SELECTED TECHNICAL PAPERS STP1596 Editors: Samuel Edgar Davis, Theodore A Steinberg Flammability and Sensitivity of Materials in OxygenEnriched Atmospheres: 14th Volume ASTM Stock #STP1596 DOI: 10.1520/STP1596-EB ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959 Printed in the U.S.A Library of Congress Cataloging-in-Publication Data ISBN: 978-0-8031-7637-9 ISSN: 0899-6652 Copyright © 2016 ASTM INTERNATIONAL, West Conshohocken, PA All rights reserved This material may not be reproduced or copied, in whole or in part, in any printed, mechanical, electronic, film, or other distribution and storage media, without the written consent of the publisher Photocopy Rights Authorization to photocopy items for internal, personal, or educational classroom use, or the internal, personal, or educational classroom use of specific clients, is granted by ASTM International provided that the appropriate fee is paid to the Copyright Clearance Center, 222 Rosewood 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Papers When citing papers from this publication, the appropriate citation includes the paper authors, “paper title,” STP title, STP number, book editor(s), ASTM International, West Conshohocken, PA, year, page range, Paper doi, listed in the footnote of the paper A citation is provided on page one of each paper Printed in Eagan, MN July, 2016 Foreword THIS COMPILATION OF Selected Technical Papers, STP1596, Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres: 14th Volume, contains peer-reviewed papers that were presented at a symposium held April 13–15, 2016, in San Antonio, Texas, USA e symposium was sponsored by ASTM International Committee G04 on Compatibility and Sensitivity of Materials in Oxygen Enriched Atmospheres T Symposium Chairpersons and STP Editors: Samuel Edgar Davis NASA, George C Marshall Space Flight Center Huntsville, AL, USA Teodore A Steinberg Queensland University of Technology Brisbane, Queensland, Australia Co n te n t s Overview ix Keynote Address Space Exploration and Fire Technology at Southwest Research Institute—Learning from the Past and Preparing for the Future Walter D Downing Test Methods A Method for Autogenous Ignition Temperature Determination of Metal Through Induction Heating 15 Statistical Considerations for Adiabatic Compression Testing 37 Improved ASTM G72 Test Method for Ensuring Adequate Fuel-to-Oxidizer Ratios 49 Autogenous Ignition Test Approach for Hyperbaric Oxygen (HBO 2) and Other Low-Pressure Oxygen Applications 62 Joel Stoltzfus and Timothy D Gallus Barry E Newton and Theodore A Steinberg Alfredo Juarez and Susana A Harper Gwenael J Chi foleau, Richard Barry, Barry E Newton, and Nicholas Linley Cleaning and Contamination Control Results of the Test Program for Replacement of AK-225G Solvent for Cleaning NASA Propulsion Oxygen Systems Nikki M Lowrey and Mark A Mitchell NASA Independent Assessment of Ambient Pressure Liquid Oxygen (LOX) Impact Testing of Halogenated Solvents H R Ross and S J Gentz v 76 109 An Improved Approach for Analyzing the Oxygen Compatibility of Solvents and Other Oxygen-Flammable Materials for Use in Oxygen Systems 137 Analysis of Risks to Oxygen Systems from Particulate and Fiber Contaminants and Derivation of Cleanliness Requirements 152 Susana A Harper, Alfredo Juarez, Stephen F Peralta, Joel Stoltzfus, Christina Piña Arpin, and Harold D Beeson Nikki M Lowrey Factors Afecting NVR Contaminant Fire Risk Bradley S Forsyth, Gwenael J A Chi foleau, Barry E Newton 185 Failure and Incident Investigations Fatal Accident from an Oxygen Fire in an Indian Steel Plant in 2012: Unresolved Questions 205 Auto Ignition Temperature Test Chamber Fire Investigation 234 Investigation of a Fire in a Liquid Oxygen Bulk Delivery Tank 246 Failure Analysis of a Liquid Oxygen Gate Valve Fire Gwenael J A Chi foleau, Elliot T Forsyth, and Barry E Newton 265 Kanchan Chowdhury Jonathan M Tylka and Timothy D Gallus Barry E Newton and John Schumacher Stainless Steel Plug Valve Incident