# Aging of NBR: Application to O-rings used in the braking system of the French high-speed train

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

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Adrien Redon, Jean-Benoit Le Cam, Mathieu Miroir, Eric Robin, Jean-Charles Fralin. Aging of NBR: Application to O-rings used in the braking system of the French high-speed train. 12th european conference on constitutive models for rubber (ECCMR), Sep 2022, Milan, Italy. pp.420-426, 10.1201/9781003310266-69 . hal-03959557

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# Aging of NBR: Application to O-rings used in the braking system of the French high-speed train

A. Redon

Université Rennes, CNRS, IPR (Institut de Physique de Rennes) - UMR 6251, Rennes, France

SNCF Voyageurs, Ingénierie du Matériel (CIM OMF), Rennes, France

M. Miroir, E. Robin & J.-B. Le Cam

Université Rennes, CNRS, IPR (Institut de Physique de Rennes) - UMR 6251, Rennes, France

J.-C. Fralin

SNCF Voyageurs, Ingénierie du Matériel (CIM OMF), Rennes, France

**ABSTRACT:** The sealing function in the braking system of the French high-speed train (TGV) is provided by elastomeric components, such as nitrile butadiene rubber (NBR) O-rings. Like most elastomers, these seals are sensitive to thermo-oxidative aging. In the current application, aging results in a strong hardening of the O-rings that alters the sealing function and impacts the lifetime of the mechanical braking system. This places severe demands on maintenance operations. Therefore, understanding the origin of this aging and studying its effects on the material properties is of significant importance to limit train maintenance operations.

In the present study, accelerated aging tests have been carried out with different NBR formulations at different temperatures. These tests allowed to reproduce ex-situ the hardening observed on the O-rings. Aging mechanisms were identified and related to the aging conditions of the NBR formulations through different characterization techniques: infrared spectroscopy, swelling tests, X-ray fluorescence spectrometry, differential scanning calorimetry, thermogravimetric analysis and micro-hardness.

## 1 INTRODUCTION

Nitrile butadiene rubber (NBR) is one of the main materials used for sealing applications with O-rings. This type of elastomer is generally employed when chemical resistance to fuels, oils and greases is required. Like most elastomers, NBRs are affected by prolonged exposition to air and heat. The material and its properties degrade over time, and this phenomenon is referred to as aging (Shelton 1957, Johlitz 2012).

In the current industrial use, NBR O-rings are found in the braking system of the French high-speed train (TGV). They are, as such, subjected during their lifetime to mechanical loads, to prolonged contact with compressed air and to atmospheric conditions. Maintenance operations occur at regular intervals to replace the elastomeric components, but recently more parts came before their due date and presented O-rings with fast and significant hardening, as well as permanent deformation (see Figure 1). This places severe demands on maintenance operations, because the O-rings have to be changed preventively for criticality reasons.

<figure><img src="image_2.png" /><figcaption>Figure 1. Cross section of an unaged (left) nitrile rubber O-ring and an aged O-ring after 9 years inside a train braking system (right).</figcaption></figure>

With the continuous increase on durability demands of elastomers in industrial applications, understanding aging and its effect on the material properties is of major importance in limiting preventive maintenance operations on the TGV.

Thermo-oxidative degradation and the associated aging mechanisms are complex phenomena, which have been the subject of various studies carried out on similar materials (Zhao et al. 2013, Pazur et al. 2014a, Qian et al. 2018). The oxidation scheme
proposed by Bolland and Gee (1946) is commonly considered to correctly describe the reaction mechanism. Due to the combined action of heat and oxygen, free radicals form along the macromolecular chains. Oxygen attaches itself to the chain, producing various oxygenated functionalities, such as hydroxyls and carbonyls. Two competing processes (scission and oxidative induced crosslinking) will then occur simultaneously, but often one becomes dominant, resulting in different mechanical characteristics: the degradation of the macromolecular network by chain scission causes a softening of the elastomer while the additional oxidative induced crosslinking cause a stiffening of the elastomer (Johlitz 2012).

In the present work, accelerated aging test are performed (Budrugeac 1995, Delor-Jestin et al. 2000) to reproduce the aging observed on used specimens, with the objective to compare O-rings aged on trains and thermally accelerated aged specimens. The experimental procedure is first presented, before studying the chemical and mechanical evolution of NBR during thermo-oxidative aging. The results provide us with a valuable insight in understanding the resulting behavior of the aged material and will help in laying the foundation of a selection methodology for O-rings used in the TGV's braking systems.

