Fire-safe polymers

Last updated

Fire-safe polymers are polymers that are resistant to degradation at high temperatures. There is need for fire-resistant polymers in the construction of small, enclosed spaces such as skyscrapers, boats, and airplane cabins. [1] In these tight spaces, ability to escape in the event of a fire is compromised, increasing fire risk. In fact, some studies report that about 20% of victims of airplane crashes are killed not by the crash itself but by ensuing fires. [2] Fire-safe polymers also find application as adhesives in aerospace materials, [3] insulation for electronics, [3] and in military materials such as canvas tenting. [4]

Contents

Some fire-safe polymers naturally exhibit an intrinsic resistance to decomposition, while others are synthesized by incorporating fire-resistant additives and fillers. Current research in developing fire-safe polymers is focused on modifying various properties of the polymers such as ease of ignition, rate of heat release, and the evolution of smoke and toxic gases. [1] Standard methods for testing polymer flammability vary among countries; in the United States common fire tests include the UL 94 small-flame test, the ASTM E 84 Steiner Tunnel, and the ASTM E 622 National Institute of Standards and Technology (NIST) smoke chamber. [1] Research on developing fire-safe polymers with more desirable properties is concentrated at the University of Massachusetts Amherst and at the Federal Aviation Administration where a long-term research program on developing fire-safe polymers was begun in 1995. The Center for UMass/Industry Research on Polymers (CUMIRP) was established in 1980 in Amherst, MA as a concentrated cluster of scientists from both academia and industry for the purpose of polymer science and engineering research. [1]

History

Early history

Controlling the flammability of different materials has been a subject of interest since 450 B.C. when Egyptians attempted to reduce the flammability of wood by soaking it in potassium aluminum sulfate (alum). Between 450 B.C. and the early 20th century, other materials used to reduce the flammability of different materials included mixtures of alum and vinegar; clay and hair; clay and gypsum; alum, ferrous sulfate, and gypsum; and ammonium chloride, ammonium phosphate, borax, and various acids. 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 patent for a mixture for controlling flammability issued to Obadiah Wyld in 1735, [4] and the first scientific exploration of controlling flammability, which was undertaken by Joseph Louis Gay-Lussac in 1821. [4]

Developments since WWII

Research on fire-retardant polymers was bolstered by the need for new types of synthetic polymers in World War II. The combination of a halogenated paraffin and antimony oxide was found to be successful as a fire retardant for canvas tenting. Synthesis of polymers, such as polyesters, with fire retardant monomers were also developed around this time. [5] Incorporating flame-resistant additives into polymers became a common and relatively cheap way to reduce the flammability of polymers, [6] while synthesizing intrinsically fire-resistant polymers has remained a more expensive alternative, although the properties of these polymers are usually more efficient at deterring combustion. [4]

Polymer combustion

General mechanistic scheme

Traditional polymers decompose under heat and produce combustible products; thus, they are able to originate and easily propagate fire (as shown in Figure 1).

Figure 1: A general scheme of polymer combustion. Disfig1.svg
Figure 1: A general scheme of polymer combustion.

The combustion process begins when heating a polymer yields volatile products. If these products are sufficiently concentrated, within the flammability limits, and at a temperature above the ignition temperature, then combustion proceeds. As long as the heat supplied to the polymer remains sufficient to sustain its thermal decomposition at a rate exceeding that required to feed the flame, combustion will continue. [7]

Purpose and methods of fire-retardant systems

The purpose is to control heat below the critical level. To achieve this, one can create an endothermic environment, produce non-combustible products, or add chemicals that would remove fire-propagating radicals (H and OH), to name a few. These specific chemicals can be added into the polymer molecules permanently (see Intrinsically Fire-Resistant Polymers) or as additives and fillers (see Flame-Retardant Additives and Fillers). [7]

Role of oxygen

Oxygen catalyzes the pyrolysis of polymers at low concentration and initiates oxidation at high concentration. Transition concentrations are different for different polymers. (e.g., polypropylene, between 5% and 15%). Additionally, polymers exhibit a structural-dependent relationship with oxygen. Some structures are intrinsically more sensitive to decomposition upon reaction with oxygen. The amount of access that oxygen has to the surface of the polymer also plays a role in polymer combustion. Oxygen is better able to interact with the polymer before a flame has actually been ignited. [7]

Role of heating rate

In most cases, results from a typical heating rate (e.g. 10℃/min for mechanical thermal degradation 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 evaporation of the products. [7]