in High Pressure Oxygen: Delrin® Seat and Silicone-Based Lubricant 286 Jared D Hooser, Bradley S Forsyth, Gwenael J A Chi foleau, and Barry E Newton Research on Materials and Operations Properties of ToughMet® Copper-Nickel-Tin Alloy for Oxygen Enriched Atmosphere Applications 296 Detailed Investigation of the Sequence of Mechanisms Participating in Metals Ignition in Oxygen Using Laser Heating and In Situ, Real-Time Diagnostics 308 Comparison of Combustion Products of Bulk Aluminum Rods Burning in High Pressure Oxygen in Normal and Reduced Gravity 326 Simulation of Cylindrical Rod Combustion in High-Pressure Oxygen by Steady-State Jet Model 338 Anand V Samant, Michael J Gedeon, Robert E Kusner, Chad A Finkbeiner, Fritz C Grensing, and W Raymond Cribb Maryse Muller, Hazem El-Rabii, Rémy Fabbro, Frédéric Coste, Jean-Christophe Rostaing, Martina Ridlova, Alain Colson, and Hervé Barthélémy Owen Plagens and Theodore A Steinberg S I Shabunya, V V Martynenko, V I Ignatenko, and J.-C Rostaing vi Evaluation of Containment Boxes as a Fire Mitigation Method in Elevated Oxygen Conditions 363 Promoted Ignition-Combustion Tests of Brazed Aluminum Heat Exchanger Samples in Cold Supercritical Oxygen 374 Oxygen Endurance Testing of Oxygen Cylinder Valves 393 Evaluation of a Near-Adiabatic Compression Process to Increase Fire Safety Within Oxygen Systems, Focusing on Non-Metals 405 Oxygen Partial Pressure and Oxygen Concentration Flammability: Can They Be Correlated? 413 Alfredo Juarez, Susana A Harper, and Horacio Perez Thomas A McNamara, Joseph F Million, Ravi Pahade, and James White T Kasch, C Binder, N Treisch, M Szypkowski, and A Woitzek Maria Ryan, Theodore A Steinberg, and Barry E Newton Susana A Harper, Alfredo Juarez, Horacio Perez III, David B Hirsch, and Harold D Beeson vii Overview STP1 596 is the fourteenth set of Special Technical Papers (STP) originating from the ASTM Committee G04 focusing on the Oxygen-Enriched Atmospheres Flammability and Sensitivity of Materials in Te thirteen previous STP volumes originating from the ASTM G04 committee are: 81 2, 91 0, 986, 040, 1 1 , 1 97, 267, 31 9, 395, 454, 479, 522, and 561 Copies of these STP volumes are available from ASTM International Te ASTM Committee G04 on Compatibility and Sensitivity of Materials in Oxygen Enriched Atmospheres continues to grow in its international appeal Te fourteenth symposium was attended by a number of professionals representing several countries Tese included the United States, Australia, Germany, Canada, France, India, the United Kingdom, and Belarus A number of professionals from other nations also attended the symposium and shared important information in person even though they were unable to submit a formal paper for publication As with the past STPs, the fourteenth volume expands upon the objectives that have been carried forward since the frst ASTM Committee G04 STP was published in 983 Tese objectives include: ? Review the current research on polymers and metals ignition and combustion; ? ? Overview principles of oxygen systems design and issues related to materials ? compatibility with oxygen; ? Contribute to the knowledge on the most current risk management concepts, ? practices, approaches, and procedures used by individuals and organization involved in the design, use, retro ftting, maintenance, and cleaning of oxygen systems; ? Review of accident/incident case studies related to oxygen systems and oxygen ? handling procedures; ? Provide research on new compounds or techniques to clean oxygen systems in ? order to make these systems safer for users; ? ? Provide the most current data related to the fammability and sensitivity of materials in oxygen-enriched atmospheres to designers, users, manufacturers and maintainers of oxygen components and systems and to support Committee G04’s Technical and Professional Training Course on Fire Hazards in Oxygen Systems; ix 414 STP 1596 On Flammability and Sensitivity ofMaterials in Oxygen-Enriched Atmospheres