Fire-safe polymers arepolymers that are resistant todegradation 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 afire is compromised, increasingfire risk. In fact, some studies report that about 20% of victims of airplane crashes are killed not by the crash itself but by ensuingfires.[2] Fire-safepolymers also find application asadhesives in aerospace materials,[3]insulation forelectronics,[3] and in military materials such as canvas tenting.[4]
Some fire-safepolymers naturally exhibit an intrinsic resistance todecomposition, while others are synthesized by incorporating fire-resistant additives and fillers. Current research in developing fire-safepolymers is focused on modifying various properties of thepolymers such as ease ofignition, rate of heat release, and the evolution of smoke and toxic gases.[1] Standard methods for testingpolymerflammability vary among countries; in the United States common fire tests include the UL 94 small-flame test, the ASTM E 84Steiner Tunnel, and the ASTM E 622National Institute of Standards and Technology (NIST) smoke chamber.[1] Research on developing fire-safepolymers with more desirable properties is concentrated at theUniversity of Massachusetts Amherst and at theFederal Aviation Administration where a long-term research program on developing fire-safepolymers 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 ofpolymer science and engineering research.[1]
Controlling theflammability of different materials has been a subject of interest since 450 B.C. whenEgyptians attempted to reduce theflammability of wood by soaking it inpotassium aluminum sulfate (alum). Between 450 B.C. and the early 20th century, other materials used to reduce the flammability of different materials included mixtures ofalum andvinegar;clay andhair;clay andgypsum;alum,ferrous sulfate, andgypsum; andammonium chloride,ammonium phosphate,borax, and variousacids. 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 firstpatent for a mixture for controlling flammability issued to Obadiah Wyld in 1735,[4] and the first scientific exploration of controlling flammability, which was undertaken byJoseph Louis Gay-Lussac in 1821.[4]
Research on fire-retardantpolymers was bolstered by the need for new types ofsynthetic polymers inWorld War II. The combination of ahalogenatedparaffin andantimony oxide was found to be successful as afire retardant for canvas tenting. Synthesis ofpolymers, such aspolyesters, withfire retardant monomers were also developed around this time.[5] Incorporating flame-resistant additives intopolymers became a common and relatively cheap way to reduce the flammability ofpolymers,[6] while synthesizing intrinsically fire-resistantpolymers has remained a more expensive alternative, although the properties of thesepolymers are usually more efficient at deterringcombustion.[4]
Traditionalpolymersdecompose under heat and produce combustible products; thus, they are able to originate and easily propagatefire (as shown in Figure 1).
Thecombustion process begins when heating apolymer yieldsvolatile products. If these products are sufficiently concentrated, within theflammability limits, and at a temperature above the ignition temperature, thencombustion proceeds. As long as the heat supplied to thepolymer remains sufficient to sustain itsthermal decomposition at a rate exceeding that required to feed the flame,combustion will continue.[7]
The purpose is to control heat below the critical level. To achieve this, one can create anendothermic environment, produce non-combustible products, or add chemicals that would remove fire-propagatingradicals (H and OH), to name a few. These specific chemicals can be added into thepolymer molecules permanently (see Intrinsically Fire-Resistant Polymers) or as additives and fillers (see Flame-Retardant Additives and Fillers).[7]
Oxygen catalyzes thepyrolysis ofpolymers at low concentration and initiatesoxidation at high concentration. Transition concentrations are different for differentpolymers. (e.g.,polypropylene, between 5% and 15%). Additionally,polymers exhibit a structural-dependent relationship withoxygen. Some structures are intrinsically more sensitive todecomposition upon reaction withoxygen. The amount of access thatoxygen has to the surface of thepolymer also plays a role inpolymercombustion.Oxygen is better able to interact with thepolymer before a flame has actually been ignited.[7]
In most cases, results from a typical heating rate (e.g. 10°C/min for mechanicalthermal 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 toevaporation of the products.[7]
Volatile products are removed more efficiently under low pressure, which means the stability of thepolymer might have been compromised. Decreased pressure also slows downdecomposition of high boiling products.[7]
Thepolymers that are most efficient at resistingcombustion are those that are synthesized as intrinsically fire-resistant. However, these types ofpolymers can be difficult as well as costly to synthesize. Modifying different properties of thepolymers can increase their intrinsic fire-resistance; increasingrigidity orstiffness, the use ofpolarmonomers, and/orhydrogen bonding between thepolymer chains can all enhance fire-resistance.[8]
Most intrinsically fire-resistantpolymers are made by incorporation of aromatic cycles or heterocycles, which lendrigidity and stability to thepolymers.[9] Polyimides, polybenzoxazoles (PBOs), polybenzimidazoles, and polybenzthiazoles (PBTs) are examples ofpolymers made with aromatic heterocycles (Figure 2).
