Accelerated thermal ageing studies on nitrile rubber O-rings

doi:10.1016/S0142-9418(02)00171-X

Polymer Testing

Volume 22, Issue 6, September 2003, Pages 651-656

https://doi.org/10.1016/S0142-9418(02)00171-X Get rights and content

Abstract

Poly(acrylonitrile-butadiene) rubber (NBR) O-rings have been aged in air whilst under 12.5% compression at temperatures up to 110 °C. The compression set properties together with oxygen uptake of the O-rings under the applied load conditions were measured and results analysed using time-temperature superposition and Arrhenius kinetics. The compression set results suggest a single degradation process with an activation energy of 81.03 kJmol⁻¹. This compares well with the activation energy of 74.3 kJmol⁻¹ obtained from oxygen consumption. Results from solvent swell and oxygen uptake tests show that the main influence on ageing of the rubber is oxidative cross-linking, leading to the material becoming hard and brittle. The approach presented here predicts that the rubber will reach full compression set by 40 (+54/−7) years based on 95% confidence bands.

Introduction

NBR O-rings have been used at AWE as compression seals on a variety of vessels over the past few years. The aim of the work presented here was to determine the useful life of these O-rings.

Work conducted by RAPRA has shown that NBR compressed to 25% for forty years, lost approximately 80% of its sealing capacity when aged in temperate conditions [1]. A further study based on an accelerated ageing programme indicated that the same materials would lose approximately 16% of their sealing force over the same time period, indicating a longer service life than that found in practice [2]. RAPRA treated the compression set results with a dose rate equation rather than the traditional Arrhenius method because of insufficient data points [2]. Work by Wise [3] et al showed that oxygen consumption experiments on NBR was a sensitive technique for determining the activation energy of the oxidative degradation of the polymer. They showed the activation energy for oxygen consumption of a nitrile rubber was approximately 83.7 kJmol⁻¹, whilst that determined from tensile elongation and surface modulus was 92 kJmol⁻¹. It was believed that the correlation was sufficiently good to conclude that the main degradative process involved is that of the oxidative degradation of the NBR. This is of significance for O-rings used to seal vessels for long time periods from the atmosphere. It was recognised that ageing in air would lead to compression set and as the material is used under load a performance criterion could be defined. In service at AWE, a number of the O-rings are compressed by 12.5%, therefore the criterion should be that the compression set on the O-ring should not equal or exceed the 12.5% compressive strain. If it does then the O-rings will no longer be under load and will therefore be less likely to maintain a seal.

The methodology of Wise et al [3] together with time-temperature superposition and the Arrhenius relationship was followed in the present work to compare the activation energy for compression set with that for oxygen consumption [2], [4]. Solvent swell was carried out to determine whether there was a link between compression set and cross-link density.

Section snippets

Material

The O-rings of interest were prepared to BS2751 BA70. RAPRA Technology Ltd were commissioned to formulate an NBR compound to BS2751 BA 70:1990 standard using a sulphur cure system (see Table 1). The rubber was cured for 15 mins at 140 °C at a pressure of 40 PSI. Both O-rings (size BS1806:1989 238) and sheets (thickness 3.5 and 1 mm) were prepared. Compression jigs and reaction vessels were manufactured to specification by Instrument Technology Ltd (UK). The drawing in Fig. 1 represents a 100 mm

Compression set

The compression set versus time isotherms are shown in Fig. 2. The data was assessed using time-temperature superposition and the Arrhenius relationship [7], [8]. Each isotherm was shifted in time (a_T) until the data superimposed on to the 55 °C reference isotherm. The resulting master curve is shown in Fig. 3. The Log of the shift factors derived from time-temperature superposition were plotted against 1/T, (T is the isotherm temperature), as shown in Fig. 4. The activation energy for the

Conclusions

1.
The results indicate the predominant reaction contributing to compression set in these studies is oxidative degradation. The Arrhenius activation energy for compression set (81 kJmol⁻¹) was similar to that determined from oxygen consumption experiments (74.3–74.6 kJmol₋¹).
2.
Solvent swell has shown a correlation between compression set and cross-link density.
3.
The oxidative process leading to compression set is therefore likely to be oxidative cross linking.
4.
Utilising time-temperature superposition

Acknowledgements

The authors are grateful to Guy Rae (AWE) for the mass spectral analysis. Mark Soames (AWE) for the design of jigs and vessels and Anthony Swain (AWE) for technical review and comments.

This document is of United Kingdom origin and contains proprietary information which is the property of the Secretary of State for Defence. It is furnished in confidence and may not be copied, used or disclosed in whole or in part without the prior written consent of the Director of Contracts (SSM), Ministry of

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There are more references available in the full text version of this article.

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Material BehaviourAccelerated thermal ageing studies on nitrile rubber O-rings

Abstract

Introduction

Section snippets

Material

Compression set

Conclusions

Acknowledgements

Polymer Degradation and Stability

POLYMER

Polymer Degradation and Stability

Natural Ageing of Rubber

Material Behaviour
Accelerated thermal ageing studies on nitrile rubber O-rings