Keywords partial pressure, gaseous oxygen, maximum oxygen concentration (MOC), normoxic, flammability, elevated oxygen, enriched oxygen, NASA Standard 6001 Test 1, propagation rate Background To safely and successfully operate in enriched oxygen conditions, understanding material flammability is critical for NASA, commercial space flight companies, and industry alike Previous space programs have acquired significant data in the space transportation system (STS) environment of 30 % O at 70.3 kPa (10.2 psia) This same environment is also currently being evaluated for the Crew Exploration Vehicle (CEV) A significant amount of additional flammability data exists at the International Space Station (ISS) worst-case cabin conditions of 24.1 % O at 191.4 kPa (27.8 psia) In a desire to leverage existing data, the question arises: Do materials perform similarly with respect to flammability as long as the partial pressure of oxygen remains equivalent? This question is not only relevant to NASA and space programs In the oxygen-related industry, the ability to apply existing flammability data to various manufacturing and operating conditions would be beneficial It is important that the question be thoroughly answered because the ability to apply existing data to alternate conditions could save significant resources financially and with respect to time and schedule The purpose of this paper is to compile relevant data to examine the dependence of flammability on partial pressure of oxygen and oxygen concentration Normoxic Conditions and Partial Pressure Normoxic conditions maintain an equivalent partial pressure of oxygen in the atmosphere as would be found in that of air at sea level This level of oxygen is important for human function; therefore, space vehicles are designed to provide normoxic or close to normoxic conditions The concept of partial pressure depends on the ideal gas law (Eq 1) PV ¼ nRT where: P ¼ pressure V ¼ volume n ¼ moles of a molecule R ¼ ideal gas constant T ¼ temperature (1) HARPER ET AL., DOI 10.1520/STP159620150081 Assuming the ideal gas law, the partial pressure of oxygen pO2 is defined as the pressure that would be exerted by nO2 moles of O2 alone in the same total volume V at the same temperature T (Eq 2) pO2 V ¼ nO2 RT (2) Dividing the second equation by the first yields the mole fraction of a given component in a mixture times the total pressure, which will give you the component’s partial pressure (Eq and Eq 4) n O2 = n ¼ pO2 = P (3) ð n O2 = n Þ ? P ¼ p O2 (4) A similar calculation can be done for the pure component volume of a mixture In an ideal gas mixture, a component’s percentage by volume is equal to its mole fraction (Eq and Eq 6) n O2 = n ¼ vO2 = V (5) n O2 = n ị ? V ẳ vO2 (6) Therefore, referring to a gas mixture by its volume percent (e.g., 21 % volume O2) is the same as referring to it by its mole percent (21 mole % O2) Assuming an ideal gas behavior, partial pressure can easily be calculated from volume fraction (Eq and Eq 8) [1] pO2 ẳ vO2 = Vị ? P (7) (8) and conversely vO2 = Vị ẳ pO2 = P Using this calculation, Table outlines examples of normoxic environmental conditions that maintain equivalent partial pressures across a range of conditions The STS and the currently in-design CEV have resided at the 30 % O2 at 70.3 kPa (10.2 psia) point of the normoxic curve The ISS aims to operate at the 21 % O2, 101.4 kPa (14.7 psia) point of the curve Nonetheless, it has obtained much of its data at 24.1 % O2, 101.4 kPa (14.7 psia) conditions due to sinusoidal fluctuations of oxygen with a mean of 21 % O2 seen on the ISS Future long-duration habitation modules might select to operate at lower pressure normoxic conditions to minimize time loss and health risks associated with repeated depressurizations when frequently exiting habitats In addition, low pressure environments also provide structural design TABLE Normoxic environmental conditions 18 21 24.1 25 30 34 36 44 40 60 100 Total Pressure Oxygen Volume % 118.6 