AGING AND SURVEILLANCE OF VITON GLT O-RINGS IN MODEL 9975 SHIPPING PACKAGES Page: 4 of 6
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SRNL-STI-2009-00420
unknown. Using 90% loss as a more conservative failure criterion, slightly reduced lifetimes are
estimated (300,000 hrs or 34 years). The statistical basis for the model is limited due to the
number of O-rings currently in test.
The "master" TTS curve in Figure 4 can be time-shifted to other temperatures. From Figure 5,
the shift factors for several temperatures ranging from 70 F to 250 F were estimated and used to
translate the time to CSR failure values at 175 F to the selected temperatures. These values are
plotted in Figure 6 to provide a seal lifetime model with two curves representing "low" and
"high" lifetime estimates based on the range of time to zero sealing force values at 175 F. For
realistic seal temperatures (<175 F), the model predicts seal lifetimes of several decades.
The model predicts a seal lifetime of approximately 3200 years at 25 C for the Viton GLT O-
rings, based on near-zero sealing force. At 90% loss, the model predicts a lifetime of
approximately 2200 years. In comparison, Reference 4 estimates a 2000 year service life (90%
loss) for EPDM O-rings at 25 C, accounting for oxygen consumption rates [5]. Assuming the
activation energy does not go below a certain level, Reference 4 estimates a respectable seal
lifetime of at least 150-200 years for EPDM at room temperature. Since Viton fluoroelastomer
is generally more resistant to aging and thermooxidation than EPDM and is rated for higher
temperatures, longer service life at comparable temperatures is expected. Using similar logic
above for the Viton GLT O-rings, a lifetime of 200-300 years at 25 C may be possible. Oxygen
consumption analysis experiments are planned to evaluate this aspect.
At the other extreme, recent leak tests of PCV fixtures aged at 400 and 450 F have shown leak
failures within 8-12 days at 450 F and within 27-45 days at 400 F. For seal lifetime defined as
time to near-zero sealing force, the current CSR model predicts a seal lifetime of -47 days at 400
F and -18 days at 450 F. For seal lifetime defined as 90% loss, the model predicts ~ 15 days at
450 F and -39 days at 400 F. Note that 400 F is the upper "continuous" service temperature
rating of the O-rings. Package designers and other end-users must recognize the limitations of
such ratings for long-term service.
O-ring life prediction is complex, as seal performance depends on many factors. The TTS curve
and life prediction model are heavily dependent on the shift factors (aT). The current model
assumes constant exposure at a given temperature. In reality, seal temperatures vary with
ambient condition and payload, though such changes do not occur rapidly in the packages due to
presence of thermal insulation and thermal mass of the facility.
The relationship between sealing force and leakage must be better understood. Sealing force is
certainly important. However, even in a highly relaxed state, the O-ring could remain leaktight if
undisturbed. Alternatively, leak rates could increase at higher sealing force values with variation
in O-ring dimensions, component fabrication or assembly methods. Investigators in Reference 7
indicate that a threshold sealing force (1 N/cm) may be needed to maintain leaktightness, though
this is not presented as an absolute value. For the 9975 PCV seals, this represents <5% of initial
sealing force.
Bonding to mating surfaces has been observed at temperatures > 300 F. Elastomer-to-metal
bonding may aid or maintain sealing at very low sealing force values. Bonding is more likely at
higher aging temperatures due to thermal expansion and faster degradation, but bond integrity
may be broken during the drop to room temperature for leak testing. At lower aging
temperatures, the drop to room temperature is less severe but the tendency for bonding to occur is
also likely reduced. Uniformity of the bond is also questionable, at least in terms of providing a
seal. Experiments to evaluate bonding behavior are in process.
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Skidmore, T. E.; Hoffman, E. N.; Daugherty, W. L. & Dunn, K. A. AGING AND SURVEILLANCE OF VITON GLT O-RINGS IN MODEL 9975 SHIPPING PACKAGES, article, June 26, 2009; South Carolina. (https://digital.library.unt.edu/ark:/67531/metadc927447/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.