Ten polymeric materials including EPDM, Nylon 6,6, Buna-N and other materials marketed* as TFE-Teflon® (PTFE), Kel-F® 81 (PCTFE), Vespel® SP-21, Viton® A, Viton® A-500, Fluorel®, and Neoprene® were systematically evaluated for their oxygen compatibility property. The specific properties examined included: (1) autoignition temperature (AIT), (2) heat of combustion, and (3) liquid-oxygen (LOX) mechanical impact sensitivity. Test methods and procedures employed for the evaluations were in compliance with those prescribed in the ASTM and BSI standards. Additionally, a BOC in-house high-pressure autoigniticn test rig (HP-AIT) was used for a detailed study on the materials' autoignition behavior. This high-pressure vessel is capable of probing the AIT of a material up to 450°C and at oxygen pressure (prior to a thermal excursion) up to 5000 psig (∼34.5 MPa).
According to the test results, fluorinated polymers including TFE®, Kel-F® 81, Viton® A, Viton® A-500 and Fluorel® exhibited superior oxygen compatibility property. Specifically, they possessed relatively high autoignition temperatures and reasonably low heat of combustion values. Among them, TFE® possessed the best property by showing the highest autoignition temperature (>450°C), lowest heat of combustion (1517 cal/g), and qualified (pass) LOX impact sensitivity. Vespel® SP-21 (15% graphite-filled polyimide) exhibited an autoignition temperature higher than fluorinated elastomers like Fluorel® and Viton® A, but lower than Kel-F® 81 as well as TFE®. The variation of oxygen pressure, ranging from 1800 psig (∼12.4 MPa) to 5000 psig (∼34.5 MPa), was found to exert little influence on the autoignition temperature of the polymers.
The autoignition behavior of Kel-F®81, Viton® A, Vespel® SP-21, and Nylon 6,6 was further investigated, using HP-AIT, as a function of sample quantity and surface area at different oxygen pressure. On the one hand, variation of sample quantity exerted a more pronounced effect on the AITs of Viton® A, Nylon 6,6 and Kel-F® 81 at various pressures than on Vespel® SP-21. A higher autoignition temperature was generally observed from samples with smaller quantities. On the other hand, changes in sample surface area had little influence on the autoignition temperature of the materials in 1800 psig (∼12.4 MPa) oxygen.