101.4 88.3 85.5 71.0 62.7 59.3 48.3 53.1 35.9 21.4 kPa (psia) (17.2) (14.7) (12.8) (12.4) (10.3) (9.1) (8.6) (7.0) (7.7) (5.2) (3.1) 21.3 21.3 21.3 21.3 21.3 21.3 21.3 21.3 21.3 Oxygen Partial Pressure kPa (psia) 21.3 21.3 (3.09) (3.09) (3.09) (3.09) (3.09) (3.09) (3.09) (3.09) (3.09) (3.09) (3.09) 415 416 STP 1596 On Flammability and Sensitivity ofMaterials in Oxygen-Enriched Atmospheres benefits for long-duration habitats Use of extravehicular activity (EVA) suits is another scenario in which a reduced pressure environment is desired Off-nominal situations can also arise in which lower pressure/higher oxygen concentration environments need to be considered Some examples may include vehicle leak emergency scenarios or decompression times before an EVA In all these cases, understanding how existing flammability data can be correlated to other environments would prove useful In doing so, care should be taken, as the relationships between material flammability with oxygen concentration and partial pressure are complex These relationships will be examined in the following sections Test Method and Environmental Conditions: NASA STD-6001, Test 1, Maximum Oxygen Concentration Self-Extinguishment Thresholds The test method used in this paper to examine flammability was the NASA STD-6001, [2] Materials slated for use in space vehicles are required to undergo this test to evaluate a material’s ability to selfextinguish in less than 15 cm (6 in.) as well as to establish its propensity to propagate to nearby materials Samples (12 in long and 2.5 in wide) are subjected to an overwhelming ignition source at their anticipated use conditions After ignition, materials are evaluated to determine if they self-extinguish in less than 15 cm (6 in.), indicating that they are not likely to create sustained fires at the given test conditions Also, paper is placed below the test apparatus during the test to evaluate if any dripping elements will ignite nearby materials, thereby evaluating the propagation risk The maximum oxygen concentration (MOC) threshold, as the name suggests, establishes the MOC at which a material will still pass the test’s pass/fail criteria This threshold value can be used successfully to compare materials’ performance across a variety of conditions [3,4] The MOCs for various aerospace materials have been previously determined across a large range of pressures (2.8–119.3 kPa [0.4–17.3 psia] ) [5–7] Much of these preexisting data have been compiled in Table to allow a comprehensive analysis with respect to the effect of oxygen concentration, total pressure, and partial pressure on overall selfextinguishing limits From MOC limits, the corresponding oxygen partial pressure limits were also calculated as maximum oxygen partial pressure (MOP) The MOPs presented at 6.2 kPa (0.9 psia) or below, however, were determined experimentally in a 99.8 % oxygen environment where pressure was increased until the threshold limit was obtained at which materials still passed the NASA-STD-6001 burn-length criteria [7] Upward Flame Propagation Flammability Test Oxygen Concentration, Total Pressure, and Partial Pressure Effects Findings OXYGEN CONCENTRATION AND TOTAL PRESSURE FINDINGS Maximum oxygen concentration data from Table were plotted against total pressure in the 48.3–1 9.3 kPa (7–17.3 psia) range in Fig Pressure data below 18 18 13 47 31 18 50 EPDM Rubber Polyethylene (PE) Delrin Zotek F30 Valox DR48 Nylon/Phenolic Armalon TG4060 Sygef 18 18 18 29 Viton-A Buna-S Buna-N 25 Zytel 42 Neoprene 28 Silicone MOC (vol %) 53 MOP (psia) 76 MOP (psia) 0.9 KelF-81 MOP (psia) MOP (psia) 0.6 PTFE Material 0.5 0.4 Determined at 99.8 % volume O 3.5 1.3 2.2 3.3 0.9 1.3 1.3 1.3 1.3 1.3 2.0 1.8 2.0 5.3 3.7 MOP (psia) 34 MOC (vol %) 10.2 3.5 MOP (psia) Psia (total pressure) 12 18 16 16 17 17 21 23 23 56 46 MOC (vol %) 12.35 1.5 2.2 2.0 2.0 2.1 2.1 2.6 2.8 2.8 6.9 5.7 MOP (psia) 34 35 17 28.1 37.5 11 17 16 15 16 16 21 23 21 53 42 MOC (vol %) 14.7 5.0 5.1 2.5 4.1 