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Fire-safe polymers
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<h2 id="history">History</h2> <h3>Early history</h3> <p>Controlling the <a href="/facts/Flammability/1QexTQ4T">flammability</a> of different materials has been a subject of interest since 450 B.C. when <a href="/facts/Egyptians/kBkt6yV8">Egyptians</a> attempted to reduce the <a href="/facts/Flammability/1QexTQ4T">flammability</a> of wood by soaking it in <a href="/facts/Potassium_aluminum_sulfate/c3NCJCa9">potassium aluminum sulfate</a> (<a href="/facts/Alum/ULAn0XRB">alum</a>). Between 450 B.C. and the early 20th century, other materials used to reduce the flammability of different materials included mixtures of <a href="/facts/Alum/ULAn0XRB">alum</a> and <a href="/facts/Vinegar/yC27CDkw">vinegar</a>; <a href="/facts/Clay/xnwouEkB">clay</a> and <a href="/facts/Hair/L7vt9FL3">hair</a>; <a href="/facts/Clay/xnwouEkB">clay</a> and <a href="/facts/Gypsum/MItYU70n">gypsum</a>; <a href="/facts/Alum/ULAn0XRB">alum</a>, <a href="/facts/Ferrous_sulfate/zyc7Fukb">ferrous sulfate</a>, and <a href="/facts/Gypsum/MItYU70n">gypsum</a>; and <a href="/facts/Ammonium_chloride/SfXdntlv">ammonium chloride</a>, <a href="/facts/Ammonium_phosphate/SAr9wdvW">ammonium phosphate</a>, <a href="/facts/Borax/IQrXRJ3N">borax</a>, and various <a href="/facts/Acids/FySU18n1">acids</a>. These early attempts found application in reducing the flammability of wood for military materials, theater curtains, and other textiles, for example. Important milestones during this early work include the first <a href="/facts/Patent/LjT2bG3A">patent</a> for a mixture for controlling flammability issued to Obadiah Wyld in 1735,<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> and the first scientific exploration of controlling flammability, which was undertaken by <a href="/facts/Joseph_Louis_Gay-Lussac/co1Ne63y">Joseph Louis Gay-Lussac</a> in 1821.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> </p> <h3>Developments since WWII</h3> <p>Research on fire-retardant <a href="/facts/Polymers/eF32E50K">polymers</a> was bolstered by the need for new types of <a href="/facts/Synthetic_polymers/Hxrpn9cN">synthetic polymers</a> in <a href="/facts/World_War_II/2efV5L1U">World War II</a>. The combination of a <a href="/facts/Halogenated/vYrGVWcJ">halogenated</a> <a href="/facts/Alkane/Qam7Hvmf">paraffin</a> and <a href="/facts/Antimony_trioxide/b3LraQvL">antimony oxide</a> was found to be successful as a <a href="/facts/Fire_retardant/RkwChTJu">fire retardant</a> for canvas tenting. Synthesis of <a href="/facts/Polymers/eF32E50K">polymers</a>, such as <a href="/facts/Polyesters/Ivp9CHpl">polyesters</a>, with <a href="/facts/Fire_retardant/RkwChTJu">fire retardant</a> monomers were also developed around this time.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> Incorporating flame-resistant additives into <a href="/facts/Polymers/eF32E50K">polymers</a> became a common and relatively cheap way to reduce the flammability of <a href="/facts/Polymers/eF32E50K">polymers</a>,<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> while synthesizing intrinsically fire-resistant <a href="/facts/Polymers/eF32E50K">polymers</a> has remained a more expensive alternative, although the properties of these <a href="/facts/Polymers/eF32E50K">polymers</a> are usually more efficient at deterring <a href="/facts/Combustion/ohQoWj8z">combustion</a>.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> </p> <h2 id="polymer-combustion">Polymer combustion</h2> <h3>General mechanistic scheme</h3> <p>Traditional <a href="/facts/Polymer/eF32E50K">polymers</a> <a href="/facts/Chemical_decomposition/4X3oxvhb">decompose</a> under heat and produce combustible products; thus, they are able to originate and easily propagate <a href="/facts/Fire/M6XwR3am">fire</a> (as shown in Figure 1). </p> <p>The <a href="/facts/Combustion/ohQoWj8z">combustion</a> process begins when heating a <a href="/facts/Polymer/eF32E50K">polymer</a> yields <a href="/facts/Volatility_(chemistry)/90r3ep6w">volatile</a> products. If these products are sufficiently concentrated, within the <a href="/facts/Flammability_limits/KkImQKor">flammability limits</a>, and at a temperature above the ignition temperature, then <a href="/facts/Combustion/ohQoWj8z">combustion</a> proceeds. As long as the heat supplied to the <a href="/facts/Polymer/eF32E50K">polymer</a> remains sufficient to sustain its <a href="/facts/Thermal_decomposition/Qqx9RqkW">thermal decomposition</a> at a rate exceeding that required to feed the flame, <a href="/facts/Combustion/ohQoWj8z">combustion</a> will continue.