Unless specially formulated, elastomers will typically soften when first exposed to high temperatures. Extended heat exposure can cause irreversible changes in tensile strength and elongation, as well as alterations in the chemical makeup of a seal such that it hardens and cracks. This hardening is the result of additional cross-linking, plasticizer evaporation, and/or oxidation. Table 7 shows how high temperatures impact a handful of the most-used materials.

There are four different ASTM test methods related to high temperature effects. All are designed to gauge the amount of material degradation that results from exposure to a heated environment. The main difference between these tests is the device used to keep pressure and heat on the specimen. D 454 uses an air-pressure chamber to mimic the degrading effects of heat and air. D 572 utilizes a similar oxygen-pressure chamber to simulate deterioration by heat and oxygen. D 573 details testing in an air oven, and D 865 describes heat and air testing within a test tube enclosure.

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