# 2 EXPERIMENTAL

## 2.1 Materials

Nitrile rubbers are butadiene ($C_4H_6$) and acrylonitrile ($C_2H_3CN$) copolymers. NBRs have a complex molecular structure (see Figure 2), because the butadiene monomer copolymerizes in three distinct configurations (*trans*, *cis* and 1,2-vinyl). Acrylonitrile is statistically inserted into the macromolecular chain and its content is adjusted depending on the target application (Pazur et al. 2014b).

Table 1 presents the eight types of industrial nitrile rubbers studied, from three different suppliers (A, B and C). They have been classified by their minimum operating temperature and as well as their original hardness (IRHD).

## 2.2 Sample types and testing procedure

Specimens were prepared from the same material batch by the suppliers into three different geometries: O-rings ($\emptyset d_1 = 10.00$ mm, $\emptyset d_2 = 2.50$ mm, NF ISO 3601-1), DRC type A compression disks (NF ISO 815-1) and sheets (150x150x2 mm) in order to perform the different tests.

Accelerated aging tests were performed at 85°C and 100°C in an air-circulating oven for 336h, 672h, 1176h and 2016h (2, 4, 7 and 12 weeks respectively). For each aging temperatures, three specimens were tested.

<figure><img src="image_3.png" /><figcaption>Figure 2. Chemical structure of NBR showing the main structural units in the chain backbone.</figcaption></figure>

<table><caption>Table 1. NBR considered in this study.</caption><thead><tr><th>Specimen code</th><th>Minimum temperature (°C)</th><th>Hardness (IRHD)</th><th>Symbol</th></tr></thead><tbody><tr><td>A1-4070</td><td>-40</td><td>70</td><td>▲</td></tr><tr><td>A2-4050</td><td>-40</td><td>50</td><td>▲</td></tr><tr><td>A3-2070</td><td>-20</td><td>70</td><td>×</td></tr><tr><td>A4-4050</td><td>-40</td><td>50</td><td>×</td></tr><tr><td>B1-4070</td><td>-40</td><td>70</td><td>■</td></tr><tr><td>B2-4050</td><td>-40</td><td>50</td><td>■</td></tr><tr><td>C1-4070</td><td>-40</td><td>70</td><td>♦</td></tr><tr><td>C2-4050</td><td>-40</td><td>50</td><td>♦</td></tr></tbody></table>

## 2.3 Micro-hardness

Micro-hardness tests were done according to the NF ISO 48-2 standard, on an automatic IRHD tester Compact III from Bareiss, equipped with a centering device for O-rings. For each specimen, at least five measures were taken.

## 2.4 Infrared spectroscopy (ATR-FTIR)

The infrared spectrometer used in the study was a Fourier Transform Infrared (FTIR) spectrometer from PerkinElmer (Spectrum Two), equipped with an Attenuated Total Reflectance (ATR) module and a germanium crystal. The measurements were taken in absorbance mode, with 16 scans at a resolution of 4 $cm^{-1}$ and a scan range from 4000 $cm^{-1}$ to 700 $cm^{-1}$

(Delor-Jestin et al. 2000, Pazur et al. 2014a, Qian et al. 2018). Reported spectra are an average of three measures on different samples.

## 2.5 Swelling tests

The aged O-rings were weighted ($m_i$) and placed in individual glass containers, before being immersed in toluene for 72h at room temperature, allowing the test specimens to expand under equilibrium conditions. After swelling, the samples were patted dry with paper towels to remove excess toluene on the surface, before being weighted to obtain the swollen weight ($m_g$). To estimate the molar mass between crosslinks, the modified Flory-Rehner equation (1943) was used:
$$M_c = \frac{\rho_r V_s (V_r^{\frac{1}{2}} - \frac{V_s}{2})}{\ln(1 - V_r) + V_r + \chi V_r^2} \quad (1)$$

with $V_r = \frac{m_r}{\frac{\rho_r}{\rho_g} \frac{m_g}{\rho_g}}$ the volume fraction of swollen elastomer in the solvent (toluene), $\rho_r$ the density of the rubber, $\rho_s$ the density of toluene ($\rho_s = 0.867 \text{ g.cm}^{-3}$), $V_s$ the solvent molar volume ($V_s = 106.27 \text{ cm}^3.\text{mol}^{-1}$ and $\chi$ the Flory-Huggins polymer-solvent interaction parameter. The latter is based on the Hildebrand solubility parameters ($\chi = \chi_{\beta} + \frac{V_s}{R^2}(\delta_s - \delta_r)^2$). 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 ($\chi_{\beta} = 0.34$), a value of $\chi = 0.393$ was used (Valentin et al. 2008, Marzocca et al. 2010). The reported values of $M_c$ are the average of three samples. Crosslinking density $\nu$ (mol.g$^{-1}$) can then be calculated using the following formula:

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

## 2.6 X-ray fluorescence spectrometry (XRF)

X-ray fluorescence was used for an elemental analysis of the different NBR specimens. The spectrometer used for this purpose was the S8 Tiger from Bruker.

