong>17.7</strong></td><td>43.2</td><td>27.2</td></tr><tr><th>P6</th><td>1.8</td><td>3.9</td><td>4.9</td><td>5.9</td><td><strong>28.7</strong></td><td><strong>21.5</strong></td><td>2.5</td><td>3.8</td></tr></tbody></table>

Note: Bold values represent the difference in carbon black content (P5) and mineral residues (P6) for the peroxide-cured NBRs (B specimens).

# 3 | RESULTS

## 3.1 | Characterizing the NBRs' composition before aging

XRF and TGA analyzes are performed before aging on the 8 NBRs. These two experiments enable us in identifying the vulcanization agents and filler types used in the 8 specimens, of unknown formulation. Tables 2 and 3 summarize the results obtained. From the XRF data, B specimens exhibit a lower sulfur content than A and C specimens. This difference is explained by the use of a different vulcanization or curing system.32,33 In fact, B specimens are vulcanized with peroxides, whereas A and C specimens are sulfur cured. Moreover, both XRF and TGA measurements show that B specimens have a higher content in mineral products (Si, Al) than specimens A and C. This can typically be explained by the use of different types of reinforcing filler. Table 3 and Figure 4 show the different stages obtained from TGA thermograms when specimens are heated, and corresponding to a mass loss. P1 and P2 are respectively the content in low molecular weight organic compounds and the pyrolizable phase of the polymer. Carbon residues are formed due to pyrolysis and correspond to the P3 content. Finally, the P5 and P6 stages stand for the carbon black content and the mineral residue content, respectively. Thus, specimens B exhibit a lower carbon black content (P5) but a higher mineral residue (P6) content than specimens A and C, showing the use of silica fillers along with carbon black fillers. To sum up, specimens A and C are carbon black filled, specimens B have coupled silica and carbon

<figure><img src="image_6.png" /><figcaption>FIGURE 4 Thermogravimetric analyzer thermograms of A1-4070, B1-4070 and C1-4070 specimens, highlighting content difference between sulfur-cured NBRs (A1-4070 and C1-4070) and a peroxide-cured NBR (B1-4070). A significant difference is noted for the P5 and P6 phase of the B1-4070 specimen, corresponding to carbon black content and mineral residues. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

black fillers network. Additional TGA data are reported in the Supporting Information file available in the online version of the article.

## 3.2 | Micro-hardness evolution

Micro-hardness measures were done on the different specimens before and after aging for various durations (336, 672, 1176, and 2016 h) and temperatures (70, 85,
<figure><img src="image_7.png" /><figcaption>FIGURE 5 Hardness ratio changes in the 50 IRHD specimens, namely A2-4050 (▲), A4-4050 (★), B2-4050 (■) and C2-4050 (◆), during aging. The dotted lines are used as a guide to the eye. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>FIGURE 6 Hardness ratio changes in the 70 IRHD specimens, namely A1-4070 (▲), A3-2070 (★), B1-4070 (■) and C1-4070 (◆), during aging. The dotted lines are used as a guide to the eye. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

and 100°C). The hardness ratio of the aged specimens H over the original hardness $H_0$ are plotted in Figures 5 and 6 for 50 and 70 IRHD specimens, respectively. It is observed that the ratio increases with aging time and temperature, for both the 50 IRHD and 70 IRHD specimens, as is reported in previous studies for NBR.34,35 Also, an increase in hardening rate is obtained for 100°C compared to 85 and 70°C, which shows that temperature has an accelerating effect on the reaction.

As previously mentioned, specimens A (▲ and ★) and C (◆) are found to resist less to thermal oxidation and thus harden more than specimen B (■), both at 100°C and at 85°C. After 2016 h, 70 IRHD A1 and C1 specimens reached similar degradation to O-rings aged

<figure><img src="image_9.png" /><figcaption>FIGURE 7 Crosslink density evolution ratio ($v^*/v_0^*$) in the 50 IRHD specimens, namely A2-4050 (▲), A4-4050 (★), B2-4050 (■) and C2-4050 (◆), during aging at 70, 85, and 100°C. The dotted lines are used as a guide to the eye. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

on trains, leading to hard and brittle material,¹ whereas specimen B1 hardened less. At 70°C, this trend is also confirmed for the 70 IRHD specimens, except for specimen C1. On the other hand, for 50 IRDH, micro-hardness measures are still very close and not yet conclusive. Specimens were thus left to age in the air-circulating oven for durations of 5040 and 10,080 h for future studies. Nevertheless, the differences spotted can again be explained by the difference in vulcanizing agents, as seen in 3.1. Indeed, peroxide crosslinks (specimen B) lead to a better resistance to thermo-oxidative degradation,32,33 compared to sulfur cured specimens (A and C).