Role of pressure

Volatile products are removed more efficiently under low pressure, which means the stability of the polymer might have been compromised. Decreased pressure also slows down decomposition of high boiling products. [7]

Intrinsically fire-resistant polymers

The polymers that are most efficient at resisting combustion are those that are synthesized as intrinsically fire-resistant. However, these types of polymers can be difficult as well as costly to synthesize. Modifying different properties of the polymers can increase their intrinsic fire-resistance; increasing rigidity or stiffness, the use of polar monomers, and/or hydrogen bonding between the polymer chains can all enhance fire-resistance. [8]

Linear, single-stranded polymers with cyclic aromatic components

Most intrinsically fire-resistant polymers are made by incorporation of aromatic cycles or heterocycles, which lend rigidity and stability to the polymers. [9] Polyimides, polybenzoxazoles (PBOs), polybenzimidazoles, and polybenzthiazoles (PBTs) are examples of polymers made with aromatic heterocycles (Figure 2).

Figure 2: Different fire-resistant polymers made with aromatic heterocycles. Beckysfigure2b.png
Figure 2: Different fire-resistant polymers made with aromatic heterocycles.


Polymers made with aromatic monomers have a tendency to condense into chars upon combustion, decreasing the amount of flammable gas that is released. Syntheses of these types of polymers generally employ prepolymers which are further reacted to form the fire-resistant polymers. [10]

Ladder polymers

Ladder polymers are a subclass of polymers made with aromatic cycles or heterocycles. Ladder polymers generally have one of two types of general structures, as shown in Figure 3.

Figure 3: Two representative structures of different types of ladder polymers. Beckysfigure3b.gif
Figure 3: Two representative structures of different types of ladder polymers.


One type of ladder polymer links two polymer chains with periodic covalent bonds. [11] In another type, the ladder polymer consists of a single chain that is double-stranded. Both types of ladder polymers exhibit good resistance to decomposition from heat because the chains do not necessarily fall apart if one covalent bond is broken. However, this makes the processing of ladder polymers difficult because they are not easily melted. These difficulties are compounded because ladder polymers are often highly insoluble.

Inorganic and semiorganic polymers

Inorganic and semiorganic polymers often employ silicon-nitrogen, boron-nitrogen, and phosphorus-nitrogen monomers. The non-burning characteristics of the inorganic components of these polymers contribute to their controlled flammability. For example, instead of forming toxic, flammable gasses in abundance, polymers prepared with incorporation of cyclotriphosphazene rings give a high char yield upon combustion. [3] Polysialates (polymers containing frameworks of aluminum, oxygen, and silicon) are another type of inorganic polymer that can be thermally stable up to temperatures of 1300-1400 °C. [12]

Flame-retardant additives and fillers

Additives are divided into two basic types depending on the interaction of the additive and polymer. [1] Reactive flame retardants are compounds that are chemically built into the polymer. They usually contain heteroatoms. Additive flame retardants, on the other hand, are compounds that are not covalently bound to the polymer; the flame retardant and the polymer are just physically mixed together. Only a few elements are being widely used in this field: aluminum, phosphorus, nitrogen, antimony, chlorine, bromine, and in specific applications magnesium, zinc and carbon. 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. [13]

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. [14] 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 (aluminum trihydrate, or ATH, magnesium hydroxide, or MDH, and boehmite), metal oxides and salts (zinc borate and zinc oxide, zinc hydroxystannate), as well as expandable graphite 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. [15] [16] Further handbooks on these topics are [17] [18] A good example for a very efficient phosphorus-based flame retardant system acting in the gas and condensed phases is aluminium diethyl phosphinate 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). [19]

Natural fiber-containing composites

Besides providing satisfactory mechanical properties and renewability, natural fibers are easier to obtain and much cheaper than man-made materials. Moreover, they are more environmentally friendly. [20] Recent research focuses on application of different types of fire retardants during the manufacturing process as well as applications of fire retardants (especially intumescent coatings) at the finishing stage. [20]

Nanocomposites

Nanocomposites have become a hotspot in the research of fire-safe polymers because of their relatively low cost and high flexibility for multifunctional properties. [21] Gilman and colleagues did the pioneering work by demonstrating the improvement of fire-retardancy by having nanodispersed montmorillonite clay in the polymer matrix. Later, organomodified clays, TiO2 nanoparticles, silica nanoparticles, layered double hydroxides, carbon nanotubes and polyhedral silsesquioxanes were proved to work as well. [21] Recent research has suggested that combining nanoparticles with traditional fire retardants (e.g., intumescents) or with surface treatment (e.g., plasma treatment) effectively decreases flammability. [21]

