## 4 Fire Retardation of Styrenic Polymers with Phosphorous-Containing Compounds
### Reactive Modifications of Styrenic Polymers
#### 4.2.2 DOPO Derivatives (P+5)
The flammability studies indicated the increase in the thermal stability of the copolymers with 10-25 wt% of AC\({}_{2}\)NP\({}_{2}\) monomeric units. The PHRR values, measured by MCC, in the chemically modified PS were significantly reduced and dependent on the AC\({}_{2}\)NP\({}_{2}\) content. The PHRR was dropped by 36%, 41%, and 42% in the syrenic copolymers with 10 wt%, 20 wt%, and 25 wt% of AC\({}_{2}\)NP\({}_{2}\), respectively [119]. However, the copolymer with 30 wt% of AC\({}_{2}\)NP\({}_{2}\) had the lower values of T\({}_{1}\) and T\({}_{\text{max}}\), the higher values of PHRR, and the greater mean mass loss rate (MMLR) compared with those measured in other copolymers. These results revealed that when the amount of the FR incorporated into the polymer exceeds a certain limit, it does not necessarily improve its fire performance, but it may have a negative impact on its properties. Meanwhile, the St/AC\({}_{2}\)NP\({}_{2}\) copolymers exhibited higher TG char residue under the air atmosphere than that formed under the inert conditions. For example, the copolymer with 10 wt% of AC\({}_{2}\)NP\({}_{2}\) had nearly 10.00 wt% of char residues in the air atmosphere, at 500 \({}^{\circ}\)C, whereas under the nitrogen atmosphere, the char residue was only 2.05 wt%. This indicated an influence of oxygenated atmosphere on the char formation and thermal stability of P-containing syrenic copolymers [119]. In another study, [PERSON] et al. (2017) used the same P-N comonomer, AC\({}_{2}\)NP\({}_{2}\), in the amounts ranging from 5 to 20 g for the preparation of styrene-based copolymer via the seeded emulsion polymerisation (Figure 26) [107]. During the emulsion polymerisation, water was used as the emulsion medium along with the emulsifiers, sodium lauryl benzene sulfate and sodium \(p\)-styrene sulfonate. The fire-retarding mechanism of the AC\({}_{2}\)NP\({}_{2}\) units was determined from various characterisation and flammability studies, clearly highlighting the synergistic P-N effects. The impact of this synergism strengthened as the content of AC\({}_{2}\)NP\({}_{2}\) in the copolymer grew up to 20 g [107]. Figure 24: Copolymerisation of St with 2-[(6-oxido-6H-dibenzo[c,e][1,2]oxaphosphphin-6-yl)-methoxy]methyl phosphate N-methyl acrylamide (DMPMA) initiated by azobisisobutyronitrile (AlBN) [48]. Table 2 summarises the data obtained from various flammability studies of the PS homopolymer and the copolymer of St with 10 wt% of AC\({}_{2}\)NP\({}_{2}\) prepared through the solution and emulsion polymerisation techniques [107; 119]. The polymeric samples prepared in the solution or emulsion showed higher char residue (CR) under the air atmosphere than that produced in the inert atmosphere. Similarly, other thermal and fire-retarding properties of the polymers have been increased after the chemical incorporation of AC\({}_{2}\)NP\({}_{2}\). Moreover, the analysis of the flammability data indicated that the emulsion polymerised sytrenic materials had better fire-retarding efficiency than that of the polymers prepared in solution. Flammability studies revealed that during heating, advanced pyrolysis of the St/AC\({}_{2}\)NP\({}_{2}\) copolymer occurred due to the breakdown of the phosphate bond, thus leading to the dehydration and degradation of AC\({}_{2}\)NP\({}_{2}\)[107]. The phosphoric acid generated by AC\({}_{2}\)NP\({}_{2}\) amounts at high temperatures produced polyphosphoric acid and polyphosphate, which further participated in the formation of protective layers, boosting the condensed phase activity of FR groups. These layers prevented the release and diffusion of flammable gases. The authors also pointed out at the gaseous phase the fire-retarding effects of the AC\({}_{2}\)NP\({}_{2}\) groups [107]. Figure 26: Emulsion polymerisation of St with AC\({}_{2}\)NP\({}_{2}\)[107]. Figure 25: The Kabachnik–Fields reaction to synthesise ethyl-\(N\),\(N\)-tetramethyl-bis(phosphonate)-bis(methylene)amine acrylate (AC\({}_{2}\)NP\({}_{2}\)) [107; 119].