Unlike the changes that result from exposure to high temperatures, changes brought about by low temperature exposure are generally not permanent and can often be reversed if heat returns. For example, extended exposure to low temperatures will increase an elastomer’s hardness, but the material can soften again if the temperature rises (due, for example, to ambient changes or frictional build-up). Of course, the ambient temperature may not increase, and frictional heat may not warm the lip sufficiently to offset the low temperature effects. In such instances, the lip will become progressively stiff and brittle. The brittleness will make it more susceptible to fracturing, which will result in immediate seal failure. Even if the lip doesn’t fracture, its modulus will continue to increase. As modulus increases, followability decreases. A stiff lip is less able to flex and follow shaft eccentricities. A gap will develop between the lip and the shaft, and leakage may result.

There are two main tests related to low temperature effects. The first is detailed in ASTM D 2137 (Method A) as a way to determine a sample’s “brittleness point,” or the lowest temperature at which the sample will not fracture or crack when struck once. The second test is described in ASTM D 1329. Better known as a “TR-10,” this temperature retraction test (see Figure 33) is considered by many within the rubber industry to be the most useful indicator of a material’s low temperature performance.

In a nutshell, the TR-10 measures material resilience. Samples are frozen in a stretched state, then gradually warmed until they lose 10% of this stretch (i.e. retract by 10%). The results of such tests are believed to provide a good basis for evaluating the effects of crystallization and the impact of low temperatures on visco-elastic properties. TR-10 results are generally thought to be consistent with the capabilities of most dynamic seals, including shaft seals.

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