### 3.3 | Effect of aging on $v^*/v_0^*$

The swelling tests performed are used to determine the evolution of crosslink density according to the formulas presented in Section 2. As explained, in the case of filled rubbers, the volume of fillers has to be removed from the total volume, as only rubber part swells. As no information on filler type and nature is available for the various formulations, the actual value of crosslinking cannot be provided, but only its evolution, given by $v^*$. The ratio of $v^*$ over its initial value $v_0^*$ is plotted in Figures 7 and 8 for 50 IRHD and 70 IRDH, respectively, as a function of aging time and for different temperatures.

From Figures 9 and 10, the increase in hardness ratio is correlated to the increase in crosslink density with aging time. This clearly illustrates the formation of extra crosslinks during thermo-oxidative aging, as previously found in Refs. 3,8,16,35. One can notice that the increase
<figure><img src="image_10.png" /><figcaption>**FIGURE 8** Crosslink density evolution ratio ($\nu^*/\nu_0^*$) in the 70 IRHD specimens, namely A1-4070 (▲), A3-2070 (★), B1-4070 (■) and C1-4070 (♦), during aging at 70, 85, and 100°C. The dotted lines are used as a guide to the eye. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**FIGURE 9** Hardness ratio changes and crosslink density evolution ($\nu^*$) for A2-4050 (▲), A4-4050 (★), B2-4050 (■) and C2-4050 (♦), during aging at 100°C. The dotted lines are used as a guide to the eye. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

in hardness happens before the exponential increase of the crosslinks density. This could be explained by the consumption of formulation additives first, such as antioxidants, or the migration and exsudation of plasticizers, before actual crosslinks form on the macromolecular chains.

Also, specimen B swells more than specimens A and C, thus exhibiting a lower value of $\nu^*$. This can again be attributed to fewer formation of new crosslinks during thermo-oxidative aging of peroxide cured NBR compared to sulfur cured ones, leading to the lower increase in hardness observed for specimen B.

<figure><img src="image_12.png" /><figcaption>**FIGURE 10** Hardness ratio changes and crosslink density evolution ($\nu^*$) for A1-4070 (▲), A3-2070 (★), B1-4070 (■) and C1-4070 (♦), during aging at 100°C. The dotted lines are used as a guide to the eye. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

Finally, one can notice that specimen A3-2070 (★) exhibits a different behavior from the other 70 IRHD specimens. Since it is the only 70 IRHD specimen with a higher Tg (see Table 1 or Table 4), this difference could impact the relationship between $\nu^*/\nu_0^*$ ratio and the hardness ratio evolution. Indeed, the A3-2070 specimen seems to exhibit the lowest value of $\nu^*/\nu_0^*$, but its hardness value is similar to specimen C1-4070 (♦), and does not match with the other specimens. Therefore, the initial Tg value could justify this result.

## 3.4 | Effect of aging on Tg

Differential scanning calorimetry measurements are performed in order to observe the variations of glass transition temperatures (Tg) during different aging times at 100°C and the results are presented in Table 4. For all the specimens, aged samples presented higher Tg values with increasing aging time.

