Fire-safe polymers
[4] Some fire-safe polymers naturally exhibit an intrinsic resistance to decomposition, while others are synthesized by incorporating fire-resistant additives and fillers.
These early attempts found application in reducing the flammability of wood for military materials, theater curtains, and other textiles, for example.
The combination of a halogenated paraffin and antimony oxide was found to be successful as a fire retardant for canvas tenting.
[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).
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.
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.
[9] Polyimides, polybenzoxazoles (PBOs), polybenzimidazoles, and polybenzthiazoles (PBTs) are examples of polymers made with aromatic heterocycles (Figure 2).
Polymers made with aromatic monomers have a tendency to condense into chars upon combustion, decreasing the amount of flammable gas that is released.
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.
For example, instead of forming toxic, flammable gasses in abundance, polymers prepared with incorporation of cyclotriphosphazene rings give a high char yield upon combustion.
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.
[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.
[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.
[19] Besides providing satisfactory mechanical properties and renewability, natural fibers are easier to obtain and much cheaper than man-made materials.
[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.
