Volume 6, Issue 8 (September 2009)
Oxygen Compatibility of Brass-Filled PTFE Compared to Commonly Used Fluorinated Polymers for Oxygen Systems
Safe and reliable seal materials for high-pressure oxygen systems sometimes appear to be extinct species when sought out by oxygen systems designers. Materials that seal well are easy to find, but these materials are typically incompatible with oxygen, especially in cryogenic liquid form. This incompatibility can result in seals that leak, or much worse, seals that easily ignite and burn during use. Scientists at the Materials Combustion Research Facility (MCRF), part of NASA/Marshall Space Flight Center, are constantly searching for better materials and processes to improve the safety of oxygen systems. One focus of this effort is improving the characteristics of polymers used in the presence of an oxygen enriched environment. Very few systems can be built which contain no polymeric materials; therefore, materials which have good impact resistance, low heat of combustion, high auto-ignition temperature, and those that maintain good mechanical properties are essential. The scientists and engineers at the MCRF, in cooperation with seal suppliers, are currently testing a new formulation of polytetrafluoroethylene (PTFE) with brass filler. This brass-filled PTFE is showing great promise as a seal and seat material for high-pressure oxygen systems. Early research has demonstrated very encouraging results, which could rank this material as one of the best fluorinated polymers ever tested. This paper will compare the data obtained for brass-filled PTFE with other fluorinated polymers, such as TFE-polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene 81, Viton® A, Viton® A-500, Fluorel®, and Algoflon®. A similar metal-filled fluorinated polymer, Salox-M®, was tested in comparison to brass-filled PTFE to demonstrate the importance of the metal chosen and relative percentage of filler. General conclusions on the oxygen compatibility of this formulation are drawn, with an emphasis on comparing and contrasting the materials performance to the performance of the current state-of-the-art oxygen compatible polymers.