## 4 Fire Retardation of Styrenic Polymers with Phosphorous-Containing Compounds  
In the past, HBCD and decabromodiphenyl oxide (DBDPO) were the most popular additives used in industry to improve the fire retardance of PS [14]. Hydrogen bromide, liberated over a temperature range specific for each of these formulations, acts as an effective free radicals scavenging agent during combustion. The negative impacts of halogenated formulations and hydrogen halides on the environment and human health prompted a search for more benign options [5,34]. The P-containing compounds, with lower toxicity levels, are as important as the halogenated FRs, and to a certain extent, they are as effective in controlling the combustion and flammability of PS [5,19,20,34].  
A large number of P-containing compounds having different chemical environments of the P atom(s) are already available on the market [1,5,6,34]. These compounds, when thermally degraded, are shown to produce phosphoric acid and similar species that could inhibit the combustion process in the condensed phase [34,35]. Depending on the chemical nature of the phosphorus compounds, they can also be active in the gaseous phase: the PO\({}^{\bullet}\) radical can be considered as one of the most important species, acting as a scavenging agent [6,34,35,43]. Some common P-containing fire retardant compositions for the styrene polymers include red phosphorus, phosphine oxides, inorganic phosphates, organophosphates, phosphonates, chlorophosphates, etc. [5,6,22,34]. Furthermore, some blends of P-containing compounds with other FRs exhibit a certain degree of synergism similar to the one observed for halogen/antimony combinations [2,5,6,34]. As phosphorus (P-) and nitrogen (N-) FRs, when combined, can also demonstrate synergism, the P/N-containing compounds, such as phosphorylaminoesters and phosphoramides, were found to enhance the polymers' fire-retardant properties [21,44,45].  
The P-containing compounds used as FRs for styrene polymers include both additive and reactive types [1,6,21]. The efficiency, mode of action, advantages, and disadvantages of each type are well-documented and discussed elsewhere [1,5,6,34,35]. Generally, the organophosphorus compounds have good thermal and hydrolytic stabilities [1,5,6,21,34,35]. The efficiency of P-based compounds often depends on a number of factors: chemical environment and the oxidation state of the P atom; volatility; nature of the degradation products formed upon the thermolysis; etc. [6,34,35,46,47,48]. The condensed-phase activity of P-compounds within styrene systems predominantly involves a char formation, which is facilitated by the dehydration of the polymeric structure, leading to cyclisation, cross-linking, aromatisation, and graphitisation [6]. The cross-linking can also be induced by the decomposition by-products of the phosphorus compounds. In the gaseous phase, these compounds act mainly by scavenging free radicals such as H\({}^{\bullet}\) and HO\({}^{\bullet}\), thus preventing further oxidation and decomposition of styrene polymers [6,34].  
Thus, it is clear that phosphorus-containing compounds can be as effective as, and in some cases, superior to many new or tried-and-tested FRs. Given the renewed interest in P-containing FRs, especially as the alternatives to environmentally harmful halogenated compounds, their desirable attributes are explored through the current review.