Determining a suitable and reliable end-of-lifetime criterion for O-ring seals is an important issue for long-term seal applications. Therefore, seal failure of ethylene propylene diene rubber (EPDM) and hydrogenated nitrile butadiene rubber (HNBR) O-rings aged in the compressed state at 125 °C and at 150 °C for up to 1.5 years was analyzed and investigated under static conditions, using both non-lubricated and lubricated seals. Changes of the material properties were analyzed with dynamic-mechanical analysis and permeability experiments. Indenter modulus measurements were used to investigate DLO effects. It became clear that O-rings can remain leak-tight under static conditions even when material properties have already degraded considerably, especially when adhesion effects are encountered. As a feasible and reliable end-of-lifetime criterion for O-ring seals under static conditions should include a safety margin for slight dimensional changes, a modified leakage test involving a small and rapid partial decompression of the seal was introduced that enabled determining a more realistic but still conservative end-of-lifetime criterion for an EPDM seal.
The
self-diffusion of neat water, dimethyl sulfoxide (DMSO), octanol,
and the molecular components in a water–DMSO solution was measured
by 1H and 2H NMR diffusion experiments for those
fluids imbibed into controlled pore glasses (CPG). Their highly interconnected
structure is scaled by pore size and shows invariant pore topology
independent of the size. The nominal pore diameter of the explored
CPGs varied from 7.5 to 72.9 nm. Hence, the about micrometer mean-square
diffusional displacement during the explored diffusion times was much
larger than the individual pore size, and the experiment yielded the
average diffusion coefficient. Great care was taken to establish the
actual pore volumes of the CPGs. Transverse relaxation experiments
processed by inverse Laplace transformation were performed to verify
that the liquids explored filled exactly the available pore volume.
Relative to the respective diffusion coefficients obtained in bulk
phases, we observe a reduction in the diffusion coefficient that is
independent of pore size for the larger pores and becomes stronger
toward the smaller pores. Geometric tortuosity governs the behavior
at larger pore sizes, while the interaction with pore walls becomes
the dominant factor at our smallest pore diameter. Deviation from
the trends predicted by the Renkin equation indicates that the interaction
with the pore wall is not a just simple steric one but is in part
dependent on the specific features of the molecules explored here.
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.
We have evaluated the potential of nuclear magnetic resonance (NMR) spectroscopy based on small portable magnets for in situ assessment of moisture protective properties of wood coatings. Low field 1 H NMR with a unilateral permanent magnet was used to monitor and map the local moisture content (MC) of wood specimens uncoated or coated with various types of commercial paint systems. The MC beneath a coating layer was measured with a penetration depth up to 5 mm and with a depth resolution of 0.2 mm. The method is quick, noninvasive, simple to perform, and does not require removing wooden parts from the structure.
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