5.5 1.6 2.5 2.4 2.2 2.4 2.4 3.1 3.4 3.1 7.8 6.2 MOP (psia) 32 33 17 28 36 MOC (vol %) 17.3 5.5 5.7 2.9 4.8 6.2 MOP (psia) TABLE Pressure effects on NASA STD-6001 Test Maximum Oxygen Concentration (MOC) flammability thresholds of materials for aerospace applications and equivalent normoxic and ISS environment conditions for comparison HARPER ET AL., DOI 10.1520/STP159620150081 417 0.6 MOP (psia) 0.6 3.1 3.5 44 50 34 24 36 20 32 37 30 MOC (vol %) 0.9 MOP (psia) 29 MOC (vol %) 0.9 MOP (psia) 0.9 10.2 3.5 2.4 3.7 2.0 3.3 3.8 3.1 3.0 MOP (psia) Psia (total pressure) 28 25 MOC (vol %) 12.35 3.5 3.1 MOP (psia) MOC ¼ Maximum oxygen concentration that consistently results in material self-extinguishment MOP ¼ Maximum oxygen partial pressure when extinguishment occurs (based on MOC with the exception of99.8 % testing) 0.5 MOP (psia) MOP (psia) 0.4 0.5 Determined at 99.8 % volume O 0.4 (Continued) Udel P1,700 Polysulfone Ultem 1,000 Melamine/Glass Melinex 515 Kydex 100 Nomex 90-40 Normoxic environment partial pressure equivalents (CEV/STS) ISS environment partial pressure equivalents Material TABLE 24.1 21 34 18.5 32 31.5 21 24 MOC (vol %) 14.7 3.5 3.1 5.0 2.7 4.7 4.6 3.1 3.5 MOP (psia) 20 28 30 18 21 33 22 MOC (vol %) 17.3 3.5 4.8 5.2 3.1 3.6 5.7 3.8 MOP (psia) 418 STP 1596 On Flammability and Sensitivity ofMaterials in Oxygen-Enriched Atmospheres FIG Pressure effects on NASA STD-6001 Test MOC flammability thresholds and best fit trendlines HARPER ET AL., DOI 10.1520/STP159620150081 419 420 STP 1596 On Flammability and Sensitivity ofMaterials in Oxygen-Enriched Atmospheres kPa (1 psia) were not plotted in Fig because only five of the 22 materials examined had data in this lower range This plot can be used to observe general effects of total pressure on MOC Each material data set was fit with the equation that provided the highest regression analysis coefficient of determination (R2 ) In the 48.3–1 9.3 kPa (7–1 7.3 psia) pressure range, most materials were best described by either linear or power curve models Also, normoxic equivalent oxygen concentrations, all equaling a partial pressure of 21 kPa (3.09 psia) at their respective set of conditions, were plotted with red stars and a corresponding curve Normoxic data provide a comparison to the trends seen for the experimental flammability data for the various aerospace materials Seventeen of the 22 materials examined (77 % of materials) exhibited very little dependence on total pressure For these materials in the central pressure range of 48.3–1 9.3 kPa (7–1 7.3 psia), MOC remained relatively constant despite pressure variations The general slope of the normoxic equivalent oxygen concentrations follows a steep decline while the general slopes of the tested materials’ MOCs follow significantly shallower paths It is noteworthy that this contrasting trend from partial pressure dependent normoxic conditions to the experimental material data suggests that propagation and self-extinguishment flammability is not driven by the partial pressure of oxygen available Oxygen concentration appears to be the major driver in propagation and selfextinguishment behavior regardless of total pressure or partial pressure of oxygen Other relevant research reiterates these conclusions These include flame spread rate testing that was performed along normoxic conditions from 18 % to 100 % O2 by Olson and Miller [8] In this work, regardless of test variable modifications, the flame spread rate increased with higher oxygen concentrations even though partial pressure of oxygen remained constant [8] In addition, authors Yang, Hamins, and Donneley [9] found that burn rates of poly(methyl methacrylate) (PMMA) spheres increased significantly as O2 percent volume was increased from 19.9 % to 30 %, although little effect was observed with increased pressures from 50.0–150 kPa (7.25–21.75 psia) Though not fully modeling normoxic partial pressure trends, a few of the mate- V V rials tested for this study (Kel-F , polytetrafluoroethene (PTFE), Zotek F30, V Armalon TG4060, and Nomex ) exhibited higher dependencies on total pressure R R R A possible theory to explain the oxygen concentration and pressure dependence difference among these materials will be discussed in a later section In Fig , MOC self-extinguishment thresholds were again plotted against pressure with the inclusion of threshold pressures obtained at 99.8 volume percent oxygen for select materials [7] These material data sets with larger data ranges were again fit with equation models that provided the highest regression analysis coefficient of determination (R2) Power equation models fit all trends very precisely It is believed that, if additional high oxygen concentration data are obtained for other materials, they will likely V Kel-F is a registered trademark of M.W Kellogg Co., Jersey City, NJ V Zotek is a registered trademark of Zotefoams PLC, Surry, UK V Nomex is a registered trademark of E I du Pont de Nemours and Co., Wilmington, DE R R R FIG MOC threshold in which NASA-STD-6001 Test will consistently self-extinguish and equivalent normoxic oxygen concentrations HARPER ET AL., DOI 10.1520/STP159620150081 421 422 STP 1596 On Flammability and Sensitivity ofMaterials in Oxygen-Enriched Atmospheres show similar power trends Flammability trends found here echo trends seen in previous ignition studies by authors Nakamura and Aoki [1 0–1 2] , with the exception that a nonignition zone is not identified in the current study From this larger range view of flammability performance, it was again shown that total pressure had a minimal effect on propagation and self-extinguishment above approximately 41 kPa (6 psia) Nonetheless, below 41 kPa (6 psia), the pressure effects became highly influential It has been proposed that different ignition models govern ignition mechanics in these two zones, with the pure diffusion model governing in the 48.3–1 9.3 kPa (7–1 7.3 psia) range and the ignition in stagnation-point flow field governing in the < 41 kPa (< psia) range [1 3] These proposed differing models for middle- and low-pressure ignition and propagation would be consistent with the findings drawn from Fig and the corresponding data set OXYGEN PARTIAL PRESSURE FINDINGS From MOC testing, equivalent MOP pressures were calculated (presented in Table 2) The MOP represents the threshold value for how much oxygen is necessary to propagate a flame yet self-extinguish within the NASA-STD-6001 cm (6 in.) burn length criterion MOP data were plotted against total pressure in Fig to examine partial pressure effects directly The clearest observation is that the required partial pressure of oxygen necessary to sustain propagation to the cm (6 in.) criterion lessens with decreasing total pressure for all 22 materials examined Therefore, despite having equal partial pressures, a lower pressure/higher oxygen concentration environment would pose a greater flammability risk These findings are consistent with partial pressure ignition data trends observed by authors Nakamura and Aoki in which partial pressure of oxygen required for ignition of cellulose material decreased as total pressure was decreased [1 0–1 2] Equations were fit to data for materials possessing full-scale pressure data Power equation models described the data excellently with all coefficients of deter2 mination (R ) calculated at higher than 0.99 values It is believed that, if additional data are obtained for other materials in the low pressure/high oxygen concentration ranges, they will likely show similar power trends With respect to how to apply existing data to alternate environmental conditions, the conclusion drawn from these data is that lower oxygen concentration/ higher pressure data (e.g., 21 % O , 01 kPa [1 4.7 psia] ) cannot be conservatively applied to higher oxygen concentration/lower pressure environments (e.g., 30 %, 70.3 kPa [1 0.2 psia] ) despite equivalent partial pressures Nonetheless, higher oxygen concentration/lower pressure