The Tg ratio of aged specimens, defined as the ratio of the current Tg over the initial one Tg₀, is presented in Figures 11 and 12 for 50 IRHD and 70 IRHD, respectively. In general, if chain scission is the dominant reaction, a decrease in polymer chains interaction, and consequently, an increase in the mobility of molecular chains is observed, thus leading to a decrease in Tg.³⁶ However, for long aging times, crosslinking reaction usually dominates chain scission. This shows that the increase in Tg observed hinders the macromolecular dynamics and that a crosslinking reaction has occurred on the different NBR specimens.¹⁰,³⁷ As new crosslinks are formed during aging, macromolecular chains mobility is restricted,
<table><caption>TABLE 4 T<sub>g</sub> of specimens for different aging times at 100°C.</caption><thead><tr><th rowspan="2">Specimen code</th><th colspan="3">T<sub>g</sub></th></tr><tr><th>Unaged (°C)</th><th>Aged for 672 h (°C)</th><th>Aged for 2016 h (°C)</th></tr></thead><tbody><tr><td>A1-4070</td><td>-54</td><td>-39</td><td>-22</td></tr><tr><td>A2-4050</td><td>-56</td><td>-44</td><td>-23</td></tr><tr><td>A3-2070</td><td>-28</td><td>-17</td><td>-9</td></tr><tr><td>A4-4050</td><td>-44</td><td>-32</td><td>-19</td></tr><tr><td>B1-4070</td><td>-49</td><td>-38</td><td>-37</td></tr><tr><td>B2-4050</td><td>-55</td><td>-50</td><td>-40</td></tr><tr><td>C1-4070</td><td>-48</td><td>-31</td><td>-17</td></tr><tr><td>C2-4050</td><td>-45</td><td>-35</td><td>-24</td></tr></tbody></table>

<figure><img src="image_13.png" /><figcaption>FIGURE 11 T<sub>g</sub>/T<sub>g0</sub> ratio changes in the 50 IRHD specimens, namely A2-4050 (▲), A4-4050 (★), B2-4050 (■) and C2-4050 (♦), during aging. The dotted lines are used as a guide to the eye. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>FIGURE 13 Hardness and T<sub>g</sub> ratio changes for A1-4070 (▲), A3-2070 (★), B1-4070 (■) and C1-4070 (♦), during aging at 100°C. The dotted lines are used as a guide to the eye. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>FIGURE 12 T<sub>g</sub>/T<sub>g0</sub> ratio changes in the 70 IRHD specimens, namely A1-4070 (▲), A3-2070 (★), B1-4070 (■) and C1-4070 (♦), during aging. The dotted lines are used as a guide to the eye. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

resulting in a “stiffening” of the material and an increase in hardness. These observations are confirmed by Figure 13, which shows the correlation between hardness ratio measurements and the T<sub>g</sub>/T<sub>g0</sub> ratio at 100°C, for 70 IRHD specimens. In this figure, it can be seen that the T<sub>g</sub>/T<sub>g0</sub> ratio decreases for aged specimens, in the same way as the hardness ratio increases.

## 3.5 Evolution of functional groups

Infrared spectroscopy (ATR-FTIR) is used to evaluate the functional groups changes on the surface of the different NBR specimens after thermo-oxidative aging. Three spectrum domains were examined, which correspond to characteristic absorption bands:

* The region of hydroxyl stretching vibrations (3600–3200 cm<sup>-1</sup>). In this absorption band, a broad
<figure><img src="image_16.png" /><figcaption>FIGURE 14 A2-4050 specimen IR spectra after aging. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>FIGURE 15 B1-4070 specimen IR spectra after aging. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

peak appears with time, centered around a value of 3300 cm⁻¹. This region corresponds to the formation of oxygenated functionalities, such as -OH alcohol groups,

* The region of carbonyl stretching vibrations (1750–1550 cm⁻¹). An increase of the bands around 1600 cm⁻¹ is observed with time, with two maxima at 1660 and 1580 cm⁻¹,

* The region of bending vibrations (1000–700 cm⁻¹). In this portion, out-of-plane vibrational bonds of C—H adjacent to unsaturations are observed (*trans*-1, 4-butadiene at 966 cm⁻¹ and vinyl-1,2-butadiene at 918 cm⁻¹). In the case of NBR, the underlying thermo-oxidative degradation mechanism is closely related to the chemical structure (see Figure 2) and thus to the unsaturation level (i.e., the butadiene content). Indeed,

<figure><img src="image_18.png" /><figcaption>FIGURE 16 Consumption of the *trans*-1,4-butadiene peak during aging at 70°C for A2-4050 and B1-4070 specimens. The dotted lines are used as a guide to the eye.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>FIGURE 17 Appearance of oxidative functionalities peaks during aging at 70°C for A2-4050 and B1-4070 specimens. The dotted lines are used as a guide to the eye. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

the energy required to break the C=C bond to form active radicals is relatively low.⁵,³⁸ Therefore the decreasing intensity of these bands with time, is due to a consumption of double bonds to form free radicals.¹⁶

