Fire-safe polymers

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]

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]

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.

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]

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]

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]

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]

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]

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.

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 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.

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 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]

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 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]

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]