Page 551 - Carrahers_Polymer_Chemistry,_Eighth_Edition

514 Carraher’s Polymer Chemistry

Another class of flame retardants is minerals such as aluminum hydroxide, boron compounds,
magnesium hydroxide, and antimony trioxide.
In general, flame retardants resist fire by several mechanisms. Some, such as magnesium hydrox-

ide and aluminum hydroxide decompose when exposed to fire through endothermic means remov-

ing heat from the fire. Inert fillers such as calcium carbonate and talc dilute the portion of material

that can burn. Thus, a home made of concrete block is less apt to burn completely than a home that
is made of wood since concrete block reduces the amount of fl ammable material.
While some polymers such as PVC are not readily ignited, most organic polymers, like hydro-

carbons, will burn. Some will support combustion, such as polyolefins, styrene–butadiene rubber
(SBR), wood, and paper, when lit with a match or some other source of fl ame. The major products
for much of this combustion are carbon dioxide (or carbon monoxide if insufficient oxygen is pre-

sent) and water.
Since many polymers are used as shelter and clothing and in household furnishing, it is essen-

tial that they have good flame resistance. Combustion is a chain reaction that may be initiated
and propagated by free radicals. Since halides and phosphorus radicals couple with free radicals
produced in combustion terminating the reaction, many flame retardant are halide (halogen) or

phosphorus-containing compounds. These may be additives; external retardants sprayed onto the
material such as antimony oxide and organic bromides; or internal retardants such as tetrabro-
mophthalic anhydride that are introduced during the polymerization process so that they are part
of the polymer chain.
Fuel, oxygen, and high temperature are essential for the combustion process. Thus, polyfl uo-
rocarbons, phosphazenes, and some composites are flame resistant because they are not good

fuels. Fillers such as alumina trihydrate (ATH) release water when heated and hence reduce the
temperature of the combustion process. Compounds such sodium carbonate, which releases car-
bon dioxide when heated, shield the reactants from oxygen. Char, formed in some combustion
processes also shield the reactants from a ready source of oxygen and retard the outward dif-
fusion of volatile combustible products. Aromatic polymers, such as PS, tend to char and some
phosphorus and boron compounds catalyze char formation aiding in controlling the combustion
process.
Synergistic flame retardants such as a mixture of antimony trioxide and an organic bromo com-

pound are more effective than single flame retardants. Thus, mixtures are often employed to protect
materials and people.

Since combustion is subject to many variables, tests for flame retardancy may not correctly pre-
dict flame resistance under unusual conditions. Thus, a disclaimer stating that flame retardancy tests

do not predict performance in an actual fire must accompany all flame-retardant products. Flame
retardants, like many organic compounds, may be toxic or may produce toxic gases when burned.
Hence, care must be exercised with using fabrics or other polymers treated with fl ame retardants.
Combustion is a chain reaction that can be initiated and propagated by free radicals such as the
hydroxyl free radical. The hydroxyl radical may be produced by reaction of oxygen with macroalkyl
radicals as shown below:
R−CH 2 + O 2 R−CHO + HO
Macroalkyl Oxygen Dead polymer Hydroxyl (15.17)
radical radical

HO + R−CH 3 R−CH + H O
2
2
(15.18)
Hydroxyl Polymer Macroradical Water
Halide-containing compounds have often been used to suppress flame propagation through

reduction of free radical concentration as follows:

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