data (e.g., 30 %, 70.3 kPa [1 0.2 psia] ) can be conservatively applied to evaluate the risk of lower oxygen concentration/higher pressure environments (e.g., 21 % O , 01 kPa [1 4.7 psia] ) Discussion: Oxygen Molecular Collision Rate Competition for Reaction Sites Oxygen molecular collision rate competition for reaction sites is proposed as a potential theory to help describe the reported experimental trends The major FIG MOP thresholds in which NASA-STD-6001 Flammability Test will consistently self-extinguish and equivalent normoxic partial pressures HARPER ET AL., DOI 10.1520/STP159620150081 423 424 STP 1596 On Flammability and Sensitivity ofMaterials in Oxygen-Enriched Atmospheres trends observed in examination of data are as follows: Oxygen concentration is the maj or driver for material flammability with little effect from total pressure in standard pressure ranges above 41 kPa (6 psia) ; decreasing amounts of oxygen partial pressures are needed to support propagation as total pressure is decreased; and certain materials such as Kel-F, PTFE, Zotek F30, Armalon TG4060, and Nomex exhibited higher flammability pressure dependencies than other materials Piloted ignition and combustion of a material are normally described as a series of three events: a material heating time resulting in pyrolysis with corresponding generation of flammable gasses; an oxidizer and flammable gas mixing time; and an induction or chemistry time [1 4] Nonetheless, in trying to not only explain rationale behind oxygen concentration as a primary flammability driver but also the rationale behind why certain classes of materials might exhibit different dependencies on pressure, an additional step may be proposed—one where the oxygen molecular collision rate competition for available reaction sites may play a role At two different conditions along the normoxic curve, partial pressure of oxygen and a therefore equivalent quantity of oxygen molecules are available Nonetheless, the normoxic point that resides at the higher oxygen concentration and lower total pressure possesses a smaller percentage of inert molecules In the test environment, molecules are continually colliding with the material surface Conditions with increased percentages of oxygen offer a higher chance of an oxidizer molecule coming into contact with a reaction site versus the corresponding normoxic condition with fewer competing inert molecules This would explain why increased oxygen concentration escalates flammability despite equivalent partial pressures of oxygen The same rationale holds true for the observation that decreasing amounts of oxygen partial pressures are needed to support propagation as total pressure is decreased Decreased total pressure means fewer molecules as a whole competing for reaction sites With decreased competition from inert molecules, it would make sense that fewer oxygen molecules are necessary to maintain similar flammability performance Recall that the materials exhibiting higher flammability pressure dependencies were Kel-F, PTFE, Zotek F30, Armalon TG4060, and Nomex All of these more highly pressure-dependent materials are highly halogenated, with the exception of Nomex Kel-F is a thermoplastic chlorofluoropolymer, polychlorotrifluoroethene n (PCTFE), with the molecular formula (CF2 CClF) Polytetrafluoroethylene is a V9 , fluoropolymer, such as Teflon R with the molecular formula (C2 F 4) n Zotek F is a closed cell polyvinylidene fluoride (PVDF) -based foam with the molecular formula n (C2 H F ) Armalon TF 4060 is a fluorocarbon fiberglass composite Because of their saturated chains of highly electronegative halogenated molecules (F, Cl), these materials are highly stable and possess few susceptible reaction sites Though Nomex is not a halogenated compound, its aramid composition provides a highly stable structure with dense electron clouds that also offers few susceptible reaction sites V Teflon is registered trademark of E I du Pont de Nemours and