Use of DSC and DMA Techniques to Help Investigate a Material Anomaly for PTFE Used in Processing a Piston Cup for the Urine Processor Assembly (UPA) on International Space Station (ISS)

摘要

Human urine and flush water are eventually converted into drinking water with the Urine ProcessorAssembly (UPA) aboard the International Space Station (ISS). This conversion is made possiblethrough the Distillation Assembly (DA) of the UPA. One component of the DA is a molded circularpiston cup made of virgin polytetrafluoroethylene (PTFE). The piston cup is assembled to a titaniumcomponent using eight fasteners and washers. Molded PTFE produced for spare piston cups in the firstquarter of 2010 was different in appearance and texture, and softer than material molded for previouscups. For the suspect newer PTFE material, cup fasteners were tightened to only one-half the requiredtorque value, yet the washers embedded almost halfway into the material.The molded PTFE used in the DA piston cup should be Type II, based on AMS 3667D and ASTMD4894 specifications. The properties of molded PTFE are considerably different between Type I and IImaterials. Engineers working with the DA thought that if Type I PTFE was molded by mistake insteadof Type II material, that could have resulted in the anomalous material properties. Typically, the vendormolds flat sheet PTFE from the same material lot used to mold the piston cups, and tensile testing as partof quality control should verify that the PTFE is Type II material. However, for this discrepant lot ofmaterial, such tensile data was not available. Differential scanning calorimetry (DSC) and dynamicmechanical analysis (DMA) were two of the testing techniques used at the NASA/Marshall Space FlightCenter (MSFC) to investigate the anomaly for the PTFE material. Other techniques used on PTFEspecimens were: Shore D hardness testing, tensile testing on dog bone specimens and a qualitativeestimation of porosity by optical and scanning electron microscopy.