Problems with additives and fillers

Although effective at reducing flammability, flame-retardant additives and fillers have disadvantages as well. Their poor compatibility, high volatility and other deleterious effects can change properties of polymers. Besides, addition of many fire-retardants produces soot and carbon monoxide during combustion. Halogen-containing materials cause even more concerns on environmental pollution. [1] [22]

See also

Related Research Articles

Fire Rapid oxidation of a material

Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products. Fire is hot because the conversion of the weak double bond in molecular oxygen, O2, to the stronger bonds in the combustion products carbon dioxide and water releases energy (418 kJ per 32 g of O2); the bond energies of the fuel play only a minor role here. At a certain point in the combustion reaction, called the ignition point, flames are produced. The flame is the visible portion of the fire. Flames consist primarily of carbon dioxide, water vapor, oxygen and nitrogen. If hot enough, the gases may become ionized to produce plasma. Depending on the substances alight, and any impurities outside, the color of the flame and the fire's intensity will be different.

Polyvinyl chloride Synthetic plastic polymer

Polyvinyl chloride is the world's third-most widely produced synthetic polymer of plastic. About 40 million tons of PVC are produced each year.

Polyurethane Polymer composed of a chain of organic units joined by carbamate (urethane) links

Polyurethane refers to a class of polymers composed of organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane is produced from a wide range of starting materials. This chemical variety produces polyurethanes with different chemical structures leading to many different applications. These include rigid and flexible foams, varnishes and coatings, adhesives, electrical potting compounds, and fibers such as spandex and PUL. Foams are the largest application accounting for 67% of all polyurethane produced in 2016.

The autoignition temperature or kindling point of a substance is the lowest temperature in which it spontaneously ignites in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion. The temperature at which a chemical ignites decreases as the pressure is increased.

Polybutylene terephthalate Chemical compound

Polybutylene terephthalate (PBT) is a thermoplastic engineering polymer that is used as an insulator in the electrical and electronics industries. It is a thermoplastic (semi-)crystalline polymer, and a type of polyester. PBT is resistant to solvents, shrinks very little during forming, is mechanically strong, heat-resistant up to 150 °C and can be treated with flame retardants to make it noncombustible. It was developed by Britain's Imperial Chemical Industries (ICI).

Flame retardant Substance applied to items to slow burning or delay ignition

The term flame retardants subsumes a diverse group of chemicals which are added to manufactured materials, such as plastics and textiles, and surface finishes and coatings. Flame retardants are activated by the presence of an ignition source and are intended to prevent or slow the further development of ignition by a variety of different physical and chemical methods. They may be added as a copolymer during the polymerisation process, or later added to the polymer at a moulding or extrusion process or applied as a topical finish. Mineral flame retardants are typically additive while organohalogen and organophosphorus compounds can be either reactive or additive.

Fire retardant Substance reducing flammability

A fire retardant is a substance that is used to slow down or stop the spread of fire or reduce its intensity. This is commonly accomplished by chemical reactions that reduce the flammability of fuels or delay their combustion. Fire retardants may also cool the fuel through physical action or endothermic chemical reactions. Fire retardants are available as powder, to be mixed with water, as fire-fighting foams and fire-retardant gels. Fire retardants are also available as coatings or sprays to be applied to an object.

Chlorendic acid Chemical compound

Chlorendic acid, or 1,4,5,6,7,7-hexachlorobicyclo[2.2.1]-hept-5-ene-2,3-dicarboxylic acid, is a chlorinated hydrocarbon used in the synthesis of some flame retardants and polymers. It is a common breakdown product of several organochlorine insecticides.

Silicone rubber is an elastomer composed of silicone—itself a polymer—containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers are widely used in industry, and there are multiple formulations. Silicone rubbers are often one- or two-part polymers, and may contain fillers to improve properties or reduce cost. Silicone rubber is generally non-reactive, stable, and resistant to extreme environments and temperatures from −55 to 300 °C while still maintaining its useful properties. Due to these properties and its ease of manufacturing and shaping, silicone rubber can be found in a wide variety of products, including voltage line insulators; automotive applications; cooking, baking, and food storage products; apparel such as undergarments, sportswear, and footwear; electronics; medical devices and implants; and in home repair and hardware, in products such as silicone sealants.

