REG NASA-LLIS-1039-2001 Lessons Learned Limitations of material procurement specifications in polytetrafluoroethylene (PTFE) resin selection.pdf

1、Lessons Learned Entry: 1039Lesson Info:a71 Lesson Number: 1039a71 Lesson Date: 2001-08-22a71 Submitting Organization: WSTFa71 Submitted by: Jess WallerSubject: Limitations of material procurement specifications in polytetrafluoroethylene (PTFE) resin selection Description of Driving Event: In order

2、to mitigate seal extrusion-related failures in the Primary Reaction Control System (PRCS) pilot-operated valve (POV), an effort was undertaken to develop a redesigned POV (RPOV) pilot seat assembly. The POV controls the flow of hypergolic liquid propellants to the Space Shuttle Orbiter attitude cont

3、rol thrusters. Specifications and selection assistance from suppliers were found to be inadequate for choosing an appropriate grade of PTFE for seal material without further testing.The two PTFE resins under consideration had similar properties per current material procurement specifications. For ex

4、ample, the dimensional stability, tensile strength, and percent elongation requirements were virtually the same. Meaningful properties such as dimensional stability under load (creep), retention of properties at higher operational temperatures, and chemical resistance to propellants were not covered

5、. Resin selection design assistance from commercial resin suppliers for specific applications such as a thruster valve seal was found to be lacking. Resin candidates were also hard to distinguish by most laboratory tests, but each candidate was later found to exhibit unique physical and mechanical p

6、roperties that made the difference between fabricating a viable versus non-viable seal.Initial RPOV seal fabrication efforts focused on using a pre-sintered, free-flowing extrusion molding PTFE grade per AMS 3658 1, sold under the trademark of Algoflon(r)-E2 2. Seals made from Algoflon-E2 exhibited

7、significant splotchiness, microcracking, and fibrillation after the hot-forming assembly process and seal recession after valve testing. To resolve these issues, an as-sintered, non-free-flowing compression molding PTFE per AMS 3660 3, sold under the trademark of Teflon(r)-7A 4, was used.Provided by

8、 IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionDFigure 1. Redesigned pilot seal showing overall splotchy appearance and fibrillation around seal id and od after trimmingGiven the limited utility of material procurement specifications, a s

9、queeze test which allowed the compressive strain to be varied from 15 to 90 percent was devised to better differentiate between resins. Specimens made from the pre-sintered, free-flowing Algoflon-E2 resin produced a lower strength PTFE that was more susceptible to fracturing (Figure 2, bottom). Sinc

10、e Algoflon-E2 consists of coarse, agglomerated particles, fracturing was attributed to adhesive failure along resin particle boundaries. By comparison, a specimen made from the as-sintered, non-free-flowing resin gradually transitioned from fully opaque to translucent material without any evidence o

11、f fracturing or splotchiness.refer to D descriptionDProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Figure 2. Polytetrafluoroethylene squeeze test specimens: Teflon-7A (top, 1 piece) and Algoflon-E2 (bottom, 2 pieces). Notice the fracturing in the Al

12、goflon-E2 specimens.References:1. Aerospace Material Specification 3658, Revision C. Polytetrafluoroethylene Extrusions, Premium Strength, Sintered and Stress-Relieved Radiographically Inspected. Society of Aerospace Engineers, Warrendale, PA, July 1993.2. Algoflon(r) is a registered trademark of Au

13、simont Montedison Group, Milan, Italy.3. Aerospace Material Specification 3660, Revision C. Polytetrafluoroethylene (PTFE) Moldings, General Purpose Grade, As Sintered. Society of Aerospace Engineers, Warrendale, PA, February 1994.4. Teflon(r) is a registered trademark of E. I. DuPont and Nemours an

14、d Company, Wilmington, DE.Lesson(s) Learned: 1. The criticality of understanding unique PTFE properties that derive from subtle resin differences can not be underestimated. Additional tests must be performed to distinguish between candidate resins, and to ensure that required properties not covered

15、by available specifications are met.2. Materials selection criteria must be constantly re-evaluated to ensure that all key properties are addressed in new applications.3. Proper resin selection also was found to partially compensate for an unoptimized seal design.4. Given the nature of the hot-formi

16、ng assembly process used, the material with the highest resistance to compressive fracturing should have been chosen initially. Switching from a free-flowing extrusion grade to a nonfree-flowing compression-molding grade of PTFE allowed fabrication of viable seals suitable for valve level testing.Re

17、commendation(s): 1. Perform comprehensive research and testing prior to manufacturing parts for unique operational environments.2. Develop more informative tests such as a squeeze test to better differentiate between candidate resins.Evidence of Recurrence Control Effectiveness: N/ADocuments Related

18、 to Lesson: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-N/AMission Directorate(s): a71 Exploration Systemsa71 Aeronautics ResearchAdditional Key Phrase(s): a71 Hardwarea71 Parts Materials & Processesa71 Research & Developmenta71 Test & VerificationAdditional Info: Approval Info: a71 Approval Date: 2001-12-12a71 Approval Name: Jan Jungewaeltera71 Approval Organization: WSTFa71 Approval Phone Number: 505-524-5161Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-