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> </p> <h3>Purpose and methods of fire-retardant systems</h3> <p>The purpose is to control heat below the critical level. To achieve this, one can create an <a href="/facts/Endothermic/06xmHw6Q">endothermic</a> environment, produce non-combustible products, or add chemicals that would remove fire-propagating <a href="/facts/Radical_(chemistry)/qFymuVoT">radicals</a> (H and OH), to name a few. These specific chemicals can be added into the <a href="/facts/Polymer/eF32E50K">polymer</a> molecules permanently (see Intrinsically Fire-Resistant Polymers) or as additives and fillers (see Flame-Retardant Additives and Fillers).<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> </p> <h3>Role of oxygen</h3> <p><a href="/facts/Oxygen/acyn6o8r">Oxygen</a> catalyzes the <a href="/facts/Pyrolysis/h1cEegI7">pyrolysis</a> of <a href="/facts/Polymers/eF32E50K">polymers</a> at low concentration and initiates <a href="/facts/Oxidation/Pyd5RVcj">oxidation</a> at high concentration. Transition concentrations are different for different <a href="/facts/Polymers/eF32E50K">polymers</a>. (e.g., <a href="/facts/Polypropylene/HM9PvoCz">polypropylene</a>, between 5% and 15%). Additionally, <a href="/facts/Polymers/eF32E50K">polymers</a> exhibit a structural-dependent relationship with <a href="/facts/Oxygen/acyn6o8r">oxygen</a>. Some structures are intrinsically more sensitive to <a href="/facts/Chemical_decomposition/4X3oxvhb">decomposition</a> upon reaction with <a href="/facts/Oxygen/acyn6o8r">oxygen</a>. The amount of access that <a href="/facts/Oxygen/acyn6o8r">oxygen</a> has to the surface of the <a href="/facts/Polymer/eF32E50K">polymer</a> also plays a role in <a href="/facts/Polymer/eF32E50K">polymer</a> <a href="/facts/Combustion/ohQoWj8z">combustion</a>. <a href="/facts/Oxygen/acyn6o8r">Oxygen</a> is better able to interact with the <a href="/facts/Polymer/eF32E50K">polymer</a> before a flame has actually been ignited.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> </p> <h3>Role of heating rate</h3> <p>In most cases, results from a typical heating rate (e.g. 10°C/min for mechanical <a href="/facts/Thermal_degradation/Qqx9RqkW">thermal degradation</a> studies) do not differ significantly from those obtained at higher heating rates. The extent of reaction can, however, be influenced by the heating rate. For example, some reactions may not occur with a low heating rate due to <a href="/facts/Evaporation/tgvIlJys">evaporation</a> of the products.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> </p> <h3>Role of pressure</h3> <p><a href="/facts/Volatility_(chemistry)/90r3ep6w">Volatile</a> products are removed more efficiently under low pressure, which means the stability of the <a href="/facts/Polymer/eF32E50K">polymer</a> might have been compromised. Decreased pressure also slows down <a href="/facts/Chemical_decomposition/4X3oxvhb">decomposition</a> of high boiling products.<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> </p> <h2 id="intrinsically-fire-resistant-polymers">Intrinsically fire-resistant polymers</h2> <p>The <a href="/facts/Polymers/eF32E50K">polymers</a> that are most efficient at resisting <a href="/facts/Combustion/ohQoWj8z">combustion</a> are those that are synthesized as intrinsically fire-resistant. However, these types of <a href="/facts/Polymers/eF32E50K">polymers</a> can be difficult as well as costly to synthesize. Modifying different properties of the <a href="/facts/Polymers/eF32E50K">polymers</a> can increase their intrinsic fire-resistance; increasing <a href="/facts/Stiffness/xlabLkfT">rigidity</a> or <a href="/facts/Stiffness/xlabLkfT">stiffness</a>, the use of <a href="/facts/Chemical_polarity/KlIJ0zjB">polar</a> <a href="/facts/Monomers/Gw3CltMb">monomers</a>, and/or <a href="/facts/Hydrogen_bonding/85V86IIg">hydrogen bonding</a> between the <a href="/facts/Polymer/eF32E50K">polymer</a> chains can all enhance fire-resistance.