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. 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. Fire-safe polymers also find application as adhesives in aerospace materials, insulation for electronics, and in military materials such as canvas tenting.
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. 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. 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.
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, and the first scientific exploration of controlling flammability, which was undertaken by Joseph Louis Gay-Lussac in 1821.
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. Incorporating flame-resistant additives into polymers became a common and relatively cheap way to reduce the flammability of polymers, 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.
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).
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.
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).