Influence of Ageing on Sealability of Elastomeric O‐Rings

Abstract

Summary At BAM, which is the federal institute for materials research and testing in Germany, it is one of our tasks to evaluate the safety of casks designed for transport and/or storage of radioactive material. This includes the assessment of the service lifetime of elastomeric seals that are part of the container lid system with regard to the requirements for long‐term safety (40 years and more) of the containers. Therefore, we started an accelerated ageing programme with selected rubbers often used for seals (HNBR, EPDM and FKM) which are aged at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) up to 1.5 years. In order to assess sealability, O‐rings are aged in compression by 25% (corresponding to the compression during service) between plates as well as in flanges that allow leakage rate measurements. For comparison, uncompressed O‐rings are aged as well. Further methods characterising seal performance are compression stress relaxation (CSR) reflecting the loss of sealing force of a compressed seal over time, and compression set (CS) which represents the recovery behaviour of a seal after release from compression. Additionally, hardness is measured for information about the change of mechanical properties. The experimental results indicate that while hardness, CSR and CS show considerable degradation effects, the leakage rate stays relatively constant or even decreases until shrinkage combined with the loss of resilience of the aged seal leads to leakage. This demonstrates that static leakage rate, which is the only available direct seal performance criterion, has only limited sensitivity towards the degradation of the seal material. CS data is extrapolated using time‐temperature shifts and Arrhenius graphs. An exemplary CS of 50% would be reached after approx. 1.2, 17 and 29 years at 60 °C for HNBR, EPDM and FKM respectively.

Erroneous or Arrhenius: A Degradation Rate-Based Model for EPDM during Homogeneous Ageing

“…Moreover, DLO of EPDM-sheet samples aged at 125 • C was detected for an ageing time of 210 d, which is not the case for our material. In our previous investigation [34], no DLO effects were detected for EPDM O-rings aged at 125 • C for 549 d, although our material is thicker, which normally enhances the appearance of DLO effects.…”

“…This value is very close to the value obtained from the experimental results with the TTS ( 91 kJ/mol, see Figure 9). 101 days (which corresponds to Rc = 0.06 from the experimental results, see Figure 1); as for these samples, no heterogeneous hardening was observed [16,34]. The ageing times at 50% force drop (Rc = 0.5) are determined from the simulation results given in Figure 8.…”

“…The EPDM compound was peroxide-cured and contained 90 phr carbon black fillers, and no plasticizer. [16,18,34]. The isothermal experimental results are presented as the normalized continuous stress relaxation R c which is defined as the ratio of the continuously measured force to the initial force for compression.…”

Scission, Cross-Linking, and Physical Relaxation during Thermal Degradation of Elastomers

“…This sample preparation yielded a very uniform geometry. Using the sheet material had the additional advantage of excluding the slight DLO effects that were detected previously in HNBR O-rings aged at 125 °C [21,27]. In order to determine possible deviations between the two sample configurations, comparative tests were made.…”

“…However, the presence of DLO effects slows the ageing rate and can distort the data for the lifetime prediction analysis. Therefore, the present study has been performed on DLO-free samples as assessed in our previous publications [21,27]. Another encountered challenge during data analysis is to separate physical and chemical relaxation processes.…”