## 4 Fire Retardation of Styrenic Polymers with Phosphorous-Containing Compounds
### Reactive Modifications of Styrenic Polymers
#### 4.2.2 DOPO Derivatives (P+5)
Another novel monomer with the structure shown in Figure 27a, acryloxyethylphenoxy phosphorodiethylamine (AEPPA), was synthesised by the esterification reaction of phenyldichlorophosphate (PDCP) with hydroxyethyl acrylate (HEA), followed by the reaction with diethylamine [120]. The AEPPA monomer was incorporated into the sytrenic chains through the bulk polymerisation technique in a two-step process. In the first step, polymerisation was carried out with AEPPA and St in the presence of AIBN at 90 \({}^{\circ}\)C. Later, the obtained viscous mixture was moulded at 80 \({}^{\circ}\)C for 16 h. The flammability studies showed that the incorporation of 10 wt% of AEPPA increased the char residue from 1.49 wt% in PS to 7.00 wt% in the St/AEPPA copolymer. Similarly, the introduction of AEPPA units into the PS chains considerably reduced the release of combustible compounds evolved and cooled the pyrolysis zone by diluting the hot atmosphere with non-flammable products such as CO\({}_{2}\), NH\({}_{3}\), N\({}_{2}\), and P-based compounds produced by the AEPPA [121]. Meanwhile, the incorporation of AEPPA in the amounts higher than 15 wt% had no significant impact on the fire-retarding performance of the copolymer [120]. The synergistic P-N effect on the flammability of sytrenic polymers was found in a comparative study carried out by [PERSON] and [PERSON] (2012) [122]. They prepared two sytrenic copolymers, one containing only P atoms (Compound (**1**), Figure 27c), and the other containing both P and N atoms, joined via the P-N bond, (Compound (**2**), Figure 27c). The TGA data suggested that the presence of the P-N-bearing units enhanced the impact of P on the thermal stability of PS in terms of the increased value of T\({}_{\rm i}\) during thermal degradation. The polymer with the integrated moieties of Compound (**1**) had the T\({}_{\rm i}\) of 416 \({}^{\circ}\)C, while in the copolymer containing monomeric units of Compound (**2**), these temperature values were about 10 \({}^{\circ}\)C higher. The synergistic P-N effect, leading to the boost of the \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Polymer**} & **Polymerisation** & **T\({}_{\rm i}\) (\({}^{\circ}\)C)** & **T\({}_{\rm max}\) (\({}^{\circ}\)C)** & **PHRR\({}^{\prime}\)** & **PHRR\({}^{\prime\prime}\)** & **CR (wt\%)** & **LOI** \\  & **Technique** & **(N2/Air)** & **(N2/Air)** & **(W/g)** & **(kW/m\({}^{2}\))** & **(N2/Air)** & **(vol\%)** \\ \hline Neat PS & Solution & 392/321 & 418/377 & 751 & - & 0.25/1.50 & 18.5 \\ St/AC\({}_{2}\)NP\({}_{2}\) & Solution & 385/363 & 423/398 & 480 & - & 2.05/9.77 & 21.0 \\ Neat PS & Emulsion & 403/319 & 428/381 & - & 1283 & 1.94/0.33 & 18.0 \\ St/AC\({}_{2}\)NP\({}_{2}\) & Emulsion & 418/321 & 447/413 & - & 824 & 5.89/6.14 & 22.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Flammability data for PS and St/AC\({}_{2}\)NP\({}_{2}\) copolymer prepared by different polymerisation techniques [107; 119]. MCC: microcone calorimetry, PHRR: peak heat release rate. Figure 27: A chemical structure of acryloxyethylphenoxy phosphorodiethylamine (AEPPA) (**a**) and 2-((6-oxidodibenzo[c,e][1,2]oxaphosphinin-6-yl)methoxy)acryloxyethyl)chlorophosphate (PACP) (**b**); syntheses of styrene-based reactive FRs (**c**) [120; 122; 123]. condensed phase activity, was evident from the higher char yields at 600 \({}^{\circ}\)C: 16.3 wt% of the residue was obtained for the polymeric material containing units of Compound **(1)**, and 26.2 wt% of the residue was obtained for the copolymer with the groups of Compound **(2)**[122]. Similarly, the copolymers with P-N-containing moieties displayed reduced PHRR values, measured in MCC, and better fire-retarding performance compared to those containing only P-containing units. Thus, all the studies discussed above clearly demonstrated the benefits of utilising the P-N synergism and its positive impact on the thermal stability and fire retardance emanating from the incorporation of P-N-containing comonomers into the PS backbone.