<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> </p> <h3>Linear, single-stranded polymers with cyclic aromatic components</h3><p> Most intrinsically fire-resistant <a href="/facts/Polymers/eF32E50K">polymers</a> are made by incorporation of aromatic cycles or heterocycles, which lend <a href="/facts/Stiffness/xlabLkfT">rigidity</a> and stability to the <a href="/facts/Polymers/eF32E50K">polymers</a>.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> Polyimides, polybenzoxazoles (PBOs), polybenzimidazoles, and polybenzthiazoles (PBTs) are examples of <a href="/facts/Polymers/eF32E50K">polymers</a> made with aromatic heterocycles (Figure 2). </p><p><a href="/facts/Polymers/eF32E50K">Polymers</a> made with aromatic monomers have a tendency to condense into chars upon <a href="/facts/Combustion/ohQoWj8z">combustion</a>, decreasing the amount of flammable gas that is released. Syntheses of these types of <a href="/facts/Polymers/eF32E50K">polymers</a> generally employ prepolymers which are further reacted to form the fire-resistant <a href="/facts/Polymers/eF32E50K">polymers</a>.<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> </p><h3>Ladder polymers</h3> <p><a href="/facts/Ladder_polymer/WwPTfsFq">Ladder polymers</a> are a subclass of <a href="/facts/Polymers/eF32E50K">polymers</a> made with aromatic cycles or heterocycles. Ladder <a href="/facts/Polymers/eF32E50K">polymers</a> generally have one of two types of general structures, as shown in Figure 3.</p><p>One type of ladder <a href="/facts/Polymer/eF32E50K">polymer</a> links two <a href="/facts/Polymer/eF32E50K">polymer</a> chains with periodic <a href="/facts/Covalent_bonds/EzMLf3Pz">covalent bonds</a>.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> In another type, the ladder <a href="/facts/Polymer/eF32E50K">polymer</a> consists of a single chain that is double-stranded. Both types of ladder <a href="/facts/Polymers/eF32E50K">polymers</a> exhibit good resistance to <a href="/facts/Chemical_decomposition/4X3oxvhb">decomposition</a> from heat because the chains do not necessarily fall apart if one <a href="/facts/Covalent_bond/EzMLf3Pz">covalent bond</a> is broken. However, this makes the processing of ladder <a href="/facts/Polymers/eF32E50K">polymers</a> difficult because they are not easily melted. These difficulties are compounded because ladder polymers are often highly <a href="/facts/Insoluble/kLE9h3Jb">insoluble</a>. </p><h3>Inorganic and semiorganic polymers</h3> <p>Inorganic and semiorganic <a href="/facts/Polymers/eF32E50K">polymers</a> often employ <a href="/facts/Silicon/cgWx0hdr">silicon</a>-<a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a>, <a href="/facts/Boron/Pmwgd7Mf">boron</a>-<a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a>, and <a href="/facts/Phosphorus/27xCeIqs">phosphorus</a>-<a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a> monomers. The non-burning characteristics of the <a href="/facts/Inorganic/TfFfzxSz">inorganic</a> components of these <a href="/facts/Polymers/eF32E50K">polymers</a> contribute to their controlled <a href="/facts/Flammability/1QexTQ4T">flammability</a>. For example, instead of forming toxic, flammable gasses in abundance, <a href="/facts/Polymers/eF32E50K">polymers</a> prepared with incorporation of cyclotriphosphazene rings give a high <a href="/facts/Char_(chemistry)/YrRbu7xh">char</a> yield upon <a href="/facts/Combustion/ohQoWj8z">combustion</a>.<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> Polysialates (<a href="/facts/Polymers/eF32E50K">polymers</a> containing frameworks of <a href="/facts/Aluminum/7wmR8jYE">aluminum</a>, <a href="/facts/Oxygen/acyn6o8r">oxygen</a>, and <a href="/facts/Silicon/cgWx0hdr">silicon</a>) are another type of <a href="/facts/Inorganic/TfFfzxSz">inorganic</a> <a href="/facts/Polymer/eF32E50K">polymer</a> that can be thermally stable up to temperatures of 1300-1400 °C.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> </p> <h2 id="flame-retardant-additives-and-fillers">Flame-retardant additives and fillers</h2> <p>Additives are divided into two basic types depending on the interaction of the additive and <a href="/facts/Polymer/eF32E50K">polymer</a>.