These three typical evolutions in the absorption bands are illustrated in Figures 14 and 15, with the stacked spectra of two different specimens, namely A2-4050 and B1-4070, at 70°C. Figures 16 and 17, which represent the evolution of peak intensity from the IR spectra, help in monitoring the consumption of *trans*-1,4-butadiene and the appearance of oxidative functionalities during aging. These results were achieved by integrating the area of each corresponding peaks and ratioing the resulting band areas
<table><caption>TABLE 5 Characteristics of O-rings aged on trains.</caption><thead><tr><th>Name</th><th>Hardness</th><th>Shape</th><th>Picture</th></tr></thead><tbody><tr><td>Specimen 1</td><td>75 IRHD</td><td>Round</td><td></td></tr><tr><td>Specimen 2</td><td>87 IRHD<br>77 IRHD</td><td>Round (side a)<br>Permanent deformation (side b)</td><td></td></tr></tbody></table>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /><figcaption>FIGURE 18 Infrared spectra of specimen 1 aged on train. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>FIGURE 20 Infrared spectra of specimen 2 aged on train, sealing side (side b). [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>FIGURE 19 Infrared spectra of specimen 2 aged on train, pressure side (side a). [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

to the nitrile band area (2235 cm⁻¹).³⁹ As such, hydroxyl and carbonyl regions absorption intensity increase with time (Figure 17), while the *trans*-1,4-butadiene peak decreases (Figure 16), which is in good agreement with

results obtained in Refs. 16,34,39,40. As mentioned above in 3.1, B specimens contain silica fillers, which can be observed in Figure 15 and the presence of a large peak around 1100 cm⁻¹. This corresponds to SiO bonds found in mineral products such as kaolin or talc.

## 4 | DISCUSSION

In this section, we discuss the differences between laboratory aged specimens and specimens aged on trains. This comparison aims at a better understanding of aging of O-rings used in the pneumatic system of the TGV. With the numerous analyzes done on accelerated aged specimens, a correlation is found between the increase in the hardness ratio (increase in stiffness) with the formation of crosslinks (increase of crosslinking density, Figure 10) and the reduction of macromolecular chain mobility (Figure 13). Also, from FTIR spectroscopy spectra, the consumption of unsaturations in the butadiene part of the copolymer and the formation of oxygenated functionalities was shown to be the principal degradation
mechanism (Figures 16 and 17). These results are in good agreement with previous results on similar material¹⁶: the degradation mechanism of the various NBRs can be observed by FTIR-ATR, with the generation of hydroxyl and carbonyl based products, and the consumption of the *trans*-1,4-butadiene part through aging. The oxidation of this area begins with a proton loss on the allylic position of the methylene group, with heat accelerating oxidation, and then follows the basic auto-oxidation scheme proposed by Bolland, Gee, Bateman et al.¹¹⁻¹⁴ All the resulting thermo-oxidative reactions occur on the 1,4-butadiene unit, resulting in the loss of *trans*-1,4-butadiene peaks during aging. The creation of alkyl radicals is also a reactive site for oxidative-induced crosslinking reactions, with C-C creation between butadiene chains, which explains why the specimens stiffen and harden extensively.

Now that the aging mechanisms are highlighted, these results can be put into perspective with sealing applications, especially with O-rings specimens aged on trains (see Figure 1 and Table 5). First of all, regarding the physical aspect, Specimen 1 kept its circular shape, with no significant increase in hardness. On the other hand, Specimen 2 presents the characteristic permanent deformation of O-rings compressed in a groove. The flat part (side b) that creates the seal remains quite elastic, as its hardness only increased from 70 IRHD to 77 IRHD. Contrastingly, the round shape (side a), which receives the highest amount of pressure, shows a notable increase in hardness, going from 70 IRDH to 87 IRHD. During maintenance testing operations, this O-ring failed its leak test and leakage was noticed in the system it equipped.

FTIR analyzes were performed with these aged O-rings used on trains. When comparing spectra with those of accelerated aged specimens, strong similarities are found: an increase in the hydroxyl and carbonyl regions, showing the formation of oxygenated functionalities during aging. A few differences can be spotted between Specimen 1 and Specimen 2. From Figure 18, showcasing Specimen 1, we can see that the oxygenated functionalities are less present, while the peak at 1538 cm⁻¹, which can be associated to formulation additives, typically anti-oxidants, remains intense. This clearly illustrates the fact that this specimen did not reach critical aging during its service life, as it shows on hardness values and its physical aspect as discussed previously.

