Significance and Use
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.
5.9 The potential for personnel injury, facility damage, product loss, or downtime occurring as a result of ignition, combustion, or catastrophic equipment failure will be least for systems or components using air and greatest for systems or components using pure oxygen.
5.10 In terms of physical and mechanical properties, aging is expected to have a greater influence on a polymer’s ultimate properties such as strength and elongation, than bulk properties such as modulus.
5.11 In terms of fire properties, aging is expected to have a greater influence on a polymer’s ignition properties (for example, autogenous ignition temperature (AIT), mechanical and pneumatic impact) than its propagation properties (for example, upward and downward flame propagation). To date, the only background on aging influences is that of the Bundesanstalt für Materialforschung und -prüfung (BAM) which has assessed the effect of aging at elevated pressure and temperature on a material’s AIT. BAM has used the AIT test results to establish maximum constraints on the use of materials at elevated pressure and temperature.5
Scope
1.1 These practices describe procedures that are used to determine the age resistance of plastic, thermosetting, elastomeric, and polymer matrix composite materials exposed to oxygen-containing media.
1.2 While these practices focus on evaluating the age resistance of polymeric materials in oxygen-containing media prior to ignition and combustion testing, they also have relevance for evaluating the age resistance of metals, and nonmetallic oils and greases.
1.3 These practices address both established procedures that have a foundation of experience and new procedures that have yet to be validated. The latter are included to promote research and later elaboration in this practice as methods of the former type.
1.4 The results of these practices may not give exact correlation with service performance since service conditions vary widely and may involve multiple factors such as those listed in subsection 5.8.
1.5 Three procedures are described for evaluating the age resistance of polymeric materials depending on application and information sought.
1.5.1 Procedure A: Natural Aging—This procedure is used to simulate the effect(s) of one or more service stressors on a material’s oxygen resistance, and is suitable for evaluating materials that experience continuous or intermittent exposure to elevated temperature during service.
1.5.2 Procedure B: Accelerated Aging Comparative Oxygen Resistance—This procedure is suitable for evaluating materials that are used in ambient temperature service, or at a temperature that is otherwise lower than the aging temperature, and is useful for developing oxygen compatibility rankings on a laboratory comparison basis.
1.5.3 Procedure C: Accelerated Aging Lifetime Prediction—This procedure is used to determine the relationship between aging temperature and a fixed level of property change, thereby allowing predictions to be made about the effect of prolonged service on oxidative degradation.
1.6 The values stated in SI units are to be regarded as the standard, however, all numerical values shall also be cited in the systems in which they were actually measured.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific precautionary statements are given in Section 10.