Low smoke zero halogen or low smoke free of halogen is a material classification typically used for cable jacketing in the wire and cable industry. LSZH cable jacketing is composed of thermoplastic or thermoset compounds that emit limited smoke and no halogen when exposed to high sources of heat.

Fire-retardant fabric Flame retardant fabric

Fire-retardant fabrics are textiles that are more resistant to fire than others through chemical treatment of flame-retardant or manufactured fireproof fibers.

Nylon 66 is a type of polyamide or nylon. It, and nylon 6, are the two most common for textile and plastic industries. Nylon 66 is made of two monomers each containing 6 carbon atoms, hexamethylenediamine and adipic acid, which give nylon 66 its name.

Filler (materials)

Filler materials are particles added to resin or binders that can improve specific properties, make the product cheaper, or a mixture of both. The two largest segments for filler material use is elastomers and plastics. Worldwide, more than 53 million tons of fillers are used every year in application areas such as paper, plastics, rubber, paints, coatings, adhesives, and sealants. As such, fillers, produced by more than 700 companies, rank among the world's major raw materials and are contained in a variety of goods for daily consumer needs. The top filler materials used are ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), kaolin, talc, and carbon black. Filler materials can affect the tensile strength, toughness, heat resistance, color, clarity etc. A good example of this is the addition of talc to polypropylene. Most of the filler materials used in plastics are mineral or glass based filler materials. Particulates and fibers are the main subgroups of filler materials. Particulates are small particles of filler which are mixed in the matrix where size and aspect ratio are important. Fibers are small circular strands that can be very long and have very high aspect ratios.

Ammonium polyphosphate Chemical compound

Ammonium polyphosphate is an inorganic salt of polyphosphoric acid and ammonia containing both chains and possibly branching. Its chemical formula is [NH4 PO3]n(OH)2 showing that each monomer consists of an orthophosphate radical of a phosphorus atom with three oxygens and one negative charge neutralized by an ammonium cation leaving two bonds free to polymerize. In the branched cases some monomers are missing the ammonium anion and instead link to three other monomers.

Polymer stabilizers are chemical additives which may be added to polymeric materials, such as plastics and rubbers, to inhibit or retard their degradation. Common polymer degradation processes include oxidation, UV-damage, thermal degradation, ozonolysis, combinations thereof such as photo-oxidation, as well as reactions with catalyst residues, dyes, or impurities. All of these degrade the polymer at a chemical level, via chain scission, uncontrolled recombination and cross-linking, which adversely affects many key properties such as strength, malleability, appearance and colour.

The limiting oxygen index (LOI) is the minimum concentration of oxygen, expressed as a percentage, that will support combustion of a polymer. It is measured by passing a mixture of oxygen and nitrogen over a burning specimen, and reducing the oxygen level until a critical level is reached.

A thermoset polymer matrix is a synthetic polymer reinforcement where polymers act as binder or matrix to secure in place incorporated particulates, fibres or other reinforcements. They were first developed for structural applications, such as glass-reinforced plastic radar domes on aircraft and graphite-epoxy payload bay doors on the Space Shuttle.

A high-refractive-index polymer (HRIP) is a polymer that has a refractive index greater than 1.50.

Triphosphorus pentanitride Chemical compound

Triphosphorus pentanitride is an inorganic compound with the chemical formula P3N5. Containing only phosphorus and nitrogen, this material is classified as a binary nitride. While it has been investigated for various applications this has not led to any significant industrial uses. It is a white solid, although samples often appear colored owing to impurities.

Aluminium diethyl phosphinate Chemical compound

Aluminium diethyl phosphinate is a chemical compound with formula Al(C
4H
10O
2P)3. It decomposes above 300 °C.