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> Reactive <a href="/facts/Flame_retardant/0dpdUCYh">flame retardants</a> are compounds that are chemically built into the <a href="/facts/Polymer/eF32E50K">polymer</a>. They usually contain <a href="/facts/Heteroatoms/QzxXyfCT">heteroatoms</a>. Additive <a href="/facts/Flame_retardant/0dpdUCYh">flame retardants</a>, on the other hand, are compounds that are not <a href="/facts/Covalently/EzMLf3Pz">covalently</a> bound to the <a href="/facts/Polymer/eF32E50K">polymer</a>; the flame retardant and the <a href="/facts/Polymer/eF32E50K">polymer</a> are just physically mixed together. Only a few <a href="/facts/Chemical_element/0jducgWD">elements</a> are being widely used in this field: <a href="/facts/Aluminum/7wmR8jYE">aluminum</a>, <a href="/facts/Phosphorus/27xCeIqs">phosphorus</a>, <a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a>, <a href="/facts/Antimony/YsA7lVgT">antimony</a>, <a href="/facts/Chlorine/y27UwYBM">chlorine</a>, <a href="/facts/Bromine/oLfj3C7D">bromine</a>, and in specific applications <a href="/facts/Magnesium/XyIhd2FG">magnesium</a>, <a href="/facts/Zinc/80b1qgk3">zinc</a> and <a href="/facts/Carbon/ukrgQexo">carbon</a>. One prominent advantage of the flame retardants (FRs) derived from these elements is that they are relatively easy to manufacture. They are used in important quantities: in 2013, the world consumption of FRs amounted to around 1.8/2.1 Mio t for 2013 with sales of 4.9/5.2 billion USD. Market studies estimate FRs demand to rise between 5/7 % pa to 2.4/2.6 Mio t until 2016/2018 with estimated sales of 6.1/7.1 billion USD.<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> </p><p>The most important flame retardants systems used act either in the gas phase where they remove the high energy radicals H and OH from the flame or in the solid phase, where they shield the polymer by forming a charred layer and thus protect the polymer from being attacked by oxygen and heat.<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> Flame retardants based on bromine or chlorine, as well as a number of phosphorus compounds act chemically in the gas phase and are very efficient. Others only act in the condensed phase such as metal hydroxides (<a href="/facts/Aluminium_hydroxide/e0pXvDGr">aluminum trihydrate, or ATH</a>, <a href="/facts/Magnesium_hydroxide/0hKR5Ica">magnesium hydroxide, or MDH</a>, and <a href="/facts/Boehmite/VakFKTtQ">boehmite</a>), metal oxides and salts (zinc borate and zinc oxide, zinc hydroxystannate), as well as <a href="/facts/Expandable_graphite/p4LpJ1A7">expandable graphite</a> and some nanocomposites (see below). Phosphorus and nitrogen compounds are also effective in the condensed phase, and as they also may act in the gas phase, they are quite efficient flame retardants. Overviews of the main flame retardants families, their mode of action and applications are given in.<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a><a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a> Further handbooks on these topics are <a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a><a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> A good example for a very efficient phosphorus-based flame retardant system acting in the gas and condensed phases is <a href="/facts/Aluminium_diethyl_phosphinate/y71sJArF">aluminium diethyl phosphinate</a> in conjunction with synergists such as melamine polyphosphate (MPP) and others. These phosphinates are mainly used to flame retard polyamides (PA) and polybutylene terephthalate (PBT) for flame retardant applications in electrical engineering/electronics (E&E).<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> </p> <h3>Natural fiber-containing composites</h3> <p>Besides providing satisfactory mechanical properties and renewability, <a href="/facts/Natural_fiber/E4eyPJ4d">natural fibers</a> are easier to obtain and much cheaper than man-made materials. Moreover, they are more environmentally friendly.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> Recent research focuses on application of different types of <a href="/facts/Fire_retardant/RkwChTJu">fire retardants</a> during the manufacturing process as well as applications of <a href="/facts/Fire_retardant/RkwChTJu">fire retardants</a> (especially <a href="/facts/Intumescent/mQw6J09S">intumescent</a> coatings) at the finishing stage.<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a> </p> <h3>Nanocomposites</h3> <p><a href="/facts/Nanocomposites/ueSgAT4o">Nanocomposites</a> have become a hotspot in the research of fire-safe <a href="/facts/Polymers/eF32E50K">polymers</a> because of their relatively low cost and high flexibility for multifunctional properties.