Co., Wilmington, DE R HARPER ET AL., DOI 10.1520/STP159620150081 If reaction sites are eliminated from the ignition initiation equation, then the oxygen molecule collision rate competition for reaction sites may no longer play a significant role (as proposed earlier) Therefore, a material may directly follow the pyrolysis, mixing, and induction ignition model without an additional oxidizer reaction site collision rate accelerating step [1 4] In these cases, a material may exhibit a higher pressure dependency (as was evident for Kel-F, PTFE, Zotek F30, Armalon TG4060, and Nomex) than what was seen for its more reaction site rich material counterparts The proposal that an oxygen molecule reaction site collision rate competition step may supplement the ignition sequence of pyrolysis, flammable gas mixing, and ignition induction appears to be successful in describing reported experimental trends Conclusions The desire to understand how to apply existing enriched oxygen environment flammability data to alternate environmental conditions arises for NASA, commercial space flight companies, and industry alike The question as to whether data can be applied to alternate environments based on partial pressure equivalents comes up frequently Existing data from various sources have been compiled and examined here to address this common question Analysis of data generated a series of observations Flammability characteristics (per NASA-STD-6001 Test MOC self-extinguishment thresholds, material ignition, and burn rates) show a strong dependence on oxygen concentration with little relation to total pressures above 41 kPa (6 psia) Below 41 kPa (6 psia) , MOCs and required oxygen partial pressures show increased dependence on total pressure Power equation models fit trends very precisely across pressure ranges spanning 2.8–1 9.3 kPa (0.4–1 7.3 psia) for both MOC and partial pressure plots against total pressures A notable finding was that required partial pressure of oxygen necessary to sustain propagation decreases with decreased total pressures This directly implies an increased flammability risk at lower total pressure conditions This method of analysis may aid in the application of existing flammability data to alternate environmental conditions and may ultimately provide guidance as to the types of tests that should be performed to yield the most useful results The conclusion drawn from these data is that lower concentration/higher pressure data (e.g., 21 % O 2, 01 kPa [1 4.7 psia] ) cannot be conservatively applied to higher oxygen concentration/lower pressure environments (e.g., 30 %, 70.3 kPa [1 0.2 psia] ) despite equivalent partial pressures Nonetheless, higher oxygen concentration/ lower pressure data (e.g., 30 %, 70.3 kPa [1 0.2 psia] ) can be conservatively applied to evaluate the risk of lower concentration higher pressure environments (e.g., 21 % O 2, 01 kPa [1 4.7 psia] ) Certain materials (Kel-F, PTFE, Zotek F30, Armalon TG4060, and Nomex) exhibited higher dependencies on total pressure and are believed to perform differently due to their limited amount of susceptible reaction sites A step for oxygen molecular collision rate competition for reaction sites was suggested as an additional 425 426 STP 1596 On Flammability and Sensitivity ofMaterials in Oxygen-Enriched Atmospheres mechanism in the ignition sequence of pyrolysis, flammable gas mixing, and ignition induction This mechanism appears to be successful in describing reported experimental trends Future planned work includes additional testing in low pressure ranges as well as acquisition ofburn-rate data ofthe presented materials at the various normoxic conditions References [1 ] Fel der, R M and Rousseau , R.W., Elementary Principles of Chemical Processes, 2nd ed , Wi l ey, Toronto, 986 [2] N ASA-STD-6001 B, Procedures, Flammability, Offgassing, and Compatibility Requirements and Test Test , “Upward Fl ame Propagati on,” N ati onal 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