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13. October 2020

Overview of ongoing long-term ageing investigations on elastomer seals

Elastomer O-rings can remain leak-tight under purely static conditions even when compression stress relaxation, compression set and mechanical properties already indicate far advanced degradation. For this reason, a modified leakage test involving a small and rapid partial decompression of the seal was developed that enables determining an end-of-lifetime criterion for O-rings with a safety margin for thermal shrinkage and vibrations.

M. Zaghdoudi, A. Kömmling, M. Jaunich, D. Wolff

One of tasks of the Division 3.4 “Safety of Storage Containers” at Bundesanstalt für Materialforschung und -prüfung (BAM), is to evaluate the safety of containers designed for disposal of low and intermediate radioactive waste. As such containers might be transported before and stored until disposal, safe enclosure of the radioactive inventory is important for this time span.

Elastomers are widely used as sealing materials due to their excellent elastic properties and easy applicability. However, elastomers can age under the separate or combined influences of temperature, oxygen, radiation and mechanical stress. This can ultimately lead to leakage and thus to seal failure. Additionally, when a seal exchange is not easily possible, as e.g. in containers for radioactive waste, it is important to predict the seal lifetime reliably and to ensure that no safety-relevant failure occurs in the estimated service period. This includes transport and storage (up to several decades) until disposal of the radioactive inventory.

Working principle of the modified apparatus for leakage rate measurement after partial releaseWorking principle of the modified apparatus for leakage rate measurement after partial release

For this purpose, an ageing programme with hydrogenated nitrile butadiene rubber (HNBR), ethylene-propylene-diene rubber (EPDM) and fluorocarbon rubber (FKM) was started with current ageing times of up to 3.7 years. Degradation and the change of mechanical properties (e.g. hardness, strain at break) were studied over several years at four different ageing temperatures (75 °C, 100 °C, 125 °C and 150 °C) as well as the change of sealing properties.

Continuous and intermittent compression stress relaxation (CSR) measurements were performed in order to investigate the respective contribution of crosslinking and chain scission to the observed degradation effects. Thus, the degradation kinetics and mechanisms could be resolved more clearly. For assessing the seal performance, compression set (CS) and leakage rate measurements were conducted.

Uncompressed and compressed O‑rings as well as sheet material were aged. Seal-related properties such as compression stress relaxation (CSR), compression set (CS) and leakage rate were determined on O‑ring samples. The sheet material was used to investigate material properties for which the O-ring geometry is not suitable because of diffusion-limited oxidation (DLO) effects, especially for analysing microstructural changes that happen during thermooxidative ageing (i.e. scission and crosslinking) and also by estimating the lifetime prediction. For determining an end-of-lifetime criterion, both for static and dynamic (with a partial seal decompression) conditions, leakage tests of the seals were conducted by using the pressure rise method.

In this contribution the authors give an overview of ongoing ageing investigations of EPDM, FKM and HNBR seals. Already published results are summarised and extended by some new results for different methods. To achieve a better understanding of the ageing state and therefore perform more realistic lifetime estimations, the combination of different methods is mandatory.

The experimental results showed that the O-rings remained leak-tight under purely static conditions even when CSR, CS and mechanical properties already indicated far advanced degradation. For this reason, a modified leakage test involving a small and rapid partial decompression of the seal was developed that enabled determining an end-of-lifetime criterion for O-rings with a safety margin for thermal shrinkage and vibrations.

Citation:
M. Zaghdoudi, A. Kömmling, M. Jaunich, D. Wolff, RFP Rubber Fibres Plastics, 03 2020, 146-151.

https://www.gupta-verlag.com/magazines/rfp-rubber-fibres-plastics-international/03-2020

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