Revelations of dimensional instability in polychlorotrifluoroethylene (PCTFE) semifinished articles and finished parts raised concerns that leaks or part failure could occur during service, leading to catastrophic component or system failure, especially in high-pressure gaseous oxygen systems where flow friction, flow resonance, or kindling chain ignition mechanisms are operative. These concerns culminated in the release of an internal NASA Kennedy Space Center Problem Advisory and a Government-Industry Data Exchange Program Problem Advisory on PCTFE. Release of the advisories coincided with a task group study examining the factors contributing to property variation in PCTFE. More specifically, the effect of resin grade, process route, annealing method, and machining on dimensional stability, molecular weight, and crystallinity was determined. To better control dimensional stability, for example, a Biot-Fourier method is described that allows calculation of the time needed for PCTFE parts of known geometry to reach thermal equilibrium during annealing. A voluntary consensus material specification was then implemented to control properties in finished PCTFE parts used in aerospace applications.
Revelations of excessive property variation in polychlorotrifluoroethylene (PCTFE) semifinished and finished parts led to concerns that leaks or part failure could occur in service, possibly leading to catastrophic component or system failure by flow friction and/or kindling chain mechanisms. Such concerns led to the issuances of an internal Kennedy Space Center (KSC) Problem Advisory and a Government-Industry Data Exchange Program Materials Advisory on PCTFE. The advisories led to an engineering analysis review of PCTFE-containing ground support equipment used in “at-risk” high-pressure oxygen and air systems at KSC Representative PCTFE replacement parts used in “at-risk” systems were removed from inventory and tested. Tests included determination of (1) dimensional stability by thermomechanical analysis and metrology, (2) percent crystallinity by specific gravity, and (3) the effect of annealing on engineering tolerances. While dimensional instability was determined not to be a major issue in existing inventories, establishing traceability back to the semifinished article (starting rod or sheet stock) was often not possible. Percent crystallinity varied widely depending on part origin and thickness. Annealing was found to lead occasionally to out-of-tolerance parts. The pneumatic impact ignition threshold of PCTFE in enriched air was also determined at pressures up to 41.4 MPa. Results show that ignition does not occur at ambient oxygen concentrations. Last, hazard analyses were performed on “at-risk” systems. Two problematic designs were discovered, but based on operational histories, a recommendation was made to redesign or repair and replace on a noninterference basis.
The oxygen compatibility of five O-ring formulations used in the Space Shuttle oxygen/hydrogen (O2/H2) fuel cell was evaluated following exposure to 450 and 6200 kPa (65 and 900 psi) oxygen at 121 °C (250 °F) for 48 h. The elastomers tested were Neoprene compound C873-70, and four fluoroelastomers: Aflas® 7182D, Kalrez® 1045, Fluorel® 2180, and compound V884-75. Post-aging changes in mass, dimension, tensile strength, elongation at break, durometer hardness, and compression set were determined for each elastomer. Aging results were compared to ignition and combustion data, namely the autogenous ignition temperature (AIT) and heat of combustion (ΔHc). Finally, the possibility of heterogeneous oxidation is discussed.
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