## 2.7 Differential Scanning Calorimetry (DSC)

Glass transition temperature ($T_g$) was measured for the different specimens using differential scanning calorimetry (DSC) on a Q20 calorimeter from TA Instruments, according to the NF T 46-050 standard. The $T_g$ values were determined using the tangent method on the first record of energy variations with respect to a reference.

## 2.8 Thermogravimetric analysis (TGA)

Compositional analysis of the elastomer's main components (volatile substances, carbon black, mineral fillers) was monitored using the Q50 thermogravimetric analyser (TGA) from TA Instruments. Specimens were heated according to the NF T 46-047 standard and specimen composition was calculated by measuring the weight of each components as they volatilize or decompose under controlled conditions.

# 3 RESULTS AND DISCUSSION

## 3.1 Characterizing the NBR specimens before aging

First of all, XRF and TGA measures were performed before accelerated aging tests, in order to identify

major compositional differences in the different NBR specimens. Table 2 and Table 3 show the XRF and TGA results respectively, for each formulation.

From the XRF data, B specimens show a lower sulfur content than A and C specimens. This difference is explained by the crosslink network type, as B specimens are vulcanized with peroxides. A and C samples are thus assumed to be sulfur cured NBRs.

<figure><img src="image_4.png" /><figcaption>Figure 3. Hardness ratio changes in the specimens during aging.</figcaption></figure>

Moreover, B specimens are found to have a higher content in mineral products (Si, Al) than specimens A and C. This is both confirmed by the XRF and TGA measurements and can be explained by the use of a different type of reinforcing filler. In Table 3, P5 and P6 stages stand for the carbon black content and the mineral residue content, respectively. Specimens B exhibit a lower carbon black content but a higher mineral residue content than specimens A and C, showing the use of silica fillers along with carbon black fillers. The effect of reinforcing fillers relative to aging behavior will be investigated in a future study.

## 3.2 Hardness evolution between the different NBR samples during aging

Micro-hardness measures were done on the different specimens after aging for various durations (336h, 672h, 1176h and 2016h). The hardness ratio of the aged specimens over the original hardness is presented in Figure 3.

It was observed that this ratio increases with aging time and temperature, which was reported in previous studies (Delor-Jestin et al. 2000) for NBR. Also an increase in hardening rate is obtained for 100°C compared to 85°C.

Specimens A and C were found to resist less to thermal oxidation than specimen B, both at 100°C and at 85°C. This can be explained by the difference in vulcanizing agents, as specimens B are vulcanized with peroxides and specimens A and C with sulfur. Peroxide crosslinks lead to a better resistance to thermo-oxidative degradation, as well as a three-dimensional network with better mechanical properties
in compression (Kruželák et al. 2016). However, these improvements come at the expense of high temperature tensile strength and mechanical fatigue resistance.

### 3.3 Chemical evolution of the different NBR samples during aging

#### ATR-FTIR spectroscopy

Infrared spectroscopy (ATR-FTIR) is used to evaluate the chemical changes of the different NBR specimens after thermo-oxidative aging (Delor-Jestin et al. 2000, Kawashima and Ogawa 2005, Pazur et al. 2014b). From these different studies, three spectrum domains were examined, which correspond to characteristic absorption bands:

* in the region of hydroxyl stretching vibrations (3600-3200 cm⁻¹), a broad absorption band 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.

* in 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 maximas at 1660 and 1580 cm⁻¹.

* in the region of bending vibrations (1000-700 cm⁻¹), out-of-plane vibrational bonds of C-H adjacent to unsaturations are observed (trans-1,4-polybutadiene at 966 cm⁻¹ and 1,2-polybutadiene at 918 cm⁻¹). The intensity of these bands decrease with time, illustrating a decrease in double bonds.

The absorption spectra of two different samples (A3-2070 and B1-4070) at 100°C are presented in Figures 4 and 5 to illustrate this behavior. As such, hydroxyl and carbonyl regions absorption intensity increase with time, while the trans-1,4-butadiene peak decreases, as described from the previous studies. Figure 4 also displays the decrease of another peak at 1540 cm⁻¹, which can be associated to a decrease in

<figure><img src="image_5.png" /><figcaption>Figure 4. A3-2070 specimen IR spectra after aging.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 5. B1-4070 specimen IR spectra after aging.</figcaption></figure>

formulation additives, such as antioxidants (Delor-Jestin et al. 2000). As was noted in 3.1, B specimens use silica fillers, which can be observed in Figure 5 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.

