5.1 This practice allows the user to evaluate the effect of service or accelerating aging on the oxygen resistance of polymeric materials used in oxygen service.
5.2 The use of this practice presupposes that the properties used to evaluate the effect of aging can be shown to relate to the intended use of the material, and are also sensitive to the effect of aging.
5.3 Polymeric materials will, in general, be more susceptible than metals to aging effects as evidenced by irreversible property loss. Such property loss may lead to catastrophic component failure, including a secondary fire, before primary ignition or combustion of the polymeric material occurs.
5.4 Polymers aged in the presence of oxygen-containing media may undergo many types of reversible and irreversible physical and chemical property change. The severity of the aging conditions determines the extent and type of changes that take place. Polymers are not necessarily degraded by aging, but may be unchanged or improved. For example, aging may drive off volatile materials, thus raising the ignition temperature without compromising mechanical properties. However, aging under prolonged or severe conditions (for example, elevated oxygen concentration) will usually cause a decrease in mechanical performance, while improving resistance to ignition and combustion.
5.5 Aging may result in reversible mass increase (physisorption), irreversible mass increase (chemisorption), plasticization, discoloration, loss of volatiles, embrittlement, softening due to sorption of volatiles, cracking, relief of molding stresses, increased crystallinity, dimensional change, advance of cure in thermosets and elastomers, chain scissioning, and crosslinking.
5.6 After a period of service, a material’s properties may be significantly different from those when new. All materials rated for oxygen service should remain resistant to ignition and combustion (primary fire risk). Furthermore, all materials rated for oxygen service should be resistant to oxidative degradation and retain relevant physical and mechanical properties during service, because part failure can indirectly lead to an unacceptable ignition or combustion risk (secondary fire risk).
5.7 In cases where aging makes a material more susceptible to fire or causes significant oxidative degradation, aging tests may be used to evaluate whether the material will become unacceptable during service. In cases where aging makes a material less susceptible to fire, aging tests may be used to evaluate whether a material can be conditioned (artificially aged) to prolong its service lifetime.
5.8 Oxygen resistance as determined by this practice does not constitute grounds for material acceptability in oxygen service. Determination of material acceptability must be performed within the broader context of review of system or component design, plausible ignition mechanisms, ignition probability, post-ignition material properties, and reaction effects such as are covered by Guide G63.
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