Sealing
Table 2 . Energy values for different end-of-life criterions
End-life criterion (%)
E a
( kJ / mol ) for CS
A B C D 30 123 85 47 79 50 170 104 62 117 80 214 120 75 151
out using the Arrhenius approach , using the end-of-life criteria at 30 , 50 and 80 %. Figure 5a shows an example of processing this data by Cetim using the ‘ power law ’ model , the use of which is necessary for extrapolation to the long term , particularly for an end-of-life criterion of 50 and 80 % for CS . Figure 5b illustrates data processing according to Arrhenius to calculate the activation energy . We note the linearity of the data . Table 2 gives the activation energy values obtained for the different members ( A , B , C , D ) for each of the end-of-life criterion values selected . A quasi-linear increase of the criterion is highlighted and a variability of the results according to the members for the same material . This observation can be explained by the ageing condition , which depends on the oven used , notably air renewal and air-circulating speed in the heating equipment .
Compression stress relaxation The analysis continued with a study of the change of CSR and the determination of the associated activation energy for a criterion of 50 %. The series of tests was performed by 3 members of the WG and only one case could be treated in its entirety with an SC-329 O-ring ( Figure 6a ). The activation energy determined for CSR was 38 kJ / mol for the test duration studied ( Figure 6b ). If we compare the activation energy between CS and CSR with the same end-of-life criterion of 50 %, a difference was observed : E a
( CS ) = 117 kJ / mol vs E a
( CSR ) = 38 kJ / mol . This difference seems to be explained by the impact of ageing conditions and probably the size of the samples . The CSR apparatus developed by Cetim simulates a normal sealing system , which limits the amount of oxygen in contact with rubber in comparison with the CS apparatus .
Conclusions
The study carried out by the working group explored the accelerated ageing of an elastomer material and highlighted the impact of environmental conditions . The results were processed for the two properties ( CS and CSR ) to determine the activation energy using the Arrhenius approach . An assessment of the end-of-life criterion was explored , highlighting the impact and the risks of extrapolation to predict lifetime . Differences were found between the members and the properties , prompting further reflection . The division continues to study the properties using the time-temperature superposition approach with Arrhenius and the possibility to implement into an FEA code to describe the permanent set observed . The differences highlighted encourage us to continue our discussions about the ageing of elastomers ( impact of thermooxidation , linearity of E a with 1 / T , the functional criterion of tightness , etc .).
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