On the other hand, Specimen 2 has a significant increase in the hydroxyl and carbonyl peaks on the side exposed to higher pressure (side a), compared to the side where the seal is made (side b). This can be noticed in Figures 19 and 20, respectively. From the spectra of Figure 19, we clearly see a more important formation of oxygenated functionalities, since this side is more

exposed to oxygen, that is, where the pressure comes from. On the contrary, the spectra in Figure 20 still hARBORS a peak around 1540 cm⁻¹, meaning that additives or unsaturations are still present in the less hardened part, that is, the zone where sealing is made. Pressure and oxygen flow have, as such, an impact on how the material ages, since side (a) is exposed to higher pressures and thus higher oxygen flow than side (b). This behavior can explain the heterogeneities observed in aged specimens on trains, as sides more exposed to oxygen will tend to age faster than sides making the seal.

Also, the resulting permanent deformation observed for Specimen 2 could be explained by both the gradual diffusion of oxygen in the material and the pressure applied. When O-rings operate under unaged conditions, the seal is done by deforming the NBR under pressure in its groove, preventing air flow inside the pneumatic system. As we have seen, when the O-rings age, oxygen diffuses in the material, creating oxygenated functionalities and new chains. These chemical modifications hinder the macromolecular mobility, stiffen the material and will gradually confine the O-ring in its deformed shape, until a certain critical aging point has been reached. At this stage, the O-rings are not capable of elastically returning to their initial shape and can therefore lose their sealing function.

5 | CONCLUSION

The effect of thermo-oxidative aging on eight different filled nitrile rubbers has been studied with accelerated aging tests and several characterization methods have been employed (mechanical and chemical ones).

The formation of oxidative functionalities and consumption of unsaturations in the butadiene part of the copolymer during aging has been highlighted, leading to a degradation of the mechanical properties (i.e., microhardness). Indeed, swelling tests show that extra crosslinks form in the elastomers, increasing the hardness and reducing the mobility of the macromolecular chains, making the elastomers hard and brittle. Comparison with aged O-rings from trains enabled us in showing how these O-rings lose their sealing function: the O-rings are “frozen” in a deformed state and lose elasticity, which leads to leaks in the systems they equip.

Additional effects such as Diffusion-Limited Oxidation (DLO) and mechanical loadings will be investigated thoroughly. As shown in previous studies,⁷,¹⁵,³⁹,⁴¹ DLO has to be taken into account when predicting lifetimes, as it is a key contributor to differences between accelerated thermal aging and actual aging behavior, as we have seen. A reliable aging factor, which accommodates for
DLO, mechanistic solicitations⁴² and non-linear temperature dependencies, is currently being investigated, to further enhance the comprehension of thermo-oxidation of NBRs coupled with mechanical loadings.

## AUTHOR CONTRIBUTIONS

**Adrien Redon:** Data curation (lead); formal analysis (lead); investigation (lead); methodology (lead); writing – original draft (lead). **Jean-Benoît Le Cam:** Supervision (lead); validation (equal); writing – review and editing (equal). **Eric Robin:** Investigation (supporting); methodology (supporting); writing – review and editing (supporting). **Mathieu Miroir:** Investigation (supporting); methodology (supporting); writing – review and editing (supporting). **Jean-Charles FRALIN:** Funding acquisition (equal); resources (lead); supervision (equal); validation (equal); writing – review and editing (supporting).

## ACKNOWLEDGMENTS

The authors gratefully acknowledge SNCF Voyageurs for their financial support and SNCF's Ingénierie du Matériel (CIM Rennes and AEF) for their technical assistance.

## DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## ORCID

Adrien Redon https://orcid.org/0000-0003-4850-7292

Jean-Benoît Le Cam https://orcid.org/0000-0002-2366-2512

Mathieu Miroir https://orcid.org/0000-0001-5884-6103

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## SUPPORTING INFORMATION

Additional supporting information can be found online in the Supporting Information section at the end of this article.

**How to cite this article:** A. Redon, J.-B. Le Cam, E. Robin, M. Miroir, J.-C. Fralin, *J. Appl. Polym. Sci.* **2023**, *140*(29), e54068. https://doi.org/10.1002/app.54068