Flame retardants generally impart their properties to plastics in the condensed or the gas phase. In the condensed phase, the additive can remove thermal energy from the substrate by functioning as a heat sink or by participating in char formation to form a barrier against heat and mass transfer. The additive can also provide flame retardancy by conduction, evaporation, or mass dilution or by participating in endothermic chemical reactions. Char-forming systems, also called intumescent systems, form a foamy, porous protecting barrier on the plastic to shield it from further pyrolysis and combustion. Most intumescent systems require an acid source (catalyst), a char-forming compound (carbonific), and a gas-evolving compound (spumific). In a typical system, a phosphorus compound promotes the charring of a substrate (typically a carbon-oxygen compound or oxygen-containing polymer) and this char is foamed by gases released during decomposition of a nitrogen compound. The acid component is typically phosphoric acid or a suitable derivative such as ammonium polyphosphate. Typical carbonifics include pentaerythritol and other polyols. Common spumifics include urea, melamine, and dicyandiamide. Commercial intumescent systems containing the catalyst, carbonific, and spumific are available.

A coordinated sequence of chemical and physical reactions is necessary for a good intumescent coating to form. The timing of acid release, degradation of the carbonific, and evolution of gas must be closely coupled. In addition, the viscosity of the mass while these processes occur must be such that the small bubbles produced result in a multicellular char that eventually gels and solidifies. Polyamides [5] and polyolefins, such as polypropylene (PP) [6], are some of the resins where additives promoting intumescence have been found to be effective flame retardants.

Other additives function as flame retardants in the condensed phase, not by forming an intumescent layer, but by depositing a surface coating which insulates the polymer from the heat source and retards the evolution of additional fuel. Some silicone flame retardants for polyolefins are thought to deposit silicon dioxide (sand) on the polymer surface [7].

Resorcinol diphenylphosphate (RDP) is an organophosphate which provides flame retardancy in the condensed phase by a unique mechanism. RDP apparently catalyzes the Fries rearrangement of several resins (PC, PC/ABS, PPO) to give phenolic-containing decomposition products. These products can undergo transesterification with RDP to form non-volatile, non-combustible, phosphorus species on the surface of the resin [8]. This “surface barrier” may inhibit the diffusion of combustible gases to the flame. Aromatic sulfonate salts are also thought to impart flame retardancy to PC by catalyzing the Fries rearrangement [9].

The largest volume flame retardant, alumina trihydrate (ATH), functions in the condensed phase, not as a char former or protective layer former, but as a heat sink and a source of a non-combustible gas (H2​O) for fuel dilution. ATH starts to decompose at 230 ∘C and eventually loses $34.5\%$ of its mass as water vapor. Magnesium hydroxide decomposes at a higher temperature (340 ∘C) with a $31\%$ mass loss. ATH and magnesium hydroxide also decompose endothermically and remove heat from the condensed phase which decreases

the rate of polymer decomposition. The enthalpy of decomposition for alumina trihydrate is $+280\text{cal/mole}$ while that for magnesium hydroxide is $+328\text{cal/mole}$. Substantial quantities of these additives are needed to impart flame retardancy to a resin. A resin formulation containing $40\text{ to }60\%$ (by weight) ATH is typical.

Melamine and some melamine derivatives appear to provide flame retardancy through a number of different mechanisms [10]. Melamine sublimes rather than melts and can cool (heat sink) a polymer such as polyethylene ($\text{PE}$) when the plastic is subjected to heat. Melamine vapor, having a high nitrogen content, can act as an inert diluent in the flame. Once in the flame, the melamine can dissociate, providing another heat sink. Melamine cyanurate also functions as a heat sink and an inert gas source in polyamides, while melamine polyphosphate provides fire retardancy in an intumescent manner [11].

The styrenic polymers melt, drip, and depolymerize to form volatile monomers, dimers, and trimers when exposed to heat. Typically, these polymers require a flame retardant that functions in the gas phase rather than the condensed phase. A possible exception is polystyrene foam where hexabromocyclododecane ($\text{HBCD}$) is a common flame retardant additive. $\text{HBCD}$ is thought to promote a decrease in the polymer molecular weight and viscosity, resulting in the foam shrinking away from the combustion source [12].

Flame retardants operating in the gas phase interrupt the combustion chemistry of the fire. During combustion, polymer fragments interact with oxygen and other highly reactive species in a chain reaction to form oxygen radicals, hydroxyl radicals, and hydrogen radicals. Certain plastic additives, mainly those containing halogen or phosphorus, can chemically interact with these radicals to form less energetic species and, in effect, interrupt the chain propagation necessary for fire initiation or continuation.

Some important reactions in the combustion of hydrocarbons involve reactions $12.1,12.2$ and $12.3$.

If the hydrogen-oxygen reaction can be interrupted, the combustion of the fuel can be retarded [13]. Halogen halides are efficient flame quenchers and their effectiveness follows the order $\text{HI}>\text{HBr}>\text{HCl}>\text{HF}$ on a molar basis. The flame quenching effects are thought to be caused by inhibition of chain branching reactions $12.1$ and $12.2$ by the action of halogen halide to produce the less reactive halogen radical (reactions $12.4$ and $12.5$).

Many chlorinated and brominated flame retardants, especially aliphatic types, release $\text{HX}$ upon heating. However, in practice, it has been found that improved flame retardancy often

results from halogenated materials when metal oxides, such as antimony oxide, are added. In World War II, a combination of chlorinated paraffin, antimony oxide, and binder was used as a fire retardant and waterproofing treatment for canvas. The use of organohalogen-antimony oxide is a true case of synergism because antimony oxide by itself imparts minimal flame retardancy to most materials.

Many investigators agree that the formation of volatile antimony species (antimony trihalide, antimony oxyhalide) is responsible in great part for the synergistic effect. The action of halogen-antimony oxide has been explained on the basis of two separate flame inhibition effects [14]. The halogen compound decomposes on heating to form $\text{HX}$, which reacts with antimony oxide to form volatile antimony halide. The antimony halide can interrupt the combustion process by removing hydrogen radicals in the flame by a multi-step process. The antimony oxide catalyzed recombination of hydrogen radicals presents the second mode of inhibiting combustion.

Organohalogen compounds that do not generate $\text{HX}$ upon heating rely on the polymer to initiate the reaction with antimony oxide. For example, high impact polystyrene ($\text{HIPS}$) formulated with decabromodiphenyloxide and antimony oxide can generate hydrocarbon radicals at high temperatures. The radical can react with the organohalogen to initiate formation of antimony tribromide, leading to combustion inhibition [15] (reactions $12.13\text{–}12.16$).

Some phosphorus flame retardants can also be effective in the gas phase. Volatile molecules such a triphenylphosphate can fragment in the flame to give small, phosphorus-containing radicals that can combine with hydrogen radicals to help quench the flame.