Evolution of compression set (CS) over time of EPDM aged for 10 d at 125 °C and its corresponding fit. The broken line represents the CS value after tempering and is the same as the one after one year at room temperature.
Source: BAM FB 3.4
BAM Division 3.4 Safety of Storage Containers performs safety evaluations of containers for interim storage and final disposal of radioactive materials as applicable to license procedures under nuclear legislation. Elastomer seals are used in these containers for operating times of several decades. Therefore, aging behaviour and lifetime prediction of these components is a relevant research topic for division 3.4 which has been investigated for several years now. This resulted in extensive knowledge about the aging processes of different elastomer materials and how the lifetime of O-ring seals can be predicted. The present paper focuses on the analysis of the elastic recovery behaviour of EPDM seals during aging.
The paper comprises parts of the results from the extensive aging investigations. The experiments were not performed on materials which are actually in use in containers for radioactive material, but on commercial standard materials of the respective material classes. O-rings or segments made of ethylene propylene diene rubber (EPDM) were aged up to 5 years at 75 °C, 100 °C, 125 °C and 150 °C. The elastic recovery was characterized mainly using the compression set method. Compression set is a measure for the permanent deformation of a rubber sample after aging in the compressed state. After compressing the seal segment by 25 %, it was stored for a certain time at elevated temperature to induce artificial aging. After aging and cooling of the sample, it is demounted from compression and starts to recover due to the elastic properties of the rubber material. According to the respective standards, the recovered height is measured 30 min after release from compression. A compression set (CS) of 0 % corresponds to a full recovery back to the initial height prior to compression, while a CS of 100 % corresponds to no recovery at all and thus indicates strong degradation. However, the recovery is time-dependent and continues after the measurement 30 min after release from compression. For this reason, CS was measured after additional times up to one year. To accelerate the recovery, samples were also tempered for one day at 100 °C. CS after tempering was approximately the same as CS after one year recovery at room temperature (see Figure). It is assumed that CS after tempering corresponds to the equilibrium CS which is influenced only by chemical changes (and not reversible or time-dependent physical processes). Thus, it is a good measure for seal degradation. Additionally, CS correlates well with sealing force and is therefore a better indicator for tightness and lifetime of the seal than other, e.g. mechanical properties. For this reason, a time-temperature superposition is performed with CS data in which data measured at different temperatures are shifted along the logarithmic time axis until the data align as a single master curve at the chosen reference temperature. Thus, data measured at higher temperatures can be extrapolated to lower temperatures. Using a suitable end-of-lifetime criterion, which was determined as 80 % CS in former studies, the lifetime can be predicted for the investigated temperatures.
Understanding the recovery behaviour and the degradative processes of EPDM during ageing
Maha Zaghdoudi, Anja Kömmling, Matthias Jaunich, Dietmar Wolff
published in Polymer Testing, Volume 121, First Page 107987
BAM Department Containment Systems for Dangerous Goods; Energy Storage
BAM Division Safety of Storage Containers