In some applications, elastomer seals have to remain leak tight over extended time periods (up to several decades) as an exchange of the seals is not practical. Therefore, it is necessary to be able to predict the lifetime of such seals reliably. For this reason, ageing experiments with O-rings and sheets based on hydrogenated acrylonitrile butadiene rubber (HNBR), ethylene propylene diene rubber (EPDM) and fluorocarbon rubber (FKM) were performed over up to two years at different temperatures. For investigating the changes of material properties during ageing, measurements of i.a. hardness and tensile behaviour as well as dynamic-mechanical and thermogravimetric analyses were conducted. For assessing the relaxation and recovery behaviour of the ageing seals, compression stress relaxation and compression set experiments were performed. Furthermore, leakage rate was measured to detect seal failure. By examining the aged samples, both the property changes and the responsible degradation mechanisms were characterised. Additionally, the most suitable approach for lifetime predictions should be determined using the large database of results from different test methods performed at several ageing times and temperatures. This included identifying an end-of-lifetime criterion that correlates with leakage and thus seal failure. Regarding the property changes and degradation mechanisms of HNBR, a pronounced embrittlement was observed due to dominant crosslinking reactions during ageing. These also resulted in a decrease of oxygen permeability, leading to a reduction of the oxygen transport into the interior sample (mainly of the thicker O-rings). This resulted in diffusion-limited oxidation effects (DLO effects), meaning that the interior of the sample aged less strongly than regions close to the surface. During ageing of EPDM, both chain scission and crosslinking occurred, which became noticeable in deteriorated tensile properties and decreased compression force as well as an increase of compression set. For strongly aged samples, crosslinking dominated as well and also led to pronounced DLO effects. Due to its excellent heat and oxidative stability, FKM exhibited comparatively minor degradation effects. Only compression stress relaxation and compression set showed notable changes. Measurements of static leakage rate revealed that O-rings became untight only after advanced degradation and that no gradual increase of leakage rate could be observed before failure. For this reason, additional leakage measurements were performed using a modified experiment that enabled a small decompression of the seal during the measurement. As this test requires a dynamic recovery of the seal and is thus more demanding, the tested EPDM O-ring failed earlier compared to the purely static test. The corresponding ageing state was defined as the point of failure and correlated to data from other test methods, corresponding e.g. to a compression set of 80 %. Based on this criterion, the lifetimes could be determined using the time-temperature superposition approach. During this work, a number of findings regarding the best way to conduct accelerated ageing experiments and lifetime predictions of O-rings, and how to avoid possible error sources, were collected and are discussed in detail.
Alterung und Lebensdauervorhersage von O-Ring-Dichtungen
Aging and lifetime prediction of O-ring seals
2017
Sonstige
Elektronische Ressource
Deutsch
aging , Verpressung , rubber , Elastomer , Degradation , Gummi , leakage , lifetime prediction , elastomer , Leckage
DDC: | 629 |
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