## 4 Fire Retardation of Styrenic Polymers with Phosphorous-Containing Compounds
### Additive Modifications of Styrenic Polymers
#### 4.1.5 Functionalisation of Inorganic FRs with Organophosphorus Compounds (P\({}^{+5}\))
of P-containing FRs with N-bearing compounds can be positive or negative, resulting in either synergistic or antagonistic effects, respectively [90]. The use of P-N synergistic FRs can exhibit the condensed phase mechanism of fire retardance, thus generating more carbonaceous char, which acts as a physical barrier and stops the further degradation of a polymeric material. Additionally, P-N-containing FRs can prevent the release of flammable and toxic volatile products [91]. However, some of the P-N-containing FRs are small molecular compounds that may have a relatively poor compatibility with the polymer matrix and hence could leach out very easily from the polymer composite over time. These problems can be solved through the use of oligomeric or polymeric FRs. For example, [PERSON] et al. (2012) synthesised the P-N-containing compounds, 2-dimethylaminoethylphenylhydroxyethylacrylate phosphate (DPHP) and its oligomer PDPHP (Figure 15a) [92]. The T\({}_{\text{max}}\) values increased from 433 \({}^{\circ}\)C in the neat PS to 446 \({}^{\circ}\)C in the PS composite with the added 30 wt% of PDPHP, while the char yield, recorded by TGA, was raised to 8.7 wt%. The flammability studies suggested that the enhancement in the fire retardance is caused by the gaseous and condensed phase actions. A macromolecular P-N-based FR, noted as MNP in Figure 15b, has been developed for the protection of the EPS foam [93]. The condensed and gaseous phase capabilities of MNP were responsible for better fire retardance of the modified foam. Even though the density of the foam almost doubled as a result of this compound being added, it did not affect the thermal conductivity of the EPS containing 30 wt% MNP. Meanwhile, the compressive strength of the modified foam was increased by 15%. [PERSON] et al. (2008) carried out a comparative study of four different polymers with the added FR based on diphenyl piperazine-1,4-di-ylbis(methylphosphinate) (DPPMP, Figure 15c) [94]. It was found that DPPMP is not suitable for the polymers such as ABS, while it was very effective for protecting charring polymers, for example, polycarbonate and polybutylene terephthalate. The DPPMP additives imparted the gaseous phase fire retardance due to the presence of the CH\({}_{3}\)-P(=O)-unit [94]. Some research groups used other elements such as Si or boron (B) to improve the efficiency of P-N-containing FRs further [95; 96; 97]. The addition of these compounds can endow the synergistic effect and thereby increase the charring and enhance the thermal performances of the polymers. For example, a novel FR containing P, N, and Si was developed from \(N\)-\(\beta\)-(aminoethyl)-\(\gamma\)-aminopropyltrimethoxysilane (NTMS, Figure 16a) reacted with phosphorus acid, H\({}_{3}\)PO\({}_{3}\) (POA), and phosphoric acid, H\({}_{3}\)PO\({}_{4}\) (PA), respectively, for incorporation into the EPS foam [95]. The obtained  
Figure 15: Synthesis of PDPHP oligomer (**a**), chemical structures of macromolecular P–N-based FR, (MNP) (**b**) and diphenyl piperazine-1,4-di-ylbis(methylphosphinate) (DPPMP) (**c**) [92; 93; 94]. PDPHP: oligomer of 2-dimethylaminoethylphenylhydroxyethylacrylate phosphate. formulations were applied to the EPS by surface coating, which did not alter the key physical properties (e.g., density and compressive strength) of the foam. However, in this study, the synergism did not lead to any reduction in the amount of FR required to achieve the desired fire performance. Indeed, the lowest loading of the FR needed for the EPS to attain the V-0 rating in the UL-94 test remained relatively high, around 40 wt% [95]. Similarly, different B-containing compounds have been used as the synergistic FR additives to enhance fire safety of a range of polymers, including PS [96]. In the study carried out by [PERSON] et al. (2012), zinc borate, 2 ZnO3B2O3.3.5H2O (ZnB), and boron phosphate, BPO4 (BP) were used as the synergists of poly(4,4'-diaminodiphenylmethanephenyldichlorophosphate) (PDMPD, Figure 16b) [97].