<table><caption>Table 2. Elemental chemical composition of the different NBR samples using XRF.</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><td>CHN</td><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><td>S</td><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><td>Zn</td><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><td>Si</td><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><td>Al</td><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>

<table><caption>Table 3. Mean content of the different levels of the NBR samples from TGA measurements.</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><td>P1</td><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><td>P2</td><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><td>P3</td><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><td>P5</td><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><td>P6</td><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>
A comparison is made from O-rings specimens aged for several years on trains (see Figure 6). First of all, regarding the physical aspect, the specimen aged for 7.5 years retains its circular shape, with no significant increase in hardness. On the other hand, the 10 year old specimen present the characteristic permanent deformation of O-rings compressed in a groove. The flat part (b) that creates the seal remains quite elastic, as its hardness only increased from 70 IRHD to 77 IRHD. Contrasting, the round shape (a) shows a notable increase in hardness, going from 70 IRDH to 87 IRHD. During maintenance testing operations, the O-ring failed in sealing properly, creating a leak in the system it equipped.

The spectra of these aged O-rings used on trains, presented in Figures 7 to 9, are similar to the accelerated aging test ones. We can also observe an increase in the hydroxyl and carbonyl regions, showing the formation of oxygenated functionalities during aging. Also, a few differences can be spotted between the specimen aged for 7.5 years and the 10 years old specimen. From Figure 7, we can see that the oxygenated functionalities are less present, while the peak at $1538 \text{ cm}^{-1}$ (formulation additives) 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, the 10 year old specimen has a significant increase in the hydroxyl and carbonyl peaks on side (a), compared to side (b). The latter still showed a peak around $1540 \text{ cm}^{-1}$, meaning that additives or unsaturations are still present in the less hardened part. Pressure and oxygen flow have, as such, an impact on how the material ages (Budrugeac 1995), since side (a) is exposed to higher pressures and thus higher oxygen flow than side (b).

This behavior can be explained by the diffusion-limited oxidation (DLO) effect and responsible for the heterogeneous aging observed in the use conditions (Celina et al. 1998, Gillen et al. 2005, Celina 2013). As was indicated by these authors, 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. The accelerated aging method relies on Arrhenius extrapolation, which do not always reproduce correctly the degradation processes and do not take account of mechanistic solicitations (Garnier et al. 2013) and non-linear temperature dependencies. All these factors will have to be considered in order to better predict the lifetime of the NBRs used in the industrial application.

### Crosslinking density

The swelling tests performed were used to determine the crosslink density according to the formulas presented in Section 2.

From Figure 10, the increase in hardness is correlated to the increase in crosslink density with aging time. This clearly illustrates the formation of extra crosslinks during thermo-oxidative aging.

Also, specimen B swells less than specimens A and C. 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.

## 3.4 Macromolecular mobility

DSC measurements were performed in order to observe the variations in $T_g$ during the different stages of aging. Indeed, the thermo-oxidation of the macro-molecular network of the elastomer influences the transient behavior of NBR. Figure 11 shows the changes in the ratio of $T_g$ measured on aged samples over the original $T_g$, for the 70 IRHD specimens at 100°C.

<figure><img src="image_7.png" /><figcaption>Figure 6. Aged specimens after 7.5 years (left) and 10 years (right).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Figure 7. 7.5 year old specimen infrared spectra.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Figure 8. 10 year old specimen infrared spectra, side (a).</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 9. 10 year old specimen infrared spectra, side (b).</figcaption></figure>

From Figure 11, it can be seen that the T<sub>g</sub>/T<sub>g0</sub> ratio decreases (T<sub>g</sub> increases) for aged samples. This increase in T<sub>g</sub> is related to the modification of the mobility of the macromolecular chains (Qian et al. 2018). As new crosslinks are formed from free radicals during thermo-oxidative aging, macromolecular

<figure><img src="image_11.png" /><figcaption>Figure 10. Hardness ratio changes and crosslinks density in the specimens during aging.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Figure 11. Hardness and T<sub>g</sub> ratio changes in the specimens during aging.</figcaption></figure>

chains mobility is restricted, resulting in an increase in T<sub>g</sub>. These measures help to confirm that the crosslink density increases.

# 4 CONCLUSION AND PERSPECTIVES

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

A decrease in mechanical properties (increase in hardness) was correlated with the formation of oxidative functionalities and the consumption of unsaturations in the butadiene part of the copolymer. 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. Thus the viscoelastic properties of the different NBR specimens change during the accelerated aging tests.

Furthermore, DLO and mechanical loadings could have possible additional effects in the aging process of the O-rings and have to be taken into account when predicting lifetimes.

# ACKNOWLEDGMENTS

The authors acknowledge SNCF Voyageurs (Rennes, France) for their financial support.

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