Testing of Polymers in Oxygen-Enriched Environments

Abstract

For over 20 years, the NASA White Sands Test Facility (WSTF) has been responsible for the testing of polymers for use in aerospace-related oxygen systems. To provide state-of-the-art test capabilities, WSTF has developed two new methods that examine ignition and flammability characteristics of polymers in oxygen-enriched environments. These methods were tested with three types of polymers (acrylonitrile butadiene, high-density polyethylene, and polytetrafluoroethylene). The first method comprises a tube furnace assembly and a quartz tube through which a test gas flows over a polymer sample located in a combustion boat. The combustion gas flowing through the infrared test cell is analysed for pyrolysis products using a Fourier transform infrared (FTIR) spectrometer. By measuring the spectral characteristics of the reactant products (spectral absorption bands) as a function of the sample temperature, it is possible to determine some of the pyrolysis properties of the materials. Inspection of the sample while the temperature increases is made possible by viewing the sample through an oblique f ield-of-view angle to the quartz sampling tube. The sample is monitored visually or recorded with a video camera. Visual monitoring for flame emissions allows for positive identification of the autoignition temperature (AIT) as indicated by a sheathed thermocouple in contact with the sample. With the exception of the high-density polyethylene, the AIT results were in good agreement with previously published values. The FTIR spectrometer was demonstrated to be extremely sensitive for characterizing the suitability of polymer materials with oxygen. The second method discussed is a modification to a Differential Scanning Calorimeter (DSC). This device, which utilizes a silicon photodiode coupled to the DSC chamber via a light pipe assembly, is used to detect radiative flame emissions from a polymer test sample. The modification provides a method of identifying the AIT threshold, while simultaneously measuring endothermic and exothermic responses. The photodiode detector assembly has been adapted for use with the high-pressure DSC cell configuration. The visible wavelength bandpass of the silicon photodiode has a weak response to the radiation heat of the sample or DSC chamber for temperatures under 500 K. Thus, flames with temperatures in excess of 500 K are easily distinguished from the radiative response of the chamber or slow reaction rates (no visible flame emissions). The testing of the DSC demonstrated the merit of using the radiative flame emissions as the primary signature for the identification of the auto-ignition condition. The method reduces or eliminates ambiguity associated with interpretation of exothermic reactions. Thus, the new instrumentation provides viable methods of determining the AIT for polymers in oxygen-enriched environments.