Polymers made with aromatic monomers have a tendency to condense into chars uponcombustion, decreasing the amount of flammable gas that is released. Syntheses of these types ofpolymers generally employ prepolymers which are further reacted to form the fire-resistantpolymers.[10]
Ladder polymers are a subclass ofpolymers made with aromatic cycles or heterocycles. Ladderpolymers generally have one of two types of general structures, as shown in Figure 3.
One type of ladderpolymer links twopolymer chains with periodiccovalent bonds.[11] In another type, the ladderpolymer consists of a single chain that is double-stranded. Both types of ladderpolymers exhibit good resistance todecomposition from heat because the chains do not necessarily fall apart if onecovalent bond is broken. However, this makes the processing of ladderpolymers difficult because they are not easily melted. These difficulties are compounded because ladder polymers are often highlyinsoluble.
Inorganic and semiorganicpolymers often employsilicon-nitrogen,boron-nitrogen, andphosphorus-nitrogen monomers. The non-burning characteristics of theinorganic components of thesepolymers contribute to their controlledflammability. For example, instead of forming toxic, flammable gasses in abundance,polymers prepared with incorporation ofcyclotriphosphazene rings give a highchar yield uponcombustion.[3]Polysialates (polymers containing frameworks ofaluminum,oxygen, andsilicon) are another type ofinorganicpolymer that can be thermally stable up to temperatures of 1300-1400 °C.[12]
Additives are divided into two basic types depending on the interaction of the additive andpolymer.[1] Reactiveflame retardants are compounds that are chemically built into thepolymer. They usually containheteroatoms. Additiveflame retardants, on the other hand, are compounds that are notcovalently bound to thepolymer; the flame retardant and thepolymer are just physically mixed together.Only a fewelements are being widely used in this field:aluminum,phosphorus,nitrogen,antimony,chlorine,bromine, and in specific applicationsmagnesium,zinc andcarbon. 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, andboehmite), metal oxides and salts (zinc borate and zinc oxide, zinc hydroxystannate), as well asexpandable 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 isaluminium 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]
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 offire retardants during the manufacturing process as well as applications offire retardants (especiallyintumescent coatings) at the finishing stage.[20]
Nanocomposites have become a hotspot in the research of fire-safepolymers 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 nanodispersedmontmorillonite clay in the polymer matrix. Later, organomodifiedclays, TiO2nanoparticles,silicananoparticles,layered double hydroxides,carbon nanotubes and polyhedralsilsesquioxanes were proved to work as well.[21] Recent research has suggested that combiningnanoparticles with traditionalfire retardants (e.g.,intumescents) or with surface treatment (e.g., plasma treatment) effectively decreasesflammability.[21]
Although effective at reducingflammability, flame-retardant additives and fillers have disadvantages as well. Their poor compatibility, highvolatility and other deleterious effects can change properties ofpolymers. Besides, addition of many fire-retardants producessoot andcarbon monoxide duringcombustion.Halogen-containing materials cause even more concerns on environmentalpollution.[1][22]