Permanent deformation of EPDM O-rings with initially circular cross-section after one year of ageing in compression at the given temperature

Permanent deformation of EPDM O-rings with initially circular cross-section after one year of ageing in compression at the given temperature

Source: BAM, Division Safety of Storage Containers

BAM's Division Safety of Storage Containers is performing 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, the ageing behaviour and lifetime prediction of these components is a relevant research topic for division 3.4 which is investigated for several years now. In this process in depth knowledge of the aging process of different elastomer materials and ways of lifetime prediction for O-Ring seals was gained. In the presented paper parts of the overall results were published. The paper documents also the cooperation with the KTH Royal Institute of Technology in Stockholm.

The investigations were not performed on materials which are currently in use in containers but commercial standard materials of the respective material classes were selected for this study. The materials were aged at four different ageing temperatures for durations of up to 1,5 a. The aim is to detect changes in the material properties by different methods and in sealing performance.

The results show that under static conditions seals remain functional despite strong changes of their material properties which resulted e.g. in a high hardness increase of the initially rubber elastic material. The spatial resolved testing of hardness with the indenter modulus enables the detection of diffusion limited diffusion effects which is an indicator for inhomogeneous ageing. A direct correlation between hardness of the material and the performance of a seal remains difficult.

Therefore, besides measurements of material properties also parameters closer related to the sealing performance were tested. An example for those parameters is the compression set value which is a measure for the potential recovery of a compressed seal after ageing under compression. Due to the correlation with sealability and it`s rather easy measurability Compression Set is often used for lifetime prediction. At the end of the paper known extrapolation methods are applied to estimate the expected lifetimes for a rather conservatively selected criterium of 50 % Compression Set at 60 °C. The values lie, depending on the material, between one and 29 years.

Influence of Ageing on Sealability of HNBR, EPDM and FKM O-rings
Anja Kömmling, Matthias Jaunich, Dietmar Wolff
Macromolecular Symposia, Special Issue: polymertec16 June 2017, Volume 373, Issue 1
BAM Department Containment systems for dangerous goods, Division Safety of Storage Containers