<a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a> Gilman and colleagues did the pioneering work by demonstrating the improvement of fire-retardancy by having nanodispersed <a href="/facts/Montmorillonite/PxKtefgA">montmorillonite</a> clay in the polymer matrix. Later, organomodified <a href="/facts/Clay/xnwouEkB">clays</a>, TiO2 <a href="/facts/Nanoparticles/k8kNxX4E">nanoparticles</a>, <a href="/facts/Silica/A3WILp6J">silica</a> <a href="/facts/Nanoparticles/k8kNxX4E">nanoparticles</a>, <a href="/facts/Layered_double_hydroxides/gWBuQLJx">layered double hydroxides</a>, <a href="/facts/Carbon_nanotubes/atuDPPaG">carbon nanotubes</a> and polyhedral <a href="/facts/Silsesquioxane/1KQM6Ase">silsesquioxanes</a> were proved to work as well.<a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a> Recent research has suggested that combining <a href="/facts/Nanoparticles/k8kNxX4E">nanoparticles</a> with traditional <a href="/facts/Fire_retardant/RkwChTJu">fire retardants</a> (e.g., <a href="/facts/Intumescent/mQw6J09S">intumescents</a>) or with surface treatment (e.g., plasma treatment) effectively decreases <a href="/facts/Flammability/1QexTQ4T">flammability</a>.<a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a> </p> <h3>Problems with additives and fillers</h3> <p>Although effective at reducing <a href="/facts/Flammability/1QexTQ4T">flammability</a>, flame-retardant additives and fillers have disadvantages as well. Their poor compatibility, high <a href="/facts/Volatility_(chemistry)/90r3ep6w">volatility</a> and other deleterious effects can change properties of <a href="/facts/Polymers/eF32E50K">polymers</a>. Besides, addition of many fire-retardants produces <a href="/facts/Soot/xzU91yKv">soot</a> and <a href="/facts/Carbon_monoxide/CKuLtEMJ">carbon monoxide</a> during <a href="/facts/Combustion/ohQoWj8z">combustion</a>. <a href="/facts/Halogen/YeFgWHeA">Halogen</a>-containing materials cause even more concerns on environmental <a href="/facts/Pollution/JRagxumQ">pollution</a>.<a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a><a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Plastics/Kla5QItn">Plastics</a></li> <li><a href="/facts/Fireproofing/t6MKThD7">Fireproofing</a></li> <li><a href="/facts/Phenol_formaldehyde_resin/poc4M0JX">Phenol formaldehyde resin</a></li> <li><a href="/facts/Pyrolysis/h1cEegI7">Pyrolysis</a></li> <li><a href="/facts/Combustion/ohQoWj8z">Combustion</a></li> <li><a href="/facts/Fire-retardant_gel/GaIrhCdT">Fire-retardant gel</a></li> <li><a href="/facts/Fire_test/5ddKlTQr">Fire test</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="http://www.fire.tc.faa.gov/">Fire-Safety Branch of the Federal Aviation Administration</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Zhang, H. Fire-Safe Polymers and Polymer Composites, Federal Aviation Administration technical report; U.S. Department of Transportation: Washington, D.C., 2004. <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Sarkos, C. P. The Effect of Cabin Materials on Aircraft Postcrash Fire Survivability. Technical Papers of the Annual Technical Conference 1996, 54 (3), 3068-3071. <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Kumar, D.; Gupta, A. D.; Khullar, M. Heat-resistant thermosetting polymers based on a novel tetrakisaminophenoxycyclotriphosphazene. J. Polym. Sci. Part A: Polym. Chem. 1993, 31 (11), 2739-2745. 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In Fire and Polymers – Hazards Identification and Prevention; Nelson, G. L., Ed.; American Chemical Society: Washington, D.C., 1990; pp 87-96. ISSN 0097-6156 <a href="/wiki/ISSN_(identifier)" target="_blank">/wiki/ISSN_(identifier)</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Zhang, H. Fire-Safe Polymers and Polymer Composites, Federal Aviation Administration technical report; U.S. Department of Transportation: Washington, D.C., 2004. <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Zhang, H. Fire-Safe Polymers and Polymer Composites, Federal Aviation Administration technical report; U.S. Department of Transportation: Washington, D.C., 2004. <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Zhang, H. 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