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By Alex Wilson

PVC and fluoropolymer resins are inherently flame resistant due to their halogen content, but the plasticizers added to PVC are not, so additional flame retardants have to be added to PVC for use in some wiring applications. FEP is more flame resistant than PVC, but the hydrofluoric acid released when it’s exposed to heat is more toxic than the hydrochloric acid released by PVC.

Three classes of flame retardants are commonly used in wire and cable insulation: halogenated compounds (based on bromine, fluorine, or chlorine), inorganic compounds (such as antimony), and phosphorous compounds. Among halogenated flame retardants, bromine-based compounds are more effective than chlorine-based compounds, because bromine forms a weaker bond to carbon and thus interferes more effectively with combustion. A number of brominated flame retardants are commonly added to polyolefin wire and cable insulation—either alone or mixed with an antimony compound.

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Chlorine is sometimes added to polyethylene insulation, but its presence can negatively affect the performance of the polymer, and as with PVC, it may release hydrogen chloride or dioxin in the event of a fire.

The most common inorganic flame retardants are metal hydrates, antimony compounds, and zinc borate. Metal hydrates work by introducing water to the fire; when used, they can be either compounded with the resin, or packed in around the wires as the cable is manufactured. Aluminum hydroxide is widely used in plastics such as polyethylene. At temperatures above 480 degrees Fahrenheit, the compound degrades into water and alumina, slowing flame spread or extinguishing the fire. Magnesium hydroxide is similar but degrades at a higher temperature; it is more commonly used with polypropylene.

Antimony flame retardants are generally most effective when combined with halogens. Antimony trioxide is commonly added to PVC, for example. Halogen acid, released during a fire, reacts with the antimony compound and produces char, which acts as a physical barrier to flame spread. Antimony-halogen reactions in a fire also keep oxygen from easily combining with the fuel contributed by the polymer.

Zinc borate, alone or in combination with aluminum hydroxide, is used as a flame retardant in a variety of halogen-free polymers. Phosphorous-containing flame retardants are very versatile; many different compounds are used, although the most common are phosphate esters (used in flexible PVC) and chlorinated phosphates (used in polyurethanes).

Other components used to make insulation and jacketing, like fillers, pigments, dyes, and lubricants, are generally of lesser environmental priority and concern than the ones mentioned above.