References

  1. 1 2 3 4 5 6 Zhang, H. Fire-Safe Polymers and Polymer Composites, Federal Aviation Administration technical report; U.S. Department of Transportation: Washington, D.C., 2004.
  2. ↑ Sarkos, C. P. The Effect of Cabin Materials on Aircraft Postcrash Fire Survivability. Technical Papers of the Annual Technical Conference1996,54 (3), 3068-3071.
  3. 1 2 3 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. doi : 10.1002/pola.1993.080311109
  4. 1 2 3 4 Hindersinn, R. R. Historical Aspects of Polymer Fire Retardance. 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
  5. ↑ Robitschek, P.; Bean, C. T. Flame-Resistant Polyesters from Hexachlorocyclopentadiene. Ind. Eng. Chem.1954,46 (8), 1628-1632. doi : 10.1021/ie50536a034
  6. ↑ Connolly, W. J.; Thornton, A. M. Aluminum Hydrate Filler in Polyester Systems. Mod. Plastics1965,43 (2), 154-202.
  7. 1 2 3 4 5 Camino, G.; Costa, L.; Luda di Cortemiglia, M. P. Overview of Fire Retardant Mechanisms. Polym. Degrad. Stabil.1991,33 (2), 131-154. [ dead link ] doi : 10.1016/0141-3910(91)90014-I
  8. ↑ Frazer, A.H. High Temperature Resistant Polymers; John Wiley & Sons: New York, 1968. doi : 10.1002/app.1969.070130822
  9. ↑ Mallakpour, S. E.; Hajipour, A.-R.; Mahdavian, A.-R.; Khoee, S. Synthesis and characterization of novel optically active and flame-retardant heterocyclic polyimides. J. Appli. Polym. Sci.2000,76 (2), 240-248. doi : 10.1002/(SICI)1097-4628(20000411)76:2<240::AID-APP13>3.0.CO;2-A
  10. ↑ Akinseye, T. D.; Harruna, I. I.; Bota, K. B. Processable Precursors .4. Synthesis and Characterization of Processable Precursors to Poly (Phenylene Bixbenzoxazoles). Polymer1997,38 (10), 2507-2513. [ dead link ] doi : 10.1016/S0032-3861(96)00778-1
  11. ↑ Sirkecioglu, O.; Tunca, A. A.; Talinli, N.; Akar, A. Ladder type polymers from dihydroxyaromatic compounds and dialdehydes. Angew. Makrom. Chem.1999,271 (1), 8-10. ISSN   0003-3146
  12. ↑ Barbosa, V. F. F.; MacKenzie, K. J. D.; Thaumaturgo, C. Synthesis and characterization of materials based on inorganic polymers of alumina and silica: sodium polysialate polymers. Int. J. Inorg. Mater.2000,2 (4), 309-317. doi : 10.1016/S1466-6049(00)00041-6
  13. ↑ Troitzsch, J.H. Flame retardants. Demands and innovations. 5th International SKZ Conference on Flame Retardant Plastics, Shanghai, China, 21 March 2014
  14. ↑ Lewin, M., Weil, E. Mechanisms and modes of action in flame retardancy of polymers, p. 31 f., in Fire retardant materials, Horrocks, R., Price, D. Ed., Woodhead Publishing, 2004
  15. ↑ Bourbigot, S., Le Bras, M. Flame retardants, p. 133 f. and Eckel, T. Flame retarded plastics, p. 158 f. in Plastics flammability handbook, 3rd Ed., Troitzsch, J. Ed., Hanser Publishers, Munich, 2004
  16. ↑ Weil, E., Levchik S. Flame retardants for plastics and textiles. Practical applications. Hanser Publishers, Munich, 2009
  17. ↑ Wilkie, C., Morgan, A. Fire retardancy of organic materials, 2nd Ed., CRC Press, 2010
  18. ↑ Morgan, A., Wilkie, C. Non halogenated flame retardant handbook, Scrivener Publishing, Wiley, 2014.
  19. ↑ Huang, K.J., Hörold, S., Dietz, M., Schmitt, E. Phosphinates as flame retardants for plastics in electronics. 1st International SKZ Conference on Flame Retardant Plastics, Shanghai, China, 21 September 2009
  20. 1 2 Kozlowski, R.; Wladyka-Przybylak, M. Flammability and fire resistance of composites reinforced by natural fibers. Polymer Advan. Technol.2008,19 (6), 446-453. doi : 10.1002/pat.1135
  21. 1 2 3 Bourbigot, S.; Duquesne, S.; Jama C. Polymer Nanocomposites: How to Reach Low Flammability? Macromol. Symp.2006,233 (1), 180-190. doi : 10.1002/masy.200650123
  22. ↑ Porter, D.; Metcalfe, E.; Thomas, M. J. K. Nanocomposite Fire Retardants – A Review. Fire Mater.2000,24 (1), 45-52. doi : 10.1002/(SICI)1099-1018(200001/02)24:1<45::AID-FAM719>3.0.CO;2-S
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.