Effects of heterogeneous aging in compressed HNBR and EPDM O-ring seals

Researchers led by Dietmar Wolff at Bundesanstalt für Materialforschung und –prüfung (BAM) in Germany studied the development and consequences of diffusion-limited oxidation DLO effects in compressed O-rings presented for two commercial seal materials as well as the limits in which extrapolation of accelerated aging data yielded useful results. The study is published in Polymer Degradation and Stability.

To conduct the study the authors used ethylene-propylene-diene rubber EPDM and hydrogenated nitrile rubber HNBR. The examined O-rings made of commercial elastomers have a cord diameter of 10 mm and inner diameter of 190 mm. Both compressed (25 %) and uncompressed O-rings were oven-aged at four different temperatures (75 ^°C, 100 ^°C, 125 ^°C and 150 ^°C) and examined after 1, 3, 10, 31, 101 and 183 days.

Hardness was measured based on DIN EN ISO 868 using type D hardness on O-ring surface and international rubber hardness degrees IRHD micro (based on DIN ISO48) on five positions on the O-ring cross-section using a linear table. Type D hardness was used for hardness measurement on the surface while the finer tip of the IRHD microprobe was used for finer resolution over the cross-section.

The results showed that surface hardness of HNBR and EPDM increased with aging time and temperature which could be attributed to cross-links reactions, higher polarity due to oxygen incorporation and plasticizer loss. Uncompressed samples showed stronger hardness increase compared to compressed samples, but only at temperatures of 125 ^°C and higher for HNBR and at 150 ^°C for EPDM. This effect could also be observed visually on the cross-section of uncompressed and compressed HNBR seals after 101 days at temperature of 150 ^°C.

IRHD micro hardness measurements showed that diffusion-limited oxidation effects occurred in HNBR already after 10 days of aging at 125 ^°C and in EPDM after 101 days at 150 ^°C. Further tests indicated that accelerated aging temperatures of 100 ^°C for HNBR and 125 ^°C for EPDM should not be exceeded for O-rings of 10 mm cord thickness in order to measurenon-DLO-affected bulk properties.

After 10 days of aging at 150 ^°C, permeability of EPDM stayed practically constant while permeability of HNBR decreased by 20%. Decrease in permeability of HNBR can be related to crosslinks as the predominant aging mechanism.

When comparing compression stress relaxation and recovery results at 100 ^°C, EPDM exhibited better performance due to its higher force retention and recovery. At 150 ^°C, EPDM exhibited better performance only for  30 days of aging but at longer aging times, sealing force of EPDM continued to decrease while that of HNBR levels up after 20 days and becomes higher compared to EPDM. Both recovery and compression stress relaxation results at 150 ^°C indicate that the degradation of HNBR slows after about 30 days which can be correlated with DLO effects occurring in HNBR seals.

Lifetime predictions using compression set data were possible using the time-temperature superposition approach. However, DLO-affected data points have to be discarded.

This study showed that DLO effects occurring during aging can lead to distorted bulk properties, such as compression stress relaxation and compression set, which are related to the seal performance. Aging temperature and oxygen permeability of the material were shown to influence the extent of DLO effects, which were more pronounced in HNBR. DLO-affected samples have to be detected and the data discarded for extrapolations to lower temperatures.

Comment from the authors:

The described investigations are part of an ongoing aging project which also includes Fluorocarbon rubber FKM and is designed for a total aging time of 5 years. In this program, the seal performance in terms of leakage rate is also measured, which allows comparing changes in material properties with component performance. A discussion of the most suitable method for lifetime predictions of O-ring seals, including the selection of an appropriate end-of-lifetime criterion, is presented in a follow-up paper that was submitted to Polymer Degradation and Stability in December 2016.